Trans-diphosphines

The formation of ester via reaction (11) of Figure 12.10 deserves some further attention as it is not one of the elementary steps discussed in Chapter 2. One possible mechanism is the direct, outer-sphere attack of an alcohol or alkoxide at the acyl carbon atom, similar to the reaction of acid halides and alcohols (17-18 in Figure 12.13). This reaction is accessible for both cis and trans diphosphine complexes 12 and 13. Since monophosphines give mainly trans acyl complexes 13, not suited for insertion reactions, they were thought to have a preference for making esters or low molecular weight products. Trans complexes do form esters in reactions with alcohols or alkoxides, but this does not give direct information about the mechanism [42,43,44],... 

Figure 5.8. Idealized geometries of binuclear-trans-diphosphine complexes.

Trans-diphosphines in Methanol Carbonylation Dinuclear Systems References... 

A further diphosphine complex, [Cu2(p.-S2CNEt2)(n-dppm)2][C104], has been prepared and crystallographically characterized (Hg. 235) (1737). It consists of two copper(I) centers in close proximity [Cu—Cu 2.711(1) A] bridged by mutually trans diphosphines and a dithiocarbamate that lies at right angles to both. 

Both cis- and (rans-structures are possible RuH2(PMe3)4 is cis (Ru-H 1.507, 1.659 A, Ru-P 2.276-2.306 A) [90] while spectra show that RuH2(PF3)4 and others have this configuration. RuH2[PPh(OEt)2]4 is definitely trans (X-ray) with Ru-H 1.6 A, Ru-P 2.272 A. Many diphosphines form dihydrides. Ru(dmpe)2H2 has been a useful starting material for the synthesis of thiolate complexes [91] such as fra s-Ru(SPh)2(dmpe)2. 

Elimination reactions have been particularly studied in the case of dialkyls. They depend on the alkyl groups being cis trans-complexes have to isomerize before they can eliminate, and a complex with a trans-spanning diphosphine ligand is stable to 100°C (Figure 3.56). 

Phosphacyclic diphosphines (73a) and (73b) with wide natural bite angles were synthesized and the effect of the phosphacyclic moieties on the coordination chemistry in the [(diphosphine) Rh(CO)2H] complexes was studied. Both NMR and IR spectroscopy showed that the phosphacyclic xantphos ligands exhibit an enhanced preference for diequatorial chelation compared to the diphenylphosphino-substituted parent compound. In the hydroformylation of 1-octene the introduction of the phosphacyclic moieties leads to higher reaction rates. The dibenzophospholyl- and phenoxaphosphino-substituted xantphos ligands exhibit a high activity and selectivity in the hydroformylation of trans-2- and 4-octene to linear nonanal. CO dissociation rates from the... 

B(3,5-(CF3)2C6H3)4-.512 Palladium complexes with a hemilabile terdentate carbene ligand, 1,3-bis(pyl)imidazol-2-ylidene, were active toward the catalytic polymerization of CO/norbornylene.513 Palladium complexes of cz s-bidentate C4-bridged diphosphines cis- and trans- 1,2-bis [(diphenylphosphino)methyl]cyclohexane, e fl o,e fl o-2,3-bis[(diphenylphosphino)methyl] norbornane,... 

Rigid diphosphines have been used to enforce trans-geometries thus with the phenanthrene-derived diphosphine (Figure 3.49, R = Et) the complexes PdLCl and PtLCl2 have closely similar geometries (Pd—P 2.307 A, Pd—Cl 2.306 A, P-Pd-P 177.4° Pt-P 2.293 A, Pt-Cl 2.304A, P-Pt-P 177.1°)... 

Although diphosphine disulfides of formula R2P(S)P(S)R2 take up a trans conformation in the solid state (see Section 5.2.3), upon coordination to a metal centre they can rearrange to adopt a cis conformation, hence facilitating bidentate chelation of the metal (e.g., Equation 78).117 In addition, a cis-bridging mode has also been proposed for these ligands in bi-metallic complexes.117... 

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