Ligand Exchange on and Allylic CâH Activation by Iron(0) Fragments

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Ligand Exchange on and Allylic C−H Activation by Iron(0) Fragments: π‑Complexes, Allyliron Species, and Metallacycles Alicia Casitas,‡ Helga Krause,‡ Sigrid Lutz,‡ Richard Goddard,‡ Eckhard Bill,∥ and Alois Fürstner*,‡ ‡ ∥

Max-Planck-Institut für Kohlenforschung, 45470 Mülheim/Ruhr, Germany Max-Planck-Institut für Chemische Energiekonversion, 45470 Mülheim/Ruhr, Germany S Supporting Information *

ABSTRACT: The complexes [(dippp)Fe(C2H4)2] (2) and [CpFe(C2H4)2][Li·(tmeda)] (5) both contain a formally zerovalent iron center but exhibit markedly different catalytic properties. Whereas 5 is able to induce a broad range of cycloisomerization and cycloaddition reactions, 2 is so far basically limited to cyclotrimerizations of alkynes and nitriles. Investigations into the behaviors of both complex vis-à-vis unsaturated substrates provided insights into the likely origins of this distinct behavior. Thus, ordinary terminal or internal alkenes were found not to replace the ligated ethylene units in 2, whereas the stronger π-acceptor ligands 1,5-cyclooctadiene, 2norbornene, and tolane afforded the corresponding π-complexes 8, 9, 10, and 13. A cyclopropene derivative engaged in oxidative cyclization with formation of the corresponding metallacycle 12. Allyl-9-BBN or alkenyl-9-BBN derivatives succumbed to allylic C−H activation with formation of the unorthodox allyliron complexes 25 and 27 featuring a bridging hydride ligand between the iron and the boron atoms. Along the same line, 1,3-dienes bind well to 2 but undergo spontaneous activation if allylic C−H bonds are present; the resulting hydride is transferred to a residual ethylene ligand, as manifest in the formation of the cyclopentadienyl ethyl complex 22. The same elementary steps surface in a remarkable reaction cascade comprising two consecutive C−H activation reactions and a stereoselective C−C bond formation, which ultimately provides the substituted cyclohexadienyl complexes 20 and 23. In contrast, the heterobimetallic complex 5 neither induces allylic C−H activation nor binds 1,3-butadiene under conditions where it proved catalytically active. The targeted butadiene complex 34 had to be made by an indirect route and is distinguished by a noteworthy “flyover” constitution. Therefore, we conclude that the known cycloaddition and cycloisomerization reactions catalyzed by 5 do not commence at a 1,3-diene motif but require an enyne entity as starter unit.

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INTRODUCTION Among its many virtues, iron-catalyzed C−C bond formation is a potentially attractive alternative to noble-metal-catalyzed methodologies that currently dominate the field.1−6 Despite significant advances in this timely research area, iron-catalyzed reactions are often poorly understood in mechanistic terms. In cases in which low-valent iron species are generated in situ in ligand-free form, their actual nature is usually difficult to determine, and details concerning the ensuing catalytic cycles necessarily remain opaque. Even the use of well-defined iron precatalysts does not always lead to unambiguous conclusions: Investigations into cross coupling reactions are deemed representative,1−7 in that one set of experiments with fully characterized Fe(+) complexes advocated for an Fe(+)/Fe(3+) mechanism,8,9 whereas data generated with a different type of Fe(+) species suggested that such a scenario, though possible, is unlikely on kinetic grounds.10 This preliminary conclusion was later confirmed, at least for selected substrate/reagent combinations, by a more integral approach based on spectroscopic, kinetic, and structural studies.11 This example showcases that large sectors of iron catalysis in general remain uncharted; © XXXX American Chemical Society

moreover, it becomes increasingly clear that even seemingly related transformations do not necessarily follow the same mechanism.12,13 The paucity of secured information about the innate reactivity of relevant low-valent iron complexes is at the very heart of the dilemma. Therefore, we see the need to study the elementary steps entertained by low-valent iron complexes of proven catalytic performance in greater detail, even though the (exceptional) sensitivity of many such species renders these investigations highly demanding. As part of our endeavor, which has already led to more than one surprise,10,13,14 we now report results on π-complex formation and the ease with which some of the resulting adducts succumb to C−H activation. These data provide better insights into the seemingly trivial first step of the cycloisomerization and cycloaddition reactions of the Alder-ene, Special Issue: Organometallic Chemistry in Europe Received: July 27, 2017

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DOI: 10.1021/acs.organomet.7b00571 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

therefore not be as easily replaced as one might expect, which in turn would imply a serious kinetic barrier for substrate binding (and, potentially, product release) during a catalytic transformation. In fact, no signs of ligand exchange were observed on treatment of 2 with excess 1-hexene or cyclohexene, whereas styrene binds readily but engages the arene ring into η6coordination with formation of the 18e complex 7 (Scheme 2

[4+2], [5+2], and [2+2+2] type, which certain low-valent iron complexes are able to catalyze very efficiently.15−18

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RESULTS AND DISCUSSION The [(Diphosphine)iron(0)] Series. In our quest for iron complexes that are catalytically competent yet structurally well defined, we turned our attention to the use of dippp (dippp = bis(diisopropylphosphino)propane) as the ancillary ligand.19 This particular diphosphine renders many iron complexes soluble in organic media, ensures a good balance between accessibility to and steric shielding of the metal center, and helps to preclude the formation of dimeric or oligomeric species. In this context, we have previously shown that treatment of [(dippp)FeCl2] (1) with EtMgBr in THF results in a near quantitative two-electron reduction (Scheme 1); when carried out under

Scheme 2. π-Complex and Metallacycle Formationa

Scheme 1. Preparation of Fe(0) Alkene Complexes by TwoElectron Reduction

a Reagents and conditions: (a) styrene, THF, −20 °C, 67%; (b) 1,5cyclooctadiene (cod), THF, RT, 52%; (c) norbornene, THF, −5 °C → RT, 93%; (d) norbornene (excess), RT, THF, 68%; (e) cyclopropenone-1,3-propanediol ketal, THF, −70 °C, 19%; (f) tolane, THF, −5 °C, 70%.

ethylene atmosphere, the resulting Fe(0) adduct 2 can be isolated on a multigram scale and stored for extended periods of time;20 this complex therefore qualifies as a basis for further investigations. We like to point out that the one carbon shorter homologue 4 can be prepared analogously from 3; however, 4 is much less stable than 2 and has to be used without delay; this comparison exemplifies the intricacies of low-valent iron chemistry. Complex 2 has proven highly effective in a wide range of cross coupling reactions, whereas its scope with regard to cycloaddition chemistry is surprisingly narrow. So far, we were only able to use 2 for the cyclotrimerization of certain alkynes and nitriles,20,21 whereas its heterobimetallic siblings 5 and 6, which also contain an iron center of the formal oxidation state zero,22 excel in a broad range of cycloisomerization and cycloaddition reactions.15 The reasons for this disparity are not intuitive. It was therefore deemed necessary to investigate the behavior of these complexes vis-à-vis prototypical unsaturated compounds to gain insights into the possible reasons for the distinct behavior.23−27 The structure of 2 in the solid state shows that the ethylene ligands stabilize the complex by accepting electron density from the metal into the antibonding π* orbitals.28 They might

Figure 1. Structure of complex 7 in the solid state.

and Figure 1). It takes the more π-acidic norbornene to replace one or both ethene ligands. Interestingly, the heteroleptic complex 9 was formed even when two equivalents of norbornene were added; actually, it required a large excess to obtain 10. The structures of these complexes in the solid state are shown in Figure 2 and Figure S1. In complex 9, the norbornene double bond, for its better π-acceptor properties, is much more elongated (C16−C21 1.430(2) Å versus 1.333 Å in free norbornene)29 than the ethylene unit (C23−C24 1.338(4) Å versus 1.3305 Å in unbound ethylene but ∼1.41 Å in 2).28,30 Ring strain in combination with the chelate effect also renders 1,5B


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coordination entailed an oxidative cyclization event, it was surprising to find that tolane binds readily but does not succumb to metallacycle formation.20 The fact that the bisalkyne Fe(0) complex 13 could be isolated in pure form is all the more astounding if one considers that complex 2 has proven catalytically competent in [2+2+2] cycloadditions of 3-hexyne or cyclo-octyne.20 In any case, the bis(alkyne) adduct 13 seems to be without precedent in the literature and can be regarded as a model of the loaded catalyst in a prereactive state. We proposed that the twist between the two alkyne units enforced by the lateral phenyl rings prevents efficient orbital overlap from occurring and therefore puts oxidative cyclization and any follow-up chemistry on hold.20 Of similar interest is the behavior of complex 2 vis-à-vis butadiene, not least because certain low-valent iron complexes are known to catalyze the cyclodimerization or polymerization of this and related 1,3-dienes.36−39 In fact, the ethylene ligands of 2 are readily replaced by butadiene: it had already previously been shown that excess butadiene engenders dissociation of one of the phosphines with formation of complex 14;40 no signs of competing (cyclo)oligomerization of the substrate had been observed (Scheme 3). Attempted preparation of complex 15

Figure 2. Structure of complex 9 in the solid state. Selected bond lengths (Å): C16−C21 1.430(2), C23−C24 1.338(4), Fe1−C16 2.089(2), Fe1−C21 2.069(2), Fe1−C23 2.131(2), Fe1−C24 2.075(2).

cyclooctadiene (cod) capable of replacing the ethylene units of 2 to give the known adduct 8.31 The electron-richness of the iron center in 2 surfaced in the reaction with a cyclopropene derivative. Although cyclopropenes are subject to ring opening with formation of metal carbene complexes on exposure to various transition metal fragments,32,33 an oxidative cyclization with formation of metallacycle 12 was observed even at −70 °C; the fact that a single stereomer was formed is ascribed to steric factors. Related reactions are known for Ni(0); as they form the basis of catalytic [2+2+1] and [2+2+2] cycloadditions between cyclopropenes and appropriate partners (CO, acetylenes etc.),34 we suppose that such reactions might also lend themselves to iron catalysis. The structure of 12 in the solid state shows an almost perfectly planar ferracycle (Figure 3) with strikingly alternating C−C

Scheme 3. Fe(0)-Butadiene Complexesa

a

Reagents and conditions: (a) (chloromethyl)cyclopropane, Mg, THF, −78 °C → −35 °C, 51%; (b) 1,3-butadiene, THF, −30 °C, 69%, ref40.

endowed with a single 1,3-butadiene ligand by strictly limiting the amount of butadiene proved erratic since coordination of a second butadiene ligand does occur even at very low temperature. A reductive approach was found to provide a much better entry: to this end, a mixture of 1 and 4-chloro-1-butene or (chloromethyl)cyclopropane in THF was treated with activated magnesium41 to give complex 15 in the form of dark-green single crystals suitable for X-ray diffraction (Figure 4). The C−C bonds of the bound 1,3-diene are surprisingly uniform in length (C9− C10 1.417(2), C10−C11 1.426(2), and C11−C12 1.414(2) Å), which indicates massive back-donation of electron density from the metal into the antibonding orbitals of the π-system.42 The level of complexity increases when the diene substrate carries allylic hydrogen atoms. As previously communicated, treatment of 4 with cyclopentadiene affords the CpFe(2+) complex 22 (Scheme 4).20 This complex is likely formed by allylic C−H activation once the diene unit has entered the ligand sphere of the metal center; the remaining ethylene ligand in 21 then undergoes migratory insertion into the transient iron hydride to form the iron ethyl unit of product 22. The same

Figure 3. Structure of complex 12 in the solid state. Selected bond lengths (Å): Fe1−C16 2.049(2), Fe1−C19 2.045(2), C16−C17 1.575(2), C17−C18 1.502(2), C18−C19 1.569(2).

bond lengths; interestingly, the bonds which are part of the cyclopropane rings (C16−C17, C18−C19) are significantly longer than the C17−C18 bond that is not. Reactions of [(Diphosphine)Iron(0)] with Tolane and 1,3-Dienes. Since acetylenes as well as 1,3-dienes tend to bind more tightly to low-valent iron than ordinary alkenes,35 it seemed likely that the ethene ligands in 2 could be replaced by such substrates. In view of the ease with which cyclopropene C


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Figure 4. Structure of complex 15 in the solid state; the butadiene ligand is disordered over two positions, only one of which is shown. Selected bond lengths (Å): C9−C10 1.417(2), C10−C11 1.426(2), C11−C12 1.414(2), Fe1−C9 2.122(1), Fe1−C10 2.059(1), Fe1−C11 2.057(1), Fe1−C12 2.121(2).

Figure 5. Structure of complex 23 in the solid state; only the iron hydride and the hydrogen atom at the branching point are shown for clarity. Selected bond lengths (Å): Fe1−H1 1.47(1).

Scheme 4. Allylic C−H Activation of 1,3-Diene Substratesa

The recorded isomer shift in the Mössbauer spectrum is also well in line with the low-spin nature of this complex (Figure 6 and Table 1 in the Experimental Section).

Figure 6. Zero-field Mössbauer spectrum of the low-spin Fe(II) complex 23 recorded at 80 K (dots). The red line represents a fit with a Lorentzian quadrupole doublet (see Table 1, Experimental Section).

C−H Activation of Allyl- and Alkenylboranes. The remarkable ease of allylic C−H activation manifest in these examples encouraged us to investigate further possibilities. Instead of transferring the transient iron hydride onto an ethylene ligand of the starting complex, we conjectured that a boron atom in the substrate might also serve as an appropriate acceptor. To test this hypothesis, the 9-alkenyl-9-BBN derivative 28 was treated with 2, even though it was not clear at the outset whether the trans-disubstituted double bond would be able to replace the ethylene moieties; we conjectured, however, that conjugation with the empty p-orbital at boron renders the olefin a sufficiently strong π-acceptor for the substitution process to occur. Gratifyingly, this turned out to be the case: the major product isolated from the crude mixture by crystallization was shown to be the allyliron complex 25 formed by allylic C−H activation of a transient π-complex 24 (Scheme 5). Interestingly, the hydride occupies a bridging position between the iron and the boron atoms. It is placed at 1.790 and 1.320 Å distance from the respective metal/metalloid centers, which suggests that the net Lewis acidity of these sites must be comparable.44,45 This hydride has been located on the Fourier map and is therefore shown in Figure 7, from which the η3-binding mode of the ironallyl unit also becomes apparent.

Reagents and conditions: (a) 1,3-cyclohexadiene, THF, −5 °C → RT, 40% (20), 75% (23); (b) cyclopentadiene, THF, −20 °C → RT, 65%.

a

elementary steps apply to cyclohexadiene as the substrate en route to the alkylated cyclohexadienyl complexes 20 and 23.20,43 The formation of these products is best explained if one assumes that a transient iron ethyl complex such as 18 undergoes reductive ligand coupling to give the corresponding Fe(0) complex 19; the now substituted diene ligand experiences a second C−H activation to give complex 20 as the final product; complex 23 is formed analogously. The endo-orientation of the ethyl group, which is evident from the projection of product 23 shown in Figure 5, is suggestive of the proposed reaction cascade comprised of two consecutive C−H activations and an interwoven stereoselective C−C bond formation in the coordination sphere of a single iron center. With 1.47(1) Å, the terminal Fe−H bond in 23 is clearly shorter than that of the bridging hydrogen atoms in complexes 25 and 27 (see below). D


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formed at low temperature. The hydride is once again interacting with the iron as well as the boron atom (Figure 8). The

Scheme 5. Allylic C−H Activation in Alkenyl- and Allylboron Derivativesa

Figure 8. Structure of the allyliron complex 27 with a hydride atom bridging the iron and the boron centers; co-crystallized solute is not shown for clarity. Selected bond lengths (Å) and angles (deg): Fe1−H1 1.71(2), B1−H1 1.32(2), Fe1−C1 2.103(2), Fe1−C2 2.064(2), Fe1− C3 2.084(2), Fe1−H1−B1 105.4.

paramagnetic character of this complex did not allow 11B NMR spectra to be recorded, from which additional conclusions about the incipient borate character could have been drawn. The moderately high Mössbauer isomer shift of 27 in conjunction with a large quadrupole splitting (δ = 0.52 mm s−1 and |ΔEQ| = 2.22 mm s−1) is typical of high-spin Fe(II) with low coordination number or covalent bonds (Figure 9). The values resemble

Reagents and conditions: (a) 28, hexane, −10 °C, 40%; (b) 29, hexane, −10 °C, 69%.

a

Figure 7. Structure of the allyliron complex 25 with a hydride atom bridging the iron and the boron centers; cocrystallized THF is not shown for clarity. Selected bond lengths (Å) and angles (deg): Fe1−H1 1.79(1), B1−H1 1.32(1), Fe1−C16 2.103(2), Fe1−C17 2.089(1), Fe1−C18 2.140(2), Fe1−H1−B1 102.2.

Figure 9. Zero-field Mössbauer spectrum of the high-spin Fe(II) complex 27 recorded at 80 K (dots). The red line represents a fit with a Lorentzian quadrupole doublet (see Table 1 in the Experimental Section). The deviations at ca. 0 and 2 mm s−1 indicate a 12% impurity with an unknown Fe(II) species.

The ease of allylic C−H activation must be seen in the context of literature reports.46,47 It is striking that the closely related Fe(0) phosphine complex [(dmpe)2Fe] (dmpe = 1,2-bis(dimethylphosphino)ethane) exclusively activates the Csp2−H bond of linear as well as cyclic alkenes while leaving the allylic Csp3−H bonds untouched.48,49 In marked contrast, Fe(0) fragments bearing strongly π-accepting CO ligands are known to cleave allylic rather than olefinic C−H bonds.50,51 It is perplexing that the behavior of 2 vis-à-vis olefinic substrates resembles that of the [Fe(CO)3] fragment rather than that of [(dmpe)2Fe]; these results can currently not be reconciled. Next, 9-allyl-9-BBN (29) was chosen as the substrate. Although the boron center and the alkene site are decoupled and regular terminal alkenes were shown not to replace ethylene in 2 (see above), the corresponding π-allyl species 27 was readily

remarkably well those of hydride-bridged [(nacnac)Fe(II)] complexes with quasi tetrahedral coordination (e.g., [LtBuFe(μH)]2, δ = 0.51 mm s−1 and |ΔEQ| = 2.05 mm s−1).52 It is important to note, however, that replacement of 9-allyl-9BBN by B-(allyl)pinacolboronate failed to afford an analogous complex. This distinctly different behavior shows that the hydride acceptor quality of the boron atom is critical for the reaction to proceed, most likely because it renders the C−H activation step irreversible. Diene Complexes of the Type [CpFe]−M+. The surprising ease of allylic C−H activation by the [(dippp)Fe(0)] fragment has almost certainly implication for catalysis. It may well be a “short circuit” and hence a major reason why applications of E


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Organometallics complex 2 are so far limited to the cyclotrimerization of unhindered alkynes or hetero-cyclotrimerization of alkynes with nitriles. In any case, we have not yet managed to use this complex for cycloisomerizations or cycloadditions of enynes or dienes, which other low-valent iron species are able to promote.15−18 It was almost three decades after the original report on the preparation of the heterobimetallic complex [CpFe(C2H4)2]Li(tmeda) (5) and close relatives such as 6 by Jonas and coworkers53 that our group recognized their potential as precatalysts for organic synthesis. In addition to their use in cross coupling chemistry,10,54 5 was successfully applied to a host of [4+2], [2+2+2], and [5+2] cycloadditions as well as Alder-ene cycloisomerization reactions;15 A detailed mechanistic investigation has conclusively ruled out that allylic C−H activation with formation of an allylmetal species is accountable for the Alder-ene reactions.15 Likewise, the representative example of a [4+2] cycloaddition of the unactivated diene-yne 30 shown in Scheme 6 implies that C−H activation of the allylic CH2 group flanking

Scheme 7. Preparation of Heterobi- and -trimetallic Butadiene Complexesa

Reagents and conditions: (a) ZnCl2, THF, −10 °C, 64%; (b) butadiene, THF, 0 °C → 60 °C, 73%; (c) Li, THF, 0 °C, then tmeda, 84%; (d) ZnCl2, THF, RT, 70%.

a

Scheme 6. Probing the Ability of the Formally Fe(0) Complexes 2 and 5 To Induce a Prototypical Intramolecular [4+2] Cycloaddition; Possible Reactive Intermediates Are Shown

ensure complete conversion. It is not necessary to isolate complex 32; rather, the sequence can also be performed in one pot. Reduction of 33 with lithium metal in THF led to the precipitation of metallic zinc and concomitant formation of the originally targeted butadiene complex 34. In contrast to the phosphine complex 15, complexes 32−35 are diamagnetic; their NMR and Mössbauer data (see Table 1 and spectra in the Supporting Information) are in accord with the proposed constitutions, which were unambiguously established by single-crystal X-ray diffraction. The trimetallic complexes 32, 33, and 35 with a Fe−Zn−Fe axis seem to be unprecedented (Figure 10, Figures S2−S4); in view of recent progress in the use

the diene unit by 5 does not occur at a competitive rate. In view of the ease of allylic activation of butadiene derivatives by 2 described above, this example showcases the strikingly different selectivities and competences of the formally zerovalent iron complexes 2 and 5. It is generally accepted that metal-catalyzed Diels−Alder reactions of unactivated substrates proceed via metallacyclic intermediates, but it is not intuitive at all whether the conversion of substrates such as 30 commence at the 1,3-diene or at the enyne subunit; either conceivable intermediate A or B could evolve via C into the final product 31 (Scheme 6). To help clarify the point, we decided to study the binding of butadiene to precatalyst 5 as a truncated model for an intermediate of type A. While butadiene−binding to 2 is fast and facile at low temperatures (see complex 14), the exchange of the ethylene ligands in 5 for 1,3-butadiene proved sluggish even under forcing conditions. Transmetalation of the heterobimetallic complex 5 with a less electropositive metal cation might reduce the ionic character, decrease back-donation of electron density from the ferrate center into the ligated olefins, and hence facilitate ligand exchange. In line with this notion and with early precedent from the Jonas group,53 complexes 5 and 6 were transmetalated with ZnCl2 (Scheme 7). Treatment of the resulting complexes 32 and 3553 with excess butadiene allowed the olefinic ligands to be replaced and the butadiene adduct 33 to be obtained even though it was still mandatory to heat the mixture to 60 °C to

Figure 10. Structure of [CpFe(ethylene)]2Zn (32) in the solid state.

of bimetallic species comprising a Fe−M (M = Zn, Cu) motif as versatile catalysts for various transformations,55 these unorthodox complexes merit investigation in their own right. Likewise, the structure of 34 in the solid state is remarkable. The projection shown in Figure 11 confirms that this complex is not a ferracycle of type A; rather, it is the lithium counterion that entertains close contacts to the terminal C-atoms C6 and C9, and it tilted away from coplanarity with the butadiene ligand by only 16.8°. This position allows it to weakly engage with the iron center too, which in turn is tightly bound to the π-system as manifest in the short Fe−C distances. A comparison with the iron butadiene complex 15 is instructive.56 Whereas the C−C bonds in 15 are quite uniform in length, 34 is distinguished by a clearly F


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raw data and to eliminate the parabolic background by using our mf.SL package (EB), which was also used to least-squares fit the sample spectra with Lorentzian doublets. The minimum experimental line width was 0.24 mms−1 (full width at half-height). The sample temperature was maintained constant in an Oxford Instruments Variox cryostat. The γsource (57Co/Rh, 1.8 GBq) was kept at room temperature. The spectrometers are calibrated by recording the Mössbauer spectrum of a 25 μm thick α-Fe foil at room temperature, with the center of the six-line pattern being taken as zero velocity. Hence, isomer shifts reported in Table 1 are quoted relative to iron metal at 300 K. Copies of the spectra are contained in the Supporting Information.

Table 1. Mössbauer Isomer Shifts (δ) and Quadrupole Splittings (ΔEQ) of Selected Fe(II) Complexes

Figure 11. Structure of complex 34 in the solid state. Selected bond lengths (Å) and angles (deg): C6−C7 1.447(2), C7−C8 1.417(2), C8− C9 1.446(2), Fe1−C6 2.050(1), Fe1−C7 1.984(1), Fe1−C8 1.979(1), Fe1−C9 2.048(1), Li1−C6 2.493(2), Li1−C9 2.465(2), Li1−Fe1 2.430(2).

alternating pattern: strikingly, the former double bonds of butadiene C6−C7 and C8−C9 have become significantly longer than the connecting single bond C7−C8. The evidently massive π-back-bonding from the electron-rich metal center into the π*orbitals imparts a “flyover” constitution on this complex in that one metal atom entertains contacts to the termini of the fourcarbon chain, whereas a second metal forms π-bonds to each individual atom of this chain.57,58 The fact that exchange of the ethylene ligands in 5 for butadiene is inefficient strongly argues against an intermediate of type A being involved in catalytic [4+2] cycloadditions of substrates of type 30, as do the structural peculiarities of the butadiene complex 34 discussed above. Furthermore, treatment of 34 with excess tolane or 3-hexyne did not lead to any noticeable cycloaddition, although we acknowledge that these intermolecular control experiments are entropically disfavored compared with the intramolecular setting in 30 → 31. We conclude that such reactions most likely proceed via metallacycles of type B (Scheme 6), even though intermediates of this sort have defied isolation and characterization.59 The notion that the enyne entity rather than the 1,3-diene constitutes the starter unit of the [4+2] cycloaddition is further advocated by the fact that all known types of cycloisomerizations catalyzed by complex 5 employ enyne derivatives as the substrates. Complementary studies into the reactivity of well-defined iron complexes vis-à-vis more highly functionalized π-systems are ongoing and will be reported in due course.

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compound

δ/mm s−1

|ΔEQ|/mm s−1

5 23 27 32 33 34

0.40 0.25 0.52 0.42 0.42 0.46

1.38 1.57 2.22 1.41 1.75 1.71

Complex [(dippp)FeCl2] (1). A flame-dried two-necked roundbottomed flask equipped with a magnetic stir bar and connected to the Ar line was charged with FeCl2·THF1,5 (4.5 g, 0.019 mol) and THF (50 mL) under Ar. A solution of bis(diisopropylphosphino)propane (dippp, 5.2 g, 0.019 mol) in THF (5 mL) was slowly added via cannula to the resulting suspension, and stirring was continued overnight. For workup, all volatile materials were distilled off in vacuum, the remaining pale-pink solid was suspended in diethyl ether (25 mL), and the suspension was stored at −30 °C overnight. The solvent was siphoned off by canula and the residue dried in vacuum to give the title complex as a crystalline material (6.9 g, 91%). Single crystals suitable for X-ray diffraction were grown from a saturated solution in THF by lowering the temperature; the structure in the solid state is contained in the Supporting Information of our Preliminary Communication.20 MS (EI): m/z (%): 404 (6) [M, 37Cl], 402 (10) [M, 35Cl], 233 (100), 148 (10), 43 (7); HRMS (EI): calcd for C15H34Cl2FeP2: 402.08622 [M, 35Cl]; found: 402.08648; Anal. Calcd (%) for C15H34Cl2FeP2: C 44.69, H 8.50, P 15.37; found: C 44.63, H 8.55, P 15.37. Complex [(dippe)FeCl2] (3).61 Prepared analogously from FeCl2· THF1,5 (2.9 g, 0.012 mol) and bis(diisopropyphosphino)ethane (dippe, 3.3 g, 0.013 mol) as a crystalline material (4.3 g, 90%). MS (EI): m/z (%): 390 (7) [M, 37Cl], 388 (11) [M, 35Cl], 219 (100), 43 (44). HRMS (EI): calcd for C14H32Cl2FeP2: 388.07057 [M, 35Cl]; found: 388.07086; Anal. Calcd (%) for C14H32Cl2FeP2: C 43.22, H 8.29, P 15.92; found: C 43.54, H 8.10, P 15.79. Complex [(dippp)(η2-C2H4)2Fe] (2). A flame-dried two-necked round-bottomed flask equipped with a magnetic stir bar, a gas inlet, and a connection to the Ar line was charged with complex 1 (1.27 g, 3.15 mmol) and THF (20 mL). Ethylene was bubbled through this mixture at −5 °C for 30 min. EtMgBr (3 M in Et2O, 3.15 mL, 9.45 mmol) was then added dropwise while keeping the ethylene flow, causing an immediate color change from pale yellow to dark turquoise-blue. The resulting mixture was stirred for 1 h under ethylene atmosphere at −5 °C. For workup, all volatile materials were removed under vacuum at −10 °C to leave a dark-brown residue. Extraction of the solid with cold hexane and filtration at −40 °C afforded a clear turquoise-blue solution, which was evaporated in vacuo at −10 °C to give the title complex as a turquoiseblue powder (0.9 g, 77%). The material is highly air sensitive and unstable at ≥0 °C, but can be stored at −80 °C under Ar for prolonged periods of time without noticeable decomposition. Single crystals suitable for X-ray diffraction were grown from a saturated solution in hexanes by lowering the temperature to −80 °C; the structure in the solid state is contained in the Supporting Information of our preliminary communication.20 Anal. Calcd (%) for C19H42FeP2: C 58.76, H 10.90, Fe 14.38, P 15.95; found: C 58.57, H 10.97, Fe 14.37, P 15.87.

EXPERIMENTAL SECTION

General. All manipulations were carried out under argon atmosphere using Schlenk techniques. The solvents were purified by distillation over the indicated drying agents and were transferred under argon: THF, Et2O (Mg-anthracene), hexane, pentane, [D8]-THF (Na/ K). NMR: Spectra were recorded on a AV 400 or AV 500 spectrometer in the solvents indicated; chemical shifts (δ) are given in ppm, coupling constants (J) in Hz. MS (EI): Finnigan MAT 8200 (70 eV), HRMS: Finnigan MAT 95, Bruker APEX III FT-ICR-MS (7 T magnet); Elemental analyses: H. Kolbe, Mülheim/Ruhr. All commercially available compounds (Lancaster, Fluka, Aldrich) were used as received unless stated otherwise. [FeCl2(THF)1,5],60 bis(diisopropylphosphino)propane (dippp), 19 [CpFe(C 2 H 4 ) 2 ][Li(tmeda)] (5),15 and [CpFe(cod)2][Li(dme)] (6)15 were prepared according to literature procedures. Mö ssbauer Spectroscopy. Mössbauer spectra were recorded on a conventional spectrometer with alternating constant acceleration of the γ-source. The spectra were folded to merge the two linear halves of the G


Article

Organometallics Complex [(dippp)Fe(styrene)] (7). Styrene (182 μL, 1.56 mmol) was added at −20 °C to a solution of complex 2 (625 mg, 0.56 mmol) in THF (8 mL), causing a color change from deep green to yellow-brown. After the solution was stirred for 30 min, all volatile materials were evaporated in vacuum, and the residue was extracted with cold pentane. The combined extracts were filtered at −20 °C, and the resulting brown filtrate was concentrated in vacuo to about one-half of the volume and then slowly cooled to −75 °C over the course of 24 h. The resulting dark crystals were analytically pure and proved suitable for single-crystal Xray diffraction (472 mg, 67%). MS (EI): m/z (%): 436 (M+, 7), 332 (12), 233 (100). Anal. Calcd (%) for C23H42FeP2: C 63.31, H 9.70; found: C 63.60, H 9.58. Complex [(dippp)Fe(cod)] (8). A flame-dried two-necked roundbottomed flask equipped with a magnetic stir bar, a gas inlet, and a connection to the Ar line was charged with 1 (594 g, 1.47 mmol) and THF (15 mL). Ethylene was bubbled through this mixture at −5 °C for 30 min. EtMgBr (3 M in Et2O, 1.47 mL, 4.42 mmol) was added dropwise while keeping the ethylene flow, causing an immediate color change from pale yellow to dark turquoise-blue. The resulting mixture was stirred for 2 h under an ethylene atmosphere at −5 °C. Bubbling of ethylene was discontinued and the mixture stirred for 30 min before 1,5cyclooctadiene (cod, 0.9 mL, 7.36 mmol) was added. The cooling bath was removed and the mixture allowed to reach ambient temperature. After 2 h, the green solution was transferred via canula into a Schlenk flask, and the solvent was evaporated in vacuo to leave a black residue. This material was triturated with hexane, the resulting suspension was filtered, and the resulting deep green filtrate was evaporated in vacuum to give the title complex in the form of a green-brown solid material. The residue was recrystallized by slowly cooling a saturated solution in THF from 0 °C to −78 °C (134 mg, 21%). Single crystals suitable for X-ray diffraction were obtained by recrystallization from hexane at low temperature. Alternatively, the complex was obtained by stirring a solution of preformed 2 (2.60 g, 6.04 mmol) and 1,5-cyclooctadiene (4 mL) in hexane (20 mL) at ambient temperature for 3 h. All insoluble material was filtered off under Ar at −30 °C, and the resulting green filtrate was evaporated at this temperature to give the title compound in the form of dark green crystals (1.38 g, 52%). MS (EI): m/z (%): 440 (10, M+), 332 (5), 248 (20), 233 (100), 148 (23). Anal. Calcd (%) for C23H46FeP2: C 62.73, H 10.53; found: C 62.40, H 10.81. Complex [(dippp)Fe(ethylene)(norbornene)] (9). Prepared analogously from (dippp)FeCl2 (582 mg, 1.45 mmol), EtMgBr (3 M in Et2O, 1.45 mL, 4.34 mmol), and norbornene (272 mg, 2.89 mmol) in the form of black-greenish crystals that were analytically pure and suitable for X-ray diffraction (607 mg, 93%). MS (EI): m/z (%): 454 (M+, 2), 426 (8), 233 (100). Anal. Calcd (%) for C24H48FeP2: C 63.43, H 10.65; found: C 63.19, H 10.83. Complex [(dippp)Fe(norbornene)2] (10). Prepared analogously from (dippp)FeCl2 (670 mg, 1.67 mmol), EtMgBr (3 M in Et2O, 1.45 mL, 4.98 mmol), and norbornene (1.14 g, 12.1 mmol) in the form of black-green crystals that were analytically pure and suitable for X-ray diffraction (0.588 mg, 68%). MS (EI): m/z (%): 520 (M+, 12), 426 (8), 332 (5), 233 (100). Anal. Calcd (%) for C29H54FeP2·(C6H14): C 69.29, H 11.30; found: C 68.96, H 11.08. Metallacycle 12. A solution of cyclopropenone-1,3-propanediol ketal (188 mg, 1.67 mmol)62 in THF (2 mL) was added at −50 °C to a solution of 2 (325 mg, 0.84 mmol) in THF (5 mL), causing a color change from turquoise-blue to brown-yellow. The resulting mixture was stirred at −70 °C for 16 h before all volatile materials were evaporated under high vacuum while keeping the temperature ≤ −35 °C. The residue was extracted at this temperature with cold hexane, and the combined extracts were filtered and evaporated. The solid material was dissolved in the minimum amount of cold (−40 °C) Et2O. The solution was layered with cold pentane and stored at −78 °C for 3 d before the formed crystals were collected and dried in vacuo. These single crystals were analytically pure and suitable for X-ray diffraction (91 mg, 19%). Anal. Calcd (%) for C27H50FeO4P2: C 58.28, H 9.06; found C 58.49, H 9.20. Complex [(dippp)Fe(1,3-butadiene)] (15). (Chloromethyl)cyclopropane (225 mg, 2.48 mmol) and activated magnesium powder

(59 mg, 2.48 mmol)41 were successively added to a solution of complex 1 (500 mg, 1.24 mmol) in THF (20 mL) at −78 °C. The temperature was raised to −35 °C and the suspension stirred for 20 h. All volatile materials were evaporated in high vacuum, and the remaining solid material was triturated with cold pentane. The slurry was filtered at −35 °C, the solid material was rinsed with cold pentane, the combined filtrates were evaporated at this temperature in high vacuum, and the residue was recrystallized by slowly cooling a solution in the miniumum amount of pentane/Et2O from −35 °C to −80 °C. The title compound was obtained in the form of green crystals that were analytically pure and suitable for X-ray diffraction (489 mg, 51%). MS (EI): m/z (%): 386 (22, M+), 332 (14, (M−C4H6)+), 233 (100), 54 (17). Anal. Calcd (%) for C19H40FeP2: C 59.07, H 10.44; found: C 58.81, H 10.60. Complex [(dippe)(η5-C5H5)Fe(Et)] (22). A flame-dried Schlenk flask, equipped with a magnetic stir bar, a gas inlet, and a connection to the Ar line, was charged with (dippe)FeCl2 (405 mg, 1.03 mmol) and THF (15 mL). Ethylene was bubbled through the resulting solution for 30 min at −20 °C. Next, EtMgBr (3 M in Et2O, 0.69 mL, 2.08 mmol) was added dropwise while keeping the ethylene flow, causing an immediate color change from pale yellow to deep blue. After the resulting solution was stirred at −20 °C for 1.5 h, freshly distilled cyclopentadiene (0.43 mL, 5.24 mmol) was added dropwise, causing another color change from deep blue to red-brown. The temperature was slowly raised to 20 °C, and stirring was continued for 1 h. For workup, the solvent was removed in vacuo, and the resulting brown residue was repeatedly extracted with pentane to give a clear red solution. The product was isolated by crystallization upon lowering the temperature from 0 °C to −85 °C over the course of 24 h. The precipitate was filtered off at −30 °C, rinsed with cold Et2O, and dried in vacuo to give the title compound as a yellow crystalline solid (277 mg, 65%). Single crystals suitable for X-ray diffraction were grown from a saturated solution in pentane upon decreasing the temperature from 0 °C to −35 °C; the structure of the complex in the solid state is contained in the Supporting Information of our preliminary communication.20 1H NMR (500 MHz, [D8]-THF, −55 °C): δ = 3.98 (5H), 2.26 (2H), 1.96 (2H), 1.56 (2H), 1.42−0.93 (29H), − 0.13 (2H); 13C NMR (126 MHz, [D8]-THF, −55 °C): δ = 76.0, 31.7, 26.8, 22.6 (t, JPC+P′C = 35.8 Hz), 21.0, 20.6, 20.4, 20.3, 19.5, −11.4 (t, JPC+P′C = 46.4 Hz); 31P{1H} NMR (202 MHz, [D8]-THF, −55 °C): δ = 110.6; MS (EI): m/z (%): 412 (14), 383 (100), 340 (23), 121 (14); HRMS (EI): calcd for C21H42FeP2: 412.21112; found: 412.21146 [M+]. Complex [(dippp)(η5-C6H7Et-endo)Fe(H)] (20). A flame-dried Schlenk flask, equipped with a magnetic stir bar, a gas inlet, and a connection to the Ar line, was charged with (dippp)FeCl2 (499.8 mg, 1.24 mmol) and THF (15 mL). Ethylene was bubbled through the stirred solution at −5 °C for 30 min before EtMgBr (3 M in Et2O, 1.24 mL, 3.72 mmol) was added dropwise while keeping the ethylene flow, thus causing an immediate color change from pale yellow to turquoiseblue. After the mixture was stirred at −5 °C for 1.5 h, freshly distilled 1,3cyclohexadiene (0.59 mL, 6.2 mmol) was slowly added, causing another color change to orange-brown. The temperature was gradually raised to 20 °C, and the resulting solution was stirred for 1 h before the solvent was removed in vacuo to leave a brown solid residue. Extractions with pentane gave a clean yellow solution from which the product was crystallized by lowering the temperature to −85 °C over the course of 24 h. The resulting solid material was collected at −30 °C, rinsed with cold Et2O, and dried in vacuo to give the title complex in the form of an orange crystalline solid (217 mg, 40%). Single crystals suitable for X-ray diffraction were grown from a saturated solution in pentane upon decreasing the temperature from 0 °C to −85 °C; the structure of the complex in the solid state is contained in the Supporting Information of our Preliminary Communication.20 1H NMR (500 MHz, [D8]-THF, −55 °C, line broadening): δ = 6.14 (1H), 4.44 (2H), 2.22 (2H), 2.04 (1H), 1.64−0.73 (39H), −2.71 (1H); 13C NMR (126 MHz, [D8]-THF, −55 °C, line broadening): δ = 85.4, 74.6, 38.5, 36.2, 32.2, 28.8, 25.3 (under solvent peak), 21.7, 20.2, 19.5, 18.9, 18.4, 12.4; 31P{1H} NMR (202 MHz, [D8]-THF, − 55 °C): δ = 64.4; IR: 1840 cm−1 (Fe−H); MS (ESI+): m/z 439.2; HRMS (ESI+): calcd for C23H45FeP2: 439.23457; found: 439.23404 [(M-H)+]. H


Article

Organometallics Complex [(dippe)(η5-C6H7Et-endo)Fe(H)] (23). Prepared analogously from (dippe)FeCl2 (498 mg, 1.28 mmol), EtMgBr (3 M in Et2O, 1.28 mL, 3.84 mmol), and 1,3-cyclohexadiene (0.61 mL, 6.40 mmol) in the form of yellow crystals (410 mg, 75%). 1H NMR (500 MHz, [D8]THF, −30 °C): δ = 4.04 (s, 5H), 2.13−2.01 (m, 2H), 2.01−1.92 (m, 2H), 1.39−1.24 (m, 2H), 1.39−1.09 (m, 12H), 1.06.-0.95 (m, 8H), 0.95−0.86 (m, 6H). 13C NMR (126 MHz, [D8]-THF, −30 °C): δ = 72.2, 29.3 (t, J = 5 Hz), 28.5 (t, J = 14 Hz), 22.3 (t, J = 18 Hz), 20.3, 19.1, 18.8, 18.6; 31P{1H} NMR (202 MHz, [D8]-THF, − 30 °C): δ = 131.8, 131.4; MS (EI): m/z 384 (100), 341 (32), 298 (12), 276 (20), 234 (51), 192 (30), 121 (18). HRMS (EI): calcd for C19H38FeP2: 384.17098; found: 384.1796; for the Mössbauer data, see Table 1. Complex 25. A solution of borane 28 (132 mg, 0.65 mmol)63 in hexane (1.5 mL) was added at −10 °C to a solution of 2 (212 mg, 0.55 mmol) in hexane (3 mL), and the resulting mixture was stirred for 7 h at this temperature. Stirring was discontinued, the red-brown precipitate was allowed to settle, and the black solution was removed via canula. The solid material was rinsed twice with cold hexane, dried in vacuo, and recrystallized by slowly cooling a solution in the minimum amount of THF from −30 °C to −80 °C. The mother liquor was siphoned off, and the red-brown crystals were dried in vacuo (117 mg, 40%). MS (EI): m/ z: 536 (M+, 4), 334 (17), 333 (100), 276 (38). Because of the exceptional sensitivity of this complex, no correct combustion analysis was obtained. Complex 27. Prepared analogously from (dippp)Fe(ethylene)2 (236 mg, 0.61 mmol) and 9-allyl-9-BBN (29) (113.3 mg, 0.70 mmol)64 in the form of red-brown crystals suitable for X-ray diffraction (207 mg, 69%). The Mössbauer data, see Table 1; the Mössbauer spectrum (Figure 9) indicated an unknown iron-containing impurity (ca. 12%). MS (EI): m/z: 494 (M+, 10), 233 (100). Because of the exceptional sensitivity of this complex, no correct combustion analysis was obtained. Complex [CpFe(ethylene)2]2Zn (32). A suspension of ZnCl2 (0.35 g, 2.58 mmol) in THF (2 mL) was added to a solution of [CpFe(ethylene)2][Li(tmeda)] (5) (1.55 g, 5.16 mmol) at −10 °C. The resulting dark red solution gradually becomes turbid. After the solution was stirred for 48 h at this temperature, the orange-red suspension was filtered at −25 °C under Ar, the collected solid material was dissolved in the minimum amount of THF, and the resulting solution stored under an atmosphere of ethylene at −25 °C. After 1 d, fine red needles had formed that were collected and stored at low temperature (693 mg, 64%). Single crystals suitable for X-ray diffraction were grown by slowly cooling a saturated solution in THF from −25 °C to −78 °C. 1H NMR (400 MHz, [D8]-THF, 193 K): δ = 4.45 (s, 5H), 2.32 (br. S, 1H), 0.77 (br. S, 1H), 0.14 (br. S, 1H9, − 1.12 (br s, 1H); 13C NMR (100 MHz, [D8]-THF, 193 K): δ = 78.8, 30.3, 8.7); for the Mössbauer data, see Table 1. Anal. Calcd (%) for C18H26Fe2Zn: C 51.54, H 6.25; found: C 51.69, H 6.47. Complex [CpFe(cod)2]2Zn (35). A suspension of ZnCl2 (0.75 g, 5.47 mmol) in THF (14 mL) was added to a solution of [CpFe(cod)2][Li(dme)] (6) (3.57 g, 10.94 mmol). After stirring for 1.5 h at ambient temperature, the turbid mixture was stored in a freezer overnight. The orange-red precipitate was filtered off under Ar at 0 °C, dispersed in toluene (5 mL), and cod (0.2 mL) was added. The suspension was warmed to 60 °C to give an almost clear mixture, which was filtered under Ar to remove insoluble impurities. The filtrate was stored at −25 °C, the red precipitate was filtered off, the mother liquor was reduced to half of its original volume, and stored again at −25 °C to give a second crop of product in the form of red crystals which were suitable for X-ray diffraction analysis (2.58 g, 70%). 1H NMR (400 MHz, [D8]-THF): δ = 4.41 (s, 10H), 2.93 (br, 8H), 2.30 (br, 8H), 1.73 (br, 4H), 1.45 (br, 4H); 13C NMR (100 MHz, [D8]-THF): δ = 77.6, 59.5, 46.4, 34.0 (br). Complex [CpFe(butadiene)]2Zn (33). In a pressure Schlenk tube, a suspension of ZnCl2 (0.265 g, 1.94 mmol) in THF (3 mL) was added to a solution of [CpFe(ethylene)2][Li(tmeda)] (5) (1.16 g, 3.88 mmol) at 0 °C. The resulting mixture was stirred under an atmosphere of 1,3butadiene for 1 h at 0 °C and for an additional 1 h at 60 °C. The suspension was filtered while hot, and the filtrate was slowly cooled to −78 °C. The precipitated red needles were collected and found suitable

for single-crystal X-ray diffraction (586 mg, 73%). 1H NMR (400 MHz, [D8]-THF): δ = 5.06 (m, 2H), 4.28 (s, 5H), 0.82 (m, 2H), −2.09 (t, 2H); 13C NMR (100 MHz, [D8]-THF): δ = 71.7, 70.1, 14.4; For the Mössbauer data, see Table 1. Anal. Calcd (%) for C18H22Fe2Zn: C 52.04, H 5.34; found: C 52.32, H 5.30. Complex [CpFe(butadiene)][Li(tmeda)] (34). Lithium sand (190 mg, 270 mmol) was added to a red-brown solution of [CpFe(butadiene)]2Zn (33) (2.54 g, 6.11 mmol) in THF (10 mL) at 0 °C, and the resulting suspension was stirred at this temperature for 1 h. Excess lithium was filtered off and rinsed with THF (3 mL), and the combined filtrates were diluted with tmeda (15 mL). After the solution was stored at 0 °C overnight, orange-brown crystals had formed that were filtered off, dried in vacuum, and stored under Ar at −20 °C (3.05 g, 84%). Crystals suitable for single-crystal X-ray diffraction were grown by slowly cooling a saturated solution in THF from 0 °C to −80 °C. A wellresolved 1H NMR could not be obtained neither for the tmeda free complex nor for the tmeda adduct due to massive line broadening. 13C NMR (100 MHz, [D8]-THF, resolved signals): δ = 68.6, 67.0, 58.6, 46.2; For the Mössbauer data, see Table 1. Anal. Calcd (%) for C15H27N2FeLi: C 60.42, H 9.13, N 9.40; found: C 60.25,H 9.34.

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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.organomet.7b00571. Additional crystallographic information showing the structures of complexes 10, 33, and 35a,b (two polymorphs), NMR spectra of new compounds, and Mössbauer spectra of complexes 5, 23, 27, 32−34, including Figures S1−S10 (PDF) Accession Codes

CCDC 1564812−1564824 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]. ORCID

Richard Goddard: 0000-0003-0357-3173 Alois Fürstner: 0000-0003-0098-3417 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous financial support by the Fundación Ramón Areces (fellowship for A C.) and the MPG is gratefully acknowledged. We thank S. Auris for assistance, B. Mienert for recording the Mössbauer spectra, and the analytical departments of our Institute for excellent support.

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REFERENCES

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