Exequiel Moraga

Complexes of cobalt(III) with D-glucosamine and amines

Abstract d -Glucosamine and ammonia, 1,2-ethylenediamine, or 1,10-phenanthroline form complexes with Co(III) * . The complexes are: [Co(NH 3 ) 3 ·H 2 O· d -glucosamine] 3+ , [Co(en) 2 · d -glucosamine] 2+ and [Co(phen) 2 · d -glucosamine] 3+ . Formation of the last two complexes is stereospecific with Co(III) with predominance of the Δ over the Λ diastereomer. The Δ and Λ diastereomers of the ethylenediamine and phenanthroline complexes with d -glucosamine were separated and their configurations about Co(III) assigned from the CD and ORD spectra. The ethylenediamine complex has p K a ∼ 3.4 and is more acidic than the other complexes, pointing to coordination of the alkoxide residue at position 1 of glucosamine.

Structures of complexes of cobalt(III) and glucosamine. Applications of molecular mechanics and NMR spectroscopy

Abstract The 1 H NMR signals of the hydrochlorides of 2-amino-2-deoxy-α- and β- d -glucose (α- and β- d -glucosamine) have been assigned and chemical shifts and coupling constants determined from their phase-sensitive COSY spectra in D 2 O. Based on the Karplus relation the observed coupling constants fit H-H dihedral angles calculated with MM2 parameters. The COSY spectrum of the Λ isomer of [Co(en) 2 · d -glucosamine] 2+ shows that it is formed from the α anomer. The coupling constants, except for J 2,3 and J 3,4 , agree qualitatively with dihedral angles predicted by MM2 parameters. The structure of triammine-Co(III)- l -malate simulated by using MM2 parameters agrees reasonably well with that from X-ray crystallography.

Complexation of amino sugars with cobalt(III)bis(phenanthroline)

Abstract Complexes of Co(III)bis(phenanthroline) with β - d -mannosamine and α - d -galactosamine can be isolated as the triiodide salts. The complex with d -mannosamine has the Δ -configuration at Co(III). Complexation with d -galactoamine gives the Δ - in excess over the Λ - complex, and the Δ -complex can be isolated chromatographically. Complexation involves cis -1-OH and 2-NH 2 groups, eq , ax respectively with β -mannosamine and ax , eq respectively, with α -galactosamine, as with α -glucosamine, based on 1 H NMR spectra. Galactosamine complexes in aqueous solution decompose to the free sugar, but the mannosamine complex is much more stable.

Optical activity of mixed complexes of copper(II) and amino acids in the visible region

Abstract Study of the visible absorption and circular dichroism spectra of mixed complexes formed by copper(II), an optically active amino acid anion and a phenanthroline or dipyridyl moiety, shows that although the absorption spectra are similar the Cotton effects differ. This behavior is analyzed in terms of the energy levels of the mixed complexes and their relationship with the sequence of levels of the corresponding bis-aminoacidates. An interpretation relating the variation of the circular dichroism bands with structural changes is proposed.

Mixed complexes of cobalt(III) with phenanthroline and galactose or arabinose

Abstract Mixed complexes of bis-phenanthroline cobalt(III) and α- d -galactose or β- d - or l -arabinose are identified in aqueous solution from their 1H NMR and circular dichroism (CD) spectra. Galactose forms only the Δ-complex, but d -arabinose gives preferentially Δ-, and l -arabinose preferentially Λ-complexes, consistent with structural optimization with PM3 (tm) parameters. The Λ-complex is formed initially from β- d -arabinose, but the Δ-complex is preferred thermodynamically. Examination of the absorption and CD spectra gives information on configuration at cobalt(III) and allows assignments of electronic transitions in Co(III) and the phenanthroline residues.

Complexation of sucrose with cobalt(III)bis(phenanthroline)

The delta-[Co(III)bis(phenanthroline)(sucrose)]3+ complex forms with little perturbation of the sucrose conformation, and complexation by HO-2(g) and HO-1(f).

Complexes of cobalt(III) with d-fructose and phenanthroline

Abstract d -Fructose and 1,10-phenanthroline form complexes with Co(III). Configurations about Co(III) are assigned from the CD and ORD spectra of the separated Λ and Δ diastereomers, and Λ strongly predominates in the mixture. Most of the 1 H NMR signals of the complexes are shifted strongly upfield, relative to those of d -fructose, due to shielding by aromatic residues, and the effect is especially strong for hydrogens at positions 1 and 6. Signals of the fructose residue are assigned for the Λ diastereomer and coupling constants estimated, but only some of the signals can be assigned for the Δ diastereomer. The marked changes in chemical shifts on formation of the complexes from fructose are rationalized in terms of predicted conformations based on molecular-mechanics calculations with MM2 parameters, which predict the higher stability of the Λ over the Δ diastereomer, and also complexation at positions 2 and 3.

COMPLEXES OF CO(III) WITH L-SORBOSE AND PHENANTHROLINE

Abstract l -Sorbose and two molecules of 1,10-phenanthroline form a complex with Co(III) which has the A configuration about Co(III), based on CD and ORD spectra. The 1 H NMR signals of α- l -sorbopyranose and its mixed Co(III)(phen) 2 complex have been assigned. Signals of H-6 and H-6′ of the sugar are shifted strongly upfield because of shielding by the aromatic residues, but shifts are much smaller for H-1 and H-1′. These observations are consistent with molecular modelling with MM2 parameters, which predicts that the most stable complex is the Λ diastereomer with complexation at positions 1 and 2. The characteristics of the NMR spectra and the predicted structure of the complex differ from those of the corresponding complex of d -fructose.

Optical activity of compounds formed by ammino complexes of cobalt(III) and carbohydrates—II. d-arabinose complexes

Abstract Two sugar-containing chiral complexes are formed by the reaction of cis-[Co(NH3)4(H2O)2]3+ and d -arabinose at pH 6. Complex I is a mixed valence cobalt(II)/cobalt(IH) paramagnetic species, whereas complex II is a mononuclear cobalt(III) complex. Their syntheses and characterization are described. Visible absorption spectra and IR spectra were recorded. Optical activity was measured via circular dichroism spectra and optical rotatory dispersion. Complex I undergoes anomerization in aqueous solution, the rate of this process being pH and chiral complex dependent. Complex II does not retain its optical activity in aqueous solution as a result of decomposition. Spectral data support the coordination scheme proposed for each complex.

Studies of Δ,Λ-[M(1,10-phen)2(S-ala)]qX diastereoisomeric systems of nickel and zinc with different counter-anions

Abstract Δ,Λ-[M(phen) 2 ( S -ala)] + (X q- ) 1/q diastereoisomeric systems, in which M = nickel or zinc, phen = 1,10-phenanthroline, S -ala − = S -alaninate ( q = 1 and 2), with a series of counteranions have been prepared as methanolic solutions and characterized by spectral (CD, ORD, UV-vis) and conductance data. In both nickel and zinc systems the CD spectra in the β′ band region of the diimine ligand exhibit exciton splittings with sign patterns corresponding to a predominance of the Λ isomers. Analysis of the ORD spectra by means of many-term Drude equations suggests the order Zn > Ni for the absolute values of excitation resonance energy. Differences between nickel and zinc systems have been observed concerning the effects of the counter-anion nature upon the formation equilibria. In the nickel systems these effects would consist mainly of outer-sphere interactions affecting the position of the [Λ]/[Δ] equilibrium, whereas in the zinc ones ligand/counter-anion exchange equilibria would also occur to a significant degree. By using the molar-ratio method the stability orders [Zn(phen)( S -ala) 2 ] ≈ [Zn(phen) 2 ( S -ala)] + > [Zn(phen) 3 ] 2+ and [Ni(phen) 3 ] 2+ > [Ni(phen) 2 ( S -ala)] + have been established. The ion-pairing and the third stepwise stability constants for [Zn(phen) 2 ( S -ala)]Cl have also been estimated. The counteranion dependence of the apparent stability constants for the zinc systems, is discussed in terms of the counter-anion basicity and the possible “discriminating qualities” of the [Zn(phen)( S -ala)] + species.