Synthesis and Structural Characteristics of Discrete Organoboron and Organoaluminum Hydrides Incorporating Bulky Eind Groups

The
bulky Eind-based aryllithium, (Eind)Li (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl), reacted with BF3·OEt2 in Et2O to form the corresponding aryldifluoroborane
(Eind)­BF2 (1), along with a trace amount of
the sterically congested diarylborane, (Eind)2BH (2). The reaction of 1 with LiAlH4 in
THF led to the isolation of the corresponding lithium trihydroborate
[Li­(thf)]2[(Eind)­BH3]2 (3), which can be transformed into the diborane(6), (Eind)­HB­(μ-H)2BH­(Eind) (4), with treatment with Me3SiCl. The Eind-based lithium trihydroaluminate [Li­(OEt2)]2[(Eind)­AlH3]2 (5) has been synthesized by the reaction of (Eind)Li with LiAlH4 in Et2O. The subsequent addition of Me3SiCl to a solution of 5 in toluene produced the dialumane(6),
(Eind)­HAl­(μ-H)2AlH­(Eind) (6), the heavier
congener of 4. The dialumane(6) 6 reacted
with lithium metal in a mixed solvent of Et2O and toluene
to give the diarylalumane, (Eind)2AlH (7),
via a disproportionation reaction along with the cleavage and recombination
of the Al–C bond. The discrete monomeric structures of 1, 2, and 7 and dimeric structures
of 3, 4, 5, and 6 have been determined by X-ray crystallography.