Transition Metal Phosphine Compounds

Metal-phosphine bonds can generally be modeled in much the same way as any other metal-heteroatom bond. The fact that phosphines participate in x-backbonding (filled dn (metal) - empty d or a (phosphorus) interaction) is only of importance for generic force field parameterization schemes, and half-integer bond orders have been used to describe the effect of x-back-donation[ 153). In the usually adopted empirical force field formalism, x-bonding effects, like most of the other structural/elec-tronic effects, are accommodated by the general parameter-fitting procedure (see Parts I and III). 

More important is the question of how the trans influence may be included in a general parameterization scheme. This is not a simple problem to solve, and at present, in the few examples reported, the trans influence has not been parameterized in a general way. That is, different parameter sets have been used for ligands cis and trans to the group causing the trans influence (see Chapter 11, Section 11.2)[30,1531. 

On this restricted basis, transition metal phosphines have been modeled with accuracies similar to those of other metal-heteroatom systems11531. A number of phosphine complexes are of interest in the area of asymmetric syntheses. Due to the requirement that the geometry and the force field of the relevant intermediates have to be known for a thorough study (see Chapter 7, Section 7.4) most of the reports in this area are only of a qualitative nature. Some of the investigations are based on molecular graphics analyses, and the evaluation of the predicted selectivity has been based on van der Waals terms alone[53,233,2341. 

There are detailed experimental studies on the conformational properties and dynamics of [(r 5-(C 5H4R)Fe(CO) (PPh3) (L)] (R = H, Bu L = COMe, I) and these have been compared with molecular mechanics calculations (Fig. 13.5)[225,227,2281. The 

A possible complication due to the jt-bonding between the metal center and the ligands is the hindered rotation around these bonds, because this is often neglected in molecular mechanics studies of metal complexes. 

That is, the two possible rotamers have an energy difference of 3.8 (calculated) or 5.5kJ mol (experimental) with a calculated barrier of 31.8 kj mol [556]. Rotations around the phosphine-Fe and the phosphorous-phenyl axes of three derivatives were 

New MM3+-based force fields for (p.3-allyl)palladium and palladium olefin complexes with various co-ligands, which are based on the points-on-a-sphere approach and do not require dummy atoms to define the connectivity, have been developed and validated with experimentally observed and quantum-mechanically computed data[454l 

The conformational space of tripodal phosphine ligands coordinated to transition metal ions has been analyzed extensively, and fascinating new techniques, involving neural networks and genetic algorithms have been used to optimize the force fields and analyze the data1203,457-4611. 

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Indeed, these reactions proceed at 25 °C in ethanol-aqueous media in the absence of transition metal catalysts. The ease with which P-H bonds in primary phosphines can be converted to P-C bonds, as shown in Schemes 9 and 10, demonstrates the importance of primary phosphines in the design and development of novel organophosphorus compounds. In particular, functionalized hydroxymethyl phosphines have become ubiquitous in the development of water-soluble transition metal/organometallic compounds for potential applications in biphasic aqueous-organic catalysis and also in transition metal based pharmaceutical development [53-62]. Extensive investigations on the coordination chemistry of hydroxymethyl phosphines have demonstrated unique stereospe-cific and kinetic propensity of this class of water-soluble phosphines [53-62]. Representative examples outlined in Fig. 4, depict bidentate and multidentate coordination modes and the unique kinetic propensity to stabilize various oxidation states of metal centers, such as Re( V), Rh(III), Pt(II) and Au(I), in aqueous media [53 - 62]. Therefore, the importance of functionalized primary phosphines in the development of multidentate water-soluble phosphines cannot be overemphasized. 

A variety of Group VIII transition metal phosphine complexes are shown to be active catalysts for hydrogenation of aliphatic nitro compounds. However, chiral phosphines have been found to be noneffective to induce asymmetric induction.110... 

Trimethylsilyl cyanide reacts with diphenylcyclopropenone in the presence of Fe2(CO)9 or PPh3 as catalyst to give the aminofuran derivative 73 (40-60%) (Eq. (10)). Other phosphines and transition metal phosphine complexes are effective catalysts. A similar reaction was achieved using cycloheptenocyclopropenone. Desilylprotonation of compound 73 was achieved in hot MeOH containing a trace of p-TsOH, but the primary amine was trapped in situ as a cycloadduct without isolation (87JOC4408). 

Preparation of Phosphines by Addition of P-H to Unsaturated Compounds. -This route has not received much attention over the past year. A stereoselective synthesis of tris(Z-styryl)phosphine is offered by the addition of phosphine to phenylacetylene in a superbasic system (HMPA-H20-K0H)." In a similar vein, the reaction of phosphine with styrene and a-methylstyrene in a superbasic medium (DMSO-KOH) provides a route to the primary phosphines, (2-phenylethyl)phosphine and (2-methyl-2-phenylethyl)phosphine, respectively. 7 Transition metal phosphine complexes have been shown to catalyse the a-hydroxylation, P-cyanoethylation, and P-alkoxycarbonylethylation of phosphine. 71 Addition of primary phosphines to acrylic esters has been used for the synthesis of the phosphines (80).7 A similar addition of diphenylphosphine to acrylic esters and amides has given a series of hydrophilic phosphines (81). 72 The bis(phosphorinanyl)ethane (82) is formed in the photochemical addition of l,2-bis(phosphino)ethane to 1,4-pentadiene. ... 

This chapter has presented a tabulation of transition-metal phosphine complexes bearing some relation to proposed intermediates in a number of catalytic reactions. It is an interesting commentary on structural inorganic chemistry that, despite the determination of approximately 2500 crystal structures of such complexes, there are so few structures of direct relevance to such processes. For example, there is but one direct structural analogue (compound 2) for any of the intermediates proposed in the hydroformyla-tion reaction, the most important and oldest industrial process based on a homogeneous transition-metal phosphine complex. Similarly, the structure of only one transition-metal allyl hydride is known,despite the fact that allyl hydrides are frequently proposed intermediates. This paucity of structural information does not necessarily result from the inherent reactivity of such intermediates—for example the synthesis of suitable analogues for the hydroformylation reaction would appear to be straightforward. There appear to be several potential routes to complexes of the type MH2(C0R)L3, and one would expect that complexes of the type A/(H)-(C0)L2 would be stable, especially for L, a bulky phosphine. 

The primary focus of research using transition metal phosphine complexes for oxidations is in the complexation and activation of molecular oxygen. These oxygen complexes have been variously regarded as complexes of coordinated peroxide, superoxide, or singlet oxygen, and their reactivity with reduced substrate has been interpreted on such a basis. In this chapter, we will focus on the chemical reactivity of these compounds for oxygen atom transfer oxidation reactions, with a particular emphasis on the mechanistic features of these processes. 

Library of Congress Cataloging in Publication Data. Pregosin, P. S. P and C NMR of transition metal phosphine complexes. (NMR, basic principles and progress 16) Bibliography p. Includes index. 1. Nuclear magnetic resonance spectroscopy. 2. Transition metal compounds. 

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. 

The stabilities of the [ML2R2] phosphines increase from Ni to Pt and for Ni" they are only isolable when R is an o-substituted aiyl. Those of Pt", on the other hand, are amongst the most stable cr-bonded organo-transition metal compounds while those of Pd" occupy an intermediate position. 

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 ... 

Liicke et al. have prepared other phosphinated POSS compounds Tg[(CH2)2-PMe2]8 and Tg[(CH2)3-PMe2]8 by treating T8[CH = CH2]8 or T8[CH2-CH = CH2]8 with H-PMe2 under UV irradiation. The former compound has shown to have good coordination properties to carbonyl transition metal complexes such as CpMn(CO)3 (Table 15). 

It has been found that certain 2 + 2 cycloadditions that do not occur thermally can be made to take place without photochemical initiation by the use of certain catalysts, usually transition metal compounds. Among the catalysts used are Lewis acids and phosphine-nickel complexes.Certain of the reverse cyclobutane ring openings can also be catalytically induced (18-38). The role of the catalyst is not certain and may be different in each case. One possibility is that the presence of the catalyst causes a forbidden reaction to become allowed, through coordination of the catalyst to the n or s bonds of the substrate. In such a case, the... 

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