Treatment of [Mn12O12(O(2)CMe)(16)(H2O)(4)] (1) with an excess of EtCO(2)H in toluene leads to high yield (similar to 70%) conversion to [Mn12O12(O(2)CEt)(16)(H2O)(3)] (3) Complex 3.4H(2)O crystallizes in the triclinic space group P1, which at -165 degrees C has a = 13.851(3) Angstrom, b = 23.431(5) Angstrom, c = 13.663(3) Angstrom, alpha = 103.76(1)degrees, beta = 111.46(1)degrees, gamma = 78.13(1)degrees, and Z = 2. The structure was refined with 8470 observed [F > 3.0 sigma(F)] reflections to give R = 6.07 and R(W) = 6.21%. The complex consists of a central [(Mn4O4)-O-IV](8+) cubane held within a nonplanar ring of eight Mn-III atoms by eight mu(3)-O2- ions. One of the Mn-III ions is five coordinate. NMR (H-1,F-19) studies on [Mn12O12(O(2)CR)(16)(H2O)(4)] (R = Me,Et, and substituted Ph) show that the dodecanuclear species retain their structural integrity upon dissolution in CD3CN. The spectra show four carboxylate resonances, consistent with effective D-2 minimal symmetry in solution. Electrochemical studies on 1-3 and a series of related species show a quasireversible reduction in the 0.00 to 0.47 V range, and a quasireversible oxidation in the 0.70 to 1.07 V range, together with additional mostly irreversible oxidation and reduction processes. Reduction of complex 3 with PP4I in CH2Cl2 leads to isolation of (PPh(4))[Mn12O12(O(2)CEt)(16)(H2O)(4)] (4) in similar to 70% yield. Complex 4 crystallizes in monoclinic space group P2(1)/c with the following cell dimensions at -168 degrees C: a 13.046(4), b = 28.223(9), c = 27.162(8) Angstrom, beta = 96.32(1)degrees, Z = 4, and V = 9940.45 Angstrom(3). The structure was refined using 9871 unique reflections with I > 3 sigma(I) to give R = 6.51 and R(W) = 6.65%, respectively. The structure shows that the added electron is localized on an outer (originally Mn-III) ion rather than an inner (cubane) Mn-IV ion, producing a trapped-valence (MnMn7Mn4IV)-Mn-II-Mn-III anion. The results of de and ac magnetic susceptibility studies are described. DC studies of complexes 3.H2O (3a) and 4 in a 10 kG field show maxima in the mu(eff)/Mn-12 vs T plots at similar to 18.5-19.0 mu(B) in the similar to 15-20 K range, consistent with ground states with relatively large spins. In order to determine the magnitude of the latter, additional de (2.0-4.0 K, 20-50 kG fields) and ac (2.0-30 K, 1.0 and 0.0050 G fields, 25-1000 Hz frequencies) data were collected. Analysis df these data show that complex 3a has an S = 9 ground state, and that complex 4 has an S = (19)/(2) groundstate. Furthermore, (chi)'T-M values for complex 4, where (chi)'(M) is the in-phase component of the ac susceptibility, exhibit a plateau: in the 6-16 K range, below which there is an abrupt decrease in (chi)'T-M. Related to this is the observation that (chi)''(M), the out-of-phase component of the ac susceptibility, has a nonzero value that is frequency dependent. This is extremely rare and unusual behavior, seen previously in discrete species only for complex 1 and its PhCO(2)(-) analogue complex 2. Frequency-dependent (chi)''(M) values are normally seen for superparamagnetic materials. Complex 3a exhibits two peaks in the (chi)''(M) VS T plot; hence, two different relaxation processes are seen. When an ac field oscillating at 500 Hz is used, for example, the two (chi)''(M) peaks occur at 2.8 and 6.4 K. Upon reexamination of the ac susceptibilities of complexes 1 and 2, it was found that they also show a second low-temperature (chi)''(M) peak. Polystyrene-doped (3.6 and 54 wt %) samples of complex 3a also show two (chi)''(M) peaks. DC susceptibility data for complexes 3a, 4, and polystyrene-doped 3a show that these complexes exhibit well-developed hysteresis loops. Furthermore, zero-field-cooled and field-cooled magnetization data for complexes 3a and 4 are not superimposed at the lowest temperatures. All of these data reflect the presence of relaxation processes. The fact that the same observations are made for the polystyrene-doped sample of complex 3a indicates that the hysteresis/relaxation processes are attributable to properties of individual molecules, rather than from a bulk effect of the material.
