Reversible Binding of Solvent to Naked PbII Centers in Unusual Homoleptic Alkynyl‐Based Pt2Pb2 Clusters

We report a series of luminescent sandwich-type clusters [Pt2Pb2(C≡CR)8] (R=Tol, 1; C6H4OMe-3, 2; C6H4OMe-4, 3) with a dynamic Pt2Pb2 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 Pt2Pb2/C≡CR nature, with a predominant lead contribution and Pb⋅⋅⋅Pb bonding character (3MLCCT/3IL). 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(Me2CO), 2(acetone)3, and 2(THFMe-2)2 allow the vapochromic responses to be ascribed to the fast creation/disruption of solvate clusters [Pt2Pb2(C≡CR)8Sx] (x≥2), with concomitant electronic and geometrical modifications within the Pt2Pb2 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 Pb2+ 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⋅CH2Cl2 and 3⋅2 CH2Cl2 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.