Bivalent and Trivalent Iron Complexes of Acyclic Hexadentate Ligands Providing Pyridyl/Pyrazine−Amide−Thioether Coordination

Acyclic pyridine-2-carboxamide- and thioether-containing hexadentate ligand 1,4-bis[o-(pyridine-2-carboxamidophenyl)]-1,4-dithiobutane (H2bpctb), in its deprotonated form, has afforded purple low-spin (S = 0) iron(II) complex [Fe(bpctb)] (1). A new ligand, the pyrazine derivative of H2bpctb, 1,4-bis[o-(pyrazine-2-carboxamidophenyl)]-1,4-dithiobutane (H2bpzctb), has been synthesized which has furnished the isolation of purple iron(II) complex [Fe(bpzctb)]·CH2Cl2 (4) (S = 0). Chemical oxidation of 1 by [(η5-C5H5)2Fe][PF6] or [Ce(NO3)6][NH4]2 led to the isolation of
low-spin (S = 1/2) green Fe(III) complexes [Fe(bpctb)][PF6] (2) or [Fe(bpctb)][NO3]·H2O (3), and oxidation of 4 by
[Ce(NO3)6][NH4]2 afforded [Fe(bpzctb)][NO3]·H2O (5) (S = 1/2). X-ray crystal structures of 1 and 4 revealed that (i)
in each case the ligand coordinates in a hexadentate mode and (ii) bpzctb2- binds more strongly than bpctb(2-),
affording distorted octahedral MIIN2(pyridine/pyrazine)N‘2(amide)S2(thioether) coordination. To the best of our
knowledge, 1 and 4 are the first examples of six-coordinate low-spin Fe(II) complexes of deprotonated pyridine/pyrazine amide ligands having appended thioether functionality. The Fe(III) complexes display rhombic EPR spectra.
Each complex exhibits in CH2Cl2/MeCN a reversible to quasireversible cyclic voltammetric response, corresponding
to the FeIII−FeII redox process. The E1/2 value of 4 is more anodic by ∼0.2 V than that of 1, attesting that compared
to pyridine, pyrazine is a better stabilizer of iron(II). Moreover, the E1/2 value of 1 is significantly higher (∼1.5 V)
than that reported for six-coordinate Fe(II)/Fe(III) complexes of the tridentate pyridine-2-carboxamide ligand
incorporating thiolate donor site.