PREPARATION AND PROPERTIES OF MODELS FOR THE PHOTOSYNTHETIC WATER OXIDATION CENTER - SPIN FRUSTRATION IN THE [MN4O2(O2CR)7(PIC)2]- ANION

Synthetic procedures are described that allow access to complexes containing the anion [Mn4O2(O2CR)7(pic)2]- (R = Me, Et, Ph; picH = picolinic acid). Reaction mixtures consisting of Mn(O2CR)2 (R = Me, Ph), RCOOH, pic-, and NBu(n)4MnO4 in MeCN give deep red solutions from which the products (NBu(n)4)[Mn4O2(O2CME)7(pic)2] (1) and (NBu(n)4)[Mn4O2(O2CPh)7(pic)2] (2) can be obtained in high yield and purity. Complex 1.MeCN crystallizes in triclinic space group P1BAR with (at -158-degrees-C) a = 12.655 (2) angstrom, b = 18.919 (4) angstrom, c = 11.951 (2) angstrom, alpha = 98.00 (1)-degrees, beta = 101.07 (1)-degrees, gamma = 77.43 (1)-degrees, Z = 2, and V = 2726.59 angstrom 3. The structure was solved by direct methods (MULTAN) and refined to values of conventional indices R (R(w)) of 7.01% (6.90%) by using 5778 unique reflections with F > 3-sigma-(F). The structure of the anion consists of an [Mn4(mu-3-O)2]8+ core with the four Mn atoms disposed in a "butterfly" arrangement and O atoms triply bridging each plane ("wing"). Peripheral ligation is provided by seven mu-AcO- and two chelating pic- groups; the overall structure has idealized C2 symmetry. The anion of 1 bears striking similarity to the cation [Mn4O2(O2CMe)7(bpy)2]+ reported previously, showing little structural consequence of the pic-for-bpy substitution. Treatment of [Mn3O(O2CR)6(py)3](ClO4) (R = Me, Et) with Na(pic).3/4H2O in MeCN leads to formation of sparingly soluble Na[Mn4O2(O2CR)7(pic)2] salts, providing an additional synthetic route to the tetranuclear anions. Complex 1 can be cleanly converted to 2 by treatment with excess PhCOOH in MeCN, demonstrating facile carboxylate exchange with more acidic acids. Variable-temperature, solid-state magnetic susceptibility studies have been performed on complex 1 in the range 5-300 K. The effective magnetic moment, mu-eff/Mn4, decreases gradually from 7.98 mu-B at 300.0 K to 6.35 mu-B at 25.00 K and then decreases more rapidly to 3.28 mu-B at 5.00 K. The data were fit to a theoretical expression derived from the appropriate spin Hamiltonian for a Mn4O2 core with C2v symmetry which also included an axial zero-field splitting term [D(S triple-over-dot-z2 - 1/3S triple-over-dot-T2)] for the S = 3 ground state. The fitting parameters are J(wb) = -5.3 cm-1, J(bb) = -24.6 cm-1, g = 1.96, and D = 3.7 cm-1 with TIP held constant at 200 x 10(-6) cgsu; J(bb) refers to the interaction between the two-body bis(mu-oxide)-bridged Mn(III) atoms, and J(wb) refers to the four mono(mu-oxide)-bridged pairs of Mn(III) atoms. The exchange parameters are very similar to those previously reported for [Mn4O2-(O2CMe)7(bpy)2](ClO4).3H2O (J(wb) = -7.8 cm-1, J(bb) = -23.5 cm-1), consistent with the structural similarities between the two complexes. The spin frustration present in these [Mn4O2]8+ complexes is examined in detail. Depending on the magnitudes of J(wb) and J(bb), different ground states are found. The presence of mu-3-oxide-bridged Mn(III)4 units in these complexes causes frustration in the spin alignment at each Mn ion. The proton NMR spectral data are reported for complex 1 in CDCl3, CD3CN, and (CD3)2SO. The spectra in all three solvents are very similar, supporting retention of the solid-state structure of the anion of 1 in good donor solvents. The only major influence of the pic-for-bpy substitution is to be found in the redox properties investigated by cyclic voltammetry. Complexes 1 and 2 display a quasi-reversible oxidation in CH2Cl2 at +0.52 and +0.54 V vs ferrocene, respectively. The latter is to be contrasted with the value of +0.87 V found previously for [Mn4O2(O2CPh)7(bpy)2](ClO4), showing that oxidation to the 3Mn(III), Mn(IV) oxidation level becomes easier when bpy is replaced by pic-.