Influence of the Diphosphine Coordination Mode on the Structural and Optical Properties of Cyclometalated Platinum(II) Complexes: An Experimental and Theoretical Study on Intramolecular Pt•••Pt and π•••π Interactions

The reaction of [Pt(C<^>N)(CF3CO2)(SMe2)] (1), in which CAN is either benzo[h]quinolinate (bhq), 1a, or 2-phenylpyridinate (ppy), 1b, with 1 equiv of bis(diphenylphosphino)methane (dppm) gave the bischelate complexes [Pt(C<^>N)dppm]CF3CO2 (2). The binuclear complexes [Pt-2(C<^>N)(2)(CF3CO2)(2)(mu-dppm)] (3) were prepared, using an unusual reaction pathway, by the addition of equimolar amount of complexes 1 and 2, through the ring opening of the chelating dppm ligand and coordination of the CF3CO2 anion to the platinum center. The proposed reaction pathway and effect of the solvent polarity were investigated by density functional theory (DFT) calculations. The crystal structure of 3a shows considerable intramolecular Pt center dot center dot center dot Pt and pi center dot center dot center dot pi interactions. The crystal structure and formation pathway toward 3 were compared with the similar analogue [Pt-2(bhq)(2)(Cl)(2)(mu-dppm)] (5). All complexes were fully characterized using multinuclear NMR spectroscopy and elemental analysis. Furthermore, the crystal structures of some complexes including 1b, 2a, 2b, 3a, and 5 were confirmed by X-ray crystallography. The effect of dimerization via a change in the coordination mode of dppm, from a chelate mode in complex 2 to a bridge mode in complexes 3 and 5, upon the excited states of the studied compounds was investigated in their distinguished absorption and emission profiles. The appearance of a remarkably low energy band in the absorption spectra of 3, which was assigned to a metal metal to ligand charge transfer [MMLCT; d sigma*(Pt-2) -> pi*(C<^>N)] transition showing negative solvatochromism, is important evidence for the Pt center dot center dot center dot Pt intramolecular interaction. The vibronically resolved and long-lifetime emission of 2a in poly(methyl methacrylate) media and powder states at 77 and 300 K, along with time-dependent DFT calculations, suggested that the triplet ligand-centered ((LC)-L-3) emission was mixed with some MMLCT character. Unstructured and short-lifetime emission in 3 refers to the phosphorescence (MMLCT)-M-3 [d sigma*(Pt-2) -> pi*(C<^>N)] transition. Although complex 5 is a binuclear compound, the long distance of the Pt center dot center dot center dot Pt interaction caused the occurrence of the (MMLCT)-M-3 transition to fade and act as a mononuclear unit, and the emission originated mostly from the (MLCT)-M-3 transition. As a result, more metal participation leads to more red-shifted absorption and emission spectra of the studied complexes upon going from LC to MLCT to MMLCT transitions (lambda(abs) and lambda(em): 3a > 3b > 5 > 2a > 2b).