Synthesis, Structure, and Hydroboration Reactivity of Anionic Nickel(0) Complexes Supported by Bidentate NHC-Pyridone Ligands

Abstract

A family of anionic, formally nickel(0) complexes supported by bidentate NHC–pyridone ligands is described. The unsymmetric chelating environment and capping [K(18-crown-6)]+ countercation allow isolation of single-component, monometallic complexes in high yield. The steric and electronic properties are assessed through a battery of experimental (NMR, IR, UV–vis, X-ray diffraction) and computational tools. Catalytic activity for highly branched-selective hydroboration of styrene with HBpin is demonstrated. Control experiments implicate an important role of the pyridone in establishing reactivity and regioselectivity, suggesting the potential to leverage secondary coordination sphere effects with these single-component precatalysts for reagent activation and delivery.

N-Heterocyclic carbenes as privileged ligands for nickel-catalysed alkene functionalisation

“…Ferrocene-containing alkene also participated well (114). Utilisation of an ortho-substituted pyridine substrate (113) which presents increased steric hindrance led to greater complexity, potentially attributable to the inhibition of coordination with the aluminium Lewis acid. Switching to a less bulky triethylaluminium Lewis acid was imperative in catalysing the required transformation, albeit with a lower yield due to formation of a competing C6-alkylated side product.…”

“…Detailed investigation into the mechanism revealed that the nickel-hydride complex [NiCp*H(IMes)] ( 278 From single-crystal diffraction analysis, the [K(18-crown-6)] + counterion of this anionic nickelate was observed to be in a contact ion pair with the pyridone oxygen, allowing for monomeric isolation of the complex instead of multimetallic aggregation and clustering induced by the pyridone. 113 The low-valent Ni 0 core is highly electron-rich, with an asymmetric electronic and steric coordination environment around the nickel centre. In their work, this class of complexes was demonstrated to exhibit exceptional catalytic activity towards the hydroboration of alkenes (with the evaluation of only styrenes 285 and bmethylstyrene 284), affording the desired hydroboration product in good yields and with excellent branched Markovnikov selectivity (Scheme 45).…”

“…In 2022, the Kennedy research group developed a class of mononuclear, anionic Ni 0 complexes 283 , stabilised by a unique bidentate NHC-pyridone ligand which exclusively coordinates to the Ni centre in a κ 2 -C,N coordination mode 113 (Scheme 45). From single-crystal diffraction analysis, the [K(18-crown-6)] + counterion of this anionic nickelate was observed to be in a contact ion pair with the pyridone oxygen, allowing for monomeric isolation of the complex instead of multimetallic aggregation and clustering induced by the pyridone.…”

“…Scheme 45 k 2 -C,N anionic NHC/pyridone Ni 0 complex for hydroboration of alkenes 113. Scheme 46 Allyl-Ni II (NHC)-catalysed hydrosilylation of styrenes 116.…”

Mechanistic insights and comparative analysis of Ru(ii)–NNC pincer complexes with anionic-, protic-, and classical-NHCs for transfer hydrogenation of ketones

“…31–38 Anionic NHCs are known to show great potential in the area of small molecule activation due to their ability to show cooperative interactions. 39–42 The more nucleophilic naked β-nitrogen offers a strong tendency towards catalytic activity in multi-metallic catalysis via an efficient substrate recognition and comparatively enhanced stability than protic NHCs. 22,34,43 The Brønsted basic nature of naked β-nitrogen may entail the re-protonation via acids, showing that the protic and anionic NHCs are interconvertible via acid/base assisted protonation/deprotonation, an important property that can be utilized in hydrogenation/dehydrogenation type catalysis.…”