Synthesis and characterization of novel cyclopentadienyl molybdenum imidazo[1,5-a]pyridine-3-ylidene complexes and their application in olefin epoxidation catalysis

Article abstract of DOI:10.1016/j.jcat.2014.08.013

Two novel cyclopentadienyl molybdenum complexes [CpMo(CO)₂(ImPyMes)Cl] (1) (ImPyMes = 2-mesitylimidazo[1,5-a]pyridine-3-ylidene) and [CpMo(CO)2(ImPyMes)(NCCH3)]BF₄(2) were employed as pre-catalysts in olefin epoxidation catalysis usi

Full text of DOI:10.1016/j.jcat.2014.08.013

Journal of Catalysis 319 (2014) 119–126
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Synthesis and characterization of novel cyclopentadienyl molybdenum
imidazo[1,5-a]pyridine-3-ylidene complexes and their application
in olefin epoxidation catalysis
Andrea Schmidt , Nidhi Grover , Teresa K. Zimmermann a,b,1, Lilian Graser , Mirza Cokoja ,
a,1
a,1
a
a
a
a,
⇑
Alexander Pöthig , Fritz E. Kühn
a
Chair of Inorganic Chemistry/Molecular Catalysis, Catalysis Research Center, Technische Universität München, Ernst-Otto-Fischer-Straße 1, 85747 Garching b. München, Germany
WACKER Institut für Siliciumchemie, Department of Chemistry, Technische Universität München, Ernst-Otto-Fischer-Straße 1, 85747 Garching b. München, Germany
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel cyclopentadienyl molybdenum complexes [CpMo(CO) (ImPyMes)Cl] (1) (ImPyMes = 2-mesi-
2
Received 15 May 2014
Revised 13 August 2014
Accepted 14 August 2014
tylimidazo[1,5-a]pyridine-3-ylidene) and [CpMo(CO)
2 3 4
(ImPyMes)(NCCH )]BF (2) were employed as
pre-catalysts in olefin epoxidation catalysis using tert-butylhydroperoxide (TBHP) as oxidant. Turnover
À1
À1
frequencies (TOFs) of 40,900 h (0.005 mol% 1) and 53,100 h (0.01 mol% 2) were achieved in cis-cyclo-
octene epoxidation in CHCl at 55 °C, outperforming most previously reported Mo epoxidation catalysts.
The synthesis of 1 proceeds via the silver carbene transmetallation route in 90% yield, and further treat-
ment of 1 with AgBF in CH CN leads to the ionic complex 2 in 87% yield. Even at very low pre-catalyst
3
Keywords:
Molybdenum
Carbene ligand
Cyclopentadienyl ligand
Epoxidation
4
3
concentrations, quantitative cis-cyclooctene conversion with high selectivity is achieved. Furthermore,
epoxidation of more challenging substrates results in good conversions and recycling experiments in
room temperature ionic liquid (RTIL) [C mim]NTf (C mim = 1-methyl-3-octylimidazolium, NTf =
8 2 8 2
Homogeneous catalysis
bis(trifluoromethanesulfonyl)imide) shows reusability of 1 in catalytic epoxidation for several runs with-
out loss of activity.
Ó 2014 Elsevier Inc. All rights reserved.
1
. Introduction
decarbonylation to give catalytically active dioxo [CpMoO
2
R] and
oxo-peroxo [CpMo(O)(O
To date, a number of
2
)R] complexes (Scheme 1) [3,5,7,8].
5
Transition metal-catalyzed epoxidation reactions are of consid-
g
-cyclopentadienyl tricarbonyl molybde-
erable interest due to the industrial demand for epoxides as inter-
num complexes with different ligand spheres have been investi-
mediates for the synthesis of pharmaceuticals, agrochemicals, and
gated as pre-catalysts in olefin epoxidation [2,9–15]. Of these,
5
5
many other compounds [1,2]. Over the past two decades,
g
-cyclo-
the ansa-bridged cyclopentadienyl Mo complex [{
g
-C
5
H
4
pentadienyl molybdenum complexes have been extensively stud-
[CH(CH ]-
2
)
3
g
1-CH}Mo(CO)
3
] [16] shows the highest TOF in cis-
À1
ied as olefin epoxidation catalysts. Bergman and Trost were the
cyclooctene epoxidation, namely 11,800 h in organic solvents
5
À1
first to report on olefin epoxidation with a
pentadienyl Mo catalyst [Cp*MoO
oxides [3]. The pre-catalysts [CpMo(CO)
Mo(CO) CH ] [6], which can be easily oxidized to their catalytically
g
-pentamethylcyclo-
[17] and TOFs up to 44,000 h in room temperature ionic liquids
2
Cl] in the presence of hydroper-
(RTILs) [18].
3
Cl] [4,5] and [Cp
The use of N-heterocyclic carbene (NHC) ligands has extended the
scope of cyclopentadienyl molybdenum complexes in olefin epoxi-
dation catalysis. As spectator ligands, they often lead to higher com-
3
3
active species using alkyl hydroperoxides, show good catalytic
À1
activities with turnover frequencies (TOFs) of up to 6000 h at a
plex stability due to their strong r-donating and poor p-accepting
pre-catalyst loading of 0.1 mol% and with cis-cyclooctene as sub-
character [19]. Royo et al. synthesized the first ansa-bridged cyclo-
strate. Cyclopentadienyl molybdenum(II) tricarbonyl complexes
pentadienyl-imidazolylidene molybdenum complexes with the
general formula [g -C R {CR CRPh-g -NHC }Mo(CO) I] (R = H,
5 4 2 2
5
1
Me
[
Cp*Mo(CO)
3
R] (R = alkyl, halide) are known to undergo oxidative
alkyl) [20]. All of these complexes are catalytically active in olefin
epoxidation; however, the catalytic reactions exhibit long
induction periods and proceed rather slowly in comparison with
tricarbonyl molybdenum complexes. On the other hand, longer
⇑
Corresponding author.
E-mail address: fritz.kuehn@ch.tum.de (F.E. Kühn).
A. S., N. G. and T. K. Z. contributed equally to this work.
1
http://dx.doi.org/10.1016/j.jcat.2014.08.013
021-9517/Ó 2014 Elsevier Inc. All rights reserved.
0

1
20
A. Schmidt et al. / Journal of Catalysis 319 (2014) 119–126
on a Varian ATR-FTIR instrument. Elemental analyses were carried
out by the microanalytical laboratory of Technische Universität
München. FAB mass spectra were obtained with a Finnigan MAT
TBHP
TBHP
Mo
Mo
R
O
OC
OC
R
O
Mo
R
O
CO
O
O
9
0 spectrometer. Catalytic runs were monitored by GC methods
on a Varian CP-3800 gas chromatograph. 2-Mesitylimidazo[1,5-
a]pyridinium chloride was prepared according to the known proce-
dure [25].
tBuOH
Epoxide
tBuOH
+ Olefin + Epoxide
2.2. Single-crystal XRD structure determination
TBHP
TBHP
+ Olefin
+
Crystallographic data (excluding structure factors) for the struc-
tures reported in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary
publication numbers CCDC-985095 (1), and CCDC-985096 (2).
Copies of the data can be obtained free of charge on application
to the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. Fax: +44
3
Scheme 1. Oxidation of [CpMo(CO) R] precatalysts with TBHP. Adapted from Kühn
et al. [8].
catalyst lifetimes are observed due to their increased stability under
oxidative conditions, and thus, higher conversions are obtained. In
2
010, Hor, Zhao et al. reported the synthesis of a series of cyclopen-
tadienyl imidazolylidene molybdenum complexes [21]. The neutral
complexes [CpMo(C O) (NHC)X] (X = halide) show comparatively
poor catalytic activities in epoxidation reactions, whereas the ionic
complex [CpMo(CO) (IMes)(NCCH )]BF (IMes = 1,3-bis(2,4,6-tri-
1
223 336 033; e-mail, deposit@ccdc.cam.ac.uk (see also Table 1).
2
2.3. Computational details
2
3
4
methy lphenyl)imidazol-2-ylidene) is quite active with a TOF of
All calculations were performed with Gaussian09 [30]. The
hybrid functional B3LYP [31–33] and the nonhybrid functional
BPW91 [34,35] were used with the split valence double-f (DZ)
6-31G** basis set [36,37] for all atoms except Mo, which was trea-
ted with the Stuttgart97-ECP with a DZ description of the valence
electrons [38]. This level of theory was used for geometry optimi-
zation and frequency calculation. Carbonyl absorption bands and
partial atomic charges (Mulliken [39] and NBO [40–46]) were
taken from the Gaussian-09 outputs of the frequency calculations
and Natural Bond Orbital Analysis, respectively. No symmetry or
coordinate constraints were applied during optimizations. All
reported optimized geometries were verified as being true minima
by the absence of eigenvalues in the vibrational frequency analysis.
À1
>34 00 h . Another type of NHC molybdenum complexes of the
formula [CpMo(CO)
,2,3-triazol-5-ylidenyl) has been recently reported by our group
22]. This triazolylidene-based Mo catalyst was applied as pre-
2
(NHC)Cl] (NHC = 1-methyl-2-phenyl-4-tolyl-
1
[
catalyst in epoxidation catalysis and features moderate catalytic
activity in ionic liquids.
Imidazo[1,5-a]pyridine-3-ylidenes (ImPy) are among the stron-
gest heteroaromatic
r-donors and are sterically more demanding
than 1,3-disubstituted imidazolylidenes. Consequently, the stereo-
electronic environment at the metal center may be manipulated
easily by varying substituents on the NHC ligand [23–25]. Several
imidazo[1,5-a]pyridine-3-ylidene transition metal complexes
based on Rh [26–28], Ir [23,26] and Pd [23,27] have been reported
previously and applied as catalysts in allylic substitution [27],
Suzuki–Miyaura cross-coupling [23] and other cross-coupling
reactions [24]. To the best of our knowledge, no reports of their
application in olefin epoxidation are known. The bicyclic structure
of ImPy ligands facilitates a unique stereoelectronic environment.
Therefore, interest in the effect of this type of ligand for molybde-
num complexes and their activities in olefin epoxidation appears to
be justified. This report discusses the synthesis and characteriza-
9
5
Calculation of Mo NMR shifts was carried out using the GIAO
method [47–51] and the B3LYP and BPW91 functionals with the
9
5
IGLOII basis set [52] instead of 6-31G** [53]. For Mo NMR shift
calculation, the BPW91 functional was found to be superior to
the popular B3LYP in previous studies, justifying its supplementary
use in the present study [53,54]. The polarizable continuum model
(PCM) [55–74] was applied for all calculations in solvent media.
2.4. Synthesis of [CpMo(CO) (ImPyMes)Cl] (1)
2
tion of two novel NHC molybdenum complexes [CpMo(CO)
Mes)Cl] (1) (ImPyMes = 2-mesitylimidazo[1,5-a]pyridine-3-
ylidene) and [CpMo(CO) (ImPyMes)(NCCH )]BF (2) and their
application as pre-catalysts in olefin epoxidation.
2
(ImPy-
2-Mesitylimidazo[1,5-a]pyridinium chloride (0.400 g, 1.5 mmol,
1.0 equiv.) and silver(I)-oxide (0.170 g, 0.7 mmol, 0.5 equiv.) were
suspended in dry dichloromethane (20 mL). The reaction mixture
was stirred at room temperature under exclusion of light for 16 h.
2
3
4
2
. Experimental section
.1. General remarks
All preparations and manipulations were performed using stan-
Table 1
Crystallographic data for complexes 1 and 2.
2
1
2
Formula
Formula wt.
Space group
a (Å)
b (Å)
c (Å)
C
24
H
22
C
l4MoN
2
O
2
C
25 4 3 2
H24BF MoN O
608.18
P
581.22
P 21/n
dard Schlenk techniques under an argon atmosphere. All reagents
were purchased from commercial sources and used without fur-
ther purification. All solvents were dried with a MBraun MB SPS
8.8892(2)
10.8399(2)
13.5088(3)
102.602(1)
93.068(1)
101.799(1)
1236.90(5)
2
1.633
4509
301
0.0236
13.5880(3)
12.2260(3)
15.0385(4)
90
95.550(1)
90
2496.59(11)
4
1.553
4567
409
0.0257
0.0585
1.020
8
00 purification system and stored under argon over molecular
sieves. Chromatographic separations were performed using silica
gel (40–63 m). All catalytic reactions were carried out under
a
(°)
b (°)
(°)
l
c
3
V (Å )
laboratory atmosphere. NMR spectra were recorded with a Bruker
Avance DPX 400 or a Bruker Avance III 400 spectrometer at a tem-
perature of 298 K. The spectra were referenced to the residual
Z
À3
D
N
N
calc (g cm
)
1
H
ref
1
3
1
and C{ H} signals of the solvents in parts per million (ppm)
29]. Abbreviations for NMR multiplicities are as follows: sin-
par
R1 (I > 2
wR2 (all data)
Goodness of fit
r
(I))
[
0.0532
1.059
glet (s), doublet (d), triplet (t), quartet (q), and multiplet (m).
Coupling constants J are given in Hz. The IR spectra were recorded

A. Schmidt et al. / Journal of Catalysis 319 (2014) 119–126
121
The suspension was filtered through Celite and dried under reduced
pressure to give a pale yellow solid. The solid was dissolved in dry
toluene (25 mL) and [CpMo(CO) Cl] (0.253 g, 0.9 mmol, 0.6 equiv.)
3
was added. The solution was heated to reflux under exclusion of
light for 1 h. The dark red solution was concentrated under reduced
pressure and the residue was purified by column chromatography
water and diluted with iso-propanol (0.3 mL). After filtration, a
mixture of indane and p-xylene (4 mg/mL each) in iso-propanol
was added as external standard, and the solution was injected into
a GC column. The conversion of cis-cyclooctene and the formation
of cyclooctene epoxide were calculated from calibration curves
2
(r = 0.999) recorded prior to the reaction course. For the recycling
(
gradient elution with hexane:ethyl acetate = 2:1). The eluting pink
8 2
experiments in RTIL [C mim]NTf , a sample was taken after 24 h
band was collected and concentrated under reduced pressure to
and treated as described above. For subsequent runs, the upper
phase was removed by means of cannulation from the reaction
vessel. Tert-butanol was removed from the remaining ionic liquid
phase under reduced pressure. After drying, the next batch of cis-
cyclooctene (1.10 g, 10 mmol) and TBHP (3.64 mL, 5.5 M in n-dec-
ane) was added to the RTIL phase at 55 °C. The procedure was
repeated 10 times. The course of the reaction in case of the other
substrates (1-octene, cis-stilbene and trans-b-methylstyrene) was
1
give the product as a pink solid (0.395 g, 90%). H NMR (400 MHz,
3
3
CDCl
Hz, 1H, H
m-HMes), 6.92 (dd, JH,H = 9.1, 6.5 Hz, 1H, H7), 6.65 (t, JH,H = 7.0 Hz,
3
): d (ppm) = 8.21 (d, JH,H = 7.5 Hz, 1H, H
5
), 7.32 (d, JH,H = 9.2
8
), 7.25 (s, 1H, H ), 7.11 (s, 1H, m-HMes), 7.07 (s, 1H,
1
3
3
1
H, H
6
), 4.94 (s, 5H, HCp), 2.43 (s, 3H, p-CH
3 3
), 1.98 (s, 6H, o-CH ).
1
3
C NMR (101 MHz, CDCl ): d (ppm) = 257.4 (CO), 252.2 (CO),
3
1
1
7
79.6 (CCarbene), 140.4 (Car), 138.0 (Car), 137.2 (Car), 135.5 (Car),
33.4 (Car), 130.7 (Car), 129.3 (Car), 129.1 (Car), 123.4 (Car), 11
1
monitored by H NMR analysis at specific time intervals. The con-
version of the olefins and the yields of epoxides were calculated
according to the internal standard.
.0 (Car), 113.4 (Car), 113.2 (Car), 95.2 (CHCp), 21.3 (p-CH
3
), 18.2
): d (ppm) = À513.
(cm ) = 1943 (CO), 1834 (CO). MS (FAB): m/z
95
(
o-CH
IR (solid):
%) = 455.1 ([M–Cl] ). EA: Anal. Calcd for C23
C 56.51, H 4.33, N 5.73. Found: C 56.49, H 4.41, N 5.71.
3 3 3
), 17.8 (o-CH ). Mo NMR (26 MHz, CDCl
À1
+
m
(
2 2
H21ClMoN O (%):
3. Results and discussion
2 3
2.5. Synthesis of [CpMo(CO) (ImPyMes)(NCCH )]BF
4
(2)
3.1. Synthesis and characterization of 1
Complex 1 (0.488 g, 1.0 mmol, 1.0 equiv.) was dissolved in dry
acetonitrile (25 mL), and AgBF (0.389 g, 2.0 mmol, 2.0 equiv.)
The ligand precursor 2-mesitylimidazo[1,5-a]pyridinium chlo-
ride was prepared according to a literature procedure via a three-
component coupling reaction between picolinaldehyde, formalin
and 2,4,6-trimethylaniline [25]. 2-Mesitylimidazo[1,5-a]pyridine-
3-ylidene Mo complex 1 was synthesized by the transmetalation
route via silver carbene of 2-mesitylimidazo[1,5-a]pyridinium
chloride according to a modified synthesis of similar compounds
[21]. The silver carbene was formed in situ by treating the
imidazo[1,5-a]pyridinium salt with silver(I)-oxide. 1 was obtained
as a pink solid in 90% yield by reacting the silver carbene with
4
was added to the pink solution. The mixture was stirred at room
temperature for 1 h to create a red suspension, which was filtered
through celite. Volatiles were removed under reduced pressure.
The red residue was purified by recrystallization from acetonitrile
and diethyl ether to obtain the product as red crystals (0.504 g,
1
3
8
1
1
7%). H NMR (400 MHz, CD
3
CN): d (ppm) = 8.07 (d, JH,H = 7.6 Hz,
3
H, H ), 7.72 (s, 1H, H
5
1
), 7.61 (d,
J
H,H = 9.3 Hz, 1H, H
8
), 7.20 (s,
3
H, m-HMes), 7.18 (s, 1H, m-HMes), 7.11 (dd, JH,H = 9.2, 6.5 Hz, 1H,
3
H
7
), 6.91 (t,
p-CH ), 2.10 (s, 3H, o-CH
o-CH
J
H,H = 6.9 Hz, 1H, H
), 1.96 (s, 3H, NCCH
NMR (101 MHz, CD CN): (ppm) = 250.8 (CO),
6
), 5.12 (s, 5H, HCp), 2.42 (s, 3H,
3
[CpMo(CO) Cl] (Scheme 2).
3
3
3
), 1.81 (s, 3H,
Complex 1 is stable towards air and moisture and can be stored
and handled in laboratory atmosphere; it is highly soluble in polar
and non-polar solvents such as methanol, acetonitrile, chloroform,
THF, toluene and benzene but poorly soluble in n-hexane and
n-pentane. TG–MS analysis confirms the stability of 1 at ambient
temperature, as decomposition starts at 241.5 °C by loss of a car-
bonyl ligand relating to a MS signal with 28 m/z (CO). Complex 1
1
3
3
).
C
3
d
2
1
4
48.7 (CO), 171.7 (CCarbene), 141.8 (Car), 138.1 (Car), 136.9 (Car),
36.5 (Car), 134.9 (Car), 130.3 (Car), 130.1 (Car), 129.1 (Car), 12
.3 (Car), 119.4 (Car), 117.6 (Car), 116.0 (Car), 95.7 (CHCp), 21.2
95
3 3 3 3
(p-CH ), 17.7 (o-CH ), 17.5 (o-CH ). Mo NMR (26 MHz, CD CN):
1
1
d (ppm) = À643. B NMR (128 MHz, CD CN): d (ppm) = À1.18.
3
1
9
10
À
1
13 95
F NMR (377 MHz, CD
3
CN): d (ppm) = À151.68 ( BF
4
), À15
has been characterized by IR, NMR ( H, C, Mo), FAB-MS and ele-
mental analysis. The successful metallation is confirmed by the
appearance of the characteristic carbene signal at 179.6 ppm in
1
À
À1
1
4
.73 (1 BF
7 (BF
). IR (solid):
m
(cm ) = 1969 (CO), 1860 (CO), 10
À
+
4
). MS (FAB): m/z (%) = 455.1 [M–NCCH
MoN (%): C 51.66, H 4.16, N 7.23. Found: C 51.41,
H 4.18, N 7.06.
3
] . EA: Anal. Calcd
1
3
for C25
H24BF
4
3
O
2
C NMR and by the shift of the Cp signal from 5.66 ppm in
1
[CpMo(CO)
of the new complex [CpMo(CO)
two carbonyl signals at 252.2 and 257.4 ppm in C NMR, indicat-
3
Cl] upfield to 4.94 ppm in H NMR of 1. The formation
(ImPyMes)Cl] is further verified by
2
1
3
2.6. Epoxidation catalysis reactions
ing a cis-configuration of the carbonyls [75,76]. The IR spectra
À1
For cis-cyclooctene: Substrate (1.10 g, 10 mmol) and pre-catalyst
show a strong shift from
m
CO = 1931, 2043 cm of [CpMo(CO)
3
Cl]
À1
[
0.1 mol%, 10
were added to the reaction vessel. The catalyses were carried out
neat, in an organic solvent, e.g. CHCl (5 mL) or in RTIL [C mim]NTf
0.5 mL) at 55 °C or at room temperature. The reaction was initiated
lmol (or: 1 mol%, 0.05 mol%, 0.01 mol%, 0.005 mol%)]
to
m
CO = 1834, 1943 cm of 1, illustrating the strong
character of the NHC ligand. In comparison with similar literature
known complexes (Fig. 1) [20–22], the imidazo[1,5-a]pyridine-3-
ylidene in 1 exhibits slightly stronger r-donor characteristics than
the triazolylidene (A: 1848, 1944 cm ) [22] and the ansa-NHC
ligands first reported by Royo et al. (B: 1851, 1945 cm ) [20]
and significantly stronger r-donor properties than the imidazoly-
r-donating
3
8
2
(
À1
by adding the oxidant TBHP (3.64 mL, 5.5 M in n-decane). For other
substrates: Olefin [1-octene (112 mg), cis-stilbene (180 mg) or
trans-b-methylstyrene (118 mg); 1 mmol], 1,2-dichloroethane
À1
(
1
99 mg, 1 mmol, internal standard) and pre-catalyst (0.1 mol%,
mol) were dissolved in CDCl (0.5 mL). The reaction was carried
out in an NMR tube at 55 °C and initiated by adding TBHP (364 L,
.5 M in n-decane).
The course of the epoxidation reaction of cis-cyclooctene was
monitored by quantitative GC analysis. Samples (0.2 mL) were
taken at specific time intervals, treated with activated MnO to
decompose excess alkyl hydroperoxide and MgSO to remove
lidenes originally described by Hor, Zhao et al. (C: 1860,
1956 cm ) [21].
The IR spectrum of 1 provides further indication of a cis-config-
uration of CO ligands with the relative intensities Iasym and Isym of
À1
l
3
l
5
m
asym(CO) and msym(CO) using the ratio Iasym/Isym = tan 2h (2h = OC–
9
5
Mo–CO bond angle) [75,76]. The Mo NMR signal is shifted from
À836 ppm in [CpMo(CO) Cl] [77] downfield to À513 ppm in 1
and provides insight into the electronic situation of the metal.
2
3
4

1
22
A. Schmidt et al. / Journal of Catalysis 319 (2014) 119–126
Scheme 2. Synthesis of complex 1.
to 171.7 ppm and of the carbonyl signals to 248.7 and
1
1
19
À
4
2
50.8 ppm. B NMR and F NMR confirm the presence of BF as
1
a counterion. The appearance of a signal in H NMR at 1.96 ppm
from acetonitrile, the elemental analysis and the MS spectrum
indicate that one solvent molecule coordinates to molybdenum,
as observed for D (Fig. 1) [21]. The Mo NMR signal appears
upfield at À643 ppm compared to 1 (À513 ppm). However, this
should not be taken to indicate lower Lewis acidity in 2, since
9
5
9
5
Mo NMR shifts are comparable only for closely related com-
pounds. The substitution of chloride for acetonitrile and the subse-
quent formation of an ionic complex render the direct comparison
9
5
of Mo NMR shifts of 1 and 2 impossible. CO absorption bands in
IR spectra, however, appear to give a better indication of the elec-
tronic situation at the metal center. The CO bands of 2 are shifted
À1
Fig. 1. Previously reported cyclopentadienyl NHC molybdenum complexes A [22], B
20], C [21], and D [21] with similar structural motifs.
to higher frequencies (1860 and 1969 cm ), indicating less
[
p-backbonding of molybdenum compared to 1 and thus higher
Lewis acidity. This assumption is supported by DFT investigations
of Mulliken and NBO charges on the B3LYP 6-31G** level of theory,
which point to a higher Lewis acidity of 2 compared to 1. The
tendencies observed for CO absorption bands in IR and signals in
Compared to the signal of the ansa-NHC Mo complex B at
9
5
À810 ppm [20], the Mo NMR signal of 1 is shifted downfield,
indicating lower electron density at the Mo center and thus higher
Lewis acidity of molybdenum. This is supported by NBO and
Mulliken charges at Mo obtained from DFT calculations on the
9
5
Mo NMR correspond well to the results of the theoretical study
of the complexes, supporting the validity of the theoretical model.
Further details on DFT studies can be found in the Supporting
9
5
13
B3LYP/6-31G** level of theory (see Table S2). Even though Mo
NMR shifts are susceptible to small changes within the ligands
sphere, they can be used as indicators of the electronic situation
at the metal center due to similar structural motifs of B and 1
Information. As in 1, C NMR and IR spectra of 2 indicate a
cis-arrangement of the carbonyl ligands.
3.3. Single-crystal XRD structure determination of 1 and 2
[
78]. This effect is counterintuitive when the r-donor properties
of the NHC-ligands in 1 and B are considered, but may be
attributed to the simultaneous variation of NHC and halide ligand.
The molecular structures of 1 and 2 were determined by single-
crystal XRD. Suitable crystals of 1 were obtained by diffusion of
n-hexane into a solution of 1 in ethyl acetate. Single crystals of 2
were obtained by diffusion of diethyl ether into a solution of 2 in
acetonitrile. The molecular structures of both complexes as well
as selected bond lengths and angles are depicted in Figs. 2 and 3.
Both structures can be described as four-legged piano stool half-
sandwich Mo(II) complexes. The L–Mo–L (L = ligand) bond angles
vary from 74.36(8)° to 82.43(6)° for 1 and from 75.55(9)° to
84.72(8)° for 2 and thus fall within the range of related compounds
[20,21]. The OC–Mo–CO bond angles of 74.67(9)° in 1 and
75.55(9)° in 2 confirm the cis-arrangement of the carbonyl ligands
in both complexes. A comparison of some selected bond distances
of 1, 2, A, C, and D is given in Table 2.
3
.2. Synthesis and characterization of 2
The synthesis of the ionic imidazo[1,5-a]pyridine-3-ylidene
molybdenum complex 2 was conducted according to a modified
literature procedure by treating 1 with AgBF in acetonitrile
Scheme 3) [21].
After recrystallization, 2 was isolated as red crystals in 87%
4
(
yield; it is less stable towards air and moisture than 1 but can be
handled in a laboratory atmosphere for a few hours. Compound 2
is highly soluble in polar solvents such as methanol and acetoni-
trile, but poorly soluble in chloroform. The ionic complex 2 has
been characterized by IR, NMR ( H, C, Mo, B, F), FAB-MS
and elemental analysis. The successful substitution of chloride is
indicated by H NMR spectroscopy. A downfield shift of the Cp sig-
3 3
nal from 4.94 ppm (1 in CDCl ) to 5.12 ppm (2 in CD CN) is
observed. C NMR shows the upfield shift of the carbene signal
1
13
95
11
19
The r-donating character of different NHC ligands is reflected in
the Mo–CCarbene bond distance. Complexes 1 and 2 feature bond
lengths (2.227(2) and 2.222(2) Å) comparable to the triazolylidene
complex A (2.221(4) Å) and shorter Mo–CCarbene distances than
C (2.244(3) Å) and D (2.249(3) Å). The Mo–CO distances of 1
1
1
3
(
(
1.961(2) and 1.956(2) Å) are on average longer than in A
1.965(4) and 1.945(4) Å) and C (1.939(5) and 1.932(4) Å), indicating
that oxidative decarbonylation might be facilitated in 1 compared
to known molybdenum NHC complexes. The Mo–CO bond lengths
are even longer in the ionic complex 2 (1.985(3) and 1.941(3) Å),
illustrating even weaker bonding of the carbonyl ligands to
molybdenum. Because of the proposed increased Lewis acidity of
2
compared to 1 and the resulting stronger
p-acceptor character,
Scheme 3. Synthesis of complex 2.
the Mo–CpCentroid distance of 2 (2.002 Å) is slightly shorter than

A. Schmidt et al. / Journal of Catalysis 319 (2014) 119–126
123
Table 2
Selected bond lengths [Å] of complexes 1 and 2 as well as of compounds A [22], C [21],
and D [21].
Bond
1
2
A
C
D
Mo–CO
1.961(2)
1.985(3)
1.941(3)
2.002
1.965(4)
1.945(4)
–
2.519(1)
2.221(4)
1.939(5)
1.932(4)
1.996(3)
1.947(3)
1.956(2)
c
c
c
Mo–CpCentroid
Mo–X
Mo–CCarbene
2.015
2.5389(5)
2.227(2)
–
–
–
a
2.177(2)b
2.222(2)
a
c
2.172(2)b
2.249(3)
2.244(3)
a
b
c
X = Cl.
X = CH
Not available.
3
CN.
mine the effect of the ImPyMes ligand on the catalytic activity of
these Mo NHC complexes. Cis-cyclooctene, 1-octene, cis-stilbene,
and trans-b-methylstyrene were employed as olefins under various
reaction conditions, varying temperature, substrate:oxidant ratio,
solvent, and pre-catalyst loading. In all catalytic experiments, TBHP
(
5.5 M in n-decane) was used as an oxidant for the sake of compar-
ison with known Mo(II) catalytic systems. Control experiments
without catalyst yielded no epoxide. In all catalytic reactions, no
significant formation of byproducts was observed; thus conversion
of substrate equals yield of epoxide. Every catalysis was performed
three times. The single data points are averaged values of these
three independent measurements.
To determine the optimum reaction conditions regarding tem-
perature, substrate:oxidant ratio, solvent, and pre-catalyst concen-
tration, several experiments were conducted using cis-cyclooctene
as a model substrate. The kinetic profiles were monitored by GC
and plotted in time-conversion graphs; activities and turnover fre-
quencies (TOFs) were determined at the steepest slope after 5 min
and/or 15 (30) min. The applied pre-catalysts 1 and 2 were oxi-
dized to their catalytically active species using TBHP. Since the
exact catalyst concentration cannot be determined due to the
in situ formation, all TOFs are referenced to the pre-catalyst
concentration.
Fig. 2. ORTEP style plot of 1. Thermal ellipsoids are shown at the 50% probability level.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°):
Mo1–C22 1.956(2), Mo1–C23 1.961(2), Mo1–C1 2.227(2), Mo1–Cl1 2.5389(5);
C22–Mo1–C23 74.67(9), C23–Mo1–C1 74.36(8), C1–Mo1–Cl1 78.58(5), C22–Mo1–
Cl1 82.43(6).
Epoxidation at room temperature with 0.1 mol% pre-catalyst
loading gave an induction period (Fig. 4). Oxidative decarbonyla-
tion of the pre-catalyst is indicated by gas evolution and a rapid
color change of the catalyst mixture from pink (1) or red (2) to col-
orless. At room temperature, oxidative decarbonylation occurs
after 25 min for 1 and after 12 min for 2. At 55 °C, oxidative
decarbonylation was observed within the first 5 min, after 4 min
for 1 and after 1 min for 2. In general, complex 2 was found to
be more active than 1 in epoxidation catalysis of cis-cyclooctene
both at room temperature and at 55 °C. This is in line with its faster
activation via oxidative decarbonylation as illustrated by the
Fig. 3. ORTEP style plot of 2. Thermal ellipsoids are shown at the 50% probability
À
level. Hydrogen atoms and BF
4
anions are omitted for clarity. Selected bond
lengths (Å) and angles (°): Mo1–C23 1.941(3), Mo1–C22 1.985(3), Mo1–
C1 2.222(2), Mo1–N3 2.177(2); C23–Mo1–C22 75.55(9), C22–Mo1–C1 76.43(9),
N3–Mo1–C1 79.05(7), C23–Mo1–N3 84.72(8).
of 1 (2.015 Å). Overall, the data obtained from the crystal struc-
tures of 1 and 2 support the structural indications and conclusions
of NMR and IR measurements.
3.4. Application in olefin epoxidation catalysis
Fig. 4. Kinetic profile of cis-cyclooctene epoxidation with pre-catalysts 1 and 2
The catalytic performance of complexes 1 and 2 as catalyst pre-
(
0.1 mol%) in CHCl
3
at room temperature and 55 °C using TBHP (pre-
cursors in olefin epoxidation catalysis was investigated to deter-
catalyst:substrate:oxidant = 1:1000:2000).

1
24
A. Schmidt et al. / Journal of Catalysis 319 (2014) 119–126
exceptionally long Mo–CO bond lengths in 2 and may be attributed
to its higher Lewis acidity as indicated by IR vibrational frequencies
of CO absorption bands and DFT investigation (see above).
In addition, epoxidation of cis-cyclooctene with 0.1 mol% of pre-
catalyst 1 was conducted in chloroform at 55 °C using different
substrate:oxidant ratios (see Supporting Info Fig. S1). In agreement
with previously published results, catalysis using a cis-cyclooc-
tene:TBHP ratio 1:2 outperforms catalysis using a ratio of 1:1. To
circumvent the induction period observed at room temperature
and for sake of comparison with other epoxidation catalysts in lit-
erature, all subsequent experiments were conducted at 55 °C and
with a substrate:TBHP ratio of 1:2. In order to identify the best sol-
vent for olefin epoxidation with catalysts of this type, a variety of
solvents were screened at 55 °C with 1 (0.1 mol%) (Table 3). Due
to their coordinating character, epoxidation in polar solvents such
as methanol, THF, and acetonitrile results in lower catalytic activ-
ities compared to chlorinated and non-polar solvents. A coordinat-
ing solvent can compete with the oxidant TBHP for coordination to
molybdenum, thus lowering catalytic activity [14]. Benzotrifluo-
ride (BTF) is in some cases a useful alternative for chlorinated sol-
Fig. 5. Kinetic profile of cis-cyclooctene epoxidation with different concentrations
of pre-catalyst 1 in CHCl and neat at 55 °C using TBHP (substrate:oxidant = 1:2).
3
reaction. This corresponds well to the observation in neat reaction
mixtures, where the induction period is still present, but dimin-
ishes significantly. A higher catalyst concentration, i.e. higher num-
ber of catalyst molecules per unit volume, will increase the rate of
the induction reaction, since more reactive encounters can take
place [84]. The same concept applies to the increased TBHP con-
centration when the reaction is performed neat. Another notewor-
thy aspect is that the reactions with a pre-catalyst concentration of
0.1 and 0.05 mol% perform with higher reaction rates in the
absence of solvent (neat), and full conversion was obtained after
5 min. In contrast, the reactions with 0.01 and 0.005 mol% pre-cat-
alyst loads are slower without solvent. This may be reasoned by a
slower formation of the catalytic active species and by catalyst
inhibition due to competitive coordination of the byproduct tert-
butanol [85]. In neat reaction mixtures, the continuous formation
of tert-butanol results in a polar reaction medium. This is analo-
gous to solvent coordination to the metal center as described above
[14].
3
vents (e.g. CHCl ), because it is a non-coordinating and
environmentally benign solvent [79–82]. However, cis-cyclooctene
conversion achieved in BTF here is quite low and compares to the
conversions in coordinating solvents. Epoxidation of cis-cyclooc-
8 2 8
tene with 1 in RTIL [C mim]NTf (C mim = 1-methyl-3-octylimid-
azolium, NTf = bis(trifluoromethanesulfonyl)-imide) is substan-
2
tially slower compared to homogeneous one-phase catalysis; this
may be attributed to mass transfer limitations of the biphasic sys-
tem [83]. The best catalyst performance is observed in toluene,
dodecane, chloroform, and without solvent (neat). Thus, further
experiments were conducted in chloroform or neat for comparison
purposes with previously published results.
Complex 1 is a prototype for studying the effect of imidazo[1,5-
a]pyridine-3-ylidene type ligands on the catalytic activities of NHC
molybdenum(II) systems toward olefin epoxidation. It is reason-
able to assume that optimum reaction conditions for 1 will be
appropriate for a catalytic application of 2.
To determine the pre-catalyst concentration yielding the high-
est conversion and TOF, catalysis was carried out using different
pre-catalyst concentrations at 55 °C both in chloroform and neat.
The kinetic profiles for 1 are shown in Figs. 5 and S2 (Supporting
Information), and the resulting TOFs and conversions after 24 h
are given in Table 4. Even at low catalyst loadings (0.005 mol%),
pre-catalyst 1 exhibits remarkably high TOFs and high conversion
rates of 97% in chloroform and 87% neat. The fact that the induction
period is prolonged at lower pre-catalyst loadings is noteworthy
In further experiments, a pre-catalyst concentration of 0.1 mol%
1 in chloroform was chosen, since these conditions yield the high-
À1
est TOF (11,100 h ) after 5 min with quantitative conversion after
30 min. However, pre-catalyst loadings of 0.01 and 0.005 mol%
perform extraordinarily well, yielding almost 100% conversion
À1
after 24 h and TOFs up to 40,900 h
.
In a separate series of experiments, the optimum pre-catalyst
concentration of 2 was determined by several catalytic runs in
chloroform at 55 °C. The kinetic profile of epoxidation reactions
is depicted in Fig. 6 and the obtained data are given in Table 5.
All four reactions, performed with different pre-catalyst loadings,
surpass the activities observed for 1. TOFs between 11,800 and
(Fig. 5: 5 min at 0.01 mol% and 15 min at 0.005 mol%, both in
CHCl ). Since the same amount of solvent (5 mL) was used for all
3
À1
pre-catalyst concentrations, this might be ascribed to an increased
dilution resulting in a decreased reaction rate of the activation
53,100 h and conversions of 100% after 30 min (0.1 mol% and
0.05 mol%) or 24 h (0.01 mol% and 0.005 mol%), respectively, were
obtained. No induction periods were observed for epoxidation
catalysis with 2.
À1
Complex 1 provides TOFs up to 40,900 h at a pre-catalyst con-
Table 3
centration of 0.005 mol%, which is the highest reported TOF of Mo
NHC complexes so far; it exhibits TOFs even higher than the struc-
Conversions of cis-cyclooctene with 1 (0.1 mol%) using TBHP (5.5 M in n-decane) as
oxidant at 55 °C in different solvents (pre-catalyst:substrate:oxidant = 1:1000:2000).
3
turally related complexes of the type [CpMo(CO) X] (X = alkyl,
Solvent
Conversion (%) after
halide) [5,6,15]. As far as we are aware, only ansa-cyclopentadienyl
molybdenum complexes in RTIL [18] exceed the activity of the
reported pre-catalyst 1 in terms of TOF. Catalytic investigations
4
h
24 h
MeOH
45
21
29
31
100
100
100
75
75
45
50
3
CH CN
À1
of 2 show even higher activities with TOFs up to 53,100 h at a
Benzotrifluoride
THF
pre-catalyst loading of 0.01 mol%. These constitute the highest
TOF of cyclopentadienyl molybdenum pre-catalysts reported to
date.
In order to determine the limitations of the catalytic system, we
employed additional substrates such as 1-octene, cis-stilbene, and
trans-b-methylstyrene in olefin epoxidation catalysis using
78
CHCl
3
100
100
100
97
Toluene
Dodecane
[
C
8
mim]NTf
2
Neat/n-decane
100
100

A. Schmidt et al. / Journal of Catalysis 319 (2014) 119–126
125
Table 4
Catalytic activities in terms of TOFs and conversions of cis-cyclooctene with different concentrations of 1 using TBHP (5.5 M in n-decane) as oxidant at 55 °C in CHCl
substrate:oxidant = 1:2). Time-conversion plots for conditions not displayed in Fig. 5 are assembled in Fig. S2 (Supporting Information).
3
or neat
(
À1
À1
Conv.a (%)
Solvent
CHCl
Pre-catalyst loading (mol%)
1
TOF5min (h
)
TOFmax (h
)
3
1200
11,100
6900
2000
1900
1200
11,100
6900
18,500
40,900
100
100
100
99
0
0
0
0
.1
.05
.01
.005
97
Neat/n-decane
0.1
12,000
23,800
4700
12,000
23,800
25,800
30,000
100
100
93
0
0
0
.05
.01
.005
2300
87
a
Conversion after 24 h.
oxidant due to the terminal position of the double bond [2,3],
lower conversion was observed.
The catalytic epoxidation of cis-cyclooctene was also carried out
in a RTIL. For comparison with known NHC molybdenum systems
[
8 2
22], [C mim]NTf was chosen as RTIL. Using 0.1 mol% 1 at 55 °C,
epoxidation proceeds with 97% conversion after 24 h in the bipha-
sic catalytic system. The kinetic profile of cis-cyclooctene epoxida-
tion with pre-catalyst 1 in RTIL is shown in Fig. S3 (Supporting
Information). However, the reaction is substantially slowed down
À1
with a TOF of 2400 h , probably due to phase transfer limitations.
The upper phase consists of substrate and n-decane, while the
lower phase contains RTIL plus catalyst; TBHP is partially soluble
in both phases. Taking into account the advantages of RTILs such
as low volatility, thermal stability, high polarity, simple catalyst
recycling, and product separation [18,86], the prolonged reaction
times are acceptable for potential applications, especially consider-
ing the obtained high conversion.
Fig. 6. Kinetic profile of cis-cyclooctene epoxidation with different concentrations
of pre-catalyst 2 in CHCl
3
at 55 °C using TBHP (substrate:oxidant = 1:2).
To investigate the stability and reusability of pre-catalyst 1 and
accordingly of the active species, recycling experiments were per-
8 2
formed with pre-catalyst loading of 0.1 mol% in [C mim]NTf at
Table 5
Catalytic activities in terms of TOFs and conversions of cis-cyclooctene epoxidation
after 24 h with different concentrations of 2 using TBHP as oxidant at 55 °C in CHCl
55 °C. The catalyst was recycled for ten catalytic runs; quantitative
conversion was observed for each run after 24 h (see Supporting
Info Table S1). Deviations lie within the experimental error.
In contrast to triazolylidene-based molybdenum complex A
3
(substrate:oxidant = 1:2).
À1
Pre-catalyst loading (mol%)
TOF5min (h
)
Conversion after 24 h (%)
[
22], complex 1 shows excellent performance in recycling experi-
0
0
0
0
.1
11,800
22,300
53,100
45,800
100
100
99
.05
.01
.005
ments. Therefore, the catalysis in [C mim]NTf indicates higher
8
2
stability of 1 compared to molybdenum NHC pre-catalysts as
known in literature. Further catalytic experiments and stability
studies of these complexes under catalytic conditions are currently
the subject of investigation in our laboratories.
99
Table 6
Conversions of different substrates after epoxidation with 1 (0.1 mol%) using TBHP as
4
. Conclusion
oxidant at 55 °C in CDCl
3
(pre-catalyst:substrate:oxidant = 1:1000:2000).
Substrate
Conversion (%) after Selectivity (%)
The synthesis of two novel cyclopentadienyl 2-mesitylimi-
4
h
24 h
dazo[1,5-a]pyridine-3-ylidene
molybdenum
complexes
is
1
-Octene
cis-Stilbene
trans-b-Methylstyrene
66
82
99
75
100
100
100
100
100
reported. Synthesis proceeds in excellent yields via the silver car-
bene transmetallation route for 1 and subsequent substitution
reaction for 2. Compounds 1 and 2 were characterized by NMR,
IR, elemental analysis, FAB-MS, and by single-crystal XRD. 1 and
2
were applied as pre-catalysts in catalytic epoxidation of aliphatic
pre-catalyst 1 under optimized reaction conditions (see above).
Quite remarkable conversions of the aforementioned olefins were
and aromatic olefins under various reaction conditions and with
TBHP as oxidant. Both compounds surpass previously reported
molybdenum epoxidation pre-catalysts in catalytic activity. Even
at low pre-catalyst concentrations (0.005 mol%), full conversion
of cis-cyclooctene was obtained after 24 h, and high TOFs up to
40,900 h (1) and up to 53,100 h (2) are observed. In addition,
more challenging substrates such as 1-octene, cis-stilbene, and
trans-b-methylstyrene were applied in olefin epoxidation with 1,
resulting in good yields and high selectivity. Pre-catalyst 1 was
obtained with a pre-catalyst loading of 0.1 mol% 1 in CDCl
3
at
1
5
5 °C (Table 6). Conversions were determined by H NMR spectros-
copy, using 1,2-dichloroethane as the internal standard. Identical
À1
À1
results for cis-cyclooctene conversions were obtained by monitor-
1
1
ing either by GC or by H NMR; thus H NMR is considered a viable
method for monitoring conversion. All catalytic reactions pro-
ceeded without significant formation of byproducts. Since 1-octene
is electronically disfavored as substrate for olefin epoxidation with
cyclopentadienyl molybdenum carbonyl catalysts using TBHP as
successfully applied in recycling experiments in RTIL [C mim]NTf2
and can be reused for at least 10 subsequent runs without loss of
8

1
26
A. Schmidt et al. / Journal of Catalysis 319 (2014) 119–126
activity, indicating high stability of the catalyst. The outstanding
catalytic activities of pre-catalysts 1 and 2 compared to closely-
related catalytic systems have been attributed to steric and
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r-donating character of imi-
dazo[1,5-a]pyridine-3-ylidenes and the high Lewis acidity of the
metal center. The higher Lewis acidity of 2 compared to 1 may
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demanding imidazo-[1,5-a]pyridine-3-ylidenes feature properties
that appear to be beneficial in epoxidation catalysis. Further stud-
ies examining the stability and active species of these systems are
currently the subject of investigation in our laboratories.
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