The chemistry of vitamin B12. Part 28. Crystal structure of dicyanocobyrinic acid heptamethyl ester and its interaction with alcohols: the effects of hydrogen bonding to co-ordinated cyanide

Abstract

An X-ray structural analysis of the PrⁱOH adduct of dicyanocobyrinic acid heptamethyl ester [monoclinic, space group P2₁, a= 16.332(6), b=13.252(5), c= 14.475(5)Å, β= 109.75(3)°, and Z= 2] has shown that one cyanide is H-bonded to the PrⁱOH while the other interacts with two methyl groups of ester side-chains on neighbouring molecules. A study of the solvent dependence of the absorption spectrum of the dicyanide has shown that the wavelength of the main (γ) band varied from ca. 374 nm in non-polar solvents (e.g. benzene, CCl₄) through ca. 371 nm in more polar solvents (MeCN, MeCO₂Me), to ca. 368 nm in protic solvents [ H₂O, ROH (R = alkyl)], accompanied by an increase in the CN stretching frequency, while the titration of a solution of the dicyanide in CCl₄ with MeOH showed the binding of one molecule of MeOH per Co with K₁= 7 dm³ mol^–1 and a further equilibrium with K₂⩽ 0.05 dm³ mol^–1. The results show that H-bonding to co-ordinated cyanide reduces the donor power of the ligand towards the metal, with effects on both the cis and trans positions, and also suggests that interaction with a methyl carrying a slight positive charge (as in MeCN and MeCO₂Me) is qualitatively similar to, though weaker than, an H-bond.