Modeling the photosynthetic water oxidation center: Chloride/bromide incorporation and reversible redox processes in the complexes Mn(4)O(3)X(OAc)(3)(dbm)(3) (X=Cl, Br) and (pyH)(3)[Mn4O3Cl7(OAc)(3)]

Synthetic procedures are described that allow conversion of [Mn4O2(OAc)(6)(py)(2)(dbm)(2)] (1, dbmH = dibenzoylmethane) to [Mn(4)O(3)X(OAc)(3)(dbm)(3)] (X = Cl, 2; X = Br, 3). Treatment of 1 with NBu(4)(n)Cl in CH2Cl2 or hot MeCN leads to 2 in 5-8% and 35-43% yields (based on dbm), respectively. A higher yield (similar to 88%) is obtained by treating 1 with 4 equiv of Me(3)SiCl in CH2Cl2. An analogous procedure with 4 equiv of Me(3)SiBr in CH2Br2 gives 3 in 55% yield. Complexes 2 and 3 are isomorphous, monoclinic space group P2(1)/n, T = -155 degrees C, Z = 4. For 2, a = 13.900(3), b = 22.038(5), and c = 16.518(5) Angstrom and beta = 107.80(1)degrees; for 3, a = 13.644(2), b = 22.190(4), and c = 16.548(3) Angstrom, and beta = 106.64(1)degrees. The structures were solved by direct methods (MULTAN78) and refined on F to R(R(w)) values of 7.85 (7.38) and 7.37 (6.89)% using 2267 and 2809 unique reflections with F > 2.33 sigma(F) for 2 and 3, respectively. Treatment of [Mn3O(OAc)(6)(py)(3)](ClO4) in MeCN with Me(3)SiCl followed by addition of H2O and acetic acid results in crystallization of (pyH)(3)[Mn4O3Cl7(OAc)(3)]. 2MeCN (4) in 75% yield (based on Mn). Complex 4 crystallizes in monoclinic space group C2/c with the following cell parameters at -157 degrees C: a = 37.420(5), b = 13.752(1), and c = 16.139(2) Angstrom, beta = 110.33(1), V = 7787.9 Angstrom(3), and Z = 8. The structure was solved by direct methods (MULTAN78) and refined on F to R(R(w)) values of 5.74 (5.78)% using 2612 unique reflections with F > 3.0 sigma(F). The complexes possess a [Mn-4(mu(3)-O)(3)(mu 3-X)] distorted cubane core and a 3Mn(III),Mn-IV trapped-valence oxidation-state description. Three AcO- groups bridge each (MnMnIV)-Mn-III pair, and a chelating dbm(-) (2 and 3) or two Cl- ions (4) on each Mn-III complete peripheral ligation. The pyridinium cations of 4 are involved in hydrogen-bonding interactions with the mu(3)-O2- and the terminal Cl- ions of the anion. Variable-temperature solid-state magnetic susceptibility studies show that the magnetic properties of 2 and 3 are very similar: mu(eff) values steadily rise from similar to 9 mu(B) at room temperature to similar to 10 mu(B) at 30.0 K and then drop rapidly to similar to 9.5 mu(B) at 5 K. Fitting of the experimental data for the two complexes to the appropriate theoretical equation yield the following fitting parameters, in the format 2/3: J = J(Mn-III... Mn-IV) = -28.4/-30.1 cm(-1), J' = J(Mn-III... Mn-III) = +8.3/+7.4 cm(-1), and g = 1.98/2.03. Both 2 and 3 have S = 9/2 ground states that are well-separated (similar to 180 cm(-1)) from an S = 7/2 first excited state. The ground state was confirmed by magnetization vs magnetic field studies at several fields and temperatures; fitting of the data allowed the zero-field splitting parameter D to be determined for both complexes. The magnetochemical properties of 4 are very similar to those of 2 and 3, and the fitting parameters were J = -29.1 cm(-1), J' = +10.2 cm(-1), and g = 1. 97, giving an S = 9/2 ground state and showing that the hydrogen-bending interactions of the mu(3)-O2- ions do not cause a significant change to the exchange parameters or to the electronic structure of the [Mn4O3Cl](6+) core. H-1 NMR spectra of 2-4 in CDCl3 or CD3CN solution at similar to 23 degrees C are similar and show that the Mn-4 complexes retain their solid-state structure on dissolution in this solvent. X-band EPR spectra of 2 and 3 in CH2Cl3/toluene (1:1) glasses at 5 K are also extremely similar, with three main features at g = 11.0, 5.2, and 1.96. Cyclic voltammetry at 100 mV/s and differential pulse voltammetry at 5 mV/s show that both 2 and 3 support a reversible oxidation and two reductions, the first of which is reversible. The reversible processes are at 1.09/1.06 and -0.25/-0.21 V vs ferrocene and show that the [Mn(4)O(3)X] core can exist at three oxidation levels spanning the 4Mn(III) to 2Mn(III), 2Mn(IV) range. The combined results from 2 and 3 show that the identity of X has minimal influence on the resultant structures, magnetic properties, H-1 NMR and EPR spectral properties, or the redox behavior. Such observations are of interest with regard to the ability of Br- to successfully substitute for Cl- at the photosynthetic water oxidation center and thus maintain the activity of the tetranuclear Mn aggregate toward oxygen evolution.