Milan Strašák

SYNTHESIS AND CHARACTERIZATION OF COLBALT(III) COMPLEXES WITH A STEREOSPECIFIC LIGAND DERIVED FROM S-PROLINE

Abstract This paper concerns the preparation, separation, and characerization of cobalt(III) complexes in which 2S, 2′S-1,1′-(ethane-1,2-diyl)bis(pyrrolidine-2-carboxylate), (pren), acts as etradentate ligand and the other two positions in the coordination sphere are occupied by monodentate or bidentate groups. The complex [Co(pren)(H2 O)2]+ exists only as the cis-α isomer, while in the case of [Co(pren)en]+ complexes, substantial yields of both cis-α and cis-β isomers have been isolated through a series of chromatographic steps and fractional crystallizations. The geometrical and optical configurations are assigned on the basis of absorption, circular dichroism, and 13C NMR spectra. In addition, the IR spectra of the complexes are discussed.

Synthesis and characterization of cobalt(III) complexes with a stereospecific ligand derived from (S)-valine

Abstract Complexes of the type [CoLX 2 ] + and [CoLXY], where L is the quadridentate ligand ethylenediamine-N,N′-di-(S)-α-isovalerate (H 2 ven) and X = H 2 O or ethylenediamine (en) and Y = NO 3 have been prepared. Three geometrical isomers, cis-α, cis-β, and trans forms, may in principle be obtained. The complex [Co(ven)(NO 3 )H 2 O] exists only as the cis-α geometrical isomer, while in the case of [Co(ven)-(H 2 O) 2 ] + and [Co(ven)en] + complexes both cis-α and cis-β are found to exist. The yields of the cis-α isomers exceeded considerably those of the cis-β isomers. 13 C-NMR and absorption spectral measurements confirm these arrangements. In addition, the IR spectra of the complexes are discussed. The compounds exhibit a string negative CD bond under the 1 A 1g → 1 T 1g cubic absorption band, which is due to the 1 A 1g → 1 E g tetragonal component, consistent with the Δ absolute configuration imposed by the (S) configuration of the asymmetric carbon atom in the (S)-valine moiety.

Kinetics of the interaction of some cobalt(II) chelates with air oxygen: formation of two-nitrogen oxygen carriers

Abstract The thermodynamics and kinetics of reversible uptake of air oxygen in aqueous solution by cobalt(II) chelates of amino acid derivatives EBAA∗∗ have been examined. Kinetic data support the mechanism reported by Wilkins [1]. Enthalpies and entropies of activation have been determined from the temperature dependence of the rate constant k1 for the oxygenation reaction CoL + O2→CoLO2. The favorable enthalpy of oxygen binding is almost entirely negated by an unfavorable entropy term. The relationship between these experimental constants, the electronic structure of the complexes, and the significance of the results in light of earlier investigations are discussed.

Spectroscopic Studies of Metal Ion Mediated Interactions with Components of Nucleic Acids. I. Esr Study of Ternary Copper(II) Complexes ofα-Amino Acids and Dipeptides with Adenosine

Abstract The formation of the title ternary complexes in aqueous solution has been investigated by electronic and ESR spectroscopy. ESR parameters have been used to compute molecular orbital coefficients. Coordination modes and their influence on metal-ligand bonding have been studied. A characteristic difference between GlyPro and other dipeptides is attributed to the lack of the peptide proton in GlyPro. It is suggested that, at near physiological pH conditions, the nucleoside binds at an equatorial site by displacing a water molecule from the Cu(II) ion.

Crystal and molecular structure of Δ-cis-α-ethylenebis-S-prolinato(1,2-diamino-ethane)cobalt(III) perchlorate dihydrate, Δ-cis-α-[Co(ss-EBP)(en)] ClO4·2H2O

Abstract The crystal and molecular structure of Δ- cis -α- ethylenebis-S-prolinato(1,2-diaminoethane)cobalt(III) perchlorate dihydrate, Δ- cis -α-[Co(SS-EBP)(en)] ClO 4 · 2H 2 O, was determined from three-dimensional X-ray diffractometer data. The complex crystallizes in the orthorhombic system, space group P 2 1 2 1 2 1 with a = 7.879(4) A, b = 13.738(9) A, c = 19.445(2) A, V = 2104(2) A 3 . With Z = 4, the observed and calculated densities are 1.60(2) and 1.605 g cm −3 , respectively. The structure was refined by the block- diagonal least-squares technique to a final R = 0.0560 for 1604 observed reflections. The geometry about the cobalt atom is roughly octahedral with the tetradentate SS-EBP (= ethylenebis-S-prolinate ion), assuming cis -α configuration in which the complex possesses two out-of-plane amino acidate (R) rings and the backbone ethylenediamine (E) ring. The E ring conformation is δ. On the other hand, the R rings have λ conformation as well as the en ring. Δ-R N R N− (δ E  λ R 1  λ R 2 )(λ en )- cis -α-[Co(SS-EBP)(en)] + is one of two possible isomers of this compound which have been isolated and whose absolute configurations have been tentatively assigned by spectroscopy. The crystal and molecular structure determination confirms these assignments.

Stereochemical control of oxygen activation in synthetic oxygen carriers

Abstract A series of mixed ligand complexes of cobalt(III) containing a linear tetradentate amino acid derivative EBAA 2 and imidazole has been synthesised and characterized. It has been established for their cobalt(II) analogues that they are able to reversibly bind atmospheric oxygen in aqueous solution. By means of the combined use of visible-UV, IR, 13C NMR and ESR spectroscopy, paper electrophoresis and elemental analyses, the tentative structure of these cobalt dioxygen complexes has been proposed. Possible stereochemical control of the uptake of molecular oxygen by cobalt(II) complexes in solution in light of earlier investigations is discussed. For ternary complexes of EBAA and a heterocyclic nitrogen base B, binuclear dioxygen complexes are formed without hydroxo bridges, presumably because all of the available coordination positions of the cobalt atom are occupied by the ligand donor groups and the dioxygen molecule, whereas in the case of binary complexes of EBAA binuclear dibridged μ(O2, OH) complexes are formed. These complexes may be used as models for active sites in some metalloproteins, including respiratory proteins.

Kinetics and mechanism of the decomposition of dibridged cobalt dioxygen complexes of amino acid derivatives in alkaline medium

Abstract The kinetics and thermodynamics of the decomposition of dibridged μ(O 2 2− , OH − )cobalt complexes with the linear tetradentate − OOCCH(R)NHCH 2 CH 2 NHCH(R)COO − (L = EBAA*) ligand have been studied in alkaline medium. Under these conditions, the dioxygen complex Co 2 L 2 O 2 OH − undergoes irreversible metal-centered autoxidation to form a CO(III) complex with the unchanged ligand and H 2 O 2 . The reaction is base-catalyzed, and a mechanism has been proposed in which deprotonation of a coordinated nitrogen activates the hydrolysis of the CO III O 2 bond. From the enthalpy—entropy relationship, the isokinetic temperature β=274 K has been found.

Kinetics of the interaction of some cobalt(II) chelates with air oxygen: mutual equatorial—axial influence of ligands on oxygen activation

Abstract Reactions of air oxygen with mixed ligand complexes of cobalt(II) containing a linear tetradentate amino acid derivative L=EBAA and a heterocyclic nitrogen base B are described. Enthalpies and entropies of activation have been determined from the temperature dependency of the rate constant k 1 for the oxygenation reaction CoL(B)+O 2 →CoL(B)O 2 . Polar and steric effects in the oxygenation have been separated by use of the linear free energy relationship; it was found that steric effects were decisive in the case of equatorial influence, while electronic effects are important in the case of mutual equatorial-axial influence. A degree of the dioxygen ligand activation is interrelated with the stereochemical conditions dictated by the actual equatorial-axial influence. The classic concept of the MIL is extended to intramolecular interligand interactions. A comparison is made of the cobalt(II)-O 2 interaction with the iron(II)- and copper(I)-O 2 systems occurring in the biologically important respiratory proteins.

Rapid optical resolution of electrically neutral cobalt(III) chelate complexes by gel permeation chromatography

Abstract Three geometrical isomers of electrically neutral [Co(EBAA)(aa)] [EBAA = ethylenebis(aminoacidate); aa = an amino acidate] complexes have been synthesized and characterized. They have been resolved into the enantiomers, except for symmetrical cis-mer-[Co (EBAA)(aa)] isomers, by gel permeation chromatography on a Sephadex G-10 column with water as the eluent, and their absolute configurations have been determined based on their circular dichroism spectra. It has been revealed that, except for [Co(EBG)(β-ala)] [EBG = ethylenebis(glycinate)], the Δ enantiomers have a substantially greater optical purity.

Diastereoselectivity in metal complexes of some phenyl-substituted ethylene-diamine-N,N,N',N'-tetraacetate derivatives

Abstract Meso-1,2-diphenylethylenediamine-N,N,N′,N′-tetraacetic acid (meso-H4DPHEDTA) and rac-1-phenyl-ethylenediamine-N,N,N′,N′-tetraacetic acid (H4PHEDTA) have been synthesized by the condensation of meso-1,2-diphenylethylenediamine or R,S-phenylethylenediamine, respectively, with bromoacetic acid and their cobalt(III) complexes prepared. The stereochemistry of both complexes was assigned on the basis of electronic, infrared, 1H and 13C NMR spectral data and elemental analyses. The stability constants of the chelating agents represented by monophenyl and both meso- and rac-diphcnylderivatives of EDTA have been determined potentiometrically and polarographically for selected metal ions. It was found that these hexadentate ligands exhibit high diastereoselectivity in metal complexes. The factors determining stereochemistry and stabilities of these complexes are mainly intramolecular hydrophobic stacking interactions which cause different conformation of the central ethylenediamine E ring.