Platinum(II)-based hydrogen-evolving catalysts linked to multipendant viologen acceptors: Experimental and dft indications for bimolecular pathways

Three new [PtCl₂(bpy)] derivatives tethered to 2, 4, and 6 dicationic viologen moieties, [PtCl₂(MV2)]⁴⁺ (1), [PtCl₂(MV4)]⁸⁺ (2), and [PtCl₂(MV6)] ¹²⁺ (3), have been synthesized (MV2⁴⁺=5-ethoxycarbamoyl- 5′-(N-R¹-carbamoyl)-2,2′-bipyridine, MV4 ⁸⁺=5,5′-bis(N-R¹-carbamoyl)-2,2′-bipyridine, and MV6¹²⁺=5,5′-bis(N-R²-carbamoyl)-2,2′- bipyridine, in which R¹=Asp(NH-VG)-NH-VG, R²=Asp(NH-VG)- Asp(NH-VG)-NH-VG, and VG=-(CH₂)₂-⁺NC ₅H₄-C₅H₄N⁺-CH ₃). In spite of the higher charge storage capacity of 2 and 3 due to the higher number of acceptor groups (VG groups), compound 1 with the lowest number of VG tethers has turned out to exhibit an outstanding catalytic performance towards the hydrogen evolution from water. Quantitative analysis of UV/Vis-NIR absorption spectral changes during the photolysis for 2 and 3 reveal that approximately 2 electrons per molecule are stored over the acceptor groups during the photolysis, and the storage events saturate after 20 min. As for 1, the total number of electrons stored per molecule increases once during the initial 10 min and then abruptly decreases down to around 0.1 electrons per molecule at 20 min, during which the storage is maximized at 10-20 min with 0.6-0.7 electrons stored per molecule, thereby indicating that the rates of radical formation and consumption are balanced during the photochemical hydrogen evolution reaction. The electrical conductivity measurements reveal that ion-pair adducts (adducts with PF₆ ^- ions in solution) are formed by 2 and 3 but are not given by 1 under the catalysis conditions. These, together with the results of molecular mechanics calculations, reveal that stack of two [PtCl₂(bpy)] units becomes unfavorable as the number of sterically bulky and highly charged VG units per molecule increases. We have therefore concluded that dimerization that leads to the formation of a PtPt association is a key step in the effective catalytic enhancement with [PtCl₂(bpy)]-type catalysts.