Ethylene reactions of a [ReH(η2-BH4)(NO)(PPh 3)2] complex: Reductive elimination of ethane and oxidative coupling to butadiene

Article abstract of DOI:10.1021/om201187w

The borohydride complex [Re^+IH(η²-BH ₄)(NO)(PPh₃)₂] (1ph) reacts with ethylene to yield [Re^+IH₂(η²-C₂H ₄)(NO)(PPh₃)₂] (2ph) and triethylborane formed by ethylene hydroboration. Subsequent ethylene insertion into the Re-H bond of 2ph and uptake of another 1 equiv of ethylene led to the kinetically stable cis-hydrido-ethyl complex [Re^+IH(Et)(η²-C ₂H₄)(NO)(PPh₃)₂] (3ph). 3ph was found to slowly reductively eliminate ethane. The rate of this process was determined by quantitative NMR spectroscopy in the temperature range from 293 to 338 K, enabling calculation of the activation parameters (ΔH = 68.7 kJ mol^-1, ΔS = -94 J mol^-1 K^-1; half-life time 1.8 h at 303 K). The reaction was found to follow first-order kinetics in c(3ph) and is zeroth order in c(C₂H₄) and c(PPh ₃), ruling out preceding ligand dissociation. The presumptive intermediate [Re^-I(η²-C₂H ₄)(NO)(PPh₃)₂] could not be traced, since it rapidly reacted further with ethylene, furnishing the stable butadiene complex [Re^-I(η²-C₂H₄) (η⁴-C₄H₆)(NO)(PPh₃)] (4ph) in 88% yield. This transformation of dehydrogenative ethylene coupling is suggested to involve the elementary steps of rhenacyclopentane formation from two coordinated ethylene ligands and then double C-H activation via β-hydride shifts to generate the butadiene unit and formal H₂ elimination from the rhenium dihydride with concomitant triphenylphosphine elimination. An X-ray crystallographic study confirmed the spectroscopically derived pentacoordinate structure of 4ph.

Full text of DOI:10.1021/om201187w

Article
pubs.acs.org/Organometallics
Ethylene Reactions of a [ReH(η2-BH4)(NO)(PPh3)2] Complex: Reductive
Elimination of Ethane and Oxidative Coupling to Butadiene
Balz Dudle, Olivier Blacque, and Heinz Berke*
Institute of Inorganic Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich
̈
̈
S
* Supporting Information
ABSTRACT: The borohydride complex [Re+IH(η2-BH4)(NO)-
(PPh3)2] (1ph) reacts with ethylene to yield [Re+IH2(η2-
C2H4)(NO)(PPh3)2] (2ph) and triethylborane formed by
ethylene hydroboration. Subsequent ethylene insertion into the
Re−H bond of 2ph and uptake of another 1 equiv of ethylene led
to the kinetically stable cis-hydrido−ethyl complex [Re+IH(Et)-
(η2-C2H4)(NO)(PPh3)2] (3ph). 3ph was found to slowly
reductively eliminate ethane. The rate of this process was
determined by quantitative NMR spectroscopy in the temperature range from 293 to 338 K, enabling calculation of the activation
parameters (ΔH⧧ = 68.7 kJ mol−1, ΔS⧧ = −94 J mol−1 K−1; half-life time 1.8 h at 303 K). The reaction was found to follow first-
order kinetics in c(3ph) and is zeroth order in c(C2H4) and c(PPh3), ruling out preceding ligand dissociation. The presumptive
intermediate [Re−I(η2-C2H4)(NO)(PPh3)2] could not be traced, since it rapidly reacted further with ethylene, furnishing the
stable butadiene complex [Re−I(η2-C2H4)(η4-C4H6)(NO)(PPh3)] (4ph) in 88% yield. This transformation of dehydrogenative
ethylene coupling is suggested to involve the elementary steps of rhenacyclopentane formation from two coordinated ethylene
ligands and then double C−H activation via β-hydride shifts to generate the butadiene unit and formal H2 elimination from the
rhenium dihydride with concomitant triphenylphosphine elimination. An X-ray crystallographic study confirmed the
spectroscopically derived pentacoordinate structure of 4ph.
Scheme 1. Generalized Re(+I)/Re(−I) and Re(+I)/Re(+III)
1. INTRODUCTION
a
Hydrogenation Schemes for ReH2(olefin) Complexes
Our efforts to develop rhenium nitrosyl based hydrogenation
catalysts culminated in recent reports on highly efficient olefin
hydrogenation catalyzes with [ReHX(L)(NO)(PR3)2] (X = F,
Cl, Br, I; R = cy, iPr) and [ReHBr(L)(NO)(P∩P)] (P∩P =
dpephos, homoxantphos, Sixantphos, dppfc, diprpfc) catalysts.1
In both cases of catalyses the reaction courses were found to
proceed along Osborn type cycles2 involving Re(+I)/Re(+III)
redox changes. The alkane release steps of these catalyses were
indeed reductive eliminations from a Re+III(H)(alkyl) species.
In contrast, the previously reported [ReH2(olefin)(NO)-
(PR3)2] (R = cy, iPr) systems3 were suggested to follow
catalytic routes involving either Re(+I)/Re(+III) or Re(+I)/
Re(−I) redox changes (Scheme 1). This capability to generate
redox couples at different oxidation state levels makes rhenium
very prone to hydrogenations with significantly differing
coordination spheres. This gave us the notion to study the
reactions of [ReH(η2-BH4)(NO)(PPh3)2] (1ph) with ethylene
in greater detail in order to explore its potential further to
establish high- or low-oxidation-state routes. The propensity of
rhenium centers to adopt various oxidation states in hydrogen
and olefin chemistry is, for instance, also reflected in a
[ReH7−nLn/2(L′)2] series4 with one to seven hydrides. Such
hydride species are often subject to catalytically relevant
Ren(H)2 ⇆ Ren−2(η2-H2) redox equilibria5,4a and are addition-
ally known for their high olefin affinity.6 In view of the ligand
tuning we anticipated that the redox properties of catalytic
rhenium centers prone to hydrogen and olefin chemistry could
a
□ = vacant coordination site or labile ligand.
be additionally fine-tuned by the type of phosphine substitution
pattern, as for example in the case of the [ReH7(PR3)2] system,
which is known to respond to subtle changes in the phosphine
stereoelectronic properties.4,5a Moreover, we expected that on
the basis of the donicities and steric demand of the phosphine
ligands, they should show also different catalytically relevant
dissociation behavior. Variations in the phosphine coordination
sphere could therefore open up additional catalytic reaction
channels, such as dehydrogenation,7 C−H activation,8,4b and
C−C bond formation9 pathways. To illustrate this important
Received: November 28, 2011
Published: February 23, 2012
© 2012 American Chemical Society
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Scheme 2
Figure 1. 1H NMR spectrum of a mixture of 2ph and 3ph (THF-d8, 500 MHz, 300 K). The ethylene ligands are fixed in an “upright” conformation
aligned with the P−Re−P axis.
point of “catalytic” rhenium, the anticipated two routes of olefin
hydrogenations with Re(−I/+I) or Re(+I/+III) centers are
sketched in Scheme 1 in generalized form for a Re(H)2(olefin)
moiety.
(2ph) and subsequently at 22 ppm (3ph). The rates of the
interconversions of 1ph into 2ph and then into 3ph were found
to be strongly dependent on the ethylene concentration and
the solvent. The ethylene concentration could not reproducibly
be controlled in the NMR tube, which prevented quantitative
investigations. However, the conversion of 1ph to 2ph was
found to be much faster in THF than in benzene, and therefore
it seemed reasonable to assume that THF facilitates BH3
dissociation from 1ph, generating an unsaturated or THF-
stabilized rhenium intermediate, both of which could not be
traced spectroscopically (Scheme 4). This also suggested that in
the formation of 2ph BH3 dissociation was rate limiting.
Subsequently 2ph transformed into 3ph by a β-hydrogen shift
step followed by the coordination of another 1 equiv of
ethylene. The rate of this transformation is comparable to the
rate of 2ph formation. Thus, after exposure of 1ph to ethylene
at ambient temperature both species 2ph and 3ph were present
in the reaction mixture for a short period of time within about
3−10 min. 3ph becomes the sole product, depending strongly
on the C2H4 pressure and the mixing process. Quick removal of
the ethylene atmosphere and of BH3/BEt3 in vacuo after
completion of the transformation of 1ph into 2ph stopped the
The reactions of the tricyclohexylphosphine and triisopro-
pylphosphine derivatives [ReH(η2-BH4)(NO)(PR3)2] (R = cy
(1cy), R = iPr (1iPr)) with ethylene were studied earlier and
furnished access to the stable dihydride olefin complexes
[ReH2(η2-C2H4)(NO)(PR3)2] (R = cy (2cy), iPr (2iPr)),3
which corresponded to a substitution process replacing the BH3
unit as BH3·THF and a slightly modified pathway along the
Re(+I)/Re(+III) cycle of Scheme 1 and according to Scheme 2.
The reaction apparently stopped at the olefin dihydride level,
which we attributed to the strongly donating property of the
phosphine ligands. The rhenium centers were thought to be too
electron rich, preventing the β-hydride shift to form an ethyl
group and subsequent reaction steps from this point. We
therefore anticipated that the less donating triphenylphosphine
substituent would be more supportive for a subsequent reaction
chemistry, eventually turning with less electron rich centers into
a Re(−I/+I) chemistry and in addition potentially permitting
also dissociation of a less strongly bound triphenylphosphine.
1
conversion of 2ph into 3ph and allowed H, 31P{1H}, and
13C{1H} NMR and IR spectroscopic characterization of 2ph in
the 2ph/3ph mixture. From this observation we concluded that
the driving force for the 2ph → 3ph conversion is the trapping
of the unsaturated [ReH(Et)(NO)(PPh3)2] intermediate by
ethylene coordination, since otherwise this intermediate would
be expected to exist even in the absence of ethylene. Thus, we
can state that the 16e complex [ReH(Et)(NO)(PPh3)2] is
thermodynamically less favored than the 18e complexes 2ph
and 3ph. 2ph and 3ph possess distinct 31P{1H} NMR spectra,
2. RESULTS
The reaction of 1ph with ethylene was found to proceed in
THF at 23 °C via the unstable intermediates [Re+IH2(η2-
C2H4)(NO)(PPh3)2] (2ph) and [Re+IH(Et)(η2-C2H4)(NO)-
(PPh3)2] (3ph) to eventually generate the stable dehydrogen-
ative ethylene coupling product [Re(η2-C2H4)(η4-C4H6)(NO)-
(PPh3)] (4ph) (Scheme 4). 2ph and 3ph were characterized in
situ by 1H, 31P{1H}, and 13C{1H} NMR and IR spectroscopy in
solution.
2.1. Reaction of 1ph with Ethylene Forming 2ph and
3ph. The less donating PPh3 of the 1ph derivative was seen to
reveal initially a reactivity similar to that of the 1cy and 1iPr
derivatives but then showed follow-up chemistry. The reaction
of 1ph with ethylene dissolved in deuterated benzene or THF
was initially studied by in situ NMR spectroscopy. In the
31P{1H} NMR spectrum the signal of the starting material at 33
ppm disappeared and new signals emerged first at 28 ppm
1
and their H and 13C{1H} spectra differ in the Re−H, Re−Et,
1
and Re−(η2-C2H4) chemical shift regions, whereas the H and
13C{1H} signals of the PPh3 ligands were seen to coincide. In
the 1H NMR spectrum of 2ph in THF of Figure 1 the
chemically different rhenium hydrides gave rise to a set of
2
2
signals at 1.51 (td, JPH = 36.5 Hz, JHH = 7.4 Hz) and −3.29
2
2
ppm (td, JPH = 27.6 Hz, JHH = 7.4 Hz). The protons of the
ethylene ligand were found to be also chemically different and
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therefore gave rise to two multiplets at 2.84 ppm (2H) and at
2.76 ppm (2H). These multiplets were broadened at room
temperature (300 K, 500 MHz). Increasing the temperature of
the sample to 320 K led to coalescence, which would be
consistent with a low barrier for ethylene rotation around the
Re−(η2-C2H4) axis. In the 31P NMR spectrum a broad virtual
triplet at 28.5 ppm originating from similar couplings to both
the Re−H moieties is observed, which suggests a symmetrical
trans phosphine arrangement, in agreement with its structure
protons of the ethyl ligand showed correlation with those of the
ethylene ligand, which undoubtedly supports that isomer A is
the structure of 3ph. On actual reaction of 2ph with ethylene
via a β-hydride shift and ethylene addition to the vacant site,
the stereochemistry of this process would be expected such that
the isomer C is created. Therefore, we have to assume an
intermittent Et group rearrangement changing sides and final
ethylene addition to form isomer A of 3ph (Scheme 4).
2.2. Reductive Elimination of Ethane from 3ph and
Dehydrogenative Ethylene Coupling To Form 4ph.
Unlike 2ph, 3ph was found to be remarkably stable in solution
at room temperature. 3ph seemed to decompose by initial
ethane reductive elimination, which however occurred only
very slowly (half-life time 1.8 h at 303 K). This encouraged us
to investigate the kinetics of of this process in the presence of
ethylene in greater detail. Expectedly, the reductive elimination
rate was found to depend first order on c(3ph) but was
independent of c(C2H4) and the type of solvent (benzene or
THF). Also, no influence of c(PPh3) could be determined,
which changes during the reaction course due to PPh3 release
concomitant with the formation of 4ph. Thus, the reductive
elimination is not induced by preceding ethylene or phosphine
ligand dissociation. We decided to determine the rates of the
reductive elimination at temperatures ranging from 293 to 338
K via 1H NMR spectroscopy and integration of the 3ph
methylene protons with the PPh3 signals as internal standard.
This also allowed determination of the activation parameters of
this process (ΔH⧧ = 68.7 kJ mol−1 ΔS⧧ = −94 J mol−1 K−1) via
an Eyring plot (Figure 2).11 The fact that reductive elimination
occurs further supports the earlier proposed structure of 3ph
featuring a cis-hydrido−alkyl arrangement, since trans positions
of the eliminating moieties would not permit reductive
eliminations.12
sketched in Scheme 4. In the 13C{1H} NMR spectrum the
13
C
nuclei cause one signal at 35.8 ppm. The assignment
ethylene
of this carbon signal to the ethylene ligand was confirmed by a
HSQC experiment.
1
The H NMR spectrum of 3ph also displayed in Figure 1
consists of two isolated spin systems attributed to the ethylene
and the ethyl ligands and a HRe signal. This latter signal at 2.91
ppm (t, 2JPH = 31.8 Hz, 1 H) shows the typical coupling pattern
originating from the coupling with two chemically equivalent
phosphorus nuclei. The methylene protons of the Re−Et unit
3
entailed a broadened resonance at −1.17 ppm (q, JHH = 7.3
Hz, 2 H). Phosphorus decoupling furnished a sharp quartet.
The adjacent Me group was found to cause a well-defined
signal at 0.68 ppm (t, 3JHH = 7.3 Hz, 3 H). The four protons of
the ethylene ligand were observed as two broad resonances,
1
which resolve in the H{31P} NMR spectrum into a set of two
sharp resonances at 2.94 ppm (d, JHH = 10.2 Hz, 2 H) and at
2.12 ppm (d, JHH = 10.2 Hz, 2 H). These ethylene signals are
not affected by dynamic exchange processes, indicating
hindered rotation of the ethylene ligand in a presumably
preferred “upright” conformation aligned with the P−Re−P
axis as the preferred rotameric conformation. This could be
confirmed applying a HSQC experiment, where both signals
were assigned to one 13C resonance at 42.9 ppm. In this
experiment the methylene and the methyl protons of the Et
group expectedly showed correlation with two 13C resonances
at 19.0 and 20.0 ppm. In the 31P NMR spectrum of 3ph a broad
Alongside with the reductive elimination of ethane, the
formation of the butadiene ligand in 4ph was observed. For this
transformation we propose a mechanism as depicted in Scheme
4 proceeding via the formation of the bis(ethylene) complex
[Re−I(η2-C2H4)2(NO)(PPh3)2] followed by coupling of the
ethylene ligands to form a redox-driven [Re+I(η2-C4H8)(NO)-
(PPh3)2] molecule with a metallacyclopentane unit and
concomitant oxidation to a 16e rhenium center. From this
Re(+I) species facile double β-hydride shifts are expected to
occur, along with H2 loss or perhaps hydrogenation of an
ethylene ligand with subsequent uptake of C2H4 to produce
4ph. None of these intermediates could be traced in the NMR
spectra of the reaction mixture, which points to their high
reactivity. Analytically pure samples of 4ph were obtained via
the reaction of 1ph in THF in the presence of C2H4. Isolation
was accomplished via filtration of the reaction solution and
evaporation of the volatiles, followed by extraction of the oily
residue with hexane in 88% yield. 4ph was characterized by
2
signal was observed at 20.4 ppm (d, JPH = 31.8 Hz). From
these data, it was also concluded that the phosphines are bound
in chemically equivalent trans positions. However, the relative
arrangement of the ethylene, the ethyl, and the hydride ligands
within the pseudo-octahedral environment could not be fully
elucidated. Therefore more detailed NMR experiments were
carried out assuming the three constitutional isomers A, B, and
C (Scheme 3) as a basis for the interpretation of a NOESY
spectrum of 3ph.
Scheme 3. The Three Possible Trans Phosphine Isomers of
3ph
1
elemental analysis, IR and H, 13C{1H}, and 31P{1H} NMR
spectroscopy. The 1H NMR spectrum of 4ph consisted of PPh3
signals in the aromatic region and of two spin systems, which
could be assigned by means of COSY, HSQC, and HMBC
spectra to the butadiene and the ethylene ligands (Scheme 5).
In the 13C{1H} NMR spectrum of 4ph, the PPh3 ligand gave
rise to the expected aromatic carbon signals in the region of
125−140 ppm. The four chemically different Cbutadiene nuclei
and the two chemically different Cethylene nuclei were assigned
by aid of HSQC and HMBC experiments as well, as depicted in
Scheme 5. The signals of the terminal butadiene carbon nuclei
and of the ethylene carbon atoms showed coupling with the
For the “equatorial” ligands the trans influence10 decreases in
the order NO > C2H4 > H > Et, from which we could predict
that A is more likely than B and C, since only in isomer A is the
strongest trans ligand (NO) trans to the weakest trans ligand
(Et). The presence of cross-peaks in the NOESY spectrum
showed correlation of the Me protons of the ethyl ligand and
the hydride ligand and cross-peaks between the methylene
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Scheme 4. Reaction of 1ph with C2H4
Scheme 6. “Top View” of the Three Conformers of 4ph
Figure 2. Eyring plot of the reductive elimination of ethane from 3f in
benzene-d6.
Scheme 5. Assignment Scheme of the H and 13C Signals of
4ph (C6D6, 500 MHz, 300 K)
1
made the metallacyclopentene structure seem unlikely. In
accordance with this, all vicinal coupling constants are
approximately 7 Hz, which indicates that the butadiene ligand
is η4 rather than η2 bound, as otherwise the coupling pattern in
a metallacyclopentene envelope structure would lead to
different coupling constants15 for the internal and the two
types of terminal protons. In accord with that, we propose a s-
cis-η4-butadiene structure for 4ph. In addition, we could also
rule out rotamer F, as in the NOESY experiment cross-peaks
between the ethylene multiplet at 1.66 ppm and the H(int1)/
H(anti1) atoms are visible. This indicated that 4ph possesses
either the structure of rotamer D or rotamer E. DFT
calculations on the PMe3 model rotamers of [Re(η2-
C2H4)(η4-butadiene)(NO)(PMe3)] DMe−FMe in ref 16 re-
vealed that the EMe rotamer is expected to be thermodynami-
cally 16.3 kJ mol−1 more favorable than the FMe rotamer and
36.0 kJ mol−1 more favorable than the DMe rotamer. To gain
further insight into the structure of 4ph, an X-ray diffraction
study was carried out. The ORTEP diagram in Figure 3 indeed
showed that (a) the butadiene ligand possesses an s-cis
phosphorus nucleus (3−7 Hz), whereas for the internal carbon
atoms this coupling was absent. In the 31P{1H} NMR spectrum
4ph gave rise to a single resonance at 22.4 ppm and the IR
spectrum an intense and sharp ν(NO) band was observed at
1641 cm−1.
The butadiene or the ethylene ligands of the pentacoordinate
d8 system did not show NMR dynamics at room temperature as
is known for other butadiene complexes.13 Therefore, these
ligands were assumed to be strongly bound in a unique, highly
asymmetric conformer, which is stable at room temperature.
Three different s-cis butadiene rotamers of 4ph are principally
conceivable as structural isomers (Scheme 6). In addition,
metallacyclopentene structures and an s-trans-η4-butadiene
complex could be envisaged as well (Scheme 6).
1
conformation, which could be derived from the H NMR
NOESY spectrum and the 1H NMR coupling patterns, and (b)
the ethylene and the butadiene ligands are arranged as was
proposed from the NOESY spectrum. Additionally the
structure of 4ph possesses conformation E (Scheme 6) with
the PPh3 ligand in the “leg in the hole” position of the
butadiene ligand.15
The formation of an s-trans butadiene complex could be
ruled out by a NOESY experiment, which confirmed the spacial
proximity of the Hsyn1 and Hsyn2 protons. Furthermore, the
geminal coupling between the syn and the anti protons is only
3 Hz, suggesting sp2 rather than sp3 carbon atoms,14 which
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4ph. This stability, however, is thought to prevent the closing
up of the catalytic cycle for dehydrogenative ethylene coupling
(Scheme 4).
3. DISCUSSION
This study revealed major differences in reactivity between the
phenyl derivative 1ph and the previously reported isopropyl/
cyclohexyl derivatives 1iPr/1cy of a [ReH(η2-BH4)(NO)-
(PR3)2] complex series which are caused mainly by differences
in the donor strengths of these ligands,20 which to a certain
extent also correlates with their binding strengths to the
rhenium center. In the case of the reactions of the 1iPr/1cy
complexes only exchange of the BH3 moiety with ethylene
could be initiated.3 This stands in contrast to the reaction of
1ph bearing less donating triphenylphosphine ligands with
ethylene, where the formation of 2ph is the starting point of a
cascade of consecutive steps leading finally to oxidative
ethylene coupling forming a butadiene ligand in 4ph. A
decreased electron density in 1ph is illustrated by a ν(NO)
band at 1666 cm−1 in comparison with the bands of 1iPr/1cy
at 1660 cm−1, which indicated the right trend in electron
richness of the rhenium centers, but the values are apparently
an overlay of counteracting electronic effects and are in sum
therefore too small to explain the differences in the observed
reactivities. However, it is indeed expected that the direct
influence of the different electron donicities of the phosphorus
ligands and the accompanying electron densities at the rhenium
centers trigger formation of the kinetically stable cis-hydrido−
alkyl complex 3ph in the reaction of 2ph with ethylene.
Moreover, such cis-hydrido−alkyl species which are formed
from a metal dihydride complex via migratory insertion of an
olefin into a M−H bond are to our knowledge usually short-
lived intermediates and are hardly traceable21 because of the
spontaneously occurring elimination of alkane. In contrast to
this, numerous cis-hydrido−alkyl complexes of varying
stabilities are known, which were formed by other reactions
such as alkylation with, for example, Grignard compounds22 or
by protonation of alkyl complexes23 or by the oxidative addition
of a C−H bond to an unsaturated electron-rich metal as is
known e.g. for numerous Ru, Rh, Pd, Os, Ir, and Pt
complexes.24 In the given case of triphenylphosphine-
substituted complexes the extraordinary stability of these
species was attributed to a reduced tendency for reductive
elimination of ethane, since an electron-rich Re(−I) state
would be reached, for which any donor phosphineeven the
comparably moderate donor strength of PPh3substitution
pattern would be counteracting. This is well reflected in the
relatively high activation barrier for this process (e.g., ΔG⧧ =
96.7 kJ mol−1 at 298 K), which is not rare for such processes.25
These findings and earlier results1a,b support the idea that in the
case of alternative reaction pathways Re(+III)/Re(+I) reductive
eliminations are preferred over related Re(+I)/Re(−I)
processes. However, another factor could influence this process
as well: often reductive eliminations are initiated and
accelerated by initial phosphine dissociation. It is a general
experience in the realm of homogeneous catalysis that lower
coordination numbers accelerate reductive eliminations and
therefore the weaker the binding strength of a phosphine, the
lower the minimal coordination number could be and the lower
the activation barrier would be. The reduced electron donation
of the triphenylphosphine ligand in 3ph might permit its
dissociation and subsequently enhance reductive elimination in
comparison with triisopropyl- or tricyclophexylphosphine.
Figure 3. ORTEP diagram of 4ph drawn at the 50% probability level.
H atoms are omitted for clarity. Selected bond lengths (Å): C1−C2 =
1.433(5), C3−C4 = 1.428(5), C4−C5 = 1.404(6), C5−C6 =
1.444(6), C1−Re1 = 2.210(3), C2−Re1 = 2.214(3), C3−Re1 =
2.229(3), C4−Re1 = 2.233(3), C5−Re1 = 2.263(4), C6−Re1 =
2.260(3), N1−O1 = 1.200(3), N1−Re1 = 1.773(2), P1−Re1 =
2.4025(7). Selected bond angles (deg): C3−C4−C5 = 116.1(3), C4−
C5−C6 = 118.2(3), C1−Re1−C3 = 145.86(13), C1−Re1−C6 =
85.48, C3−Re1−N1 = 101.90(13), C6−Re1−N1 = 167.33(12), C2−
Re1−P1 = 86.42(9), C3−Re1−P1 = 85.26(9), C6−Re1−P1 =
94.40(9), O1−N1−Re1 = 177.0(2).
A closer look into the structure of 4ph revealed that all Re−
C bonds are in the range of 2.210−2.263 Å, which is typical for
Re−C single bonds. The C−C bond lengths (1.404−1.444 Å)
are between single (1.53 Å) and double bonds (1.34 Å)17 and
thus indicate strong (back) bonding18 in this Re(−I) system.
Furthermore, the very uniform C−C bond lengths in the
butadiene ligand speak for strong electron delocalization. The
NO ligand is almost linearly bound (∠O1−N1−Re1 =
177.0(2)°) and thus acts as a 3e ligand with typically short
Re−NO (1.773(2) Å) and elongated N−O (1.200(3) Å)
bonds, also indicating strong back-donation from the electron
rich Re(−I) center. Overall the structure can be described as a
trigonal pyramid with the NO, ethylene, and butadiene ligands
in the basal plane and the phosphine ligand in an apical
position.
2.3. Attempts To Catalyze Dehydrogenative Ethylene
Coupling Forming Butadiene. We also probed the
capability of 4ph to catalyze dehydrogenative ethylene coupling
along eq 1 as occurs stoichiometrically in the formation of 4ph.
The ethylene side of eq 1 is thermodynamically favored by ΔH
= −5.2 kJ mol−1.17 The entropy ΔS for eq 1 should be close to
0. Therefore, coupling of this reaction with hydrogenation of
either ethylene (ΔH = −136.3 kJ mol−1) or butadiene (ΔH =
−108.4 kJ mol−1) would generate a more negative free enthalpy
driving force, sufficiently high for exergonic dehydrogenative
ethylene coupling.17
2C H ⇄ H + C H
4 6
(1)
2
4
2
Pursued by NMR, the reaction of 4ph in THF solutions with
ethylene (2 bar) did not show free butadiene19 even after 1
week of exposure at 60 °C. Employing even harsher conditions,
charging with 60 bar of ethylene for 24 h at 100 °C (pressure
rises to 100 bar), also did not result in C4H6 release. While 4ph
was found to be stable at 60 °C under 2 bar of ethylene for 1
week, it decomposed under the more harsh conditions given,
resulting in various unidentified decomposition products. The
butadiene ligand in 4ph is apparently quite strongly bound to
the rhenium center, and its strong binding is presumably part of
the thermodynamic driving force for the facile conversion to
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However, the transformation of 3ph to 4ph has been found to
be independent of c(PPh3), which indeed rules out this
possibility.
neously with further ethylene uptake to the ethylene butadiene
Re(−I) complex 4ph. This revealed an unexpected facet of
Re(−I/+I)NO chemistry, demonstrating reactivity closely
related to those of chromium-based olefin oligomerization
catalysts29 encompassing oxidative coupling of ethylene by
metallacyclization, double β-hydride elimination steps, and
reductive elimination of an H2 equivalent. The availability of
the given redox-driven steps was attributed to the medium
strength in donicity of PPh3 still permitting access to highly
reduced Re(+I) species and providing a labile phosphine for
vacant sites required at some stages of the transformations. The
application of the PPh3 ligand can thus be viewed as part of a
rhenium-based tuning effort, which specifically opened up the
quite unusual reaction channel of dehydrogenative ethylene
coupling.
The structure of the olefin/alkyl complex 3ph can also be
compared with those of low valent Ni-based olefin oligomeriza-
tion/isomerization catalysts, for which cis-alkyl−ethylene
complexes were proposed to be important intermediates.26
However, olefin oligomerization was not observed during the
NMR experiments in solutions of 3ph, which we interpret in
terms of a hindered alkyl migration onto bound ethylene. For
this process the ethylene ligand of 3ph had to be aligned with
the NO, Et, H plane so that the relatively high barrier for the
ethylene ligand rotation in 3ph would add to the overall barrier
of alky migration. Basically it is again an overly high electron
density on the metal center which contributes in various ways
to hindrance of repetitive olefin insertions in the chain
propagation process of olefin polymerizations.26b,c
5. EXPERIMENTAL SECTION
Nevertheless, the unsaturated intermediate [Re(η2-C2H4)-
(NO)(PPh3)2] complex resulting from ethane elimination of
3ph proved to be capable of an alternative reaction path. First,
another ethylene adds to the rhenium center and in the
dehydrogenative coupling of ethylene to butadiene a
rhenacyclopentane is then formed, with release of H2 and
PPh3 to eventually form 4ph after C2H4 addition (Scheme 4).
The oxidative coupling of ethane furnishing butadiene
complexes is rather uncommon,27 whereas reductive elimi-
nation of ethenyl moieties furnishing butadiene complexes or
the synthesis of butadiene complexes by the reduction of a
precursor complex in the presence of butadiene are well-
known.28 In this reaction course hydride olefin intermediates
are likely to be generated. It should be mentioned that this
occurs somewhat analogously to catalytic cycles of Ni-based
olefin oligomerizations,26 where β-hydride shifts were seen to
participate in the processes, as well. At the intermittent stage of
the second β-hydride shift from the rhenacyclopentane, PPh3
dissociation is required, another condition which makes it
necessary to have a labile phosphine within the coordination
sphere. It is also noteworthy that olefin oligomerization
pathways via the metallacycloalkene route were postulated for
Cr-based catalysts.29 However, in the NMR pursuits to generate
4ph, neither a rhenacyclopentane species and elimination of
butenes nor the formation of higher olefins could be observed.
The thermodynamic driving force for the formation of 4ph is
the strong butadiene binding to the rhenium center; for that
reason the rhenium-based catalysis of dehydrogenative ethylene
coupling apparently could not be achieved.
General Considerations. All manipulations were carried out
under an atmosphere of dry nitrogen using either standard Schlenk
techniques or an MBraun glovebox. All solvents were dried, distilled,
and degassed according to standard laboratory procedures. [ReH(η2-
BH4)(NO)(PPh3)2] was prepared according to published methods31
and stored at −30 °C. NMR spectra were recorded on a Bruker AV-2
500 spectrometer. Chemical shifts are expressed in parts per million
(ppm) and referenced to the solvent’s residual signals32 or in the case
of 31P{1H} NMR spectra to an external standard (85% H3PO4 at δ 0.0
ppm). IR spectra were recorded on a BioRad Excalibur spectrometer.
Microanalyses were carried out at the Anorganisch-Chemisches
Institut of the University of Zurich.
̈
[ReH2(η2-C2H4)(NO)(PPh3)2] (2ph) and [ReH(Et)(η2-C2H4)(NO)-
(PPh3)2] (3ph). In an NMR tube with a Young Teflon cap a
suspension of [ReH(η2-BH4)(NO)(PPh3)2] (0.010 g, 0.0013 mmol)
in C6D6 (0.6 mL) or alternatively THF (0.6 mL) was degassed using
three freeze−pump−thaw cycles before the tube was charged with
ethylene (1−2 bar). The NMR tube was shaken vigorously. The
sample so prepared was used for NMR spectroscopic characterization
of 2ph and 3ph. For IR spectroscopy the volatiles were quickly
removed in vacuo and the oily residue was used directly for the
preparation of a KBr pellet.
NMR Spectroscopic Data of [ReH2(η2-C2H4)(NO)(PPh3)2] (2ph). 1H
NMR (C6D6, 300 K, 500 MHz): δ 7.86 (m, 12 H, PPh3), 7.25−6.90
(m, 18 H, PPh3), 2.84 (m, 2 H, C2H4), 2.76 (m, 2 H, C2H4), 1.51 (td,
JPH = 36.5 Hz, JHH = 7.4 Hz, 1 H, ReH), −3.29 (td, JPH = 27.6 Hz, JHH
= 7.4 Hz, 1 H, ReH) ppm. 13C{1H} NMR (C6D6, 295 K, 125 MHz): δ
= 136.9 (t, JPC = 25.0 Hz, PPh3), 134.6 (t, JPC = 5.0 Hz, PPh3), 130.0,
128.2−127.5 (m, PPh3 and C6D6), 35.8 (s, C2H4) ppm. 31P NMR
(C6D6, 300 K, 200 MHz): δ 28.5 (dd, JHP = 36.5 Hz, JHP = 27.6 Hz,
PPh3) ppm.
NMR Spectroscopic Data of [ReH(Et)(η2-C2H4)(NO)(PPh3)2] (3ph).
1H NMR (C6D6, 300 K, 500 MHz): δ 7.96 (m, 12 H, PPh3), 7.25−
6.90 (m, 18 H, PPh3), 2.94 (d, JHH = 10.2 Hz, 2 H, C2H4), 2.91 (t, JPH
= 31.8 Hz, 1 H, ReH), 2.12 (d, JHH = 10.2 Hz, 2 H, C2H4), 0.68 (t,
3JHH = 7.3 Hz, 3 H, ReEt), −1.17 (t br, 3JHH = 7.3 Hz, 2 H, ReEt) ppm.
13C{1H} NMR (C6D6, 295 K, 125 MHz): δ 136.9 (t, JPC = 25.0 Hz,
PPh3), 134.8 (t, JPC = 5.0 Hz, PPh3), 131.2 (s, PPh3), 128.2−127.5 (m,
PPh3 and C6D6), 42.9 (s, C2H4), 20.0 (s, ReEt), 19.0 (s, ReEt) ppm.
31P NMR (C6D6, 300 K, 200 MHz): δ = 20.4 (d, JHP = 31.8 Hz, PPh3)
4. CONCLUSIONS
This study on the hydrogen−ethylene chemistry of triphenyl-
phosphine nitrosyl rhenium complexes demonstrated that the
complex [Re(H)2(η2-C2H4)(NO)(PPh3)2] (2ph) induces
ethylene hydrogenation and was seen to yield ethane. This
transformation follows the mechanistic lines of an Osborn
type30 olefin hydrogenation catalysis but was stoichiometric and
much slower in the olefin insertion step to yield eventually the
cis-hydrido−ethyl rhenium(+I) species 3ph.1 The relaxed
positions of the trans-disposed monophosphines is thought to
significantly contribute to the stabilization of 3ph, but the
retardation of this process is anticipated to be due to the
destabilization of the final low oxidation state of the [Re−I(η2-
C2H4)(NO)(PPh3)2] species appearing after the rate-limiting
reductive elimination of ethane. The subsequent diethylene
[Re−I(η2-C2H4)2(NO)(PPh3)2] intermediate reacted instanta-
ppm.
IR Spectroscopic Data of [ReH2(η2-C2H4)(NO)(PPh3)2] (2ph) and
[ReH(Et)(η2-C2H4)(NO)(PPh3)2] (3ph). IR (cm−1, KBr pellet): 1959 (m,
ν(ReH)), 1897 (m, ν(ReH)), 1815 (m, ν(ReH)), 1630 (s, ν(NO)).
[Re(η2-C2H4)2(η4-butadiene)(NO)(PPh3)] (4ph). A suspension of
[ReH(η2-BH4)(NO)(PPh3)2] (1; 0.100 g, 0.13 mmol) in THF (5 mL)
was placed in a Young Schlenk tube with a magnetic stirring bar. The
Schlenk tube was degassed with three freeze−thaw−pump cycles and
charged with 2 bar of C2H4. The suspension was stirred for 16 h,
giving a pale yellow solution. After filtration through a short plug of
Celite the volatiles were removed and the crude was triturated with
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hexane (3 × 1 mL), yielding analytically pure 4ph (65 mg, 88%).
Crystals suitable for X-ray diffraction were obtained by slowly
evaporating a concentrated toluene solution of 4ph.
Sun, M.; Yuan, X.; Zheng, N.; Li, P. J. Organomet. Chem. 2009, 694,
3343−3348.
(4) (a) Chatt, J.; Coffey, R. S. J. Chem. Soc. A 1969, 1963−1972.
(b) Bradley, M. G.; Roberts, D. A.; Geoffroy, G. L. J. Am. Chem. Soc.
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1
IR (cm−1, KBr pellet): 1640 (s, ν(NO)). H NMR (C6D6, 300 K,
500 MHz): δ 7.62 (m, 6 H, PPh3), 7.00 (m, 9 H, PPh3), 5.75 (q, JHH
=
6 Hz, 1 H, CH2CHCHCH2), 5.42 (q, JHH = 6.5 Hz, 1 H, CH2
CHCHCH2), 2.50 (t, JHH = 9.5 Hz, 1 H, C2H4), 2.23 (m, 2 H,
CH2CHCHCH2, C2H4), 1.66 (d, JHH = 7.7 Hz, 2 H, C2H4), 0.15
(m, 1 H, CH2=CHCHCH2), −0.53 (dd, JHH = 6.5, JHH = 3.5 Hz, 1
H, CH2=CHCHCH2), −1.55 (dd, JHH = 7.1, JHH = 3.9 Hz, 1 H,
CH2CHCHCH2) ppm. 13C{1H} NMR (C6D6, 300 K, 125
MHz): δ 134.2 (d, JPC = 20 Hz, PPh3), 133.6 (d, JPC = 10 Hz, PPh3),
129.9 (d, JPC = 3 Hz, PPh3), 128.9−128 (overlapping m, PPh3, C6D6),
100.8 (s, CH2CHCHCH2), 83.0 (s, CH2CHCHCH2), 43.5
(d, JPC = 3.8 Hz, CH2CHCHCH2), 35.7 (d, JPC = 6.3 Hz, CH2
CHCHCH2), 31.3 (d, JPC = 7.5 Hz, C2H2), 25.3 (d, JPC = 3.8 Hz,
C2H2) ppm. 31P{1H} NMR (C6D6, 300 K, 200 MHz): δ = 22.4 (s,
PPh3) ppm. Anal. Calcd for C24H25NOPRe (560.64): C, 51.42; H,
4.49; N, 2.50. Found: C, 51.47; H, 4.49; N, 2.28.
(5) (a) Zeiher, K. E. H.; DeWit, D. G.; Caulton, K. G. J. Am. Chem.
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Adv. Synth. Catal. 2005, 347, 1435−1446.
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ed.; HarperCollins College: New York, 1993.
X-ray Diffraction Analysis. Intensity data were collected at
183(2) K on a Xcalibur diffractometer (Agilent Technologies, four-
circle kappa platform, Ruby CCD detector, and a single wavelength
Enhance X-ray source with Mo Kα radiation, λ = 0.710 73 Å).26 The
selected suitable single crystal was mounted using polybutene oil on
the top of a glass fiber fixed on a goniometer head and immediately
transferred to the diffractometer. Pre-experiment, data collection, data
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reduction, and analytical absorption correction27 were performed with
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120−126.
(14) Hesse, M.; Meier, H.; Zeeh, B. Spektroskopische Methoden in der
organischen Chemie, 7th ed.; Thieme: Stuttgart, Germany, 2005.
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Press: Boca Raton, FL, 1993.
28
the program suite CrysAlisPro
.
The crystal structure was solved with
SHELXS-9729 using direct methods. The structure refinement was
performed by full-matrix least squares on F2 with SHELXL-97.29 All
programs used during the crystal structure determination process are
included in the WINGX software.30 The program PLATON31 was
used to check the result of the X-ray analysis. In the crystal structure of
4ph, all hydrogen positions were calculated after each cycle of
refinement using a riding model, with C−H = 0.93 Å and Uiso(H) =
1.2[Ueq(C)] for aromatic H atoms, with C−H = 0.97 Å and Uiso(H) =
1.2[Ueq(C)] for olefinic H atoms. CCDC-854185 contains the
supplementary crystallographic data (excluding structure factors) for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/
data_request/cif.
(18) Kim, C. K.; Lee, K. A.; Kim, C. K.; Lee, B.-S.; Lee, H. W. Chem.
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1
1
(19) Reference H and 13C{1H} NMR spectra of 1,2-butadiene:. H
NMR (THF-d8, 293.15 K, 500 MHz) δ 6.32 (m, 2 H, int H), 5.17 (m,
2 H, syn H), 5.05 (m, 2 H, anti H) ppm; 13C{1H} NMR (THF-d8,
293.15 K, 125 MHz) δ 138.9 (s, 1,4-butadiene CH), 118.0 (s, 1,4-
butadiene CH2) ppm.
ASSOCIATED CONTENT
■
S
* Supporting Information
(20) Tolman, C. A. J. Am. Chem. Soc. 1970, 92, 2953−2956.
(21) (a) Chan, A. S. C.; Halpern, J. J. Am. Chem Soc. 1980, 102, 838−
840. (b) Landis, C. R.; Halpern, J. J. Am. Chem. Soc. 1987, 109, 1746−
1754. (c) Hussey, A. S.; Takeuchi, Y. J. Am. Chem. Soc. 1969, 91, 672−
675.
A CIF file of 4ph giving further crystallographic data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(22) (a) Halpern, J.; Ayusman, L. A. J. Am. Chem. Soc. 1978, 2915−
2916. (b) Crumpton-Bregel, D. M.; Goldberg, K. J. Am. Chem. Soc.
2003, 125, 9442−9456.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: hberke@aci.uzh.ch.
(23) Jenkins, H. A.; Yap, G. P. A; Puddephatt, R. J. Organometallics
1997, 16, 1946−1955.
Notes
(24) (a) Davis, F. A.; Mancinelli, P. A.; Balasubramanian, K.; Nadir,
U. K. J. Am. Chem. Soc. 1979, 101, 1045−1047. (b) Milstein, D. J. Am.
Chem. Soc. 1982, 104, 5227−5228. (c) Tal, D.; Keinan, E.; Mazur, Y. J.
Am. Chem. Soc. 1979, 101, 503−505.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(25) (a) Northcutt, T. O.; Wick, D. D.; Vetter, A. J.; Jones, W. D. J.
Am. Chem. Soc. 2001, 123, 7257−7270. (b) Parkin, G. J. Labelled
Compd. Radiopharm. 2007, 50, 1088−1114.
We thank the Swiss National Science Foundation and the
University Zurich for financial support.
̈
(26) (a) Jenkins, J. C.; Brookhart, M. J. Am. Chem. Soc. 2004, 126,
5827−5842. (b) Svejda, S. A.; Brookhart, M. Organometallics 1999, 18,
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