Synthesis and characterization of pyridine amide cation radical complexes of iron: stabilization due to coordination with low-spin iron(III) center

Abstract

We reported the synthesis and characterization of peptide complexes of low-spin iron(III) [Fe(bpb)(py)₂][ClO₄] (1) and Na[Fe(bpb)(CN)₂] (2) [H₂bpb = 1,2-bis(pyridine-2-carboxamido)benzene; py = pyridine], where iron is coordinated to four nitrogen donors in the equatorial plane with two amide nitrogen anions and two pyridine nitrogen donors (Ray, M.; Mukherjee, R.; Richardson, J. F.; Buchanan, R. M. J. Chem. Soc., Dalton Trans. 1993, 2451). Chemical oxidation of 2 and a new low-spin iron(III) complex Na[Fe(Me₆bpb)(CN)₂]·H₂O (4) [synthesized from a new iron(III) complex [Fe(Me₆bpb)(py)₂][ClO₄] (3) (S =½)] [H₂Me₆bpb = 1,2-bis(3,5-dimethylpyridine-2-carboxamido)-4,5-dimethylbenzene) by (NH₄)₂Ce(NO₃)₆ afforded isolation of two novel complexes [Fe(bpb)(CN)₂] (5) and [Fe(Me₆bpb)(CN)₂]·H₂O (6). All the complexes have been characterized by physicochemical techniques. While 1-4 are brown/green, 5 and 6 are violet/bluish violet. The collective evidence from infrared, electronic, Mossbauer, and ¹H NMR spectroscopies, from temperature-dependent magnetic susceptibility data, and from cyclic voltammetric studies provides unambiguous evidence that 5 and 6 are low-spin iron(III) ligand cation radical complexes rather than iron(IV) complexes. Cyclic voltammetric studies on isolated oxidized complexes 5 and 6 display identical behavior (a metal-centered reduction and a ligand-centered oxidation) to that observed for complexes 2 and 4, respectively. The Mossbauer data for 6 are almost identical with those of the parent compound 4, providing compelling evidence that oxidation has occurred at the ligand in a site remote from the iron atom. Strong antiferromagnetic coupling (|-2J| ≥ 450 cm^-1) of the S = ½ iron atom with the S = ½ ligand π-cation radical leads to an effectively S = 0 ground state of 5 and 6. The oxidized complexes display ¹H NMR spectra (in CDCl₃ solution), characteristic of diamagnetic species.