C–H Bond Activation by a Mononuclear Nickel(IV)-Nitrate Complex

Abstract

The recent focus on developing high-valent non-oxo-metal complexes for late transition metals has proven to be an effective strategy to study the rich chemistry of these high-valent species while bypassing the synthetic challenges of obtaining the oxo-metal counterparts. In our continuing work of exploring late transition metal complexes of unusually high oxidation states, we have obtained in the present study a formal mononuclear Ni(IV)-nitrate complex (2) upon 1-e − oxidation of its Ni(III) derivatives (1-OH and 1-NO 3 ). Characterization of these Ni complexes by combined spectroscopic and computational approaches enables deep understanding of their geometric and electronic structures, bonding interactions, and spectroscopic properties, showing that all of them are square planar complexes and exhibit strong πcovalency with the amido N-donors of the N3 ligand. Furthermore, results obtained from X-ray absorption spectroscopy and density functional theory calculations provide strong support for the assignment of the Ni(IV) oxidation state of complex 2, albeit with strong ligand-to-metal charge donation. Notably, 2 is able to oxidize hydrocarbons with C−H bond strength in the range of 76−92 kcal/mol, representing a rare example of high-valent late transition metal complexes capable of activating strong sp 3 C−H bonds.

Synthesis and Characterization of a Masked Terminal Nickel‐Oxide Complex

“…The measured Ni−O distance is in closest agreement (±0.08 Å) with [Ni III (OH)(L2)] 2− (L2=N,N’‐(pyridine‐2,6‐diyl bis (phenylmethylene)) bis (2,6‐dimethylaniline)) by Shanmugam et al. that is currently one of the few structurally characterized Ni III −OH complex [9h,i] …”

Preparation and Characterization of a Formally NiIV–Oxo Complex with a Triplet Ground State and Application in Oxidation Reactions

“…For example, in a recent study, the preparation of a Ni(IV)-nitrato complex was claimed. 52 Although the authors note a ∼0.7 eV blue-shift of Ni K-edge XAS features of this complex relative to the Ni(III) starting material, they also acknowledge the difficulties of making oxidation state assignments on the basis of XANES edge-shifts alone. Furthermore, the authors highlight that both formal Ni(III) and Ni(IV) complexes possess significant ligand noninnocence.…”

“…Correspondingly, claims of Ni­(IV) complexes in the literature have to be taken with some skepticism. For example, in a recent study, the preparation of a Ni­(IV)-nitrato complex was claimed . Although the authors note a ∼0.7 eV blue-shift of Ni K-edge XAS features of this complex relative to the Ni­(III) starting material, they also acknowledge the difficulties of making oxidation state assignments on the basis of XANES edge-shifts alone.…”

Rational Design of First-Row Transition Metal Complexes as the Catalysts for Oxidation of Arenes: A Homogeneous Approach

“…In nature, no nickel-containing enzyme is found to activate the C–H bonds in its active site. However, inspired by iron enzymes in oxygen activation chemistry (Cyt P450, sMMO etc.,) (vide supra), researchers have recently used several bioinspired nickel complexes as efficient catalysts for the oxidation of alkanes, olefins, sulfides, phosphines, etc., ,,− However, the activation of strong C–H bonds using nickel catalysts is less explored.…”