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)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}2Mg·THF}2],
2, and
[{{(PhCH2)2N}2Mg·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
[{(PhCH2)2N}2Mg·2THF], 3, and
[{(PhCH2)2N}2Mg·2HMPA],
5. The chelating ligand TMEDA gives the
monomer
[{(PhCH2)2N}2Mg·TMEDA],
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
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.