High-spin molecules: (NB un4) [Mn8O6Cl6(O2CPh)7(H 2O)2] (S = 11) and [Mn9Na2O7(O2CPh) 15(MeCN)2] (S = 4)

Article abstract of DOI:10.1021/ja00114a013

The syntheses and electrochemical and magnetochemical properties of (NBuⁿ₄)[Mn₈O₆Cl₆(O ₂CPh)₇-(H₂O)₂]·xCH ₂Cl₂ (2·xCH₂Cl₂) and [Mn₉Na₂O₇(O₂CPh) ₁₅(MeCN)₂]·3MeCN (3·3MeCN) are reported. Both complexes were prepared from reactions involving (NBuⁿ₄) [Mn₄O₂(H₂O)(O₂CPh)₉] (1). Complex 2 was obtained in 45-60% yield on treatment of complex 1 with 4 equiv of Me₃SiCl in CH₂Cl₂. Complex 2·3/2CH₂Cl₂·2H₂O crystallizes in triclinic space group P1 with the following cell parameters at -169 °C: a = 16.104(4) A?, b = 21.501(6) A?, c = 14.843(4) A?, α = 94.24(1)°, β = 105.96(1)°, γ = 89.07(1)°, V = 4927.79 A?³, and Z = 2. The structure was refined by employing 10 108 unique reflections with F > 3.0σ(F) to give R = 0.0912 and R_w = 0.0944. Complex 2 contains a [MH₈O₆Cl₄]⁸⁺ core (8Mn^III) that may be conveniently described as resulting from the fusion of two [MiLtO2J butterfly units by sharing one body or hinge Mn. An eighth Mn^III ion is connected to the resultant [Mn₇O₄] unit by two additional bridging O^2- ions. Complex 3 was obtained in 31% yield from the treatment of complex 1 with 1 equiv of benzoyl peroxide in MeCN, followed by addition of NaClO₄. Complex 3·3MeCN crystallizes in triclinic space group P1 with the following cell parameters at -170 °C: a = 15.116(2) A? = 27.903(4) A?, c = 15.007(2) A?, α = 102.40(1)°, β = 112.36(1)°, γ = 84.17(1)°, V = 5715.26 A?³, and Z = 2. The structure was refined by employing 12 020 unique reflections with F > 3σ(F) to give R = 0.0514 and A_w = 0.0525. Complex 3 possesses a mixed-metal undecanuclear [Mn₉Na₂O₇]¹⁵⁺ core (9Mn^III); the Mn₉O₇ subcore again comprises a [Mn₇O₄] unit constructed from the fusion, in the same manner as for complex 2, of two [Mn₄O₂] butterfly units. There are now two additional Mn^III ions connected to the [Mn₇O₄] unit, by three additional bridging O^2- ions. The Na⁺ ions are bound to bridging O^2- ions of the [Mn₉O₇] core, supporting a heterometallic-aggregate description rather than an ion-pairing description. Complex 2 displays reversible redox couples when examined by cyclic voltammetry in CH₂Cl₂; an oxidation and a reduction are observed at 0.91 and 0.12 V, respectively, vs ferrocene. Complex 3 displays no reversible processes. Variable-temperature and variable-field dc magnetic susceptibility data were collected for polycrystalline samples of complexes 2 and 3. In a 10.0 kG field, _χMT for complex 2 increases with decreasing temperature from 23.7 cm³ K mol^-1 (μ_eff = 13.8 μ_B) at 300.0 K to a maximum of 53.2 cm³ K mol^-1 (μ_eff = 20.6 μ_B) at 15.0 K, whereupon there is a decrease to 39.3 cm³ K mol^-1 (μ_eff = 17.7 μ_B) at 5.01 K. The shape of this _χMT vs emperature curve reflects the population of a S = 11 ground state at low temperature. Least-squares fitting of reduced magnetization vs H/T data for complex 2 in the 2.0-30.0 K and 5.00-50.0 kG field range confirms that complex 2 has a 5 = 11 ground state with g = 1.92 and axial zero-field splitting of D = -0.04 cm^-1. The ac susceptibility data measured in zero external field for complex 2 in the 2.0-30.0 K range also indicate a 5 = 11 ground state. No out-of-phase ac magnetic susceptibility signal was observed for complex 2 in the 2.0-30.0 K range in spite of its large-spin ground state. The absence of an out-of-phase ac signal is attributable to a very small zero-field splitting in the S = 11 ground state, and the small D-value results from a near cancellation of single-ion zero-field interactions at the eight Mn^III ions in complex 2. Complex 3 exhibits dc and ac magnetic susceptibility data consistent with a 5 = 4 ground state. In a 10.0 kG field μ_eff decreases gradually from 12.23 μ_B at 320 K to 6.94 μ_B at 5.01 K. Even though there are some similarities in molecular structure between complexes 2 and 3, differences in the nature of spin frustration result in a S = 11 ground state for complex 2 and a S = 4 ground state for complex 3.