A mononuclear MnIII/‘bis-tris’ complex and its conversion to a mixed-valence MnII/III5cluster

Abstract

The use of the anion of 2,2-bis(hydroxymethyl)-2,2′,2″-nitrilotriethanol (‘bis-tris’ or LH₅) as a chelate in Mn cluster chemistry is reported, and two products [Mn(N₃)(LH₃)] (1) and [Mn₅(LH₂)₃(LH₅)(MeOH)_2.5]Cl₄ (2) are described. The reaction of Mn(ClO₄)₂·6H₂O and NaN₃ with LH₅ and NEt₃ in a 1 : 1 : 1 : 2 molar ratio in DMF gave complex 1. The reaction of 1, NEt₃ and MnCl₂·4H₂O in a 2 : 2 : 1 ratio in MeOH gave the pentanulear complex 2. Complex 1 contains a distorted-octahedral Mn^III atom exhibiting a Jahn–Teller axial elongation. Complex 2 has a mixed-valence Mn^II₂Mn^III₃ trigonal bipyramidal Mn₅ topology, with the apical positions of the [Mn₅(μ-OR)₇]⁶⁺ core occupied by the Mn^II atoms. Variable-temperature, solid-state dc and ac magnetization studies are reported for 1 and 2 in the 1.8–300 K range. The data for 1 are as expected for a high-spin d⁴ Mn^III complex with S = 2; at low temperature, the effects of zero-field splitting (ZFS) and intermolecular antiferromagnetic interactions viaOH⋯O hydrogen bonds become evident. Fitting of magnetization vs. field (H) and temperature (T) data by matrix diagonalization gave S = 2, D = −3.25 cm⁻¹ and |E| = 0.32 cm⁻¹, with g fixed at 2.00, where D and E are the axial and rhombic ZFS parameters, respectively. The analogous fit for 2 gave S = 3 and D = −0.68 cm⁻¹. The combined results demonstrate the usefulness of ‘bis-tris’ as a new poly-alkoxide chelate for the synthesis of new Mn complexes.