Unexpected Reduction of a Coordinated Diazapyridinophane Ligand Bound to Chromium(III) Ion Leading to Delocalization of the Unpaired Electron across Two Isolated Pyridine Units

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. Pyridine moieties, which are electronically isolated from each other by saturated atom chains, can be potentially redox‐active in complexes. Thus, structural and spectroscopic methods in combination with theoretical calculations provide unequivocal evidence that the coordinated tetraazamacrocyclic ligand L‐N4Me2 in the stable neutral bis(acetato)chromium complex indeed contains anionic pyridine radical units coordinated to a chromium(III) ion.