Experimental and computational study of alkane dehydrogenation catalyzed by a carbazolide-based rhodium PNP pincer complex

Article abstract of DOI:10.1039/c5sc04794c

A rhodium complex based on the bis-phosphine carbazolide pincer ligand was investigated in the context of alkane dehydrogenation and in comparison with its iridium analogue. (carb-PNP)RhH₂ was found to catalyze cyclooctane/t-butylethylene (COA/TBE) transfer dehydrogenation with a turnover frequency up to 10 min^-1 and turnover numbers up to 340, in marked contrast with the inactive Ir analogue. TONs were limited by catalyst decomposition. Through a combination of mechanistic, experimental and computational (DFT) studies the difference between the Rh and Ir analogues was found to be attributable to the much greater accessibility of the 14-electron (carb-PNP)M(i) fragment in the case of Rh. In contrast, Ir is more strongly biased toward the M(iii) oxidation state. Thus (carb-PNP)RhH₂ but not (carb-PNP)IrH₂ can be dehydrogenated by sacrificial hydrogen acceptors, particularly TBE. The rate-limiting segment of the (carb-PNP)Rh-catalyzed COA/TBE transfer dehydrogenation cycle is found to be the dehydrogenation of COA. Within this segment, the rate-determining step is calculated to be (carb-PNP)Rh(cyclooctyl)(H) undergoing formation of a β-H agostic intermediate, while the reverse step (loss of a β-H agostic interaction) is rate-limiting for hydrogenation of the acceptors TBE and ethylene. Such a step has not previously been proposed as rate-limiting in the context of alkane dehydrogenation, nor, to our knowledge, has the reverse step been proposed as rate-limiting for olefin hydrogenation.

Full text of DOI:10.1039/c5sc04794c

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Experimental and computational study of alkane
dehydrogenation catalyzed by a carbazolide-based
rhodium PNP pincer complex†
Cite this: DOI: 10.1039/c5sc04794c
a
b
b
b
David Bezier, Changjian Guan, Karsten Krogh-Jespersen, Alan S. Goldman*
´
a
and Maurice Brookhart*
A rhodium complex based on the bis-phosphine carbazolide pincer ligand was investigated in the context
of alkane dehydrogenation and in comparison with its iridium analogue. (carb-PNP)RhH was found to
catalyze cyclooctane/t-butylethylene (COA/TBE) transfer dehydrogenation with a turnover frequency up
2
ꢀ1
to 10 min and turnover numbers up to 340, in marked contrast with the inactive Ir analogue. TONs
were limited by catalyst decomposition. Through a combination of mechanistic, experimental and
computational (DFT) studies the difference between the Rh and Ir analogues was found to be
attributable to the much greater accessibility of the 14-electron (carb-PNP)M(I) fragment in the case of
Rh. In contrast, Ir is more strongly biased toward the M(III) oxidation state. Thus (carb-PNP)RhH
2
but not
(
carb-PNP)IrH can be dehydrogenated by sacrificial hydrogen acceptors, particularly TBE. The rate-
2
limiting segment of the (carb-PNP)Rh-catalyzed COA/TBE transfer dehydrogenation cycle is found to be
the dehydrogenation of COA. Within this segment, the rate-determining step is calculated to be (carb-
PNP)Rh(cyclooctyl)(H) undergoing formation of a b-H agostic intermediate, while the reverse step (loss
of a b-H agostic interaction) is rate-limiting for hydrogenation of the acceptors TBE and ethylene. Such
a step has not previously been proposed as rate-limiting in the context of alkane dehydrogenation, nor,
to our knowledge, has the reverse step been proposed as rate-limiting for olefin hydrogenation.
Received 11th December 2015
Accepted 20th January 2016
DOI: 10.1039/c5sc04794c
www.rsc.org/chemicalscience
rhodium-based systems were developed independently by the
groups of Saito, Tanaka and Goldman which exhibited high
Introduction
3
4
5
Olens are highly versatile intermediates which can be con- TONs for alkane dehydrogenation; however, formation of the
verted to a wide array of products such as detergents, pharma- active species, Rh(Cl)(PR , could only be achieved photo-
atmosphere, limiting the utility of
3 2
)
1
6
ceutical intermediates, lubricants, fuels and polymers.
Accordingly, there has been growing interest in the homoge- these systems.
chemically or under H
2
tBu
4
neous dehydrogenation of alkanes as a potential highly atom-
The development of the iridium pincer complex ( PCP)
economic route to olens.
IrH by Kaska and Jensen was a breakthrough for the achieve-
2
7
The rst catalytic transfer alkane dehydrogenations were ment of high TONs in the benchmark COA/TBE system. More
reported independently by Felkin and Crabtree using phos- active and stable iridium complexes were next developed
phine-based rhenium, ruthenium and iridium catalysts for through modication of the pincer ligand. Catalysts based on
12
8
9
10
10a,11
a reaction that has become standard for screening transfer the PCP, POCOP, PCOP, anthraphos
(Fig. 1) and other
dehydrogenations, the use of t-butylethylene (TBE) as frameworks have since been used and studied extensively.
2
a hydrogen acceptor to dehydrogenate cyclooctane (COA).
The mechanism of the transfer dehydrogenation of COA with
13
8a,b
Turnover numbers (TONs) in these systems were limited TBE using PCP and POCOP
iridium pincer complexes has
(
<100 TO) by low catalyst stability. Following these reports, been thoroughly investigated. The overall catalytic cycle is
shown in Scheme 1.
a
Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel
Hill, North Carolina 27599, USA
b
Department of Chemistry and Chemical Biology, Rutgers, The State University of New
Jersey, New Brunswick, New Jersey 08903, USA
†
Electronic supplementary information (ESI) available: General experimental
details, procedures, and data; images of NMR spectra for new compounds;
computational details; structures, energies, and .mol les for computational
species. See DOI: 10.1039/c5sc04794c
Fig. 1 Examples of active PCP iridium pincer complexes for alkane
dehydrogenation.
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t
resulting Ir(III) alkyl hydride, 1-H(C
2
H
4
Bu), was thermody-
namically very unfavorable (eqn (2)). Thus, compared with PCP
ligands, the carb-PNP ligand was indeed found to strongly favor
the Ir(III) alkyl hydride, as well as the Ir(III) dihydride, relative to
the 14-electron Ir(I) fragment. But while C–H addition and
alkane dehydrogenation by the 14-electron Ir species were fav-
oured by the carb-PNP ligand, the hydrogenation segment of the
cycle was disfavored so strongly that catalytic transfer-dehy-
drogenation was precluded.
(2)
Scheme 1 Mechanism of transfer dehydrogenation of COA with TBE
using PCP- and POCOP–iridium complexes.
8
a
It was previously shown that in the case of PCP-type pincer
ligands, the Rh(III) state was not sufficiently accessible to allow
an effective catalytic cycle based on the Rh(I)/Rh(III) couple.
Based on the conclusions reached in the studies with Ir(I), we
considered that for carb-PNP complexes of rhodium, the Rh(III)
state should be relatively more favorable and thus the system
might be active for alkane dehydrogenation.
The mechanisms for these two systems are similar. Begin-
ning with the 16-electron iridium dihydride complex, insertion
of TBE yields the alkyl hydride complex which undergoes
reductive elimination to form the Ir(I) 14-electron species. This
complex activates the C–H bond of cyclooctane, followed by
b-hydride elimination to yield cyclooctene and regenerate the
iridium dihydride. At low concentration of TBE, hydrogenation
is turnover-limiting for the (PCP)Ir system, and the resting state
Here we report that we have synthesized the rhodium
complexes (carb-PNP)Rh(ethylene), 2-C
Rh(H) , 2-H , and studied, experimentally and computationally,
their hydrogenation of ethylene and TBE, in analogy with the
2 4
H , and (carb-PNP)
2
2
15
previous study of the (carb-PNP)Ir complexes.
These
is (PCP)IrH , while at high [TBE], COA dehydrogenation is
2
complexes were also investigated for catalytic alkane transfer
dehydrogenation. In contrast to the (carb-PNP)Ir analogues, and
in accord with the hypothesis proposed above, we nd them to
be quite active as catalysts for COA/TBE transfer-
dehydrogenation.
turnover-limiting and the resting state is the vinyl hydride
complex. For the (POCOP)Ir system, dehydrogenation is turn-
over-limiting and alkene (TBE and COE) complexes are the
resting states.
DFT calculations have been conducted which indicate that
more weakly s-donating groups at the central position of the
pincer ligand favor the thermodynamics of C–H (and H–H)
addition to the 14e pincer-Ir fragments. Intrigued by the
14
Results and discussion
possible implications for alkane dehydrogenation, we recently Synthesis of (carb-PNP)Rh(ethylene) 2-C H
2 4
synthesized 1-C H , an Ir complex of the bis-phosphine carba-
2
4
The bis-phosphine carbazole ligand was synthesized following
our previously reported procedure. Aer deprotonation of the
zolide pincer, carb-PNP, in which the central coordinating
15
2
group is an sp nitrogen which is much less s-donating than the
ligand with LiN(TMS)2 and addition of [(C H ) RhCl] , the
2
4 2
2
2
sp carbon of PCP pincer ligands. In a previous study, however,
solution turned dark brown. Filtration, followed by evaporation
of the solvent gave a brown solid which was washed multiple
times with cold n-octane and dried under vacuum to yield (carb-
1
-C
2 4
H was found to be ineffective as a catalyst for alkane
15
transfer dehydrogenation (eqn (1)).
3
1
1
PNP)Rh(ethylene), 2-C
2
H
4
, as a yellow solid ( P{ H} NMR: d ¼
16
44.12 (d, JP–Rh ¼ 130 Hz)) (eqn (3)).
(1)
(3)
Experimental and computational studies indicated that
hydrogenation of TBE was the rate-limiting step for 1-catalyzed
COA/TBE transfer dehydrogenation. TBE did insert into an Ir–H
Addition of CO to 2-C H at rt resulted in quantitative
2
4
formation of the monocarbonyl complex (carb-PNP)Rh(CO),
3
1
1
2
1
-CO ( P{ H} NMR: d ¼ 54.68 (d, JP–Rh ¼ 125 Hz); IR n(CO) ¼
2
bond of 1-H , but reductive elimination of alkane from the
ꢀ1
954 cm ) (eqn (4)).
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Under an atmosphere of dihydrogen, a solution of 2-H
TBE showed no reaction aer 3 h at room temperature (eqn
10)). This behaviour contrasts with the iridium dihydride 1-H2
2
and
(
(
4)
which, despite of the lack of reaction in the absence of H2,
rapidly catalyzes the hydrogenation of TBE to TBA at room
temperature. This hydrogenation was demonstrated to proceed
15
2
via an Ir(III)/Ir(V) catalytic cycle (eqn (11)) with H playing
Hydrogenation of ethylene and TBE using complexes 2-C H
and 2-H2
2
4
a critical role of promoting reductive elimination of alkane from
the iridium center through formation of an Ir(V) trihydride
intermediate. An analogous mechanism is apparently not
operative for the rhodium system.
Purging complex 2-C H with H at rt for 10 min resulted in the
2
4
2
3
1
1
complete conversion to (carb-PNP)Rh(H) , 2-H ( P{ H} NMR:
2
2
1
d ¼ 65.70 (d, J
¼ 121 Hz); H NMR: ꢀ19.69 (q, J
¼ 16 Hz,
P–Rh
P–Rh
2
H)) (eqn (5)).
(10)
(5)
2
Under 1 atm of ethylene, 2-H was rapidly converted to
2
-C at rt (eqn (6)). This behaviour is in marked contrast to the
2 4
H
ꢁ
iridium analogue 1-H
with a half-life of 45 min for the analogous reaction (eqn (7)).
2
which requires a temperature of 70 C
(11)
15
(
(
6)
7)
The mechanism of the hydrogenation of ethylene by 2-H
2
eqn (6)) was investigated. Addition of excess ethylene to
(
ꢁ
rhodium dihydride 2-H
2
at ꢀ88 C gave Rh(III) ethylene cis-(di-
3
1
1
1
hydride) complex 2-H (C H ) ( P{ H} NMR: d ¼ 84.23 (bs); H
2
2
4
NMR: ꢀ11.07 (bs, 1H), ꢀ20.22 (bs, 1H)) (eqn (12)). Binding of
ethylene to 2-H to give 2-H (C ) is reversible and thermo-
dynamically favored at low temperature (ꢀ88 C to ꢀ70 C). The
2 4
free energy barrier for exchange of free ethylene with 2-H (C H )
2
2
2 4
H
ꢁ
ꢁ
2
ꢀ
1
was estimated to be z9 kcal mol based on NMR line broad-
ꢁ
ening of the Ir–H signals at ꢀ80 C. The dissociation of ethylene
is easier from the rhodium ethylene dihydride complex than
from the iridium analogue for which the barrier to dissociation
2
We next investigated the hydrogenation of TBE by 2-H (eqn
(
8)). No reaction was detected aer 3 h at rt. However, aer
ꢀ
1
ꢁ
15
was found to be z14 kcal mol at 0 C.
ꢁ
heating at 80 C for 3 h, 2-H
2
was converted to (carb-PNP)
3
1
1
Rh(TBE), 2-TBE, ( P{ H} NMR: d ¼ 26.21 (dd, JP–P ¼ 346 Hz, JP–Rh
17
¼
140 Hz), d ¼ 13.26 (dd, JPP ¼ 347 Hz, JP–Rh ¼ 130 Hz)). Thus
(12)
hydrogenation of TBE, like ethylene, by 2-H is much more facile
2
than by iridium dihydride 1-H , the latter showing no reactivity
2
ꢁ
15
aer 10 h at 100 C under the same conditions (eqn (9)).
The rate of the stoichiometric hydrogenation of C H by 2-H
2
2
4
ꢁ
was measured at different concentrations of C
eqn (13)). The rate was found to be rst-order in 2-H
but otherwise independent of the concentration of C H
4
2
H
4
at ꢀ30 C
(C ),
in the
(
(
8)
9)
(
2
2 4
H
2
ꢀ
4
ꢀ1
range 0.05–0.5 M. A rst-order rate constant, k, of 2.4 ꢂ 10
s
ꢁ
‡
ꢀ1
was obtained, corresponding to DG ¼ 18 kcal mol at ꢀ30 C.
(13)
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In the case of the hydrogenation of ethylene by the iridium Table 1 TONs for the transfer dehydrogenation of COA and TBE
a
‡
catalyzed by 2-H2
complex 1-H (C H ), DG was determined to be ca. 26
2
2
4
ꢀ1
ꢁ
kcal mol at a temperature of 75 C. In contrast with the
reaction of 2-H (C H ), the rate of reaction of 1-H (C H ) was
2
2
4
2
2
4
2 4
found to be dependent on the concentration of C H ; this
implied that another molecule of ethylene was involved in the
reductive elimination of ethane from 1-(H)(Et), analogous to the
promotion of reductive elimination by H
species.
2
from the same
ꢁ
Entry
Catalyst loading (mol%)
T ( C)
t [min]
TON
15
Deuterium labelling experiments were conducted to deter-
mine whether the rate-limiting step during the hydrogenation of
ethylene (eqn (14)) was the migratory insertion, converting
1
2
3
4
5
6
7
8
9
0.3
0.3
0.3
0.3
0.3
0.3
0.15
0.15
0.3
0.3
200
200
200
200
200
200
200
200
150
150
5
10
30
47
80
149
213
245
260
285
340
12
60
2 2 4
2-H (C H ) to 2-H(Et), or the reductive elimination (2-H(Et) to
120
180
120
240
60
2
-C
2
H
4
). To a solution of deuterium-labeled 2-D
2
(C
2
H
4
), an excess
ꢁ
of ethylene was added at ꢀ80 C and the reaction was gradually
ꢁ
warmed up to ꢀ30 C while monitored by NMR spectroscopy
(eqn (15)). No evidence of H exchange into the Rh-D positions of
1
0
7200
220
2
-D (C H ) was detected prior to formation of 2-C H plus
2 2 4 2 4
a
ethane, suggesting that the migratory insertion is irreversible and
TONs were calculated based on conversion of TBE determined by GC
‡
ꢀ1 18
2
analysis. COA (2.33 mmol), TBE (2.33 mmol), 2-H (3.42–6.83 mmol).
is the rate-limiting step with DG ¼ 18 kcal mol
.
2 2 2
and (carb-PNP)Rh(H ) (2-H ) with a 2-TBE/2-H ratio of 3 : 1.
The catalytic activity decreased over time with TONs of 149 aer
14) 30 min (entry 3) and 260 aer 3 h (entry 6) corresponding to
(
44% and 76% conversion respectively. Longer reaction times do
not afford increased TONs which suggests that the catalyst had
3
1
decomposed. Accordingly, no P NMR signals were detected
aer 3 h. With a catalyst loading of 0.15 mol%, slightly higher
ꢁ
TONs were obtained, up to 340 aer 4 h at 200 C (entry 8). The
rate of the reaction dropped dramatically when the temperature
ꢁ
was decreased to 150 C, with TONs of 12 and 220 obtained aer
(
15) 1 h (entry 9) and 120 h (entry 10), respectively. For all these
reactions the same rates and TONs obtained using 2-H
also obtained with the use of 2-C as a catalyst precursor.
-H2 was signicantly less effective for the transfer dehy-
drogenation of n-octane (Table 2) than for COA. A 1 : 1 n-octa-
ne : TBE solution of 2-H (0.3 mol%) gave only 4 TOs aer 5 min
2
were
2 4
H
2
Since the Rh(III) intermediate 2-H(Et) was not detected, the
barrier to reductive elimination of ethane from 2-H(Et) must be
less than 18 kcal mol . In fact this energy barrier was calcu-
2
ꢁ
ꢀ
1
ꢀ1
(
TOF z 1 min ) at 200 C (entry 1). The resting states detected
3
1
‡
ꢀ1
by P NMR aer 5 min were (carb-PNP)Rh(TBE) 2-TBE and
lated by DFT to be quite low (DGcalc ¼ 6 kcal mol ). As antic-
ipated, the reductive elimination of ethane from the rhodium
complex is more facile than for the iridium analogue which has
‡
ꢀ1 15
Table 2 TONs for the transfer dehydrogenation of n-octane and TBE
a high kinetic barrier (DG ¼ 20–25 kcal mol ).
a
2
catalyzed by 2-H
Alkane dehydrogenation using rhodium dihydride complex 2-
H
2
The reaction of (carb-PNP)RhH
behaviour of the Ir analogue which showed no reaction with Entry
2
with TBE contrasted with the
ꢁ
T ( C)
t [min]
TON
TBE in the absence of a H
2
atmosphere. This led us to test
1
2
3
4
5
6
200
200
200
150
150
150
5
4
complex 2-H as a catalyst for transfer dehydrogenation using
2
30
60
60
120
180
13
14
<1
1
the benchmark COA/TBE system (Table 1).
In contrast with the inactive iridium analogue, 2-H showed
2
high activity for COA/TBE transfer-dehydrogenation at 200 C.
ꢁ
3
With 0.3 mol% catalyst loading of 2-H , 47 TONs were obtained
2
ꢀ
1
a
aer 5 min (TOF z 10 min , entry 1) and the resting states
TONs were calculated based on conversion of TBE determined by GC
3
1
ꢁ
analysis. n-Octane (2.33 mmol), TBE (2.33 mmol), 2-H (6.83 mmol).
detected by P NMR at 200 C were (carb-PNP)Rh(TBE) 2-TBE
2
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0
0
(
carb-PNP)RhH
2
(2-H
2
) with a ratio 2-TBE/2-H
2
of 3 : 1, the same complex 2 -H
2
(C
2
H
4
) to give the three-coordinate (carb-PNP )Rh
0
ꢀ1
as observed in the COA/TBE system. The complex (carb-PNP) (2 ) and ethane is calculated to be 16.1 kcal mol , in good
Rh(1-octene) was not detected, arguing against product inhibi- agreement with the experimental value (for 2-H (C H )) of
2
2
4
‡
ꢀ1
tion as the explanation underlying the slow rate with n-octane. DG ¼ 18 kcal mol
.
0
Decomposition of the catalyst limited the TON to 14 aer 1 h
As indicated in eqn (15), H/D exchange of 2 -D
2
(C
2
H
4
) with
(entry 3). The 1-octene isomer represented 16% of all octenes
2 4
C H is not observed in the course of the hydrogenation reac-
aer 1 h. Calculations indicate a very slight kinetic preference tion. This would typically be interpreted to suggest that migra-
ꢀ
1
ꢁ
for formation of 1-octene (0.7 kcal mol at 200 C) so this tory insertion of ethylene is irreversible and is the rate-limiting
suggests that olen isomerization is competitive with alkane step of the reaction, followed by fast elimination of ethane. The
ꢁ
dehydrogenation. Lowering the temperature to 150
decreased the reaction rate to z0.5 TON per h (entry 5).
C
calculations, however, yield a somewhat different explanation.
0
Insertion of C H into a Rh–H bond of 2 -H leads to a b-agostic
2
4
2
2
0
2
ethyl complex (PNP)RhH(h -Et), 2 -H(h -Et), with a nearly fully
1
9
˚
Computational results
formed C–H bond (d ¼ 1.23 A; see Fig. 3 and 4). The TS for this
ꢁ
ꢀ1
insertion process at ꢀ30 C has a free energy 9.7 kcal mol
DFT calculations were conducted on the reactions discussed
above using the M06-L density functional and valence basis sets
of triple-zeta plus polarization quality (see ESI†). We used
a model ligand in which the two i-Pr groups on each P atom
0
above that of 2 -H
2
(C
2
H
4
ꢀ
) while the free energy of the agostic
1
0
product is 10.6 kcal mol higher than 2 -H
2
(C
2
H
4
) (although it
has a lower free energy, G, the electronic energy, E, of the TS
leading to the agostic intermediate is higher than that of the
agostic intermediate, as required of a proper TS on the potential
energy surface). Accordingly, the barrier to the back-reaction of
were replaced with a t-Bu and a methyl group to give a C
2
i
symmetric diastereomer. Since metal-bound P Pr
2
groups typi-
cally adopt a conformation in which one of the two methine
C–H bonds points toward the metal center while the other
points away, the P BuMe group mimics the steric effect of the
2
P Pr group. The P BuMe group, however, offers the advantage
of avoiding the many local (non-global) conformational minima
which we have encountered in calculations of pincers with P Pr
0
2
0
this process (i.e. 2 -H(h -Et) / 2 -H
2
(C
˚
2
H
4
)) is negligible.
0
2
t
The short Rh–H distance of 1.75 A in 2 -H(h -Et) indicates
a very strong agostic interaction. The H atom is located trans to
the weak-trans-inuence carbazole nitrogen, while the a-carbon
i
t
i
2
groups (see ESI† for a computational assessment of this model).
In addition, our model does not include the two methyl groups
at the positions para to the carbazolide N atom. We refer to this
0
ligand as carb-PNP and the model compounds as derivatives of
0
2
to distinguish them from the experimental complexes of 2.
The results of the calculations proved quite valuable in
attempting to interpret the experimental results. A free energy
0
diagram for the reaction of dihydride 2 -H with ethylene at
2
ꢁ
ꢁ
ꢀ
30 C is shown in Fig. 2. Note that at ꢀ80 C, the calculations
0
0
indicate that ethylene binds to 2 -H , to give 2 -H (C H ),
2 2 2 4
ꢀ1
exoergically (DG ¼ ꢀ0.7 kcal mol ), in agreement with the
ꢁ
observation illustrated in eqn (12). At ꢀ30 C, the observed
equilibrium suggests that DG is slightly positive and indeed, the
ꢀ1
calculated free energy of binding is DG ¼ 1.5 kcal mol . At
ꢁ
ꢀ
30 C, the barrier to the reaction of ethylene dihydride
Fig. 3 Structural parameters, in the plane bisecting the P–Rh–P axis,
0
along the pathway for the insertion of ethylene into a Rh–H bond of 2 -
2 2 4
H (C H ).
0
2
Fig. 4 “3-D” models of (a) agostic intermediate 2 -H(h -Et) and (b)
ꢀ1
0
0
2
0
Fig. 2 Calculated free energies (kcal mol ) for reaction of 2 -H
ethylene at ꢀ30 C.
2
with ring-opening transition state TS(2 -H(h -Et)/2 -H(Et)). Peripheral
atoms omitted for clarity. Rh–H distances in A˚ .
ꢁ
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is trans to a strong-trans-inuence hydride ligand; this result is
consistent with conclusions of an earlier study of the relation-
ship between agostic bond strengths and the respective trans
20
inuences of ancillary ligands.
Loss of the agostic interaction in 2 -H(h -Et) has a barrier
0
2
‡
ꢀ1
DG ¼ 5.5 kcal mol (Fig. 2). The product of this ring-opening,
0
ꢀ1
0
2
-H(Et), is 18.9 kcal mol lower in free energy than agostic 2 -
H(h -Et). Since this is formally only a bond breaking reaction,
with no concomitant bond making, a negative value of DG, and
2
ꢀ1
particularly such a strongly negative value (ꢀ18.9 kcal mol ), is
quite striking. This result can be explained, however, in terms of
0
2
the geometry of reactant and product. In 2 -H(h -Et), the strong-
trans-inuence agostic ethyl group a-carbon is positioned trans
to the strong-trans-inuence hydride ligand. In contrast, 2 -
H(Et) adopts a so-called Y-geometry, in which the Ca–Rh–H
angle, instead of being ca. 180 (mutually trans) is only 72 ,
while the N–Rh–Ca angle (106 in 2 -H(h -Et)) is 149.0 (Fig. 3).
As a result, 2 -H(Et) has a very short Rh–C bond (2.095 A vs.
.175 A in 2 -H(h -Et)) and a much shorter Rh–H bond (1.546 A
0
21
ꢁ
ꢁ
ꢁ
0
2
ꢁ
ꢀ
1
Fig. 5 Calculated free energies (kcal mol ) for reaction of 2 with n-
0
˚
ꢁ
octane (blue) and with cyclooctane (red) at 200 C (gas phase, pres-
sures corresponding to molarity of pure liquid).
0
2
˚
˚
2
0
2
˚
vs. 1.640 A) than is found in 2 -H(h -Et).
The barrier to elimination of ethane from 2 -H(Et) is only
0
‡
ꢀ1
ꢀ1
DG ¼ 6.0 kcal mol as compared with 24.4 kcal mol for the with pressures that correspond to the molarity of the respective
0
reverse reaction, i.e. re-formation of the agostic bond to give 2 - pure liquid alkanes). Oxidative addition of the C–H bond of COA
2
0
2
ꢀ1
H(h -Et). Thus the “ring-opening” of 2 -H(h -Et) is the rate- has a calculated barrier ca. 4 kcal mol higher than that of
determining step for the overall loss of ethane from dihydride n-octane. However, the TS for formation of the b-agostic species
ethylene complex 2 -H (C H ). Attempts to locate a TS for rota- (carb-PNP )RhH(h -1-octyl), which is rate-determining for
tion around the ethylene C–C bond of agostic complex 2 -H(h - n-octane dehydrogenation, is 5.4 kcal mol higher than the TS
Et) only led to loss of the agostic interaction to give 2 -H(Et). for formation of the corresponding b-agostic cyclooctyl complex.
0
0
2
2
2
4
0
2
ꢀ1
0
0
Thus, although insertion of ethylene into a Rh–H bond of 2 - This may be explained in terms of the eclipsed interactions
H (C H ) is fully reversible, the calculations predict that it required by the formation of agostic complex (carb-PNP )RhH(h -
2 2 4
0
2
should not lead to exchange between hydride (or deuteride) and 1-octyl) (see Fig. 4a for the ethyl analogue). Such unfavorable
ethylene H atoms, in accord with the observed lack of H/D interactions are also present in the TS for formation of (carb-
0
2
exchange between 2-D
2
and C
2
H
4
.
PNP )RhH(h -cyclooctyl). However, in the case of COA, unlike n-
The reaction of dihydride 2-H with TBE, as noted above, octane, these eclipsed interactions are already present in the
2
does not proceed at room temperature in contrast with the alkane substrate (being responsible for the well known ring
ꢁ
reaction with ethylene, which occurs at ꢀ30 C. The TBE reac- strain of COA) as well as in the non-agostic C–H addition product.
ꢁ
tion proceeds slowly at 80 C; the timescale corresponds to Thus, relative to these free species and the non-agostic alkyl
ꢀ1
a free energy barrier of ca. 26–27 kcal mol , about 8–9 kcal hydride, the TS for agostic bond formation for COA is signi-
ꢀ
1
mol greater than the reaction with ethylene. The reaction is cantly lower in energy that that for n-octane. It may be relevant in
calculated to proceed via a pathway analogous to that for this context that unlike the case for n-octane or ethane, the
0
2
0
2
ethylene. An agostic analogue to 2 -H(h -Et) is calculated to agostic cyclooctyl complex (the analogue of 2 -H(h -Et)) appears
ꢀ
1
0
form with a free energy 24.2 kcal mol higher than 2 -H plus to be a distinct minimum on the free energy surface (Fig. 5), and
2
TBE, followed by a rate-determining ring-opening with a TS that not only a minimum on the electronic energy surface.
ꢀ
1
is 3.3 kcal mol higher. The overall barrier for the reaction is
On the dehydrogenation pathway the step subsequent to
‡
ꢀ1
ꢀ1
thus DG ¼ 27.5 kcal mol , about 10 kcal mol greater than formation of the agostic intermediate is b-H-elimination. In the
0
the reaction barrier with ethylene, in very good agreement with case of n-octane dehydrogenation the TS for this step, TS(2 -
2
0
experiment. While the Ir analogue was previously shown to H(h -1-Oc)/2 -H(1-Oc)), has a much lower free energy (23.2
react with TBE via an Ir(III)/Ir(V) pathway requiring the presence kcal mol ) than the TS for the (rate-determining) formation of
, no acceleration by H is observed in the present Rh the agostic complex (28.5 kcal mol ). In contrast, b-H elimi-
2 2
system. This is consistent with the calculated barrier for elim- nation of the agostic cyclooctyl complex (carb-PNP )RhH(h -
ꢀ1
ꢀ1
of H
0
2
0
‡
ination of neo-hexane from (carb-PNP )Rh(t-butylvinyl)(H), DG
cyclooctyl) is calculated to have a TS slightly higher in free
ꢀ
1
ꢀ1
¼
5.9 kcal mol , which is far lower than the barrier calculated energy (24.5 kcal mol ) than the TS for formation of the agostic
‡
ꢀ1
ꢀ1
for the back-reaction (DG ¼ 28.6 kcal mol ).
complex (23.1 kcal mol ), although this difference is quite
The calculations also provide insight into the much greater small (and probably too small to be meaningful for the
rate of dehydrogenation of COA compared with n-octane (free comparison of such different species).
0
energy values are shown in Fig. 5, expressed relative to 2 plus
Interestingly, the free energy of the TS for b-H elimination of
ꢀ
1
the free alkane and calculated for T ¼ 473 K in the gas phase the cyclooctyl complex (24.5 kcal mol ) is higher than that for
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ꢀ1
the 1-octyl complex (23.2 kcal mol ). Likewise (but not
surprisingly), as noted above, the TS for C–H addition of COA is
of higher free energy than that for n-octane. These steps, C–H
addition and b-H elimination, are the steps most commonly
considered in the context of alkane dehydrogenation (while their
microscopic reverse reactions are regarded similarly for olen
hydrogenation). But although the higher reactivity of COA vs. n-
alkanes is a staple of organometallic-catalyzed alkane dehydro-
genation, in the present system the TSs of both of these steps are
calculated to be higher in free energy for the dehydrogenation of
COA than of n-octane. The higher reactivity of COA vs. n-octane
in the present system, according to our calculations, is a result of
only the lower energy of the unanticipated transition state for the
formation of an agostic interaction in the case of COA.
Notes and references
1 K. Weissermel and H.-J. Arpe, in Industrial Organic Chemistry,
Wiley-VCH Verlag GmbH, 2008, p. 59.
2 (a) M. Findlater, J. Choi, A. Goldman and M. Brookhart, in
Alkane C–H Activation by Single-Site Metal Catalysis, ed. P. J.
P ´e rez, Springer, Netherlands, 2012, vol. 38, ch. 4, p. 113;
(b) J. Choi, A. H. R. MacArthur, M. Brookhart and
A. S. Goldman, Chem. Rev., 2011, 111, 1761; (c) D. Morales-
Morales, Iridium Complexes Org. Synth., 2009; (d)
R. H. Crabtree, J. Chem. Soc., Dalton Trans., 2001, 2437.
3 K. Nomura and Y. Saito, J. Chem. Soc., Chem. Commun., 1988,
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4 T. Sakakura, T. Sodeyama, Y. Tokunaga and M. Tanaka,
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5
(a) J. A. Maguire, W. T. Boese and A. S. Goldman, J. Am. Chem.
Soc., 1989, 111, 7088; (b) J. A. Maguire, W. T. Boese,
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Conclusions
2
The iridium dihydride complex 1-H based on the carbazole bis-
phosphine ligand was previously reported to be ineffective as
a transfer-dehydrogenation catalyst. This was found to be ulti-
mately attributable to the very high energy of the (carb-PNP)Ir(I)
complex relative to (carb-PNP)Ir(III). Thus potential hydrogen
acceptors such as TBE inserted into an Ir–H bond (maintaining
the Ir(III) oxidation state), but the barrier to subsequent elimi-
nation to give the Ir(I) product was prohibitively high while
6 A. D. Chowdhury, N. Weding, J. Julis, R. Franke, R. Jackstell
and M. Beller, Angew. Chem., Int. Ed., 2014, 53, 6477.
deinsertion was much more favorable. Hydrogenation by H
was effected, but this was found to proceed via an Ir(III)/Ir(V)
2
7 M. Gupta, C. Hagen, R. J. Flesher, W. C. Kaska and
C. M. Jensen, Chem. Commun., 1996, 2083.
pathway involving addition of H to the Ir(III) alkyl hydride; such
a path is not viable for alkane dehydrogenation.
8 (a) W.-W. Xu, G. P. Rosini, K. Krogh-Jespersen,
A. S. Goldman, M. Gupta, C. M. Jensen and W. C. Kaska,
Chem. Commun., 1997, 2273; (b) M. Gupta, W. C. Kaska and
C. M. Jensen, Chem. Commun., 1997, 461; (c) D. W. Lee,
W. C. Kaska and C. M. Jensen, Organometallics, 1998, 17, 1;
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A. S. Goldman, Organometallics, 2009, 28, 5432; (f) B. Punji,
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2702; (g) J. J. Adams, N. Arulsamy and D. M. Roddick,
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Y. V. Kissin, A. E. Cherian, G. W. Coates and
A. S. Goldman, Chem. Commun., 2005, 3388; (k)
S. A. Kuklin, A. M. Sheloumov, F. M. Dolgushin,
M. G. Ezernitskaya, A. S. Peregudov, P. V. Petrovskii and
A. A. Koridze, Organometallics, 2006, 25, 5466.
2
As the M(I)/M(III) thermodynamics are biased more towards M(I)
22
in the case of Rh than Ir, we suspected the relatively high stability
of a Rh(III) analogue would not preclude, and might even favor,
2
transfer dehydrogenation. Indeed the complex 2-H is found to be
an active catalyst for the dehydrogenation of COA with TBE
ꢀ
1
tBu
4
achieving TOFs up to 10 min , similar to the catalyst ( PCP)
7
IrH . To our knowledge this is the rst example of a highly active
2
rhodium-based alkane transfer-dehydrogenation catalyst that does
2
not require light or H atmosphere. However, decomposition of
ꢁ
the catalyst at 200 C limits the catalyst efficiency.
n-Octane dehydrogenation proceeded more slowly than COA
dehydrogenation. DFT calculations indicate that the slower rate
for n-octane is attributable to the barrier to a rate-determining
step not heretofore given consideration in the context of alkane
dehydrogenation (or its microscopic reverse, in the case of
alkene hydrogenation), namely the formation of an agostic
0
2
intermediate, (carb-PNP )RhH(h -1-octyl), subsequent to C–H
addition. Even so the reaction is not prohibitively slow;
however, the combination of relatively rapid decomposition at
ꢁ
2
00 C and the relatively slow dehydrogenation rate leads to very
limited TONs. The development of more stable rhodium pincer
complexes based on a similar framework is currently underway.
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Acknowledgements
We gratefully acknowledge the nancial support of this work by
NSF under the CCI Center for Enabling New Technologies
through Catalysis (CENTC) Phase II Renewal, CHE-1205189.
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ꢁ
2
ꢁ
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, 1768.
18 Since exchange with excess free C
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(d ) complex with incorporation of H into the hydride
positions as indicated in eqn (15).
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H
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1
1
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