Cobalt complexes phosphorus ligands

Cyclic silylphosphanes, see Silylphosphanes, phosphorus-rich, cyclic Cyclic sulfur-nitrogen compounds, see Sulfur-nitrogen compounds, cyclic Cyclic trithiolate ligand, 38 8-9 Cyclic voltammetry A. chroococcum Fd 1, 38 130-131 fullerene adducts, 44 19 nickel(ll) macrocyclic complexes, 44 112 Rieske proteins, 47 138, 139 Cyclidenes, as cobalt complex ligands, 44 282-284... 

Improved hydrocyanation, as with catalysts discussed elsewhere in this section, follows from the use of Lewis acid promoters and excess phosphorus ligand. Low-valent cobalt complexes may be prepared by reduction of cobalt(II) chloride with zinc metal in the presence of phosphorus ligand generating in situ the requisite Lewis acid promoter, ZnClj. 

The hydrolysis of ethylene phosphate in hydroxide ion solution proceeds with a rate constant of 5 x 10" L-mol" s" (100). The O-P-0 angle in the ring of ethylene phosphate of 99° is expected to be rather similar to that for the four-membered ring incorporating the cobalt and phosphorus centers. Therefore it is likely that reaction at the strained P center of the complex is eclipsed by a more rapid metal-ligand cleavage reaction. This problem can be circumvented by the use of metal ion complexes of Ir(III) where the metal-ligand bonds are more inert as the locus for the reaction. 

The mechanisms of the oxidation of phosphines and arsines by chromium(VI) have been examined both in solution and on a diatomite support. Kinetic parameters are presented for both supported and solution reactions. A ruthenium complex of 1,4,8,1 l-tetramethyl-l,4,8,ll-tetraazacyclotetradecane has been utilized to oxidize triphenylphosphine in acetonitrile. Although a limited temperature range was utilized, a AH value of 8.7 0.8 kcal mor and a A5 value of -20 2 cal K mor were calculated. The secondary phosphine oxides, HP(0)R (R = n-butyl, isobutyl, cyclohexyl) and 9H-9-phosphabicyclononane-9-oxide, react with cobaltocene to yield dihydrogen and cobalt(I) compounds. With the less bulky phosphorus ligands at elevated temperatures trinuclear cobalt(III, II) complexes may be obtained. Arsenious acid may be utilized to catalyze the oxygen atom... 

Typical examples of different behavior in relation to the metal are trivalent phosphorus ligands. Thus, trials to modify cobalt complexes with PPhg proved rather problematic, due to the shift of the equilibrium to the left-hand side, especially under increased CO pressure (Scheme 1.7). As a consequence, the hydroformylation is catalyzed by the unmodified Co complex. Diphosphines of the type Ph2PZPPh2 (Z = (CH2)2, (CH2)4, CH=CH) cause a dramatic decrease in reactivity [19]. Also, phosphites do not form active hydroformylation catalysts with cobalt. It seems that only basic trialkyl phosphines are suitable for the generation of stable Co phosphine hydroformylation catalysts. 

The [2 -I- 2 -I- 2] cycloaddition reaction can give rise to chiral compounds, especially biaryls [3q]. Control of the enantioselectivity in such transformations is of prime importance, notably because biaryls can be used as ligands in asymmetric catalysis. This topic is covered in detail in Chapter 9. Nowadays, cobalt still looks like a poor relation in this field, which is largely dominated by rhodium. Nevertheless, a report from Heller et al. shows for the first time that phosphorus-bearing axially chiral biaryls 9 can be formed by enantioselective benzene formation using the neomenthyl-indenyl cobalt complex II as a catalyst (Scheme 1.3) [7]. Good yields... 

The metal-vapor technique was applied to cobalt atoms and r-BuC = P (01JOM(635)212). The mixture of products that resulted includes the mixed-ligand sandwiches 170 and 171. Further interaction of complex 170 with [W(C0)5(THF)] leads to the coordination of the W(CO)5-group via the phosphorus heteroatom of the four-membered ring to yield 172. 

Although trialkyl- and triarylbismuthines are much weaker donors than the corresponding phosphorus, 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. 

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