Remote Substituent Effects on the Structures and Stabilities of P E Ï

Article pubs.acs.org/IC

Remote Substituent Effects on the Structures and Stabilities of PE π‑Stabilized Diphosphatetrylenes (R2P)2E (E = Ge, Sn) Keith Izod,* Peter Evans, Paul G. Waddell, and Michael R. Probert Main Group Chemistry Laboratories, School of Chemistry, Newcastle University, Newcastle upon Tyne NE1 7RU, U.K. S Supporting Information *

ABSTRACT: A rare P−E π interaction between the lone pair of a planar P center and the vacant p orbital at the Ge or Sn center provides efficient stabilization for P-substituted tetrylenes (R2P)2E (E = Ge, Sn) and enables isolation of the first example of a compound with a crystallographically authenticated PSn bond. Subtle changes in the electronic properties of the bulky aryl substituents in these compounds change the preference for planar versus pyramidal P centers in the solid state; however, variabletemperature NMR spectroscopy indicates that in solution these species are subject to a dynamic equilibrium, which interconverts the planar and pyramidal P centers. Consistent with this, density functional theory studies suggest that there is only a small energy difference between the planar and pyramidal forms of these compounds and reveal a small singlet−triplet energy separation, suggesting potentially interesting reactivities.

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INTRODUCTION

interactions. Thus, stable phosphacarbenes should be accessible where a planar geometry at P can be achieved. Since Bertrand’s initial report,2a examples of several classes of stable (or at least persistent) P-substituted carbenes have been reported. These include the following: (i) silyl-substituted push−pull phosphacarbenes, e.g., (iPr2N)2P−C−SiMe3 (1; Chart 1),2a in which the carbene center is stabilized by a

III

Despite the fact that singlet phosphacarbenes, in which a P center lies directly adjacent to a divalent C atom, were among the first stable carbenes to be isolated, their chemistry is significantly less well-established than that of their Nsubstituted analogues.1−3 This is particularly striking given the widespread use of diaminocarbenes (such as N-heterocyclic carbenes, NHCs) as supporting ligands in transition-metalbased catalysis and organocatalysis and the major advances made in the manipulation of the steric and electronic properties of these compounds.1,4 The stability of diaminocarbenes may be attributed to the presence of electronegative N atoms adjacent to the carbene center, which increase the s character of the carbene lone pair and the π donation of the electron density from the N lone pairs into the vacant p-orbital at the carbene C atom, which mitigates the electron deficiency of this center. In this context, while it is a commonly held belief that pπ−pπ overlap will be poorer for P than N because of the larger, more diffuse valence orbitals of the former, calculations suggest that the inherent πdonor capacity of a planar P center is equal to, or better than, that of N. 5 As a consequence, the paucity of stable phosphacarbenes may be attributed to the high energetic barrier to planarization of P, a key requirement for efficient stabilization of the carbene center through P−C pπ−pπ © XXXX American Chemical Society

Chart 1. Selected Examples of (Di)phosphacarbenes and Diphosphatetrylenes

Received: July 21, 2016

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DOI: 10.1021/acs.inorgchem.6b01566 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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RESULTS AND DISCUSSION Synthesis and Structural Characterization. The reaction between SnCl2 and 2 equiv of [(Dipp)2P]Li in tetrahydrofuran (THF) gives a clear yellow solution (Scheme 1). Removal of the solvent and extraction of the residue into toluene give a deep-purple solution containing pale solids. Filtration, concentration, and cooling of the toluene solution yield deep-purpleblack crystals of the diphosphastannylene 6Sn. The formation of a yellow solution upon the initial reaction between [(Dipp)2P]Li and SnCl2 contrasts with the immediate formation of a deep-red solution during the corresponding reaction between [(Dipp)2P]Li and GeCl2(1,4-dioxane).14 We attribute the pale color observed in the former case to the initial formation of an ate complex of the form {(Dipp)2P}2SnClLi(THF)n, which would be electron-precise and in which the P− Sn pπ−pπ interactions would be absent. This degrades to 6Sn and LiCl in nondonor solvents, and the deep color associated with a PE π−π* transition becomes apparent (see below). Formation of a similar ate complex during the synthesis of 6Ge is disfavored because of the smaller size of the GeII atom and the consequently increased steric hindrance about this center. X-ray crystallography reveals that 6Sn is isostructural and isomorphous with 6Ge (Figure 1), with a V-shaped P−Sn−P

combination of P−C pπ−pπ overlap (push) and delocalization of the carbene lone pair into vacant C−Si σ* orbitals (pull); aryl-substituted push−spectator phosphacarbenes, e.g., (iPr2N)2P−C(mes) (2; mes = 2,4,6-Me3C6H2),2i where stabilization is afforded solely by the P−C pπ−pπ interaction; (iii) cyclic diphosphacarbenes (PHCs), e.g., 3,3 in which both P centers are engaged in pπ−pπ interactions with the vacant p orbital at the carbene center. Somewhat surprisingly, P-substituted heavier group 14 carbene analogues (phosphatetrylenes, (R2P)EX, and diphosphatetrylenes, (R2P)2E; E = Si, Ge, Sn, Pb) are also poorly represented, in spite of the relative accessibility of the 2+ oxidation state for many of the heavier group 14 elements.6−12 While the first crystallographically characterized two-coordinate diphosphatetrylenes [{(Tripp)(tBu)(F)Si}(iPr3Si)P]2E [E = Sn (4Sn), Pb (4Pb); Tripp = 2,4,6-iPr3C6H2] were reported by Driess and co-workers in 1995,8 further examples of such species were, until recently, limited to a small number of compounds whose monomeric nature has been inferred from spectroscopic and/or cryoscopic data.9 In both 4Sn and 4Pb, the two P centers adopt a trigonal-pyramidal geometry; there is no evidence for the π-type electron donation usually associated with the stabilization of NHCs and related compounds. Simple diphosphatetrylenes with less sterically demanding substituents, e.g., [(iPr2P)2Ge]2 (5),7g are found to dimerize both in the solid state and in solution, while the remaining (di)phosphatetrylenes possess three-coordinate tetrel centers and so may be considered electron-precise.10,11 As part of an extended program investigating the chemistry of heavier group 14 carbene analogues,7h,12,13 we recently reported the synthesis of the first two-coordinate diphosphatetrylene to exhibit significant pπ−pπ interactions between a planar P center and GeII, {(Dipp)2P}2Ge [Dipp = 2,6iPr2C6H3] (6Ge; Chart 2).14 This compound crystallizes with one planar and one pyramidal P atom and exhibits a substantially shorter P−Ge distance for the former than the latter.

Figure 1. Molecular structure of 6Sn with 40% probability ellipsoids and with H atoms omitted for clarity. Selected bond lengths (Å) and angles (deg): Sn(1)−P(1) 2.4458(8), Sn(1)−P(2) 2.5757(7), P(1)− C(1) 1.843(3), P(1)−C(13) 1.846(3), P(2)−C(25) 1.860(3), P(2)− C(37) 1.860(3); P(1)−Sn(1)−P(2) 106.20(3).

Chart 2

core [P−Sn−P 106.20(3)°] and with one nearly planar and one pyramidal P center [sum of the angles at P(1) 355.53° and P(2) 311.58°]. The slightly more pyramidal nature of the P(1) center in 6Sn compared to that in 6Ge [sum of the angles at P(1) 358.35° and P(2) 311.53°]14 is consistent with poorer pπ−pπ overlap between the lone pair at P and the vacant Sn 5p orbital in the former compared to the less diffuse 4p orbital in 6Ge (see the DFT Calculations section). The P(1)−Sn(1)

In order to probe this chemistry further, we investigated the synthesis of, and bonding in, the corresponding tin homologue {(Dipp) 2 P} 2 Sn (6Sn) and the analogous compounds {(Tripp)2}2E [E = Ge (7Ge), Sn (7Sn)]. This study, detailed below, reveals that relatively small changes in the steric and electronic properties of the ligands may have a profound effect on the structures and properties of their group 14 compounds. Scheme 1. Synthesis of 6/7Ge and 6/7Sn

B


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Inorganic Chemistry

suggests a different structure for the former in which pπ−pπ interactions are absent. This was confirmed by X-ray crystallography. While compound 7Ge adopts a structure that is very similar to that of 6Ge [P(1)−Ge−P(2) 103.98(8)°; sum of the angles at P(1) 360.0° and P(2) 311.5° (see the Supporting Information)], 7Sn adopts a rather different configuration. Although 7Sn crystallizes as discrete monomers with a V-shaped geometry, both P centers in this compound are pyramidal [sum of the angles at P(1) 308.62° and P(2) 299.74°] (Figure 2); the two

distance of 2.4458(8) Å is approximately 5% shorter than the P(2)−Sn(1) distance [2.5757(7) Å], the latter of which is typical of SnII−P distances.6 It has been noted previously15 that a portion of the decrease in the P−E (E = P, B) distances upon going from a pyramidal to a planar P center may be attributed to the change in the hybridization of the P atom from sp3 to sp2, and it is likely that such a rehybridization effect contributes to the short P−Ge/Sn distances in 6Ge/Sn. However, the presence of a genuine P−E pπ−pπ interaction is strongly supported by both theoretical calculations and spectroscopic data (see below). While there is substantial precedent for P−M π interactions in complexes of phosphide ligands with group 13 elements and transition metals,16 examples of compounds exhibiting P−Ge/ Sn pπ−pπ interactions are extremely rare; previously reported diphosphagermylenes and -stannylenes possess either pyramidal or bridging P centers (see above),6−12 while low-oxidationstate group 14 phosphinediides (phosphinidenes, [E(PR)]n) usually form clusters with μ3-PR ligands in which P−E pπ−pπ interactions are not possible.6 In this regard, Power and coworkers recently reported the solid-state structures of sterically encumbered, dimeric phosphinediide complexes [2,6-{2,6iPr2C6H3}2C6H3}P]2E2 [E = Ge, Sn, Pb].17 While the tin compound [2,6-{2,6-iPr2C6H3}2C6H3}P]2Sn2 exhibits one rather short and one typical Sn−P distance [2.4134(18) and 2.5192(17) Å, respectively], the corresponding Ge−P and Pb− P distances are typical for such species and the P atoms adopt a pyramidal geometry in all three compounds, and so it was concluded by the authors that there were minimal P−E pπ−pπ interactions in these compounds. In both 6Ge and 6Sn, only one of the two P centers adopts a planar configuration. This is a direct consequence of the barrier to planarization of trigonal-pyramidal phosphorus: if both P centers adopted a planar configuration, there would be competition for the p orbital at the tetrel center, resulting in a weaker P−E π interaction, which would not compensate for the energy required to planarize both P atoms. In order to explore the effect of substitution on the structures and properties of diphosphatetrylenes, we sought to prepare the corresponding p-iPr-substituted compounds {(Tripp)2P}2E [E = Ge (7Ge), Sn (7Sn)]. We expected that, given that the p-iPr groups should be directed away from the P−E−P core, the steric properties of (Tripp)2P would be similar to those of (Dipp)2P but that the electron-releasing p-iPr groups would potentially perturb the electronic structures of the diphosphatetrylene products. The reaction between 2 equiv of in situ prepared [(Tripp)2P] Li and GeCl2(1,4-dioxane) in THF gives a deep-red-black solution. Removal of the solvent, followed by extraction of the residue into methylcyclohexane and cooling, gives red-black crystals of the solvate {(Tripp)2P}2Ge.C7H14 (7Ge·C7H14). The solvent of crystallization is only very weakly held and rapidly lost under vacuum to give an amorphous red solid with elemental analyses and NMR spectra corresponding to the solvent-free {(Tripp)2P}2Ge (7Ge). In contrast, the reaction between 2 equiv of [(Tripp)2P]Li and SnCl2 in THF gives a yellow solution; removal of the solvent and extraction of the residue into n-hexane give a deep-purple solution, which yields yellow crystals of {(Tripp)2P}2Sn (7Sn) upon cooling. Once again, we attribute the initial formation of a yellow solution in the synthesis of 7Sn to the formation of an ate complex, which degrades to LiCl and 7Sn in nonpolar solvents. However, the difference in color between isolated crystals of 7Sn and 6Sn

Figure 2. Molecular structure of 7Sn with 40% probability ellipsoids and with H atoms omitted for clarity. Selected bond lengths (Å) and angles (deg): Sn(1)−P(1) 2.5836(6), Sn(1)−P(2) 2.5673(7), P(1)− C(1) 1.862(2), P(1)−C(16) 1.870(2), P(2)−C(31) 1.856(2), P(2)− C(46) 1.876(2); P(1)−Sn(1)−P(2) 89.62(2).

ligands adopt a syn,syn conformation. Consistent with this, the Sn(1)−P(1) and Sn(1)−P(2) distances [2.5836(6) and 2.5673(7) Å, respectively] are rather similar and are typical of P−SnII distances.6 In addition, the P−Sn−P angle in 7Sn is close to 90° and significantly more acute than the corresponding angle in 6Sn [P(1)−Sn(1)−P(2) 106.20(3)° (6Sn) and 89.62(2)° (7Sn)]. Compounds exhibiting GeP or SnP multiple-bond character are extremely rare.6 Only three phosphagermenes RR′GeP(mes*) [R, R′ = mes (8a);18 R = mes, R′ = tBu (8b);19 R, R′ = tBu2MeSi (8c);20 mes* = 2,4,6-tBu3C6H2], containing true GeP double bonds, have been structurally characterized. There are no previous crystallographically authenticated examples of a SnP double bond, although the phosphastannenes {(Me3Si)2CH}2SnP(mes*) (9a) and (Tripp)2SnP(mes*) (9b) have been assigned structures containing a SnP double bond on the basis of their NMR spectra and reactions.21 Thus, 6Ge, 7Ge·C7H14, and 7Sn represent some of the first examples of compounds exhibiting a GeP or SnP π interaction and are the first examples of such species where the tetrel center is in the 2+ oxidation state. The moderate isolated yields of crystalline 6/7Ge and 6/7Sn may be attributed to their high solubility even in nonpolar hydrocarbon solvents such as n-hexane; 31P{1H} NMR spectra of the crude reaction solutions indicate that these compounds are formed essentially quantitatively. Compounds 6Ge and 7Ge are stable in the absence of air and moisture, both in the solid state and in solution. In contrast, both 6Sn and 7Sn decompose over a period of several days at room temperature to give the diphosphines (Ar)2PP(Ar)2 [Ar = Dipp (10), Tripp (11)22] C


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Inorganic Chemistry

pyramidal P centers, respectively. However, the loss of solvent from 7Ge·C7H14 under vacuum yields an amorphous solid, the solid-state 31P{1H} MAS NMR spectrum of which exhibits broad peaks with multiple spinning side bands centered on 3.3 and 5.6 ppm, clearly inconsistent with the presence of a planar P center (see the Supporting Information). The absence of such a planar center, and hence the absence of PGe π interactions, is consistent with the significantly paler color of this amorphous material in comparison to 7Ge·C7H14, although the deep color of this compound is restored upon dissolution in toluene. Given the foregoing, we attribute signals B and E to the planar and pyramidal P centers observed in the solid state for crystalline 7Ge. The identities of the species responsible for peaks C and D remain elusive. The inequivalent intensity of these signals and lack of 31P−31P coupling indicate that these are separate species, rather than arising from the terminal and bridging P centers in a dimer similar to 5; in support of this, examination of a space-filling model of 7Ge reveals that the Ge center is shrouded by the Tripp groups, making dimerization through either a Ge2P2 core or formation of a GeGe double bond highly unlikely. The foregoing, in conjunction with coalescence at room temperature of all four of the signals observed at −90 °C, suggests that peaks C and D may be due to alternative configurations of 7Ge in which the two P centers are pyramidal. Attempts to locate geometries corresponding to such configurations by DFT calculations have revealed two minima (see below), which possess two pyramidal P centers; the calculated chemical shifts for these centers are −9 and −41 ppm in one (syn,anti) conformer and −15 ppm for both P centers in the second (syn,syn) conformer. The variable-temperature 31P{1H} NMR spectra of 6Sn and 7Sn in toluene-d8 are very similar to each other but are somewhat different from those of 6Ge/7Ge. The 31P{1H} NMR spectrum of 7Sn at room temperature exhibits a singlet (F) at −19.5 ppm [full-width at half-maximum (fwhm) ca. 100 Hz] displaying tin satellites (JPSn = 1420 Hz; Figure 4; see the Supporting Information for the corresponding spectra of 6Sn). As the temperature is reduced, this peak broadens and moves to higher field, such that, at −20 °C, the spectrum consists of a very broad singlet at −35.6 ppm (fwhm ca. 1200 Hz) on which tin satellites are not resolved. As the temperature is decreased further, this signal begins to sharpen and decoalesce until, at −40 °C, the spectrum consists of a broad singlet (G) at −47.4 ppm (fwhm ca. 270 Hz) exhibiting poorly resolved tin satellites (JPSn = ca. 960 Hz), along with a very broad, low-intensity singlet (H) at 1.3 ppm (fwhm ca. 1500 Hz) on which tin satellites are not resolved; signals G and H have an approximate ratio of 14:1 at this temperature. Further reduction of the temperature leads to the disappearance of signal H and the broadening of peak G, such that, at −80 °C, the spectrum consists of a broad singlet at −60.8 ppm (fwhm ca. 640 Hz). Below this temperature, this peak sharpens and continues to move to higher field until, at −100 °C, the spectrum consists of a broad singlet at −62.5 ppm (J; fwhm ca. 140 Hz) with wellresolved tin satellites (JPSn = 820 Hz). The corresponding room temperature 31P{1H} NMR spectrum of 6Sn consists of a singlet at −25.1 ppm (JPSn = 1300 Hz), and this decoalesces at −60 °C to give two signals at −57.6 ppm (JPSn = 850 Hz) and 0.8 ppm in an approximately 90:1 ratio. At the lower temperature limit (−90 °C), the signal at 0.8 ppm disappears and the spectrum consists of a singlet at −61.8 ppm (JPSn = 820 Hz).

and elemental Sn (compound 10 was synthesized deliberately for confirmation; see the Supporting Information). This decomposition is accelerated by exposure to ambient light, especially for solution samples: solutions of 6Sn or 7Sn in toluene degrade over a period of several hours, while solutions in THF degrade rapidly, completely decomposing to elemental Sn and 10 or 11 within 30 min. Solution-State Behavior. The room temperature 1H NMR spectra of 6Sn and 7Ge exhibit a single set of ligand signals in each case, consistent with rapid exchange between the planar and pyramidal centers and with rapid rotation about the P−C bonds. At lower temperatures, these signals broaden and decoalesce to give complex spectra with many broad and overlapping peaks, which are not possible to assign unambiguously. Similarly, the room temperature 1H NMR spectrum of 7Sn consists of a single set of ligand resonances that broaden and decoalesce as the temperature is reduced, giving a highly complex spectrum at low temperature. The variable-temperature 31P{1H} NMR spectra of 7Ge follow a pattern similar to that of the corresponding spectra of 6Ge. At room temperature, a single, broad singlet is observed at 3.6 ppm (A, Figure 3); as the temperature is reduced, this signal

Figure 3. Variable-temperature toluene-d8.

31

P{1H} NMR spectra of 7Ge in

broadens and shifts to slightly higher field, such that, at −20 °C, the spectrum consists of a broad singlet at −0.6 ppm. Below this temperature, this signal broadens further and decoalesces, until, at −90 °C (the lowest temperature we were able to attain), the spectrum exhibits signals at 101.5 (B), 7.2 (C), −43.3 (D), and −84.7 ppm (E, doublet JPP = 145.8 Hz) in the approximate ratio of 1.0:1.5:1.3:1.0. The signals corresponding to B and E in the low-temperature 31 1 P{ H} NMR spectrum of the analogous compound 6Ge were attributed to the planar and pyramidal P centers observed in the solid-state structure of this compound, on the basis of their chemical shifts and the correspondence between these and both the observed chemical shifts of these two centers in the solidstate 31P{1H} magic-angle-spinning (MAS) NMR spectrum of 6Ge and with the calculated chemical shifts from density functional theory (DFT) studies. It is also notable that the P centers in the phosphagermenes 8a and 8c are highly deshielded, resulting in 31P chemical shifts for these species of 175.4 and 416.3 ppm, respectively.18,20 In accordance with the foregoing, for 7Ge, DFT calculations (see below) yield chemical shifts of 103 and −55 ppm for the planar and D


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Inorganic Chemistry

Figure 4. Variable-temperature 31P{1H} NMR spectra of 7Sn in toluene-d8.

between 6/7SnplanA and 6/7SnplanB to be relatively rapid.23 Thus, at room temperature, a single 31P signal (F) is observed that corresponds to a weighted average of the chemical shifts of the planar and pyramidal P centers in 6/7Snplan and the two pyramidal P centers in 6/7Snpyr and a weighted average of their 31 P−117/119Sn coupling constants. This leads to a moderately low-field 31 P chemical shift and a moderately large 31 P−117/119Sn coupling constant. As the temperature is reduced, the rate of exchange of process b decreases and the signal decoalesces to give separate, broad peaks for the averaged signal for 6/7Snplan (H), which is still subject to dynamic process a, and for 6/7Snpyr (G). However, configuration 6/7Snpyr is favored at low temperatures, and so this species increases in concentration at the expense of 6/7Snplan. Thus, as 6/7Snpyr begins to predominate, the 31P signal moves to higher field and exhibits a significantly smaller 31P−117/119Sn coupling constant until, at −95 °C, the signal (J) is essentially that of 6/7Snpyr. Consistent with this, the 31P−117/119Sn coupling constants in the low-temperature 31P{1H} NMR spectra of both 6Sn and 7Sn (820 and 950 Hz, respectively) lie close to those observed in the solid-state 31P{1H} MAS NMR spectrum of the latter (850 and 900 Hz). These 31P−117/119Sn coupling constants are typical for compounds where a pyramidal P center is bound to SnII; for example, the 31P−117/119Sn coupling constant for the electron-precise compound [{(Me3Si)2CH}(C6H4-2-NMe2)P]2Sn is 1050 Hz.12c The rapid interconversion of the pyramidal and planar P centers in 6/7Ge and 6/7Sn is reminiscent of dynamic processes observed in early-transition-metal complexes such as Cp2M(PR2)2 [M = Zr, Hf; R = Et, Cy, Ph], which also contains one planar and one pyramidal P center in the solid state, and Cp2Nb(CO)(PiPrPh), for which 1H and 31P NMR spectroscopy suggests a rapid dynamic equilibrium between a planar and pyramidal geometry at the P center at room temperature.24 Because of the observed dynamic processes and the inherently broad nature of the 119Sn signals in these species, no 119Sn NMR signal could be found at room temperature for either compound; at −95 °C, compound 6Sn exhibits an extremely broad signal at 440 ppm (fwhm ca. 2000 Hz), while no signal could be found for 7Sn even at this low temperature.

The solid-state 31P{1H} MAS NMR spectrum of 6Sn (Figure 5a) exhibits signals at 95.8 ppm (JPSn = 2620 Hz) and −66.2 ppm (JPSn = 1180 Hz), with the former exhibiting significant chemical shift anisotropy (with CSAs of ca. −350 and ±50 for the two signals, respectively). These signals may be attributed to the planar and pyramidal P centers, respectively, observed in the crystal structure. Consistent with this, DFT calculations suggest 31P{1H} chemical shifts for the planar and pyramidal P centers in 6Sn of 94 and −67 ppm, respectively. The solid-state 31P{1H} MAS NMR spectrum of 7Sn (Figure 5b) differs markedly from that of 6Sn, consisting of wellresolved doublets at −48.8 and −58.7 ppm (JPP = 160.0 Hz) exhibiting poorly resolved tin satellites (JPSn = ca. 900 and 850 Hz, respectively); both signals have few spinning side bands, typical of a low CSA in each case. This is consistent with the presence of two similar, but distinct, pyramidal P environments, as observed in the crystal structure. In support of this, DFT calculations indicate a 31P{1H} NMR chemical shift of −41 ppm for the two pyramidal P centers in 7Sn. From the solid-state NMR data, we conclude that, in diphosphastannylenes, planar P centers are associated with lowfield chemical shifts, large 31P−119Sn coupling constants (JPSn > 1300 Hz), and a substantial CSA, while pyramidal P centers are associated with high-field chemical shifts, reduced coupling constants (JPSn < 1200 Hz), and a small CSA. This is also consistent with the low-field 31P chemical shifts and large 31 P−119Sn coupling constants observed for the P centers in the phosphastannenes 9a (204.7 ppm and 2295 Hz) and 9b (170.7 ppm and 2208 Hz).21 We therefore interpret the solution behavior of 6Sn and 7Sn according to Scheme 2. At room temperature, both compounds are subject to two separate dynamic equilibria: one (a) interconverts the planar and pyramidal P centers (i.e., 6/ 7SnplanA ↔ 6/7SnplanB), while the second (b) interconverts the configuration with one planar and one pyramidal P center with a configuration in which both P centers are pyramidal (i.e., 6/ 7SnplanA + 6/7SnplanB ↔ 6/7Snpyr). It has been established that the barrier to inversion of P centers directly bonded to electropositive elements and/or bulky groups is reduced, and so we would expect the exchange E


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Inorganic Chemistry

Figure 5. Solid-state 31P{1H} MAS NMR spectra of (a) 6Sn and (b) 7Sn (spin rate 10 kHz).

Compounds 6/7Ge and 6/7Sn are highly colored (deep red or purple) in solution. For both 6Ge and 7Ge, the deep-red color is associated with absorptions at 424 nm (ε = 5340 and 5130 M−1 cm−1 for 6Ge and 7Ge, respectively) and 548 nm (ε = 1080 and 1060 M−1 cm−1 for 6Ge and 7Ge, respectively), although the UV−visible spectra of both compounds additionally exhibit absorptions at approximately 304 nm, which are partially obscured by strong absorptions in the ultraviolet from the solvent (toluene) and the aromatic rings of the ligands. Time-dependent DFT (TD-DFT) calculations indicate that the absorptions at 424 and 548 nm are largely due to PGe π → π* transitions in the planar forms 6/7Geplan. Unfortunately, both 6Sn and 7Sn decompose rapidly to elemental Sn and

Scheme 2. Dynamic Equilibria Exhibited by 6/7Sn in Solution

We were also unable to observe a compound in the solid state.

119

Sn NMR signal for either F


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Inorganic Chemistry either 10 or 11 upon exposure to ambient light, and so we were not able to obtain UV−visible spectra of these compounds. Somewhat surprisingly, the UV−visible spectra of solutions of 6/7Ge in toluene and THF are essentially identical (Figure S2), indicating that the PGe π interaction is not perturbed by the strong donor solvent THF. We also note that cooling toluene solutions of 6/7Ge and 6/7Sn in liquid nitrogen leads to a significant change in the intensity of the color of the samples (Figure S3). This is consistent with these compounds favoring the pyramidal forms 6/7Gepyr and 6/7Snpyr at low temperature, for which TD-DFT calculations indicate extinction coefficients for the major transition at approximately 420 nm, which are an order of magnitude less than that of the corresponding planar forms 6/7Geplan and 6/7Snplan (see the Supporting Information). DFT Calculations. In order to gain insight into the relative stabilities of the two different configurations of 6/7Ge and 6/ 7Sn and to better understand the bonding in these compounds, we have carried out a DFT study (preliminary results for 6Ge were reported earlier).14 Minimum-energy geometries were located at the B97D/6311G(2d,p) level of theory25 for both the planar and pyramidal configurations of 6/7Ge′ and 6/7Sn′ [the relativistic LANL2DZ effective core potential basis set26 was applied to the Sn atoms; primes are used to denote calculated geometries throughout]. In addition to 6Ge′pyr, in which the two pyramidal phosphide ligands adopt a syn,syn conformation, we have also located a minimum-energy geometry for the corresponding syn,anti conformation, 6Ge′pyr(alt), which lies 50.9 kJ mol−1 higher in energy. Attempts to locate geometries with two planar P centers were unsuccessful; the relative energies for all configurations are given in Table 1, and the corresponding

Figure 6. Minimum-energy geometries for the three principal configurations of 6Ge′.

Table 2. Experimental and Calculated Bond Lengths (Å) and Angles (deg) for the Configurations of 6Ge′, 6Sn′, 7Ge′, and 7Sn′ 6Gea 6Ge′plan 6Ge′pyr 6Ge′pyr(alt) 6Sna 6Sn′plan 6Sn′pyr 7Gea 7Ge′plan 7Ge′pyr 7Sn′plan 7Sn′pyr 7Sna

Table 1. Calculated Relative Free Energies, Singlet−Triplet Energy Gaps, and HOMO−LUMO Separationsa compound 6Ge′plan 6Ge′pyr 6Ge′pyr(alt) 6Sn′plan 6Sn′pyr 7Ge′plan 7Ge′pyr 7Sn′plan 7Sn′pyr

ΔG

b

13.8 64.7 −2.5 −3.0 −4.2

ΔS−Tc

ΔH−Ld

162.4 148.6 97.7 80.0 82.5 76.2 79.2 76.3 80.4

3.58

a

3.38

E−Pplanar

E−Ppyramidal

P−E−P

2.2337(11) 2.2571

2.3823(12) 2.3912 2.4118 2.4062 (anti), 2.4569 (syn) 2.5780(6) 2.5964 2.6009 2.367(2) 2.3996 2.4063 2.5816 2.5992 2.5673(7), 2.5836(6)

107.40(4) 99.38 90.34 107.409 106.001(12) 96.736 88.239 103.98(8) 99.420 91.038 95.550 86.217 89.62(2)

2.4478(6) 2.4561 2.231(2) 2.2519 2.4535

From X-ray crystallography.

crystallographically for 7Sn. For the syn,anti conformer 6Ge′pyr(alt), the Ge−P distances differ significantly [2.4062 Å (anti) and 2.4569 Å (syn)]. Consistent with the above, the Wiberg bond indices (WBIs) for the E−P(1) bonds are significantly greater than 1 in 6/ 7Ge′plan and 6/7Sn′plan (Table 3), implying a significant degree of multiple-bond character, whereas the WBIs for the E−P(2) bonds in these compounds lie in the range 0.7−0.9, which is typical for a Ge−P or Sn−P σ bond. As expected, the WBIs for the E−P(1) bond in 6/7Sn′plan are lower than those for the corresponding bond in 6/7Ge′plan. For the pyramidal configurations 6/7Ge′pyr and 6/7Sn′pyr, the E−P WBIs lie in the range of 0.70−0.73. For 6Ge′, the planar form 6Ge′plan is calculated to be 13.8 kJ mol−1 more stable than 6Ge′pyr and 64.7 kJ mol−1 more stable than 6Ge′pyr(alt), consistent with the observed preference for the planar configuration 6Geplan in the solid state. In contrast, DFT calculations suggest that 7Ge′plan and 7Ge′pyr are almost isoenergetic, with the pyramidal form 7Ge′pyr more stable by 2.5 kJ mol−1. Similarly, for both 6Sn′ and 7Sn′, the pyramidal form 6/7Sn′pyr is favored by just a few kilojoules per mole over 6/7Sn′plan. For 6/7Sn′, this is consistent with the variabletemperature 31P{1H} NMR spectra, which indicate a dynamic

3.57 2.91

a

B97D/6-311G(2d,p) (for C, H, P, and Ge) and LANL2DZ (for Sn). Free energy of alternative form − free energy of planar form (kJ mol−1). cSinglet−triplet free-energy separation (kJ mol−1). dHOMO− LUMO gap (eV). b

minimum-energy geometries for 6Ge′ (taken as a representative example) are shown in Figure 6. There is a good correspondence between the calculated bond lengths and those determined for 6Sn, 7Ge, and 7Sn by X-ray crystallography; however, DFT calculations significantly underestimate the P− E−P angle in all cases, especially for 6Ge′plan, 6Sn′plan, and 7Ge′plan (Table 2). For both 6/7Ge′plan and 6/7Sn′plan, the E−P(2) distance is approximately 5.0−6.5% longer than the E−P(1) distance, consistent with the crystallographic data for 6/7Ge and 6Sn. The E−P distances in 6/7Ge′pyr and 6/7Sn′pyr are identical by symmetry, and those for 7Sn′pyr are similar to those determined G


Article

Inorganic Chemistry

which the Ge atom is substituted by two strongly σ-electrondonating boryl groups. The low value of ΔS−T calculated for 6/7Sn′plan may explain the limited tolerance of 6/7Sn to elevated temperatures and ambient light. In each case, the triplet-state geometry possesses two pyramidal P centers. The singly occupied molecular orbitals of each triplet state consist of an orbital that largely comprises the p orbital perpendicular to the P−E−P plane and an orbital with antibonding character that lies across the two P and the group 14 atoms (see the Supporting Information). Analyses of the molecular orbitals of 6/7Ge′plan and 6/ 7Sn′plan indicate that, while, for 6Ge′plan, HOMO−1 and LUMO+1 comprise the P−Ge π and π* orbitals, for 7Ge′plan and 6/7Sn′plan, the P−E π and π* orbitals are the HOMO and LUMO, respectively, in each case (Figure 7 and the Supporting

Table 3. NBO Analyses and E−P WBIs of 6Ge′, 6Sn′, 7Ge′, and 7Sn′ WBIb compound

% s E(lp)a

E−P(1)

E−P(2)

E(2) for E−P(2)c

6Ge′plan 6Ge′pyr 6Ge′pyr(alt) 6Sn′plan 6Sn′pyr 7Ge′plan 7Ge′pyr 7Sn′plan 7Sn′pyr

83.8 90.3 85.9 86.4 88.9 83.8 90.2 86.1 88.6

1.333 0.732 0.943 1.131 0.700 1.343 0.731 1.138 0.704

0.890 0.732 0.780 0.774 0.700 0.884 0.731 0.787 0.704

55.0

45.1 54.8 46.5

a

Pecentage of s character in the E lone pair. bWiberg bond indices of E−P(1) and E−P(2) bonds, respectively. cE(2) energy for the E(lone pair) → [P(2)−C(Me) σ*] delocalization (kJ mol−1).

equilibrium between 6/7Snpyr and 6/7Snplan in toluene solution (see above). We attribute the fact that the two different configurations 6Snplan and 7Snpyr are isolated in the solid state to the very small differences in energy between the two forms of each of these compounds and to the resulting influence of crystal packing effects on determining solid-state structural preferences. This is also consistent with the observation that 7Ge crystallizes as the methylcyclohexane solvate in the planar form 7Geplan but that this reverts to an amorphous solid upon exposure to vacuum and loss of methylcyclohexane, which, from 31P{1H} MAS NMR experiments, appears to contain two pyramidal P centers. Calculations have shown that the highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gaps (ΔH−L) and singlet−triplet energy separations (ΔS−T) in tetrylenes play key roles in their reactions with small molecules such as H2 and NH3, with smaller HOMO−LUMO and singlet−triplet gaps leading to reduced activation barriers for these reactions.27−29 For the prototypical N-heterocyclic germylene {CHNH}2Ge, ΔH−L and ΔS−T have been calculated to be 4.15 eV and 195.0 kJ mol−1, respectively, at the B3LYP/6311+G** level of theory.29 In contrast, the HOMO−LUMO gaps in 6/7Ge′plan and 6/7Sn′plan range from 2.91 to 3.58 eV, closer to the HOMO−LUMO gaps calculated for the hypothetical germylenes H2Ge and Me2Ge (3.41 and 3.43 eV, respectively).27 This results in rather low ΔS−T values (Table 1), suggesting that these diphosphatetrylenes will exhibit enhanced reactivities in comparison to related amidotetrylenes. This is especially the case for 7Ge′plan, 6Sn′plan, and 7Sn′plan, for which ΔS−T is calculated to be 76.2, 80.0, and 76.3 kJ mol−1, respectively. For comparison, ΔS−T for the recently reported phosphanylidene−phosphiranesubstituted germylene 11 (Chart 3), which also contains a P− Ge π interaction, is calculated to be 165 kJ mol−1 at the M062X/6-311+G(d,p) level of theory.30 The calculated values of ΔS−T for 7Ge′plan and 6/7Sn′plan are closer to that calculated for the germylene {(CHNDipp)2B}2Ge (43.9 kJ mol−1),29 in

Figure 7. (a) HOMO and (b) LUMO of 6Sn′plan.

Information). For 6/7Sn′pyr, the HOMO is centered on the aromatic rings of the Dipp and Tripp substituents, while for 6/ 7Ge′pyr, the HOMO lies across the Ge and P centers; in each of 6/7Ge′pyr and 6/7Sn′pyr, the LUMO is based on the vacant p orbital at the tetrel center (see the Supporting Information). Natural bond orbital (NBO) analyses of 6/7Ge′plan and 6/ 7Sn′plan reveal that the lone pair on the Ge or Sn atom has substantial s character (Table 3). The s character of this lonepair orbital is slightly higher in the pyramidal forms 6/7Ge′pyr and 6/7Sn′pyr. NBO analysis also reveals that, in addition to stabilization by the P−E π interaction, each of 6/7Ge′plan and 6/7Sn′plan is further stabilized by the donation of a Ge or Sn lone pair into one of the P−C σ* orbitals of the pyramidal P center. Secondorder perturbation theory analysis shows that the approximate stabilization afforded by these interactions [the E(2) energy] ranges from 44.1 to 55.0 kJ mol−1, with the higher values calculated for 6/7Ge′plan; such delocalizations are absent in the pyramidal forms 6/7Ge′pyr and 6/7Sn′pyr.

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CONCLUSIONS In conclusion, we have synthesized several new diphosphatetrylenes, including the first structurally authenticated example of a compound that exhibits a P−Sn π interaction. We have demonstrated that stabilization of these diphosphatetrylene species by such P−E π interactions is dependent upon the nature of the substituents at the P centers and the tetrel center. While {(Dipp)2P}2E (6Ge/Sn) and {(Tripp)2}2Ge (7Ge· C7H14) possess a planar P center in the solid state and thus benefit from stabilizing P−E π interactions, for {(Tripp)2}2Sn (7Sn), such interactions are absent, and this compound has two pyramidal P centers. In solution, 6/7Ge and 6/7Sn are subject to dynamic equilibria, which interconvert both the pyramidal and planar P centers within a molecule (6/7EplanA ↔ 6/

Chart 3

H


Article

Inorganic Chemistry

4H, ArH). 13C{1H} NMR (CDCl3): δ 24.14 (o-CHMeMe), 24.27 (pCHMeMe), 24.30 (p-CHMeMe), 24.54 (o-CHMeMe), 33.08 (d, JPC = 12.8 Hz, o-CHMeMe), 34.54 (p-CHMeMe), 121.84 (d, JPC = 2.8 Hz, Ar), 130.23 (d, JPC = 16.5 Hz, Ar), 149.40 (Ar), 152.65 (d, JPC = 10.9 Hz, Ar). 31P NMR (CDCl3): δ −102.6 (d, JPH = 231.7 Hz). Synthesis of {(Dipp)2P}2Ge (6Ge). This is a modified synthesis of this compound, based on our previously published procedure.14 A solution of nBuLi in hexanes (1.2 mL, 3.00 mmol) was added to a solution of Dipp2PH (1.01 g, 2.85 mmol) in THF (15 mL). The resulting red solution was stirred for 30 min and then added, dropwise, to a solution of GeCl2(1,4-dioxane) (0.334 g, 1.44 mmol) in THF (10 mL) at room temperature, and this mixture was stirred for 1 h. The solvent was removed in vacuo from the resulting dark-red solution to give a sticky red solid. The product was extracted into n-hexane (30 mL) to give a dark-red solution with pale solids. The solution was filtered, and the dark-red solution was reduced to 10 mL and stored at −25 °C. Dark-red crystals of 6Ge were isolated after 1 week. Yield: 0.39 g, 34%. 31P and 1H NMR data match those previously reported. Synthesis of {(Dipp)2P}2Sn (6Sn). A solution of n-BuLi in hexanes (1.0 mL, 2.50 mmol) was added to a solution of Dipp2PH (0.80 g, 2.27 mmol) in THF (15 mL). The resulting red solution was stirred for 1 h and then added, dropwise, to a solution of SnCl2 (0.216 g, 1.14 mmol) in THF (15 mL) at room temperature, and this mixture was stirred for 1 h in the absence of light. The solvent was removed in vacuo from the resulting dark-yellow solution to give a sticky orange solid. The product was extracted into toluene (15 mL) to give a purple solution containing pale solids. The solution was filtered, and the darkpurple filtrate was reduced to 5 mL and stored at −25 °C. Dark-purple crystals of 6Sn suitable for X-ray crystallography were isolated after 1 week. Yield: 0.37 g, 39%. Anal. Calcd for C48H68P2Sn (825.73): C, 69.80; H, 8.30. Found: C, 69.70; H, 8.33. 1H NMR (toluene-d8, 298 K): δ 1.13 (d, JHH = 7.0 Hz, 48H, CHMe2), 3.88 (m, 8H, CHMe2), 7.05 (d, JHH = 7.5 Hz, 8H, ArH), 7.12 (m, ArH). 13C{1H} NMR (toluene-d8, 298 K): δ 26.26 (CHMe2), 35.02 (m, CHMe2), 125.90 (Ar), 129.72 (Ar), 138.08 (m, Ar), 155.22 (m, Ar). 31P{1H} NMR (toluene-d8, 298 K): δ −25.1 (br s, JPSn = 1300 Hz). Synthesis of {(Tripp)2P}2Ge (7Ge). A solution of n-BuLi in hexanes (1.4 mL, 3.5 mmol) was added to a solution of Tripp2PH (1.55 g, 3.53 mmol) in THF (20 mL). The resulting dark-red solution was stirred for 30 min and then added, dropwise, to a solution of GeCl2(1,4-dioxane) (0.415 g, 1.79 mmol) in THF (20 mL), and this mixture was stirred for 1 h. The solvent was removed in vacuo from the resulting dark-red solution to give a sticky red solid. The product was extracted into methylcyclohexane (20 mL) to give a dark-red solution with pale solids. The solution was filtered, and the dark-red filtrate was reduced to 10 mL and stored at −25 °C. Dark-red crystals of 7Ge·C7H14 suitable for X-ray crystallography were isolated after 1 week and washed with hexamethyldisiloxane (2 mL). The isolated compound, which lost crystallinity due to solvent loss, was brown. The following data relate to the solvent-free form 7Ge. Yield: 0.52 g, 31%. Anal. Calcd for C60H92P2Ge (947.97): C, 76.02; H, 9.78. Found: C, 75.83; H, 9.89. 1H NMR (toluene-d8, 298 K): δ ca. 1.2 (br s, 48H, oCHMe2), 1.21 (d, JHH = 7.0 Hz, 24H, p-CHMe2), 2.76 (sept, JHH = 7.0 Hz, 4H, p-CHMe2), 4.07 (br s, 8H, o-CHMe2), 7.07 (s, 8H, ArH). 13 C{1H} NMR (toluene-d8, 298 K): δ 25.05 (p-CHMe2), 26.10 (br s, o-CHMe2), 35.54 (o-CHMe2), 36.69 (p-CHMe2), 123.69, 136.82, 150.51, 154.65 (Ar). 31P{1H} NMR (toluene-d8, 298 K): δ 3.6 (br s). Synthesis of {(Tripp)2P}2Sn (7Sn). A solution of n-BuLi in hexanes (1.1 mL, 2.75 mmol) was added to a solution of Tripp2PH (1.18 g, 2.69 mmol) in THF (20 mL). The resulting dark-red solution was stirred for 1 h and then added, dropwise, to a solution of SnCl2 (0.256 g, 2.69 mmol) in THF (15 mL) at room temperature, and this mixture was stirred for 1 h in the absence of light. The solvent was removed in vacuo from the resulting dark-yellow solution to give a sticky orange solid. The product was extracted into n-hexane (15 mL) to give a purple solution containing pale solids. The solution was filtered, and the dark-purple filtrate was reduced to 5 mL and stored at −25 °C. Dark-yellow crystals of 7Sn suitable for X-ray crystallography were isolated after 2 days. Yield: 0.39 g, 29%. Anal. Calcd for C60H92P2Sn (994.05): C, 72.50; H, 9.33. Found: C, 72.40; H, 9.14. 1H

7EplanB) and the two different forms (6/7Eplan ↔ 6/7Epyr). DFT calculations indicate that, for 7Ge and 6/7Sn, there is little difference in energy between the latter two forms. NBO calculations reveal that stabilization of diphosphatetrylenes possessing a planar P center (6/7Ge and 6Sn) may be characterized as of the push−pull type, with the push arising from the π donation of the electron density from P(1) into the vacant p orbital at the tetrel center and the pull arising from delocalization of the Sn or Ge lone pair into a P−C σ* orbital of the pyramidal P(2) center. In contrast, stabilization of diphosphatetrylenes possessing two pyramidal P centers (7Sn) appears to be largely steric in origin.

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EXPERIMENTAL SECTION

General Procedures. All manipulations were carried out using standard Schlenk and drybox techniques under an atmosphere of dry nitrogen or argon. THF, diethyl ether, toluene, methylcyclohexane, and n-hexane were dried prior to use by distillation under nitrogen from sodium, potassium, or sodium/potassium alloy, as appropriate; hexamethyldisiloxane was distilled from CaH2. THF and hexamethyldisiloxane were stored over activated 4A molecular sieves; all other solvents were stored over a potassium film. Deuterated toluene was distilled from potassium, and CDCl3 was distilled from CaH2 under nitrogen; all NMR solvents were deoxygenated by three freeze− pump−thaw cycles and were stored over activated 4A molecular sieves. (Dipp)2PH was prepared by a previously published procedure;14 nbutyllithium was purchased from Aldrich as a 2.5 M solution in hexanes. All other compounds were used as supplied by the manufacturer. 1 H and 13C{1H} NMR spectra were recorded on a Bruker Avance III 500 MHz spectrometer operating at 500.16 and 125.65 MHz, respectively, or a Bruker Avance III 300 MHz spectrometer operating at 300.15 and 75.47 MHz, respectively; chemical shifts are quoted in ppm relative to tetramethylsilane. 31P{1H} and 119Sn{1H} NMR spectra were recorded on a Bruker Avance III 500 MHz spectrometer operating at 202.35 and 186.59 MHz, respectively; chemical shifts are quoted in ppm relative to external 85% H3PO4 and Me4Sn, respectively. UV−visible spectra were recorded as 0.15 mM solutions in either toluene or THF in 1 cm matched quartz cells on a Shimadzu UB-1800 spectrophotometer. Solid-state 31P{1H} NMR spectra were recorded at 161.99 MHz using a Bruker Avance III 400 MHz spectrometer and a 4 mm (rotor o.d.) MAS probe. Spectra were obtained using cross-polarization with a 1 s recycle delay and a 1 ms contact time at ambient probe temperature (∼25 °C) and at a sample spin rate of either 10 or 12 kHz. Spectral referencing was with respect to an external sample of 85% H3PO4 (carried out by setting the signal from brushite to 1.0 ppm). Simulation of the spinning-side-band structure for 6Sn yields chemical shift anisotropies (σzz−σiso) of approximately ±50 and −350 ppm for the signals at −66.2 and 95.8 ppm, respectively. Elemental analyses were obtained by the Elemental Analysis Service of London Metropolitan University. Synthesis of (Tripp)2PH. This is a modification of a previously reported procedure.31 To a cold (0 °C) solution of TrippLi·OEt2 (5.67 g, 19.9 mmol) in Et2O (40 mL) was added PCl3 (0.9 mL, 10 mmol) dropwise, and this mixture was allowed to warm to room temperature with stirring for 1 h. Solid LiAlH4 (0.378 g, 10.0 mmol) was added in portions, and the mixture was stirred for 1 h before being quenched by the gradual addition of degassed water (30 mL). The organic phase was extracted into Et2O (3 × 20 mL) and dried over activated 4 Å molecular sieves. The solution was filtered, and the solvent was removed in vacuo from the filtrate to give a viscous pale-yellow oil (4.28 g, 9.75 mmol). Recrystallization from isopropyl alcohol (30 mL) at −78 °C gave (Tripp)2PH as a white crystalline solid. Yield: 2.19 g, 50%. 1H NMR (CDCl3): δ 0.99 (d, JHH = 6.9 Hz, 12H, o-CHMeMe), 1.03 (d, JHH = 6.9 Hz, 12H, o-CHMeMe), 1.20 (d, JHH = 6.9 Hz, 12H, p-CHMe2), 2.82 (sept, JHH = 6.9 Hz, 2H, p-CHMe2), 3.49 (m, 4H, oCHMeMe), 5.42 (d, JPH = 231.7 Hz, 1H, PH), 6.93 (d, JPH = 2.4 Hz, I


Article

Inorganic Chemistry NMR (toluene-d8, 298 K): δ 1.20 (br s, 24H, o-CHMe2), 1.23 (d, JHH = 6.9 Hz, 12H, p-CHMe2), 2.74 (sept, JHH = 6.9 Hz, 4H, p-CHMe2), 4.02 (br s, 8H, o-CHMe2), 7.08 (s, 8H, ArH). 13C{1H} NMR (toluene-d8, 298 K): δ 25.06 (p-CHMe2), 26.29 (o-CHMe2), 35.21 (oCHMe2), 35.52 (p-CHMe2), 123.87 (Ar), 135.97 (m, Ar), 149.85 (Ar), 155.30 (m, Ar). 31P{1H} NMR (toluene-d8, 298 K): δ −19.5 (br s, JPSn = 1420 Hz). Synthesis of (Dipp)2PP(Dipp)2 (10). A solution of n-BuLi (1.0 mL, 2.3 mmol, 2.3 M) was added to a solution of Dipp2PH (0.84 g, 2.37 mmol) in THF (10 mL). The resulting red solution was stirred for 15 min and then added rapidly to a suspension of PbI2 (0.544 g, 1.20 mmol) in THF (10 mL). The mixture immediately formed a black precipitate and was stirred for 30 min. The solvent was removed in vacuo, and the product was extracted into Et2O (25 mL). The solution was filtered, and the solvent was removed in vacuo from the clear yellow filtrate to give a pale solid and a red oil. The colored impurities were removed by heating the mixture in degassed ethanol (10 mL). The mixture was then cooled to 0 °C, and the supernatant solution was decanted. This was repeated three more times until the supernatant was colorless. Removal of the residual solvent in vacuo from the solid gave 10 as a pale-yellow powder. Yield: 0.39 g, 47%. 1H NMR (CDCl3): δ 0.04 (d, JHH = 6.9 Hz, 6H, CHMe2), 0.14 (d, JHH = 6.4 Hz, 6H, CHMe2), 0.42 (d, JHH = 6.8 Hz, 6H, CHMe2), 0.49 (d, JHH = 6.4 Hz, 6H, CHMe2), 1,13 (d, JHH = 6.5 Hz, 6H, CHMe2), 1.14 (d, JHH = 6.5 Hz, 6H, CHMe2), 1.22 (d, JHH = 6.7 Hz, 6H, CHMe2), 1.25 (d, JHH = 6.5 Hz, 6H, CHMe2), 3.61 (sept, JHH = 6.6 Hz, 2H, CHMe2), 3.74 (sept, JHH = 6.1 Hz, 2H, CHMe2), 3.96 (sept, JHH = 5.8 Hz, 2H, CHMe2), 4.90 (m, 2H, CHMe2), 6.96 (m, 4H, ArH), 7.02 (dd, JHH = 7.8 Hz, JHH = 1.4 Hz, 2H, ArH), 7.08 (dd, JHH = 7.5 Hz, JHH = 1.4 Hz, 2H, ArH), 7.16 (t, JHH = 7.8 Hz, 2H, ArH), 7.18 (t, JHH = 7.8 Hz, 2H, ArH). 13C{1H} NMR (CDCl3): δ 22.58, 22.85, 23.94, 24.01, 24.03, 24.41, 25.48 (CHMe2), 31.57 (t, JPC = 18.7 Hz, CHMe2), 32.19 (t, JPC = 20.5 Hz, CHMe2), 32.63 (t, JPC = 19.2 Hz, CHMe2), 32.83 (CHMe2), 123.95 (t, JPC = 3.1 Hz, Ar), 124.24 (t, JPC = 3.9 Hz, Ar), 126.15, 126.51, 128.67, 129.72 (Ar), 134.07 (t, JPC = 14.7 Hz, Ar), 134.99 (t, JPC = 18.9 Hz), 154.27 (m, Ar), 154.77 (t, JPC = 4.2 Hz, Ar), 155.65 (t, JPC = 19.3 Hz, Ar). 31P NMR (CDCl3): δ −38.1 (s). X-ray Crystallography. Crystal structure data sets for all compounds were collected on an Xcalibur, Atlas, Gemini ultradiffractometer using an Enhance Ultra X-ray source (λCu Kα = 1.54184 Å) for 7Ge·C7H14 and 10 and a fine-focus sealed X-ray tube (λMo Kα = 0.71073 Å) for 7Sn, 7Sn at room temperature, and 6Sn. Using an Oxford Cryosystems CryostreamPlus open-flow nitrogen cooling device, data for all structures were collected at 150 K, with the exception of 7Sn at room temperature, which was collected at 290 K. Cell refinement, data collection, and data reduction were undertaken using CrysAlisPro.32 For 7Ge·C7H14, 10, and 6Sn, an analytical numeric absorption correction was applied using a multifaceted crystal model based on expressions derived by Clark and Reid.33 For 7Sn and 7Sn at room temperature, intensities were corrected for absorption empirically using spherical harmonics.The structures were solved using SHELXT34 and refined by SHELXL35 through the Olex2 interface.36 H atoms were positioned with idealized geometry, and their displacement parameters were constrained using a riding model. Disorder in the structure of 10 was modeled using restraints and constraints where appropriate to maintain a physically meaningful refinement. Structure solution for 6Sn was originally performed using data collected at the Diamond Light Source facility; however, subsequent crystallizations provided samples of higher quality. These were suitable for in-house instrumentation and provided the data for full structural analysis presented herein. DFT Calculations. Geometry optimizations were performed with the Gaussian09 suite of programs (revision D.01).37 The pure B97D functional,25 which includes a correction for dispersion effects, was employed throughout; the 6-311G(2d,p) all-electron basis set25 was used on all atoms except Sn, for which the relativistic LANL2DZ basis set26 was used (default parameters were used throughout). The identities of minima were confirmed by the absence of imaginary vibrational frequencies in each case. Automatic density fitting was employed for all geometry optimizations and frequency calculations.

NMR shielding tensors were calculated using the GIAO method38 at the B97D/LANL2DZ,6-311++G(2d,p)//LANL2DZ,6-311G(2d,p) level of theory, and chemical shifts are quoted in ppm relative to H3PO4 (based on PMe3 at −61 ppm calculated at the same level of theory); TD-DFT calculations were carried out at the B97D/ LANL2DZ,6-311G(2d,p) level of theory with solvation by toluene using the IEF polarizable continuum model implemented in Gaussian09.39 NBO analyses were performed using the NBO 3.1 module of Gaussian09.40

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01566. Data supporting this publication are openly available under an ‘Open Data Commons Open Database License’. Additional metadata are available at: http://dx.doi. org/10.17634/154300-31. Please contact Newcastle Research Data Service at [email protected] for access instructions. Details of structure determination, atomic coordinates, bond lengths and angles, 1H, 13C{1H}, and 31P{1H} NMR spectra of 6Sn, 7Ge, 7Sn, and 10, variabletemperature 31P{1H} NMR spectra of 7Sn, solid-state MAS 31P{1H} NMR spectra of 7Ge, UV−visible spectra of 6/7Ge, and details of DFT calculations, final atomic coordinates, and energies of all located minima for 6/ 7Ge′ and 6/7Sn′ (PDF) Displacement parameters in CIF format (CIF) Computed Cartesian coordinates of all of the molecules reported in this study, which may be opened as a text file to read the coordinates or opened directly by a molecular modeling program such as Mercury (version 3.3 or later, http://www.ccdc.cam.ac.uk/pages/Home.aspx) for visualization and analysis (XYZ)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the use of the EPSRC U.K. National Service for Computational Chemistry Software (NSCCS) at Imperial College London in carrying out this work and thank the Diamond Light Source for access to synchrotron facilities (I19) and Prof W. Clegg for the initial structure solution of 6Sn. Solid-state NMR spectra were obtained at the EPSRC U.K. National Solid-state NMR Service at Durham, UK. This work was partly funded by the U.K. Engineering and Physical Sciences Research Council (EPSRC; Grant EP/L5048281).

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REFERENCES

(1) For reviews, see: (a) Vignolle, J.; Cattoën, X.; Bourissou, D. Stable Noncyclic Singlet Carbenes. Chem. Rev. 2009, 109, 3333−3384. (b) Martin, D.; Melaimi, M.; Soleilhavoup, M.; Bertrand, G. A Brief Survey of Our Contribution to Stable Carbene Chemistry. Organometallics 2011, 30, 5304−5313. (c) Melaimi, M.; Soleilhavoup, M.; Bertrand, G. Stable Cyclic Carbenes and Related Species Beyond Diaminocarbenes. Angew. Chem., Int. Ed. 2010, 49, 8810−8849. (2) (a) Igau, A.; Grützmacher, H.; Baceiredo, A.; Bertrand, G. Analogous α,α′-Bis-Carbenoid Triply Bonded Species: Synthesis of a Stable λ3-Phosphinocarbene-λ5-Phosphaacetylene. J. Am. Chem. Soc. 1988, 110, 6463−6466. (b) Gillette, G.; Baceiredo, A.; Bertrand, G. J


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Inorganic Chemistry

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