Orientation‐ and Temperature‐Dependent Rotational Behavior of Imidazole Ligands (L) in β‐[Ru(azpy)2(L)2](PF6)2 Complexes

Abstract

AbstractThe synthesis and characterization of the cis bifunctional coordinated ruthenium(II) complexes β‐[Ru(azpy)2(MeIm)2](PF6)2 (β‐MeIm) and β‐[Ru(azpy)2(MeBim)2](PF6)2 (β‐MeBim) (azpy = 2‐phenylazopyridine, MeIm = 1‐methylimidazole and MeBim = 1‐methylbenzimidazole) is reported. In β‐MeIm the two MeIm ligands can both freely rotate around the Ru−N axes on the NMR timescale. In β‐MeBim the two MeBim ligands appear restricted in their rotation around the Ru−N axes, which becomes slow on the NMR timescale at low temperatures. In contrast to the analogous complexes α‐[Ru(azpy)2(MeBim)2](PF6)2 and cis‐[Ru(bpy)2(MeBim)2](PF6)2, only one atropisomer is observed for the two MeBim ligands in β‐MeBim. The orientation of the MeBim ligands appears to correspond to an HT isomer which is similar to the orientation of the MeBim ligands in the most abundant atropisomer found in the related α‐[Ru(azpy)2(MeBim)2](PF6)2. A stacking interaction between the phenyl ring of one azpy and one MeBim ligand is likely to stabilize the observed atropisomer of β‐MeBim, and is such that the rotation of the phenyl ring of one of the two azpy ligands is restricted. At very low temperatures this rotation, or flipping of the phenyl ring between two identical positions, is in the slow‐exchange range on the NMR timescale. (© Wiley‐VCH Verlag GmbH & Co KGaA, 69451 Weinheim, Germany, 2003)