Catalytic CO2 activation assisted by rhenium hydride/B(C 6F5)3 frustrated Lewis pairs - Metal hydrides functioning as FLP bases

Article abstract of DOI:10.1021/ja402381d

Reaction of 1 with B(C₆F₅)₃ under 1 bar of CO₂ led to the instantaneous formation of the frustrated Lewis pair (FLP)-type species [ReHBr(NO)(PR₃)₂(η²-O=C= O-B(C₆F₅)₃)] (2, R = iPr a, Cy b) possessing two cis-phosphines and OCO₂-coordinated B(C₆F ₅)₃ groups as verified by NMR spectroscopy and supported by DFT calculations. The attachment of B(C₆F₅)₃ in 2a,b establishes cooperative CO₂ activation via the Re-H/B(C ₆F₅)₃ Lewis pair, with the Re-H bond playing the role of a Lewis base. The Re(I) η¹-formato dimer [{Re(μ-Br)(NO)(η¹-OCH=O-B(C₆F₅) ₃)(PiPr₃)₂}₂] (3a) was generated from 2a and represents the first example of a stable rhenium complex bearing two cis-aligned, sterically bulky PiPr₃ ligands. Reaction of 3a with H₂ cleaved the μ-Br bridges, producing the stable and fully characterized formato dihydrogen complex [ReBrH₂(NO) (η¹-OCH=O-B(C₆F₅)₃)(PiPr ₃)₂] (4a) bearing trans-phosphines. Stoichiometric CO ₂ reduction of 4a with Et₃SiH led to heterolytic splitting of H₂ along with formation of bis(triethylsilyl)acetal ((Et ₃SiO)₂CH₂, 7). Catalytic reduction of CO ₂ with Et₃SiH was also accomplished with the catalysts 1a,b/B(C₆F₅)₃, 3a, and 4a, showing turnover frequencies (TOFs) between 4 and 9 h^-1. The stoichiometric reaction of 4a with the sterically hindered base 2,2,6,6-tetramethylpiperidine (TMP) furnished H₂ ligand deprotonation. Hydrogenations of CO₂ using 1a,b/B(C₆F₅)₃, 3a, and 4a as catalysts gave in the presence of TMP TOFs of up to 7.5 h^-1, producing [TMPH][formate] (11). The influence of various bases (R₂NH, R = iPr, Cy, SiMe₃, 2,4,6-tri-tert-butylpyridine, NEt₃, PtBu ₃) was studied in greater detail, pointing to two crucial factors of the CO₂ hydrogenations: the steric bulk and the basicity of the base.

Full text of DOI:10.1021/ja402381d

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Article
Catalytic CO2 Activation Assisted by “Rhenium Hydride/B(C6F5)3”
Frustrated Lewis Pairs – Metal Hydrides Functioning as FLP Bases
Yanfeng Jiang, Olivier Blacque, Thomas Fox, and Heinz Berke
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja402381d • Publication Date (Web): 25 Apr 2013
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Yanfeng Jiang, Olivier Blacque, Thomas Fox, Heinz Berke*
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Anorganisch-chemisches Institut, Universität Zürich, Winterthurerstr. 190, CH-8037, Zürich, Switzerland.
Supporting Information Placeholder
ABSTRACT: Reaction of 1 with B(C6F5)3 under 1 bar of CO2 led to the instantaneous formation of the FLP type species
[ReHBr(NO)(PR3)2(η2-O=C=O-B(C6F5)3)] (2, R = iPr a, Cy b) possessing two cis-phosphines and OCO2 coordinated B(C6F5)3 groups as
verified by NMR spectroscopy and supported by DFT calculations. The attachment of B(C6F5)3 in 2a,b establishes cooperative CO2 activa-
tion via the “Re-H/B(C6F5)3” Lewis pair with the Re-H bond taking the role of a Lewis base. The Re(I) η1-formato dimer [{Re(μ-
Br)(NO)(η1-OCH=O-B(C6F5)3)(PiPr3)2}2] (3a) was generated from 2a and represents the first example of a stable rhenium complex bearing
two cis-aligned, sterically bulky PiPr3 ligands. Reaction of 3a with H2 cleaved the μ-Br bridges producing the stable and fully characterized
formato dihydrogen complex [ReBrH2(NO)(η1-OCH=O-B(C6F5)3)(PiPr3)2] (4a) bearing trans phosphines. The stoichiometric CO2 reduc-
tion of 4a with Et3SiH led to heterolytic splitting of H2 along with formation of bis(triethylsilyl)acetal, (Et3SiO)2CH2 (7). Catalytic reduction
of CO2 with Et3SiH was also accomplished with the catalysts “1a,b/B(C6F5)3”, 3a and 4a showing TOFs between 4 and 9 h-1. The
stoichiometric reaction of 4a with the sterically hindered base TMP furnished H2 ligand deprotonation. Hydrogenations of CO2 using
“1a,b/B(C6F5)3”, 3a and 4a as catalysts gave in the presence of TMP TOFs of up to 7.5 h-1 producing [TMPH][formate] (11). The influence
of various bases (R2NH, R = iPr, Cy, SiMe3; 2,4,6-tri-tert-butylpyridine, NEt3, PtBu3) was studied in greater detail pointing to two crucial
factors of the CO2 hydrogenations: the steric bulk and the basicity of the base.
1. INTRODUCTION
with an external Lewis acid to form a “M-H/LA” FLP. In addition
this structural arrangement would allow subsequent reactivity with
hydride transfer as demonstrated by reports of Zr and Ru involved
FLPs.14 Based on this idea, this paper demonstrates that rhenium
hydride bonds can indeed act as a Lewis base in “Re-H/B(C6F5)3”
FLPs to activate and reduce CO2.
CO2 activation and reduction for alternative energy sources is a
topic of current interest.1 The major challenge originates from the
high thermodynamic stability of CO2, which calls for a remarkable
driving force to ensure irreversible fixation. The currently devel-
oped chemical methods for CO2 reduction could be classified into
2-4
Scheme 1. "Isolobal" analogy between a Lewis base and a metal hydride
in the bifunctional or FLP activation process of CO2 (or generally a XY
molecule). Left: classical CO2 bifunctional activation by M-H. Right:
FLP type CO2 activation by a M-H/LA system with the M-H unit as
the Lewis base component.
two categories based on either transition metals or main-group
5-12
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elements
using dihydrogen,2 hydrosilanes and boranes as
oxygen scavengers. One of the most prominent examples is the
cooperative activation of CO2 by frustrated Lewis pairs (FLPs),
which consist of non- or weakly interacting Lewis acid and base
pairs of the B/P,10 B/N,11 or Al/P type.12 FLPs are thus a milestone
in the metal-free bifunctional activation of small molecules,8-9 but in
addition they provide a non-classical way of activation using the
polarizing power of electrostatic fields. In this context, the classical
CO2 insertion into a transition metal hydride bond could be viewed
as a bifunctional activation or a FLP type activation process show-
ing for the earlier process a vacant site at the metal center as the
Lewis acid and for the latter process the hydride as the Lewis base.
The FLP arrangement actually emphasizes the crucial role of the
hydride component, which must be hydridic and consequently
nucleophilic in character to enable interaction with the electro-
philic carbon atom of CO2.13
2. RESULTS AND DISCUSSIONS
In line with the FLP notion we thus recognized that a transition
metal hydride bond can be taken as “isolobal” to the free electron
pair of a Lewis base, as depicted schematically in Scheme 1. The
shape of the σ orbital of a M-H bond with sufficiently hydridic
character resembles that of the lone pair electron of an organic
Lewis base, which offers opportunity for activation by cooperation
2.1. Activation of CO2 by the “Re-H/B(C6F5)3” pair. The chemistry
of the five-coordinate Re(I) hydride complexes [ReHBr(NO)-
(PR3)2] (1, R = iPr a, Cy b) was developed particularly with respect
to catalysis.15 Due to its unsaturated 16e nature 1 shows strong
affinities toward 2e donors, such as H2, CO, O2, ethylene, and
carbenes.15b However, no reaction was observed when for instance
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the benzene-d6 solution of 1a or 1b was exposed to 1 bar of CO2
with the CCO2 atom and with the distantly coordinated B(C6F5)3
moiety.
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with spectroscopic monitoring. This reflected not only weak coor-
dinating ability of CO2, but maybe also insufficient hydridic and
concomitant insufficient nucleophilic character of the Re-H bond
for a secondary coordination sphere hydride transfer to CO2. The
position of the vacant site trans to the hydride also hampered the
primary coordination sphere activation pathway. As verified addi-
tionally by 1H, 31P and 19F NMR spectroscopy, no reaction oc-
curred between the pairs “1a,b and B(C6F5)3”, or “CO2 and
B(C6F5)3”.
The CO2 capture could be shown to be reversible, since exposure
of the benzene solution of 2a,b to N2 or vacuum led to the quantita-
tive regeneration of the free molecules 1a,b, B(C6F5)3 and CO2, as
evidenced for 1a,b and B(C6F5)3 by NMR spectroscopy. Addition
of acetonitrile to the benzene solution of 2b afforded instantane-
ously the CH3CN·B(C6F5)3 adduct concomitantly with the
15b
1b/CH3CN adduct [ReHBr(NO)(PCy3)2(CH3CN)]
possess-
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ing two trans phosphine ligands, further emphasizing the instability
of 2a,b envisaged to possess loosely bound CO2.
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Scheme 2. CO2 insertion into the Re-H bond of 1a,b with B(C6F5)3.
The singlet observed in the 31P NMR spectrum was assigned to un-
reacted 1a.
The type 2 complexes appeared to be only intermittently stable
even under 1 bar of CO2 and gradually evolved into several new
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species within 4 h at 23 C. The H NMR spectrum then exhibited
a new singlet at 9.61 ppm, which was correlated to a new singlet at
3.9 ppm in the 31P NMR spectrum. The 19F NMR spectrum re-
vealed a new set of resonances at -133.62, -159.55 and -166.07 ppm.
These were interpreted in terms of formation of a rhenium η1-
formato species with the terminal oxygen atom functionalized by
B(C6F5)3. This species was again seen to be transient, and gradually
transformed into two other species revealing two singlets at 8.36
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and 7.77 ppm in the H NMR spectrum accompanied by two new
singlets at 52.4 and 28.6 ppm in the 31P NMR and two new sets of
signals at -134.18, -159.32, -166.06 ppm and at -133.68, -158.82, -
165.07 ppm in the 19F NMR spectrum. Evidently, these data point
to formation of two closely related rhenium formate isomers of as
yet unknown detailed structures.
Interestingly, red-crystals gradually precipitated out from the
benzene solution of 2a along with the disappearance of 2a. X-ray
diffraction studies revealed a μ-Br bridged Re(I) dimer η1-formato
structure [{Re(μ-Br)(NO)(η1-OCH=O-B(C6F5)3)(PiPr3)2}2] (3a)
(Figure 1). Each center of the dimer adopts a pseudo-octahedral
geometry. The B(C6F5)3-coordinated formato moieties are located
trans to the linear nitrosyl as a result of the strong π-donating/π-
accepting push-pull effect. The most prominent structural feature is
that the two phosphine ligands are cis showing a P-Re-P angle of
103.78(2)o. To the best of our knowledge, this is the first example
of a cis-alignment of two sterically bulky PiPr3 ligands within the
realm of rhenium complexes. Rare such cases of pseudo octahedral
complexes with two cis PiPr3 ligands were observed previously in
Os and Rh complexes.17 Noteworthy is the fact that in the solid
state structure of 3a the boron attached C-O bond (1.268(3) Å) is
longer than that bonded to the Re center (1.246(2) Å), which is in
accord with the conjugated character of the coordinated formato
group characterized in solution.
In contrast, when the 1a/B(C6F5)3 or 1b/B(C6F5)3 mixtures (1:1)
were exposed to 1 bar of CO2 in benzene-d6, the original purple
solutions immediately turned light brown affording within 20 min
the six-coordinate 18e species 2 (R = iPr a, Cy b) in 85 % in-situ
yield, which were identified in solution as the Re(I) hydride struc-
tures of the type [ReHBr(NO)(PR3)2(η2-O=C=O-B(C6F5)3)]
possessing two cis-phosphines and a B(C6F5)3-attached η2-CO2
ligand (Scheme 2). The 1H NMR spectrum revealed doublet of
doublet resonances at 5.72 (2a) and 6.11 ppm (2b), which were
assigned to the HRe atoms. Surprisingly, two 2JPH coupling constants
of 18 and 48 Hz were observed, which indicated the presence of
phosphine ligands cis and trans to the hydride. In the 31P NMR
spectra, two doublets were observed at 39.6 and 33.0 ppm for 2a,
and at 30.0 and 22.1 ppm for 2b, respectively. In comparison with
2
the JPP values of ca. 120 Hz observable for trans-bisphosphine
ligands, the relatively small coupling constants of ca. 80 Hz in 2
further supported cis-alignment of the two phosphine ligands.15
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o
The H,31P-correlation spectrum of 2a recorded at -60 C revealed
only correlation between the phosphorus signal at 39.6 ppm and
that of the hydride indicative of their trans positions. The other
phosphorus signal at 33.0 ppm showed no correlation to the hy-
dride apparently suggesting their cis position. The 19F NMR spectra
exhibited a new set of resonances at -134.97 (o-F), -160.07 (p-F), -
166.55 (m-F) ppm for 2a, and at -134.16 (o-F), -159.99 (p-F), -
166.47 (m-F) ppm for 2b providing evidence for the formation of
the O-B(C6F5)3 moiety.16 In the long range 13C, 1H correlation
spectrum of 2a (HMBC) scalar couplings were observed between
the hydride signal and the carbon resonances in the range of δ 150
to 135 ppm. This implied η2 coordination of CO2 cis to the Re-H
bond, which enabled simultaneous interaction of the Re-H moiety
Figure 1. Molecular structure of 3a with 30 % probability displacement
ellipsoids. All hydrogen atoms were omitted for clarity except for the
formate moieties. Selected bond lengths (Å ) and angles (deg): N(1)-
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O(1), 1.192(2); O(2)-Re(1), 2.1164(15); Re(1)-P(1), 2.4675(7);
Lewis acid free [M-OCHO] species and is expected to have low-
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Re(1)-P(2), 2.4713(6); O(3)-B(1), 1.553(3); C(19)-O(2), 1.246(2);
C(19)-O(3), 1.268(3); O(1)-N(1)-Re(1), 174.63(18); C(19)-O(2)-
Re(1), 144.74(16); N(1)-Re(1)-O(2), 178.66(8); P(1)-Re(1)-P(2),
103.78(2); C(19)-O(3)-B(1), 126.52(18).
ered v(OC=O) wavenumbers due to π-electron withdrawal in-
duced by the B(C6F5)3 attachment.18 For 3a a satisfactory elemental
analysis was obtained. NMR spectra of 3a could not be recorded,
due to insufficient solubility of this compound in non-coordinating
solvents.
o
Similarly, 2b slowly evolved under CO2 atmosphere at 23 C into
three new rhenium formate species showing singlets at 9.57, 8.36
The structures of 2a,b and related species along the CO2 activa-
tion course were modelled by DFT calculations using PMe3 model
ligands. Based on the [ReHBr(NO)(PMe3)2] system with trans
and cis PMe3 ligands (denoted as “transRe” or “cisRe”), the reac-
tions with CO2 and BF3 were explored. All optimized structures
were found to be local energy minima. The model of 2a,b (V)
shows a pseudo-pentagonal bipyramidal geometry with a Br-Re-
NO axis (Figure 2). CO2 is η2-coordinated to the Re center and BF3
weakly attached to the OCO2 atom showing a B-O distance of 1.633
Å. The relatively long Re-O distance of 2.243 Å speaks for a weakly
bound CO2 ligand, indeed consistent with the experimental obser-
vations for 2a,b. The two PMe3 ligands are disposed cis with a P-
Re-P angle of 102.3o, which is quite comparable to that of the solid
state structure of 3a.
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and 8.14 ppm in the H NMR spectrum, which are correlated to
three singlets at 25.1, 20.2 and 7.0 ppm in the 31P NMR spectrum.
The species corresponding to the signal at 8.36 ppm remained as
the major component after 2b had completely disappeared within
15 h. In contrast to the case of 2a, no precipitate was eventually
formed. Particularly the 31P NMR spectrum of the solution indi-
cated a complex mixture, from which attempts to isolate stable
products proved to be unsuccessful. We however assume that the
2b reactions proceeded along similar lines as that of 2a.
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Complex 3a could be prepared in 72 % isolated yield from the
reaction of 1a/B(C6F5)3 (1:1.5) with 1 bar of CO2 in benzene after
o
stirring at 23 C for 24 h. In the IR spectrum, the expected v(NO)
absorption is split revealing bands at 1708 and 1695 cm-1. The
v(OC=O) band was observed at 1592 cm-1, which compared to
Figure 2. DFT calculated intermediates possibly involved in the CO2 activation course with energies denoting local energy minima with respect to the
reference (∆E, kcal/mol)
The three free molecules of the transRe/CO2/BF3 arrangement I
were taken as the energetic reference (0.0 kcal/mol). The energy of
the mainly electrostatic binding of BF3 to the Re-H bond in II was
slightly endothermic (0.5 kcal/mol) showing a B-H bond distance
of 2.341 Å. In the arrangement II’ a B-O distance of 2.788 Å was
observed suggesting weak electrostatic interactions with -1.2
kcal/mol of released energy. The BF3-functionalized CO2 can
weakly coordinate to transRe showing in V’ only -1.6 kcal/mol of
stabilization energy. The two PMe3 ligands are bent toward the
hydride with a P-Re-P angle of 143.3o, which supposedly prevents
the insertion of the C=O bond into the trans-Re-H bond. On the
other hand, the free cisRe molecule, CO2 and BF3 (III) are 11.3
kcal/mol higher in energy than the reference. The interaction
between the BF3 and the Re-H unit of cisRe is much stronger than
that of II since a much shorter B-H distance of 1.428 Å was ob-
served in IV showing an energy decrease of 4.8 kcal/mol relative to
III. Coordination of CO2 to the Re center of IV resulted in IV’,
which lies 3.4 kcal/mol lower in energy than IV. The interaction
between BF3 and the Re-H unit in IV’ became further enhanced
showing a B-H distance of 1.394 Å. The CO2 ligand remained
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linear and was weakly bound to the Re center displaying a Re-O
which definitely indicated that cleavage of the Re-O bond in 4a by
Et3SiH had occurred.
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distance of 2.454 Å. Coordination of the BF3-functionalized CO2 to
cisRe afforded V, which is 3.8 kcal/mol higher in energy than the
reference, and -7.5 kcal/mol below III. Following the activation
course of CO2 by Re-H and BF3 further afforded the model formato
species VI releasing 12.9 kcal/mol in energy with respect to V. The
isomerization of VI to VII bearing trans-aligned formato and ni-
trosyl ligands was found to be thermodynamically downhill by -2.5
kcal/mol. Furthermore, the dimerization via the bromo bridges led
to a substantial thermodynamic stabilization of -15.5 kcal/mol.
Scheme 3. Formation of the “compressed dihydride” complex 4a and
its subsequent reaction with Et3SiH
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Based on these calculational results, we are inclined to propose
that substitution of BF3 by B(C6F5)3 would additionally support the
geometry change from the trans-phosphine to the cis-phosphine
arrangements. Subsequently cooperative activation of CO2 by the
Re-H·····B (C6F5)3 FLP occurs in a similar fashion as for the Stephan
type LB·····B(C6F5)3 FLPs by insertion of the substrate molecule in
between the Lewis pair encounter complex. Such an activation
course could initially be assisted by a 2-CO2  type interaction
with the rhenium center, which similar as in V or V’ helps to draw
CO2 closer to the Re-H and B(C6F5)3 moieties.
The 29Si NMR spectrum revealed the presence of four silicon
species showing singlets at 37.05, 30.01, 18.90 and 9.56 ppm.
Among them the signal at 9.56 ppm was assigned to the [Et3Si-H-
B(C6F5)3] adduct.20 The resonance at 18.90 ppm represents the
most prominent component, which corresponds to a singlet at 4.96
2.2. Catalytic reduction of CO2 with Et3SiH. The μ-Br bridges can
be cleaved by 2e donating molecules. For instance, under 1 bar of
o
H2 the suspension of 3a in toluene afforded at 60 C within 1 h the
complex [ReBrH2(NO)(η1-OCH=OB(C6F5)3)(PiPr3)2] (4a) in
99 % isolated yield (Scheme 3). The IR spectrum revealed two
characteristic intense bands: a ν(NO) band at 1741 cm-1 and a
ν(OC=O) band at 1595 cm-1. The reaction with H2 is assumed to
ppm in the H NMR as revealed by the H, 29Si correlation spec-
trum. This evidently pointed to the formation of the
bis(triethylsilyl)acetal (Et3SiO)2CH2 (7), which was also observed
by other CO2 reduction systems using Et3SiH as reducing agents,
such as the “Zr/B(C6F5)3” system reported by Matsuo and Kawa-
guchi,3b the Iridium pincer catalyst by Brookhart et al.,3c and the
“TMP/B(C6F5)3” system by Piers et al.11b It is proposed that in a
similar “tandem reaction” 4a undergoes Et3Si+ replacement of
B(C6F5)3 to form the [HB(C6F5)3]- and 8 (Scheme 3). The strong
oxophilicity of the [Et3Si]+ cation attached to the OCO2 atom wea-
kens the Re-O bond in 8, which then reacts further with Et3SiH to
afford 7 accompanied by H-H heterolytic cleavage yielding 5 and 6.
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lead to a “compressed dihydride” complex. The corresponding H
NMR resonance was observed as a relatively sharp triplet at 3.05
ppm, while the proton signal of the formato unit was found at 7.71
ppm in a typical region for Hformato substituents. The 31P NMR
spectrum revealed a singlet resonance at 28.5 ppm suggesting two
chemically equivalent trans-phosphine ligands. The presence of a
O-B(C6F5)3 moiety was indicated by the set of signals at -134.81, -
157.74 and -164.98 ppm in the 19F NMR spectrum. As indicated by
1
the H NMR and by analogy to the reported dibromo dihydrogen
Re(I) complexes,19 the H2 ligand in 4a is anticipated to also belong
to the class of “compressed dihydrides” with a considerably elon-
gated practically non-bonding HH distance. This was supported
further by DFT calculations on the model complex [ReBrH2(η1-
OCH=OBF3)(NO)(PMe3)2], where the H···H distance turned
out to be non-bonding at 1.50 Å.
In an attempt to accomplish catalytic reduction of CO2 we used
the “1a/B(C6F5)3” system (1:1.5) and triethylsilane as a reductant
under various conditions. In certain cases the silane functioned also
as an oxygen scavenger (Table 1). Applying a catalyst loading of 1.0
mol% of 1a and 1.5 mol% of B(C6F5)3 in benzene-d6 under 1 bar of
o
Addition of 2 equiv. Et3SiH to the light yellow toluene solution of
CO2 at 80 C, 35 % of the Et3SiH was converted to (Et3SiO)2CH2
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4a afforded at 100 C after 5 min a mixture in purple showing no
(7) within 4 h corresponding to a TON of 35 and a TOF of 8.8 h-1.
In this experiment no other silyl containing product could be ob-
trace of formato protons by 1H NMR. Instead formation of the
phosphonium borate [HPiPr3][HB(C6F5)3] (5)20 was observed
exhibiting in the 1H NMR spectrum a characteristic quartet of
doublet signal at 4.40 ppm (1JHP = 453 Hz), which correlated with a
singlet at 42.6 ppm in the 31P NMR spectrum formed in 32 % yield.
The presence of the [HB(C6F5)3]- anion of 5 was confirmed by
both a doublet signal at -19.45 ppm in the 11B NMR and the set of
resonances at -134.19, -164.48 and -167.95 ppm in the 19F NMR
spectrum. Interestingly, a doublet signal at -7.61 ppm (2JHP = 22 Hz)
in the 1H NMR spectrum could be spotted, which correlated with a
phosphorus signal at 71.5 ppm in 25 % yield. This signal was as-
signed to a monophosphine hydride moiety [ReHBr(NO)(PiPr3)]
presumably stabilized by an η4 toluene ligand (6)21 formed via
heterolytic cleavage of the H-H bond in 4a by dissociating a neigh-
boring basic phosphine and deprotonation of the H2 ligand.22 The
31P NMR spectrum also revealed the formation of 1a in 43 % yield,
1
served by H NMR spectroscopy.3b,3c,11b GC-MS revealed the pres-
ence of small amounts of (Et3Si)2O, which however was anticipated
to originate from the reaction of Et3SiH with H2O involved in the
GC-MS manipulation. The “1b/B(C6F5)3” system proved to be less
efficient affording under the same conditions compound 7 in a
TON of 18.
Significantly, the reaction catalyzed by 1 mol% of 4a applying now
5 bar of CO2 afforded at 80 oC within 13 h the silyl acetal 7 in 89 %
yield, which corresponds to a TON of 89 and a TOF of 6.8 h-1. A
very small amount (3 %) of Et3SiOCH3 (9) was observed, which
1
was recognized in the H NMR spectrum as a singlet resonance at
3.32 ppm. We rationalized that 9 could be generated by the reac-
tion of 7 with Et3SiH. In addition the reaction produced (Et3Si)2O
(10) in equivalent amounts to 9.3b,3c,11b The catalytic reduction of
CO2 using 1 mol% of 3a and Et3SiH produced 7 within 15 h in 87 %
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Journal of the American Chemical Society
yield corresponding to a TON of 87 and a TOF of 5.8 h-1. Under
the same conditions 3a showed a slightly lower catalytic activity
than 4a, which was attributed to the lower solubility of 3a in ben-
zene. In the case of the catalysis with 3a, product 9, which requires
an additional reduction step, was formed in only 1 % yield together
with an equivalent (1 %) of 10.
1
2
3
4
5
6
7
Table 1. Catalytic reduction of CO2 by Et3SiH
8
9
10
11
12
13
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2.3. Catalytic CO2 hydrogenation in the presence of base. Catalytic
hydrogenation of CO2 was carried out with the “1/B(C6F5)3” sys-
tem, but additionally in presence of the sterically hindered Lewis
base TMP (2,2,6,6-tetramethylpiperidine). Under 20 bar of CO2
and 40 bar of H2 in THF with 1 mmol of TMP, the “1a/B(C6F5)3”
The molecular structure of 11 was established by both NMR
spectroscopy and an X-ray diffraction study, as depicted in Figure 3.
Moderately strong intermolecular hydrogen bonding was observed
between the NH proton and the formate oxygen atom.24
o
system (0.5 mol%, 1:2) afforded at 80 C within 15 h the piperidi-
nium formate salt [H-TMP]+[O-CH=O]- (11) in 52 % yield cor-
responding to a TON of 104 (entry 2, Table 2). Formation of the
related formamide could not be observed.23 The same reaction
afforded within 2 h a TON of 16 indicating a slow turnover rate
(entry 3, Table 2). A blank reaction in the absence of “1/B(C6F5)3”
did not afford hydrogenation at all (entry 4, Table 2). THF turns
out to be an appropriate solvent for these catalyses, most probably
due to its efficient solvation effect. The reaction in chlorobenzene
or methanol showed under the same condition lower activities
(entries 1 and 12, Table 2). At 23 oC hydrogenation of CO2 did not
o
take place (entry 5, Table 2). Increasing the temperature to 120 C
resulted also in a poor TON (entry 6, Table 2), which was attri-
buted to increased decomposition of the active species. Decreasing
the H2 and CO2 pressures to 20 and 10 bar resulted in a very small
TOF (entry 7, Table 2), which was interpreted in terms of the
inability to accumulate sufficient kinetically relevant concentra-
tions of 4a.
Figure 3. Molecular structure of 11 with 30 % probability displacement
ellipsoids. All hydrogen atoms were omitted for clarity except for the
formate and NH2 moieties.
To obtain further insight into the reaction mechanism, stoichi-
ometric reactions of the catalyst were investigated. When 4a was
o
mixed with 2 equiv. of TMP at 23 C in THF-d8, the solution in-
Remarkably, when 4a was used as a catalyst in the absence of
additional B(C6F5)3, the hydrogenation of CO2 in THF with 1
stantaneously turned purple with a white precipitate formed. In the
1H NMR spectrum a triplet resonance at -6.53 ppm (JHP = 15 Hz)
and a broad singlet at -15.22 ppm were observed, which correlated
with two singlets at 41.8 and 40.8 ppm in the 31P NMR spectrum.
The triplet signal was assigned to a trans diphosphine Re-H species
generated via deprotonation of the H2 moiety. The broad singlet at
-15.22 ppm was characteristic for the hydride 1a.15a Both observa-
tions implied TMP assisted H2 heterolytic cleavage followed by
dissociation of the formato ligand from the Re center of 4a. The
white precipitate could be ascertained to be the piperidinium for-
mate 11, which was partly still present in solution as confirmed by
1H NMR spectroscopy. Products relating back to reactions with the
“TMP/B(C6F5)3” FLP and CO2 or H2 could not be detected.11,8
o
mmol of TMP at 80 C afforded within 15 h a TON value of 92
(entry 10, Table 2), which is quite comparable to that of the
“1a/B(C6F5)3” experiment. This indicated that complexes of type
4a might be involved as intermediates in the various reaction
courses. In comparison to 4a, the activity of 3a was found to be
slightly lower giving under the same conditions a TON of 72,
presumably caused by the low solubility of 3a in THF (entry 9,
Table 2).
Table 2. Catalytic hydrogenation of CO2 in the presence of TMP
The above results pointed to a CO2 hydrogenation mechanism in
presence of B(C6F5)3, as proposed in Scheme 4. Like in a FLP the
activation pathway starts with the interaction of the hydride ligand
of the “1/B(C6F5)3” pair with CO2 to give 3, which further reacts
with H2 to afford the formato dihydrogen complexes of type 4. It
should be noted at this point that the cycle is assumed to be driven
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Page 6 of 22
by complexes with trans phosphine arrangements. The cis given “compressed dihydrides” are more acidic. Therefore the
1
2
3
4
5
6
7
8
phosphine complexes of type 3 are assumed to function as resting
states outside the catalytic cycle. In the presence of the sterically
hindered Lewis base TMP, deprotonation of the H2 ligand occurs
affording the [HTMP]+ cation and the anionic Re-H formato
complex 12 in full agreement with the observations of the de-
scribed stoichiometric reaction.22 12 is prepared for formate elimi-
nation and exchange with H2 via a 16e rhenium hydride species 13
to form 14, since the  donor formate feels strong repulsion from
the filled d orbitals of the rhenium center. The repulsion is even
enhanced by the fact that the complex is anionic and loaded with
charge. Subsequently the trans dihydrogen hydride complex 14 is
anticipated to rearrange to the isomeric “compressed dihydride
hydride 15 via a trihydride transition state.25 Related rhenium
complexes showed in H2/D2 scrambling experiments similar dihy-
drogen hydride exchange courses.15e DFT calculations using the
[ReHBr(NO)(PMe3)2] model fragment suggest a practically ther-
moneutral process for the conversion of 13 to 14 revealing the
minute energy difference of -0.1 kcal/mol between the model
complexes of 13/H2 and 14. Similarly the isomerization step from
14 to 15 can be anticipated to be mildly “exothermic”, since the
calculations of the corresponding model complexes brought about
a -6.0 kcal/mol energy difference.
regeneration step of 4(4’) preceding the deprotonation is pre-
sumed to be rate determining. In the earlier context the
stoichiometric reaction sequence of 1a/B(C6F5)3 with CO2 and
subsequently with H2 has been shown to be overall very slow at
room temperature. The related transformation, which constitutes
insertion of CO2 into the Re-H bond, is expected to be slow even at
the higher temperature of 80 oC of the catalyses. Therefore we
rationalized that this step would be the slowest of all steps of the
catalytic cycle and rate determining.
9
10
11
12
13
14
15
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17
18
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It should also be noted that the 1/B(C6F5)3/CO2/H2 reaction
mixture contains not only the 1/B(C6F5)3 FLP, but also the “classi-
cal” metal-free TMP/B(C6F5)3 FLP. Competitive FLP reactivity
can be envisaged to occur with simultaneous capture of CO2 and
H2 by this FLP leading to the formation of a [H-TMP][O=CH-
OB(C6F5)3] product or by H2 embracement to the [H-
8
TMP][HB(C6F5)3] salt.11, In the presence of 1a however both
these FLP reaction channels seemed not to be competitive with the
reactions of the 1/B(C6F5)3 FLP, as implied by entry 4 in Table 2.
Table 3. Comparison of the performance of different bases in catalytic
hydrogenation of CO2
Scheme 4. Sketch of a catalytic cycle for CO2 hydrogenation in pres-
ence of TMP displaying the type 1, 3 and 4 compounds in presence
and absence of B(C6F5)3. Those species appearing in absence of
B(C6F5)3 are denoted as 4’ and 12’.
It should be noted at this point that H2 ligands trans to a strong
donor, like the hydride ligand and trans to a strong  acceptor are
expected to possess a H2 ligand structure, while H2 ligands trans to
a  donor like halogen ligands should reveal an elongated dihydro-
gen or compressed dihydride structure.26 In the catalytic reaction
course 1 could possess higher concentrations similar to a resting
state, since it precedes the rate determining step.15b According to
Scheme 4 15 and B(C6F5)3 constitute the catalytically crucial “Re-
H/B(C6F5)3” FLP, which activates CO2 and regenerates the type 4
complexes. The deprotonation step of 4(4’) could principally be
slow and rate determining, since the “compressed dihydrides” of
type 4 are expected to be less acidic than dihydrogen complexes.
Nevertheless, the stoichiometric deprotonation of 4a proceeded
instantaneously at ambient temperature, which indicates that the
The potential of the catalytic CO2 hydrogenation applying 1a was
additionally explored in the absence of B(C6F5)3, which surpris-
ingly afforded under the same conditions as in the presence of
B(C6F5)3 a TON of 78 (entry 11, Table 2). Thus, 1a alone ap-
peared to be a good, but somewhat less active catalyst than the co-
catalytic system 1a/B(C6F5)3. We assume that this catalysis follows
a similar reaction course with formation of the B(C6F5)3 free for-
mato rhenium species 4’. The secondary coordination sphere
hydride transfer from 15 onto CO2 does not contrast the observa-
tion that 1 does not react with CO2, since the reactive species 14
are isomeric to 1. Structures of type 14 are expected to possess a
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degassed by freeze-thaw cycles prior to use. The deuterated solvent
more hydridic character in the Re-H bonds than 1, which originates
1
2
3
4
5
6
7
8
CD2Cl2 was dried over molecular sieves, benzene-d6 and toluene-d8
were dried with sodium/benzophenone and vacuum transferred for
storage in a Schlenk flask fitted with Teflon valves. 1H NMR, 13C{1H}-
NMR, 31P{1H}-NMR, 19F NMR and 11B NMR data were recorded on a
Varian Mercury 300 spectrometer using 5 mm diameter NMR tubes
equipped with Teflon valves, which allow degassing and further intro-
duction of gases into the probe. Chemical shifts are expressed in parts
from the trans influence of the trans positioned NO ligand. The H2
ligand of 4’ is however expected to be less acidic than that of 4,
since the B(C6F5)3 substituent of 4 leads to additional electron
withdrawal affecting also the H2 ligand.
In a tuning effort different bases of various base strengths were
then tested for the performance in the hydrogenation of CO2.
Exclusive formation of the corresponding formate salts was invaria-
bly observed instead of the alternative formation of formamide
compounds (Table 3). In the case of the sterically less demanding
base HNiPr2, the catalytic system of “1a/B(C6F5)3” afforded the
formate salt [HNiPr2]+[O-CH=O]- (16) within 34 h in 87 % yield
corresponding to a TON of 174 and a TOF of 5.1 h-1 (entry 1,
Table 3). The “1b/B(C6F5)3” system was found here to be slightly
less active. Within 36 h a TON of 134 was accomplished (entry 2,
Table 3). The reaction with the least bulky amine NEt3 afforded
under the same conditions poorer TONs of up to 10 within 28 h
(entries 4 and 5, Table 3). By comparison, HNCy2 as a base fur-
nished a conversion of 61 % and a TON of 122 using the
“1a/B(C6F5)3” co-catalytic system with formation of the [HN-
Cy2]+[O-CH=O]- (17) salt (entry 8, Table 3). These results evi-
dently pointed out that steric hindrance of the employed base is
key to the efficiency of the CO2 hydrogenation. Another crucial
factor is apparently the basicity of the base. In the case of PtBu3,
which is sterically bulky, but much less basic than the secondary
amines, nearly no catalytic activity was observed (entry 7, Table 3).
The NMR spectrum revealed a complex mixture possibly contain-
ing the CO2 captured product [tBu3P-C(=O)-O-B(C6F5)3] origi-
nating from the reaction of CO2 with the “PtBu3/B(C6F5)3” FLP.10a
However, the H2 activated product [HPtBu3][HB(C6F5)3] was not
observed. The crucial role of the basicity of the base was further
supported by the reaction using HN(SiMe3)2, which is bulkier, but
less basic than HNiPr2. Indeed, a very poor TON of 4 was obtained
within 16 h under the same catalytic conditions as for the other
base reactions (entry 10, Table 3). 2,4,6-tri-tert-butylpyridine also
showed no conversion in the CO2 hydrogenation attributed to its
too low basicity (entry 11, Table 3). Finally it should be mentioned
that in all cases of secondary amines the CO2 hydrogenations cata-
lyzed by 1a were also examined in the absence of B(C6F5)3. Invaria-
bly inferior catalytic performances compared to those in the pres-
ence of B(C6F5)3 were observed.
1
per million (ppm). H and 13C{1H}-NMR spectra were referenced to
the residual proton or 13C resonances of the deuterated solvent. All
chemical shifts for the 31P{1H}-NMR data are reported downfield
in ppm relative to external 85 % H3PO4 at 0.0 ppm. Signal patterns are
reported as follows: s, singlet; d, doublet; t, triplet; m, multiplet. IR
spectra were obtained by using ATR methods with a Bio-Rad FTS-45
FTIR spectrometer. Complexes 1 were prepared according to reported
procedures.15a
9
10
11
12
13
14
15
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60
[{Re(μ-Br)(NO)(η1-OCH=OB(C6F5)3)(PiPr3)2}2] (3a): In a 30 mL
Young-tap Schlenk vessel, 62.0 mg [ReHBr(NO)(PiPr3)2] (1a, 0.10
mmol) was mixed in a glove-box with 71.5 mg B(C6F5)3 (0.14 mmol)
in 5 mL benzene. The N2 atmosphere was replaced with 1.0 bar of CO2
using the freeze-pump-thaw cycle. After warming to room temperature,
the originally violet solution instantaneously turned brown. The mix-
ture was kept stirring at room temperature for overnight leading to the
formation of a large amount of a pale-red precipitate, which was iso-
lated and further washed with benzene (1 x 1 mL) and pentane (3 x 3
mL), dried in vacuo affording a pale-red solid: 84.0 mg. Yield: 72 %. IR
(cm-1, ATR): ν (NO) 1708, 1695; ν (OC=O) 1592. Anal. Calcd for
C74H86B2Br2F30N2O6P4Re2 (2347.18): C, 37.87; H, 3.69; N, 1.19.
Found: C, 37.48; H, 3.85; N, 1.09.
Reaction of 1a with B(C6F5)3 and CO2 in the NMR tube. In a 3 mL
Young-tap NMR-tube, 12.4 mg 1a (0.02 mmol) was mixed with 30.2
mg B(C6F5)3 (0.06 mmol, 3 equiv.) in 0.6 mL benzene-d6 to give a
deep-purple solution. Although the 19F NMR spectrum implied the
presence of free B(C6F5)3, the 31P NMR resonance at 43.4 ppm became
1
broadened, and the Re-H resonance in the H NMR spectrum disap-
peared. This all indicated a weak interaction between the Re-H and the
boron atom of B(C6F5)3 in solution. Formate formation was not ob-
served. The N2 atmosphere was then replaced with 1.0 bar of CO2
using the freeze-pump-thaw cycle. The purple solution turned immedi-
ately brown. After 20 min, 31P NMR spectroscopy confirmed forma-
tion of the intermediate species 2a in 85 % yield as the only new spe-
cies. 1H NMR (300.08 MHz, benzene-d6, ppm): δ 5.72 (dd, 2J(HP) = 48
Hz, 2J(HP) = 18 Hz, 1H, ReH), 2.75 (m, 3H, PCH(CH3)2), 2.50 (m, 3H,
PCH(CH3)2), 1.31 (m, 9H, PCH(CH3)2), 1.12 (m, 9H, PCH(CH3)2),
0.93 (m, 18H, PCH(CH3)2). 13C{1H} NMR (75.47 MHz, benzene-d6,
ppm): δ 27.1 (d, J(PC) = 28 Hz, P-CH), 24.6 (d, J(PC) = 24 Hz, P-CH),
19.2 (s), 19.1 (s), 18.3 (m), 18.0 (s). 31P{1H} NMR (121.47 MHz,
3. CONCLUSIONS
In conclusion, we demonstrated that cooperative FLP type activa-
tion of CO2 can be accomplished by “Re-H/B(C6F5)3” systems
with the Re-H bond operating as the Lewis base component. These
FLPs rendered subsequent catalytic reduction of CO2 by a hydrosi-
lane, and catalytic hydrogenation of CO2 in the presence of sterical-
ly hindered strong bases. This work therefore not only emphasized
the possibility for extending the scope of the FLP concept into
transition metal hydrides as bases components to get involved in
small molecules activations, but also demonstrated that FLP tuning
of non-platinum-group transition metals may provide ample op-
portunity for catalytic CO2 reduction. Explorations of related but
more efficient catalytic CO2 hydrogenation processes are currently
underway in our group.
2
2
benzene-d6, ppm): δ 39.62 (d, J(PP) = 84 Hz, 1P), 33.05 (d, J(PP) = 82
Hz, 1P). 19F NMR (282.33 MHz, benzene-d6, ppm): δ -134.97 (m, 6F,
ortho-C6F5), -160.07 (t, JCF = 20 Hz, 3F, para-C6F5), -166.55 (m, 6F,
1
meta-C6F5). No signals were observed in 11B NMR spectroscopy.
Reaction of 1b with B(C6F5)3 and CO2 in the NMR tube. In a 3 mL
Young-tap NMR-tube, 17.2 mg 1b (0.02 mmol) was mixed with 30.1
mg B(C6F5)3 (0.06 mmol, 3 equiv.) in 0.6 mL benzene-d6 to give a
deep-purple solution. 1H, 31P and 19F NMR spectroscopy indicated that
reaction had not occurred between 1b and B(C6F5)3, as the original
resonances (for instance Re-H signal at -17.0 ppm in the 1H NMR) still
remained, and no new species formed. The N2 atmosphere was then
replaced with 1.0 bar of CO2 using a freeze-pump-thaw cycle. Immedi-
ately the purple solution turned light brown indicative of formation of
the 18e rhenium species 2b. After 30 min, 31P NMR spectroscopy
confirmed the quantitative formation of 2b. Formate was not generated
at this stage. 1H NMR (300.08 MHz, benzene-d6, ppm): δ 6.11 (dd,
General Experimental. All manipulations were carried out under an
atmosphere of dry nitrogen using standard Schlenk techniques or in a
glovebox (M. Braun 150B-G-II) filled with dry nitrogen. Solvents were
freshly distilled under N2 by employing standard procedures and were
2
2J(HP) = 48 Hz, J(HP) = 18 Hz, 1H, ReH), 1.03-2.70 (m, 66H,
P(C6H11)3). 13C{1H} NMR (75.47 MHz, benzene-d6, ppm): δ 37.2 (d,
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J(PC) = 26 Hz, P-CH), 33.9 (d, J(PC) = 23 Hz, P-CH), 29.6 (s), 29.0 (s),
NMR spectroscopy. The color of solution gradually turned darker, and
27.5 (m), 27.0 (s), 26.2 (s). 31P{1H} NMR (121.47 MHz, benzene-d6,
ppm): δ 30.00 (d, 2J(PP) = 79 Hz, 1P), 22.13 ppm (d, 2J(PP) = 79 Hz, 1P).
19F NMR (282.33 MHz, benzene-d6, ppm): δ -134.16 (m, 6F, ortho-
eventually became purple. Alternatively in a 50 mL autoclave vessel, 4a
(6.0 mg, 0.005 mmol) or 3a (6.0 mg, 0.0025 mmol) were mixed with
0.5 mmol of Et3SiH (76 μL) in 6 mL of benzene-d6. Then 5 bar of CO2
was charged and the reaction was kept at 80 oC for 13 h. The TON and
TOF values were determined based on the consumption of the hydro-
1
2
3
4
5
6
7
8
1
C6F5), -159.99 (t, JCF = 22 Hz, 3F, para-C6F5), -166.47 (m, 6F, meta-
C6F5). 11B NMR signals could not be observed. The transient species is
only temporarily stable under CO2 atmosphere. Exposure of the solu-
tion to vacuum leads to back-reaction affording the starting materials
1b/B(C6F5)3 in over 90 % yield.
1
silane. The composition of the products was determined by H NMR
and GC-MS. Characteristic NMR resonances:
1
(Et3SiO)2CH2 (7). H NMR (300.08 MHz, benzene-d6, ppm): δ 5.07
Addition of 5 drops of acetonitrile to the benzene solution of the in-
termediate leads to the quantitative formation of the trans-phosphine
Re(I) hydride species [ReHBr(NO)(PCy3)2(CH3CN)]15b (31P NMR:
δ 16.6 ppm) and CH3CN·B(C6F5)3 (19F NMR in benzene-d6: δ -130.84
(s), 1.05 (t, J = 9 Hz, CH3), 0.67 (q, J = 6 Hz, SiCH2). 29Si NMR (90
MHz, benzene-d6, ppm): δ 18.90 (s);
9
10
11
12
13
14
15
16
17
18
19
20
21
22
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27
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43
44
45
46
47
48
49
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53
54
55
56
57
58
59
60
Et3SiOCH3 (9). 1H NMR (300.08 MHz, benzene-d6, ppm): δ 3.32 (s).
Hydrogenation of CO2 catalyzed by “1/B(C6F5)3” in the presence of
bases. In a 50 mL autoclave vessel, 0.005 mmol of [Re] (1a, 3.1 mg; 1b,
4.3 mg; 3a, 5.9 mg; 4a, 5.9 mg) and 0.01 mmol of B(C6F5)3 (5.4 mg)
were mixed with 1.0 mmol of base (TMP, 170 μL; HNiPr2, 140 μL;
NEt3, 150 μL; HNCy2, 200 μL; PtBu3, 250 μL; HN(SiMe3)2, 208 μL;
2,4,6-tri-tert-butylpyridine, 250 mg) in 1 mL of solvent (THF or chlo-
robenzene). Afterwards the autoclave was charged with 20 bar of CO2
and 40 bar of H2. The mixture was kept at 80 oC for overnight. After
cooling to room temperature, 10 μL of DMF (0.13 mmol) was added
to the resultant solution as an internal standard. An aliquot of the
mixture (50 μL) was mixed in 0.5 mL of D2O or CDCl3 and examined
1
(m, 6F, ortho-C6F5), -151.68 (t, JCF = 20 Hz, 3F, para-C6F5), -159.48
(m, 6F, meta-C6F5), or 19F NMR in CD2Cl2: δ -136.22 (m, 6F, ortho-
1
C6F5), -158.74 (t, JCF = 20 Hz, 3F, para-C6F5), -165.92 (m, 6F, meta-
C6F5).)).
[ReBrH2(NO)(η1-OCH=OB(C6F5)3)(PiPr3)2] (4a): In
a 30 mL
Young-tap Schlenk vessel placed in glove-box, 3a (60 mg, 0.025 mmol)
was mixed in 2 mL of toluene. The N2 atmosphere was replaced with
1.0 bar of H2 using a freeze-pump-thaw cycle. The mixture was kept at
60 oC for 1 h affording a clear light brown solution. After filtration
through Celite, the solvent was evaporated in vacuo, and the residue
was washed with pentane (3 x 2 mL), and dried giving a light brown
solid: 61 mg, 99 %. IR (cm-1, ATR): ν (NO) 1741, ν (OC=O) 1595. 1H
NMR (300.08 MHz, toluene-d8, ppm): δ 7.71 (s, 1H, OCHO), 3.05 (t,
2J(HP) = 18 Hz, 2H, η2-H2), 2.20 (m, 6H, PCH(CH3)2), 1.15 (m, 18H,
PCH(CH3)2), 0.82 (m, 18H, PCH(CH3)2)). 13C{1H} NMR (75.47
MHz, toluene-d8, ppm): δ 173.0 (s, OCHO), 150.3 (s), 147.1 (s),
1
by H NMR spectroscopy. The yield was calculated from the integra-
tion of the signal of the formato proton (δ 8.5, s) and that of DMF (δ
7.9, s). After removing the solvent, the residue was washed with Et2O
(3 x 2 mL), dried giving off-white solids, which were identified to be
the formate salts.
[H-TMP]+[O-CH=O]- (11). Isolated: 37 mg. Yield: 20 %. 1H NMR
(300.08 MHz, D2O, ppm): δ 8.44 (s, 1 H, CHO), 1.75 (m, 2 H), 1.65
(m, 4 H), 1.39 (s, 12 H). 1H NMR (300.08 MHz, CDCl3, ppm): δ 8.70
(s, 1 H, CHO), 3.75 (s, 2H, NH), 2.17 (m, 2 H), 1.69 (m, 4 H), 1.45 (s,
12 H). 13C NMR (75.47 MHz, D2O, ppm): δ 171.4 (s), 57.2 (s), 34.9
(s), 27.1 (s), 16.1 (s). IR (cm-1, ATR): 2947 (s), 2644 (s), 2596 (br),
2502 (br), 1587 (s, C=O). MS (ESI): m/z: 142.2 ([H-TMP]+). Anal.
Calcd for C10H21NO2 (187.16): C, 64.13; H, 11.30; N, 7.48. Found: C,
64.39; H, 11.25; N, 7.30.
139.0 (s), 136.2 (s), 23.6 (t, J(PC)
= 12 Hz, P-CH), 18.6 (s,
PCH(CH3)2). 31P{1H} NMR (121.47 MHz, toluene-d8, ppm): δ 28.5
(s). 19F NMR (282.33 MHz, toluene-d8, ppm): δ -134.81 (m, 6F,
1
ortho-C6F5), -157.74 (t, JCF = 20 Hz, 3F, para-C6F5), -164.98 (m, 6F,
meta-C6F5). Anal. Calcd for C37H45BBrF15NO3P2Re (1175.60): C,
37.80; H, 3.86; N, 1.19. Found: C, 38.01; H, 3.74; N, 1.11.
Reaction of 4a with Et3SiH. In a 3 mL Young-tap NMR-tube, 12 mg of
4a (0.01 mmol) was mixed with 3.2 μL Et3SiH (0.02 mmol) in 0.6 mL
o
of toluene-d8. After being kept at 100 C for 5 min, the solution turned
[H2NiPr2]+[O-CH=O]- (16). Isolated: 18 mg. Yield: 12 %. 1H NMR
(300.08 MHz, CDCl3, ppm): δ 8.58 (s, 1 H, CHO), 3.75 (br, 2H, NH),
from light brown to bright pink. NMR spectroscopy indicated the
disappearance of the starting materials (absence of a formate OCHO
signal at 7.7 ppm), and the formation of several species.
3
3
3.42 (m, JHH = 6 Hz, 2H), 1.44 (d, JHH = 6 Hz, 12H). 13C NMR
(75.47 MHz, CDCl3, ppm): δ 168.1 (s), 47.3 (s), 19.2 (s). IR (cm-1,
ATR): 2979 (s), 2864 (br), 2676 (s), 2492 (br), 1626 (s, C=O), 1555
(s). MS (ESI): m/z: 102.4 ([H2NiPr2]+).
1
1) [HPiPr3][HB(C6F5)3] (5, 32 %). H NMR (300.08 MHz, toluene-
3
1
d8, ppm): δ 4.40 (dxq, 1H, JHH = 6 Hz, JHP = 453 Hz, PH). 31P{1H}
NMR (121.47 MHz, toluene-d8, ppm): δ 42.6 (s). 11B NMR (96.28
[H2NCy2]+[O-CH=O]- (17). Isolated: 30 mg. Yield: 13 %. 1H NMR
(300.08 MHz, D2O, ppm): δ 8.31 (s, 1 H, CHO), 3.06 (m, 2H, N-CH),
1.16-1.91 (m, 20 H). H NMR (300.08 MHz, CDCl3, ppm): δ 8.60 (s,
MHz, toluene-d8, ppm): δ -19.45 (d, JHB = 96 Hz). 19F NMR (282.33
1
1
MHz, toluene-d8, ppm): δ -134.19 (m, 6F, ortho-C6F5), -164.48 (m, 3F,
para-C6F5), -167.95 (m, 6F, meta-C6F5);
1 H, CHO), 3.74 (br, 2H, NH), 3.00 (m, 2H, N-CH), 1.23-2.09 (m,
20H). 13C NMR (75.47 MHz, D2O, ppm): δ 171.4 (s), 53.6 (s), 29.9
(s), 25.2 (s), 24.5 (s). ). 13C NMR (75.47 MHz, CDCl3, ppm): δ 168.1
(s), 52.8 (s), 29.2 (s), 25.1 (s), 24.8 (s). IR (cm-1, ATR): 2936 (s),
2857(s), 2756 (s), 2671 (s), 1630 (s, C=O), 1548 (s). MS (ESI): m/z:
182.1 ([H2NCy2]+). Anal. Calcd for C13H25NO2 (227.19): C, 68.68; H,
11.08; N, 6.16. Found: C, 68.72; H, 10.98; N, 6.05.
2) 1a (43 %), characterized by its purple color in solution. 31P{1H}
NMR (121.47 MHz, toluene-d8, ppm): δ 41.1 (s);
3) [ReHBr(NO)(PiPr3)(η4-toluene)] (6, 25 %). 1H NMR (300.08
2
MHz, toluene-d8, ppm): δ -7.61 (d, 1H, JHP =22 Hz, ReH). 31P{1H}
NMR (121.47 MHz, toluene-d8, ppm): δ 71.5 (s);
4) (Et3SiO)2CH2 (7). 1H NMR (300.08 MHz, toluene-d8, ppm): δ
4.96 (s); 5) C=O-B(C6F5)3. 19F NMR (282.33 MHz, toluene-d8, ppm):
δ -134.19 (m, 6F, ortho-C6F5), -159.81 (m, 3F, para-C6F5), -164.76 (m,
6F, meta-C6F5).
Computational Methods. All calculations were performed with the
28
Gaussian03 program package 27 using the functional B3LYP in com-
bination with the Stuttgart/Dresden effective core potentials (SDD)
29
Reduction of CO2 with Et3SiH catalyzed by “1a/B(C6F5)3”. In a 3 mL
Young-NMR-tube, 0.25 mmol of Et3SiH (38 μL) was mixed with 0.6
mL of benzene-d6. Then the combination of the catalysts 1a (1.5 mg,
0.0025 mmol) or 1b (2.1 mg, 0.0025 mmol) and B(C6F5)3 (2.0 mg,
0.0038 mmol) were put on the top of the tube avoiding early mixing
with the hydrosilane before charging CO2. The N2 atmosphere was
replaced with 1.0 bar of CO2 using a freeze-pump-thaw cycle. Mixing
all the components afforded immediately a light-yellow solution. The
basis set for Re and the standard 6-31+G(d)30 for the remaining
atoms. Sum of electronic and zero-point energies are taken as relative
energies.
Crystallographic studies of compounds 3a and 11. Single-crystal X-ray
diffraction data were collected at 183(2) K on a Xcalibur diffractome-
ter (Agilent Technologies, Ruby CCD detector) using a single wave-
length Enhance X-ray source with MoK radiation (= 0.71073 Å )
for 3a and on a Supernova area-detector diffractometer using a high
intensity copper (Cu) X-ray micro-source (= 1.54184 Å ) for 11.31
o
mixture was kept at 80 C and the reaction progress was monitored by
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3347.; (e) Deglmann, P.; Ember, E.; Hofmann, P.; Pitter, S.; Walter, O.
The selected suitable single crystals were mounted using polybutene
Chem. Eur. J. 2007, 13, 2864.; (f) Jessop, P. G.; Ikariya, T.; Noyori, R.
Chem. Rev. 1999, 99, 475.
1
2
3
4
5
6
7
8
oil on the top of a glass fiber fixed on a goniometer head and imme-
diately transferred to the diffractometer. Pre-experiment, data collec-
32
(4) For “M/Borane”: (a) Laitar, D. S.; Müller, P.; Sadighi, J. P. J. Am.
Chem. Soc. 2005, 127, 17196.; (b) Chakraborty, S.; Zhang, J.; Krause, J. A.;
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tion, data reduction and analytical absorption corrections
were
performed with the program suite CrysAlisPro 31
.
The crystal structures
33
were solved with SHELXS97 using direct methods. The structure
refinements were performed by full-matrix least-squares on F2 with
SHELXL97.33 All programs used during the crystal structure determi-
nation process are included in the WINGX software.34 PLATON 35 was
used to check the result of the X-ray analyses. For more details about
9
the
refinements,
see
the
_exptl_special_details
and
10
11
12
13
14
15
16
17
18
19
20
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23
24
25
26
27
28
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45
46
47
48
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iucr_refine_instructions_details sections in the Crystallographic
Information files (Supporting Information). CCDC-927599 (for 3a)
and CCDC-927600 (for 11) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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8999-9045.
Supporting Information. Crystallographic details and the CIF file of 3a
and 11, computational details, and NMR spectra for CO2 activation
and reduction courses. This material is available free of charge via the
Internet at http://pubs.acs.org.
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hberke@aci.uzh.ch
The authors declare no competing financial interest.
Financial support from the Swiss National Science Foundation,
Lanxess AG, Leverkusen, Germany, the Funds of the University of
Zurich, the DFG and SNF within the project 'Forschergruppe
1175 - Unconventional Approaches to the Activation of Dihydrogen'
are gratefully acknowledged.
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Figure 1. Molecular structure of 3a with 30 % probability dis-placement ellipsoids. All hydrogen atoms were
omitted for clarity except for the formate moieties. Selected bond lengths (Å) and angles (deg): N(1)-O(1),
1.192(2); O(2)-Re(1), 2.1164(15); Re(1)-P(1), 2.4675(7); Re(1)-P(2), 2.4713(6); O(3)-B(1), 1.553(3);
C(19)-O(2), 1.246(2); C(19)-O(3), 1.268(3); O(1)-N(1)-Re(1), 174.63(18); C(19)-O(2)-Re(1), 144.74(16);
N(1)-Re(1)-O(2), 178.66(8); P(1)-Re(1)-P(2), 103.78(2); C(19)-O(3)-B(1), 126.52(18).
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H
III
Br
P
Re
P
+ BF3 + CO2
NO
F3B
1.428 Å
11.3
"FLP"
1.633 Å
2.419 Å
2.200 Å
9
H
Re
P
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F3B
Br
P
1.242 Å
1.249 Å
+ CO2
2.075 Å
O
NO
H
C
IV
H
102.3o
Br
P
2.243 Å
6.5
P
Br
P
Re
P
O
NO
V
Re
IV'
NO
3.8
+ BF3 + CO2
3.1
I
F3B
II -0.5
0.0
II' -1.2
H
O
V (model of 2)
V'
-1.6
Br
HC
O
BF3
2.341 Å
F3B
P
Br
H
O
Re
P
P
Br
P
Re
P
NO
O
HC
O
Re
+ CO2
P
C
O
NO
VI
-9.1
BF3
NO
Br
1.587 Å
Re
P
II
ON
P
1.394 Å
H
BF3
2.788 Å
P
O
BF3
Br
VII
-11.6
1.777 Å
102.1o
1.394 Å
Re
O C O
C
176.4o
P
NO
H
O
2.454 Å
Br
P
2.454 Å
BF3
BF3
Re
NO
O
C
O
O
P
IV'
O C
IV'
II'
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Figure 3. Molecular structure of 11 with 30 % probability dis-placement ellipsoids. All hydrogen atoms were
omitted for clarity except for the formate and NH2 moieties.
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O
LA
-
+
O
C
-
O
-
+
-
H
C
O
C
+
M
-
H
-
H
+
+
M
O
+
M
O
Primary
coordination sphere
LB XY LA
Secondary
M
H
LA
XY
"M-H/LA" FLP
"LB/LA" bifunctional activation
LA = Lewis acid, LB = Lewis base; M = metal center
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(C6F5)3B
CO2
benzene-d6
H
O
PR3
NO
Br
B(C6F5)3
+
H
Re
C
R3P
Br
N2 or vacuum
- CO2
O
Re
1
R3P
NO
R = iPr a, Cy b
PR3
2
2JPP = 80 Hz
trans-P
cis-P
R = iPr
ppm
2JHP
=
5.1
5.2
5.3
5.4
5.5
5.6
5.7
(C6F5)3B
18 Hz, 48 Hz
O
O
H
NO
Re-H
iPr3P
Br
PiPr3
PiPr3
Re
O
Re
1/2
iPr3P
Br
H
NO
45 44 43 42 41 40 39 38 37 36 35 34 33
ppm
31P,1H-correlation of 2a
O
3a
B(C6F5)3
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(C6F5)3B
O
Br
iPr3P
H
Re
H
O
PiPr3
H
NO
Br
1 bar H2
2eq. Et3SiH
Re
3a
6 (25 %)
toluene
60 oC, 1 h
99 %
toluene-d8
100 oC
iPr3P
H
NO
H
5 min
4a
PiPr3
Br
Re
Et3SiH
iPr3P
NO
1a
(43 %)
[HB(C6F5)3]-
Et3Si
O
[HPiPr3][HB(C6F5)3]
(32 %)
H
5
O
Et3SiH
PiPr3
Br
O
O
Re
H
Et3Si
C
SiEt3
Re-O, H-H
cleavage
iPr3P
H2
H
NO
7
8
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9
Hydride FLP formation in case B(C6F5)3 is present
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H
PR3
Br
"1/B(C F ) "
FLP
5 3
6
(C6F5)3B
+ CO2
B(C6F5)3
H
Re
O
R3P
NO
CO2
H
H
H
O
H2
PR3
H
1 + H2
Br
PR3
Br
3
Re
Re
NO
H
- H2
Rate
determining
step
R3P
R3P
H
H
NO
TMP
15
4
(4')
[H-TMP]+
(C6F5)3B
O
H
H
PR3
H
Br
H
O
Re
PR3
Br
R3P
Re
NO
14
R3P
H
NO
12
(12')
PR3
Re
H
Br
R3P
H2
O
NO
B(C6F5)3
H
O
13
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cat.
1.0 mol%
(Et3SiO)2CH2
+ Et3SiOMe
(Et Si) O
Et3SiH + CO2
+
3
2
benzene
80 oC
7
10
9
Yield (%)a
PCO
TOF
(h-1)
2
t
(h)
cat.
Entry
TON
(bar)
7 / 9 / 10
35 / 0 / 0
18 / 0 / 0
87 / 1 / 1
89 / 3 / 3
b
1
2
3
4
1a/B(C6F5)3
1
1
5
5
4
35
18
89
95
8.8
4.5
5.9
7.3
b
1b/B(C6F5)3
4
3a
4a
15
13
a Determined by GC-MS. b In a ratio of 1:1.5
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O
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[Re]/B(C6F5)3
(1:2)
CO2
+ H2
+
H2N
N
H
H
O
(0.5 mol%)
1 mmol
11
T(oC)
PH /CO
t(h) Conv.
TOF
(h-1)
Entry [Re] solvent
2
2
TON
(%)a
(bar)
1
2
3
4
5
6
1a
1a
1a
--
C6H5Cl
THF
80
80
80
80
23
120
40/20
40/20
40/20
40/20
40/20
40/20
15
15
2
8
15
104
16
0
1.0
6.9
7.5
0
52b
THF
8
THF
24
18
18
0
1a
1a
THF
0
0
0
THF
3
6
0.3
7
8
9c
1a
1b
3a
4a
1a
1a
THF
THF
80
80
80
80
80
80
20/10
40/20
40/20
40/20
40/20
40/20
15
16
15
15
15
15
4
8
0.4
3.4
4.8
6.1
5.2
3.3
28
36
46
39
25
55
72
92
78
50
THF
10c
11c
12c
THF
THF
MeOH
a Determined by referring to the internal standard DMF in 1H NMR
spectra. b in 20 % isolated yield. c in the absence of B(C6F5)3
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0.5 mol% [Re]
1.0 mol% B(C6F5)3
O
CO2
20 bar 40 bar
+
H2
base
+
H-base
H
O
THF, 80 oC
product
9
1 mmol
10
11
12
13
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t(h) Conv.
Entry [Re]
TON TOF
(h-1)
base
(%) a
1
1a
1b
1a
34
36
15
87b 174 5.1
O
2
67
27
134 3.7
54 3.6
N
N
H
H
O
H2
3c
16
4
1a
1b
1a
22
28
18
3
5
3
7
0.3
O
NEt3
5
10 0.4
H-NEt3
H
O
6c
6
0.3
--
7
1a
---
16
<1
--
PtBu3
61d 122 4.1
O
O
8
1a
1a
30
20
Cy Cy
N
Cy Cy
N
9c
34
68 3.4
H2
H
H
O
H
17
H2N(SiMe3)2
10
1a HN(SiMe3)2
16
6
2
4
0.2
--
O
tBu
11e 1a
--
--
tBu
N
---
tBu
a Determined by referring to the internal standard DMF in 1H NMR
spectra. b in 12 % isolated yield. c in the absence of B(C6F5)3. d In 13
% isolated yield. e with 10 mol% of 1a/B(C6F5)3 (1:2)
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(C6F5)3B
-
O
Et3SiH
-
(Et3SiO)2CH2
+
C
O
H
+
[Re-H]
Br
Re
Catalysis
CO2
R3P
NO
R1
+
PR3
B(C6F5)3
H2
HNR1R2
H
H
[O-CH=O]-
N
Re
H
B(C6F5)3
·····
R2
"Re-H/B(C6F5)3" FLP
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