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Toward Tunable Immobilized Molecular Catalysts: Functionalizing the Methylene Bridge of Bis(N-heterocyclic carbene) Ligands [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) A new immobilization mode for methylene-bridged bis(NHC) (NHC= N-heterocyclic carbene) ligand systems is presented, allowing fine-tuning of the steric and electronic properties of the bidentate ligand. For example, four hydroxymethyl-functionalized imidazolium salts (1a-c and 2a) and three bis(NHC) Pd complexes 3a, 3b, and 4a are described. The chloro-functionalized bis(NHC) Pd complex 4a was obtained quantitatively by conversion of the hydroxyl substituent of complex 3a into a chloro substituent by employing thionyl chloride. All three bis(NHC) complexes 3a, 3b, and 4a were characterized by NMR spectroscopy, elemental analysis, mass spectrometry, and single-crystal X-ray diffraction. Two different synthetic routes were applied to immobilize the bis(NHC) Pd complex 3a on polystyrene. The obtained heterogeneous catalyst 5b was utilized for Suzuki-Miyaura cross-coupling reactions and could be recycled without significant activity loss in four runs. Furthermore, the water-soluble homogeneous catalyst 3a itself could be employed for Suzuki-Miyaura cross-coupling reactions in water. Keywords : bridging ligands · carbene ligands · cross- coupling · immobilization · palladium Introduction Bidentate methylene-bridged bis(NHC)s (NHC = N-heterocyclic carbene) are a unique type of chelating NHC ligand,[1-7] which improve the complex stability compared to monodentate NHCs by forming a six-membered metallacycle. The respective bis(NHC) palladium(II) complex was even found to be stable in trifluoroacetic acid solution[8-10] and recently Strassner et al. employed this ligand class under these harsh conditions for the oxidation of propane using air as the oxidant.[11] Generally, these bis(NHC)s have been attached to a variety of metal cen- ters, as NHCs are a versatile ligand class for a wide range of metals.[2, 7] This not only demonstrates the versatility of this ligand motif from a synthetic point of view, but also shows the good applicability of its stable complexes both for catalytic ap- plications (e.g. , CH activation,[11] hydrosilylation,[12] cross-cou- pling,[13] and transfer hydrogenation[14]) and material develop- ment (e.g. , organic light-emitting diodes[15, 16] and metal-organ- ic frameworks[17]). However, methylene-bridged bis(NHC) compounds are still far from being fully explored, especially in how to introduce special properties into the ligand system and to immobilize it on carrier material without sacrificing its stability and versatili- ty. Most of the studies concerning the functionalization and immobilization of such bis(NHC) systems have focused on wingtip modification, which is so far the easiest access towards immobilized catalysts (Figure 1 a).[13,16,18] Unfortunately, the wingtip immobilization intrinsically limits the possible substitu- ent modifications. These, however, are very important for fine- tuning the properties of the complexes by changing the steric hindrance and the electronic characteristics.[19-23] Furthermore, this approach can change the behavior of the molecular cata- lyst in comparison with the respective homogeneous com- pound owing to the proximity of the metal center to the solid support.[24] More recently, modifying the imidazole backbone of the ligand (Figure 1 b) has also been attempted, but this approach is still in its infancy and apparently not easy.[25, 26] The work presented herein focuses on developing a method to functionalize the methylene bridge of the bis(NHC) system (Figure 1 c) while retaining the elegant modular approach of the known methylene-bridged bis(NHC)s. Up until now there exists only a patent by K^hl[27] and a method by Peris et al. ,[28-30] which yield bis(NHC) ligand with a functionalized methylene bridge. However, these methods still have limita- tions, particularly when it comes to wingtip variation, because of their special synthetic routes, especially if using bis(imidazol- yl) ketone and aldehyde to synthesize tripodal bis(imidazolyli- dene) ligand. Therefore, a more versatile approach to achieve a tunable methylene-bridged bis(NHC) system is required. Herein, a facile and useful method to obtain functionalized methylene-bridged bis(NHC) systems is presented. Four hy- droxyl-functionalized bisimidazolium salts 1a-c and 2a were synthesized. Moreover, the hydroxyl group of complex 3a could be transferred to the chloro-functionalized bis(NHC) Pd complex 4a. Additionally, two methods were applied to syn- thesize the polymer-supported Pd catalyst 5b. Catalyst 3a and catalyst 5b were used for Suzuki-Miyaura reactions in aqueous phase and organic solvents, respectively. When applied for the cross-coupling of aryl bromides with phenylboronic acid in tol- uene, compound 5bcould be recycled. Results and Discussion Synthesis and characterization of ligands Ligands 1a-c with hydroxyl group bridge-functionalized bisi- midazolium salts were prepared by the nucleophilic substitu- tion of dichloroethanol with three substituted imidazoles (1- methylimidazole, 1-isopropylimidazole, and 1-tert-butylimida- zole) in toluene (Scheme 1). Dichloromethane derivatives are normally difficult to react relative to their iodine and bromine analogues, so a higher reaction temperature and longer reac- tion time tend to be necessary. In our case using toluene as solvent, we could obtain the products in a cleaner and safer way (110-130 8C, pressure tube) compared with the use of sol- vents such as CH3CN and THF. 1H and 13C NMR spectra, as well as elemental analysis, con- firmed the synthesis of ligands 1a-c. HMQC spectra confirm that the signals at 6.47 (1a), 6.42 (1b), and 6.41 ppm (1c) are those of the hydroxyl protons and therefore display the suc- cessful synthesis of the functionalized methylene-bridged bisi- midazolium salts, which was furthermore confirmed by crystal- lographic evidence for 1b(see the Supporting Information). All three ligands are stable in air, but moisture sensitive. Moreover, we obtained ligand 2a in good yield by anion exchange of li- gand 1a with ammonium hexafluorophosphate. Note that li- gand 2a dissolves well in CH3CN, which is not the case for li- gands 1a-c. Synthesis and characterization of bis(carbene) complexes With the ligand precursors in hand, further complexation of the hydroxyl-functionalized bisimidazolium salt system was tested. As shown in Scheme 2, compounds 1a and 1b were used to synthesize the bis(NHC) Pd complexes according to the typical synthesis method by using Pd(OAc)2 as the metal precursor.[31, 32] The palladium complexes 3a and 3b were ob- tained in good yield by heating the reactants in dimethyl sulf- oxide (DMSO) at 55 8C for 24 hours. Complexes 3a and 3b were characterized by means of NMR spectroscopy, elemental analysis, mass spectrometry, and single-crystal X-ray diffraction studies. The 1H NMR spectra of 3a and 3b show the absence of the downfield-shifted NCHN resonance, thus indicating the deprotonation of the acidic protons of 1a and 1b. The 13C NMR spectra provide further evidence of the metalation, as seen by the signals at d = 154.99 and 154.92 ppm, assigned to the Pd-Ccarbene carbon atoms of 3a and 3b, respectively, which are consistent with those observed in previous studies.[10] Ob- taining complexes 3a and 3b shows that the acidity of the hy- droxyl group is lower than that of the proton located at the designated carbene carbon atom.[33, 34] Further 1H NMR experi- ments employing an excess of different bases, such as NaOAc, K2CO3, and KOH, to react with 1a confirmed this result, as no deprotonation of the hydroxyl group was observed. Consequently, the hydroxyl group would not influence the reactivity of the imidazolium moieties in regard to the complexation. We consider the hydroxyl as a synthetically flexible group, which could be transformed to other functional groups such as carbonyl, nitrile, and halogen, so we envisioned that a fur- ther variation of the hydroxyl group on complex 3a should be possible. Therefore, thionyl chloride was chosen as the chlori- nation reagent to react with complex 3a, with several drops of dimethylformamide (DMF) as a catalyst (Scheme 2).[35] As ex- pected, the chloro-functionalized bis(NHC) Pd complex 4a was synthesized successfully from complex 3a and obtained in quantitative yield. Moreover, this synthesis proves that the complexes 3a and 4a are both stable even when refluxed in thionyl chloride. Complex 4a was also fully characterized by NMR spectroscopy, elemental analysis, mass spectrometry, and single-crystal X-ray diffraction studies. The 1H NMR spectrum of 4a shows the absence of the hydroxyl group as well as all the signals being shifted to the high field relative to complex 3a, thus confirming the substitution by chlorine. Single crystals were obtained by slow vapor diffusion of tet- rahydrofuran (THF) into a DMSO solution of the complexes 3a, 3b (see the Supporting Information), and 4a. Figure 2 shows the molecular structures of 3a and 4a together with selected distances and angles. As can be seen in Figure 2 on the bottom right, the basic structural features of the functionalized complexes 3a and 4a are retained relative to the unsubstitut- ed methylene-bridged complex 3'.[10] The geometrical parame- ters of the central six-membered palladacycle are almost iden- tical within the range of statistical uncertainty, as in the unsub- stituted case the metallacycle shows a boat conformation in the solid state. Interestingly, the substituents are located at the axial posi- tion of the methylene bridge, both for 3a and 4a, such that one hydrogen atom of the hydroxymethyl group (or chloro- methyl group, respectively) is pointing towards the metal (Figure 3). The distance between this H atom and the Pd center is 2.7546(2) ^ for 3a (2.7684(2) ^ for 4a) and the angle is 1268, which corresponds to an anagostic interaction.[36, 37] This goes along with the downfield shift of the signal for the CH2 group in the 1H NMR spectrum from 4.38 (1a)to 4.65 ppm. Moreover, the variable-temperature 1H NMR studies of 3a and 4a (see the Supporting Information) show the as- sumed rotation can be stopped below ^10 8C for both com- pounds, which confirms the proton-Pd interaction preventing the rotation of the hydroxymethyl group and chloromethyl group, respectively. DFT calculations of 3a and 4a in the gas phase were con- ducted to get a better idea of the energetic nature of possible conformational isomers and to ensure that packing effects in the single-crystalline material do not cause the proton-Pd in- teraction. Therefore, the axis between the bridge-head sub- stituent's carbon atom C10 and the methylene bridge carbon atom C4 is rotated for both 3a and 4a to identify possible ground states with either proton-Pd contacts or oxygen-Pd and chlorine-Pd interactions, respectively, and the correspond- ing rotational barriers. For both complexes an energetic mini- mum for the previously described anagostic proton-Pd contact is confirmed ; however, rotation around the C10-C4 axis in the case of 3a does not reveal an additional stable conformational isomer, but rather shows a rotational barrier of 8.8 kcal mol^1 (all energies given as Gibbs free energies). Interestingly, for 4a, in addition to the favored conformer with proton-Pd interac- tion, a rotational conformer with a chlorine-Pd contact was also identified. It is 9.0 kcal mol^1 higher in energy and the ro- tational barrier between the two conformers of 4a adds up to 10.8 kcal mol^1. Additionally, for both complexes the energetic states with the substituents standing either towards or away (exo) from the Pd center (3 a exo and 4a exo) were deter- mined by DFT calculations. The results showed that the confor- mation that allows proton-Pd interaction is favored by 5.8 kcal mol^1 for 3aand 6.4 kcal mol^1 for 4a. Thus, all energy values correspond well to the X-ray diffraction results (see the Sup- porting Information for schematic structures and their ener- gies). Synthesis and characterization of polymer-supported catalysts With the functionalized com- plexes in hand, we decided to investigate a new general method to immobilize our hy- droxyl-functionalized bis(NHC) system, since we considered that it has good prospects for catalyt- ic applications. First, two model reactions were conducted to confirm that our bis(NHC) system was able to attach to the solid surface through the hydroxyl group (Scheme 3). After several types of possible reactions with the hydroxyl group had been tested, benzoyl chloride was finally found to react effectively with both the ligand 2a and com- plex 3a by an esterification. The reactions proceeded at room temperature using CH3CN or N-methyl-2-pyrrolidone (NMP) as solvents. The target compounds 2b and 4b were obtained in quantitative yields and confirmed by NMR spectroscopy, IR spectroscopy, and mass spectrometry. The reason for using these polar solvents in the reactions was to dissolve the high- est possible amount of ligand 2a and complex 3a, such that the subsequent heterogeneous immobilization process would also work under the same reaction conditions. As the model reaction processed well, benzoyl chloride-func- tionalized polystyrene (loading of benzoyl chloride, 0.8- 1.0 mmol g^1) was chosen as the solid support to conduct the immobilization. As Scheme 4 shows, two routes were tested to obtain the supported Pd catalyst 5b. With respect to route 1, complex 3a was used to react directly with benzoyl chloride- functionalized polystyrene to synthesize compound 5b. In this reaction, complex3a(0.05mmol, 0.02g), a catalytic amount of dry pyridine, and benzoyl chloride-functionalized polystyrene (50 mg) were suspended in dry NMP (1 mL). The reaction mix- ture was then stirred for 24 hours. A variation of reaction tem- perature from room temperature to 150 8C (increased gradually in 30 8C steps) was then conducted to determine the highest possible loading of the complex. However, atomic absorption spectroscopy (AAS) analysis showed that all obtained com- pounds contained less than 0.1 % of Pd, which was far below the theoretically highest possible Pd loading of 6.5 %. On em- ploying different temperatures and different solvents, such as acetonitrile and pyridine, no Pd content was detected from the obtained compounds. Therefore, we considered that the im- mobilization route 1 was not suitable for complex 3a, because of the poor solubility of complex 3a in most organic solvents. Nevertheless, for complexes with the general ligand motif of 3a, which have different wingtip moieties or in general pos- sess a better intrinsic solubility, this immobilization route may likely be applicable. To circumvent the solubility issues of immobilization route 1, we tried another method to synthesize the targeted heteroge- neous catalyst 5b (route 2, Scheme 4). In this synthetic ap- proach, ligand 2a was immobilized onto the polymer prior to complexation with the Pd precursor. At the first ligand immo- bilization step, benzoyl chloride-functionalized polystyrene (0.05 g) and a catalytic amount of pyridine were added to a so- lution of ligand 2a (0.10 mmol, 0.05 g) in dry acetonitrile. The reaction mixture was stirred and heated for 24 hours. To ensure that the highest possible amount of ligand was at- tached to the polymer, a variation of the reaction temperature was also performed at this step (see the Supporting Informa- tion). As shown by the elemental analysis results, the highest nitrogen content (1.93 %) of the obtained polymer was ach- ieved after the reaction at 100 8C, whereas a decrease of nitro- gen content (0.79 %) was observed on further elevating the temperature to 120 8C, which may stem from a possible de- composition of the ligand precursor at higher temperatures. Therefore, 100 8C was selected as the optimized temperature for further large-scale synthesis. The theoretically highest possi- ble nitrogen content of 3.69 % is difficult to obtain in the actual immobilization reaction, most likely because the benzoyl chloride groups attached to the polymer resin are presumably not completely exposed to the surface if the reaction is per- formed in a slurry. Further up-scaling of the reaction (1.0 g of benzoyl chloride- functionalized polystyrene resin was used) led to a decreased nitrogen content (1.40%) of the ligand-loaded polymer 5a, which we believe is acceptable, since still 40 % of the theoreti- cally reactive positions were substituted. In general, with re- spect to the low absolute values, the error of the elemental analysis of the nitrogen content is comparably high. Hence, we tried to get independent proof and quantification of the amount of the immobilized ligand 2a. The fluorine content was found to be 1.9 %, which was lower than expected from the corresponding nitrogen content (5.7 %). However, consider- ing that the counter anions PF6^ of the ligand can be ex- changed by chloride anions, which are generated from ester bond formation during the reaction, a reduction of the fluorine content is likely to happen. In addition, the solid-state 13C cross-polarization magic angle spinning (CP-MAS) NMR spectrum of the ligand-loaded polymer 5a supports the pres- ence of the methylene-bridged bis(imidazolium) ligand system (see the Supporting Information) ; the characteristic resonance for the methylene carbon (NCH2N) bridge (d =66.8 ppm) ap- pears close to those found in the molecular analogues 1a, 2a, and 2b (d = 69.8, 71.2, and 69.0 ppm, respectively). The IR spectrum of 5ashows an identifiable absorption band attribut- ed to the C=C bond of the imidazole ring at 1544 cm^1, which also exists in the molecular analogues 1a, 2a, and 2b (n = 1547, 1548, and 1546 cm^1, respectively), and which is not present in the pure polymer resin. To obtain our targeted Pd-loaded compound 5b, the ligand- loaded polymer 5a was further treated with PdCl2 in DMSO ac- cording to well-established literature procedures.[10] Afterwards, the polymer beads were collected and washed with DMSO, H2O, and MeOH five times each and then dried in vacuum overnight (16 h) for further analysis and catalytic tests. The solid-state 13C CP-MAS NMR experiment of compound 5b re- sulted in a quite similar spectrum to that of compound 5a, with a resonance at d = 67.6 ppm. Owing to the coverage of signals from the polymer and the relatively low resolution of solid-state 13C NMR spectroscopy, it is difficult to attribute the other specific resonances such as carbene carbon (d = 155.6 ppm, 4b) in this spectrum. Nevertheless, the IR spectrum of 5b shows an identifiable absorption band at 1264 cm^1, which also exists in the molecular analogues 3a and 4b (n = 1262 and 1264 cm^1, respectively), whereas this absorption was not observed for the pure polymer resin. The AAS analysis as well as photometric analysis, which are more accurate than the nitrogen determination, showed that the palladium con- tent was 5.6 % (5.8 % respectively). This corresponds to a Pd content of 0.52 mmol Pd g^1. The catalytic activity and reusability of the Pd-loaded com- pound 5b was then examined in the Suzuki-Miyaura cross- coupling reaction (Figure 4). To find the best combination of base and solvent for catalyst 5b, an optimization of reaction conditions was conducted by employing the molecular ana- logue 3a in catalyzing the cross-coupling of 4-bromoanisole and phenylboronic acid (see the Supporting Information). Thereby, three substrates, 4-bromobenzaldehyde, 4'-bromoace- tophenone, and 4-bromoanisole, were tested for the recycling experiments of catalyst 5b under the optimized conditions for five runs each. As shown in Figure 4, catalyst 5bcould be recy- cled in four runs for all three substrates without significant loss of activity. To confirm the heterogeneous nature of the cata- lyst, a filtration test was then conducted. The reaction using 4- bromoanisole as the substrate was run to a conversion of 9 % half an hour after starting the reaction. At this time, the stirring was stopped and half of the clear supernatant solution was fil- tered with a membrane filter into another reaction tube loaded with the same amount of substrates. The new reaction mixture was then stirred at 110 8C, as was the remaining reaction mixture. It was found that an increased conversion of 19 % was obtained from the reaction in the presence of 5b after 1 hour, whereas the reaction with the supernatant solu- tion showed no further conversion of the substrate even after 24 hours. This indicates that only the heterogeneous species of catalyst 5b is responsible for the catalytic activity rather than a homogeneous species, such as a leached Pd species. Water solubility and catalysis in water In addition to enabling a synthetic access to the immobiliza- tion of the complex, we considered the hydroxyl group of 3a itself to increase the polarity of the molecular complex. There- fore, we speculated that our hydroxyl-functionalized bis(NHC) system could also be soluble in water. An initial test showed that complex 3a was not dissolved in D2O at room tempera- ture, but we observed trace signals of 3a in the 1H NMR spec- trum, which indicated that at higher temperatures the solubili- ty of 3a in water could possibly be increased. Thus, we gradu- ally elevated the temperature and finally found the solubility of 3a in water to be limited to 5.5 mg mL^1 in D2O at 100 8C. Furthermore, the 1H NMR spectrum of 3a in D2O also showed the complex was not destroyed even after heating to 100 8C under air exposure. Given that complex 3a could be dissolved in water, we de- cided to preliminarily evaluate its catalytic reactivity under aqueous conditions, by employing again the Suzuki-Miyaura cross-coupling reaction as a standard tool to test the potential of Pd catalysts. To optimize the reaction conditions, the con- version of 4-bromoanisole and phenylboronic acid was initially tested in pure water (see Table 1). Four water-soluble bases were applied (Table 1, entries 1-4), out of which K2CO3 was found to be the base of choice, which is consistent with previ- ous studies by Wang and co-workers.[38] Owing to the fact that the obtained yields were low, tetrabutylammonium bromide (TBAB) was introduced as a phase-transfer reagent (Table 1, en- tries 5, 7-15). As expected, an increment in yields was ob- served when one equivalent of TBAB was employed as an ad- ditive under the same conditions (Table 1, entries 4 and 5). In addition, two experiments at a lower temperature of 60 8C (Table 1, entries 6 and 7) showed that an excellent yield could be obtained when TBAB was added (entry 7), even if only 31 % yield was observed initially (entry 6). As we further reduced the loading of catalyst and the reaction time, a quantitative yield could still be obtained by using only 0.1 mol % of catalyst in 2 hours (Table 1, entries 8-10). To gain further insight into the reactivity of complex 3a, four other aryl halide substrates were also tested (Table 1, entries 11-15). For the two nonhindered aryl bromides, complex 3a reached excellent yields by using 0.1 mol % of catalyst in 2 hours. However, with respect to the hindered aryl bromides (2-bromomesitylene) and the aryl chlorides (4'-chloroacetophenone), the yields were still low even upon extending the reaction time to four hours and in- creasing the loading of catalyst to 1.0 mol % (Table 1, en- tries 13-15). From this exemplary catalytic study, we found the catalytic performance of complex 3a was moderate compared to other Pd catalysts used in aqueous Suzuki-Miyaura cross-coupling re- actions.[39] One of the reasons for not achieving an excellent performance of complex 3a was possibly the Pd center's less bulky surroundings, which are important for the reductive elimination of the catalytic recycling. Nevertheless, if we take into account the possibility of changing the substituents of NHC rings and the coordination metals, our hydroxyl-function- alized bis(NHC) system could be a good choice to develop water-soluble catalysts. Further investigations on water-soluble catalysts based on this hydroxyl-functionalized system are cur- rently under way in our group. Conclusion A new facile method to functionalize methylene-bridged bis- (NHC) systems with a hydroxymethyl group and further con- version to a chloromethyl group has been developed. The suc- cessful synthesis of the different bis(imidazolium) salts 1a-c ex- emplarily confirms the possibility to change the wingtip sub- stituents of the imidazole ring. Furthermore, the synthesis of the hydroxymethyl-functionalized bis(NHC) Pd complex 3a and the chloromethyl-functionalized bis(NHC) Pd complex 4a proves that the functionalization has no influence on the met- alation of the imidazolium salts and therefore displays similar properties to the unsubstituted complex. In addition, the appli- cation of complex 3a in catalyzing Suzuki-Miyaura cross-cou- pling in water and the successful synthesis of heterogeneous Pd catalyst 5b shows that the introduction of the hydroxy- methyl group provides a general access towards tunable meth- ylene-bridged bis(NHC) complexes for heterogeneous applica- tions or reactions. Experimental Section General experimental methods Oxygen-free solvents were obtained by an MBraun MB SPS purifi- cation system. Benzoyl chloride polystyrene resin (0.8-1.0 mmol g^1 substitution loading) was purchased from Alfa-Aesar. Other chemi- cals were purchased from commercial suppliers and used without further purification. Liquid NMR spectra were recorded on Bruker DRX 400 and Bruker ultrashield 400 spectrometers (1H NMR, 400.13MHz;13CNMR, 100.53MHz) at 298K.The spectra were cali- brated by using the residual solvent shifts as internal standards. Chemical shifts were referenced in parts per million (ppm). Abbre- viations for signal multiplicities are as follows : singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m), and broad (br). Solid-state NMR spectra were recorded on a Bruker Avance 300 spectrometer equipped with a 4 mm BBMAS probe head and referenced to ada- mantane as an external standard (1H NMR, 300.13 MHz ; 13C NMR, 75.47 MHz) at 298 K. A Varian 670 FTIR spectrometer was used to conduct IR measurements. ESI-HRMS analyses were performed on a Thermo Scientific LTQ Orbitrap XL instrument by Thermo Fisher Scientific. Elemental analyses were performed by the microanalyti- cal laboratory of the Technische Universit^t M^nchen (TUM). Synthesis of the ligands 1a: In a sealed tube, 1-methylimidazole (20 mmol, 1.64 g) and di- chloroethanol (6.5 mmol, 0.75 g) were dissolved in toluene (4 mL) andheated at1108C for 2days, then at1308C for7days. Thepre- cipitate was isolated by filtration, washed with THF (3 ^ 5 mL) and CH2Cl2 (2 ^5 mL), and dried in vacuum to obtain the ligand. Yield: 36% (0.65 g). 1HNMR ([D6]DMSO, 258C): d=9.91 (s, 2H; NCHN), 8.25 (s, 2H; NCH), 7.85 (s, 2H; NCH), 7.32 (t, J=4.0 Hz, 1H; NCHCH2), 6.47 (t, J=4.0 Hz, 1H; OH), 4.38 (t, J=4.0 Hz, 2H; CHCH2OH),3.90ppm (s, 6H; NCH3);13CNMR([D6]DMSO, 258C): d= 137.7 (NCHN), 124.1 (NCH), 121.2 (NCH), 69.8 (NCHCH2), 59.9 (CHCH2OH), 36.2 ppm (NCH3); IR: ñ=3414, 3178, 3034, 2857, 1648, 1581, 1547, 1421, 1339, 1278, 1168, 1093, 1084, 860, 802, 778, 648, 618 cm^1; HRMS (ES +): m/z calcd for C10H16Cl2N4O[M^Cl]+: 243.1007; found : 243.1005; elemental analysis calcd (%) for C10H16Cl2N4O: C 43.02, H 5.78, N 20.07; found: C 42.63, H 5.86, N 19.71. 1b: In a sealed tube, 1-isopropylimidazole (9 mmol, 1.00 g) and di- chloroethanol (3.76 mmol, 0.31 g) were dissolved in toluene (4 mL) andheated at1108C for 2days, then at1308C for7days. Thepre- cipitate was isolated by filtration, washed with THF (3 ^ 5 mL) and CH2Cl2 (2 ^ 5 mL), and dried in a vacuum to obtain the ligand. Yield: 25% (0.65 g).1HNMR ([D6]DMSO, 258C): d=10.02 (s, 2H; NCHN), 8.28 (s, 2H; NCH), 8.07 (s,2H;NCH), 7.18(t, J=4.0Hz, 1H; NCHCH2), 6.42 (t, J=4.0Hz, 1H; OH), 4.69 (sept., J=6.5 Hz, 2H; CH3CHCH3), 4.41 (t, J=4.0 Hz, 2H; CHCH2OH), 1.51 ppm (d, J= 6.5Hz, 12H; CH3CHCH3); 13C NMR ([D6]DMSO, 258C): d=136.1 (NCHN), 121.6 (NCH), 121.1 (NCH), 70.1 (NCHCH2), 59.9 (CHCH2OH), 52.9 (CH3CHCH3), 22.1, 22.1 ppm (CH3CHCH3). 1c: In a sealed tube, 1-tert-butylimidazole (10 mmol, 1.24 g) and di- chloroethanol (5.25 mmol, 0.60 g) were dissolved in toluene (4 mL) andheated at1108C for 2days, then at 1308C for 7days. Thepre- cipitate was isolated by filtration, washed with THF (3 ^ 5 mL) and CH2Cl2 (2 ^ 5 mL), and dried in a vacuum to obtain the ligands. Yield: 33% (0.63 g). 1H NMR ([D6]DMSO, 258C): d=10.19 (s, 2H; NCHN), 8.45 (s, 2H; NCH), 8.19 (s,2H;NCH), 7.26(t, J=5.6Hz, 1H; NCHCH2), 6.41 (t, J=5.6 Hz, 1H; OH), 4.50 (t, J=5.6 Hz, 2H; CHCH2OH), 1.63ppm (s, 18H; C(CH3)3);13CNMR ([D6]DMSO, 258C): d = 136.3 (NCHN), 122.2 (NCH), 121.2 (NCH), 70.4 (NCHCH2), 60.87 (NCCH3), 60.2 (CHCH2OH), 29.4 ppm (C(CH3)3); HRMS (ES +): m/z calcd for C16H28ClN4O[M^Cl]+ : 327.1946 ; found : 327.1945 ; elemen- tal analysis calcd (%) for C16H28Cl2N4O·0.5H2O : C 51.61, H 7.85, N 15.05 ; found : C 51.76, H 7.62, N 15.35. 2a: Ammonium hexafluorophosphate (8.9 mmol, 1.46 g) in water (3 mL) was added dropwise to a solution of 1a (3.6 mmol, 1.0 g) in water (5 mL). A brown precipitate was formed, which was isolated by filtration. The solid was washed with water (1 mL twice) and dried in vacuo to give a brown powder. Yield: 73 % (1.31 g). 1H NMR (CD3CN, 258C): d = 8.81 (s, 2 H; NCHN), 7.60 (s, 2 H; NCH), 7.45 (s, 2H; NCH), 6.64 (t, J=4.0 Hz, 1H; NCHCH2), 4.35 (d, J= 4.0Hz, 2H; CHCH2OH), 4.22 (s, 1H; OH), 3.89 ppm (s, 6H; NCH3); 13C NMR (CD3CN, 25 8C): d = 137.9 (NCHN), 125.4 (NCH), 122.5 (NCH), 71.3 (NCHCH2), 61.7 (CHCH2OH), 37.4 ppm (NCH3); 31P NMR (CD3CN, 161 MHz): d=^131.53 to ^157.73 ppm (sep); IR: ñ=3583, 3172, 3122, 1589, 1581, 1548, 1423, 1274, 1226, 1166, 1087, 811, 738, 686, 622, 553 cm^1; HRMS (ES +): m/z calcd for C10H16F12N4OP2 [M^PF6] + : 353.0960 ; found : 352.8200. 2c: Benzoyl chloride (1.07 mmol, 0.15 g), pyridine (0.05 mL), and 2a (0.21 mmol, 0.10 g) were dissolved in CH3CN (1 mL). The reac- tion mixture was stirred for 12 h at room temperature. After the re- action, the solvent was evaporated and the solid washed with CH2Cl2 (3 ^5 mL) and Et2O (5 mL). Then it was dried in vacuo to obtain the product as a brownish powder. Yield: 97 %. 1H NMR (CD3CN,258C):d=9.35(s, 2H;NCHN), 7.98 (d, J=8.0Hz,2H; Ph), 7.80(s,2H; NCH), 7.69 (t,J=8.0Hz,1H;Ph), 7.53 (t,J=8.0Hz,2H; Ph), 7.49 (s, 2H; NCH), 7.45 (t, J=4.0 Hz, 1H; NCHN), 5.20 (d, J= 8.0Hz, 2H;CHCH2O),3.90ppm(s,6H; NCH3);13CNMR([D6]DMSO, 25 8C): d=165.8 (C=O), 138.4 (NCHN), 135.0 (Ph), 130.7 (Ph), 129.8 (Ph), 129.4 (Ph), 125.9 (NCH), 122.1 (NCH), 69.0 (NCHCH2), 62.8 (CHCH2O), 37.5 ppm (NCH3); 31PNMR (CD3CN, 161 MHz): d= ^131.54 to ^157.72 ppm (sep); IR: ñ=3160, 1727, 1588, 1546, 1451, 1423, 1266, 1226, 1173, 1109, 828, 712, 622, 556 cm^1; HRMS (ES +): m/z calcd for C17H20F12N4O2P2 [M^PF6]+: 457.1223; found: 456.8100. Synthesis of complexes 3a: Pd(OAc)2 (1.34 mmol, 0.300 g) and 1a (1.34 mmol, 0.373 g) were dissolved in DMSO (10 mL) and heated for 24 h at 55 8C. During the reaction the solution turned from dark red to yellow and an off-white precipitate was generated. After the reaction, the precipitate was isolated by filtration and washed with methanol (3 ^ 5 mL) and Et2O (5 mL). Then it was dried in vacuo to obtain the product as an off-white powder. Yield: 62 % (0.317 g). 1H NMR ([D6]DMSO, 25 8C): d = 7.56 (s, 2H; NCH), 7.32 (s, 2H; NCH), 6.46 (t, J=4.0 Hz, 1H; NCHCH2), 5.91 (t, J=4.0 Hz, 1H; OH), 4.65 (t, J= 4.0Hz, 2H; CHCH2OH), 3.92 ppm (s, 6H; NCH3); 1HNMR (D2O, 258C): d=7.54 (s, 2H; NCH), 7.23 (s, 2H; NCH), 6.51 (t, J=4.0 Hz, 1H;NCHCH2), 4.84 (t, J=4.0Hz, 2H; CHCH2OH), 3.95ppm (s, 6H; NCH3); 13C NMR ([D6]DMSO, 258C): d= 154.9 (NCHN), 123.1 (NCH), 121.9 (NCH), 74.1 (NCHCH2), 66.4 (CHCH2OH), 38.1 ppm (NCH3); IR: ñ=3458, 3131, 2969, 1566, 1465, 1443, 1415, 1331, 1300, 1262, 1217, 1198, 1087, 1063, 1044, 955, 836, 821, 758, 729, 693, 676, 660, 609, 504 cm^1; ESI-MS ([M] +): m/z (%): 387.8 [3a^Cl^+ MeCN] +; elemental analysis calcd (%) for C10H14Cl2N4OPd·0.4DMSO : C 31.27, H 3.98, N 13.51; found : C 31.28, H 3.90, N 13.08. 3b: Pd(OAc)2 (1.34 mmol, 0.300 g) and 1b (1.34 mmol, 0.588 g) were dissolved in DMSO (10 mL) and heated for 24 h at 55 8C. During the reaction the solution turned from dark red to yellow and an off-white precipitate was generated. After the reaction, the precipitate was isolated by filtration and washed with methanol (3^5mL) and Et2O (5mL). Then it was dried invacuoto obtain the product as an off-white powder. Yield: 46 %. 1H NMR ([D6]DMSO, 258C): d=7.59 (s, 2H; NCH), 7.53 (s, 2H; NCH), 6.44 (t, J=4.0 Hz, 1H;NCHCH2), 5.93 (t, J=4.0 Hz, 1H; OH), 5.56 (sept., J=7.3 Hz, 2H; CH3CHCH3), 4.71 (t, J=4.0 Hz, 2H; CHCH2OH), 1.43 (d, J= 6.5 Hz, 6H; CH3CHCH3), 1.24 ppm (d, J=6.5 Hz, 6H; CH3CHCH3); 13C NMR ([D6]DMSO, 25 8C): d=154.9 (NCHN), 122.7 (NCH), 118.0 (NCH), 74.4 (NCHCH2), 66.1 (CHCH2OH), 52.1 (CH3CHCH3), 23.9, 21.6 ppm (CH3CHCH3); ESI-MS ([M] +): m/z (%): 404.89 [3b^Cl^]+; elemental analysis calcd (%) for C10H14Cl2N4OPd·0.2CH2Cl2 : C 37.35, H 4.94, N 12.27; found : C 37.17, H 5.12, N 12.25. 4a: Pd complex 3a (0.26 mmol, 0.100 g) was suspended in SOCl2 (3 mL) and five drops of DMF were added. The reaction mixture was stirred for 24 h at 90 8C by using a reflux condenser protected under argon. After the reaction, the mixture was added to THF (20 mL), then the precipitate was isolated by filtration and washed with CH2Cl2 (3^5 mL) and Et2O (5 mL). The obtained solid was dried in vacuo to give complex 4a as a gray powder. Yield: 98 % (0.102g). 1HNMR ([D6]DMSO, 258C): d=7.62 (s, 2H; NCH),7.37 (s, 2H;NCH), 6.87 (t, J=4.0Hz, 1H; NCHCH2), 4.99 (2, J=8.0Hz, 2H; CHCH2Cl), 3.93ppm (s, 6H; NCH3);13CNMR ([D6]DMSO, 258C): d= 155.4 (NCHN), 123.4 (NCH), 122.1 (NCH), 72.3 (NCHCH2), 47.5 (CHCH2Cl), 38.2 ppm (NCH3); ESI-MS ([M] +): m/z (%): 367.1 [4a^Cl^]+ ; elemental analysis calcd (%) for C10H13Cl3N4Pd : C 29.88, H 3.26, N 13.94; found : C 28.53, H 3.07, N 13.08, S 0.26. (Residual 0.1 equiv SOCl2: elemental analysis calcd (%) for C10H13Cl3N4Pd·0.1SOCl2 : C 29.02, H 3.17, N 13.54, S 0.77.) 4b: Benzoyl chloride (8.65 mmol, 1.21 g), pyridine (1 mL), and 3a (0.21 mmol, 0.08 g) were dissolved in N-methyl-2-pyrrolidone (NMP, 1 mL). The reaction mixture was stirred for 24 h at room tempera- ture. After the reaction, the precipitate was isolated by filtration and washed with methanol (3 ^5 mL) and Et2O (5 mL). Then it was dried in vacuo to obtain the product as an off-white powder. Yield : 96%. 1H NMR ([D6]DMSO, 258C): d=7.99 (d, J=8.0 Hz, 2H; Ph), 7.76 (s, 2H;NCH),7.71 (t,J=8.0Hz,1H;Ph),7.57 (t,J=8.0Hz,2H; Ph),7.41(s,2H; NCH), 6.38 (t, J=4.0Hz, 1H;NCHCH2), 5.48 (d, J= 8.0Hz,2H;CHCH2O),3.95ppm(s,6H; NCH3);13CNMR([D6]DMSO, 258C): d= 164.7 (C=O), 155.6 (NCHN), 134.1 (Ph), 129.5 (Ph), 128.5 (Ph), 123.6 (NCH), 121.9 (NCH), 70.7 (NCHCH2), 67.7 (CHCH2O), 38.3 ppm (NCH3); IR: ñ=3500, 3441, 3164, 3077, 1720, 1599, 1571, 1456, 1434, 1399, 1352, 1307, 1263, 1203, 1180, 1122, 1089, 1072, 1024, 870, 806, 768, 744, 713, 693, 678, 623, 517 cm^1; ESI-MS ([M] +): m/z (%): 451.2 [4b^Cl^]+; elemental analysis calcd (%) for C17H18Cl2N4O2Pd·DMSO : C 40.33, H 4.28, N 9.9; found : C 39.95, H 3.84, N 10.27. Preparation of heterogeneous catalyst Route 1: Benzoyl chloride-functionalized polystyrene (0.05 g) and 3a (0.05 mmol, 0.02 g) were suspended in dry NMP (1 mL), then three drops of dry pyridine were added as a catalyst. The reaction conditions were varied from room temperature to 80, 100, 120, and 150 8C for 24 h. After the reaction, the solid was isolated by fil- tration and washed with DMSO (3^ 5 mL) and MeOH (3^ 5 mL), then dried in vacuo to obtain immobilized Pd catalyst as brownish beads. The loadings of immobilized Pd compounds obtained at dif- ferent temperatures were all below 0.1 wt %. Route 2 : Benzoyl chloride-functionalized polystyrene (1.00 g), dry pyridine (0.05 mL), and 2a (2.01 mmol, 1.00 g) were charged into a 50 mL Schlenk tube. CH3CN (10 mL) was then added to the reac- tion mixture, which was stirred at room temperature for 16 h, fol- lowed by 48 h at 100 8C. After the reaction, the solid was isolated by filtration and washed with CH3CN (5 ^3 mL), MeOH (5^ 3 mL), and Et2O (5 mL). Then it was dried in vacuo to obtain 5a as brown- ish beads. Then compound 5a, palladium(II) chloride (0.56 mmol), sodium chloride (65 mg, 1.12 mmol), and sodium acetate (152 mg, 1.12 mmol) were dissolved in dry DMSO (2 mL). The reaction mix- ture was firstly stirred at room temperature for 16 h, followed by 24 h at 50 8C, and was subsequently heated for 4 h at 90 8C. After the reaction, the solid was isolated by filtration and washed with DMSO (5^ 3 mL), water (5^3 mL), and MeOH (5 ^3 mL), then dried in vacuo to obtain immobilized Pd catalyst 5b as brownish beads. The temperature variation for synthesis of 5a was done according to the procedures described above, only with smaller amounts of substrates used as follows : benzoyl chloride-functionalized poly- styrene (0.05 g), three drops of pyridine, and 2a (0.10 mmol, 0.05 g) were reacted in CH3CN (0.5 mL). Compound 5a: 13C NMR (solid state, 75 MHz, nr=10 kHz, 25 8C): d= 167.2 (C=O), 146.2 (polystyrene, Ph), 128.8 (polystyrene, Ph), 66.8 (NCHCH2), 41.2 ppm (polystyrene, -CH2CH-); IR: ñ=3024, 2919, 1710, 1698, 1603, 1544, 1491, 1451, 1416, 1375, 1271, 1177, 1105, 1009, 905, 842, 757, 699, 620, 557 cm^1; elemental analysis : C 77.33, H 7.01, N 1.40, F 1.90. Compound 5b: 13C NMR (solid state, 75 MHz, nr =10 kHz, 25 8C): d= 167.2 (C=O), 146.2 (polystyrene, Ph), 128.8 (polystyrene, Ph), 67.6 (NCHCH2), 41.2 ppm (polystyrene, -CH2CH-); IR: ñ=3024, 2920, 1712, 1601, 1546, 1491, 1449, 1372, 1263, 1178, 1070, 1009, 962, 904, 818, 753, 696, 537 cm^1; elemental analysis : C 72.26, H 6.32, N 1.33, Pd 5.6. General Suzuki-Miyaura cross-coupling procedure in water The Pd complex 3a (0.01 or 0.02 mmol), phenylboronic acid (2.00 mmol), base (2.00 mmol), and the aryl halide (1.00 mmol) were added to a Schlenk flask containing a magnetic stirrer bar. The vial was then purged twice with argon and a solvent (1.5 mL) was added. The reaction mixture was stirred at a certain tempera- ture for the desired period. After reaction, the mixture was extract- ed by CH2Cl2 (2^ 5 mL) and filtered. The solvent of the filtrate was removed in a rotary evaporator (40 8C, 100 mbar) and the crude product was used directly for chromatographic separation or dis- solved by CDCl3 with a standard (1,3,5-trimethoxybenzene, 0.3 mmol) for 1H NMR analysis. Recycling of immobilized catalyst 5 b in Suzuki-Miyaura cross-coupling The immobilized catalyst 5b (10 mg, 0.25 mol %), phenylboronic acid (0.12 g, 1.00 mmol), CS2CO3 (1.00 mmol), and the aryl halide (0.50 mmol) were added to a Schlenk flask containing a magnetic stirrer bar. The vial was then purged twice with argon and toluene (1.5 mL) was added. The reaction mixture was stirred at 100 8C for 4 or 12 h. After reaction, the mixture was extracted with toluene (5 mL) and ethyl acetate (5 mL) separately. The solvent was collect- ed by centrifugation and removed in a rotary evaporator (40 8C, 100 mbar). Next, the crude product was dissolved by CDCl3 with a standard (1,3,5-trimethoxybenzene, 0.3 mmol) for 1H NMR analy- sis to determine the yield of the reaction. Moreover, the residue from centrifugation was dried in vacuo and used for the next cycle of catalysis. Computational methods All calculations used DFT methodology as implemented in Gaussi- an 09[40] using the density functional hybrid model B3LYP[41-43] to- gether with 6-311 + G(d) as the basis set on nonmetals[44-49] and the Stuttgart RSC 1997 ECP as the basis set on Pd.[50] No symmetry or internal coordinate constraints were applied during optimiza- tions. 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Received : May 6, 2014 Published online on June 24, 2014 Rui Zhong, Alexander Pçthig,* Stefan Haslinger, Benjamin Hofmann, Gabriele Raudaschl- Sieber, Eberhardt Herdtweck, Wolfgang A. Herrmann, and Fritz E. K^hn*[a] [a] R. Zhong, Dr. A. Pçthig, S. Haslinger, B. Hofmann, Dr. G. Raudaschl-Sieber, Dr. E. Herdtweck, Prof. Dr. W. A. Herrmann, Prof. Dr. F. E. K^hn Chair of Inorganic Chemistry, Molecular Catalysis Catalysis Research Center Technische Universit^t M^nchen Ernst-Otto-Fischer Strasse 1, 85748 Garching bei M^nchen (Germany) Fax: (+ 49) 89-289-13473 E-mail : [email protected] [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201402135. (c) 2014 Blackwell Publishing Ltd. |