Transition tertiary phosphines

With certain transition metals, eg, Ru(II)-tertiary phosphine complexes, the principal products are bis(epoxides) (82). 

Although trialkyl- and triarylbismuthines are much weaker donors than the corresponding phosphoms, arsenic, and antimony compounds, they have nevertheless been employed to a considerable extent as ligands in transition metal complexes. The metals coordinated to the bismuth in these complexes include chromium (72—77), cobalt (78,79), iridium (80), iron (77,81,82), manganese (83,84), molybdenum (72,75—77,85—89), nickel (75,79,90,91), niobium (92), rhodium (93,94), silver (95—97), tungsten (72,75—77,87,89), uranium (98), and vanadium (99). The coordination compounds formed from tertiary bismuthines are less stable than those formed from tertiary phosphines, arsines, or stibines. 

Coordination-catalyzed ethylene oligomerization into n-a-olefins. The synthesis of homologous, even-numbered, linear a-olefins can also be performed by oligomerization of ethylene with the aid of homogeneous transition metal complex catalysts [26]. Such a soluble complex catalyst is formed by reaction of, say, a zero-valent nickel compound with a tertiary phosphine ligand. A typical Ni catalyst for the ethylene oligomerization is manufactured from cyclo-octadienyl nickel(O) and diphenylphosphinoacetic ester ... 

Tertiary phosphines - and the analogous arsines - are able to stabilize transition metals in a variety of oxidation states and coordination geometries. Investigations of complexation with P-ligands were promoted by the high stabilization of metal by P ligands, which is mainly due to n-back bonding. 

In the examples above, one or both of the reaction centers are already attached to the metal center. In many cases, the reactants are free before reaction occurs. If a metal ion or complex is to promote reaction between A and B, it is obvious that at least one species must coordinate to the metal for an effect. It is far from obvious whether both A and B enter the coordination sphere of the metal in a particular instance. A number of metal-oxygen complexes can oxygenate a variety of substrates (SOj, CO, NO, NO2, phosphines) in mild conditions. Probably the substrate and O2 are present in the coordination sphere of the metal during these so-called autoxidations. In the reaction of oxygen with transition metal phosphine complexes, oxidation of metal, of phosphine or of both, may result. The initial rate of reaction of O2 with Co(Et3P)2Cl2 in tertiary butylbenzene. 

Non-phosphine type ligands are studied time by time with the aim to obtain water-soluble transition metal complexes with catalytic properties. However, with the exception of a few specific reaction types (e.g. oxidations) these catalysts cannot cope with tertiary phosphines - with the ligands on Figure 20 this has been found once again. 

Although the most versatile hydrogenation catalysts are based on tertiary phosphines there is a continuous effort to use transition metal complexes with other type of ligands as catalysts in aqueous systems some of these are listed in Table 3.3. 

Hydrogen phosphonates [(R0)2P(0)H] and secondary phosphine oxides R2P(0)H exist in equilibrium with their P(III) tautomers, (RO)2P(OH) and R2P(0H), respectively, the P(V) tautomers being more favored under ambient conditions. As ligands, they coordinate, like tertiary phosphines, to transition metals to form complexes, which have been used as catalysts for organic reactions. However, catalytic addition reactions of P(V)-H bonds have not been scrutinized until recently. 

The two-coordinate phosphorus atom of a phosphonio-phospholide moiety is generally a weaker Lewis-base than a tertiary phosphine as a consequence, the complexes are thermally less stable (this holds in particular for metal fragments with low back donation capability, e.g. Mn(CO)4Cl) [35, 43], benzophospholide ligands are easily displaced by tertiary phosphines such as PhsP [27, 44], and bidentate ligands comprising a phosphonio-ben-zophospholide and a phosphine site react with transition metal fragments preferentially at the phosphine site coordination of both phosphorus atoms is only observed if the substrate offers two vacant coordination sites [27, 47]. 

With iodine, the imidazol-2-ylidenes (IV) form stable adducts (Scheme 8.22), in which the carbene clearly acts as a basic a donor, just like a tertiary phosphine. Interestingly, the molecular structure of this adduct may be considered as an isolated transition state that models the nucleophilic attack of the carbene on the iodine molecule. 

It is generally accepted that C02 is somewhat unreactive towards transition metal complexes and that the metal C02 linkage is favoured by a nucleophilic metal centre such as a tertiary phosphine metal(O) moiety. 

Phospholes can behave as simple two electron donors, in the same way as tertiary phosphines, and most of the transition metals have been complexed to phospholes. For example, ruthenium(II) forms a series of complexes [(Phole)2 Ru(CO)2C12] and [(Phole)3 Ru(CO)C12]. The formation of the tris phosphole complex attests to their small size. Because of the ring structure an unusual isomerism has been observed, with the rings either in the basal plane of the square pyramidal complex or normal to the basal plane (Figure 23). 

---