Nickel( II ) Complexes Incorporating Pyridyl, Imine and Amino Chelate Ligands: Synthesis, Structure, Isomer Preference, Structural Transformation and Reactivity Towards Nickel( III ) Derivatives

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, L 1 ) and 2,7‐bis(2‐pyridyl)‐3,6‐diazaocta‐2,6‐diene (tetradentate, L 3 ), respectively. Replacing the ketone with 2‐pyridinecarboxaldehyde gave a similar set of condensates (L 2 and L 4 ). The tridentate mono‐Schiff bases (L 1 and L 2 ) react smoothly with Ni(ClO 4 ) 2 ·6H 2 O furnishing the brown bis‐chelate complexes ( 1a and 1b ) with an NiN 6 coordination sphere, while the tetradentate bis‐Schiff bases (L 3 and L 4 ) form green pseudo‐octahedral complexes ( 2a and 2b ) in which Ni II is present in an N 4 O 2 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 3d 7 system. A colour change phenomenon has been observed for type 2 complexes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)