Dual cobalt – copper light-driven catalytic reduction of aldehydes and aromatic ketones in aqueous media

We present an efficient, general, fast, and robust light-driven methodology based on earth-abundant elements to reduce aryl ketones, and both aryl and aliphatic aldehydes (up to 1400 TON). The catalytic system consists of a robust and well-defined aminopyridyl cobalt complex active for photocatalytic water reduction and the [Cu(bathocuproine)(Xantphos)](PF6) photoredox catalyst. The dual cobalt–copper system uses visible light as the driving-force and H2O and an electron donor (Et3N or iPr2EtN) as the hydride source. The catalytic system operates in aqueous mixtures (80–60% water) with high selectivity towards the reduction of organic substrates (>2000) vs. water reduction, and tolerates O2. High selectivity towards the hydrogenation of aryl ketones is observed in the presence of terminal olefins, aliphatic ketones, and alkynes. Remarkably, the catalytic system also shows unique selectivity for the reduction of acetophenone in the presence of aliphatic aldehydes. The catalytic system provides a simple and convenient method to obtain α,β-deuterated alcohols. Both the observed reactivity and the DFT modelling support a common cobalt hydride intermediate. The DFT modelled energy profile for the [Co–H] nucleophilic attack to acetophenone and water rationalises the competence of [CoII–H] to reduce acetophenone in the presence of water. Mechanistic studies suggest alternative mechanisms depending on the redox potential of the substrate. These results show the potential of the water reduction catalyst [Co(OTf)(Py2Tstacn)](OTf) (1), (Py2Tstacn = 1,4-di(picolyl)-7-(p-toluenesulfonyl)-1,4,7-triazacyclononane, OTf = trifluoromethanesulfonate anion) to develop light-driven selective organic transformations and fine solar chemicals

Design of Zn-, Cu-, and Fe-Coordination Complexes Confined in a Self-Assembled Nanocage

The encapsulation of coordination complexes in a tetragonal prismatic nanocage (1·(BArF)8) built from Zn-porphyrin and macrocyclic Pd-clip-based synthons is described. The functional duality of the guest ligand L1 allows for its encapsulation inside the cage 1·(BArF)8, along with the simultaneous coordination of ZnII, CuII, or FeIII metal ions. Remarkably, the coordination chemistry inside the host–guest adduct L1⊂1·(BArF)8 occurs in both solution solution and solid state. The resulting confined metallocomplexes have been characterized by means of UV-vis, ESI-HRMS, NMR, and EPR techniques. Furthermore, the emission of the Zn-porphyrin fluorophores of 1·(BArF)8 is strongly quenched by the encapsulation of paramagnetic complexes, representing a remarkable example of guest-dependent tuning of the host fluorescence

Spectroelectrochemical analysis of the water oxidation mechanism on doped nickel oxides

Metal oxides and oxyhydroxides exhibit state-of-the-art activity for the oxygen evolution reaction (OER); however, their reaction mechanism, particularly the relationship between charging of the oxide and OER kinetics, remains elusive. Here, we investigate a series of Mn-, Co-, Fe-, and Zn-doped nickel oxides using operando UV–vis spectroscopy coupled with time-resolved stepped potential spectroelectrochemistry. The Ni2+/Ni3+ redox peak potential is found to shift anodically from Mn- < Co- < Fe- < Zn-doped samples, suggesting a decrease in oxygen binding energetics from Mn- to Zn-doped samples. At OER-relevant potentials, using optical absorption spectroscopy, we quantitatively detect the subsequent oxidation of these redox centers. The OER kinetics was found to have a second-order dependence on the density of these oxidized species, suggesting a chemical rate-determining step involving coupling of two oxo species. The intrinsic turnover frequency per oxidized species exhibits a volcano trend with the binding energy of oxygen on the Ni site, having a maximum activity of ∼0.05 s–1 at 300 mV overpotential for the Fe-doped sample. Consequently, we propose that for Ni centers that bind oxygen too strongly (Mn- and Co-doped oxides), OER kinetics is limited by O–O coupling and oxygen desorption, while for Ni centers that bind oxygen too weakly (Zn-doped oxides), OER kinetics is limited by the formation of oxo groups. This study not only experimentally demonstrates the relation between electroadsorption free energy and intrinsic kinetics for OER on this class of materials but also highlights the critical role of oxidized species in facilitating OER kinetics

Electrocatalytic water oxidation with alpha-[Fe(mcp)(OTf)(2)] and analogues

The complex alpha-[Fe(mcp)(OTf)(2)] (mcp = N,N'-dimethyl-N,N'-bis(pyridin-2-ylmethyl)-cyclohexane-1,2-diamine and OTf = trifluoromethanesulfonate anion) was reported in 2011 by some of us as an active water oxidation (WO) catalyst in the presence of sacrificial oxidants. However, because chemical oxidants are likely to take part in the reaction mechanism, mechanistic electrochemical studies are critical in establishing to what extent previous studies with sacrificial reagents have actually been meaningful. In this study, the complex alpha-[Fe(mcp)(OTf)(2)] and its analogues were investigated electrochemically under both acidic and neutral conditions. All the systems under investigation proved to be electrochemically active toward the WO reaction, with no major differences in activity despite the structural changes. Our findings show that WO-catalyzed by mcp-iron complexes proceeds via homogeneous species, whereas the analogous manganese complex forms a heterogeneous deposit on the electrode surface. Mechanistic studies show that the reaction proceeds with a different rate-determining step (rds) than what was previously proposed in the presence of chemical oxidants. Moreover, the different kinetic isotope effect (KIE) values obtained electrochemically at pH 7 (KIE similar to 10) and at pH 1 (KIE = 1) show that the reaction conditions have a remarkable effect on the rds and on the mechanism. We suggest a proton-coupled electron transfer (PCET) as the rds under neutral conditions, whereas at pH 1 the rds is most likely an electron transfer (ET).Catalysis and Surface ChemistryMetals in Catalysis, Biomimetics & Inorganic Material

Improved Electro- and Photocatalytic Water Reduction by Confined Cobalt Catalysts in Streptavidin

Incorporation of biotinylated aminopyridine cobalt complexes derived from the triazacyclononane scaffold into the streptavidin protein leads to formation of artificial metalloenzymes for water reduction to hydrogen. The synthesized artificial metalloenzymes have lower overpotential (at the half-peak up to 100 mV) and higher photocatalytic hydrogen evolution activity (up to 14- and 10-fold increase in TOF and TON, respectively, at pH 12.5) than the free biotinylated cobalt complexes. 1H-NMR, EPR and XAS highlight the presence of the metal complexes upon supramolecular attachment to the streptavidin. pHdependent catalytic studies and molecular dynamics (MD) simulations suggest that the increase in the catalytic activity could be induced by the protein residues positioned close to the metal centers. These findings illustrate the ability of the biotin−streptavidin technology to produce artificial metalloproteins for photo- and electrocatalytic hydrogen evolution reaction

Oxidant-Free Au(I)-Catalyzed Halide Exchange and C_sp2–O Bond Forming Reactions

Au has been demonstrated to mediate a number of organic transformations through the utilization of its π Lewis acid character, Au­(I)/Au­(III) redox properties or a combination of both. As a result of the high oxidation potential of the Au­(I)/Au­(III) couple, redox catalysis involving Au typically requires the use of a strong external oxidant. This study demonstrates unusual external oxidant-free Au­(I)-catalyzed halide exchange (including fluorination) and C_sp2–O bond formation reactions utilizing a model aryl halide macrocyclic substrate. Additionally, the halide exchange and C_sp2–O coupling reactivity could also be extrapolated to substrates bearing a single chelating group, providing further insight into the reaction mechanism. This work provides the first examples of external oxidant-free Au­(I)-catalyzed carbon–heteroatom cross-coupling reactions

CCDC 1060000: Experimental Crystal Structure Determination

Related Article: Teresa Corona, Florian F. Pfaff, Ferran Acuña-Pares, Apparao Draksharapu, Christopher J. Whiteoak, Julio Lloret Fillol, Wesley R. Browne, Kallol Ray, Anna Company|2015|Chem.-Eur.J.|21|15029|doi:10.1002/chem.201501841,An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Octahedral iron(IV)–tosylimido complexesexhibiting single electron-oxidation reactivity

International audienceHigh valent iron species are very reactive molecules involved in oxidation reactions of relevance to biology and chemical synthesis. Herein we describe iron(iv)-tosylimido complexes [Fe-IV(NTs)(MePy(2)tacn)](OTf)(2) (1((IV))& xe001;NTs) and [Fe-IV(NTs)(Me-2(CHPy2)tacn)](OTf)(2) (2((IV))& xe001;NTs), (MePy(2)tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane, and Me-2(CHPy2)tacn = 1-(di(2-pyridyl)methyl)-4,7-dimethyl-1,4,7-triazacyclononane, Ts = Tosyl). 1((IV))& xe001;NTs and 2((IV))& xe001;NTs are rare examples of octahedral iron(iv)-imido complexes and are isoelectronic analogues of the recently described iron(iv)-oxo complexes [Fe-IV(O)(L)](2+) (L = MePy(2)tacn and Me-2(CHPy2)tacn, respectively). 1((IV))& xe001;NTs and 2((IV))& xe001;NTs are metastable and have been spectroscopically characterized by HR-MS, UV-vis, H-1-NMR, resonance Raman, Mossbauer, and X-ray absorption (XAS) spectroscopy as well as by DFT computational methods. Ferric complexes [Fe-III(HNTs)(L)](2+), 1((III))-NHTs (L = MePy(2)tacn) and 2((III))-NHTs (L = Me-2(CHPy2)tacn) have been isolated after the decay of 1((IV))& xe001;NTs and 2((IV))& xe001;NTs in solution, spectroscopically characterized, and the molecular structure of [Fe-III(HNTs)(MePy(2)tacn)](SbF6)(2) determined by single crystal X-ray diffraction. Reaction of 1((IV))& xe001;NTs and 2((IV))& xe001;NTs with different p-substituted thioanisoles results in the transfer of the tosylimido moiety to the sulphur atom producing sulfilimine products. In these reactions, 1((IV))& xe001;NTs and 2((IV))& xe001;NTs behave as single electron oxidants and Hammett analyses of reaction rates evidence that tosylimido transfer is more sensitive than oxo transfer to charge effects. In addition, reaction of 1((IV))& xe001;NTs and 2((IV))& xe001;NTs with hydrocarbons containing weak C-H bonds results in the formation of 1((III))-NHTs and 2((III))-NHTs respectively, along with the oxidized substrate. Kinetic analyses indicate that reactions proceed via a mechanistically unusual HAT reaction, where an association complex precedes hydrogen abstraction