Encapsulation of thallium(I) by tetranuclear rhodium or iridium complexes:: Synthesis and molecular structure of heterobimetallic complexes stabilized by s2-d8 bonding interactions

Reactions of [M-4(mu-pyS(2))(2)(cod)(4)] (M = Rh, Ir; pyS(2) = 2,6-pyridinedithiolate, cod = 1,5-cyclooctadiene) with equimolar amounts of TlPF6 give the heterobimetallic complexes [TlM4(mu-pyS(2))(2)(cod)(4)] [PF6]. An outstanding change in the coordination modes of the bridging ligands occurs on the incorporation of the Tl atom into the tetranuclear complexes with the concomitant formation of two unprecedented TL-Rh bonds. The X-ray structure of 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 calculations 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 ppm) 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 anodic processes at 0.52 and 0.93 V (CH2Cl2, 0.1 V s(-1)). These waves are shifted 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 irreversible reduction process at -1.48 V in which the Tl+ ion is extruded. Interestingly, upon excitation in the solid state at 77 K, complex 3 shows strong luminescence which can be related to the Rh-Tl-Rh chromophore.