Synthesis of Paramagnetic Tetranuclear Rhodium and Iridium Complexes with the 2,6-Pyridinedithiolate Ligand. Redox-Induced Degradation to Diamagnetic Triiridium Compounds

The tetranuclear complexes [M4(μ-PyS2)2(diolefin)4] [PyS2 = 2,6-pyridinedithiolate; M = Rh, diolefin = cod (1,5-cyclooctadiene) (1), tfbb (tetrafluorobenzo[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 electrode in dichloromethane at potentials accessible by chemical reagents. The rhodium tetranuclear complexes were selectively oxidized to the monocationic complexes [Rh4(μ-PyS2)2(diolefin)4]+ (1+, 2+) by mild one-electron oxidants such as [Cp2Fe]+ or [N(C6H4Br-4)3]+ and isolated as the PF6-, BF4-, and ClO4- salts. Silver salts behave as noninnocent one-electron oxidants for the reactions with the rhodium complexes 1 and 2 since they give sparingly soluble coordination polymers. The complex [Ir4(μ-PyS2)2(cod)4]+ (3+) was obtained as the tetrafluoroborate salt by reaction of 3 with 1 molar equiv of AgBF4, but the related complex 4+ could not be isolated from the chemical oxidation of [Ir4(μ-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 triiridium cations [Ir3(μ-PyS2)2(diolefin)3]+. The molecular structure of [Ir3(μ-PyS2)2(cod)3]BF4, determined by X-ray diffraction methods, showed the three metal atoms within an angular arrangement. Both 2,6-pyridinedithiolate tridentate ligands bridge two metal−metal bonded d7 centers in pseudo octahedral environments and one d8 square-planar iridium center. An interpretation of the EPR spectra of the 63-electron mixed-valence paramagnetic tetranuclear complexes suggests that the unpaired electron is delocalized over two of the metal atoms in the complexes 1+3+.