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

Abstract

AbstractIn 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.