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

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}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.

Advances in alkaline earth-nitrogen chemistry

“…Addition of excess HMPA or THF affords the monomeric Mg[N(CH 2 Ph) 2 ] 2 (donor) 2 as evidenced by NMR spectroscopy. Demonstrating the propensity for four-coordinate species, addition of two molar equivalents of TMEDA yielded the monomeric Mg[N(CH 2 Ph) 2 ] 2 (tmeda) [81]. Similar results were obtained in the presence of dicyclohexylamine, HN(Cy) 2 , affording the threecoordinate magnesium dimer, [Mg(N(Cy) 2 ) 2 ] 2 , a rare example of a homoleptic dialkylmagnesium amide [82].…”

“…In the absence of polar solvents, a threecoordinate magnesium dimer, [Mg{N(CH 2 Ph) 2 } 2 ] 2 is isolated [81]. Addition of 2 molar equivalents of either THF or HMPA led to the coordination of one donor molecule per magnesium resulting in the dimeric [Mg{N(CH 2 Ph) 2 } 2 (donor)] 2 (donor = THF, HMPA) [81]. Addition of excess HMPA or THF affords the monomeric Mg[N(CH 2 Ph) 2 ] 2 (donor) 2 as evidenced by NMR spectroscopy.…”

Selective Colorimetric Detection of Hg2+ and Mg2+ with Crown Ether Substituted N‐Aryl‐9‐aminobenzo[b]quinolizinium Derivatives

“…The redistribution of amide ligands in Mg complexes has been shown to operate in solution, and the driving force for this process may be the strong N-Mg interaction. [24] The blueshift of the absorption and emission maxima indicates the reduced donor ability of the nitrogen atom in this complex, which is likely to result from steric strain and a twist of the benzo[b]quinolizinium unit about the C-N bond such that conjugation with the nitrogen atom at C-9 is significantly reduced. The increased emission intensity may be the result of reduced conformational flexibility of the complex, because it has been shown that the low emission quantum yield of 9-amino-N-arylbenzo[b]quinolizinium derivatives is mainly caused by the torsional relaxation.…”

Magnesium Bisamides as Reagents in Synthesis

“…[11] However, when bulky amides are utilised, the aggregation of the complexes is generally limited to either monomers or dimers. [13] Indeed, this specific feature will be advantageous in the use of these complexes as selective reagents, since easily reproducible solution aggregation is possible. In turn, this will allow transformations to be accomplished in an identical physical environment for each molecule.…”

“…Scheme 2). [13] In general, monomers are prevalent in highly polar solvent media or in the presence of two or more equivalents of strong Lewis bases such as hexamethylphosphoramide (HMPA). Higher aggregation of magnesiumnitrogen complexes can however be achieved using specialised ligand systems such as dianionic amides and imides (RN 2À ).…”