Ni 2,2'-bipyridine (bpy) complexes are commonly employed photoredox catalysts of bondforming reactions in organic chemistry. However, the mechanisms by which they operate are still under investigation. One potential mode of catalysis is via entry into Ni(I)/Ni(III) cycles, which can be made possible by light-induced, excited state Ni(II)-C bond homolysis. Here we report experimental and computational analyses of a library of Ni(II)-bpy aryl halide complexes, Ni( R bpy)( R′ Ph)Cl (R = MeO, t-Bu, H, MeOOC; R′ = CH3, H, OMe, F, CF3), to illuminate the mechanism of excited state bond homolysis. At given excitation wavelengths, photochemical homolysis rates span two orders of magnitude across these structures and correlate linearly with Hammett parameters of both bpy and aryl ligands, reflecting structural control over key metal-to-ligand charge transfer (MLCT) and ligand-to-metal charge transfer (LMCT) excited state potential energy surfaces (PESs). Temperature-and wavelength-dependent investigations reveal moderate excited state barriers (ΔH ‡ ~4 kcal mol -1 ) and a minimum energy excitation threshold (~55 kcal mol -1 , 525 nm), respectively. Correlations to electronic structure calculations further support a mechanism in which repulsive triplet excited state PESs featuring a critical aryl-to-Ni LMCT lead to bond rupture. Structural control over excited state PESs provides a rational approach to utilize photonic energy and leverage excited state bond homolysis processes in synthetic chemistry.

Coordination of tris(pentafluorophenyl)borane to hexacyanoferrate shifts the formal potential by over 2.1 V while maintaining electrochemical reversibility.

One of the most oft-employed methods for C-C bond formation involving the coupling of vinyl-halides with aldehydes catalyzed by Ni and Cr (Nozaki-Hiyama-Kishi, NHK) has been rendered more practical using an electroreductive manifold. Although early studies pointed to the feasibility of such a process those precedents were never applied by others due to cumbersome setups and limited scope. Here we show that a carefully optimized electroreductive procedure can enable a more sustainable approach to NHK, even in an asymmetric fashion on highly complex medicinally relevant systems. The e-NHK can even enable non-canonical substrate classes, such as redox-active esters, to participate with low loadings of Cr when conventional chemical techniques fail. A combination of detailed kinetics, cyclic voltammetry, and in situ UV-vis spectroelectrochemistry of these processes illuminates the subtle features of this mechanistically intricate process. File list (2)download file view on ChemRxiv e-NHK_TEXT Final_03192021.pdf (2.63 MiB) download file view on ChemRxiv e-NHK_SI_Final YG03192021.pdf (33.98 MiB)

We report the facile photochemical generation of a library of Ni(I)−bpy halide complexes (Ni(I)( R bpy)X (R = t-Bu, H, MeOOC; X = Cl, Br, I) and benchmark their relative reactivity toward competitive oxidative addition and off-cycle dimerization pathways. Structure−function relationships between the ligand set and reactivity are developed, with particular emphasis on rationalizing previously uncharacterized ligand-controlled reactivity toward high energy and challenging C(sp 2 )−Cl bonds. Through a dual Hammett and computational analysis, the mechanism of the formal oxidative addition is found to proceed through an S N Ar-type pathway, consisting of a nucleophilic two-electron transfer between the Ni(I) 3d(z 2 ) orbital and the C aryl −Cl σ* orbital, which contrasts the mechanism previously observed for activation of weaker C(sp 2 )−Br/I bonds. The bpy substituent provides a strong influence on reactivity, ultimately determining whether oxidative addition or dimerization even occurs. Here, we elucidate the origin of this substituent influence as arising from perturbations to the effective nuclear charge (Z eff ) of the Ni(I) center. Electron donation to the metal decreases Z eff , which leads to a significant destabilization of the entire 3d orbital manifold. Decreasing the 3d(z 2 ) electron binding energies leads to a powerful two-electron donor to activate strong C(sp 2 )−Cl bonds. These changes also prove to have an analogous effect on dimerization, with decreases in Z eff leading to more rapid dimerization. Ligand-induced modulation of Z eff and the 3d(z 2 ) orbital energy is thus a tunable target by which the reactivity of Ni(I) complexes can be altered, providing a direct route to stimulate reactivity with even stronger C−X bonds and potentially unveiling new ways to accomplish Ni-mediated photocatalytic cycles.

While the icosahedral closo-[B 12 H 12 ] 2cluster does not display reversible electrochemical behavior, perfunctionalization of this species via substitution of all twelve B-H vertices with alkoxy or benzyloxy (OR) substituents engenders reversible redox chemistry, providing access to clusters in the dianionic, monoanionic, and neutral forms. Here, we evaluated the electrochemical behavior of the electron-rich B 12 (O-3-methylbutyl) 12 (1) cluster and discovered that a new reversible redox event that gives rise to a fourth electronic state is accessible through one-electron oxidation of the neutral species. Chemical oxidation of 1 with [N(2,4-Br 2 C 6 H 3) 3 ] •+ afforded the isolable [1] •+ cluster, which is the first example of an open-shell cationic B 12 cluster in which the unpaired electron is proposed to be delocalized throughout the boron cluster core. The oxidation of 1 is also chemically reversible, where treatment of [1] •+ with ferrocene resulted in its reduction back to 1. The identity of [1] •+ is supported by EPR, UV-vis, multinuclear NMR (1 H, 11 B), and X-ray photoelectron spectroscopic characterization. and characterization data for all new compounds is available free of charge via the Internet at http://pubs.acs.org.