Molecular and electronic structures of bis(pyridine-2,6-diimine)metal complexes [ML2](PF6)n (n = 0, 1, 2, 3; M = Mn, Fe, Co, Ni, Cu, Zn)

Abstract

A series of mononuclear, octahedral first-row transition metal ion complexes mer-[M(II)L0(2)](PF6)2 containing the tridentate neutral ligand 2,6-bis[1-(4-methoxyphenylimino)ethyl]pyridine (L0) and a Mn(II), Fe(II), Co(II), Ni(II), Cu(II), or Zn(II) ion have been synthesized and characterized by X-ray crystallography. Cyclic voltammetry and controlled potential coulometry show that each dication (except those of Cu(II) and Zn(II)) can be reversibly one-electron-oxidized, yielding the respective trications [M(III)L0(2)]3+, and in addition, they can be reversibly reduced to the corresponding monocations [ML2]+ and the neutral species [ML2]0 by two successive one-electron processes. [MnL2]PF6 and [CoL2]PF6 have been isolated and characterized by X-ray crystallography; their electronic structures are described as [Mn(III)L1(2)]PF6 and [Co(I)L0(2)]PF6 where (L1)1- represents the one-electron-reduced radical form of L0. The electronic structures of the tri-, di-, and monocations and of the neutral species have been elucidated in detail by a combination of spectroscopies: UV-vis, NMR, X-band EPR, Mossbauer, temperature-dependent magnetochemistry. It is shown that pyridine-2,6-diimine ligands are noninnocent ligands that can be coordinated to transition metal ions as neutral L0 or, alternatively, as monoanionic radical (L1)1-. All trications are of the type [M(III)L0(2)]3+, and the dications are [M(II)L0(2)]2+. The monocations are described as [Mn(III)L1(2)]+ (S = 0), [Fe(II)L0L1]+ (S = 1/2), [Co(I)L0(2)]+ (S = 1), [Ni(I)L0(2)]+ (S = 1/2), [Cu(I)L0(2)]+ (S = 0), [Zn(II)L1L0]+ (S = 1/2) where the Mn(II) and Fe(II) ions are low-spin-configurated. The neutral species are described as [Mn(II)L1(2)]0, [Fe(II)L1(2)]0, [Co(I)L0L1]0, [Ni(I)L0L1]0, and [Zn(II)L1(2)]0; their electronic ground states have not been determined.