Disubstituted Imidazolium-2-Carboxylates as Efficient Precursors to N-Heterocyclic Carbene Complexes of Rh, Ru, Ir, and Pd Adelina M. Voutchkova, Leah N. Appelhans, Anthony R. Chianese, and Robert H. Crabtree* Chemistry Department, Yale UniVersity, 225 Prospect Street, New HaVen, Connecticut 06520-8107 Received October 4, 2005; E-mail: robert.crabtree@yale.edu While N-heterocyclic carbene (NHC) complexes have provided many of the most recent powerful homogeneous catalysts, 1 methods of synthesizing them have advanced more slowly. Two common routes are deprotonation of a precursor imidazolium salt (1), either by a strong base 2a or by a basic ligand, 2b and oxidative addition of the C-H bond of 1. 2c-d The free carbene intermediate of the first route necessitates dry, air-free conditions and provides limited tolerance of other functionalities, while direct oxidative addition is known only for a number of specific cases. In an important advance, Lin et al. showed that Ag 2 O can be used to form a Ag- NHC complex from the imidazolium salt which readily transfers the NHC to palladium and gold. 3 Transmetalation to various metal species can also give a wide variety of NHCs of rhodium, copper, ruthenium, and iridium. 4,5 Failures can still be encountered, 6 however, and Ag-induced oxidative degradation of the imidazolium precursor has also been noted. 7 Further methods 8 are therefore eagerly sought. We now report that N,N′-disubstituted imidazolium-2-carbox- ylates, air- and moisture-stable species, can transfer NHCs to a variety of metal salts with release of CO 2 . For example, the N,N′- dimethyl imidazolium-2-carboxylate (2) reacts rapidly with [Rh(cod)Cl] 2 to produce Rh(cod)(NHC)Cl in 93% isolated yield (Scheme 1, cod ) 1,5-cyclooctadiene) to give 3. Carboxylate 2 also transfers the NHC to several other metal salts of Rh, Ir, Ru, and Pd (Table 1). Reactions of entries 1 and 2 are complete in as little as 20 min, while those of entries 3-6, in 2-12 h, to give mono-, bis-, or tris-NHC complexes in high yields. In addition to the cleavage of chloride-bridged dimers (Table 1, entries 1-3), displacement of neutral ligands such as triphenylphosphine (entry 4) and pyridine (entry 5) as well as anionic ligands such as acetate (entry 6) was also found. The reaction with Pd(OAc) 2 affords an unusual tris-NHC derivative in 71% yield (Figure 1). This method affords products, such as 8 and 10, which have not previously been synthesized. The remaining product complexes in Table 1 have previously been synthesized by the free carbene route 9 or by oxidative addition of 1, 5 but the present method is consider- ably faster and affords comparable or better yields. This method does not require dry or air-free conditions for the NHC transfer, provided the substrate metal salt is not itself affected by air or water, as is the case for entries 2, 4, 5, and 6. The reaction in Scheme 1 was even carried out in 90:10 (v/v) H 2 O-MeCN and afforded the same yield with no observable imidazolium salt. Conversion also even occurs at room temperature but, depending on the solvent, is limited by the low solubility of 2 under these conditions. The utility of this procedure requires a generally applicable synthesis of the NHC carboxylates. The known 10 synthesis of 2 by alkylation of methyl imidazole with (MeO) 2 CO (Scheme 2) is of limited generality. Louie and co-workers 11 have reported a synthesis of the N,N′-di(mesityl)imidazolium-2-carboxylate using KOBu t /CO 2 (Scheme 3). Scheme 1 Table 1. NHC Metal Complexes Obtained from Reaction of Metal Salts with N,N′-Dimethyl Imidazolium Carboxylate (2) a,b entry reactant product time (h) yield % (lit.) 1 [Rh(cod)Cl]2 Rh(cod)(NHC)Cl(3) 0.3 93(91) 9 2 [Ir(cod)Cl]2 Ir(cod)(NHC)Cl (4) 0.3 82(90) 9 3 [(ArH)RuCl2]2 (5) (ArH)RuCl2(NHC) (6) 2 85(90) 9 4 [Ir(cod)(PPh3)2]PF6 (7) [Ir(cod)(NHC)2]PF6 (8) 2 84 5 Ir(cod)(py)2]PF6(9) [Ir(cod)(NHC)2]PF6 (8) 2 76 6 Pd(OAc)2 [Pd(NHC)3(OAc)]OAc (10) 12 71 a Conditions: MeCN, 75 °C. b NHC ) 1,3-dimethylimidazole-2-ylidene; cod ) 1,5-cyclooctadiene; ArH ) η 6 -p-cymene; py ) pyridine. Figure 1. An ORTEP diagram of the cation of 10. Selected bond lengths (Å) and angles (deg): Pd(1)-C(6), 1.968(3); Pd(1)-C(1), 2.036(3); Pd(1)- C(11), 2.049(3); Pd(1)-O(1), 2.096(2); C(6)-Pd(1)-C(1), 91.39(13); C(6)-Pd(1)-C(11), 89.72(13); C(1)-Pd(1)-C(11), 177.84(13); C(6)- Pd(1)-O(1), 175.81(11). Published on Web 11/22/2005 17624 9 J. AM. CHEM. SOC. 2005, 127, 17624-17625 10.1021/ja056625k CCC: $30.25 © 2005 American Chemical Society