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

We previously reported that the tetraazamacrocyclic Schiff base complex [Co-III(CR14)(X)(2)](n+) (CR14 = 2,12-dimethyl-3,7,11,17-tetraazabicyclo [11.3.1] heptadeca-1 (17),2,11,13,15-pentaene, X = Cl (n = 1) (1-Cl-2) or H2O (n = 3) (1-(H2O)(2))) is a very efficient H-2-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 H-2-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 H-2-evolving performances of a series of compounds from a close-related family, the pentaaza-macrocyclic cobalt [Co-II(CR15)(H2O)(2)]Cl-2 complex (2, CR15 = 2,13-dimethy1-3,6,9,12,18-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 H-2 formation via proton-coupled-electron transfer (PCET) reactions. Besides, the iron, manganese, and zinc analogues, [Fe-II(CR15)(X)(2)](n+) (X = Cl (n = 0) or H2O (n = 2)) (3), [Mn-II(CR15)(CH3CN)(2)](PF6)(2) (4), and {[Zn-II(CR15)Cl](PF6)}(n) (5) were also synthesized and investigated. The photocatalytic activity of 2-5 toward proton reduction was then evaluated in a tricomponent system containing the [Ru-II(bpy)(3)]Cl-2 photosensitizer and ascorbate, in fully aqueous solution. The photocatalytic activity of 2 was also compared with that of 1 in the same experimental conditions. It was found that the number of catalytic cycles versus catalyst for 2 are slightly lower than that for 1, suggesting that if the amine released upon reduction of 2 plays a role in promoting the H-2-evolving catalytic activity, other factors balance this effect. Finally, photophysical and nanosecond transient absorption spectroscopies were used to investigate the photocatalytic system.