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 . TH F}(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, o f THF to 1 causes deaggregation of the dimer to the monomeric bis-solv ates [{{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. Conce ntration studies on solutions of monosolvated dimer 3 show it to be in equilibrium with both the bis-solvated monomer 5 and the unsolvated m onomer 7. X-ray crystallographic determinations have been carried out on complexes 1, 2, and 4-6, and a comparative analysis of their struct ures is detailed. The bis(dibenzylamido)magnesium system has shown rem arkable structural flexibility with di-, tri-, and tetracoordination a t the metal.