Trans-cis isomerization and structure of [R2Ga(mu-NHSiEt3)](2) (R = Me, Et)

Abstract

Reactions of Et3SiNH2 With R3Ga in a 1:1 ratio have produced dimeric silylamidogallanes, [R2Ga(mu-NHSiEt3)](2) (1, R= Me, trans:cis = 1.5:1; 2, R = Et, trans:cis = 1.3:1), as a mixture of trans (a) and cis (b) isomers. Purification of 1 by either recrystallization or sublimation gives only trans isomer la as colorless crystals. Colorless liquid 2 has been obtained only as a mixture of the two isomers. The trans --> cis isomerization of 1 has been studied by H-1 NMR spectroscopy. The equilibrium has been observed to follow reversible first-order kinetics with Delta H degrees = - 0.64 +/- 0.03 kJ mol(-l) and Delta S degrees = - 4.4 +/- 0.2 J K-1 mol(-1). The activation parameters for the trans (1a) --> cis (1b) conversion are Delta H-1(double dagger) = 72.4 +/- 1.3 kJ mol(-1) and Delta S-1(double dagger) = - 38.8 +/- 4.5 J K-1 mol(-1), and those for the reverse process are Delta H--1(double dagger) = 73.0 +/- 0.4 kJ mol(-1) and Delta S--1(double dagger) = - 34.4 +/- 1.7 J K-1 mol(-1). The isomerization is markedly accelerated in the presence of Lewis bases. A crossover experiment indicates that the isomer interconversion is a unimolecular process. The thermodynamic and kinetic data have been explained based on the solvent effect and the silyl substituent effect on the nitrogen atom. The molecular structure of 1a has been determined by a single-crystal X-ray diffraction study. The molecular geometry of la consists of a centrosymmetric and dimeric unit with two bridging (triethylsilyl)amido groups and two terminal methyl groups bound to each gallium atom. The two silyl groups are trans to each other with respect to the planar (Ga-N)(2) ring framework. The coordination geometry of both gallium and nitrogen atoms is distorted tetrahedral.