Synthesis and Reactivity of Diiminopyridine and Iminopyridine Cobalt Complexes

Alkylphosphine- and alkylpyridine-substituted 2,6-bis(imino)pyridines (pyridine diimines, PDI) have recently been used as polydentate, redox non-innocent ligands that support the development of highly active catalysts. The alkyl phosphine-substituted ligand, Ph2PPrPDI, was added to (Ph3P)3CoCl and subsequent reduction using excess sodium amalgam yielded (Ph2PPrPDI)Co. Electronic structure analysis revealed a cobalt(I) complex that features a singly reduced PDI chelate. Additionally, low valent Ph2PPrPDI complexes of Fe and Ni were synthesized and structurally characterized. Furthermore, a series of Ph2PPrPDI Mn, Fe, Co, and Ni complexes were investigated to evaluate ligand denticity and redox activity. Finally, the catalytic hydrosilylation of carbonyls was investigated to compare the activity of the series and determine whether electron count plays a role in catalysis. An analogous ligand system featuring alkylpyridine substituents, PyEtPDI, was added to CoCl2, affording a cobalt dichloride complex with the formula [(PyEtPDI)CoCl][Cl]. Single crystal X-ray diffraction revealed a high-spin cobalt(II) center that possesses an octahedral geometry and an outer-sphere chloride ion. Further treatment using 2 equivalents of NaEt3BH resulted in the formation of (κ4-N,N,N,N-PyEtIPCHMeNEtPy)Co, which has been verified by multinuclear NMR spectroscopy and single crystal X-ray diffraction. (κ4-N,N,N,N-PyEtIPCHMeNEtPy)Co was then used in the catalytic hydroboration of nitriles at ambient conditions to yield the corresponding N,N-diborylamines, which were used as precursors for amide formation.