Synthesis and Structure of Triphenylmethylphosphonium Tetrakis(tetrahydroborato)aluminate, (Ph3MeP)(Al(BH4)4), an Example of Eight-Coordinate Aluminum(III)

摘要

Complex cation, (Ph3P)2N(+) and Ph3MeP(+), salts of the tetrakis(tetrahydroborato)aluminate anion Al(BH4)4(-) were prepared as white crystalline solids in good yield from the reactions of Ph3MePBH4 and (Ph3P)2NBH4 with Al(BH4)3 in CH2Cl2 solutions. These salts are thermally stable above 100 deg C. They have been characterized by (1)H, (11)B, and (27)Al NMR spectroscopy, and the structure of Ph3MePAl(BH4)4 has been determined by single crystal X-ray analysis. Four BH4(-) units form eight apparent 3c-2e Al-H-B bonds with the Al(3+) center. This is the first molecular structure of an aluminum compound which contains an eight coordinate aluminum atom. The four BH4 units are arranged in a distorted tetrahedral fashion around the aluminum atom. Each BH4 unit contributes two hydrogen bridges to the aluminum. The eight bridging hydrogens form a slightly distorted dodecahedral coordination geometry around the aluminum center. Other previously reported salts MAlH(BH4)3, MAlH2(BH4)2, and MAlH3(BH4) (M+ approx. Li+, Bu4N+) were studied further in solution by NMR spectroscopy. jg p.1.