Highly selective binding of nitric oxide by Co(III) and Fe(III) complexes

In order to construct compounds with highly selective binding activity for NO, two Co(III) and two Fe(III) complexes with square-planar N(2)O(2)-type donor sets, N-[2-(2-hydroxybenzylamino)ethyl]-2-hydroxybenzamide (H(3)L1) and 1,2-bis(2-hydroxybenzoylamino)ethane (H(4)L2), [Co(III)(L1)] (1), Na[Co(III)(L2)] (2), [Fe(III)(L1)] (3), and (PPh(4))[Fe(III)(L2)] (4), were designed and synthesized. These compounds were characterized by electronic absorption, FT-IR, (1)H-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 NO(2)(-), NO(3)(-), CO, and O(2) was also studied. None of the complexes react with CO and O(2). Co(III) complexes 1 and 2 react with NO2(-), while Fe(III) complexes, 3 and 4, do not react with small amounts of NO(2)(-). Complex 3 reacts with NO(2)(-) at concentrations above 100 equiv. of NO(2)(-). We succeeded in preparing complexes with highly selective reactivity for NO.