Ladder-like heteropolynuclear assemblies via cyanido bridges and platinum(II)-thallium(I) bonds: structural and photophysical properties

We describe the mononuclear anionic cyanido-pentafluorophenyl complexes, (NBu₄)[Pt(C^N)(C₆F₅)(CN)] [C^N = 7,8-benzoquinolate (bzq) 1, 2-(2,4-difluorophenyl)pyridinate (dfppy) 2] and the heteropolynuclear derivatives [{Pt(C^N)(C₆F₅)(CN)}Tl] (C^N = bzq 3, dfppy 4). These complexes were synthesized via a two-step modular synthesis by reaction of the corresponding potassium salts K[Pt(C^N)(C₆F₅)(CN)], prepared in situ from [Pt(C^N)(C₆F₅)(DMSO)] and KCN in acetone/H₂O, with TlPF₆. The structures of {[Pt(bzq)(C₆F₅)(CN)Tl]·THF}_n (3·THF)_n and [{Pt(dfppy)(C₆F₅)(CN)}Tl]₄·dioxane [4]4·dioxane, determined by X-ray crystallography, confirm the presence of Pt(II)-Tl(I) bonds [2.9795(6)-3.0736(3) Å], but in the dfppy complex, the incorporation of dioxane, causes a significant structural change. Thus, whereas [3·THF]_n achieves a bent-ladder shape extended double chain Tl⋯[Pt⋯Tl]_n⋯Pt supported by lateral bridging [Pt](μ-CN)[Tl] ligands, [4]4·dioxane is formed by discrete Pt₄Tl₄ rectangular aggregates stabilized by [Pt](μ-CN)[Tl] and Pt⋯Tl bonds, which are connected by dioxane bridging molecules through Tl⋯O(dioxane) additional contacts. Solid state emissions are redshifted compared with the mononuclear derivatives 1 and 2 and have been assigned, with the support of theoretical calculations on Pt₄Tl₄ models, to metal-metal'-to-ligand charge transfer (³MM'LCT [d/s σ*(Pt, Tl) → π*(C^N)]) for 3 and mixed ³MM'LCT/³IL for 4. In fluid THF solution, the complexes are not emissive. At 77 K, 3 and 4 exhibit bright emissions attributed to the formation of bimetallic [{Pt(C^N)(C₆F₅)(CN)}Tl(THF)_x], and anionic [Pt(C^N)(C₆F₅)(CN)]^- fragments. Furthermore, both 3 and 4 exhibit a reversible mechanochromism with a red shift of the emissions upon crushing, suggesting some degree of shortening of metal-metal separation. Finally, complex 3 shows solvatochromic behavior with color/luminescence changes by treatment with a drop of MeOH, CH₂Cl₂, THF or Et₂O, with shifts from 583 in 3-MeOH to 639 nm in 3-THF. However, 4 only demonstrates a bathochromic response to MeOH.