Solid-state 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)2N}2Mg}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)2N}2MgâTHF}2], 2, and [{{(PhCH2)2N}2MgâHMPA}2], 4 respectively, which maintain the dimeric framework but increase the metal's coordination number to 4. Addition of 4 molar equiv of HMPA, or a 20-fold excess, of THF to 1 causes deaggregation of the dimer to the monomeric bis-solvates [{(PhCH2)2N}2- Mgâ2THF], 3, and [{(PhCH2)2N}2Mgâ2HMPA], 5. The chelating ligand TMEDA gives the monomer [{(PhCH2)2N}2MgâTMEDA], 6, on mixing with 1. 1H and 13C NMR spectroscopic studies reveal that dimer 1 is partially retained in arene solution but is in equilibrium with the unsolvated monomer [{(PhCH2)2N}2Mg], 7. Concentration studies on solutions of monosolvated dimer 4 show it to be in equilibrium with both the bissolvated 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