Iron(III) Chelation: Tuning of the pH Dependence by Mixed Ligands

Abstract

The iron(III) chelating properties of two heteropodands with 8-hydroxyquinoline and catechol binding groups were examined and compared to those of the corresponding homopodal analogues, O-TRENSOX and TRENCAMS. Like the parent homopodands, the two heteropodands are based on the TREN scaffold and the chelating units are connected by amide groups, TRENSOX2CAMS having two 8-hydroxyquinoline and one catechol arms and TRENSOXCAMS2 one 8-hydroxyquinoline and two catechol moieties. The aqueous coordination chemistry of these ligands was examined by po-

Iron(III) Exchange Process between Hexadentate Tripodal Ligands: Models for the Ternary Complexes

“…The UV/Vis spectra were recorded as a function of the time and at the end of the kinetics. The structure of the final complex was supported by its absorption spectrum on the basis of our previous studies, particularly for the mixed coordination around Fe III [9] related to one catechol and two 8-hydroxyquinoline groups as in FeL 1 or two catechol and one 8-hydroxyquinoline groups as in FeL 2 (see the λ max and ε values in the Experimental Section).…”

“…For the exchanges FeL 0 /L 1 and FeL 0 /L 2 , the nature of the entering group was clearly identified as a catechol group on the basis of the spectrum change. In a previous paper, [9] we determined the UV/Vis parameters for the ferric complexes with a mixed catechol/8-hydroxyquinoline coordina-tion sphere. In addition, the values of k 1 Table 2 summarizes the values of k 1 for the three series of iron exchange collected according to the nature of the chelating group that enters in the iron coordination sphere.…”

“…This is clearly evidenced for the iron exchange between FeL 3 and L 0 since the formation of an intermediate ternary complex having a mixed coordination 8-hydroxyquinolinate/catecholate could be identified from the UV/Vis spectra as previously published. [9] A probable mechanism is depicted in Figure 3. The same process is assumed to occur for the exchange between FeL 0 and L 3 , but the coordination of the second and the third catechol groups is fast relative to the first one.…”

“…Indeed, a negative deprotonated hydroxy is assumed to be more reactive than the protonated one. For the catechol groups in L 1 , L 2 , and L 3 the lower pK a is 5.62, [9] 5.55, [9] and 5.57, [8] respectively, indicating that the hydroxy group of one arm is deprotonated at pH 7.4, whereas for the 8-hydroxyquinoline groups the lower pK a is 7.44 [6] for L 0 and 6.68 [7] for L Ј0 . For the sake of comparison, the numbers of deprotonated OH groups in the catechol and 8-hydroxyquinoline moieties are reported in Table 2.…”

“…[5] Using tripodal ligands as tools and starting from the hypothesis of a step-by-step ("arm-by-arm") exchange, we investigated the kinetics of the iron removal from the ferric complexes of the tripodal tris(hydroxyquinoline) ligands O-TRENSOX (L 0 ), [6] and LЈ 0 , [7] where L 0 is anchored to a tertiary amine and LЈ 0 to a tertiary carbon (Figure 1), and a tris(catechol) analogue built from a tertiary amine [8] TRENCAMS (L 3 ). Two hexadentate heterotripodal ligands were also examined, TRENSOX2CAMS (L 1 ) with two hydroxyquinoline units and one catechol [9] unit and TREN-SOXCAMS2 (L 2 ) with two catechol moieties and one hydroxyquinoline [9] building block ( Figure 1). Our aim was to gain insight into the step-by-step ligand exchange processes via kinetic intermediates called "ternary complexes".…”