Remarkable Influence of the Cyclometalating Ligand on the Nuclearity and Properties of Hetero-bridged (m-X)(m-CCR) Pt(II) Complexes.

Abstract

Partial dehalogenation reaction of [Pt(C^N)(μ-Cl)] 2 [C^N = 2,6-diphenylpyridinate (dppyH), 7,8-benzoquinolate (bzq)] with NaC=C tBu (or HC=C tBu/NEt 3/CuI) proceeds with formation of binuclear ([{Pt(dppyH)} 2(μ-Cl)(μ-C=C tBu)], 1) or tetranuclear ([Pt 2(bzq) 2(μ-Cl) (μ-C=C tBu)] 2, 4) complexes, depending on the nature of the cyclometalated ligand. Similar halide-bridged complexes 2, 3, 5, and 6 have also been prepared. The X-ray structures of 1, 4, and 5 reveal the selective formation in all cases of the isomer having the bridging chlorine atom located in a trans position to the metalated carbon atoms of two Pt(C^N) units. In 4 and 5, the tetrahedral Pt 4 core is additionally stabilized by short Pt•••Pt and π•••π intramolecular bonding interactions, which have influence on their photophysical properties. The lower lying absorption bands are ascribed to mixed 1MLCT [Pt→dppyH]/ 1LL′CT [C=CR/X→dppyH] (complexes 1-3) or to 1MMLL′CT transitions (L = C=CR, L′ = bzq) (complexes 4-6, TD-DFT). Complexes 1-3 are nonemissive. By contrast, the tetranuclear clusters are brightly emissive at 77 K (solid state, frozen CH 2Cl 2), exhibiting an unstructured orange emission (560-580 nm). This emission has been attributed, according to computational studies of the frontier orbitals in the optimized T 1 state of 5, to an excited state of large [Pt(d)/π(C=CR)/Pt(d)→bzq/bzq] character, with the LUMO delocalized on the adjacent Pt(bzq) fragments. In frozen CH 2Cl 2, an additional low-energy shoulder (∼640-675 nm) is also detected, probably due to the presence of close distinct emissive states originating in small modifications in the ground-state structure in frozen solution. © 2011 American Chemical Society.