Lithium, sodium and potassium picolyl complexes: syntheses, structures and bonding

Synthetically important for introducing a picolyl scaffold into a molecular construction, alkali metallated picoline (methylpyridine) complexes are also interesting in their own right for the diversity of their ligand-metal bonding possibilities. Here the syntheses of seven new such complexes are reported: namely three 4-picoline derivatives 4-picLi·Me₆TREN, 1, 4-picNa·Me₆TREN, 2, and [4-picK·2(4-picH)]_∞, 3; and four 2-picoline derivatives, 2-picLi·Me₆TREN, 4, 2-picLi·PMDETA, 4′, 2-picNa·Me₆TREN, 5, and [2-picK·PMDETA]₂, 6′ [where pic = NC₅H₄(CH₂); Me₆TREN = tris(N,N-dimethyl-2-aminoethyl)amine, (Me₂NCH₂CH₂)₃N; PMDETA = N,N,N′,N′′,N′′-pentamethyldiethylenetriamine, (Me₂NCH₂CH₂)₂NMe]. X-ray crystallographic studies establish that the lighter alkali metal complexes 1, 2, 4′ and 5 adopt monomeric structures in contrast to the polymeric and dimeric arrangements adopted by potassium complexes 3 and 6′ respectively. All complexes have also been characterized by solution NMR spectroscopy (¹H, ¹³C, and where relevant ⁷Li). This study represents the first example of sodium and potassium picolyl complexes to be isolated and characterized. DOSY (Diffusion-Ordered Spectroscopy) experiments performed on 4 and 4′ suggest both compounds retain their monomeric constitutions in C₆D₆solution. Discussion focuses on the influence of the metal and neutral donor molecule on the structures and the nature of the ligand-metal (enamido versus aza-allylic) interactions.