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

Abstract

One new heterometallic nickel(II)–manganese(II) complex [(NiL¹)₂Mn₃(C₆H₅CO₂)₆] (1) and two new copper(II)–manganese(II) complexes[(CuL¹)₂Mn₃(m-(NO₂)C₆H₄CO₂)₆] (2) and [{CuL²(H₂O)}₂Mn₃(m-(NO₂)C₆H₄CO₂)₆] (3) have been synthesized and structurally characterized using [NiL¹]/[CuL¹]/[CuL²] as a metalloligand (where H₂L¹ = N,N′-bis(2-hydroxynaphthyl-methylidene)-1,3-propanediamine, H₂L² = N,N′-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^IIcentres of a linear trinuclear Mn^II₃ core i.e. [Mn₃(C₆H₅CO₂)₆] (for 1)/[Mn₃(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 [Mn₃(C₆H₅CO₂)₆] unit are antiferromagnetically 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 (J₁ = −28.5 cm⁻¹) and also between the metal centres in the Mn^II₃ core unit (J₂ = −2.11 cm⁻¹).