Cis Effects in the Cobalt Corrins. 1. Crystal Structures of 10-Chloroaquacobalamin Perchlorate, 10-Chlorocyanocobalamin, and 10-Chloromethylcobalamin
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- 作者：Kenneth L. Brown ; Shifa Cheng ; Xiang Zou ; Jeffrey D. Zubkowski ; Edward J. Valente ; Leanne Knapton ; and Helder M. Marques
- 刊名：Inorganic Chemistry
- 出版年：1997
- 出版时间：August 13, 1997
- 年：1997
- 卷：36
- 期：17
- 页码：3666 - 3675
- 全文大小：260K
- 年卷期：v.36,no.17(August 13, 1997)
- ISSN：1520-510X
文摘
The crystal structures of10-chloroaquacobalamin perchlorate hydrate(10-Cl-H2OCbl·ClO4) (Mo K
,0.710 73 Å,monoclinic system, P21, a =11.922(4) Å, b = 26.592(10) Å, c= 13.511(5) Å, 
ddle"> = 93.05(3)
deg.gif">, 10 535independentreflections, R1 = 0.0426),10-chlorocyanocobalamin-acetone hydrate (10-Cl-CNCbl) (Mo K,0.710 73 Å, orthorhombicsystem, P212121,a = 16.24(3) Å, b = 21.85(5) Å,c = 26.75(8) Å, 7699 independent reflections,R1 = 0.0698), and10-chloromethylcobalamin-acetone hydrate (10-Cl-MeCbl) (Mo K,0.71073 Å, orthorhombic system,P212121, a=16.041(14) Å, b = 22.13(2) Å, c =26.75(4) Å, 6792 independent reflections,R1 = 0.0554), in which the C10 mesoHis substituted by Cl, are reported. An unusual feature of thestructures is disorder in the C ring, consistent with atwo-site occupancy in which the major conformation has the C46 methylgroup in the usual position, "upwardly" axial,and the C47 methyl group equatorial, while in the minorconformation both are pseudoequatorial, above and belowthecorrin ring. 13C NMR chemical shifts of C46, C47, C12,and C13 suggest that the C ring disorder may persist insolution as a ring flip. Since molecular dynamics simulations failto reveal any population of the minor conformation,the effect is likely to be electronic rather than steric. Theaxial bond lengths in 10-Cl-MeCbl are very similar tothosein MeCbl (dCo-C =1.979(7) vs 1.99(2); to 5,6-dimethylbenzimidazole,dCo-NB3 =2.200(7) vs 2.19(2)), but the bondsto the four equatorial N donors,dCo-N(eq), are onaverage 0.05 Å shorter. In 10-Cl-CNCbl,dCo-C anddCo-NB3 arelonger(by 0.10(2) and 0.03(1) Å, respectively) than the bondlengths observed in CNCbl itself, while conversely, theC-Nbond length is shorter by 0.06(2) Å, but there is littledifference indCo-N(eq). TheCo-O bond length to coordinatedwater in 10-Cl-H2OCbl+ is very similarto that found in H2OCbl+ itself, but thedCo-NB3 bond is longer(1.967 vs1.925(2) Å), while the averagedCo-N(eq) is verysimilar. The coordinated water molecule in10-Cl-H2OCbl+ is hydrogenbonded to the c side chain carbonyl oxygen, as inH2OCbl+. NMR observations indicatethat the H bond betweencoordinated H2O and the c side chain amidepersists in solution. The equilibrium constant,KCo, for coordination ofbzm to Co(III) is smaller in 10-Cl-MeCbl and 10-Cl-CNCbl than intheir C10-unsubstituted analogs (181 vs 452;4.57× 103 vs 3.35 × 105), but couldnot be determined for 10-Cl-H2OCbl because hydrolysisof the phosphodiester iscompetitive with the establishment of the base-off equilibrium.Substitution of H by Cl at C10 causes the bands intheelectronic spectrum of 10-Cl-XCbl complexes to move to lower energy,which is consistent with an increase in electrondensity in the corrin 
-conjugated system. This increasedelectron density is not due to greater electron donationfromthe axial ligand as bonds between these and the metal are either longer(not shorter) or unchanged, and it most probablyarises from -donation to the corrin by Cl at C10. As the donorpower of X increases (H2O < CN- < Me), thecorrinring becomes more flexible to deformation, and the number of bondlengths and bond angles that are significantlydifferent in XCbl and 10-Cl-XCbl increases; importantly, the C10-Clbond length, dC10-Cl,increases as well. Thus,despite the fact that chlorine is an inductively electron withdrawingsubstituent, its resonance electron donation is themore 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-Clanalogs at the crystal structure geometry areshown to correlate reasonably well with 13C NMR shifts andmay be used to determine the pattern of electrondistributionin these complexes. Substitution by Cl at C10 causes an increasein charge density at Co when X = H2O andCN-,while the charge density on the four equatorial N donors remainsvirtually unchanged, but a decrease when X = Me,while the charge density on the equatorial N donors also decreases.In response,dCo-NB3 increases inthe first twocomplexes but the equatorial bond lengths remain virtually unchanged,while dCo-NB3 remainsunchanged and the averagedCo-N(eq) decreases in10-Cl-MeCbl. Furthermore, the partial charge on chlorine increasesas the donor power of Xincreases. The small decrease in the pKa ofcoordinated H2O in10-Cl-H2OCbl+ compared toH2OCbl+ itself (7.65vs8.09) is due to a decreased charge density on oxygen in 10-Cl-OHCblcompared to OHCbl. The picture that emerges,therefore, is of competitive electron donation by X and Cl toward thecorrin system. In 10-Cl-CNCbl, the decrease inthe C
d1.gif">N bond length as Co-C increases compared to CNCbl suggeststhat d-p bonding between cobalt and cyanideis important. 13C and 15N NMR observationson 10-Cl-13C15NCbl are consistent withthese effects.
,0.710 73 Å,monoclinic system, P21, a =11.922(4) Å, b = 26.592(10) Å, c= 13.511(5) Å, ddle"> = 93.05(3)deg.gif">, 10 535independentreflections, R1 = 0.0426),10-chlorocyanocobalamin-acetone hydrate (10-Cl-CNCbl) (Mo K,0.710 73 Å, orthorhombicsystem, P212121,a = 16.24(3) Å, b = 21.85(5) Å,c = 26.75(8) Å, 7699 independent reflections,R1 = 0.0698), and10-chloromethylcobalamin-acetone hydrate (10-Cl-MeCbl) (Mo K,0.71073 Å, orthorhombic system,P212121, a=16.041(14) Å, b = 22.13(2) Å, c =26.75(4) Å, 6792 independent reflections,R1 = 0.0554), in which the C10 mesoHis substituted by Cl, are reported. An unusual feature of thestructures is disorder in the C ring, consistent with atwo-site occupancy in which the major conformation has the C46 methylgroup in the usual position, "upwardly" axial,and the C47 methyl group equatorial, while in the minorconformation both are pseudoequatorial, above and belowthecorrin ring. 13C NMR chemical shifts of C46, C47, C12,and C13 suggest that the C ring disorder may persist insolution as a ring flip. Since molecular dynamics simulations failto reveal any population of the minor conformation,the effect is likely to be electronic rather than steric. Theaxial bond lengths in 10-Cl-MeCbl are very similar tothosein MeCbl (dCo-C =1.979(7) vs 1.99(2); to 5,6-dimethylbenzimidazole,dCo-NB3 =2.200(7) vs 2.19(2)), but the bondsto the four equatorial N donors,dCo-N(eq), are onaverage 0.05 Å shorter. In 10-Cl-CNCbl,dCo-C anddCo-NB3 arelonger(by 0.10(2) and 0.03(1) Å, respectively) than the bondlengths observed in CNCbl itself, while conversely, theC-Nbond length is shorter by 0.06(2) Å, but there is littledifference indCo-N(eq). TheCo-O bond length to coordinatedwater in 10-Cl-H2OCbl+ is very similarto that found in H2OCbl+ itself, but thedCo-NB3 bond is longer(1.967 vs1.925(2) Å), while the averagedCo-N(eq) is verysimilar. The coordinated water molecule in10-Cl-H2OCbl+ is hydrogenbonded to the c side chain carbonyl oxygen, as inH2OCbl+. NMR observations indicatethat the H bond betweencoordinated H2O and the c side chain amidepersists in solution. The equilibrium constant,KCo, for coordination ofbzm to Co(III) is smaller in 10-Cl-MeCbl and 10-Cl-CNCbl than intheir C10-unsubstituted analogs (181 vs 452;4.57× 103 vs 3.35 × 105), but couldnot be determined for 10-Cl-H2OCbl because hydrolysisof the phosphodiester iscompetitive with the establishment of the base-off equilibrium.Substitution of H by Cl at C10 causes the bands intheelectronic spectrum of 10-Cl-XCbl complexes to move to lower energy,which is consistent with an increase in electrondensity in the corrin -conjugated system. This increasedelectron density is not due to greater electron donationfromthe axial ligand as bonds between these and the metal are either longer(not shorter) or unchanged, and it most probablyarises from -donation to the corrin by Cl at C10. As the donorpower of X increases (H2O < CN- < Me), thecorrinring becomes more flexible to deformation, and the number of bondlengths and bond angles that are significantlydifferent in XCbl and 10-Cl-XCbl increases; importantly, the C10-Clbond length, dC10-Cl,increases as well. Thus,despite the fact that chlorine is an inductively electron withdrawingsubstituent, its resonance electron donation is themore 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-Clanalogs at the crystal structure geometry areshown to correlate reasonably well with 13C NMR shifts andmay be used to determine the pattern of electrondistributionin these complexes. Substitution by Cl at C10 causes an increasein charge density at Co when X = H2O andCN-,while the charge density on the four equatorial N donors remainsvirtually unchanged, but a decrease when X = Me,while the charge density on the equatorial N donors also decreases.In response,dCo-NB3 increases inthe first twocomplexes but the equatorial bond lengths remain virtually unchanged,while dCo-NB3 remainsunchanged and the averagedCo-N(eq) decreases in10-Cl-MeCbl. Furthermore, the partial charge on chlorine increasesas the donor power of Xincreases. The small decrease in the pKa ofcoordinated H2O in10-Cl-H2OCbl+ compared toH2OCbl+ itself (7.65vs8.09) is due to a decreased charge density on oxygen in 10-Cl-OHCblcompared to OHCbl. The picture that emerges,therefore, is of competitive electron donation by X and Cl toward thecorrin system. In 10-Cl-CNCbl, the decrease inthe Cd1.gif">N bond length as Co-C increases compared to CNCbl suggeststhat d-p bonding between cobalt and cyanideis important. 13C and 15N NMR observationson 10-Cl-13C15NCbl are consistent withthese effects.