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

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Controlled Dissociation of Iron and Cyclopentadienyl from a Diiron Complex with a Bridging C3 Ligand Triggered by One-Electron Reduction Gabriele Agonigi,† Gianluca Ciancaleoni,† Tiziana Funaioli,† Stefano Zacchini,‡ Francesco Pineider,† Calogero Pinzino,§ Guido Pampaloni,† Valerio Zanotti,‡ and Fabio Marchetti*,† †

Dipartimento di Chimica e Chimica Industriale, University of Pisa, Via G. Moruzzi 13, I-56124 Pisa, Italy Dipartimento di Chimica Industriale “Toso Montanari”, University of Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy § Area della Ricerca, ICCOM-CNR, Via G. Moruzzi 1, I-56124 Pisa, Italy

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‡

S Supporting Information *

ABSTRACT: The one-electron reduction of a diiron cationic complex revealed unique features: cleavage of the diiron structure occurred despite a multidentate bridging C3 ligand and was accompanied by the clean dissociation of one η5cyclopentadienyl ring and one iron as isolated units. Thus, the iron(II)−iron(II) μ-vinyliminium complex [Fe2Cp2(CO)(μCO){μ-η1:η3-C3(Et)C2HC1N(Me)(Xyl)}][SO3CF3] ([1a]SO3CF3) reacted with cobaltocene in tetrahydrofuran (THF), affording the iron(II) vinylaminoalkylidene [FeCp(CO){C1N(Me)(Xyl)C2HC3(Et)C(O)}] (2a) in 77% yield relative to the C3 ligand. Analogously, [FeCp(CO){C1N(Me)(Xyl)C2HC3(CH2OH)C(O)}] (2b) was obtained in 64% yield from the appropriate diiron precursor and CoCp2. The formation of 2a is initiated by the one-electron reduction of [1a]+, followed by a reversible intramolecular rearrangement terminating with the irreversible release of CpH (NMR and gas chromatography−mass spectrometry) and Fe [electron paramagnetic resonance (EPR) and magnetometry]. The key intermediate iron(I) ferraferrocene (3) was detected by EPR and IR spectroelectrochemistry, while the related species 3-H-3 was isolated after the addition of a hydrogen source and then identified by X-ray diffraction. A plausible mechanism for the route from [1a]+ to 3 was ascertained by density functional theory calculations. The dication [1a]2+, displaying both carbonyl ligands in terminal positions, and the anion [3]− were electrochemically generated. The functionalized diiron compounds 4 (52% yield) and 5 (62%) were afforded through the activation of O2 and S8 by a radical intermediate along the reductive pathway of [1a]+. The reaction of [Fe2Cp2(CO)(μ-CO){μ-η1:η3-C(SiMe3)CHCN(Me)(Xyl)}][SO3CF3] ([1c]SO3CF3) with CoCp2 in THF afforded [Fe2Cp2(CCSiMe3)(CO)(μ-CO){μ-CNMe(Xyl)}] (6) in 65% yield.

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INTRODUCTION

currently being done in the direction of designing suitable diiron complexes able to mimic the relevant biological systems,3 i.e., enzymes based on a fundamental [FeFe] skeleton. 4 Besides, a large variety of derivatives of [Fe2Cp2(CO)4], a milestone of organometallic chemistry,5 are accessible through the replacement of CO ligands and have progressively emerged as convenient and versatile scaffolds to

Dimetal complexes constitute the most simple framework able to provide multisite coordination for the ligands and cooperative effects between the metal centers, thus allowing reactivity patterns that are otherwise not viable on mononuclear species.1 In this setting, diiron complexes have conquered a central position in sustainable synthetic chemistry, in view of replacing precious elements with nontoxic and earthabundant, cost-effective counterparts in metal-mediated processes.2 In particular, an impressive research effort is © XXXX American Chemical Society

Received: August 29, 2018

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DOI: 10.1021/acs.inorgchem.8b02445 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Four-Step Synthesis of the Vinyliminium Complex (Xyl = 2,6-C6H3Me2; TfO = CF3SO3)a

a

Inset: secondary isomer.

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RESULTS AND DISCUSSION Synthesis and Chemical Reduction of a Diiron Vinyliminium Complex. The new μ-vinyliminium compound [1a]SO3CF3 was obtained from Fe2Cp2(CO)4 using previously reported synthetic protocols (Scheme 1).19,20 The 1 H NMR spectrum of [1a]SO3CF3 in a CDCl3 solution shows a mixture of two isomers, in a ca. 3:1 ratio. On the basis of a comparison of the NMR data with those related to a library of compounds,20 the prevalent isomer displays an E configuration of the iminium substituents and cis geometry of the Cp ligands, while the opposite stereochemistry (Z)-trans is adopted in the secondary isomer.21 The density functional theory (DFT)calculated structure of (E)-cis-[1a]SO3CF3 is supplied in Figure S1. According to the electrochemical outcomes (vide infra), CoCp2 appeared to be a suitable one-electron reductant for [1a]SO3CF3.22 The reaction was carried out in tetrahydrofuran (THF) and allowed to isolate the monoiron complex 2a (CCDC 1855386) in 77% yield (relative to the C3 unit; Scheme 2). We successfully extended this result to a similar system containing a hydroxyl group (synthesis of 2b from [1b]SO3CF3;20b Scheme 2).

explore unconventional reaction mechanisms and synthetic routes.6,7 Metal-centered electron exchange in dinuclear complexes based on 3d elements often results in the homolytic rupture of the single metal−metal bond in the absence of a tightly bound bridging ligand.8 In other terms, fragmentation generally takes place with a distribution of ligands and electrons between the two separating metal fragments.9 For instance, the one-electron oxidation/reduction of the iron(I) prototype [Fe2Cp2(CO)4] gives rise to single mononuclear products [oxidation, iron(II); reduction, iron(0)], favored by the fluxionality of the carbonyl ligands.10 As a useful comparison, the 4d and 5d homologues [M2Cp2(CO)4] (M = Ru,11 Os12) maintain their dinuclear structure upon one-electron oxidation. The introduction of a multidentate bridging ligand is often crucial to providing stability to the fundamental metal (3d)− metal (3d) single-bond skeleton when the complexes are involved in electron transfer.13 With specific reference to Fe− Fe compounds, the retention of the dinuclear structure across different oxidation states is essential to the catalytic behavior of bioinspired species. 14 Also, complexes based on the [Fe2Cp2(CO)3] frame and containing a bridging multidentate hydrocarbyl ligand do not undergo Fe−Fe cleavage upon electron exchange, although this process may significantly enhance the reactivity of the system.15 For instance, the μthiocarbyne cation [Fe2Cp2(CO)3(μ-CSMe)]+ is subjected to one-electron reduction to form a fairly stable dinuclear radical, and the weakening of the Fe−CO bonds favors the replacement of one carbonyl with various organic groups.15a After many years of investigations on the reactivity of diiron complexes by some of us,16 we became interested in the electrochemical behavior of complexes with a vinyliminium C3 ligand coordinated to the frame [Fe2Cp2(CO)2] via an unusual η1:η3 fashion.17 Previous reactivity studies have evidenced that this coordination type is robust but versatile; thus, several derived organometallic structures supported on the Fe−Fe core have been obtained.18 We report herein that the vinyliminium three-carbon chain adapts across different oxidation states, and following reduction, it is incorporated in a stable monoiron structure via unprecedented heterolytic metal−metal cleavage, cleanly releasing isolated iron (Fe) and cyclopentadienyl (Cp) units. These results will be discussed with reference to multiple techniques; moreover, strategies for vinyliminium functionalization (including O2 activation) will be described.

Scheme 2. Synthesis of Monoiron Compounds by the Reaction of Diiron Vinyliminium Complexes with Cobaltocene

A few compounds analogous to 2a and 2b were obtained in the past, in an attempt to deprotonate the C2−H function belonging to the respective diiron vinyliminium parent species.23 The new complexes 2a and 2b were characterized by elemental analysis and IR and NMR spectroscopy. The 1H NMR spectrum of 2a consisted of two sets of signals attributed to E and Z isomers, while 2b was detected exclusively in the E configuration. The 13C NMR resonance of C1 occurred around B


Article

Inorganic Chemistry 265 ppm, which is typical for an aminoalkylidene carbon;24 on the other hand, C2 and C3 resonate at ca. 145 and 180 ppm, respectively, with these values indicating alkenyl character, which is manifested also by the low-field 1H NMR resonance of C2−H (ca. 6.5 ppm). The molecular structure of (E)-cis-2a was ascertained by an X-ray diffraction study, and a view of the structure is shown in Figure 1. The 1-metalla-2-aminocyclopenta-1,3-dien-5-one

(compound 3 in Scheme 3). Although this spectral pattern indicates a monoiron motif resembling that of 2a, the addition to the reaction mixture of an excess of I2, acting as an oxidant, resulted in almost a quantitative recovery of [1a]+. This reversibility suggests that the “FeCp” fragment is still incorporated in 3. Because of these observations and in light of the X-ray determination of the related compound 3-H-3 (CCDC 1855387; vide infra), 3 was identified as the diiron radical [FeCp(CO){CN(Me)(Xyl)CHC(Et)CO}FeCp]*. Figure 2 gives a view of the DFT-calculated structure of 3. The computer-simulated IR spectrum of 3 well matches the experimental spectrum in a THF solution (bands at 1949 and 1554 cm−1).26 Moreover, the EPR spectrum (Figure S4) and the calculated spin-density distribution (Figure S5) agree in that the unpaired electron is mainly localized on the Fe2 atom. The intermediate 3 was revealed to be strongly air-sensitive and unstable at ambient temperature, quantitatively converting to 2a in ca. 10 h. In the course of the 3-to-2a transformation, slight variations in the position of the IR band related to the carbonyl ligand were recorded. However, we found that the addition to the reaction solution containing 3 at −50 °C of a small amount (1−2 equiv with respect to the initial [1a]+) of a H-atom source [i.e., H2O, dichloromethane (CH2Cl2), or butylated hydroxytoluene] led to a clear change in the IR spectrum (formation of 3-H-3; Scheme 3), with the carbonyl band shifting from 1949 to 1941 cm−1.27 The corresponding EPR spectrum matches that of 3 because of the unpaired electron being substantially localized on iron (Figure S6). Like 3, 3-H-3 converts to 2a in THF at ambient temperature (we propose that 3-H-3 is intermediate along the 3 → 2a pathway, vide infra). A small amount of 3-H-3 could successfully be isolated using a quick workup procedure: the cold reaction solution was filtered through a short silica pad to remove impurities and dried under vacuum. The residue was dissolved in CH2Cl2, and a few extremely air-sensitive crystals suitable for X-ray analysis were collected by slow diffusion of hexane into this solution at −30 °C. The crystals of 3-H-3 were found in an admixture with 2a. The molecular structure of 3-H-3 is composed of two symmetry-related (by an inversion center) and H-bonded monomers (3), bearing exactly the same structures. Each monomeric unit is composed of the same five-membered ring as that of 2a, which is η5-coordinated to the Fe(2)Cp fragment. In other terms, the overall structure might be viewed as a functionalized ferraferrocene.28 The O-bonded H atom is disordered over the two symmetry-related positions, and it has been refined with a 0.5 occupancy factor. The monomeric unit of 3-H-3 is shown in Figure 3, while the H-bonded dimer is represented in Figure 4, with the related H-bonded parameters being O(1)−H(1) 0.84(2) Å, H(1)···O(1)#1 1.87(4) Å, O(1)···O(1)#1 2.688(7) Å, and ∠O(1)−H(1)−O(1)#1 166(12)° (symmetry transformation used to generate equivalent atoms: #1, −x, −y, −z + 1). The main distances and angles within each monomer are reported in Table 1, where they are compared to the corresponding values for 2a. The Fe(1)−Fe(2) distance [2.5046(10) Å] is typical of a metal−metal bond, as previously reported for many other Fe−Fe-bonded diiron carbonyl cyclopentadienide complexes.16,29 The Fe(1), C(1), C(2), C(3), and C(4) atoms composing the metallacyclopentadiene-like ring, η5coordinated to Fe(2), may still be considered coplanar [mean deviation from the Fe(1)−C(1)−C(2)−C(3)−C(4) least-

Figure 1. Molecular structure of 2a. Displacement ellipsoids are at the 30% probability level. H atoms have been omitted for clarity. Main bond distances (Å) and angles (deg): Fe(1)−C(1) 1.918(3), Fe(1)− C(11) 1.741(3), C(1)−C(2) 1.478(4), C(2)−C(3) 1.342(4), C(3)− C(4) 1.512(4), C(1)−N(1) 1.328(3), Fe(1)−C(4) 1.943(3), C(11)−O(11) 1.154(4), C(4)−O(1) 1.229(3), N(1)−C(8) 1.477(3); C(1)−Fe(1)−C(4) 82.86(11), C(1)−C(2)−C(3) 114.6(2), C(3)−C(4)−Fe(1) 113.68(18), Fe(1)−C(1)−C(2) 115.29(19), C(2)−C(3)−C(4) 113.2(2), Fe(1)−C(11)−O(11) 178.2(3).

five-membered ring is essentially coplanar (mean deviation from the Fe(1)−C(1)−C(2)−C(3)−C(4) least-squares plane = 0.0264 Å), and the bonding distances are in good agreement with the vinylaminoalkylidene nature evidenced by NMR spectroscopy. The synthesis of 2a and 2b from [1a]+ and [1b]+ deserves some comments. Noticeably, it implies the accommodation of the vinyliminium C3 chain within a peculiar five-membered metallacycle that is not obtainable starting from monoiron precursors. Conversely, basic aminoalkylidene cyclic structures of the type [FeCp(CO){CN(R)2(CH2)3}] were previously prepared via the sequential modification of a cyclobutene ligand coordinated to [FeCp(CO)2]+.25 Moreover, no reduction apparently occurs upon going from [1a]+ and [1b]+ to 2a and 2b because the Fe center in 2a and 2b retains the 2+ oxidation state. In order to shed light on the mechanism of the unusual C3-ligand rearrangement and to clarify the destiny of the missing “FeCp” unit (Scheme 2), the reaction of [1a]SO3CF3 with CoCp2 was studied by multiple techniques. First, this reaction was conducted at −50 °C and monitored by IR and electron paramagnetic resonance (EPR) spectroscopy. In principle, the initial CoCp2 to [1a]+ one-electron transfer is expected to generate the elusive radical species 1a, which actually was not experimentally observed (Scheme 3). Views of the DFT-optimized structure of 1a and the spin-density distribution are shown in Figures S2 and S3. According to the calculations, the spin density is mainly localized on the C1bound Fe atom (see the Supporting Information for details). The first species that could be readily detected along the pathway to 2a exhibited two IR bands at 1949 and 1554 cm−1 C


Article

Inorganic Chemistry Scheme 3. Oxidation and Reduction Pathways of a Diiron μ-Vinyliminium Cationic Complex and Functionalization Reactionsa

Fe atoms with formal oxidation state ≠ 2+ are in green. Red steps: confirmed by spectroelectrochemistry. Blue steps: via electrochemistry only.

a

similar “sliding” of the bridging vinyliminium ligand over a diiron frame was previously observed, although not resulting in disruption of the dinuclear structure.30 The transition state (TS1; ΔE⧧ = 23.6 kcal·mol−1) for the 1am-to-1am* modification was located through an extensive description of the potential energy surface (Figure S7); TS1 evidences a concerted mechanism whereby C1 approaches the distal Fe atom, while the distance between the latter and C3 increases. An intramolecular attack of C3 to the bridging carbonyl ligand (CO insertion into the Fe−C3 bond) is viable in 1am*, affording 3m; the transition state of this step (TS2) requires 26.3 kcal·mol−1.

squares plane = 0.0906 Å], even if to a lesser extent with respect to 2a. A DFT study was carried out to elucidate the mechanism of the intramolecular rearrangement leading from 1a to 3 (Figure 5). In order to save computational resources, the Et and Xyl groups were replaced with H and Me, respectively (model compound: 1am); unless otherwise specified, the following energy values will be referred to 1am. The first likely molecular event is the isomerization of 1am to 1am* (ΔG = +18.9 kcal· mol−1). The salient aspect of this transformation is the exchange between C3 and C1 in the occupancy of the nearly equidistant site between the two Fe atoms (this site is occupied by C3 in 1am, as well as in [1a]+, and by C1 in 1am*). A D


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Inorganic Chemistry

Table 1. Comparative View of Selected Bond Distances (Å) and Angles (deg) for 2a and 3-H-3 2a

Figure 2. DFT-optimized structure of 3. H atoms are not displayed for clarity. Selected bond distances (Å) and angles (deg): Fe1−C1 1.980, Fe1−C4 2.014, C1−N 1.384, C1−C2 1.449, C2−C3 1.435, C3−C4 1.476, Fe1−Fe2 2.514, Fe2−C1 2.122, Fe2−C2 2.053, Fe2− C3 2.076, Fe2−C4 2.254, C4−O1 1.265, C5−O2 1.200; N−C1−C2 119.2, C1−C2−C3 116.5.

3-H-3

Fe(1)−C(1) Fe(1)−C(4) Fe(1)−C(11) Fe(1)−Cpav Fe(1)−Fe(2) Fe(2)−C(1) Fe(2)−C(2) Fe(2)−C(3) Fe(2)−C(4) Fe(2)−Cpav C(1)−N(1) C(1)−C(2) C(2)−C(3) C(3)−C(4) C(4)−O(1) C(11)−O(11)

1.918(3) 1.943(3) 1.741(3) 2.123(7)

1.328(3) 1.478(4) 1.342(4) 1.512(4) 1.229(3) 1.154(4)

2.032(5) 1.979(5) 1.751(6) 2.126(11) 2.5046(10) 2.041(5) 2.043(5) 2.033(5) 2.002(5) 2.084(11) 1.388(6) 1.437(7) 1.423(7) 1.443(7) 1.312(6) 1.158(6)

C(1)−Fe(1)−C(4) Fe(1)−C(1)−C(2) C(1)−C(2)−C(3) C(2)−C(3)−C(4) C(3)−C(4)−Fe(1) Fe(1)−C(11)−O(11)

82.86(11) 115.29(19) 114.6(2) 113.2(2) 113.68(18) 178.2(3)

82.8(2) 110.4(3) 116.1(4) 114.4(4) 113.0(3) 178.1(4)

Characterization of Fragmentation Coproducts: NMR, Magnetometric, and EPR Analyses. The calculated ΔG variation upon going from 1a to 3 is slightly positive (5.6 kcal·mol−1); thus, the irreversible release of the “FeCp” moiety from 3 is reasonably the driving force allowing the quantitative formation of 2a (Scheme 3). In order to elucidate the destiny of “FeCp”, we carried out different experiments. The reaction of [1a]SO3CF3 with CoCp2, in deuterated THF, afforded a final solution whose NMR analysis pointed out the presence of cyclopentadiene (CpH, but not CpD), in an equimolar amount with respect to 2a (no other Cp signals were recognized). In diluted solutions, CpH is expected to be relatively stable in its monomeric form.31 The formation of CpH was confirmed by gas chromatography (GC)−mass spectrometry (MS) analysis on the stripped solution (see Figure S9 and the Experimental Section for details).

Figure 3. Molecular structure of the monomeric unit [FeCp(CO){C1N(Me)(Xyl)C2HC3(Et)CO}FeCp] within the H-bonded dimer of 3-H-3. Displacement ellipsoids are at the 30% probability level. H atoms have been omitted for clarity.

The preliminary variation in the coordination mode of the vinyliminium ligand (from 1am to 1am*) seems necessary to access the final species 3m. Otherwise, direct CO insertion into the Fe−C3 bond of 1am is featured by a more prohibitive activation barrier (33.8 kcal·mol−1) and leads to an isomer of 3m (3mi). This alternative but unlikely pathway has been explored and is presented in detail in Figure S8.

Figure 4. View of the H-bonded dimer 3-H-3. Displacement ellipsoids are at the 30% probability level. H atoms, except H(1), have been omitted for clarity. Symmetry transformation used to generate equivalent atoms: #1, −x, −y, −z + 1. E


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Inorganic Chemistry

Figure 5. Calculated pathway for a model reaction and optimized geometries of intermediate species.

Figure 6. Analyses of an iron-loaded silica sample. Left: variable-temperature magnetization versus applied magnetic field plots. Right: EPR spectrum (a, experimental, 298 K; b, calculated).

The yellowish-red fine suspension produced in the reaction mixture [1a]SO3CF3/NaH32 was dispersed on a silica matrix, which was repeatedly washed with MeCN to remove organic/ organoiron species (see the Experimental Section for details). The resulting silica material underwent IR, magnetometric, and EPR studies (Figure 6). The IR spectrum was perfectly superimposable on that of blank silica, confirming the absence of organoiron compounds. Although no quantitative information could be extracted from the magnetometric data,33 the magnetization curves clearly indicate temperature-dependent paramagnetic behavior, thus ruling out the presence of ferromagnetic Fe0 clusters or highly ordered spin structures34 and otherwise suggesting iron(III)-based oxido species. Accordingly, the EPR spectrum (see also Figure S10 and Table S1 for details) evidenced a population of noninteracting FeIII sites (lines at g ≈ 2.1 and 2.4). In conclusion, although the mechanism of the “FeCp” elimination accompanying the formation of 2a appears to be difficult to define in detail, it is evident that this process proceeds with the clean and separated release of the Fe and Cp units. According to Scheme 3, we tentatively propose that the

radical 3 is prone to capturing a H atom from the reaction medium, converting into 3-H-3 (the formation of 3-H-3 is fast adding a suitable H-atom source; see above); subsequent intramolecular hydrogen migration from the O atom to the leaving Cp ligand35 would produce 2a, CpH, and atomic iron. Iron is presumably oxidized to iron(III) at the surface of silica, despite working under an inert atmosphere. It has to be noted that dissociation of a η5-coordinated Cp ligand from a lowvalent late transition metal is an exceedingly rare reaction.36 To the best of our knowledge, the present case is a unique example of straightforward Cp release from an iron compound in mild conditions.37 Electrochemical and IR Spectroelectrochemistry Studies. The electrochemical investigation of [1a]SO3CF3 was carried out in THF/[NnBu4]PF6 0.2 M by cyclic voltammetry (CV) at a platinum working electrode and by in situ IR spectroelectrochemistry in an optically transparent thin-layer electrochemical (OTTLE) cell. The potential of the working electrode was swept between the selected potentials at a scan rate of 1.0 mV·s−1, and a sequence of vibrational spectra in the 2100−1500 cm−1 region was collected at regular time F


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Inorganic Chemistry intervals. By a comparison between the sequences of IR spectra recorded during the slow potential scan and the profile of the i/E curve, the spectra could be separated into groups, each of them attributable to an electron exchange. The CV profile shown in Figure 7 exhibits a chemically reversible oxidation at the formal electrode potential E°′ =

Figure 8. IR spectral changes of a THF solution of [1a]SO3CF3 recorded in an OTTLE cell during the progressive reduction of the potential from 0.0 to −1.4 V. [NnBu4]PF6 (0.2 mol·dm−3) was used as the supporting electrolyte. The absorptions of the solvent and supporting electrolyte have been subtracted.

resembling that of 3; however, slight structural differences can be observed upon going from 3 to [3]−, in particular the lengthening of the C1−N (from 1.384 to 1.420 Å), Fe1−Fe2 (from 2.514 to 2.578 Å), and C5−O2 bonds (from 1.200 to 1.283 Å) and the shortening of the Fe2−C1 (from 2.122 to 2.007 Å) and Fe2−C4 bonds (from 2.254 to 2.123 Å). The anion [3]− appears to be rather chemically stable considering that, during the reverse oxidation step, first 3 and then [1a]+ were stepwise recovered. Moreover, the final IR spectra, recorded after the respective backward oxidation steps to 0.0 V following the first and second reduction processes, showed comparable relative intensities of the absorptions related to [1a]+ and 2a (Figure S12). Also, the oxidation behavior of [1a]+ was spectroelectrochemically investigated. When the potential of the working electrode was swept from 0.0 and +0.8 V (Figure 9), during the

Figure 7. Differential-pulse (a) and double-cycle (b) voltammograms of [1a]SO3CF3 recorded at a platinum electrode in 0.2 M [NnBu4]PF6/THF solutions (black line, first cycle; red line, second cycle).

+0.62 V (ΔEp = 92 mV) and two reductions at E°′ = −1.31 (ΔEp = 120 mV) and E°′ = −1.48 V (ΔEp = 95 mV), respectively.38 The latter are complicated by subsequent chemical reactions, as shown by the appearance of new oxidation processes during the back scan toward positive potentials, in the second cycle of the voltammetric experiment (red line). The reversible oxidation marked with an asterisk was attributed to 2a by a comparison with the electrochemical response of a sample of this compound in the same electrolyte (Figure S11).39 The IR spectroelectrochemical in situ experiment confirmed the electrochemical quasi-reversibility of the reduction process and major changes in the [1a]+ structure, in agreement with Scheme 3. When the potential of the working electrode was progressively decreased from 0.0 to −1.4 V, the νCO bands of [1a]+ (1988 and 1807 cm−1) were replaced by a single absorption at 1949 cm−1 because of the clean formation of 3; accordingly, the NC1C2 and aromatic CC bands (at 1635 and 1587 cm−1, respectively) were replaced by a NC1C2 C3 stretch at 1553 cm−1 (Figure 8). During the backward oxidation step, [1a]+ was partially recovered, and new bands characteristic of 2a (1919, 1626, and 1606 cm−1) clearly appeared (Figure S12). A further change in the IR pattern was observed when the working electrode potential was lowered to −1.9 V (Figure S13), indicating the probable formation of [3]−. New bands appeared at 1895, 1671, and 1605 cm−1 in correspondence with the second reduction step observed in CV (E°′ = −1.48 V; Figure 7) and were assigned respectively to the terminal carbonyl, acyl group, and aminoalkylidene moiety of [3]−. The DFT-calculated structure of [3]− is shown in Figure S14,

Figure 9. IR spectral changes of a THF solution of [1a]SO3CF3 recorded in an OTTLE cell during the progressive increase of the potential from 0.0 to +0.8 V. [NnBu4]PF6 (0.2 mol·dm−3) was used as the supporting electrolyte. The absorptions of the solvent and supporting electrolyte have been subtracted.

chemically reversible one-electron oxidation of [1a]+, the IR bands typical of [1a]+ were gradually replaced by higher frequency absorptions [2069 cm−1 (CO), 1989 cm−1 (CO), 1656 cm−1 (NC1C2), and 1587 cm−1 (aromatic CC)], attributed to the dicationic species [1a]2+. The new set of signals indicates a rearrangement of the carbonyl ligands, both occupying terminal sites in [1a]2+. This stereochemical modification renders the oxidation electrochemically quasireversible, in alignment with the peak-to-peak separation value G


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Inorganic Chemistry provided by the CV experiment (ΔEp = 90 mV). The starting compound [1a]+ was almost completely recovered in the backward potential scan from +0.8 to 0.0 V, suggesting a certain stability of [1a]2+ on the spectroelectrochemistry time scale. The structure of [1a]2+ was optimized by DFT calculations, showing a significant accordance with the electrochemical studies. In particular, the DFT-optimized geometry bears two terminal carbonyl ligands (Figure 10), and the computed IR

Scheme 4. Resonance Forms of the O-Derivatized Vinyliminium Compound 4

afforded the zwitterionic complex 5 in about 60% yield (Scheme 3). A similar result (40% yield) was achieved by adding S8 to a solution of freshly prepared 3. As for 4, the formation of 5 is accompanied by inversion of the stereochemical configuration at the iminium moiety (from E to Z). Some complexes analogous to 5 were previously obtained from the reactions of the parent diiron vinyliminium precursors with S8 and sodium hydride, with the latter supposed to behave as a deprotonating agent.18b Indeed, the syntheses of 4 and 5 are formally deprotonation reactions, proceeding via one-electron reduction.23 In order to deepen this point, we treated [Fe2Cp2(CO)(μ-CO){μ-η1:η3-C(SiMe3)CHCN(Me)(Xyl)}][SO3CF3] ([1c]SO3CF3) with CoCp2 in THF. Interestingly, this reaction afforded the aminocarbyne acetylide complex [Fe2Cp2(CCSiMe3)(CO)(μ-CO){μ-CNMe(Xyl)}] (6) as the main product (Scheme 5). Compound 6 is the

Figure 10. DFT-optimized geometry of [1a]2+. H atoms have been omitted for clarity. Selected distances (Å) and angles (deg): Fe1−Fe2 2.748, Fe1−C1 1.920, Fe1−C3 2.102, Fe2−C3 2.073, C1−N 1.304, C1−C2 1.424, C2−C3 1.417, Fe1−C4 2.501, F2−C4 1.795; N−C1− C2 133.04; C2−C3−C4 119.41; Fe2−C4−O1 168.03.

Scheme 5. Cobaltocene-Induced Deprotonation of the Vinyliminium Complex

wavenumbers are in excellent agreement with the experimental data.40 The movement of one carbonyl ligand from a bridging ([1a]+) to a terminal ([1a]2+) position allows one to minimize electrostatic repulsions, with the newly generated positive charge being localized at the Fe atom not directly connected to the iminium moiety (Scheme 3). Trapping of Chalcogens by Radical Species: Functionalization of the Vinyliminium Ligand. In an attempt to study the reversibility of the [1a]+ → 3 process using an oxidant different from I2 (see above), dry air was bubbled into a THF solution of freshly prepared 3 at −50 °C. The IR spectrum of the resulting mixture indicated the minor recovery of [1a]+ and, surprisingly, the generation of a new product. After workup, the diiron complex 4 was isolated in 52% yield (Scheme 3). Compound 4 was obtained in a lower yield when O2 was used in place of dry air. The product was purified by alumina chromatography and unambiguously characterized by MS, elemental analysis, and IR and NMR spectroscopy. The IR and NMR data of 4 closely resemble those of the X-raycharacterized compound (Z)-cis-[Fe2{μ-η1:η3-C(Me)C(O)CN(Me)(Xyl)}(μ-CO)(CO)(Cp)2], obtained in the past via a low-yield route, and suggest a hybrid bisalkylidene/ zwitterionic structure (Scheme 4).18b The formation of 4 seems to be the result of 3-to-1a reconversion, followed by oxygenation of the latter from O2 through C−H bond activation.41 The involvement of traces of H2O as an O source should be excluded because it has been demonstrated that the reaction of 3 with H2O has a different outcome (i.e., formation of 3-H-3; see above). The synthetic strategy, leading to functionalization of the vinyliminium ligand with an O atom, was extended to the synthesis of an analogous S derivative.42 Thus, the reaction of [1a]SO3CF3 with CoCp2 was carried out directly in the presence of elemental S and

deprotonated derivative of [1c]+ and can be straightforwardly obtained by the reaction of [1c]SO3CF3 with NaH.23 In conclusion, the one-electron-reduction mechanism might be involved in all of the previously reported NaH deprotonation reactions of diiron vinyliminium complexes.18b,23,43

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CONCLUSIONS We have described the redox chemistry of a cationic diiron complex based on the [Fe2Cp2(CO)2] frame, which is governed by the reactivity of the bridging vinyliminium ligand. In particular, one-electron reduction promotes a straightforward C−C coupling rearrangement retaining the C3 moiety, which is finally incorporated in a stable five-membered metallacycle not accessible from any iron precursor. Cleavage of the diiron core takes place with the clean release of the η5Cp ligand and iron as isolated units. These features do not find correspondence in the known reactivity of dinuclear transitionmetal complexes that, when involved in electron-transfer processes, may either be reluctant to metal−metal bond cleavage or undergo rupture with a distribution of ligands and electrons between the two separating metal fragments. It is remarkable that dissociation of a Cp ligand from low-valent H


Article

Inorganic Chemistry

The solvent was removed under vacuum; thus, CH2Cl2 (5 mL) and petroleum ether (30 mL) were added to the residue in the order given. The obtained dark-brown powder was isolated and dried under vacuum (Chart 1). Yield: 333 mg, 80%. Anal. Calcd for

late transition metals is a very rare reaction and has never been observed to date from iron complexes unless under drastic and complicated conditions. The reductive pathway presented herein proceeds through intermediate radical complexes that can be exploited for functionalization purposes. In other words, it is possible to regulate the destiny of the reactive radical iron species, with some of them being relatively long-lived, turning off the Fe/Cp elimination and obtaining instead diiron complexes with a derivatized C3 ligand.

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Chart 1. Structure of [1a]+

EXPERIMENTAL SECTION

Materials and Methods. All of the reactions were routinely carried out under a nitrogen atmosphere using standard Schlenk techniques. The reaction vessels were oven-dried at 140 °C prior to use, evacuated (10−2 mmHg), and then filled with nitrogen. Organic reactants (Sigma-Aldrich) and CoCp2 (Strem) were commercial products of the highest purity available. Compounds [Fe2Cp2(CO)3{μ-CN(Me)(Xyl)}][SO3CF3] (Xyl = 2,6-C6H3Me2)19 and [Fe 2 Cp 2 (CO)(μ-CO){μ-η 1 :η 3 -C(R)CHCN(Me)(Xyl)}][SO3CF3] [R = CH2OH (1b), SiMe3 (1c)]20b were prepared according to the literature. Solvents were distilled before use under nitrogen from the appropriate drying agents. Chromatography separations were carried out under nitrogen on columns of deactivated alumina (Sigma-Aldrich, 4% w/w H2O) or silica (TCI Europe). IR spectra of solid samples were recorded on a PerkinElmer Spectrum One Fourier transform infrared (FT-IR) spectrometer, equipped with a UATR sampling accessory. IR spectra of solutions were recorded on a PerkinElmer Spectrum 100 FT-IR spectrometer with a CaF2 liquid transmission cell (4000−1000 cm−1 range). NMR spectra were recorded at 298 K on a Bruker Avance II DRX400 instrument equipped with a BBFO broad-band probe. Chemical shifts (expressed in parts per million) are referenced to the residual solvent peaks (1H and 13C).44 Spectra were assigned with the assistance of 1 H−13C (gs-HSQC and gs-HMBC) correlation experiments.45 NMR signals due to a second isomeric form (where it has been possible to detect them) are italicized. Nuclear Overhauser effect (NOE) measurements were recorded using the DPFGSE-NOE sequence.21 Magnetometric experiments were carried out on a Quantum Design (Sacramento, CA) MPMS SQUID magnetometer operating in the 1.8−350 K temperature range with an applied field of up to 5.0 T, with the powder sample pressed into a pellet under an inert atmosphere. All data were corrected for the diamagnetic contribution of the silica matrix. EPR spectra were recorded at 298 K on a Varian (Palo Alto, CA) E112 spectrometer operating at X band, equipped with a Varian E257 temperature control unit and interfaced to an IPC 610/P566C industrial-grade Advantech computer, using an acquisition board46 and a software package specially designed for EPR experiments.47 Experimental EPR spectra were simulated by the WINSIM 32 program.48 Elemental analyses were performed on a Vario MICRO cube instrument (Elementar). GC−MS analyses were performed on a HP6890 instrument, interfaced with a MSD-HP5973 detector, and equipped with a Phenonex Zebron column. MS analysis was carried out in a flow-injection analyzer on a triple quadrupole PE Sciex API 365 instrument, equipped with a Turbospray source, and interfaced to an Agilent 1100 high-performance liquid chromatography (HPLC) system with a binary pump, high-pressure mixing, and autosampling. Synthesis of [Fe2Cp2(CO)(μ-CO){μ-η1:η3-C3(Et)C2HC1N(Me)(Xyl)}][SO3CF3] ([1a]SO3CF3). A solution of [Fe2Cp2(CO)2(μCO){μ-CNMe(Xyl)}]SO3CF3 (400 mg, 0.644 mmol) in MeCN (10 mL) was treated with Me3NO (58 mg, 0.772 mmol). The mixture was stirred for 30 min, during which time progressive darkening was observed. The volatiles were removed under vacuum, and then the residue was dissolved in CH2Cl2 (20 mL). An excess of 1-butyne was bubbled into the solution, which was stirred at ambient temperature for 48 h. The final solution was charged on an alumina column. CH2Cl2 and CH2Cl2/THF mixtures were used to elute impurities. A major brown band was collected by using neat MeCN as the eluent.

C27H28F3Fe2NO5S: C, 50.10; H, 4.36; N, 2.16; S, 4.95. Found: C, 50.17; H, 4.21; N, 2.00; S, 4.83. IR (CH2Cl2; ν/cm−1): 1999vs (CO), 1813s (CO), 1634m (NC1C2), 1585w (CC). IR (THF; ν/cm−1): 1988vs (CO), 1807s (CO), 1635m (NC1C2), 1587w (CC). 1H NMR (CDCl3): δ 7.48−7.35, 7.20−7.12, 6.96−6.94 (3 H, C6H3Me2), 5.41, 5.20, 4.72, 4.58 (s, 10 H, Cp), 4.27, 3.88 (m, 2 H, CH2CH3), 4.20, 3.73 (s, 3 H, NMe), 3.95 (s, 1 H, CβH), 2.53, 2.30, 2.02, 1.77 (s, 6 H, C6H3Me2), 1.52 (t, 3 H, CH2CH3). (E)-cis/(Z)-trans ratio ≅ 3. 13 C{1H} NMR (acetone-d6): δ 254.9 (μ-CO), 233.8 (C1), 219.8 (C3), 210.7 (CO), 145.2 (ipso-C6H3Me2), 132.1, 131.3, 129.5, 129.2 (C6H3Me2), 91.2, 87.9 (Cp), 50.7 (C2), 45.7 (NMe), 40.2 (CH2), 19.3 (CH2CH3), 17.3, 16.3 (C6H3Me2). Reactions of Vinyliminium Compounds with CoCp2: Synthesis of [FeCp(CO){C1N(Me)(Xyl)C2HC3(R)C(O)}] [R = Et (2a), CH2OH (2b)]. [FeCp(CO){C1N(Me)(Xyl)C2HC3(Et)C(O)}] (2a). A solution of [1a]SO3CF3 (160 mg, 0.247 mmol) in THF (15 mL) was treated with CoCp2 (68 mg, 0.36 mmol), and the mixture was stirred at ambient temperature for 18 h. The mixture was then filtered through a short alumina pad. The filtered solution was dried under vacuum; the residue was dissolved in CH2Cl2/diethyl ether (OEt2; 1:1, v/v) and charged on an alumina column. A brown band was collected by using neat CH2Cl2 as the eluent. The product was isolated as a brown solid upon removal of the solvent under vacuum. Yield: 72 mg, 77% (relative to Et). Crystals suitable for X-ray analysis were obtained by slow diffusion of hexane into a CH2Cl2 solution of 2a at −30 °C (Chart 2). Anal. Calcd for C21H23FeNO2: C, 66.86; H,

Chart 2. Structure of 2a

6.15; N, 3.71. Found: C, 66.97; H, 6.30; N, 3.62. IR (CH2Cl2; ν/ cm−1): 1915vs (CO), 1622s (COacyl), 1598m (C1N). IR (THF; ν/ cm−1): 1919vs (CO), 1626s (COacyl), 1606m (C1N). 1H NMR (CDCl3): δ 7.29−7.19 (m, 3 H, C6H3Me2), 6.49 (s, 1 H, C2H), 4.64, 3.99 (s, 5 H, Cp), 3.85, 3.68 (s, 3 H, NMe), 2.40, 2.37, 2.22, 2.12 (s, 6 H, C6H3Me2), 2.16 (q, 2 H, CH2), 1.13, 0.86 (t, 3 H, CH2CH3). E/Z ratio = 1.8. 13C{1H} NMR (CDCl3): δ 270.5 (CO), 266.4 (C1), 221.4 (CO), 179.2, 176.9 (C3), 146.6 (C2), 145.0 (ipso-C6H3Me2), 134.1, 133.3, 132.7, 132.1, 129.2, 128.9, 128.6, 128.0 (C6H3Me2), 85.2, 84.9 (Cp), 48.8, 44.3 (NMe), 20.8, 20.5 (CH2), 18.4, 17.7, 17.4 (C6H3Me2), 12.4, 12.2 (CH2CH3). I


Article

Inorganic Chemistry [FeCp(CO){C1N(Me)(Xyl)C2HC3(CH2OH)C(O)}] (2b). This compound was obtained by using a procedure analogous to that described for 2a, from [Fe2Cp2(CO)(μ-CO){μ-η1:η3-C3(CH2OH)C2HC1N(Me)(Xyl)}]SO3CF3 ([1b]SO3CF3; 230 mg, 0.354 mmol), and CoCp2 (135 mg, 0.712 mmol; Chart 3). Yield: 86 mg, 64% (relative

1941vs (CO), 1525w (NC1C2). IR (CH2Cl2; ν/cm−1): 1953vs (CO), 1622w (COacyl), 1530m (NC1C2). When a THF solution of 3-H-3 was stirred at ambient temperature, the complete disappearance of 3H-3 was detected by IR in 3−4 h, and 2a was the prevalent product (Chart 5).

Chart 3. Structure of 2b

Chart 5. Structure of 3-H-3

to CH2OH). Anal. Calcd for C20H21FeNO3: C, 63.34; H, 5.58; N, 3.69. Found: C, 63.03; H, 5.50; N, 3.78. IR (CH2Cl2; ν/cm−1): 1922vs (CO), 1602w-sh (C1N), 1579m (COacyl). IR (solid state; ν/ cm−1): 3403br (OH), 1913s (CO), 1603w-m, 1574m. 1H NMR (DMSO-d6): δ 7.27 (m, 3 H, C6H3Me2), 6.55 (s, 1 H, C2H), 4.82 (br, 1 H, OH), 4.63 (s, 5 H, Cp), 4.07, 3.92 (dd, 2JHH = 20 Hz, 2 H, CH2), 3.79 (s, 3 H, NMe), 2.18, 2.05 (s, 6 H, C6H3Me2). 13C{1H} NMR (DMSO-d6): δ 268.6 (COacyl), 262.5 (C1), 222.7 (CO), 176.8 (C3), 146.2 (C2), 145.5 (ipso-C6H3Me2), 132.7, 132.3, 129.5, 129.3, 129.0 (C6H3Me2), 85.3 (Cp), 58.3 (CH2), 49.5 (NMe), 17.7, 17.2 (C6H3Me2). Study of the Reaction of [1a]SO3CF3 with CoCp2. IR and EPR Detection of [FeCp(CO){C1N(Me)(Xyl)C2HC3(Et)CO}FeCp]* (3). A solution of [1a]SO3CF3 (120 mg, 0.185 mmol) in THF (12 mL), cooled to ca. −50 °C, was treated with CoCp2 (51 mg, 0.270 mmol). The mixture was stirred at ca. −50 °C. After 10 min, the IR spectrum of the red solution clearly evidenced the consumption of the starting material and the presence of two new bands at 1949 cm−1 (vs, CO) and 1554 cm−1 (m, NC1C2C3), attributed to 3 (Chart 4). An

NMR and GC−MS Identification of CpH. A solution of [1a]SO3CF3 (78 mg, 0.12 mmol) in THF-d8 (5 mL) was treated with CoCp2 (18 mg, 0.095 mmol), and then the mixture was stirred at ambient temperature for 1 week. NMR spectra were recorded on the final solution. 1H NMR for 2a (THF-d8): δ 7.27−7.21 (m, 3 H, C6H3Me2), 6.51 (s, 1 H, CβH), 4.61 (s, 5 H, Cp), 2.23, 2.10 (s, 6 H, C6H3Me2), 2.06 (q, 2 H, CH2), 0.80 (t, 3 H, CH2CH3). N−Me signal of 2a covered by a ondeuterated solvent peak. 1H NMR for CpH (THF-d8): δ 6.52, 6.44 (m, 4 H, CH), 2.95 (m, 2H, CH2). 2a/CpH ratio = 1.0. 13C{1H} NMR for CpH (THF-d8): δ 130.7, 130.0 (CH), 39.3 (CH2).31a The volatiles were stripped under vacuum and thus condensed into a Schlenk flask immersed in liquid nitrogen. Subsequent GC−MS (Figure S9) and NMR analyses of the liquid confirmed the presence of CpH. EPR and Magnetometric Measurements. Compound [1a]SO3CF3 (0.90 mmol), dissolved in a THF solution (5 mL), was allowed to react with NaH (1.35 mmol) overnight. The final mixture was filtered through a short silica pad (height ≅ 1.5 cm; diameter = 2 cm), and the filter was exhaustively washed with MeCN. The filtered solution was dried under vacuum, and then IR (in CH2Cl2) and 1H NMR (in CDCl3) analyses of the residue evidenced the presence of 2a as largely prevalent species. The orange silica material was dried under vacuum, stored in a sealed glass tube under nitrogen, and then analyzed by IR, EPR, and magnetometry. Reaction of [1a]SO3CF3 with CoCp2/Air: Synthesis of [Fe2{μη1:η3-C3(Et)C2(O)C1N(Me)(Xyl)}(μ-CO)(CO)(Cp)2] (4). A solution of 3 in THF was obtained at −50 °C from [1a]SO3CF3 (0.250 mmol), according to the procedure described above. Dry air was bubbled into the cooled THF solution for 1 min. The resulting solution was stirred for an additional 20 min, during which time it was allowed to warm to ambient temperature. The IR spectrum recorded on an aliquot of the resulting mixture indicated the presence of 4 in an admixture with a minor amount of [1a]SO3CF3 (Chart 6). The solution was charged on an alumina column. Elution with CH2Cl2/THF (3:1, v/v) allowed one to collect a green band corresponding to 4. The title product was isolated as an air-sensitive green solid upon removal of the solvent under vacuum. Yield: 68 mg, 52%. Anal. Calcd for C26H27Fe2NO3: C, 60.85; H, 5.30; N, 2.73; O, 9.35. Found: C, 60.68; H, 5.21; N, 2.78; O, 9.27. IR (CH2Cl2; ν/cm−1): 1946vs (CO), 1777s (CO), 1548w (C1N). 1H NMR (CDCl3): δ 7.34−7.24 (m, 3 H, C6H3Me2), 4.94,

Chart 4. Structure of 3

aliquot of the same solution was analyzed by EPR spectroscopy (Figures S4 and S5). Quantitative conversion of 3 to 2a was achieved after overnight stirring at ambient temperature. The addition of I2 (60 mg, 0.236 mmol) to the cooled solution of 3 resulted in clean regeneration of the cation [1a]+ after 10 min, according to IR spectroscopy. Isolation and Characterization of 3-H-3. A solution of compound 3 at −50 °C, obtained with the procedure described above from [1a]SO3CF3 (0.15 mmol), was added of an excess of H2O (ca. 2 mmol). The resulting mixture was stirred for an additional 20 min at −50 °C (an aliquot of the solution was analyzed by EPR spectroscopy; see Figure S6), and then it was quickly passed through a short silica pad. Removal of the solvent under vacuum afforded an air-sensitive, brown solid. Yield: 28%. Crystals suitable for X-ray analysis were obtained by slow diffusion of hexane into a CH2Cl2 solution at −30 °C. Anal. Calcd for C52H57Fe4N2O4: C, 62.62; H, 5.76; N, 2.81. Found: C, 61.70; H, 5.91; N, 2.60. IR (THF; ν/cm−1): J


Article

Inorganic Chemistry Chart 6. Structure of 4


4.28 (s, 10 H, Cp), 4.26, 3.66 (m, 2 H, CH2), 3.44 (s, 3 H, NMe), 2.36, 2.13 (s, 6 H, C6H3Me2), 1.83 (t, 3 H, CH2CH3). 13C{1H} NMR (CDCl3): δ 270.3 (μ-CO), 258.0 (C1), 214.1 (CO), 194.2 (C3), 153.8 (C2), 143.9 (ipso-C6H3Me2), 137.3, 134.5, 129.3, 128.5, 128.2 (C6H3Me2), 88.3, 86.7 (Cp), 46.3 (NMe), 42.9 (CH2), 18.1, 17.4 (C6H3Me2), 17.4 (CH2CH3). ESI-MS(+). Found: m/z 514 ([M + H]+), 486 ([M + H − CO]+), 458 ([M + H − 2CO]+). Reactions of [1a]SO3CF3 with CoCp2/S8: Synthesis of [Fe2{μη1:η3-C3(Et)C2(S)C1N(Me)(Xyl)}(μ-CO)(CO)(Cp)2] (5). S8 (160 mg, 0.624 mmol) and then CoCp2 (116 mg, 0.613 mmol) were added to a solution of [1a]SO3CF3 (220 mg, 0.340 mmol) in THF (20 mL) at ambient temperature. The resulting mixture was allowed to stir for 24 h. The final mixture was filtered through a short alumina pad using MeCN as the eluent. The filtered solution was dried under vacuum, then the residue was dissolved in CH2Cl2, and the new solution was charged on an alumina column. A green band corresponding to 5 was separated by using neat THF as the eluent (Chart 7). The product

5 °C). HPLC-grade THF (Sigma-Aldrich) was distilled from and stored under argon over 3-Å molecular sieves. Electrochemical-grade [NnBu4]PF6 and FeCp2 (Fluka) were used without further purification. CV was performed in a homemade three-electrode cell, having a platinum-disk working electrode and a platinum-spiral counter electrode, both sealed in a glass tube. A quasi-reference platinum was employed as the reference electrode. The cell was predried by heating under vacuum and filled with argon. The Schlenktype construction of the cell maintained anhydrous and anaerobic conditions. The solution of the supporting electrolyte, prepared under argon, was introduced into the cell, and the CV of the solvent was recorded. The analyte was then introduced, and the voltammograms were recorded; a small amount of ferrocene was added to the solution, and a further voltammogram was repeated. Under the present experimental conditions, the one-electron oxidation of ferrocene occurred at E° = +0.39 V versus saturated calomel electrode. IR spectroelectrochemical measurements were carried out using an OTTLE cell equipped with CaF2 windows, platinum mini-grid working and auxiliary electrodes, and a silver wire pseudoreference electrode.49 During the microelectrolysis procedures, the electrode potential was controlled by a PalmSens4 instrument interfaced to a computer employing PSTrace5 electrochemical software. Argonsaturated THF solutions of the compound under study, containing 0.2 M [NnBu4]PF6 as the supporting electrolyte, were used. The in situ spectroelectrochemical experiments were performed by collecting IR spectra at constant time intervals during the oxidation or reduction obtained by continuously increasing or lowering the initial working potential at a scan rate of 1.0 mV·s−1. IR spectra were recorded on a PerkinElmer FT-IR 1725X spectrophotometer and UV−vis spectra on a PerkinElmer Lambda EZ201 spectrophotometer. X-ray Crystallography. Crystal data and collection details for 2a and 3-H-3 are reported in Table S2. Data were recorded on a Bruker APEX II diffractometer equipped with a CCD detector using Mo Kα radiation. Data were corrected for Lorentz polarization and absorption effects (empirical absorption correction with SADABS).50 The structures were solved by direct methods and refined by fullmatrix least squares based on all data using F2.51 H atoms were fixed at calculated positions and refined by a riding model. The asymmetric unit of the unit cell of 3-H-3 contains one 3-H molecule, which forms a H bond with a symmetry-related (by an inversion center) molecule. The O-bonded H atom was located in the Fourier difference map and refined isotropically with a 0.5 occupancy factor, using the 1.5-fold Uiso value of the parent O atom with a restrained O−H distance. The O(1)···O(1)#1 distance is 2.688(7) Å (symmetry transformation used to generate equivalent atoms: #1, −x, −y, −z + 1). This indicates a H bond between these two symmetry-related molecules. Therefore, it must be assumed that the O-bonded H atom is disordered over two symmetry-related positions, and it has been refined with a 0.5 occupancy factor. Overall, 3-H and 3 have exactly the same structures because they are generated by an inversion center. Computational Studies. All of the geometries were optimized with ORCA 4.0.1.2,52 using the B97 functional53 in conjunction with a triple-ζ-quality basis set (def2-TZVP). The dispersion corrections were taken into account using the Grimme D3-parametrized correction and the Becke−Jonhson damping to the DFT energy.54 All of the structures were confirmed to be local energy minima (no


was isolated as an air-sensitive, green solid upon removal of the solvent under vacuum. Yield: 112 mg, 62%. Anal. Calcd for C26H27Fe2NO2S: C, 59.00; H, 5.14; N, 2.65; S, 6.06. Found: C, 59.10; H, 4.98; N, 2.67; S, 6.21. IR (CH2Cl2; ν/cm−1): 1961vs (CO), 1790s (CO), 1600w (C1N), 1582w (Xyl). 1H NMR (CDCl3): δ 7.38−7.23 (3 H, C6H3Me2), 5.03, 3.90 (m, 2 H, CH2), 5.01, 4.34 (s, 10 H, Cp), 3.61 (s, 3 H, NMe), 2.65, 2.12 (s, 6 H, C6H3Me2), 1.85 (t, 3 H, CH2CH3). 13C{1H} NMR (CDCl3): δ 264.6 (μ-CO), 233.9 (C1), 212.4 (CO), 208.3 (C3), 142.3 (ipso-C6H3Me2), 135.7, 134.8, 129.0, 128.9, 128.6 (C6H3Me2), 110.2 (C2), 89.9, 89.0 (Cp), 45.7 (NMe), 44.5 (CH2), 18.4, 18.0 (C6H3Me2), 16.4 (CH2CH3). Reaction of [1c]SO 3 CF 3 with CoCp 2 : Formation of [Fe2Cp2(C2C3SiMe3)(CO)(μ-CO){μ-C1NMe(Xyl)}] (6).23 A solution of [1c]SO3CF3 (120 mg, 0.174 mmol) in THF (12 mL) was cooled to −50 °C and then treated with CoCp2 (50 mg, 0.264 mmol). After 5 min, the IR spectrum of the solution evidenced the presence of several bands in the region of 1700−2050 cm−1. The mixture was allowed to warm to ambient temperature and stirred for an additional 18 h. The final solution was filtered through a Celite pad, and then the volatiles were removed under reduced pressure. The residue was washed with OEt2 (20 mL) and then dried under vacuum, thus affording 6 as an ochre-yellow solid in an admixture with minor side products (Chart 8. Yield: ca. 55%. IR (CH2Cl2; ν/cm−1): 2012 (C C), 1973vs (CO), 1792s (CO), 1506w (C1N). 1H NMR (CDCl3): δ 7.35−7.18 (3 H, C6H3Me2), 4.81, 4.30 (s, 10 H, Cp), 4.40 (s, 3 H, NMe), 2.65, 2.23 (s, 6 H, C6H3Me2), −0.18 (s, 9 H, SiMe3). Spectroelectrochemical Studies. Electrochemical measurements were recorded on a PalmSens4 instrument interfaced to a computer employing PSTrace5 electrochemical software and performed in THF solutions containing [NnBu4]PF6 (0.2 mol dm−3) as the supporting electrolyte at ambient temperature (20 ± K


Article

Inorganic Chemistry imaginary frequencies) for intermediate species and saddle points (one imaginary frequency) for transition states. In the case of [1a]2+, self-consistent-field convergence was not obtained; therefore, the geometry and vibrations were computed with the ADF package (version 2014.09),55 using the same functional, Slater-type triple-ζquality basis set (TZ2P), frozen-core approximation, and ZORA Hamiltonian to account for scalar relativistic effects (numerical integration grid = 6.0).56

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Organic Synthesis: A Critical Assessment of What It Takes To Make This Base Metal a Multitasking Champion. ACS Cent. Sci. 2016, 2, 778−789. (c) Topics in Organometallic Chemistry; Iron Catalysis II;Bauer, E., Ed.; Springer, 2016; Vol. 50. (d) Enthaler, S.; Junge, K.; Beller, M. Angew. Chem., Int. Ed. 2008, 47, 3317−3321. (e) Bisz, E.; Szostak, M. ChemSusChem 2017, 10, 3964−3981. (f) Bleith, T.; Wadepohl, H.; Gade, L. H. Iron Achieves Noble Metal Reactivity and Selectivity: Highly Reactive and Enantioselective Iron Complexes as Catalysts in the Hydrosilylation of Ketones. J. Am. Chem. Soc. 2015, 137, 2456−2459. (3) Selected references: (a) Jasniewski, A. J.; Que, L. Dioxygen Activation by Nonheme Diiron Enzymes: Diverse Dioxygen Adducts, High-Valent Intermediates, and Related Model Complexes. Chem. Rev. 2018, 118, 2554−2592. (b) Trehoux, A.; Mahy, J.-P.; Avenier, F. A growing family of O2 activating dinuclear iron enzymes with key catalytic diiron(III)-peroxo intermediates: Biological systems and chemical models. Coord. Chem. Rev. 2016, 322, 142−158. (c) Artero, V.; Berggren, G.; Atta, M.; Caserta, G.; Roy, S.; Pecqueur, L.; Fontecave, M. From Enzyme Maturation to Synthetic Chemistry: The Case of Hydrogenases. Acc. Chem. Res. 2015, 48, 2380−2387. (d) Darensbourg, M. Y.; Bethel, R. D. Nat. Chem. 2012, 4, 11−13. (e) Friedle, S.; Reisner, E.; Lippard, S. J. Current challenges of modeling diiron enzyme active sites for dioxygen activation by biomimetic synthetic complexes. Chem. Soc. Rev. 2010, 39, 2768− 2779. (4) (a) Kurtz, D. M.; Boice, E.; Caranto, J. D.; Frederick, R. E.; Masitas, C. A.; Miner, K. D. Iron: Non-Heme Proteins with DiironCarboxylate Active Sites. Encyclopedia of Inorganic and Bioinorganic Chemistry 2014, 1−18. (b) Wang, V. C.-C.; Maji, S.; Chen, P.-Y.; Lee, H. K.; Yu, S.-F.; Chan, S. I. Alkane Oxidation: Methane Monooxygenases, Related Enzymes, and Their Biomimetics. Chem. Rev. 2017, 117, 8574−8621. (c) Senger, M.; Laun, K.; Wittkamp, F.; Duan, J.; Haumann, M.; Happe, T.; Winkler, M.; Apfel, U.-P.; Stripp, S. T. Proton-Coupled Reduction of the Catalytic [4Fe−4S] Cluster in [FeFe]-Hydrogenases. Angew. Chem., Int. Ed. 2017, 56, 16503− 16506. (d) May, B.; Young, L.; Moore, A. L. Structural insights into the alternative oxidases: are all oxidases made equal? Biochem. Soc. Trans. 2017, 45, 731−740. (5) For instance, see: (a) Labinger, J. A. Does cyclopentadienyl iron dicarbonyl dimer have a metal−metal bond? Who’s asking? Inorg. Chim. Acta 2015, 424, 14−19. (b) Jung, T. C.; Argouarch, G.; van de Weghe, P. Cyclopentadienyliron dicarbonyl dimer: A simple tool for the hydrosilylation of aldehydes and ketones under air. Catal. Commun. 2016, 78, 52−54. (c) Bitterwolf, T. E. Photochemistry and reaction intermediates of the bimetallic Group VIII cyclopentadienyl metal carbonyl compounds, (η5-C5H5)2M2(CO)4 and their derivatives. Coord. Chem. Rev. 2000, 206−207, 419−450. (6) Selected references: (a) He, J.; Deng, C.-L.; Li, Y.; Li, Y.-L.; Wu, Y.; Zou, L.-K.; Mu, C.; Luo, Q.; Xie, B.; Wei, J.; Hu, J.-W.; Zhao, P.H.; Zheng, W. A New Route to the Synthesis of PhosphineSubstituted Diiron Aza- and Oxadithiolate Complexes. Organometallics 2017, 36, 1322−1330. (b) Tong, P.; Yang, D.; Li, Y.; Wang, B.; Qu, J. Hydration of Nitriles to Amides by Thiolate-Bridged Diiron Complexes. Organometallics 2015, 34, 3571−3576. (c) Chen, Y.; Liu, L.; Peng, Y.; Chen, P.; Luo, Y.; Qu, J. Unusual ThiolateBridged Diiron Clusters Bearing the cis-HNNH Ligand and Their Reactivities with Terminal Alkynes. J. Am. Chem. Soc. 2011, 133, 1147−1149. (d) Busetto, L.; Maitlis, P. M.; Zanotti, V. Bridging vinylalkylidene transition metal complexes. Coord. Chem. Rev. 2010, 254, 470−486. (e) Marchetti, F.; Zacchini, S.; Zanotti, V. Carbon monoxide−isocyanide coupling promoted by acetylide addition to a diiron complex. Chem. Commun. 2015, 51, 8101−8104. (f) Doherty, S.; Elsegood, M. R. J.; Clegg, W.; Ward, M. F.; Waugh, M. New Reaction Pathways for μ-η1,η2-Allenyl Ligands: On−Off Allenyl Coordination and CO Insertion into the Hydrocarbyl Bridge in Ru2(CO)6(μ-PPh2){μ-η1,η2α,β-C(Ph)CCPh2}. Organometallics 1997, 16, 4251−4253. (g) Casey, C. P.; Crocker, M.; Vosejpka, P. C.; Rheingold, A. L. Reactions of organocopper reagents with the cationic bridging acylium complex [C5H5(CO)Fe]2(.mu.-CO)(.mu.-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02445. DFT-optimized geometries, details of DFT calculations, EPR, GC−MS, and NMR spectra, and spectroelectrochemical graphs (PDF) Crystallographic data (XYZ) Accession Codes

CCDC 1855386−1855387 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]. Webpage: http://www. dcci.unipi.it/fabio-marchetti.html. ORCID

Gianluca Ciancaleoni: 0000-0001-5113-2351 Stefano Zacchini: 0000-0003-0739-0518 Francesco Pineider: 0000-0003-4066-4031 Guido Pampaloni: 0000-0002-6375-4411 Valerio Zanotti: 0000-0003-4190-7218 Fabio Marchetti: 0000-0002-3683-8708 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Universities of Bologna and Pisa for financial support. REFERENCES

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