Synthesis, crystal structure, solution and spectroscopic properties, and hydrogen-evolving activity of [K(18-crown-6)][Pt(ii)(2-phenylpyridinato)Cl2]

A water-soluble ortho-metalated platinum(ii) complex [K(18-crown-6)][Pt(ppy)Cl2]·0.5H2O (1) (ppy = phenylpyridinate, 18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane) was prepared from a 1: 1 reaction of [K(18-crown-6)]2[PtCl4] and 2-phenylpyridine in methanol. The compound crystallizes in the triclinic system, space groupP1̄ (No. 2). A solvolysis product of 1, Pt(ppy)Cl(DMF) (DMF = N,N-dimethylformamide) (10), was also isolated and structurally characterized by X-ray diffraction (monoclinic system, space groupP21/n (No. 14)), revealing the lability of the coordination bond trans to the C(ppy) donor. Ligand substitution reactions of 1 in water were investigated by means of UV-visible absorption spectroscopy and time-dependent DFT calculations. The initial hydrolysis product in aqueous media was suggested to be Pt(ppy)Cl(OH2), in which the chloride ion trans to the C(ppy) donor in [Pt(ppy)Cl2]− is replaced with an aqua ligand due to the stronger trans effect originated by the C(ppy) donor. It was found that the ligand substitution reactions of [Pt(ppy)Cl2]− is extremely complicated. Leaving the complex in solution for a prolonged time resulted in deposition of a yellowish green chloride-bridged dimer Pt2(ppy)2(μ-Cl)2. Moreover, an unidentified blue species, ascribable to a Pt(ii)/Pt(iii) mixed-valence species, started to form while the initial hydrolysis reactions proceeded. Photochemical hydrogen production from water catalyzed by [Pt(ppy)Cl2]− was examined using a photosystem made up of Ru(bpy)32+ (bpy = 2,2′-bipyridine), methylviologen, and a sacrificial electron donor (EDTA) in an acetate buffer (pH 5). It is shown that the H2-evolving activity of [Pt(ppy)Cl2]− is higher that those of the hydrolysis products of [Pt(ppy)Cl2]−, but are comparable to those of other common Pt(ii) complexes having a cis-Pt(ii)Cl2 unit, such as cis-PtCl2(NH3)2, PtCl2(4,4′-dicarboxy-bpy), and PtCl2(2,2′-bipyrimidine).