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)] (<b>1</b>), in which C^N is either benzo­[<i>h</i>]­quinolinate (bhq), <b>1a</b>, or 2-phenylpyridinate (ppy), <b>1b</b>, with 1 equiv of bis­(diphenylphosphino)­methane (dppm) gave the bischelate complexes [Pt­(C^N)­dppm]­CF₃CO₂ (<b>2</b>). The binuclear complexes [Pt₂(C^N)₂(CF₃CO₂)₂(μ-dppm)] (<b>3</b>) were prepared, using an unusual reaction pathway, by the addition of equimolar amount of complexes <b>1</b> and <b>2</b>, 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 <b>3a</b> shows considerable intramolecular Pt···Pt and π···π interactions. The crystal structure and formation pathway toward <b>3</b> were compared with the similar analogue [Pt₂(bhq)₂(Cl)₂(μ-dppm)] (<b>5</b>). All complexes were fully characterized using multinuclear NMR spectroscopy and elemental analysis. Furthermore, the crystal structures of some complexes including <b>1b</b>, <b>2a</b>, <b>2b</b>, <b>3a</b>, and <b>5</b> 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 <b>2</b> to a bridge mode in complexes <b>3</b> and <b>5</b>, 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 <b>3</b>, which was assigned to a metal–metal to ligand charge transfer [MMLCT; dσ*­(Pt₂) → π*­(C^N)] transition showing negative solvatochromism, is important evidence for the Pt···Pt intramolecular interaction. The vibronically resolved and long-lifetime emission of <b>2a</b> 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 (³LC) emission was mixed with some ³MLCT character. Unstructured and short-lifetime emission in <b>3</b> refers to the phosphorescence ³MMLCT [dσ*­(Pt₂) → π*­(C^N)] transition. Although complex <b>5</b> is a binuclear compound, the long distance of the Pt···Pt interaction caused the occurrence of the ³MMLCT transition to fade and act as a mononuclear unit, and the emission originated mostly from the ³MLCT 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 (λ_abs and λ_em: <b>3a</b> > <b>3b</b> > <b>5</b> > <b>2a</b> > <b>2b</b>)