Remarkable Influence of the Cyclometalating Ligand on the Nuclearity and Properties of Heterobridged (μ-X)(μ-CCR) Platinum(II) Complexes

Partial dehalogenation reaction of [Pt(C∧N)(μ-Cl)]2 [C∧N = 2,6-diphenylpyridinate (dppyH), 7,8-benzoquinolate (bzq)] with NaCCtBu (or HCCtBu/NEt3/CuI) proceeds with formation of binuclear ([{Pt(dppyH)}2(μ-Cl)(μ-CCtBu)], 1) or tetranuclear ([Pt2(bzq)2(μ-Cl)(μ-CCtBu)]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 Pt4 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 CH2Cl2), exhibiting an unstructured orange emission (560−580 nm). This emission has been attributed, according to computational studies of the frontier orbitals in the optimized T1 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 CH2Cl2, 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.