Synthesis and reactivity of an isolable cobalt(I) complex containing a β-diketiminate-based acyclic tetradentate ligand

A model for cobalamin was synthesized using a new monoanionic tetradentate nitrogen donor ligand; 2-(4- tolyl)-1,3-bis(2-isopropylpyridyl)propenediimine (Tol- BDI ^(2-pp)2H) (1), which utilizes isopropylpyridines as pendant arms on a β-diketiminate (BDI) backbone. During the synthesis of 1, the rearrangement product, Tol-BDI ^(2-pp)(4-pp)H (2) was observed. Metalation of 1 with zinc iodide and cobalt chloride yielded the corresponding Tol-BDI ^(2-pp)2ZnI (3) and Tol-BDI ^(2-pp)2CoCl (4) complexes. The redox properties of 4 in comparison to cobalamin were examined through electrochemical studies. Electrochemical and bulk reduction of complex 4 gave a diamagnetic cobalt(I) complex, Tol-BDI ^(2-pp)2Co (5). Reactivity of 5 toward C-X bonds was investigated using methyl iodide and 1-iodo-2-(trimethylsilyl)acetylene, yielding Tol-BDI ^(2-pp)2Co(CH ₃)I and Tol-BDI ^(2-pp)2Co(C ₂Si(CH ₃) ₃)I respectively. Synthesis and characterization details for these complexes, including the crystal structure of 3, are reported.