The chemistry of vitamin B12. Part X. cis- and trans-Effects

Abstract

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 cyanocomplexes, 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 21 32 cm.-l when X = benzimidazole to 2082 cm.-l when X = CH3CH2-. All the evidence relating to cis-and trans-effects in corrinoids which contain only the light ligand atoms C, N. and 0 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 H, O fall to values more typical of ion-pairs. It is concluded that the main factor is the amount of negative charge donated to the cobalt atom through the o-bond.WE have been studying the interactions of ligands in cobalt (111) corrinoids, which are complexes related to vitamin B,, (for a diagram of the structure see Part VI 2).These cis-and trans-effects have been divided into three categories:(1) ground-state effects, e g . , effects on bond lengths and angles, bending and stretching force constants; (2) thermodynamic effects, i.e., equilibria;(3) kinetic effects, eg., rates of ligand substitution. The ease of interpretation clearly falls in the order 1 > 2 > 3, and we have concentrated on obtaining data on the ground-state and thermodynamic effects.I n previous papers 1*3-5 we have studied the effect of changing one axial ligand X t on the equilibrium constants between ligands Y and 2 in the trans-position (thermodynamic trans-effect) and on the spectra of the equatorial corrin ring (cis-effect). In this paper we t The axial ligands are designated as follows. X is the ligand whose effect we are studying. For the ground-state effect Y is the ' probe,' e.g., CN-when we study the dependence of the CN stretching frequency on the nature of X. For the thermodynamic effects Y and 2 are the ligands involved in the equilibrium; Y is the common denominator, if any, e.g., H,O in the aquocobinamides and aquocobalamin, Bz in the cobalamins.The following abbreviations are used: Bz = the chelating heterocyclic base 5,6-dimethylbenzimidazole, present in the nucleoside side-chain of the cobalamins ; DBC, 5'-deoxyadenosylcobalamin ; DMSO, dimethyl sulphoxide. report experiments on the effect of X on (1) the stretching frequency of cyanide co-ordinated in the trans-position Y (ground-state trans-effect), and (2) the spectrum when Y = CN-, together with (3) a few additional stability constants for the substitution of one axial ligand Y by another 2. Taken together with previous formation constants and spectra when Y = H20 and Bz, this provides a large amount of data on the ground...

TiO2-assisted Photocatalytic Degradation of Vitamin B12 in Aqueous Solution:Influencing Factors, Kinetics, and Reaction Investigations

“…It is reported that band A is attributable to the C-N stretching of vitamin B 12 with respect to the trans axial ligand. 37 The results suggest that the C-N bond in the vitamin B 12 molecule was cleaved within 80 min. Band B was shifted from 1666 to 1638 cm -1 , which is a suitable marker for monitoring the reactions of the side chains of vitamin B 12 during photodegradation.…”

Synthesis and Properties of Formylcobalamin and Propionylcobalamin, Novel Acylcobalamins

“…3, a six-coordinate aqua complex may also be involved in a formal sense as an intermediate in the equilibrium of DBI displacement upon acidification of cobalamins, but Firth et al reported that the proportion of a six-coordinate aqua complex in neutral aqueous solution is negligible with AdoCbl and ethylCbl (15). They demonstrated as well with organo-Cbls that a strong -donor ligand increases an equilibrium constant (K)-that is, it favors the formation of a fi vecoordinate complex (14). Therefore, the above-mentioned order of apparent pKa values of acyl-Cbls seems to be reasonable by assuming the equilibrium shown in Eq.…”

“…The net charges of alkyl-and acyl-Cbls refl ect their baseon → ← base-off equilibria (Eq. 3) at this pH, suggesting that the so-called trans-effect (14) or the -donor strength of an upper axial ligand increases in the order: formylϽacetylϽpropionyl. Although the methyl group is a stronger -donor than acyl groups, methyl-Cbl showed much smaller mobility (Table 2).…”

The chemistry of vitamin B12. Part 21. Ethynylaquocobinamide: novel reaction of diaquocobinamide with acetylene catalysed by copper ions

“…(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(4.2 x lod2) 4.5 x dm3 mol-'; i.e. logl& 2 22.7 (3)…”