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)

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-bis1-(4-methoxyphenylimino)ethylpyridine (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 ML20 by two successive one-electron processes. MnL2PF6 and CoL2PF6 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)L0L10, Ni(I)L0L10, and Zn(II)L1(2)0; their electronic ground states have not been determined.