Pentanuclear MII-MnII (M = Ni and Cu) complexes of N2O2 donor ligands with a variation of carboxylate anions: syntheses, structures, magnetic properties and catecholase-like activities

One new heterometallic nickel(II)-manganese(II) complex [(NiL1)(2)Mn-3(C6H5CO2)(6)] (1) and two new copper(II)-manganese(II) complexes[(CuL1)(2)Mn-3(m-(NO2)C6H4CO2)(6)] (2) and [{CuL2(H2O)}(2)Mn-3(m-(NO2)C6H4CO2)(6)] (3) have been synthesized and structurally characterized using [NiL1]/[CuL1]/[CuL2] as a metaoligand (where H2L1 = N,N'-bis(2-hydroxynaphthyl-methylidene)-1,3-propanediamine, H2L2 = N,Ni-bis(methyl-2-hydroxynaphthyl-methylidene)-1,3-propanediamine). Crystal structure analyses show that in all three pentanuclear complexes two [NiL1]/[CuL1]/[CuL2] units are coordinated to the terminal Mn(II)centres of a linear trinuclear Mn-3(II) core i.e. [Mn-3(C6H5CO2)(6)] (for 1)/[Mn-3(m-(NO2)C6H4CO2)(6)] (for 2 and 3) through bis-phenoxido bridges. All three complexes (1-3) mimic catecholase-like activity in the oxidation of 3,5-di-tert-butyl catechol (3,5-DTBC) as the substrate with turnover numbers (k(cat)) of 734, 450 and 358 h(-1), respectively. A plausible mechanism of this catalytic oxidase reaction is proposed on the basis of mass spectral evidence. The temperature-dependent (2-300 K) dc molar magnetic susceptibility measurements of 1 reveal that the Mn-Mn centres of the [Mn-3(C6H5CO2) 6 ] unit are antiferromagneticay coupled with an exchange coupLing constant (J) value of -0.70 cm(-1). Isothermal magnetization experiments of 1 at 2 K suggest a change in the ground spin state from S = 5/2 to S = 7/2 upon increasing the magnetic field at 5 T because of the presence of low-lying excited spin states due to the low value of the coupling constant (J). Complexes 2 and 3 have similar structures and thus magnetic characterization of 2 has only been performed. It shows antiferromagnetic exchange interactions between the Cu-Mn centres (J(1) = -28.5 cm(-1)) and also between the metal centres in the Mn-3(II) core unit (J(2) = -2.11 cm(-1)).

Automated summary

One new heterometallic nickel(II)-manganese(II) complex and two new copper(II)-manganese(II) complexes have been synthesized and structurally characterized. These complexes exhibit catecholase-like activity in the oxidation reaction and show antiferromagnetic exchange interactions.