Nature uses an Mn cluster for water oxidation, and thus, water oxidation using Mn clusters is interesting when used in artificial water-splitting systems. An important question is whether an Mn cluster is a true catalyst for water oxidation or not. Herein, an Mn–K cluster was investigated for electrochemical water oxidation to find the true and the kinetically dominant catalyst using X-ray absorption spectroscopy, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and electrochemical methods. The experiments showed that conversion into nanosized Mn oxide occurred for the cluster, and the nanosized Mn oxides are the true catalyst for water oxidation

Chernev, Petko

Dau, Holger

Hołyńska, Małgorzata

Mohammadi, Mohammad Reza

Najafpour, Mohammad Mahdi

Safdari, Rasoul

Institutional Repository of the Freie Universität Berlin

1Electronic Supplementary InformationElectronic Supplementary Material (ESI) for Dalton Transactions.This journal is © The Royal Society of Chemistry 20182A mononuclear cobalt complex for water oxidation:  New controversies and puzzlesRasoul Safdari,a Mohammad Reza Mohammadi,b,c Małgorzata Hołyńska,d Petko Chernev,b,e Holger Daub and  Mohammad Mahdi Najafpour*a,f,gaDepartment of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, 45137-66731, Iran bFreie Universität Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin, GermanycUniversity of Sistan and Baluchestan, Department of Physics, Zahedan, Iran, 98167-45845dFachbereich Chemie and Wissenschaftliches Zentrum für Materialwissenschaften (WZMW), Philipps-Universität Marburg, Hans-Meerwein-Straße, D-35032 Marburg, GermanyeUppsala University, Department of Chemistry - Ångströmlaboratoriet, Lägerhyddsvägen 1, 75120 Uppsala, SwedenfCenter of Climate Change and Global Warming, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, 45137-66731, IrangResearch Center for Basic Sciences & Modern Technologies (RBST), Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran*Corresponding author; Phone: (+98) 24 3315 3201; E-mail: mmnajafpour@iasbs.ac.ir3ExperimentalMaterials All reagents and solvents were purchased from commercial sources and were used without further purification. Di(2-pyridyl)ketone, cobalt(II) perchlorate hexahydrate and fluorine-doped tin oxide coated glass (FTO) were purchased from the Sigma-Aldrich  Company. Sodium carbonate was purchased from Merck Company.Synthesis of 11 was synthesized by the previously reported method [1]. Solution 1: Co(ClO4)2·6H2O (0.5 mmol) was dissolved in 4 mL of acetonitrile. Solution 2: Na2CO3 (0.5 mmol) and di(2-pyridyl)ketone (1.0 mmol) (1:2) were dissolved in a mixture of distilled water and acetonitrile (Vwater:Vacetonitrile= 1:1, 8mL) and the mixture was stirred until a clear solution was formed. Subsequently, the solution 1 was slowly added dropwise to the solution 2 under strong stirring. The solution was strongly stirred for 3 hours and then filtered. The filtrate was kept in a 20 mL flask to allow slow evaporation at room temperature and the dark-red crystals of 1 were obtained after 5 days.CharacterizationElectrochemical experiments were performed using an EmStat3+ device from PalmSens (Netherlands). Cyclic voltammetry studies were carried out with a conventional three-electrode setup in which FTO, Ag|AgCl|KClsat and a platinum rod served as the working, 4reference and auxiliary electrodes, respectively. The distance between the two opposite sides of the FTO electrode was measured with a digital caliper MarCal 16ER model (Mahr, Germany). The temperature was measured with the Laserliner 082 device (Germany). XAS experimentsAfter the bulk electrolysis of 1 for one hour at 1.20 V (vs. Ag|AgCl|KClsat) using an FTO electrode we investigated this FTO electrode (FTO-A) in XAS experiments. The electrode was frozen by immersion in liquid nitrogen.XAS measurements at the cobalt K-edge were performed at the KMC-3 beamline at the BESSY II synchrotron facility (Helmholtz-Zentrum Berlin, Germany) at 20 K in a liquid-helium cooled cryostat (Oxford-Danfysik). The angle between the film surface and the incident beam was approximately 45°. Fluorescence-detected X-ray absorption spectra at the cobalt K-edge were collected using a 13-element Ge detector (Ultra-LEGe, Canberra) installed perpendicular to the X-ray beam.X-ray diffraction studiesSingle crystal in form of an orange block was mounted on a Bruker Quest D8 diffractometer with CMOS detector. A multi-scan absorption correction was applied. Basic crystallographic data are collected in Table S1.5RefinementThe crystal structure was solved by direct methods in SHELXS and refined with a full-matrix technique in SHELXL-2014. O-bonded H atoms were found on difference Fourier map and subsequently, the cation hydroxyl H atoms were constrained with AFIX 147 constraints. Additional maxima appearing in the vicinity of perchlorate O atoms were interpreted as a disorder of this anion. In this disorder the Cl1-O8 moiety position does not change, whereas the remaining three O atoms adopt two positions related by a rotation along the Cl1-O8 axis. The refined occupancies are 0.89(1) and 0.11(1), respectively. The minor-occupancy component was refined isotropically. SADI restraints were applied to keep the Cl-O bond lengths similar. On the final difference Fourier map the highest maximum of 0.48 e/Å3 is located in the middle of C12-C13 bond.6Fig. S1 ESI-Mass spectrum for 1. 7Fig. S2 Cation columns formed along the a direction. The disordered anion minor component and H atoms are omitted for clarity.8Fig. S3 SEM images for the fresh FTO.9Fig. S4 SEM images for the film formed on the FTO-A electrode after 5.0 hours of electrolysis of 1 at (1.20 V vs. vs. Ag/AgCl) in 100.0 mL of sodium borate buffer solution  (pH  9.0,  80  mM).10Fig. S5 EDX-SEM results for the film formed on the FTO-A electrode after 5.0 hours of electrolysis of 1 at (1.20 V vs. vs. Ag/AgCl) in 100.0 mL of sodium borate buffer solution  (pH  9.0,  80  mM).11Fig. S6 1HNMR spectra for 1 before (below) and after 5.0 hours (top) amperometry (1.20 V vs. Ag/AgCl) of 1 (20.0 mg) in 100.0 mL of sodium borate buffer solutions (pH 9.0, 80 mM).123 4 5 6 7 8 9 10 11 12FTO-A11 on FTO10k3(k)[Å-3]k [Å-1]Fig. S7 k3-weighted χ(k) of 1 and 1 adsorbed on FTO as well as the operated compound on FTO (FTO-A) at 1.20 V for 5.0 hour in 100.0 mL of sodium  borate  buffer  solutions  (pH  9.0,  80  mM) and the simulation results (red lines). XANES of adsorbed 1 on FTO and FTO-A at 1.2 V in 100.0 mL of sodium borate buffer solutions (pH 9.0, 80 mM). The fit parameters are given in Table 1.EXAFS simulationsSimulations were performed using the in-house software package SimX[2] and SimXLite.An EXAFS spectrum 𝜒 (𝑘)  is given by the sum of the contributions of nshell ‘atomic shells’. A ‘shell’ is a group of elements with identical atomic number and similar distances from the X-ray absorbing atom (e.g., six oxygen atoms surrounding the absorbing manganese ion). The EXAFS equation is mathematically defined by the following equation [3, 4]:where 𝑆ₒ² is the amplitude reduction factor, 𝐴(𝑅𝑖𝑖,𝑘)𝑖  is a factor that includes the scattering amplitude and mean-free-path of the photo-electron, 𝜙𝑖 the phase correction, 𝑁𝑖 the number of atoms in the 𝑖th  atomic shell, 𝜎𝑖 the Debye-Waller parameter of the 𝑖th atomic shell, and 𝑅𝑖 the (average) distance between the X-ray 13absorbing atom and the atoms of the 𝑖th atomic shell. The functions  𝐴 and 𝜙  were obtained herein from ab-initio calculations using Feff 9.05 [5], using coordinates from the molecular structure of 1.For conversion of the energy axis to a k-vector axis, an E0 of 7709 eV was used. Curve-fitting of the data was accomplished within a k-range of 2.5 Å-1 to 13 Å-1. The amplitude reduction factor, 𝑆ₒ², was 0.7. Parameter error estimation was performed as described in Ref. 6.As it is well known, distance R is not an independent parameter but strongly coupled with the E0 value. Similarly, the coordination numbers N and Debye-Waller parameters σ are also coupled and need to be separated from each other. Therefore, we first determined E0 and σ from a fit to the initial compound 1 in which the coordination numbers were fixed to the values expected from the molecular structure of 1 (sum of N for Co-Oshort/Nshort shells equal to 6, and sum of N for Co-C shells equal to 10). We then used these E0 and σ values for the fit of the sample after operation. 14Table S1 Basic crystallographic data.1Formula C22H18CoN4O4·ClO4·2(H2O)Formula weight 596.81Temperature [K] 100(2)λ [Å] 0.71073Crystal system TriclinicSpace group P1̅a [Å] 8.445 (3)b [Å] 11.908 (3)c [Å] 12.414 (4)α [°] 99.74 (3)β [°] 103.51 (3)γ [°] 90.24 (3)V [Å3] 1195.1 (7)Z, ρcalc [g cm-3] 2, 1.658μ [mm-1] 0.90F(000) 612Crystal size [mm] 0.64 × 0.24 × 0.15θ range[º] 2.2–30.1rflns: total /unique 60965/7036Abs. corr. multi-scanMin., max. transmission factors0.600, 0.876Data/restraints/params 7036/21/376GOF on F2 1.02R1 [I > 2σ(I)] 0.026wR2 (all data) 0.076Max., min. Δρelect [e Å-3] 0.48, -0.5415Table S2 Selected bond lengths and bond angles [Å, °].Co1—O3 1.8859 (11) Co1—N3 1.9302 (12)Co1—O1 1.8902 (11) O1—C6 1.4047 (14)Co1—N4 1.9133 (12) O2—C6 1.3759 (14)Co1—N1 1.9159 (11) O3—C17 1.4062 (14)Co1—N2 1.9238 (12) O4—C17 1.3799 (14)O3—Co1—O1 178.44 (3) N4—Co1—N2 178.45 (4)O3—Co1—N4 83.33 (5) N1—Co1—N2 87.92 (5)O1—Co1—N4 95.36 (5) O3—Co1—N3 82.91 (5)O3—Co1—N1 95.90 (5) O1—Co1—N3 97.93 (5)O1—Co1—N1 83.26 (5) N4—Co1—N3 88.82 (5)N4—Co1—N1 90.78 (5) N1—Co1—N3 178.78 (4)O3—Co1—N2 97.65 (5) N2—Co1—N3 92.49 (5)O1—Co1—N2 83.64 (5)16Table S3 Hydrogen bonding parameters [Å, °].D—H···A D—H H···A D···A D—H···AO1W—H1W1···O8i 0.81 (3) 2.15 (3) 2.942 (2) 167 (2)O2W—H1W2···O3i 0.78 (2) 1.90 (2) 2.6706 (16) 172 (2)O2W—H2W2···O8 0.77 (3) 2.12 (3) 2.8938 (18) 179 (3)O2—H2O···O1ii 0.84 1.78 2.6181 (14) 174O4—H4O···O2W 0.84 1.76 2.5929 (15) 173C2—H2···O1Wi 0.95 2.65 3.344 (2) 130C4—H4···O2iii 0.95 2.63 3.225 (2) 121C10—H10···O6i 0.95 2.62 3.142 (3) 115C11—H11···O6i 0.95 2.42 3.048 (2) 124C12—H12···O7iv 0.95 2.63 3.529 (2) 158C12—H12···O66iv 0.95 2.59 3.110 (14) 115C12—H12···O2ii 0.95 2.59 3.2171 (17) 124C15—H15···O4v 0.95 2.60 3.498 (2) 159C19—H19···O1Wv 0.95 2.66 3.252 (2) 121C22—H22···O7vi 0.95 2.37 3.314 (2) 174C22—H22···O77vi 0.95 2.45 3.356 (13) 158(i) -x+1, -y+1, -z+1(ii) -x+1, -y+2, -z+2(iii) -x+2, -y+2, -z+2(iv) x, y, z+1(v)  -x, -y+1, -z+1(vi) -x+1, -y+2, -z+117Reference[1] Y. Zhao, J. Lin, Y. Liu, B. Ma, Y. Ding and M. Chen, Chem. Commun., 2015, 51, 17309.[2] J. Dittmer, Ph.D. Thesis, Christian-Albrechts-Universität (Kiel, Germany), 1999.[3] J. E. Penner-Hahn, Coord. Chem. Rev., 1999, 1101, 190.[4] J. J. Rehr and R. C. Albers, Rev. Mod. Phys., 2000, 72, 621.[5] a) A. L. Ankudinov, B. Ravel, J. J. Rehr and S. D. Conradson, Phys. Rev. B, 1998, 58, 7565; b) J. J. Rehr, J. J. Kas, M. P. Prange, A. P. Sorini, Y. Takimoto and F. Vila, C. R. Physique, 2009, 10, 548.[6] M. Risch, K. Klingan, J. Heidkamp, D. Ehrenberg, P. Chernev, I. Zaharieva and H. Dau, Chem. Commun., 2011, 47, 11912.

new controversies and puzzles

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new controversies and puzzles

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