Nature of the dimethylaluminum, -gallium, and -indium methylphenylamide dimers in solution and the molecular structure of [(CH3)2InN(CH3)(C6H 5)]2

Article abstract of DOI:10.1021/ic50222a012

The solution properties of [(CH₃)₂MN(CH₃)(C₆H ₅)]₂ (M = Al, Ga, In) and the crystalline state of the indium derivative have been investigated. All compounds exist in solution as mixtures of cis and trans geometrical isomers. The cis isomer is the predominant species for the aluminum and gallium compounds, whereas the trans isomer is more abundant in the case of indium. An X-ray structural study of the indium derivative identified the trans isomer in the solid state. The complex [(CH₃)₂InN(CH₃)(C₆H ₅)]₂ crystallizes in the centrosymmetric triclinic space group P1 (No. 2) with a = 7.3202 (15) ?, b = 7.6095 (21) ?, c = 8.9800 (27) ?, α = 83.194 (24)°, β = 81.800 (21)°, γ = 81.986 (19)°, V = 487.8 (2) ?³, and ρ(calcd) = 1.71 g cm^-3 for Z = 1 (dimeric unit) with molecular weight 502.1. Diffraction data were collected with a Syntex P2₁ automated four-circle diffractometer, and the structure was solved by using Patterson, Fourier, and full-matrix, least-squares refinement techniques. The resulting discrepancy indices were R_F = 2.7% and R_wF = 3.3% for all 2259 reflections with 2θ = 3.0-55.0° (Mo Kα radiation). The dimeric molecule lies on a crystallographic center of symmetry. The In-N (bridging) distances are In-N(1) = 2.280 (2) ? and In-N(1′) = 2.284 (2) ?, the In?In′ distance is 3.363 ?, and the indium-methyl bond lengths are defined by In-C(1) = 2.156 (4) ? and In-C(2) = 2.149 (4) ?. The dimerization and/or isomerization reactions in solution were investigated by ¹H NMR spectroscopy by evaluating the effects of solvent and temperature on the cis/trans isomer ratios. All data are consistent with the hypotheses that the aluminum-nitrogen dimer is formed by a concerted π-cycloaddition reaction, but the gallium and indium dimers are formed by a series of metal-nitrogen bond forming reactions. The influence of these proposed dimerization reactions on the potential for polymer formation are discussed.