Modeling the Photosynthetic Water Oxidation Center: Chloride/Bromide Incorporation and Reversible Redox Processes in the Complexes Mn4O3X(OAc)3(dbm)3 (X = C

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• 作者：Sheyi Wang ; Hui-Lien Tsai ; Eduardo Libby ; Kirsten Folting ; William E. Streib ; David N. Hendrickson ; and George Christou
• 刊名：Inorganic Chemistry
• 出版年：1996
• 出版时间：December 18, 1996
• 年：1996
• 卷：35
• 期：26
• 页码：7578 - 7589
• 全文大小：514K
• 年卷期：v.35,no.26(December 18, 1996)
• ISSN：1520-510X

文摘

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 of1 with NBuⁿ₄Cl inCH₂Cl₂ or hotMeCN leads to 2 in 5-8% and 35-43% yields (based ondbm), respectively. A higher yield (~88%) isobtainedby treating 1 with 4 equiv of Me₃SiCl inCH₂Cl₂. An analogous procedure with 4equiv of Me₃SiBr inCH₂Br₂gives 3 in 55% yield. Complexes 2 and3 are isomorphous, monoclinic space groupP2₁/n, T = -155 mages/entities/deg.gif">C,Z =4. For 2, a = 13.900(3), b= 22.038(5), and c = 16.518(5) Å and mages/gifchars/beta2.gif" BORDER=0 ALIGN="middle"> =107.80(1)mages/entities/deg.gif">; for 3, a = 13.644(2),b =22.190(4), and c = 16.548(3) Å, and mages/gifchars/beta2.gif" BORDER=0 ALIGN="middle"> =106.64(1)mages/entities/deg.gif">. 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 uniquereflections withF > 2.33mages/gifchars/sigma.gif" BORDER=0 >(F) for 2 and3, respectively. Treatment of[Mn₃O(OAc)₆(py)₃](ClO₄)in MeCN with Me₃SiCl followedby addition of H₂O and acetic acid results incrystallization of(pyH)₃[Mn₄O₃Cl₇(OAc)₃]·2MeCN(4) in 75%yield (based on Mn). Complex 4 crystallizes inmonoclinic space group C2/c with the followingcell parametersat -157 mages/entities/deg.gif">C: a = 37.420(5), b =13.752(1), and c = 16.139(2) Å, mages/gifchars/beta2.gif" BORDER=0 ALIGN="middle"> =110.33(1), V = 7787.9 ų, andZ = 8.The structure was solved by direct methods (MULTAN78) and refinedon F to R(R_w) values of5.74 (5.78)%using 2612 unique reflections with F >3.0mages/gifchars/sigma.gif" BORDER=0 >(F). The complexes possess a[Mn₄(mages/entities/mgr.gif">₃-O)₃(mages/entities/mgr.gif">₃-X)]distorted cubanecore and a 3Mn^III,Mn^IV trapped-valenceoxidation-state description. Three AcO^- groupsbridge each Mn^IIIMn^IVpair, and a chelating dbm^- (2 and3) or two Cl^- ions (4) on eachMn^III complete peripheral ligation. Thepyridiniumcations of 4 are involved in hydrogen-bonding interactionswith the mages/entities/mgr.gif">₃-O^2- and theterminal Cl^- ions of theanion. Variable-temperature solid-state magnetic susceptibilitystudies show that the magnetic properties of 2and 3 are very similar: mages/entities/mgr.gif">_eff values steadilyrise from ~9 mages/entities/mgr.gif">_B at room temperature to ~10mages/entities/mgr.gif">_B at 30.0 K and thendrop rapidly to ~9.5 mages/entities/mgr.gif">_B at 5 K. Fitting of theexperimental data for the two complexes to the appropriatetheoreticalequation yield the following fitting parameters, in the format2/3: 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 arewell-separated (~180 cm^-1) from anS = ⁷/₂ first excited state.The ground state was confirmed by magnetizationvs magnetic field studies at several fields and temperatures; fittingof the data allowed the zero-field splittingparameter D to be determined for both complexes. Themagnetochemical properties of 4 are very similartothose of 2 and 3, and the fitting parameters wereJ = -29.1 cm^-1, J'= +10.2 cm^-1, and g = 1.97,giving an S= ⁹/₂ ground state and showing that thehydrogen-bonding interactions of themages/entities/mgr.gif">₃-O^2- ions do not cause asignificantchange to the exchange parameters or to the electronic structure of the[Mn₄O₃Cl]⁶⁺ core.¹H NMR spectra of2-4 in CDCl₃ orCD₃CN solution at ~23 mages/entities/deg.gif">C are similar and show thatthe Mn₄ complexes retain their solid-statestructure on dissolution in this solvent. X-band EPR spectra of2 and 3 inCH₂Cl₂/toluene (1:1) glasses at 5Kare also extremely similar, with three main features at g= 11.0, 5.2, and 1.96. Cyclic voltammetry at 100mV/sand differential pulse voltammetry at 5 mV/s show that both2 and 3 support a reversible oxidation andtworeductions, the first of which is reversible. The reversibleprocesses are at 1.09/1.06 and -0.25/-0.21 V vsferrocene and show that the [Mn₄O₃X] core canexist at three oxidation levels spanning the 4Mn^III to2Mn^III,2Mn^IV range. The combined results from 2 and3 show that the identity of X has minimal influence ontheresultant structures, magnetic properties, ¹H NMR and EPRspectral properties, or the redox behavior. Suchobservations are of interest with regard to the ability ofBr^- to successfully substitute for Cl^- atthe photosyntheticwater oxidation center and thus maintain the activity of thetetranuclear Mn aggregate toward oxygen evolution.