Reaction of GaMe3 with H2NCH2CH2NMe2:  Synthesis and Characterization of Adducts and Imidogallane

Reaction of GaMe3 with N,N-dimethylethylenediamine (DMEDA) produces adducts, Me3Ga:NH2C2H4NMe2 (1:1, 1) and Me3Ga:NH2C2H4NMe2:GaMe3 (2:1, 2). A fast concerted intermolecular exchange of the two inequivalent GaMe3 moieties occurs in 2 at room temperature. Thermolysis of 2 at 210 °C in a sealed ampule produces a mixture of cis and trans isomers of tetrametallic imidogallane, (3). Slow recrystallization of 3 affords only the cis isomer as colorless crystals, but in solution the cis isomer equilibrates with the trans isomer. The cis → trans isomerization of 3 has been studied by 1H NMR spectroscopy. The equilibrium has been observed to follow reversible first-order kinetics with ΔH° = − 3.35 ± 1.70 kJ mol-1 and ΔS° = − 10.4 ± 1.7 J mol-1 K-1. The activation parameters for the cis → trans conversion are Δ = 144.6 ± 6.2 kJ mol-1 and Δ = 105.6 ± 1.5 J mol-1 K-1, and those for the reverse process are Δ = 147.6 ± 6.4 kJ mol-1 and Δ = 115.4 ± 1.5 J mol-1 K-1. The isomerization process has been proposed to be assisted by the NMe2 end of DMEDA as an internal Lewis base. The molecular structures of 2 and cis-3 have been determined by single-crystal X-ray diffraction studies. The geometry about the gallium atom of 2 is distorted tetrahedral with a Ga−C (av) distance of 1.98(1) Å and a Ga−N distance of 2.16(1) Å. The molecular geometry of cis-3 involves one four-membered Ga2N2 ring and two five-membered GaN2C2 rings of cis conformation, and it has approximate C2 symmetry with the C2 axis going through the center of the Ga2N2 ring.