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Synthesis, Geometry, and Bonding Nature of Heptacoordinate Compounds of Silicon and Germanium Featuring Three Phosphine Donors Hajime Kameo,*,† Tatsuya Kawamoto,‡ Shigeyoshi Sakaki,§ Didier Bourissou,∥,⊥ and Hiroshi Nakazawa*,‡ †

Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Gakuen-cho 1-1, Naka-ku, Sakai, Osaka 599-8531, Japan ‡ Department of Chemistry, Graduate School of Science, Osaka City University, Sugimoto 3-3-138, Sumiyoshi-ku, Osaka 558-8585, Japan § Fukui Institute for Fundamental Chemistry, Kyoto University, Takano-nishihiraki-cho 34-4, Sakyo-ku, Kyoto 606-8103, Japan ∥ Laboratoire Hétérochimie Fondamentale Appliquée, Université de Toulouse, UPS, 118 Route de Narbonne, F-31062 Toulouse, France ⊥ CNRS, LHFA UMR 5069, F-31062 Toulouse, France S Supporting Information *

ABSTRACT: Structural studies were performed on heptacoordinate compounds of silicon {(o-Ph2P)C6H4}3SiX (X = F (1), Cl (3), H (5)) and germanium {(o-Ph2P)C6H4}3GeX (X = F (2), Cl (4), H (6), Me (7)) compounds featuring three phosphine donors. We found that 5, 6, and 7 have approximately a C3 symmetry similar to Corriu’s compounds (heptacoordinate silane {(o-Me2NCH2)C6H4}3SiX (X = F (8), H) and germane {(o-Me2NCH2)C6H4}3GeX (X = H, F) with three nitrogen donors coordinating to the central Si/Ge trans to the Cipso atoms). In contrast, the Si compounds 1 and 3 and the Ge compounds 2 and 4 have novel heptacoordinate geometries; the incorporation of such electronegative substituents as F and Cl results in the change of one phosphine donor from the position trans to the Cipso atom to that trans to the X atom. Compounds 1−4 retain this unprecedented geometry in solution but show dynamic behavior. The structural modification observed upon changing the substituent at Si and Ge is rationalized by electrostatic and charge transfer interactions.

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σ*(E−Cipso) orbitals acted as electron acceptors. There is no donor group at the positions trans to the E−H or E−F bond, and the σ*(E−F) or σ*(E−H) MO is not involved in the formation of the dative N→E interactions. Although SN2-type reactions generally take place at the position trans to the Si−F bond, these results indicate that the reaction can occur at a different site in case the steric factors are important. Heptacoordinate species bearing O→E interactions were also structurally elucidated using an L3X-type ligand by some groups (type II in Chart 1).3b,d,e From these earlier studies, it is known that the second-row element donors are useful for the formation of heptacoordinate species. In contrast, there is, to the best of our knowledge, no example of heptacoordinate silane or germane compounds containing dative interactions from third-row elements.4,5 This may be attributed, at least to some extent, to the larger atomic radii of third-row elements as

INTRODUCTION

Hypercoordinate compounds involving heavier group 14 elements have been a curiosity to many chemists due to their versatile structures, and they have attracted considerable attention as models for transition states or intermediates of SN2-type reactions.1 Although neutral penta- and hexacoordinate compounds have been well explored, heptacoordinate species remain scarce.2,3 Corriu et al. made a significant contribution to the chemistry of heptacoordinate group 14 compounds.2 Their pioneering works started from the observation of a heptacoordinate intermediate (or transition state) in the reaction involving a nucleophilic attack on a hexacoordinate silicon atom.2a Their research evolved into the synthesis and structural analysis of heptacoordinate species.2b−d For example, they employed LX-type bidentate ligands and reported silane and germane compounds having four covalent bonds and three N→E (E = Si, Ge) interactions (type I in Chart 1).2b The dative N→E bonds (E = Si, Ge) took place at the face opposite the three covalent bonds (E−Cipso), whose © XXXX American Chemical Society

Received: September 2, 2014

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dx.doi.org/10.1021/om500906f | Organometallics XXXX, XXX, XXX−XXX

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Chart 1. Neutral Heptacoordinate Compounds of Silicon and Germanium

One phosphorus atom coordinates to the silicon or germanium center opposite the fluorine atom (P1−Si1−F1: 156.29(8)°, P3−Ge1−F1: 158.04(7)°). The other two phosphorus atoms occupy positions trans to ipso-carbons of the ortho-phenylene groups. This geometry is named “form a” hereafter. The structural features of 1a and 2a are different from that of Corriu’s nitrogen donor system {(o-Me2NCH2)C6H4}3Si(F) (8), in which no nitrogen donor is located at the position trans to the Si−F bond, but all nitrogen atoms occupy positions trans to the ipso-carbons of the phenylene moieties. All E−C bonds in 1a (1a: 1.881(3), 1.877(3), 1.876(3) Å; 2a: 1.955(3), 1.959(3), 1.959(3) Å) are assignable to typical single bonds.8 The lone pairs of electrons on the phosphorus atoms are directed toward regions of σ*(E−X) orbitals (X = F, Cipso) at the silicon or germanium center. In addition, the three P−E distances (1a: 3.3797(13), 3.3575(12), 3.3899(11) Å; 2a: 3.3150(10), 3.3603(10), 3.2768(11) Å) are significantly shorter than the sum of van der Waals radii (Si−P: 4.05 Å; Ge−P: 4.05 Å),9 supporting the presence of P→E interactions. The three Si−P distances in 1a are comparable, while the Ge−P distance (3.2768(11) Å) involving the P atom trans to the F atom is shorter than the other two Ge−P distances (3.3150(10) and 3.3603(10) Å). The P−Ge distance trans to F in 2a is shorter than the Si−P distances in 1a, despite the larger atomic radius of Ge relative to Si.10 Further, the Si−F distance of 1a (1.606(2) Å) is comparable with that of Ph3SiF (1.603 Å),11 while the Ge−F distance is slightly longer in 2a (1.762(2) Å) than in Ph3GeF (1.749 Å).12 Probably, the electron donation from the phosphorus atom trans to the Ge−F bond weakens somewhat the Ge−F bond through P→σ*(Ge−F) interaction. The nature and magnitude of P→E interactions will be evaluated later on by DFT calculations. In the 29Si{1H} NMR spectrum of 1a recorded at ambient temperature, a doublet of quartets (1JSi−F = 288 Hz, 1JSi−P = 7.6 Hz) is observed at δ −5.0 ppm, as compared with the doublet signal (1JSi−F = 281 Hz) found at δ −3.2 ppm for Ph3SiF.13 Consistently, the coordination of phosphine donors in 1a induces little change in the Si−F distance and in the tetrahedral arrangement of the silicon atom (∑(C−Si−C) = 340.0° (1a), 336.7° (Ph3SiF)).11 The presence of JSi−P coupling constants indicates the presence of P→Si interactions. Such JSi−P coupling was not observed by Bourissou and Gabbai ̈ in the related diphosphine silane {(o-iPr2P)C6H4}2Si(F)(Ph),14 and the larger isopropyl groups probably prevent the coordination of the phosphine groups to Si in this case. At low temperature (around −100 °C), the 31P{1H} NMR spectra of 1a and 2a display three signals, in line with structures of low symmetry (Figures 2 and 3, left). Interestingly, one of the three signals was, in both cases, observed as a singlet without JP−F coupling (1a: −15.05 ppm, 2a: −12.52 ppm),

compared to second-row elements, which may disfavor multiple D→E interactions (D = electron donor) due to steric constraints. In this contribution, we report phosphine-based heptacoordinate species of silicon and germanium: fluoro {(o-Ph2P)C6H4}3EF (E = Si (1a), Ge (2a)), chloro {(o-Ph2P)C6H4}3ECl (E = Si (3a), Ge (4a)), hydride {(o-Ph2P)C6H4}3EH (E = Si (5b), Ge (6b)), and methyl {(o-Ph2P)C6H4}3GeMe (7b) compounds. The substituent at Si and Ge has a noticeable structural impact. The fluoro (1a and 2a) and chloro (3a and 4a) compounds adopt a novel heptacoordinate geometry, whose nature and origin are discussed based on DFT calculations.

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RESULTS AND DISCUSSION The ability of o-phenylene spacers to induce donor−acceptor interactions involving not only P and Si atoms4d but also P and Sn atoms6 has been reported. The rigidity of this building block makes it ideally suited to form three P→Si or P→Ge interactions. First, we targeted the fluoro silane and germane {(o-Ph2P)C6H4}3E(F) (1a: E = Si, 2a: E = Ge) because the strong electron-withdrawing character of the fluorine atom increases the accepting character of Si and Ge.7 We synthesized {(o-Ph2P)C6H4}3Si(OMe) (its purity was estimated about 95% by 31P NMR spectroscopy) by treating tetramethoxysilane, Si(OMe)4, with 3 equiv of o-lithiated(diphenylphosphino)benzene (Scheme 1). Subsequent methoxy−fluorine exchange Scheme 1. Synthesis of Heptacoordinate Fluoro Species of Silicon 1a and Germanium 2a

using HF-pyridine imparted a high stability to the compound, resulting in the isolation of the desired 1a in 90% yield after purification. Similarly, the germane analogue {(o-Ph2P)C 6 H 4 } 3 Ge(OMe) was synthesized by the reaction of tetramethoxygermane, Ge(OMe)4, with 3 equiv of o-lithiated(diphenylphosphino)benzene. Further, the reaction of {(oPh2P)C6H4}3Ge(OMe) with a slightly excess of HF-pyridine and subsequent purification provided 2a in 91% yield. The X-ray crystallographic analyses of silane 1a and germane 2a revealed the presence of three interactions between the phosphorus atoms and the central group 14 element E (Table 1). Compounds 1a and 2a adopt similar structures (Figure 1). B


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Table 1. Selected Distances (Å) and Angles (deg) of 1a, 2a, 4a, 5b, 6b, and 7b X=F E−Cipso E−Cipso E−Cipso E1−P1 E1−P2(P1i) E1−P3(P1ii) E1−X Cipso−E− Cipso

X = Cl

X=H

X = Me

1a (E = Si)

2a (E = Ge)

4a (E = Ge)

5b (E = Si)

6b (E = Ge)

7b (E = Ge)

1.881(3) 1.877(3) 1.876(3) 3.3797(13) 3.3575(12) 3.3899(11) 1.606(2) 111.16(13) 118.58(13) 110.28(13)

1.955(3) 1.959(3) 1.959(3) 3.3150(10) 3.3603(10) 3.2768(11) 1.762(2) 115.66(14) 116.38(13) 111.00(14)

1.956(2) 1.962(2) 1.965(2) 3.4193(7) 3.4945(6) 3.3140(7) 2.2353(6) 115.73(10) 114.13(9) 108.20(10)

1.895(4) 1.893(4) 1.873(4) 3.3161(15) 3.3229(16) 3.2280(15) 1.36(3) 103.09(7) 103.73(7) 108.04(7)

1.969(3) 1.955(3) 1.961(3) 3.3548(19) 3.3547(19) 3.258(2) 1.44(2) 105.05(11) 105.19(11) 109.57(11)

1.969(3) 1.980(3) 1.976(3) 3.3728(11) 3.4345(9) 3.4388(10) 1.949(3) 107.50(11) 106.15(12) 106.40(12)

Figure 1. Molecular structures of heptacoordinate (fluoro)silane 1a (left) and -germane 2a (right). Hydrogen atoms, phenyl groups, and solvent molecules are omitted for clarity. Thermal ellipsoids are set at 40% probability.

Figure 2. Experimental (left) and simulated (right) variable-temperature 31P{1H} NMR spectra (162 MHz, 4:1 mixture of THF-d8 and toluene-d8) of 1a. Enthalpy and entropy of activation: ΔH⧧ = 7.6 ± 0.1 kcal/mol, ΔS⧧ = −7.5 ± 0.8 eu.

= 59.7 Hz)).15 The three 31P{1H} signals of 1a coalesced at around −70 °C and were observed as a doublet (JP−F = 62.4 Hz) at ambient temperature due to the dynamic behavior. The

while the other two signals were doublets with fairly large JP−F coupling (1a: −10.03 ppm (JP−F = 97.3 Hz), −14.16 ppm (JP−F = 64.3 Hz); 2a: −8.72 ppm (JP−F = 90.4 Hz), −12.92 ppm (JP−F C


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Figure 3. Experimental (left) and simulated (right) variable-temperature 31P{1H} NMR spectra (162 MHz, 4:1 mixture of THF-d8 and toluene-d8) of 2a. Enthalpy and entropy of activation: ΔH⧧ = 7.6 ± 0.4 kcal/mol, ΔS⧧ = −5.2 ± 2.2 eu. 31

P{1H} signals of 2a coalesced at a slightly lower temperature, around −80 °C. Rate constants of the site exchange were estimated on the basis of line-shape analysis with a g-NMR program package (Figures 2 and 3, right). The site exchange at all measurement temperatures was about 3 times faster in 2a than in 1a. The corresponding enthalpy and entropy of activations are in the same range (1a: ΔH⧧ = 7.6 ± 0.1 kcal/ mol, ΔS⧧ = −7.5 ± 0.8 eu; 2a: ΔH⧧ = 7.6 ± 0.4 kcal/mol, ΔS⧧ = −5.2 ± 2.2 eu), indicating that the site exchange proceeds similarly in both systems. Faster site exchange in 2a presumably results from the longer Ge−C bonds relative to Si−C bonds, which facilitate the rotation of ortho-diphenylphosphinophenyl groups. The 19F{1H} NMR spectra of 1a at −105 °C and 2a at −100 °C provide a doublet of doublets at −145.3 ppm (JP−F = 95.4, 67.1 Hz) and −173.9 ppm (JP−F = 103.0, 54.9 Hz), respectively. These resonances gradually broadened upon increasing the temperature (Figures 4 and 5). The spectra of

Figure 5. Variable-temperature 19F{1H} NMR (376 MHz, 4:1 mixture of THF-d8 and toluene-d8) spectra of 2a.

2), leading to intermediate A of appropriate C3 symmetry. As discussed later on in the DFT calculation section, intermediate A is close in energy to 1a and 2a. Alternatively, dissociation of one of the phosphines trans to the ipso-carbons may lead to intermediate B bearing two phosphines trans to the fluorine atom. Comparison of the fluoro compounds (1a and 2a) with analogous compounds featuring a different substituent at E would be informative. The chloro derivatives {(o-Ph2P)C6H4}3E(Cl) (3a: E = Si, 4a: E = Ge) were prepared in moderate yields by treating the corresponding tetrachloride precursors ECl 4 (E = Si, Ge) with o-lithiated(diphenylphosphino)benzene (Scheme 3). The hydride analogues {(o-Ph2P)C6H4}3E(H) (5b: E = Si, 6b: E = Ge) were already reported by Peters et al.16 and us,17a respectively, and were prepared in accordance with literature procedures. The methyl germane {(o-Ph2P)C6H4}3Ge(Me) (7b) was also synthesized in good yield using MeGeCl3 instead of GeCl4. However, all attempts to prepare the methyl silane {(oPh2P)C6H4}3Si(Me) were unsuccessful. The shorter Si−C bonds in {(o-Ph2P)C6H4}3Si(Me) relative to Ge−C bonds in 7b probably lead to severe steric repulsions between the substituents on silicon. The structures of 4a, 5b, 6b, and 7b were confirmed by X-ray diffraction studies (Figure 6). The structural analysis on 3a was impracticable due to the poor quality of the crystals. Slow

Figure 4. Variable-temperature 19F{1H} NMR (376 MHz, 4:1 mixture of THF-d8 and toluene-d8) spectra of 1a.

1a at 60 °C and 2a at 20 °C provide a quartet at −141.4 (JP−F = 64.9 Hz) and −173.9 (JP−F = 53.4 Hz) ppm, respectively, as a result of the spin−spin coupling with three equivalent phosphorus atoms. These observations are well consistent with the results of VT-31P{1H} NMR measurements. Site exchange of 1a and 2a is likely initiated by the dissociation of the phosphines trans to the E−F bond (Scheme D


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Scheme 2. Plausible Mechanisms for the Site Exchange Process of {(o-Ph2P)C6H4}3E(F) (E = Si (1a), Ge (2a))

Scheme 3. Synthesis of the Heptacoordinate Silanes and Germanes Bearing Chloro (E = Si, Ge) or Methyl (Ge) Substituents

Figure 6. Molecular structures of heptacoordinate compounds of silicon and germane. (a) 4a. (b) 5b. (c) 6b. (d) 7b. Hydrogen atoms except for E− H, phenyl groups, and solvent molecules are omitted for clarity. Thermal ellipsoids are set at 40% probability.

coordinating to Ge trans to the Ge−Cl bond and two phosphine donors occupying the positions trans to the ipsocarbons of phenylene groups (form a). The shortest Ge−P distance (3.3140(7) Å) in 4a is the one trans to the Ge−Cl

diffusion of n-hexane into a dichloromethane solution provided single crystals of 4a, 6b, and 7b. Single crystals of 5b17b were prepared by slow diffusion of n-hexane into a benzene solution. Similar to 1a and 2a, compound 4a has one phosphine donor E


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Figure 7. Optimized structures of the (fluoro)silane 1 and -germane 2 compounds with forms a and b at the M06 level of theory. (a) 1a. (b) 1b. (c) 2a. (d) 1b. The relative free energies are given in parentheses followed by numbers of compounds. Hydrogen atoms and phenyl groups (except for ipso-carbons) are omitted for clarity.

interacts with the silicon or germanium center trans to ipsocarbons of the phenylene groups. The lone pairs of the phosphorus atoms are directed toward regions of the σ*(E− Cipso) orbitals at the silicon or germanium center. The Si−P distances range from 3.228 to 3.323 Å in 5b and are significantly shorter than the sum of van der Waals radii (4.05 Å), supporting some degree of P→Si donation. The P− Ge distances in 6b (3.3548(19), 3.3547(19), 3.258(2) Å) and 7b (3.3728(11), 3.4345(9), 3.4388(10) Å) are also much shorter than the sum of van der Waals radii (4.05 Å), supporting the presence of dative P→Ge interactions. A comparison of P−Ge distances in 6b and 7b reveals that the replacement of the hydride atom in 6b for a methyl group results in slight elongation of the Ge−P distances. Probably,

bond. It is almost equal to the Ge−P distance trans to the Ge− F bond (3.2768(11) Å) in 2a, suggesting the presence of a comparably strong P→Ge interaction. The other two Ge−P distances in 4a (3.4193(7) and 3.4945(6) Å) are significantly longer but are shorter than the sum of van der Waals radii (4.05 Å), supporting the presence of dative P→Ge interactions. Although the chloro compound 3a could not be characterized by X-ray diffraction, the variable-temperature NMR data (vide infra) indicate it adopts form a as 4a. In contrast to the fluoro and chloro compounds, the hydride (5b, 6b) and methyl (7b) derivatives adopt an approximate C3 symmetrical geometry in the solid state (for convenience, this geometry is named as form b), which is well consistent with the NMR data (vide infra). In 5b and 6b, each phosphine atom F


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Figure 8. Optimized structures of the (hydro)silane 5 and -germane 6 compounds with forms a and b at the M06 level of theory. (a) 5a. (b) 5b. (c) 6a. (d) 6b. The relative free energies are given in parentheses followed by numbers of compounds. Hydrogen atoms and phenyl groups (except for ipso-carbons) are omitted for clarity.

broadened and finally split into three singlets. The broadening in 4a started at higher temperature in comparison with 3a, and the site exchange of phosphorus atoms proceeded more readily for the germane 4a than the silane 3a, similar to that encountered in the fluoro derivatives (1a and 2a). Further, the exchange process was faster for the fluorine systems than for the corresponding chlorine systems. The rate constant of the phosphine site exchange at −60 °C was 45 times larger for 1a (1800 s−1) than for 3a (40 s−1). Similarly, the rate constant at −70 °C was much larger for 2a (2000 s−1) than for 4a (18 s−1). Larger values of the enthalpy of activation in the chloro compounds (3a: 9.5 ± 0.7 kcal/mol; 4a: 11.2 ± 0.2 kcal/mol) relative to the fluoro derivatives (1a: 7.6 ± 0.1 kcal/mol; 2a: 7.6 ± 0.4 kcal/mol) are the dominant factor slowing down the exchange process. The larger atomic radius of chlorine relative to fluorine18 presumably disfavored the hexacoordinated transition state forming intermediate A (in Scheme 2) due to larger steric repulsions.19

steric repulsion by the larger methyl group weakens the coordination of the phosphine groups. It is also reasonable that stronger electron donation from the methyl group relative to hydrogen weakens the charge transfer from the phosphines to the germanium center. The fact that compounds 5b, 6b, and 7b all adopt form b (C3 symmetric geometry with P→E interactions trans to Cipso), whatever the size of the substituent at Si/Ge (H is smaller than F, but the methyl group is larger), suggests that steric factors do not play a prominent role. The way in which the phosphorus atoms interact with the Si/Ge center and the structure of the resulting heptacoordinate silanes/germanes are probably more influenced by orbital interactions and electrostatic effects. Variable-temperature 31P{1H} NMR analyses of the chloro compounds 3a and 4a revealed dynamic behavior similar to that observed for 1a and 2a (Figures S1 and S2 in the Supporting Information). At ambient temperature, the spectrum exhibited one singlet at δ = −12.4 (3a) and −11.5 (4a) ppm. When the temperature decreased, the resonance G


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Figure 9. Antibonding orbitals of Si−X bonds in Ph3SiX (X = F (9a), H (11a)), where PPh2 moieties in 1a and 5a were replaced by hydrogens. (a) LUMO of 9a (−0.89 eV); (b) LUMO+8 of 11a (−0.42 eV). The contour value was set to 0.04.

NMR spectroscopic analyses on 3a and 4a demonstrate the presence of a dynamic behavior similar to 1a and 2a and support the structural similarity of compounds 1a−4a. In contrast, the NMR measurements performed on H (5b and 6b) and Me (7b) analogues provided no evidence for dynamic behavior. The 31P{1H} NMR spectra of 5b−7b at ambient temperature provided only one equivalent 31P{1H} NMR signal around −10 ppm (5b: δ = −11.8 ppm; 6b: δ = −10.5 ppm; 7b: δ = −9.7 ppm), which remained unchanged from −110 to 100 °C. These results indicate that 5b−7b retain the C3 symmetrical structure in solution. To better understand the origin of the different conformations adopted by the heptacoordinate compounds, the fluoro (1a and 2a) and hydride (5b and 6b) derivatives were investigated with density functional theory (DFT) calculations with the M06 functional.20 The optimized geometries of 1a, 2a, 5b, and 6b fit nicely with the crystallographic data; see Tables S7 and S8 in the Supporting Information for the E−P, E−C, and E−F distances and C−E−C angles. Among key geometrical parameters, the maximum deviation concerns the Ge− P1 distance in 2a (Δ = 0.0607 Å). For a clear comparison, we also optimized 1b, 2b, 5a, and 6a, which are isomers of 1a, 2a, 5b, and 6b, respectively. Note that the isomers 1b and 2b, with the F atom in the axial position, adopt a conformation similar to 5b and 6b, while the isomers 5a and 6a, with H in the axial position, adopt a conformation similar to 1a and 2a. As shown in Figures 7 and 8, 1b and 2b are less stable than 1a and 2a by 2.4−3.0 kcal/mol, whereas 5a and 6a are less stable than 5b and 6b by 5.3−5.5 kcal/mol, which is consistent with our experimental observations. Natural bond orbital (NBO) analyses were then performed to assess the charge transfer stabilization associated with P→E interactions. In 1b, three charge transfers (CTs) from LP(P) to σ*(Si− Cipso) associated with stabilization energies from 2.0 to 3.1 kcal/ mol are observed, in line with the presence of three weak P→Si interactions (Figure 7b). On the other hand, 1a has two CTs (2.5 and 2.2 kcal/mol) from LP(P) to σ*(Si−Cipso) and one CT (4.5 kcal/mol) from LP(P) to σ*(Si−F) (Figure 7a). The latter LP(P)−σ*(Si−F) interaction is larger than the LP(P)−σ*(Si−Cipso) interaction in 1a and 1b, which plays a role in stabilizing 1a over 1b. Similar to the Si system, the Ge system 2a has a substantial LP(P)−σ*(Ge−F) interaction (7.1 kcal/mol), which is larger than the LP(P)−σ*(Ge−Cipso) interactions (1.5−3.9 kcal/mol) in 2a and 2b (Figure 7c and d). As a result, the a conformation is the most stable for the fluoro Si and Ge compounds. In the Si and Ge compounds with a H atom on the axial position, 5b has three similar CTs (3.6−3.7 kcal/mol) from LP(P) to σ*(Si−Cipso), which is consistent with the C3

symmetrical geometry (Figure 8b). On the other hand, 5a has two LP(P)−σ*(Si−Cipso) CT interactions (3.3 kcal/mol) and one CT (2.4 kcal/mol) from LP(P) to the σ*(Si−H) orbital (Figure 8a). The latter is weaker than the LP(P)−σ*(Si−Cipso) CT interactions of 5b. Similarly, a CT (2.5 kcal/ mol) from LP(P) to σ*(Ge−H) in 6a is weaker than those of LP(P)−σ*(Ge−Cipso) CT interactions (3.6−3.7 kcal/mol) in 6b (Figure 8c and d). Hence for the hydride, form b is more stable than form a. On the basis of these computational results, it is concluded that the difference in geometry between the F and H compounds arises from the stronger CT interactions from LP(P) to σ*(E−F) than from LP(P) to σ*(E−H). To elucidate the origin of this difference, we compared the MO energy of the σ*(E−F) orbitals in {(o-Ph2P)C6H4}3EF (1a: E = Si, 2a: E = Ge) and the σ*(E−H) orbitals in {(oPh2P)C6H4}3EH (5a: E = Si, 6a: E = Ge). To suppress the influence of the phosphine lone pair on the MO energy of these antibonding orbitals, we calculated hypothetical structures {(oH)C6H4}3EF (9a: E = Si, 10a: E = Ge) and {(o-Ph2P)C6H4}3EH (11a: E = Si, 12a: E = Ge), in which we substituted H atoms for the PPh2 groups but employed the same geometry for the remaining moiety as those in 1a, 2a, 5a, and 6a (Figure 9 and Figure S1321). The σ*(E−F) orbital energies (−0.89 eV for E = Si (9a) and −0.93 eV for E = Ge (10a)) are much lower than the σ*(E−H) orbital energies (2.2 eV for E = Si (11a) and 2.5 eV for E = Ge (12a)). The σ*(E−F) orbital at lower energy than the σ*(E−H) orbital leads to the formation of the stronger LP(P)→σ*(E−F) CT interaction than in the case of the LP(P)→σ*(E−H). The next question is why the σ*(E−F) exists at lower energy than the σ*(E−H). The σ*(E− F) and σ*(E−H) orbitals consist of an antibonding overlap between the E sp3 orbital and the σ orbital of the F/H atom. As shown in Scheme 4, the valence orbital of F is much lower in energy than that of H. This is the reason for the low σ*(E−F) orbital energy. Scheme 4. MO Diagrams Showing σ Si−X Bonds (F, H)

H


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Besides CT interactions, electrostatic effects should also be considered, because the lone pair on the phosphine moiety may experience different electrostatic interactions in the fluoro and hydride compounds. We employed the hypothetical structures {(o-H)C6H4}3SiX (9a: X = F, 11a: X = H) in a way similar to the calculation of CTs to avoid the influence of the phosphine lone pair on the electrostatic potential. The electrostatic potentials of 9a and 11a at the positions trans to the Si−X bond were much more positive in 9a than in 11a; they are +0.0432 (2.0 Å), +0.0183 (2.5 Å), +0.0102 (3.0 Å) in 9a and +0.0276 (2.0 Å), +0.0079 (2.5 Å), +0.0027 (3.0 Å) in 11a (Figure 10).

Å). Generally, CT interactions strongly depend on energies and sizes of lone pair orbitals. The lone pair orbital of NMe3 (−5.8 eV) is slightly higher in energy than that of PPh3 (−6.0 eV), which is inconsistent with our experimental result that phosphine donors induce stronger CT interactions (Figure 12). On the other hand, the lone pair orbital of PPh3 is more

Figure 10. Electrostatic potentials of {(o-H)C6H4}3SiX (9: X = F, 11: X = H) at the positions trans to the Si−X bond.

Figure 12. Lone pair orbitals in (a) NMe3 and (b) PPh3. The contour value was set to 0.04.

These results indicate that the lone pair of the phosphine is more stabilized electrostatically in the fluoro compound than in the hydride compound.22 This is another factor that may explain why the fluoro silane adopts preferentially form a whereas the corresponding hydride adopts form b. Essentially the same features are obtained for the Ge−F and Ge−H compounds; see page S20 in the Supporting Information. A comparison of structures between Corriu’s nitrogen-based (fluoro)silane 8 and our phosphine-based (fluoro)silane 1a provides a better understanding of these heptacoordinate compounds (Figure 11). LP(N2)→(Si−C ipso) and LP(N3)→(Si−Cipso) interactions (1.7 and 1.2 kcal/mol, respectively) are much smaller than LP(P)→(Si−Cipso) interactions in 1a (2.2−4.5 kcal/mol); note that no LP(N1)→(Si−Cipso) CT is detected. The very weak donor−acceptor interactions in 8 are associated with long Si−N distances (3.490, 3.308, and 3.004

diffuse than that of NMe3, suggesting that the lone pair of the phosphines in 1a can overlap more efficiently with σ*(Si−X) orbitals (X = F, C) than that of the amines in 8. This is probably why stronger CT interactions are observed with the phosphines. In summary, saturated quaternary silane and germane compounds bearing one X atom (X = F, Cl, H, Me) and three phenyl derivatives with three pendant phosphine donors were prepared. In these compounds, three P→E (E = Si, Ge) CT interactions are demonstrated by crystallographic analyses and DFT calculations. The structures of these heptacoordinate compounds depend on the substituent X; with X = F (1a and 2a) or Cl (3a and 4a), one phosphorus atom coordinates to the Si and Ge centers at the position trans to the E−X bond (E = Si, Ge; X = F, Cl), while the other two phosphorus atoms occupy positions trans to ipso-carbons of the phenyl derivatives (form a). Variable-temperature 31P{1H} NMR measurements indicate dynamic behavior of 1a−4a. The geometry adopted by 1a−4a is different from that of Corriu’s nitrogen-based compound 8, where the donor N moieties do not coordinate to the Si center at the position trans to the X atom (X = F, H). In contrast to the fluoro and chloro derivatives, the H (5b and 6b) and Me (7b) compounds adopt an approximately C3 symmetric structure, in which the three phosphorus atoms are located trans to ipso-carbons (form b). These geometric features are similar to that of 8. DFT calculations rationalize why for {(o-Ph2P)C6H4}3EF compounds form a is favored over form b. Strong LP(P)−σ*(E−F) CT interactions (E = Si, Ge) and electrostatic effects are responsible for the higher stability of the new heptacoordinate form.

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EXPERIMENTAL SECTION

General Procedures. All experiments were performed under a dry nitrogen atmosphere using standard Schlenk techniques. Benzene-d6, toluene-d8, tetrahydrofuran-d8, and diethyl ether were dried over sodium benzophenone ketyl and distilled under a dinitrogen atmosphere. Tetrahydrofuran and n-hexane were purified using a two-column solid-state purification system. Chloroform-d and dichloromethane were dried over P2O5 and stored over 4 Å molecular sieves. The other reagents used in this study were purchased from commercial sources and used without further purification. 1H, 13C{1H}, 19F{1H}, 29Si{1H}, and 31P{1H} NMR spectra were recorded

Figure 11. Molecular structure of Corriu’s compound 8. NBO analysis was performed at the M06 level of theory. Hydrogen atoms and phenyl groups (except for ipso-carbons) are omitted for clarity. Distances are in angstroms. I


Organometallics

Article

with a JEOL JNM-AL 400 spectrometer. The 1H and 13C{1H} NMR data were analyzed with reference to the residual peaks of the solvent, and 19F{1H}, 29Si{1H}, and 31P{1H} NMR chemical shifts were referenced to external hexafluorobenzene (−164.9 ppm), tetramethylsilane (0 ppm), and 85% H3PO4 (0 ppm) samples, respectively. Elemental analyses were conducted using a J-Science Lab JM-10 or FISONS Instrument EA108 elemental analyzer. {o-(Ph2P)C6H4}3Si(H) (5b),5 {o-(Ph2P)C6H4}3Ge(H) (6b),17 and {o-PPh2(C6H4)}Li· Et2O17 were prepared as described in the literature. Preparation of {(o-Ph2P)C6H4}3Si(F) (1a). A Schlenk tube was charged with 958 mg of {o-PPh2(C6H4)}Li·Et2O (2.80 mmol) and 10 mL of THF. TMEDA (0.46 mL, 3.08 mmol) was added to the solution. After the solution was cooled to −78 °C, Si(OMe)4 (125 μL, 0.840 mmol) was added slowly to the prepared solution. The mixture was allowed to warm to room temperature and was stirred at 60 °C for 30 h. After removing the volatile materials under vacuum, benzene (15 mL) was added. The resulting solution was filtered through a Celite pad. Removal of the volatile materials from the filtrate in vacuo gave a white solid. The solid was washed with hexane (2 mL × 2) and dried under vacuum to afford {o-(Ph2P)C6H4}3Si(OMe) (294 mg, 0.349 mmol) in 42% yield as a white powder (its purity was estimated to be about 95% by 31P NMR spectroscopy). {o-(Ph2P)C6H4}3Si(OMe): 1H NMR (400 MHz, C6D6): δ 3.02 (s, 3H, OCH3), 7.00−7.07 (m, 9H, Harom), 7.15−7.23 (m, 21H, Harom), 7.30−7.34 (m, 9H, Harom), 7.67− 7.69 (m, 3H, Harom). 13C{1H} NMR (100 MHz, CDCl3): δ 52.7, 127.8, 128.3, 128.8, 129.7, 133.5, 133.8, 135.3, 138.9, 143.4, 144.6. 31 1 P{ H} NMR (162 MHz, C6D6): δ −11.7 (s). A Schlenk tube was charged with 398 mg of {o-PPh2(C6H4)}3Si(OMe) (0.472 mmol) and 5 mL of benzene, and then hydrogen fluoride−pyridine (288 μL, 2.40 mmol) was added to the benzene solution. After the reaction mixture was stirred at ambient temperature for 16 h, the volatile materials were removed in vacuo. The reaction mixture was extracted with CH2Cl2 (20 mL), and the subsequent removal of the volatile materials in vacuo gave a white solid. The solid was washed with hexane (2.5 mL × 2) and dried under vacuum to afford 1a (352 mg, 0.423 mmol) in 90% yield as a white powder. 1H NMR (400 MHz, C6D6): δ 6.95−7.08 (m, 27H, Harom), 7.25−7.42 (m, 12H, Harom), 8.17−8.20 (m, 3H, Harom). 13 C{1H} NMR (100 MHz, C6D6): δ 128.2, 128.4, 128.5, 130.6, 133.9, 135.2, 138.7, 139.1, 144.0, 144.6. 19F{1H} NMR (376 MHz, C6D6): δ −142.3 (br s, w1/2 = 221.3 Hz). 29Si{1H} NMR (79 MHz, CDCl3): δ −5.0 (dq, 1JSi−F = 288.1 Hz, JSi−P = 7.6 Hz). 31P{1H} NMR (162 MHz, C6D6): δ −10.8 (d, JP−F = 62.4 Hz). Anal. Calcd for C54H42FP3Si: C, 78.06; H, 5.09. Found: C, 78.16; H, 5.28. [M + H]+ calcd for C54H43FP3Si 831.2331; found 831.2338. Preparation of {(o-Ph2P)C6H4}3Ge(F) (2a). A Schlenk tube was charged with 980 mg of {o-PPh2(C6H4)}Li·Et2O (2.86 mmol) and 10 mL of THF. TMEDA (0.47 mL, 3.15 mmol) was added to the solution. After the solution was cooled to −78 °C, Ge(OMe)4 (130 μL, 0.875 mmol) was added slowly to the prepared solution. The mixture was allowed to warm to room temperature and was stirred at room temperature for 17 h. After removing the volatile materials under vacuum, benzene (16 mL) was added. The resulting solution was filtered through a Celite pad. Removal of the volatile materials from the filtrate in vacuo gave a white solid. The solid was washed with hexane (3 mL × 3) and dried under vacuum to afford analytically pure {o-(Ph2P)C6H4}3Ge(OMe) (716 mg, 0.806 mmol)14 in 92% yield as a white powder. A Schlenk tube was charged with 615 mg of {oPPh2(C6H4)}3Ge(OMe) (0.693 mmol) and 3 mL of CH2Cl2. Hydrogen fluoride−pyridine (121 μL, 1.01 mmol) was added to the prepared solution. The reaction mixture was stirred at room temperature for 15 h. The resulting solution was filtered through a Celite pad. Removal of the volatile materials in vacuo gave a white solid. The solid was washed with hexane (2 mL × 2) and dried under vacuum to afford 2a (553 mg, 0.632 mmol) in 91% yield as a white powder. 1H NMR (400 MHz, CDCl3): δ 7.01−7.49 (m, 39H, Harom), 7.90 (m, 3H, Harom). 13C{1H} NMR (100 MHz, CDCl3): δ 128.1, 128.2, 128.8, 130.1, 133.4, 134.9, 136.8, 138.0, 147.1, 147.6. 19F{1H} NMR (376 MHz, C6D6): δ −172.8 (q, JF−P = 53.9 Hz). 31P{1H} NMR (162 MHz, CDCl3): δ −9.7 (d, JP−F = 53.9 Hz). Anal. Calcd for C54H42FP3Ge: C, 74.08; H, 4.84. Found: C, 74.20; H, 4.84.

Preparation of {(o-Ph2P)C6H4}3SiCl (3a). A Schlenk tube was charged with 170 mg of {o-PPh2(C6H4)}Li·Et2O (0.498 mmol) and 2 mL of toluene. The resulting solution was cooled to −78 °C. A 1 M toluene solution of SiCl4 (150 μL, 0.150 mmol) was added slowly to the solution, and the mixture was then allowed to warm to room temperature. The reaction mixture was stirred at 100 °C for 17.5 h. After the mixture was allowed to cool to room temperature, the resulting solution was filtered through a Celite pad. The solvent was then removed under reduced pressure to give a white solid. The solid was washed with hexane (2 mL × 2) and benzene (1 mL) and dried under vacuum to afford 3a (84.2 mg, 0.0994 mmol) in 66% yield as a white powder. 1H NMR (400 MHz, CDCl3): δ 6.91−7.03 (m, 12H, Harom), 7.13−7.19 (m, 21H, Harom), 7.28−7.31 (m, 6H, Harom), 8.08− 8.12 (m, 3H, Harom). 13C{1H} NMR (100 MHz, CDCl3): δ 127.9, 128.0, 128.2, 130.3, 133.3, 135.9, 138.4, 139.8, 143.3, 143.8. 29Si{1H} NMR (79 MHz, CDCl3): δ −1.6 (q, JSi−P = 10.3 Hz) 31P{1H} NMR (162 MHz, CDCl3): δ −12.7 (s). Anal. Calcd for C54H42ClSiP3: C, 76.54; H, 5.00. Found: C, 76.14; H, 5.22. Preparation of {(o-Ph2P)C6H4}3GeCl (4a). A Schlenk tube was charged with 412 mg of {o-PPh2(C6H4)}Li·Et2O (1.20 mmol) and 5 mL of toluene. The resulting solution was cooled to −78 °C. GeCl4 (66.0 μL, 0.581 mmol) was added slowly to the solution, and the mixture was then allowed to warm to room temperature. The reaction mixture was stirred at 100 °C for 24 h. After the mixture was allowed to cool to room temperature, the resulting solution was filtered through a Celite pad. The solvent was then removed under reduced pressure to give a white solid. The solid was washed with Et2O (10 mL × 3) and toluene (1 mL × 2) and dried under vacuum to afford 4a (271 mg, 0.304 mmol) in 52% yield as a white powder. 1H NMR (400 MHz, CDCl3): δ 6.98−7.02 (m, 12H, Harom), 7.10−7.21 (m, 21H, Harom), 7.28−7.30 (m, 6H, Harom), 7.99−8.02 (m, 3H, Harom). 13C{1H} NMR (100 MHz, CDCl3): δ 128.0, 128.1, 128.4, 130.1, 133.2, 133.4, 135.8, 137.4, 138.2, 142.6. 31P{1H} NMR (162 MHz, CDCl3): δ −10.9 (s). Anal. Calcd for C54H42ClGeP3: C, 72.72; H, 4.75. Found: C, 73.03; H, 5.10. Preparation of {(o-Ph2P)C6H4}3GeMe (7b). A Schlenk tube was charged with 954 mg of {(o-Ph2P)C6H4}Li·Et2O (2.788 mmol) and 15 mL of toluene. The resulting solution was cooled to −78 °C. MeGeCl3 (1.0 M in toluene, 836 μL, 0.836 mmol) was added slowly to the solution, and the mixture was then allowed to warm to room temperature. The reaction mixture was stirred at 100 °C for 24 h. After the mixture was allowed to cool to room temperature, the resulting solution was filtered through a Celite pad. The solvent was then removed under reduced pressure to give a white solid. The solid was washed with a mixed solvent (4 mL × 3) of hexane/Et2O (3:1) and dried under vacuum to afford 7b (589 mg, 0.675 mmol) in 81% yield as a white powder. 1H NMR (400 MHz, CDCl3): δ 0.10 (s, 3H, GeCH3), 7.04−7.09 (m, 3H, Harom), 7.12−7.16 (m, 12H, Harom), 7.22−7.29 (m, 24H, Harom), 7.39−7.41 (m, 3H, Harom). 13C{1H} NMR (100 MHz, CDCl3): δ 10.4, 128.1, 128.3, 128.4, 128.9, 133.5, 135.7, 137.6, 138.7, 142.8, 149.7. 31P{1H} NMR (162 MHz, CDCl3): δ −9.2 (s). Anal. Calcd for C55H45GeP3: C, 75.80; H, 5.20. Found: C, 76.24; H, 5.42. Structure Determination by X-ray Diffraction. Single crystals suitable for X-ray diffraction analysis were obtained as described above. Diffraction intensity data were collected with a Rigaku/MSC Mercury CCD diffractometer at 200 K(2), and a semiempirical multiscan absorption23 correction was performed. The space groups were chosen based on the systematic absences in the diffraction data. The structures were solved using SIR9724 by subsequent difference Fourier synthesis and refined by full matrix least-squares procedures on F2. All nonhydrogen atoms were refined with anisotropic displacement coefficients. The hydrogen atoms were treated as idealized contributions and refined in a rigid group model. All software and sources of scattering factors are contained in the SHELXL97 program package.25 CCDC 1001527, 1001528, 1001529, 1001530, 1001531, and 1001532 contain supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif. J


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(6) Lin, T.-P.; Gualco, P.; Ladeira, S.; Amgoune, A.; Bourissou, D.; Gabbaï, F. P. C. R. Chim. 2010, 13, 1168. (7) For a detailed study on the influence of fluorine on the Lewis acidity of silanes in the context of σ-acceptor ligands, see: Gualco, P.; Mercy, M.; Ladeira, S.; Coppel, Y.; Maron, I.; Amgoune, A.; Bourissou, D. Chem.Eur. J. 2010, 16, 10808. (8) The sum of covalent radii of sp2-carbon and E atoms (E = Si, Ge) are 2.12 (Si−C) and (Ge−C) Å, which are comparable to E−C bond distances in 1−4. Cordero, B.; Gómez, V.; Platero-Prats, A. E.; Revés, M.; Echeverría, J.; Cremades, E.; Barragán, F.; Alvarez, S. Dalton Trans. 2008, 37, 2832. (9) Batsanov, S. S. Inorg. Mater. 2001, 37, 871. (10) Covalent atomic radii of silicon and germane are 1.11 and 1.20 Å, respectively. These values are taken from ref 9. (11) Brendler, E.; Heine, T.; Seichter, W.; Wagler, J.; Witter, R. Z. Anorg. Allg. Chem. 2012, 638, 935. (12) Prince, P. D.; McGrady, G. S.; Steed, J. W. New J. Chem. 2002, 26, 457. (13) Farooq, O. J. Chem. Soc., Perkin Trans. 1 1998, 661. (14) Gualco, P.; Lin, T.-P.; Sircoglou, M.; Mercy, M.; Ladeira, S.; Bouhadir, G.; Pérez, L. M.; Amgoune, A.; Maron, L.; Gabbaï, F. P.; Bourissou, D. Angew. Chem., Int. Ed. 2009, 48, 9892. (15) Two doublets and one singlet would be assignable to the phosphines trans to the Cipso atoms and that trans to the E−F bond, respectively, because observed JP‑F coupling constants are probably attributed to direct P−F interactions. The presence of donor→ acceptor F→P interactions involving the phosphines trans to the Cipso atom was confirmed at second perturbation energy (1a: 0.55, 0.65 kcal/mol; 2a: 0.54, 0.70 kcal/mol), while F→P interaction involving the phosphine trans to the E−F bond was not detected. A similar situation was observed in the diphosphine−silane gold complexes: only the F atom position cis to the P atoms gives rise to JP−F coupling. See ref 7. (16) Although Peters et al. reported the structural data for the isopropyl analogue {(o-iPr2P)C6H4}3Si(H) of 5a in ref 5, we performed, for clear comparison, the structural analysis of 5a bearing Ph substituents. (17) (a) Kameo, H.; Ishii, S.; Nakazawa, H. Organometallics 2012, 31, 2212. (b) When the manuscript was in preparation, structural analysis on 5 was reported. Herrmann, R.; Braun, T.; Mebs, S. Eur. J. Inorg. Chem. 2014, 4826. (18) Covalent atomic radii of fluorine and chlorine are 0.57 and 1.02 Å, respectively. These values are taken from ref 9. (19) Besides steric factors, one may consider the magnitude of the P→E interaction to be dissociated to reach intermediate A. However, shorter Ge−P distances in 2a relative to those in 4a imply stronger P→Si interactions in 2a, which is inconsistent with a faster site exchange process of 2a. (20) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215. (21) Pictures of the σ*(Ge−H) in 11a and the σ*(Ge−F) in 12a are shown in Figure S13 in the Supporting Information. (22) We confirmed that the lone pairs of P atoms are associated with negative electronic potentials. See page S20 in the Supporting Information. (23) Rigaku. REQAB, Version 1.1; Rigaku Coporation: Tokyo, Japan, 1998. (24) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo, C.; Guagliard, A.; Moliterni, A. G. G.; Spagna. J. Appl. Crystallogr. 1999, 32, 115. (25) Sheldrick, G. M. SHELXL97: Program for the Refinement of Crystal Structures; University of Göttingen: Germany, 1997.

ASSOCIATED CONTENT

S Supporting Information *

Variable-temperature 31P{1H} NMR spectra, Eyring plots of dynamic behavior, crystallographic data, details of DFT calculations. Crystallographic data are also available in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a Challenging Exploratory Research grant (No. 25620048), a Grant-in-Aid for Young Scientists (B) (No. 23750064) from the Japan Society of the Promotion of Science, and a Grant-in-Aid for Scientific Research on Innovative Areas “Stimuli-Responsive Chemical Species” (No. 25109538) from MEXT, Japan. S.S. acknowledges Grants-in-Aid for Specially Promoted Science and Technology (No. 22000009). H.K. acknowledges the financial support from the Kurata Memorial Hitachi Science and Technology Foundation. Dr. Amos Rosenthal is thanked for critical reading of the manuscript.

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REFERENCES

(1) (a) Akiba, K.-y. In Chemistry of Hypervelent Compounds; John Wiley & Sons: New York, 1999. (b) Baukov, Y. I.; Tandura, S. N. Hypervalent Compound of Organic Germanium, Tin and Lead Derivatives. In The Chemistry of Organic Germanium, Tin and Lead Compounds, Vol. 2; Rappoport, Z., Ed.: Wiley: Chiichester, 2002; p 963. (c) Kost, D.; Kalikhman, I. Hypervalent Silicon Compounds. In The Chemistry of Organic Silicon Compounds, Vol. 2; Rappoport, Z.; Apeloig, Y., Eds.; Wiley,: New York, 1998; p 1339. (d) Brook, M. A. In Silicon in Organic, Organometallic, and Polymer Chemistry; John Wiley & Sons: New York, 2000. p 97. (2) (a) Breliére, C.; Carré, F.; Corriu, R. J. P.; Royo, G. Organometallics 1988, 7, 1006. (b) Carré, F.; Chuit, C.; Corriu, R. J. P.; Mehdi, A.; Reyé, C. Angew. Chem., Int. Ed. Engl. 1994, 33, 1097. (c) Breliére, C.; Carré, F.; Corriu, R. J. P.; Royo, G.; Man, M. W. C. Organometallics 1994, 13, 307. (d) Carré, F.; Chuit, C.; Corriu, R. J. P.; Fanta, A.; Mehdi, A.; Reyé, C. Organometallics 1995, 14, 194. (3) (a) Auner, N.; Probst, R.; Hahn, F.; Herdweck, E. J. Organomet. Chem. 1993, 459, 25. (b) Dostal, S.; Stoudt, S. J.; Fanwick, P.; Sereatan, W. F.; Kahr, B.; Jackson, J. E. Organometallics 1993, 12, 2284. (c) Kawachi, A.; Tanaka, Y.; Tamao, K. Organometallics 1997, 16, 5102. (d) Kobayashi, J.; Ishida, K.; Kawashima, T. Silicon Chem. 2002, 1, 351. (e) Iwanaga, K.; Kobayashi, J.; Kawashima, T.; Takagi, N.; Nagase, S. Organometallics 2006, 25, 3388. (4) Examples of silane and germane with third-row element donors are rare even in hepta- and hexacoordinate species. See: (a) Toshimitsu, A.; Saeki, T.; Tamao, K. J. Am. Chem. Soc. 2001, 123, 9210. (b) Grobe, J.; Hildebrandt, W.; Martin, R.; Walter, A. Z. Anorg. Allg. Chem. 1991, 592, 121. (c) Quintard, D.; Keller, M.; Breit, B. Synthesis 2004, 905. (d) Sanji, T.; Naito, K.; Kashiwabara, T.; Tanaka, M. Heteroat. Chem. 2012, 23, 520. (5) Although Peters et al. reported the structural data for {(o-iPr2P) C6H4}3Si(H) bearing three LX-type bidentate ligands, they did not demonstrate the presence of three P→Si interactions. Whited, M. T.; Mankad, N. P.; Lee, Y.; Oblad, P. F.; Peters, J. C. Inorg. Chem. 2009, 48, 2507. K


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