Coordination Chemistry and Methylation of Mixed-Substituted Tetraphosphetanes (RP-PtBu)2 (R=Ph, Py).

Synthesis of mixed-substituted tetraphosphetanes (RP-PtBu)2 (R=Ph (4), Py (5); Py=2-pyridyl) is achieved from the condensation of dipyrazolylphosphanes RPpyr2 (R=Py (1), Ph (3); pyr=3,5-dimethylpyrazolyl) as P1 -building block (R-P) and tBuPH2 in an equimolar ratio. Compound 5 is of special interest since the presence of two pyridyl-substituents as well as the P4 -core allows for a rich coordination chemistry with coinage metal salts [Cu(MeCN)4 ][OTf], Ag[OTf] and in situ formed [Au(tht)][OTf] (tht=tetrahydrothiophene). Both tetraphosphetanes undergo alkylation reaction with MeOTf to give a series of tetraphosphetanium and tetraphosphetanediium triflate salts with additional methylation of the pyridyl-moiety in case of 5 resulting in interesting novel cyclic trications. Harsh reaction condition and an excess of MeOTf converts 5 into the cyclic trication [-P(Me Py)PMe2 P(Me Py)PtBu-]3+ (133+ ; Me Py=1-methylpyridiniumyl) through the elimination of isobutene. This salt undergoes a complicated rearrangement reaction involving a P-P/P-P bond metathesis to form trication [-P(MePy)3 PtBu-]3+ (173+ ) when reacted with Me2 PPMe2 .

Introduction

Cyclic polyphosphanes are a well‐known substance class in poly‐phosphorus chemistry.1 Most commonly, these compounds are synthesized by condensation2 or reduction3 of corresponding dihalophosphanes RPX2 (X=halogen) yielding symmetrical, mono‐substituted derivatives of type (RP) (n=3–6), which undergo rearrangement to the respective thermodynamically favored ring‐sizes (typically tetraphosphetanes or pentaphospholanes, which are also known as cyclotetraphosphanes and cyclopentaphosphanes, respectively; both nomenclatures are equivalent).1a This sort of scrambling reaction strongly depends on the substituents and the polarity of the used solvents.1, 4 Dihalophosphanes featuring sterically demanding substituents give access to diphosphenes RP=PR,5 which under certain circumstances dimerize to the respective tetraphosphetanes (RP)4.6 However, classical routes for the formation of mixed‐substituted tetraphosphetanes remain very often unselective, thus, there are only a few reports on mixed‐substituted tetraphosphetanes using specialized synthetic protocols.6, 7, 8 Noteworthy, reacting the sterically encumbered phosphane Mes*PH2 (Mes*=2,4,6‐tri‐tert‐butylphenyl) with PCl3 in the presence of a base, Schulz and co‐workers succeeded in the synthesis of the dichloro‐substituted tetraphosphetane (Mes*P‐PCl)2,8a showing a remarkable follow‐up chemistry, such as the reduction to bicyclic tetraphosphane Mes*P4Mes*.8 However, a more general route would certainly be beneficial for further exploration of the chemistry of this type of compounds.

As part of our ongoing development of methodologies using pyrazolyl‐substituted phosphanes as P1‐building blocks in P−P bond forming reactions by condensation or P−N/P−P bond metathesis,9, 10 we recently reported on the targeted synthesis of a series of polyphosphorus compounds including acyclic and cyclic polyphosphanes (Figure 1).11 The controlled reactions of dipyrazolylphosphanes RPpyr2 1 and 2 (pyr=3,5‐dimethylpyrazolyl; R=Py (1), BTz (2); Py=2‐pyridyl, BTz=2‐benzothiazolyl) with secondary phosphanes (R’2PH; PhPH(CH2)2PHPh; R’=Cy, tBu; Figure 1) yields triphosphanes () and a triphospholane () by condensation or pentaphospholanes () through a P−N/P−P bond metathesis reaction.11 Our protocol allows for the synthesis of these compounds on a multigram scale and, thus, offers their use as multidentate ligands in coordination chemistry as well as follow‐up chemistry, such as alkylation reactions. Extending this protocol towards primary phosphanes (RPH2) should generally allow the formation of cyclophosphanes. Thus, we selected tBuPH2 as sterically demanding primary phosphane and reacted it with dipyrazolylphosphanes RPpyr2 (R=Py (1), Ph (3)) as P1‐building block (R−P) in a 1:1 ratio in order to selectively access tetraphosphetanes 4 and 5 (Figure 1).

Figure 1

Previous work: Synthesis of triphosphanes (), triphospholane () and pentaphospholanes () from dipyrazolylphosphanes RPPyr2.3 This work: Synthesis of mixed‐substituted tetraphosphetanes 4 und 5 from RPpyr2 and tBuPH2 and selected follow‐up chemistry.

Particularly, compound 5 is of special interest since the presence of two pyridyl‐substituents as well as the P4‐core should allow for a rich coordination chemistry which we probed in the reaction with the coinage metal salts [Cu(MeCN)4][OTf], Ag[OTf] and in situ prepared [Au(tht)][OTf] (tht=tetrahydrothiophene). Both tetraphosphetanes were excessively alkylated with MeOTf in order to investigate their capability in the formation of multiply charged cations, related to those reported by Burford and co‐workers (IV‐VI, Figure 2).12 Equimolar reactions of 4 or 5 with MeOTf yield the mono‐methylated salts 6[OTf] and 8[OTf], whereas the formation of dicationic salts 7[OTf]2 and 9[OTf]2, similar to V, are not observed if the amount of MeOTf is increased. The presence of the pyridyl‐substituents in 5 offers an additional alkylation site, thus, multiply charged cations are accessible. Increasing the amount of MeOTf, we observe a preference for the alkylation of pyridyl‐substituents over the P4‐core. However, mono‐methylation to salt 10[OTf] is not observed since the formation of a mixture of di‐methylated 11[OTf]2 and trimethylated 12[OTf]3 is preferred. Harsh reaction condition and a large excess of MeOTf converts 5 into tricationic salt 13[OTf]3 which undergoes a complicated rearrangement reaction to salt 17[OTf]3 when reacted with Me2PPMe2, by means of a P−P/P−P bond metathesis reaction. A detailed discussion of our findings is presented in the following.

Figure 2

Several triflate salts derived from tetraphosphetanes 4 and 5 compared to established salts IV–VI.12

Results and Discussion

The mixed‐substituted tetraphosphetanes 4 (R=Ph) and 5 (R=Py) form readily from the reaction of tBuPH2 and the respective dipyrazolylphosphanes RPpyr2 (R=Py (1), Ph (3)) when mixed in equimolar ratios in CH3CN at −30 °C (Scheme 1).13

Scheme 1

Preparation of tetraphosphetanes 4 and 5; i) −2 pyrH, MeCN, −30 °C, 16 h, 69 % (4), 53 % (5).

After evaporating the solvent and subsequent sublimation of the by‐product 3,5‐dimethylpyrazole, both compounds are obtained as colorless powders in quantitative yields and an acceptable purity of >90 %, deduced from the integral ratios in the 31P NMR spectra.14 Washing of the crude products with cold MeCN gives analytically pure materials, however, reduces the isolated yield significantly (4: 69 %, 5: 53 %).14

Notably, the formation of other ring sizes is not observed and for both compounds the observation of an A2X2 spin system in the 31P NMR spectrum [4: δ(PA)=−88.4 ppm, δ(PX)=−15.5 ppm; 1 J(PAPX)=−130 Hz; 5: δ(PA)=−81.9 ppm, δ(PX)=−18.8 ppm; 1 J(PAPX)=−131 Hz] confirms the C 2 symmetry of the molecules. The structural connectivity is moreover confirmed by single‐crystal X‐ray analysis and the molecular structures of 4 and 5 are depicted in Figure 3. The structural parameters of the P4‐cores of 4 and 5 are listed in Table 3 and compare well with known symmetrically, homosubstituted tetraphosphetanes such as (PhP)4 15 and (CyP)4 16 which made us abstain from a detailed discussion. While the coordination behavior of tetraphosphetanes of type (RP)4 (R=Me, Et, Ph) has been extensively explored,17 corresponding investigations with mixed‐substituted derivatives are scarce and, to the best of our knowledge, did not involve derivatives with pyridyl‐substituents. The additional nitrogen‐based donor site of the pyridiyl‐units in 5 makes it a suitable multidentate ligand for metal coordination with selected CuI, AgI and AuI triflate salts. Thus, we reacted 5 in a 2:1 ratio in CH2Cl2 with [Cu(MeCN)4][OTf] and Ag[OTf] and in the case of AuI, with the corresponding triflate salt which was in situ prepared from (tht)AuCl (tht=tetrahydrothiophene) and Me3SiOTf (Scheme 2). Vapor diffusion of n‐pentane into the reaction mixtures at −30 °C yields crystals of [Cu5 2][OTf]*CH2Cl2, [Ag5 2][OTf]*CH2Cl2 and [Au5 2][OTf]*CH2Cl2 in good to very good yields (74–89 %). Crystals of [Cu5 2][OTf]*CH2Cl2 and [Au5 2][OTf]*CH2Cl2 are suitable for X‐ray analysis, however, better quality crystals of [Ag5 2][OTf] are obtained as MeCN monosolvate by recrystallization from MeCN/Et2O. The molecular structures are depicted in Figure 4 and structural parameters are summarized in Table 1. In all three cases the molecular structures reveal mononuclear metal complexes where the metal center MI is coordinated by two tetraphosphetanes through one tBuP‐moiety and the N atom of one pyridyl‐substituent each. The P−P bond lengths in the complexes remain virtually unchanged compared to the free tetraphosphetane 5. The average P−M distances in [M5 2][OTf] [2.2621 Å (Cu), 2.4100 Å (Ag) and 2.2982 Å (Au)] are comparable to those reported for other coinage metal complexes of polyphosphanes.11 [Cu5 2]+ shows a distorted tetrahedral geometry around the copper atom with a N‐Cu‐N angle of 94.55(6)° and a P‐Cu‐P angle of 127.26(2)°. This distortion causes a slight elongation of the Cu−N bond lengths in [Cu5 2]+ (av. 2.1124 Å) compared to tetrakis(pyridine)copper(I) hexafluorophosphate (Cu−N 2.061(3) Å), which shows an almost perfect tetrahedral coordination geometry.18

Figure 3

Molecular structures of tetraphosphetanes 4 (left) and 5 (right) (hydrogen atoms are omitted for clarity; thermal ellipsoids are displayed at 50 % probability); selected bond lengths and angles are given in Table 3.

Table 3

Selected geometrical parameters of crystallographically characterized 4, 5, 6[OTf], 8[OTf], 11[OTf]2, 12[OTf]3⋅2 MeNO2, 13[OTf]⋅3 MeNO2 and 17[OTf]; bond lengths are given in Å, angles are given in °; entries in blue indicate λ3P−λ4P+ bond lengths and λ3P‐λ4P+‐λ3P bond angles.

	4	5	6[OTf]	8[OTf]	11[OTf]2	12[OTf]3	13[OTf]3	17[OTf]3
P1−P2	2.2236(4)	2.2226(4)	2.1983(6)	2.1899(6)	2.2187(5)	2.2034(5)	2.247(2)	2.2398(7)
P2−P3	2.2222(4)	2.2232(5)	2.2341(6)	2.2419(6)	2.2345(5)[a]	2.2120(5)	2.238(2)	2.2194(7)
P3−P4	2.2198(4)	2.2195(4)	2.2333(5)	2.2397(6)	–	2.2420(5)	2.203(2)	2.2224(7)
P4−P1	2.2313(4)	2.2191(4)	2.1938(6)	2.1861(6)	–	2.2351(5)	2.204(2)	2.2257(7)
P1‐P2‐P3	87.87(1)	85.55(2)	83.97(2)	82.46(2)	86.30(2)[a]	90.33(2)	83.61(6)	82.92(3)
P2‐P3‐P4	85.94(1)	85.14(2)	90.28(2)	85.89(2)	87.21(2)[a]	84.12(2)	81.51(6)	84.44(3)
P3‐P4‐P1	87.73(1)	85.72(2)	84.09(2)	82.59(2)	–	88.76(2)	85.44(6)	83.17(9)
P4‐P1‐P2	85.63(1)	85.16(2)	92.77(2)	88.49(2)	–	84.48(2)	81.29(5)	83.88(2)

[a] P1’≙P3, P2’≙P4.

Scheme 2

Reaction of 5 with selected coinage metal triflate salts “MI[OTf]”; i) CH2Cl2, r.t., 1 h; M=Cu, M[OTf]=[Cu(MeCN)4][OTf], −4 MeCN, 89 %; M=Ag, M[OTf]=Ag[OTf], 76 %; M=Au, M[OTf]=(tht)AuCl+Me3SiOTf, ‐Me3SiCl, ‐tht, 74 %.

Figure 4

Molecular structures of [Cu5 2]+ in [Cu5 2][OTf]*CH2Cl2 (left), [Ag5 2]+ in [Ag5 2][OTf]*MeCN (middle) and [Au5 2]+ in [Au5 2][OTf]*CH2Cl2 (right) (hydrogen atoms, anions and solvate molecules are omitted for clarity, thermal ellipsoids are displayed at 50 % probability); selected bond lengths (Å) and angles (°) are given in Table 1.

Table 1

Selected geometrical parameters of crystallographically characterized coinage metal complexes [Cu5 2][OTf]⋅CH2Cl2, [Ag5 2][OTf]⋅MeCN and [Au5 2][OTf]⋅CH2Cl2.

	[Cu5 2][OTf]	[Ag5 2][OTf]	[Au5 2][OTf]
P‐Pa in Å	2.2204	2.2183	2.2191
P‐Ma in Å	2.2621	2.4100	2.2982
N‐Ma in Å	2.1124	2.4458	2.6262
N‐M‐N in °	94.55(6)	85.18(5)	75.42(6)
P‐M‐P in °	127.26(2)	141.14(2)	154.71(2)

[a] average bond lengths and angles are given.

Cations [Ag5 2]+ and [Au5 2]+ show wider P‐M‐P angles ([Ag5 2]+: 141.14(2)° and [Au5 2]+ 154.71(2)°) and more acute N‐M‐N angles ([Ag5 2]+: 85.18(5)° and [Au5 2]+ 75.42(6)°), causing a further elongation of the N−M distances ([Ag5 2]+: 2.4458 Å and [Au5 2]+ 2.6262 Å (average values). This states a decreasing participation of the pyridine nitrogen and vice versa an increasing involvement of the phosphorus in the coordination of silver and gold, which is in consistency with Pearsons’ concept.19 We further analyzed coordination complexes [Cu5 2][OTf], [Ag5 2][OTf] and [Au5 2][OTf] by multinuclear NMR spectroscopy at various temperatures. All three complexes [Cu5 2][OTf], [Ag5 2][OTf] and [Au5 2][OTf] show a dynamic behavior in CD2Cl2 solution at 290 K. In the 1H NMR spectra four resonances for the pyridyl moieties are observed in each case of the three coordination complexes, stating the fast exchange of the pyridyl moieties coordinating the respective metal cation. Upon cooling this exchange is slowing down, leading to eight different resonances for the pyridyl moieties. While two pyridyl groups are coordinating the metal cation, the other two are not, making them chemically inequivalent. The 1H NMR spectra of [Cu5 2][OTf], [Ag5 2][OTf] and [Au5 2][OTf] at temperatures from 290‐190 K are depicted in the supporting information,20 showing coalescence temperatures of around 260 K for [Cu5 2][OTf] and of 210 K for [Ag5 2][OTf] and [Au5 2][OTf]. These findings are also observed in the 31P NMR spectra.20 [Au5 2][OTf] and [Cu5 2][OTf] show three broadened resonances at 300 K due to dynamic processes and additionally for [Cu5 2][OTf] due to the fast quadrupole relaxation of the 63Cu nucleus.21 For [Ag5 2][OTf] two resonances are observed at 300 K. Measuring the 31P NMR spectra at 190 K reveals an AA'BB'MM'XX’ spin system for each coinage metal complex [M5 2][OTf] (see Figure 5; Table 2). Details on the coupling constants for [Au5 2][OTf], acquired by iteratively fitting the spectrum, are reported in the Supporting Information. Severe line broadening in the spectra of [Cu5 2][OTf] due to the fast quadrupole relaxation of the 63Cu nuclei,21 and further line splitting in the spectra of [Ag5 2][OTf] as a result of the complexation with the 107Ag/109Ag nuclei, made us refrain from iteratively fitting these spectra. It is noteworthy that the resonance of the tBu−P moiety coordinating the metal cation shows a significant shift compared to the tBu−P moiety of the free ligand 5. Coordination to CuI and AgI cause upfield shifts of Δδ=26.3 ppm and Δδ=13.7 ppm, respectively. Yet coordination to AuI is observed by a downfield shift of Δδ=38.0 ppm (Figure 5; Table 2).

Figure 5

31P{1H} spectra of 5 (CD2Cl2, 300 K) and [Cu5 2][OTf], [Ag5 2][OTf] and [Au5 2][OTf] (CD2Cl2, 190 K); insets show the AA'BB'MM'XX’ spin system of the experimental (upwards) and fitted spectra (downwards) of [Au5 2][OTf].

Table 2

Chemical shifts of 5, [Cu5 2][OTf]*CH2Cl2, [Ag5 2][OTf]*MeCN and [Au5 2][OTf]*CH2Cl2 in ppm; entries in blue indicate resonances of the tBu‐P moiety coordinating the metal(I) cation.

	5	[Cu5 2][OTf]	[Ag5 2][OTf]	[Au5 2][OTf]
δ(PA)	−81.9	−81.4	−87.6	−77.3
δ(PB)	–	−68.9	−82.4	−73.0
δ(PM)	–	−45.1	−32.5	−17.5
δ(PX)	−18.8	−15.4	−12.4	19.2

–

–

We further investigated the donor ability of compounds 4 and 5 by methylation reactions with an equimolar ratio of MeOTf in Et2O (Scheme 3). For both compounds, colorless precipitates are obtained which after filtration and subsequent recrystallization from MeCN/Et2O were identified as tetraphosphetane‐1‐ium triflate salts 6[OTf] and 8[OTf]. Both salts are obtained in very good yield (6[OTf]: 87 %; 8[OTf]: 91 %) and their 31P{1H} NMR spectra display an A2MX spin system each [6[OTf]: δ(PA)=−81.2 ppm, δ(PM)=−39.8 ppm, δ(PX)=22.0 ppm; 1 J(PAPX)=−248 Hz, 1 J(PAPM)=−127 Hz, 2 J(PMPX)=23 Hz; 8[OTf]: δ(PA)=−70.5 ppm, δ(PM)=−24.9 ppm, δ(PX)=24.2 ppm; 1 J(PAPX)=−225 Hz, 1 J(PAPM)=−132 Hz, 2 J(PMPX)=15 Hz] as expected for the mono‐methylation of one of the tBu−P moieties. X‐ray analysis of both salts confirmed our findings and the molecular structures of cations 6 + and 8 + are shown in Figure 6 and their structural parameters are in good agreement with those reported for related mono‐methylated tetraphosphetanium cations (Table 3).12b, 12c

Scheme 3

Mono‐methylation reaction of 4 and 5; i)+MeOTf, Et2O, r.t., 16 h, 87 % (6[OTf]), 91 % (8[OTf]).

Figure 6

Molecular structures of 6[OTf] (left) and 8[OTf] (right) (hydrogen atoms and anions are omitted for clarity; thermal ellipsoids are displayed at 50 % probability); selected bond lengths and angles are given in Table 3.

–

–

–

–

To access compounds of type V (Figure 2) we attempted harsher methylation conditions of 4 and 6 by reacting them in a slurry of a fivefold excess of MeOTf similarly as reported by Burford and co‐workers.12d In case of 4, only the exclusive formation of the mono‐methylated salt 6[OTf] is observed with no indication for the formation of other compounds. However, in case of 5 the formation of two distinct cations are observed as indicated by the 31P{1H} NMR spectrum of the heterogeneous reaction mixture, dissolved in CD3NO2 (Figure 7).

Figure 7

31P{1H} NMR spectrum of the reaction of 5 with 5 equiv. MeOTf (CD3NO2; 300 K); small amounts of unidentified compounds are indicated by asterisks.

The presence of an A2X2 spin system suggests the formation of the C 2 symmetric cation 11 2+ [δ(PA)=−96.4 ppm, δ(PX)=−4.3 ppm; 1 J(PAPX)=−132 Hz] and the observed A2MX spin system [δ(PA)=−89.8 ppm, δ(PM)=−17.4 ppm, δ(PX)=42.6 ppm; 1 J(PAPX)=−249 Hz, 1 J(PAPM)=−129 Hz, 2 J(PMPX)=22 Hz] is significantly different from that of the mono‐methylated cation 8 + suggesting the formation of tricationic 12 3+ (Scheme 4). We justify our findings by the observed shifts of the A2X2 spin system which are in the region of tri‐coordinated phosphorus atoms, whereas the resonance at δ(PX)=42.6 ppm for cation 12 3+ is typical for a tetra‐coordinated phosphorus.22 Our assumption was confirmed by X‐ray analysis since we were able to crystalize the salts, however, repeatedly as mixtures. After removal of the excess MeOTf from the reaction mixture, crystals of 11[OTf]2 suitable for X‐ray analysis grew from a MeCN solution by slow vapor diffusion of Et2O at −30 °C next to the deposition of copious amounts of amorphous material. If the reaction mixture is dissolved with excess of MeOTf in MeNO2 followed by Et2O addition by slow vapor diffusion at −30 °C, suitable crystals of 12[OTf]3*2 MeNO2 could be harvested.

Scheme 4

Reaction of 5 with excess MeOTf; i) 5 equiv MeOTf, neat, r.t., 16 h; unbalanced equation.

The separation of 11[OTf]2 from 12[OTf]3 by fractional crystallization was not possible so far, thus, hampering the isolation of analytically pure salts. Figure 8 displays the molecular structures of the cations 11 2+ and 12 3+. The P−P bond lengths in dicationic 11 2+ are comparable to those of tetraphosphetanes 4 and 5 (Table 3). The P‐P‐P angles differ only slightly indicating only a minor influence of the methylation of the pyridyl‐substituents to the structural parameters of the P4‐core.

Figure 8

Molecular structures of 11 2+ in 11[OTf]2 (left) and 12 3+ in 12[OTf]3*2 MeNO2 (right) (hydrogen atoms, anions and solvate molecules are omitted for clarity, thermal ellipsoids are displayed at 50 % probability); selected bond lengths and angles are given in Table 3.

Consistent with our findings, the structural parameters of trication 12 3+ are very similar to those of mono‐methylated tetraphosphetanium cations 6 + and 8 (Table 3). Our attempts to selectively synthesize 12[OTf]3 included the reaction of 5 in a 22‐fold excess of MeOTf at elevated temperature (80 °C; microwave). After 4 h, a red‐colored solution was obtained, and, although the 31P{1H} NMR spectrum showed a rather complex mixture of compounds, one very dominant A2MX spin system [δ(PA)=−73.5 ppm, δ(PM)=−12.0 ppm, δ(PX)=19.9 ppm; 1 J(PAPX)=−228 Hz, 1 J(PAPM)=−118 Hz, 2 J(PMPX)=31 Hz] indicated the formation of a new compound in high yields. After the addition of Et2O to the reaction mixture, copious amounts of a colorless precipitate are obtained. Washing with CH2Cl2 gives the analytically pure product in high yields (91 %) which was identified as 13[OTf]3 after recrystallization from a MeNO2 solution and slow vapor diffusion of Et2O or CH2Cl2 at −30 °C (Figure 9). We reason the formation of cation 13 3+ according to Scheme 5 through a series of alkylation and dealkylation reactions in which the first step is the formation of the tri‐methylated trication 12 3+. Due to the high charge of cation 12 3+, the nucleophilic attack of one triflate anion and elimination of tBuOTf is supported and leads to the formation of dication 14 2+. At temperatures above −30 °C tBuOTf is known to decompose to isobutene and HOTf.23 The formation of the latter can be traced by multi nuclear NMR spectroscopy of the reaction mixture (δ(H)=15.42 ppm (br), δ(F)=−76.0 ppm (s)).23b In the last step, dication 14 2+ is regioselectively methylated due to the excess of MeOTf to trication 13 3+. The molecular structure of 13 3+ is shown in Figure 9. The λ3P−λ4P+ bond lengths in 13 3+ (P3−P4 2.203(2) Å, P4−P1 2.204(2) Å) are marginally shorter compared to the λ3P−λ4P+ bond lengths in 12 3+ (P1−P2 2.2034(5) Å and P2−P3 2.2120(5) Å). Moreover, they are considerably shorter than the other P−P bond lengths in 13 3+ (P1−P2 2.247(2) Å, P2−P3 2.238(2) Å) which is caused by the bond polarization in the λ3P‐λ4P+ bond. This leads to a large λ3P‐λ4P+‐λ3P bond angle (P3‐P4‐P1 85.44(6)°) compared to the smaller P‐P‐P angles in 13 3+ (P1‐P2‐P3 83.61(6)°, P2‐P3‐P4 81.51(6)°, P4‐P1‐P2 81.29(5)°), which is consistent with our findings for the tetraphosphetan‐1‐ium salts 6[OTf] and 8[OTf] (Table 3).

Figure 9

Molecular structure of 13 3+ in 13[OTf]3*MeNO2 (hydrogen atoms, solvate molecules and anions are omitted for clarity, thermal ellipsoids are displayed at 50 % probability). Selected bond lengths and angles are given in Table 3.

Scheme 5

Proposed mechanism of the formation of 13[OTf]3 from the reaction of 5 with MeOTf at elevated temperatures; i) 22 equiv MeOTf, neat, 80 °C, 4 h, ‐ butene, ‐HOTf, 91 %.

Part of our investigation was, if ring contraction of 13 3+ by phosphenium abstraction gives access to the three‐membered dication 15 2+. Related approaches have been demonstrated by the Burford group which reacted tetraphosphetanium salts of type IV (Figure 2) with strong bases such as PMe3 and observed the formation of the respective ring contracted triphosphirane and phosphanylphosphonium salts.12a In our reactions we chose the more basic diphosphane Me2PPMe2 and added an equimolar amount to a solution of 13[OTf]3 in MeCN. A deep red‐colored reaction mixture was observed immediately which showed after a reaction time of 4 h at ambient temperature three main products as judged from the 31P{1H} NMR spectrum.24 Hexamethyltriphosphan‐2‐ium cation 16 2+ can be identified by its characteristic A2X spin system [δ(PA)=−59.2 ppm, δ(PX)=10.5 ppm; 1 J(PAPX)=298 Hz]12a and is the product of the successful phosphenium abstraction from 13 3+.24 In addition, the presence of a known A2X2 spin system identifies the formation of dicationic 11 2+ [δ(PA)=−96.4 ppm, δ(PX)=−4.3 ppm; 1 J(PAPX)=−132 Hz] and a new A2MX spin system [δ(PA)=−68.3 ppm, δ(PM)=−51.1 ppm, δ(PX)=2.7 ppm; 1 J(PAPX)=−123 Hz, 1 J(PAPM)=−100 Hz, 2 J(PMPX)=91 Hz] can be assigned to tricationic 17 3+ (Scheme 6).

Scheme 6

Suggested mechanism of the formation of 17[OTf]3 from the reaction of 13[OTf]3 with Me2PPMe2 by P−P/P−P bond metathesis (I) and a rearrangement reaction of the in situ generated triphosphirane 15 2+ (II); i) MeCN, r.t., 4 h, 45 %.

At first glance, these findings are different from our assumption, which does not agree with the observation of cation 11 2+ and the new cation 17 3+. In order to shed light on this outcome, we performed the reaction at −30 °C and monitored the progress of the reaction during the warm‐up to room temperature. At −30 °C the 31P{1H} NMR spectrum shows mainly the resonances for cation 16 + and an a characteristic A2X spin system [δ(PA)=−138.3 ppm, δ(PX)=−106.5 ppm; 1 J(PAPX)=−199 Hz] which is consistent with a triphosphirane, and thus, we assigned it to dication 15 2+ (Scheme 6, I).24 During warm‐up, the resonances for 15 2+ completely vanish and the resonances of cation 11 2+ and 17 3+ appear. It is known that cyclophosphanes can undergo interconversion reactions in protic solvents to give thermodynamically more favored ring sizes.4 Therefore, we assume that triphosphirane 15 2+ undergoes a ring‐interconversion as a result of a P−P/P−P bond metathesis to form cations 17 3+ and 11 2+ (Scheme 6, II).3, 25 We have been able to separate salt 17[OTf]3 from the reaction mixture in yields of 45 % by slow vapor diffusion of CH2Cl2 into the reaction mixture at −30 °C. The obtained clear, yellow‐colored crystals were suitable for X‐ray analysis and the molecular structure of 17 3+ is depicted in Figure 10. The P−P bond lengths in 17 3+ are in the expected range of P−P single bonds and comparable to the P−P bonds in 4, 5 and 11 2+ (Table 3). Compared to 11 2+ (P1‐P2‐P1’ 86.30(2)°, P2‐P1‐P2’ 87.21(2)°) the exchange of one tert‐butyl moiety for one methylpyridiniumyl‐substituent leads to smaller P‐P‐P angles in 17 3+ (P1‐P2‐P3 82.92(3)°, P2‐P3‐P4 84.44(3)°, P3‐P4‐P1 83.17(9)°, P4‐P1‐P3 83.88(2)°), as the methylpyridiniumyl group is sterically less demanding.

Figure 10

Molecular structure of 17 in 17[OTf]3 (hydrogen atoms, solvate molecules and anions are omitted for clarity, thermal ellipsoids are displayed at 50 % probability); selected bond lengths and angles are given in Table 3.

Conclusions

In summary, we could show that reacting dipyrazolylphosphanes PhPpyr2 and PyPpyr2 with tBuPH2 is a convenient and effective way to synthesize mixed‐substituted tetraphosphetanes 4 and 5 in good yields. Bearing additional donor sites, we eagerly investigated the diverse coordination behavior of pyridyl‐substituted tetraphosphetane 5 towards coinage metal triflate salts. The synthesized coordination complexes were studied by X‐ray analysis and multinuclear NMR spectroscopy at various temperatures. Further reactivity studies of the mixed‐substituted tetraphosphetanes 4 and 5 focused on the reactivity towards electrophilic MeOTf. Next to the expected formation of mono‐methylated tetraphosphetane‐1‐ium triflate salts 6[OTf] and 8[OTf], methylation reactions of 5 showed a rich and diverse chemistry due to the additional donor site of the pyridyl‐substituents. While treatment of 5 with excess MeOTf yielded a mixture of di‐ and trimethylated 11 and 12 3+, additional heating allowed for the synthesis of tricationic 13 3+ through unprecedented alkylation and dealkylation steps. Treating 13 3+ with Me2PPMe2 leads to 11 2+ and 17 3+ via triphosphirane 15 2+, which is formed in a P−P/P−P bond formation reaction and rearranges to form the thermodynamically favored products 11 2+ and 17 3+. This reaction sequence was intensively studied by multinuclear (2D) NMR spectroscopy. Additionally, all of the methylated tetraphosphetanes were studied by X‐ray analysis, confirming their structural connectivity.

Experimental Section

Crystallographic data: Deposition numbers 1990335, 1990336, 1990337, 1990338, 1990339, 1990340, 1990341, 1990342, 1990343, 1990344, and 1990345 contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.

Conflict of interest

The authors declare no conflict of interest.

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Supplementary

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