Steric control of polynuclear manganese complexes by the use of tripodal tetradentate ligands

Article abstract of DOI:10.1246/bcsj.71.2873

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 Mn(III) complexes, the other ligands afforded mixed-valence trinuclear complexes with an Mn(III)- Mn(III)-Mn(III) 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 Mn(III) and a five-coordinated Mn(II) site are bridged by a phenolate and an alkoxo oxygen donor. The tetranuclear complex, [Mn₄(L⁵)₂(ba)₆], including an Mn(III)-Mn(II)-Mn(II)-Mn(III) 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 Mn(III) and Mn(II) ions. On the other hand, the antiferromagnetic interactions stem from carboxylato bridges.