Reversible Binding of Solvent to Naked Pb^II Centers in Unusual Homoleptic Alkynyl‐Based Pt₂Pb₂ Clusters

Abstract

We report a series of luminescent sandwich-type clusters [Pt2 Pb2 (C≡CR)8] (R=Tol, 1; C6 H4 OMe-3, 2; C6 H4 OMe-4, 3) with a dynamic Pt2 Pb2 metallic core, which is key to their intriguing stimuli-responsive photophysical properties. The solvent-free solids 1-3 display an orange emission ascribed to charge transfer from Pt-alkynyl fragments to a delocalized orbital with mixed Pt2 Pb2 /C≡CR nature, with a predominant lead contribution and Pb⋅⋅⋅Pb bonding character ((3) MLCCT/(3) IL). They exhibit mechanical, color, and luminescence changes that are reversible and perceivable with the naked eye, which are attributed to small inter- and intramolecular structural modifications induced by gentle grinding. Interestingly, 1 and 2 also exhibit remarkable and fast reversible vapochromic responses to donor solvent vapors (acetone, THFMe-2: yellow; NCMe: green, vs. dry solids: orange). The structures of 1(acetone)2 ⋅2(Me2 CO), 2(acetone)3 , and 2(THFMe-2)2 allow the vapochromic responses to be ascribed to the fast creation/disruption of solvate clusters [Pt2 Pb2 (C≡CR)8 Sx] (x≥2), with concomitant electronic and geometrical modifications within the Pt2 Pb2 core, which are easily accessible through a slight change in the stereochemical activity of the lone pair. The binding of one (or two) solvent molecules to Pb(2+) increases the Pb⋅⋅⋅Pb separation in the metallic core, causing a destabilization of the target orbital and larger energy gaps of the transitions. All the solvates exhibit remarkable rigidochromism upon a decrease in temperature, which is also associated with the gradual increase in the transannular Pb⋅⋅⋅Pb separation, as revealed by X-ray crystallography of 1(acetone)2 at different temperatures. Investigation of the crystal lattice of 1⋅CH2 Cl2 and 3⋅2 CH2 Cl2 further suggests that the lack of vapor stimuli response of complex 3 could be attributed to the presence of competitive additional secondary intermolecular Pb⋅⋅⋅O(OMe) contacts, which give rise to a more compact network built up from extended chains of clusters.

Vapochromic Platinum(II) Complexes: Crystal Engineering toward Intelligent Sensing Devices

“…In this context, Berenguer et al recently reported an interesting vapochromic heterometallic tetranuclear complex, [Pt 2 Pb 2 (C≡C–R) 8 ] ( 19 ‐R; Figure 22). 58 The cluster is composed of two dianionic square‐planar Pt II ‐alkynyl complex molecules connected by two Pb II ions through two Pt II →Pb II dative bonds and several π‐bonding interactions between Pb II ions and surrounding alkynyl ligands, thereby resulting in the formation of a unique rhombic tetranuclear Pt 2 Pb 2 core. The sandwiched Pb II ions can bind solvent molecules by weak coordination bonds; X‐ray structural analysis revealed that the Pb II ion is bound by one or two acetone molecules in the acetone‐bound form, 19 ‐PhOMe · (acetone) 3 .…”

“…(b) Photographic images under UV light and (c) emission spectra of 19 ‐PhOMe under exposure to CH 3 CN (light blue), 2‐MeTHF (green), acetone (yellow), and air (orange). Reprinted with permission from the literature 58. Copyright 2014 Wiley‐VCH Verlag GmbH & Co. KGaA.…”

Luminescent Thermochromism in a Gold(I)–Copper(I) Phosphine–Pyridine Complex

“…This phenomenon is commonly seen in many luminescent transition-metal complexes. [19,20] Upon heating from 298 to 343 K, complex 3 exhibits a remarkable, reversible emission change from yellow to aquamarine. The high-temperature luminescent thermochromism of 3 is not easily understood.…”

“…Like 1, two molecules of 3·CH 2 Cl 2 appear in the asymmetric unit, but they only differ in the twisting of a phenyl ring. As noted in 3, the structure of 3·CH 2 (19) P3-Au2-Cu2-N3 45.26(12) P2-Au1-Cu1-N2 44.29 (13) P4-Au2-Cu2-N4 46.25 (13) The emission of compounds 1-3 were characterized in solution and the solid-state (Supporting Information). The absorption spectra of 1 and 2 in acetonitrile show broad and featureless bands deep into the UV.…”

“…These emission intensities at each temperature are maintained throughout several (Ͼ 5) heating/cooling cycles. To probe the nature of the reversible, high-temperature luminescent thermochromism, 3 was characterized by differential scanning calorimetry (DSC) and 19 …”