Electrochemical and Theoretical Studies of the Impact of the Chelating

Article pubs.acs.org/Organometallics

Electrochemical and Theoretical Studies of the Impact of the Chelating Ligand on the Reactivity of [Fe2(CO)4(κ2-LL)(μ-pdt)]+ Complexes with Different Substrates (LL = IMe-CH2-IMe, dppe; IMe = 1Methylimidazol-2-ylidene) Dounia Chouffai,† Giuseppe Zampella,*,‡ Jean-François Capon,*,† Luca De Gioia,*,‡ Alan Le Goff,†,§ François Y. Pétillon,† Philippe Schollhammer,† and Jean Talarmin*,† †

UMR CNRS 6521 “Chimie, Electrochimie Moléculaires et Chimie Analytique”, Université de Bretagne Occidentale, UFR Sciences et Techniques, CS 93837, 29238 Brest-Cedex 3, France ‡ Department of Biotechnology and Bioscience, University of Milano-Bicocca, Piazza della Scienza 1, 20126 Milan, Italy S Supporting Information *

ABSTRACT: The reactivity of [Fe2(CO)4(κ2-LL)(μ-pdt)]+ complexes (pdt = S(CH2)3S, propanedithiolate) with different substrates L′ (L′ = CO, MeCN, P(OMe)3) was investigated electrochemically in order to assess the influence of the chelating ligand κ2-LL (LL = IMe-CH2-IMe (1+), dppe (2+); IMe = 1-methylimidazol-2-ylidene). This latter ligand is effectively shown to affect the reactivity of the cations in different ways: when L′ = CO, the adduct [Fe2(CO)4(μ-CO)(κ2-dppe)(μ-pdt)]+ (2-CO+) was clearly observed by cyclic voltammetry, whereas [Fe2(CO)4(μ-CO)(κ2-IMe-CH2-IMe)(μ-pdt)]+ (1-CO+) was not detected, although DFT calculations show that the energies of the products and the activation barriers to their formation are similar. When L′ = MeCN, the adducts X-MeCN+ with X = 1, 2 are both observed, but the formation is easier when LL = dppe. Finally, the reaction of [Fe2(CO)4(κ2-IMe-CH2-IMe)(μ-pdt)]+ (1+) with P(OMe)3 produces the disubstituted dication [Fe2(CO)2{P(OMe)3}2(κ2-IMe-CH2-IMe)(μ-CO)(μ-pdt)]2+ (42+) via a disproportionation reaction, while previous studies demonstrated that monocationic derivatives were obtained when LL = dppe. Complex 4[PF6]2 was fully characterized, and its X-ray crystal structure confirms the presence of a carbonyl ligand in a bridging position, which did not exist in the related P(OMe)3-substituted κ2-dppe cations.

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that convert to the more stable bridging isomers.5−7 In contrast, protonation at the Fe−Fe bond of the biological site is prevented by the presence of the bridging or semibridging CO, while protonation at the vacant apical position (Scheme 1) provides a terminal hydride. Previous studies revealed that terminal hydrides reduce at a less negative potential, and are more reactive toward protons, than their bridging counterpart.7,8 Consequently, thermodynamic and kinetic advantages would be associated with the occurrence of the former rather than the latter in proton reduction processes. The interest in diiron dithiolate models showing an inverted geometry analogous to that found at the active site of the [FeFe]H2ases is thus clearly apparent.2 Pioneering work of Pickett, Darensbourg, and Rauchfuss and their co-workers showed that one-electron oxidation of

INTRODUCTION Diiron dithiolate complexes derived from [Fe2(CO)6(μ-SR)2] have been thoroughly investigated during the past decade due to their resemblance to the active site of the [FeFe]hydrogenases. Most of the synthetic mimics of the biological site are able to electrocatalyze the H+ → H2 reduction, although much less efficiently than the enzyme in both thermodynamic and kinetic terms.1,2 In the meantime, a major difference between the natural and synthetic 2Fe2S cores is the arrangement of the ligand about the metal centers (Scheme 1).3,4 None of the FeIFeI synthetic models known to date shows the inverted geometry found at the enzyme’s active site, which is likely to affect the efficiency of the proton reduction process catalyzed by the model compounds. Although terminal hydrides were detected upon protonation of [Fe2(CO)4(κ2LL)(μ-pdt)] (LL = chelating ligand), which suggested the occurrence of an accessible rotated state in dissymmetrically disubstituted compounds, these are short-lived intermediates © 2012 American Chemical Society

Received: November 16, 2011 Published: February 1, 2012 1082

dx.doi.org/10.1021/om201143p | Organometallics 2012, 31, 1082−1091

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Scheme 1. Schematic Representation of the Active Site of the [FeFe] Hydrogenases3,4 (Left) and of a Model Complex with Unspecified Terminal Ligands (Right)

[Fe2(CO)6−n(L)n(μ-dithiolate)] complexes with the usual eclipsed pyramidal arrangement of the ligands (Scheme 1) induced an isomerization to the rotated geometry of the cations that was also investigated by theoretical methods.9−11 Recently, we investigated electrochemically and theoretically the oxidatively induced geometry change in [Fe2(CO)4(κ2dppe)(μ-pdt)] (dppe = Ph2PCH2CH2PPh2) as well as the reactivity of the electrogenerated vacant site with trimethyl phosphite. Although [Fe2(CO)4(κ2-dppe)(μ-pdt)]+ could not be isolated, DFT calculations showed that the most stable cation is rotated at the disubstituted iron center. Nevertheless, the isomer with the rotated Fe(CO)3 center is also easily accessible and the reaction of P(OMe)3 with the cation was shown to occur at this metal center.12 In the present study, we have investigated the reactivity of two [Fe2(CO)4(κ2-LL)(μ-pdt)]+ derivatives (LL = IMe-CH2-IMe (1+), dppe (2+); IMe = 1-methylimidazol-2-ylidene) with different substrates in order to assess whether the chelating ligand affects the formation of a rotated cation and to what extent it influences the reactivity at the neighboring metal center.

Figure 1. Cyclic voltammetry of [Fe2(CO)4(κ2-IMe-CH2-IMe)(μ-pdt)] (1; 1.78 mM) in CH2Cl2−[NBu4][PF6] (vitreous carbon electrode; v = 0.2 V s−1; potentials in V vs Fc+/Fc). The red trace shows that the second oxidation and the reduction at −0.15 V constitute a chemically reversible system.

significant chemical reversibility of the overall process (oxidation at 0.42 V and reduction at −0.15 V; see discussion in the Supporting Information, Digital CV simulations section). This is quite similar to the second (irreversible) oxidation of 2, which affords an oxidized species characterized by an irreversible reduction around −0.05 V. Several experimental and theoretical studies have previously established that the one-electron oxidation of diiron dithiolate analogues of 1 and 2 induces a geometry change that produces a cation with an inverted pyramid about one Fe center, the socalled rotated state.9−11 The structure change may follow the electron transfer (EC mechanism) or be concerted with it (quasi-reversible process, QR).15−18 The experimental study of the first oxidation of 1 did not allow us to discriminate between the above possibilities, since the increase of the scan rate (v ≤ 20 V s−1) only results in an increase of the peak separation ΔEp, as already noted for 2.12 Digital CV simulations19 of the oxidation of 1 according to either an EC or a QR mechanism suggest that both are consistent with the experimental data (Supporting Information, Figure S1).20 DFT calculations favor an EC mechanism for the one-electron oxidation of 1, since a 1+ structure characterized by an eclipsed geometry resembling the neutral parent 1 does correspond to an energy minimum. However, the eclipsed 1+ isomer would rapidly rearrange to thermodynamically more stable isomers characterized by a rotated geometry (Supporting Information, Scheme S1). Controlled-potential electrolysis performed at the potential of the first oxidation at −10 °C produced the expected cation after the transfer of ca. 1 F mol−1 1. Infrared monitoring of the chemical oxidation of 1 at −10 °C in CH2Cl2 showed the formation of 1+ (ν(CO): 2059 (s), 1997 (s) cm−1) upon addition of 1 equiv of ferrocenium hexafluorophosphate. Because of the thermal sensitivity of the cation, neither single crystals nor a satisfactory elemental analysis has been obtained thus far. Other NHC-substituted complexes (NHC = N-heterocyclic carbene), namely [{Fe(CO)2PMe3}{Fe(CO)2(NHC)}(μpdt)]0/+, have been reported recently.10 Despite the fact that their reduction potentials range from −0.24 to −0.47 V, the cations all have their highest ν(CO) band, assigned to the νsym stretching mode of the carbonyls on the {Fe(CO)2PMe3}

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RESULTS AND DISCUSSION 1. Electrochemical Oxidation of [Fe2(CO)4(κ2-IMe-CH2IMe)(μ-pdt)] (1) in CH2Cl2. The CV of 1 in CH2Cl2− [NBu4][PF6] shows that the complex undergoes two oxidation steps (Figure 1). From the usual electrochemical criteria (ΔEp, ipc/ipa),13,14 the first oxidation is a quasi-reversible one-electron process, with E1/2 = −0.44 ± 0.01 V. The negative shift of the oxidation of 1 with respect to the first, quasi-reversible, oneelectron oxidation of [Fe2(CO)4(κ2-dppe)(μ-pdt)] (2; E1/2 = −0.24 ± 0.01 V) indicates that the bis-carbene ligand is a better electron donor than dppe. This is in agreement with the shift to lower wavenumbers of the ν(CO) bands of 1 (ν(CO) in CH3CN: 1996, 1920, 1872 cm−1)5b compared to those in 2 (ν(CO) in CH2Cl2: 2019, 1949, 1904 cm−1).5a The second oxidation of 1 (Ep = 0.42 V), irreversible for scan rates up to 20 V s−1, is a one-electron step (by comparison of the second oxidation peak current, ipox2, to that of the first oxidation, i p ox1 ) corresponding to a Fe IFe II → Fe IIFe II transition. The second oxidation, which generates a product detected by an irreversible reduction at −0.15 V (Figure 1), is thus followed by a fast chemical reaction (EC process).13,14 The fact that the product of the EC process reduces around −0.15 V suggests that a substantial modification of the FeIIFeII species took place in the chemical step. Nevertheless, the reduction at −0.15 V regenerates the initial FeIFeII cation, as shown by the 1083


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Figure 2. Cyclic voltammetry of [Fe2(CO)4(κ2-dppe)(μ-pdt)] (2; 1.13 mM) in CH2Cl2−[NBu4][PF6] under dinitrogen and under CO (vitreous carbon electrode; potentials in V vs Fc+/Fc). For the peak marked with an arrow, see text.

Figure 3. Cyclic voltammetry of [Fe2(CO)4(κ2-dppe)(μ-pdt)] (2) (1.5 mM) in MeCN−[NBu4][PF6] under argon (black line) and under CO (red line) (vitreous carbon electrode; potentials in V vs Fc+/Fc).

center, at 2036 cm−1.10c By analogy, the highest ν(CO) band of 1+ is tentatively assigned to the νsym stretching mode of the Fe(CO)3 carbonyls. The fact that this band is observed here at 2059 cm−1 is consistent with a weaker back-donation from the Fe(CO)3 center than from the {Fe(CO)2PMe3} one. In contrast with what was observed for the [{Fe(CO)2PMe3}{Fe(CO)2(NHC)}(μ-pdt)]0/+ compounds,10c the shift of the redox potentials is paralleled by a shift of the ν(CO) bands in the case of dissymmetrically disubstituted complexes (1+, E1/2 = −0.44 V, ν(CO) 2059, 1997 cm−1; 2+, E1/2 = −0.24 V, ν(CO) 2079, 2021 cm−1). On this basis, it appears that the change of the chelating ligand affects the electronic properties at the neighboring, unsubstituted metal center. 2.1. Comparison of the Reactivities of the [Fe2(CO)4(κ2-LL)(μ-pdt)]+ (1+, 2+) Complexes with CO and MeCN: Influence of the Chelating Ligand (LL). Reactivity of 1+ and 2+ toward CO. When CO is present, the cyclic voltammetry of 2 in CH2Cl2−[NBu4][PF6] shows a new quasi-reversible oxidation at E1/2 = 0.10 V in addition to that of the parent complex (Figure 2). The scan rate dependence of the ratio of the second to the first oxidation peak currents demonstrates that the oxidation of 2 in the presence of CO occurs according to an EC mechanism,13,14 which proves the binding of CO to the electrogenerated cation. The supple-

mentary system is thus attributed to the oxidation of the CO adduct, [Fe2(CO)5(κ2-dppe)(μ-pdt)]+ (2-CO+). The 2-CO+/2+ redox system is not fully reversible, and a supplementary reduction peak (see arrows in Figure 2) is present on the return scan, in particular at slow scan rates. The fact that the potential of this peak is the same as that of a reduction observed in the absence of CO after the second oxidation of 2 has been traversed (Supporting Information, Figure S2) suggests that 2CO2+ undergoes the loss of a CO ligand to produce the dication also formed at the second (irreversible) oxidation of 2. Digital CV simulations19 are consistent with these reactions (see the Supporting Information, Table S2 and Figure S3). In contrast, the cyclic voltammetry of 1 is exactly the same under CO and under N2 at scan rates from 0.05 to 1 V s−1 in CH2Cl2−[NBu4][PF6] (Supporting Information, Figure S4). In the absence of a supplementary redox system that would have evidenced the formation of an adduct, no reaction of 1+ with CO could thus be detected by CV. DFT dissection of the reaction pathway X+ + CO → X-CO+ (X = 1, 2) resulted in reaction energies and barriers differing by less than 2 kcal/mol (Supporting Information, Schemes S2− S4) and, therefore, did not permit us to reach decisive conclusions to rationalize the different reactivities of 1+ and 2+ with CO. 1084


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Scheme 2. Simplified Oxidation Mechanism of [Fe2(CO)4(κ2-LL)(μ-pdt)] in the Presence of L′ = CO, MeCNa

a

1-L′+/2+ is observed only for L′ = MeCN.

2.2. Reactivity of 1+ and 2+ with MeCN. The CV of 2 in MeCN is substantially different from that recorded in CH2Cl2. The oxidation, which is partially reversible at fast scan rates (Figure 3), involves the transfer of two electrons in MeCN− [NBu4][PF6]. This is demonstrated by the comparison of the oxidation peak current of 2 with that of [Fe2(CO)4(κ2dppe)(μ-pdt)(μ-H)]+, which was shown to undergo a diffusion-controlled one-electron oxidation in MeCN−[NBu4][PF6]21 (Supporting Information, Figure S5). The fact that 2 undergoes a quasi-reversible one-electron oxidation in CH2Cl2 and a two-electron-oxidation process in MeCN strongly suggests that MeCN binding is involved, as previously reported for other diiron complexes.10c,11a,c The two-electron oxidation of 2 in MeCN is consistent with the coordination of an acetonitrile molecule at the FeIFeII level, followed by the oxidation of the resulting MeCN adduct, 2MeCN+, at a potential more negative than (or equal to) the oxidation of 2 (ECE process,13,14 Scheme 2, E1 − E2(L′) ≥ 0). The effect of CO on the oxidation of 2 in MeCN is also shown in Figure 3. The enhanced chemical reversibility of the oxidation ((ipc/ipa)ox 22 increases from ca. 0.3 to 0.5 (v = 0.5 V s−1) and from 0.6 to 0.9 (v = 5 V s−1) upon switching the atmosphere from Ar to CO) and the suppression of the small oxidation around −0.03 V under CO indicate that a reversible CO loss from the dicationic MeCN species is also occurring. In order to inspect further the reaction of 2+ with MeCN, we investigated the CV of 2 in CH2Cl2−[NBu4][PF6] with added acetonitrile. The formation of a MeCN adduct is confirmed by the presence of a new couple whose current increases and whose peak shifts negatively with successive additions of MeCN (Figure 4A). The fact that a single two-electron transfer is observed in neat MeCN is in accord with the negative shift of the potential (see E2(L′) in Scheme 2) in CH2Cl2 upon increasing the concentration of MeCN. The different solvent dependences of the potentials of the dication/cation couple may also contribute to the occurrence of two separate oxidation steps in dichloromethane.15,16,23−25 In contrast to that of 2, the oxidation of 1 remains a oneelectron step, with E1/2ox1 = −0.38 V, in MeCN−[NBu4][PF6]. However, a second, quasi-reversible oxidation, assigned to 1MeCN+, is observed at E1/2ox2 = −0.15 V in this solvent. The formation of 1-MeCN+ is confirmed by the CV in CH2Cl2− [NBu4][PF6] with added MeCN. The MeCN adduct gives rise

Figure 4. Cyclic voltammetry of (A) [Fe2(CO)4(κ2-dppe)(μ-pdt)] (2; 1.5 mM) and (B) [Fe2(CO)4(κ2-IMe-CH2-IMe)(μ-pdt)] (1; 1.7 mM) in CH2Cl2−[NBu4][PF6] in the presence of MeCN (vitreous carbon electrode, v = 0.2 V s−1, potentials in V vs Fc+/Fc).

to a supplementary, quasi-reversible system whose oxidation current and potential are dependent on the amount of MeCN present (Figure 4B). Although this is qualitatively similar to what is observed for the κ2-dppe analogue, the concentration of MeCN required to produce a detectable amount of X-MeCN+ is much larger for X = 1 than for X = 2 (Figure 4). The enhanced reactivity of the cations 1+ and 2+ with substrates (CO, MeCN) compared to that of the neutral precursors is consistent with an oxidatively induced structure change that leads to the exposure of a vacant coordination site at a rotated metal center (Scheme 2). Digital simulations19 of the CVs of 1 and 2 in CH2Cl2 in the presence of MeCN are presented as Supporting Information. Although the set of constants that give rise to a reasonable agreement between the simulated and experimental CVs is probably not unique, it suggests that both the equilibrium (Keq = [X-MeCN+]/ [X+][MeCN]) and rate constants for MeCN binding are smaller for 1+ than for 2+ (Supporting Information, Tables S3 and S4). Another noticeable difference between the (κ2-dppe) and 2 (κ -IMe-CH2-IMe) derivatives is the fact that the oxidation of 2 changes from a one- to a two-electron (ECE) process on going 1085


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from CH2Cl2 to MeCN while the oxidation of 1 is a oneelectron step in both solvents. Thus, the separation of the redox potentials of the neutral 1 and 2 species (ΔE1/2ox1 = 0.2 V) and of their cations (ΔEpox2 ≈ 0.28 V) in CH2Cl2 is not conserved for the MeCN adducts, since ΔE1/2 for X-MeCN+/2+ is about 70 mV according to the CV simulations (Supporting Information, Tables S3 and S4; X = 1, 2, respectively). Therefore, the electronic impact of the substitution of IMe-CH2IMe for dppe on the redox potentials is lessened when MeCN is bound at the neighboring iron center of the cation. The comparative analysis of frontier molecular orbitals computed for X and X-MeCN+ (X = 1, 2) species can contribute to explaining why the separation of the redox potentials of the neutral species 1 and 2 is not conserved in the MeCN adducts. In the neutral species 1 and 2 the HOMO is mainly localized on the iron atom proximal to the chelating LL ligand, as well as on the chelating LL ligand itself: i.e., on the portion of the molecule which differs in 1 and 2 (Figure 5). Instead, in 1-

On a longer time scale, the increase of the oxidation peak current of 1 when P(OMe)3 is present (Figure 6, v = 0.05 V s−1, and Supporting Information, Figure S8) indicates that a supplementary electron transfer occurs in a second stage. The final product shows an irreversible reduction at −0.75 V (v = 0.05 V s−1). These results thus suggest that the product of the EC process (1-P+) is involved in a slow reaction that produces a species that is easier to oxidize than 1 or disproportionates. Similar results are observed in MeCN. Addition of 1 equiv of P(OMe)3 to 1+ produced either by electrochemical means or by chemical oxidation (1 equiv of FcPF6) in CH2Cl2 at −10 °C provided evidence for a disproportionation, since 1, characterized by its infrared spectrum and by its redox potentials, was recovered in about 50% yield after reaction (Supporting Information, Figure S9). The second product was identified as [Fe 2 (CO) 2 {P(OMe)3}2(κ2-IMe-CH2-IMe)(μ-CO)(μ-pdt)]2+ (42+), by comparison of its redox potentials and infrared spectrum with those of an authentic sample of this dication (see section 3.2). The formation of the neutral, unsubstituted parent and of a bis-P(OMe)3 dication indicates that ligand loss and ligand binding steps accompany the disproportionation of 1-P(OMe)3+ (Scheme 3). Controlled-potential oxidation of 1 in the presence of P(OMe)3 (3 equiv) performed at room temperature yielded a mixture of products, while experiments carried out at −10 °C produced only 42+ (Figure 7, blue trace) after the passage of 1.7 F mol−1 1. Similarly, complex 42+ was synthesized by chemical oxidation of 1 by ferrocenium (2 equiv) in the presence of P(OMe)3 (3 equiv) at −10 °C. Infrared monitoring of the reaction performed at room temperature demonstrated that unidentified side products were present, even though the major product was still 42+. The oxidation of complexes 1 and 2 in the presence of P(OMe)3 appear to follow different courses, since it respectively produces dicationic and cationic compounds. Indeed, controlled-potential electrolysis of 2 in the presence of P(OMe)3 (3 equiv) was previously shown to afford [Fe2(CO)2{P(OMe)3}2(κ2-dppe)(μ-pdt)]+.12 However, the experiments were run under different conditions, since the reactions were carried out at room temperature in the case of 2. In order to assess to what extent the chelating ligand (dppe or IMe-CH2-IMe) affects the course of the reaction, the electrochemical oxidation of 2 plus P(OMe)3 (3 equiv) was performed at −10 °C. The cyclic voltammetry of the electrolyzed solution (Figure 8, red trace) shows that two different products are formed under these conditions. One of them is [Fe2(CO)2{P(OMe)3}2(κ2-dppe)(μ-pdt)]+, as evidenced by comparison of the redox potentials of the product with those of an authentic sample of the disubstituted dppe neutral complex (Figure 8, trace b).26 Although the other product reduces at the same potential as [Fe2(CO)3{P(OMe)3}(κ2-dppe)(μ-pdt)]+,12 it is a different species, since it does not show the oxidation of the latter (Figure 8, trace c). The second product formed by electrolysis at low temperature is tentatively assigned as [Fe2(CO)2{P(OMe)3}2(κ2-dppe)(μ-CO)(μ-pdt)]2+, which is the dppe analogue of 42+. A tentative explanation of the different reactivities of the cationic species 1+ and 2+ with P(OMe)3 is that the intermediate X-P(OMe)3+ is longer lived and can disproportionate when X = 1 (Scheme 3). When X = 2, it is possible that a carbonyl is lost too rapidly for 2-P(OMe)3+ to disproportionate significantly.

Figure 5. Highest occupied molecular orbitals computed for 1, 2, 1MeCN+, and 2-MeCN+. Orbital phases are depicted in red and blue. Structures correspond to the lowest energy isomers.

MeCN+ and 2-MeCN+ the HOMOs are extremely similar and, most importantly, mainly localized on the two iron atoms and the MeCN ligand, while the atoms of the LL ligands do not contribute significantly. 3.1. Reaction of 1+ with P(OMe)3. Oxidation of 1 in the Presence of P(OMe)3. Cyclic voltammetry shows that the reaction of 1+ with P(OMe)3 proceeds in two stages in CH2Cl2−[NBu4][PF6]. First, the fact that the addition of P(OMe)3 does not affect the peak current on a short time scale (v = 1 V s−1) but decreases the reversibility of the oxidation of 1 ((ipc/ipa)ox1 < 1) is typical of an EC process whose product is characterized by a reduction at E1/2red = −0.67 V (Figure 6, v = 1 V s−1; the reversibility of the reduction at −0.67 V is not shown in the figure). The first steps of the process thus produce a cationic P(OMe)3 derivative (1-P+) that may either be the 1-P(OMe) 3 + adduct or the substituted cation [Fe2(CO)3{P(OMe)3}(κ2-IMe-CH2-IMe)(μ-pdt)]+, analogous to that formed in the reaction of 2+ with P(OMe)3.12 1086


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Figure 6. Cyclic voltammetry of [Fe2(CO)4(κ2-IMe-CH2-IMe)(μ-pdt)] (1; 1.8 mM) in CH2Cl2−[NBu4][PF6] in the absence and in the presence of P(OMe)3 (vitreous carbon electrode; v = 0.2 V s−1; potentials in V vs Fc+/Fc).

Scheme 3.

a

a

LL = IMe-CH2-IMe.

The IR spectrum showed two strong ν(CO) bands at 2036 and 2021 cm−1, accompanied by a weak and broad band at 1912 cm−1 (see the Supporting Information, Figure S10), the latter being assigned to a bridging CO. NMR data clearly indicated that the reaction produced only one of the possible isomers of the dication. The 31P NMR spectrum featured two doublets at 140.5 and 156.3 ppm with a coupling constant 2JPP of 129 Hz, due to the P(OMe)3 ligands (see the Supporting Information, Figure S11). The 13C NMR spectrum showed in the low-field region a doublet of doublets at 227.8 ppm (2JPC = 19 and 9 Hz) attributable to a bridging carbonyl and a singlet at 205.9 ppm and a doublet of doublets at 206.3 ppm (2JPC = 25 and 46 Hz) due to two terminal COs. These attributions are consistent with the 13C NMR data reported for the diferrous dithiolate complex [Fe2(CO)(PMe 3)4(MeCN)(μ-CO)(μS2C2H4)][PF6]2, which displayed a low-field multiplet at 223.6 ppm assigned to a bridging CO and another multiplet at 212.3 ppm due to the terminal carbonyl.27 Two characteristic carbene signals were observed at 171.9 and 177.3 ppm. The structure of 4[PF6]2 in the solid state, determined by Xray diffraction (Figure 9), confirms the presence of two phosphite ligands bound to the same iron atom. The carbonyl C3O3 is semibridging,28 with a Fe2−C3 distance of 2.21 Å vs an Fe1−C3 distance of 1.87 Å, as observed in other nonsymmetric diferrous compounds.27,29 The shorter Fe−μCO bond is trans to a NHC ligand, which is a better σ-donor than a P(OMe)3 group. Both of these ligands are in a basal− apical position, while the terminal carbonyl ligands adopt a basal−basal (trans) configuration.

Figure 7. Cyclic voltammetry of [Fe2(CO)4(κ2-IMe-CH2-IMe)(μ-pdt)], (1; ca. 1.5 mM) in CH2Cl2−[NBu4][PF6] at −10 °C before (black line) and after (red line) the addition of 3 equiv of P(OMe)3, and (blue line) after controlled-potential electrolysis (Eel = −0.4 V; graphite anode; n = 1.7 F mol−1 1) (vitreous carbon electrode; v = 0.2 V s−1; potentials in V vs Fc+/Fc).

3.2. Synthesis and Characterization of [Fe2(CO)2{P(OMe)3}2(κ2-IMe-CH2-IMe)(μ-CO)(μ-pdt)][PF6]2 (4[PF6]2). The reaction of [Fe2(CO)4(κ2-IMe-CH2-IMe)(μ-pdt)] (1) with 2 equiv of FcPF6 in the presence of an excess of P(OMe)3 gave a good yield (76%) of [Fe2(CO)2{P(OMe)3}2(κ2-IMe-CH2-IMe)(μ-CO)(μ-pdt)][PF6]2 (4[PF6]2); 4[PF6]2 was characterized by elemental analysis, IR and NMR spectroscopy, and X-ray crystallography. 1087


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Figure 8. Cyclic voltammetry of [Fe2(CO)4(κ2-dppe)(μ-pdt)] (2; ca. 1.5 mM) in CH2Cl2−[NBu4][PF6] after controlled-potential electrolysis in the presence of 3 equiv of P(OMe)3 at −10 °C (Eel = −0.03 V; graphite anode; n = 1.23 F mol−1 2) (trace a) and comparison with the CVs of [Fe2(CO)2{P(OMe)3}2(κ2-dppe)(μ-pdt)]+ (trace b) and of [Fe2(CO)3{P(OMe)3}(κ2-dppe)(μ-pdt)]+ (trace c) (vitreous carbon electrode; v = 0.2 V s−1; potentials in V vs Fc+/Fc).

Scheme 4.

a

a

LL = IMe-CH2-IMe (1+), dppe (2+); L′ = CO, MeCN, P(OMe)3.

= P(OMe)3, the disubstituted, CO-bridged dication 42+ is formed when LL = IMe-CH2-IMe, while [Fe2(CO)4−n{P(OMe)3}n(κ2-dppe)(μ-pdt)]+ monocationic derivatives with terminal CO ligands were obtained for the dppe analogue (n = 1, 2).12 This feature is tentatively assigned to a different lability of the bridging CO in the corresponding adducts. In the case where L′ = MeCN, the electrochemical EC process shows that an adduct is formed for both LL ligands. In this case, the LL effect is manifested by the different magnitudes of the ratio [MeCN]/[X+] required to form detectable amounts of XMeCN+ (Figure 4). When L′ = CO, the impact of the LL ligand that is observed experimentally could not be rationalized by DFT calculations. The fact that no CO binding could be detected when LL = IMeCH2-IMe while this reaction was clearly observed for the dppe analogue is not likely to result from a higher lability of the bridging CO in a putative {[Fe2(CO)4(μ-CO)(κ2-IMe-CH2IMe)(μ-pdt)]+} adduct (Schemes S2−S4, Supporting Information). Alternatively, 1+, which appears to be less reactive than 2+ toward MeCN, could also be less reactive than 2+ toward CO. In this case, it is possible that the minimum [CO]/[1+] ratio required to form detectable amounts of the 1-CO+ adduct could not be reached due to an insufficient solubility of CO in dichloromethane.

Figure 9. ORTEP view of the dication of 4[PF6]2. The thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms and two PF6− counterions were omitted for clarity. Selected bond distances (Å) and angles (deg): C3−Fe1 = 1.868(5), C3−Fe2 = 2.212(5), C1− O1 = 1.139(5), C8−Fe1 = 1.997(5), C7−Fe1 = 1.987(5), Fe1−Fe2 = 2.5943(10), Fe2−P2 = 2.1662(16), Fe2−P1 = 2.2023(16); Fe2−S1− Fe1 = 69.69(4), Fe2−S2−Fe1 = 69.03(4).

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CONCLUSION In addition to the influence on the redox potentials and on the infrared wavenumbers for 1 and 2 showing that the bis-NHC carbene chelate is more electron releasing than dppe, the chelating ligand has also a marked effect on the reactivity of [Fe2(CO)4(κ2-LL)(μ-pdt)]+ with substrates whose electronic properties range from π accepting to σ donating. Theoretical (DFT) calculations showed that the one-electron oxidation of the neutral complexes with the eclipsed geometry produced different isomers, among which rotated species provide exposed sites for substrate binding (Scheme 4; LL = dppe, IMe-CH2-IMe; L′ = CO, MeCN, P(OMe)3). When L′ = P(OMe)3, MeCN, the nature of, respectively, the isolated product (42+) or the calculated adduct (X-MeCN+) indicates that substrate binding occurred at the rotated Fe(CO)3 center. Since differences are observed depending on the chelating ligand, it can be said that the latter affects the reactivity at the neighboring metal center. In the case where L′

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EXPERIMENTAL SECTION

Reactions were carried out under nitrogen using standard Schlenk techniques. All reagents were used as purchased (Sigma-Aldrich). The solvents were predried using conventional methods and were distilled prior to use. Deuterated dichloromethane was stored under argon over molecular sieves before use. The compounds [Fe2(CO)4(κ2-IMe-CH21088


Organometallics

Article

P(OCH3)3), 4.12 (s, 3H, NCH3), 5.71 (d, JAB = 14 Hz, HA, N−CH2− N), 6.29 (d, JAB = 14 Hz, HB, N−CH2−N), 7.11 (s, 1H, CH), 7.33 (s, 1H, CH), 7.54 (s, 1H, CH), 7.74 (s, 1H, CH). 13C{1H} NMR (500 MHz, CD2Cl2): δ 21.10, 21.75, 25.51 (SCH2CH2CH2S), 39.62 (NCH3), 39.69 (NCH3), 56.03 (d, 2JPC = 11 Hz, P(OMe)3), 56.39 (d, 2JPC = 11 Hz, P(OMe)3), 63.46 (NCH2N), 125.42 (CH), 125.51 (CH), 126.15 (CH), 126.90 (CH), 171.92 (C(carbene)), 177.28 (C(carbene)), 205.94 (dd, 2JPC = 46 Hz, 2JPC = 25 Hz, CO), 206.3 (s, CO), 227.76 (dd, 2JPC = 19 Hz, 2JPC = 9 Hz, μCO). 31P{1H} NMR (CD2Cl2, 300 MHz): δ −144.80 (heptet, JPF = 711 Hz, 2P, 2PF6), 140.46 (d, 2JPP = 129 Hz, 1P, P(OMe)3), 156.29 (d, 2 J P P = 129 Hz, 1P, P(OMe) 3 ). Anal. Calcd for C21H36Fe2S2P4N4O9F12: C, 24.82; H, 3.57; N, 5.51. Found: C, 24.33; H, 3.74; N, 5.02. DFT Calculations. Density functional theory (DFT) calculations have been carried out by adopting the TURBOMOLE suite of programs,34 at the B-P86/TZVP35,36 level of theory, which was previously shown to be suitable for investigating [FeFe]hydrogenase models.37−39 Stationary points on the PES have been determined by means of energy gradient techniques, and a full vibrational analysis has been carried out to further characterize each point. Transition state structures have been searched by means of a procedure based on a pseudo-Newton−Raphson algorithm. As a preliminary step, the geometry optimization of a guess transition state structure is carried out by freezing the molecular degrees of freedom corresponding to the reaction coordinate (RC). After performing the vibrational analysis of the constrained minimum energy structures, the negative eigenmode associated to the RC is followed to locate the true transition state structure, which corresponds to the maximum energy point along the trajectory which joins two adjacent minima. Free energy (G) values have been obtained from the electronic SCF energy considering three contributions to the total partition function (Q), namely qtranslational, qrotational, and qvibrational, under the approximation that Q may be written as the product of such terms. Evaluation of H and S contributions has been made by setting T and P values at 298.15 K and 1 bar, respectively. An implicit treatment of solvent effects (COSMO:40 ε = 9.1, dichloromethane; ε = 37.5, acetonitrile) has been used. The presence of MeCN as a coordinating solvent has been treated explicitly. In light of available experimental data and considering the chemical nature of the ligands, only low-spin complexes have been considered.

IMe)(μ-pdt)] (1) and [Fe2(CO)4(κ2-dppe)(μ-pdt)] (2) were prepared as previously reported.5a,b The preparation and the purification of the supporting electrolyte [NBu4][PF6] were as described previously.30 The electrochemical equipment consisted of a PGSTAT 12 or a μ-AUTOLAB (Type III) driven by the GPES software. A GCU potentiostat and a IG5-N integrator (Tacussel/Radiometer) were used for controlled-potential electrolyses and coulometry. All the potentials (text, figures) are referred to the ferrocene−ferrocenium couple; ferrocene was added as an internal standard at the end of the experiments. NMR spectra were recorded on Bruker AMX 3-400 and Bruker DRX 500 spectrometers with chemical shifts reported in δ values relative to residual protonated solvents for 1H NMR spectra and to the solvent for 13C NMR spectra. Infrared spectra were recorded in the range 2300−1600 cm−1 in CH2Cl2 solution with a Nicolet NEXUS FT-IR spectrometer. Elemental analyses were performed by the

Table 1. Crystallographic Data for 4[PF6]2·CH2Cl2 empirical formula formula wt temp (K) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z ρcalcd (Mg m−3) μ (mm−1) cryst size (mm) range of θ (deg) no. of rflns measd Rint no. of unique data/params R1 (I > 2σ(I)) R1 (all data) wR2 (all data) goodness of fit on F2 Δρmax, Δρmin/e Å−3

C22H38Cl2F12Fe2N4O9P4S2 1101.16 170(2) monoclinic P21/c 11.1098(6) 27.5014(12) 13.7020(6) 90.515(5) 4186.3(3) 4 1.747 1.176 0.17 × 0.10 × 0.03 2.78−26.37 32 052 0.1303 8547/522 0.0579 0.1475 0.1072 0.829 0.577, −0.361

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Service Central d′Analyses du CNRS. Crystal data (Table 1) for compound 4[PF6]2 were collected on a Oxford Diffraction X-calibur-2 CCD diffractometer, equipped with a jet cooler device and graphitemonochromated Mo Kα radiation (λ = 0.710 73 Å). The structure was solved and refined by standard procedures.31−33 Digital Simulations. All the simulations were performed with DigiElch Special Build Version 3 (Build SB3.600).19 Details of the procedure are given as Supporting Information. Synthesis of 4[PF6]2: Reaction of 1 with FcPF6 in the Presence of P(OMe)3. Into a 100 mL round-bottom flask containing 70.7 mg (0.140 mmol) of 1 and 92.5 mg (0.280 mmol) of FcPF6, immersed in a cold bath at −10 °C was transferred 40 mL of CH2Cl2 cooled to −10 °C. After the mixture was stirred for 5 min, P(OMe)3 (0.050 mL, 0.40 mmol) was added. The reaction mixture was stirred vigorously for 35 min at −10 °C. Addition of Et2O (40 mL) led to the precipitation of a brown solid. The orange supernatant was filtered off. The precipitate was washed with hexane (3 × 20 mL), and 4[PF6]2 was obtained as a brown solid after filtration (108 mg, yield 76%). Data for 4[PF6]2 are as follows. IR (CH2Cl2, cm−1): ν(CO) 2036 (s), 2021 (s), 1912 (w). 1H NMR (CD2Cl2, 500 MHz): δ 1.24 (m, 1H, SCH2CH2CH2S), 1.50 (m, 1H, SCH2CH2CH2S), 1.68 (m, 1H, SCH2CH2CH2S), 2.02 (m, 1H, SCH2CH2CH2S), 2.25 (m, 1H, SCH2CH2CH2S), 2.85 (m, 1H, SCH2CH2CH2S), 3.93 (d, 3JPH = 11 Hz, 9H, P(OCH3)3), 3.97 (s, 3H, NCH3), 4.00 (d, 3JPH = 11 Hz, 9H,

ASSOCIATED CONTENT

S Supporting Information *

Text, tables, figures, and a CIF file giving digital CV simulations of the first and second oxidations of 1 in the absence of substrate, cyclic voltammetry of 2 under N2 and under CO and the corresponding digital simulations, cyclic voltammetry of 1 under N2 and under CO, scan rate dependence of the current function for the oxidation of 2 and for the oxidation of [Fe2(CO)4(κ2-dppe)(μ-pdt)(μ-H)]+, digital CV simulations of oxidation of 1 and 2 in the presence of MeCN, scan rate dependence of the current function for the oxidation of 1 in the absence and in the presence of P(OMe)3, cyclic voltammetry of 1 before and after controlled-potential electrolysis at −10 °C, computed energy differences between 1+ isomers, computed energy differences between transition states corresponding to CO binding to 1+, computed energy differences between transition states corresponding to CO binding to 2+, computed energy differences between CO adducts which are formed upon CO binding to 1+ and 2+, spectroscopic data of 42+, and crystal data for 4[PF6]2·CH2Cl2. This material is available free of charge via the Internet at http://pubs.acs.org. 1089


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(7) Barton, B. E.; Rauchfuss, T. B. Inorg. Chem. 2008, 47, 2261− 2263. (8) van der Vlugt, J. I.; Rauchfuss, T. B.; Whaley, C. M.; Wilson, S. R. J. Am. Chem. Soc. 2005, 127, 16012−16013. (9) Razavet, M.; Borg, S. J.; George, S. J.; Best, S. P.; Fairhurst, S. A.; Pickett, C. J. Chem. Commun. 2002, 700−701. (10) (a) Liu, T.; Darensbourg, M. Y. J. Am. Chem. Soc. 2007, 129, 7008−7009. (b) Singleton, M. L.; Bhuvanesh, N.; Reibenspies, J. H.; Darensbourg, M. Y. Angew. Chem., Int. Ed. 2008, 47, 9492−9495. (c) Thomas, C. M.; Liu, T.; Hall, M. B.; Darensbourg, M. Y. Inorg. Chem. 2008, 47, 7009−7024. (d) Thomas, C. M.; Liu, T.; Hall, M. B.; Darensbourg, M. Y. Chem. Commun. 2008, 1563−1565. (e) Thomas, C. M.; Darensbourg, M. Y.; Hall, M. B. J. Inorg. Biochem. 2007, 101, 1752−1757. (11) (a) Justice, A. K.; Rauchfuss, T. B.; Wilson, S. R. Angew. Chem., Int. Ed. 2007, 46, 6152−6154. (b) Justice, A. K.; De Gioia, L.; Nilges, M. J.; Rauchfuss, T. B.; Wilson, S. R.; Zampella, G. Inorg. Chem. 2008, 47, 7405−7414. (c) Justice, A. K.; Nilges, M. J.; Rauchfuss, T. B.; Wilson, S. R.; De Gioia, L.; Zampella, G. J. Am. Chem. Soc. 2008, 130, 5293−5301. (12) Chouffai, D.; Zampella, G.; Capon, J.-F.; De Gioia, L.; Gloaguen, F.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J. Inorg. Chem. 2011, 50, 12575−12585. (13) The parameters ip and Ep are respectively the peak current and the peak potential of a redox process. E1/2 = (Epa + Epc)/2. Epa, ipa and Epc, ipc are respectively the potential and the current of the anodic and of the cathodic peak of a reversible process. ΔEp = Epa − Epc. CV stands for cyclic voltammetry. v (V s−1) is the scan rate in CV experiments. An ECE process consists of a chemical reaction (C) occurring between two electron transfer steps (E). (14) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley, New York, 1980; Chapter 11, pp 429−485. (b) Savéant, J.-M. Elements of Molecular and Biomolecular Electrochemistry - An Electrochemical Approach to Electron Transfer Chemistry; Wiley: New York, 2006; Chapter 2, pp 78−181. (15) (a) Evans, D. H. Chem. Rev. 2008, 108, 2113−2144. (b) MaciasRuvalcaba, N. A.; Evans, D. H. Chem. Eur. J. 2007, 13, 4386−4395. (c) Evans, D. H.; O’Connell, K. M. in Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1986; Vol. 14, pp 113−207. (16) (a) Geiger, W. E. Organometallics 2007, 26, 5738−5765. (b) Geiger, W. E. In Progress in Inorganic Chemistry; Lippard, S. J., Ed.; Wiley: New York, 1985; Vol. 33, pp 275−352. (17) Bond, A. M.; Colton, R. Coord. Chem. Rev. 1997, 166, 161−180. (18) Pombeiro, A. J. L.; Guedes da Silva, M. F. C.; Lemos, M. A. N. D. A. Coord. Chem. Rev. 2001, 219−221, 53−80. (19) Digital CV simulations were done with DigiElch Special Build Version 3; see www.elchsoft.com and the following publications: (a) Rudolph, M. J. Electroanal. Chem. 2003, 543, 23−29. (b) Rudolph, M. J. Electroanal. Chem. 2004, 571, 289−307. (c) Rudolph, M. J. Comput. Chem. 2005, 26, 619−632. (d) Rudolph, M. J. Comput. Chem. 2005, 26, 633−641. (e) Rudolph, M. J. Comput. Chem. 2005, 26, 1193−1204. (20) (a) It is likely that the increase of the peak separation (ΔEp) with increasing scan rate is partially due to uncompensated solution resistance. In order to limit the effect of the ohmic drop, we used the values of ΔEp measured at slow scan rates (0.04 V s−1 ≤ v ≤ 0.4 V s−1) to estimate20b the apparent heterogeneous rate constant for a quasireversible electron transfer: ksapp = (8 ± 1) 10−3 cm s−1. This is similar to ksapp for 2 (ksapp = (5 ± 1) × 10−3 cm s−1).12 (b) Nicholson, R. S. Anal. Chem. 1965, 37, 1351−1355. (21) (a) The deduction of the number of electrons involved in the oxidation of 1 is made by comparison of the peak currents for equimolar solutions of 1 and of [Fe2(CO)4(κ2-dppe)(μ-pdt)(μ-H)]+ (one-electron oxidation21b). This requires that both complexes have identical diffusion coefficients, which is a reasonable assumption. (b) Ezzaher, S.; Capon, J.-F.; Dumontet, N.; Gloaguen, F.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J. J. Electroanal. Chem. 2009, 626, 161−170.

AUTHOR INFORMATION

Present Address

§ Département de Chimie Moléculaire, UMR CNRS 5240, ICMG FR-2607, CNRS, Université Joseph Fourier, BP, 38041 Grenoble, France.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The CNRS (Centre National de la Recherche Scientifique), ANR (Programme “PhotoBioH2” and “CatH2”), Université de Bretagne Occidentale, and University of Milano-Bicocca are acknowledged for financial support. We are grateful to Dr. F. Michaud for the crystallographic measurements of 4[PF6]2. D.C. is grateful to the Ministère de l′Education Nationale, de l′Enseignement Supérieur et de la Recherche, for providing a studentship.

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