Lingjing Chen

Chemical and Visible‐Light‐Driven Water Oxidation by Iron Complexes at pH 7–9: Evidence for Dual‐Active Intermediates in Iron‐Catalyzed Water Oxidation

In recent decades, tremendous efforts have been devoted by chemists to develop efficient, cost-effective catalytic methods for solar-driven water oxidation, which would provide an unlimited source of protons and electrons for the production of hydrogen and other renewable fuels. However, to be economically viable, the catalysts for water oxidation should be made from earth-abundant materials; so far only a few cobalt, manganese, iron, and copper water-oxidation catalysts (WOCs) have been developed. Among the first-row transition metals, iron is probably the most desirable to be used in WOCs because it is the most abundant and relatively nontoxic. Collins, Bernhard, and co-workers recently reported the use of an iron(III) complex bearing a tetraamido macrocyclic ligand (Fe-TAML) to catalyze water oxidation by Ce at approximately pH 1 with a turnover number (TON) of 18 and turnover frequency (TOF) of greater than 1.3 s . Subsequently, Fillol and Costas et al. , also reported that a number of iron complexes bearing tetradentate Ndonor ligands can catalyze water oxidation at low pH with TON> 350 and > 1000 using Ce and IO4 , respectively. Herein, we report chemical and light-driven water oxidation catalyzed by a number of iron complexes and iron salts at pH 7–9 in borate buffer. We provide evidence that at this pH range, Fe2O3 particles are produced, which are the actual catalyst for water oxidation. The catalytic activity of various iron complexes towards water oxidation at pH 7–9 was investigated by a chemical method using [Ru(bpy)3](ClO4)3 (bpy = bipyridine) as the terminal oxidant (Table 1). [Ru(bpy)3] 3+ is commonly used as an oxidant in this pH range because of its relative stability and high redox potential (E = 1.21 V), whereas Ce, the other commonly used oxidant, is stable only at low pH values. Initially we investigated the complex cis-Fe(mcp)Cl2 (mcp = N,N’-dimethyl-N,N’-bis(2-pyridylmethyl)cyclohexane-1,2-diamine; Figure S1), because it was recently reported to be a highly active catalyst for water oxidation by Ce or IO4 at low pH. In our hands, when we used this complex as a catalyst and (NH4)2Ce(NO3)6 as the oxidant in unbuffered water, we obtained a TON of 290 15, in reasonable agreement with the value of 320 15 reported in the literature. However, when we used [Ru(bpy)3](ClO4)3 as the oxidant at pH 7–9 in borate buffer, no oxygen was produced (after subtracting the background signal) from this complex (Table 1, entry 2 and Figure S2). On the other hand, when other iron complexes such as [Fe(bpy)2Cl2]Cl, [Fe(tpy)2]Cl2, cis-[Fe(cyclen)Cl2]Cl, and trans-Fe(tmc)Br2 (where tpy = 2,2’:6’,2’’-terpyridine, cyclen = 1,4,7,10-tetraazacyclodecane, and tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) were used as catalysts, oxygen evolution readily occurred, with TON ranging from 19 to 108 (Table 1, entries 3–6). Notably, simple iron salts such as Fe(ClO4)3 is even more active than the other iron complexes (Table 1, entry 7; Table S1 and Figure S3). The maximum TOF of 9.6 s 1 is higher than those of other earth-abundant artificial catalysts such as [Co4(H2O)2(aPW9O34)2] 10 (5 s ) and Fe-TAML (1.3 s ). In the absence of an iron catalyst, 8% yield of oxygen was also detected (Table 1, entry 1), which comes from background oxidation of water by [Ru(bpy)3] . No oxygen could be detected when the reaction was carried out in phosphate buffer (Table S1, entry 8), which is attributed to the formation of the very insoluble FePO4 (Ksp = 9.92 10 ). On the other Table 1: Iron-catalyzed water oxidation by [Ru(bpy)3](ClO4)3 at pH 8.5 in borate buffer.

Photocatalytic oxidation of alkenes and alcohols in water by a manganese(V) nitrido complex

Mn(v) nitrido complex [Mn(N)(CN)4](2-) is an efficient catalyst for visible-light induced oxidation of alkenes and alcohols in water using [Ru(bpy)3](2+) as a photosensitizer and [Co(NH3)5Cl](2+) as a sacrificial oxidant. Alkenes are oxidized to epoxides and alcohols to carbonyl compounds.

Oxidation of Alkanes by Periodate Using a Mn(V) Nitrido Complex as Catalyst.

The design of catalytic systems that can selectively oxidize unactivated C-H bonds under mild conditions is a challenge to chemists. We report here that the manganese(V) nitrido complex [MnV (N)(CN)4 ]2- is a highly efficient catalyst for the oxidation of alkanes by periodate (IO4- ) at ambient conditions. Excellent yields of alcohols and ketones (>95 %) are obtained with a maximum turnover number (TON) of 3000.

Mechanism of Water Oxidation by Ferrate(VI) at pH 7 - 9

The kinetics of water oxidation by K2 FeO4 has been reinvestigated by UV/Vis spectrophotometry from pH 7-9 in 0.2 m phosphate buffer. The rate of reaction was found to be second-order in both [FeO4 2- ] and [H+ ]. These results are consistent with a proposed mechanism in which the first step involves the initial equilibrium protonation of FeO4 2- to give FeO3 (OH)- , which then undergoes rate-limiting O-O bond formation. Analysis of the O2 isotopic composition for the reaction in H2 18 O suggests that the predominant pathway for water oxidation by ferrate is intramolecular O-O coupling. DFT calculations have also been performed, which support the proposed mechanism.