Reactive Pendant Mn O in a Synthetic Structural Model of a Proposed

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A reactive pendant Mn=O in a synthetic structural model of a proposed S state in the photosynthetic oxygen evolving complex. 4

Shivaiah Vaddypally, Sandeep K. Kondaveeti, Santosh Karki, Megan M. Van Vliet, Robert J. Levis, and Michael J. Zdilla J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b05906 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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Journal of the American Chemical Society

A reactive pendant Mn=O in a synthetic structural model of a proposed S4 state in the photosynthetic oxygen evolving complex. Shivaiah Vaddypally, Sandeep K. Kondaveeti, Santosh Karki, Megan M. Van Vliet, Robert J. Levis, Michael J. Zdilla* Department of Chemistry, Temple University, 1901 N. 13th Street, Philadelphia, PA 19122 ABSTRACT: The molecular mechanism of the Oxygen Evolving Center of photosystem II has been under debate for decades. One frequently cited proposal is the nucleophilic attack by water hydroxide on a pendant Mn=O moiety, though no chemical example of reactivity at a manganese cubane cluster has been reported. We describe here the preparation, characterization, and a reactivity study of a synthetic manganese cubane cluster with a pendant manganese-oxo moiety. Reaction of this cluster with alkenes results in oxygen and hydrogen atom transfer reactions to form alcohol- and ketonebased oxygen-containing products. Nitrene transfer from core imides is negligible. The inorganic product is a cluster identical to the precursor, but with the pendant Mn=O moiety replaced by a hydrogen abstracted from the organic substrate, and is isolated in quantitative yield. 18O and 2H isotopic labelling studies confirm the transfer of atoms between the cluster and the organic substrate. The results suggest that the core cubane structure of this model compound remains intact, and that the pendant Mn=O moiety is preferentially reactive.

Introduction. The rise of oxygenic photosynthesis during the Archean eon marked the start of an explosion of life that terraformed the surface of the earth into a lush biological landscape. The key biological innovation that made this possible is the use of solar energy to excite and extract electrons from water for synthesis of cellular fuel. Water oxidation generates dioxygen as a byproduct, the sole known biological catalyst for which is photosystem II (PSII),1 a multi-subunit, membrane-bound enzyme present in all known oxygenic photosynthetic organisms. The water oxidation half-reaction is catalyzed by the Oxygen Evolving Complex (OEC, Figure 1) of photosystem II, a CaMn4 cluster. This cluster is itself oxidized by the redoxactive chlorophyll P680 following each successful photon absorption event, advancing the OEC through the Kok cycle (Figure 2) one electron at a time (S0-S4). In the highly oxidized S4 state, the enzyme reductively eliminates O2 and returns to the fully reduced S0 state.1

Figure 1. Left: X-ray crystallographic structural model of the 2,3 OEC. Right: nucleophilic attack hypothesis for O-O bond formation.

hν

P680+

O2 2 H2O

S0

P680

S4

S1 P680+

P680 hν P680+ S3

S2

P680 hν

P680 P680+ hν

Figure 2. Kok cycle for oxygen evolution by the OEC.(1)

The structure of the OEC in the dark-adapted S1 state has been reproduced in several crystal structures,2,3 described as a Mn3CaO4 heterocubane cluster with a 4th “dangler” or “pendant” manganese atom (Figure 1, left). Though there has been much progress on the structures of the intermediate S2 and S3 states4-15 at present there is not consensus on the structure of S4 state, nor the identity of the oxide ligands responsible for O2 formation. Numerous mechanistic proposals for O-O bond formation have been published, though theoretical investigations of the OEC tend to favor a description whereby formation of an oxyl radical monoanion ligand facilitates O-O coupling, 7,16-19 though there is not agreement on which oxygen atoms are involved. The generation of an obligatory reactive oxyl radical as part of the catalytic cycle has been proposed to play a role in enzyme degradation by abstraction of nearby protein-based hydrogen atoms17, and the involvement of radical oxygen character in water splitting is supported in part by the wide success of cobalt-based water oxidation catalysts, which impart radical character to oxo ligands via occupation of antibonding M-O orbitals

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by d-electrons.20-23 One early proposal16,24-26 that has gained much attention since the first high resolution structure of PSII3 is that a the pendant manganese atom facilitates the formation of a terminal Mn=O moiety with an electron-poor oxide which can undergo a nucleophilic attack from a Ca-bound hydroxo ligand (Figure 1, right).8,27 The appeal of this proposal stems from the known reactivity of Mn=O terminal oxide ligands from synthetic chemistry, and the known role of Lewis acidic sblock cations like Ca for activating water for nucleophilic attack.28 Whether the reacting terminal MnO moiety is best described as a manganese(V) oxo or a manganese(IV) oxyl radical has been a topic of debate. Electronic structure investigations on mononuclear species from the group of Borovik indicate a lack of electron spin character on terminal oxo ligands at manganese,29 and biomimetic chemistry from the group of Åkermark30 showed that the attack of solution phase hydroxide on a pendant Mn=O liberated O2. Studies from the groups of Goldberg, AbuOmar, Fukuzumi, and Nam have showed that valence tautomerism between metals and ancillary ligands and M=O bond order changes induced by Lewis Acids can tune reactivity of Mn=O centers,31-34 illustrating that proposals of Mn(V)=O vs. Mn(IV)-O· involvement in the S4 state are not necessarily mutually exclusive. Several groups have specifically proposed proton and redox tautomerism to be involved in the PSII mechanism.8,35,36

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since three of the quaternary carbons reproducibly exhibit non-positive displacement parameters. This behavior indicates the presence of a heavy atom at this location. Further, the C-C bonds in the assigned tert-butyl group are unreasonably long, at 1.65-1.66 Å, a bond length more akin to a terminal oxo complex of a first-row transition metal. The structure was successfully modeled as a compositionally disordered mixture of 2, and a cluster where three of the four bridging tert-butylimido groups are partially substituted by nitridotrioxomanganate(VII), [NMnO3]2-, a metalloligand with similar shape, size, and identical charge to the tert-butylimido ligand (R1(obs) = 2.8%). The occupancies of tert-butyl imide on the three compositionally disordered ligands containing N(1), N(2) and N(3) refine to 0.86, 0.86, and 0.89 respectively (Figure 3). Scheme 1. Reactivity of cluster 1

While there are several examples of structurally accurate OEC models37-40 or multinuclear MnO clusters that facilitate O2 evolution,39,41-46 detailed mechanistic information is lacking. Although involvement of specific MnO moieties in this reaction chemistry has been inferred, to our knowledge there are no isolated examples of manganese cubane species with “pendant” multiply bonded Mn=O groups. We report here the isolation of a pentamanganese cubane cluster with a pendant manganese trioxide group with structural similarity to the proposed S4 state, but with a more highly oxidized pendant MnVII. Its reactivity with two cycloalkenes is described. Results and Discussion The current report is a continuation of our work in manganese-tert-butylimido cubane cluster chemistry, which began with a report of a biologically inspired manganese-tert-butylimido cubane cluster synthesis that generates two products, both of which are Mn4Li4(N)(NtBu)10 cluster systems (1), but which can have either a 4th lithium ion or a Mn=N at the apex of the cluster (Scheme 1).47 When lithium ion is removed from these clusters, these compounds have been shown to undergo a 4-electron reductive elimination of azo-tert-butane, a nitrene analogue of the OEC product O2, generating a simple Mn4(µ3NtBu)4(NtBu)4 cubane cluster (2) in 80% yield (Scheme 1, left).48 When lithium ions are removed by 12-crown-4 in the presence of a substoichiometric amount of water, crystals with identical morphology to 2 are obtained. Despite morphological similarity, these cannot be crystallographically modeled using the simple formula for 2, Mn4(NtBu)8

Figure 3. Thermal ellipsoid plot of [2]5·[3]3 cocrystal showing two-part model with symmetry independent Mn, N, and O t 2atomic labels. Left: all bridging ligands modeled as [N Bu] with occupancies of 0.86, and 0.89 for the imides containing


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N(1) (two symmetry equivalent atoms) and N(2) respectively. 2Right: three of four bridging ligands modeled as [NMnO3] with occupancies of 0.14, and 0.11 for the metalloligands containing N(1) and N(2) respectively. The bridging tert-butyl imide containing N(3) is not compositionally disordered. Ellipsoids set at 50% probability level. Hydrogen atoms omitted for clarity.

While the crystallographic two-component model used for structural refinement is illustrated as the two molecules in Figure 3, in reality a binomial distribution of all possible ligand combinations is expected. Based on the observed occupancies favoring tert-butylimido ligation, the binomial distribution would be predominantly 2 (about 60%), and the formulation containing a single [NMnO3]2metalloligand, Mn4(µ3-NtBu)(µ3t NMnO3)(N Bu)4 (3) (about 26%). The remaining clusters containing two, three, and four [NMnO3]2- metalloligands represent a minority of the mixture. Thus, for simplicity, we consider this mixture as an approximate 5:3 ratio of 2 and 3 respectively (Scheme 1, right), and label this material as [2]5·[3]3 for simplicity, though ~14% of the crystalline material most likely contains other ligand mixtures. Compound 3 forms when water protolyzes tertbutylimido ligands of 1, generating tBuNH2 and manganese-oxo complexes, but the more inert nitrido ligands are are not protolyzed due to the stronger and less labile metal-nitride bond.49 In compositionally disordered [2]5·[3]3, crystallography alone cannot distinguish between the assignment of the bridging “nitrides” as nitrogen vs. oxygen atoms. The crystallographic assignment of nitride is favored for two reasons: One is that the alternative oxygen-containing permanganate ion (MnO4-) is a poor ligand, and while discrete MnO4 units are common internal components of clusters,50-54 permanganate would not be expected to bridge three metal ions in this fashion. Second, the formation of nitridotrioxomanganate(VII) from the precursor’s pendant [Mn(N)(NtBu)3]2- fragment in 1Mn=N or central [Mn(N)(NtBu)3]2- fragment in 1-Li (Scheme 1) is logical according to the following reaction:

trospray, we were able to measure a spectrum of one sample that showed a parent ion peak consistent with 3. Laser electrospray mass spectrometry (LEMS)56 proved to be a superior technique for reproducible detection of 3, and showed the expected isotopic distribution (Figure S1). In the LEMS measurement, a femtosecond laser pulse vaporizes analyte directly from the condensed phase into an electrospray stream. The vaporized analytes then selectively remain on the electrospray droplet surface where the charge resides, and hence the analysis is not limited by the solubility of the analyte in a particular solvent. LEMS analysis reveals a parent ion signal for 3 at m/z = 834.1928, and for 3 + H+ at m/z = 835.2091. (Figure S1), confirming the presence of 3 in this mixture, and positively identifying nitride as the ligand bridging to the pendant manganese atom. Preparation of 3 using isotopically labeled 18OH2 water gave a high resolution ESI mass spectrum with the expected 6 additional mass units, consistent with isotopic labelling of all three oxygen sites with 18O (Figure S1). Compound 3 is of fundamental interest due to its topological similarity to the proposed terminal oxo model of the S4 state of photosystem II (Figure 4). Though it must be acknowledged that the structures differ in that the pendant manganese atom in 3 is formally MnVII, while that of the proposed S4 state is formally MnV, the structural and ligand analogy makes the reactivity of 3 of fundamental interest. With 3 positively identified as a component of the cocrystal, we sought to explore reactivity of 3. Of particular interest are oxidation reactions with analogy to the action of the OEC. Imido and oxo complexes of high-valent 1st row middle-transition metals such as manganese are known to effect analogous nitrene57-60 and oxene,61-65 and proton coupled electron transfer (PCET)6671 reactions that bear analogy to the action of the OEC, which converts oxide to neutral oxene (in the form of O2) and mediates PCET as photons drive the coordinated motion of protons and electrons, driving the OEC through the Kok cycle to generate reducing equivalents, pumping protons to the lumen.1

[Mn(N)(NtBu)3]2- + 3 H2O [Mn(N)O3]2- + 3 H2NtBu

(1)

While the X-ray crystallographic model describing cluster 3 is sufficient to explain the X-ray data, X-ray diffraction on a compositionally disordered mixture is insufficient to be confident in the assignment of 3. We therefore turned to other spectroscopies to confirm the identity of 3. Infrared spectroscopy confirms the presence of a Mn=O stretch at 963 cm-1 (Figure S4) comparable to that of permanganate.55 This stretch is not seen in the identical reaction carried out in the absence of water, from which only 2 is isolated.48 IR spectroscopy is therefore consistent with the assignment of 3. Use of isotopically labelled 18OH2 for the synthesis of 3 gave an appropriate shift of the Mn=O stretch from 963 to 915 cm-1 (Figure S4). Though obtaining a high-resolution mass spectrum of 3 using standard ESI was challenging due to the insolubility of the compounds in the polar solvents needed for elec-

24

Figure 4. Comparison of proposed S4 structure to the structure of 3.

In contrast to the terminal manganese oxo complex of Åkermark, our systems did not show detectable oxidation of water or hydroxide to form O2, as measured via residual gas analysis mass spectrometry. This may be due to the



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fact that in the Åkermark system and in most proposed OEC mechanisms, the most oxidized manganese is Mn(V), while in our system, the pendant metal is a permanganate-like Mn(VII). This oxidation state may result in strong Mn=O bonds which are kinetically inert, or which cannot impart necessary radical character to the oxide ligand. Alternatively, protolysis of the cluster may be more rapid than redox chemistry, precluding water oxidation. We examined the reactivity of [2]5·[3]3 with cyclooctene, a commonly investigated nucleophilic substrate for nitrene and oxene transfer reactions.72-77 Since Mn(IV) and Mn(VII) imido and oxo complexes could inherently react via nitrene transfer or oxene transfer, the more reactive molecular moiety of 3 is of interest. Since crystals of [2]5·[3]3 are inherently a mixture, careful control experiments are necessary to distinguish the reactivity of Mn-N based 2 from the terminal manganese-oxocontaining 3. Mixing of pure isolated crystals of 2 with excess cyclooctene in pentane solvent with heating resulted in no detectable reaction products by 1H NMR, suggesting that MnIV-NtBu moieties are unreactive toward the olefin. However, the [2]5·[3]3 mixture reacts with cyclooctene generating colorless organic products, and a black, metal-containing product which can be separated from the organic products by precipitation with acetonitrile. Analysis of the organic products by 1H NMR and GC indicates a complex mixture of products: likely oligomers and polymers resulting from radical chemistry. Infrared spectroscopy reveals significant formation of ketones and alcohols by the appearance of bands at ~1700 cm-1 and ~ 3000 cm-1 respectively (Figure S6). GCMS positively identifies cyclooctane-1,2-dione and cylcooctane-1,2-diol (or tautomers thereof) as trace products that are not present in the starting materials (Figure S2). Among the numerous peaks in the GCMS chromatogram, No tert-butylnitrene-containing products, such as amines, diamines, imines, or aziridines were detected by, suggesting that nitrene transfer reactions are not significant. The formation of oxygen-containing products suggests oxene transfer is occurring from the pendant manganese group. Reactivity of cyclooctene with 18O isotopically labelled 3 gave the expected shifts in the IR frequency of ketonic products from 1670 to 1643 cm-1 (Figure S6) and isotopically labelled oxygenated organic diol products were observed by mass spectrometry (Figure S2). The formation of diketone suggests loss of olefinic hydrogen atoms during the reaction, indicating C-H bond activation by hydrogen atom abstraction is also mediated by 3.

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order on a special position for one of the two molecules in the asymmetric unit. The isolated crystalline material is obtained in near quantitative yielda, and is pure based powder X-ray diffraction of the bulk isolate, indicating only one crystallographic phase in the product, which matches the theoretical powder pattern of 4 (Figure S7). The presence of an N-H stretch in the IR spectrum at 3508 cm-1 supports the assignment of the [µ3-NH]2- ligand (Figure S5). Finally, the 1H NMR spectrum of 4 exhibits the expected four resonances, including a broad signal integrating to 1H, assigned to the bridging N-H ligand. The NMR is sharp and indicative of a S = 0 cluster as has previously been observed for Mn-NtBu cubane clusters.48 The observed inorganic and organic products and isotopic labelling experiments are consistent with oxene transfer as evidenced by the loss of oxo ligands from 3 and the oxygenation of cyclooctene. Further, the organic and inorganic products are consistent with hydrogen atom abstraction as evidenced by the appearance of a proton on the nitride ligand and the loss of olefinic hydrogens in the formation of ketones as organic products. Reaction of 3 with deuterated cis-cyclohexene results in the formation of 4 but with a deuterated μ3-N ligand evidenced by the isotopically shifted infrared band at 2276 cm-1, redshifted by 1232 cm-1 in comparison to the protonated 4, supporting the direct transfer of hydrogen from alkene to the cluster during the reaction. Reaction of protio-alkenes in deuterated solvents did not result in a deuterated μ3-N bridge in 4, suggesting hydrogen atom transfer occurs from the substrate, not from the solvent. Finally, the loss of the pendant manganese moiety of 3 in the formation of 4 is consistent with this metal atom being the site of reactivity. The overall reaction is illustrated in Scheme 2. Scheme 2. Reactivity of 3 with cyclooctene.

The metal-containing byproduct is less soluble in organic solvents than 2 and 3, permitting easy separation of the reaction product from unreacted 2. Crystallographic analysis reveals the metal-containing product as the cubane cluster Mn4(µ3-NtBu)3(µ3-NH)(NtBu)4, (4, Figure 5) identical to 3 except that the pendant MnO3 moiety has been replaced by a proton. The N-H proton is identified as an electron density peak in the crystallographic Fourier difference map on both molecules of 4 in the asymmetric unit, and refined freely. The crystal structure of 4 is excellent, with an R1(obs) of 2.15%, despite whole-molecule dis-


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Figure 5. Thermal ellipsoid plot of 4 with symmetry independent Mn, N, and H atomic labels. Ellipsoids set at 50% probability level. C-H hydrogen atoms omitted for clarity.

Conclusion We report here a manganese cubane cluster molecule containing a pendant Mn=O moiety. The Mn-N core of the cubane cluster is relatively stable, as the all-imido cluster 2 shows no reactivity with cyclooctene, while the pendant manganese trioxo moiety of 3 is reactive, generating oxygenated organic products and an inorganic cubane cluster in which the pendant manganese moiety has been lost. In photosystem II, the reactive moiety is most often described as a Mn(V)=O or a Mn(IV)-O· moiety, and since our pendant manganese is formally d0 Mn(VII), the attached oxo ligands may be more electrophilic due to strong π-donation into the Mn(VII) center, and since the pendant Mn atom itself has no unpaired electrons, it may not impart radical character to the terminal oxo ligands. In essence, we see that this cluster’s reactivity is more reminiscent of permanganate- than the OEC based on oxidation of olefins and undetectable O2 products from nucleophilic attack by water, despite the electrophilicity of Mn(VII)=O ligands. While the illustration of the reaction in Scheme 2 depicts the oxene transfer originating from cluster 3, we cannot rule out the possibility that a more complex mechanism is responsible, such as the preequilibrium dissociation of the MnO3 unit. Conversely, it may be the case that the attachment of the Mn(VII) moiety to the core of the cubane cluster may enhance its reactivity, as it has been previously shown that ancillary attachments to permanganate can have that effect.78,79 However, we have not been able to show that this activation facilitates water oxidation by nucleophilic attack specifically, despite the presumed electrophilicity of the oxo ligands. Future efforts will probe the electronic structure, redox chemistry, and mechanistic elucidation of the reaction chemistry of these cluster systems. Experimental General Methods

All manipulations were performed under a rigorous dry, anaerobic atmosphere of nitrogen gas using standard Glove Box and Schlenk line techniques. All reagents were purchased from commercial sources (Aldrich, Strem). Anhydrous solvents such as benzene and pentane were purified using an Innovative Technology, Inc. Pure Solv.TM system. Tetrahydrofuran and hexamethyldisiloxane were distilled from sodium benzophenone ketyl under a nitrogen atmosphere. BuLi, tBuNH2, and cyclooctene were purchased from Aldrich. MnF3 was purchased from Strem chemicals. These reagents were used without further purification. 12-crown-4 was purchased from Aldrich and used without purification (wet) for the synthesis of 3. t BuNHLi,80 1,47, and 2 48 were synthesized according to literature protocol. It is particularly important to note that the quality of commercial MnF3 is highly variable, and different lots gave different results. An alternative synthetic protocol for preparation of 1 has been provided depending on the quality of MnF3 obtained. Anhydrous solvents were used throughout all experiments. 1H-NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer. Values for chemical shifts (ppm) are referenced to the residual protiosolvent resonances (C6D6 7.16 ppm). FT-IR spectra were recorded in the range of 400-4000 cm-1 on a Nicolet iS5 FT-IR under nitrogen atmosphere using a KBr pellet press. UV-visible spectra were recorded on a Shimadzu UV-1800 UV spectrophotometer in the range of 200-900 nm. High resolution mass spectrometry was performed on a Bruker MicrOTOF-Q II, Bruker Daltonics GmbH, Bremen, Germany. GCMS was performed on a Agilent Technologies 5977B MSD GCMS. Mass Spectrometry The electrospray solution was prepared by dissolving compound [2]5[3]3 in acetonitrile and tetrahydrofuran. A gas tight syringe was used to inject the sample to the mass spectrometer at a voltage of -4.5kV and a flow rate of the electrospray solvent at 3 µL/min using a syringe pump (Harvard Apparatus, Holliston, MA). A countercurrent of hot nitrogen gas (180oC) was flowed at 4 L/min at assist the desolvation process. The spectrometer was tuned for m/z below 1000 using a solution of 99:1 (v/v) acetonitrile:ESI calibrant (#63606-10 ML, Fluka Analytical/ Sigma Aldrich, Buchs, Switzerland). The in-source collision region (ISCID) and collision cell (CID) voltages were set to 0 and 10eV, respectively. For laser electrospray mass spectrometry (LEMS), the femtosecond laser system used for vaporization of the sample consists of a Ti:sapphire laser oscillator (KM Laboratories, Inc., Boulder, CO) and a regenerative amplifier (Coherent, Inc., Santa Clara, CA) to create 75 fs, 0.6 mJ laser pulses centered at 800 nm. The laser, operated at 10 Hz to couple with the electrospray (ES) ion source, was focused to a spot size of ~250 μm in diameter with an incident angle of 45o respect to the sample using a 16.9 cm focal length lens, with an approximate intensity of 1 x 1013 W/cm2. The ES needle was maintained at ground while the inlet capillary was biased to -4.5 kV to operate in positive ion mode. The area sampled was 6.4 mm below and approximately 1 mm in front of the ES needle. An aliquot



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of the sample was deposited onto the sample stage and was immediately vaporized into the ES droplets containing acetonitrile for capture and ionization. The flow rate for ES solvent was set at 3 μL/min by a syringe pump (Harvard Apparatus, Holliston, MA). X-Ray Diffraction X-ray data were collected on a Bruker KAPPA APEX II DUO diffractometer. Single crystals were mounted on a MiTeGen loop using paratone-N oil. For powder XRD, samples were caked onto the end of a glass fiber using paratone-N oil. Single crystal data were obtained using Mo Kα radiation from a sealed tube equipped with a TRIUMPH monochromator, and processed using the APEX2 software suite. Structures were solved using direct methods and refined using full-matrix least-squares minimization using the SHELX crystallography package81 with OLEX2 as a GUI and graphics program.82 Powder patterns were obtained with a 180° φ scan using Cu Kα radiation from a sealed tube. Powder patterns were simulated using Mercury (CCDC).83 Synthesis Mn4Li4(N)(NtBu)10·Mn5Li3(N)2(NtBu)10 (1), was prepared according to literature procedure.47 However, recent lots of MnF3 we have purchased from Strem appeared contaminated by water according to analytical reports from the vendor, and led to problems with synthesis. Under these circumstances, the heating the reaction at 135° instead of 120° improved the reaction product yield and quality. Synthesis of 5 [Mn4(µ3-NtBu)3(NtBu)4] · 3 [Mn4(µ3N Bu)3(µ3-NMnO3)(NtBu)4] ([2]5·[3]3) t

Cocrystalline [2]5·[3]3 was prepared according to a modification of literature protocol48 in which we replaced chloride ion with undried 12-crown-4 as a means to remove Li+ and introduce trace water. n [Mn4Li4(μ6-N)(μ3NtBu)6(μ-NtBu)3(NtBu)] · (1-n) [Mn5Li3(μ6-N)(N)(μ3NtBu)6(μ-NtBu)3(NtBu)] (1, 0.210 g 0.216 mmol) was dissolved in 20mL of diethyl ether followed by the addition of 12-crown-4 (0.265g, 1.50 mmol). This reaction mixture was stirred for 5 hours at room temperature resulting in a dark brown solution. The reaction mixture is filtered to remove a solid byproduct. The filtrate is dried under vacuum and dried compound is extracted with 20mL of diethyl ether and filtered. Crystals are obtained by vapor diffusion of this solution with hexamethyldisiloxane at room temperature under closed and anaerobic conditions over two days. Yield of [2]5·[3]3 is 0.065g; 38% based on n = 0.9, for the component ratio in 1 as previously determined.47 Unit Cell: Orthorhombic C, a = 17.2315(15) Å b = 12.7392(11) Å c = 19.4043(16) Å V = 4259.5(6) Å3. FT-IR: [cm-1] 2967, 2916, 2849 (C-H stretch); 1465, 1452, 1375, 1352 (C-H bend), 1217 (M=NR stretch), 964 (M=O stretch). LEMS [m/z] (m/z theory): C28H63Mn5N8O3 (3) 834.1928 (834.1925), C28H64Mn5N8O3 (3+H) 835.2091 (835.2003). Synthesis of 18O-isotopically labelled [Mn4(µ3-NtBu)3(µ3NMnO3)(NtBu)4]

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0.5 μL of 18OH2 (95% isotopic purity) was stirred overnight in 0.265 g of dry 12-crown-4. n [Mn4Li4(μ6N)(μ3-NtBu)6(μ-NtBu)3(NtBu)] · (1-n) [Mn5Li3(μ6-N)(N)(μ3NtBu)6(μ-NtBu)3(NtBu)] (1, 0.210 g 0.216 mmol) was dissolved in 20mL of diethyl ether followed by the addition of the 18OH2-spiked 12-crown-4 (0.265g, 1.50 mmol). This reaction mixture was stirred for 5 hours at room temperature resulting in a dark brown solution. The reaction mixture is filtered to remove a solid byproduct. The filtrate is dried under vacuum and used without further purification. FT-IR: [cm-1] 2967, 2916, 2849 (C-H stretch); 1465, 1452, 1375, 1352 (C-H bend), 1261, 1136. (M=NR stretch), 916 (M=18O stretch). LEMS [m/z] (m/z theory): C28H63Mn5N8O3 (3) 834.1928 (834.1925), C28H64Mn5N8O3 (3+H) 840.2169 (840.2053). Synthesis of Mn4(µ3-NtBu)(µ3-NH)(NtBu)4 (4). 0.015g of cocrystalline [2]5[3]2 was dissolved in 3mL of pentane and followed by the addition of 0.1g of ciscylcoctene The reaction mixture is stirred for 4 hours at 1100 C in a sealed, heavy walled pressure flask. The solvent is dried under vacuum to yield solid which was washed with acetonitrile and dried in vacuo and redissolved in a minimum of pentane. Crystals are obtained by vapor diffusion of this solution with hexamethyldisiloxane at room temperature under closed and anaerobic conditions for 24 hours. Yield of [Mn4(μ3-NtBu)4(NtBu)4(NH)] (3): 0.0056g (Quantitative yield based on compound 3 and assuming an approximate formula of [2]5[3]2). Unit Cell: Rhombohedral, a = b = 34.7647(11) c = 10.8425(4) V = 11348.5(7) Å3. 10.78 1H NMR [ppm] (400 MHz, 293 K C6D6): δ 10.78 (br, 1H, NH), 1.731 (s, 18H, tBu), 1.528 (s, 9H, tBu), 1.417 (s, 18H, t Bu). FT-IR: [cm-1] 3508 (N-H stretch) 3079, 3037, 3013, 2981, 2945 (C-H stretch); 1489, 1399, 1384 (C-H bend) 1249, 1216 (M=NR stretch). Synthesis of isotopically ND)(NtBu)4 (4).

labelled

Mn4(µ3-NtBu)(µ3-

4 was prepared as described in the preceeding synthesis, except that cyclohexene-d10 was substituted for cyclooctene. The resulting product was dried and analyzed as a crude product since additional extraction and purification procedures resulted in the deuteron exchanging with trace contaminant water to give protio product. The crude dried product was analyzed by FTIR. Organic products in synthesis of 4. Compound 4 was prepared in situ as described above. The organic products extracted into acetonitrile, and the acetonitrile layer is either concentrated and analyzed by GCMS, or dried to give a white solid which is analyzed by FTIR, or 1HNMR. Analysis of this mixture by Infrared spectroscopy (Fig. S6) indicates the presence of ketones and alcohols based upon the observation of bands in the region of 3300 and 3150 (for alcohol) and 1672 (for ketone). 1H NMR and GCMS analysis indicate a vast mixture of products and GCMS ientifies the presence of cyclooctane dione ([C8H12O2H+] theor 141 m/z) and cyclooctane diol([C8H16O2H+] theor 145 Fig. S2) as organic products. No tert-butyl nitrene containing products are detected.


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ASSOCIATED CONTENT Materials and Methods, Mass, NMR, FTIR spectra, Powder Xray patterns, Single crystal X-ray data, Crystallographic information files. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected].

Author Contributions The manuscript was written through contributions of all authors.

Funding Sources Support of this research by the National Science Foundation under Award CHE- 1254545 is gratefully acknowledged. LEMS experiments were supported by the National Science Foundation under awards CHE-1362890, CHE-0957694. Elemental analytical data were obtained from the CENTC Elemental Analysis Facility at the University of Rochester, funded by NSF CHE-0650456.

Notes a

. The % yield of 4 is calculated according to the assumption that unreactive 2 and reactive 3 exist in a 5:2 ratio. Thus, the theoretical yield is based upon the moles of 3 in the weighed sample, as described in the experimental section.

ABBREVIATIONS OEC, Oxygen Evolving Complex; PSII, Photosystem II;

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SYNOPSIS TOC


Reactive Pendant Mn O in a Synthetic Structural Model of a Proposed

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