Mn(ii) complexes of different nuclearity: synthesis, characterization and catecholase-like activity

Abstract

Two “end-off” compartmental ligands, 2-formyl-4-chloro-6-N-ethylmorpholine-iminomethyl-phenol (HL1) and 2-formyl-4-methyl-6-N-ethylpyrrolidine-iminomethyl-phenol (HL2) have been designed and three complexes of Mn(II), one mono-, one di- and a polynuclear, namely Mn(L1)(SCN)₂(H₂O)] (1), [Mn₂(L1)(OAc)₂](BPh₄)] (2), and [Mn₂(L2)(OAc)₂(dca)]_n (3) have been synthesized and structurally characterized. Variable temperature magnetic studies of 2 and 3 have been performed and data analyses reveal that Mn centers are antiferromagnetic coupled with J = −9.15 cm⁻¹ and J = −46.89, respectively. Catecholase activity of all the complexes has been investigated using 3,5-di-tert-butyl catechol (3,5-DTBC). All are highly active and the activity order on the basis of the k_cat value is 2 > 1 > 3. In order to unveil whether the metal centered redox participation or the radical pathway is responsible for the catecholase-like activity of the complexes, detailed EPR and cyclic voltammetric (CV) studies have been performed. In addition to the six-line EPR spectrum characteristic to Mn(II), an additional peak at g ∼ 2 is observed when the EPR study is done with the mixture of 3,5-DTBC and the catalyst, suggesting the formation of an organic radical, most likely ligand centered. The CV experiment with the mixture of 3,5-DTBC and the catalyst reveals ligand centered reduction rather than reduction of Mn(II) to Mn(I). It is thus inferred that complexes 1–3 show catecholase-like activity due to radical generation.