Cyclometalated Complexes of Palladium(II) and Platinum(II):  cis-Configured Homoleptic and Heteroleptic Compounds with Aromatic C⌒N Ligands

The palladium(II) and platinum(II) bis-homoleptic complexes M(C⌒N)2, where C⌒N is benzo[h]quinoline (H-bhq), 2-phenylpyridine (H-phpy), 2-(2‘-benzothienyl)pyridine (H-bthpy), 2-(2‘-thienyl)quinoline (H-thq), and 2-(2‘-thienyl)pyridine (H-thpy), were prepared by metal exchange of the lithiated ligands C⌒N with M(Et2S)2Cl2. The palladium(II) bis-heteroleptic complexes, Pd(C⌒N)(C‘⌒N‘), were synthesized from Pd(C⌒N)2 bis-homoleptic complexes, which were converted by HCl into the dichloro-bridged compounds [Pd(C⌒N)Cl]2. By addition of Et2S, the Pd(C⌒N)(Et2S)Cl complexes were formed, which were allowed to react with Li(C‘⌒N‘), yielding M(C⌒N)(C‘⌒N‘) compounds. An alternative way for obtaining the bis-heteroleptic molecules is by ligand exchange, according to the equilibrium M(C⌒N)2 + M(C‘⌒N‘)2 = 2M(C⌒N)(C‘⌒N‘). The crystal structures of Pt(bhq)2 (1) and Pt(thq)2 (3) present an important distortion of the square planar (SP-4) geometry toward a two-bladed helix. Bis-homoleptic and some bis-heteroleptic complexes of palladium(II) have also been synthesized. In both cases, the steric interactions between the two ligands cause again a helical distortion rather than yielding trans compounds. For cis-bis(benzo[h]quinoline)platinum(II) (1), in the crystal (monoclinic, space group P21/n, a = 13.728(3) Å, b = 6.9537(15) Å, c = 19.701(5) Å, β = 106.17(2)°, Z = 4, ρcalcd = 2.028 g·cm-3; diffractometer measurements, block-matrix least-squares refinement, R = 0.043, R w = 0.047) the average Pt−N and Pt−C distances are 2.151(6) and 1.988(7) Å, respectively. One benzo[h]quinoline ligand is significantly less planar than the other. For cis-bis[2-(2‘-thienyl)quinoline]platinum(II) (3), in the crystal (trigonal, space group P3221, a = b = 9.373(1) Å, c = 20.152(3) Å, Z = 3, ρcalcd = 2.022 g·cm-3; diffractometer measurements, full-matrix least-squares refinement, R = 0.010, R w = 0.010) the molecule has C 2 symmetry and is chiral. The Pt−N and Pt−C bond lengths are 2.156(2) and 1.984(3) Å, respectively. The quinoline moitey is not planar but bent about the fused bond by 6.8°. The thiophene moiety is inclined to the best plane through the quinoline moiety by 24.4°.