Highly selective binding of nitric oxide by CoIII and FeIII complexes

Abstract

In order to construct compounds with highly selective binding activity for NO, two Co^III and two Fe^III complexes with square-planar N₂O₂-type donor sets, N-[2-(2-hydroxybenzylamino)ethyl]-2-hydroxybenzamide (H₃L1) and 1,2-bis(2-hydroxybenzoylamino)ethane (H₄L2), [Co^III(L1)] (1), Na[Co^III(L2)] (2), [Fe^III(L1)] (3), and (PPh₄)[Fe^III(L2)] (4), were designed and synthesized. These compounds were characterized by electronic absorption, FT-IR, ¹H-NMR spectroscopies, ESI-mass spectrometry, and elemental analyses. The redox potentials of the Co^III and Fe^III complexes with L1, 1 and 3, have quasi-reversible waves at −0.51 and −0.49 V, respectively, and those with L2, 2 and 4, afforded reversible and irreversible waves at −0.96 and −1.04 V, respectively. Interestingly, all complexes quickly react with NO under an Ar atmosphere to form nitrosyl complexes, as monitored by UV-vis spectroscopy. The formation of nitrosyl complexes was confirmed by the appearance of the N–O stretching vibration at about 1650 cm⁻¹; 1649 for 1, 1651 for 2, 1648 for 3, and 1650 cm⁻¹ for 4. The reactivity of each of these complexes with other small molecules such as NO₂⁻, NO₃⁻, CO, and O₂ was also studied. None of the complexes react with CO and O₂. Co^III complexes 1 and 2 react with NO₂⁻, while Fe^III complexes, 3 and 4, do not react with small amounts of NO₂⁻. Complex 3 reacts with NO₂⁻ at concentrations above 100 equiv. of NO₂⁻. We succeeded in preparing complexes with highly selective reactivity for NO.