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The Effect of Carboxylate Ligands on Alkane Dehydrogenation with ( Phebox)Ir Complexes dm
Hongmei Yuan, William W. Brennessel, and William D. Jones ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04057 • Publication Date (Web): 03 Feb 2018 Downloaded from http://pubs.acs.org on February 4, 2018
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ACS Catalysis
The Effect of Carboxylate Ligands on Alkane Dehydrogenation with (dmPhebox)Ir Complexes Hongmei Yuan, William W. Brennessel, and William D. Jones* Department of Chemistry, University of Rochester, Rochester, NY 14627 ABSTRACT: A series of carboxylate ligated iridium complexes (dmPhebox)Ir(O2CR)2(H2O) (R = -CH3, -CH2CH3, -CMe3, -CH2C6H5, -CH=CMe2) were designed and synthesized to understand the carboxylate ligand effects on the reactivity of the complex for alkane dehydrogenation. Kinetic studies showed that the different R groups of the carboxylate iridium complexes can affect the reactivity with octane in the β–H elimination step. The rate constants for octene formation with different carboxylate ligands follow the order R = -CH=CMe2 > CMe3 > -CH2CH3 > -CH3 > -CH2C6H5. In contrast, there is no significant effect of carboxylate ligand on the rate of the C-H activation step at 160 °C. These experimental results support the findings in the previously reported density functional theory (DFT) study of the (dmPhebox)Ir complex in alkane C-H activation. Keywords: C-H activation, iridium, concerted-metallation-deprotonation, dehydrogenation, kinetics β–H elimination step. 13 Another potential way to lower the INTRODUCTION energy barrier is to change the carboxylate group coordinated The dehydrogenation of alkanes and alkyl groups is of great to the metal center, since the affinity for carboxylate binding importance in the production of fuels and the synthesis of fine to iridium is affected and the pKa is changed. chemicals. Transition-metal catalysts have been intensively studied in selective activation and functionalization of alkane C-H bonds with promising advantages including affording high turnover numbers and the selective dehydrogenation of the terminal position.1-4 Such catalysts generally involved unsaturated 14-electron metal centers in low oxidation states. The active site for C-H activation is often inhibited by coordination of the oxidized product or water.5-7 The Nishiyama group reported that RhIII complex dm ( Phebox)Rh(OAc)2(OH2) (dmPhebox = 2,6-bis(4,4dimethyloxazolinyl)-3,5-dimethylphenyl) and the IrIII analog (dmPhebox)Ir(OAc)2(OH2) (1a) could activate C(sp2)-H and C(sp3)-H bonds to form aryl-MIII (M = Rh, Ir) or alkyl-IrIII complexes at 160 °C in the presence of potassium carbonate (Scheme 1).8-10 The same complex 1a was later studied in the Goldberg group for reactivity of arenes and alkanes. In contrast to the Nishiyama group’s results, while the same Irphenyl complex was observed in the C-H activation of benScheme 1. Octane dehydrogenation by (dmPhebox)Irzene, reaction in octane at 200 °C produced octene rather than (OAc)2(OH2). an Ir-octyl complex from simple alkane C-H activation.11 A novel IrIII-hydride complex and free alkene resulted from alRESULTS AND DISCUSSION kane dehydrogenation. They found this iridium complex can Compound Synthesis and Characterization. In this report, stoichiometrically activate and dehydrogenate alkanes to prowe describe the synthesis and reactivity of IrIII carboxylates of duce alkenes. The C-H activation and functionalization by the the type (dmPhebox)Ir(O2CR)2(H2O) (1a: R = -CH3; 1b: R= IrIII center of 1a was not inhibited by the common small moleCH2CH3; 1c: R= -CMe3; 1d: R= -CH2C6H5; 1e: R= cules nitrogen, water, or alkene. The reaction was also accelCH=CMe2) (see Supporting Information Figure S1 for details). erated in the presence of water. The Goldberg group reported The single crystal X-ray structures of complexes 1b, 1c, 1d, later that 1a can be regenerated from reaction of IrIII-hydride and 1e were determined. While complexes 1c, 1d, and 1e complex with O2 in the presence of acetic acid.12 showed a structure similar to 1a10 with a bound water and two Very recently, the Goldman group reported that Na+ can κ1-carboxylate groups, the structure of 1b showed one κ1catalyze reaction of 1a as a Lewis acid in β–H elimination of carboxylate and one κ2-carboxylate with no water (Figures 1(dmPhebox)Ir(OAc)(n-alkyl) 2a to give (dmPhebox)Ir(OAc)(H), 2). The reactivity of these carboxylate complexes with octane 3a, greatly lowering the energy barrier of the rate determining
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ACS Catalysis for the C-H activation step and the β–H elimination step at different temperatures was studied and compared.
method would give similar results as before. To measure the small, in-situ amounts of the octene resonances, a solventsuppression pulse sequence (Figure S2) was used to reduce the size of the octane peak in the 1H-NMR spectrum to avoid problems with dynamic range. Table 1. Comparision of octene isomer distribution using catalyst 1a after 72 h at 200 °C in different reactors octene
Figure 1. Thermal ellipsoid plot of 1b. Selected bond distances (Å): Ir(1)-O(1), 2.056 (4); Ir(1)-O(2), 2.359 (3); Ir(1)-O(3), 2.055 (3); Ir(1)-O(4), 3.040 (4). (one of 3 molecules in asymmetric unit)
trans-2 trans-3 trans-4 cis-2 % % % %
cis-3 %
reactor
Exp.
35
26
19
11
8
NMR tube
Lit.
32
28
20
11
8
glass vessel
With this in-situ NMR monitoring method, the increasing total amount of octene relative to an internal standard (mesitylene) with time (see Figure S3) was used to compute the rate constants of the reaction of the different (dmPhebox)Ir carboxylate complexes with octane at 200 °C. As shown in Figure 3, the complexes show a first-order approach to completion, and the data were fit to equation 2, where Ct is the % complete at time t and C∞ is the final % of octenes (100%).
Ct = C∞ (1 – e–kt)
(2)
100
80
% NMR yield
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60 MeCO2 40
EtCO2 t-BuCO2
20
Figure 2. Thermal ellipsoid plot of 1e. Selected bond distances (Å): Ir(1)-O(1), 2.070 (4); Ir(1)-O(3), 2.037 (3); Ir(1)-O(7), 2.216 (4).
Kinetics of Octane Dehydrogenation. 1a was synthesized according to the reported procedure.9 The octane dehydrogenation reaction (eq 1) was carried out under the conditions reported by Goldberg et al.13 in a medium-wall NMR tube with a Teflon valve rather than with a glass Schlenk vessel so that the reaction progress could be monitored by NMR spectroscopy. After 72 hours at 200 °C, the octene isomer distribution was quantified by GC analysis. Compared to the results reported by Goldberg, a similar distribution of octene isomers was observed (Table 1), which validated that this NMR-monitoring
(1)
Me2C=CHCO2
0 0
1000
2000 3000 time, min
4000
5000
Figure 3. Kinetic plots for the dehydrogenation of octane to form octenes by 1a-e.
The rate constants differ by almost an order of magnitude for the various (dmPhebox)Ir carboxylates, indicating that the R group of the carboxylate has a strong effect on the overall rate of the dehydrogenation reaction (Table 2). To our surprise, there is no detectable octene peak with 1d (R= -CH2C6H5) after several days at 200 °C. A small quantity of octene appeared after 11 days, which was about 30% of the maximum possible octene based upon the GC analysis. The (dmPhebox)Ir carboxylate 1e (R= -CH=CMe2) is almost six times faster than (dmPhebox)Ir carboxylate 1a (R = CH3). The rate constants with different carboxylate ligands follow the order R = CH=CMe2 > -CMe3 > -CH2CH3 > -CH3 > -CH2C6H5. This order follows the pKa values of the corresponding RCOOH acids, and this order supports the hypothesis that a Lewis acid effects the β-H elimination step.13 Both the Lewis acid and an R group with larger pKa facilitate the transformation of the carboxylate ligated to iridium center from κ2-coordination to κ1-coordination, generating an empty site which is essential
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ACS Catalysis for the β–H elimination step. Also, the low reactivity of 1d, with its low pKa of 4.31, fits nicely into this correlation. Table 2. Rate constants for the octane dehydrogenation reactions by complexes 1a-1e at 200 °C.a R group
k, min-1 b
σ, min-1
pKa14
-CH2Ph, 1d
v. slow
-
4.31
-CH3, 1a
4.78 x 10-4
8.3 x 10-5
4.76
7.79 x 10
-4
-5
4.87
-CMe3, 1c
8.98 x 10
-4
-4
5.05
-CH=CMe2, 1e
3.11 x 10-3
-CH2CH3, 1b
7.0 x 10 1.1 x 10
2.3 x 10-4
a
5.12 b
rate for 1d was too slow to measure reliably. k is the rate constant for eq 1. σ is the standard deviation. However, the effect of the carboxylic acid on the overall rate is also consistent with a variation of the rate of the C–H activation step. Previous reported DFT calculations by the Cundari group have shown that the identity of the carboxylate does not reduce the activation energy barrier of the C-H activation step. 15 Therefore, we also used complexes 1a-1e to determine the effect of the carboxylate on just the C–H activation step. To do this, the kinetic studies for C-H activation of octane were performed at 160 °C in the presence of potassium carbonate, conditions that allow activation but not β-hydrogen elimination (eq 3; the carbonate is present to remove the HOAc that is produced). The disappearance of the (dmPhebox)Ir carboxylates were monitored with time by the UV-vis measurement using an apparatus that connected a high-pressure bulb to a 2 mm UV cell (see Figure S4 for apparatus).
At = (A0–A∞)e–kt + A∞
(4)
Finally, we have also examined the reaction of 1e and octane in the presence of NaBArF4, which has been reported to lower the barrier for the β-H elimination step.13 This was postulated to be due to the conversion of the κ2-acetate to the κ1 form upon coordination of the Na+ to the keto-oxygen. The high-pressure NMR tube with a Teflon valve was charged with 1e, NaBArF4, and octane. The reaction was heated at 50 °C and monitored over time by 1H NMR spectroscopy. Due to the poor solubility of the solids, the reaction could not be quantified, but from the 1H NMR spectra and the GC analysis after 60 hours, octene formation and isomerization was observed (see Figures S20, S21). A control experiment without added NaBArF4 showed no octenes. Consequently, use of 1e and NaBArF4 allows for facile octane dehydrogenation under mild conditions.
CONCLUDING REMARKS In summary, five (dmPhebox)Ir carboxylates were designed and synthesized to understand the effect of the carboxylate ligand on alkane C-H activation and the β-H elimination. The capability of carboxylate ligands to adjust the reactivity of the β-H elimination step is in accordance with the observed Lewis acid effects, both of which allow for more facile formation of an empty site on the iridium metal center both opening the carboxylate coordination from κ2 to κ1. The experimental results agree with the prior DFT calculations that showed that the carboxylate ligands have no influence on the C-H activation step, but have a significant impact on the β-elimination step.
AUTHOR INFORMATION Corresponding Author
(3) *E-mail:
[email protected] ASSOCIATED CONTENT
The UV absorbance was measured periodically by tipping the contents of the high-pressure portion of the apparatus into the UV cell. Absorbance (375 nm) was plotted according to equation 4 to obtain the first-order rate constants (Table 3). The rate data show that there is no significant effect of the carboxylate ligand on the C-H activation step, which experimentally supports the conclusions from the earlier DFT calculations.15 Consequently, all of the observed variation in rate can be ascribed to the β-hydride elimination step of the reaction. Table 3. Rate constants for the C-H activation step by complexes 1a-1e at 160 °C R group
k, min-1 a
σ
R2
-CH3, 1a
0.0117
4.6 x 10-3
0.974
-CH2CH3, 1b
0.0116
2.0 x 10-3
0.981
-CMe3, 1c
0.0114
2.2 x 10-3
0.967
- CH2Ph, 1d
0.0120
1.72 x 10-3
0.974
0.0115
-3
0.996
-CH=CMe2, 1e a
0.68 x 10
k is the rate constant for eq 3. σ is the standard deviation.
Supporting Information. Experimental details for synthesis and characterization data of (dmPhebox)Ir carboxylate complexes; details of the kinetics experiments; X-ray crystallographic data for (dmPhebox)Ir(O2CCH2CH3)2, 1b, (dmPhebox)Ir(O2CCMe3)2(H2O), 1c, (dmPhebox)Ir(O2CCH2Ph)2(H2O), 1d, and (dmPhebox)Ir(O2CCH=CMe2)2(H2O), 1e, (CCDC# 1582712, 1586732, 1586733, 1582713). This material is free of charge via the Internet.
ACKNOWLEDGMENT This work was supported by the National Science Foundation through the CCI center for Enabling New Technologies through Catalysis (CENTC) Phase II Renewal, Grant CHE-1205189. Prof. Tom R. Cundari, Dr. Dale Pahls, Prof. Melanie S. Sanford, Prof. Elon A. Ison, Prof. Alan S. Goldman and Prof. Karen I. Goldberg are thanked for helpful discussions.
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(6) Dobereiner, G. E.; Crabtree, R. H. Chem. Rev. 2010, 110, 681703. (7) Kumar, A.; Goldman, A. S. In Topics in Organometallic Chemistry.; van Koten, G., Gossage, R. A., Eds.; Springer International Publishing: Cham, Switzerland, 2016; Vol. 54, pp 307. (8) Ito, J.; Nishiyama, H. Eur. J. Inorg. Chem. 2007, 1114-1119. (9) Ito, J.; Shiomi, T.; Nishiyama, H. Adv. Synth. Catal. 2006, 348, 1235-1240. (10) Ito, J.; Kaneda, T.; Nishiyama, H. Organometallics 2012, 31, 4442-4449.
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