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.

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Reaction of the simple alane adduct [Me,Al . HN(CH,Ph),] existence here can be attributed to the stabilising effect of (2) with the lithium amide [(PhCH,),NLi] leads to the forma-the attached Me3Al ligand. Crystalline 2 adopts the classical, tion of the mixed adduct [Me3A1 . (PhCH2),NLi . distorted-tetrahedral arrangement of simple monomeric HN(CH,Ph),] (1). The crystal structures of 1 and 2 are re-alane adducts. Ab initio MO calculations on model systems ported. Exhibiting a four-membered, mixed-metal, mixed-show that niixed adduct formation from Me3Al and Me,NLi anion = N ring-core, the structure of 1 is unusual in con-is exothermic, while further reaction to give Me,AlNMe, and taining a monomeric lithium dibenzylamide fragment. Such MeLi (i.e., complete transmetallation) is endothermic. fragments generally convert to aza-ally1 derivatives, so its Lithium dibenzylamide has been a key compound in the development of lithium amide structural chemistry. Its crystal structure (solvent-free) ['] and those of a series of solvated derivatives (containing dioxanel'l, ether"], hexamethylphosphoramide[ll or tetrahydrofuran[2]) have contributed much to the understanding of how lithium amide structures self-assemble, for which there are now well-establishedIn addition, a novel form containing a phosphorus ylide molecule as a ligand has been characterisedl'll. Recently, we have introduced this lithium amide into the arena of heterobimetallic chemistry by synthesizing a mixed Group 1 (dilithium-dis~dium) [~] and a pair of mixed Group 1 -Group 2 (monolithium-monomagnesium and dilithium-monomagnesium)[61 derivatives.In this paper, we extend this heterobimetallic work to Group 3 in reporting the trimethylaluminium adduct [Me3AI * (PhCH2)'NLi . HN(CH2Ph)2] (l), which contains the first example of dibenzylamine acting as a ligand in a lithium dibenzylamide structure. As described below, other examples of lithium amide amine interactions are known. They are probably much more widespread than so far appreciated as conceivably they are formed but go undetected during the preparation of lithium amides from amines, either when the amine is genuinely in excess, or in equimolar reactions when the lithium reagent is added to the amine solution, a situation creating a temporary excess. Formally 1 was synthesised by insertion of the lithium amide into the N+AI dative bond of the simple amine adduct [Me3Al .