Molybdenum(0) tricarbonyl and tetracarbonyl complexes with a cationic pyrazolylpyridine ligand: Synthesis, crystal structures and catalytic performance in olefin epoxidation

Article abstract of DOI:10.1039/c8ra01687a

The synthesis of molybdenum(0) tricarbonyl and tetracarbonyl complexes of the form [Mo(CO)₃(ptapzpy)Br] (1) and cis-[Mo(CO)₄(ptapzpy)]Br (2) is reported, where ptapzpy = 2-(1-propyltrimethylammonium-3-pyrazolyl)pyridine. Preparation of these derivatives was accomplished either through thermal replacement of CO in Mo(CO)₆ (for 1) or substitution under milder conditions of piperidine ligands in the precursor cis-[Mo(CO)₄(pip)₂] (for 2). The crystal structures of the ligand [ptapzpy]Br and complexes 1 and 2 were determined. Thermal treatment of 2 at 125-150 °C leads to mono decarbonylation and formation of 1. On the other hand, oxidative decarbonylation of 1 and 2 by reaction with tert-butylhydroperoxide (TBHP, 10 equiv.) gives a molybdenum oxide hybrid material formulated as [Mo₃O₉([ptapzpy]Br)₂]·nH₂O (3), which was characterised by FT-IR and Raman spectroscopy, thermogravimetric analysis, and ¹³C{¹H} CP MAS NMR spectroscopy. Compounds 1-3 were effective (pre)catalysts for the epoxidation of cis-cyclooctene at 55 °C with aqueous H₂O₂ or TBHP (slightly better results were obtained with the former). The characterisation of the Mo-containing solids isolated after the catalytic reaction showed that poorly soluble β-octamolybdate salts, (L)_x[Mo₈O₂₆], were formed from 1-3 with TBHP and from 1 with H₂O₂, while soluble oxoperoxo species were formed from 3 with H₂O₂. These findings helped to explain the different catalytic performances obtained.

Full text of DOI:10.1039/c8ra01687a

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Molybdenum(0) tricarbonyl and tetracarbonyl
complexes with a cationic pyrazolylpyridine ligand:
synthesis, crystal structures and catalytic
performance in olefin epoxidation†
Cite this: RSC Adv., 2018, 8, 16294
a
a
a
a
Lucie S. Nogueira,
Patrıcia Neves,
Ana C. Gomes,
*
Pedro Lavrador,
´
b
a
a
Luıs Cunha-Silva,
Anabela A. Valente,
Isabel S. Gonçalves
´
a
*
and Martyn Pillinger
The synthesis of molybdenum(0) tricarbonyl and tetracarbonyl complexes of the form [Mo(CO)3(ptapzpy)
Br] (1) and cis-[Mo(CO)4(ptapzpy)]Br (2) is reported, where ptapzpy ¼ 2-(1-propyltrimethylammonium-3-
pyrazolyl)pyridine. Preparation of these derivatives was accomplished either through thermal
replacement of CO in Mo(CO)6 (for 1) or substitution under milder conditions of piperidine ligands in the
precursor cis-[Mo(CO)4(pip)2] (for 2). The crystal structures of the ligand [ptapzpy]Br and complexes 1
and 2 were determined. Thermal treatment of 2 at 125–150 ꢀC leads to mono decarbonylation and
formation of 1. On the other hand, oxidative decarbonylation of 1 and 2 by reaction with tert-
butylhydroperoxide (TBHP, 10 equiv.) gives
a molybdenum oxide hybrid material formulated as
[Mo3O9([ptapzpy]Br)2]$nH2O (3), which was characterised by FT-IR and Raman spectroscopy,
thermogravimetric analysis, and 13C{1H} CP MAS NMR spectroscopy. Compounds 1–3 were effective
(pre)catalysts for the epoxidation of cis-cyclooctene at 55 ꢀC with aqueous H2O2 or TBHP (slightly better
results were obtained with the former). The characterisation of the Mo-containing solids isolated after
the catalytic reaction showed that poorly soluble b-octamolybdate salts, (L)x[Mo8O26], were formed from
1–3 with TBHP and from 1 with H2O2, while soluble oxoperoxo species were formed from 3 with H2O2.
These findings helped to explain the different catalytic performances obtained.
Received 26th February 2018
Accepted 24th April 2018
DOI: 10.1039/c8ra01687a
rsc.li/rsc-advances
molecular structure of this complex was determined only
recently.3 Since the report by Stiddard, much attention has been
Introduction
given to molybdenum(0)-diimine-tetracarbonyl complexes con-
taining bidentate ligands (N–N) such as bipyridines,3,4 phe-
nanthrolines,5 1,4-diazabutadienes,6 iminopyridines,7 and
pyrazolylpyridines.8 Further substitution of the carbonyl ligands
is possible (without alteration of the formal charge on the
central Mo atom), e.g. of one carbonyl ligand by a neutral (L) or
The organometallic chemistry of molybdenum dates back to the
mid 1930s when Hieber and co-workers described the thermal
substitution of CO ligands in Mo(CO)6 to give octahedral Mo0
derivatives of the type Mo(CO)x(L)y.1 The physical properties of
one of these compounds, cis-[Mo(CO)4(bipy)] (bipy ¼ 2,20-
bipyridine), were investigated much later by Stiddard,2 and the
anionic coligand (Xꢁ) to give tricarbonyl complexes
[Mo(CO)3(N–N)(L/X)]0/ꢁ
,
or of two carbonyl ligands by
6a,9
aDepartment of Chemistry, CICECO – Aveiro Institute of Materials, University of
a
second diimine ligand to give dicarbonyl complexes
´
Aveiro, Campus Universitario de Santiago, 3810-193 Aveiro, Portugal. E-mail:
[Mo(CO)2(N–N)2].10 Although anionic complexes [Mo(CO)3(N–
N)(X)]ꢁ were described by Behrens et al. in 1970,9a molecular
structures for this family of compounds were only described six
years ago.6a
mpillinger@ua.pt, atav@ua.pt
bREQUIMTE/LAQV, Department of Chemistry and Biochemistry, Faculty of Sciences,
University of Porto, 4169-007 Porto, Portugal
† Electronic supplementary information (ESI) available: Experimental details
(chemicals, physical measurements), X-ray crystallography (crystal and structure
renement data, discussion of supramolecular interactions and crystal
packing), TGA curves, powder XRD patterns, FT-IR and FT-Raman spectra, 13C
{1H} CP MAS NMR spectra, comparison of catalytic performance of 1 and 2 with
literature data, comparison of FT-IR bands of recovered solids with literature
data for octamolybdate salts, and elemental analysis results. CCDC 1523642
([ptapzpy]Br), 1817124 (1$CH3CN) and 1523641 (2$CH3CN). For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c8ra01687a
Complexes of the type [Mo(CO)3(N–N)(X)]ꢁ can be prepared
from cis-[Mo(CO)4(N–N)]9b but not in the reverse sense from the
pentacarbonyl complexes [Mo(CO)5(X)]ꢁ, since reaction of the
latter with the ligand N–N gives instead the tetracarbonyl
derivatives cis-[Mo(CO)4(N–N)].11 This was attributed to prefer-
ential halogen displacement (rather than CO substitution) in
the intermediate [Mo(CO)4(N–N)(X)]ꢁ owing to the fact that
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bidentate diimines such as bipy are relatively poor p-acceptors,
thereby strengthening the remaining Mo–C bonds in the
intermediate.11 To the best of our knowledge, the direct
synthesis of anionic complexes [Mo(CO)3(N–N)(X)]ꢁ from
molybdenum hexacarbonyl has never been reported.
Carbonyl–diimine complexes of Mo0, especially the tetra-
carbonyl derivatives, have been intensively studied due to their
rich electronic absorption spectra, photophysics, and photo-
chemistry.12 Tetracarbonyl derivatives are strongly colored due
to an intense Mo / diimine metal-to-ligand charge-transfer
(MLCT) absorption band in the visible spectral region. The
low-lying MLCT excited state is associated with several inter-
esting phenomena such as negative solvatochromism,6a,12c–e
second-order nonlinear optical responses,12f,g luminescence,12a–c
and photochemically induced CO substitution.12a,h,i Aside from
their photo-properties, Mo carbonyl diimine complexes possess
other interesting features and have been studied as CO-
releasing molecules for therapeutic applications,13a catalysts
for the electrochemical reduction of CO2,13b–d initiators for the
polymerisation of methyl methacrylate,13e alkylation of allylic
acetates,5b and precatalysts for the epoxidation of olens.4a,8 In
the latter case, the catalyst precursors undergo oxidative
decarbonylation (OD) by reaction with an oxidant to give oxo-
molybdenum(VI)-diimine compounds, the structures of which
vary according to the nature of the organic ligand. With tetra-
carbonyl precursors, structurally characterised OD products
include tetranuclear [Mo4O12(pzpy)4] (pzpy ¼ 2-[3(5)-pyrazolyl]
pyridine),8a octanuclear [Mo8O24(di-tBu-bipy)4] (di-tBu-bipy ¼
4,40-di-tert-butyl-2,20-bipyridine),4a and the one-dimensional
(1D) molybdenum oxide/bipyridine polymer [MoO3(bipy)].4a
As part of our continuing interest in pyrazolylpyridine
ligands, we have prepared the cationic derivative 2-(1-
Scheme 1 Preparation of [ptapzpy]Br, complexes 1 and 2, and the
oxidised compounds formed by OD of the carbonyl complexes by
reaction with 10 eq. of TBHP or a large excess of TBHP under the
catalytic epoxidation reaction conditions (denoted Epox).
pyrazolyl)pyridine derivatives.14 [ptapzpy]Br is highly hygro-
scopic, soluble in water, ethanol, acetonitrile, dichloromethane
and chloroform, and insoluble in acetone, toluene, 1,2-dichlo-
roethane and diethyl ether.
propyltrimethylammonium-3-pyrazolyl)pyridine
bromide
([ptapzpy]Br). The introduction of cation-bearing ligands into
transition metal complexes can help improve solubility in polar
solvents or target the complexes for immobilisation in solid
supports with charged frameworks and/or functional groups.
The reaction of [ptapzpy]Br with Mo(CO)6 unexpectedly yielded
the charge-neutral zwitterionic complex [Mo(CO)3(ptapzpy)Br]
rather than the tetracarbonyl derivative cis-[Mo(CO)4(ptapzpy)]
Br. For comparison, the latter complex was prepared by
Reaction of pyrazolylpyridine ligands with 1 equivalent of
Mo(CO)6 in reuxing toluene usually gives tetracarbonyl
complexes of the type cis-[Mo(CO)4(N–N)].8 However, in the case
of [ptapzpy]Br, this reaction led to the tricarbonyl derivative
[Mo(CO)3(ptapzpy)Br] (1) (Scheme 1). The unusual direct
synthesis of the tricarbonyl complex from the hexacarbonyl and
the ligand results from the presence of the terminal trimethy-
lammonium group in the ligand together with the bromide
counterion, which favour the formation of the charge-neutral
zwitterionic complex. The tetracarbonyl derivative cis-
[Mo(CO)4(ptapzpy)]Br (2) could be prepared by treatment of the
precursor cis-[Mo(CO)4(pip)2] (prepared by the literature
method;15 pip ¼ piperidine) with 1 equivalent of [ptapzpy]Br in
a
different route and the molecular structures of both
complexes have been determined. OD of the complexes with
tert-butylhydroperoxide (TBHP) is shown to give a hybrid
molybdenum oxide/organic material. The catalytic behavior and
chemical transformations of all compounds in the epoxidation
of cis-cyclooctene have been studied using TBHP or H2O2 as
oxidant.
ꢀ
ethanol at 50 C. Complexes 1 and 2 are at best only sparingly
Results and discussion
Synthesis and characterisation
soluble in nonpolar solvents such as chloroform. Dissolution of
the compounds in polar, coordinating solvents such as water,
acetonitrile and dimethyl sulfoxide is accompanied by solvol-
ysis (especially for 1, as reported previously in ref. 9b for other
tricarbonyl anions [Mo(CO)3(N–N)(X)]ꢁ) and fast degradation
involving decarbonylation. While complex 2 displays extended
stability in the solid-state if stored cold in the dark and under
The ligand [ptapzpy]Br was prepared by deprotonation of 2-(3-
pyrazolyl)pyridine followed by addition of (3-bromopropyl)tri-
methylammonium bromide and reux for 4 h (Scheme 1).
Characterisation data (FT-IR, 1H and 13C solution NMR) were in
line with those reported previously for related 2-(1-alkyl-3-
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inert atmosphere, complex 1 is unstable and undergoes decar-
bonylation over a period of a few days.
The FT-IR spectra of freshly prepared 1 and 2 conrmed the
formation of tricarbonyl and tetracarbonyl complexes, respec-
tively (Fig. S4 in the ESI†). In the carbonyl stretching region
(1700–2020 cmꢁ1), complex 1 displays two overlapping strong
bands at 1742 and 1764 cmꢁ1, and one very strong band at
1895 cmꢁ1, in a pattern that is typical of fac-[Mo(CO)3(N–N)(L/
X)]0/ꢁ complexes.6a,9b The two lower energy bands arise from
splitting of the E mode, while the high-energy band is associ-
ated with the A1 mode. Four carbonyl stretching bands are
observed for complex 2 as expected for cis-substituted tetra-
carbonyl complexes. These are assigned as A21 (medium,
2012 cmꢁ1), B1 (shoulder, 1888 cmꢁ1), A11 (very strong,
1869 cmꢁ1) and B2 (very strong, 1815 cmꢁ1).16 From 300 to
1700 cmꢁ1 the vibrational spectra of 1 and 2 exhibit numerous
ligand (ptapzpy) modes. Upon complexation the ligand
undergoes structural changes which affect the 1550–1650 cmꢁ1
region.8b,14a,17 In particular, the pyridyl C–N stretching mode
shis from 1590 cmꢁ1 for the free ligand to 1604–1608 cmꢁ1 for
1 and 2.
Thermogravimetric analysis (TGA) of 2 revealed a weight loss
step of 4.2% between 90 and 125 ꢀC (Fig. S2 in the ESI†). This is
close to the calculated value of 5.0% for the removal of one CO
ligand. Accordingly, ATR FT-IR spectra (not shown) of 2 heated
at different temperatures (15 min at each temperature) showed
ꢀ
that the compound is stable at 75 C but is converted at 125–
150 ꢀC to the tricarbonyl complex 1 (Scheme 1). Notably,
complex 1 differs from 2 in not displaying a resolved weight loss
step around 100 ꢀC. Above 150 ꢀC the TGA curves for 1 and 2 are
simiꢀlar, showing two major decomposition steps in the 200–
300 C and 475–575 ꢀC intervals.
Single-crystal X-ray structure analyses
Fig. 1 Representations of (a) the molecular structure of the cationic
ligand in [ptapzpy]Br, (b) the molecular structure of [Mo(CO)3(ptapzpy)
Br] in 1$CH3CN, (c) the molecular structure of [Mo(CO)4(ptapzpy)]+ in
2$CH3CN, (d) and (e) the Mo0 coordination centres found in
complexes 1 and 2, respectively. Thermal ellipsoids are drawn at the
Single-crystals of [ptapzpy]Br, [Mo(CO)3(ptapzpy)Br]$CH3CN
(1$CH3CN) and [Mo(CO)4(ptapzpy)]Br$CH3CN (2$CH3CN) suit-
able for X-ray diffraction (XRD) were obtained (Table S1 in the
ESI†). The cationic organic ligand in [ptapzpy]Br crystallised
with the counterion Brꢁ in the monoclinic space group P21/c 50% probability level and the labelling scheme is provided for all non
H-atoms. 1$CH3CN: selected bond lengths [A˚]: Mo1–C15 1.935(6),
Mo1–C16 1.894(7), Mo1–C17 2.072(9), Mo1–N1 2.293(5), Mo1–N2
2.259(6), Mo1–Br1 2.7476(10). Selected bond angles [ꢀ]: C16–Mo1–
C15 85.2(3), N2–Mo1–N1 72.13(19), C17–Mo1–Br1 174.63(19), C15–
Mo1–Br1 93.0(2), N1–Mo1–Br1 84.49(13), C17–Mo1–N1 94.6(2).
(Fig. 1a). In 1, the neutral tricarbonyl complex crystallised with
one CH3CN molecule of crystallisation in the monoclinic space
group P21 (Fig. 1b), while in 2 the cationic tetracarbonyl
complex crystallised jointly with one Brꢁ counterion and one
ꢀ
2$CH3CN: selected bond lengths [A˚]: Mo1–C15 1.946(2), Mo1–C16
2.042(3), Mo1–C17 1.954(2), Mo1–C18 2.026(3), Mo1–N1 2.2689(17),
Mo1–N2 2.2576(17). Selected bond angles [ꢀ]: C15–Mo1–C17
89.13(10), N1–Mo1–N2 71.71(6), C18–Mo1–C16 165.75(9), C15–Mo1–
C16 86.81(10), C16–Mo1–N1 93.09(8), C18–Mo1–N1 95.96(8).
uncoordinated CH3CN molecule in the triclinic space group P1
(Fig. 1c).
A striking feature of the crystal structure of [ptapzpy]Br is
that the pyrazolylpyridine group adopts a cisoid conformation
with respect to the pyridine and pyrazole nitrogen atoms. In
analogy with the situation usually encountered with uncoordi-
nated 2,20-bipyridine derivatives in the solid-state, uncoordi-
nated pyrazolylpyridine ligands typically adopt a transoid
conformation, i.e. the two rings are rotated along the central
C5–C6 bond, thereby minimising repulsion between the lone
pairs of the nitrogen atoms and between the protons H-4 and H-
8. Crystal packing effects together with a complex network of
weak intermolecular interactions (involving the bromide anion)
may be responsible for this rare conformation of ptapzpy in the
solid-state.
In complex 1, the Mo0 centre coordinates to three carbonyl
groups, one N,N-chelating pyrazolylpyridine ligand and one
bromide anion, leading to a distorted octahedral coordination
geometry with wide ranges for the bond lengths and internal
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˚
bond angles (Fig. 1d). The bond lengths range from 1.894(7) A having the general composition [Mo3O9(N–N)2] have been re-
(Mo–C) to 2.7476(10) A (Mo–Br). Furthermore, the cis angles ported previously, namely the 1D materials with N–N ¼ bipy19a
˚
range between 72.13(19) and 101.9(3)ꢀ, while the trans angles and 1,10-phenanthroline.19b The Raman spectrum of [Mo3O9(-
are found in the range of 172.9(3)–174.63(19)ꢀ.
bipy)2] was reported by Twu et al. and found to exhibit three
The main crystallographic features of the {MoC4N2} core of Mo–O bands at 893, 923 and 946 cmꢁ1 which coincide quite
complex 2 are consistent with those observed in the handful of closely with the three bands observed for 3 at 895, 924 and
structures reported containing molybdenum tetracarbonyl 962 cmꢁ1, suggesting that the two materials may possess the
complexes with chelating 2-(3-pyrazolyl)pyridine residues.18 The same type of molybdenum oxide substructure.20 The structure
Mo0 centre is coordinated by four carbonyl groups and one N,N- of the bipy hybrid consists of 1D chains built up from alter-
chelating pyrazolylpyridine ligand, originating a distorted nating {MoO4} tetrahedra and pairs of corner-linked {MoO4N2}
pseudo-octahedral geometry as conrmed by the distinct Mo–C octahedra.19a
and Mo–N bond lengths (Fig. 1e). The Mo–C bond lengths of the
˚
axial carbonyl groups (2.026(3), 2.042(3) A) are longer than those
Catalytic epoxidation
˚
for the equatorial groups (1.946(2), 1.954(2) A), most probably as
Compounds 1–3 were tested for catalytic epoxidation with TBHP
a consequence of distinct p back-bonding, Mo(d) / C^O(p*).
The equatorial CO groups are competing with the less p-acidic
N-donor atoms from the pyrazolylpyridine ligand, while the
axial groups compete with each other.18c The geometric defor-
mation of the octahedral coordination of the metal centre is
further veried by an inspection of the internal bond angles: the
cis angles are found between 71.71(6) and 101.82(8)ꢀ, while the
trans angles are found in the range of 165.75(9)–173.41(8)ꢀ.
A full account of the supramolecular interactions (hydrogen
bonding, p/p contacts, C–H/acceptor) and crystal packing
arrangements in the crystal structures of [ptapzpy]Br, 1$CH3CN
and 2$CH3CN is given in the ESI.†
[decane solution (TBHPD) or aqueous solution (TBHPA)], at
ꢀ
55 C, using cis-cyclooctene (Cy) as a benchmark substrate for
olens (Scheme 2). The three compounds led to 1,2-epox-
ycyclooctane (CyO) as the main product (Table 1). With 1,2-
dichloroethane (DCE) as cosolvent and TBHPD as oxidant,
complex 1 led to a higher conversion at 24 h than 2 or 3 (39%
compared with 22 and 17%, respectively). On the other hand, 1
led to lower CyO selectivity (71%) than 2 or 3 (86–88%), at 17–
22% conversion (Fig. 2); byproducts included 2-cycloocten-1-
one and others that were not clearly identied. Without cata-
lyst, conversion was less than 10% at 24 h. The system TBHPD/
DCE led to better results than TBHPA/CH3CN, in the presence of
1: 27% and 17% CyO yield, respectively, at 24 h. These con-
trasting performances may be partly due to differences in
dissolution rate and solubility of the molybdenum compounds.
The catalytic results were improved when no organic cosol-
vent was added: 87–95% CyO selectivity at 40–54% conversion,
24 h. Under these conditions, 1 and 2 display similar perfor-
mance, leading to about 48% CyO yield at 24 h. For comparison,
the complex cis-[Mo(CO)4(pzpy)] containing the unsubstituted
organic ligand led to 78% epoxide yield under equivalent reac-
tion conditions.8a The catalytic results for 1 and 2 are interme-
diate between those reported in the literature (for similar
Oxidative decarbonylation
Oxidative decarbonylation of 1 was carried out by the dropwise
addition of TBHP (10 equiv.) to a suspension of the complex in
CH2Cl2. Aer stirring at room temperature for 4 h, an off-white
solid (3) was recovered by ltration. Thermogravimetric analysis
(TGA; Fig. S2 in the ESI†) and CHN microanalyses for 3 were
consistent with the composition [Mo3O9([ptapzpy]Br)2]$3H2O.
Powder XRD (PXRD) showed that the solid was amorphous, with
only a few very broad overlapping diffraction peaks being
observed in the 2q range of 5–30ꢀ (Fig. S3†). The FT-IR and
Raman spectra for 3 conrmed that complete decarbonylation
had occurred during the reaction of 1 with the oxidant since no
bands were observed in the carbonyl stretching region (1700–
2020 cmꢁ1; Fig. S4†). On the other hand, the spectra retained
the characteristic ligand modes of ptapzpy in the range of 1000–
1650 cmꢁ1, matching closely the bands observed for 1 and
therefore indicating that the ligand is coordinated in a biden-
tate fashion to Mo centres. The structural integrity of the
organic ligand in 3 was veried by comparing its 13C{1H} CP
MAS NMR spectrum with that of the free ligand [ptapzpy]Br
(Fig. S5†). Treatment of complex 2 with TBHP under the same
conditions as used for 1 gave rise to the same molybdenum
oxide hybrid material 3 (in a comparable yield) on the basis of
the similar TGA, PXRD and spectral data (Figures S2–S5†).
New vibrational bands for 3 that arise in the 850–1000 cmꢁ1
interval are assigned to Mo–O vibrations. A very broad absorp-
tion band centred at 609 cmꢁ1 in the IR spectrum is assigned to
n(Mo–O–Mo) and points towards a polynuclear or polymeric
ꢀ
reaction conditions: no cosolvent, 55 C) for complexes of the
type [Mo(CO)n(L)] containing chelating N-heterocyclic carbene
(L) ligands (entries 6 and 7 of Table S3 in the ESI†), which led to
9–16% conversion at 24 h,21 and those for [Mo(CO)4(pyim)]
(pyim ¼ N-(n-propyl)-2-pyridylmethanimine), which led to 100%
conversion at 5 h (entry 5 of Table S3†).22
The catalytic system 2/TBHP was further investigated for the
epoxidation of styrene (which possesses a terminal C]C bond)
Scheme 2 Reaction of cis-cyclooctene with TBHP, at 55 ꢀC, in the
structure. Molybdenum oxide-organonitrogen hybrid materials presence of 1–3.
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Table 1 cis-Cyclooctene epoxidation in the presence of 1–3a
Compound
Oxidant
Cosolvent
Conv.b (%)
Yieldb (%)
—
1
1
1
1
TBHPD
TBHPD
TBHPD
TBHPD
TBHPD
TBHPD
TBHPD
TBHPD
TBHPD
TBHPD
TBHPD
TBHPD
TBHPD
TBHPD
TBHPA
H2O2
None
DCE
None
[bmim]NTf2
[bmim]PF6
DCE
None
None
-/-/4
-/-/4
3/18/39
9/35/54
6/25/46
7/18/29
2/10/22
5/31/52
-/31/48
4/16/37
3/9/17
5/23/40
3/13/34
3/29/47
13/49/62
-/-/21
3/13/27
9/31/47
6/22/40
7/15/24
2/9/19
5/30/48
-/28/45
4/16/35
3/8/15
5/22/38
3/13/31
3/26/41
13/46/58
-/-/17
2
2
2c
2
[bmim]NTf2
DCE
None
3
3
3
[bmim]NTf2
[bmim]PF6
[bmim]PF6
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
3
3d
1
1
11
20
67
56
9
16
66
56
1d
2
H2O2
H2O2
H2O2
H2O2
3
3d
51
51
a
Reaction temperature ¼ 55 ꢀC and initial Mo : olen : oxidant molar
ratio
¼
1 : 113 : 172 (unless otherwise indicated), initial Cy
concentration ¼ 1.0 M (organic cosolvent and TBHPA/D), 1.7 M (ionic
liquid and TBHPD), 1.2 M (CH3CN and H2O2) or 2.4 M (no cosolvent).
b
Cy conversion and CyO yield at 1 h/6 h/24 h. Values for H2O2 as
c
oxidant are for 24 h. Mo : olen : oxidant molar ratio ¼ 1 : 75 : 114.
d
Reaction carried out at 70 ꢀC.
and DL-limonene (which possesses endocyclic and exocyclic
C]C bonds), at 55 ꢀC, without additional solvent. Compound 2
led to 20%/39% styrene conversion at 6 h/24 h, and the main
reaction products were styrene oxide and benzaldehyde formed
with 52%/64% and 42%/31% selectivity, respectively. These
results demonstrate the ability of 2/TBHP for the epoxidation of
terminal C]C bonds, which are electron-richer.
The reaction of DL-limonene gave mainly limonene-1,2-oxide Fig. 2 cis-Cyclooctene epoxidation with TBHPD at 55 ꢀC (DCE as
cosolvent) using compounds 1 (,), 2 (B) and 3 (O): (A) conversion of
Cy as a function of time and (B) selectivity of CyO as a function of Cy
conversion. The dashed lines are visual guides.
formed with 70%/71% selectivity at 23%/43% conversion, aer
6 h/24 h, respectively. These results suggest regioselectivity
effects in favour of the epoxidation of the endocyclic C]C bond
over the exocyclic one. Other reaction products were formed via
allylic oxidation, leading to alcohol and carbonyl derivatives of
p-mentha-1,8-diene.
(which lead to biphasic solid–liquid reaction mixtures), the
reaction mixtures are saturated with metal species.
The use of the complex cis-[Mo(CO)4(pzpy)] in Cy epoxidation
Characterisation of the off-white solid phases i-S-TBHPD (i ¼
with TBHPD led to a biphasic solid–liquid mixture and the solid
1–3; cosolvent ¼ DCE) by ATR FT-IR spectroscopy indicated that
phase was identied as the tetranuclear species [Mo4O12(-
1–3 were converted to different types of metal species (Fig. 3).
pzpy)4].8a Biphasic solid–liquid mixtures were also obtained for
Comparison of these spectra with literature data for poly-
oxomolybdates of the type (L)x[Mo8O26] (where L is an organic
cation such as pyridinium and imidazolium) indicates that
a similar type of poorly soluble b-octamolybdate salt is formed
the systems (1, 2 or 3)/TBHPD. Attempts to isolate metal species
from the colourless liquid phases were unsuccessful, suggesting
that the concentration of dissolved species aer a 24 h batch
run was either negligible or very low. The low solubility of the
in the catalytic reaction systems of 1–3 with TBHPD. In the
metal species is probably one reason for the poor catalytic
region of the Mo–O vibrations (<1000 cmꢁ1), the solids exhibi-
results obtained with these systems. Accordingly, for the system
ted bands at ca. 942, 910, 840, 705 and 659 cmꢁ1, which match
2/TBHPD (no cosolvent), a 1.5-fold increase in the catalyst
closely with literature values for several (L)x[Mo8O26] salts (Table
S4 in the ESI†).23 The recovered solids also exhibited a band at
amount (i.e. from ca. 0.9 mol% to 1.3 mol%) did not improve
the epoxide yield (Table 1). This is because the catalytic reaction
767 cmꢁ1 and several bands between 950 and 1650 cmꢁ1
is homogeneous in nature and, for both catalyst amounts
attributed to the charge-balancing organic cation ptapzpy. CHN
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reaction kinetics. In comparison to the volatile organic solvent
DCE, the reaction using [bmim]NTf2 as cosolvent led to superior
catalytic results for all three compounds (Table 1). CyO selec-
tivity of at least 87% was reached at 34–46% conversion (31–
40% CyO yield). Complex 1 with [bmim]NTf2 led to superior
catalytic results than those with [bmim]PF6, whereas the
opposite was observed for 3. These results may be due to
differences in solubility and rate of formation of the molyb-
denum species in the different ILs, although this is not trivial to
demonstrate since it would require tracking the molybdenum
species formed over time, and assessing their intrinsic activi-
ties. For the best performing system 3/[bmim]PF6, increasing
the reaction temperature from 55 to 70 ꢀC led to an increase in
the CyO yield at 24 h from 41% to 58%.
Aer 24 h-batch runs at 55 ꢀC, the reaction mixtures were
liquid (IL)-liquid (organic phase)-solid(S). The ATR FT-IR
spectra of the recovered solids i-S-TBHPD-[bmim]NTf2 were
similar to the spectra of i-S-TBHPD (i ¼ 1–3) with respect to Mo–
O vibrations (700–1000 cmꢁ1), but different in the region of the
ligand modes (>1000 cmꢁ1) (Fig. 3). Similar results were ob-
tained for i-S-TBHPD-[bmim]PF6 (not shown). The spectra very
closely match that reported previously for the solid obtained
upon OD of the tetracarbonyl complex cis-[Mo(CO)4(ppzpy)]
(ppzpy ¼ 2-(1-pentyl-3-pyrazolyl)pyridine) during Cy epoxida-
tion with TBHPD in the presence of [bmim]NTf2.8b CHN
microanalyses (Table S5 in the ESI†) for the solid 2-S-TBHPD-
[bmim]NTf2 were consistent with the formulation (bmim)3(H3-
O)[Mo8O26]. PXRD data for this solid, designated as compound
5 in Scheme 1, showed that the material is microcrystalline
(Fig. S6 in the ESI†). A comparison of the PXRD pattern of 5 with
a simulated pattern calculated using crystal structure data re-
ported for (bmim)3NH4[Mo8O26] (6) suggests that the two salts
may have similar structures.24 Accordingly, FT-IR spectra for the
two compounds display a striking correspondence in the region
350–1000 cmꢁ1 (Fig. S7 in the ESI†). For 1–3 (Scheme 1), the
precipitation of the b-octamolybdate salt with [bmim]+ coun-
terions (rather than a cation derived from the ligand ptapzpy) is
due to the large excess of IL present in the reaction mixture.
Compounds 1–3 were further studied for Cy reaction with
H2O2 at 55 ꢀC. Results with 2 and 3 (56–66% CyO yield at 24 h)
were superior to those with 1 (9%) (Table 1). With the exception
of complex 1, the catalytic results are generally better with H2O2
as oxidant than with TBHP. Notably, the CyO yield of 66% ob-
tained with 2 surpasses the 41% yield obtained with cis-
[Mo(CO)4(ppzpy)] under identical reaction conditions.8b
Fig. 3 ATR FT-IR spectra of (a) 1, (e) 2 and (h) 3, and the recovered
solids (b) 1-S-TBHPD, (c) 1-S-H2O2, (d) 1-S-TBHPD-[bmim]NTf2, (f) 2-
S-TBHPD, (g) 2-S-TBHPD-[bmim]NTf2, (i) 3-S-TBHPD, (j) 3-S-TBHPD-
[bmim]NTf2, and (k) 3-L-H2O2.
microanalyses (Table S5 in the ESI†) for the solid 2-S-TBHPD
were consistent with the formulation (ptapzpy)4[Mo8O26]. PXRD
data for this solid, designated as compound 4 in Scheme 1,
showed that the material is microcrystalline (Fig. S6 in the
ESI†).
Whereas an octamolybdate salt was formed from 1–3 under
the catalytic reaction conditions with TBHPD, the oxidative
decarbonylation of 1 and 2 with TBHPD in the absence of
substrate gave 3. These contrasting outcomes may be attributed
to the different reaction conditions used (10 eq. TBHPD, room
temperature/4 h and no substrate for the preparation of 3; 172
eq. TBHPD, 55 ꢀC/24 h and presence of Cy for the catalytic
reaction). It is still possible that in the catalytic reaction the
hybrid material 3 may be an intermediate in the conversion of 1
and 2 to species of the type (L)x[Mo8O26].
Iodometric titrations performed for 3/TBHPD/55 ꢀC and 3/
H2O2/55 ꢀC (without substrate) showed that the decomposition
of TBHP was not considerable (31% TBHP conversion at 24 h),
whereas that of H2O2 was very signicant (ca. 98% decomposi-
tion). Hence, the epoxidation process with TBHP does not seem
to be compromised by side-reactions of the oxidant (i.e.
unproductive decomposition into molecular oxygen and tert-
butanol). On the other hand, the unproductive decomposition
of H2O2 (into molecular oxygen and H2O) may compete with use
of the oxidant for the catalytic reaction. Increasing the reaction
temperature from 55 to 70 ꢀC for the systems (1 or 3)/H2O2 was
only slightly benecial for 1 and actually led to poorer results
Compounds 1–3 were further tested for Cy epoxidation with
TBHPD using the ionic liquid (IL) [bmim]NTf2 (bmim ¼ 1-butyl-
3-methylimidazolium; NTf2
¼
bis(triuoromethylsulfonyl)
imide) as cosolvent at 55 ꢀC (Table 1). The use of an IL may
enhance the solubility of the metal species, and thus the overall
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with 3 (Table 1), presumably due to enhanced decomposition of (90 mL) was added dropwise and the mixture stirred under reux
the oxidant. for 4 h, resulting in a yellow solution and a white precipitate. The
The differences in catalytic results for TBHP or H2O2 as mixture was evaporated to dryness under reduced pressure. The
oxidant may also be partly due to differences in catalyst residue was extracted with CHCl3 (4 ꢂ 50 mL) and the combined
stability. The system 1/H2O2 consisted initially of a yellow extracts were evaporated to dryness under reduced pressure, giving
solution which changed with time into a biphasic solid–liquid a brown oil. The oil was washed with diethyl ether (50 mL) and
mixture, where the liquid phase was colourless (attempts to then dissolved in acetone (50 mL). Diethyl ether (50 mL) was added
isolate metal species from this phase were unsuccessful) and to precipitate the product, which was ltered, washed with diethyl
the solid was off-white. The FT-IR spectrum of the recovered ether/acetone, and nally vacuum-dried to give the ligand [ptapzpy]
solid 1-S-H2O2 (at 24 h) indicated that it was a salt of the type Br as a cream solid. Yield: 2.0 g, 45%. Anal. calcd for C14H21-
(L)x[Mo8O26] (similar to that veried for the systems (1–3)/ BrN4$1.6H2O (354.07): C, 47.49; H, 6.89; N, 15.82. Found: C, 47.60;
TBHP) (Fig. 3c). For 2/H2O2 and 3/H2O2 the reaction mixture H, 6.45; N, 15.74%. FT-IR (KBr, cmꢁ1): 404 (m), 470 (w), 524 (w), 619
consisted of yellow liquid phases and no undissolved solid. The (m), 630 (m), 698 (m), 728 (m), 750 (w), 777 (vs), 871 (m), 923 (m),
conversion of 2 and 3 to a different, highly soluble Mo- 962 (s), 970 (s), 991 (w), 1037 (w), 1060 (m), 1095 (m), 1147 (w), 1160
containing species may explain the superior catalytic results (m), 1189 (w), 1240 (s), 1282 (w), 1303 (w), 1330 (w), 1359 (s), 1403
obtained for these systems. For 3/H2O2, a yellow solid was iso- (m), 1432 (s), 1450 (w), 1465 (w), 1490 (s), 1519 (m), 1567 (m), 1590
lated from the liquid phase (3-L-H2O2), which exhibited (s) (nC]N), 1649 (w), 2954 (m), 3002 (m), 3024 (m), 3083 (m), 3118
a different ATR FT-IR spectrum from those of 3 and the recov- (w). 1H NMR (300.1 MHz, 295 K, CDCl3): d ¼ 8.63 (d, 1H, H-11), 7.83
ered solids discussed above (Fig. 3k). The yellow colour is (td, 1H, H-8), 7.73 (dt, 1H, H-9), 7.64 (d, 1H, H-5), 7.24 (ddd, 1H, H-
typically indicative of oxoperoxomolybdenum(VI) species. 10), 6.84 (d, 1H, H-4), 4.41 (t, 2H, N–CH2), 3.80 (m, 2H, N–CH2),
Accordingly, bands in the IR spectrum of the recovered solid 3.39 (s, 9H, N–CH3), 2.55 (m, 2H, CH2) ppm (see Fig. S5 in the ESI†
may be assigned as n(Mo]O) at 938 cmꢁ1, n(O–O) at 855 cmꢁ1
,
for atom numbering). 13C NMR (75.4 MHz, 295 K, CDCl3): d ¼
and n(Mo(O2)2) at 663, 585 and 530 cmꢁ1. An additional strong 152.51 (C-7), 152.07 (C-11), 149.76 (C-3), 136.92 (C-9), 132.32 (C-5),
and broad band at 732 cmꢁ1 may be due to a Mo–O–Mo 122.82 (C-8), 120.37 (C-10), 104.91 (C-4), 64.48 (N–CH2), 53.88 (N–
stretching vibration, suggesting the presence of a polynuclear CH3), 48.73 (N–CH2), 24.26 (CH2) ppm. 13C{1H} CP MAS NMR: d ¼
species. From the characterisation studies, it seems that 1 151.3 (C-7), 149.6 (C-11 and C-3), 138.0 (C-9), 129.3 (C-5), 124.4 (C-
possesses a different reactivity with H2O2 than 3, being con- 8), 121.1 (C-10), 107.4 (C-4), 61.2 (N–CH2), 53.7 (N–CH3), 47.8 (N–
verted to different types of metal species.
CH2), 24.7 (CH2) ppm.
[Mo(CO)3(ptapzpy)Br] (1). In a Schlenk tube, Mo(CO)6
(0.20 g, 0.76 mmol) and [ptapzpy]Br (0.25 g, 0.77 mmol) were
added to toluene (20 mL) and the mixture was reuxed under N2
Conclusions
Two distinct synthetic pathways (both starting with Mo(CO)6) for 30 min, resulting in an orange solid and solution. Hexane
have been established for the synthesis of molybdenum(0) tri- (20 mL) was added to promote product precipitation. The
carbonyl and tetracarbonyl complexes containing a bidentate- solution was ltered off and the solid washed with hexane (2 ꢂ
coordinated pyrazolylpyridine ligand. The complexes display 20 mL), diethyl ether (2 ꢂ 20 mL), and nally vacuum-dried.
moderate activity when applied as (pre)catalysts for the epoxi- Yield: 0.34 g, 83%. Anal. calcd for C17H21BrMoN4O3$2H2O
dation of cis-cyclooctene with TBHP, which can be attributed to (541.25): C, 37.72; H, 4.66; N, 10.35. Found: C, 37.58; H, 4.81; N,
the in situ formation of poorly soluble b-octamolybdate salts. 10.1%. TGA (Fig. S2†) showed a mass loss of 6.2% at 100 ꢀC
Better results were obtained with H2O2 as oxidant and the tet- (calcd for loss of 2H2O: 6.6%). Selected FT-IR (KBr, cmꢁ1): 494
racarbonyl precatalyst, despite signicant non-productive (w), 629 (w), 771 (m), 873 (w), 960 (m), 1095 (w), 1157 (w), 1239
decomposition of the oxidant. The active species are yellow (m), 1365 (m), 1439 (m), 1477 (m), 1604 (w) (nC]N), 1742 (vs)
oxoperoxo-molybdenum(VI) species formed by the oxidative (nCO), 1764 (vs) (nCO), 1895 (vs) (nCO).
decarbonylation of the precursor. Treatment of the carbonyl
[Mo(CO)4(ptapzpy)]Br (2). [Mo(CO)4(pip)2] (0.11 g, 0.30
precursors with TBHP in the absence of olen gave a molyb- mmol) was added to a solution of [ptapzpy]Br (0.10 g, 0.30
denum oxide–organonitrogen hybrid material with a substruc- mmol) in ethanol (10 mL) and the mixture was heated at 50 ꢀC,
ture that is proposed to consist of {MoO4} tetrahedra and with stirring, for 30 min. The solvents were evaporated under
{MoO4N2} octahedra. Work is ongoing to isolate a more crys- reduced pressure, and the resultant red solid was washed with
talline form of this hybrid and determine its structure.
hexane (2 ꢂ 7 mL) and ethanol (2 ꢂ 7 mL). Yield: 0.13 g, 79%.
Anal. calcd for C18H21BrMoN4O4$1.5H2O (560.25): C, 38.59; H,
4.32; N, 10.00. Found: C, 38.90; H, 4.58; N, 10.14%. TGA
(Fig. S2†) showed a mass loss of 4.2% at 90 ꢀC (calcd for loss of
1.5H2O: 4.8%). Selected FT-IR (KBr, cmꢁ1): 364 (s), 471 (w), 580
Experimental
Synthesis
2-(1-Propyltrimethylammonium-3-pyrazolyl)pyridine bromide (m), 650 (m), 765 (s), 962 (m), 1097 (w), 1241 (m), 1365 (m), 1438
([ptapzpy]Br). 2-[3(5)-Pyrazolyl]pyridine (2.00 g, 13.78 mmol) was (m), 1482 (w), 1608 (w) (nC]N), 1815 (vs) (nCO), 1869 (vs) (nCO),
added slowly to a suspension of NaH (0.5 g, 20.8 mmol) in THF (40 1888 (sh) (nCO), 2012 (s) (nCO).
mL), resulting in a yellow mixture. A solution of (3-bromopropyl)
[Mo3O9([ptapzpy]Br)2] (3). In a Schlenk tube, 5–6 M TBHP in
trimethylammonium bromide (3.60 g, 13.79 mmol) in acetonitrile decane (0.35 mL, ca. 2 mmol) was added to a mixture
16300 | RSC Adv., 2018, 8, 16294–16302
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comprising complex 1 (0.10 g, 0.20 mmol) and CH2Cl2 (10 mL), by GC. The reactant/products were identied using GC-MS
and stirring was continued for 4 h under N2 at room tempera- (Trace GC 2000 Series Thermo Quest CE Instruments GC;
ture. The resultant off-white solid was recovered by ltration, Thermo Scientic DSQ II), using He as the carrier gas.
washed with diethyl ether (2 ꢂ 10 mL), and nally vacuum-
Iodometric titrations were carried out in order to quantify
dried. Yield: 0.04 g, 53%. Anal. calcd for C28H42Br2Mo3N8O9- the non-productive decomposition of the oxidants (TBHP or
$3H2O (1136.36): C, 29.59; H, 4.26; N, 9.86. Found: C, 29.51; H, H2O2). The reactors containing the (pre)cataꢀlyst, solvent and
4.47; N, 9.99%. TGA (Fig. S2†) showed a mass loss of 5.5% at oxidant, without substrate, were heated at 55 C for 24 h. Aer
150 ꢀC (calcd for loss of 3H2O: 4.8%). Selected FT-IR cooling the reactors to ambient temperature, liquid samples
(KBr, cmꢁ1): 326 (w), 399 (w), 426 (w), 609 (s), 631 (s), 777 (m), were withdrawn for titration.
846 (w), 895 (s), 924 (s), 962 (w), 1022 (w), 1076 (w), 1099 (w),
For reaction mixtures which were biphasic solid–liquid, the
1161 (w), 1242 (m), 1371 (m), 1439 (m), 1479 (m), 1610 (m) (nC] solid phase was separated from the catalytic reaction mixture by
N), 2850 (w), 3026 (w). Selected FT-Raman (cmꢁ1): 641 (w), 727 centrifugation (3500 rpm), washed with organic solvents
(w), 752 (w), 783 (m), 893 (w), 923 (vs), 963 (m), 1020 (m), 1053 (pentane, hexane, ethanol and/or acetone), dried overnight
(w), 1159 (w), 1242 (w), 1287 (w), 1346 (m), 1372 (m), 1422 (m), under atmospheric conditions, ꢀ and subsequently under
1532 (m), 1568 (m), 1609 (m) (nC]N), 2826 (w), 2966 (m), 3022 vacuum (ca. 4 mbar) for 1 h at 60 C. The recovered solids are
(m), 3065 (m). 13C{1H} CP MAS NMR: d ¼ 146.8 (C-7, C-11 and C- denoted i-S-oxid where oxid is the oxidant (TBHP or H2O2) and i
3), 140.5 (C-9), 134.7 (C-5), 124.8 (C-8 and C-10), 105.2 (C-4), 63.8 is compound 1, 2 or 3. For epoxidation systems using ionic
(N–CH2), 53.6 (N–CH3), 48.9 (N–CH2), 25.2 (CH2) ppm.
liquids, the recovered solids are denoted i-S-TBHPD-IL {IL ¼
[bmim]NTf2 or [bmim]PF6}. For the system 3/H2O2, metal
species could be isolated from the liquid phases of the catalytic
reactions by precipitation of solids aer addition of an appro-
Catalytic tests
The typical epoxidation experiments with TBHP (in decane or priate organic solvent. The precipitated solid was washed and
aqueous solution) were carried out in 10 mL borosilicate batch dried as described above for the recovery of the undissolved
reactors possessing a valve for sampling. Reaction mixtures solids and denoted 3-L-H2O2.
were stirred magnetically (1000 rpm, PTFE-coated stirring bar)
and heated to 55 ꢀC with a thermostatically controlled oil bath.
The reactors were charged with the (pre)catalyst (amount
Conflicts of interest
equivalent to 16 mmol Mo), cis-cyclooctene (1.8 mmol), and There are no conicts to declare.
a cosolvent [1 mL of 1,2-dichloroethane or CH3CN, or 0.3 mL of
an ionic liquid (IL)]. The reactor containing the catalyst, olen
and cosolvent was preheated for 10 min at the reaction
Acknowledgements
temperature. In a separate ask, the oxidant was preheated in This work was developed in the scope of the projects (Associate
a similar fashion, and subsequently added to the reactor to give Laboratories) CICECO-Aveiro Institute of Materials-POCI-01-
˜
ˆ
a Mo : Cy : oxidant molar ratio of 1 : 113 : 172; this was marked 0145-FEDER-007679 [FCT (Fundaçao para a Ciencia e a Tecno-
as the initial instant of the catalytic reaction. logia) ref. UID/CTM/50011/2013] and REQUIMTE-LAQV-POCI-
Catalytic tests using 30 wt% aq. H2O2 as oxidant were 01-0145-FEDER-007265 (FCT ref. UID/QUI/50006/2013),
carried out using tubular borosilicate batch reactors with nanced by national funds through the FCT/MEC and when
pear-shaped bottoms (ca. 12 mL capacity), equipped with applicable co-nanced by FEDER (Fundo Europeu de Desen-
a PTFE-coated magnetic stirring bar (1000 rpm) and a valve volvimento Regional) under the PT2020 Partnership Agreement.
for (un)charging of the reactor. Catalyst (16 mmol Mo), Cy (1.8 The FCT and the European Union are acknowledged for a PhD
mmol), CH3CN (1 mL) and H2O2 (Mo : Cy : oxidant molar grant to L. N. (PD/BD/109666/2015), and post-doctoral grants to
ratio of 1 : 113 : 172) were added to the reactor, which was P. N. (SFRH/BPD/110530/2015), A. C. G. (SFRH/BPD/108541/
ꢀ
subsequently immersed in an oil bath heated to 55 or 70 C. 2015) and L. C-S. (SFRH/BPD/111899/2015), co-funded by
Separate catalytic experiments were carried out for each MCTES and the European Social Fund through the program
reaction time.
POPH of QREN.
The reaction mixtures were analysed by using a Varian 3900
GC equipped with a DB-5 capillary column (30 m ꢂ 0.25 mm ꢂ
0.25 mm) and a FID detector, with H2 as the carrier gas. The
concentrations of reactant and products were determined using
the internal calibration method, i.e. based on calibration curves
with undecane as internal standard. The FID response was
linear in the range of concentrations used for the calibration
curves and sample analysis. Using the internal calibration
method the determined concentrations of Cy and CyO are
reliable (experimental range of error of ca. 6%), i.e. the
conversion values account for substrate consumption irre-
spective of the types of products formed being detected or not
Notes and references
¨
1 W. Hieber and F. Z. Muhlbauer, Z. Anorg. Allg. Chem., 1935,
221, 337–348.
2 M. H. B. Stiddard, J. Chem. Soc., 1962, 4712–4715.
3 S. S. Braga, A. C. Coelho, I. S. Gonçalves and F. A. Almeida
Paz, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2007, 63,
m780–m782.
4 (a) T. R. Amarante, P. Neves, A. C. Coelho, S. Gago,
A. A. Valente, F. A. A. Paz, M. Pillinger and I. S. Gonçalves,
Organometallics, 2010, 29, 883–892; (b) A. C. Gomes,
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 16294–16302 | 16301

View Article Online
RSC Advances
Paper
S. M. Bruno, A. A. Valente, I. S. Gonçalves and M. Pillinger, J.
Organomet. Chem., 2013, 744, 53–59.
5 (a) G. A. Ardizzoia, M. Bea, S. Brenna and B. Therrien, Eur. J.
F. Franco, C. Cometto, F. Sordello, C. Minero, L. Nencini,
J. Fiedler, R. Gobetto and C. Nervi, ChemElectroChem, 2015,
2, 1372–1379; (e) A. Mente¸s, M. Sarbay, B. Hazer and
H. Arslan, Appl. Organomet. Chem., 2005, 19, 76–80.
¨
Inorg. Chem., 2016, 3829–3837; (b) M. P. T. Sjogren, H. Frisell,
˚
B. Akermark, P.-O. Norrby, L. Eriksson and A. Vitagliano, 14 (a) T. R. Amarante, P. Neves, F. A. A. Paz, A. A. Valente,
Organometallics, 1997, 16, 942–950.
6 (a) B. Oelkers, A. Venker and J. Sundermeyer, Inorg. Chem.,
2012, 51, 4636–4643, and references cited therein; (b)
M. Pillinger and I. S. Gonçalves, Dalton Trans., 2014, 43,
6059–6069; (b) W. R. Thiel, M. Angstl and T. Priermeier,
Chem. Ber., 1994, 127, 2373–2379.
R. S. Herrick, C. J. Ziegler, H. Bohan, M. Corey, 15 D. J. Darensbourg and R. L. Kump, Inorg. Chem., 1978, 17,
M. Eskander, J. Giguere, N. McMicken and I. E. Wrona, J.
Organomet. Chem., 2003, 687, 178–184.
2680–2682.
ˇ
16 A. Vlcek, Jr., F. Baumann, W. Kaim, F.-W. Grevels and
´
´
7 (a) R. Garcıa-Rodrıguez and D. Miguel, Dalton Trans., 2006,
F. Hartl, J. Chem. Soc., Dalton Trans., 1998, 215–220.
´
1218–1225; (b) M. Karakus, G. Davulga, S. Gomez-Ruiz, 17 A. C. Coelho, M. Nolasco, S. S. Balula, M. M. Antunes,
S. Tschirschwitz and E. Hey-Hawkins, Polyhedron, 2009, 28,
91–94.
8 (a) P. Neves, T. R. Amarante, A. C. Gomes, A. C. Coelho,
C. C. L. Pereira, F. A. A. Paz, A. A. Valente, M. Pillinger,
P. Ribeiro-Claro, J. Klinowski and I. S. Gonçalves, Inorg.
Chem., 2011, 50, 525–538.
S. Gago, M. Pillinger, I. S. Gonçalves, C. M. Silva and 18 (a) W. R. Thiel, T. Priermeier, D. A. Fiedler, A. M. Bond and
A. A. Valente, Appl. Catal., A, 2011, 395, 71–77; (b)
L. S. Nogueira, P. Neves, A. C. Gomes, A. A. Valente,
M. Pillinger and I. S. Gonçalves, J. Organomet. Chem., 2017,
846, 185–192.
9 (a) H. Behrens, E. Lindner and G. Lehnert, J. Organomet.
Chem., 1970, 22, 665–676; (b) B. J. Brisdon, D. A. Edwards
and J. W. White, J. Organomet. Chem., 1978, 156, 427–437.
M. R. Mattner, J. Organomet. Chem., 1996, 514, 137–147; (b)
¨
K. Robler, T. Kluge, A. Schubert, Y. Sun, E. Herdtweck and
W. R. Thiel, Z. Naturforsch., B: J. Chem. Sci., 2004, 59,
1253–1263; (c) A. C. Coelho, F. A. A. Paz, J. Klinowski,
M. Pillinger and I. S. Gonçalves, Molecules, 2006, 11, 528–
˜
538; (d) M. Arroyo, D. Miguel, F. Villafane, E. C. B. Alegria
and A. J. L. Pombeiro, Dalton Trans., 2012, 41, 7017–7025.
10 (a) M. H. Chisholm, J. A. Connor, J. C. Huffman, E. M. Kober 19 (a) P. J. Zapf, R. C. Haushalter and J. Zubieta, Chem. Mater.,
and C. Overton, Inorg. Chem., 1984, 23, 2298–2303; (b)
H. Behrens and N. Harder, Chem. Ber., 1964, 97, 426–432;
1997, 9, 2019–2024; (b) Y. Zhou, L. Zhang, H.-K. Fun and
X. You, Inorg. Chem. Commun., 2000, 3, 114–116.
(c) Y. J. Han and A. J. Lees, Inorg. Chim. Acta, 1988, 147, 20 J. Twu, Y.-Y. Yu, C.-W. Tang, G.-J. Wang and K.-H. Chen,
45–49. Appl. Spectrosc., 1999, 53, 1083–1086.
11 H. D. Murdoch, R. Henzi and F. Calderazzo, J. Organomet. 21 Z. Wang, S. Li, W. J. Teo, Y. T. Poh, J. Zhao and T. S. A. Hor, J.
Chem., 1967, 7, 441–448. Organomet. Chem., 2015, 775, 188–194.
12 (a) A. Vlcek, Jr., Coord. Chem. Rev., 2002, 230, 225–242; (b) 22 A. C. Gomes, S. M. Bruno, S. Gago, R. P. Lopes,
ˇ
K. A. Rawlins and A. J. Lees, Inorg. Chem., 1989, 28, 2154–
2160; (c) D. M. Manuta and A. J. Lees, Inorg. Chem., 1986,
25, 1354–1359; (d) P. N. W. Baxter and J. A. Connor, J.
D. A. Machado, A. P. Carminatti, A. A. Valente, M. Pillinger
and I. S. Gonçalves, J. Organomet. Chem., 2011, 696, 3543–
3550.
Organomet. Chem., 1995, 486, 115–121; (e) I. Veroni, 23 (a) C. A. Gamelas, P. Neves, A. C. Gomes, A. A. Valente,
˜
A. Rontoyianni and C. A. Mitsopoulou, Dalton Trans., 2003,
255–260; (f) M. D. Zidan, M. M. Al-Ktaifani and
A. Allahham, Optik, 2016, 127, 2570–2574; (g) L.-T. Cheng,
W. Tam and D. F. Eaton, Organometallics, 1990, 9, 2856–
2857; (h) W.-F. Fu and R. van Eldik, Inorg. Chem., 1998, 37,
C. C. Romao, I. S. Gonçalves and M. Pillinger, Catal. Lett.,
2012, 142, 1218–1224; (b) C. J. Carrasco, F. Montilla,
E. Alvarez, M. Herbert and A. Galindo, Polyhedron, 2013,
54, 123–130; (c) C. Yang, Q. Jin, H. Zhang, J. Liao, J. Zhu,
B. Yu and J. Deng, Green Chem., 2009, 11, 1401–1405; (d)
M.-D. Zhou, M.-J. Liu, L.-L. Huang, J. Zhang, J.-Y. Wang,
´
ˇ
1044–1050; (i) A. Vlcek, Jr., Coord. Chem. Rev., 1998, 177,
¨
219–256.
X.-B. Li, F. E. Kuhn and S.-L. Zang, Green Chem., 2015, 17,
13 (a) R. D. Rimmer, A. E. Pierri and P. C. Ford, Coord. Chem.
1186–1193.
Rev., 2012, 256, 1509–1519; (b) J. Tory, B. Settereld-Price, 24 S. L. Linguito, X. Zhang, M. Padmanabhan, A. V. Biradar,
R. A. W. Dryfe and F. Hartl, ChemElectroChem, 2015, 2,
213–217; (c) D. Sieh, D. C. Lacy, J. C. Peters and
C. P. Kubiak, Chem.–Eur. J., 2015, 21, 8497–8503; (d)
T. Xu, T. J. Emge, T. Asefa and J. Li, New J. Chem., 2013,
37, 2894.
16302 | RSC Adv., 2018, 8, 16294–16302
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