title: Synthesis, Characterization, and Photocatalytic H2-Evolving Activity of a Family of [Co(N4Py)(X)](n+) Complexes in Aqueous Solution.
abstract: A series of [Co(III)(N4Py)(X)](ClO4)n (X = Cl(-), Br(-), OH(-), N3(-), NCS(-)-κN, n = 2: X = OH2, NCMe, DMSO-κO, n = 3) complexes containing the tetrapyridyl N5 ligand N4Py (N4Py = 1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine) has been prepared and fully characterized by infrared (IR), UV-visible, and NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HRESI-MS), elemental analysis, X-ray crystallography, and electrochemistry. The reduced Co(II) and Co(I) species of these complexes have been also generated by bulk electrolyses in MeCN and characterized by UV-visible and EPR spectroscopies. All tested complexes are catalysts for the photocatalytic production of H2 from water at pH 4.0 in the presence of ascorbic acid/ascorbate, using [Ru(bpy)3](2+) as a photosensitizer, and all display similar H2-evolving activities. Detailed mechanistic studies show that while the complexes retain the monodentate X ligand upon electrochemical reduction to Co(II) species in MeCN solution, in aqueous solution, upon reduction by ascorbate (photocatalytic conditions), [Co(II)(N4Py)(HA)](+) is formed in all cases and is the precursor to the Co(I) species which presumably reacts with a proton. These results are in accordance with the fact that the H2-evolving activity does not depend on the chemical nature of the monodentate ligand and differ from those previously reported for similar complexes. The catalytic activity of this series of complexes in terms of turnover number versus catalyst (TONCat) was also found to be dependent on the catalyst concentration, with the highest value of 230 TONCat at 5 × 10(-6) M. As revealed by nanosecond transient absorption spectroscopy measurements, the first electron-transfer steps of the photocatalytic mechanism involve a reductive quenching of the excited state of [Ru(bpy)3](2+) by ascorbate followed by an electron transfer from [Ru(II)(bpy)2(bpy(•-))](+) to the [Co(II)(N4Py)(HA)](+) catalyst. The reduced catalyst then enters into the H2-evolution cycle.
authors.affiliation: Department of Chemistry, University of Otago , P. O. Box 56, Dunedin 9054, New Zealand.
authors.forename: Warrick K C
authors.initials: WK
authors.lastname: Lo
authors.affiliation: Département de Chimie Moléculaire, CNRS, Université Grenoble Alpes , F-38000 Grenoble, France.
authors.forename: Carmen E
authors.initials: CE
authors.lastname: Castillo
authors.affiliation: Département de Chimie Moléculaire, CNRS, Université Grenoble Alpes , F-38000 Grenoble, France.
authors.forename: Robin
authors.initials: R
authors.lastname: Gueret
authors.affiliation: Département de Chimie Moléculaire, CNRS, Université Grenoble Alpes , F-38000 Grenoble, France.
authors.forename: Jérôme
authors.initials: J
authors.lastname: Fortage
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: 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: Département de Chimie Moléculaire, CNRS, Université Grenoble Alpes , F-38000 Grenoble, France.
authors.forename: Fabrice
authors.initials: F
authors.lastname: Thomas
authors.affiliation: Department of Chemistry, University of Otago , P. O. Box 56, Dunedin 9054, New Zealand.
authors.forename: C John
authors.initials: CJ
authors.lastname: McAdam
authors.affiliation: Institute of Fundamental Sciences, Massey University , P. O. Box 11-222, Palmerston North 4442, New Zealand.
authors.forename: Geoffrey B
authors.initials: GB
authors.lastname: Jameson
authors.affiliation: Department of Chemistry, University of Otago , P. O. Box 56, Dunedin 9054, New Zealand.
authors.forename: David A
authors.initials: DA
authors.lastname: McMorran
authors.affiliation: Department of Chemistry, University of Otago , P. O. Box 56, Dunedin 9054, New Zealand.
authors.forename: James D
authors.initials: JD
authors.lastname: Crowley
authors.affiliation: Département de Chimie Moléculaire, CNRS, Université Grenoble Alpes , F-38000 Grenoble, France.
authors.forename: Marie-Noëlle
authors.initials: MN
authors.lastname: Collomb
authors.affiliation: School of Applied Sciences, Auckland University of Technology , Private Bag 92006, Auckland 1142, New Zealand.
authors.forename: Allan G
authors.initials: AG
authors.lastname: Blackman