Chemical Oxidation of a Coordinated PNP-Pincer Ligand Forms Unexpected Re–Nitroxide Complexes with Reversal of Nitride Reactivity

Abstract

Because of the thermodynamic demands of N2 cleavage, N2-derived nitride complexes are often unreactive. The development of multistep N2 functionalization reactions hinges on methods for modulating nitride reactivity with supporting ligands. Here, we describe the reactions of N2-derived Re–nitride complexes, including the first Re nitrides supported by a nitroxide-containing pincer ligand, and unusual examples of Re6+–nitride complexes. The previously reported N2-derived complex (PNP)Re(N)(Cl) (PNP = N(CH2CH2PtBu2)2) can be oxidized by O atom transfer to the backbone amide to form a novel nitroxide-pincer complex or by 1e– to form a rare S = 1/2 Re6+–nitride complex. The Re–nitrido interaction in a series of Re- and ligand-oxidized complexes is characterized using 15N NMR spectroscopy, IR spectroscopy, and DFT calculations, and shows changes in the Re–N bond order from both ligand- and metal-centered oxidations. Chemical oxidation of the supporting ligand to form a nitroxide-pincer ligand results in subtle electronic changes at Re and a more electron-deficient nitride ligand. Combined ligand- and metal-centered oxidation to form a Re6+–nitroxide complex results in a reversal of reactivity at the nitride ligand from nucleophilic to electrophilic. These systematic electronic structure and reactivity studies demonstrate methods for inducing reactivity in N2-derived nitride complexes.