Annulated N-Heterocyclic Carbenes: 1,3-Ditolylphenanthreno[9,10-d

Apr 2, 2009 - Figure 2. Molecular structure of the formula unit of 7b·CH2Cl2 (ellipsoids with 50% probability). Selected bond lengths (Å) and angles...
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Organometallics 2009, 28, 2441–2449

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Annulated N-Heterocyclic Carbenes: 1,3-Ditolylphenanthreno[9,10-d]imidazol-2-ylidene and Transition Metal Complexes Thereof Farman Ullah,† Markus K. Kindermann,† Peter G. Jones,‡ and Joachim Heinicke*,† Institut fu¨r Biochemie-Anorganische Chemie, UniVersita¨t Greifswald, Felix-Hausdorff-Strasse 4, 17487 Greifswald, Germany, and Institut fu¨r Anorganische and Analytische Chemie, Technische UniVersita¨t Braunschweig, Hagenring 30, 38106 Braunschweig, Germany ReceiVed January 7, 2009

The N,N′-bis(tolylamino)phenanthrenes 1a and 1b were synthesized by reductive cyclization of benzilbis(tolylimines) and cyclized with triethyl orthoformate in the presence of NH4PF6 to give the corresponding bis(tolyl)phenanthreno[9,10-d]imidazolium hexafluorophosphates 2a,b. These were deprotonated with excess KH in THF. Whereas the p-tolyl-substituted phenanthrene-annulated imidazol-2-ylidene (phenimy) 3a proved to be unstable and dimerized to 4a, the bulkier o-tolyl derivative is isolable as the monomer carbene 3b. Cationic silver complexes were prepared by the method of Wang and Lin (6a, 6b) or directly (6b), rhodium and palladium complexes 7b-9b by reaction of 3b and the respective metal precursors. Detailed structural information is given by X-ray crystal structure analyses of 6a and 7b and by HH- and CH-COSY NMR measurements of 1b, 2a, 2b, 3b, and 7b, representing the various compound types. The phenimy ligands are distinguished from benzo- or linear naphtho-annulated imidazol-2-ylidene ligands by higher basicity and free 3b by lower deshielding of the 13CII nuclei. Introduction The discovery of stable, highly nucleophilic N-heterocyclic carbenes (NHCs) by Arduengo in 19911 has revitalized the interest in these divalent carbon compounds. Their unique electronic and steric ligand properties2 have inspired burgeoning research activities and led to many applications in organometallic3 and organic chemistry.4 The extensive stabilization of transition metal fragments by NHC makes them ideal tools for transition metal catalysis.5 Tuning of the electronic ligand properties is possible by varying substituents and saturation2-5 * To whom correspondence should be addressed. Tel: (+)49 3834 864318. Fax: (+)49 3834 864377. E-mail: [email protected]. † Universita¨t Greifswald. ‡ Technische Universita¨t Braunschweig. (1) (a) Arduengo, A. J., III.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361–363. (b) Arduengo, A. J., III. Acc. Chem. Res. 1999, 32, 913–921. (2) (a) Dı´ez-Gonza´lez, S.; Nolan, S. P. Coord. Chem. ReV. 2007, 251, 874–883. (b) Cavallo, L.; Correa, A.; Costabile, C.; Jacobsen, H. J. Organomet. Chem. 2005, 690, 5407–5413. (3) Recent reviews e.g.: (a) Hahn, F. E.; Jahnke, M. Angew. Chem., Int. Ed. 2008, 47, 3122–3172. (b) Peris, E. Top. Organomet. Chem. 2007, 21, 83–116. (c) Praetorius, J. M.; Crudden, C. M. Dalton Trans. 2008, 4079– 4094. (d) Lin, I. J. B.; Vasam, C. S. Coord. Chem. ReV. 2007, 251, 642– 670. (e) Garrison, J. C.; Youngs, W. J. Chem. ReV. 2005, 105, 3978–4008. (f) Bourissou, D.; Guerret, O.; Gabbaı¨, F. P.; Bertrand, G. Chem. ReV. 2000, 100, 39–91. (g) Herrmann, W. A.; Ko¨cher, C. Angew. Chem., Int. Ed. 1997, 36, 2163–2187. (4) Recent reviews on NHC organocatalysis: (a) Marion, N.; Dı´ezGonza´lez, S.; Nolan, S. P. Angew. Chem., Int. Ed. 2007, 46, 2988–3000. (b) Enders, D.; Niemeier, O.; Henseler, A. Chem. ReV. 2007, 107, 5606– 5655. (5) Reviews on NHC transition metal catalysis, e.g.: (a) Marion, N.; Nolan, S. P. Acc. Chem. Res. 2008, 41, 1440–1449. (b) Wuertz, S.; Glorius, F. Acc. Chem. Res. 2008, 41, 1523–1533. (c) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem., Int. Ed. 2007, 46, 2768–2813. (d) Gade, L. H.; Bellemin-Laponnaz, S. Top. Organomet. Chem. 2007, 21, 117–157. (e) Zapf, A.; Beller, M. Chem. Commun. 2005, 431–440. (f) Peris, E.; Crabtree, R. H. Coord. Chem. ReV. 2004, 248, 2239–2246. (g) Perry, M. C.; Burgess, K. Tetrahedron: Asymmetry 2003, 14, 951–961. (h) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290–1309.

and, particularly for unsaturated NHC, also by annulation.6-18 The first stable annulated NHC was dineopentylbenzimidazol(6) For example: (a) O’Brien, C. J.; Kantchev, E. A. B.; Chass, G. A.; Hadei, N.; Hopkinson, A. C.; Organ, M. G.; Setiadi, D. H.; Tang, T.-H.; Fang, D.-C. Tetrahedron 2005, 61, 9723–9735. (b) Hadei, N.; Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Org. Lett. 2005, 7, 1991–1994. (c) Saravanakumar, S. PhD thesis, Greifswald, 2006. (7) (a) Hahn, F. E.; Wittenbecher, L.; Boese, R.; Bla¨ser, D. Chem.sEur. J. 1999, 5, 1931–1935. (b) Boesveld, W. M.; Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F.; Schleyer, P. v. R. Chem. Commun. 1999, 755–756. (c) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. J. Chem. Soc., Dalton Trans. 2000, 3094–3099. (d) Khramov, D. M.; Bielawski, C. W. J. Org. Chem. 2007, 72, 9407–9417 (Supporting Information). (e) Ku¨hl, O.; Saravanakumar, S.; Ullah, F.; Kindermann, M. K.; Jones, P. G.; Ko¨ckerling, M.; Heinicke, J. Polyhedron 2008, 27, 2825–2832. (8) (a) Glorius, F.; Altenhoff, G.; Goddard, R.; Lehmann, C. Chem. Commun. 2002, 2704–2705. (b) Lohre, C.; Froehlich, R.; Glorius, F. Synthesis 2008, 2221–2228. (9) (a) Arduengo, A. J., III.; Tapu, D.; Marschall, W. J. Angew. Chem., Int. Ed. 2005, 44, 7240–7244. (b) Arduengo, A. J., III.; Tapu, D.; Marschall, W. J. J. Am. Chem. Soc. 2005, 127, 16400–16401. (10) Khramov, D. M.; Boydston, A. J.; Bielawski, C. W. Angew. Chem., Int. Ed. 2006, 45, 6186–6189. (11) (a) Alcarazo, M.; Roseblade, S. J.; Cowley, A. R.; Ferna´ndez, R.; Brown, J. M.; Lassaletta, J. M. J. Am. Chem. Soc. 2005, 127, 3290–3291. (b) Burstein, C.; Lehmann, C. W.; Glorius, F. Tetrahedron 2005, 61, 6207– 6217. (c) Weiss, R.; Reichel, S. Eur. J. Inorg. Chem. 2000, 1935–1939. (d) Nonnenmacher, M.; Kunz, D.; Rominger, F.; Oeser, T. J. Organomet. Chem. 2005, 690, 5647–5653. (12) Ullah, F.; Bajor, G.; Veszpre´mi, T.; Jones, P. G.; Heinicke, J. Angew. Chem., Int. Ed. 2007, 46, 2697–2700. (13) (a) Herrmann, W. A.; Schu¨tz, J.; Frey, G. D.; Herdtweck, E. Organometallics 2006, 25, 2437–2448. (b) Burbiel, J. C.; Hockemeyer, J.; Mu¨ller, C. E. ArkiVoc 2006, 77–82. (c) Kascatan-Nebioglu, A.; Panzner, M. J.; Garrison, J. C.; Tessier, C. A.; Youngs, W. J. Organometallics 2004, 23, 1928–1931. (14) Saravanakumar, S.; Kindermann, M. K.; Heinicke, J.; Ko¨ckerling, M. Chem. Commun. 2006, 640–642. (15) Saravanakumar, S.; Oprea, A. I.; Kindermann, M. K.; Jones, P. G.; Heinicke, J. Chem.sEur. J. 2006, 12, 3143–3154. (16) (a) Sanderson, M. D.; Kamplain, J. W.; Bielawski, C. W. J. Am. Chem. Soc. 2006, 128, 16514–16515. (17) Tapu, D.; Owens, C.; VanDerveer, D.; Gwaltney, K. Organometallics 2009, 28, 270–276.

10.1021/om9000132 CCC: $40.75  2009 American Chemical Society Publication on Web 04/02/2009

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2-ylidene,7 originally synthesized by thione reduction using potassium-sodium alloy7a or KC8,7b,c later also by deprotonation.7d,e Subsequently, functionally substituted benzimidazol-2-ylidenes6 and a variety of further annulated NHCs were reported, including electron-rich oxazolino and thiazolo,8 metalloceno,9 benzimidazol-2-ylideno,10 N-heterocyclic pyrido,11,12 pyrimidino,13 quinolino,11a and quinoxalino14 as well as more extended carbocyclic naphtho,15 naphthochinono,16 and very recently also a phenanthreno17 annulated imidazol-2-ylidene. A few six- and seven-membered annulated NHCs are also known.18 One goal of our recent research was a comparison of carbo- (benzo, naphtho) and heterocyclic (pyrido, quinoxalino) annulated dineopentylimidazol-2-ylidenes7c,12,14,15 with homologous heterocyclic silylenes, germylenes, and stannylenes.19,20 To study the effect of more extended carbocyclic annulation, we also investigated phenanthreno[9,10-d]imidazol-2-ylidenes21 and present here our results.

Ullah et al. Scheme 1. Synthesis of N,N′-Ditolylphenanthreno[9,10-d]imidazolium Salts 2

Scheme 2. Deprotonation of 2a,b to Phenimy 3b and Dimer 4a, and Conversion of 3b to 5b

Results and Discussion Synthesis of Precursors. In most of our earlier work on annulated NHCs or their homologues we used N,N′-dineopentyl substituents for comparability and estimation of annulation effects. With few exceptions,14 these provide sufficient steric bulk to obtain isolable compounds without hindering cyclization reactions. However, attempts to condense phenanthrene quinone with alkyl amines even under harsh conditions (200 °C, 10 bar) failed in our hands to give defined diimines as starting materials for phenanthrene-annulated imidazoliums salts. Whereas Tapu et al. were able to obtain the latter by stepwise N,N′-dialkylation of phenanthreno[9,10-d]imidazole,17 we had turned to N,N′diarylphenanthrene diimines, which can be obtained by reductive cyclodehydrogenation of benzil-bis(arylimines), as reported so far for N-phenyl22 and N-3,5-dimethylphenyl groups.23 To provide a NMR probe and indicator of steric effects, we chose p- and o-tolyl groups. The corresponding benzil-bis(tolylimines) formed in high yields by condensation of benzil and p- or o-toluidine in the presence of p-toluenesulfonic acid (2 day reflux) or more efficiently with TiCl4 (2-3 days 20 °C) as acid catalyst. The diimines were cycloreduced by 4-6 equiv of lithium and hydrolytically worked up to give 9,10-bis(tolylamino)phenanthrenes 1a,b. Subsequent cyclocondensation with triethyl orthoformate in the presence of ammonium hexafluorophosphate provided the ditolylphenanthreno[9,10-d]imidazo(18) (a) Bazinet, P.; Yap, G. P. A.; Richeson, D. S. J. Am. Chem. Soc. 2003, 125, 13314–13315. (b) Scarborough, C. S.; Grady, M. J. W.; Guzei, I. A.; Gandhi, B. A.; Bunel, E. B.; Stahl, S. S. Angew. Chem., Int. Ed. 2005, 44, 5269–5272. (19) (a) Ullah, F.; Kühl, O.; Bajor, G.; Veszpre´mi, T.; Jones, P. G.; Heinicke, J. Eur. J. Inorg. Chem. 2009, 221–229. (b) Ullah, F.; Oprea, A. I.; Kindermann, M. K.; Bajor, G.; Veszpre´mi, T.; Heinicke, J. J. Organomet. Chem. 2009, 694, 397–403. (c) Ku¨hl, O.; Lo¨nnecke, P.; Heinicke, J. New J. Chem. 2002, 26, 1304–1307. (d) Ku¨hl, O.; Lo¨nnecke, P.; Heinicke, J. Polyhedron 2001, 20, 2215–2222. (e) Heinicke, J.; Oprea, A.; Kindermann, M. K.; Karpati, T.; Nyula´szi, L.; Veszpre´mi, T. Chem.-Eur. J. 1998, 4, 541–545. (f) Heinicke, J.; Oprea, A. I. Heteroat. Chem. 1998, 9, 439–444. (g) Gehrhus, B.; Lappert, M. F.; Heinicke, J.; Boese, R.; Bla¨ser, D. J. Chem. Soc., Chem. Commun. 1995, 1931–1932. (20) (a) Zabula, A. V.; Hahn, F. E. Eur. J. Inorg. Chem. 2008, 5165– 5179. (b) Ku¨hl, O. Cent. Eur. J. Chem. 2008, 6, 365–372. (c) Ku¨hl, O. Coord. Chem. ReV. 2004, 248, 411–427. (d) Hill, N. J.; West, R. J. Organomet. Chem. 2004, 689, 4165–4183. (e) Gehrhus, B.; Lappert, M. F. J. Organomet. Chem. 2001, 617-618, 209–223. (21) Ullah, F. Ph.D. thesis, University Greifswald, May 2008. (22) (a) MacPherson, E. J.; Smith, J. G. Tetrahedron 1971, 27, 2645– 2649. (b) MacPherson, E. J.; Smith, J. G. Chem. Commun. 1970, 1552– 1553. (23) Ketterer, A.; Ziller, J. W.; Rheingold, A. L.; Heyduk, A. F. Organometallics 2007, 26, 5330–5338.

lium salts 2a,b (Scheme 1). Attempts to extend the synthesis to the more bulky and symmetric N-mesityl derivative failed at the stage of the reductive cyclization. A similar observation was made in an attempt to synthesize 9,10-bis(2,6-diisopropylphenylimino)phenanthrene via reductive cyclization of the corresponding benzildiimine and air oxidation of the intermediate phenanthrene diamide.24 Attempts to introduce electron-donating methoxy groups into the phenanthrene moiety starting from bis(p-methoxy)benzil also failed. Reactions of Phenanthreno[9,10-d]imidazolium Salts and Carbene Characterization. The deprotonation of the pand o-tolyl-substituted phenanthreno[9,10-d]imidazolium salts 2a and 2b with potassium hydride in THF furnished different results. The less bulky p-tolyl compound did not afford the monomer “phenimy” 3a but instead the dimer 4a, whereas the one o-methyl group per N-aryl group in 3b provides sufficient steric bulk to allow isolation of the monomer as a viscous oil. For monomer benzimidazol-2-ylidenes, stronger hindrance is required to prevent dimerization or monomer-dimer equilibria.25 3b can be stored at room temperature, at least for some days, but is sensitive to air and moisture and cannot be purified by chromatography on silica gel. In the mass spectrum the molecular cation is the base peak if the sample is supplied under inert conditions. The dimer 4a does not show a molecular cation but undergoes fragmentation with the base peak at [M + 2]+ (M - monomer). Apart from mass and NMR spectra the carbene 3b was characterized by addition of selenium, yielding the phenanthrenoimidazol-2-selone 5b (Scheme 2). Synthesis of Phenimy Transition Metal Complexes. In contrast to free 3a, stable transition metal complexes thereof (24) VanBelzen, R.; Klein, R. A.; Smeets, W. J. J.; Spek, A. L.; Benedix, R.; Elsevier, C. J. Recl. TraV. Chim. Pays-Bas 1996, 275–285. (25) (a) Hahn, F. E.; Wittenbecher, L.; Le Van, D.; Fro¨hlich, R. Angew. Chem. 2000, 112, 551–554; Angew. Chem. Int. Ed. 2000, 39, 541-544. (b) Liu, Y.; Lindner, P. E.; Lemal, D. M. J. Am. Chem. Soc. 1999, 121, 10626–10627.

Annulated N-Heterocyclic Carbenes

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Scheme 3. Syntheses of Phenimy Transition Metal Complexes 6-9

are easily accessible using the method of Wang and Lin.26 An example is the cationic silver complex 6a(PF6), obtained in high yield by reaction of the phenimy precursor 2a with silver oxide in methylene chloride and trapping of water by molecular sieves A3. The related o-tolyl-substituted phenimy silver complex 6b(PF6) was obtained in the same way, whereas 6b(OTf) and 6b(SbF6) were synthesized directly from the respective silver salt and carbene 3b, previously generated in THF, and separated from KPF6 and excess KH by filtration. Chlororhodium(I)(phenimy)(COD) (7b), allylpalladium(II)(phenimy) chloride (8), and palladium(0)(phenimy)(dvds) (9) were also prepared from free 3b and the corresponding precursor complexes (Scheme 3). To compare 3b with higher group 14 homologues, an attempt was made to synthesize the corresponding germylene, which should be easily accessible and stable as a monomer in contrast to the more easily associating annulated N-heterocyclic stannylenes.19f,27 The reaction of dilithium phenanthrene-9,10 diamide 1bLi with GeCl2 dioxane solvate in THF led indeed to the germylene 10b, which, after removal of THF and extraction with diethyl ether, forms an orange viscous oil, stable at room temperature. The compound is however highly sensitive to any trace of moisture and polar groups in silica gel, which easily regenerates 1b and prevented further purification by column chromatography.

Structure and Properties. All compounds were characterized by conclusive 1H and 13C NMR data, complemented by HRMS data or elemental analysis. Two of the phenanthrene-annulated NHC complexes formed single crystals by slow diffusion of hexane into the CH2Cl2 solution. 6a(PF6) · 3CH2Cl2 (Figure 1) is monoclinic (P21/c); the cation displays no crystallographic symmetry but approximates closely to 2-fold symmetry (rms deviation 0.06 Å). The anion and all solvent molecules are wellordered. 7b · CH2Cl2 (Figure 2) is also monoclinic (P21/n). Like for the related (di-n-butyl-phenimy)Rh(COD)Cl complex,17 the annulated ring system is to a good approximation planar; rms (26) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972–975. (27) Hahn, F. E.; Wittenbecher, L.; LeVan, D.; Zabula, A. V. Inorg. Chem. 2007, 46, 7662–7667. (b) Braunschweig, H.; Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. Z. Anorg. Allg. Chem. 1995, 621, 1922–1928.

deviations from the best plane are 0.05 and 0.04 Å for 6a(PF6) · 3CH2Cl2 (whereby the atoms C35 and C36 lie ca. 0.25 Å out of the plane and were omitted in its calculation) and 0.04 Å for 7b · CH2Cl2. The π-planes of the N-tolyl groups are markedly rotated with respect to the π-planes of the annulated imidazole rings, with interplanar angles between the phenimy ring and the toluene rings of 59° to 85° (in 6a(PF6) · 3CH2Cl2) and 78.5(1)° and 68.5(1)° (in 7b · CH2Cl2). In the crystals of 7b · CH2Cl2 both o-methyl groups point to the same side of the phenimy plane, while in solution two forms were indicated, suggesting rotamers with cis- and trans-orientation of the o-methyl groups, whose signals do not average out within the NMR time scale at 25 °C. The C(II)-Ag-C(II) angle of 6a(PF6) · 3CH2Cl2 deviates from a linear arrangement by only about 10°, and the two phenimy planes around silver subtend an interplanar angle of 62.1(1)°. Other structural features, bond lengths, and angles are similar to those reported for other NHC rhodium3c,17 and silver3d,e,17 complexes. The average value of the Rh-(CdC) distances (2.2048(18) Å) trans to the CII atom and thus the trans-influence of the NHC ligand are almost the same as in (nBu2phenimy)Rh(COD)Cl (2.213(3)17 Å). For solution NMR characterization HH- and CH-COSY experiments were carried out with representatives of the new

Figure 1. Molecular structure of the cation of 6a(PF6) · 3CH2Cl2 (ellipsoids with 50% probability). Selected bond lengths (Å) and angles (deg): Ag-C(1) 2.0827(18), Ag-C(31) 2.0809(18), C(1)-N(1) 1.356(2), C(1)-N(2) 1.353(2), N(1)-C(2) 1.399(2), N(2)-C(3) 1.395(2), C(2)-C(3) 1.379(3), C(3)-C(4) 1.438(3), C(9)-C(10) 1.461(3); C(31)-Ag-C(1) 169.57(7), N(2)C(1)-N(1) 105.44(16), N(2)-C(1)-Ag 124.83(13), N(1)-C(1)-Ag 128.99(14), C(1)-N(1)-C(2) 111.06(16), C(3)-C(2)-N(1) 106.02(16), C(1)-N(1)-C(16) 120.98(15), C(2)-N(1)-C(16) 127.42(15).

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Figure 2. Molecular structure of the formula unit of 7b · CH2Cl2 (ellipsoids with 50% probability). Selected bond lengths (Å) and angles (deg): Rh-C(1) 2.0167(16), Rh-Cl(1) 2.4115(5), C(1)-N(1) 1.364(2), C(1)-N(2) 1.361(2), N(1)-C(3) 1.400(2), N(2)-C(2) 1.405(2), C(2)-C(3) 1.371(2), C(3)-C(4) 1.435(2), C(9)-C(10) 1.464(2); C(1)-Ag-Cl(1) 86.61(5), N(2)-C(1)-N(1) 104.96(14), N(2)-C(1)-Rh 131.68(12), N(1)-C(1)-Rh 122.81(11), C(1)-N(1)-C(3) 111.16(13), N(1)-C(3)-C(2) 106.49(14), C(1)-N(1)-C(21) 119.99(13), C(3)-N(1)-C(21) 128.01(14).

phenanthrene diamines, phenanthrene-imidazolium salts, and phenimy complexes (1b Figure 3, 3b Figure 4; 2a, 2b, and 7b see Supporting Information) to allow conclusive assignment of the proton and 13C NMR signals, which except for methyl signals are all localized in the aryl region. For the phenanthrene diamines 1a,b a strong downfield doublet is assigned to the H-4 protons in the o,o′-positions of the planarized biphenyl substructure. H-1 (weak 5J cross-peak in the HH-COSY spectrum of 1b) is also downfield shifted. The o-tolyl signals appear all relatively upfield, indicating +M interactions with the lone electron pair at the amino group, particularly for the H-6′ doublet. The assignment of the CH carbon nuclei is based on the cross-peaks in the CH-COSY spectrum. The quaternary carbon nuclei were assigned analogously to 6a and 6b, where two of these nuclei couple with silver. The spectra of the phenanthreno[9,10-d]imidazolium salts 2a and 2b differ strongly from those of 1a,b by the cyclodelocalized π-electron system, involving also the electron lone pairs at nitrogen. Hence the tolyl group can no longer profit from the +M effect of the amino group, and the proton and carbon signals are downfield shifted, especially the o-protons (-I effect). The fixation of the N-C(tolyl) bond in the NHC-ring plane and hindrance of the tolyl rotation around the N-C axis exposes H-6 to the anisotropy cone of the toluene π-system and causes a strong upfield shift compared to this proton (H-1) in 1a,b. The carbon signal (C-6 of 2a,b versus C-1 of 1a,b) is less upfield shifted. The spectra of the o-tolyl compound are rendered more complicated by hindrance of the N-tolyl rotation and occurrence of two isomers with the o-methyl groups at the same or different sides. This can be seen, for example, in the 2D NMR spectra of 3b (Figure 4) (for others see Supporting Information). The RhI(phenimy)(COD)Cl complex 7b displays only one of the expected two rotamers, but the general features of the spectra do not differ strongly from those of the salt 2b (the greatest difference being a small downfield shift of H-6′ by δ ca. 0.5/ 0.4 ppm). The one-bond couplings of CII with rhodium (7a) and with silver (6a,b) display typical values (1J(103Rh13C) )

Ullah et al.

50.4, 1JAgC ) 185.1-188.2, 213.7-218.1 Hz).3c-e,17,28 The bis(p-tolyl) phenimy complex 6a, which provides only one set of well-resolved signals, shows additionally small coupling to two further quarternary 13C carbon nuclei, assigned according to increasing distance to Cq-4 (3JAgC ) 5.9 Hz) and Cq-5 (4JAgC ) 1.1 Hz). In the bis(o-tolyl)phenanthreno-imidazol-2-ylidene complexes one of the two (7b-9b) or four (6b) rotamers, which make the spectra more complex, is usually favored and displays more intense signals than the others. The most characteristic chemical shift values of NHCs and their metal complexes are those of the 13CII signals. Whereas increasing linear annulation causes downfield shift, for benzoto naphtho[b]-annulated N,N′-dineopentylimidazol-2-ylidenes at δ ) 231.87 to δ ) 239.9,15 the Y-shaped phenanthreneannulated imidazolylidenes 3b (δ ) 224.5, 224.9) and nBu2phenimy (δ ) 225.1)17 display the CII resonance more upfield, between benzo- and nonannulated imidazol-2-ylidenes. The coordination chemical shifts of the complexes depend on the metal, its oxidation state, and slightly on the annulation (6b ∆δ ) -38, 8b and 9b ∆δ ) -33.6 and -13.6, cf. 29). For (NHC)Rh(COD)Cl complexes a (small) response to annulation is known rather for the downfield CdC-nuclei trans to CII of the 1,5-COD ligand than for ∆δCII,15 and these resonate at δ ) 97.2, close to those of nonannulated R2imy-Rh(COD)Cl complexes (e.g., R ) cHex δ ) 97.530). This indicates strong donor properties of the phenanthrene-annulated imidazol-2ylidenes. For (nBu2phenimy)Rh(CO)2Cl17 this was detected also via the influence on the CO stretching frequency. The high donor strength and the unexpected relatively upfield 13CII resonance may be due to partial electronic decoupling of the imidazol-2ylidene ring from the benzene rings in the Y-shaped π-system, reflected in rather short C(2)-C(3), elongated C(3)-C(4)/ C(2)-C(15), and even longer C(9)-C(10) bonds, i.e., alternate bond lengths in the central six-membered ring of 6a and 7b (see above). This may be expressed by a high weight of a resonance structure with strong 6π-delocalization within the two outer benzene rings and the five-membered imidazole ring but weak mutual coupling. The different electronic structure of 1 and 2 is evident also in the UV and emission spectra. The long-wavelength absorption of 1 (333 nm), caused by the tolylamino group, is lacking in 2, which displays only the strong band at 254 nm and shoulders at 247, 277, 287, and 195 nm. The maxima of the o- and p-tolyl compounds are very similar; only the extinctions are slightly different. In the fluorescence spectra 2a and 2b display strong emission bands (max. 355 nm, 6-8 times stronger than phenanthrene) in the same region as phenanthrene,31 whereas the emission is strongly red-shifted in 1b (460 nm). In the Pd0 complex 9b the emission (355 nm) is strongly reduced, ca. 20% relative to phenanthrene, but less strongly than in the nBu2phenimy Rh(COD)Cl complex of Tapu et al.17

Conclusion N,N′-Diaryl-substituted phenanthreno[9,10-d]imidazolium salts are accessible by reductive cyclization of not too bulky benzil(28) Arduengo, A. J., III.; Dias, H. V. R.; Calabrese, J. C.; Davidson, F. Organometallics 1993, 12, 3405–3409. (29) (a) Mihai, S. V.; Germaneau, F. G.; Navarro-Fernandez, O.; Stevens, E. D.; Nolan, S. P. Organometallics 2002, 21, 5470–5472. (b) Jackstell, R.; Harkal, S.; Jiao, H.; Spannenberg, A.; Borgmann, C.; Ro¨ttger, D.; Nierlich, F.; Cavell, K.; Nolan, S. P.; Beller, M. Chem.-Eur. J. 2004, 10, 3891–3900. (30) Herrmann, W. A.; Ko¨cher, C.; Goossen, C. J.; Artus, G. R. J. Chem.-Eur. J. 1996, 2, 1627–1636. (31) Dawson, W. R.; Windsor, M. W. J. Phys. Chem. 1968, 72, 3251– 3259.

Annulated N-Heterocyclic Carbenes

Organometallics, Vol. 28, No. 8, 2009 2445

Figure 3. HH-COSY (left) and CH-COSY (right) spectra of 1b.

Figure 4. HH-COSY (left) and CH-COSY (right) spectra of 3b.

Experimental Section

compounds were conducted under an argon atmosphere using Schlenk techniques. Benzil bis(arylimines) were prepared by variation of a known procedure32 from benzil (ca. 30 mmol) and excess anilines in toluene at 0-20 °C (3 days) using TiCl4 (ca. 2 mL) as a catalyst and were purified after removal of toluene by extraction with diethyl ether and precipitation with ethanol, yields 72-86%. Other chemicals were used as purchased. NMR spectra were recorded on a multinuclear FT-NMR ARX300 or AVANCE II 300 spectrometer (Bruker) at 300.1 (1H), 75.5 (13C), and 121.5 (31P) MHz. Shift references are tetramethylsilane for 1H and 13C and H3PO4 (85%) for 31P. Assignment numbers are given in Scheme 1. Coupling constants refer to JHH unless stated otherwise. Assignments are based on HH- and CH-COSY NMR experiments and DEPT 13C NMR spectra for selected compounds. HRMS measurements were carried out in Go¨ttingen with a double-focusing sectorfield instrument MAT 95 (Finnigan) with EI (70 eV, PFK as reference substances) or with ESI in MeCN, MeOH, or MeOH/ NH4OAc using a 7 T APEX IV Fourier transfrom ion cyclotron resonance mass spectrometer (Bruker Daltonics) or in Bielefeld with EI using an Autospec X VG sectorfield mass spectrometer. UV spectra were measured with a UV/vis Lambda 800 (Perkin-Elmer) spectrometer, and emission spectra with a Varioskan fluorescence spectrometer (Thermo Electron), using λexc ) 256 nm for excitation.

All solvents except methanol and ethanol were carefully dried and distilled prior to use. Reactions with air- or moisture-sensitive

(32) Belzer, R. V.; Klein, R. A.; Smeets, W. J. J.; Spek, A. L.; Benedix, R.; Elsevier, C. J. Recl. TraV. Chim. Pays-Bas. 1996, 115, 275–285.

bis(arylimines) with 4-6 equiv of lithium in THF and subsequent cyclization with excess trialkyl orthoformate and an ammonium salt. The route is limited to aryl groups with not more than one o-substituent. Deprotonation with KH in THF provides the free N,N′-diarylphenanthreno[9,10-d]imidazol-2ylidenes, which are stable in the presence of sterically stabilizing o-substituents but dimerize in its absence to the corresponding entetramine. The 13C NMR spectrum of the isolated carbene shows a surprising upfield chemical shift of the CII resonance relative to linear carbocyclic annulated imidazol-2-ylidenes. This hints at partial decoupling of the cyclodelocalized fivemembered ring from the planar tetracyclic π-system, which causes higher donor strengths than for linear carbocyclic annulated imidazol-2-ylidenes. Transition metal complexes of the isolable o-substituted N,N′-diarylphenanthreno[9,10-d]imidazol-2-ylidenes can be obtained directly from the ligand and the corresponding metal precursor, whereas the p-tolyl isomers will be accessible by the known transmetalation route of the silver complex, obtained in high yield by the method of Wang and Lin.

2446 Organometallics, Vol. 28, No. 8, 2009 Fluorescence units of emission bands are given relative (FUrel ) RFUλ/RFUphen360) to the RFU of the emission band of phenanthrene at 360 nm (for 10-4 M solution in CH2Cl2). Melting points (uncorrected) were determined with a Sanyo Gallenkamp melting point apparatus, and elemental analysis was determined with a CHNS-932 analyzer from LECO using standard conditions or a combustion aid (V2O5 for 6a(PF6), 6b(PF6), 8b, 9b). Precursors for Phenanthrene-Annulated Imidazol-2-ylidenes. N,N′-Di(p-tolyl)-1,2-diphenylethane-1,2-diimine. TiCl4 (about 2 mL, used as catalyst) was added dropwise through a septum to a cooled solution (0 °C) of benzil (10.0 g, 47.6 mmol) and excess o-toluidine (30 mL, 6 equiv) in toluene (150 mL). The solution was stirred at room temperature for 3 days. Diethyl ether (150 mL) was added, and the solution was filtered to remove solid products. The solvent was evaporated under vacuum to give an orange-red liquid crude product, which was precipitated by addition of ethanol (50 mL). The precipitate was collected by filtration, washed with ethanol, and dried in vacuum to give 15.7 g (85%) of NMR pure product. 1 H NMR (CDCl3): δ 2.24 (s, 6 H, Me-4′), 6.51 (dd, 3J ) 8.4, 4J ) 1.8 Hz, 4 H, H-2′/6′), 6.87 (d, 3J ) 8.4 Hz, 4 H, H-3′/5′), 7.39 (m, 6 H, H-3/5, H-4), 7.86 (dd, 3J ) 8.0, 4J ) 1.6 Hz, 4 H, H-2/6). 13 C{1H} NMR (CDCl3): δ 20.92 (Me-4′), 120.30 (C-2′/6′), 128.21 128.67,128.98 (CH-2/6, CH-3/5, CH-3′/5′), 130.90 (CH-4), 134.56 (Cq-4′), 137.54 (Cq-1), 146.76 (Cq-1′), 163.77 (CqdN). MS (EI, 70 eV, 345 °C): m/z (%) 389 (2), 388 (10) [M+], 373 (6), 195 (18), 194 (100), 106 (6), 105 (6), 90 (38), 66 (19), 57 (16). Anal. Calcd for C28H24N2 (388.19): C, 86.56; H, 6.23; N, 7.21. Found: C, 85.72; H, 6.24; N, 6.94. N,N′-Di(o-tolyl)-1,2-diphenylethane-1,2-diimine. TiCl4 (2 mL) was added at 0 °C to a solution of benzil (6.0 g, 28.5 mmol) and excess o-toluidine (25 mL, ca. 8 equivalents) in toluene (100 mL). Workup by using the procedure described above gave an orangered liquid crude product, which was precipitated by addition of ethanol (50 mL). The precipitate was collected by filtration, washed with ethanol, and dried under vacuum, yielding 9.5 g (86%) of NMR-pure yellow solid di(o-tolyl)-1,2-diphenylethane-1,2-diimine. 1 H NMR (CDCl3): δ 1.34 (s, 6 H, 2 Me), 6.55 (d, 3J ) 7.7 Hz, 2 H, H-6′), 6.83 (td, 2 H, H-4′), 6.96 (m, 4 H, H-3′/5′), 7.42-7.51 (m, 6 H, H-3, H-4, H-5), 8.00 (dd, J ) 8.1, J ) 1.8 Hz 4 H, H-2, H-6). 13C{1H} NMR (CDCl3): δ 16.59 (Me-2′), 116.70 (C-6′), 125.03, 125.51(C-4′, C-5′), 128.40, 128.75 (CH-2/6, CH-3/5), 129.89 (CH-3′), 130.92 (CH-4), 132.26 (Cq-2′), 138.30 (Cq-1), 147.85 (Cq-1′), 162.47 (CqdN). MS (EI, 70 eV, 345 °C): m/z (%) 389 (4), 388 (15) [M+], 348 (4), 319 (21), 195 (18), 194 (100), 105 (21), 91 (37), 77 (20), 66 (19), 57 (13). Anal. Calcd for C28H24N2 (388.19): C, 86.56; H, 6.23; N, 7.21. Found: C, 86.20; H, 6.19; N, 6.92. N,N′-Di-p-tolylphenanthrene-9,10-diamine (1a). Di(p-tolyl)-1,2diphenylethane-1,2-diimine (4.1 g, 10.6 mmol) was dissolved in THF (100 mL), and Li suspension (440 mg, 63.4 mmol, excess, about 6 equiv) was added at room temperature. Reaction (dissolution) of Li was started by sonification and allowed to complete by stirring at room temperature for 20 h. Excess lithium was removed by filtration through glass wool. Methanol was added dropwise to the filtrate with continued stirring until the evolution of hydrogen ceased. Then water was added, and the product was extracted with diethyl ether. After drying with MgSO4 and removal of ether a yellow-orange viscous liquid was obtained. A small amount of diethyl ether was added, and the resulting precipitate was separated by filtration, washed with few milliliters of diethyl ether, and dried under vacuum to give 2.75 g (67%) of NMR-pure off-white solid 1a. 1H NMR (CDCl3): δ 2.23 (s, 6 H, Me-4′), 5.74 (br s, 2 H, NH), 6.55 (d, 3J ) 8.4 Hz, 4 H, H-2′/6′), 6.93 (d, 3J ) 8.1 Hz, 4 H, H-3′/5′), 7.50 (ddd, 3J ) 8.1, 3J ) 7.0, J ) 1.0 Hz, 2 H, H-2), 7.61 (ddd, 3J ) 8.2, 3J ) 7.0, 4J ) 1.2 Hz, 2 H, H-3), 8.01 (dd, 3J ) 8.2, 4J ) 1.1 Hz, 2 H, H-1), 8.72 (d br, 3J ) 8.1 Hz, 2 H, H-4). 13 C{1H} NMR (CDCl3): δ 20.48 (Me-4′), 115.38 (CH-2′/6′), 122.85

Ullah et al. (CH-4), 124.96 (CH-1), 126.12 (CH-3), 126.79 (CH-2), 128.78 (Cq4′), 129.80 (CH-3′/5′), 129.83, 130.04, 131.08 (Cq-4a, Cq-8a, Cq9), 144.28 (Cq-1′). MS (EI, 70 eV, 250 °C): m/z (%) 389 (30) [M+ + H], 388 (100), [M+], 373 (35), 283 (25), 282 (23), 281 (26), 91 (20), 57 (23). Anal. Calcd for C28H24N2 (388.50): C, 86.56; H, 6.23; N, 7.21. Found: C, 86.67; H, 6.51; N, 6.92. N,N′-Di-o-tolylphenanthrene-9,10-diamine (1b). Di(o-tolyl)-1,2diphenylethane-1,2-diimine (8.15 g, 21.0 mmol) was dissolved in THF (150 mL) and reduced with Li (874 mg, 125.9 mmol, excess, about 6 equiv) using the same procedure and workup as described above for 1a, yielding 5.52 g (68%) of NMR-pure off-white solid 1b. UV (10-5 M in MeOH): λmax () sh 247 (31200), 251.5 (33 700), 333 (4700) nm. Emission (10-4 M in CH2Cl2): λmax (FUrel) 458 nm (2.6), sh 485 (2.2). 1H (HH-COSY) NMR (CDCl3): δ 2.13 (s, 6 H, 2 Me), 5.44 (br s, 2 H, NH), 6.27 (d br, 3J ) 7.8 Hz, 2 H, H-6′), 6.73 (ddd, 3J ) 7.4, 3J ) 7.3, 4J ) 0.9 Hz, 2 H, H-4′), 6.87 (ddd, 3 J ) 7.7, 3J ) 7.6, 4J ) 1.2 Hz, 2 H, H-5′), 7.13 (d br, 3J ) 7.3 Hz, 2 H, H-3′), 7.51 (ddd, 3J ) 8.1, 3J ) 7.1, 4J ) 1.1 Hz, 2 H, H-2), 7.63 (ddd, 3J ) 8.1, 3J ) 7.1, 4J ) 1.2 Hz, 2 H, H-3), 8.02 (dd, 3J ) 8.1, 4J ) 1.0 Hz, 2 H, H-1), 8.74 (d br, 3J ) 8.1 Hz, 2 H, H-4). 13C{1H} (CH-COSY) NMR (CDCl3): δ 17.40 (Me-2′), 113.32 (CH-6′), 119.15 (CH-4′), 122.90 (CH-4), 123.57 (Cq-2′), 124.62 (CH-1), 126.33 (CH-3), 126.92, 126.93 (CH-5′, CH-2), 130.43 (CH-3′), 129.87, 130.44 (sh), 131.16 (Cq-4a, Cq-8a, Cq-9), 144.57 (Cq-1′). MS (EI, 70 eV, 250 °C): m/z (%) 389 (35), 388 (100) [M+], 373 (13), 283 (38), 282 (33), 281 (31), 280 (30), 267 (16), 165 (22), 106 (15), 65 (16), 57 (15). Anal. Calcd for C28H24N2 (388.50): C, 86.56; H, 6.23; N, 7.21. Found: C, 86.77; H, 6.56; N, 7.19. Synthesis of Phenanthreno-Annulated Imidazolium Salts. 1,3Di-p-tolyl-1H-phenanthreno[9,10-d]imidazolium hexafluorophosphate (2a). Compound 1a (1.43 g, 3.68 mmol), NH4PF6 (628 mg, 3.85 mmol), and triethyl orthoformate (15 mL) were heated for 6 h at 140 °C in a rectification apparatus for removal of the ethanol generated in the reaction. The solution was allowed to cool to room temperature. The product precipitated was collected by filtration and washed with diethyl ether and n-hexane (3 × 10 mL), respectively, and then dried under vacuum to give 1.6 g (80%) of pale brown solid 2a. UV (10-5 M in MeOH): λmax () sh 247 (45 400), 253.6 (60 200), sh 277 (11 600), sh 287 (9200), 195 (6000) nm. Emission (10-4 M in CH2Cl2): λmax (FUrel) 355 nm (8). 1H NMR (HH-COSY) (DMSO-d6): δ 2.57 (s, 6 H, 2 Me), 7.36 (dd, 3J ) 7.8, 4J ) 0.6 Hz, 2 H, H-6), 7.61 (t, 3J ) 7.8, 7.5 Hz, 2 H, H-7), 7.68 (d, 3J ) 8.2 Hz, 4 H, H-m′), 7.83 (td superimposed, 3J ≈ 8, 4 J ) 0.9 Hz, 2 H, H-8), 7.86 (d, 3J ) 8 Hz, 4 H, H-o’), 9.10 (d, 3J ) 8.4 Hz, 2 H, H-9), 10.26 (s, 1 H, H-2). 13C{1H} NMR (CHCOSY) (DMSO-d6): δ 21.03 (Me-p’), 120.15 (Cq-10), 120.95 (CH6), 125.00 (CH-9), 125.96 (Cq-4), 127.15 (2 CH-3′,5′), 128.29, 128.37 (CH-7, 8), 129.63 (Cq-4′), 131.10 (2 CH-2′,6′), 132.46 (Cq5), 141.91 (Cq-1′), 142.55 (br, CH-2). 31P{1H} NMR (DMSO-d6): δ -143.2 (hept, 1JPF ) 711 Hz). MS (EI, 70 eV, 345 °C): m/z (%) 400 (10), 399 (18) [M+ - PF6], 284 (10), 283 (60), 106 (23), 91 (13), 57 (18), 44 (100). Anal. Calcd for C29H23N2PF6 (544.47): C, 63.97; H, 4.26; N, 5.15. Found: C, 63.74, H, 4.60; N, 5.14. 1,3-Di-o-tolyl-1H-phenanthreno[9,10-d]imidazolium hexafluorophosphate (2b). Compound 1b (1.5 g, 3.86 mmol), NH4PF6 (692 mg, 4.25 mmol), and triethyl orthoformate (15 mL) were heated for 7 h at 140 °C in a rectification apparatus. Workup in the same way as described for compound 2a gave 1.65 g (78%) of pale brown solid 2b, forming a mixture of two axial isomers, E/Z-2b, with the o-methyl groups on opposite or the same side. Signals with subscript index a are slightly stronger (2′-Me proton intensity ratio 55:45%) than those indexed b; no index indicates complete superimposition. UV (10-5 M in MeOH): λmax () sh 246 (46 000), 253.5 (62 600), sh 270 (14 800), sh 277 (12 300), sh 287 (10 600), 295 (6800) nm. Emission (10-4 M in CH2Cl2): λmax (FUrel) 355 nm (6.9). 1H NMR (HH-COSY) (DMSO-d6): δ 2.19, 2.23 (s, 6 H, 2 Me), 7.19, 7.21

Annulated N-Heterocyclic Carbenes

Organometallics, Vol. 28, No. 8, 2009 2447 Table 1. Crystal Data and Structure Refinement

empirical formula fw temperature wavelength cryst syst space group unit cell dimens volume Z density (calculated) absorp coeff F(000) cryst size θ range for data collection index ranges reflns collected indep reflns completeness to given θ absorp corr max. and min. transmn refinement method data/restraints/params goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole

6a(PF6) · 3CH2Cl2

7b · CH2Cl2

C61H50AgCl6F6N4P 1304.59 100(2) K 0.71073 Å monoclinic P21/c a ) 13.0015(4) Å, R ) 90° b ) 19.2789(8) Å, β ) 90.098(3)° c ) 22.4380(11) Å, γ ) 90° 5624.2(4) Å3 4 1.541 Mg/m3 0.737 mm-1 2648 0.27 × 0.15 × 0.15 mm3 1.39 to 32.06° -19 e h e 18, -28 e k e 28, -33 e l e 33 161 379 19 460 [R(int) ) 0.0487] 99.2% to 31.5° semiempirical from equivalents 0.8975 and 0.7723 full-matrix least-squares on F2 19 460/0/716 1.050 R1 ) 0.0435, wR2 ) 0.1079 R1 ) 0.0627, wR2 ) 0.1219 2.300 and -1.812 e · Å-3

C38H36Cl3N2Rh 729.95 133(2) K 0.71073 Å monoclinic P21/n a ) 14.0608(11) Å, R ) 90° b ) 14.9417(11) Å, β ) 106.750(4)° c ) 16.1758(12) Å, γ ) 90° 3254.2(4) Å3 4 1.490 Mg/m3 0.802 mm-1 1496 0.30 × 0.25 × 0.20 mm3 1.69 to 30.51° -20 e h e 20, -21 e k e 21, -23 e l e 23 68 990 9919 [R(int) ) 0.0327] 99.9% to 30° semiempirical from equivalents 0.8561 and 0.7331 full-matrix least-squares on F2 9919/6/415 1.057 R1 ) 0.0290, wR2 ) 0.0689 R1 ) 0.0422, wR2 ) 0.0774 0.631 and -0.467 e · Å-3

(each dd, 3J ) 8.3, 4J ) 0.8 Hz, 2 H, H-6a,b), 7.62 (t br, 3J ) 7-8 Hz, 2 H, H-7a slightly upfield from H-7b), 7.65-7.88 (m, 8 H), assigned by HH-COSY: 7.69 (t) H-4′b, 7.72 (t) H-4′a, 7.75 (d) H-3′b, 7.77 (d) H-3′a, 7.82 (t) H-5′b, 7.84 (t) H-5′a, 7.86 (t) H-8), 7.95, 8.07 (each dd, 3J ) 7.8, 4J ) 0.9 Hz, 2 H, H-6′a,b), 9.13 (d, 3J ) 8.4 Hz, 2 H, H-9), 10.29, 10.33 (2 s, 1 H, H-2a,b). 13C{1H} NMR (CH-COSY) (DMSO-d6): δ 16.80, 16.99 (2′-Mea,b), 120.02 (CH6), 120.18, 120.34 (Cq-10a,b), 125.06, 125.10 (CH-9b,a), 125.81, 125.97 (Cq-4a,b), 127.82, 128.32 (CH-6′a,b), 128.27, 128.49 (CH4′b,a), 128.72, 128.74 (CH-7, CH-8), 129.76, 129.81 (Cq-2′b,a), 132.05, 132.15 (CH-3′b,a), 132.25, 132.35 (CH-5′b,a), 134.01 (Cq5), 135.36, 135.56 (Cq-1′a,b), 141.46, 141.79 (CH-2b,a). 31P{1H} NMR (DMSO-d6): δ -143.2 (sept, 1JPF ) 711.4 Hz). MS (EI, 70 eV, 330 °C): m/z (%) 401 (19), 399 (87) [M+ - PF6], 388 (100), 397 (8), 384 (12), 309 (20), 308 (11), 282 (12), 281 (35), 279 (12), 107 (32), 91 (11), 65 (15). Anal. Calcd for C29H23N2PF6 (544.47): C, 63.97; H, 4.26; N, 5.15. Found: C, 63.78, H, 4.20, N, 5.01. Deprotonation of Phenanthrenoimidazolium Salts and Carbene Selenium Adduct. 1,3-Di-o-tolylphenanthreno[9,10-d]imidazol-2-ylidene (3b). A suspension of 2b (300 mg, 0.55 mmol) in THF was added at room temperature to a suspension of 30% KH in mineral oil (56 mg, 1.4 mmol, 2.5 equiv), washed before use with THF. The mixture was stirred 2 h at room temperature. Hydrogen evolved, and the color of the solution became red-brown on stirring after 30 min. After filtration the solvent was removed under vacuum, and the residue was extracted with benzene. Evaporation of the solvent under vacuum gave 185 mg (84%) of a red-brown viscous oil, stable at room temperature for a long time under inert atmosphere. NMR spectra in C6D6 showed complete conversion of the starting material to the carbene 3b containing only traces of an impurity (