Remarkable Acid Catalysis in Proton-Coupled Electron-Transfer Reactions of a Chromium(III)-Superoxo Complex

Abstract

Much enhanced acid catalysis was observed in oxygen atom transfer (OAT) reactions by a mononuclear nonheme Cr(III)-superoxo complex, [(Cl)(TMC)Cr(O)] (1), in the presence of triflic acid. In the acid-catalyzed reactions, the reactivity of 1 in OAT of thioanisole was enhanced significantly, showing more than 10-fold acceleration in rate. Electron transfer (ET) from electron donors to 1 also occurred only in the presence of HOTf. The enhanced reactivity of 1 by HOTf was explained by proton-coupled electron transfer from electron donors, such as ferrocene, to 1 in light of the Marcus theory of ET. The present study reports for the first time the dramatic proton effect on the chemical properties of metal-superoxo species.

Acid Catalysis via Acid‐Promoted Electron Transfer

“…•-)] + and outer-sphere one-electron donors, which are the coordinatively saturated metal complexes. 126 The difference of 3.5 in the log K values between thioanisole and [Fe II L 3 ] 2 + was determined, which agrees that the observed log k ox value of thioanisole (red point no. 11) is 3.5 times bigger than the log k et value in blue line (Figure 16).…”

“…•-)] + in the presence of HOTf (1.0 mM) in CH 3 CN at 233 K was determined to be 3.5(3) M −1 s −1 , which is 10 4 times bigger than that in the absence of HOTf (3.6 × 10 −4 M −1 s −1 ). 126,127 As shown in Figure 14, the k ox value increased with an increase in HOTf concentration, which is proportional to [HOTf] 2 [Eq. (13)].…”

“…•-)] + is remarkably shifted from −0.52 V to 1.12 V in the absence and presence of HOTf (2.5 mM), respectively. 126 The rate constants (k et ) of HOTf-promoted ET from the one-electron donors to [(Cl)(L3)Cr III (O 2…”

“…Acid Catalysis via Acid-Promoted Electron Transfer BULLETIN OF THE KOREAN CHEMICAL SOCIETY two protons, following the formation of the precursor complex and the subsequent O O bond cleavage by thioanisole radical cation to form the sulfoxide and Cr IV -oxo species, accompanied by release of two protons to complete the acid catalysis (Scheme 8). 126 ET from Me 8 Fc to [(Cl)(L3)Cr III (O 2 •-)] + is also accelerated not only by H + but also by metal ions such as Ca 2+ (Figure 17). 130 , Al 3+ , Sc 3+ and Ce 4+ ) (dpaq = 2-[bis(pyridin-2-ylmethyl)] amino-N-quinolin-8-yl-acetamidate) in CH 3 CN.…”

Cooperative activating effects of metal ion and Brønsted acid on a metal oxo species

“…11 The strong Lewis acid Sc 3+ and the strong Brønsted acid CF 3 SO 3 H have been used to activate Fe(IV) and Mn(IV) oxo complexes, 12,13 as well as metal superoxo species. 14,15 There is also much interest in the use of the relatively weak Lewis acid, Ca 2+ , to activate M=O because of its relevance to the CaMn 4 O 5 cluster in Photosystem II; although as expected, its effect is usually much smaller than those of Sc 3+ or BF 3 . For example, Agapie et al reported that Ca 2+ and other metal ions can increase the reduction potentials (E 0 ) of manganese oxo clusters, and there is a linear correlation between E 0 and pK a of the metal ions.…”