Tertiary phosphine-transition metal complexes

This chapter is concerned with the effects of nonbonded interactions in tertiary phosphine-transition-metal complexes on (a) cyclometallation and C-H activation and (b) the stability and conformations of large chelate rings. It also deals with how nonbonded interactions might be used in other areas of chemistry. 

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 ... 

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. 

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. 

The Preparation, Characterization, and Properties of Highly Soluble Transition-Metal Complexes of Long-Chain Tertiary Phosphines... 

Five series of transition-metal complexes of the new tertiary phosphines have been prepared cis-[Pt(PR3)2Cl2], trans-[Pd(PR3)2Cl2], trans- [Pt(PR3 )2HCl], [Pt(PR3)4], and trans- [Rh(PR3)2Cl(CO)] where R = n-alkyl or p-alkylaryl and R = n-alkyl (5). All of the complexes displayed essentially the same solubility characteristics as the parent phosphines. Accordingly the synthetic routes that were used were chosen because they gave the cleanest reactions with largely volatile side products and a minimum of isomers. 

Mohr, F., Priver, S.H., Bhargava, S.K. and Bennett, M.A. (2006) Ortho-metallated transition metal complexes derived from tertiary phosphine and arsine ligands. Coord. Chem. Rev., 250,... 

Several systematic experimental and computational studies have compared the sigma-donating abilities of NHCs and tertiary phosphines for a variety of transition-metal complexes [8-17]. As illustrative examples, analyses of the nickel-carbonyl complex 1 and iridium carbonyl complex 2 (Fig. 1) re-... 

Phosphines and their derivatives are known to be very useful ligands toward transition metals and a variety of complexes with phosphine as a ligand have been prepared for all kinds of transition metals. If one of the substituents on a coordinating tertiary phosphorus compound is abstracted as an anion, it would form a cationic phosphenium complex. Actually this strategy has been widely used, and halide, hydride, and alkoxide have been abstracted as an anion by an appropriate Lewis acid. An alternative method to prepare cationic phosphenium complexes is a direct reaction of a phosphenium cation with a transition metal complex having appropriate... 

The development of the catalytic hydrogenation system based on RhCl(PPh3)3 and methods for the resolution of optical isomers of tertiary phosphines occurred around the same time (1965), and this led to the possibility of asymmetric catalytic hydrogenation of prochiral unsaturated substances with C=C, C=0, and C=N bonds using transition metal complexes with chiral phosphine ligands. Such tertiary phosphines are of three types ... 

The catalysts for transfer hydrogenations are usually late transition metal complexes with tertiary phosphine ligands or bidentate nitrogen ligands, and the donors are usually organic compounds whose oxidation potential is sufficiently low to tolerate hydrogen transfer under mild conditions. Suitable donors are secondary alcohols such isopropanol. This alcohol is the most convenient since it is stable, non-toxic, environmentally friendly, easy to handle (bp 82°C), inexpensive and dissolves many organic compounds. 

Trialkyl and triphenyl substituted tertiary arsines and stibines are, like the phosphine analogues, suitable as ligands in transition metal complexes in several oxidation states, due to their low reduction potentials (cf Table 7 in Section III.A.l) and relatively high oxidation potentials (cf Table 14 in Section V.A). 

The idea that transition-metal complexes can act as PT agents was presented in some early studies of metal-complex-catalyzed reactions under PTC conditions. Lately, several approaches to the design of bifunctional catalysts have been made. Organometallic complexes capable of effecting phase transfer reactions were prepared for the first time in 1982 [192, 193]. Tertiary phosphines containing polyether substituents react with Pd(PhCN)2Cl2 to afford complexes capable of catalyzing the reduction of bromobenzene with NaH suspended in toluene. 

One of the many important differences between phosphorus and nitrogen chemistry is the relative strengths of their bonds to hydrogen. The relatively weak P—H bond means that this functionality can be added across a wide variety of unsaturated molecules (alkenes, alkynes, carbonyls) and hence this represents an excellent method for preparing tertiary phosphines. The addition of P 11 compounds to C=0 and C=N has been described in detail by Gilheany and Mitchell.2 The reaction can be catalyzed by base (potassium hydroxide, butyllithium), acid (HC1, carboxylic acids, sulfonic acids, boron trifluoride), free radical (uv, organic peroxides, AIBN) or metal (simple metal salts, late transition-metal complexes). In some circumstances no catalyst is required at all for P 11 additions to proceed.60... 

A variety of transition-metal complexes with Schiff base tertiary phosphines, (64)-(68) (Scheme 5), are known, including those of Cr,125 Mo,125,126 131 W,12 Ru,132 Ni, 24,127,128... 

---