Catalytic Polymer membranes under forcing conditions: reduction of nitrobenzene by CO/H2O catalyzed by ruthenium bis(arylimino) acenaphthene complexes

Polymeric membranes embedding a metal complex have been previously employed as reusable catalysts under relatively mild conditions. Herein, the first example of a polymeric catalytic membrane employed under very forcing conditions (160°C and 5 MPa CO) is reported. The reaction investigated was the reduction of nitrobenzene to aniline by CO/H2O, catalyzed by ruthenium bis(arylimino)acenaphthene (Ar-BIAN) complexes. To better retain the complex in the membrane, a modified ligand, with long alkyl chains in the para positions of the aryl rings, was prepared. Among several polymers tested as membranes, PEEK-WC, a modified polyether ether ketone, gave the best results. Attempts to embed the ligand only in the membrane and functionalize it with the metal later, analogously to the in situ generation of the active species practiced for the homogeneous system, failed and it was necessary to synthesize new complexes that could be reduced under the reaction conditions. Best results were obtained using [Ru(Ar-BIAN)(CO)2Cl2]+Et3N. During the reaction, the complex is transformed into a mixture of [Ru3(CO)12] and a reduced form of the ligand, Ar-BIANH2. The latter was independently prepared and shown to be able to reduce nitrobenzene even in the absence of any metal. A new kind of support was designed to allow the placing of the membrane in a stirred autoclave. Using only water as solvent, no metal or ligand leaching was detected and several recycles were performed. All employed membranes were thoroughly characterized by different techniques.