Nickel (II) complexes incorporating pyridyl, imine and amino chelate ligands: synthesis, structure, isomer preference, structural transformation and reactivity towards nickel (III) derivatives

Abstract

Facile condensation of 2-acetylpyridine with ethylenediamine in a 1:1 or 2:1 molar ratio yielded two neutral ligands with different denticity: 1-amino-4-(2-pyridyl)-3-azapent-3-ene (tridentate, L¹) and 2,7-bis(2-pyridyl)-3,6-diazaocta-2,6-diene (tetradentate, L³), respectively. Replacing the ketone with 2-pyridinecarboxaldehyde gave a similar set of condensates (L² and L⁴). The tridentate mono-Schiff bases (L¹ and L²) react smoothly with Ni(ClO₄)₂•6H₂O furnishing the brown bis-chelate complexes (1a and 1b) with an NiN₆ coordination sphere, while the tetradentate bis-Schiff bases (L³ and L⁴) form green pseudo-octahedral complexes (2a and 2b) in which Ni^II is present in an N₄O₂ coordination environment. The isomer specificity for both types of complexes is conspicuous from the representative X-ray structures of 1a and 2a. cis-trans-cis and cis-cis-trans isomers are found exclusively for 1 and 2, respectively. The crystal structure of 2a reveals O−H•••O hydrogen bond interactions assembling alternating cations and anions in an infinite chain-like array. Cyclic voltammetric measurements of 1 and 2 in MeCN solution show a quasi-reversible one-electron oxidation near 0.95 and 0.87 V (vs. SCE), respectively, attributed to a Ni^III-Ni^II redox couple. Another irreversible Ni^IV-Ni^III redox response was observed at higher potential near 1.70 and 1.80 V (vs. SCE) for 1 and 2, respectively. Complexes 1 and 2 display a weak, broad d-d transition band along with a charge-transfer transition. Magnetic susceptibility measurements (at 298 K) confirmed that the spin states of the Ni^II centres in 1 and 2 are same, S = 1, in agreement with an octahedral configuration. Complexes 1 form stable reddish-brown Ni^III complexes (3) under exhaustive constant-potential electrolysis treatment. The Ni^III complexes display axial EPR spectra both at 298 K and 77 K with g_⊥ > g_|| indicating the presence of an unpaired electron primarily on the metal centre. Complexes of type 1 and 3 exhibit virtually superimposable cyclic voltammograms in a reverse electrode scan study which confirms no change in the coordination sphere on going from 1 to 3. The effective magnetic moment values (ca. 2.10 μ_B) of 3 suggest a significant orbital contribution to the paramagnetism, consistent with a tetragonally distorted low spin 3d⁷ system. A colour change phenomenon has been observed for type 2 complexes.