Reactions of chelating alkali metal amides with the Grignard reagent nBuMgCl: crystal structures and solution 1H NMR spectroscopic studies of the expected mono(amido)-product [PhCH2(Me2NCH2CH2)NMgnBu]2 and the unexpected bis(amido)-product {[Ph(2-Pyr)N]2M9 · (THF)2}

Chelating alkali metal amides derived from metallation of the secondary amines PhCH2(Me2NCH2CH2)NH and Ph(2-Pyr)NH, react with the Grignard reagent (n)BuMgCl in 1:1 molar ratios to afford mono(amido) [PhCH2(Me2NCH2CH2)NMgnBu]2 1 and bis(amido) {[Ph(2-Pyr)N]2Mg.(THF)2} 2 (where THF = tetrahydrofuran) respectively. X-Ray crystallography shows 1 to be dimeric having an (amido N-Mg)2 ring, the central one of a fused tricyclic core, with the other two arising from internal complexation by the tertiary amine nitrogen atoms. Butyl ligands complete the pseudo-tetrahedral coordination of the magnesium atoms. Variable temperature H-1 NMR spectroscopic studies indicate that this structure retains its compact, highly rigid nature in arene solution, rendering it resistant to external complexation by THF. In contrast, the crystal structure of 2 is monomeric, with a pseudo-octahedral magnesium centre coordinated by two bidentate Ph(2-Pyr)N- anions and two THF molecules. Bis(amido) 2 appears to result from the THF-induced disproportionation of alkyl(amido) [Ph(2-Pyr)NMgnBu]2, a dimer akin to 1, but decidedly more flexible sterically and therefore susceptible to attack by external donor molecules.