title: Cobalt(III) tetraaza-macrocyclic complexes as efficient catalyst for photoinduced hydrogen production in water: Theoretical investigation of the electronic structure of the reduced species and mechanistic insight.
abstract: We recently reported a very efficient homogeneous system for visible-light driven hydrogen production in water based on the cobalt(III) tetraaza-macrocyclic complex [Co(CR)Cl2](+) (1) (CR=2,12-dimethyl-3,7,11,17-tetra-azabicyclo(11.3.1)-heptadeca-1(17),2,11,13,15-pentaene) as a noble metal-free catalyst, with [Ru(II)(bpy)3](2+) (Ru) as photosensitizer and ascorbate/ascorbic acid (HA(-)/H2A) as a sacrificial electron donor and buffer (PhysChemChemPhys 2013, 15, 17544). This catalyst presents the particularity to achieve very high turnover numbers (TONs) (up to 1000) at pH 4.0 at a relative high concentration (0.1mM) generating a large amount of hydrogen and having a long term stability. A similar activity was observed for the aquo derivative [Co(III)(CR)(H2O)2](3+) (2) due to substitution of chloro ligands by water molecule in water. In this work, the geometry and electronic structures of 2 and its analog [Zn(II)(CR)Cl](+) (3) derivative containing the redox innocent Zn(II) metal ion have been investigated by DFT calculations under various oxidation states. We also further studied the photocatalytic activity of this system and evaluated the influence of varying the relative concentration of the different components on the H2-evolving activity. Turnover numbers versus catalyst (TONCat) were found to be dependent on the catalyst concentration with the highest value of 1130 obtained at 0.05 mM. Interestingly, the analogous nickel derivative, [Ni(II)(CR)Cl2] (4), when tested under the same experimental conditions was found to be fully inactive for H2 production. Nanosecond transient absorption spectroscopy measurements have revealed that the first electron-transfer steps of the photocatalytic H2-evolution mechanism with the Ru/cobalt tetraaza/HA(-)/H2A system involve a reductive quenching of the excited state of the photosensitizer by ascorbate (kq=2.5×10(7) M(-1) s(-1)) followed by an electron transfer from the reduced photosensitizer to the catalyst (ket=1.4×10(9) M(-1) s(-1)). The reduced catalyst can then enter into the cycle of hydrogen evolution.
authors.affiliation: Univ. Grenoble Alpes, DCM, F-38000 Grenoble, France; CNRS, DCM, F-38000 Grenoble, France.
authors.forename: Robin
authors.initials: R
authors.lastname: Gueret
authors.affiliation: Univ. Grenoble Alpes, DCM, F-38000 Grenoble, France; CNRS, DCM, F-38000 Grenoble, France.
authors.forename: Carmen E
authors.initials: CE
authors.lastname: Castillo
authors.affiliation: Laboratoire de Spectrochimie Infrarouge et Raman, UMR 8516 CNRS-Université Lille 1 Sciences et Technologies, 59655 Villeneuve d'Ascq Cedex, France.
authors.forename: Mateusz
authors.initials: M
authors.lastname: Rebarz
authors.affiliation: Univ. Grenoble Alpes, DCM, F-38000 Grenoble, France; CNRS, DCM, F-38000 Grenoble, France.
authors.forename: Fabrice
authors.initials: F
authors.lastname: Thomas
authors.affiliation: Univ. Grenoble Alpes, DCM, F-38000 Grenoble, France; CNRS, DCM, F-38000 Grenoble, France.
authors.forename: Aaron-Albert
authors.initials: AA
authors.lastname: Hargrove
authors.affiliation: Univ. Grenoble Alpes, INAC-SCIB, F-38000 Grenoble, France; CEA, INAC-SCIB, Reconnaissance Ionique et Chimie de Coordination, F-38000 Grenoble, France.
authors.forename: Jacques
authors.initials: J
authors.lastname: Pécaut
authors.affiliation: Laboratoire de Spectrochimie Infrarouge et Raman, UMR 8516 CNRS-Université Lille 1 Sciences et Technologies, 59655 Villeneuve d'Ascq Cedex, France.
authors.forename: Michel
authors.initials: M
authors.lastname: Sliwa
authors.affiliation: Univ. Grenoble Alpes, DCM, F-38000 Grenoble, France; CNRS, DCM, F-38000 Grenoble, France. Electronic address: jerome.fortage@ujf-grenoble.fr.
authors.forename: Jérôme
authors.initials: J
authors.lastname: Fortage
authors.affiliation: Univ. Grenoble Alpes, DCM, F-38000 Grenoble, France; CNRS, DCM, F-38000 Grenoble, France. Electronic address: marie-noelle.collomb@ujf-grenoble.fr.
authors.forename: Marie-Noëlle
authors.initials: MN
authors.lastname: Collomb
mesh_terms: D002384:Catalysis; D003035:Cobalt / Q000737:chemistry* / Q000378:metabolism; D056831:Coordination Complexes / Q000737:chemistry* / Q000378:metabolism; D003460:Crystallization; D004579:Electron Transport / Q000502:physiology; D006859:Hydrogen / Q000737:chemistry* / Q000378:metabolism; D008962:Models, Theoretical; D010084:Oxidation-Reduction; D010775:Photic Stimulation / Q000379:methods; D017319:Photosensitizing Agents / Q000737:chemistry* / Q000378:metabolism; D014867:Water / Q000737:chemistry* / Q000378:metabolism
chemical_list: D056831:Coordination Complexes; D017319:Photosensitizing Agents; D014867:Water; D003035:Cobalt; D006859:Hydrogen
keywords: Cobalt; Hydrogen; Macrocyclic ligand; Photocatalysis; Water chemistry