The low-temperature polymorph of tetramethyldiphosphine-bis(monoborane): Insight into the stabilization of different rotational isomers in the solid state

摘要

The low-temperature (LT) polymorph of tetramethyldiphosphine-bis(monoborane), Me-2(H3B)P-P(BH3)Me-2, was obtained by crystallization from diethyl ether at 4 degreesC. It crystallizes in the monoclinic space group P2(1)/c, a = 6.464(1), b = 7.605(1), c = 11. 867(2) Angstrom, beta = 119.99(1)degrees (at 153 K) with 2 molecules per unit cell. This implies that the individual molecules have crystallographic inversion symmetry and a strict anti arrangement with respect to the central B-P-P-B skeleton. At 87.6 degreesC (DTA) the LT polymorph transforms to the high-temperature (HT) modification which contains the anti and gauche conformers in a 1:2 ratio (P2(1)/c, Z = 6; H. L. Carrell, J. Donohue, Acta Crystallogr. B24, 699 (1968)). This strongly suggests that the gauche conformer is higher in energy and stabilized by the crystal packing of the HT modification. The P-P-B angle in the anti LT form (113.91(6)degrees) ascertains the value of the anti conformer in the HT form (114.4(6)degrees) which was found to be significantly different from the gauche conformer which centered around 110 degreesC.