Route to Metallacrowns: The Mechanism of Formation of a Dinuclear Iron(III)-Salicylhydroxamate Complex

Abstract

The equilibria and the kinetics of the binding of Iron(III) to salicylhydroxamic (SHA) and benzohydroxamic (BHA) acids have been investigated in aqueous solution (I = 1 M (HClO(4)/NaClO(4)), T = 298 K) using spectrophotometric and stopped-flow methods. Whereas Iron(III) forms a 1:1 complex (ML) with BHA, it forms both ML and M(2)L complexes with SHA. The presence of M(2)L in aqueous medium is corroborated by FTIR measurements. The reactive form of Iron(III) is the hydrolyzed species FeOH(2+), which binds to the O,O site in ML and to the O,O and O(P),N (P = phenolate) sites in M(2)L, inducing full deprotonation of the latter. The reaction pathway is discussed in terms of a multistep mechanistic scheme in which the metal-ligand interaction is coupled to hydrolysis and self-aggregation steps of Iron(III). The observation and characterization of M(2)L as a stable species is important because it contains the -Fe-O-N-Fe- sequence, which constitutes the repetitive motif of the SHA-based metallacrown ring and provides the rationale for 12-MC-4 metallacrowns. In the framework of this study, the kinetics of the Iron(III) dimerization and trimerization have also been investigated using the stopped-flow method to perform dilution jumps. The reaction scheme put forward involves two parallel steps (FeOH(2+) + FeOH(2+) and Fe(3+) + FeOH(2+)) that lead to formation of the Fe(2)(OH)(2)(4+) dimer and a slower step (FeOH(2+) + Fe(2)(OH)(2)(4+)) to form the trimer species. The kinetics of the last step have been investigated here for the first time, and the results deduced indicate that, of the two possible trimer structures reported in the literature, Fe(3)(OH)(3)(6+) and Fe(3)(OH)(4)(5+), the latter prevails by far.

The mechanism of the Cu2+[12-MCCu(Alaha)-4] metallacrown formation and lanthanum(iii) encapsulation

“…At the best of our knowledge, the only characterized dinuclear complex of a hydroxamic acid in aqueous solution derives from the complexation of Fe(III) by salicylhydroxamic acid. 34 The formation of a dinuclear complex as a precursor of Cu 2+ [12-MC-4] formation is here confirmed by ITC experiments. It should also be noted that single point energy calculations, that we have performed by using the HyperChem software, have also shown that the transition from the dimer to the dinuclear complex (second step of process (18)) is energetically favoured (ΔE = −6.3 kcal mol −1 ).…”

Poly(amidoxime) from Polymer-Grafted Khaya Cellulose: An Excellent Medium for the Removal of Transition Metal Cations from Aqueous Solution

“…Indeed, in this slow step the breaking of the M-OH2 bond occurs and, therefore, this stage must have many characteristics in common with the solvent exchange process in the metal coordination sphere. The variation is found for the logarithm of KH2O with the number of d electrons in (Beccia et al 2011). In this study, identical hydroxamic acid ligand supported by copolymers is involved in chelation process and also ligand exhibit highest affinity towards d electrons of Cu 2+ ion (Fig.…”

Uncovering the Roles of Oxygen in Cr(III) Photoredox Catalysis

“…Possible explanations for this nonevent are as follows: first, the diffusion rates of large ionic species are expected to be dramatically slower than those of oxygen and redox congeners (e.g., superoxide); separately or in conjunction, cation−cation repulsion may prevent radical cation approach to the photoreduced [Cr] 2+ complex. 65,66 Related, we do not believe that a chromyl species (formed when divalent 1 is exposed to excess oxygen bubbled through solution) is actively involved in the regeneration of the catalyst. We observe spectroscopic evidence for this species only when the divalent complex 1 is exposed to oxygen, not when the trivalent complex 2 is used.…”

“…However, no product is formed without oxygen, so the kinetics of the air-free route must be relatively slow. Possible explanations for this nonevent are as follows: first, the diffusion rates of large ionic species are expected to be dramatically slower than those of oxygen and redox congeners (e.g., superoxide); separately or in conjunction, cation–cation repulsion may prevent radical cation approach to the photoreduced [Cr] 2+ complex. , …”