Solvent dependence of the solid-state structures of salicylaldiminate magnesium amide complexes

There are challenges in using magnesium coordination complexes as reagents owing to their tendency to adopt varying aggregation states in solution and thus impacting the reactivity of the complexes. Many magnesium complexes are prone to ligand redistribution via Schlenk equilibrium due to the ionic character within the metal-ligand interactions. The role of the supporting ligand is often crucial for providing stability to the heteroleptic complex. Strategies to minimize ligand redistribution in alkaline earth metal complexes could include using a supporting ligand with tunable sterics and electronics to influence the degree of association to the metal atom. Magnesium bis(hexamethyldisilazide) was reacted with salicylaldimines [¹L = N-(2,6-diisopropylphenyl)salicylaldimine and ²L = 3,5-di-tert-butyl-N-(2,6-diisopropylphenyl)salicylaldimine] in either nondonor (toluene) or donor solvents [tetrahydrofuran (THF) or pyridine]. The structures of the magnesium complexes were studied in the solid state via X-ray diffraction. In the nondonor solvent, i.e. toluene, the heteroleptic complex bis{μ-2-[(2,6-diisopropylphenyl)iminomethyl]phenolato}-κ³N,O:O;κ³O:N,O-bis[(hexamethyldisilazido-κN)magnesium(II)], [Mg₂(C₁₉H₂₂NO)₂(C₆H₁₈NSi₂)₂] or [¹LMgN(SiMe₃)₂]₂, (1), was favored, while in the donor solvent, i.e. pyridine (pyr), the formation of the homoleptic complex {2,4-di-tert-butyl-6-[(2,6-diisopropylphenyl)iminomethyl]phenolato-κ²N,O}bis(pyridine-κN)magnesium(II) toluene monosolvate, [Mg(C₂₇H₃₈NO)₂(C₅H₅N)₂]·C₅H₅N or [{²L₂Mg₂(pyr)₂}·pyr], (2), predominated. Heteroleptic complex (1) was crystallized from toluene, while homoleptic complexes (2) and the previously reported [¹L₂Mg·THF] [Quinque et al. (2011). Eur. J. Inorg. Chem. pp. 3321-3326] were crystallized from pyridine and THF, respectively. These studies support solvent-dependent ligand redistribution in solution. In-situ ¹H NMR experiments were carried out to further probe the solution behavior of these systems.