Stereoselective Cyclometalation of Planar Pro-chiral (6-Arene)tricarbonylchromium Complexes with OC-6-[Ru(CO)2Cl2]n
Résumé
Ortho-metalation of ligand-appended and planar pro-chiral (6-arene)tricarbonylchromium complexes has been attempted with octahedral [Ru(CO)2Cl2]n. While amino- and oxazolyl-appended substrates displayed no reactivity, 2-[tricarbonyl(6-phenyl)chromium]pyridine was readily converted into the corresponding cycloruthenated product consisting of a mixture of chloro-bridged heterotetranuclear dimers, which was analyzed by CPMAS 13C NMR and 2D-DOSY 1H NMR techniques in the solid state and in solution, respectively. The main side-products of this cyclometalation reaction were identified as novel dimers containing a chloro and a hydroxo bridge, according to spectroscopic and X-ray diffraction structural determination. The relative configuration of the produced tetranuclear chloro-bridged (Cr,Ru) dimers was assessed by chemical derivatization into (Cr,Ru) monomers, of which the structures were established by X-ray diffraction analysis. Consistent results as to the stereoselectivity of the cycloruthenation reaction were obtained with 2-ferrocenylpyridine. Further reactivity studies of racemic planar-chiral (Cr,Ru) and (Fe,Ru) dimeric ruthenacycles toward anionic bidentate ligands indicated that chelation of the Ru center was rather stereoselective although probably subject to polytopal rearrangements. It was established that the chelation of the ruthenium(II) center by anionic homobidentate ligands was sensitive to steric hindrance, which favors the coordination of the anionic bidentate ligand at the Ru center in an anti fashion with respect to both Cr(CO)3 and FeCp. This property was applied to the synthesis of the first examples of scalemic ruthenacycles possessing ruthenium-centered and planar chiralities. This study is supported by 12 X-ray diffraction structures of relevant new complexes, among which are two unprecedented examples of chloro,hydroxo-bridged dicarbonylruthenium(II) [C,N] chelates.