Oxidomolybdenum complexes for acid catalysis using alcohols as solvents and reactants

Article abstract of DOI:10.1039/c5cy02052b

The application field of dichloridodioxidomolybdenum(vi) chelate complexes, which have been intensively investigated as catalysts for liquid-phase olefin epoxidation, is broadened to encompass acid catalysis, in particular, alcoholysis and acetalisation reactions. Complex [MoO₂Cl₂(L)] (1) with L = 4,4′-di-tert-butyl-2,2′-bipyridine was chosen as (pre)catalyst. Depending on the reaction conditions, 1 either remained structurally intact or was transformed into different metal species, the structures of which were determined on the basis of single-crystal X-ray diffraction, spectroscopic techniques (FT-IR and ¹H NMR), and elemental analysis. The first example of a mixed-ligand complex of the type [MoO₂X(OR)(L)] (X = halide) is disclosed. This complex is one of only a handful of complexes known to date that exhibit an all-cis configuration instead of the usual cis-oxido, trans-X, cis-L configuration (X = anionic ligand). Mechanistic considerations of the formation of the metal species are made.

Full text of DOI:10.1039/c5cy02052b

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Oxidomolybdenum complexes for acid catalysis
using alcohols as solvents and reactants†
Cite this: DOI: 10.1039/c5cy02052b
a
a
b
a
*
Ana C. Gomes, Patrícia Neves, Luís Cunha-Silva, Anabela A. Valente,
a
a
*
Isabel S. Gonçalves and Martyn Pillinger
The application field of dichloridodioxidomolybdenumIJVI) chelate complexes, which have been intensively
investigated as catalysts for liquid-phase olefin epoxidation, is broadened to encompass acid catalysis, in
particular, alcoholysis and acetalisation reactions. Complex [MoO2Cl2(L)] (1) with L = 4,4′-di-tert-butyl-2,2′-
bipyridine was chosen as (pre)catalyst. Depending on the reaction conditions, 1 either remained structurally
intact or was transformed into different metal species, the structures of which were determined on the ba-
sis of single-crystal X-ray diffraction, spectroscopic techniques (FT-IR and 1H NMR), and elemental analysis.
The first example of a mixed-ligand complex of the type [MoO2X(OR)(L)] (X = halide) is disclosed. This
complex is one of only a handful of complexes known to date that exhibit an all-cis configuration instead
of the usual cis-oxido, trans-X, cis-L configuration (X = anionic ligand). Mechanistic considerations of the
formation of the metal species are made.
Received 27th November 2015,
Accepted 18th March 2016
DOI: 10.1039/c5cy02052b
www.rsc.org/catalysis
obtained through optimisation of the type of organic ligand
and the associated substituents.2f
Introduction
The oxychloride MoO2Cl2 has been intensively studied over
the years as a catalyst for a wide variety of organic transforma-
tions involving, for example, oxidations, activation of X–H (X =
Si, B, P and H) bonds, C–X bond forming reactions, and in
the reduction of a variety of functional groups.1 In many
cases the catalyst used is a solvent adduct of the type
[MoO2Cl2(S)2] (S = DMF, THF, DMSO). The solvent molecules
in these adducts are readily replaced by Lewis base ligands
(L) to give complexes of the type [MoO2Cl2(L)n]. A wide
variety of complexes containing bidentate nitrogen-donor
ligands have been prepared and studied, such as those with
2,2′-bipyridine (bipy) and its derivatives,2 1,4-diazabutadi-
enes,2a,3 ethylenediimines,4 oxazolines,5 pyridylimines,6
pyrazolylpyridines,7 and pyrazolylpyrimidines.8 Catalytic stud-
ies with these complexes have essentially been limited to
liquid-phase olefin epoxidation using alkylhydroperoxides as
oxidants. Very active and selective catalysts have been
In the present work, we set out to widen the field of organic
transformations catalysed by dichloridodioxidomolybdenumIJVI)
complexes. The complex [MoO2Cl2(di-tBu-bipy)] (1) (di-tBu-bipy =
4,4′-di-tert-butyl-2,2′-bipyridine) was chosen as the (pre)cata-
lyst. Several published studies demonstrate that di-tBu-bipy
is a good choice as a ligand for oxometal complexes to be
used as catalysts for organic transformations.9 For example,
in addition to the mononuclear complex 1, which was first
reported as an olefin epoxidation catalyst in 2000,2a the
complexes [MoO(O2)2(di-tBu-bipy)],9a [MoO2(NCS)2(di-tBu-
bipy)],9b–9e [Mo2O4(μ2-O)X2(di-tBu-bipy)2] (X = Cl, NCS),9a,e
[Mo2O6(di-tBu-bipy)2],9f and [V2O2(μ-MeO)2(μ-MO4)2(di-tBu-
bipy)2] (M = Mo, W)9k are effective as oxygen atom transfer
agents and/or (pre)catalysts for alcohol oxidation and olefin
epoxidation. Kodama et al. concluded that the affinity of the
heterotetranuclear V2W2 complex to organic substrates was im-
proved by the selection of di-tBu-bipy as the organic ligand.9k
Complexes of the type [MoO2Cl2(L)n] possess Lewis acid
character, which plays a key role in defining the reaction
mechanism for olefin epoxidation with alkylhydro-
peroxides.1a,b We therefore chose to study two reactions typi-
cally catalysed by Brønsted or Lewis acids, namely the
acetalisation of benzaldehyde with ethanol, and the ring-
opening reaction of styrene oxide with ethanol. Acetalisation
is often used in organic synthesis to protect the carbonyl
group of aldehydes and ketones, since 1,3-dioxolanes are sta-
ble under neutral and basic conditions.10 Besides the interest
in acetals as protecting groups, they are interesting for their
a Department of Chemistry, CICECO - Aveiro Institute of Materials, University of
Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal.
E-mail: atav@ua.pt, mpillinger@ua.pt
b REQUIMTE/LAQV, Department of Chemistry, Faculty of Sciences, University of
Porto, 4169-007 Porto, Portugal
† Electronic supplementary information (ESI) available: Powder XRD patterns,
FT-IR and GC–MS data, crystal packing descriptions, tabulated crystallographic
data and additional figures. CCDC 1435952 (2), 1435953 (4·5CHCl3) and
1435954 (4·2EtOH). For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c5cy02052b
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use in pharmaceuticals and as fragrances in soaps and per-
fumes.10,11 Regarding the second reaction studied, epoxide
ring-opening is a frequently used transformation in organic
synthesis.12 In particular, ring-opening reactions of epoxides
with alcohols provide a synthetically important path to
β-alkoxyalcohols, which are important moieties in many bio-
logically active molecules, and can be employed as valuable
organic solvents, natural products, or intermediates, in or-
ganic and inorganic synthesis.13
Although Lewis base adducts of the type [MoO2Cl2(L)]
bearing bidentate nitrogen-donor ligands are generally
considered to be quite stable, the Mo–Cl bond is susceptible
to cleavage, especially in the presence of water.7c,d,9h In a re-
cent study, the oxido-bridged dinuclear complex [Mo2O4(μ2-
O)Cl2(pypzEA)2] (pypzEA = ethyl[3-(pyridin-2-yl)-1H-pyrazol-1-
yl]acetate) was obtained in significant yield upon treatment of
[MoO2Cl2(pypzEA)] with ethanol.14 Hence, in the present
work, a particular emphasis was placed on examining the sta-
bility of 1 in alcohol solvents in order to identify the main
species responsible for the catalytic reactions. We show that,
depending on the reaction conditions, 1 is transformed into
other species, including an unprecedented molybdenum(VI)
oxidoalkoxido complex, dinuclear oxido-bridged complexes,
and an octanuclear complex. The crystal structures of some
of these species have been determined.
Results and discussion
Stability experiments in ethanol
In parallel with the catalytic studies (described below), con-
tact tests (CTs) were performed to investigate the stability of
the mononuclear MoVI complex [MoO2Cl2(di-tBu-bipy)] (1) in
the presence of alcohols (ethanol, tert-butanol, and
1-phenylethanol) at different temperatures (without organic
substrate). Studies on the stability of 1 in the presence of eth-
anol solutions containing organic substrates (benzaldehyde,
styrene oxide, 1-phenylethanol and 2-phenylethanol) are de-
scribed below in the catalysis sections. The CT performed un-
der conditions equivalent to those used in the catalytic tests
(20 μmol 1, 1 mL EtOH, 55 °C, 24 h) gave a suspension. The
solid (1/CT-sol) was isolated by filtration, washed with penta-
ne, and dried. Addition of pentane to the filtrate, followed by
storage of the mixture at 4 °C overnight, led to a colourless
precipitate (1/CT-liq), which was filtered, washed with penta-
ne, and dried. The ATR FT-IR spectra of 1/CT-sol and 1/CT-liq
matched that for 1 (Fig. 1) suggesting that the complex is
quite stable under these conditions. Similar results were
obtained when the contact test was performed on a larger
scale (0.32 mmol 1, 20 mL EtOH).
Fig. 1 ATR FT-IR spectra in the range 350–1750 cm−1 of (a) complex 1,
(b) 1/CT-liq, (c) 1/CT-sol, (d) 1/BzA-liq, (e) 1/BzA-sol, (f) 1/PhEtOH-liq,
(g) 1/PhEtOH-sol, (h) 1/StyOx-liq, (i) complex 6, (j) 6/StyOx-liq, (k)
5/StyOx-liq, (l) 2/StyOx-liq, (m) 4/StyOx-liq, and (n) complex 4.
below). Vapour diffusion of pentane into the mother liquor
obtained in this reaction gave a small number of yellow and
orange crystals, which were mechanically separated and
subjected to X-ray diffraction analyses (see below). The yellow
crystals were identified as the MoVI oxidoalkoxido complex
[MoO2Cl(OEt)(di-tBu-bipy)] (2), while the orange crystals were
found to comprise the MoV oxido-bridged dinuclear complex
[Mo2O2(μ2-O)2Cl2(di-tBu-bipy)2]·2EtOH (4·2EtOH). In addition,
a small amount of a bordeaux-coloured solid precipitated,
which was identified as a mixture comprising complexes 1
(ca. 10 mol%), 2 (ca. 80 mol%) and 3 (ca. 10 mol%) by 1H
In contrast to the results at 55 °C, complex 1 underwent
fairly rapid transformations when treated with ethanol under
reflux (large-scale contact test, LSCT). A bordeaux-coloured
precipitate formed during 24 h, which was isolated and iden-
tified as the MoV oxido-bridged dinuclear complex [Mo2O2(μ2-
O)Cl4(di-tBu-bipy)2] (3) (Scheme 1) on the basis of FT-IR, 1H
NMR and elemental analysis data (discussed in more detail
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Scheme 1 Summary of the complexes isolated in this work starting from 1.
NMR (combined yield of 15% based on Mo). A comparison of
the powder X-ray diffraction (PXRD) pattern of the bordeaux
precipitate with a simulated pattern calculated from the
single-crystal X-ray structural data for 2 confirmed that the
major component was complex 2 (ESI,† Fig. S1).
edge, complex 2 is the first example of a mixed-ligand complex
of the type [MoO2X(OR)(L)] (X = halide). Furthermore, as will
be described below in the section devoted to the discussion of
the crystal structures, the complex exhibits the extremely rare
all-cis configuration instead of the far more common cis-oxido,
trans-X, cis-L configuration.
MoVI oxidoalkoxido complexes of the type [MoO2(OR)2(L)]
are quite scarce, even though complexes with R = iPr and tBu,
and L = bipy, were reported in 1984 by Chisholm et al.,15a
followed later by reports describing the complexes with OR =
allyloxo and L = bipy,15b and OR = diacetoneglucose and L =
1,10-phenanthroline (1,10-phen).15c To the best of our knowl-
Mechanistic considerations of the transformation of 1
Complex 3 was previously isolated from the reduction of 1
with H2 in toluene.16a Key IR bands for 3 at 964 cm−1
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[ν(MoO)] and 786 cm−1 [ν(Mo–O–Mo)] (ESI,† Fig. S2) are in
agreement with those reported by Reis and Royo,16a and with
values reported for analogous complexes containing other
bidentate heterocyclic amines, e.g. 966 and 801 cm−1 for
[Mo2O2(μ2-O)Cl4(bpym)2] (bpym = 2,2′-bipyrimidine),16b and
966 and 798 cm−1 for [Mo2O2(μ2-O)Cl4(bipy)2].16c The forma-
tion of 3 from 1 in significant amount (63% yield for the re-
action performed under reflux) is somewhat surprising but
can be attributed to the propensity of ethanol to function as
both a solvent and a reducing agent. Indeed, reduced meta-
llo–organic complexes are often prepared by reacting a com-
nuclear complex [Mo2O4(μ2-O)Cl2(di-tBu-bipy)2] (5) (obtained
in a low yield of 7%) by FT-IR spectroscopy and PXRD. The
PXRD pattern of the bulk product 5 was in good agreement
with a simulated pattern calculated from the single-crystal
X-ray structural data published for the hydrate 5·0.2H2O
(ESI,† Fig. S1).21 The formation of 5 in a low yield can be at-
tributed to the presence of a residual amount of water in the
solvent. Taking into account the above discussion concerning
the reducing behaviour of ethanol, the failure to observe the
formation of either of the MoV dinuclear complexes 3 or 4
upon treatment of 1 with tert-butanol is not surprising since
the latter has no α-hydrogen available for abstraction,
preventing any subsequent reduction step. Accordingly, in
contrast to ethanol, tert-butanol was found to be ineffective
for the preparation of colloidal rhodium through the reduc-
tion of rhodium(III) chloride.18a Additional reactions were
performed in which complex 5 was treated with ethanol at ei-
ther 55 °C or under reflux overnight. At the lower tempera-
plex with the metal in
a higher oxidation state with
ethanol17a (e.g., PtIV → PtII,17b RhIII → RhI,17c RuIII
→
RuII,17d). Also, alcohols are widely used in nanoparticle prepa-
rations.18 As noted previously,16a the formation of 3 is likely
the result of the reduction of 1 to the oxidomolybdenum(IV)
species [MoOCl2(di-tBu-bipy)], followed by comproportiona-
tion. This is consistent with the well known tendency of
oxidomolybdenum(IV) and dioxidomolybdenum(VI) species to
associate leading to stable comproportionation products of
oxidomolybdenum(V), which are more resistant to reduc-
tion.19 We may further propose that the MoVI oxidoalkoxido
complex 2 could be an intermediate in the formation of
[MoOCl2(di-tBu-bipy)] (Scheme 2). H-abstraction from the co-
ordinated ethoxide could lead to liberation of acetaldehyde
and formation of [MoO(OH)Cl(di-tBu-bipy)], which in the
presence of HCl may give [MoOCl2(di-tBu-bipy)] and water.
Such a mechanism parallels that generally presented for the
oxidation of ethanol to acetaldehyde over supported molybde-
num oxide catalysts.20 The water formed in this reaction will
likely contribute to the eventual hydrolysis of 3 to give 4. In-
deed, as noted above, single crystals of the ethanol solvate
4·2EtOH were isolated from the mother liquor of the reaction
of 1 in refluxing ethanol.
ture,
5 was recovered unchanged, while at the higher
temperature an orange solid (identified as 4) was isolated. We
may propose that at the higher temperature 5 reacts with one
molecule of ethanol to give 2 and an intermediate of the type
[MoO2(OH)Cl(di-tBu-bipy)]. In a manner equivalent to that
proposed above for the formation of 3, 2 could give acetalde-
hyde and [MoO(OH)Cl(di-tBu-bipy)], which may react with
[MoO2(OH)Cl(di-tBu-bipy)] to give the MoV complex 4 and one
molecule of water.
To study the stability of 1 in alcohol media at higher
temperatures, 1-phenylethanol was used at 55 and 130 °C (for
24 h). At the lower temperature, a bordeaux precipitate (3)
and an orange solution (containing 4) were obtained, similar
to the behaviour with ethanol under reflux. At 130 °C, an
orange-red solution was obtained with no undissolved solid.
Addition of pentane led to the precipitation of the MoV dinu-
clear complex 4 as an orange solid in 73% yield. The
remaining 1-phenylethanol/pentane solution was analysed by
GC–MS, which revealed the presence of acetophenone (ESI,†
Fig. S3). Hence, like ethanol, 1-phenylethanol can function
as both a solvent and a reducing agent, being oxidised in
this case to acetophenone. This has been noted in other works
such as the preparation of palladium organosols by reduction
of palladium acetate in the presence of polyvinylpyrrolidone.22
Key IR bands for 4 at 956 cm−1 [ν(MoO)], 935 cm−1
[ν(MoO)] and 728 cm−1 [ν(Mo–O–Mo)] (ESI,† Fig. S2) are in
agreement with those reported for analogous complexes
containing other bidentate heterocyclic amines, e.g. 955, 934
and 735 cm−1 for [Mo2O2(μ2-O)2Cl2(bpym)2],16b and 954, 935
and 738 cm−1 for [Mo2O2(μ2-O)2Cl2(1,10-phen)2].23 Recrystalli-
sation of 4 from chloroform/hexane gave orange crystals of
the chloroform solvate 4·5CHCl3 suitable for structure deter-
mination by X-ray diffraction (see below).
Some further support for the mechanistic considerations
discussed above was obtained by treating complex 1 in
refluxing tert-butanol. Under these conditions, no bordeaux-
coloured solid formed during 24 h (the undissolved solid
phase was identified as 1 by FT-IR spectroscopy), and the
pink-coloured solid obtained after addition of pentane to the
reaction solution was identified as the MoVI oxido-bridged di-
Crystal structures of complexes 2 and 4
Compounds 2 and 4 were isolated as single-crystals appropri-
Scheme
2 Possible pathways for the sequential formation of
complexes 2–4 from 1 (L = di-tBu-bipy) in ethanol.
ate for XRD analysis, being unequivocally formulated as
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[MoO2Cl(OEt)(di-tBu-bipy)] (2), [Mo2O2(μ2-O)2Cl2(di-tBu-bipy)2]
·2EtOH (4·2EtOH) and [Mo2O2(μ2-O)2Cl2(di-tBu-bipy)2]·5CHCl3
(4·5CHCl3) (see Experimental section for details). Complex 2
crystallised in the monoclinic space group P21/c with the
asymmetric unit (asu) only including the mononuclear com-
plex [MoO2Cl(OEt)(di-tBu-bipy)], while complex 4 crystallised
in two distinct crystal systems depending on the nature of
the solvent used: 4·2EtOH crystallised in the triclinic space
gles are found in the 69.79(11)–106.8(2)° interval, while the
trans angles range between 154.9(2) and 161.09(9)° (Table
S2†). The Mo1 atom is displaced by ca. 0.24 and 0.50 Å from
the equatorial plane and geometrical centre of the coordina-
tion polyhedron, respectively.
The molecular structure of complex 2 is highly unusual
since the ligands adopt an all-cis configuration, that is,
cis-oxido, cis-X, cis-L (X = Cl, OEt). Complexes of the type
[MO2X2(L)n], where L is either a neutral bidentate (n = 1) or
monodentate (n = 2) ligand, display almost exclusively a cis,
trans, cis arrangement in which the weaker donor atoms
belonging to L are found to be trans to the terminal oxygen
atoms, where they are not directly competing for the
available empty metal d orbitals.24 Only a handful of com-
plexes with the all-cis configuration have been reported,
namely [MO2Cl2(tmen)] (M = Mo, W; tmen = N,N,N′,N′-
tetramethylethylenediamine),25a,b [MoO2Cl2(tmc)] (tmc = tetra-
methylcyclam),25c dinuclear [Mo2O4Cl4{o-C6H4(CH2SMe)2}2],25d
the oxido-bridged dinuclear complex [Mo2O4(μ2-O)Cl2(pyrazole)4],25e
[MoO2(L)] (L = 1,4-diazepane-based bis(phenolate) ligand),25f
and [MoO2(L)2] (L = aryloxide-pyrazole ligand).25g A computa-
tional investigation of the relative stabilities of the two
different isomers of [MoO2Cl2(tmen)] led to the conclusion
that steric effects (involving repulsive interactions between
the chelating ligand and the two ligands perpendicular to
the N–Mo–N plane) could override the electronic preference
for the cis, trans, cis isomer.24a For complex 2, steric
destabilisation of the cis, trans, cis isomer may arise from
unfavourable interactions between the ethoxido group and
tert-butyl groups in the same molecule and/or in neighbouring
molecules.
¯
group P1 with one dinuclear complex [Mo2O2(μ2-O)2Cl2(di-
tBu-bipy)2] and two EtOH molecules of crystallisation com-
prising the asu; 4·5CHCl3 crystallised in the cubic space
¯
group Fd3c in which the asu is composed of half of the com-
plex unit and 2.5 molecules of CHCl3 (ESI,† Fig. S4).
The molecular structure of the MoVI oxidoalkoxido com-
plex 2 is depicted in Fig. 2. The unique MoVI centre has
distorted octahedral geometry due to coordination of two
N-atoms of one di-tBu-bipy molecule, two terminal oxido
groups, one ethoxido ligand and one chlorido ligand,
{MoClN2O3}. As usual, the two MoO bonds [1.633(3) and
1.699(5) Å] are considerably shorter than the remaining Mo–
O, Mo–N and Mo–Cl, and markedly exert their trans-effect on
the opposite single bonds (ESI,† Table S2). This is evident in
the two different bond lengths found for Mo–N1 [2.253(4) Å]
and Mo–N2 [2.316(3) Å]. The high distortion of the octahedral
coordination environment of the MoVI centre is clearly appar-
ent from an analysis of the internal bond angles: the cis an-
The crystal structure of complex 4, [Mo2O2(μ2-O)2Cl2(di-
tBu-bipy)2], reveals a MoV oxido-bridged dinuclear complex
(Fig. 2b and ESI,† Fig. S4). The MoV centres are inter-
connected by two μ2-bridging oxido groups, imposing
Mo⋯Mo distances of 2.5625(2) and 2.5686(2) Å for the com-
pounds 4·2EtOH and 4·5CHCl3, respectively. These distances
are indicative of Mo–Mo single bonds resulting from a d1−d1
(MoV–MoV) interaction.26a,b The Mo–O–Mo angles involving
the μ2-bridging oxido groups are 83.050(5) and 83.205(5)° for
4·2EtOH, and 83.06(1)° for 4·5CHCl3, which are in good
agreement with those previously reported for similar
complexes.19b,26b–d For the complex in 4·2EtOH, the two crys-
tallographically independent MoV centres have identical
distorted octahedral coordination environments, being
connected to two N-atoms of one di-tBu-bipy ligand, two μ2-
bridging oxido groups, one terminal oxido group and one
chlorido ligand, {MoClN2O3}. The significant degree of distor-
tion verified in the coordination geometries of Mo1 and Mo2
is confirmed by the bond lengths and internal octahedral an-
gles (ESI,† Table S3): as expected, the MoO distances [Mo1–
O1 = 1.687(3) Å, Mo2–O4 = 1.687(3) Å] are shorter than the
Mo–O distances for the μ2-O bridges, which range from
1.913(3) to 1.947(3) Å; the distances of the Mo–N bonds in
the position trans relative to the terminal MoO bonds
[2.319(3) and 2.303(3) Å] are considerably longer than those
Fig.
2 Molecular structures of the (a) mononuclear complex
[MoO2Cl(OEt)(di-tBu-bipy)] (2) and (b) dinuclear complex [Mo2O2(μ2-
O)2Cl2(di-tBu-bipy)2] (4; present in compound 4·2EtOH), showing the
labelling scheme for all atoms composing the Mo coordination
environments. H-atoms have been omitted for clarity. Detailed infor-
mation about the bond lengths and angles of the Mo coordination
centres is summarised in Tables S2 and S3 in the ESI.†
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which are trans to the μ2-O bridges [2.235(3) and 2.229(3) Å],
as a consequence of the trans effect of MoO; the cis and
trans internal octahedral (Cl,N,O)–Mo–(Cl,N,O) angles range
from 70.61(12) to 112.04(13)°, and from 156.45(13) to
160.30(9)°, respectively, revealing considerable dispersion
around the expected values for regular octahedral environ-
ments. In the complex of 4·5CHCl3, the unique crystallo-
graphically independent Mo centre (Mo1) has a similar envi-
ronment to that described for Mo1 and Mo2 in 4·2EtOH, also
coordinating to two N-atoms of one di-tBu-bipy ligand, two
μ2-bridging oxido groups, one terminal oxido group, and one
chlorido ligand, {MoClN2O3} (ESI,† Fig. S4). In fact, the Mo1
coordination centre exhibits a highly distorted overall octahe-
dral geometry with the Mo1–Cl,N,O bond lengths and (Cl,N,
O)–Mo–(Cl,N,O) angles being comparable to those of 4·2EtOH
(ESI,† Table S4).
to the right by continuously removing water from the reac-
tion medium, activated 3 Å molecular sieves were added to
the reaction mixture together with 1. The reaction stopped at
10 min and conversion was only slightly higher (89%) than
that reached in the absence of molecular sieves (86%). More
efficient removal of water from the reaction medium may
lead to further improvements in conversion.
The system of 1/benzaldehyde/ethanol consisted of a bi-
phasic mixture, specifically a colourless transparent liquid
phase and a solid phase. The solids recovered after 24 h (as
described in the Experimental section) were characterised by
ATR FT-IR spectroscopy and PXRD; 1/BzA-sol and 1/BzA-liq
denote the metal species which were undissolved and
dissolved in the reaction medium, respectively. As found for
the contact tests (CTs) performed in the absence of substrate
(1/CT-sol and 1/CT-liq, discussed above), the recovered solids
and original complex 1 exhibited similar FT-IR spectra
(Fig. 1) and PXRD patterns (ESI,† Fig. S10), suggesting that
complex 1 was essentially the active species responsible for
the acetalisation reaction.
A full account of the crystal packing for 2, 4·2EtOH and
4·5CHCl3, including figures and details of hydrogen bonding
geometries, is provided in the ESI.†
Acid-catalysed acetalisation of benzaldehyde using 1
Acid-catalysed alcoholysis of styrene oxide using 1
Complex 1 was tested for the acetalisation of benzaldehyde
with ethanol at 55 °C (Scheme 3). To the best of our knowl-
edge, the use of Mo-based complexes in this reaction has not
been previously reported, and there are no studies involving
different transition metal complexes bearing bipyridine-type
ligands. The catalytic reaction was very fast, reaching ca. 86%
conversion at 10 min, and benzaldehyde diethyl acetal was
formed with 100% selectivity. Without catalyst the reaction
was sluggish (4% conversion at 10 min). These results point
to the role of 1 as a Lewis acid catalyst in the acetalisation
mechanism. According to studies reported in the literature
for Lewis acid-catalysed acetalisation reactions, the overall
mechanism may involve the intermediate formation of a
hemiacetal.11d,27 No intermediates were detected for the cata-
lytic system 1/benzaldehyde/ethanol, which may be due to
their relatively high reactivity under the catalytic reaction
conditions used.
Complex 1 was further explored for the acid-catalysed ring-
opening reaction of styrene oxide (StyOx) with ethanol at
55 °C (Scheme 4). The corresponding β-alkoxyalcohol prod-
uct, namely 2-ethoxy-2-phenylethanol (EPE), was formed with
100% selectivity at 98% conversion, 24 h reaction (Fig. 3).
Without catalyst, the conversion was only 5% at 24 h. Hence,
the alcoholysis reaction was promoted by Lewis acid molyb-
denum species. To the best of our knowledge, there are only
two studies reported in the literature on the use of
molybdenum-based homogeneous catalysts for this reac-
tion,28 and there are no studies involving different transition
metal complexes bearing bipyridine-type ligands. Specifi-
cally, the ethanolysis of StyOx was carried out in the pres-
ence of [MoO2Cl2], which led to 92% EPE yield after 2 h re-
action at room temperature (conversion and selectivity were
not indicated, and catalyst stability was not addressed).28a
In the other study, cyclopentadienyl molybdenum carbonyl
complexes were tested as pre-catalysts for ethanolysis of
StyOx at 55 °C (using higher catalyst concentration), which
led to 100% EPE selectivity, and in terms of reaction rate,
the best result was reported for [(η5-C5H4COOH)Mo(CO)2(η3-
Benzaldehyde conversion reached a plateau at ca. 10 min,
which can be attributed to thermodynamic limitations associ-
ated with the reversible acetalisation process, i.e. water is
formed as a co-product and the acetal can be hydrolysed back
to the aldehyde.11a,d,27a In an attempt to shift the equilibrium
−1
C3H5)] which gave a turnover frequency of 61 mol molMo
Scheme 3 Acid-catalysed acetalisation of benzaldehyde.
Scheme 4 Acid-catalysed ethanolysis of styrene oxide.
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catalytic system benzaldehyde/ethanol/1, ATR FT-IR (Fig. 1)
and PXRD (ESI,† Fig. S10) data for the solids recovered from
the liquid and solid phases of the reaction of 1-phenylethanol
(denoted as 1/PhEtOH-liq and 1/PhEtOH-sol, respectively)
were similar to those for 1.
In contrast to the biphasic systems 1/benzaldehyde/etha-
nol and 1/phenylethanol/ethanol, the system StyOx/ethanol/1
consisted of a transparent homogeneous pale yellow solution.
The solution species in the StyOx reaction system were iso-
lated by precipitation with pentane (as detailed in the Experi-
mental section), and the resultant solid (1/StyOx-liq) was
characterised by ATR FT-IR spectroscopy and PXRD. The
solid 1/StyOx-liq was X-ray amorphous (PXRD not shown),
whereas 1 was crystalline, and the FT-IR spectra of 1/StyOx-
liq and 1 were very different (Fig. 1). Hence, 1 was converted
to different metal species during the StyOx catalytic process.
Interestingly, the ATR FT-IR spectra of the solids (n/StyOx-liq
with n = 2, 4 or 5) isolated from the liquid phase of the cata-
lytic systems (2, 4 or 5)/StyOx/ethanol were practically identi-
cal to that for 1/StyOx-liq (Fig. 1). These results suggest that
the same molybdenum-containing species was formed from
the different complexes, which may partly explain the fact
that the kinetic curves tended to similar conversion at 24 h
reaction.
Fig. 3 Acid-catalysed alcoholysis of styrene oxide with ethanol at
55 °C in the presence of compounds 1 (◊), 2 (×), 4 (△), 5 (□) and 6 (+).
h−1, which is similar to that for compound 1 (56 mol
molMo h−1).28b
−1
For comparison, compounds 2, 4 and 5 were also exam-
ined for the reaction of StyOx under similar conditions to
those used for 1 (Fig. 3). For the tested compounds, EPE was
always the only reaction product, and initial reaction rate was
highest for 1. The differences in catalytic results may be due
to various factors, including differences in intrinsic activities,
initial dissolution rates and solubility of the starting com-
pound, catalyst stability (discussed ahead) and kinetics of for-
mation/conversion of metal species, which can influence the
overall kinetics of the catalytic reaction in complex fashions.
Nevertheless, all kinetic curves tended to similar conversion
at 24 h reaction (94–98%).
For the catalytic system using 1, no condensation prod-
ucts (ethers) of the alcohol solvent were detected. The same
was observed when 1-phenylethanol or 2-phenylethanol were
used (instead of StyOx) as model hydroxylated substrates,
i.e., no reaction took place during 24 h at 55 °C. These re-
sults, together with the excellent regioselectivity toward the
β-alkoxyalcohol product formed from the epoxide substrate
StyOx, suggest that the catalytic system can exhibit a carbon
atom efficiency of 100% (since all the carbon atoms present
in the reactants are incorporated into the product). A nucle-
ophilic substitution mechanism may be involved in the
conversion of StyOx to the corresponding β-alkoxyalcohol.
According to the literature, the interaction between the
oxygen atom of StyOx and the Lewis acid metal center
(acid–base interaction) can lead to enhanced electrophilicity
of the carbon atoms of the oxirane ring, facilitating the nu-
cleophilic attack of an incoming alcohol molecule at the
more substituted carbon atom to give a β-alkoxyalcohol
product.13b,d,29
After comparing the FT-IR spectra of n/StyOx-liq (n = 1, 2,
4 and 5) with spectra reported for other oxomolybdenum
complexes containing the ligand di-tBu-bipy, including
those obtained in the present work, it became clear that
the spectra match very well with that exhibited by the
octanuclear complex [Mo8O24(di-tBu-bipy)4] (6) (Fig. 1). Previ-
ously, complex 6 had only been isolated after oxidative
decarbonylation of molybdenum carbonyl complexes with
tert-butylhydroperoxide, namely the complexes [Mo(CO)4(di-
tBu-bipy)],9g
[Mo(CO)3I2(di-tBu-bipy)],30
and
[Mo(η3-C3-
H5)Cl(CO)2(di-tBu-bipy)].9a The molecular structure of 6 con-
sists of a central cubane-type Mo4(μ3-O)4 core (Scheme 1).9a,g
Four peripheral [MoO2(di-tBu-bipy)]2+ units cap the long
edges of the Mo4 tetrahedron of the central cubane to form
the final windmill-type octanuclear complex. The formation
of 6 in the StyOx/ethanol system at 55 °C is in stark contrast
to the apparent stability of 1 in the benzaldehyde/ethanol
and phenylethanol/ethanol systems at the same temperature,
and is evidently associated with the rapid solubilisation of
species in the StyOx/ethanol system to give a homogeneous
solution. The presence of StyOx in excess at the beginning of
the reaction (StyOx : Mo molar ratio of 20) is likely to be an
important factor, possibly affecting the polarity of the me-
dium. The conversion of 1 to 6 may involve dissociation of
the chlorido ligands, and condensation reactions of mono-
and dinuclear intermediates possibly related to complexes
2–5. When complex 6 was examined for the reaction of StyOx
under similar conditions to those used for 1, 2, 4 and 5, EPE
was again obtained with 100% selectivity (Fig. 3). The initial
reaction rate was much lower for 6 than for 1, but conversion
at 6 h was highest for 6 (96% for 6 vs. 62% for 4 and 71–75%
for 1, 2 and 5). For the reaction using 6, the ATR FT-IR
With the phenylethanol model substrate, the reaction mix-
tures were biphasic solid–liquid. As found for the biphasic
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spectrum of the solid isolated from the liquid phase (denoted
6/StyOx-liq) was practically identical to that for 6 (Fig. 1).
The reaction of StyOx in the presence of physically mixed
MoO3 and di-tBu-bipy (each in equivalent molar amounts to
that used for 1) led to very poor catalytic results: 18%
conversion at 24 h, and 92% EPE selectivity. The isolated
metal species exhibited a similar ATR FT-IR spectrum to that
of MoO3, suggesting that in situ complexation reactions be-
tween MoO3 and di-tBu-bipy did not occur. On the other
hand, while complex 1 was in situ converted to 6, this was
activity and selectivity, and could be recovered unchanged at
the end of the reaction.
In future work we plan to further study the catalytic per-
formance of 6 and related oxidomolybdenum complexes in
other acid-catalysed organic transformations. The impact of
known and as yet undiscovered oxidomolybdenum complexes
in the field of acid catalysis may supersede that of oxidation
catalysis for which they have been essentially investigated to
date.
not the case for the physical mixture MoO3/di-tBu-bipy. It has Experimental section
been previously reported that harsh hydrothermal conditions
General procedures and materials
are required to convert MoO3 and di-tBu-bipy into 6 (160 °C,
3 days, with agitation; 70% synthesis yield).9h
Solvents were dried by standard procedures, distilled under ni-
trogen and stored over 4 Å molecular sieves. MoO2Cl2, 4,4′-di-
tert-butyl-2,2′-bipyridine, hexane (99%), pentane (99%), ace-
tone (99.5%), diethyl ether (99.8%), styrene oxide (98%), benz-
aldehyde (≥99%), 1-phenylethanol (>98%), 2-phenylethanol
(>98%), 1-octene (98%), and nonane (99%) were purchased
from Sigma-Aldrich and used as received. Anhydrous abso-
lute ethanol (99.9%) was acquired from Carlo Erba and used
as received. The complexes [MoO2Cl2(di-tBu-bipy)] (1) and
[Mo8O24(di-tBu-bipy)4] (6) were prepared as described
previously.9a,e The large-scale contact tests were performed
under nitrogen.
Elemental analysis for C, H, and N was performed by M.
Marques at the University of Aveiro with a Leco TruSpec 630-
200-200 analyser. PXRD data were collected at ambient tem-
perature using a PANalytical Empyrean instrument equipped
with a PIXcel 1D detector set at 240 mm from the sample.
Cu-Kα1,2 X-radiation (λ1 = 1.540598 Å; λ2 = 1.544426 Å) filtered
with a nickel foil was used along with a standard reflection
sample holder. Working operating conditions for the
X-ray tube: 45 kV and 40 mA. Intensity data were collected in
continuous mode in the ca. 3.5 ≤ 2θ ≤70° range.
Transmission FT-IR spectra were measured as KBr pellets
on a Unican Mattson 7000 spectrophotometer equipped with a
DTGS CsI detector (resolution 4 cm−1, 128 scans). Attenuated
total reflectance (ATR) FT-IR spectra were measured using a
Specac Golden Gate Mk II ATR accessory having a diamond
top plate and KRS-5 focusing lenses (resolution 4 cm−1, 256
scans). 1H NMR spectra in solution were recorded using a
Bruker CPX 300 spectrometer at ambient temperature. Chemi-
cal shifts are given in ppm relative to tetramethylsilane.
Conclusions
In this work a number of valuable developments have been
made concerning the chemistry and catalytic application of
oxidomolybdenum complexes. It has been shown that the
MoV oxido-bridged dinuclear complexes [Mo2O2(μ2-O)Cl4(di-
tBu-bipy)2] (3) and [Mo2O2(μ2-O)2Cl2(di-tBu-bipy)2] (4) can be
isolated in good yields by treatment of the MoVI complex
[MoO2Cl2(di-tBu-bipy)] (1) with the alcohols ethanol and
1-phenylethanol, which function simultaneously as reducing
agents and solvents. In the presence of ethanol, the MoVI
oxidoalkoxido complex [MoO2Cl(OEt)(di-tBu-bipy)] (2) was
also isolated and structurally characterised. The formation of
this complex is significant from a number of viewpoints. To
the best of our knowledge, it is the first example of a mixed-
ligand complex of the type [MoO2X(OR)(L)], where X = halide
or alkyl group. From a structural perspective the complex is
remarkable in presenting an all-cis configuration rather than
the far more common cis-oxido, trans-X, cis-L configuration
normally exhibited by complexes of the type [MoO2X2(L)]. The
complex could, in principle, be an intermediate in the con-
version of 1 to 3, a pathway that would have clear parallels
with that generally presented for the oxidation of ethanol to
acetaldehyde over supported molybdenum oxide catalysts. In-
deed, complex 2 can be viewed as a valuable model com-
pound for the latter catalytic process.
To date, catalytic studies with complexes of the type
[MoO2X2(L)] (L = bidentate organic ligand) have essentially
been limited to liquid-phase olefin epoxidation using alkyl-
hydroperoxides as oxidants. In the present work, complex 1
was studied as a (pre)catalyst in acid-catalysed reactions
performed under mild conditions, namely the acetalisation
of benzaldehyde and the ethanolysis of styrene oxide. Good
results were obtained for both reactions with selective forma-
tion of the target products benzaldehyde diethyl acetal and
2-ethoxy-2-phenylethanol. Whereas complex 1 appears to be
essentially the active species responsible for the acetalisation
reaction, in the presence of styrene oxide the complex un-
Large-scale contact tests (LSCT)
Complex 1, ethanol, reflux/24 h. A mixture comprising the
mononuclear complex 1 (0.50 g, 1.07 mmol) and anhydrous
ethanol (40 mL) was heated under reflux for 24 h. The resul-
tant bordeaux coloured precipitate was filtered, washed with
ethanol (3 × 20 mL), pentane (3 × 20 mL), and diethyl ether
(3
× 20 mL), and finally vacuum-dried to give the
molybdenumIJV) dinuclear complex [Mo2O2(μ2-O)Cl4(di-tBu-
bipy)2] (3) (0.31 g, 63%). Anal. calcd for C36H48Cl4Mo2N4O3
(918.48): C, 47.08; H, 5.27; N, 6.10. Found: C, 46.95; H, 5.41;
N, 5.95. FT-IR (KBr, cm−1): ν = 2960 (m), 2908 (w), 2870 (w),
dergoes
a transformation to the octanuclear complex
[Mo8O24(di-tBu-bipy)4] (6). In separate assays, complex 6 was
shown to catalyse the ethanolysis of styrene oxide with high
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1617 (vs), 1544 (m), 1486 (m), 1467 (sh), 1413 (s), 1369 (m),
1307 (w), 1251 (m), 1203 (m), 1122 (w), 1081 (w), 1031 (sh),
1022 (m), 964 (vs), 902 (m), 840 (s), 786 (m), 740 (w), 717 (w),
605 (m), 551 (w), 447 (m), 320 (m). 1H NMR (300 MHz,
Complex 1, tert-butanol, reflux. A mixture comprising the
mononuclear complex 1 (0.15 g, 0.32 mmol) and tert-butanol
(15 mL) was stirred under reflux for 24 h, and then filtered to
separate the resultant pink-coloured solution from unreacted 1.
Addition of pentane (20 mL) to the filtrate gave a pink precipi-
tate, which was filtered, washed with pentane (3 × 10 mL),
vacuum-dried, and identified as the molybdenumIJVI) oxido-
bridged dinuclear complex [Mo2O4(μ2-O)Cl2(di-tBu-bipy)2] (5)
(10 mg, 7%) by FT-IR spectroscopy and PXRD.
3
CD3COCD3): δ = 9.12 (d, 2H, H6/6′, JH–H = 6 Hz), 8.92 (d, 2H,
3
4
H6/6′, JH–H = 6 Hz), 8.67 (d, 2H, H3/3′, JH–H = 2 Hz), 8.61 (d,
2H, H3/3′
,
4JH–H = 2 Hz), 7.80 (dd, 2H, H5/5′
,
3JH–H = 6 Hz,
4JH–H = 2 Hz), 7.75 (dd, 2H, H5/5′, JH–H = 6 Hz, JH–H = 2 Hz),
3
4
1.52 (s, 18H, C(CH3)3), 1.44 (s, 18H, C(CH3)3).
After the reaction of 1 in ethanol, the filtrate (mother li-
quor) was subjected to vapour diffusion by pentane, which
gave small crops of yellow crystals of the MoVI oxidoalkoxido
complex [MoO2Cl(OEt)(di-tBu-bipy)] (2) and orange crystals of
the MoV dinuclear complex [Mo2O2(μ2-O)2Cl2(di-tBu-bipy)2]
·2EtOH (4·2EtOH). These were mechanically separated and
found to be suitable for structure determination by X-ray dif-
fraction. In addition to these crystals, a small amount of bor-
deaux precipitate (0.08 g) was obtained, which was identified
as a mixture comprising complexes 1 (ca. 10 mol%), 2 (ca. 80
Single-crystal X-ray diffraction studies
Crystalline material of the compounds [MoO2Cl(OEt)(di-tBu-
bipy)] (2), [Mo2O2(μ2-O)2Cl2(di-tBu-bipy)2]·2EtOH (4·2EtOH)
and [Mo2O2(μ2-O)2Cl2(di-tBu-bipy)2]·5CHCl3 (4·5CHCl3) were
collected from the crystallisation vials and immediately im-
mersed in highly viscous FOMBLIN Y perfluoropolyether vac-
uum oil (LVAC 140/13, Sigma-Aldrich). Selected single-crystals
of each compound were mounted on CryoLoops with the as-
sistance of a stereomicroscope.31 Diffraction data sets were
obtained at 150(2) K on a Bruker X8 APEX II CCD area-
detector diffractometer (Mo Kα graphite-monochromated ra-
diation, λ = 0.71073 Å) controlled by the APEX2 software
package,32 and equipped with an Oxford Cryosystems Series
700 cryostream. Images were processed using the software
package SAINT+,33 and the absorption effects were corrected
by the multi-scan semi-empirical method implemented in
SADABS.34 The structures were solved using the algorithms
implemented in SHELXT-2014,35,36 allowing the direct loca-
tion of most of the heaviest atoms. The remaining non-H
atoms were located from difference Fourier maps calculated
from successive full-matrix least squares refinement cycles on
F2 using SHELXL-v.2014,35,37 and all of them were success-
fully refined using anisotropic displacement parameters.
H-atoms bound to carbon were located at model geometri-
cal positions using suitable HFIX instructions in SHELXL: 43
for the aromatic, 13 for CH–, 23 for –CH2– and 137 for the
terminal –CH3 methyl groups belonging to the di-tBu-bipy
and OEt ligands, as well as to the solvent molecules of
crystallisation, EtOH and CHCl3. All of these atoms were in-
cluded in subsequent refinement cycles in riding-motion ap-
proximation with isotropic thermal displacement parameters
(Uiso) fixed at 1.2 (aromatic, CH– and –CH2– groups) or 1.5
(–CH3 groups) times Ueq of the attached carbon atom. In
4·2EtOH, the H-atoms of the OH groups in the EtOH mole-
cules of crystallisation were clearly visible in the difference
Fourier maps, included in the final structure model during sub-
sequent refinement stages with the O–H distances restrained to
0.95(1), and further treated by a riding-motion approximation
with an isotropic thermal displacement parameter (Uiso) fixed
at 1.5 × Ueq of the parent oxygen atom. In the structure of
compound 2, a central {MoO2Cl} group is disordered over
two positions with occupancy factors of 15% and 85%. Table
S1 (ESI†) summarises relevant information about the crystal
data, single-crystal X-ray data collection and structure refine-
ment details of 2, 4·2EtOH and 4·5CHCl3.
1
mol%) and 3 (ca. 10 mol%) by H NMR and PXRD (combined
yield of 15% based on Mo): characterisation data for 2: Anal.
calcd for C20H29ClMoN2O3 (476.84): C, 50.38; H, 6.13; N, 5.87.
Found: C, 50.33; H, 5.76; N, 5.76. FT-IR (KBr, cm−1): ν = 2968
(m), 2908 (w), 2869 (w), 1614 (s), 1548 (m), 1486 (m), 1465
(m), 1409 (s), 1365 (m), 1307 (w), 1252 (m), 1203 (w), 1160
(w), 1122 (w), 1089 (w), 1057 (s), 1032 (m), 931 (m), 905 (vs),
889 (s), 865 (m), 750 (w), 737 (w), 721 (w), 607 (m), 553 (w),
1
482 (w), 404 (w), 376 (w), 343 (w). H NMR (300 MHz, CDCl3):
δ = 8.99 (d, 2H, H6/6′, 3JH–H = 6 Hz), 8.14 (d, 2H, H3/3′, 4JH–H = 2
3
4
Hz), 7.54 (dd, 2H, H5/5′, JH–H = 6 Hz, JH–H = 2 Hz), 3.72 (q,
3
2H, OCH2CH3, JH–H = 7 Hz), 1.48 (s, 18H, C(CH3)3), 1.24 (t,
3H, OCH2CH3, 3JH–H = 7 Hz).
Complex 1, 1-phenylethanol, 130 °C. A mixture comprising
the mononuclear complex
1 (0.50 g, 1.07 mmol) and
1-phenylethanol (20 mL) was heated at 130 °C for 24 h. A
dark red-orange solution was obtained. Addition of pentane
(40 mL) resulted in the precipitation of an orange solid,
which was filtered, washed with pentane (3 × 20 mL) and
diethyl ether (3 × 20 mL), and finally vacuum-dried to give
the molybdenumIJV) dinuclear complex [Mo2O2(μ2-O)2Cl2(di-
tBu-bipy)2] (4) in 73% yield (0.34 g). Anal. calcd for
C36H48Cl2Mo2N4O4 (863.57): C, 50.07; H, 5.60; N, 6.49. Found:
C, 49.75; H, 5.51; N, 6.25. FT-IR (KBr, cm−1): ν = 2964 (m),
2908 (w), 2871 (w), 1616 (vs), 1547 (m), 1486 (m), 1465 (w),
1412 (s), 1367 (m), 1305 (w), 1292 (w), 1251 (m), 1201 (w),
1157 (w), 1124 (w), 1082 (w), 1027 (m), 956 (vs), 935 (sh), 900
(m), 848 (m), 728 (m), 606 (m), 551 (w), 485 (m). 1H NMR
3
(300 MHz, CDCl3): δ = 9.88 (d, 2H, H6/6′, JH–H = 6 Hz), 8.24
4
4
(d, 2H, H3/3′, JH–H = 2 Hz), 8.04 (d, 2H, H3/3′, JH–H = 2 Hz),
7.80 (dd, 2H, H5/5′, 3JH–H = 6 Hz, 4JH–H = 2 Hz), 7.35 (d, 2H, H6/6′
,
3JH–H = 6 Hz), 6.67 (dd, 2H, H5/5′, JH–H = 6 Hz, JH–H = 2 Hz),
3
4
1.57 (s, 18H, C(CH3)3), 1.38 (s, 18H, C(CH3)3).
Recrystallisation of 4 from chloroform/hexane (liquid–liq-
uid slow diffusion) resulted in orange crystals of [Mo2O2(μ2-
O)2Cl2(di-tBu-bipy)2]·5CHCl3 (4·5CHCl3) suitable for structure
determination by X-ray diffraction.
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Catalytic tests
Notes and references
The reactions were carried out with magnetic stirring (1000
rpm), under air, in closed borosilicate micro-reactors (5 mL)
equipped with a valve to allow sampling. Typically, the reac-
tor was charged with (pre)catalyst (equivalent to 20 μmol of
Mo), substrate (0.4 mmol) and ethanol (1 mL), and immersed
in a temperature-controlled oil bath.
The reaction mixtures were analysed by using a Varian
3900 GC equipped with a DB-5 capillary column (30 m × 0.25
mm × 0.25 μm) and a FID detector, with H2 as the carrier
gas. The reaction products were identified by GC–MS (Trace
GC 2000 Series Thermo Quest CE Instruments GC; Thermo
Scientific DSQ II), using He as the carrier gas. 1-Octene and
nonane were used as internal standards added after the reac-
tion for the substrates styrene oxide and benzaldehyde,
respectively.
After a batch run, the reaction mixture was cooled to am-
bient temperature and the undissolved solid was separated
from the liquid phase by centrifugation (3500 rpm), washed
with pentane, and vacuum-dried (ca. 0.1 bar) at 55 °C for 1 h.
The liquid phase was filtered through a 0.2 μm PTFE mem-
brane and, after addition of pentane, stored at 4 °C over-
night. Any resultant precipitate was separated by centrifuga-
tion (3500 rpm), washed with pentane or diethyl ether, and
vacuum-dried (ca. 0.1 bar) at 55 °C for 1 h. Contact tests (CT)
were carried out for different complexes as follows: a mixture
of complex and alcohol solvent was stirred at 55 °C for 24 h.
Afterwards, the mixture was cooled to ambient temperature,
centrifuged (3500 rpm), and the dissolved (denoted 1/CT-liq)
and undissolved (1/CT-sol) metal species were isolated as de-
scribed above for the normal catalytic batch runs. The recov-
ered solids were characterised by ATR FT-IR spectroscopy and
PXRD.
1 (a) K. Jeyakumar and D. K. Chand, J. Chem. Sci., 2009, 121,
111–123; (b) R. Sanz and M. R. Pedrosa, Curr. Org. Synth.,
2009, 6, 239–263; (c) S. C. A. Sousa, I. Cabrita and A. C.
Fernandes, Chem. Soc. Rev., 2012, 41, 5641–5653; (d) R. G.
de Noronha and A. C. Fernandes, Curr. Org. Chem., 2012, 16,
33–64.
2 (a) F. E. Kühn, A. D. Lopes, A. M. Santos, E. Herdtweck, J. J.
Haider, C. C. Romão and A. G. Santos, J. Mol. Catal. A:
Chem., 2000, 151, 147–160; (b) F. E. Kühn, M. Groarke, É.
Bencze, E. Herdtweck, A. Prazeres, A. M. Santos, M. J.
Calhorda, C. C. Romão, I. S. Gonçalves, A. D. Lopes and M.
Pillinger, Chem. – Eur. J., 2002, 8, 2370–2383; (c) A. M. Al-
Ajlouni, A. A. Valente, C. D. Nunes, M. Pillinger, A. M.
Santos, J. Zhao, C. C. Romão, I. S. Gonçalves and F. E. Kühn,
Eur. J. Inorg. Chem., 2005, 1716–1723; (d) A. M. Al-Ajlouni, A.
Günyar, M.-D. Zhou, P. N. W. Baxter and F. E. Kühn, Eur. J.
Inorg. Chem., 2009, 1019–1026; (e) A. Günyar, M.-D. Zhou, M.
Drees, P. N. W. Baxter, G. Bassioni, E. Herdtweck and F. E.
Kühn, Dalton Trans., 2009, 8746–8754; ( f ) A. Günyar, D.
Betz, M. Drees, E. Herdtweck and F. E. Kühn, J. Mol. Catal.
A: Chem., 2010, 331, 117–124; (g) A. Günyar and F. E. Kühn,
J. Mol. Catal. A: Chem., 2010, 319, 108–113.
3 (a) A. A. Valente, J. Moreira, A. D. Lopes, M. Pillinger, C. D.
Nunes, C. C. Romão, F. E. Kühn and I. S. Gonçalves, New J.
Chem., 2004, 28, 308–313; (b) S. Gago, J. E. Rodríguez-
Borges, C. Teixeira, A. M. Santos, J. Zhao, M. Pillinger, C. D.
Nunes, Z. Petrovski, T. M. Santos, F. E. Kühn, C. C. Romão
and I. S. Gonçalves, J. Mol. Catal. A: Chem., 2005, 236, 1–6.
4 (a) Ž. Petrovski, M. Pillinger, A. A. Valente, I. S. Gonçalves, A.
Hazell and C. C. Romão, J. Mol. Catal. A: Chem., 2005, 227,
67–73; (b) S. M. Bruno, S. S. Balula, A. A. Valente, F. A. A.
Paz, M. Pillinger, C. Sousa, J. Klinowski, C. Freire, P. Ribeiro-
Claro and I. S. Gonçalves, J. Mol. Catal. A: Chem., 2007, 270,
185–194; (c) G. Chahboun, J. A. Brito, B. Royo, M. A. E.
Amrani, E. Gómez-Bengoa, M. E. G. Mosquera, T. Cuenca
and E. Royo, Eur. J. Inorg. Chem., 2012, 2940–2949.
Acknowledgements
We acknowledge funding by FEDER (Fundo Europeu de
Desenvolvimento Regional) through COMPETE (Programa
Operacional Factores de Competitividade). National funding
through the FCT (Fundação para a Ciência e a Tecnologia)
within the project FCOMP-01-0124-FEDER-029779 (FCT ref.
PTDC/QEQ-SUP/1906/2012, including the research grant with
ref. BPD/UI89/4864/2014 to A. C. G.) is thanked. This work
was developed in the scope of the projects CICECO – Aveiro
Institute of Materials, POCI-01-0145-FEDER-007679 (FCT ref.
UID/CTM/50011/2013), and REQUIMTE-LAQV (UID/QUI/
50006/2013), financed by national funds through the FCT/
MEC and co-financed by FEDER under the PT2020 Partner-
ship Agreement. The FCT and the European Union are ac-
knowledged for post-doctoral grants to A. C. G. (SFRH/BPD/
108541/2015) and P. N. (SFRH/BPD/73540/2010) co-funded by
MCTES and the European Social Fund through the program
POPH of QREN. The FCT and CICECO are acknowledged for
financial support toward the purchase of the single-crystal
diffractometer.
5 (a) S. M. Bruno, B. Monteiro, M. S. Balula, F. M. Pedro, M.
Abrantes, A. A. Valente, M. Pillinger, P. Ribeiro-Claro, F. E.
Kühn and I. S. Gonçalves, J. Mol. Catal. A: Chem., 2006, 260,
11–18; (b) J. A. Brito, S. Ladeira, E. Teuma, B. Royo and M.
Gómez, Appl. Catal., A, 2011, 398, 88–95.
6 S. S. Balula, S. M. Bruno, A. C. Gomes, A. A. Valente, M.
Pillinger, I. S. Gonçalves, D. J. Macquarrie and J. H. Clark,
Inorg. Chim. Acta, 2012, 387, 234–239.
7 (a) S. M. Bruno, C. C. L. Pereira, M. S. Balula, M. Nolasco,
A. A. Valente, A. Hazell, M. Pillinger, P. Ribeiro-Claro and
I. S. Gonçalves, J. Mol. Catal. A: Chem., 2007, 261, 79–87; (b)
S. M. Bruno, J. A. Fernandes, L. S. Martins, I. S. Gonçalves,
M. Pillinger, P. Ribeiro-Claro, J. Rocha and A. A. Valente,
Catal. Today, 2006, 114, 263–271; (c) A. C. Coelho, M.
Nolasco, S. S. Balula, M. M. Antunes, 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; (d)
T. R. Amarante, P. Neves, F. A. A. Paz, A. A. Valente, M.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2016

View Article Online
Catalysis Science & Technology
Paper
Pillinger and I. S. Gonçalves, Dalton Trans., 2014, 43,
6059–6069.
1992, 114, 1501–1502; (c) U. Piarulli, D. N. Williams, C.
Floriani, G. Gervasio and D. Viterbo, J. Chem. Soc., Dalton
Trans., 1995, 3329–3334.
8 S. Gupta, S. Pal, A. K. Barik, S. Roy, A. Hazra, T. N. Mandal,
R. J. Butcher and S. K. Kar, Polyhedron, 2009, 28, 711–720.
9 (a) C. A. Gamelas, A. C. Gomes, S. M. Bruno, F. A. A. Paz,
A. A. Valente, M. Pillinger, C. C. Romão and I. S. Gonçalves,
Dalton Trans., 2012, 41, 3474–3484; (b) H. Arzoumanian, G.
Agrifoglio, H. Krentzien and M. Capparelli, J. Chem. Soc.,
Chem. Commun., 1995, 655–656; (c) H. Arzoumanian, G.
Agrifoglio and H. Krentzien, New J. Chem., 1996, 20,
699–705; (d) H. Arzoumanian, L. Maurino and G. Agrifoglio,
J. Mol. Catal. A: Chem., 1997, 117, 471–478; (e) B. Monteiro,
S. Gago, P. Neves, A. A. Valente, I. S. Gonçalves, C. C. L.
Pereira, C. M. Silva and M. Pillinger, Catal. Lett., 2009, 129,
350–357; ( f ) T. R. Amarante, P. Neves, F. A. A. Paz, M.
Pillinger, A. A. Valente and I. S. Gonçalves, Inorg. Chem.
Commun., 2012, 20, 147–152; (g) 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; (h)
T. R. Amarante, P. Neves, C. Tomé, M. Abrantes, A. A.
Valente, F. A. A. Paz, M. Pillinger and I. S. Gonçalves, Inorg.
Chem., 2012, 51, 3666–3676; (i) P. Ferreira, W.-M. Xue, É.
Bencze, E. Herdtweck and F. E. Kühn, Inorg. Chem.,
2001, 40, 5834–5841; ( j) F. E. Kühn, W.-M. Xue, A. Al-
Ajlouni, A. M. Santos, S. Zang, C. C. Romão, G. Eickerling
and E. Herdtweck, Inorg. Chem., 2002, 41, 4468–4477; (k) S.
Kodama, A. Nomoto, S. Yano, M. Ueshima and A. Ogawa,
Inorg. Chem., 2011, 50, 9942–9947; (l) S. Kodama, S.
Hashidate, A. Nomoto, S. Yano, M. Ueshima and A. Ogawa,
Chem. Lett., 2011, 40, 495–497.
16 (a) P. M. Reis and B. Royo, Tetrahedron Lett., 2009, 50,
949–952; (b) D. B. Soria, M. Barquín, M. J. G. Garmendia and
G. Estiu, J. Coord. Chem., 2008, 61, 3815–3828; (c) P. C. H.
Mitchell, J. Inorg. Nucl. Chem., 1963, 25, 963–970.
17 (a) T. Zweifel, J.-V. Naubron, T. Büttner, T. Ott and H.
Grützmacher, Angew. Chem., Int. Ed., 2008, 47, 3245–3249;
(b) D. Steinborn, Dalton Trans., 2005, 2664–2671; (c) J.
Chatt and B. L. Shaw, J. Chem. Soc. A, 1966, 1437–1442; (d)
M. O. Albers, T. V. Ashworth, H. E. Oosthuizen, E.
Singleton, J. S. Merola and R. T. Kacmarcik, Inorg. Synth.,
1989, 26, 68–77.
18 (a) H. Hirai, Y. Nakao and N. Toshima, J. Macromol. Sci.,
Chem., 1979, A13, 727–750; (b) Y.-C. Hsieh, Y. Zhang, D. Su,
V. Volkov, R. Si, L. Wu, Y. Zhu, W. An, P. Liu, P. He, S. Ye,
R. R. Adzic and J. X. Wang, Nat. Commun., 2013, 4, 2466; (c)
J.-L. Mi, Y. Shen, J. Becker, M. Bremholm and B. B. Iversen,
J. Phys. Chem. C, 2014, 118, 11104–11110.
19 (a) J. H. Enemark, J. J. A. Cooney, J.-J. Wang and R. H. Holm,
Chem. Rev., 2004, 104, 1175–1200; (b) R. Aguado, J.
Escribano, M. R. Pedrosa, A. D. Cian, R. Ranz and F. J.
Arnáiz, Polyhedron, 2007, 26, 3842–3848.
20 C. Caro, K. Thirunavukkarasu, M. Anilkumar, N. R. Shiju
and G. Rothenberg, Adv. Synth. Catal., 2012, 354, 1327–1336.
21 A. C. Gomes, J. A. Fernandes, C. A. Gamelas, I. S. Gonçalves
and F. A. A. Paz, Acta Crystallogr., Sect. E: Struct. Rep. Online,
2011, 67, m1738–m1739.
22 K. Esumi, T. Itakura and K. Torigoe, Colloids Surf., A,
1994, 82, 111–113.
23 L. Xu, E. Wang, C. Hu and R. Huang, Transition Met. Chem.,
2001, 26, 563–565.
10 (a) F. A. J. Meskens, Synthesis, 1981, 501–522; (b) S.
Krompiec, M. Penkala, K. Szczubiałka and E. Kowalska,
Coord. Chem. Rev., 2012, 256, 2057–2095.
11 (a) G. D. Yadav and A. A. Pujari, Can. J. Chem. Eng., 1999, 77,
489–496; (b) M. J. Climent, A. Velty and A. Corma, Green
Chem., 2002, 4, 565–569; (c) B. Thomas, S. Prathapan and S.
Sugunan, Microporous Mesoporous Mater., 2005, 80, 65–72;
(d) A. S. Poyraz, C.-H. Kuo, E. Kim, Y. Meng, M. S. Seraji and
S. L. Suib, Chem. Mater., 2014, 26, 2803–2813.
24 (a) G. Barea, A. Lledos, F. Maseras and Y. Jean, Inorg. Chem.,
1998, 37, 3321–3325; (b) V. L. Abramenko and V. S.
Sergienko, Russ. J. Inorg. Chem., 2009, 54, 2031–2053.
25 (a) K. Dreisch, C. Andersson and C. Stalhandske, Polyhedron,
1992, 11, 2143–2150; (b) K. Dreisch, C. Andersson and C.
Stalhandske, Polyhedron, 1993, 12, 303–311; (c) A. Lachgar,
P. Farrall and J. M. Mayer, Polyhedron, 1993, 12, 2603–2609;
(d) M. F. Davis, W. Levason, M. E. Light, R. Ratnani, G. Reid,
K. Saraswat and M. Webster, Eur. J. Inorg. Chem.,
2007, 1903–1910; (e) C. C. L. Pereira, S. S. Balula, F. A. A.
Paz, A. A. Valente, M. Pillinger, J. Klinowski and I. S.
Gonçalves, Inorg. Chem., 2007, 46, 8508–8510; ( f ) R.
Mayilmurugan, B. N. Harum, M. Volpe, A. F. Sax, M.
Palaniandavar and N. C. Mösch-Zanetti, Chem. – Eur. J.,
2011, 17, 704–713; (g) J. A. Schachner, P. Traar, C. Sala, M.
Melcher, B. N. Harum, A. F. Sax, M. Volpe, F. Belaj and N. C.
Mösch-Zanetti, Inorg. Chem., 2012, 51, 7642–7649.
12 C. Schneider, Synthesis, 2006, 3919–3944.
13 (a) R. García, M. Martínez and J. Aracil, Chem. Eng. Technol.,
1999, 22, 987–990; (b) A. Dhakshinamoorthy, M. Alvaro and
H. Garcia, Chem. – Eur. J., 2010, 16, 8530–8536; (c) A. S.
Capes, A. T. Crossman, L. A. Webster, M. A. J. Ferguson and
I. H. Gilbert, Tetrahedron Lett., 2011, 52, 7091–7094; (d) P.
Silva, F. Vieira, A. C. Gomes, D. Ananias, J. A. Fernandes,
S. M. Bruno, R. Soares, A. A. Valente, J. Rocha and F. A. A.
Paz, J. Am. Chem. Soc., 2011, 133, 15120–15138; (e) Y.-X.
Zhou, Y.-Z. Chen, Y. Hu, G. Huang, S.-H. Yu and H.-L. Jiang,
Chem. – Eur. J., 2014, 20, 14976–14980.
14 T. R. Amarante, A. C. Gomes, P. Neves, F. A. A. Paz, A. A.
Valente, M. Pillinger and I. S. Gonçalves, Inorg. Chem.
Commun., 2013, 32, 59–63.
15 (a) M. H. Chisholm, K. Folting, J. C. Hoffman and C. C.
Kirkpatrick, Inorg. Chem., 1984, 23, 1021–1037; (b) J.
Belgacem, J. Kress and J. A. Osborn, J. Am. Chem. Soc.,
26 (a) J. I. Dulebohn, T. C. Stamatakos, D. L. Ward and D. G.
Nocera, Polyhedron, 1991, 10, 2813–2820; (b) M. Majumdar,
S. K. Patra and J. K. Bera, Polyhedron, 2007, 26, 1597–1602;
(c) V. S. Joshi, M. Nandi, H. Zhang, B. S. Haggerty and A.
Sarkar, Inorg. Chem., 1993, 32, 1301–1303; (d) B. Modec and
J. Zubieta, Materials, 2010, 3, 150–157.
This journal is © The Royal Society of Chemistry 2016
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
27 (a) M. B. Smith and J. March, Reactions, Mechanisms, and
Structure, March's Advanced Organic Chemistry, John Wiley &
Sons, Inc., New Jersey, 2007, pp. 1270–1271; (b) P. Chaloner,
Organic Chemistry: A Mechanistic Approach, CRC Press, 2015.
28 (a) K. Jeyakumar and D. K. Chand, Synthesis, 2008, 807–819;
(b) S. M. Bruno, A. C. Gomes, M. Abrantes, A. A. Valente, M.
Pillinger and I. S. Gonçalves, J. Organomet. Chem.,
2015, 799–800, 179–183.
29 (a) M. Mirza-Aghayan, M. Alizadeh, M. M. Tavana and R.
Boukherroub, Tetrahedron Lett., 2014, 55, 6694–6697; (b)
B. H. Kim, F. Piao, E. J. Lee, J. S. Kim, Y. M. Jun and B. M.
Lee, Bull. Korean Chem. Soc., 2004, 25, 881–888.
31 T. Kottke and D. Stalke, J. Appl. Crystallogr., 1993, 26,
615–619.
32 APEX2 Data Collection Software, Version 2012.4, Bruker AXS,
Delft, The Netherlands.
33 SAINT+, Data Integration Engine, version 8.27b, Bruker AXS,
Madison, Wisconsin, USA, 1997–2012.
34 G. M. Sheldrick, SADABS, version 2012/1, Bruker AXS Area
Detector Scaling and Absorption Correction Program, Bruker
AXS, Madison, Wisconsin, USA.
35 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr.,
2008, 64, 112–122.
36 G. M. Sheldrick, SHELXT-2014, Program for Crystal Structure
Solution, University of Göttingen, 2014.
37 G. M. Sheldrick, SHELXL, version 2014/3, Program for Crystal
Structure Refinement, University of Göttingen, 2014.
30 C. A. Gamelas, P. Neves, A. C. Gomes, A. A. Valente, C. C.
Romão, I. S. Gonçalves and M. Pillinger, Catal. Lett.,
2012, 142, 1218–1224.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2016