Palladium(II) and platinum(II) analogues of luminescent diimine triangulo complexes supported by triply bridging sulfide ligands: Structural and spectroscopic comparisons

Reaction of [M(L-L)Cl-2] [M = Pd, Pt; L-L = 4,4'-di-tert-butyl-2,2'-bipyridine ((t)Bu(2)bpy), 4,4'-dimethyl-carboxylate-2,2'-bipyridine ((CO2Me)(2)bpy), bis(diphenylphosphino)methane (dppm)] with Na2S in refluxing methanol afforded [M-3((t)Bu(2)bpy)(3)(mu (3)-S)(2)](2+) [M = Pd (1a), Pt (2a)], [M-3((CO2Me)(2)bpy)(3)(mu (3)-S)(2)](2+) [M = Pd (1b), Pt (2b)], and [Pt-3(dppm)(3)(mu (3)-S)(2)](2+) (3) as perchlorate salts. X-ray crystal analysis revealed that la, 1b, 2a, and 3 have triangular M3S2 core structures. The three metal atoms in la, 2a, and 3 form virtual equilateral triangles with intramolecular Pd-Pd and Pt-Pt separations of 3.027(1)-3.065(1) and 3.104(1)-3.154(1) Angstrom, respectively. An isosceles triangle of Pd-3 atoms is observed in the molecular structure of lb. The (MLCT)-M-1 absorption of 2a and 2b appears at 415 and 448 run, respectively, in dichloromethane and is significantly red-shifted from the lowest energy absorption band of the Pd3 analogues. Complex la exhibits weak photoluminescence in the solid state at 77 and 298 K (uncorrected lambda (max) 760 and 730 nm, respectively) while the 77 K solid-state emission of lb (uncorrected lambda (max) 760 nm) is also weak. At 77 K, complexes 2a, 2b, and 3 display broad unstructured emissions at lambda (max) 616-630 nm in the solid state. Ligand-field excited states are tentatively assigned for these emissions.