Electrophilic RhI catalysts for arene H/D exchange in acidic media: Evidence for an electrophilic aromatic substitution mechanism

Article abstract of DOI:10.1016/j.molcata.2016.07.045

A series of new rhodium (I) complexes supported by bidentate nitrogen-donor ligands with varying electronic and steric properties were synthesized in situ and evaluated for catalytic arene C–H/D activation. In trifluoroacetic acid (HTFA), these complexes are proposed to mediate H/D exchange of arene C–H/D bonds by an electrophilic aromatic substitution mechanism that involves Rh-mediated activation of HTFA (or DTFA). DFT calculations support the proposed pathway for the H/D exchange reactions.

Full text of DOI:10.1016/j.molcata.2016.07.045

Accepted Manuscript
Title: Electrophilic RhI catalysts for arene H/D exchange in
acidic media: evidence for an electrophilic aromatic
substitution mechanism
Author: Michael S. Webster-Gardiner Paige E. Piszel Ross Fu
Bradley A. McKeown Robert J. Nielsen William A. Goddard
III T. Brent Gunnoe
PII:
S1381-1169(16)30295-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.07.045
MOLCAA 9979
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
27-6-2016
23-7-2016
25-7-2016
Please cite this article as: Michael S.Webster-Gardiner, Paige E.Piszel, Ross Fu, Bradley
A.McKeown, Robert J.Nielsen, William A.Goddard, T.Brent Gunnoe, Electrophilic
RhI catalysts for arene H/D exchange in acidic media: evidence for an electrophilic
aromatic substitution mechanism, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.07.045
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Electrophilic RhI catalysts for arene H/D exchange in acidic media:
evidence for an electrophilic aromatic substitution mechanism
This paper is dedicated to Professor Georgiy B. Shul'pin whose research has profoundly impacted the
field of C–H activation and functionalization.
Michael S. Webster-Gardiner,a Paige E. Piszel,a Ross Fu,b Bradley A. McKeown,a Robert J. Nielsen,b
William A. Goddard III,*b and T Brent Gunnoe*a
a Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, USA
b Materials and Process Simulation Center, Department of Chemistry, California Institute of Technology,
Pasadena, California 91125, USA
*Corresponding Author
E-mail address: tbg7h@virginia.edu
E-mail address: wag@wag.caltech.edu

Highlights
- Rh(I) catalyzed H/D exchange
- Ligand effects
- Electrophilic aromatic substitution

TOC Graphic

Abstract
A series of new rhodium (I) complexes supported by bidentate nitrogen-donor ligands with varying
electronic and steric properties were synthesized in situ and evaluated for catalytic arene C−H/D
activation. In trifluoroacetic acid (HTFA), these complexes are proposed to mediate H/D exchange of
arene C−H/D bonds by an electrophilic aromatic substitution mechanism that involves Rh-mediated
activation of HTFA (or DTFA). DFT calculations support the proposed pathway for the H/D exchange
reactions.
Keywords Rhodium, C–H Activation, Arene, Acid, Electrophilic

1. Introduction
Efficient and selective functionalization of hydrocarbon C−H bonds has been an area of intense study
[1-14], but many examples of transition metal mediated C−H bond functionalization rely on directing
groups to promote selectivity and activity [15-18]. Thus, developing catalysts that functionalize
unactivated hydrocarbons (e.g., arenes and alkanes) remains challenging [2, 19-23]. The discovery by
Shilov and coworkers that simple PtII salts can activate C–H bonds resulted in the demonstration that
electrophilic metals are viable catalysts for alkane and arene C–H functionalization [4, 24-26]. The
primary drawback of the initial Shilov process for alkane functionalization was the use of PtIV as a
stoichiometric oxidant.
Studies of Shilov type catalysts have proposed that the rate limiting step is C−H activation through an
electrophilic substitution mechanism (Scheme 1) [8, 14]. Since the discovery that electrophilic PtII can
catalytically functionalize hydrocarbons, new catalysts based on electrophilic HgII[27], PtII[28], PdII [29,
30], AuI/III[31] and I2[32] have been reported. Importantly, over oxidation can be avoided by protecting
the functionalized hydrocarbyl with electron-withdrawing groups. For example, in one variant a Pt
catalyst is used to functionalize methane in oleum to produce methylbisulfate, which is protected against
further oxidation by the electron-withdrawing bisulfate group through an increase in the activation barrier
toward electrophilic C–H activation for CH3OSO3H [28, 33]). These catalysts appear to only operate
efficiently in concentrated and oxidizing super acids, such as oleum, which makes product separation and
solvent recycling difficult[34]. In addition, these electrophilic catalysts are readily poisoned by water (or
other Lewis bases) and can suffer from product inhibition [33]. More recently, main group elements and
transition metal complexes have been shown to activate and functionalize hydrocarbons in less acidic,
trifluoroacetic acid [35-41].
Scheme 1. Activation of C–H bonds by electrophilic substitution.
Extension of the general strategy for Shilov type hydrocarbon functionalization would benefit from
catalysts that are less susceptible to inhibition by Lewis bases, which could allow chemistry in non-super
acidic media. Developing reactivity with earlier transition metals (i.e., earlier than the Ni, Pd and Pt triad)
could provide more tolerance of Lewis basic groups, but major challenges include avoiding oxidation of
the metal to higher valent states that are incapable of C–H activation and developing complexes that
possess electrophilic hydrocarbyl ligands (after C–H activation) [42-53]. Recently, a number of catalysts
based on Ir and Rh have been shown to be active for benzene C−H activation [50-61]. Rhodium
complexes are particularly attractive due to the possibility of C−H activation by RhI [62] or RhIII [55].
Further, oxidation of the rhodium center can be achieved with air recyclable Cu(II) salts, which provides a
strategy for indirect use of dioxygen or air as the terminal oxidant [60, 63].
A common method for generating an active electrophilic metal center is to generate a coordinatively
unsaturated complex through halide abstraction from the metal with a silver salt of the form AgX (X =

triflate, acetate, tetrafluoroborate, etc.). The recent report that Lewis acids can facilitate H/D exchange
between arenes and acidic media complicates analysis of transition metal-mediated C–H activation [64].
Thus, we sought a method to generate active catalysts in situ without the use of an additive Lewis acid,
such as AgX.
Recently, two RhI complexes bearing neutral bidentate nitrogen donors were shown to be catalysts for
the rapid H/D exchange between benzene and trifluoroacetic acid [62]. The most active catalyst for
aromatic H/D exchange, (FlDAB)Rh(COE)(TFA) (FlDAB = N,N'-bis-(pentafluorophenyl)- 2,3-dimethyl-
1,4-diaza-1,3-butadiene, COE = cyclooctene, TFA = trifluoroacetate) possesses easily modulated aryl
groups and therefore was chosen for a study of the impact of substituent variation on catalytic benzene
H/D exchange reactions. This report focuses on understanding the impact of the diimine ligand on
catalytic H/D exchange activity of arenes in DTFA (Scheme 2). By varying the electronic character of the
ligand, the influence of electron donating versus withdrawing is studied. Further, the impact of sterics on
catalysis is reported. These catalysts were then explored for C−H functionalization of benzene in acidic
and neutral media.
Scheme 2. Diimine proligands designed to vary the electron donor ability and steric profile of RhI
catalysts (iPr = isopropyl).
2. Experimental
2.1 Materials and instruments
Unless otherwise noted, all synthetic procedures were performed under anaerobic conditions in a
nitrogen-filled glovebox or by using standard Schlenk techniques. Glovebox purity was maintained by
periodic nitrogen purges and was monitored by an oxygen analyzer (O2 < 15 ppm for all reactions).
Tetrahydrofuran, toluene, and n–pentane were dried by distillation from sodium. Benzene was purified by

passage through a column of activated alumina. Hexamethylbenzene was used as received. C6D6 and
toluene-d8 were stored under a N2 atmosphere over 4Å molecular sieves. Trifluoroacetic-d1 acid was used
as received. Nitrogen (99.99%) was purchased from GTS Welco and used as purchased. GC/MS analysis
was performed using a Shimadzu GCMS–QP2010 Plus system with a 30 mm × 025 mm RTx-Qbond
column with 8 µm thickness using electron impact ionization. All other reagents were used as purchased
from commercial sources.
2.2 Ligand and complex synthesis
2.2.1 Ligand synthesis
Most of the diimine ligands (diazbutadienes, or DABs) were synthesized following a modified
literature procedure [65]: To a round-bottomed flask, 2.1 equivalents of the aniline, 1 equivalent of 2,3-
butanedione, 0.1 equivalents of p-toluenesulfonic acid, and 4 equivalents of trimethylorthoformate were
combined in 10 mL of anhydrous methanol under air. The reaction mixture was stirred for 8 hours
accompanied by the precipitation of a yellow solid. The solid was collected by filtration, washed with 5
mL of cold methanol, and dried under vacuum. The identity of the DABs was confirmed by comparison
to reported NMR data for 3,5-diMeDAB = N,N'-bis-(3,5-dimethylphenyl)-2,3-dimethyl-1,4-diaza-1,3-
butadiene)[66], 2,6-dippDAB = N,N'-bis-(2,6-diisopropylphenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene)
[67], 2,6-diClDAB = N,N'-bis-(2,6-dichlorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene) [68], 3,5-
diCF3DAB = N,N'-bis-(3,5di(trifluoromethyl)phenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene) [69], 2,6-
diFlDAB = N,N'-bis-(2,6-difluorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene)[70] (2,4-diFlDAB =
N,N'-bis-(2,4-difluorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene),
(4-FlDAB
=
N,N'-bis-(4-
fluorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene)[71], (FlDAB = N,N'-bis-(pentafluorophenyl)-2,3-
dimethyl-1,4-diaza-1,3-butadiene) [65]. The ligands (2,6-dip-4-nitroDAB = N,N'-bis-(2,6-diisopropyl,4-
nitrophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene) and (4-nitroDAB = N,N'-bis-(4-nitrophenyl)-2,3-
dimethyl-1,4-diaza-1,3-butadiene) were prepared according to literature procedures [72], 4-nitroDAB was
confirmed to reported NMR data [73].
2.2.2 Procedure for synthesis of in situ generated diimine rhodium complexes
Under nitrogen, the diimine ligand (2 equiv, 414.5 μmol) in THF (10 ml) was added to a stirring
solution of [Rh(-TFA)(2-C2H4)2]2 (1 equiv, 207 μmol) in THF (15 mL). The solution immediately
became deep purple to black. After 1 hour the solvent was removed in vacuo, and the resulting solid was
washed with n-pentane (20 mL) and then dried under vacuum to yield the corresponding in situ rhodium
catalyst.
2.3. General Procedure for benzene H/D exchange
Stainless steel reactors were used for all H/D exchange reactions. Prior to use, the interior of the
reactor and stir bars were treated with 35% hydrogen peroxide, washed with acetone and dried at 180 °C.
A stock solution of catalyst was made by placing 0.02 mmol of the Rh complex into a glass vial and
dissolving in 6.2 mL (80.4 mmol) of trifluoroacetic acid-d1. C6H6 (360 μL, 4 mmol) was then added to the
stock solution. Then, 1 mL of the stock solution was added to each pressure reactor, which was sealed
under N2, pressurized with 800 psi N2, and placed in a heating block set to 120 °C, 150 °C or 180 °C. At

the end of the reaction, the pressure reactor was allowed to cool to room temperature, slowly vented, and
opened. An aliquot (1 μL) of the reaction mixture was placed into a vial that contained 1 mL of acetone.
This vial was then analyzed by GC-MS. The extent of H/D exchange was analyzed by deconvolution of
the GC-MS spectrum using an excel sheet developed by Periana, Goddard and coworkers [74].
2.4 General Procedure for toluene H/D exchange
To a glass vial, a stock solution was made by dissolving 12 μmol of Rh catalyst in 7.8 mL of
trifluoroacetic acid and 240 μL of toluene-d8. Then, 0.8 mL of stock solution was placed into a J-young
NMR tube. The J-Young NMR tube was then placed in an oil bath at 120 °C, 150 °C or 180 °C. After 15
minutes, the J-Young NMR tube was removed from the high temperature oil bath, allowed to cool to
room temperature, and a capillary tube filled with DMSO-d6 was added. Then the reaction mixture was
analyzed by 1H NMR spectroscopy.
2.5 General Procedure for aliphatic H/D exchange
Stainless steel reactors were used for H/D exchange reactions. All pressure reactors and stir bars were
treated with 35% hydrogen peroxide, washed with acetone and dried at 180 °C before use. For liquid
aliphatics (cyclohexane) the same procedure described for benzene (see above) was used. For methane,
the following procedure was followed: A stock solution was made by placing 0.02 mmol of Rh catalyst
into a glass vial and dissolved into 6.2 mL (80.4 mmol) of trifluoroacetic acid-d1. Then, 1 mL of the stock
solution was added to each reactor, which was sealed under N2, pressurized with 100 psi of methane, and
placed in a heating block set to 120 °C, 150 °C or 180 °C. At the end of the reaction, the reactor was
allowed to cool to room temperature. Then, 2 uL of reactor headspace was analyzed by GC-MS. Extent of
H/D exchange was analyzed by deconvolution of the GC-MS spectrum using an excel sheet developed by
Periana, Goddard and coworkers [74] .
2.6 General Procedure for arene functionalization in trifluoroacetic acid
A solution was made by placing 0.02 mmol of Rh catalyst into a glass pressure tube and dissolved in
6.125 mL (80 mmol) of trifluoroacetic acid. Benzene (90 μL, 1 mmol) and hexamethylbenzene (65mg,
0.4 mmol) was added to the solution. Then, 2 mmol of oxidant was added to the glass pressure tube. The
pressure tube was sealed and then heated at 150 °C or 180 °C. At the end of the reaction, the reactor was
allowed to cool to room temperature. An aliquot (1 μL) of the reaction mixture was placed into a vial with
1 mL of acetone. The solution was analyzed by GC-MS.
2.7 General Procedure for arene functionalization in non-acidic media
A solution was made by placing 0.02 mmol of Rh catalyst and hexamethylbenzene (65mg, 0.4 mmol)
into a glass pressure tube and dissolved into 6.5 mL (80 mmol) of benzene. Then, 2 mmol of oxidant was
added to the glass pressure tube. The pressure tube was sealed and then heated at 150 °C or 180 °C. At the
end of the reaction, the reactor was allowed to cool to room temperature. An aliquot (1 μL) of the reaction
mixture was placed into a vial with 1 mL acetone. The solution was then analyzed by GC-MS.
2.8 Calculations for mechanistic investigation

DFT calculations were performed using the M06 functional [75, 76] and the triple-zeta basis set 6-
311G**++ [77, 78] for all atoms except for rhodium. For rhodium, the triple-zeta basis set and
pseudopotential LACV3P**++ [79] was used. Further details may be found in the supporting
information.
3. Results and discussion
3.1 Synthesis of (DAB)Rh(TFA)(2-C2H4) complexes
Diimine proligands were coordinated to Rh by reaction of [Rh(-TFA)(2-C2H4)2]2 with the diimine
in THF (Scheme 3). To a yellow-brown stirring solution of [Rh(-TFA)(2-C2H4)2]2 in THF, the
appropriate diimine ligand was added. Almost instantaneously, the solutions turn deep purple/black. The
Rh complexes are not isolated, but rather were used after in situ generation.
Scheme 3. General in situ synthesis of rhodium catalyst precursors from diimines and [Rh(-TFA)(2-
C2H4)2]2.
3.2 Catalytic Benzene H/D Exchange
In 1958 H/D exchange of arenes with trifluoroacetic acid-d1 was shown to occur, albeit quite slowly
(especially at lower temperatures) [80]. Therefore, for all reactions reported herein, we determined the
extent of uncatalyzed H/D exchange between the substrates and trifluoroacetic acid-d1. The background
H/D exchange reaction increased with temperature and was determined to be the equivalent of 7(1),
15(4), and 96(9) turnovers (TO) (relative to standard amount of Rh catalyst, see below) at 120 °C, 150 °C
and 180 °C, respectively (numbers in parenthesis are standard deviations from multiple experiments).
These TO were subtracted from the results for catalytic H/D exchange using the various Rh catalysts. Our
standard screening conditions incorporated 0.5 mol % (relative to benzene) of Rh catalyst with 20
equivalents of trifluoroacetic acid-d1 relative to benzene for 4 hours (Scheme 4).
Scheme 4. Rhodium catalyzed H/D exchange between C6H6 and DTFA

Figure 1 shows the results of the catalytic H/D exchange between benzene and trifluoroacetic acid-d1.
The effect of temperature was studied for all catalysts. The optimal temperature for catalysts 1, 5, and 7-
11 is 150 °C. Complexes 2-4 and 6 showed minimal catalytic activity at all temperatures studied.
Catalysts 1, 5, and 7-11 are all active at 120 °C; however, only complexes 5, 7, 9 and 11 show activity at
180 °C. The inactivity of complexes 1, 8, and 10 at 180 °C is likely due to catalyst decomposition. In
general, more electron-withdrawing ligands appear to enhance catalysis. Catalyst 11 shows the highest
TO at 150 °C with 140(14) TO after 4 hours, which corresponds to an apparent turnover frequency (TOF,
calculated by TO/time) of 9.7(9) x 10-3 s-1 (Table 1). Relative to the catalyst with a phenyl substituent
(complex 1), which gives 25(7) TO after 4 hours (TOF ~ 1.7(5) x 10-3 s-1), the apparent activity of 5 is
approximately 5.5 times greater. The complexes 8-10 all show TO after 4 hours between catalyst
precursors 1 and 11. This is expected due to the increased number of electron withdrawing fluorine
substituents on the aryl ring relative to 1 but decreased relative to 11. Comparison of data using
complexes 8-10 reveals that the position of fluorine substituents on the phenyl rings makes little
difference to catalytic activity. Comparison of data using catalytic precursors 2 and 5 highlights the
impact of electron-withdrawing groups. Complex 2 possesses electron-donating methyl groups in the 3
and 5 positions on the phenyl ring whereas 5 has trifluoromethyl groups in the 3 and 5 positions on the
ring. No TO above background reactivity is observed using 2; however, complex 5 is an active precatalyst
with 55(2) TO which corresponds to an apparent TOF of 3.8(1) x 10-3 s-1 at 150 °C. Furthermore,
installing electron-withdrawing nitro groups in the para position of the the DAB phenyl results in the
second most active catalyst of the series, as catalyst as 7 gives 82(2) TO (apparent TOF of 5.7(1) x 10-3 s-
1).
We propose three possible rationalizations for increased efficacy for benzene/trifluoroacetic acid-d1
H/D exchange using Rh complexes with electron-withdrawing substituents on the DAB ligand: 1) The C–
H activation occurs at the RhI oxidation state, and less donating ligands stabilize RhI in the presence of
oxidizing trifluoroacetic acid whereas more donating ligands result in oxidation to RhIII; 2) the C–H
activation occurs by an electrophilic substitution process (regardless of the exact mechanism, which could
be "classic" electrophilic substitution, see Scheme 1, or concerted-metalation deprotonation) [81] and is
enhanced by less electron-rich Rh catalysts; 3) the C–H "activation" occurs by an electrophilic aromatic
substitution pathway, as we have previously demonstrated for other metals in acidic media [64], and more
electrophilic Rh complexes enhance reactivity. We have no evidence that the DAB-RhI complexes are
oxidized in the presence of trifluoroacetic acid. For example, no color changes are observed as would be
expected for oxidation from RhI to RhIII. As discussed below, the results of H/D exchange with toluene is
most consistent with the electrophilic aromatic substitution pathway.

H/D exchange of benzene
160
140
120
100
80
120 °C
150 °C
180 °C
60
40
20
0
1
2
3
4
5
6
7
8
9
10 11
Catalyst
Fig. 1. Results from Rh catalyzed H/D exchange between benzene and trifluoroacetic acid-d1 using
complexes 1-11 (see Scheme 2). Conditions: 0.5 mol % Rh relative to benzene, 20 equivalents of
trifluoroacetic acid-d1 relative to benzene for 4 hours at 120 °C, 150 °C or 180 °C.
Increased steric bulk around the rhodium metal inhibits the catalytic H/D exchange. At all
temperatures studied, complexes 2 and 4 show minimal to no activity above the background reactions.
There is a possibility that the alkyl groups are too electron-donating resulting in a rhodium center that is
not sufficiently electrophilic. Installing a nitro group should mitigate the electron-donating effects of the
alkyl groups. However, even installing a nitro group on the ligand, as in the comparison of 4 and 6, does
not result in catalytic H/D exchange. This observation provides support that alkyl groups on the aryl 2,6-
positions sterically hinder access to the rhodium center, which likely inhibits coordination of benzene or
trifluoroacetic acid.

Table 1. Rhodium catalyst precursors and apparent TOFs (s-1) after 4 hours at 120 °C, 150 °C and 180 °C
for H/D exchange between benzene (C6H6) and trifluoroacetic acid-d1.a
TOF (s-1)
150 °C
Catalyst Precursor
120 °C
180 °C
6(3) x 10-4 1.7(9) x 10-3
0
1
2
0
0
0
0
0
0
0
0
0
3
4
2.5(2) x 10-3 3.8(2) x 10-3 1.8(8) x 10-3
5
0
0
0
6
2.3(1) x 10-3 5.7(2) x 10-3 3.3(7) x 10-3
1.3(5) x 10-3 4(1) x 10-3
1.5(7) x 10-3 2.9(6) x 10-3 1.6(8) x 10-3
7
0
8
9
2(1) x 10-3
5(2) x 10-3
3(1) x 10-3
9(2) x10-3
0
10
11
5.2(7) x 10-3
a
Reactions performed using 1 mL of a solution containing 80.4 mmol (6.2 mL) of trifluoroacetic acid
with 4 mmol (360 L) of benzene with the Rh catalyst (0.5 mol % relative to benzene).
3.3 Attempted H/D exchange with toluene, cyclohexane and methane
The Rh catalyzed H/D exchange between toluene-d8 and trifluoroacetic acid was studied. The
1
regioselectivity of the reactions was determined using H NMR spectroscopy (Scheme 5). For reactions
with toluene, electrophilic aromatic protonation mechanisms favor the ortho and para positions whereas
organometallic complexes often favor activation of the meta and para positions [64, 82]. The Rh
complexes (0.5 mol % relative to toluene-d8) were heated (120 °C, 150 °C, or 180 °C) in trifluoroacetic
1
acid with toluene-d8. After 15 minutes, the solutions were analyzed by H NMR spectroscopy to
determine o:m:p ratio for proton exchange (Table 2). All of the Rh catalysts exhibited selectivity for H/D
exchange at the ortho and para positions with (ortho + para) to meta ratios > 6:1, which is very similar to
the ratio for trifluoroacetic acid electrophilic aromatic substitution (Table 2, Entry 1). None of the Rh
catalysts activate the methyl group of toluene. These results point to a likely reaction pathway that
involves protic electrophilic aromatic substitution rather than Rh mediated C–H activation. In this
proposed reaction pathway, the Rh catalyst activates HTFA to liberate H+ (from HTFA) or D+ (from
DTFA), which then adds to the arene to form a Wheland type arenium intermediate. We have previously
proposed a similar reaction pathway for Lewis acid mediated H/D exchange between arenes and acidic
media [64].

Scheme 5. Rhodium catalyzed H/D exchange between toluene-d8 and HTFA.
Table 2. Evaluation of Rh catalyst precursors 1-11 for the H/D exchange of toluene-d8 with
trifluoroacetic acid and ortho:meta:para selectivity.a
Ratio of o:m:p
Entry
Catalyst Precursor
120 °C
150 °C
180 °C
1
2
HTFA
8(2) :1 : 7(2)
8(0.3) : 1 : 7(0.3)
12(3) : 1 : 16(3)
5.8(1) : 1 : 7(2)
4(2) : 1 : 3(2)
12(1) : 1 : 11(1)
14(0.7) : 1 : 13(0.5)
5(1) : 1 : 6.4(1)
9(2) : 1 : 9(2)
9(0.2) : 1 : 7(0.4)
12(1) : 1 : 9(1)
8(2) : 1 : 8(2)
[Rh(-TFA)(2-C2H4)2]2
3
1
2
4
12(4) : 1 : 10(3)
11(1) : 1 : 10(1)
7(1) : 1 : 6(1)
5
9.1(1) : 1 : 9(1)
8(2) : 1 : 8(1)
3
6
5(2) : 1 : 6(2)
4
7
9(0.2) : 1 : 8(0.2)
9(0.1) : 1 : 9(1)
7(2) : 1 : 8(0.3)
6(1) : 1 : 7(0.01)
9(0.04) : 1 : 9(0.1)
4(2) : 1 : 3(1)
16(2) : 1 : 14(2)
10(0.4) :1 : 10(0.3)
7(0.03) : 1 : 7(0.2)
8(0.3) : 1 : 8(0.3)
8(1) : 1 : 7(1)
14(2) : 1 : 10(1)
10(1) : 1 : 10(0.3)
14(2) : 1 : 15(2)
9(0.2) : 1 : 10(1)
9(1) : 1 : 7(1)
5
8
6
9
7
10
11
12
13
8
9
9(0.7) : 1 : 9(1)
10(0.3) : 1 : 9(0.3)
11(1) : 1 : 10(1)
12(2) : 1 : 10(1)
10
11
9(2) : 1 : 8(2)
a
Reactions performed using 0.8 mL of a solution containing 101 mmol (7.8 mL) of trifluoroacetic acid
with 2.26 mmol (240 L) of toluene with the Rh catalyst (0.5 mol % relative to toluene).
A recent report by Ison and coworkers disclosed that the mechanism for catalytic H/D exchange
between arenes and acids using Cp*Ir(III) complexes varies as a function of acid identity [82]. The study
showed that electrophilic aromatic protonation dominates in trifluoroacetic acid whereas an
organometallic mechanism, where an Ir−Ph is formed and is subsequently protonated by the deuterated

solvent, occurs in methanol and acetic acid. Under our standard conditions with rhodium complexes 7 and
11, no catalytic H/D exchange was observed between benzene and acetic acid, trifluoroethanol or
methanol. These weaker acids (compared to trifluoroacetic acid) are most likely insufficiently acidic to
promote the protic electrophilic aromatic substitution. These results suggest that Rh−Ph bonds are not
likely formed under these conditions.
C−H activation reactivity with other hydrocarbon substrates was attempted with the most active Rh
catalyst precursors. Solutions of 0.5 mol % of 7 or 11 in trifluoroacetic acid-d1 were pressurized with 100
psi of CH4 and heated to 120 °C, 150 °C or 180 °C. After 4 hours, the headspace of the reactors was
analyzed by GC/MS. Analysis revealed no deuterium incorporation into methane beyond the natural
isotope abundance. Under the same conditions C−H activation of cyclohexane was also examined, but
again no deuterium incorporation was detected. This lack of reactivity is consistent with the proposed
mechanism for arenes in trifluoroacetic acid which relies on the formation of an arenium ion.
3.4 Attempted benzene oxidation in trifluoroacetic acid
Rhodium complexes 7 and 11 were examined for benzene oxidation with a number of chemical
oxidants. In previous work, we have shown (FlDAB)Rh(-TFA)(2-C2H4) catalyzes the oxidative
hydrophenylation of ethylene (using Cu(II) salts as oxidant) to produce styrene [60]. Therefore attempts
to generate other oxidation products from benzene were undertaken. A 0.5 mol % solution of 11 in
trifluoroacetic acid with 50 equivalents of benzene and 100 equivalents of copper(II) acetate was heated
to 150 °C for 4 hours. The reaction mixture was then analyzed by GC-MS. However, no evidence of
benzene oxidation was obtained (i.e., no observation of PhOAc, PhTFA or biphenyl). Silver oxidants
(AgTFA, Ag2O) and hyper-valent iodine (III) compounds, such as (Ph)I(OAc)2, were also used as
potential oxidants, but the only functionalized arene products detected were from direct reaction with the
oxidant alone.
3.5 Benzene oxidation in non-acidic media
The lack of benzene oxidation in HTFA (see above) could result from the failure of the Rh complexes
to mediate benzene C–H activation in trifluoroacetic acid. We speculated that non-acidic solvents might
allow arene functionalization chemistry (Scheme 6). A 0.5 mol % solution of 7 or 11 in benzene with 100
equivalents of copper (II) acetate was heated to 150 °C and 180 °C for 4 hours. Analysis by GC/MS
revealed no benzene functionalization at 150 °C. However, at 180 °C the production of biphenyl and a
minor quantity of PhOAc was observed when using Cu(OAc)2. Control experiments at 180 °C without
rhodium catalyst produced the same quantity of phenyl acetate; however, no biphenyl was observed. The
formation of PhOAc presumably is due to reaction with Cu(OAc)2. This was confirmed by running a
control reaction in toluene and producing significantly more benzylic acetate than tolylacetate (see
Supporting Information, Scheme S1). The production of biphenyl was also observed with catalyst 4, 7, 11
and [Rh(-TFA)(2-C2H4)2]2 in similar yields.
Scheme 6. Attempted catalytic oxidation of benzene.

3.6 Computational investigation of mechanism
Our assertion that H/D exchange occurs by a Rh-mediated electrophilic aromatic substitution pathway
is supported by DFT calculations. Table shows the calculated change in Gibbs free energy for protonation
of arenes (benzene and toluene) by either HTFA or H2TFA+. H2TFA+ is formed upon Rh-mediated
activation of HTFA: [Rh]n+ + 2HTFA  {[Rh]-TFA}(n-1)+ + H2TFA+. In Table 3 column 3, we see that
HTFA solvent can indeed support the formation of arenium cations, since they are less acidic than
+
TFAH2 . Also, protonation of the meta position in toluene is comparable to that of benzene since there is
no stabilization of resonance structures due to a tertiary carbocation. Protonation of the ortho or para
positions is more favorable due to the resonance structure with a tertiary carbocation. However, due to the
high self-ionization energy of HTFA, formation of the arenium cation in neutral HTFA solution is quite
uphill (Table 3 column 4).
Our assertion is that our Rh complexes act as Lewis acids, coordinating to HTFA and enhancing its
Brønsted acidity. We performed calculations for this process with 11 and our results are shown in Scheme
7. The calculations reveal that (FlDAB)Rh+ is a very strong Lewis base and will coordinate two
+
equivalents of TFAH in solution, forming (FlDAB)Rh(TFAH)2 (top right). It is only 12.3 kcal/mol uphill
+
to produce TFAH2 from this species, much lower than the 45.8 kcal/mol required by HTFA self-
ionization. These results are provide an explanation for the catalytic ability of 11 and related complexes to
help arenes undergo H/D exchange.

Table 3. Calculated free energies of the formation of various arenium cations in uncatalyzed TFAH
solution. In all cases, the temperature was set to 150°C. Note that the difference in ΔG's for the two
rightmost columns is due to the self-ionization free energy of TFAH: 45.8 kcal/mol for
→
at 150 °C.
ΔG (kcal/mol)
ΔG (kcal/mol)
Entry
Reaction
X = TFAH
X = TFA-
1
-9.7 kcal/mol
-2.8 kcal/mol
-12.1 kcal/mol
-8.1 kcal/mol
-13.4 kcal/mol
36.0 kcal/mol
43.0 kcal/mol
33.6 kcal/mol
37.6 kcal/mol
32.4 kcal/mol
2
(ipso)
3
(ortho)
4
(meta)
5
(para)
Scheme 7. Energetics of (11)Rh+ complexing with TFAH. All free energies calculated at 150 °C and in
+
kcal/mol. Note that the lowest energy species is (11)Rh(TFAH)2 (top right) and that release of a proton
from this species to form (11)Rh(TFAH)(TFA) (bottom right) is uphill by only 12.3 kcal/mol.

4. Conclusion
Eleven new RhI diimine complexes have been generated in situ and examined for catalytic H/D
exchange between arenes and acidic media. The most active catalyst precursors possess electron-
withdrawing substituents on the diimine ligands. The mechanism of the arene H/D exchange reactions
most likely involves protic electrophilic aromatic substitution with Rh acting as a Lewis acid to activate
DTFA and provide access to D2TFA+. This was shown by monitoring the selectivity for H/D exchange of
toluene, which revealed selectivity for the ortho and para positions over the meta position. DFT
calculations demonstrate the viability of our proposed mechanism of H/D exchange. Attempts to extend
catalysis to other solvents and aliphatic hydrocarbons were unsuccessful, which is consistent with the
proposed protic electrophilic aromatic substitution. Although, attempts to functionalize benzene in acidic
media were unsuccessful with a range of chemical oxidants, catalysis in neat arene was successful, but
with low turnover number of ~2.
Acknowledgements
The authors acknowledge the National Science Foundation (CHE-1465145, TBG; NSF CHE-1214158,
WAG) for financial support. M.S.W-G. acknowledges support from AES for a graduate student
fellowship, the Jefferson Scholars Foundation for a dissertation year fellowship and Junqi Chen for
assistance with an experimental procedure.

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