Isolation and Characterization of Reactive Intermediates and Active Catalysts in Homogeneous Catalysis

A new method has been developed for the isolation and characterization of highly reactive intermediates and of active catalysts in homogeneous catalysis by transition-metal complexes. Using the principles of steric exclusion type chromatography, a method has been devised for the isolation of highly reactive intermediates on the surface of porous polymer films. Analysis of these surface-isolated intermediates by X-ray photoelectron spectroscopy (ESCA) provided detailed information about the transition-metal complexes that resided on the surface of the polymer film. The utility of this process was demonstrated by a reanalysis of the decarbonylation of acid chlorides using chlorotris(triphenylphosphine)rhodium(I) (Wilkinson's catalyst). Through the utilization of the concepts and techniques described above, the active catalytic intermediate involved in the metathesis-promoted polymerization of cyclopentene to polypentenomer by oxotetrachlorotungsten-ethylaluminum dichloride was isolated on the surface of porous polystyrene film. This technique was viable because the active catalyst had to be attached to the end of the growing polymer chain. Analysis of the surface-isolated catalyst by X-ray photoelectron spectroscopy showed a ratio of W:0:A1:C1 of 1:1:1:4-5. The tungsten showed a binding energy of 36.0 and 38.2 eV [W(4f_7/2) and W(4f_5/2), respectively). This intermediate species was very labile. On treatment with trimethylphosphine, this catalyst was converted into a new tungsten complex (noncatalytic) that showed binding energies of 34.0 and 36.2 eV for W(4f_7/2) and W(4f_5/2), respectively. These values can be compared to values of 34.1 and 36.3 eV for Schrock's stable tungsten—alkylidene complex, W(O)(CHCMe₃)Cl₂(PEt₃)₂.