Noncovalent Anchoring of Organometallic Complexes via Adsorption

The same authors compared catalysts prepared from these precursors and [Ru(BINAP)Cl2]2 adsorbed on MCM-41 (with 26 and 37 A pores) and an amorphous mesoporous silica (with 68 A pores) all treated with combinations of SiPh2Cl2 and Si(CH2)3X (X = NH2, CO2H). Catalysts were also prepared in which the organometallic precursors were immobilized by entrapment into silica (using sol-gel techniques). This is one of the few studies in which the performance of chiral phosphine catalysts immobilized by covalent and noncovalent procedures are compared directly. The materials were examined as catalysts for the hydrogenation of sodium a-acetamidocinnamate and of a-acetamidocinnamic acid under similar conditions to those used for the catalysts on unmodified MCM-41. The catalysts 

A further example of ion-exchange of an organometallic complex onto a layered support has been provided by the anion exchange of a sulfonated ruthenium BINAP complex onto the external surface of layered double hydroxides [119]. Although achvihes and enantioselechvities for the hydrogenation of dimethyl itaconate were comparable to the homogeneous catalyst, and catalyst deactivation was not detected, with geraniol as substrate, catalyst deactivation was unavoidable. 

The SHB concept was expanded to chiral phosphine catalysts by de Rege et al., who reacted the trifluoromethanesulfonate (triflate) counter anion of the cationic complex [Rh(COD)((R,Rj-MeDuPhos)] with the surface hydroxyl groups of the silaceous mesoporous material MCM-41 [122]. The complex was loaded to a level of 1.03 wt% Rh. A decrease in support surface area and pore volume is consistent with the complex being located within the support pores. The counterion is very important in this process if the anion of the homogeneous catalyst precursor is altered to BArp no adsorption of the catalyst is observed. It is postulated that the mechanism of triflate binding is hydrogen bonding with the support, and that the 

The use of surface bound triflate ions has been exploited by Raja et al. to immobilize the complexes [Rh(COD) fSj-(-i-)-PMP ], [Pd(allyl) fSj-(-i-)-PMP ], [Rh(COD) fSj-(-)-AEP rand[Rh(COD) flR,2Rj-(-t)-DED ] in the pores of silicas possessing various pore sizes with narrow distributions [128]. These constrained chiral catalysts were then tested for the asymmetric hydrogenation of methyl ben-zoylformate to its corresponding methyl mandelate (40°C, methanol, 2 MPa H2). In the homogenous form, only the catalysts [Rh(COD) fSj-(-i-)-PMP ], [Pd(allyl) (Sj-(-i-)-PMP ] exhibit any signiflcant e.e.s under the reaction conditions (53% 

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