Bivalent and trivalent iron complexes of acyclic hexadentate ligands providing Pyridyl/Pyrazine-Amide-Thioether coordination

Abstract

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 =½) 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 = ½). 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^IIN₂(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 Fe^III-Fe^II redox process. The E_½ 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 E_½ 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.