Rhenium in homogeneous catalysis: [ReBrH(NO)(labile ligand)(large-bite- angle diphosphine)] complexes as highly active catalysts in olefin hydrogenations

Article abstract of DOI:10.1021/ja107245k

The reaction of [ReBr₂(MeCN)(NO)(P∪P)] (P∪P = 1,1′-bisdiphenylphosphinoferrocene (dppfc) (1a), 1,1′- bisdiisopropylphosphinopherrocene (diprpfc) (1b), 2,2′- bis(diphenylphosphino)diphenyl ether (dpephos) (1c), 10,11-dihydro-4,5- bis(diphenylphosphino)dibenzo[b,f]oxepine (homoxantphos) (1d) and 4,6-bis(diphenylphosphino)-10,10-dimethylphenoxasilin (Sixantphos) (1e)) led in the presence of HSiEt₃ and ethylene to formation of the ethylene hydride complexes [ReBrH(η²-C₂H₄)(NO) (P∪P)] (3a,b,d), the MeCN ethyl complex [ReBr(Et)(MeCN)(NO)(dpephos)] (5c) and two ortho-metalated stereoisomers of [ReBr(η²-C ₂H₄)(NO)( η³-o-C₆H ₄-Sixantphos)] 8e(up) and 8e(down). The complexes 3a,b,d, and 5c and the isomers of 8e showed high catalytic activity (TOFs ranging from 22 to 4870 h^-1, TONs up to 24000) in the hydrogenation of monosubstituted olefins. For 8e(down) and 8e(up) a remarkable functional group tolerance and catalyst stability were noticed. Kinetic experiments revealed k_obs to be first order in c(cat) and c(H₂) and zeroth order in c(olefin). Mechanistic studies and DFT calculations suggest the catalysis to proceed along an Osborn-type catalytic cycle with olefin before H₂ addition. The unsaturated key intermediates [ReBrH(NO)(P∪P)] (2a-e) could be intercepted with MeCN as [ReBrH(MeCN)(NO)(P∪P)] (10a-d) complexes or isolated as dimeric μ²-(H)₂ complexes [{ReBr(μ²-H)(NO) (P∪P)}₂] (9b and 9e). Variation of the bidentate ligand demonstrated a crucial influence of the (large)-bite-angle on the catalytic performance and reactivity of 3a,b,d, 5c, and 8e.

Full text of DOI:10.1021/ja107245k

ARTICLE
pubs.acs.org/JACS
Rhenium in Homogeneous Catalysis: [ReBrH(NO)(labile ligand)
large-bite-angle diphosphine)] Complexes as Highly Active
Catalysts in Olefin Hydrogenations
(
Balz Dudle, Kunjanpillai Rajesh, Olivier Blacque, and Heinz Berke*
Institute of Inorganic Chemistry, University of Z €u rich, Winterthurerstrasse 190, CH-8057 Z €u rich
S Supporting Information
b
0
ABSTRACT: The reaction of [ReBr (MeCN)(NO)(P∩P)] (P∩P = 1,1 -bisdiphe-
2
0
nylphosphinoferrocene (dppfc) (1a), 1,1 -bisdiisopropylphosphinopherrocene (diprpfc)
1b), 2,2 -bis(diphenylphosphino)diphenyl ether (dpephos) (1c), 10,11-dihydro-4,5-
0
(
bis(diphenylphosphino)dibenzo[b,f]oxepine (homoxantphos) (1d) and 4,6-bis-
diphenylphosphino)-10,10-dimethylphenoxasilin (Sixantphos) (1e)) led in the
(
presence of HSiEt and ethylene to formation of the ethylene hydride com-
3
2
plexes [ReBrH(η -C H )(NO)(P∩P)] (3a,b,d), the MeCN ethyl complex
2
4
[
ReBr(Et)(MeCN)(NO)(dpephos)] (5c) and two ortho-metalated stereoisomers
2
3
of [ReBr(η -C H )(NO)(η -o-C H ꢀSixantphos)] 8e(up) and 8e(down). The complexes 3a,b,d, and 5c and the isomers of 8e
2
4
6 4
ꢀ
1
showed high catalytic activity (TOFs ranging from 22 to 4870 h , TONs up to 24000) in the hydrogenation of monosubstituted
olefins. For 8e(down) and 8e(up) a remarkable functional group tolerance and catalyst stability were noticed. Kinetic experiments
revealed kobs to be first order in c(cat) and c(H ) and zeroth order in c(olefin). Mechanistic studies and DFT calculations suggest the
2
catalysis to proceed along an Osborn-type catalytic cycle with olefin before H addition. The unsaturated key intermediates
2
[
ReBrH(NO)(P∩P)] (2aꢀe) could be intercepted with MeCN as [ReBrH(MeCN)(NO)(P∩P)] (10aꢀd) complexes or isolated
2
2
2 2
as dimeric μ -(H) complexes [{ReBr(μ -H)(NO)(P∩P)} ] (9b and 9e). Variation of the bidentate ligand demonstrated a crucial
influence of the (large)-bite-angle on the catalytic performance and reactivity of 3a,b,d, 5c, and 8e.
1. INTRODUCTION
nitrosyl complexes also on the basis of their isoelectronic
relationship to ruthenium carbonyl or phosphine or rhodium
halide fragments, which comprise essential building blocks of
Catalytic Wilkinson and Osborn-type hydrogenations of un-
saturated organic compounds are widely applied. Their mech-
8
9
1
effective hydrogenation and hydroformylation catalysts.
Our research group has accumulated considerable experience
in the realm of rhenium nitrosyl chemistry, and we could recently
anisms were studied in detail and are well understood. Highly
active and selective catalysts are available to hydrogenate various
unsaturated substrates and make it a method of choice fo2r
demonstrated that trans-[ReH (η -C H )(NO)(PR ) ] (R = ipr,
2
2
2
4
3 2
environmentally benign and enantioselective reductions.
cy) complexes catalyze the hydrogenation of simple olefins and
Although the field of catalytic hydrogenations seems to be quite
mature, fundamental challenges remained. The fact that most of
the homogeneous hydrogenations require platinum group metal
centers may cause future resource problems and makes it
necessary to fully recycle the catalyst in industrial processes. In
10
ketones, as well as hydrogen-related reactions, such as hydrosilyla-
11
tions, dehydrogenative silylations, and dehydrogenative aminobor-
12
ane coupling reactions. We expected improvement of the catalytic
performance of these complexes by forcing the phosphines into cis
positions by applying chelate diphosphines. Therefore, we ap-
proached the preparation of rhenium complexes of the type
[ReHBrL(NO)(P∩P)] with L, representing a labile ligand, and
P∩P, a large-bite-angle chelate diphosphine, and their use as pre-
catalysts in olefin hydrogenations.
3
addition, because of toxicity reasons, pharmaceutical products
produced by such catalyzes have to be freed from the precious
4
metals to the ppm level. Often precious metal catalysis also
suffers from reduction to the metals under hydrogenation
5
conditions resulting in loss of activity and/or selectivity. These
latter disadvantages were anticipated to be circumvented by the
use of transition metal catalysts less “noble” in character. The
element rhenium being border to the precious metals has
preserved at least some of the precious metal’s character, as for
2
. RESULTS AND DISCUSSION
2.1. Preparation of Ethylene Hydride Complexes 3aꢀd. From
6
7
instance the preference for interaction with H2 and olefins.
our work on the bromide complexes [ReBr (MeCN)(NO)(P∩P)]
2
These chemical properties made us believe that appropriate
rhenium complexes could be efficient hydrogenation catalysts.
We therefore targeted the development of new rhenium
Received: August 23, 2010
Published: April 25, 2011
r 2011 American Chemical Society
8168
dx.doi.org/10.1021/ja107245k
|J. Am. Chem. Soc. 2011, 133, 8168–8178

Journal of the American Chemical Society
ARTICLE
1
4
(
(
P∩P = dppfc (1a), diprpfc (1b), dpephos (1c), homoxantphos
ꢀ
spectroscopic features 3a, b, and d were assigned pseudo-octahe-
dral structures with trans-Br and -NO ligands stabilized by a strong π
pushꢀpull interaction. The X-ray diffraction study of 3d confirmed
the proposed structure (Figure 1). It displayed NO/Br disorder,
which indicated the presence of two isomeric forms of 3d in the solid
state. This isomerism turned out to be of negligible importance for
the catalytic performance of these species (vide infra). The geome-
trical aspects of this isomerism are related to those of
13
1d), or Sixantphos (1e)) we learned that the unsupported 16e
species [ReBrH(NO)(P∩P)] (2aꢀe) are unstable, hence not suited
as catalyst precursors and needed to be generated in situ for catalytic
applications. We reckoned that these unsaturated complexes could be
reversibly generated from [ReBrH (SiEt )(NO)(P∩P)] in the
2
3
presence of HSiEt . However, with the exception of the diprpfc
3
derivative, these complexes were still too reactive to be employed as
13
isolable precatalysts. Therefore, we attempted the reaction of 1aꢀe
[ReBr (MeCN)(NO)(P∩P)] systems and were discussed in depth
2
13
with HSiEt in the presence of ethylene as an additional auxiliary
along with the stereochemistry of these complexes.
3
(
Scheme 1). Ethylene was considered an ideal ligand for the
Unlike in the cases of 1a, b, and d, the reaction of 1c with
ꢀ
stabilization of the 16e species 2aꢀe, because of its small size and
HSiEt and ethylene did not afford the ethylene hydride complex
3
7,10
appropriate binding capabilities to electron-rich Re(I) centers.
3c, rather the MeCN ethyl derivative [ReBr(Et)(MeCN)(NO)-
2
14
Additionally, in view of the application of the expected [ReBrH(η -
C H )(NO)(P∩P)] (3aꢀe) compounds to function as a precatalyst
(dpephos)] (5c), which could be rationalized on the basis of
the initial formation of 3c, which according to Scheme 1 under-
2
4
ꢀ
in hydrogenation catalyzes, the ethylene ligand was anticipated to be
initially hydrogenated to ethane, releasing the desired highly reactive
goes a β-hydride shift to form the 16e intermediate
[ReBr(Et)(NO)(dpephos)] (4c). The stabilization arising from
the addition of MeCN to 4c forming 5c is in the case of the
dpephos derivative sufficiently high to shift the equilibrium of
eq 1 quantitatively to the right side.
2
aꢀe intermediates, which could then drive the catalytic cycle
(Scheme 1).
The reactions of 1a, 1b, and 1d with an excess of HSiEt3
proceeded smoothly in the presence of 2 bar of ethylene forming
2
3c þ MeCN a 5c
Reacting 1e in the presence of ethylene with HSiEt produced
ð1Þ
the isolable [ReBrH(η -C H )(NO)(P∩P)] complexes 3a, 3b,
2
4
31
and 3d. Monitoring the reactions by P NMR showed almost
quantitative conversions. The isolated yields were however some-
what lower (3a: 70%; 3b: 61%; 3d: 81%). On the basis of their
3
2
3
two ortho metalated isomers [ ReBr(η -C H )(NO)(η -C H -
2
4
6 4
o-Sixantphos)] 8e(up) and 8e(down), which were characterized
structurally. Unlike in the previous cases, where an up/down
Scheme 1. Synthetic Access to and Reactivity of the Mono-
hydride Species [ReHBr(NO)(P∩P)]
14
isomerism was proposed on the basis of X-ray structures and
31
1
13,14
P{ H} NMR spectra,
the 8(up/down) isomers could be
separated and were individually characterized. 8e(up/down)
proved to be kinetically stable in solution. We propose the ortho
ꢀ
metalation to proceed via the unsaturated 16e species
[
ReBrH(NO)(Sixantphos)] (2e). This species is not sufficiently
stabilized by the binding of C H or MeCN, but it is reactive
2
4
enough to activate the o-CꢀH bond of one Sixanphos phenyl
3
units forming the [ReBr(H )(NO)(η -C H -o-Sixantphos)]
2
6
4
species (6e), which in turn can undergo a H /C H ligand
2
2
4
3
exchange via [ReBr(NO)(η -C H -o-Sixantphos)] (7e) re-
6
4
sulting in the 8e(up/down) isomers (Scheme 2).
.2. Hydrogenation Using 3a,b,d, 5c, and 8e(up/down) as
Catalysts. 3a,b,d, 5c, and 8e(up/down) turned out to be
pre)catalysts for the hydrogenation of olefins comparable in
2
(
1,15
activity to Wilkinson or Osborn-type Rh catalysts. To explore
Figure 1. ORTEP diagram of 3d(down) (left) is 85% populated, and 3d(up) (right) is 15% populated drawn at 50% probability. Selected bond lengths
Å]: Br1AꢀRe1 2.5546(4), C1ꢀC2 1.401(4), C1ꢀRe1 2.242(3), C2ꢀRe2 2.216(2), HꢀRe1 1.59, N1AꢀRe1 1.771(4), N1AꢀO1A 1.184(4),
P1ꢀRe1 2.4597(7), P2ꢀRe1 2.5691(6). Selected bond angles [deg]: Br1AꢀRe1ꢀN1A 175.5(4), Br1AꢀReꢀBr1B 168.82(7), C1ꢀReꢀC2
6.63(10), C1ꢀReꢀP2 83.63(7), C2ꢀReꢀH 65, HꢀRe1ꢀP1 80, P1ꢀRe1ꢀP2 95.22(2).
[
3
8
169
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Journal of the American Chemical Society
ARTICLE
1
b
the catalytic capabilities of 3a,b,d, 5c, and 8e in the catalytic
hydrogenations of olefins, we employed a press gas flow con-
troller for quantitative kinetic monitoring. The hydrogenations
of 1-hexene and styrene were selected as benchmark processes to
enable comparison in catalytic activities. Kinetic isotope effects
were also probed for 3aꢀd in the hydrogenations/deuterations
of 1-hexene using the same set up. The found reaction rates are
listed in the form of TOFs in Table 1 together with the
conditions of the catalytic transformations.
observed for hydrogenation catalysts. Moreover, probing
the kinetic isotope effect (KIE) with D revealed k and kH2
2
D2
to be practically identical (KIE = 1), which is not unusual for
catalytic hydrogenation processes proceeding with homolytic
splitting of H /D (for the Wilkinson catalysts k /k was
2
2
H2 D2
1
reported to be 0.9, whereas for the Osborn catalyst it was
16
reported to be 1.04 ). From this result it seems improbable that
a unique elementary step with prominent H/D participation,
such as the oxidative addition of H or D or the reductive
2
2
The reaction rates were found to be zeroth order in c(olefin).
This implies relatively strong olefin binding and a high olefin
affinity of the rhenium center, which leads at already very low
c(olefin) values to quantitative olefin saturation of the catalyst.
Furthermore, the hydrogenation rate was found to be first order
in p . This implies that H saturation cannot be achieved under
elimination of the product or the β-hydride shift, is uniquely rate
limiting, since all these elementary-type reactions would be
17
associated with a significantly higher KIE. One aspect, which
even further complicates the interpretation of the KIE, is the fact
that the ethylene hydride complexes 3aꢀd undergo fast rever-
sible β-hydride shift reactions, and can therefore scramble the
D(Re) content into the coordinated olefin (eq 2). In the case of
H
2
2
the employed conditions (Table 1). This behavior is often
3
a (vide infra) this reaction is approximately 20 times faster than
Scheme 2. Transformation of 8e(up/down) into 3e(up/
down)
the hydrogenation of 1-hexene (Table 1).
½
ReBrDðH2C¼CHRÞðNOÞðP∩PÞꢁ a
½
ReBrHðHDC¼CHRÞðNOÞðP∩PÞꢁ
ð2Þ
Since we could envisage that non-hydrogen-related steps, such
as ligand exchanges, are also rate limiting or that KIE cancella-
tions of hydrogen-related steps would occur, we cannot draw a
clear overall picture of the components contributing to the KIE.
In the case of the hydrogenations of 1-hexene, very slow time-
dependent decreases in activity were observed under the given
conditions, indicating the catalytic systems to be fairly stable. In
the case of 3a,b and 5c, replacement of THF with DCM led to a
much faster decrease of the reaction rates, which was interpreted
in terms of a progressing catalyst decomposition presumably via
Table 1. Catalytic Hydrogenation Using 3a,b,d, 5c, and 8e(up/down) as Catalysts
ꢀ1
catalyst
loading (mol %)
substrate (mL)
solvent (mL)
temp. (ꢀC)
p 2
H
(bar)
TOF (h
)
TON
a
a
b
a
a
a
a
a
a
a
a
a
a
e
3
3
3
3
3
3
3
3
3
8
8
8
8
8
8
8
8
8
8
a
0.14
1-hexene (1)
THF (2)
23
23
10
10
10
10
10
10
10
10
10
10
10
10
10
50
10
10
10
10
50
290
386
700
900
a
0.10
styrene (1)
THF (2)
b
0.50
1-hexene (1)
toluene (8)
THF (10)
THF (10)
THF (2)
23
22
500
b
0.42
1-hexene (2)
60
144
240
b
0.42
1-hexene (2)
80
750
240
c/5c
c/5c
0.08
1-hexene (1)
23
1248
823
1200
1000
1600
1100
5000
24100
10200
20000
24400
6300
24100
6300
20000
31500
0.09
styrene (1)
THF (2)
23
c
d
0.06
1-hexene (1)
THF (2)
23
1050
1020
4120
2961
1230
1940
600
c
d
0.09
styrene (1)
THF (2)
23
d
d
d
d
d
e(up)
e(up)
e(up)
e(up)
e(up)
0.005
0.004
0.004
0.005
0.003
0.005
0.004
0.004
0.005
0.003
hexene (15)
toluene (10)
toluene (10)
toluene (10)
toluene (10)
toluene (5)
toluene (10)
toluene (10)
toluene (10)
toluene (10)
toluene (5)
80
styrene (15)
80
cyclohexene (15)
R-methylstyrene (15)
dimethylitaconate (5)
hexene (15)
120
120
140
80
d
d
d
d
d
a
a
a
a
e
e(down)
e(down)
e(down)
e(down)
e(down)
4870
2414
600
styrene (15)
80
cyclohexene (15)
R-methylstyrene (15)
dimethylitaconate (5)
120
120
140
1820
500
a
The experiments were conducted with a B €u chi press gas flow controller. The TOFs where determined in the first 10 min of the experiments. The
b
products were identified by NMR spectroscopy. The TOF was measured in situ by ATR-IR spectroscopy in the range from 40 to 100 bar and
extrapolated to 10 bar. 3 μL MeCN were added to the reaction mixture. 100 μL HSiEt were added to the reaction mixture. The reactions were
c
d
e
3
carried out in a closed autoclave system without monitoring during the reaction. Instead, the TOFs were calculated after the pressure did not further
decline in the autoclave. The conversion was determined by GCꢀMS.
8
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Journal of the American Chemical Society
ARTICLE
the reaction of unsaturated catalytic species with this solvent. For 3d
it was found that the addition of small amounts (5ꢀ10 equiv) of
MeCN is crucial for proper catalytic performance. However, a too
large amount of acetonitrile led also to lower TONs and TOFs,
presumably caused by competitive substrate blocking with the
acetonitrile ligand. This demonstrated that the activities of the given
rhenium catalysts can be quite sensitive to the choice of the reaction
medium. However, in contrast to this, catalytic reactions with 8e
were found to be less dependent on the type of solvent. TONs of
more than 24000 could be reached for styrene hydrogenations
without decrease of activity until the substrate was fully consumed.
Despite this quite intriguing result, we had to realize that the 8e(up)
and 8e(down) catalysts were not as efficient in the hydrogenation of
Scheme 3. Proposed Osborn-Type Catalytic Cycle for
Catalytic Hydrogenations with 3a,b,d, 5c and 8e
1-hexene. This, however, could not be explained by missing catalyst
stabilities alone. Rather, we observed a concomitant and intervening
isomerization reaction transforming 1-hexene into Z/E-2-hexene,
which was found to proceed at a rate comparable to that of
hydrogenations under catalytic conditions (80 ꢀC, 10 bar H ).
starting from 3aꢀe as precatalysts. The crucial steps are the
formation of the alkyl complexes of type 4 proceeding via the
transition state TSa and the subsequent hydrogenolysis step of
the alkylꢀrhenium bonds, which is expected to occur via the
dihydrogen complexes of type 12 or/and the dihydride species of
type 13 via TSd (vide infra DFT calculations). From kinetic
measurements we concluded that the overall rate of the hydro-
genation is first order in c(cat) and pH2 but zeroth order in
c(olefin) and no significant kinetic isotope effect could be
determined. These observations would not contradict the pro-
posed cycle but would, on the other hand, also not particularly
support it. This made it necessary to devise further detailed
mechanistic experiments to acquire more information on unique
and separately conducted elementary steps of the catalyzes with
3aꢀe. The results of these studies are presented in the subse-
quent sections.
2
We assume that the Z/E-2-hexenes bind less strongly to rhenium,
and thus, the substrate stabilization is lowered as soon as all of the
1
-hexene substrate is either hydrogenated or isomerized. As a
consequence, early decay sets in, since the stabilization of the
catalyst by the formation of stable olefin complexes seems no longer
possible. The catalytic performance of the two isomers, 8e(up) and
8
e(down), in hydrogenations was found to differ, pointing to the
13
fact that under catalytic conditions an upꢀdown isomerization
does not take place. In absolute numbers, however, these differences
were small (Table 1). In addition to this, we explored the catalytic
potential of 3a,b,d, 5c, andof8e(up/down) in the hydrogenation of
the sterically more demanding disubstituted substrates cyclohexene,
R-methylstyrene, and dimethyl itaconate. 3a,b,d and 5c showed
virtually no activity for these substrates, while 8e(up) and 8e-
(down) were found to be moderatly active (Table 1).
Attempts to identify the nature of the catalyst’s remains from
the reaction solutions after the hydrogenation experiments with
a,b,d, 5c and 8e failed. Moreover, these remains were not active
3. MECHANISTIC INVESTIGATIONS OF THE CATALYTIC
HYDROGENATION
3
in subsequent hydrogenation experiments. From this observa-
tion we conclude that these catalyzes apparently do not consti-
tute “living” systems. This could be explained on the basis of
formation of the compounds [ReBrH(MeCN)(NO)(P∩P)]
3.1. Studies on the β-Hydride Shift Step. The feasibility of
the β-hydride shift step as an initial step of the catalytic cycle was
indicated by the fact that the equilibrium of eq 1 could be
1
1
0aꢀd described in the next sections, which were found to be
observed. The line shapes of the H NMR spectrum of 3c
unstable. In the case of 8e(up/down) the hydrogenations of
non-styrenic substrates proceeded under generally lower
TOFs and TONs. They were limited by a progressive catalyst
deactivation. It is presumed that the crucial catalytic species are
indicated at room temperature fast ReꢀH/C H proton ex-
2
4
change caused by the reversible β-hydride shift reaction con-
necting 3c with the unsaturated ethyl complex 4c (vide infra).
1
Because the line shapes of the H NMR spectrum of 3c were
ꢀ
the 16e complexes 2aꢀe, and it is their missing or too weak
found to be independent of the c(3c) and c(MeCN) concentra-
tions, the nonobservable species 4c connecting 3c with 5c is
generated from 3c via an intramolecular reaction (β-hydride
shift). In order to exclude that this equilibrium is a peculiarity of
the 3c/5c system, the MeCN association equilibria were deter-
mined quantitatively also for 3a and 3b (Table 2).
stability that restricts the TONs. The unsaturated Sixantphos
complex 2e is stabilized via ortho metalation and concomitant
H release to form 7e. This could be substantiated further
2
by deuteration experiments, which revealed that, under catalytic
conditions using D , the phenyl groups of the Sixantphos
2
ligand of 8e became deuterated in the ortho positions. The
reversibility of the ortho metalation is the basis for its protection
function to the unsaturated [ReBrH(NO)(P∩P)] species. The
deactivation of the catalysts was found to be substrate and
catalyst precursor dependent and followed for the olefinic
substrates approximately the order of their metal-binding cap-
abilities: 1-hexene < styrene , cyclohexene, R-methylstyrene,
dimethylitaconate.
It was noticed that the β-hydride shift reaction causes ex-
change of the H(Re) and the ethylene protons according to the
mechanism depicted in Scheme 4, which became nicely visible in
1
14
the H NMR spectra of compounds 3aꢀd (500 MHz, 300 K).
In Scheme 4 this reversible process was denoted as hydride
exchange. It could be observed for all derivatives 3aꢀd with
coalescing signals of the H(Re) moieties and the CH protons in
2
1
2
the vicinity to the ReꢀH group. In the case of 3aꢀc the H /H
At this point, summarizing the facts accumulated from the
hydrogenation experiments, we propose an Osborn-type cycle
proceeding along the catalytic pathway depicted in Scheme 3
nuclei display well-defined individual signals. This indicates that
3aꢀc undergo exclusively the reversible hydride exchange. For
3a the rate of this exchange could be quantified in a temperature
8
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Journal of the American Chemical Society
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Table 2. K , ΔH, and ΔS of the Equilibrium 5aꢀc / 3aꢀc þ
Scheme 5. Reaction of Type 3 Compounds with H , MeCN
d
2
MeCN
and C H
2
4
ꢀ1
ꢀ1 ꢀ1
reaction
K
d
(300 K)
ΔH [kJ mol
]
ΔS [J mol
K ]
5
5
5
a/3a
b/3b
c/3c
0.9
29.8
0.04
18.1
12.5
30.8
60
71
74
Scheme 4. Schematic Sketch of the Hydride and the
Phosphine Exchange Mechanism
2
fragments were converted to the form of μ -H bridged
[
{ReBrH(NO)(P∩P)} ] dimers 9b and 9e obtained as poorly
2
soluble single crystals suitable for structural characterization. In
contrast to this 5c produced the moderatly stable MeCN complex
2
[
ReBrH(MeCN)(NO)(dpephos)] (10c) instead of the μ -H
bridged dimer. 10c was characterized by NMR spectroscopy. As
2
expected the hydride ligand gave rise to a resonance at 1.95 ppm ( J-
2
(
H-trans-P) = 96.0 Hz, J(H-cis-P) = 25.5 Hz) with distinct coupling
31
constants to the cis and the trans phosphorus nuclei. In the P NMR
spectrum a resonance at 18.1 ppm ( J(H-cis-P) = 25.5 Hz, coupling
2
2
not fully resolving) and doublet at ꢀ5.1 ppm ( J(H-trans-P) =
range of 240ꢀ300 K in 10 K intervals by means of an NMR
96.0 Hz) were observed strongly supporting the proposed structure
with chemically inequivalent phosphorus nuclei (Scheme 1). Unlike
q
simulation, from which the activation parameters (ΔH = 62
ꢀ
1
q
ꢀ1 ꢀ1
kJ mol and ΔS = 31 J mol K ) were calculated using
for the reactions of 3b, 5c, and 8e with H , the same reactions with 3a
and 3d led to noncharacterizable, insoluble precipitates. But reaction
2
18
transition state theory. In the case of 3b the signal shapes of the
involved protons suggested that the process proceeded with a rate
comparable to that of 3a. In the cases of 3c and 3d the exchange
proceeded evenfaster. Quantificationof theexchange was inthese
cases not possible, because unfavorable signal overlaps of the 5c
of 3a,3b,and3d with additional amounts of MeCN and H furnished
2
the acetonitrile hydride complexes 10a, 10b, and 10d with spectro-
14
scopic features similar to those of 10c. Since complexes of type 9
and 10 represent also intercepted forms of the highly reactive
intermediates of type 2, which are produced under hydrogenation
conditions from type 3and 5complexes, it seems plausible that type 2
complexes are intermediates in the catalytic hydrogenation reactions
of type 3 and 5 complexes. In contrast to the intermediates of type 2
/
3c þ MeCN equilibrium or the lack of suitable solvents
prevented scrutiny. In addition to the hydride exchange of
Scheme 4, the process denoted as phosphine exchange caused
1
2
1
2
coalescing H /H and P /P resonances, only observed for 3d at
room temperature. In neither case was evidence provided for an
exchange involving all of the C H protons and the H(Re) atom.
and 4, the intermediate H or dihydride complexes 12 or 13 could not
2
be traced spectroscopically, nor could they be intercepted. Thus,
structures of type 12 and 13 are presumably located at higher energies
getting involved in the rate-limiting part of the catalytic cycle.
3.3. Isolated Reaction Steps Supporting the Mechanism of
the Catalytic Cycle. 3.3.1. Reactions of 3aꢀd and 5aꢀd with
2
4
This indicated that the C H ligand is tightly bound on the NMR
2
4
time scale and does not undergo free rotation or fast reversible
dissociation, since this would be expected to go along with the
exchange of all C H proton signals. It was concluded from these
2
4
observations that the β-hydride shift process is the primary
H , MeCN, and C H . Characterization of the most abundant
2 2 4
elementary step of the given Osborn catalytic cycles.
reaction intermediates 3aꢀd and 5aꢀd and also of the hydride
3
.2. Interception of Intermediates of the Catalytic Cycle.
compounds 10aꢀd allows subdivision of the catalytic cycle into
Pursuing the hydrogenation of ethylene with 3a, 3b, 3d, and 5c by
NMR spectroscopy revealed that these ethylene or the related ethyl
complexes are the only observable reaction intermediates. Further
support of the proposed mechanism was envisaged to come from
the detection of type 2, 4, 12, and 13 intermediates. In the previous
section the dynamics of the β-hydride shift step in 3aꢀd and of the
MeCN complexes 5aꢀc were analyzed in detail. They were
considered as intercepted forms of the [ReBr(Et)(NO)(P∩P)]
a H -splitting reaction and an olefin-binding reaction (Scheme
2
5). For instance, in NMR experiments the reaction of 3aꢀd
and 5aꢀd and H
led in the presence of MeCN via type 2
2
intermediates to ethane and 10aꢀd, which were found to
regenerate 3aꢀd and 5aꢀd upon exposure to ethylene.
3.3.2. Catalytic Olefin Isomerization Reactions. The courses of
Osborn type hydrogenations often are accompanied by olefin
1b
isomerization. Therefore, the catalytic olefin isomerization in some
cases side-tracking the olefin hydrogenation catalyis had to be probed.
“Ex-situ” catalytic olefin isomerization was attempted with 3a,b,d
(
4) species, and since they are resting states of the catalytic cycles,
they could be suitable entries into catalysis. As mentioned before the
ꢀ
1
6e complexesoftheform[ReBrH(NO)(P∩P)] (2) are assumed
and 5c with 1-hexene in the absence of H . The premises of such
2
experiments were the following:
to be very unstable and could not be prepared directly from type 1
13
complexes. Therefore, we attempted studies of the reactions of 3a,
1. Olefin isomerization would require olefin exchange at the
rhenium center and would thus provide a measure for the
stability of the Re-olefin complexes. So far we had experi-
enced that olefin exchange is not observable in solutions
b,d, 5c, and 8e with H in the absence of an olefinic substrate
2
(
Scheme 5). This did not, however, allow isolation of type 2
complexes. In the case of 3b and 8e the [ReBrH(NO)(P∩P)]
8
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Journal of the American Chemical Society
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with 3 and 5 complexes in the time frame of the NMR time
scale (section 3.1).
Scheme 6. Proposed Mechanism for the Observed HD
Exchange
2
. The binding preferences for certain olefin isomers will
become visible. Olefins with favorable binding properties
will bind to type 2 complexes and can be isomerized.
Olefins with unfavorable binding properties will be ex-
empted from binding to the rhenium centers and could
thus not be isomerized. The final product distribution of
the olefin isomerization process was expected to show
accumulation of the isomer with the least binding affinity
unless the thermodynamics of the isomerization process
predetermine other preferences.
Exemplarily the isomerization of 1-hexene was investigated.
The order in thermodynamic stability of the hexenes is as
2
7
o
ꢀ1
follows: E-3-hexene (ΔH = ꢀ86.1 kJ mol ) < E-2-hexene
f
o
ꢀ1
o
f
(
ΔH f = ꢀ85.5 kJ mol ) < Z-2-hexene (ΔH = ꢀ83.9
ꢀ1 o ꢀ1
kJ mol ) < Z-3-hexene (ΔH = ꢀ78.9 kJ mol ) < 1-hexene
f
o
ꢀ1
(
ΔH = ꢀ74.2 kJ mol ). Therefore, E-3-hexene would be the
f
nature of the underlying detailed isomerization mechanism is still
unclear. Two alternative mechanisms could be envisaged:
product of a thermodynamically driven isomerization process. In
NMR experiments applying 3aꢀd (5 mg) as a catalyst and
1
. Isomerization via reversible β-hydride shift reactions in
combination with rotation of the β-carbon moiety similar
to the process observed for 3aꢀd (Scheme 7). Such a
mechanism was found to be responsible for olefin isomer-
1
-hexene (50 μL ) as a substrate in CDCl (500 μL) solutions at
3
2
95 K, the following order of activity (TOF/rate of 1-hexene
ꢀ
1
ꢀ1
consumption) was observed: 3d (14 h ) > 3c (5 h ) . 3a,b
ꢀ
1
(0.3 h ). The reactions were found to be first order in c([Re])
1b
izations in conjunction with Rh-catalyzed hydrogenations
and zeroth order in c(1-hexene). The initial products were Z-2-
hexene and E-2-hexene in an approximate 1:1 ratio, which could
20
and hydroformylations of olefins.
2
. Isomerization via the formation of an allyl complex. This
mechanism would require the generation of an unsaturated
intermediate (e.g., dissociation of one phosphorus donor)
prior to the formation of an allyl ligand presumably
established via oxidative addition of a CꢀH bond adjacent
to the olefinic bond. Such a mechanism was discussed for
the olefin isomerization concomitant with Ni-catalyzed
13
19
be verified by quantitative C NMR spectroscopy. However,
exclusively E-2-hexene was observed after prolonged reaction
times. Isomers of the 3-hexenes were not detectable. Further-
3
1
1
more, the P{ H} spectra of the reaction solutions revealed
signals in addition to those of the catalysts, which were associated
with the corresponding hexene complexes. Finally, the ethylene
complexes 3aꢀd were recovered as the only metal-containing
products after the removal of the volatiles. This reaction course
allowed the following conclusions:
21a,b
hydrocyanation
of olefins (along with other mecha-
21c
nisms) or for some Rh- and Ir-catalyzed hydrogenations.
The two mechanisms cannot be distinguished with the available
experimental data. However, DFT calculations support mechanism
1
Because the thermodynamic product E-3-hexene was not
found, the product formation should be kinetically
determined.
1
3
(vide infra). It is noteworthy that the isomerization processes of
a,b,d and 5c catalysts are at 295 K several orders of magnitude
2
Dissociations of the 2-hexenes from 3 proceed with com-
parable rates. Compared to ethylene both 2-hexene isomers
bind weaker to 2aꢀd otherwise 3aꢀd could not be
recovered as the sole rhenium containing product.
Terminal alkenes coordinate stronger than internal ones.
Otherwise the isomerization rate would not be zeroth order
in c(1-hexene) and the overall reaction rate would also depend
on the changing concentrations of the other hexene isomers.
The initially formed mixture of Z/E 2-hexene is eventually
isomerized to the thermodynamically more favorable E-2-
hexene. This indicates that Z-2-hexene can still coordinate
to Re centers.
slower than the rate of the catalytic hydrogenation. This circum-
stance supports the assumption of a very strong Reꢀolefin bond
and is in agreement with the zeroth order dependence of c(olefin)
on the hydrogenation rate. This is in contrast to rhodium-based
Osborn-type hydrogenation catalysts, where the Rhꢀolefin bonds
are weaker and the olefin isomerization and hydrogenation rates
3
4
1b,c
were reported to be first order in c(olefin).
3.3.3. H /D Isotope Scrambling. Another type of experiment,
2
2
which was carried out to support the proposed catalytic cycle were
H /D scrambling reactions. They were thought to provide strong
2
2
evidence for the existence of type 2 and type 11 complexes.
Exposing a mixture of H /D to 10aꢀd results in isotope scram-
2
2
1
In this context the principal question arises whether the
observed olefin isomerization rates can be related to the exchange
rate of the olefins and to the ethylene dissociation pre-equilib-
rium. To get further insight into this reaction the activation
parameters for the exemplary isomerization reaction of 1-hexene
applying 3c were determined in the temperature range
bling within minutes to form HD, which was pursued by H
NMR spectroscopy. We propose the trihydride complexes
[
ReBrH (NO)(P∩P)] 11aꢀd as key intermediates, which are
3
structurally related to the alkyl intermediates of type 13 (Scheme 6).
2
4
. DFT STUDIES ON THE CIS-PHOSPHINE [ReBrH(η -C H )-
ꢀ
1
q
2 4
of 295.15ꢀ318.15 K (ΔH = 94 kJ mol , ΔS = 90 J
(
NO)(PMe ) ] MODEL SYSTEM
ꢀ
1
ꢀ1
q
3 2
mol
K ). The large ΔS indeed supports the hypothesis that
olefin dissociation is rate limiting and that the observed isomer-
ization rates reflect dissociative olefin exchange. However, the
4.1. The Hydrogenation Cycle. The hydrogenolysis step of
the Reꢀalkyl bond in type 4 complexes occurring via type 12 and
8
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Journal of the American Chemical Society
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Figure 2. Calculated free energies (and electronic energies in brackets) of the structurally optimized model species along an Osborn-type reaction path.
ꢀ
1
1
3 species (Scheme 3) could not be observed spectroscopically.
achieve, because the H complex 12f is 22.5 kJ mol higher in
2
Therefore, we attempted elucidation of this part of the catalytic cycle
by DFT calculations using a model system, in which the diphos-
energy than the most abundant reaction intermediate 3f. This
explains the observed independence of c(olefin) and the linear
phines of the Re catalysts were replaced by cis-arranged PMe
dependence of c(H ) of the hydrogenation rate employing
3
2
22
ligands. The employed PMe model and the methodology (the
3a,b,d, 5c, and 8e as catalysts. In the case of 3b the rate of the
1-hexene hydrogenation was linearly dependent on pH2 up to a
pressure of 100 bar.
3
aptitude of which is discussed in ref 23) are the same as Liu et al.
23
used in their thorough investigation of the hydrogenation catalysis
2
with the trans-phosphine [ReH (η -C H )(NO)(PMe ) ] com-
Comparing the course of the catalytic reaction along the
2
2
4
3 2
2
pound. The PMe -substituted structures of types 2ꢀ4, 12, and13 of
calculated intermediates of the cis-[ReBrH(η -C H )(NO)-
3
2 4
the ethylene hydrogenation course of Scheme 3 were denoted as
(PMe ) ] model catalyst with the course of the trans-[ReH -
3 2 2
2 23
2
fꢀ4f(ag), 12f, and 13f and the related transition states as TSa
(η -C H )(NO)(PMe ) ] catalyst revealed that both systems
2 4 3 2
connecting 3f with 4f(ag), TSb connecting 4f(ag) with 4f, TSc
connecting 12f with 13f, and TSd connecting 13f with 2f. These
structures were optimized without constraints (Figure 3). Their
relative energies along the Osborn-type reaction path are shown in
follow basically the same Osborn-type mechanism, but in the
case of the cis-PMe system the involved species are all at lower
3
energies. The energy span between the transition state of the
highest energy and the intermediate of the lowest energy is
ꢀ
1
Figure 2. The calculations revealed two substantial barriers ΔE
49.3 kJ mol in the cis-PMe system compared to 106.3 kJ
3
2
1
ꢀ1
ꢀ1
(
(
39.1 kJ mol β-hydride shift via TSa/ethylene insertion) and ΔE2
18.8 kJ mol reductive elimination of ethane from 13f). The
mol (path 1, span between trans-[ReH (η -C H )(NO)-
2 2 4
ꢀ
1
ꢀ1
(PMe ) ] and oxidative addition of H ) or 121.3 kJ mol
3 2 2
2
barrier ΔE corresponds to the β-hydride shift reaction observed in
(path 2, span between trans-[ReH (η -C H )(NO)(PMe ) ]
2 2 4 3 2
23
1
1
the H NMR spectra of 3aꢀddiscussed in section 3.1. For the PMe
and reductive elimination of ethane) in the trans-PMe system.
3
3
ꢀ
1
model system this barrier was found to be lower (39.1 kJ mol )
Furthermore the barrier of the initial β-hydride shift step was
ꢀ
1
ꢀ1
23
compared to the barrier in 3a (62 kJ mol ). However, because the
exchange in 3a is the slowest of the 3aꢀd series and type 3
complexes were found to be quite sensitive to the nature of the
employed diphosphine, this is not surprising and it is still in line with
the proposed catalytic cycle. Smaller barriers were found for the
breakage of the β-agostic Re H(ethyl) interaction in 4f(ag) to
found to decline from 87.0 kJ mol (trans-PMe3 ) to 39.1 kJ
ꢀ
1
mol (cis-PMe ). Thus, our initial assumption that changing the
3
phosphine arrangement from trans to cis would increase the
activity of the catalysts is also reflected in the DFT calculations.
A discussion of the structural changes along the cis-PMe catalytic
3
cycle may commence with 3f, in which the ethylene is bound slightly
asymmetric with the ReꢀC bonds in the typical range of single
bonds (C1ꢀRe 2.25 Å, C2ꢀRe 2.29 Å). The C1ꢀC2 bond is
considerably stretched to 1.41 Å compared to typical olefinic
3
3 3
form 4f via TSb and for the oxidative addition of the H ligand of the
2
H complex 12f to form the dihydride 13f via TSc. Atthelevelofthe
2
free energies of these interconversions at 298.15 K the relative free
energies of the involved species change such that TSb and TSc lose
27
bonds (1.35 Å) indicating strong back bonding. This is in
reasonable agreement with the ReꢀC bond lengths (2.216(2)
and 2.242(3) Å) and the CꢀC bond length (1.401(4) Å) observed
their barrier character due to favorable contributions of E , Etrans,
rot
and E . According to the relative energies of the catalytically
vib
relevant species, 3f is the most abundant reaction intermediate
in the structure of 5d. The geometry of the PMe ligands (ReꢀP1
3
(which is also in agreement with the experiment), and TSd is the
2.479 Å, ReꢀP2 2.541 Å, —P1ꢀReꢀP2 95.1ꢀ) in the model is also
in very good agreement with the structure of 5d (ReꢀP1 2.4597
(7) Å, ReꢀP2 2.5691(6) Å, —P1ꢀReꢀP2 95.22(2)ꢀ). The
ReꢀH bond is 1.68 Å, the C1ꢀH(Re) is 2.35 Å and the
HꢀReꢀC1 angle is 71.8ꢀ. A comparison of the IR spectra of
highest energy transition state. The free energy span between these
ꢀ
1
24
two species is 49.3 kJ mol . Employing Shaik’s kinetic model for
this reaction cycle reveals a TOF control of 3f and TSd of XTOF,I = 1
and XTOF,T = 0.92, whereas the initial β-hydride shift step and the
25
26
oxidative addition of H are of minor importance for the TOF.
3aꢀd with the calculated spectrum (scaled by 0.9614 ) of 3f
2
ꢀ
1
Moreover, the free energy profile in Figure 2 reveals a H binding
revealed fairly good agreement of the ν(NO) (3a 1685 cm ; 3b
2
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
energy of ꢀ8.9 kJ mol for 4f, while the unsaturated complex 2f
1678 cm ; 3c 1704, 1683 cm ; 3d 1673 cm ; 3f 1687 cm ),
ꢀ
1
ꢀ1
ꢀ1
displays an ethylene binding energy of ꢀ90.1 kJ mol . Therefore,
the model catalyst would be saturated with the olefin already at
lowest concentrations, forming the most abundant reaction inter-
whereas the ν(ReH) (3a 1960 cm ; 3b 1974 cm ; 3c 2014,
1
ꢀ
ꢀ1
ꢀ1
1954 cm ; 3d 2009 cm ; 3f 1889 cm ) of the model appears
somewhat to much red-shifted. Going from 3f to TSa, the ReꢀH
distance and the HꢀReꢀC1 angle are reduced to 1.51 Å and 40.5ꢀ
mediate 3f. Saturation of the system with H would be difficult to
2
8
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Journal of the American Chemical Society
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Figure 3. DFT optimized structures of the model compounds 2ꢀ5f, 10fꢀ13f, TSa, TSc, and TSd. Selected bond lengths [Å]: 2f H1ꢀRe 1.69;
3
f C1ꢀC2 1.41, C1ꢀRe 2.25, C2ꢀRe 2.29, H1ꢀRe 1.68, P1ꢀRe 2.54, P2ꢀRe 2.48; 4f C1ꢀRe 2.18, C1ꢀC2 1.54; 5f CꢀRe 2.23, NꢀRe 2.08; 10f
H1ꢀRe 1.69, N1ꢀRe 2.09; 11f H1ꢀRe 1.68, H2ꢀRe 1.66, H3ꢀRe 1.68; 12f C1ꢀRe 2.22, H1ꢀH2 0.84, H1ꢀRe 1.85, H2ꢀRe 1.84; 13f C1ꢀRe 2.25,
H1ꢀH2 1.43, H1ꢀRe 1.68, H2ꢀRe 1.65; TSa C1ꢀC2 1.45, C1ꢀH1 1.51, C1ꢀRe 2.32, C2ꢀRe 2.22, H1ꢀRe 1.75; TSc C1ꢀRe 2.24, H1ꢀH2 1.24,
H1ꢀRe 1.70, H2ꢀRe 1.67; TSd C1ꢀRe 2.39, C1ꢀH2 1.63, H1ꢀH2 1.75, H1ꢀRe 1.64, H2ꢀRe 1.68. Selected bond angles [deg]: 2f P1ꢀReꢀP2
1
03.5; 3f C1ꢀReꢀH1 71.8, P1ꢀReꢀP2 95.1; 4f P1ꢀReꢀP2 102.4; 5f CꢀReꢀN 85.8, CꢀReꢀP2 84.9, NꢀReꢀP1 88.9, P1ꢀReꢀP2 99.3; 10f
P1ꢀReꢀP2 99.2; 11f:H1ꢀReꢀH2 60.0, H2ꢀReꢀH3 60.6, P1ꢀReꢀP3 94.7; 12f C1ꢀReꢀH2 74.5, H1ꢀReꢀH2 26.4, P1ꢀReꢀP2 99.8; 13f
CꢀReꢀH2 64.6, CꢀReꢀP2 79.1, H1ꢀReꢀH2 50.9, H1ꢀReꢀP1 68.5, P1ꢀReꢀP2 95.9; TSa C1ꢀReꢀC2 37.1, C1ꢀReꢀH1 40.5, P1ꢀReꢀP2
1
01.5; TSc CꢀReꢀH1 68.8, CꢀReꢀP2 79.9, H1ꢀReꢀH2 43.3, H2ꢀReꢀP1 70.7, P1ꢀReꢀP2 96.3; TSd C1ꢀReꢀH2 42.9, H2ꢀReꢀH1 63.6,
P1ꢀReꢀP2 95.5.
(
significantly below the ca. 2.8 Å of the combined van der Waals
second PMe ligand in the apical position. The coordination site trans
3
27
radii of C and H ). At the same time the ReꢀH (1.75 Å/þ0.06 Å),
the ReꢀC1 (2.32 Å/þ0.07 Å) and the C1ꢀC2 (1.45 Å/þ0.04 Å)
bonds are slightly elongated compared to the respective parameters of
to the apical PMe ligand remains unoccupied and is well accessible.
3
The ReꢀC1 (2.18 Å) and the C1ꢀC2 (1.54 Å) bonds are both in
the typical range of ReꢀC and CꢀC single bonds. 4f then binds H
2
3
f. This suggests that TSa retains most of the ethylene hydride
to the vacant coordination site, forming the H complex 12f with
2
character of 3f and therefore can be called an early transition state.
The PꢀReꢀP angle in TSa is widened by 6ꢀ to 101.5ꢀ, favoring in
this elementary step the presence of the large-bite-angle diphosphines,
since these would perfectly fit TSa, but not the starting structure 3f,
and thus are anticipated to promote the β-hydride shift step. This
result would explain the large-bite-angle influence of the diphosphines
on the real catalytic reaction course, provided that the β-hydride shift
step would contribute to the overall barrier height of the catalytic
process. From TSa the hydride would finally be transferred from the
relatively long ReꢀH distances (1.84 and 1.85 Å) and an elongated
H1ꢀH2 distance (0.84 Å). Compared to 4f the 12f PꢀReꢀP and
the C1ꢀReꢀP1 angles are slightly (2.6ꢀ; 3.4ꢀ) compressed to 99.8ꢀ
and 91.2ꢀ, respectively, reflecting the “steric pressure” originating
from the binding of the H molecule. The oxidative addition of the H
2
2
ligand to form the dihydride complex 13f proceeds via TSc.
Compared to those of 12f the ReꢀH bonds of 13f are much shorter
(1.68/1.67 Å), and the HꢀH distance (1.43 Å) is significantly
28
increased and may be denoted as a “compressed” dihydride species.
ꢀ
Re atom to the C H ligand, resulting in the formation of a 16e ethyl
The transition state TSc connecting 12f with 13f can be characterized
as a late transition state, since it is structurally much closer to 13f than
to 12f, which is reflected in short ReꢀH bonds (1.70/1.67 Å) and a
significantly elongated H1ꢀH2 distance (1.24 Å) still in the range of
a “compressed” dihydride structure. Finally, ethane is reductively
eliminated from 13f via TSd, which structurally resembles 13f as a
pentagonal bipyramidal structure. Compared to that in 13f the
ReꢀH1 bond in TSd is shortened to 1.63 (ꢀ0.04) Å, the ReꢀC
bond is elongated to 2.39 (þ0.13) Å, and the CꢀReꢀH1 angle and
2
4
species, for which two energetic minima (4f(ag) and 4f) were found.
As already mentioned 4f(ag) is stabilized by a β-agostic interaction
between the Me(ethyl) group and the Re center. 4f was calculated as
an unsaturated ethyl complex without agostic interactions. These
species are energetically almost equivalent (4f(ag) is favored by
ꢀ1
1
.3 kJ mol over 4f in free energy), and their interconversion via TSb
is practically barrierless. 4f possesses a square pyramidal structure with
the Et, PMe , NO, and the Br ligands in the basal plane and the
3
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Scheme 7. Mechanism of the Olefin Isomerization via the
β-Hydride Shift Pathway
Table 3. Olefin Dissociation Free Energies (D ) of Various
Olefin Complexes
e
ꢀ1
R
1
R
2
R
3
R
4
D
e
(olefin) [kJ mol
]
d(CꢀC) [Å]
3f
3f
3f
3f
3f
3f
3f
3f
3f
H
H
H
H
H
H
H
H
H
H
90.1
73.4
77.7
73.2
63.6
57.3
44.2
67.1
65.1
1.4097
1.4140
1.4115
1.4124
1.4140
1.4141
1.4160
1.4151
1.4159
1
2
3
4
5
6
7
8
Me
H
Me
H
H
Me
H
H
H
Me
H
Me
H
Me
H
H
the CꢀH1 distance are reduced from 64.6ꢀ and 2.15 Å in 13f to 42.9ꢀ
and 1.63 Å. This can be interpreted as a beginning H transfer to the
ReꢀC bond. At the same time the ReꢀH2 follows this movement
Me Me
Me
H
H
Me
H
H
Me
Me
approaching its final position in 2f trans to the PMe ligand. Upon
3
continuing on the reaction path from TSd no further stable inter-
mediate (for instance an agostic structure) could be found, which
indicates that TSd converts to the product ethane and 2f, finishing
this reductive elimination process or it converts back to 13f. Overall in
these transformations the PꢀReꢀP angle adjusts freely to the Re
center’s binding needs (2f 103.5ꢀ, 3f 95.1ꢀ, 4f(ag) 102.1ꢀ, 4f 102.4ꢀ,
However, since this rotation occurs around a ReꢀC σ-bond, no
substantial barriers are expected. Therefore, we expected the
olefin complexes of type 3 to be the prevailing reaction inter-
mediates and the unsaturated 16e Re-hydride 2f with the freed
olefin to be the highest energy state defining the overall free
energy span of the catalytic cycle.
ꢀ
12f 99.8ꢀ, 13f 95.6ꢀ, TSa 101.5ꢀ, TSb 102.6ꢀ, TSc 96.3ꢀ, TSd 95.5ꢀ),
Because the stability of olefin complexes is known to be sensitive
to the substitution pattern of the olefin even for alkyl-substituted
while the BrꢀReꢀNO axis remains linear and structurally almost
unaffected by any change in the coordinative environment.
29
ones, we calculated all possible propene and 2-butene derivatives
4
.2. Stabilization of 2f by H , MeCN, and C H Coordina-
2 2 4
of 3f as PMe -substituted model compounds with the intention of
tion. The DFT calculations also allowed estimation of the stabiliza-
tion of 2f by the addition of H or MeCN. When H is added to the
3
explaining the observed selectivity of the isomerization and hydro-
genation reactions. The relative stability of these compounds
compared to those of 2f and the free olefins are summarized in
Table 3, revealing two important aspects.
2
2
rhenium center, the trihydride 11f is formed, which results in a
ꢀ
1
stabilization energy of ꢀ38.0 kJ mol with respect to 2f. In the case
of the MeCN addition forming 10f, the stabilization energy amounts
2
9
ꢀ
1
1
. Theinductive effectof the alkyl substituents together with
the steric effect of the olefin substitution pattern causes a
decline in the relative olefin affinities in the order ethylene >
propene > 2-butenes. Although the olefin affinity was found
to ꢀ73.6 kJ mol . However, for the addition of ethylene transform-
ꢀ
1
ing 2f into 3f, the stabilization energy amounts to ꢀ90.1 kJ mol ,
exceeding the stabilization energies from H and MeCN complexa-
2
tions so that on thermodynamic grounds H and MeCN can be
2
ꢀ
1
to vary from ꢀ44.2 (3f ) and ꢀ57.3 kJ mol (3f ) for
displaced by ethylene. The stabilization energy of 2f from its
dimerization to a hypothetical 9f species was not investigated, since
the results were anticipated to be rather artificial and not matching the
reality well. It was assumed that the steric congestion of the diphos-
6
5
ꢀ
1
Z-butene to ꢀ90.1 kJ mol for ethylene, the CꢀC bond
length remained close to 1.41 Å. Thus, it seems that the
degree of back-donation is not affected by the variation of
the olefin, while the overall olefin affinity varies substantially
phines would not be appropriately modeled by PMe ligands. More-
3
(
Table 3).
over, the H affinity experiments (section 3.3) provided also evidence
2
2
. Steric effects are crucial to olefin binding at rhenium. In
that the MeCN complexes of type 10 are thermodynamically favored
over the formation of the dinuclear compounds of type 9(Scheme5).
This further rendered computational model evaluations of the di-
nuclear structures less significant for the real catalytic reaction course.
However formation of 5f from 4f by the addition of MeCN seemed to
be an appropriate model process to mimic real catalysis and was
ꢀ
1
particular the large energy difference of 22.9 kJ mol
between the most favored (3f ) and the most disfavored
7
(
3f ) 2-butene derivatives illustrates that sterics greatly
6
influence also the total energies of the PMe model system.
3
Steric influence can be traced with short distances (below
the van der Waals radii) between the Me groups of the
olefins and the NO/Br ligands. In the real systems with the
more bulky diphosphine ligands, this aspect of the olefin
binding might even be more pronounced.
ꢀ
1
calculated revealing a ΔE of ꢀ26.2 kJ mol corresponding to a ΔH
ꢀ
1
of 28.7 kJ mol in the gas phase, which would be in the range of the
ꢀ1
experimentally obtained values for 5a (ΔH = ꢀ18.1 kJ mol ), 5b
ꢀ1
ꢀ1
(
ΔH = ꢀ12.5 kJ mol ), and 5c (ΔH = ꢀ30.8 kJ mol ).
4
.3. DFT Studies on Olefin Binding in 3f and on the Olefin
The calculated thermodynamics of the olefin complexes in
Table 3 should be interpreted with caution, since the applied “plain”
Isomerization Catalysis. The first three steps in the free energy
diagram of Figure 2 can also be envisaged as a part of the olefin
isomerization reaction proceeding via a reversible β-hydride shift
mechanism. In addition to these reaction steps rotation of the
alkyl moiety of substituted olefin derivatives of 4f would be
required in order to effect isomerization as sketched in Scheme 7.
22
DFT method leads to an incorrect prediction of the thermo-
dynamic stabilities of the butene isomers. The calculated thermo-
ꢀ
ꢀ1
2
1
dynamic order of the butene isomers is Z-2-butene (0.0 kJ mol ) >
ꢀ
1
E-2-butene (þ12.8 kJ mol ) > 1-butene (þ12.9 kJ mol ),
7
whereas the experimental order under standard conditions is
8
176
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Journal of the American Chemical Society
ARTICLE
4
5
. dynamic NMR analysis of the β-hydride shift step, and
. accompanying DFT studies.
The basis for this work is the enhanced affinity of appropriately
substituted rhenium centers to hydrogen and olefins similar to
that of the neighboring group of precious metals. These affinity
properties could be exploited for hydrogenation catalyzes only if
vacant coordination sites were made temporarily available during
6
the course of the catalytic reaction. Octahedral Re(I) d com-
plexes normally possess high kinetic barriers for such ligand-
exchange steps. In the presented systems this problem could be
circumvented by appropriately tuned ligand sets enabling a facile
β-hydride shift step, which subsequently opens a required vacant
coordination site for the oxidative addition of H . The employed
2
large-bite-angle diphosphines also support Re(I)/Re(III) redox
changes favoring the generation of seven-coordinate Re(III)
intermediates required in oxidative addition processes. On the
basis of the excellent performance of the given, structurally
simple rhenium catalysts, we are presently exploring the scope
of functional group tolerance in rhenium-based olefin hydro-
genation and also enantioselective hydrogenations.
Figure 4. Free energy profile for the isomerization of olefins along the
allyl mechanism.
o
ꢀ1
o
E-2-butene (ΔH = ꢀ11.4 kJ mol ) > Z-2-butene (ΔH = ꢀ7.1
f
f
ꢀ
1
o
f
ꢀ1
kJ mol ) > 1-butene (ΔH = þ0.1 kJ mol ). This is not
surprising, since DFT calculations are known to be inaccurate if
30
nonbonding interactions come into play.
’ ASSOCIATED CONTENT
To summarize the most important findings we can safely state
that the energetic distinction of various isomeric olefin com-
plexes is substantial with a pronounced preference for terminal
olefins over internal ones. This also explains the selectivity of the
catalysts 3a,b,d and 5c for terminal monosubstituted olefins. The
differences in olefin binding arise from electronic and from steric
factors. Furthermore, the TOF of the olefin isomerization is
determined by the energy span between the most stable olefin
S
Supporting Information. Spectroscopic data and experi-
b
mental procedures for the synthesis of 3a, 3b, 3d, 5c, 8e(up/down),
b, 9e, 10a, 10b, 10c, and 10d; additional crystallographic data
9
including ORTEP diagrams of 8e(up), 8e(down), 9b and 9e;
NMR-simulation data and rates of the 3a/4a exchange process; and
equilibrium data for the (3aꢀc þ MeCN) / 5aꢀc equilibrium.
This material is available free of charge via the Internet at http://
pubs.acs.org.
ꢀ
complex and the separate 16e complex 2f and the correspond-
ing olefin. The reversible β-hydride shift reactions and the
rearrangement of the corresponding alkyl complexes of type 4
are anticipated to be much faster and can therefore be considered
as a fast pre-equilibrium step.
’ AUTHOR INFORMATION
Corresponding Author
hberke@aci.uzh.ch
As an alternative to the isomerization reaction via repetitive
reversible β-hydride shift steps, a mechanism proceeding via allyl
complexes needs to be considered as well. Therefore, we modeled
’
ACKNOWLEDGMENT
2
the unsaturated complex [ReBrH(η -propene)(NO)(PMe )]
3
3
We thank the Swiss National Science Foundation and the
(
14f), the allyl complex [ReBrH (η -allyl)(PMe )] (15f), and
2
3
University Z €u rich for financial support.
the transition state TSe connecting 14f with 15f by means of the
same DFT method as applied before. The free energy profile of this
sequence is also shown in Figure 4 and revealed a free energy span of
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