Phosphine chelating

However, when either P(CgH )(CH2)2 or P(CgH )2(CH2) is used to form cis- or /n j -M(N2)2(PR3)4j M = Mo or W, respectively, followed by treatment with acid, ammonia yields of about 2 mol or 0.7 mol pet mole of complex for M = W and Mo, respectively, are produced (193,194). These and related data have been used to suggest a possible stepwise sequence for the reduction and protonation of N2 on a single molybdenum atom ia nitrogeaase (194). However, acidificatioa leads to complete destmctioa of the complex. Using both the stabilizing effect of the chelating phosphine triphos,... 

Heteroleptic complexes of uranium can be stabilized by the presence of the ancillary ligands however, the chemistry is dominated by methyl and benzyl ligands. Examples of these materials include UR4(dmpe) (R = alkyl, benzyl) and U(benzyl)4MgCl2. The former compounds coordinate "soft" chelating phosphine ligands, a rarity for the hard U(IV) atom. 

Using the chelating phosphine bis(diphenylphosphino)ethane (dppe) a related complex Ir(dppe)Jcan be made [123]... 

With chelating phosphine and arsine ligands, two types of complex have been isolated ... 

Cements, polyester, 30 CFCs. See Chlorofluorocarbons (CFCs) Chain conformation, 54 Chain extenders, 213-214 structure of, 219 Chain extension, 216 Chain-growth polymerizations, 4 Char formation, 421, 423 Chelated phosphine ligands, 488 Chemical recycling, 208 Chemical structure... 

Scheme 13.10 Decomposition of Pd-NHC complexes bearing chelating phosphine ligands...

Chelating phosphines are effective ligands for Co11. The [Co(mtriphos)2]2+ cation (85) was formed by controlled potential electrolysis of its trivalent relative and characterized by EPR... 

Dioxygen and its ions can bind in mononuclear and dinuclear structures in a number of ways,962 as illustrated in Scheme 1. The typical reaction of dioxygen with Co compounds involves a number of these binding forms, outlined in Scheme 2. Mononuclear Co111—peroxo complexes are relatively rare, but yellow trigonal bipyramidal complexes [Co(02)L2]+ (L = chelating phosphines dppe or dppp) have been characterized structurally where the 022 is bonded to the Co in the side-on r]2 form (Co—O 1.858(7) 1.881(4) A), with O—O stretching frequencies ( 870 cm-1) consistent with Coin-peroxo speciation.963... 

Early work in the field of asymmetric hydroboration employed norbornene as a simple unsaturated substrate. A range of chiral-chelating phosphine ligands were probed (DIOP (5), 2,2 -bis(diphenyl-phosphino)-l,l -binaphthyl (BINAP) (6), 2,3-bis(diphenylphosphino)butane (CHIRAPHOS) (7), 2,4-bis(diphenylphosphino)pentane (BDPP) (8), and l,2-(bis(o-methoxyphenyl)(phenyl)phos-phino)ethane) (DIPAMP) (9)) in combination with [Rh(COD)Cl]2 and catecholborane at room temperature (Scheme 8).45 General observations were that enantioselectivities increased as the temperature was lowered below ambient, but that variations of solvent (THF, benzene, or toluene) had little impact. 

An extensive array of chiral phosphine ligands has been tested for the asymmetric rhodium-catalyzed hydroboration of aryl-substituted alkenes. It is well known that cationic Rh complexes bearing chelating phosphine ligands (e.g., dppf) result in Markovnikoff addition of HBcat to vinylarenes to afford branched boryl compounds. These can then be oxidized through to the corresponding chiral alcohol (11) (Equation (5)) ... 

Intramolecular hydrosilylation of the fe-alkenyl silane yields the chiral spirosilane with high diastereoselectivity (Scheme 30). With 0.3-0.5 mol.% of catalyst consisting of [Rh(hexadiene)Cl]2 and a range of chelating phosphines P-P (P-P = (R)-BINAP (6), (R,R)-DIOP (5)), a maximum chemical yield of spirosilane of 82% was found with 83% enantiomeric excess. These values were improved considerably by the use of the new ligand... 

Table 10 Impact of the chelating phosphine on levels of enantioselectivity in rhodium-catalyzed intramolecular hydrosilylation with [Rh(P-P)(acetone)2]+.

Amphiphilic resin supported ruthenium(II) complexes similar to those displayed in structure 1 were employed as recyclable catalysts for dimethylformamide production from supercritical C02 itself [96]. Tertiary phosphines were attached to crosslinked polystyrene-poly(ethyleneglycol) graft copolymers (PS-PEG resin) with amino groups to form an immobilized chelating phosphine. In this case recycling was not particularly effective as catalytic activity declined with each subsequent cycle, probably due to oxidation of the phosphines and metal leaching. 

Fig. 10 Chelating phosphine ligands (a-g) with varying steric and electronic properties...

Zero-valent nickel-chelating phosphine complexes are used at 120°C under C02 pressure. 

The number of phosphine ligands on the active catalyst system is also subject to speculation. In Scheme 9 Hata postulated an active complex consists of only one chelating phosphine. However, he (66) and others (70, 71, 83) also observed that 2 moles of the bisphosphine 32 per mole of Co are needed for best selectivity. Sarafidis (55) suggested that a more desired structure might consist of two bisphosphines, with one of the Co—P bonds having the ability to dissociate to provide coordination sites for incoming monomers (see structure 34). 

The Co system is more reactive as well as much more selective than the Ni and Rh catalyst systems (Table XVII). The best systems allow almost 100% conversion with almost 100% yield of c -l,4-hexadiene. The best of the Ni and Rh systems known so far are still far from such amazing selectivity. The tremendous difference between the Ni system and the Co or Fe system must be linked to the difference in the nature of the coordination structures of the complexes, i.e., hexacoordinated (octahedral complexes) in the case of Co and Fe and tetra- or penta-coordinated (square planar or square pyramidal) complexes in the case of Ni. The larger number of coordination sites allows the Co and Fe complex to utilize chelating phosphines which are more effective than monodentate phosphines for controlling the selectivity discussed here. These same ligands are poison for the Ni (and Rh) catalyst system, as shown earlier. 

Some experimental evidences are in agreement with this proposed mechanism. For example, coordinating solvents like diethyl ether show a deactivating effect certainly due to competition with a Lewis base (149). For the same reason, poor reactivity has been observed for the substrates carrying heteroatoms when an aluminum-based Lewis acid is used. Less efficient hydrovinylation of electron-deficient vinylarenes can be explained by their weaker coordination to the nickel hydride 144, hence metal hydride addition to form key intermediate 146. Isomerization of the final product can be catalyzed by metal hydride through sequential addition/elimination, affording the more stable compound. Finally, chelating phosphines inhibit the hydrovinylation reaction. 

Milstein and colleagues [100] have developed very efficient methods using basic, chelating phosphine ligands. Even aryl chlorides underwent reductive dechlorination to the corresponding arenes with ](dippp)2Pd] as catalyst (dippp,... 

The reactivity order Ni>Pd>Pt has been found for the oxidative addition of aryl halides. Steric and electronic properties, and the numbers of L as well as chelate effects, play an important role [65, 194—196]. For example, Pd(0) complexes of basic chelating phosphines react substantially more easily with chlorobenzenes than their nonchelating analogues (see Section 18.2.4) [2, 100, 196]. 

The trap (trans-chelating phosphines) ligands developed by Ito and co-workers [33] form nine-membered metallocycles where trans-chelation is possible. However, it is not clear whether the cis isomer which has been shown to be present in small amounts or the major trans isomer is responsible for the catalytic activ-... 

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