Redox-active ligands possess the ability to accept or donate electrons while remaining bound to a metal center. Ruthenium complexes have demonstrated augmented reactivity when using fully oxidized, acceptor type redox-active ligands, however, the cases involving the reduced type analogues are sparse. A novel, electronically rich tetradentate redox-active ligand based around an ortho-phenylenediamine (OPDA) backbone featuring pendant diphenylphosphino arms was developed and successfully bound to a ruthenium II center. The resulting complexes of the form (N₂P₂)RuL(L = PPh₃, CO, and MeCN) were fully characterized and their reactivity towards various substrates were characterized. Addition of iodine to reduced type complexes (L = PPh₃, and MeCN) yielded the corresponding (N₂P₂)RuI₂ species which was fully characterized by x-ray diffraction (XRD), nuclear magnetic resonance spectroscopy (NMR), and cyclic voltammetry (CV). Analysis of the data supports the idea that the oxidative addition proceeds via participation from the ligand-based electrons rather than a formal two-electron oxidation of the metal-center. Likewise, addition of an equivalent of methyl iodide yielded the (N₂P₂)RuCH₃I complex in a similar manner. Surprisingly, the addition of MeOTf or primary alkyl bromides to the (N₂P₂)RuMeCN complex did not alkylate the metal, but rather attacked the ligand’s nitrogen center. The influences of this divergent reactivity including effects of the ancillary ligand, leaving group and reaction conditions will be discussed along with the implications for future research involving reduced-type redox-active ligands for investigating fundamental organometallic reactions.

Alan Heyduk

Univ of California

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