Mono- and dibridged peroxo complex of cobalt(III)

Abstract

plexes. The overall titration equation for 2:1 solutions is 2Con(H2L2)2+ + > , + 60H--> 2CoIHL2" + 7H20 where L represents GlyGly with an ionized amide nitrogen coordinated to metal ion. The product Co(III) complexes for both 2:1 and 1:1 solutions exhibit an absorption maximum at 520 nm, identical with earlier results with excess ligand and consistent with four nitrogen donors.2'8 The nearly half absorption magnitude for the 1:1 solution suggests disproportionation to yield the same Co(III) dipeptide complex as in the 2:1 solution. The disproportionation observed in the 1:1 solutions confirms earlier suggestions that at least three nitrogen donors are required for oxygenation and oxidation of Co(II) complexes in the presence of amine and peptide ligands.2'3 Because ionization of amide hydrogens in a 2:1 dipeptide-cobalt(II) complex has been shown to be cooperative in the presence of oxygen,2'8 both amide hydrogens undergo deprotonation in formation of binuclear peroxo and final red Co(III) complexes. Experimental SectionCommercial dipeptides were weighed out as required and the purity was checked by titration on a Radiometer combination titrimeter-titrigraph. Analytical reagent grade Co(N03)2-6H20 and Co(CH3COO)2-4H20 were employed. Titrations under nitrogen were performed as described previously.8 CD spectra were recorded on a Durrum-Jasco 5 instrument. All molar absorptivities and differential molar absorptivities between left and right circularly polarized light are based upon the molar concentration of cobalt ion. Pmr spectra were recorded on a Varían HA-100 spectrometer and are reported in hertz downfield from external TMS. ABC pmr spectra were analyzed by the program LAOCOON II.22 All pH values recorded are meter readings uncorrected for the presence of any D20.Two to one molar ratios of dipeptide and Co(II) (at 0.1 or 0.2 M) were used for the pmr experiments. Usually the D20 solution for the pmr experiments was diluted with H20 to 0.02 or 0.04 M cobalt for CD and absorption spectra. For the Gly-L-His studies cobalt concentrations of 2.5-10 mM were employed for each ratio of ligand to metal ion. All experiments were performed throughout at room temperature, near 25°.Efforts were made to account quantitatively for dipeptides as Co(III) complexes by utilizing Co(CH3COO)2-4H20 as the Co(II) starting material with twice as much dipeptide. Results of the comparisons of areas under the methyl peaks of dipeptides with that due to a known amount of acetate are not completely satisfactory but do appear to indicate some loss of dipeptide. For instance, in the first three solutions of Table I only 75-90% of the alanyl methyl groups are accounted for as Co(III) products yet no other peaks appear in the well-defined pmr spectra. Lower percentages were obtained by this method for Gly-L-Ala. Lower percentages are accompanied by a reduced molar absorptivity at 520 nm. In comparing CD results of dipeptide Co(III) complexes, the magnitudes should probably be scaled to the same e value at 520 nm.

Reactions of auranofin ((1-thio-.beta.-D-glucopyranose 2,3,4,6-tetraacetato-S)(triethylphosphine)gold(I)) in aqueous hydrochloric acid

“…relatively small activation energies and substantial negative entropies), suggesting that similar mechanisms are operating in both solvent media. It should be noted that AEa = 5 kJ/mol for reaction 1 in water, which indicates a very small dependence of £, upon temperature, in agreement with the similarity in the equilibrium constants determined spectrophotometrically for this reaction at ambient temperature and at 37 °C in a separate experiment. We calculate from the kinetic results that Aj = 4.5 X "4 M"1 at 37 °C and µ = 1.0 as compared to the value of 4.6 X 10"4 M-1 obtained at the same temperature and ionic strength using spectrophotometric techniques (vide supra).…”

Electronic spectra of metal-dioxygen complexes

“…In CD spectra, this feature is quite clearly observed with a rotation that is opposite to that of the lower energy band. 31 The peak intensities in the doubly bridged systems appear always to be lower, and sometimes much lower, than observed with the corresponding LMCT bands in singly bridged systems. This may be associated with the increasing distortion (twisting) of the ring.…”

“…The two transitions are magnetic-dipole-allowed; consistent with theory, intense CD absorption is observed with shoulders being evident. However, the two main components are not resolved.11, 31 No other absorption bands attributable to transitions involving dioxygen are expected nor observed, in the conventional region, because of the closed-shell nature of the coordinated peroxo group.…”

Molecular Structure of the Bis[tris(2‐aminoethyl)aminecyano]dicobalt(III) Ion and its UV/VIS Spectrum

“…By torsion of the Co-0 bonds around the 0-0 axis (Fig.4) the distinction between z ; and z t is lost, both become strongly bonding and can give rise to a n*-+d,* LMCT transition of similar intensity. Spectra of type ii have also been reported of (his),CoOOCo(his), [27] and (dap),CoOOCo(dap), [2] (dap = 2,3-diaminopropionic acid). However, in none of these cases, a structure determination has been carried out.…”

“…Type ii spectrum is exhibited whenever the coordination spheres containing the amines dien or tren are completed with negatively charged ligands such as oxalate, HPO:-, CN-or NO;. A similar absorption is observed on oxygenation of Co(his), [27j and Co(dap), [2] leading to singly bridged p-peroxo-…”