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

Abstract

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 groupP (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).