A tricarboxylated PtCl(terpyridine) derivative exhibiting pH-dependent photocatalytic activity for H₂ evolution from water

The first negatively charged PtCl(tpy) (tpy = 2,2′:6′,2′′-terpyridine) derivative, formulated as Na₂[PtCl(tctpy)]·5H₂O (tctpy = 2,2′:6′,2′′-terpyridine-4,4′,4′′-tricarboxylate), was prepared, characterized, and investigated in detail for its activity as a single-component photocatalyst that drives water reduction to H₂ in the presence of a sacrificial electron donor (EDTA). This compound was confirmed to exist in its fully deprotonated form [PtCl(tctpy)]^2- in aqueous media at pH > 4.4. Despite its dianionic character, [PtCl(tctpy)]^2- was found to form a specific adduct with anionic EDTA (i.e., YH₂^2- and YH^3-, where YH₄ is a fully protonated form of EDTA), enabling reductive quenching of the triplet metal-to-ligand charge transfer excited state within the adduct, leading to subsequent electron transfer steps correlated with Pt(ii)-catalyzed H₂ evolution from water. Electrochemical studies also reveal that the compound exhibits a unique pH-dependent first reduction (i.e., tctpy-centered reduction), leading to our realization of the first example of a Pt(ii)-based molecular system that photocatalyzes the H₂ evolution reaction accompanied by a ligand-based proton-coupled electron transfer (PCET) process.