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 (H(2)bpctb), in its deprotonated form, has afforded purple low-spin (S = 0) iron(II) complex [Fe(bpctb)] (1). A new ligand, the pyrazine derivative of H(2)bpctb, 1,4-bis[o-(pyrazine-2-carboxamidophenyl)]-1,4-dithiobutane (H(2)bpzctb), has been synthesized which has furnished the isolation of purple iron(II) complex [Fe(bpzctb)].CH(2)Cl(2) (4) (S = 0). Chemical oxidation of 1 by [(eta(5)-C(5)H(5))(2)Fe][PF(6)] or [Ce(NO(3))(6)][NH(4)](2) led to the isolation of low-spin (S = 1/2) green Fe(III) complexes [Fe(bpctb)][PF(6)] (2) or [Fe(bpctb)][NO(3)].H(2)O (3), and oxidation of 4 by [Ce(NO(3))(6)][NH(4)](2) afforded [Fe(bpzctb)][NO(3)].H(2)O (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) bpzctb(2-) binds more strongly than bpctb(2-), affording distorted octahedral M(II)N(2)(pyridine/pyrazine)N'(2)(amide)S(2)(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 CH(2)Cl(2)/MeCN a reversible to quasireversible cyclic voltammetric response, corresponding to the Fe(III)-Fe(II) redox process. The E(1/2) value of 4 is more anodic by approximately 0.2 V than that of 1, attesting that compared to pyridine, pyrazine is a better stabilizer of iron(II). Moreover, the E(1/2) value of 1 is significantly higher (approximately 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.