Cobalt addition initiated

For simple acidic extractants, such as carboxylic or sulfonic acids, the similarity in formation constants does not produce cobalt-nickel separation factors (Sn° 2) sufficiently large for commercial operation (Fig. 11.4). Data for pH versus extraction for some chelating acid extractants does seem to offer the possibility of separation [e.g., for the hydroxyoxime Acorga P50, the pHso for nickel(II) is 3.5 and for cobalt(II) 5.0]. Normally, this pH difference would be suitable for a separation process, but this particular system has hidden complications. The rate of nickel extraction is very slow compared with cobalt and, in addition, although cobalt is initially extracted... 

The cobalt(III) initiation and catalysis pathways are very effective in many oxidations but suffer some limitations, e.g., Co " is strongly inhibited by cobalt(II) ions, which seem to form dimers with Co ". Such dimers are only weak catalysts in arene oxidations. As a result the rate of oxidations is inversely dependent on the concentration of Co " in the reaction mixture thus the cleavage of such dimers by addition of small amounts of co-catalysts will attain the reaction rate [11c, 12]. Additionally in the case of deactivated, electron-poor systems such as toluic acid or p-nitrotoluene, cobalt(III) alone is not an efficient catalyst - synergistic co-catalysts are necessary to achieve good results. 

Theoretical work has also been devoted to examine the influence of reactions that interrupt the often repeated cycles of radical formation (activation) and cross-termination to dormant species (deactivation). For the nitroxide- and the cobalt-mediated systems, such reactions are the formation of R(—H) and YH by a usual radical disproportionation, which competes with the coupling of R and Y or by a direct fragmentation of R—Y to the hydroxylamine or a hydridocobalt complex and the alkene.22-33a-35-47-51 57 Even rather small fractions of these processes limit the maximum conversion and stop the polymerization prematurely in nearly indistinguishable ways, because they lead to an exponential decay of the dormant species. Before the end of conversion this does not affect the linear dependence of An on conversion and causes only minor increases of the polydispersity.57 To some extent the deteriorating effect of these reactions can be compensated by the rate enhancement through an additional initiation.50... 

Polyether Polyols. Polyether polyols are addition products derived from cyclic ethers (Table 4). The alkylene oxide polymerisation is usually initiated by alkah hydroxides, especially potassium hydroxide. In the base-catalysed polymerisation of propylene oxide, some rearrangement occurs to give aHyl alcohol. Further reaction of aHyl alcohol with propylene oxide produces a monofunctional alcohol. Therefore, polyether polyols derived from propylene oxide are not truly diftmctional. By using sine hexacyano cobaltate as catalyst, a more diftmctional polyol is obtained (20). Olin has introduced the diftmctional polyether polyols under the trade name POLY-L. Trichlorobutylene oxide-derived polyether polyols are useful as reactive fire retardants. Poly(tetramethylene glycol) (PTMG) is produced in the acid-catalysed homopolymerisation of tetrahydrofuran. Copolymers derived from tetrahydrofuran and ethylene oxide are also produced. 

A Belgian patent (178) claims improved ethanol selectivity of over 62%, starting with methanol and synthesis gas and using a cobalt catalyst with a hahde promoter and a tertiary phosphine. At 195°C, and initial carbon monoxide pressure of 7.1 MPa (70 atm) and hydrogen pressure of 7.1 MPa, methanol conversions of 30% were indicated, but the selectivity for acetic acid and methyl acetate, usehil by-products from this reaction, was only 7%. Ruthenium and osmium catalysts (179,180) have also been employed for this reaction. The addition of a bicycHc trialkyl phosphine is claimed to increase methanol conversion from 24% to 89% (181). 

Use of CD30D or methyl tetrahydrofuran solvents to encourage electron capture, resulted in a complex set of reactions for methyl cobalamine. Initial addition occurred into the w corrin orbital, but on annealing a cobalt centred radical was obtained, the e.s.r. spectrum of which was characteristic of an electron in a d z.y orbital (involving the corrin ring) rather than the expected d2z orbital. However, the final product was the normal Co species formed by loss of methyl. Formally, this requires loss of CH3 , but this step seems highly unlikely. Some form of assisted loss, such as protonation, seems probable. 

Use of less sterically hindered examples of 5 in combination with MAO allows for active catalysts for the linear (head-to-head) dimerisation of a-olefins such as 1-butene, 1-hexene, 1-decene and Chevron Phillips C20-24 a-olefin mixture (Scheme 4) [47], The mechanism for dimerisation is thought to involve an initial 1,2-insertion into an iron-hydride bond followed by a 2,1-insertion of the second alkene and then chain transfer to give the dimers. Structurally related cobalt systems have also been shown to promote dimerisation albeit with lower activities [62], Oligomerisation of the a-olefms propene, 1-butene and 1-hexene has additionally been achieved with the CF3-containing iron and cobalt systems 5j and 6j yielding highly linear dimers [23],... 

Studies conducted to examine the mode of activation of MAO with bis(imino) pyridine cobalt halide systems have shown some intriguing findings. With regard to 6a/MAO, initial reduction of the cobalt(II) precatalyst to cobalt halide followed by conversion to a cobalt methyl and ultimately to a cobalt cationic species has been demonstrated (see Sect. 2.6) [108, 109], Addition of ethylene affords an eth-... 

It was shown in the previous section that hydrocarbon oxidation catalyzed by cobalt salts occurs under the quasistationary conditions with the rate proportional to the square of the hydrocarbon concentration and independent of the catalyst (Equation [10.9]). This limit with respect to the rate is caused by the fact that at the fast catalytic decomposition of the formed hydroperoxide, the process is limited by the reaction of R02 with RH. The introduction of the bromide ions into the system makes it possible to surmount this limit because these ions create a new additional route of hydrocarbon oxidation. In the reactions with ROOH and R02 the Co2+ ions are oxidized into Co3+, which in the reaction with ROOH are reduced to Co2+ and do not participate in initiation. 

These reactions result in an additional route of chain propagation, which allows one to exceed the rate limit due to the mechanism of action of only variable-valence ions. In fact, the initial rate of RH transformation in the presence of the cobalt bromide catalyst is determined by the rate of two reactions, namely, R02 with RH (kp) and R02 with Co2+ (kp), followed by the reactions of Co3+ with Br and Br with RH. The general scheme proposed by Zakharov includes the following steps (written in the simplified form) [206] ... 

A comparison of the initial rates obtained with various cobalt complexes (Table I) reveals that the chelate complexes of Co(II) are more efficient than the simple salts, the catalytic activity of Co(III) is lower than that of Co(II) and the reaction becomes slower by increasing the number of N atoms in the coordination spheres in both oxidation states. In general, the addition of amine derivatives increased the activity of the catalysts. 

Not unexpectedly, alkylation of the double carbonylated complex proceeds via a base-catalysed interfacial enolization step, but it is significant that the initial double carbonylation step also involves an interfacial reaction, as it has been shown that no pyruvic acid derivatives are obtained at low stirring rates. Further evidence comes from observations of the cobalt-catalysed carbonylation of secondary benzyl halides [8], where the overall reaction is more complex than that indicated by Scheme 8.3. In addition to the expected formation of the phenylacetic and phenylpyruvic acids, the reaction with 1-bromo-l-phenylethane also produces 3-phenylpropionic acid, 2,3-diphenylbutane, ethylbenzene and styrene (Scheme 8.4). The absence of secondary carbonylation of the phenylpropionylcobalt tetracarbonyl complex is consistent with the less favourable enolization of the phenylpropionyl group, compared with the phenylacetyl group. 

Tridentate ligands for cobalt and iron catalysts. The catalysts discussed earlier in the section on ethene oligomerisation can also be used for making polymers, provided that they are suitably substituted. In Figure 10.30 we have depicted such a catalyst, substituted with isopropyl groups at the aryl substituents on the imine group, as in Brookhart s catalysts [49], The initiation is now carried out by the addition of MAO to a salt of the cobalt or iron complexes. The catalysts obtained are extremely active, but they cannot be used for polar substrates. 

Control experiments establish that the initial process converting the dibromide 275 to 276 takes place in two steps with E1/2 for extrusion of the first and second axial ligands occurring at —0.36 and —1.08 V, respectively [75]. After reaction with the alkyl halide, the resulting octahedral complex 277 is further reduced in the range of -1.4 to -1.7 V to form a cobalt (II) complex which decays via the addition of an additional electron, cleavage of the C-Co and Co-Y bonds, and reaction with the Michael acceptor. 

From the data presented in Table 4 it may be concluded that the porous nature of the chemically modified silica remains more or less the same after immobilization of the cobalt(lll) complexes. In addition, there is a decrease in the surface area of the support following the incorporation of the metal complexes. The AAS data on Co(III)-CMS2 and Co(lll)-CMS3 appear to suggest that the extent of cobalt loading is dependent upon the initial amount of cobalt used. The cobalt loadings obtained for the catalysts prepared by H2O2 oxidation of CMS suspensions in 1 and 2 mmol cobalt(ll) solutions (in presence of 1 and 2... 

Remarkably, the catalytic cycle is not controlled by the presence of phosphine ligands, but it is controlled by the organo group Y at the cobalt the neutral ligand L is displaced by the substrates in the initial step. Oxidative addition of two acetylenes results in a cobaltacycle that reacts with the nitrile to give the pyridine derivative with regeneration of the active [YCo] species. 

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