Note pubs.acs.org/Organometallics
Inhibited Catalyst Activation in (N-Heterocyclic carbene)PdCl2(diethylamine) Complexes by Intramolecular Hydrogen Bonding Ming-Tsz Chen,† David A. Vicic,†,§ William J. Chain,† Michael L. Turner,‡ and Oscar Navarro†,* †
Department of Chemistry, University of Hawaii at Manoa, 2545 McCarthy Mall, Honolulu, Hawaii 96825, United States Organic Materials Innovation Centre, School of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.
‡
S Supporting Information *
ABSTRACT: The synthesis and characterization of two new (N-heterocyclic carbene)PdCl 2(diethylamine) complexes is reported. When compared to the more σ-donating triethylamine ligand, diethylamine is more tightly bound to Pd due to a rare intramolecular hydrogen bonding interaction with an adjacent chloride. The effect of this stronger coordination in the activation and in the activity of these complexes as precatalysts in the Suzuki−Miyaura cross-coupling reaction is discussed.
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Pd and the consequent weakening of the N−H bond was confirmed by infrared spectroscopy: the ν(N−H) for DEA7 is 3294 cm−1, whereas it was found at 3227 cm−1 for 3a and at 3221 cm−1 for 3b. The complexes were fully characterized by elemental analysis and 1H and 13C NMR spectroscopy. Interestingly, DEA-bearing complexes displayed an unexpected downfield shift in the 13C{1H} NMR of the carbene carbon signal when compared to the TEA-bearing counterparts, implying a stronger σ-donation (Table 1).
INTRODUCTION Our group recently reported on the synthesis of (NHC)PdCl2(TEA) complexes1 1 (NHC = N-heterocyclic carbene, TEA = triethylamine)2 and their use as precatalysts for Suzuki− Miyaura3 and Buchwald−Hartwig4 cross-coupling reactions.5 At low temperature, these complexes exhibit higher activity than the corresponding 3-Cl-pyridine-bearing counterparts ((NHC)Pd(PEPPSI), 2)6 due to an easier departure of the “throw-away” ligand TEA during the activation process (transition from the stable Pd(II) complex to the active [(NHC)-Pd(0)] species) and a higher tendency of TEA to recoordinate to [(NHC)-Pd(0)], conserving the active species in solution. To obtain more information on these two phenomena and to examine which factor could have a higher impact on the activity of the catalyst, we decided to substitute the TEA ligand for a weaker σ-donor, DEA (DEA = diethylamine), in order to enhance the departure of the “throw-away” ligand, potentially leading to a much faster activation. At the same time though, the use of a weaker σdonor should be detrimental to the lifetime of the catalyst, due to a lesser tendency to recoordinate to the active [(NHC)Pd(0)] species. (NHC)PdCl 2(DEA) complexes 3 were prepared by following a similar procedure to that for the synthesis of (NHC)PdCl2(TEA) complexes, combining the corresponding [(NHC)PdCl2]2 dimer with an excess of DEA in CH2Cl2 at room temperature. After 1 h, removal of the solvent afforded the desired complexes as pale yellow solids, which were then washed with pentane. The coordination of DEA to © 2011 American Chemical Society
Table 1. Comparison of δCcarbene and Relevant Bond Distances of 1a, 1b, 3a, and 3b complex
δCcarbene (ppm)
Pd−Ccarbene (Å)
Pd−N(3) (Å)
1a 3a 1b 3b
154.2 159.5 185.9 190.0
1.968(4) 2.1182(16) 1.970(3) 2.106(2)
2.205(4) 1.9869(17) 2.219(2) 1.965(3)
The coordination of the DEA ligand was examined by singlecrystal X-ray diffraction of 3a and 3b. As expected, both complexes show a slightly distorted square-planar geometry, with the chlorides perpendicular to the plane of the NHC and DEA trans to it. The stronger σ-donation and larger trans influence of the DEA ligand were confirmed by the observation Received: September 2, 2011 Published: November 29, 2011 6770
dx.doi.org/10.1021/om200828a | Organometallics 2011, 30, 6770−6773
Organometallics
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Figure 1. (NHC)PdCl2 complexes with different “throw-away” ligands.
that the Pd−Ccarbene distances for the DEA complexes 3a and 3b are considerably longer than those for the TEA complexes (1a and 1b). The Pd−N(3) distances are also considerably shorter, as expected for a stronger bond. In both DEA complexes, the hydrogen (3a H38, 3b H32), nitrogen (N3), palladium, and chloride (3a Cl1, 3b Cl2) are all in the same plane, perpendicular to the NHC ring, with a very small torsion angle of 1.12(1.64)° for 3a and 0.89(2.22)° for 3b. The H−Cl distance for both complexes is very similar (2.612 Å for 3a and 2.652 Å for 3b) and significantly shorter than the sum of the van der Waals radii of H and Cl atoms, 2.95 Å,8 suggesting an intramolecular hydrogen bonding between the DEA ligand and one of the chlorines attached to the palladium.9 The interaction of the hydrogen atom with the chlorine would explain an enhancement of σ-donor character of the nitrogen, which ultimately results in a stronger than expected Pd−N bond and a higher trans influence on the NHC. In addition to these intramolecular N(H)−Cl interactions, intermolecular N(H)− Cl interactions can also be observed in the solid state (2.752 Å for 3a and 2.876 Å for 3b, both below the sum of the van der Waals radii of H and Cl), resulting in the formation of dimers (Figures 4 and 5).10
Figure 3. Crystal structure of (SIPr)PdCl2(DEA) (3b) with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity except those in the backbone of the NHC and the nitrogen of the DEA ligand.
solution10 have a direct influence on the activation of the complex to generate the active species [(NHC)-Pd(0)] required in a cross-coupling reaction. Table 2 shows a comparison in activity between complexes 1a and 3a for the Suzuki−Miyaura coupling of 2,6-dimethylphenyl chloride and phenylboronic acid. Under the same reaction conditions, it can be seen how there is a clear difference in performance between the two complexes, which can be attributed only to the presence of a different “throw-away” ligand. While complex 1a allows for the coupling to take place at 25 °C in a relatively short time (Table 2, entry 1), complex 3a has a much more modest activity even at mild temperatures, requiring an increase in the reaction temperature to 60 °C (Table 2, entry 5) to perform more or less at the same level as 1a at room temperature. The gap in performance decreases as the temperature increases further (Table 2, entries 6, 7), although a significant difference can still be observed by decreasing the catalyst loading, suggesting the activation of complex 3a is much slower than that for 1a even at high temperature (Table 2, entries 8−11). The greater stability of the DEA complexes over the TEA counterparts is confirmed by the ligand exchange reaction depicted in Scheme 1. Complex 1b can be easily converted into 3b at room temperature in a short time using an excess of DEA. However, attempting the opposite conversion affords only a small amount of 1b even at higher temperature. In conclusion, we have synthesized and fully characterized two new (NHC)PdCl2(DEA) complexes. These complexes display an unexpected stability due to a rare intramolecular hydrogen bonding between DEA and a chloride attached to the Pd center. When compared to the triethylamine-bearing counterparts, this greater stability has a clear effect on their activation and activity as precatalysts for the Suzuki−Miyaura cross-coupling reaction.
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Figure 2. Crystal structure of (IPr)PdCl2(DEA) (3a) with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity except those in the backbone of the NHC and the nitrogen of the DEA ligand.
EXPERIMENTAL SECTION
General Considerations. All aryl halides and boronic acids were used as received. Technical grade ethanol was used to carry out Suzuki−Miyaura reactions. Sodium hydroxide was stored under nitrogen in a glovebox. All reactions were carried out under an atmosphere of nitrogen in screw cap vials. 1H and 13C NMR were
The stronger than expected coordination of DEA and the potential for those intermolecular interactions to be present in 6771
dx.doi.org/10.1021/om200828a | Organometallics 2011, 30, 6770−6773
Organometallics
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Figure 4. Intermolecular hydrogen bonding between two (IPr)PdCl2(DEA) units. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity except those in the backbone of the NHC and the nitrogen of the DEA ligand.
Figure 5. Intermolecular hydrogen bonding between two (SIPr)PdCl 2(DEA) units. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity except those in the backbone of the NHC and the nitrogen of the DEA ligand.
Table 2. Activity Comparison for the Suzuki−Miyaura Coupling of 2,6-Dimethylphenyl Chloride and Phenylboronic Acid
entry 1 2 3 4 5 6 7 8 9 10 11
[Pd] (mol %) (1a) (1a) (3a) (1a) (3a) (1a) (3a) (1a) (3a) (1a) (3a)
(1.0) (1.0) (1.0) (1.0) (1.0) (1.0) (1.0) (0.5) (0.5) (0.25) (0.25)
temperature (°C)
time (min)
yield (%) a,b
25 40 40 60 60 80 80 80 80 80 80
90 20 120 5 60 1 5 1 150 15 240
96 >99 38 >99 97 >99 >99 >99 85 >99 23c
a
Reaction conditions: aryl chloride, 0.50 mmol; phenylboronic acid, 0.55 mmol; NaOH, 0.6 mmol; ethanol, 1 mL. bConversion to coupling product based on aryl chloride, determined by GC using hexamethylbenzene as internal standard; average of two runs. c27% conversion after 21 h. 6772
dx.doi.org/10.1021/om200828a | Organometallics 2011, 30, 6770−6773
Organometallics
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Scheme 1. Interconversion of 1b and 3b
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recorded on a 300 or 500 MHz spectrometer at ambient temperature in CDCl3. General Procedure for the Suzuki−Miyaura Reaction. In a glovebox, complex, NaOH (0.6 mmol), hexamethylbenzene (0.1 mmol), and phenylboronic acid (0.55 mmol) were added in turn to a vial equipped with a magnetic bar and sealed with a screw cap fitted with a septum. Outside the glovebox, technical grade ethanol (1 mL) and aryl chloride (0.5 mmol) were injected into the vial, and the reaction mixture was allowed to stir on a magnetic plate. The reaction was monitored by gas chromatography and allowed to continue until all the chloride was consumed or no further conversion was observed. The amount of product shown is the average of two runs. Synthesis of (IPr)PdCl2(DEA) (3a). [Pd(μ-Cl)Cl(IPr)]2 (226 mg, 0.2 mmol) was charged in a vial equipped with a magnetic stirring bar and suspended in dichloromethane (1 mL). An excess of diethyl amine (0.5 mL) was added, and the solution was allowed to stir at room temperature for 1 h. Removal of the solvent afforded a pale yellow solid, which was washed with pentane. Crystals suitable for X-ray diffraction were grown from a solution in DCM/hexane. Yield =74% (190 mg). 1H NMR(CDCl3, 300 MHz): δ 0.83 (t, J = 7.2 Hz, N(CH2CH3)3, 6H), 1.06 (d, J = 6.9 Hz, CHCH3, 12H), 1.41 (d, J = 6.6 Hz, CHCH3, 12H), 2.52 (q, J = 7.1 Hz, N(CH2CH3)2, 6H), 3.16 (sep, J = 6.8 Hz, CHCH3, 4H), 7.11 (s, CH, 2H), 7.34 (d, J = 7.8 Hz, CH, 4H), 7.48 (t, J = 7.8 Hz, CH, 2H). 13C{1H} NMR (CDCl3, 125 MHz): δ 14.3 (s, NH(CH2CH3)2), 22.6 (s, iPr), 26.4 (s, iPr), 28.7 (s, CHiPr), 45.9 (s, NH(CH2CH3)2), 123.6 (s, CHCH), 124.7 (s, CH aromatic), 130.0 (s, CH aromatic), 135.1 (s, CH aromatic), 146.8 (s, C aromatic), 159.5 (s, C carbene). Anal. Calcd for C33H47Cl2N3Pd: C, 58.26; H, 7.41; N, 6.58. Found: C, 57.85; H, 7.76; N, 6.61. Synthesis of (SIPr)PdCl2(DEA) (3b). [Pd(μ-Cl)Cl(SIPr)]2 (57 mg, 0.05 mmol) was charged in a vial equipped with a magnetic stirring bar and suspended in dichloromethane (1 mL). An excess of diethyl amine (0.5 mL) was added, and the solution was allowed to stir at room temperature for 1 h. Removal of the solvent afforded a pale yellow solid, which was washed with pentane. Crystals suitable for Xray diffraction were grown from a solution in DCM/hexane. Yield = 93% (60 mg). 1H NMR(CDCl3, 300 MHz): δ 0.86 (t, J = 8.4 Hz, NH(CH2CH3)2, 6H), 1.22 (d, J = 6.9 Hz, CHCH3, 12H), 1.48 (d, J = 6.6 Hz, CHCH3, 12H), 2.08 (m, NH(CH2CH3)2, 2H), 2.56 (m, NH(CH2CH3)2, 2H), 3.52 (sep, J = 6.2 Hz, CHCH3, 4H), 4.08 (s, CH2, 4H), 7.28 (d, J = 7.2 Hz, CH, 4H), 7.43 (t, J = 8.6 Hz, CH, 2H), 9.14 (br, NH, 1H). 13C{1H} NMR (CDCl3, 125 MHz): δ 14.1 (s, N(CH2CH3)2), 23.5 (s, iPr), 27.1 (s, iPr), 28.7 (s, CHiPr), 45.7 (s, N(CH2CH3)2), 53.3 (s, CH2), 124.0 (s, CH aromatic), 129.2 (s, CH aromatic), 135.2 (s, C aromatic), 148.1 (s, C aromatic), 190.0 (C carbene). Anal. Calcd for C33H49Cl2N3Pd: C, 58.08; H, 7.70; N, 6.55. Found: C, 58.04; H, 7.99; N, 6.43. Synthesis of (SIPr)PdCl2(DEA) (3b) from (SIPr)PdCl2(TEA) (1a). A vial equipped with a magnetic stirring bar was loaded with 1a (17 mg, 0.025 mmol), DCM (1 mL), and an excess of DEA (0.5 mL). The mixture was allowed to stir at room temperature for 1 h. Removal of the solvent afforded a pale yellow solid, which was washed with cold hexane. 1H NMR of the solid showed a conversion of >95% into 3b.
AUTHOR INFORMATION Corresponding Author *E-mail
[email protected]. ACKNOWLEDGMENTS
O.N. gratefully acknowledges the National Science Foundation for funding (CHE-0924324). M.L.T. acknowledges the NWDA and the EPSRC for financial support. The authors also thank Dr. Lee M. Daniels (Rigaku Americas Corp.) for assistance solving the crystal structure of 3a and Dr. Richard Staples (Michigan State University) for helpful discussions.
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REFERENCES
(1) Chen, M.-T.; Vicic, D. A.; Turner, M. L.; Navarro, O. Organometallics 2011, 30, 5052−5056. (2) For recent reviews on NHCs, see: (a) Cazin, C. S. J., Ed. NHeterocyclic Carbenes in Transition Metal Catalysis and Organocatalysis; Springer: London, 2010. (b) Glorius, F., Ed. N-Heterocyclic Carbenes in Transition Metal Catalysis; Springer-Verlag: Berlin, 2007. (3) Reviews on the Suzuki−Miyaura reaction: (a) Miyaura, N. Top. Curr. Chem. 2002, 219, 11−59. (b) Kotha, S.; Lahiri, K.; Kashinath, D. Tetrahedron 2002, 58, 9633−9695. (c) Bellina, F.; Carpita, A.; Rossi, R. Synthesis 2004, 2419−2440. (4) Reviews on the Buchwald−Hartwig reaction: (a) Muci, A. R.; Buchwald, S. L. Top. Curr. Chem. 2002, 219, 131−209. (b) Hartwig, J. F. In Handbook of Organopalladium Chemistry for Organic Synthesis; Negishi, E.-i, Ed.; Wiley: New York, 2002. (5) General reviews on cross-coupling reactions: (a) Tsuji, J. Palladium Reagents and Catalysis, 2nd ed.; Wiley: West Sussex, England, 2004. (b) de Meijere, A., Diederich, F., Ed. Metal-Catalyzed Cross-Coupling Reactions, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2004. (c) Navarro, O.; Nolan, S. P. C-C Bond Formation by CrossCoupling. In Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, M. P., Eds.; Elsevier: New York, 2007; Vol. 11, Chapter 1. (6) Organ, M. G.; Avola, S.; Dubovyk, I.; Hadei, N.; Kantchev, E. A. B.; O’Brien, C. J.; Valente, C. Chem.Eur. J. 2006, 12, 4749−4755. (7) Chowdhury, P. K. J. Phys. Chem. A 2003, 107, 5692−5696. (8) Average van der Waal radii: Bondi, A. J. Phys. Chem. 1964, 68, 441−451. (9) For another example of an intramolecular hydrogen bonding in a Pd complex, see: (a) Andrieu, J.; Camus, J.-M.; Dietz, J.; Richard, P.; Poli, R. Inorg. Chem. 2001, 40, 1597−1605. For a recent example in Pt, see: (b) Dub, P. A.; Daran, J.-C.; Levina, V. A.; Belkova, N. V.; Shubina, E. S.; Poli, R. J. Organomet. Chem. 2011, 1174−1183. (10) We thank one of the reviewers for pointing out these intermolecular interactions in the solid state and their potential stabilizing role in solution.
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ASSOCIATED CONTENT S Supporting Information * Characterization and crystallographic information files (CIF) of complexes 3a and 3b. This material is available free of charge via the Internet at http://pubs.acs.org. 6773
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