Iron coordination chemistry of N-(bis(2-pyridyl)methyl)pyridine-2-carboxamide

The amidate function participates in the coordination chemistry of iron containing biomolecules such as the anti-tumor drug bleomycin and the enzyme nitrile hydratase. Our interest in amidate coordination prompted an investigation of the iron complexes of the potentially tetradentate ligand N-(bis(2-pyridyl)methyl)pyridine-2-carboxamide (HL). A number of complexes have been isolated and structurally characterized, including [FeII(L)2] (1), [FeIII(LOCH3)Br2(CH3OH)] (3), [FeIII 2(μ-OH)2(LOCH3)2Br2] (4), and [FeIII 4(μ-OCH3)2(LO)2Br6] (5). In these complexes L acts as a meridional tridentate ligand, as previously observed for the corresponding [Cu(L)Cl(CH3OH)] complex (Inorg. Chem. 39 (2000) 5326). In the cases of 3 and 4, the hydrogen of the tertiary carbon has been replaced by a methoxy group in the course of complex synthesis. In the case of 5, the tertiary hydrogen is replaced by hydroxide, and this oxygen and the dangling pyridine act as a bidentate ligand to a second iron ion. When the reaction of FeBr3 and HL was carried out in acetonitrile in the presence of base but in the absence of air, the ligand was cleaved into two pieces, affording [FeIIBr2(pyridine-2-carboxamide)(di-2-pyridylketone)] (6). It is proposed that the coordination of the amide nitrogen of HL to an iron(III) center as an amidate activates the α-CH bond and results in the oxidation of the α-CNamide bond to an imine.