Reversible Carboxylate Shift in a μ-Oxo Diferric ... - ACS Publications

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Reversible Carboxylate Shift in a μ‑Oxo Diferric Complex in Solution by Acid-/Base-Addition Thomas Philipp Zimmermann, Thomas Limpke, Anja Stammler, Hartmut Bögge, Stephan Walleck, and Thorsten Glaser* Lehrstuhl für Anorganische Chemie I, Fakultät für Chemie, Universität Bielefeld, Universitätsstrasse 25, D-33615 Bielefeld, Germany S Supporting Information *

ABSTRACT: A reversible carboxylate shift has been observed in a μ-oxo diferric complex in solution by UV−vis−NIR and FTIR spectroscopy triggered by the addition of a base or an acid. A terminal acetate decoordinates upon the addition of a proton, resulting in a shift of the remaining terminal acetato to a μ−η1:η1 bridge. The addition of a base restores the original structure containing only terminal acetates. The implications for metalloenzymes with carboxylate-bridged nonheme diiron active sites are discussed.

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OAc)FeIII}]3+ (Figure 1a,c) with a bridging acetate by the addition of a base.

INTRODUCTION Carboxylate-bridged nonheme diiron centers form the active sites of many metalloenzymes catalyzing various essential biological processes. This family includes bacterial multicomponent monooxygenases (BMMs), ribonucleotide reductase (RNR), Δ9 desaturase, and human deoxyhypusine hydroxylase (h DoHH).1−6 Besides at least one bridging carboxylate, additional carboxylates are coordinated in various coordination modes. It has been proposed that these carboxylate ligands can change their coordination modes7−12 in a dynamic process termed carboxylate shift.7,8 This carboxylate shift can open coordination sites for substrate or O2 binding, store and release protons in the enzyme’s active site, or modulate the molecular and thus electronic structure for function.13−15 An example for the carboxylate shift in a peroxobridged diferric biomimetic complex was reported,16 although other assignments have been made in the literature.11,17,18 Furthermore, a dynamic carboxylate shift in the solid state has been established by temperature-dependent X-ray crystallography in a pyrazolate-based diferrous complex.19 In solution, a carboxylate shift has been reported for tetranuclear zinc complexes by NMR spectroscopy.20 Here we report to the best of our knowledge the first direct observation of a reversible carboxylate shift in solution for a biomimetic diferric complex. This carboxylate shift between a μ−η1:η1 bridging and a monodentate terminal coordination mode is reminiscent to glutamate 115 in ribonucleotide reductase and is adjustable by acid/base addition and verified by single-crystal X-ray diffraction. This carboxylate shift can be induced starting either from [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}]2+ (Figure 1a,b) with two terminal acetates by the addition of a proton or from [(susan){FeIII(μ-O)(μ© XXXX American Chemical Society

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EXPERIMENTAL SECTION

Solvents and starting materials were of the highest commercially available purity and used as received. The syntheses of the ligand susan (= 4,7-dimethyl-1,1,10,10-tetra(2-pyridylmethyl)-1,4,7,10-tetraazadecane) and of [(susan){FeCl(μ-O)FeCl}](ClO4)2 were previously reported.21 Although we experienced no problems, the use of perchlorate salts is potentially hazardous and should only be handled in small quantities and with adequate precautions. [(susan){Fe(OAc)(μ-O)Fe(OAc)}](ClO4)2·2.5H2O. A solution of susan (132 mg, 0.245 mmol, 1 equiv) in acetone (20 mL) was added to a solution of Fe(ClO4)2·6H2O (185 mg, 0.510 mmol, 2.08 equiv) and (Bu4N)OAc (182 mg, 0.604 mmol, 2.46 equiv) in EtOH (20 mL). The resulting brown solution was treated with Et3N (0.2 mL, 1.4 mmol, 5.9 equiv) and stirred for 1 h. Diffusion of MTBE resulted after 1 week in deep-brown crystals, which were filtered off, washed three times with MTBE, and dried under reduced pressure. Yield: 144 mg (140 mmol, 57%). IR (KBr): ν̃/cm−1 = 3432 s, 3074 w, 2924 w, 2878 w, 2817 w, 2014 w, 1711 w, 1639 s, 1607 s, 1572 m, 1473 m, 1447 m, 1434 m,1372 m, 1309 s, 1273 m, 1089 vs, 1018 s, 947 m, 927 w, 864 w, 814 s, 773 m, 734 w, 722 w, 637 m, 624 s, 506 w, 471 w, 416 w. ESI−MS (MeCN): m/z = 392.1 [(susan){Fe(OAc)(μ-O)Fe(OAc)}]2+, 380.1 [(susan){Fe(OAc)(μ-O)Fe(Cl)}]2+, 371.2 [(susan){Fe(OAc)(μ-O)Fe(OH)}]2+, 359.2 [(susan){Fe(OH)(μ-O)FeCl}]2+. Anal. Found: C 41.91; H 5.08; N 10.82. Calcd for [(susan){Fe(OAc)(μ-O)Fe(OAc)}](ClO4)2·2.5H2O: C 42.04; H 5.19; N 10.90. [(susan){Fe(μ-O)(μ-OAc)Fe](ClO4)3·3H2O. A solution of susan (196 mg, 0.36 mmol, 1 equiv) in acetone (20 mL) was added to a solution of Fe(ClO4)2·6H2O (293 mg, 0.81 mmol, 2.24 equiv) and (Bu4N)OAc (117 mg, 0.39 mmol, 1.07 equiv) in EtOH (20 mL). The addition of NEt3 (230 mg, 2.27 mmol, 6.3 equiv) (2 mL) resulted in a Received: February 9, 2018

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DOI: 10.1021/acs.inorgchem.8b00376 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

One acetone molecule was found in the asymmetric unit of [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}](ClO4)2·acetone, which, however, suffered from strong disorder and could not be refined properly. Thus its scattering power was removed from the data set using the PLATON/SQUEEZE routine,26 which finds approximately 40 electrons per complex molecule in the solvent region. This corresponds to approximately one solvent molecule. Acetone had been used together with EtOH during the crystallization process and is therefore included in the given sum formula and derived quantities. It cannot be excluded that small quantities of EtOH are (disordered with acetone) also present in the unit cell. Crystal Data for [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}](ClO4)2· acetone (M = 1041.50): monoclinic, space group P21/c (no. 14), a = 19.7554(7), b = 14.2438(5), c = 16.1598(6) Å, β = 101.171(2)°, V = 4461.1(3) Å3, Z = 4, T = 100(2) K, μ(MoKα) = 0.846 mm−1, ρcalc= 1.551 g/cm3, crystal size = 0.44 × 0.27 × 0.17 mm3, 78 353 reflections measured (5.08 ≤ 2Θ ≤ 50.00°), 7830 unique reflections (Rint = 0.0235) used in the refinements. The final R1 values (600 refined parameters) were 0.0464 for 7002 reflections with I > 2σ(I) and 0.0510 for all data. Crystal Data for [(susan){FeIII(μ-O)(μ-OAc)FeIII}](ClO4)3·1/ 2EtOH·1/4H2O (M = 1051.37): orthorhombic, space group Pna21 (no. 33), a = 22.0987(6), b = 19.4499(5), c = 10.4588(3) Å, V = 4495.4(2) Å3, Z = 4, T = 100(2) K, μ(Cu Kα) = 7.481 mm−1, ρcalc= 1.553 g/cm3, crystal size = 0.33 × 0.04 × 0.03 mm3, 33 810 reflections measured (6.06 ≤ 2Θ ≤ 134.22°), 6921 unique reflections (Rint = 0.0464) used in the refinements. The final R1 values (640 refined parameters) were 0.0454 for 6065 reflections with I > 2σ(I) and 0.0553 for all data. The structure was refined as a two-component (ratio approximately 1/1) inversion twin. CCDC-1817685 and -1817686 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam. ac.uk/data_request/cif. Other Physical Measurements. Infrared spectra (400−4000 cm−1) of solid samples were recorded on a Shimadzu FTIR 8300 or a Bruker Vertex 70 as KBr disks. Infrared spectra of solutions were measured on a Bruker Vertex 70 using a MIRacle-ATR unit equipped with a triple reflection diamond/ZnSe crystal plate by placing a drop of the solution on the diamond/ZnSe crystal. The spectra were corrected for the background of the solvent used. The ATR spectra are shown as received and have not been corrected for the wavelength. UV− vis−NIR absorption spectra were measured on a Shimadzu UV3101 PC spectrophotometer at ambient temperature, on a JASCO V770-ST UV−vis−NIR spectrophotometer at 20 °C, or on an AGILENT 8453 UV−vis spectrophotometer.

Figure 1. (a) Ligand used. Molecular structures of (b) [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}]2+ in single-crystals of [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}](ClO4)2·acetone and (c) [(susan){FeIII(μ-O)(μ-OAc)FeIII}]3+ in single-crystals of [(susan){FeIII(μO)(μ-OAc)FeIII}](ClO4)3·1/4H2O·1/2EtOH. Hydrogen atoms have been omitted for clarity.

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RESULTS AND DISCUSSION The ligand susan (= 4,7-dimethyl-1,1,10,10-tetra(2-pyridylmethyl)-1,4,7,10-tetraazadecane, Figure 1a) is a member of a dinucleating ligand family providing two tetradentate ligand compartments covalently bridged by a flexible ethylene linker with varying terminal donors.27,21,28,29 The two complexes [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}](ClO4)2 and [(susan){FeIII(μ-O)(μ-OAc)FeIII}](ClO4)3 have been synthesized using the ligand susan [FeII(H2O)6](ClO4)2, and NEt3, combined with the addition of either 2.5 or 1.1 equiv of TBA(OAc), respectively. The molecular structures of both complexes have been determined by single-crystal X-ray diffraction, and the two cations are shown in Figure 1b,c. Selected interatomic distances and angles are provided in Table 1. In [(susan){FeIII(OAc)(μO)FeIII(OAc)}]2+ (Figure 1b), the two terminal acetates coordinate trans to each other with respect to the Fe(μ-O)Fe axis as in [(susan){FeIIICl(μ-O)FeIIICl}]2+,21 while the bridging acetate in [(susan){FeIII(μ-O)(μ-OAc)FeIII}]3+ coordinates cis. The strongest difference in bond distances between the two complexes is for the acetato ligands. In [(susan){FeIII(OAc)(μ-

dark suspension. Stirring for 2 h provided a solution that was filtered. Diffusion of MTBE initiated the deposition of dark-brown crystals, which were washed three times with MTBE and dried under reduced pressure. Yield: 247 mg (0.24 mmol, 67%). IR (KBr): ν̃/cm−1 = 3438 m, 3079 w, 2930 w, 2880 w, 2021 w, 1608 s, 1572 w, 1536 m, 1468 m, 1439 s, 1366 w, 1310 w, 1295 w, 1272 w, 1095 vs, 1022 m, 942 w, 924 w, 821 w, 765 s, 735 w, 721 w, 655 w, 624 s, 486 w, 417 w. ESI−MS (MeCN/acetone) m/z = 923.0 {[(susan){Fe(μ-O)(μ-OAc)Fe](ClO4)2}+. Anal. Found: C 37.71; H 4.63; N 10.21. Calcd for [(susan){Fe(μ-O)(μ-OAc)Fe](ClO4)3·3H2O C 37.89; H 4.77; N 10.40. Crystal Structure Determination. Single crystals of [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}](ClO4)2·acetone and of [(susan){FeIII(μ-O)(μ-OAc)FeIII}](ClO4)3·1/2EtOH·1/4H2O were removed from the mother liquor, coated with oil, and immediately cooled to 100(2) K on a Bruker Kappa-APEX-II and a Bruker X8-Prospector Ultra diffractometer with 4K CCD detector, respectively. SADABS2012/1 (Bruker, 2012) was used for multiscan absorption correction;22 solution and refinement with SHELXS/L23,24 using OLEX2.25 B

DOI: 10.1021/acs.inorgchem.8b00376 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Selected Interatomic Distances (Å) and Angles(deg) Obtained by Single-Crystal X-ray Diffraction [(susan){Fe(OAc)(μ-O)Fe(OAc)}](ClO4)2 Fe1−O3 Fe2−O3 Fe1−O51 Fe2−O61/52 Fe1−N1 Fe2−N41 Fe1−N2 Fe2−N42 Fe1−N3 Fe2−N43 Fe1−N4 Fe2−N44 O51−C51 O61−C61 O52−C51 O62−C61 O62B−C61 O53−C51 C51−C52 C61−C62 Fe1···Fe2 Fe2−O3−Fe1

1.7973(19) 1.7958(19) 1.931(2) 1.928(2) 2.238(3) 2.298(3) 2.206(3) 2.189(3) 2.145(3) 2.186(3) 2.226(2) 2.214(3) 1.286(4) 1.291(4) 1.218(4) 1.287(10) 1.101(19)

[(susan){Fe(μ-O)(μ-OAc)Fe}](ClO4)2 1.804(4) 1.792(4) 1.973(6) 2.066(5) 2.264(5) 2.208(5) 2.188(6) 2.232(5) 2.152(5) 2.153(5) 2.183(5) 2.125(5) 1.288(9) 1.245(8)

1.489(6) 1.485(6) 3.5794(6) 169.97(14)

1.492(10)) 3.2823(13) 131.8(3)

O)FeIII(OAc)}]2+ the Fe−OAc bond length is 1.93 Å, which increases to 1.97 and 2.07 Å in [(susan){FeIII(μ-O)(μOAc)FeIII}]3+. This bond length increase is in line with changing the coordination mode of a ligand from terminal to bridging.30 The asymmetry in the bond lengths of the bridging acetate is attributed to the different trans effects of the pyridine N44 and t-amine N2 donors. FTIR spectroscopy provides clear signatures of these two structural motifs not only in the solid state but also in solution. Spectra measured on KBr pellets (Figure 2a) exhibit the νas(FeOFe) stretch at 814 cm−1 in [(susan){FeIII(OAc)(μO)FeIII(OAc)}]2+, which almost coincides with 816 cm−1 in [(susan){FeIIICl(μ-O)FeIIICl}]2+ (Figure 2a) as a signature for the monobridged μ-oxo structure.21 In [(susan){FeIII(μ-O)(μOAc)FeIII}]3+ νas(FeOFe) shifts to 764 cm−1, as expected for the decreased Fe−O−Fe angle in doubly bridged μ-oxo diferric complexes.31,32 The carboxylate shift that causes the different bridging modes, can be detected by the change in the asymmetric νas(CO2) and symmetric νs(CO2) stretching modes of the acetates.33 The terminal acetates in [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}]2+ exhibit one νas(CO2) at 1639 cm−1 and two intense bands at 1372 and 1309 cm−1 for the νs(CO2) stretch. The μ−η1:η1 bridging mode in [(susan){FeIII(μ-O)(μ-OAc)FeIII}]3+ results in a smaller energy difference with νas(CO2) and νs(CO2) at 1535 and 1439 cm−1, respectively. A comparison of the FTIR spectra measured on KBr pellets or on a powder by ATR for [(susan){FeIII(OAc)(μO)FeIII(OAc)}](ClO4)2 and [(susan){FeIII(μ-O)(μ-OAc)FeIII}](ClO4)3 provides no significant differences (Figures S2 and S3).33 In general, dissolution of complexes might result in ligand substitutions or structural rearrangements. Thus spectroscopic methods that can be applied to the solid state and to solutions are of pivotal interest to correlate structures in solution to the solid state. FTIR spectroscopy is a readily available method for the solid-state (KBr pellets) and for solutions (ATR). In [(susan){FeIIICl(μ-O)FeIIICl}]2+, a strong νas(FeOFe) at 812

Figure 2. Sections of the FTIR spectra measured on (a) KBr pellets and (b) CH3CN solutions. The arrows highlight specific vibrations discussed in the text. (c) UV−vis−NIR spectra on CH3CN solutions. The spectra of [(susan){FeIIICl(μ-O)FeIIICl}](ClO4)2 have been added for comparison.21

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DOI: 10.1021/acs.inorgchem.8b00376 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry cm−1 confirms the μ-oxo diferric structure in solution (Figure 2b). For [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}]2+, almost the same stretches as in the solid state were found: 816 cm−1 νas (FeOFe), 1307 and 1373 cm−1 νs(CO2), 1639 cm−1 νas(CO2). For the acetato-bridged [(susan){FeIII(μ-O)(μOAc)FeIII}]3+, νas(FeOFe) at 766 cm−1 and νas(CO2) at 1567 cm−1 clearly confirm the doubly bridged structure in solution. Thus, using FTIR spectroscopy, we have shown that both acetato complexes retain their core structures in solution. The UV−vis−NIR spectra on CH3CN solutions (Figure 2c) are dominated by strong μ-oxo → FeIII LMCT transitions above 23 000 cm−1. [(susan){FeIIICl(μ-O)FeIIICl}]2+ shows additional bands at 17 340 and 10 540 cm−1, which were assigned to the 6A1 → 4T2 and 6A1 → 4T1 transitions, respectively, based on the work of Solomon et al. for μ-oxo diferric complexes.34 Such intense absorptions for FeIII h.s. d−d transitions can only arise in strongly coupled μ-oxo diferric complexes, thus confirming the persistence of the μ-oxo bridge in solution. [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}]2+ exhibits the analogous transitions at 17 700 and 10 300 cm−1 (Figure 2c insets). This demonstrates that the terminal ligands have no strong influence on the electronic absorption spectra in these monobridged μ-oxo complexes. In contrast, the doubly bridged [(susan){FeIII(μ-O)(μ-OAc)FeIII}]3+ exhibits besides the 6A1 → 4T1 transition around 10 000 cm−1 several stronger transitions around 20 000 cm−1 and a resolved band at 14 000 cm−1. These different signatures were already observed for mono- and doubly bridged complexes of tpa ligands.35 Thus, from the combination of X-ray diffraction, solid and solution FTIR, and solution UV−vis−NIR spectroscopies we can unambiguously conclude that both complexes [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}]2+ and [(susan){FeIII(μ-O)(μOAc)FeIII}]3+ dissolve in CH3CN without a severe change of their molecular structures. Moreover, we can easily use FTIR and UV−vis−NIR spectroscopies to follow structural rearrangements in solution. Figure 3a shows the UV−vis−NIR spectra of [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}]2+ in CH3CN at −40 °C upon the addition of 1 equiv HClO4. The band at 17 700 cm−1 disappears, while two new absorptions around 14 000 and 20 000 cm−1 appear, which are characteristic for the acetatobridged complex [(susan){FeIII(μ-O)(μ-OAc)FeIII}]3+. This indicates that one terminal acetato ligand in [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}]2+ is protonated and substituted by the noncoordinated oxygen atom of the other acetato ligand. Interestingly, solutions of the doubly bridged complex [(susan){FeIII(μ-O)(μ-OAc)FeIII}]3+ in acetone at 0 °C (Figure 3b) lose their bands at 14 000 and 20 000 cm−1 and gain a new band around 17 700 cm−1 upon the addition of NEt3. This indicates the formation of a mono-μ-oxo-bridged complex with one terminal ligand per FeIII site. Thus the bridging acetato ligand opens one coordination site and becomes a terminal acetato ligand on one FeIII site. This process is probably initiated by the nucleophilic attack of a ligand generated by the addition of NEt3. This ligand is most likely OH− from the deprotonation of H2O in the not-dried acetone, as a corresponding complex [(susan){FeIII(OH)(μO)FeIII(OH)}]2+ can be isolated from CH3CN solutions containing only susan, [FeII(H2O)6](ClO4)2, and NEt3 under aerobic conditions.36 To test the reversibility of these transformations, we have measured time-dependent UV−vis−NIR spectra upon the sequential addition of HClO4 and NEt3 to an acetone solution

Figure 3. (a) UV−vis−NIR spectra of [(susan){FeIII(OAc)(μO)FeIII(OAc)}]2+ in CH3CN at −40 °C and upon the addition of 1 equiv HClO4. (b) UV−vis−NIR spectra of [(susan){FeIII(μ-O)(μOAc)FeIII}]3+ in acetone at 0 °C and upon the addition of NEt3 (the number of equivalents NEt3 is the total number added).

of [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}]2+ at 0 °C (Figure 4a,b). The times of addition of HClO4 and NEt3 are indicated in Figure 4b, which shows the time traces of selected wave numbers and indicates the fast response after the addition of either HClO4 or NEt3. The spectra in Figure 4a are taken in between the additions of HClO4 and NEt3. The times provided in Figure 4a correlate with the x axis in Figure 4b. The addition of 1 equiv HClO4 results in the fast appearance of the 13 700 and 20 500 cm−1 bands corresponding to [(susan){FeIII(μO)(μ-OAc)FeIII}]3+. The addition of one equivalent NEt3 results in the reformation of the starting spectrum. It must be noted that this is not necessarily [(susan){FeIII(OAc)(μO)FeIII(OAc)}]2+ as the mono-μ-oxo bridged species with one terminal ligand per FeIII ion do not show significant differences in their absorption spectra (vide supra). However, with respect to the synthetic conditions, a re-entering of acetate is quite plausible. The addition of another equivalent NEt3 initiates no significant changes. Interestingly, the addition of 1 HClO4 also provides no changes, but the addition of another HClO4 brings back the spectrum of the doubly bridged species. This indicates that the one equivalent of NEt3, which had no influence, must be neutralized by one HClO4 first before the next HClO4 is effective again in the formation of the μ-acetato bridge. Analogously, the addition of another HClO4 makes no D

DOI: 10.1021/acs.inorgchem.8b00376 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

restores the spectrum of [(susan){Fe III (OAc)(μ-O)FeIII(OAc)}]2+. In summary, we have presented the direct observation of a reversible carboxylate shift between a bridging μ−η1:η1 and a monodentate terminal carboxylate coordination mode. The protonation of [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}]2+ results in the decoordination of an acetate presumable in the form of acetic acid. In the environment of a metalloprotein, this can open a coordination site combined with the storage of a proton close to the active site. In our model complex, the free coordination site becomes coordinated by a terminal acetate that becomes bridging. Interestingly, this bridging carboxylate can release an open coordination site upon the addition of a base. Thus open coordination sites in carboxylate-coordinated μ-oxo diiron active sides can be realized not only by the addition of an acid but also by a base. To the best of our knowledge, this is the first observation of such a reversible carboxylate shift in solution triggered by acid/base addition.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00376. Thermal ellipsoid plots and comparison of solid-state FTIR spectra KBr pellets versus ATR. (PDF) Accession Codes

CCDC 1817685−1817686 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax + 49-521-1066003.

Figure 4. (a) UV−vis−NIR spectra of a 0.27 mM solution of [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}](ClO4)2 in acetone at 0 °C by the consecutive addition of HClO4 and NEt3. Please note that the specified equivalents of HClO4 and NEt3 represent the total number added at the given time. (b) Time traces of the spectra in panel a at selected wavenumbers. The times of HClO4 and NEt3 additions are indicated. (c) Solution FTIR spectra in CH3CN of [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}]2+ and after the consecutive addition of HClO4 and NEt3. The solution FTIR spectrum of [(susan){FeIII(μO)(μ-OAc)FeIII}]3+ is added for comparison. The asterisk indicates solvent bands.

ORCID

Thorsten Glaser: 0000-0003-2056-7701 Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Solomon, E. I.; Brunold, T. C.; Davis, M. I.; Kemsley, J. N.; Lee, S.-K.; Lehnert, N.; Neese, F.; Skulan, A. J.; Yang, Y.-S.; Zhou, J. Geometric and Electronic Structure/Function Correlations in NonHeme Iron Enzymes. Chem. Rev. 2000, 100, 235−349. (2) Ray, K.; Pfaff, F. F.; Wang, B.; Nam, W. Status of reactive nonheme metal-oxygen intermediates in chemical and enzymatic reactions. J. Am. Chem. Soc. 2014, 136, 13942−13958. (3) Tinberg, C. E.; Lippard, S. J. Dioxygen activation in soluble methane monooxygenase. Acc. Chem. Res. 2011, 44, 280−288. (4) Kurtz, D. M. Oxo- and hydroxo-bridged diiron complexes. Chem. Rev. 1990, 90, 585−606. (5) Nordlund, P.; Eklund, H. Structure and function of the Escherichia coli ribonucleotide reductase protein R2. J. Mol. Biol. 1993, 232, 123−164. (6) Logan, D. T.; Su, X.-D.; Aberg, A.; Regnstrom, K.; Hajdu, J.; Eklund, H.; Nordlund, P. Structure 1996, 4, 1053−1064. (7) Rardin, R. L.; Tolman, W. B.; Lippard, S. J. Monodentate carboxylate complexes and the carboxylate shift - implications for

difference, and one NEt3 is needed again for neutralization before the next NEt3 opens the μ-acetato bridge to the mono-μoxo bridge. Thus the addition of NEt3 to [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}]2+ or of HClO4 to [(susan){FeIII(μ-O)(μOAc)FeIII}]3+ causes no changes. In summary, there is a protonation/deprotonation-induced carboxylate shift from bridging to terminal by the addition of a base and from terminal to bridging by the addition of a proton. This reversible carboxylate shift can also be observed by FTIR in solution (Figure 4c). Starting with [(susan){FeIII(OAc)(μ-O)FeIII(OAc)}]2+, the addition of one equivalent HClO4 provides a spectrum identical to that of [(susan){FeIII(μ-O)(μ-OAc)FeIII}]3+.The addition of NEt3 E

DOI: 10.1021/acs.inorgchem.8b00376 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00376 Inorg. Chem. XXXX, XXX, XXX−XXX