Synthesis, structure and magnetism of new single molecule magnets composed of (Mn2Mn2III)-Mn-II alkoxo-carboxylate bridged clusters capped by triethanolamine ligands

A family of tetranuclear mixed-valent Mn(II)(2)/Mn(III)(2) complexes of type [Mn-4(LH2)(2)(LH)(2)(H2O)(x)(RCO2)(2)](Y)(2).nS has been synthesised and structurally characterised, where LH3 = triethanolamine (N(CH2CH2OH)(3)), {R = CH3, x = 2, Y = CH3CO2-, n= 2, S = H2O; 1}, {R = C6H5, x = 0, Y = C6H5CO2-, n= 1, S = CH3CN; 2}, {R = C2H5, x = 0, Y = ClO4-, 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 mu(3)-oxygen atoms from a deprotonated alcohol 'arm' of each tridentate LH2- ligand and by mu(2)-oxygen atoms from each tetradentate LH2- ligand. The four nitrogen atoms from LH2- and LH2- 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 LH2- 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 3 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 1 and 2. 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), 1 and 2. The activation energies were compared with the potential energy barrier height, U, for magnetisation direction reversal (U=DS2) 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.