A new tetraaza-pyridinophane macrocycle (L1) N-alkylated with two isopropyl and one methyl groups symmetrically disposed has been prepared and its behavior compared with those of the unsubstituted pyridinophane (L3) and the related compound with three methyl groups (L2). The protonation studies show that, first, a proton binds to the central methylated amine group of L1, while, second protonation leads to a reorganization of the protons that are at this stage attached to the lateral isopropylated amines. The X-ray structure of [HL1] agrees with the UV-vis and NMR studies as well as with the results of DFT calculations. The stability of the Cu complexes decreases on increasing the bulkiness of the alkyl substituents of the amine groups. The crystal structures of [CuL1Cl](ClO) and [CuL1(HO)](ClO)·HO show square pyramidal coordination geometries with the ligands disposed in a bent L-shaped conformation. Kinetic studies indicate that the rates of both complexation and ligand dissociation decrease with the bulkiness of the substituents, so that the stability changes are surely the results of compensating effects, complex formation dominating over complex dissociation. The pH dependence of the rate constants for complex formation cannot be explained by consideration of rapid pre-equilibria involving the different protonated forms of the ligand, and it has been interpreted in terms of a mechanism involving an acid-base equilibrium for a reaction intermediate. NBT SOD studies show that the Cu complex of the bulkiest L1 ligand is the one having the highest activity (IC = 0.26(5) μM, k = 13.7 × 10 M s) which can be associated with the poorer σ-donor ability of the tertiary amino groups, and the rigidity of the system, caused by the bulky isopropyl groups.
A series of iron(IV) oxo complexes, which differ in the donor (CH 2 py or CH 2 COO − ) cis to the oxo group, three with hemilabile pendant donor/second coordination sphere base/acid arms (pyH/py or ROH), have been prepared in water at pH 2 and 7. The ν FeO values of 832 ± 2 cm −1 indicate similar Fe IV O bond strengths; however, different reactivities toward C−H substrates in water are observed. HAT occurs at rates that differ by 1 order of magnitude with nonclassical KIEs (k H /k D = 30−66) consistent with hydrogen atom tunneling. Higher KIEs correlate with faster reaction rates as well as a greater thermodynamic stability of the iron(III) resting states. A doubling in rate from pH 7 to pH 2 for substrate C−H oxidation by the most potent complex, that with a cis-carboxylate donor, [Fe IV O(Htpena)] 2+ , is observed. Supramolecular assistance by the first and second coordination spheres in activating the substrate is proposed. The lifetime of this complex in the absence of a C−H substrate is the shortest (at pH 2, 3 h vs up to 1.3 days for the most stable complex), implying that slow water oxidation is a competing background reaction. The iron(IV)O complex bearing an alcohol moiety in the second coordination sphere displays significantly shorter lifetimes due to a competing selective intramolecular oxidation of the ligand.
ABTS (2,2′-azinobis(3-ethylbenzothiazoline)-6-sulfonic
acid) oxidation to form its radical cation in the presence of H2O2 is frequently used as a test for determining
the peroxidase activity of enzyme mimics. Detailed studies using salen-type
Mn(III) complexes show that photochemical processes involving H2O2, ABTS, and the complex itself can lead to erroneous
results. The capability of the complexes to act as •OH scavengers can be also relevant when the mechanism of their biological
activity is considered.
The synthesis, acid-base behavior and Pb coordination chemistry of the new aza-scorpiand like ligand 5-[2-(N-2-fluorenyl)ethylamino]-2,5,8-triaza[9]-2,6-pyridinophane (L1) have been studied by potentiometry, NMR and spectrofluorimetric titrations, and the results are compared with those obtained for the related compounds L2, lacking the fluorenyl group, and L3, the macrocycle lacking the pendant arm. The crystal structures obtained for complexes [PbL1][PbL1Cl](NO)Cl·4HO (1) and [PbL3](ClO) (2) reveal that the metal ion is located over the plane defined by the nitrogen atoms of the macrocyclic core due to its inability to accommodate the large Pb ion in the macrocyclic cavity. For L1, the secondary amino group of the pendant arm is implicated in the coordination of the metal ion, although the stereoactive lone pair of Pb prevents the closed conformation associated with the coordination of metal ions in aza-scorpiand derivatives. The kinetics of the acid-promoted dissociation of the ligand from the Pb complexes with the three ligands have been studied using stopped-flow with simultaneous absorbance and fluorescence detection. The results indicate that in spite of their similarity, the dissociation of the metal ion occurs with very different rates in the three complexes. During the course of the kinetic studies evidence was obtained for the occurrence of a photochemical process that leads to ligand degradation with the unexpected elimination of one CHCH fragment from the macrocyclic core.
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