Preparation and properties of models for the photosynthetic water oxidation center : spin frustration in the [Mn4O2(O2Cr)7(pic)2]- anion

structure was solved by direct methods (MULTAN) and refined to values of conventional indices R (R,) of 7.01% (6.90%) by using 5778 unique reflections with F > 349. The structure of the anion consists of an [Mn4(p3-0),l8+ core with the four Mn atoms disposed in a 'butterfly" arrangement and 0 atoms triply bridging each plane ('wing"). Peripheral ligation is provided by seven p-AcO- and two chelating pic- groups; the overall structure has idealized C, symmetry. The anion of 1 bears striking similarity to the cation [Mn402(02CMe)7(bpy)2]+ reported previously, showing little structural consequence of the pic-for-bpy substitution. Treatment of [Mn30(02CR),(py),](C104) (R = Me, Et) with Na(pic)-'/,HzO in MeCN leads to formation of sparingly soluble Na[ Mn402(02CR)7(pi~)2] 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, pen/Mn4, decreases gradually from 7.98 pB at 300.0 K to 6.35 pB at 25.00 K and then decreases more rapidly to 3.28 pB at 5.00 K. The data were fit to a theoretical expression derived fromthe appfopriate spin Hamiltonian for a Mn4O2 core with C, symmetry which also included an axial zero-field splitting term [D(S: - 1/3ST2)] for the S = 3 ground state. The fitting parameters are Jwb = -5.3 cm-I, Jbb = -24.6 cm-l, g = 1.96, and D = 3.7 cm-l with TIP held constant at 200 X lod cgsu; Jbb refers to the interaction between the two-body bis(p-oxide)-bridged Mn"' atoms, and Jwb refers to the four mono(poxide)-bridged pairs of Mdl' atoms. The exchange parameters are very similar to those previously reported for (Mn402(02CMe)7(bpy)2](C104).3H,0 (Jwb = -7.8 cm-l, JN = -23.5 cm-I), consistent with the structural similarities between the two complexes. The spin frustration present in these [Mn4O2I8+ complexes is examined in detail. Depending on the magnitudes of Jwb and Jw, different ground states are found. The presence of p3-oxide-bridged Mn1114 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 CDCI,, CD3CN, and (CD3)*S0. 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 CH2C12 at +OS2 and +0.54 V vs ferrocene, respectively. The latter is to be contrasted with the value of +0.87 V found previously for [Mn402(02CPh)7(bpy)2](C104), showing that oxidation to the 3Mn'l',Mn'V oxidation level becomes easier when bpy is replaced by pic-.