The synthesis and magnetic properties of the high-spin tetranuclear cluster [Mn(III)(2)Mn(II)(2)(O(2)CC(CH(3))(3))(2)(teaH(2))(2)(teaH)(2)](O(2)CC(CH(3))(3))(2) (1) (where teaH(3) = triethanolamine) is described. Complex 1 is the pivalate analogue of our previously reported family of tetranuclear mixed-valence carboxylate clusters. The teaH(2)(-) and teaH(2-) anions in complex 1 act as oxygen donors in the {Mn(III)(2)Mn(II)(2)O(2)} "butterfly" core. Detailed dc and ac magnetic susceptibility measurements and magnetisation isotherms have been made and show that intra-cluster ferromagnetic coupling is occurring between the S = 2 Mn(III) and S = 5/2 Mn(II) ions to yield a S = 9 ground state and the g, J(bb) and J(wb) parameters have been deduced (b = body, w = wingtip). Incorporation of the acetylacetonate (acac(-)) ligand has led to three new clusters: [Mn(III)(2)Mn(II)(2)(O(2)CPh)(4)(teaH)(2)(acac)(2)].MeCN (2), [Mn(III)(2)Mn(II)(2)(teaH)(2)(acac)(4)(MeOH)(2)](ClO(4))(2) (3) and [Mn(III)(2)Mn(II)(2)(bheapH)(2)(acac)(4)(MeOH)(2)](ClO(4))(2) (4) (where bheapH(3) = 1-[N,N-bis(2-hydroxyethyl)amino]-2-propanol). Unlike any previously reported tetranuclear clusters containing the Mn(II)(2)Mn(III)(2) core, 2, 3, and 4 exhibit a reversal in their Mn(II)(2)Mn(III)(2) oxidation state distribution. In these clusters, the "wing-tip" Mn atoms exhibit Mn(III) (S = 2) oxidation states while the Mn(II) ions occupy the central "body" positions. Furthermore, the cores in 2, 3, and 4 contain at least one mu(2)-oxygen based bridging ion as opposed to the standard two mu(3)-oxygen bridges previously reported. More precisely, cluster 2 exhibits one mu(3)-O bridge and two mu(2)-bridges in a {Mn(II)(2)Mn(III)(2)O(3)} core while clusters 3 and 4 exhibit two mu(2)-O linkers within the {Mn(II)(2)Mn(III)(2)O(2)} core. All display trigonal prismatic coordination around the Mn(II) centres. These structural and oxidation state differences lead to very different magnetic coupling interactions between the four Mn(II/III) centres compared to 1. Direct current magnetic susceptibility measurements and magnetisation isotherms show that clusters 3 and 4 have ground states of S = 1. The g, J(bb) and J(wb) parameters have been deduced.

A family of tetranuclear mixed-valent Mn(II)(2)/Mn(III)(2) complexes of type [Mn(4)(LH(2))(2)(LH)(2)(H(2)O)(x)(RCO(2))(2)](Y)2.nS has been synthesised and structurally characterised, where LH(3) = triethanolamine (N(CH(2)CH(2)OH)(3)), (R=CH(3), x=2, Y = CH(3)CO(2)-, n=2, S = H(2)O; 1), (R=C(6)H(5), x=0, Y=C(6)H(5)CO(2)-, n=1, S = CH(3)CN; 2), (R=C(2)H(5), x=0, Y=ClO(4)(-), n=0; 3). A common structural core was deduced from X-ray crystallography and consists of a rhomboidal (planar-diamond) array with two 7-coordinate Mn(II) "wingtip (w)" centres and two 6-coordinate Mn(III) "body (b)" centres. The Mn(III) ions are bridged to the Mn(II) ions by mu3-oxygen atoms from a deprotonated alcohol "arm" of each tridentate LH(2-) ligand and by mu2-oxygen atoms from each tetradentate LH(2)(-) ligand. The four nitrogen atoms from LH(2-) and LH(2)(-) groups, together with bridging and terminal carboxylates oxygens complete the outer coordination sites around the Mn atoms. A feature of these clusters is that they are linked together in the crystal lattice by hydrogen-bonding interactions involving a non-coordinated hydroxyl arm on each LH(2-) group. Detailed DC and AC magnetic susceptibility measurements and magnetisation isotherms have been made on the three complexes and show that intra-cluster ferromagnetic coupling is occurring between the S = 2 Mn(III) and S = 5/2 Mn(II) ions to yield S = 9 ground states. The g, J(bb) and J(wb) parameters have been deduced. Inter-cluster antiferromagnetic coupling was noted in and this influences the magnetisation versus field behaviour and the temperature and magnitude of the out-of-phase AC chi"M maxima in comparison to those observed for and. An Arrhenius plot of the reciprocal temperature of the maxima in chi"M obtained at different frequencies (10 to 1500 Hz), in the range 1.75 K to 4 K, against the natural logarithm of the magnetization relaxation rate (1/tau) yielded values of the activation energies and pre-exponential factors for two of these new tetranuclear single-molecule magnets (SMMs), and. The activation energies were compared with the potential energy barrier height, U, for magnetisation direction reversal (U = DS(2)) using the axial zero-field splitting parameter, D, deduced from the DC M/H isotherm analysis for these S = 9 species. The very small separation of S = 9 and 8 levels for these clusters highlights the limitations in the determination of D values from M/H data at low temperatures.

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...