Effect of Redox-Inactive Metal Ion–Nickel(III) Interactions on the Redox Properties and Proton-Coupled Electron Transfer Reactivity

Abstract

Mononuclear nickel­(II) and nickel­(III) complexes of a bisamidate-bisalkoxide ligand, (NMe4)2[NiII(HMPAB)] (1) and (NMe4)­[NiIII(HMPAB)] (2), respectively, have been synthesized and characterized by various spectroscopic techniques including X-ray crystallography. The reaction of redox-inactive metal ions (M n+ = Ca2+, Mg2+, Zn2+, Y3+, and Sc3+) with 2 resulted in 2-M n+ adducts, which was assessed by an array of spectroscopic techniques including X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and reactivity studies. The X-ray structure of Ca2+ coordinated to Ni­(III) complexes, 2-Ca2+T, was determined and exhibited an average Ni–Ca distance of 3.1253 Å, close to the metal ions’ covalent radius. XAS analysis of 2-Ca2+ and 2-Y3+ in solution further revealed an additional coordination to Ca and Y in the 2-M n+ adducts with shortened Ni–M distances of 2.15 and 2.11 Å, respectively, implying direct bonding interactions between Ni and Lewis acids (LAs). Such a short interatomic distance between Ni­(III) and M is unprecedented and was not observed before. EPR analysis of 2 and 2-M n+ species, moreover, displayed rhombic signals with g av > 2.12 for all complexes, supporting the +III oxidation state of Ni. The NiIII/NiII redox potential of 2 and 2-M n+ species was determined, and a plot of E 1/2 of 2-M n+ versus pK a of [M­(H2O) n ] m+ exhibited a linear relationship, implying that the NiIII/NiII potential of 2 can be tuned with different redox-inactive metal ions. Reactivity studies of 2 and 2-M n+ with different 4-X-2,6-ditert-butylphenol (4-X-DTBP) and other phenol derivatives were performed, and based on kinetic studies, we propose the involvement of a proton-coupled electron transfer (PCET) pathway. Analysis of the reaction products after the reaction of 2 with 4-OMe-DTBP showed the formation of a Ni­(II) complex (1a) where one of the alkoxide arms of the ligand is protonated. A pK a value of 24.2 was estimated for 1a. The reaction of 2-M n+ species was examined with 4-OMe-DTBP, and it was observed that the k 2 values of 2-M n+ species increase by increasing the Lewis acidity of redox-inactive metal ions. However, the obtained k 2 values for 2-M n+ species are much lower compared to the k 2 value for 2. Such a variation of PCET reactivity between 2 and 2-M n+ species may be attributed to the interactions between Ni­(III) and LAs. Our findings show the significance of the secondary coordination sphere effect on the PCET reactivity of Ni­(III) complexes and furnish important insights into the reaction mechanism involving high-valent nickel species, which are frequently invoked as key intermediates in Ni-mediated enzymatic reactions, solar-fuel catalysis, and biomimetic/synthetic transformation reactions.

Quadruple bridged-carbonate supported dodecanuclear [Ni8Ca4] coordination cluster

“…Selected bond lengths and angles are listed in Tables S3 and S4 (ESI †). These Ca-O bond distances are similar to those found in the only eight reported clusters with Ni-O-Ca bonds, where Ca-O bond lengths vary in the range of 2.32-2.36 Å for the only example with coordination number six, 60 [57][58][59]61 and 2.34-2.55 Å in the only example with coordination number nine. 62 The two independent deprotonated Schiff base ligands present in the structure of the Ni 8 Ca 4 ring show the same coordination mode.…”

“…In fact, a search in the CCDC database (July 2023) shows that there are only eight examples with a Ni II -O-Ca II moiety. [56][57][58][59][60][61][62] Among these eight complexes, there are four NiCa dimers, 56,58,59,62 one Ni 2 Ca trimer, 57 one Ni 3 Ca tetramer 60 and two Ni 5 Ca hexamers, 61 showing that the field of polynuclear Ni-Ca clusters remains almost unexplored.…”

Structural and Spectroscopic Characterization of Copper(III) Complexes and Subsequent One-Electron Oxidation Reaction and Reactivity Studies

“…It has been shown recently that Ni III -alkoxide complexes can participate in PCET reactions, where the alkoxide arm works as the proton acceptor. , It was shown in a study that Ni II –O­( H )­R possesses a high BDFE value of 91 kcal/mol, which is higher than many terminal Mn n + (O) and Fe n + (O) complexes, thus implying that Ni III –O alkoxide can work as a potent oxidizing agent. Understanding the reactivity of high-valent metal-oxo (M n + (O)) species are often correlated with the ground-state thermodynamic properties.…”

“…Spectrophotometric titration experiments of 1a using 1,5,7-triazabicyclo[4.4.0]­dec-5-ene (TBD, p K a = 26.03) base revealed a p K a value of ∼25.4 (Figure S35). The value is significantly higher than the conjugate acid of a Cu III –OH 55 or a (ligand radical)­Cu II –OH complex but comparable to the p K a value of the alkoxide arm-protonated Ni II of a bisamidate–bisalkoxide ligand or bispyridine–bisalkoxide ligand . To get an estimation of the BDFE of the O–H bond (i.e., in 1a ) formed upon transfer of H + /e – to 3 , we envisioned determining the value using the E 1/2 of the Cu III /Cu II redox couple and the p K a value according to eq …”

“…41,55−57 It has been shown recently that Ni III -alkoxide complexes can participate in PCET reactions, where the alkoxide arm works as the proton acceptor. 58,59 It was shown in a study that Ni II − O(H)R possesses a high BDFE value of 91 kcal/mol, 59 However, such information about the reactivity of mononuclear Cu(III) complexes is limited. Thus, we have undertaken to investigate the PCET reactivity study of the Cu(III) complex ( 3).…”