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

• Published in: Inorganic chemistry Vol. 36; no. 17; pp. 3666 - 3675
• Main Authors: Brown, Kenneth L, Cheng, Shifa, Zou, Xiang, Zubkowski, Jeffrey D, Valente, Edward J, Knapton, Leanne, Marques, Helder M
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 13.08.1997
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic9615077

The crystal structures of 10-chloroaquacobalamin perchlorate hydrate (10-Cl-H2OCbl·ClO4) (Mo Kα, 0.710 73 Å, monoclinic system, P21, a = 11.922(4) Å, b = 26.592(10) Å, c = 13.511(5) Å, β = 93.05(3)°, 10 535 independent reflections, R 1 = 0.0426), 10-chlorocyanocobalamin−acetone hydrate (10-Cl-CNCbl) (Mo Kα, 0.710 73 Å, orthorhombic system, P212121, 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 Kα, 0.71073 Å, orthorhombic system, P212121, 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. 13C 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-H2OCbl+ is very similar to that found in H2OCbl+ 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-H2OCbl+ is hydrogen bonded to the c side chain carbonyl oxygen, as in H2OCbl+. NMR observations indicate that the H bond between coordinated H2O 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 × 103 vs 3.35 × 105), but could not be determined for 10-Cl-H2OCbl 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 π-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 π-donation to the corrin by Cl at C10. As the donor power of X increases (H2O < 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 13C 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 = H2O 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 H2O in 10-Cl-H2OCbl+ compared to H2OCbl+ 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⋮N bond length as Co−C increases compared to CNCbl suggests that dπ−pπ bonding between cobalt and cyanide is important. 13C and 15N NMR observations on 10-Cl-13C15NCbl are consistent with these effects.