Systematic Electronic Tuning on the Property and Reactivity of Cobalt–(Hydro)peroxo Intermediates

Abstract

A series of cobalt(III)–peroxo complexes, [CoIII(R2-TBDAP)(O2)]+ (1 R2 ; R2 = Cl, H, and OMe), and cobalt(III)–hydroperoxo complexes, [CoIII(R2-TBDAP)(O2H)(CH3CN)]2+ (2 R2 ), bearing electronically tuned tetraazamacrocyclic ligands (R2-TBDAP = N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)-p-R2-pyridinophane) were prepared from their cobalt(II) precursors and characterized by various physicochemical methods. The X-ray diffraction and spectroscopic analyses unambiguously showed that all 1 R2 compounds have similar octahedral geometry with a side-on peroxocobalt(III) moiety, but the O–O bond lengths of 1 Cl [1.398(3) Å] and 1 OMe [1.401(4) Å] were shorter than that of 1 H [1.456(3) Å] due to the different spin states. For 2 R2 , the O–O bond vibration energies of 2 Cl and 2 OMe were identical at 853 cm–1 (856 cm–1 for 2 H ), but their Co–O bond vibration frequencies were observed at 572 cm–1 for 2 Cl and 550 cm–1 for 2 OMe , respectively, by resonance Raman spectroscopy (560 cm–1 for 2 H ). Interestingly, the redox potentials (E 1/2) of 2 R2 increased in the order of 2 OMe (0.19 V) < 2 H (0.24 V) < 2 Cl (0.34 V) according to the electron richness of the R2-TBDAP ligands, but the oxygen-atom-transfer reactivities of 2 R2 showed a reverse trend (k 2: 2 Cl < 2 H < 2 OMe ) with a 13-fold rate enhancement at 2 OMe over 2 Cl in a sulfoxidation reaction with thioanisole. Although the reactivity trend contradicts the general consideration that electron-rich metal–oxygen species with low E 1/2 values have sluggish electrophilic reactivity, this could be explained by a weak Co–O bond vibration of 2 OMe in the unusual reaction pathway. These results provide considerable insight into the electronic nature–reactivity relationship of metal–oxygen species.

Mechanistic Insights into Nitrile Activation by Cobalt(III)–Hydroperoxo Intermediates: The Influence of Ligand Basicity

“…The significantly different p K a values could be mainly rationalized by the compression of Co–N ax distances in Me 3 -TPADP for the peroxo case, which could explain its higher basicity due to a strong ligand field. 44 , 57 Furthermore, the decreased redox potential of 2 (0.12 V vs Fc +/0 ) compared to that of 2 TBDAP (0.24 V vs Fc +/0 ) indicated that 2 was a more electron-rich complex ( Figure S24 ), 73 which was consistent with the previous studies that reported the decreases in redox potential values by the introduction of less bulky ligands. 50 − 53 These results were further supported by DFT calculations (vide infra).…”

“…The p K a value of 2 was estimated as 7.7, which was higher than that of 2 TBDAP (p K a = 2.1) (Figures S22 and S23). The significantly different p K a values could be mainly rationalized by the compression of Co–N ax distances in Me 3 -TPADP for the peroxo case, which could explain its higher basicity due to a strong ligand field. , Furthermore, the decreased redox potential of 2 (0.12 V vs Fc +/0 ) compared to that of 2 TBDAP (0.24 V vs Fc +/0 ) indicated that 2 was a more electron-rich complex (Figure S24), which was consistent with the previous studies that reported the decreases in redox potential values by the introduction of less bulky ligands. − These results were further supported by DFT calculations (vide infra). Thus, the substantially different p K a values and redox potentials of 2 and 2 TBDAP clearly illustrated that the control of basicity by ligand substitution greatly affected the nucleophilicity of the corresponding cobalt­(III)–hydroperoxo complexes and regulated the reactivity of nitrile activation.…”

Synthetic Advances for Mechanistic Insights: Metal–Oxygen Intermediates with a Macrocyclic Pyridinophane System

“…The cobalt­(III)–peroxo complexes, [Co III (R 2 -TBDAP)­(O 2 )] + ( 3 R2 ; R 2 = Cl, H, and OMe), were synthesized by reacting cobalt­(II) complexes, [Co II (R 2 -TBDAP)] 2+ , with H 2 O 2 in the presence of TEA . In the crystal structures, all 3 R2 exhibit octahedral geometries binding the peroxo ligands in a side-on (η 2 ) fashion to the cobalt center (Figure B).…”

“…10.1021/acs.inorgchem.3c0082620236271417149 Electronic tuning of a pyridinophane ligand was evaluated in properties and reactivities of the cobalt­(III)–peroxo and–hydroperoxo intermediates.…”

“…In contrast to 3 R2 , the cobalt­(III)–hydroperoxo complexes, [Co III (R 2 -TBDAP)­(O 2 H)­(CH 3 CN)] 2+ ( 10 R2 ), exhibited no spin crossover behavior. Instead, a linear trend was observed depending on the electronic nature of substituents . For example, cyclic voltammetry of 10 R2 showed that the redox potentials ( E 1/2 ) increase in the order of 10 OMe (0.19 V) < 10 H (0.24 V) < 10 Cl (0.34 V) (Table ).…”