N-(Aryl)pyrrole-2-aldimine complexes of ruthenium: synthesis, structure, and spectral and electrochemical properties

Abstract

Reaction of [Ru(dmso)₄Cl₂] with N-(4′-R-phenyl)pyrrole-2-aldimines (abbreviated as HL-R; where H depicts the dissociable pyrrole N–H proton and R = OCH₃, CH₃, H and Cl) in refluxing toluene in the presence of triethylamine has afforded a family of complexes of type [Ru(L-R)₂(dmso)₂]. The crystal structures of all four complexes have been determined, which show that the aldimine ligands are chelated to the metal center as an N,N-donor and the dmso's are coordinated through sulfur. The relative thermodynamic stability of the five possible geometric isomers of these complexes has been assessed with the help of DFT calculations, and the tcc-isomer (tcc depicting the pyrrole-nitrogens to be trans, the imine-nitrogens cis, and the S-coordinated dmso's cis) is found to be the most stable isomer. The isolated complexes also have the tcc geometry, as observed in their crystal structures. The coordinated dmso's are found to be displaceable by chelating bidentate ligands (depicted as L-L) to furnish mixed-tris complexes of type [Ru(L-R)₂(L-L)], as demonstrated through isolation of two such complexes (with R = OCH₃, and L-L = 2,2′-bipyridine or 1,10-phenanthroline). Optimization of their structures and probing into the thermodynamics of their formation have been done by the DFT method. Electronic spectra of all six complexes, recorded in dichloromethane solution, show intense absorptions in the visible and ultraviolet regions, which have been analyzed by the TDDFT method. Cyclic voltammetry of all the complexes in acetonitrile solution shows ruthenium-based oxidative response(s) within 0.77–1.24 V vs. SCE, and ligand (bpy/phen) based reductions within −1.06 to −1.70 V vs. SCE in the case of the [Ru(L-R)₂(L-L)] complexes.