N − H bond dissociation free energy of a terminal iron phosphinimine

Abstract

The transfer of protons and electrons to metal-ligand multiply-bonded species is a key step in the mechanism of many bond activation processes. In this regard, terminally-bonded phosphinimides (PN) are isolobal analogues of oxos and imidos and should enable late transition metal complexes featuring terminal PNs to engage in H+/e− transfer processes. Recently, we developed a rigid, multidentate PN framework intended to stabilize terminally bonded PN ligands at late, first row transition metals. Herein, we report the synthesis, structure, and spectroscopic characterization of mononuclear FeIII complexes featuring exclusively terminal PN coordination. Magnetic susceptibility and EPR spectroscopic data are consistent with S = 5/2 spin ground states. Proton transfer reactivity both via 1,2-addition across a PN − FeIII moiety and 1,2-elimination from a ferric phosphinimine (PNH) moiety is observed. Hydrogen atom transfer reactivity is observed upon oxidation of the PN − FeIII complex, resulting in formation of a PNH − FeIII species. A combination of electrochemical, pK a measurement, and DFT studies support an N − H bond dissociation free energy of ∼90 kcal/mol for a PNH − FeIII species, and serve to highlight a reactive, transient PN − FeIV species. Combined, our results highlight terminal, late transition metal-PN moieties as redox-active superbases competent for efficient hydrogen atom abstraction processes.