The chemistry of vitamin B12. Part 27. The cage phosphite ester 4-ethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane and the anionic dimethyl phosphite as ligands for cobalt(III) corrinoids

Susan M. Chemaly, Eric A. Betterton, John M. Pratt

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The equilibrium constant for the substitution of co-ordinated H 2O in aquocobalamin by 4-ethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2] octane (L4) has been determined in aqueous solution at 25 °C by u.v.-visible spectrophotometry to be log K = 7.2; comparison with the estimated pK of 1.5 for the protonation of L4 shows the marked preference of L4 for CoIII over the proton. The cobinamide and acidified cobalamin (with protonated dbzm, the nucleotide base) containing anionic dimethyl phosphite (L5) show a reversible temperature-dependent change in spectrum, ascribed to an equilibrium between five- and six-co-ordinate forms with axial ligands L5-Co and L5-Co-OH2 respectively. Axial ligands L4 and L5 exert a similar cis effect (based on wavelength of the γ band), viz. H2O < dbzm < CN- < SO32- ≈ L4 ≈ L5 < HC≡C- < CH2CH- < CH3-, but a very different trans effect (based mainly on equilibrium constants), viz. H2O < dbzm < L4 < CN- < SO32- ≈ HC≡C- < L5 ≈ CH2=CH- < CH3 -.

Original languageEnglish (US)
Pages (from-to)761-767
Number of pages7
JournalJournal of the Chemical Society, Dalton Transactions
Issue number4
StatePublished - Jan 1 1987
Externally publishedYes


ASJC Scopus subject areas

  • Chemistry(all)

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