An Unusual Hydride-Bridged Aluminum Complex with a Square-Planar Tetraaluminum Core Stabilized by 2,5-Bis((Dimethylamino)methyl)pyrrole Ligands

[[abstract]]The novel hydride-bridged tetranuclear aluminum complex {AlH2[C4H2N(CH2NMe2)2-2,5]AlH3}2 (2) and the related deuterium complex {AlD2[C4H2N(CH2- NMe2)2-2,5]AlD3}2 (2-D) have been synthesized by reaction of AlCl2[C4H2N(CH2NMe2)2-2,5] (1) with 2 equiv of LiAlH4 or LiAlD4. The crystal structure of complex 2-D reveals an unusual structure in which the aluminum atoms are in a square-planar arrangement. Reaction of 2 with 2 equiv of H2O in diethyl ether affords the aluminum dihydride complex [C4H2N(CH2NMe2)2-2,5]- AlH2 (3). Metal hydrides1 play an important role in catalytic reactions due to their highly reactive metal-hydrogen bonds. Among the metal hydrides, aluminum hydrides are well-known and the topic has been reviewed.2 Hydride ions bound to aluminum atoms have been observed in either terminal or bridging positions in accord with varied auxiliary ligands in mononuclear,3 dinuclear,4 or trinuclear5 aluminum hydride complexes. However, a hydride-bridged tetranuclear6 aluminum complex has not been reported thus far. Currently, we are interested in using the tridentate substituted pyrrole7 [C4H3N(CH2NMe2)2-2,5] as an auxiliary ligand in early-transition-metal8 or group 13 metal complexes.9 We now report the synthesis and structural characterization of a novel, hydride-bridged, tetranuclear aluminum complex, which contains bis((dimethylamino)-methyl)pyrrole as a ligand. The reaction of compound 19 with 2 equiv of LiAlH4 in diethyl ether at 0 �C generates compound 210 in 84% yield (eq 1)