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

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(CH₂CH₂P^tBu₂)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.