pubs.acs.org/Organometallics Published on Web 12/11/2009 r 2009 American Chemical Society

134 Organometallics 2010, 29, 134–141

DOI: 10.1021/om900894k

Synthesis of Electron-Rich CNN-Pincer Complexes, with N-Heterocyclic

Carbene and (S)-Proline Moieties and Application to

Asymmetric Hydrogenation

M�erce Boronat,† Avelino Corma,† Camino Gonz�alez-Arellano,‡,§ Marta Iglesias,*,‡ andF�elix S�anchez*,§

†Instituto deTecnologıaQuımica,CSIC-UPV,Avenida de losNaranjos s/n, 46022Valencia, Spain, ‡Institutode Ciencia de Materiales de Madrid, CSIC, C/ Sor Juana In�es de la Cruz 3, Cantoblanco 28049 Madrid,

Spain, and §Instituto de Quımica Org�anica, CSIC, C/ Juan de la Cierva 3, 28006 Madrid, Spain

Received October 13, 2009

New chiral CNN-pincer-type gold, palladium, and rhodium complexes containing N-heterocycliccarbene substituent and (S)-N-tert-butyl-methylpyrrolidine-2-carboxamide as chiral auxiliary havebeen synthesized and studied for asymmetric hydrogenation. The complexes were prepared by the silvercarbene transfer route from the respective silver complex. The reaction with [RhCl(cod)]2 (cod =cycloocta-1,5-diene), PdCl2(CH3CN)2, or K[AuCl4] affords the corresponding cationic [Rh(cod)-(ligand)]Cl, [PdCl(ligand)]Cl, and [AuCl(ligand)]Cl2 complexes in which the ligand functions effectivelyin a CNN coordination mode. The complexes catalyze the enantioselective hydrogenation of prochiralalkenes. Enantioselectivity is very sensitive to the NHCN-substituent, resulting in a useful switch in thepredominant enantiomer.

Introduction

The chemistry of N-heterocyclic carbenes (NHCs) hasdeveloped significantly over recent years, encompassingtheir synthesis, reactivity, coordination chemistry, and ap-plication.1 Furthermore, the ease of functionalization ofthe imidazolium salt proligands led to incorporation ofN-heterocyclic carbene donors in polydentate ligand struc-tures, usually in combination with other classical donors.

Among the polydentate ligand structures with carbene do-nors, tripodal2 and pincer systems3 have attracted muchattention. Seminal work, with pincer ligands bearing phos-phine and amine donors was carried out by Shaw,4 van derBoom and Milstein,5 Albrecht and van Koten,6 and others.The incorporation of NHC donors led to the synthesisof several kinds of NHC-containing pincer-type ligands:neutral3a,7 andmonoanionic,8 with theNHCmoiety as lateraldonor function or as the backbone.9 The synthesis of newrigid tridentate ligands with modified topologies, functionalgroups, chirality, and hemilability is a challenge for futuredevelopments in these areas, and there are relatively few chiralpincer ligands known.10 In most of the reported systems oneor more C-centered stereogenic centers are incorporated intothe pincer framework via a chiral auxiliary.Gold complexes can have high activity in either homo-

geneous or heterogeneous catalysis, and abetter understanding

*Corresponding authors. (M.I.) E-mail: marta.iglesias@icmm.csic.es. Tel: þ34 913349000. Fax: þ34 913720623. (F.S.) E-mail: felix-iqo@iqog.csic.es. Tel: þ34 915622900. Fax: þ34 915644853.(1) (a)Glorius, F.N-heterocyclicCarbenes (NHC) in TransitionMetal

Catalysis (Topics in Organometallic Chemistry); Springer-Verlag: Berlin,2006. (b) Nolan, S. P. N-Heterocyclic Carbenes in Synthesis; Wiley-VCH,2006. (c) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev.2000, 100, 39. (d) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008,47, 3122. (e) Nair, V.; Bindu, S.; Sreekumar, V. Angew. Chem., Int. Ed.2004, 43, 5130. (f) Marion, N.; Diez-Gonzalez, S.; Nolan, I. P. Angew.Chem., Int. Ed. 2007, 46, 2988. (g) Fremont, P. de; Marion, N.; Nolan, S. P.Coord. Chem. Rev. 2009, 253, 862.(2) (a) Hu, X. L.; Castro-Rodrıguez, I.; Meyer, K. Organometallics

2003, 22, 3016. (b) Mas-Marza, E.; Poyatos, M.; Sanau, M.; Peris, E.Organometallics 2004, 23, 323. (c) Hahn, F. E.; Langenhahn, V.; L€ugger,T.; Pape, T.; LeVan, D. Angew. Chem., Int. Ed. 2005, 44, 3759–3763. (d)McKie, R.; Murphy, J. A.; Park, S. R.; Spicer, M. D.; Zhou, Sh.-Z. Angew.Chem., Int. Ed. 2007, 46, 6525–6528. (e) Hahn, F. E.; Radloff, Ch.Chem.-Eur. J. 2008, 14, 10900–10904. (f) Kaufhold, O.; Stasch, A.; Pape, T.; Hepp,A.; Edwards, P. G.; Newman, P. D.; Hahn, F. E. J. Am. Chem. Soc. 2009,131, 306–317.(3) (a) Peris, E.; Loch, J. A.; Mata, J.; Crabtree, R. H. Chem.

Commun. 2001, 201. (b) Tulloch, A. A. D.; Danopoulos, A. A.; Tizzard,G. J.; Coles, S. J.; Hursthouse,M. B.; Hay-Motherwell, R. S.;Motherwell,W.B.Chem.Commun. 2001, 1270. (c) Chen, J. C. C.; Lin, I. J. B.DaltonTrans.2000, 839–840. (d) Corber�an, R.; Mas-Marz�a, E.; Peris, E. Eur. J. Inorg.Chem. 2009, 1700–1716. (e) Hahn, F. E.; Jahnke, M. C.; Gomez-Benitez, V.;Morales-Morales, D.; Pape, T. Organometallics 2005, 24, 6458–6463. (f)Hahn, F. E.; Jahnke, M. C.; Pape, T.Organometallics 2006, 25, 5927–5936.

(4) Moulton, C. J.; Shaw, B. L. Dalton Trans. 1976, 1020–1024.(5) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759–

1792.(6) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40,

3750–3781.(7) Gr€undemann, S.; Albrecht, M.; Loch, J. A.; Faller, J. W.;

Crabtree, R. H. Organometallics 2001, 20, 5485–5488.(8) (a) Andavan, G. T. S.; Bauer, E. B.; Letko, C. S.; Hollis, T. K.;

Tham, F. S. J. Organomet. Chem. 2005, 690, 5938–5947. (b) Rubio, R. J.;Andavan, G. T. S.; Bauer, E. B.; Hollis, T. K.; Cho, J.; Tham, F. S.;Donnadieu, B. J. Organomet. Chem. 2005, 690, 5353–5364. (c) Hahn, F.E.; Jahnke, M. C.; Pape, T. Organometallics 2007, 26, 150–154.

(9) Pugh, D.; Danopoulos, A. A. Coord. Chem. Rev. 2007, 251, 610–641.

(10) (a) The Chemistry of Pincer Compounds; Morales-Morales, D.,Jensen, C.M., Eds.; Elsevier: Amsterdam, 2007. (b) Gosiewska, S.; Herreras,S. M.; Lutz, M.; Spek, A. L.; Havenith, R. W. A.; van Klink, G. P. M.; vanKoten, G.; Gebbink, R. J. M. K. Organometallics 2008, 27, 2549–2557.

Article Organometallics, Vol. 29, No. 1, 2010 135

of the role of ligands for these metal ions is required.11 Inparticular, ligands are needed that can prevent easy decom-position to gold metal while maintaining high catalyticactivity and that can facilitate mechanistic studies ofthe catalytic reactions. For gold(III) catalysts, these ligandsshould also stabilize the higher oxidation state, thus rulingout soft donor ligands such as tertiary phosphines that tendto favor gold(I).12 Many simple nitrogen-donor ligands areeasily displaced from gold(III), and decomposition to me-tallic gold tends to occur.12 Gold chemistry has the potentialfor reactions such as hydrogenation of unsaturated groups(ketones, imines, etc.) or C-H activation.11,13 However, todate no studies of gold complexes with pincer ligands bearingan N-heterocyclic carbene moiety have been conducted.Our research is focused on the synthesis of neutral

unsymmetrical pyridine pincer-type ligands with a lateral(S)-N-tert-butyl-methylpyrrolidine-2-carboxamide donorfunction and an N-heterocyclic carbene moiety. Pre-viously, symmetrical NCN-pincer complexes with twoester prolinates or prolinols as the nitrogen donor sub-stituents have been described.14,10b We also have reportedthe synthesis of chiral tridentate (1a, 1b in Figure 1)15 andpincer-type ONN (2a, 2b) (Figure 1), which containOphenol, Npyridil, and chiral Ncarboxamide donor groupderivatives of the natural amino acid (S)-proline asN-donor groups on the pincer backbone, resemblingcoordination environments present in previously de-scribed Schiff base ligands (1a, 1b) (Figure 1).16 Theseare commercially available and enantiomerically purebuilding blocks for the synthesis of chiral ligands, andtheir stereogenic information can be maintained bychoosing the appropriate reaction conditions during thesynthesis and purification of the complexes. The stereo-genic center (SC) at the prolinate ring plays an importantrole, together with the bulkiness of the ester group, inthe introduction of stereogenicity on the nitrogen atoms(SN or RN) upon coordination to the metal center.We have now incorporated an N-heterocyclic carbenemoiety (Figure 1, ligands 3, 4) into the CNN-pincerbackbone. The purpose of incorporating a NHC func-tionality is to obtain unsymmetrical CNN pincer-typeligands, 2-[(3-aryl-2,3-dihydro-1H-imidazol-1-yl)methyl]-6-(pyrrolidin-1-ylmethyl)pyridine (Figure 1, 3a, 4a) andchiral (2S)-N-tert-butyl-1-((6-((3-aryl-2,3-dihydro-1H-imida-zol-1-yl)methyl)pyridin-2-yl)methyl)pyrrolidine-2-carboxamide

(Figure 1, 3b, 4b). These ligands react with gold, palladium, andrhodium precursors to afford square-planar gold(III),palladium(II), and pentacoordinate rhodium(I) complexes.We have also studied the catalytic activity of the resultingproducts for hydrogenation reactions.

Results and Discussion

Preparation of Imidazolium Salts. The new neutralN-heterocyclic carbene pincer-type ligands were designed withthe objective of stabilizing metal complexes with enhancedcatalytic properties. In this regard two anionic ONN-triden-tate unsymmetrical pincer-type ligands, 2-(6-(pyrrolidin-1-ylmethyl)pyridin-2-yl)phenol (2a) and N-tert-butyl-1-((6-(2-hydroxyphenyl)pyridin-2-yl)methyl)pyrrolidine-2-carboxa-mide (2b),16 have been recently reported, and their gold andPd complexes showed activity toward several organic reac-tions. In this work we have obtained the unsymmetricalCNN pincer-type ligands 2-[(3-aryl-2,3-dihydro-1H-imida-zol-1-yl)methyl]-6-(pyrrolidin-1-ylmethyl)pyridine (3a, 4a)and (2S)-N-tert-butyl-1-((6-((3-aryl-2,3-dihydro-1H-imida-zol-1-yl)methyl)pyridin-2-yl)methyl)pyrrolidine-2-carboxa-mide (3b, 4b). Specifically, the new 1-aryl-3-{[6-(pyrrolidin-1-ylmethyl)pyridin-2-yl]methyl}-2,3-dihydro-1H-imidazol-3-iumbromide ([3a]Br, [3b]Br) were prepared in two successivenucleophilic substitutions with 1-aryl-1H-imidazole and 2,6-bis(bromomethyl)pyridine and the corresponding pyrro-lidine in 70%yieldwith full retention of theSC configurationof the carbon stereogenic center (Scheme 1). The 1H NMRspectra (CDCl3) of compounds [3a]Br, [3b]Br, [4a]Br, and[4b]Br show that imidazolium C(2)-H characteristically re-sonates at 10.10 and 10.40 ppm. The bridging methylene(NimCH2C) moiety appeared as a singlet at 5.99 ([3a]Br) and6.23 and 5.98 ([3b]Br) ppm, CCH2Npyrr ([3a]Br) at 3.73, andCCH2Npro ([3b]Br) at 3.77 and 3.67 ppm in the 1H NMRspectrum and at 59.77 (CCH2Npyrr); 51.44 (NimCH2C)([3a]Br) or 61.44 (CCH2Npro); and 54.25 (NimCH2C)([3b]Br) ppm in the 13C NMR spectrum. In the electrospraymass spectrum the imidazolium cation appeared as molecu-lar peaks at m/z 361 ([3a]Br) or 460 ([3b]Br), respectively, in100% abundance.Synthesis of Complexes. Salts [3]Br and [4]Br are precur-

sors to NHCs, and their gold, rhodium, and palladiumcomplexes were synthesized by Lin’s method of transmetala-tion from intermediate silver(I) complexes 3Ag and 4Ag

Figure 1

(11) (a) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed.2006, 45, 7896–7936. (b) Bond, G. C.; Louis, C.; Thompson, D. T.Catalysisby Gold; Imperial College Press: London, 2006. (c) Dyker, G. Angew.Chem., Int. Ed. 2000, 39, 4237-4239, and references therein. (d) Ivanova,S.; Petit, C.; Pitchon, V. Gold Bull. 2006, 39, 3–8. (e) Corma, A.; Serna, P.Science 2006, 313, 332. (f) Gonz�alez-Arellano, C.; Corma, A.; Iglesias, M.;S�anchez, F. Chem. Commun. 2005, 3451–3453.(12) (a) Schmidbaur, H. Gold: Progress in Chemistry, Biochemistry

and Technology; Wiley: Chichester, U.K., 1999. (b) Puddephatt, R. J. TheChemistry of Gold; Elsevier: Amsterdam, 1978.(13) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev.

2004, 248, 2201–2237.(14) (a) Gosiewska, S.; Veld, M. H.; Pater, J. J. M. de; Bruijnincx, P.

C. A.; Lutz, M.; Spek, A. L.; van Koten, G.; Gebbink, R. J. M. K.Tetrahedron: Asymmetry 2006, 17, 674–686. (b) Gosiewska, S.; Martinez,S. H.; Lutz, M.; Spek, A. L.; van Koten, G.; Gebbink, R. J. M. K. Eur. J.Inorg. Chem. 2006, 22, 4600–4607.(15) Gonz�alez-Arellano, C.; Guti�errez-Puebla, E.; Iglesias, M.;

S�anchez, F. Eur. J. Inorg. Chem. 2004, 1955–1962.(16) (a) Debono, N.; Iglesias, M.; S�anchez, F. Adv. Synth. Catal.

2007, 349, 2470–2476. (b) del Pozo, C.; Debono, N.; Corma,A.; Iglesias,M.;S�anchez, F. ChemSusChem 2009, 2, 650–657.

136 Organometallics, Vol. 29, No. 1, 2010 Boronat et al.

(Scheme 2).17 Metalation of the imidazolium salts [3]Br and[4]Br using Ag2O was monitored by 1H NMR spectroscopy,but the silver complexes were not isolated. Instead, PdCl2-(CH3N)2 or [RhCl(cod)]2 (cod = cycloocta-1,5-diene) wasadded directly to the dichloromethane solution of 3Ag or4Ag. Precipitation of silver chloride was observed immedi-ately, but the mixtures were allowed to stir at room tempera-ture for 2-24 h before workup. Consistent with theformation of the Ag complex, the 1H NMR spectra showedthe absence of an imidazolium (NCHN) resonance at∼10.10ppm owing to the loss of the acidic imidazolium proton as aresult of the reactionwithAg2Oalongwith the appearance ofa diagnostic silver-bound carbene (NCN-Ag) peak at ∼174ppm in the 13C NMR spectrum of 3Ag and 4Ag.PalladiumComplexes.Addition of an equal molar amount

of PdCl2(CH3N)2 to a CH2Cl2 solution of 3Ag or 4Ag

immediately afforded a precipitate (75%, 95% yield). The1H and 13C NMR spectra obtained for complexes 3bPd and

4bPd displayed only one set of signals and indicated thepresence of a single diastereoisomer in solution. In the1H NMR spectrum of the Pd complexes, the bridg-ing methylene (CH2) moieties appeared at 6.19 ppm(CCH2Nim), 4.33 ppm (CCH2Npyrr) (3aPd), 5.67 ppm(CCH2Nim), 4.37 ppm (3bPd), 5.6 ppm (CCH2Nim), 4.05ppm (CCH2Npyrr) (4aPd), and 5.83 ppm (CCH2Nim), 5.46ppm (CCH2Npro) (4bPd). The characteristic palladium-bound carbene (NNC-Pd) peak appeared at 160.25 (3aPd),169.23 ppm (3bPd), 160.85 (4aPd), and 175.63 ppm (4bPd) inthe 13C NMR and falls well within the range, ca. 175-145ppm, observed for other reported Pd-NHC complexes. Un-fortunately we have not obtained crystals of adequate size todetermine the structure by X-ray. In order to study thenature of the ligand-metal interactions, the molecularstructure of complexes 3bPd and 4bPd was obtained fromDFT calculations (see Figure 2). In both pincer-type com-plexes the Pd center is bonded to the imino and pyrrolidineNatoms, to the carbene ligand (with Pd-Ccarb distances of2.060 and 2.070 A, respectively), and to a chloride atom. Ahydrogen bond is formed between theCl atomand the aminoproton of the CONHtBu group, which preferentially stabi-lizes the exo isomer in the transmetalation step.Rhodium Complexes. Reaction of 3Ag and 4Ag with

[RhCl(cod)]2 at 40 �C gave the rhodium complexes (3Rh,4Rh) in high yields. The 1H and 13C NMR spectra are asexpected for complexes of the general formula Rh(NHC-ligand)(cod)Cl. In the 1H NMR spectra the cyclooctadieneresonances are observed as four broad lines between 1.8 and4.8 ppm due to fluxionality in the conformation of the codchelate. Complexes 3Rh and 4Rh exhibit a doublet carbenesignal in the 13C spectrum at 171-181 ppm in addition tofour other signals attributable to the carbon atoms of theCOD ligand. Elemental analysis and the ESI-MS spectrumare consistent with the proposed formulation shown inScheme 2, but a single crystal structure has yet to beobtained.Gold Complexes.Chelated organogold(III) complexes can

readily be prepared by transmetalation using organo-mercury(II) reagents.18 We have used a similar route fromthe silver compounds described before. Treatment of thesilver complexes 3Ag and 4AgwithKAuCL4 in ethanol givesthe chlorogold(III) complexes 3Au(III) and 4Au(III), respec-tively, in>90%yield, as yellow-orange solids. The 1HNMRspectra showed the absence of an imidazolium (NCHN)

Scheme 1. Synthesis of Ligand Precursors

Scheme 2. Synthesis of Complexes

Figure 2. Optimized geometry of complexes 3bPd (left) and4bPd (right).

(17) (a) Garrison, J. C.; Youngs, W. J. Chem. Rev. 2005, 105, 3978–4008. (b) Chianese, A. R.; Li, X.W.; Janzen,M. C.; Faller, J. W.; Crabtree, R.H. Organometallics 2003, 22, 1663–1667. (c) Simons, R. S.; Custer, P.;Tessier, C. A.; Youngs, W. J. Organometallics 2003, 22, 1979–1982. (d)Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972–975.

(18) (a) Li, C. K.-L.; Sun, R. W.-Y.; Kui, S. Ch.-F.; Zhu, N.; Che,Ch.-M.Chem.-Eur. J. 2006, 12, 5253–5266. (b) Parish, R. V.;Wright, J. P.;Pritchard, R. G. J. Organomet. Chem. 2000, 596, 165–176. (c) Bonnardel, P.A.; Parish, R. V.; Pritchard, R. G.DaltonTrans. 1996, 3185–3193. (d)Wong,K. H.; Cheung, K. K.; Chang, M. C. W.; Che, C. M.Organometallics 1998,17, 3505–3511.

Article Organometallics, Vol. 29, No. 1, 2010 137

resonance at 10.11 ([3a]Br), 10.39 ([3b]Br), 10.20 ([4a]Br), or10.42 ([4b]Br) ppm owing to the loss of the acidic imidazo-lium proton along with the appearance of a diagnostic gold-bound carbene (NCN-Au) peak at 192.18 ppm (3aAu(III)),181.29 ppm (3bAu(III)), or 197.62 (4bAu(III)) in the 13CNMR spectrum, although it was possible to obtain thedesired gold(III) complex by direct reaction of the imidazo-lium salt [3a]Br or [3b]Br with the potassium gold salt(KAuCl4) (Scheme 3). The 1H NMR spectra showed thepresence of an imidazolium (NCHN) resonance at ∼10.00ppm owing to the acidic imidazolium proton of [3a]Br and[3b]Br. This intermediate, in the presence of a base such as[(CH3)3Si]2NK yields the formation of 3aAu(III) and 3bAu-

(III), respectively. Transmetalation from the silver salt wasmuch more efficient, giving cleaner and quicker reactions.

Subsequent abstraction of the chloride anion from allcomplexes by treatment with AgPF6 yielded the cationiccomplexes in good yields. One new stereogenic center wasgenerated upon coordination of ligand 3b or 4b to metal(palladium, gold, rhodium). In all cases the cationic com-plexes were obtained as single diastereoisomers.Catalytic Activity: Hydrogenation of Olefins. Table 1

shows data for the hydrogenations of diethyl itaconate anddiethyl 2-benzylidenesuccinate with Au, Rh, and Pd com-plexes carried out under standard conditions (EtOH as thesolvent, 4 atm PH2, 40 �C). The nature of the metal centerinfluences the catalytic activities. In general, rhodium(I)catalyst is more active than gold(III) or palladium(II) withthe same ligand. In all cases nometal was detected during thehydrogenation reaction. Low enantiomeric excesses wereachieved with the 3bPd compound (<40%), but 3bRh(I)and 3bAu(III) show an ee>80 and 70%, respectively, for thehydrogenation of diethyl 2-benzylidenesuccinate, as wecould detect from the HPLC analysis of hydrogenatedproducts.

Table 1 shows the effect that ligand substituents have onactivity and enantioselectivity. Thus, when we have a fairlyrigid skeleton (ligand 1b), resulting complexes are very activecatalysts but lead to poor enantioselectivity.When we have arigid skeleton, such as pyridine, selectivity in the case ofpalladium does not improve, but we have observed a sig-nificant increase in the gold derivative case.When we replacethe phenolate group by a NHC carbene substituent, thecatalysts show a significant improvement on all metals.Table 1 also shows the effect of modifying the NHCN-substituent, indicating that the rate and particularly theenantioselectivity are very sensitive to changes at this posi-tion. The catalytic activity diminishes when the bis-3,5-diisopropyl phenyl derivative (4b) is used as catalyst, butthis decrease is accompanied by a significant increase of theenantioselectivity (>90%) and inversion of stereochemistry.Hydrogenation of methylenesuccinates catalyzed by deriva-tives 3b (N-mesityl) leads to theS isomer, while the derivative

4b (N-2,6-diiosopropylphenyl) gives rise to the R isomer.This fact was also recently observed with NHC-phenolimineiridium complexes for the asymmetric transfer hydrogena-tion and suggests decisive alkyl-aryl interactions.19 Thesubstitution of the functional group in the substrate hasa substantial influence on the enantioselectivity. The mostenhanced selectivity was observed with bulkier diethyl2-benzylidenesuccinate.

In view of these results, we are studying the applicability ofthese complexes as catalysts in the hydrogenation of othersubstrates and other reactions such as C-C coupling(Suzuki, Sonogashira) and hydrosilylation.

Conclusions

In conclusion, a novel family of mixed carbene/pyridine/prolinamide ligands has been developed. The modular syn-thetic strategy gives facile access to several new imidazoliumsalts. The coordination behavior of the corresponding car-benes has been investigated by preparing gold, rhodium, andpalladium complexes as single diastereoisomers. These com-plexes were shown to be active and selective catalysts for theenantioselective hydrogenation of diethyl 2-benzylidenesuc-cinates with high reactivity in mild conditions. Enantioselec-tivity is very sensitive to the NHCN-substituent, resulting ina useful switch in the predominant enantiomer, which per-mits the preparation of both isomers. Studies of the immo-bilization of these pincer complexes on a support such assilica or mesoporous materials and their use as recoverablecatalyst are in progress.

Experimental Section

General Remarks. All preparations of metal complexes werecarried out under dinitrogen by conventional Schlenk-tubetechniques. Solvents were carefully degassed before use. C, H,and N analyses were carried out by the analytical department ofthe Instituto de Quımica Org�anica (CSIC) with a Lecco appa-ratus.Metal contents were analyzed by atomic absorption usinga Perkin-Elmer AAnalyst 300 atomic absorption apparatus andplasma ICP Perkin-Elmer OPTIMA 2100 DV. IR spectra wererecorded on a Bruker IFS 66v/S spectrophotometer (range4000-200 cm-1) inKBr pellets. 1HNMRand 13CNMRspectra

Scheme 3. Synthesis of Gold(III) Complexes, Route A Table 1. Catalytic Hydrogenation of Diethyl Itaconate

and Diethyl 2-Benzylidenesuccinate with Rh(I), Pd(II),and Au(III) Catalystsa

diethyl itaconate diethyl 2-benzylidene succinate

entry catalyst TOFb ee %c TOFb ee %d

1 3bAu 4470 10 (S) 1000 70 (S)2 3bRh 4962 <10 (S) 829 82 (S)3 3bPd 3372 <5 (S) 1064 41 (S)4 4bAu 3500 15 (R) 480 90 (R)5 4bRh 4000 18 (R) 500 99 (R)6 4bPd 2000 10 (R) 450 99 (R)7 2bAu 3200 8 (S) 580 80 (S)8 2bPd 2800 <5 (S) 565 15 (S)9 1bAu 4920 0 (S) 2040 15 (S)10 1bPd 3368 6 (S) 2316 12 (S)

aConditions: 4 atm, 40 �C, S/C ratio 1000:1. bTOF: h-1. cMeasuredby HPLC (λ: 230 nm, hexane/iPrOH: 98:2, column chiralcel AD-H).dMeasured by HPLC (λ: 254 nm, hexane/iPrOH: 95:5, column chiralcelOD).

(19) Dyson, G.; Frison, J.-C.; Whitwood, A. C.; Douthwaite, R. E.Dalton Trans. 2009, 7141–7151.

138 Organometallics, Vol. 29, No. 1, 2010 Boronat et al.

were recorded on Varian XR300 and Bruker 200 spectrometers.Chemical shifts are referenced to tetramethylsilane (internalstandard). Optical rotation values were measured at thesodium-D line (589 nm) with a Perkin-Elmer 241 MC polari-meter. Gas chromatography analysis was performed using aHewlett-Packard 5890. The enantiomeric excess was measuredby HPLC (Agilent 1200) using chiral column chiralcel OD(diethyl 2-benzylidene succinate (λ: 254 nm, hexane/iPrOH.95:5, 0.5 mL/min) and chiralcel AD-H (diethyl itaconate, λ:230 nm, hexane/iPrOH, 98:2, 0.4 mL/min.).Preparation of Precursors. 1-Mesityl-3-{[6-(amine-1-ylmethyl)-

pyridin-2-yl]methyl}-2,3-dihydro-1H-imidazol-3-ium Bromide

([3]Br). 3-{[6-(Bromomethyl)pyridin-2-yl]methyl}-1-mesityl-1H-imidazol-3-ium Bromide. A mixture of 2,6-bis(bromo-methyl)pyridine (6 mmol, 1.59 g) and 1-mesityl-1H-imidazole(4 mmol, 750 mg) in acetone was refluxed for 16 h. The crudeproduct was purified by chromatography on silica gel elutingwith acetone/ethanol (6:1) to yield a white solid (706 mg, 40%).Mp = 207.5-209.0 �C. Anal. Calcd for C19H21N3Br2 (451): C:50.4; H: 4.9; N: 9.3. Found: C: 50.3; H: 4.8; N: 9.4. IR (KBr,cm-1): ν 3158, 3086 (CHCdC); 1591, 1565, 1549 (CdC, CdN);1458 (C-C). 1HNMR(CDCl3, ppm):δ 10.20 (1H,NCHN); 8.06(1H, d, J=1.7Hz,Himi); 7.60 (2H,Hpy, d, J=7.57Hz, J=7.82Hz, J=0.74Hz); 7.71 (1H, t,Hpy); 7.10 (1H,d,J=1.7Hz,Himi);6.99 (2H, s, Hmes); 6.18 (2H, s, NimCH2C); 4.41 (2H, s, CH2Br);2.30 (3H, s, p-CH3); 2.08 (6H, s, o-CH3).

13C NMR (CDCl3,ppm): δ 156.69 (Cpy); 152.48 (Cpy); 141.19 (NCHN); 138.59(Cpy); 137.99 (Cmes); 134.22 (Cmes); 130.57 (Cmes); 129.69(Cmes); 123.96 (CHimi); 123.35 (Cpy); 123.11 (CHimi); 122.44(Cpy); 53.42 (NimCH2C); 33.27 (CH2Br); 20.97 (p-CH3); 17.55(o-CH3). EM (m/z, %): 370 (Mþ, 100).1-Mesityl-3-{[6-(pyrrolidin-1-ylmethyl)pyridin-2-yl]methyl}-1H-

imidazol-3-ium Bromide ([3a]Br). A mixture of 1-[(6-bromopyr-idin-2-yl)methyl]-3-mesityl-1H-imidazol-1-ium bromide (5 mmol,2.25 g), sodium carbonate (75 mmol, 7.9 g), and pyrrolidine(6 mmol, 0.5 mL) in acetone was stirred at room temperaturefor 12 h. The crude product was washed with diethyl ether toafford the product as a beige solid (1.65 g, 75%). Mp =148.0-151.0 �C. Anal. Calcd for C23H29N4Br (441): C: 62.6; H:6.6; N: 12.7. Found: C: 62.4; H: 6.5; N: 12.5. IR (KBr, cm-1):ν 3050 (CHarom.); 1700, 1663 (CdC, CdN). 1H NMR (CDCl3,ppm): δ 10.11 (1H, br s, NCHN); 8.10 (1H, d, J= 1.7 Hz, Himi);7.73 and 7.62 (2H, d, Hpy, J=7.66Hz, J=7.42Hz,); 7.31 (1H, t,Hpy); 7.10 (1H, d, J=1.7Hz,Himi); 6.92 (2H, s,Hmes); 5.99 (2H, s,NimCH2C); 3.73 (2H, s, CCH2Npyrr); 2.55 (2H, s, CH2pyrr); 2.25(3H, s, p-CH3); 2.06 (6H, s, o-CH3); 1.80 (2H, s, CH2pyrr).

13CNMR (CDCl3, ppm): δ 157.13 (Cpy); 149.98 (Cpy); 139.43(NCHN); 137.20 (Cpy); 136.11 (Cmes); 132.34 (Cmes); 132.11(Cmes); 127.94 (Cmes); 122.12 (Cimi); 121.43 (Cpy); 120.64 (Cimi);120.52 (Cpy); 59.77 (CCH2Npyrr); 52.02 (CH2pyrr); 51.44(NimCH2C); 21.65 (CH2pyrr); 19.18 (p-CH3); 15.71 (o-CH3). EM(m/z, %): 361 (Mþ - Br, 100).(S)-3-((6-((2-(tert-Butylcarbamoyl)pyrrolidin-1-yl)methyl)pyridin-

2-yl)methyl)-1-mesityl-1H-imidazol-3-ium Bromide ([3b]Br). A mix-ture of 1-[(6-bromopyridin-2-yl)methyl]-3-mesityl-1H-imidazol-1-ium bromide (5 mmol, 2.25 g), sodium carbonate (10 mmol,1.06g), and (S)-(tert-butyl)prolinamide (5mmol, 846mg) inacetonewas stirred at room temperature for 12 h. The crude product waswashed with diethyl ether to afford the product as a yellow solid(2.16 g, 80%). Mp = 121.0-126.0 �C. Anal. Calcd for C28H38-N5OBr (540.5): C: 62.2; H: 7.1; N: 13.0. Found: C: 62.4; H: 7.0; N:12.6. IR (KBr, cm-1): ν 3433 (CHarom.); 1666 (CdC, CdN).UV-vis (λ, nm): 261, 252. 1H NMR (CDCl3, ppm): δ 10.39 (1H,NCHN);8.12 (1H,d,JCHimi,Himi=1.74Himi); 7.87 (2H,d,J=7.35Hz, J=0.76 Hz, Hpy); 7.73 (1H, t, Hpy); 7.30 (1H, d, J=7.73Hz,Hpy); 7.08 (1H, br s,Himi); 6.99 (2H, s,Hmes); 6.10 (2H, d, J=14.79Hz, NimCH2C); 3.72 (2H, d, J=13.0Hz, CCH2Npro); 3.17 (1H, q,J = 3.6 Hz, CH2CHNpro); 3.13-3.09 (1H, m, CH2CH2Npro);2.51-2.40 (1H, m, CH2CH2Npro); 2.32 (3H, s, p-CH3); 2.19-2.00(1H, m, CH2CH2Npro); 1.98 (9H, s, o-CH3); 1.84-1.61 (3H, m,

CH2CH2Npro; CH2CH2CH); 1.11 (9H, s, C(CH3)3).13C NMR

(CDCl3, ppm): δ 174.35 (CdO); 158.65 (Cpy); 152.66 (Cpy); 141.71(Cmes); 138.69 (NCHN); 138.65 (Cpy); 134.61 (Cmes); 131.15 (Cmes);130.26 (Cmes); 124.74 (Cimi); 123.72 (Cpy); 123.12 (Cpy); 122.88(Cimi); 67.86 (CH2CHNpro); 61.44 (CCH2Npro); 55.56 (CH2-CH2Npro); 54.25 (NimCH2C); 50.31 (C(CH3)3); 31.30 (CH2-CH2Npro); 29.00 (C(CH3)3); 24.39 (CH2CH2CH); 21.48 (p-CH3);18.01 (o-CH3). EM (m/z, %): 460 (Mþ - Br, 100).

1-(2,6-Diisopropylphenyl)-3-{[6-(amine-1-ylmethyl)pyridin-2-yl]-methyl}-1H-imidazol-3-ium Bromide ([4]Br)}. 3-{[6-(Bromo-

methyl)pyridin-2-yl]methyl}-1-(2,6-diisopropylphenyl)-1H-imida-

zol-3-ium Bromide. A mixture of 2,6-bis(bromomethyl)pyridine(11.32 mmol, 3 g) and 1-(2,6-diisopropylphenyl-1H-imidazole(7.59 mmol, 1.73 g) in acetone was refluxed for 18 h. The crudeproduct was purified by chromatography on silica gel elutingwith acetone/ethanol (10:1) to yield a white solid (1.49 g, 40%).Mp = 200.5-202.0 �C. Anal. Calcd for C22H27N3Br2 (493): C:53.5; H: 5.7; N: 8.5. Found: C: 53.1; H: 5.2; N: 8.1. IR (KBr,cm-1): ν 3047 (CHCdC.); 1594, 1574, 1546 (CdC, CdN); 1458(C-C). 1H NMR (CDCl3, ppm): δ = 10.08 (1H, NCHN); 8.16(1H, Himi); 7.84 (1H, Hpy, d, J= 7.7 Hz), 7.71 (1H, t, Hpy, J=7.60, 7.7 Hz); 7.48 (1H, Harom, t, J = 7.8, 7.7 Hz), 7.35 (1H, d,Hpy, J = 7.5 Hz); 7.26-7.22 (2H, m, Harom,); 7.10 (1H, Himi);6.21 (2H, s, NimCH2C); 4.39 (2H, s, CH2Br); 2.34-2.21 (2H, m,CHiPr); 1.18, 1.15 (6H, d, J = 6.8 Hz, CH3); 1.09, 1.07 (6H, d,J = 6.8 Hz, CH3).

13C NMR (CDCl3, ppm): δ 156.30 (Cpy);152.10 (Cpy); 144.93 (NCHN); 138.27 (Cpy); 137.71 (Carom);131.45 (Carom); 129.63 (Carom); 124.20 (Carom); 123.64 (CHimi);123.03 (Cpy); 122.91 (CHimi); 53.07 (NimCH2C); 32.87 (CH2Br);28.13 (CHiPr); 23.84, 23.78 (CH3). EM (m/z, %): 413 (Mþ- Br,100).

1-(2,6-Diisopropylphenyl)-3-((6-(pyrrolidin-1-ylmethyl)pyridin-2-yl)methyl)-1H-imidazol-3-ium Bromide ([4a]Br). A mixture of3-{[6-(bromomethyl)pyridin-2-yl]methyl}-1-(2,6-diisopropyl-phenyl)-1H-imidazol-3-ium bromide (1 mmol, 493 mg), potas-sium carbonate (12 mmol, 1.66 g), and pyrrolidine (1 mmol,82 μL) in acetone was stirred at room temperature for 24 h. Thecrude product was washed with diethyl ether to afford theproduct as a beige solid (362 mg, 75%). Mp = 165-168 �C.Anal. Calcd for C26H35N4Br (483.5): C: 64.6; H: 7.3; N: 11.6.Found: C: 64.1; H: 7.0; N: 11.2. IR (KBr, cm-1): ν 3050(CHarom.); 1593.6, 1574.7, (CdC, CdN). 1H NMR (CDCl3,ppm): δ 10.20 (1H, NCHN); 8.16 (1H, Himi); 7.80 (1H, Hpy, d,J=7.8Hz), 7.70 (1H, t,Hpy, J=7.82, 7.57Hz); 7.48 (1H,Harom,t, J= 7.81, 7.81 Hz), 7.35 (1H, d, Hpy, J= 7.82 Hz); 7.26-7.20(2H, m, Harom,); 7.04 (1H, Himi); 6.14 (2H, s, NimCH2C); 3.66(2H, s, CH2pyrr); 2.48 (2H, s, CH2pyrr); 2.32-2.14 (2H,m,CHiPr);1.73 (2H, s, CH2pyrr), 1.16 (6H, d, J=6.84Hz,CH3); 1.07 (6H, d,J = 6.80 Hz, CH3).

13C NMR (CDCl3, ppm): δ 158.56 (Cpy);152.94 (Carom); 150.85 (Cpy); 144.36 (NCHN); 137.15(Carom);137.09 (Cpy); 130.90 (Carom); 129.16 (Carom); 123.66 (Carom);123.11 (CHimi); 122.44 (CHimi); 122.14 (Cpy); 121.37 (Cpy);60.97 (CCH2Npyrr); 53.29 (NCH2Pyrr); 52.92; (NimCH2C); 27.58(CHiPr); 23.36, 23.20 (CH2Pyrr); 23.00; 22.52 (CH3). EM(m/z,%):403 (Mþ - Br, 100).

(S)-3-((6-((2-(tert-Butylcarbamoyl)pyrrolidin-1-yl)methyl)-pyridin-2-yl)methyl)-1-(2,6-diisopropylphenyl)-1H-imidazol-3-iumBromide ([4b]Br). A mixture of 3-{[6-(bromomethyl)pyridin-2-yl]methyl}-1-(2,6-diisopropylphenyl)-1H-imidazol-3-ium bro-mide (1 mmol, 493 mg), potassium carbonate (12 mmol, 1.66g), and (S)-(tert-butyl)prolinamide (1 mmol, 169.2 mg) inacetone was stirred at room temperature for 24 h. The crudewas washed with diethyl ether to afford the product as beigesolid (460 mg, 80%). Mp = 110-113 �C. Anal. Calcd forC31H44N5BrN5O (582.6): C: 63.9; H: 7.6; N: 12.0. Found: C:63.5; H: 7.2; N: 11.5. IR (KBr, cm-1): ν 3050 (CHarom.); 1664.3(CdO); 1593.8, 1574.8, (CdC, CdN). 1H NMR (CDCl3, ppm):δ 10.42 (1H, NCHN); 8.26 (1H, Himi); 7.85 (1H, Hpy, d, J=7.8Hz), 7.74 (1H, t, Hpy, J=7.80, 7.41Hz); 7.51 (1H, Harom, t, J=7.80, 7.80Hz), 7.29 (1H, d,Hpy, J=7.41Hz); 7.24-7.23 (2H,m,

Article Organometallics, Vol. 29, No. 1, 2010 139

Harom,); 7.10 (1H, Himi); 6.35 (1H, d, NimCH2C, J = 15.82Hz); 6.03 (1H, d, NimCH2C, J = 14.82 Hz); 3.77 (1H, d,CCH2Npro, J=13.87 Hz); 3.65 (1H, d, CCH2Npro, J=12.87Hz); 3.48-3.42 (1H, m, CH2CHNpro); 3.19-3.12 (2H,m, CHiPr); 2.48-2.42 (1H, m, CH2CH2Npro); 2.36-2.27(1H, m, CH2CH2Npro); 2.16-2.05 (1H, m, CH2CH2Npro);1.80-1.66 (3H, m, CH2CH2Npro, CH2CH2CH); 1.25-1.20 (6H, m, CH3); 1.13-1.11 (6H, CH3tBu); 1.09 (9H, s,CH3).

13C NMR (CDCl3, ppm): δ 174.33 (CdO); 158.54(Cpy); 152.85 (Cpy); 145.80 (NCHN); 138.72, (Carom); 137.09(Cpy); 132.27 (Carom); 130.73 (Carom); 125.09 (Carom); 124.87(Carom); 123.82 (CHimi); 123.67 (CHimi); 123.25 (Cpy); 122.81(Cpy); 69.13 (CH2CHNpro); 61.55 (CCH2Npro); 55.71(CH2CH2Npro); 54.03(NimCH2C); 50.30 (C(CH3)3); 31.44(CH2CH2Npro); 29.05 (C(CH3)3); 28.97 (CHiPr); 24.88 (CH2-CH2CH); 24.76, 24.72, 24.68, 24.41 (CH3). EM (m/z, %): 503(Mþ - Br, 100).Synthesis of Palladium(II) and Rhodium(I) Complexes. The

complexes were synthesized following the general method:a solution of ([3a]Br) (2 mmol, 884 mg) or ([3b]Br) (2 mmol,1.08 g) and Ag2O (1 mmol, 231 mg) in dichlorometane wasstirred at room temperature under a N2 atmosphere. Themixturewas filtered throughCelite in order to remove unreactedAg2O and other insoluble solids. PdCl2(CH3CN)2 (1 mmol,259 mg) or [RhCl(cod)]2 (0.5 mmol, 246 mg) was added to thesolution of the resulting silver salt in CH2Cl2. After stirringovernight at room temperature, the solution was filteredthrough Celite. The solvents were removed in vacuo, and theresidue was washed several times with diethyl ether. Cationiccomplexes with PF6 as counterion were generated by halideabstraction via addition of AgPF6 in a CH2Cl2/water solventsystem.[Pd(3a)Cl]Cl (3aPd): orange-brown. Yield: 75%. Mp:

170-175 �C. ΛM = 100Ω-1 cm2 mol-1 (acetone). Anal. Calcdfor C23H28N4Cl2Pd (537.8): C: 51.3; H: 5.2; N: 10.4; Pd: 19.8.Found C: 51.7; H: 5.6; N: 10.1; Pd: 20.6. IR (KBr, cm-1): ν 3423(CHarom.); 1613 (CdC, CdN). UV-vis (λ, nm): 442, 272, 259.1H NMR (CDCl3, ppm): δ 8.47 (1H, d, J = 9.3 Hz, Hpy); 8.22(1H, d, J=1Hz, Himi); 8.11 (1H, t, J=8.5 Hz, Hpy); 7.86 (1H,d, J = 8.5 Hz, Hpy); 6.92 (2H, s, Hmes); 6.79 (1H, d, J = 1 Hz,Himi); 6.19 (2H, s, CCH2Nim); 4.33 (2H, s, CCH2Npyrr); 3.50(2H, br s, CH2CH2Npyrr); 2.72 (2H, br s, CH2CH2Npyrr); 2.32(3H, s, p-CH3); 2.10 (6H, s, o-CH3); 2.05 (2H, br s,CH2CH2Npyrr); 1.86 (2H, br s, CH2CH2Npyrr).

13C NMR(CDCl3, ppm): δ 160.25 (C-Pd); 152.76 (Cpy); 149.55 (Cpy);141.90 (Cpy); 139.14 (Cmes); 135.90 (Cmes); 129.78 (Cmes); 126.22(Cpy); 124.46 (Cimi); 124.08 (Cpy); 123.31 (Cimi); 66.74(CCH2Npyrr); 58.61 (CH2CH2Npyrr); 54.25 (CCH2Nim); 22.91(CH2CH2Npyrr); 20.92 (p-CH3); 18.53 (o-CH3). EM (m/z): 503(Mþ - Cl, 106Pd).[Rh(3a)(cod)]Cl (3aRh): yellow. Yield: 88%. Mp: 135-

140 �C. ΛM = 91 Ω-1cm2 mol-1 (Acetone). Anal. Calcd forC31H40N4ClRh (607): C: 61.3; H: 6.6; N: 9.2; Rh: 17.0. Found:C: 61.3; H: 6.7; N: 9.6; Rh: 17.6. IR (KBr, cm-1): ν 3445(CHarom.); 1612, 1549 (CdC, CdN); 567 (Rh-C). UV-vis (λ,nm): 394, 350, 268, 258. 1H NMR (CDCl3, ppm): δ 7.69 (1H, t,J=6.8Hz,Hpy); 7.47 (1H, d, J=8.6Hz,Hpy); 7.36 (1H, d, J=8.6Hz,Hpy); 7.10 (1H, br s,Himi); 6.91 (2H, br s,Hmes); 6.75 (1H,d, J = 1.5 Hz, H0

imi); 6.41 and 5.89 (2H, d, J = 15.4 Hz,CCH2Nim); 4.95-4.80 (2H, m, CHcod); 4.05-3.94 (1H, m,CCH2Npyrr); 3.66-3.54 (1H, m, CCH2Npyrr); 3.43-3.32 (2H,m, CHcod); 2.57-2.52 (2H, m, CH2CH2Npyrr); 2.46 (3H, s,p-CH3); 2.10-2.00 (8H, m, CH2(cod)); 1.97-1.96 (1H, m,CH2CH2Npyrr); 1.80-1.74 (4H, m, CH2CH2Npyrr, CH2CH2-Npyrr); 1.59 (6H, s, o-CH3).

13C NMR (CDCl3, ppm): δ 173.54(C-Rh); 154.90 (Cpy); 153.84 (Cpy); 138.50 (Cmes); 137.25 (Cmes);137.17 (Cpy); 134.12 (Cmes); 128.12 (Cmes); 123.06 (Cimi); 122.05(Cpy); 121.72 (Cpy); 121.16 (Cimi); 97.34 (CHcod); 68.93 (CHcod);67.53 (CH2CH2Npyrr); 62.18 (CCH2Npyrr); 56.58 (CCH2Nim);54.32 (CH2CH2Npyrr); 33.78 (CH2(cod)); 29.09 (CH2(cod)); 23.56

(CH2CH2Npyrr); 21.07 (CH2CH2Npyrr); 19.75 (p-CH3); 17.76(o-CH3). EM (m/z): 571 (Mþ - Cl).

[Pd(4a)Cl]PF6 (4aPd): yellow. Yield: 95%. Mp: 140-143 �C.ΛM = 128 Ω-1cm2 mol-1 (acetone). Anal. Calcd forC26H34N4ClF6PPd (689.4): C: 45.2; H: 5.1; N: 8.1; Pd: 15.4.Found: C: 44.9; H: 4.7; N: 7.6; Pd: 15.0. IR (KBr, cm-1): ν 3445(CHarom.); 1622, 1569 (CdC, CdN); 840 (P-F); 570 (Rh-C).1H NMR (CDCl3, ppm): δ 8.05 (1H, t, J= 6.70 Hz, Hpy); 7.85(1H, d, J=8.5Hz,Hpy); 7.60 (1H, d, J=8.5Hz,Hpy); 7.35 (1H,br, Harom); 7.10 (3H, br, Harom, Himi); 6.80 (1H, br, Himi); 5.60(2H, d, J = 14.8 Hz, CCH2Nim); 4.05 (2H, m, CCH2Npyrr);3.50-3.35 (2H, m, CH2CH2Npyrr); 2.60-2.45 (2H, m,CH2CH2Npyrr); 2.35-2.30 (2H, m, CHiPr); 1.95-1.93 (1H, m,CH2CH2Npyrr); 1.80-1.75 (3H, m, CH2CH2Npyrr, CH2CH2-CH); 1.40 (6H, d, CH3); 1.10 (6H, d, CH3).

13C NMR (CDCl3,ppm): δ 160.85 (C-Pd); 152.80 (Cpy); 149.50 (Cpy); 141.90 (Cpy);138.50 (Carom); 137.21 (Carom); 137.20 (Cpy); 134.10 (Carom);127.96 (Carom); 123.10 (Cimi); 122.00 (Cpy); 120.98 (Cimi); 67.24(CH2CH2Npyrr); 61.98 (CCH2Npyrr); 56.30 (CCH2Nim); 53.82(CH2CH2Npyrr); 27.80 (CHiPr); 23.56 (CH2CH2Npyrr); 21.07(CH2CH2Npyrr); 19.60 (CH3); 17.60 (CH3). EM (m/z): 567([Mþ þ Na] - PF6); 545 (Mþ - PF6).

[Rh(4a)(cod)]PF6 (4aRh): yellow. Yield: 89%. Mp: 170-172 �C. ΛM = 58 Ω-1 cm2 mol-1 (acetone). Anal. Calcd forC34H46N4F6PRh (758.6): C: 53.9; H: 6.0; N: 7.4; Rh: 13.6.Found: C: 53.4; H: 5.6; N: 7.0; Rh: 13.1. IR (KBr, cm-1): ν3143 (CHarom.); 1623, 1564 (CdC, CdN); 844 (P-F); 559(Rh-C). 1H NMR (CDCl3, ppm): δ 7.90 (1H, t, J = 6.7 Hz,Hpy); 7.7 (1H, d, J=8.5Hz,Hpy); 7.39 (1H, d, J=8.5Hz,Hpy);7.30 (1H, br s, Harom); 7.20 (2H, br s, Harom, Himi); 5.65 (2H, d,J=15.4 Hz, CCH2Nim); 4.90-4.79 (2H, m, CHcod); 4.50-4.40(1H, m, CCH2Npyrr); 3.83-3.74 (1H, m, CCH2Npyrr);3.40-3.30 (2H, m, CHcod); 2.6-2.50 (2H, m, CH2CH2Npyrr);2.10-2.00 (8H,m,CH2(cod)); 1.98-1.96 (1H,m,CH2CH2Npyrr);1.80-1.70 (3H, m, CH2CH2Npyrr, CH2CH2CH); 1.20 (6H, d,CH3); 1.10 (6H, d, CH3).

13C NMR (CDCl3, ppm): δ 171.34 (C-Rh); 156.93 (Cpy); 152.94 (Carom); 150.84 (Cpy); 137.50 (Carom);137.25 (Carom); 130.87 (Carom); 129.12 (Carom); 123.70 (Carom);123.06 (Cimi); 122.05 (Cpy); 121.72 (Cpy); 121.09 (Cimi); 96.84(CHCOD); 68.63 (CHcod); 66.53 (CH2CH2Npyrr); 61.98(CCH2Npyrr); 56.43 (CCH2Nim); 54.32 (CH2CH2Npyrr); 33.48(CH2(cod)); 29.29 (CH2(COD)); 27.58 (CHiPr); 23.56 (CH2CH2-Npyrr); 21.07 (CH2CH2Npyrr); 23.00; 22.52 (CH3). EM (m/z): 613(Mþ - PF6).

[Pd(3b)Cl]Cl (3bPd): dark orange-brown. Yield: 95%. Mp:195-199 �C. [R]Hg

435 þ10.9 (c 0.1, EtOH). C28H37N5Cl2OPd(637): C: 52.8; H: 5.9; N: 11.0; Pd: 16.7. Found: C: 52.6; H: 5.8;N: 11.1; Pd: 16.5. IR (KBr, cm-1): ν 3436 (CHarom.); 1676(CdO), 1593 (CdC, CdN); 567 (Rh-C). UV-vis (λ, nm): 541,423, 323, 272, 248. 1HNMR (CDCl3, ppm): δ 8.56-8.46 (1H,m,Hpy); 8.41-8.30 (1H, m, H0

imi); 8.15-8.06 (1H, m, Hpy);7.95-7.86 (1H, m, Himi); 7.53 (1H, d, J = 8.8 Hz, Hpy); 6.94(2H, s, Hmes); 5.67 (2H, s, CCH2Nim); 4.37 (1H, d, J=16.9 Hz,CCH2Npro); 4.26-4.20 (1H, m, CCH2Npro); 2.70-2.60 (2H, m,CH2CHNpro, CH2CH2Npro); 2.43-2.35 (1H,m, CH2CH2Npro);2.32 (3H, s, p-CH3); 2.17-2.13 (1H,m,CH2CH2Npro); 2.02 (6H,s, o-CH3); 1.86-1.78 (3H, m, CH2CH2Npro); 1.24 (9H, s, C-(CH3)3).

13C NMR (CDCl3, ppm): δ 169.23 (C-Pd); 167.62(CdO); 159.70 (Cpy); 153.23 (Cpy); 148.52 (Cmes); 141.84 (Cpy);138.98 (Cmes); 134.50 (Cmes); 129.08 (Cmes); 126.42 (H0

imi);124.70 (Cpy); 124.11 (Cpy); 122.55 (Himi); 67.09 (CH2CHNpro);60.89 (CCH2Npro); 58.42 (CH2CH2Npro); 54.98 (CCH2Nim);51.53 (C(CH3)3); 28.58 (C(CH3)3); 25.58 (CH2CH2Npro); 21.54(CH2CH2CH); 21.12 (p-CH3); 18.57 (o-CH3). EM (m/z): 598(Mþ - Cl, 102Pd).

[Rh(3b)(cod)]Cl (3bRh): yellow. Yield: 90%. Mp: 117-120 �C. [R]D25 -1.9 (c 1, EtOH). Anal. Calcd for C36-H49N5ClORh (706.2): C: 61.2; H: 7.0; N: 9.9; Rh: 14.6. Found:C: 61.5; H: 7.5; N: 9.5; Rh: 14.8. IR (KBr, cm-1): ν 3459(CHarom.); 1675 (CdO), 1595 (CdC, CdN); 579 (Rh-C).

140 Organometallics, Vol. 29, No. 1, 2010 Boronat et al.

UV-vis (λ, nm): 394, 347, 270, 257. 1H NMR (CDCl3, ppm): δ7.71 (1H, t, J=7.1Hz, Hpy); 7.50 (1H, d, J=7.4Hz, Hpy); 7.23(1H, d, J=7.1 Hz, Hpy); 7.14-7.10 (1H, m, Himi); 6.96 (2H, br.s, Hmes); 6.76 (1H, d, J = 3.2 Hz, H0

imi); 6.41 and 5.91 (2H, d,J = 9.9 Hz, CCH2Nim); 4.85-4.80 (2H, m, CHcod); 3.96 and3.62 (2H, d, J = 12.8 Hz, CCH2Npro); 3.22-3.15 (2H, m,CHcod); 3.08-3.04 (1H, m, CH2CH2Npro); 3.03-2.98 (1H, m,CH2CHNpro); 2.66-2.58 (1H, m, CH2CH2Npro); 2.37 (3H, s, p-CH3); 2.26-2.13 (1H, m, CH2CH2Npro); 2.06-1.95 (8H, m,CH2cod); 1.85 (6H, s, o-CH3); 1.53-1.37 (3H, m, CH2CH2Npro;CH2CH2CH); 1.28 (9H, s, C(CH3)3).

13C NMR (CDCl3, ppm):δ 174.05 (C-Rh); 158.98 (CdO); 158.85 (Cpy); 157.02 (Cpy);139.04 (Cmes); 137.85 (Cpy); 137.52 (Cmes); 134.65 (Cmes); 128.56(Cmes); 123.64 (Cimi); 122.23 (Cpy); 121.98 (Cpy); 121.92 (Cimi);97.68 (CHcod); 68.04 (CHcod); 67.29 (CH2CHNpro); 61.10(CCH2Npro); 56.60 (CCH2Nim); 54.10 (CH2CH2Npro); 50.12(C(CH3)3); 33.83 (CH2(cod)); 29.00 (C(CH3)3); 28.37 (CH2(cod));24.51 (CH2CH2Npro); 21.48 (CH2CH2CH); 20.15 (p-CH3);18.19 (o-CH3). EM (m/z): 671 (Mþ - Cl).[Pd(4b)Cl]PF6 (4bPd): yellow. Yield: 70%. Mp: 160-163 �C.

Anal. Calcd for C31H43N5ClF6OPPd (788.5): C: 47.2; H: 5.6; N:8.9; Pd, 13.5. Found: C: 47.2; H: 5.5; N: 8.5; Pd: 13.8. IR (KBr,cm-1): ν 3156 (CHarom.); 1672 (CdO); 1613 (CdC, CdN); 845(P-F); 560. 1H NMR (CDCl3, ppm): δ 8.09 (1H, t, J= 8.0 Hz,Hpy); 7.90 (1H, d, J= 7.5 Hz, Hpy); 7.81-7.79 (1H, m, Harom);7.52-7.50 (1H, m, Himi); 7.49-7.47 (3H, m, Hpy, Harom);7.20-7.18 (1H, m, Harom); 6.89 (1H, d, J = 2.1 Hz, Himi);5.83, 5.79 (1H, d, J = 15.93 Hz, NimCH2C); 5.46, 5.42 (1H, d,J = 15.38 Hz, NimCH2C); 4.51, 4.48 (1H, d, J = 15.57 Hz,CCH2Npro); 4.40, 4.36 (1H, d, J = 15.56 Hz, CCH2Npro);3.20-3.16 (1H, m, CH2CHNpro); 3.11-3.06 (2H, m, CHiPr);2.79-2.76 (1H, m, CH2CH2Npro); 2.54-2.20 (1H, m,CH2CH2Npro); 2.1-1.90 (2H, m, CH2CH2Npro); 1.53-1.37(2H, m, CH2CH2CH); 1.24, 1.14, 1.3 (12H, m, CH3); 0.96(9H, s, C(CH3)3).

13C NMR (CDCl3, ppm): δ = 175.63(C-Pd); 168.72 (CdO); 159.90 (Cpy); 153.01 (Cpy); 150.91(Carom); 141.47 (Cpy); 134.48 (Carom); 130.27 (Carom); 125.73(Carom); 125.55 (Carom); 124.36 (Carom); 123.82 (Cimi); 123.67(Cpy); 123.04 (Cpy); 122.18 (Cimi); 67.40 (CH2CHNpro); 61.48(CCH2Npro); 58.88 (CCH2Nim); 54.45 (CH2CH2Npro); 51.67(C(CH3)3); 29.26 (CHiPr); 28.76 (CH2CH2Npro); 28.57(CH2CH2CH); 28.28 (C(CH3)3); 28.19 (CH3). EM (m/z): 644(Mþ - PF6); 502 (4b).[Rh(4b)(cod)]PF6 (4bRh): yellow. Yield: 65%. Mp: 120-

123 �C. Anal. Calcd for C39H55F6N5OPRh (857.8): C: 54.7; H:6.4; N: 8.2; Rh: 12.0. Found: C: 54.3; H: 6.1; N: 7.8; Rh: 11.6. IR(KBr, cm-1): ν 3157 (CHarom.); 1657 (CdO); 1607 (CdC,CdN); 850 (P-F). 1H NMR (CDCl3, ppm): δ 8.20 (1H, Himi);7.94 (Hpy), 7.75 (Hpy); 7.55 (Harom), 7.35 (Hpy, Harom,); 7.03 (1H,Himi); 6.71 (1H, d,NimCH2C); 5.94 (1H, d,NimCH2C); 4.88 (2H,m, CHcod); 3.85 (1H, d, CCH2Npro); 3.46 (1H, d, CCH2Npro);3.27-3.24 (1H, m, CH2CHNpro); 2.90-2.80 (2H, m, CHcod);2.68-2.62 (2H, m, CHiPr); 2.49-2.37 (1H, m, CH2CH2Npro);2.15-2.05 (1H, m, CH2CH2Npro); 2.03-1.87 (8H, m, CH2cod);1.70-1.62 (3H,m,CH2CH2Npro, CH2CH2CH); 1.58-1.47 (6H,CH3); 1.34-1.26 (6H, CH3); 1.18 (9H, s, CH3).

13C NMR(CDCl3, ppm): δ 181.28 (C-Rh); 159.61 (CdO); 157.41 (Cpy);153.01 (Cpy); 145.16 (Carom); 140.14 (Cpy); 139.58 (Carom);130.49 (Carom); 125.98 (Carom); 124.68 (Carom); 124.00 (CHimi);123.55 (CHimi); 123.21 (Cpy); 122.65 (Cpy); 97.88 (CHcod); 68.71(CHcod); 68.37 (CH2CHNpro); 60.69 (CCH2Npro); 58.21(CH2CH2Npro); 55.33 (NimCH2C); 52.96 (C(CH3)3); 34.06(CH2(cod)); 31.30 (CH2CH2Npro); 28.98 (CHiPr); 28.64(C(CH3)3); 28.49 (CH2(cod)); 27.85 (CH3); 26.44 (CH2CH2-CH). EM (m/z): 712 (Mþ - PF6); 502 (4b).Synthesis of Gold Complexes

(a) A solution of [3a]Br (1 mmol, 442 mg) or [3b]Br (1 mmol,504 mg) and KAuCl4 (1 mmol, 378 mg) in ethanol was stirred atreflux temperature under N2 atmosphere. After stirring over-night, the solution was filtered, the solvents were removed in

vacuo, and the residue was washed several times with diethylether. A solution of KHMDS in toluene (0.5M, 19.6 μL,1.02 mmol) was added to a Au(III) complex in dry tetrahydro-furane (5 mL). The resulting suspension was stirred for 2 h. Themixture was filtered and evaporated. Since the desired complexdissolved in CH2Cl2, ethyl ether was used to precipitate. Thecomplex was filtered off as a green powder in 70% yield.

(b) The milder conditions of the transmetalation pathwaymake it an attractive choice also for the synthesis of gold(III)complexes. A solution of ligand (2 mmol) and Ag2O (1 mmol,231 mg) in dichloromethane was stirred at room temperatureunder a N2 atmosphere. The mixture was filtered through Celitein order to remove unreacted Ag2O and other insoluble solids.An ethanolic solution of KAuCl4 (1 mmol) was added to thesolution of the silver salt in CH2Cl2. After stirring overnight atroom temperature, the solution was filtered through Celite. Thesolvents were removed in vacuo, and the residue was washedseveral times with diethyl ether.

[Au(3a)Cl][Cl]2 (3aAu): green. Yield: 98%. Anal. Calcd forC23H28N4AuCl3 (663.8): C: 41.6; H: 4.2; N: 8.4; Au: 29.7.Found: C: 41.3; H: 4.2; N: 8.3; Au: 29.3. IR (KBr, cm-1):ν 3435 (CHarom.); 1640, 1600 (CdC, CdN). 1H NMR (CDCl3,ppm): δ 7.90 (1H, t, J= 17.2 Hz, H6); 7.59-7.46 (2H, m, Himi,Hpy); 7.20-7.18 (1H,m,Hpy); 6.97 (2H, s,Hmes); 6.90-6.87 (3H,m, Himi, CCH2Nim); 3.62 (2H, s, CCH2Npyrr); 2.38 (3H, s,p-CH3); 2.25 (4H, br s, CH2CH2Npyrr); 2.13 (6H, s, o-CH3);1.95 (4H, br s, CH2CH2Npyrr).

13C NMR (CDCl3, ppm): δ192.18 (C-Au); 151.30 (Cpy); 150.46 (Cpy); 139.57 (Cmes);134.6743 (Cpy); 130.42 (Cmes); 130.28 (Cmes); 128.33 (Cmes);125.08 (Cpy); 124.96 (Cimi); 124.68 (Cimi); 122.59 (Cpy); 60.93(CCH2Npyrr); 55.16 (CCH2Nim); 54.39 (CH2CH2Npyrr); 45.35(CH2CH2Npyrr); 24.73 (CH2CH2Npyrr); 23.94 (CH2CH2-Npyrr); 21.47 (p-CH3); 18.13 (o-CH3). EM (m/z): 664 (Mþ),629 (Mþ - Cl).

[Au(3b)Cl][Cl]2 (3bAu): orange. Yield: 85%. Anal. Calcd forC28H37N5AuCl3O (763): C: 44.1; H: 4.9; N: 9.2; Au: 25.8.Found: C: 43.5; H: 4.5; N: 8.7; Au: 25.2. IR (KBr, cm-1): ν3427 (CHarom.); 1669 (CdO); 1610 (CdC, CdN). 1H NMR(CDCl3, ppm): δ 7.98-7.96 (1H, m, H0

imi); 7.90-7.86 (1H, m,Hpy); 7.75-7.56 (1H, m, Hpy); 7.40-7.32 (1H, m, Hpy);7.15-6.99 (1H, m, Himi); 6.97 (2H, s, Hmes); 5.79 (2H, s,CCH2Nim); 3.73-3.71 (2H, m, CCH2Npro); 3.45-3.42 (3H, m,CH2CHNpro; CH2CH2Npro); 2.35 (3H, s, p-CH3); 2.12 (6H, s, o-CH3); 1.95-1.78 (4H, m, CH2CH2Npro; CH2CH2CH); 1.29(9H, s, C(CH3)3).

13C NMR (CDCl3, ppm): δ 191.29 (C-Au);177.50 (CdO); 159.30 (Cpy); 152.15 (Cpy); 145.60 (Cmes); 138.50(Cpy); 136.37 (Cmes); 135.72 (Cmes); 131.62 (Cmes); 123.32 (Cimi);119.36 (Cpy); 117.43 (Cpy); 115.09 (Cimi); 68.36 (CH2CHNpro);61.30 (CCH2Npro); 57.99 (CH2CH2Npro); 54.70 (CCH2Nim);53.60 (C(CH3)3); 29.28 (CH2CH2Npro); 29.15 (C(CH3)3); 26.01(CH2CH2CH); 21.56 (p-CH3); 19.05 (o-CH3). EM (m/z): 692(Mþ - 2Cl).

[Au(4a)Cl][PF6]2 (4aAu): yellow. Yield: 60%. Mp: 110-112 �C. Anal. Calcd for C26H34N4AuClF12P2 (924.9): C: 33.7;H: 3.8; N: 6.1; Au: 21.3. Found: C: 33.4; H: 3.4; N: 5.6; Au: 20.8.IR (KBr, cm-1): ν 3135 (CHarom.); 1635, 1600 (CdC, CdN). 1HNMR (CDCl3, ppm): δ 7.90 (1H, t, J= 6.6 Hz, Hpy); 7.80 (1H,Hpy); 7.70 (1H, Hpy); 7.40 (2H, br, Harom); 7.3-7.10 (2H, br,Harom, Himi); 6.90 (1H, br, Himi); 5.9-5.7 (2H, br, CCH2Nim);4.70-4.50 (1H, m, CCH2Npyrr); 3.90-3.60 (1H, m,CCH2Npyrr); 3.3-3.1 (2H, m, CH2CH2Npyrr); 2.40-2.20(2H, m, CHiPr) 1.95-1.90 (6H, m, CH2CH2Npyrr, CH2CH2-Npyrr, CH2CH2Npyrr); 1.30-1.10 (12H, d, CH3). EM (m/z): 635(Mþ - 2Cl).

[Au(4b)Cl][PF6]2 (4bAu): yellow. Yield: 73%. Mp: 108-110 �C. Anal. Calcd for C31H43N5AuClF12OP2 (1024): C:36.3; H: 4.3; N: 6.8; Au: 19.2. Found: C: 36.7; H: 4.4; N: 7.1;Au: 19.8. IR (KBr, cm-1): ν 3127 (CHarom.); 1680 (CdO); 1599(CdC, CdN); 839 (P-F). 1H NMR (CDCl3, ppm): δ 7.95 (1H,

Article Organometallics, Vol. 29, No. 1, 2010 141

Himi); 7.83 (1H, Hpy,), 7.69(H, Hpy); 7.57-7.44 (1H, Harom),7.36-7.28 (H, Hpy); 7.25-7.16 (2H, m, Harom,); 6.99 (1H, Himi);6.00, 5.97 (2H, NimCH2C); 4.89-4.64 (1H, CCH2Npro);4.61-4.41 (1H, CCH2Npro); 4.25-3.91 (1H, m, NCHproCO);3.41-3.25 (2H, m, CHiPr); 2.80-2.44 (2H, m, NCH2CH2);2.42-1.94 (2H, m, CH2CH2CH); 1.27-1.09 (21H, CH3).

13CNMR (CDCl3, ppm): δ 197.62 (C-Au); 167.11 (CdO); 152.80(Cpy); 149. 92; 146.04; 145.44; 139.14, 126.66; 124.81; 124.68;124.35 (CHimi); 122.08 (CHimi); 68.23 (CH2CHNpro); 59.64(CCH2Npro); 57.26 (CH2CH2Npro); 52.23 (NimCH2C); 51.04(C(CH3)3); 31.01 (CH2CH2Npro); 28.62 (C(CH3)3); 28.44

(CHiPr); 26.62 (CH2CH2CH); 24.30, 24.23, 24.11, 22.69 (CH3).EM (m/z): 734 (Mþ - 2PF6), 698 (Mþ - 2PF6 - Cl), 502 (4b).

Computational Details. Calculations were carried out bymeans of the program package Gaussian0320 at density func-tional theory (DFT) level using the hybrid B3PW91 function.21

The standard 6-31G(d,p) basis set22 was used for N, O, C, Cl,and H atoms, and Pd is described by means of the LANL2DZ23

pseudopotential and its associated basis set for the valenceelectrons.

Catalytic Activity. Hydrogenation of Alkenes. The catalyticproperties, in hydrogenation reactions, of the complexeswere examined under conventional conditions for batch reac-tions in a reactor (Autoclave Engineers) of 100 mL capacity at40 �C temperature, 4 atm dihydrogen pressure, and 1:1000metal/substrate molar ratio. The evolution of the reactionof hydrogenated product was monitored by gas chromato-graphy.

Acknowledgment. We thank the Direcci�on General deInvestigaci�on Cientıfica y T�ecnica of Spain (ProjectMAT2006-14274-C02-02) for financial support. Theauthors thank J. A. Esteban for lab assistance.

(20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;Barone, V.;Mennucci, B.; Cossi,M.; Scalmani, G.; Rega, N.; Petersson,G. A.; Nakatsuji, H.; Hada, M.; Ehara, M., Toyota, K., Fukuda, R.;Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,H.;Klene,M.; Li,X.;Knox, J. E.;Hratchian,H. P.; Cross, J. B.; Bakken,V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.W.; Ayala, P. Y.;Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakr-zewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.;Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;Ortiz, J. V.; Cui,Q.; Baboul,A.G.;Clifford, S.; Cioslowski, J.; Stefanov,B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;Challacombe,M.; Gill, P.M.W.; Johnson, B.; Chen,W.;Wong,M.W.;Gonz�alez, C.; Pople, J. A.Gaussian 03, Revision C.02; Gaussian, Inc.:Wallingford, CT, 2004.

(21) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Perdew, J. P.;Wang, Y. Phys. Rev. B 1992, 45, 13244.

(22) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.(23) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.