Synthesis of paramagnetic tetranuclear rhodium and iridium complexes with the 2,6-pyridinedithiolate ligand. Redox-induced degradation to diamagnetictriiridium compounds

The tetranuclear complexes [M-4(mu -PYS2)(2)(diolefin)(4)] [PyS2 = 2,6-pyri dinedithiolate; M = Rh, diolefin cod (1,5-cyclooctadiene) (1), tfbb (tetraf luorobenzo[5,6]bicyclo[2.2.2]octa-2,5,7-triene) (2); M = Ir, diolefin cod ( 3), tfbb (4)] exhibit two one-electron oxidations at a platinum disk electr ode in dichloromethane at potentials accessible by chemical reagents. The r hodium tetranuclear complexes were selectively oxidized to the monocationic complexes [Rh4(mu -PyS2)(2)(diolefin)(4)](+) (1(+), 2(+)) by mild one-elec tron oxidants such as [CP2Fe](+) or [N(C6H4Br-4)(3)](+) and isolated as the PF6-, BF4-, and ClO4- salts. Silver salts behave as noninnocent one-electr on oxidants for the reactions with the rhodium complexes 1 and 2 since they give sparingly soluble coordination polymers. The complex [Ir-4(mu -PyS2)( 2)(cod)(4)](+) (3(+)) was obtained as the tetrafluoroborate salt by reactio n of 3 with 1 molar equiv of AgBF4, but the related complex 4(+) could not be isolated from the chemical oxidation of [Ir4(mu -PyS2)(2)-tfbb)(4)] (4) with AgBF4. Oxidation of 3 and 4 with 2 molar equiv of common silver salts resulted in the fragmentation of the complexes to give the diamagnetic trii ridium cations [Ir-3(mu -PyS2)(2)(diolefin)(3)](+). The molecular structure of [Ir-3(mu -PYS2)(2)(cod)(3)]BF4, determined by X-ray diffraction methods , showed the three metal atoms within an angular arrangement. Both 2,6-pyri dinedithiolate tridentate ligands bridge two metal-metal bonded d(7) center s in pseudo octahedral environments and one d(8) square-planar iridium cent er. An interpretation of the EPR spectra of the 63-electron mixed-valence p aramagnetic tetranuclear complexes suggests that the unpaired electron is d elocalized over two of the metal atoms in the complexes 1(+)-3(+).