Elucidating the Mechanism of Excited State Bond Homolysis in Nickel–Bipyridine Photoredox Catalysts

Abstract

Ni 2,2’–bipyridine (bpy) complexes are commonly employed photoredox catalysts of bond-forming 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(Rbpy)(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.

The Role of Excited States of LNiII/III(Aryl)(Halide) Complexes in Ni–Halide Bond Homolysis in the Arylation of Csp3–H Bonds

“…No significant differences in the character of excited states responsible for the dissociation were observed when using dtbbpy compared to the bpy ligand (see Table S18), which validates using bpy as the ligand in these calculations. This result is also consistent with the similar behavior of the bpy and dtbbpy ligands in the MR calculations reported by Hadt et al 40,41 Multireference calculations were subsequently performed using MC-PDFT. 60 The total MC-PDFT electronic energy is the sum of the kinetic energy, nuclear attraction, and classical approximation to the electron−electron Coulomb energy from the MCSCF reference, and the exchange and correlation energies are obtained from an MC-PDFT on-top functional.…”

“…This hypothesis was revised by Hadt et al on the basis of photochemical experiments and multireference calculations. 40,41 The main conclusion was that the photoexcited 1 MLCT state can undergo intersystem crossing to repulsive triplet surfaces corresponding to the 3 MLCT state and to a ligand-to-metal charge transfer (LMCT) state. This LMCT state was suggested to be responsible for bond homolysis.…”

“…Despite all these remarkable advances, our understanding of the Ni−halide bond dissociation step is incomplete. Recent studies using multireference methods focused on the dissociation of the Ni−aryl 40,41 or Ni−ligand 42 bonds. To our knowledge, no studies focused on the dissociation of the Ni− halide bond with multireference methods.…”

Zinc(II) Complexes with Triplet Charge-Transfer Excited States Enabling Energy-Transfer Catalysis, Photoinduced Electron Transfer, and Upconversion

“…The ILCT states of our Zn II complexes complement the different types of photoactive excited states reported recently for first-row and other Earth-abundant transition-metal complexes, 28 which includes the classical MLCT states for d 6 complexes (Cr 0 , Mn I , Fe II , Co III ), 19,24,25,115−117 square-planar d 8 compounds (Ni II ) 26,118 and four-coordinate d 10 complexes (Cu I ), 3,78 ligand-to-metal charge-transfer (LMCT) states for Ti IV , Zr IV , 119,120 Mn IV , Fe III and Co III , 121−126121−126 metalcentered (MC) states for V III , Cr III and Co III , 127−132 as well as ligand-to-ligand charge-transfer (LLCT) excited states for twocoordinate Cu I complexes. 29−33 Given these findings, it seems reasonable to conclude that Zn II complexes with chargetransfer and triplet excited states would perhaps deserve greater attention in future studies aiming to discover new photophysics and photochemistry in first-row transition-metal complexes.…”

Overcoming Photochemical Limitations in Metallaphotoredox Catalysis: Red-Light-Driven C–N Cross-Coupling

“…Furthermore, they demonstrate that the rate of aryl−nickel bond homolysis decays with increasingly longer wavelengths, leading to decreased quantities of reduced arenes and biaryl byproducts. 17 We hypothesized that we could decrease the amount of hydrodehalogenation observed in dual Ni/PC by using lowerenergy light to decrease the rate of aryl−Ni bond homolysis. The utility of near-infrared (NIR) and deep red (DR) light for photoredox catalysis has recently been demonstrated by our group 24−26 and others.…”