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

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 Co^III 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^- < SO₃^2- ≈ L⁴ ≈ L⁵ < HC≡C^- < CH₂CH^- < CH₃^-, but a very different trans effect (based mainly on equilibrium constants), viz. H₂O < dbzm < L⁴ < CN^- < SO₃^2- ≈ HC≡C^- < L⁵ ≈ CH₂=CH^- < CH₃ ^-.