Lessons from Isolable Nickel(I) Precursor Complexes for Small Molecule Activation

Small-molecule activation by transition metals is essential to numerous organic transformations, both biological and industrial. Creating useful metal-mediated activation systems often depends on stabilizing the metal with uncommon low oxidation states and low coordination numbers. This provides a redox-active metal center with vacant coordination sites well suited for interacting with small molecules. Monovalent nickel species, with their d(9) electronic configuration, are moderately strong one-electron reducing agents that are synthetically attractive if they can be isolated. They represent suitable reagents for closing the knowledge gap in nickel-mediated activation of small molecules. Recently, the first strikingly stable dinuclear beta-diketiminate nickel(I) precursor complexes were synthesized, proving to be suitable promoters for small-molecule binding and activation. They have led to many unprecedented nickel complexes bearing activated small molecules in different reduction stages. In this Account, we describe selected achievements in the activation of nitrous oxide (N2O), O-2, the heavier chalcogens (S, Se, and Te), and white phosphorus (P-4) through this beta-diketiminatonickel(I) precursor species. We emphasize the reductive activation of O-2, owing to its promise in oxidation processes. The one-electron-reduced O-2 activation product, that is, the corresponding beta-diketiminato-supported Ni-O-2 complex, is a genuine superoxonickel(II) complex, representing an important intermediate in the early stages of O-2 activation. It selectively acts as an oxygen-atom transfer agent, hydrogen-atom scavenger, or both towards exogenous organic substrates to yield oxidation products. The one-electron reduction of the superoxonickel(II) moiety was examined by using elemental potassium, beta-diketiminatozinc(II) chloride, and beta-diketiminatoiron(I) complexes, affording the first heterobimetallic complexes featuring a [NiO2M] subunit (M is K, Zn, or Fe). Through density functional theory (DFT) calculations, the geometric and electronic structures of these complexes were established and their distinctive reactivity, including the unprecedented monooxygenase-like activity of a bis(mu-oxo)nickel-iron complex, was studied. The studies have further led to other heterobimetallic complexes containing a [NiO2M] core, which are useful for understanding the influence of the heterometal on structure-reactivity relationships. The activation of N2O led directly to the hydrogen-atom abstraction product bis(mu-hydroxo)nickel(II) species and prevented isolation of any intermediate. In contrast, the activation of elemental S, Se, and Te with the same nickel(I) reagent furnished activation products with superchalcogenido E-2(-) (E is S, Se, one) and dichalcogenido E-2(2-) ligand in different activation stages. The isolable supersulfidonickel(II) subunit may serve as a versatile building block for the synthesis of heterobimetallic disulfidonickel(II) complexes with a [NiS2M] core. In the case of white phosphorus, the P-4 molecule has been coordinated to the nickel(I) center of dinuclea beta-diketiminatonickel(I) precursor complexes; however, the whole P-4 subunit is a weaker electron acceptor than the dichalcogen ligands E-2, thus remaining unreduced. This P-4 binding mode is rare and could open new doors for subsequent functionalization of P-4. Our advances in understanding how these small molecules are bound to a nickel(I) center and are activated for further transformation offer promise for designing new catalysts. These nickel-containing complexes offer exceptional potential for nickel-mediated transformations of organic molecules and as model compounds for mimicking active sites of nickel-containing metalloenzymes.