Identification, Characterization, and Electronic Structures of Interconvertible Cobalt–Oxygen TAML Intermediates
Deesha D. Malik, Wooyeol Ryu, Yujeong Kim, Gurjot Singh, Junhyeong Kim, Muniyandi Sankaralingam, Yong‐Min Lee, Mi Sook Seo, Mahesh Sundararajan, Daniel Ocampo, Michael Roemelt, Kiyoung Park, Sun Hee Kim, Mu‐Hyun Baik, Jason Shearer, Kallol Ray, Shunichi Fukuzumi, Wonwoo Nam
Abstract
The reaction of Li[(TAML)Co III ]·3H 2 O (TAML = tetraamido macrocyclic tetraanionic ligand) with iodosylbenzene at 253 K in acetone in the presence of redox-innocent metal ions (Sc(OTf) 3 and Y(OTf) 3 ) or triflic acid affords a blue species 1, which is converted reversibly to a green species 2 upon cooling to 193 K. The electronic structures of 1 and 2 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 ab initio theoretical studies. Complex 1 is best represented as an S = 1/2 [(Sol)(TAML •+ )Co III ---OH(LA)] − species (LA = Lewis/Brønsted acid and Sol = solvent), where an S = 1 Co(III) center is antiferromagnetically coupled to S = 1/2 TAML •+, which represents a one-electron oxidized TAML ligand. In contrast, complex 2, also with an S = 1/2 ground state, is found to be multiconfigurational with contributions of both the resonance forms [(H-TAML)Co IV ═O(LA)] − and [(H-TAML •+ )Co III ═O(LA)] −; H-TAML and H-TAML •+ represent the protonated forms of TAML and TAML •+ ligands, respectively. Thus, the interconversion of 1 and 2 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 ( S Co = 0) Co(III) and doublet ( S Co = 1/2) Co(IV) characters. The reactivities of 1 and 2 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 1 and 2 .