Heteropolynuclear Pt(II)–M(I) Clusters with a C^∧N^∧C Biscyclometalated Ligand

The mononuclear complex [(EtO₂C-C^∧N^∧C)­Pt­(dmpyz)] (<b>1</b>) (dmpyz = 2,5-dimethylpyrazine) has been synthesized by reaction of [(EtO₂C-C^∧N^∧C)­Pt­(dmso)] (<b>A</b>) with dmpyz in a 1:1 molar ratio in dichloromethane. Complex <b>1</b> is the precursor for preparing the homodinuclear complex [{(EtO₂C-C^∧N^∧C)­Pt}₂(μ-dmpyz)] (<b>2</b>) and the heterotrinuclear clusters [{(EtO₂C-C^∧N^∧C)­Pt­(dmpyz)}₂M]­X (M = Cu, X = PF₆ (<b>3</b>); M = Ag, X = BF₄ (<b>4</b>)). Compounds <b>1</b>, <b>2</b>, and <b>4</b> were studied by X-ray diffraction methods. In the crystal packing of <b>1</b> and <b>2</b>, the molecules display short intermolecular π···π contacts, which control the solid-state emissive behavior. X-ray study on <b>4</b> shows two [Pt₂Ag] sandwich-type clusters in the asymmetric unit, both with the two square-planar “(R-CNC)­Pt­(dmpyz)” moieties stabilized by two Pt → Ag donor–acceptor bonds as well as by η¹- and η²-Ag–C interactions. Intramolecular π–π contacts were found between the pyridine rings of the CNC ligands within the same Pt₂Ag cluster. ¹H and ¹⁹⁵Pt NMR studies confirm that the Pt₂M cluster is also retained in solution at room temperature. ¹⁹⁵Pt NMR spectra of <b>3</b> and <b>4</b> show signals shifted significantly downfield when comparing with that for the monomer (<b>1</b>), which is attributed to the presence of Pt–M dative bonds. At lower temperatures (<i>T</i> = 193 K), the copper derivative definitely falls apart, whereas the silver one still holds up unbroken. In the solid state at 77 K, compounds <b>1</b>–<b>4</b> give red emissions arising from ³ππ excited states due to the intra- or intermolecular π–π contacts observed in the crystal structures. As expected, in glassy solutions (77 K), compound <b>3</b> displays analogous emissions to those from the starting material (<b>1</b>). Complexes <b>1</b> and <b>2</b> show structured emission bands that are particularly sensitive to the λ_ex (HE and LE). In contrast, <b>4</b> displays an unstructured emission at 680 nm with a shoulder at 556 nm; both are not dependent on the λ_ex. DFT and TDDFT computational studies have been performed on <b>1</b> and <b>2</b>, which support the conclusions drawn from the photophysical studies