Ruthenium Complexes with Novel Tridentate N<i>,</i>P<i>,</i>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 NOPONiPr and referred to as bis(oxazolinyl) phenylphosphonite, toward Ru(II) with that of NOPONMe2 (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 [RuCl2(η6-p-cymene)]2 with IV afforded the pentacoordinated complex [RuCl2(NOPONMe2)] 1, the chiral ligand V led instead to the dinuclear, chloride-bridged complex [Ru(μ-Cl)Cl(NOPONiPr)]2 (3). The crystal structures of 1 and 3 have been determined by X-ray diffraction. The mer arrangement of the NOPONMe2 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 RuCl2P 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 RuCl2P 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.