Reversible Reductive Dimerization of Diiron μ-Vinyl Complex via C–C

ARTICLE pubs.acs.org/Organometallics

Reversible Reductive Dimerization of Diiron μ-Vinyl Complex via C
C Coupling: Characterization and Reactivity of the Intermediate Radical Species Adriano Boni,†,^ Tiziana Funaioli,† Fabio Marchetti,*,† Guido Pampaloni,† Calogero Pinzino,‡ and Stefano Zacchini§ †

Dipartimento di Chimica e Chimica Industriale, Universit
a di Pisa, Via Risorgimento 35, I-56126 Pisa, Italy CNR-Consiglio Nazionale delle Ricerche, ICCOM, Area della Ricerca, Via G. Moruzzi 1, I-56124 Pisa, Italy § Dipartimento di Chimica Fisica e Inorganica, Universit
a di Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy ^ Scuola Normale Superiore, Piazza dei Cavalieri, I-56126 Pisa, Italy ‡

bS Supporting Information ABSTRACT: The diiron μ-vinyl complex [Fe2Cp2(CO)2(μ-CO){μ-η1:η2-CHdCH(Ph)}][BF4], [1][BF4], reacted with CoCp2 affording selectively the C
C coupling product [Fe2Cp2(CO)2(μ-CO){μ-CHCH(Ph)}]2, 2. The cation [1]+ was regenerated from 2 in good yield by I2-induced oxidative cleavage. The cation [1]+ underwent two sequential monoelectron reductions, the first one being an electrochemically reversible process that generated the radical species [Fe2Cp2(CO)2(μ-CO){μ-CHCH(Ph)}], [1]•. The latter was characterized by EPR spectroelectrochemistry. The structures of [1]+, [1]•, and 2 were optimized for the gas phase by DFT calculations. The reaction of [1][BF4] with NEt3 in the presence of excess PhSSPh gave [Fe2Cp2(CO)2(μ-CO){μ-CHCH(Ph)(SPh)}], 3. The new compounds 2 and 3 were fully characterized by IR and NMR spectroscopy, elemental analysis, and X-ray diffraction studies.

’ INTRODUCTION Carbon
carbon couplings on small transition metal units have attracted considerable attention since they may serve as models for related catalytic processes.1 In this context, the reactivity of hydrocarbyl units, bridging coordinated in dinuclear iron or ruthenium complexes bearing ancillary cyclopentadienyl and/or carbonyl ligands, has been widely investigated.2 In principle, dinuclear metal species may provide nonconventional reactivity patterns to bridging ligands and offer the possibility of stabilizing coordination fashions, as a consequence of the cooperativity effects of the two metal centers working in concert.3 A series of intermolecular C
C bond forming reactions between hydrocarbyl fragments coordinated to metal complexes have been documented, and they generally lead to an increase of nuclearity with no reversible character.4 The series includes the coupling of two radical units based on the [Fe2Cp2(CO)2(μCO)] frame to give a Fe4 derivative.5 Other relevant examples are the reductive dimerization of [Co4(CO)3(μ3-CO)3(μ3-C7H7)(η5-C7H9)] and that of the 19-electron complex FeCp(η6C6H6), affording C
C bridged dimers, involving respectively the cycloheptatrienyl ligand6 and the aromatic C6 ring.7 In the present paper, we report on the redox chemistry (straightforward reversible reduction to a fairly stable radical r 2011 American Chemical Society

species, which dimerizes to a C
C bridged Fe4 compound) of the diiron μ-vinyl complex [Fe2Cp2(CO)2(μ-CO){μ-η1:η2CHdCH(Ph)}][BF4], [1][BF4], obtained in two steps from the commercial Fe2Cp2(CO)4 according to Scheme 1.8

’ RESULTS AND DISCUSSION Our interest in the chemistry of hydrocarbyl ligands coordinated in diiron and diruthenium complexes9 prompted us to investigate the reactivity of the formerly described μ-vinyl [Fe 2 Cp 2 (CO)2 (μ-CO){μ-η 1 :η 2 -CHdCH(Ph)}][BF 4 ], [1][BF4]. The nucleophilic additions of a restricted series of nucleophiles (e.g., H
, CN
) to [1][BF4] were reported to occur at the phenyl-substituted carbon, to afford alkylidene derivatives.8c Surprisingly, all our attempts to extend this chemistry to other nucleophiles (e.g., PhLi, LiCtCPh, NEt3) resulted in the formation of a unique product, which was identified as the tetrairon complex [Fe2Cp2(CO)2(μ-CO){μ-CHCH(Ph)}]2, 2. On account of the fact that the formation of 2 appears to be the result of the C
C homocoupling of a diiron complex derived from the monoelectron reduction of [1][BF4], the best conditions for the synthesis of 2 were found by using a typical monoelectron Received: May 19, 2011 Published: July 06, 2011 4115

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Scheme 1. Synthesis of the μ-Vinyl Complex [1][BF4]

Scheme 2. Reductive Dimerization of [1][BF4]

Table 1. Selected Bond Lengths (Å) and Angles (deg) for 2 Fe(1)
Fe(2)

2.5135(12)

Fe(3)
Fe(4)

2.5148(12)

Fe(1)
C(11)

1.737(5)

Fe(3)
C(31)

1.750(8)

Fe(1)
C(12)

1.893(5)

Fe(3)
C(32)

1.924(7)

Fe(1)
C(14)

2.021(5)

Fe(3)
C(17)

2.008(5)

Fe(2)
C(12)

1.929(5)

Fe(4)
C(32)

1.898(7)

Fe(2)
C(13) Fe(2)
C(14)

1.732(6) 1.992(5)

Fe(4)
C(33) Fe(4)
C(17)

1.730(7) 1.999(5)

C(14)
C(15)

1.539(7)

C(16)
C(17)

1.541(6)

C(15)
C(16)

1.583(7)

O(11)
C(11)

1.159(7)

O(31)
C(31)

1.139(8)

O(12)
C(12)

1.179(6)

O(32)
C(32)

1.158(8)

O(13)
C(13)

1.169(7)

O(33)
C(33)

1.143(8)

Fe(1)
C(12)
Fe(2)

82.3(2)

Fe(2)
C(14)
Fe(1)

77.54(17) Fe(4)
C(17)
Fe(3)

C(14)
C(15)
C(16) 112.6(4)

Figure 1. Molecular structure of 2 with key atoms labeled. H atoms, except H(14), H(15), H(16), and H(17), have been omitted for clarity. Thermal ellipsoids are at the 30% probability level.

reducing agent such as CoCp210 (see Scheme 2). The formation of the dimerization product 2 resembles the previous finding that [Fe2Cp2(CO)2(μ-CO){μ-η1:η2-CHdCH(CO2Et)}][PF6] may convert into [Fe2Cp2(CO)2(μ-CO){μ-CHCH2(CO2Et)}]2 upon treatment with aqueous NaHCO3.5 Compound 2 was fully characterized by elemental analysis, IR, NMR and UV
vis spectroscopy, and X-ray diffraction. The

Fe(4)
C(32)
Fe(3)

82.3(3) 77.75(19)

C(17)
C(16)
C(15) 110.5(4)

molecular structure of 2 is represented in Figure 1, whereas most relevant bonding parameters are reported in Table 1. The tetranuclear 2 comes from the dimerization of two μcarbene diiron fragments via the formation of a Csp3
Csp3 bond. The two diiron units of 2 display almost identical bonding parameters, being both composed by a Fe2 core bonded to two terminal Cp and CO ligands, one bridging CO, and a μ-carbene ligand. The equivalent C(14)
C(15) [1.539(7) Å] and C(16)
C(17) [1.541(6) Å] interactions are as expected for Csp3
Csp3 single bonds, whereas C(15)
C(16) [1.583(7) Å], which joins the two units, is rather elongated probably because of steric repulsion between the bulky substituents on C(15) and C(16). The C(15)
C(16) bond displays an s-cis configuration with a dihedral angle C(14)
C(15)
C(16)
C(17) =
159.3(4). Compound 2 crystallizes as the solvato species 2 3 C6H14 in the chiral space group P212121 with the chiral carbon atoms C(15) and C(16) showing the S configuration. Coherently with the structure determined for the solid state, the IR spectrum of 2 (in CH2Cl2 solution) shows three absorptions in the carbonyl stretching region, attributed to terminal (1973, 1936 cm
1) and bridging (1774 cm
1) CO ligands. The NMR spectra exhibit single sets of resonances accounting for the two equivalent [Fe2] units of a unique diastereoisomeric form. In other terms, it is presumable that compound 2 exists in solution as a couple of (R,R) and (S,S) enantiomers, the latter being the (S,S) enantiomer found in the single-crystal structure (see above). Otherwise the possible diastereoisomer (R,S) 4116

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Table 2. Formal Electrode Potentials (E0 , V, vs FeCp2) and Peak-to-Peak Separations (mV) for the Redox Changes Exhibited by [1]+ and 2 in 0.2 M [NnBu4][PF6]/CH2Cl2 Solution reduction processes compound [1][BF4] 2

0

E

oxidation


0.92b

+1.17c c

c

+1.24, +0.55,

+0.29,c +0.23c a

Figure 2. UV
vis spectra (CH2Cl2) of [Fe2Cp2(CO)2(μ-CO){μCHCH(Ph)}]2, 2 (solid line), and [Fe2Cp2(CO)2(μ-CO){μ-η1:η2CHdCH(Ph)}][BF4], [1][BF4] (dotted line). +

Scheme 3. Formation of [1] by Oxidation of 2

appears not to be formed, probably as a consequence of the disfavoring steric repulsions that would be established between the Cp rings and the phenyl ones. The hypothesis has been confirmed by DFT calculations (vide infra), indicating that the RR/SS form is more stable than the RS/SR one by 52.3 kJ/mol. Salient NMR features of 2 are represented by the low-field resonances at 12.10 ppm (1H) and 178.7 ppm (13C), ascribable to the μ-carbene moiety; the Ph-substituted carbon nuclei resonate at 77.5 ppm. The UV
vis spectrum (in CH2Cl2) of 2 exhibits absorptions at 498 and 330 nm (Figure 2, solid line) assigned to metal
ligand charge-transfer processes.11 Otherwise the spectrum of the μ-vinyl precursor [1][BF4] consists of absorptions at 562, 450, 327, and 275 nm (Figure 2, dotted line), in agreement with the spectrum calculated on the basis of the DFT-optimized structure (the calculated spectrum of [1]+ is given as Supporting Information, Figure S1). Interestingly, the treatment of a tetrahydrofuran solution of 2 with elemental iodine resulted in clean recovery of the vinyl complex [1]+, Scheme 3. This evidence points out that the reductive dimerization of [1]+ to 2 is a reversible C
C bond forming process, which is a quite unusual feature for organometallic systems.6,7 It should be remarked here that the reductive dimerization of the μ-vinyl complex [Fe2Cp2(CO)2(μ-CO){μη1:η2-CHdCH(CO2Et)}]+, analogous to [1]+, was not reported to hold reversible character (see Introduction).5 Electrochemistry. The electrochemical properties of [1][BF4] and 2 were studied by cyclic voltammetry, and the formal electrode potentials for the observed electron transfers are compiled in Table 2. Compound [1][BF4] in CH2Cl2/[NnBu4][PF6]

c


1 b


2.10,

ΔEpa 70

E0

ΔEpa


1.73a,c 255

c


2.35,c
2.50c

Measured at 0.1 V s . Coupled to relatively fast chemical reactions. Peak potential value for irreversible or quasireversible processes.

solution undergoes two reduction processes respectively at
0.92 and
1.73 V and one irreversible, presumably multielectronic, oxidation at +1.17 V. The analysis of the cyclic voltammetric response, related to the reductions with scan rates ranging between 0.02 and 1.00 V 3 s
1, confirms that the first reduction is an electrochemically reversible, diffusion-controlled process (the peak-to-peak separation, ΔEp, approaches the theoretical value of 59 mV, and the (ip)red/ν1/2 remains almost constant12), complicated by a subsequent chemical reaction (ipc/ipa = 0.7 at 0.10 V 3 s
1). The coupled chemical complications are supported by the appearance, in the back scan toward positive potentials, of oxidation processes that have been attributed to 2, the product arising from the coupling of two [1]• radicals. The coupling reaction is quite fast in the time scale of the cyclic voltammetry, as suggested by the fact that the peak
current ratio remains considerably lower than 1 on increasing the scan rate up to 1 V 3 s
1. According to the shape of the peaks and the peak-to-peak separation, the reduction process occurring at the more negative potential (
1.73 V) appears as a quasi-reversible one (Figure 3). It was formerly reported that the electrochemical pattern of diiron μ-alkylidene complexes of the type [Fe2Cp2(CO)2(μCO){μ-C(CN)(X)}]n+ (X = H, C-, N-, O-, or P-substituent) showed a one-electron reduction to the corresponding paramagnetic derivatives, whose stability depended on X; moreover EPR spectroscopy suggested that the unpaired electron resided mainly on the Fe
Fe moiety.13 Instead the cyclic voltammetric profile exhibited by the diiron μ-alkylidene 2 in CH2Cl2/[NnBu4][PF6] is quite different: in addition to three irreversible reductions at rather negative potentials (
2.10,
2.35, and
2.50 V) (Figure 4A), some not well resolved, irreversible oxidation processes have been observed at potentials higher than +0.20 V. The three successive anodic waves at +0.23, +0.29, and +0.55 V look irreversible also when the scans are reversed at lower potentials (Figure 4B). Moreover, the closeness of the first two oxidations (at +0.23 and +0.29 V) and the similar shape of the related peaks suggest the occurrence of a two-electron oxidation step. The bielectronic character of the process has been verified by comparison of the peak's height with that related to an equimolar amount of decamethylferrocene added as internal standard. The appearance of a reduction at
0.92 V when cycling the potential several times between positive (+0.45) and negative (
1.25 V) values suggests the formation of the cation [1]+ (Figure 4B); this points out that the electrochemical oxidation of 2 generates unstable cation [2]n+, which quickly breaks up into the dinuclear complex [1]+. The electrochemical reversibility of the reduction process [1]+ f [1]• occurring at
0.92 V foreshadows essentially the 4117

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Figure 3. Cyclic voltammograms of [1][BF4] (10
3 M) recorded at a platinum electrode in CH2Cl2 solution containing [NnBu4][PF6] 0.2 M. Scan rate = 0.1 V s
1. The solid line was obtained starting the scan toward positive potentials; the dashed line was obtained after one scan at negative potentials.

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Figure 5. EPR spectrum of [1]• (A: experimental; B: calculated).

Figure 6. DFT-calculated positive spin density surface (0.004 electron/au3) of [Fe2Cp2(CO)3(μ-CHCHPh)]•, [1]• (spin states A, B, C).

Figure 4. Cyclic voltammograms of 2 (10
3 M) recorded at a platinum electrode in CH2Cl2 solution containing [NnBu4][PF6] 0.2 M. Scan rate = 0.1 V s
1. (A) Cyclic voltammetry obtained starting the scan toward negative potentials. (B) The dashed and the dotted line voltammograms were obtained starting the scan toward positive potentials; the solid line voltammogram was obtained starting the scan toward negative potentials.

same geometry for the monocationic [1]+ and the redox derivative [1]•.12 An EPR experiment aimed at intercepting the radical [1]• was carried out by generating the radical directly in the EPR cavity. Thus a 2.5  10
3 M solution of [1][BF4] in CH 2 Cl 2 /[N n Bu 4 ][PF 6 ] was introduced into a EPR

spectroelectrochemical cell under an argon atmosphere, and the solution was electrolyzed at constant potential (Ew =
1.0 V vs FeCp2). Actually the analysis evidenced the roomtemperature formation of the radical compound [1]•, whose EPR spectrum is shown in Figure 5. The EPR spectrum shows a signal at giso = 2.0067 with evidence of hyperfine structure characterized by a hyperfine coupling constant of 4.90 G, suggesting an interaction between the unpaired electron and a carbon-bonded hydrogen atom. DFT outcomes indicate that the radical species [1]• can be represented by three distinct spin states (A, B, C), differing in the arrangement of the vinyl moiety; see Figure 6. The spin state B, characterized by an important spin localization on the substituted benzyl carbon C15 (Fe1 = 0.28, Fe2 = 0.28, C15 = 0.39 electron/ au3), is 7.9 kJ/mol more stable than the two degenerate spin states A and C, presenting a spin density mainly localized on the iron centers (A: Fe1 = 0.60, Fe2 = 0.38, C15 = 0.01 electron/au3; C: Fe1 = 0.38, Fe2 = 0.60, C15 = 0.01 electron/au3). The DFTcalculated B structure of [1]• (gas-phase) is provided as Supporting Information (Figure S2). Moreover Figure S3 (Supporting Information) shows a view of the DFT-calculated negative spin density surface in [1]•; the presence of negative spin density at H16 is the reason for the observed value of the hyperfine coupling constant (4.90 G, see above). Probably, the delocalization of the unpaired electron to the benzyl moiety is responsible for the additional stability of [1]• with respect to the possible carboxylato-substituted [Fe2Cp2(CO)2(μ-CO){μ-CHCH(CO2Et)}]•,5 which was not observed in the course of the nonreversible reductive dimerization of the vinyl complex [Fe2Cp2(CO)2(μ-CO){μ-η1:η2-CHdCH(CO2Et)}]+ 4118

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Table 3. Selection of Bond Distances (Å) and Angles (deg) for Compounds [1]+, [1]•, and 2

a

Gas-phase (DFT). b Solid state (X-ray).

(see Introduction).14 In other terms, the contribution of the phenyl group to the stability of [1]• may be considered crucial for the detection of this radical at room temperature and in order to establish a chemically reversible 2[1]+ + 2e
/ 2 transformation.15 For a better understanding of the structural modifications of the bridging hydrocarbyl ligand in the course of the sequence [1]+ f [1]• f 2, we calculated the gas-phase molecular structures of compounds [1]+ and 2 (see Supporting Information, Figures S4 and S5, respectively). A selection of calculated bond distances and angles for [1]+, [1]•, and 2 is reported in Table 3, together with the experimental values available from the X-ray characterization of 2.16 Substantial agreement is seen between the computed geometric parameters of 2 (gas-phase) and the corresponding X-ray data (solid-state). Elongation of the CR
Cβ and Cβ
Cγ distances and decrease of the CR
Cβ
Cγ angle occur from [1]+ to 2 (see Table 3), as a consequence of the change in the hybridization of the phenyl-substituted carbon Cβ (sp2 in [1]+, sp3 in 2). The values of such parameters obtained for the three forms of [1]• are very close to the corresponding values related to [1]+, thus indicating that Cβ maintains basically the sp2 hybridization in [1]•. In other words, the calculations agree with the electrochemistry investigations in that [1]+ and [1]• have similar structures (see above). Otherwise the Ph-substituted carbon Cβ gets far away from one iron (Fe2) on passing from [1]+ (2.349 Å) to 2 (3.156 Å), and an intermediate situation is found on average in the radical species [1]•. On considering that the reduction process leading to 2 occurs with intermediate formation of the reactive radical [1]•, in principle the same reaction may be used in order to functionalize the bridging hydrocarbyl ligand by trapping [1]• with opportune reactants. A significant parallelism is represented by the chemistry of the diiron μ-vinyliminium complexes [Fe2(Cp)2(CO)(μ-CO){μ-η1:η3-C(R)dCHCdN(Me)(R0 )}]+ (R = Me, CO2Me, Tol, SiMe3; R0 = Me, 2,6-Me2C6H4).17 These may be easily converted into highly reactive neutral derivatives by treatment with sodium hydride,18 and when this reaction is performed in the presence of appropriate molecules (e.g., chalcogens, diphenyl disulfide, isocyanides, diazocompounds), diiron derivatives comprising unusual functionalized organic fragments are produced selectively.19

Scheme 4. Synthesis of the Alkylidene Compound 3

Figure 7. Molecular structure of 3 with key atoms labeled. H atoms, except H(14) and H(15), have been omitted for clarity. Thermal ellipsoids are at the 30% probability level.

Preliminary investigations in this direction led us to obtain the thioether-containing alkylidene compound [Fe2Cp2(CO)2(μ-CO){μ-CHCH(Ph)(SPh)}], 3. This forms in high yield when [1][BF4] is treated with NEt3 in the presence of excess PhSSPh; see Scheme 4. 4119

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Table 4. Selected Bond Lengths (Å) and Angles (deg) for 3 Fe(1)
Fe(2)

2.500(3)

C(12)
O(12)

1.150(17)

C(11)
Fe(1)

1.79(2)

C(13)
Fe(2)

1.686(19)

C(12)
Fe(1)

1.911(16)

C(12)
Fe(2)

1.904(14)

C(14)
Fe(1)

1.952(13)

C(14)
Fe(2)

1.906(17)

C(11)
O(11) C(14)
C(15)

1.13(2) 1.55(3)

C(13)
O(13) C(15)
C(16)

1.18(2) 1.48(2)

C(15)
S(3)

1.818(18)

C(22)
S(3)

1.74(2)

Fe(2)
C(12)
Fe(1)

81.9(6)

Fe(2)
C(14)
Fe(1)

80.7(6)

C(16)
C(15)
C(14)

108.6(16)

C(14)
C(15)
S(3)

106.5(11)

C(16)
C(15)
S(3)

115.5(12)

C(22)
S(3)
C(15)

107.4(8)

Compound 3 was fully characterized by analytical and spectroscopic techniques; moreover the molecular structure was ascertained by X-ray diffraction analysis. The molecular structure of 3 is depicted in Figure 7, whereas its most relevant bonding parameters are reported in Table 4. This compound can be described as a thioether-containing alkylidene complex closely related to the two diiron units composing the tetranuclear species 2. Also in this case the bonded Fe2 core is coordinated by one terminal CO and two terminal Cp ligands, one bridging CO, and a μ-carbene (alkylidene) ligand. All bonding parameters are as expected for this class of compounds, and C(14)
C(15) [1.55(3) Å] and C(15)
C(16) [1.48(2) Å] interactions are in the usual ranges for single Csp3
Csp3 and Csp3
Csp2 bonds, respectively. Similarly, C(15)
S(3) [1.818(18) Å] and C(22)
S(3) [1.74(2) Å] are typical Csp3
S and Csp2
S single bonds. With reference to the asymmetric carbon C(15), both R and S stereoisomers are present in the crystal examined (space group P21/c). The IR spectrum of 3 (in CH2Cl2) shows absorptions due to the terminal carbonyl ligands (1972, 1934 cm
1) and the bridging one (at 1774 cm
1). The NMR spectra (in CDCl3) contain single sets of resonances and resemble those of the relevant complex 2. In particular, the μ-alkylidene gives raise to resonances at 11.28 ppm (1H) and 180.3 ppm (13C), respectively; moreover, the carbon with the
Ph and
SPh substituents resonates at 66.9 ppm.

’ CONCLUDING REMARKS A cationic diiron complex bearing a Ph-substituted bridging vinyl ligand is susceptible to monoelectron reductive dimerization occurring via carbon
carbon bond coupling. The process may be reversed by C
C cleavage oxidation, providing a redox cycle approaching the concept of “molecular battery” proposed by Floriani and coworkers,20 i.e., a molecular device capable of storing/releasing electrons through the formation/cleavage of chemical bonds. Spectroscopic, electrochemical, and DFT studies agree that the reductive dimerization reaction proceeds with the formation of a radical intermediate; the contribution of the phenyl group to the stability of this radical species is crucial in order to observe reversibility. Moreover, the reductive reaction may be exploited in order to obtain functionalized organic fragments, by trapping the reactive intermediate with a radical capturer such as diphenyl disulfide. Work is in progress in order to use this approach for extending the chemistry of diiron μ-vinyl complexes. ’ EXPERIMENTAL SECTION General Considerations. All reactions were carried out under a nitrogen atmosphere, using standard Schlenk techniques. Solvents were

distilled before use under nitrogen from appropriate drying agents. All reactants were commercial products (Aldrich) of the highest purity available and used as received. Compound [1][BF4] was prepared from commercial [Fe2Cp2(CO)4] (Strem) by a published procedure.8 Ferrocene (FeCp2) was prepared according to the literature21 and purified by sublimation. [NnBu4][PF6] (Fluka, puriss. electrochemical grade) was used as purchased. Chromatography separations were carried out on columns of alumina (Fluka, Brockmann Activity I). Glassware was oven-dried before use. Infrared spectra were recorded at 298 K on a FT-IR Perkin
Elmer spectrometer equipped with a UATR sampling accessory (solid samples). UV
vis spectra were recorded with a PerkinElmer Lambda EZ201 spectrophotometer. NMR measurements were performed at 298 K on a Bruker Avance DRX400 instrument equipped with a probe BBFO broadband. The chemical shifts for 1H and 13C NMR spectra were referenced to the nondeuterated aliquot of the solvent; the spectra were fully assigned via DEPT experiments and 1 13 H, C correlations measured through gs-HSQC and gs-HMBC experiments.22 EPR analyses were recorded at 298 K by a Varian (Palo Alto, CA, USA) 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 board23 and software package especially designed for EPR experiments.24 Experimental EPR spectra were simulated by the WINSIM 32 program.25 DFT geometry optimization, calculation of the UV
vis spectra, and calculation of the electron spin density distribution were performed by the parallel Linux version of the Spartan '10 software.26 We adopted the B3LYP (Becke, three-parameter, Lee
Yang
Parr)27,28 exchange
correlation functional formulated with the Becke 88 exchange functional,29 the correlation functional of Lee, Yang, and Parr,30 and the 6-31G** base functions set, which is appropriate for calculations of split-valence plus polarization quality. Electrochemical measurements were performed in 0.2 M CH2Cl2 solutions of [NnBu4][PF6] as supporting electrolyte. Cyclic voltammograms were performed with a Princeton Applied Research (PAR) 273A potentiostat/galvanostat, interfaced to a computer employing PAR M270 electrochemical software. All measurements were carried out in a three-electrode home-built cell at room temperature (20 ( 5 C). The working and the counter electrode consisted of a platinum disk electrode and a platinum wire spiral, respectively, both sealed in a glass tube. A quasi-reference electrode of platinum was employed as reference. The Schlenk-type construction of the cell maintained anhydrous and anaerobic conditions. The cell was predried by heating under vacuum and filled with argon. A 0.2 M CH2Cl2 solution of [NnBu4][PF6] prepared under an atmosphere of argon was introduced into the cell, and the working electrode was cycled several times between the anodic and the cathodic limits of interest until there was no change in the charging current. The substrate was then introduced to obtain a 1 mM solution, and voltammograms were recorded at a sweep rate of 100 mV/s. After several voltammograms were obtained on the substrate solution, a small amount of ferrocene was added, and the voltammogram was repeated. The E values of the compounds were then determined placing the E1/2 of the ferrocene couple at 0.0 V. Synthesis of [Fe2Cp2(CO)2(μ-CO){μ-CHCH(Ph)}]2 (2). Compound [Fe2Cp2(CO)2(μ-CO){μ-η1:η2-CHdCH(Ph)}][BF4] ([1][BF4], 0.350 g, 0.678 mmol) was dissolved in CH2Cl2 (15 mL) and then treated with CoCp2 (0.150 g, 0.793 mmol). The mixture was stirred for 1 h at room temperature. Thus the volatile materials were removed in vacuo; the resulting red residue was dissolved in Et2O (20 mL) and charged on an alumina column. A red fraction corresponding to 2 was collected by using neat CH2Cl2 as eluent. Compound 2 was obtained as a microcrystalline solid upon removal of the solvent and recrystallized from CH2Cl2/pentane. Yield: 0.239 g, 82%. Crystals suitable for X-ray analysis were obtained from a CH2Cl2 solution layered with hexane at
30 C. Anal. Calcd for C42H34Fe4O6: C, 58.79; H, 3.99. Found: C, 4120

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Table 5. Crystal Data and Experimental Details for 2 3 C6H14 and 3 2 3 C6H14

3

formula

C48H48Fe4O6

C27H22Fe2O3S

fw T, K

944.26 295(2)

538.21 296(2)

λ, Å

0.71073

0.71073

cryst syst

orthorhombic

monoclinic

space group

P212121

P21/c

a, Å

10.815(3)

18.936(15)

b, Å

17.560(6)

8.902(5)

c, Å

23.070(7)

14.751(9)

β, deg cell volume, Å3

90 4381(2)

112.461(8) 2298(3)

Z

4

4

Dc, g cm
3

1.431

1.556

μ, mm
1

1.345

1.381

F(000)

1952

1104

cryst size, mm

0.23  0.21

0.19  0.17

 0.14

 0.11

θ limits, deg reflns collected

1.46
25.20 41 987

1.49
25.02 19 984

indep reflns

7880 (Rint =

4024 (Rint = 0.1101)

0.0588) data/restraints/params

7880/240/493

4024/150/299

goodness on fit on F2

1.084

1.315

R1 [I > 2σ(I))

0.0503

0.1178

wR2 (all data)

0.1414

0.3530

largest diff peak and hole, e Å
3

0.662/
0.358

2.256/
1.196

58.34; H, 4.11. IR (CH 2Cl 2): ν CO 1973 vs, 1936 m, 1774 m cm
1. 1 H NMR (CDCl 3): δ 12.10 (d, 1 H, 3 JHH = 12.5 Hz, μ-CH); 7.67
6.87 (5 H, Ph); 4.91 (d, 1 H, 3 JHH = 12.5 Hz, CHPh); 4.77, 4.30 (s, 10 H, Cp). 13C{1H} NMR (CDCl3): δ 274.2 (μ-CO); 213.4, 212.9 (CO); 178.7 (μ-C); 146.6 (ipso-Ph); 134.3, 131.2, 128.3, 126.6, 125.4 (Ph); 88.2 (Cp); 77.5 (CPh). UV
vis: λ max (CH2Cl 2, 298 K) 610, 398 nm.

Reaction of [Fe2Cp2(CO)2(μ-CO){μ-CHCH(Ph)}]2 (2) with I2: Formation of [Fe2Cp2(CO)2(μ-CO){μ-η1:η2-CHdCH(Ph)}]+.

The treatment of a solution of compound [Fe2Cp2(CO)2(μ-CO){μ-η2CHCH(Ph)}]2 (0.100 g, 0.117 mmol) in tetrahydrofuran (15 mL) with I2 (0.075 g, 0.295 mmol) resulted in a slight color change. 1H NMR spectrum (in CDCl3), performed after 8 h on an aliquot of the mixture priorly dried in vacuo, indicated the clean formation of [Fe2Cp2(CO)2(μ-CO){μ-η1:η2-CHdCH(Ph)}]+, [1]+.

Electrochemical Synthesis and EPR Characterization of [Fe2Cp2(CO)2(μ-CO){μ-CHCH(Ph)}], [1]•. Cathodic reduction of [1][BF4] was carried out directly in the spectrometer cavity on a platinum foil placed in the flat region of a quartz solution rectangular cell (Wilmad Glass WG-808-Q). A platinum quasi-reference electrode and a platinum wire counter electrode, placed in the upper part of the cell, near the working electrode, were used. The connecting wires from the three electrodes were sheathed in PTFE tape so that contact was avoided in the narrow 3 mm i.d. tube and electrolysis only occurred in the flat portion of the cell. The quartz cell was customized in-house by supplying its upper part with a Schlenk-type constructed head with ground-glass joints as inlet for the three platinum electrodes. A 2.5  10
3 M solution of [1][BF4] in CH2Cl2/[NnBu4][PF6] was used for EPR studies. The solution was transferred by syringe into the cell

previously thoroughly deoxygenated by evacuation and filling with argon gas. EPR spectra were recorded at room temperature during an electrolysis experiment at constant potential (Ew=
1.0 V, vs FeCp2) using a BAS CV-27 electrochemical analyzer as polarizing unit.

Synthesis of [Fe2Cp2(CO)2(μ-CO){μ-CHCH(Ph)(SPh)}] (3).

To a solution of compound [Fe2Cp2(CO)2(μ-CO){μ-η1:η2-CHdCH(Ph)}][BF4] (1; 0.200 g, 0.388 mmol) in CH2Cl2 (15 mL) were added PhSSPh (0.840 g, 3.85 mmol) and NEt3 (0.17 mL, 1.2 mmol) in the order given. The mixture was stirred for 1 h at room temperature. Then the volatile materials were removed in vacuo; the resulting red residue was washed with pentane (3  10 mL), dissolved in Et2O (15 mL), and chromatographed on alumina. Compound 3 was collected as an orange band by using a mixture of CH2Cl2 and THF (5:1 v/v) as eluent and obtained as a microcrystalline solid upon removal of the solvent. Yield: 0.163 g, 78%. Crystals suitable for X-ray analysis were obtained from a CH2Cl2 solution layered with pentane, at
30 C. Anal. Calcd for C27H22Fe2O3S: C, 60.25; H, 4.12. Found: C, 60.02; H, 4.26. IR (CH2Cl2): νCO 1972 vs, 1934 m, 1774 m cm
1. 1H NMR (CDCl3): δ 11.28 (d, 1 H, 3JHH = 12.5 Hz, μ-CH); 7.73
7.11 (10 H, Ph); 5.12 (d, 1 H, 3JHH = 12.5 Hz, CHPh) 4.94, 4.93 (s, 10 H, Cp). 13C{1H} NMR (CDCl3): δ 266.7 (μ-CO); 218.0, 212.4 (CO); 180.3 (μ-C); 139.6
124.7 (Ph); 88.0, 87.4 (Cp); 66.9 (CPh). X-ray Crystallography. Crystal data and collection details for 2 3 C6H14 and 3 are reported in Table 5. The diffraction experiments were carried out on a Bruker Apex II diffractometer equipped with a CCD detector using Mo KR radiation. Data were corrected for Lorentz polarization and absorption effects (empirical absorption correction SADABS).31 Structures were solved by direct methods and refined by full-matrix least-squares based on all data using F2.32 All non-hydrogen atoms were refined with anisotropic displacement parameters, apart from the C6H14 molecule in 2 3 C6H14, which was treated isotropically. H atoms were placed in calculated positions and treated isotropically using the 1.2-fold Uiso value of the parent C atoms. Similar U restraints (s.u. 0.01) were applied to the C and O atoms. The crystals of 3 are pseudomerohedrally twinned with twin matrix [1 0 1 0
1 0 0 0
1]. Thus, refinement was performed using the instruction TWIN with the appropriate twin law in SHELXL and one BASF parameter, which refined as 0.465(5). The C
C distances related to the C6H14 molecule in 2 3 C6H14 were restrained to 1.53 Å (s.u. = 0.01) during refinement. The crystal of 2 3 C6H14 is chiral with refined Flack parameter 0.07(2).

’ ASSOCIATED CONTENT

bS

Supporting Information. Figure S1, showing the calculated UV
vis spectrum of [1]+. Figures S2, S4, and S5, showing the calculated structures of compounds [1]•, [1]+, and 2, respectively. Figure S3, showing the DFT-calculated negative spin density surface (0.001 electron/au3) in [1]• (form B). Figure S6, showing the DFT-calculated positive spin density surface (0.004 electron/ au3) in [Fe2Cp2(CO)3(μ-CHCHCO2Et)]•. Crystallographic data for compounds 2 and 3 in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: int. code +050 2219 245. Fax: int. code +050 2219 260.

’ ACKNOWLEDGMENT We thank the Ministero dell’Istruzione, dell’Universit
a e della Ricerca, for financial support. 4121

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Organometallics

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ARTICLE

(13) Bordoni, S.; Busetto, L.; Calderoni, F.; Carlucci, L.; Laschi, F.; Zanello, P.; Zanotti, V. J. Organomet. Chem. 1995, 496, 27. (14) DFT calculations on the hypothetical species [Fe2Cp2(CO)2(μ-CO){μ-CHCH(CO2Et)}]• indicate, like [1]•, the existence of three spin states with different spin distributions (A: Fe = 0.59, 0.40, C-CO2Et = 0.01 electron/au3; B: Fe = 0.30, 0.30, C-CO2Et = 0.39 electron/au3; C: Fe = 0.40, 0.60, C-CO2Et = 0.01 electron/au3); see Figure S6. In contrast to [1]•, the two degenerate spin states A and C, presenting negligible spin density on the hydrocarbyl ligand, are 11.4 kJ/mol more stable than B. (15) The calculated ΔH for the dimerization reactions 2[Fe2Cp2(CO)2(μ-CO){μ-CHCH(R)}]• f [Fe2Cp2(CO)2(μ-CO){μ-CHCH(R)}]2 are
12.25 kJ mol
1 (R = Ph) and
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