Meridional vs. facial coordination geometries of a dipodal ligand framework featuring a secondary coordination sphere

The synthesis of a novel dipodal ligand framework, H₂[^MeN(pi^Cy)₂], is summarized. Upon metalation with MCl₂ salts (M = Fe, Cu), the ligand undergoes a conformational change, resulting in the formation of a trigonal bipyramidal metal center with a pseudoplanar, meridionally-bound ligand framework. This tautomerization positions pendant amines in the metal's secondary coordination sphere. Metalation with M(OTf)₂ in coordinating solvent yields octahedral metal complexes, where two solvent molecules bind in the apical positions with one outer sphere counter ion. Reactivity of these complexes, (^MeN(afa^Cy)₂)M(X)₂ (X = Cl, OTf), with 2,2′-bypyridine results in ligand reorganization, yielding a facial coordination geometry of the dipodal framework. The described complexes have been characterized by ¹H NMR, EPR, IR, Mössbauer and electronic absorption spectroscopies as well as X-ray crystallography.