Siu-Mui Ng

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 Conversion of CO2 to CO by a Copper(II) Quaterpyridine Complex

The invention of efficient systems for the photocatalytic reduction of CO2 comprising earth-abundant metal catalysts is a promising approach for the production of solar fuels. One bottleneck is to design highly selective and robust molecular complexes that are able to transform the CO2 gas. The CuII quaterpyridine complex [Cu(qpy)]2+ (1) is found to be a highly efficient and selective catalyst for visible-light driven CO2 reduction in CH3 CN using [Ru(bpy)3 ]2+ (bpy: bipyridine) as photosensitizer and BIH/TEOA (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole/triethanolamine) as sacrificial reductant. The photocatalytic reaction is greatly enhanced by the presence of H2 O (1-4 % v/v), and a turnover number of >12 400 for CO production can be achieved with 97 % selectivity, which is among the highest of molecular 3d CO2 reduction catalysts. Results from Hg poisoning and dynamic light scattering experiments suggest that this photocatalyst is homogenous. To the best of our knowledge, 1 is the first example of molecular Cu-based catalyst for the photoreduction of CO2 .

Oxygen atom transfer from a trans-dioxoruthenium(VI) complex to nitric oxide.

In aqueous acidic solutions trans-[Ru(VI)(L)(O)(2)](2+) (L=1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) is rapidly reduced by excess NO to give trans-[Ru(L)(NO)(OH)](2+). When ≤1 mol equiv NO is used, the intermediate Ru(IV) species, trans-[Ru(IV)(L)(O)(OH(2))](2+), can be detected. The reaction of [Ru(VI)(L)(O)(2)](2+) with NO is first order with respect to [Ru(VI)] and [NO], k(2)=(4.13±0.21)×10(1) M(-1) s(-1) at 298.0 K. ΔH(≠) and ΔS(≠) are (12.0±0.3) kcal mol(-1) and -(11±1) cal mol(-1) K(-1), respectively. In CH(3)CN, ΔH(≠) and ΔS(≠) have the same values as in H(2)O; this suggests that the mechanism is the same in both solvents. In CH(3)CN, the reaction of [Ru(VI)(L)(O)(2)](2+) with NO produces a blue-green species with λ(max) at approximately 650 nm, which is characteristic of N(2)O(3). N(2)O(3) is formed by coupling of NO(2) with excess NO; it is relatively stable in CH(3)CN, but undergoes rapid hydrolysis in H(2)O. A mechanism that involves oxygen atom transfer from [Ru(VI)(L)(O)(2)](2+) to NO to produce NO(2) is proposed. The kinetics of the reaction of [Ru(IV)(L)(O)(OH(2))](2+) with NO has also been investigated. In this case, the data are consistent with initial one-electron O(-) transfer from Ru(IV) to NO to produce the nitrito species [Ru(III)(L)(ONO)(OH(2))](2+) (k(2)>10(6) M(-1) s(-1)), followed by a reaction with another molecule of NO to give [Ru(L)(NO)(OH)](2+) and NO(2)(-) (k(2)=54.7 M(-1) s(-1)).