Phosphine-metal complexes nickel

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 ... 

Layered inorganic solids have been used for site isolation, for example, nickel phosphine complexes confined within the interlayer spaces of sepiolite have been used as olefin hydrogenation catalysts [63], and similarly there has been the encapsulation of metal complexes into zirconium phosphates [64], The principal idea is illustrated in Figure 5.8. The metal complex can be encapsulated by covalent means (a) or by non-covalent interactions (b). 

Among transition metal complexes used as catalysts for reactions of the above-mentioned types b and c, the most versatile are nickel complexes. The characteristic reactions of butadiene catalyzed by nickel complexes are cyclizations. Formations of 1,5-cyclooctadiene (COD) (1) and 1,5,9-cyclododecatriene (CDT) (2) are typical reactions (2-9). In addition, other cyclic compounds (3-6) shown below are formed by nickel catalysts. Considerable selectivity to form one of these cyclic oligomers as a main product by modification of the catalytic species with different phosphine or phosphite as ligands has been observed (3, 4). 

In the case of phosphine, especially tri-n-butyl and triphenyl phosphines, an active phosphine complex is formed in the reaction medium via reaction with nickel carbonyl. This complex is a very active species provided that the optimum concentration of phosphine is used. Low phosphine concentration results in a loss of the effective nickel concentration through the formation of nickel tetra-carbonyl, nickel metal or nickel iodide. The absolute concentration of phosphine is less important than the P/Ni ratio. In addition to form the stable Ni-P catalyst, the phosphine has to compete with other ligands in the reaction mixture for nickel. With high carbon monoxide partial pressure, there is more CO in solution to compete with phosphine favoring the formation of the carbonyl, which is inactive under the reaction conditions. Hence with high carbon mon-... 

Another simple oligomerization is the dimerization of propylene. Because of the formation of a relatively less stable branched alkylaluminum intermediate, displacement reaction is more efficient than in the case of ethylene, resulting in almost exclusive formation of dimers. All possible C6 alkene isomers are formed with 2-methyl-1-pentene as the main product and only minor amounts of hexenes. Dimerization at lower temperature can be achieved with a number of transition-metal complexes, although selectivity to 2-methyl-1-pentene is lower. Nickel complexes, for example, when applied with aluminum alkyls and a Lewis acid (usually EtAlCl2), form catalysts that are active at slightly above room temperature. Selectivity can be affected by catalyst composition addition of phosphine ligands brings about an increase in the yield of 2,3-dimethylbutenes (mainly 2,3-dimethyl-1-butene). 

The most common nickel(O) complexes are those containing phosphorus, arsenic and antimony as donor atoms. Besides the Malatesta and Cenini book/1 which specifically deals with metal(O) complexes, nickel(O) complexes have been summarized in books and review articles which report complexes with phosphine, arsine and stibine ligands.31-37 Actually the nickel(O) complexes with these ligands amount to hundreds and the number of new complexes which are synthesized is increasing very rapidly, making nickel(0) phosphine chemistry a very extensive topic. 

A number of X-ray crystal structures of nickel(O) complexes containing both alkenes and phosphines have been reported to date with the aim of gaining more information on the bonding in metal alkene complexes. Structural data for the most relevant nickel(O) phosphine alkene complexes are reported in Table 9. 

Neutral phosphates (RO)sPO, phosphonates (RO)2R PO and phosphinates (RO)R2PO are well known as extracting agents for metal ions.1823 The isolation of their metal complexes as crystalline compounds is, in general, more difficult than the preparation of complexes with other substituted phosphoryl compounds. It is often essential to reflux solutions of the reactants with dehydrating agents such as triethyl orthoformate or 2,2 -dimethoxypropane. In some cases the neutral phosphoryl ligands or triethyl orthoformate by themselves act as the reaction media in the synthesis of the nickel(II) complexes. 

Several groups have screened a variety of transition metal complexes for activity in the double silylation system, but only compounds of nickel, palladium, and platinum appear to be viable catalysts. The key factor appears to be the involvement of a M(0) species, although certain M(II) complexes can also be used, presumably with in situ reduction to M(0). Generalizations regarding the activity of the different transition metal complexes are difficult, as many variables exist in each system. However, the most active complexes seem to combine palladium metal centers with dba, small basic phosphine, or isocyanate ligands. 

Template syntheses of P macrocycles are a new area. In fact, a 1978 review93 of template synthesis made no mention of P macrocycles. Template syntheses have been developed by Stelzer and co-workers.94 Firstly, two molecules of the bidentate secondary phosphine are complexed with a nickel(II) or palladium(II) salt (Scheme 6) and the resultant secondary phosphine complex is then condensed with a diketone to form the macrocyclic metal complex. Unfortunately, these macrocycles are strong field ligands and no method has yet been devised to remove the metal from the ring. On the other hand, Cooper and co-workers95 have used a template synthesis to produce a [l4]aneP2N2 macrocycle (Scheme 7). 

Silylene 59 also behaves somewhat like a phosphine in its interactions with metal carbonyls98,149-376. Typical reactions involve substitution of silylene for CO, to give a silylene-metal complex. Three examples are shown in Scheme 20, and the structure of the nickel complex 75 is displayed in Figure 7149. This complex is both the first silylene-nickel complex, and the first example of a bis-silylene-metal complex free of stabilization by Lewis base donors. 

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-... 

PMe2(Ment), a mixture of the corresponding diastereoisomers (all-trans-Ci2H18)Ni[PMe2(Ment)] was obtained, which could be separated by fractional crystallization. Abstraction of the chiral phosphine by the TT-allyl nickel(II) dimer [(r 3-C3H5)NiBr]2 led to the two enantiomers of 12, which do not race-mize at room temperature [103], They are probably the simplest optically active transition metal complexes known. 

Many P(CH3)3 complexes of the first-row transition metals may be conveniently prepared by direct reaction with the appropriate anhydrous chromium (II), (III),3 cobalt(II),4 iron(II)s or hydrated iron(II),s nickel(II)4a>6,7 salt. The compound [(CH3)3P] 2FeCl2 is the starting material for a variety of phosphine iron complexes.4,8,9... 

Metal complex chemistry, homogeneous catalysis and phosphane chemistry have always been strongly connected, since phosphanes constitute one of the most important families of ligands. The catalytic addition of P(III)-H or P(IV)-H to unsaturated compounds (alkene, alkyne) offers an access to new phosphines with a good control of the regio- and stereoselectivity [98]. Hydrophosphination of terminal nonfunctional alkynes has already been reported with lanthanides [99, 100], or palladium and nickel catalysts [101]. Ruthenium catalysts have made possible the hydrophosphination of functional alkynes, thereby opening the way to the direct synthesis of bidentate ligands (Scheme 8.35) [102]. 

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