Reactivity control in iron(III) amino triphenolate complexes: comparison of monomeric and dimeric complexes

Iron(III) amino triphenolate complexes with different substituents in the ortho-position of the phenolate moiety (R = H, Me, tBu, or Ph) have been synthesized by the reaction of iron(III) chloride and the sodium salt (Na(3)L(R)) of the requisite ligand. The complexes have been shown to be of either monomeric ([FeL(R)(THF)]) or dimeric ([FeL(R)](2)) nature by a combination of X-ray diffraction, (1)H NMR, solution magnetic susceptibility, and cyclic voltammetry studies. These analytical studies have shown that the monomeric and dimeric [FeL(R)] complexes behave distinctively, and that the dimer stability is a function of the ortho-positioned groups. Both the dimeric as well as monomeric complexes were tested as catalysts for the catalytic cycloaddition of carbon dioxide to oxiranes, and the data show that the monomeric complexes are able to mediate this conversion with significantly higher activities than the dimeric complexes. This difference in reactivity is controlled by the substitution pattern on the ligand L(R), and is in line with the catalytic requisite of binding of the epoxide substrate by the iron(III) center.