Metal-Metal Interactions in Stacked Mononuclear and Dinuclear Rhodium 2,2′-Biimidazole Carbonyl Complexes

Noncovalent metal-metal interactions in the solid state were studied by investigating the stacking of the cationic mononuclear [Rh(H(2)bim)(CO)(2)](+) (H(2)bim = 2,2'-biimidazole) and neutral dinuclear [Rh-2(R(2)bim)Cl-2(CO)(4)] (R = Me, methyl or Bn, benzyl) complexes. In both cases, the d(8) rhodium atoms possessed square planar coordination geometries. The cationic [Rh(H(2)bim)(CO)(2)](+) complexes with a chelating H(2)bim ligand all formed polymeric stacks with variable Rh center dot center dot center dot Rh distances (3.2743(3)-3.430(6) angstrom), depending on the counter-anions. In the solid state, the neutral dinuclear complexes with bridging Bn(2)bim formed isolated complexes with no metal-metal interactions between the dinuclear [Rh-2(Bn(2)bim)Cl-2(CO)(4)] motifs. The use of the Me(2)bim ligand led to a tetranuclear complex [Rh-2(Me(2)bim)C-2(CO)(4)](2) with an intermolecular Rh center dot center dot center dot Rh distance of 3.4104(7) angstrom. A neutral polymeric stack of [Rh-2(Me(2)bim)Cl-2(CO)(4)](n) was obtained only in crystals with ethanol of crystallization. The effect of the intermolecular metal-metal interactions on the absorption properties of the compounds was studied using time-dependent density functional theory (TD-DFT) methods. A bathochromic shift was observed in the lambda(max) metal-to-ligand charge transfer (MLCT) absorption with decreasing Rh center dot center dot center dot Rh distance. The shift was also observed spectroscopically. In the case of [Rh(H(2)bim)(CO)(2)][BF4], a clear dependence of color on temperature was also observed in the solid state due to the changes in Rh center dot center dot center dot Rh distance. Furthermore, the formation of an infinite linear metal chain generated metallic luster on the crystals.