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

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)] (), [Mn2(L1)(OAc)2](BPh4)] (), and [Mn2(L2)(OAc)2(dca)]n () have been synthesized and structurally characterized. Variable temperature magnetic studies of and 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 kcat value is > > . 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 show catecholase-like activity due to radical generation.