Platinum Complexes of Tetradentate <i>NCCN</i> -Coordinating Ligands: Structures and Photophysical Properties of Pt ^II , Pt ₂ ^III and Pt ^IV 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, PtL^1-3, featuring <i>NCCN</i>-coordinated ligands. For H₂L² and H₂L³, the corresponding Pt(IV) compounds PtL^2-3Cl₂ were initially isolated, from which PtL^2-3 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 PtL^2-3 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.