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]

Synthetic procedures are described that allow conversion of [Mn 4 O 2 (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 n 4 Cl in CH 2 Cl 2 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 Me 3 SiCl in CH 2 Cl 2 . An analogous procedure with 4 equiv of Me 3 SiBr in CH 2 Br 2 gives 3 in 55% yield. Complexes 2 and 3 are isomorphous, monoclinic space group P 2 1 / 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 ( R w ) 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 [Mn 3 O(OAc) 6 (py) 3 ](ClO 4 ) in MeCN with Me 3 SiCl followed by addition of H 2 O and acetic acid results in crystallization of (pyH) 3 [Mn 4 O 3 Cl 7 (OAc) 3 ]·2MeCN ( 4 ) in 75% yield (based on Mn). Complex 4 crystallizes in monoclinic space group C 2/ 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 Å 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 > 3.0σ( F ). The complexes possess a [Mn 4 (μ 3 -O) 3 (μ 3 -X)] distorted cubane core and a 3Mn III,Mn IV trapped-valence oxidation-state description. Three AcO - groups bridge each Mn III Mn IV 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 μ 3 -O 2 - 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: μ eff values steadily rise from ∼9 μ B at room temperature to ∼10 μ B at 30.0 K and then drop rapidly to ∼9.5 μ 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 (∼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 μ 3 -O 2 - ions do not cause a significant change to the exchange parameters or to the electronic structure of the [Mn 4 O 3 Cl] 6+ core. 1 H NMR spectra of 2 − 4 in CDCl 3 or CD 3 CN solution at ∼23 °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 CH 2 Cl 2 /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, 1 H 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.