Temperature Dependence of the Catalytic Two- versus Four-Electron Reduction of Dioxygen by a Hexanuclear Cobalt Complex

Abstract

The synthesis and characterization of a hexanuclear cobalt complex 1 involving a nonheme ligand system, L1, supported on a SnO stannoxane core are reported. Complex 1 acts as a unique catalyst for dioxygen reduction, whose selectivity can be changed from a preferential 4e/4H dioxygen-reduction (to water) to a 2e/2H process (to hydrogen peroxide) only by increasing the temperature from -50 to 25 °C. A variety of spectroscopic methods (Sn-NMR, magnetic circular dichroism (MCD), electron paramagnetic resonance (EPR), SQUID, UV-vis absorption, and X-ray absorption spectroscopy (XAS)) coupled with advanced theoretical calculations has been applied for the unambiguous assignment of the geometric and electronic structure of 1. The mechanism of the O-reduction reaction has been clarified on the basis of kinetic studies on the overall catalytic reaction as well as each step in the catalytic cycle and by low-temperature detection of intermediates. The reason why the same catalyst can act in either the two- or four-electron reduction of O can be explained by the constraint provided by the stannoxane core that makes the O-binding to 1 an entropically unfavorable process. This makes the end-on μ-1,2-peroxodicobalt(III) intermediate 2 unstable against a preferential proton-transfer step at 25 °C leading to the generation of HO. In contrast, at -50 °C, the higher thermodynamic stability of 2 leads to the cleavage of the O-O bond in 2 in the presence of electron and proton donors by a proton-coupled electron-transfer (PCET) mechanism to complete the O-to-2HO catalytic conversion in an overall 4e/4H step. The present study provides deep mechanistic insights into the dioxygen reduction process that should serve as useful and broadly applicable principles for future design of more efficient catalysts in fuel cells.

Cobalt‐Exchanged Poly(Heptazine Imides) as Transition Metal–Nx Electrocatalysts for the Oxygen Evolution Reaction

“…Moreover, when comparing the IR spectra with polymeric carbon nitride (CN), first the vibration of NH x groups decreases in both PHI and PHI‐Co, while several new peaks emerge in the range of 865–1158 cm −1 , due to the intercalation of metal ions . Pulse EPR spectroscopy at 9.7 GHz (X band) and 5 K yields a 300 mT broad field swept echo spectrum of the Co 2+ (Figure S4a, Supporting Information), which can be assigned to Co 2+ in S = 3/2 state . To investigate a specific binding between the Co 2+ and PHI, electron spin echo envelope modulation (ESEEM) spectroscopy was performed at the peak position (310 mT) of the EPR spectrum.…”

“…[28] Pulse EPR spectroscopy at 9.7 GHz (X band) and 5 K yields a 300 mT broad field swept echo spectrum of the Co 2+ ( Figure S4a, Supporting Information), which can be assigned to Co 2+ in S = 3/2 state. [36] To investigate a specific binding between the Co 2+ and PHI, electron spin echo envelope modulation (ESEEM) spectroscopy was performed at the peak position (310 mT) of the EPR spectrum. The echo decay with increasing pulse separation τ in a 2-pulse π/2-τ-echo sequence is monotonous without any detectable modulation effect (Figure 2f), which can be corroborated by a 2D stimulated echo π/2-τ-π/2-T-π/2-echo experiment.…”

Controlling Oxygen Reduction Selectivity through Steric Effects: Electrocatalytic Two‐Electron and Four‐Electron Oxygen Reduction with Cobalt Porphyrin Atropisomers

“…[8][9][10] Recently, many molecular ORR catalysts have been reported. [1][2][3][4][5][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Although achievements have been made in improving the 4 e À ORR by providing rapid electron and proton transfers, [17,29,30] introducing hydrogenbonding [31,32] and electrostatic interactions, [33] and using dinuclear cooperation, [34][35][36][37][38] realizing these functionalities is challenging from both design and synthesis points of view. Unlike 4 e À ORR, few studies have been dedicated to improving the 2 e À ORR.…”

Acid Catalysis via Acid‐Promoted Electron Transfer

“…In the case of Brønsted acid, proton-promoted ET is often defined as proton-coupled electron transfer (PCET). [43][44][45][46][47][48][49][50][51][52][53] However, in the original definition of PCET, ET form DH to A is accompanied by simultaneous proton transfer (PT) from DH to A to result in an hydrogen atom transfer (HAT) from DH to A to produce D • and AH • . [43][44][45][46][47][48][49][50][51][52][53] In this feature article, ET promoted by binding of proton (PCET) and metal ions (MCET: metal ion-coupled electron transfer) to A •is defined as acid-promoted electron transfer (APET).…”

“…In the case of Brønsted acid, proton‐promoted ET is often defined as proton‐coupled electron transfer (PCET) 43–53 . However, in the original definition of PCET, ET form DH to A is accompanied by simultaneous proton transfer (PT) from DH to A to result in an hydrogen atom transfer (HAT) from DH to A to produce D • and AH • 43–53 .…”