Complexes of Rhodium(III) with Two Chelating C⁁N Ligands and One Diimine Ligand

The synthesis of two series of cyclometalated Rh^III compounds is described, namely of 5 dinuclear chlorobridged species {Rh(C?N)₂} (μ-Cl)₂] A and of 15 mononuclear complexes Rh(C?N)₂(N?N)]⁺ B ; C?N stands for five different cyclometalating ligands, i.e. deprotonated phenylpyridine, deprotonated 2-(thienyl)pyridine, and three deprotonated 1-aryl-1H-pyrazoles, and N?N for six diimine ligands such as 2,2-bihyridine, 2,2′-bi-1H-imidazole, and 2,2′-bipyrimidine. For (2,2′-bipyridine)bis2-(thien-2-yl)pyridinato-N,C^3′]rhodium(III) chloride, an X-ray structure determination was carried out. In the other cases, ¹H-NMR spectra established the configuration of the complexes. All mononuclear and dinuclear complexes show a C,C cis-configuration. The UV/VIS-absorption bands at longest wavelength are most likely due to metal-to-ligand charge-transfer (MLCT) transitions, depending on the nature of the cyclometalating ligand C?N and on the diimine ligand N?N. The receptor orbital is in some cases on the cyclometalating ligand, in others on the diimine. All monomer complexes exhibit at least one reversible reduction wave in the cyclovoltammogram in dimethylformamide solutions, attributable to a ligand-centered reduction. It is, therefore, concluded that the LUMO in Rh(C?N)₂(N?N)]⁺ is of L(π*) character, as opposed to Rh(bpy)₃]³⁺, where it is a metal d-orbital. The crystal system of (2,2′-bipyridine)bis2-(thien-2-yl)pyridinato-N,C_3′]rhodium(III) chloride—water (1/2.125) is tetragonal (space group P⁴; R = 0.036, R_w = 0.040). The Rh-atom has slightly distorted octahedral environment; the average distances are (Rh? N/thienyl? pyridine) = 2.060 (3), Rh? C = 1.9885 (3), and Rh? N(bipyridine) = 2.1415 (3) Å. Of the three ligands the 2,2′-bipyridine is the most planar.