Synthesis and Characterization of a Masked Terminal Nickel‐Oxide Complex

Abstract

AbstractIn exploring terminal nickel‐oxo complexes, postulated to be the active oxidant in natural and non‐natural oxidation reactions, we report the synthesis of the pseudo‐trigonal bipyramidal NiII complexes (K)[NiII(LPh)(DMF)] (1[DMF]) and (NMe4)2[NiII(LPh)(OAc)] (1[OAc]) (LPh=2,2’,2’’‐nitrilo‐tris‐(N‐phenylacetamide); DMF=N,N‐dimethylformamide; −OAc=acetate). Both complexes were characterized using NMR, FTIR, ESI‐MS, and X‐ray crystallography, showing the LPh ligand to bind in a tetradentate fashion, together with an ancillary donor. The reaction of 1[OAc] with peroxyphenyl acetic acid (PPAA) resulted in the formation of [(LPh)NiIII−O−H⋅⋅⋅OAc]2−, 2, that displays many of the characteristics of a terminal Ni=O species. 2 was characterized by UV‐Vis, EPR, and XAS spectroscopies and ESI‐MS. 2 decayed to yield a NiII‐phenolate complex 3 (through aromatic electrophilic substitution) that was characterized by NMR, FTIR, ESI‐MS, and X‐ray crystallography. 2 was capable of hydroxylation of hydrocarbons and epoxidation of olefins, as well as oxygen atom transfer oxidation of phosphines at exceptional rates. While the oxo‐wall remains standing, this complex represents an excellent example of a masked metal‐oxide that displays all of the properties expected of the ever elusive terminal M=O beyond the oxo‐wall.