Kristin Krämer, Markus Schmitz, Harald Kelm, Christoph Van Wüllen, Hans-Jörg Krüger

• In the tetraazamacrocyclic ligand N,N’-dimethyl-2,11-diaza-[3.3](2,6)pyridinophane (L-N4Me2), the two pyridine units are separated from each other by sp3-hybridized triatomic bridges. Such electronically isolated pyridine moieties are considerably less prone to reductions than di- or triimines. A detailed structural, magnetic, and spectroscopic investigation of the complexes [Cr(L-N4Me2)(OAc)2] and [Cr(L-N4Me2)(OAc)2](PF6), in combination with theoretical calculations, reveals that the reduced complex must be described as a chromium(III) ion coordinated to the anionic radical ligand (L-N4Me2)⋅− rather than a low-spin chromium(II) ion bound to closed-shell ligands. Thus, it is, to the best of our knowledge, only the second example of a stable and structurally characterized metal complex containing a reduced isolated pyridine unit. The stability is attributed to the delocalization of the unpaired electron across the two pyridine units, mediated by their interaction to the metal ion.