HIGH-SPIN MOLECULES - IRON(III) INCORPORATION INTO [MN12O12(O(2)CME)(16)(H2O)(4)] TO YIELD [MN8FE4O12(O(2)CME)(16)(H2O)(4)] AND ITS INFLUENCE ON THE S=10 GROUND-STATE OF THE FORMER

The synthesis, single-crystal X-ray structure, and electrochemical, magnetochemical and Mossbauer properties are reported for [Mn8Fe4O12(O(2)CMe)(16)(H2O)(4)] (3) as its 2MeCO(2)H.4H(2)O solvate. Complex 3 represents the partially Fem-substituted form of [Mn12O12(O(2)CMe)16(H2O)4] (1), for which the benzoate analogue, [Mn12O12(O-2-CPh)(16)(H2O)(4)] (2) is also known. Treatment of Fe(O(2)CMe)2 with KMnO4 (16.3:6.4 molar ratio) in 60% aqueous acetic acid followed by slow heating to 60 degrees C, cooling to room temperature and layering of the golden brown solution with acetone, leads to black crystals of 3.2MeCO(2)H.4H(2)O in similar to 85% yield. The crystals are isomorphous with 1.2MeCO(2)H.4H(2)O, with the following unit cell parameters at -158 degrees C: tetragonal, I (4) over bar, a = b = 17.169(4), 12.258(3), V = 3612.9 Angstrom(3) and Z = 2. The structure was solved (MULTAN) and refined employing 3211 unique reflections with F > 3 sigma(F) to final values of R = 0.0768 and R(w) = 0.0768. The molecule consists of a central [(Mn4O4)-O-IV](8+) cubane held within a nonplanar ring of eight alternating Mn-III and Fe-III ions by eight mu 3-O-2-ions. Peripheral ligation is provided by sixteen mu-MeCO(2)(-) and four terminal H2O groups, the latter being ligated one each on the four Fe-III ions. The identification of the Fe-III ions was facilitated by the absence of a Jahn-Teller axial elongation as seen for the Mn-III ions. Elemental analysis data suggest a small fraction of molecules contain Fe-III ions at the Mn-III sites; Fe:Mn analysis ratios are approximately 4.37:7.63. Electrochemical studies in MeCN solution using cyclic voltammetry reveal a quasireversible oxidation at 0.81 V vs ferrocene and a quasireversible reduction at 0.17 V, in addition to irreversible oxidation and reduction features. The reversible processes occur at essentially identical potentials as for 1 suggesting the reduction and oxidation processes are occurring at manganese centers. Fe-57 Mossbauer spectra are reported for 3.2MeCO(2)H.4H(2)O at 300 and 120 K. At both temperatures there are two doublets present. A fit of the 300 K spectrum shows that the area associated with the main doublet is 82.6% of the total. This doublet has a quadrupole splitting (Delta E(Q)) of 0.459(4) mm/s and is attributable to the four high-spin FeIII ions identified in the X-ray structure. The other doublet with Delta E(Q) = 1.061(2) mm/s at 300 K is assignable to the excess high-spin FeIII which is disordered throughout the Mn-III sites. For a polycrystalline sample of 3.2MeCO(2)H.4H(2)O embedded in parafilm ina 10.0 kG field the value of mu(eff) per molecule is 11.18 mu(B) at 300.0 K and decreases gradually with decreasing temperature to 4.85 mu(B) at 5.00 K. Least-squares fitting of 0.50-50.O kG data in the 2.0-30 K range with a full matrix diagonalization approach shows that this Fe4Mn8 complex has a well-isolated S = 2 ground state with an axial zero-field splitting of D = -1.8 cm(-1). The origin of the change from a S = 10 ground state for the (Mn4Mn8III)-Mn-IV complex 1 to a S = 2 ground state for (Mn4Mn4Fe4III)-Mn-IV-Fe-III complex 3 is discussed.