Palladium(II) and Platinum(II) Complexes Featuring a Nitrile

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Organometallics 2010, 29, 570–581 DOI: 10.1021/om9008372

Palladium(II) and Platinum(II) Complexes Featuring a Nitrile-Functionalized N-Heterocyclic Carbene Ligand 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 28, 2009

The transmetalation reaction of 2 equiv of trans-PdCl2(CH3CN)2 or Pd(cod)Cl2 with a nitrilefunctionalized N-heterocyclic carbene complex of silver(I), bis[1-(2-cyanophenyl)-3-methylimidazol2-ylidene]silver(I) tetrafluoroborate ([Ag(m-CN)2]BF4, 1), and 1 equiv of AgBF4 afforded a bimetallic complex formulated as [(m-CN)2Pd(μ-Cl)2Pd(CH3CN)2](BF4)2 (2a). The interesting trimetallic complex 2b was obtained during an attempt to crystallize 2a from wet diethyl ether and acetonitrile. The solid-state structure of 2b reveals the presence of a novel C-N-N-C donor ligand providing two bridging imido nitrogens to adjacent palladium(II) centers in the trimetallic complex [{Pd(CH3CN)2}3(C-N-N-C)](BF4)4 (2b). The C-N-N-C ligand, which has a central -NdC-O-CdN- linkage, was formed from the hydrolysis and condensation of the nitrile groups of two carbene ligands. Palladium(II) and platinum(II) complexes bearing chelating olefin and N-heterocyclic carbene ligands were synthesized by transmetalation reactions of N-heterocyclic carbene complexes of silver(I), complex 1 and bis[1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]silver(I) tetrafluoroborate ([Ag(IMes)2]BF4, 7), with [Pd(η1:η2-coe-OMe)(μ-Cl)]2 and Pt(cod)Cl2 (coe-OMe = 2-methoxycyclooct-5-enyl, cod = 1,5-cyclooctadiene) or by addition of a methoxide anion to the coordinated diolefin ligand of the novel platinum(II) complex, chloro[1-(2-cyanophenyl)3-methylimidazol-2-ylidene](η4-1,5-cyclooctadiene)platinum(II) tetrafluoroborate ([Pt(m-CN)(cod)Cl]BF4, 4). The complexes bearing 1-(2-cyanophenyl)-3-methylimidazol-2-ylidene and 2-methoxycyclooct5-enyl ligands ([M(m-CN)(η1:η2-coe-OMe)]þ: 3, M = Pd; 6, M = Pt) exists in both monomeric and dimeric forms depending on the choice of recrystallization solvents. All of these complexes were isolated and studied by NMR and infrared spectroscopy. Different ratios of rotamers were observed for the complexes bearing a 2-methoxycyclooct-5-enyl ligand owing to its orientation relative to the carbene ligand. Rotamers are believed to form because of steric restrictions to rotation about the M-Ccarbene bond (M = Pd, Pt).

Introduction Donor functionalized N-heterocyclic carbenes (NHC) have exceptional utility in the synthesis of novel and highly active homogeneous catalysts in the fields of organometallic and synthetic organic chemistry.1,2 The versatility in the mode of coordination of these ligands and, more importantly, their ability to promote oxidative addition, reductive elimination, and dehydrohalogenation reactions in catalytic reactions have been extensively reviewed in the literature in recent years.3 Our group has been interested in the development of catalysts for the hydrogenation of polar bonds, including those of ketones, imines, and nitriles. High activities were *To whom correspondence should be addressed. E-mail: rmorris@ chem.utoronto.ca. (1) (a) Liddle, S. T.; Edworthy, I. S.; Arnold, P. L. Chem. Soc. Rev. 2007, 36, 1732–1744. (b) Kuhl, O. Chem. Soc. Rev. 2007, 36, 592–607. (2) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122– 3172. (3) (a) Crudden, C. M.; Allen, D. P. Coord. Chem. Rev. 2004, 248, 2247–2273. (b) Lee, H. M.; Lee, C. C.; Cheng, P. Y. Curr. Org. Chem. 2007, 11, 1491–1524. (c) Normand, A. T.; Cavell, K. J. Eur. J. Inorg. Chem. 2008, 2781–2800. (d) Corberan, R.; Mas-Marza, E.; Peris, E. Eur. J. Inorg. Chem. 2009, 1700–1716. pubs.acs.org/Organometallics

Published on Web 12/23/2009

reported for ruthenium(II)-4 and iron(II)-based5 catalysts bearing phosphino-amino ligands. We have reported the syntheses of novel coordinatively unsaturated hydridoruthenium(II) complexes bearing N-heterocyclic carbene ligands6 and, more recently, nitrile-functionalized N-heterocyclic carbene ligands and their metal complexes of silver(I) and rhodium(I) (Figure 1).7 The nitrile functionality on these carbene ligands can bridge to metal centers and can be hydrolyzed with the assistance of metal ions, giving primary (4) (a) Guo, R.; Lough, A. J.; Morris, R. H.; Song, D. T. 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. (c) Guo, R.; Morris, R. H.; Song, D. J. Am. Chem. Soc. 2005, 127, 516–517. (5) Morris, R. H. Chem. Soc. Rev. 2009, 38, 2282–2291 and references therein. (6) Abdur-Rashid, K.; Fedorkiw, T.; Lough, A. J.; Morris, R. H. Organometallics 2004, 23, 86–94. (7) O, W. W. N.; Lough, A. J.; Morris, R. H. Organometallics 2009, 28, 853–862. (8) (a) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev. 2004, 248, 2201–2237. (b) Ikariya, T.; Murata, K.; Noyori, R. Org. Biomol. Chem. 2006, 4, 393–406. (c) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300–1308. (d) O, W. W. N.; Lough, A. J.; Morris, R. H. Organometallics 2009, 28, 6755–6761. r 2009 American Chemical Society

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Scheme 1. Synthesis of Pd(II) and Pt(II) Complexes (2, 3a, and 4) Starting from a Nitrile-Functionalized N-Heterocyclic Carbene Complex of Ag(I) (1)

Figure 1. Nitrile-functionalized N-heterocyclic carbene complexes of Ag(I) and Rh(I) and the general structures of the complexes reported in this work (M = Pd(II), Pt(II); D = olefin or alkyl ligand; L = nitrile donor or a chloro group).

amido-functionalized N-heterocyclic carbene ligands. The nitrile moiety might be reduced at a metal center, producing a M-NH2CH2- group, which is known to be important in the metal-ligand bifunctional catalytic reduction of polar bonds.8 In our continuing investigation of the coordinating ability and reactivity of these nitrile-functionalized N-heterocyclic carbene ligands, and N-heterocyclic carbene ligands in general and their late-transition-metal complexes, we report herein the synthesis and studies of novel palladium(II) and platinum(II) complexes bearing N-heterocyclic carbene and olefin ligands, which predominantly have the general structures shown in Figure 1. Of particular interest is the reactivity of the coordinated olefin and nitriles in these metal complexes.

Results and Discussion Transmetalation of a Nitrile-Functionalized N-heterocyclic Carbene Ligand from Silver(I) to Palladium(II). The transmetalation reaction9 of 1 ([Ag(m-CN)2]BF4) with 2 equiv of trans-PdCl2(CH3CN)2 and 1 equiv of AgBF4 afforded the bimetallic complex 2a as a yellow powder in 62% yield. Alternatively, the complex could also be prepared using Pd(cod)Cl2 in higher yields (79%) (Scheme 1). The reaction did not proceed cleanly when only 1 equiv of the palladium(II) precursor was used. Although crystals suitable for X-ray diffraction were not obtained successfully, the structure of 2a can be elucidated by spectroscopic data and elemental analysis. The 13C{1H} NMR spectrum in acetonitrile-d3 solution shows a singlet at 141.4 ppm, which was assigned to the carbene carbon (Pd-Ccarbene) according to a 1H-13C HMBC experiment. Although it is shifted upfield compared to most Pd(NHC)2X2 (X = halide) systems,10-13 it is in the expected range when compared to cationic cis-[Pd(NHC)2(CH3CN)X]þ or cis-[Pd(NHC)2(CH3CN)2]2þ systems.14 The nitrile stretching absorptions in the infrared spectrum suggested the presence of coordinated acetonitrile15,16 and (9) Lin, I. J. B.; Vasam, C. S. Coord. Chem. Rev. 2007, 251, 642–670. (10) Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.; Artus, G. R. J. Angew. Chem., Int. Ed. 1995, 34, 2371–2374. (11) Ku, R. Z.; Huang, J. C.; Cho, J. Y.; Kiang, F. M.; Reddy, K. R.; Chen, Y. C.; Lee, K. J.; Lee, J. H.; Lee, G. H.; Peng, S. M.; Liu, S. T. Organometallics 1999, 18, 2145–2154. (12) Magill, A. M.; McGuinness, D. S.; Cavell, K. J.; Britovsek, G. J. P.; Gibson, V. C.; White, A. J. P.; Williams, D. J.; White, A. H.; Skelton, B. W. J. Organomet. Chem. 2001, 617-618, 546–560. (13) Grundemann, S.; Albrecht, M.; Kovacevic, A.; Faller, J. W.; Crabtree, R. H. Dalton Trans. 2002, 2163–2167. (14) Scherg, T.; Schneider, S. K.; Frey, G. D.; Schwarz, J.; Herdtweck, E.; Herrmann, W. A. Synlett 2006, 2894–2907. (15) Andrews, M. A.; Chang, T. C. T.; Cheng, C. W. F.; Emge, T. J.; Kelly, K. P.; Koetzle, T. F. J. Am. Chem. Soc. 1984, 106, 5913–5920. (16) Mitsudo, K.; Kaide, T.; Nakamoto, E.; Yoshida, K.; Tanaka, H. J. Am. Chem. Soc. 2007, 129, 2246–2247.

noncoordinated nitrile groups of carbene ligands (2329 and 2237 cm-1, respectively). Of note, the nitrile stretching frequency on the carbene ligand is significantly higher compared to a rhodium(I) complex bearing the same ligand with bridging nitrile groups (2183 cm-1).7 The complex is formulated as [Pd(m-CN)Cl(CH3CN)(BF4)]n according to elemental analysis. The ion [Pd(m-CN)2Cl]þ is the major species revealed by electrospray-ionization mass spectrometry (ESI-MS). We therefore propose that the compound has the structure [(mCN)2Pd(μ-Cl)2Pd(CH3CN)2](BF4)2, where the chloro ligands bridge between two palladium(II) centers and the two carbene ligands are coordinated to one metal center. Hydrolysis of Nitrile-Functionalized N-Heterocyclic Carbene Ligands on Palladium(II) Centers. In an attempt to obtain a crystal structure of 2a using wet diethyl ether layered on top a saturated solution of the complex in acetonitrile, the nitrilefunctionalized NHC ligands were hydrolyzed, leading to formation of very small amounts of crystals, which in turn were characterized by X-ray diffraction as the trimetallic complex 2b (Figure 2, Table 1). Complex 2b contains a novel C-N-N-C donor ligand providing two bridging imido nitrogens in the trimetallic complex [{Pd(CH3CN)2}3(C-N-N-C)](BF4)4; the C-N-N-C ligand, which has a central -NdC-O-CdN- linkage, is formed from the partial hydrolysis of the nitrile moiety to a primary amido group7 followed by a condensation reaction with elimination of water. Each square-planar palladium(II) center has two cis acetonitrile ligands. The coordination geometry around the central palladium(II) center comprises of a six-membered ring with a -Pd-NdC-O-CdN- linkage and resembles that of a β-diketiminato complex.17 To our knowledge, there (17) Hadzovic, A.; Song, D. Organometallics 2008, 27, 1290–1298. (18) (a) Gramstad, T.; Husebye, S.; Saebo, J. Tetrahedron Lett. 1983, 24, 3919–3920. (b) Hitzler, M. G.; Lutz, M.; Shrestha-Dawadi, P. B.; Jochims, J. C. Liebigs Ann. Chem. 1996, 247–257.

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Figure 2. ORTEP diagram of 2b depicted with thermal ellipsoids at the 50% probability level. The counteranions, hydrogens, and solvent molecules have been omitted for clarity. Selected bond distances (A˚) and bond angles (deg): Pd(1)-C(1), 1.969(6); Pd(3)-C(20), 1.957(6); Pd(1)-N(3), 2.015(5); Pd(3)-N(4), 2.018(5); Pd(2)-N(3), 1.976(5); Pd(2)-N(4), 1.972(5); Pd(1)-N(8), 2.094(6); Pd(1)-N(7), 2.020(5); Pd(3)-N(11), 2.059(6); Pd(3)-N(12), 2.018(6); C(10)-N(3), 1.272(8); C(11)-N(4), 1.265(8); C(10)-O(1), 1.361(7); C(11)-O(1), 1.382(7); N(3)-Pd(2)-N(4), 91.9(2); C(1)-Pd(1)-N(3), 85.4(2); C(9)-C(10)-O(1), 107.2(5); C(10)-O(1)-C(11), 126.7(5); C(10)-N(3)-Pd(2), 125.3(4).

are only few examples of organic compounds with the general formula R-NdCR0 -O-R0 CdN-R reported in the literature (R = H, alkyl).18 The outer palladium(II) cations are coordinated to the bridging imido nitrogen and the carbene carbon of the N-heterocyclic carbene, thus forming a seven-membered ring (Figure 2). Bridging groups with this type of nitrogen are rare. Shriver and co-workers have reported the structural characterization of metalcarbonyl cluster complexes with an acetamido ligand bridging between three heterometal centers (Ru, Mn, Re) via nitrogen and coordination to ruthenium via its oxygen.19 Mehrotra and Verkade have reported structures with trialkylstannates bridged by acetamido ligands.20 More recently, Besenyei have reported the synthesis of some bridging phenylacetamido complexes of palladium(II) from reactions of benzoyl azides with [PdCl(dppm)]2 (dppm = 1,1-bis(diphenylphosphino)methane).21 Complex 2b crystallizes in the triclinic space group P1 with two units residing in the unit cell. Despite the presence of disordered diethyl ether molecules, this contribution to the electron density was removed from the observed data and resulted in a significant increase in the precision of the geometric parameters (see the Supporting Information). The structure shows square-planar geometries about the metal centers, and the phenyl rings are twisted with respect to the imidazolidene rings at dihedral angles 41.00 and 41.13° to facilitate chelation. The C(11)-N(3) and C(10)-N(4) bond distances (1.272(8) and 1.265(8) A˚) reveal a doublebond character between carbon and nitrogen atoms, forming (19) (a) Voss, E. J.; Sabat, M.; Shriver, D. F. Inorg. Chem. 1991, 30, 2705–2707. (b) Voss, E. J.; Sabat, M.; Shriver, D. F. Inorg. Chim. Acta 1995, 240, 49–61. (20) (a) Sharma, K. K.; Mehrotra, S. K.; Mehrotra, R. C. J. Organomet. Chem. 1977, 142, 165–169. (b) Geetha, S.; Ye, M. C.; Verkade, J. G. Inorg. Chem. 1995, 34, 6158–6162. (21) Besenyei, G.; Parkanyi, L.; Szalontai, G.; Holly, S.; Papai, I.; Keresztury, G.; Nagy, A. Dalton Trans. 2004, 2041–2050.

an imido linkage (cf. CPh-C(sp2)dN-Cavg = 1.279 A˚; C(sp3)-Navg = 1.46-1.48 A˚).22 On the other hand, the C-O distances (1.361(7) and 1.382(7) A˚) are shorter than a C(sp3)-O bond (C(sp3)-Oavg = 1.42-1.45 A˚ for ethers) and slightly longer than the C(sp2)-O bonds of organic amides (C(sp2)-Oavg = 1.23 A˚).22 These all suggest delocalization of electrons through the π-orbitals of the N-C-O-C-N bonds. In addition, the Pd(2)-N(3) and Pd(2)-N(4) bond distances (1.976(5) and 1.972(5) A˚) are significantly shorter than a Pd-RNdCR2 motif (Pd-Navg = 2.037 A˚),23 reflecting the presence of highly charged bridgingimido nitrogen atoms. The Pd-Ccarbene bond distances lie within the typical range for Pd(NHC) complexes.10,13,14,24,25 The carbene ligand has a stronger trans influence than the bridging-imido groups, as reflected by the Pd-Nacetonitrile bond lengths (Pd(1)-N(8)trans to C = 2.094(6), Pd(1)-N(7)trans to N = 2.020(5) A˚). The mechanism in the formation of complex 2b from a solution of 2a is not well understood. It is believed that the carbene ligands on the palladium(II) center of 2a serve as a base in the deprotonation of water in the cooperative hydrolysis of the nitrile moieties. Cooperative hydrolysis of nitrile was observed in cis-dialkylcyanamide complexes of platinum(IV) (cis-PtCl4(NCNR2)2, R = CH3, C2H5, C5H10) to a diimino complex of platinum(IV) (cis-PtCl4(NC-R2N-μ-O-NR2-CN)) under base-free conditions.26 All attempts to synthesize 2b independently to obtain spectroscopic information, by stoichiometric (22) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1–S19. (23) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D. G.; Taylor, R. Dalton Trans. 1989, S1–S83. (24) (a) Jensen, D. R.; Sigman, M. S. Org. Lett. 2003, 5, 63–65. (b) Kantchev, E. A. B.; Ying, J. Y. Organometallics 2009, 28, 289–299. (25) Broggi, J.; Clavier, H.; Nolan, S. P. Organometallics 2008, 27, 5525–5531. (26) Bokach, N. A.; Pakhomova, T. B.; Kukushkin, V. Y.; Haukka, M.; Pombeiro, A. J. L. Inorg. Chem. 2003, 42, 7560–7568.

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Table 1. Selected Crystal Data and Data Collection and Refinement Parameters for 2b, 3a, 4, 5, 6a, and 8a 2b

3a

empirical formula

4

5

C34H36B4F16N12 C22H27BF4 C19H21BClF4 C20H24ClN3 OPd3 3 1.5CH3CN N4OPd N3Pt OPt FW 1356.77 556.69 608.74 552.96 lattice type triclinic triclinic monoclinic triclinic P1 P21/n P1 space group P1 T, K 150 150 150 150 a, A˚ 11.9154(5) 8.0035(4) 10.3133(3) 7.8325(8) b, A˚ 14.7371(5) 9.9223(3) 18.4483(6) 10.1620(7) c, A˚ 17.2324(7) 16.6421(8) 11.0279(3) 14.0270(16) R, deg 81.452(2) 95.775(3) 90 69.530(6) β, deg 85.5340(19) 103.2560(18) 105.5170(16) 85.660(4) γ, deg 72.647(2) 107.764(2) 90 68.290(5) 2854.37(18) 1204.13(9) 2021.72(10) 969.92(16) V, A˚3 Z 2 2 4 2 -3 1.579 1.535 2.000 1.893 Fcalcd/Mg m 1.028 0.823 7.119 7.385 μ(Mo KR), mm-1 F(000) 1334 564 1168 546 3 0.16 0.06 0.30 0.30 0.22 0.18 0.08 cryst size, mm 0.03 0.20 0.12 0.18 0.04 θ range collected, deg 2.62-27.54 2.73-27.53 2.63-27.53 2.80-25.00 no. of rflns collected/unique 29 793/12677 10 858/5379 11 163/4574 6774/3278 abs cor semiempirical from equivalents max and min 0.993 and 0.841 0.909 and 0.738 0.276 and 0.235 0.750 and 0.441 transmissn coeff no. of params refined 685 309 263 237 goodness of fit 1.046 1.044 1.038 1.074 R1 (I > 2σ(I)) 0.0661 0.0478 0.0356 0.0584 wR2 (all data) 0.1861 0.1272 0.0912 0.1521 1.360 and -0.702 0.779 and -1.031 2.720 and -1.870 2.382 and -2.622 peak and hole, e A˚-3 P P P P a Definition of R indices: R1 = (Fo - Fc)/ (Fo); wR2 = [ [w(Fo2 - Fc2)2]/ [w(Fo2)2]]1/2.

6a

8

C22H27BF4 N4OPt 645.38 triclinic P1 150 8.0398(2) 9.8791(3) 16.6686(5) 95.7990(17) 102.5620(18) 108.2830(16) 1206.35(6) 2 1.777 5.868 628 0.24 0.14 0.08 2.72-27.51 16 993/5481

C28H36BClF4 N2Pt 3 0.5CH2Cl2 772.41 monoclinic Cc 150 23.9715(5) 8.2260(1) 30.7848(7) 90 99.7030(8) 90 5983.6(2) 8 1.715 4.916 3048 0.16 0.14 0.12 2.62-27.46 17 716/9190

0.629 and 0.427

0.562 and 0.423

300 1.123 0.0376 0.0832 2.130 and -2.020

724 1.087 0.0497 0.1155 2.403 and -2.315

reaction of trans-PdCl2(CH3CN)2, [Ag(m-CN)2]BF4 (1), silver tetrafluoroborate, and water in acetonitrile failed. Palladium(II) Complex Bearing Nitrile-Functionalized NHeterocyclic Carbene and Methoxycyclooctenyl Ligands. In order to obtain more well-defined palladium(II) complexes with the nitrile-functionalized NHC ligand, the reactions of other palladium(II) precursors with the silver(I) complex 1 were explored. Attempts using PdCl2(en),27 [Pd(CH3CN)4](BF4)2,28 and [Pd(cod)(μ-Cl)]2(BF4)229 (en = ethylenediamine, cod = 1,5-cyclooctadiene) as starting materials failed. The palladium(II) precursor [Pd(η1:η2-coe-OMe)(μ-Cl)]2

(coe-OMe = 2-methoxycyclooct-5-enyl) can be conveniently prepared as the exo isomer by the reaction of Pd(cod)Cl2 and sodium methoxide in methanol.30 When this was reacted with 1 equiv of 1 and AgBF4, a white complex 3a was isolated in 67% yield. The product exists as mixture of diastereomers, which includes rotamers A and B and each of their enantiomers, with chiral centers at the C(1) and C(2) carbons (see Figure 3 for the numbering schemes). The rotamers are believed to form via restricted rotation about the M-Ccarbene bond (M = Pd, Pt; vide infra) owing to the steric crowd of the neighboring 2-methoxycyclooct-5-enyl ligand. Grubbs and co-workers showed that a ruthenium alkylidene complex with a saturated N-heterocyclic carbene with o-tolyl groups on the nitrogens has a restricted rotation around the Ru-Ccarbene bond. The o-tolyl groups have a smaller barrier to rotation than the Ru-Ccarbene bond but do cause the formation of rotamers about the CPh-Nimid bonds.31 A second explanation of the data is that there are geometric isomers with the carbene ligand trans to the chelating olefin and the carbene ligand trans to the σ-alkyl group. These, however, are unlikely, as the 13C{1H} NMR resonances of these carbons fortuitously have the same chemical shifts (with a difference less than 10 Hz), whereas a chemical shift difference of up to 150 Hz was observed if these geometric isomers do exist.32 Slow diffusion of diethyl ether into a saturated solution of 3a afforded white needles which were then characterized by X-ray diffraction. The square-planar complex crystallizes in the chiral triclinic space group P1 with the exo-S,S rotamer A being observed (Figures 3 and 4 and Table 1). The carbene

(27) Hohmann, H.; Van Eldik, R. Inorg. Chim. Acta 1990, 174, 87–92. (28) Thomas, R. R.; Sen, A.; Beck, W.; Leidl, R. Inorg. Synth. 1989, 26, 128–134. (29) Eaborn, C.; Farrell, N.; Pidcock, A. Dalton Trans. 1976, 289–292.

(30) Bailey, C. T.; Lisensky, G. C. J. Chem. Educ. 1985, 62, 896–897. (31) Stewart, I. C.; Benitez, D.; O’Leary, D. J.; Tkatchouk, E.; Day, M. W.; Goddard, W. A.; Grubbs, R. H. J. Am. Chem. Soc. 2009, 131, 1931–1938. (32) Cooper, D. G.; Powell, J. Inorg. Chem. 1977, 16, 142–148.

Figure 3. Diastereomers and rotamers (A and B) of Pd(II) (3a) and Pt(II) (6a) complexes bearing 2-methoxycyclooct-5-enyl and nitrile-functionalized N-heterocyclic carbene ligands.

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O et al. Scheme 2. Interconversion of 3a to 3b and 6a to 6b in Different Recrystallization Solvents

Figure 4. ORTEP diagram of 3a ([Pd(m-CN)(η1:η2-coe-OMe)(CH3CN)]BF4) depicted with thermal ellipsoids at the 50% probability level. The counteranion and hydrogens have been omitted for clarity. Selected bond distances (A˚) and bond angles (deg): Pd(1)-C(1), 2.012(4); Pd(1)-C(19), 2.040(4); Pd(1)-N(4), 2.164(3); Pd(1)-coe(trans to C)cent, 2.161; C(11)-N(3), 1.148(6); C(18)-O(1), 1.441(5); C(1)-Pd(1)-coe(trans to C)avg, 161.2; C(19)-Pd(1)-N(4), 177.2(9); C(1)-Pd(1)-N(4), 89.5(8); C(1)-Pd(1)-C(19), 87.7(1).

ligand is oriented trans to the cyclooctene ligand. The Pd-C bond distances of the carbene12,13,33 (Pd-Ccarbene) and methoxycyclooctenyl34-36 (Pd-C(1)) ligands are within expected ranges. On the other hand, the acetonitrile ligand is weakly coordinated, owing to the strong trans influence of the σ-alkyl group (2.164(3) A˚; cf. 2.06 A˚ in PdCl2(CH3CN)(μ-dpmp)PdCl237 and 2.01-2.09 A˚ in 2b; dpmp = bis((diphenylphosphino)methyl)phenylphosphine). The 2-cyanophenyl group is rotated so that the nitrile group is directed away from the metal center, in contrast to those of the dimeric rhodium(I) complex (Figure 1) that are rotated 180° about the C-N bond to bridge to the other rhodium center.7 The dihedral angle between the phenyl and the imidazolidene rings of the carbene ligand is 59.38°, large enough to relieve steric repulsion between the coordinated ligands. The infrared spectrum of 3a displays two nitrile stretches at 2277 cm-1 for the carbene ligand and 2229 cm-1 for the coordinated acetonitrile. In nitromethane-d3 solution, the carbene atom (Pd-Ccarbene) of the NHC ligand was observed as a singlet (33) Li, D. C.; Liu, D. J. J. Chem. Crystallogr. 2003, 33, 989–991. (34) Hoel, G. R.; Stockland, R. A.; Anderson, G. K.; Ladipo, F. T.; Braddock-Wilking, J.; Rath, N. P.; Mareque-Rivas, J. C. Organometallics 1998, 17, 1155–1165. (35) Binotti, B.; Bellachioma, G.; Cardaci, G.; Macchioni, A.; Zuccaccia, C.; Foresti, E.; Sabatino, P. Organometallics 2002, 21, 346–354. (36) (a) Macchioni, A.; Bellachioma, G.; Cardaci, G.; Travaglia, M.; Zuccaccia, C.; Milani, B.; Corso, G.; Zangrando, E.; Mestroni, G.; Carfagna, C.; Formica, M. Organometallics 1999, 18, 3061–3069. (b) Binotti, B.; Bellachioma, G.; Cardaci, G.; Carfagna, C.; Zuccaccia, C.; Macchioni, A. Chem. Eur. J. 2007, 13, 1570–1582. (37) Olmstead, M. M.; Guimerans, R. R.; Farr, J. P.; Balch, A. L. Inorg. Chim. Acta 1983, 75, 199–208.

at 174.2 ppm in the 13C{1H} NMR spectrum. All the proton and carbon resonances of the methoxycyclooctenyl ligand were assigned by 1H-1H COSY, 1H-13C HSQC, and 1 H-13C HMBC experiments based on the distinctive resonance33-35 of the C(2) carbon in the 13C{1H} NMR spectrum. The two enantiomers of a single rotamer are not distinguishable by one-dimensional 1H NMR spectroscopy. However, the rotamers A and B of 3a in nitromethane-d3 solutions are distinguishable in the 13C{1H} NMR spectrum, where the resonance of each carbon appears in a 1:4 ratio, and in the 1H NMR spectrum, where there are two resonances of the H(1) proton of the methoxycyclooctenyl ligand. The other peaks of the methoxycyclooctenyl ligand were overlapping with each other so that the exact ratio could not be determined. As no single crystal was isolated as the rotamer B, it is therefore expected, on the basis of the NMR data, that there exists approximately 20 mol % of the isolated product containing the rotamer B. The 1H and 13C{1H} NMR spectra of the platinum(II) analogues also provide evidence for the existence of these rotamers (vide infra). When 3a was recrystallized in dichloromethane and diethyl ether mixtures, the acetonitrile ligand was lost and the corresponding dimer 3b was isolated (Scheme 2). The rhodium(I) complex bearing the same bridging nitrile-functionalized NHC ligand was isolated as a dimer (Figure 1).7 The identity of 3b was established for its nitromethane-d3 solution by the absence of the resonance at 2.01 ppm for coordinated acetonitrile ligand in the 1H NMR spectrum. The nitrile stretch of the acetonitrile ligand was absent in the infrared spectrum. There is a slight decrease in the stretching frequency of the nitrile moiety of the carbene ligand on going from complex 3a (2277 cm-1) to 3b (2260 cm-1), signaling the effect of coordination to palladium(II) center. Complex 3a can be crystallized from a solution of 3b in acetonitrile and diethyl ether, suggesting the lability of the nitrile groups of both the acetonitrile and carbene ligand. Of note, the 2-cyanophenyl group of the carbene ligand has to rotate about the C-N bond by 180° to allow this interconversion of the structures. It should be noted that solutions of 3a and 3b are extremely light sensitive and are prone to β-hydride elimination to release cyclooctadienyl methyl ethers and then reductive elimination of the palladium hydride to release the imidazolium salt with the formation of black palladium(0) metal. Reductive elimination of N-heterocyclic carbene ligands can occur when they are not tethered to the metal by a second donor group from the carbene ligand.2 The organic products were identified by 1H NMR.38 In addition, complex 3a is also light sensitive in its solid state to form palladium metal. Pure (38) Anderson, C. B.; Burreson, B. J.; Michalowski, J. T. J. Org. Chem. 1976, 41, 1990–1994.

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Figure 5. ORTEP diagram of 4 ([Pt(m-CN)(cod)Cl]BF4) depicted with thermal ellipsoids at the 50% probability level. The counteranion and hydrogens have been omitted for clarity. Selected bond distances (A˚) and bond angles (deg): Pt(1)-C(1), 2.011(5); Pt(1)-Cl(1), 2.317(6); Pt(1)-cod(trans to C)cent, 2.152; Pt(1)-cod(cis to C)cent, 2.048; C(11)-N(3), 1.134(7); C(1)-Pt(1)-cod(trans to C)avg, 162.3; Cl(1)-Pt(1)-cod(cis to C)avg, 161.0; C(1)-Pt(1)-Cl(1), 86.3(3); C(1)-Pt(1)-cod(cis to C)avg, 94.0.

3a can be recovered by filtration of the palladium metal from a solution of the compound exposed to light and then recrystallization in acetonitrile and diethyl ether solution. Complex 3b in its solid state is less sensitive to light and can be stored without further decomposition. Nitrile-Functionalized N-Heterocyclic Carbene Complex of Platinum(II). Initial attempts using both isomers of PtCl2(CH3CN)239 as starting materials in the transmetalation reaction with 1 and AgBF4 failed. Only starting materials were recovered, even if the reactions were carried out at elevated temperatures. On the other hand, the reaction of 1 with 2 equiv of Pt(cod)Cl2 and 1 equiv of AgBF4 in acetonitrile and dichloromethane mixtures afforded complex 4 ([Pt(m-CN)(cod)Cl]BF4) as a white solid in 59% yield. The complex, in turn, was characterized by use of X-ray diffraction (Figure 5 and Table 1) and NMR spectroscopy. The slightly distorted square-planar complex crystallizes in the monoclinic space group P21/n with four units residing in the unit cell. The cyclooctadiene ligand is coordinated to the metal center with bond distances from the centroid of each olefin being 2.152 and 2.048 A˚. The longer distance (2.152 A˚) corresponds to that for the olefin trans to the carbene ligand, and the shorter CdC bond distance of this olefin compared to that trans to the chloro is consistent with a higher trans influence of the carbene ligand (C(16)-C(17)trans to C, 1.372(8) A˚; C(12)-C(13)trans to Cl, 1.408(7) A˚). The carbene ligand that is bonded to the metal center is oriented with a dihedral angle between the phenyl and imidazolidene ring of 51.70°, and the nitrile moiety is directed away from the metal center. The Pt-Ccarbene bond distance is in the expected range for platinum(II) compounds bearing nonchelating (39) Fraccarollo, D.; Bertani, R.; Mozzon, M.; Belluco, U.; Michelin, R. A. Inorg. Chim. Acta 1992, 201, 15–22.


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NHC ligands.40-43 A similar cationic platinum(II) complex with a diolefin and an N-heterocyclic carbene ligand with a cyclometalated phenyl ring of the carbene ligand, [Pt(dpim)(cod)]ClO4 (dpim = 1,3-diphenyl-2-imidazolidinylidenato-2-C,20 -C), was reported by Hiraki and co-workers.44 The 13C{1H} NMR spectrum in acetonitrile-d3 solution of 4 shows a singlet for the carbene carbon (Pt-Ccarbene) at 151.4 ppm; this is high compared to the case for most Pt(NHC)2X2 systems.11,41-44 Platinum satellites were observed at the four olefinic carbons bonded to the metal center (1JPt-C = 31.54, 31.92, 75.12, and 80.20 Hz) and for two olefinic protons (2JPt-H = 30.72 and 32.61 Hz). The resonances with smaller 1JPt-C coupling constants are assigned to the olefinic C-H trans to the carbene ligand. Similar observations were reported and discussed by Clark and Manzer,45 Appleton,46 Anderson,47 and Klein.48 Attempts to remove the cyclooctadiene and the chloro ligands by reduction of the olefin and reaction with silver(I) salts, respectively, failed. Nucleophilic Attack of Methoxide on the Coordinated 1,5Cyclooctadiene Ligand of [Pt(m-CN)(cod)Cl]BF4 (4). Coordinated olefin ligands on palladium and platinum metal complexes are prone to nucleophilic attack by alkoxides,49,50 malonates,51 and amines52 to give σ-alkyl complexes or complexes with a bidentate η1:η2-alkenyl ligand if the olefin is a chelating diene. In order to demonstrate the reactivity on the coordinated 1,5-cyclooctadiene ligand in 4, we reacted 4 with sodium methoxide in methanol to deliver a methoxide group onto the olefin ligand. The reaction was thwarted, however, by instant decomposition of the platinum(II) compound in basic solution, leading to intractable products. The in situ generation of the methoxide anions by use of potassium acetate in a methanol solution of 4 under reflux generated the neutral square-planar complex 5 with 2-methoxycyclooct-5-enyl, carbene, and chloro ligands (Scheme 3). This occurs via an exo attack of the methoxide anion onto the coordinated diolefin ligand. While the structure of the isolated crystals of 5 was characterized as rotamer A, solutions of 5 reveal the presence of rotamers A and B in equal amounts, as evidenced by NMR spectroscopy (see below). The retention of the chloro ligand and the formation of both rotamers A and B are unusual (40) Baker, M. V.; Brown, D. H.; Simpson, P. V.; Skelton, B. W.; White, A. H.; Williams, C. C. J. Organomet. Chem. 2006, 691, 5845– 5855. (41) Brissy, D.; Skander, M.; Retailleau, P.; Marinetti, A. Organometallics 2007, 26, 5782–5785. (42) Bacciu, D.; Cavell, K. J.; Fallis, I. A.; Ooi, L. L. Angew. Chem., Int. Ed. 2005, 44, 5282–5284. (43) Fantasia, S.; Petersen, J. L.; Jacobsen, H.; Cavallo, L.; Nolan, S. P. Organometallics 2007, 26, 5880–5889. (44) Hiraki, K.; Onishi, M.; Ohnuma, K.; Sugino, K. J. Organomet. Chem. 1981, 216, 413–419. (45) Clark, H. C.; Manzer, L. E. J. Organomet. Chem. 1973, 59, 411– 428. (46) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev. 1973, 10, 335–422. (47) Anderson, G. K.; Clark, H. C.; Davies, J. A. Inorg. Chem. 1981, 20, 1636–1639. (48) Klein, A.; Klinkhammer, K. W.; Scheiring, T. J. Organomet. Chem. 1999, 592, 128–135. (49) Stille, J. K.; Morgan, R. A.; Whitehur, D. D.; Doyle, J. R. J. Am. Chem. Soc. 1965, 87, 3282. (50) Stille, J. K.; Morgan, R. A. J. Am. Chem. Soc. 1966, 88, 5135– 5141. (51) Tsuji, J.; Takahash, H. J. Am. Chem. Soc. 1965, 87, 3275–3276. (52) (a) Cope, A. C.; Kliegman, J. M.; Friedrich, E. C. J. Am. Chem. Soc. 1967, 89, 287–291. (b) Åkermark, B.; B€ackvall, J. E.; Hegedus, L. S.; Zetterberg, K.; Siirala-Hansen, K.; Sj€oberg, K. J. Organomet. Chem. 1974, 72, 127–138.

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Figure 6. ORTEP diagram of 5 (Pt(m-CN)(η1:η2-coe-OMe)Cl) depicted with thermal ellipsoids at the 50% probability level. Hydrogens have been omitted for clarity. Selected bond distances (A˚) and bond angles (deg): Pt(1)-C(1), 1.969(1); Pt(1)-C(12), 2.095(1); Pt(1)-Cl(1), 2.430(4); Pt(1)-coe(trans to C)cent, 2.102; C(11)-N(3), 1.135(7); C(19)-O(1), 1.418(2); C(1)-Pt(1)-coe(trans to C)avg, 160.7; C(12)-Pt(1)-Cl(1), 178.4(4); C(1)-Pt(1)-Cl(1), 92.4(4); C(1)-Pt(1)-C(12), 88.9(5). Scheme 3. Nucleophilic Attack of Methoxide at the Coordinated Cyclooctadiene Ligand of 4 to 5 and Chloride Abstraction of 5 with AgBF4 in CH3CN to 6a

observations for such reactions of palladium(II) and platinum(II) diolefin complexes.34,50 Complex 5 was unambiguously characterized by X-ray diffraction (Figure 6 and Table 1). The square-planar complex as the exo-R,R rotamer A crystallizes in the chiral triclinic space group P1. The relative orientation of carbene and methoxycyclooctenyl ligands is similar to that of its palladium analogue (3a). The dihedral angle between the phenyl and the imidazolidene rings of the carbene ligand is 58.77°, to relieve sterics between the coordinated ligands. The Pt-C bond distances of the carbene11,43,53 (Pt-Ccarbene) and methoxycyclooctenyl35,54-56 (Pt-C(1)) ligands are within expected ranges, while the Pt-Ccarbene bond distance (1.970(1) A˚) is shorter than that of 4 (2.011(5) A˚).

(53) Ahrens, S.; Herdtweck, E.; Goutal, S.; Strassner, T. Eur. J. Inorg. Chem. 2006, 1268–1274. (54) Angurell, I.; Martı´ nez-Ruiz, I.; Rossell, O.; Seco, M.; G omez-Sal, P.; Martı´ n, A.; Font-Bardia, M.; Solans, X. J. Organomet. Chem. 2007, 692, 3882–3891. (55) Aucott, S. M.; Slawin, A. M. Z.; Woollins, J. D. Dalton Trans. 2000, 2559–2575. (56) Goel, A. B.; Goel, S.; Vanderveer, D. G. Inorg. Chim. Acta 1981, 54, L169–L170.

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Figure 7. ORTEP diagram of 6a ([Pt(m-CN)(η1:η2-coe-OMe)(CH3CN)]BF4) depicted with thermal ellipsoids at the 50% probability level. The counteranion and hydrogens have been omitted for clarity. Selected bond distances (A˚) and bond angles (deg): Pt(1)-C(1), 2.001(5); Pt(1)-C(17), 2.044(5); Pt(1)-N(4), 2.122(5); Pt(1)-coe(trans to C)cent, 2.100; C(10)-N(3), 1.145(8); C(16)-O(1), 1.449(6); C(1)-Pt(1)-coe(trans to C)avg, 161.0; C(17)-Pt(1)-N(4), 178.3(0); C(1)-Pt(1)-N(4), 89.4(2); C(1)-Pt(1)-C(17), 88.9(7).

Complex 5 is soluble in polar solvents such as methanol, acetonitrile, dichloromethane, and tetrahydrofuran, as well as in aromatic solvents such as benzene and toluene. The absence of the tetrafluoroborate counteranion is established by the lack of requisite signals in the 19F NMR and infrared spectra. A dichloromethane-d2 solution of 5 at 298 K shows a significant downfield shift of the carbene resonance (Pt-Ccarbene) to 175.8 ppm in the 13C{1H} NMR spectrum compared to the signal in 4. In addition, the 1H NMR spectrum shows a 1:1 ratio of the methyl resonances for the carbene (4.05 and 4.02 ppm) and methoxycyclooctenyl ligands (3.05 and 2.95 ppm). In addition, the H(3) proton of the phenyl ring of the carbene ligand appears as a pair of doublets (3JHH = 7.86 Hz) at 8.66 and 8.39 ppm. Therefore, the complex exists as a 1:1 mixture of rotamers A and B because of the restricted rotation about the Pt-Ccarbene bond similar to complex 3a. The barrier to rotation must be high, because heating a solution of 5 in nitromethane-d3 at 323 K afforded no change in the 1H NMR spectrum in comparison to that acquired at 298 K. A close examination of the 13 C{1H} NMR spectrum of 5 showed a doubling of all of the resonances in a 1:1 ratio. This also indicates the presence of roughly equal amounts of rotamers A and B of 5 in solution. Rotamer B is more abundant than that of the palladium(II) complex 3a. Attempts to establish by 1H-1H NOESY NMR spectroscopy the proximity of groups of each rotamer of 5 failed, as no cross-peaks of the relevant protons were observed (see the Supporting Information). In order to synthesize the platinum(II) analogues of complexes 3a and 3b, we first attempted to react the silver(I) complex 1 with 1 equiv of [Pt(η1:η2-coe-OMe)(μ-Cl)]254

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Scheme 4. Reaction Pathways Leading to Rotamers A and B of Complexes 3, 5, and 6

and AgBF4 in acetonitrile solutions. These, however, led to intractable products along with decomposition. On the other hand, adding 1 equiv of AgBF4 to an acetonitrile-d3 solution of 5 afforded the immediate precipitation of silver chloride. Subsequent filtration gave the white acetonitrile complex 6a (Scheme 3). The carbene carbon (Pt-Ccarbene) of the NHC ligand was observed as two singlets at 168.9 and 168.7 ppm in a 1:4 ratio in its 13C{1H} NMR spectrum in dichloromethane-d2. All the proton and carbon resonances of the methoxycyclooctenyl ligand were assigned similarly as with complex 3a on the basis of the resonance of the C(2) carbon.35,54,55 In general, the resonances of the platinum(II) compound appeared at lower frequencies compared to those of its palladium(II) analogue, consistent with the observation reported by Macchioni and co-workers.35 Again, the presence of both rotamers A and B in solution are detectable by NMR techniques. In accordance with the previous assignments made to complexes 3a and 5, we believe there exists at least 20 mol % of rotamer B in the solution sample of 6a. This is much smaller than the amounts of rotamer B in a solution for a sample of 5. Evaporation of the deuterated solvent and slow diffusion of diethyl ether into a solution of 6a in acetonitrile provided crystals suitable for X-ray diffraction (Figure 7 and Table 1). The square-planar complex crystallizes in the chiral triclinic space group P1 with the exo-R,R rotamer A being observed. The phenyl ring is twisted at a dihedral angle of 61.36° to the plane of the imidazolidene ring of the carbene ligand. The acetonitrile ligand is weakly coordinated (Pt(1)-N(4), 2.122(5) A˚) owing to the strong trans influence of the σ-alkyl group (cf. the average Pt-N distances of other acetonitrile complexes, 2.003 A˚23). A comparison of the M-C and M-N distances (M = Pd, Pt) of 3a and 6a reveal similarities in their magnitudes. When 6a was resynthesized in a larger scale and recrystallized in dichloromethane and diethyl ether mixtures, the corresponding dimer 6b was obtained, analogous to complex 3b (Scheme 2). The loss of the acetonitrile ligand was identified by the absence of the resonance at 1.94 ppm in the 1H NMR spectrum in dichloromethane-d2 and the nitrile stretch of acetonitrile in the infrared spectrum (2227 cm-1). As for the palladium(II) complexes 3a and 3b, there is a decrease in the nitrile stretching frequency on going from 6a

to 6b (2282 to 2262 cm-1). The acetonitrile complex 6a and the dimeric complex 6b, like 3a and 3b, are interconvertible by using different recrystallization solvents (Scheme 2). Unlike the palladium(II) complexes, the platinum(II) complexes are stable enough for storage without any precautions taken. In order to account for the presence of both rotamers A and B in complexes 3, 5, and 6, and the difference in the distributions of rotamers of complexes 5 and 6, it is proposed that the carbene ligand can freely rotate in a pentacoordinate complex produced before the solvent molecule or a chloride ligand that occupies the fifth coordination site of the squarepyramidal complex dissociates. The transmetalation reaction of complex 1 with [Pd(η1:η2-coe-OMe)(μ-Cl)]2 first forms the transient complex [Pd(η1:η2-coe-OMe)(m-CN)Cl], where m-CN is the nitrile-functionalized N-heterocyclic carbene ligand. The solvent/nitrile ligand then occupies the fifth coordination site of the square-planar complex, forming a pentacoordinate, possibly square-pyramidal complex with the carbene ligand occupying the axial position. The carbene ligand of this intermediate might freely rotate, as the steric congestion brought about by the 2-methoxycyclooct-5enyl ligand is relieved. Silver chloride precipitates from acetonitrile/dichloromethane solutions, forming the squareplanar complex 3a. The presence of 80 mol % of rotamer A in solution and the fact that only the single rotamer A was found in the crystal suggest that this is the thermodynamically preferred product (Scheme 4). On the other hand, an exo attack by methoxide onto the diolefin of complex 4 occurs at the double bond that is trans to the chloride ligand. This displaces the chloride ligand and forms a similar transient complex as the [Pt(η1:η2-coe-OMe)(m-CN)(CH3OH)]þ cation, or it dimerizes with a bridging group50 such as the carbene ligand. In this case the coordination of solvated chloride gives the same pentacoordinate intermediate. Upon dissociation of the coordinated methanol, which is a poor ligand compared to chloride, this gave the same amounts of rotamers A and B of 5 in solution. In fact, removal of the chloride ligand from 5 by precipitation using silver tetrafluoroborate in acetonitrile gives the same distribution of rotamers (favoring A over B) of complex 6a as for complex 3a (Scheme 4). Platinum(II) Complex with Coordinated 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene ligand (IMes). For comparison,

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Scheme 5. Synthesis of Pt(II) Complex 8 from an N-Heterocyclic Carbene Complex of Ag(I) (7)

the analogous coordination chemistry of platinum(II) was investigated with 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes). The silver(I) precursor complex [Ag(IMes)2]BF4 (7) was prepared by an established method7 and was reacted with 2.5 equiv of Pt(cod)Cl2 and 1 equiv of AgBF4 under reflux overnight (Scheme 5). The corresponding complex 8, with coordinated chloro, 1,5-cyclooctadiene, and IMes ligands, was isolated in 85% yield. Reactions carried out at room temperature did not proceed and recovered exclusively the starting materials. The carbene carbon (Pt-Ccarbene) was observed as a singlet at 151.4 ppm in the 13C{1H} NMR spectrum in acetonitrile-d3 solution, consistent with that of complex 4 with a carbene bearing an electron-withdrawing group. Platinum satellites were also observed at the four olefinic protons and carbons bonded to the metal center (2JPt-H = 22.03 and 31.55 Hz; 1JPt-C = 33.11 and 76.88 Hz), whereas the protons and carbons with smaller coupling constants are bonded trans to the carbene ligand.45-48 A small coupling of the imidazolidene carbon with platinum satellites was also observed (3JPt-C = 16.14 Hz). When complex 8 was subjected to similar reaction conditions for the synthesis of 5, namely nucleophilic attack of the coordinated diolefin by methoxide anion, only a trace amount of the desired product was observed along with the generation of imidazolium salt, 1,3-bis(2,4,6-trimethylphenyl)imidazolium. On the other hand, reaction of the silver(I) complex 7 with 1 equiv of [Pt(η1:η2-coe-OMe)(μ-Cl)]2 and AgBF4 in refluxing acetonitrile overnight afforded complex 8, along with trace amounts of the desired product (9), imidazolium salt, and other decomposition products (Scheme 5). All these results can be attributed to the steric bulk of the IMes ligand directed toward the metal center and to the methoxycyclooctenyl ligand, therefore destabilizing the metal complex and inducing β-elimination of H(1) and methoxide of the olefin ligand, generating imidazolium salts and complex 8, respectively (Scheme 5). Attempts to prepare palladium(II) analogues of complex 3 with IMes from the reaction of 7, 1 equiv of [Pd(η1:η2-coe-OMe)(μ-Cl)]2, and AgBF4 was thwarted by instant decomposition under light and in solutions, in particular, chlorinated solvents. Slow diffusion of diethyl ether solutions into the aforementioned reaction mixture in dichloromethane afforded crystals of complex 8, which were thereby characterized by X-ray diffraction (Figure 8 and Table 1). The slightly distorted square-planar complex crystallizes in the monoclinic

Figure 8. ORTEP diagram of 8 ([Pt(IMes)(cod)Cl]BF4) depicted with thermal ellipsoids at the 50% probability level. The counteranion, hydrogens, and solvent molecules have been omitted for clarity. Only one asymmetric unit is shown. Selected bond distances (A˚) and bond angles (deg): Pt(1A)-C(1A), 2.037(1); Pt(1A)-Cl(1A), 2.315(3); Pt(1A)-cod(trans to C)cent, 2.173; Pt(1A)-cod(cis to C)cent, 2.084; C(1A)-Pt(1A)-cod(trans to C)avg, 162.3; Cl(1)-Pt(1)-cod(cis to C)avg, 160.9; C(1)-Pt(1)-Cl(1), 89.8(4); C(1)-Pt(1)-cod(cis to C)avg, 95.1.

space group Cc with four pairs of asymmetric units residing in the unit cell. The carbene ligand that is bonded to the metal center is oriented with the dihedral angles between the mesityl and imidazolidene rings of 80.78 and 97.40°. The Pt-Ccarbene bond distance is in the expected range for most platinum(II) compounds bearing IMes ligands42,43 but is slightly longer than that of 4 because of its bulkiness. A high trans influence of the carbene ligand is demonstrated by the lengthening of bond distance from the centroid of the olefin trans to it (Pt-cod(trans to C)cent, 2.173 A˚; Pt-cod(cis to C)cent, 2.084 A˚). Overall, the metal-carbon bonds of 8 are slightly longer compared to those of 4.

Conclusion In summary, we have synthesized and fully characterized new palladium(II) and platinum(II) complexes bearing chelating olefin and N-heterocyclic carbene ligands. Initial attempts using silver(I) carbene complex 1 and PdCl2(CH3CN)2 led to the dimeric complex 2a, along with the partially hydrolyzed trimetallic complex 2b as a side product when wet diethyl ether and acetonitrile were used as recrystallization solvents. The structure of 2b reveals the presence of a novel C-N-N-C donor ligand, comprised of a central -NdC-O-CdN- linkage and two bridging imido nitrogens. Platinum(II) complexes with 1,5-cyclooctadiene ligands and N-heterocyclic carbene ligands (4 and 8) were prepared by transmetalation of the corresponding silver(I) carbene complexes (1 and 7). The reaction of methoxide with the coordinated diolefin ligand of 4 afforded the neutral complex 5 and complex 6 upon removal of the chloro ligand. The palladium(II) analogue, complex 3, was independently

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prepared by direct transmetalation of 1 and [Pd(η1:η2-coeOMe)(μ-Cl)]2. Both the monomer and dimer of complexes 3 and 6 were isolated and studied by NMR and infrared spectroscopy. In addition, reactions leading to complexes 3, 5, and 6 produced different amounts of rotamers A and B, owing to the orientation of carbene ligand relative to the 2-methoxycyclooct-5-enyl ligand, and restricted rotation about the M-Ccarbene bond originated from the steric crowding of the 2-methoxycyclooct-5-enyl ligand by the carbene ligand. These are the first examples of structurally characterized N-heterocyclic carbene complexes of palladium(II) and platinum(II) bearing 1,5-cyclooctadiene and 2-methoxycyclooct-5-enyl ligands.

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. Deuterated solvents were purchased from Cambridge Isotope Laboratories and Sigma Aldrich and degassed and dried over activated molecular sieves prior to use. NMR spectra were recorded on a Varian 400 spectrometer operating at 400 MHz for 1H, 100 MHz for 13C, and 376 MHz for 19F. The 1H and 13C{1H} NMR spectra 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. The 1H-1H NOESY NMR spectrum of complex 5 was recorded on a Varian 600 spectrometer operating at 600 MHz for 1H. 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 KappaCCD diffractometer with Mo KR radiation (λ = 0.710 73 A˚). The CCD data were integrated and scaled using the DenzoSMN 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 given in Table 1. The synthesis of [Ag(m-CN)2]BF4 (1) was previously reported.7 The complex [Ag(IMes)2]BF4 (7) was prepared using procedures analogous to those for [Ag(m-CN)2]BF4 (1) from (IMes-H)BF4, which was prepared by counteranion metathesis of (IMes-H)Cl57 and NH4BF4 in water. The characterization data are similar to those reported in the literature.58 The syntheses of PdCl2(CH3CN)2,15 Pd(cod)Cl2, Pt(cod)Cl2,59 [Pd(η1:η2-coe-OMe)(μCl)]2,30 and [Pt(η1:η2-coe-OMe)(μ-Cl)]253 were reported in the literature. All other reagents were purchased from commercial sources and were used as received. Synthesis of Bis(1-(2-cyanophenyl)-3-methylimidazol-2-ylidene)palladium(II)(μ-dichloro)bis(acetonitrile)palladium(II) Tetrafluoroborate [(m-CN)2Pd( μ-Cl)2Pd(CH3CN)2](BF4)2, 2a). A solution of 1 (39 mg, 0.07 mmol) and silver tetrafluoroborate (14 mg, 0.07 mmol) in acetonitrile (2 mL) was added to a solution of PdCl2(CH3CN)2 (36 mg, 0.14 mmol) in acetonitrile (4 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 yellow solution. The volume of solvent was reduced (ca. 1 mL), and diethyl ether (57) Arduengo, A. J.; Krafczyk, R.; Schmutzler, R.; Craig, H. A.; Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron 1999, 55, 14523–14534. (58) Yu, X. Y.; Patrick, B. O.; James, B. R. Organometallics 2006, 25, 2359–2363. (59) Drew, D.; Doyle, J. R. Inorg. Synth. 1990, 28, 346–349.

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(8 mL) was added. The solution was kept at -10 °C to give a yellow oil. The supernatant liquid was decanted, and the oil was triturated with diethyl ether (3 3 mL) to give a yellow powder, which was collected and dried in vacuo. Yield: 39 mg, 62%. Alternatively, the title compound can be prepared from 1 (75 mg, 0.13 mmol), silver tetrafluoroborate (26 mg, 0.13 mmol), and Pd(cod)Cl2 (77 mg, 0.27 mmol) using the same procedure. Yield: 96 mg, 79%. 1H NMR (CD3CN, δ): 8.03 (dd, JHH = 1.58, 7.70 Hz, 3-CH of Ph, 1H), 8.00 (dt, JHH = 1.58, 7.85 Hz, 5-CH of Ph, 1H), 7.89 (dd, JHH = 1.22, 7.85 Hz, 6-CH of Ph, 1H), 7.84 (dt, JHH = 1.22, 7.70 Hz, 4-CH of Ph, 1H), 7.56 (d, JHH = 2.10 Hz, 4-CH of imid, 1H), 7.48 (d, JHH = 2.10 Hz, 5-CH of imid, 1H), 4.15 (s, CH3 of m-CN, 3H), 1.96 (s, integration cannot be determined because of exchange with CD3CN, CH3 of CH3CN). 19 F NMR (CD3CN, δ): -152.2 (s), -152.3 (s). 13C{1H} NMR (CD3CN, δ): 141.4 (Pd-C), 140.7 (CPh), 135.4 (CPh), 135.1 (CPh), 131.9 (CPh), 130.7 (CPh), 126.5 (Cimid), 126.2 (Cimid), 118.4 (CN of CH3CN), 116.2 (CN of m-CN), 112.1 (CPh), 38.6 (CH3 of m-CN), 1.69 (CH3 of CH3CN). IR (KBr, cm-1): 2329 (v(CN) of CH3CN), 2237 (v(CN) of m-CN). MS (ESI, methanol/ water; m/z): 507.0 [Pd(m-CN)2Cl]þ, 471.1 [Pd(m-CN)2]•þ. Anal. Calcd for C26H24B2Cl2F8N8Pd2: C, 34.47; H, 2.67; N, 12.37. Found: C, 34.74; H, 2.58; N, 12.00. Synthesis of (Acetonitrile)(1-(2-cyanophenyl)-3-methylimidzol2-ylidene)(η1:η2-2-methoxycyclooct-5-enyl)palladium(II) Tetrafluoroborate ([Pd(m-CN)(η1:η2-coe-OMe)(CH3CN)]BF4, 3a). A solution of 1 (69 mg, 0.12 mmol) and silver tetrafluoroborate (24 mg, 0.12 mmol) in acetonitrile (4 mL) was added to a solution of [Pd(η1:η2-coe-OMe)(μ-Cl)]2 (70 mg, 0.12 mmol) in dichloromethane (6 mL). A white precipitate was formed instantaneously. The reaction mixture continued to be stirred for 2 h, protected from light. It was then filtered through a pad of Celite to give a golden yellow solution. The solvent was then removed in vacuo, and the crude product was recrystallized with acetonitrile (2 mL) and diethyl ether (10 mL) at -25 °C to give white needles. The needles were then collected on a glass frit and dried in vacuo. Yield: 92 mg, 67%. Alternatively, the title compound can be obtained by recrystallization of 3b in acetonitrile and diethyl ether. Suitable crystals for an X-ray diffraction study were obtained by slow diffusion of diethyl ether into a saturation solution of 3a in acetonitrile. In addition to a peak at 2.01 ppm in the 1H NMR spectrum (CD3NO2), all other spectroscopic information are identical with those of compound 3b. IR (KBr, cm-1): 2277 (v(CN) of m-CN), 2229 (v(CN) of CH3CN). MS (ESI, methanol/water; m/z): 428.1 [Pd(m-CN)(η1:η2-coe-OMe)]þ. Anal. Calcd for C22H27BF4N4OPd: C, 47.46; H, 4.89; N, 10.06. Found: C, 47.18; H, 4.04; N, 10.26. Synthesis of Bis[(1-(2-cyanophenyl)-3-methylimidazol-2ylidene)(η1:η2-2-methoxycyclooct-5-enyl)palladium(II)] Tetrafluoroborate ([Pd(m-CN)(η1:η2-coe-OMe)]2(BF4)2, 3b). A solution of 1 (148 mg, 0.26 mmol) and silver tetrafluoroborate (52 mg, 0.27 mmol) in acetonitrile (4 mL) was added to a solution of [Pd(η1:η2-coe-OMe)(μ-Cl)]2 (149 mg, 0.27 mmol) in dichloromethane (10 mL). A white precipitate was formed instantaneously. The reaction mixture continued to be stirred for 2 h, protected from light. The solvent was then removed in vacuo. The residue was extracted with dichloromethane (10 mL) and filtered through a pad of Celite to give a golden yellow solution. The solvent was removed, and the crude product was recrystallized with dichloromethane (2 mL) and diethyl ether (10 mL) to give a white precipitate. The precipitate was filtered and dried in vacuo to give a white powder. Yield: 210 mg, 77%. Alternatively, the title compound can be obtained by recrystallization of 3a in dichloromethane and diethyl ether. 1H NMR (CD3NO2, δ): 8.24 (d, JHH = 7.90 Hz, 3-CH of Ph, 1H), 8.06 (t, JHH = 7.88 Hz, 5-CH of Ph, 1H), 7.88 (t, JHH = 7.90 Hz, 4-CH of Ph, 1H), 7.78 (d, JHH = 7.80 Hz, 6-CH of Ph, 1H), 7.66 (d, JHH = 1.86 Hz, 4-CH of imid, 1H), 7.54 (d, JHH = 1.86 Hz, 5-CH of imid, 1H), 6.39 (m, 6-CH of olefinic CH, 1H), 6.10 (m, 5-CH of olefinic CH, 1H), 3.91 (d, CH3 of m-CN, 3H), 3.29 (m, 2-CH of coe-OMe,

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1H), 2.95 (d, CH3 of coe-OMe, 3H), 2.65 (m, 1A-CH or 1B-CH and 4-CH of coe-OMe, 1.5H), 2.36 (m, 1A-CH or 1B-CH of coe-OMe, 0.5H), 2.15 (m, 4-CH and 7-CH of coe-OMe, 2H), 1.91 (m, 3-CH2 of coe-OMe, 2H), 1.77 (m, 7-CH of coe-OMe, 1H), 1.42 (m, 8-CH of coe-OMe, 1H), 0.42 (m, 8-CH of coeOMe, 1H). 19F NMR (CD3NO2, δ): -152.9 (s), -153.0 (s). 13 C{1H} NMR (CD3NO2, δ): 174.6, 174.2 (Pd-CB/A), 144.5, 144.4 (CPh,B/A), 136.4, 136.0 (CPh,B/A), 134.8, 134.3 (CPh,A/B), 130.5, 130.2 (CPh,B/A), 128.3, 127.9 (CPh,B/A), 124.4, 124.2 (Cimid,B/A), 123.4, 123.2 (Cimid,A/B), 119.5, 119.4 (6-CA/B of coeOMe), 117.7, 117.4 (CN, B/A), 115.4, 115.2 (5-CA/B of coe-OMe), 109.4, 109.1 (CPh,A/B), 81.9, 80.8 (2-CA/B of coe-OMe), 54.9, 54.8 (CB/A -CH3 of coe-OMe), 41.4, 41.2 (1-CA/B of coe-OMe), 37.5, 37.4 (CA/B-CH3 of m-CN), 33.5, 33.5 (8-CB/A of coe-OMe), 30.1, 29.4 (3-CA/B of coe-OMe), 28.0, 27.6 (4-CA/B of coe-OMe), 24.8, 24.4 (7-CB/A of coe-OMe). IR (KBr, cm-1): 2260 (v(CN) of m-CN). MS (ESI, methanol/water; m/z): 428.1 [Pd(m-CN)(η1:η2-coe-OMe)]þ. Anal. Calcd for C40H48B2F8N6O2Pd2: C, 46.58; H, 4.69; N, 8.15. Found: C, 45.90; H, 5.18; N, 7.82. Synthesis of Chloro[1-(2-cyanophenyl)-3-methylimidazol-2ylidene](η4-1,5-cyclooctadiene)platinum(II) Tetrafluoroborate ([Pt(m-CN)(cod)Cl]BF4, 4). A solution of 1 (109 mg, 0.19 mmol) and silver tetrafluoroborate (38 mg, 0.19 mmol) in acetonitrile (4 mL) was added to a solution of Pt(cod)Cl2 (145 mg, 0.39 mmol) in dichloromethane (10 mL). A white 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 pale yellow solution. The solvent was then removed in vacuo, and the crude product was recrystallized with dichloromethane (3 mL) at -25 °C to give white crystalline solids. The product was then collected on a glass frit, rinsed with dichloromethane (2 3 mL) and diethyl ether (3 3 mL), and dried in vacuo. Yield: 140 mg, 59%. Suitable crystals for an X-ray diffraction study were obtained by slow diffusion of diethyl ether into a saturation solution of 4 in acetonitrile. 1H NMR (CD3CN, δ): 8.08 (dd, JHH = 1.23, 8.03 Hz, 3-CH of Ph, 1H), 8.03 (dd, JHH = 1.53, 7.70 Hz, 6-CH of Ph, 1H), 8.01 (dt, JHH = 1.53, 8.03 Hz, 4-CH of Ph, 1H), 7.84 (dt, JHH = 1.23, 7.70 Hz, 5-CH of Ph, 1H), 7.68 (d, JHH = 2.10 Hz, 4-CH of imid, 1H), 7.54 (d, JHH = 2.10 Hz, 5-CH of imid, 1H), 5.94 (m, olefinic CH of codtrans to C, 2H), 5.32 (m, JPt-H = 32.61 Hz, olefinic CH of codtrans to Cl, 1H), 4.35 (m, JPt-H = 30.72 Hz, olefinic CH of codtrans to Cl, 1H), 4.00 (s, CH3, 3H), 2.69 (m, CH2 of cod, 1H), 2.56 (m, CH2 of cod, 1H), 2.38 (m, CH2 of cod, 1H), 2.24 (m, CH2 of cod, 1H), 2.07 (m, CH2 of cod, 1H), 1.72 (m, CH2 of cod, 1H). 19F NMR (CD3CN, δ): -152.2 (s), -152.3 (s). 13C{1H} NMR (CD3CN, δ): 151.4 (Pt-C), 140.1 (CPh), 135.8 (CPh), 135.6 (CPh), 132.0 (CPh), 130.1 (CPh), 126.5 (Cimid), 125.8 (Cimid), 119.5 (JPt-C = 31.54 Hz, olefinic C of codtrans to C), 119.4 (JPt-C = 31.92 Hz, olefinic C of codtrans to C), 115.9 (CN), 111.3 (CPh), 97.7 (JPt-C = 75.12 Hz, olefinic C of codtrans to Cl), 94.9 (JPt-C = 80.20 Hz, olefinic C of codtrans to Cl), 38.6 (CH3), 33.1 (CH2 of cod), 31.7 (CH2 of cod), 29.1 (CH2 of cod), 28.5 (CH2 of cod). IR (KBr, cm-1): 2231 (v(CN)). MS (ESI, methanol/water; m/z): 521.1 [M]þ, 517.2 [Pt(m-CN)(cod)(MeOH)]•þ, 503.1 [Pt(m-CN)(cod)(H2O)]•þ. Anal. Calcd for C19H21BClF4N3Pt: C, 37.49; H, 3.48; N, 6.90. Found: C, 37.68; H, 3.55; N, 7.14. Synthesis of Chloro[1-(2-cyanophenyl)-3-methylimidazol-2ylidene)(η1:η2-2-methoxycyclooct-5-enyl)platinum(II) (Pt(m-CN)(η1:η2-coe-OMe)Cl, 5). A mixture of 4 (100 mg, 0.16 mmol) and potassium acetate (19 mg, 0.19 mmol) were suspended in methanol (12 mL). The reaction mixture was then refluxed under an argon atmosphere for 3 h to a golden yellow solution, and all solids were completely dissolved. The solvent was then removed in vacuo. The residue was extracted with dichloromethane (3 mL) and filtered through a pad of Celite. The crude product was then recrystallized overnight at -25 °C with the addition of n-pentane (15 mL) to the dichloromethane solution to give white crystalline solids. The product was then collected on a glass frit and dried in vacuo. Yield: 63 mg, 69%. Suitable

O et al. crystals for an X-ray diffraction study were obtained by slow diffusion of diethyl ether into a saturation solution of 5 in dichloromethane. 1H NMR (CD2Cl2, δ): 8.66 (d, JHH = 7.86 Hz, 3A-CH or 3B-CH of Ph, 0.5H), 8.39 (d, JHH = 7.86 Hz, 3A-CH or 3B-CH of Ph, 0.5H), 7.85 (t, JHH =7.86 Hz, 4-CH of Ph, 1H), 7.80 (d, JHH = 7.75 Hz, 6-CH of Ph, 1H), 7.61 (dd, JHH = 7.75, 14.37 Hz, 5-CH of Ph, 1H), 7.42 (d, JHH = 1.94 Hz, 4-CH of imid, 1H), 7.23 (d, JHH = 1.94 Hz, 5-CH of imid, 1H), 5.39 (m, JPt-H = 24.46 Hz, 5-CH of olefinic CH, 1H), 5.26 (m, JPt-H = 31.14 Hz, 6-CH of olefinic CH, 1H), 4.05 (s, CH3-A or CH3-B of m-CN, 1.5H), 4.02 (s, CH3-A or CH3-B of m-CN, 1.5H), 3.20 (m, 2A-CH or 2B-CH of coe-OMe, 0.5H), 3.05 (s, CH3-A or CH3-B of coe-OMe, 1.5H), 2.95 (s, CH3-A or CH3-B of coeOMe, 1.5H), 2.71 (m, 4A-CH or 4B-CH of coe-OMe, 1H), 2.57 (m, 4A-CH or 4B-CH of coe-OMe, 1H), 2.28 (m, 1A-CH or 1BCH of coe-OMe, 0.5H), 2.11 (m, 7A-CH or 7B-CH of coe-OMe, 1H), 1.97 (m, 1A-CH or 1B-CH, 2A-CH or 2B-CH, and 8ACH or 8B-CH of coe-OMe, 2H), 1.74 (m, 7A-CH or 7B-CH, and 8A-CH or 8B-CH of coe-OMe, 2H), 1.59 (m, 3A-CH or 3B-CH of coe-OMe, 2H). 13C{1H} NMR (CD2Cl2, δ): 176.2, 175.8 (Pt-CA/B), 142.0, 141.7 (CPh,A/B), 133.9, 133.8 (CPh,A/B), 133.6, 133.5 (CPh,A/B), 130.9, 130.6 (CPh,A/B), 129.8, 129.6 (CPh,A/B), 123.5, 123.3 (Cimid,A/B), 122.2, 122.1 (Cimid,A/B), 116.1, 116.0 (CN, A/B), 109.6, 109.5 (CPh,A/B), 102.9, 101.4 (6-CA/B of coe-OMe), 101.9, 101.2 (5-CA/B of coe-OMe), 83.9, 83.1 (2-CA/B of coe-OMe), 55.7, 55.6 (CA/B-CH3 of coe-OMe), 38.2, 38.1 (CA/B-CH3 of m-CN), 35.4, 34.1 (3-CA/B of coe-OMe), 29.9, 29.7 (4-CA/B of coe-OMe), 29.6, 29.4 (7-CA/B of coe-OMe), 27.4, 26.9 (8-CA/B of coe-OMe), 22.2, 20.1 (1-CA/B of coe-OMe). IR (KBr, cm-1): 2232 (v(CN)). MS (ESI, methanol/water; m/z): 517.2 [Pt(m-CN)(η1:η2-coe-OMe)]þ. Anal. Calcd for C20H24ClN3OPt: C, 43.44; H, 4.37; N, 7.60. Found: C, 43.14; H, 4.95; N, 7.92. Synthesis of (Acetonitrile)(1-(2-cyanophenyl)-3-methylimidazol-2-ylidene)(η1:η2-2-methoxycyclooct-5-enyl)platinum(II) Tetrafluoroborate ([Pt(m-CN)(η1:η2-coe-OMe)(CH3CN)]BF4, 6a). Complex 5 (11 mg, 0.08 mmol) was dissolved in acetonitrile-d3 (3 mL). Silver tetrafluoroborate (15 mg, 0.08 mmol) was then added to the reaction mixture, whereupon a pale brown precipitate was formed instantaneously. The solution was then stirred for 1 h. The reaction mixture was filtered through a pad of Celite to give a golden yellow solution. 1H NMR showed quantitative conversion of complex 5 to the title compound. The solution was then evaporated to dryness to give a yellow oil as the crude product. Suitable crystals for an X-ray diffraction study were obtained by slow diffusion of diethyl ether into a saturated solution of the crude product in acetonitrile. In addition to a peak at δ 1.99 ppm in the 1H NMR spectrum (CD2Cl2), all other spectroscopic data are identical with those for compound 6b. IR (KBr, cm-1): 2282 (v(CN) of m-CN), 2227 (v(CN) of CH3CN). MS (ESI, methanol/water; m/z): 517.2 [Pt(m-CN)(η1:η2-coe-OMe)]þ. HRMS (ESI, methanol/ water; m/z): calcd for C20H24N3OPtþ [M - BF4 - CH3CN]þ 517.1561, found 517.1537. Synthesis of Bis[(1-(2-cyanophenyl)-3-methylimidazol-2-ylidene)(η1:η2-2-methoxycyclooct-5-enyl)platinum(II)] Tetrafluoroborate ([Pt(m-CN)(η1:η2-coe-OMe)]2(BF4)2, 6b). Complex 5 (42 mg, 0.08 mmol) was dissolved in acetonitrile (5 mL). Silver tetrafluoroborate (15 mg, 0.08 mmol) was then added to the reaction mixture, whereupon a pale brown precipitate was formed instantaneously. The solution was then stirred for 1 h. The reaction mixture was filtered through a pad of Celite to give a golden yellow solution, and the solvent was evaporated to dryness. The crude product was then recrystallized with dichloromethane (1 mL) and diethyl ether (8 mL) to give a white precipitate. The precipitate was then collected and dried in vacuo to give a white powder. Yield: 40 mg, 87%. 1H NMR (CD2Cl2, δ): 8.24 (dd, JHH = 1.56, 7.98 Hz, 3-CH of Ph, 1H), 7.96 (dt, JHH = 1.56, 7.80 Hz, 5-CH of Ph, 1H), 7.85 (dt, JHH = 1.18, 7.98 Hz, 4-CH of Ph, 1H), 7.83 (d, JHH =1.88 Hz, 4-CH of

Article imid, 1H), 7.60 (dt, JHH = 1.18, 7.80 Hz, 6-CH of Ph, 1H), 7.55 (d, JHH = 1.88 Hz, 5-CH of imid, 1H), 5.95 (m, JPt-H = 29.49 Hz, 6-CH of olefinic CH, 1H), 5.57 (m, JPt-H = 27.08 Hz, 5-CH of olefinic CH, 1H), 3.80 (d, CH3 of m-CN, 3H), 3.01 (d, CH3 of coe-OMe, 3H), 2.90 (m, 2-CH of coe-OMe, 1H), 2.81 (m, 4ACH or 4B-CH of coe-OMe, 1H), 2.52 (m, 4A-CH or 4B-CH of coe-OMe, 1H), 2.16 (m, 8A-CH or 8B-CH of coe-OMe, 1H), 1.87 (m, 1-CH, 7A-CH or 7B-CH, and 8A-CH or 8B-CH of coeOMe, 3H), 1.62 (m, 3-CH2 and 7A-CH or 7B-CH of coe-OMe, 3H). 19F NMR (CD3CN, δ): -152.2 (s), -152.3 (s). 13C{1H} NMR (CD2Cl2, δ): 168.9, 168.7 (Pt-CB/A), 141.5, 141.3 (CPh,B/A), 136.7, 136.6 (CPh,A/B), 136.1, 136.0 (CPh,A/B), 131.6, 131.2 (CPh,B/A), 128.8, 128.5 (CPh,B/A), 126.1, 125.7 (Cimid,B/A), 123.8, 123.6 (Cimid,A/B), 118.3, 117.9 (CN, B/A), 110.5, 110.4 (CPh,A/B), 109.1, 108.9 (JPt-C = 48.51, 28.31 Hz, 6-CA/B of coe-OMe), 105.7, 104.0 (5-CA/B of coe-OMe), 83.1, 81.5 (2-CA/B of coe-OMe), 56.0, 54.8 (CA/B-CH3 of coe-OMe), 38.3, 38.1 (CA/B-CH3 of m-CN), 34.4, 33.9 (3-CB/A of coe-OMe), 29.5 (4-CA/B of coeOMe), 28.9, 28.2 (7-CA/B of coe-OMe), 26.6, 26.1 (8-CB/A of coe-OMe), 24.1, 23.9 (1-CA/B of coe-OMe). IR (KBr, cm-1): 2262 (v(CN) of m-CN). MS (ESI, methanol/water; m/z): 517.2 [Pt(m-CN)(η1:η2-coe-OMe)]þ. Anal. Calcd for C40H48B2F8N6O2Pt2: C, 39.75; H, 4.00; N, 6.95. Found: C, 39.36; H, 3.91; N, 6.92. Synthesis of Chloro[1,3-bis-(2,4,6-trimethylphenyl)imidazol-2ylidene)](η4-1,5-cyclooctadiene)platinum(II) Tetrafluoroborate ([Pt(IMes)(cod)Cl]BF4, 8). A solution of 7 (106 mg, 0.13 mmol) and silver tetrafluoroborate (26 mg, 0.13 mmol) in acetonitrile (6 mL) was added to a solution of Pt(cod)Cl2 (123 mg, 0.33 mmol) in dichloromethane (6 mL). A white precipitate was formed instantaneously. The reaction mixture was then refluxed under an argon atmosphere overnight. The solvent was removed in vacuo after the reaction was complete. The residue was extracted with acetonitrile (6 mL) and filtered through a pad of Celite to give a golden yellow solution. The solvent was then removed from vacuum, extracted with dichloromethane (3 mL), and filtered through a pad of Celite. The crude product was


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recrystallized at -25 °C with the addition of diethyl ether (15 mL) to the dichloromethane solution to give tan crystalline solids. The product was then collected on a glass frit and dried in vacuo. Yield: 163 mg, 85%. 1H NMR (CD3CN, δ): 7.52 (s, CH of imid, 2H), 7.16 (d, 2-CH of Ph, 4H), 5.61 (m, JPt-H = 22.03 Hz, olefinic CH of codtrans to C, 2H), 4.87 (m, JPt-H = 31.55 Hz, olefinic CH of codtrans to Cl, 2H), 2.39 (s, o-CH3 of Ph,12H), 2.25 (s, p-CH3 of Ph, 6H), 2.25-2.20 (m, CH2 of cod, 8H). 19F NMR (CD3CN, δ): -152.2 (s), -152.3 (s). 13C{1H} NMR (CD3CN, δ): 151.4 (Pt-C), 142.5 (CPh), 137.3 (CPh), 135.5 (CPh), 134.6 (CPh), 130.8 (CPh), 130.1 (CPh), 127.3 (JPt-C = 16.14 Hz, Cimid), 119.1 (JPt-C = 33.11 Hz, olefinic C of codtrans to C), 96.6 (JPt-C = 76.88 Hz, olefinic C of codtrans to Cl), 32.6 (CH2 of cod), 28.4 (CH2 of cod), 21.1 (o-C-CH3 of Ph), 19.4 (p-C-CH3 of Ph). MS (ESI, methanol/water; m/z): 642.2 [M]þ, 638.3 [Pt(IMes)(cod)(MeOH)]•þ, 624.3 [Pt(IMes)(cod)(H2O)]•þ. Anal. Calcd for C29H36BClF4N2Pt 3 CH2Cl2: C, 44.22; H, 4.70; N, 3.44. Found: C, 44.44; H, 4.61; N, 3.70.

Acknowledgment. The NSERC Canada is thanked for a Discovery Grant to R.H.M. and a postgraduate scholarship to W.W.N.O. Professor Scott Prosser and Dr. Ferenc Evanics are acknowledged for the acquisition of the 1 H-1H NOESY NMR spectrum of complex 5. Note Added after ASAP Publication. The version of this paper that was published on the web on Dec 23, 2009, had Supporting Information that incorrectly included an alternate synthesis of compound 2b. The correct version of the Supporting Information that appears on the web as of Jan 7, 2010 does not include this information. Supporting Information Available: CIF files giving X-ray structural data for compounds 2b, 3a, 4, 5, 6a, and 8, and figures giving 1H-1H NOESY NMR spectra of complex 5. This material is available free of charge via the Internet at http://pubs.acs.org.

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