Novel Chemistry of Carbon Bound to Cobalt in Organocobalt Complexes Related to B12

Synthesis and crystal structures of a new series of intramolecular bridged alkylcobaloxime complexes

A new series of intramolecular bridged alkylcobaloxime complexes were prepared by heating a solution of aqua-(3-bromopropyl)cobaloxime (1) through an intramolecular nucleophilic substitution. They are characterized by EA, ESI-MS, and (1)H NMR. Crystal structures of Br(O-C(3)H(6)-(dmgH))(dmgH)Co(III) (2), [H(2)O(O-C(3)H(6)-(dmg))(dmgH(2))Co(III)]ClO(4) (3), [Py(O-C(3)H(6)-(dmg))(dmgH(2))Co(III)]ClO(4) (4), [Bzm(O-C(3)H(6)-(dmg))(dmgH(2))Co(III)]ClO(4) (5), and Im(O-C(3)H(6)-(dmg))(dmgH)Co(III) (6') were determined by X-ray diffraction. It is revealed that an intramolecular tri-methylene bridge exists between cobalt and one of the equatorial oxime oxygens forming a six-membered ring in the complexes. Additionally, the axial bases have some influences in their structure parameters, for example, Co-L bond lengths; folding angles in equatorial ligands; hydrogen atom positions in O-H-O bridge; and the orientations of those L planes with respect to the equatorial planes.

New β Cis Folded Organocobalt Derivatives with a Salen-Type Ligand

The reduction of [Co(III)(tmsalen)py(2)](+)ClO(4)(-), where tmsalen = 4,4',7,7'-tetramethylsalen, with NaBH(4)/PdCl(2) in alkaline methanolic solution, followed by the oxidative addition of CH(2)ClI, leads to the expected trans organometallic dimeric species 1, [CH(2)ClCo(tmsalen)](2), provided that the product is recovered from the reaction mixture immediately after the completion of the reaction. If 1 is left for longer time in contact with the reaction mixture, the intramolecular reaction of the axial chloromethyl group with the equatorial chelate leads to the formation of the monocationic complex 2, containing a seven-membered ring. In this complex the novel tetradentate ligand coordinates Co in a cis fashion, the other two positions being occupied by one py and one water molecule. The resulting complex is chiral, even if the reaction product is a racemic compound. The unidentate ligands of 2 have been exchanged quantitatively for N-MeIm, and the resulting complex 3 still maintains the beta cis geometry. Therefore, 2 may be considered the precursor of a new class of organocobalt derivatives with a folded tetradentate ligand and two adjacent exchangeable sites. On the basis of the geometry of the tetradentate Schiff bases in complexes, where they adopt a planar geometry, it was suggested that there is a significant electron density delocalization involving the metal center over the two chemically equivalent moieties of Co(chel). Comparison of the geometry of the planar salicylaldiminate (sal) moiety with that of the cyclized methoxy-iminate one (imi) in 2 and 3 strongly supports that the delocalization, still present in sal, is essentially either lost or strongly reduced in imi.

Dichloroacetylene is not the precursor to dichlorinated vinylcobaloxime and vinylcobalamin in cobalt catalyzed dechlorination of perchloro- and trichloroethylene

Vitamin B(12) catalyzes the reductive dechlorination of perchloroethylene (PCE), a process for which vinylcobalamins have been proposed as intermediates. Previous model studies have shown that PCE and trichloroethlylene (TCE) react with cob(I)aloxime to form cis-1,2-dichlorovinylcobaloxime (1). This compound could be formed by nucleophilic vinylic substitution of cob(I)aloxime on TCE or its syn-addition to dichloroacetylene. To evaluate the latter possibility, dichloroacetylene was reacted in this study with cob(I)aloxime. The major product was not complex 1 but a novel cobalt complex, indicating that dichloroacetylene is not involved in the reductive dechorination of PCE catalyzed by cob(I)aloxime. An X-ray structure of the major product was obtained showing an unexpected tricyclic structure in which one of the carbons of dichloroacetylene is a ligand to the metal and the second carbon has formed a C-C bond to one of the oxime carbons. This arrangement connects the axial and equatorial ligands. The cathodic peak potential of this complex is significantly more negative than that of previously characterized chlorinated vinylcobaloximes. Cob(I)alamin also reacts with chloroacetylene to provide cis-chlorovinylcobalamin in analogy to cob(I)aloxime, but it does not provide dichlorinated vinylcobalamins in the reaction with dichloroacetylene. Hence, dichlorinated vinylcobalt complexes detected in the reductive dechlorination of PCE catalyzed by cobaloximes or vitamin B(12) are not derived from a dichloroacetylene intermediate.

Protonation state of the equatorial ligands and dynamics of the OH...O units in a cobaloxime biomimetic

The protonation state of the dimethylglyoxime ligands in the B(12) coenzyme biomimetic [Co(CCl=CHCl)(dmgH)(2)(py)].CHCl(3) was investigated by using first-principles molecular dynamics. Our simulations at 173 and 300 K reveal that one of the oxime protons remains bonded to a nitroxyl group, while the proton of the second NO...H-ON unit is essentially shared with similar probability between the two oxygen atoms. This reconciles the results of the experimental determination (Jones, P. G.; Yang, L.; Steinborn, D. Acta Cryst. 1996, C52, 2399), showing all N-O distances as equivalent, with the commonly accepted rule that the protonation state of the dimethylglyoxime ligands can be identified by the different N-O distances. Further aspects of the dynamics of the OH...O units, in relation to the occurrence of weak CH...O intermolecular interactions, are analyzed.

New alkyl-cobalt(III) complexes containing chiral centers in the chelating system

The complex mer-[Co(III)(L(1)Npy)(2)](+) (1') where the L(1)Npy(-) is the tridentate 3-[(2-pyridyl)methylimino]butan-2-one oximate ligand, gives alkyl-cobalt derivatives after reduction with NaBH(4)/Pd(2+) to the Co(I) and alkylation. The formation of the cobalt-carbon bond is accompanied by the reduction to the amino form of one or both imino ligands (depending on the experimental conditions) initially present in 1'. In one series of experiments, complexes of the type fac-[RCo(III)(L(1)Npy)(H-L(1)NHpy)](+) (R = Me, i-Pr, CH(2)Cl, CH(2)Br, CH(2)CF(3), and Bz) were obtained, in which only one of the two ligands was reduced to the amino form (H-L(1)NHpy). The saturation of one azomethine group causes the products to assume a fac configuration and induces the formation of one asymmetric carbon and one asymmetric nitrogen center in the chelating system. When an excess of reducing agent is used, both azomethine groups may be saturated, causing the introduction of one pair of chiral carbons and one pair of chiral nitrogens. Two isomers of the methyl derivative [MeCo(III)(L(1)NHpy)(H-L(1)NHpy)](+) were isolated. The X-ray analysis reveals that these isomers differ from one another in configuration of the C and N chiral centers. Possible reaction mechanisms leading to these different types of complexes are proposed.

A new organocobalt complex containing a CO-N-C three membered ring

Treatment of R = -CH(2)X (X = halogen) derivatives of the type [RCo(III)(LNH-py)(HLNH-py)](+), where HLNH-py = 2-(2-pyridyl-ethyl)amino-3-butanone oxime and LNH-py its conjugated base with diluted NaOH, afforded a new complex containing a three-membered ring by a pathway involving the intramolecular nucleophilic addition of an equatorial nitrogen donor to the axial carbon. The X-ray analysis reveals a highly distorted structure. The C-Co-N angle is acute (42.8 degrees ) with the distortion of the coordination sphere concentrated in the Co-C axial and Co-N bonds, which move away from the pseudo-octahedral positions in the CH(2)X parent complex to form the C-N bond of the metallocycle. Kinetic studies of the formation of this novel complex starting from [(XCH(2)Co(III)(LNH-py)(HLNH-py)](+) (X = Cl,Br,I) showed that the metallocycle formation rates increase in the order Cl < Br < I. Kinetic data are consistent with a mechanism involving an intermediate species resulting from the deprotonation of an amine equatorial nitrogen in a rapid preequilibrium, followed by the slow step of the ring closure.

Structural and electronic properties of Co-corrole, Co-corrin, and Co-porphyrin

A quantitative study of the structure and electronic properties of Co-corrole, Co-corrin, and Co-porphyrin, using density functional theory, is reported. The structure of each macrocycle is optimized, with no symmetry constraints, by considering different spin states. The ground-state structures and spin states (S = 1 for Co-corrole, S = 0 for Co-corrin and S = 1/2 for Co-porphyrin) are in good agreement with the experimental data available. The trends in the sizes of the coordination cavities upon varying the inner metal atom and/or the macrocycle are analyzed and compared with those for the Fe-porphyrin we studied previously. Our results reveal that most of the distortion of the Co-corrin core in the B12 coenzyme is due to the inherent properties of Co-corrin. Quite different behaviors are found between corrinoids and porphyrins upon varying the spin state. While an increase in the metal-nitrogen (M-N) distance with spin state occurs in the porphyrins, the corrinoids show little variation in the M-N distance and, in some cases, undergo small structural changes in the ring structure. These results aid in understanding the often discussed question of why nature has chosen corrin/porphyrin for carrying out the particular biological functions identified in the B12 coenzyme.

The Co-CH(3) Bond in Imine/Oxime B(12) Models. Influence of the Orientation and Donor Properties of the trans Ligand As Assessed by FT-Raman Spectroscopy

Near-IR FT-Raman spectroscopy was used to probe the properties of three types of methyl imine/oxime B(12) model compounds in CHCl(3) solution. These types differ in the nature of the 1,3-propanediyl chain and were selected to test the influence of electronic and steric effects on the Co-CH(3) stretching (nu(Co)(-)(CH)()3) frequency, a parameter related to Co-C bond strength. For the first type studied, [LCo((DO)(DOH)pn)CH(3)](0/+) ((DO)(DOH)pn = N(2),N(2)(')-propane-1,3-diylbis(2,3-butanedione 2-imine 3-oxime)), nu(Co)(-)(CH)()3 decreased from 505 to 455 cm(-)(1) with stronger electron-donating character of the trans axial ligand, L, in the order Cl(-), MeImd, Me(3)Bzm, 4-Me(2)Npy, py, 3,5-Me(2)PhS(-), PMe(3), and CD(3)(-). This series thus allows the first assessment of the effect of negative axial ligands on nu(Co)(-)(CH)()3; these ligands (L = Cl(-), 3,5-Me(2)PhS(-), CD(3)(-)) span the range of trans influence. The CH(3) bending (delta(CH3)) bands were observed at 1171, 1159, and 1150/1105 cm(-)(1), respectively. The decrease in C-H stretching frequencies (nu(CH)) of the axial methyl suggests that the C-H bond strength decreases in the order Cl(-) > 3,5-Me(2)PhS(-) > CD(3)(-). This result is consistent with the order of decreasing (13)C-(1)H NMR coupling constants obtained for the axial methyl group. The trend of lower nu(Co)(-)(CH)()3 and nu(CH) frequencies and lower axial methyl C-H coupling constant for stronger electron-donating trans axial ligands can be explained by changes in the electronic character of the Co-C bond. The symmetric CH(3)-Co-CH(3) mode (nu(CH)()3(-)(Co)(-)(CH)()3) for (CH(3))(2)Co((DO)(DOH)pn) was determined to be 456 cm(-)(1) (421 cm(-)(1) for (CD(3))(2)Co((DO)(DOH)pn). The L-Co-CH(3) bending mode (delta(L)(-)(Co)(-)(CH)()3) was detected for the first time for organocobalt B(12) models; this mode, which is important for force field calculations, occurs at 194 cm(-)(1) for ClCo((DO)(DOH)pn)CH(3) and at 186 cm(-)(1) for (CH(3))(2)Co((DO)(DOH)pn. The nu(Co)(-)(CH)()3 frequencies were all lower than those reported for the corresponding cobaloxime type LCo(DH)(2)CH(3) (DH = monoanion of dimethylglyoxime) models for planar N-donor L. This relationship is attributed to a steric effect of L in [LCo((DO)(DOH)pn)CH(3)](+). The puckered 1,3-propanediyl chain in [LCo((DO)(DOH)pn)CH(3)](+) forces the planar L ligands to adopt a different orientation compared to that in the cobaloxime models. The consequent steric interaction bends the equatorial ligand toward the methyl group (butterfly bending); this distortion leads to a longer Co-C bond. In a second imine/oxime type, a pyridyl ligand is connected to the 1,3-propanediyl chain and oriented so as to minimize butterfly bending. The nu(Co)(-)(CH)()3 frequency for this new lariat model was close to that of pyCo(DH)(2)CH(3). In a third type, a bulkier 2,2-dimethyl-1,3-propanediyl group replaces the 1,3-propanediyl chain. The nu(Co)(-)(CH)()3 bands for two complexes with L = Me(3)Bzm and py were 2-5 cm(-)(1) lower in frequency than those of the corresponding [L(Co((DO)(DOH)pn)CH(3)](+) complexes. The decrease in the axial nu(Co)(-)(CH)()3 frequencies is probably due to the steric effect of the equatorial ligand. Thus, the nu(Co)(-)(CH)()3 frequency can be useful for investigating both steric and electronic influences on the Co-C bond.