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.