Metallophosphines

Beside these catalytically active metallophosphine dendrimers (see above), preliminary studies on the chemical properties of phoshorus-based dendrimers complexed to metals such as platinum, palladium and rhodium have been described by Majoral, Caminade and Chaudret [21], They showed that these macromolecules (see Scheme 13) could be useful for the (in situ) generation of metallodendrimer catalysts. 

The reaction products are altered when the transformations outlined in Scheme 2 are carried out starting from phosphito derivatives [L = P(OMe)3 or PPh(OMe)2j. Phosphonato complexes are isolated either as the sole products (M = Nb, R = Me M = Ta, R = Ph) or together with the expected metallophosphines in variable yields (M = Nb, R = Ph). A complete transformation from metallophosphine to the corresponding phosphonato complex occurs over 24 h. 

Open chain bimetallic complexes containing a phosphido bridged ligand are easily synthesized by reacting metallophosphines l(Nb, Ta, Mo, W) with [M Ln] species (Fig. 

Commercial development of calcium polyphosphate fibres Discovery of new class of aluminophosphate molecular sieves Preparation of first carbophosphene - (bis methylene phosphorane) Discovery of Phospha-Cope rearrangement Synthesis of first compound with an As=P bond Cyclo-P, triple decker compound made First asymmetric hydrogenation catalyst discovered (a metallophosphine complex)... 

The numerous applications of phosphines include (1) synthetic reagents, (2) ligands in metallo-phosphorus compounds, (3) catalysts, (4) metal deposition agents, (5) electron-rich compounds. Metallophosphines (metal phosphides) types MPR2 and M2PR (M=Li, Na, K) are especially useful in synthesis (Chapter 8.8) (Table 6.8). 

The simplest examples of compounds with X-P-M bridges are provided by the metallophosphines (8.27a) and (8.28a), where M is usually a group I or group II metal. Some of these compounds, however, exist as metal cations and (organo) phosphide anions (8.27b) and (8.28b), the adopted configuration being influenced both by M and the nature of R. 

In other metallophosphine derivatives with M = Na, Mg, Zn, Cd, solid state studies have revealed structural features similar to those possessed by the covalent structures above. An unequivocal assignment of covalent character to the M-P bonds in these structures has not proved possible however. Metal silicophosphides have similar structural features, for example, Li+[P(SiH3)2] has an ionic structure (8.46g) in the presence of solvent [65], but LiP(SiMe3)2 adopts a ladder-like hexa-meric solid state structure in the absence of solvent [66] (8.46h) and a tetrameric structure for the phenyl derivative with THF (8.46i). White P is said to react with Na in liquid ammonia to produce the ionic compound NaCNHj) [-PH-PH-PH ] [67]. 

In addition to metallophosphines (8.52), there are also many phosphite (8.53a), phosphonite (8.53b), phosphinite (8.53c) and other complexes which have been studied. Phosphite, phosphonito and phosphinito complexes, which contain M-O-P as well as M-P linkages have also been prepared (Table 8.16). 

This is currently a field of intensive investigation, particularly for asymmetric catalysts capable of promoting asymmetric syntheses. Metallophosphine complexes (favourite metals studied being Rh, Ru, Re, Ir, Pt, Pd, Co, Ni) are remarkable not only for the variety of reactions they may catalyse, but also in some cases for their high specificity of action. Transition metal pincer complexes are particularly fashionable since they are very stable and can impose unusual reaction pathways [9,31]. These properties have greatly stimulated molecular structure studies, but their reaction mechanisms often remain speculative. 

Numerous isomerisations are promoted by metallophosphine catalysts, but these are often very specific. Reaction (12.369), for example, is effectively promoted by RuCl2(PPh3)3, but the use of Rh(PPh3)3Cl leads to decomposition of the aldehyde product. In reaction (12.370), trans-IrCKCO) (PPh3)2 is very effective but RuCl2(PPh3)3 is quite ineffective. 

Heterogeneous catalytic reactions usually involve the interaction of gaseous or liquid compounds in the presence of a solid or solid-supported catalyst. Homogeneous catalysts on the other hand are present in the same phase as the reacting components - usually in a solution when metallophosphine catalysts are involved. Difficulties frequently arise in the latter case, with the separation of the reaction products. With polymer-supported catalysts, however, these difficulties are usually removed. 

Heterogeneous metallophosphine catalysts can be obtained by chemical bonding to an organic or inorganic support, preferably one with a relatively large surface area. Inorganic supports which have been used include silica, alumina, and various zeolites. Organic supports which can be used include polystyrene, polybutadiene, polyvinyl alcohol and cellulose. 

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