Extended Structures containing Pt(II)-Tl(I) Bonds. Effect of these interactions on the Luminescence of cyclometalated Pt(II) compounds

Resumen

Neutralization reactions of the appropriate precursors (NBu 4)[Pt(bzq)(CC-R)2] and (NBu4)[Pt(CN)(CN) 2] (CN = bzq, ppy) with TlI salts afford [{PtTl(bzq)(CC-R)2}2] [R = Ph (1), C5H 4N-2 (2)] and [PtTl(CN)(CN)2] [CN = bzq (3), ppy (4)], respectively. X-Ray diffraction studies of complexes 2-4 show the existence of PtII-TlI bonds. In 2·CH2Cl2 the platinum-thallium units are associated in tetranuclear Pt2Tl 2 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(CN)(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: λmax 625 nm), orange-red (2: λmax 640 nm) or yellow (3: λmax 582 nm) luminescence that changes to yellowish-green (1: λmax 532 nm) or green [2: λmax 524 nm; 3: λmax 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*(CN)] mixed, as in the corresponding precursors, with some intraligand 3IL[π(CN′) →π*(CN)] 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 PtII-TlI bond breaks down in solution. © 2009 The Royal Society of Chemistry.