Alkyl derivatives of [B6H6](2-): NMR and vibrational spectra and crystal structure of (Ph4P)[B6H6CH2Ph]

In the reaction of [B6H6](2-) with benzyl bromide in dichloromethane the pr otonated anion [B(6)H(5)HfacCH(2)Ph](-) is formed. The crystal structure of (Ph4P)[(B6H5HCH2Ph)-C-fac] has been determined by single crystal diffracti on analysis: monoclinic, space group P2(1)/c with a = 13.795(5), b = 11.133 (5), c = 19.231(5) Angstrom, beta = 110 659(5)degrees, Z = 4, R = 0.056. Ty pical distances of 1.59, 1.73 and 1.13 Angstrom are averaged out for B-C, B -B and B-H, respectively. One of the four facets, which shares the ipso bor on atom, is significantly expanded suggesting that the additional proton H- fac is fixed above this facet by a 2e4e bond in the solid state. In solutio n and at room temperature the proton is fluctuating across the four ipso fa cets as shown by variable temperature B-11 NMR spectroscopy, and the point of coalescence for its fixation is found at 234 K in dichloromethane soluti on in a held of 9.4 T. A second point of coalescence according to the fixat ion of the benzyl group is observed at 225 K. Additionally, a nuclear Overh auser interaction between H-fac and the alkyl H atoms has been proven by H- 1-H-1 NOESY below the first coalescence temperature. All data from vibratio nal and H-1, B-11, C-13 NMR spectroscopy are consistent with the structural features. (C) 1999 Elsevier Science SA. All rights reserved.