CIS TRANS ISOMERIZATION AND STRUCTURE OF DIMERIC [ME2GA(MU-NH(T)BU)]2

Abstract

The reaction of GaMe3 with tBuNH2 at 110-degrees-C affords a mixture of trans- (1a) and cis-[Me2Ga(mu-NHtBu)]2 (1b) which crystallizes as pure 1a in 75% yield. H-1 NMR study shows that the dimeric amido complex undergoes a cis-trans isomerization (1a --> 1b) in solution. The equilibrium has been observed to follow reversible first-order kinetics with DELTAH-degrees = 3.12 +/- 0.04 kJ mol-1 and DELTAS-degrees = 4.56 +/- 0.06 J mol-1 K-1. The activation parameters for the conversion 1a --> 1b are DELTAH-1 = 120.8 +/- 7.4 kJ mol-1 and DELTAS1 = 41.4 +/- 2.6 J mol-1 K-1 and for the reverse reaction 1b --> 1a are DELTAH-1 = 117.8 +/- 7.9 kJ mol-1 and DELTAS-1 = 37.3 +/- 2.5 J mol-1 K-1. The isomerization is markedly accelerated in the presence of pyridine. A crossover experiment indicates that the isomer interconversion is a unimolecular process. On the basis of these data, a possible pathway for the isomerization is proposed. The molecular structure of 1a has been determined by a single-crystal X-ray diffraction study. The molecular geometry of 1a consists of a centrosymmetric and dimeric unit with two bridging amido groups and two terminal methyl groups bound to each gallium atom. The two N-(t)Bu groups are trans to each other with respect to the (Ga-N)2 ring. The coordination geometry of both the gallium and nitrogen atoms is distorted tetrahedral.