Cobaloxime Complexes Research Articles

Synthesis of (chlorovinyl)cobaloxime complexes, model complexes of proposed intermediates in the B12-catalyzed dehalogenation of chlorinated ethylenes.

(Chlorovinyl)cobaloxime complexes, (dmgH)2(py)CoR (R = cis-dichlorovinyl, trans-2-chlorovinyl or 1-chlorovinyl, dmgH = dimethylglyoximate) were synthesized from the reaction of chlorinated ethylenes with in situ generated (dmgH)2Co(I) in the presence of pyridine.

The reactivities of dehydroalanine derivatives towards 1,3-dienyl cobaloxime complex: new routes to functionalised carbocyclic amino acids

The reactivities and selectivities of the Diels–Alder reactions of acyclic dehydroalanine esters and cyclic dehydroalanines derived from piperazine-2,5-diones with 1,3-dienyl cobaloxime complex are reported. From our studies we demonstrate the use of the activated organocobalt diene in a ‘combinatorial’ approach to the synthesis of functionalised cyclic amino acids and derivatives from a single cycloaddition reaction.

Electrocatalytic activity of cobaloxime complexes adsorbed on glassy carbon electrodes toward the reduction of dioxygen

We report here a detailed study concerning the electrochemical behavior of two cobaloxime complexes adsorbed on glassy carbon electrodes and their applications to electrocatalytic activity for dioxygen reduction. The complexes were adsorbed irreversibly and strongly on the surface of preanodized glassy carbon electrodes. The electrochemical behavior and stability of the modified electrodes as well as the two-electron reduction of O 2 at the electrodes were investigated using cyclic voltammetry, chronoamperometry and rotating disk electrode methods. At the modified electrodes, the reduction of dioxygen to hydrogen peroxide occurs at potentials where it is not observed at a bare glassy carbon electrode. The modified electrodes exhibited potent and persistent electrocatalysis for O 2 reduction in acetate buffer solutions of pH 4.0 with the domination of an overpotential of about 950 mV and an increase in the peak current. The heterogeneous rate constants for the reduction of O 2 at the surface of modified electrodes were determined by rotating disk electrode voltammetry using the Koutecký–Levich plots. A possible catalytic mechanism is proposed and discussed.

Solid-State Specific and Unidirectional Photoisomerization of 3-Substituted Propyl to 1-Substituted Propyl Cobaloxime Complexes via 2-Substituted Propyl Complexes

Abstract Solid state-specific (γ→β→α)photoisomerization of 3-cyanopropyl and 3-(methoxycarbonyl)propyl cobaloxime complexes were found to occur on visible-light irradiation. There exist considerable differences in the reaction rates, which have no relation to the basicity of the axial ligand. In the former series of complexes, the rate constant (k2) for the second step (β→α) is much faster than that (k1) for the first step. However, in the latter series of complexes, the rate constant (k2) for the second step tends to decrease extremely, and thus, k2 become smaller than k1.

Electrocatalytic reduction of dioxygen on a glassy carbon electrode modified with adsorbed cobaloxime complex.

The preparation and electrochemical properties of a glassy carbon (GC) electrode modified with cobaloxime complex were investigated. The complex of the type [CoIII(DO)(DOH)pn)Cl2] where (DO)(DOH)pn = N2,N2'-propanediylbis-2,3-butanedione-2-imine-3-oxime) was adsorbed irreversibly and strongly on the surface of preanodized glassy carbon electrode. Electrochemical behavior and stability of modified GC electrode were investigated by cyclic voltammetry. The electrocatalytic reduction of dioxygen has been studied using this modified glassy carbon electrode by cyclic voltammetry, chronoamperometry and rotating disk electrode voltammetry as diagnostic techniques. The modified electrode showed excellent eletrocatalytic ability for the reduction of dioxygen to hydrogen peroxide in acetate buffer (pH 4.0) with overpotential 1.0 V lower than the plain glassy carbon electrode. The formal potential for this modified electrode is not shifted to more negative potentials by repeated reduction-oxidation cycles in oxygen-saturated supporting electrolyte solution. The apparent electron transfer rate constant (kS), the transfer coefficent (alpha) and the catalytic rate constant of O2 reduction at a GC modified electrode were determined by cyclic voltammetry and rotating disk electrode voltammetry and were found to be around 2.6 s(-1), 0.33 and 2.25 x 10(4) M(-1) s(-1). Based on the results, a catalytic mechanism is proposed and discussed.

Direct observation of deuterium migration in crystalline-state reaction by single-crystal neutron diffraction. III. Photoracemization of 1-cyanoethyl cobaloxime complexes.

The H atoms bonded to the chiral C atoms (stereogenic center) of the 1-cyanoethyl groups in two cobalt complexes, [(R)-1-cyanoethyl]bis(dimethylglyoximato)(pyridine)cobalt(III) (2) and [(R,S)-1-cyanoethyl]bis(dimethylglyoximato)(piperidine)cobalt(III) (3), were replaced with D atoms, such as Co--C*D(CH(3))CN. The crystals of the two cobalt complexes were irradiated with a xenon lamp for 72 h and 27 d, respectively. The unit-cell dimensions were gradually changed with retention of the single-crystal form. The crystal structures after irradiation were determined by neutron diffraction. In each crystal the chiral 1-cyanoethyl group of one of the two crystallographically independent molecules was partly inverted to the opposite configuration, whereas that of the other molecule kept the original configuration. The C*--D bond in the inverted group was completely conserved in the process of the inversion of the chiral alkyl group. This suggests that the inversion of the chiral 1-cyanoethyl group proceeds with the rotation of the cyanoethyl radical after the Co--C bond cleavage by photo-irradiation so that the opposite side of the radical faces the Co atom. This is followed by recombination of the Co--C bond to form the inverted 1-cyanoethyl group.

Direct Observation of Steric Hindrance in the Process of Photoisomerization of Cobaloxime Complexes

The cobaloxime complex crystals with the β-cyanoethyl group as an axial group and the biphenylboron group substituted to the equatorial ligand were prepared. When the crystals were irradiated with a xenon lamp, the β-cyanoethyl groups were changed to the α-cyanoethyl group with retention of the single crystal forms. The produced α-cyanoethyl groups were not racemic but chiral depending on the asymmetry around the β-cyanoethyl groups in the original crystals. It was made clear from the structure analyses before and after the reaction that the steric repulsion of the biphenylboron group with the produced α-cyanoethyl group is responsible to the generation of chirality.

Successive Crystalline-State Photoisomerization of the 4-Cyanobutyl Group in a Cobaloxime Complex

Abstract The 4-cyanobutyl group bonded to the cobalt atom in a cobaloxime complex crystal was isomerized to the 2-cyanobutyl group through the 3-cyanobutyl group on exposure to a xenon lamp with retention of the single crystal form. The X-ray crystal structure analysis of the initial crystal, (I), and the crystal after 66 hours exposure, (II), revealed that the produced 3-cyanobutyl group at one site in (II) is not disordered racemates but one enantiomer. The occupancy factors of the 4-cyanobutyl and the produced 3-cyanobutyl group were 0.786(7) and 0.214(7), respectively in (II). Although the structure of the produced 2-cyanobutyl group was not located on the difference electron density map, the NMR measurement clearly indicated that a small amount of the 2-cyanobutyl complex (ca. 6%) was produced.

The Role of Hydrogen Bond and Crystal Solvent in the Solid-state (β-α) Photoisomerization of Novel 2-(N-phenylcarbamoyl)ethyl Cobaloxime Complexes [1

The 2-(phenylcarbamoyl)ethyl group bonded to the cobalt atom in cobaloxime complexes was isomerized on exposure to visible light in the solid-state. The three complexes with different amines as axial base ligands were prepared and their crystal structures were determined by X-ray analysis. The reaction rates in the solid-state were mainly affected by the hydrogen bonds of the reactive group with the neighboring molecules and the occupation of crystal solvent molecules around the reactive group.

Synthesis of a clathrochelate complex with an appended pyridine and its coordination to a cobaloxime complex.

An iron clathrochelate complex with an appended pyridyl group is synthesized and characterized. The ability of this complex to act as a ligand is demonstrated via its coordination to cobaloxime com...

Asymmetric Induction in Cobaloxime Complex Crystals Due to Chiral Crystal Environment

The achiral 2-cyanoethyl group of cobaloxime complexes was isomerized to the chiral 1-cyanoethyl group on exposure to a xenon lamp in the solid state. From the crystal structures and the enantiomer or diastereomer excesses of the seven cobaloxime complexes with different axial base ligands, it was found that the chirality of the produced 1-cyanoethyl group depends on the asymmetric shape of the reaction cavity for the 2-cyanoethyl group before irradiation.

(2-Carbamoylethyl)bis(dimethylglyoximato-N, N')

The 2-carbamoylethyl and 2-(methylcarbamoyl)ethyl groups in the title cobaloxime complexes, [Co(C(4)H(7)N(2)O(2))(2)(C(3)H(6)NO)(C(12)H(13)N)].C(2)H(6)O and [Co(C(4)H(7)N(2)O(2))(2)(C(4)H(8)NO)(C(10)H(13)NO(2))], were isomerized to 1-carbamoylethyl and 1-(methylcarbamoyl)ethyl groups, respectively, on exposure to visible light in the solid state. Although both the crystal structures and the intermolecular hydrogen bonds are different in the two crystals, similar reaction rates were observed.

Reactions of Cobaloximes with Alkenyl Triflates and Halides as a New Method for the Preparation of Cobalt−sp2Carbon Bonds

Cobaloxime complex [(pyridine)2(dimethylglyoxime)2Co(II)] (generated in situ) reacts with a number of alkenyl triflates and halides in the presence of zinc, providing a new, simple route to alkenyl...

Direct observation of deuterium migration in crystalline-state reaction by single-crystal neutron diffraction. II. 3-1 photoisomerization of a cobal-oxime complex

Single crystal neutron diffraction analysis of photo-exposed (3-cyanopropyl-d2(alpha,alpha))-[(R)-1-phenylethylamine-d11]bis(dimethylglyoximato-d14)cobalt(III) was carried out in order to clarify the mechanism of the crystalline-state photoisomerization of the 3-cyanopropyl group bonded to the Co atom in some cobaloxime complexes. Before irradiation the two H atoms bonded to the C1 atom of the 3-cyanopropyl group were exchanged with the D atoms such as --CH2CH2CD2CN. On exposure to a xenon lamp, the cell dimensions of the crystal were gradually changed. After 7 d exposure the change became insignificantly small. The structure was analyzed by neutron diffraction. The 3-cyanopropyl group was transformed to the 1-cyanopropyl group such as --CD(CN)C(H1/2,D1/2)2CH3 with retention of the single-crystal form. This indicates that one of the D atoms bonded to C1 migrates to either position bonded to C2. The other atoms of the complex remained unchanged. These results indicate that photoisomerization proceeded in two steps: the 3-cyanopropyl group was isomerized to the 2-cyanopropyl group in the first place and then the 2-cyanopropyl group was transformed to the 1-cyanopropyl group. Moreover, it was made clear that the second-step isomerization was irreversible, since one of the D atoms was retained. The disordered structure at C2 is estimated to be caused by the interconversion between the 1-cyanopropyl group produced and its dehydrogenated olefin after the photoisomerization.

Optical enrichment in chiral crystal environment of [(R,S)-1-cyanoethyl][methyl (S)-alaninate]cobaloxime complex only by photoirradiation

Optical enrichment in chiral crystal environment of [(R,S)-1-cyanoethyl][methyl (S)-alaninate]cobaloxime complex only by photoirradiation

Mechanism of deuterium–hydrogen exchange in the photoisomerization of cobalt complex

The 3-cyanopropyl(–CH 2CH 2CH 2CN) group bonded to the cobalt atom in cobaloxime complexes is isomerized to the 1-cyanopropyl{–CH(CN)CH 2CH 3} group with retention of the single crystal form by photo irradiation. When the dideuterated 3-cyanopropyl group (–CH 2CH 2CD 2CN) was used, the crystal structure analysis by neutron diffraction revealed that the produced 1-cyanopropyl group was not {–CD(CN)CHDCH 3} but the disordered {–CD(CN)C(H 1/2D 1/2) 2CH 3} group. This indicates that the reaction proceeds through the 2-cyanopropyl group and that the olefinic molecule and the cobalthydride may exist in the intermediate stages of the reaction.

Racemic-to-Chiral Transformation in Cobaloxime Complex Crystals Only by Photoirradiation

Abstract The cobaloxime complex crystal with the racemic-1-cyanoethyl group and piperidine as axial ligands, which has a chiral space group of P212121, changed its cell dimensions with retention of the single crystal form on exposure to visible light. The chloroform solution of the crystals after irradiation showed the optical rotation ([α]D = 30°). This indicates the racemic-to-chiral transformation occurred in the crystal only by photoirradiation. The crystal before irradiation, analyzed by X-rays, has two complexes with R- and S-1-cyanoethyl groups in the asymmetric unit. After irradiation, one of the complexes with the S group, which has a larger reaction cavity than the R group, was partly inverted to create a disordered structure. The R : S ratio in the crystal changed from 50 : 50 to 70 : 30. The final ratio of 70 : 30 is well explained by the symmetric shape of the reaction cavity after the change. For the cobaloxime complex crystal with the racemic-1-cyanoethyl group and pyrrolidine as axial ligands, a similar transformation was observed. The racemic crystal with the space group P1 has four complexes in a unit cell, two of which have a R-1-cyanoethyl group while the other two complexes have a S-1-cyanoethyl group. The complexes with the S group, which have larger cavities than those with the R group, were partly inverted to create the opposite configuration. This caused optical enrichment in the crystal. Such a racemic-to-chiral transformation will be observed not rarely but commonly if the racemic crystals with chiral space groups will be examined cautiously.

Crystalline-State Racemization of Cobaloxime Complexes. 21. Racemization of a Bulky (R)-1,2-Bis(ethoxycarbonyl)ethyl Group

Abstract Three cobaloxime complexes with a chiral 1,2-bis(ethoxycarbonyl)ethyl group bonded to the cobalt atom: [(R)-1,2-bis(ethoxycarbonyl)ethyl]bis(dimethylglyoximato-N,N′)[(S)-1-phenylethylamine]cobalt(III), 1, [(R)-1,2-bis(ethoxycarbonyl)ethyl]bis(dimethylglyoximato-N,N′)(pyridine)cobalt(III), 2, and [(R)-1,2-bis(ethoxycarbonyl)ethyl]bis(dimethylglyoximato-N,N′)(propylamine)cobalt(III), 3, were analyzed by X-rays. The cell dimensions of 2 were remarkably changed when the crystal was irradiated with a xenon lamp. The structure after the irradiation indicated that the chiral 1,2-bis(ethoxycarbonyl)ethyl group was changed to the disordered racemates retaining the single crystal form. The other two crystals showed no change after the irradiation in the same conditions. Comparing the reaction cavities among four crystals analyzed so far, the shape rather than size of the cavity may play an important role in the crystalline-state racemization.

Use of 13C−1H NMR Coupling Constants To Assess the Binding of Imidazole Ring Ligands to Metals

In this report we initiate analysis of imidazole rings bound to metals using one−bond 1H-13C couplings. Shift trends in the metal binding of N-donor heterocyclic ligands have been difficult to interpret since signals of close-in nuclei, particularly the carbon in the CN2 grouping of imidazole rings, often have erratic or small shift changes. We exploit the well-studied trans influence of a series of unidentate axial R and X ligands in cobaloxime complexes (LCo(DH)2(R or X)) since shifts of carbons remote from Co have interpretable trends that correlate with Co−N bond lengths. We compare information obtained from 1JCH values for two series, one with L = a small ligand, N-methylimidazole (N-MeImd), and one with L = a large ligand, 1,5,6-trimethylbenzimidazole (Me3Bzm). For comparison to our spectroscopic measurements, we have determined X-ray crystal structures for N-MeImdCo(DH)2(R or X) (where DH = monoanion of dimethylglyoxime, R or X = CH2CH3 (1), CH3 (2), CH2CN (3), and Cl (4)) doubling the number of structures reported for cobaloximes containing N-MeImd. The observation of the B-type orientation of the planar N-MeImd axial ligand in structures 1 and 2 almost certainly results from the small size of N-MeImd. This B-orientation is rare for cobaloximes, which normally have an axial ligand in the A-orientation. The t scores obtained from the principal component analysis of cobaloxime data [Randaccio, L.; Geremia, S.; Zangrando, E.; Ebert, C. Inorg. Chem. 1994, 33, 4641] were applied for the first time in the analysis of intraligand coupling constants. The use of these parameters allowed us to identify steric effects on the 13C shifts and 1JCH values for the bulky, lopsided Me3Bzm axial ligand in the Me3BzmCo(DH)(R or X) series; similar steric effects on spectra were not observed for N-MeImdCo(DH)(R or X). The intraligand 1JCH values for the close-in N2CH carbon of Me3Bzm and N-MeImd were found to reflect the ligand-to-metal binding even better than the more commonly used 13C shifts of carbons remote from the metal. Thus, the use of 1JCH values holds considerable promise in metallobiochemistry.

Photoisomerization of (2-cyanoethyl)(isonicotinic acid)cobaloxime complex in a series of isostructural host–guest complex crystals

A series of host–guest complex crystals has been formed between (2-cyanoethyl)(isonicotinic acid)cobaloxime as a guest and dicyclohexylamine, cycloheptylcyclohexylamine and cyclohexylcyclooctylamine as hosts. The X-ray analyses revealed that the three crystals are isostructural to each other. There are two crystallographically independent molecules in an asymmetric unit of each crystal. One of the cobaloxime molecules, A, has the 2-cyanoethyl group with gauche conformation around the central C–C bond while that of the B molecule has the trans conformation. When the powdered samples of the crystals were irradiated with a xenon lamp, the 2-cyanoethyl groups were isomerized to the 1-cyanoethyl group. The reaction rate was estimated from the change in the FT-IR spectra of the stretching vibration mode of the C–N triple bond. The reaction rates of the 2-cyanoethyl groups with gauche and trans conformations were estimated separately from the spectra. The rate constant of the 2-cyanoethyl group with the gauche conformation is significantly greater than that with the trans conformation in each crystal. The isomerization rate was accelerated as the size of the cycloalkane ring increased. The reaction cavity for the 2-cyanoethyl group expands as the ring size increases. This suggests that it is possible to control the reaction rate if a series of the host–guest complexes are formed and the size of the host molecule is systematically changed.