Topics in Catalysis

, Volume 55, Issue 7, pp 565–570

Rh(III) Pyridinium Substituted Bipyridine Complexes as Catalysts for Arene H/D Exchange

Authors

  • J. Brannon Gary
    • Department of ChemistryUniversity of Michigan
  • Tyler J. Carter
    • Department of ChemistryUniversity of Michigan
    • Department of ChemistryUniversity of Michigan
Original Paper

DOI: 10.1007/s11244-012-9825-z

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

Abstract

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.

Keywords

C−H activationH/D exchangeRhodiumBipyridine

1 Introduction

The development of new transition metal catalysts for the activation and functionalization of carbon-hydrogen bonds remains an important goal in catalysis [118]. Based on pioneering work by Shilov starting in the late 1960s, PdII and PtII-based catalysts have been a major focus of efforts in the literature [1928]. As one example, our group recently reported the synthesis and catalytic studies of PtII and PdII complexes containing the electron deficient dicationic bipyridine ligand 1 (Fig. 1) [29]. Relative to neutral analogues like 2,2′-di-tert-butylbipyridine and/or 2,2′-bipyrimidine, ligand 1 significantly enhanced the reactivity of these group 10 metal centers towards catalytic arene H/D exchange reactions [29]. Furthermore, Pd complexes of ligand 1 also showed activity as catalysts for benzene C–H acetoxylation.
https://static-content.springer.com/image/art%3A10.1007%2Fs11244-012-9825-z/MediaObjects/11244_2012_9825_Fig1_HTML.gif
Fig. 1

Cationic Bipyridine Ligand Investigated in Current Studies

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, 3062]. 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

2.1 Instrumentation

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, [29] 2-(2-chlorophenyl)pyridine [63], and [Rh(COE)2Cl]26 [64] were prepared according to literature procedures. All liquid reagents were dispensed by difference using a gas-tight Hamilton syringe.

2.3 Synthesis

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 2a2b 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 [65]. 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

Our initial synthetic efforts focused on accessing RhIII complexes containing ligand 1. Heating a methanol solution of RhCl3·H2O with 1 at 80 °C for 2 h resulted in the precipitation of 2 as an orange solid (64 % yield). High resolution mass spectrometric analysis of this solid showed a mass of 1417.5255, consistent with a product of stoichiometry Rh(Cl)3(1). While it has not proven possible to definitely assign the solid state structure of 2, the chloride bridged dimer 2a (Scheme 1) is consistent with this stoichiometry and has a stable 18 e configuration about each RhIII center. NMR characterization of this product is complicated by the fact that it is only soluble in coordinating solvents such as dimethyl sulfoxide (DMSO). Under these conditions, 1H NMR spectroscopic analysis showed a mixture of two compounds in a 6:1 ratio. The major species possesses a single set of pyridine resonances, consistent with the symmetry of the chloride bridged dimer 2a. The minor compound shows two different environments for the pyridine protons, consistent with monomeric solvento complex 2b.
https://static-content.springer.com/image/art%3A10.1007%2Fs11244-012-9825-z/MediaObjects/11244_2012_9825_Sch1_HTML.gif
Scheme 1

RhIII Complex Synthesis

We studied the reactivity of complex 2 towards C−H activation of benzene using a catalytic H/D exchange assay recently developed in our laboratory [29, 6568]. Two different deuterium sources (CD3CO2D and CD3OD) were examined under standard conditions (2 mol% [Rh] catalyst, 25 equiv of D-source, 1 equiv of benzene, 150 °C, 24 h), and the turnover numbers for H/D exchange were determined using GCMS. As shown in Table 1, entries 1 and 2, complex 2 alone did not catalyze H/D exchange under these conditions. However, significant H/D exchange between C6H6 and CD3CO2D was observed upon the addition of 6 mol% of AgOAc (entry 3). We hypothesize that this additive generates a more reactive catalyst by replacing one or more of the chloride ligands of 2 with acetates. Notably, similar results have recently been observed with RhIII-pincer complexes in which the chloride ligands were replaced with acetate to achieve C–H activation [34].
Table 1

Benzene H/D Exchange in Various Solvents https://static-content.springer.com/image/art%3A10.1007%2Fs11244-012-9825-z/MediaObjects/11244_2012_9825_Figa_HTML.gif

Entry

D-Source

Additive

TON

1

CD3CO2D

None

1 ± 1

2

CD3OD

None

0

3

CD3CO2D

AgOAc

41 ± 15

4

CD3OD

AgOAc

0

The use of 2 as a catalyst for H/D exchange between CD3CO2D and 2-phenylpyridine was also investigated. At 130 °C, complex 2 afforded 14 turnovers for this transformation (Table 2, entry 1), while, under otherwise identical conditions, the combination of 2 (2 mol%) and AgOAc (6 mol%) provided 38 turnovers (entry 2). In both cases this reaction proceeded with high selectivity for H/D exchange at the ortho-position, and minimal (<5 % based on 1H NMR integration error) deuterium incorporation was observed at any other C–H site on the molecule. This suggests strongly that the C–H activation event is directed by the pyridine moiety in this substrate. As above, CD3OD was an ineffective deuterium source for this transformation (entries 3 and 4).
Table 2

H/D Exchange of 2-Phenylpyridine https://static-content.springer.com/image/art%3A10.1007%2Fs11244-012-9825-z/MediaObjects/11244_2012_9825_Figb_HTML.gif

Entry

D-Source

Additive

TON

1

CD3CO2D

None

14 ± 3

2

CD3CO2D

AgOAc

38 ± 1

3

CD3OD

None

0

4

CD3OD

AgOAc

0

We hypothesized that a cyclometalated complex like 3 might be an intermediate in the H/D exchange of 2-phenylpyridine discussed in Table 2. Thus, we next sought to independently synthesize this compound and confirm its catalytic competence in H/D exchange reactions. As shown in Scheme 2, the addition of 2 equiv of 1 to [Rh(COE)2Cl]2 followed by addition of 2-(2-chlorophenyl)pyridine resulted in the formation of 3 in 82 % yield. Complex 3 was characterized by 1H and 13C NMR spectroscopy along with high resolution mass spectrometry. Importantly, all of the aromatic rings in 3 are inequivalent by 1H and 13C NMR spectroscopy. This is clearly manifest in four distinct signals for the hydrogens on the pyridinium rings at 8.53, 8.50, 8.47, and 8.39 ppm. We assign 3 as the structure shown in Scheme 2 on the basis of the fact that the ortho-aryl proton (HPh) is shifted significantly upfield from the aromatic region, appearing at 5.90 ppm. Prior reports of related PdIV complexes have shown that such upfield shifts are indicative of the geometry shown in Scheme 2, in which HPh is oriented below the π-system of the bipyridine ligand [69, 70].
https://static-content.springer.com/image/art%3A10.1007%2Fs11244-012-9825-z/MediaObjects/11244_2012_9825_Sch2_HTML.gif
Scheme 2

Synthesis of Complex 3

As shown in Table 3, complex 3 showed nearly identical reactivity to 2 (TON = 16 and 14, respectively) for H/D exchange between 2-phenylpyridine and CD3CO2D. This data is consistent with the possible intermediacy of a cyclometalated species in this transformation.
Table 3

H/D Exchange of 2-Phenylpyridine https://static-content.springer.com/image/art%3A10.1007%2Fs11244-012-9825-z/MediaObjects/11244_2012_9825_Figc_HTML.gif

Entry

[Rh]

TON

1

2

14 ± 3

2

2/AgOAc

38 ± 1

3

3

16 ± 1

4

4

8 ± 2

Finally, we sought to compare the H/D exchange activity of complexes containing cationic ligand 1 to that of analogous species containing neutral 2,2′-di-tert–butylbipyridine (dtbpy). Attempts to synthesize dtbpy-containing analogues of 2 resulted in the formation of inseparable mixtures of products. However, gratifyingly, the dtbpy analogue of 3 could be prepared by reacting [Rh(COE)2Cl]2 with 2 equiv of dtbpy followed by the addition of 2-(2-chlorophenyl)pyridine. The product (4) was characterized by 1H and 13C NMR spectroscopy along with elemental and mass spectrometry analysis. Similar to complex 3 above, the observation of a highly upfield aromatic proton resonance at 6.26 ppm (HPh) is consistent with the structural isomer shown in Scheme 3.
https://static-content.springer.com/image/art%3A10.1007%2Fs11244-012-9825-z/MediaObjects/11244_2012_9825_Sch3_HTML.gif
Scheme 3

Synthesis of Complex 4

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.

4 Conclusions

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.

Acknowledgments

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.

Supplementary material

11244_2012_9825_MOESM1_ESM.docx (1.6 mb)
Supplementary material 1 (DOCX 1662 kb)

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