Efficient Lewis acid promoted alkene hydrogenations using dinitrosyl rhenium(-I) hydride catalysts

Article abstract of DOI:10.1021/om400733u

Highly efficient alkene hydrogenations were developed using NO-functionalized hydrido dinitrosyl rhenium catalysts of the type [ReH(PR ₃)₂(NO)(NO(LA))][Z] (2, LA = B(C₆F ₅)₃; 3, LA = [Et]⁺, Z = [B(C₆F ₅)₄]^-; 4, LA = [SiEt₃]⁺, Z = [HB(C₆F₅)₃]^-; R = iPr a, Cy b). Lewis acid attachment to the NO ligand was found to facilitate bending at the N_OLA atom and concomitantly to open up a vacant site at the rhenium center. According to DFT calculations, the ability to bend follows the order 4 > 3 > 2, which did not match with the order of increasing hydrogenation activities: 3 > 4 > 2. The main factor spoiling catalytic performance was catalyst deactivation by detachment of the LA group occurring during the catalytic reaction course, which was found to go along with the decrease in order of DFT-calculated strengths of the O_NO-LA bonds. LA detachment from the O_NO atom could at least partly be prevented by LA addition as cocatalysts, which led to an extraordinary boost of the hydrogenation activities. For instance the 1/hydrosilane/B(C₆F ₅)₃ (1:5:5) system exhibited the highest performance, with TOFs up to 1.2 × 10⁵ h^-1 (1-hexene, 1-octene, cyclooctene, cyclohexene). The cocatalyst [Et ₃O][B(C₆F₅)₄] showed the smallest effect, presumably due to the strong Lewis acidic character of the reagent causing side-reactions before reacting with 1a,b. The catalytic reaction course crucially involves not only reversible bending at the N_OLA atom but also loss of a PR₃ ligand, forming 16e or 14e monohydride reactive intermediates, which drive an Osborn-type hydrogenation cycle with olefin before H₂ addition.

Full text of DOI:10.1021/om400733u

Article
pubs.acs.org/Organometallics
Efficient Lewis Acid Promoted Alkene Hydrogenations Using
Dinitrosyl Rhenium(−I) Hydride Catalysts
Yanfeng Jiang, Wenjing Huang, Helmut W. Schmalle, Olivier Blacque, Thomas Fox, and Heinz Berke*
Anorganisch-Chemisches Institut, Universitat Zurich, Winterthurerstraße 190, CH-8037, Zurich, Switzerland
̈
̈
S
* Supporting Information
ABSTRACT: Highly efficient alkene hydrogenations were developed using NO-
functionalized hydrido dinitrosyl rhenium catalysts of the type [ReH-
(PR3)2(NO)(NO(LA))][Z] (2, LA = B(C6F5)3; 3, LA = [Et]+, Z =
[B(C6F5)4]−; 4, LA = [SiEt3]+, Z = [HB(C6F5)3]−; R = iPr a, Cy b). Lewis
acid attachment to the NO ligand was found to facilitate bending at the NOLA
atom and concomitantly to open up a vacant site at the rhenium center.
According to DFT calculations, the ability to bend follows the order 4 > 3 > 2,
which did not match with the order of increasing hydrogenation activities: 3 > 4
> 2. The main factor spoiling catalytic performance was catalyst deactivation by
detachment of the LA group occurring during the catalytic reaction course, which was found to go along with the decrease in
order of DFT-calculated strengths of the ONO−LA bonds. LA detachment from the ONO atom could at least partly be prevented
by LA addition as cocatalysts, which led to an extraordinary boost of the hydrogenation activities. For instance the “1/
hydrosilane/B(C6F5)3” (1:5:5) system exhibited the highest performance, with TOFs up to 1.2 × 105 h−1 (1-hexene, 1-octene,
cyclooctene, cyclohexene). The cocatalyst [Et3O][B(C6F5)4] showed the smallest effect, presumably due to the strong Lewis
acidic character of the reagent causing side-reactions before reacting with 1a,b. The catalytic reaction course crucially involves not
only reversible bending at the NOLA atom but also loss of a PR3 ligand, forming 16e or 14e monohydride reactive intermediates,
which drive an Osborn-type hydrogenation cycle with olefin before H2 addition.
Scheme 1. NO Function in Transition Metal Based Catalysis
1. INTRODUCTION
In recent years tuning of middle transition metal complexes for
homogeneous catalysis became a major research challenge,
avoiding the use of platinum group metals.1 Middle transition
metals in general tend to obey the 18e rule and are reluctant to
form complexes with low coordination numbers; consequently
they show little tendency for ligand exchanges as crucial
properties for catalytic reaction courses. Tuning of the ancillary
ligand sphere by ligand effects for self-labilization or labilization
of other ligands turns out to be the only way to eventually
establish appropriate conditions for a well-functioning catalytic
cycle and to render high catalytic activities. Ligand properties
used to stabilize 16- or 14-electron intermediates are the cis
labilization effect,2 the trans influence and trans effect,3
noninnocent ligand properties,4 and adjustment of the π-
accepting properties of ligands and of their steric properties via
incorporation of large Tolman cone angle ligands and/or large
bite angle chelate ligands.5 In line with some of the listed
properties is the nitrosyl ligand, which possesses three
prominent functions in transition metal based catalysis
(Scheme 1).6 Two of these functions (1 and 2 of Scheme 1)
became the main focus of our group in the exploration of
efficient catalyses based on middle transition metal complexes.
(1) First, the nitrosyl ligand can serve as an ancillary ligand
exhibiting strong electron-withdrawing σ- and π-effects/
influences to activate the trans ligand.7 For σ-donors, such as
the hydride ligand, and π-acceptors such as a H2 ligand trans to
NO, NO may cause enhancement of the hydridic or acidic
property, respectively, prominent in secondary coordination
sphere activation processes.8 For π-donors trans to NO, such as
halide ligands, appropriate Lewis acidic cocatalysts often can
support the elimination of these ligands and facilitate
generation of highly electron-deficient metal centers with one
vacant site. This was exemplified by the boron Lewis acid
promoted catalytic activities of mononitrosyl rhenium bromo
complexes in alkene hydrogenation reactions, where the
trapping of the bromo ligand by external boron Lewis acid
serves as the key initiating step.1a
Received: July 25, 2013
Published: November 4, 2013
© 2013 American Chemical Society
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(2) The nitrosyl ligand can be considered as a “noninnocent”
ligand capable of supporting different oxidation states of metal
centers via either linear (M−N−O of 160−180°) or bent (M−
N−O of 110−140°) coordination modes.9 Bending triggers a
NO+/NO− transformation, which is equivalent to a formal 2e
oxidation of the metal center and often accompanied by a
change in the coordination geometry with formation of a vacant
site.10 This redox change induced by NO bending was also
termed “stereochemical control of valence” by Enemark and
Feltham.11 It is anticipated to have great impact on the
activation of molecules in catalytic reaction courses, allowing
them “silent” addition to metal centers without ligand
exchange. Exploiting this property we recently reported a
silylium cation triggered nitrosyl bending, boosting perform-
ance of mononitrosyl rhenium diiodo catalysts to high
efficiencies in alkene hydrogenations, denoted as the “catalytic
nitrosyl effect”.12 In situ NMR spectroscopy and DFT
calculations verified the facilitation of NO bending upon
attachment of the silylium cation to the ONO atom. This newly
formed intermediate in turn promoted heterolytic H−H
splitting and subsequent protonation of a phosphine ligand.
(3) The nitrosyl ligand can serve as a “nitrosyl/nitro” redox
couple, supporting secondary oxo- or oxene-transfer reac-
tions,13 which however are not targets of this paper.
Scheme 2. Synthesis of Lewis Acid Functionalized Dinitrosyl
Rhenium(−I) Hydrido Complexes
low NO bond order, which may be taken as an indication of
enforced strong electron back-donation from the metal to the
π* orbital of the NO ligand.
In order to take advantage of properties 1 and 2 of the NO
ligand in catalytic reaction courses (Scheme 1), the attachment
of the Lewis acids (LA) to the NO ligand was intended to be
probed, however not by starting from a cocatalytic mixture of
the complex and LA, rather than by creating a single-
component catalyst attaching the Lewis acid to the ONO
atom prior to catalysis. The halide-free hydrido dinitrosyl
rhenium complexes [ReH(PR3)2(NO)2] (1, R = iPr a, Cy b),
which are five-coordinate 18e, d8 Re(−I) complexes possessing
a TBP coordination geometry,14 are eventually used as base
components in homogeneous alkene hydrogenations. The ONO
atoms were expected to be Lewis basic enough to enable
attachment of Lewis acids.15 ONO functionalization of 1a,b was
carried out with the following Lewis acids, the neutral
B(C6F5)3, and the cationic [Et]+ and [SiEt3]+ moieties16 to
induce the expected effects in catalytic hydrogenations.
Similarly, the reactions of 1a,b with 1 equiv of [Et3O][B-
(C6F5)4] in diethyl ether afforded at 23 °C within 1 h the
cationic ethyl-derivatized dinitrosyl complexes [ReH-
(PR3)2(NO)(NOEt)][B(C6F5)4] (3, R = iPr a, Cy b) in 93%
1
(3a) and 86% (3b) yield, respectively. In the H NMR spectra
the hydride resonances were found shifted low-field to 5.50
(3a) and 5.41 (3b) ppm in comparison to those of 2a and 2b,
indicating higher electron deficiencies at the rhenium centers,
which seemed reasonable mainly as a consequence of the
positive charge of 3a,b. The signal of the ethyl group was in
either case observed as a quartet at 4.77 ppm (2JHH = 7 Hz) and
a triplet at 1.44 ppm (2JHH = 7 Hz) for 3a and at 4.76 ppm
(2JHH = 6 Hz) and 0.90 ppm (2JHH = 6 Hz) for 3b. In the
31P{1H} NMR spectra singlet resonances appeared at 59.2 ppm
for 3a and at 49.1 ppm for 3b, shifted low-field with respect to
those of 1a and 1b.14a In the IR spectra the vibrations of the
“ethyl-free” NO groups were observed at 1664 for 3a and at
1670 cm−1 for 3b. The vibrations of the ethyl-bonded NO
groups were found at the very low wavenumbers of 1252 (3a)
and 1249 cm−1 (3b), indicative of a strongly decreased N−O
bond order.
2. RESULTS AND DISCUSSION
2.1. Synthesis and Characterization of NO-Function-
alized Dinitrosyl Hydrido Rhenium(−I) Complexes. The
reactions of 1a,b with various Lewis acids led indeed to
formation of the isolable and stable ReNO-LA adducts 2a,b,
3a,b, and 4a,b, as depicted in Scheme 2. The preparations of
compounds 2a and 3a were described in a previous short
communication by our group.17 The [ReH(PR3)2(NO)(NO-
B(C6F5)3)] derivatives (2, R = iPr a, Cy b) were obtained in
quantitative yields by treating 1a,b with 1 equiv of B(C6F5)3 in
toluene at 23 °C within 30 min. The 1H NMR spectra revealed
characteristic triplets for the hydride ligand, which were shifted
slightly high-field to 4.07 (2a) and 4.17 (2b) ppm in
comparison with those of 1a and 1b.14a The 31P{1H} NMR
spectra exhibited a singlet at 53.5 for 2a and at 41.3 ppm for 2b,
indicative of two chemically equivalent phosphorus nuclei in
each coordination environment. In the IR spectra the ν(NO)
vibration of the B(C6F5)3-bonded ligand appeared at the low
wavenumbers of 1340 for 2a and 1339 cm−1 for 2b, shifted by
over 300 cm−1 to lower wavenumbers in comparison with those
of the “free” nitrosyl ligands at 1641 (2a) and 1644 cm−1 (2b).
The lower wavenumbers of B(C6F5)3-attached ligands reflect a
It has been reported by Piers and Oestreich that the reaction
of B(C6F5)3 with hydrosilanes affords a reactive adduct
described by the B··H−Si ↔ B−H··Si hyperconjugative
resonance structures.18 These reagents constitute crypto
silylium cation sources capable of derivatizing particularly
oxygen functions due to the high oxophilicity of the silicon
atom. When a mixture of 1.0 equiv of B(C6F5)3 and 1.5 equiv of
Et3SiH in toluene was treated with 1a,b at 23 °C, ONO
functionalization took place within 5 min with formation of
the [ReH(PR3)2(NO)(NOSiEt3)][HB(C6F5)3] salts (4, R =
iPr a, Cy b) in quantitative in situ yields and in 53% (4a) and
67% (4b) isolated yields. Silylium binding to the ONO atom was
confirmed also via 29Si NMR spectroscopy, since singlets were
observed for both 4a and 4b at 41.0 ppm, which appeared in
the typical chemical shift range of [R3SiO]+ compounds.19 In
1
the H/29Si correlation spectra the 29Si NMR singlets of both
derivatives showed correlation with the 1H NMR resonances of
the ethyl groups appearing at 0.56 (t) and 0.33 (q) ppm for 4a
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Figure 1. Molecular structures of 2a, 2b, 3a, and 3b with 30% probability displacement ellipsoids. All hydrogen atoms were omitted for clarity except
for the HRe atoms. The solvent molecules and the [B(C6F5)4]− counteranion in 3a and 3b were also omitted for clarity.
and at 0.83 (t) and 0.76 (q) ppm for 4b. The HRe signals were
slightly shifted low-field to 4.38 for 4a and to 4.71 ppm for 4b
in comparison with those of 1a and 1b. Similar downfield
chemical shifts were observed in the 31P NMR spectra,
revealing singlets at 57.7 for 4a and at 47.0 ppm for 4b,
implying also electron deshielding of the rhenium centers.
Moreover, evidence for the presence of the [HB(C6F5)3]−
counterions was provided by the 11B NMR spectra, showing a
doublet resonance at −24.5 for 4a and at −24.8 ppm for 4b.
The structure of 4a was further substantiated by a NOE
experiment, wherein dipolar H···H couplings were detected
between the HRe atom and the SiEt3 group established through
irradiation of the isopropyl CH signal of 4a at 1.98 ppm. ESI-
MS spectroscopy confirmed for 4a and 4b the presence of both
the Re(−I) cations and the borate anions, respectively. In the
IR spectra of 4a and 4b ν(NO) vibrations of the silylium-
derivatized NO groups appeared at the low wavenumbers of
1340 (4a) and 1296 cm−1 (4b) apparently due to drainage of
electron density from the rhenium to the NO ligand. It should
also be noted at this point that the structurally related
complexes bearing [BAr′4]− anions could previously be
accessed from the reaction of the cationic 16e complexes
[Re(PR3)2(NO)2][BAr′4] with Et3SiH through H−Si bond
heterolytic cleavage.14b
Further structural proof for these Lewis acid adducts was
established by X-ray diffraction studies of single crystals of 2a,
2b, 3a, and 3b. Structural models are displayed in Figure 1. In
comparison to 1a,b,14a the coordination of the Lewis acid to the
ONO atom of the nitrosyl ligand only slightly changes the
coordination geometries at the rhenium center. Distorted TBP
geometries were invariably seen to be adopted in the four
molecular structures bearing the two types of NO groups and
one hydride in the trigonal plane and the two phosphine
ligands occupying apical positions. The P−Re−P angles in the
range ca. 135−143° deviate strongly from linearity (Table 1).
These moieties show even stronger bending toward the HRe
atom than in the structure of 1a (154°).14a This can be
interpreted in terms of enhancement of π-back-donation from
the rhenium center to the NO ligand.20 The bending of the
phosphines polarizes the rhenium dxz orbital, providing better
dxz−π*NO overlap. Steric repulsion of the bulky B(C6F5)3
groups with the phosphines in 2a,b might also contribute to
the decrease in P−Re−P angles. Evidently the Lewis acid
coordinates to the nitrosyl ligand in an attracto fashion, leaning
over toward the other NO ligand. Noteworthy is the fact that
the Lewis acid functionalized nitrosyl ligands remained
essentially linear in all four cases of structures, showing Re−
Table 1. Selected Bond Distances (Å) and Angles (deg) for
Complexes 2a, 2b, 3a, and 3b
parameter
2a
2b
3a
3b
Re−N1
Re−N2 (LA)
Re−P1
Re−P2
N1−O1
1.792(4)
1.794(3)
2.4274(10)
2.4252(10)
1.186(5)
1.261(4)
134.77(3)
167.9(4)
175.0(3)
126.09(16)
0.46
1.771(7)
1.763(7)
2.439(2)
2.430(2)
1.232(9)
1.294(9)
141.71(6)
168.3(6)
171.1(5)
126.6(3)
0.63
1.776(3)
1.755(3)
2.4169(10)
2.4249(9)
1.197(4)
1.319(4)
143.39(3)
164.5(3)
172.5(3)
126.11(16)
0.62
1.788(3)
1.775(3)
2.4295(9)
2.4185(10)
1.180(4)
1.317(4)
139.60(3)
167.4(3)
171.3(3)
129.39(16)
0.59
N2−O2(LA)
P1−Re−P2
O1−N1−Re
O2−N2−Re
N1−Re−N2
a
τ
a
τ = (β − α)/60; see text.
N−O angles in the range ca. 171−175°. The Re−N bond
distances are short, lying in the range ca. 1.76−1.79 Å, similar
to those of linear nitrosyls.21 The “Lewis acid free” nitrosyls
possess structures with NO double-bond character, showing
distances of ca. 1.18−1.23 Å.21 In comparison, the Lewis acid
coordinated N−O bonds are lengthened (range of ca. 1.26 to
1.32 Å), approaching N−O single-bond character. This is again
an indication of strong π-back-donation from the Re dxz orbital
to the π*NO orbital, which is also reflected in the shortened
Re−N bonds of the Re-NOLA moieties in comparison with the
“Lewis acid free” compounds 1a and 1b. This change in the
Re−N and N−O bond distances upon Lewis acid attachment is
more pronounced in the structures of 3a,b than in those of
2a,b, emphasizing the stronger electron-withdrawing effect of
the [Et]+ cation than of the neutral B(C6F5)3 moiety. As the
linear nitrosyl is formally viewed as a [NO]+ unit, an 18e
configuration was assigned to the d8 Re(−I) centers.
In order to put these structural results in more systematic
context for pentacoordinate structures, we employed the
structural index parameter τ, which is an angular parameter
introduced by Addison et al.22 as τ = (β − α)/60 to
differentiate between the TBP and SP geometries. As depicted
in Scheme 3, the Lewis acid functionalized nitrosyls take the
axial position, the largest basal angle (P−Re−P) is defined as β,
and the second largest basal angle (H−Re−NO) is defined as
α. The parameter τ is formulated as τ = (β − α)/60. In a
perfect TBP with β = 180° and α = 120° τ amounts to 1. On
the other hand, an ideal square pyramid has α = β = 180°, and
its angular parameter τ is 0. Therefore, any structural
intermediate between SP and TBP must have an angular
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energies of the ONO-LA moieties were found to possess the
order [Et]+ > [SiMe3]+ > BF3. Coordination of BF3 to the ONO
atom of [Re] is accompanied by an energy release of 11.3 kcal/
mol. In comparison, [Et]+ and [SiMe3]+ are more strongly
bound to the ONO atom, showing an energy release of −83.3
and −64.9 kcal/mol, respectively.
Scheme 3. Definition of the Parameter τ
Subsequently the energy required for NO bending was
studied. The optimized ground-state structures possess linear
Re−N−OLA arrangements. Structures kept artificially at a Re−
N−OLA bending angle of 125° at the N atom are shown in
Figure 2. The “Lewis acid free” complex [Re] requires an
energy of +7.7 kcal/mol to adopt a Re−N−O bending angle of
135°. Decreasing the Re−N−O angle further to 125° resulted
in the higher energy of +12.9 kcal/mol. In comparison, the
[Re-BF3] complex required a lower bending energy of +6.4
kcal/mol to reach an angle of 135°, and further bending to 125°
indeed resulted in the relatively low-energy demand of +9.7
kcal/mol. This Lewis acid triggered nitrosyl bending became
even more facilitated in the cationic [Re-Et]+ and [Re-SiMe3]+
model complexes, where the small energy demands of +4.3 or
+3.3 kcal/mol were calculated upon Re−N−O bending to 135°
for [Re-Et]+ or [Re-SiMe3]+, respectively. For the enhanced
bending angle of 125° the relatively low energies of +5.4 kcal/
mol for [Re-Et]+ and +5.1 kcal/mol for [Re-SiMe3]+ were
required. The more facile bending of the NO ligand with the
[Et]+ and [SiMe3]+ cations attached is interpreted in terms of a
stronger shift of electron densities from the rhenium to the
NNO atom, forming a lone pair at this atom, facilitating the
bending.
parameter τ in the range 0 ≤ τ ≤ 1. Thus, the angular
parameter τ tells us how far or close a particular structure is
from the ideal TBP or SP. Applying this definition, the
parameter τ for 1a is calculated to be 0.53, which can be taken
as a reference. In comparison, τ values of 0.46, 0.63, 0.62, and
0.59 were obtained for 2a, 2b, 3a, and 3b, respectively. These
data emphasize the presence of geometries in between TBP and
SP, but except for 2a the adopted geometries are slightly closer
to TBPs on going from 1a,b to the Lewis acid derivatized
nitrosyl ligands, which is in agreement with the observation that
generally TBPs of five-coordinate d8 complexes are more stable
than the corresponding SP geometries.23
2.2. DFT Evaluations of Lewis Acid Induced Nitrosyl
Bending. In order to further evaluate the bonding between the
ONO atom and the Lewis acid, DFT calculations were carried
out at the B3LYP level with an SDD basis set for Re and 6-
31+G(d) for the other atoms using a PMe3 model complex
ReH(NO)2(PMe3)2 ([Re]) and BF3, [Et]+, and [SiMe3]+
attached to one ONO atom in an attracto fashion. The geometry
optimizations led to local and global minima of [Re-BF3], [Re-
Et]+, and [Re-SiMe3]+ structures. In general, the association
Figure 2. Optimized ground states of [Re], [Re-BF3], [Re-Et]+, and [Re-SiMe3]+ upon bending of the Re−N−OLA angle to 125° with relative
+
energies of the local minima (ΔE, kcal/mol). All the carbon and hydrogen atoms except for the C atoms of the Et+ and SiMe3 groups and the
hydride ligand are omitted for clarity.
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Table 2. Relative Energies, Geometry Parameters of the Local Minima, and τ Values Obtained upon Bending of Nitrosyl Ligand
[Re]
[Re-BF3]
[Re-Et]+
[Re-SiMe3]+
Re−N−O (deg)
172.9
135.0
125.0
177.4
135.0
125.0
177.3
135.0
125.0
177.2
135.0
125.0
ΔE (kcal/mol)
H−Re−N1 (deg)
P1−Re−P2 (deg)
N1−Re−N2 (deg)
Re−N1 (Å)
Re−N2 (LA) (Å)
N1−O1 (Å)
N2−O2 (LA) (Å)
ν(NO)
0.0
+7.7
+12.9
146.7
161.8
105.6
1.809
1.879
1.206
1.220
0.0
+6.4
+9.7
0.0
+4.3
+5.4
0.0
+3.3
+5.1
116.8
157.8
126.4
1.815
1.815
1.205
1.205
143.7
159.4
108.4
1.806
1.852
1.207
1.213
108.8
160.6
126.9
1.807
1.790
1.195
1.257
1744
1511
0.86
138.4
160.9
109.1
1.799
1.806
1.200
1.270
1716
1371
0.38
145.2
162.5
105.2
1.806
1.825
1.198
1.280
1718
1312
0.29
103.6
147.5
127.0
1.804
1.789
1.186
1.302
1784
1313
0.73
137.5
150.3
107.3
1.797
1.796
1.189
1.325
1765
1171
0.21
146.0
152.2
103.8
1.808
1.808
1.188
1.337
1765
1098
0.10
105.8
146.6
126.2
1.808
1.783
1.188
1.302
1774
1374
0.68
140.4
149.2
107.2
1.803
1.793
1.190
1.321
1757
1215
0.15
146.5
151.2
104.0
1.811
1.806
1.189
1.334
1758
1138
0.08
ν(N-OLA)
τ
0.68
0.26
0.25
a
Table 3. Hydrogenation of Alkenes Catalyzed by Rhenium Dinitrosyl Complexes with or without Cocatalysts
entry
alkene (mmol)
[Re]
cocatalyst (5 equiv)
time (h)
conversion (%)
TON
2405
TOF (h−1
)
1
1-hexene (20)
1-hexene (20)
1-hexene (20)
1-hexene (20)
1-hexene (20)
1-hexene (20)
1-hexene (20)
1-hexene (20)
1-hexene (20)
1-hexene (20)
1-hexene (20)
1-hexene (20)
1-hexene (20)
1-hexene (20)
cyclooctene (16)
cyclooctene (16)
cyclooctadiene (16)
1,7-octadiene (14)
1-hexene (24)
1-hexene (24)
1-hexene (24)
1-hexene (24)
1-octene (16)
1-octene (16)
cyclooctene (16)
cyclooctene (16)
cyclohexene (25)
cyclohexene (25)
1a
1b
2a
2a
2b
2b
3a
3a
3b
3b
4a
4a
4b
4b
3a
3b
3b
3b
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
15.0
39.0
4.0
24
160
2
94
9400
241
3
83
8260
2065
9778
2222
2.5 × 104
5000
4
B(C6F5)3
1.0
98
9778
5
4.5
100
100
100
92
1.0 × 104
1.0 × 104
1.0 × 104
9240
6
B(C6F5)3
24/60
2.0
7
8
[Et3O][B(C6F5)4]
[Et3O][B(C6F5)4]
Et3SiH/B(C6F5)3
Et3SiH/B(C6F5)3
2.0
4600
9
1.7
100
99
1.0 × 104
9930
5882
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
1.5
6620
3.0
76
7600
2533
45/60
2.5
100
80
1.0 × 104
8022
1.3 × 104
3209
18/60
1.5
100
100
100
1.0 × 104
8000
3.3 × 104
5333
1.0
8000
8000
b
7.0
66
5280
754
16.0
8/60
8/60
20/60
22/60
5/60
4/60
30/60
20/60
40/60
45/60
100
76
7000
437
Me2PhSiH/B(C6F5)3
Me2PhSiH/B(C6F5)3
Et3SiH/B(C6F5)3
9176
6.9 × 104
8.2 × 104
3.5 × 104
3.2 × 104
9.6 × 104
1.2 × 105
1.4 × 104
1.7 × 104
1.5 × 104
1.4 × 104
91
1.1 × 104
1.2 × 104
1.2 × 104
8000
97
Et3SiH/B(C6F5)3
99
Me2PhSiH/B(C6F5)3
Me2PhSiH/B(C6F5)3
Me2PhSiH/B(C6F5)3
Me2PhSiH/B(C6F5)3
Me2PhSiH/B(C6F5)3
Me2PhSiH/B(C6F5)3
100
100
85
8000
6792
71
5714
82
1.0 × 104
1.1 × 104
86
a
Reactions were performed in a steel autoclave using 0.002 mmol of rhenium complex under 40 bar of H2 at 100 °C. In the case of application of
cocatalysts, these were added in a 5:1 molar ratio with respect to the Re catalyst. The reaction progress was monitored by the decrease in H2
pressure. With a distribution of 61% of cyclooctene and 39% of cyclooctene.
b
The geometric parameters of each structural state are listed
in Table 2. Importantly, along the bending course the H−Re−
NO angles open up drastically with concomitant decrease of
the N−Re−N angles, indicating a geometric transformation
from TBP (ground state) toward SP at the NO bending angle
of 125°. In comparison, the P−Re−P angle displayed an only
slight increase during the bending course. The geometry change
became still more evident when comparing the structural index
parameter τ of each state. When BF3 is coordinated, the τ value
sharply decreases from 0.86 in the ground state to 0.29 in the
bent form (125°), approaching indeed a distorted SP geometry.
In the case of [Re-Et]+ and [Re-SiMe3]+, the bent form at 125°
is much closer to an ideal SP coordination, exhibiting a τ value
of 0.10 and 0.08, respectively. The distortion toward the SP
geometry triggered by the bending of the apical NOLA ligand
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Scheme 4. Proposed Catalyst Activation Pathways
creates a vacant site opposite of this ligand, expected for
complexes with a d6 configuration.
(entry 3). Further improvement of the activities was
accomplished with [Et3Si]+ functionalization of 4a,b, which
afforded TONs of 7600 for 4a and 8022 for 4b within 3.0 and
2.5 h, corresponding to TOFs of 2533 h−1 (4a) and 3209 h−1
(4b), respectively. The [Et]+-functionalized complexes 3a,b
proved to be the most active ones. A TON of 1.0 × 104 and a
TOF of 5000 h−1 were achieved with 3a within 2.0 h at full
conversion (entry 7). 3b showed slightly better performance
than 3a, affording 100% conversion within 1.7 h, corresponding
to a TOF of 5882 h−1 (entry 9). Other alkenes, such as
cyclooctene, cyclooctadiene, and 1,7-octadiene, were also tested
as substrates in the hydrogenations with 3a,b as catalysts,
performing best in the given series of experiments. Surprisingly
the reactions of the disubstituted olefin cyclooctene turned out
to be very efficient. Within 1.5 h for 3a and 1.0 h for 3b not
only were full conversions accomplished but also a maximum
TOF of 8000 h−1 (entry 16) was reached. In contrast, the
related hydrogenations of dienes required a relatively long
reaction time to reach reasonable TONs (entries 17 and 18).
Generally, no alkene isomerized or polymerized product was
observed in the case of full conversion, and only trace amounts
(<1%) of isomerized alkenes were detected when the
hydrogenations were not completed.
2.3. NO-Functionalized Hydrido Complexes as Cata-
lysts in Hydrogenations of Alkenes. The generation of a
vacant site via Re−N−O or Re−N−OLA bending was
anticipated to be beneficial or even crucial for the catalytic
performance of rhenium compounds, which normally prefer
18e coordination spheres and show low tendency for ligand
exchange reactions. Assuming the capability of facile Re−N−
OLA bending of 1a,b−4a,b, the catalytic potential in
hydrogenation of alkenes was still to be manifested.24
For the different series of catalysis experiments conditions
were chosen to be as much as possible comparable with each
other to allow comparison. Typically solvent-free conditions
were applied using 2.5 mL of 1-hexene (20 mmol) with 0.01
mol % of the rhenium catalyst under 40 bar of H2 at 100 °C.
The results for 1-hexene, cyclic alkenes (standing for
disubstituted alkenes), and dienes are listed in Table 3. The
non-LA-functionalized complexes 1a,b showed low hydro-
genation activities, despite their relative moderate energy
demand for NO bending to offer vacant sites. Applying 1a as
catalyst in 1-hexene hydrogenation, a conversion of 24% was
accomplished within 15 h, corresponding to a TOF of 160 h−1
(entry 1). 1b afforded a TON of 9400, but only after the long
reaction time of 39 h (entry 2). In contrast to this, the NO-
functionalized complexes 2a,b−4a,b demonstrated significant
catalytic accelerations. For instance, the hydrogenation with 2b
proceeded quickly, affording a TON of 1.0 × 104 and a TOF of
2222 h−1 within 4.5 h (entry 5). 2a turned out to perform
slightly less well than 2b and furnished a TON of 8260 at 83%
conversion after 4 h, corresponding to a TOF of 2065 h−1
The order of the activities of the catalysts was found to be
3a,b > 4a,b > 2a,b, correlating with the calculated order of the
ONO−LA bond strength of the [Re-Et]+, [Re-SiMe3]+, and
[Re-BF3] species. For 2a,b, showing the weakest Lewis acid
attachment, we anticipated a too easy loss of the coordinated
Lewis acid approaching the poor catalytic level of 1a,b.
Therefore, we reckoned that addition of a stoichiometric
excess of B(C6F5)3 (5 equiv) as a cocatalyst to 2a,b could
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improve the performance of the olefin hydrogenations by
pushing the dissociation equilibrium of the Lewis acid to the
side of 2a,b. Much to our pleasure, the reaction catalyzed with
0.01 mol % of the “2b/B(C6F5)3” system (1:5) afforded full
conversion within 24 min, corresponding to a TON of 1.0 ×
104 and a TOF of 2.5 × 104 h−1 (entry 6), which suggested an
11 times acceleration with respect to the case of no extra
B(C6F5)3 (entry 5). Similarly, the “2a/B(C6F5)3” system (1:5)
showed a 5 times increase in activity for the hydrogenation of
1-hexene, affording a TOF of 9778 h−1 within 1.0 h (entry 4). It
should be mentioned that formation of the PR3/B(C6F5)3
addition products [(R3P)(C6F4)BF(C6F5)2] (R = iPr, Cy)
reported by Stephan et al.25 was not observed after catalyses,
excluding the ability of B(C6F5)3 to abstract a phosphine ligand
from the rhenium center. These results strongly suggested that
the lower activities of the 2a,b catalysts with no extra B(C6F5)3
added were mainly due to ONO−B(C6F5)3 bond cleavages and
subsequent B(C6F5)3 scavenging in the catalytic course by
various nucleophiles evolved in the catalytic reactions.
The same trick to add extra Lewis acid was applied using 4a,b
as catalysts. The addition of 5 equiv of “Et3SiH/B(C6F5)3”
(1:1) to 4b revealed a ca. 10 times acceleration in the
hydrogenation of 1-hexene, showing a TON of 1.0 × 104 and a
TOF of 3.3 × 104 h−1 within 18 min (entry 14). The
promotion effect of the added cocatalytic system was less
pronounced with 4a. A ca. 5 times acceleration was observed
upon addition of 5 equiv of “Et3SiH/B(C6F5)3” as cocatalyst to
4a (entry 12). Unfortunately, such methodology to improve
activities could not be applied to catalyses with 3a,b. The
additional presence of [Et3O][B(C6F5)4] did not significantly
improve the hydrogenation performance in comparison with
the catalyses using solely 3a,b. Among other effects this was
attributed not only to the poor solubility of [Et3O][B(C6F5)4]
in 1-hexene but also to the highly reactive character of this
species, reacting not only with 1a,b in the reaction mixture.
When the cationic systems of 3b, 4b, “4b/Et3SiH/B(C6F5)3”,
and “1b/Et3SiH/B(C6F5)3” were applied as catalysts, the
phosphonium cation [HPCy3]+ combined with the [B-
(C6F5)4]− or [HB(C6F5)3]− anions was invariably observed as
the major phosphorus-containing species after catalysis.
[HPCy3]+ was previously identified in the hydrogenation
system of “Re(I) diiodo/hydrosilane/B(C6F5)3”.12 In compar-
ison, this phosphonium cation was not observed in hydro-
genations catalyzed by 1b, 2b, or the “2b/B(C6F5)3” system.
These observations not only suggested that the higher catalytic
activities can be achieved by removal of at least one phosphine
ligand from the rhenium center of the catalyst but also implied
that different activation pathways are operative for functional-
ization with the neutral Lewis acid B(C6F5)3 and the cationic
Lewis acids [Et]+ and [SiEt3]+. It should be noted that it is the
same as the previous case12 that the reaction of free PCy3 and
B(C6F5)3 with 1-hexene and H2 under the reaction conditions
did not reveal the phosphonium salt [HPCy3][HB(C6F5)3].
Therefore a frustrated Lewis pair (FLP)-type hydrogenation as
a competitive reaction step seemed less plausible.
alternative pathways are followed depending on the type of
Lewis acid functionalization.
1. Path 1. The generated vacant site of I-1 gets occupied by
an alkene ligand, affording intermediate I-2, possessing an 18e
pseudo-octahedral structure. The alkene ligand is presumably
orientated in the H−Re−NO plane, prone to facile alkene
insertion into Re−H when entering the Osborn-type hydro-
genation cycle,26 which includes elementary steps of alkene
insertion into the M−H bond, coordination of H2, and
reductive elimination. DFT geometry optimizations on the
[ReH(PMe3)2(NO)(NOSiMe3)(η2-C2H4)]+ model complex of
I-1 however showed no energetic minimum for the ethylene
coordination, and furthermore the full energy hypersurface to
simultaneously add the olefin and transfer the hydride onto it
could not be calculated. Despite this, this pathway looks
reasonable from a more empirical point of view and cannot be
excluded, particularly in the case of 1, where low catalytic
activities were recognized.
2. Path 2. The excellent performance exhibited by type 2 or
“2/B(C6F5)3” catalysts in comparison with 1 strongly suggested
different activation pathways involving reversible phosphine
dissociation from the rhenium center. In this way the “double
Lewis acidic” 14e species I-3 bearing two vacant sites is
assumed to be generated. Taking into account Negishi’s notion
of catalytic enhancement by formation of “double Lewis acidic”
“superelectrophiles”,27 the Lewis acid promoted NO bending
together with the generation of another vacant site is essential
for excellent catalytic performance, since the “double Lewis
acid” will “melt” the two Lewis centers temporarily together to
form a Negishi-type “superelectrophile”. The generated super
vacant site of I-3 is then occupied by an alkene ligand, resulting
in the 16e Re(I) species I-4, which can enter the Osborn-type
hydrogenation cycle. The highly electron-deficient nature of I-4
is expected to lower the barrier for the insertion reaction of the
olefin into the Re−H bond, contributing to the overall
acceleration in the hydrogenation catalyses. However, it can
be argued that the 14e species I-3 is too reactive to exist. This
view could be compromised by arguing that this species is
expected to be very short-lived, and its “resting state” is the
linear NO-LA form I-5 with a 16e Re(-I) configuration and a
TBP geometry and just one vacant site. A related intra-
molecular redox process with NO bending was also proposed
for ruthenium complexes by Eisenstein and Caulton.28
Coordination of the alkene would lead to formation of I-6
ready for entering the Osborn-type catalytic cycle.26 The 18e
Re(−I) species I-6 is supposed to equilibrate with the 16e
Re(I) species I-4. The proposed unsupported reversible
phosphine dissociations look reasonable for the cases of
initiation courses of 2a,b or the B(C6F5)3 attached cases 2a,b,
since the formation of phosphonium cations or R3P-B(C6F5)3
adducts could not be observed.
3. Path 3. The formation of the phosphonium cation as a
byproduct in the hydrogenations catalyzed by the cationic type
3 and 4 complexes or those obtained in situ with the
“hydrosilane/B(C6F5)3” reagents as cocatalysts suggests the
existence of the third activation pathway, involving irreversible
phosphine dissociation from the rhenium center. Ethylene and
H2 have similar binding affinities to transition metal centers.29
Therefore instead of ethylene, the small H2 molecule could
enter the coordination sphere of I-1 to occupy the vacant site,
affording the 18e Re(I) species I-7.30 Heterolytic cleavage of
the H−H bond could occur by deprotonation with a
neighboring phosphine to form [HPR3]+ phosphonium salts
2.4. The “Catalytic Nitrosyl Effect”: Initiation of
Catalysis via NO Bending. The mechanism for catalyst
activation was proposed in Scheme 4. The crucial step
corresponds to the NO bending assisted by the coordinated
Lewis acid, denoted as the “catalytic nitrosyl effect”. The 18e
TBP Re(−I) monohydride complexes of types 2−4 form first a
distorted square pyramidal Re(I) 16e intermediate I-1
possessing a vacant site trans to the bent NO moiety. Three
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and a dihydride of type I-8,31 which are neutral species when
the attached LAs are cations. Subsequent coordination of the
alkene to the vacant site possibly oriented in plane with either
the H−Re−NO(linear) or H−Re−NO(bent) arrangement
would allow this species to enter the Osborn-type hydro-
genation cycle.26 Path 3, involving cationic Lewis acids, seems
to also get support from the positive charge enhancing the H−
H heterolytic cleavage by PR3 so that catalysis could also get
more efficient than those following path 2. This is in accord
with experimental observations that 3, 4, and the “4 or 1/
hydrosilane/B(C6F5)3” systems are more active than 2 and the
“2/B(C6F5)3” systems.
The monohydride alkene complexes I-2, I-4, I-6, and I-9
could join into the Osborn-type hydrogenation cycle, under-
going the elementary steps of alkene insertion into the M−H
bond, subsequent coordination and oxidative addition of H2,
and reductive elimination of the alkane product.26 Generally,
the considerable enhancement of the hydrogenations upon
addition of a Lewis acidic cocatalyst, such as B(C6F5)3 or
hydrosilane/B(C6F5)3, was interpreted in terms of a shift of the
dissociation equilibrium of the coordinated Lewis acid toward
higher concentrations of the adduct. The central adduct species
I-1 of the initiation course was attempted to be traced by
various NMR techniques, which however proved to be
unsuccessful.
spectrometer. Microanalyses were carried out at the Anorganisch-
Chemisches Institut of the University of Zurich. Complexes 1 were
prepared according to reported procedures.14a 1-Hexene, 1-octene,
cyclooctene, and 1,5-cyclooctadiene were purified by distillation.
General Procedure for Hydrogenation of Alkenes Catalyzed
by the Rhenium Dinitrosyl Complexes. In a 30 mL steel autoclave
equipped with a stirring bar, a certain amount of substrate alkene (e.g.,
1-hexene: 2.5 mL, 20 mmol; 1-octene: 2.5 mL, 16 mmol; cyclooctene:
1 mL, 8 mmol; 1,5-cyclooctadiene: 1 mL, 8 mmol; cyclohexene: 2.5
mL, 25 mmol; 1,7-octadiene: 1 mL, 7 mmol), 0.002 mmol of rhenium
catalyst 1−8 with or without 0.01 mmol of B(C6F5)3 (5.2 mg), and
0.01 mmol of hydrosilane (Et3SiH: 1.6 μL; Me2PhSiH: 1.6 μL) were
mixed. The system was charged with 40 bar of H2 and kept stirring at
100 °C. The progress of the reactions was monitored by the decreased
pressure of the autoclave. The hydrogenation products were
1
characterized by H NMR spectroscopy in CDCl3.
[ReH(PR3)2(NO)(NOB(C6F5)3)] (2, R = iPr a, Cy b). In a 20 mL
vial in a glovebox, 1a (56.8 mg, 0.10 mmol) and B(C6F5)3 (51.2 mg,
0.10 mmol) were mixed in 3 mL of toluene. A yellow solution
immediately formed, and after stirring at room temperature for 30 min,
the solution was filtered off and the filtrate was dried in vacuo. The
residue was washed with pentane (2 × 1 mL) and dried, giving a
yellow solid, 2a. Yield: 98 mg, 91%. IR (cm−1, ATR): ν (NO) 1641, ν
1
(N-OB) 1340. H NMR (300.08 MHz, toluene-d8, ppm): δ 4.07 (t,
2
1H, JHP = 41 Hz, Re-H), 1.79 (m, 6H, P(CHMe2)3), 0.77 (m, 36H,
P(CHMe2)3). 13C{1H} NMR (75.5 Hz, toluene-d8, ppm): δ 148.8
1
1
(dm, JCF = 242 Hz, o-C6F5), 137.7 (dm, JCF = 244 Hz, m-C6F5),
140.4 (dm, 1JCF = 235 Hz, p-C6F5), 120 (m, ipso-C6F5), 28.4 (t, 1JCP
=
14 Hz), 19.4 (s), 18.9 (s). 31P{1H} NMR (121.5 MHz, toluene-d8,
ppm): δ 53.5 (s). 19F NMR (282.3 MHz, toluene-d8, ppm): δ −131.3
3. CONCLUSIONS
3
3
(d, 6F, JFF = 21 Hz, o-C6F5), −159.6 (t, 3F, JFF = 20 Hz, p-C6F5),
−165.8 (m, 6F, m-C6F5). Anal. Calcd for C36H43BF15N2O2P2Re
(1079.68): C, 40.05; H, 4.01; N, 2.59. Found: C, 40.17; H, 3.85; N,
2.63. In a similar procedure, the orange-brown-colored 2b was
prepared from 1b (40.6 mg, 0.05 mmol) and B(C6F5)3 (25.9 mg, 0.05
mmol) in 2 mL of toluene within 1 h. Yield: 58 mg, 88%. IR (cm−1,
ATR): ν (NO) 1644, ν (N-OB) 1339. 1H NMR (300.08 MHz,
In summary, we have presented the Lewis acid functionalized
dinitrosyl rhenium hydride complexes as highly efficient
catalysts for alkene hydrogenations. NO bending and opening
up of a vacant site for reversible substrate binding are the main
effects to promote catalysis summarized under the term
“catalytic nitrosyl effect”. NO bending was found to be
facilitated by Lewis acid coordination to the ONO atom.
Different activation pathways were proposed for functionaliza-
tion with the neutral or cationic Lewis acid. This work also
demonstrated ways of how to tune “uncatalytic” middle
transition metal centers for high catalytic performance in olefin
hydrogenations using the structural flexibility of NO ligands.
Detailed DFT evaluations on the catalyst activation pathway
and further exploitation of the “catalytic nitrosyl effect” of
various other nitrosyl complexes are currently under way in our
group.
2
toluene-d8, ppm): δ 4.17 (t, 1H, JHP = 42 Hz, Re-H), 1.04−1.95 (m,
66H, PCy3). 13C{1H} NMR (75.5 Hz, toluene-d8, ppm): δ 38.4 (t, 1JCP
= 14 Hz), 30.2 (s), 30.0 (s), 27.4 (m), 26.3 (s). 31P{1H} NMR (121.5
MHz, toluene-d8, ppm): δ 41.3 (s). 19F NMR (282.3 MHz, toluene-d8,
ppm): δ −131.7 (d, 6F, 3JFF = 20 Hz, o-C6F5), −159.7 (t, 3F, 3JFF = 20
Hz, p-C6F5), −166.1 (m, 6F, m-C6F5). Anal. Calcd for
C54H67BF15N2O2P2Re (1320.41): C, 49.13; H, 5.12; N, 2.12. Found:
C, 49.01; H, 5.32; N, 2.26.
[ReH(PR3)2(NO)(NOEt)][B(C6F5)4] (3, R = iPr a, Cy b). In a 20 mL
vial in a glovebox, 1a (56.8 mg, 0.10 mmol) and [Et3O][B(C6F5)4]
(79.0 mg, 0.10 mmol) were mixed in 5 mL of ether. After stirring for 1
h the formed yellow solution was filtered off and dried in vacuo. The
residue was washed with ether and pentane (1:10) (3 × 3 mL) and
dried, giving a yellow powder, 3a. Yield: 118.2 mg, 93%. IR (cm−1,
EXPERIMENTAL SECTION
■
1
ATR): ν (NO) 1664, ν (N-OEt) 1252. H NMR (300.1 MHz, THF-
General Experimental Procedures. 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 degassed by freeze−thaw cycles prior to
use. The deuterated solvents were dried with sodium/benzophenone
(toluene-d8, benzene-d6, THF-d8) or P2O5 (CD2Cl2, C6D5Cl) and
vacuum transferred for storage in Schlenk flasks fitted with Teflon
2
2
d8, ppm): δ 5.50 (t, 1H, JHP = 38 Hz, Re-H), 4.77 (q, 2H, JHH = 7
2
Hz, NOCH2), 2.71 (m, 6H, P(CHMe2)3), 1.44 (t, 3H, JHH = 7 Hz,
NOCH2CH3), 1.36 (m, 36H, P(CHMe2)3). 13C{1H} NMR (75.5
MHz, THF-d8, ppm): δ 81.0 (s, NOCH2CH3), 28.2 (t, 1JCP = 15 Hz),
19.7 (s), 18.5 (s), 13.0 (s, NOCH2CH3). 31P{1H} NMR (121.5 MHz,
THF-d8, ppm): δ 59.2 (s). 19F NMR (282.3 MHz, THF-d8, ppm): δ
3
3
−135.1 (d, 8F, JFF = 20 Hz, o-C6F5), −167.8 (t, 4F, JFF = 21 Hz, p-
C6F5), −171.3 (m, 8F, m-C6F5). Anal. Calcd for C44H48BF20N2O2P2Re
(1275.80): C, 41.42; H, 3.79; N, 2.20. Found: C, 41.75; H, 3.43; N,
2.28. In a similar procedure, the orange-colored 3b was prepared from
1b (81.0 mg, 0.10 mmol) and [Et3O][B(C6F5)4] (79.0 mg, 0.10
mmol) in 5 mL of ether within 1 h. Yield: 131 mg, 86%. IR (cm−1,
ATR): ν (NO) 1670, ν (N-OEt) 1249. 1H NMR (300.08 MHz, THF-
1
valves. H NMR, 13C{1H} NMR, 31P{1H} NMR, and 11B NMR data
were recorded on a Varian Gemini-300, Varian Mercury 200, or Bruker
DRX 500 spectrometer using 5 mm diameter NMR tubes equipped
with Teflon valves, which allow degassing and further introduction of
gases into the probe. Chemical shifts are expressed in parts per million
(ppm). 1H 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 a KBr pellet with a Bio-Rad FTS-45 FTIR
2
3
d8, ppm): δ 5.41 (t, 1H, JHP = 39 Hz, Re−H), 4.76 (q, JHH = 6 Hz,
3
NOCH2CH3), 1.35−2.40 (m, 66H, P(C6H11)3), 0.90 (t, JHH = 6 Hz,
NOCH2CH3). 13C{1H} NMR (75.5 MHz, THF-d8, ppm): δ 80.0 (s,
NOCH2CH3), 37.2 (t, 1JCP = 14 Hz), 30.2 (s), 28.9 (s), 26.9 (m), 25.9
(s), 13.4 (s, NOCH2CH3). 31P{1H} NMR (121.47 MHz, THF-d8,
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ppm): δ 49.1 (s). 19F NMR (282.3 MHz, THF-d8, ppm): δ −133.9
2b), and CCDC-919261 (for 3b) contain the supplementary
crystallographic data (excluding structure factors) 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.
3
(m, 8F, ortho-C6F5), −166.3 (t, JFF = 20 Hz, 4F, para-C6F5), −168.4
(m, 8F, meta-C6F5). Anal. Calcd for C62H72BF20N2O2P2Re (1516.44):
C, 49.11; H, 4.79; N, 1.85. Found: C, 49.41; H, 4.98; N, 1.67.
[ReH(PR3)2(NO)(NOSiEt3)][HB(C6F5)3] (4, R = iPr a, Cy b). In a 3
mL Young NMR tube, B(C6F5)3 (18.7 mg, 0.033 mmol) and Et3SiH
(6.5 μL, 0.045 mmol) were mixed in 0.5 mL of toluene-d8. Then 1a
(17.0 mg, 0.03 mmol) or 1b (24.4 mg, 0.03 mmol) was added, and the
tube was vigorously shaken for 5 min. A brown oil precipitate was
formed with a light yellow supernatant solution. When the same
reaction was carried out in chlorobenzene-d5, the solution remained
homogeneous. NMR spectroscopy indicated 100% in situ formation of
the silylium-coordinated nitrosyl complex salt. After removal of the
solvent in vacuo, the residue was washed with pentane (4 × 2 mL)
through vigorous shaking and dried, affording 4a as a light yellow oil or
4b as light brown powders. 4a: 19.0 mg. Yield: 53%. IR (cm−1, ATR):
ASSOCIATED CONTENT
* Supporting Information
■
S
The NMR spectra of 4a, comparison of catalytic activities of 2−
4 with or without cocatalyst, IR spectra of Lewis acid
functionalized rhenium dinitrosyl complexes, crystallographic
details and CIF files, and computational details. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: hberke@aci.uzh.ch.
1
■
ν (NO) 1641, ν (N-OSi) 1340. H NMR (300.08 MHz, C6D5Cl,
ppm): δ 4.38 (t, 1H, 2JHP = 41 Hz, Re-H), 1.98 (m, 6H, PCH(CH3)2),
3
0.83 (m, 36H, PCH(CH3)2), 0.56 (t, JHH = 8 Hz, SiCH2CH3), 0.33
3
(q, JHH = 8 Hz, SiCH2CH3). 13C{1H} NMR (75.47 MHz, C6D5Cl,
Notes
ppm): δ 28.6 (t, J(PC) = 15 Hz, P-CH), 19.8 (s, PCH(CH3)2), 19.3 (s,
PCH(CH3)2), 6.3 (s, SiCH2CH3), 4.0 (s, SiCH2CH3). 31P{1H} NMR
(121.47 MHz, C6D5Cl, ppm): δ 57.7 (s). 11B NMR (96.28 MHz,
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
1
C6D5Cl, ppm): δ −24.47 (d, JHB = 91 Hz). 19F NMR (188.13 MHz,
■
1
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 “Forscher-
gruppe 1175 - Unconventional Approaches to the Activation of
Dihydrogen” are gratefully acknowledged.
toluene-d8, ppm): δ −133.96 (m, 6F, ortho-C6F5), −165.59 (t, JCF
=
19 Hz, 3F, para-C6F5), −168.32 (m, 6F, meta-C6F5). 29Si NMR (99.4
MHz, toluene-d8, ppm): 41.00 (s). ESI-MS: m/z 683.1 M+
{[ReH(PiPr3)2(NO)(NOSiEt3)]+}, 567.1 (M − SiEt3)+·, 512.8
[HB(C6F5)3]−, 542.8 [CH3OB(C6F5)3]−. Anal. Calcd for
C42H59BF15N2O2P2ReSi (1195.97): C, 42.18; H, 4.97; N, 2.34.
Found: C, 41.86; H, 4.79; N, 2.29. 4b: 29.1 mg. Yield: 67%. IR
(cm−1, ATR): ν (NO) 1657, ν (N-OSi) 1296. 1H NMR (300.08 MHz,
REFERENCES
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2
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