Solid State Structures and Dynamic Solution Equilibria of Bis(dibenzylamido)magnesium Complexes: Aggregation Dependence on Stoichiometry and Denticity of Donor Solvent

Reaction of commercial Bu 2 Mg with 2 molar equiv of dibenzylamine gives the bis(amido)magnesium complex [{{(PhCH 2 ) 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 [{{(PhCH 2 ) 2 N} 2 Mg·THF} 2 ], 2, and [{{(PhCH 2 ) 2 N} 2 Mg·HMPA} 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 [{(PhCH 2 ) 2 N} 2 Mg·2THF], 3, and [{(PhCH 2 ) 2 N} 2 Mg·2HMPA], 5 . The chelating ligand TMEDA gives the monomer [{(PhCH 2 ) 2 N} 2 Mg·TMEDA], 6, on mixing with 1 . 1 H and 13 C NMR spectroscopic studies reveal that dimer 1 is partially retained in arene solution but is in equilibrium with the unsolvated monomer [{(PhCH 2 ) 2 N} 2 Mg], 7 . Concentration studies on solutions of monosolvated dimer 4 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.