Ruthenium Complexes with Novel Tridentate N,P,N Ligands Containing a Phosphonite Bridge between Two Chiral Oxazolines. Catalytic Activity in Cyclopropanation of Olefins and Transfer Hydrogenation of Acetophenone

We compare the coordination behavior of the new chiral ligand ( S, S )-bis[1-(4-isopropyl-4,5-dihydrooxazol-2-yl)-1-methylethyl] phenylphosphonite ( V ), abreviated NOPON iPr and referred to as bis(oxazolinyl) phenylphosphonite, toward Ru(II) with that of NOPON Me 2 ( IV ). Although the similarity between IV and V led to the expectation that they would coordinate in a similar manner, considerable differences were observed. Whereas reaction of [RuCl 2 (η 6 - p -cymene)] 2 with IV afforded the pentacoordinated complex [RuCl 2 (NOPON Me 2 )] 1, the chiral ligand V led instead to the dinuclear, chloride-bridged complex [Ru(μ-Cl)Cl(NOPON i Pr )] 2 ( 3 ). The crystal structures of 1 and 3 have been determined by X-ray diffraction. The mer arrangement of the NOPON Me 2 ligand in 1 results from the presence of the four methyl groups, two of which would point toward each other and lead to a steric clash if a fac geometry were adopted. Preliminary theoretical calculations on 1 at the EHT level indicated that coordination of ethylene parallel to the RuCl 2 P plane would result in a stable situation and it is for steric reasons that ethylene does not coordinate to the metal, either parallel or perpendicular to the RuCl 2 P plane. Coordination of CO to 1 was found to be reversible, whereas bulkier ligands do not coordinate to Ru. Complex 3 is a rare example of a fully characterized complex in which a tridentate chiral bis(oxazoline)-type ligand coordinates in a non- mer fashion. It was evaluated in catalytic reactions such as asymmetric cyclopropanation of styrene with ethyl diazoacetate and transfer hydrogenation of acetophenone in propan-2-ol.