Stereoisomers and functional groups in oxidorhenium(v) complexes: Effects on catalytic activity

Article abstract of DOI:10.1039/c9dt01352k

The syntheses of oxidorhenium(v) complexes [ReOCl(L1a-c)₂] (3a-c), equipped with the bidentate, mono-anionic phenol-dimethyloxazoline ligands HL1a-c are described. Ligands HL1b-c contain functional groups on the phenol ring, compared to parent ligand 2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-phenol H1a; namely a methoxy group ortho to the hydroxyl position (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-6-methoxyphenol, H1b), or a nitro group para to the hydroxyl position (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-4-nitrophenol, H1c). Furthermore, oxidorhenate(v) complexes (NBu₄)[ReOCl₃(L1a-b)] (2a-b) were synthesized for solid state structural comparisons to 3a-b. All novel complexes are fully characterized including NMR, IR and UV-Vis spectroscopy, MS spectrometry, X-ray crystallography, elemental analysis as well as cyclic voltammetry. The influence of functional groups (R = -H, -OMe and -NO₂) on the catalytic activity of 3a-c was investigated in two benchmark catalytic reactions, namely cyclooctene epoxidation and perchlorate reduction. In addition, the previously described oxidorhenium(v) complex [ReOCl(oz)₂] (4), employing the phenol-oxazoline ligand 2-(4,5-dihydro-2-oxazolyl)phenol Hoz, was included in these catalysis studies. Complex 4 is a rare case in oxidorhenium(v) chemistry where two stereoisomers could be separated and fully characterized. With respect to the position of the oxazoline nitrogen atoms on the rhenium atom, these two stereoisomers are referred to as N,N-cis and N,N-trans isomer. A potential correlation between spectroscopic and structural data to catalytic activity was evaluated.

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Stereoisomers and functional groups in
oxidorhenium(V) complexes: effects on
catalytic activity†
Cite this: DOI: 10.1039/c9dt01352k
J. A. Schachner, * B. Berner, F. Belaj
and N. C. Mösch-Zanetti
*
The syntheses of oxidorhenium(V) complexes [ReOCl(L1a–c)2] (3a–c), equipped with the bidentate,
mono-anionic phenol–dimethyloxazoline ligands HL1a–c are described. Ligands HL1b–c contain func-
tional groups on the phenol ring, compared to parent ligand 2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-
yl)-phenol H1a; namely a methoxy group ortho to the hydroxyl position (2-(4,4-dimethyl-4,5-dihydro-
1,3-oxazol-2-yl)-6-methoxyphenol, H1b), or a nitro group para to the hydroxyl position (2-(4,4-dimethyl-
4,5-dihydro-1,3-oxazol-2-yl)-4-nitrophenol, H1c). Furthermore, oxidorhenate(V) complexes (NBu4)
[ReOCl3(L1a–b)] (2a–b) were synthesized for solid state structural comparisons to 3a–b. All novel com-
plexes are fully characterized including NMR, IR and UV-Vis spectroscopy, MS spectrometry, X-ray crystal-
lography, elemental analysis as well as cyclic voltammetry. The influence of functional groups (R = –H,
–OMe and –NO2) on the catalytic activity of 3a–c was investigated in two benchmark catalytic reactions,
namely cyclooctene epoxidation and perchlorate reduction. In addition, the previously described oxidor-
henium(V) complex [ReOCl(oz)2] (4), employing the phenol–oxazoline ligand 2-(4,5-dihydro-2-oxazolyl)
phenol Hoz, was included in these catalysis studies. Complex 4 is a rare case in oxidorhenium(V) chemistry
where two stereoisomers could be separated and fully characterized. With respect to the position of the
oxazoline nitrogen atoms on the rhenium atom, these two stereoisomers are referred to as N,N-cis and
N,N-trans isomer. A potential correlation between spectroscopic and structural data to catalytic activity
was evaluated.
Received 29th March 2019,
Accepted 17th April 2019
DOI: 10.1039/c9dt01352k
rsc.li/dalton
lysts. The first two examples of such rhenium(V) complexes
again came from the group of Herrmann, using tetra- and
Introduction
The interest in the chemistry of high oxidation state rhenium bidentate Schiff-base ligands.9,10 Over the last few years the
oxides was certainly triggered by the two seminal publication chemistry of oxidorhenium(V) complexes and their application
on methyltrioxorhenium(VII) (MTO) by Herrmann in 1991.1 in homogeneous catalytic epoxidation of cyclooctene further
After finding an elegant synthesis of MTO, the group of developed.6,11–18 Compared to the impressive activities of MTO
Herrmann and others demonstrated the impressive catalytic however, oxidorhenium(V) complexes had been inferior epoxi-
activities in oxidation catalysis by MTO, including catalytic dation catalysts, with the published complexes until 2014 only
epoxidation of a wide variety of olefins.2,3–5 To this day, MTO reaching turnovers of TON < 75.17 Our initial research efforts
remains the most active rhenium-based epoxidation catalyst, concentrated on phenol–pyrazole (HpyzR) ligands, equipped
reaching turnover numbers (TONs) of >20 000.3,6,7 However, with various substituents R (R = H, Me, OMe, Br, NO2) on the
MTO suffered from epoxide hydrolysis, when acid-sensitive phenol moiety (Fig. 1).14,16
epoxides are formed.8 This observed drawback, as well as
The introduction of functional groups on the phenol
decomposition reactions of MTO, prompted the investigation moiety was found to have a beneficial effect on epoxidation
of oxidorhenium(V) complexes as potential epoxidation cata- activity of the resulting oxidorhenium(V) complexes [ReOCl
(pyzR)2].14 Especially the set of complexes with R = OMe, Br
and NO2 showed enhanced activities (TON > 95) in the epoxi-
dation of cyclooctene (1 mol% cat. loading, 3 equiv. TBHP) as
Institute of Chemistry, University of Graz, Schubertstr. 1, 8010 Graz, Austria.
E-mail: joerg.schachner@uni-graz.at
well as activity with the green oxidant H2O2 (TONs between 55
†Electronic supplementary information (ESI) available: Details on synthesis of
to 61).14 In contrast, complexes [ReOCl(pyzR)2] with R = H and
ligand H1c as well as crystallographic data of complexes 2a–b and 3a–c. CCDC
Me showed either reduced epoxidation activity (TON < 60, with
TBHP) or none at all (TON = 0, with H2O2) under the same con-
1895410, 1850856 and 1562677–1562679. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c9dt01352k
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equatorial plane. Both isomers contain an element of sym-
metry (σ or C2) and are therefore also referred to as “symmetric
isomers”. Depending on the relative position of the nitrogen
donor atom, isomer A is referred to as an N,N-cis isomer, B as
an N,N-trans isomer. In isomers C to F, ligands O and X adopt
a mutual cis orientation to each other, resulting in total loss of
symmetry. Most of the isolated oxidorhenium(V) complexes
belong to asymmetric isomers C and D.17 Again C constitutes
an N,N-cis and D an N,N-trans isomer. Isomers E and F have
not been observed yet in rhenium chemistry. This might be
due to the strong trans influence of the oxido ligand, making
the coordination of the neutral nitrogen donor in this position
unlikely.
For parent complex [ReOCl(oz)2] 4, both the N,N-cis (C, cis-
4) and N,N-trans (D, trans-4) isomer are formed in the synthesis
(Scheme 4). It could be shown that trans-4 is more active in
perchlorate reduction compared to cis-4.12 The initial step of
the catalytic cycle is loss of the chlorido ligand, creating a
vacant site on the rhenium atom. In cis-4, the neutral nitrogen
atom of the oxazoline moiety is trans to the chlorido ligand,
exerting a weaker trans-influence compared to trans-4, where
the phenolate oxygen atom is trans to the chlorido ligand
(Scheme 2). This situation elongates the Re–Cl bond in trans-4
(2.4093(10) Å), compared to cis-4 (2.383(3) Å).12 Hence the loss
of chlorido ligand requires less energy for trans-4. The same is
true for the subsequent steps in the catalytic cycle. Those are
OAT from the substrate to the rhenium atom and subsequent
OAT from rhenium to the sulfide acceptor. For all these steps,
the energy barriers are higher for cis-4 compared to trans-4.12
In the synthesis of complexes 3a–c, special attention was paid
on the potential formation of N,N-cis or N,N-trans isomers
(Scheme 4), because of their significant influence on catalytic
activity.12,22,26,27 Stereocontrol in the synthesis of oxidorhe-
nium(V) complexes to obtain the desired N,N-trans isomer is
therefore highly desired. In literature, it was shown that a
single methyl group on the oxazoline moiety is sufficient for
the isomerically pure formation of an N,N-trans isomer.26,27
The resulting steric hindrance in an hypothetical N,N-cis
complex was identified as one source of stereo-control.27 The
observation that complexes cis/trans-4, without a substituent
on the oxazoline moiety, are obtained as a mixture of N,N-cis
and N,N-trans isomers supports this conclusion. Within this
manuscript we would like to present an additional element of
stereocontrol, based on the coordination chemistry of the
Hdmoz ligands HL1a–c.
Fig. 1 Phenol–pyrazole and phenol–oxazoline ON-bidentate ligands
used in oxidorhenium(V) chemistry, shown with their commonly used
abbreviations.
ditions.14 Whereas MTO and related oxidorhenium(VII) com-
plexes were shown to form side-on coordinated mono-peroxo
and bis-peroxo complexes as the active catalyst,4,5,18,19 no such
structures have been observed or isolated for oxidorhenium(V)
complexes like 3a–c. A peroxide activation mechanism via
deprotonation of the incoming peroxide similar to dioxidomo-
lybdenum(VI) complexes20 seems less likely for oxidorhenium(V)
complexes. Here, the oxido ligand is much less nucleophilic
due to strong π-bonding.21
A second type of catalytic reaction effected by an oxido-
rhenium(V) complex was first described in 2000. The group of
Abu-Omar reported the capability of complex [ReOCl(oz)2] (4),
equipped with the phenol–oxazoline ligand Hoz (Fig. 1), to cat-
alytically reduce perchlorate anions to chloride.22–24 We also
became interested in this remarkable chemistry of the Hoz
ligand, and began to investigate the oxazoline–dimethyl
version of the Hoz ligand, the Hdmoz ligand (HL1a, Fig. 1), in
perchlorate reduction chemistry.12 Based on the ground laying
work of the Abu-Omar group, a dissociative oxygen atom trans-
fer (OAT) mechanism of redox-catalysis was postulated, with
the Re atom cycling between Re(V) and Re(VII) (Scheme 1).22–25
In perchlorate reduction catalysis, we could show that
isomers play an important role for catalyst activity.11,12 For an
octahedral complex with two ON-bidentate and two monoden-
tate ligands like 3a–c, six stereoisomers could form in prin-
ciple (Scheme 2). Isomers A and B have a trans arrangement of
the oxido and X ligand, with the bidentate ON-ligand in the
With this manuscript, we would like to present results
obtained for the set of oxidorhenium(V) complexes [ReOCl
(L1a–c)2] in epoxidation and perchlorate reduction catalysis. By
systematic introduction of electron-donating (–OMe, HL1b)
and an electron-withdrawing (–NO2, HL1c) groups (Fig. 1) we
are relating structural and spectroscopic factors to catalytic
activity. In addition factors that control the stereoselectivity of
the synthesis of these complexes were investigated with the
help of model complexes 2a–b. Finally the influence of stereoi-
somerism on catalytic epoxidation activity was tested with
complexes cis- and trans-4.
Scheme 1 Proposed dissociative oxygen atom transfer mechanism of
perchlorate reduction via Re(V)/(VII) redox catalysis; solv = solvent.
Scheme 2 Possible stereoisomers A–F for octahedral [ReOX(ON)2]
complexes (E and F have not been observed yet).
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structural data obtained by single-crystal X-ray analyses (Fig. 3)
confirmed the exclusive formation of N,N-trans isomers.
Results and discussion
Synthetic procedures
Ligands H1a–b were synthesized according to literature,12,28,29
HL1c based on established literature procedures30 with modifi-
cations (see ESI†). Anionic oxidorhenate(V) complexes (NBu4)
[ReOCl3(L1a–b)] 2a–b were obtained by reaction of precursor
complex (NBu4)[ReOCl4] with one equivalent of ligand HL1a or
HL1b at room temperature in EtOH (Scheme 3). Only this pre-
cursor, together with short reaction times (0.5–1 h) at room
temperature, allows for isolation of mono-ligated complexes
2a–b. Under the same reaction conditions, precursor
[ReOCl3(OPPh3)(SMe2)] gives mixtures with bis-ligated com-
plexes 3a and 3b, respectively.
Complexes [ReOCl(L1a–c)2] 3a–c were obtained by reaction
of precursor complex [ReOCl3(OPPh3)(SMe2)] with two equi-
valent of ligands HL1a–c respectively, under refluxing con-
ditions in CH3CN (Scheme 4). Complete dissolution of the
poorly soluble precursor complex as well as a typical color
change of the reaction mixtures to green (3a and 3c) or green-
ish-brown (3b) indicate successful synthesis of the respective
complexes. The isolated complexes are stable to air and moist-
ure, and are in general insoluble in non-polar solvents like
pentane or heptane. Based on their substituents on the phenol
moiety, complex 3b (–OMe) shows the highest solubility in
polar solvents like CH2Cl2, CHCl3, or CH3CN, followed by 3a
(–H) and 3c (–NO2) with the lowest solubility. Also alcohols like
CH3OH and EtOH or CH3CN/H2O mixtures are possible sol-
vents. When using the HdmozR ligands HL1a–c in the syn-
thesis of complexes 3a–c, 1H NMR spectroscopy and solid state
Crystallography
Single crystals for all five novel complexes 2a–b and 3a–c were
obtained, and their solid state structure could be solved by
X-ray crystallography. The molecular structures of 2a and 2b
revealed the expected anionic oxidorhenate(V) complexes with
(NBu4)+ as counterion (Fig. 2). Both complexes adopt a slightly
distorted octahedral conformation with similar bond lengths
and angles (Table 1). The presence of the methoxy group in 2b
does not result in a significantly different coordination around
the rhenium atom, compared to 2a. Several other anionic tri-
halo oxidorhenate(V) complexes have been structurally charac-
terized and show the same coordination pattern.9,26,31–33 In all
cases, the alkoxide oxygen of the ligand moiety coordinates
trans to the oxido ligand, resulting in an axial–equatorial
coordination of the bidentate ligand, with the three chlorido
ligands in a meridional coordination. This preferred coordi-
nation might also explain the absence of isomers E and F
(Scheme 2) in rhenium chemistry.
Single crystals of complex 3a were obtained from an EtOH
solution at 8 °C, of complex 3b from an EtOH/EtOAc mixture
at room temperature, and of complex 3c from a saturated
EtOH solution by slow evaporation at 8 °C. X-ray diffraction
analysis revealed quite similar solid state structures, with the
rhenium centers in a distorted octahedral coordination. In all
Fig. 2 Molecular views (50% level) of 2a and 2b (H atoms, solvent and
(NBu4)+ molecules omitted for clarity).
Scheme 3 Synthesis of (NBu4)[ReOCl3(L1a–b)] 2a–b.
Table 1 Selected bond lengths [Å] and angles [°] of 2a–b
[Å]
2a
2b
Re1vO1
Re1–N13
Re1–O21
Re1–Cl1
Re1–Cl2
Re1–Cl3
1.690(7)
2.172(8)
2.011(8)
2.381(3)
2.377(2)
2.420(3)
1.6801(16)
2.1386(16)
1.9800(16)
2.3909(4)
2.3717(4)
2.3891(4)
[°]
Scheme 4 Synthesis (isolated yield) of single isomer of trans-3a–c and
O1–Re1–O21
Cl1–Re1–Cl3
N13–Re1–Cl2
171.4(3)
171.51(10)
169.7(2)
171.18(9)
169.71(3)
171.40(6)
isomeric mixture of complexes cis/trans-4 ([Re]
= [ReOCl3(OPPh3)
(SMe2)]; n. ob. = not observed).
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cases the chlorido ligand is located cis to the oxido ligand. plexes 3a–c show that the solid state structures are sensitive to
Furthermore, the N,N-trans orientation of the two bidendate the substituents on the ligand moieties L1a–c. However, a
ligands was confirmed for all three complexes (Fig. 3).
clear trend is not easily extracted from the data at hand. For
A summary of selected bond angles and distances for 3a–c example, 3a displays the longest Re–Cl1 distance, indicating a
can be found in Table 2. The observed bond lengths in com- stronger trans influence of the L1a ligand moiety, compared to
L1b–c. Also, complex 3a displays the shortest RevO bond,
compared to complexes 3b–c, which show comparable RevO
bond lengths, regardless of the electron-donating or -with-
drawing nature of the phenol substituents. The rhenium–
oxido and rhenium–chlorido bond lengths in 3c are actually
very similar to 3b, equipped with the electron-donating
ligand L1b.
Stereo-control of isomers
Switching from the Hoz to the Hdmoz ligand class, with two
methyl groups on the oxazoline moiety, only leads to the
respective N,N-trans complexes (Scheme 4). In order to study
this stereoselective formation of N,N-trans isomers of 3a–c in
more detail, the two mono-ligated, anionic oxidorhenate(V)
complexes (NBu4)[ReOCl3(L1a)] (2a) and (NBu4)[ReOCl3(L1b)]
(2b) were synthesized (Scheme 3). The formation of mono-
ligated complexes 2a–b provides evidence that the synthesis of
bis-ligated complexes [ReOCl(L1a–c)2] is a step-wise process.
Thus, whether the N,N-trans or N,N-cis isomer is formed in the
next step depends on the respective trans influence of the
initially coordinated ligand moiety (Scheme 5).
Complex 2a can be compared to previously published
complex (H2oz)[ReOCl3(oz)] (2-oz).26 As mentioned above, the
respective bis-ligated complex 3a is isomerically pure, whereas
4 is isolated as an isomeric N,N-trans and N,N-cis mixture. As
summarized in Table 3, for complex 2a, the two trans co-
ordinated chlorido ligands Cl1 and Cl3 exhibit longer bond
distances to the Re center (2.381(3) and 2.420(3) Å, respect-
ively) compared to chlorido ligand Cl2, which is trans to the
oxazoline nitrogen N13. Here the Re–Cl bond distance is at
Fig. 3 Molecular views (50% probability level) of complex 3a (top), 3b
(middle) and 3c (bottom). Hydrogen atoms were omitted for clarity.
Scheme 5 Schematic representation of step-wise coordination of a
bidentate ligand H(ON) (e.g. HL1a–c), with formation of N,N-trans or
Table 2 Selected bond distances [Å] and angles [°] for complexes 3a–c
N,N-cis isomers. Arrows indicate point of substitution of incoming
phenolate oxygen of the second H(ON) ligand and resulting
stereoisomer.
[Å]
3a
3b
3c
Re1vO1
Re1–Cl1
Re1–N13
Re1–N33
Re1–O21
Re1–O41
1.682(6)
2.440(2)
2.203(5)
2.058(5)
2.056(7)
1.944(7)
1.757(4)
2.400(2)
2.118(5)
2.096(4)
1.987(4)
1.999(4)
1.719(4)
2.4087(18)
2.205(3)
2.037(3)
2.033(3)
1.962(3)
Table 3 Comparison of Re–Cl bond lengths [Å] of 2-oz, 2a and 2b
Re1–Cl1
Re1–Cl2
Re1–Cl3
2a
2.381(3)
2.377(2)
2.3717(4)
2.3690(6)
2.420(3)
2.3891(4)
2.3621(7)
[°]
2b
2.3909(4)
2-oz26
2.4701(6)a
O1–Re1–O21
Cl1–Re1–O41
N13–Re1–N33
173.2(3)
163.8(6)
165.9(2)
177.3(3)
168.07(18)
165.4(3)
172.91(17)
166.62(10)
168.63(13) a Elongated because of H-bonding to neighboring (H2oz)+ cations.
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2.377(2) Å, as expected based on the weaker trans influence of
N13. The same observations are made in case of 2b, where bis-
ligated complex 3b also only shows the N,N-trans isomer.
Complex 2-oz displays an unusually long Re–Cl1 bond
(2.4701(6) Å), which is caused by two H-bonds to neighboring
(H2oz)+ cations in the unit cell.26 Therefore this bond has to be
disregarded for the structural discussion. The remaining two
bonds Re–Cl2 (2.3690(6) Å) and Re–Cl3 (2.3621(7) Å) are of very
similar distances. Such a similar bonding situation could
explain the observed mixtures of N,N-cis and N,N-trans isomers
for 4, as there is no preference for the point of coordination of
the second incoming Hoz ligand (Scheme 5). The shorter
and therefore stronger Re–Cl2 bonds observed in 2a and 2b
would explain the stereoselective synthesis of N,N-trans
complexes 3a–b.
Cyclic voltammetry
In order to evaluate the electronic situation on the Re center
with respect to the ligand substituents, cyclic voltammetry
experiments on bis-ligated complexes 3a–c were performed. In
addition, also the two isomers of N,N-cis/trans 4 were investi-
gated. Analyte solutions for cyclic voltammetry were near
1 mM in acetonitrile, with (NBu4)PF6 used as supporting elec-
trolyte (0.1 M). The recorded currents Ip [µA] were divided by
the actual concentration of analyte to normalize the voltammo-
grams for better comparability. All five complexes displayed
quasi-reversible Re(V)/(VI) redox couples at
a
positive
Fig. 4 Top: Comparison of Re(V)/Re(VI) redox couples of complexes
3a–c; bottom: comparison of Re(V)/Re(VI) redox couples of complexes
cis/trans-4.
potential,14,34 which also proved to be scan-rate independent.
At negative potentials only irreversible redox events occurred.
Half-wave potentials E1/2 (E1/2 = (Ep,c + Ep,a)/2) are given in
Table 4 (referenced to ferrocene/ferrocenium).
For complexes 3a–c (Table 4 und Fig. 4), the half-wave
potential of 3c is shifted by 280 mV to higher potential, com-
pared to unsubstituted 3a, as expected for the electron-with-
drawing nature of the nitro groups on the ligand moiety of
L1c. Methoxy substituted complex 3b on the other hand is
only shifted by 29 mV to lower potential, compared to 3a, indi-
cating a small electron-donating effect of the methoxy substitu-
ents in 3b. The influence of the methyl groups on the oxazo-
line moiety can be compared between 3a and trans-4. Here, the
Re(V)/(VI) redox couple for trans-4 is 60 mV lower than for 3a,
indicating an electron-richer metal center in trans-4. The data
in Table 4 and Fig. 4 show that the two stereoisomer of 4 have
virtually identical redox potentials. Obviously, stereoisomerism
does not have an effect on the redox potential of the Re center.
Scheme 6 Epoxidation of cyclooctene using tert-butylhydroperoxide
(TBHP).
effect of the different isomers cis/trans-4 and the introduction
of electron-donating (–OMe in 3b) and withdrawing (–NO2 in
3c) groups on catalytic activity was of particular interest.
Complexes 2a and 2b were also tested for potential catalytic
activity, as other mono-ligated complexes have proven to be
active epoxidation catalysts.14,15 In typical catalytic experi-
ments 3 equiv. of tert-butylhydroperoxide (TBHP) as oxidant
with a 1 mol% catalyst loading in CHCl3 at 50 °C and the sub-
strate cyclooctene were used (Scheme 6). Reaction progress was
monitored by withdrawing aliquots during the reaction and
analysis with GC-MS. In general, all six tested complexes are
catalytically active without induction period. A summary of
Epoxidation of cyclooctene
Complexes 2a–b, 3a–c and cis/trans-4 were evaluated as cata-
lysts in the epoxidation of standard substrate cyclooctene. The
Table 4 Redox potentials E1/2 [V] of complexes 3a–c and N,N-cis/trans turnover numbers (TON) with the respective turnover fre-
4 at 200 mV s−1 in CH3CN
quency (TOF [h−1]) is given in Table 5. The lowest activity was
displayed by complex 2a, the highest by 3c. Complexes 3a and
3b showed activities in a similar range as cis/trans-4. All com-
[V]
3a
3b
3c
cis-4
trans-4
E1/2
0.642
0.613
0.922
0.576
0.582
plexes tested showed a significant brightening to complete dis-
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Table 5 TON (TOF [h−1]) for catalysts 2a–b, 3a–c and cis/trans-4
TON (TOF)
2a
2b
<10
23 (0.96)
37 (1.5)
34 (1.4)
80 (9.4)
40 (1.7)
30 (1.3)
3a
3ba
3c
cis-4
trans-4
TOFs [h−1] were calculated at time of maximum yield of epoxide, not
maximum activity of catalyst. If not mentioned otherwise, this was
after 24 h. a Maximum yield of epoxide reached after 8.5 h.
coloration from the initially green solutions during catalytic
experiments, hinting at the formation of catalytically inactive
Re(VII) complexes.31
Fig. 6 Yield of epoxide for isomeric complexes cis/trans-4.
Time-conversion plots of cyclooctene epoxidation are given
in Fig. 5.
activity (TON = 80) of all seven complexes (Fig. 5). In this case,
the same positive effect on catalytic activity was observed as
with [ReOCl(pyzNO2)2].14 Obviously, the electron-withdrawing
property of a nitro group results in enhanced epoxidation
activity.
Data in Fig. 6 reveals a slight difference in catalytic per-
formance of isomeric complexes cis- and trans-4, with cis-4
(TON = 40) performing slightly better compared to trans-4
(TON = 30).
With the results from epoxidation catalysis in hand, a
potential correlation to structural and spectroscopic data can
be probed. Table 6 shows a summary of such data together
with TONs in cyclooctene epoxidation. By comparing spectro-
scopic data of complexes 3a–c and cis/trans-4 no obvious corre-
lation however between electronic properties and catalyst
activity can be drawn.12,22 Neither the RevO IR absorption fre-
quency nor the RevO bond lengths show a clear trend corre-
lating to epoxidation activity. For example, cis-4 and 3c show
similar RevO IR absorptions (957 vs. 963 cm−1), but very
different activities in epoxidation (TON 40 vs. 80). Similarly,
the RevO bond distance of 3c does not vary considerably
from the other six RevO bond distances. Nevertheless, 3c is
the most active epoxidation catalyst tested in this series. The
Data in Fig. 5 and Table 5 show complexes 2a and 2b to
exhibit the lowest activities (2a: TON < 10; 2b: TON = 23) of all
seven investigated complexes. The few examples of oxido-
rhenate(V) complexes, that were also tested in epoxidation,
were of similar low activity.31,33 Also the bis-ligated complexes
3a (TON = 37) and 3b (TON = 34) showed low activities in the
epoxidation of cyclooctene. In the cases of complexes 2b and
3b, significant discoloration after addition of oxidant TBHP
could be observed, indicating a fast oxidation to unproductive
perrhenate(VII) salts.31 Hence, ligand L1b seems to result in
unstable complexes under strong oxidative conditions. This is
quite in contrast to the OMe-substituted pyrazole–phenol
complex [ReOCl(pyzOMe)2], which showed excellent activities
in cyclooctene epoxidation for both TBHP and H2O2.14
A
potential reason for the lack of activity for 2b and 3b could be
ring-opening or decomposition reactions of the non-aromatic
oxazoline moiety in L1b, a known problem in oxazoline chem-
istry.24 In contrast to 3b, complex 3c, coordinated by the elec-
tron-withdrawing ligand moiety L1c, showed the highest
Table 6 Comparison of selected analytical and epoxidation data for
complexes 2a–b, 3a–c and cis/trans-4
b
ν(CvN) ν(RevO) λmax (ε)a
d(RevO)
[Å]
E1/2
[V]
TONb
[]
[cm−1
]
[cm−1
]
[]
2ab
2bb
3a
1611
1609
1604
1582
1588
962
969
955
955
963
972
957
710 (73)
580 (56)
1.690(7)
Irr.c
<10
23
37
34
80
30
40
1.6801(16) Irr.c
635 (69)b 1.682(6)
0.642
0.613
0.922
0.582
0.576
3bb
3cb
660 (65)
670 (85)
665 (87)
650 (69)
1.757(4)
1.719(4)
1.693(2)
1.689(8)
trans-422 1630
cis-412
1620
a In CH2Cl2, ε = molar extinction coefficient [dm−3 mol−1 cm−1]. b This
Fig. 5 Comparison of yield of epoxide for complexes 2b and 3a–c (2a
is not shown due to low activity).
work. c Only irreversible redox waves observed.
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Table 7 Yield of DMSO [%] from catalytic perchlorate reduction after
24 h
only property of 3c that is significantly different compared to
the other six complexes is the measured redox potential E1/2 of
0.922 V. This seems to indicate a more active catalyst if an elec-
tron poor rhenium center is present. The similarly active cata-
lyst [ReOCl(pyzNO2)2] also showed a similar high redox poten-
tial E1/2 of 0.992 mV.14
To summarize, stereoisomers as in cis/trans-4 do not play a
significant role in epoxidation activity. The observed differ-
ences in electronic and steric properties are most likely too
small to cause a significant difference in catalytic activity.
Electron-withdrawing substituents seem to have a beneficial
effect on epoxidation, as both [ReOCl(dmozNO2)2] 3c as well as
the previously published complex [ReOCl(pyzNO2)2] showed
enhanced catalytic activity.14 These results indicate that an
electron-deficient rhenium center results in higher catalyst
activity and must therefore facilitate the oxygen atom transfer
from TBHP to the olefin. The exact mode of interaction
between the rhenium center and TBHP remains unclear at this
point. The redox potential E1/2 might have predictive power
concerning catalyst activity.
DMSO [%]
5 vol% D2O
50 vol% D2O
2a–b
3a
3b
3c
cis-4
0
75
0
32
64
0b
58
91
78
48a
33
trans-4
>9922
Conditions: 3.2 mol% catalyst, 4 equiv. of DMS, CD3CN/D2O, 25 °C.
a Only partly soluble. b Insoluble.
Results are summarized in Table 7. While mono-ligated
rhenate complexes 2a–b are not active, complexes 3a–c and cis/
trans-4 were capable to reduce perchlorate at both water con-
tents (5 and 50 vol%), with the exception of nitro-substituted
complex 3c. Complex 3c showed complete loss of catalytic
activity at 50 vol% water. In this case however, we attribute this
to the low solubility of 3c in aqueous media. In both solvent
mixtures, solid 3c can be observed throughout the 24 h experi-
ment time. Similar to our previous findings in 5 vol% water,12
trans-4 remains the more active complex also at 50 vol% water
(91%), compared to cis-4 (58%). It is interesting to observe
though that only for cis-4 the catalytic activity increases with
increasing water content (33% at 5 vol% vs. 58% at 50 vol%).
At 5 vol% water, 3a and 3b show essentially the same catalytic
activity (75 and 78% respectively, Table 7). At 50 vol% water
however, 3a shows a significantly decreased activity (75 vs.
32%). With 3b, the difference is not as pronounced (78 vs.
64%).
For all active complexes, catalyst decomposition can be
observed by 1H NMR spectroscopy, as more and more free
ligand is visible over the 24 h reaction time. A discoloration of
the initially green solutions points to decomposition to perrhe-
nate.31 The inactivity of complexes 2a–b is consistent to pre-
viously tested, neutral mono-ligated complexes.12 The observed
differences in catalytic activity in regard to water content are
subject to current investigations in our lab.
Perchlorate reduction
Complexes 2a–b, 3a–c and cis/trans-4 were tested in the chal-
lenging reduction of perchlorate to chloride, with a 3.2 mol%
catalyst loading in the presence of four equivalents of the
sulfide acceptor SMe2 (DMS) (Scheme 7). Reactions were con-
ducted in CD3CN/D2O mixtures of 95/5 or 50/50 vol%, and the
conversion of SMe2 to OSMe2 (DMSO) was monitored by 1H
NMR spectroscopy over 24 h. Whereas the 95/5 vol% mixture is
necessary to fully dissolve the LiClO4 substrate, we were inter-
ested if higher water contents in the reaction affect catalyst
activity. This could either occur by potential hydrolytic
decomposition of the catalyst or coordination of the H2O mole-
cule to the vacant site on the rhenium thereby inhibiting cata-
lysis. From reaction of 4 with AgOTf (OTf = SO2CF3−) it is
known that H2O will coordinate to the resulting cationic oxi-
dorhenium(V) triflate complex. The solid state structure of
complex [ReO(oz)2(H2O)]OTf revealed the formation of a sym-
metric N,N-trans stereoisomer (isomer B, Scheme 7), with the
water oxygen atom coordinated trans to the oxido ligand and
the oz ligands in equatorial position.23 From DFT calculations
it was shown that a symmetric cation B is not catalytically
active in perchlorate reduction, as the coordination site trans
to the oxido ligand is deactivated for perchlorate coordi-
nation.12 The catalytically active species therefore should
remain an isomer D (Scheme 7), with the vacant coordination
site cis to the oxido ligand.
Conclusions
Oxidorhenium(V) complexes equipped with phenol–dimethlox-
azoline ligands remain as a unique class of complexes capable
to both catalytically epoxidize cyclooctene and reduce the chal-
lenging substrate perchlorate to harmless chloride under mild,
aqueous conditions. In olefin epoxidation, stereoisomers do
not play a significant role in catalyst activity, as demonstrated
by the very similar TONs of cis/trans-4 (40/30). Electron with-
drawing substituents rather seem to result in more active cata-
lysts. Nitro-substituted complex 3c showed the highest activity
with a TON of 80 for cyclooctene. This is line with the nitro-
substituted complex [ReOCl(pyzNO2)2], which also proved to
be a similarly active epoxidation catalyst.14 From comparison
of spectral and structural data, it seems the redox potential
Scheme 7 Left: catalytic perchlorate reduction by oxidorhenium(V)
complexes; right: catalytically inactive and active stereoisomers in per-
chlorate reduction; solv = solvent.
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E1/2 obtained from cyclic voltammetry measurements corre- analyses were carried out using a Heraeus Vario Elementar
lates with enhanced activity in epoxidation, in contrast to IR automatic analyzer at the University of Technology Graz.
absorptions or bond lengths.
X-ray structure determinations
In contrast to epoxidation catalysis, stereoisomers play a
vital role for catalyst activity in perchlorate reduction. Here, Crystallographic data (excluding structure factors) for 2a,
only N,N-trans isomers are active. The formation of such N,N- 2b, 3a, 3b, and 3c were deposited with the Cambridge
trans isomers is a unique feature of the oxazoline ligand Crystallographic Data Center as supplementary publication no.
moiety. All related pyrazole based complexes [ReOCl(pyzR)2] 1895410 (2a), 1850856 (2b), 1562677 (3a), 1562678 (3b) and
adopted the N,N-cis conformation in the solid state, which 1562679 (3c).†
also explains their inactivity in perchlorate reduction cataly-
Epoxidation of cyclooctene
sis.16 For the observed stereoselective synthesis of complexes
3a–c, a novel mechanism is proposed, based on the two-step A Heidolph Parallel Synthesizer 1 was used for all epoxidation
coordination of the respective HL1a–c ligands. Coordination experiments. In a typical experiment, 2–3 mg of catalyst
of the first ligand gives intermediate oxidorhenate(V) com- (1 mol%) were dissolved in 0.5 mL CHCl3 and mixed with
plexes [ReOCl3(L1a–c)]−, with a strengthened Re–Cl2 bond cyclooctene (1 equiv.) and 50 µL of mesitylene (internal stan-
thereby directing the second incoming ligand to form an N,N- dard) and heated to the respective reaction temperature
trans complex. Finally, H2O seems to play a far more impor- (50 °C). Then the oxidant (3 equiv.) was added. Aliquots for
tant role in perchlorate catalysis than simply dissolving the GC–MS (20 μL) were withdrawn at given time intervals,
perchlorate salt. It potentially can also act as an inhibitor by quenched with MnO2 and diluted with HPLC grade ethyl
competing with substrate for the vacant coordination site on acetate. The reaction products were analysed by GC–MS
rhenium and favoring the formation of inactive cationic (Agilent 7890 A with an Agilent 19091J–433 column coupled to
stereoisomers. Complexes 3a and 3b showed a diminished a mass spectrometer type Agilent 5975 C), and the epoxide pro-
activity at 50 vol% H2O compared to 5 vol%, whereas conver- duced from each reaction mixture was quantified vs. mesityl-
sion to DMSO actually increased for less active stereoisomer ene as the internal standard. Experimental error of GC-MS
cis-4. The exact role of H2O however is subject to ongoing measurements is 5%.
investigations in our lab.
Synthesis of complex (NBu4)[ReOCl3(L1a)] 2a
Ligand HL1a (0.030 g, 0.16 mmol, 1 equiv.) and (NBu4)
[ReOCl4] (0.10 g, 0.16 mmol, 1 equiv.) were dissolved in 20 ml
EtOH and stirred at rt for 1 h. The mixture turns green within
5 minutes. After 1 h, a small amount of white precipitate
Experimental section
General
The rhenium precursors [ReOCl3(OPPh3)(SMe2)]35 and (probably NBu4Cl) is filtered off and the solvent removed com-
(NBu4)[ReOCl4]36 were prepared according to previously pub- pletely. The crude product is a mixture of 2a and bis-ligated
lished methods. The ligands HL1a,27,29 HL1b28 as well as 3a, which can be separated by dissolving the better soluble 3a
complexes 3a12 and cis/trans-412 were prepared via published in small amounts of EtOAc, leaving behind analytical pure
routes. Synthesis of HL1c can be found in the ESI.† Chemicals yellowish–greenish 2a (62 mg, 52%). Recrystallization from
1
were purchased from commercial sources and used without EtOH/EtOAc (1/1 vol%) yielded 2a as single crystals. H NMR
further purification. NMR spectra were recorded with a Bruker (300 MHz, chloroform-d) δ 7.61 (dd, J = 7.9, 1.8 Hz, 1H), 7.12
Avance (300 MHz) instrument. Chemical shifts are given in (ddd, J = 8.7, 7.1, 1.8 Hz, 1H), 7.00–6.90 (m, 1H), 6.90 (ddd, J =
ppm and are referenced to residual protons in the solvent. 8.2, 7.1, 1.2 Hz, 1H), 4.51 (s, 2H), 3.35–3.23 (m, 8H), 1.74
Coupling constants (J) are given in Hertz (Hz). Mass spectra (s, 6H), 1.72–1.61 (m, 8H), 1.53–1.38 (m, 8H), 1.00 (t, J = 7.3
were recorded with an Agilent 5973 MSD – Direct Probe using Hz, 12H). 13C NMR (75 MHz, chloroform-d) δ 136.33, 131.32,
+
the EI ionization technique. Samples for infrared spectroscopy 118.80, 116.96, 108.59, 78.98 (CH2), 73.54, 59.41 (NBu4 ), 26.78
+
+
+
were measured on a Bruker Optics ALPHA FT-IR Spectrometer (Me2), 24.39 (NBu4 ), 19.95 (NBu4 ), 13.90 (NBu4 ), (two
equipped with an ATR diamond probe head. GC-MS measure- C obscured); ATR-IR νmax/cm−1: 2960, 2930, 2871, 1612 (CvN),
ments were performed on an Agilent 7890 A with an Agilent 1482, 1378, 1323, 1271, 1082, 962 and 955 (RevO), 879, 860,
19091J–433 column coupled to a mass spectrometer type 756, 616; EI-MS (m/z): 497.9 (M+ − NBu4); UV λmax(CH2Cl2)/nm:
Agilent 5975 C. Electrochemical measurements were per- 710 (ε/dm3 mol−1 cm−1 73); elemental analysis calculated for
formed under an inert N2 atmosphere in a glove box in dry C27H48Cl3N2O4Re (741.3 g mol−1): C 43.75, H 6.53, N 3.78;
acetonitrile (stored over molecular sieve) with
a Gamry found C 42.83, 6.51 H, 3.30 N.
Instruments Reference 600 Potentiostat using a three electrode
setup. Platin was used as working electrode, Pt wire (99.99%)
Synthesis of complex (NBu4)[ReOCl3(L1b)] 2b
as supporting electrode; the reference electrode was a Ag wire Ligand HL1b (0.083 g, 0.375 mmol, 1.1 equiv.) and
immersed in a solution of 0.01 M AgNO3 and 0.1 M (NBu4)PF6 (NBu4)[ReOCl4] (0.20 g, 0.34 mmol, 1 equiv.) were dissolved in
in CH3CN separated from the solution by a Vycor® tip. EtOH and stirred at rt for 0.5 h. An intense green color formed
Supporting electrolyte used was (NBu4)PF6 (0.1 M). Elemental moments after dissolution. Upon concentration unreacted
Dalton Trans.
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(NBu4)[ReOCl4] precipitated and was removed by filtration. (M+); UV λmax(CH2Cl2)/nm: 670 (ε/dm3 mol−1 cm−1 85);
Removal of solvent yielded crude 2b. Recrystallization from elemental analysis calculated for C22H22ClN4O9Re·0.5CH2Cl2
EtOH/EtOAc (1/1 vol%) yielded 2b as single crystals (105 mg, (708.06 g mol−1): C 36.00, H 3.09; N 7.46; found C 36.01;
42%). 1H NMR (300 MHz, CDCl3) δ 7.30 (dd, J = 5.6, 4.1 Hz, H 2.98; N 7.00.
1H), 6.81–6.77 (m, 2H), 4.51 (s, 2H), 3.83 (s, 3H), 3.26–3.15
(m, 8H), 1.74 (s, 6H), 1.65–1.52 (m, 8H), 1.37 (m, 8H), 0.95 (t,
J = 7.3 Hz, 12H); 13C NMR (75 MHz, CDCl3) δ 166.53, 157.48, Conflicts of interest
148.69, 124.86, 123.98, 116.77, 109.78, 100.11, 78.91 (CH2),
+
+
There are no conflicts to declare.
73.66, 59.54 (OCH3), 58.83 (NBu4 ), 26.81 (Me2), 24.25 (NBu4 ),
+
+
19.83 (NBu4 ), 13.89 (NBu4 ); ATR-IR νmax/cm−1
: 2959,
2933, 1609 (CvN), 1582, 1458, 1379, 1254, 1055, 969 and
957 (RevO), 845, 737, 670, 567, 435; EI-MS (m/z): 528.1
(M+ − NBu4); UV λmax(CH2Cl2)/nm: 580 (ε/dm3 mol−1 cm−1 56);
elemental analysis calculated for C28H50Cl3N2O4Re (771.3 g mol−1):
C 43.60, H 6.53, N 3.63; found C 43.78, H 6.48, N 3.66.
UV-Vis data of complex 3a. UV λmax(CH2Cl2)/nm: 650
(ε/dm3 mol−1 cm−1 70); other analytical data has been pre-
viously published.12
Acknowledgements
The organizers of the 7th EuCheMS conference on N-ligands in
September 2018, Lisbon, where parts of this research was pre-
sented, are thankfully acknowledged. Financial support from
NAWI Graz is gratefully acknowledged.
Notes and references
Synthesis of complex [ReOCl(L1b)2] 3b
Ligand HL1b (0.63 g, 2.85 mmol, 2 equiv.) and precursor
[ReOCl3(OPPh3)(SMe2)] (0.93 g, 1.43 mmol, 1 equiv.) was
heated under reflux in 30 ml CH3CN for 3 h. Analytically pure
3b precipitated upon cooling and was isolated by filtration
(495 mg, 51%). Single crystals suitable for X-ray diffraction
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1
analysis were obtained from EtOH/EtOAc. H NMR (300 MHz,
chloroform-d) δ 7.53 (dd, J = 8.3, 1.6 Hz, 1H), 7.34 (dd, J = 8.0,
1.6 Hz, 1H), 6.92–6.82 (m, 2H), 6.76 (dd, J = 8.0, 1.6 Hz, 1H),
6.67 (t, J = 8.1 Hz, 1H), 4.67–4.51 (m, 4H), 3.73 (s, 3H), 3.67 (s,
3H), 2.00 (s, 3H), 1.90 (s, 6H), 1.64 (s, 3H); 13C NMR (75 MHz,
CDCl3) δ 150.22, 121.98, 121.33, 118.65, 117.54, 116.48, 115.70,
110.92, 81.89, 79.47, 78.75, 72.92, 55.86 (OCH3), 55.43 (OCH3),
27.61, 27.10, 27.07, 25.97 (some C are obscured); ATR-IR
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EI-MS (m/z): 678.2 (M+); UV λmax(CH2Cl2)/nm: 660
(ε/dm3 mol−1 cm−1 65); elemental analysis calculated for
C24H28ClN2O7Re (678.15 g mol−1): C 42.51, H 4.16, N 4.13;
found C 42.74, H 4.06, N 4.08.
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