Pt–Ag clusters and their neutral mononuclear Pt(ii) starting complexes: structural and luminescence studies

Abstract

The mononuclear complexes [Pt(bzq)(S⁁S)] [S⁁S = pyrrolidinedithiocarbamate (pdtc 1), dimethyldithiocarbamate (dmdtc 2)] were prepared by reaction of [Pt(bzq)(NCMe)₂]ClO₄ with an equimolecular amount of [NH₄(pdtc)] and [Na(dmdtc)·2H₂O] respectively in MeOH. Reactions of 1 and 2 with AgClO₄ in 1 : 1 and 2 : 1 molar ratios rendered the heteropolinuclear compounds [{Pt(bzq)(S⁁S)Ag}₂](ClO₄)₂ (S⁁S = pdtc 3, dmdtc 4) and [{Pt(bzq)(S⁁S)}₂Ag](ClO₄) (S⁁S = pdtc 5, dmdtc 6) respectively. The X-ray studies on single crystals of 3 and 4 showed that both consist of tetranuclear [Pt₂Ag₂] clusters with the Pt–Ag and the Ag–Ag distances in the range of those corresponding to Pt–Ag dative bonds and argentophilic interactions. In 3 the tetranuclear [Pt₂Ag₂] clusters are connected into infinite polymeric chains by Pt⋯Pt metallophilic interactions (Pt⋯Pt = 3.1890(7) Å). The X-ray study on a single crystal of 5 showed that it is a polymer based on trinuclear [Pt₂Ag] clusters containing two unsupported Pt–Ag dative bonds and connected by Ag–S bonds in such a way that the “Pt–Ag–S–Pt–Ag–S” atoms draw a zigzag polymeric chain. TD-DFT calculations carried out for 1 indicate that the lowest energy absorption band in CH₂Cl₂ can be described as a mixture of ¹MLCT, ¹IL and ¹L′LCT transitions. Powdered samples of 1 at 298 K and 77 K show a green-yellow emission band coming mainly from a ³LC excited state. However complex 2 shows “luminescence thermochromism”: the colour of its luminescence changes from green-yellow at 77 K to orange-red at 298 K. The emission of the Pt–Ag clusters, 3–6, in the solid state, are due to excimeric ³ππ and/or ³MMLCT (dσ* → π*) low-lying excited states, indicating that the presence of silver in the clusters makes the “Pt(bzq)(S⁁S)” fragments interact to a large extent through Pt⋯Pt and/or π–π interactions. Solid 3 is a highly selective vapochromic compound towards acetonitrile although this behaviour is not fully reversible.