An extended chain and trinuclear complexes based on unsupported Pt(II)-M (M = Tl(I), Pb(II)) Bond: Contrasting Photophysical Behavior

Abstract

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.