Modeling the Photosynthetic Water Oxidation Center: Chloride/Bromide Incorporation and Reversible Redox Processes in the Complexes Mn4O3X(OAc)3(dbm)3 (X = Cl, Br) and (pyH)3[Mn4O3Cl7(OAc)3

Abstract

Synthetic procedures are described that allow conversion of [Mn4O2(OAc)6(py)2(dbm)2] (1, dbmH = dibenzoylmethane) to [Mn4O3X(OAc)3(dbm)3] (X = Cl, 2; X = Br, 3). Treatment of 1 with NBun4Cl in CH2Cl2 or hot MeCN leads to 2 in 5−8% and 35−43% yields (based on dbm), respectively. A higher yield (∼88%) is obtained by treating 1 with 4 equiv of Me3SiCl in CH2Cl2. An analogous procedure with 4 equiv of Me3SiBr in CH2Br2 gives 3 in 55% yield. Complexes 2 and 3 are isomorphous, monoclinic space group P21/n, T = −155 °C, Z = 4. For 2, a = 13.900(3), b = 22.038(5), and c = 16.518(5) Å and β = 107.80(1)°; for 3, a = 13.644(2), b = 22.190(4), and c = 16.548(3) Å, and β = 106.64(1)°. The structures were solved by direct methods (MULTAN78) and refined on F to R(Rw) values of 7.85 (7.38) and 7.37 (6.89)% using 2267 and 2809 unique reflections with F > 2.33σ(F) for 2 and 3, respectively. Treatment of [Mn3O(OAc)6(py)3](ClO4) in MeCN with Me3SiCl 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 °C: a = 37.420(5), b = 13.752(1), and c = 16.139(2) Å, β = 110.33(1), V = 7787.9 Å3, and Z = 8. The structure was solved by direct methods (MULTAN78) and refined on F to R(Rw) values of 5.74 (5.78)% using 2612 unique reflections with F > 3.0σ(F). The complexes possess a [Mn4(μ3-O)3(μ3-X)] distorted cubane core and a 3MnIII,MnIV trapped-valence oxidation-state description. Three AcO- groups bridge each MnIIIMnIV pair, and a chelating dbm- (2 and 3) or two Cl- ions (4) on each MnIII complete peripheral ligation. The pyridinium cations of 4 are involved in hydrogen-bonding interactions with the μ3-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: μeff values steadily rise from ∼9 μB at room temperature to ∼10 μB at 30.0 K and then drop rapidly to ∼9.5 μB 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(MnIII···MnIV) = −28.4/−30.1 cm-1, J‘ = J(MnIII···MnIII) = +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 (∼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-bonding interactions of the μ3-O2- ions do not cause a significant change to the exchange parameters or to the electronic structure of the [Mn4O3Cl]6+ core. 1H NMR spectra of 2−4 in CDCl3 or CD3CN solution at ∼23 °C are similar and show that the Mn4 complexes retain their solid-state structure on dissolution in this solvent. X-band EPR spectra of 2 and 3 in CH2Cl2/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 [Mn4O3X] core can exist at three oxidation levels spanning the 4MnIII to 2MnIII, 2MnIV range. The combined results from 2 and 3 show that the identity of X has minimal influence on the resultant structures, magnetic properties, 1H 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.