The First Phenanthrene-Fused Imidazol-2-ylidene ... - ACS Publications

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Organometallics 2009, 28, 270–276

The First Phenanthrene-Fused Imidazol-2-ylidene and Its Transition-Metal Complexes Daniela Tapu,*,† Clayton Owens,† Donald VanDerveer,‡ and Kevin Gwaltney† Department of Chemistry and Biochemistry, Kennesaw State UniVersity, 1000 Chastain Road, Kennesaw, Georgia 30144, and X-ray Crystallography Laboratory, Department of Chemistry, Clemson UniVersity, Clemson, South Carolina 29634 ReceiVed August 22, 2008

A new phenanthrene-fused N-heterocyclic carbene was generated and characterized in solution. The synthesis of air-stable Rh, Ir, and Ag complexes, supported by the new phenanthrene-fused imidazol2-ylidene ligand, is described. Deprotonation of phenanthrene-fused imidazolium salt 4 in the presence of [M(COD)Cl]2 (M ) Rh, Ir) afforded complexes 6 and 7 in excellent yields. The silver complex was obtained by reaction of 4 with Ag2O. The solid state molecular structures of these complexes have been determined by X-ray diffraction studies. The fluorescence emission spectra of 4, 6, and 7 are compared to that of phenanthrene. Introduction Since the first stable crystalline carbene was reported in 1991, “Arduengo”-type carbenes have emerged as a powerful class of carbon-based ligands.1 Owing to their unique electronic and steric properties, they have been incorporated in a large variety of catalytically active metal complexes.2 The demand for carbenes with different 3D shapes and/or substitution patterns has rapidly increased. Therefore, it has become increasingly important to determine what effect the modification of the carbene’s architecture has on its electronic properties, which largely determine the ligand behavior.3 One of the strategies that have been used to modify the ligand properties of imidazol2-ylidenes is annulation with different carbo- and heterocyclic groups. Annulation is possible in the 4-5, 1-5, and 3-4 positions of the imidazole ring. The first stable 4,5-annulated imidazol-2-carbene reported was benzimidazol-2-ylidene, Ia.4 This carbene exhibits the topology of an unsaturated Nheterocyclic carbene, but shows spectroscopic and structural properties, as well as the reactivity, of saturated imidazolidin2-ylidenes. A few other examples of 4,5-fused unsaturated “Arduengo-type” carbenes and/or their corresponding metal complexes were reported recently (Chart 1).5 These studies have * To whom correspondence should be addressed. Tel: 1-678-797-2259. Fax: 1-770-423-6744. E-mail: [email protected]. † Kennesaw State University. ‡ Clemson University. (1) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361. (2) Herrmann, W. A.; Weskamp, T.; Bohm, V. P. W. AdV. Organomet. Chem. 2001, 48, 1. (3) Diez-Gonza´les, S.; Nolan, S. P. Coord. Chem. ReV. 2007, 251, 874. (4) Hahn, F. E.; Wittenbecher, L.; Boese, R.; Bla¨ser, D. Chem.-Eur. J. 1999, 5, 1931. (5) (a) Saravanakumar, S.; Kindermann, M. K.; Heinicke, J.; Ko¨ckerling, M. Chem. Commun. 2006, 640. (b) Saravanakumar, S.; Oprea, A. I.; Kindermann, M. K.; Jones, P. G.; Heinicke, J. Chem.-Eur. J. 2006, 12, 3143. (c) Sanderson, M. D.; Kamplain, J. W.; Bielawski, C. W. J. Am. Chem. Soc. 2006, 128, 16514. (d) Ullah, F.; Bajor, G.; Veszpre´mi, T.; Jones, P. G.; Heinicke, J. W. Angew. Chem., Int. Ed. 2007, 46, 2697. (e) Arduengo, A. J., III; Tapu, D.; Marshall, W. J. Angew. Chem., Int. Ed. 2005, 44, 7240. (f) Arduengo, A. J., III; Tapu, D.; Marshall, W. J. J. Am. Chem. Soc. 2005, 127, 16400. (g) Khramov, D. M.; Boydston, A. J.; Bielawski, C. W. Angew. Chem., Int. Ed. 2006, 45, 6186. (h) Li, W. Ph.D.Thesis, University of Alabama, Tuscaloosa, 2004.

Chart 1. Examples of 4-5 Annulated Imidazol-2-ylidenes

shown that annulation significantly influences the stability and the σ-donor/π-acceptor properties of the carbene species, and this may be used as a versatile tool for the fine-tuning of their electronic properties. Interest in the development of new polycyclic aromatic annulated imidazol-2-ylidenes and their corresponding transitionmetal complexes has arisen due to their potential application in catalysis and in fluorescent devices. Phenanthrene and its derivatives are a class of aromatic compounds whose photophysical properties have been widely studied.6 A phenanthrenefused imidazol-2-ylidene provides a promising framework in which the carbene center is a component of an electron-rich, extended aromatic system. This feature not only tunes the donor properties of the carbene center but also imposes geometric constraints on the N-substituents, influencing their steric impact. Herein, the synthesis and structural characterization of four metal complexes derived from 1,3-dibutylphenanthro[9,10-d]imidazol2-ylidene are reported, as well as the in situ characterization of (6) (a) Birks, J. B. Photophysics of Aromatic Molecules; WileyInterscience: London, 1970. (b) Zhang, P.; Winnik, A.; Wang, Z. Y. J. Photochem. Photobiol., A 1995, 89, 13.

10.1021/om800822m CCC: $40.75  2009 American Chemical Society Publication on Web 11/21/2008

First Phenanthrene-Fused Imidazol-2-ylidene Scheme 1. Synthesis of Imidazolium Salt 4 and Carbene 5

the free carbene. Some preliminary studies of their photophysical properties are also reported.

Results and Discussion Synthesis of the Carbene Precursor. A general method for accessing N-heterocyclic carbenes is the deprotonation of the appropriate imidazolium salt precursors. Four steps were necessary to access imidazolium salt 4 (Scheme 1).7 Initially, 1Hphenanthro[9,10-d]imidazole (1) was prepared by a multicomponent condensation starting from the commercially available 9,10-phenanthrenequinone, formaldehyde, and ammonium acetate in acetic acid. Although a photochemical synthesis of 1 was reported previously,8 an improved general synthesis would facilitate studies and applications of these materials. Our method yields 1 almost quantitatively. Subsequent alkylation of 1 with n-butyl bromide in the presence of sodium hydroxide yields 1-butylphenanthro[9,10-d]imidazole (2). A second alkylation of 2 with butyl iodide at 100 °C provided imidazolium iodide 3. The tetrafluoroborate 4 was prepared to circumvent potential halide exchange and any subsequent separation problems in reactions involving metal chlorides. The 13C and 1H NMR spectra of 3 and 4 are virtually identical and consistent with the proposed structures. In the 1H NMR spectrum, the imidazolium proton appears at δ 9.82 ppm in d6DMSO (δ 9.73 ppm in d8-THF), while the 13C NMR shift of C2 appears at δ 141.6 ppm (d6-DMSO). A characteristic feature in the NMR spectra of 3 and 4 is the strong downfield shift of an NCH2 proton signal (δ 4.92 ppm), which can be attributed to the diamagnetic ring current of the fused phenanthrene. Generation of the Free Carbene. Our attempt to generate the free carbene through deprotonation of 4 with potassium tertbutoxide in THF resulted in rapid decomposition. However, in an NMR experiment treatment of 4 with NaH/DMSO(cat) in dry THF proceeded cleanly to afford free carbene 5. The free carbene was sufficiently stable to be analyzed by NMR spectroscopy. The 13C NMR spectrum of 5 revealed a signal at δ 225.10 ppm. This chemical shift is higher than that of nonannulated imidazol-2-ylidene (δ 13C2 ) 217 ppm)5a but lower than that of benzo- and naphtha-annulated imidazol-2ylidenes (δ 13C2 ) 231.84,9 and 239.9 ppm).5b These values (7) A similar carbene precursor, 1,3-dimethylphenanthro[9,10-c]imidazolium iodide, has been reported (without characterization) as a catalyst for benzoin condensation: Miyashita, A.; Suzuki, Y.; Kobayashi, M.; Kuriyama, N.; Higashino, T. Heterocycles 1996, 43, 509. (8) Purushothaman, E.; Rajasekharan-Pillai, V. N. Indian J. Chem. B 1989, 28, 290. (9) (a) Boesveld, W. M.; Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F.; Schleyer, P. V. R. Chem. Commun. 1999, 755. (b) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. J. Chem. Soc., Dalton Trans. 2000, 3094.

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suggest a decreased π-charge density at C2 of 5 in comparison to the nonannulated counterpart and an increased π-charge density at the carbene center compared with benz- and naphthaannulated imidazol-2-ylidenes.10 Unfortunately, 5 was found to decompose upon concentration, which precluded its isolation. However, 5 was found to persist in solution for days at room temperature.11 Synthesis of Metal Complexes. Having demonstrated that free carbene 5 can be generated in situ, subsequent efforts were focused on the coordination of this carbene to various transition metals. Access to metal complexes of imidazol-2-ylidenes is mainly based on three different synthetic routes: (a) the complexation of the free, preisolated carbene, (b) the in situ deprotonation of carbene precursors (carbenium ions), and (c) the use of a metal-carbene complex as a transfer agent. Rhodium(I) and iridium(I) complexes 6 and 7 were prepared by in situ deprotonation of 4 with potassium tert-butoxide in the presence of [M(COD)Cl]2. The reactions were conducted at room temperature. Both complexes are quite stable. They can be handled in air and purified by column chromatography on silica gel. They are soluble in THF and halogenated solvents (dichloromethane, chloroform). Both 6 and 7 are stable indefinitely in solution (dichloromethane) at room temperature, but decompose in air at higher temperatures. The identity of the compounds was confirmed by 1H and 13C NMR spectroscopy as well as elemental analysis. These metal complexes exhibit 13 C chemical shifts and coupling constants that are comparable to those of other reported rhodium-carbene complexes.12 The chemical shift for C2 in 6 is 190.9 ppm with a characteristic coupling constant 1JRh-C of 51.6 Hz. The carbons in the two COD double bonds are coupled with the rhodium atom differently (1JRh-C ) 6.8, 14.5 Hz), which is consistent with their placement trans to different groups. The downfield shift of the NCH2 protons in complexes 6 and 7 is even stronger than in the annulated imidazolium salts 3 and 4. The anisotropy of the 1,5-COD ligand in 6 and 7 shifts the second NCH2 signal further downfield. When carbon monoxide was bubbled through a solution of 6 in dichloromethane, the complex 8 was obtained in excellent yield. The carbonyl stretching frequencies for 8 were found to be νCO ) 2074 (sym.) and 1994 (asym.) cm-1. These results indicate that carbene 5 is among the strongest σ-donors in the unsaturated series of Arduengo-type carbenes, but still weaker than the best known C-ligands.13 Its σ-donating power is similar to that of the recently reported N,N-disubstituted diaminocarbene containing a 1,1′-disubstituted ferrocene moiety in its back(10) Arduengo, A. J., III; Dixon, D. A.; Kumashiro, K. K.; Lee, C.; Power, W. P.; Zilm, K. W. J. Am. Chem. Soc. 1994, 116, 6361. (11) To avoid concentration, we attempted to crystallize carbene 5 by slow diffusion of hexane into a saturated THF solution of 5. No crystals were obtained. After 6 days the obtained brown solution was treated with sulfur. The resulting thione was recovered in 76% yield. Further details on the synthesis of this thione and its coordination properties will be reported elsewhere. (12) (a) C¸etinkaya, B.; Hitchcock, P. B.; Lampert, M. F.; Shaw, D. B.; Spyropoulos, K.; Warhurst, N. J. W. J. Organomet. Chem. 1993, 459, 311. (b) Va´zquez-Serrano, L. D.; Owens, B. T.; Buriak, J. M. Chem. Commun. 2002, 2518. (c) Hillier, A. C.; Lee, H. M.; Stevens, E. D.; Nolan, S. P. Organometallics 2001, 20, 4246. (d) Chianese, A. R.; Li, X.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H. Organometallics 2003, 22, 1663. (e) Tu¨rkmen, H.; Pape, T.; Hahn, F. E.; C¸etinkaya, B. Organometallics 2008, 27, 571. (13) (a) Hermannn, W. A.; Schu¨tz, J.; Frey, G. D.; Herdtweck, E. Organometallics 2006, 25, 2437. (b) Mayr, M.; Wurst, K.; Ongania, K.; Buchmeister, M. R. Chem.-Eur. J. 2004, 10, 1256. (c) Lavallo, V.; Mafhouz, J.; Canac, Y.; Dannadieu, B.; Schoeller, W.; Bertrand, G. J. Am. Chem. Soc. 2004, 126, 8670. (d) Bazinet, P.; Yap, G. P. A.; Richeson, D. S. J. Am. Chem. Soc. 2003, 125, 13314.

272 Organometallics, Vol. 28, No. 1, 2009 Scheme 2.

Synthesis of Rhodium, Iridium, and Silver Complexes 6, 7, 8, and 9

bone.14 The synthesis and characterization of rhodium and iridium complexes 6 and 7 establishes the fundamental properties and applicability of the carbene 5 as a ligand in organometallic chemistry. The generation of metal complexes from 4 is also possible with basic metal precursors. An example is the reaction of 4 with silver oxide, which gives the cationic bis(1,3-dibutylphenanthro[9,10-d]imidazol-2-ylidene) silver complex 9 in good yields. However, this reaction takes place above 80 °C and only in the presence of 3 Å molecular sieves. Since imidazol-2-ylidene silver complexes are capable of transmetalation,15 this points the way to other transition-metal complexes of 5. The silver complex 9 is insoluble in all common organic solvents except DMSO, in which it is sparingly soluble and slowly decomposes. A spectroscopically pure sample of 9 in DMSO shows decomposition to imidazolium salt 4 within days at room temperature. The carbene carbon signal for 9 was not found, which is not unusual.16 The dynamic behavior as well as the poor relaxation of the quaternary carbenic carbon could be factors contributing to the absence of the C2 resonance. Structural Studies. A single crystal of 6 suitable for X-ray crystallographic measurements was grown by slow diffusion of hexane into a saturated chloroform solution of 6. Crystal data and details of the crystal structure determination are presented in Table 1. Selected bond lengths and angles are given in Table 2. Rhodium complex 6 crystallizes in the monoclinic space group P2(1)/n. Rhodium is coordinated in a square-planar fashion by the carbene, the centers of the two CdC bonds of COD ligand, and the chloride (Figure 1). The plane of the carbene is oriented almost perpendicular (interplanar angle 87.8°) to the rhodium coordination plane. The C2-Rh bond distance of 2.031(3) Å is typical for this type of carbene coordination.17 The average distance between rhodium and olefinic carbons trans to the carbene [rav(Rh-C28(29) ) 2.213(3) Å] is slightly larger than the average distance between rhodium and double(14) Khramov, D. M.; Rosen, E. L.; Lynch, V. M.; Bielawski, C. W. Angew. Chem., Int. Ed. 2008, 47, 1. (15) (a) Herrmann, W. A.; Ko¨cher, C.; Grossen, L. J. Chem.-Eur. J. 1996, 2, 1627. (b) Seo, H.; Kim, B. Y.; Lee, J. H.; Park, H.-J.; Son, S. U.; Chung, Y. K. Organometallics 2003, 22, 4783. (16) Alcarazo, M.; Roseblade, S. J.; Cowley, A. R.; Ferna´ndez, R.; Brown, J. M.; Lassaletta, J. M. J. Am. Chem. Soc. 2005, 127, 3290. (17) (a) Herrmann, W. A.; Baskakov, D.; Herdtweck, E.; Hofmann, S. D.; Bunlaksananusorn, T.; Rampf, F.; Rodefeld, L. Organometallics 2006, 25, 2449. (b) Cavell, K. J.; Elliott, M. C.; Nielsen, D. J.; Paine, J. S. Dalton Trans. 2006, 4922.

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bond carbons trans to the chlorine atom [rav(Rh-C24(25) ) 2.111(3) Å]. This indicates a stronger trans-influence (effect) of the carbene moiety and a weaker rhodium-(CdC) bond for the carbons trans to C2 than trans to chloride. Slow diffusion of hexane into a saturated dichloromethane solution of 7 allowed crystals suitable for X-ray structure determination to be grown. Complex 7 crystallizes in the monoclinic space group P2(1)/n, and it is nearly isostructural with rhodium complex 6. Selected bonds and angles for 7 are given in Table 2. The asymmetric unit contains two unique molecules. As shown in the ORTEP plot (Figure 2), complex 7 adopts square-planar coordination geometry around the iridium center. The C2-Ir bond distance is 2.022(6) and 2.034(6) Å, respectively. The same trans influence is observed for the carbene center. The CdC bond of COD trans to C2 is shorter and displays longer Ir-C distances. The solid state structure of 9 was identified by X-ray crystallography. X-ray quality crystals were obtained by recrystallization in DMSO. The silver complex 9 displays crystallographic 2-fold symmetry. As depicted in Figure 3, the coordination geometry of the silver atom is essentially linear, with a C2-Ag-C2′ bond angle of 178.45(17)° (Table 2). The C-Ag bond distance of 2.100(3) Å and the internal ring angle (N1-C2-N3) at the carbene center of 106.6(3)° are within the range of other known NHC silver complexes.18 Spectral Studies. Absorption maxima in dichloromethane are 256, 263, and 254 nm for 4, 6, and 7, respectively. In dichloromethane, 4, 6, and 7 exhibit fluorescence emission in the region 330-430 nm, similar to phenanthrene. The fluorescence emission spectra of 4, 6, 7, and phenanthrene are shown in Figure 4. With excitation at 256 nm, imidazolium salt 4 exhibits over 3 times stronger emission than phenanthrene, while the emission intensities for 6 and 7 are 0.7% and 9% compared to phenanthrene. The quantum yields of 4, 6, and 7 are 0.18, 2.0 × 10-3, and 0.012, respectively, as determined by integration of their emission spectra and comparison to the emission spectrum of phenanthrene.19 As observed in other covalent complexes, fluorescence quenching is likely caused by the heavy atom effect, where singlet excited state population decreases due to intersystem crossing.20 Additionally, it is evident that the electronic structure is affected by both substitution of the phenanthryl moiety and metal complexation since emission peaks for 4, 6, and 7 are blue-shifted relative to phenanthrene and since the relative intensities of the bands are different (Table 3). Further studies on these new fluorescent materials are currently under investigation in our laboratory. In conclusion, a novel phenanthrene-fused imidazol-2-ylidene was prepared and characterized. This carbene was characterized in solution, and it has a decreased π-charge density at C2 in comparison to the nonannulated counterpart and an increased π-charge density at the carbene center compared with other polyaromatic fused carbenes. As determined by IR spectroscopy, 5 is among the strongest σ-donors in the unsaturated series of Arduengo-type carbenes. Furthermore, details on the chemistry of this fused carbene with respect to its ability to support catalytically relevant metal complexes were provided. Four metal complexes that incorporate this ligand have been syn(18) For a review on silver complexes see: Garrison, J. C.; Youngs, W. J. Chem. ReV. 2005, 105, 3978. (19) Dawson, W. R.; Windsor, M. W. J. Phys. Chem. 1968, 72, 3251. (20) Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F. Inorg. Chem. 1986, 25, 3858.

First Phenanthrene-Fused Imidazol-2-ylidene

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Table 1. Selected X-ray Crystallographic Data for Complexes 6, 7, and 9 empirical formula fw cryst size (mm) temp (K) cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dc (g cm -3) diffractometer/scan radiation/wavelength (Å) θ max (deg) index range (hkl) reflections measd independent reflns (Rint) data/restrains/params max., min. transmn final R indices (I > 2σ(I)) R indices (all data) goodness-of-fit on F2 peak/hole (e Å-3)

6

7

9

C31H38ClN2Rh 577.02 0.43 × 0.22 × 0.17 153(2) monoclinic P2(1)/n 9.1656(18) 18.482(4) 18.705(4) 90 101.36(3) 90 3106.5(11) 4 1.489 Rigaku AFC8S/CCD/ω 0.71073 25.15 -10 e h e 10 -14 e k e 22 -22 e l e 22 20 913 5538 (0.0259) 5538/7/388 0.8595, 0.6935 R1 ) 0.0354, wR2 ) 0.0803 R1 ) 0.0414, wR2 ) 0.0852 1.121 1.102/-0.632

C31H38ClN2Ir 666.28 0.48 × 0.29 × 0.24 153(2) monoclinic P2(1)/n 25.476(5) 9.0377(18) 25.771(5) 90 113.07(3) 90 5459.2(19) 8 1.621 Rigaku AFC8S/CCD/ω 0.71073 25.04 -30 e h e 30 -7 e k e 10 -26 e l e 30 30 382 9619 (0.0489) 9619/0/635 0.3793, 0.1971 R1 ) 0.0432, wR2 ) 0.1026 R1 ) 0.0570, wR2 ) 0.1133 1.074 2.278/-1.825

C46H52N4AgBF4 844.60 0.34 × 0.17 × 0.17 153(2) monoclinic C2/c 19.426(4) 15.038(3) 15.379 90 94.84(3) 90 4476.9(16) 4 1.269 Rigaku AFC8S/CCD/ω 0.71073 25.03 -23 e h e 21 -17 e k e 17 -18 e l e 18 16 258 3959 (0.0396) 3959/0/255 0.9196, 0.8480 R1 ) 0.0491, wR2 ) 0.1274 R1 ) 0.0548, wR2 ) 0.1337 1.078 0.683/-0.681

Table 2. Selected Bond Lengths (Å) and Angles (deg) for 6, 7, and 9 M-C2 M-X M-C24 M-C25 M-C28 M-C29 C2-N1 C2-N3 C3A-C15A C15A-N1 C3A-N3 M-C2-N1 M-C2-N3 N1-C2-N3 Cl1-M-C2 Cl1-M-C24 Cl1-M-C25 Cl1-M-C28 Cl1-M-C29 C2-M-C2′ a

6

7a

2.031(3) 2.3676(11) 2.104(3) 2.118(3) 2.202(3) 2.224(3) 1.358(4) 1.355(4) 1.382(4) 1.396(4) 1.399(4) 127.7(2) 126.0(2) 106.3(2) 90.02(8) 156.12(8) 163.97(8) 89.92(8) 92.28(8)

2.022(6)/2.034(6) 2.3684(17)/2.3703(16) 2.101(7)/2.130(7) 2.119(7)/2.105(7) 2.180(7)/2.202(7) 2.205(7)/2.192(7) 1.359(8)/1.359(8) 1.370(8)/1.364(8) 1.381(8)/1.393(9) 1.391(8)/1.387(8) 1.401(8)/1.399(8) 124.8(4)/106.3(5) 129.4(4)/128.3(5) 105.7(5)/106.3(5) 88.97(17)/88.43(17) 158.3(2)/163.4(2) 162.5(2)/157.4(3) 91.71(19)/91.3(2) 93.34(18)/89.6(2)

9 2.100(3)

1.345(4) 1.350(4) 1.380(5) 1.401(4) 1.403(4) 126.4(2) 126.9(2) 106.6(3)

Synthesis of 1H-Phenanthro[9,10-d]imidazole (1). A mixture of 9,10-phenanthrenequinone (30 g, 0.144 mol), formaldehyde (23.1 mL, 37 wt %), glacial acetic acid (560 mL), and ammonium acetate (230.7 g, 2.99 mol) was refluxed for 4 h. After cooling, the reaction mixture was diluted with water (600 mL) and neutralized with concentrated aqueous ammonia (28-30% wt) to pH 7. A light cream precipitate formed. It was filtered, washed with water, acetone, dichloromethane, and ethyl ether, and then dried to give 31.3 g of product (99% yield). The spectral data were in accordance with those described in the literature.21 Synthesis of 1-Butylphenanthro[9,10-d]imidazole (2). To a mixture of 1 (20 g, 0.092 mol) and sodium hydroxide (4 g, 0.1 mol) was added DMSO (200 mL). The reaction mixture was stirred at room temperature for 2 h. After addition of butyl bromide (10 mL, 0.092 mol), the reaction was stirred at room temperature for

178.45(17)

The two sets correspond to two unique molecules in the unit cell.

thesized and fully characterized. The solid state molecular structures of these complexes have been confirmed by X-ray diffraction studies. The fluorescence emission spectra of 4, 6, and 7 were determined and compared to those of phenanthrene. Further studies on these new fluorescent compounds and their analogues are underway and will be reported in the future.

Experimental Section All reactions were carried out without any special need for inert conditions, except where indicated. Reagents were purchased from commercial sources and used as supplied. [Rh(COD)Cl]2 and [Ir(COD)Cl]2 were purchased from Strem. NMR spectra were recorded on a Bruker DPX 300 (1H, 300 MHz; 13C, 75.5 MHz) or a Bruker AMX 500 (1H, 500 MHz; 13C, 125.7 MHz). Chemical shifts are described in parts per million downfield shifted from SiMe4.

Figure 1. Molecular structure of complex 6 showing 50% probability ellipsoids. Hydrogen atoms have been omitted for clarity.

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Figure 2. Molecular structure of complex 7 showing 50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. There are two crystallographically independent complexes.

Figure 3. Molecular structure of complex 9 showing 50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Table 3. Emission Peak Positions for 4, 6, 7, and Phenanthrene (Ph) in Dichloromethane maximum emission (nm) [relative intensity]a

compound 4 7 6 Ph

342 342 341 348

[1.00] [0.00732] [0.00204] [0.297]

358.5 [0.946] 361 [0.0270] 359 [0.00204] 365.5 [0.298]

376.5 [0.387] 395 sh [0.117] 378 [0.0211] 378 [0.00145] 385.5 [0.139] 407 sh [0.0384]

a Excitation at 256 nm, 0.5 s integration time, 300 points, 1.24 mm slit width; sh ) shoulder; emission maximum indicated in bold.

Figure 4. Emission spectra of 4, 6, 7, and phenanthrene (standard) at 256 nm in CH2Cl2 (1.0 × 10-5 mol L-1). another 25 min. The temperature was increased to 37 °C, and the reaction mixture was stirred at this temperature overnight. The solvent was removed; the residue was taken up in acetonitrile and filtered. The volatiles were removed from the filtrate, and the solid residue was further purified by trituration with ethyl ether to give 23.84 g of solid. (94.8% yield).

1 H NMR (d6-DMSO, 300 MHz): δ 8.89 (d, 1H, 3J ) 8.1 Hz, ArH), 8.79 (d, 1H, 3J ) 8.2 Hz, ArH), 8.57 (d, 1H, 3J ) 7.9 Hz, ArH), 8.31 (d, 1H, 3J ) 8.1 Hz, ArH), 8.25 (s, 1H, C2-H), 7.71 (q, 2H, 3J ) 7.9 Hz, ArH), 7.60 (q, 2H, 3J ) 7.9 Hz, ArH), 4.65 (t, 2H, 3J ) 7.0 Hz, NCH2), 1.83 (pentet, 2H, 3J ) 7.3 Hz, NCH2CH2), 1.30 (sextet, 2H, 3J ) 7.4 Hz, CH2CH3), 0.85 (t, 3H, 3J ) 7.4 Hz (CH3). 13C NMR (d6-DMSO, 75.5 MHz): δ 143.72 (NCN), 138.46 (ArC), 128.48 (ArC), 127.83 (ArC), 127.68 (ArC), 125.76 (ArC), 125.47 (ArC), 124.92 (ArC), 124.83 (ArC), 123.98 (ArC), 123.30 (ArC), 122.24 (ArC), 121.65 (ArC), 47.18 (NCH2), 32.32 (NCH2CH2), 19.51 (CH2CH3), 13.91 (CH3). Anal. Calcd for C19H18N2 (274.37): C, 83.17; H, 6.61; N, 10.21. Found: C, 83.06; H, 6.63; N, 10.22.

First Phenanthrene-Fused Imidazol-2-ylidene Synthesis of 1,3-Dibutylphenanthro[9,10-d]imidazolium Iodide (3). Compound 2 (17.57 g, 64 mmol) and 75 mL of n-butyl iodide were added into a pressure flask. The flask was sealed, and the reaction mixture was stirred at 100 °C for 38 h. The volatiles were removed under vacuum, and the residue was triturated with ethyl acetate to leave, after evaporation, 27.5 g of pure product (yield 93%). 1H NMR (d6-DMSO, 300 MHz): δ 9.82 (s, 1H, C2H), 9.11 (d, 2H, 3J ) 8.4 Hz, ArH), 8.55 (d, 2H, 3J ) 8.3 Hz, ArH), 7.91 (m, 4H, ArH), 4.93 (t, 4H, 3J ) 7.1 Hz, NCH2), 2.01 (p, 4H, 3 J ) 7.4 Hz, NCH2CH2), 1.48 (sextet, 4H, 3J ) 7.4 Hz, CH2CH3),0.976 (t, 6H, 3J ) 7.4 Hz, CH3). 13C NMR (d6-DMSO, 75.5 MHz): δ 141.57 (NCN), 129.81 (ArC), 129.24 (ArCH), 128.62 (ArC), 126.31(ArC), 125.25 (ArC), 122.89 (ArC), 120.92 (ArC), 50.47 (NCH2), 30.99 (NCH2CH2), 19.37 (CH2CH3), 13.97 (CH3). Anal. Calcd for C23H27N2I (458.38): C, 60.26; H, 5.93; N, 6.11. Found: C, 60.42; H, 5.94; N, 6.15. Synthesis of 1,3-Dibutylphenanthro[9,10-d]imidazolium Tetrafluoroborate (4). Imidazolium salt 3 (2.5 g, 5.45 mmol) was dissolved in about 300 mL of acetone. An aqueous solution of Pb(BF4)2 (1.22 mL, 50% wt) was added to immediately form a yellow precipitate. The mixture was stirred at room temperature for 2 h and filtered. The filtrate was evaporated to give 2.096 g of pure product (yield 92%). The spectral data were identical to those for imidazolium salt 3. Anal. Calcd for C23H27N2BF4 (418.29): C, 66.04; H, 6.50; N, 6.69. Found: C, 66.03; H, 6.49; N, 6.67. NMR Tube Reaction for the Generation of 1,3-Dibutylphenanthro[9,10-d]imidazol-2-ylidene (5). Imidazolium salt 4 and sodium hydride (1:4 mol ratio) were suspended in d8-THF under nitrogen. A catalytic amount of d6-DMSO was added and the NMR tube was sealed. The resulting solution was subjected to 1H NMR and 13 C NMR measurements, which indicated the formation of the carbene 5 within 20 min. This solution did not show major decomposition overnight, as indicated by 1H NMR. 1H NMR (d8THF, 300 MHz): δ 8.74 (d, 2H, 3J ) 8.1 Hz, ArH), 8.34 (d, 2H, 3 J ) 8.1 Hz, ArH), 7.56 (t, 2H, 3J ) 7.0 Hz, ArH), 7.46 (t, 2H, 3J ) 7.0 Hz, ArH), 4.70 (t, 4H, 3J ) 7.1 Hz, NCH2) 1.84 (p, 4H, 3J ) 7.1 Hz NCH2CH2), 1.39 (sextet, 4H, 3J ) 7.3 Hz, CH2CH3), 0.86 (t, 6H, 3J ) 7.3 Hz, CH3). 13C NMR (d8-THF, 300 MHz): δ 225.10 (NCN), 129.50 (ArC), 128.61(ArC), 128.01 (ArC), 125.69 (ArC), 125.09 (ArC), 124.05 (ArC), 122.71 (ArC), 52.16 (NCH2), 33.95 (NCH2CH2), 20.73 (CH2CH3), 14.37 (CH3). Synthesis of (1,3-Dibutylphenanthro[9,10-d]imidazol-2-ylidene)rhodium(1,5-cyclooctadiene) Chloride (6). To a mixture of imidazolium salt 4 (0.50 g, 1.2 mmol), [Rh(COD)Cl]2 (0.295 g, 0.6 mmol), and KOt-Bu (0.134 g 1.2 mmol) was added dry THF (50 mL) under inert conditions. The reaction mixture was stirred at room temperature for 12 h. The solvent was removed in Vacuo, and the residue was purified by flash chromatography (silica, eluted with dichloromethane) to give 6 as a yellow solid. Yield: 0.65 g, 94%. 1H NMR (CDCl3, 300 MHz): δ 8.80 (d, 2H, 3J ) 8.1 Hz, ArH), 8.34 (d, 2H, 3J ) 8.1 Hz, ArH), 7.73 (t, 2H, 3J ) 7.0 Hz, ArH), 7.66 (t, 2H, 3J ) 7.0 Hz, ArH), 5.80 (dd, 1H, 2J )12.3 Hz, 3J ) 5.3 Hz, NCH2), 5.76 (dd, 1H, 2J ) 12.3 Hz, 3J ) 5.3 Hz, NCH2) 5.27 (dd, 1H, 2J ) 12.2 Hz, 3J ) 4.2 Hz, NCH2) 5.22 (dd, 1H, 2J ) 12.2 Hz, 3J ) 4.2 Hz, NCH2) 5.20 (bs, 2H, CHCOD), 3.46 (bs, 2H, CHCOD), 2.52 (m, 4H, (CH2)COD), 2.33 (m, 2H, NCH2CH2), 2.04 (m, 2 H, (CH2)COD), 1.98 (m, 2H, NCH2CH2), 1.76 (sextet, 4H, 3J ) 7.3 Hz, CH2CH3), 1.18 (t, 6H, 3J ) 7.3 Hz, CH3). 13C NMR (CDCl3, 75.5 MHz): δ 190.9 (d, 1JRh-C ) 51.56 Hz, NCN), 128.30 (ArC), 128.03 (ArC), 127.40 (ArC), 125.56 (ArC), 124.05 (ArC), 121.60 (ArC), 121.41 (ArC), 98.85 (d, 1JRhC ) 6.84 Hz, CHCOD), 68.8 (d, 1JRhC ) 14.76 Hz), 51.76 (NCH2), 32.99 (NCH2CH2), 31.72 ((CH2)COD), 28.99 ((CH2)COD), 20.41 (CH2CH3), 13.85 (CH3). Anal. Calcd for C31H38N2RhCl (577.02): C, 64.52; H, 6.63; N, 4.85. Found: C, 64.58; H, 6.69; N, 4.83.

Organometallics, Vol. 28, No. 1, 2009 275 Synthesis of (1,3-Dibutylphenanthro[9,10-d]imidazol-2-ylidene)iridium(1,5-cyclooctadiene) Chloride (7). To a mixture of imidazolium salt 4 (0.41 g, 0.98 mmol), [Ir(COD)Cl]2 (0.33 g, 0.49 mmol), and KOt-Bu (0.135 g 1.2 mmol) was added dry THF (50 mL) under inert conditions. The reaction mixture was stirred at room temperature for 12 h. The solvent was removed under vacuum, and the residue was purified by flash chromatography (silica, eluted with dichloromethane) to give 7 as a yellow solid. Yield: 0.48 g, 73%. 1H NMR (CDCl3, 300 MHz): δ 8.81 (d, 2H, 3J ) 8.4 Hz, ArH), 8.34 (d, 2H, 3J ) 8.1 Hz, ArH), 7.73 (t, 2H, 3J ) 7.0 Hz, ArH), 7.67 (t, 2H, 3J ) 7.0 Hz, ArH), 5.61 (dd, 1H, 2J ) 12.4 Hz, 3J ) 5.4 Hz, NCH2), 5.56 (dd, 1H, 2J )12.4 Hz, 3J ) 5.3 Hz, NCH2) 5.12 (dd, 1H, 2J )12.1 Hz, 3J ) 4.1 Hz, NCH2) 5.07 (dd, 1H, 2J )12.1 Hz, 3J ) 4.2 Hz, NCH2) 4.77 (m, 2H, CHCOD), 3.05 (m, 2H, CHCOD), 2.32 (m, 4H, (CH2)COD), 2.21 (m, 2H, NCH2CH2), 1.99 (m, 2H, NCH2CH2), 1.85 (m, 2H, (CH2)COD), 1.75 (m, 2H, (CH2)COD), 1.69 (sextet, 4H, 3J ) 7.3 Hz, CH2CH3), 1.14 (t, 6H, 3J ) 7.3 Hz, CH3). 13C NMR (CDCl3, 90.5 MHz): δ 187.32 (NCN), 128.48 (ArC), 127.94 (ArC), 127.47 (ArC), 125.63 (ArC), 124.12 (ArC), 121.79 (ArC), 121.47 (ArC), 84.99 (CHCOD), 52.53(CHCOD), 51.41 (N CH2), 33.67 (NCH2 CH2), 31.71 ((CH2)COD), 29.64 ((CH2)COD), 20.31 (CH2CH3), 13.79 (CH3). Anal. Calcd for C31H38N2IrCl (666.33): C, 55.87; H, 5.74; N, 4.20. Found: C, 56.00; H, 5.79; N, 4.16. Synthesisof(1,3-Dibutylphenanthro[9,10-d]imidazol-2-ylidene)rhodium(CO)2 Chloride (8). Complex 6 (100 mg, 0.17 mmol) was dissolved in dichloromethane (20 mL) and placed under an atmosphere of CO(g) for 2 h. The remaining solvent was then removed, and the resulting solid was triturated with pentane to remove residual COD. The remaining solid was dried in Vacuo to yield 0.09 mg (99% yield) of 8 as a light yellow powder. 1H NMR (CDCl3, 300 MHz): δ 8.78 (d, 2H, 3J ) 7.7 Hz, ArH), 8.29 (d, 2H, 3 J ) 7.7 Hz, ArH), 7.71 (m, 4H), 5.29 (dd, 1H, 2J )11.2 Hz, 3J ) 5.5 Hz, NCH2), 5.25 (dd, 1H, 2J )11.2 Hz, 3J ) 5.5 Hz, NCH2) 5.08 (dd, 1H, 2J )11.2 Hz, 3J ) 4.9 Hz, NCH2) 5.03 (dd, 1H, 2J )11.2 Hz, 3J ) 4.9 Hz, NCH2) 2.18 (m, 2H, NCH2CH2), 1.99 (m, 2H, NCH2CH2), 1.68 (m, 4H, NCH2CH3), 1.10 (t, 6H, 3J ) 7.35 Hz, CH3). 13C NMR (CDCl3, 75.5 MHz): δ 185.4 (d, 1JRh-C ) 53.97 Hz), 182.77 (d, 1JRh-C ) 74.43 Hz,), 181.13 (d, 1JRh-C ) 43.94 Hz), 128.89 (ArC), 128.03 (ArC), 127.75 (ArC), 126.27 (ArC), 124.25 (ArC), 121.53 (ArC), 121.42 (ArC), 52.07 (NCH2), 31.59 (NCH2CH2), 20.12 (CH2CH3), 13.85 (CH3). IR νCO (cm-1): 2074, 1994. Anal. Calcd for C25H26N2O2RhCl (524.84): C, 57.21; H, 4.99; N, 5.34. Found: C, 57.05; H, 4.97; N, 5.28. Synthesis of Bis(1,3-dibutylphenanthro[9,10-d]imidazol-2ylidene)silver Tetrafluoroborate (9). A pressure vessel was charged with 4 (0.56 g, 1.34 mmol), Ag2O (0.31 g, 1.34 mmol), 3 Å molecular sieves (0.8 g), and 15 mL of dry dichloromethane. The flask was sealed and the reaction mixture was stirred at 80 °C for 24 h. The volatiles were removed, and the solid residue was extracted with DMSO and filtered. The filtrate was evaporated and triturated with ethyl acetate to give 0.462 g of 7 (80% yield). 1 H NMR (d6-DMSO, 300 MHz): δ 9.08 (d, 4H, 3J ) 8.2 Hz, ArH), 8.56 (d, 4H, 3J ) 8.1 Hz, ArH), 7.90 (t, 4H, 3J ) 7.5, ArH), 7.82 (t, 8H, 3J ) 7.5, ArH), 5.12 (t, 4H, 3J ) 6.8 Hz, NCH2), 2.00 (p, 8H, 3J ) 7.2 Hz, NCH2CH2), 1.55 (sextet, 8H, 3J ) 7.4 Hz, CH2CH3), 0.97 (t, 12H, 3J ) 7.3 Hz, CH3). 13C NMR (d6-DMSO, 75.5 MHz): δ C2 not detected, 129.23 (ArC), 128.84 (ArC), 127.44 (ArC), 127.40 (ArC), 125.11 (ArC), 122.52 (ArC), 121.48 (ArC), 52.87 (NCH2), 32.67 (NCH2CH2), 20.07 (CH2CH3), 14.24 (CH3). Further purification was not possible due to decomposition. Anal. Calcd for C46H52N4AgBF4 (855.62): C, 64.57; H, 6.12; N, 6.54. Found: C, 62.57; H, 6.24; N, 6.32.

Acknowledgment. A collaboration with Professor A. J. Arduengo III and his group is gratefully acknowledged. This work has been supported by internal grants from Kennesaw

276 Organometallics, Vol. 28, No. 1, 2009

State University (Mentor Prote´ge´ grant no. DT01FY200801 and a 2008-2009 CETL Incentive Grant) and the National Science Foundation (CHE-0413521, CHE-0115760, and CHE-0342921).

Tapu et al. tables of fractional coordinates, isotopic and anisotropic thermal parameters, and bond distances and angles, as well as fluorescence emission spectra of 4, 6, 7, and phenanthrene. This material is available free of charge via the Internet at http://pubs.acs.org. OM800822M

Supporting Information Available: A complete description of the X-ray crystallographic determination on 6, 7, and 9 including

(21) Krebs, F. C.; Spanggaard, H. J. Org. Chem. 2002, 67, 7185–7192.