A mononuclear Mn^III/‘bis-tris’ complex and its conversion to a mixed-valence Mn^II/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 x 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 x 4H2O in a 2:2:1 ratio in MeOH gave the pentanulear complex 2. Complex 1 contains a distorted-octahedral MnIII atom exhibiting a Jahn-Teller axial elongation. Complex 2 has a mixed-valence MnII2MnIII3 trigonal bipyramidal Mn5 topology, with the apical positions of the [Mn5(micro-OR)7](6+) core occupied by the MnII 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 MnIII complex with S = 2; at low temperature, the effects of zero-field splitting (ZFS) and intermolecular antiferromagnetic interactions via OHO 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.

A rare ferromagnetic manganese(III) hexanuclear cluster

“…The in-phase χ M ′T vs. T data are powerful complement for determining the ground state of a system, extrapolation of the plot to 0 K from above 6 K gives a value of χ M ′ T ∼ 10.0 cm 3 K mol − 1 . This indicates an S = 4 ground state with a g ∼ 2, this conclusion provides an independent confirmation that complex 1 possesses an S T = 4 ground state [28]. The shift of peak temperature (T f ) of χ M ′ is measured by a parameter φ = (ΔT f / T f ) / Δ(logf) = 0.17, and which is in the range of normal superparamagnets [29] and excludes the possibility of a spin glass (0.01 b φ b 0.1) [30,31].…”

Probing the Metal-Ion-Binding Strength of the Hydroxyl Group

“…There is a further interesting X-ray structure 126 of a Mn(III)-Bistris complex, [Mn(BistrisÀ2H)(N 3 )], containing a 2-fold deprotonated Bistris. The Mn(III) ion is coordinated by the NO 4 Àpentadentate chelating (BistrisÀ2H) 2À ligand.…”

“…The other distances are two short MnÀO bonds (1.867 and 1.907 Å), the MnÀN bond of 2.063 Å, and a further MnÀN bond of 1.973 Å involving the terminal N of the azide ion. 126 Only highly polarizing metal ions like Mn(III) or Cu(II) (with Tris) are able to deprotonate HOÀCH 2 groups via MÀO coordination. In fact, in aqueous solution this is expected to occur only at the upper end of the physiological pH range.…”

Structural motifs and topological representation of Mn coordination clusters

“…Recently, the utilization of 2,2-bis(hydroxymethyl)-2,2 0 ,2 0 0nitrilotriethanol (H 5 L) along with MnCl 2 Á4H 2 O afforded a new pentanuclear cluster with molecular formula [Mn II 2 Mn III 3 (H 2 L) 3 (H 5 L)(MeOH) 2.5 ]Cl 4 (6). 77 Compound 6 consists of a mixed-valence Mn II 2 Mn III 3 metallic core and possesses a structural motif similar to that of 5 (Fig. 3, right).…”