Synthesis, Crystal Structures, and Magnetic Properties of a Series of Linear Pentanickel(II) Complexes: [Ni5(μ5-tpda)4X2] (X = Cl-, CN-, N3-, NCS-) and [Ni5(μ5-tpda)4(CH3CN)2]- (PF6)2 (tpda2- = the Tripyridyldiamido Dianion)

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Abstract
The synthesis, crystal structures, and magnetic properties of linear pentanuclear complexes, [Ni5(μ5-tpda)4X2]n+ [tpdaH2 = tripyridyldiamine, with different axial ligands X = Cl- (1), CN- (2), N3- (3), and NCS- (4) (n = 0) and CH3CN (5) (n = 2)], are reported. All of the [Ni5(tpda)4]2+ moieties are isostructral and involve a Ni5 linear chain unit with all of the ∠Ni−Ni−Ni being nearly 180°, terminated by the two axial ligands. The pentanuclear linear metal chain is helically wrapped by four syn−syn−syn−syn type tpda2- ligands. There are two types of Ni−Ni distances existing in these complexes. The terminal Ni−Ni distances bonded with the axial ligand are longer (2.35−2.40 Å) and affected by various axial ligands. The inner Ni−Ni distances are very short and remain constant (∼2.30 Å). The relationship between terminal Ni−Ni distances and properties of the axial ligands on 1−5 will be discussed. Two terminal Ni(II) ions bonded with the axial ligands are in a square-pyramidal (NiN4X) environment and exhibit long Ni−N bonds (∼2.10 Å) which are consistent with a high-spin Ni(II) configuration. The inner three Ni(II) ions display short Ni−N (∼1.90 Å) bond distances which are consistent with a square-planar (NiN4), diamagnetic arrangement of a low-spin Ni(II) configuration. All compounds exhibit similar magnetic behavior, indicating an antiferromagnetic interaction of two terminal high-spin Ni(II) ions in these complexes. The XANES (X-ray absorption near-edge absorption spectroscopy) spectra also confirm the existence of the high-spin and low-spin Ni(II) ions in these complexes.

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