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Royal Society of Chemistry, RSC Advances, 63(5), p. 51290-51301

DOI: 10.1039/c5ra05776k

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Solvent dependent ligand transformation in a dinuclear copper(ii) complex of a compartmental Mannich-base ligand: synthesis, characterization, bio-relevant catalytic promiscuity and magnetic study

This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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Abstract

An “end-off” pentadentate compartmental ligand HL has been synthesized by Mannich base condensation using p-cresol and 2-benzyl amino ethanol and structurally characterized. A dinuclear copper(II) complex, namely [Cu2(L)(µ-OH)(H2O)(ClO4)2], has been prepared by treating HL with Cu(ClO4)2.6H2O in methanolic solution with the aim to investigate its catalytic promiscuity. Single crystal structural analysis reveals that Cu-Cu separation is of 2.9 Å. Catecholase activity of the complex has been investigated in anhydrous DMSO as well as in DMSO-water mixture with progressive increasing quantity of water up to a 1:1 volume ratio in order to assess the bio compatibility of the catalyst using 3,5-DTBC as model substrate. In anhydrous DMSO the catalytic activity reaches its peak and decreases with increasing the water concentration, a feature most likely due to insolubility of 3,5-DTBQ, the product formed in the catalysis, in water. The complex also shows excellent phosphatase-like activity by exploiting the Lewis acidity, the necessary requirement for that activity, under different pH. Thorough investigation reveals that no activity is observed at pH 6 but the activity increases with increasing pH and attains maximum at pH 9. Variable temperature magnetic study shows that the two Cu centers are antiferromagnetically coupled at low temperature with J value –78.63±1.30cm−1. In acetonitrile medium the complex shows very exciting behavior. A new transformed ligand is generated that has been assigned as a Schiff-base ligand, 2,6-bis-[(2-hydroxy-ethylimino)-methyl]-4-methylphenol. The genesis of the new ligand is a consequence of dealkylation from HL followed by oxidation. This oxidation is counterbalanced by reduction of Cu(II) to Cu(I) as it is evidenced from isolation of [Cu(MeCN)4](ClO4) from the mixture followed by X-ray structural characterization of the species.