New water soluble and luminescent platinum(II) compounds, vapochromic behavior of [K(H2O)][Pt(bzq)(CN)2], new examples of the influence of the counterion on the photophysical properties of d8 square-planar complexes

This work describes the synthesis of compounds [Pt(C boolean AND N)(NCMe)(2)]ClO4 (C boolean AND N = 7,8-benzoquinolinato (bzq), 2-phenylpyridinato (ppy)) and their use as precursors for the preparation of the cyanido complexes [Pt(C boolean AND N)(CN)2](-), which were isolated as the potassium, [K(H2O)][Pt(C boolean AND N)(CN)(2)] [C boolean AND N = bzq (3a), ppy (4a)], and the tetrabutylammonium, NBu4[Pt(C boolean AND N)(CN)(2)] [C boolean AND N = bzq (5), ppy (6)], salts. The difference in the cation has an influence on the solubility, color, and emission properties of these compounds. Compounds 5 and 6 are yellow and soluble in organic solvents, while the potassium salts are also soluble in water and exhibit two forms: the water-containing [K(H2O)][Pt(C boolean AND N)(CN)(2)] [C boolean AND N = bzq (3a), ppy (4a)] complexes and the anhydrous ones K[Pt(C boolean AND N)(CN)(2)] [CAN = bzq (3b), ppy (4b)], the former being strongly colored [red (3a) or purple (4a)) and the latter being yellow. Compounds 3a and 4a transform reversibly into the yellow, 3b and 4b, compounds upon desorption/reabsorption of water molecules from the environment. The red solid, 3a, also exhibits vapochromic behavior when it is exposed to volatile organic compounds, the shortest response times being those observed for methanol and ethanol. UV-vis and emission spectra of all compounds were recorded both in solution and in the solid state. In methanol solution, the difference in the cation causes no differences in the absorption nor in the emission spectra, which is as expected for the monomer species. However, in the solid state, the differences are notable. For both the red (3a) and purple (4a) compounds, a prominent absorption, which has maxima at about 550 nm and is responsible for their intense colors, as well as a structureless emission at lambda > 700 nm that suffers a significant red-shift upon cooling, are due to (MMLCT)-M-1,3 (= metal-metal-to-ligand charge transfer) [d sigma*(Pt) -> pi*(C boolean AND N)] transitions characteristic of linear-chain platinum complexes with short Pt...Pt contacts. Time-dependent density-functional theory calculations on complex 5 and the X-ray diffraction study on compound [K(OCMe2)(2)][Pt(PPY)(CN)(2)] (4c) are also included.