The electronic and steric effects in the stoichiometric dehydrocoupling of secondary and primary phosphine-boranes H3B·PR2H [R = 3,5-(CF3)2C6H3; p-(CF3)C6H4; p-(OMe)C6H4; adamantyl, Ad] and H3B·PCyH2 to form the metal-bound linear diboraphosphines H3B·PR2BH2·PR2H and H3B·PRHBH2·PRH2, respectively, are reported. Reaction of [Rh(L)(η(6)-FC6H5)][BAr(F)4] [L = Ph2P(CH2)3PPh2, Ar(F) = 3,5-(CF3)2C6H3] with 2 equiv of H3B·PR2H affords [Rh(L)(H)(σ,η-PR2BH3)(η(1)-H3B·PR2H)][BAr(F)4]. These complexes undergo dehydrocoupling to give the diboraphosphine complexes [Rh(L)(H)(σ,η(2)-PR2·BH2PR2·BH3)][BAr(F)4]. With electron-withdrawing groups on the phosphine-borane there is the parallel formation of the products of B-P cleavage, [Rh(L)(PR2H)2][BAr(F)4], while with electron-donating groups no parallel product is formed. For the bulky, electron rich, H3B·P(Ad)2H no dehydrocoupling is observed, but an intermediate Rh(I) σ phosphine-borane complex is formed, [Rh(L){η(2)-H3B·P(Ad)2H}][BAr(F)4], that undergoes B-P bond cleavage to give [Rh(L){η(1)-H3B·P(Ad)2H}{P(Ad)2H}][BAr(F)4]. The relative rates of dehydrocoupling of H3B·PR2H (R = aryl) show that increasingly electron-withdrawing substituents result in faster dehydrocoupling, but also suffer from the formation of the parallel product resulting from P-B bond cleavage. H3B·PCyH2 undergoes a similar dehydrocoupling process, and gives a mixture of stereoisomers of the resulting metal-bound diboraphosphine that arise from activation of the prochiral P-H bonds, with one stereoisomer favored. This diastereomeric mixture may also be biased by use of a chiral phosphine ligand. The selectivity and efficiencies of resulting catalytic dehydrocoupling processes are also briefly discussed.
The electronic apan class="Chemical">nd steric effects in the stoichiometric dehydrocoupling of secondn class="Chemical">ary and primn class="Chemical">ary phosphine-boranesH3B·PR2H [R = 3,5-(CF3)2C6H3; p-(CF3)C6H4; p-(OMe)C6H4; adamantyl, Ad] and H3B·PCyH2 to form the metal-bound lineardiboraphosphinesH3B·PR2BH2·PR2H and H3B·PRHBH2·PRH2, respectively, are reported. Reaction of [Rh(L)(η(6)-FC6H5)][BAr(F)4] [L = Ph2P(CH2)3PPh2, Ar(F) = 3,5-(CF3)2C6H3] with 2 equiv of H3B·PR2H affords [Rh(L)(H)(σ,η-PR2BH3)(η(1)-H3B·PR2H)][BAr(F)4]. These complexes undergo dehydrocoupling to give the diboraphosphine complexes [Rh(L)(H)(σ,η(2)-PR2·BH2PR2·BH3)][BAr(F)4]. With electron-withdrawing groups on the phosphine-borane there is the parallel formation of the products of B-P cleavage, [Rh(L)(PR2H)2][BAr(F)4], while with electron-donating groups no parallel product is formed. For the bulky, electron rich, H3B·P(Ad)2H no dehydrocoupling is observed, but an intermediate Rh(I) σ phosphine-borane complex is formed, [Rh(L){η(2)-H3B·P(Ad)2H}][BAr(F)4], that undergoes B-P bond cleavage to give [Rh(L){η(1)-H3B·P(Ad)2H}{P(Ad)2H}][BAr(F)4]. The relative rates of dehydrocoupling of H3B·PR2H (R = aryl) show that increasingly electron-withdrawing substituents result in faster dehydrocoupling, but also suffer from the formation of the parallel product resulting from P-B bond cleavage. H3B·PCyH2 undergoes a similar dehydrocoupling process, and gives a mixture of stereoisomers of the resulting metal-bound diboraphosphine that arise from activation of the prochiral P-H bonds, with one stereoisomer favored. This diastereomeric mixture may also be biased by use of a chiral phosphine ligand. The selectivity and efficiencies of resulting catalytic dehydrocoupling processes are also briefly discussed.
The development of
efficient catalytic methods for the formation
of bonds between main group elements is of considerable interest for
the continued development of main group chemistry. Such processes
enable new discoveries to be made in the promising application areas
that main group species are now occupying, such as high performance
polymers, emissive materials, etch resists for lithography, and precursors
to ceramic thin films or devices.[1−6] However, the development of this field lags substantially behind
the advances made in catalytic C–C and C–X bond formation,
for which there are now a myriad of efficient ways to promote such
unions that are important for the construction of new molecules. Catalytic
dehydrocoupling[5,7,8] of
amine– and phosphine–boranes is one method that has
emerged for the formation of B–N and B–P bonds, and
development in the area has been spurred on by the potential for ammonia–borane
to act as a hydrogen carrying vector.[9−11] In addition, polymeric
materials that can arise from dehydropolymerization of primary analogues
are also of significant interest as they are valence isoelectronic
with technologically ubiquitous polyolefins. Although the metal catalyzed
formation of polyaminoboranes has attracted recent attention,[12−18] catalytic routes to polyphosphinoboranes have also been known since
1999.[19] Perhaps the best example is that
of the [Rh(COD)2][OTf] catalyzed dehydrocoupling of secondary,
H3B·PR2H, and primary, H3B·PRH2, phosphine–boranes to give oligomeric and polymeric
materials (Scheme 1).[19−21]
Scheme 1
Phosphine–Borane
Dehydrocoupling
In copan class="Chemical">ntrast to n class="Chemical">amine–n class="Chemical">borane
dehydrocoupling,[8,10,15,22−24] the mechanism of catalytic
dehydrocoupling of phosphine–boranes
has received less attention. Although initial reports demonstrated
that catalysis using [Rh(COD)2][OTf] was a homogeneous
process (i.e., not colloidal),[25] there
has been only sporadic further work on elucidating the mechanistic
details.[26−29] Progress has no doubt been slowed due to the fact that the reaction
conditions reported for phosphine–borane dehydrocoupling often
require melt conditions, thus making interrogation of the catalytic
cycle problematic. Recently, we have reported that the Rh(I) complexes
[Rh(PtBu2H)2(η6-FC6H5)][BArF4],[30] and [Rh(L)(η6-FC6H5)][BArF4],[31] [L
= Ph2P(CH2)3PPh2] are
particularly well-suited to the study of the dehydrocoupling mechanism
of secondary phosphine–boranes in solvents such as fluorobenzene;
and on the basis of the observation of intermediates, kinetic studies,
and H/D exchange experiments we have proposed a catalytic cycle for
the dehydrocoupling of H3B·PR2H (R= Ph, tBu; Scheme 2). For this cycle, intermediate
species were isolated, but their structures could not be confirmed
by X-ray crystallography. In particular for R = Ph, a β-B-agostic
σ complex B, and the product of dehydrocoupling F, that is proposed to sit off cycle, could be isolated and
spectroscopically characterized. Under stoichiometric conditions the
observation that B transforms into F on
gentle heating allowed for kinetic parameters to be determined that
suggested that the rate-determining step(s) for dehydrocoupling were
located within the transformations B to D. In solution phase the turnover limiting step for catalysis is proposed
to be the displacement of the lineardiboraphosphine product (i.e., F to A), although under the melt conditions used
for efficient catalysis this may well be different. Further insight
comes from the observations that for R = tBu the barrier
to dehydrocoupling is higher (70 °C versus 25 °C for reaction),
P–H activation appears also to be a higher energy process,
different intermediates (A and E) are observed,
and the turnover limiting process in catalysis is now suggested to
be the P–H activation/dehydrocoupling steps. Prior work has
demonstrated a similar difference in relative rates of dehydrocoupling
of secondary H3B·PR2H [R = p-(CF3)C6H4, Ph, tBu, iBu] and primary H3B·PRH2 [R = Ph, tBu, iBu] phosphine–boranes using the [Rh(COD)2][OTf] catalyst, and this was suggested to be due to a combination
of steric and electronic (relative P–H bond strengths) factors,[21,32,33] although the mechanism of dehydrocoupling
of phosphine–boranes using this catalyst is currently not known.[20,25,30] Interestingly, the related dehydrogenation
of arylamine–boranes shows that the activity of the N–H
bond is such that spontaneous dehydrocoupling occurs in the absence
of catalyst, with electron-withdrawing aryl groups [p-(CF3)C6H4] undergoing faster reaction
than electron-donating [p-(OMe)C6H4].[34] Very recent work has shown
that paramagnetic Ti(III) centers might also be involved in dehydrocoupling
of phosphine– and amine–boranes when using Cp2Ti-based catalysts,[35] while oligomerization
of base-stabilized phosphino–boranes at Cp2Ti centers
has been described.[29] Likely decomposition
routes in Rh-systems for phosphine–borane dehydrocoupling to
form bis(phosphine)boronium salts have also recently been discussed.[36]
Scheme 2
Proposed Catalytic Cycle for the Dehydrocoupling
of H3B·PR2H To Give H3B·PR2BH2·PR2H
[BArF4]− anions are not shown.
Proposed Catalytic Cycle for the Dehydrocoupling
of H3B·PR2H To Give H3B·PR2BH2·PR2H
n class="Chemical">[BArF4]− anions pan> class="Chemical">are not shown.
In this papan class="Chemical">n class="Chemical">Article, we report anclass="Chemical">pan> extension of our
investigations into
the mechanism of n class="Chemical">phosphine–borane dehydrocoupling using the
{Rh(Ph2P(CH2)3PPh2)}+ fragment, by varying the electronic and steric profile of
the secondary phosphine–n class="Chemical">boranes H3B·PR2H [R = 3,5-(CF3)2C6H3; p-(CF3)C6H4; p-(OMe)C6H4; adamantyl],
as well as investigations with the primary phosphine–boraneH3B·PCyH2. Dehydrocoupling forms the corresponding
metal–bound lineardiboraphosphinesH3B·PR2BH2·PR2H and H3B·PRHBH2·PRH2, respectively. These studies provide
insight into the determining role of the electronics and sterics of
the phosphine–borane in the dehydrocoupling process, as well
as providing as yet unreported examples of the solid-state structures
of the intermediates related to the catalytic cycle. We also report
for the first time the partial control of diastereoselectivity in
dehydrocoupling of primary phosphine–boranes, that can additionally
be biased by use of a chiral chelating phosphine on the rhodium center.
Results
and Discussion
Phosphine–Borane and Diboraphosphine
Starting Materials
A range of secopan class="Chemical">ndary n class="Chemical">phosphine–boranes
with differing electronic
and steric properties have been used in this study (1, 2, 3, and 4, Figure 1), which also provide comparison with the previously
reported Ph, 6, and tBu, 7, analogues.[31] The primary phosphine–borane 5 has also been used.[37] Compounds 2(33) and 3(38) are known adducts and offer electron-withdrawing
and donating aryl groups, respectively. Bis-CF3-substituted 1 is a new complex and offers an alternative to 2. The synthesis of adamantyl-substituted phosphine, 4, an analogue of 7, has been reported in the patent
literature.[39] Compared with the tButyl group, adamantyl has a greater steric bulk due to its larger
volume and rigid structure.[40,41] The new lineardiboraphosphines, 10–13, have also been synthesized to aid
in the identification of final dehydrocoupling products. Complexes 10–12 are synthesized by a Rh-catalyzed process from
the corresponding phosphine–boranes, while primary phosphine
containing 13 has been synthesized in good isolated yield
(85%) by addition of [NBu4][BH4] to the bis(phopshine)boronium
[(CyH2P)2BH2]Br.[36]
Figure 1
Phosphine–boranes 1–7 and
diboraphosphines 8–13.
Phosphine–class="Chemical">pan> class="Chemical">boranes 1–7 and
diboraphosphines 8–13.
Stoichiometric Dehydrocoupling of Secondary
Phosphine–Boranes
Addition of 2 equiv of 1 to [papan class="Chemical">n class="Chemical">Rhclass="Chemical">pan> class="Chemical">(L)(η6-FC6H5)][BArF4] [L =
Ph2P(CH2)3PPh2] in 1,2-F2C6H4 solution at 25 °C rapidly
(on time of mixing) resulted in the formation of [Rh(L)(H)(σ,η-PR2BH3)(η1-H3B·PR2H)][BArF4], 14 [R = 3,5-(CF3)2(C6H3), Scheme 3], which was characterized by NMR spectroscopy,
ESI-MS (electrospray ionization mass spectrometry), and single crystal
X-ray diffraction. Likewise, the use of 2 equiv of phosphine–borane 2 or 3 results in the formation of the analogous
complexes 15 [R = p-(CF3)C6H4] and 16 [R = p-(OMe)C6H4], respectively, which were fully
characterized using solution techniques. All these complexes proceed
to dehydrocouple (vide infra), and only for 14 was an
analytically pure crystalline solid obtained. Even so, dissolution
of crystalline material of 14 resulted in the observation
of small amounts (approximately 5–10%) of the associated dehydrocoupling
product in the solution NMR spectra after short periods of time. Complexes 15 and 16 could only be isolated as oils, but
their characterization by NMR spectroscopy and ESI-MS was fully consistent
with their formulation.
Scheme 3
Synthesis of Complexes 14, 15, and 16
[BArF4]− anions are not shown.
Synthesis of Complexes 14, 15, and 16
n class="Chemical">[BArF4]− anions pan> class="Chemical">are not shown.
The solutionpan class="Chemical">NMR spectra for 14, 15,
and 16 are very similn class="Chemical">ar to those previously reported
for [Rh(L)(H)(σ,η-PPh2BH3)(η1-H3B·PPh2H)][BArF4] (i.e., B, Scheme 2(31)), and data for 14 is discussed
in detail. The 31P{1H} NMR spectrum of 14 shows four different phosphorus environments. Two of the
resonances are broadened significantly compared to the other two,
suggesting these phosphorus atomsare bound to a quadrupolarboron
center. One of these shows both a large trans PP
coupling [J(PP) 244 Hz] and coupling to 103Rh [J(RhP) 75 Hz], while the other is a broad singlet.
The other two signals are sharper and are assigned to the two 31P environments of the Ph2P(CH2)3PPh2 ligand. One of these sharper resonances [δ
29.5, ddd, J(RhP) 130, J(PP) 35, J(PP) 21 Hz] is assigned to the phosphorus atom trans to the weakly bound β-B-agostic interaction
on the basis of the larger 103Rh coupling constant, while
the other signal [δ 11.3, ddd, J(RhP) 103, J(PP) 244, J(PP) 35 Hz] is assigned to
the phosphorus atom trans to the coordinated phosphido
ligand. In the 1H NMR spectrum of 14 one broad,
relative integral 3H, signal is observed at δ −0.78,
indicative of a Rh···H3B σ interaction
in which the B–H bonds are undergoing rapid site exchange on
the NMR spectroscopic time scale between terminal and bridging sites.[42] A broad, relative integral 1H, resonance at
δ −6.12 is assigned to a static β-B-agostic B–H
interaction. Cooling of the solution to 0 °C led to the resolution
of this signal as doublet [J(PH) = 65 Hz], fully
consistent with its trans disposition to a phosphine.
The remaining BH(terminal) signals are not observed, and it is likely
they are coincident with the {CH2}3 signals.
A sharper signal at δ −16.21, relative integral 1H, is
assigned to a metal–hydride resonance, in which the coupling
to both 103Rh and 31P is clearly small and unresolved.
The PH group is observed at δ 5.81 that collapses into a singlet
in the 1H{31P} NMR spectrum. The 11B NMR spectrum shows a broad signal centered at δ −39.8,
which is not shifted significantly from that of free phosphine–borane 1 (δ −42.0). This is assigned to a coincidence
of the η1 β-B–H···Rh
agostic and σ Rh···H3B signals, as has been noted previously.[31,43] Complexes 15 and 16 have similar1H, 11B, and 31P NMR spectra, and thus
we assign very similar structures.
Crystals of complex 14 of suitable quality for analysis
by X-ray diffractiopan class="Chemical">n were obtained by layering of a n class="Chemical">1,2-F2C6H4 solution with n class="Chemical">pentane at −26 °C.
The structure of 14 in the solid-state (Figure 2) is fully consistent with the structure deduced
from the solution NMR spectroscopic data. The formally Rh(III) center
adopts a pseudo-octahedral geometry, with the chelating phosphine
ligand and the hydride located on one of the faces of the octahedron.
Two of the three remaining coordination sites are occupied by a phosphine–borane
unit that has undergone P–H activation, and is bound to the
metal via a phosphido bond [Rh1–P3, 2.3045(10)
Å] and a β-B-agostic bond [Rh1–B1, 2.515(4) Å].
The other phosphine–borane unit occupies the last coordination
site via a σ η1-Rh···H–B
interaction.[42] All the hydrides (B–H
and Rh–H) were located in the final difference map. The structure
is in full accord with the solution NMR spectroscopic data, confirming
the spectroscopic assignments that have been made previously.[31] β-B-agostic interactions are known,[35,44,45] and we have recently reported
[Rh(κ1,η-PPh2BH2·PPh3)(PPh3)2][BArF4] in which a base-stabilized phosphine–borane adopts a β-B-agostic
interaction with the Rh-center.[36] σ
phosphine–boranesare also known,[42,46,47] and bimetallic complexes showing both B-agostic
and σ borane coordination modes have been reported.[48] Compared to a Rh(I) complex that shows a bidentate
η2-coordination mode for the σ borane, [Rh(PtBu2H)2(η2-H3B·PtBu2H)][BArF4],[30] the Rh···B distance
for the η1-interaction in 14 is considerably
longer [2.188(3) Å versus 2.740(4) Å, respectively], consistent
with this different binding motif. Similar changes in M···B
distance have been noted on moving between η1 and
η2 coordination modes in chelating phosphine–boranes.[43]
Figure 2
Molecular structure of the cation of 14.
Displacement
ellipsoids are drawn at 50% probability level. Some hydrogen atoms
are omitted for clarity. Selected bond lengths (Å) and angles
(deg): Rh1–P1, 2.778(10); Rh1–P2, 2.3163(9); Rh1–P3,
2.3045(10); P3–B1, 1.913(4); P4–B2, 1.918(4); Rh1···B1,
2.515(4); Rh1···B2, 2.740(4); Rh1–P3–B1,
72.54(14); Rh1–B2–P4, 121.3(2).
Moleculn class="Chemical">arpan> structure of the cation of 14.
Displacement
ellipsoids n class="Chemical">are drawn at 50% probability level. Some hydrogen atoms
are omitted for cln class="Chemical">arity. Selected bond lengths (Å) and angles
(deg): Rh1–P1, 2.778(10); Rh1–P2, 2.3163(9); Rh1–P3,
2.3045(10); P3–B1, 1.913(4); P4–B2, 1.918(4); Rh1···B1,
2.515(4); Rh1···B2, 2.740(4); Rh1–P3–B1,
72.54(14); Rh1–B2–P4, 121.3(2).
Complexes 14–16 undergo
spopan class="Chemical">ntaneous
dehydrocoupling (25 °C) to form products of the general formula
[n class="Chemical">Rh(n class="Chemical">L)H(σ,η2-PR2·BH2PR2·BH3)][BArF4]: 17, R = 3,5-(CF3)2C6H3; 18, R = p-(CF3)C6H4; 19, R = p-(OMe)C6H4 (Scheme 4). This process
also results in the liberation of H2 (observed, 1H NMR spectroscopy). For 17 and 18 there
are additional products formed, assigned as [Rh(L)(PR2H)2][BArF4], 21 and 22, respectively, on the basis of NMR spectroscopic data.
These complexes are formed in parallel to 17 and 18, as preformed 17 (vide infra) does not proceed to form 21. Complex 21 has been independently prepared by addition of two equivalents of
HP((CF3)2C6H3)2 to [Rh(L)(η6-FC6H5)][BArF4].
Scheme 4
Dehydrocoupling of Complexes 14–16
[BArF4]− anions are not shown. Time = 6 h 17/21, 18/22 (25 °C);
8
h 16/19 (35 °C).
Dehydrocoupling of Complexes 14–16
n class="Chemical">[BArF4]− anions pan> class="Chemical">are not shown. Time = 6 h 17/21, 18/22 (25 °C);
8
h 16/19 (35 °C).
This mixture of products observed for the electron-withdrawipan class="Chemical">ng
n class="Chemical">phosphine substituents (i.e., 1 anclass="Chemical">pan>d 2) contrasts
with that found for when R = Ph[31] and n class="Chemical">p-(OMe)C6H4, which yield the dehydrocoupled
(e.g., 19 and F, Scheme 2) product in essentially quantitative form (∼95% by 31P{1H} NMR spectroscopy). Complex 17 has been synthesized cleanly from direct addition of the preformed
dehydrocoupled diboraphosphine product, 10, to [Rhn class="Chemical">(L)(η6-FC6H5)][BArF4], Scheme 5. It was from this reaction that
material of 17 suitable for single crystal X-ray diffraction
was obtained.
Scheme 5
Synthesis of 17 by Direct Addition of
the Linear Diboraphosphine 10
[BArF4]− anions are not shown.
Synthesis of 17 by Direct Addition of
the Linear Diboraphosphine 10
n class="Chemical">[BArF4]− anions pan> class="Chemical">are not shown.
Figure 3 shows the solid-state
structure
of 17, in which the papan class="Chemical">n class="Chemical">diboraphosphine acts as a chelate
to the class="Chemical">pan> class="Chemical">Rh(III) center, via a phosphido group and
two B-agostic interactions: [Rh(L)H(σ,η2-PR2·BH2PR2·BH3)][BArF4] [R = 3,5-(CF3)2C6H3]. All the hydride ligands (B–H and Rh–H)
were located in the final difference map. The Rh(III) center has pseudo-octahedral
geometry, in which the oligomeric phosphine–borane is bound
tridentate to the metal through η2-BH2···Rh [B2–Rh1, 2.280(5) Å] and phosphido
[P3–Rh1, 2.3925(10) Å] interactions. The hydride ligand
is positioned trans to one of the B–H···Rh
interactions. The Rh···B distance is considerably shorter
than those observed in 14, consistent with the η2-bidentate binding mode of the borane. This distance is similar
to others reported for chelating phosphine–borane complexes
with Rh.[49−52]
Figure 3
Molecular
structure of the cation of 17. Displacement
ellipsoids are drawn at 50% probability level. Some hydrogen atoms
are omitted for clarity. Selected bond lengths (Å) and angles
(deg): Rh1–P1, 2.3241(11); Rh1–P2, 2.2650(11); Rh1–P3,
2.3925(10); Rh1···B2, 2.280(5); Rh1–P3–B1,
110.88(15); B1–P4–B2, 107.5(2).
Moleculn class="Chemical">arpan>
structure of the cation of 17. Displacement
ellipsoids n class="Chemical">are drawn at 50% probability level. Some hydrogen atoms
are omitted for cln class="Chemical">arity. Selected bond lengths (Å) and angles
(deg): Rh1–P1, 2.3241(11); Rh1–P2, 2.2650(11); Rh1–P3,
2.3925(10); Rh1···B2, 2.280(5); Rh1–P3–B1,
110.88(15); B1–P4–B2, 107.5(2).
The NMR spectroscopic data for 17 papan class="Chemical">n class="Chemical">are fully
conpan>sistent
with the solid-state structure being retained in solution and are
also very similar to that reported for the analogous complex formed
from the deydrocoupling of 6 (R = Ph).[31] The 31P{1H} NMR spectrum shows four
different phosphorus environments. Two of these signals are well-resolved
and show coupling to 103Rh, δ 46.6 [J(RhP) 111 Hz] and δ 12.8 [J(RhP) 91 Hz], and
are attributed to the chelating phosphine ligand. One of these signals
(δ 12.8) also shows large 31P–31P coupling [J(PP) 260 Hz] suggesting a trans position relative to the phosphido center. The other two environments
are broad, typical of those observed when coupling to a quadrupolarboron center. For one of these trans J(PP) coupling
is also observed. The 1H NMR spectrum shows three different
broad, relative integral 1H, environments assigned to the BH3 moiety [δ −4.54, −1.20, and 4.37]. This indicates
that the BH3 unit is not undergoing exchange on the NMR
spectroscopic time scale, as noted previously for similar η2-M···H3B systems.[31,43,50,52] The Rh–H
signal is observed at δ −13.98 as a sharper signal, although
this also shows unresolved coupling. The 11B NMR spectrum
shows two different environments [δ −27.1 and 0.21] for
the two boron atoms present in the diboraphosphine, with the latter
assigned to the η2-H3B unit on the basis
of the large downfield shift from free ligand (Δδ = +36.8).[43] Spectroscopic data for complexes 18 and 19, that are produced by the direct dehydrocoupling
route are similar, although for 18 this is also formed
as a mixture with 22.
The dehydrocoupling reactiopan class="Chemical">n
(i.e., 14 to 17) shows a dependence on the
substituents on the n class="Chemical">phosphine. For electron-withdrawing
n class="Chemical">aryl groups (e.g., p-CF3), it is faster
when compared with electron rich groups (i.e., p-OMe).
Following these processes in situ using NMR spectroscopy
demonstrated that these dehydrocoupling reactions follow a first order
rate profile for the consumption of the starting material over at
least three half-lives (see Supporting Information): 1 3 h (25 °C); 2 3 h (25 °C); 6 14 h (25 °C);[31]3 8 h (35 °C), ∼120 h (25 °C). That the parallel
products 21 and 22 are formed in approximately
equal ratio to the dehydrocoupled product (17, 18, respectively)) suggests that k1 ≈ k2 (Scheme 4). In addition to this parallel process, direct comparison
of the rate constants is further complicated by the fact that 16 → 19 required heating to 35 °C
to make the reaction run over a convenient time scale for analysis
by NMR spectroscopy. Nevertheless these relative rates reflect previous
observations on the rate of catalytic dehydrocoupling when the electronics
of a system are changed, in as much as electron-withdrawing groups
promote the reaction.[21] Interestingly,
for all the aryl complexes initial P–H activation to form a
phosphido hydride complex (i.e., 14) is very rapid, occurring
on time of mixing. This suggests that for aryl-substituted phosphine–boranes
it is not initial P–H activation that is rate-determining for
the dehydrocoupling event, as we have commented on for R = Ph.[31] In this study we suggested that B–H activation/reorganization
in intermediates such as B (Scheme 2) prior to P–B bond formation might be the rate limiting process.[31] This might well be promoted by a weaker B–H
bond, and calculations on analogous H3B·L (L = Lewis
base) systems show that the B–H bond is considerably weaker
when there are electron-withdrawing groups on the Lewis base.[53] However, we cannot rule out that the relative
P–H bond strengths in intermediates such as 14 also might play a role, or that there is a change in the rate determining
step on changing the phosphine–borane ligand, as the intimate
details of the mechanism leading to P–B formation still remain
to be resolved. The observation that for an electron-withdrawing phosphine
there is a significant proportion of parallel product formed that
results from P–B bond cleavage is consistent with the weakening
of the P–B bond with increasingly electron-withdrawing aryl
substiutents.[8,54] P–B bond cleavage has
been noted previously in σ phosphine–borane complexes
to give either simple adducts[47] or further
reaction to yield bis(phosphine)boronium salts.[30]
Prior to the formatiopan class="Chemical">n of the parallel product 21 (R
= n class="Chemical">3,5-(CF3)2C6H3) an intermediate
is observed that has been characterized by 1H and 31P{1H} NMR spectroscopy as [Rh(L)H(σ,η-PR2·BH3)(PR2H)][BArF4] 20, i.e., a complex that sits directly between 14 and 21 by loss of one “BH3” fragment (Scheme 6). Complex 20 results from P–B bond cleavage, formally of the
σ-H3B·PR2H ligand, to afford a complex
with a β-B-agostic interaction from a phosphide borane ligand
(as for 14) and a simple PR2H ligand trans to a hydride. Complex 20 was not isolated
in pure form, being observed alongside 14 and the final
products 17/21. However, after 2 h reaction
a significant proportion of 20 is present (∼20%
by 31P NMR spectroscopy), allowing for its identification
aided by comparison with the NMR spectroscopic data for 14 (Supporting Information). In particular
four environments are observed in the 31P NMR spectrum,
with only one of these broadened significantly by coupling to quadrupolarboron. This signal also shows a large, mutual, trans J(PP) coupling with another phosphine environment. In the high-field
region of the 1H NMR spectrum a broad doublet is observed
at δ −7.06 [J(HP) = 76 Hz] which is
assigned to the β-B-agostic interaction, while there is a relatively
sharper one at δ −9.61 [J(HP) = 165
Hz] assigned to Rh–H, and again 103Rh coupling is
not resolved. These assignments were confirmed by 1H{31P}, 1H{11B}, and 1H/31P correlation experiments.
Scheme 6
Formation of the
Parallel Products 17 and 21 from 14
[BArF4]− anions are not shown.
Formation of the
Parallel Products 17 and 21 from 14
n class="Chemical">[BArF4]− anions pan> class="Chemical">are not shown.
Addition of 2 equiv of the bulky apan class="Chemical">nd electron rich n class="Chemical">phosphine–n class="Chemical">borane
H3B·P(adamantyl)2H, 4, to
[Rh(L)(η6-FC6H5)][BArF4] in 1,2-F2C6H4 solution
at 25 °C rapidly results in a color change from orange to purple
and the formation of the new σ bound Rh(I)phosphine–borane
complex [Rh(L)(η2-H3B·P(adamantyl)2H)][BArF4], 23, which was
characterized in situ by NMR spectroscopy. This complex
could not be isolated as it undergoes further reaction, by P–B
bond cleavage at room temperature, to form 24 (Scheme 7). Addition of 1 equiv of 4 resulted
in a final mixture of 24 and [Rh(L)(η6-FC6H5)][BArF4].
Scheme 7
Synthesis of Complex 24 by Direct and Indirect Routes
[BArF4]− anions are not shown.
Synthesis of Complex 24 by Direct and Indirect Routes
n class="Chemical">[BArF4]− anions pan> class="Chemical">are not shown.
The n class="Chemical">1Hpan> NMR spectrum of complex 23 immediately
after prepn class="Chemical">aration shows a n class="Chemical">broad, relative integral 3H, signal at δ
−1.36 characteristic of a σ-bound phosphine–borane
that is undergoing site exchange between the coordinated and uncoordinated
B–H environments.[42] Two signals
are observed in the 31P{1H} NMR spectrum, in
a 2:1 ratio at δ 35.1 [J(RhP) 167 Hz] and δ
30.1 (br). Over time (1 h), complex 23 disappears to
be replaced by a new complex that has been characterized by NMR spectroscopy
and a solid-state X-ray diffraction experiment as [Rh(L)(PHR2)(η1–H3B·PHR2)][BArF4] (24, R = adamantyl). Figure 4 shows the structure of the cation present in 24 in the solid-state. A Rh(I) center is in a pseudo-square-planar
geometry with a chelating ligand, and the other two coordination sites
are occupied by P(adamantyl)2H and a η1-H3B·P(adamantyl)2H [Rh···B,
2.457(7) Å] ligands, respectively. The BH and PH hydrogen atoms
were located in the final difference map. The solution NMR spectroscopic
data for 24 are fully consistent with the solid-state
structure, and in particular the trans disposition
of P1 and P3, and the η1-H3B·PR2H ligand.
Figure 4
Molecular structure of the cation of 24.
Displacement
ellipsoids are drawn at 50% probability level. Some hydrogen atoms
are omitted for clarity. Selected bond lengths (Å): Rh1–P1,
2.2262(16); Rh1–P2, 2.2861(16); Rh1–P3, 2.3568(15);
Rh1···B1, 2.457(7); B1–P4, 1.936(7).
Moleculn class="Chemical">arpan> structure of the cation of 24.
Displacement
ellipsoids n class="Chemical">are drawn at 50% probability level. Some hydrogen atoms
are omitted for cln class="Chemical">arity. Selected bond lengths (Å): Rh1–P1,
2.2262(16); Rh1–P2, 2.2861(16); Rh1–P3, 2.3568(15);
Rh1···B1, 2.457(7); B1–P4, 1.936(7).
A significant amount of P–B bond cleavage
product is thus
observed for both electron poor n class="Chemical">aryl n class="Chemical">phosphine–boranes (e.g., 14) and very bulky electron rich phosphine–boranes
(e.g., 24), but not the electron rich
aryl phosphine 3 or H3B·PPh2H (6).[31] Interestingly we
have recently reported that for H3B·PtBu2H P–B bond cleavage is also observed during dehydrocoupling
catalysis being accompanied by a further dehydrocoupling step, through
which bis(phosphine)boronium saltsare ultimately formed.[30,36] Similar complexes can be prepared on rhodium using H3B·PPh2H and PPh3 under stoichiometeric
conditions.[36] One suggested mechanism for
this process is the reaction of a short-lived phosphino–borane
(or its masked equivalent) with coordinated phosphine, not dissimilar
to the mechanism suggested for the formation of diaminoboranes from
amine–boranes and amines catalyzed by alkaline earth catalysts.[55] Complexes 20 and 24 serve as models for intermediates in this process [Rh(III) and Rh(I),
respectively], although we do not observe the formation of corresponding
bis(phosphine)boronium salts in this case.
Stoichiometric Dehydrocoupling
of Primary Phosphine–Boranes
The dehydrocoupling of
primpapan class="Chemical">n class="Chemical">ary class="Chemical">pan> class="Chemical">phosphine–boranes can yield
polyphosphinoboranes, rather than the simple oligomers observed with
secondary phosphine–boranes (Scheme 1). With an appreciation of the intermediate metal complexes formed
with secondary phosphine–boranes from this and previous work,[30,31,36] it was of interest to explore
whether the proposed dehydrocoupling mechanism for secondary phosphine–boranes
using [Rh(L)(η6-FC6H5)][BArF4] could be applied to primary analogues. Such
insight into the mechanism of dehydropolymerization of phosphine–boranes
is important, as these processes currently remain unresolved due to
the melt conditions employed that make following intermediates or
kinetics problematic.[20,28,33]
In situ ipan class="Chemical">nvestigations using stoichiometric
quantities of primary n class="Chemical">phosphine–boranesH3B·PPhH2 resulted in immediate reaction when combined with [Rh(L)(η6-FC6H5)][BArF4], but a number of products were formed which we have not been able
to convincingly characterize. This mixture of species observed is
in contrast with H3B·PPh2H where single
products are formed analogous to 14–16.[31] However, reaction of [Rh(L)(η6-FC6H5)][BArF4] with a slight excess of H3B·PCyH2 (5) in 1,2-F2C6H4 solution
at 25 °C led to the instantaneous formation of only two complexes
in a 1:1 ratio, 25a and 25b, [Rh(L)H(σ,η-PCyH·BH3)(η1-H3B·PCyH2)][BArF4], as a proposed diastereomeric pair
(Scheme 8). This stereoisomerism comes from
P–H activation at the prochiral primary phosphine. These new
products are directly analogous to those formed with secondary phosphine–boranes
(i.e., 14), and the NMR spectroscopic data match closely.
The 31P{1H} NMR spectrum from this reaction
shows 8 resonances, in addition to a broad peak at δ −35.5
due to excess phosphine–borane, as each diastereomer contains
four distinct phosphorus environments. Signals centered at δ
31.7 and 30.5 are assigned to one of the chelating phosphine ligand 31P environments in each diastereoisomer, and show characteristic J(RhP) coupling constants consistent with a Rh(III) center.
Complex overlapping multiplets at δ 11.8 [2 × ddd] represent
the resonances for both diastereomers of the second chelated phosphorus
center, which is trans to the phosphide position,
displaying a large trans PP coupling constant [J(PP) ∼ 200 Hz] in addition to coupling to 103Rh and cis-31P. The remaining 4 signals
are broad indicating the phosphorus centers are bound to a quadrupolar11B nucleus. Of these, peaks at δ −11.0 and −32.1
are assigned to the phosphide centers of each diastereomer trans to the chelating phosphine [J(PP)
∼200 Hz], and resonances at δ −39.8 and δ
−44.2 as assigned to phosphorus centers in the σ-bound
phosphine–borane unit. These large differences in chemical
shift of the phosphido signal (Δδ 21.2) might reflect
significant local difference in steric pressure between 25a and 25b at this group. Interestingly, a much smaller
difference is observed with the dehydrocoupled products (26a/b, Δδ 3.5) in which the phosphide group is part of a chelate
ring. The 1H NMR spectrum does not have the necessary resolution
to separate out the diastereomers in the hydride region, with broad
resonances observed at δ −2.3 (3 H, BH3),
δ −7.9 (1 H, Rh–H–B), δ −17.5
(Rh–H).
Scheme 8
Synthesis of 25a, 25b, and
the Dehydrocoupled
Products 26a and 26b
[BArF4]− anions are not
shown.
Synthesis of 25a, 25b, and
the Dehydrocoupled
Products 26a and 26b
n class="Chemical">[BArF4]− anions pan> class="Chemical">are not
shown.
Complexes 25a/b canpan class="Chemical">not
be isolated in pure form, and
chn class="Chemical">aracterization by NMR spectroscopy is best performed on freshly
prepared samples, as after 1 h (25 °C) they have undergone dehydrocoupling
to give a mixture of two resolvable diastereomers 26a and 26b, with one of the diastereomers present in a
significantly larger amount ∼6:1 (Scheme 8), indicating that the dehydrocoupling step occurs with some stereocontrol.[56] The decomposition product [Rh(L)(PH2Cy)2]+, analogous to 21/22, was also observed. NMR spectroscopic and ESI-MS analysis
suggests that the dehydrocoupling products formed are direct analogues
of 17. This mixture of diastereomers can also be synthesized
cleanly by direct reaction of [Rh(L)(η6-FC6H5)][BArF4] with the preformed diaboraphosphineH3B·PCyHBH2·PCyH2 (13) in 1,2-F2C6H4 solution
at 25 °C (Figure 5 for the solid-state
structure). Immediate measurement of the 31P{1H} NMR spectrum after mixing showed clean conversion to complexes 26a and 26b in an approximate 1:1 ratio, interestingly
different from the 1:6 ratio observed from dehydrocoupling.
Figure 5
Molecular structure
of 13. Displacement ellipsoids
are drawn at 50% probability level. Selected bond lengths (Å)
and angles (deg): P1–B1 1.9267(18), B1–P2 1.9381(18),
P2–B2 1.926(2); P1–B1–P2 108.34(9), B1–P2–B2
113.32(9).
Moleculn class="Chemical">arpan> structure
of 13. Displacement ellipsoids
are drawn at 50% probability level. Selected bond lengths (Å)
and angles (deg): P1–B1 1.9267(18), B1–P2 1.9381(18),
P2–B2 1.926(2); P1–B1–P2 108.34(9), B1–P2–B2
113.32(9).
Resonapan class="Chemical">nces in the n class="Chemical">31P{n class="Chemical">1H} NMR spectrum of 26 can, again, be assigned
aided by reference to those of
structurally characterized 17. Peaks centered at δ
37.9 and 34.5 result from the chelated phosphorus trans to the B-agostic site, while the signals for the phosphorus trans to the phosphido group overlap at δ 10.7, and
display characteristic J(PP) trans coupling [255 Hz]. The broad resonances of the diboraphosphineare
observed at δ 19.8 and 16.2 for the phosphido center [J(PP) 255] and δ −14.9 and −16.6 ppm
for the remaining site. The high-field region of the 1H
NMR spectrum of 26a/26b shows a slight downfield
shift of the Rh–H hydride resonance to δ −16.1,
when compared to 25a/25b, while the η2-BH2···Rh units are observed as two broadened
resonances at δ −2.98 (1H) and δ −5.98 (1H).
For these hydride signals the separate signals are not resolved for
each diasteroisomer, although each resonance is rather asymmetric
suggesting two overlapping environments.
A n class="Chemical">31pan class="Chemical">P{class="Chemical">pan> class="Chemical">1H} NMR spectrum taken of this mixture
after 18 h at 25 °C showed a significant change in the ratios
of the diastereomers 26a/26b (Scheme 9). The peaks for one isomer at [δ 34.5, 16.2, 10.7,
and −14.9] have reduced relative area, giving an approximate
ratio of 6:1 for the two diastereoisomers. This ratio is similar to
that found from direct dehydrocoupling in 25a/25b after
1 h (vide supra), underscoring the stereocontrol
occurring in the P–B bond forming process. Leaving this solution
for one week resulted in no significant change to this ratio, suggesting
equilibrium had been reached. We suggest that the mechanism for equilibration
involves reductive elimination of the phosphido and hydride ligands
to form a Rh(I) σ phosphine–borane complex,[30] similar to E in Scheme 2, which then undergoes rapid oxidative addition
of the other P–H bond. This must be a reversible process, leading
to a thermodynamic ratio of the diastereoisomers and the resulting
selectivity. Unfortunately we were unable to deduce the stereochemistry
of the preferred isomer using ROESY experiments or a solid-state structure.
However, inspection of models leads us to propose that the thermodynamic
product is likely to have the cyclohexyl group pointing away from
the chelating phosphine ligand’s phenyl groups, i.e., 26b. That these diastereoisomers are a result of the metal
activation of the prochiral terminal P–H bonds in 13 is shown by addition of an excess of dppe to 26a/b.[56] This affords [Rh(dppe)(L)][BArF4][31] with the concomitant formation
of free 13 (Scheme 9).
Scheme 9
Change
in Diastereoisomeric Ratio and Release of the Diboraphosphine
[BArF4]− anions are not shown.
Change
in Diastereoisomeric Ratio and Release of the Diboraphosphine
n class="Chemical">[BArF4]− anions pan> class="Chemical">are not shown.
We have n class="Chemical">brpan>iefly explored the use of a chiral n class="Chemical">metal/ligand fragment
in dehydrocoupling, [n class="Chemical">Rh(BDPP)]+ [S,S-BDPP = (2S,4S)-2,4-bis(diphenylphosphino)pentane].
This chiral ligand was chosen as electronically and sterically (i.e.,
bite angle) it is similar to Ph2P(CH2)3PPh2. Addition of H3B·PCyH2, 5, to [Rh(BDPP)(η6-FC6H5)][BArF4] results in the immediate
formation of two diastereoisomers of [Rh(BDPP)H(σ,η-PCyHBH3)(η1-H3B·PCyH2)][BArF4], 27, in a 3:1 ratio
(Scheme 10). Although we are unable to comment
on the absolute configuration of these isomers, it is interesting
to note that this is biased away from the 1:1 ratio observed in the
achiral system. Compounds 27a/b proceed on to dehydrocouple
to form diastereoisomers of [Rh(BDPP)H(σ,η2-PRH·BH2PRH·BH3)][BArF4], 28, 1:5:3:0 ratio. The same mixture of
diasteroisomers can be formed by direct addition of 13 to [Rh(BDPP)(η6-FC6H5)][BArF4]. Initially a 2:1:2:1 ratio of 4 isomers is observed,
that changes to a 5:1:3:0 ratio after 18 h. We are unable to comment
in more detail on the conformation of these isomers, although the
observation of stereocontrol in the direct dehydrocoupling is similar
to that observed for the achiral system. Addition of excess dppe to
this mixture forms a product identified by ESI-MS as [Rh(BDPP)(dppe)]+ and free 13 (by 31P and 11B NMR spectroscopy). We have not explored whether there is enantiocontrol
at the central {PCyH unit} arising from this PB coupling event on
release from the metal.
Scheme 10
Use of a Chiral Ligand in Dehydrocoupling
[BArF4]− anions are not shown.
Use of a Chiral Ligand in Dehydrocoupling
n class="Chemical">[BArF4]− anions pan> class="Chemical">are not shown.
For these experiments with papan class="Chemical">n class="Chemical">H3B·class="Chemical">pan> class="Chemical">PCyH2 it
is interesting to note that P–H activation is rapid and
reversible with the Rh(I) precursor. This is in contrast to results
obtained with secondary phosphine–boranesH3B·PtBu2H and H3B·PtBu2BH2·PtBu2H, which on
addition to [Rh(L)(η6-FC6H5)][BArF4] gave the corresponding Rh(I) σ-phosphine–borane
complexes with no P–H activation.[31] Such selectivity for primary over secondary phosphines in P–H
activation at a metal center has been described previously for both
phosphine[57] and phosphine–borane
ligands.[27] In particular it has been shown
that addition of H3B·PPhH2 to Pt(PEt3)2H(PPh2·BH3) results
in exchange of the metal bound phosphide complex to give the primary
phosphido–borane complex.[26] Here
it was suggested that the greater thermodynamic driving force for
formation of the primary phosphido–borane complex comes from
steric effects, as M–P bonds to smaller primary phosphido ligands
are likely to be stronger.
Catalytic Dehydrocoupling of Secondary Phosphine–Boranes
Under the stapan class="Chemical">ndn class="Chemical">ard catalytic melt conditionpan>s (90 °C, 5 mol
%),[20] [n class="Chemical">Rhn class="Chemical">(L)(η6-FC6H5)][BArF4] will dehydrocouple
the secondary arylphosphine–boranes used in this study to
form the corresponding lineardiboraphosphines 10–12, although we have not explored in detail the temporal evolution
of these systems due to the problems associated with directly interrogating
the melt. However, trends can be observed. For electron-withdrawing
groups (1 and 2), complete consumption of
starting material occurs in 4 h (Table 1).
The reaction at this temperature is not selective, and although the
main product is the lineardiboraphosphine, there are products that
we tentatively identify as the cyclic oligomers (BH2PR2) (n = 3, 4).[20,33] Our results are broadly in line with the previously reported catalyzed
dehydrocoupling of 2 using [Rh(COD)Cl]2, which,
at a slightly lower temperature (60 °C, 16 h, melt), affords
the lineardiboraphosphine product in 69% isolated yield, while at
100 °C only the cyclic oligomers are isolated. The mechanism
of formation of the higher cyclic oligomers, (BH2PR2), remains to be resolved.[20] For electron-donating 3 the reaction
is slower using the [Rh(L)(η6-FC6H5)][BArF4] catalyst (8 h) but overall
is more selective. For R = Ph we have previously shown that [Rh(L)(η6-FC6H5)][BArF4] catalyzes dehydocoupling to give the corresponding lineardiboraphosphine
in greater than 95% conversion after 4 h.[31] For secondary phosphine–boranes, H3B·PPh2H thus offers balance between overall rate and selectivity.
Table 1
Conversion of H3B·PR2H
with Timea
H3B·PR2H
time/h
H3B·PR2H/%
H3B·PR2BH2PR2H/%
(BH2PR2)n/%
1
1
10
55
<5
4
<5
45
10
8
<5
35
50
2
1
10
70
10
4
<5
70
15
8
<5
70
15
3
1
50
30
<5
4
30
45
5
8
20
60
5
R = aryl, see Figure 1. [Cat.] = [Rh(L)(η6-FC6H5)][BArF4], L = Ph2P(CH2)3PPh2. Conversions calculated from 31P{1H} NMR spectra.
Conditions: [Rh(L)(η6-FC6H5)][BArF4], 5 mol %, 90°C, melt.
R = aryl, see Figure 1. [Cat.] = [Rh(L)(η6-FC6H5)][BArF4], L = Ph2P(CH2)3PPh2. Conversions calculated from 31P{1H} NMR spectra.
Conditions: [Rh(L)(η6-FC6H5)][BArF4], 5 mol %, 90°C, melt.Given the product distributions
and likely decomposition pathways
in the melt it is inappropriate to comment in detail on the nature
of the rate-determining steps during catalysis under these conditions.
However, on the basis of the solution studies, P–B bond formation,
(dehydrocoupling) is faster with electron-withdrawing groups. The
temporal differences in observed product conversion in the melt could
reflect a difference in the rate of the P–B bond forming event,
or alternatively, they could reflect the ease at which the bound product
is substituted on the n class="Chemical">metal center, i.e., a turnpan>over limiting step.
To probe this latter scenario, reaction between 19 (aryl-OMe)
and diboraphosphine 11 (aryl-CF3) to form 18 and free 12 demonstrates that an equilibrium
is established slightly in favor of 18 (Scheme 11). This suggests that there is not a strong inherent
difference in binding strengths between the two products, with the
implication being that the observed rate differences in the
melt arise from the dehydrocoupling step. Although this is
different from what is observed in solution at room temperature, in
which release of the product is likely the turnover limiting step,
it is consistent with the high local concentration of H3B·PR2H that being under melt conditions (90 °C)
would promote such a substitution.
Scheme 11
Competition Experiments
between Linear Diboraphosphines
[BArF4]− anions are not shown.
Competition Experiments
between Linear Diboraphosphines
n class="Chemical">[BArF4]− anions pan> class="Chemical">are not shown.
Catalytic Dehydrocoupling of Primary Phosphine–Boranes
[n class="Chemical">Rhpan>n class="Chemical">(L)(η6-n class="Chemical">FC6H5)][BArF4] also acts as a catalyst for the dehydrocoupling
of primary phosphine–boranes. Under melt conditions (90 °C,
5 mol %, 4 h) H3B·PPhH2 is dehydrocoupled
to give a major product which is identified by 31P NMR
spectroscopy as being polymeric (BH2PPhH) by comparison with previously reported[19,20] data for purified material coming from the [Rh(COD)2][OTf]
catalyzed process [δ −49.3, d, J(PH)
∼350 Hz, 1,2–F2C6H4; lit.: δ −48.9, δ, J(PH) 360
Hz, CDCl3]. There were also other species observed ∼δ
−55, which could be reduced in relative concentration (to ∼10%)
by precipitation into hexanes. Such species have been previously suggested
to be short-chain oligomers.[20] Interestingly,
these proposed shorter chain oligomers are present in a greater proportion
at shorter reaction times, which might suggest that polycondensation
is occurring to give higher molecular weight polymer. Under non-melt
conditions[20] (toluene heated to reflux,
0.5 mol %, 16 h) these shorter oligomers are by far the dominant species
(Supporting Information). It thus appears
that a high local concentration of phosphine–borane is necessary
for productive dehydropolymerization. Positive mode ESI-MS (electrospray
mass spectrometry) of the melt reaction product demonstrated polymerization,
showing repeat units of [H(PPhHBH2)PPhH2]+ up to n =
10 (Supporting Information). Similar analyses
have been reported for amine–borane dehydropolymerization.[12,14,58] That these polymersare terminated
by {PPhH2} groups rather than {BH3} is confirmed
by inspection of the corresponding isotopomer patterns. This formulation
also argues against cyclic oligomers being observed by ESI-MS, and
presumably the additional phosphinearises from P–B bond cleavage.
Use of H3B·PCyH2 under these conditions
afforded significantly more complex mixtures that we were unable to
resolve.
Conclusions
The solid-state structures
of the intermediates ipan class="Chemical">n the dehydrocoupling
of secondn class="Chemical">ary n class="Chemical">phosphine–boranes using the {Rh(Ph2PCH2CH2CH2PPh2)}+ fragment have been determined. This demonstrates that the
complex that precedes dehydrocoupling to form a lineardiboraphosphine
has σ bound and P–H activated phosphine–borane
ligands, while the product has a lineardiboraphosphine bound to the
metal center. For arylphosphine–boranes, electron-withdrawing
groups (CF3) promote stoichiometric dehydrocoupling faster
than for more electron-donating (OMe) groups. This increase in rate
is accompanied by a significant degree of parallel and competitive
P–B bond cleavage to afford metal complexes with two monodentate
phosphine ligands, which we suggest is due to a weakening of the P–B
bond with electron-withdrawing aryl groups. These systems also turnover
catalytically under melt conditions, with the overall rate of conversion
broadly following the relative dehydrocoupling rates observed in the
stoichiometric studies, suggesting that the dehydrocoupling step under
melt conditions might also be the turnover limiting step. P–B
bond cleavage also occurs for very bulky electron rich adamantylphosphine–boranes,
to such an extent that stoichiometric dehydrocoupling is not observed.
For this phosphine–borane we suggest that sterics play a role
in this process.
A significapan class="Chemical">nt observation is that, for primary
n class="Chemical">phosphine–boranes,
which are precursors to polyphosphinoboranes, use of the {Rh(Ph2PCH2CH2CH2PPh2)}+ fragment results in some apparent diastereoselectivity
in the dehydrocoupling step, at least in the stoichiometric reactions
that produce metal-bound diboraphosphines. Such selectivity could
well have implications in the control of the stereochemistry of polymer
that would result from further insertion events. A significant future
challenge is to harness any inherent bias in each dehydrocoupling
insertion step productively while also developing the necessary spectroscopic
and physical characterization markers to interrogate the oligomer
and polymer stereochemistry.
Experimental Section
All manipulatiopan class="Chemical">ns, unless otherwise stated, were performed under
an atmosphere of n class="Chemical">argon, using standn class="Chemical">ard Schlenk and glovebox techniques.
Glassware was oven-dried at 130 °C overnight and flamed under
vacuum prior to use. Hexane and pentane were dried using a Grubbs
type solvent purification system (MBraun SPS-800) and degassed by
successive freeze–pump–thaw cycles.[59] CD2Cl2, C6H5F, and 1,2-F2C6H4 were distilled
under vacuum from CaH2 and stored over 3 Å molecular
sieves, 1,2-F2C6H4 was stirred over
alumina for 2 h prior to drying. Bis-1,3-(diphenylphosphino)propane
(dpp3) and (2S,4S)-2,4-bis(diphenylphosphino)pentane(BDPP) were purchased from Aldrich. [Rh(nbd)Cl]2[60] and [Rh(nbd)(dpp3)][BArF4][16] were prepared as previously described.
(4-Methoxyphenyl)2HP·BH3 (3), (adamantyl)2HP·BH3 (4),
and CyH2P·BH3 (5) were prepared
by the same method as Me3P·BH3[61] but with the phosphine changed. (4-Trifluoromethylphenyl)2PH·BH3 (2) and (3,5-bis(trifluoromethyl)phenyl)2PCl were prepared according to literature procedures reported
by Clark et al.[33] NMR spectra were recorded
on a Bruker AVD 500 MHz spectrometer at room temperature unless otherwise
stated. In 1,2-C6H4F2, 1H NMR spectra were referenced to the center of the downfield solvent
multiplet (δ 7.07), and 31P and 11B NMR
spectra were referenced against 85% H3PO4 (external)
and BF3·OEt2 (external), respectively.
The spectrometer was prelocked and preshimmed using a C6D6 (0.1 mL) and 1,2-C6H4F2 (0.3 mL) sample. Chemical shifts are quoted in ppm and coupling
constants in Hz. ESI-MS were recorded on a Bruker micrOTOF instrument.[62] In all ESI-MS spectra there was a good fit to
both the principal molecular ion and the overall isotopic distribution.
Signals in the 31P{1H} NMR spectra were integrated
relative to those in similar environments (i.e., Rh–P or B–P)
to obtain the relative ratios of products, and data was acquired with
a pulse repetition time of 1 s. This avoids potential problems with
different relaxation times for different phosphorus environments.
Nevertheless, the quoted relative ratios based upon this data should
be treated as qualitative rather than quantitative.
Synthesis and Characterization
of New Complexes
Synthesis of H3B·PR2H [R = 3,5-Bis(trifluoromethyl)phenyl]
(1)
A solution of papan class="Chemical">n class="Chemical">(3,5-bis(trifluoromethyl)phenyl)2PCl (1.48 g, 3.0 mmon class="Chemical">l) in diethyl ether (5 mL) was added dropwise
to a diethyl ether (20 mL) suspension of LiBH4 (0.070 g,
3.21 mmol) cooled to 5 °C with an ice bath. The mixture became
cloudy immediately and was allowed to stir for 30 min. The diethyl
ether was removed in vacuo, and the residue was dissolved
in hexanes (30 mL) and filtered through Celite. The hexanes were reduced in vacuo to ∼10 mL, and the solution was placed in
the freezer (−20 °C) overnight yielding colorless crystals.
Excess hexanes were decanted, and crystals were dried to afford a
fine white powder which was subsequently washed with 2 × 3 mL
of cold hexanes. Removal of all volatiles under reduced pressure yielded
630 mg of fine white powder (1).
1H
NMR (300 MHz, CDCl3): δ 8.14 (br s, 1 H, p-Ar-H), 8.09 (br s, 2 H, o-Ar-H), 6.58
(dm, 1JHP = 388 Hz, 1 H, PH),
0.3–2.0 (br m, 3 H, BH). 31P{1H} NMR
(121 MHz, CDCl3): δ 4.7 (br s, PH). 11B{1H} NMR (160 MHz, CDCl): δ −41.7 (br s, BH3). 19F NMR (282 MHz, CDCl3): δ −62.9 (s, CF3). EI-MS (70 eV) m/z (%):
458 (62) [M+ – BH3]. Anal. Found: C 40.71%,
H 2.02%. Calcd for C16H10BF12P: C
40.68%, H 2.14%.
Synthesis of (Adamantyl)2PH·BH3 (4)
(Adamantyl)2PH·class="Chemical">pan> class="Chemical">BH3 was
prepared under the same conditions as Me3P·BH3[61] but with (adamantyl)2PH instead of PMe3.
1H NMR (300 MHz,
CDCl3): δ 3.61 (dm, 1 H, 1JHP = 379 Hz, PH), 2.11 to 1.83 (30 H, adamantyl-H), 0.41
to −0.15 (br m, 3 H, BH). 31P{1H} NMR
(121 MHz, CDCl3): δ 40.1 (br m, PH). 11B{1H} NMR (160 MHz, CDCl3): δ −44.8
(br d, BH3). Anal. Found: C 75.78%, H 10.71%. Calcd for
C20H34BP: C 75.89%, H 10.84%.
Method A follows. To a Youngs flask charged
with class="Chemical">pan> class="Chemical">[Rh(dpp3)(C6H5F)][BArF4] (50 mg, 0.034 mmol) and 2 equiv of H3B·PR2H (32 mg of 1, 23 mg of 2, 18 mg of 3 0.068 mmol) was added 1,2-F2C6H4 (5 mL). The solution was stirred at room temperature for
24 h. The formation of H2 gas is also observed. Complex 19 was isolated as yellow oil (37 mg, 61%). Complexes 17 and 18 could not be isolated cleanly; they
were observed with 22 and 23, respectively.
Method B follows.
To a Youngs flask charged with class="Chemical">pan> class="Chemical">[Rh(dpp3)(C6H5F)][BArF4] (50 mg, 0.034
mmol) and 1 equiv of 10 (32 mg, 0.068 mmol) was added
1,2-F2C6H4 (4 mL). Complex 17 was isolated as yellow solid (65 mg, 82%).
Details
follow for 17. Slow diffusion of pentane (10
mclass="Chemical">pan> class="Chemical">L) over a solution of 17 in1,2-F2C6H4 at −24 °C afforded yellow crystals (one
of which was employed for an X-ray diffraction study).
n class="Chemical">1Hpan> NMR (500 MHz, n class="Chemical">1,2-F2C6H4): δ 8.32 (s, 8H, n class="Chemical">BArF4), 7.69
(s, 4H, BArF4), 4.40 (vbr, 1H, BH) 3.10–2.12
(8H, 3CH dpp3 + BH2), −1.20 (vbr, 1H, BH), −4.54
(vbr, 1H, BH), −13.98 (s, 1H, Rh–H). Signals from aromatics
not observed due to being overlapped by signals from 1,2-F2C6H4. 31P{1H} NMR (202
MHz, 1,2-F2C6H4): δ 46.6 (dd, JRh–P= 111, JP2–P1(cis)= 36,
Ph2P(CH2)3PPh2), 29.5 (m, JP–P(trans) = 260, Rh-PR2BH3PR2HBH3), 12.8 (ddd,, JP–P(trans) = 260, JRh–P= 91, JP–P(cis) = 33, Ph2P(CH2)3PPh2), −2.7 (s, Rh-PR2BH3PR2HBH3). 11B{1H} NMR (160 MHz, 1,2-F2C6H4): δ 0.21 (br), −6.2 (s, BArF4), −27.1 (br). ESI-MS (1,2-F2C6H4, 60 °C) positive ion: m/z = 1457.09 (calcd 1457.12, M). Anal. Found: C 47.15%, H 2.34%. Calcd for C91H56B3F48P4Rh: C 47.07%,
H 2.43%.
Details follow for 18. n class="Chemical">1Hpan> NMR
(500 MHz,
n class="Chemical">1,2-F2C6H4): δ 8.32 (s, 8 H,
n class="Chemical">BArF4), 7.69 (s, 4 H, BArF4), 4.24 (v br, 1 H, BH), 2.61–1.72 (8 H, 3CH dpp3 + BH2), −1.29 (v br, 1 H, BH), −4.65
(v br, 1 H, BH), −14.90 (s, 1 H, Rh–H). Signals from
aromatics not observed due to being overlapped by signals from 1,2-F2C6H4. 31P{1H}
NMR (202 MHz, 1,2-F2C6H4): δ
42.5 (dd, JPRh = 106 Hz, JPP(cis) = 34 Hz, Ph2P(CH2)3PPh2), 28.6 (m, JPP(trans) = 272 Hz, Rh-PR2BH3PR2HBH3), 15.3 (ddd, JPP(trans) = 272 Hz, JPRh = 101 Hz, JPP(cis) = 33 Hz, Ph2P(CH2)3PPh2), −6.4 (s, Rh-PR2BH3PR2HBH3). 11B{1H} NMR (160 MHz, 1,2-F2C6H4): δ 0.4 (br), −6.2 (s, BArF4),
−25.9 (br). ESI-MS (1,2-F2C6H4) positive ion: m/z = 1185.17 (calcd
1185.18, M).
Details follow for 19. n class="Chemical">1Hpan> NMR (500 MHz,
n class="Chemical">1,2-F2C6H4): δ 8.32 (s, 8 H,
n class="Chemical">BArF4), 7.69 (s, 4 H, BArF4), 3.77 (s, 3 H, −OCH3), 3.76 (s, 3 H, −OCH3), 3.69 (s, 6 H, −OCH3), 2.85–1.72
(8 H, 3CH dpp3 + BH2), −1.03
(v br, 1 H, BH), −4.00 (v br, 1 H, BH), −14.55 (s, 1
H, Rh–H). Signals from aromatics not observed due to being
overlapped by signals from 1,2-F2C6H4. 31P{1H} NMR (202 MHz, 1,2-F2C6H4): δ 42.7 (ddd, JPRh = 109 Hz, JPP(cis) = 35 Hz, JPP(cis) = 12 Hz Ph2P(CH2)3PPh2), 28.1 (br m, JPP(trans) = 279 Hz, Rh-PR2BH3PR2HBH3), 12.5 (ddd, JPP(trans) = 279 Hz, JPRh =
88 Hz, JPP(cis) = 12 Hz, Ph2P(CH2)3PPh2), −11.7 (br s,
Rh-PR2BH3PR2HBH3). 11B{1H} NMR (160 MHz, C6H4F2): δ 3.42 (br), −6.2 (s, BArF4), −27.7 (br). ESI-MS (1,2-F2C6H4) positive ion: m/z = 1033.27 (calcd 1033.27, M).
Synthesis of [Rh(dpp3)(PHR2)2][BArF4] [R = 3,5-Bis(trifluoromethylphenyl) (21)]
Method A follows. To a Youngs flask charged with class="Chemical">pan> class="Chemical">[Rh(dpp3)(C6H5F)][BArF4] (50 mg, 0.034
mmol) and 2 equiv of H3B·PR2H (32 mg of 1) was added 1,2-F2C6H4 (5
mL). The solution was stirred at room temperature for 24 h. The formation
of H2 (gas) is also observed. Complex 21 could
not be isolated cleanly as 17 was also observed.
Method B follows. To a Youngs flask charged with class="Chemical">pan> class="Chemical">[Rh(dpp3)(C6H5F)][BArF4] (20 mg, 0.034
mmol) and 2 equiv of PHR2 (31 mg, 0.068 mmol, R = 3,5-bis(trifluoromethyl)phenyl)
was added 1,2-F2C6H4 (1 mL). After
stirring for 10 min the solution was evaporated to dryness and the
solid washed with pentane (2 mL). Complex 21 was isolated
as yellow solid (yield 17.8 mg, 57%).
n class="Chemical">1Hpan> NMR (500
MHz, n class="Chemical">1,2-F2C6H4): δ 8.33 (s,
8 H, n class="Chemical">BArF4), 7.69
(s, 4 H, BArF4), 6.41 (dm, JHP = 375 Hz, 2 H, PH), 2.62 (br, 4 H, 2 CH dpp3), 2.17 (m, 2 H CH2, dpp3). Signals
from aromatics not observed due to being overlapped by signals from
1,2-F2C6H4. 31P{1H} NMR (202 MHz, 1,2-F2C6H4): δ
9.6 (m, 2P, AA′BB′M), 5.5 (m, 2P, AA′BB′M). ESI-MS (1,2-F2C6H4) positive ion: m/z = 1431.04 (calcd
1431.07, M). Anal. Found: C 47.71%, H
2.21%. Calcd for C91H52BF48P4Rh: C 47.60%, H 2.28%.
Complex 22 was chn class="Chemical">arpan>acterized
by in situ NMR spectroscopy. n class="Chemical">1H NMR (500
MHz, 1,2-F2C6H4): δ 8.33 (s,
8 H, n class="Chemical">BArF4), 7.69 (s, 4 H, BArF4), 5.91 (dm, JHP = 359 Hz, 2 H,
PH), 2.57 (br, 4 H, 2 ×
CH2dpp3), 2.06 (m, 2 H × CH2, dpp3). Signals
from aromatics not observed due to being overlapped by signals from
1,2-F2C6H4. 31P{1H} NMR (202 MHz, 1,2-F2C6H4): δ
12.2 (m, 2P, AA′BB′M), 5.2 (m, 2P, AA′BB′M).
Synthesis of [Rh(dpp3)(η2-H3B·PR2H)][BArF4] [R = Adamantyl (23)]
To a Youngs flask charged with class="Chemical">pan> class="Chemical">[Rh(dpp3)(C6H5F)][BArF4] (50 mg, 0.034 mmol)
and H3B·PR2H (4) (11 mg, 0.034
mmol) was added 1,2-F2C6H4 (5 mL).
The solution was stirred at room temperature for 1 min, and a change
in color from pale orange to purple was observed. Complex 23 could not be isolated because further reaction to form 24 occurred.
n class="Chemical">1Hpan> NMR (500 MHz, n class="Chemical">1,2-F2C6H4): δ 8.32 (s, 8 H, n class="Chemical">BArF4), 7.68 (s, 4 H, BArF4), 3.45 (d, 1
H, 1JHP = 380 Hz, B-PH), 2.52–1.12
(36 H, adamantyl-H + dpp3 CH2), −1.36 (br, 3 H,
BH3). Signals from aromatics not observed due to being
overlapped by signals from 1,2-F2C6H4. 31P{1H} NMR (202 MHz, 1,2-F2C6H4): δ 35.1 (d, JPRh = 167 Hz, dpp3), 30.5 (br, B–P). 11B NMR (160
MHz, 1,2-F2C6H4): δ −0.8
(br), −6.0 (s, BArF4). ESI-MS (1,2-F2C6H4) positive ion: m/z = 629.09 (calcd 629.08, [Rh(dpp3)(C6H4F2)]+). The weakly bound σ-complex
could not be observed.
Synthesis of [Rh(dpp3)(PR2H)(H3B·PR2H)][BArF4] [R =
Adamantyl (24)]
Method A follows. To a Youngs
flask charged with class="Chemical">pan> class="Chemical">[Rh(dpp3)(C6H5F)][BArF4] (50 mg, 0.034
mmol) and 2 equiv of H3B·PR2H (4) (22 mg, 0.068 mmol) was added 1,2-F2C6H4 (5 mL). The solution was stirred at room temperature for
24 h. Complex 24 was isolated as orange solid.
Method B follows. To a Youngs flask charged with class="Chemical">pan> class="Chemical">[Rh(dpp3)(C6H5F)][BArF4] (50 mg, 0.034
mmol) and 1 equiv of PHR2 (10 mg, 0.034 mmol) was added
1,2-F2C6H4 (5 mL). The solution was
stirred for 5 min, and then 1 equiv of H3B·PR2H (4) (11 mg, 0.034 mmol) was added. The solution
was stirred for 5 min, and complex 24 was isolated as
orange solid (yield 48 mg, 71%). Slow diffusion of pentane (10 mL)
into a solution of 24 in 1,2-F2C6H4 at −24 °C afforded yellow crystals (one
of which was employed for X-ray diffraction studies).
n class="Chemical">1Hpan> NMR (500 MHz, n class="Chemical">1,2-F2C6H4): δ
8.33 (s, 8 H, n class="Chemical">BArF4), 7.69
(s, 4 H, BArF4), 3.30 (d, 1JHP = 362 Hz, 1 H, B-PH), 2.87 (d, 1JHP = 412 Hz, 1 H, PH), 2.45–1.56
(66 H, dpp3 CH2 and adamantyl-H), −0.24 (br, 3 H,
BH3). 31P{1H} NMR (202 MHz, 1,2-F2C6H4): δ 60.4 (ddd, JPP(trans) = 266 Hz, JPRh =
142 Hz, JPP(cis) = 30 Hz, Ph2P(CH2)3PPh2), 30.5 (s, PHR2BH3), 22.8 (dd, JPRh = 163
Hz, JPP(cis) = 30 Hz, PPh2(CH2)3PPh2), 6.6 (ddd, JPP(trans) = 270 Hz, JPRh =
144 Hz, JPP(cis) = 30 Hz, Rh-PR2BH3). 11B{1H} NMR (160 MHz, 1,2-F2C6H4): δ −6.0 (s, BArF4), −42.2 (br, BH3). ESI-MS (1,2-F2C6H4) positive ion: m/z = 956.30 (unidentified fragment). Anal. Found:
C 59.38%, H 4.99%. Calcd for C99H103B2F24P4Rh: C 59.50%, H 5.20%.
Synthesis
of [Rh(dpp3)H(PCyH·BH3)(H3B·PCyH2)][BArF4] (25a and 25b)
To a Youngs flask chpapan class="Chemical">n class="Chemical">arged with class="Chemical">pan> class="Chemical">[Rh(dpp3)(C6H5F)][BArF4] (50 mg, 0.034
mmol) was added 1,2-F2C6H4 (5 mL).
A 2 equiv portion of H3B·PH2Cy (5) (0.68 mL, 0.1 M solution in 1,2-F2C6H4, 0.068 mmol) was then added. The solution was stirred at
room temperature for 1 min, and a change in color from orange to pale
yellow was observed. Complexes 25a and 25b were observed as an approximate 1:1 ratio of isomers and were characterized in situ by NMR spectroscopy. Complexes 25a and 25b could not be isolated as they reacted quickly to form
complexes 26a and 26b. The 31P{1H} NMR spectrum of this reaction mixture indicates
that 2 diastereomers are present; while we were able to identify the
2 sets of 4 resonances each (labeled † and §, based on
coupling constants and approximate integrations) it was not possible
to determine which set of signals belonged to which diastereomer.
See Figure S4, Supporting Information,
for more detail.
n class="Chemical">1Hpan> NMR (500 MHz, n class="Chemical">1,2-F2C6H4): δ 8.32 (s, 8 H, n class="Chemical">BArF4), 7.69 (s, 4 H, BArF4), 2.73–0.32
(32 H, 3 CH dpp3 + BH2, CyH,
PH), −2.29 (v br, 3 H, BH), −7.92 (br d, 1 H, BH-Rh),
−17.51 (s, 1 H, Rh–H). Signals from aromatics not observed
due to being overlapped by signals from 1,2-F2C6H4. 31P{1H} NMR (202 MHz, 1,2-F2C6H4): δ 31.7 (dm, JPRh = 134 Hz, Ph2P1†(CH2)3PPh2), 30.5 (dm, JPRh = 129 Hz, Ph2P1§(CH2)3PPh2), 11.8 (overlapping ddd, JPP(trans) = approximately 200 Hz, JPRh = approximately 104 Hz, JPP(cis) = approximately 25 Hz, Ph2P(CH2)3PPh2), −11.0 (br d, JPP(trans) = approximately 200 Hz, Rh-PHCy-B), −32.1 (br d, JPP(trans) = approximately 200 Hz, Rh-PHCy-B),
−39.8 (br s, Rh–H3BP4†H2Cy), −44.2 (br s, Rh–H3BP4H2Cy). ESI-MS (1,2-F2C6H4, 60 °C) positive ion: m/z = 747.20 (calcd 747.21, M – 2BH3).
Synthesis of [(CyH2P)2BH2]Br
To a stirred solution
of papan class="Chemical">n class="Chemical">PCyH2 (3.40 mL, 10% wt in class="Chemical">pan> class="Chemical">hexane,
2.0 mmol) in dichloromethane (20 mL) was added BrH2B·SMe2 (1.0 mL, 1.0 M in CH2Cl2, 1.0 mmol)
and the solution stirred at room temperature for 2 h. The resulting
colorless solution was concentrated in vacuo to approximately
3 mL, and diethyl ether (30 mL) was added to precipitate a white solid.
The solvent was decanted off and the solid washed with a further 10
mL of diethyl ether. The solid was redissolved in dichloromethane,
filtered, and recrystallized from a mixture of dichloromethane and
diethyl ether at −18 °C to yield white crystals (first
crop 0.190 g, second crop 0.041 g, overall yield 71%). At room temperature
in CD2Cl2 [(CyH2P)2BH2]Br undergoes a degenerate exchange reaction; [(CyH2P)2BH2]Br is in equilibrium with CyH2PBH2Br + PH2Cy. This exchange process does
not occur at −60 °C on the NMR time scale, and each of
these species can be observed. In the solid-state the complex exists
as [(CyH2P)2BH2]Br.
1H NMR (500 MHz, −60 °C, CD2Cl2):
δ 5.46 (dm, JHP = 429 Hz, [(CyH2P)2BH2]+), 4.75 (dm, JHP = 388 Hz, CyH2PBH2Br), 2.52 (dm, JHP = 196 Hz,
CyH2P), 2.37–1.16 (CyH and BH). 31P{1H} NMR (202 MHz, −60 °C, CD2Cl2): δ −35.5 (br s, [(CyH2P)2BH2]+), −38.2 (br s, CyH2PBH2Br), −107.5 (s, CyH2P). Anal. Found:
C 44.35%, H 8.74%. Calcd for C12H28BBrP2: C 44.30%, H 8.68%.
Synthesis of CyH2P·BH2PCyH·BH3 (13)
[(CyH2P)2BH2]Br (0.150 g, 0.461
mmoclass="Chemical">pan> class="Chemical">l) (prepared as above) and [NnBu4][BH4] (0.119 g, 0.461 mmol) were
added to a Schlenk tube. Dichloromethane (10 mL) was added, and effervescence
was observed; the solution was stirred at room temperature for 1 h.
The solvent was removed in vacuo, and n-hexane (20 mL) was added to the white solid. The solution was filtered
to remove [NnBu4]Br and concentrated to approximately
2 mL. Storage of this solution for 16 h at −18 °C yielded
colorless crystals of 13 (0.102 g, 86%).
1H NMR (500 MHz, 25 °C, CD2Cl2): δ
4.68 (dm, JHP = 386 Hz, CyH2P·BH2PHCy·BH3), 3.50 (dm, JH = 326 Hz, CyH2P·BH2PHCy·BH3), 2.17–0.02
(CyH and BH). 31P{1H} NMR (202 MHz, 25 °C,
CD2Cl2): δ −36.6 (br, CyH2P·BH2PHCy·BH3),
−45.1 (br, CyH2P·BH2PHCy·BH3). Anal. Found: C 55.79%, H 11.82%. Calcd
for C12H30B2P2: C 55.77%,
H 11.71%.
Synthesis of [Rh(dpp3)H(PHCy·BH2PHCy·BH3)][BArF4] (26a and 26b)
Method A follows. To a
Youngs flask chpapan class="Chemical">n class="Chemical">arged
with class="Chemical">pan> class="Chemical">[Rh(dpp3)(C6H5F)][BArF4] (50 mg, 0.034 mmol) was added 1,2-F2C6H4 (5 mL) followed by 2 equiv of H3B·PCyH2 (5) (0.68 mL, 0.1 M solution in 1,2-F2C6H4, 0.068 mmol). The solution was stirred
at room temperature for 12 h. The formation of H2 is also
observed. Complexes 26a and 26b were characterized
as a mixture in solution by NMR spectroscopy and ESI-MS. The 31P{1H} NMR spectrum of this reaction mixture indicates
that 2 diastereomers are present; we were able to identify the 2 sets
of 4 resonances each (labeled 26a and 26b, based on coupling constants and approximate integrations) and tentatively
assigned the individual diastereomers (Scheme S5, Supporting Information) by inspection of a model.
Method
B follows. To a Youngs flask charged with class="Chemical">pan> class="Chemical">[Rh(dpp3)(C6H5F)][BArF4] (50 mg, 0.034 mmol) and 1
equiv of CyH2P·BH2PCyH·BH3 (13) (8.8 mg, 0.034 mmol) was added 1,2-F2C6H4 (4 mL). Complexes 26a and 26b were characterized as a mixture in solution by NMR spectroscopy
and ESI-MS.
n class="Chemical">1Hpan> NMR (500 MHz, n class="Chemical">1,2-F2C6H4): δ 8.32 (s, 8 H, n class="Chemical">BArF4), 7.69
(s, 4 H, BArF4), 4.35–3.01 (PH and BH),
2.92–0.92 (30 H, 3CH dpp3, BH2, CyH), −2.98 (br, 1 H, BH-Rh), −5.98 (br, 1
H, BH-Rh), −16.08 (s, 1 H, Rh–H). Signals from aromatics
not observed due to being overlapped by signals from 1,2-F2C6H4. 31P{1H} NMR (202
MHz, 1,2-F2C6H4): δ 37.9 (dm, JPRh = 102 Hz, Ph2P(CH2)3PPh2), 34.5 (dm, JPRh = 102 Hz, Ph2P(CH2)3PPh2), 19.8 (br d, JPP(trans) = approximately 255 Hz, Rh-PHCyB), 16.2 (br d, JPP(trans) = approximately 255 Hz, Rh-PHCy-B), 10.7 (overlapping
dm, JPP(trans) = 255 Hz, JPRh = 88 Hz, JPP(cis) = approximately
25 Hz, Ph2P(CH2)3PPh2), −14.9 (br
s, Rh-PHCyBH2PHCyBH3), −16.6 (br s, Rh-PHCyBH2PHCyBH3). ESI-MS (1,2-F2C6H4, 60 °C)
positive ion: m/z = 773.26 (calcd
773.24, M).
Synthesis of [Rh(BDPP)(nbd)][BArF4]
n class="Chemical">[Rh(nbd)2][BArF4]pan> was synthesized
by an adaptation of the prepn class="Chemical">aration of [Rh(cod)2][BArF4].[63] [Rh(nbd)2][BArF4] (0.100 g, 0.0869 mmol) was dissolved
inn class="Chemical">CH2Cl2 (10 mL) to produce a deep red solution.
(2S,4S)-2,4-Bis(diphenylphosphino)pentane(BDPP) (0.0383 g, 0.0869 mmol) was added, and a color change to red
was observed. The solution was stirred for 1 h at room temperature
before the solvent was removed in vacuo. n-Pentane (20 mL) was added, and the solution was sonicated
to produce an orange powder. The solvent was decanted and the solid
washed with pentane (2 × 20 mL). The product was dried in vacuo and isolated (yield 0.102 g, 78%).
1H NMR (300 MHz, CD2Cl2): δ 7.72 (s, 8
H, BArF4), 7.56 (s, 4 H, BArF4), 7.70–7.36 (m, P-Ph), 4.86 (m, 2 H, =C—H),
4.30 (m, 2 H, =C—H), 3.90 (m, 2 H, nbd C—H),
2.77 (m, 2 H, P—C—H), 1.84 (tt, 3JHH = 6.45 Hz, 3JHP = 19.95 Hz, 2 H, CH2), 1.58 (m, 2 H, nbd
CH2), 1.13 (m, 6 H, CH3). 31P{1H} NMR (121.6 MHz, CD2Cl2): δ
27.4 (d, JPRh = 149 Hz). Anal. Found:
C 54.32%, H 3.18%. Calcd for C68H50BF24P2Rh: C 54.47%, H 3.36%.
Synthesis of [Rh(BDPP)(C6H5F)][BArF4]
[Rh(BDPP)(nbd)][BArF4] preppan> class="Chemical">ared as above (0.020 g, 0.0133 mmol) was
added to a
high-pressure NMR tube and dissolved in fluorobenzene (0.5 mL). The
sample was degassed by the freeze–pump–thaw method and
hydrogen gas (4 atm) introduced. The sample was mixed for 30 min and
then degassed by the freeze–pump–thaw method and placed
under argon.
n class="Chemical">1Hpan> NMR (500 MHz, n class="Chemical">C6H5F): δ 8.32 (s, 8 H, n class="Chemical">BArF4), 7.61
(s, 4 H, BArF4), 5.63 (m, 2 H, Ar–H),
5.43 (m, 2 H, Ar–H), 5.10 (m, 1 H, Ar–H), 2.38 (m, 2
H, P-CH), 1.40 (m, 2 H, CH2), 0.78 (m, 6 H, CH3). Signals from P-Ph not observed due to being overlapped by signals
from C6H5F. Signals from norbornane (from hydrogenation
of norbornadiene) can also be observed at δ 2.12 (m, 2 H, nba
C–H), 1.40 (m, 2 H, nba CH2, overlapped), 1.11 (m,
2 H, nba CH2). 31P{1H} NMR (202 MHz,
C6H5F): δ 39.9 (d, JPRh = 194 Hz). ESI-MS (C6H5F, 60 °C)
positive ion: m/z = 639.13 (calcd
639.12, M+).
Synthesis of [Rh(BDPP)H(PCyH·BH3)(H3B·PCyH2)][BArF4] (27a and 27b)
n class="Chemical">[Rh(BDpan class="Chemical">PP)(C6H5F)][BArF4] (26) was prepared in
a high-pressure NMR tube as above. To this was added 2 equiv of H3B·PH2Cy (5) (0.27 mL, 0.1 M solution
in 1,2-F2C6H4, 0.027 mmol) and the
solution mixed for 1 min. Complexes 27a and 27b were observed as ratio of isomers and were characterized in situ by NMR spectroscopy. Complexes 27a and 27b could not be isolated as they reacted quickly to form
complexes of 28; signals for 28 can be observed
in both 1H and 31P{1H} NMR spectra
of 27a and 27b which, along with the presence
of H2, show that the complexes rapidly undergo dehydrocoupling
to form complexes of 28. The 31P{1H} NMR spectrum of this reaction mixture indicates that 2 diastereomers
are present; while we were able to identify the 2 sets of 4 resonances
(labeled † and §, based on coupling constants and approximate
integrations), it was not possible to determine which set of signals
belonged to which diastereomer. See Figure S6, Supporting Information.
n class="Chemical">1Hpan> NMR (500 MHz,
n class="Chemical">C6H5F + n class="Chemical">1,2-F2C6H4): δ 8.32 (s, 8 H, BArF4), 7.69
(s, 4 H, BArF4), 4.19 to 0.32 (CH, CH, CH, BDPP, BH2, CyH, PH), −1.80 to −3.31 (v br, 3 H, BH3), −7.54 to −8.86 (br d, 1 H, BH-Rh), −17.73
(br s, 1 H, Rh–H). Signals from aromatics not observed due
to being overlapped by signals from C6H5F and
1,2-F2C6H4. H2 can be
observed in the NMR spectrum as a sharp singlet at δ 4.52 ppm
suggesting that some dehydrocoupling to complex 28 has
occurred. This is further evidenced by the small Rh–H hydride
signal at δ −16.07 ppm which is observed for complex 28. 31P{1H} NMR (202 MHz, C6H5F + 1,2-F2C6H4): δ
42.5 (dm, JPRh = 129 Hz, Ph2P(CH2)3PPh2), 32.6 (dm, JPRh = 119 Hz, Ph2P(CH2)3PPh2), 28.3 (overlapping ddd, JPP(trans) =
approximately 208 Hz, JPRh = approximately
98 Hz, JPP(cis) = approximately 32 Hz,
Ph2P(CH2)3P2†§Ph2), 3.6 (br d, JPP(trans) = approximately 208 Hz, Rh-PHCy-B), −12.1
(br d, JPP(trans) = approximately 208
Hz, Rh-PHCy-B), −42.1 (br s, Rh–H3BPH2Cy), −44.2 (br s, Rh–H3BPH2Cy).
Synthesis of [Rh(BDPP)H(PCyH·BH2PCyH·BH3)][BArF4] (28a, 28b, 28c, and 28d)
Method
A follows. n class="Chemical">[Rh(BDpan class="Chemical">PP)(C6H5F)][BArF4] was prepared on an NMR scale from [Rh(BDPP)(nbd)][BArF4] (0.020 g, 0.0133 mmol) as above. This solution
was transferred by cannula to an NMR tube containing CyH2P·BH2PHCy·BH3 (13) (3.4
mg, 0.0133 mmol), and the solution was mixed briefly to yield a pale
yellow solution.
Method B follows. [Rh(BDPP)(C6H5F)][BArF4] was preppan> class="Chemical">ared on an NMR scale
from [Rh(BDPP)(nbd)][BArF4] (0.020 g, 0.0133
mmol) as above. To this was added 2 equiv of H3B·PH2Cy (5) (0.27 mL, 0.1 M solution in 1,2-C6H4F2, 0.027 mmol) and the solution mixed
for 18 h.
The n class="Chemical">31pan class="Chemical">P{class="Chemical">pan> class="Chemical">1H} NMR spectrum arising
from method
A indicates that 4 diastereomers are present; while we were able to
identify some of the signals from the 4 sets of 4 resonances (labeled
†, §, $, and & based on coupling constants and approximate
integrations), it was not possible to determine which set of signals
belonged to which of the diastereomers. See Figure S7, Supporting Information. Method B affords, essentially,
only one diastereoisomer.
n class="Chemical">1Hpan> NMR (500 MHz, n class="Chemical">C6H5F): δ
8.32 (s, 8 H, n class="Chemical">BArF4), 7.69 (s, 4 H, BArF4), 5.47–2.46 (PH), 2.11–0.63 (CH,
CH2, and CH3 BDPP, BH2, CyH), −3.21
(br, 1 H, BHRh), −5.65 to −6.85 (br, 1 H, BHRh), −16.09,
−16.20, and −17.04 (s, 1 H, Rh–H). Signals from
aromatics not observed due to being overlapped by signals from C6H5F and 1,2-C6H4F2. 31P{1H} NMR (202 MHz, C6H5F): δ 58.9 (ddd, JPRh =
107 Hz, JPP(cis) = 30 Hz, JPP(cis) = 12 Hz, Ph2P(CH2)3PPh2), 58.0 (dm, JPRh = 102
Hz, Ph2P1(CH2)3PPh2), 46.6 (dm, JPRh = 100 Hz, Ph2P (CH2)3PPh2), 43.6
(dm, JPRh = 102 Hz, Ph2P1&(CH2)3PPh2), 28.7 (ddd, JPP(trans) = 254 Hz, JPRh = 90 Hz, JPP(cis) = 32 Hz, Ph2P(CH2)3PPh2), 27.5 (ddd, JPP(trans) = 250 Hz, JPRh = 92 Hz, JPP(cis) = 32 Hz, Ph2P(CH2)3PPh2), 24.8–17.7 (overlapping m, JPP(trans) = approximately 254 Hz, JPRh = approximately 87 Hz, JPP(cis) = approximately 26 Hz, Ph2P(CH2)3P2†§Ph2 and Rh-P3HCy-B), −13.5 to −20.6 (br s
of 4 isomers, Rh-PHCyBH2PHCyBH3). ESI-MS (1,2-C6H4F2, 60
°C) positive ion: m/z = 801.29
(calcd 801.29, M).
In order to
establish that dehydrocouplipan class="Chemical">ng had occurred when method
B was used to form complexes 28, excess (10 equiv) of
n class="Chemical">1,2-bis(diphenylphosphino)ethane was added to the reaction mixture
to release the dehydrocoupled product, n class="Chemical">CyH2P·BH2PHCy·BH3, from the metal center. The 31P{1H} NMR spectrum showed two broad signals at
δ −37.9 and −43.9 which are in agreement with
those found for compound 13.
Attempted Polymerization
of PhH2P·BH3 in Solution
In a
procedure similpapan class="Chemical">n class="Chemical">ar to that reported by
Manclass="Chemical">pan>ners et al.,[20] n class="Chemical">PhH2P·BH3 (0.248 g, 2.0 mmon class="Chemical">l) was dissolved in toluene (10 mL) either
in the presence of [Rh(dpp3)(C6H5F)][BArF4] (14.7 mg, 0.01 mmol) or with no catalyst present.
The solution was heated to reflux for 16 h before cooling to room
temperature. The solution was concentrated in vacuo and added to stirred hexane (100 mL) to produce a white precipitate.
The solvent was decanted and the solid washed with hexane (2 ×
50 mL). The solid was dried in vacuo and isolated
in air (yield 0.110 g Rh catalyzed, 0.101 g uncatalyzed). The 31P{1H} NMR spectrum of the isolated solids produced
by the different methods are very similar with several very broad
peaks from δ −45 to −57 and very broad peaks of
lower intensity from δ −72 to −87 . See Figure
S8, Supporting Information. This is in
agreement with the results obtained by Manners et al. for uncatalyzed
polymerization of PhH2P·BH3.[20]
Melt Polymerization of PhH2P·BH3
A Youngs flask chpapan class="Chemical">n class="Chemical">arged with class="Chemical">pan> class="Chemical">PhH2P·BH3 (31 mg, 0.25 mmol) and 5 mol % of [Rh(dpp3)(C6H5F)][BArF4] (18.4 mg, 0.0125 mmol)
was heated
to 90 °C for 4 h in melt conditions. The resulting solid was
dissolved in 1,2-difluorobenzene and analyzed by NMR spectroscopy.
The 31P{1H} NMR spectrum shows a peak at δ
−49.3 ppm in agreement with that observed by Manners et al.[7] for polymeric material and a lower intensity
resonance at δ −55.0 ppm. See Supporting
Information Figure S9. In the 31P{1H}
NMR spectrum the peak at δ −49.3 ppm split into a broad
doublet with a JPH coupling constant of
approximately 350 Hz. Analysis by ESI-MS of the reaction mixture showed
a repeating pattern corresponding to the polymeric repeat unit −[PhHP·BH2]–, see Figure S10, Supporting
Information.
Authors: Holger Helten; Alasdair P M Robertson; Anne Staubitz; James R Vance; Mairi F Haddow; Ian Manners Journal: Chemistry Date: 2012-03-05 Impact factor: 5.236
Authors: Timothy J Clark; José M Rodezno; Scott B Clendenning; Stephane Aouba; Peter M Brodersen; Alan J Lough; Harry E Ruda; Ian Manners Journal: Chemistry Date: 2005-07-18 Impact factor: 5.236
Authors: Heather C Johnson; Alasdair P M Robertson; Adrian B Chaplin; Laura J Sewell; Amber L Thompson; Mairi F Haddow; Ian Manners; Andrew S Weller Journal: J Am Chem Soc Date: 2011-07-01 Impact factor: 15.419
Authors: Nicolas Merle; Gabriele Koicok-Köhn; Mary F Mahon; Christopher G Frost; Giuseppe D Ruggerio; Andrew S Weller; Michael C Willis Journal: Dalton Trans Date: 2004-10-25 Impact factor: 4.390
Authors: Andrew R Jupp; Gemma Trott; Éléonore Payen de la Garanderie; James D G Holl; Duncan Carmichael; Jose M Goicoechea Journal: Chemistry Date: 2015-04-17 Impact factor: 5.236
Authors: Christian Marquardt; Titel Jurca; Karl-Christian Schwan; Andreas Stauber; Alexander V Virovets; George R Whittell; Ian Manners; Manfred Scheer Journal: Angew Chem Int Ed Engl Date: 2015-10-02 Impact factor: 15.336
Scheme 1
Scheme 2
Figure 1
Scheme 3
Figure 2
Scheme 4
Scheme 5
Figure 3
Scheme 6
Scheme 7
Figure 4
Scheme 8
Figure 5
Scheme 9
Scheme 10
Scheme 11