Heteroleptic binuclear palladium(ii) and platinum(ii) complexes containing 1,2-bis(diphenylphosphino)acetylene and 1,2-benzenedithiolates: Syntheses, crystal structures, electrochemistry and photoluminescence properties

A series of heteroleptic binuclear Pd(ii) and Pt(ii) complexes, [M(bdts)]₂(μ-dppa)₂ (M = Pd (3) and Pt (4); dppa = 1,2-bis(diphenylphosphino)acetylene = Ph₂PCCPPh₂; bdts = 1,2-benzenedithiolate (bdt: a), 3,4-toluenedithiolate (tdt: b) and 1,4-dichloro-2,3-benzenedithiolate (Cl₂bdt: c), containing two square-planar MP₂S₂ cores were prepared using (MCl ₂)₂(μ-dppa)₂ (M = Pd (1) and Pt (2)) and the corresponding 1,2-benzenedithiols, and characterized by spectroscopic methods including FT-IR, Raman, UV-vis, MALDI-TOF-MS, ³¹P{¹H} and/or ¹⁹⁵Pt{¹H} NMR spectroscopy. X-Ray crystal structure analyses for complexes 3 and 4 revealed that C1C2C4C3 is twisted in two ways with a torsion angle of 21.6-30.7° for 3a, 3b, 4a and 4b and about 42° for 3c and 4c, and that their crystals are racemic mixtures. Due to the more electronegative chloride atoms in the ligand, complexes 3c and 4c show higher ν(M-S) vibrational frequencies in their Raman spectra, smaller spin-spin coupling constants (J_Pt-P) in their ¹⁹⁵Pt{³¹P} NMR spectra and higher anodic potentials (E_pa) in their cyclic voltammograms than complexes 3a, 3b, 4a and 4b. Moreover, only complex 4c containing the chlorinated ligand and Pt(ii) ion exhibits luminescence (λ_ob = 610 nm and λ_ex = 440 nm) in the solid state at 298 K. This emissive transition can be assigned as the d-π*_dithiolate metal-to-ligand charge transfer (MLCT) and the feasibility of this spin-forbidden transition is ascribed to the effective spin-orbit coupling of ligand c containing heavy chloride atoms and the large spin-orbit coupling in Pt(ii).