Synthesis and Characterization of Nitrile-Functionalized N

Jan 6, 2009 - Anthony R. Chianese , Paul T. Bremer , Christina Wong and Rae Julienne Reynes. Organometallics 2009 28 (17), 5244-5252. Abstract | Full ...
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Organometallics 2009, 28, 853–862

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Synthesis and Characterization of Nitrile-Functionalized N-Heterocyclic Carbenes and Their Complexes of Silver(I) and Rhodium(I) Wylie W. N. O, Alan J. Lough, and Robert H. Morris* DaVenport Laboratory, Department of Chemistry, UniVersity of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada ReceiVed September 26, 2008

A series of unsymmetrical nitrile-functionalized N-heterocyclic carbene precursors were synthesized by N-alkylation and N-arylation of 2-cyanophenylimidazole. These imidazolium salts were used to synthesize N-heterocyclic carbene complexes of silver(I), all of which were characterized by NMR and Fourier transform infrared (FT-IR) spectroscopies and elemental analyses. The compound bis[1-(2cyanophenyl)-3-methylimidazol-2-ylidene]silver(I) tetrafluoroborate ([Ag(m-CN)2]BF4, 2a) was structurally characterized by single-crystal X-ray diffraction. Interestingly, the preparation of the silver(I) carbene complex of 3-(cyanomethyl)-1-(2-cyanophenyl)imidazolium (1b) with excess silver(I) oxide leads to the formation of the desired bis-carbene complex, and the bis-carbene complex with hydrolyzed cyanomethyl groups on the ligands. The selectivity in hydrolysis of the cyanomethyl group over the cyanophenyl group on the ligand, as evident from the NMR data, suggests that the process must be mediated by silver(I) centers. The use of an N-heterocyclic carbene complex of silver(I) as a carbene transfer reagent was demonstrated by the reaction of 2a with [Rh(cod)Cl]2. The crystal structure of bis[(1-(2-cyanophenyl)3-methylimidazol-2-ylidene)-(η4-1,5-cycloctadiene)rhodium(I) tetrafluoroborate ([Rh(m-CN)(cod)]2(BF4)2, 3a) revealed the dimeric nature of the complex, of which the nitrile nitrogen and the carbene carbon of the bridging ligand were coordinated to two different rhodium(I) centers, with a slight distortion of the Rh-N≡C bond angles. This is the first dimeric N-functionalized N-heterocyclic carbene complex of rhodium(I) that has been structurally characterized. Introduction The use of N-heterocyclic carbenes as ligands in homogeneous catalysis has been extensively explored in the past decades because of the higher stability and reactivity they impart to homogeneous catalysts and their lower toxicity relative to phosphine ligands.1 The carbene ligands, in particular, imidazol2-ylidene-type ligands isolated by Arduengo and co-workers,2,3 have comparable or even stronger donor capacity compared to phosphine ligands, and are capable of stabilizing metal centers with different oxidation states.4,5 With different synthetic routes

* To whom correspondence should be addressed. E-mail: rmorris@ chem.utoronto.ca. (1) (a) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18–29. (b) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290–1309. (c) Perry, M. C.; Burgess, K. Tetrahedron: Asymmetry 2003, 14, 951–961. (d) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122–3172. (2) (a) Arduengo, A. J.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361–363. (b) Arduengo, A. J.; Goerlich, J. R.; Marshall, W. J. J. Am. Chem. Soc. 1995, 117, 11027–11028. (c) Arduengo, A. J. Acc. Chem. Res. 1999, 32, 913–921. (3) Arduengo, A. J.; Krafczyk, R.; Schmutzler, R.; Craig, H. A.; Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron 1999, 55, 14523–14534. (4) (a) Furstner, A.; Ackermann, L.; Gabor, B.; Goddard, R.; Lehmann, C. W.; Mynott, R.; Stelzer, F.; Thiel, O. R. Chem. Eur. J. 2001, 7, 3236– 3253. (b) Crudden, C. M.; Allen, D. P. Coord. Chem. ReV. 2004, 248, 2247– 2273. (c) Scott, N. M.; Nolan, S. P. Eur. J. Inorg. Chem. 2005, 181, 5– 1828. (5) (a) Leuthausser, S.; Schwarz, D.; Plenio, H. Chem. Eur. J. 2007, 13, 7195–7203. (b) Fantasia, S.; Petersen, J. L.; Jacobsen, H.; Cavallo, L.; Nolan, S. P. Organometallics 2007, 26, 5880–5889.

reported for these imidazolium salt precursors,3,6 the steric bulk of the ligand can be tuned either on the imidazole backbone, or on the N,N-substituted groups. This is beneficial in order to tune the activity and selectivity of a catalyst. In recent years, N-heterocyclic carbenes are viewed not only as spectator monodentate ligands in homogeneous catalysis, but also, when suitably functionalized at nitrogen, as useful chelating synthons in the design of new homogeneous catalysts. These chelating N-functionalized ligands with tethering groups show coordination versatility and mediate the coordination or activation of substrates.7,8 Examples include those with amino-,9-11 imino-,10,12,13 amido-,14-16 pyridyl-,17-20 and oxazolinyl21,22-groups, and many others. Our group has been interested in the hydrogenation of polar bonds including ketones, imines, and nitriles catalyzed by ruthenium(II)- and iron(II)-based systems, including RuHCl(H2NCR2CR2NH2)(phosphine)2,23,24 chelating systems such as RuHCl(phosphino-amine)2,25 and tetradentate systems such as RuHCl(P-N-N-P)26 and [Fe(CH3CN)2(P-N-N-P)]2+.27 In the continuous pursuit of active catalysts for polar bond hydrogenation, we are interested in the use of N-heterocyclic (6) (a) Furstner, A.; Alcarazo, M.; Cesar, V.; Lehmann, C. W. Chem. Commun. 2006, 2176–2178. (b) Bon, R. S.; deKanter, F. J. J.; Lutz, M.; Spek, A. L.; Jahnke, M. C.; Hahn, F. E.; Groen, M. B.; Orru, R. V. A. Organometallics 2007, 26, 3639–3650. (c) Struble, J. R.; Bode, J. W. Tetrahedron 2008, 64, 6961–6972. (d) Kuhn, K. M.; Grubbs, R. H. Org. Lett. 2008, 10, 2075–2077. (7) VanVeldhuizen, J. J.; Campbell, J. E.; Giudici, R. E.; Hoveyda, A. H. J. Am. Chem. Soc. 2005, 127, 6877–6882. (8) Liddle, S. T.; Edworthy, I. S.; Arnold, P. L. Chem. Soc. ReV. 2007, 36, 1732–1744.

10.1021/om8009377 CCC: $40.75  2009 American Chemical Society Publication on Web 01/06/2009

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carbene systems to enhance catalytic activities, so as to replace traditional phosphine donors in the construction of our ligands. From our initial report on the synthesis of novel coordinatively unsaturated hydridoruthenium(II) NHC systems,28 we are interested in constructing new N-functionalized NHC ligands that would assist in the bifunctional MH/HX catalysis involved in the hydrogenation of polar bonds.24 The nitrile group, when hydrogenated at ruthenium or rhodium, could produce a H-M-NH2CH2- group known to be very effective at the hydrogenation of ketones and imines.24 In this present contribution, we report the synthesis and characterization of a series of novel nitrile-functionalized N-heterocyclic carbene precursors (Figure 1), and their corresponding metal complexes of silver(I) and rhodium(I). Although (9) (a) Mungur, S. A.; Blake, A. J.; Wilson, C. Angew. Chem., Int. Ed. 2003, 42, 5981–5984. (b) Douthwaite, R. E.; Houghton, J.; Kariuki, B. M. Chem. Commun. 2004, 698–699. (c) Spencer, L. P.; Fryzuk, M. D. J. Organomet. Chem. 2005, 690, 5788–5803. (d) Edworthy, I. S.; Rodden, M.; Mungur, S. A.; Davis, K. M.; Blake, A. J.; Wilson, C.; Schro¨der, M.; Arnold, P. L. J. Organomet. Chem. 2005, 690, 5710–5719. (e) Busetto, L.; Cassani, M. C.; Femoni, C.; Macchioni, A.; Mazzoni, R.; Zuccaccia, D. J. Organomet. Chem. 2008, 693, 2579–2591. (10) Bonnet, L. G.; Douthwaite, R. E.; Hodgson, R.; Houghton, J.; Kariuki, B. M.; Simonovic, S. Dalton Trans. 2004, 3528–3535. (11) (a) Jong, H.; Patrick, B. O.; Fryzuk, M. D. Can. J. Chem. 2008, 86, 803–810. (b) Jimenez, M. V.; Perez-Torrente, J. J.; Bartolome, M. I.; Gierz, V.; Lahoz, F. J.; Oro, L. A. Organometallics 2008, 27, 224–234. (12) (a) Bonnet, L. G.; Douthwaite, R. E.; Kariuki, B. M. Organometallics 2003, 22, 4187–4189. (b) Coleman, K. S.; Dastgir, S.; Barnett, G.; Alvite, M. J. P.; Cowley, A. R.; Green, M. L. H. J. Organomet. Chem. 2005, 690, 5591–5596. (c) Houghton, J.; Dyson, G.; Douthwaite, R. E.; Whitwood, A. C.; Kariuki, B. M. Dalton Trans. 2007, 3065–3073. (13) Dyson, G.; Frison, J. C.; Simonovic, S.; Whitwood, A. C.; Douthwaite, R. E. Organometallics 2008, 27, 281–288. (14) Legault, C. Y.; Kendall, C.; Charette, A. B. Chem. Commun. 2005, 3826–3828. (15) Samantaray, M. K.; Pang, K.; Shaikh, M. M.; Ghosh, P. Inorg. Chem. 2008, 47, 4153–4165. (16) Liao, C. Y.; Chan, K. T.; Chiu, P. L.; Chen, C. Y.; Lee, H. M. Inorg. Chim. Acta 2008, 361, 2973–2978. (17) Chen, J. C. C.; Lin, I. J. B. Organometallics 2000, 19, 5113–5121. (18) Catalano, V. J.; Etogo, A. O. J. Organomet. Chem. 2005, 690, 6041– 6050. (19) (a) Grundemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.; Crabtree, R. H. J. Am. Chem. Soc. 2002, 124, 10473–10481. (b) Mas-Marza, E.; Sanau, M.; Peris, E. Inorg. Chem. 2005, 44, 9961–9967. (c) Xi, Z.; Zhang, X.; Chen, W.; Fu, S.; Wang, D. Organometallics 2007, 26, 6636– 6642. (20) Wang, C. Y.; Liu, Y. H.; Peng, S. M.; Liu, S. T. J. Organomet. Chem. 2006, 691, 4012–4020. (21) (a) Powell, M. T.; Hou, D.-R.; Perry, M. C.; Cui, X.; Burgess, K. J. Am. Chem. Soc. 2001, 123, 8878–8879. (b) Perry, M. C.; Cui, X. H.; Powell, M. T.; Hou, D. R.; Reibenspies, J. H.; Burgess, K. J. Am. Chem. Soc. 2003, 125, 113–123. (c) Chen, D.; Banphavichit, V.; Reibenspies, J.; Burgess, K. Organometallics 2007, 26, 855–859. (22) Ren, L.; Chen, A. C.; Decken, A.; Crudden, C. M. Can. J. Chem. 2004, 82, 1781–1787. (23) (a) Abdur-Rashid, K.; Lough, A. J.; Morris, R. H. Organometallics 2001, 20, 1047–1049. (b) Abdur-Rashid, K.; Faatz, M.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2001, 123, 7473–7474. (c) Guo, R. W.; Elpelt, C.; Chen, X. H.; Song, D. T.; Morris, R. H. Chem. Commun. 2005, 3050– 3052. (d) Hadzovic, A.; Lough, A. J.; Morris, R. H.; Pringle, P. G.; Zambrano-Williams, D. E. Inorg. Chim. Acta 2006, 359, 2864–2869. (24) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. ReV. 2004, 248, 2201–2237. (25) (a) Guo, R.; Lough, A. J.; Morris, R. H.; Song, D. Organometallics 2004, 23, 5524–5529. (b) Abdur-Rashid, K.; Guo, R. W.; Lough, A. J.; Morris, R. H.; Song, D. T. AdV. Synth. Catal. 2005, 347, 571–579. (26) (a) Li, T.; Churlaud, R.; Lough, A. J.; Abdur-Rashid, K.; Morris, R. H. Organometallics 2004, 23, 6239–6247. (b) Li, T.; Bergner, I.; Haque, F. N.; Iuliis, M. Z.-D.; Song, D.; Morris, R. H. Organometallics 2007, 26, 5940–5949. (c) Haque, F. N.; Lough, A. J.; Morris, R. H. Inorg. Chim. Acta 2008, 361, 3149–3158. (27) (a) Sui-Seng, C.; Freutel, F.; Lough, A. J.; Morris, R. H. Angew. Chem., Int. Ed. 2008, 47, 940–943. (b) Mikhailine, A. A.; Kim, E.; Dingels, C.; Lough, A. J.; Morris, R. H. Inorg. Chem. 2008, 47, 6587–6589. (28) Abdur-Rashid, K.; Fedorkiw, T.; Lough, A. J.; Morris, R. H. Organometallics 2004, 23, 86–94.

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Figure 1. Novel nitrile-functionalized imidazolium salt precursors 1a-1d.

Figure 2. Dyson’s nitrile-functionalized N-heterocyclic carbene complex of palladium(II).

an analogous nitrile-functionalized NHC complex of palladium(II) was reported by Dyson and co-workers,29 the nitrile moiety on the ligand (Figure 2) was remote from the metal center. The nitrile group on our novel ligands 1a-1d, on the other hand, shows coordination versatility and reactivity with metal centers. The hydrolysis of the nitrile-functionalized carbene ligand mediated by silver(I) centers is reported here.

Results and Discussion Synthesis of Imidazolium Salt Precursors. Initial attempts utilized the standard protocol for imidazolium salt synthesis starting from glyoxal and 2-aminobenzonitrile to synthesize 1,3bis(2-cyanophenyl)imidazolium halide. These failed because the nucleophilic character of the amine-nitrogen of 2-aminobenzonitrile is diminished due to the presence of an ortho-cyano group and so did not condense with glyoxal to yield the glyoxal diimine, even under templating conditions in the presence of nickel(II) or zinc(II) cations.3 Therefore we developed a synthesis of the desired unsymmetrical imidazolium salt precursors of carbenes using the N-alkylation and N-arylation of 2-cyanophenylimidazole. The starting imidazole compound is conveniently prepared in a large scale with very good yields using literature methods.22,30 The syntheses of the new imidazolium salt precursors were conducted by means of electrophilic alkylation with trimethyloxonium tetrafluoroborate (Meerwein’s salt) to give 1a and SN2 reactions with bromoacetonitrile or picolyl chloride to yield 1b and 1c, respectively, in good yields (Schemes 1-3). N-Arylation of 2-cyanophenylimidazole with phenyl halides, on the other hand, did not proceed, even at elevated temperatures.31 Under similar conditions reported in the literature for the preparation of N-functionalized (2pyridyl)imidazolium salts,17,32 the compound 1d was prepared in 54% yield starting from 2-bromopyridine (Scheme 3). The crude halide salts of 1b, 1c, and 1d have limited solubility in (29) Fei, Z.; Zhao, D.; Pieraccini, D.; Ang, W. H.; Geldbach, T. J.; Scopelliti, R.; Chiappe, C.; Dyson, P. J. Organometallics 2007, 26, 1588– 1598. (30) Johnson, A. L.; Kauer, J. C.; Sharma, D. C.; Dorfman, R. I. J. Med. Chem. 1969, 12, 1024–1028. (31) (a) Chan, B.; Chang, N.; Grimmett, M. Aust. J. Chem. 1977, 30, 2005–2013. (b) Harlow, K. J.; Hill, A. F.; Welton, T. Synthesis 1996, 697. (32) Tulloch, A. A. D.; Danopoulos, A. A.; Winston, S.; Kleinhenz, S.; Eastham, G. J. Dalton Trans. 2000, 4499–4506.

Nitrile-Functionalized N-Heterocyclic Carbenes Scheme 1. Synthesis of the Imidazolium Salt Precursor 1a and Its Ag(I) and Rh(I) Complexes (2a and 3a)

Scheme 2. Synthesis of Imidazolium Precursor 1b and the Ag(I) complexes (2b, 2e)

Scheme 3. Synthesis of Imidazolium Precursors 1c and 1d and Their Ag(I) Complexes (2c, 2d)

most organic solvents, except for water and dimethyl sulfoxide (DMSO). Counter anion metathesis with NH4PF6 was therefore performed in water to improve solubilities. In general, these hexafluorophosphate salts were soluble in alcohols, acetonitrile, and water, but not in chlorinated solvents.

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These new imidazolium salt precursors were unambiguously characterized by NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS). The formation of 1a to 1d was indicated by a singlet in the 1H NMR spectra in the region between δ 9.6 and 10.6 ppm due to the presence of the acidic hydrogen attached to the ipso-carbon C1. The presence of nitrile groups in the imidazolium precursors was signaled by a peak at about 2200 cm-1 in the IR-spectra. In addition, the structures of 1a and 1b (Figures 3 and 4) have been characterized by single-crystal X-ray diffraction study. The bond distances and angles are in the expected range compared to those of known imidazolium salts, for example 1,3-dimesitylimidazolium chloride ([IMesH]Cl),33 and the bond distances in 1a are also similar to those of 1-(4-cyanophenyl)-3-methylimidazolium.34 Several attempts to isolate the free carbenes of 1a-1d by treating the corresponding imidazolium salts with KH or KN(SiMe3)2 in THF solutions at -78 °C failed. Isolation of the free carbene was apparently difficult due to its reactivity toward its own electrophilic nitrile functionality. Nitrile Functionalized N-Heterocyclic Carbene Complexes of Silver(I). Ever since the first report by Wang and Lin,35 N-heterocyclic carbene complexes of silver(I) have served as convenient carbene transfer reagents, providing a wide range of platinum metal group complexes.36,37 These silver(I)-NHC complexes were prepared by the treatment of the imidazolium salt precursors with mildly basic Ag2O or Ag2CO3,7,32 normally under mild conditions without the generation of the free carbene. Given the ease of their preparation, their relative stability toward air and moisture, and their structural diversity, we were prompted to prepare analogous silver(I) complexes bearing cyano-substituted N-heterocyclic carbene ligands. The preparations of the novel silver(I) complexes, 2a-2d, were conducted by the reaction of excess silver(I) oxide with the imidazolium salt precursors 1a-1d, respectively, under reflux for overnight with protection from light (Schemes 1-3). Stirring the aforementioned solutions at room temperature did not work efficiently. In addition, an excess of silver(I) oxide, preferably 1.5 equiv with respect to the imidazolium salt, is required for the successful generation of these bis-silver(I)-NHC complexes. A full conversion under the same reaction conditions could not be achieved if a stoichiometric amount of base was used. The use of molecular sieves was crucial for obtaining a pure product by preventing the hydrolysis of the nitrile group in the presence of silver hydroxide and water (Vide infra). The formation of these silver(I) complexes was established by observing a sharp singlet above 182 ppm in the 13C{1H} NMR spectra which was assigned to the carbene carbon attached to the silver center (Ccarbene-Ag). No one-bond coupling 1 107 J( Ag-13C) or 1J(109Ag-13C) was observed, even at -40 °C for 2a in [D3]acetonitrile solution.37 This may be the result of a rapid exchange of carbene ligands between silver ions. On the other hand, the absence of a peak in the region of δ 9.6-10.6 ppm in the 1H NMR spectra further suggested full conversion to silver(I) NHC complexes. The silver(I) complex 2a has been structurally characterized by use of X-ray diffraction (Figure 5). The complex crystallizes in the monoclinic space group P21/n with four (33) Cole, M. L.; Junk, P. C. CrystEngComm 2004, 6, 173–176. (34) Hatzidimitriou, A.; Gourdon, A.; Devillers, J.; Launay, J. P.; Mena, E.; Amouyal, E. Inorg. Chem. 1996, 35, 2212–2219. (35) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972–975. (36) (a) Lin, I. J. B.; Vasam, C. S. Comments Inorg. Chem. 2004, 25, 75–129. (b) Lin, I. J. B.; Vasam, C. S. Coord. Chem. ReV. 2007, 251, 642– 670. (37) Garrison, J. C.; Youngs, W. J. Chem. ReV. 2005, 105, 3978–4008.

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Figure 3. ORTEP diagram of 1a (m-CN-BF4) depicted with thermal ellipsoids at 50% probability. The counteranion has been omitted for clarity. Selected bond distances (Å) and bond angles (deg): C(1)-N(1), 1.340(3); C(1)-N(2), 1.324(3); C(2)-C(3), 1.342(3); C(11)-N(3), 1.139(3); N(1)-C(1)-N(2), 108.2(2); C(6)-C(11)-N(3), 177.7(3).

Figure 4. ORTEP diagram of 1b (CH2CN-CN-PF6) depicted with thermal ellipsoids at 50% probability. The counteranion has been omitted for clarity. Selected bond distances (Å) and bond angles (deg): C(1)-N(1), 1.342(5); C(1)-N(2), 1.328(5); C(2)-C(3), 1.349(6); C(12)-N(4), 1.134(6); C(5)-N(3), 1.142(6); N(1)-C(1)-N(2), 107.8(4); C(7)-C(12)-N(4), 179.1(5); C(4)-C(5)-N(3), 179.1(6).

units residing in the unit cell. The structure shows a slightly distorted geometry from linearity about the silver(I) center, and the C-Ag-C bond angle of 171.1(2)° is smaller than those of typical [Ag(NHC)2]+ complexes, at around 174-177°. The average silver-carbene distance (Ccarbene-Ag) is 2.095 Å, which is in the expected range of typical [Ag(NHC)2]+ complexes. The decrease in the C-N bond lengths C(1)-N(1) and C(1)-N(2) and the bond angle N(1)-C(1)-N(2) in the imidazol-2-ylidene ring compared to the imidazolium precursor suggests an increase in p-character at the carbene carbon.15,16,32,35,37,38 Interestingly, it is expected that the phenyl rings on the carbene ligand should be directed toward the methyl groups to avoid steric hindrance; instead, the phenyl rings are directed toward each other, with the cyano group oriented away from the metal center. In fact, given that a facile phenyl ring rotation could take place about the (38) Paas, M.; Wibbeling, B.; Fro¨hlich, R.; Hahn, F. Eur. J. Inorg. Chem. 2006, 15, 8–162.

N(4)-C(15) and N(2)-C(7) axes, the phenyl rings are twisted with respect to the imidazole rings in opposite enantiomeric configurations with dihedral angles of 43.18° and 51.05°, in order to facilitate stacking of phenyl and imidazole rings in the crystal lattice, with the contact distance between them being 3.40 Å, which is comparable to the sum of van der Waals radius of carbon atoms. This orientation also explains the slight deviation from linearity of the C(1)-Ag(1)-C(4) angle to relieve the interactions between the two phenyl rings (Figure 5). The complexes 2c and 2d showed different structural motifs compared to 2a. We would expect the structures of these compounds would be similar to those of other pyridyl-functionalized N-heterocyclic carbene complexes of silver(I).18,32,39,40 However, the analytical data (NMR, Fourier transform infrared (FT-IR), and elemental analysis) suggest the presence of a [Ag(NHC)](PF6) unit, which could exist in a monomeric, dimeric, or trimeric structure. Although crystals suitable for X-ray diffraction were not obtained successfully, the structures

Nitrile-Functionalized N-Heterocyclic Carbenes

Figure 5. ORTEP diagram of 2a ([Ag(m-CN)2]BF4) depicted with thermal ellipsoids at 50% probability. The counteranion has been omitted for clarity. Selected bond distances (Å) and bond angles (deg): Ag(1)-C(1), 2.094(5); Ag(1)-C(4), 2.095(5); C(1)-N(1), 1.359(6); C(1)-N(2), 1.362(6); C(13)-N(5), 1.140(7); C(1)-Ag(1)C(4), 171.1(2); N(1)-C(1)-N(2), 104.4(4); N(5)-C(13)-C(12), 178.3(6).

can be partially assigned on the basis of NMR data. Only one set of signals are present in the 1H NMR spectra, indicative of a symmetric single component complex. A monomeric structure is highly unlikely, as the Ccarbene-Ag-Npy angle will be bent away from linearity, if the pyridyl group were to bond to the metal center. A trimeric structure is not possible, as metal-metal interactions of the silver(I) clusters will give rise to extensive Ccarbene-Ag couplings at the carbene carbon in the 13C{1H} NMR spectrum. Only a sharp singlet was observed in our system at δ 183 ppm. This signal is more downfield than those of silver(I) cluster systems, such as [Ag3((pyCH2)2imid)3](BF4)3 and [Ag3((quinCH2)2imid]3](BF4)3 (py ) 2′-pyridyl; quin ) 2′quinolyl; imid ) imidazole).39 Dimeric structures are most probable, with either the nitrile or the pyridyl group bound to the silver(I) center to produce a dication with a 10- or 12-membered macrometallocyclic ring. Silver(I)-NHC complexes forming a 12-membered ring macrometallocycle are known, at least with N-functionalized NHC ligands of alkylated amides14,15 and pyrazoles.41 The resulting complexes are neutral and cationic, respectively. Bridging via the nitrile group on the carbene is not likely because the wavenumbers of the nitrile stretch of these compounds and their imidazolium precursors observed in the IR spectra are comparable (see the Experimental Section). The only additional binding site is through the pyridyl group of the ligand. Possible structures are given in Figure 6, with each silver(I) center bonding to the carbene carbon and nitrogen (2c-1, 2d-1), or with one silver(I) center bonding to two carbene carbons, and the other bonding to two nitrogens (2c-2, 2d-2), similar to those reported with pyrazole-functionalized NHC ligands.41 (39) (a) Catalano, V. J.; Malwitz, M. A. Inorg. Chem. 2003, 42, 5483– 5485. (b) Catalano, V. J.; Malwitz, M. A.; Etogo, A. O. Inorg. Chem. 2004, 43, 5714–5724. (40) Catalano, V. J.; Moore, A. L. Inorg. Chem. 2005, 44, 6558–6566. (41) (a) Chiu, P. L.; Chen, C. Y.; Lee, C.-C.; Hsieh, M.-H.; Chuang, C.-H.; Lee, H. M. Inorg. Chem. 2006, 45, 2520–2530. (b) Zhou, Y. B.; Zhang, X. M.; Chen, W. Z.; Qiu, H. Y. J. Organomet. Chem. 2008, 693, 205–215.

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Figure 6. Proposed structures of Ag(I) complexes 2c and 2d forming 12- and 10-membered rings, respectively.

Figure 7. 1H NMR spectra (400 MHz) in the amide-NH region of (a) acetamide, (b) acetamide prepared from hydrolysis of acetonitrile with silver oxide and water, and (c) 2e in CD3CN.

Hydrolysis of Nitrile-Functionalized N-Heterocyclic Carbene Complexes of Silver(I). In the preparation of the silver(I) complex using the imidazolium salt 1b, a mixture of products 2b and 2e was always observed, even when molecular sieves were used in the syntheses. The appearance of two broad peaks at δ 6.12 and 6.49 ppm in the 1H NMR spectrum of the compounds in [D3]acetonitrile suggested the presence of an amide group (Figure 7c), and this is further confirmed by their disappearance upon the addition of D2O to the NMR sample. We initially believed that these peaks were due to the presence of acetamide formed by the hydrolysis of acetonitrile during the course of reaction, as similar peaks were observed in the preparation of 2a without using molecular sieves. In order to identify these products that resulted from hydrolysis, we attempted to synthesize acetamide under the same conditions that were used in the preparation of 2a-2d. The 1H NMR (400 MHz) of the reaction mixture resulting from refluxing a solution of acetonitrile, water, and silver(I) oxide in [D3]acetonitrile showed two broad peaks at δ 6.03 and 6.64 ppm (Figure 7b). This was compared to the 1H NMR (400 MHz) of pure acetamide in [D3]acetonitrile, which gave two broad peaks at δ 6.18 and 6.51 ppm (Figure 7a). The peak separations of these

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Scheme 4. Proposed Reaction Pathways to Hydrolysis of Ag(I) Complexes 2b to 2e

amide-NH peaks are quite different for these compounds. In particular, it is smaller in pure acetamide (0.33 ppm) than in the ones prepared from silver(I) oxide (0.61 ppm). Even though the peak separation between the amido-protons of 2e are small (0.37 ppm) like that of acetamide, the peak width of these protons are the smallest among all. These observations suggest the NH2 group in 2e has a higher barrier to rotation about the C-N bond compared to free acetamide. We postulate that a weak silver-oxygen electrostatic interaction may contribute to the observed NMR behavior, and this could also explain the smaller peak separation for acetamides prepared from silver(I) oxide when compared to pure acetamide (Figure 7). Further evidence for the formation of the hydrolyzed complex 2e is provided from NMR data acquired from the reaction mixture. The appearance of new sets of aromatic peaks in the 1 H NMR spectrum suggests the formation of a new compound, whereas the integration of each aromatic peak to each amidoproton is 1 to 1. A more diagnostic piece of evidence derived from the 13C{1H} NMR spectrum is a peak at δ 169.4 ppm for the mixture in [D3]acetonitrile, which corresponds to the carbonyl carbon of the primary amide. In addition, all the aromatic signals in the 13C{1H} NMR doubled up, except for the nitrile carbon attached to the methylene linker. However, we do observe the formation of acetamide as minor broad peaks in the 1H NMR at δ 5.86 and 6.21 ppm in the sample in [D3]acetonitrile. The new amido-NHC silver(I) complex 2e, compared to alkylated amido-functionalized NHC complexes of silver(I),14-16 are not stable in solutions and in solid state, even when protected from light, as facile decomposition takes place. In an attempt to convert 2b to 2e by treating aqueous potassium hydroxide at room temperature, the product decomposed readily after 2 h of reaction. We believe that the nitrile group attached to the methylene linker is preferentially hydrolyzed over that of attached to the phenyl ring. The cyanophenyl and the carbonyl carbon of 2e were identified in the 13C{1H} NMR spectrum, and the assignment was further established by two-dimensional 1H-13C HSQC and HMBC NMR experiments. In addition, under identical reaction conditions in the preparation of 2a and 2b when molecular sieves were used, the amido-protons were absent in 2a but not in 2b, even though hydrolysis can be promoted without the use of molecular sieves in the preparation of 2a.

This selectivity is reversed compared to conventional base hydrolysis where the benzonitrile is hydrolyzed more readily. This suggests the hydrolysis is mediated by the metal center. In fact, as there are more degrees of freedom of the cyanomethyl group compared to the cyanophenyl group, the nitrogen on the nitrile group can be activated by two silver(I) cations to facilitate attack by the hydroxide ion on the electrophilic carbon, which leads to the formation of a six-membered ring intermediate 2e-i (Scheme 4). A similar behavior was observed by Mao and coworkers, when the nitrogen on the acetonitrile was activated by two silver(I) centers of a silver(I) cryptate system, and thus underwent catalytic hydrolysis,42 although the nitrile was hydrolyzed stoichiometrically by silver hydroxide in our system. We propose that the hydrolysis occurs during the preparation of 2b according to Scheme 4. The first equivalent of the imidazolium salt reacts with silver(I) oxide, leading to the formation of the monocarbene complex 2b-i and silver hydroxide. Silver hydroxide and a second equivalent of the imidazolium salt react to generate 2b. However, as water is generated and in the presence of silver(I) oxide, more silver hydroxide is generated and this starts to hydrolyze the cyanomethyl group on the ligand, forming the six-member ring intermediate 2e-i. Further hydrolysis lead to the final product 2e. An alternative proposal is that the ligand could be hydrolyzed prior to the formation of 2b at 2b-i. The formation of the bis-carbene complex could involve stepwise deprotonation of the imidazolium salt by silver(I) oxide and silver hydroxide, of which silver(I) oxide is more basic than silver hydroxide. The intermediate 2b-i is readily converted to the thermodynamically more stable 2b, as supported by theoretical calculations.43 Thus the hydrolysis should occur after the formation of 2b. Nitrile Functionalized N-Heterocyclic Carbene Complex of Rhodium(I). To demonstrate the use of a silver(I)-NHC complex as a carbene transfer reagent, we reacted 2a with [Rh(cod)Cl]2 and 1 equiv of AgBF4 in acetonitrile to afford 3a in 71% yield. This yellow compound is very soluble in acetonitrile, and moderately soluble in acetone, alcohols, and chlorinated solvents. Interestingly, the compound crystallized (42) Luo, R. S.; Mao, X. A.; Pan, Z. Q.; Luo, Q. H. Spectrochim. Acta, Part A 2000, 56, 1675–1680. (43) Hayes, J. M.; Viciano, M.; Peris, E.; Ujaque, G.; Lledos, A. Organometallics 2007, 26, 6170–6183.

Nitrile-Functionalized N-Heterocyclic Carbenes

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Figure 8. ORTEP diagram of 3a ([Rh(m-CN)(cod)]2(BF4)2) depicted with thermal ellipsoids at 30% probability. The counteranions and hydrogens have been omitted for clarity. Selected bond distances (Å) and bond angles (deg): Rh(1)-C(16), 2.040(5); Rh(1)-N(1), 2.064(4); Rh(1)-cod(trans to N)cent, 2.016; Rh(1)-cod(trans to C)cent, 2.100; C(9)-N(1), 1.139(6); C(9)-N(1)-Rh(1), 172.0(5); C(16)-Rh(1)-N(1), 89.9(2); N(1)-Rh(1)-cod(trans to N)avg, 160.6; N(1)-Rh(1)-cod(cis to N)avg, 91.15; C(16)-Rh(1)-cod(trans to C)avg, 162.1; C(16)-Rh(1)cod(cis to C)avg, 92.37. Table 1. Selected Crystal Data, Data Collection, and Refinement Parameters for 1a, 1b, 2a, and 3aa compound empirical formula FW lattice type Space group T, K a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z Fcalc/Mg m-3 µ(Mo, KR), mm-1 F(000) cryst. size, mm3 range θ collected, deg reflns collected/unique abs cor max and min transmission coeff parameters refined goodness of fit R1(I > 2σ (I)) wR2 (all data) peak and hole, e Å-3 a

1a 1b C11H10BF4N3 C12H9F6N4P 271.03 354.20 monoclinic monoclinic P 21/c P 21 150 150 12.8385(5) 7.7595(4) 10.2371(6) 10.3099(7) 9.7995(6) 9.0340(5) 90 90 111.308(3) 92.098(4) 90 90 1199.90(11) 722.23(7) 4 2 1.500 1.629 0.135 0.261 552 356 0.18 × 0.12 × 0.04 0.19 × 0.14 × 0.140 2.62 to 27.46 2.63 to 27.46 8249/2739 5873/2949 semiempirical from equivalents 1.001 and 0.914 0.967 and 0.878 173 209 0.974 1.038 0.0532 0.0504 0.1369 0.1087 0.240 and -0.293 0.309 and -0.363

2a C22H18AgBF4N6 561.10 monoclinic P 21/n 150 11.6459(3) 7.6957(4) 25.4018(12) 90 96.613(3) 90 2261.45(17) 4 1.648 0.947 1120 0.08 × 0.06 × 0.05 2.77 to 27.54 15849/5160

3a C38H42B2F8N6Rh2 962.22 orthorhombic Fdd2 150 25.3729(3) 30.2083(7) 9.9140(7) 90 90 90 7598.8(6) 8 1.682 0.946 3872 0.18 × 0.12 × 0.12 2.70 to 27.47 14622/4015

0.973 and 0.847 327 0.992 0.0583 0.1116 1.508 and -1.180

0.944 and 0.844 253 1.081 0.0398 0.0590 1.450 and -0.593

Definition of R indices: R1 ) ∑(Fo - Fc)/∑(Fo); wR2 ) [∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]]1/2.

as a dimer and was structurally characterized by X-ray diffraction (Figure 8, Table 1). This is the first dimeric N-functionalized N-heterocyclic carbene complex of rhodium(I) that has been structurally characterized. The structure shows a slightly distorted square planar geometry about the rhodium(I) center. The Rh-Ccarbene11,13,20,38,44 and Rh-Nnitrile45 distances are in the expected range for analogous compounds. The Rh-cod bond distances measured from the centroid of the olefin are different with respect to its coordination sphere, the one trans to the carbene (2.100 Å) being longer than the one trans to nitrile (2.016 Å). The carbene ligand should impart a stronger trans influence compared to the nitrile group, and this is also seen in the olefinic bond distances of the cyclooctadiene ligand:

(C(5)-C(6)trans to carbene: 1.361(8) Å; C(1)-C(2)trans to nitrile: 1.389(7) Å). On the other hand, the C(9)-N(1)-Rh(1) bond angle is bent away from linearity at 172.0(5)°, indicating strain in the 14-membered Rh-Ccarbene-N-Rh-Ccarbene-N ring. The dihedral angle between the phenyl ring and the imidazole group is 51.05°, and the traverses of the skewed groups have the same chirality. Additional spectroscopic evidence supports the coordination of nitrile as well as the carbene carbon. The 13C{1H} NMR shows a doublet at δ 178.4 ppm (1JRh,C ) 50.47 Hz), which is typical for most Rh(NHC) systems.11,13,20,38,44 A doublet is also located at δ 108.0 ppm (2JRh,C ) 7.60 Hz) for the carbon on the nitrile group coordinated to rhodium(I). This coupling is small

860 Organometallics, Vol. 28, No. 3, 2009

compared to ones in systems such as the pincer-type compound Rh(PCN)(PhCN). (PCN ) PiPr2CH2-2-(3,5-(CH3)2-C6H)-6CH2NEt2, 2JRh,C ) 19.1 Hz)45 Further, the nitrile stretching frequency observed in the IR spectrum (2183 cm-1) is significantly lower than that of the imidazolium salt (2283 cm-1), due to donation of electron density from the nitrile moiety to the rhodium(I) center.

Conclusion We have reported the synthesis of unsymmetrical nitrilefunctionalized N-heterocyclic carbene precursors from 2-cyanophenylimidazole. The reactions of these novel imidazolium salts with silver(I) oxide yielded novel silver(I) carbene complexes 2a-2d. Unlike the preparation of other Ag(NHC)2+ or Ag(NHC)X type complexes reported, the preparations of these complexes were sensitive to the presence of water. In particular, the reaction of 3-(cyanomethyl)-1-(2-cyanophenyl)imidazolium (1b) with excess silver(I) oxide leads to the formation of the desired bis-carbene complex, but also leads to the hydrolysis of the cyanomethyl group of the carbene ligand. This is the first report of the observation of a primary-amidofunctionalized NHC complex of silver(I). The selectivity in hydrolysis of cyanomethyl group over the cyanophenyl group suggests the process must be mediated by metal centers. We have observed hindered rotation about the C-N bond of the amide by means of NMR, and suggested that an interaction between the silver(I) center and the oxygen of the amido-group was present. We further postulated that the hydrolysis of the nitrile on the ligand occurred during the formation of a silver(I) carbene complex, and was mediated by two silver(I) centers. Mechanistic and computational studies of the hydrolysis of the nitrile-functionalized NHC ligand are currently underway. The use of these novel N-heterocyclic carbene complexes of silver(I) as a carbene transfer reagent was further demonstrated by the reaction of 2a with [Rh(cod)Cl]2. This novel nitrilefunctionalized NHC ligand has shown versatility in its coordination in 3a. This is the first dimeric N-functionalized N-heterocyclic carbene complex of rhodium(I) that has been structurally characterized. This complex with bridging carbene ligands crystallized as a dimer where the nitrile on the ligand and the carbene ligand were coordinated to two different rhodium(I) centers.

Experimental Section General Considerations. All of the preparations and manipulations, except where otherwise stated, were carried out under an argon or nitrogen atmosphere using standard Schlenk-line and glovebox techniques. Dry and oxygen-free solvents were always used. The syntheses of (2-cyanophenyl)imidazole22 and [RhCl(cod)]246 were reported in the literature. All other reagents were purchased from commercial sources and were used as received. Deuterated solvents were purchased from Cambridge Isotope Laboratories and degassed and dried over activated molecular sieves prior to use. NMR spectra were recorded on a Varian 400 (44) (a) Neveling, A.; Julius, G. R.; Cronje, S.; Esterhuysen, C.; Raubenheimer, H. G. Dalton Trans. 2005, 181–192. (b) Dastgir, S.; Coleman, K. S.; Cowley, A. R.; Green, M. L. H. Organometallics 2006, 25, 300–306. (c) Messerle, B. A.; Page, M. J.; Turner, P. Dalton Trans. 2006, 3927–3933. (d) Blum, A. P.; Ritter, T.; Grubbs, R. H. Organometallics 2007, 26, 2122–2124. (45) Cohen, R.; Rybtchinski, B.; Gandelman, M.; Shimon, L. J. W.; Martin, J. M. L.; Milstein, D. Angew. Chem., Int. Ed. 2003, 42, 1949– 1952. (46) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1990, 28, 88–90.

O et al. spectrometer operating at 400 MHz for 1H, 100 MHz for 13C, and 376 MHz for 19F. The 1H and 13C{1H} NMR were measured relative to partially deuterated solvent peaks but are reported relative to tetramethylsilane (TMS). All 19F chemical shifts were measured relative to trichlorofluoromethane as an external reference. All infrared spectra were recorded on a Nicolet 550 Magna-IR spectrometer. The elemental analysis was performed at the Department of Chemistry, University of Toronto, on a Perkin-Elmer 2400 CHN elemental analyzer. Samples were handled under argon where it was appropriate. Single-crystal X-ray diffraction data were collected using a Nonius Kappa-CCD diffractometer with Mo KR radiation (λ ) 0.71073 Å). The CCD data were integrated and scaled using the Denzo-SMN package. The structures were solved and refined using SHELXTL V6.1. Refinement was by full-matrix least-squares on F2 using all data. Details are listed in Table 1. Synthesis of 1-(2-Cyanophenyl)-3-methylimidazolium Tetrafluoroborate (m-CN-BF4, 1a). A Schlenk flask was charged with (2cyanophenyl)imidazole (1.69 g, 10 mmol) and trimethyloxonium tetrafluoroborate (Meerwein’s salt; 2.22 g, 15 mmol). Dry acetonitrile (20 mL) was then added to the reaction mixture, for which a yellow color was observed. The solution was stirred at ambient temperature for 2 h under argon, and the solvent was removed in vacuo. The crude product was recrystallized with hot methanol to give a white crystalline solid. Yield: 2.45 g, 90%. 1H NMR (DMSOd6, δ): 9.66 (s, 2-CH of imid., 1H), 8.27 (t, JHH ) 1.84 Hz, 5-CH of imid., 1H), 8.20 (dd, JHH ) 1.03, 7.63 Hz, 3-CH of Ph, 1H), 8.03 (dt, JHH ) 1.03, 8.83 Hz, 5-CH of Ph, 1H), 8.00 (t, JHH ) 1.84 Hz, 4-CH of imid., 1H), 7.88 (dd, JHH ) 1.45, 8.83 Hz, 6-CH of Ph, 1H), 7.86 (dt, JHH ) 1.45, 7.63 Hz, 4-CH of Ph, 1H), 4.01 (s, CH3, 3H).13C{1H} NMR (DMSO-d6, δ): 137.7 (NCN), 136.1 (CPh), 134.9 (CPh), 134.3 (CPh), 131.1(CPh), 127.1(CPh), 124.1 (Cimid.), 123.2 (Cimid.), 114.9 (CN), 108.4 (CPh), 36.1 (CH3). IR (KBr, cm-1): 2238 (V(CN)). MS (ESI, methanol/water; m/z): 184.1 [M]+. Anal. Calcd for C11H10BF4N3: C, 48.75; H, 3.72; N, 15.50. Found: C, 48.45; H, 3.74; N, 15.43. Synthesis of 3-(Cyanomethyl)-1-(2-cyanophenyl)imidazolium Hexafluorophosphate (CH2CN-CN-PF6, 1b). A two-necked Schlenk flask was charged with (2-cyanophenyl)imidazole (1.69 g, 10 mmol) in dry acetonitrile (20 mL). Bromoacetonitrile (1.80 g, 15 mmol) was added through a syringe to the refluxing solution during the course of 0.5 h under argon. The reflux was continued for 3.5 h, whereupon the bromide salt of the imidazolium precipitated out from the reaction mixture. After the reaction mixture was cooled to ambient temperature, the white solids were filtered and rinsed with diethyl ether. The crude bromide salt of the imidazolium was dissolved in water (10 mL), and was added to a saturated aqueous solution (10 mL) of ammonium hexafluorophosphate (2.45 g, 15 mmol). The white precipitate was then filtered, rinsed with cold water, and recrystallized with hot ethanol to give a white crystalline solid. Yield: 2.96 g, 84%. 1H NMR (DMSO-d6, δ): 9.87 (s, 2-CH of imid., 1H), 8.39 (t, JHH ) 1.83 Hz, 4-CH of imid., 1H), 8.23 (t, JHH ) 1.83 Hz, 5-CH of imid., 1H), 8.22 (dd, JHH ) 1.37, 7.67 Hz, 3-CH of Ph, 1H), 8.04 (dt, JHH ) 1.37, 8.09 Hz, 5-CH of Ph, 1H), 7.92 (dd, JHH ) 1.22, 8.09 Hz, 6-CH of Ph, 1H), 7.87 (dt, JHH ) 1.22, 7.67 Hz, 4-CH of Ph, 1H), 5.74 (s, CH2, 2H).13C{1H} NMR (DMSO-d6, δ): 138.9 (NCN), 136.0 (CPh), 134.9 (CPh), 134.4 (CPh), 131.4 (CPh), 127.3 (CPh), 124.2 (Cimid), 123.2 (Cimid), 115.0 (PhCN), 114.3 (CH2CN), 108.7 (CPh), 37.3 (CH2). IR (KBr, cm-1): 2236 (V(CN)). MS (ESI, methanol/water; m/z): 209.1 [M]+. Anal. Calcd for C12H9F6N4P: C, 40.69; H, 2.56; N, 15.82. Found: C, 40.68; H, 2.51; N, 15.63. Synthesis of 1-(2-Cyanophenyl)-3-(2-pyridinylmethyl)imidazolium Hexafluorophosphate (CH2Py-CN-PF6, 1c). A solution of 2-picolyl chloride hydrochloride (1.23 g, 7.5 mmol) and potassium carbonate (0.95 g, 11.3 mmol) in degassed 95% ethanol (10 mL) was stirred at ambient temperature for 15 min under argon. The reddish solution was added through a syringe, to a refluxing

Nitrile-Functionalized N-Heterocyclic Carbenes solution of (2-cyanophenyl)imidazole (0.846 g, 5 mmol) in degassed 95% ethanol (10 mL). The solution was then refluxed for overnight under argon until a deep red color solution was obtained. The solvent was removed in vacuo, and the residue was dissolved in water (10 mL) and filtered to remove any unreacted (2-cyanophenyl)imidazole. The aqueous solution containing the crude chloride salt of the imidazolium salt was then added to a saturated aqueous solution (10 mL) of ammonium hexafluorophosphate (1.22 g, 7.5 mmol). The red-brown precipitate was then filtered, rinsed with cold water, recrystallized with hot ethanol, and chilled at - 78 °C to give a tan solid. Yield: 1.01 g, 50%. 1H NMR (DMSO-d6, δ): 9.92 (s, 2-CH of imid., 1H), 8.60 (m, 6-CH of Py, 1H), 8.32 (t, JHH ) 1.84 Hz, 5-CH of imid., 1H), 8.21 (dd, JHH ) 1.53, 7.65 Hz, 3-CH of Ph, 1H), 8.12 (t, JHH ) 1.84 Hz, 4-CH of imid., 1H), 8.03 (dt, JHH ) 1.53, 7.80 Hz, 5-CH of Ph, 1H), 7.94 (dd, JHH ) 1.11, 7.65 Hz, 6-CH of Ph, 1H), 7.92 (m, 4-CH of Py, 1H), 7.86 (dt, JHH ) 1.11, 7.80 Hz, 4-CH of Ph, 1H), 7.57 (m, 3-CH of Py, 1H), 7.44 (m, 5-CH of Py, 1H), 5.73 (s, CH2, 2H).13C{1H} NMR (DMSO-d6, δ): 152.9 (CPy), 149.6 (CPy), 138.1 (NCN), 137.5 (CPy), 136.2 (CPh), 134.9 (CPh), 134.4 (CPh), 131.2 (CPh), 127.3 (CPh), 123.9 (Cimid.), 123.7 (Cimid), 123.6 (CPy), 122.6 (CPy), 115.0 (CN), 108.7 (CPh), 53.6 (CH2). IR (KBr, cm-1): 2236 (V(CN)). MS (ESI, methanol/water; m/z): 261.1 [M]+. Anal. Calcd for C16H13F6N4P: C, 47.30; H, 3.23; N, 13.79. Found: C, 47.03; H, 3.29; N, 13.22. Synthesis of 1-(2-Cyanophenyl)-3-(2-pyridinyl)imidazolium Hexafluorophosphate (Py-CN-PF6, 1d). A 10 mL round-bottom flask was charged with (2-cyanophenyl)imidazole (0.846 g, 5 mmol) and 2-bromopyridine (1.19 g, 7.5 mmol), and was heated neat at 140 °C under argon for 1 day with vigorous stirring. The liquid was cooled down to give a deep red solid, which was extracted with water and filtered through activated charcoal to give a bright red solution. The red solution containing the crude bromide salt of the imidazolium salt was then added to a saturated aqueous solution (10 mL) of ammonium hexafluorophosphate (1.22 g, 7.5 mmol). The red precipitate was filtered, rinsed with cold water, and was recrystallized with hot ethanol and chilled at - 78 °C to give a red solid. Yield: 1.05 g, 54%. 1H NMR (DMSO-d6, δ): 10.63 (s, 2-CH of imid., 1H), 8.83 (dd, JHH ) 1.72, 2.12 Hz, 5-CH of imid., 1H), 8.72 (m, 3-CH of Py, 1H), 8.58 (dd, JHH ) 1.72, 2.12 Hz, 4-CH of imid., 1H), 8.30 (m, 5-CH of Py, 1H), 8.26 (dd, JHH ) 1.55, 8.06 Hz, 3-CH of Ph, 1H), 8.14 (m, 6-CH of Py, 1H), 8.08 (dd, JHH ) 1.40, 7.52 Hz, 6-CH of Ph, 1H), 8.04 (dt, JHH ) 1.40, 8.06 Hz, 5-CH of Ph, 1H), 7.91 (dt, JHH ) 1.55, 7.52 Hz, 4-CH of Ph, 1H), 7.73 (m, 4-CH of Py, 1H).13C{1H} NMR (DMSO-d6, δ): 149.3 (CPy), 146.0 (CPy), 140.7 (CPy), 136.6 (NCN), 136.0 (CPh), 134.9 (CPh), 134.4 (CPh), 131.5 (CPh), 127.4 (CPh), 125.7 (CPy), 124.5 (Cimid.), 119.7 (Cimid.), 115.0 (CN), 114.7 (CPy), 108.7 (CPh). IR (KBr, cm-1): 2242 (V(CN)). MS (ESI, methanol/water; m/z): 247.1 [M]+. Anal. Calcd for C15H11F6N4P: C, 45.93; H, 2.83; N, 14.28. Found: C, 46.03; H, 3.05; N, 14.51. Synthesis of Bis[1-(2-cyanophenyl)-3-methylimidazol-2-ylidene]silver(I) Tetrafluoroborate ([Ag(m-CN)2]BF4, 2a). A Schlenk flask was charged with 1a (542 mg, 2 mmol) and silver(I) oxide (695 mg, 3 mmol) in dry acetonitrile (15 mL). The reaction mixture was refluxed over 3 Å molecular sieves under argon overnight, protected from light. It was then filtered through a pad of Celite to give a gold-yellow solution. The volume of solvent was reduced to 3 mL ca., and diethyl ether (10 mL) was added, which caused fine tan crystals to precipitate from the reaction mixture. The crystals were then collected on a glass frit and dried in vacuo. Yield: 456 mg, 81%. Suitable crystals for an X-ray diffraction study were obtained by slow diffusion of diethyl ether into a saturated solution of 2a in acetonitrile. 1H NMR (CD3CN, δ): 7.88 (dd, JHH ) 1.63, 7.68 Hz, 3-CH of Ph, 1H), 7.77 (dt, JHH ) 1.63, 7.84 Hz, 5-CH of Ph, 1H), 7.68 (dt, JHH ) 1.26, 7.68 Hz, 4-CH of Ph, 1H), 7.55 (dd, JHH ) 1.26, 7.84 Hz, 6-CH of Ph, 1H), 7.47 (d, JHH ) 1.86 Hz, 5-CH of imid., 1H), 7.39 (dt, JHH ) 1.86 Hz, 4-CH of imid., 1H),

Organometallics, Vol. 28, No. 3, 2009 861 3.85 (s, CH3, 3H).19F NMR (CD3CN, δ): -152.2 (s), -152.3 (s). 13 C{1H} NMR (CD3CN, δ): 182.6 (C-Ag), 142.1 (CPh), 135.0 (CPh), 134.6 (CPh), 130.6 (CPh), 128.5 (CPh), 124.1 (Cimid.), 123.5 (Cimid.), 116.1 (CN), 110.7 (CPh), 39.0 (CH3). IR (KBr, cm-1): 2242, 2227 (V(CN)). Anal. Calcd for C22H18AgBF4N6: C, 47.09; H, 3.23; N, 14.98. Found: C, 46.95; H, 3.39; N, 14.75. Observation of Silver(I) Complex of (3-(Cyanomethyl)-1-(2cyanophenyl)imidazol-2-ylidene (2b). A Schlenk flask was charged with 1b (354 mg, 1 mmol) and silver(I) oxide (348 mg, 1.5 mmol) in dry acetonitrile (10 mL). The reaction mixture was refluxed over 3Å molecular sieves under argon overnight, protected from light. It was then filtered through a pad of celite to give a gold-yellow solution. The volume of solvent was reduced to 0.5 mL ca., and 3 mL of [D3]acetonitrile was added to prepare samples for NMR analysis. The compound was too unstable to be isolated in its solid state, and prone to decomposition under light in both solution and solid state. 1H NMR (CD3CN, δ): 7.88 (dd, JHH ) 1.57, 7.70 Hz, 3-CH of Ph, 1H), 7.77 (dt, JHH ) 1.57, 7.76 Hz, 5-CH of Ph, 1H), 7.70 (dt, JHH ) 0.96, 7.70 Hz, 4-CH of Ph, 1H), 7.56 (m, CH of imid., 2H), 7.55 (dd, JHH ) 0.96, 7.76 Hz, 6-CH of Ph, 1H), 5.23 (s, CH2, 2H). 19F NMR (CD3CN, δ): -72.4 (s), -74.3 (s). 13C{1H} NMR (CD3CN, δ): 184.4 (C-Ag), 142.0 (CPh), 135.6 (CPh), 135.1 (CPh), 131.5 (CPh), 128.8 (CPh), 125.0 (Cimid.), 123.8 (Cimid.), 116.4 (PhCN), 116.1 (CH2CN), 110.9 (CPh), 40.4 (CH2). Observation of Silver(I) Complex of (3-(Carbomoylmethyl)1-(2-cyanophenyl)imidazol-2-ylidene (2e). The amide complex of 2b was generated in situ in small quantities during the preparation of 2b, and was observed by NMR as with 2b. 1H NMR (CD3CN, δ): 7.86 (m, 3-CH of Ph, 1H), 7.76 (m, 5-CH of Ph, 1H), 7.70 (m, 4-CH of Ph, 1H), 7.55 (m, 6-CH of Ph, 1H), 7.48 (d, JHH ) 1.86 Hz, 5-CH of imid., 1H), 7.48 (d, JHH ) 1.86 Hz, 4-CH of imid., 1H), 6.49 (s, br, NH2, 1H), 6.12 (s, br, NH2, 1H), 4.89 (s, CH2, 2H). 19F NMR (CD3CN, δ): -72.4 (s), -74.3 (s). 13C{1H} NMR (CD3CN, δ): δ 183.9 (C-Ag), 169.4 (CONH2), 142.3 (CPh), 135.6 (CPh), 135.1 (CPh), 131.2 (CPh), 128.7 (CPh), 124.6 (Cimid.), 123.9 (Cimid.), 116.6 (PhCN), 110.8 (CPh), 54.2 (CH2). Synthesis of Bis{[1-(2-cyanophenyl)-3-(2-pyridinylmethyl)imidazol-2-ylidene]silver(I)} Hexafluorophosphate ([Ag(CH2PyCN)]2(PF6)2, 2c). A Schlenk flask was charged with 1c (203 mg, 0.5 mmol) and silver(I) oxide (174 mg, 0.75 mmol) in dry acetonitrile (10 mL). The reaction mixture was refluxed over 3 Å molecular sieves under argon for overnight, protected from light. It was then filtered through a pad of Celite to give a gold-yellow solution. The volume of solvent was reduced to 1 mL ca., and diethyl ether (8 mL) was added for which tan solids were precipitated from the reaction mixture. The precipitate was filtered and dried under vacuo to give a tan powder. Yield: 174 mg, 68%. 1 H NMR (CD3CN, δ): 8.44 (m, 3-CH of Py, 1H), 7.79 (dd, JHH ) 1.16, 7.76 Hz, 3-CH of Ph, 1H), 7.75 (m, 5-CH of Py, 1H), 7.69 (dt, JHH ) 1.16, 8.07 Hz, 5-CH of Ph, 1H), 7.61 (dt, JHH ) 1.23, 7.76 Hz, 4-CH of Ph, 1H), 7.52 (dd, JHH ) 1.23, 8.07 Hz, 6-CH Ph, 1H), 7.49 (d, JHH ) 1.86 Hz, 4-CH of imid., 1H), 7.48 (d, JHH ) 1.86 Hz, 5-CH of imid., 1H), 7.31 (m, 4-CH of Py, 1H), 7.27 (m, 6-CH of Py, 1H), 5.41 (s, CH2, 2H). 19F NMR (CD3CN, δ): -72.4 (s), -74.3 (s).13C{1H} NMR (CD3CN, δ): 183.5 (C-Ag), 156.1 (CPy), 151.0 (CPy), 142.5 (CPh), 138.8 (CPy), 135.5 (CPh), 135.1 (CPh), 131.1 (CPh), 128.9 (CPh), 124.7 (CPy), 124.4 (Cimid), 124.1 (Cimid.), 123.6 (CPy.), 116.6 (CN), 111.1 (CPh), 57.8 (CH2). IR (KBr, cm-1): 2236 (V(CN)). Anal. Calcd for C32H24Ag2F12N8P2: C, 37.45; H, 2.36; N, 10.91. Found: C, 38.04; H, 2.83; N, 10.42. Synthesis of Bis{[1-(2-cyanophenyl)-3-(2-pyridinyl)imidazol2-ylidene]silver(I)} Hexafluorophosphate ([Ag(Py-CN)]2(PF6)2, 2d). A Schlenk flask was charged with 1d (189 mg, 0.48 mmol) and silver(I) oxide (168 mg, 0.72 mmol) in dry acetonitrile (10 mL). The reaction mixture was refluxed over 3 Å molecular sieves under argon overnight, protected from light. It was then filtered through a pad of Celite to give a gold-yellow solution. The volume

862 Organometallics, Vol. 28, No. 3, 2009 of solvent was reduced to ca. 1 mL, and diethyl ether (8 mL) was added for which beige solids were precipitated from the reaction mixture. The precipitate was filtered and dried under vacuo to give a beige powder. Yield: 150 mg, 63%. 1H NMR (CD3CN, δ): 8.19 (m, 3-CH of Py, 1H), 8.01 (d, JHH ) 1.44 Hz, 4-CH of imid., 1H), 7.88 (m, 5-CH of Py, 1H), 7.86 (d, JHH ) 7.40 Hz, 3-CH of Ph, 1H), 7.80 (m, 6-CH of Py, 1H), 7.72 (t, JHH ) 7.79 Hz, 5-CH of Ph, 1H), 7.68 (d, JHH ) 1.44 Hz, 5-CH of imid., 1H), 7.67 (t, JHH ) 7.40 Hz, 4-CH of Ph, 1H), 7.62 (d, JHH ) 7.79 Hz, 6-CH of Ph, 1H), 7.41 (m, 4-CH of Py, 1H). 19F NMR (CD3CN, δ): -72.4 (s), -74.3 (s).13C{1H} NMR (CD3CN, δ): 183.8 (C-Ag), 151.2 (CPy), 149.9 (CPy), 142.7 (CPh), 140.9 (CPy), 135.5 (CPh), 135.1 (CPh), 131.4 (CPh), 128.9 (CPh), 125.5 (Cimid.), 125.2 (CPy), 121.6 (Cimid.), 116.5 (CPy.), 116.4 (CN), 111.1 (CPh). IR (KBr, cm-1): 2232 (V(CN)). Anal. Calcd for C30H20Ag2F12N8P2: C, 36.10; H, 2.02; N, 11.23. Found: C, 37.17; H, 2.26; N, 11.30. Synthesis of Bis[(1-(2-cyanophenyl)-3-methylimidazol-2ylidene)-(η4-1,5-cycloctadiene)rhodium(I)]Tetrafluoroborate([Rh(mCN)(cod)]2(BF4)2, 3a). A solution of 2a (56 mg, 0.1 mmol) and silver tetrafluoroborate (10 mg, 0.05 mmol) in acetonitrile (3 mL) was added to a solution of [RhCl(cod)2]2 (57 mg, 0.115 mmol) in acetonitrile (2 mL). A pale-brown precipitate was formed instantaneously. The reaction mixture continued to be stirred for 2 h. It was then filtered through a pad of Celite to give a bright-yellow solution. The solvent was then removed in vacuo, and the residue was dissolved in dichloromethane (1 mL). Addition of diethyl ether (8 mL) caused precipitation of a bright-yellow precipitate. The precipitate was then filtered, rinsed with diethyl ether (2 mL), and dried in vacuo. Yield: 75 mg, 71%. Suitable crystals for an X-ray diffraction study were obtained by slow diffusion of diethyl ether into a saturated solution of 3a in dichloromethane. 1H NMR (CD2Cl2, δ): 8.21 (dd, JHH ) 1.40, 7.80 Hz, 3-CH of Ph, 1H), 8.00 (dt, JHH ) 1.40, 7.97 Hz, 5-CH of Ph, 1H), 7.90 (dt, JHH ) 1.13,

O et al. 7.80 Hz, 4-CH of Ph, 1H), 7.61 (d, JHH ) 2.00 Hz, 5-CH of imid., 1H), 7.55 (dd, JHH ) 1.13, 7.97 Hz, 6-CH of Ph, 1H), 7.41 (d, JHH ) 2.00 Hz, 4-CH of imid., 1H), 5.16 (m, olefinic-CH of cod, 1H), 4.76 (m, olefinic-CH of cod, 1H), 4.07 (m, olefinic-CH of cod, 1H), 3.90 (s, CH3, 3H), 2.88 (m, olefinic-CH of cod, 1H), 2.51 (m, CH2 of cod, 1H), 2.24 (m, CH2 of cod, 1H), 2.18 (m, CH2 of cod, 1H), 2.04 (m, CH2 of cod, 1H), 1.94 (m, CH2 of cod, 1H), 1.88 (m, CH2 of cod, 1H), 1.65 (m, CH2 of cod, 1H), 1.40 (m, CH2 of cod, 1H). 19F NMR (CD2Cl2, δ): -152.3 (s), -152.4 (s). 13C{1H} NMR (CD2Cl2, δ): 178.4 (C-Rh, 1JRh,C ) 50.47 Hz), 142.56 (CPh), 136.7 (CPh), 136.3 (CPh), 131.3 (CPh), 128.6 (CPh), 125.9 (Cimid.), 123.9 (Cimid.), 110.5 (CPh), 108.0 (CN, 2JRh,C ) 7.60 Hz), 100.1 (olefinic-C of cod, 1JRh,C ) 7.20 Hz), 99.2 (olefinic-C of cod, 1JRh,C ) 6.71 Hz), 83.3 (olefinic-C of cod, 1JRh,C ) 13.30 Hz), 79.1 (olefinic-C of cod, 1JRh,C ) 12.69 Hz), 38.3 (CH3), 34.2 (CH2 of cod), 30.4 (CH2 of cod), 29.9 (CH2 of cod), 27.8 (CH2 of cod). IR (KBr, cm-1): 2183 (V(CN)). MS (ESI, methanol/water; m/z): 394.1 [Rh(m-CN)(cod)]+. Several attempts at elemental analyses failed to give an acceptable carbon content, while hydrogen and nitrogen content are in the acceptable range. Typical results: Anal. Calcd for C38H42B2F8N6Rh2: C, 47.43; H, 4.40; N, 8.73. Found: C, 45.49; H, 4.39; N, 8.51.

Acknowledgment. The NSERC Canada is thanked for a Discovery Grant to R.H.M. and a postgraduate scholarship to W.W.N.O. Supporting Information Available: X-ray structural data crystallographic file (CIF) format for compounds 1a, 1b, 2a, and 3a. This material is available free of charge via the Internet at http://pubs.acs.org. OM8009377