Oxidorhenium(V) Complexes with Tetradentate Iminophenolate Ligands: Influence of Ligand Flexibility on the Coordination Motif and Oxygen-Atom-Transfer Activity

Article abstract of DOI:10.1021/acs.inorgchem.6b00466

The synthesis of oxidorhenium(V) complexes 1-3 coordinated by tetradentate iminophenolate ligands H₂L1-H₂L3 bearing backbones of different rigidity (alkyl, cycloalkyl, and phenyl bridges) allows for the formation of distinct geometri

Full text of DOI:10.1021/acs.inorgchem.6b00466

Article
pubs.acs.org/IC
Oxidorhenium(V) Complexes with Tetradentate Iminophenolate
Ligands: Influence of Ligand Flexibility on the Coordination Motif
and Oxygen-Atom-Transfer Activity
Niklas Zwettler, Jo
̈
̈
rg A. Schachner, Ferdinand Belaj, and Nadia C. Mosch-Zanetti*
Institute of Chemistry, University of Graz, Schubertstrasse 1, 8010 Graz, Austria
S Supporting Information
*
ABSTRACT: The synthesis of oxidorhenium(V) complexes 1−3
coordinated by tetradentate iminophenolate ligands H L1−H L3
2
2
bearing backbones of different rigidity (alkyl, cycloalkyl, and
phenyl bridges) allows for the formation of distinct geometric
isomers, including a symmetric trans-oxidochlorido coordination
motif in complex 3. The complex employing a cycloalkyl-bridged
ligand (2) of intermediate rigidity exhibits an interesting solvent-
and temperature-dependent equilibrium between a symmetric
(
trans) isomer and an asymmetric (cis) isomer in solution. The
occurrence of a symmetric isomer for 2 and 3 is confirmed by single-crystal X-ray diffraction analysis. Chlorido abstraction from
with AgOTf yields the corresponding cationic complex 2a, which does not exhibit an isomeric equilibrium in solution but
2
adopts the isomeric form predominant for 2 in a given solvent. All complexes were, furthermore, employed in three benchmark
oxygen-atom-transfer (OAT) reactions, namely, the reduction of perchlorate, the epoxidation of cyclooctene, and OAT from
dimethyl sulfoxide (DMSO) to triphenylphosphane (PPh ), to assess the influence of the isomeric structure on the reactivity in
3
these reactions. In perchlorate reduction, a clear structural influence was observed, where the trans arrangement in 3 led to the
complete absence of activity. In the epoxidation reaction, all complexes led to comparable epoxide yields, albeit higher catalytic
activity but lower overall stability of the catalysts with a trans arrangement was observed. In OAT from DMSO to PPh , also a
3
clear structural dependence was observed, where the trans complex 3 led to full phosphane conversion with an excess of oxidant,
while the cis compound 1 was completely inactive.
17,18
INTRODUCTION
reactions.
Consequently, oxidorhenium(V) complexes with
■
various ligand systems were investigated for their applicability
in the epoxidation of olefins, albeit with varying success.
Generally, the investigated complexes were coordinated by bi-
or tetradentate chelating N,O ligands. With bidentate ligands,
Over the past decades, rhenium has become a very important
element for oxygen-atom-transfer (OAT) reactions, with the
1
major interest focusing on oxidation states V+ and VII+. One
of the rhenium compounds most intensely investigated for its
application in oxidation reactions is the rhenium(VII) complex
methyltrioxidorhenium (MTO). MTO has been successfully
employed as a catalyst in a variety of transformations such as
monosubstituted complexes of the type [ReOCl
P, As) and disubstituted complexes of the type [ReOXL1
L(EPh
)] (E =
] (X =
2
2
3
2
9−22
monoanionic, monodentate ligand) were investigated.
A
aldehyde olefination, olefin metathesis, and epoxidation of
comprehensive survey of previous work was recently published
by Machura. In our group, very promising results were
3
23
olefins by using of hydrogen peroxide as the oxidant. To date,
4
−8
research interest in MTO and its catalysis remains high. The
benefits of MTO as an epoxidation catalyst are its ease of
preparation, broad range of feasible substrates like cycloalkenes,
allyl alcohols, and n-alkenes, and its broad activity range in both
obtained by employing bidentate pyrazole phenolate ligands,
where full conversion of cyclooctene within 3 h and a
noteworthy tolerance toward aqueous H O as the oxidant
2
2
were observed for the first time in oxidorhenium(V)-catalyzed
9
catalyst loading and reaction conditions. Nevertheless,
24,25
oxidation reactions.
sensitive epoxides are sometimes found to be catalytically
hydrolyzed to the respective diols or follow-up reaction
An OAT reaction of growing importance is the homoge-
neous reduction of the persistent perchlorate anion. Because
perchlorate has been identified as a dangerous pollutant
incorporating severe health risks, the development of robust
and stable catalyst systems suitable for the clean reduction of
9
−14
products.
ligands often prevents hydrolysis.
beneficial use of ionic liquids as well as an ionic rhenium(VII)
The addition of nitrogen-containing Lewis base
4
,6,7,10−12,14,15
Also the
16
system has recently been explored. However, there is still
interest in robust catalyst alternatives. In this endeavor, the
attention turned to rhenium(V) compounds because of their
high stability under ambient conditions. They were at first
investigated for their catalytic activity in several other OAT
26,27
perchlorate to harmless chloride ions is of high importance.
Received: February 24, 2016
©
XXXX American Chemical Society
A
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Article
In such a reaction, perchlorate acts as an oxygen donor and is
stepwise reduced according to Scheme 1.
oxazoline- and thiazolinephenolate-based oxidorhenium(V)
complexes.
32
Considering the importance of the coordination motif on the
activity and bearing in mind that complexes employing
tetradentate iminophenolate ligands exhibit significant catalytic
activity in the reduction of perchlorate as well as olefin
epoxidation, we decided to continue our investigation of the
influence of the coordination motif on these OAT reactions in a
systematic fashion. To do so, oxidorhenium(V) complexes with
tetradentate iminophenolate ligands incorporating bis(imine)
backbones of different rigidity were synthesized, in an attempt
to enforce specific coordination motifs, i.e., ligand arrangements
at the metal center. Here we present the successful synthesis of
a series of monomeric oxidorhenium(V) complexes coordi-
nated by tetradentate ligands with increasing rigidity in the
backbone. Furthermore, the cationic analogue of a complex
employing a cyclohexyl-bridged ligand was prepared via
chlorido abstraction using AgOTf. In that course, we present
two new rare examples of oxidorhenium(V) complexes
Scheme 1. Stepwise Reduction of Perchlorate via Catalytic
OAT to an Organic Sulfide as an Oxygen Acceptor
Similar to epoxidation reactions, rhenium(V) systems have
also been investigated for perchlorate reduction. Abu-Omar and
co-workers showed three oxidorhenium(V) compounds to be
active catalysts in perchlorate reduction with organic sulfides as
2
8
oxygen acceptors. They investigated neutral and cationic
oxidorhenium(V) complexes with two bidentate oxazolinephe-
nolato ligands (hoz) (Figure 1) as well as an oxidorhenium(V)
3
3,34
employing a trans-ORe−Cl moiety,
with one being a
chiral oxidorhenium(V) catalyst structurally related to
Jacobsen’s well-known manganese(III) species. The pre-
viously published complex 1 (Scheme 2) is included in the
35
21
study, for which a more atom-efficient synthesis is reported.
Reactivity trends in three benchmark OAT reactions
perchlorate reduction, epoxidation, and OAT from dimethyl
sulfoxide (DMSO) to a phosphaneare discussed with respect
to the coordination motif.
Figure 1. Stereoisomers of the catalyst [ReOCl(hoz) ] and oxygen-
acceptor conversion in a perchlorate reduction according to Scheme 1.
2
Conversions correspond to diphenyl sulfide as the oxygen acceptor
28,29
(
3.2 mol % catalyst at room temperature vide infra).
RESULTS AND DISCUSSION
■
Ligand Synthesis. Ligands H L1−H L3 were prepared
2
2
21,36,37
according to published procedures in high yield.
The
complex employing a tetradentate iminophenolate ligand
ligands differ in their backbone, i.e., the unit bridging the two
phenolate moieties. For this study, alkyl, cycloalkyl, and aryl
moieties were used as backbones. To increase the solubility of
the envisioned oxidorhenium complexes, the phenolate
moieties were equipped with tBu substituents. The ligand
related to H L1 (Figure 2), first reported by Herrmann and
2
2
1
co-workers. The neutral complexes [ReOCl(hoz) ] and its
2
cationic analogue [ReO(hoz) ](OTf) were able to reduce
2
perchlorate quantitatively after 4 h with Me S and Et S as
2
2
oxygen acceptors. The compound employing the tetradentate
ligand, on the other hand, reached 57% and 69% conversion
H L2 was used in both its racemic and enantiopure R,R forms.
The ligands, ordered by increasing rigidity, are depicted in
Figure 2.
2
28
after 48 h, with Ph S and Me S as substrates, respectively.
2
2
The catalytic pathway of the reaction employing an oxazoline-
based catalyst has been thoroughly investigated by Abu-Omar
Complex Synthesis. For the synthesis of oxidorhenium(V)
complexes with ligands H L1−H L3, the metal precursor
2
2
2
9−31
38,39
and co-workers as well as our group.
Within our
[ReOCl (Me S)(OPPh )]
was used and a method
3
2
3
21
investigations, a stereoisomer of the originally reported catalyst
could be isolated. We subsequently found that the isomeric
form of the employed catalyst is of crucial importance for
catalytic activity and is presumably retained during the course
according to a published protocol was employed. Heating a
solution of 2 equiv of the respective ligand (1 equiv acts as HCl
scavenger) and [ReOCl (Me S)(OPPh )] in anhydrous
3
2
3
acetonitrile (MeCN) under a N atmosphere resulted in a
2
29
of the reaction (Figure 1). Research interest in oxazoline-
based systems remains high, as demonstrated by a recent study
about the configuration dynamics and catalytic activities of
color change to dark brown (1 and 2) or deep red (3),
respectively, indicating the formation of complexes. Careful
workup of the resulting crude products (vide infra) led to the
Figure 2. Ligands H L1−H L3 investigated within this study. The chiral ligand H L2 was used in both its racemic and enantiopure R,R forms.
2
2
2
B
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a
Scheme 2. Synthesis of Complexes 1−3
a
Complex 2 is isolated as a mixture of two isomers (cis and trans, with respect to the oxido and chlorido ligands) that are in dynamic equilibrium in
solution (vide infra).
Table 1. Selected Crystallographic Data and Structure Refinement for Complexes 1−3
1
trans-2
3
empirical formula
C H ClN O Re
C H ClN O Re
C H ClN O Re
3
5
52
2
3
36 52
2
3
36 46
2
3
fw
770.43
block, yellow
782.45
776.40
cryst description
needle, orange
plate, red
cryst system, space group
monoclinic, P2 /c
triclinic, P1
11.9835(8)
12.9480(9)
13.1585(9)
70.315(2)
81.226(2)
72.538(2)
1830.8(2)
2
̅
orthorhombic, Pbca
10.8495(8)
1
a, Å
16.4474(5)
11.7992(4)
18.4902(5)
b, Å
20.5248(16)
30.165(3)
c, Å
α, deg
β, deg
101.6019(11)
γ, deg
volume, Å3
3515.01(19)
4
6717.3(9)
Z
8
reflns collected/unique
R(int), R(σ)
35370/10249
0.0253, 0.0235
100.0 (30.0)
10249/412/0
9300
12947/7159
17726/6588
0.0556, 0.1407
99.7 (26.0)
0.0292, 0.0695
99.9 (26.0)
completeness, % (Θmax, deg)
data/param/restraints
signif unique reflns [I > 2σ(I)]
GOF on F2
7159/421/19
5748
6588/324/9
3731
1.048
1.142
1.029
final R indices [I > 2σ(I)]
R indices (all data)
largest difference peak and hole, e/Å
CCDC no.
R1 = 0.0168, wR2 = 0.0403
R1 = 0.0204, wR2 = 0.0416
1.227 and −0.712
1439655
R1 = 0.0450, wR2 = 0.0869
R1 = 0.0642, wR2 = 0.0913
0.789 and −1.052
1439656
R1 = 0.0661, wR2 = 0.1071
R1 = 0.1301, wR2 = 0.1172
1.425 and −2.776
1439657
3
mononuclear oxidorhenium(V) complexes 1−3 (Table 1) in
moderate-to-good yield as brown-to-red solids. The synthesis
gave the same product and similar yields for both racemic and
In principle, there are four possible coordination modes of a
tetradentate ligand (Figure 3). Structure A corresponds to a
trans isomer with respect to the ORe−Cl moiety, whereas in
structures B−D, the ORe−Cl moiety is arranged in a cis
fashion, differing in the overall ligand orientation. Structures of
enantiopure H L2. Substitution of the second equiv of the
2
ligand by 2 equiv of NEt also led to the desired complex 1;
3
3
3,34
attempts to synthesize 2 or 3 in that manner did not yield the
type A are uncommon,
and the vast majority of
targeted species. However, the readily precipitating H L·2HCl
2
oxidorhenium(V) complexes bearing tetradentate ligands
could easily be recycled via treatment with, e.g., Na CO .
2
3
Complexes 1−3 are stable toward air and moderately stable
toward moisture. They are well soluble in most polar and
apolar organic solvents including benzene and heptane. Direct
exposure to hydrous solvents as well as prolonged standing in
solution at ambient atmosphere slowly led to decomposition to
a free ligand and NMR-inactive rhenium species due to
hydrolysis. Scheme 2 depicts the synthesized complexes 1−3
and their structural motifs.
Figure 3. Possible coordination motifs at the oxidorhenium(V) metal
center with a tetradentate O,N,N,O ligand.
C
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employ a cis-ORe−Cl motif and are of the structure type B.
Structure types C and D have not been described in the
literature. It has also been observed that complexes of the
structure type A tend to dimerize in the presence of water,
yielding μ-oxido-bridged dinuclear complexes of the type
S10), the thermodynamic equilibrium parameters ΔH = (8.48
± 0.18) kJ·mol− and ΔS = (24.3 ± 0.6) J·mol ·K were
calculated. From the data, it is evident that the cis isomer is
thermodynamically favored and the conversion of cis isomer to
trans isomer is thus endothermic. The increase of the entropy
could be the result of easier chloride dissociation in trans-2.
Compound 2 also exhibits an interesting solvent-dependent
dynamic behavior in solution, which can be nicely studied via
1
−1 −1
3
3,34,40−42
[
(ReOL) (μ-O)].
2
1
The H NMR spectrum of complex 1 is consistent with the
2
1
literature and shows two distinct sets of aromatic and imine
protons, thus pointing toward an asymmetric coordination
motif. Subsequent single-crystal X-ray diffraction analysis of 1
confirmed the asymmetric structure type B in the solid state
1
H NMR spectroscopy in different solvents. Whereas the cis/
trans ratio in the apolar solvent C D is approximately 1:5, the
6
6
equilibrium shifts toward the cis isomer with increasing solvent
(
vide infra). In contrast, the rigid ligand H L3 enforces the
2
polarity, leading to a ratio of 2:1 in CD CN (Figure 5).
3
1
symmetric coordination motif A, as demonstrated by the H
NMR spectrum of 3, showing only signals for a symmetric
The arrangement of the chlorido and oxido ligands leads to a
higher overall dipole moment of cis-2 in comparison to the
trans isomer. Polar solvents thus possibly stabilize the more
polar isomeric form, leading to the observed solvent depend-
ency.
To further our understanding of the isomeric form of 2
during catalytic processes, the cationic complex 2a of the
formula [ReO(L2)(MeCN)](OTf) was synthesized. This
compound was obtained via the addition of 1 equiv of
AgOTf to a solution of 2 in MeCN and subsequent removal of
the insoluble AgCl precipitate, according to Scheme 3. The
macroscopic behavior of complex 2a is comparable to that of
the related compound 2, with a noteworthy difference being a
generally lower solubility and pronounced sensitivity in
solution. This sensitivity is also reflected in the electrospray
ionization mass spectriometry (ESI-MS) spectrum of 2a, where
ligand. With the cyclohexyl-bridged ligand H L2, the resulting
2
complex 2 exists as a mixture of two isomers, supposedly
symmetric trans (structure type A) and asymmetric cis
(
structure type B) isomers. For 2, the respective resonances
1
are distinguishable by H NMR spectroscopy. Interestingly, the
aromatic region shows six resonances for each isomer (two
imine and four aromatic protons). Thus, also in the proposed
trans isomer, all protons are nonhomotopic because of the
asymmetric cyclohexyl bridge. The proposal of such a trans
isomer is corroborated by the fact that there are two sets of
aromatic signals with only minor differences in chemical shifts
(Figure 5) and single-crystal X-ray diffraction analysis, which
confirmed the existence of the trans conformation in the solid
state (vide infra). The postulation is, furthermore, in
compliance with the literature, where van Bommel et al.
suggest an equilibrium between the structure types A and B for
a significant peak for the oxidized rhenium(VII) species
+
[
ReO (L2)] is observed (Figure S9).
2
4
1
related alkoxide complexes.
For complex 2a, only signals corresponding to one isomer
1
1
H NMR measurements of a sample of 2 at two different
are observed in H NMR spectroscopy, which is in contrast to
temperatures in CD CN revealed a temperature dependence of
3
the dynamic isomerism observed for compound 2. In CDCl ,
3
1
the isomeric ratio. The experiment showed an increase of the
trans isomer from approximately 35% at 25 °C to 45% at 60 °C
the H NMR spectrum of cationic species 2a nicely resembles
the signal set for the major symmetric isomer trans-2,
corroborating this arrangement to be sterically favored after
chlorido abstraction. Because of the conditions employed in
(Figure 4).
To provide the thermodynamic parameters of the isomeric
equilibrium in MeCN, we additionally performed a variable-
1
perchlorate reduction (vide infra), H NMR data for 2a have
temperature NMR experiment in CD CN (−30 to +60 °C in
3
also been collected in CD CN. Here a situation different from
3
steps of 10 °C) and determined the isomeric ratios via
that in CDCl is found: the two imine protons as well as all four
3
1
integration of the respective H NMR resonances (Figure S9).
aromatic signals are distinct, which hints toward an asymmetric
arrangement, suggesting that the cationic species 2a exclusively
2
With the help of a van’t Hoff plot (linear fit, R = 0.996; Figure
adopts the isomeric cis form in CD CN, which is also
3
predominant for 2 in the same solvent (Figure 6). These
findings are in accordance with an enhanced dynamic behavior
in five-coodintate complexes. In the cationic species 2a, where
the solvent molecule is only weakly bound, isomerization
occurs readily. Similarly, also in 2, dissociation of chloride
enables isomerization albeit to a weaker extent, giving rise of
two isomers.
1
Additionally, in the H NMR spectrum of 2a, a broad peak
denoting the coordinating MeCN solvent was present in the
aliphatic region. Whereas the peaks integrated for roughly three
protons in CDCl , in CD CN, the integral was lower, likely
3
3
because of the exchange of coordinating CH CN with CD CN
3
3
(
Figures S6−S8).
Molecular Structures. Solid-state molecular structures of
complexes 1, 2 (trans isomer; crystals from a batch employing
racemic H L2), and 3 were obtained by single-crystal X-ray
2
1
diffraction analysis. Molecular views are given in Figure 7, and
selected angles and bond lengths are provided in Table 2. In
general, all bond distances are within the expected range of
Figure 4. H NMR spectra of 2 in CD CN at 25 °C (bottom) and 60
°
3
C (top) showing a change in the ratio between isomers cis-2 (●) and
trans-2 (○).
D
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N
43
Figure 5. Ratio of the coordination isomers cis-2 (●) and trans-2 (○) depending on the solvent polarity (E values according to Reichardt).
T
13
Because of the low solubility in C D , the C satellites of the solvent residual signal are visible as two peaks that are unaccounted for. The additional
6
6
peaks in the spectrum recorded in (CD ) CO most likely correspond to a μ-oxido-bridged dimer originating from H O traces in the solvent.
3
2
2
a
Scheme 3. Synthesis of the Cationic Complex 2a
a
The vacant coordination site originating from abstraction of the chlorido ligand is occupied by an MeCN molecule. Whereas compound 2 exists as
an isomeric mixture in solution, 2a shows no such behavior and adopts a single isomer in a given solvent (vide infra).
this arrangement, the cycloalkyl-bridged ligand is more flexible
and leads to a partial rearrangement to cis-2 in solution, a
putative structural isomer of 1 (vide supra). These coordination
modes lead to a cis-ORe−Cl arrangement in 1 (and cis-2)
and to an unusual trans-ORe−Cl arrangement in trans-2 and
3
. This results in an elongated Re−Cl bond in the symmetric
complexes trans-2 and 3 in comparison to that of 1. In 3, also
the ReO bond length is elongated. Furthermore, the O
Re−Cl bond angles show significant deviations from 180° in
the symmetric structures of trans-2 and 3, which is in
agreement with a generally favorable cis-ORe−Cl arrange-
ment (Table 2).
Perchlorate Reduction. OAT from perchlorate ions to
1
Figure 6. Aromatic region of the H NMR spectra of 2a in CD CN
3
organic sulfides R S (R = Me, Ph) according to Scheme 4, using
2
(
top) and CDCl (bottom), featuring resonances corresponding to
asymmetric cis (CD CN, ●) or symmetric trans (CDCl , ○)
conformation in solution. One aromatic signal in the bottom spectrum
is obscured by the residual solvent peak.
3
oxidorhenium(V) complexes 1−3 and 2a as OAT catalysts, was
investigated.
3
3
Catalytic data are summarized in Table 3. Yields of sulfoxide
1
were determined via H NMR measurements at given times
and integration of the respective sulfide and sulfoxide peak
areas with the solvent residual signal as internal standards. Only
compound 1 exhibits a significant catalytic reactivity, while
compounds 2 and 2a react very little and compound 3 is
inactive. Generally, obtained yields were found to be low at
room temperature but increase at 50 °C. Nevertheless, after 24
h of reaction time at 50 °C, a significant discoloration was
observable, likely due to catalyst decomposition to perrhenate
because of oxidation and hydrolysis. This is also reflected in
only a minute increase in the sulfoxide yield from 24 to 60 h.
The activity of 1 is significantly lower than that obtained with
related previously reported oxidorhenium(V) com-
plexes.
2
1,33,34,42
In all complexes, the rhenium atoms are coordinated in a
distorted octahedral fashion by the dianionic tetradentate Schiff
base ligand and by the terminal oxido and chlorido ligands. The
2
‑
alkyl-bridged ligand L1 is arranged in an asymmetric fashion
around the metal center (structure type B, Figure 3) in complex
1
, while the ligands in complexes trans-2 and 3 are arranged in a
symmetric fashion around the metal center (structure type A,
Figure 3). Whereas the phenyl-bridged ligand only allows for
E
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a
Table 3. Yields (%) of Sulfoxide after 4/24/60 h
substrate
1 (cis)
2 (cis/trans)
2a (cis)
3 (trans)
Me2S
Ph2S
2/8/15
1/3/5
1/3/5
5/9/9
1/3/4
1/2/4
1/3/3
0/0/0
0/0/0
0/2/2
1/1/1
2/7/13
b
Ph2S
13/28/31
8/24/48
c
Me2S
a
Conditions: 3.2 mol % catalyst, 4 equiv of R S, 95:5 (v/v) CD CN/
2
3
b
c
D O, room temperature. Reaction temperature: 50 °C. AgClO as
2
4
the oxygen source.
all tested complexes the tetradentate ligand framework restricts
the arrangement of the two nitrogen donors in the cis
28,29
position.
However, the reactivity of 1 is also lower
compared to not only the N,N-cis isomer of the oxazoline
coordinated catalyst (Figure 1) but also a rhenium(V) catalyst
coordinated by a tetradentate iminophenolate ligand, where
6
4
9% (Me S) and 57% (Ph S) conversions were observed after
2 2
28
8 h at room temperature. The only structural differences
between 1 and the latter compound are the tert-butyl
substituents in 1. The introduction of tert-butyl groups has a
significant impact on the steric and electronic situations, both
presumably reducing the catalytic activity. Whereas steric
hindrance obviously hampers substrate coordination, the
additional electron density at the metal center possibly
stabilizes the rhenium(VII) dioxido intermediate, thus decreas-
27−31
ing the catalytic activity.
peak for [ReO L] in the ESI-MS spectrum of 2a substantiates
The occurrence of a significant
+
2
this assumption (Figure S9).
The inactivity of 3, where the oxido group and the potential
active site remain in the trans position to each other, is
interesting and delivers ample evidence for the previously
reported mechanistic details with the oxazoline system where
the active site remains cis to oxido throughout the catalytic
cycle, avoiding a highly unfavorable rhenium(VII) trans-dioxido
27−30,32
intermediate.
However, an explanation of the low
Figure 7. Molecular views (50% probability level) of 1 (top), trans-2
reactivity of complexes 2 and 2a remains elusive. Although
found in an isomeric mixture in solution, the predominant
isomer of 2 in the reaction solvent exhibits an active site cis to−
the oxido ligand (vide supra). Furthermore, abstraction of Cl
leads solely to the cis isomer in the reaction solvent. This would
suggest that higher reactivity should be observable. Thus, at the
current stage of research, we attribute the very low activity of 2
and 2a to the pronounced sensitivity of 2a under reaction
conditions.
(
middle), and 3 (bottom). Hydrogen atoms as well as solvent
molecules are omitted for clarity reasons. For disordered fragments,
only atoms with a higher site occupation factor are depicted.
Table 2. Selected Bond Lenghts [Å] and Angles [deg] for
Complexes 1, trans-2, and 3
1
trans-2
3
Re−O1
1.6952(11)
1.9935(11)
2.0078(11)
2.3814(4)
2.0773(13)
2.0773(13)
95.65(4)
1.664(8)
1.983(3)
1.993(3)
2.518(3)
2.014(4)
2.078(3)
171.6(3)
1.721(9)
1.973(5)
1.999(5)
2.494(3)
2.042(7)
2.059(6)
165.3(3)
Re−O2 (O11)
Re−O3 (O21)
Re−Cl1
Re−N1
Re−N2
Catalytic Epoxidation. Complexes 1−3 and 2a were tested
in the catalytic epoxidation of cyclooctene according to Scheme
5
. Conditions used were 1 mol % of catalyst with 3 equiv of tert-
Scheme 5. Rhenium(V)-Catalyzed Oxidation of Cyclooctene
O1−Re1−Cl1
Scheme 4. Reduction of Lithium Perchlorate to Lithium
Chloride via Catalytic OAT Using an Organic Sulfide as the
Oxygen Acceptor
butyl hydroperoxide (TBHP) in three different solvents at 50
°
C. The solvents were chosen for their decreasing polarity,
namely, chloroform, toluene, and n-heptane.
All tested complexes were active in cyclooctene epoxidation,
albeit with moderate yields. As summarized in Table 4, the
maximum conversion to epoxide was reached within the first 7
h, in essentially all catalytic experiments. Prolonged reaction
times did not lead to a significant increase in the epoxide yield.
Conversions between 50 and 70%, as reported herein, were
complexes coordinated by bidentate oxazoline phenolate
ligands in the N,N-trans position (Figure 1), supporting the
importance of the N,N-trans position in the catalyst, because in
F
DOI: 10.1021/acs.inorgchem.6b00466
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Table 4. Yields (%) of Epoxide in the Epoxidation of cis-Cyclooctene with TBHP after 1/4/7/24 h
solvent
1 (cis)
2 (cis/trans)
2a (trans)
3 (trans)
chloroform
toluene
16/52/54/52
17/35/43/65
7/19/32/43
50/59/68/62
31/46/51/52
15/28/32/38
39/28/24/8
0/45/47/40
0/51/52/46
42/48/50/44
37/49/51/50
18/30/34/37
n-heptane
a
Conditions: 1 mol % catalyst, 3 equiv of TBHP, 50 °C.
observed for the vast majority of oxidorhenium(V) com-
Scheme 6. Rhenium(V)-Catalyzed OAT from DMSO to
PPh3
2
0,22,25,44−46
plexes.
This limitation in the catalytic activity can
be attributed to two main effects: oxidation to unreactive
perrhenate(VII) species by the oxidant TBHP as well as
inhibition by accumulating tBuOH, the follow-up product after
2
1,22,45,46
TBHP oxidation.
However, by a direct comparison of the performance of
complexes 1−3 and 2a in catalysis, interesting trends regarding
the influence of the isomeric form could be observed.
Compound 1 was previously used in the same epoxidation
asymmetric cis complex 1 was completely inactive, as reflected
by no observable amount of phosphane oxide after 24 h, the
cis/trans dynamic complex 2 showed mediocre conversion and
complex 3 fully converted PPh to its oxide within the reaction
3
21
time if 10 equiv of DMSO was used. After 5 days (120 h) of
reaction time, complex 2 reached full conversion under the
same conditions. All results are summarized in Table 5.
reaction in chloroform. In our hands, a similar yield was
obtained, but we could not detect an induction period of 70
min without catalysis. Nonetheless, conversion is low (16%)
within the first hour for 1, whereas complexes 2 and 3 showed
the highest catalytic activity within this period. No significant
catalytic activity was observed after 1 h (compounds 2 and 3)
and 4 h (compound 1). The cationic complex 2a was practically
inactive after 4 h of reaction time in toluene and heptane,
whereas in chloroform, the epoxide yield decreased from 39%
after 1 h to 8% after 24 h, hinting toward fast decomposition
and follow-up reactions in the chlorinated solvent (Table 4).
These results clearly indicate a higher activity but lower stability
of complexes with a trans arrangement in comparison to the cis-
ORe−Cl arrangement in compound 1. In all reactions, no
significant amount of substrate was found after 24 h of reaction
time, but several new peaks, corresponding to side and follow-
up products, could be observed in the gas chromatography−
mass spectrometry (GC−MS) spectra. Similarly, in catalysis
experiments run at 80 °C in toluene, heptane, and 1,2-
dichloroethane, full substrate conversion but only a minute
yield of epoxide were observed with all tested compounds after
Table 5. Yields (%) of OPPh
DMSO
after 24 h of OAT from
3
a
DMSO
1 (cis)
2 (cis/trans)
3 (trans)
100
b
1
0 equiv
equiv
0
0
38 (100 )
c
1
15
31 (7 )
a
b
Conditions: 5 mol % catalyst, room temperature, C D . Full
6
6
c
conversion after 120 h of reaction time. 1 mol % catalyst.
The significant activity difference of 1 and 3 is likely a result
of the readily dissociating oxido trans chloride in complex 3,
which provides a vacant coordination site for the DMSO
18
molecule.
CONCLUSION
■
The synthesis of oxidorhenium(V) complexes with three
tetradentate iminophenolate ligands of increasing rigidity is
described. While the previously reported compound 1 exhibits
a cis-ORe−Cl arrangement, 2 and 3 belong to the group of
uncommon rhenium complexes with a trans-ORe−Cl
moiety. Furthermore, in solution, 2 has been found to exhibit
a solvent- and temperature- dependent equilibrium of
symmetric trans and asymmetric cis isomers, likely caused by
an interplay of steric strain and the disfavored trans-ORe−Cl
arrangement. In contrast, cationic 2a was found to be
isomerically pure in solution, adopting the conformation
found to be major for 2 in a given solvent, thus symmetric in
CDCl and asymmetric in CD CN. In perchlorate reduction, an
7
h. All tested complexes were inactive in the epoxidation of
styrene and 1-octene.
To confirm the suggested stability difference of compounds
1−3 in epoxidation catalysis, at the end of a catalytic run in
toluene (after 24 h), a new aliquot of cyclooctene and TBHP
was added to each catalyst solution, respectively, and samples
were withdrawn to determine conversion via GC−MS.
Interestingly, all three catalysts were still active in the
epoxidation of cyclooctene, albeit with lower selectivity than
that in the first 24 h. A total of 9 h after the second addition of
cyclooctene and TBHP, complexes 1−3 reached respective
yields of epoxide of 48%, 46%, and 41%. At prolonged reaction
time (33 h after the second addition of cyclooctene and
TBHP), complexes 2 and 3 showed no more conversion of
substrate and complex 1 showed an increase from 48% to 53%
epoxide. These results are in agreement with a higher stability
of the cis complex 1.
3
3
influence of the coordination motif on the reactivity could be
observed. Only the asymmetric complex 1 exhibits significant
activity, whereas 3 remains completely inactive, substantiating
that species with an active site trans to the oxido moiety do not
28,29
partake in catalytic perchlorate reduction.
Also, a clear
influence of the tert-butyl substituents’ electronic and steric
properties in perchlorate reduction was observed. In the
epoxidation of cyclooctene, all complexes were found to be
catalytically active but suffer from chemoselectivity problems,
especially at high temperatures. Interestingly, higher catalytic
activity but lower stability of the symmetric trans conformation
over the asymmetric cis conformation could be observed. With
the cationic compound 2a, catalysis experiments in toluene and
OAT from DMSO to Triphenylphosphane (PPh3).
Complexes 1−3 were tested for their ability to catalyze the
OAT reaction from DMSO to PPh according to Scheme 6.
3
The conditions used were 5 mol % catalyst with 1 or 10 equiv
of DMSO in deuterated benzene at room temperature.
The activity of complexes 1−3 in the investigated OAT
reaction is strongly dependent on their structures. Whereas the
G
DOI: 10.1021/acs.inorgchem.6b00466
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
heptane led to epoxide yields comparable to those observed
small amount of cold dry methanol. The ligands match analytical data
3
7,48,49
provided in the literature.
Complex Synthesis. It is important to note that the metal
with 2 and 3, whereas in CHCl , decomposition was
3
predominant for the cationic compound. The most significant
structure-related activity difference was observed in the catalytic
3
9
precursor has to be purified properly; traces of residual HCl or other
impurities tend to severely interfere with the complex syntheses
disclosed herein.
OAT reaction from DMSO to PPh , where the cis compound 1
3
was completely inactive, whereas the trans compound 3 led to
full phosphane oxidation under the employed reaction
conditions with an excess of DMSO.
21
Synthesis of [ReOCl(L1)] (1). For the synthesis of 1, 1 equiv of
[
ReOCl (OPPh )(SMe )] (250 mg, 0.39 mmol) and 1 equiv of H L1
3
3
2
2
(206 mg, 0.39 mmol) were dissolved in 10 mL of dry MeCN under a
N atmosphere and refluxed for 1 h. Then 2 equiv of NEt (78 mg,
2
3
0
.77 mmol) was added, whereupon the greenish suspension
EXPERIMENTAL SECTION
■
immediately turned dark brown. The resulting dark suspension was
stirred under reflux for an additional 4 h. After cooling, the reaction
mixture was filtered and the filtrate evaporated in vacuo. The crude
product was redissolved in a minimal amount of CHCl3 and
subsequently stored at 5 °C for several days. The precipitate was
filtered and washed thoroughly with dry pentane to obtain 1 as a
brownish crystalline solid (195 mg, 66%). Crystals suitable for single-
crystal X-ray diffraction analysis were obtained via recrystallization
from dry MeCN. Analytical data are in compliance with the
Caution! Perchlorate salts of complexes with organic ligands are
potentially explosive. Personal safety equipment has to be worn at all
times when handling perchlorate salts in OAT reactions. Unless specified
otherwise, experiments were performed under inert conditions using
standard Schlenk equipment. Commercially available chemicals were
purchased from Sigma-Aldrich and used as received. No further
purification or drying operations were performed. The metal precursor
[
ReOCl (OPPh )(SMe )] was synthesized according to known
3 3 2
39
21
1
procedures. Solvents were purified via a Pure Solv MD-4-EN
solvent purification system from Innovative Technology, Inc.
Methanol was refluxed over activated magnesium for at least 24 h
literature. Mp: 320 °C (dec). H NMR (CDCl , 300 MHz): δ
3
7.76 (s, 1H, CHN), 7.67 (d, 1H, Ar−H), 7.43 (s, 1H, CHN),
7.22 (d, 1H, Ar−H), 7.14 (d, 1H, Ar−H), 7.00 (d, 1H, Ar−H), 4.38
(q, 2H, CH ), 3.59 (q, 2H, CH ), 1.56 (s, 9H, tBu), 1.33 (s, 9H, tBu),
1
13
19
and then distilled prior to use. The H, C, and F HSQC NMR
spectra were recorded on a Bruker Optics instrument at 300, 75, and
82 MHz, respectively. Peaks are denoted as singlet (s), broad singlet
bs), doublet (d), doublet of doublets (dd), triplet (t), pseudodoublet
“d”), pseudotriplet (“t”), and multiplet (m). Solvents used and the
2
2
1.29 (s, 9H, tBu), 1.12 (s, 9H, tBu), 1.11 (s, 3H, CH ), 0.89 (s, 3H,
3
CH3). 13C NMR (CDCl , 75 MHz): δ 173.58, 173.49 (CHN),
2
(
(
3
170.31, 161.21, 141.54, 141,51, 140.16, 137.83, 133.11, 129.93, 129.70,
119.02, 117.12 (Ar), 36.04, 35.59, 35.25, 34.63, 34.11 (CH , q-C),
2
peak assignment are mentioned at the specific data sets. Electron
impact mass spectrometry (EI-MS) measurements were performed
with an Agilent 5973 MSD mass spectrometer with a push rod. High-
31.49, 31.36, 30.43, 29.82 (tBu), 26.23, 26.16, 19.92, 13.85 (CH , q-
3
−1
C). IR (ATR, cm ): ν
̃
2953 (m), 1605 (m, CHN), 1248 (s), 944
(s, ReO), 836 (m), 748 (s), 531(s), 483 (s).
+
resolution mass spectrometry (HR-MS; ESI ) measurements were
Synthesis of [ReOCl(L2)] (cis/trans-2). For the synthesis of 2, 1
equiv of [ReOCl (OPPh )(SMe )] (523 mg, 0.85 mmol) and 2 equiv
performed at the Department of Analytical Chemistry, University of
Graz, using a Thermo Scientific Q-Exactive mass spectrometer in
positive-ion mode. Peaks are denoted as cationic mass peaks, and the
unit is the corresponding mass/charge ion ratio. GC−MS measure-
ments were performed with an Agilent 7890 A gas chromatograph
3
3
2
of H L2 (930 mg, 1.70 mmol) were dissolved in 20 mL of dry MeCN
2
under a N atmosphere and stirred under reflux for 5 h. The formed
2
precipitate was filtered off and the dark-brown filtrate evaporated in
vacuo. After the addition of 5 mL of dry MeCN and sonication, the
resulting solid was filtered off and washed twice with a little cold dry
pentane to afford 2 as a brick-red powder (366 mg, 54%). Orange
crystalline needles of the trans isomer suitable for single-crystal X-ray
(
column type, Agilent 19091J-433), coupled to an Agilent 5975 C
mass spectrometer. Samples for IR spectroscopy were measured on a
Bruker Optics Alpha FT-IR spectrometer. IR bands are reported with
−1
wavenumbers (cm ) and intensities (s, strong; m, medium; w, weak).
UV−vis spectra were recorded with a Varian Cary 50 in absorption
scan mode using a Hellma Analytics High Precision Quartz cell with a
diffraction analysis were obtained via vapor diffusion of Et O into a
2
saturated solution of rac-2 in MeCN. Compound 2 exists in a solvent-
and temperature-dependent equilibrium of isomers in solution (vide
1
2
mm light path. A Heidolph Parallel Synthesizer 1 was used for all
supra). Mp: 281 °C (dec). H NMR (minor cis isomer, CDCl , 300
3
epoxidation experiments. Elemental analyses were measured at the
Microanalytical Laboratory, University of Vienna.
MHz): δ 8.53 (s, 1H, CHN), 7.94 (s, 1H, CHN), 7.78 (d, 1H,
Ar−H), 7.38 (d, 1H, Ar−H), 7.32 (d, 1H, Ar−H), 7.05 (d, 1H, Ar−
Single-Crystal X-ray Diffraction Analyses. Single-crystal X-ray
diffraction analyses were measured on a Bruker AXS SMART Apex II
diffractometer equipped with a CCD detector. All measurements were
performed using monochromatized Mo Kα radiation from a fine-focus
sealed tube at 100 K (cf. Table 2). Absorption corrections were
semiempirically performed from equivalents. Structures were solved by
H), 4.02 (“t”, 1H, CH), 3.06 (“t”, 1H, CH), 2.56 (m, 2H, CH ), 2.08
2
(“t”, 2H, CH ), 1.85−1.46 (m, 4H, CH ), 1.63 (s, 9H, tBu), 1.34 (s,
2
2
1
9H, tBu), 1.31 (s, 9H, tBu), 1.26 (s, 9H, tBu). H NMR (major trans
isomer, CDCl , 300 MHz): δ 8.93 (s, 1H, CHN), 8.65 (s, 1H,
3
CHN), 7.65 (d, 2H, Ar−H), 7.12 (“t”, 2H, Ar−H), 4.30 (“t”, 1H,
CH), 3.67 (“t”, 1H, CH), 2.86 (“d”, 1H, CH ), 2.70 (“d”, 1H, CH ),
2
2
47
direct methods (SHELXS-97) and refined by full-matrix least-squares
2.08 (“t”, 2H, CH ), 1.85−1.46 (m, 4H, CH ), 1.55 (“d”, 18H, tBu),
2
2
47
techniques against F2 (SHELXL-2014/6). CCDC 1439655−
13
1.31 (s, 18H, tBu). C NMR (minor cis isomer, CDCl , HSQC 300/
3
1
439657 contain the supplementary crystallographic data for this
75 MHz, q-C obscured): δ 173.08, 170.93 (CHN), 134.92, 132.92,
130.56, 128.51 (Ar), 82.52, 80.64 (CH), 31.21, 31.17, 30.37, 29.83
(tBu), 29.93, 29.70, 25.11, 24.71 (CH2). 13C NMR (major trans
paper. These data can also be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Full experimental details for single-crystal X-ray
diffraction analyses of all compounds are provided in the Supporting
Information.
isomer, CDCl , HSQC 300/75 MHz, q-C obscured): δ 177.33, 176.14
3
(CHN), 133.97, 131.72 (Ar), 82.62, 81.61 (CH), 31.21 (2 × tBu),
30.03 (tBu), 30.01 (CH ), 29.98 (tBu), 28.67, 25.11, 24.71 (CH ). IR
2
2
−
1
Ligand Synthesis. Ligands H L1−H L3 were synthesized
(ATR, cm ): ν
̃
2952 (m), 1605 (s, CHN), 1531 (m), 1242 (s),
2
2
2
1
according to known procedures with slight modifications. The
syntheses were performed at ambient atmosphere using standard
laboratory glassware. A total of 2 equiv of the respective 2-
hydroxybenzaldehyde derivative was dissolved in dry methanol.
Subsequently, 1 equiv of the diamine was added and the reaction
1167 (m), 947 (s, ReO), 833 (m), 570 (s), 547 (s). EI-MS: m/z
+
782.6 ([M ]). Anal. Calcd for C H ClN O Re (782.48): C, 55.26;
36
52
2
3
H, 6.70; N, 3.58. Found: C, 54.99; H, 6.74; N, 3.81.
Synthesis of [ReO(L2)](OTf) (2a). For the synthesis of 2a, 1 equiv of
[ReOCl(L2)] (40 mg, 0.05 mmol) and 1 equiv of AgOTf (13 mg, 0.05
mmol) were dissolved in 3 mL of dry MeCN. The resulting orange-
brownish solution was stirred for 1 h. After filtration and evaporation
mixture stirred for several hours [for enantiopure H L2, the tartrate
2
salt of (R,R)-cyclohexyldiamine was used, following Jacobsen’s
48
procedure ]. After concentration in vacuo, the desired product was
of the filtrate in vacuo, 2a was obtained as a brown powder in
1
obtained in high yield by filtration and subsequent washing with a
quantitative yield (42 mg, 99%). Mp: 155 °C (dec). H NMR (CDCl ,
3
H
DOI: 10.1021/acs.inorgchem.6b00466
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3
00 MHz): δ 8.90 (s, 1H, CHN), 8.62 (s, 1H, CHN), 7.76 (m,
H, Ar−H), 7.27 (m, 2H, Ar−H), 4.19 (“t”, 1H, CH), 3.69 (“t”, 1H,
X-ray crystallographic data in CIF format (CIF)
2
CH), 3.11 (bs, 3H, CH CN), 2.99 (m, 2H, CH ), 2.17−1.95 (m, 4H,
3
2
AUTHOR INFORMATION
CH ), 1.75−1.55 (m, 2H, CH ), 31.54 (s, 9H, tBu), 1.52 (s, 9H, tBu),
■
2
2
13
−
1
.34 (“d”, 18H, tBu). C NMR (CDCl , 300 MHz, OTf and MeCN
Corresponding Author
E-mail: nadia.moesch@uni-graz.at.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
3
signals obscured): δ 175.57, 174.65 (CHN), 167.19 (Ar−O),
*
1
3
2
42.62, 140.66, 135.60, 132.48, 121.56 (Ar), 81.22, 80.15, (CH),
6.22, 34.25 (q, tBu), 31.31 (tBu), 29.94 (tBu), 28.99, 28.09, 24.84,
19
4.19 (CH ). F NMR (CDCl , 282 MHz): δ −78.32. IR (ATR,
2
3
−1
cm ): ν
̃
2951 (m), 1599 (s, CHN), 1536 (s), 1242 (s), 1163 (s),
026 (s), 987 (m, ReO), 832 (m), 750 (m), 636 (s), 574 (m), 546
m), 414 (m). HR-MS (ESI ). Calcd for C H N O Re: m/z
36 52 2 3
1
(
+
Notes
+
7
47.3532 ([M] ). Found: m/z 747.3572.
Synthesis of [ReOCl(L3)] (3). For the synthesis of 3, 1 equiv of
ReOCl (OPPh )(SMe )] (266 mg, 0.43 mmol) and 2 equiv of H L3
The authors declare no competing financial interest.
[
(
3
3
2
2
ACKNOWLEDGMENTS
The authors gratefully acknowledge support from NAWI Graz.
460 mg, 0.85 mmol) were dissolved in 20 mL of dry MeCN under a
■
N atmosphere and stirred at 60 °C for 4 h. The resulting dark
2
suspension was concentrated in vacuo and stored at −25 °C for 48 h.
Subsequently, the precipitate was filtered and washed thoroughly with
dry pentane to obtain pure 3 as a dark-red powder (193 mg, 58%).
REFERENCES
■
(
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1
and Applications; Wiley-VCH: Weinheim, Germany, 2010.
3
(
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(
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1
1
1
.59 (s, 18H, tBu), 1.36 (s, 18H, tBu). 13C NMR (CDCl , 75 MHz): δ
3
(
64.84 (CHN), 158.70 (Ar−O), 142.88, 140.43, 137.31, 128.30,
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̈
hn, F. E. Green Chem. 2007, 9, 1296−1298.
−
1
(tBu). IR (ATR, cm ): ν
̃
2955 (m), 1602 (m, CHN), 1578 (m),
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1
7
525 (s), 955 (s, ReO), 830 (m), 751 (s), 543 (s). EI-MS: m/z
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+
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364.
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̈
hn, F. E. Eur. J. Inorg. Chem. 2012,
36
46
2
3
H, 5.97; N, 3.61. Found: C, 55.62; H, 5.61; N, 3.58.
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̈
̈
hn, F. E. Chem. Rec. 2016, 16, 349−
AgClO (0.1 M) and an organic sulfide (Me S or Ph S, 0.4 M) were
dissolved in 0.4 mL of 95:5 (v/v) CD CN/D O in a standard NMR
tube. After the addition of 3.2 mol % catalyst and the calculated
amount of perchlorate from a 0.5 M stock solution in CD CN/D O,
4
2
2
̈
hn, F. E. J. Mol. Catal. A: Chem. 2013,
3
2
368-369, 145−151.
(7) Li, S.; Zhang, B.; Ku
̈
hn, F. E. J. Organomet. Chem. 2013, 730,
3
2
1
the tubes were sealed, and H NMR spectra were recorded after 7/24/
0 h. The yield of sulfoxide formed was determined via integration of
132−136.
6
́
(8) (a) Valla, M.; Stadler, D.; Mougel, V.; Coperet, C. Angew. Chem.,
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Vives, A.; Conley, M. P.; Verel, R.; Coperet, C.; Sautet, P. J. Am. Chem.
́
Catalytic Epoxidation. In a typical experiment, 2−3 mg of catalyst
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(
1 mol %) was dissolved in 0.5 mL of the respective solvent and mixed
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with 1 equiv of substrate. Then 50 μL of mesitylene was added as an
internal standard, and the reaction mixtures were heated to the
respective temperatures, whereupon the oxidant (3 equiv) was added
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́
M.; Coperet, C. Angew. Chem., Int. Ed. 2007, 46, 3870−3873.
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Ed. Engl. 1991, 30, 1638−1641.
2
grade ethyl acetate. The reaction products were analyzed by GC−MS
(
Agilent Technologies 7890 GC system), and the epoxide produced
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internal standard.
(11) Coperet, C.; Adolfsson, H.; Sharpless, K. B. Chem. Commun.
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Catalytic OAT. In a typical experiment, ∼4 mg of catalyst (5 mol
%
) was dissolved in 0.5 mL of C D in an oven-dried NMR tube.
(12) Kiersch, K.; Li, Y.; Junge, K.; Szesni, N.; Fischer, R.; Kuhn, F. E.;
6
6
̈
Subsequently, 1 equiv of a substrate (PPh ) and 1 or 10 equiv of
Beller, M. Eur. J. Inorg. Chem. 2012, 2012, 5972−5978.
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ASSOCIATED CONTENT
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