Cobalt ligand reactions

The relatively scanty information available and the limited research effort devoted to the study of ligand reactions cannot be attributed to the relative youth of the experimental area, for the literature contains scattered observations dating back to the earliest possible time at which such reactions could be recognized. Indeed, Werner (68, 76) utilized a ligand reaction in his classic demonstration of the manner of attachment of thiocyanate to cobalt (III). In his view, the conversion of thiocyanate to ammonia within the coordination sphere could only mean that SCN is attached to cobalt through the nitrogen atom (Equation 1). 

Reaction of Cytochrome cIinn with Bis(ferrozine)copper(II) Knowledge of the redox properties of cytochrome c was an encouragement to initiate a kinetics investigation of the reduction of an unusual copper(II) complex species by cyt c11. Ferrozine (5,6-bis(4-sulphonatophenyl)-3-(2-pyridyl)-1,2.4-triazine)286 (see Scheme 7.1), a ligand that had come to prominence as a sensitive spectrophotometric probe for the presence of aqua-Fe(II),19c,287 forms a bis complex with Cu(II) that is square pyramidal, with a water molecule in a fifth axial position, whereas the bis-ferrozine complex of Cu(I) is tetrahedral.286 These geometries are based primarily upon analysis of the UV/visible spectrum. Both complexes are anionic, as for the tris-oxalato complex of cobalt in reaction with cytochrome c (Section 7.3.3.4), the expectation is that the two partners will bind sufficiently strongly in the precursor complex to allow separation of the precursor formation constant from the electron transfer rate constant, from the empirical kinetic data. 

The first calculation of the complete hydroformylation cycle with Rh-phosphine catalysts (substrate = ethylene, model ligand = PH3) was published in 1997 [3]. The QM methods used are HF and MP2, respectively (cf. Section 3.1.2.1). Hybrid DFT methods such as B3LYP [4], however, are more appropriate in terms of both accuracy and efficiency [5, 6] (cf. Section 3.1.2.1). Therefore, the same model system was recalculated [7] on the level B3LYP functional/DZVP basis set [8]/quasi-relativistic pseudopotentials on rhodium [9]. Since homologous Ir catalysts are interesting alternatives from an economic point of view [10], calculations with the central metal Ir were also made. This comparative treatment is supported by the experimental assumption of a common mechanism [11], which equals the Heck-Breslow mechanism of the cobalt-catalyzed reaction [12],... 

Starting from optically active nitriles, Botteghi and co-workers [32] have applied the cobalt-catalyzed reaction for the prepartion of optically active 2-substituted pyridines (eq. (8)). The chiral center is maintained during the alkyne-nitrile co-cyclization reaction. This reaction has recently been extended to the synthesis of bipyridyl compounds having optically active substituents [33] and provides an access to chiral ligands of potential interest in transition metal-catalyzed asymmetric synthesis. 

The cobalt-catalyzed reaction of carbon monoxide and hydrogen with an alkene, hydroformylation, is an extremely important industrial process, but it occurs under vigorous conditions (200-400 bar, 150-200 °C) and is not a particularly selective reaction. In the presence of ligand-modified rhodium catalysts, however, hydroformylation can be carried out under extremely mild conditions (1 bar, 25 C). The catalytic activity of such rhodium complexes is in fact lO -Ky times greater than that of cobalt complexes and side reactions, such as hydrogenation, are significantly reduced. The reactivity of alkenes in hydroformylation follows a similar pattern to that observed in other carbonylation reactions, i.e. linear terminal alkenes react more readily than linear internal alkenes, which in turn are more reactive than branched... 

The less favorable entropies of the polyamine ligand reactions are attributed to the greater loss of rotational freedom about the C N bonds of the aliphatic bases upon metal-ion coordination. It was observed that the coordinated polyamine of a cobalt dioxygen complex undergoes oxidative dehydrogenation under anaerobic conditions to form an imine with the double bond conjugated to the heterocyclic ligand.32... 

The cobalt—hydride intermediate has not been isolated and is virtually undetected. Cobalt hydride reaction with an olefin by first migrating the hydrogen atom from cobalt to a porphyrin nitrogen atom (78) or carbon atom (79) is precedented by the isomerization of a benzylcobalt chelate. In that case, the benzyl migrates reversibly from the cobalt to the carbon atom of the equatorial ligand.253... 

Other reactions are alkane formation by hydrogenation, ketone formation (especially with ethylene ), ester formation through hydrogen transfer and formate ester synthesis. An improved catalyst system in which one CO ligand of CoH(CO)4 is substituted with a trialkylphosphine ligand , was disclosed by Shell workers in the early 1960s. With this catalyst, which is more thermally stable than the unsubstituted cobalt carbonyl, reaction proceeds at 140-190 C with 3-7 MPa of CO and Hj. Additionally, mostly linear aldehydes are obtained from linear terminal and internal olefins. This remarkable result arises from the high preference for the terminal addition to an a-olefin, and the isomerization of the olefinic position which occurs simultaneously with hydroformyiation. 

This chapter, in previous volumes, concentrated on mechanistic studies of the stoichiometric reactions of coordinated a- and tt-hydrocarbons with nucleophiles and electrophiles. In order to provide a more comprehensive overview of the reactivity of coordinated ligands in general, related ligand reactions in classical coordination complexes are now also included. The stereospecificity of such processes and their potential for asymmetric synthesis has continued to attract increasing attention, and it is therefore appropriate to collect them all together in one chapter. There are three subsequent sections. The first is concerned with cobalt(III) complexes, and the second with complexes of other metals. The last section deals with the ligand reactivity of organometallic compounds. 

This chapter is concerned with coordinated ligand reactions, and notably enhanced reactivity resulting from coordination. It is divided into three sections. The first two deal with coordination complexes of cobalt(III) and other metal centers, while the last section discusses ligand reactivity of organometallic compounds. The emphasis is on stoichiometric reactions catalytic processes are covered in Chapter 14. 

Cobalt(II) (F).— The reaction of cobalt(ii) with pan (5) is slightly slower than that with par (6), allegedly owing to the greater bulk of the pan ligand. Several complex formation reactions show complicated kinetic patterns due to parallel redox processes. This is true for the cobalt(ii) reactions mentioned above ... 

Other interesting reports have appeared dealing with reactions of ligands coordinated to cobalt(iii). Reaction of 2-mercaptoethylamine in (7) with one equivalent of Np or Co in aqueous HCIO4 gives the radical (8) which dimerizes and finally produces the disulphide-bridged mononuclear product (9), ... 

In contrast to their effect of rhodium-catalyzed hydroformylation, organic ligands effect on the cobalt-catalyzed reaction is less pronounced. Moreover, a desired regiodirecting effect strongly depends on the equilibrium between the modified and unmodified catalyst. 

The cobalt-catalyzed reaction is applicable to intramolecular cyclization of 6-halo-1-hexene derivatives (Table 2). In the intramolecular version, dppb, l,4-bis(diphenylphosphino)butane, is the best ligand, and a higher temperature is necessary. The same system also effects the conversion of aryl iodides having an olefinic moiety at a proper position into the cyclized product (eq33). 

An example of a kinetic study of a coordinated ligand reaction at ruthen-ium(III) is provided by that of hydrolysis of p-nitrophenylacetate catalyzed by [Ru(NH3)5(im)] . This complex is a very effective catalyst, being 10,000 times more effective than its cobalt(III) equivalent.Acetylacetone exchange at [Ru(acac)3] will be discussed under cobalt(III) (see Section 5.7.5.2). The rate law for ruthenium trichloride catalysis of oxidation of (substituted) phenols by periodate in alkaline aqueous solution is claimed to indicate preequilibrium formation of a ruthenium(III)-phenol complex. This is assumed to arise from re action between the phenol and RuOHaq present in aqueous solutions of ruthenium trichloride.The substitution reaction... 

It is not clear, at present, whether the spectrum (Figure 2) may be attributed to the penta-ammino-complex or to the ruthenium atoms formed by the rapid dissociation of the complex. There is no evidence, however, for the reduction of the ligand nitrogen under these conditions. Hydrated electrons generated in the pulse radiolysis of aqueous solutions containing cobalt(m) cyanide complexes react very rapidly (jk 10 —10 1 mol s ) with the cobalt solutes. Reactions of the type... 

Catalytic carbonylation of arynes was first reported by Chatani et al. in 2001 [18], As depicted in Scheme 28.14, the cobalt-catalyzed reaction of arynes with CO provided anthraquinones 37 in 80% yield. Furthermore, the three-component coupling of arynes, CO, and allyl acetates 38 was promoted efficiently in the presence of a palladium catalyst, giving 2-methyleneindanones 39 in good yield. Direct construction of 2-methyleneindanone skeletons (39 and 41) was also achieved by employing methyl allyl carbonates 40 instead of allyl acetates (Scheme 28.15), where the regios-electivities were totally dependent on the ligands [PPhs or P(o-tol)3] [19]. 

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