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Pyrazolato-Bridged Dinuclear Complexes of Ruthenium(II) and Rhodium(III) with N‑Heterocyclic Carbene Ligands: Synthesis, Characterization, and Electrochemical Properties Stefan A. Reindl, Alexander Pöthig, Markus Drees, Bettina Bechlars, Eberhardt Herdtweck, Wolfgang A. Herrmann,* and Fritz E. Kühn* Department of Chemistry, Technische Universität München, Chair of Inorganic Chemistry/Molecular Catalysis, Catalysis Research Center, Ernst-Otto-Fischer Straße 1, D-85747 Garching bei München, Germany S Supporting Information *

ABSTRACT: Pyrazolato-bridged dinuclear complexes of ruthenium and rhodium were synthesized from N-heterocyclic carbene (NHC) precursors, 3,5-bis[(methylimidazolium-1yl)methyl]-1H-pyrazole bis(hexafluorophosphate), and the metal precursors [Ru(p-cymene)Cl2]2 and [Rh(η5-C5Me5)Cl2]2. Depending on the reaction conditions, dinuclear bis(imidazolium) complexes or the corresponding bis(NHC) complexes were formed. These complexes were characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. The metal−metal distances are in the range 3.85− 3.92 Å. Accordingly, a metal−metal bond can be excluded in all cases. The electronic properties were examined by cyclic voltammetry (CV) to detect possible electronic coupling between the metal centers. In the case of the imidazolium complexes irreversible processes are observed in CV, indicating decomposition. The Ru−bis(NHC) complexcoordinatively saturated with six acetonitrile molecules instead of p-cymene ligandsshows three reversible redox processes. Density functional theory (DFT) calculations were used to verify the processes during CV. The Rh−bis(NHC) complex decomposes through irreversible reductions.

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of metal−NHC complexes in the field of molecular electronics has attracted much less attention than that in catalysis.8 Nevertheless, several complexes with potential metal−metal interactions were reported recently (Chart 1) and investigated toward electronic coupling. Those complexes can be classified

INTRODUCTION Complexes of N-heterocyclic carbenes (NHCs) with transition metals are used in a wide variety in homogeneous catalytic reactions.1 Great ligand potential for transition metals is associated with excellent σ-donor properties as well as strong M−C bonds.2 While in most cases monometallic complexes are used as catalysts, dinuclear complexes can provide cooperative effects enhancing catalytic activity.3 In general, complexes without metal−metal interactions with lower chances for cooperative effects can be distinguished from complexes with an interaction of the two metal centers. Several possibilities exist for metal−metal interactions in dinuclear complexes. The most obvious are direct metal−metal bonds or short metal− metal distances, where the metals interact directly with each other. Another way is electronic communication via the ligand. The latter case requires a metal−ligand electron transfer. Metal−NHC complexes displaying a significant π-bond character of the metal−carbene interaction4 can mediate an electron transfer.5 Furthermore, dinuclear bis(NHC) complexes with appropriate bridges such as aromatic rings and πconjugated systems can provide electron transfer through the ligand.5 Therefore, electronic coupling of the metal centers becomes possible, being essential for generating mixed-valent species.6 This offers advantages for investigating the electronic properties for application in molecular electronics: for instance, as switches or junctions in molecular wires.7 So far, application © 2013 American Chemical Society

Chart 1. Dinuclear Bis(NHC) Complexes with Bridges That Permit Electron Transfer

Received: February 7, 2013 Published: July 17, 2013 4082

dx.doi.org/10.1021/om400106f | Organometallics 2013, 32, 4082−4091

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Scheme 1. Synthesis of the Ru−Bis(imidazolium) Complex 1 and Ru−Bis(NHC) Complex 2

according to the system by Robin and Day,9 which divides the complexes into three different classes, depending on the strength of the electronic coupling: class I with metal centers being almost independent from each other, class II with moderate interaction, and class III with strong electronic coupling between the metal centers. The Janus-type benzannulated bis(NHC) complexes of the type A (Chart 1), which were first reported by Bielawski et al.,10 only show weak intermetallic coupling,11 corresponding to class II of the Robin and Day classification. The coupling becomes stronger (class III) by inserting an azido linker between the carbene carbon and the metal atom.12 In contrast, the metals in complexes of type B behave completely independently (class I).13 Albrecht and co-workers investigated the phenyl-bridged bis(NHC) complexes C, determining also weak intermetallic coupling for the respective iron complexes (class II).14 For the pyridazine-bridged bis(NHC) ligand D, several metal complexes are known.15 Among them is a recently reported ruthenium complex. The electrochemical properties were determined, showing strong intermetallic coupling of the ruthenium atoms (310 mV).15e Even for the simplest methylene-bridged bis(NHC) complexes E a moderate intermetallic coupling (class II) was detected, although they do not possess a π-conjugated system.16 In this case, the large electron-tunneling attenuation factor for methylene units (β ≈ 0.85−1.0)17 is assumed to enable the electron transfer over the bridging unit. Finally, complexes of the type F, bearing a pyrazolato-bridged ligand, are well established for a variety of transition metals. Out of the initial silver complexes,18 several complexes with the metals Rh,19 Ir,19 Pd,20 Ni,19,21 and Cu22 were synthesized. Nevertheless, electrochemical analysis was only performed for a structurally related copper-coordinated pyrazolato-bridged ligand without NHC ligands being involved. For this system, two reversible one-electron oxidations were observed to be separated by 230 mV, indicating strong intermetallic communication between the metals.23 As we are interested in dinuclear NHC complexes with metal centers that are in close proximity or electronically coupled, the pyrazolatobridged ligand system seems to be beneficial. In this work the synthesis, characterization, and electrochemical properties of new pyrazolato-bridged ruthenium(II) and rhodium(III) complexes are reported.

trile generates the silver−NHC complex [Ag4L2](PF6)2.18 No deprotonation of the imidazolium salt [H3L](PF6)2 was observed when the reaction was carried out in a suspension in dichloromethane. However, a one-pot synthesis in dichloromethane with [H3L](PF6)2, Ag2O, and the ruthenium precursor [Ru(p-cymene)Cl2]2 yields the new pyrazolatobridged dinuclear Ru(II)−bis(imidazolium) complex [LRu2(μ-Cl)Cl2(p-cymene)2](PF6)2 (1; Scheme 1) after 36 h at room temperature. Although an excess of Ag2O (2.5 equiv) was used, only the pyrazole ring was deprotonated. Using a stoichiometric amount of Ag2O (0.5 equiv), the same complex was synthesized. However, the reaction time had to be increased up to several days to achieve complete conversion. In the 1H NMR spectrum of 1, the signals of the acidic protons of the imidazolium salts are still visible at 8.71 ppm and shifted downfield, while the signal of the pyrazole amine proton at 2.23 ppm vanished. Additionally, the signals of the CH2 linker are observed as two doublets (2J = 14.2 Hz) at room temperature. This indicates free rotation around the Cpyrazole−CH2 bond being hindered, because of the sterically demanding p-cymene ligands on the ruthenium atoms. The initial efforts to prepare a pyrazolato-bridged dinuclear ruthenium bis(NHC) complex by transmetalation of the silver−NHC complex [Ag4L2](PF6)2 with [Ru(p-cymene)Cl2]2 in acetonitrile under mild conditions did not yield any defined product. Similar results were obtained when conversion of the bis(imidazolium) complex 1 into a bis(NHC) complex by adding further amounts of Ag2O in acetonitrile was attempted. However, when the temperature was increased to 110 °C for 2 h, the dinuclear ruthenium−bis(NHC) complex [LRu2(μCl)(MeCN)6](PF6)2 was obtained (2; Scheme 1). The direct synthesis of [LRu2(μ-Cl)(MeCN)6](PF6)2 was also successful using the imidazolium salt [H3L](PF6)2, Ag2O, and [Ru(pcymene)Cl2]2 (Scheme 1, bottom). The best results were obtained when the silver−NHC complex was generated in acetonitrile at 80 °C with 1.5 equiv of Ag2O in a first step. Then, after 2 h, a acetonitrile solution containing [Ru(pcymene)Cl2]2 was added to the silver−NHC solution, followed by a fast increase of the temperature up to 110 °C and stirring for another 2 h. After filtration through Celite, the bis(NHC) complex 2 was isolated as a light yellow solid by precipitation using diethyl ether. In the 1H NMR spectrum, the signal of the acidic protons disappeared, indicating a successful deprotonation of the imidazolium salts. Additionally, the signals of the pcymene ligands disappeared and the two doublets of the methylene linker appeared as a singlet at 5.13 ppm. In the 13C NMR spectrum, the carbene signal was characteristically shifted downfield to 179.69 ppm. The sterically demanding p-cymene

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RESULTS AND DISCUSSION Synthesis of Ruthenium Complexes. The treatment of the pyrazole-bridged NHC precursor 3,5-bis[(methylimidazolium-1-yl)methyl]-1H-pyrazole bis(hexafluorophosphate) ([H3L](PF6)2) with Ag2O in acetoni4083


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exclusion of light. The insoluble silver−NHC complex slowly disappeared, and the typical white precipitate of AgCl was formed. Although the reaction time was increased to several days, small amounts of the bis(imidazolium) complex 4 were found in the AgCl residue. Therefore, a possible deprotonation of the bis(imidazolium) complex 4, to convert it to the bis(NHC) complex 3, was investigated. Addition of further amounts of Ag2O and several external bases was tested, but no selective formation of the Rh−bis(NHC) complex 4 was possible. The addition of AgPF6, to hopefully abstract the terminal chloro ligands, led to decomposition of the complex within hours. In the 1H NMR spectrum of complex 3 the signals for the bridging methylene groups appeared as two doublets at 4.61 and 5.02 ppm (2J = 15.0 Hz) and were shifted upfield in comparison to those of the ligand [H3L](PF6)2 (5.34 ppm). Therefore, a nonplanar structure is assumed for [LRh2(μCl)Cp*2](PF6)2 (3) because the bridging methylene groups do not appear as a single signal in the 1H NMR spectrum. A possible reason for that is the pentamethylcyclopentadienyl (Cp*) ligands, which prevent the formation of a planar structure still being coordinated to the rhodium and appearing as a singlet at 1.68 ppm. The signals for the carbene carbons were shifted upfield in comparison to those of the Ru− bis(NHC) complex 2 and appeared as a doublet at 167.98 ppm (1JC−Rh = 52.3 Hz) in the 13C NMR spectrum. The 1H NMR spectrum of the Rh−bis(imidazolium) complex 4 was very similar to that of the Ru−bis(imidazolium) complex 1. The two acidic protons appeared at 8.90 ppm, and most likely because of the hindered rotation around the Cpyrazole−CH2 bond, the methylene groups appeared as two doublets (2J = 14.0 Hz) at 5.41 and 5.90 ppm. The signal of the Cp* ligand was shifted upfield to 1.26 ppm in comparison to the Rh precursor [RhCp*Cl2]2 (1.55 ppm) or the Rh−bis(NHC) complex [LRh2(μ-Cl)Cp*2Cl2](PF6)2 (4). Single-Crystal X-ray Determination. All complexes could be characterized by single-crystal X-ray diffraction experiments. The imidazolium rings of the bis(imidazolium) complex [LRu2(μ-Cl)Cl2(p-cymene)2](PF6)2 (1) are not bound to the ruthenium atoms and point away from the metal centers (Figure 1), as was expected from the 1H NMR spectrum. The p-cymene ligands are located in front of and behind the plane of the pyrazole ring. Unlike the ruthenium precursor, one bridging chloro ligand is replaced by the pyrazolato bridge. The central structural motif is a five-membered ring including the two nitrogen atoms of pyrazolato, two ruthenium atoms, and a bridging chloro ligand. Such a motif can only be found in a few complexes.15e,24 For two of them, even direct Ru−Ru bonds with lengths of 2.79424a and 2.917 Å24b were reported. In the bis(imidazolium) complex 1 the observed Ru···Ru distance is significantly longer (3.9172(5) Å), which is why we can exclude a direct metal−metal bond. However, the bridging as well as the terminal Ru−Cl bonds (2.398−2.407 Å) lie in the range of those of similar complexes.24 The bis(NHC) complex [LRu2(μ-Cl)(MeCN)6](PF6)2 (2) is not planar in the solid state (Figure 2). According to the 1H NMR spectrum this should not be the case, but due to the bridging CH2 groups appearing as a singlet, we think that the NHC ligand is flipping in solution to be planar on the NMR time scale. The bound acetonitriles provide an octahedrally coordinated Ru atom. At 3.865 Å, the distance between the ruthenium atoms is slightly shorter in comparison to that of bis(imidazolium) complex 1.

ligands might prevent the NHCs from coordinating the ruthenium atoms. This is a possible reason for the imidazolium moieties still being protonated in complex 1. Such an assumption is supported by the fact that at high temperatures the arene ligands are removed from the ruthenium atoms, allowing the coordination of the NHCs. To saturate the coordination sphere of ruthenium, acetonitriles are then coordinated, which can be seen in the 1H NMR spectrum at 2.20 and 2.50 ppm. These acetonitriles are not bonded very strongly and can be replaced with CD3CN, as these signals disappear in the 1H NMR spectrum after several hours. This also explains why the Ru−bis(NHC) complex 2 cannot be synthesized in dichloromethane, since it is a noncoordinating solvent and has a boiling point which is far too low for removal of the arene. Synthesis of Rhodium Complexes. With the structurally related rhodium(III) precursor [RhCp*Cl2]2, synthesis of the analogous Rh complexes using the same reaction conditions was attempted. In contrast to the case for the ruthenium precursor, the one-pot reaction of the imidazolium salt [H3L](PF6)2, 2.5 equiv of Ag2O, and [Cp*RhCl2]2 in dichloromethane yielded the Rh−bis(NHC) complex [LRh2(μ-Cl)Cp*2](PF6)2 as the major product (3; Scheme 2). This complex could be easily isolated by filtering the Scheme 2. Synthesis of the Rh−Bis(NHC) Complex 3 and Rh−Bis(imidazolium) Complex 4

reaction mixture through Celite and precipitating the bis(NHC) complex out of dichloromethane solution by adding diethyl ether. The residue of the filtration contained unreacted Ag2O as well as AgCl and was extracted with acetonitrile in order to isolate byproducts that are insoluble in dichloromethane. Using this procedure, the rhodium bis(imidazolium) complex [LRh2(μ-Cl)Cp*2Cl2](PF6)2 (4) could be separated from the bis(NHC) complex 3, since unlike the ruthenium complex 1, the rhodium complex 4 is insoluble in dichloromethane. The selective synthesis of the Rh−bis(imidazolium) complex 4 worked very well in acetonitrile solution, when only 0.5 equiv of Ag2O was used. In contrast, the synthesis of the Rh−(bis)NHC complex 3 always created a small amount of the bis(imidazolium) complex 4. An excess of Ag2O always created a mixture of bis(imidazolium) and bis(NHC) complexes. The best yields for 3 were achieved when the silver−NHC complex [Ag4L2](PF6)2 was first synthesized and isolated by evaporating the solvent followed by subsequent addition of a solution of [Cp*RhCl2]2 in dichloromethane and stirring under the 4084


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ruthenium atoms, and a bridging chloro ligand are present, the Ru···Ru distance is somewhat shorter (3.671−3.785 Å).15e,24j The Ru−Cl bonds (2.4797(8) and 2.4795(8) Å) as well as the Ru−C bonds (2.015(3) and 2.007(3) Å) are within the range of other Ru−NHC complexes.10,16,24,25 The structure of the Rh−bis(NHC) complex [LRh2(μCl)Cp*2](PF6)2 (3) is different from that of the distorted planar Ru−bis(NHC) complex 2. Both NHCs are located on the same side of the pyrazolato ring, forming a cavity with the ruthenium atoms and the bridging μ-Cl ligand (Figure 3). The

Figure 1. ORTEP style plot of the cationic fragment of the ruthenium−bis(imidazolium) complex [LRu2(μ-Cl)Cl2(p-cymene)2](PF6)2 (1). Ellipsoids are shown at the 30% probability level. Hydrogen atoms as well as PF6 counterions and solvent molecules are omitted for clarity. Selected bond lengths (Å), bond angles (deg), and torsion angles (deg): Ru···Ru = 3.9172(5); Ru1−N1 = 2.137(3), Ru2−N2 = 2.121(3), Ru1−Cl1 = 2.4021(10), Ru1−Cl2 = 2.4004(11), Ru2−Cl2 = 2.4024(10), Ru2−Cl3 = 2.4045(11); Ru1−Cl2−Ru2 = 109.22(4); Ru1−N1−N2−Ru2 = −51.4(3).

Figure 3. ORTEP style plot of the cationic fragment of the rhodium− bis(NHC) complex [LRh2(μ-Cl)Cp*2](PF6)2 (3). Ellipsoids are shown at the 50% probability level. Hydrogen atoms as well as PF6 counterions and solvent molecules are omitted for clarity. Selected bond lengths (Å), bond angles (deg), and torsion angles (deg): Rh1···Rh2 = 3.9541(6); Rh1−C1 = 2.020(5), Rh2−C9 = 2.036(5), Rh1−N3 = 2.122(3), Rh2−N4 = 2.120(4), Rh1−Cl1 = 2.4720(12), Rh2−Cl1 = 2.4827(12); Rh1−Cl1−Rh2 = 105.89(4); Rh1−N3−N4− Rh2 = 3.8(5).

Cp* ligands, which cannot be replaced by acetonitrile molecules, are located on the other side. The Rh···Rh distance (3.9541(6) Å) is somewhat longer than the metal−metal distance in the ruthenium complexes. In a previously reported Rh(I)−bis(NHC) complex19 the intermetallic distance is longer than that of complex 3. This possibly arises from the absence of a bridging chloro ligand as can be found in the Rh(III)−bis(NHC) complex 3. The different ionic radii of the metals due to the different oxidation states might be other reasons for this. The Rh−C bonds (2.020(5) and 2.036(5) Å) as well as the Rh−Cl bonds are within the expected ranges.19,26 In principle, the Rh−bis(imidazolium) complex [LRh2(μCl)Cp*2Cl2](PF6)2 (4) (Figure 4) has the same structural features as the related Ru−bis(imidazolium) complex 1. In contrast to the case for complex 1, the asymmetric unit contains only half of the cationic fragment of 4 as well as one PF6 anion. The second half of the dinuclear complex is generated through rotation around a C2 axis through C6 and Cl2. In addition, the metal−metal distance in the bis(imidazolium) complex 4 (3.8753(4) Å) is shorter than that in the corresponding bis(NHC) complex 3. There is only little precedent of a fivemembered ring with two rhodium and two nitrogen atoms as well as a bridging ligand.27 Both complexes with additional bridging groups such as a carbonyl or another pyrazole (3.26027a and 3.593 Å27b) and complexes without an additional bridging group, which is realized in an azido-bridged dinuclear

Figure 2. ORTEP style plot of the cationic fragment of the ruthenium−bis(NHC) complex [LRu2(μ-Cl)(MeCN)6](PF6)2 (2). Ellipsoids are shown at the 50% probability level. Hydrogen atoms as well as PF6 counterions and solvent molecules are omitted for clarity. Selected bond lengths (Å), bond angles (deg), and torsion angles (deg): Ru···Ru = 3.8653(4); Ru1−C1 = 2.015(3), Ru2−C11 = 2.007(3), Ru1−N3 = 2.068(2), Ru2−N4 = 2.067(2), Ru1−Cl1 = 2.4797(8), Ru2−Cl1 = 2.4795(8); Ru1−Cl1−Ru2 = 102.41(3); Ru1− N3−N4−Ru2 = 44.4(3).

In general, first-row transition metals have shorter metal− metal distances in pyrazolato-bridged bis(NHC) complexes (Ni···Ni: = 3.712−3.873 Å;19,20 Cu···Cu = 3.350 Å22). However, this is the shortest distance of two transition metals of the second row for pyrazolato-bridged bis(NHC) complexes (Pd···Pd = 3.95−4.05 Å,20 Rh···Rh = 4.213 Å19). In related pyridazine-bridged complexes coordinating ruthenium, where also a five-membered ring including two nitrogen atoms, two 4085


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Figure 4. ORTEP style plot of the cationic fragment of the rhodium− bis(imidazolium) complex [LRh2(μ-Cl)Cp*2Cl2](PF6)2 (4). Ellipsoids are shown at the 50% probability level. Hydrogen atoms as well as PF6 counterions are omitted for clarity. Selected bond lengths (Å), bond angles (deg), and torsion angles (deg): Rh···Rh = 3.8753(4); Rh1−N1 = 2.148(2), Rh1−Cl1 = 2.4183(7), Rh1−Cl2 = 2.4343(5); Rh1−Cl2− Rh1a = 105.50(3); Rh1−N1−N1a−Rh1a = 64.2(2). Symmetry code: (a) −x, y, 1/2 − z. Figure 5. Cyclic voltammogram (top) and differential pulse voltammogram (bottom) of the Ru−bis(NHC) complex 2 (5 mM in MeCN with 0.1 M of [Bu4N][PF6]).

rhodium complex (3.739 Å27c), show shorter Rh···Rh distances than in complexes 3 and 4. The repulsion of the Cp* ligands might cause the longer Rh···Rh distance. Electrochemical Properties and DFT Calculations. Possible electronic coupling between the two metal centers of these new bimetallic complexes was evaluated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) and spectroelectrochemical analysis. The Ru−bis(imidazolium) complex 1 only shows an irreversible oxidation at 1.5 V. No reduction peak was observed in the potential range between −1.0 and 2.0 V. The voltammetric response for the Ru−bis(NHC) complex 2 shows three oxidation/reduction processes in the potential range between 0 and 1.8 V (Figure 5, top). A summary of the electrochemical data is shown in Table 1. The first oxidation process at Epa1 = 0.61 V vs Ag/AgCl reference electrode presents a fully reversible one-electron process with an ipc/ipa ratio close to 1 and a peak separation of the cathodic and anodic response of ΔEp = 56 mV. In contrast, the second and third redox processes located at Epa2 = 0.85 V and Epa3 = 1.23 V correspond to quasi-reversible one-electron processes with an ipc/ipa ratio lower than 1 (E2, ipc/ipa = 0.80; E3, ipc/ipa = 0.73) and peak separations of 80 mV (E2) and 107 mV (E3), respectively. These potentials lie in the range of the oxidation potentials of related ruthenium complexes, containing a NHC ligand as well as an N-donor ligand.11a,23,28 DPV was used to check if there are overlapping peaks in the CV of Ru bis(NHC) complex 2. On the basis of the fact that three symmetrical peaks are observed in the differential pulse voltammogram (see Figure 5, bottom), there are no overlapping peaks and only three electrochemical events. The thermodynamic stability Kc toward comproportionation of the oxidized species can be calculated from the separation of the voltammetric response ΔE1/2 with the equation

Table 1. Summarized Peak Potentials of the CV and DPV of [LRu2(μ-Cl)(MeCN)6](PF6)2 (2) and the Supposed Oxidation States peak

Epa, V

ΔEp, mV

E1/2(DPV), V

oxidn state

.

1 2 3

0.61 0.85 1.23

56 80 107

0.54 0.79 1.18

(L)RuII/RuII (L)RuII/RuIII (L)RuIII/RuIII

4.0 6.4

Kc = 10ΔE1/2 /59 mV

The relatively high value of log Kc = 4.0 indicates good thermodynamic stability of the species generated after the first oxidation.31 The stability of the mixed-valence species, generated after the second oxidation, is even higher, with a comproportionation constant of log Kc = 6.4. Therefore, in principle it should be possible to isolate the mixed-valence species, but all of our attempts failed. The mesomerism of the oxidized species is an adequate explanation for the good thermodynamic stability of the mixedvalence species. Starting from complex 2, the consecutive oneelectron processes lead to oxidized species, for which different formal resonance structures can be postulated (Scheme 3). According to DFT calculations (B3LYP/6-31G*), the first redox process (E1) corresponds to a ligand-based oxidation. The HOMO of the starting complex (Figure 7a) is located at the NHC fragments, the Ru atoms, and the bridging chlorine. The calculated partial atomic charge in the starting complex is −0.23 for both Ru centers, which can be formally attributed to Ru(II). The strongly σ donating NHC ligands as well as the 4086


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Scheme 3. Possible Mesomeric Structures of Oxidized Species during CV Experiments Starting from [LRu2(μCl)(MeCN)6](PF6)2 (2)

with a potential of 1.10 V a new charge transfer band is observed at λmax 631 nm. This corresponds to 15848 cm−1, which is within the range of typical absorption bands between 14000 and 27000 cm−1 for class II systems.9 In addition, the bandwidth is broader than the calculated maximum for a class III system (Δν̃1/2(obsd) = 7800 cm−1, Δν̃1/2(calcd) = 6051 cm−1), thus supporting the classification as class II.30 According to the DFT calculations, the triplet is more stable than the singlet state by 14.4 kcal/mol after the second oxidation (E2). The two unpaired electrons of the biradical complex [LRu2]4+ are located at the HOMO (Figure 7c) and HOMO-1 (Figure 7d), which are delocalized over nearly the entire complex. As in the case of the product of the first oxidation step, the spin density of the triplet state is mostly localized at the metal centers (refer to the Supporting Information, page S21) but is partially on the carbene carbon atoms and is not as symmetric

anionic bridging pyrazolato moiety might be the reason for the partial negative charge. After the first oxidation (E1), the calculated charge of the two ruthenium atoms in the resulting complex [LRu2]3+ only rises insignificantly (to −0.07), indicating that the abstracted electron most probably originates from the ligand. The singly occupied MO (SOMO) of the resulting radical complex is located at the pyrazole fragment and the ruthenium atoms (Figure 7b). Spin density analysis shows that the unpaired electron is delocalized over the ruthenium atoms (refer to the Supporting Information, page S17), implying that the electron is present at each of the metal centers with the same probability. Consequently, if generated, a charge-delocalized mixed-valence species should be present. However, spectroelectrochemical analysis (Figure 6) of [LRu2]3+, carried out by

Figure 6. Enhanced excerpts of UV−vis spectra of Ru bis(NHC) complex 2 at different potentials (for complete spectra, see the Supporting Information, Figure S14).

oxidizing complex 2 for 60 s at a potential of 0.75 V, resulted in no additional charge transfer (CT) band in the UV−visible spectrum, which would be expected for a mixed-valence species. Therefore, since it is known that DFT calculations may overdelocalize an unpaired electron,29 it is likely that the latter is located at the pyrazole ring, while the ruthenium atoms both remain in the same oxidation state ([(L)RuIIRuII]3+). The second oxidation (E2) leads to a considerable increase of the calculated ruthenium charges, which are not equally distributed (+1.67 and +1.48) in [LRu2]4+, indicating the generation of a mixed-valence species. Additionally, the spectroelectrochemical data suggest the formation of a charge-localized class II system, since after oxidation for 60 s

Figure 7. Computed HOMOs of the species generated during the CV: (a) ground state [LRu2]2+; (b) first oxidation [LRu2]3+; (c) second oxidation [LRu2]4+ (SOMO); (d) HOMO-1(SOMO) of [LRu2]4+; (e) third oxidation [LRu2]5+ (further orbitals can be found in the Supporting Information). 4087


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as was the case for [LRu2]3+. Thus we propose that the ligand is uncharged, while the ruthenium atoms rest in the formal oxidation states +II and +III ([(L)RuIIRuIII]4+). Interestingly, further oxidation (E3) then causes a decrease of the calculated charges at the ruthenium atoms. They still remain in an unequal state, but with less separated values (+1.45 and +1.34). Furthermore, the HOMO of the resulting complex [LRu2]5+ (Figure 7e) has an unsymmetrical profile. In the spectroelectrochemical experiments, the absorption band at 631 nm shifts to 636 nm and grows when the complex is oxidized for 60 s at a potential of 1.55 Va strong indication for the complex being still in a mixed-valence charge-localized state.9 According to the DFT calculations, the most stable spin configurations for [LRu2]5+ are a pseudo doublet and a quartet state, which only differ by 0.2 kcal/mol in energy. For both states, the spin density analysis shows that the unpaired electrons are located mostly at the pyrazole ring and both ruthenium atoms (refer to Supporting Information, pages S23 and S25). It also implies that both configurations have three unpaired electrons, which all have the same spin in the quartet state, while in the doublet state the spin of one of the ruthenium-centered electrons is different. Especially in this case, the assignment to one of the possible resonance structures of [LRu2]5+ (Scheme 3, right) is difficult, since although both the calculations as well as the spectroelectrochemical measurements support the existence of a class II mixed-valence species, an exclusive use of one mesomeric form does not agree with all the observed and calculated results. Hence, the most likely explanations for the mixed-valence state after the third oxidation are possible mesomeric structures with different oxidation states at the ruthenium atoms (+III/+IV or +II/+IV). In contrast, the Rh−bis(imidazolium) complex 4 undergoes only irreversible oxidation/reduction processes. In the range between 0 and 2 V three oxidations at 1.40, 1.56, and 1.78 V were visible in the cyclic voltammogramm. Reductions were observed at −0.71 and −1.58 V. The Rh−bis(NHC) complex 3 also behaved in a similar way, and irreversible reductions were observed at potentials below −0.6 V. The coordinated pentamethylcyclopentadienyl ligands as six-electron donors might prevent reversible redox processes. While rhodium(III) is reduced to rhodium(I), the complex would have too many electrons to be structurally stable. At least one two-electron donor would have to leave the coordination sphere of the rhodium(I) to generate a 18-electron species, which would explain the decomposition of the complex.

for all known complexes with this pyrazolato bridging ligand, forming five-membered rings with two nitrogens, two metal atoms, and the bridging chloro ligand. The electrochemical properties were determined by cyclic voltammetry. Only the dinuclear Ru−bis(NHC) complex shows reversible redox processes in the cyclic voltammogram. Here, three electrochemical events can be detected, all of which represent reversible or quasi-reversible one-electron processes. According to spectroelectrochemical experiments and DFT calculations, the first oxidation is mostly ligand-based. The second and third oxidations then lead to relatively stable mixed-valence species, which could be classified as charge-localized class II systems.

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EXPERIMENTAL SECTION

General Comments. All reactions were performed under an argon atmosphere using standard Schlenk techniques. All solvents were dried in a solvent purification system (SPS by M. Braun) and used without further purification. Deuterated solvents for NMR experiments were purchased from Eurisotop and dried over common agents and distilled before use. The bis(imidazolium) salts 3,5-[bis(methylimidazolium-1yl)methyl]-1H-pyrazole bis(hexafluorophosphate) was synthesized according to the literature procedure.18 All other reagents were commercially available and purchased from Alfa Aesar, ABCR, Merck, Fluka, Acros Organics, or Sigma Aldrich. NMR spectra were measured on a Bruker AV-400 spectrometer (1H, 400.13 MHz; 13C, 100.50 MHz). Chemical shifts (δ) are given in parts per million (ppm) and coupling constants (J) in hertz (Hz). Elemental analyses were performed by the Microanalytical Laboratory of the TUM. Cyclic Voltammetry. Cyclic voltammograms were measured using a Gamry potentiostat employing a gastight three-electrode cell under an argon atmosphere. A platinum-disk electrode (1 mm diameter) was used as the working electrode and was polished before each measurement. A Ti/Pt electrode was used as the counter electrode. The potential was measured against Ag/AgCl (3.4 M KCl, 0.205 V vs NHE) with a scan rate of 100 mV/s. [Bu4N][PF6] (0.1 M in MeCN) was used as base electrolyte, and the concentrations of the complexes were about 5 mM. Spectroelectrochemistry. Spectroelectrochemical measurements were carried out on an AUTOLAB PGSTAT101: 2.131.0101 potentiostat connected with an AvaSpec-2048 spectrometer (Metrohm GmbH) employing a three-electrode cell (quartz cuvette, layer thickness 1 mm), which was implemented in a Faraday cage under an argon atmosphere in a glovebox (M. Braun). A platinum lattice was used as the working electrode with a mesh size of 177 μm. As counter electrode a platinum wire (0.3 mm) was used. An Ag/AgNO3 (0.01 mol/L, 0.544 V vs NHE32) reference electrode was applied. The measurements were performed on a 1 mM solution of complex 2 in acetonitrile with [Bu4N][PF6] as additional electrolyte (0.1 M) and a scan rate of 25 mV/s. For comparable results, the potentials were calibrated to the Ag/AgCl reference electrode. DFT Calculations. All calculations were performed with GAUSSIAN-0933 using the density functional/Hartree−Fock hybrid model Becke3LYP34 and the split valence double-ζ (DZ) basis set 631G*.35 The Ru atoms were described with a Hay−Wadt ECP with a DZ description of the valence electrons.36 No symmetry or internal coordinate constraints were applied during optimizations. All reported intermediates were verified as being true minima by the absence of negative eigenvalues in the vibrational frequency analysis. XYZ coordinates for all calculated compounds can be found in the Supporting Information. The values for the partial atomic charges come from the atomic polar tensor (APT) derived charges37 reported in the Gaussian-09 outputs of the frequency calculations. These charges are more basis set independent than the pure Mulliken charges. Synthesis of [LRu2(μ-Cl)Cl2(p-cymene)2](PF6)2 (1). A mixture of 200 mg (0.365 mmol) of [H3L](PF6)2, 126.8 mg (0.547 mmol, 1.5 equiv) of Ag2O and 223.4 mg (0.365 mmol, 1 equiv) of [Ru(pcymene)Cl2]2 was suspended in 25 mL of dichloromethane and stirred

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CONCLUSION New pyrazolato-bridged ruthenium(II) and rhodium(III) complexes were synthesized with the NHC precursor 3,5[bis(methylimidazolium-1-yl)methyl]-1H-pyrazole bis(hexafluorophosphate) and characterized via single-crystal Xray diffractometry. Depending on the solvent and the reaction conditions two different types of complexes can be synthesized using the metal precursors [Ru(p-cymene)Cl 2 ] 2 and [RhCp*Cl2]2. Because of the sterically demanding ligands on the metals (p-cymene and Cp*) a new type of complex can be synthesized, with only one bridging chloro ligand being substituted by the deprotonated pyrazole ring. The imidazolium rings are still protonated and are not coordinated to the metal centers. The other types of complexes are pyrazolato-bridged bis(NHC) complexes, which are unknown with Ru and Rh(III) as metal centers. Additionally, there is a remaining bridging chloro ligand in all synthesized complexes, which is not the case 4088


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the mixture was filtered through Celite and concentrated and 40 mL of diethyl ether was added to precipitate an orange solid. The solution was filtered off, and the residue was dried under vacuum to yield 131.8 mg (0.117 mmol, 32%) of [LRh2(μ-Cl)Cp*2Cl2](PF6)2 (4). Slow diffusion of diethyl ether into a solution of complex 4 in CH3CN yielded crystals suitable for XRD. 1H NMR (400 MHz, CD3CN): δ (ppm) 1.26 (s, 30H, (CH3)5C5), 3.82 (s, 6H, NCH3), 5.42 (d, 2H, 2 JHH = 14.0 Hz, CH2), 5.89 (d, 2H, 2JHH = 14.0 Hz, CH2), 6.95 (s, 1H, CHpyrazole), 7.29 (2H, dd, 3JHH = 1.0 Hz, CHimidazole), 7.74 (2H, dd, 3 JHH = 1.0 Hz, CHimidazole), 8.89 (s, 2H, NCHN). 13C NMR (100 MHz, CD3CN): δ (ppm) 156.31 (NCHN), 136.99 (CCHC), 124.77 (CH imidazole ), 123.30 (CH imidazole ), 112.61 (CH pyrazole ), 96.28 (C5(CH3)5), 48.21 (CH2), 37.02 (NCH3), 9.40 (C5(CH3)5). Anal. Calcd for C33H47Cl3F12N6P2Rh2: C, 35.08; H, 4.19; N, 7.44. Found: C, 34.67; H, 4.08; N, 7.43. Single-Crystal X-ray Structure Determination: Crystal Data and Details of the Structure Determination. Compound 1: formula C33H45Cl3N6Ru2·2PF6, Mr = 1124.18; crystal color and shape orange fragment, crystal dimensions 0.20 × 0.20 × 0.20 mm; crystal system monoclinic; space group C2/c (No. 15); a = 21.4773(4) Å, b = 32.3885(6) Å, c = 16.0489(6) Å, β = 107.321(1)°; V = 10657.6(5) Å3; Z = 8; μ(Mo Kα) = 0.847 mm−1; ρcalcd = 1.401 g cm−3; θ range 1.54− 25.37°; T = 173 K; data collected 76171; independent data (Io > 2σ(Io)/all data/Rint) 7934/9733/0.037; data/restraints/parameters 9733/0/604; R1 (Io > 2σ(Io)/all data) 0.0410/0.0541; wR2 (Io > 2σ(Io)/all data) 0.1048/0.1096; GOF = 1.032; Δρmax/min = 1.48/−0.62 e Å−3. Compound 2: formula C25H33ClN12Ru2 ·2F6P·2C2H3N, Mr = 1111.27; crystal color and shape golden needle, crystal dimensions 0.10 × 0.10 × 0.61 mm; crystal system orthorhombic; space group P212121 (No. 19); a = 12.4308(2) Å, b = 14.2124(3) Å, c = 24.7579(5) Å, V = 4374.02(15) Å3; Z = 4; μ(Mo Kα) = 0.917 mm−1; ρcalcd = 1.688 g cm−3; θ range 1.65−25.36°; T = 173 K; data collected 32003; independent data (Io > 2σ(Io)/all data/Rint) 7488/7987/0.025; data/ restraints/parameters 7987/0/551; R1 (Io > 2σ(Io)/all data) 0.0241/ 0.0275; wR2 (Io > 2σ(Io)/all data) 0.0572/0.0589; GOF = 1.022; Δρmax/min = 0.51/−0.34 e Å−3. Compound 3: formula C33H45ClN6Rh2·2F6P·CH2Cl2, Mr = 1141.89; crystal color and shape orange plate, crystal dimensions 0.05 × 0.25 × 0.30 mm; crystal system monoclinic; space group P21/c (No. 14); a = 19.4408(12) Å, b = 14.6358(9) Å, c = 17.6206(12) Å, β = 114.909(2)°, V = 4547.2(5) Å3; Z = 4; μ(Mo Kα) = 1.055 mm−1; ρcalcd = 1.668 g cm−3; θ range 1.15−25.45°; data collected 34415; independent data (Io > 2σ(Io)/all data/Rint) 7314/8084/0.028; data/ restraints/parameters 8084/0/544; R1 (Io > 2σ(Io)/all data) 0.0414/ 0.0460; wR2 (Io > 2σ(Io)/all data) 0.0978/0.1021; GOF = 1.058; Δρmax/min = 1.41/−0.69 e Å−3. Compound 4: formula C33H47Cl3N6Rh2·2PF6, Mr = 1129.88; crystal color and shape red fragment, crystal dimensions 0.18 × 0.42 × 0.71 mm; crystal system monoclinic; space group C2/c (No. 15); a = 24.8183(12) Å, b = 8.8631(4) Å, c = 22.7017(11) Å, β = 114.723(2)°, V = 4374.02(15) Å3; Z = 4; μ(Mo Kα) = 1.056 mm−1; ρcalcd = 1.655 g cm−3; θ range 1.81−25.35°; T = 173 K; data collected 44055; independent data (Io > 2σ(Io)/all data/Rint) 3994/4145/0.043; data/ restraints/parameters 4145/0/269; R1 (Io > 2σ(Io)/all data) 0.0287/ 0.0295; wR2 (Io > 2σ(Io)/all data) 0.0835/0.0843; GOF = 1.091; Δρmax/min = 0.78/−0.53 e Å−3. For more detailed information see the Supporting Information. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 921676 (1), CCDC-921677 (2), CCDC-921678 (3), and CCDC921679 (4). Copies of the data can be obtained free of charge on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax, (+44)1223-336-033; e-mail, [email protected]).

with exclusion of light at room temperature. After 36 h the mixture was filtered through Celite and 40 mL of diethyl ether was added to precipitate a dark orange solid. The solution was filtered off, and the residue was redissolved in 10 mL of dichloromethane. Addition of pentane precipitated a bright orange solid, which was collected by filtration and dried under vacuum to yield 274.3 mg (0.244 mmol, 67%) of [LRu2(μ-Cl)Cl2(p-cymene)2](PF6)2 (1). Single crystals suitable for XRD were obtained by cooling a saturated solution of complex 1 in dichloromethane. 1H NMR (400 MHz, CD2Cl2): δ (ppm) 1.11 (d, 6H, 3JHH = 6.9 Hz, CH(CH3)2), 1.15 (d, 6H, 3JHH = 6.9 Hz, CH(CH3)2), 1.51 (s, 3H, CH3cymene), 1.52 (s, 3H, CH3cymene), 2.75 (m, 2H, 3JHH = 6.9 Hz, CH(CH3)2), 3.95 (s, 6H, NCH3), 5.12 (2H, d, 3JHH = 6.0 Hz, CHcymene), 5.135 (2H, d, 3JHH = 5.8 Hz, CHcymene), 5.48 (2H, d, 3JHH = 6.0 Hz, CHcymene), 5.56 (2H, d, 3JHH = 5.8 Hz, CHcymene), 5.66 (2H, d, 2JHH = 14.2 Hz, CH2), 5.97 (2H, d, 2 JHH = 14.2 Hz, CH2), 6.81 (s, 1H, CHpyrazole), 7.14 (s, 2H, CHimidazole), 7.62 (s, 2H, CHimidazole), 8.71 (s, 2H, NCHN). 13C NMR (100 MHz, CD2Cl2): δ (ppm) 155.57 (NCHN), 136.79 (CCHC), 123.77 (CHimidazole), 123.71 (NCimidazole), 111.39 (CHpyrazole), 105.24 (CCH(CH3)2), 99.72 (CCH3), 85.95 (CHcymene), 83.19 (CHcymene), 81.55 (CHcymene), 81.54 (CHcymene), 50.23 (CH2), 37.21 (NCH3), 31.00 (CCH(CH3)2), 23.76 (CCH(CH3)2), 20.81 (CCH(CH3)2), 17.58 (CCH3). Anal. Calcd for C33H45Cl3F12N6P2Ru2: C, 35.26; H, 4.03; N, 7.48. Found: C, 34.99; H, 4.10; N, 7.63. Synthesis of [LRu2(μ-Cl)(MeCN)6](PF6)2 (2). An acetonitrile solution (15 mL) of 200 mg (0.365 mmol) of [H3L](PF6)2 was stirred with 126.8 mg (0.547 mmol, 1.5 equiv) of Ag2O at 80 °C with exclusion of light. After 2 h, a solution of 223.4 mg (0.365 mmol, 1 equiv) of [Ru(p-cymene)Cl2]2 in 10 mL of acetonitrile was added. The temperature was increased to 110 °C, and the mixture was stirred for 2 h at this temperature. Subsequently, the mixture was filtered through Celite and diethyl ether was added to precipitate an off-white solid. The solid was filtered off and dried under vacuum to yield 236.9 mg (0.230 mmol, 63%) of [LRu2(μ-Cl)(MeCN)6](PF6)2 (2). Slow diffusion of diethyl ether into a saturated CH3CN solution of complex 2 yielded crystals suitable for XRD. 1H NMR (400 MHz, CD3CN): δ (ppm) 2.20 (s, 12H, CH3CN), 2.501 (s, 6H, CH3CN), 3.87 (s, 6H, NCH3), 5.13 (s, 4H, CH2), 6.19 (s, 1H, CHpyrazole), 7.17 (d, 2H, 3JHH = 2.0 Hz, CHimidazole), 7.27 (d, 2H, 3JHH = 2.0 Hz, CHimidazole). 13C NMR (100 MHz, CD3CN): δ (ppm) 179.69 (NCN), 147.94 (Cpyrazole), 124.43 (CHimidazole), 122.95 (CHimidazole), 102.75 (CHpyrazole), 48.36 (CH2), 37.93 (NCH3). Anal. Calcd for C25H33ClF12N12P2Ru2·0.15CH3CN: C, 29.35; H, 3.26; N, 16.44. Found: C, 29.69; H, 3.25; N, 16.09. Synthesis of [LRh2(μ-Cl)Cp*2](PF6)2 (3). A solution of 200 mg (0.365 mmol) of [H3L](PF6)2 in 15 mL of acetonitrile was stirred with 126.8 mg (0.547 mmol, 1.5 equiv) of Ag2O at room temperature with exclusion of light. After 24 h, the solvent was removed and a solution of 225.6 mg (0.365 mmol, 1 equiv) of [Cp*RhCl2]2 in 20 mL of dichloromethane was added. After it was stirred for 24 h with exclusion of light, the mixture was filtered through Celite and diethyl ether was added to precipitate a yellow solid. The solid was dried under vacuum to yield 262.1 mg (0.248 mmol, 68%) of [LRh2(μ-Cl)Cp*2](PF6)2 (3). Slow diffusion of diethyl ether into a CH3CN solution of complex 3 yielded crystals suitable for XRD. 1H NMR (400 MHz, CD3CN): δ (ppm) 1.68 (s, 30H, C5(CH3)5), 3.59 (s, 6H, NCH3), 4.61 (d, 2H, 2 JHH = 15.0 Hz, CH2), 5.02 (d, 2H, 2JHH = 15.0 Hz, CH2), 6.37 (s, 1H, CHpyrazole), 7.01 (2H, d, 3JHH = 1.9 Hz, CHimidazole), 7.23 (2H, d, 3JHH = 1.9 Hz, CHimidazole). 13C NMR (100 MHz, CD3CN): δ (ppm) 167.98 (d, 1JCRh = 52.3 Hz, NCN), 153.39 (Cpyrazole), 125.28 (CHimidazole), 124.63 (CHimidazole), 109.36 (CHpyrazole), 100.90 (C5(CH3)5), 49.26 (CH 2 ), 37.77 (NCH 3 ), 10.76 (C 5 (CH 3 ) 5 ). Anal. Calcd for C33H45ClF12N6P2Rh2·0.5CH2Cl2: C, 36.60; H, 4.22; N, 7.64. Found: C, 36.68; H, 4.07; N, 7.67. Synthesis of [LRh2(μ-Cl)Cp*2Cl2](PF6)2 (4). An acetonitrile solution (15 mL) of 200 mg (0.365 mmol) of [H3L](PF6)2 and 42.3 mg (0.183 mmol, 0.5 equiv) of Ag2O was stirred with exclusion of light at room temperature. After 24 h a solution of 225.6 mg (0.365 mmol, 1 equiv) of [Cp*RhCl2]2 in 10 mL of acetonitrile was added. Stirring with exclusion of light was continued for another 16 h. Then, 4089


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ASSOCIATED CONTENT

S Supporting Information *

Figures, tables, and CIF files giving 1H and 13C NMR spectra and crystallographic data of the synthesized compounds as well as DFT computed data (geometries, energies, spin densities) and graphical expressions of more MOs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS S.A.R. thanks the Karl-Max von Bauernfeind Verein (Ph.D. fellowship) and the TUM graduate school for financial support. S.A.R. also thanks Dr. Osnat Younes-Metzler for support with CV measurements and Dr. Markus Finger for support with spectroelectrochemistry. M.D. thanks the Leibniz Rechenzentrum of the Bavarian Academy of Science for the provision of computing time.

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Article

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