The reaction of Mn(O2CPh)2·2H2O and PhCO2H in EtOH/MeCN with NBu4nMnO4 gives (NBu4n)[Mn4O2(O2CPh) 9-(H2O)] (4) in high yield (85-95%). Complex 4 crystallizes in monoclinic space group P21/c with the following unit cell parameters at -129 °C: a = 17.394(3) Å, b = 19.040(3) Å, c = 25.660(5) Å, β = 103.51(1)°, V = 8262.7 Å3, Z = 4; the structure was refined on F to R (Rw) = 9.11% (9.26%) using 4590 unique reflections with F > 2.33σ(F). The anion of 4 consists of a [Mn4(μ3-O)2]8+ core with a "butterfly" disposition of four MnIII atoms. In addition to seven bridging PhCO2- groups, there is a chelating PhCO2- group at one "wingtip" Mn atom and terminal PhCO2- and H2O groups at the other. Complex 4 is an excellent steppingstone to other [Mn4O2]-containing species. Treatment of 4 with 2,2-diethylmaionate (2 equiv) leads to isolation of (NBu4n)2[Mn8O4(O 2CPh)12(Et2mal)2(H 2O)2] (5) in 45% yield after recrystallization. Complex 5 is mixed-valent (2MnII,6MnIII) and contains an [Mn8O4]14+ core that consists of two [Mn4O2]7+ (MnII,3MnIII) butterfly units linked together by one of the μ3-O2- ions in each unit bridging to one of the body Mn atoms in the other unit, and thus converting to μ4-O2- modes. The MnII ions are in wingtip positions. The Et2mal2- groups each bridge two wingtip Mn atoms from different butterfly units, providing additional linkage between the halves of the molecule. Complex 5·4CH2Cl2 crystallizes in monoclinic space group P21/c with the following unit cell parameters at -165 °C: a = 16.247(5) Å, b = 27.190(8) Å, c = 17.715(5) Å, β = 113.95(1)°, V = 7152.0 Å3, Z = 4; the structure was refined on F to R (Rw) = 8.36 (8.61%) using 4133 unique reflections with F > 3σ(F). The reaction of 4 with 2 equiv of bpy or picolinic acid (picH) yields the known complex Mn4O2(O2CPh)7(bpy)2 (2), containing MnII,3MnIII, or (NBu4n)[Mn4O2(O2CPh) 7(pic)2] (6), containing 4MnIII. Treatment of 4 with dibenzoylmethane (dbmH, 2 equiv) gives the mono-chelate product (NBu4n)[Mn4O2(O2CPh) 8(dbm)] (7); ligation of a second chelate group requires treatment of 7 with Na(dbm), which yields (NBu4n)[Mn4O2(O2CPh) 7(dbm)2] (8). Complexes 7 and 8 both contain a [Mn4O2]8+ (4MnIII) butterfly unit. Complex 7 contains chelating dbm- and chelating PhCO2- at the two wingtip positions, whereas 8 contains two chelating dbm- groups at these positions, as in 2 and 6. Complex 7·2CH2Cl2 crystallizes in monoclinic space group P21 with the following unit cell parameters at -170 °C: a = 18.169(3) Å, b = 19.678(4) Å, c = 25.036(4) Å, β = 101.49(1)°, V = 8771.7 Å3, Z = 4; the structure was refined on F to R (Rw) = 7.36% (7.59%) using 10 782 unique reflections with F > 3σ(F). Variable-temperature magnetic susceptibility studies have been carried out on powdered samples of complexes 2 and 5 in a 10.0 kG field in the 5.0-320.0 K range. The effective magnetic moment (μeff) for 2 gradually decreases from 8.61 μB per molecule at 320.0 K to 5.71 μB at 13.0 K and then increases slightly to 5.91 μB at 5.0 K. For 5, μeff gradually decreases from 10.54 μB per molecule at 320.0 K to 8.42 μB at 40.0 K, followed by a more rapid decrease to 6.02 μB at 5.0 K. On the basis of the crystal structure of 5 showing the single MnII ion in each [Mn4O2]7+ subcore to be at a wingtip position, the MnII ion in 2 was concluded to be at a wingtip position also. Employing the reasonable approximation that Jwb(MnII/MnIII) = Jwb(MnIII/MIII, where Jwb is the magnetic exchange interaction between wingtip (w) and body (b) Mn ions of the indicated oxidation state, a theoretical χM vs T expression was derived and used to fit the experimental molar magnetic susceptibility (χM) vs T data. The obtained fitting parameters were Jwb = -3.9 cm-1, Jbb = -9.2 cm-1, and g = 1.80. These values suggest a ST = 5/2 ground state spin for 2, which was confirmed by magnetization vs field measurements in the 0.5-50.0 kG magnetic field range and 2.0-30.0 K temperature range. For complex 5, since the two bonds connecting the two [Mn4O2]7+ units are Jahn-Teller elongated and weak, it was assumed that complex 5 could be treated, to a first approximation, as consisting of weakly-interacting halves; the magnetic susceptibility data for 5 at temperatures ≥40 K were therefore fit to the same theoretical expression as used for 2, and the fitting parameters were Jwb = -14.0 cm-1 and Jbb = -30.5 cm-1, with g = 1.93 (held constant). These values suggest an ST = 5/2 ground state spin for each [Mn4O2]7+ unit of 5, as found for 2. The interactions between the subunits are difficult to incorporate into this model, and the true ground state spin value of the entire Mn8 anion was therefore determined by magnetization vs field studies, which showed the ground state of 5 to be ST = 3. The results of the studies on 2 and 5 are considered with respect to spin frustration effects within the [Mn4O2]7+ units. Complexes 2 and 5 are EPR-active and -silent, respectively, consistent with their ST = 5/2 and ST = 3 ground states, respectively.
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