Quantitative assay for the direct comparison of platinum catalysts in benzene H/D exchange

Article abstract of DOI:10.1021/om900495n

This paper describes a protocol for the direct comparison of diverse Pt catalysts in the H/D exchange between C₆H₆ and TFA-d ₁, CD₃CO₂D, and TFE-Od₃ using turnover number (TON) as a standar

Full text of DOI:10.1021/om900495n

5
316 Organometallics 2009, 28, 5316–5322
DOI: 10.1021/om900495n
Quantitative Assay for the Direct Comparison of Platinum Catalysts in
Benzene H/D Exchange
Amanda J. Hickman, Janette M. Villalobos, and Melanie S. Sanford*
Department of Chemistry, University of Michigan, 930 N. University Avenue, Ann Arbor, Michigan 48109
Received June 11, 2009
This paper describes a protocol for the direct comparison of diverse Pt catalysts in the H/D
exchange between C H and TFA-d , CD CO D, and TFE-d using turnover number (TON) as a
6
6
1
3
2
3
standard metric. An initial survey of Pt complexes, including commercial Pt salts (PtCl , K PtCl )
2
2
4
and Pt chloride complexes containing bidentate and tridentate nitrogen donor ligands, has been
conducted. These studies have established that the addition of AgOAc (in TFA-d ) or AgBF4
1
(in CD CO D and TFE-d ) displaces the Cl ligands on the Pt precatalyst, which leads to dramatically
increased turnover numbers. In general, PtCl and K PtCl provided the fewest turnovers, and
3 2 3
2
2
4
species containing bidentate ligands afforded higher turnover numbers than those with tridentate
ligands. A diimine Pt complex was found to be a top performing catalyst for H/D exchange with all
deuterium sources examined. Interestingly, the relative reactivity of many of the catalysts varied
dramatically upon changing the deuterium source, highlighting the need to thoroughly assay
potential catalysts under a variety of conditions.
Introduction
Scheme 1. Proposed Mechanism of Platinum-Catalyzed R-H
2,5
Oxidation
The development of Pt-based catalysts for the oxidation of
alkanes/arenes (R-H) to alcohols has been an important
area of research for nearly 40 years. Pioneering studies by
Shilov in the early 1970s demonstrated that a combination of
*
Corresponding author. E-mail: mssanfor@umich.edu.
1) Goldshleger, N. F.; Eskova, V. V.; Shilov, A. E.; Shteinman, A. A.
Zh. Fiz. Khim. 1972, 46, 1353.
2) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed.
998, 37, 2181.
3) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.;
Fujii, H. Science 1998, 280, 560.
4) Conley, B. L.; Tenn, W. J.; Young, K. J. H.; Ganesh, S. K.; Meier,
(
(
1
(
II
IV
simple Pt and Pt salts promotes the oxidation of methane
to methanol, and subsequent work in this field has aimed to
improve this system through variation of the ancillary ligands
(
S. K.; Ziatdinov, V. R.; Mironov, O.; Oxgaard, J.; Gonzales, J.;
Goddard, W. A.; Periana, R. A. J. Mol. Catal., A 2006, 251, 8.
II
1,2
(
5) Kua, J.; Xu, X.; Periana, R. A.; Goddard, W. A. Organometallics
002, 21, 511.
6) (a) Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 1997, 119, 848. (b) Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc.
997, 119, 10235. (c) Holtcamp, M. W.; Henling, L. M.; Day, M. W.;
Labinger, J. A.; Bercaw, J. E. Inorg. Chim. Acta 1998, 270, 467.
d) Johansson, L.; Ryan, O. B.; Tilset, M. J. Am. Chem. Soc. 1999, 121,
974. (e) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am.
at Pt (Scheme 1). In one of the most important advances,
2
II
Periana demonstrated that (bpym)Pt Cl (bpym = κ-2,2 -
0
2
(
bipyrimidine) catalyzes the conversion of methane to methyl-
bisulfate in fuming sulfuric acid with a 72% first-pass yield
3,4
and 81% selectivity.
1
(
1
II
Carbon-hydrogen bond activation at Pt centers of gen-
Chem. Soc. 2000, 122, 10846. (f) Heiberg, H.; Johansson, L.; Gropen, O.;
Ryan, O. B.; Swang, O.; Tilset, M. J. Am. Chem. Soc. 2000, 122, 10831.
eral structure A (Scheme 1, step 1) has been proposed as a key
step in Pt-catalyzed alkane/arene oxidation chemistry and has
therefore been an important focus of recent studies. The most
common literature assay for C-H activation involves the
(
1
(
g) Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2002, 124,
378. (h) Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc. 2001, 123, 5100.
i) Johansson, L.; Ryan, O. B.; Romming, C.; Tilset, M. J. Am. Chem. Soc.
e
II
2
001, 123, 6579. (j) Iverson, C. N.; Carter, C. A. G.; Baker, R. T.; Scollard, J.
D.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2003, 125, 12674.
k) Gerdes, G.; Chen, P. Organometallics 2003, 22, 2217. (l) Driver, T. G.;
Day, M. W.; Labinger, J. A.; Bercaw, J. E. Organometallics 2005, 24, 3644.
stoichiometric reaction of an alkane or arene with a Pt alkyl
complex (exemplified by B in eq 1).
containing diverse bi- and tridentate nitrogen donor ligands
participate in this model reaction, and studies of these systems
6-8
II
Many Pt complexes
(
(
(
m) Thomas, C. M.; Peters, J. C. Organometallics 2005, 24, 5858.
n) Karshtedt, D.; McBee, J. L.; Bell, A. T.; Tilley, T. D. Organometallics
2006, 25, 1801. (o) Owen, J. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 2006, 128, 2005. (p) Driver, T. G.; Williams, T. J.; Labinger, J. A.;
Bercaw, J. E. Organometallics 2007, 26, 294. (q) Williams, T. J.; Caffyn, A.
J. M.; Hazari, N.; Oblad, P. F.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 2008, 130, 2418.
(7) Zhang, F.; Kirby, C. W.; Hairsine, D. W.; Jennings, M. C.;
Puddephatt, R. J. J. Am. Chem. Soc. 2005, 127, 14196.
(8) Ong, C. M.; Burchell, T. J.; Puddephatt, R. J. Organometallics
2004, 23, 1493.
pubs.acs.org/Organometallics
Published on Web 08/24/2009
r 2009 American Chemical Society

Article
Organometallics, Vol. 28, No. 18, 2009 5317
Figure 1. Pt Catalysts Examined for H/D Exchange Reactions
9
have provided valuableinsightsintoreactivity andselectivity.
Table 1. Background H/D Exchange between C
a
as a Function of Time
H
6 6
and TFA-d
1
However, their relevance to the mechanism in Scheme 1
remains unclear, as C-H activation at Pt alkyl complexes of
general structure B is unlikely to be a key step within this
2
,5
catalytic cycle.
additive
none
2 h
6 h
12 h
24 h
1 ( 1
1 ( 1
1 ( 1
4 ( 1
4 ( 1
3 ( 1
7 ( 1
5 ( 1
5 ( 1
16 ( 3
13 ( 1
11 ( 1
4
4
mol % AgOAc
mol % AgCl
a
Conditions: benzene (23.2 μL, 0.26 mmol), AgOAc (1.7 mg,
A second method for studying C-H activation involves
Pt -catalyzed arene or alkane H/D exchange (eq 2). In
10 μmol) or AgCl (1.4 mg, 10 μmol) in TFA-d1 (0.5 mL, 6.5 mmol,
25 equiv relative to benzene) at 150 °C. Data are reported as % D
incorporation ( standard deviation.
II
contrast to the model systems in eq 1, H/D exchange does
not rely on the ability to isolate stable organometallic com-
plexes and is believed to proceed via intermediates directly
relevant to those in catalytic R-H oxidation (Scheme 1). The
1
reverse (complex 1 ), and/or to catalyze H/D exchange
6
3
13 11
12
reactions (complexes 2, 3, 4, and 7 ). However, despite
many literature studies of these and related complexes, their
reactivity has not been systematically compared by any
metric. The chloride complexes were utilized because they
can be conveniently synthesized in a single step from readily
II
10
catalytic activity of simple Pt salts as well as of nitrogen-
II
ligated Pt complexes
11-14
in H/D exchange reactions with
both C H and CH has been examined. However, all of
6
6
4
these studies used significantly different reaction conditions,
with variable times, temperatures, concentrations, catalyst
loadings, additives, and deuterium sources. Thus, there is
II
17
available Pt precursors. Catalytic experiments were con-
ducted with analytically pure samples of complexes 1-8,
which were stored at -35 °C in vials sealed with Teflon-lined
currently insufficient data in the literature to systematically
II
1
8
caps.
The H/D exchange between C H and trifluoroacetic acid-
d (TFA-d ) was selected as an initial test reaction. We felt
compare the reactivity of different Pt
Herein, we present a standard assay of Pt-catalyzed H/D
complexes.
6
6
exchange between C H and CF CO D (TFA-d1),
6
1
1
6
3
2
that this would be an ideal benchmark because analogues of
, 4, and 7 have been previously reported to catalyze this
CD CO D, and trifluoroethanol-d (TFE-d ) and utilize it
3
2
3
3
2
transformation under several different conditions.
to assess the reactivity (as measured by turnover numbers) of
11,12
II
a series of Pt complexes containing common nitrogen
donor ligands.
The
H/D exchange reactions were conducted in 4 mL Teflon-
sealed Schlenk tubes that were fully submerged in an oil bath.
At the completion of the reactions, samples were subjected to
a basic workup and then analyzed by GC-MS. Isotope
distributions were determined using a recently published
worksheet that allows for deconvolution of mass spectral
1
2
data for benzene isotopologues within ∼5% error.
Results
Throughout this paper, the % deuterium incorporation is
defined as the % of C-H bonds converted to C-D bonds.
Turnover numbers (TONs) are calculated as moles of D
incorporated per mole of catalyst. The reported error is the
standard deviation over at least two trials.
II
Our investigations focused on comparing Pt chloride
complexes 1-8 (Figure 1) as catalysts for H/D exchange
between C H and various deuterium sources. Complexes
II
6
6
1
-8 were selected because Pt species containing analogous
ligands are known to participate in stoichiometric R-H
activation (complexes 5, 6, and 8 ) or its microscopic
7
7
15
(
16) Ong, C. M.; Jennings, M. C.; Puddephatt, R. J. Can. J. Chem.
2
003, 81, 1196.
(17) Complex 1: Vicente, J.; Gonzalez-Herrero, P.; Perez-Cadenas,
(
9) For a review of C-H activation at Pt(II), see: Lersch, M.; Tilset,
M. Chem. Rev. 2005, 105, 2471, and references therein.
10) (a) Hodges, R. J; Webster, D. E.; Wells, P. B. J. Chem. Soc.,
Chem. Commun. 1971, 462. (b) Hodges, R. J; Webster, D. E.; Wells, P. B.
J. Chem. Soc. A 1971, 3230. (c) Hodges, R. J; Webster, D. E.; Wells, P. B.
J. Chem. Soc., Dalton Trans. 1972, 2571.
M.; Jones, P. G.; Bautista, D. Inorg. Chem. 2007, 46, 4718. Complex 2:
Neumann, R.; Bar-Nahum, I.; Khenkin, A. M. J. Am. Chem. Soc. 2004,
126, 10236. Complex 3: See Supporting Information. Complex 4:
Annibale, G.; Cattalini, L.; Chessa, G.; Marangoni, G.; Pitteri, B.; Tobe,
M. L. Gazz. Chim. Ital. 1985, 115, 279. Complex 5: Rauterkus, M. J.;
Krebs, B. Angew. Chem., Int. Ed. 2004, 43, 1300. Complex 6: Tu, C.; Wu,
X.; Liu, Q.; Wang, X.; Xu, Q.; Guo, Z. Inorg. Chim. Acta 2004, 357, 95.
Complex 7: ref 12. Complex 8: Peters, J. C.; Harkins, S. B.; Brown, S.
D.; Day, M. W. Inorg. Chem. 2001, 40, 5083.
(
(
11) Ziatdinov, V. R.; Oxgaard, J.; Mironov, O. A.; Young, K. J. H.;
Goddard, W. A.; Periana, R. A. J. Am. Chem. Soc. 2006, 128, 7404.
12) Young, K. J. H.; Meier, S. K.; Gonzales, J. M.; Oxgaard, J.;
Goddard, W. A.; Periana, R. A. Organometallics 2006, 25, 4734.
(
(13) Gerdes, G.; Chen, P. Organometallics 2004, 23, 3031.
(14) Kloek, S. M.; Goldberg, K. I. J. Am. Chem. Soc. 2007, 129, 3460.
(15) Harkins, S. B.; Peters, J. C. Organometallics 2002, 21, 1753.
(18) Analytically pure samples of 4 were challenging to obtain. Thus,
2 4
samples of 4 used in catalysis contained 0.05 mol % of residual K PtCl
as well as some residual DMF from recrystallization.

5
318 Organometallics, Vol. 28, No. 18, 2009
Hickman et al.
Figure 3. Comparison of turnover numbers for 1/AgOAc ver-
II
Figure 2. Turnover numbers for H/D exchange between ben-
zene and TFA-d catalyzed by 1 and 2 as a function of
sus 1-TFA and for 2/AgOAc versus 2-TFA. Conditions: Pt
catalyst (2 mol %, 5 μmol), benzene (23.2 μL, 0.26 mmol),
1
II
equivalents of AgOAc. Conditions: Pt catalyst (2 mol %, 5
1
AgOAc (2 equiv, 1.7 mg, 10 μmol) in TFA-d (0.5 mL, 6.5 mmol,
μmol), benzene (23.2 μL, 0.26 mmol), AgOAc (1 equiv, 0.85 mg, 5
μmol; 2 equiv, 1.7 mg, 10 μmol) in TFA-d (0.5 mL, 6.5 mmol, 25
1
equiv relative to benzene) at 150 °C for 24 h. Reported as TON (
standard deviation.
2
5 equiv relative to benzene) at 150 °C for 24 h. Reported as
TON ( standard deviation.
II
We examined Pt complexes 1 and 2 as catalysts for
H/D exchange between C H and TFA-d at 150 °C under
Scheme 2. Isolation of 1-TFA and 2-TFA
6
6
1
standard conditions (2 mol % of [Pt], 0.5 M C H in TFA-d ).
The parent chloride complexes exhibited modest activity
6
6
1
1
9
(
TON < 25) after 24 h. However, the addition of AgOAc
dramatically increased catalyst turnover, and optimal results
TON = 181 for 1 and 202 for 2 after 24 h) were obtained
(
2
2
with 4 mol % of AgOAc (Figure 2).
II
To determine the identity of the Pt species generated
under these conditions, 1 and 2 were treated with 2 equiv
of AgOAc in trifluoroacetic acid, and the Pt-containing
products were isolated (Scheme 2). After filtration to remove
AgCl, the reaction mixture was concentrated and diluted
We first examined the background reaction between ben-
zene and TFA-d under standard conditions (0.50 M benzene
with CH Cl (for 1) or MeCN (for 2). The products were then
2 2
1
in TFA-d ). At 150 °C, negligible (<2%) H/D exchange
precipitated by the addition of diethyl ether. This procedure
provided the bis-trifluoroacetate complexes 1-TFA and
2-TFA in 80% and 75% isolated yield, respectively. Analy-
tically pure samples of 1-TFA and 2-TFA were then used as
1
occurred after 2 h; however, after 24 h, 16 ( 3% D incor-
poration was observed (Table 1). The background reaction
19
was also investigated in the presence of AgOAc and AgCl.
The former is a common additive in metal-catalyzed C-H
while the latter is the byproduct of
catalysts for H/D exchange between C
as summarized in Figure 3, essentially identical turnover
numbers were obtained compared to (N-N)PtCl /2 equiv of
H and TFA-d , and,
6 1
6
11,20,21
activation reactions,
II
chloride abstraction from Pt chloride complexes. The addi-
tion of 4 mol % of AgOAc or AgCl led to comparable results to
the reactions without Ag (with 13 ( 1% and 11 ( 1% D
2
AgOAc. These results strongly suggest that the AgOAc
serves to abstract the chloride ligands, but does not play an
additional role in promoting H/D exchange. As such, the in
2
2
incorporation, respectively, after 24 h at 150 °C) (Table 1). At
lower temperatures (between 75 and 125 °C), <5% D incor-
poration was observed in the presence of the Ag salts.
II
situ generation of Pt H/D exchange catalysts by reaction of
halide precatalysts with AgOAc appears to be a convenient
means of assaying TONs.
II
On the basis of these findings, the Pt complexes 1-8 were
(19) A number of additional Ag salts were screened in this reaction
see Supporting Information for details). Notably, AgTFA afforded
evaluated as precatalysts for H/D exchange between C H6
and TFA-d in the presence of 4 mol % of AgOAc under the
1
6
(
essentially identical results to AgOAc; therefore we utilized the less
expensive Ag salt for these transformations.
II
standard reaction conditions. The simple Pt salts PtCl and
2
(
20) Wong-Foy, A. G.; Henling, L. M.; Day, M.; Labinger, J. A.;
Bercaw, J. E. J. Mol. Catal. A 2002, 189, 3.
21) Corberan, R.; Sanau, M.; Peris, E. J. Am. Chem. Soc. 2006, 128,
974.
22) The raw H/D exchange data for all catalytic transformations
with TFA-d were corrected by subtraction of the % deuterium incor-
poration observed in the background reaction between TFA-d and
in the presence of AgCl.
10
K PtCl were also studied for comparison. As summarized
2
4
in Figure 4, complexes 1, 2, 3, and 6 afforded nearly identical
TON (∼200) after 24 h. Importantly, 207 is the statistical
maximum TON under these conditions.
While the data in Figure 4 offer a valuable initial metric
for comparing these C-H activation catalysts, the results
(
3
(
1
1
6 6
C H

Article
Organometallics, Vol. 28, No. 18, 2009 5319
II
Figure 4. Turnover numbers for H/D exchange between C
catalyst (2 mol %, 5 μmol), benzene (23.2 μL, 0.26 mmol), AgOAc (1.7 mg, 10 μmol) in TFA-d
6
H
6
and TFA-d
1
catalyzed by 1-8/AgOAc after 24 h. Conditions: Pt
(0.5 mL, 6.5 mmol, 25 equiv relative to
1
benzene) at 150 °C for 24 h. Reported as TON ( standard deviation. The statistical maximum TON corrected for the background
reaction is 207 under these conditions (see Supporting Information for details).
(
25 °C intervals from 75 to 150 °C). As summarized in
Figure 6, catalyst 4 provided the highest TON at 75 and
1
precipitation of platinum black began to occur within 2 h.
This illustrates the careful balance between activity and
stability required in catalyst design. Notably, complex 3
performed significantly better at lower temperatures than 1,
00 °C; however, even at 75 °C, decomposition of 4 and
1
1
2
, or 6 and far surpassed all catalysts at 125 and 150 °C.
Finally, we examined H/D exchange between C H and the
lessacidicdeuterium sources CD CO D andtrifluoroethanol-
6
6
3
2
Figure 5. Turnover number versus time for H/D exchange
between C and TFA-d catalyzed by 1-3 and 6. Conditions:
Pt catalyst (2 mol %, 5 μmol), benzene (23.2 μL, 0.26 mmol),
AgOAc (1.7 mg, 10 μmol) in TFA-d (0.5 mL, 6.5 mmol,
5 equiv relative to benzene) at 150 °C. The statistical maximum
TON at each time point is corrected for the background reac-
tion; therefore, this value decreases with time (see Supporting
Information for details).
d (TFE-d ). C-H activation is expected to be more challen-
3
3
6
H
6
1
ging under these conditions, because the solvents can compete
with C-H bonds for coordination to the metal center. To our
knowledge, there is only one reported example of Pt-catalyzed
II
1
2
1
3,24
H/D exchange between benzene and CD CO D.
3
Thus, we
2
aimed to establish whether catalysts 1-8 displayed activity
with these deuterium sources and to determine how this
reactivity compared to that with TFA-d1.
provide only a snapshot of catalyst performance under a
single set of conditions and at a single time point. We next
followed the progress of H/D exchange as a function of time
for the best performing catalysts (1-3 and 6) to gain further
insights into catalyst activity. As illustrated in Figure 5, the
H/D exchange between TFA-d1 and benzene proceeded
fastest with diimine complex 3. This catalyst displayed an
The background reaction was negligible with both
CD
was observed after 24 h at 150 °C in the presence or absence of
4 mol % of AgOAc, AgCl, or AgBF . Modest H/D exchange
was observed in CD CO D upon the addition of 2 mol % of 1
CO D and TFE-d ; for example, <1% H/D exchange
3
2 3
4
3
2
or 2 along with 4 mol % of AgOAc (TON = 24 ( 8 and 26 (
9, respectively). Furthermore, TONs could be dramatically
increased through the substitution of AgBF for AgOAc
4
-
1
initial turnover frequency (TOF) of 0.05 s after 15 min at
1
2
50 °C and achieved the statistical maximum TON after only
(TON = 144 ( 12 and 94 ( 13, respectively, Figure 7).
With theseconditionsinhand, weinvestigated the reactivity
2
3
h at 150 °C. The bipyrimidine catalyst 2 was slower (TOF =
.01 s ), and the bipyridine and dipyridylamine complexes 1
-1
0
and 6 displayed comparable but significantly lower TOFs of
of 1-6 as well as K
between C and CD
and TFE-d (Figure 9) in the presence of 4 mol % of AgBF
2
PtCl
CO
4
and PtCl
D (Figure 8) and between C H
6 6
2
for H/D exchange
H
6
6
3
2
-1
25
.
0.009 and 0.007 s , respectively.
3
4
In the case of catalyst 4 (Figure 4, column D), the
precipitation of Pt black was observed after 24 h at 150 °C.
This suggests that catalyst decomposition is occurring under
the reaction conditions. In order to obtain a better bench-
mark for 4, we examined the performance of catalysts 1-4
and 6 after a shorter time (2 h) and at lower temperatures
With both deuterium sources, negligible reaction (<8 turn-
overs) was observed with the commercial Pt chloride salts.
In contrast, all of the complexes 1-6 showed good to excellent
levels of catalytic activity after 24 h at 150 °C, with TON
1
1
ranging from 65 to 230 using CD
CO D and from 50 to 233
2
3
using TFE-d . Interestingly, the relative TONs obtained with
3
2 4 2
(24) K PtCl and PtCl also catalyze this reaction, but the addition of
a mineral acid is required. See ref 10 for details.
(23) The apparently lower TONs observed at extended times are an
artifact of subtraction of the AgCl background reaction at the respective
time points.
(25) Complexes 7 and 8 showed negligible activity for H/D exchange
3 2 3
with CD CO D or TFE-d under our standard conditions.

5
320 Organometallics, Vol. 28, No. 18, 2009
Hickman et al.
Figure 6. Turnover numbers for H/D exchange between C H and TFA-d catalyzed by 1-4 and 6 as a function of temperature.
6
6
1
II
Conditions: Pt catalyst (2 mol %, 5 μmol), benzene (23.2 μL, 0.26 mmol), AgOAc (1.7 mg, 10 μmol) in TFA-d
1
(0.5 mL, 6.5 mmol,
2
5 equiv relative to benzene) for 2 h. Reported as TON ( standard deviation.
each catalyst changed significantly as a function of deuterium
source; for example, 3 and 5 were the most active catalysts in
CD CO D, while 1 and 3 were optimal in TFE-d .
3
2
3
A time study was conducted with the three top-performing
catalysts in CD CO D to assess the reactivity profiles of
3
2
2
6
these species. As summarized in Figure 10, 3 was the most
-1
active catalyst, with TOF = 0.2 s after 15 min at 150 °C! In
contrast, 5 showed an order of magnitude lower initial rate
(TOF after 15 min = 0.01 s ); however, after 24 h, the
statistical maximum TON was achieved with this catalyst.
Finally, 1 was the most sluggish, with initial TOF = 0.003 s
and a TON of only 170 after 54 h.
-1
-1
Diimine complex 3 was consistently a top-performing
catalyst with all three deuterium sources, and most H/D
exchange reactions with 3 proceeded to equilibrium (within
error) at significantly faster rates than with the other Pt
complexes. Thus, we sought to challenge catalyst 3 in order
to establish the upper limits of its reactivity. Catalysts 3 and 5
(the two best catalysts in CD CO D) were therefore exam-
ined at 4-fold reduced Pt loading (0.5 mol % of Pt/1 mol % of
3 2
AgBF ) under otherwise identical conditions to the previous
4
experiments. Catalyst 3 remained highly active under these
conditions and afforded 633 ( 43 turnovers for C H /
6
6
Figure 7. Turnover numbers for H/D exchange between C H
6
6
CD CO D H/D exchange after 2 h. Complex 5 showed
almost 3-fold lower activity with TON = 223 ( 3.
and CD CO D catalyzed by 1 and 2 as a function of Ag salt.
3 2
3
2
II
Conditions: Pt catalyst (2 mol %, 5 μmol), benzene (23.2 μL,
0
1
.26 mmol), AgOAc (1.7 mg, 10 μmol) or AgBF4 (1.9 mg,
0 μmol) in CD CO D (0.37 mL, 6.5 mmol, 25 equiv relative
Discussion
3
2
to benzene) at 150 °C for 24 h. Reported as TON ( standard
deviation.
Previous studies have examined the catalytic activity of
II
simple Pt salts and of nitrogen-ligated Pt complexes
10
II
9-12
in benzene H/D exchange. However, these investigations were
all conducted under different reaction conditions, with vari-
able times, temperatures, concentrations, catalyst loadings,
additives, and deuterium sources. In this work, we have
established a first-pass protocol to assay Pt catalysts for
benzene H/D exchange by comparing catalyst turnover num-
bers under a standard set of conditions. In addition, we have
reported reaction profiles (turnovers versus time) for the
catalysts that provided the highest TONs in the initial assay.
Together, these two studies allow the first direct comparison
of the activity of catalysts 1-8, and they should serve as useful
benchmarks against which to measure future catalysts.
There are three discernible trends in the data reported
II
herein. First, Pt complexes containing bidentate N-donor
ligands (1-6) were superior catalysts to simple Pt chloride
salts under all of the H/D exchange conditions examined.
In reactions with K PtCl or PtCl , rapid formation of a
(
26) Notably, an analogous study was not undertaken in TFE-d
3
because this solvent is prohibitively expensive.
2
4
2

Article
Organometallics, Vol. 28, No. 18, 2009 5321
II
Figure 8. Turnover numbers for H/D exchange between benzene and CD
(
3
CO
2
D catalyzed by 1-6 after 24 h. Conditions: Pt catalyst
4 3 2
2 mol %, 5 μmol), benzene (23.2 μL, 0.26 mmol), AgBF (1.9 mg, 10 μmol) in CD CO D (0.37 mL, 6.5 mmol, 25 equiv relative to
benzene) at 150 °C for 24 h. Reported as TON ( standard deviation. The statistical maximum TON corrected for the background
reaction is 241 under these conditions (see Supporting Information for details).
II
Figure 9. Turnover numbers for H/D exchange between C H and TFE-d catalyzed by 1-3, 5, and 6 after 24 h. Conditions: Pt
6
6
3
catalyst (2 mol %, 5 μmol), benzene (23.2 μL, 0.26 mmol), AgBF
4
3
(1.9 mg, 10 μmol) in TFE-d (0.47 mL, 6.5 mmol, 25 equiv relative to
benzene) at 150 °C for 24 h. Reported as TON ( standard deviation. The statistical maximum TON corrected for the background
reaction is 241 under these conditions (see Supporting Information for details).
black precipitate was observed, suggesting that decompo-
sition to platinum black contributes to low TONs in these
systems.
unfavorable ΔH associated with the C-H activation reac-
11
tion.
Third, complex 3 consistently showed high TONs for C H6
6
1
0
Second, complexes containing bidentate ligands (1-6)
generally afforded higher TONs for H/D exchange relative
to those with tridentate ligands (7 and 8). This is clear from
Figure 4 as well as from the fact that 7 and 8 showed
negligible turnovers with either CD CO D or TFE-d as a
H/D exchange at temperatures above 100 °C. This was true
over a range of different deuterium sources and reaction
conditions. However, complex 3 does appear to suffer from
some decomposition over the course of these transformations,
as the catalytic solutions darkened considerably (changing
color from orange todark grayor black) after 24h at150 °C in
all of the solvents examined. These results suggest that the
3
2
3
deuterium source. Computational studies in the literature
have suggested that the low reactivity of 7 is due to a highly

5
322 Organometallics, Vol. 28, No. 18, 2009
Hickman et al.
Intriguingly, the relative activity of the other catalysts was
found to vary substantially depending on the nature of the
deuterium source. The latter results reveal the importance of
comparing new catalysts in several different assays in order
to fully evaluate their reactivity. We anticipate that the
conditions reported here will serve as a valuable method to
rapidly screen and benchmark new transition metal catalysts
for C-H activation as a first step in developing new catalytic
C-H oxidation reactions. Ongoing studies in our group aim
to examine correlations between the results presented herein
and the reactivity of 1-8 in catalytic arene and alkane
oxidation.
Figure 10. Turnover number versus time for H/D exchange
between benzene and CD CO D catalyzed by 1, 3, and 5.
Conditions: Pt catalyst (2 mol %, 5 μmol), benzene (23.2 μL,
3
2
II
Experimental Section
0
.26 mmol), AgBF (1.9 mg, 10 μmol) in CD CO D (0.37 mL,
4 3 2
6
.5 mmol, 25 equiv relative to benzene) at 150 °C.
All glassware and stir bars were treated with aqua regia,
washed with copious water and acetone, and dried before each
use. A representative example of the H/D exchange procedure is
shown below. More details are provided in the Supporting
Information.
Representative H/D Exchange Procedure. To a 4 mL reseal-
able Schlenk tube was added a small stir bar and complex 3
1
(3.2 mg, 5 μmol, 2 mol %). TFA-d (0.25 mL) was then added to
the Schlenk tube, and the sides of the vessel were carefully rinsed
future development of related catalysts should focus on limit-
ing decomposition pathways.
It is more difficult to make generalizations among the
complexes containing bidentate N-donor ligands, as both the
relative and absolute TON varied significantly upon chan-
ging the deuterium source. For example, the observed trends
in TON after 24 h were as follows: 3 ≈ 2 ≈ 6 ≈ 1 > 5 > 4
to wash all of the catalyst into the tube. A stock solution of silver
acetate (13.6 mg in 2 mL of TFA-d ) was prepared immediately
(
5
TFA-d ), 5 ≈ 3 > 1 ≈ 4 > 2 > 6 (CD CO D), and 1 ≈ 3 >
1
3
2
1
> 6 ≈ 2 (TFE-d ). This does not appear to track well with
3
prior to use. A portion of this stock solution (0.25 mL, 1.7 mg,
10 μmol, 4 mol % of AgOAc) was then added to the vessel such
the reactivity of related complexes in stoichiometric C-H
activation. There is also no clear correlation with the
electronic character of the ligands, although protonation of
the basic nitrogens of 2 and 6 in TFA-d may be partially
7
that a total of 25 equiv of TFA-d
was delivered relative to
benzene. The resulting suspension was stirred for 1 min. Benzene
1
˚
(
23.2 μL, 0.26 mmol), stored over 4 A molecular sieves, was then
1
2
7
added. The reaction vessel was sealed and completely sub-
merged in a preheated oil bath, and the mixture was heated with
vigorous stirring for the appropriate time. At the end of the
reaction, the vessel was cooled to room temperature, and the
reaction mixture was filtered through a plug of Celite and
washed with ethyl acetate (2 mL) into a 20 mL scintillation vial.
responsible for obscuring the expected trends. These dif-
ferences may reflect differences in the H/D exchange me-
chanism as a function of catalyst, a change in mechanism
with the different deuterium sources, and/or competing
electronic requirements of various steps of the H/D exchange
process as a function of catalyst/solvent. In general, a more
systematic and detailed examination of ligand electronic and
steric characteristics as well as reaction mechanism will be
essential to definitively establish the parameters responsible
for catalyst activity in these systems.
A saturated aqueous solution of K
2
CO
3
(2 ꢀ 1 mL) was added
carefully to the vial to quench the residual acid. The organic
layer was separated and diluted to afford a 12.8 mM solution
Go fC b- Me n Sz e. ne in EtOAc (∼1 mg/mL), which was analyzed by
Acknowledgment. We thank the Department of Energy
DE-FG02-08ER 15997) and the NSF Center for Enab-
Conclusion
(
This paper has described a protocol for the direct, quanti-
tative comparison of Pt-based catalysts for the H/D ex-
change between C H and TFA-d , CD CO D, and TFE-
d3. An initial survey of 10 Pt catalysts under these conditions
has established that diimine complex 3 is highly active for
benzene H/D exchange with all three deuterium sources.
ling New Technologies through Catalysis (CENTC) for
partial support of this research. We also thank Karen
Goldberg, Bill Jones, and Mike Heinekey for helpful
discussions.
6
6
1
3
2
Supporting Information Available: Complete experimental
details, catalyst characterization, and H/D exchange data are
provided. This material is available free of charge via the
Internet at http://pubs.acs.org.
(27) Xu, X.; Kua, J.; Periana, R. A.; Goddard, W. A. Organometallics
2003, 22, 2057.