Electrochemical Formation of Fe^V(O) and Mechanism of Its Reaction with Water During O−O Bond Formation

Abstract

A detailed electrochemical investigation of a series of iron complexes (biuret-modified tetraamido iron macrocycles Fe -bTAML), including the first electrochemical generation of Fe (O), and demonstration of their efficacy as homogeneous catalysts for electrochemical water oxidation (WO) in aqueous medium are reported. Spectroelectrochemical and mass spectral studies indicated Fe (O) as the active oxidant, formed due to two redox transitions, which were assigned as Fe (O)/Fe (OH ) and Fe (O)/Fe (O). The spectral properties of both of these high-valent iron oxo species perfectly match those of their chemically synthesised versions, which were thoroughly characterised by several spectroscopic techniques. The O-O bond-formation step occurs by nucleophilic attack of H O on Fe (O). A kinetic isotope effect of 3.2 indicates an atom-proton transfer (APT) mechanism. The reaction of chemically synthesised Fe (O) in CH CN and water was directly probed by electrochemistry and was found to be first-order in water. The pK value of the buffer base plays a critical role in the rate-determining step by increasing the reaction rate several-fold. The electronic effect on redox potential, WO rates, and onset overpotential was studied by employing a series of iron complexes. The catalytic activity was enhanced by the presence of electron-withdrawing groups on the bTAML framework. Changing the substituents from OMe to NO resulted in an eightfold increase in reaction rate, while the overpotential increased threefold.

The Equatorial Ligand Effect on the Properties and Reactivity of Iron(V) Oxo Intermediates

“…A 0.45 m m solution of [Fe III (Cl)(Y‐bTAML)] 2− ( 1 Y ) reacted to form 2 Y on addition of 1.2 equiv of NaOCl in acetonitrile . Quantitative conversion of 1 Y to 2 Y was confirmed by UV/Vis spectroscopy.…”

“…A0 .45 mm solution of [Fe III (Cl)(Y-bTAML)] 2À (1 Y )r eacted to form 2 Y on addition of 1.2 equiv of NaOCl in acetonitrile. [14,16,17,33] Quantitative conversion of 1 Y to 2 Y was confirmed by UV/Vis spectroscopy. The substrate was added through ag as-tight microsyringe under an argon atmosphere after the formation of 2 Y was complete, and the reaction was monitored by UV/Vis spectroscopy.T he change in adsorption was plotted versus the concentration of substrate and enabled us to determine the second-order rate constants of the reaction.…”

“…The iron(III)c hlorido complexes [Fe III (Cl)(Y-bTAML)] 2À (1 Y )w ere synthesized as described in the Experimental Section and characterized by single-crystal XRD (X-ray diffraction), HRMS( highresolution mass spectrometry), UV/Vis spectroscopy,e lectrochemistry,a nd EPR spectroscopy. [17] Additiono fa ne quimolar amount of aqueous sodium hypochlorite to as olution containing 1 Y (Y = H, F, Cl, CN, NO 2 )r esulted in ac hange of color to dark violet due to ab road absorption around8 90-950 nm. Subsequently,t his speciesd ecays into an amber-colored species for Y = NO 2 and ag reen species for all other oxidants.…”

Quantum Chemical Study of the Mechanism of Water Oxidation Catalyzed by a Heterotrinuclear Ru2Mn Complex

“…[1][2][3][4][5] During the past few decades, extraordinary efforts have been dedicated to the synthesis of manganese-based molecular complexes to mimic the structure and function of the OEC. [6][7][8][9][10] Other transition metals were also used for the synthesis of water oxidation catalysts, including Ru, [11][12][13][14][15][16][17][18][19][20][21][22][23][24] Ir, [25,26] Fe, [27][28][29][30][31] Co, [32][33][34] Ni, [35][36][37] and Cu. [38][39][40] Recently,the Llobet group disclosed the synthesisofaheterotrinuclear Ru 2 Mn complex A 0 {[Ru II (tpy)] 2 (m-[Mn II (bpp) 2 ]) (OAc) 2 } 2 + [tpy = 2,2':6',2''-terpyridine, bpp = 3,5-bis(2-pyridyl)pyrazolate].…”

O−O Bond Formation and Liberation of Dioxygen Mediated by N5‐Coordinate Non‐Heme Iron(IV) Complexes

“…[44] Tw op lausible pathways of the iron-borne O À O coupling are shown in Scheme 2. Theo xoiron(V) path (a) alludes to ideas as expressed by Costas and others, [17][18][19]45,46] whereas the concerted O-atom transfer (c) adopts the mechanistic paradigm of mCPBA-driven olefin epoxidation. [47,48] Thelatter concerted pathway invokes essentially simultaneous peroxo OÀOb ond breaking and O 2 formation within ac yclic intermediate (Scheme 2c).…”