Search

The reduction, methylation, and photolysis of cobalt phthalocyanine solutions have been studied by visible spectrophotometry, e.s.r., n.m.r., and mass spectrometry. Analogies are drawn with the reactions of vitamin B12.

The n.m.r. spectra of a number of cobalt(III) dimethylglyoxime complexes of the type Co(DH)2LX where L is triphenylphosphine or pyridine have been measured. The chemical shifts of the dimethylglyoxime methyl hydrogens and the hydrogen-bonded hydrogens depend on the axial ligands and a correlation has been found between the chemical shift of the methyl hydrogens and the Hammett σpara-function of the ligand X. This correlation is interpreted as lending support to the suggestion that the ground-state cis-effect in these complexes reflects changes in the electron density in the cis-ligand.

Formation constants have been determined for the substitution of co-ordinated H2O in aquocobalamin in aqueous solution (ionic strength usually 0·5) at room temperature by the following ligands: CH3·CO2–, 4·5 l./mole; N3–, 7·2 × 104; NO2–, 2·3 × 105; CH3·NC, 7 × 104; SO32–, 2·2 × 107; thiourea, 13. No complex was detected with the ligands CO, O2, NO, (CH3)2S, or Ph3P. Additional formation constants relating to the thermodynamic trans-effect have been determined (i) when the trans-ligand is SO32–, and (ii) for the substitution of H2O by N3–, CH3·NC, and SO32– with different trans-ligands. It is shown that SO32– is comparable with HCC– and CH2CH– in its cis- and trans-effects. The absence of CO complexes and the close parallel between formation constants involving CH3·NC and N3– are evidence for the absence of any significant π-bonding between the cobalt(iii) ion and the ligand.

Intermolecular complexes between several metal porphyrins and series of both conventional acceptors and donors have been studied in organic solvents. The stability constants and absorption spectra of the complexes have been measured. It is suggested that one of the factors contributing to stability is the matching of the sizes of the porphyrin and the donor or acceptor, but that this is not always dominant.

The syn- and anti-isomers of a cis-co-ordinated dithioether ligand in the complex Pd(EtS·CH2·CO·N·[CH2]3·-N·CO·CH2·SPh) have been identified by 1H n.m.r. spectroscopy and the thermodynamic parameters of isomerisation have been measured. The 1H n.m.r. spectra showed the presence of two isomers in the complexes Pd[CH2(CH2·N·-CO·CH2·SPh)2] and Pd[CH2(CH2·N·CO·CH2·SEt)2].

The proton magnetic resonance spectra of a number of cobalamins and cobinamides have been measured and prominent resonances assigned. The dependence of the resonances on the ligands attached to the cobalt is discussed. The rate of proton-exchange of the C(10) hydrogen has been measured.

Solutions of organocobinamides show reversible changes in the optical spectrum and in the 1H n.m.r. of the H-10 of the corrin ring on varying the temperature. Similar reversible changes in the optical spectrum are observed in the solid state on varying the water content. The simplest explanation is that the observed equilibria represent the reversible removal of co-ordinated water to form a five-co-ordinate complex: [CoIII·corrin·X·H2O]=[CoIII·corrin·X]+ H2O The magnitude of the equilibrium constant depends on the nature of the trans-ligand X, the ease of removing H2O increasing in the order cyanide vinyl < methyl < ethyl < isopropyl. The following values were obtained for the enthalpy and entropy changes in methanol: X = vinyl, ΔH=+4·5 ± 2·0 kcal./mole, ΔS=+15·5 ± 7 e.u; X = methyl, ΔH= 4·4 ± 2·0 kcal./mole, ΔS= 16 ± 7 e.u. On varying the temperature of the solution or the water content of the solid, ethylcobalamin and 5′-deoxyadenosylcobalamin also show changes in spectrum, caused by the reversible co-ordination of the benzimidazole to the cobalt.

The reduction of nitrobenzene, nitrosobenzene, and azoxybenzene by potassium borohydride is catalysed by hydroxocobalamin. The products of the reaction with nitrobenzene include aniline and azobenzene. The cobalt(I) cobalamin, vitamin B12s, reacts with nitrobenzene, nitrosobenzene, azoxybenzene, and azobenzene but not with phenylhydroxylamine or hydrazobenzene. A mechanism for the reduction is proposed.

Controlled potential coulometric analysis has been used to measure the number of electrons involved in the reduction of some vitamin B12 compounds. The reductions of B12a to B12r, and B12r to B12s, each require one electron. Analyses of the numbers of protons involved in the reactions show that B12s is not a hydride. The rate constants for the reductions are analysed in detail since a catalytic current is observed in the case of reduction to B12s. The rate constant for the decomposition of B12s in the absence of oxygen has been determined from these data and agrees with that determined by a polarographic method. The rate of decomposition depends on the pH of the solution and on the buffer anions present. It is suggested that B12s can catalyse the reduction of some anions or of water.

Formation constants have been determined for the substitution of co-ordinated water in diaquocobinamide by ammonia and imidazole, in aquocobalamin by ammonia, and in cyanoaquocobinamide by ammonia, pyridine, imidazole, benzimidazole, and adenine; the last three bases can co-ordinate in the conjugate anion form. It is shown that the formation constants for the binding of ammonia are more sensitive to the nature of the trans-ligand than those for the binding of imidazole, and that the relative order of their formation constants can change as the trans-ligand is varied. Pseudo-first-order rate constants for the substitution of co-ordinated ammonia by water at 30° have been obtained for corrinoids with the following trans-ligands: H2O, 1·4 × 10-5 s–1; 5,6-dimethylbenzimidazole, 8·6 × 10-5 s–1; CN–, SO32–, and CH3–, 3 × 10–1 s–1. The order of ligands observed in the kinetic trans-effect parallels that for the thermodynamic trans-effect.

Present experiments support Friedrich's conclusion that corrinoids in which the axial ligands are H2O and CN–, e.g., aquocyanocobinamide (Factor B) and aquocyanocobyric acid (Factor V1a), exist in solution as a mixture of stereoisomers. It is shown that thin-layer chromatography is better than column or paper chromatography in separating the isomers. Mercury(II) salts inhibit the isomerisation and decompose the dicyanide complexes to the aquocyano-complexes without decomposing the latter; it is concluded that isomerisation requires the presence of free cyanide and proceeds through the intermediate formation of the dicyanide. The probable identity of the isomers has been established in the case of Factor V1a, whose structure is known in the solid state. Organo-aquocobinamides, where the axial ligand cannot exist free in solution, do not isomerise

The ability of palladium to promote the ionisation of amide hydrogen atoms is demonstrated by the preparation of a series of neutral complexes of quadridentate ligands containing two deprotonated amide groups. The other groups are either both primary amine, both secondary amine, or both thioether groups. The names of the ligands and the formulae of the complexes are NN′-di-(2-aminoethyl)malondiamide, Pd[CH2(CO·N·CH2·CH2NH2)2],3H2O, NN′-di-(3-benzylaminopropyl)oxamide, Pd(–CO·N·[CH2]3·NH·CH2Ph)2, NN′-di-(S-ethylmercaptoacetyl)-1,3-di-aminopropane, Pd[CH2(CH2·N·CO·CH2·S·CH2·CH3)2]. With ligands of general formula RNH·CO·CH2·S·-[CH2]n·S·CH2·CO·NHR, when R = H, n= 2 or 3, neutral complexes are formed, Pd{[CH2]n(S·CH2·CO·NH)2},-xH2O (n= 2, x= 2; n= 3, x= 3). When R = CH2·CH2·OH, for steric reasons only one of the two amide groups can co-ordinate through nitrogen and the complexes formed are PdCl(HO·CH2·CH2·N·CO·CH2·S·[CH2]2·-S·CH2·CO·NH·CH2·CH2·OH) and {Pd(HO·CH2·CH2·N·CO·CH2·S·[CH2]3·S·CH2·CO·NH·CH2·CH2·OH)}+Cl–.

Controlled potential coulometric analysis has been used to investigate the reduction of alkyl halides in the presence of vitamin B12a. At voltages greater than the half-wave potential of the alkyl halide the reduction is a two-electron process. Below this half-wave potential the reduction is a one-electron reaction. The importance of these observations lies in considerations of the reaction mechanisms of reductions catalysed by vitamin B12 in both model and biological systems.

The 1H n.m.r. spectra of a series of para-substituted phenyl derivatives of bae-cobalt complexes ([H2bae = bis(acetylacetone)ethylenedi-imine] have been measured. The chemical shifts of all the hydrogens in the planar ligand are dependent on the axial ligand. It is concluded that through-space effects are mainly responsible for the dependence on the axial ligand of the difference in the chemical shifts of the A2 and B2 hydrogens of the ethylene-di-imine bridge system whilst changes in electron density transmitted by way of the bonding electrons are mainly responsible for the variations in the chemical shifts of the methene and methyl hydrogens which are related to the Hammett σ-para-functions of the para-substituent in the axial ligand. A similar correlation is observed with the formation constants of the six-co-ordinate pyridine and piperidine complexes.

We have studied the effect of varying one axial ligand (X), usually an organo-ligand, on the stretching frequency of cyanide co-ordinated in the trans-position (Y) and on the spectrum of the equatorial corrin ring in these cyano-complexes, and have measured several additional formation constants for substitution of the axial ligands. The cyanide stretching frequency is very dependent on the nature of X, falling from 2132 cm.–1 when X = benzimidazole to 2082 cm.–1 when X = CH3CH2–. All the evidence relating to cis- and trans-effects in corrinoids which contain only the light ligand atoms C, N, and O is compared. It is shown that there is a correlation between cis- and trans-effects and between ground-state and thermodynamic effects, and that, in general, as the ligand X becomes more polarisable, so the other metal–ligand bond lengths increase, cyanide co-ordinated in the trans-position becomes more ionic, and the formation constants for the substitution of H2O fall to values more typical of ion-pairs. It is concluded that the main factor is the amount of negative charged donated to the cobalt atom through the σ-bond.