Polyphosphine Homogeneous Catalysts

For metal-phosphine catalysts of the types RhCl(PPh3)3, RuHCl(PPh3)3, and RhH(CO)(PPh3)3, it has become accepted that dissociation of triphenylphosphine is a necessary condition for hydrogenation or hydrofor-mylation catalysis by such complexes. In the sections to follow, examples have been selected to show that three phosphino groups are bonded to the metal centers of the active species yet rapid hydrogenation and selective hydroformylation of terminal olefins still occur. 

Hydrogenation Catalysis with Chelating Triphosphine Ligands 

Proton and P H NMR spectra of toluene solutions of RhCl(ttp) and AlEt3 (or AlEt2Cl) indicate that RhH(ttp) is formed rapidly and cleanly in the solutions and that addition of terminal olefins results in very rapid conversion of RhH(ttp) to RhR (ttp). The nmr data also indicate that one ethylaluminum species is bonded directly to the rhodium and another is bridged via the Rh-H linkage.The exact nature of the ethylaluminum species is not known because of exchange between coordinated and free ethylaluminum units and the likelihood that AlEt2Cl is the product from the first step of the reaction between RhCl(ttp) and AlEt3. Hence, the 

Formation of the hydride complex RhH(ttp) in toluene results from (3-hydride elimination of ethylene from RhEt(ttp), which is formed by an initial alkylation of RhCl(ttp) by either AlEt3 or AlEt2Cl. 

On the basis of the P H and the nmr spectra, the kinetic data, and the fact that solutions of RhCl(ttp) and either AlEt3 or AlEt2Cl rapidly catalyze the hydrogenation of olefins at ambient conditions, the following mechanism involving Rh/ (ttp), RhH(ttp) associated with an ethylaluminum species, and H2 was proposed.(Association of the ethylaluminum species with the rhodium complexes is omitted for clarity.) 

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