HIGH-SPIN MOLECULES - (NBU(4)(N))[M N(8)O(6)CL(6)(O(2)CPH)(7)(H2O)(2)] (S=11) AND [MN9NA2O7(O(2)CPH)(15)(MECN)(2)] (S=4)

The syntheses and electrochemical and magnetochemical properties of (NBu(4)(II)) [Mn8O6Cl6(O(2)CPh)(7)(H2O)(2)]. xCH(2)Cl(2) (2 . xCH(2)Cl(2)) and [Mn9Na2O7(O(2)CPh)(15) (MeCN)(2)]. 3MeCN (3 . 3MeCN) are reported. Both complexes were prepared from reactions involving (NBu(4)(II))[Mn4O2(H2O)(O(2)CPh)(9)] (1). Complex 2 was obtained in 45-60% yield on treatment of complex 1 with 4 equiv of Me(3)SiCl in CH2Cl2. Complex 2 . 3/2CH(2)Cl(2) . 2H(2)O crystallizes in triclinic space group P (1) over bar with the following cell parameters at -169 degrees C: a 16.104(4) Angstrom, b= 21.501(6) Angstrom, c = 14.843(4) Angstrom, alpha = 94.24(1)degrees, beta = 105.96(1)degrees, gamma = 89.07(1)degrees, V = 4927.79 Angstrom(3), and Z = 2. The structure was refined by employing 10 108 unique reflections with F > 3.0 sigma(F) to give R = 0.0912 and R(w) = 0.0944. Complex 2 contains a [Mn8O6Cl4](8+) core (8Mn(III)) that may be conveniently described as resulting from the fusion of two [Mn4O2] 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 O2- 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 P (1) over bar with the following cell parameters at -170 degrees C: a = 15.116(2) Angstrom b = 27.903(4) Angstrom, c = 15.007(2) Angstrom, alpha = 102.40(1)degrees, beta = 112.36(1)degrees, gamma = 84.17(1)degrees, V = 5715.26 Angstrom(3), and Z = 2. The structure was refined by employing 12 020 unique reflections with F > 3 sigma(F) to give R = 0.0514 and R(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 O2- ions. The Na+ ions are bound to bridging O2- 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, chi(M)T for complex 2 increases with decreasing temperature from 23.7 cm(3) K mol(-1) (mu(eff) = 13.8 mu(B)) at 300.0 K to a maximum of 53.2 cm(3) K mol(-1) (mu(eff) = 20.6 mu(B)) at 15.0 K, whereupon there is a decrease to 39.3 cm(3) K mol(-1) (mu(eff) = 17.7 mu(B)) at 5.01 K. The shape of this chi(M)T vs temperature 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 S = 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 S = 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 S = 4 ground state. In a 10.0 kG field mu(eff) decreases gradually from 12.23 mu(B) at 320 K to 6.93 mu(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.