Organocobalt B12 model complexes of the imine/oxime-type containing R groups with good trans influence were prepared using the isolated CoI reagent (CO)CoI((DO)(DOH)pn). Complexes were of the type [LCo((DO)(DOH)pn)R]PF6 (or ClO4) (L = H2O, py, Me3Bzm for R = CH2OCH3; L = H2O, Me3Bzm for R = CH2SCH3). The −CH2SCH3 group is a new ligand for a B12 model system. Different synthetic routes were used to prepare other new complexes of the type [LCo((DO)(DOH)pn)R(or X)]PF6 (or ClO4) (L = H2O, N-MeImd, Me3Bzm for X = Cl; L = py, Me3Bzm for X = N3; L = H2O, py, Me3Bzm for R = CH2Cl, CH2NO2; L = py for R = CH2CN). X-ray crystal structures were determined for [Me3BzmCo((DO)(DOH)pn)CH2OCH3]PF6 (1), [pyCo((DO)(DOH)pn)CH2OCH3]PF6 (2), and [Me3BzmCo((DO)(DOH)pn)CH2Cl]PF6 (3). Combined with previous results, the structures of the new Me3Bzm complexes create a series of five imine/oxime structures. The trend of axial Co−N bond lengthening with increasing electron-donating ability of the R group found for [Me3BzmCo((DO)(DOH)pn)R]PF6 [CH2CF3 < CH2Cl < CH3 < CH2CH3 < CH2OCH3] was similar to that of the well-studied cobaloximes LCo(DH)2R (DH = monoanion of dimethylglyoxime). The axial Co−N and Co−C bonds of the imine/oxime-type complexes were slightly longer compared to the respective bonds in the corresponding cobaloximes. Such a slight dependence of Co−C bond length on the nature of the equatorial ligand has not been established previously. For some carbons, 13C NMR chemical shifts for [Me3BzmCo((DO)(DOH)pn)R(or X)]ClO4 in CDCl3 correlated linearly with the electronic parameter (EP), a spectroscopic measure of the electronic trans influence. The shifts for [Me3BzmCo((DO)(DOH)pn)CH2SCH3]ClO4 allowed us to calculate the EP value for the −CH2SCH3 group. The value indicates a trans influence for −CH2SCH3 similar to that of −CH3. Shifts for some types of carbons in [Me3BzmCo((DO)(DOH)pn)R(or X)]ClO4 did not correlate well. This lack of correlation can be attributed, in part, to steric strain in the Me3Bzm induced by non-bonded repulsions of this axial ligand with the equatorial ligand. The influence of such steric strain is not so evident in the shift trends of the corresponding carbons in cobaloxime analogs. This result suggests less ligand repulsion in the cobaloximes since the orientation of the plane of the Me3Bzm ligand differs by 60−90° from the analogous plane in the imine/oxime model system.

A rare Co−N−C three-membered ring has been formed by a novel pathway involving a facile interligand nucleophilic addition of an equatorial nitrogen donor to a ligated axial carbon in some organocobalt complexes. The highly distorted structure of a typical resulting complex provides clear experimental evidence useful in assessing hypotheses and computations concerning Co−C bond activation in B12 chemistry. Treatment of R = −CH2X (X = halogen) derivatives of imine/oxime B12 models of the type [LCo((DO)(DOH)pn)R]+ with base afforded major products with striking NMR spectral features in common, e.g., two one-proton olefinic doublets (J ≈ 3 Hz) and two one-proton singlets. The X-ray structure for the pyridine (py) derivative [pyCo(N-CH2-CHEL)]+ (1) reveals two unexpected features:  a three-membered metallocycle containing an η2-aminomethylene group, and an enamine (N−CCH2) in place of one imine moiety (NC−CH3). The C−Co−N ring angle is acute (43.7°) with the distortion of the coordination sphere concentrated in the Co−C and the Co−N ring linkages, which move away from the normal pseudooctahedral positions. Studies of the formation of the aqua analog of 1, [H2OCo(N-CH2-CHEL)]+ (3), in aqueous solution suggest that the initial intermediate formed is deprotonated at one imine methyl. Ring closure in the short-lived deprotonated intermediate is facile. Published calculations predict that such ring closure could occur and that, of two processes which could facilitate Co−C bond cleavage (θ-bending of the N−Co−C angle and φ-bending of the Co−C−C angle), the latter factor should be more significant in weakening the Co−C bond. There is large θ-bending in 1; thus, the new metallocyclic B12 analogs afford the first experimental test of these calculations. Our findings that the Co−C bond in 1 is short and that the bond did not readily cleave support the predictions of the calculations.

Enzyme-bound methyl-B 12 transfers its methyl group to homocysteine during methionine synthesis. However, treatment of several types of organocobalt B 12 models with arene-and alkanethiolates under ambient conditions leads only to thiolate ligation. The structure of [AsPh 4 ][EtSCo(DH) 2 CH 3 ] (DH ) monoanion of dimethylglyoxime), the first characterization by X-ray crystallography of an organocobalt complex containing a unidentate coordinated thiolate, demonstrates unambiguously the S-ligation of ethanethiolate to Co, trans to the CH 3 ligand. This compound contains a very long Co-S bond (2.342(2) Å). However, the length of the Co-C bond (2.005(7) Å) is typical; this result strongly supports reported FT-Raman spectroscopic data indicating that the thiolate-type ligand does not have a strong trans influence and does not significantly weaken the Co-C bond in the ground state. Since a strong trans influence alkyl ligand weakens the trans Co-C bond, we examined the effect of EtSon Co-((DO)(DOH)pn)(CH 3 ) 2 [(DO)(DOH)pn ) N 2 ,N 2′ -propanediylbis(2,3-butanedione 2-imine 3-oxime) is an imine/ oxime quadridentate ligand]. Even for this compound, no attack on the Co-C bond was observed, although independently synthesized EtSCo((DO)(DOH)pn)CH 3 was stable. Furthermore, thiolate did not cleave the Co-C bond of an organocobalt complex with a highly distorted Co-C group. Several new spectroscopic and ligandexchange reactions were observed in this study. Ligand-responsive NMR shift trends in these other new complexes also indicate that thiolate ligands have a weak trans influence.

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 correspon...