Nickel-assisted oxidative C-C coupling and subsequent cleavage to C=O of active methylene group in a tetradentate ligand system: Di- and mononuclear complexes with transformed ligands

Abstract

Treatment of pyrazolines (isolated or prepared in situ from. 4R-benzoylhydrazines and acetylacetone in a 2:1 mol ratio) with Ni(O 2CCH3)2-4H2O in boiling methanol results in the formation of nickel(II) complexes of formula [Ni(HRL)] [1 (R = H) and 2 (R = OMe)], where (HRL)2- represents the deprotonated Schiff base acetylacetone bis(4-R-benzoylhydrazone), Despite several attempts, the analogous nickel(II) complex with R = NMe2 could not be isolated. However, stirring the above described reaction mixtures in air at room temperature for approximately 5 h provides dinuclear paramagnetic species [Ni(RL-RL)Ni] [3 (R = H), 4 (R = OMe), and 5 (R = NMe 2)]. Here (RL-RL)6- represents the dinucleating ligand formed by oxidative C-C coupling involving the central methylene group of the acetylacetone fragment of (HRL)2-, The room-temperature (300 K) magnetic moments of 3-5 indicate an S = 1/2 spin state of each, metal center in these dinuclear complexes, Cryomagnetic measurements with 3 reveal its weak antiferromagnetic nature, The EPR spectral features suggest that 3-5 are on the borderline between genuine nickel(III) species and nickel(II)-stabilized ligand radical systems. In wet dichloromethane/hexane mixture under aerobic conditions, each of 3-5 forms mononuclear nickel(II) complexes of formula [Ni(RLO)] [6 (R = H), 7 (R = OMe), and 8 (R = NMe2)], where (RLO)2- represents the tetradentate ligand formed from. (RL- RL)6- by C-C bond cleavage followed by oxidation to the keto group. The molecular structures of 1, 2, 5, 7, and 8, determined by X-ray crystallography, are reported. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.