Catalytic hydrogenation of acrylonitrile-butadiene copolymers by a series of osmium complexes

The activity of a series of osmium complexes as catalysts for the hydrogena tion of nitrile-butadiene copolymers (NBR) was studied under 3.42 MPa hydro gen pressures and at 130 degrees C in monochlorobenzene. On the basis of th e correlation of phosphine ligand properties with the catalytic activities of complexes, the complexes studied are divided into three classes: class I (bulky monophosphines with Tolman's cone angle greater than or equal to 16 0 degrees) OsHCl(CO)(O-2)(PR3)(2) (PR3 = (PPr3)-Pr-1, PCy3, PCy2Ph); class LI (small monophosphines) OsHCl(CO)(PR3)(3) (PR3 = PPh3, P(m-C6H4Me)(3)); c lass III (diphosphines) OsHCl(CO)(PPh3)(dppp) (dppp = Ph2P(CH2)(3)PPh2). In general, the activity of these complexes decreases as follows: class I > c lass LI > class III. This trend is mainly attributed to the ease of dissoci ation of a ligand from an 18-electron complex to generate a 16-electron spe cies (O-2 > PR3 > dppp) in the catalytic process. In the case of complexes of class I as catalysts, their activities increase in the order: PCy2Ph << (PPr3)-Pr-1 < PCy3. While this activity trend does not appear to correlate with the steric effect based on Tolman's cone angles, it is in good agreeme nt with the electronic effect, which is evaluated based on the infrared nu( CO) values of these complexes. The catalytic activity of there complexes in creases with the decrease of nu(CO) value, which is consistent with the inc rease of the donor power of phosphine ligands. This indicates that the acti vity of these complexes are predominately dependent on the electronic prope rties of phosphines and, in conjunction with the kinetic isotope effect, th e rare determining step of the catalytic process would be an H-2 bond break ing related process or an olefin insertion into an Os-H bond. With complexe s of classes II and III, a complex containing a bulky, strong a-donor and w eak pi-acceptor phosphine is a good catalyst whereas that containing a chel ating phosphine ligand would he a poor catalyst. (C) 2000 Elsevier Science B.V. All rights reserved.