Class

Article pubs.acs.org/Organometallics

From Molecular Wires to Molecular Resistors: TCNE, a Class-III/Class-II Mixed-Valence Chemical Switch Alexandre Burgun,†,‡ Benjamin G. Ellis,‡ Thierry Roisnel,† Brian W. Skelton, and Claude Lapinte*,†

§

Michael I. Bruce,*,‡

†

Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, F-35042 Rennes, France School of Chemistry and Physics, University of Adelaide, South Australia 5005, Australia § Centre for Microscopy, Characterization and Analysis, University of Western Australia, Crawley, Western Australia 6009, Australia

‡

S Supporting Information *

ABSTRACT: The binuclear complexes {Cp*(dppe)M}2{μCCC[C(CN)2]C[C(CN)2]CC} (2Fe, M = Fe; 2Ru, M = Ru) and {Cp*(dppe)Fe}{CCC[C(CN)2]C[C(CN)2]CC}{Ru(dppe)Cp*} (2FeRu) were obtained by treatment of the binuclear precursors Cp*(dppe)M-C CCCCC-M′(dppe)Cp* (M = M′ = Fe, 1Fe; M = M′ = Ru, 1Ru; M = Fe, M′ = Ru, 1FeRu) with TCNE in CH2Cl2 at 20 °C. Complexes 2Fe and 2FeRu were isolated as deep purple powders (82% and 87% yields, respectively), and 2Ru was isolated as a brown-yellow solid (55%). The paramagnetic salt [2FeRu][C3(CN)5] was also isolated in 51% yield. The structural and electronic properties of the new compounds were investigated by 1H, 13C, and 31P NMR, XRD analysis, cyclic voltammetry, IR and UV−vis, EPR, and NIR spectroscopies. The experimental data clearly show that the insertion of the central −CC− triple bond of [M]−C6−[M] into tetracyanoethene (TCNE) dramatically decreases the electronic interaction between the metal termini. NIR spectroscopy of the salt [2FeRu][C3(CN)5] demonstrated that the coupling between the iron and ruthenium centers is not completely removed by addition of TCNE to [1FeRu]+, but has produced a very strong attenuation of the electronic coupling from Hab = 0.50 eV to Hab = 0.03 eV.

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(TCNE).21 This interesting finding opens the possibility to convert delocalized class-III mixed-valence complexes, which behave as molecular wires, into localized class-I MV derivatives, which can be considered as molecular insulators. Further, provided that in some instances addition of TCNE onto the carbon chain leads to a strong attenuation of the electronic coupling but does not completely interrupt it, one can expect transformation of a delocalized class-III MV into a class-II MV, which can be regarded as possible precursors of QCA.10 Thus, it is possible to imagine a wider range of applications from the same precursors by synthetic efforts. It is noteworthy that TCNE, discovered in the middle of the previous century, is a popular reagent that has provided numerous examples of novel addition and substitution reactions.22 The formation of π complexes between TCNE and a variety of unsaturated hydrocarbons affords materials with intense charge-transfer absorptions in their visible−UV spectra. Several reviews of the chemistry of this remarkable olefin are available.23 The transition-metal chemistry of TCNE is also very rich and has afforded original results. Coordination of CN groups to metal centers is the more commonly observed bonding mode and was the origin of several complexes. This

INTRODUCTION Molecules with mixed-valence (MV) states are compelling targets of study.1−3 These molecules provide important model systems for intramolecular charge transfer,4−6 and their electronic and optical properties are of potential use in materials and devices.7 One of the major interests in these systems stems from the possibility of exploiting their chargetransfer properties to create molecular components functioning as wires8 or usable in electronic devices based upon the quantum cellular automata (QCA) architecture.3,9,10 In these applications, understanding and control of charge mobility within molecules and molecular assemblies is important. A large variety of molecules with a donor− bridge−acceptor arrangement have been investigated to understand the factors affecting intramolecular charge transfer (CT), including thermodynamic driving force, attenuation by the bridge, distance dependence, salt effect, and magnetic interactions.5,11 In particular, bimetallic compounds with an allcarbon bridge, namely [M]−(CC)m−[M], often exhibit exceptional CT efficiencies for a wide variety of [M] as complexes of Fe,12,13 Mn,14 Mo,15 W,16 Re,17 Ru,18,19 and Pt.20 An interesting contribution from the groups of Ren and Crutchley has shown that the electronic coupling across the [M]−(CC)m−[M] framework can be completely interrupted by the insertion of a −CC− group into tetracyanoethene © 2014 American Chemical Society

Received: May 6, 2014 Published: August 4, 2014 4209

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remarkable olefin also provided metal π complexes by coordination of the CC double bond to a transition metal.24 [2 + 2]-Cycloaddition of TCNE to give cyclobutenes was reported by Hopf and co-workers,25 and more recently, the elegant studies of Diederich and his group have given a series of compounds with original push−pull electronic properties.26 Similar tetracyanocyclobutenyls were obtained by one of us several years earlier from reactions of transition-metal alkynyls, and these derivatives were observed to undergo more or less ready ring-opening reactions to form tetracyanobutadienyl complexes, which are capable of a range of further reactions.27−29 We have recently reported the synthesis and the original properties of two families of complexes, [Cp*(dppe)Fe-(C C)3-Fe(dppe)Cp*]n+(PF6−)n (1Fe(PF6)n, Cp* = η5-C5(CH3)5, dppe = 1,2-bis(diphenylphosphino)ethane) and [Cp*(dppe)Fe-(CC)3-Ru(dppe)Cp*]n+(X−)n (1FeRu(PF6)n), which were isolated in three and two different oxidation states, respectively. For the MV derivatives 1Fe(PF6) and 1FeRu(PF6), we found that the charge is fully delocalized over the whole molecule for both compounds even at the very fast IR time scale.30 We now report the synthesis of the diruthenium complex Cp*(dppe)Ru-(CC)3-Ru(dppe)Cp* (1Ru) and the preparation of the homobinuclear TCNE adducts {Cp*(dppe)M}2{μ-CCC[C(CN)2]C[C(CN)2]CC} (M = Fe, 2Fe; M = Ru, 2Ru)) and the heterobinuclear adduct {Cp*(dppe)Fe}{CCC[C(CN)2]C[C(CN)2]CC}{Ru(dppe)Cp*} (2FeRu). In addition, the heterobinuclear radical cation [{Cp*(dppe)Fe-CCC[C(CN)2]C[C(CN) 2 ]CC-Ru(dppe)Cp*}][C 3 (CN) 5 )] (2FeRu[C3(CN)5)]) was also synthesized and fully characterized. The structural and electronic properties of the new compounds were investigated by 1H, 13C, and 31P NMR, XRD analysis, cyclic voltammetry (CV), IR and UV−vis, EPR, and NIR spectroscopies in order to examine how the electronic coupling between the redox termini is perturbed by the addition of TCNE to the −C6− carbon bridge.

Scheme 1. Synthesis of the Diruthenium Complex 1Ru

2. Preparation of the TCNE Adducts. When 1 equiv of TCNE reacts with 1 equiv of the binuclear complexes 1Fe and 1FeRu in dichloromethane, the color of the solutions changed immediately from orange to deep purple. At the same time the insoluble precursor dissolved. After purification complexes {Cp*(dppe)Fe}2{μ-CCC[C(CN)2]C[C(CN)2]CC} (2Fe) and {Cp*(dppe)Fe}{CCC[C(CN) 2 ]C[C(CN)2]CC}{Ru(dppe)Cp*} (2FeRu) were isolated as deep purple powders in 82% and 87% yields, respectively. Performed in the same conditions, the reaction between 1Ru and TCNE resulted in an immediate color change from yellow to deep blue, followed by a change to red over a period of 30 min. After purification, yellow-brown crystalline {Cp*(dppe)Ru}2{μ-CCC[C(CN)2]C[C(CN)2]CC} (2Ru) was isolated in 55% yield. In accord with related systems,34,35 the reaction is anticipated to proceed via a paramagnetic intermediate resulting from the oxidation of the binuclear complex by TCNE, which then undergoes a [2 + 2]cycloaddition to give a tetracyanocyclobutenyl complex (Scheme 2). Although this latter complex has been isolated in some cases, ring-opening usually occurs to relieve the strained four-membered ring and give the butadienyl systems 2.27,29 This sequence of reactions has recently been termed a cycloaddition−retro-electrocyclic (CA-RE) process.36 The new products were initially characterized as the 1/1 adducts 2 from high-resolution ESI-mass spectra and were identified by their IR spectra recorded in solution (CH2Cl2). The IR spectra of 2Fe and 2Ru display characteristic νCN at 2206 and 2208 cm−1, respectively, and νCC bands at 1950 and 1959 cm−1, respectively. The IR spectrum of the nonsymmetrical complex 2FeRu also contains νCN (2209 cm−1) and νCC bands (1956 cm−1) at very similar frequencies. The TCNE adducts 2Ru and 2FeRu, which have good kinetic stability, have been characterized by elemental analyses and all the usual spectroscopic methods, while 2Fe, which is more difficult to handle, was briefly characterized by IR, 31P NMR, and single-crystal X-ray diffraction. The 1H NMR spectrum of 2Ru shows a triplet at δ 1.51 (4JHP = 2 Hz)

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RESULTS AND DISCUSSION 1. Synthesis of {Cp*(dppe)Ru}2(μ-CCCCCC) (1Ru). The syntheses of the complexes 1Fe and 1FeRu have been previously reported.30 In the homobinuclear ruthenium series, the syntheses of the complexes {Cp*(dppe)Ru}2(μ-C2x) (2x = 2,31 4,32 6, 833) have also been described elsewhere, but the complex with the hexatriynediyl bridge is still unpublished. This complex can be readily obtained following the procedure used for the preparation of 1Fe.30 Treatment of RuCl(dppe)Cp* with half an equivalent of Me3Si(CC)3SiMe3 in MeOH in the presence of KF, which acts as a desilylating reagent, gave 1Ru in 57% yield (Scheme 1). The complex 1Ru was characterized by elemental analysis along with various spectroscopic techniques. The IR spectrum contained three νCC bands at 2129, 2057, and 1959 cm−1. In the 1H NMR spectrum, a singlet at δ 1.61 (Cp*) is accompanied by the CH2 protons of the dppe signals in the expected regions (2−3 ppm range). In the 31P NMR spectrum, a sharp singlet at δ 79.4 was observed, characteristic of the Ru(dppe)Cp* fragment. A well-resolved 13C NMR spectrum of 1Ru was not obtained, due to its poor solubility in all available solvents. The electrospray mass spectrum (ES-MS) of 1Ru contained a strong M+ ion at m/z 1342. Although the complex was readily purified by crystallization (CH2Cl2), crystals suitable for X-ray diffraction studies have not been obtained. 4210

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Scheme 2. Reaction of TCNE with the Dimetalla-hexatriyndiyl Complexes

assigned to the Cp* ligands, while other resonances at δ 1.98 and 2.44 and in the range δ 6.77−7.71 were assigned to the CH2 and Ph protons, respectively, of the dppe ligands. 13C NMR spectroscopy revealed two CN groups at δ 115.61 and 116.97, while a resonance at δ 122.25 could not be unambigously assigned, possibly being due to C3 or C(CN)2. The remaining carbon resonance was not observed and was possibly masked by the phenyl signals. The twisting of the cyanocarbon fragment about the central C−C bond renders the two phosphorus atoms of the dppe ligand magnetically nonequivalent. This is evidenced by the observation of two distinct doublets (δ 92.9 and 97.7, 2JPP = 7 Hz; δ 76.0 and 79.9, 2JPP = 13 Hz) in the 31P NMR spectra of 1Fe and 1Ru, respectively. These resonances are significantly shifted downfield by ca. 3−7 ppm for the iron complex and 1− 4 ppm for the ruthenium analogue with respect to the usual values expected for neutral derivatives containing the Cp*(dppe)M (M = Fe, Ru) organometallic fragments. Indeed, chemical shifts close to 100 and 80 ppm are usually found for the Fe and Ru cationic species, while the resonances of the neutral compounds are found around 90 and 70 ppm, respectively. These data clearly evidence the strong electronwithdrawing ability of the cyanocarbon fragments and their impact on the electronic structure of the terminal metal-alkyne. Intramolecular charge transfer occurs between the electron-rich

organometallic moieties and the electron-poor cyanocarbon, as depicted in Scheme 3 by the canonical electronic structure B. Scheme 3. Selected Possible Mesomeric Structures for 2Fe (M = M′ = Fe), 2Ru (M = M′ = Ru), and 2FeRu (M = Fe, M′ = Ru)

The relative weights of the A and B mesomeric forms depend on the metal atoms, and the A/B ratio is probably smaller in the iron series than in the ruthenium one. Considering the electron richness of the Cp*(dppe)Fe-CC fragment, participation of the canonical form D in the description of the electronic structure of 2Fe could not be excluded and may 4211

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explain why a resolved 1H NMR spectrum has not been obtained for this complex. Comparison of the IR frequencies of the νCC bands of the symmetrical dimetallahexatriyndiyl compounds and their TCNE adducts shows a lowering of almost 100 cm−1, which also greatly supports contribution of structure B in the description of the electronic structure of these compounds. The spectroscopic properties of the heterobinuclear complex 2FeRu present some peculiarities. The 31P NMR spectrum displays two doublets (δ 74.2 and 79.3, 2JPP = 14 Hz), which can be assigned to the nonequivalent phosphorus nuclei of the dppe ligand coordinated to ruthenium. Surprisingly, the resonances corresponding to the diphosphane bound to the iron center were not observed. In the 1H NMR spectrum, the resonances corresponding to the iron moiety are much less intense and much broader than the resonances assigned to the protons of the ruthenium fragment. In particular, the peak corresponding to the protons of the Cp* ligand of the iron site is shifted downfield (δ 1.07) and is at least 10 times broader than the signal of the protons of the Cp* on the ruthenium atom (δ 1.26). Examination of the 13C resonances of the iron Cp* ligand leads to the same observation (see Figure S1). On several occasions, it has been established that large line broadening and downfield shift of the proton NMR signal of the Cp* ligand are diagnostic of partial oxidation of the iron site with a significant spin density on the iron nucleus.37,38 As the Cp*(dppe)Fe moiety is more electron-rich than its ruthenium analogue (Cp*(dppe)Fe is easier to oxidize than Cp*(dppe)Ru by almost 1 V in 2FeRu; see below), an intramolecular electron transfer can take place. Consequently, the electronic structure of 2FeRu can be partially described by the canonical structure B and also by the nonsymmetrical mesomeric form C (Scheme 3). However, further efforts to evidence the intramolecular redox process were unsuccessful, since compound 2FeRu, like its relative 2Fe, is EPR silent at 67 K and ligand field transitions characteristic of the Cp*(dppe)Fe cation were not observed in the NIR range. 3. Redox Properties of 2Fe, 2Ru, and 2FeRu and Preparation of [2FeRu][C3(CN)5]. The initial scans of the cyclic voltammograms were recorded for the TCNE adducts 2Fe, 2Ru, and 2FeRu from −1.5 to +1.5 V vs SCE. The redox potentials of the reversible processes are collected in Table 1, and the CV of 2FeRu is shown in Figure 1. For purpose of comparison, the potentials of 1Fe and 1FeRu, which were measured with the same conditions and have been recently

Figure 1. Cyclic voltammogram of 2FeRu (10−3 M solution in CH2Cl2 at 298 K, 0.1 M [Bun4N]PF6, scan rate 0.100 V s−1, potentials in V vs SCE with reference to the ferrocene−ferrocenium couple (0.460 V vs SCE)).

reported, are also given in Table 1.30 In addition, the CV of 1Ru was also measured. The CVs of the complexes 1Fe and 1FeRu display three reversible waves, while the CV of the bis(ruthenium) complex 1Ru shows four redox events. The first three waves are fully reversible, while the fourth one is only partially reversible. These properties compare well with previous observations for the homologous complexes {Cp*(dppe)M}2(μ-CCCC) (M = Fe, Ru, and Fe/Ru).12,18,39 The CVs of the homobinuclear TCNE adducts 2Fe and 2Ru show two fully reversible oxidation waves, while no reduction waves, usually observed with TCNE adducts, were found within the solvent-imposed limits.21,29,40 The presence of the four strongly electron-withdrawing cyano groups on the −C6− carbon chain anodically shifts the first oxidation potentials of the complexes 2Fe and 2Ru by ca. 0.7 V with respect to the parent derivatives 1Fe and 1Ru. This very large shift suggests that the metal centers are carrying an almost full positive charge, as depicted in canonical structure B (Scheme 3). In accord with the electronic structures of these compounds, the two one-electron redox events 2/2+ and 2+/22+ can be seen as the successive oxidation of the two Cp*(dppe)M-CCC(C(CN)2 moieties of the molecules. Moreover, the wave separations between the two redox events decrease by ca. 0.3 V with respect to the parent complexes 1Fe and 1Ru, indicating that the insertion of the [M]−C6−[M] into TCNE dramatically decreases the electronic interaction between the metal termini. The CV of the heterobinuclear complex 2FeRu also diplays two well-separated and reversible oxidation processes with potentials of 0.35 and 0.88 V, which match well with those found for the homobinuclear homologues (Figure 1). In addition, a reversible reduction wave is observed at −1.31 V, a potential close to the solvent front. This particular behavior of the heterobinuclear complex, which corresponds to the formation of [2FeRu]− on the platinum electrode, is consistent with a contribution from mesomeric structure C to the description of the electronic structure of 2FeRu (Scheme 3). Moreover, the potential difference between the two oxidation processes (E03 − E02 = 0.53 V), which is not far from the potential difference usually observed for a pair of iron and ruthenium mononuclear complexes bearing the same ligands,41−43 suggests that the two moieties of the molecule behave quasi-independently, and as a consequence, the electronic coupling between the iron and ruthenium centers has been severely reduced by the insertion of the CC triple bond into TCNE (see Section 6). One recalls that a full disruption of the electronic coupling has been observed in a related example of TCNE addition.21,29,40

Table 1. Comparison of the Electrochemical Potentials for 2Fe, 2Ru, and 2FeRu with Those of Closely Related Compoundsa cmpd

E01

E02

E03

E04

1Fe 1Ru 1FeRu 2Fe 2Ru 2FeRu

−0.42 −0.15 −0.28 0.28 0.64 −1.31

0.13 0.33 0.36 0.48 0.84 0.35

1.00 1.05 1.03

1.33c

ref 30 b

30 b b

0.88

b

Potentials in CH2Cl2 (0.1 M [NBun4]PF6, 25 °C, platinum electrode, sweep rate 0.100 V s−1) are given in V vs SCE; the ferrocene− ferrocenium couple (0.460 V vs SCE) was used as an internal reference for potential measurements. bThis work. cWave partially reversible. a

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structure C depicted in Scheme 4 in the description of the electronic structure of 2FeRu. 4. Molecular Structures of 2Fe, 2Ru, 2FeRu, and [2FeRu][C3(CN)5]. The Cp*(dppe)M-C6-M(dppe)Cp* (M = Fe, Ru) compounds could not be crystallized due to their low solubilities, which strongly contrasts with those containing butadiyndiyl and octatetrayndiyl bridges, which were structurally characterized.32,45 Compounds 2Fe, 2Ru, and 2FeRu are far more soluble than their parent compounds and were crystallized from CH2Cl2/pentane together with the salt [2FeRu][C3(CN)5], and their structures have been determined by X-ray diffraction. Molecular plots are given in Figure 2, while selected key metric parameters are collected in Table 2. The crystallographic parameters are collected in the Experimental Section and also in Table S1. The X-ray diffraction studies confirmed that TCNE has added to the central CC triple bond to form the ring-opened products. The unit cell of 2Fe contains one molecule of complex, one molecule of CH2Cl2, and half a molecule of 1,2-bis(diphenylphosphino)ethane oxide, formed during the workup as an impurity, which helps the crystallization process. In the symmetrical complexes 2Fe and 2Ru, the electronwithdrawing cyanocarbon group significantly affects bond lengths around the metal centers. The M−P distances are longer than in typical neutral Cp*(dppe)Fe(II)12,38,42 or Cp*(dppe)Ru(II)18,46 fragments, while the M−(C1,6) distances are slightly shorter. In both series, the metal−ligand distances are comparable with those found in cationic complexes, suggesting that a partial positive charge has developed on the metal centers as depicted on canonical form B of Scheme 3. In line with this bonding description, the CC triple bonds are also significantly longer, while the C(3)−C(4) and C(3,4)−C(31,41) distances are consistent with single and double bonds, respectively. These are comparable to those found for other compounds derived from TCNE addition to a hexatriyne fragment.21,29 Moreover, it is interesting to note that the dihedral angle between the two planes containing the CC(CN)2 fragments is very close to 90°. In the heterobinuclear complexes 2FeRu and [2FeRu][C3(CN)5] the ruthenium and iron atoms are disordered. In the neutral compound, the two sites have the same occupancy, and the two metal sites and distances about the metal atoms are similar, as expected for disordered metal sites. In the salt [2FeRu][C3(CN)5], site occupancies for M(1) and M(2) refine to 0.790(2) for Ru(1) and Fe(2), those for Fe(1) and Ru(2) being the complement. The major metal contribution in both Cp*(dppe)M fragments obtained from refinement are consistent with the bond lengths (M−P, M−C, and M− Cp*cent) at the major iron site being slightly shorter than those at the major ruthenium site. However, the structural parameters of the neutral and oxidized complexes 2FeRu and [2FeRu][C3(CN)5] are not significantly different, although in both cases the average M−P distances (Fe and Ru) are elongated in comparison with typical M(II)−P average distances.30,39 The structural parameters are fully consistent with the IR data and confirm the participation of the C(CN)2 groups in the oxidation process. In other words, comparison of the experimental data obtained for 2FeRu and [2FeRu]+ suggests that the electron (hole) in the mixed-valence compound has a significant bridge character. In addition, average M−C(1,6) bond distances are shorter, due to the electron-withdrawing cyanocarbon ligand, as found

Intrigued by the original spectroscopic properties of 2FeRu, which indicate a particular electronic structure, and with the intent of gaining additional insight into the electronic properties of the associated radical cation 2FeRu+, we have prepared the mono-oxidized species [2FeRu][C3(CN)5]. This compound was obtained by reacting 1FeRu or 2FeRu with a large excess of TCNE (10 equiv) in dichloromethane. The salt was isolated as a dark brown solid in 51% yield after purification by preparative TLC. The proposed mechanism for the formation is illustrated in Scheme 4. Initially, TCNE oxidizes Scheme 4. Preparation of [2FeRu][C3(CN)5]

2FeRu to give [2FeRu][C2(CN)4], which in the presence of dioxygen during purification on silica gel is converted into the air-stable complex [2FeRu][C3(CN)5]. Decomposition of the (TCNE)•− radical anion to form the C3(CN5)− anion in the presence of dioxygen is well known.44 The use of a large excess of TCNE shortens the reaction time and makes the access to [2FeRu][C3(CN)5] straightforward and rapid. Upon oxidation, the IR spectrum of 2FeRu is only weakly modified. The νCC band remains at the same place (1957 cm−1), while the frequency of the νCN band decreases by 10 cm−1. Apparently, the loss of one electron has more effect on the C(CN)2 groups than on the alkynyl iron center. These data are also consistent with a significant weight of the mesomeric 4213

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for 2Fe and 2Ru. Similar to homobinuclear complexes, the C(3)−C(4) [1.517(14) and 1.516(6) Å] and C(3,4)− C(31,41) [1.367(13), 1.389(13) Å and 1.376(6), 1.370(6) Å] distances are consistent with single and double bonds, respectively. 5. UV−Vis Spectroscopy of 2Fe, 2Ru, 2FeRu, and [2FeRu][C3(CN)5]. The UV−vis spectra of 2Fe, 2Ru, 2FeRu, and [2FeRu][C3(CN)5] were recorded at 20 °C in CH2Cl2 (Figure 3). The spectra of these deeply colored complexes are not very different. Besides intense absorptions below 350 nm corresponding to intraligand transitions involving the Cp* and the dppe ligands, the electronic spectra reveal three almost resolved transitions in the visible range. These bands are more intense and blue-shifted in the complex 2Ru than in the complex 2Fe, in accord with the colors of these compounds, which are yellow-brown and purple, respectively. As the visible spectra of the related mononuclear alkynyl derivatives have similar absorptions,35,43,47 they are tentatively assigned to dπ(M) → π*(CC) metal-to-ligand charge transfer (MLCT) transitions. The spectrum of the heterobinuclear complex 2FeRu shows absorptions on the low-energy side that match well with those of 2Fe, while on the high-energy side its shape is close to that of the spectrum of 2Ru. As a result, the spectrum is close to the sum of the individual contributions of the iron and ruthenium fragments, suggesting that the electronic interactions between the two moieties of the molecules are rather weak. The spectrum of the salt [2FeRu][C3(CN)5] is less intense on the low-energy side than that of its neutral parent, in accord with an iron-centered oxidation. One can anticipate that oxidation centered on the iron center has switched off the dπ(Fe) → π*(CC) MLCT transition. 6. Glass EPR and NIR Spectroscopies for [2FeRu][C3(CN)5]. The EPR parameters of organometallic compounds are related to the metal spin density distribution;48 therefore in order to shed light on the electronic structure of the radical cation [2FeRu][C3(CN)5], its X-band EPR spectrum was run at 67 K in a rigid glass (CH2Cl2). The spectrum displays the three features corresponding to the three components of the gtensors (g1 = 2.480, g2 = 2.023, g3 = 1.974), as expected for d5 low-spin metal complexes in a pseudo-octahedral environment. The anisotropy tensor is very large (Δg = 0.506) and can be compared with the anisotropy of mononuclear iron complexes such as [Cp*(dppe)Fe-CC-SiMe3](PF6) (Δg = 0.505).41 These Δg values are among the largest values found in the [Cp*(dppe)Fe-CC-R](PF6) series. The influence of the R group on the main g-tensor components is important, and it has been established that the presence of conjugated R substituents strongly diminishes Δg.41 Consequently, the EPR parameters indicate that the unpaired electron has a very strong iron character, and the C4(CN)4 fragment acts as an insulating linker developing almost no π−π interactions with the two alkynyl metal moieties. The NIR spectrum of the radical cation [2FeRu][C3(CN)5] run in CH2Cl2 at 25 °C displays a broad band of weak intensity with a maximum centered around 8000 cm−1 (Figure 4). Deconvolution using Gaussian functions revealed the presence of two overlapping transitions. The low-energy component (νmax = 5100 cm−1, ε = 50 dm3 mol−1 cm−1, Δν1/2 = 1000 cm−1) is characterized by parameters typical of the iron(III) radical cation in the [Cp*(dppe)FeCC-R]•+ series and can be confidently ascribed to a ligand-field (LF) transition in the mixed-valence species,49 also called an interconfigurational (IC)

Figure 2. From top to bottom, views of the molecules of 2Fe, 2Ru, 2FeRu, and [2FeRu][C3(CN)5]. Hydrogen atoms have been omitted for clarity. 4214

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Table 2. Selected Bond Lengths (Å) and Angles (deg) for Compounds 2Fe, 2Ru, 2FeRu, and [2FeRu][C3(CN5)] Bonds M(1)−P(1,2) M(2)−P(3,4) M(1,2)−Cp*(1,2)cent M(1,2)−C(1,6) C(1,6)−C(2,5) C(2,5)−C(3,4) C(3)−C(4) C(3,4)−C(31,41) C(31,41)−C(32,42) C(31,41)−C(33,43) Angles P(1,3)−M(1,2)−P(2,4) C(1,6)−M(1,2)−P(1,3) C(1,6)−M(1,2)−P(2,4) M(1,2)−C(1,6)−C(2,5) C(1,6)−C(2,5)−C(3,4) C(2,5)−C(3,4)−C(4,3) C(2,5)−C(3,4)−C(31,41)

2Fe

2Ru

2FeRu

[2FeRu](C3(CN5))

2.1966(7), 2.2012(8) 2.2140(8), 2.1909(7) 1.750, 1.756 1.840(3), 1.840(3) 1.242(4), 1.243(4) 1.392(4), 1.385(4) 1.509(3) 1.386(4), 1.388(4) 1.432(4), 1.436(4) 1.443(4), 1.431(4)

2.2729(6), 2.3042(6) 2.2808(6), 2.2971(6) 1.911, 1.911 1.939(2), 1.956(2) 1.236(3), 1.232(3) 1.389(3), 1.396(3) 1.497(3) 1.390(3), 1.377(3) 1.428(3), 1.435(5) 1.430(3), 1.426(3)

2.262(3), 2.233(3) 2.247(3), 2.220(3) 1.846, 1.820 1.882(9), 1.900(9) 1.257(13), 1.215(12) 1.396(13), 1.393(13) 1.517(14) 1.367(13), 1.389(13) 1.448(14), 1.442(14) 1.405(14), 1.408(13)

2.2985(12), 2.2866(12) 2.2517(13), 2.2752(12) 1.895, 1.801 1.924(5), 1.895(5) 1.248(6), 1.229(6) 1.372(6), 1.391(6) 1.516(6) 1.376(6), 1.370(6) 1.431(5), 1.439(6) 1.428(6), 1.432(7)

85.27(3), 87.15(8), 86.76(8), 174.3(2), 174.2(3), 117.4(2), 126.3(2),

83.02(2), 84.82(6), 88.79(6), 171.6(2), 175.4(2), 117.9(2), 125.6(2),

84.00(10), 84.02(10) 86.7(3), 87.5(3) 87.1(3), 87.5(3) 174.4(8), 170.7(9) 167.8(10), 168.4(10) 117.4(9), 118.6(9) 123.9(9), 122.0(10)

81.74(4), 83.11(4) 83.12(13), 85.29(14) 95.55(13), 93.79(13) 168.0(4), 169.4(4) 172.1(5), 170.3(5) 118.7(4), 117.0(4) 125.3(4), 125.0(4)

85.54(3) 89.51(8) 85.33(8) 174.6(2) 177.0(3) 118.8(2) 125.6(2)

84.12(2) 89.03(6) 89.47(6) 168.5(2) 173.4(2) 116.4(2) 125.4(2)

The main component of the NIR spectrum (νmax = 8100 cm−1, ε = 360 dm3 mol−1 cm−1, Δν1/2 = 3400 cm−1) was assigned to an intervalence charge-transfer transition (IVCT). The full width at half-height is in line with the calculated value based on the Hush model for nonsymmetrical MV systems (eq 1).2,6 In eq 1, ΔG0 is the free energy difference between the redox isomers A and B (Scheme 5) and, as proposed for other Scheme 5. Selected Redox Isomers for [2FeRu][C3(CN)5]

Figure 3. UV−vis spectra of 2Fe (red plain line), 2Ru (blue plain line), 2FeRu (black dotted line), and [2FeRu][C3(CN)5] (purple dotted line) in CH2Cl2.

Figure 4. NIR spectrum of [2FeRu][C3(CN)5] in CH2Cl2 and proposed deconvolution.

transition by Meyer et al.,50 by analogy to the absorption invariably found in related mononuclear relatives. In full accord with the EPR data, the presence of this transition in the NIR spectrum confirms the localization of the electron vacancy at the iron center. Indeed, similar LF transitions also take place at the ruthenium(III) radical cations in the same ligand environment, but the corresponding transitions are shifted to the 7000−8000 cm−1 spectral range.43

examples,39,51 can be approximated by the difference in oxidation potentials between the mononuclear alkynyl iron and ruthenium complexes Cp*(dppe)MCCH (ΔG0 = 0.43 V or 3500 cm−1).41 The value obtained ((Δν1/2)calc = 3260 cm−1) matches pretty well with the experimental data. Considering the approximations made and the experimental accuracy of the numbers, this provides a valuable validation for 4215

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multinuclear spectrometer at 20 °C or on a 300 MHz Varian Gemini 2000 instrument. Chemical shifts are reported in parts per million (δ) relative to tetramethylsilane (TMS) for 1H and 13C spectra and external 85% aqueous H3PO4 for 31P NMR spectra. Coupling constants (J) are reported in hertz (Hz), and integrations are reported as the number of protons. Mass spectra were run on an HP 5971/A/ 5890-II GC/MS coupling spectrometer. EPR spectra were recorded on a Bruker EMX-8/2.7 (X-band) spectrometer. Electrospray mass spectra in the positive-ion mode were obtained from samples dissolved in MeOH unless otherwise indicated. Solutions were injected into a Varian Platform II spectrometer via a 10 mL injection loop. Nitrogen was used as the drying and nebulizing gas. Chemical aids to ionization were used as required.22 Electrochemical samples (1 mM) were dissolved in CH2Cl2 containing 0.5 M [NBu4]BF4 as the supporting electrolyte for the spectro-electrochemical experiments. Cyclic voltammograms were recorded using a PAR model 263 apparatus, with a saturated calomel electrode, with ferrocene as internal calibrant (FeCp2/[FeCp2]+ = +0.46 V).53 Elemental analyses were conducted on a Thermo-Finnigan Flash EA 1112 CHNS/O analyzer by the Microanalytical Service of the Centre Régional de Mesures Physiques de l’Ouest (CRMPO) at the University of Rennes 1, France, and CMAS, Belmont, Vic., Australia. Starting Reagents. Literature methods were used to make 1Fe,30 1FeRu,30 RuCl(dppe)Cp*,32 and Me3Si(CC)3SiMe3.30 TCNE (Aldrich) was sublimed before use. Synthesis of {Cp*(dppe)Ru}2(μ-CCCCCC) (1Ru). Me3Si(CC)3SiMe3 (50 mg, 0.23 mmol) was added to a suspension of RuCl(dppe)Cp* (321 mg, 0.48 mmol) and KF (28 mg, 0.48 mmol) in MeOH (25 mL), and the mixture was stirred at 25 °C for 12 h. The yellow precipitate was filtered off, washed with acetone (2 × 10 mL) and pentane (5 × 10 mL), and dried to give the crude product (250 mg, 81%). An analytical sample was obtained by passing a CH2Cl2 solution through a squat column of basic alumina, eluting with CH2Cl2. Concentration to ca. 5 mL afforded a yellow crystalline solid, which was centrifuged, washed several times with Et2O, and dried to give pure 1Ru (175 mg, 57%). Anal. Calcd (C78H78P4Ru2·0.5CH2Cl2): C, 68.12; H, 5.75; M (unsolvated), 1342. Found: C, 68.51; H, 5.17. IR (Nujol, cm−1): νCC 2129 w, 2057 s, 1959 m. 1H NMR: δ 1.61 (s, 30H, Cp*), 2.16, 2.81 (2m, 2 × 4H, CH2), 7.20−7.88 (m, 40H, Ph). 31 P NMR: δ 79.4 (s, dppe). ESI-MS (m/z): 1342, M+; 635, [Ru(dppe)Cp*]+. Synthesis of {Cp*(dppe)Fe}2{μ-CCC[C(CN)2]C[C(CN)2]C C} (2Fe). Complex 1Fe (50 mg, 0.040 mmol) and TCNE (5 mg, 0.040 mmol) were dissolved in dichloromethane (5 mL). After stirring for 2 h at room temperature, pentane (50 mL) was added to the solution. The resulting precipitate was filtered off and washed with pentane (3 × 5 mL) to afford {Cp*(dppe)Fe}2{μ-CCC[C(CN)2]C[C(CN)2]CC} (2Fe) (45 mg, 82%) as a deep purple powder. IR (CH2Cl2): νCN 2206, νCC 1950, νCC 1599 cm−1. 31P NMR (CDCl3, 121 MHz): δ 97.7 (d (br), 2JPP = 7 Hz) and 92.9 (d (br), 2JPP = 7 Hz). ESI-MS (m/z): calcd for C84H78Fe2N4P4 1378.3870, found 1378.3873 [M]+. Synthesis of {Cp*(dppe)Ru}2{μ-CCC[C(CN)2]C[C(CN)2]C C} (2Ru). TCNE (9.5 mg, 0.074 mmol) was added to a suspension of {Ru(dppe)Cp*}2{μ-(CC)3} (50 mg, 0.037 mmol) in CH2Cl2 (20 mL), resulting in an immediate color change from yellow to blue and then over 1 h to deep red. After removal of solvent, the residue was extracted into the minimum amount of CH2Cl2 and purified by preparative TLC (hexane−acetone, 3/1) to give a red band (Rf = 0.5), which afforded yellow-brown crystals of {Ru(dppe)Cp*}2{μ-C CC[C(CN)]C[C(CN)2]CC} (2) (30 mg, 55%). Anal. Calcd (C84H78N4P4Ru2.0.5CH2Cl2): C, 67.12; H, 5.26; N, 3.70; M, 1470. Found: C, 66.88; H, 5.30; N, 3.74. IR (Nujol, cm−1): νCN 2208 w, 2193 w; νCC 1973 (sh), 1959 m. 1H NMR: δ 1.51 [t, 4JHP 2 Hz, 30H, Cp*], 1.98, 2.44 (2m, 2 × 4H, CH2), 6.77−7.71 (m, 40H, Ph). 13C NMR: δ 10.23 (s, C5Me5), 29.26 (s, CH2), 96.07 [t, JCP = 2 Hz, C5Me5], 115.61, 116.97 (2s, CN), 122.25 [s, C3 or C(CN)2], 127.42− 138.28 (m, Ph), 148.82 [t, JCP 2 Hz, C2], 204.94 [t, JCP 21 Hz, C1]. 31P NMR: δ 76.0 [d, J(PP) = 13 Hz, dppe], 79.9 [t, J(PP) = 13 Hz, dppe]. ESI-MS (m/z): 1493, [M + Na]+.

the use of the Hush model to extract the electronic coupling Hab with eq 2.52 (Δν1/2)calc = [2310(νmax − ΔG 0)]1/2

(1)

Hab = 0.0205[(εmax νmax Δν1/2)1/2 ]/dab

(2)

The electronic coupling parameter Hab, which physically represents the electronic coupling of the redox isomers A and B (Scheme 5), was computed taking the through-space iron− ruthenium distance in the crystal structure of [2FeRu][C3(CN)5] (dab = 8.61 Å). The obtained value (Hab = 0.03 eV or 242 cm−1) is a typical value for a weakly coupled class-II MV complex. In comparison with the electronic coupling found for the class-III parent complex [1FeRu](PF6) (Hab = 0.50 eV or 4025 cm−1), this value is small and demonstrates that insertion of TCNE may in some circumstances be regarded as a class-II/class-III MV switch. It is interesting to emphasize that the modulation of the electronic coupling by insertion of TCNE into a carbon linker depends on the nature of the redox termini. Indeed, it was nicely shown by the Ren group that insertion of TCNE in [Ru2(Xap)4]2(μ-C6) (Xap = 2-anilinopyridinate) completely switches off the electronic coupling between the two Ru2 centers.21

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CONCLUSION In this contribution, the binuclear complexes {Cp*(dppe)M}2{μ-CCC[C(CN)2]C[C(CN)2]CC} (2Fe, M = Fe; 2Ru, M = Ru) and {Cp*(dppe)Fe}{CCC[C(CN)2]C[C(CN)2]CC}{Ru(dppe)Cp*} (2FeRu) were obtained in good yield by treatment of the binuclear precursors Cp*(dppe)M-CCCCCC-M′(dppe)Cp* (M = M′ = Fe, 1Fe; M = M′ = Ru, 1Ru; M = Fe, M′ = Ru, 1FeRu) with TCNE in CH2Cl2 at 20 °C. The radical cation [2FeRu][C3(CN)5] was also prepared by treatment of 2FeRu with a large excess of TCNE. The structural and electronic properties of the new compounds were investigated by 1H, 13C, and 31P NMR, XRD analysis, cyclic voltammetry, IR and UV−vis, EPR, and NIR spectroscopies. The experimental data clearly show that the insertion of the [M]−C6−[M] arrangement into TCNE dramatically decreases the electronic interaction between the metal termini. NIR spectroscopy studies on the salt [2FeRu][C3(CN)5] demonstrate that the very large electronic coupling which characterized the heterobinuclear MV complex [1FeRu](PF6) (Hab = 4025 cm−1 = 0.50 eV) is dramatically reduced by addition of TCNE to the −C6− chain, but not completely interrupted, and a sizable coupling still exists (Hab = 242 cm−1 = 0.03 eV). Finally, in this example, TCNE acts as a chemical switch, transforming a delocalized class-III MV into a class-II MV complex in a very simple way. Addition of TCNE to binuclear MV complexes with an allcarbon bridge is very subtle, since in a closely related example, it was found that a class-III/class-I switch occurred.21

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EXPERIMENTAL SECTION

General Data. Manipulations of air-sensitive compounds were performed under an argon (for iron-containing complexes) or nitrogen atmosphere using standard Schlenk techniques or in an argon-filled Jacomex 532 drybox. All glassware was oven-dried and vacuum or argon/nitrogen flow-degassed before use. Fourier transform infrared (FT-IR) spectra were recorded using a Bruker IFS28 spectrophotometer (range 4000−400 cm−1) as solids dispersed in KBr pellets. UV−visible spectra were recorded on a CARY 5000 spectrometer. 1H, 13 C, and 31P NMR spectra were recorded on a Bruker AVIII 400 NMR 4216

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= 102.530(3)°, volume = 7729.5(4) Å3; Z = 4, ρc = 1.367 g cm−3, μ = 4.29 mm−1; reflections collected = 41 334, unique = 13 713, Rint = 0.075, data/restraints/parameters = 13 713/26/975, R1(I > 2σ(I)) = 0.051, wR2 (all data) = 0.129; refinement showed that each of the metal sites were a mixture of Ru and Fe. The site occupancies of Ru(1) and Fe(2) refined to 0.790(2), with those for Fe(1) and Ru(2) being the complement.

Synthesis of {Cp*(dppe)Fe}{CCC(C(CN)2) C(C(CN)2)C C}{Ru(dppe)Cp*} (2FeRu). Complex 1FeRu (44 mg, 0.034 mmol) and TCNE (4 mg, 0.034 mmol) were dissolved in dichloromethane (5 mL). After stirring for 2 h at room temperature, hexane (50 mL) was added to the solution. The resulting precipitate was filtered off and washed with hexane (3 × 10 mL) to afford {Cp*(dppe)Fe}{-C CC(C(CN)2) C(C(CN)2)CC}{Ru(dppe)Cp*} (2FeRu) (42 mg, 87%) as a deep purple powder: Anal. Calcd (C84H78FeN4P4Ru): C, 70.83; H, 5.52; N, 3.93. Found: C, 71.14; H, 6.00; N, 3.77. IR (CH2Cl2): νCN 2209, νCC 1956, νCC 1604 cm−1. 1H NMR (CDCl3): δ 1.07 (s, 15H, Cp*Fe), 1.26 (s, 15H, Cp*Ru), 1.95−2.14 (m, 4H, 2 × CH2), 2.52, 2.59 (2m, 4H, 2 × CH2), 6.78−7.74 (m, 40H, Ph). 13C NMR (CDCl3): δ 10.32 (s, C5Me5), 10.40 (s, C5Me5), 29.07−30.46 (m, dppe), 96.26 (s, C5Me5), 96.46 (s, C5Me5), 114.19, 117.02 (2s, 2 × CN), 115.68 (s(br), 2 × CN), 122.51 (s), 127.33− 139.41 (m, Ph), 205.64 [t(br), M-C]. 31P NMR (CDCl3): δ 79.3 [d (br), 2JPP = 14 Hz, Ru(dppe)], 74.2 [d, 2JPP = 14 Hz, Ru(dppe)]. ESIMS (m/z): calcd for C84H78FeN4P4Ru 1424.3570, found 1424.5621 [M]+. Synthesis of [{Cp*(dppe)Fe}{CCC(C(CN)2)C(C(CN)2) C C}{Ru(dppe)Cp*}][C3(CN)5], [2FeRu][C3(CN)5]. Complex 1FeRu (41 mg, 0.032 mmol) and TCNE (40 mg, 0.32 mmol) were dissolved in dichloromethane (8 mL). After stirring for 2 h at room temperature, the solvent was removed under reduced pressure. The residue was then purified by preparative TLC (acetone/hexane, 1:1), and the brown band (Rf = 0.50) was collected to afford [2FeRu][C3(CN)5] (26 mg, 51%) as a dark brown powder. IR (CH2Cl2): νCN 2199, νCC 1957, νCC 1592, νCCC 1506 cm−1. ESI-MS (m/z): calcd for C84H78FeN4P4Ru 1424.3570, found 1424.3601 [M]+. Structure Determinations. Crystallographic data for the structures were collected at 100(2) K (150 K for 2Fe and 2Ru) on CCD diffractometers fitted with Mo Kα radiation, λ = 0.71073 Å for 2Fe and 2Ru, and Cu Kα radiation, λ = 1.54178 Å, for 2FeRu and [2FeRu][C3(CN5)]. Following multiscan or analytical absorption corrections and solution by direct methods, the structures were refined against F2 with full-matrix least-squares using the program SHELXL97.54 Anisotropic displacement parameters were employed for the non-hydrogen atoms. All H atoms were added at calculated positions and refined by use of riding models with isotropic displacement parameters based on those of the parent atom. Results are given below and also in Figure 2, which show non-hydrogen atoms with 50% probability amplitude displacement envelopes, and in Tables 2 and S1. Crystal Data and Refinement Details. 2Fe. Molecular formula: 2(C84H78Fe2N4P4), C26H24O0.75P2, 2(CH2Cl2); molecular weight = 3338.25; λ = 0.71073 Å; triclinic, space group P1,̅ a = 13.0215(4) Å, b = 15.5412(4) Å, c = 26.2434(6) Å, α = 105.340(2)°, β = 91.571(2)°, γ = 113.160(2)°; volume = 4656.9(2) Å3; Z = 1, ρc = 1.19 g cm−3, μ = 0.50 mm−1. Reflections collected = 52 853, unique = 19 800, Rint = 0.037, data/restraints/parameters = 19 800/0/1020, R1(I > 2σ(I)) = 0.052, wR2(all data) = 0.134. The site occupancy of the oxygen atom of the ethane-1,2-bis(diphenylphosphine oxide) “solvent” molecule refined to 0.374(9). 2Ru. Molecular formula: C84H78N4P4Ru2; molecular weight = 1469.52; λ = 0.71073 Å; monoclinic, space group P21/n, a = 14.8635(8) Å, b = 18.5242(10) Å, c = 25.6428(13) Å, β = 98.600(1)°, volume = 6981.0(6) Å3; Z = 4; ρc = 1.398 g cm−3, μ = 0.57 mm−1; reflections collected = 102 192, unique = 26 030, Rint = 0.067, data/ restraints/parameters = 26 030/0/857, R1(I > 2σ(I)) = 0.039, wR2(all data) = 0.081. 2FeRu. Molecular formula: C84H78FeN4P4Ru, CH2Cl2; molecular weight = 1509.25; λ = 1.54178 Å; orthorhombic, space group P212121, a = 15.3752(7) Å, b = 15.6268(8) Å, c = 30.2427(13) Å, volume = 7266.3(6) Å3; Z = 4, ρc = 1.380 g cm−3, μ = 5.17 mm−1; reflections collected = 21 889, unique = 11 712, Rint = 0.105, data/restraints/ parameters = 11 712/0/884, R1(I > 2σ(I)) = 0.064, wR2(all data) = 0.173. The Fe and Ru atoms are disordered between the two metal sites with site occupancies constrained to 0.5 after trial refinement. 2FeRu[C3(CN)5]. Molecular formula: C84H78FeN4P4Ru, C8N5; molecular weight = 1590.43; λ = 1.54178 Å; monoclinic, space group P21/c, a = 18.1725(6) Å, b = 22.6960(7) Å, c = 19.1979(5) Å, β

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data, details of data collection and structure refinement parameters (Table S1), selected NMR spectra for 2FeRu (Figure S1), and a CIF file giving crystallographic data for 2Fe, 2Ru, 2FeRu, and [2FeRu][C3(CN)5] are supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Full details of the structure determinations have also been deposited with the Cambridge Crystallographic Data Centre as CCDC 994998, 830964, 994999, and 995000, respectively. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Street, Cambridge CB2 1EZ, U.K. (fax, +441223-336-033; e-mail, [email protected]).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The MNERT (Ph.D. grant to A.B.) and the Université Européenne de Bretagne (UEB, travel grant to A.B.) are acknowledged for financial support. We also thank the Centre National de la Recherche Scientifique (CNRS) and the Australian Research Council (ARC). Johnson Matthey plc, Reading, is thanked for a generous loan of RuCl3·nH2O.

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REFERENCES

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Organometallics

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Organometallics

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