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

Banerjee, Suparna ; Gangopadhyay, Jaydip ; Lu, Can-Zhong ; Chen, Jiu-Tong ; Ghosh, Ashutosh (2004) Nickel (II) complexes incorporating pyridyl, imine and amino chelate ligands: synthesis, structure, isomer preference, structural transformation and reactivity towards nickel (III) derivatives European Journal of Inorganic Chemistry, 2004 (12). pp. 2533-2541. ISSN 1434-1948

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Official URL: http://onlinelibrary.wiley.com/doi/10.1002/ejic.20...

Related URL: http://dx.doi.org/10.1002/ejic.200300865

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 NiII 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 NiIII-NiII redox couple. Another irreversible NiIV-NiIII 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 NiII centres in 1 and 2 are same, S = 1, in agreement with an octahedral configuration. Complexes 1 form stable reddish-brown NiIII complexes (3) under exhaustive constant-potential electrolysis treatment. The NiIII 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 μB) 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.

Item Type:Article
Source:Copyright of this article belongs to John Wiley and Sons, Inc.
Keywords:Coordination Modes; Isomer Specificity; N Ligands; Nickel; Structural Transformation
ID Code:108396
Deposited On:13 Oct 2017 12:40
Last Modified:13 Oct 2017 12:40

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