Bis(4′-(4-pyridyl)-2,2′:6′,2′′-terpyridine)ruthenium(ii) complexes and their N-alkylated derivatives in catalytic light-driven water oxidation

Abstract

Hydrogen sulfate salts of [Ru(1)₂]²⁺ where 1 = 4′-(4-pyridyl)-2,2′:6′,2′′-terpyridine and four N-alkylated derivatives [Ru(L)₂]⁴⁺ were used as photosensitizers (λ_max ∼510 nm) for water oxidation in light driven reactions with peroxydisulfate as a sacrificial electron acceptor and Na₁₀[Co₄(H₂O)₂(α-PW₉O₃₄)₂] (Co₄POM) as the catalyst in sodium borate buffers at pH 8.0 and 9.0. The N-substituents investigated were benzyl (L⁺ = 2⁺), ethyl (L⁺ = 3⁺), allyl (L⁺ = 4⁺) and 4-cyanobenzyl (L⁺ = 5⁺). The O₂ yield in the presence of [Ru(L)₂]⁴⁺ (L⁺ = 2⁺–4⁺) was comparable to that obtained in the presence [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) using light sources with λ_max ≈ 490 nm. The ruthenium(III) complexes [Ru(1)₂]³⁺ and [Ru(L)₂]⁵⁺ (L⁺ = 2⁺–5⁺) are rather unstable in acidic conditions and could not be isolated. The most efficient photosensitizers [Ru(L)₂]⁵⁺ (L⁺ = 2⁺ and 4⁺) were the least stable under weakly basic conditions (pH 9.0) with a half-life τ_1/2 ∼ 10 ms. The stability of the complexes under photocatalytic turnover conditions is probably controlled by the rate at which ligand L⁺ is oxidized by Co₄POM in its highest oxidation state.