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 (Na3LR) of the requisite ligand. The complexes have
been shown to be of either monomeric ([FeLR(THF)]) or dimeric
([FeLR]2) nature by a combination of X-ray diffraction, 1H NMR, solution magnetic susceptibility, and cyclic voltammetry
studies. These analytical studies have shown that the monomeric and
dimeric [FeLR] 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 LR, and is in
line with the catalytic requisite of binding of the epoxide substrate
by the iron­(III) center.