Highly Stabilized Low-Spin Iron(III) and Cobalt(III) Complexes of a Tridentate Bis-Amide Ligand 2,6-Bis(N-phenylcarbamoyl)pyridine. Novel Nonmacrocyclic Tetraamido-N Coordination and Two Unusually Short Metal−Pyridine Bonds

Two novel low-spin complexes, [Et4N][FeL2]·1.5H2O (1) and [Et4N][CoL2]·2H2O (2) (L is a deprotonated bis-amide ligand), have been synthesized and characterized. Four relatively longer equatorial M−Namide bonds and two significantly shorter axial M−Npy bonds are the noteworthy features of their X-ray structures. EPR and (34−300 K) magnetic studies of 1 confirm its low−spin character. Cyclic voltammetric studies reveal a highly stabilized M(III) state. For 1 a linear correlation between the Fe(III)−Fe(II) reduction potentials and the reciprocal of solvent dielectric constants is obtained.