Redox-Active Ligands Facilitate Bimetallic O₂Homolysis at Five-Coordinate Oxorhenium(V) Centers

Abstract

Five-coordinate oxorhenium(V) anions with redox-active catecholate and amidophenolate ligands are shown to effect clean bimetallic cleavage of O(2) to give dioxorhenium(VII) products. A structural homologue with redox-inert oxalate ligands does not react with O(2). Redox-active ligands lower the kinetic barrier to bimetallic O(2) homolysis at five-coordinate oxorhenium(V) by facilitating formation and stabilization of intermediate O(2) adducts. O(2) activation occurs by two sequential Re-O bond forming reactions, which generate mononuclear eta(1)-superoxo species, and then binuclear trans-mu-1,2-peroxo-bridged complexes. Formation of both Re-O bonds requires trapping of a triplet radical dioxygen species by a cis-[Re(V)(O)(cat)(2)](-) anion. In each reaction the dioxygen fragment is reduced by 1e(-), so generation of each new Re-O bond requires that an oxometal fragment is oxidized by 1e(-). Complexes containing a redox-active ligand access a lower energy reaction pathway for the 1e(-) Re-O bond forming reaction because the metal fragment can be oxidized without a change in formal rhenium oxidation state. It is also likely that redox-active ligands facilitate O(2) homolysis by lowering the barrier to the formally spin-forbidden reactions of triplet dioxygen with the closed shell oxorhenium(V) anions. By orthogonalizing 1e(-) and 2e(-) redox at oxorhenium(V), the redox-active ligand allows high-valent rhenium to utilize a mechanism for O(2) activation that is atypical of oxorhenium(V) but more typical for oxygenase enzymes and models based on 3d transition metal ions: O(2) cleavage occurs by a net 2e(-) process through a series of 1e(-) steps. The implications for design of new multielectron catalysts for oxygenase-type O(2) activation, as well as the microscopic reverse reaction, O-O bond formation from coupling of two M=O fragments for catalytic water oxidation, are discussed.

Nickel(ii) radical complexes of thiosemicarbazone ligands appended by salicylidene, aminophenol and aminothiophenol moieties

“…[5][6][7][8][9][10][11]23 The phenoxyl/phenolate redox couple is unexpectedly irreversible, in contrast with what is commonly observed in Ni(II) salen complexes involving sterically hindered phenolates. 23 This behaviour most likely results the higher lability of the proton of the Cβ (C7 in Fig.…”

Reverse Catalase Reaction: Dioxygen Activation via Two-Electron Transfer from Hydroxide to Dioxygen Mediated By a Manganese(III) Salen Complex

“…21 It was also reported that redox-active ligands promote air oxidation of an oxorhenium-(V) complex to a dioxorhenium(VII) product. 22 In the case of photoactive iron(III) or manganese(III) complexes such as porphyrin complexes, the photoirradiation was employed for O 2 activation. 23−28 The present study describes another attractive route of O 2 activation, in which aqueous alkaline solution is employed for O 2 activation.…”

O2 Activation and Catalytic Alcohol Oxidation by Re Complexes with Redox-Active Ligands: A DFT Study of Mechanism

“…Neese and co-workers applied Density Functional Theory (DFT) and ab initio methods for examining various trends in the experimental spectra of transition metal-complexes involving noninnocent ligands, 32 and for exploring systems of bioinorganic interest. 33,39 Recent experimental studies on the oxidation catalysis by the complex [Re V (O)(cat) 2 ] − with redox-active catecholate ligands showed 12,40 41 oxo−molybdenum, 42,43 and oxo− rhenium). 44 The characterization of the redox-active catecholate as electron-reservoir for O 2 homolysis was supplemented by computational studies.…”

“…44 The characterization of the redox-active catecholate as electron-reservoir for O 2 homolysis was supplemented by computational studies. 40 Yet, several mechanistic details remained unclear, especially regarding the kinetics: (1) How does the trans−cis isomerization occur in the initial phase? (2) What is the role of O 2 during this isomerization?…”

“…These computational results corroborate a facile radical pathway for O 2 activation, as previously proposed on the basis of experimental findings. 40 The entropy correction applied is described in Section 4.2, Computational Details. Figure S1 of the Supporting Information (SI) provides comparative energy profiles for systems in the gas phase and in methanol.…”