An Extended Chain and Trinuclear Complexes Based on
Pt(II)–M (M = Tl(I), Pb(II)) Bonds: Contrasting Photophysical
Behavior
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Abstract
The
syntheses and structural characterizations of a Pt–Tl chain
[{Pt(bzq)(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>}Tl(Me<sub>2</sub>CO)]<sub><i>n</i></sub> <b>1</b> and two trinuclear
Pt<sub>2</sub>M clusters (NBu<sub>4</sub>)[{Pt(bzq)(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>}<sub>2</sub>Tl] <b>2</b> and [{Pt(bzq)(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>}<sub>2</sub>Pb] <b>3</b> (bzq
= 7,8-benzoquinolinyl), stabilized by donor–acceptor Pt →
M bonds, are reported. The one-dimensional heterometallic chain <b>1</b> is formed by alternate “Pt(bzq)(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>” and “Tl(Me<sub>2</sub>CO)”
fragments, with Pt–Tl bond separations in the range of 2.961(1)–3.067(1)
Å. The isoelectronic trinuclear complexes <b>2</b> (which
crystallizes in three forms, namely, <b>2a</b>, <b>2b</b>, and <b>2c</b>) and <b>3</b> present a sandwich structure
in which the Tl(I) or Pb(II) is located between two “Pt(bzq)(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>” 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 CH<sub>2</sub>Cl<sub>2</sub> and tetrahydrofuran (THF) of <b>1</b>, associated with <sup>1</sup>MLCT/<sup>1</sup>L′LCT <sup>1</sup>[5d<sub>π</sub>(Pt) → π*(bzq)]/<sup>1</sup>[(C<sub>6</sub>F<sub>5</sub>) → 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 <b>2</b> and <b>3</b>, it shows
a blue shift in THF and a red shift in CH<sub>2</sub>Cl<sub>2</sub>, supporting a more extensive cleavage of the Pt–M bonds in
THF solutions than in CH<sub>2</sub>Cl<sub>2</sub>, where the trinuclear
entities are predominant. The Pt–Tl chain <b>1</b> displays
in solid state a bright orange-red emission ascribed to <sup>3</sup>MM′CT (M′ = Tl). It exhibits remarkable and fast reversible
vapochromic and vapoluminescent response to donor vapors (THF and
Et<sub>2</sub>O), 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 CH<sub>2</sub>Cl<sub>2</sub>) <b>1</b> shows interesting
luminescence thermochromism with emissions strongly dependent on the
solvent, concentration, and excitation wavelengths. The Pt<sub>2</sub>Tl complex <b>2</b> shows an emission close to <b>1</b>, ascribed to charge transfer from the platinum fragment to the thallium
[<sup>3</sup>(L+L′)MM′CT]. <b>2</b> also shows
vapoluminescent behavior in the presence of vapors of Me<sub>2</sub>CO, THF, and Et<sub>2</sub>O, although smaller and slower than those
of <b>1</b>. The trinuclear neutral complex Pt<sub>2</sub>Pb <b>3</b> displays a blue-shift emission band, tentatively assigned
to admixture of <sup>3</sup>MM′CT <sup>3</sup>[Pt(d) →
Pb(sp)] with some metal-mediated intraligand (<sup>3</sup>ππ/<sup>3</sup>ILCT) contribution. In contrast to <b>1</b> and <b>2</b>, <b>3</b> does not show vapoluminescent behavior
