Dioxomolybdenum(VI) compounds with Α-amino acid donor ligands as catalytic precursors for the selective oxyfunctionalization of olefins

Article abstract of DOI:10.1016/j.mcat.2017.12.018

A series of cis-dioxomolybdenum(VI) α-amino acid containing compounds I–V has been investigated as potential catalyst precursors for the mild and selective oxyfunctionalization of conjugated or unconjugated olefins like styrene, α-methylstyrene, cis-β-methylstyrene, cyclohexene and cyclooctene, using tert-butyl hydroperoxide (TBHP) as main oxidant. All the I–V complexes behaved as active heterogeneous and recyclable catalysts, showing good to quantitative conversion values of the substrate. In all cases, high selectivity toward the corresponding epoxide formation was detected. No substantial difference in terms of efficiency has been observed among the different catalysts I–V, thus confirming that the different nature of the amino acidic side-chain does not strictly affect the catalytic process. Insights into mechanistic details and reaction free energy profile of catalytic oxidations by means of quantum chemical calculations have been discussed.

Full text of DOI:10.1016/j.mcat.2017.12.018

Molecular Catalysis 446 (2018) 39–48
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Dioxomolybdenum(VI) compounds with ␣-amino acid donor ligands
as catalytic precursors for the selective oxyfunctionalization of olefins
Issam Abdalghani , Lorenzo Biancalana , Massimiliano Aschi , Guido Pampalonib,c,
Fabio Marchetti
a
a
b,c
a
b,c,∗∗
a,∗
, Marcello Crucianelli
Department of Physical and Chemical Sciences, University of L’Aquila, via Vetoio, I-67100 L’Aquila, Italy
Department of Chemistry and Industrial Chemistry, University of Pisa, via G. Moruzzi 13, I-56124 Pisa, Italy
CIRCC, via Celso Ulpiani 27, I-70126 Bari, Italy
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 September 2017
Received in revised form
A series of cis-dioxomolybdenum(VI) ␣-amino acid containing compounds I–V has been investigated
as potential catalyst precursors for the mild and selective oxyfunctionalization of conjugated or uncon-
jugated olefins like styrene, ␣-methylstyrene, cis-␤-methylstyrene, cyclohexene and cyclooctene, using
tert-butyl hydroperoxide (TBHP) as main oxidant. All the I–V complexes behaved as active heterogeneous
and recyclable catalysts, showing good to quantitative conversion values of the substrate. In all cases,
high selectivity toward the corresponding epoxide formation was detected. No substantial difference in
terms of efficiency has been observed among the different catalysts I–V, thus confirming that the dif-
ferent nature of the amino acidic side-chain does not strictly affect the catalytic process. Insights into
mechanistic details and reaction free energy profile of catalytic oxidations by means of quantum chemical
1
2 December 2017
Accepted 13 December 2017
Keywords:
cis-Dioxomolybdenum
␣
-Amino acid
Olefin oxidation
Heterogeneous catalysis
tert-Butyl hydroperoxide
calculations have been discussed.
© 2017 Elsevier B.V. All rights reserved.
Introduction
valuable chemical that has widespread application in perfumery,
pharmaceuticals, dyestuffs, and agrochemicals; it is the second
Molybdenum occurs in a wide range of metalloenzymes cov-
ering key positions both in biochemical cycles of carbon, nitrogen
and sulfur and in the metabolism of living organisms [1]; its very
rich redox chemistry might explain why it is the only member
of the second transition series with known biological functions
most important aromatic molecule (after vanillin) used in the cos-
metics and flavor industries [5]. On the other hand, styrene oxide
can be used for producing epoxy resin diluting agents, ultraviolet
absorbents, flavoring agents, etc., and it is also an important inter-
mediate for organic synthesis, pharmacochemistry and perfumery
[6].
[2]. In this context, the tremendous impetus arisen in the syn-
thesis of a number of model complexes mimicking oxotransferase
molybdoenzymes is not surprising [3], finding application in cat-
alytic oxygen atom transfer (OAT) reactions [4]. Since many years,
catalytic oxidation has established to be a key technology for
converting petroleum feedstocks into useful chemicals such as
alcohols, carbonyl compounds and epoxides.
Epoxides are, in turn, important synthetic intermediates for
the synthesis of oxygen containing derivatives. Styrene oxide and
benzaldehyde are among the most important products obtained
from the catalytic oxidation of styrene. Benzaldehyde is a very
In the light to develop sustainable catalytic processes based
on non toxic metal species, molybdenum appears a good choice.
In particular, Mo(VI) compounds are able to activate mild oxi-
dants such as H O , alkyl hydroperoxides or air, thus overcoming
the severe environmental and health restrictions connected to the
use of peracids [7]. Especially, molybdenum(VI) dioxo species are
highly active and selective in the epoxidation of internal aliphatic
alkenes (e.g. cyclooctene or cyclohexene) [7a,8], while the epoxida-
tion of terminal alkenes (e.g. styrene) remains challenging, because
frequently the formation of ring-opening products comes favoured
2
2
[9]. As a matter of fact, only a limited number of effective molyb-
denum(VI) dioxo complexes have appeared in the literature as
effective catalysts for styrene epoxidation [10].
The study of the interaction between molybdenum compounds
and naturally-occurring molecules is intended to contribute to the
coordination chemistry of Mo(VI), particularly in water, and every-
∗
Corresponding author.
Corresponding author at: Department of Chemistry and Industrial Chemistry,
∗
∗
University of Pisa, via G. Moruzzi 13, I-56124 Pisa, Italy.
E-mail addresses: fabio.marchetti1974@unipi.it (F. Marchetti),
marcello.crucianelli@univaq.it (M. Crucianelli).
https://doi.org/10.1016/j.mcat.2017.12.018
2
468-8231/© 2017 Elsevier B.V. All rights reserved.

4
0
I. Abdalghani et al. / Molecular Catalysis 446 (2018) 39–48
Chart 1. Molybdenum(VI) complexes, with l-Methionine (I), l-Phenylalanine (II), N,N-dimethyl-l-Phenylalanine (III), Glycine (IV) and l-Proline (V) as ␣-amino acid ligands.
thing that ensues from it for catalytic purposes. In this respect,
-amino acids are attracting ligands due to easy availability, low
state infrared spectra were recorded on a Perkin Elmer Spectrum
One FT-IR spectrometer, equipped with a UATR sampling acces-
sory. Infrared spectra of CDCl3 solutions were recorded on a Perkin
␣
toxicity and the usual presence of an asymmetric center, and indeed
their coordination to a variety of metal ions has been extensively
investigated [11]. The chemistry of oxomolybdenum (V) [12] or
Mo(VI) species with ␣-amino acids in aqueous medium has aroused
huge interest, as witnessed by several publications [13]. In spite of
this, only sparse information have appeared in the literature on
the use of Mo(VI)-amino acid compounds in the oxidative derivati-
zation of synthetically important olefins [14]. Some of us, recently
described a detailed solid-state characterization of a series of poorly
soluble Mo(VI) compounds isolated after the reaction of molyb-
date salts with ␣-amino acids from acidic aqueous solutions [15].
Solid-state IR and NMR spectroscopy, corroborated by DFT studies,
indicated the formation of polymeric species of general formula
Elmer Spectrum 100 FT-IR spectrometer with a CaF liquid trans-
2
−
1
mission cell in the 4000–1000 cm
range. UV–vis spectra were
recorded on a Ultraspec 2100 Pro spectrophotometer with 0.1 cm
quartz cuvettes in the 190–900 nm range. Gas chromatographic
analyses (GC-FID) were performed by means of a Hewlett–Packard
6890 series instrument equipped with a flame ionization detector,
using a 30m × 0.32mm × 0.25 ␮m film thickness (crosslinked 5%
phenylmethylsiloxane) column and chromatography grade helium,
as carrier gas. In GC calculations, all peaks amounting to at least 0.5%
of the total products were considered. 1H and C NMR analyses
of products were performed after purification, and compared with
authentic samples. Mass spectra were recorded with an electron
beam of 70 eV.
13
{
Mo O5(OH) (␣-amino acid)}n whose repeating unit featured cis-
2
2
MoO fragment bridged by a zwitterionic bidentate ␣-amino acid
2
donor ligand (see Chart 1).
Catalytic oxidations
Herein we describe a study on these Mo(VI) ␣-amino acid
complexes I–V (Chart 1) as potential catalysts for the mild and
selective catalytic oxyfunctionalization of conjugated or uncon-
jugated olefins (styrene, ␣-methylstyrene, cis-␤-methylstyrene,
cyclohexene and cyclooctene), using tert-butyl hydroperoxide
All catalytic experiments were carried out in a carousel reactor
−
3
fitted with a water condenser. In a typical experiment, 2·10 mmol
1.0 mol% vs. olefin) of the selected catalyst was suspended in
.0 mL of chloroform and 0.4 mmol (2.0 equivalents vs. olefin) of
(
1
(
TBHP) as oxygen source. The results of quantum chemical cal-
TBHP (5.5 M in n-decane) were added, followed by 0.2 mmol of the
selected olefin. The reaction mixture was stirred at 50 C for the
culations will be discussed to give insights into mechanistic and
thermodynamic aspects.
◦
chosen time and the reaction progress was monitored and evalu-
ated by GC analysis by taking, at regular time intervals, aliquots
of the crude. n-Hexadecane or mesitylene (in the case of cyclohex-
ene or cyclooctene, respectively) were used as internal standard. At
the end of reaction, the mixture was quenched with 1.0 mL of a 5%
aqueous solution of Na S O5 and the organic phase recovered with
Experimental section
Materials and methods
2
2
1
.0 mL of chloroform, dried over anhydrous MgSO4 and analyzed
All reagents and solvents were purchased from Sigma-Aldrich
Italy) in the highest purity grade available and were used as
by GC.
(
such. A 5.5 M solution in n-decane (or 70% w/w aqueous solu-
tion) of tert-butyl hydroperoxide (TBHP), or a 35% w/w aqueous
solution of H O , were used as primary oxidants. Compounds I–V
Reactivity of compound I with H O or TBHP.
2
2
2
2
were prepared according to the literature [15]. Molar concentra-
General procedure
−
2
tions of aqueous solutions of, respectively, 70% w/w of TBHP and
compound I [Mo O (OH) (Met)] (19 mg, 4·10 mmol), CDCl3
2
4
4
1
−2
3
5% w/w of H O , were determined via H NMR using Me SO2 as
(4.0 mL), PhNO2 (5.0 ␮L, 4.9·10 mmol) as NMR internal standard
and the selected oxidant (ca. 200 eq.) were introduced in a test tube.
The reaction mixture, equipped with a reflux condenser, was stirred
2
2
2
internal standard, in D O solution [␦ = 3.05 (Me SO ), 1.15 (TBHP)
2
2
2
ppm] (for TBHP) or THF solution [␦ = 9.59 (H O ), 3.09 (Me SO )
2
2
2
2
◦
ppm] (for H O ). NMR spectra were recorded at 298 K, respectively,
for 4 h at 50 C. The organic phase was periodically sampled (t = 0, 1,
2
2
1
on a Bruker Avance II DRX400 equipped with a BBFO broadband
probe, or with a Bruker Avance III 400 MHz instruments. Solid-
2, and 3 h) for IR, H NMR and UV–vis analysis (see below), showing
no evidence for the formation of a soluble Mo-methionine species.

I. Abdalghani et al. / Molecular Catalysis 446 (2018) 39–48
41
Then, molybdenum-containing compounds I-A and I-B were iso-
lated as described below.
All the calculations were carried out in the framework of Den-
sity Functional Theory with the CAM-B3LYP functional [18] in
conjunction with 6–31G* basis set and Los Alamos Effective Core
Potential in conjunction with Double Zeta basis set, LAN-L2DZ [19],
for molybdenum.
Compound I and H O : organic phase monitoring and isolation of
2
2
I-A
The reaction was performed with 35% w/w H O2 (0.72 mL,
Although the effect of the functional is a well assessed problem
in particular when transition metals are concerned, in the present
study because of the dimension of the investigated systems and the
related cost of the calculations, we did not undergo a systematic
sensitivity analysis.
2
8
(
(
.03 mmol). Reaction mixture (1–3 h): yellow aqueous solution
␭max = 307 nm)/colorless organic solution. 1H NMR (CDCl ): H O
3
2
␦ = 1.60 ppm) and H O2 (␦ = 5.88-5.26 ppm [16a]) signals were
2
identified. IR (CDCl ): variations due to H O/H O bands ( ꢀ˜ : 3600-
3
2
2
2
−
200br, 1711, 1364, 1224 cm ) were observed. UV–vis (CDCl ): no
3
1
3
absorptions (0–3 h, ␭cutoff = 300 nm). At the end of the reaction, the
aqueous phase was separated. Few mg of KI were added and the
solution was stirred for 1 h, at room temperature, to decompose
Results and discussion
◦
excess H O . Volatiles were then removed under vacuum (40 C)
Optimization of the reaction conditions for the epoxidation of
styrene with TBHP
2
2
affording a yellow solid (I-A).
I-A: IR (solid state): ꢀ˜ (cm−1) = 3600-3300m-br, 3216-3160m-
br, 3017m, 2930m, 1675m-sh, 1621m, 1506m, 1444m-sh, 1405m,
At the beginning, the optimal experimental conditions in terms
of temperature, solvent, oxidant and catalyst loadings were eval-
uated by using styrene as a challenging olefin (characterized by a
relatively low electron rich double bond), tert-butyl hydroperoxide
(TBHP, 5.5 M in n-decane) as oxidant, and catalyst I. After sev-
eral preliminary experiments, the higher conversions and reaction
1
369m, 1323m-sh, 1281s, 1193w, 1127s, 1073m, 960s, 909w, 857s,
8
05w, 772w, 735w, 658w.
1
H NMR (D O): ␦ = 3.95 (br, 1H), 3.40 (br, 2H), 3.08 (s, 3H),
2
1
3
1
2
2
.35 (br, 1H) ppm. C{ H} NMR (D O): ␦ = 172.5, 52.7, 49.9, 39.8,
2
3.1 ppm.
Compound I-A is stable in the solid state and is soluble in acetone
rates achieved with CHCl at moderate temperatures, prompted us
3
and water. The stability of I-A in aqueous solution without excess
to select it as the best solvent for the present study, in compar-
ison with acetonitrile or toluene. Indeed, a limited activity using
coordinating solvents (e.g. acetonitrile) due to their ability to com-
pete with TBHP for the binding to the Mo center, was previously
described [20] and also observed in our case during the oxida-
of H O is limited (see Supporting Information).
2
2
Compound I and TBHP: organic phase monitoring and isolation of
I-B
◦
The reaction was performed with 70% w/w aqueous solution
of TBHP (1.06 mL, 8.01 mmol). Reaction mixture (0–3 h): colorless
solid dispersed in the aqueous phase/colorless organic solution.
tion of styrene (about 30–35% of substrate conversion, at 60 C,
after 8 h). Diversely, with toluene, moderate conversions of styrene
(about 50–60%) were observed but only after a longer reaction
1
◦
H NMR (CDCl ): H O (␦ = 2.6–2.8 ppm), TBHP [␦ = 8 (br), 1.23 (s)
time (12 h) and at higher temperatures (80 C). Anyway, in both
3
2
ppm] and tBuOH [␦ = 1.25 (s) ppm, t = 3 h] signals were identi-
cases a very low selectivity toward the formation of the epoxide
was observed, being benzaldehyde the main product. Concern-
ing the optimization of the catalyst amount (1 mol%, 2 mol% or
3 mol% ratios), we observed an increase in the starting material
(s.m.) conversion on increasing the loading (2 and 3 mol%), during
the first hour (within the 30–35% range); nevertheless, the con-
version values reached at the end of reaction (4–8 h) were rather
comparable in all cases, thus justifying our choice to work with
1 mol% of catalyst. Lower amounts (0.5 mol%) proved to be unsatis-
factory due to longer reaction times required to obtain acceptable
conversion values. Concerning the oxidant/starting material molar
ratio, the optimal compromise, in terms of catalytic efficiency, was
1
3
1
fied. C{ H} NMR (CDCl ): TBHP (␦ = 80.6, 25.7 ppm) and tBuOH
3
(
␦ = 69.7, 26.5 ppm, t = 3 h) were identified. IR (CDCl ): variations
3
−
1
due to tBuOH bands ( ꢀ˜ : 3605, 1149 cm , t = 3 h) were observed.
UV–vis (CDCl ): no absorptions (0–3 h, ␭ = 300 nm). At the end
3
cutoff
of the reaction, the suspension was filtered. The resulting color-
less solid (I-B) was washed with CH Cl , acetone and dried under
2
2
◦
vacuum (40 C).
I-B. Yield = 15 mg. IR (solid state): ꢀ˜ = 1290w, 1280w, 1125w,
1
8
09 w cm− in addition to the bands of compound I.
Computational study
2
.0 equiv., since we observed that higher molar ratios (3.0 equiv.)
All the calculations were carried out with the Gaussian09 pack-
age RevD.01 [16b]. The structures were optimized in vacuo (fully
or in the presence of constraints, see below) using a threshold of
improved the conversion only marginally, if any. By taking constant
the amount of styrene (0.2 mmol), we observed that the increase
of dilution, from 1.0 to 3.0 mL of CHCl , led to a gradual decrease
3
0
.008 atomic units (a.u.) for the maximum forces. Subsequently the
in substrate conversion. This result may be explained on consider-
ing that, the formation of active peroxo species at the molybdenum
center, upon close interaction with TBHP at the solid/liquid inter-
face, could be less probable in more diluted solutions. Generally
speaking, a reduction in the volume of solvents is appealing, when
harmonic frequencies were calculated from the eigenvalues of the
mass-weighted Hessian matrix.
The frequencies and the obtained structures were then utilized
◦
for calculating gas-phase free energy at 50 C, hence mimicking
the temperature of the experiment. These calculations were per-
formed assuming a concentration of 1.0 M for all the species taken
as standard state.
The frequencies were also utilized for ascertaining the nature of
the transition state (see below).
practicable, as witnessed by some recent reports on [MoO L]n pro-
2
moted oxidative catalysis in solvent free conditions [21]. Anyway,
in the case of styrene oxidation, we didn’t observe substantial
advantages, working under solventless conditions, neither in terms
of yield nor in the selectivity toward epoxide formation. Thus, we
A rough estimation of the excess free energy in solution (CHCl3)
was finally carried out using Polarizable Continuum Method as
implemented in the same package [17].
further decided to work with 1.0 mL of CHCl , also for sake of safety
3
in the handling of TBHP. The working temperature was settled at
◦
50 C as a right compromise: no significant advancement of both
Note that for these calculations we utilized the dielectric con-
stant and the cavitation free energies as obtained at 298 K. Thus,
data obtained from the calculations should be utilized at a qualita-
tive or semi-quantitative level of interpretation.
yield and conversion values was observed under refluxing CHCl3,
while the kinetic of reaction, at room temperature, was rather slow.
In summary, after the optimization study, we opted for the
following experimental conditions: 0.2 mmol of substrate, 1.0 mL

4
2
I. Abdalghani et al. / Molecular Catalysis 446 (2018) 39–48
Table 1
a
Oxidation of styrene with TBHP and catalysts I–V.
Catalystb
Conv.c
%
Productsd
TONe
EP: %Y (%S)f
BA: %Y (%S)f
PA: %Y (%S)f
I
83
88
89
96
94
35 (56)
30 (50)
33 (50)
37 (49)
39 (57)
12 (19)
10 (17)
11 (17)
16 (21)
10 (14)
15 (24)
20 (33)
22 (33)
22 (29)
20 (29)
83
88
89
96
94
II
III
IV
V
a
◦
Exp. conditions: s.m. (0.2 mmol), TBHP (5.5 M in n-decane; 2.0 eq.), cat. (1.0 mol%), CHCl3 (1.0 mL), 50 C, 8 h.
I = Mo-Met; II = Mo-Phe; III = Mo-dmPhe; IV = Mo-Gly; V = Mo-Pro.
GC yields based on the starting olefin.
EP = epoxide; BA = benzaldehyde; PA = phenylacetaldehyde.
TON = turnover number: mmol of converted s m./mmol of catalyst.
Y = yield and S = selectivity, after 4 h.
b
c
d
e
f
90
80
70
60
50
40
30
20
10
O
EP
CHO
I
R
Catalysts
I-V)
BA
PA
(
II
III
IV
V
2
CH CHO
R'
0
0
2
4
6
8
O
Time (h)
Chart 2. Graphical sketch of the main oxidation products detected in the oxidation
Fig. 1. Comparison between catalysts I–V, in terms of substrate conversion, for the
of styrene (R = R’ = H), ␣-methylstyrene (R = CH , R’ = H) and cis-␤-methylstyrene
3
oxidation of styrene with TBHP, after 2, 4, 6 and 8 h.
(R = H, R’ = CH ), respectively.
3
working with high valent molybdenum and vanadium based cata-
lysts [23a,d].
◦
of CHCl , 1.0 mol% of catalyst and 2.0 equiv. of TBHP, at 50 C,
3
In addition, after prolonged times, the rate of the catalytic oxi-
dation (promoted by TBHP) considerably slows down since an
increasing number of tert-butyl alcohol molecules become com-
petitive with TBHP toward the access to the molybdenum center,
thus making it more probable the occurrence of side-reactions with
recalcitrant substrates [24].
in accordance with the literature for similar catalytic systems
[9a,22]. Unless otherwise specified, these conditions have been
applied to all substrates, namely styrene, ␣-methylstyrene, cis-␤-
methylstyrene, cyclohexene and cyclooctene.
We first evaluated the activity of catalysts I–V in the oxidation
of styrene and the main results are showed in Table 1 and Fig. 1.
The corresponding epoxide (EP), benzaldehyde (BA) and phenylac-
etaldehyde (PA) were detected as the main products (Chart 2).
All catalysts proved to be active to a comparable degree, afford-
ing satisfactory conversion values, ranging from 66 to 78%, after
only 4 h (Fig. 1). The highest selectivity was observed toward the EP
formation, within the range of 49–57%, while PA was obtained in a
slightly predominant way over BA (15–22% vs 10–16%, respectively,
Table 1).
As known, the formation of BA and PA may be normally observed
during styrene oxidation, their ratio depending on the specific
properties of the catalyst [9a,23]. By prolonging the reaction time
up to 8 h, almost quantitative conversions (83–96%) were observed,
accompanied by an increase in BA yield thus resulting in a conse-
quential reduction (around 15–20%) of the selectivity toward EP.
As a matter of fact, in a control catalytic oxidation performed on
styrene oxide, under otherwise identical experimental conditions,
a substantial stability of such epoxide toward further oxidation was
observed, being the BA formation detectable only in traces. This
suggests that the increasing conversion of styrene, upon prolonged
reaction times, favors its competitive C C double bond oxidative
cleavage, thus lowering the selectivity toward EP in favor of BA
formation. On the other hand, this type of selectivity during the
catalytic oxidation of styrene, has been commonly observed when
Substantial differences, in terms of both conversion and yield
values, have not been observed among the different catalysts I–V
(Fig. 1), being catalysts IV and V only moderately more active.
This fact may be tentatively justified on considering that, in the
proximity of the catalytic metal center, the different amino acidic
side-chain does not strictly affect the formation of active species
(i.e. Adduct-1 or Peroxo-2, see onward Fig. 5), thus preventing its
discriminating role in influencing the selectivity of the oxidation
and the stereoselectivity of the epoxide product (see onward).
Blank runs (without catalyst) were performed and, as expected,
no significant substrate conversion (<10%) was observed under oth-
erwise similar conditions.
Substrate scope and selectivity in the oxidation of other olefins
with catalysts I–V
Encouraged from good results obtained in the oxidation
of
a reluctant olefin such styrene, we decided to investi-
gate catalysts I–V for the selective oxyfunctionalization of ␣-
and ␤-methylstyrene (Tables 2 and 3). In the oxidation of ␣-
methylstyrene, under optimized conditions, EP and acetophenone
(AP, see Chart 2) were the main products, EP being obtained
with higher yields (35–48%, Table 2) with respect to AP (25–30%,

I. Abdalghani et al. / Molecular Catalysis 446 (2018) 39–48
43
Scheme 1. Sketch of the probable structure of water soluble Mo complex [MoO(O2)2(aaH)(H2O)] I-A.
Table 2
Oxidation of ␣-methylstyrene with TBHP and catalysts I–V.a
Table 3
a
Oxidation of cis-␤-methylstyrene with TBHP and catalysts I–V.
Catalystb
Conv.c
%
Productsd
TONe
Catalystb
Conv.c
Productsd
TONe
%
EP: %Y (%S)f
AP: %Y (%S)f
EP: %Y (%S)f
BA: %Y (%S)f
I
98
89
90
92
90
48 (62)
37 (58)
35 (54)
39 (61)
38 (58)
30 (38)
27 (42)
30 (46)
25 (39)
27 (42)
98
89
90
92
90
I
96
95
98
90
93
84 (88)
85 (89)
90 (92)
77 (86)
82 (88)
12 (12)
10 (11)
8 (8)
13 (14)
11 (12)
96
95
98
90
93
II
III
IV
V
II
III
IV
V
a
a
Exp. conditions: s.m. (0.2 mmol), TBHP (5.5 M in n-decane; 2.0 eq.), cat.
Exp. conditions: s.m. (0.2 mmol), TBHP (5.5 M in n-decane; 2.0 eq.), cat.
◦
◦
(
1.0 mol%), CHCl3 (1.0 mL), 50 C, 6 h.
(1.0 mol%), CHCl3 (1.0 mL), 50 C, 2.5 h.
b
b
I = Mo-Met; II = Mo-Phe; III = Mo-dmPhe; IV = Mo-Gly; V = Mo-Pro.
GC yields based on the starting olefin.
EP = epoxide; AP = acetophenone.
TON = turnover number: mmol of converted s m./mmol of catalyst.
I = Mo-Met; II = Mo-Phe; III = Mo-dmPhe; IV = Mo-Gly; V = Mo-Pro.
GC yields based on the starting olefin.
EP = epoxide; AP = acetophenone.
TON = turnover number: mmol of converted s m./mmol of catalyst.
c
c
d
d
e
e
f
f
Y = yield and S = selectivity, after 4 h.
Y = yield, S = selectivity.
Table 2). As expected, due to the positive role exerted by the
methyl group in the alpha position of the double bond (in terms of
electron density raising) [23b], the conversion values were higher
than those observed with styrene and almost quantitative conver-
sion (89–98%) was achieved after shorter reaction times (6 vs 8 h,
Table 2). Again, a prolonged reaction time led to a slight reduction
of EP yield along with an increase in the AP yield (within the 5–8%
range of variation).
UV–vis, IR and 1H NMR analysis of the chloroform solution
showed no evidence for the formation of a soluble molybdenum
complex. In fact, in the case of TBHP, a colorless solid deriving
from I and dispersed in the aqueous phase was present throughout
the experiment. On the other hand, compound I dissolved almost
immediately when heated in the presence of H O , with formation
of a yellow color in the aqueous phase. According to these observa-
tions, it may be concluded that we are dealing with different types
of heterogeneous catalytic systems, depending on the selected oxi-
2
2
Better results have been obtained from the oxidation of cis-
␤
-methylstyrene, where almost quantitative conversions (ranging
from 90% up to 98%) were observed after shorter reaction times
2.5 h) (Table 3). EP was obtained with high yield and selectivity,
dants: liquid/liquid when using H O2 and solid/liquid in the case
2
of TBHP.
(
At the end of the reaction, a yellow solid was isolated from the
aqueous phase containing H O after solvent evaporation (product
while lower yields (8–13%) were ascertained for BA.
2
2
The usual higher reactivity of unconjugated olefins as cyclo-
hexene and cyclooctene toward catalytic oxidation was verified
also with our catalysts I–V. Total selectivity toward epoxide for-
mation and quantitative conversions (TON = 100) were observed
with cyclooctene after only 1 h, while 2 h were required to
obtain the same results with cyclohexene. These results are at all
comparable with those previously reported for other types of cis-
dioxomolybdenum based catalysts [9a,25].
I-A), while a white solid (product I-B) was recovered by filtra-
tion, in the TBHP experiment (see Schemes 1 and 2). The yellow
water-soluble compound I-A was identified by means of IR and
NMR spectroscopy. The FT-IR spectrum of this product is very dif-
ferent with respect to that of the starting compound I, displaying
−
1
bands typical for the stretching of Mo = O (960 cm ), Mo (O O)
−
1
−1
−1
(
857 cm ) and SO2 (1281 cm , ꢀasym; 1127 cm , ꢀsym) moieties
(
See Figures SI1-SI2 and Table SI2). The oxidation of the thioether
13
moiety to sulfone is supported also by C NMR data (see Table
SI1). An increase in the frequency of the antisymmetric carboxylate
stretching and a decrease in the symmetric one was observed on
Comparative study to evaluate the catalytic activity of I with
TBHP or H O2
2
−
1
going from I to I-A (Table SI2). A value of ꢁꢀ = 216 cm between
−
1
No significative results have been previously observed by us, in
these two bands in I-A (instead of ꢁꢀ = 148 cm in I [15]), sug-
gests a change in the coordination mode of the ␣-amino acid, from
bidentate bridging (I) to monodentate (I-A) (Scheme 1) [26a].
Yellow mononuclear Mo(VI)-oxido/peroxido complexes have
already been reported in the literature for a number of ␣-amino
acids and their crystal structures are known: they all feature a
[MoO(O ) (aaH)(H O)] structure, with a zwitterionic (␬-O) mon-
chloroform, when H O2 was used in the place of TBHP. Therefore,
2
we became interested in evaluating the nature of the interaction
between the molybdenum complexes and the two different oxi-
dant species. We decided to investigate, in a comparative study,
the behavior of the selected catalyst I in the presence of a large
excess of H O or TBHP, respectively, under the same conditions
2
2
2
2
2
␩
2
used for the catalytic reactions, but without the substrate.
odentate amino acid and two
-peroxo ligands [26b]. It is

4
4
I. Abdalghani et al. / Molecular Catalysis 446 (2018) 39–48
Scheme 2. Sketch of the probable structure of white insoluble Mo complex I-B.
Fig. 2. Schematic representation of the octanuclear fragment HO-(Mo2O7H2·Met)4-H used in the present work for mimicking the catalyst I.
Table 4
(
less than 5% of conversion) after this experiment proved that the
a
Catalytic recycling experiments with catalyst IV.
catalytic activity stems from the surface-fixed species in a hetero-
geneous system and not from the leaching of active species into the
solution. Moreover, a series of experiments were performed with
the selected compound IV in order to evaluate catalyst recyclability
in the oxidation of styrene and cyclooctene, under the optimized
conditions.
After the first run, the solid recovered and washed with chloro-
form, was analyzed by FT-IR (KBr disc) to confirm its unmodified
nature. We observed that the catalyst recovered after the first cycle
continued to be active, under the same conditions, at least for three
consecutive cycles, with comparable yield and selectivity values
Cycle
Styrene
Cyclooctene
Conv.% (Sel.%)
Conv.% (Sel.%)b
b
I run
II run
III run
67 (58)
65 (56)
62 (55)
>99 (100)
96 (100)
96 (100)
a
Exp. conditions: s m. (0.2 mmol), TBHP (5.5 M in n-decane; 2.0 eq.), cat.
◦
(
1.0 mol%), CHCl3 (1.0 mL), 50 C; reaction times: 4 h for styrene or 1 h for
cyclooctene. Catalyst IV = Mo-Gly.
b
GC conversions based on the starting olefin; selectivity values referred to EP.
(Table 4).
reasonable from IR and NMR data that compound I-A has an analo-
Finally, the catalysts I–V were also studied in order to establish
gous [MoO(O ) (aaH)(H O)] structure (Scheme 1). In this respect,
2
2
2
the role, if any, of the ␣-amino acidic ligand, as a chiral inducer,
for the formation of the corresponding enantioenriched epoxide,
starting from prochiral olefins. In all cases, low enantioselectivity
values were detected, for the oxidation of styrene, the best e.e.
values ranging from 10 to 13%. This result, could be tentatively
explained by supposing that the chiral ␣-amino acid ligand, even
if having an important stabilizing effect for the overall stability of
compounds I–V through the occurrence of an intramolecular H-
further insights have been provided by quantum-chemical calcula-
tions (see below).
On the other hand, the white solid quantitatively recovered from
the TBHP experiment (I-B) showed an almost identical FT-IR spec-
trum to the starting compound I (See Figures SI3 and SI4). The
−
1
appearance of two new weak bands (1125 and 1280–1290 cm
)
indicates the occurrence of thioether to sulfone oxidation within
the amino acidic side-chain (Scheme 2 and Table S2). Their low
intensity can be justified on considering that only some of the
+
bond between −NH3 and (O O) peroxo moieties (see onward, the
Peroxo-2 species in Figs. 4 and 5), has the carbon stereocenter lying
too far from the catalytically active metal center, thus exercising its
role as chiral inducer only to a very small extent.
S
Me groups were oxidized. On the other hand, the Mo O back-
bone and the amino acid coordination were preserved during
the experiment, indicating that the metal center has a much
weaker interaction with TBHP than with H O . As a matter
2
2
of fact, dioxomolybdenum(VI)-alkylperoxo complexes have been
observed in solution, but no one of them has been isolated so far
Results from the computational study
[26c–e].
Selection of the model
In order to confirm the absence of any catalyst leaching, the solu-
The reaction mechanism operative during the oxidation of
olefins mediated by dioxomolybdenum(VI) catalysts, using TBHP as
main oxidant, is still under study. Even if there is a general accord
on the opinion that the catalyst first activates the oxidant molecule
followed by oxygen atom transfer to the olefin, there is no uniform
tion recovered after filtration of catalyst I (from the above described
experiment with TBHP) was checked for activity after addition of
fresh reagent (styrene) and TBHP, under the usual conditions, for
3
h. The absence of activity for the oxidative conversion of styrene

I. Abdalghani et al. / Molecular Catalysis 446 (2018) 39–48
45
Fig. 3. Overlap of the calculated IR spectra for FO (red) and CO (black) of HO-(Mo2O7H2·Met)4-H. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
Fig. 4. Schematic representation of the likely catalytic cycle with octanuclear HO-(Mo2O7H2·Met)4-H model fragment and TBHP (R = t-But) or H2O2 (R = H).
agreement on the details of the oxidant activation and oxygen atom
transfer. To this scope several detailed theoretical calculations have
been published since last decade [21b,27].
ous study carried out by some of us [15], we decided to utilize the
cis-dioxomolybdenum(VI) octanuclear fragment depicted in Fig. 2.
The structure of the polynuclear compound in Fig. 2, hereafter
termed as HO-(Mo O7H ·Met) -H, was obtained in two differ-
As a model system for our calculations, we decided to focus our
2
2
4
attention on the oxidation of ethylene, in CHCl , in the presence of
ent computational routes: (i) a Full-Optimization (FO) and (ii) a
Constrained-Optimization (CO) in which we maintained rigid the
Mo-O framework while relaxing the Methionine moieties.
3
the selected catalyst I, with TBHP or H O as primary oxidant. The
2
2
main difficulties associated to the computational elucidation of the
catalytic process involving catalyst I are: (i) the prohibitively large
dimension of the system and (ii) the absence of a precise chem-
ical formula due to its polymeric nature. For the first point, we
decided to adopt a cluster-based approach, extensively used in this
kind of systems [28]; for the second point, inspired by the previ-
The calculated IR spectra of CO and FO structures of HO-
(Mo O7H ·Met) -H, both reported in Fig. 3, essentially differ for
2
2
4
−
1
the presence of an intense band at 2390 cm (corresponding to
a Mo
O H O stretching red-shifted because of the occurrence of
H-bond) which is absent in the experimental spectrum (Figure SI5).

4
6
I. Abdalghani et al. / Molecular Catalysis 446 (2018) 39–48
◦
Fig. 5. Standard free energy diagram for the formation of Peroxo-2 intermediate with TBHP (R = t-But) and H2O2 (R = H), at 50 C. For sake of clarity, we only reported the
carboxyl group of methionine fragment into Adduct-1 and TS structures.
This finding led us to select the CO-based HO-(Mo O7H ·Met) -H
2
2
4
as the most plausible structure to be utilized in the modeling of the
catalytic mechanism (see below).
Additional information concerning the IR spectrum calculated
for CO HO-(Mo O7H ·Met) -H, are reported in the Supplemen-
2
2
4
tary Information (Figure SI6) together with assignment of the most
relevant signals, on the basis of the Hessian eigenvectors atomic
composition (Table SI3).
Comparison of the calculated spectra with the experimental
one, considering all the heavy approximations necessary for the
computational modeling, indicates that the selected octanuclear
molybdenum model might be utilized for capturing the essential
chemical information we are interested in, as below reported.
Fig. 6. Enlarged view of the intramolecular H-bond as extracted from Peroxo-2
optimized structure. All the hydrogen atoms have been omitted for the sake of
clarity.
Mechanism of the oxidation of I
As previously reported, CO HO-(Mo O7H ·Met) -H has been uti-
2
2
4
lized for modeling the oxidation of I representing the first step of the
slightly endergonic (1.0 kcal/mol, corresponding to an equilibrium
constant of 0.2 at 50 C) reaction was obtained hence indicating
◦
catalytic cycle reported in Fig. 4. The addition of TBHP (or H O ) to
2
2
the starting representative octanuclear molybdenum fragment HO-
that, upon thermodynamic control, TBHP results much more effi-
cient than H O . Thus, if the formation of peroxo species is generally
(
Mo O7H ·Met) -H leads to the formation of heptavalent Adduct-1
2
2
4
2
2
which, in turns by interaction with ethylene, affords the heptava-
accepted when H O is used as oxidant, our simulation clearly indi-
2 2
lent Peroxo-2 intermediate, after the elimination of tert-butanol
cates that also in the case of TBHP its formation cannot be ruled out,
(
or water). Finally, the formation of ethylene oxide regenerates the
at least in the presence of our octanuclear HO-(Mo O7H ·Met) -H
2
2
4
starting catalytic species (see Fig. 4).
model catalyst.
The full structures of intermediates Adduct-1 and Peroxo-2 are
schematically reported in the Supplementary Information (Figure
SI7) together with all the details of the presented structures.
Both TBHP and H O types of oxidants have been utilized in our
It is also interesting to remark the role of the amino acidic
moiety which, as shown in Fig. 6, provides a stabilizing intramolec-
+
ular H-bond between −NH3 and (O O) peroxo moieties probably
important for the negative free energy accompanying the elimina-
tion of ROH from the Adduct-1 (Fig. 4). This effect is likely to be the
most remarkable one played by the amino acidic moiety along the
whole process, and may be related to the small enantioselection
observed for the formation of epoxide (see above).
2
2
model. The schematic representation of the standard free energy
◦
diagram at T = 50 C for the formation of the key-intermediate
Peroxo-2 is reported in Fig. 5.
Note that, despite several attempts, we were not able to locate
a transition structure connecting CO HO-(Mo O7H ·Met) -H and
Moreover, the barrier for the elimination of the ROH group (indi-
cated as TS in Fig. 5), representing the rate determining step of the
whole process, appears fairly lower for TBHP probably because of
the higher proton affinity of the leaving group (t-BuO− vs HOO−).
We wish to underline that this latter observation, although appar-
ently able to justify the observed higher efficiency of the reaction
when using TBHP, is probably well beyond the chemical accuracy
of the theoretical model and the level of calculation.
2
2
4
Adduct-1.
Although alternative mechanisms have been hypothesized in
the literature when TBHP is concerned, in the present study we have
focused only on the plausible occurrence, if any, of peroxo inter-
mediate in both the investigated reactions (with TBHP or H O ). In
2
2
fact, results reported in Fig. 5 clearly indicate that not only Peroxo-
species is a true minimum on the investigated potential energy
2
surface, but also that its formation, when TBHP is involved, is an
Briefly, we can summarize that quantum-chemical calculations
confirm the presence of stable species resembling the intermedi-
ates hypothesized in the catalytic cycle (see Fig. 4). At the same time
our results reveal a possible strategic role of the ␣-amino acid moi-
exergonic reaction (−2.0 kcal/mol, corresponding to an equilibrium
◦
constant of 22 at 50 C) probably because of the higher ability of t-
ButOH to behave as leaving group. Diversely, in the case of H O
a
2
2

I. Abdalghani et al. / Molecular Catalysis 446 (2018) 39–48
47
Fig. 7. Calculated IR spectrum for I-A species.
Table 5
Experimental and calculated relevant frequencies and normal modes assignment
for I-A species.
−
1
Experimental Frequencies
cm (assignments)
Calculated Frequencies cm
(assignments)
−
1
a
a
3
3
3
2
1
1
1
1
1
1
1
1
1
1
1
9
9
8
8
600-3300m-br [ꢀ(OH)]
216-3160m-br
017m
930m
675m-sh [␦(H2O)]
621m
506m
444m-sh
405m
3335s [ꢀ(OH)]
2897 [ꢀ(NH)]
1685 [␦(H2O)]
1679 [ꢀ(CO2−)]
1579 [␦(NH3)]
1481 [␦(CH2)]
369m
323m-sh
281s [ꢀ(SO2)]
193w
127s [ꢀ(SO2)]
073m
60s [ꢀ(Mo = O)]
09w
57s [ꢀ(O-O)]
05w
1100 [ꢀ(SO2)]
◦
Fig. 8. Free energy diagram (50 C) for the addition of ethene to I-A in CHCl3. Details
of the calculations are the same as reported in the Experimental Section.
1023 [ꢀ(Mo = O)]
950 [ꢀ(O-O)]
ety providing the stabilization of the key-intermediate Peroxo-2
through an intramolecular H-bond (Fig. 6).
772w
7
6
35w
58w
a
Only the most relevant signals have been reported. For the complete spectrum
see Fig. 7.
Characterization of complex I-A
As a final step of the computational study, we have addressed the
structural and spectral analysis of the water-soluble molybdenum
complex I-A (see Scheme 1). Moreover, we have also analyzed the
kinetic role of I-A with respect to its plausible involvement in the
olefin oxidation, in solution. Details of the geometrical features of
I-A are reported in the Supplementary Information (see Figures SI1-
SI2). The IR spectral features are collected in Fig. 7 and in Table 5.
Comparison with the relevant experimental IR absorptions found
for I-A indicates a good overlap, hence reinforcing the hypothesis
about its plausible structure (Scheme 1).
We also evaluated the reactivity of I-A with ethylene as selected
olefin, in CHCl3 solution. To this purpose, we searched for the TS
possibly involved in the addition of olefin. The obtained diagram,
reported in Fig. 8, reveals that I-A is basically inert with respect
to olefin oxidation, thus suggesting a possible explanation of the
Conclusions
In this work, the application of
a
series of cis-
dioxomolybdenum(VI)-based compounds I–V containing a chiral
bidentate ␣-amino acid donor ligand, as potential catalysts for the
selective oxidative functionalization of conjugated or unconju-
gated olefins (like styrene, ␣-methylstyrene, cis-␤-methylstyrene,
cyclohexene and cyclooctene), using tert-butyl hydroperoxide
(TBHP) as oxygen source, has been studied. The compounds
I–V have proven to be insoluble in the catalytic system when
using TBHP as oxidant, confirming that we are dealing with a
heterogeneous solid/liquid catalytic process. Their activity in the
catalytic oxyfunctionalization of different types of olefins, runs
from excellent to very good levels if we move from unconjugated
olefins as cyclohexene and cyclooctene toward the less reactive
styrene, always showing higher selectivity over the corresponding
epoxides.
experimentally observed scarce catalytic activity of I when H O is
2
2
employed as oxidant.

4
8
I. Abdalghani et al. / Molecular Catalysis 446 (2018) 39–48
The absence of relevant differences in terms of both conver-
[10] (a) M.E. Judmaier, C. Holzer, M. Volpe, N.C. Mösch-Zanetti, Inorg. Chem. 51
2012) 9956–9966;
b) P. Neves, S. Gago, C. Pereira, S. Figueiredo, A. Lemos, A. Lopes, I. Gon c¸ alves,
(
(
sion of substrates and product yields among the different catalysts
I–V, suggests that, in the proximity of the catalytic metal center,
the variable amino acidic side-chain, does not strictly affect the
activation process, thus preventing a possible discriminant role
in the selectivity of the oxidation, including the stereoselectiv-
ity of the epoxide product. Nevertheless, a possible active role of
the amino acidic moiety in the stabilization of the key intermedi-
ate through an intramolecular hydrogen bond, as suggested from
quantum-chemical calculations, cannot be ruled out. Interestingly,
the catalyst IV showed to be recyclable (at least for three consec-
utive cycles), during the oxidation of styrene and cyclooctene, as
representative substrates.
Quantum-chemical calculations, carried out on a model sys-
tem whose reliability has been based by comparing the related IR
spectrum with the experimental one, have given insights into the
structure and energetic of the intermediates possibly involved in
the catalytic route.
M. Pillinger, C. Silva, A. Valente, Catal. Lett. 132 (2009) 94–103;
(c) S.M. Bruno, S.S. Balula, A.A. Valente, F.A. Almeida Paz, M. Pillinger, C. Sousa,
J. Klinowski, C. Freire, P. Ribeiro-Claro, I.S. Gon c¸ alves, J. Mol. Catal. A 270
(
(
2007) 185–194;
d) M. Abrantes, A.M. Santos, J. Mink, F.E. Kühn, C.C. Romão, Organometallics
22 (2003) 2112–2118.
[11] (a) M. Bortoluzzi, M. Hayatifar, F. Marchetti, G. Pampaloni, S. Zacchini, Inorg.
Chem. 54 (2015) 4047–4055;
(
b) K. Vassilev, M. Dimitrova, S. Turmanova, R. Milina, Synth. React. Inorg.
Metal-Org. Chem. 43 (2013) 243–249;
(c) W.J. Shaw, Catal. Rev. Sci. Eng. 54 (2012) 489–550;
(
d) D. Desbouis, I.P. Troitsky, M.J. Belousoff, L. Spiccia, B. Graham, Coord.
Chem. Rev. 256 (2012) 897–937;
e) K. Micskei, T. Patonay, L. Caglioti, G. Palyi, Chem. Biodivers. 7 (2010)
1660–1669;
f) K. Severin, R. Bergs, W. Beck, Angew. Chem. Int. Ed. 37 (1998) 1635–1654.
(
(
[
[
12] M. Ta sˇ ner, B. Prugovecki, Z. Soldin, S. Prugovecki, L. Rukavina, D.
Matkovic-Calogovic, Polyhedron 52 (2013) 268–275, and literature cited
therein.
13] (a) W.S. Zhu, H. Li, Q.Q. Gu, P. Wu, G. Zhu, Y. Yan, G.J. Chen, J. Mol. Catal. A 336
(
2011) 16–22;
On the other hand, when H O2 was employed as the oxidant, a
2
(b) M.L. Ramos, L.L.G. Justino, H.D. Barroiws, Dalton Trans. 40 (2011)
4374–4383;
soluble Mo-peroxido species I-A was isolated and characterized by
IR/NMR spectroscopy and DFT calculations. These last, carried out
at the same level of theory, indicate that such species I-A is scarcely
reactive towards ethylene epoxydation, thus confirming the exper-
imentally observed absence of catalytic activity with compounds
I–V in the presence of H O .
(
c) E. Cartuyvels, K. VanHecke, L. VanMeervelt, C. Görller-Walrand, T.N.
Parac-Vogt, J. Inorg. Biochem. 102 (2008) 1589–1598;
d) U. Kortz, J. Vaissermann, R. Thouvenot, P. Gouzerh, Inorg. Chem. 42 (2003)
(
1135–1139;
(e) U. Kortz, M.G. Savelieff, F.Y.A. Ghali, L.M. Khalil, S.A. Maalouf, D.I. Sinno,
Angew. Chem. Int. Ed. 41 (2002) 4070–4073.
14] K. Vassilev, S. Turmanova, E. Ivanova, V. Trifonova, J. Biomater.
Nanobiotechnol. 4 (2013) 28–36.
2
2
[
In our opinion, this study confirms the remarkable catalytic
potential of Mo(VI) systems as valid candidates for sustainable pro-
cesses, shedding light on a topic that, while covering a considerable
synthetic interest [29], has been poorly studied from the point of
view of its application in heterogeneous oxidative catalysis [30].
[15] L. Biancalana, M. Bortoluzzi, C. Forte, F. Marchetti, G. Pampaloni, RSC Adv. 5
(
2015) 9010–9018.
[
16] (a) N.A. Stephenson, A.T. Bell, The 1H NMR signal of H2O2 is
concentration-dependent, Anal. Bioanal. Chem. 381 (2005) 1289–1293;
(b) M.J. Frisch, et al., Gaussian, Inc, Wallingford, CT, 2013.
[
[
17] J. Tomasi, B. Mennucci, R. Cammi, Chem. Rev. 105 (2005) 2999–3093.
18] T. Yanai, D. Tew, N. Handy, Chem. Phys. Lett. 393 (2004) 51–57.
Acknowledgements
[19] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 299–310.
[
20] (a) F.E. Kuhn, M. Groarke, E. Bencze, E. Herdtweck, A. Prazeres, A.M. Santos,
M.J. Calhorda, C.C. Romao, I.S. Goncalves, A.D. Lopes, M. Pillinger, Chem. Eur. J.
The Universities of Pisa and L’Aquila are gratefully acknowl-
edged for financial support. I.A. thanks to the Phoenix Study
Programme (PX14DF0786 code) for his PhD scholarship.
8
(
(2002) 2370–2383;
b) M. Minelli, J.H. Enemark, R.T.C. Brownlee, M.J. O’Connor, A.G. Wedd, Coord.
Chem. Rev. 68 (1985) 169–278.
[
[
[
21] (a) M. Cindric, G. Pavlovic, R. Katava, D. Agustin, New J. Chem. 41 (2017)
5
94–602;
(b) J. Morlot, N. Uyttebroeck, D. Agustin, R. Poli, ChemCatChem 5 (2013)
01–611.
Appendix A. Supplementary data
6
22] (a) M. Bagherzadeh, R. Latifi, L. Tahsini, V. Amani, A. Ellern, L.K. Woo,
Polyhedron 28 (2009) 2517–2521;
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.mcat.2017.12.
18.
(
b) A. Rezaeifard, I. Sheikhshoaie, N. Monadi, H. Stoeckli-Evans, J. Inorg. Chem.
0
2010 (2010) 799–806.
23] (a) G. Romanowski, J. Kira, Polyhedron 117 (2016) 352–358;
(
b) S. Rayati, N. Rafiee, A. Wojtczak, Inorg. Chim. Acta 386 (2012) 27–35;
(c) J. Valand, H. Parekh, H.B. Friedrich, Catal. Commun. 40 (2013) 149–153;
d) F. Marchetti, C. Pettinari, C. Di Nicola, R. Pettinari, A. Crispini, M.
Crucianelli, A. Di Giuseppe, Appl. Catal. A 378 (2010) 211–220.
References
(
[
[
[
1] E.I. Stiefel, Metal Ions Biol. Syst. 39 (2002) 1–29.
2] R.R. Mendel, Dalton Trans. (2005) 3404–3409.
3] (a) A. Majumdar, S. Sarkar, Coord. Chem. Rev. 255 (2011) 1039–1054;
[24] J. Zhao, X. Zhou, A.M. Santos, E. Herdtweck, C.C. Romão, F.E. K u¨ hn, Dalton
Trans. (2003) 3736–3742.
[25] M. Bagherzadeh, L. Tahsini, R. Latifi, L.K. Woo, Inorg. Chim. Acta 362 (2009)
3698–3702.
[26] (a) G.B. Deacon, R.J. Phillips, Coord. Chem. Rev. 33 (1980) 227–250;
(b) C. Djordjevic, N. Vuletic, B.A. Jacobs, M. Lee-Renslo, E. Sinn, Inorg. Chem.
36 (1997) 1798–1805;
(
2
(
b) R.H. Holm, E.I. Solomon, A. Majumdar, A. Tenderholt, Coord. Chem. Rev.
55 (2011) 993–1015;
c) S. Metz, W. Thiel, Coord. Chem. Rev. 255 (2011) 1085–1103.
[
[
[
[
4] R.H. Holm, Chem. Rev. 87 (1987) 1401–1449.
5] A. Patel, S. Pathan, Ind. Eng. Chem. Res. 51 (2012) 732–740.
6] L. Shechter, J. Wynstra, R. Kurkjy, Ind. Eng. Chem. 49 (1957) 1107–1109.
7] (a) M. Amini, M.M. Haghdoost, M. Bagherzadeh, Coord. Chem. Rev. 257 (2013)
(c) E.P. Talsi, O.V. Klimov, Z.I. Zamaraev, J. Mol. Catal. 83 (1993) 329–346;
(d) M. Groarke, I.S. Gon c¸ alves, W.A. Herrmann, F.E. Kühn, J. Organomet. Chem.
649 (2002) 108–112;
(e) C. Bhaumik, E. Manoury, J.-C. Daran, P. Sözen-Aktas, F. Demirhan, R. Poli, J.
Organomet. Chem. 760 (2014) 115–123.
1
093–1121;
(
(
(
(
b) K.C. Gupta, A.K. Sutar, C.C. Lin, Coord. Chem. Rev. 253 (2009) 1926–1946;
c) K.C. Gupta, A.K. Sutar, Coord. Chem. Rev. 252 (2008) 1420–1450;
d) S. Liu, J. Xiao, J. Mol. Catal. A 270 (2007) 1–43;
[27] (a) L.F. Veiros, A. Prazeres, P.J. Costa, C.C. Romão, F.E. Kühn, M.J. Calhorda,
Dalton Trans. (2006) 1383–1389;
e) Q.-H. Xia, H.-Q. Ge, C.-P. Ye, Z.-M. Liu, K.-X. Su, Chem. Rev. 105 (2005)
1
603–1662;
(b) P.J. Costa, M.J. Calhorda, F.E. Kühn, Organometallics 29 (2010) 303–311;
(c) A. Comas-Vives, A. Lledós, R. Poli, Chem. Eur. J. 16 (2010) 2147–2158.
[28] H. Noh, Y. Cui, A.W. Peters, D.R. Pahls, M.A. Ortu n˜ o, N.A. Vermeulen, C.J.
Cramer, L. Gagliardi, J.T. Hupp, O.K. Farha, J. Am. Chem. Soc. 138 (2016)
14720–14726.
(
(
(
(
(
f) B.S. Lane, K. Burgess, Chem. Rev. 103 (2003) 2457–2474;
g) C.-M. Che, J.-S. Huang, Coord. Chem. Rev. 242 (2003) 97–113;
h) D. Sherrington, Catal. Today 57 (2000) 87–104;
i) M.H. Dickman, M.T. Pope, Chem. Rev. 94 (1994) 569–584;
j) K.A. Jørgensen, Chem. Rev. 89 (1989) 431–458.
[29] (a) S.-M. Chen, C.-Z. Lu, Y.-Q. Yu, Q.-Z. Zhang, X. He, Acta Cryst. C60 (2004)
m437–m439;
[
[
8] H. Noh, Y. Cui, A.W. Peters, D.R. Pahls, M.A. Ortu n˜ o, N.A. Vermeulen, C.J.
Cramer, L. Gagliardi, J.T. Hupp, O.K. Farha, J. Am. Chem. Soc. 138 (2016)
(b) T. Yamase, M. Inoue, H. Naruke, K. Fukaya, Chem. Lett. (1999) 563–564.
[30] M. Najafi, A. Abbasi, M. Masteri-Farahani, H. Shahbaazi, M. Ahmadniai
Motlaghd, J. Janczak, RSC Adv. 6 (2016) 29944–29949.
1
4720–14726.
9] (a) J.A. Schachner, P. Traar, C. Sala, M. Melcher, B.N. Harum, A.F. Sax, M. Volpe,
F. Belaj, N.C. Mösch-Zanetti, Inorg. Chem. 51 (2012) 7642–7649;
(
b) K. Jeyakumar, D.K. Chand, Synthesis (2008) 807–819.