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(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.

本研究通过考察阳离子单核配合物[Rh(H2bim)(CO)2]+（其中H2bim代表2,2'-联咪唑）与中性双核配合物[Rh2(R2bim)Cl2(CO)4]（R为甲基（methyl, Me）或苄基（benzyl, Bn））的堆叠行为，探究了固态环境中的非共价金属-金属相互作用。两类体系中的d8构型铑原子均采用平面正方形配位几何构型。带有螯合型H2bim配体的阳离子配合物[Rh(H2bim)(CO)2]+均会形成金属-金属间距可变（范围为3.2743(3)~3.430(6) 埃）的聚合物堆叠结构，其间距随抗衡阴离子的不同而发生变化。在固态环境中，带有桥联Bn2bim配体的中性双核配合物会形成分立的配合物结构，双核结构基元[Rh2(Bn2bim)Cl2(CO)4]之间不存在金属-金属相互作用。当使用Me2bim配体时，可得到四核配合物[Rh2(Me2bim)Cl2(CO)4]2，其分子间Rh···Rh间距为3.4104(7) 埃。仅在含有结晶乙醇的晶体中，才能得到[Rh2(Me2bim)Cl2(CO)4]n的中性聚合物堆叠结构。本研究采用含时密度泛函理论（time-dependent density functional theory, TD-DFT）方法，探究了分子间金属-金属相互作用对配合物吸收光谱性质的影响。随着Rh···Rh间距的缩短，金属到配体电荷转移（metal-to-ligand charge transfer, MLCT）吸收峰的最大吸收波长λmax出现了红移现象。该红移现象在光谱实验中也得到了观测验证。对于配合物[Rh(H2bim)(CO)2][BF4]，由于Rh···Rh间距随温度发生变化，其固态样品的颜色也表现出明显的温度依赖性。此外，无限线性金属链的形成会使晶体呈现出金属光泽。