Facile syntheses and characterization of pentaamminechromium(III) complexes with neutral O- and N-bound ligands

Isomeric Distribution and Catalyzed Isomerization of Cobalt(III) Complexes with Pentadentate Macrocyclic Ligands. Importance of Hydrogen Bonding

We have investigated the isomeric distribution and rearrangement of complexes of the type [CoXLn]2+,3+ (where X = Cl-, OH-, H2O, and Ln represents a pentadentate 13-, 14-, and 15-membered tetra-aza or diaza-dithia (N4 or N2S2) macrocycle bearing a pendant primary amine). The preparative procedures for chloro complexes produced almost exclusively kinetically preferred cis isomers (where the pendant primary amine is cis to the chloro ligand) that can be separated by careful cation-exchange chromatography. For L13 and L14 the so-called cis-V isomer is isolated as the kinetic product, and for L15 the cis-VI form (an N-based diastereomer) is the preferred, while for the L14(S) complex both cis-V and trans-I forms are obtained. All these complexes rearrange to form stable trans isomers in which the pendent primary amine is trans to the monodentate aqua or hydroxo ligand, depending on pH and the workup procedure. In total 11 different complexes have been studied. From these, two different trans isomers of [CoClL14(S)]2+ have been characterized crystallographically for the first time in addition to a new structure of cis-V-[CoClL14(S)]2+; all were isolated as their chloride perchlorate salts. Two additional isomers have been identified and characterized by NMR as reaction intermediates. The remaining seven forms correspond to the complexes already known, produced in preparative procedures. The kinetic, thermal, and baric activation parameters for all the isomerization reactions have been determined and involve large activation enthalpies and positive volumes of activation. Activation entropies indicate a very important degree of hydrogen bonding in the reactivity of the complexes, confirmed by density functional theory studies on the stability of the different isomeric forms. The isomerization processes are not simple and even some unstable intermediates have been detected and characterized as part of the above-mentioned 11 forms of the complexes. A common reaction mechanism for the isomerization reactions has been proposed for all the complexes derived from the observed kinetic and solution behavior.

Metal ion-biomolecule interactions. Part 22.1 Cr(III)-catalyzed isotopic hydrogen exchange in methylimidazoles

The detritiation kinetics of the Cr(III) complexes 1 and 3-6 have been studied in aqueous buffers at 35°C, using the liquid scintillation counting technique. Results for 5 and 6 provide benchmark kinetic data for N-T/N-H exchange under the experimental conditions of the study and have aided in the delineation of N-T/N-H from C-T/C-H exchange in the parallel reactions observed experimentally. Curved first-order plots obtained for 1, 3, and 4 were treated to yield rate constants for two parallel exchange reactions kobsA  and kobsB  for the "fast" and "slow" processes, respectively. The "fast" process has been assigned to C(2)-H exchange in 1, competing N-H and C(2)-H exchange in 3, and N-H exchange in 4. In all cases, the "slow" process is associated with C(4,5)-H exchange. Identification of exchange sites in 1, 3, and 4 was made possible by the consideration of the results of an IR spectroscopic study of hydrogen-deuterium exchange, comparison of the extent of 3H incorporatation in different complexes in the tritiation experiments, and a careful analysis of the exchange kinetic data. Analysis of the rate data indicates that Cr(III) significantly enhances C(2)-H exchange in 1 and 3, while C(4,5)-H exchange, hitherto reported in the literature only under drastic reaction conditions, was observed for 1, 3, and 4 under the mild conditions of the present study. Quantitation of the effect of Cr(III) coordination on 3H exchange in imidazole-type nuclei was achieved fully in 1; giving kM+ values of 6 × 103 and 7 × 102 M-1s-1 for C(2)-H and C(4,5)-H exchange, respectively. Using the literature value for kH+, the second-order rate constant for C(2)-H exchange under H+ catalysis, 2.9 × 102 M-1s-1, it follows that Cr(III) is ca. 20 times better as a catalyst for C(2)-H exchange in 1-methylimidazole than H+, providing the first example of a metal ion being more effective than a proton in these processes. Comparison of the results obtained with 1 with literature results for 2 shows a very large (ca. 3 × 105-fold) difference in the catalytic activities of Cr(III) and Co(III), favouring the former. The dichotomy in the effectiveness of the two metal ions in catalyzing 3H exchange in the imidazole nucleus has been ascribed to differences in (i) extent of Mn+—N bond polarization (and the consequent effect on ligand C-H acidity); (ii) electronic configuration; (iii) crystal field stabilization and activation energies; and (iv) importance of metal-ligand π back-bonding. The study highlights the diversity of factors and complexity of interactions involved in determining the role of metal ions in biological systems, especially where such processes involve complex formation between metal ions and heterocyclic fragments of biomolecules.Key words: metal ion-biomolecule interactions, methylimidazole, isotopic hydrogen exchange, catalysis by chromium.

(Pyridine)pentaamminechromium(III). Synthesis, Characterization, and Photochemistry

The Cr(NH(3))(5)(py)(3+) ion has been obtained by metathesis of Cr(NH(3))(5)(Me(2)SO)(3+) in pyridine, isolated as the perchlorate salt, and characterized by absorption (lambda(max) at 467, 352, and 260 nm) and emission spectra (lambda(max) at 668 nm, tau = 2.0 &mgr;s at 20 degrees C in water) and by the py aquation rate (k = 5 x 10(-)(4) s(-)(1) at 80 degrees C). Ligand-field (LF) band irradiation in acid aqueous solution (10(-)(2) M HClO(4)) induces photoaquation of py (Phi = 0.26) and NH(3) (Phi = 0.16). HPLC indicates that the latter reaction gives rise to both cis- and trans-Cr(NH(3))(4)(py)(H(2)O)(3+), with the predominance of the cis isomer. This is the first Cr(NH(3))(5)X(z+)() species where Phi(x) > Phi(NH)3: the result is compared with the predictions of various photolysis models and is taken as chemical evidence for pi-acceptance by the py ligand. The photostereochemistry is also discussed. The phosphorescence is totally quenched by Cr(C(2)O(4))(3)(3)(-) (k(q) = 2.7 x 10(9) M(-)(1) s(-)(1)), while the photoreactions are only in part. On 470-nm excitation, the Phi(py)/Phi(NH)()3 ratio is approximately 1 and approximately 2 for the unquenchable and the quenchable contributions, respectively. Such a difference, suggesting at least two reactive precursors, can be interpreted in terms of the photochemistry proceeding from either the lowest doublet and quartet excited states or, alternatively, from the (4)E and (4)B(2) states. Irradiation of the very distinct absorption of coordinated pyridine results in both doublet-state emission and loss of py and NH(3). Comparison of this photobehavior with the LF results gives an efficiency of 0.7 for conversion of the py-localized pipi states into the Cr-localized LF states, confirmed by the wavelength dependence of the relative emission yields. Some py release (Phi = 0.03) is concluded to originate in the pipi states.