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

Clegg, W. and Craig, F.J. and Henderson, K.W. and Kennedy, A.R. and Mulvey, R.E. and O'Neil, P.A. and Reed, D. (1997) Solid-state and dynamic solution equilibria of bis(dibenzylamido) magnesium complexes: aggregation dependence on stoichiometry and denticity of donor solvent. Inorganic Chemistry, 36 (27). pp. 6238-6246. ISSN 0020-1669

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Abstract

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.