The chemistry of vitamin B12. Part IX. Evidence for five-co-ordinate cobalt(III) complexes

Firth, R. A.; Hill, H. A. O.; Mann, Brian E.; Pratt, J. M.; Thorp, R. H.; Williams, R. J. P.

doi:10.1039/j19680002419

Cited by 70 publications

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“…This is expected, since the presence of CH 3 in the axial position exerts a strong trans influence and the coordination of the other axial ligand (H 2 O) is very weak. A similar observation was made in the case of alkylcobaloximes and alkylcobinamide COENZYME B 12 MODELS 555 [7,29,30]. In the case of cobinamide, increasing pressure of decreasing temperature shifts the equilibrium towards the 6-coordinate species.…”

Section: Resultssupporting

confidence: 64%

Equilibrium and Kinetic Studies on Ligand Substitution Reactions of Trans-[CO^III(en)₂(Me)H₂O]²⁺: A Model for Coenzyme B₁₂

Hamza

2003

Journal of Coordination Chemistry

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Ligand substitution of trans-[Co III (en) 2 (Me)H 2 O] 2þ was studied for pyrazole, 1,2,4-triazole and N-acetylimidazole as entering nucleophiles. These displace the coordinated H 2 O molecule trans to the methyl group to form trans-[Co(en) 2 (Me)azole]. Stability constants at 18 C for the substitution of H 2 O by pyrazole, 1,2,4-triazole and N-acetylimidazole are 0.7 AE 0.1, 13.8 AE 1.4 and 1.7 AE 0.2 M À1 , respectively. Second order rate constants at the same temperature for the reaction of trans-[Co III (en) 2 (Me)H 2 O] 2þ with pyrazole, 1,2,4-triazole and N-acetylimidazole are 161 AE 12, 212 AE 11 and 12.9 AE 1.6 M À1 s À1 , respectively. Activation parameters (ÁH 6 ¼ , ÁS 6 ¼ ) are 67 AE 6 kJ mol À1 , þ 27 AE 19 J K À1 mol À1 ; 59AE 2 kJ mol À1 , þ 1 AE 6 J K À1 mol À1 and 72 AE 4 kJ mol À1 , þ 23 AE 14 J K À1 mol À1 for reactions with pyrazole, 1,2,4-triazole and N-acetylimidazole, respectively. Substitution of coordinated H 2 O by azoles follows an I d mechanism.

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Section: Resultssupporting

confidence: 64%

Equilibrium and Kinetic Studies on Ligand Substitution Reactions of Trans-[CO^III(en)₂(Me)H₂O]²⁺: A Model for Coenzyme B₁₂

Hamza

2003

Journal of Coordination Chemistry

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“…in the cobinamides. 2 The other axial position can be occupied by any of a number of ligands (sixth ligand). Coenzyme B12 contains a carbon-bound 5'-deoxyadenosyl ligand, 3 and complexes with other carbon ligands can also be prepared.…”

Section: Introducilonmentioning

confidence: 99%

Visible and near‐ultraviolet resonance Raman spectra of photolabile vitamin B₁₂ derivatives with a rapid‐flow technique

Salama

Spiro

1977

J Raman Spectroscopy

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Resonance Raman spectra, obtained with 5145,4880 and 3638 excitation, are reported for aquo-, methyl-, 5'-deoxyadenosyl-(coenzyme B,,), thiosolfate and cysteinyl cobdamin, of methyl cobmamide, and of Co(XI) cobalamin (Blzr). Laser-induced photodecomposition was held to acceptable levels with a rapid-%ow technique. A rich assortment of corrin ring vibrations are observed above 600 an-', but the frequencies are constant (within *3cm-') for all the derivatives. Some Raman bands show differential enhancement by the visible and near-ultraviolet electronic transitions, reflecting their different polarizntions. B,, shows large relative intensity changes in bands at 1500 and 1600 cm-', witb respect to the Co(1II) derivatives. Despite an absorption spectrum resembling that of B,,, methyl cobmamide shows Raman intensity behavior typical of the other Co(II1) derivatives. INTRODUCIlONThere is much current interest in the chemistry and biochemistry of vitamin B12 and its derivatives.' All of these contain a cobalt atom coordinated to four nitrogen atoms of the corrin ring. One axial coordinate position is occupied by benzimidazole, bound covalently to the ring, although this can be removed chemically, e.g. in the cobinamides.2 The other axial position can be occupied by any of a number of ligands (sixth ligand). Coenzyme B12 contains a carbon-bound 5'-deoxyadenosyl ligand,3 and complexes with other carbon ligands can also be prepared.The corrins have strong absorption bands in the visible and near-u.v. regions, which are due to T-T* electronic transitions. As with the porphyrins, excitation in these bands produces strong resonance enhancement of Raman bands associated with corrin ring vibrations. Several resonance Raman (RR) studies of B12 derivatives have been rep~rted.~" These have shown very similar RR spectra for cobalamins, the Co(II1) B12 derivatives, regardless of the nature of the sixth ligand. Somewhat different spectra are observed for the reduced forms, B12r, which contains Co(I1) and BlZs, which contains Co(1).Wozniak and Spiro reported,6 however, that methyl cobinamide and 'base off methyl cobalamin, in which the bond between benzimidazole and cobalt is dissociated by protonation, both have spectra identical with B12r, rather than other Co(II1) derivatives. They suggested that the loss of benzimidazole might allow the corrin ring to relax to a conformation close to that of B12r. In retrospect, an alternate explanation is that the observed spectra were in fact due to Co(I1) species t Abbreviations: B,, = Vitamin B,, [cr-(5,6-dimethylbenzimidazolyl)cobamide]; BIZ, = Co(I1) Cobalamin; Coenzyme-B,, = 5'-deoxyadenosyl-B,,; Methyl-cobin = Methyl-cobinamide. @ Heyden & Son Ltd, 1977 generated by photolysis in the laser beam. Photolysis of B12 derivatives is particularly facile when they contain ~a r b o n ,~-~ and to a lesser extent sulfur, ligands, and under anaerobic conditions, the product is B12r. Detection of this artifact is made difficult by the similarity of the B12r and methyl cobinamide (Me-Cobin) or base-off ...

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“…The electron demand of the central Co-atom can principally be satisfied from either apical direction. Under conditions guaranteeing an unprotonated benzimidazole, literature data [9] show that the ribonucleotide from secondary alkylcobalamins does not coordinate the Co-atom. According to this evidence, we assume that *) Co(d6), ovcr all 16 electrons.…”

Section: Scheme 4 )mentioning

confidence: 97%

“…workup of the remaining mixture gave 65.5 mg of a mixture (93.5% by weight, see above). GC: 3 (22%), 6 (1.8%), 7 (47.9, 8 (13%), 9 (1 %), 5 (8%). For the data of 2 , 4 CJ [la].…”

Section: Experimental Partmentioning

confidence: 97%

Oxidative Cyclopropanol Opening by Cob(III)alamin

Wan

Fischli

1984

Helvetica Chimica Acta

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SummaryStarting from the cyclopropanol 2, the isomeric cyclopropanol 4 and the / 3, y-unsaturated aldehydes 7 and 8 have been produced by a cobalamin-dependant transformation. In traces, the two acetoxycyclopropanes 3 and 6, the saturated aldehydes 5 and 11 and the P,y-unsaturated aldehyde 9 could be detected (cf. Structural Formulae and Table). Starting from 4 the same products in a rather similar distribution were obtained. The isomerization 2+4 as well as the transformations leading to 7, 8, and 9 are shown to be mediated by cob(II1)alamin (l(II1)). The results are explained on the basis of rearranging Co-complexes. The migrations might be driven by the electrophilic nature of the central Co(d6)-atom.

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The chemistry of vitamin B12. Part IX. Evidence for five-co-ordinate cobalt(III) complexes

Cited by 70 publications

References 0 publications

Equilibrium and Kinetic Studies on Ligand Substitution Reactions of Trans-[CO^III(en)₂(Me)H₂O]²⁺: A Model for Coenzyme B₁₂

Equilibrium and Kinetic Studies on Ligand Substitution Reactions of Trans-[CO^III(en)₂(Me)H₂O]²⁺: A Model for Coenzyme B₁₂

Visible and near‐ultraviolet resonance Raman spectra of photolabile vitamin B₁₂ derivatives with a rapid‐flow technique

Oxidative Cyclopropanol Opening by Cob(III)alamin

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The chemistry of vitamin B12. Part IX. Evidence for five-co-ordinate cobalt(III) complexes

Cited by 70 publications

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Equilibrium and Kinetic Studies on Ligand Substitution Reactions of Trans-[COIII(en)2(Me)H2O]2+: A Model for Coenzyme B12

Equilibrium and Kinetic Studies on Ligand Substitution Reactions of Trans-[COIII(en)2(Me)H2O]2+: A Model for Coenzyme B12

Visible and near‐ultraviolet resonance Raman spectra of photolabile vitamin B12 derivatives with a rapid‐flow technique

Oxidative Cyclopropanol Opening by Cob(III)alamin

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Equilibrium and Kinetic Studies on Ligand Substitution Reactions of Trans-[CO^III(en)₂(Me)H₂O]²⁺: A Model for Coenzyme B₁₂

Equilibrium and Kinetic Studies on Ligand Substitution Reactions of Trans-[CO^III(en)₂(Me)H₂O]²⁺: A Model for Coenzyme B₁₂

Visible and near‐ultraviolet resonance Raman spectra of photolabile vitamin B₁₂ derivatives with a rapid‐flow technique