SYNTHESIS AND CHARACTERIZATION OF LUMINESCENT SQUARE-PLANAR PLATINUM(II) COMPLEXES CONTAINING DITHIOLATE OR DITHIOCARBAMATE LIGANDS

The synthesis, characterization, and emission properties of a series of Pt(L2)(S-S) complexes are reported. The(L2) ligands include 4,7-diphenyl-1,10-phenanthroline (Ph2phen), 4,4'-dimethyl-2,2'-bipyridine (Me2bpy), 1,5-cyclooctadiene (COD), trimethyl phosphite (P(OMe)3), and the alpha-C-deprotonated form of 2-phenylpyridine (2-phpy). The (S-S) ligands include 1-(tert-butoxycarbonyl)-1-cyanoethylene-2,2-dithiolate(tbcda), 1-(diethoxyphosphinyl)-1-cyanoethylene-2, 2-dithiolate (cpdt), cis-1,2-dicarbomethoxyethylene-1,2-dithiolate (met), and N,N-diethyldithiocarbamate (Et2dtc). The complexes are readily synthesized by the addition of the dithiolate or thiolate ligand to Pt(L2)Cl2 except for Pt(P(OMe)3)2(met), which is prepared by the addition of 2 equiv of trimethyl phosphite to Pt(COD)(met). On the basis of characterizations using NMR and infrared spectroscopies, all of the complexes are assigned square=planar coordination geometries with varying degrees of distortion determined by ligand steric and electronic effects. The assignment of square-planar coordination is confirmed by single-crystal structure determinations of Pt(Me2bpy)(met) (5) and Pt(P(OMe)3)2(met) (7). Purple crystals of Pt(Me2bpy)(met).CHCl3 (C19H19Cl3N2O4PtS2) are monoclinic, of space group P2(1)/n (No. 14), with a = 11.402(1) angstrom, b = 9.085(8) angstrom, c = 23.041(2) angstrom, beta = 97.83 (5)degrees, V = 2364.70 angstrom3, Z = 4, and final R = 0.024 (R(w) = 0.030) for 3123 unique reflections. Pale yellow crystals of Pt(P(OMe)3)2(met) (7) (C12H24O10P2PtS2) are monoclinic, of space group P2(1)/c (No. 14), with a = 11.209(3) angstrom, b = 12.250(7) angstrom, c = 16.558(4) angstrom, beta = 102.97(3)degrees, V = 2215.80 angstrom3, Z = 4, and final R = 0.047 (R(w) = 0.061) for 4092 unique reflections. The average Pt-S distance of 5 is shorter than that of 7, being 2.244(7) and 2.301(4) angstrom, respectively. The average Pt-N distance of 2.055(1) angstrom for 5 and the average Pt-P distance of 2.238(9) angstrom for 7 agree well with previously reported structural results. The metrical parameters of the metchelate ring indicate some degree of metal-ligand delocalization in both structures. All of the complexes containing alpha,alpha'-diimine ligands are solvatochromic and exhibit strong luminescence in solution at room temperature. In particular, the complexes containing 1,1-dithiolates, Pt(Ph2phen)(tbcda) (1) and Pt(Ph2phen) (cpdt) (2), have a single featureless emission band in solution at room temperature at approximately 600 nm. Upon cooling of the samples to 77 K, two new higher energy bands emerge at 535 and 578 nm for 1 and 2 while the emission band at approximately 600 nm shifts to lower energy, approximately 650 nm. The emission and excitation spectra at 77 K are dependent upon the excitation wavelength. The cationic complex [Pt(Me2bpy)(Et2dtc)]PF6 (3) exhibits similar emissive properties at room temperature with an emission band at 590 nm and at 77 K with two higher energy bands at 474 and 506 nm and a lower energy band at 564 nm. The Pt(2-phpy)(Et2dtc) (4) complex is solution luminescent with a highly structured emission spectrum at both room temperature and 77 K. In contrast to those of the complexes containing 1,1-dithiolates, the emission and excitation spectra of Pt(Me2bpy)(met)(5) do not vary with excitation wavelength. Also, the emission spectrum of 5 is structured with a vibrational progression of 1175 cm-1. Pt(COD)(met)(6) and Pt(P(OMe)3)2(met) (7) exhibit strong emission in the solid state and in frozen glasses at 77 K with less resolved vibrational structure than is seen for complex 5.