Steric effects on the stability and reactivity of model complexes of non-heme diiron proteins and ferritin.

摘要

The effects of bulky ligands on the stability and reactivity of a series of μ-hydroxo bis(carboxylato) bridged diiron(II) complexes were investigated. The complexes [Fe2(μ-OH)(O₂CR)₂(Bn₃tacn) ₂]OTf (O₂CR = p-F-PhCO₂, p-CF₃O-PhCO₂ or AdaCO₂; Bn ₃tacn = 1,4,7-tribenzyl-1,4,7-triazacyclononane) and [Fe₂(μ-OH)(AdaCO ₂)₂(Me₃tacn)₂]OTf (Me₃tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) were synthesized and their X-ray crystal structures reported. NMR spectroscopic studies provide solution characterization, and the results correspond well with their solid state description. NMR results reveal the ligands to be kinetic labile at room temperature. The reactivity of these complexes toward dioxygen at room temperature was monitored over time by electronic absorption spectroscopy. The formation and decay of an absorbance maximum in the range 670–680 nm was observed for each complex. The lifetime (t_max) of the absorbance maxima varies and reflects the bulk properties of the respective complex. Their redox properties were explored using cyclic voltammetry. Results here revealed increasing stability of the mixed valence species with increasing bulk properties. The mixed valence complexes [Fe₂(μ-OH)(AdaCO₂)₂(R₃ tacn)₂]OTf₂ and [Fe₂(μ-O)(AdaCO ₂)₂(R₃tacn)₂]OTf (R = Bn or Me) were synthesized by the chemical oxidation of the parent diiron(II) complexes. The X-ray crystal structures of the MC₃tacn complexes are reported. 1H NMR studies showed these species to be stable in solution.; Bulky ligands were incorporated to study the reactivity of the “paddle-wheel” complexes [M₂(O₂CR)₄(L)₂] (M = Fe or Co; O₂CR = piv, Ph₃C₂O₂ or AdaCO₂; L = py-d5 (pyridine-d5), 2AP (2-aminopyridine) or 26AP (2,6-diaminopyridine)). The X-ray crystal structures of these complexes are reported. 1H/2H NMR studies show fast exchange between the systems. Reaction of the diiron(II) complexes with dioxygen under varying conditions generated new trimeric and tetrameric iron(II,III) aggregates.; Ligands with steric constraint were used to synthesize new tetranuclear iron(II) and heptanuclear iron(II,III) complexes. The X-ray crystal structures and 1H/2H NMR spectra of [Fe₄(μ ₃-OMe)₄(OTf)₃(MeCN)₉]OTf, [Fe ₇(thme)₂(μ-OPh)₆(MeCN)₁₂](OTf) ₃ and [Fe₇(thme)₂(μ-OPh)₆(MeCN) ₁₂](OTf)₃ (cht = cis,cis-cyclohexanetriol; thme = 1,1,1-tris(hydroxymethyl)ethane) are reported.