Steric Control of Polynuclear Manganese Complexes by the Use of Tripodal Tetradentate Ligands

Abstract

Abstract Tripodal tetradentate ligands (H2L1: N,N-bis(2-hydroxybenzyl)-N′,N′-dimethylethylenediamine, H2L2: N-(3,5-di-t-butyl-2-hydroxybenzyl)-N-(2-hydroxybenzyl)-N′,N′-dimethylethylenediamine, H2L3: N,N-bis(3,5-di-t-butyl-2-hydroxybenzyl)-N′,N′-dimethylethylenediamine, H2L4: N,N-bis(2-hydroxy-3-methoxybenzyl)-N′,N′-dimethylethylenediamine, H2L5: N-(3,5-di-t-butyl-2-hydroxybenzyl)-N-(2-hydroxy-3-methoxybenzyl)-N′,N′-dimethylethylenediamine) made it possible to prepare sterically controlled polynuclear manganese complexes. In the presence of carboxylate ligands, in contrast to the L3 ligand which gave mononuclear MnIII complexes, the other ligands afforded mixed-valence trinuclear complexes with an MnIII–MnII–MnIII arrangement. In methanol, the general formula of the products is [Mn3(L)2(carboxylato)2(OCH3)2]. While preparations in acetonitrile generated [Mn3(L1)2(ba)4] (ba = benzoate(1−)) and [Mn3(L2)2(ba)2(OH)2]. The structure of [Mn3(L1)2(ba)4] was determined by X-ray analysis. The three manganese cores are arranged linearly, and the central and terminal ions are bridged by a phenolate and two carboxylate groups. In the case of the L5 ligand, dinuclear and tetranuclear complexes were also obtained and structurally characterized. In the dinuclear complex, [Mn(L5)(CH3OH)(OCH3)MnCl2], a distorted octahedral MnIII and a five-coordinated MnII site are bridged by a phenolate and an alkoxo oxygen donor. The tetranuclear complex, [Mn4(L5)2(ba)6], including an MnIII–MnII–MnII–MnIII arrangement, is regarded as a carboxylato bridged dimer of a dinuclear unit, [Mn(L5)(ba)2Mn]+. Because the bulkiness of the L5 ligand lies between the L2 and L3 ligands, the dinuclear unit is favorable. Variable-temperature magnetic susceptibility measurements showed the ferromagnetic spin-exchange coupling for [Mn(L5)(CH3OH)(OCH3)MnCl2] and the antiferromagnetic one for [Mn3(L1)2(ba)4]. The phenolato and alkoxo bridges give rise to the ferromagnetic exchange interactions between MnIIIand MnII ions. On the other hand, the antiferromagnetic interactions stem from carboxylato bridges.