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Ghosh M, Weyhermuller T, Wieghardt K: Electronic structure of the members of the electron transfer series [NiL](z) (z = 3+, 2+, 1+, 0) and [NiL (X)](n) (X = Cl, CO, P (OCH (3))(3)) species containing a tetradentate, redox-noninnocent, Schiff base macrocyclic ligand L: an experimental and density functional theoretical study. Dalton Trans. 2010 Feb 28;39(8):1996-2007. Epub 2010 Jan 14.
The electronic structure of the four members of the electron transfer series [NiL](z) (z = 3+, 2+, 1+, 0) have been established experimentally (EPR spectroscopy and X-ray crystallography) and by density functional theoretical (DFT) calculations using the B3LYP functional in conjunction with a conductor-like screening model (COSMO) for acetonitrile solvent effects. L represents a generic designation of the tetradentate macrocycle 2,12-dimethyl-3,7,11,17-tetraazabicyclo [11.3.1]-heptadeca-1 (17),2,11,13,15 -pentane where the true oxidation level is not specified; (L (Ox))(0) represents its neutral form, (L )(1-) is the one-electron reduced pi radical anion, and (L (Red))(2-) is the singlet (or triplet) diradical dianion of this ligand. It is shown that the above series consists of square planar [Ni (III)(L (Ox))](3+) (S = 1/2), [Ni (II)(L (Ox))](2+) (S = 0), [Ni (II)(L )](1+) (S = 1/2), [Ni (II)(L (Red))](0) (S = 0). The structure of [Ni (II)(L (Red))](0) has been determined by X-ray crystallography. The electrochemistry of [Ni (II)(L (Ox))](PF (6))(2) in the presence of hard chloride anions shows the presence of trans-[Ni (III)(L (Ox)) Cl (2)](+), the EPR spectrum of which has been recorded and calculated, and of trans-[Ni (II)(L (Ox)) Cl (2)](0) (S = 1). Upon further reduction the coordinated Cl (-) ligands dissociate and [Ni (II)(L )](1+) and [Ni (II)(L (Red))](0) are successively generated. Similarly, in the presence of good pi-acceptor ligands such as CO or P (OCH (3))(3) the following five-coordinate, square base pyramidal species are found to be stable: [Ni (I)(L (Ox))(X)](1+) (S = 1/2), [Ni (I)(L )(X)](0) (S = 0, 1) (X = CO, P (OCH (3))(3)). As shown by EPR spectroscopy in the work of J. Lewis and M. Schroder, J. Chem. Soc., Dalton Trans., 1982, 1085, the monocations consist of a central nickel (i) ion (d (9), S (Ni) = 1/2). These spectra have been faithfully reproduced by the calculations. The neutral complexes [Ni (I)(L )(X)](0) are singlet or triplet diradicals comprising a central nickel (i) ion and a pi radical anion (L )(1-). Interestingly, six-coordinate species trans-[Ni (L)(X)(2)](n) (n = 2+, 1+, 0) are computationally not stable in the gas phase or in solution. No experimental evidence has been found for their existence. |
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