Orientation and Restricted Rotation of Lopsided N-Donor Heterocyclic Bioligands in Octahedral Ruthenium Complexes

Abstract

In order to analyze the roles of steric and electrostatic factors in determining the orientation and dynamic behavior of benzimidazole-type ligands in RuII(dimethyl sulfoxide) complexes, compounds of the general formula RuX2(dmso-S)2−n(CO)n(L)2 [X = Cl, Br; L = 1,5,6-trimethylbenzimidazole (Me3Bzm), benzimidazole (Bzm); n = 0−2] with cis and trans arrangements of the X ligands, have been prepared and structurally characterized both in solution and in the solid state, by NMR and X-ray techniques, respectively. New descriptors, based on the torsion angles about the Ru−N bonds, are introduced for a more efficient definition of the head-to-head (HH) and head-to-tail (HT) orientations of the heterocyclic nitrogen bases. Similar descriptors, based on the torsion angles about the Ru−S bonds, are used to define the orientation of the dimethyl sulfoxide ligands. The experimental molecular conformations are discussed on the basis of molecular mechanics (MM) calculations. The electrostatic interaction between the halide ligands and the positively charged N2CH proton on the imidazole ring has been found to be the essential factor in determining the specific orientation of one Me3Bzm base, both in the solid state and in solution. The different stabilities of the HH and HT base conformations observed in solution, in the solid state, and in the isolated molecules are rationalized in terms of solvation and packing effects. The larger solvent-accessible surface area in the HH conformation as compared with that in the HT form leads to more effective intermolecular interactions, which are responsible for stabilizing the former geometry in the condensed phase.