Highly effective pincer-ligated iridium catalysts for alkane dehydrogenation. DFT calculations of relevant thermodynamic, kinetic, and spectroscopic properties

Article abstract of DOI:10.1021/ja047356l

The p-methoxy-substituted pincer-ligated iridium complexes, (MeO- ^tBuPCP)IrH₄ (^RPCP = κ³-C ₆H₃-2,6-(CH₂PR₂)₂) and (MeO-^iPrPCP)IrH₄, are found to be highly effective catalysts for the dehydrogenation of alkanes (both with and without the use of sacrificial hydrogen acceptors). These complexes offer an interesting comparison with the recently reported bis-phosphinite POCOP (^RPOCOP = κ³-C₆H₃-2,6-(OPR₂) ₂) pincer-ligated catalysts, which also show catalytic activity higher than unsubstituted PCP analogues (Goettker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804). On the basis of ν_CO values of the respective CO adducts, the MeO-PCP complexes appear to be more electron-rich than the parent PCP complexes, whereas the POCOP complexes appear to be more electron-poor. However, the MeO-PCP and POCOP ligands are calculated (DFT) to show effects in the same directions, relative to the parent PCP ligand, for the kinetics and thermodynamics of a broad range of reactions including the addition of C-H and H-H bonds and CO. In general, both ligands favor (relative to unsubstituted PCP) addition to the 14e (pincer)Ir fragments but disfavor addition to the 16e complexes (pincer)IrH₂ or (pincer)-Ir(CO). These kinetic and thermodynamic effects are all largely attributable to the same electronic feature: O → C(aryl) π-donation, from the methoxy or phosphinito groups of the respective ligands. DFT calculations also indicate that the kinetics (but not the thermodynamics) of C-H addition to (pincer)Ir are favored by σ-withdrawal from the phosphorus atoms. The high ν_CO value of (POCOP)Ir(CO) is attributable to electrostatic effects, rather than decreased Ir-CO π-donation or increased OC-Ir σ-donation.

Full text of DOI:10.1021/ja047356l

Published on Web 09/17/2004
Highly Effective Pincer-Ligated Iridium Catalysts for Alkane
Dehydrogenation. DFT Calculations of Relevant
Thermodynamic, Kinetic, and Spectroscopic Properties
Keming Zhu, Patrick D. Achord, Xiawei Zhang, Karsten Krogh-Jespersen,* and
Alan S. Goldman*
Contribution from the Department of Chemistry and Chemical Biology, Rutgers,
The State UniVersity of New Jersey, New Brunswick, New Jersey 08903
Received May 5, 2004; E-mail: agoldman@rutchem.rutgers.edu; krogh@rutchem.rutgers.edu
Abstract: The p-methoxy-substituted pincer-ligated iridium complexes, (MeO-tBuPCP)IrH4 (RPCP ) κ3-
C6H3-2,6-(CH2PR2)2) and (MeO-iPrPCP)IrH4, are found to be highly effective catalysts for the dehydroge-
nation of alkanes (both with and without the use of sacrificial hydrogen acceptors). These complexes offer
an interesting comparison with the recently reported bis-phosphinite “POCOP” (RPOCOP ) κ3-C6H3-2,6-
(OPR2)2) pincer-ligated catalysts, which also show catalytic activity higher than unsubstituted PCP analogues
(Go¨ttker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804). On the basis of
νCO values of the respective CO adducts, the MeO-PCP complexes appear to be more electron-rich than
the parent PCP complexes, whereas the POCOP complexes appear to be more electron-poor. However,
the MeO-PCP and POCOP ligands are calculated (DFT) to show effects in the same directions, relative
to the parent PCP ligand, for the kinetics and thermodynamics of a broad range of reactions including the
addition of C-H and H-H bonds and CO. In general, both ligands favor (relative to unsubstituted PCP)
addition to the 14e (pincer)Ir fragments but disfavor addition to the 16e complexes (pincer)IrH2 or (pincer)-
Ir(CO). These kinetic and thermodynamic effects are all largely attributable to the same electronic feature:
O f C(aryl) π-donation, from the methoxy or phosphinito groups of the respective ligands. DFT calculations
also indicate that the kinetics (but not the thermodynamics) of C-H addition to (pincer)Ir are favored by
σ-withdrawal from the phosphorus atoms. The high νCO value of (POCOP)Ir(CO) is attributable to electrostatic
effects, rather than decreased Ir-CO π-donation or increased OC-Ir σ-donation.
Introduction
experimental and computational studies of the effect of p-
methoxy and other p-substituents on the thermodynamics of
The development of systems for the selective catalytic func-
tionalization of alkanes, and “unactivated” C-H bonds more
generally, is one of the most significant challenges in modern
catalysis. Organometallic systems have demonstrated great pro-
mise in this context. Particular progress has been made over
the past two decades in the development of catalysts for the
dehydrogenation of alkanes, either with or without the use of
sacrificial olefins as hydrogen acceptors.1,2 Presently, the most
promising systems seem to be (κ3-P,C,P-pincer)-ligated iridium
catalysts (e.g., complexes of the form (X-RPCP)Ir (RPCP )
κ3-C6H3-2,6-(CH2PR2)2)), first introduced as catalysts for transfer
dehydrogenation3 and later found to be highly effective for
acceptorless dehydrogenation as well.4
addition reactions.5-7 DFT calculations predicted that additions
of C-H bonds and H2 to the 14-electron fragments (X-PCP)-
Ir are favored by π-donating X groups such as methoxy. The
mechanism of dehydrogenation by “(tBuPCP)Ir” precursors8 has
been shown to operate via C-H addition to this 14e fragment
(which is rate-determining under certain conditions) and to
proceed via formation of (tBuPCP)IrH2.9 Thus, factors that favor
addition of either C-H bonds or H2 are of obvious relevance
in this context. Herein we report that p-methoxy-PCP deriva-
tives, in particular the previously unreported complex (MeO-
iPrPCP)Ir,10 afford unprecedented levels of catalytic dehydro-
genation activity under suitable conditions.
(2) (a) Maguire, J. A.; Goldman, A. S. J. Am. Chem. Soc. 1991, 113, 6706-
6708. (b) Maguire, J. A.; Petrillo, A.; Goldman, A. S. J. Am. Chem. Soc.
1992, 114, 9492-9498. (c) Wang, K.; Goldman, M. E.; Emge, T. J.;
Goldman, A. S. J. Organomet. Chem. 1996, 518, 55-68.
(3) (a) Gupta, M.; Hagen, C.; Flesher, R. J.; Kaska, W. C.; Jensen, C. M. Chem.
Commun. 1996, 2083-2084. (b) Liu, F.; Pak, E. B.; Singh, B.; Jensen, C.
M.; Goldman, A. S. J. Am. Chem. Soc. 1999, 121, 4086-4087.
(4) Xu, W.; Rosini, G. P.; Gupta, M.; Jensen, C. M.; Kaska, W. C.; Krogh-
Jespersen, K.; Goldman, A. S. Chem. Commun. 1997, 2273-2274.
(5) Krogh-Jespersen, K.; Czerw, M.; Zhu, K.; Singh, B.; Kanzelberger, M.;
Darji, N.; Achord, P. D.; Renkema, K. B.; Goldman, A. S. J. Am. Chem.
Soc. 2002, 124, 10797-10809.
(6) Kanzelberger, M.; Singh, B.; Zhu, K.; Goldman, A. S. Abstracts of Papers;
219th National Meeting of the American Chemical Society, San Francisco,
CA; American Chemical Society: Washington, DC, 2000; INOR-366 AN
2000:331138.
We recently reported the synthesis of iridium complexes
bearing a p-methoxy-substituted PCP ligand (MeO-tBuPCP) and
(1) (a) Burk, M. J.; Crabtree, R. H.; Parnell, C. P.; Uriarte, R. J. Organometallics
1984, 3, 816-817 (and references therein for stoichiometric dehydroge-
nations). (b) Burk, M. J.; Crabtree, R. H. J. Am. Chem. Soc. 1987, 109,
8025-8032. (c) Felkin, H.; Fillebeen-Khan, T.; Holmes-Smith, R.; Lin,
Y. Tetrahedron Lett. 1985, 26, 1999-2000.
9
13044
J. AM. CHEM. SOC. 2004, 126, 13044-13053
10.1021/ja047356l CCC: $27.50 © 2004 American Chemical Society

Pincer-Ligated Ir Catalysts for Alkane Dehydrogenation
A R T I C L E S
Table 1. Acceptorless Dehydrogenation of CDA (bp 201 °C)a
Very recently, Brookhart and co-workers reported that the
bis-phosphinite catalysts (X-tBuPOCOP)IrH2 (RPOCOP ) κ3-
C6H3-2,6-(OPR2)2) are more effective than (tBuPCP)IrH2 for
cyclooctane/tert-butylethylene (COA/TBE) transfer dehydro-
genation.11 The metal centers of the POCOP complexes might
Catalyzed by (X-RPCP)IrHn
a
catalyst; total turnovers (
MeO;
t-Bu
) mM)
X
)
t-Bu
H;
X
)
X
)
MeO;
i-Pr
time/h
R
)
R
)
R )
1
2
4
60
158
357
110
170
220
360
360
-
275
430
575
820
820
-
450
714
868
2120
2970
3050
6
24
48
78
be expected to be less electron-rich than their PCP analogues.
This expectation was supported by measurement of the C-O
stretching frequencies of the respective CO adducts,12 the
standard indicator for electron richness of metal centers. These
observations seemed at odds with the greater catalytic effective-
ness of the p-methoxy-PCP derivatives (as described below),
since the latter complexes are more electron-rich than the parent
catalysts. Computational results, however, resolve this apparent
contradiction. We demonstrate in this work that key thermo-
dynamic and kinetic properties are affected similarly by the
presence of p-methoxy substituents or by the substitution of
the phosphorus-linked methylenes of PCP by oxygen atoms (to
give POCOP), even though these substitutions affect νCO values
in opposite directions. We believe these kinetic and thermody-
namic similarities are closely linked to the greater catalytic
activity of both POCOP and MeO-PCP catalysts.
a Conditions: catalyst, 1.0 mM; 1.5 mL of CDA; 250 °C oil bath;
concentrations determined by GC.
tBuPCP complex (158 turnovers (TO) vs 60 TO after 1 h15).
After 24 h, the p-methoxy-substituted catalyst afforded 820 TO
as compared with 360 TO from the parent catalyst. These values
obtained using the MeO-tBuPCP catalyst are essentially equal
(within20%)tothosepreviouslyreportedusingthebis(di-isopropyl-
phosphino) catalyst (with no methoxy substituent), (iPrPCP)-
IrH4. The latter complex was reported to give turnover numbers
(TONs) higher than any previously reported alkane dehydro-
genation catalyst.14
Since the p-methoxy substitution of the aryl ring and the
presence of i-Pr groups on phosphorus (in lieu of t-Bu) each
lead to significant improvements in rate and TONs, we naturally
wished to explore the effect of both these substitutions on the
same complex. (MeO-iPrPCP)IrH4 was synthesized (see Ex-
perimental Section) and indeed found to be extremely effective
for CDA dehydrogenation. Under conditions identical to those
described above, 360 TO are obtained with (MeO-iPrPCP)-
IrH48 after 1 h of reflux. After 24 h, 2100 TO are obtained, and
after 72 h a total of 3050 TO is observed (see Table 1). These
are the highest turnover numbers, by a factor of ca. 3,
reported to date for homogeneous acceptorless alkane dehy-
drogenation.
Results and Discussion
1. Experiments with (X-RPCP)Ir Catalysts. 1.1. Accep-
torless Dehydrogenation of Cyclodecane. Acceptorless dehy-
drogenation is a reaction that is potentially even more valuable
than transfer dehydrogenation.4,13,14 Cyclodecane (CDA) is a
convenient substrate for screening this reaction owing to the
equivalence of all HCCH units and its high boiling point (201
°C).4,14 CDA solutions of (tBuPCP)IrH2 and (MeO-tBuPCP)-
IrH2 (1.0 mM) were prepared and subjected to vigorous reflux
with argon flowing above the condenser to permit loss of H2,
as described previously.4,14
1.2. Acceptorless Dehydrogenation of an n-Alkane. The
superiority of (MeO-tBuPCP)IrH2 over (tBuPCP)IrH2 was not
reproduced with the acceptorless dehydrogenation of n-undecane
(bp 196 °C). There was little difference in rates at early reaction
times (18 vs 23 TO, respectively, after 0.5 h) and only a slight
advantage at longer times (108 vs 83 turnovers after 24 h). Even
more disappointing, (MeO-iPrPCP)IrH2 gave significantly
poorer results for acceptorless n-undecane dehydrogenation than
did either of the tBuPCP-based catalysts (see Table 2).
As seen in Table 1, the initial rate of dehydrogenation was
greater with the MeO-tBuPCP complex than with the parent
1.3. Transfer Dehydrogenation of n-Alkanes. Despite the
poor results obtained for acceptorless dehydrogenation of
n-alkane, (MeO-iPrPCP)IrH2 has proven to be a highly effective
catalyst for transfer dehydrogenation of n-alkanes (and also for
other n-alkyl group containing species16). For example, in runs
with 0.79 M norbornene (NBE) added to n-octane catalyst
(7) Goldman, A. S.; Czerw, M.; Renkema, K. B.; Singh, B.; Zhu, K.; Krogh-
Jespersen, K. Abstracts of Papers; 222nd ACS National Meeting, Chicago,
IL; American Chemical Society: Washington, DC, 2001; INOR-016 AN
2001:639203.
(8) Both (X-tBuPCP)IrH4 and (X-tBuPCP)IrH2 are suitable precursors of
“(X-tBuPCP)Ir”. The tetrahydrides lose H2 reversibly and rapidly under
even ambient conditions. We have never observed any difference in catalytic
activity with the two precursors (although presumably an additional mole
of sacrificial acceptor is consumed with the tetrahydrides). The tetrahydrides
are more stable and are stored more conveniently. The difference in ease
of isolation is more pronounced for the X-iPrPCP complexes; preliminary
attempts to isolate the (MeO-iPrPCP)IrH2 were unsuccessful. Thus, all
reactions of “(MeO-iPrPCP)Ir” reported in this work use the tetrahydride
precursor.
(11) Go¨ttker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004,
126, 1804-1811.
(12) Go¨ttker-Schnetmann, I.; White, P. S.; Brookhart, M. Organometallics 2004,
23, 1766-1776.
(13) (a) Fujii, T.; Saito, Y. J. Chem. Soc., Chem. Commun. 1990, 757-758. (b)
Fujii, T.; Higashino, Y.; Saito, Y. J. Chem. Soc., Dalton. Trans. 1993, 517-
520. (c) Aoki, T.; Crabtree, R. H. Organometallics 1993, 12, 294-298.
(14) Liu, F.; Goldman, A. S. Chem. Commun. 1999, 655-656.
(15) One TO ) 1 mol product/mol iridium; products are cis- and trans-
cyclodecene and 1,2-diethylcyclohexane, which is formed via dehydroge-
nation/Cope rearrangement/hydrogenation, as reported previously (ref
14).
(9) Renkema, K. B.; Kissin, Y. V.; Goldman, A. S. J. Am. Chem. Soc. 2003,
125, 7770-7771.
(10) Zhu, K.; Emge, T. J.; Zhang, X.; Goldman, A. S. Abstracts of Papers;
226th National Meeting of the American Chemical Society, New York,
September 7-11; American Chemical Society: Washington, DC, 2003;
AN 2003:632907.
9
J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004 13045

A R T I C L E S
Zhu et al.
Table 2. Acceptorless Dehydrogenation of n-Undecane (bp 196
above, Brookhart et al. have used it for screening (POCOP)Ir
catalysts.
°C)a Catalyzed by (X-RPCP)IrHn
a
catalyst; total turnovers () mM)
X
)
H;
t-Bu
X
R
)
MeO;
X
R )
)
MeO;
i-Pr
time/h
R
)
)
t-Bu
0.5
1
2
4
17
23
18
21
44
66
76
83
42
66
91
31
-
30
-
108
a Conditions: catalyst, 1.0 mM; 1.5 mL of n-undecane; 250 °C oil bath;
concentrations determined by GC.
When 1:1 (mol:mol) COA/TBE solutions of catalyst (1.2
mM) were simultaneously heated for 8 min in a 200 °C oil bath,
the respective TONs were as follows: (tBuPCP)IrH2, 47; (MeO-
tBuPCP)IrH2, 79; (MeO-iPrPCP)IrH2, 542. Thus, the p-methoxy
group enhances the reaction rate slightly while the substitution
of the t-butyl groups by i-propyl groups has a more significant
favorable effect. On the basis of the relative TONs vs (tBuPCP)-
IrH2 (1.0:1.7:11.5), for this particular substrate couple under
this particular set of conditions, (MeO-tBuPCP)IrH2 appears
somewhat less effective than (tBuPOCOP)IrH2, while (MeO-
iPrPCP)IrH2 appears comparable to the tBuPOCOP complex.17
Using NBE as acceptor gave somewhat greater TONs than TBE
in the same time interval with use of the bis(t-butyl)phosphino
catalysts: (tBuPCP)IrH2, 180; (MeO-tBuPCP)IrH2, 211. (It was
not possible to use NBE for transfer dehydrogenation catalyzed
by (MeO-iPrPCP)IrH2 under these conditions.18)
1.5. Comparisons between the Various Catalysts. There
appears to be no simple unifying explanation for all of the above
results. Consider, for example, that (MeO-iPrPCP)IrH2 is highly
effective for transfer dehydrogenation of n-alkane and accep-
torless dehydrogenation of cyclodecane, but not for the accep-
torless dehydrogenation of n-alkane. It is not surprising that these
derivatizations of the catalyst do not yield a single, simple
outcome. Even in work with a single catalyst, “(tBuPCP)Ir”, and
a single donor/acceptor pair (COA/TBE), we find that the
catalyst resting state is dependent upon TBE concentration and
temperature.9 At high [TBE] the major resting state is (tBuPCP)-
Ir(CHdCHtBu)(H), while at low [TBE] it is (tBuPCP)IrH2 (high
temperature favors the dihydride).19 Obviously, the identity of
the resting state will be dependent on the nature of the acceptor
as well as the dehydrogenated products present in the mixture.
Likewise, the nature of the transition state will be variable. In
the (tBuPCP)IrH2/COA/TBE system, we found that the transition
states for C-H elimination (of TBA) and C-H addition (of
COA) were rate-determining at low and high TBE concentra-
tions, respectively. Furthermore, the transition state (TS) for
Table 3. n-Octane Transfer Dehydrogenation Catalyzed by
(X-RPCP)IrHn (150 °C)a
(tBuPCP)Ir
(MeO
−
tBuPCP)Ir
(MeO
−
iPrPCP)Ir
time/min
octenes NBE
octenes
NBE
octenes
NBE
0
10
20
30
60
120
180
0
163
234
277
393
502
536
790
615
550
503
378
272
215
0
113
167
211
352
511
790
638
580
540
390
247
0
234
435
612
787
790
560
350
181
0
a Conditions: catalyst, 15.0 mM; initial [NBE], 0.79 M; 0.5 mL of
n-octane; concentrations determined by GC.
solutions (15 mM), (MeO-iPrPCP)IrH2 afforded 100% hydro-
genation of NBE within 60 min (150 °C) as compared with
65% NBE conversion after 120 min with (tBuPCP)IrH2 (or 68%
after 120 min with (MeO-tBuPCP)IrH2) (Table 3).
The new catalyst (MeO-iPrPCP)IrH4 has proven to be
particularly effective in yielding high (and unprecedented)
conversions of n-alkanes. For example, in a run with 0.37 g of
NBE (3.9 mmol) added to a solution of 4.2 mg of (MeO-
iPrPCP)IrH4 (0.0075 mmol) in 0.5 mL of n-octane (3.6 mmol),
essentially all (>99%) NBE and 40% of the n-octane was
consumed after 12 h at 150 °C. The mixture of products was
complex, including products suspected to result from dehydro-
cyclization. Accordingly, the product distribution was simpler
when n-hexane was the substrate. In a run with 0.5 mL of
n-hexane (3.9 mmol), 4.5 mg of (MeO-iPrPCP)IrH4, and 0.48
g of NBE (5.1 mmol), the conversion of n-hexane was 43%
after 30 h at 150 °C. Of the various products, 78.5% was linear
hexenes (including dienes and triene) and 5.4% was benzene-
derived from cycloaromatization (confirmed by comparison with
authentic benzene by GC, GC-MS, and NMR). The remaining
16% was high boiling point compounds, predominantly with
MW 176; these were presumed to be NBE/hexadiene adducts.
(16) Using (MeO-iPrPCP)IrH4, much higher yields have been obtained for the
dehydrogenation of tertiary amines to give enamines (Zhang, X.; Goldman,
A. S. Unpublished results) and the partial dehydrogenation of saturated
polyolefins (Ray, A.; Goldman, A. S. Unpublished results, 2003).
(17) Brookhart has found (ref 11) that (tBuPOCOP)IrH2 gives TONs 5.9 times
greater than “(tBuPCP)Ir” generated in situ from dehydrohalogenation of
the hydrido chloride. Presumably this value would be no greater (and
possibly less) by comparison with the (tBuPCP)IrH2; this compares with
the factor of 11.5 obtained for (MeO-iPrPCP)IrH2. However, the absolute
TONs obtained by Brookhart et al. are roughly double those reported herein
for (MeO-iPrPCP)IrH2. Given the lack of reliability in comparing runs
conducted using different sets of apparatus (aluminum block vs oil bath)
in different labs, it seems reasonable to simply consider these catalysts as
being “comparably” effective.
1.4. Transfer Dehydrogenation of Cyclooctane. The pro-
totypical substrate-acceptor couple for organometallic-catalyzed
transfer dehydrogenation is COA/TBE, first introduced by
Crabtree for the purpose of screening and developing such
catalysts. The COA/TBE couple is of particular interest in the
context of the following section of this article, since, as noted
(18) Attempts to obtain comparable values for (MeO-iPrPCP)IrH2-catalyzed
COA/NBE dehydrogenation were stymied by polymerization of NBE by
the iridium catalyst. This surprising reactivity is currently under further
investigation. Zhang, X.; Goldman, A. S. Unpublished observation, 2003.
(19) The temperature dependence of the resting state in (tBuPCP)Ir-catalyzed
COA/TBE transfer dehydrogenation can be understood by considering that
the formation of (tBuPCP)Ir(CHdCHtBu)(H) and TBA, from (tBuPCP)Ir and
2 mol TBE is a reaction in which 3 mol reagent gives 2 mol product.
9
13046 J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004

Pincer-Ligated Ir Catalysts for Alkane Dehydrogenation
A R T I C L E S
Table 4. Calculated and Experimental C-O Stretching Frequencies (νCO, cm-1) and Ir-CO Bond Dissociation Energies (BDE, kcal/mol) for
Pincer Complexes
1
ν
CO (cm-
)
expt (R
)
t-Bu)
calcd (R
)
Me)
∆νexpt
∆νcalcd
BDE
∆BDE
(H-RPCP)Ir(CO)
(H-RPOCOP)Ir(CO)
(MeO-RPCP)Ir(CO)
(MeO-RPOCOP)Ir(CO)
1a-CO
1927.7a
1949b
2015.4
2030.1
2012.7
2026.5
2029.1
2024.4
0.0
21.3
-2.2
19.3
0.0
14.6
-2.7
11.1
13.6
9.0
62.2
63.1
63.3
64.1
60.7
60.9
0.0
0.9
1.1
1.9
-1.5
-1.3
1925.5a
1947b
1b-CO
a Cyclooctane solution, ref 5. b Pentane solution, ref 12.
TBE insertion into an Ir-H bond was fairly close in energy.9
The same derivatizations that might favor one particular TS or
resting state, operative under a particular set of conditions, could
very easily disfaVor a different rate-determining TS or a different
resting state that may be operative under a different set of
conditions.
groups (as in POCOP) but in which phosphorus remains bound
to the aryl through a methylene unit (as in PCP).
There are two possible stereoisomeric fragments of the
composition {2,6-C6H3[CH2PMe(OMe)]2}Ir: 1a and 1b; 1a
possesses a meso-type structure of near Cs symmetry, whereas
1b attains rigorous C2 symmetry.
Consequently, the effect of catalyst derivatizations on catalytic
activity is not easily rationalized. Nevertheless, the increased
electron richness of the p-methoxy complexes appeared to be a
factor generally favorable to catalytic activity. We were therefore
intrigued by the recent discovery, by Brookhart et al., that the
POCOP complexes were more catalytically active than their PCP
analogues despite being apparently less electron-rich.11 We have
therefore undertaken a comparative study of these two catalyst
classes using computational (DFT) methods. As elaborated
below, we find that the catalytically relevant kinetics and
thermodynamics of the (MeO-PCP)Ir and (POCOP)Ir frag-
ments (as compared with unsubstituted (PCP)Ir) bear very strong
similarities.
We did not expect to find any major differences in the
electronic properties of the two stereoisomers; but independent
calculations of the properties for both species provided a self-
consistency check and some indication of the level of “random
error” in the calculations.
The C-O stretching frequencies of the CO adducts of 1a
and 1b (1a-CO and 1b-CO) were calculated to be close to each
other (2029.1 and 2024.4 cm-1, respectively; Table 4) and close
to that of (MePOCOP)Ir(CO) (2030.1 cm-1). These values are
2. Computational Studies of (PCP)Ir and (POCOP)Ir
Complexes. DFT calculations were conducted on all pincer
complexes in both PCP and POCOP ligand classes with methyl
groups on phosphorus in place of the t-butyl (or i-propyl) groups
in the experimentally studied catalysts. Although such trunca-
tions of alkyl groups can dramatically affect absolute energies,
largely because of steric factors, they do allow us to elucidate
substituent electronic effects with high accuracy.5
all higher than that calculated for (MePCP)Ir(CO), 2015.4 cm-1
,
in accord with the experimental tBuPCP and tBuPOCOP12 values.
In contrast, the presence of a p-methoxy substituent on the aryl
ring of PCP (or added to POCOP) leads to a slightly lowered
ν
CO value (2026.5 and 2012.7 cm-1, respectively),12 attributable
2.1. C-O Stretching Frequencies and Ir-CO Bond
Dissociation Energies. The experimentally observed greater
C-O stretching frequency of the POCOP carbonyl complexes
vis-a`-vis the PCP analogues is reproduced by the calculations
(Table 4), though the computed shift (∼13 cm-1) is not quite
as large as the measured shifts (∼20 cm-1). The calculations
reproduce the slightly lowered C-O stretching frequency of
(MeO-PCP)Ir(CO) relative to that of (PCP)Ir(CO) extremely
well (computed ∆νCO ) -2.7 cm-1; measured ∆νCO ) -2.2
cm-1). Thus, the calculations appear to successfully capture the
major effects responsible for the shifted νCO values. In the case
of POCOP, this is presumed to be the greater positive charge
on the iridium center engendered by the greater electron-
withdrawing ability of oxygen relative to methylene. However,
we should also consider the possibility that the oxygen atoms
would donate electron density to the pincer aryl ring and,
through that, further on to Ir; this is the factor presumably
responsible for the low-energy shift in νCO in the case of (p-
MeO-PCP)Ir(CO). In an effort to assess the relative contribu-
tions of these two effects in the POCOP complexes, we ran
calculations on several complexes that contained phosphinite
to O f aryl π-donation. Thus, the greater νCO value of
(POCOP)Ir(CO) appears to reflect effects exerted largely
through the phosphorus atoms rather than through the aryl ring.
However, these results certainly do not rule out O f aryl
π-donation in (POCOP)Ir; although this would contribute to a
lowering of νCO, the effect might easily be offset by other
factors.
Support for the importance of O f aryl donation in
(POCOP)Ir is obtained from calculations on the thermodynamics
of Ir-CO bond formation (Table 4). The relative values of the
Ir-CO bond dissociation energies (BDEs) show no correlation
with the νCO values. For example, whereas the POCOP iridium
carbonyl shows a higher νCO value than (MePCP)Ir(CO) and the
p-methoxy-substituted PCP complex shows a slightly lower νCO
,
both the POCOP and MeO-PCP complexes show similarly high
Ir-CO BDEs, which are, respectively, 0.9 and 1.1 kcal/mol
greater than that of (MePCP)Ir. In contrast, the Ir-CO BDEs of
1a-CO and 1b-CO are weaker (by ∼1.4 kcal/mol) than that of
the parent complex, even though their νCO values are similar to
that of (MePOCOP)Ir(CO). Thus, O-substitution on phosphorus
(as seen with 1a and 1b) raises νCO and lowers the Ir-CO BDE.
9
J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004 13047

A R T I C L E S
Zhu et al.
Table 5. Reaction and Activation Energies (kcal/mol) for Addition
of Methane and Propane to Pincer Complexes
M
+
CH4 f M(CH3)(H)
M
+
C3H8 f M(C3H7)(H)
TS product
Eq ∆∆Eq
TS
Eq ∆∆Eq
product
pincer complex (M)
∆
∆E
∆∆
E
∆
∆E
∆∆
E
(
(
MePCP)Ir
4.0
0.0 -1.6
0.0 6.5
0.0 -1.3
0.0
MePOCOP)Ir
1.2 -2.8 -4.6 -3.0 3.7 -2.8 -4.8 -3.5
3.1 -0.9 -2.9 -1.3 5.7 -0.8 -2.7 -1.4
(MeO-MePCP)Ir
Figure 1. Labeling of axes for (pincer)Ir complexes.
(MeO-MePOCOP)Ir 0.4 -3.6 -6.2 -4.6 2.8 -3.7 -6.2 -4.9
1a
1b
4.5
3.6 -0.4
0.5 -0.4
0.3
1.2 6.7
1.9 5.7 -0.8
0.2
0.6
0.8
1.9
2.1
The effects of O-substitution at the aryl para position of (pincer)-
Ir are opposite; however, the magnitude of the BDE effect is
greater (per substituent), while the effect on νCO is much less.
Assuming that ortho and para aryl effects are similar, the
properties of the POCOP carbonyl are consistent with the
combination of aryl and phosphorus substituent effects that
would be expected on the basis of the above observations: a
significantly raised νCO value and a higher Ir-CO BDE.20
2.2. Thermodynamics of C-H and H-H Addition (Ir(I)
f Ir(III)). We have previously demonstrated that aryl f Ir
π-donation into the d orbitals of the xz plane (i.e., the plane
orthogonal to and bisecting the approximately collinear P-Ir-P
linkage; see Figure 1) strongly influences the thermodynamics
of C-H and H-H addition to either (PCP)Ir or (PCP)IrH2.5
This follows seminal work by Eisenstein and co-workers, who
demonstrated the critical role of π-bonding from the X ligand
in 5-coordinate d6 complexes of the form L2IrXRH (R ) H or
hydrocarbyl). We found the thermodynamic effects from
σ-donation along the z-axis plane to be significantly less
pronounced.5,21,22 The effect of π-donation into the x,y or y,z
planes, or σ-donation along the y-axis, has not previously been
investigated systematically, although replacing PH3 ligands with
PMe3 was calculated to favor addition of C-H or H2 to Ir(PR3)2-
Cl.22
We have calculated the reaction and activation energies for
addition of CH3-H and n-C3H7-H to several fragments,
including (MePCP)Ir, (MeO-MePCP)Ir, (MePOCOP)Ir, (MeO-
MePOCOP)Ir, 1a, and 1b. As noted above, the calculated νCO
values of the CO adducts of the complexes with P-bound O
atoms (POCOP and 1) are nearly equal to each other and
significantly higher than those of the phosphino complexes, in
accord with the experimental νCO values of (tBuPOCOP)Ir(CO)
and (tBuPCP)Ir(CO). By contrast, the p-methoxy-substituted PCP
and POCOP carbonyls (experimental and computational) have
Table 6. Reaction Energies (kcal/mol) for Addition of H2 and
Benzene to Pincer complexes
M
+
H
2 f M(H)2
M + PhH f M(Ph)(H)
pincer complex (M)
∆E
∆∆E
∆E
∆∆
E
(
(
MePCP)Ir
-28.8
-31.7
-30.5
-33.7
-27.8
-27.9
0.0
-2.9
-1.7
-4.9
1.0
-11.6
-13.3
-12.7
-14.9
-8.5
0.0
MePOCOP)Ir
-1.7
-1.1
-3.3
3.1
(MeO-MePCP)Ir
(MeO-MePOCOP)Ir
1a
1b
0.9
-9.0
2.6
by the POCOP ligand is in the same direction but considerably
larger than the effect of the p-methoxy substituent calculated
for (MeO-MePCP)Ir (∆∆E ) -1.3 and -1.4 kcal/mol). Thus,
the same computational model that correctly produces a higher
νCO value for (MePOCOP)Ir(CO) and a lower νCO value for
(MeO-MePCP)Ir yields thermodynamics of C-H addition that
are more favorable for both complexes than for the parent
complex.
The calculated effects of a p-methoxy substituent on PCP
and POCOP ligands are the same: reduced νCO values and
thermodynamically more favorable additions of Me-H and Pr-
H. It bears note in this context that Brookhart and co-workers
have found that elimination of benzene from (MeO-tBuPO-
COP)Ir(Ph)(H) is slower than elimination from (tBuPOCOP)Ir-
(Ph)(H).12
In contrast to the thermodynamically favorable effect on C-H
addition engendered by the POCOP bis(phosphinite) ligand, the
bis(phosphinite) complexes 1a and 1b are calculated to add
methane and propane less favorably than the PCP analogues
(∆∆E ≈ +1 to +2 kcal/mol). Thus, the favorable thermody-
namics of addition afforded by POCOP can apparently be
attributed to effects transmitted through the aryl ring, not through
the O-P linkage.
νCO values lower than the parent complexes. We find, however,
that the energetics of C-H and H-H addition conform to a
pattern completely different from that displayed by the νCO
values.
Looking first at the thermodynamics of C-H addition, the
results in Table 5 show that the effect of the O-for-CH2
substitution to give (MePOCOP)Ir (using (MePCP)Ir as the
reference point) favors Me-H and Pr-H addition by ∆∆E )
-3.0 and -3.5 kcal/mol, respectively. This product stabilization
The trend for the thermodynamic effects calculated for Me-H
and n-Pr-H addition to the pincer fragments is reproduced for
addition of H-H and for addition of phenyl-H as well (Table
6).
The computed exoergicities (-∆E) for all four addition
reactions (methane, propane, benzene, dihydrogen) show the
following order:
MeO-MePOCOP > MePOCOP > MeO-MePCP >
MePCP > 1a ≈ 1b
(20) In fact, if we rather crudely assume additivity and equal para and ortho
effects, we could expect the Ir-CO bond (MePOCOP)Ir(CO) to be about
0.7 kcal/mol stronger than that of (MePCP)Ir(CO) (-1.5 kcal/mol (O on P,
meso) + 2 × 1.1 kcal/mol (O on aryl)); the computed value is 0.9 kcal/
mol (Table 2). Similarly, the expected νCO shift in (MePOCOP)Ir(CO) would
be about 8 cm-1 (13.6 cm-1 (O on P, meso) + 2 × (-2.7 cm-1) (O on
aryl)), and the computed ∆νCO is 14.6 cm-1 (Table 4).
(21) Krogh-Jespersen, K.; Goldman, A. S. In Transition State Modeling for
Catalysis; Truhlar, D. G., Morokuma, K., Eds.; ACS Symposium Series
721; American Chemical Society: Washington, DC, 1999; pp 151-162.
(22) Rosini, G. P.; Liu, F.; Krogh-Jespersen, K.; Goldman, A. S.; Li, C.; Nolan,
S. P. J. Am. Chem. Soc. 1998, 120, 9256-9266.
Essentially the same order is found for CO addition as well.
As previously demonstrated,5 C-H and H-H additions to
(PCP)Ir fragments (addition to Ir(I) to afford Ir(III)) are
thermodynamically favored by increasing π-donation from the
pincer aryl group. The marked difference between the thermo-
dynamic effects of POCOP versus the phosphinite ligands of 1
9
13048 J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004

Pincer-Ligated Ir Catalysts for Alkane Dehydrogenation
A R T I C L E S
Table 7. Reaction Energies (kcal/mol) for Addition of H2 and CO
to (Pincer)IrH2 Complexes To Afford 18e Complexes
least favorable for the MeO-PCP and POCOP complexes, while
methoxy substituents on phosphorus exert a relatively small
effect.
MH2
+
H2 f MH4 MH2 + CO f MH2(CO) M(CO) + H2 f MH2(CO)
pincer complex (M)
∆E
∆∆E
∆E
∆∆E
∆E
∆∆E
Thus, for all three addition reactions examined (eqs 4-6),
formation of the 18-electron species is disfavored by POCOP
as well as by the p-methoxy derivatization. We propose that
this may be very significant in the context of catalysis: increased
π-donation to the metal is likely to disfavor (kinetically and/or
thermodynamically) the formation of inactive out-of-cycle 18e
resting states. At the very least, this result offers additional
evidence that (POCOP)Ir bears a closer resemblance to (MeO-
PCP)Ir than to the parent (PCP)Ir fragment.
(
MePCP)Ir
-19.8
-16.4
-18.4
0.0 -46.7
3.4 -43.6
1.4 -45.4
5.4 -41.6
0.1 -45.7
0.0
3.1
1.3
5.1
1.0
-13.4
-12.3
-12.6
-11.1
-12.8
-14.3
0.0
1.1
0.8
2.3
0.6
(
MePOCOP)Ir
(MeO-MePCP)Ir
(MeO-MePOCOP)Ir -14.4
1a
1b
-19.7
-20.4 -0.6 -47.4
-0.7
-0.9
can accordingly be attributed to O f aryl π-donation. The
magnitudes of the thermodynamic POCOP/PCP differences are,
very roughly, twice the effects of a single p-methoxy substituent.
The fact that such effects must presumably counteract unfavor-
able effects transmitted via phosphorus suggests that the total
O f aryl π-donation operative in POCOP is at least twice that
of the MeO-PCP ligand. This conclusion is further supported
by electronic population analyses (see Section 2.5).
2.4. Kinetics of C-H Addition and Elimination. We have
thus far focused on the thermodynamics of C-H addition,
although catalysis is intrinsically a kinetic phenomenon. The
kinetics of C-H addition, however, only impact catalytic rates
if the TS of the rate-determining reaction step is that of C-H
addition. At least in the case of (tBuPCP)Ir-catalyzed transfer
dehydrogenation of COA/TBE, this appears to be the case. In
the limit of low [TBE] the rate-determining step is C-H
elimination, which, of course, proceeds via the same TS as that
of addition. In the limit of high [TBE] we infer that C-H
addition is rate-determining.9,25
2.3. Thermodynamics of H-H Addition to (Pincer)IrH2
(Ir(III) f Ir(V)). The Formation of (Pincer)IrH4 and Other
18-Electron Products. (tBuPCP)IrH2 readily adds H2 to give
(
tBuPCP)IrH4, a classical (i.e., Ir(V)) tetrahydride.23
(pincer)IrH2 + H2 ) (pincer)IrH4
(4)
The kinetics of methane and propane C-H bond addition to
either (MePOCOP)Ir or (MeO-MePCP)Ir are calculated to be
more favorable than to (MePCP)Ir (Table 5). This correlates with
the thermodynamics and is in accordance with the Hammond
Postulate. Consequently, with respect to both thermodynamic
and kinetic parameters studied in the course of this work, the
POCOP and MeO-PCP ligands show, in all cases, the same
effects relative to unsubstituted PCP.
While we are aware of no isolated (pincer)Ir(V) species other
than the tetrahydrides, (pincer)Ir(V) species have been clearly
implicated as intermediates, for example, in the H/D exchange
reactions of (tBuPCP)IrH2 with either hydrocarbon solvent or
with the phosphino tert-butyl groups.5,24
Although increased π-donation by substituents on the aryl
ring favors addition to pincer-ligated iridium(I), we have
previously shown, both by experimental and computational
means, that this same factor disfaVors addition to the Ir(III)
dihydrides (eq 4).5 We have also shown (both experimentally
and computationally) that the exothermicity of H2 addition to
(X-RPCP)Ir(CO) (eq 5) decreases with increasing π-donation
from X.5 Coordination of CO to the Ir(III) species (X-PCP)-
IrH2 (eq 6) is similarly calculated to be disfavored by π-donation
from X.5
Unlike the Hammond behavior calculated for POCOP and
MeO-PCP, the kinetics and thermodynamics for C-H addition
to complexes 1a and 1b correlate poorly. Although the
thermodynamics of C-H addition are distinctly disfavored by
the phosphinite ligands (Tables 5 and 6), the kinetics of addition
are comparable to that of (MePCP)Ir (Table 5). The values in
question are small, but we believe they do reflect a real effect.
If we view the TS for C-H addition as possessing some alkane
σ-complex character, we would expect that lowering the energy
2
of the dz (σ) acceptor orbital should favor formation of the TS.
(pincer)Ir(CO) + H2 ) (pincer)Ir(CO)H2
(pincer)IrH2 + CO ) (pincer)Ir(CO)H2
(5)
(6)
TS formation should also be favored by raising the energy of
the dxz orbital by promoting π-donation into the C-H σ* orbital.
Thus, the favorable effects of both the π-donating p-methoxy
group on the aryl ring and the σ-withdrawing methoxy groups
on phosphorus are not at all inconsistent.
The favorable thermodynamics of addition to 14e (POCOP)-
Ir, as well as the charge distributions discussed in Section 2.5,
suggest that the POCOP aryl ring is in fact more π-electron-
donating than that of PCP. If so, then (POCOP)IrH2 should add
H2 and CO less favorably than the PCP analogues. This is indeed
calculated to be the case (Table 7).
We have begun to test this hypothesis further using fluoro-
substituted phosphines. This will be elaborated upon in a
subsequent publication, but preliminary results support the
notion of an inverse correlation between kinetic and thermo-
dynamic effects resulting from σ-withdrawing groups on
the pincer phosphorus atoms. We calculate that H3C-H
additions to fluorinated derivatives 2a and 2b are 0.5 and 1.0
kcal/mol kinetically more favorable than H3C-H addition to
MePCP (independent of this work, the Brookhart lab has initiated
The thermodynamic substituent effects calculated for addition
to (pincer)IrH2 (Table 7) are approximately the opposite of those
observed for addition to the 14e Ir(I) fragments (Table 6).
Addition to the Ir(III) dihydrides and to the 16e carbonyls is
(23) Evidence for the classical (Ir(V)) nature of (tBuPCP)IrH4 has been obtained
from T1(min) measurements (Gupta, M. Ph.D. Thesis, University of Hawaii,
1997) and more recently, crystallographic data (Zhang, X.; Emge, T.;
Goldman, A. S. Unpublished results, 2003).
(25) Goldman, A. S.; Renkema, K. B.; Czerw, M.; Krogh-Jespersen, K. Alkane
Transfer-Dehydrogenation Catalyzed by a Pincer-Ligated Iridium Complex.
In ActiVation and Functionalization of C-H Bonds; Goldberg, K. I.,
Goldman, A. S., Eds.; ACS Symposium Series 885; American Chemical
Society: Washington, DC, 2004; pp 198-215.
(24) Lee, D. W.; Kaska, W. C.; Jensen, C. M. Organometallics 1998, 17, 1-3.
9
J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004 13049

A R T I C L E S
Zhu et al.
Table 8. Atomic Net Charges and Natural Atomic Orbital Populations
(pincer)Ir
(pincer)Ir(CO)
atomic net charges
(PCP)Ir(CO)
(PCP)Ir
(C2v
(MeO
−
PCP)Ir
(POCOP)Ir
(C2v
(MeO
−
(Cs)
POCOP)Ir
(MeO
−
PCP)Ir(CO)
(Cs)
(POCOP)Ir(CO)
(C2v
(MeO
−
POCOP)Ir(CO)
(Cs)
)
(Cs)
)
(C2v
)
)
Ir
-0.344
0.937
-0.344
0.937
-0.116
-0.219
-0.048
-0.290
0.350
-0.353
1.255
-0.234
-0.808
0.316
-0.283
-0.207
-0.355
1.252
-0.264
-0.804
0.336
-0.353
0.364
-0.264
0.988
-0.262
0.988
-0.394
-0.214
-0.013
-0.300
0.369
-0.574
-0.230
0.483
-0.299
1.301
-0.475
-0.809
0.349
-0.290
-0.187
-0.295
1.298
-0.504
-0.804
0.367
-0.360
0.382
-0.568
-0.232
0.477
-0.527
-0.050
P
C2
CH2 or O
C7
C8
C9
-0.086
-0.220
-0.070
-0.219
-0.225
-0.364
-0.215
-0.034
-0.229
-0.202
O(Me)
C(Me)
C
-0.577
-0.229
-0.573
-0.230
0.487
-0.539
-0.052
0.482
-0.523
-0.041
O
-0.542
-0.059
CO total
natural atomic orbital populations
POCOP)Ir (PCP)Ir CO (MeO
(PCP)Ir
(MeO
−
PCP)Ir
(POCOP)Ir
(MeO
−
−
−
PCP)Ir
(Cs)
−CO
(POCOP)Ir(CO)
(MeO
−
POCOP)Ir(CO)
(Cs)
(C2v)
(Cs)
(C2v
)
(Cs)
(C2v)
(C2v)
Ir(6s)
Ir(dxy)
Ir(dxz)
Ir(dyz)
0.762
1.859
1.885
1.901
1.780
1.176
0.768
0.761
1.856
1.900
1.898
1.778
1.170
0.769
0.757
1.879
1.900
1.862
1.783
1.189
0.759
1.867
1.059
1.019
1.071
1.021
0.756
1.877
1.913
1.859
1.782
1.185
0.760
1.865
1.095
1.008
1.128
0.994
1.856
0.657
1.903
1.711
1.769
1.801
1.432
0.768
0.655
1.901
1.710
1.769
1.805
1.433
0.769
0.665
1.914
1.728
1.757
1.805
1.438
0.759
1.865
1.051
0.992
1.073
0.995
0.664
1.913
1.727
1.756
1.805
1.438
0.760
1.864
1.090
0.977
1.130
0.972
1.848
0.685
0.656
0.882
1.582
1.563
1.651
2
2
)
Ir(dx -y
2
Ir(dz )
P(px)
O(px)
C2(px)
C7(px)
C8(px)
C9(px)
O(px)MeO
C(px)CO
C(py)CO
C(pz)CO
O(px)CO
O(py)CO
O(pz)CO
0.958
1.044
1.018
1.036
0.992
1.031
1.073
1.006
1.861
0.941
1.015
1.020
1.007
0.978
1.003
1.076
0.983
1.854
0.687
0.654
0.876
1.589
1.569
1.652
0.682
0.655
0.876
1.586
1.570
1.652
0.678
0.657
0.882
1.578
1.563
1.651
the synthesis of fluoroaryl derivatives of POCOP26). Additions
of the more electron-rich n-Pr-H bond to 2a and 2b are
calculated to be even more kinetically favorable relative to
MePCP, by 1.4 and 2.8 kcal/mol, respectively. These favorable
kinetics are in contrast with the thermodynamics, which, in all
cases, are less favorable for the fluoro analogues than for
addition to (MePCP)Ir.
are fully supportive of the conclusions drawn above strictly on
the basis of thermodynamic considerations, namely that π-dona-
tion by the aryl-bound oxygen atoms of MeO-MePCP, MePO-
COP, and MeO-MePOCOP plays a dominant role in the relative
thermodynamics and kinetics of the corresponding (pincer)Ir
complexes. However, a more detailed analysis reveals additional
aspects of interest.
We first consider the p-methoxy derivatization. Formally, the
methoxy oxygen donates about 0.14e to the aryl π-system, and
thus, predictably, the most marked effect of the p-methoxy group
is an increase in π-density at the relative ortho (C8, ∼0.055e)
and para (C2, ∼0.035e) positions (see Figure 2). This places an
2.5. Natural Atomic Charges and Atomic Orbital Oc-
cupations. Atomic net charges and natural atomic orbital
populations were calculated for several (MePCP)Ir and (MePO-
COP)Ir complexes as well as for the methoxy derivatives. The
effects of the various derivatizations on electronic populations
are found to be generally similar for the (pincer)Ir, (pincer)Ir-
(CO), and (pincer)IrH2 complexes; results for the former two
sets of complexes are shown in Table 8. Overall, the calculations
Figure 2. Numbering of carbon atoms for PCP and POCOP complexes
(C5 methylene group replaced with O in POCOP complexes), with axis
labels shown.
additional total charge of 0.030e on C2 in (MeO-MePCP)Ir vs
(
MePCP)Ir; the increase in electron population on C2 is similar
for the corresponding carbonyl complexes. The electronic
structure of the P atoms is essentially unperturbed by the
presence of a p-methoxy group on the aryl ring. The Ir(d) orbital
conjugating with the aryl π-system, Ir(dxz), is calculated to have
(26) Brookhart, M., University of North Carolina, Chapel Hill, NC, 2003,
personal communication.
9
13050 J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004

Pincer-Ligated Ir Catalysts for Alkane Dehydrogenation
A R T I C L E S
a slightly increased population of 0.015e, but the overall charge
on Ir remains unchanged. The general picture reflects significant
MeO f C(aryl) π-donation. Figure 2 shows the numbering of
the carbon atoms for PCP and POCOP complexes.
approximation that C-O stretching frequencies afford as a gauge
of “electron-density” on a metal center; unfortunately, we are
not aware of any other experimental tool that conveniently offers
more precise information.
The effects of the oxygen atoms present in POCOP are more
complex. Formally, each O in POCOP donates about 0.13e to
the aryl π-system, just as the single p-methoxy oxygen does.
Thus, the conclusion drawn above, based on thermodynamics,
that the total O f C(aryl) π-donation is significantly greater in
(POCOP)Ir than in (p-MeO-PCP)Ir, is readily substantiated.
The greatest difference in net charge is found at the aryl C atoms
located ortho to Ir (C7; +0.386e), a result of C(aryl) f O
σ-donation. The increased π-donation from o-positions leads
to a significant buildup of negative charge, -0.148e, on C2 in
Concluding Remarks
We report two new, p-methoxy-substituted, PCP-pincer
catalysts, which are found to give unprecedented TONs for
dehydrogenation of alkanes (both with and without sacrificial
acceptors) under several different sets of conditions. The
generally favorable effect of the methoxy group is demonstrated
by comparisons with the parent complex, (tBuPCP)IrH2. Isopro-
pyl groups on phosphorus, which were previously demonstrated
to yield more favorable results for acceptorless dehydrogena-
tion,14 are found to afford an excellent catalyst for n-alkane
transfer dehydrogenation when combined with the p-methoxy
substituent on the aryl ring. The effect of the methoxy groups
is undoubtedly electronic, engendered largely by O f C(aryl)
π-donation. We presume that the effects of isopropyl groups
on phosphorus, in place of t-butyl groups, are largely steric in
nature.
It is very difficult to elucidate with any certainty the factor
directly responsible for the generally “favorable” effect that the
p-methoxy substituent has upon catalytic activity. Any simple
explanation should be generally applicable (e.g., for all alkanes),
but we find that in several cases (e.g., acceptorless dehydro-
genation of n-undecane) the new catalysts do not produce
particularly good results. Given that the nature of the resting
states (and possibly TSs as well) is expected to depend on the
particular set of conditions, this is not too surprising. However,
computational studies of key parameters, particularly the kinetics
and thermodynamics of C-H addition to Ir(I) and the thermo-
dynamics of H2 addition to both Ir(I) and Ir(III), afford insight
into the properties that are likely to play a role in leading to
improved catalytic activity.
During the course of this work, Brookhart and co-workers
discovered that another variation on the PCP motif (replacement
of the methylene linkage with O atoms) also resulted in excellent
catalytic activity.11,12 However, on the basis of νCO values of
the respective CO adducts, the POCOP catalysts appeared to
be relatively electron-poor in contrast with the electron-rich
p-methoxy-substituted catalysts. We were intrigued by this
contrast and included the POCOP system in our computational
studies. The POCOP and MeO-PCP ligands show effects in
the same direction, relative to the parent PCP ligand, for the
entire range of kinetic and thermodynamic parameters studied.
We believe this is not coincidental. The aryl groups of both
ligands are more π-donating than the parent, and we believe
this property is closely tied to the improved catalytic activity
displayed by both families of complexes.
The high νCO value of (POCOP)Ir(CO) is attributable to
electrostatic effects, rather than decreased M-CO π-donation.
A very different distribution of charge is calculated for the
(POCOP)Ir fragment vis-a`-vis (PCP)Ir, as reflected in a much
greater dipole moment (3.9 D vs 1.9 D). This results in
polarization of a CO ligand, to give greater C-O triple-bond
character, as was shown previously for cationic metal centers.27
The thermodynamics and kinetics of C-H addition to the
14e “(pincer)Ir” fragment are favored by both the POCOP and
MeO-PCP ligands, relative to the parent. However, likely
(
(
(
MePOCOP)Ir relative to (MePCP)Ir. The total population of the
MePOCOP)Ir C2(px) orbital is 0.101e greater than that of
MePCP)Ir; a difference of only 0.034e is computed for the
analogous C2(px) orbital of (MeO-MePCP)Ir. Substantial P f
O σ-donation leads to a large positive net charge on the P atoms,
+0.318e. Consistent with the opposing effects of increased
positive charge on P and increased negative charge on C2, the
overall charge on the iridium center of (POCOP)Ir is nearly
the same as that in the (PCP)Ir complexes.
The calculated POCOP atomic charges and orbital occupan-
cies appear consistent with the calculated energetics of the
various addition reactions; in particular, the greater π-electron
density at the Ir-bound aryl carbon has been demonstrated to
lead to more favorable energetics of C-H and H-H addition.5
Experimentally and computationally, a significant increase
of the CO stretching frequency results in POCOP complexes
(relative to analogous PCP complexes). The underlying “mech-
anism” for this is less obvious than that behind the thermody-
namic effects. Detailed examination of the population data
indicates that the O-for-CH2 substitution engenders only minimal
changes in Ir orbital populations or in net charge on the CO
unit. Net Ir f CO charge transfer appears miniscule. However,
changes occur internally in the C-O charge distribution. The
major difference when comparing the CO ligands in (MePO-
COP)Ir(CO) and (MePCP)Ir(CO) is that the former shows more
polarization toward carbon.
This polarization of the (MePOCOP)Ir(CO) carbonyl ligand
is most easily explained in terms of an electrostatic effect. The
calculated dipole moment of (MePOCOP)Ir is 3.9 D (pointing
along the z-axis), while that of (MePCP)Ir is only 1.9 D and
that of (MeO-MePCP)Ir is 1.7 D (1.5 D along the z-axis). Thus,
although the p-methoxy and O-for-CH2 derivatizations similarly
affect the orbital occupancies of iridium (and the kinetics and
thermodynamics of C-H and H-H additions), they have
opposite effects on the magnitude of the dipole moment. It has
been previously demonstrated that increased polarization of a
carbonyl ligand (toward carbon), operating via purely electro-
static effects, can dramatically increase C-O stretching frequen-
cies.27 The large calculated difference in dipole moment (∼2
D) can easily explain the observed (and calculated) (POCOP)Ir
blue shift of approximately 20 cm-1 27
Calculations on model
.
dipolar systems, in which there is no covalent bonding to CO,
support this contention.28 This result highlights the crude
(27) Goldman, A. S.; Krogh-Jespersen, K. J. Am. Chem. Soc. 1996, 118, 12159-
12166.
(28) Achord, P.; Goldman, A. S.; Krogh-Jespersen, K. To be submitted for
publication.
9
J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004 13051

A R T I C L E S
Zhu et al.
including those on the pincer ligand, were assigned a double-ú quality
21G basis set.36
resting states (e.g., (pincer)IrH2) are also thermodynamically
favored by the substitutions; this factor should inhibit catalytic
activity. A comparison of relative stabilization of resting states
vs stabilization of TSs by the different ligands is probably not
meaningful. Such differences in relatiVe stabilization are
calculated to be very small (ca. several tenths of a kilocalorie
per mole) and probably too small to be meaningful. Extrapolat-
ing these difference to the alkanes, acceptors, and phosphi-
noalkyl groups (t-butyl or i-propyl) used experimentally, seems
particularly unreasonable. It is especially important to note in
this context that completely nonanalogous resting states might
be operative in different systems. Only a very detailed compara-
tive kinetic study (including the determination of resting states)
will allow the key differences to be elucidated.
Resistance to the formation of catalytically inactive species
(either reversible deactivation or decomposition) may be as
important as the relative kinetics of various steps within the
catalytic cycle. It seems likely that potential inactive species
would typically have an 18e configuration. Both POCOP and
MeO-PCP ligands are calculated to disfavor all reactions that
yield 18e species which we have studied, including the addition
of either H2 or CO to (pincer)IrH2 and the addition of H2 to
(pincer)Ir(CO).
Finally, we note that the calculations, in combination with
the experimental observations, offer indications that the kinetics
of C-H addition are favored by both O f C(aryl) π-donation
to the aryl ring and by σ-withdrawal from the phosphorus atoms.
In reference to POCOP, it is noteworthy that the oxygen atoms
presumably afford both these effects. The effect of σ-withdrawal
from the phosphorus atoms, if indeed it favors the TS for C-H
addition and elimination while disfavoring resting states such
as C-H addition products and dihydrides, is of particular interest
with respect to the design of future pincer catalysts.
Reactant, transition state, and product geometries were fully
optimized using gradient methods with the ECP/basis set combination
described above. The exact nature of a particular stationary point on
the potential energy surface was ascertained via standard vibrational
frequency/normal-mode analysis. Additional single-point calculations
used a more extended basis set for Ir in which the default LANL2DZ
functions for the Ir(6p) orbital were replaced by the functions
reoptimized by Couty and Hall,37 and sets of diffuse d functions
(exponent ) 0.07) and f functions (exponent ) 0.938) were added as
well. All computed energy values discussed in the text or presented in
the tables are based on data from the extended basis set calculations.
The parent (POCOP)Ir complex optimizes to a structure of C2V
symmetry as do the simple dihydride and carbonyl complexes. The
(p-MeO-POCOP)Ir complexes tend to possess Cs symmetry. The
complexes containing PCP pincer ligands optimize so that the aryl group
is canted away from alignment with the P-Ir-P axis to give structures
of C2 molecular symmetry for the parent, the carbonyl, and the dihydride
complexes; most substituted (PCP)Ir complexes possess no symmetry
(C1). On all pincer ligands, methyl groups were attached to the
phosphorus atoms. This represents a compromise between the use of
hydrogen atoms and the alkyl groups (i-Pr or t-Bu) that are typically
employed in experimental systems. Methyl groups capture most of the
electronic effects of the larger alkyl groups, but do not fully represent
the steric bulk exerted by those systems. However, steric effects are
not expected to play a role in the comparisons of reactions considered
here.
Electronic population analysis was carried out with the NBO module
incorporated into Gaussian98.38 Higher symmetry (Cs, C2V) was imposed
on the (PCP)Ir complexes to more clearly illustrate the σ- vs π-effects.
The energetic cost of regularizing the structures of these molecules
was in all cases less than 1.5 kcal/mol, and a comparison of net atomic
charges on corresponding symmetric (constrained) and asymmetric
(relaxed) structures showed a maximum difference of 0.01e.
Experimental Section
Computational Methods
General Experimental. (tBuPCP)IrH4 and (MeO-tBuPCP)IrH4
3a
5
All calculations used DFT methodology29 as implemented in the
Gaussian98 series of computer programs.30 We have made use of the
three-parameter exchange functional of Becke31 and the correlation
functional of Lee, Yang, and Parr (B3LYP).32 The Hay-Wadt
relativistic, small core ECP, and corresponding basis sets (split valence
double-ú) were used for the Ir atom (LANL2DZ model);33 all-electron,
full double-ú plus polarization function basis sets were applied to the
elements C, O, and P (Dunning-Huzinaga D95(d)).34 Hydrogen atoms
in H2 or in a hydrocarbon, which formally become hydrides in the
product complexes, were described by the triple-ú plus polarization
311G** basis set;35 other hydrogen atoms in alkyl or aryl groups,
were prepared as described previously. All manipulations were
conducted under an argon atmosphere (note: dinitrogen will poison
all catalysts in this work24) either in a glovebox or using standard
Schlenk techniques. All solvents (COA, n-octane, n-hexane) were
distilled under vacuum from Na/K alloy. NBE was purified by vacuum
sublimation. Catalytic reactions were monitored using a Varian 3400
gas chromatograph with a 60 m × 0.32 mm SUPELCO SPB-5 capillary
column. Calibration curves were prepared using authentic samples.
GC-MS was performed using a Hewlett-Packard 5980 Series II gas
chromatograph with an HP5971 mass spectrometer. All NMR spectra
were recorded with Varian Mercury and Inova spectrometers operating
at 300 or 400 MHz, respectively.
(29) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules;
Oxford University Press: Oxford, 1989.
Transfer Dehydrogenation. Alkane transfer dehydrogenation ex-
periments were typically conducted as follows. A 5-mL reactor vessel
was fitted with a Kontes high-vacuum stopcock, which allows freeze-
pump-thaw cycling and addition of argon, and an Ace Glass
“Adjustable Electrode Ace-Thred Adapter”, which allows removal of
0.5-µL samples. In the argon-atmosphere glovebox, 0.5 mL of alkane
solution (15 mM catalyst, and acceptor) was charged into the reactor.
The charged apparatus was removed from the glovebox, and additional
argon was added on a vacuum line to give a total pressure of 800 Torr.
The reactor was put into a GC oven at the desired temperature. Samples
were periodically taken by microliter syringe for GC analysis. After
(30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.9; Gaussian,
Inc.: Pittsburgh, PA, 1998.
(31) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(32) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(33) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(34) Dunning, T. H.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H.
F., Ed.; Plenum: New York, 1976; pp 1-28.
(36) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980, 102,
939.
(37) Couty, M.; Hall, M. B. J. Comput. Chem. 1996, 17, 1359.
(35) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980,
72, 650.
(38) Carpenter, J. E.; Weinhold, F. J. Mol. Struct. (THEOCHEM) 1988, 169,
41.
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13052 J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004

Pincer-Ligated Ir Catalysts for Alkane Dehydrogenation
A R T I C L E S
the reactions were monitored in this manner, the apparatus was typically
returned to the glovebox where the solution was transferred to an NMR
tube. 1H NMR was used to confirm the identity and approximate
concentration of the products.
2H, 2 Ar-H), 3.40 (s, 3H, CH3O), 2.68 (s, 4H, 2 CH2), 1.61 (m, 4H,
4 CH), 1.02(d, JHH ) 7.2 Hz, 12H, 4 CH3), 0.98 (d, JHH ) 7.2 Hz,
12H, 4 CH3). MS: m/z 368.
Synthesis of (MeO-iPrPCP)IrHCl. To 0.21 g of MeO-iPrPCP-H
(0.57 mmol) in 10 mL of toluene was added 0.21 g of [IrCl(1,5-
cyclooctadiene)]2 (0.31 mmol) at room temperature. This mixture was
heated to 70 °C and stirred for 1 h under H2 atmosphere; the solvent
was then removed in vacuo overnight, giving 0.24 g (94.7%) of (MeO-
Acceptorless Dehydrogenation of Alkanes. In a typical experiment,
in the argon-atmosphere glovebox, 1.5 mL of catalyst solution (1 mM)
was charged into a reactor consisting of a 5-mL round-bottom
cylindrical flask fused to a water-jacketed condenser (ca. 15 cm). The
top of the condenser was fused to two Kontes high-vacuum valves and
an Ace Glass “Adjustable Electrode Ace-Thred Adapter”. The solution
was refluxed in an oil bath held ca. 50 °C above the alkane boiling
point: 200 °C (COA) or 250 °C (CDA or n-undecane). Escape of H2
from the system is facilitated by a continuous argon stream above the
condenser. The reaction was monitored by GC.
1
iPrPCP)IrHCl as a dark-red solid. 31P{1H} NMR (C6D6): δ 58.68. H
NMR (C6D6): δ 6.72 (s, 2H, Ar-H), 3.56 (s, 3H, CH3O), 2.82 (dvt,
2
downfield signal of AB pattern, JHH ) 18.0 Hz, JHP ) 3.3 Hz, 4H, 2
2
CH2), 2.71 (dvt, upfield signal of AB pattern, JHH ) 18.0 Hz, JHP
)
3.3 Hz, 4H, 2 CH2), 2.11 (m, 2H, 2 CH), 1.97 (m, 2H, 2 CH), 1.22 (m,
12H, 4 CH3), 0.92 (m, 12H, 4 CH3), -37.06 (t, JHP ) 12.6 Hz, 1H,
Ir-H).
Synthesis of Diisopropylphosphine. A solution of ClPiPr2 (20 g,
0.131 mol) in 100 mL of degassed dry ether was added dropwise to a
slurry of LiAlH4 (4.0 g, 0.421 mol) in 250 mL of degassed dry ether
in ice/water bath. This mixture was then stirred overnight at room
temperature until the reaction was complete as indicated by 31P NMR
and GC. The excess LiAlH4 was quenched following a literature
procedure,39 and the gray precipitate was removed by filtration and
washed with dry ether and dried by MgSO4. Distillation (bp ) 118
°C) yielded 14.35 g of product (92.8%) as a clear liquid (d ) 0.800
Synthesis of (MeO-iPrPCP)IrH4. A quantity of 0.17 g of (MeO-
iPrPCP)IrHCl (0.285 mmol) was dissolved in 70 mL of pentane. A
volume of 0.29 mL of 1 M LiBEt3H in THF (0.29 mmol) was added
dropwise to the solution at room temperature under H2 atmosphere.
The solution turned lighter color, and some white precipitate formed
at the bottom of the flask. After the addition of LiBEt3H was completed,
the solution was stirred for 1 h at room temperature under H2, and the
precipitate was then filtered out. The solvent was removed by purging
the solution with a stream of H2 and then dried by vacuum, giving
0.08 g (50%) of (MeO-iPrPCP)IrH4 as brown solid. NMR data for
(MeO-iPrPCP)IrH4: 31P{1H} NMR (C6D6): δ 54.60 (s). 1H NMR
(C6D6): δ 6.82 (s, 2H, Ar-H), 3.59 (s, 3H, CH3O), 3.11 (vt, JHP ) 3.6
Hz, 4H, CH2), 1.53 (m, 4H, 4 CH), 1.00 (app. quart., JHP ) 7.2 Hz,
12H, 4 CH3), 0.93 (app. quart., JHP ) 6.8 Hz, 12H, 4 CH3), -9.32 (t,
JHP)10.0 Hz, 4H, IrH4).
1
g/mL). 31P{1H} NMR (C6D6): δ -16.49. H NMR (C6D6): δ 2.91
(dt, JHP ) 192.3 Hz, 3JHH ) 6.0 Hz, 1H, P-H), 1.78 (m, 2H, 2 C-H),
1.02 (m, 12H, 4 CH3). MS: m/z 118.
Synthesis of 1,3-Bis-[(di(isopropyl)phosphino)methyl]-5-methoxy-
benzene (MeO-iPrPCP-H). To 7.0 g of 1,3-bis-bromomethyl-5-
methoxy-benzene (23.81 mmol), prepared as described previously,5 in
100 mL of degassed acetone was added 6.18 g of HPiPr2 (52.38 mmol)
at room temperature. This mixture was heated under reflux with stirring
for 8 h under argon atmosphere, and the solvent was removed in vacuo
(note: precipitated gel rapidly expands to fill the flask under vacuum).
The solid was dissolved in degassed methanol (50 mL) and treated
with triethylamine (8.28 mL, 59.4 mmol). The product was crystallized
out in cold methanol and filtrated and vacuum-line dried overnight,
giving 7.58 g (86.5%) of the ligand as white crystals. 31P{1H} NMR
Acknowledgment. We are very grateful to Prof. M. Brookhart
and Dr. I. Go¨ttker-Schnetmann for sharing unpublished results
and for extensive and productive discussions. Financial support
by the Department of Energy, Division of Chemical Sciences,
Office of Basic Energy Sciences, Office of Energy Research
(DE-FG02-93ER14353; synthetic and experimental parts of this
work) and by the National Science Foundation (Grant CHE-
0316575; computational work) is gratefully acknowledged.
1
(C6D6): δ 9.23 (s). H NMR (C6D6): δ 7.04 (s, 1H, Ar-H), 6.88 (s,
(39) Fieser, L. F.; Fieser, M. Reagents for Organic Synthesis; Wiley & Sons:
New York, 1967; Vol. 1, p 584.
JA047356L
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J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004 13053