Atropisomeric discrimination in new Ru^II complexes containing the C₂-symmetric didentate chiral phenyl-1,2-bisoxazolinic ligand

A new family of RuII complexes containing the tridentate meridional 2,2′:6′,2″-terpyridine (trpy) ligand, a C 2-symmetric didentate chiral oxazolinic ligand 1,2-bis[4′- alkyl-4′,5′-dihydro-2′-oxazolyl]benzene (Phbox-R, R = Et or iPr), and a monodentate ligand, of general formula [Ru(Y)-(trpy)(Phbox-R)] n+ (Y = Cl, H2O, py, MeCN, or 2-OH-py (2-hydroxypyridine)) have been prepared and thoroughly characterized. In the solid state the complexes have been characterized by IR spectroscopy and by X-ray diffraction analysis in two cases. In solution, UV/Vis, cyclic voltammetry (CV), and one-dimensional (1D) and two-dimensional (2D) NMR spectroscopy techniques have been used. We have also performed density functional theory (DFT) calculations with these complexes to interpret and complement experimental results. The oxazolinic ligand Phbox-R exhibits free rotation along the phenyloxazoline axes. Upon coordination this rotation is restricted by an energy barrier of 26.0 kcal mol-1 for the case of [Ru-(trpy)(Phbox-iPr)(MeCN)]2+ thus preventing its potential interconversion. Furthermore due to steric effects the two atropisomers differ in energy by S.7 kcal mol-1 and as a consequence only one of them is obtained in the synthesis. Subtle but important structural effects occur upon changing the monodentate ligands that are detected by NMR spectroscopy in solution and interpreted by using their calculated DFT structures. © 2006 Wiley-VCH Verlag GmbH & Co. KGaA.