Abstract
The reaction of (C6F5)3BOPPh2OH (prepared in situ from the rather weak Brønsted acid Ph2PO2H and the strong Lewis acid B(C6F5)3) with (t-Bu2SnO)3 provided the dinuclear complex [t-Bu2Sn(OH)OPPh2O)]2, which was investigated by X-ray crystallography.
The acidity of Brønsted acid can be significantly increased by Lewis acids. This principle was recently exemplified for the acid combination Ph2PO2H/B(C6F5)3, which forms, upon complexation, the metastabile acid (C6F5)3BOPPh2OH (1) in solution (Kather et al., 2016). Despite its limited life span, (C6F5)3BOPPh2OH (1) has been used to protonate the substrates (Me2SnO)n and Ph4Sn, which afforded the organotin products [Me2Sn(OPPh2O)2SnMe2][HOB(C6F5)3]2 (2) and Ph3SnOPPh2OB(C6F5)3 (3), respectively. In the context of this work, we also investigated the reaction of (C6F5)3BOPPh2OH (1) with (t-Bu2SnO)3, which provided the dinuclear product [t-Bu2Sn(OH)OPPh2O]2 (4) in 73% yield (Scheme 1). We speculate that the reaction might proceed via the initial formation of the putative intermediate t-Bu2Sn(OH)OPPh2OB(C6F5)3 (5), a functionalized analog of 3, which, however, releases B(C6F5)3 upon intramolecular hydrogen bond formation. Compound 4 belongs to the well-known family of dinuclear di-tert-butyltin species [t-Bu2Sn(OH)X]2, whereby examples with X=F, Cl, Br (Puff et al., 1985), OH (Beckmann et al., 1998), O3SR (Sakamoto et al., 1999, 2000; Kundu et al., 2014), NO3 (Sakamoto et al., 2000; Reuter and Reichelt, 2014), TcO4 (Oehlke et al., 2010), O2CR, and OP(S)(OMe)2 (Mokal et al., 1992; Mokal and Jain, 1995) can already be found in the literature. It is also related to the trinuclear cation [Ph2P(OSnt-Bu2)2O·t-Bu2Sn(OH)2]+, which was obtained by the reaction of the acid combination Ph2PO2H/F3CSO3H with (t-Bu2SnO)3 (Beckmann et al., 2003). Compound 4 is air stable and shows low solubility in common organic solvents like acetonitrile, dichloromethane, or toluene. The 31P-NMR spectrum (CDCl3) of 4 shows a signal at δ=31.7 ppm that is similar to [Me2Sn(OPPh2O)2SnMe2][HOB(C6F5)3]2 (δ=31.2, 31P-MAS NMR), Ph3SnOP(Ph2)(O)B(C6F5)3 (δ=31.3, CDCl3) and slightly shifted upfield to the parent Ph2PO2H (δ=33.9 ppm) (Kather et al., 2016). No 119Sn-NMR signal could be observed probably due to the low solubility and/or due to the dynamic equilibria between four- and five-coordinated species in solution that have been observed for the same compound class previously (Beckmann et al., 1998; Beckmann 2005). Colorless crystals of 4 suitable for X-ray could be obtained by recrystallization from dichloromethane/n-hexane solution. The molecular structure of 4 (Figure 1) reveals centrosymmetric units of [Ph2(O)POSn(t-Bu2)OH]2 having a Sn2O2 core in which the Sn-O bond distances of 2.039(2) Å (Sn1-O3) and of 2.177(2) Å (Sn1-O3a) are more unsymmetrical compared to the Sn2O2 core of [Bu2Sn(OH)(O3SCF3)]2 (Sn-O=2.090(4) and 2.120(4) Å) (Lee et al., 2004) or [Ph2(HO)SnOSn(O3SCF3)Ph2]2 (Sn-O=2.064(3) and 2.117(3) Å) (Beckmann et al., 2004a). The Sn1-O1 bond distance of 2.159(2) Å is significantly longer compared to 3 (Sn-O=2.058(1) Å) (Kather et al., 2016). The two symmetrically equal tin atoms of 4 adopt a distorted trigonal bipyramidal geometry defined by a C2O3 donor set. For Sn1 the C1, C5, and O3 atoms occupy the equatorial positions, whereas O3 also coordinates to Sn1a occupying with one OP atom the axial positions. The distortion is expressed by the O1-Sn1-O3a angle of 154.5(5)°, deviating significantly from the ideal angle of 180°. The Sn1 atom is marginally displaced by 0.054(1) Å from the trigonal plane defined by C1, C5, and O3 in the direction of O1. The O3-Sn1-C bond angles of 117.5(1)° and 118.6(1)° are slightly compressed compared to the C-Sn1-C angle of 123.7(2)°. The P1-O1 bond distance of 1.534(2) Å is longer compared to [Ph2Sn(CH2)SnPh2(O2PPh2)](O3SCF3) (P-O=1.524(2) and 1.517(2) Å) (Beckmann et al. 2004b), and the P1-O1-Sn1 bond angle of 132.9(1)° is significantly smaller compared to 3 (P-O-Sn=142.29(7)°) (Kather et al., 2016). Compound 4 shows intramolecular hydrogen bonds with a non-bonding O···O distance of 2.595(3) Å indicative for medium strength hydrogen bonding (Steiner, 2002).
![Scheme 1: Formation of [t-Bu2Sn(OH)OPPh2O]2 (4) via the possible intermediate t-Bu2Sn(OH)OPPh2OB(C6F5)3 (5).](/document/doi/10.1515/mgmc-2017-0017/asset/graphic/j_mgmc-2017-0017_scheme_001.jpg)
Formation of [t-Bu2Sn(OH)OPPh2O]2 (4) via the possible intermediate t-Bu2Sn(OH)OPPh2OB(C6F5)3 (5).
![Figure 1: Molecular structures of 4 showing 30% probability ellipsoids and the crystallographic numbering scheme. Selected bond parameters of 4 [Å, °]: P1-O1 1.534(2), P1-O2 1.500(2), Sn1-O1 2.159(2), Sn1-O3 2.039(2), O2-O3 2.595(3), P1-O1-Sn1 132.9(1), O1-Sn1-O3 85.2(1).](/document/doi/10.1515/mgmc-2017-0017/asset/graphic/j_mgmc-2017-0017_fig_001.jpg)
Molecular structures of 4 showing 30% probability ellipsoids and the crystallographic numbering scheme. Selected bond parameters of 4 [Å, °]: P1-O1 1.534(2), P1-O2 1.500(2), Sn1-O1 2.159(2), Sn1-O3 2.039(2), O2-O3 2.595(3), P1-O1-Sn1 132.9(1), O1-Sn1-O3 85.2(1).
Experimental section
General
Starting material diphenylphosphinic acid (Acros Organics, GB) was obtained commercially. Tris(pentafluorophenyl)borane (Massey and Park, 1964; Kuprat et al., 2010) and di-t-butyltin oxide (Puff et al., 1985) were prepared according to literature procedures. Dry dichloromethane, n-hexane, and toluene were collected from a SPS800 mBraun solvent system. NMR spectra (1H at 360.1 MHz, 13C at 90.6 MHz, and 31P at 145.7 MHz) were recorded in CDCl3 at room temperature using a Bruker Avance-360 spectrometer and are referenced to tetramethylsilane (1H, 13C) and phosphoric acid (85% in water) (31P). Chemical shifts are reported in parts per million (ppm), and coupling constants (J) are given in hertz (Hz). Electron impact mass spectrometry was carried out using a Finnigan MAT 95. IR spectra were recorded on a Perkin Elmer Spectrum 1000 FT-IR spectrometry as KBr discs and are reported in cm−1.
Synthesis of [t-Bu2Sn(OH)OPPh2O]2 (4):
Tris(pentafluorophenyl)borane (100 mg, 0.195 mmol), diphenylphosphinic acid (42.6 mg, 0.195 mmol), and di-tert-butyltin oxide (48.6 mg, 0.065 mmol) were stirred in 10 mL toluene at room temperature for 3 days. The reaction mixture is allowed to stand for about 12 h, during which time a white solid settles. The precipitate was isolated and washed with toluene and n-hexane providing 4 as colorless solid (66.6 mg, 0.071 mmol, 73%). To obtain crystals suitable for X-ray diffraction, 4 was recrystallized by dichloromethane/n-hexane.
1H-NMR (CDCl3): δ=7.78–7.67 (m, 8H, CH), 7.51–7.38 (m, 12H), 3.64 (s, 2H, OH), 1.31 (s, 36H, CH3). 13C{1H}-NMR (CDCl3): δ=133.8 (d, 1J(31P-13C)=133 Hz, i-C), 132.5 (d, 4J(31P-13C)=2 Hz, p-CH), 131.2 (d, 2J(31P-13C)=11 Hz, o-CH), 129.9 (d, 3J(31P-13C)=13 Hz, m-CH), 30.5 (s, CH3). 31P{1H}-NMR (CDCl3): δ=31.7 (s). MS (EI+): m/z: 336.9 [Ph2P(O)OSn]+, 57.1 [(CH3)3C]+. IR (KBr, cm−1):
X-ray crystallography:
Intensity data of 4 were collected on a Siemens P4 diffractometer at 173 K with graphite-monochromated Mo-Kα (0.7107 Å) radiation. All structures were solved by direct methods and refined based on F2 by use of the SHELX program package as implemented in WinGX (Farrugia, 1999). All non-hydrogen atoms were refined using anisotropic displacement parameters. Hydrogen atoms attached to carbon atoms were included in geometrically calculated positions using a riding model. Crystal and refinement data are collected in Table 1. Figures were created using DIAMOND (Brandenburg and Putz, 2006). Crystallographic data (excluding structure factors) for the structural analyses have been deposited with the Cambridge Crystallographic Data Centre. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
Crystal data and structure refinement of 4.
| 4 | |
|---|---|
| Formula | C40H58O6P2Sn2 |
| Formula weight (g mol−1) | 934.18 |
| Crystal system | Triclinic |
| Crystal size (mm) | 0.07×0.05×0.03 |
| Space group | P1̅ |
| a (Å) | 8.881 (1) |
| b (Å) | 9.112 (1) |
| c (Å) | 13.737 (1) |
| α (deg) | 75.87 (1) |
| β (deg) | 88.30 (1) |
| γ (deg) | 85.61 (1) |
| V (Å3) | 1074.8 (2) |
| Z | 1 |
| ρcalcd (Mg m−3) | 1.443 |
| μ (Mo Kα), mm−1 | 1.278 |
| F(000) | 476 |
| θ range (deg) | 2.75–27.49 |
| Index ranges | −5≤h≤11 |
| −11≤k≤11 | |
| −17≤l≤17 | |
| No. of reflns collected | 4901 |
| Completeness to θmax | 99.4% |
| No. indep. Reflns | 4901 |
| No. obsdreflns with [I>2σ(I)] | 4604 |
| No. refined parameters | 236 |
| GooF (F2) | 1.055 |
| R1 (F) [I>2σ(I)] | 0.0237 |
| wR2 (F2) (all data) | 0.0595 |
| Largest diff peak/hole, e Å−3 | 0.383/−0.404 |
| CCDC number | 1541491 |
Acknowledgments
The Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged for financial support.
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