Cobalt complexes isomerisation

The intermolecular Pauson-Khand reaction of the resulting S/P-cobalt complexes with norbornadiene was studied under thermal and A -oxide activation conditions. Thus, heating the diastereomerically pure complex (R = Ph, R = Cy) with ten equivalents of norbornadiene at 50 °C in toluene afforded the corresponding exo-cyclopentenone in a quantitative yield and with an enantio-selectivity of 99% ee. Under similar conditions, the analogous trimethylsilyl complex (R = TMS, R = Cy) afforded the expected product in a high yield but with a lower enantioselectivity of 57% ee. In order to increase this enantio-selectivity, these authors performed this reaction at room temperature in dichloromethane as the solvent and in the presence of NMO, which allowed an enantioselectivity of 97% ee to be reached. These authors assumed that the thermal activation promoted the isomerisation of the S/P ligand leading to a nonstereoselective process. 

With a view to determining the equilibrium constant for the isomerisation, the rates of reduction of an equilibrium mixture of cis- and rra/i5-Co(NH3)4(OH2)N3 with Fe have been measured by Haim S . At Fe concentrations above 1.5 X 10 M the reaction with Fe is too rapid for equilibrium to be established between cis and trans isomers, and two rates are observed. For Fe concentrations below 1 X lO M, however, equilibrium between cis and trans forms is maintained and only one rate is observed. Detailed analysis of the rate data yields the individual rate coefficients for the reduction of the trans and cis isomers by Fe (24 l.mole sec and 0.355 l.mole .sec ) as well as the rate coefficient and equilibrium constant for the cw to trans isomerisation (1.42 x 10 sec and 0.22, respectively). All these results apply at perchlorate concentrations of 0.50 M and at 25 °C. Rate coefficients for the reduction of various azidoammine-cobalt(lll) complexes are collected in Table 12. Haim discusses the implications of these results on the basis that all these systems make use of azide bridges. The effect of substitution in Co(III) by a non-bridging ligand is remarkable in terms of reactivity towards Fe . The order of reactivity, trans-Co(NH3)4(OH2)N3 + > rra/is-Co(NH3)4(N3)2" > Co(NH3)sN3 +, is at va-... 

There is no firm evidence for the structure of the initial protonated species, but, bearing in mind that the HOMO of a /<-peroxo complex is essentially a jTg orbital of dioxygen (Sect. D), and on the basis of other structural evidence, it seems probable that the proton is bound to one oxygen only. The structure of the final isomerised product has been determined by X-ray crystallography Protonation of a /<-peroxo complex of cobalt with tertiary arsine ligands has also been reported ... 

Isomerisation of olefins catalysed 11 by palladium and other transi- (34) tion-metal complexes Hydrogenation reaction with 10 cobalt carbonyl hydride as a (29) hydrogenation agent 7r-Complex adsorption in hydrogen 27 exchange on Group VIII transi- (45) tion metal catalysts... 

There is an interesting contrast in the substitution kinetics of plati-num(II) complexes and the complexes of cobalt(III) and chromium(III), in relation to the two solvents DMF and DMSO. As emphasised by solvolysis and isomerisation studies with both cobalt(III) and chro-mium(III), these two solvents differ in only minor ways. DMSO is a slightly stronger ligand, based on its resistance to substitution replacement by anions, but this difference is small as are the differences in their mutual interchange rates. - " ... 

Chromium(m) and Cobalt(iii) Complexes.—Just as copper(ii) catalyses aquation of oxalato-complexes of chromium(iii) (cf. Section 3, ref. 117), so do a variety of cations catalyse isomerisation of fra 5-[Cr(oxalate)2(OH2)2] . Rate constants are a function of the charge, radius, and structure of the... 

Isomerisation studies of cobalt(iii) complexes cover a variety of compounds. The rate law for isomerisation of [Co(dien)(OH2)a] + indicates some isomerisation via the hydroxo-cation [Co(OH)dien(OHa)2] +. Rates and activation parameters for direct isomerisation of the tris-aquo-complex were determined. Further kinetic results for isomerisation of cis- and rra 5 -[Co(OH)2 en2]+ in strongly basic solution indicate an intramolecular mechanism. Isomerisation of cw-[CoCl2(diars)2] in methanol is also intramolecular, but isomerisation of cw-[Co(02C CH3)2en2]+ in acetic acid occupies an intermediate position between intra- and inter-molecu-larity, for the essential step is solvent-assisted acetate exchange within ion-pairs. This assignment of mechanism results from consideration of kinetics of acetate exchange as well as of isomerisation. ... 

The majority of catalysts mentioned so far have been complexes of rhodium or of iridium. Compounds of the other member of this triad, cobalt, may also catalyse alkene isomerisation. A recent example is provided by the cluster compound [Co(CO)2(PRs)] the catalytically... 

Photoinduced cycloisomerisation can also be carried out in the presence of other metal ions serving as templates. For example, secocorrin coordinated to metal ions such as lithiiun(I), magnesirun(II), zinc(II) or cadmium(II) readily cyclises in the absence of oxygen in almost quantitative yield at room temperature [441, 444]. Platinum(II) and palladium(II) complexes (quantum )deld 0.008 in chloroform at 20°C) also cyclise, but more slowly. However, nickel(II), cobalt(III) and copper(II) secocorrinates do not undergo such a transformation [441, 444]. If, however, the nickel(II) l-methylidene-l,19-secocorrinate is subjected to one-electron electrochemical oxidation, followed by direct reduction of the isomerised cation-radical, then such a transformation is realisable [450]. 

Cobalt(III) chemistry is dominated by Co(III) complexes. A large number of these have been prepared and studied during this century and their structure and bonding have been elucidated with the advance in instrumentation. Since most of these complexes are octahedral, low spin, diamagnetic and kinetically inert, their reactions can be conveniently studied. They undergo numerous reactions acid and base hydrolysis, ligand exchange, reduction and, where applicable, isomerisation. 

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