Mechanistic insight into olefin epoxidation catalyzed by rhenium(V) oxo complexes that contain pyridazine-based ligands

Article abstract of DOI:10.1021/ic200756r

Three novel tridentate pyridazine phenolate ligands were prepared in high yields by Schiff-base condensation of salicylic aldehyde with various pyridazine hydrazines (substituent R in the 6 position: R = Cl (HL^Cl), ^tBu (HL^t

Full text of DOI:10.1021/ic200756r

ARTICLE
pubs.acs.org/IC
Mechanistic Insight into Olefin Epoxidation Catalyzed by Rhenium(V)
Oxo Complexes That Contain Pyridazine-Based Ligands
Katrin R. Gr€unwald, Gerald Saischek, Manuel Volpe, and Nadia C. M€osch-Zanetti*
Institut f€ur Chemie, Bereich Anorganische Chemie, Karl-Franzens Universit€at Graz, Schubertstrasse 1, A-8010 Graz, Austria
S Supporting Information
b
ABSTRACT: Three novel tridentate pyridazine phenolate ligands were
prepared in high yields by Schiff-base condensation of salicylic aldehyde
with various pyridazine hydrazines (substituent R in the 6 position:
t
t
R = Cl (HLCl), Bu (HL Bu), or tol (HLtol)). They react with
[ReOCl3(OPPh3)(SMe2)] to form rare mononuclear trans-dichloro
oxo complexes of general formula [ReOCl2(LR)] with R = tol (1),
tBu (2), or Cl (3) as confirmed by single-crystal X-ray diffraction
analyses of 1 and 2. They were found to be catalysts for oxidation of cyclooctene to the corresponding epoxide by tert-butyl
hydroperoxide (TBHP). Extensive UVꢀvis and NMR spectroscopic investigations followed by evaluation using the powerful
Mauser method revealed mechanistic details. This showed the catalyst precursor [ReOCl2(L)] (2) to be transformed into the
rhenium(VII) compound [ReO3L] (4) in a two-step reaction via intermediate INT which is tentatively assigned to [ReO2L].
Confirmation gave the isolation of 4 by reaction of 2 with excess of TBHP. Monitoring the catalytic oxidation reaction by UVꢀvis
spectroscopy clearly excludes the two rhenium(V) compounds 2 and INT from being the catalytically active species as their
formation is several orders of magnitude faster than the observed catalytic epoxidation reaction.
’ INTRODUCTION
lone pair located at the noncoordinating nitrogen atom may
additionally interact as it provides electrons for hydrogen bond-
ing and may thus help in stabilizing intermediate species. Herein,
we report the preparation of air-stable Re(V) complexes of the
type [ReOCl2(L)] exhibiting a rare trans-dichloro configuration
and their application in epoxidation catalysis. During our studies
we gained further insight into the role of Re(V) as a precatalyst in
oxidation reactions.
Rhenium oxo complexes made their way from mere curiosities
to catalysts for various organic transformations.1ꢀ3 In particular,
methylrhenium trioxide (MTO) was found to be, after its
serendipitous discovery,4 a highly versatile, robust, and active
Re(VII) oxidation catalyst.5,6 Applying various oxygen donors
such as bis(trimethylsilyl) peroxide,7 a ureaꢀhydrogen peroxide
complex,8 or substituted pyridines9 in combination with the
green oxidant H2O2 led to the huge success of MTO and
derivatives thereof.10 Furthermore, side reactions11 as well as
the influence of inorganic and metalꢀorganic cocatalysts on the
catalytic oxidation processes applying MTO are thoroughly
investigated.
Rhenium(V) oxo complexes are far less explored, although
their use provides several advantages. They are prepared in a
straightforward manner and are usually air-stable solids.
Rhenium(V) oxo complexes are active catalysts in a number of
oxidation and reduction processes.12ꢀ14 In particular, several
oxygen-atom transfer (OAT)15,16 and epoxidation reactions
attracted significant interest.17ꢀ24 First, rhenium(V) oxidation
catalysts feature salen-derived ligands,25 which inspired the
development of related systems with high catalytic activities.26
However, the investigated Re(V) epoxidation catalysts do not
reach as yet the high activity observed with Re(VII). Further
exploration of novel rhenium(V) compounds in oxidation reac-
tions is thus in need. Here, we present a series of salen-derived
monoanionic ligands containing a pyridazine moiety, similar to
those we reported recently.27 The heterocycle belongs to the
electron-deficient group, in contrast to pyridine, so that we
expected unusual electronic influences on the coordinating
capability as well as on the catalytic activity. Furthermore, the
’ RESULTS AND DISCUSSION
Synthesis of the Ligands. The three tridentate pyridazine-
containing phenol ligands HLR (R = p-tolyl, HLtol, R = tert-butyl,
HLtBu, R = chloro, HLCl) employed in this report were prepared
by Schiff-base condensation of unsymmetrically substituted
pyridazine hydrazines with salicylic aldehyde in ethanol in
quantitative yields and high purities. The pyridazine hydrazine
precursors were synthesized by a convenient method we recently
described.27 Ligands HLtol, HLtBu, and HLCl were characterized
by NMR spectroscopy, which confirmed the structure shown in
Scheme 1. The ligands were obtained as off-white crystalline
solids, which were well soluble in polar solvents at elevated
temperatures but displayed limited solubility at room tempera-
ture or in apolar solvents.
Synthesis of the Complexes. Complex synthesis was carried
out by stirring an acetone suspension of stoichiometric amounts
of the respective ligand and [ReOCl3(OPPh3)(SMe2)]28 at
50 °C. After completion of the reaction, indicated by complete
dissolution of the starting material, the deep red solutions were
Received: April 12, 2011
Published: July 07, 2011
r
2011 American Chemical Society
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Inorganic Chemistry
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Scheme 1. Synthesis of Ligands HLtol, HLtBu, and HLCl and
Complexes of the Type [ReOCl2(LR)] (R = tol (1), tBu (2),
Cl (3))
Figure 2. Molecular structures of complex 1 (50% probability). Hydro-
gen atoms and solvent molecules have been omitted for clarity.
are shifted downfield upon coordination (Figure 1, a). The two
corresponding doublets of the pyridazine ring are shifted apart
between 0.2 and 0.4 ppm in the uncoordinated ligand, whereas
coordination leads to increased shift differences between the two
(Δδ = 0.8 to 1 ppm) induced by coordination to the metal
(Figure 1, b). All protons of the phenolate rings experience a
downfield shift except one doublet of doublets, whose shift
remains almost constant for all complexes upon coordination to
the metal center.
Molecular Structures of the Complexes. For complexes 1
and 2 single crystals suitable for X-ray diffraction analysis were
obtained by recrystallization from acetonitrile and dichloro-
methane, respectively. A representative molecular view of com-
plex 1 is given in Figure 2 (a molecular view of complex 2 is
included in the Supporting Information, Figure S1), selected
bond lengths and angles are in Table 1, and crystallographic data
are in Table 2.
Complexes 1 and 2 are isostructural and show distorted
octahedral geometries around the Re centers. The planar ligands
coordinate in a tridentate meridional mode, while the oxo groups
are trans to the imino nitrogen (N4) atoms completing the basal
plane. The chlorine atoms occupy the apical positions of the
octahedron and are thus trans to each other, albeit distorted
(Cl1ꢀRe1ꢀCl2 166.60(2)° in 1 and 165.72(8)° in 2). This
configuration is rare. To the best of our knowledge, this is the first
oxo trans-dichloro rhenium crystal structure featuring a triden-
tate N2O ligand. Comparable systems featuring a N2O donor set
form mononuclear29ꢀ32 as well as oxo-bridged dimeric struc-
tures.29,30,33,34 All of these systems involve simple heterocyclic
nitrogen donors such as pyridine or pyrimidine. In addition, one
Figure 1. 1H NMR spectra (aromatic region in DMSO-d6) of HLtol
,
[ReOCl2(Ltol)] (1), HLtBu
,
[ReOCl2(LtBu)] (2), HLCl, and
[ReOCl2(LCl)] (3): (a) imino proton resonance, (b) pyridazine reso-
nances, other resonances can be assigned to the protons of the
phenolate ring.
filtered and allowed to cool to room temperature. Upon evapora-
tion of the solvent dark auburn crystalline materials were
obtained. Recrystallization from hot acetonitrile or dichloro-
t
methane gave complexes [ReOCl2(LR)] (R = tol (1), Bu (2),
and Cl (3)) in good yields as dark red crystals. Those of
compounds 1 and 2 were suitable for X-ray diffraction analysis
(vide infra). All complexes are moderately soluble in a number of
common solvents such as methanol, acetonitrile, or acetone,
fairly soluble in THF or dichloromethane, and insoluble in
diethyl ether and pentane. The tert-butyl group in compound 2
increases the solubility.
35
bidentate N2 and one tetradentate N4 ligand36 are reported,
which form complexes with the trans-dichloro oxo configuration. In
the starting material [ReOCl3(OPPh3)(SMe2)]28 the three chlorine
atoms are coordinated in a meridional mode.37 Thus, this precursor
presents easy access to rare ReV trans-dichloro oxo complexes. In
complexes 1 and 2 the N2ꢀRe1ꢀO2 angle deviates significantly
from the ideal geometry, being 155.18(7)° and 155.68(18)°,
respectively. Although the structures were measured at different
temperatures (100 K for 1 and 293 K for 2), the rhenium oxygen
bonds Re1ꢀO1 are comparable, being 1.6734(17) and 1.656(3) Å
for 1 and 2, respectively, confirming a triple-bond character.38 The
ReꢀO2 bonds are 1.9358(15) and 1.930(3) Å for 1 and 2,
respectively, similar to those reported in the literature.17,18,39,40 The
pyridazine N2ꢀRe1 bond is shorter than the imino N4ꢀRe1 bonds,
which is in agreement with the trans influence induced by O1. Both
apical chloro ligands display bond distances to the central metal
similar to those reported in the literature.29ꢀ32
The compounds were characterized by 1H NMR spectrosco-
py. Most protons in the molecules are found in the aromatic
region (phenol, pyridazine, and imine) which leads to rather
crowded spectra in this region. However, in the 1H NMR spectra
the two doublets of the pyridazine protons and the singlet of the
imino proton are distinct as shown in Figure 1. Furthermore,
those resonances are significantly shifted upon coordination of
the ligand, rendering them a convenient tool for characterization.
Figure 1 shows a comparison of the aromatic regions of the
uncoordinated ligand and of the corresponding rhenium com-
pound for each individual couple. Inall cases, theimino resonances
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Table 1. Selected Bond Lengths [Å] and Angles [deg] for Complexes [ReOCl2(Ltol)] (1) and [ReOCl2(LtBu)] (2)
1
2
1
2
Re1ꢀO1
1.6734(17)
1.9358(15)
2.0909(18)
113.46(7)
91.19(8)
1.656(3)
1.930(3)
Re1ꢀN4
2.2056(19)
2.3746(6)
2.3787(6)
94.44(5)
82.63(5)
96.61(7)
85.04(5)
89.42(5)
86.32(5)
166.60(2)
2.201(3)
2.337(5)
2.379(5)
92.3(4)
Re1ꢀO2
Re1ꢀCl1
Re1ꢀN2
2.095(3)
Re1ꢀCl2
O1ꢀRe1ꢀO2
O1ꢀRe1ꢀN2
O2ꢀRe1ꢀN2
O1ꢀRe1ꢀN4
O2ꢀRe1ꢀN4
N2ꢀRe1ꢀN4
O1ꢀRe1ꢀCl1
O2ꢀRe1ꢀCl1
112.22(14)
91.56(14)
155.68(18)
164.00(13)
83.73(13)
72.45(12)
96.0(5)
N2ꢀRe1ꢀCl1
N4ꢀRe1ꢀCl1
O1ꢀRe1ꢀCl2
O2ꢀRe1ꢀCl2
N2ꢀRe1ꢀCl2
N4ꢀRe1ꢀCl2
Cl1ꢀRe1ꢀCl2
84.6(3)
155.18(7)
163.42(8)
83.02(7)
97.5(5)
80.3(4)
92.0(3)
72.48(7)
83.7(3)
96.13(6)
165.72(8)
86.10(5)
90.3(4)
Table 2. Crystallographic Data and Structure Refinement for
Complexes 1 and 2
butyl group increases the electron density at the metal, rendering
the latter more easy to oxidize. The reversibility prompted us to
investigate chemical oxidations of 1ꢀ3 with various strong one-
electron oxidation agents ([NO]+, [thianthrene]+, or antimony
halide),41 forming a potential ReVI species. However, only
decomposition products were obtained, presumably due to side
reactions in the ligand backbone.
All complexes show irreversible reductions waves at ꢀ1.159 V
for 1, ꢀ1.179 for 2, and ꢀ1.102 V for 3 following the expected
trend of the potentials (chloride (3) < tolyl (1) < tert-butyl (2)),
where the electron-donating tert-butyl system is more difficult to
reduce.
Catalytic Epoxidations. Our ongoing interest in epoxidation
reactions employing Re(V) compounds17ꢀ19 led us to explore
also compounds 1ꢀ3 as catalysts in the epoxidation of cis-
cyclooctene with tert-butyl hydroperoxide (TBHP) as the oxi-
dant. This model reaction (Scheme 2) allowed us to compare
the performance the pyridazine complexes to the few previously
investigated Re(V) systems.17ꢀ24 Rhenium(V) compounds are
potentially ideal catalysts as they are conveniently prepared and
are easy to handle in ambient conditions. In particular, the latter
would be advantageous over Re(VII) systems that tend to be
more difficult to handle.
1 2C2H3N
2 CH2Cl2
3
3
empirical formula
ReC19H17Cl2N4O2 ReC16H19Cl2N4O2
3
3
2(C2H3N)
672.57
CH2Cl2
641.38
Mr, g/mol
cryst description
cryst syst
tablet, black
monoclinic
C2/c
tablet, red
orthorhombic
Pna21
space group
a, Å
29.2128(15)
13.7694(7)
14.0722(7)
90.00
18.639(3)
15.233(2)
7.9877(11)
90.00
b, Å
c, Å
R, deg
β, deg
114.230(2)
90.00
90.00
γ, deg
90.00
volume, Å3
5161.8(5)
8
2267.9(5)
4
Z
T, K
100(2)
293(2)
Dc, g/cm3
1.731
1.878
μ, mmꢀ1
1.407
1.407
F(000)
2624
1240
Typically, the catalyst (1 equiv) and cis-cyclooctene (100 equiv)
were mixed in chloroform and heated to 50 °C, whereupon
the catalyst slowly dissolved. The oxidant (TBHP in n-decane,
200 equiv) was then added, which initiated the reaction. The
reaction performance was monitored by periodically taking
samples for GC-MS analysis (Table 4). Complexes 1ꢀ3 convert
the substrate into epoxide; no other products were analyzed.
Despite the limited solubility of the complexes in chloroform at
room temperature, good performances could be achieved at
50 °C. At temperatures lower than 50 °C no conversion was
observed. Employing solvents in which compounds 1ꢀ3 are
more soluble, such as acetonitrile or acetone, did not yield any
product. Furthermore, with other potential substrates such as
cyclohexene or 4-phenyl butane no conversion was observed.
The performance of catalysts 1ꢀ3 is in the same range as
previously investigated active ReV systems.17ꢀ24 The lower
conversion of the olefin after 0.25 h employing complex 3 can
be attributed to its lower solubility compared to 1 and 2. In order
to obtain more insights into the mechanism we decided to explore
the catalyst in more detail. Whereas mechanistic information has
elegantly been elucidated for Re(VII) catalysts,42ꢀ44 no informa-
tion onthecatalyticallyactive species of theRe(V) systemsisasyet
reflns collected
unique reflns
reflns with I g 2 σ (I)
R(int), R(σ)
no. of parameters/restraints
final R1 a, wR2 b (I g 2 σ)
R indices (all data)
GOF on F2
32 563
26 521
7516
6200
6499
5048
0.0453, 0.0302
311/0
0.0430, 0.0377
228/7
0.0206, 0.0488
0.0283, 0.0532
1.060
0.0301, 0.0659
0.0441, 0.0715
1.016
largest diff. peak and hole, e/Å3 1.603, ꢀ1.110
1.271, ꢀ0.875
a R1 = Σ||Fo| ꢀ |Fc||/Σ|Fo|. b wR2 = {Σ[w(Fo ꢀ Fc2)2]2/Σ[w(Fo ) }
.
2
2 2 1/2
Electrochemical Studies. Redox properties of complexes
1ꢀ3 were investigated by cyclic voltammetry in dry acetonitrile
solution (Table 3). All complexes show fully reversible oxidation
waves with half-wave potentials of 0.869 V for 1, 0.851 V for 2,
and 0.932 V for 3, which can be assigned to one-electron
oxidations ReV f ReVI. The trend of the potentials (tert-butyl
(2) < tolyl (1) < chloride (3)) is consistent with the electronic
nature of the substituents at the pyridazine ring. Electron-with-
drawing groups (Cl, tol) decrease and the electron-donating tert-
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Table 3. Electrochemical Data for 1ꢀ3 in Dry Acetonitrile
Scheme 2. Catalytic Epoxidation of Cyclooctenea
Solution at Room Temperaturea
compound
Ep
E1/2
ΔEp
process
red
ox
[ReOCl2(Ltol)] 1
ꢀ1.159
ꢀ1.179
ꢀ1.102
ReV f ReIV
ReV f ReVI
ReV f ReIV
ReV f ReVI
ReV f ReIV
ReV f ReVI
0.869
0.851
0.932
75
77
71
tBu
a Conditions: 1 mol % Re catalyst 1ꢀ3, 1 equiv of cyclooctene, 2 equiv of
tBuOOH (5.5M in decane), 50 °C, CHCl3.
[ReOCl2(L )] 2
[ReOCl2(LCl)] 3
Table 4. Catalytic Performance of Complexes 1ꢀ3a
a Measured by cyclic voltammetry in CH3CN at 50 mV/s. E vs
Ag/AgNO3. Conditions: Pt disk (working), Pt wire (counter), and
Ag/AgNO3 (reference) electrodes; supporting electrolyte 0.05 M TBAP
solution in CH3CN in an Ar atmosphere.
catalyst
time [h] cyclooctene [%] cyclooctene oxide [%]
[ReOCl2(Ltol)] (1)
0.25
4
67
55
45
72
58
44
87
35
34
33
45
55
28
42
56
13
65
66
24
0.25
4
[ReOCl2(LtBu)] (2)
[ReOCl2(LCl)] (3)
available. Such information would be helpful for development of
future ReV precursors that show superior performance.
UVꢀVis Spectroscopy Under Catalytic Conditions. In
order to gain insight into the formation of the catalytically active
species time-resolved UVꢀvis measurements were performed.
Only the tert-butyl-substituted compound 2 exhibits solubility in
chloroform which was high enough for the investigations. With
the help of stock solutions, 0.2 μmol of compound 2 (1 equiv),
20 μmol of cis-cyclooctene (100 equiv), and 2 g of chloroform
were mixed in a quartz cuvette, which was placed into a spectro-
meter equipped with a holder thermostat controlled at 50 °C.
In order to increase the rate of the diluted reaction, excess
oxidation reagent was added (800 μmol of TBHP, 4000 equiv).
Immediately after addition of TBHP the first UVꢀvis spectrum
was measured, and thereafter, every 5 min additional spectra
were acquired for up to 9 h. Figure 4 shows UVꢀvis spectra
between 300 and 400 nm. The first spectrum (in bold) shows
a strong absorption at 355 nm which decreases with time.
New absorptions at 307 and 335 nm appeared during the first
5 min, after which they decrease to give a final spectrum after
3 h without distinct absorption bands in this region (see insert,
Figure 4).
These results suggest that the catalyst precursor 2 converts
quickly into at least one intermediate species INT (absorptions
at 307 and 335 nm) but with time slowly reacts further to a final
product. To monitor epoxide formation during the reaction,
GC-MS samples were taken periodically (Figure 4 b). Both the
starting material 2 and INT do not seem to be the catalytically
active compounds as no correlation to the rate of the catalytic
reaction is observable.
We were interested in the nature of the formed compounds, in
particular in the intermediate species immediately formed after
addition of the peroxide as well as in the species formed after
prolonged reaction time. By comparing literature data, it seems
reasonable to assign the UVꢀvis absorption at 335 nm and thus
the intermediate species INT to a rhenium trans-dioxo com-
pound, as comparable complexes absorb in the same wavelength
range ([Re(O)2(py)4]+ λmax = 331 nm,45 [Re(O)2(4-MeOPy)4]+
and [Re(O)2(4-pyrrolidinopyridine)4]+ λmax = 334 nm,46
[Re(O)2(acyclic tetraamine)]+ λmax =350nm,47 [Re(O)2(tridentate
SO2 ligand)] λmax = 300ꢀ350 nm,48 or [Re(O)2(bis-(3,5-
dimethylpyrazol-1-yl)methane)]+ λmax = 303 nm49). In contrast,
[Re(O)3Br(phen)], an analogue to a potentially oxidized pro-
duct, shows exclusively absorption below 300 nm.50
24
0.25
4
24
a Reactions were performed in chloroform at 50 °C, 1 equiv of substrate
(cyclooctene), 1 mol % catalyst, 2 equiv of TBHP. GC-MS analysis,
internal standard n-decane.
without substrate was investigated by 1H NMR, IR, and UVꢀvis
spectroscopies.
Oxidation of Compound 2: NMR and IR Spectroscopies.
Reaction of complex 2 with TBHP in the absence of substrate
1
was investigated by H NMR spectroscopy. In a vial, 20 mg
of complex 2 (36 μmol) was reacted with TBHP (10 equiv,
360 μmol) in 2 mL of chloroform at 50 °C. After 8 h product 4
was isolated as a pale orange solid material by removing all
volatiles. Its UVꢀvis spectrum was identical to the final
spectrum of the UVꢀvis experiment shown in Figure 4 a.
The 1H NMR spectrum in DMSO-d6 showed one new set of
downfield-shifted ligand signals with respect to the reso-
nances of the starting complex 2 (aromatic region shown in
Figure 5).
The two doublets marked with (a) are typical for the two
pyridazine protons whereas (b) corresponds to the imino
resonance. Those three signals are remarkably deshielded by
Δδ = 1.0, 0.5, and 2.25 ppm, respectively, in comparison to 2.
The strong downfield shift of an imino resonance has previously
been explained by the strong trans influence of an oxo ligand.51
Due to solubility reasons, no meaningful 13C NMR spectrum
could be obtained. Furthermore, in the IR spectrum of the
isolated material of complex 4 a very strong absorption at
ν = 888 cmꢀ1 was observed. Very strong absorptions in this
region are typical for rhenium trioxo complexes (e.g.,
[ReO3Br(phen)], [ReO3Cp], and [ReO3(tacn)]+ at 906, 887,
and 878 cmꢀ1).50,52 Thus, NMR and IR spectroscopies let us
believe complex 4 to be the oxidized rhenium trioxo complex
[ReO3(LtBu)], which is formed via the proposed intermediate
INT (eq 1).
t
t
þ BuOOH
þ BuOOH
½ReOCl2ðLtBuÞꢁ
½ReO ðLtBuÞꢁ
½ReO ðLtBuÞꢁ
f
f
2
3
4
2
proposed INT
In order to gain further insight into the nature of the
compounds the reactivity of compound 2 with excess TBHP
ð1Þ
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Figure 3. Cyclic voltammogram of complexes 1ꢀ3 in acetonitrile solution (scan rate 50 mV/s): (a) reversible oxidations and (b) irreversible
reductions.
Figure 4. Catalytic epoxidation of cyclooctene, 1 mol % catalyst 2, 1 equiv of cyclooctene, 40 equiv of TBHP. (a) UVꢀvis spectra measured every 5 min.
The initial spectrum is marked with a bold line. The insert shows the absorbance at 335 nm. (b) Product formation (GC-MS) and UVꢀvis absorbance at
335 nm.
Figure 5. 1H NMR spectra (aromatic region in DMSO-d6) of [ReOCl2(LtBu)] (2) and the crude product (4) obtained after reaction with 10 equiv of
TBHP after 8 h: (a) pyridazine resonances, (b) imino signal, (c) aromatic phenolate signals. Asterisks (*) marked unidentified impurities.
Oxidation of Compound 2: UVꢀVis Spectroscopy. Oxida-
tion of compound 2 with excess TBHP was followed by UVꢀvis
spectroscopy. This allows further insight into the reaction
mechanism by quantifying the number of reaction components
in solution without the need to isolate the individual species.
When we performed time-resolved UVꢀvis spectroscopy of
the reaction solution with an excess of TBHP (19 equiv) in the
absence of the substrate (Figure 6), an identical behavior was
observed as in the catalytic reaction (see Figure 4). These data
were subjected to graphical and mathematical assessment in
order to elucidate the number of linearly independent reaction
steps involved in formation of complex 4.
For detailed investigation the region from 300 to 350 nm was
considered, as there the largest absorption changes occur during
the reaction. Prior to the rate equation establishment the sum of
independent reactions in the system has to be determined. This
analysis was done graphically according to Mauser as well as
numerically with singular value decomposition (SVD)53,54 of the
experimental matrix Aλt. Processing details are summarized in
the Supporting Information.
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Graphical Assessment According to Mauser.53,54 The A
diagram (A/A) and the A difference diagram (AD/AD) for
selected wavelength combinations show curved lines as depicted
in Figure 7a and 7b, whereas the absorbanceꢀdifference plot
(ADQ) represents straight lines with high correlation to linear
fits as demonstrated in Figure 8.
The three-dimensional representation of the absorbanceꢀ
absorbanceꢀabsorbance correlation results in a curve laying
onto a plane. Rotation of the coordinate system leads to
projection of the plane as shown in Figure 9.55 Graphical
evaluation of the data according to Mauser thus results in a
reaction system of two independent steps.
Numerical Assessment.56,57 The number of the linear in-
dependent reactions is determined by the rank of the data matrix
A applying the SVD method. The diagonal matrix S obtained
from the transformation A = USVT consists of a series of singular
values (Figure 10). As the calculations are based on real data,
introduction of a tolerance (0.005) is essential. Thus, on applying
SVD to the six selected wavelengths (305ꢀ340 nm), two singular
values can be obtained, which clearly indicate a reaction system
consisting of two independent steps.
Both evaluations agree; thus, an irreversible reaction system
with two steps is anticipated for the present process. From the
chemistry involved, it is also clear that the reaction must be a
consecutive reaction. A reaction in two parallel steps is excluded.
Due to the excess of TBHP as the oxidizing agent pseudo-first-
order conditions are obeyed.
A þ ½Bꢁ f C f D
Fitting experimental data58 (e.g., 315 nm) to equation S6
(Supporting Information) provided the reaction constants
k1 = 1.047 ꢂ 10ꢀ2 sꢀ1 and k2 = 1.770 ꢂ 10ꢀ4 sꢀ1. Experimental
data and calculated values agree well as shown in Figure 11.
Additionally, experimental data with strong absorption varia-
tion (from 305 to 360 nm) were fitted applying the Levenbergꢀ
Marquardt algorithm (least-squares fit), yielding experimental
rate constants k1 = 1.06 ( 0.02 ꢂ 10ꢀ2 sꢀ1 and k2 = 2.3 ( 0.8 ꢂ
Figure 6. Oxidation of complex 2 (1 equiv) with 19 equiv of TBHP.
UVꢀvis spectra measured every 8 s. The initial spectrum is marked with
a bold line. (Insert) Absorption at 335 nm vs time.
10ꢀ4 sꢀ1
.
Figure 7. (a) Extinctions plot. (b) Differential extinctions plot.
Figure 8. ADQ plot for selected wavelengths; full scale (left) and zoom (right).
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Figure 9. (a) Three-dimensional absorptionꢀabsorptionꢀabsorption correlations and (b) projection.
Figure 10. Diagonal matrix S indicating two singular values.
Figure 12. Species distribution curves of compounds 2, INT, and 4.
’ CONCLUSION
In this work the straightforward synthesis of a series of ReV
complexes bearing novel tridentate pyridazine-containing li-
gands is presented. X-ray crystallography of complexes 1 and 2
revealed a rare trans-dichloro oxo configuration at the rhenium
centers. Complexes 1ꢀ3 were fully characterized including
NMR and IR spectroscopy as well as EI mass spectrometry
and cyclic voltammetry. They were successfully applied in the
catalytic oxidation of cyclooctene with TBHP, yielding ∼60%
cyclooctene oxide after 24 h. Furthermore, we could clearly show
that the initial precatalyst 2 is converted to 4 via one intermediate
INT. This supports our proposal where 2 quickly reacts to INT
(presumably [ReO2(LtBu)]), which is subsequently slowly con-
verted to [ReO3(LtBu)] (4) (eq 1). The constants obtained for
formation of INT and the obtained rate constant of the epoxida-
tion process deviate several orders of magnitude. Thus, neither 2
nor INT is the catalytically active species. The rate-determining
step represents formation of compound 4, which is the entry
point into the catalytic cycle.
Figure 11. Measured and calculated extinction curve at 315 nm with
A0 = 1.8 ꢂ 10ꢀ4 M.
The concentration curves shown in Figure 12 were obtained
by implementing the obtained coefficients into the differential
equation system shown in Supporting Information equations
S2ꢀS4.
Catalytic Performance of Compound 4. The isolated orange
material described above for which we have good evidence that is
represents compound [ReO3(LtBu)] (4) was investigated as
catalyst in the oxidation of cis-cyclooctene. Identical reaction
conditions were applied as in the experiment with catalyst 2
(in chloroform at 50 °C, 1 mol % catalyst, 2 equiv of TBHP). The
progress of the reaction was monitored by GC-MS analysis.
Comparing the performance of compound 4 to that of 2 showed
very similar results: after 0.25 h 23% (28% with 2), after 4 h 35%
(42% with 2), and after 24 h 50% (56% with 2). This provides
further evidence that the rhenium(V) catalyst precursor is
converted to 4, which is responsible for the catalytic conversion
of the olefin to the epoxide.
’ EXPERIMENTAL SECTION
General Considerations. Substituted hydrazinopyridazines27 and
[ReOCl3(OPPh3)(SMe2)]28 were prepared according to literature
procedures. All other chemicals were purchased from commercial
sources and used without further purification. All NMR spectra were
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measured on a Bruker Avance III spectrometer (300 MHz for 1H and 75
MHz for 13C NMR). The 1H NMR spectroscopic data are reported as
s = singlet, d = doublet, t = triplet, m = multiplet or unresolved, and
br = broad signal, coupling constant(s) are given in Hertz, and shifts are
given in ppm relative to the solvent residual peak. Infrared spectra were
measured on a Bruker ALPHA-P Diamant ATR-FTIR spectrometer.
Mass spectroscopy was performed on an Agilent Technologies 5973D
inert XL MSD direct insertion probe spectrometer applying electron
impact ionization. GC-MS analyses were performed on an Agilent
7890A with an Agilent 19091J-433 column coupled to a spectrometer-
type Agilent 5975C. UV/vis spectra were acquired with a Varian Cary 50
spectrophotometer with a thermostatted cuvette holder.
X-ray Diffraction Analyses. Intensity data sets for compounds
1 and 2 were collected using a Bruker Smart Apex II diffractometer
equipped with a graphite monochromator (Mo KR radiation,
λ = 0.71073 Å) and a CCD detector. Compound 1 was measured at
100 K, whereas compound 2 showed a disruptive phase transition upon
cooling and was therefore measured at 293 K. The structures were solved
by direct methods (SHELXS-97) and refined by full-matrix least-squares
techniques against F2 (SHELXL-97). Although systematic absences and
intensity statistics seemed to indicate for complex 2 the presence of a
glide plane m, a meaningful solution could only be found for the
noncentrosymmetric space group Pna21 (no. 33) instead of the apparent
Pnma (no. 63). All non-hydrogen atoms were refined with anisotropic
displacement parameters. Data were corrected for absorption and Lp
factors with SADABS.59 Hydrogen atoms were placed geometrically and
refined using a riding model. CCDC 809604 (for 1) and 809605 (for 2)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from the Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Electrochemical Methods. Redox chemical properties of com-
plexes 1ꢀ3 were carried out in an argon-filled glovebox using a Gamry
Reference 600 potentiostat connected to a personal computer. Cyclic
voltammograms were obtained in 0.05 M solution [nBu4N][PF6]/
CH3CN at 25 °C. The experiments were carried out in a three-electrode
glass cell with a platinum disk (d = 3 mm) working electrode, a platinum
wire auxiliary electrode, and a [Ag/AgNO3 0.01 M in CH3CN, 0.1 M
[nBu4N][ClO4]] reference electrode. Under these conditions, oxidation
of ferrocene occurs at 0.095 V.
General Procedure for Ligand Syntheses. A suspension of
para substituted 3-(1-methylhydrazino)pyridazine (1 equiv) in ethanol
(100 mL) was heated to reflux. Salicylaldehyde (1 equiv) was added to
the resulting solution. A white precipitate started to form after a few
minutes. The reaction mixture was heated to reflux for an additional
10 h, after which full conversion was evidenced by TLC. After cooling
the solution to room temperature the white precipitate was filtered off
and washed with ice-cooled ethanol to give pure products.
(s, 1H, OH). 13C NMR (75 MHz, DMSO-d6, 298 K): δ 30.1 (Cꢀ
(CH3)3), 30.3 (Cꢀ(CH3)3), 36.5 (NꢀCH3), 115.3, 116.6, 119.8, 121.7,
125.9, 127.0, 130.3, 134.8, 156.2, 158.0, 163.5 (imino and aromatic
carbons).
Synthesis of HLCl. General synthesis using 3-(1-methylhydrazino)-
6-(4-chloro)pyridazine (500 mg, 3.15 mmol) and salicylaldehyde (385
mg, 336 μL, 3.15 mmol) afforded 760 mg (92%) of pure HLCl. 1H NMR
(300 MHz, DMSO-d6, 298 K): δ 3.68 (s, 3H, NCH3), 6.84ꢀ6.95
(m, 2H, ArH), 7.20 (m, 1H, ArH), 7.67 (d, J = 9.5 Hz, 1H, ArH), 7.81 (d,
J = 7.5 Hz, 1H, ArH), 7.96 (d, 1H, ArH), 8.16 (s, 1H, NdCH), 10.12
(s, 1H, OH). 13C NMR (75 MHz, DMSO-d6, 298 K): δ 30.2 (NꢀCH3),
116.6, 118.7, 119.8, 121.6, 126.6, 130.2, 130.8, 135.6, 148.6, 156.4, 159.1
(imino and aromatic carbons).
Synthesis of [ReOCl2(Ltol)] (1). HLtol (100 mg, 0.31 mmol) and
[ReOCl3(OPPh3)(SMe2)] (199 mg, 0.31 mmol) were suspended in
acetone (20 mL) and heated to 50 °C. After a few seconds, a color
change to red occurred and the reagents slowly dissolved. The resulting
clear deep red solution was evaporated to dryness. Recrystallization from
hot acetonitrile afforded 145 mg (78%) of pure 1 as red crystals suitable
for X-ray diffraction analysis. ESI-MS (m/z (%)): 590 [M]+ (60); 556
[M ꢀ Cl]+ (100); 521 [M ꢀ :Cl2]+ (85). 1H NMR (300 MHz, DMSO-
d6, 298 K): δ 2.09 (s, 3H, CH3), 4.01 (s, 3H, NCH3), 7.21 (m, 1H, ArH),
7.44 (d, J = 8.1 Hz, 2H, ArH), 7.52 (m, 2H, ArH), 7.80 (d, J = 9.7 Hz, 1H,
ArH), 7.93 (d, J = 7.6 Hz, 1H, ArH), 8.16 (d, J = 8.1 Hz, 2H, ArH), 8.28
(s, 1H, NdCH), 8.45 (d, J = 9.7 Hz, 1H ArH,). 13C NMR (75 MHz,
DMSO-d6, 298 K): δ 21.5 (CH3), 34.9 (N-CH3), 100.0, 111.8, 114.8,
119.0, 119.6, 126.8, 130.4, 131.4, 1834.1, 135.2, 140.9, 147.0, 152.6,
156.7, 164.7 (imino and aromatic carbons). IR: (cmꢀ1, ATR) 1707 (m),
1599 (m), 1499 (s), 961 (s, νas RedO), 805 (s), 760 (s), 620 (m). Anal.
Calcd for ReC19H17N4O2Cl2 (Found): C, 38.65 (38.06); H, 2.90
(2.91); N, 9.49 (9.45).
Synthesis of [ReOCl2(LtBu)] (2). HLtBu (100 mg, 0.78 mmol) and
[ReOCl3(OPPh3)(SMe2)] (223 mg, 0.78 mmol) were suspended in
acetone (20 mL) and heated to 50 °C. After a few seconds, a color
change to red occurred and the reagents slowly dissolved. The resulting
clear deep red solution was evaporated to dryness. Recrystallization from
dichloromethane afforded 125 mg (64%) of pure 2 as red crystals
suitable for X-ray diffraction analysis. ESI-MS (m/z (%)): 556 [M]+
(100); 522 [M ꢀ Cl + H]+ (95); 486 [M ꢀ Cl2]+ (25). 1H NMR (300
MHz, DMSO-d6, 298 K): δ 1.46 (s, 9H, Cꢀ(CH3)3), 3.95 (s, 3H,
NCH3), 7.18 (m, 1H, ArH), 7.21 (d, J = 9.7 Hz, 1H, ArH), 7.50 (m, 2H,
ArH), 7.91 (m, 1H, ArH), 8.22 (s, 1H, NdCH), 8.34 (d, J = 9.7, 1H,
ArH). 13C NMR (75 MHz, DMSO-d6, 298 K): δ 29.2 (Cꢀ(CH3)3),
29.6 (Cꢀ(CH3)3), 37.4 (NꢀCH3), 111.9, 114.6, 118.8, 119.6, 132.8,
133.9, 135.1, 146.5, 156.4, 163.6, 164.6 (imino and aromatic carbons).
IR (cmꢀ1, ATR): 1600 (s), 1505 (s), 1435 (m), 1363 (m), 1149 (m),
982 (s), 964 (s, νas RedO), 763 (s). Anal. Calcd for ReC16H19N4O2Cl2
(Found): C, 34.54 (34.65); H, 3.44 (3.43); N, 10.07 (10.12).
Synthesis of HLtol. General synthesis using 3-(1-methylhydrazino)-
6-(4-para-methylphenyl)pyridazine (500 mg, 2.33 mmol) and salicylal-
dehyde (285 mg, 249 μL, 2.33 mmol) afforded 735 mg (99%) of pure
HLtol. 1H NMR (300 MHz, DMSO-d6, 298 K): δ 2.35 (s, 3H, CH3), 3.76
(s, 3H, NCH3), 6.87ꢀ6.95 (m, 2H, ArH), 7.21 (m, 1H, ArH), 7.30 (d, J =
8.1 Hz, 1H, ArH), 7.81 (m, 1H, ArH), 7.87 (d, J = 9.5 Hz, 1H, ArH), 7.96
(d, J = 8.1 Hz, 2H, ArH), 8.00 (d, J = 9.5 Hz, 1H, ArH), 8.16 (s, 1H,
NdCH), 10.17 (s, 1H, OH). 13C NMR (75 MHz, DMSO-d6, 298 K): δ
21.3 (CH3), 30.1 (NꢀCH3), 115.4, 116.6, 119.9, 121.7, 126.1, 126.2,
126.9, 129.9, 130.5, 133.9, 135.2, 139.0, 152.8, 156.3, 158.4 (imino and
aromatic carbons).
Synthesis of [ReOCl2(LCl)] (3). HLCl (100 mg, 0.38 mmol) and
[ReOCl3(OPPh3)(SMe2)] (241 mg, 0.38 mmol) were suspended in
acetone (20 mL) and heated to 50 °C. After a few seconds, a color
change to red occurred and the reagents slowly dissolved. The resulting
clear deep red solution was evaporated to dryness. Recrystallization from
hot acetonitrile afforded 170 mg (83%) of pure 3 as red crystals. ESI-MS
(m/z (%)): 534 [M]+ (80); 500 [M ꢀ Cl + H]+ (100); 465 [M ꢀ Cl2]+
(30). 1H NMR (300 MHz, DMSO-d6, 298 K): δ 4.00 (s, 3H, NCH3),
7.24 (m, 1H, ArH), 7.42 (d, J = 9.7 Hz, 1H, ArH), 7.53 (m, 2H, ArH),
7.94 (d, J = 7.8 Hz, 1H, ArH), 8.31 (s, 1H, NdCH), 8.45 (d, J = 9.7, 1H,
ArH). 13C NMR (75 MHz, DMSO-d6, 298 K): δ 35.3 (N-CH3), 111.49,
117.0, 119.2, 119.6, 134.4, 135.5, 135.7, 147.3, 148.2, 157.1, 164.8
(imino and aromatic carbons). IR (cmꢀ1, ATR): 1594 (m), 1504 (s),
1394 (m), 1296 (m), 1151 (m), 982 (m), 963 (s, νas RedO), 821 (m),
771 (s), 708 (m), 466 (m). Anal. Calcd for ReC16H19N4O2Cl2 (Found):
C, 26.95 (26.76); H, 1.88 (1.84); N, 10.48 (10.36).
Synthesis of HLtBu. General synthesis using 3-(1-methylhydrazino)-
6-(4-tert-butyl)pyridazine (500 mg, 2.77 mmol) and salicylaldehyde (339
mg, 296 μL, 2,77 mmol) afforded 710 mg (90%) of pure HLtBu. 1H NMR
(300 MHz, DMSO-d6, 298 K): δ 1.33 (s, 9H, Cꢀ(CH3)3), 3.70 (s, 3H,
NCH3), 6.87ꢀ6.93 (m, 2H, ArH), 7.18 (m, 1H, ArH), 7.64 (d, J = 9.5 Hz,
1H, ArH), 7.76 (d, J = 9.3 Hz, 2H, ArH), 8.11 (s, 1H, NdCH), 10.26
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Synthesis of [ReO3(LtBu)] (4). To a solution of complex 2 (20 mg,
36 μmol) in chloroform (2 mL) was added TBHP (5.5 M solution in
decane, 75 μL, 360 μmol). The mixture was heated to 50 °C and stirred
for 8 h. Subsequently, all volatiles were removed under reduced pressure
to yield a pale orange material, which was triturated with diethyl ether for
10 min to remove most of the decane. The material could as yet not be
obtained in an analytically pure form. 1H NMR (300 MHz, DMSO-d6,
298 K): δ 1.37 (s, 9H, Cꢀ(CH3)3), 4.35 (s, 3H, NCH3), 7.06 (t, J = 7.5
Hz, 1H, ArHPh), 7.13 (d, J = 8.1 Hz, 1H, ArHPh), 7.61 (m, 2H, ArH),
8.48 (d, J = 9.9 Hz, 1H, ArHPdz), 8.96 (d, J = 9.9 Hz, 1H, ArHPdz), 10.50
(s, 1H, NdCH). IR (cmꢀ1, ATR): 1369 (w), 1130 (w), 888 (vs), 828
(w), 759 (w).
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and substrate (1 mmol) were dissolved in CHCl3 (2 mL) at 50 °C. To start
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’ ASSOCIATED CONTENT
S
Supporting Information. Molecular view of complex 2
b
and processing details for graphical and mathematical assess-
ment. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: nadia.moesch@uni-graz.at.
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’ ACKNOWLEDGMENT
The authors acknowledge ForteꢀWissenschafterinnenkolleg
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