Diphosphines with Longer Chain Lengths

Complexes of Diphosphines with Longer Chain Lengths 

The chelated complex [HRh((-)-diop)2] is active for the hydrogenation of terminal olefins or unsaturated carboxylic acids. From kinetic data and spectroscopic evidence an active intermediate containing a monoden-tate (-)-diop ligand has been postulated, analogous to a known carbonyl complex. Polymeric species containing bridging diphosphine ligands were postulated to explain inhibition of catalytic activity in the presence of added (-)-diop or dppb, but not added However, proof of 

The ligand p-C6H4[CH2P(CH2CH2PPh2)2]2(=i ) is able to chelate two separate metal atoms, thereby combining chelation and bridging features.The catalytic activity of the binuclear complex [RhCl(L)RhCl] is said to be due to the independent activity of each metal center, maintained at a distance from one another. Comparison with the mononuclear analog, [RhCl(triphos)], was not attempted. 

The mechanisms of hydrogenation by neutral or cationic mononuclear complexes of rhodium are reasonably well understood and are discussed in detail in Chapter 2. The system is, however, sufficiently complex that new information is constantly being obtained. For example, it has recently been shown that a solvated complex is kinetically important when benzene is the solvent. This may have major implications in studies of complexes of aryl-phosphines or diphosphines, such as Ph2P(CH2)nPPh2 or diop. It should be noted that for complexes of chelating chiral diphosphines (see Chapter 4), the enantioselective step involves the least stable of two possible catalytic intermediates the complex which is stable, isolable, and present in highest concentration is not as important in determining the reaction rate and does not yield the major product.  

Homogeneous and heterogeneous hydrogenation catalysts have been compared, but multinuclear homogeneous processes have frequently not been considered. This is surprising when one considers the often proposed multicenter mechanism for heterogeneous catalysis  

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