Encapsulation of thallium(I) by tetranuclear rhodium or iridium complexes:Synthesis and molecular structure of heterobimetallic complexes stabilizedby s(2)-d(8) bonding interactions

Reactions of [M-4(mu-pyS(2))(2)(cod)(4)] (M = Rh, Ir; pyS(2) = 2,6-pyridine dithiolate, cod = 1,5-cyclooctadiene) with equimolar amounts of TlPF6 give the heterobimetallic complexes [TlM4(mu-pyS(2))(2)(cod)(4)] [PF6]. An outst anding change in the coordination modes of the bridging ligands occurs on t he incorporation of the Tl atom into the tetranuclear complexes with the co ncomitant formation of two unprecedented TL-Rh bonds. The X-ray structure o f the cation [TlRh4(mu-pyS(2))(2)(cod)(4)](+) (3(+)) shows an almost linear Rh-Tl-Rh bonded subunit with Tl-Rh separations of 2.7686(9) and 2.7706(9) Angstrom. An unusual sawhorse coordination environment of the thallium atom is completed by binding to two pyridine nitrogen atoms. Theoretical calcul ations on this Rh-Tl interaction also support the formation of Rh-Tl bonds, which is accompanied by a slight electron transfer from thallium to both " Rh(cod)" fragments. An additional partial oxidation of the thallium center agrees with the large shift low field of the resonance (observed at 2926 pp m) in the Tl-205{H-1} NMR spectrum of [TlRh4(mu-pyS(2))(2)(cod)(4)][PF6] (3 ) at 273 K. Moreover, complex 3 undergoes two irreversible one-electron ano dic processes at 0.52 and 0.93 V (CH2Cl2, 0.1 V s(-1)). These waves are shi fted to higher potential than those of the parent complex [Rh-4(mu-pyS(2))( 2)(cod)(4)]. In addition, the cathodic response of this complex shows an ir reversible reduction process at -1.48 V in which the Tl+ ion is extruded. I nterestingly, upon excitation in the solid state at 77 K, complex 3 shows s trong luminescence which can be related to the Rh-Tl-Rh chromophore.