Synthesis, structure and magnetism of manganese amidinate clusters

Abstract

Since the discovery of Mn12-acetate in the early 80s and the subsequent determination of its single molecule magnet (SMM) characteristics, interest in the area of large metal clusters, particularly Mn cluster chemistry, has continued to grow. To date, several SMMs have been identified including Fe8, V4, Mn4 and Mn12 clusters but it has been found that Mn clusters often possess large values of ground state spin -an important requirement for SMM behaviour. Our group is interested in the synthesis of new Mn clusters and the investigation of their structural and magnetic properties. The focus currently is on the incorporation of bridging ligands other than O-donor ligands such as benzamidinate (bza-), which is an N-donor, isoelectronic analogue of benzoate. A [Mn4O(bza)6] tetramer was formed by the reaction of a MnII salt with deprotonated ligand under an inert atmosphere. The core consists of four MnII atoms arranged tetrahedrally around an oxygen atom. Each metal atom has a pseudo-tetrahedral arrangement of nitrogen atoms around it with each of the six bza-groups forming a bridge between two metal centres. A [Mn3O(bza)6(pyr)3] trimer was formed from a comproportionation reaction between a MnII salt and MnO4 -in the presence of bza. The cluster is mixed valent (2MnIII, MnII) and possess an oxo-centred Mn3O unit with 6 bzagroups providing peripheral ligation and three pyridine groups in terminal positions. The magnetic properties are similar to those of the benzoate analogue. Work is in progress on larger clusters of benzamidinate and with other bridging ligands. (C6H15N4O2) + (H2PO4) -(H2O) was analysed from six atomic displacement parameters (ADP) sets obtained from X-ray and neutron diffraction data at two temperatures. Special care was put in obtaining high quality ADPs. In the case of X-ray data a pseudoatom model of the electron density was used in order to obtain a good deconvolution between static electron density and thermal vibration. In the thermal vibration analysis, the molecules were considered as rigid bodies vibrating independently. An additional internal vibration was supposed present in the organic cations. Using this approach, molecular normal mode frequencies were obtained from each ADP set. The coherence of the molecular modes calculated from each ADP set is discussed, showing than the molecular modes are very sensitive to the quality of the ADPs. The molecular mode frequencies are compared with the crystal normal mode frequencies observed in far-IR spectra for both crystals. This comparison can be helpful in the interpretation of the far-IR spectra, even when there is no direct equivalence between molecular and crystal normal modes. Non-analyticity of interatomic interactions (II) in crystals is one of the fundamental problems concerning II, which do not decrease with distance. The discrete crystal structure of a short-range-ordered solid solution with anisotropy of its elastic properties can be adequately studied by the radiation diffuse scattering (DS) [1], if one take into account the non-a...