An Extended Chain and Trinuclear Complexes Based on Pt(II)–M (M = Tl(I), Pb(II)) Bonds: Contrasting Photophysical Behavior

The syntheses and structural characterizations of a Pt–Tl chain [{Pt­(bzq)­(C6F5)2}­Tl­(Me2CO)]n 1 and two trinuclear Pt2M clusters (NBu4)­[{Pt­(bzq)­(C6F5)2}2Tl] 2 and [{Pt­(bzq)­(C6F5)2}2Pb] 3 (bzq = 7,8-benzoquinolinyl), stabilized by donor–acceptor Pt → M bonds, are reported. The one-dimensional heterometallic chain 1 is formed by alternate “Pt­(bzq)­(C6F5)2” and “Tl­(Me2CO)” fragments, with Pt–Tl bond separations in the range of 2.961(1)–3.067(1) Å. The isoelectronic trinuclear complexes 2 (which crystallizes in three forms, namely, 2a, 2b, and 2c) and 3 present a sandwich structure in which the Tl­(I) or Pb­(II) is located between two “Pt­(bzq)­(C6F5)2” subunits. NMR studies suggest equilibria in solution implying cleavage and reformation of Pt–M bonds. The lowest-lying absorption band in the UV–vis spectra in CH2Cl2 and tetrahydrofuran (THF) of 1, associated with 1MLCT/1L′LCT 1[5dπ(Pt) → π*­(bzq)]/1[(C6F5) → bzq], displays a blue shift in relation to the precursor, suggesting the cleavage of the chain maintaining bimetallic Pt–Tl fragments in solution, also supported by NMR spectroscopy. In 2 and 3, it shows a blue shift in THF and a red shift in CH2Cl2, supporting a more extensive cleavage of the Pt–M bonds in THF solutions than in CH2Cl2, where the trinuclear entities are predominant. The Pt–Tl chain 1 displays in solid state a bright orange-red emission ascribed to 3MM′CT (M′ = Tl). It exhibits remarkable and fast reversible vapochromic and vapoluminescent response to donor vapors (THF and Et2O), related to the coordination/decoordination of the guest molecule to the Tl­(I) ion, and mechanochromic behavior, associated with the shortening of the intermetallic Pt–Tl separations in the chain induced by grinding. In frozen solutions (THF, acetone, and CH2Cl2) 1 shows interesting luminescence thermochromism with emissions strongly dependent on the solvent, concentration, and excitation wavelengths. The Pt2Tl complex 2 shows an emission close to 1, ascribed to charge transfer from the platinum fragment to the thallium [3(L+L′)­MM′CT]. 2 also shows vapoluminescent behavior in the presence of vapors of Me2CO, THF, and Et2O, although smaller and slower than those of 1. The trinuclear neutral complex Pt2Pb 3 displays a blue-shift emission band, tentatively assigned to admixture of 3MM′CT 3[Pt­(d) → Pb­(sp)] with some metal-mediated intraligand (3ππ/3ILCT) contribution. In contrast to 1 and 2, 3 does not show vapoluminescent behavior.