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

Treatment of pyrazolines (isolated or prepared in situ from 4‐R‐benzoylhydrazines and acetylacetone in a 2:1 mol ratio) with Ni(O 2 CCH 3 ) 2 · 4H 2 O in boiling methanol results in the formation of nickel(II) complexes of formula [Ni(H R L)] [ 1 (R = H) and 2 (R = OMe)], where (H R L) 2– represents the deprotonated Schiff base acetylacetone bis(4‐R‐benzoylhydrazone). Despite several attempts, the analogous nickel(II)complex with R = NMe 2 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( R L– R L)Ni] [ 3 (R = H), 4 (R = OMe), and 5 (R = NMe 2 )]. Here ( R L– R L) 6– represents the dinucleating ligand formed by oxidative C–C coupling involving the central methylene group of the acetylacetone fragment of (H R L) 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( R LO)] [ 6 (R = H), 7 (R = OMe), and 8 (R = NMe 2 )], where ( R LO) 2– represents the tetradentate ligand formed from ( R L– R L) 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)