Structural Diversity of Lithium Oligo-α-Pyridylamides

Lithium oligo-alpha-pyridylamides are useful intermediates in coordination chemistry. Upon trans-metalation they have afforded a variety of extended metal atom chains (EMACs), which are currently investigated as molecular wires and single-molecule magnets. However, structural information on this class of compounds is scarce. Two trilithium salts of a new, sterically encumbered oligo-alpha-pyridylamido ligand were isolated in crystalline form and structurally characterized in the solid state and in solution. Lithiation of N-2-(trimethylsilyl)-N-6-{6-[(trimethylsilyl)amino]pyridin-2-yl}pyridine-2,6-diamine (H3L) with n-BuLi in thf yielded dimeric adduct [Li6L2(thf)(6)] (1), which was crystallized from n-hexane/thf as 1 center dot C6H14. Crystals of a tetra-thf solvate with formula [Li6L2(thf)(4)] (2) were also obtained. The compounds feature two twisted L3- ligands exhibiting a cis-cis conformation and whose five nitrogen donors are all engaged in metal coordination. The six Li+ ions per molecule display coordination numbers ranging from 3 to 5. Compound 1 center dot C6H14 was investigated by multinuclear 1D and 2D NMR spectroscopy, including H-1 DOSY experiments, which indicated retention of the dimeric structure in benzene-d(6) solution. To the best of our knowledge, 1 and 2 are the longest-chain lithium oligo-alpha-pyridylamides structurally authenticated so far, thereby qualifying as appealing intermediates to access high-nuclearity EMACs by trans-metalation.

Automated summary

Lithium oligo-alpha-pyridylamides are useful intermediates in coordination chemistry, and can be used to synthesize extended metal atom chains with variable coordination numbers.