Solid state structures and dynamic solution equilibria of bis(dibenzylamido)magnesium complexes: Aggregation dependence on stoichiometry and denticity of donor solvent

Reaction of commercial Bu2Mg with 2 molar equiv of dibenzylamine gives the bis(amido)magnesium complex [{{(PhCH2)(2)N}(2)Mg}(2)], 1 Compound 1 is dimeric with three-coordinate magnesium in the crystalline state. Addition of 2 molar equiv of the monodentate donor solvents THF and HMPA to solutions of 1 affords the complexes [{{(PhCH2)(2)N}(2)Mg . THF}(2)], 2, and [{{(PhCH2)(2)N}(2)Mg . HMPA}(2)], 4 respectively, which maintain the dimeric framework but increase the metal's coordination number to 4. Addition of 3 molar equiv of HMPA, or a 20-fold excess, of THF to 1 causes deaggregation of the dimer to the monomeric bis-solvates [{{PhCH2)(2)N}(2)-Mg . 2THF], 3, and [{{PhCH2)(2)N}(2)Mg . 2HMPA], 5. The chelating ligand TMEDA gives the monomer [{(PhCH2)(2)N}(2)Mg . TMEDA], 6, on mixing with 1. H-1 and C-13 NMR spectroscopic studies reveal that dimer 1 is partially retained in arene solution but is in equilibrium with the unsolvated monomer [{{PhCH2)(2)N}(2)Mg], 7. Concentration studies on solutions of monosolvated dimer 3 show it to be in equilibrium with both the bis-solvated monomer 5 and the unsolvated monomer 7. X-ray crystallographic determinations have been carried out on complexes 1, 2, and 4-6, and a comparative analysis of their structures is detailed. The bis(dibenzylamido)magnesium system has shown remarkable structural flexibility with di-, tri-, and tetracoordination at the metal.