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New Polynuclear Manganese Clusters from the Use of the Hydrophobic Carboxylate Ligand 2,2-Dimethylbutyrate

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journal contribution
posted on 27.06.2005, 00:00 by Nicole E. Chakov, Lev N. Zakharov, Arnold L. Rheingold, Khalil A. Abboud, George Christou
The syntheses, structures, and magnetic properties are reported of [Mn12O12(O2CPet)16(MeOH)4] (4), [Mn6O2(O2CH2)(O2CPet)11(HO2CPet)2(O2CMe)] (5), [Mn9O6(OH)(CO3)(O2CPet)12(H2O)2] (6), and [Mn4O2(O2CPet)6(bpy)2] (7, bpy = 2,2‘-bipyridine), where Pet = tert-pentyl (PetCO2H = 2,2-dimethylbutyric acid). These complexes were all prepared from reactions of [Mn12O12(O2CPet)16(H2O)4] (3) in CH2Cl2. Complex 4·2MeCN crystallizes in the triclinic space group P1̄ and contains a central [MnIV4O4] cubane core that is surrounded by a nonplanar ring of eight alternating MnIII and eight μ3-O2- ions. This is only the third Mn12 complex in which the four bound water molecules have been replaced by other ligands, in this case MeOH. Complex 5·1/2CH2Cl2 crystallizes in the monoclinic space group P21/c and contains two [Mn33-O)]7+ units linked at two of their apexes by two PetCO2- ligands and one μ4-CH2O22- bridge. The complex is a new structural type in Mn chemistry, and also contains only the third example of a gem-diolate unit bridging four metal ions. Complex 6·H2O·PetCO2H crystallizes in the orthorhombic space group Cmc21 and possesses a [MnIII93-O)6(μ-OH)(μ3-CO3)]12+ core. The molecule contains a μ3-CO32- ion, the first example in a discrete Mn complex. Complex 7·2H2O crystallizes in the monoclinic space group P21/c and contains a known [MnIII2MnII23-O)2]6+ core that can be considered as two edge-sharing, triangular [Mn3O] units. Additionally, the synthesis and magnetic properties of a new enneanuclear cluster of formula [Mn9O7(O2CCH2But)13(THF)2] (8, THF = tetrahydrofuran) are reported. The molecule was obtained by the reaction of [Mn12O12(O2CCH2But)16(H2O)4] (2) with THF. Complexes 2 and 4 display quasireversible redox couples when examined by cyclic voltammetry in CH2Cl2; oxidations are observed at −0.07 V (2) and −0.21 V (4) vs ferrocene. The magnetic properties of complexes 48 have been studied by direct current (DC) and alternating current (AC) magnetic susceptibility techniques. The ground-state spin of 4 was established by magnetization measurements in the 1.80−4.00 K and 0.5−7 T ranges. Fitting of the reduced magnetization data by full matrix diagonalization, incorporating a full powder average and including only axial anisotropy, gave S = 10, g = 2.0(1), and D = −0.39(10) cm-1. The complex exhibits two frequency-dependent out-of-phase AC susceptibility signals (χM‘ ‘) indicative of slow magnetization relaxation. An Arrhenius plot obtained from χM‘ ‘ vs T data gave an effective energy barrier to relaxation (Ueff) of 62 and 35 K for the slower and faster relaxing species, respectively. These studies suggest that complex 4 is a single-molecule magnet (SMM). DC susceptibility studies on complexes 58 display overall antiferromagnetic behavior and indicate ground-state spin values of S ≤ 2. AC susceptibility studies at <10 K confirm these small values and indicate the population of low-lying excited states even at these low temperatures. This supports the small ground-state spin values to be due to spin frustration effects.