Theory and experiment indicate that the protonation of reduced NiFe dithiolates proceeds via a previously undetected isomer with enhanced basicity. In particular, it is proposed that protonation of (OC)3Fe(pdt)Ni(dppe) (1; pdt(2-) = (-)S(CH2)3S(-); dppe = Ph2P(CH2)2PPh2) occurs at the Fe site of the two-electron mixed-valence Fe(0)Ni(II) species, not the Fe(I)-Ni(I) bond for the homovalence isomer of 1. The new pathway, which may have implications for protonation of other complexes and clusters, was uncovered through studies on the homologous series L(OC)2Fe(pdt)M(dppe), where M = Ni, Pd (2), and Pt (3) and L = CO, PCy3. Similar to 1, complexes 2 and 3 undergo both protonation and 1e(-) oxidation to afford well-characterized hydrides ([2H](+) and [3H](+)) and mixed-valence derivatives ([2](+) and [3](+)), respectively. Whereas the Pd site is tetrahedral in 2, the Pt site is square-planar in 3, indicating that this complex is best described as Fe(0)Pt(II). In view of the results on 2 and 3, the potential energy surface of 1 was reinvestigated with density functional theory. These calculations revealed the existence of an energetically accessible and more basic Fe(0)Ni(II) isomer with a square-planar Ni site.
Theory and experiment indicate that the protonation of reduced NiFe dithiolates proceeds via a previously undetected isomer with enhanced basicity. In particular, it is proposed that protonation of (OC)3Fe(pdt)Ni(dppe) (1; pdt(2-) = (-)S(CH2)3S(-); dppe = Ph2P(CH2)2PPh2) occurs at the Fe site of the two-electron mixed-valence Fe(0)Ni(II) species, not the Fe(I)-Ni(I) bond for the homovalence isomer of 1. The new pathway, which may have implications for protonation of other complexes and clusters, was uncovered through studies on the homologous series L(OC)2Fe(pdt)M(dppe), where M = Ni, Pd (2), and Pt (3) and L = CO, PCy3. Similar to 1, complexes 2 and 3 undergo both protonation and 1e(-) oxidation to afford well-characterized hydrides ([2H](+) and [3H](+)) and mixed-valence derivatives ([2](+) and [3](+)), respectively. Whereas the Pd site is tetrahedral in 2, the Pt site is square-planar in 3, indicating that this complex is best described as Fe(0)Pt(II). In view of the results on 2 and 3, the potential energy surface of 1 was reinvestigated with density functional theory. These calculations revealed the existence of an energetically accessible and more basicFe(0)Ni(II) isomer with a square-planar Ni site.
The protonation of
metalcomplexes and clusters continues to attract
much attention because of its relevance to catalysis involving proton
and hydride transfer reactions. In organometallic and heterogeneous
catalysis, protonation is a step in substrate hydrogenation and hydrogen
evolution.[1,2] In biology, metal protonation is implicated
in the reduction of CO2 and N2, not to mention
hydrogen evolution.[2,3] For all of these reactions, a
central question is the regiochemistry of the protonation. For compounds
with metal–metal bonds, protonation can in principle occur
at a single metal or at the metal–metal bond.The regiochemistry
of protonation is relevant to a mechanistic
understanding of the [NiFe]- and [FeFe]-hydrogenases, which also feature
Ni–Fe and Fe–Fe bonding interactions.[4,5] By
proton transfer, these complexes equilibrate protons and dihydrogen,
i.e., 2H+ + 2e– ⇌ H2 in accordance with the local redox environment and activities of
H+ and H2. Functional models for the active
sites of these enzymes present opportunities for translating biological
insights into practical catalytic systems.[6] In addition to being fast and operating at low overpotentials, the
hydrogenases are attractive targets for functional modeling because
they utilize earth abundant metals.[7] For
the above reasons, much effort has been directed, both computationally[8] and experimentally,[9] at understanding the details of proton transfer to and from syntheticNiFe and FeFecomplexes.While many complexes engage separately
in acid–base and
redox reactions, few exhibit both kinds of reactivity, providing a
possible explanation for the paucity of functional hydrogenase (H2ase) models. With respect to the [NiFe]-H2ases
(Figure 1, left), the premier example of such
a model is (OC)3Fe(pdt)Ni(dppe) (1).[10] Tricarbonyl 1 and analogous phosphine-substituted
derivatives undergo: (i) protonation to afford rare examples of nickel–ironhydrides and (ii) 1e– oxidation to give Fe(I)Ni(II)
derivatives (Figure 1, right). Several examples
of charge-neutral Fe(I)Ni(I) species, as well as their related Fe(II)Ni(II)
hydrides[11−13] and Fe(I)Ni(II)cations,[14,15] have been characterized.
Figure 1
Active site of [NiFe]-H2ase, a bidirectional
catalyst,
participates in redox and acid–base chemistry (left). Similar
behavior is observed for the prototypical model complex 1 (right).
Active site of [NiFe]-H2ase, a bidirectional
catalyst,
participates in redox and acid–base chemistry (left). Similar
behavior is observed for the prototypical model complex 1 (right).The [NiFe]-H2ase active
site features Ni bound to four
Cys ligands in a distorted tetrahedral (SF4-like) arrangement.
Two of the thiolates link to an Fecenter whose coordination sphere
includes the unusual CO and CN– cofactors, the latter
H-bonding to neighboring Ser and Arg residues.[4] In the active states of the enzyme, the FeNicore appears to be
relatively conformationally rigid, with the Fe···Ni
distance and coordination geometry being rather insensitive both to
redox state and the presence/absence of H– substrate.[16] The rigidity of the active site is undoubtedly
a factor in the high rates and low overpotentials at which the interconversion
of H+ and H2 is mediated.Incontrast to the rigid active site, small molecule mimics
are
conformationally dynamic. For example, 1 undergoes a
tetrahedral → square-planar twist at Ni upon oxidation or protonation
(Figure 1). While the structures of 1, [1]+, and [1H]+ are established,[10−15] those of any intermediates are not. Such transformations may, for
example, involve “pre-twisting” of the Ni coordination
in 1 (Scheme 1, top).
Scheme 1
Distortion of the ground-state structure of 1 could
well affect oxidation states by inducing 2e– mixed-valency.[17] Furthermore, the degree of Fe–Ni bonding
and, most importantly, reactivity would also be perturbed. While ligand-centered
isomerism is a common theme in the chemistry of multimetalliccompounds,
drastic differences in coordination geometry and metal–metal
bonding between two (or more) interconverting species are extremely
rare. They are, however, not without precedent; for example, Jones
and co-workers reported X-ray structures for two complexes of formula
(OC)2Rh(μ-PR2)2Rh(CO)2 (Scheme 1, bottom).[18] One species takes the form of a red compound featuring Rh–Rh
bonding (2.761 Å), in which one Rhcenter is square-planar and
the other tetrahedral. A second (orange) isomer was also observed,
in which non-interacting Rhcenters (3.717 Å) adopt square-planar
geometries.The present study involves parallel experimental
and density functional
theory (DFT) investigations to obtain a complete structural, energetic,
and mechanistic picture of complexes derived from FeNicomplex 1. This work was extended to FePd and FePt species such that
a homologous series can be interrogated for information regarding
conformational motions and metal–metal bonding.
Results and Discussion
The major routes to low-valent FeNicomplexes of type (OC)3Fe(pdt)Ni(diphosphine) involve treatment
of (pdt)Ni(diphosphine)
with either (i) Fe2(CO)9 or (ii) FeI2(CO)4/CoCp2. As applied to the Pd and Pt derivatives,
the second route proved more attractive, giving the diamagnetic heterobimetallics(OC)3Fe(pdt)Pd(dppe) (2) and (OC)3Fe(pdt)Pt(dppe) (3) (Scheme 2).[19]
Scheme 2
Isolated as dark-green needles, 2 is sensitive to
O2 (decomposing to CO-free paramagnetic species) but is
thermally stable in the solid state under N2. The energies
of the two νCO bands (A + E modes) compare well with
those observed for 1 (Table 1).
Table 1
IR Data (νCO/cm–1) for FeM (M = Ni, Pd, Pt) Compounds in CH2Cl2 Solution
compound
FeNi
FePd
FePt
(OC)3Fe(pdt)M(dppe)
2028, 1952[20]
2024, 1955
1962, 1883, 1870
[(OC)3FeH(pdt)M(dppe)]BF4
2082, 2024[20]
2081, 2020
2081, 2022
[(OC)3Fe(pdt)M(dppe)]BF4
2057, 1986[13]
2058, 1986
2058,
1987
[Cy3P(OC)2Fe(pdt)M(dppe)]BF4
1966, 1899[12]
1964, 1900
1966, 1900
The FePd species 2 is
similar to 1 in
that its room temperature 31P{1H} NMR spectrum
features a broad singlet resonance (δ 51.3) for the dppe ligand.
At lower temperature, the resonance for 1 decoalesces
into two peaks (well-resolved at −68 °C), consistent with
a dynamic process that interconverts the 31P sites. The
previously reported coalescence temperature (−30 °C)[12] corresponds to a free energy barrier of 9.5
kcal/mol for the FeNi species 1. This process presumably
occurs via an intermediate or transition state with square-planar
Ni. No decoalescence was observed for 2 even at −85
°C, behavior suggestive of either: (i) a tetrahedral Pd ground
state with a particularly low rotational barrier or (ii) a square-planar
Pd ground state. The second possibility is ruled out based on the
similarity of the IR data for 1 and 2. The
structure of 2 in the solid state was confirmed crystallographically
(Figure 2).
Figure 2
ORTEP of 2 with ellipsoids
drawn at the 50% probability
level and H atoms omitted for clarity. Selected distances (Å)
(mean DFT values in parentheses): Pd1–Fe1, 2.561 (2.56); Pd1–P1,
2.249; Pd1–P2, 2.271 (2.26); Pd1–S1, 2.573; Pd1–S2,
2.402 (2.49); Fe1–S1, 2.299; Fe1–S2, 2.275 (2.30); Fe1–C30,
1.795; Fe1–C31, 1.793; Fe1–C32, 1.798 (1.77). Selected
angles (deg) (mean DFT values in parentheses): P1–Pd1–S1,
135.2; P1–Pd1–S2, 118.83; P2–Pd1–S1, 115.7;
P2–Pd1–S2, 122.5 (135.1); S1–Fe1–C30,
162.2; S2–Fe1–C32, 163.1 (161.1).
ORTEP of 2 with ellipsoids
drawn at the 50% probability
level and H atoms omitted for clarity. Selected distances (Å)
(mean DFT values in parentheses): Pd1–Fe1, 2.561 (2.56); Pd1–P1,
2.249; Pd1–P2, 2.271 (2.26); Pd1–S1, 2.573; Pd1–S2,
2.402 (2.49); Fe1–S1, 2.299; Fe1–S2, 2.275 (2.30); Fe1–C30,
1.795; Fe1–C31, 1.793; Fe1–C32, 1.798 (1.77). Selected
angles (deg) (mean DFT values in parentheses): P1–Pd1–S1,
135.2; P1–Pd1–S2, 118.83; P2–Pd1–S1, 115.7;
P2–Pd1–S2, 122.5 (135.1); S1–Fe1–C30,
162.2; S2–Fe1–C32, 163.1 (161.1).Given the covalent radii of Pd (1.39 Å) and low-spin
Fe (1.32
Å),[21] the Fe–Pd distance of
2.561 Å suggests the presence of bonding between the metalcenters
in 2. The intermetallic separation is somewhat shorter
in 1 (2.467 Å). Metal–metal interactions
aside, the Pd and Fecenters in 2 exist in roughly tetrahedral
and square-pyramidal (τ = 0.01 indicates a low degree of trigonality)[22] coordination geometries, respectively. The complex
is isostructural to its Fe(I)Ni(I)congener, and it is expected that
an Fe(I)Pd(I) description is appropriate, particularly considering
the tetrahedral Pdcoordination, as such a geometry is unlikely for
Pd(II).The structures of several complexes reported here were
calculated
using DFT, and the optimized structures were in excellent agreement
with available crystal structures (selected metrics are presented
in Tables S3 and S4). In the case of 2, calculations accurately predicted the geometry of Pd, as
well as its proximity to Fe. Additional isomers characterized as minima
with DFT will be discussed below. NBO (natural bond orbital)[23] analysis revealed the Fe–Pd bond to arise
from overlap of an Fe-centered d(z2) orbital with a Pd-centered orbital with d character (Figure S56b). NBO analysis
of FeNicongener 1 also revealed metal–metal bonding
(Figure S56a) consistent with previous
DFT analyses of this system.[10] NBO analysis
of other Fe–M complexes discussed herein did not indicate mixing
of metal d orbitals; hence, no Fe–M bonding
is expected for these species.
Mixed-Valence Derivatives
The cyclic
voltammogram of 2 features reversible and quasi-reversible
oxidations at E1/2 = −0.87 and
+0.26 V, respectively
(vs ferrocene/ferrocenium, Fc0/+; Figure 3). The reversible wave, assigned to the [2]0/+ couple, is 300 mV more negative than the wave for [1]0/+,[15] a result reflective
of the more reducing nature of the heavier group 10 metal.
Figure 3
Cyclic voltammograms
of 2 (dotted trace) and [2′]BF4 (solid trace) acquired in CH2Cl2 with
100 mM NBu4PF6.
Potentials (V vs Fc0/+) were swept at 100 mV s–1.
Cyclic voltammograms
of 2 (dotted trace) and [2′]BF4 (solid trace) acquired in CH2Cl2 with
100 mM NBu4PF6.
Potentials (V vs Fc0/+) were swept at 100 mV s–1.DFT was used to calculate the
potentials associated with the two
couples for each [L(CO)2Fe(pdt)M(dppe)]0/+/2+ (L = CO, PCy3; M = Ni, Pd, Pt) system. The calculated
potentials are in excellent agreement with the experimental values
for the neutral/monocation couples, while the values for the monocation/dication
couples are in qualitative but not quantitative agreement. The latter
discrepancies are mainly a result of these more anodiccouples being
only partially reversible, the associated potentials being less thermodynamically
reliable.ΔEp(n class="Chemical">Fc0/+) = 0.064 V under these n class="Chemical">conditions.
Anodin class="Chemical">c potential for irreversible
oxidation.
Data from ref (15).Calculated and experimental values
for [1′]BF4 agree by construction and
were used as references for the corresponding [1] waves.Calculated and experimental
values
for [2′]BF4 agree by construction and
were used as references for the corresponding [2], [3], and [3′] waves.This value was not determined as
difficulties arising from ln class="Chemical">ocal minima led to inconclusive results.
This wave is assigned to a
n class="Chemical">Fe0PtII/FeIPtII couple
(vide infra).
Chemical oxidation of 2 with FcBF4 afforded
[(OC)3Fe(pdt)Pd(dppe)]BF4 ([2]BF4), isolated as an olive-brown powder. Positive ion ESI-MS
analysis (m/z 749.8) was consistent
with the formula, and the lability of [(OC)3Fe(pdt)Pd(dppe)]+ was evidenced by observation of CO dissociation products
(m/z 721.8, 693.8). The νCO bands for [2]+ (2058, 1986 cm–1) are only modestly shifted from those of 2, implicating oxidation at the Pdcenter. EPR measurements also indicated
an Fe(I)Pd(II) description for [2]+, the assignment
being based on the magnitude of the g shifts and
the absence of 31P hyperfine coupling (Figure 4, top; for simulated EPR spectra and parameters
see Figure S23 and Table S1, respectively).
The data are similar to those for Fe(I)Ni(II)complexes, for which
theory indicated an Fe-centered d(z2) doublet ground-state singly occupied molecular orbital
(SOMO).[15] These computational results,
as well the implied absence of spin–orbit coupling expected
for a d(z2) ground state,
are consistent with a resonance in the vicinity of ge. Owing to the inversion of the pdt2– ring, the [(OC)3Fe(pdt)M(dppe)]+ radicals
exist as two conformers, one giving rise to an axial pattern and the
other to a rhombic pattern.
Figure 4
X-band EPR spectra (CH2Cl2/PhMe, 110 K) of
[2]BF4 and [2′]BF4.
X-band EPR spectra (CH2Cl2/PhMe, 110 K) of
[2]BF4 and [2′]BF4.DFT analysis of spin densities
was used to determine the localization
of the unpaired electron in [2]+ as well as
all other mixed-valence bimetallics (Table 3). In each case, the spin is entirely Fe-centered, consistent with
the Fe(I)M(II) assignments from EPR analyses. The spin density distribution
is unaffected by inversion of the pdt2– ring, as
observed for related FeNicomplexes.[14,15]
Table 3
Calculated Spin Densities on Metal
Centers in Mixed-Valence Fe(I)M(II) Species
complex
ρ(M)
ρ(Fe)
[(OC)3Fe(pdt)Ni(dppe)]+ ([1]+)
0.04
1.03
[Cy3P(OC)2Fe(pdt)Ni(dppe)]+ ([1′]+)
0.01
1.14
[(OC)3Fe(pdt)Pd(dppe)]+ ([2]+)
0.03
1.04
[Cy3P(OC)3Fe(pdt)Pd(dppe)]+ ([2′]+)
0.03
1.06
[(OC)3Fe(pdt)Pt(dppe)]+ ([3]+)
0.03
1.03
[Cy3P(OC)3Fe(pdt)Pt(dppe)]+ ([3′]+)
0.04
1.05
Cation [2]+ undergoes substitution with
PCy3 to afford [Cy3P(OC)2Fe(pdt)Pd(dppe)]BF4 ([2′]BF4) as a yellow-brown
solid. Similar to its tricarbonyl precursor, [2′]+ is also described as an Fe(I)Pd(II)compound, the rhombic
signals split by interaction of the SOMO with 31PCy3 (A(31P) = 54, 61, 57 MHz). The
data are distinct from those of the green Fe(I)Ni(II) analog [1′]+, which show 31P hyperfine
to be absent due to structural distortions at Fe.As with 2, the cyclic voltammogram of [2′]BF4 features two waves, the events at E1/2 = −1.14 and −0.20 V being assigned to
[2′]0/+ and [2′]+/2+ couples, respectively. The waves are cathodically
shifted relative to the tricarbonyl parent, with a smaller shift in
the first wave (ΔE1/2 = −0.27
V) than in the second (ΔE1/2 = −0.46
V). These trends were also reproduced by DFT calculations (Table 2). The fact that ligand substitution at Fe perturbs
the anodic wave to a greater degree points to this redox event being
Fe-centered. These data are consistent with the sequence: Fe(I)Pd(I)
⇌ Fe(I)Pd(II) ⇌ Fe(II)Pd(II). The reversibility of the
Fe(I)Pd(II)/Fe(II)Pd(II) ([2′]+/2+)
couple is greater than that of [2]+/2+, the
ligands in the former case being better suited to stabilize the ferrous
state.
Table 2
Redox Potentials (V vs Fc0/+,
Experimental and DFT-Calculated) and Associated Eletrochemical
Data (Recorded at 0.1 Vs–1) for FeNi, FePd, and
FePt Complexes (1 mM) in NBu4PF6 Electrolyte
(100 mM CH2Cl2 Solution)
E1/2 (FeIMII/FeIIMII) (|ipc/ipa|, |ΔEp|)
E° (FeIMII/FeIIMII) DFT
E1/2 (FeIMI/FeIMII) (|ipa/ipc|, |ΔEp|)a
E° (FeIMI/FeIMII)
DFT
1
+0.34b
+0.52
–0.59 (0.90, 0.059)
–0.62
[1′]BF4
–0.14 (0.68, 0.076)b
–0.14d
–0.92 (1.00, 0.072)c
–0.92d
2
+0.26 (0.29, 0.115)
+0.39
–0.87 (0.98, 0.061)
–0.92
[2′]BF4
–0.20 (0.67, 0.080)
–0.20e
–1.14
(0.83, 0.122)
–1.14e
3
+0.33 (0.49, 0.168)
NDf
–0.93 (0.97, 0.105)g
–0.91g
[3′]BF4
–0.19 (0.50, 0.076)
–0.24
–1.37 (0.66, 0.067)g
–1.38
ΔEp(Fc0/+) = 0.064 V under these conditions.
Anodic potential for irreversible
oxidation.
Data from ref (15).
Calculated and experimental values
for [1′]BF4 agree by construction and
were used as references for the corresponding [1] waves.
Calculated and experimental
values
for [2′]BF4 agree by construction and
were used as references for the corresponding [2], [3], and [3′] waves.
This value was not determined as
difficulties arising from local minima led to inconclusive results.
This wave is assigned to a
Fe0PtII/FeIPtII couple
(vide infra).
Palladium–Iron Hydride
The protonation of 2 with HBF4 affords the hydride salt[2H]BF4, the formation of which is signaled by the appearance
of high energy νCO bands (2081, 2020 cm–1) suggestive of an Fe(II)HPd(II) product. A high-field 1H NMR signal (δ −4.3) and a single downfield 31P NMR resonance (δ 62.8) are consistent with [2H]+ adopting a Cs-symmetric
structure in solution. When the acidification of 2 was
performed in the presence of CD3OD, ESI-MS and 2H NMR spectroscopy confirmed formation of the deuteride isotopologue
[2D]+. The evidence for the solution structure
of [2H]+ is complemented by crystallographic
analysis (Figure 5).
Figure 5
ORTEP of one of two independent
complexes in [2H]BF4·THF·0.5Et2O with ellipsoids drawn at
the 50% probability level. The solvate molecules, BF4– anion and H atoms (except the H– ligand) are omitted for clarity. Mean distances in the complexes
(Å) (mean DFT values in parentheses): Pd2–Fe2, 2.882 (2.92);
Pd2–H2, 2.173 (2.19); Pd2–P3, 2.253; Pd2–P4,
2.270 (2.30); Pd2–S3, 2.367; Pd2–S4, 2.363 (2.39); Fe2–S3,
2.330; Fe2–S4, 2.338 (2.34); Fe2–H2, 1.483 (1.54). Mean
angles in the complexes (deg) (mean DFT values in parentheses): Pd2–H2–Fe2,
102.4 (101.2); P3–Pd2–S4, 171.5; P4–Pd2–S3,
177.7 (177.6); S3–Fe2–C35, 165.4; S4–Fe2–C33,
167.1 (165.8).
ORTEP of one of two independent
complexes in [2H]BF4·THF·0.5Et2O with ellipsoids drawn at
the 50% probability level. The solvate molecules, BF4– anion and H atoms (except the H– ligand) are omitted for clarity. Mean distances in the complexes
(Å) (mean DFT values in parentheses): Pd2–Fe2, 2.882 (2.92);
Pd2–H2, 2.173 (2.19); Pd2–P3, 2.253; Pd2–P4,
2.270 (2.30); Pd2–S3, 2.367; Pd2–S4, 2.363 (2.39); Fe2–S3,
2.330; Fe2–S4, 2.338 (2.34); Fe2–H2, 1.483 (1.54). Mean
angles in the complexes (deg) (mean DFT values in parentheses): Pd2–H2–Fe2,
102.4 (101.2); P3–Pd2–S4, 171.5; P4–Pd2–S3,
177.7 (177.6); S3–Fe2–C35, 165.4; S4–Fe2–C33,
167.1 (165.8).By single crystal X-ray
diffraction, two structurally similar FePdhydrides were found in the asymmetric unit of [2H]BF4, in each case with the Fe–Pd distance (2.845, 2.918
Å) being greater than the sum of the covalent radii (2.71 Å).[21] Protonation at Fe is accompanied by rotation
of the Pd(dppe)center, affording square-planar Pd linked by the pdt2– group to octahedral Fe. Indeed, the arrangement of
metalcenters in [2H]+ is analogous to that
in [(OC)3FeH(pdt)Ni(dppe)]+ ([1H]+). The H– ligand in [2H]+ was located in the difference map and allowed to refine;
its final location was significantly closer to the Fecenter. Considering
the covalent radius of H (0.31 Å),[21] the H– ligand in [2H]+ is formally bonded to Fe (1.469 Å), whereas its interaction
with Pd is weak (2.168 Å). DFT calculations also predict unsymmetrical
binding of the hydride ligand, which, despite being poised between
the metals, can be viewed as a terminal ligand.
Platinum–Iron
Complexes
The first indication
of the unusual structure of (CO)3Fe(pdt)Pt(dppe) (3), which is red-brown, is that its UV–vis spectrum
features two absorptions (480, 360 nm), whereas single bands were
observed for the green complexes 1 (398 nm) and 2 (376 nm). The IR spectrum of 3 is also distinctive,
with the νCO bands (1962, 1883, 1870 cm–1) observed at energies ∼70 cm–1 lower than
those for Fe(I)Ni(I) and Fe(I)Pd(I) analogs. The 31P{1H} NMR spectrum for 3 consists of a singlet (and 195Pt-coupled doublet) and is temperature-invariant down to
−85 °C. The chemical shift (δ 44.1) and 1JPPt (3277 Hz) are close to those of
the (pdt)PtII(dppe) precursor (δ 44.5, 1JPPt = 2717 Hz). While NMR data are in
line with Ptcoordination being either (i) highly fluxional tetrahedral
or (ii) square-planar, the latter is most consistent with low-energy
νCO bands. Square planarity at Pt is corroborated
by DFT geometry optimization (Figure 6).
Energy-minimized
structure of 3 obtained with DFT.
Selen class="Chemical">cted distances (Å): Fe–Pt, 2.87; Fe–C1, 1.75;
Fe–C2, 1.75; Fe–C3, 1.76; Fe–S1, 2.34; Fe–S2,
2.37; Pt–S1, 2.37; Pt–S2, 2.38; Pt–P1, 2.28;
Pt–P2, 2.28. Selected angles (deg): C1–Fe–S2,
136.8; C2–Fe–S1, 169.4; S1–Pt–P2, 176.1;
S2–Pt–P1, 177.7.
Indeed, rather than being isostructurual to 1 and 2, 3 is predicted to feature a square-planar
Pt site, with the angle between the PtP2 and PtS2 planes being 4°. The calculated Fe–Pt distance (2.87
Å) exceeds the sum of the covalent radii (2.68 Å),[21] suggesting an absence of Fe–Pt bonding
and a purely pentacoordinate Fe. This site, in contrast to the square-pyramidal
Fe(I) in 2, is distorted trigonal bipyramidal (τ
= 0.54). As mentioned previously, NBO analysis did not reveal d orbital mixing, which was observed in 1 and 2. Overall, the coordination geometries are consistent with
an Fe(0)Pt(II) description for 3.Quasi-reversible
oxidations for (CO)3Fe(pdt)Pt(dppe)
were observed at −0.93 and +0.33 V (Figure
S29), potentials similar to those for the FePd analog 2. The proposed sequence of 1e– events:
Fe(0)Pt(II) ⇌ Fe(I)Pt(II) ⇌ Fe(II)Pt(II). This sequence
is supported by the IR spectrum for [(OC)3Fe(pdt)Pt(dppe)]BF4 ([3]BF4, prepared from 3 and FcBF4), in which the νCO bands (2058,
1987 cm–1) are at virtually identical energies to
the bona fide Fe(I)M(II) species [1]+ and [2]+. While the EPR spectrum
of [3]+ indicates some unpaired spin resides
on Pt (A(195Pt) ≈ 97 MHz, A(31P) ≈ 11 MHz), the g-shifts suggest an Fe(I)Pt(II) description to be appropriate, as
mirrored in the spin density calculations (Table 3). The paramagnetic salt
[3]BF4 undergoes ligand substitution with
PCy3 to afford dicarbonyl [Cy3P(OC)2Fe(pdt)Pt(dppe)]BF4 ([3′]BF4), the characterization for which mirrors that for [2′]BF4.As expected, reaction of low-valent 3 with HBF4 furnished the hydride salt[(OC)3FeH(pdt)Pt(dppe)]BF4 ([3H]BF4), for which a 1H NMR resonance at δ −3.57
is particularly diagnostic.
The small coupling observed for the satellites (1JHPt = 173 Hz) indicates that the H···Pt
interactions are rather weak. For comparison, square-planar platinumhydrides exhibit 1JHPt from
∼650 to 1700 Hz,[24] with 1JHPt being at the low end of this range
for five-coordinate complexes of the type [Pt(PR3)4H]+.[25] Thus, [3H]+ is similar to its FePd analog in that the
H– ligand can be considered terminally Fe-bound,
as also supported by the 31P NMR resonances for 3 being insensitive to protonation at Fe (for [3H]+ δ 46.4, 1JPPt = 3220 Hz). In contrast, the energies of the νCO bands for the FePtconjugate acid and base species are very different–those
for [3H]+ (2081, 2022 cm–1) are greatly shifted relative to those for 3 (the average
shift, taking into account the A + E patterns was ΔνCO = 137 cm–1). A similar shift (139 cm–1) was observed by Angelici and co-workers upon protonation
of the Fe(0) species Fe(dppe)(CO)3 to give the ferrous
hydride [FeH(dppe)(CO)3]+.[26] Taken together, these data are consistent with Fe(0)Pt(II),
Fe(I)Pt(II), and Fe(II)HPt(II) assignments for 3, [3]BF4, and [3H]BF4, respectively.While only a handful of FePtcomplexes bearing H– and CO ligands exist, the compounds (CO)3FeH(μ-PR2)Pt(PR′3)2 are particularly pertinent.[27] These complexes interconvert between isomers
with terminal ((H)FeIIPt0) and bridging (Fe0(μ-H)PtII) hydride ligands. When these FePt
species feature the substituents R = Ph, R′ = OPh, a non-fluxional
terminal hydride results (1JHPt = 28 Hz), whereas when R = Cy, R′ = Et the hydride is evenly
shared (1JHPt = 520 Hz). One
would expect values double the latter (∼1000 Hz) for purely
terminal Pt–H moieties. (28)
Comparison
of CO Frequencies
Good agreement was found
between experimental and calculated shifts in weighted average CO
vibrational frequency (ΔνCO) induced by protonation
and oxidation of bimetallics 1–3 (Scheme 3). Protonation of the FeNi and FePdcomplexes is
predicted to shift νCO by ∼50 cm–1. In contrast, the shift induced by protonation of the FePt analog
is much greater, with ΔνCO ∼110 cm–1. These ΔνCO values correlate
with the formal changes in Fe oxidation state: from +I to +II upon
protonation of 1 and 2 as well as from 0
to +II upon protonation of 3. Trends in ΔνCO for the couples [1]0/+, [2]0/+, and [3]0/+ are similar to
those seen for protonation, the effects again being greater for the
Ptcase. The ΔνCO values for these oxidations
are consistent with their occurring remote from the Fe(CO)3center in [1] and [2] but directly at
this center in [3].
Scheme 3
Stereochemical Non-Rigidity
As discussed above, the
fluxionality of 1 and 2, evidenced by the
broad singlet in their 31P{1H} NMR spectra,
implicate the presence of a transient or intermediate species featuring
square-planar Ni/Pdcenters (Scheme 1, top). This conformational motion is likely to perturb both the oxidation
states and intermetallic bonding of the Fe and M centers. Two optimized
DFT structures characterized as minima were calculated for each bimetallic 1–3: one in which the (pdt)M(dppe) fragment
is tetrahedral, and one in which it is square-planar. Selected bond
lengths and angles are provided in Table S5. In each case, the Fecenter is within bonding distance of M when
the latter adopts a tetrahedral geometry, while the Fe–M distance
is elongated when M is planar. The Fe–M elongation is subtle
in the FeNicase (0.13 Å) but more pronounced for FePd and FePt
(∼0.30 Å). The relative stability of the two isomers was
computed, and the reaction free energies (ΔG°) and free energy barriers (ΔG†) associated with tetrahedral to square-planar isomerization are
presented in Table 4.
Table 4
Calculated
Reaction Free Energies
and Free Energy Barriers (kcal/mol) for Rotation from Tetrahedral
to Square-Planar Geometry at the M(dppe) Site in Compounds 1–3
complex
ΔG° DFT
ΔG† DFTa
ΔG† exptc
(OC)3Fe(pdt)Ni(dppe) (1)
–0.67
6.66
9.5
(OC)3Fe(pdt)Pd(dppe) (2)
+0.99
3.29
<7.2
(OC)3Fe(pdt)Pt(dppe) (3)
–30.42
NDb
ND
The potential energy surface was
found to be extremely flat along the isomerization pathway. As a result,
the single imaginary frequency for the transition state (TS) was <20
cm–1 for the two barriers given in this table. Each
TS was verified to lead to the relevant tetrahedral and square-planar
geometries by following the intrinsic reaction coordinate (IRC) for
4 or 5 steps in both directions and subsequently optimizing the geometries.
As the imaginary frequencies may be below the numerical accuracy of
the methodology, and the complete IRC was not obtained, the free energy
barriers should be viewed with caution. Moreover, multiple TSs were
found for 1, suggesting a ruffled potential energy surface
connecting the two isomers, and only the highest free energy barrier
is reported.
The TS was
not determined for 3 as the isomerization was found to
be significantly exergonic.
Estimated according to the Gutowsky–Holm
relation using the coalescence temperature of 243 K (detailed in SI), but the DFT free energies were calculated
at 298 K for consistency with other experiments. Note that the calculated
and experimental free energy barriers are not exactly equivalent.
The potential energy surface was
found to be extremely flat along the isomerization pathway. As a result,
the single imaginary frequency for the transition state (TS) was <20
cm–1 for the two barriers given in this table. Each
TS was verified to lead to the relevant tetrahedral and square-planar
geometries by following the intrinsic reaction coordinate (IRC) for
4 or 5 steps in both directions and subsequently optimizing the geometries.
As the imaginary frequencies may be below the numerical accuracy of
the methodology, and the complete IRC was not obtained, the free energy
barriers should be viewed with caution. Moreover, multiple TSs were
found for 1, suggesting a ruffled potential energy surface
connecting the two isomers, and only the highest free energy barrier
is reported.The TS was
not determined for 3 as the isomerization was found to
be signifin class="Chemical">cantly exergonin class="Chemical">c.
Estimated according to the Gutowsky–Holm
relation using the coalescence temperature of 243 K (detailed in SI), but the DFT free energies were calculated
at 298 K for consistency with other experiments. Note that the calculated
and experimental free energy barriers are not exactly equivalent.For the FeNi and FePdcongeners,
the isomerization is close to
thermoneutral (ΔG° = −0.67 and
+0.99 kcal/mol, respectively). Given the error associated with the
calculations, it is not possible to determine a thermodynamic preference
for the coordination geometry at Ni/Pd. The reaction free energies
calculated using various other density functionals and basis sets
with geometry optimization in CH2Cl2 solvent
are qualitatively consistent with the results presented here, although
certain levels of theory do not predict a stable square-planar intermediate
for geometry optimizations in the gas phase (see Tables S6–S8). The calculated free energy barriers
are relatively low, consistent with the fluxionality of 1 and 2 observed in their room temperature NMR spectra.
Because the potential energy surfaces for 1 and 2 were found to be extremely flat along the isomerization
pathway, the calculated free energy barriers may not be quantitatively
accurate and can be analyzed only qualitatively. The calculated free
energy barrier of 6.66 kcal/mol for 1 is qualitatively
consistent with, although somewhat lower than, the free energy barrier
of 9.5 kcal/mol estimated from the coalescence temperature in the
NMR measurement. Moreover, the lower calculated free energy barrier
of 3.29 kcal/mol for 2 is consistent with the lack of
decoalescence observed in the NMR experiments for 2.
The observation of a particularly flat potential energy surface along
the isomerization pathway has implications for the enzymatic system,
where the geometry at the Ni site is between tetrahedral and square-planar.The similarity in free energies and the relatively low free energy
barriers suggest that the two isomers of 1 and 2 may interconvert. In addition, the free energy change associated
with oxidation of 1 and 2 is similar for
the two different isomers, with the calculated reduction potentials
of the square-planar and tetrahedral isomers differing by only ∼0.03
V, which is within the estimated error of the methodology. It is thus
a distinct possibility that the Ni/Pdcenters in 1 and 2 “preorganize” into square-planar geometries
prior to 1e– loss. In the case of FePtcomplex 3, the square-planar isomer is overwhelmingly favored, consistent
with the high stability characteristic of square-planar Pt(II) centers.
Calculations indicate that νCO energies for the tetrahedral
isomer of 3, which was found to be a local minimum, are
within 10 cm–1 of those for 1 and 2. Square-planar isomers of 1 and 2, while not observed experimentally, have calculated νCO bands 55–65 cm–1 lower in energy
than the respective tetrahedral conformers. This νCO shift is consistent with the change in conformation inducing a change
in oxidation state.Given the dynamics in the neutral species 1 (as well
as 2), we set about investigating hydride [1H]+ to yield further insight into the mechanism of protonation.
At room temperature, [(CO)3FeH(pdt)Ni(dppe)]+ displays a single 31P NMR peak owing to its Cs-symmetry. In view of the difficulty with determining
whether or not Ni(dppe) twisting is occurring here, we interrogated
an analogous hydride of lower symmetry, which entailed preparation
of the new bimetallic(CO)3Fe(pdt)Ni(S,S-chiraphos) (S,S-chiraphos = 2S,3S-bis(diphenylphosphino)butane,
Scheme 4). Analogous to 1, this
species gave rise to a single 31P NMR resonance at room
temperature, with decoalescence observed on cooling to −90
°C, at which temperature Ni(S,S-chiraphos) twisting is slowed (Figure S49).
Scheme 4
While (CO)3Fe(pdt)Ni(S,S-chiraphos) twists rapidly at room temperature, its conjugate
acid
does not; the nonequivalent31P nuclei
in the chiral hydride [(CO)3FeH(pdt)Ni(S,S-chiraphos)]+ (Scheme 4, center) give rise to two broad signals (resolved into two 31P-coupled doublets at −70 °C). If twisting and
equivalencing of the 31P sites were to be facile, it would
be necessary for a tetrahedral hydride (Scheme 4, right) to be energetically accessible, which appears not to be
the case. In general, it is proposed that twisting of neutral species
is associated with “on/off” switching of Fe–M
bonding in the FeNi and FePd (but not FePt) complexes. It is now evident
that the twisting of hydride species is slower, perhaps reflecting
the rigidifying influence of the H···M interactions
(despite their weakness).These observations are supported by
computational work: the twisting
of hydride [1H]+ was investigated, and two
optimized DFT structures characterized as minima were calculated for
the isomers of [1H]+. The relative free energies
of the structures, which feature either square-planar or tetrahedral
(pdt)Ni(dppe) fragments (Figure 7), were also
calculated.
Figure 7
Energy-minimized structures obtained with DFT for [1H]+ with the (pdt)Ni(dppe) moiety either square-planar
(red) or distorted tetrahedral (blue). The optimized geometries are
superimposed and presented in two views. Other (higher energy) isomers/tautomers
proposed can be found in Figure S57.
Energy-minimized structures obtained with DFT for [1H]+ with the (pdt)Ni(dppe) moiety either square-planar
(red) or distorted tetrahedral (blue). The optimized geometries are
superimposed and presented in two views. Other (higher energy) isomers/tautomers
proposed can be found in Figure S57.Twisting of the square-planar
(pdt)Ni(dppe) site in [1H]+ to adopt a tetrahedral
geometry was calculated to
be significantly endergonic (ΔG° = +11.47
kcal/mol). The tetrahedral conformer is predicted by DFT to be far
(∼108 ×) more acidic than the square-planar
state (the experimental pKa(MeCN) for
[1H]+ is 10.7).[11] Considering the conjugate bases for the two isomers of [1H]+, these results suggest that 1 has greatly
enhanced basicity when in the square-planar form, consistent with
its description as Fe(0)Ni(II) (Scheme 1).
Thus, it is reasonable that this isomer is responsible for the rich
acid–base and redox chemistry exhibited by 1 and
related complexes.
Conclusions
Numerous studies have
examined the protonation and 1e– oxidation of complexes
of the type (CO)3Fe(pdt)Ni(diphosphine)
and substituted derivatives thereof. This paper presents evidence
that the protonations, and possibly the oxidations, proceed via a
latent 2e– mixed-valence intermediate (Scheme 5).
Scheme 5
Despite previous computational
analyses,[10,13,15] this key square-planar
species had not been
identified as an energetically accessible isomer of 1. The first clue for this unsuspected structure was provided by the 31P NMR properties of 1, which implicate a transient
or intermediate square-planar species that interconverts the 31P sites. The new and compelling evidence for the intermediate
comes from (i) DFT calculations that suggested thermodynamically accessible
rotation at the non-Fecenter and (ii) spectroscopic and electrochemical
properties of the FePtcomplex, the protonation and redox behavior
of which mirrors that of Fe(0) diphosphines.[26,29] The 2e– mixed valency of 1 contrast
with that for (cymene)Ru(pdt)Ni(dppe), which features a rigidly tetrahedral
Ni(0) center.[30]The rotation-induced
2e– mixed-valence behavior
also has implications for understanding active site mimics of the
[FeFe]-H2ases. Prior to protonation, model complexes almost
invariably feature Fe2(SR)2L6 cores
wherein terminal coordination sites are unavailable
for H+ binding. Protonation of these Fe(I)Fe(I)complexes
has been proposed to proceed via a “rotated intermediate”
(Scheme 6).[31] Models
for this rotated state have been crystallized, revealing that Fe–Fe
bonding is weakened and a terminal site on one Fecenter is vacant.[32] By analogy to the results in this paper, such
rotated diironcomplexes should perhaps be described as Fe(II)Fe(0),
thus underlining the role of latent, but highly reactive intermediates.
Scheme 6
Overall, the work reported here shows that the geometry
at Ni decisively
influences the reactivity of a neighboring Fe site. For small molecule
[NiFe]-H2ase mimics, it has been demonstrated that perturbation
of the Ni site greatly alters the acid–base properties of these
complexes. With respect to the hydrogenases themselves, the results
suggest that the distorted nature of the Ni(Cys)4 site
is likely an important factor in tuning basicity and redox potentials,
thereby enabling their remarkable activity.
Experimental
Section
Unless otherwise stated, chemicals were purchased
from commercial
sources and used as received. Chromatography was performed using SiO2 (40–63 μm, 230–400 mesh) as the stationary
phase. CD2Cl2 was distilled from CaH2. The complexes 1,[20] FeI2(CO)4, Cl2Ni(S,S-chiraphos),[33] (pdt)Pd(dppe),
and (pdt)Pt(dppe)[34] were prepared according
to the literature methods. All reactions and purifications were conducted
in an MBraun glovebox equipped with a solvent purification system;
the concentrations of O2 and H2O in the N2 atmosphere were typically no higher than 2 and 0.2 ppm, respectively.
Glassware used in the preparation of deuteride [2D]BF4 was washed with D2O and dried prior to use. Solution
IR spectra were recorded on a PerkinElmer Spectrum 100 FTIR spectrometer.
A Waters Micromass Quattro II spectrometer was used to acquire ESI-MS
data for analytes in dilute CH2Cl2 or CD2Cl2 solution. Analytical data were acquired using
an Exeter Analytical CE-440 elemental analyzer. Unless otherwise stated,
NMR data were acquired at room temperature, with samples under an
atmosphere of N2. 1H and 31P{1H} NMR spectra were recorded on a Varian VXR500 or U500 spectrometer
at 500 and 202 MHz, respectively. 2H NMR spectra were recorded
on a Varian UI 500NB spectrometer at 77 MHz. Chemical shifts (δ/ppm)
are referenced to CHDCl2/CH2Cl2 (5.32
ppm for 1H) and external 85% H3PO4 (0 ppm for 31P). Cyclic voltammetry was carried out in
a single compartment glass cell using a CH Instruments CHI630D electrochemical
analyzer. The working, counter, and pseudoreference electrodes were
glassy C, Pt, and Ag, respectively. The analyte (1 mM) and NBu4PF6 (100 mM) were dissolved in CH2Cl2, and potentials (reported here relative to internal Fc/Fc+) were swept at 0.1 V s–1. Crystallographic
data were collected using a Siemens SMART diffractometer equipped
with a Mo Kα source (λ = 0.71073 Å) and
an Apex II detector. EPR spectra of complexes (∼1 mM in CH2Cl2/PhMe, 1:1) were recorded on a Varian E-line
12″ Century Series X-band CW spectrometer.
(OC)3Fe(pdt)Pd(dppe)
(2)
At
−28 °C, a mixture of FeI2(CO)4 (29.5
mg, 70 μmol) and (pdt)Pd(dppe) (42.8 mg, 70 μmol) were
dissolved in CH2Cl2 (1 mL) with stirring. After
1 min, the solution was treated with CoCp2 (26.5 mg, 140
μmol) in CH2Cl2 (1 mL) and allowed to
warm to room temperature, before it was concentrated to ∼0.5
mL and chromatographed (∼10 cm SiO2, CH2Cl2 eluent). The fourth band, deep green in color, was
concentrated to ∼2 mL, layered with pentane (∼20 mL),
and allowed to stand overnight at −28 °C. The solids that
formed were isolated by filtration, washed with pentane (2 ×
1 mL), and dried briefly to afford the title compound as dark-green
plates (9.0 mg, 12 μmol, 17%). 1H NMR (CD2Cl2): δ 7.74 (m, 8H, o-Ph), 7.43
(m, 8H, m-Ph), 7.42 (m, 4H, p-Ph),
2.59 (ddd, 2JHH = 13.5 Hz, 3JHH = 5.8 Hz, 3JHH = 3.3 Hz, 2H, equatorial CH2CH2CH2), 2.35
(m, 2H, PCH2CH2P), 2.22 (m, 2H, PCH2CH2P), 1.94 (m, 2H, axial CH2CH2CH2), 1.29 (m, 2H, CH2CH2CH2). 31P{1H} NMR (CD2Cl2): δ 51.3.
ESI-MS: m/z 749.8 [M]+, 721.8 [M – CO]+, 693.8 [M – 2CO]+ (ionization induced by addition of Fc[B(C6H3-3,5-(CF3)2)4]). Anal. calcd for
C32H30O3S2P2NiPd·0.2CH2Cl2: C, 50.36; H, 3.99; N,
0.00. Found: C, 50.35; H, 3.57; N, 0.00. Green plates of 2 formed upon slow diffusion of pentane layered onto a concentrated
CH2Cl2 solution of the title compound at −28
°C. One crystal (0.377 × 0.232 × 0.058 mm3) was subjected to X-ray diffraction at 168 K. Its space group was
determined to be triclinic P-1 with cell parameters: a 8.900 Å, b 12.883 Å, c 14.778 Å, α 104.64°, β 106.73°,
γ 90.98°. Integration of 6798 reflections and solution
by direct methods using SHELXTL V6.12 afforded a model with R1 = 0.0285 and wR2 = 0.0684.
[(OC)3Fe(pdt)Pd(dppe)]BF4 ([2]BF4)
A stirred solution of 2 (11.3
mg, 15 μmol) in CH2Cl2 (1 mL) was treated
with FcBF4 (4.1 mg, 15 μmol) in CH2Cl2 (1 mL). After 1 min, pentane (20 mL) was added, and the mixture
allowed to stand overnight at −28 °C. The solids were
isolated by filtration, washed with pentane (2 × 1 mL), and dried
briefly to afford the title compound as an olive-brown powder (3.5
mg, 4.2 μmol, 28%). ESI-MS: m/z 749.8 [M – BF4–]+, 721.8 [M – CO – BF4–]+, 693.8 [M – 2CO – BF4–]+. Anal. calcd for C32H30FeO3P2PdS2BF4·0.5CH2Cl2: C, 44.35; H, 3.55; N, 0.00.
Found: C, 44.26; H, 3.19; N, 0.54.
[(OC)3FeH(pdt)Pd(dppe)]BF4 ([2H]BF4)
A solution of 2 (22.5 mg,
30 μmol) in CH2Cl2 (1 mL) was treated
with HBF4 (54% Et2O solution, 14.6 mg, 90 μmol)
in CH2Cl2 (1 mL). The solution was evaporated
to dryness, and the oily residue triturated with Et2O (3
mL). The resulting solid was isolated by filtration, washed with Et2O (2 × 1 mL), and dried briefly to afford the title compound
as an orange powder (21.1 mg, 25 μmol, 84%). 1H NMR
(CD2Cl2): δ 7.70 (m, 8H, o-Ph), 7.64 (t, 2JHH = 6.7
Hz, 4H, p-Ph), 7.57 (m, 8H, m-Ph),
2.87 (m, 4H, PCH2CH2P), 2.63 (m, 1H, equatorial CH2CH2CH2), 2.30 (m, 4H, CH2CH2CH2), 1.29 (m, 1H,
axial CH2CH2CH2),
−4.3 (s, 1H, FeH). 31P{1H} NMR (CD2Cl2): δ 62.8. ESI-MS: m/z 750.8 [M – BF4–]+. Orange blocks of [2H]BF4·THF·0.5Et2O formed upon slow diffusion
of Et2O vapor into a THF solution of the title compound
at −28 °C. One crystal (0.377 × 0.232 × 0.058
mm3) was subjected to X-ray diffraction at 178 K. Its space
group was determined to be monoclinic P21/n with cell parameters: a 11.466
Å, b 20.692 Å, c 34.720
Å, α 90.00°, β 97.41°, γ 90.00°.
Integration of 9965 reflections and solution by direct methods using
SHELXTL V6.12 afforded a model with R1 = 0.0369 and wR2 = 0.0754.
[(OC)3FeD(pdt)Pd(dppe)]BF4 ([2D]BF4)
A solution of 2 (22.5 mg,
30 μmol) in CH2Cl2 (1 mL) and CD3OD (0.1 mL) was treated with HBF4 (54% Et2O
solution, 14.6 mg, 90 μmol) in CD3OD (0.3 mL). The
solution was evaporated to dryness, and the oily residue triturated
with Et2O (3 mL). The resulting solid was isolated by filtration,
washed with Et2O (2 × 1 mL), and dried briefly to
afford the title compound as an orange powder (19.6 mg, 23 μmol,
78%). 2H NMR (CH2Cl2): δ −4.3
(m). 31P{1H} NMR (CH2Cl2): δ 64.3. ESI-MS: m/z 751.9
[M – BF4–]+. Anal.
calcd for C32H30DFeO3P2PdS2BF4·0.25CH2Cl2: C, 44.99; H, 3.69; N, 0.00. Found: C, 45.03; H, 3.82; N, 0.00.
[Cy3P(OC)2Fe(pdt)Pd(dppe)]BF4 ([2′]BF4)
Compound 2 (15.0 mg, 20 μmol) and FcBF4 (5.4 mg, 20
μmol) were dissolved in CH2Cl2 (1 mL)
with rapid stirring. After 1 min the solution was added dropwise to
PCy3 (28.0 mg, 100 μmol) in CH2Cl2 (0.5 mL). The solution was stirred for a further 0.5 min,
and pentane (−28 °C, 15 mL) was added and the mixture
allowed to stand overnight at −28 °C. The solids were
isolated by filtration, washed with pentane (−28 °C, 2
× 2 mL), and dried briefly to afford the title compound as a
yellow-brown powder (5.9 mg, 5.4 μmol, 27%). ESI-MS: m/z 1002.4 [M – BF4–]+, 974.3 [M – CO – BF4–]+. Anal. calcd for C49H63FeO2P3PdS2BF4·0.25CH2Cl2: C, 53.23; H, 5.76; N, 0.00.
Found: C, 53.16; H, 5.82; N, 0.20.
(OC)3Fe(pdt)Pt(dppe)
(3)
A
mixture of solid FeI2(CO)4 (25.3 mg, 60 μmol)
and (pdt)Pt(dppe) (42.0 mg, 60 μmol) was cooled to −28
°C and dissolved in CH2Cl2 (2 mL) with
stirring. After 1 min, the solution was treated with CoCp2 (22.7 mg, 120 μmol) in CH2Cl2 (1 mL)
and allowed to warm to room temperature before it was concentrated
to ∼0.5 mL and chromatographed (∼10 cm SiO2, CH2Cl2 eluent). The olive-brown band (following
the rapidly eluting orange band containing Fe2(pdt)(CO)6) was concentrated to ∼2 mL, layered with pentane (∼20
mL), and allowed to stand overnight at −28 °C. The supernatant
was removed, the dark residue triturated with pentane (2 × 5
mL), and dried briefly to afford the title compound as a brown powder
(17.0 mg, 20.2 μmol, 34%). 1H NMR (CD2Cl2): δ 7.86 (m, 8H, o-Ph), 7.48
(m, 8H, m-Ph), 7.46 (m, 4H, p-Ph),
2.57 (m, 2H, CH2CH2CH2), 2.22 (m, 4H, PCH2CH2P), 1.58 (m, 2H, CH2CH2CH2), 1.29
(m, 2H, CH2CH2CH2). 31P{1H} NMR (CD2Cl2): δ 44.1 (s, d, 1JPPt = 3280 Hz). ESI-MS: m/z 838.9
[M]+ (ionization induced by addition of Fc[B(C6H3-3,5-(CF3)2)4]). Anal.
calcd for C32H30FeO3P2PtS2: C, 45.78; H, 3.60; N, 0.00. Found: C, 46.36; H,
3.22; N, 0.05.
[(OC)3Fe(pdt)Pt(dppe)]BF4 ([3]BF4)
This compound was prepared
analogously
to [2]BF4, instead using 3 as
the precursor. Yield: 81%, yellow-green powder. ESI-MS: m/z 838.9 [M – BF4–]+.
This compound
was prepared analogously to [2′]BF4, instead using 3 as the precursor. Yield: 72%, yellow
powder. ESI-MS: m/z 1091.7 [M –
BF4–]+. Anal. calcd for C49H63FeO2P3PtS2BF4·0.75CH2Cl2: C, 48.09; H,
5.23; N, 0.00. Found: C, 48.06; H, 5.42; N, 0.52.
(pdt)Ni(S,S-chiraphos)
This compound was
prepared analogously to (pdt)Ni(dppe), instead
using Cl2Ni(S,S-chiraphos)
as the precursor.[33] Yield: 95%, orange
powder. 1H NMR (CD2Cl2): δ
8.15 (m, 4H, p-Ph), 7.63–7.47 (m, 16H, o,m-Ph), 2.19 (m, 4H, CH2CH2CH2), 2.03
(m, 2H, CH), 1.81 (qu, 3JHH = 6.3 Hz, 4H, CH2CH2CH2), 0.96 (dd, 3JPH =
10.6 Hz, 3JHH = 6.2 Hz, 6H,
CH3). 31P{1H} NMR (CD2Cl2): δ 56.1. ESI-MS: m/z 590.4 [M]+. Anal. calcd for C31H34S2P2Ni: C, 62.96; H, 5.79; N, 0.00.
Found: C, 62.22; H, 5.69; N, 0.41.
(CO)3Fe(pdt)Ni(S,S-chiraphos)
The complexes (pdt)Ni(S,S-chiraphos) (118.3 mg, 200 μmol)
and Fe2(CO)9 (72.8 mg, 200 μmol) were
dissolved in CH2Cl2 (3 mL) with stirring. After
18 h, the deep
green solution was concentrated to ∼0.5 mL and chromatographed
(∼10 cm SiO2, CH2Cl2 eluent).
An olive-green band was collected, treated with MeCN (∼5 mL),
concentrated to ∼1 mL and cooled to −28 °C. The
solids were isolated by filtration, washed with MeCN (−28 °C,
2 mL) and pentane (−28 °C, 2 × 2 mL), and dissolved
in CH2Cl2 (1 mL) and MeCN (5 mL). The solution
was concentrated to ∼1 mL and cooled to −28 °C.
The crystals that formed were isolated by filtration, washed with
MeCN (−28 °C, 2 mL) and pentane (−28 °C, 2
× 2 mL), and dried to afford the title complex as dark green
crystals (46.9 mg, 64.1 μmol, 32%). 1H NMR (CD2Cl2): δ 8.11 (m, 4H, Ph), 7.62 (m, 4H, Ph),
7.52–7.39 (m, 12H, Ph), 2.57 (ddd, 2JHH = 13.4 Hz, 3JHH = 7.1 Hz, 3JHH = 7.1 Hz,
1H, equatorial CH2CH2CH2), 2.17 (ddd, 2JHH =
13.2 Hz, 3JHH = 7.3 Hz, 3JHH = 7.1 Hz, 1H, equatorial CH2CH2CH2), 1.98 (m, 2H,
CH), 1.91 (m, 1H, equatorial CH2CH2CH2), 1.82 (m, 2H, axial CH2CH2CH2), 1.31 (m, 1H,
axial CH2CH2CH2),
0.87 (m, 6H, CH3). 31P{1H} NMR (CD2Cl2): δ 63.8. 31P{1H} NMR (CD2Cl2, −90 °C): δ
75.0, 52.6. FTIR (CH2Cl2): νCO = 2026, 1953 cm–1. ESI-MS: m/z 729.3 [M]+ (ionization induced by addition
of Fc[B(C6H3-3,5-(CF3)2)4]). Anal. calcd for C34H34O3S2P2NiFe: C, 55.84; H, 4.69; N, 0.00.
Found: C, 55.15; H, 4.53; N, 0.57.
DFT calculations were
performed using
the B3P86 density functional with the SDD pseudopotential and basis
set[35] for the Fe, Ni, Pd, and Pt atoms,
the 6-31G** basis set[36] for μ-H ligands,
and the 6-31G* basis set[37,38] for all other atoms.
The starting geometries for 1,[10] [1H]+,[20]2, and [2H]+ were obtained from crystal
structures. The starting coordinates for atoms in 3 and
[3H]+ were obtained from the crystal structure
of [2H]+, where the metalcenter and hydride
were manually altered prior to optimization. Solvation effects were
included using the conductor-like polarizable continuum model[39,40] with Bondi atomic radii[41] and included
the non-electrostaticcontributions of dispersion,[42,43] repulsion,[42,43] and cavitation energies.[44]For the results presented in the main
paper, the geometry optimizations were performed in the gas phase.
Geometry optimizations were also performed in solution and were found
to be consistent with the gas phase optimizations; these results are
provided in the Supporting Information.
In all cases, the minimum-energy structures were confirmed to have
no imaginary frequencies. The νCO values were calculated
with DFT and were scaled by the standard factor of 0.9850.[45] The transition states (TSs) for the tetrahedral
to square-planar isomerization were identified using the synchronous
transit-guided quasi-Newton method,[46,47] and the resulting
structures were confirmed to have only a single imaginary frequency.
Each TS was verified to lead to the relevant tetrahedral and square-planar
geometries by following the IRC using the local quadratic approximation[48] for 4 or 5 steps in both directions and subsequently
optimizing the geometries. Because the potential energy surface was
found to be very flat, the single imaginary frequencies were small,
and the complete IRCconnecting the two isomers through the TS could
not be obtained.Thermochemical data were calculated at T = 298.15
K. The reaction free energies (ΔG°) and
free energy barriers (ΔG†) associated with the tetrahedral to square-planar isomerization
in solution included zero-point energy, entropiccontributions, and
solvation effects. The relative reduction potentials and pKa’s were calculated from the corresponding
reaction free energies using methodology described elsewhere.[49] Chemical bonding analysis was performed using
NBO.[23] All calculations were performed
using the Gaussian 09 electronic structure program.[50] Structures and energies of the systems studied herein are
provided in the Supporting Information.
Authors: Jillian L Dempsey; Arthur J Esswein; David R Manke; Joel Rosenthal; Jake D Soper; Daniel G Nocera Journal: Inorg Chem Date: 2005-10-03 Impact factor: 5.165
Authors: Wenguang Wang; Thomas B Rauchfuss; Curtis E Moore; Arnold L Rheingold; Luca De Gioia; Giuseppe Zampella Journal: Chemistry Date: 2013-10-15 Impact factor: 5.236
Authors: Sabrina Munery; Jean-François Capon; Luca De Gioia; Catherine Elleouet; Claudio Greco; François Y Pétillon; Philippe Schollhammer; Jean Talarmin; Giuseppe Zampella Journal: Chemistry Date: 2013-10-14 Impact factor: 5.236
Authors: Wenfeng Zhu; Andrew C Marr; Qiang Wang; Frank Neese; Douglas J E Spencer; Alexander J Blake; Paul A Cooke; Claire Wilson; Martin Schröder Journal: Proc Natl Acad Sci U S A Date: 2005-12-13 Impact factor: 11.205
Authors: David Schilter; James M Camara; Mioy T Huynh; Sharon Hammes-Schiffer; Thomas B Rauchfuss Journal: Chem Rev Date: 2016-06-29 Impact factor: 60.622
Authors: Olbelina A Ulloa; Mioy T Huynh; Casseday P Richers; Jeffery A Bertke; Mark J Nilges; Sharon Hammes-Schiffer; Thomas B Rauchfuss Journal: J Am Chem Soc Date: 2016-07-18 Impact factor: 15.419
Authors: Geoffrey M Chambers; Mioy T Huynh; Yulong Li; Sharon Hammes-Schiffer; Thomas B Rauchfuss; Edward Reijerse; Wolfgang Lubitz Journal: Inorg Chem Date: 2015-09-30 Impact factor: 5.165
Figure 1
Scheme 1
Scheme 2
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Scheme 3
Scheme 4
Figure 7
Scheme 5
Scheme 6