Getting in tune: Systematic tuning of the electronic properties of modular non-heme iron coordination complexes can be used to extract important information on the reaction mechanism and intermediates, which, in turn, help to explain the activity of these systems as water oxidation catalysts.

Density functional theory (DFT) is employed to: 1) propose a viable catalytic cycle consistent with our experimental results for the mechanism of chemically driven (Ce(IV) ) O2 generation from water, mediated by nonheme iron complexes; and 2) to unravel the role of the ligand on the nonheme iron catalyst in the water oxidation reaction activity. To this end, the key features of the water oxidation catalytic cycle for the highly active complexes [Fe(OTf)2 (Pytacn)] (Pytacn: 1-(2'-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane; OTf: CF3 SO3 () ) (1) and [Fe(OTf)2 (mep)] (mep: N,N'-bis(2-pyridylmethyl)-N,N'-dimethyl ethane-1,2-diamine) (2) as well as for the catalytically inactive [Fe(OTf)2 (tmc)] (tmc: N,N',N'',N'''-tetramethylcyclam) (3) and [Fe(NCCH3 )((Me) Py2 CH-tacn)](OTf)2 ((Me) Py2 CH-tacn: N-(dipyridin-2-yl)methyl)-N',N''-dimethyl-1,4,7-triazacyclononane) (4) were analyzed. The DFT computed catalytic cycle establishes that the resting state under catalytic conditions is a [Fe(IV) (O)(OH2 )(LN4 )](2+) species (in which LN4 =Pytacn or mep) and the rate-determining step is the OO bond-formation event. This is nicely supported by the remarkable agreement between the experimental (ΔG(≠) =17.6±1.6 kcal mol(-1) ) and theoretical (ΔG(≠) =18.9 kcal mol(-1) ) activation parameters obtained for complex 1. The OO bond formation is performed by an iron(V) intermediate [Fe(V) (O)(OH)(LN4 )](2+) containing a cis-Fe(V) (O)(OH) unit. Under catalytic conditions (Ce(IV) , pH 0.8) the high oxidation state Fe(V) is only thermodynamically accessible through a proton-coupled electron-transfer (PCET) process from the cis-[Fe(IV) (O)(OH2 )(LN4 )](2+) resting state. Formation of the [Fe(V) (O)(LN4 )](3+) species is thermodynamically inaccessible for complexes 3 and 4. Our results also show that the cis-labile coordinative sites in iron complexes have a beneficial key role in the OO bond-formation process. This is due to the cis-OH ligand in the cis-Fe(V) (O)(OH) intermediate that can act as internal base, accepting a proton concomitant to the OO bond-formation reaction. Interplay between redox potentials to achieve the high oxidation state (Fe(V) O) and the activation energy barrier for the following OO bond formation appears to be feasible through manipulation of the coordination environment of the iron site. This control may have a crucial role in the future development of water oxidation catalysts based on iron.

Herein, we report the formation of a highly reactive nickel-oxygen species that has been trapped following reaction of a Ni(II) precursor bearing a macrocyclic bis(amidate) ligand with meta-chloroperbenzoic acid (HmCPBA). This compound is only detectable at temperatures below 250 K and is much more reactive toward organic substrates (i.e., C-H bonds, C=C bonds, and sulfides) than previously reported well-defined nickel-oxygen species. Remarkably, this species is formed by heterolytic O-O bond cleavage of a Ni-HmCPBA precursor, which is concluded from experimental and computational data. On the basis of spectroscopy and DFT calculations, this reactive species is proposed to be a Ni(III) -oxyl compound.

Silver is extensively used in homogeneous catalysis for organic synthesis owing to its Lewis acidity, and as a powerful one-electron oxidant. However, two-electron redox catalytic cycles, which are most common in noble metal organometallic reactivity, have never been considered. Here we show that a Ag(I)/Ag(III) catalytic cycle is operative in model C–O and C–C cross-coupling reactions. An aryl-Ag(III) species is unequivocally identified as an intermediate in the catalytic cycle and we provide direct evidence of aryl halide oxidative addition and C–N, C–O, C–S, C–C and C–halide bond-forming reductive elimination steps at monometallic silver centres. We anticipate our study as the starting point for expanding Ag(I)/ Ag(III) redox chemistry into new methodologies for organic synthesis, resembling well-known copper or palladium cross-coupling catalysis. Furthermore, findings described herein provide unique fundamental mechanistic understanding on Ag-catalysed cross-coupling reactions and dismiss the generally accepted conception that silver redox chemistry can only arise from one-electron processesThis work was supported by grants from the European Research Council (Starting Grant Project ERC-2011-StG-277801), the Spanish MINECO (CTQ2012-37420-C02-01/BQU, CTQ2012-32436, Consolider-Ingenio CSD2010-00065, INNPLANTA project INP-2011-0059-PCT-420000-ACT1) and the Catalan DIUE of the Generalitat de Catalunya (2009SGR637

A dual catalytic system based on earth-abundant elements reduces aromatic ketones and aldehydes to alcohols in aqueous media under visible light. An unprecedented selectivity for the reduction of aromatic ketones versus aliphatic aldehydes is reported.

Abstract.Terminal high-valent metal-oxygen species are key reaction intermediates in the catalytic cycle of both enzymes (e.g., oxygenases) and synthetic oxidation catalysts. While tremendous efforts have been directed towards the characterization of the biologically relevant terminal manganese-oxygen and iron-oxygen species, the corresponding analogues based on late-transition metals such as cobalt, nickel or copper are relatively scarce. This is in part related to the "Oxo Wall" concept, which predicts that late transition elements cannot support a terminal oxido ligand in a tetragonal environment. Here, the nickel(II) complex (1) of the tetradentate macrocyclic ligand bearing a 2,6-pyridinedicarboxamidate unit is shown to be an effective catalyst in the chlorination and oxidation of C-H bonds with sodium hypochlorite as terminal oxidant in the presence of acetic acid (AcOH). Insight into the active species responsible for the observed reactivity was gained through the study of the reaction of 1 with ClO -at low temperature by UV/Vis absorption, resonance Raman, EPR, ESI-MS, and XAS analyses. DFT calculations aided the assignment of the trapped chromophoric species (3) as a nickel-hypochlorite species. Despite the fact that the formal oxidation state of the nickel in 3 is +4, experimental and computational analysis indicate that 3 is best formulated as a Ni III complex with one unpaired electron delocalized in the ligands surrounding the metal center. Most remarkably, 3 reacts rapidly with a range of substrates including those with strong aliphatic C-H bonds, indicating the direct involvement of 3 in the oxidation/chlorination reactions observed in the 1/ClO -

The formation and spectroscopic characterization of a superoxido cobalt(iii) and a peroxido dicobalt(iii) species formed in the temperature dependent reversible reaction of a cobalt(ii) precursor with O is described. The electronic nature of each species is explored in their reactivity with organic substrates.

The nature of the oxidizing species in water oxidation reactions with chemical oxidants catalyzed by α-[Fe(OTf)2(mcp)] (mcp = N, N′-dimethyl-N,N′-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine, OTf = trifluoromethanesulfonate anion), (1α) and β-[Fe(OTf)2(mcp)], (1β) has been investigated. Mössbauer spectroscopy provides definitive evidence that 1α and 1β generate oxoiron(IV) species as the resting state. Decomposition paths of the catalysts have been investigated by identifying and quantifying ligand fragments that form upon degradation. This analysis correlates the water oxidation activity of 1α and 1β with stability against oxidative damage of the ligand via aliphatic C-H oxidation. The site of degradation and the relative stability against oxidative degradation is shown to be dependent on the topology of the catalyst. Furthermore, the mechanisms of catalyst degradation have been rationalized by computational analyses, which also explain why the topology of the catalyst enforces different oxidation sensitive sites. This information has served for creating catalysts where sensitive C-H bonds have been replaced by C-D bonds. Deuterated analogs, D4-α-[Fe(OTf)2(mcp)] (D4-1α), D4-β-[Fe(OTf)2(mcp)] (D4-1β) and D6-β-[Fe(OTf)2(mcp)] (D6-1β) were prepared, and their catalytic activity has been studied. D4-1α proves to be an extraordinarily active and efficient catalyst (up to 91% of O2 yield); it exhibits initial reaction rates identical to its protio analogue, but it is substantially more robust towards oxidative degradation and yields more than 3400 TON (n(O2)/n(Fe)). Altogether evidence that the water oxidation catalytic activity is performed by a well-defined coordination complex and not by iron oxides formed after oxidative degradation of the ligands.