Synthesis and catalytic activity of Mo(II) complexes of α-diimines intercalated in layered double hydroxides

Article abstract of DOI:10.1016/j.ica.2018.10.062

The two layered double hydroxides ZnAl-LDH and MgAl-LDH were functionalized with bis(4-HOOC-phenyl)-acenaphthenequinonediimine) (H₂BIAN), leading to the intercalation of its dianion, which in a second step reacted with [Mo(CO)₃X₂(NCMe)₂] (X = I, Br), affording four new materials. These materials and the two complexes [Mo(CO)₃X₂(H₂BIAN)₂] (X = I, Br) were tested in the olefin epoxidation reaction with substrates cis-cyclooctene, styrene, 1-octene, trans-hex-3-en-1-ol, and R-(+)-limonene, using tert-butylhydroperoxide (tbhp) as oxidant. The new catalysts were particularly good for cis-cyclooctene and styrene (100% conversions) and at least one heterogeneous catalyst was comparable to the homogeneous ones in the epoxidation of 1-octene and trans-hex-3-en-1-ol. The homogeneous catalysts were the best to oxidize R-(+)-limonene (higher conversions).

Full text of DOI:10.1016/j.ica.2018.10.062

Accepted Manuscript
Research paper
Synthesis and catalytic activity of Mo(II) complexes of α-diimines intercalated
in Layered Double Hydroxides
João Marreiros, Maria Diaz-Couce, Maria João Ferreira, Pedro D. Vaz, Maria
José Calhorda, Carla D. Nunes
PII:
S0020-1693(18)30941-1
https://doi.org/10.1016/j.ica.2018.10.062
ICA 18608
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
19 June 2018
9 October 2018
27 October 2018
Please cite this article as: J. Marreiros, M. Diaz-Couce, M.J. Ferreira, P.D. Vaz, M. José Calhorda, C.D. Nunes,
Synthesis and catalytic activity of Mo(II) complexes of α-diimines intercalated in Layered Double Hydroxides,
Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.10.062
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Synthesis and catalytic activity of Mo(II) complexes of -diimines intercalated in
Layered Double Hydroxides
João Marreirosa, Maria Diaz-Coucea, Maria João Ferreirab, Pedro D. Vazc d, Maria José
Calhordaa,e*, Carla D. Nunesa,b*
a Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Campo Grande,
1749-016 Lisboa, Portugal
b Centro de Química Estrutural, Faculdade de Ciências e Instituto Superior Técnico, Universidade
de Lisboa, 1049-001 Lisboa, Portugal
c ISIS Neutron & Muon Source, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire
OX11 0QX, United Kingdom
d CICECO – Aveiro Institute of Materials, Department of Chemistry, University of Aveiro, 3810-
193 Aveiro, Portugal
e BioISI -Biosystems & Integrative Sciences Institute, Departamento de Química e Bioquímica,
Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
E-mail addresses: mjcalhorda@fc.ul.pt, cmnunes@fc.ul.pt
Dedicated to Malcolm Green
Abstract
The two double layer hydroxides ZnAl-LDH and MgAl-LDH were functionalized with bis(4-
HOOC-phenyl)-acenaphthenequinonediimine) (H2BIAN), leading to the intercalation of its dianion,
which in a second step reacted with [Mo(CO)3X2(NCMe)2] (X = I, Br), affording four new
materials. These materials and the two complexes [Mo(CO)3X2(H2BIAN)2] (X = I, Br) were tested
in the olefin epoxidation reaction with substrates cis-cyclooctene, styrene, 1-octene, trans-hex-3-en-
1-ol, and R-(+)-limonene, using tert-butylhydroperoxide (tbhp). The new catalysts were particularly
good for cis-cyclooctene and styrene (100% conversions) and at least one heterogeneous catalyst
was comparable to the homogeneous ones in the epoxidation of 1-octene and trans-hex-3-en-1-ol.
The homogeneous catalysts were the best to oxidize R-(+)-limonene (higher conversions).
1

Keywords
Catalysis ; Molybdenum ; BIAN ; epoxidation ; Layered Double Hydroxides
1. Introduction
The development of better catalysts can contribute to prevent environment damage by increasing
the efficiency of the processes, generating less secondary products and using less solvent. In
particular, heterogeneous catalysts may combine the ease of recovering catalyst and product, with
specific advantages of homogeneous catalysts, such as selectivity [1,2]. This happens, for instance,
by immobilizing a homogeneous catalyst in a suitable support by a strong interaction, covalent or
electrostatic [3,4,5]. The first is achieved, among others, by reaction of functionalized compounds
on silica based materials [6], while the second can be obtained by intercalating a charged species
within the layers of clays, namely layered double hydroxides (LDH) [7,8]. LDHs may be
considered as deriving from a solid M2+ cation hydroxide (Mg2+, Zn 2+, Ni2+) with some M2+
substituted by a M3+ cation (Al3+, Cr3+, Ga3+). Therefore the layer formed by edge-sharing
octahedral M(OH)6 units acquires a positive charge that must be compensated by introduction of
anions. The general formula [M2+ M3+ (OH)2][Am-]x/m.nH2O allows a wide variety of
1-x
x
compositions, and the x value, reflecting the relative amount of M2+ and M3+ cations, controls the
distance between layers [9,10,11,12]. These hosts can be functionalized with anionic guests which
will occupy the inter layer space and may be designed to perform different tasks, from controlled
drug delivery [13,14,15,16,17] to environmental applications [18,19] and catalysis
[20,21,22,23,24,25]. We and other authors have applied these ideas in the field of catalytic olefin
epoxidation [26,27,28,29,30]. This functionalization reaction may proceed by direct anion exchange
with the Am- initially present in the clay, or, when the anion is carbonate, by calcination and
rehydration. When calcination occurs, the loss of the anion leads to the collapse of the structure,
however, immersion in a solution containing another anion reestablishes the layer structure (the so-
called memory effect) [9,10,12,31]. This anion may be a metal complex or, more efficiently, a
molecule that attaches the clay by some groups leaving others free to bind a metal center. An
alternative procedure consists of synthesizing directly the LDH with the desired anions [32,33].
We have used Zn/Al and Mg/Al combinations of M2+ and M3+ cations to synthesize two of these
LDH hosts and have chosen as catalyst Mo(II) complexes, [Mo(3-C3H5)X(CO)2(L)2][34,35] and
[MoI2(CO)3(L)2][36]. The L ligands are bidentate -diimines with two nitrogen donor atoms
bearing substituents that can be deprotonated and yield anionic species able to bind the metal and
intercalate the clays. They must remain strongly attached through electrostatic interactions to avoid
leaching of the catalyst. In a recent work we used 4,4’-(COOH)2-2,2’-bipyridyl [37]. We now study
the related bis(4-HOOC-phenyl)-acenaphthenequinonediimine) (H2BIAN, Scheme 1), since the
2

different  system and substitution pattern are expected to modify the catalytic activity of their
[MoI2(CO)3(L)2] derivatives and acenaphthenequinonediimine ligands have been widely used as
ligands in transition metal catalysts [38].
Scheme 1. H2BIAN, its dianion and [MoI2(CO)3(H2BIAN)]
2. Experimental
2.1 General
All reagents, including the Mg/Al clay (MgAl-LDH-CO3), were obtained from Aldrich and used as
received. Commercial grade solvents were dried and deoxygenated by standard procedures
(dichloromethane, and dimethylformamide (dmf) over calcium hydride), distilled under nitrogen,
and kept over 4 Å molecular sieves. The Zn/Al clay (ZnAl-LDH-CO3) was synthesized by the
methodology previously described [39], [Mo(CO)3X2(NCMe)2] [36] and bis(4-HOOC-phenyl)-
acenaphthenequinonediimine [40] were prepared by reported procedures. All preparations and
manipulations were handled under nitrogen inert atmosphere.
2.2. Catalysts characterization
FTIR spectra were obtained by Diffuse Reflectance (DRIFT) measurements on a Nicolet 6700 in
the 400–4000 cm−1 range with 4 cm−1 resolution. Solid state 13C NMR measurements were
performed at room temperature on a Tecmag Redstone/Bruker 300WB spectrometer operating at
75.47 MHz. Samples (~ 200 mg) were packed into 7 mm o.d. zirconia rotors, equipped with Kel-F
caps. The standard magic angle spinning (MAS) cross polarization–dipolar decoupling RF pulse
sequence (CP–DD) was used under about 3-4 kHz spinning rate. 13C NMR spectra were recorded
3

with 2 ms contact time, 90˚ RF pulses of around 4 µs, 5 s recycling delay and a number of scans
higher than 900. The Hartmann–Hahn condition was optimized using glycine, which is also used as
the external reference to set the chemical shift scale (13CO at 176.03 ppm). Powder XRD
measurements were taken on a Philips Analytical PW 3050/60 X’Pert PRO (theta/2 theta) equipped
with X’Celerator detector and with automatic data acquisition (X’Pert Data Collector (v2.0b)
software), using a monochromatized Cu-Kα radiation as incident beam, 40 kV − 30 mA. TGA
studies were performed using a Netzsch F3 Jupiter system at ISIS, Neutron & Muon Source,
Rutherford Appleton Laboratory, UK. The measurements were done in the 300–1073 K temperature
range using a heating rate of 10 K min−1 under N2 atmosphere. Microanalyses for CHN and Mo
quantitation were performed at CACTI, University of Vigo. CHN analyses were performed on a
Fisons EA 1108; Mo quantification was performed on a Perkin Elmer Optima 4300DV using In as
internal standard.
2.3 Catalytic tests
The materials were tested in epoxidation of olefins and alcohols, namely cis-cyclooctene, styrene,
R-(+)-limonene, 1-octene and trans-hex-3-en-1-ol, using tert-butylhydroperoxide (tbhp) as oxidant
(5.5 M in n-decane). The catalytic oxidation tests were carried out at 383 K, using toluene as
solvent. The reactions occurred under air in a reaction vessel equipped with a magnetic stirrer and a
condenser. In a typical experiment, the vessel was loaded with catalyst (1 mol% for complexes and
150 mg for materials), olefin or alcohol (100 mol%), oxidant (200 mol%) and 3 mL of solvent. The
final volume of the reaction was ca. 6 mL. The addition of the oxidant determined the initial time of
the reaction. Conversion, product yields and stereochemistry were monitored by sampling
periodically and analyzing the products on a Shimadzu QP2010-Plus GC/MS system and a capillary
column (Teknokroma TRB-5MS, TRB-1MS) operating in the linear velocity mode. Recycling tests
were carried out as described above using the recycled materials as catalyst; conversion and product
yields were monitored as described above. After each cycle (24 h) the catalyst was filtered, washed
which CH2Cl2 several times and dried prior to reuse in a new catalytic cycle.
2.4 Synthesis of catalysts
2.4.1 Synthesis of complexes
[Mo(CO)3X2(H2BIAN)] (X=Br, I)
A solution of the ligand H2BIAN (421 mg, 1.0 mmol) in acetonitrile was added to a stirred solution
of [Mo(CO)3X2(NCMe)2] (X=Br, I) (1.0 mmol) in 10 mL of acetonitrile. The mixture was left
4

stirring in inert atmosphere at room temperature for 18 hours. The precipitate formed was filtered,
washed with dichloromethane and dried under vacuum.
[Mo(CO)3I2(H2BIAN)]
Yield: 626 mg, 72.8 % C29H16I2MoN2O7 (854.195 g.mol-1)
Elemental analysis (%): Exp C 40.78, N 3.28, H 1.89; Found C 40.74, N 3.20, H 1.61
FTIR (KBr υ / cm-1): 3425 (m), 3332 (m), 3321 (m), 2015 (m), 1932 (s), 1927 (s), 1711 (vs), 1618
(vs), 1438 (s).
1H NMR (400,10 MHz, DMSO, 298 K, δ ppm TMS): 6.58 (d, 4H, H10), 7.64 (d, 4H, H11), 7.93 (t,
2H, H3), 8.09 (d, 2H, H2), 8.44 (d, 2H, H4), 10.90 (s, 1H, H13), 11.30 (s, 1H, H13).
13C NMR (100.61 MHz, DMSO, 298 K, δ ppm TMS): 112 (C10), 116 (C1), 127 (C3), 129 (C11),
131 (C4), 150 (C9), 163 (C7), 166 (C12), 186 (C13).
[Mo(CO)3Br2(H2BIAN)]
Yield: 645 mg, 85.0 % C29H16Br2MoN2O7 (760.194 g.mol-1)
Elemental analysis (%): Exp: C 45.98, N 3.99, H 1.86; Found C 45.86, N 3.44, H 2.25.
FTIR (KBr υ / cm-1): 3420 (m), 3338 (m), 3321 (m), 2013 (m), 1930 (s), 1927 (s), 1708 (vs), 1620
(vs), 1435 (s).
1H NMR (400,10 MHz, DMSO, 298 K, δ ppm TMS): 6.60 (d, 4H, H10), 7.62 (d, 4H, H11), 7.90 (t,
2H, H3), 8.09 (d, 2H, H2), 8.40 (d, 2H, H4), 10.92 (s, 1H, H13), 11.28 (s, 1H, H13).
13C NMR (100.61 MHz, DMSO, 298 K, δ ppm TMS): 112 (C10), 116 (C1), 127 (C3), 129 (C11),
131 (C4), 150 (C9), 163 (C7), 166 (C12), 186 (C13).
2.4.2. Preparation of MgAl-LDH material
The commercial MgAl-LDH layered material was calcined at 823 K for 4 hours, before the
intercalation experiments, to eliminate carbonate anions in the interlayer region [30]. This
calcination step leads to the collapse of the layers, but the deprotonated ligand BIAN in dmf, at 343
K, is able to intercalate, forcing the reconstruction of the intergallery space [32].
2.4.3. Preparation of ZnAl-LDH material
The synthesis was carried out in a two-neck flask (equipped with a reflux condenser) under a
nitrogen flow. In a typical procedure, Zn(NO3)2·6H2O, Al(NO3)3·9H2O, and urea were dissolved in
1 L of deionized water to give the final concentrations of 10, 5, and 35 mM, respectively. Then, the
mixed solution was heated at the refluxing temperature (about 370 K) under continuous magnetic
5

stirring for 2 days. The resulting white solid product was filtered, washed with deionized water and
anhydrous ethanol several times, and finally air-dried at room temperature, yielding the ZnAl-LDH
-CO3 material. Decarbonation was carried out by treatment with a salt-acid mixed solution.
Typically, 1.0 g of the LDH material was dispersed into 1 L of a 1 M NaCl and 3.3 mM HCl
aqueous solution. The reactor was sealed after purging with N2, and then stirred for 24 h. The
resulting Cl-exchanged material, ZnAl-LDH-Cl, was filtered, washed several times with deionized
water and anhydrous ethanol, and finally air-dried at room temperature. The subsequent ion
exchange of Cl by OH, using a 1 M NaOH aqueous solution, led to ZnAl-LDH-OH.
2.4.4. Synthesis of MAl-LDH materials-BIAN (M = Zn, Mg)
A mixture of BIAN (7.30 mmol) in deionized type II water (20 mL) with KOH (7.3 mmol) was
stirred until complete solubilization. This solution was then added to a suspension of the LDH
(ZnAl-LDH or MgAl-LDH) clay (1.00 g) in freshly distilled dmf (25 mL) at 343 K, and the reaction
mixture was stirred at the same temperature for 48 h.
All the manipulations were carried out under a dry N2 atmosphere to prevent the uptake of
carbonate anions. The resulting materials were then filtered off, washed with deionized type II
water (3x20 mL), and dried in a desiccator under vacuum.
ZnAl-LDH-BIAN
Elemental analysis (%): found C 20.52, H 1.50, N 0.85.
DRIFT (KBr ν/cm-1): 3448 (vs), 1730 (s), 1600 (vs), 1542 (m), 1500 (m), 1344 (m), 1283 (s), 1097
(m), 775 (w).
XRD (2θ, planes): 4.58 (003), 9.66 (006), 11.09 (003*), 15.27 (009).
MgAl-LDH-BIAN
Elemental analysis (%): found C 20.05, H 3.07, N 1.30.
DRIFT (KBr υ / cm-1): 3448 (vs), 1728 (m), 1611 (s), 1542 (w), 1500 (m), 1365 (w), 1097 (w), 773
(w).
XRD (2θ, planes): 4.25 (003), 8.73 (006), 12.29 (009).
2.4.5 Synthesis of LDH materials-BIAN-MoX2
[Mo(CO)3X2(NCMe)2] (0.3 g) were added to a suspension of MAl-LDH-BIAN (0.75 g) in
dichloromethane (10 mL). The mixture was stirred under a N2 atmosphere, at room temperature, for
24 h. The resulting material was then filtered off, and dried in a desiccator under vacuum.
6

ZnAl-LDH-BIAN-MoI2
Elemental analysis (%): found C 8.78, H 1.07, N 0.5. Mo 3.74.
DRIFT (KBr υ / cm-1): 3445 (vs), 1705 (s), 1590 (vs), 1542 (w), 1500 (m), 1344 (m), 1283 (s), 1097
(w), 773 (w).
XRD (2θ, planes): 3.86 (003), 8.93 (006), 15.10 (009).
ZnAl-LDH-BIAN-MoBr2
Elemental analysis (%): found C 17.68, H 1.76, N 0.28. Mo 9.52.
DRIFT (KBr υ / cm-1): 3432 (vs), 1944 (vs), 1901 (vs), 1794 (m), 1717 (s), 1590 (s), 1542 (w),
1511 (w), 1363 (m), 1280 (s), 1097 (w), 769 (w).
XRD (2θ, planes): 4.46 (003), 9.61 (006), 15.59 (009).
MgAl-LDH-BIAN-MoI2
Elemental analysis (%): found C 9.25, H 2.79, N 0.85, Mo 7.72.
DRIFT (KBr υ / cm-1): 3446 (vs), 2004 (w), 1915 (w), 1707 (m), 1589 (s), 1542 (w), 1500 (m),
1394 (w), 1097 (w), 765 (w).
XRD (2θ, planes): 4.34 (003), 7.83 (006), 11.86 (009).
MgAl-LDH-BIAN-MoBr2
Elemental analysis (%): found C 12.22, H 2.69, N 1.01, Mo 6.45.
DRIFT (KBr υ / cm-1): 3410 (vs), 1705 (m), 1611 (s), 1542 (w), 1500 (m), 1398 (w), 1097 (w), 779
(w).
XRD (2θ, planes): 4.34 (003), 7.80 (006), 11.25 (009).
Results and Discussion
The precursor complexes [Mo(CO)3X2(NCMe)2] (X = I, Br)[36] reacted with the H2BIAN ligand in
a 1:1 ratio, to afford [MoI2(CO)3(H2BIAN)] and [MoBr2(CO)3(H2BIAN)], respectively (Scheme 1),
by substitution of two NCMe ligands. The 1H NMR spectra of [Mo(CO)3X2(H2BIAN)] (X = I, Br)
show clear differences from those of the precursor complexes [Mo(CO)3X2(NCMe)2], namely the
absence of a resonance at ca. 2.0 ppm assigned to the acetonitrile ligands. After coordination, the
signals of the ligand in the 1H and 13C NMR spectra are shifted from those of the original free
ligand. The vibrational spectroscopy was also useful to confirm the proposed formulation of the
complexes, since in both [Mo(CO)3I2(H2BIAN)] and [Mo(CO)3Br2(H2BIAN)] the strong
absorptions of the C≡O modes are clearly observed confirming the presence of the
7

[Mo(II)(CO)3X2] core, while the C≡N modes are not observed reflecting the displacement of the
labile NCMe ligands from [Mo(CO)3X2(NCMe)2] (X = I, Br).
New heterogeneous metal catalysts were obtained from two clay-based hybrid materials after
immobilization of the BIAN ligand and further reaction with each Mo(II) precursor, as shown in
Scheme 2.
Scheme 2. Schematic representation of the synthesis pathways.
The first material, ZnAl-LDH, was synthesized by a co-precipitation method as reported [39],
leading to the carbonate clay (ZnAl-LDH-CO3), which after ion-exchange with chloride anions
yields ZnAl-LDH-Cl. Finally, chloride was exchanged with hydroxyl anions affording the ZnAl-
LDH-OH material. This lengthy procedure makes it easier to prepare the organic-LDH materials by
a further ion exchange. Therefore, introduction of the BIAN ligand is achieved by anion exchange
with hydroxide, in the presence of KOH,[39] yielding the ZnAl-LDH-BIAN material. Based on the
N-content from elemental analysis, the loaded amount of ligand is calculated as 0.3 mmol·g-1 in
ZnAl-LDH-BIAN. The high content of hydrogen found maybe probably explained by the uptake of
OH groups. In this way, on the basis of the amount of nitrogen for the material, the contents of
BIAN ligand incorporated was 25% of the clay total ion exchange capacity, the rest being fulfilled
by hydroxide anions from the solution [26]. In this way, the composition of the intercalated clay can
be rationalized as [Zn6Al2(OH)16](BIAN)0.25(OH)1.75. The high OH– content in the final hybrid clay
may be explained by the too large size of the BIAN ligand (compared to OH–), which makes it hard
8

to incorporate. This conclusion is corroborated by the powder diffraction pattern, showing a small
peak identified as corresponding to the ZnAl-LDH-OH at 2 = 11.09 .
Reaction with [Mo(CO)3X2(NCMe)2] (X = I, Br) afforded the new ZnAl-LDH-BIAN-MoX2 (X = I,
Br) hybrid materials. The Mo loading was found to be 3.74 wt.% corresponding to 0.39 mmolMo·g-1
and 9.52 wt.% corresponding to 1.00 mmolMo·g–1 for the I and Br containing materials,
respectively.
The synthetic approach to obtain the MgAl-LDH materials was based on the reconstruction method,
by taking advantage of the memory effect displayed by the hydrotalcite material. In this way, the
MgAl-LDH lamellar material was first submitted to a calcination process at 823 K for 4 h to
eliminate the CO32− anions from the interlayer region, as described in a previous work [39]. During
this process, the structure of the lamellar material collapses, and is thereafter rebuilt when the
deprotonated BIAN ligand is intercalated inside the interlayer space in dmf at 343 K [26,41]. The
deprotonation of H2BIAN is carried out in deionized water by KOH and the resulting solution is
then added to the MgAl-LDH suspension leading to the preparation of the composite MgAl-LDH-
BIAN material. Results from elemental analyses for this material show that the N content (1.30%)
yields a ligand loading of 0.46 mmol·g-1, corresponding to 27% of the clay total ion exchange
capacity. This allowed the clay composition to be rationalized as
[Mg6Al2(OH)16](BIAN)0.27(OH)1.73. This formula is very similar to the Zn-based one (see above)
with a high content of OH– anions. This suggests that the same factors must be responsible for the
small uptake of the BIAN ligand in the interlayer space.
After being dehydrated for some time the MgAl-LDH-BIAN hybrid clay materials reacted with the
precursor complexes [Mo(CO)3X2(NCMe)2] (X= I, Br) in dry CH2Cl2 during 24 h at room
temperature, under inert atmosphere, affording the new hybrid materials MgAl-LDH-BIAN-MoI2
and MgAl-LDH-BIAN-MoBr2 after filtering and drying. The molybdenum loadings for the
composite materials were found to be 7.72 wt.% and 6.45 wt.% (0.81 mmolg−1 and 0.68 mmolg−1),
respectively, showing similar values to those reported earlier for related materials [26].
The XRD powder patterns obtained for these lamellar materials are typical of this type of
reasonably well-ordered materials, exhibiting sharp and symmetric 003 reflections (Figures 1 and 2)
[12]. The XRD powder patterns of materials ZnAl-LDH-BIAN and MgAl-LDH-BIAN are similar
to those of the parent material with the 003 reflection at lower angles. The calculated distances
between layers (d) for ZnAl-LDH-BIAN and MgAl-LDH-BIAN are 19.3 and 20.4 Å, respectively.
Considering the thickness of the LDH sheets to be 4.8 Å,[9,10] the estimated gallery heights of
ZnAl-LDH-BIAN and MgAl-LDH-BIAN become 14.5 Å and 15.6 Å, respectively. On the other
hand, the BIAN molecule fits a box with 14.4 × 12.1 × 8.3 Å dimensions (Scheme 2), suggesting
that BIAN will most probably occupy a near vertical position inside the gallery. This arrangement is
9

the expected one, which approaches the negative carboxylate groups from the positively charged
LDH sheet, maximizing their electrostatic interaction and suggesting that most of the immobilized
complexes should be inside the materials.
Figure 1. XRD powder patterns of materials ZnAl-LDH-BIAN, ZnAl-LDH-BIAN-MoI2
and ZnAl-LDH-BIAN-MoBr2.
After treatment of the materials ZnAl-LDH-BIAN and MgAl-LDH-BIAN with complexes
[MoX2(CO)3(NCMe)2] (X= I, Br), the positions of the 003 reflections change towards wider
openings, reflecting the expansion required by the coordination of the Mo organometallic cores to
the intercalated BIAN ligand. Furthermore, the intensity of the 003 reflections is significantly
reduced in all the obtained materials. The presence of the Mo-centers in high loading (ca. 3.7–9.5
wt.%) at the midpoint of the interlayers may be responsible for a marked increase in the electron
density, as reported elsewhere [6c,26,42].
Figure 2. XRD powder patterns of MgAl-LDH-BIAN, MgAl-LDH-BIAN-MoI2
and MgAl-LDH-BIAN-MoBr2 materials.
10

The FTIR spectrum (Figure 3) of the ZnAl-LDH-OH host material is similar to that of other
materials of the same type. The high frequency region (4000–3450 cm−1) is dominated by broad
bands at about 3460-3356 cm−1, assigned to the O–H stretching modes of weakly hydrogen bonded
water molecules and layer hydroxyl groups. Some carbonate groups, from the air and humidity,
may be responsible for the band at 1344 cm−1. For the calcined material MgAl-LDH (Figure 4),
bands assigned to the O-H from water and hydroxyl groups were observed at 3437 cm−1, while the
band corresponding to some carbonate anions inside the material appears at 1375 cm−1.
Figure 3. DRIFT spectra of ZnAl-LDH materials.
The derivatized materials ZnAl-LDH-BIAN (Figure 3) and MgAl-LDH-BIAN (Figure 4) present
the band assigned to the νC=N mode of BIAN at 1730 and 1728 cm−1, respectively. Characteristic
bands of the COO− group observed at 1365 and 1352 cm−1 for both materials, respectively, indicate
the intercalation of BIAN. Treatment of the composite materials ZnAl-LDH-BIAN and MgAl-
LDH-BIAN with the organometallic complex [Mo(CO)3X2(NCMe)2] (X= I, Br) led to the new
materials ZnAl-LDH-BIAN-MoX2 (X = I, Br) and MgAl-LDH-BIAN-MoX2 (X = I, Br). In ZnAl-
LDH-BIAN-MoX2 (X = I, Br) (Figure 3), the metal carbonyl groups νC≡O stretching vibrations
were observed at 1944 cm−1 and 1901 cm−1 for X = Br, and at 2026 and 1959 cm-1 for X = I.
11

Figure 4. DRIFT spectra of MgAl-LDH materials.
For materials MgAl-LDH-BIAN-MoX2 (X = I, Br) (Figure 4), the νC≡O modes were assigned at
1915 and 2004 cm−1 for the iodide material and at 1995 and 1977 cm−1 for the bromide one. These
are hardly observed due to the small loading of the [MoI2(CO)3] cores within the interlayer space of
the composite materials. No bands that might be assigned to the νC≡N modes corresponding to the
acetonitrile ligand from the organometallic precursor were detected. The characteristic peaks of the
BIAN ligand are present in the spectra of the Mo(II) containing materials, with minimal shifts.
The 13C CP MAS NMR spectra of the lamellar materials ZnAl-LDH-BIAN and MgAl-LDH-BIAN
(Figure 5) exhibit at least two peaks below 100 ppm along with several resonances in the 120 < δ <
150 ppm range assigned to carbon atoms of the aromatic moieties, and a signal at δ ca. 187.3 ppm
and 187.7 ppm, respectively for ZnAl-LDH-BIAN and MgAl-LDH-BIAN, attributed to the
carboxylate carbon atoms. This result indicates that the species inside the interlamellar space of the
clay material is the deprotonated amino acid ligand.
12

Figure 5. 13C CP-DD NMR spectra for materials ZnAl-LDH (*denotes spinning side-bands).
The signals observed for material ZnAl-LDH-BIAN are similar to those found in solution for the
carbon atoms of the H2BIAN ligand, being assigned as δ 187.0 (COO−), 176.3, 170.9, 167.2, 163.4,
133.5, 128.0, 125.0, and 121.2 ppm.
Figure 6. 13C CP-DD NMR spectra for materials MgAl-LDH (*denotes spinning side-bands).
13

The material MgAl-LDH-BIAN (Figure 6) exhibits three peaks at δ ~ 170.0-150.0 ppm, that are
attributed to carbons 9-12, and the carbons 1-7 of the BIAN ligand appear at δ ~ 135.0-110.0 ppm.
In the Mo-containing materials ZnAl-LDH-BIAN-MoX2 (X = I, Br), no resonances of the
acetonitrile ligands of precursor complex are detected, and signals from the C≡O ligands are not
visible, since these nuclei have a long relaxation time. On the other hand, the signals of the BIAN
ligand exhibit little or no shift compared to the precursor material ZnAl-LDH-BIAN. Signals for
ZnAl-LDH-BIAN-MoBr2 appear at 187.7 (COO−), 174.4, 169.4, 165.7, 161.8, 134.1, 129.1, 125.4,
121.6 ppm. For the material MgAl-LDH-BIAN-MoI2, only four signals are resolved, namely the
signals at 187.4 (COO−), 171.4, 168.2, and 125.1 ppm.
Figure 7 depicts the thermogravimetric analyses (TGA) of the synthesized metal containing clay
materials, MgAl-LDH-BIAN-MoX2 and LDHZn/Al-BIAN-MoX2 (X = Br, I). All materials
evidenced similar overall mass loss profiles confirming the presence of several components. In this
way, all materials displayed a first mass loss due to water (till ca. 500 K), followed by a second
major mass loss till ca. 800 K accounting for release of the BIAN ligand and MoX2 moieties. The
remaining mass loss observed above that temperature was most probably due to dehydroxylation of
the clay structure [43].
Figure 7. TGA profiles of ZnAl-LDH-BIAN-MoX2 (top) and MgAl-LDH-BIAN-MoX2 (bottom)
materials (X = I, Br).
14

Catalytic studies on epoxidation of olefins
The activity of the Mo derivatized materials, ZnAl-LDH-BIAN-MoX2 and MgAl-LDH-BIAN-
MoX2 (X = I, Br), and of the complexes [Mo(CO)3X2(H2BIAN)] (X = I, Br) as catalyst precursors
for olefin epoxidation was tested in order to compare the performance of heterogeneous vs.
homogeneous systems. The liquid-phase epoxidation of olefins was investigated on substrates cis-
cyclooctene (cy8), styrene (sty), R-(+)-limonene (R-lim), 1-octene (1-oct), and trans-hex-3-en-1-ol
(trans), with tert-butylhydroperoxide (tbhp) as the oxygen source, at 383 K, in air, and 3 mL of
toluene as solvent (Table 1).
No reaction took place neither without metal containing catalyst nor in the presence of a catalyst
without tbhp. All homogenous catalysts exhibited initial kinetic rates very high in the case of
cis-cyclooctene and R-(+)-limonene but the results are significantly different for the heterogeneous
catalysts. The observed conversions are diverse ranging from ca. 17% to as much as 100%,
stressing the efficiency of some of the Mo(II) based catalysts. Recycling experiments were also
carried out for all the clay materials with all the substrates. The performance after 48 h reaction was
found to be similar in most tests to the first 24 h. All catalytic systems led to oxidation products,
though in moderate to higher yields as evidenced in Table 1.
All materials and complexes catalyzed selectively the oxidation of cis-cyclooctene to the
corresponding epoxide, without formation of any by-products (Table 1, entries 1-6). The
conversions reached 100% globally. Exceptions were found for the Mg/Al materials, with 72 and
90% in the first and second runs, respectively, with the Br derivative. The I derivative reached 98%
in the first run and 100% in the second one. The recyclability of these catalysts was evaluated for
cis-cyclooctene (Table 1, entries 1-6, Figures 8 and 9). The performance was the same or better in
the second run. This may be related to the activation of the catalyst, which may reach its optimal
conditions in the second run [44]. Indeed, the Mo(II) catalyst precursor undergoes an initial
oxidation step.
The reaction kinetics between the two (Figure 9) clay materials were a bit different between
themselves. Both MgAl-LDH materials (Figure 8) exhibited a slow beginning, more pronounced for
the I containing material than for the Br. On the other hand, the ZnAl-LDH clay with I reached
100% conversion in the first minutes of the reaction, the second run being only marginally faster
than the first. In the presence of the immobilized Br complex, 100% conversion was achieved after
~8 hours in the first run and ~1 hour in the second. This kinetic behavior was similar to others
found in the literature for related systems, based on complexes with different bidentate ligands
supported in the same clays.[28,29]
15

Table 1. Oxidation of cis-cyclooctene (cy8), styrene (sty), 1-octene (1-oct), trans-hex-3-en-1-ol
(trans) and R-(+)-limonene (R-lim) catalyzed by all the catalysts.
Conv. % a,b
Entry
Substrate
cy8
Catalyst
Select. %
100
1
2
3
4
5
6
[Mo(CO)3I2(H2BIAN)]
[Mo(CO)3Br2(H2BIAN)]
ZnAl-LDH-BIAN-MoI2
ZnAl-LDH-BIAN-MoBr2
MgAl-LDH-BIAN-MoI2
MgAl-LDH-BIAN-MoBr2
100
100
100
100/100d
100/100d
72/90d
98/100d
100/100d
100/100d
100/100d
100/100d
7
sty
[Mo(CO)3I2(H2BIAN)]
[Mo(CO)3Br2(H2BIAN)]
ZnAl-LDH-BIAN-MoI2
ZnAl-LDH-BIAN-MoBr2
MgAl-LDH-BIAN-MoI2
MgAl-LDH-BIAN-MoBr2
100
50
8
100
40
100/100d
100/100d
100/100d
98/100d
63/55d, e
62/58d, e
73/73d, e
78/77d, e
9
10
11
12
13
14
15
16
17
18
1-oct
trans
R-lim
[Mo(CO)3I2(H2BIAN)]
[Mo(CO)3Br2(H2BIAN)]
ZnAl-LDH-BIAN-MoI2
ZnAl-LDH-BIAN-MoBr2
MgAl-LDH-BIAN-MoI2
MgAl-LDH-BIAN-MoBr2
30
99
86
99
81/89d
32/43d
32/57d
17/60d
92/98d
99/99d
81/93d
93/100d
19
20
21
22
23
24
[Mo(CO)3I2(H2BIAN)]
[Mo(CO)3Br2(H2BIAN)]
ZnAl-LDH-BIAN-MoI2
ZnAl-LDH- BIAN-MoBr2
MgAl-LDH-BIAN-MoI2
MgAl-LDH-BIAN-MoBr2
40
74
100
100
23/98d
45/63d
39/43d
100/100d
100/99d
100/100d
75/89d
100/100d
25
26
27
28
29
30
[Mo(CO)3I2(H2BIAN)]
[Mo(CO)3Br2(H2BIAN)]
ZnAl-LDH-MoI2
100
96
98
96
94/98d
80/85d
64/82d
79/83d
97/98d
89/95d
93/91d
93/95d
ZnAl-LDH-MoBr2
MgAl-LDH-BIAN-MoI2
MgAl-LDH-BIAN-MoBr2
aAll reactions were carried out in the presence of 200 mol % oxidant (tbhp), 1 mol % of Mo catalyst and 100 mol % of substrate.
bCalculated after 24 h, unless otherwise stated.
cCalculated as ‘‘Yield of epoxide”/‘‘Conversion” X 100%.
d1 run/2 run.
eBenzaldehyde formed as by-product.
16

Figure 8. Kinetics for the 1st and 2nd runs of cy8 epoxidation catalyzed by materials MgAl-LDH-
BIAN-MoI2 (top) and MgAl-LDH-BIAN-MoBr2 (bottom).
17

Figure 9. Kinetics for the 1st and 2nd runs of cy8 epoxidation catalyzed by materials ZnAl-LDH-
BIAN-MoI2 (top) and ZnAl-LDH-BIAN-MoBr2 (bottom).
Styrene conversion was very efficient for all the tested catalysts. Both homogeneous and
heterogeneous catalysts reached very high conversion yields for styrene, 98% being the lowest
value found (Table 1, entry 12). The selectivity for the epoxide reaches only 50% and 40% after 24
h, when the complexes [Mo(CO)3I2(H2BIAN)] and [Mo(CO)3Br2(H2BIAN)], respectively, were
used as catalysts. The heterogeneous catalysts (Table 1, entries 9-12) led to epoxide selectivity in
the range of 55 – 78% after 24 h reaction time. This is a remarkable achievement, since in all cases
high substrate conversion is also observed, showing that the use of the clays is beneficial for this
particular process. Benzaldehyde was the other product formed in competition with the epoxide,
being the major product in reactions with [Mo(CO)3Br2(H2BIAN)]. As reported before [29], styrene
oxide yield reaches a maximum during the reaction, decreasing swiftly afterwards as it converts into
benzaldehyde. This product interconversion proceeds through an oxidative cleavage mechanism
from over-oxidation of styrene epoxide. The oxidation of styrene evidences how the catalyst can
tune product selectivity, depending on whether it is in homogeneous form or in the heterogeneous
form: the [Mo(CO)3Br2(H2BIAN)] complex yields benzaldehyde and the materials prefer styrene
oxide.3
It is also remarkable that when the catalysts were the MgAl-LDH clays (Table 1), the epoxide
selectivity reached the highest values (73-78%, entries 7-12) and it even increased in the second
run. The ZnAl-LDH clay materials (Table 1) had a similar behavior but with lowest epoxide yields.
Both clays are stable heterogeneous catalysts more selective than their homogeneous counterparts.
1-Octene is an unbranched terminal olefin and therefore harder to oxidize. However, some of the
catalysts display higher to outstanding substrate conversion values reaching 86% when
[Mo(CO)3Br2(H2BIAN)] is used. The best heterogeneous catalyst, ZnAl-LDH-BIAN-MoI2, is also
remarkable in achieving 81% conversion in the first cycle and 89% in the second one. All the other
heterogeneous clay catalysts (ZnAl-LDH-BIAN-MoBr2, MgAl-LDH-BIAN-MoI2, MgAl-LDH-
BIAN-MoBr2) led to low conversions in the first cycle (17-32%) and moderate in the second cycle
(43-60%). Despite this, epoxide selectivity remains high, reaching 99% when the complexes in
homogeneous phase are used. Selectivity is also high for the clay derivatized materials, with values
between 81-100%, which is an excellent result.
trans-Hex-3-en-1-ol is an alcohol and therefore a substrate with functional groups competing with
the C=C bond, making the results once more worth of note in what concerns both substrate
conversion and product selectivity. Indeed, the present catalysts were found to yield the epoxide
18

with outstanding selectivity (100%), except for entries 4 and 23, with 74% and 75%, respectively.
This shows that the catalytic systems reported here are very active for the epoxidation of the C=C
bond and do not affect the alcohol moiety except when the iodide derivatives were used.
The R-(+)-limonene substrate has two unsaturated C=C bonds, meaning that two different epoxides,
the endo- and exo-cyclic isomers, can be formed. The former was the only epoxide observed in all
tests (Table 1, entries 25-30) with selectivity above 89%, which is an outstanding result. This
selectivity was accompanied by very high substrate conversion rates, reaching almost 100% for the
complexes, and moderate to high conversions for the clay material, the lowest being 64% for the
first cycle and 82% in the second one for MgAl-LDH-BIAN-MoI2. The highest conversions, 94% in
the first cycle and 98% in the second cycle, were achieved by the ZnAl-LDH-BIAN-MoI2 material.
Stereoselectivity was also assessed for the R-(+)-limonene-1,2- epoxide, relevant differences being
found between homogeneous and heterogeneous catalysts (Table 2). While the former shows a clear
preference for the trans diastereomer, the same does not happen with the heterogeneous
counterparts. There is no defined preference with three of the catalysts that yield both diastereomers
roughly in a 50/50 ratio. The ZnAl-LDH-BIAN-MoBr2 has an intermediate behavior with a 28/72
ratio in favor of trans.
As has been mentioned above and reports previously, in many reactions the catalysts performance
was better (or similar) in the second run than in the first one [44]. Interestingly, the difference is
more evident in the substrate conversion than in the selectivity.
Table 2. Selectivity toward R-(+)-limonene-1,2-epoxide in the R-(+)-limonene epoxidation
catalyzed by all the catalysts.
Isomera
Catalyst
cis (%)
11
trans (%)
89
[Mo(CO)3I2(H2BIAN)]
[Mo(CO)3Br2(H2BIAN)]
ZnAl-LDH-MoI2BIAN
ZnAl-LDH-MoBr2BIAN
MgAl-LDH-MoI2BIAN
MgAl-LDH-MoBr2Bian
a Diastereomeric ratios; b 1st run / 2nd run.
12
88
42 / 32b
28 / 43b
45 / 44 b
49 / 53b
58 / 68b
72 / 57b
55 / 56b
51 / 47b
The above results can be directly compared with those obtained with a related system, differing in
the ligand 4,4’-(COOH)2-2,2’-bipyridyl [37]. In the epoxidation of cy8 and sty, the catalysts
described in this work are better or at least comparable, in some cases, both concerning conversions
19

(almost always 100% now) and selectivity (100% for both). For 1-oct, the previous catalysts
achieved better selectivity (100%) and slightly better conversions, the best heterogeneous catalyst
being the I-containing complex in the Zn/Al clay, instead of the Br complex in the Mg/Al clay. The
reactions with trans showed a worse performance of all the I-containing new catalysts, with
conversions dropping from 91-100 to 23-40%, and for Br complex in the Zn/Al clay, the other two
remaining similar. The selectivity was in general similar. The epoxidation of R-lim was achieved
very similarly by the two complexes with bpy and H2BIAN and the Br complex in Zn/Al. The I
complex in Mg/Al showed higher conversion (same selectivity) with the bpy ligand, while the
opposite was observed for the other two materials (better with BIAN).
The differences between the two systems are two small and random in general, with the exception
of cy8 and sty, for which the substitution of bpy by BIAN constitutes an improvement of the
catalyst.
Conclusions
The two ZnAl-LDH and MgAl-LDH materials were functionalized with BIAN, the dianion of
bis(4-HOOC-phenyl)-acenaphthenequinonediimine (H2BIAN) and in a second step with
[Mo(CO)3X2(NCMe)2] (X = I, Br). These materials were tested in the olefin epoxidation reaction in
the presence of tbhp, and compared with the related homogeneous complexes
[Mo(CO)3I2(H2BIAN)] and [Mo(CO)3Br2(H2BIAN)]. They all practically convert 100% of cy8 with
100% selectivity toward the epoxide and 100% of sty with less selectivity, being therefore more
efficient than the analogous catalysts described in a previous work, with a HCOO derivatized 2,2’-
bipyridyl ligand. Some of the new catalysts exhibit good performance for specific substrates, being
comparable in average with the previously studied system.
Acknowledgments
The authors thank Fundação para a Ciência e a Tecnologia (UID/MULTI/00612/2013 and
UID/QUIM/00100/2013) for financial support, M.J.F. for grant RECI/QEQ-QIN/0189/2012 and
M.J.C. for grant SFRH/BSAB/135473/2017.
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Graphical Abstract
22

Highlights
 Acenaphthenequinonediimine Mo(II) complexes were intercalated in Layered Double
Hydroxides.
 The complexes and materials were active in epoxidation of several olefins.
 The materials competed effectively with the complexes in the epoxidation of most
substrates.
23