Kinetics of substitution reactions of alpha, beta, gamma, delta-tetra-(4-N-methylpyridyl) porphinediaquocobalt (III)

The inorganic chemistry of the cobalt corrinoids - an update

The inorganic chemistry of the cobalt corrinoids, derivatives of vitamin B₁₂, is reviewed, with particular emphasis on equilibrium constants for, and kinetics of, their axial ligand substitution reactions. The role the corrin ligand plays in controlling and modifying the properties of the metal ion is emphasised. Other aspects of the chemistry of these compounds, including their structure, corrinoid complexes with metals other than cobalt, the redox chemistry of the cobalt corrinoids and their chemical redox reactions, and their photochemistry are discussed. Their role as catalysts in non-biological reactions and aspects of their organometallic chemistry are briefly mentioned. Particular mention is made of the role that computational methods - and especially DFT calculations - have played in developing our understanding of the inorganic chemistry of these compounds. A brief overview of the biological chemistry of the B₁₂-dependent enzymes is also given for the reader's convenience.

Cobalt(III)porphyrin to target G-quadruplex DNA

G-quadruplex DNA ligands attract much attention because of their potential use in biology. Indeed they may interfere with G-quadrulex nucleic acid function in cells. Most of the G-quadruplex ligands so far reported (including also metal complexes) are large planar aromatic compounds that interact by π-π stacking with an external G-quartet of quadruplex. Porphyrins are well-known G-quadruplex ligands. We report herein a new porphyrin scaffold (meso-tetrakis(4-(N-methyl-pyridinium-2-yl)phenyl)porphyrin) able to strongly and selectively bind to G-quadruplex DNA. We show that even when this porphyrin is metallated with cobalt(III), i.e. it carries two water molecules as axial ligands on the cobalt ion, on each face of the porphyrin, the interaction occurs by a π-stacking-like mode with an external G-quartet of quadruplex DNA.

Mechanistic Analysis of Reductive Nitrosylation on Water-Soluble Cobalt(III)-Porphyrins

The reactions of NO and/or NO2- with three water-soluble cobalt porphyrins [Co(III)(P)(H2O)2]n, where P = TPPS, TCPP, and TMPyP, were studied in detail. At pH < 3, the reaction with NO proceeds through a single reaction step. From the kinetic data and activation parameters, the [Co(III)(P)(NO)(H2O)]n complex is proposed to be the primary product of the reaction with NO. This complex reacts further with a second NO molecule through an inner-sphere electron-transfer reaction to generate the final product, [Co(III)(P)(NO-)](n-1). At pH > 3, although a single reaction step is also observed, a systematic study as a function of the NO and NO2- concentrations revealed that two reaction steps are operative. In the first, NO2- and NO compete to substitute coordinated water in [Co(III)(P)(H2O)2]n to yield [Co(III)(P)(NO)(H2O)]n and [Co(III)(P)(NO2-)(H2O)](n-1) as the primary reaction products. Only the nitrite complex could be detected and no final product formation was observed during the reaction. It is proposed that [Co(III)(P)(NO)(H2O)]n rapidly reacts with NO2- to form the nitrite complex, which in the second reaction step reacts with another NO molecule to generate the final product through an inner-sphere electron-transfer reaction. The reported results are relevant for the interaction of vitamin B(12a) with NO and NO2-.

Probing the nature of the Co(III) ion in cobalamins: a comparison of the reaction of aquacobalamin (vitamin B12a) and aqua-10-chlorocobalamin with some anionic and N-donor ligands

To probe the cis effect of the corrin macrocycle in vitamin B12 derivatives, equilibrium constants for the substitution of coordinated H2O in aquacobalamin (vitamin B12a, H2OCbl+) and in aqua-10-chlorocobalamin, H2O-10-ClCbl+, (in which Cl has replaced the C10 H) by an exogenous ligand, L (L = an anion, NO2-, SCN-, N3-, OCN-, S2O3(2-), NCSe- or a neutral N-donor, CH3NH2, pyridine, imidazole) have been determined. The cis influence reported in the electronic spectra of the cobalamins is observed in the spectra of L-10-ClCbls as well. Anionic ligands bind more strongly to H2O-10-ClCbl+ than to H2OCbl+ with log K values between 0.10 and 0.63 (average 0.26) larger; the converse is true for the neutral N-donor ligands, where log K is between 0.17 and 0.3 (average 0.25) smaller. Semi-empirical molecular orbital (SEMO) calculations using the ZINDO/1 model on the hydroxo complexes show that charge density is delocalised from the axial donor atom to the metal and Cl. This explains why coordinated OH- is a poorer base in HO-10-ClCbl than in HOCbl and the pK(a) of H2O-10-ClCbl+ is lower than that of H2OCbl+. It further explains why, because of the ability of the metal in concert with the C10 Cl to accept charge density from the ligand, an anionic ligand will bind more strongly to Co(III) in H2O-10-ClCbl+ than in H2OCbl+. The kinetics of the replacement of coordinated H2O by two probe ligand, pyridine and azide, were determined. The rate constants for substitution of H2O in H2O-10-ClCbl+ by pyridine show saturation, whilst those for substitution by N3- do not; this is inconsistent with a purely dissociative mechanism and the reactions proceed through an interchange mechanism. The values of the activation parameters are more positive for the reaction between these ligands and H2OCbl+, than for their reaction with H2O-10-ClCbl+. This is interpreted to mean that the transition state in the reaction of H2O-10-ClCbl+ occurs earlier along the reaction coordinate. In the temperature range studied, H2O-10-ClCbl+ reacts more slowly with py and N3- than does H2OCbl+. SEMO calculations indicate that as the Co-O bond to the departing H(2)O molecule is stretched, the charge density on Co in H2OCbl+ is always lower than on Co in H2O-10-ClCbl+. This suggests that the former is a better electrophile towards the incoming ligand, and offers an explanation for the kinetics observations.

Ligand substitution behavior of a simple model for coenzyme B12

The ligand substitution reactions of trans-[CoIII(en)2(Me)H2O]2+, a simple model for coenzyme B12, were studied for cyanide and imidazole as entering nucleophiles. It was found that these nucleophiles displace the coordinated water molecule trans to the methyl group and form the six-coordinate complex trans-[Co(en)2(Me)L]. The complex-formation constants for cyanide and imidazole were found to be (8.3 +/- 0.7) x 10(4) and 24.5 +/- 2.2 M-1 at 10 and 12 degrees C, respectively. The second-order rate constants for the substitution of water were found to be (3.3 +/- 0.1) x 10(3) and 198 +/- 13 M-1 s-1 at 25 degrees C for cyanide and imidazole, respectively. From temperature and pressure dependence studies, the activation parameters delta H++, delta S++, and delta V++ for the reaction of trans-[CoIII(en)2(Me)H2O]2+ with cyanide were found to be 50 +/- 4 kJ mol-1, 0 +/- 16 J K-1 mol-1, and +7.0 +/- 0.6 cm3 mol-1, respectively, compared to 53 +/- 2 kJ mol-1, -22 +/- 7 J K-1 mol-1, and +4.7 +/- 0.1 cm3 mol-1 for the reaction with imidazole. On the basis of reported activation volumes, these reactions follow a dissociative mechanism in which the entering nucleophile could be weakly bound in the transition state.

Cis Effects in the Cobalt Corrins. 1. Crystal Structures of 10-Chloroaquacobalamin Perchlorate, 10-Chlorocyanocobalamin, and 10-Chloromethylcobalamin

The crystal structures of 10-chloroaquacobalamin perchlorate hydrate (10-Cl-H(2)OCbl.ClO(4)) (Mo Kalpha, 0.710 73 Å, monoclinic system, P2(1), a = 11.922(4) Å, b = 26.592(10) Å, c = 13.511(5) Å, beta = 93.05(3) degrees, 10 535 independent reflections, R(1) = 0.0426), 10-chlorocyanocobalamin-acetone hydrate (10-Cl-CNCbl) (Mo Kalpha, 0.710 73 Å, orthorhombic system, P2(1)2(1)2(1), a = 16.24(3) Å, b = 21.85(5) Å, c = 26.75(8) Å, 7699 independent reflections, R(1) = 0.0698), and 10-chloromethylcobalamin-acetone hydrate (10-Cl-MeCbl) (Mo Kalpha, 0.71073 Å, orthorhombic system, P2(1)2(1)2(1), a = 16.041(14) Å, b = 22.13(2) Å, c = 26.75(4) Å, 6792 independent reflections, R(1) = 0.0554), in which the C10 meso H is substituted by Cl, are reported. An unusual feature of the structures is disorder in the C ring, consistent with a two-site occupancy in which the major conformation has the C46 methyl group in the usual position, "upwardly" axial, and the C47 methyl group equatorial, while in the minor conformation both are pseudoequatorial, above and below the corrin ring. (13)C NMR chemical shifts of C46, C47, C12, and C13 suggest that the C ring disorder may persist in solution as a ring flip. Since molecular dynamics simulations fail to reveal any population of the minor conformation, the effect is likely to be electronic rather than steric. The axial bond lengths in 10-Cl-MeCbl are very similar to those in MeCbl (d(Co)(-)(C) = 1.979(7) vs 1.99(2); to 5,6-dimethylbenzimidazole, d(Co)(-)(NB3) = 2.200(7) vs 2.19(2)), but the bonds to the four equatorial N donors, d(Co)(-)(N(eq)), are on average 0.05 Å shorter. In 10-Cl-CNCbl, d(Co)(-)(C) and d(Co)(-)(NB3) are longer (by 0.10(2) and 0.03(1) Å, respectively) than the bond lengths observed in CNCbl itself, while conversely, the C-N bond length is shorter by 0.06(2) Å, but there is little difference in d(Co)(-)(N(eq)). The Co-O bond length to coordinated water in 10-Cl-H(2)OCbl(+) is very similar to that found in H(2)OCbl(+) itself, but the d(Co)(-)(NB3) bond is longer (1.967 vs1.925(2) Å), while the average d(Co)(-)(N(eq)) is very similar. The coordinated water molecule in 10-Cl-H(2)OCbl(+) is hydrogen bonded to the c side chain carbonyl oxygen, as in H(2)OCbl(+). NMR observations indicate that the H bond between coordinated H(2)O and the c side chain amide persists in solution. The equilibrium constant, K(Co), for coordination of bzm to Co(III) is smaller in 10-Cl-MeCbl and 10-Cl-CNCbl than in their C10-unsubstituted analogs (181 vs 452; 4.57 x 10(3) vs 3.35 x 10(5)), but could not be determined for 10-Cl-H(2)OCbl because hydrolysis of the phosphodiester is competitive with the establishment of the base-off equilibrium. Substitution of H by Cl at C10 causes the bands in the electronic spectrum of 10-Cl-XCbl complexes to move to lower energy, which is consistent with an increase in electron density in the corrin pi-conjugated system. This increased electron density is not due to greater electron donation from the axial ligand as bonds between these and the metal are either longer (not shorter) or unchanged, and it most probably arises from pi-donation to the corrin by Cl at C10. As the donor power of X increases (H(2)O < CN(-) < Me), the corrin ring becomes more flexible to deformation, and the number of bond lengths and bond angles that are significantly different in XCbl and 10-Cl-XCbl increases; importantly, the C10-Cl bond length, d(C10)(-)(Cl), increases as well. Thus, despite the fact that chlorine is an inductively electron withdrawing substituent, its resonance electron donation is the more important effect on electron distribution in the corrin ring. Mulliken charges obtained from semiempirical RHF-SCF MO calculations using the ZINDO/1 model on XCbl and their 10-Cl analogs at the crystal structure geometry are shown to correlate reasonably well with (13)C NMR shifts and may be used to determine the pattern of electron distribution in these complexes. Substitution by Cl at C10 causes an increase in charge density at Co when X = H(2)O and CN(-), while the charge density on the four equatorial N donors remains virtually unchanged, but a decrease when X = Me, while the charge density on the equatorial N donors also decreases. In response, d(Co)(-)(NB3) increases in the first two complexes but the equatorial bond lengths remain virtually unchanged, while d(Co)(-)(NB3) remains unchanged and the average d(Co)(-)(N(eq)) decreases in 10-Cl-MeCbl. Furthermore, the partial charge on chlorine increases as the donor power of X increases. The small decrease in the pK(a) of coordinated H(2)O in 10-Cl-H(2)OCbl(+) compared to H(2)OCbl(+) itself (7.65 vs 8.09) is due to a decreased charge density on oxygen in 10-Cl-OHCbl compared to OHCbl. The picture that emerges, therefore, is of competitive electron donation by X and Cl toward the corrin system. In 10-Cl-CNCbl, the decrease in the C&tbd1;N bond length as Co-C increases compared to CNCbl suggests that dpi-ppi bonding between cobalt and cyanide is important. (13)C and (15)N NMR observations on 10-Cl-(13)C(15)NCbl are consistent with these effects.

Kinetics of the substitution reaction of thiocyanate with tetrakis (3-N-methylphyridyl)porphinecobalt(III)

The equilibrium constant for the first thiocyanate addition to Co(III)-tetra(3-N-methylpyridyl)porphine is a factor of two smaller than for the less basic 4-N-methylporphine isomer. This effect is primarily due to a larger Co-SCN dissociation rate constant for the 3-isomer.