Trans−Cis Isomerization and Structure of [R₂Ga(μ-NHSiEt₃)]₂ (R = Me, Et)

Abstract

Reactions of Et3SiNH2 with R3Ga in a 1:1 ratio have produced dimeric silylamidogallanes, [R2Ga(μ-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 1a 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 1H NMR spectroscopy. The equilibrium has been observed to follow reversible first-order kinetics with ΔH° = − 0.64 ± 0.03 kJ mol-1 and ΔS° = − 4.4 ± 0.2 J K-1 mol-1. The activation parameters for the trans (1a) → cis (1b) conversion are ΔH1⧧ = 72.4 ± 1.3 kJ mol-1 and ΔS1⧧ = − 38.8 ± 4.5 J K-1 mol-1, and those for the reverse process are ΔH-1⧧ = 73.0 ± 0.4 kJ mol-1 and ΔS-1⧧ = − 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 1a 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.