Cobalt substitution reactions

Arthur Adamson I think Dr. Wilmarth and co-workers have probably supplied the better available evidence for the pentacoordinated intermediate in a cobalt substitution reaction. 

Octahedral substitution reactions (e.g. those involving cobalt(III) complexes) may proceed by both Sf l or 8 2 reactions. In the S l case a slow dissociative mechanism (bond breaking) may take place. Reaction with the substituting... 

Delmas and his co-workers have done extensive work on pyroaurite-type materials which has recently been reviewed [73], In addition to precipitation methods, they have prepared the materials by mild oxidative hydrolysis of nickelates that were prepared by thermal methods similar to those used for the preparation of LiNiOz [74]. A cobalt-substituted material NaCoA ( Ni( A02) was prepared by the reaction of Na20, Co304 and NiO at 800 °C under a stream of oxygen. The material was then treated with a 10 molL-1 NaCIO +4 molL 1 KOH solution for 15h to form the oxidized y -oxyhydroxide. The pyroau-... 

Kinetics and mechanisms of substitution reactions of cobalt(III) frans-dioximes, in non-aqueous media. N. M. Samus and A. V. Ablov, Coord. Chem. Rev., 1979, 28,177-203 (47). 

Trans activation and limiting N1 mechanisms for substitution reactions of cobalt(III) complexes in aqueous solution. J. E. Byrd and W. K. Wilmarth, Inorg. Chim. Acta, Rev., 1971, 5, 7-18 (42). 

One of the commonest reactions in the chemistry of transition-metal complexes is the replacement of one ligand by another ligand (Fig. 9-3) - a so-called substitution reaction. These reactions proceed at a variety of rates, the half-lives of which may vary from several days for complexes of rhodium(iii) or cobalt(m) to about a microsecond with complexes of titanium(iii). 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

It will not have escaped the reader s attention that the kinetically inert complexes are those of (chromium(iii)) or low-spin d (cobalt(iii), rhodium(iii) or iridium(iii)). Attempts to rationalize this have been made in terms of ligand-field effects, as we now discuss. Note, however, that remarkably little is known about the nature of the transition state for most substitution reactions. Fortunately, the outcome of the approach we summarize is unchanged whether the mechanism is associative or dissociative. 

It is interesting to note that the products of these reactions obey the 18-electron rule. Cobalt has 27 electrons, and it acquires 6 from the three CO ligands and 3 from NO, which gives a total of 36. It is easy to see that Fe(CO)2(NO)2 and Mn(CO)4NO also obey the 18-electron rule. Because NO is considered as a donor of three electrons, two NO groups usually replace three CO ligands. This may not be readily apparent in some cases because metal-metal bonds are broken in addition to the substitution reactions. 

As indicated in Chapter 8, the production of alkanes, as by-products, frequently accompanies the two-phase metal carbonyl promoted carbonylation of haloalkanes. In the case of the cobalt carbonyl mediated reactions, it has been assumed that both the reductive dehalogenation reactions and the carbonylation reactions proceed via a common initial nucleophilic substitution reaction and that a base-catalysed anionic (or radical) cleavage of the metal-alkyl bond is in competition with the carbonylation step [l]. Although such a mechanism is not entirely satisfactory, there is no evidence for any other intermediate metal carbonyl species. 

Non-Marcusian linear free energy relationships (if I may again be permitted that barbarism) provide direct evidence for this type of potential surface in octahedral ligand substitution reactions. Both dissociative (e.g., the chloropentaamine of cobalt(III)) and associative systems (e.g., chloropentaaquo chromium(III)) may have values of slopes for the linear free energy relationships indicating non-Marcusian behavior. 

As we have seen, an area of major importance and of considerable interest is that of substitution reactions of metal complexes in aqueous, nonaqueous and organized assemblies (particularly micellar systems). The accumulation of a great deal of data on substitution in nickel(II) and cobalt(II) in solution (9) has failed to shake the dissociative mechanism for substitution and for these the statement "The mechanisms of formation reactions of solvated metal cations have also been settled, the majority taking place by the Eigen-Wilkins interchange mechanism or by understandable variants of it" (10) seems appropriate. Required, however, are more data for substitution in the other... 

The aquated Co(III) ion is a powerful oxidant. The value of E = 1.88 V (p = 0) is independent of Co(III) concentration over a wide range suggesting little dimer formation. It is stable for some hours in solution especially in the presence of Co(II) ions. This permits examination of its reactions. The CoOH " species is believed to be much more reactive than COjq Ref. 208. Both outer sphere and substitution-controlled inner sphere mechanisms are displayed. As water in the Co(H20) ion is replaced by NHj the lability of the coordinated water is reduced. The cobalt(III) complexes which have been so well characterized by Werner are thus the most widely chosen substrates for investigating substitution behavior. This includes proton exchange in coordinated ammines, and all types of substitution reactions (Chap. 4) as well as stereochemical change (Table 7.8). The CoNjX" entity has featured widely in substitution investigations. There are extensive data for anation reactions of... 

For lead references on other transition metal-catalyzed allylic substitution reactions, see (a) Cobalt Bhatia, B. Reddy, M.M. Iqbal, J. Tetrahedron Lett. 1993, 34, 6301. 

Electron-donating substituents make the aromatic subsU ate more reactive than benzene and lead to o,/ -orientation, while electron-withdrawing substituents decrease the reactivity and give mostly m-orientation products. The detailed mechanism of the formation of the a complex has been studied by oxygen-18 labeling of the sulfonyl oxygen in p-nitrobenzenesulfonyl peroxide. The ionic mechanism for aromatic substitution by sulfonyl peroxides has been confirmed by carrying out the substitution reaction in the presence of redox catalysts such as copper and cobalt salts and aluminum chloride. Small differences in the rate of the products can be found in the presence or absence of these additives, and it has been concluded that the ionic mechanism accounts satisfactorily for these results. ... 

Pyridine is converted into perfluoropiperidine (82) in low yield by reaction with fluorine in the presence of cobalt trifluoride (50JCS1966) quinoline affords (83) under similar conditions (56JCS783). Perfluoropiperidine can be obtained electrochemically. This is useful, as it may be readily aromatized to perfluoropyridine by passing it over iron or nickel at ca. 600 °C (74HC(14-S2)407). Recently, pyridine has been treated with xenon difluoride to yield 2-fluoropyridine (35%), 3-fluoropyridine (20%) and 2,6-difluoropyridine (11%), but it is not likely that this is simply an electrophilic substitution reaction (76MI20500). 

This is another of the very interesting contributions in Tobe s paper. Tobe has studied substitution reactions of the dichloro-bis(ethylenediamine)cobalt(III) ion in methanol, reported the preparation of the supposed solvo intermediate that would be required, and studied the rate of the chloride anion entry into this supposed solvo intermediate. He reports that the lability of methanol in this complex is insufficient to allow the complex to be an intermediate in a substitution process of the dichloro complex. Yet it is possible to obtain, in the case of the dichloro-chloride exchange, a term in the rate law for the free ion. This leads to the conclusion that, in fact, one has a genuinely unimolecular substitution process. 

Mechanisms of Substitution Reactions of Cobalt (III) Cyanide Complexes... 

Wayne K. Wilmarth It is a pleasure to report the work carried out by Albert Haim and Robert Grassi dealing with substitution reactions of the pentacyano-cobalt(III) complexes. 

Therefore in an attempt to distinguish among mechanisms A, B, and C the acetylacetonates of chromium(III), cobalt(III), and rhodium(III) were partially resolved and the optically active chelates were then subjected to several electrophilic substitution reactions. 

A mechanism represented by Equations 5, 6, 7, and 8 could be applied to cobalt (III), but the rate-limiting step would have to be the first substitution reaction to account for the experimental rate equation (Equation 2). It is known that cobalt (III) complexes are substitution inert (6, 23) unless significant amounts of cobalt(II) are present (I, 8, 23), and hence one could visualize the first and slow step as follows ... 

---