Platinum Complexes of Tetradentate NCCN-Coordinating Ligands: Structures and Photophysical Properties of PtII, Pt2III and PtIV Compounds.

Tetradentate proligands based on biphenyl appended at the 3 and 3' positions with pyridin-2-yl rings (H2L1), quinoline-8-yl (H2L3) heterocycles, or one such pyridine and quinoline (H2L2), have been synthesized by palladium-catalyzed cross-coupling. These compounds were expected to undergo cyclometalation with K2PtCl4 to generate Pt(II) complexes, PtL1-3, featuring NCCN-coordinated ligands. For H2L2 and H2L3, the corresponding Pt(IV) compounds PtL2-3Cl2 were initially isolated, from which PtL2-3 could be obtained by reduction with zinc. In contrast, H2L1 gave an unusual Pt(III) dimer, Pt2L12Cl2, and subsequently PtL1 upon reduction. All of the complexes have been structurally characterized in the solid state by X-ray diffraction. The quinoline-containing complexes of L2 and L3 feature one or two 6-membered chelates, respectively, with an accompanying twisting of the quinoline ring(s) relative to the biphenyl unit to allow the metal ion to achieve its preferred square-planar coordination. PtL1 displays intense green phosphorescence in solution at room temperature with a quantum yield of 67%. The deep-red emission of PtL2-3 is weaker due to suppressed triplet radiative rate constants as well as faster nonradiative decay. The Pt(IV) complexes emit only at 77 K, with long lifetimes of around 300 μs, while the Pt(III) dimer shows no detectable emission.