Anion mediated formation of different Schiff bases from the same precursors and their nickel(II) complexes with different nuclearity

Abstract

Two nickel(II) complexes, [Ni₂ (H₂O)₄](NO₃)₂ (1) and [Ni₅ (OAc)₆(OH)₂] · 5.5H₂O (2), have been synthesized and structurally characterized by X-ray crystallographic study, where L¹ and L² represent 2-formyl-4-methyl-6-(1-(2-aminomethyl)piperidine)-iminomethylphenolate and 4-methyl-2,6-bis(1-(2-aminomethyl)piperidine)-iminomethylphenolate, respectively. One-pot reactions between 2,6-diformyl-4-methylphenol with 1-(2-aminoethyl)piperidine, in the presence of nickel(II) nitrate and nickel(II) acetate, afforded in situ [1 + 1] and [1 + 2] condensation leading to Schiff-bases HL¹ and HL², and to 1 and 2 of different nuclearity. Both processes were conducted under the same reaction conditions in anhydrous EtOH. Preferential attainment of bi- or pentanuclear compounds is mediated by the anion of the Ni(II) salt. Complex 1 is formed by two unsymmetrical tridentate L¹ chelating in a head-tail fashion to make a centrosymmetric phenoxido-bridged Ni(II) dimer. Two waters complete octahedral coordination. Conversely in 2, two [Ni₂(OAc)L²]⁺ units, having a syn-syn bidentate bridging acetate, embrace a third Ni through bridging acetate and μ₃-hydroxide, giving rise to an adduct of C₂ symmetry where the compartmental ligand planes form a dihedral angle of ca. 82°. The catecholase activity of these complexes were explored and both complexes effectively catalyze the conversion of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ).

Keywords:

• Polynuclear Ni(II) complex
• Catecholase activity
• Crystal structure
• Schiff-base ligand

Acknowledgments

The authors wish to thank the University Grants Commission, New Delhi [UGC Major Research Project, F. No. 34-308\2008 (SR) Dated: 31.12.2008 (DD)] for financial support. Averi Guha is thankful to Shailabala Biswas Research Foundation, University of Calcutta, for financial support.