A Remarkable Family of Rhodium Acetonitrile Compounds Spanning Three Oxidation States and with Nuclearities Ranging from Mononuclear and Dinuclear to One-Dimensional Chains

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Abstract

A series of homoleptic cations of Rh(I,II), Rh(II), and Rh(III) have been synthesized and characterized in the solid-state and in solution. Three new dinuclear compounds of dirhodium(II,II) were prepared by treatment of Rh2(O2CCH3)4(L)2 with Et3OBF4 or Me3Si(CF3SO3) in acetonitrile or propionitrile. The cations in [Rh2II,II(MeCN)10][BF4]4 (1), [Rh2II,II(MeCN)10][SO3CF3]4 (2), and [Rh2II,II(EtCN)10][BF4]4 (3) contain eight equatorial RCN groups oriented in an approximately square planar arrangement around the two Rh atoms and two axial RCN molecules. The redox properties of 1−3 were investigated by cyclic voltammetry, which revealed the presence of one or two irreversible reduction(s) but no oxidations. Although there was no electrochemical evidence for an accessible oxidation, it was found that treatment of [Rh2II,II(MeCN)10][BF4]4 with NOBF4 occurs to yield another member of the homoleptic acetonitrile family, namely, the octahedral d6 cation [RhIII(MeCN)6][BF4]3, (4). The corresponding one-electron reduction product was isolated by a slow galvanostatic reduction of [Rh2II,II(MeCN)10][BF4]4 (1) in MeCN at a Pt electrode. The crystals harvested from the cathode were found to be the unprecedented mixed-valence 1-D chain compound [RhI,II(MeCN)4(BF4)1.5]x (5), which result from a radical polymerization of the unstable Rh2I,II dinuclear cation. In an effort to access the final member of this series, namely, the homoleptic RhI species, the dicarbonyl compound [RhI(CO)2(MeCN)2][BF4] (6) was prepared, but all thermal and photochemical attempts to remove the CO ligands led to the conclusion that only one CO is labile. The mixed-ligand, square planar cation [RhI(CO)2(MeCN)2]+ was found to form a 1-D stack in the solid state, unlike previously reported salts with bulky counterions. For all of the compounds under investigation, infrared spectroscopy and X-ray studies were performed. The mixed-valence product was also characterized by EPR spectroscopy and SQUID magnetometry.

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