Synthesis and luminescence properties of cyclopalladated complexes with S⁁N and O⁁N donor ligands

Abstract

The direct reaction between the hydroxo-complexes [{Pd(μ-OH)(C⁁N)}₂] and protonated S⁁N-donor ligands to yield new dinuclear cyclometallated palladium(II) complexes containing –NCS– bridging heterocyclic thionates of general formula [{Pd(μ-N⁁S)(C⁁N)}₂] (C⁁N = 2-(2-pyridyl)phenyl (Phpy) I; = 7,8-benzoquinolyl (Bzq) II; N⁁S = pyrimidine-2-thionate (Spym) 1, benzoimidazolidine-2-thionate (Sbimid) 2, 1-methylimidazoline-2 thionate (Smeimid) 3) is described. The related mononuclear complexes [Pd(N⁁S)(C⁁N)] N⁁S = 2-aminothiophenolate (Atph) 4) were prepared in a similar way. Photophysical properties were investigated together with those of complexes with related ligands pyridin-2-thionate (Spy) 5 and pyrrole-2-carboxaldehydate (2-pcal) 6. All the compounds absorb intensely below 300 nm via¹LC transitions located in Bzq or Phpy ligands, and display additional low energy absorptions of mixed ¹MLCT-¹LC character. In the solid state diffuse reflectance UV-vis spectra, the differences are notable for dinuclear complexes. The complexes under study are quite unusual in terms of luminescence behaviour, since most of them are emissive in solution at room temperature and all display intense emissions in frozen CHCl₃ solution, but also in the solid state at 298 and 77 K. Emission quantum yields are in the range 2.52–0.14%, similar to that of related complexes. Structural characterisation by X-ray diffraction of complexes II3 and II6 confirmed the proposed formula.