Isomerism of dimeric cis-ethene-1,2-di(alkylamido)gallium hydrides and chlorides

Dichlorogallane [HGaCl2](2) 1 can be prepared in quantitative yield from eq uimolar quantities of GaCl3 and Et3SiH at -13 degrees C in the absence of a solvent. Treatment of 1 with dilithium N,N'-di-tert-butyl-cis-ethenediamid e at -78 degrees C leads to high yields of [((Bu2N2)-Bu-t(CH)(2))GaH](2) 2. According to IR and NMR observations, compound 2 exists in the form of two isomers in benzene solution. Crystallization from pentane affords only one isomer which was identified as a centrosymmetrical dimer with a four-membe red ring (Ga-N)(2) as the core unit, two hydrogen atoms in trans position a nd two five-membered rings fused to opposite edges also in trans position ( type B). The analogous reaction of [(Bu2N2)-Bu-t(CH)(2)]Li-2 with anhydrous [GaCl3](2) gives the corresponding chloride 3 which exists as only one iso mer in solution. Crystals obtained from hexane contain four independent dim eric molecules, all of which have the same structure with inequivalent gall ium atoms owing to a redistribution of the chloride ligands. The central fo ur-membered ring (Ga-N)(2) has the two five-membered rings fused to neighbo uring edges in trans position, and the two chlorine atoms are associated wi th the same gallium atom. This isomer [(Bu2N2)-Bu-t(CH)(2)]Ga(GaCl2) 3 (typ e D) is unprecedented in the structural chemistry of the "diazabutadiene" c omplexes of Group 13 elements. The analogous cyclohexyl compound was prepar ed in high yield following the same route. It is formed as only one isomer which has the centrosymmetrical structure of type B, [((c)Hex(2)N(2)(CH)(2) )(2)GaCl](2) ((c)Hex = cyclohexyl). The isomerism is probably governed by s teric effects.