Stable host–guest complexes of bis-2,6-pyridinedicarboxylate iron(III) with dihydroxybenzenes

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The high reactivity of iron(III) complexes towards phenolic compounds, such as catechol, is well known. We show here that concealing the iron(III) site by ligands helps to obtain stable host–guest iron(III) complexes with phenolic compounds. Host–guest complexes of Htriam[Fe(2,6-pdc)2] with catechol and resorcinol (where 2,6-pdc=2,6-pyridinedicarboxylate and Htriam is mono-protonated 6-phenyl-1,3,5-triazine-2,4-diamine) were prepared and characterized. The observed color for the inclusion complex of Htriam[Fe(2,6-pdc)2] with catechol in the solid state has a distinction from the colored species generated in solution by interactions between the same components. The solid state structures of these two host–guest complexes have differences in the CO distances of dihydroxybenzene ligands as well as the arrangements of π-stacks; in the case of resorcinol, phenolic character is retained. In the catechol host–guest complex the CO distances in catechol are 1.310(7) and 1.370(6)Å, whereas in the resorcinol host–guest complex similar CO distances are 1.368(4) and 1.374(3)Å. The dissimilarity in the CO bond distances of catechol in the host–guest complex suggests charge-transfer through a semi-quinone cation radical type interactions with the iron ion showing an absorption at 570nm for the solid sample. A prior activation of catechol in the second coordination sphere through a ligand–catechol interaction is depicted.