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

Abstract

Noncovalent metal−metal interactions in the solid state were studied by investigating the stacking of the cationic mononuclear [Rh(H2bim)(CO)2]+ (H2bim = 2,2′-biimidazole) and neutral dinuclear [Rh2(R2bim)Cl2(CO)4] (R = Me, methyl or Bn, benzyl) complexes. In both cases, the d8 rhodium atoms possessed square planar coordination geometries. The cationic [Rh(H2bim)(CO)2]+ complexes with a chelating H2bim ligand all formed polymeric stacks with variable Rh···Rh distances (3.2743(3)−3.430(6) Å), depending on the counter-anions. In the solid state, the neutral dinuclear complexes with bridging Bn2bim formed isolated complexes with no metal−metal interactions between the dinuclear [Rh2(Bn2bim)Cl2(CO)4] motifs. The use of the Me2bim ligand led to a tetranuclear complex [Rh2(Me2bim)Cl2(CO)4]2 with an intermolecular Rh···Rh distance of 3.4104(7) Å. A neutral polymeric stack of [Rh2(Me2bim)Cl2(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 λmax metal-to-ligand charge transfer (MLCT) absorption with decreasing Rh···Rh distance. The shift was also observed spectroscopically. In the case of [Rh(H2bim)(CO)2][BF4], a clear dependence of color on temperature was also observed in the solid state due to the changes in Rh···Rh distance. Furthermore, the formation of an infinite linear metal chain generated metallic luster on the crystals.