Structural and spectroscopic evidence for linkage isomerism of bound nitrite in a {Fe–NO}6 nitrosyl derived from a tetradentate dicarboxamide ligand: More parallels between heme and non-heme systems

The ambidentate ligand nitrite (NO2) binds to transition metal centers through the N (nitro) or O (nitrito) atom. In metal porphyrin complexes, the energy difference between the two linkage isomers is small and hence slight differences in reaction conditions and/or ligand design give rise to formation of the isomers in different ratios. In the present work, similar behavior has been observed in case of the {Fe–NO}6 nitrosyl [(Me2bpb)Fe(NO)(NO2)] (2), derived from a non-heme planar dicarboxamide ligand N,N′-bispyridinecarboxamido-4,5-dimethylbenzenediamine (H2Me2bpb). Under anaerobic conditions, reaction of the Fe(III) complex [(Me2bpb)Fe(py)2]ClO4 (1) with NO(g) in MeCN affords 2, a product that contains both the N- and O-bound isomer in different ratios depending on the reaction conditions. In protic solvents, the same reaction affords the {Fe–NO}7 nitrosyl [(Me2bpb)Fe(NO)] (3). Both nitrosyls have been characterized by infrared spectroscopy and X-ray diffraction studies.