Improved photocatalytic hydrogen evolution driven by chloro(terpyridine)platinum(II) derivatives tethered to a single pendant viologen acceptor

Three chloro(4′-(N-methylpyridinium)-2,2′:6′,2′′-terpyridine)platinum(ii) (abbreviated as PtL²⁺) derivatives tethered to a single alkyl viologen unit (-(CH₂)_n-CH₂-N⁺C₅H₄-C₅H₄N⁺-CH₃; abbreviated as -Cn-MV²⁺, where n = 1, 3, and 4), i.e., PtL²⁺-Cn-MV²⁺, have been synthesized and investigated in detail. It is shown that the turnover number (TON) for the photocatalytic H₂ evolution from water in the presence of a sacrificial electron donor EDTA (ethylenediaminetetraacetic acid disodium salt) is dramatically improved by the attachment of a single alkyl MV²⁺ unit (TON = 21.5-25.2, 12 h). Spectrophotometric studies reveal that the photoirradiation of PtL²⁺-Cn-MV²⁺ in the presence of EDTA initially leads to the formation of a 1-electron-reduced species, and then to a 2-electron-reduced species, where reductive quenching of a photoexcited species is a major path to the reduced photoproduct in each step. Electrochemical studies show that two consecutive 1-electron reductions at the PtL²⁺ unit are nearly overlapped with the corresponding reductions at the MV²⁺ unit. The 1-electron-reduced species can be thus expressed as either PtL⁺-Cn-MV²⁺ or PtL²⁺-Cn-MV⁺, while the 2-electron-reduced one as PtL⁺-Cn-MV⁺. Moreover, the latter products behave as stacked species involving three types of π-dimer sites, (PtL⁺)₂, (MV⁺)₂, and (PtL⁺)(MV⁺), and do not drive thermal H₂ evolution according to the reaction: PtL⁺-Cn-MV⁺ + 2H⁺ → PtL²⁺-Cn-MV²⁺ + H₂. The H₂ evolution from water photocatalyzed by PtL²⁺-Cn-MV²⁺ has been found to occur via formation of 3-electron-reduced species; PtL⁺-Cn-MV⁺ + EDTA + hν → PtL⁰-Cn-MV⁺ (or PtL⁺-Cn-MV⁰) + EDTA(ox), and PtL⁰-Cn-MV⁺ (or PtL⁺-Cn-MV⁰) + 2H⁺ → PtL⁺-Cn-MV²⁺ (or PtL²⁺-Cn-MV⁺) + H₂.