Rh(III) Pyridinium Substituted Bipyridine Complexes as Catalysts for Arene H/D Exchange
- First Online:
- Cite this article as:
- Gary, J.B., Carter, T.J. & Sanford, M.S. Top Catal (2012) 55: 565. doi:10.1007/s11244-012-9825-z
- 436 Views
This report describes the synthesis of Rh(III) complexes containing a pyridinium-substituted bipyridine ligand. The catalytic activity of these complexes in H/D exchange reactions between arenes and acetic acid-d4 has been evaluated.
KeywordsC−H activationH/D exchangeRhodiumBipyridine
Given the interesting reactivity of PdII and PtII catalysts derived from ligand 1, we sought to investigate the influence of this ligand on other metal catalysts for C–H activation. Recent reports have shown that RhIII complexes can participate in both stoichiometric C–H activation and catalytic C–H functionalization reactions [12, 30–62]. As such, we targeted complexes of general structure RhIII(Cl)3(1) and several derivatives thereof. This report describes the synthesis of these compounds as well as preliminary studies of their catalytic activity in H/D exchange reactions.
2 Experimental Section
NMR spectra were recorded on Varian Inova 500, Varian vnmrs 500 MHz, Varian MR400 400 MHz, or Varian vnmrs 700 MHz NMR spectrometers with the residual solvent peak (CD3CO2D: 1H: δ = 11.53, 2.03 ppm, 13C: δ = 178.4, 20.0 ppm; CD3CN: 1H: δ = 1.94 ppm, 13C: δ = 118.2, 1.3 ppm; C6D6: 1H: δ = 7.15 ppm, 13C: δ = 128.0 ppm; DMSO-d6: 1H: δ = 2.49 ppm) as the internal reference unless otherwise noted. 19F NMR spectra are referenced to the residual solvent signal in the 1H NMR. Chemical shifts are reported in parts per million (ppm) (δ). Multiplicities are reported as follows: br (broad resonance), s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), dd (doublet of doublets), td (triplet of doublets). Coupling constants (J) are reported in Hz. Elemental analyses were performed by Atlantic Microlab, Inc., Norcross, Georgia. High-resolution mass spectrometry was performed on an Agilent Q-TOF HPLC–MS.
2.2 Materials and Methods
All reactions were conducted without rigorous exclusion of air and moisture unless noted otherwise. CD3CO2D was purchased from Cambridge Isotope Laboratories and stored in Schlenk tube under N2. CD3CN, CD3OD, and DMSO-d6 were purchased from Cambridge Isotope Laboratories and used as received. Dichloromethane, acetonitrile, methanol, ethyl acetate, 1,1,2-trichloroethane, and pentane were obtained from Fisher Scientific or Aldrich and used as received. Benzene for H/D exchange was obtained from Aldrich and stored over 4 Å molecular sieves. RhCl3·3H2O was purchased from Pressure Chemical Company. 4,4-di-t-butyl-2,2-bipyridine was purchased from Aldrich. Silver acetate and 2-phenylpyridine were obtained from Alfa Aesar. Ligand 1,  2-(2-chlorophenyl)pyridine , and [Rh(COE)2Cl]26  were prepared according to literature procedures. All liquid reagents were dispensed by difference using a gas-tight Hamilton syringe.
2.3.1 RhCl3(1) (2)
RhCl3·3H2O (25.0 mg, 94.9 μmol, 1.00 equiv) and ligand 1 (121.5 mg, 94.9 μmol, 1.00 equiv) were combined in a 20 mL vial. Methanol (4 mL) was added, the vial was sealed with a Teflon-lined cap, and the reaction mixture was heated at 80 °C for 2 h. A yellow solid precipitated from solution and was collected by vacuum filtration and washed with diethyl ether (2 × 5 mL) to yield the product 2 as a yellow solid (90.1 mg, 64 % yield). The 1H NMR spectrum of the product shows signals consistent with a mixture of dimer 2a and monomer [Rh(1)Cl3(Solv)] (Solv = H2O or DMSO-d6) (2b). The ratio of 2a–2b is ~6:1 at room temperature. This monomer/dimer mixture precluded 13C NMR analysis. 1H NMR of 2a (DMSO-d6, 499.904 MHz): δ 9.47 (d, J = 6.4 Hz, 4H), 8.80 (s, 8H), 8.49 (d, J = 1.9 Hz, 4H), 8.38 (d, J = 8.6 Hz, 8H), 7.77 (dd, J = 6.1 Hz, 1.9 Hz, 4H), 7.69 (d, J = 8.6 Hz, 8H), 7.40 (d, J = 8.8 Hz, 16H), 7.35 (d, J = 8.8 Hz, 16H), 1.36 (s, 36H), 1.20 (s, 72H). 19F NMR (DMSO-d6, 376.836 MHz): δ –148.2 (10B), –148.3 (11B). 1H NMR of 2b (DMSO-d6): Unique signals observed at δ 9.30 (d, J = 6.4 Hz, 1H), 8.84 (s, 2H), 8.83 (s, 2H), 8.68 (d, J = 6.4 Hz, 1H), 8.56 (br, 1H), 8.52 (br, 1H), 7.87 (br, 1H), 7.81 (br, 1H), 1.37 (s, 18H), 1.20 (s, 36H) as well as signals that overlap with 2a at 8.38, 7.69, 7.40, and 7.35 ppm. HRMS electrospray (m/z): [M–BF4]+ calcd. for [RhCl3C80H90ON4BF4]+ 1417.5259; found 1417.5255.
2.3.2 [Rh(1)(phpy)Cl2] (3)
In a nitrogen filled glove box, ligand 1 (356.6 mg, 0.279 mmol, 1.96 equiv) was dissolved in dry methylene chloride (10 mL) and added to [Rh(COE)2Cl]2 (102.0 mg, 0.142 mmol, 1.00 equiv). The resulting mixture was stirred for 1 min, leading to a dark orange solution. To this solution was added 2-(2-chlorophenyl)pyridine (59.2 mg, 0.310 mmol, 2.18 equiv), and this mixture was stirred for 1 h. The reaction was removed from the glove box, and pentane (40 mL) was added to precipitate the product. The precipitate was collected by filtration, washed with pentane (2 × 10 mL), and dried under vacuum to afford 3 as a yellow solid (341.2 mg, 82 % yield). 1H NMR (CD3CN, 499.904 MHz): δ 9.69 (d, J = 5.9 Hz, 1H), 9.50 (d, J = 6.2 Hz, 1H), 8.53 (d, J = 2.3 Hz, 1H), 8.50 (d, J = 2.0 Hz, 1H), 8.47 (d, J = 2.0 Hz, 1H), 8.39 (d, J = 2.1 Hz, 1H), 8.15 (m, 2H), 8.08 (m, 2H), 7.99 (m, 2H), 7.94 (d, J = 2.1 Hz, 1H), 7.81 (d, J = 2.3 Hz, 1H), 7.74 (m, 3H), 7.69 (m, 2H), 7.57 (dd, J = 6.2 Hz, 2.1 Hz, 1H), 7.52 (m, 2H), 7.45 (m, 2H), 7.41 (m, 4H), 7.35 (m, 1H), 7.31 (m, 2H), 7.08 (m, 8H), 6.90 (td, J = 6.0 Hz, 2.3 Hz, 1H), 6.85 (dd, J = 6.2 Hz, 2.3 Hz, 1H), 5.90 (d, J = 7.8 Hz, 1H), 1.39 (s, 9H), 1.37 (s, 9H), 1.32 (s, 9H), 1.31 (s, 9H), 1.29 (s, 9H), 1.15 (s, 9H). 13C NMR (CD3CN, 175.974 MHz): δ 165.55, 164.36 (d, J = 28 Hz), 159.15, 159.13, 158.67, 158.66, 157.68, 157.47, 157.22, 157.20, 156.99, 156.97, 156.21, 155.97, 155.93, 155.89, 154.40, 153.63, 152.05, 150.10, 149.67, 145.25, 140.64, 132.58, 132.11, 132.02, 131.50, 131.37, 131.16, 131.06, 131.04, 130.66, 130.22, 130.18, 130.13, 130.08, 129.95, 128.55, 128.38, 128.35, 127.92, 127.54, 127.11, 126.93, 126.84, 126.76, 126.64, 126.46, 126.34, 125.42, 124.58, 124.52, 124.10, 121.27, 36.31, 36.28, 36.14, 36.12, 36.09, 35.93, 31.98, 31.92 (2 carbons), 31.82, 31.64, 31.61. Two 13C NMR signals in the aromatic region are coincidentally overlapping. 19F NMR (CD3CN, 376.836 MHz): δ –151.6 (10B), –151.7 (11B). HRMS electrospray (m/z): [M-BF4]+ calcd. for [RhCl2C91H96N5BF4]+ 1518.6127; found 1518.6158.
2.3.3 [Rh(dtbpy)(phpy)Cl2] (4)
In a nitrogen filled glove box, 4,4′-di-tert-butyl-2,2′-bipyridine (190.0 mg, 0.708 mmol, 2.03 equiv) was dissolved in dry benzene (10 mL) and added to [Rh(COE)2Cl]2 (250.0 mg, 0.348 mmol, 1.00 equiv). The resulting solution was stirred for 5 min, leading to a color change from red–orange to dark blue. To this solution was added 2-(2-chlorophenyl)pyridine (146.1 mg, 0.766 mmol, 2.20 equiv), and the mixture was stirred for 1 h, during which time a pale yellow precipitate formed. The reaction was removed from the glove box, and pentane (40 mL) was added to fully precipitate the product. The precipitate was collected by filtration, washed with pentane (2 × 10 mL), and dried under vacuum to afford 4 as a yellow solid (370.9 mg, 89 % yield). 1H NMR (CD3CN, 699.765 MHz): δ 9.97 (m, 1H), 9.59 (m, 1H), 8.41 (d, J = 2.0 Hz, 1H), 8.28 (d, J = 2.1, 1H), 8.09 (m, 1H), 8.05 (td, J = 8.2, 1.7 Hz, 1H), 8.84 (dd, J = 6.2, 2.1 Hz, 1H), 7.78 (dd, J = 7.5, 1.4 Hz, 1H), 7.49 (m, 1H), 7.29 (m, 1H), 7.24 (dd, J = 6.2, 2.0 Hz), 6.99 (td, J = 7.3, 1.0 Hz, 1H), 6.87 (td, J = 7.5, 1.4 Hz, 1H), 6.26 (d, J = 7.9 Hz, 1H), 1.52 (s, 9H), 1.31 (s, 9H). 13C NMR (CD3CN, 175.974 MHz): δ 165.32, 164.95 (d, J = 21.1 Hz), 164.66, 164.27, 156.67, 156.60, 153.06, 151.73, 149.72, 145.06, 139.69, 133.50, 130.66, 125.37, 125.33, 124.95, 124.39, 124.05, 121.84, 121.74, 120.56, 36.42, 36.19, 30.43, 30.14. HRMS electrospray (m/z): [M–Cl]+ calcd. for [RhClC29H32N3]+ 560.1334; found 560.1335. Anal. calcd. for C40H45N2BF4: C, 58.40; H, 5.41; N, 7.05. Found: C, 58.21; H, 5.56; N, 6.97.
2.4 Reaction Details
2.4.1 General Procedure for H/D Exchange Reactions with Benzene
To a 4 mL resealable Schlenk tube was added catalyst (5.0 μmol, 2.0 mol %), AgOAc (if applicable) (2.5 mg, 15 μmol, 6.0 mol%), and a Teflon stirbar. Deuterium source (6.25 mmol, 25 equiv.; CD3CO2D 357.9 μL; CD3OD 253.5 μL) was added. Benzene (22.3 μL, 19.5 mg, 0.250 mmol, 1.00 equiv) was then added to the reaction vessel, which was subsequently sealed. The vessel was completely submerged in a preheated oil bath at 150 °C. After 24 h, the vessel was cooled to room temperature. The reaction mixture was then filtered through a plug of Celite to remove any particulates and rinsed with EtOAc (1 × 2 mL) into a 20 mL scintillation vial. A saturated aqueous solution of K2CO3 (9 M in deionized H2O, 2 × 1 mL) was added to the vial to quench the acid. The organic layer was carefully separated and diluted with additional EtOAc to give an approximately 13 mM solution of benzene (~1 mg/mL) for analysis by GCMS. The percent deuterium incorporation was defined as the percent of C–H bonds converted to C–D bonds. The background reaction (in the absence of the catalyst) at 150 °C is minimal, as has been described in detail in a previous publication . Turnover numbers (TONs) were calculated as mole deuterium incorporated per mole of catalyst. Reported values have been corrected for the background reaction in the presence of AgCl, which is formed in situ.
2.4.2 General Procedure for H/D Exchange Reactions with 2-Phenylpyridine
To a 4 mL resealable Schlenk tube was added catalyst (5.0 μmol, 2.0 mol%) and AgOAc (if applicable) (2.5 mg, 15 μmol, 6.0 mol%), and a Teflon stirbar. Deuterium source (6.25 mmol, 25 equiv.; CD3CO2D 357.9 μL; CD3OD 253.5 μL) was added. 2-Phenylpyridine (35.8 μL, 38.8 mg, 0.250 mmol, 1.00 equiv) was added to the reaction vessel, which was subsequently sealed. The vessel was completely submerged in a preheated oil bath at 130 °C. After 24 h, the vessel was cooled to room temperature. 1,1,2-Trichloroethane (23.2 μL, 0.250 mmol, 1.00 equiv) was added, and the contents were mixed and transferred to an NMR tube. The percent deuterium incorporation was calculated as the percent of C–H bonds converted to C–D bonds and was determined by the loss of signal integration in the 1H NMR spectrum as compared to an independent sample of a 1:1 mixture of 2-phenylpyridine and 1,1,2-trichloroethane. 1H NMR spectra were recorded using a single scan with the gain set to zero to minimize relaxation delay errors. Turnover numbers (TON) were calculated as total percent deuterium incorporation divided by catalyst loading.
3 Results and Discussion
Evaluation of complex 4 as a catalyst for H/D exchange between 2-phenylpyridine and CD3CO2D revealed that it afforded 8 turnovers after 24 h at 130 °C (compared to TON = 16 for 3 under analogous conditions). We note that activity is modest in both cases; thus ongoing studies are focused on designing more active Rh-based catalysts containing both of these ligands in order to achieve a detailed assessment of catalyst TON, TOF, and lifetime as a function of bipyridine ligand structure.
This report demonstrates the synthetic accessibility of several RhIII complexes containing the cationic bipyridine ligand 1. These complexes (2 and 3) catalyze H/D exchange reactions between aromatic substrates and CD3CO2D. Furthermore, comparison of complex 3 to analogue 4 (containing a neutral bipyridine ligand) reveals that the former affords higher turnover numbers in H/D exchange between 2-phenylpyridine and CD3CO2D. While the C–H activation reactivity of 2/3 remains modest, the results presented herein suggest that ligand 1 and analogues thereof may hold some promise for developing RhIII-based catalysts for C–H activation/functionalization transformations.
We thank the NSF for support of this work through the Center for Enabling New Technologies through Catalysis (CENTC). In addition, JBG gratefully acknowledges the National Science Foundation (Graduate Research Fellowship) and Rackham Graduate School (Murrill Memorial Scholarship) for financial support.