Reduction potentials cobalt complexes

The reduction potentials indicate that the cobalt(iii) aqua complex is unstable with respect to the cobalt(ii) state, whereas the cobalt(iii) ammine complex is... 

A number of metal porphyrins have been examined as electrocatalysts for H20 reduction to H2. Cobalt complexes of water soluble masri-tetrakis(7V-methylpyridinium-4-yl)porphyrin chloride, meso-tetrakis(4-pyridyl)porphyrin, and mam-tetrakis(A,A,A-trimethylamlinium-4-yl)porphyrin chloride have been shown to catalyze H2 production via controlled potential electrolysis at relatively low overpotential (—0.95 V vs. SCE at Hg pool in 0.1 M in fluoroacetic acid), with nearly 100% current efficiency.12 Since the electrode kinetics appeared to be dominated by porphyrin adsorption at the electrode surface, H2-evolution catalysts have been examined at Co-porphyrin films on electrode surfaces.13,14 These catalytic systems appeared to be limited by slow electron transfer or poor stability.13 However, CoTPP incorporated into a Nafion membrane coated on a Pt electrode shows high activity for H2 production, and the catalysis takes place at the theoretical potential of H+/H2.14... 

The cobalt mediated homo Diels-Alder reaction of norbomadiene (560) with phenyl acetylene (568a), affording a phenyl substituted deltacyclene, demonstrated the potential of low-valent cobalt complexes as catalysts332. Lautens and coworkers327 extended the scope of this reaction and were able to synthesize a wide range of substituted deltacyclenes from alkynes 568 (equation 164, Table 33). The low-valent cobalt or cobalt(O) species to be used was prepared in situ by reduction of Co(acac)3 with Et2AlCl. Monosubstituted... 

Cobalt complexes with square planar tetradentate ligands, including salen, cor-rin, and porphyrin types, all catalyse the reduction of alkyl bromides and iodides. Most preparative and mechanistic work with these reactions has used cobalamines, including vitamin-B,. A generalised catalytic cycle is depicted in Scheme 4.10 [219]. At potentials around -0.9 V vs. see, the parent ligated Co(lll) compound un-... 

Table 1 lists some of the binding constants and rate constants measured for the reaction of CO2 with redox-active molecules. Various techniques have been used to measure these constants including cyclic voltammetry, pulsed radiolysis, and bulk electrolysis followed by UV-visible spectral measurements. The binding constants span an enormous range from less than 1 to 10 M [13-17]. Co(I) and Ni(I) macrocyclic complexes have been studied in some detail [13-16]. For the cobalt complexes, the CO2 binding constants K) and second-order rate constants for CO2 binding (kf) are largely determined by the Co(II/I) reduction potentials... 

Reduction potentials of hexaaminecobalt(III) complexes span a range of more than 0.9 V, with the lowest potential (-0.63 V) exhibited by rCo(tra J-diammac)]3+/2+ and the highest potential (+0.28 V) found for [Co(tmen)3]3+/2+ (for ligand structures see Table 10.1 below), /ra/is-diammac leads to relatively short metal-ligand bonds and therefore stabilizes the cobalt(III) state, tmen leads to relatively long cobalt-amine... 

The reaction of a Co(I) nucleophile with an appropriate alkyl donor is used most frequently for the formation of a Co-C bond, which also can be formed readily by addition of a Co(I) complex to an acetylenic compound or an electron-deficient olefin (5). The nu-cleophilicity of Co(I) in Co(I)(BDHC) is expected to be similar to that in the corrinoid complex, as indicated by their redox potentials. The formation of Co-C a-bond is the attractive criterion for vitamin Bi2 models. Sodium hydroborate (NaBH4) was used for the reduction of Co(III)(CN)2(BDHC) in tetrahydrofuran-water (1 1 or 2 1 v/v). The univalent cobalt complex thus obtained, Co(I)(BDHC), was converted readily to an organometallic derivative in which the axial position of cobalt was alkylated on treatment with an alkyl iodide or bromide. As expected for organo-cobalt derivatives, the resulting alkylated complexes were photolabile (17). 

The photoreduction of cobalt(III) complexes by these complexes takes place catalytically, in which one-electron oxidized [Ru(( — )-menbpy)3]3+ and [Ru(S( —) PhEtbpy)3]3 + are reduced by ethanol in the solvent [26]. This is because these complexes have more positive reduction potential than that of [Ru(bpy)3]2+. 

Reduction potentials for /z-superoxo//z-peroxo-cobalt(III) couples have recently been obtained by cyclic voltammetry.739,740 Decomposition or dissociation of one or the other of the components of the couple, which frustrated measurements by other techniques, may be overcome by the use of fast scan rates. Protonation of the /r-peroxo group stabilizes the complex and EB values are pH dependent below pH 3. A similar stabilization on protonation of the peroxo bridge has been noted in the Ct2+-, V2+- and Eu2+-promoted reduction705 of the [(NHj)5Co(02)Co(NH3)5]4+ ion. The protonated species has Kh 10 dm3 mol-1 at 25 °C.705... 

Redox properties of (borabenzene)cobalt complexes have been investigated electrochemically. The reduction potentials are displaced uniformly to more positive values by about 0.5 V per borabenzene ligand. The existence of the... 

In general the extreme sensitivity of the whole class of cobalt(II) amine complexes toward dioxygen underscores their reducing character. A quantitative measure of this property is the reduction potential ... 

Complexes of sexadentate ligands display a wide range of reduction potentials. The lowest reduction potentials reported for any cobalt(III) hexaamine are -0.61 V for the diamcyclam complex and -0.62 V for/be 063 [Co(diAmpnsar) Both of these complexes are expected to have unusually short Co-N bond lengths this has been confirmed for the diamcyclam complex 149). 

Two perfectly reversible one-electron reduction steps are observed for Co.S ", at —0.53 V (Co +/+) and 1.205 V (Co+/ ). Comparison of the electrochemical properties of the cobalt catenate with the previously reported data for cobalt bpy or phen complexes [31, 32] shows here again a strong stabilization of the reduced states, Co and Co". Other sterically constrained polyimine ligands also lead to stabilized monovalent cobalt complexes [33-37]. Remarkable also is the drastic structural effect of the catenate on the Co +/-+ redox potential whereas bpy or phen complexes of cobalt(II) can easily be oxidized to octahedral cobalt(III) [36, 38], the redox potential value of the Co + -+ couple being close to 0 V, no oxidation peak is observed for Co.5 + prior to ligand oxidation (Ep > 1.6 V). The destabilization effect of Co " due to tetrahedral environment provided by the entwined and interlocked structure of 5 is thus very large (>1.5 V). 

The simple but rather useful molecular mechanics calculations were applied to both electron transfer kinetics and reduction potentials for a wide range of hexamine cobalt(III/II) complexes with primary, secondary, tertiary, and macrobicyclic amine ligands [237]. The redox potentials of the Co +/2+ couples varied from... 

The electrochemical data for cobalt(III) sarcophaginates with apical aromatic substituents are listed in Table 49 [137]. The nature of the aromatic substituents for the most part did not greatly affect the Co3+/2+ reduction potentials. The anthraquinone imine complex, however, showed a two-electron wave due to reduction of the anthraquinone substituent at ca 230 mV and a one-electron wave of... 

The electrochemical data for cobalt(III) sarcophaginates with pendant aromatic substituents [137] showed that the reduction potential of the anthraquinone-containing complex is more positive than that of Co3+ 2+ couples in these complexes. Hence, they may be... 

Knowing and understanding standard reduction potentials, E°, also helps determine whether a particular oxidation state will be stable and appropriate oxidants and reductants to use in a synthetic scheme. In reviewing the cobalt complex syntheses in Chapters 2 and 6, for example, complex ions are formed by oxidizing cobalt(II) salts to the more stable +3 state. 

In this experiment, the electrochemistry of both [Co(en)3]3+/2+ and [Co(ox)3]3+/2+ will be investigated using cyclic voltammetry, and the standard reduction potential (E°, V) for the [Co(en)3]3+/2+ couple will be measured. For metal complex stability reasons discussed below, it is not possible to use this technique to obtain reduction potentials for the mixed ligand cobalt systems an exercise at the end of this experiment helps to estimate these. The E° values obtained will be important for experiment 5.6, in which outer-sphere electron transfer rate constants between [Co(en)3)]2+ and [Co(en)2)(ox)]+ will be mathematically modeled using Marcus theory. 

---