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-[(ML20)-L-II](PF6)(2) containing the tridentate neutral ligand 2,6-bis[1 -(4-methoxyphenylimino)ethyl]pyridine (L-0) 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 [(ML20)-L-III](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 [(MnL21)-L-III]PF6 and [(CoL20)-L-I]PF6 where (L-1)(1-) represents the one-electron-reduced radical form of LO. 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 L-0 or, alternatively, as monoanionic radical (L-1)(1-). All trications are of the type [(ML20)-L-III](3+), and the dications are [(ML20)-L-II](2+). The monocations are described as [Mn-L(2)III(1)](+); (S = 0), [(FeLL1)-L-II-L-0](+) (S = 1/2), [(COL20)-L-I](+) (S = 1), [(NiL20)-L-I](2+) (S = 1/2), [(CuL20)-L-I](+) (S = 0), [(ZnLL0)-L-II-L-1](+) (S = 1/2) where the Mn-III and Fe-II ions are low-spin-configurated. The neutral species are described as [(MnL21)-L-II](0), [(FeL21)-L-II](0), [(CoLL1)-L-I-L-0](0), [(NiLL1)-L-I-L-0](0), and [(ZnL21)-L-II](0); their electronic ground states have not been determined.