The CIS influence of the corrin ring in cobalt corrins
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. December 2015. === It is well-established that there is electronic communication between the equatorial and axial ligands in the cob...
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| Format: | Others |
| Language: | en |
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2017
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| Online Access: | Ghadimi, Nafise (2016) The CIS influence of the corrin ring in cobalt corrins, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/21738> http://hdl.handle.net/10539/21738 |
| Summary: | A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. December 2015. === It is well-established that there is electronic communication between the equatorial and axial
ligands in the cobalt corrins. It can therefore be anticipated that the electronic structure of the corrin
ligand will affect the chemistry of the axial coordination sites of Co(III) in these complexes. To
probe this cis-influence the electronic structure of the corrin was perturbed by substituting the H
atom at C10 by Br (which is π electron-donating towards the corrin) in aquacobalamin
([H2OCbl]+), and by NO2 (which is strongly electron-withdrawing) and NH2 (which is strongly
electron-donating) in aquacyanocobester ([ACCbs]+). The first part of this study was dedicated to
aqua-10-bromocobalamin ([H2O-(10-Br)Cbl]+) and the second part to aquacyano-10-nitrocobester
([AC-(10-NO2)Cbs]+) and aquacyano-10-aminocobester ([AC-(10-NH2)Cbs]+).
The successful synthesis of [H2O-(10-Br)Cbl]+, was verified by ESI-MS, 1H and 13C NMR, uv-vis
spectroscopy and XRD.
The stability constants for the substitution of coordinated H2O by a series of anionic (N3
–, NO2
–,
SCN–, SO3
2–) and neutral N-donor ligands (imidazole, DMAP) were obtained for [H2OCbl]+,
[H2O-(10-Br)Cbl]+ and [H2O-(10-Cl)Cbl]+ under the same conditions. Substitution of the C10 H
by Cl or Br favours the coordination of anionic ligands, but discriminates against the binding of
neutral N-donor ligands. The anionic ligands bind more strongly to [H2O-(10-Br)Cbl]+ than to
[H2OCbl]+ with log K values between 0.05 and 0.62 (average 0.33) larger. Conversely, neutral
ligands bind less strongly to [H2O-(10-Br)Cbl]+ than to [H2OCbl]+ with log K values between 0.29
and 0.36 (average 0.33) smaller. DFT (BP83/TZVP) calculations were used to rationalise these
observations. When H is changed to Cl or Br, the metal ion becomes less positive. When the β
ligand changes from a neutral to an anionic ligand, the partial charge on the C10 substituent
becomes more negative. Replacing C10 H by Cl or Br discriminates against a neutral ligand
because of the greater electron richness of the metal. If the ligand is an anion, however, the charge
donation can be accepted by delocalisation onto the C10 substituent.
The reaction kinetics of the substitution of H2O in [H2O-(10-Br)Cbl]+ were determined for the
ligands N3
– and imidazole and were compared with values available for [H2OCbl]+ and [H2O-(10-
Cl)Cbl]+. The results showed that both N3
– and imidazole react more slowly with [H2O-(10-
Br)Cbl]+ than with [H2OCbl]+, consonant with the previous observations for [H2O-(10-Cl)Cbl]+.
Although ΔH‡ values are smaller, they do not compensate for significantly more negative values
of ΔS‡, indicative of a transition state that occurs earlier along the reaction coordinate in [H2O-
(10-Br)Cbl]+ and [H2O-(10-Cl)Cbl]+ whereas the transition state occurs later along the reaction
coordinate with [H2OCbl]+. It is argued that this is a consequence of the lower charge density on
the metal, making it a better electrophile both towards the incoming and the departing ligand.
Dicyano-10-nitrocobester ([DC-(10-NO2)Cbs]) and dicyano-10-aminocobester ([DC-(10-
NH2)Cbs]) were synthesised from dicyanocobester [DCCbs] by established methods and
converted to the aquacyano form so that the thermodynamics and kinetics of the substitution of
coordinated H2O by a variety of ligands could be investigated.
The stability constants for the substitution of coordinated H2O by a number of neutral (imidazole,
DMAP, methylamine) and anionic (N3
–, NO2
–, SCN–, SO3
2–, CN–) ligands were determined for
[ACCbs]+, [AC-(10-NO2)Cbs]+ and [AC-(10-NH2)Cbs]+ in 50% isopropanol. The soft anions
(SO3
2– and CN–) bind better to the softer Co(III) metal centre in [AC-(10-NH2)Cbs]+ and [ACCbs]+
than in [AC-(10-NO2)Cbs]+ and the converse is true for the hard anions (N3
–, NO2
– and SCN–).
The case is less clear for the N-donor ligands; DMAP clearly has a higher affinity for [AC-(10-
NH2)Cbs]+ and [ACCbs]+ than for [AC-(10-NO2)Cbs]+, but there is little discrimination in the case
of imidazole and methylamine.
This implies that the affinity of the metal for an exogenous ligand depends on the electron density
at the metal centre. DFT calculations showed that as the C10 substituent is changed from NH2 to
H to NO2, the charge density on the metal centre decreases and the metal becomes harder.
The kinetics of the substitution of H2O by CN– in [ACCbs]+, [AC-(10-NO2)Cbs]+ and[AC-(10-
NH2)Cbs]+ in 50% isopropanol were determined. The results showed that the substitution of
coordinated H2O proceeded with biphasic kinetics and through a dissociative interchange (Id)
mechanism where there is nucleophilic participation of the entering ligand in the transition state.
The slower phase corresponds to the substitution of coordinated H2O trans to OH– in the aqua
hydroxo species, which, together with the dicyano species, is inevitably present in solutions of
[ACCbs]+, and the faster phase corresponds to the substitution of the coordinated H2O trans to
CN– in the aquacyano species. The difference in rate of the reaction of the [AC-(10-Z)Cbs] (Z =
H, NH2 and NO2) was not very large, the ratio between the largest (for Z = H) and the smallest
(for Z = NO2) is just over 40, and does not follow the electron donor properties of Z. This is
misleading, however, because of a compensation effect between ΔH‡ and ΔS‡. As values of ΔH‡
become smaller, which causes an increase in the reaction rate, ΔS‡ becomes less positive (or more
negative), which causes a decrease in the reaction rate. Hence, comparing rate constants at any
particular temperature is not very informative and the compensation effect masks the very
significant differences in the reactivity of the metal ion towards the entering CN– ligand. The
compensation effect is attributed to the position of the transition state along the reaction coordinate,
which depends on the charge density on the metal ion. Indeed, if all three reactions had the same
value of ΔS‡ then the values of the rate constant would be in the approximate ratio 109:106:1 for Z
= NH2, H and NO2, respectively.
This study shows that how profoundly the perturbation of the electronic structure of the corrin
affects the thermodynamic and kinetic properties of the Co(III) ion, and provides further evidence
that the unusual chemistry of Co(III) in the cobalt corrins is a consequence of the cis-influence of
the equatorial macrocyclic ligand. === LG2017 |
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