N-Heterocyclic Carbene Complexes of Mercury, Silver, Iridium

Oct 14, 2009 - †Division of Organic Chemistry I, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany, and ... [email protected]...
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Organometallics 2009, 28, 6183–6193 DOI: 10.1021/om900642x

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N-Heterocyclic Carbene Complexes of Mercury, Silver, Iridium, Platinum, Ruthenium, and Palladium Based on the Calix[4]arene Skeleton Tilmann Fahlbusch,† Markus Frank,† Gerhard Maas,† and J€ urgen Schatz*,†,‡ †

Division of Organic Chemistry I, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany, and Organic Chemistry 1, Department of Chemistry and Pharmacy, University of Erlangen, Henkestrasse 42, 91054 Erlangen, Germany

‡

Received July 21, 2009

Two independent pathways to Ag(I)-N-heterocyclic-calix[4]arene complexes (5a,b and 6-8) starting from three calix[4]arenes bearing one or two (distal or proximal) imidazolium substituents attached at the upper rim via a methylene linker are described. Complexes 5 were synthesized using the free carbene route, and complexes 6-8 were obtained via in situ formation of the N-heterocyclic carbene by deprotonation and reaction with Ag2O. The structure of 5a could be verified by an X-ray single-crystal structure determination. 1H NMR studies of the compounds 5a,b showed dynamic behavior in solution, similar to a conformational change observed for known phosphine-based calixarene ligands. Using the imidazolium-calixarenes as precursors, NHC complexes of mercury, iridium, platinum, and ruthenium have been prepared and characterized. Additionally, cis and trans Pd(II)-N-heterocyclic-calix[4]arene complexes (3 and 13) were synthesized independently by in situ deprotonation of 2 with Pd(OAc)2 and by transmetalation using Ag(I) complex 6 as a supramolecular carbene transfer reagent, respectively. Both isomeric complexes 3 and 13 were tested as catalysts in the Suzuki-Miyaura reaction and compared to previously reported in situ systems. Both isolated complexes exhibit, independently of their geometry, the same catalytic activity, which is superior to the in situ system used for comparison. Introduction During the past decade, N-heterocyclic carbenes1 (NHCs) have attracted considerable attention as ligands in coordination and organometallic chemistry.2 NHCs have already been studied as reaction intermediates by Wanzlick3 and 4 € in the 1960s, but nearly 30 years passed until the Ofele isolation of a crystalline, stable carbene by Arduengo5 in 1991. From there on, enormous activity came into the field of *To whom correspondence should be addressed. E-mail: juergen. [email protected]. (1) (a) Wanzlick, H.-W.; Schikora, E. Chem. Ber. 1961, 94, 2388– 2394. (b) Regitz, M. Angew. Chem. 1996, 108, 791-794; Angew. Chem., Int. Ed. 1996, 35, 725-728 (c) Herrmann, W. A.; K€ocher, C. Angew. Chem. 1997, 109, 2256-2282; Angew. Chem., Int. Ed. 1997, 36, 2162-2187 (d) Arduengo, A. J.III Acc. Chem. Rev. 1999, 32, 913–921. (e) Arduengo, A. J.III; Krafczyk, R.; Schmutzler, R. Tetrahedron 1999, 55, 14523–14534. (f) Bourissou, D.; Guerret, O.; Gabbai, F.; Bertrand, G. Chem. Rev. 2000, 100, 39–91. (g) Scott, N. M.; Nolan, S. P. Eur. J. Inorg. Chem. 2005, 1815– 1828. (h) Crudden, C. M.; Allen, D. P. Coord. Chem. Rev. 2004, 248, 2247– 2273. (i) Perry, M. C.; Burgess, K. Tetrahedron: Asymmetry 2003, 14, 951– 961. (j) Nolan, S. P. N-Heterocyclic Carbenes in Synthesis; Wiley-VCH: Weinheim, Germany, 2006. (k) N-Heterocyclic Carbenes in Transition Metal Catalysis. Advances in Organometallic Chemistry; Glorius, F., Ed.; Springer: Berlin, Heidelberg, 2007; Vol. 21. (2) (a) Herrmann, W. A. Angew. Chem. 2002, 114, 1342-1363; Angew. Chem., Int. Ed. 2002, 41, 1290-1309. (b) Marion, N.; Nolan, S. P. Acc. Chem. Res. 2008, 41, 1440–1449. (c) Hahn, F. E.; Jahnke, M. C. Angew. Chem. 2008, 120, 3166-3216; Angew. Chem., Int. Ed. 2008, 47, 3122-3172. (d) de Fremont, P.; Marion, N.; Nolan, S. P. Coord. Chem. Rev. 2009, 253, 862–892. (3) Wanzlick, H.-W.; Steinmaus, H. Chem. Ber. 1968, 101, 244–251. € (4) Ofele, K. J. Organomet. Chem. 1968, 12, P42–P43. (5) Arduengo, A. J.III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361–363. r 2009 American Chemical Society

coordination chemistry of NHCs. Because they strongly bind and stabilize transition metals, a large variety of NHC-containing complexes and catalysts have been prepared in which NHCs replaced, for example, the air-sensitive and less σ-donating phosphine ligands.6 Recently, we have shown7 that calix[4]arenes bearing imidazolium groups;rare examples of such types of compounds8;at a distal position of the wide rim do form stable NHC complexes with palladium when salts such as 2 are treated with Pd(OAc)2 in DMSO at elevated temperatures. Corresponding chelates Pd(L-L)Cl2 such as 3 could be isolated. The structure of 3 was proven by an X-ray crystal structure determination, revealing a cis configuration around the palladium center. On the basis of these preliminary results and our general interest in the coordination properties of imidazolium-substituted calix[4]arenes, we decided to study whether our supramolecular ligand precursors are also capable of forming stable complexes with a variety of transition metals and to elucidate their properties. In addition we wanted to explore whether silver complexes can be used as supramolecular carbene transfer reagents as a common intermediate for other transitionmetal complexes. (6) (a) Heinemann, C.; M€ uller, T.; Apeloig, T.; Schwarz, H. J. Am. Chem. Soc. 1996, 118, 2023–2038. (b) Crabtree, R. H. J. Organomet. Chem. 2005, 690, 5451–5457. (c) Gusev, D. G. Organometallics 2009, 28, 763–770. (7) Frank, M.; Maas, G.; Schatz, J. Eur. J. Org. Chem. 2004, 607–613. (8) (a) Dinares, I.; Garcia de Miguel, C.; Font-Bardia, M.; Solans, X.; Alcalde, E. Organometallics 2007, 26, 5125–5128. (b) Dinares, I.; Garcia de Miguel, C.; Mesquida, N.; Alcalde, E. J. Org. Chem. 2009, 74, 482–485. Published on Web 10/14/2009

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Results and Discussion The calix[4]arene-imidazolium salts 2 which were used as ligand precursors are easily available as white solids by simple alkylation of the appropriate N-substituted imidazoles with the known chloromethyl-calix[4]arene 1.7,9 The salts are thermally stable up to their melting points but are very hygroscopic; a C2v-symmetrical cone conformation of the macrocycle in solution could be deduced from the 1H and 13 C NMR spectra. The imidazolium C(2)-H showed a typical absorption in the 1H NMR spectra at approximately δ 10.5, which is consistent with those of other reported imidazolium salts. Mercury Complex. To investigate whether the NHC precursors 29 do form stable chelating complexes with linearly coordinating metals, the imidazolium salt 2a was reacted in a first experiment with Hg(OAc)2 at 90 C in DMSO, a protocol which had already been used by Wanzlick for the synthesis of the first ever reported metal NHC complex.3 As a simple indicator for the success of the formation of NHC complex 4, the disappearance of imidazolium C(2) protons in the 1H NMR spectrum could be used, proving that during the reaction the acidic protons of the heterocycles10,11 were deprotonated by the basic ligand of the metal salt and coordination to the transition metal occurred. Additionally, the 13C resonance of the C(2) of the imidazolium moiety exhibited a shift from δ 157 ppm for 2a to 177 ppm, a resonance typical for metal complexes such as 4.12 A molecular peak at m/z 1017 in the MALDI-TOF mass spectrum of the product corresponds to the complex cation [M - Cl]þ. The isotopic pattern was consistent with that expected for a monomeric complex; dimeric or trimeric species were not detected. In addition, the 1H NMR spectrum suggested a C2v-symmetrical conformation in solution with linear bridging of the NHC-Hg-NHC fragment over the macrocyclic cavity. In comparison to our previously reported cis [Pd(L-L)Cl2] complex 3,7 the unusually high yield of 4 (62%) might therefore be a result of the less strained bridging due to the linear coordination. Silver Complexes. Among the transition-metal-NHC complexes, the silver-NHC compounds are of great interest due to their unique properties. NHC silver compounds are known to adopt different architectures, depending on the synthetic conditions employed, but above all they are the most popular complexes used in NHC transfer reactions which can be used to gain access to a variety of other latetransition-metal-NHC complexes.13 In order to obtain silver complexes of a N-heterocyclic carbene based on calix[4]arenes, the free carbene route was chosen.13a Instead of the direct reaction of the NHC precursors with a basic metal salt such as Ag2O or Ag2CO3, the imidazolium salts 2 were deprotonated using potassium tertbutoxide in THF to generate the corresponding imidazol-2ylidene ligands first before quenching the free carbene with (9) Brendgen, T.; Frank, M.; Schatz, J. Eur. J. Org. Chem. 2006, 2378–2383. (10) Brendgen, T.; Frank, M.; Schatz, J.; Sch€ uhle, D. J. Org. Chem. 2006, 71, 1688–1691. (11) Bartz, S.; Blumenr€ oder, B.; Kern, A.; Fleckenstein, J.; Frohnapfel, S.; Schatz, J.; Wagner, A. Z. Naturforsch. 2009, 64b, 629–638. (12) Tapu, D.; Dixon, D. A.; Roe, C. Chem. Rev. 2009, 109, 3385– 3407. (13) (a) Garrison, J. C.; Youngs, W. J. Chem. Rev. 2005, 105, 3978– 4008. (b) Ramírez, J.; Corberan, R.; Sanau, M.; Peris, E.; Fernandez, E. Chem. Commun. 2005, 3056–3058. (c) Sentman, A. C.; Csihony, S.; Waymouth, R. M.; Hedrick, J. L. J. Org. Chem. 2005, 70, 2391–2393.

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AgBF4. The yields of the obtained calix[4]arene-NHCsilver complexes 5a,b were fair to good (38-61%); the formation of 5 was again indicated by the absence of the acidic imidazolium proton in the 1H NMR spectrum. Further proof was gained from 13C NMR spectra, showing couplings between the 13Ccarbene and the 107/109Ag nuclei. In CDCl3 solution, the resonance for the 13Ccarbene appears for both 5a and 5b as a pair of doublets centered at δ 181.3 (5a) and 179.0 (5b). The coupling constants 1J(107Ag-13C) (180.0 Hz) and 1J(109Ag-13C) (207.8 Hz) are well within the characteristic range of coordinated N-heterocyclic carbenes (180-234 Hz for 107Ag and 204-270 Hz for 109Ag),13,14 thus giving evidence for stable silver-carbon bonds. Additionally, small 3J(107/109Ag-13C) couplings to the imidazole-C4 and -C5 carbon atoms of 5.1 and 5.9 Hz could be detected. Upon slow evaporation of a solution of complex 5a from a mixture of p-xylene and CH2Cl2, colorless crystals of 5a suitable for single-crystal X-ray determination were obtained (Figure 1). Outside the mother liquor the crystals deteriorate within less than 1 min at room temperature due to a loss of solvent molecules incorporated in the crystal lattice. It was found that the asymmetric unit of the triclinic cell contains four calixarene-BF4 units, 4.5 CH2Cl2 molecules, and two p-xylene molecules. Figure 1a shows the structure of one of the calixarene molecules which is representative for all of them. It can be seen that the position of some carbon atoms in the propyloxy chains is not well-defined, a not uncommon feature in solid-state calixarene structures. The bond geometry around the silver ion is characterized by the data given in Table 1. The Ag-Ccarbene distances are rather long, compared to the usual range (2.01-2.07, mean distance 2.11 A˚ for 372 observations in the Cambridge Crystal Structure Database);15 the two imidazolylidene ligands are virtually coplanar, and the Ccarbene-Ag-Ccarbene angle is nearly linear with angles of 175.5-177.9. The angle between the planes defined by the four methylene bridges of the calixarene skeleton and the heterocyclic ligands is 29. The complexation of the imidazolylidene units at the upper rim generates a distorted pinchedcone conformation of the calixarene scaffold: the two unsubstituted propyloxyphenyl rings are bent outward with angles of 140 relative to the plane of the methylene bridges, whereas the NHC-substituted phenyl rings form angles of ca. 85 with the same reference plane. All eight calixarene molecules in the unit cell form dimeric aggregates via a face-to-face orientation of their NHCAg-NHC moiety (Figure 1b). Two pairs connected in this manner are centrosymmetric by space group symmetry; the other two pairs are in a noncrystallographic pseudocentrosymmetric relationship. The Ag-Ag contacts (3.61-3.75 A˚) fall in the range of published intermolecular NHC-Ag-Ag contacts (2.90-3.88 A˚, mean value 3.46 A˚ for 40 observations) which can be found in the Cambridge Crystal Structure Database.15 The face-to-face orientation of the NHC-Ag-NHC moieties in the dimeric units is quite unusual; it seems to be rare and is supposed to be repulsive.16 Therefore, the rather long (14) de Fremont, P.; Scott, N. M.; Stevens, E. D.; Ramnial, T.; Lightbody, O. C.; Macdonald, C. L. B.; Clyburne, J. A. C.; Abernethy, C. D.; Nolan, S. P. Organometallics 2005, 24, 6301–6309. (15) Allen, F. H.; Kennard, O. Chem. Design Automation News 1993, 8, 31–37. (16) (a) Janiak, C. Dalton Trans. 2000, 21, 3885–3896. (b) Quezada, C. A.; Garrision, J. C.; Panzner, M. J.; Tessier, C. A.; Joungs, W. J. Organometallics 2004, 23, 4846–4848. (c) Zhang, J.-P.; Wang, Y. B.; Huang, X.-C.; Lin, Y.-Y.; Chen, X.-M. Chem. Eur. J. 2005, 11, 552–561.

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Scheme 1. Synthesis of Silver, Mercury, and Palladium Complexes of NHC-Substituted Calix[4]arenesa

Reagents and conditions: (i) 1-alkylimidazole, CHCl3, 61 C, 2 days and Et2O, 35 C, 4 h; (ii) Pd(OAc)2, DMSO, 90 C; (iii) Hg(OAc)2, DMSO, 90 C; (iv) (1) KO-t-Bu, THF, 0 C and (2) AgBF4, room temperature. a

Ag-Ag distance of 3.6 A˚ seems to be a fair compromise between attractive metallophilic17 and repulsive stacking interactions. Owing to the discrepancy of the results obtained from the crystal structure of 5a (C1 symmetry in the solid state) with those from the 1H and 13C NMR spectra at room temperature (C2v symmetry in solution), we decided to investigate the silver-NHC-calix[4]arene complexes 5 by variable-temperature 1H NMR spectroscopy. As representatively shown in Figure 2, lowering the temperature of a CD2Cl2 solution of complex 5b resulted in a strong broadening of the proton signals of the NHC-substituted phenol units. The singlet at δ 5.78 (T = 294 K) split into two equally intense signals at low temperature, indicating a decrease in symmetry. The C1 symmetry of the solid-state structure of 5a was not observed, but the latter might be a result of further loss of symmetry due to crystal-packing effects. Raising of the temperature back to 294 K revealed the reversibility of this dynamic process for both isomers 5a and 5b. Accurate measurements resulted in coalescence temperatures of 210 K for 5a and 241 K for 5b. Using the Eyring theory and under the assumption of a firstorder exchange, the free energy parameters ΔGq were estimated to be ΔGq210 K = 39.3 kJ mol-1 for complex 5a (R1 = CH3) and ΔGq241 K = 45.2 kJ mol-1 for compound 5b (R1 = iPr). It is a well-known feature of upper-rim-metalated calix[4]arenes to show dynamic behavior in solution. Although all the results reported up to now are based on calixarenes (17) Katz, M. J.; Sakai, K.; Leznoff, D. B. Chem. Soc. Rev. 2008, 37, 1884–1895.

bearing phosphine ligands, the investigations presented by Tsuji et al.18 and Matt and co-workers19 are comparable to the same extent. They both describe the dynamic process of upper-rim distally functionalized calix[4]arenes. The free energy values ΔGq which they have found by temperaturedependent 31P NMR measurements or computational investigations (ΔGq = 23-47 kJ mol-1) are in good agreement with our results obtained from the 1H NMR measurements. Because of these findings, a rollover/twist motion for complexes 5 similarly to the reported ones18 presumably links two identical Cs-symmetrical molecules in solution, as shown in Figure 2. In our first attempts to use complexes 5 as carbene transfer agents, it turned out that these silver complexes were not able to transfer their ligand to another transition metal. This incapability can be explained by the strength of the Ag-Ccarbene bond. In the case of BF4 complex 5, a strong Ag-Ccarbene bond was indicated, as mentioned before, by the 107/109Ag-13C couplings. It is known from the literature that silver complexes which lack these Ag-carbene couplings are typically useful carbene transfer reagents.13 Therefore, it was necessary to synthesize different N-heterocyclic carbene silver complexes as carbene transfer reagents. Instead of preparing the free carbene and reacting it with a silver salt, we followed the procedure reported by Lin and (18) Takenaka, K.; Obora, Y.; Jiang, L. H.; Tsuji, Y. Organometallics 2002, 21, 1158–1166. (19) Bagatin, I. A.; Matt, D.; Thoennessen, H.; Jones, P. G. Inorg. Chem. 1999, 38, 1585–1591.

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Figure 1. Molecular structure of 5a: (a) ORTEP plot, showing two different views of one of the four independent molecules (thermal ellipsoids drawn at the 20% probability level); (b) two perspective views showing the Ag-Ag and π-π stacking interactions between the two independent molecules. Table 1. Selected Geometrical Data for the Four Independent Calixarene Molecules in the Crystal Structure of 5a calixarene 1

calixarene 2

calixarene 3

calixarene 4

2.069(12) 2.096(11) 177.9(4) 3.749(2) (Ag3-Ag3II)

2.069(11) 2.081(11) 178.5(4) 3.678(2) (Ag4-Ag4III)

Bond Distances and Angles Ag-Ccarbene (A˚) — CNHC-Ag-CNHC (deg) Ag-Aga (A˚)

2.093(9) 2.095(9) 175.5(4) 3.607(1) (Ag1-Ag2I)

2.067(9) 2.074(10) 175.7(4) 3.607(1) (Ag2-Ag1I) Angles between Planes

b

C2-C8-C14-C20 C52-C58-C64-C70c ref plane (CH2)4 angle with aryl ring planes (deg) 85.0(2), 39.2(3), 85.3(2), 39.2(3) 86.7(2), 35.4(3), 83.0(2), 31.1(3) a Symmetry codes: (I) x, y, z; (II) -x, 1 - y, -z; (III) 1 - x, -y, 1 - z. b Deviations from least-squares plane: (0.046 A˚. c Deviations from least-squares plane: (0.071 A˚.

co-workers,20 who prepared carbene transfer agents by using Ag2O as a basic silver reagent in an in situ deprotonation of imidazolium salts in CH2Cl2 at room temperature. Our first results after reacting imidazolium salt 2a with Ag2O under these conditions for 2 h were rather discouraging. The 1 H NMR spectrum of the isolated product still showed the downfield imidazolium C(2)-H, indicating that actually no reaction had taken place. This result could be explained by the high steric demand of the calix[4]arene next to the imidazolium cation, which affects the ability of the silver oxide to effectively (20) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972–975.

deprotonate the imidazolium salt. Similarly, Tulloch and coworkers21 reported that silver oxide reactions with bulky imidazolium salts often require elevated reaction temperatures. Heating a solution of imidazolium salt 2a in CH2Cl2 with Ag2O for 3 days resulted in a brown solution, indicating the uptake of the insoluble silver oxide. Subsequent workup gave a pale brown solid. The 1H NMR spectrum showed no peak in the downfield area of the acidic imidazolium C(2)-H, which suggested the formation of the new (21) Tulloch, A. A. D.; Danopoulos, A. A.; Scott, W.; Kleinhenz, S.; Eastham, G. Dalton Trans. 2000, 4499–4506.

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Figure 2. Part of the VT 1H NMR spectra of 5b and proposed motion in solution. Scheme 2. Synthesis of Silver-Based Carbene Transfer Agentsa

a

Reagents and conditions: (i) Ag2O, CH2Cl2, 40-65 C, 3-5 days.

NHC-calix[4]arene-silver complex 6, the composition of which was confirmed by elemental analysis.22 The yield of the obtained silver complex was very good (80%). In comparison with complex 5, the 13C NMR spectrum of complex 6 in CDCl3 lacks the signal for the Ccarbene atom, and therefore also no couplings between 13Ccarbene and the 107/109Ag nuclei (22) For a comparable complex see for example: Huang, W.; Zhang, R.; Zou, G.; Tang, J.; Sun, J. J. Organomet. Chem. 2007, 692, 3804–3809.

could be observed. This identified complex 6 as a potential candidate as a carbene transfer agent. The spectrum observed from MALDI-TOF mass spectrometry showed a base peak at m/z 889.1 which corresponds to the complex cation ([Ag(Ligand)]þ). Under similar conditions it was also possible to transform imidazolium salts 2c,d to the corresponding silver-NHC complexes 7 and 8 in good yields. Iridium Complex. To test the scope and limitation of the synthesized supramolecular carbene transfer reagents 6-8,

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additional transition-metal complexes were targeted. First, an iridium complex was achieved by the reaction of the chloro-1,5-cyclooctadiene iridium(I) dimer23 with the silver complex 6 (Scheme 3). After the mixture was heated in dichloromethane, a precipitate of AgCl indicated the formation of the desired iridium complex; however, purification of the complex 9 was not possible by column chromatography. Only the molecular fragment [M - Cl]þ at m/z 1081.5 in the MALDI-TOF mass spectra hinted at the formation of the desired compound. Therefore, the synthetic strategy was changed and the monoligated iridium complex 10 was prepared in 47% yield by the reaction of bis(mono-NHCcalixarene)-silver complex 7 with the iridium precursor for 24 h in dichloromethane (Scheme 3).23a Platinum Complexes. For the syntheses of a platinum cisdi-NHC complex, the silver complex 8 was reacted at elevated temperature in dichloromethane with cis-bis(acetonitrile)dichloroplatinum(II). Again, after 24 h precipitation of AgCl indicated successful transformation and the cis complex 11 could be obtained in excellent yield. A nonsymmetrical cis configuration was indicated in the 1H NMR spectrum by the splitting of the proton signals of the methylene bridges between the calixarene skeleton and the metalorganic framework into two AB system with J = 14.7 and 14.9 Hz, respectively. Additionally, the resonance of the carbene carbon atom at 160.4 ppm is in the usual region of cis platinum complexes.12 Ruthenium Complexes. The synthesis of a mono-NHC ruthenium complex was also possible by following the carbene transfer route (Scheme 3). Again, silver complex 7 was reacted with bis(μ-chloro)bis[η6-(p-cymene)dichlororuthenium(II)] as ruthenium precursor24 in boiling dichloromethane. Nonreacted ruthenium precursor was removed by standard column chromatography, and the pure orange, air-stable ruthenium calixarene complex 12 could be isolated in 84% yield. It was not possible to grow crystals of the complex 12 which were suitable for X-ray analysis, but temperature-dependent proton NMR spectra of this complex indicated, as expected, a highly dynamic structure with complex intermolecular moments of motion, mainly of the η-bound cymene. Interestingly, the methyl group of this aromatic ligand is shifted to higher field and exhibits a resonance at δ 1.87 ppm vs 2.29 ppm of free cymene.25 This can be explained smoothly by self-complexation of the methyl group in the cavity of the calixarene skeleton.26 Furthermore, the aromatic protons of the benzene ring of the cymene ligand are shifted to higher field as well and exhibit resonances at δ 4.78 and 5.19 ppm, respectively. In contrast, the isopropyl group is not affected, which proved the preferential orientation of the p-cymene ring as shown in Scheme 3, similar (23) (a) Chianse, A. R.; Li, X.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H. Organometallics 2003, 22, 1663–1667. (b) Chianese, A. R.; Kovacevic, A.; Zeglis, B. M.; Faller, J. W.; Crabtree, R. H. Organometallics 2004, 23, 2461–2468. (c) Mas-Marza, M.; Sanau, M.; Peris, E. Inorg. Chem. 2005, 44, 9961–9967. (d) Herrmann, W. A.; Baskakov, D.; Herdtweck, E.; Hoffmann, S. D.; Bunlaksananusorn, T.; Rampf, F.; Rodefeld, L. Organometallics 2006, 25, 2449–2456. (24) (a) Arnold, P. L.; Scarisbrick, A. C. Organometallics 2004, 23, 2519–2521. (b) Csabai, P.; Joo, F. Organometallics 2004, 23, 5640–5643. (c) Poyatos, M.; Maisse-Francois, A.; Bellemin-Laponnaz, S.; Peris, E.; Gade, L. H. J. Organomet. Chem. 2006, 691, 2713–2720. (25) Coates, R. M.; Ho, Z.; Zhu, L. J. Org. Chem. 1996, 61, 1184– 1186. (26) Rehm (nee Baur), M.; Frank, M.; Schatz, J. Tetrahedron Lett. 2009, 50, 93–96. (27) Lejeune, M.; Jeunesse, C.; Matt, D.; Kyritsakas, N.; Welter, R.; Kintzinger, J.-P. Dalton Trans. 2002, 1642–1650.

Fahlbusch et al. Scheme 3. Carbene Transfer Reactions of Silver Complexes 6-8a

a Reagents and conditions: (i) [Ir(COD)Cl]2, CH2Cl2, 55 C, 3 days; (ii) [Ir(COD)Cl]2, CH2Cl2, 55 C, 1 day; (iii) cis-[Pt(MeCN)2Cl2]CH2Cl2, 55 C, 24 h; (iv) [Ru(p-cymene)Cl2]2, CH2Cl2, 55 C, 2 days; (v) transPd(MeCN)2Cl2, CH2Cl2, 55 C, 3 days.

to a mono-NHC ruthenium diphenylphosphino calixarene complex studied by Matt and co-workers.27 Unfortunately, the ruthenium complex 12 showed no activity in Grubbs-type metathesis under various conditions tried, although we could recently show that addition of imidazolium-substituted calixarenes do exhibit a considerable effect on metathesis reactions using Grubbs-type catalysts in aqueous media.28 (28) Brendgen, T.; Fahlbusch, T.; Frank, M.; Sch€ uhle, D. T.; Sessler, M.; Schatz, J. Adv. Synth. Catal. 2009, 351, 303–307.

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Palladium Complexes. In the next step we investigated the ability of complex 6 to transfer its ligand to another transition metal. Because our goal was to synthesize trans-substituted palladium complexes, we reacted at first complex 6 with trans-[Pd(MeCN)2]Cl220 in CH2Cl2, again under heating conditions. The reaction proceeded smoothly, and complex 13 was obtained in a yield (38%) 3 times higher than the yield obtained for the analogous cis Pd complex 3. The trans configuration of complex 13 was verified by 1H NMR, 13C NMR, and IR spectroscopy. In comparison to complex 3 the 1 H NMR spectrum of dynamic complex 13 exhibits one broad singlet at 5.48 ppm for the protons of the CH2 groups, which link the NHCs to the calix[4]arene, instead of two individual sharp doublets. This is due to the conformational mobility of the trans configuration which leads to averaged, chemically equivalent protons of the CH2 groups, whereas in complex 3 the chemical equivalence of these protons is canceled out by virtue of the cis configuration leading to a geminal coupling. Both complexes can also be identified using 13C NMR on account of the number of peaks: e.g., 14 peaks of aromatic carbons for complex 3 and only 8 peaks of aromatic carbons of complex 13, indicating higher symmetry. The 13C signal of the Ccarbene appears for trans complex 7 at δ 171.5 ppm and at 160.2 ppm for cis complex 3. These chemical shifts are consistent with those in the literature for other cis/trans isomeric NHC-Pd complexes.29,30 Herein the Ccarbene signal of the cis isomer was found in general to be around 160 ppm; that of the trans isomer was found at around 170 ppm. The upfield shift of the Ccarbene signal for cis Pd complex 3 relative to the Ccarbene signal in the trans Pd complex 13 can be easily related to the trans effect of the N-heterocyclic carbene as a strong binding σ-donor ligand in square-planar complexes.6c Vibrational spectroscopy is also a useful method to distinguish between cis and trans isomers. As expected, the IR spectrum of the trans isomer 13 exhibits only one Pd-Cl vibration at 360 cm-1, while the cis isomer 3 exhibits two at 353 and 331 cm-1. Owing to the fact that NHC-Pd complexes31 are wellknown catalysts in the Suzuki-Miyaura reaction32 over the past few years, many different catalysts of this kind have been applied to this cross-coupling reaction; meanwhile, not only isolated NHC-Pd complexes but also in situ systems are known.33,34 To evaluate their catalytic activity, the Pd complexes 3 and 13 were tested in the Suzuki cross-coupling reaction of 4-chlorotoluene and phenylboronic acid to yield 4-methylbiphenyl using a standard setup:9 All components, including the aromatic compounds, Cs2CO3 as the base, the respective catalyst, and dioxane as solvent, were loaded at once in a (29) Clyne, D. S.; Jin, J.; Genest, E.; Gallucci, J. C.; RajanBabu, T. V. Org. Lett. 2000, 2, 1125–1128. (30) Bonnet, L. G.; Douthwaite, R. E.; Hodgson, R. Organometallics 2003, 22, 4384–4386. (31) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem. 2007, 119, 2824-2870; Angew. Chem., Int. Ed. 2007, 46, 2768-2813. (32) (a) Gst€ ottmayr, C. W. K.; B€ ohm, V. P.; Herdtweck, E.; Grosche, M.; Herrmann, W. A. Angew. Chem. 2002, 114, 1421-1423; Angew. Chem., Int. Ed. 2002, 41, 1363-1365. (b) Marion, N.; Navarro, O.; Mei, J.; Stevens, E. D.; Scott, N.; Nolan, S. P. J. Am. Chem. Soc. 2006, 128, 4101– 4111. (c) Krenzow, D.; Seidel, G.; Lehmann, C. W.; F€urstner, A. Chem. Eur. J. 2005, 11, 1833–1853. (d) Navarro, O.; Kaur, H.; Mahjoor, P.; Nolan, S. P. J. Org. Chem. 2004, 69, 3173–3180. (e) Singh, R.; Viciu, M. S.; Kramareva, N.; Nolan, S. P. Org. Lett. 2005, 7, 1829–1832. (33) Zhao, Y.; Zhou, Y.; Ma, D.; Liu, J.; Li, L.; Zhang, T. Y.; Zhang, H. Org. Biomol. Chem. 2003, 1, 1643–1646. (34) Grasa, G. A.; Viciu, M. S.; Huang, J.; Zhang, C.; Trudell, M. L.; Nolan, S. P. Organometallics 2002, 21, 2866–2873.

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Scheme 4. Suzuki Coupling Catalyzed by Isolated Pd-NHCcalix[4]arene Complexes 3 and 13a

a Reaction conditions: 0.79 mmol of 4-chlorotoluene, 1.19 mmol of phenylboronic acid, 0.54 mmol of Cs2CO3, 2.5 mol % catalyst, 6 mL of dioxane, 80 C. Conversions were determined by integration of the 1H NMR spectra of the reaction mixture after 48 h reaction time (averaged yields from three independent runs).

reaction vessel, and the reaction mixture was heated for 48 h at 80 C. The conversions were monitored by 1H NMR spectroscopy. The results are summarized in Scheme 4. Furthermore, it was proven by AAS that the isolated Pd complexes 3 and 13 did not contain any remaining silver ions stemming from the synthesis that might influence the catalytic activity. Obviously, the configuration of the two complexes did not have any influence on the catalytic activity. Both complexes yielded about 30% of biaryl (Scheme 4). This finding led us to assume that both isomers generate the same catalytically active species. Starting from the isolated cis or trans Pd complexes, two pathways for the formation of the same catalytic species are likely: on the first pathway, a NHC unit simply dissociates from the palladium center, forming a monoligated species which would be the same starting from either the cis or the trans complex. Alternatively, a cis/trans isomerization of the highly strained cis isomer 3 to the less strained trans isomer 13 before dissociation of the carbene unit could occur. In order to exclude one of the two possible pathways, we tested whether the cis complex 3 can undergo cis f trans isomerization.35 Therefore, a solution of complex 3 was treated with 2.2 equiv of Bu4NCl in dioxane-d8. Even after 7 days at room temperature followed by an additional 2 h at 80 C no isomerization could be observed by NMR spectroscopy. A similar treatment with 2.2 equiv of pyridine did not affect the cis complex 3 either. Therefore, it is likely that the active species is formed via direct dissociation of a carbene fragment. A trans f cis isomerization would lead to an increase in steric strain and is therefore not likely. Accordingly, heating the trans complex 13 (80 C, 72 h) did not indicate any trans f cis isomerization; only a high extent of decomposition could be observed, without formation of any identifiable compounds. Additionally, we wanted to compare the catalytic activity of the isolated complexes with in situ systems based on N-methylimidazolium-calix[4]arenes, Pd(OAc)2, and Cs2CO3. In addition to the already reported bis-1-methylimidazolium salt 2a, we decided to include also the mono-1-methylimidazolium salt 2c into the study. The latter would be an ideal candidate for the formation of monoligated catalytic species known to be useful catalysts in cross-coupling reactions.36,37 To test the performance of the imidazolium salts 2a,c as ligand precursors in in situ systems and in order to compare their catalytic activities with those of the isolated complexes 3 and 13, the Suzuki-Miyaura reaction of 4-chlorotoluene (35) Cross, R. J. Adv. Inorg. Chem. 1989, 34, 219. (36) Christmann, U.; Vilar, R. Angew. Chem. 2005, 117, 370-378; Angew. Chem., Int. Ed. 2005, 44, 366-374. (37) Phan, N. T. S.; Van der Sluys, M.; Jones, C. W. Adv. Synth. Catal. 2006, 348, 609–679.

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was chosen as a test again. Using the same conditions as reported in Scheme 4 as a standard setup, the bis-imidazolium salt 2a and mono salt 2c gave identical yields of 4-methylbiphenyl within experimental error (2a, 8%; 2c, 10%). Again, the formation of a monoligated Pd-NHC complex as the active species in the catalytic process seems to be obvious. If a chelate or bis-NHC complex would have been necessary, the yields in the case of ligand 2c would have dropped significantly. This is in agreement with the reported fact that an imidazolium salt to Pd ratio of 1:1 gave the best results in the in situ catalyses.34 The yields obtained by the in situ system were about 3 times lower than the yields achieved by the isolated complexes. Alcalde, Dinares and co-workers came basically to similar results, with comparable calixarenes bearing imidazolium salts directly attached at the upper rim of the calixarene skeleton. Here, the isolated palladium complexes are superior to the in situ system starting from the imidazolium salts as ligand precursors.8 One reason for this decrease in activity might be the presence of water in the in situ systems due to the hygroscopic imidazolium salts. Recently, we could prove that the yields of the Suzuki reaction decrease by adding water to an imidazolium salt/Pd(OAc)2 catalytic system. Although our macrocyclic systems were also able to catalyze the reaction in pure water, a drop in the yield was observed when the solvent was changed from pure dioxane to dioxane/water mixtures.9 However, the catalytic cycle seems to be unchanged by trace water; only the efficiency is affected.

Conclusion In this paper we have reported the first synthesis of imidazol-2-ylidene-calix[4]arene complexes of mercury, silver, iridium, platinum, ruthenium, and palladium. We have shown that imidazolium salts based on the calix[4]arene skeleton 2 are ideal precursors both for linear NHC complexes of Hg(II) and Ag(I) owing to the spatial arrangement of the ligands provided by the calix[4]arene scaffold. The yields of the macrocyclic complexes are much higher than those of our previously reported cis-configured square-planar palladium complex and might therefore be a result of the less strained bridging. The single-crystal X-ray structure determination of 5a together with variable-temperature 1H NMR studies enabled us additionally to prove a dynamic behavior of the bis-NHC-silver-calix[4]arene complexes 5. Further development led to the supramolecular Ag-NHCcalix[4]arene transfer agents 6-8 in high yield which can be used without further purification for the carbene transfer reaction forming novel NHC complexes with iridium, platinum, ruthenium, and palladium. Compound 6 was successfully used to transfer the supramolecular NHC ligand to palladium, resulting in the transconfigured complex 13. In comparison to the yield for our previously reported cis Pd complex 3, the yield of the trans complex 13 is 4 times higher. The new trans Pd(II)-NHCcalix[4]arene complex 13 and the cis-Pd(II)-NHC-calix[4]arene complex 3 were tested subsequently as catalysts in the Suzuki-Miyaura cross-coupling reaction of 4-chlorotoluene. The complexes are inert concerning cis/trans isomerization and develop both the same catalytically active species which is supposed to be a monoligated Pd complex. In comparison to the in situ systems both isolated complexes exhibit independently of their configuration a 3 times higher catalytic activity.

Fahlbusch et al.

Experimental Section General Methods. Melting points were determined on a B€ uchi B-540 apparatus and are uncorrected. Infrared (IR) spectra were obtained on a Bruker Vector 22 instrument from KBr pellets unless otherwise stated. Absorptions (ν) are given in wavenumbers (cm-1). NMR spectra were recorded on a Bruker DRX 400 instrument (400.13 MHz for 1H and 100.62 MHz for 13C) at 300 K unless otherwise stated. Tetramethylsilane was used for the 1H NMR spectra as internal standard (δ 0.00 ppm), the solvent signals for the 13C NMR spectra (δ(CDCl3) 77.0, δ(DMSO-d6) 39.5, δ(methanol-d4) 49.3 ppm) and C6F6 (δ(C6F6) = -162.90 ppm) as external standard for 19F NMR spectra. Chemical shifts (δ) are given in ppm and coupling constants (J) in Hz. Assignments of 13C chemical shifts are based on proton-decoupled, C/H-correlation, and DEPT-135 spectra. All mass spectra (MALDI-TOF) were obtained on a Bruker Daltonics Reflex III instrument using DHB as matrix. Solvents were dried by standard procedures. All reaction mixtures were stirred magnetically, unless otherwise noted. Imidazolium salts 2a-c were prepared as reported before.7,10 {5,17-Bis[(3-methylimidazol-2-ylidene)methyl]-25,26,27,28-tetrapropyloxycalix[4]arene]}mercury(II) Dichloride (4). To a solution of 2a (1.50 g, 1.76 mmol) in DMSO (10 mL) was added Hg(OAc)2 (616 mg, 1.93 mmol) under argon. After the mixture was stirred at 50 C for 30 min, the temperature was raised to 90 C for an additional 3 h, before the solvent was removed under vacuum. The obtained residue was then recrystallized twice from CHCl3/n-hexane to yield a colorless product. Yield: 1.14 g (1.08 mmol, 62%). Mp: >295 C. IR (KBr): ν 3097 (w) (Ar-H); 2960 (s), 2931 (s), 2873 (s) (C-H); 1568 (m) (CdC and CdN); 1462 (s), 1385 (m), 1305 (w) (C-H); 1281 (m), 1218 (s), 1169 (w), 1129 (m), 1039 (m), 1006 (s) (Ar-O-C); 889 (w), 836 (w), 761 (m) (Ar-H). 1H NMR (DMSO-d6, 373 K): δ 0.90 (t, J = 7.5 Hz, 6 H, CH2CH3), 1.08 (t, J = 7.5 Hz, 6 H, CH2CH3), 1.79-1.86 (m, 4 H, CH2CH3), 1.90-1.97 (m, 4 H, CH2CH3), 3.10 (d, J = 13.1 Hz, 4 H, ArCH2Ar), 3.66 (t, J = 6.5 Hz, 4 H, ArOCH2), 4.03 (t, J = 7.9 Hz, 4 H, ArOCH2), 4.23 (s, 6 H, NCH3), 4.38 (d, J = 13.1 Hz, 4 H, ArCH2Ar), 5.10 (s, 4 H, ArCH2Im), 5.76 (s, 4 H, ArH), 6.93 (t, J = 7.5 Hz, 2 H, ArH), 7.02 (d, J = 7.5 Hz, 4 H, ArH), 7.29 (t, J = 1.9 Hz, 2 H, Im), 7.61 (t, J = 1.9 Hz, 2 H, Im). 13C NMR (DMSO-d6): δ 9.83, 10.84 (CH2CH3), 22.59, 23.22 (CH2CH3), 30.22 (ArCH2Ar), 38.26 (NCH3), 48.76 (ArCH2Im), 76.11, 77.12 (OCH2), 122.54, 123.71 (ArC), 123.96, 125.95 (ImC), 128.88, 129.82, 133.31, 136.07, 154.01, 157.07 (ArC), 177.55 (ImC). MS (MALDI-TOF): calcd for C50H60N4O4ClHg 1017.4, found m/z 1017.3 [(M-Cl)þ]. Anal. Calcd for C50H60Cl2Hg N4O4 3 1.3CHCl3 3 1.1(n-hexane): C, 53.39; H, 5.94; N, 4.30. Found: C, 53.25; H, 5.74; N, 4.58. General Procedure for the Preparation of NHC-calix[4]arene-silver(I) Tetrafluoroborate Complexes (5). Under argon, a suspension of the appropriate calixarene imidazolium salt 2 in THF was cooled to 0 C, before KO-t-Bu was added. After the ice bath was removed, the mixture was stirred with warming to room temperature, before AgBF4 was added. After additional stirring at room temperature for 4 h, the solvent was removed under vacuum. The dark brown residue was then dissolved in dichloromethane, filtered over Celite, and evaporated to dryness. Purification was achieved by flash chromato graphy over silica, using acetone as eluent. {5,17-Bis[(3-methylimidazol-2-ylidene)methyl]-25,26,27,28tetrapropyloxycalix[4]arene}silver Tetrafluoroborate (5a). The product was obtained as a light brown solid from the reaction of 2a (1.50 g, 1.76 mmol), KO-t-Bu (404 mg, 3.60 mmol), and AgBF4 (1.03 g, 5.27 mmol) in THF (100 mL). Yield: 647 mg, 0.66 mmol, 38%. Mp: >206 C dec. IR (KBr): ν 3165 (w), 3136 (w) (Ar-H); 2962 (s), 2932 (m), 2875 (m) (C-H); 1777 (w), 1609 (w), 1586 (w) (CdC and CdN); 1466 (s), 1436 (m), 1407 (w), 1386 (m), 1351 (w) (C-H); 1282 (m), 1245 (m), 1218 (s), 1165

Article (m), 1058 (s), 1008 (s) (Ar-O-C); 889 (w), 854 (w), 769 (w) (Ar-H). 1H NMR (CDCl3): δ 0.89 (t, J = 7.6 Hz, 6 H, CH2CH3), 1.06 (t, J = 7.3 Hz, 6 H, H2CH3), 1.82-1.90 (m, 4 H, CH2CH3), 1.96-2.06 (m, 4 H, CH2CH3), 3.06 (d, J = 13.1 Hz, 4 H, ArCH2Ar), 3.61 (t, J = 6.8 Hz, 4 H, ArOCH2), 4.00 (s, 6 H, NCH3), 4.01 (t, J = 8.3 Hz, 4 H, ArOCH2), 4.40 (d, J = 12.8 Hz, 4 H, ArCH2Ar), 4.76 (s, 4 H, ArCH2Im), 5.78 (s, 4 H, ArH), 6.78 (t, J = 1.6 Hz, 2 H, Im), 6.92 (t, J = 7.3 Hz, 2 H, ArH), 7.01 (d, J = 7.3 Hz, 4 H, ArH), 7.19 (t, J = 1.5 Hz, 2 H, Im). 13C NMR (CDCl3): δ 9.71, 10.68 (CH2CH3), 22.82, 23.41 (CH2CH3), 30.72 (ArCH2Ar), 38.55 (NCH3), 53.99 (ArCH2Im), 76.45 and 77.36 (OCH2), 121.64 (d, 3J109/107Ag-13C = 5.9 Hz, ImC), 122.13 (ArC), 123.79 (d, 3J109/107Ag-13C = 5.9 Hz, ImC), 124.11, 128.81, 129.04, 133.72, 136.42, 154.60, 157.40 (ArC), 181.32 (2  d, 1J107Ag-13C = 180.0 Hz and 1J109Ag-13C = 207.8 Hz, AgC). 19F NMR (CDCl3): δF -154.11 (BF4-). MS (MALDITOF): calcd for C50H62AgN4O4 889.4, found m/z 889.4 [(M - BF4)þ]. Suitable crystals were obtained by slow evaporation of a p-xylene/CH2Cl2 solution of 5a at room temperature. A crystal was taken from the mother liquor, coated with a perfluorocarbon oil, and directly mounted on the diffractometer device cooled at -80 C. Data collection: imaging-plate diffractometer (IPDS, Stoe), monochromatized Mo KR radiation (λ=0.710 73 A˚), T = 193(2) K; crystal size 0.58  0.27  0.15 mm; 67 735 reflections collected in the range 2.50 e θ e 23.26; 30 861 independent reflections (Rint = 0.0792, -12 e h e 12, -28 e k e 28, -46 e l e 46). Crystal data: C50H60AgBF4N4O4 3 1.125CH2Cl2 3 0.5C8H10 (=p-xylene), fw 1124.32; triclinic space group P1, a = 11.021(3) A˚, b = 26.011(7) A˚, c = 41.515(9) A˚, R = 105.39(3), β = 97.43(3), γ=93.75(3)o, V=11316(5) A˚3; Z=8, F(calcd) =1.32 Mg m-3, μ(Mo KR) = 0.523 mm-1. Structure solution and refinement: calculations were done using the program package SHELX-97.38 Full-matrix least-squares refinement, using two parameter blocks (2482 parameters, 15 restraints), converged to R values R1 = 0.0714 (I > 2σ(I)), R1=0.1718 (all data), and wR2 = 0.2240 (all data). Some carbon atoms in the propyloxy side chains were positionally ill-defined due to static and/or dynamic disorder; they were treated with isotropic thermal parameters, and the bonded hydrogen atoms were not included. The dichloromethane and pxylene solvate molecules were also not well localized; therefore, restraints were imposed on some C-Cl bond lengths and a rigidbody refinement was used for one of the xylene rings. An occupancy factor of 0.5 was given to one of the five CH2Cl2 molecules in the asymmetric unit. {5,17-Bis[(3-isopropylimidazol-2-ylidene)methyl]-25,26,27,28tetrapropyloxycalix[4]arene}silver Tetrafluoroborate (5b). The product was obtained from the reaction of 2b (1.59 g, 1.75 mmol), KO-t-Bu (404 mg, 3.60 mmol), AgBF4 (1.03 g, 5.27 mmol), and THF (100 mL). Light brown product. Yield: 1.10 g, 1.07 mmol, 61%. Mp: >258 C dec. IR (KBr): ν 3161 (w), 3134 (w) (Ar-H); 2963 (s), 2931 (m), 2874 (m) (C-H); 1775 (w), 1675 (w), 1586 (w), 1561 (w) (CdC and CdN); 1464 (s) (C-H); 1383 (m) (CH(CH3)2); 1346 (w) (C-H); 1284 (m), 1220 (s), 1192 (m), 1128 (m), 1055 (s) (Ar-O-C); 889 (w), 857 (w), 836 (w), 768 (m), 734 (w) (Ar-H). 1H NMR (CDCl3): δ 0.88 (t, J = 7.5 Hz, 6 H, CH2CH3), 1.08 (t, J = 7.3 Hz, 6 H, CH2CH3), 1.69 (d, J = 6.8 Hz, 12 H, CH(CH3)2), 1.82-1.93 (m, 4 H, CH2CH3), 1.96-2.05 (m, 4 H, CH2CH3), 3.06 (d, J = 13.1 Hz, 4 H, ArCH2Ar), 3.61 (t, J = 6.8 Hz, 4 H, ArOCH2), 4.02 (t, J = 8.3 Hz, 4 H, ArOCH2), 4.40 (d, J = 12.9 Hz, 4 H, ArCH2Ar), 4.70 (sept, J = 6.7 Hz, 2 H, CH(CH3)2), 4.79 (s, 4 H, ArCH2Im), 5.76 (s, 4 H, ArH), 6.81 (t, J = 1.6 Hz, 2 H, Im), 6.85 (t, J = 7.5 Hz, 2 H, ArH), 6.98 (d, J = 7.3 Hz, 4 H, ArH), 7.21 (t, J = 1.8 Hz, 2 H, Im). 13C NMR (CDCl3): δ 9.68, 10.68 (CH2CH3), 22.83, 23.40 (CH2CH3), 24.06 (CH(CH3)2), 30.74 (ArCH2Ar), 53.97 (CH(CH3)2), 54.51 (38) Sheldrick, G. M. SHELX-97, Program for Crystal Structure Determination from Single-Crystal Diffraction Data; University of G€ ottingen, G€ ottingen, Germany, 1997.

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(ArCH2Im), 76.46, 77.20 (OCH2), 119.73 (d, 3J109/107Ag-13C = 5.9 Hz, ImC), 121.49 (d, 3J109/107Ag-13C = 5.1 Hz, ImC), 122.05, 124.07, 128.71, 129.16, 133.65, 136.51, 154.58, 157.49 (ArC), 178.99 (2  d, 1J107Ag-13C = 180.0 Hz and 1J109Ag-13C = 207.8 Hz, AgC). 19F NMR (CDCl3): δF -154.12 (BF4-). MS (MALDI-TOF): calcd for C54H68AgN4O4 945.4, found m/z 945.4 [(M - BF4)þ]. {5,17-Bis[(3-methylimidazol-2-ylidene)methyl]-25,26,27,28tetrapropyloxycalix[4]arene}silver(I) Dichloroargentate (6). To a solution of 2a (500 mg, 0.59 mmol) in CH2Cl2 (30.0 mL) was added an excess of Ag2O (273 mg, 1.18 mmol). After the mixture was heated under reflux for 3 days, the reaction mixture was cooled to room temperature, filtered through Celite, and dried over Na2SO4. Subsequent evaporation of the solvent gave the carbene-transfer agent as a brown solid. The product could be used without further purification. For a suitable elemental analysis the product was recrystallized from CH2Cl2/n-hexane. Yield: 505 mg, 0.47 mmol, 80%. Mp: >160 C dec. IR (KBr): ν 3093 (m) (Ar-H); 2960 (s), 2930 (s), 2873 (s) (C-H); 1586 (w) (CdC); 1460 (s), 1385 (m), 1353 (w), 1303 (m) (C-H); 1280 (m), 1218 (s), 1171 (m), 1129 (m), 1080 (m), 1038 (m) (Ar-O-C); 1006 (s); 963 (m), 888 (w), 763 (m) (Ar-H). 1H NMR (CDCl3): δ 0.93 (t, J = 7.5 Hz, 6 H, CH2CH3), 1.03 (t, J = 7.5 Hz, 6 H, CH2CH3), 1.92 (m, 8 H, CH2CH3), 3.13 (d, J = 13.4 Hz, 4 H, ArCH2Ar), 3.74 (t, J = 7.2 Hz, 4 H, ArOCH2), 3.84 (s, 6 H, NCH3), 3.92 (t, J = 7.8 Hz, 4 H, ArOCH2), 4.42 (d, J = 13.1 Hz, 4 H, ArCH2Ar), 4.82 (s, 4 H, ArCH2Im), 6.34 (s, 4 H, ArH), 6.78 (d, J = 1.8 Hz, 2 H, Im), 6.80 (m, 2 H, ArH), 6.86 (d, J = 6.8 Hz, 4 H, ArH), 6.97 (d, J = 1.8 Hz, 2 H, Im). 13C NMR (CDCl3): δ 9.97, 10.44 (CH2CH3), 22.98, 23.29 (CH2CH3), 30.82 (ArCH2Ar), 38.78 (NCH3), 55.16 (ArCH2Im), 76.56 (OCH2), 121.08, 122.38 (ImC), 122.54, 127.19, 128.48, 128.72, 135.10, 135.37, 156.23, 156.78 (ArC). MS (MALDI-TOF): calcd for C50H60N4O4Ag 889.4, found m/z 889.1 [(Ag(ligand))þ]. Anal. Calcd for C50H60Ag2Cl2N4O4: C, 56.25; H, 5.66; N, 5.25. Found: C, 56.37; H, 5.78; N, 4.99. Bis{5-[(3-methylimidazolin-2-ylidene)methyl]-25,26,27,28tetrapropyloxycalix[4]arene}silver(I) Dichloroargentate (7). To a solution of 5-[(3-methylimidazolio)methyl]-25,26,27,28-tetrapropyloxycalix[4]arene (2c; 100 mg, 0.14 mmol) in CH2Cl2 (10.0 mL) was added Ag2O (16.2 mg, 0.07 mmol), and the resulting mixture was stirred at 50 C under the exclusion of light. After 3 days further Ag2O (32.0 mg, 0.14 mmol) was added and the temperature was raised to 65 C. After it was heated for another day, the solution was filtered through Celite, the solvent removed, and the resulting brown solid dried in vacuo. Yield: 150 mg (0.09 mmol, 65%). Mp: >243 C dec. IR (KBr): ν 3058 (w) (Ar-H); 2961 (s), 2931 (s), 2872 (s) (C-H); 1586 (w) (CdC); 1459 (s), 1385 (m), 1282 (m) (C-H); 1259 (s), 1215 (s), 1194 (s), 1127 (m), 1087 (s), 1039 (s) (Ar-O-C); 1008 (s); 965 (m); 889 (w), 773 (s) (Ar-H) cm-1. 1H NMR (400 MHz, CDCl3): δ 0.88 (t, J = 7.5 Hz, 6 H, CH2CH3), 0.96 (t, J = 7.5 Hz, 3 H, CH2CH3), 0.96 (t, J = 7.5 Hz, 3 H, CH2CH3), 1.84 (m, 8 H, CH2CH3), 3.07 (pseudo t, J = 13.3 Hz, 2 H, ArCH2eqAr), 3.70 (m, 4 H, OCH2), 3.72 (s, 3 H, NCH3), 3.84 (m, 4 H, OCH2), 4.36 (d, J = 13.4 Hz, 2 H, ArCH2axAr), 4.38 (d, J = 13.1 Hz, 2 H, ArCH2axAr), 4.71 (s, 2 H, Ar CH2Im) 6.17 (s, 2 H, ArH), 6.36 (d, J = 1.5 Hz, 1 H, ImH), 6.38 (m, 1 H, ArH), 6.44 (m, 2 H, ArH), 6.62 (t, J = 7.6 Hz, 2 H, ArH), 6.68 (d, J = 7.3 Hz, 1 H, ArH), 6.68 (d, J = 7.6 Hz, 1 H, ArH), 6.74 (m, 3 H, ArH and ImH). 13C NMR (100 MHz, CDCl3): δ 10.09, 10.45 (CH2CH3), 23.08, 23.30, 23.32 (CH2CH3), 30.88, 30.94 (ArCH2Ar), 38.38 (NCH3), 55.29 (ArCH2Im), 76.61, 76.96 (OCH2), 121.25, (ImC), 121.44 (ArC),.121.76 (ImC), 122.07, 127.34, 127.81, 128.11, 128.40 128.53, 134.56, 135.30, 135.38, 135.77, 156.21, 156.44, 156.90 (ArC). MS (MALDI-TOF): calcd for C45H54O4N2Ag: 793.3, found m/z 792.8 [Ag(Ligand)]þ. Anal. Calcd for C90H108Ag2Cl2N4O8 (1660.50): C, 65.10; H, 6.56; N, 3.37. Calcd for C90H108Ag2Cl2N4O8 3 0.12CH2Cl2 (1670.69): C, 64.79; H, 6.53; N, 3.35. Found: C, 64.57; H, 6.34; N, 3.05.

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5,11-Bis[(3-methylimidazolio)methyl]-25,26,27,28-tetrakis(ethoxyethyl)calix[4]arene (2d). A solution of 5,11-bis(chloromethyl)-25, 26,27,28-tetrakis(ethoxyethyl)calix[4]arene39 (300 mg, 0.37 mmol) and 1-methylimidazole (117 μL, 1.47 mmol) in dry CHCl3 (5.00 mL) was heated under reflux for 3 days. After removal of the solvent the resulting hygroscopic solid was digested with Et2O (30 mL) and dried in vacuo at 50 C. Yield: 308 mg (0.32 mmol, 85%). Mp: 258 C. IR (KBr): ν 3075 (m) (Ar-H); 2974 (s), 2925 (s), 2865 (s) (C-H); 1663 (w) (CdN); 1569 (m) (CdC); 1456 (s), 1384 (m), 1353 (m), 1286 (m) (C-H); 1249 (m), 1219 (m), 1198 (m) (Ar-O-C); 1163 (s), 1120 (s) (C-O-C); 1053 (s) (Ar-O-C); 840 (w) 763 (m) (Ar-H) cm-1. 1H NMR (CDCl3): δ 1.11 (t, J = 6.8 Hz, 6 H, CH2CH3), 1.13 (t, J = 6.9 Hz, 6 H, CH2CH3), 3.08 (d, J = 13.6 Hz, 4 H, ArCH2eqAr), 3.96 (s, 6 H, NCH3), 3.46 (q, J = 6.9 Hz, 8 H, OCH2), 3.75 (m, 8 H, OCH2), 4.05 (m, 8 H, OCH2), 4.43 (m, 4 H, ArCH2axAr), 5.15 (bs, 4 H, ArCH2Im), 6.53 (t, J = 7.2 Hz, 2 H, ArH), 6.61 (m, 4 H, ArH), 6.67 (m, 4 H, ArH), 6.95 (s, 2 H, ImH), 7.57 (s, 2 H, ImH), 10.33 (s, 2 H, ImH). 13C NMR (CDCl3): δ 15.17 (CH2CH3), 30.35, 30.59, 30.77 (ArCH2Ar), 36.48 (NCH3) 66.21, 66.25, 69.46, 69.52, 73.14, 73.42 (OCH2), 121.75 (ImC), 122.23 (ArC), 123.69 (ImC), 126.91, 128.17, 128.35, 128.47, 128.66, 134.46, 135.10, 135.80, 136.02 (ArC), 137.40 (ImC), 156.10, 156.93 (ArC). MS (MALDI-TOF): calcd for C54H70ClO8N4 937.5, found m/z 937.4 [M - Cl]þ. Anal. Calcd for C54H70Cl2N4O8 (974.07): C, 66.59; H, 7.24; N, 5.75. Calcd for C54H70Cl2N4O8 3 2.1H2O (1011.90): C, 64.10; H, 7.39; N, 5.54. Found: C, 63.98; H, 7.59; N, 5.78. {5,11-Bis[(3-methylimidazolin-2-ylidene)methyl]-25,26,27,28tetrakis(ethoxyethyl)calix[4]arene}silver(I)dichloroargentate (8). To a solution of 5,11-bis[(3-methylimidazolio)methyl]-25, 26,27,28-tetrakis(ethoxyethyl)calix[4]arene (2c; 100 mg, 0.10 mmol) in CH2Cl2 (10 mL) was added Ag2O (16.2 mg, 0.07 mmol), and the resulting mixture was stirred at 50 C for 3 days under the exclusion of light. The solution was filtered through Celite, the solvent removed, and the resulting brown solid dried in vacuo. Yield: 100 mg (0.08 mmol, 84%). Mp: >120 C dec. IR (KBr): ν 2965 (m), 2924 (m), 2862 (m) (C-H); 1586 (w) (CdC); 1455 (m), 1352 (m), 1262 (s) (C-H); 1102 (s), 1020 (s) (Ar-O-C and C-O-C); 870 (m), 800 (s) (Ar-H) cm-1. 1H NMR (400 MHz, CDCl3): δ 1.11 (t, J = 7.1 Hz, 6 H, CH2CH3), 1.12 (t, J=7.1 Hz, 6 H, CH2CH3), 3.06 (pseudo t, J = 13.8 Hz, 4 H, ArCH2eqAr), 3.46 (m, 8 H, OCH2), 3.75 (m, 14 H, OCH2 and NCH3), 4.03 (m, 8 H, OCH2), 4.40 (m, 4 H, ArCH2axAr), 4.84 (pseudo t, J = 13.4 Hz, 2 H, ArCH2Im), 4.95 (d, J = 14.7 Hz, 1 H, ArCH2Im), 4.97 (d, J = 14.7 Hz, 1 H, ArCH2Im), 6.37 (s, 2 H, ArH), 6.50-6.59 (m, 8 H, ArH), 6.77 (d, J = 1.5 Hz, 2 H, ImH), 6.88 (d, J = 1.8 Hz, 2 H, ImH). 13C NMR (100 MHz, CDCl3): δ 15.25 (CH2CH3), 30.59, 30.77, 30.84 (ArCH2Ar), 39.94 (NCH3), 55.24 (ArCH2Im), 66.30, 69.58, 69.65, 73.15, 73.30 (OCH2), 121.52, 122.08 (ImC), 127.63, 127.90, 128.19, 128.29, 129.47, 134.64, 135.12, 135.54, 135.82, 156.32, 156.58 (ArC). MS (MALDI-TOF): calcd for C54H68AgN4O8 1007.4, found m/z 1007.5 [Ag(Ligand)]þ. Anal. Calcd for C54H68Ag2Cl2N4O8 (1187.79): C, 54.61; H, 5.77; N, 4.71. Calcd for C54H68Ag2Cl2N4O8 3 1.2H2O (1209.41): C, 53.63; H, 5.87; N, 4.63. Found: C, 53.63; H, 6.12; N, 4.45. Chloro(1,5-cyclooctadiene){5-[(3-methylimidazolin-2-ylidene)methyl]-25,26,27,28-tetrapropyloxycalix[4]arene}iridium(I) (10). To [Ir(COD)Cl]2 (20.2 mg, 0.03 mmol), kept under an argon atmosphere, was added a solution of bis{5-[(3-methylimidazolin-2-ylidene)methyl]-25,26,27,28-tetrapropyloxycalix[4]arene}silver(I) dichloroargentate (7; 50.0 mg, 0.03 mmol) in dry CH2Cl2 (10 mL), and the mixture was heated for 24 h at 55 C. During that period of time the color changed from yellow to orange and a precipitate developed. After filtration over Celite, the solvent was removed and the resulting orange crude material was digested with cold pentane. Yield. 14.4 mg

(0.01 mmol, 47%). Mp: >147 C dec. IR (KBr): ν 3058 (m), (Ar-H); 2961 (s), 2928 (s), 2873 (s) (C-H); 1586 (w) (CdC); 1458 (s), 1385 (m), 1350 (w) (C-H); 1260 (m), 1214 (s), 1194 (s), 1126 (m), 1085 (s), 1038 (s) (Ar-O-C); 1006 (s); 965 (s) (C-H)COD; 759 (s) (Ar-H) cm-1. 1H NMR (400 MHz, CDCl3): δ 0.89 (t, J = 7.5 Hz, 6 H, CH2CH3), 0.95 (t, J = 7.5 Hz, 6 H, CH2CH3), 1.41 (m, 1 H, (CH)COD), 1.59 (m, 2 H, (CH)COD), 1.71 (m, 1 H, (CH)COD), 1.85 (m, 8 H, CH2CH3), 1.97 (m, 1 H, (CH)COD), 2.16 (m, 3 H, (CH)COD), 2.73 (m, 1 H, (CH)COD), 2.93 (m, 1 H, (CH)COD), 3.07 (pseudo t, J = 14.7 Hz, 4 H, ArCH2eqAr), 3.71 (m, 4 H, OCH2), 3.82 (m, 7 H, OCH2 and NCH3), 4.37 (pseudo t, J = 11.4 Hz, 4 H, ArCH2axAr), 4.47 (m, 1 H, (CH)COD), 4.55 (m, 1 H, (CH)COD), 4.96 (d, J = 14.4 Hz, 1 H, ArCH2Im), 5.24 (d, J = 14.7 Hz, 1 H, ArCH2Im), 6.16 (d, J = 1.8 Hz, 1 H, ArH), 6.31 (m, 2 H, ArH and ImH), 6.45 (m, 3 H, ArH), 6.54 (m, 1 H, ArH), 6.59 (m, 2 H, ArH), 6.68 (m, 4 H, ArH and ImH). 13C NMR (100 MHz, CDCl3): δ 10.15, 10.44 (CH2CH3), 23.13, 23.30, (CH2CH3), 29.09, 30.03 ((CH)COD), 30.94, (ArCH2Ar), 33.01, 34.16 ((CH)COD), 37.37 (NCH3), 51.09, 51.53 ((CH)COD), 53.85 (ArCH2Im), 76.62, 76.83, 76.88 (OCH2), 83.77, 84.60 ((CH)COD), 120.12, 121.37, 121.62, 122.04, 127.89, 127.92, 128.39, 128.41, 129.20, 134.67, 134.96, 135.04, 135.37, 135.40, 135.59, 156.13, 156.27, 156.83 (ArC and ImC), 180.03 (IrC). MS (MALDI-TOF): calcd for C53H66IrN2O4 987.5, found m/z 987.6 [M - Cl]þ. Anal. Calcd for C53H66ClIrN2O4 (1022.78): C, 62.24; H, 6.50; N, 2.74. Calcd for C53H66ClIrN2O4 3 0.25COD (1049.83): C, 62.92; H, 6.62; N, 2.67. Found: C, 62.94; H, 6.77; N, 2.67. cis-{5,11-Bis[(3-methylimidazolin-2-ylidene)methyl]-25,26,27,28tetrakis(ethoxyethyl)calix[4]arene}dichloroplatinum(II) (11). A mixture of {5,11-bis[(3-methylimidazolin-2-ylidene)methyl]-25, 26,27,28-tetrakis(ethoxyethyl)calix[4]arene}silver(I) dichloroargentate (8; 80.0 mg, 0.07 mmol) and cis-[Pt(MeCN)2Cl2]40 was dissolved in dry CH2Cl2 (15 mL) and stirred for 24 h at 55 C. The reaction mixture was filtered over Celite, and the solvent was removed, yielding the product as a slightly yellow solid. Yield: 73.5 mg (0.06 mmol, 90%). Mp: >150 C dec. IR (KBr): ν 3100 (w) (Ar-H); 2970 (m), 2922 (m), 2862 (m) (C-H); 1587 (w) (CdC); 1458 (s), 1406 (m), 1352 (m), 1282 (m) (C-H); 1260 (m), 1219 (m) 1115 (s), 1046 (s) (Ar-O-C and C-O-C); 801 (m), 765 (m) (Ar-H). 1H NMR (400 MHz, DMSO-d6): δ 1.12 (t, J = 7.1 Hz, 6 H, CH2CH3), 1.13 (t, J = 6.8 Hz, 6 H, CH2CH3), 3.07 (d, J = 13.1 Hz, 2 H, ArCH2eqAr), 3.13 (d, J = 13.1 Hz, 2 H, ArCH2eqAr), 3.49 (m, 8 H, OCH2), 3.77 (m, 8 H, OCH2), 3.83 (s, 6 H, NCH3), 3.98 (m, 8 H, OCH2), 4.39 (pseudo t, J = 12.6 Hz, 4 H, ArCH2axAr), 4.88 (d, J = 14.7 Hz, 2 H, ArCH2Im), 5.03 (d, J = 14.9 Hz, 2 H, ArCH2Im), 6.41 (s, 1 H, ArH), 6.55 (m, 4 H, ArH), 6.63 (m, 3 H, ArH), 6.70 (m, 2 H, ArH) 7.24 (s, 1 H, ImH), 7.42 (d, J = 1.5 Hz, 2 H, ImH), 7.47 (s, 1 H, ImH). 13C NMR (125 MHz, DMSO-d6): δ 15.13, 15.16 (CH2CH3), 30.06 (ArCH2Ar), 35.89 (NCH3), 54.88 (ArCH2Im), 65.53, 65.61, 68.98, 69.15 (OCH2), 121.83, 123.80, 126.31, 128.07, 128.27, 129.71, 134.30, 134.61, 134.99, 135.02, 136.24, 155.37, 155.87, 156.07 (ImC and ArC), 160.41 (PtC). MS (MALDI-TOF): calcd for C54H68O8N4Pt 1094.5, found m/z 1094.4 [M - 2 Cl]þ. Anal. Calcd for C54H68Cl2O8N4Pt (1167.13): C, 55.57; H, 5.87; N, 4.80. Calcd for C54H68Cl2O8N4Pt 3 2.5CH2Cl2 (1379.47): C, 49.19; H, 5.33; N, 4.06. Found: C, 49.10; H, 5.54; N, 4.19. Dichloro(η6-p-cymene){5-[(3-methylimidazolin-2-ylidene)methyl]-25,26,27,28-tetrakis(propyloxy)calix[4]arene}ruthenium(II) (12). To a solution of bis{5-[(3-methylimidazolin-2-ylidene)methyl]-25,26,27,28-tetrakis(propyloxy)calix[4]arene}silver(I) dichloroargentate (7; 50.0 mg, 0.03 mmol) in CH2Cl2 (5.00 mL) was added a solution of [RuCl2(p-cymene)]2 (20.0 mg, 0.03 mmol) in CH2Cl2 (5.00 mL). The resulting mixture was heated under the exclusion of light for 2 days at 55 C. After filtration over Celite to remove precipitated AgCl, the solution was

(39) Cacciapaglia, R.; Casnati, A.; Mandolini, L.; Reinhoudt, D. N.; Salvio, R.; Sartori, A.; Ungaro, R. J. Org. Chem. 2005, 70, 624–630.

(40) Fraccarollo, D.; Bertani, R.; Mozzon, M. Inorg. Chim. Acta 1992, 201, 15–22.

Article concentrated in vacuo and hexane was added to precipitate the orange crude material. Further purification was performed by flash chromatography on silica using acetone as eluent. Yield: 25 mg (0.03 mmol, 84%). Mp: >116 C dec. IR (KBr): ν 2960 (s), 2923 (s), 2872 (s) (C-H); 1586 (m) (CdC); 1458 (s), 1383 (s) (C-H); 1261 (s) 1214 (s), 1195 (s), 1160 (m), 1086 (s), 1036 (s) (Ar-O-C); 1009 (s); 1042 (s); 965 (m); 801 (s), 760 (s) (Ar-H) cm-1. 1H NMR (400 MHz, CDCl3): δ 0.88 (t, J = 7.5 Hz, 6 H, CH2CH3), 0.97 (t, J = 7.3 Hz, 3 H, CH2CH3), 0.98 (t, J = 7.3 Hz, 3 H, CH2CH3), 1.19 (d, J = 6.8 Hz, 6 H, (CH(CH3)2)p-cymene), 1.79-1.95 (m, 8 H, CH2CH3), 1.87 (s, 3 H, (CH3)p-cymene), 2.85 (pseudo q, J = 6.9 Hz, 1 H, (CH(CH3)2)p-cymene), 3.04 (d, J = 13.1 Hz, 2 H, ArCH2eqAr), 3.10 (d, J = 13.4 Hz, 2 H, ArCH2eqAr), 3.68 (m, 4 H, OCH2), 3.86 (m, 4 H, OCH2), 3.93 (s, 3 H, NCH3), 4.39 (d, J = 13.1 Hz, 4 H, ArCH2axAr), 4.78 (d, J = 6.1 Hz, 2 H, (ArH)p-cymene), 5.19 (bs, 1 H, (ArH)p-cymene), 5.22 (s, 2 H, ArCH2Im), 6.14 (s, 2 H, ArH), 6.24 (d, J = 1.8 Hz, 1 H, ImH), 6.34 (m, 3 H, ArH), 6.64 (t, J = 7.3 Hz, 2 H, ArH), 6.71 (d, J = 6.8 Hz, 2 H, ArH), 6.75, (d, J = 1.8 Hz, 1 H, ImH), 6.81 (d, J = 7.1 Hz, 2 H, ArH). 13C NMR (100 MHz, CDCl3): δ 10.02, 10.50, 10.54 (CH2CH3), 18.58 ((CH3)p-cymene), 21.18 ((CH(CH3)2)p-cymene), 23.06, 23.34, (CH2CH3), 23.73 ((CH (CH3)2)p-cymene), 30.94, ((CH(CH3)2)p-cymene), 30.99 (ArCH2Ar), 39.57 (NCH3), 54.82 (ArCH2Im), 76.58, 77.07 (OCH2), 99.03, 108.22 ((ArC)p-cymene), 121.75, 122.11 (ArC), 123.06, 123.13 (ImC), 126.74, 127.72, 128.36, 128.65, 130.69, 130.83, 134.26, 135.65, 136.09, 155.58, 155.97, 157.02 (ArC), 173.86 (RuC). MS (MALDI-TOF): calcd for C45H54ClO4N2Ru 823.3, found m/z 823.4 [M - Cl - (p-cymene)]þ. Anal. Calcd for C55H68Cl2N2O4Ru (993.12): C, 65.51; H, 6.90; N, 2.82. Calcd for C55H68Cl2N2O4Ru 3 0.8(acetone) (1039.59): C, 66.32; H, 7.06; N, 2.69. Found: C, 66.02; H, 7.05; N, 2.43. trans-{5,17-Bis[(3-methylimidazol-2-ylidene)methyl]-25,26,27, 28-tetrakis(propyloxy)calix[4]arene}palladium(II) Chloride (13). Silver complex 6 (200 mg, 0.19 mmol) was dissolved in CH2Cl2 (30 mL) and treated with a solution of trans-[PdCl2(CH3CN)2] (48.6 mg, 0.19 mmol) in CH2Cl2 and afterward heated for 3 days at 55 C. When the mixture was cooled to room temperature, silver chloride precipitated, which was removed by filtration through Celite. The filtrate was concentrated to dryness and passed over a short column of silica 60 (230 mesh) with acetone as eluent. Evaporation of the solvent gave the product as a pale yellow solid. Yield: 70.0 mg, 0.07 mmol, 38%. Mp: >290 C dec. IR (KBr): ν 2960 (s), 2928 (s), 2873 (s) (C-H); 1587 (w) (CdC); 1465 (s), 1436 (m), 1407 (m), 1384 (m) (C-H); 1345 (w), 1280 (m), 1239 (m), 1215 (s), 1127 (m), 1079 (m), 1039 (m) (Ar-O-C); 1006 (s); 760 (m), 704 (m) (Ar-H), 360 (m)

Organometallics, Vol. 28, No. 21, 2009

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(Pd-Cl). 1H NMR (CDCl3): δ 0.86 (t, J = 7.5 Hz, 6 H, CH2CH3), 1.09 (t, J = 7.3 Hz, 6 H, CH2CH3), 1.92 (m, 8 H, CH2CH3), 3.10 (d, J = 12.6 Hz, 4 H, ArCH2Ar), 3.64 (t, J = 7.1 Hz, 4 H, ArOCH2), 4.02 (s, 6 H, NCH3), 4.07 (t, J = 8.3 Hz, 4 H, ArOCH2), 4.45 (d, J = 12.6 Hz, 4 H, ArCH2Ar), 5.48 (bs, 4 H, ArCH2Im), 6.10 (s, 4 H, ArH), 6.58 (d, J = 1.8 Hz, 2 H, Im), 6.75 (d, J = 1.8 Hz, 2 H, Im), 6.94 (t, J = 7.5 Hz, 2 H, ArH), 7.06 (d, J = 7.6 Hz, 4 H, ArH). 13C NMR (CDCl3): δ 9.73, 10.80 (CH2CH3), 22.70, 23.51 (CH2CH3), 31.17 (ArCH2Ar), 37.29 (NCH3) 53.50 (ArCH2Im), 76.29, 77.12 (OCH2), 120.27, 122.18 (ImC), 123.02, 125.63, 129.27, 129.91, 133.53, 136.03, 154.68, 157.28 (ArC), 171.52 (PdC). MS (MALDI-TOF): calcd for C50H60ClN4O4Pd 921.3, found m/z 921.6 [(M - Cl)þ]. Anal. Calcd for C50H60Cl2N4O4Pd 3 0.6(acetone): C, 62.64; H, 6.45; N, 5.64. Found: C, 62.59; H, 6.47; N, 5.34. General Protocol Used for the Suzuki-Miyaura Reaction with Isolated Cis and Trans Pd Complexes. A reaction vessel was charged with the Pd-NHC-calix[4]arene complex (3 or 13; 2.5 mol %), phenylboronic acid (145 mg, 1.19 mmol), Cs2CO3 (600 mg, 1.84 mmol), 1,4-dioxane (6 mL), 4-chlorotoluene (94.0 μL, 0.79 mmol), and a magnetic stirring bar. The vessel was sealed and then placed in a heating block, and the reaction mixture was stirred at 80 C. After 48 h the reaction was stopped and the product ratio was determined. For this, an aliquot was withdrawn directly and subjected to NMR analysis. No volatiles were removed, because such a procedure changed the product ratio, as proven by blank experiments. The yields quoted are the averaged yields obtained in two to three independent runs. General Protocol Used for the Suzuki-Miyaura Reaction with in Situ Systems. A reaction vessel was charged with the imidazolium salt (2a or 2c, 2.5 mol %), Pd(OAc)2 (4.49 mg, 0.02 mmol, 2.5 mol %), Cs2CO3 (600 mg, 1.84 mmol), and 1,4dioxane (6 mL) and heated for 30 min to 80 C. After that, phenylboronic acid (145 mg, 1.19 mmol) and 4-chlorotoluene (94.0 μL, 0.79 mmol) were added and the reaction mixture was heated in a heating block at 80 C for 48 h. The product ratio was determined as described before. Supporting Information Available: A CIF file giving crystal data for 5a. This material is available free of charge via the Internet at http://pubs.acs.org. CCDC 606 776 also contains the supplementary crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/ retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, U.K.; fax (internat.) þ44-1223/336-033; e-mail [email protected]).