Controlling the selectivity of 4e–/4H+ reduction of oxygen over 2e–/2H+ reduction is a key challenge in making efficient catalysts for fuel cell cathodes. A tyrosine residue poised over the active site of cytochrome c oxidase (CcO) has been demonstrated to control the hydrogen atom transfer reactions and cleavage of the O–O bond of a Fe–O–O–Cu moiety to yield water. In a couple of small-molecule iron complexes supported by porphyrin derivatives, it was shown that the presence of protonation sites at the secondary coordination sphere plays an important role in directing the selectivity and rate of the oxygen reduction reaction (ORR). In this study, we designed and synthesized a mononuclear CoIII complex (1) of a bis-pyridine-bis-oxime ligand where the oxime site can participate in reversible proton exchange reactions. Electrocatalytic ORR of 1 was investigated in aqueous buffer solutions and acetonitrile containing trifluoroacetic acid as the proton source. We observed that in a 0.1 M phosphate buffer solution (PBS), 1 is selective for 4e–/4H+ reduction of O2 at pH 4, and the selectivity decreases with increasing the buffer medium’s pH, producing ca. 75% H2O at pH 7. However, in a 0.1 M acetate buffer solution (ABS), 1 remained highly selective for the cleavage of the O–O bond to produce H2O at pH 4 and pH 7. The overpotential (η) of H2O formation (ca. 0.8–0.65 V) decreased proportionally with increasing pH in PBS and ABS. In acetonitrile, 1 remained highly selective for 4e–/4H+ reduction for electrocatalytic and chemical ORR. An overpotential of 760 mV was estimated for H2O production in acetonitrile. Kinetic analysis suggests the first-order dependence of catalyst concentration on the reaction rate at 25 °C. However, the formation of a peroxo-bridged dinuclear cobalt­(III) complex was noted as a reaction intermediate in the ORR pathway in acetonitrile at −40 °C. We conjecture that the oxime scaffold of the ligand works as a proton exchanging site and assists in the proton-coupled electron transfer (PCET) reactivity to cleave the O–O bond in the acidic buffer solutions and acetonitrile, further corroborated by theoretical studies. Density functional theory (DFT) calculation suggests that the acetate ion works as a mediator at pH 7.0 for transferring a proton from the oxime scaffold to the distal oxygen of the CoIII(OOH) intermediate, responsible for high selectivity toward 4e–/4H+ reduction of O2.

The formation of Cu(III) species are often invoked as the key intermediate in Cu-catalyzed organic transformation reactions. In this study, we synthesized Cu(II) (1) and Cu(III) (3) complexes supported by a bisamidate−bisalkoxide ligand consisting of an ortho-phenylenediamine (o-PDA) scaffold and characterized them through an array of spectroscopic techniques, including UV−visible, electron paramagnetic resonance, X-ray crystallography, and 1 H nuclear magnetic resonance (NMR) and X-ray absorption spectroscopy. The Cu−N/O bond distances in 3 are ∼0.1 Å reduced compared to 1, implying a significant increase in 3's overall effective nuclear charge. Further, a Cu(III) complex (4) of a bisamidate−bisalkoxide ligand containing a transcyclohexane-1,2-diamine moiety exhibits nearly identical Cu−N/O bond distances to that of 3, inferring that the redox-active o-PDA backbone is not oxidized upon one-electron oxidation of the Cu(II) complex (1). In addition, a considerable difference in the 1s → 4p and 1s → 3d transition energy was observed in the X-ray absorption near-edge structure data of 3 vs 1, which is typical for the metal-centered oxidation process. Electrochemical measurements of the Cu(II) complex (1) in acetonitrile exhibited two consecutive redox couples at −0.9 and 0.4 V vs the Fc + /Fc reference electrode. One-electron oxidation reaction of 3 further resulted in the formation of a ligand-oxidized Cu complex (3a), which was characterized in depth. Reactivity studies of species 3 and 3a were explored toward the activation of the C−H/O−H bonds. A bond dissociation free energy (BDFE) value of ∼69 kcal/mol was estimated for the O−H bond of the Cu(II) complex formed upon transfer of hydrogen atom to 3. The study represents a thorough spectroscopic characterization of high-valent Cu complexes and sheds light on the PCET reactivity studies of Cu(III) complexes.

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.

Understanding the effect of the local electrical field around the reaction center in enzymes and molecular catalysis is an important topic of research. Herein, we explored the electrostatic field exerted by the alkaline earth metal ions (M2+ = Mg2+, Ca2+, Sr2+, and Ba2+) around Fe in FeIII(Cl) complexes by experimental and computational investigations. M2+ coordinated dinuclear FeIII(Cl) complexes (1 2M) were synthesized and characterized by X-ray crystallography and different spectroscopic techniques. EPR and magnetic moment measurements exhibited the presence of high-spin FeIII centers in the 1 2M complexes. Electrochemical investigations revealed FeIII/FeII reduction potential values shifted anodically in 1 2M complexes compared to 1. Likewise, 2p3/2 and 2p1/2 peaks in the XPS data were found to shift positively in the 1 2M complexes, demonstrating that redox-inactive metal ions make FeIII more electropositive. However, nearly similar λmax values in the UV–vis spectra were observed in 1 and 1 2M complexes. The first-principles-based computational simulations further revealed the impact of M2+ on stabilizing 3d-orbitals of Fe. The distortion in Laplacian distribution (∇2ρ(r)) of electron density around M2+ also indicates the possibility of having Fe–M interactions in these complexes. The absence of a bond critical point between FeIII and M2+ ions in the 1 2M complexes indicates dominant through-space interaction between these metal centers. Experimental and computational studies collectively imply that the installation of internal electrostatic fields exerted by M2+ ions in 1 2M complexes alters the electronic structure of FeIII.