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BY-NC-ND 3.0 license Open Access Published by De Gruyter September 26, 2014

Synthesis and structure of an acyclic dialkylstannylene

Keith Izod EMAIL logo , Corinne Wills EMAIL logo , Michael R. Probert and Ross W. Harrington

Abstract

The reaction between iPr2 PCl and one equivalent of MeMgBr, followed by one equivalent of BH3·SMe2, gives the phosphine-borane iPr2 P(BH3)Me (4). Treatment of 4 with nBuLi, followed by Me3 SiCl, gives the sterically demanding phosphine-borane {iPr2 P(BH3)}(Me3 Si)CH2 (5) in good yield. Metalation of 5 with nBuLi yields {iPr2 P(BH3)}(Me3 Si)CHLi (6), which was crystallised as the TMEDA adduct [{iPr2 P(BH3)}(Me3 Si)CH]Li(TMEDA) (6a) and characterised by X-ray crystallography [TMEDA = N,N,N′,N′-tetramethylethylenediamine]. The reaction between two equivalents of in situ-generated 6 and Cp2 Sn in toluene gives the dialkylstannylene [{iPr2 P(BH3)}(Me3 Si)CH]2 Sn (7), which was crystallised from methylcyclohexane/THF as the rac diastereomer. X-ray crystallography reveals that stabilisation of the Sn centre in 7 is afforded by two agostic-type B-H…Sn interactions, one from each BH3 group.

Keywords: lithium; tin; X-ray

Introduction

There is continuing interest in the chemistry of low oxidation state compounds of the heavier group 14 elements, formal analogues of carbenes, alkenes, and alkynes, due to their unusual structures and exceptional reactivities (for reviews see Barrau and Rima, 1998; Weidenbruch, 1999; Tokitoh and Okazaki, 2000; Gehrhus and Lappert, 2001; Klinkhammer, 2002; Hill and West, 2004; Kira, 2004; Kühl, 2004; Zabula and Hahn, 2008; Mizuhata et al., 2009). In the vast majority of cases, heavier group 14 carbene analogues R2 E (E=Si, Ge, Sn, Pb) are stabilised either by π-donation from the lone pair on an adjacent heteroatom into the vacant p-orbital at the tetrel centre [e.g., diamidotetrylenes (R2 N)2 E] or by the use of bulky substituents R, which sterically occlude the electron-deficient tetrel centre.

Over the last few years, we have developed a new approach to the stabilisation of heavier group 14 carbene analogues through agostic-type B-H…E contacts, which mitigate the electron deficiency of the tetrel centre, and have isolated a number of compounds of this type (1–3) (Izod et al., 2006a, 2008, 2009a,b, 2013). We are continuing to explore the limits of this approach, particularly with respect to the steric properties of the substituents in these systems and the effect that the substituents at phosphorus have on the B-H…E contacts. In this contribution, we describe the synthesis of a new phosphine-borane, its deprotonation to give the corresponding phosphine-borane-stabilised carbanion, and the use of this latter species for the synthesis of an agostically stabilised dialkylstannylene.

Results and discussion

The phosphine-borane adduct iPr2 P(BH3)Me (4) is readily prepared by the treatment of iPr2 PCl with one equivalent of MeMgBr, followed by one equivalent of BH3·SMe2 (Scheme 1). Treatment of 4 with one equivalent of nBuLi in THF, followed by one equivalent of Me3 SiCl, yields {iPr2 P(BH3)}(Me3 Si)CH2 (5). Compounds 4 and 5 were isolated as air-stable, colourless oils and their composition was confirmed by multinuclear (1H, 13C{1H}, 11B{1H}, and 31P{1H}) magnetic resonance spectroscopy. Treatment of 5 with one equivalent of nBuLi in THF gives the lithium salt {iPr2 P(BH3)}(Me3 Si)CHLi (6) in good yield. We were unable to obtain crystals of this lithium salt but isolated the corresponding TMEDA adduct [{iPr2 P(BH3)}(Me3 Si)CH]Li(TMEDA) (6a) as colourless blocks from cold (-30°C) diethyl ether. The 1H, 13C{1H}, 11B{1H}, 31P{1H}, and 7Li{1H} NMR spectra of 6a are typical of such species: the 31P{1H} NMR spectrum consists of a broad quartet at 33.4 ppm, while the 11B{1H} NMR spectrum consists of a broad doublet at -39.6 ppm. We have previously noted that α-metalation of phosphine-borane adducts results in a significant increase in the 11B-31P coupling constant; consistent with this, the 11B-31P coupling constants of 5 and 6a are 59 and 90 Hz, respectively (Izod et al., 2004, 2006b,c, 2007, 2010, 2014).

Scheme 1 Reagents and conditions: (i) MeMgBr, Et2 O, -78°C; (ii) BH3·SMe2; (iii) nBuLi, THF; (iv) Me3 SiCl, -78°C, THF; (v) nBuLi, THF; (vi) TMEDA, THF/Et2 O; (vii) Cp2 Sn, toluene.
Scheme 1

Reagents and conditions: (i) MeMgBr, Et2 O, -78°C; (ii) BH3·SMe2; (iii) nBuLi, THF; (iv) Me3 SiCl, -78°C, THF; (v) nBuLi, THF; (vi) TMEDA, THF/Et2 O; (vii) Cp2 Sn, toluene.

X-ray crystallography reveals that 6a adopts a monomeric structure in the solid state (Figure 1). The lithium ion is bound by the carbanion centre and one of the hydrogen atoms of the BH3 group to give a pseudo-four-membered chelate ring, and the coordination of Li is completed by two nitrogen atoms from a molecule of TMEDA, affording a pseudo-tetrahedral geometry about the lithium ion. The Li-C(1) distance [2.213(3) Å] is similar to the corresponding distances in previously reported lithium complexes of phosphine-borane-stabilised carbanions (Izod et al., 2004, 2006c, 2007, 2010, 2014), while the Li…H(1B) distance [2.02(2) Å] lies in the normal range for η1-BHn…Li contacts (Kimblin et al., 2000; Franz et al., 2011), although we note that such contacts are rare in comparison with η2- and η3-BHn…Li interactions. The carbanion centre adopts a pyramidal geometry (sum of angles in the CHSiP framework=350.7°).

Figure 1 Molecular structure of 6a with 40% probability ellipsoids and with disorder component and C-bound H atoms omitted for clarity.Selected bond lengths (Å) and angles (deg): Li-C(1) 2.213(3), Li-N(1) 2.115(3), Li-N(2) 2.139(3)(4), Li-H(1B) 2.02(2), Li…B 2.633(3), P-C(1) 1.7374(15), Si-C(1) 1.8178(15), P-B 1.9322(19), C(1)-Li…B 76.06(10), N(1)-Li-N(2) 87.02(11), P-C(1)-Si 133.09(9), P-C(1)-Li 91.30(10), Si-C(1)-Li 122.35(10).
Figure 1

Molecular structure of 6a with 40% probability ellipsoids and with disorder component and C-bound H atoms omitted for clarity.

Selected bond lengths (Å) and angles (deg): Li-C(1) 2.213(3), Li-N(1) 2.115(3), Li-N(2) 2.139(3)(4), Li-H(1B) 2.02(2), Li…B 2.633(3), P-C(1) 1.7374(15), Si-C(1) 1.8178(15), P-B 1.9322(19), C(1)-Li…B 76.06(10), N(1)-Li-N(2) 87.02(11), P-C(1)-Si 133.09(9), P-C(1)-Li 91.30(10), Si-C(1)-Li 122.35(10).

The reaction between freshly sublimed Cp2 Sn (Fischer and Grubert, 1956) and two equivalents of in situ-generated 6 in toluene gives the dialkylstannylene [{iPr2 P(BH3)}(Me3 Si)CH]2 Sn (7), along with the insoluble CpLi by-product, the latter of which was removed by filtration. The 31P{1H} NMR spectrum of the crude reaction mixture exhibits a single, broad quartet at 38.5 ppm (JPB=76 Hz) due to the rac diastereomer, as confirmed by X-ray crystallography, with no evidence for the corresponding meso isomer; a similar absence of the meso diastereomer was observed in the synthesis of 3 (Izod et al., 2009a). The 119Sn chemical shift of 403 ppm observed for 7 is comparable with those observed for 3a and 3b (377 and 375 ppm, respectively), suggesting significant stabilisation of the electron-deficient tin centre by agostic-type B-H…Sn contacts; no evidence for 31P-119Sn coupling was observed in either spectrum. The broadness of the 119Sn signal for 7 (FWHM=380 Hz) appears to be a recurring feature of stannylenes with phosphine-borane-substituted alkyl substituents; while we do not have a conclusive explanation for this line-broadening, it appears not to be associated with chemical exchange or quadrupolar broadening but may be due to significant 119Sn chemical shift anisotropy and/or the population of a low-lying triplet excited state (Izod et al., 2008).

Both the isopropyl groups in 7 and the methyl groups within them are diastereotopic, by virtue of the chiral carbanion centre and the prochiral phosphorus atom, and so the 1H NMR spectrum of 7 exhibits four doublets of doublets for these methyl groups, with this splitting pattern arising from coupling of the protons in each methyl group both to the methine proton of the same isopropyl group and to the phosphorus centre. The isopropyl methine protons are also diastereotopic and give rise to complex multiplets in the 1H NMR spectrum at 1.87 and 2.14 ppm.

Compound 7 was crystallised from cold (-30°C) methylcyclohexane containing a few drops of THF as yellow blocks suitable for X-ray crystallography. Compound 7 crystallises as a discrete molecular species; the shortest Sn…Sn distance is 6.934 Å (cf. Sn…Sn distances of 6.685 and 4.401 Å in 3a and 3b, respectively) (Izod et al., 2009a) (Figure 2). The Sn-C distances of 2.306(2) and 2.293(2) Å are similar to the corresponding distances in 3a and 3b, which range from 2.2864(16) to 2.3149(16) Å; the C-Sn-C angle [100.43(7)°] is also similar to those of 3a and 3b [98.26(6)° and 99.60(17)°, respectively].

Figure 2 Molecular structure of 7 with 40% probability ellipsoids and with C-bound H atoms omitted for clarity.Selected bond lengths (Å) and angles (deg.): Sn-C(1) 2.306(2), Sn-C(11) 2.293(2), C(1)-P(1) 1.802(2), C(11)-P(2) 1.789(2), C(1)-Si(1) 1.881(2), C(11)-Si(2) 1.879(2), P(1)-B(1) 1.905(3), P(2)-B(2) 1.924(3), Sn…H(1C) 2.36(2), Sn…H(2C) 2.32(2), C(1)-Sn-C(11) 100.43 (7).
Figure 2

Molecular structure of 7 with 40% probability ellipsoids and with C-bound H atoms omitted for clarity.

Selected bond lengths (Å) and angles (deg.): Sn-C(1) 2.306(2), Sn-C(11) 2.293(2), C(1)-P(1) 1.802(2), C(11)-P(2) 1.789(2), C(1)-Si(1) 1.881(2), C(11)-Si(2) 1.879(2), P(1)-B(1) 1.905(3), P(2)-B(2) 1.924(3), Sn…H(1C) 2.36(2), Sn…H(2C) 2.32(2), C(1)-Sn-C(11) 100.43 (7).

The solid-state structure of 7 exhibits short contacts between one hydrogen of each borane group and the tin atom [B-H…Sn=2.32(2) and 2.36(2) Å]; these distances compare with the corresponding distances in 3a and 3b of 2.38(2)/2.29(2) and 2.41(8)/2.35(6), respectively. The short B-H…Sn contacts cause the stannylene to adopt a syn, syn-configuration, as the alkyl substituents are essentially locked into place.

Although 7 has very approximate C2 symmetry, the two B-H…Sn contacts lock the alkyl substituents into differing conformations in the solid state. This places one of the methyl groups of an isopropyl substituent in close proximity to the tin centre: the Sn…C(6) distance [3.652(2) Å] is substantially shorter than the remaining Sn…Me(Pr) distances, which are all in excess of 4.7 Å, and is significantly less than the sum of the van der Waals radii of tin and carbon (3.87 Å). This structural feature appears to be preserved in solution: three of the four magnetically inequivalent isopropyl methyl groups have very similar 1H chemical shifts (1.06, 1.07 and 1.08 ppm), while a fourth is shifted slightly downfield and is observed at 1.26 ppm, which we attribute to its proximity to the tin centre. A similar effect is seen in the 13C{1H} NMR spectrum of 7: three of the isopropyl methyl groups give rise to signals between 17.34 and 18.01 ppm, while the fourth is shifted downfield to 19.13 ppm.

Experimental

All manipulations were carried out using standard Schlenk techniques under an atmosphere of dry nitrogen or in a nitrogen-filled glove-box. Toluene, THF, methylcyclohexane, and diethyl ether were distilled under nitrogen from sodium, potassium, or sodium/potassium alloy and were stored over either a potassium film or activated 4 Å molecular sieves. Deuterated toluene was distilled under nitrogen from potassium and was deoxygenated by three freeze-pump-thaw cycles and was stored over activated 4 Å molecular sieves. Cp2 Sn was prepared by a previously published procedure (Fischer and Grubert, 1956); nBuLi was purchased as a 2.5 m solution in hexanes but was standardised prior to each use; MeMgBr was purchased from Sigma-Aldich (Gillingham, UK) as a 3.0 m solution in diethyl ether and BH3·SMe2 as a 2.0 m solution in THF; and all other chemicals were used as supplied with the exception of TMEDA, which was distilled from CaH2 and stored over activated 4 Å molecular sieves.

1H, 7Li{1H}, 11B{1H}, 13C{1H}, 31P{1H}, and 119Sn{1H} NMR spectra were recorded on a JEOL Eclipse500 Spectrometer operating at 500.16, 194.38, 160.35, 125.65, 202.35, and 186.50 MHz, respectively (JEOL UK, Welwyn Garden City, UK); chemical shifts are quoted in ppm relative to tetramethylsilane (1H and 13C), aqueous LiCl (1M) (7Li), external BF3(OEt2) (11B), external 85% H3 PO4 (31P), and external Me4 Sn (119Sn), as appropriate. The positions of the BH3 signals in the 1H NMR spectra and JPH for these signals were determined using selective 1H{11B} experiments. Selective proton decoupling and 1H{31P} NMR experiments were used to determine the coupling constants for the iPr groups in 7. Elemental analyses were obtained by the Elemental Analysis Service of London Metropolitan University.

iPr2 P(BH3)Me (4)

To a cold (-78°C) solution of iPr2 PCl (5 mL, 31.4 mmol) in diethyl ether (60 mL), MeMgBr (10.5 mL, 31.4 mmol) was added. The resulting solution was allowed to warm to room temperature and was stirred for 3 h, and then BH3·SMe2 (15.7 mL, 31.4 mmol) was added. After 2 h stirring, water (40 mL) was added, and the organic layer was extracted into diethyl ether (2×30 mL). The combined organic extracts were dried over MgSO4. The solution was filtered and the solvent was removed in vacuo to yield iPr2 P(BH3)Me as a colourless oil. Isolated yield: 3.64 g, 79%. 1H{11B} NMR (CDCl3, 25°C): δ=0.23 (d, JPH=15.1 Hz, 3H, BH3), 1.06 (m, 15H, CHMe2+PMe), 1.88 (m, 2H, CHMe2). 13C{1H} NMR (CDCl3, 25°C): δ=3.11 (d, JPC=33.6 Hz, CHMeMe), 16.42 (CHMeMe), 16.71 (CHMeMe), 21.69 (d, JPC=34.5 Hz, PMe). 11B{1H} (CDCl3, 25°C): δ=-44.1 (d, JPB=60 Hz). 31P{1H} (CDCl3, 25°C): δ=28.1 (q, JPB=60 Hz).

{iPr2 P(BH3)}(Me3 Si)CH2 (5)

To a solution of iPr2 P(BH3)Me 4 (3.03 g, 20.8 mmol) in THF (20 mL), nBuLi (8.3 mL of a 2.5 m solution, 20.8 mmol) was added. The resulting solution was stirred for 2 h then added to a cold (-78°C) solution of Me3 SiCl (3.2 mL, 25.2 mmol) in THF (10 mL). The resulting solution was allowed to warm to room temperature and was stirred for 16 h. The solvent was removed in vacuo, water (40 mL) was added, and the organic layer was extracted into diethyl ether (2×30 mL). The combined organic extracts were dried over MgSO4. The solution was filtered and the solvent was removed in vacuo to yield {iPr2 P(BH3)}(Me3 Si)CH2 (5) as a colourless oil. Isolated yield: 3.44 g, 76%. 1H{11B} NMR (d8-toluene, 25°C): δ=0.18 (s, 9H, SiMe3), 0.43 (d, JPH=14.2 Hz, 2H, CH2), 0.92 (dd, JPH=13.7 Hz, JHH=7.1 Hz, 6H, CHMeMe), 0.96 (dd, JPH=14.1 Hz, JHH=7.1 Hz, 6H, CHMeMe), 0.96 (d, JPH=13.9 Hz, 3H, BH3), 1.58 (m, 2H, CHMeMe). 13C{1H} NMR (d8-toluene, 25°C): δ=0.75 (SiMe3), 5.08 (d, JPC=17.7 Hz, CH2), 16.58 (CHMeMe), 16.72 (CHMeMe), 23.96 (d, JPC=32.3 Hz, CHMeMe). 11B{1H} (d8-toluene, 25°C): δ=-41.9 (d, JPB=59 Hz). 31P{1H} NMR (d8-toluene, 25°C): δ=32.1 (q, JPB=59 Hz).

[{iPr2 P(BH3)}(Me3 Si)CH]Li(TMEDA) (6a)

To a solution of 5 (0.73 g, 3.35 mmol) in THF (30 m), nBuLi (1.45 mL of a 2.3 m solution, 3.35 mmol) was added. The resulting solution was stirred for 30 min and TMEDA (0.39 g, 3.35 mmol) was then added. After 3 h stirring, the solvent was removed in vacuo and the resulting oily solid was dissolved in diethyl ether (20 mL). This solution was cooled to -30°C for 48 h to obtain colourless block crystals of 6a suitable for X-ray crystallography. Isolated yield: 0.64 g, 56%. Anal. calcd. for C16 H42 BLiN2 PSi: C 56.63, H 12.48, N 8.26. Found: C 56.48, H 12.38, N 8.16. 1H{11B} NMR (d8-toluene, 21°C): δ=-1.70 (d, JPH=6.6 Hz, 1H, CHLi), 0.25 (s, 9H, SiMe3), 0.53 (d, JPH=10.4 Hz, 3H, BH3), 1.23 (dd, JPH=14.2 Hz, JHH=7.0 Hz, 6H, CHMeMe), 1.26 (dd, JPH=14.2 Hz, JHH=7.0 Hz, 6H, CHMeMe), 1.79 (br, 4H, NCH2), 1.98 (s, 12H, NMe2), 2.01 (m, 2H, CHMeMe). 13C{1H} NMR (d8-toluene, 21°C): δ=-7.74 (br, CHLi), 5.71 (d, JPC=3.3 Hz, SiMe3), 17.49 (CHMeMe), 17.64 (CHMeMe), 23.94 (d, JPC=32.6 Hz, CHMeMe), 45.89 (NMe2), 56.52 (NCH2). 7Li{1H} NMR (d8-toluene, 21°C): δ=0.6. 11B{1H} NMR (d8-toluene, 21°C): δ=-39.6 (d, JPB=90 Hz). 31P{1H} NMR (d8-toluene, 21°C): δ=33.4 (q, JPB=90 Hz).

[{iPr2 P(BH3)}(Me3 Si)CH]2 Sn (7)

To a solution of 5 (1.25 g, 5.73 mmol) in THF (30 mL), nBuLi (2.73 mL of a 2.1 m solution, 5.73 mmol) was added. This mixture was stirred for 1 h, and then the solvent was removed in vacuo. The resulting colourless oil was dissolved in toluene (20 mL) and added to a solution of freshly sublimed Cp2 Sn (0.71 g, 2.86 mmol). This solution was stirred for 30 min and then filtered. The solvent was removed in vacuo from the filtrate to yield a yellow solid, which was dissolved in methylcyclohexane (20 mL) containing a few drops of THF and cooled to -30°C for 16 h to yield 7 as yellow blocks. Isolated yield: 0.92 g, 58%. Anal. calcd. for C20 H54 B2 P2 Si2 Sn: C 43.43, H 9.84. Found: C 43.53, H 9.90. 1H{11B} NMR (d8-toluene, 25°C): δ=0.33 (s, 9H, SiMe3), 0.79 (d, JPH=8.8 Hz, 3H, BH3), 1.06 (dd, JPH=13.6 Hz, JHH=7.1 Hz, 3H, CHMeMe), 1.07 (dd, JPH=14.8 Hz, JHH=7.0 Hz, 3H, CHMeMe), 1.08 (dd, JPH=14.5 Hz, JHH=7.2 Hz, 3H, CHMeMe), 1.22 (d, JPH=13.5 Hz, 1H, SnCH), 1.26 (dd, JPH=15.1 Hz, JHH=7.3 Hz, CHMeMe), 1.87 (m, 1H, CHMeMe), 2.14 (m, 1H, CHMeMe). 13C{1H} NMR (d8-toluene, 25°C): δ=3.09 (d, JPC=2.1 Hz, SiMe3), 17.34 (CHMeMe), 17.78 (CHMeMe), 17.84 (SnCH), 18.01 (CHMeMe), 19.13 (CHMeMe), 26.53 (d, JPC=32.3 Hz, CHMeMe), 26.84 (d, JPC=27.8 Hz, CHMeMe). 11B{1H} NMR (d8-toluene, 25°C): δ=-37.6 (d, JPB=76 Hz). 31P{1H} NMR (d8-toluene, 25°C): δ=38.5 (q, JPB=76 Hz). 119Sn{1H} NMR (d8-toluene, 25°C): δ=403 (br, FWHM=380 Hz).

Crystal Structure Determination of 6a and 7

All data were collected at 150 K on an Oxford Diffraction Gemini A Ultra diffractometer using graphite-monochromated Mo-Kα radiation (λ=0.71073 Å). Cell parameters were refined from the observed positions of all strong reflections. An empirical absorption correction was applied to the final data for each compound, based on symmetry equivalent and repeated reflections. The structures were solved by direct methods and refined on F2 values for all unique data. Further details are given in Table 1. All nonhydrogen atoms were refined anisotropically, and C-bound H atoms were constrained with a riding model, while B-bound H atoms were freely refined; U(H) was set at 1.2 (1.5 for methyl groups) times Ueq for the parent atom. The TMEDA ligand in 6a is disordered over two positions with equal occupancy. Data were collected and processed using Oxford Diffraction CrysAlisPro and structure solution and refinement completed using the OLEX2 interface to the SHELXTL suite (Sheldrick, 2008; Dolomanov et al., 2009; CrysAlisPro, 2010).

Table 1

Crystallographic data for compounds 6a and 7.

Compound6a7
FormulaC16 H42 BLiN2 PSiC20 H54 B2 P2 Si2 Sn
M339.33553.06
Crystal systemMonoclinicMonoclinic
Space groupP21/nP21/n
a, Å9.4986(4)14.3694(6)
b, Å16.8836(6)11.8551(5)
c, Å14.7898(5)17.8704(8)
β, Å103.494(4)98.335(4)
V, Å32306.37(15)3012.1(2)
Z44
μ/mm-10.9771.039
Data collected1561730055
Unique data49977505
Rint0.02590.0523
Data with F2>2σ40995289
Refined parameters251276
R (on F, F2>2σ)0.0410.031
Rw (on F2, all data)0.1070.052
Goodness of fit on F21.0330.897
Min, max electron density, eÅ-30.42, -0.380.49, -0.44

Corresponding authors: Keith Izod and Corinne Wills, Main Group Chemistry Laboratories, Department of Chemistry, Bedson Building, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK, e-mail: ,

Acknowledgments

The authors are grateful to the EPSRC for support.

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Received: 2014-7-17
Accepted: 2014-8-20
Published Online: 2014-9-26
Published in Print: 2014-12-1

©2014 by De Gruyter

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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