Rhodium Complexes Bearing 1,10-Phenanthroline Analogue Bis

Article pubs.acs.org/Organometallics

Rhodium Complexes Bearing 1,10-Phenanthroline Analogue Bis-NHC Ligands Are Active Catalysts for Transfer Hydrogenation of Ketones Verena Gierz, Aurelija Urbanaite, Alexander Seyboldt, and Doris Kunz* Institut für Anorganische Chemie, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Germany S Supporting Information *

ABSTRACT: Synthesis of a new example of the pyridazine annelated bis(N-heterocyclic carbene) ligand 3b (vegi) bearing benzyl substituents is reported, as well as the synthesis of its cationic Rh(cod) complex 5b. The mechanism of formation of the [Rh(cod)(vegi)]+ complexes was investigated, showing a stepwise deprotonation of the imidazolium moieties via the mono-carbene imidazolium Rh(cod)Cl species 4. The [Rh(cod)(vegi)]+ complexes 5a,b show catalytic activity in the transfer hydrogenation of even sterically hindered ketones.

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INTRODUCTION Bidentate N-heterocyclic carbenes are not only useful chelating but also bridging ligands for Rh complexes.1,2 The coordination mode depends strongly on the flexibility of the ligand, influenced by the linker length between the NHC moieties, the bulkiness of the N substituents, and the readiness of rotation about the linker bonds. Even in the case of the directly N−N connected NHC ligand A (bitz) (Figure 1), an analogue of 2,2′-dipyridine, rotation about the N−N bond can lead to a bridging coordination mode for Rh and other metal complexes,3 while its isomer, the i-bitz ligand B, is chelating to a Rh(cod) fragment.4 Recently, we introduced the pyridazine annelated bis-carbene ligand 3a (vegiPr), in which rotation about the N−N bond is blocked due to annelation with pyridazine.5 Although a bridging coordination mode is found for coinage metal complexes, this 1,10-phenanthroline analogue ligand coordinates to a Rh(cod) fragment in a chelating fashion. Interestingly, the geometry of the resulting complex is almost identical with that of the respective 1,10-phenanthroline complex C with respect to bite angle, as well as the distance of the coordinating atoms.6 Another 1,10-phenanthroline analogue, carbene ligand D bearing only one carbene unit, was introduced by Monkowius this year.7 We now report an Nbenzyl-substituted example of the vegi ligand and its Rh(cod) complex and an intermediate of the formation of [Rh(cod)(vegi)]+ complexes as well as their catalytic properties in the transfer hydrogenation of ketones.

raphy. Formylation of the amine is achieved with formic acid activated by acetic anhydride. After chromatographic workup, the 1H NMR spectrum shows formamide 2 as a mixture of three stereoisomers (two symmetric, one unsymmetric) due to different conformations of the amide groups. Cyclization to the imidazolium moieties is carried out with POCl3 followed by anion exchange with KPF6 to obtain the bis-imidazolium salt 3b·2HPF6 in a 30% yield over three steps. Recently, we described the synthesis of complex 5a by in situ deprotonation of the imidazolium salt 3a·2HPF6 with 2 equiv of potassium acetate and reaction with [Rh(μ-Cl)(cod)]2 in acetonitrile at room temperature (Scheme 2).5b We now monitored the course of this reaction by NMR spectroscopy, which reveals two different signal sets after 2 h, while the signals of the starting material are no longer detectable. Due to the coupling pattern of the pyridazine signals, one signal set can be attributed to an unsymmetrical product and the second signal set to the product complex 5a (Figure 2). After further reaction the signals of the unsymmetrical intermediate 4 are gone and only signals of complex 5a can be detected. Using only 1 equiv of KOAc in acetonitrile-d3 led almost exclusively to formation of the unsymmetrical complex 4. In the 1H NMR spectrum a broad signal at 12.7 ppm is assigned to imidazolium proton 8H. The chemical shift is found at lower field than that of the bis-imidazolium salt 3a·2HPF6, which could be indicative of an intramolecular Cl−H interaction. Although no carbene signal was detected in the 13C NMR spectrum, formation of the unsymmetrical Rh complex 4 is indicated by three signals for the olefinic cod groups (102.5 and 101.4 ppm, each with 1JRhC = 7.1 Hz, and a very broad signal for two C atoms at 74.1 ppm (checked by 1H,13C HMBC)) and four signals for the CH2 groups of the cod ligand. The imidazolium carbon (C8) signal has a typical chemical shift of

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RESULTS AND DISCUSSION Synthesis of the imidazolium precursor 3b·2HPF6 of the biscarbene ligand vegi Bn (3b) starts from 3,6-bis(chloromethyl)pyridazine (Scheme 1). This compound reacts readily at room temperature with a 5-fold excess (10 equiv) of benzylamine upon formation of the bis-amine 1 along with benzylammonium chloride, which can be removed by filtration. The excess benzylamine is removed by column chromatog© 2012 American Chemical Society

Received: August 24, 2012 Published: October 30, 2012 7532

dx.doi.org/10.1021/om3008103 | Organometallics 2012, 31, 7532−7538

Organometallics

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Figure 1. 2,2′-Bipyridine (A, B) and 1,10-phenanthroline (D, 3a) analogue NHC ligands and the parent complex [Rh(COD)(phen)]+ (C).

Scheme 1

Scheme 2

Figure 2. Aromatic region of the 1H NMR spectrum (CD3CN) of the reaction of 3a·2HPF6 with [Rh(μ-Cl)(COD)]2 with KOAc after 2 h, showing the signals of the unsymmetric intermediate 4 and of the product 5a (4:5a = 2:3).

126.0 ppm (127.5 ppm in 3a5b). While the other signals of the carbene and imidazolium moiety in 4 differ only slightly, the signals of the pyridazine CC carbons (C1, 117.3 ppm; C2,

111.9 ppm) show a difference of 5 ppm. Although under the applied conditions (room temperature, KOAc as base) an exclusive single deprotonation of the bis-imidazolium salt and 7533


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The bonding situation of the ligand is identical with that of complex C6 (Figure 1). The small N−C−N angles at the carbene atoms of only 100.1° (mean) are lower than those typically found for free carbenes and are similar to the calculated values of the free vegi ligand (99.9°).5b Usually, upon coordination to a metal the N−C−N angle increases by about 2−3°. A small carbene angle is indicative of higher s character of the carbene σ orbital. In complex 5b this might be caused by the chelating coordination that requires a strong deviation of the metal−carbene bond from the (ideal) bisecting line of the N−C−N angle. In complexes 5a,b this deviation is 20°. As a higher s character leads to a more spherical shape of the σ orbital, a better overlap with the metal d orbitals might be achieved in this way. As already found for complex 5a, the C− C−C angles at the annelated rings (C1−C10a−C10 and C2− C2a−C3) are very large (140.2(2) and 140.7(2)°) and the carbene−carbene distance is very short (2.699 Å). Strikingly, the distance is almost identical with the nitrogen−nitrogen distance (mean 2.685 Å) in the 1,10-phenanthroline complex C (Figure 1). Due to the high s character of the carbene σ orbital in the Rh complex as well as in the calculated free ligand,5b we expect a reduced nucleophilicity and σ donor character of the ligand. However at the same time, the HOMO-1 shows π symmetry and is only 0.08 eV lower in energy than the σ type HOMO, most likely due to a stabilization by the conjugated π system of the heterocyclic moiety. Therefore, a π donating ability could contribute to the overall donor character of the carbene ligand and balance out any weak π acceptor character found for imidazole-derived carbenes. A rough evaluation of the overall electron-donor character of ligands can be made by comparing the stretching frequencies of the respective carbonyl complexes.9 We synthesized the respective Rh carbonyl complex 6 by bubbling CO through a solution of 5a in dichloromethane (Scheme 2). The solution changed from orange to yellow, and after workup the brownish solid contained the desired product along with 7% (NMR) of starting material. The 13C NMR spectrum showed a characteristic doublet for the carbonyl C atom at 188.1 ppm with a coupling constant of 1JRhC = 59.8 Hz and one for the carbene C atom at 163.0 ppm with a direct Rh−C coupling constant of 48.5 Hz. The solution of 6 in dichloromethane revealed the IR frequencies of the CO stretching modes at 2084 and 2029 cm−1. These wavenumbers are about 9 cm−1 higher than those found for the respective (ibitz) (B) Rh complex (Figure 1), and therefore ligand 3a is considered to be less electron donating than the i-bitz ligand.4 Comparable Rh carbonyl complexes of bipyridyl ligands show higher values for the stretching frequencies (2099 and 2042 cm −1 for [Rh(4,4′-dimethyl-2,2′-bipyridyl)(CO) 2 ] + in CH2Cl2), which confirms our expectations of a less electron donating bipyridyl ligand.10 Although we expect a nucleophilic character of the vegi ligand lower than that of the parent imidazolin-2-ylidene derived bis-carbene ligands (bis(NHC)), the CO stretching frequencies of 6 have values identical with those found for some of these bis-NHC Rh complexes.11 However, in this comparison, geometric influences on the stretching modes resulting from ligand substituents close to the metal center cannot be ruled out. In this respect a comparison with the mono-carbene phenanthroline analogue D7 (Figure 1) would be interesting. We chose the transfer hydrogenation of ketones in 2propanol to test our new complexes 5a,b in catalysis and to compare the results with those for other Rh complexes,

formation of the unsymmetrical complex is possible, all attempts to isolate complex 4 were unsuccessful. Upon workup at room temperature HCl elimination already takes place, which leads to formation of equimolar amounts of complex 5a and the bis-imidazolium salt 3a·2HPF6 along with [Rh(μCl)(cod)]2. The presence of acetic acid formed upon deprotonation seems to be crucial in stabilizing intermediate 4 by suppressing HCl elimination. Proof that complex 4 is an intermediate in the formation of complex 5a was gained by adding an additional 1 equiv of KOAc to the reaction mixture containing 4, which led exclusively to formation of complex 5a, as shown by NMR analysis. In the literature very few Rh(cod) or Ir(cod) complexes containing carbene ligands with pendant imidazolium moieties are known,2e,8 and only in one case was structural proof obtained.8a They all were found to be intermediates for dinuclear species2e,8a,b or oxidatively add to the imidazolium moiety to give the respective Rh(III) or Ir(III) HCl complexes.8c,d Considering the restricted flexibility of our ligand, oxidative addition to yield a Rh(III) species would be most likely; however, it is not observed. Synthesis of the rhodium complex 5b containing the benzylsubstituted ligand vegiBn was straightforward, on applying the conditions reported for complex 5a.5b Complex 5b was isolated in 61% yield as an analytically pure bright orange solid. The complex is symmetric, as indicated not only by the two singlets in the 1H NMR spectrum (CD2Cl2) for the heterocyclic moiety at 7.18 (3/10-H) and 7.10 ppm (1/2-H) and one singlet for the benzylic CH2 groups at 5.25 ppm but also by one singlet at 5.37 ppm corresponding to the four olefinic cod protons. This is confirmed in the 13C NMR spectrum with two signals for the cod C atoms at 87.6 (1JRhC = 9.0 Hz) and 31.5 ppm. The carbene signal is detected at 161.7 ppm with a typical Rh−C coupling constant of 54.2 Hz. If the NMR spectra are measured in acetonitrile-d3, formation of a new complex and free cod in a 1:1 ratio is observed, indicating an easy ligand exchange of cod by acetonitrile. By slow diffusion of pentane into a solution of 5b in dichloromethane we obtained single crystals suitable for X-ray analysis. The molecular structure (Figure 3) confirms the

Figure 3. Molecular structure of complex 5b with anisotropic displacement parameters given at the 50% probability level. Hydrogen atoms, cocrystallized dichloromethane, and the counterion are omitted for clarity.

chelating binding mode of the ligand at a square-planar coordination sphere of the rhodium atom. The two benzyl substituents point into the same direction perpendicular to the Rh coordination plane. The bite angle (79.94(8)°) and the carbene−Rh distances of 2.106(2) and 2.095(2) Å are similar to those found in complex 5a. 7534


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Table 1. Conditions and Results of the Catalytic Transfer Hydrogenation of Various Ketones with the [Rh(cod)(vegi)]+ Catalyst Precursors 5a,b

a

entry

cat.

1 2 3

5a 5a 5a

4 5 6

amt of cat. (mol %)

T (°C)

base (10 mol %)

ketone (1 mmol)

1 1 1

KOtBu KOH (85%) K2CO3

acetophenone acetophenone acetophenone

80 80 80

5a 5a

1 1

KOtBu KOtBu KOtBu

acetophenone cyclohexanone benzophenone

80 80 80

7

5a

1

KOtBu

pinacolone

80

8

5b

1

KOtBu

acetophenone

80

9

5b

1

KOtBu

pinacolone

80

10

5b

1

KOtBu

acetophenone

room temp

11

5b

0.1

KOtBu

acetophenone

room temp

time (h)

yield (%)a

6 7 6 9 16 6 6 8 25 6 24.5 72 6 8 6 9 24.5 16 24 48 24

98 98 71 94 7 >99 95 98 >99 35 86 98 96 98 15 21 38 57 81 97 3

determined by GC.

especially those with N-heterocyclic carbene ligands.12 As a benchmark substrate we used acetophenone and first optimized the reaction conditions (base, temperature, catalyst loading). For monitoring the reaction we chose both NMR techniques as well as GC analysis calibrated to a suitable internal standard. As has been sometimes described, a sample was filtered through silica gel with diethyl ether as solvent, the filtrate was dried in vacuo, and the residue was dissolved in CDCl3 for NMR analysis. However, strong deviations from the residual starting material and product yield with respect to the added standard (1,3,5-trimethoxybenzene, (bp 255 °C)) indicated that removing 2-propanol at the oil pump vacuum leads to severe loss of both acetophenone (bp 202 °C) and phenylethanol (bp 204 °C). After this procedure was repeated at 40 °C and 50 mbar at the rotary evaporator, we still found loss of both starting material and product in comparison with the added standard. At even weaker vacuum, however, complete removal of the 2-propanol could not be achieved, thus leading to larger errors in the integration due to the large solvent peak of the nondeuterated 2-propanol in the NMR spectrum. Therefore, we finally chose GC as the analytical method of choice, as no solvent removal is necessary after filtration of the sample over silica gel. The most reliable results were achieved if the GC instrument was calibrated to a solution containing the starting material and the product in presence of the standard (ndodecane). After evaluation of a suitable method for the analysis of the reaction mixture we tested the added base (Table 1, entries 1−3). A comparison of the course of the reactions is depicted in Figure 4. We found that with KOtBu the transfer hydrogenation proceeds fastest without any pronounced induction period at the beginning, while with

Figure 4. Comparison of different bases in the transfer hydrogenation of acetophenone with complex 5a (80 °C, 2-propanol (10 mL), acetophenone (1 mmol):base:5a = 100:10:1).

potassium carbonate the reaction rate is significantly slower. The activity is comparable to that of the most active bis-carbene Rh complex reported by Kühn.12j In contrast to literature reports,12d,f,j,k preactivation of the catalyst by reaction with the base in 2-propanol for 30 min at 80 °C and subsequent addition of the substrate did not lead to enhancement of the reaction rate. Without addition of the catalyst, a 7% yield was obtained after 16 h (Table 1, entry 4). The range of substrates is quite broad. While the reaction proceeds fastest with acetophenone and cyclohexanone (entries 1 and 5), also the sterically more demanding benzophenone gets completely reduced to diphenylmethanol within 25 h (entry 6). Even with pinacolone 98% 1-tert-butylethan-1-ol is obtained after 72 h (entry 7). To our knowledge this is the first reported example of a Rh-NHC catalyst suitable for this 7535


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substrate that is usually tested with the more active Ru-NHC13a and Ir-NHC13b,c catalysts. The nitrogen substituents of the vegi ligand also influence the stability and activity of the complexes. Figure 5 shows the

stretching frequencies confirm a stronger electron donor effect for the veginPr ligand than for 2,2′-dipyridine and a weaker donor effect than found for the triazole-derived bis-carbene ibitz. Complex 5a is a suitable catalyst precursor in the transfer hydrogenation of ketones at elevated temperatures, while the catalyst generated from complex 5b seems to be temperature sensitive. However, 5b can catalyze the transfer hydrogenation of acetophenone at room temperature.

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EXPERIMENTAL SECTION

Unless otherwise noted, all reactions were carried out under an argon atmosphere in dried and degassed solvents using the Schlenk technique. Pentane, acetonitrile, toluene, and dichloromethane were purchased from Sigma Aldrich, dried, and degassed using an MBraun SPS-800 solvent purification system. 2-Propanol was dried over CaCl2, distilled, and degassed prior to use. Liquid ketones for the catalyic studies were degassed. All other chemicals used were purchased from commercial suppliers and used without further purification. Air- and moisture-sensitive compounds were handled in an MBraun glovebox under argon. 3,6-Bis(chloromethyl)pyridazine and the veginPr chelated rhodium complex 5a were synthesized according to our published procedure.5b NMR spectra were recorded using a Bruker AVII+400 spectrometer. 1H and 13C{1H} NMR spectra were calibrated to TMS on the basis of the relative chemical shift of the solvent as an internal standard. Elemental analyses were carried out at the Institut für Anorganische Chemie at Tübingen University. Mass spectra were recorded using a Bruker Daltonics APEX II FT-ICR instrument. All measurements were carried out at Tübingen University. IR spectra were recorded on a Bruker Vertex70 instrument as dichloromethane solutions. Synthesis of 3,6-Bis(benzylaminomethyl)pyridazine (1). 3,6Bis(chloromethyl)pyridazine (1.88 g, 10.6 mmol) was dissolved in dry and degassed CH3CN (30 mL) under an argon atmosphere. Then, benzylamine (11.5 mL, 11.3 g, 106 mmol) was added and after 10 min precipitation of benzylammonium chloride started. The mixture was stirred for 2 h at room temperature, and diethyl ether (50 mL) was then added to complete precipitation. The salts were filtered off, and the filtrate was evaporated in vacuo to give an oily residue, which was purified by flash column chromatography (silica gel, ethanol/toluene 1/20). The product was obtained as an off-white oil (2.41 g, 71%) and stored at −30 °C under an argon atmosphere to avoid decomposition. 1 H NMR (DMSO-d6, 400.13 MHz): δ 7.73 (s, 2H, 4/5-H), 7.29−7.37 (m, 8H, 11/11′/12/12′-H), 7.21−7.25 (m, 2H, 13/13′-H), 3.95 (s, 4H, 7/7′-H), 3.71 (s, 4H, 9/9′-H), 2.84 (s, br, 2H, NH). 1H NMR (CD3CN, 400.13 MHz): δ 7.57 (s, 2H, 4/5-H), 7.29−7.36 (m, 8H, 11/11′/12/12′-H), 7.21−7.25 (m, 2H, 13/13′-H), 4.00 (s, 4H, 7/7′H), 3.77 (s, 4H, 9/9′-H), 2.30 (s, 2H, NH). 13C{1H} NMR (CD3CN, 100.61 MHz): δ 162.1 (C3/6), 141.7 (C10/10′), 129.3, 129.2 (C11/ 11′/12/12′), 127.8 (C13/13′), 127.3 (C4/5), 53.8 (C9/9′), 53.3 (C7/7′). FT-ICR HR-ESI+ (MeOH): m/z 319.191 43 [M + H]+, calculated 319.191 72 (relative error 0.91 ppm). Synthesis of 3,6-Bis(benzylformamidomethyl)pyridazine (2). Under an argon atmosphere a mixture of formic acid (grade 99+%, 10.3 mL) and acetic anhydride (1.6 mL, 17 mmol) was added at once under vigorous stirring to 3,6-bis(benzylaminomethyl)pyridazine (1; 2.35 g, 7.39 mmol). Warming of the reaction mixture and weak gas formation were observed. The solution was stirred at room temperature for 1.5 h. To hydrolyze any possibly unreacted mixed anhydride, water (5 mL) was added and the solvent mixture evaporated in vacuo. The oily residue was purified by flash column chromatography (silica gel, acetone/petroleum ether 1.5/1). After evaporation of the solvent the product was obtained as a yellowish, oily mixture of three conformers (1.66 g, 60%). The product still containing traces of formic acid was used without further purification. 1 H NMR (DMSO-d6, 400.13 MHz) (mixture of three conformers): δ 8.51, 8.50, 8.49, 8.48 (each s, 6H, CHO), 7.54 (s, 2H, 4/5-H), 7.53 (d, 1H, 3JHH = 8.4 Hz, 4/5-H), 7.39 (d, 1H, 3JHH = 8.4 Hz, 4/5-H), 7.39 (s, 2H, 4/5-H), 7.20−7.38 (m, 24H, 11/11′/12/12′-H), 7.16 (d, br,

Figure 5. Comparison of the benzyl- and propyl-substituted rhodium complexes 5a,b in the transfer hydrogenation of acetophenone (80 °C, 2-propanol (10 mL), acetophenone (1 mmol):KOtBu:cat. = 100:10:1).

course of the reaction with the catalyst precursors bearing the N-n-propyl (3a)- and the N-benzyl-substituted vegi ligand (3b). While with complex 5a a longer induction period is observed, it seems to outperform complex 5b at the end of the reaction, while 5b shows a high initial reaction rate but seems to lose reactivity toward the end of the transfer hydrogenation of acetophenone. Transfer hydrogenation of pinacolone confirms this hypothesis. While 5a leads to 35% yield after 6 h and 86% after 25 h, the reaction with catalyst precursor 5b is more than twice as slow, giving 15% yield after 6 h and 38% after 25 h (entries 7 and 9). We suspect that the N-benzyl substituent, a well-established protecting group for amines that can be removed by hydrogenation,14 is reactive at elevated temperature, resulting in catalyst deactivation. Therefore, we reacted acetophenone in the presence of catalyst precursor 5b at room temperature: 57% yield was obtained after 16 h (vs 98% after 8 h at 80 °C) and 97% after 48 h (entry 10). At room temperature the catalyst was not fully soluble; therefore, we reduced the catalyst loading to 0.1 mol %, but the reaction rate was retarded and led to only 3% yield after 24 h (entry 11). In the literature there is only one report of a successful transfer hydrogenation of acetophenone with a Rh(NHC) complex at room temperature by Crabtree and Peris, who obtained a yield of 17% after 10 h. However, a quantitative comparison is difficult, as they used 50 mol % of base.2a The catalytic experiments show that complexes 5a,b are active catalyst precursors in the transfer hydrogenation of various ketones, including the sterically demanding pinacolone. While the catalyst precursor 5a bearing N-n-propyl substituents is suitable for reactions at 80 °C, the catalyst generated from precursor 5b containing vegiBn ligand 3b is likely temperature sensitive; however, it shows good activity at room temperature.

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CONCLUSION Synthesis of a new vegi type ligand bearing N-benzyl subsitutents is reported. The respective Rh(cod) complex 5b could be synthesized and structurally characterized. In situ characterization of the unsymmetric mono-carbene complex 4 confirms a stepwise mechanism for the formation of [Rh(cod)(vegi)]+ complexes. Complex 5a was subjected to ligand exchange with CO, leading to carbonyl complex 6. The IR 7536


Organometallics

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6H, 3JHH = 7.0 Hz, 13/13′-H), 4.68, 4.66, 4.58, 4.56 (each s, 12H, 7/ 7′-H), 4.51, 4.36 (each s, 12H, 9/9′-H). 13C{1H} NMR (DMSO-d6, 100.61 MHz): δ 163.7, 163.5, 163.5, 163.3 (CHO), 158.2, 158.1, 157.9, 157.8 (C3/6), 136.5 (C10/10′), 128.7, 128.5, 128.1 (2 signals), 128.0 (2 signals), 127.8, 127.3 (C11/11′/12/12′13/13′), 126.7 (2 signals), 126.4, (C4/5), 50.7, 50.6 (C9/9′), 50.0, 49.9 (C7/7′), 45.1, 45.0 (C9/9′), 44.9, 44.8 (C7/7′). FT-ICR HR-ESI+ (CH3CN + NaCl): m/z 397.163 233 [M + Na]+, calculated 397.163 497 (relative error 0.67 ppm). Synthesis of the Benzyl-Substituted Bis-Imidazolium Salt 3b·2HPF6. To 3,6-bis(benzylformamidomethyl)pyridazine (2; 1.45 g, 3.89 mmol) were added dry toluene (100 mL) and POCl3 (0.8 mL, 1.3 g, 8.6 mmol) under an argon atmosphere. The mixture was stirred overnight at 85 °C. After evaporation of the solvent, the resulting brown oil was washed three times (20 mL each) with diethyl ether. The residue was dissolved in water (40 mL) and treated with a saturated aqueous solution of KPF6 (1.43 g, 7.77 mmol) at 50 °C. A sudden precipitation was observed. The suspension was stirred at 50 °C for 2 h and slowly cooled to room temperature for full precipitation of the product. The solid was then filtered off and washed two times with water (20 mL each), three times with dichloromethane (20 mL each), and five times with pentane (20 mL each). After it was dried in vacuo at 40 °C for several hours, the bis-imidazolium salt was obtained as a white solid (1.93 g, 66%). 1H NMR (DMSO-d6, 400.13 MHz): δ 10.64 (s, br, 2H, 5/8-H), 8.56 (s, 2H, 3/10-H), 7.73 (s, 2H, 1/2-H), 7.43−7.52 (m, 10H, 13/13′/14/14′/15/15′-H), 5.84 (s, 4H, 11/11′H). 13C{1H} NMR (DMSO-d6, 100.61 MHz): δ 133.8 (C12/12′), 129.3 (C15/15′), 129.2, 128.7 (C13/13′ and C14/14′), 127.8 (C5/8), 124.2 (C2a/10a), 118.1 (C3/10), 115.0 (C1/2), 54.0 (C11/11′). Anal. Calcd for C22H20N4P2F12: C, 41.92; H, 3.20; N, 8.89. Found: C, 42.06; H, 2.70; N, 8.94. FT-ICR HR-ESI+ (CH3CN): m/z 485.132 518 [M + PF6]+, calculated 485.132 429 (relative error 0.18 ppm). NMR Experiment: in Situ Generation of Complex 4. The bisimidazolium salt 3a·2HPF6 (10.0 mg, 18.7 μmol), [Rh(μ-Cl)(cod)]2 (4.6 mg, 9.3 μmol), and KOAc (1.8 mg, 19 μmol) were suspended in CD3CN (0.5 mL). A yellow suspension was formed, consisting of precipitated KPF6 and the unsymmetrical complex 4, as indicated in the 1H NMR spectrum after 3 h. 1H NMR (CD3CN, 400.13 MHz): δ 12.63 (s, br, 1H, 8-H), 7.83 (d, 1H, 4JHH = 1.8 Hz, 10-H), 7.67 (s, 1H, 3-H), 7.25 (d, 1H, 3JHH = 10.3 Hz, 1-H), 7.12 (d, 1H, 3JHH = 10.3 Hz, 2-H), 5.18−5.28 (m, 2H, CHcod), 4.96 (ddd, 2JHH = 13.2 Hz, 3JHH = 8.8 Hz, 3JHH = 6.3 Hz, 1H, 11-H), 4.59 (ddd, 2JHH = 13.2 Hz, 3JHH = 8.7 Hz, 3JHH = 6.4 Hz, 1H, 11-H), 4.45−4.58 (m, 2H, 11′-H), 3.61− 3.69 (m, 1H, CHcod), 3.49−3.57 (m, 1H, CHcod), 2.37−2.60 (m, 4H, CH2cod), 2.02−2.23 (m, 8H, CH2cod, 12/12′-H), 1.08 (t, 3H, 3JHH = 7.4 Hz, 13-H), 1.05 (t, 3H, 3JHH = 7.4 Hz, 13′-H). 13C{1H} NMR (CD3CN, 100.61 MHz): δ 127.2 (C10a), 127.1 (C2a), 126.0 (C8), 119.0 (C3), 117.9 (C10), 117.3 (C1), 111.9 (C2), 102.5 (d, 1JRhC = 7.1 Hz, CHcod), 101.4 (d, 1JRhC = 7.1 Hz, CHcod), 74.1 (s, br, CHcod), 56.2 (C11), 54.2 (C11′), 34.1 (CH2cod), 32.4 (CH2cod), 30.0 (CH2cod), 29.4 (CH2cod), 25.2 (C12), 24.5 (C12′), 11.5 (C13), 10.9 (C13′). A signal for the carbene atom C5 was not observed. Synthesis of [Rh(cod)(vegiBn)]PF6 (5b). The benzyl-substituted bis-imidazolium salt 3b·2HPF6 (100 mg, 0.159 mmol), KOAc (31.1 mg, 0.317 mmol), and [Rh(μ-Cl)(cod)]2 (39.1 mg, 0.0793 mmol) were suspended in dry and degassed CH3CN (8 mL). The mixture was stirred at room temperature and turned orange. After 80 min, the mixture was filtered and the orange solution dried in vacuo. The residue was extracted with CH2Cl2 (15 mL) and the solution evaporated to dryness. The residue was washed with small amounts of THF and pentane (5 mL) and dried in vacuo to give the product as an orange solid (66.9 mg, 61%). Crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of pentane into a solution of the complex in CH2Cl2. 1H NMR (CD2Cl2, 400.13 MHz): δ 7.39− 7.47 (m, 6H, 14/14′/15/15′-H), 7.23−7.28 (m, 4H, 13/13′-H), 7.18 (s, 2H, 3/10-H), 7.10 (s, 2H, 1/2-H), 5.37 (s, br, 4H, CHcod), 5.25 (s, 4H, 11/11′-H), 2.25−2.39 (m, 4H, CH2cod), 2.12−2.22 (m, 4H, CH2cod). 13C{1H} NMR (CD2Cl2, 100.61 MHz): δ 161.7 (d, 1JCRh = 54.2 Hz, C5/8), 135.2 (C12/12′), 129.9 (C14/14′), 129.6 (C15/15′), 127.8 (C13/13′), 122.2 (C2a/10a), 117.6 (C3/10), 114.5 (C1/2),

87.6 (d, 1JCRh = 9.0 Hz, CHcod), 54.5 (C11/11′), 31.5 (CH2cod). Anal. Calcd for C30H30F6N4PRh: C, 51.89; H, 4.35; N, 8.07. Found: C, 51.45; H, 4.21; N, 7.96. FT-ICR HR-ESI+ (CH3CN): m/z 549.152 469 [M]+, calculated 549.152 003 (relative error 0.85 ppm) Synthesis of [Rh(CO)2(vegiPr)] PF6 (6). CO gas was passed through a solution of [Rh(cod)(vegiPr)]PF6 (5a; 30.0 mg, 50.1 μmol) in CH2Cl2 (25 mL) for 3 h. Immediately, the color changed from orange to yellow. After evaporation of the solvent, the solid obtained was twice recrystallized from CH2Cl2 (5 mL) with pentane (20 mL). The brownish solid was filtered off, washed with pentane (5 mL), and dried in vacuo (26.0 mg). The product complex 6 still containing 7% (calculated from the 1H NMR spectrum) of unreacted starting material [Rh(cod)(vegiPr)]PF6 (5a) was analyzed without further purification. 1H NMR (CD2Cl2, 400.13 MHz): δ 7.60 (s, 2H, 3/10-H), 7.33 (s, 2H, 1/2-H), 4.37 (t, 3JHH = 7.3 Hz, 4H, NCH2), 2.04 (sext, 3 JHH = 7.4 Hz, 4H, CH2), 1.04 (t, 3JHH = 7.4 Hz, 6H, CH3). 13C{1H} NMR (CD2Cl2, 100.61 MHz): δ 188.1 (d, 1JRhC = 59.8 Hz, CO), 163.0 (d, 1JRhC = 48.5 Hz, C5/8), 122.4 (C2a/10a), 118.7 (C3/10), 115.2 (C1/2), 54.6 (NCH2), 26.3 (CH2), 11.2 (CH3). FT-ICR HR-ESI+ (CH3CN): m/z 414.087 02 [M − CO + CH3CN]+, calculated 414.079 56 (no internal calibration was used; this explains the quite large deviation from the calculated value of 18 ppm). IR (2% in CH2Cl2, cm−1): ν 2084 (CO), 2029 (CO). Transfer Hydrogenation of Ketones with Complexes 5a,b. Catalyst 5a or 5b (0.01 mmol, 1 mol %), n-dodecane as internal standard for GC (0.5 mmol), base (0.1 mmol, 10 mol %), and substrate (1 mmol) were added to a Schlenk tube consecutively. All compounds were weighed and then added to the Schlenk tube as a solution or a suspension in 2-propanol. Acetophenone was added via Eppendorf pipet. The mixture was suspended in 2-propanol (10 mL combined), and the Schlenk tube was closed with a Teflon septum and placed in a preheated oil bath. The mixture was then stirred at the given temperature. After the time given, 1 mL samples were taken from the reaction mixture and filtered through a short pad of silica gel with diethyl ether (4 mL). The filtrate was analyzed by GC on a Trace GC Ultra chromatograph from Thermo Scientific equipped with an FID detector using a Hewlett-Packard HP-INNOWax column (crosslinked poly(ethylene glycol), length 30 m, diameter 0.25 mm, film thickness 0.5 μm). 2-Phenylethanol and pinacolyl alcohol were analyzed with 80 kPa constant hydrogen pressure (H2 5.0) and cyclohexanol and benzhydrol with 100 kPa. Other conditions: injector temperature, 280 °C; oven start temperature, 60 °C; ramp, 10 °C/min to 180 °C (2-phenylethanol and pinacolyl alcohol) or 200 °C (cyclohexanol and benzhydrol); end temperature was held until all product signals were detected; FID temperature, 250 °C. Crystallographic Data. Data collection for 5b was performed on a STOE IPDS II diffractometer using Mo Kα radiation (λ = 0.710 73 Å) and a graphite monochromator. Corrections for absorption effects were applied using MULABS.15 The structure was solved by direct methods using SHELXS and SHELXL for structure solution and refinement.16 Further details of the refinement and crystallographic data are given in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

A CIF file giving experimental and crystal data for 5b and figures giving NMR spectra of all new compounds. This material is available free of charge via the Internet at http:// pubs.acs.org. CCDC 886517 also contains supplementary crystallographic data for compound 5b. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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Corresponding Author

*Tel: +49 7071 29-72063. Fax: +49 7071 29-2436. E-mail: [email protected]. 7537


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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Deutsche Forschungsgemeinschaft (KU1437/2-3), the BMBF/MWK-BW (Professorinnenprogramm), the Landesgraduiertenförderung BW (fellowship for V.G.), and the Erasmus Programme of Vilnius University, Lithuania (LT VILNIUS01, internship for A.U.) for financial support. We also acknowledge Theresa Schmidt for assisting in the syntheses.

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REFERENCES

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