Identification, Characterization, and Electronic Structures of Interconvertible Cobalt-Oxygen TAML Intermediates.

The reaction of Li[(TAML)Co]·3HO (TAML = tetraamido macrocyclic tetraanionic ligand) with iodosylbenzene at 253 K in acetone in the presence of redox-innocent metal ions (Sc(OTf) and Y(OTf)) or triflic acid affords a blue species , which is converted reversibly to a green species upon cooling to 193 K. The electronic structures of and have been determined by combining advanced spectroscopic techniques (X-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), X-ray absorption spectroscopy/extended X-ray absorption fine structure (XAS/EXAFS), and magnetic circular dichroism (MCD)) with theoretical studies. Complex is best represented as an = 1/2 [(Sol)(TAML)Co---OH(LA)] species (LA = Lewis/Brønsted acid and Sol = solvent), where an = 1 Co(III) center is antiferromagnetically coupled to = 1/2 TAML, which represents a one-electron oxidized TAML ligand. In contrast, complex , also with an = 1/2 ground state, is found to be multiconfigurational with contributions of both the resonance forms [(H-TAML)Co═O(LA)] and [(H-TAML)Co═O(LA)]; H-TAML and H-TAML represent the protonated forms of TAML and TAML ligands, respectively. Thus, the interconversion of and is associated with a LA-associated tautomerization event, whereby H shifts from the terminal -OH group to TAML with the concomitant formation of a terminal cobalt-oxo species possessing both singlet ( = 0) Co(III) and doublet ( = 1/2) Co(IV) characters. The reactivities of and at different temperatures have been investigated in oxygen atom transfer (OAT) and hydrogen atom transfer (HAT) reactions to compare the activation enthalpies and entropies of and .