The Low-Temperature Polymorph of Tetramethyldiphosphine- bis(monoborane): Insight into the Stabilization of Different Rotational Isomers in the Solid State

The low-temperature (LT) polymorph of tetramethyldiphosphine-bis(monoborane), Me2(H3B)PP(BH3)Me2, was obtained by crystallization from diethyl ether at 4 ◦C. It crystallizes in the monoclinic space group P21/c, a = 6.464(1), b = 7.605(1), c = 11.867(2) Å, β = 119.99(1)◦ (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 ◦C (DTA) the LT polymorph transforms to the high-temperature (HT) modification which contains the anti and gauche conformers in a 1:2 ratio (P21/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)◦) ascertains the value of the anti conformer in the HT form (114.4(6)◦) which was found to be significantly different from the gauche conformer which centered around 110 ◦C.