Cobalt(II) Pentaaza-Macrocyclic Schiff Base Complex as Catalyst for Light-Driven Hydrogen Evolution in Water: Electrochemical Generation and Theoretical Investigation of the One-Electron Reduced Species

Abstract

We previously reported that the tetraazamacrocyclic Schiff base complex [Co^III(CR14)(X)₂]^<i>n</i>+ (CR14 = 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2,11,13,15-pentaene, X = Cl (<i>n</i> = 1) (<b>1</b>-Cl₂) or H₂O (<i>n</i> = 3) (<b>1</b>-(H₂O)₂)) is a very efficient H₂-evolving catalyst (HEC) in fully aqueous solutions at pH 4.0-4.5 when used in a photocatalytic system including a photosensitizer and ascorbate as sacrificial electron donor. The excellent H₂-evolving activity of this complex, compared to other cobalt and rhodium catalysts studied in the same photocatalytic conditions, can be related to the high stability of its two-electron reduced form, the putative "Co(I)" state. These very interesting results led us to investigate the H₂-evolving performances of a series of compounds from a close-related family, the pentaaza-macrocyclic cobalt [Co^II(CR15)(H₂O)₂]Cl₂ complex (<b>2</b>, CR15 = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene), which comprises a larger macrocycle with five nitrogen atoms instead of four. Electrochemical as well as spectroscopic investigations in CH₃CN coupled to density functional theory (DFT) calculations point to decoordination of one of the amine upon reduction of Co(II) to the low-valent "Co(I)" form. The resulting unchelated amine could potentially act as a proton relay promoting the H₂ formation via proton-coupled-electron transfer (PCET) reactions. Besides, the iron, manganese, and zinc analogues, [Fe^II(CR15)(X)₂]^<i>n</i>+ (X = Cl (<i>n</i> = 0) or H₂O (<i>n</i> = 2)) (<b>3</b>), [Mn^II(CR15)(CH₃CN)₂](PF₆)₂ (<b>4</b>), and {[Zn^II(CR15)Cl](PF₆)}<i>_n</i> (<b>5</b>) were also synthesized and investigated. The photocatalytic activity of <b>2</b>-<b>5</b> toward proton reduction was then evaluated in a tricomponent system containing the [Ru^II(bpy)₃]Cl₂ photosensitizer and ascorbate, in fully aqueous solution. The photocatalytic activity of <b>2</b> was also compared with that of <b>1</b> in the same experimental conditions. It was found that the number of catalytic cycles versus catalyst for <b>2</b> are slightly lower than that for <b>1</b>, suggesting that if the amine released upon reduction of <b>2</b> plays a role in promoting the H₂-evolving catalytic activity, other factors balance this effect. Finally, photophysical and nanosecond transient absorption spectroscopies were used to investigate the photocatalytic system.