Hydrogen Atom Transfer Thermodynamics of Homologous Co(III)- and Mn(III)-Superoxo Complexes: The Effect of the Metal Spin State

Abstract

Systematic investigations on H atom transfer (HAT) thermodynamics of metal O 2 adducts is of fundamental importance for the design of transition metal catalysts for substrate oxidation and/or oxygenation directly using O 2 . Such work should help elucidate underlying electronic-structure features that govern the OO–H bond dissociation free energies (BDFEs) of metal-hydroperoxo species, which can be used to quantitatively appraise the HAT activity of the corresponding metal-superoxo complexes. Herein, the BDFEs of two homologous Co III - and Mn III -hydroperoxo complexes, 3-Co and 3-Mn , were calculated to be 79.3 and 81.5 kcal/mol, respectively, employing the Bordwell relationship based on experimentally determined p K a values and redox potentials of the one-electron-oxidized forms, 4-Co and 4-Mn . To further verify these values, we tested the HAT capability of their superoxo congeners, 2-Co and 2-Mn , toward three different substrates possessing varying O–H BDFEs. Specifically, both metal-superoxo species are capable of activating the O–H bond of 4-oxo-TEMPOH with an O–H BDFE of 68.9 kcal/mol, only 2-Mn is able to abstract a H atom from 2,4-di- tert -butylphenol with an O–H BDFE of 80.9 kcal/mol, and neither of them can react with 3,5-dimethylphenol with an O–H BDFE of 85.6 kcal/mol. Further computational investigations suggested that it is the high spin state of the Mn III center in 3-Mn that renders its OO–H BDFE higher than that of 3-Co , which features a low-spin Co III center. The present work underscores the role of the metal spin state being as crucial as the oxidation state in modulating BDFEs.

Oxygen versus Sulfur Coordination in Cobalt Superoxo Complexes: Spectroscopic Properties, O2 Binding, and H-Atom Abstraction Reactivity

“…However, a couple of studies are worth highlighting. A comparison of structurally analogous M III (O 2 · – ) complexes with different metal ions (M = Co, Mn) in the same [N 3 O 2 ] 2– ligand set found that the nature of M, and the concomitant ground spin states and metal–ligand covalencies, correlated with only small changes in the driving force for H-atom abstraction . In contrast, a series of Cr III (O 2 · – ) complexes with a tetraazacyclotetradecane macrocyclic chelate and different exogenous (Cl – ) or tethered (imidazole, pyridine) axial ligands trans to the O 2 · – ligand exhibited markedly different rates of HAT with C–H substrates …”

“…) complexes with different metal ions (M = Co, Mn) in the same [N 3 O 2 ] 2− ligand set found that the nature of M, and the concomitant ground spin states and metal−ligand covalencies, correlated with only small changes in the driving force for Hatom abstraction. 34 In contrast, a series of Cr III (O 2…”

“…For example, two structurally related dicopper superoxo complexes react with O–H bonds to generate Cu­(OO–H) species, but the difference in driving force was >10 kcal mol –1 for these two complexes. , Similarly, one example of a nonheme iron Fe III (O 2 · – ) complex was reported to be able to oxidize only weak O–H bonds (BDE < 70 kcal mol –1 ), while another example of a different Fe III (O 2 · – ) complex was reported to cleave C–H bonds with BDE > 90 kcal mol –1 . Recently, a pair of well-characterized Co and Mn hydroperoxo complexes bearing the same dialkoxide ligand framework were shown to have similar O–H bond strengths (79.3 vs 81.5 kcal mol –1 , respectively) despite having different metal ion identities and overall spin states . Computational studies have suggested various factors that may influence the H-atom abstraction reactivity of metal-superoxo species, including electrophilicity, thermodynamic driving force, and electronic coupling between the metal center and superoxo ligand. − Experimental studies are clearly needed to examine the influence of these different factors on the inherent oxidative reactivity of M­(O 2 · – ) species, as well as the effects of donor type, coordination number/geometry, steric profile, reaction medium, and redox potentials on H-atom transfer reactivity.…”

“…Recently, a pair of well-characterized Co and Mn hydroperoxo complexes bearing the same dialkoxide ligand framework were shown to have similar O–H bond strengths (79.3 vs 81.5 kcal mol –1 , respectively) despite having different metal ion identities and overall spin states . Computational studies have suggested various factors that may influence the H-atom abstraction reactivity of metal-superoxo species, including electrophilicity, thermodynamic driving force, and electronic coupling between the metal center and superoxo ligand. − Experimental studies are clearly needed to examine the influence of these different factors on the inherent oxidative reactivity of M­(O 2 · – ) species, as well as the effects of donor type, coordination number/geometry, steric profile, reaction medium, and redox potentials on H-atom transfer reactivity.…”

Electronic structure contributions to O–O bond cleavage reactions for MnIII-alkylperoxo complexes

“…One of the key gaps in knowledge hindering our understanding of the O-O cleavage step of these catalysts is the scarcity of Mn III -hydroperoxo complexes that have been trapped and spectroscopically characterized. [13][14][15][16][17] Nam and co-workers have reported Mn III -hydroperoxo complexes supported by tetramethylcyclam (TMC) and a ring-contracted derivative. 14,15 These complexes have intense electronic absorption bands in the UV region (∼400-300 nm) and resonance Raman data for [Mn III (OOH)(TMC)] 2+ showed an 16 O/ 18 O-sensitive band at 792 cm −1 .…”