Extended structures containing Pt(ii)–Tl(i) bonds. Effect of these interactions on the luminescence of cyclometalated Pt(ii) compounds

Abstract

Neutralization reactions of the appropriate precursors (NBu₄)[Pt(bzq)(CC–R)₂] and (NBu₄)[Pt(C⁁N)(CN)₂] (C⁁N = bzq, ppy) with Tl^I salts afford [{PtTl(bzq)(CC–R)₂}₂] [R = Ph (1), C₅H₄N-2 (2)] and [PtTl(C⁁N)(CN)₂] [C⁁N = bzq (3), ppy (4)], respectively. X-Ray diffraction studies of complexes 2–4 show the existence of Pt^II-Tl^I bonds. In 2·CH₂Cl₂ the platinum–thallium units are associated in tetranuclear Pt₂Tl₂ entities which generate a 3-D network through short Tl⋯π(2-py) and π⋯π(bzq) contacts and additional weak Cl₂HC–H⋯π(CC) nonclassical interactions. Compounds 3 and 4 show extended 2-D networks by connection of the organometallic “PtTl(C⁁N)(CN)₂” units, through secondary Tl⋯NC contacts and moderate π⋯π(bzq) interactions in the case of 3. Complexes 1–3 containing the bzq group exhibit in the solid state “luminescence thermochromism” associated to dual emission. At room temperature they show an intense, visible orange (1: λ_max 625 nm), orange-red (2: λ_max 640 nm) or yellow (3: λ_max 582 nm) luminescence that changes to yellowish-green (1: λ_max 532 nm) or green [2: λ_max 524 nm; 3: λ_max 512 nm] upon cooling to 77 K. The unstructured low energy (LE) bands attributed to ³π–π* excimeric emissions due to extensive π–π interactions are dominant at room temperature. By contrast, the high energy (HE) bands are highly structured and predominant at 77 K. Due to the presence of Pt–Tl bonds these HE emissions are bathochromically shifted in relation to the precursors′ ones and have been tentatively assigned to a metal-metal′-to-ligand (bzq) charge transfer MM′LCT [d/s σ*(Pt,Tl) →π*(C⁁N)] mixed, as in the corresponding precursors, with some intraligand ³IL[π(C⁁N′) →π*(C⁁N)] in 3 and 4 or ligand-to-ligand charge transfer (alkynyl to bzq) ³LL′CT in complexes 1 and 2. Complex [PtTl(ppy)(CN)₂] 4, which does not show short contacts between the phenylpyridinate groups in solid state, only shows the HE green structured band both at 298 K and at 77 K. Only the cyanide derivatives are soluble and both spectroscopic (NMR and UV-Vis) and emission data (MeOH, 298 K and 77 K) indicate that the Pt^II–Tl^I bond breaks down in solution.