New molybdenum(II) complexes with α-diimine ligands: Synthesis, structure, and catalytic activity in olefin epoxidation

Article abstract of DOI:10.3390/molecules24030578

Three new complexes [Mo(η³-C₃H₅)Br(CO)₂{ⁱPrN=C(R)C₅H₄N}], where R = H (IMP = N-isopropyl 2-iminomethylpyridine), Me, and Ph, were synthesized and characterized, and were fluxional in solution. The most interesting feature was the presence, in the crystal structure of the IMP derivative, of the two main isomers (allyl and carbonyls exo), namely the equatorial isomer with the Br trans to the allyl and the equatorial with the Br trans to one carbonyl, the position trans to the allyl being occupied by the imine nitrogen atom. For the R = Me complex, the less common axial isomer was observed in the crystal. These complexes were immobilized in MCM-41 (MCM), following functionalization of the diimine ligands with Si(OEt)₃, in order to study the catalytic activity in olefin epoxidation of similar complexes as homogeneous and heterogeneous catalysts. FTIR,¹³C- and²⁹Si-NMR, elemental analysis, and adsorption isotherms showed that the complexes were covalently bound to the MCM walls. The epoxidation activity was very good in both catalysts for the cis-cyclooctene and cis-hex-3-en-1-ol, but modest for the other substrates tested, and no relevant differences were found between the complexes and the Mo-containing materials as catalysts.

Full text of DOI:10.3390/molecules24030578

molecules
Article
New Molybdenum(II) Complexes with α-Diimine
Ligands: Synthesis, Structure, and Catalytic Activity
in Olefin Epoxidation
1
1
1
2
2
Maria Vasconcellos-Dias , João Marreiros , Rita Sales , Vitor Félix , Paula Brandão ,
Carla D. Nunes 1 and Maria José Calhorda
,3
1,4,
*
1
Centro de Química e Bioquímica, DQB, Faculdade de Ciências, Universidade de Lisboa, Campo Grande,
749-016 Lisboa, Portugal; mariavasconcellosdias@gmail.com (M.V.-D.); jtvrmarreiros@gmail.com (J.M.);
1
ritansales@gmail.com (R.S.); cmnunes@fc.ul.pt (C.D.N.)
Department of Chemistry, CICECO—Aveiro Institute of Materials, University of Aveiro,
2
3
4
3
810-193 Aveiro, Portugal; vitor.felix@ua.pt (V.F.); pbrandao@ua.pt (P.B.)
Centro de Química Estrutural, DQB, Faculdade de Ciências, Universidade de Lisboa,
049-001 Lisboa, Portugal
1
BioISI—Biosystems & Integrative Sciences Institute, DQB, Faculdade de Ciências, Universidade de Lisboa,
Campo Grande, 1749-016 Lisboa, Portugal
*
Correspondence: mjc@fc.ul.pt; Tel.: +351-21-750-0196
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Agustín Galindo del Pozo and Francisco J. Montilla Ramos
Received: 9 January 2019; Accepted: 1 February 2019; Published: 6 February 2019
ꢀ
ꢁꢂꢃꢄꢅꢆ
Abstract: Three new complexes [Mo( 3
η
-C H )Br(CO) { PrN=C(R)C H N}], where R = H (IMP =
i
3
5
2
5
4
N-isopropyl 2-iminomethylpyridine), Me, and Ph, were synthesized and characterized, and were
fluxional in solution. The most interesting feature was the presence, in the crystal structure of the
IMP derivative, of the two main isomers (allyl and carbonyls exo), namely the equatorial isomer
with the Br trans to the allyl and the equatorial with the Br trans to one carbonyl, the position trans
to the allyl being occupied by the imine nitrogen atom. For the R = Me complex, the less common
axial isomer was observed in the crystal. These complexes were immobilized in MCM-41 (MCM),
following functionalization of the diimine ligands with Si(OEt) , in order to study the catalytic
3
activity in olefin epoxidation of similar complexes as homogeneous and heterogeneous catalysts.
FTIR, 13C- and Si-NMR, elemental analysis, and adsorption isotherms showed that the complexes
were covalently bound to the MCM walls. The epoxidation activity was very good in both catalysts for
the cis-cyclooctene and cis-hex-3-en-1-ol, but modest for the other substrates tested, and no relevant
differences were found between the complexes and the Mo-containing materials as catalysts.
29
Keywords: molybdenum; allyl complexes; isomers; crystal structures; iminomethylpyridine;
epoxidation
1
. Introduction
i
N-isopropyl 2-iminomethylpyridine ( PrN=CHC H N, IMP) is an
α
-diimine ligand with lower
5
4
0
symmetry than 2,2 -bipyridyl and, therefore, a promising candidate in the design of selective
catalysts. Several authors have described the synthesis and characterization of complexes, such
3
as [Mo(VI)O Cl (IMP)] [
1
], [Mo(II)(
η
-C H )Cl(CO) (R ,R -IMP)], or [Mo(0)(CO) (R ,R -IMP)], with
2
2
3
5
2
1
2
4
1
2
several R and R groups (Scheme 1) [
2,3
]. A few crystal structures have been determined by X-ray
1
2
diffraction for the latter. The [Mo(CO) (R ,R -IMP)] complexes with R = H, Me, Ph and R =
4
1
2
1
2
2
-X-Ph (X = COOH or SO Na) were catalyzed the polymerization of methyl methacrylate (MMA) [
4].
3
[
Mo(CO) (IMP)] was also immobilized in MCM-41 (MCM) with a spacer, and both the complex and
4
the porous material were tested in the catalytic epoxidation of cis-cyclooctene [5]. More interestingly,
Molecules 2019, 24, 578; doi:10.3390/molecules24030578
www.mdpi.com/journal/molecules

Molecules 2019, 24, 578
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[
MoO Cl (IMP)] was a catalyst in the epoxidation of several substrates, including R-(+)-limonene, but
2 2
selectivity issues were not addressed [1].
Scheme 1. N-isopropyl 2-iminomethylpyridine (IMP) ligands.
n
W(IV) complexes, [WBr (R ,R -IMP)] (R = H, R = Me, Et, Pr, tBu), were also synthesized and
4
1
2
1
2
1
their magnetic properties studied by H-NMR [
6
]. Other metals were chosen to prepare complexes,
namely Cu(I) with R = H and several R chains, which were active in MMA polymerization [
7
].
Palladium nanoparticles with IMP supported in functionalized silica were characterized and shown to
be active in Suzuki-Miyaura cross coupling reactions and hydrogenation in water [ ]. Another recent
report addressed the selective oxidative cleavage of 1-octene catalyzed by arene Ru(II) complexes
of R ,R -IMP (R = H, R = functionalized chains) [ ]. Besides catalysis, IMP ligands have also
been studied owing to the photochemical properties exhibited by some of their derivatives, namely
Os (CO) (IMP)] clusters [10].
1
2
8
9
1
2
1
2
[
3
19
We synthesized Mo(II) complexes [Mo(X) (CO) (IMP)] (X = I, Br) and studied their catalytic
2
3
activity in olefin epoxidation in the homogeneous phase and after their immobilization in MCM,
having found an improved efficiency in the immobilized catalysts [11]. Since the complexes
3
[
[
Mo(
η
-C H )X(CO) (L )] (X = halide, L = bidentate ligand) are often more active catalysts than
3
5
2
2
2
Mo(X) (CO) (L )], we decided to prepare several derivatives of IMP with R = Me or Ph, as shown in
2
3
2
1
Scheme 1 and designated as Me-IMP and Ph-IMP for facility. They were also immobilized in MCM,
and the activity of the complexes and obtained materials was studied.
2
. Results and Discussion
2
.1. Chemical Studies: Complexes
Three new molybdenum complexes (1, 2, and 3) were obtained from reaction of [Mo(η
3-
C H )X(CO) (MeCN) ] (X = Br, Cl) with ligands IMP, Me-IMP, and Ph-IMP, respectively, in
3
5
2
2
dichloromethane after 6 h (Scheme 2). All the new complexes were characterized by elemental
1
13
analysis, FTIR, H- and C-NMR spectroscopy, and complexes
diffraction (vide infra).
1 and 2 also by single crystal X-ray
Scheme 2. The three complexes (1, 2, and 3) with ligands IMP, Me-IMP, and Ph-IMP, respectively, with
carbon, and respective hydrogens, numbering scheme (NMR).
The FTIR spectra of complexes 1–3 showed the two characteristic νC≡O stretching modes of cis
−
1
−1
−1
carbonyls at 1845 and 1942 cm
(1
), 1853 and 1936 cm
(
2
), 1848 and 1924 cm
(
3
), having slightly
3
−1
shifted from those of the parent compound [Mo(
No peaks from
in complexes
η
-C H )Br(CO) (CH CN) ] (1850 and 1947 cm ).
3
5
2
3
2
−
1
ν
C
≡
N stretching mode, present in the latter complex at 2287 cm , were observed
1
–
3
. The frequency deviations of the C=Nmodes of the bidentate ligands reflect the
ν

Molecules 2019, 24, 578
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coordination to the metal. They vary from 1655 to 1647 cm 1(
−
1
), 1638 to 1593 cm
−1
(2), and 1629 and
−
1
1
586 cm (3).
1
The H-NMR spectrum of
1
shows at least two isomers in solution. Two sets of peaks for Hanti
protons at 1.25 and 1.44 ppm, two sets for the Hsyn protons at 3.95 and 4.00 ppm, and one large peak at
3
4.46 assigned to Hmeso were associated to the
η
-C H group. Three peaks at 1.09, 2.31, and 4.24 ppm
3
5
were assigned to the hydrogens of the propyl chain of the coordinated IMP ligand, and those at 7.69
(
(
H6), 7.86 (H7), 7.48 (H8), and 8.48 (H9) ppm, shifted from their position relative to the free ligand (7.87
H6), 7.61 (H7), 7.98 (H8), and 8.70 (H9) ppm), belong to the pyridine ring. The signal of the H3 atom
was observed at 8.24 ppm. Other signals were weaker and could not be completely assigned. Selected
spectra for this and the other complexes are shown in Figures S1–S5 and Table S1 in Supporting
Information (SI).
The signals for the carbon atoms in the 13C-NMR were also observed: 11.6 (C1), 23.5 (C2), and
7
2.7 (C3) for the propyl chain; 55.1–68.4 (Callyl) for the allyl; and 160.2 (C4), 152.7 (C5), 126.3 (C6), 137.7
(
C7), 127.6 (C8), 150.2 (C9) for the iminopyridine.
1
The isomers observed in the H-NMR spectrum arose from the fluxional behavior of the allyl
ligand, as sketched in Scheme 3. The two isomers present in higher amounts should be the equatorial
and the axial ones (exo) and their interconversion proceeded by allyl rotation [12
,13]. Their ratio
depended on the steric and electronic effects of the substituents.
3
Scheme 3. Main isomers of [Mo(η -C H )Br(CO) (L )] complexes.
3
5
2
2
1
The H-NMR spectrum of
2 is very similar—with two sets of three signals corresponding to
the allyl protons at 4.41 and 4.80 ppm (two multiplets), 4.26 and 4.11 ppm (two doublets), and 1.61
and 1.49 ppm (two doublets), which were assigned to Hmeso, Hsyn, and Hanti protons—, indicating
the presence of two isomers in solution. The signal of H4 was replaced by the signal of the three
methyl hydrogens.
The H and 13C{1H} NMR were studied in more detail using COSY and HMQC experiments
1
(
SI, Figures S3–S5). The COSY spectrum allowed the identification of three sets of signals, from the four
doublets (1.16, 1.25, 1.34, and 1.48 ppm Hanti), three signals (2.21, 3.14, and 3.28 ppm Hsyn), and three
multiplets (3.58, 3.76, and 3.95 ppm Hmeso). From the correlation, it was possible to assign one set for
the equatorial isomer (exo), another for the equatorial isomer (endo), and the third—comprising two
doublets, for the Hanti—for the axial isomer (Scheme 2). Indeed, the axial isomer was less symmetric
and the two Hanti were more inequivalent [14].
The crystal structures of [MoBr( 3 ) and [MoBr( 3
η -C H )(CO) (IMP)] (1 η -C H )(CO) (Me-IMP)] (2)
3 5 2 3 5 2
were determined by single crystal X-ray diffraction, and are shown in Figures 1 and 2, respectively.
Selected bond distances and angles for 1 and 2 are given together in Table 1.

Molecules 2019, 24, 578
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(
b)
(
a)
Figure 1.
Molecular structures of the isomers found in the crystal structure of
3
[Mo(η
-C H )Br(CO) (IMP)] (
3
1
) with the thermal ellipsoids drawn at the 50% probability
5
2
level: (a) axial-exo; (b) equatorial-exo.
3
Figure 2. View of the asymmetric unit of [Mo(η -C H )Br(CO) (Me-IMP)] (2) showing the exo-axial
3 5 2
binding arrangement adopted and thermal ellipsoids drawn at the 50% probability level.
◦
Table 1. Selected bond distances (Å) and angles ( ) for the two independent molecules of
3
3
[
Mo(η -C H )Br(CO) (IMP)] (1) and for [Mo(η -C H )Br(CO) (Me-IMP)] (2).
3
5
2
3
5
2
Complex Isomer
1 (Equatorial)
1 (Axial)
2
Bonds
Mo-C(1)
Mo-C(2)
Mo-C(3)
Mo-Br
Mo-N(11)
Mo-N(18)
Mo-C(4)
Mo-C(6)
2.2995(16)
2.2072(15)
2.3273(16)
2.6961(2)
2.2833(12)
2.2031(12)
1.9398(16)
1.9695(16)
73.36(4)
2.3266(16)
2.2136(14)
2.3182(16)
2.6408(2)
2.2371(12)
2.2636(12)
1.9680(16)
1.9541(16)
72.74(4)
2.3442(14)
2.2148(14)
2.3163(15)
2.71031(19)
2.2652(12)
2.2049(11)
1.9561(15)
1.9468(15)
71.89(4)
N(11)-Mo-N(18)
N(11)-Mo-Br
Angles
81.22(3)
81.49(3)
85.15(3)
N(18)-Mo-Br
C(4)-Mo-C(6)
C(4)-Mo-Br
C(6)-Mo-Br
C(4)-Mo-N(11)
C(4)-Mo-N(18)
C(6)-Mo-N(11)
C(6)-Mo-N(18)
83.46(3)
80.84(7)
86.62(5)
89.00(5)
167.69(5)
103.47(6)
100.74(6)
171.03(6)
82.59(3)
78.35(6)
168.52(5)
96.79(4)
100.68(5)
87.28(6)
166.00(5)
93.26(5)
80.67(3)
76.26(6)
166.23(4)
96.86(4)
98.56(5)
87.86(5)
165.34(5)
94.04(5)

Molecules 2019, 24, 578
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The selected bond distances and angles for
1
and
2
are given together in Table 1. In both complexes,
the allyl ligand adopted an exo conformation, with the terminal allylic carbons over the carbonyls,
as sketched in Scheme 3. The coordination geometry around the molybdenum center can be described
as pseudo octahedral, with the centroid of the allyl ligand and the two carbons from the cis-carbonyl
groups determining a fac arrangement, as previously found in many structures [14]. Moreover,
the asymmetric unit of 1 is composed of two geometric isomers with the ligands exhibiting different
spatial dispositions in the metal coordination sphere. Thus, in the axial isomer, a nitrogen donor of
IMP ligand occupied an axial coordination site (Figure 1, left side), while in the equatorial isomer,
the two nitrogen donors of this chelating ligand defined the equatorial coordination plane together
with the two carbonyl ligands (Figure 1, right side). The existence of these two isomers in the solid
1
state were in agreement with the H-NMR structural findings observed in the solution.
The Mo-N(11) and Mo-N(18) distances in the equatorial isomer were 2.2995(16) and 2.2072(15) Å,
◦
respectively. These three atoms, with an N(11)-Mo-N(18) angle of 73.36(4) , and two other carbon
atoms formed a five-membered ring. A bromine atom occupied the remaining position in the metal
coordination sphere, trans to the allyl ligand, with a Mo-Br distance 2.6961(2) Å. The Mo-N(11) and
Mo-N(18) distances increased to 2.2371(12) and 2.3266(16) Å, respectively, in the axial isomer, leading
◦
to an N(11)-Mo-N(18) angle of 72.74(4) . The Mo-N(18) distance became significantly longer when
the N(18) atom was trans to the allyl instead of trans to one CO. The bromine atom, which occupied
now a position trans to one CO, made a shorter Mo-Br bond of 2.6408(2) Å. The spatial disposition
of the ligands in complex
analogous , as indicated by the comparable bond lengths and angles presented in the Table 1 for these
two complexes.
Moreover, the values of Mo-N distances and the N-Mo-N chelating angle determined for
2 (Figure 2) was equivalent to that observed in the axial exo isomer of the
1
1
3
and
2 agreed with those found for analogous Mo(η -C H )XCO) complexes (axial isomers) with
3 5 2
bis(4-Y-phenyl)-acenaphthenequinone diimine ligands: 2.277(4) and 2.229(4) Å, respectively, and an
◦
N(11)-Mo-N(18) bite angle of 72.7(1) , for the derivative with X = Cl and Y = Cl [15]; and 2.2863(17),
2
◦
.2948(16) Å and 2.2298(16), 2.2314(16) Å and a bite angle of 73.41(6), 73.33(6) in the two enantiomers
of the unit cell of the derivative with X = SCN and Y = H [16].
2
.2. Chemical Studies: Materials
The three new complexes, 1, 2 and 3, were immobilized in MCM-41 (MCM) according to the reported
procedure [16] described in Scheme 4. The MCM-41 was synthesized by a template approach [17].
In the first step, the silylated ligands (IMP-Si, Me-IMP-Si, and Ph-IMP-Si) reacted with the MCM
surface OH groups to afford the ligand functionalized materials MCM-IMP-Si, MCM-Me-IMP-Si, and
MCM-Ph-IMP-Si. The second step consisted of the reaction between these materials and the precursor
3
complex [Mo(
η
-C H )Br(CO) (MeCN) ] with formation of three Mo-containing materials (MCM-1,
3
5
2
2
MCM-2, and MCM-3).
CHN analyses of MCM-1, MCM-2, and MCM-3 materials showed 6.01% C, 1.92% H, and 1.64%
N for MCM-1; 5.08% C, 1.90% H, and 2.03% N for MCM-2; and 14.16% C, 2.26% H, and 2.68% N for
MCM-3. Based on the N content, these results indicated that the loading of the Y-IMP ligand inside
−
1
the pores of the different materials was found to be 0.59, 0.72, and 0.96 mmol
·
g
, for the MCM-1,
MCM-2, and MCM-3 materials, respectively. In addition, the Mo contents were determined to be
4
0
.90, 4.79, and 3.45% for MCM-1, MCM-2, and MCM-3, corresponding to loadings of 0.51, 0.50, and
−
1
.36 mmolMo
·
g
, respectively. The latter results showed that all Mo cores were coordinated to the
surface-grafted ligands, and not directly to the MCM wall, as the loading of ligands was higher in all
materials, as shown in Scheme 4.
All materials were characterized by powder XRD, Diffuse Reflectance Infrared (DRIFT), and
29Si and 13
C CP MAS NMR. Sorption/desorption N isotherms were also determined for estimation
2
of textural parameters. All characterization features are consistent with those reported in related
mesoporous materials [17].

Molecules 2019, 24, 578
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Scheme 4. Procedure for the immobilization of complexes 1–3 in MCM.
The powder X-ray patterns of the materials containing the ligands IMP-Si, Me-IMP-Si, and
Ph-IMP-Si affording materials MCM-IMP-Si, MCM-Me-IMP-Si, and MCM-Ph-IMP-Si, respectively,
are presented in Figure 3 for the IMP group. The powder pattern of the calcined parent material
◦
MCM-41 clearly shows four reflections in the 2
θ
range 2–10 , indexed to the (100), (110), (200), and (210)
reflections of a hexagonal unit cell [18]. The constant of the lattice was obtained from the d value of
√
the (100) reflection (35.6 Å) and found to be a = 41.1 (2d100/ 3). The functionalization of the MCM-41
material walls with the IMP-Si ligand did not significantly change the 2θ angles characteristic of the
more important reflections, confirming that the material retained the long-range hexagonal symmetry
present in the parent MCM-41. The same trend was observed for materials MCM-Me-IMP-Si and
MCM-Ph-IMP-Si. After functionalization of the MCM-Me-IMP-Si and MCM-Ph-IMP-Si materials with
the Mo(II) core, the materials MCM-1, MCM-2, and MCM-3 still showed three reflections.
Figure 3. XRD powder patterns of materials MCM, MCM-IMP-Si, and MCM-1.

Molecules 2019, 24, 578
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There was a slight deviation of the maxima toward lower 2
θ
values, as compared to the
parent MCM. In material MCM-IMP-Si, the d100 value and the related a parameter increased with
incorporation of the ligand (d100 = 36.5 Å and a = 42.1 Å for MCM-IMP-Si) and the metal fragment (d100
=
36.2 Å and a = 41.8 Å for MCM-1). Similar results were obtained for the materials MCM-Me-IMP-Si,
MCM-Ph-IMP-Si, MCM-2, and MCM-3. Other data characterizing the materials are included in Table 2,
summarizing the evolution of their relevant textural properties until the immobilization of Mo(II)
precursor complex. The observed peak intensity reduction is common to all materials, being even
more significant in the materials with the Mo cores. This is not due to a crystallinity loss, but rather to
an X-ray scattering contrast reduction between the silica walls and the pore filling material. This has
been observed for other types of materials and has been well described in the literature [19,20].
Nitrogen sorption/desorption studies at 77 K were performed and revealed that all materials
exhibited a reversible type IV isotherm (Figure 4), typical of mesoporous solids (pore width between
2
nm and 50 nm, according to IUPAC) [21]. The calculated textural parameters (SBET and V ) of the
P
synthesized materials (Table 2) agreed with the literature data [22,23]. The nitrogen adsorption curves
displayed a steep and sharp capillary condensation/evaporation step in the parent MCM-41 at 0.35–0.4
relative pressure range, indicating a uniform pore size distribution (PSD). After functionalization with
the ligand, the isotherm of the new MCM-IMP-Si showed a much smaller step. The lower N uptake
2
reflected a decrease (Table 2) in both the volume V (57%) and surface area S
(46%). In the case
BET
P
of MCM-Me-IMP-Si, the obtained decrease was similar to that achieved for the previous material,
with changes in SBET and V of 35% and 57%, respectively. For MCM-Ph-IMP-Si, similar results
P
were also obtained. These findings indicated that the ligand immobilization on the internal silica
surface was accomplished (Figure 4, Table 2). For the materials with the Mo(II) core, the SBET and VP
decrease relative to MCM was 47% and 63%, 50% and 74%, and 47% and 65% for MCM-1, MCM-2,
and MCM-3, respectively.
(
a)
(b)
Figure 4.
(a
) Nitrogen adsorption isotherms at 77 K and (
b
) pore size distribution (PSD) curves of
MCM, MCM-IMP, and MCM-1.
These results agreed with the p/p coordinate decrease in the isotherm inflection points after
0
post-synthesis treatments [24]. Furthermore, the maximum of the PSD curve (Figure 4) determined by
the BJH method, dBJH, for MCM materials, decreased from 36.0 Å to 28.5, 26.7 Å, and from 32.8 Å to
26.6 Å (Table 2) for MCM-1, MCM-2, and MCM-3, respectively. These values were in line with those
described above for the other set of materials. Textural parameters for MCM materials (Table 2) were
also found to match values reported in the literature for related systems [24].
The DRIFT spectrum (Figure 5) of the parent MCM shows a broad band at 3600–3000 cm 1, which
−
can be assigned to the O–H stretching vibrations of hydrogen bonded silanol groups. Other important
−
1
features comprised the band at ca. 1634 cm due to OH bending modes and an intense broad band at

Molecules 2019, 24, 578
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240–950 cm 1 associated with asymmetric stretching vibration modes of the mesoporous framework
−
1
(νSi–O–Si) [25].
Table 2. Textural parameters of host and composite materials from powder XRD and N isotherms at
2
7
7 K, for all prepared materials.
2
−1
3
−1
∆Vp b/%
Materials
d100 (Å)
a (Å)
SBET/m ·g
∆SBET/%
Vp/cm ·g
dBJH/(Å)
MCM
MCM-IMP-Si
MCM-1
MCM
MCM-Me-IMP-Si
MCM-2
MCM
MCM-Ph-IMP-Si
MCM-3
35.6
36.5
36.2
35.6
36.2
36.0
36.0
36.7
37.2
41.1
42.1
41.8
41.1
41.8
41.6
41.6
42.4
42.9
1037
558
553
1037
674
523
1031
683
547
-
0.97
0.42
0.36
0.97
0.42
0.25
0.88
0.46
0.31
-
36.0
32.6
28.5
36.0
28.7
26.7
32.8
28.5
26.6
−46
−47
-
−35
−50
-
−57
−63
-
−57
−74
-
−33.8
−48
−47.0
−65
The introduction of the IMP-SI ligand to obtain the functionalized material MCM-IMP-Si did
not significantly change the absorption profile of the mesoporous host material, but new bands
characteristic of the ligand present inside the pores were now detected. Grafting of the IMP ligand
was monitored by probing its νC=N mode, observed in the pyridine moiety of the free IMP ligand at
1
655 cm 1. After grafting, this mode was blue shifted to 1652 cm , as expected after the formation
− −1
of silyl esters and documented in the literature [26]. After binding the molybdenum complex
3
[
ν
MoBr(
≡
η
-C H )(CO) (MeCN) ] to afford material MCM-1, two new bands corresponding to the
3
5
2
2
−
1
C O stretching modes were observed in the DRIFT spectra, at 2031 and 1934 cm , having shifted
from 1942 and 1854 cm in 1.
−
1
Figure 5. Infrared spectrum of the material MCM-1.
Additionally, the absence of the bands due to the
MeCN) ligands showed that they had been replaced by the immobilized ligand. Similar results were
ν
C
≡
N vibrational modes from the acetonitrile
(
obtained for materials with ligands Me-IMP-Si and Ph-IMP-Si, after the functionalization with the
Mo(II) core, as well as for the new materials MCM-2 and MCM-3.
1
3
29
C and Si solid-state NMR spectroscopy studies were also performed on the materials and the
13
resonances observed were assigned according to the literature [26]. C CP MAS solid-state NMR
spectra for MCM-IMP-Si and MCM-1 are presented in Figure 6. 13C CP MAS solid-state NMR spectra
for MCM-Me-IMP-Si, MCM-2, MCM-Ph-IMP-Si, and MCM-3 are not presented, nevertheless the
results were similar to those obtained for the previous ones and were in accordance with results
described in the literature.

Molecules 2019, 24, 578
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Figure 6. 13C CP MAS NMR spectra of materials MCM-IMP-Si and MCM-1.
In the 13C solid-state spectra of the IMP-Si ligand (Figure 6), three strong signals were observed
in the 0–100 ppm range, corresponding to the aliphatic carbons closer to the SiOR group of IMP-Si.
The signal at the lower
δ was attributed to the carbon linked to Si, at 9.7 ppm, while those at 21.2 and
42.6 ppm were assigned to the other two methylene carbon atoms of the aliphatic chain. Three signals
observed at 125.6, 141.2, and 148.1 ppm correspond to the aromatic carbon atoms of the IMP-Si ligand.
In general, the same signals were observed in the 13C solid-state NMR spectra of the material after
the reaction of the Mo(II) core with precursor materials. It was possible to assign the signals at 10.8,
2
2.3, and 44.0 ppm to the aliphatic part of the ligand, and those at 126.6, 141.8, and 149.1 ppm to the
aromatic part of the ligand, in MCM-1. No carbon signals from the carbonyls of this material could
be observed. Figure 7 shows the 29Si CP MAS NMR spectra for pristine MCM and the derivatized
MCM-IMP-Si and MCM-1 materials. The parent MCM material displayed three broad convoluted
resonances in the 29Si CP MAS NMR spectrum at
species of the silica framework, respectively [Q = –Si(OSi)
−
99.9 and
(OH) ]. A weak shoulder was also
−
109.8ppm, assigned to Q and Q
3
4
n
n
4
−n
2
29
observed at
also displayed two broad signals at
−
90.4 ppm, due to the Q species. The Si CP MAS spectrum of MCM-IMP-Si material
3
4
−
101.0 and
−
109.0 ppm, assigned to Q and Q organosilicon
2
species, respectively, and it was also possible to observe a small shoulder at
−
92.1 assigned to the Q
1
2
3
species. New signals corresponding to the T , T , and T species of the Si(OR) groups of the IMP-Si
3
ligand were also observed at −49.7, −57.8, and −65.4 ppm, respectively.
Figure 7. 29Si CP MAS NMR spectra for the materials MCM, MCM-IMP-Si, and MCM-1.

Molecules 2019, 24, 578
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The 29Si CP MAS spectra in Figure 7 show that reaction of MCM-IMP-Si with the organometallic
core complex Mo(II) did not significantly change the environment of Si, as expected, indicating that
the metal fragment reacted with the immobilized ligand and did not interact with the wall surface.
2
3
It was possible to assign signals at
Q , T , T and T species, respectively.
−
106.5,
−
107.8,
−
102.6,
−
67.0,
−
58.8, and
−
47.8 ppm to the Q , Q ,
4
1
2
3
The previously described characterization techniques indicated that the surface of the MCM
porous material was functionalized with the three ligands, IMP-Si, Me-IMP-Si, and Ph-IMP-Si, which
in turn reacted with the Mo(II) metal fragments to yield metal-containing materials.
2
.3. Catalytic Studies
The catalytic activity of complexes 1, 2, and 3 (Table 3) and materials MCM-1, MCM-2, and MCM-3
(Table 4) was studied in the olefin epoxidation reaction with tert-butyl hydroperoxide (tbhp) as the
oxidant. Different substrates were chosen in order to gain insight on selectivity, namely stereoselectivity:
cis-cyclooctene (cy8), styrene (sty), cis-hex-3-en-1-ol (cis), trans-hex-3-en-1-ol (trans), geraniol (ger),
1-octene (1-oct), and S-(+)-limonene (S-lim). No conversion of any substrate was observed in blank
runs without a catalyst and in the presence of an oxidizing agent.
All of the complexes were able to oxidize cy8 and cis with 99–100% conversion, and selectivity
for the respective epoxide and yield (entries 1–3, 7–9), as had been observed for many related Mo(II)
complex precursors. Very high conversions (98%) were determined for ger, but two products were
formed in ~1:1 ration, the epoxide (2,3-oxyrane) and 6,7-oxyrane (entries 13–15). Relatively high
conversions (90 and 82%) with
trans (entries 10–12). In all cases, the selectivity for the epoxide was 86–89%, but a second product
aldehyde) was also formed. 1-oct was not easily oxidized, and conversions reached 55, 43, and
2% with , and , respectively, with epoxide selectivity in the range of 96–100% (entries 16–18).
1 and 2, and medium (50%) with 3 were found in the oxidation of
(
7
1,
2
3
The second by-product (1-oct-2-ketone) was formed in very small amounts. Sty conversions did not go
beyond 40%, and two products, the epoxide and benzaldehyde, were formed in ~3:2 ratios (entries
4
–6). The differences between the activity of the three complexes were very small, suggesting that the
change in substituent (H, Me, and Ph) had negligible effect. Only for sty was complex less active
1
than the others. The complexes oxidized cis more efficiently than trans, with higher conversion and
selectivity for the epoxide, although the differences were small. On the other hand, we showed in a
0
recent work [26] that, for a series of
α
-diimine (derivatives of 2,2 -bipyridyl and 1,10-phenantroline)
3
complexes of MoBr(
η
-C H )(CO) , the selectivities were higher for cis but the conversions were lower.
3
5
2
These ligands were more symmetrical than Y-IMP, and the approach of the trans substrate to the active
species may have been more restricted than that of cis.
Table 3. Olefin epoxidation of cis-cyclooctene (cy8), styrene (sty), cis-hex-3-en-1-ol (cis), trans-hex-3-
en-1-ol (trans), geraniol (ger), and 1-octene (1-oct) catalyzed by complexes 1–3.
Conv. % b
Select.% c
Entry
Reaction a
Catalyst
Yield %
1
2
3
1
2
3
99
99
100
100
100
99
99
100
100
17/12 d
20/13 d
4
5
1
2
29
33
58
41
26/14 d
99
6
7
3
1
40
99
36
100
8
9
2
3
99
99
100
100
99
99

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Table 3. Cont.
Conv. % b
Select.% c
Entry
Reaction a
Catalyst
Yield %
1
0
1
1
90
89
80/10 e
1
2
82
86
70/12 e
1
2
3
3
1
50
98
89
49
45/5 e
50/48 f
1
49/49 f
1
1
4
2
98
50
50/48 f
5
3
98
49
g
1
1
6
7
1
2
55
43
98
96
54/1
41/2
g
18
3
72
100
72
a
All reactions were carried out in dichloromethane in the presence of 2 eq. of oxidant tert-Butyl hydroperoxide
tbhp) and 175 mg of catalyst at 328 K; calculated after 24 h; c calculated as “Yield of epoxide”/“Conversion”
b
(
100%; d benzaldehyde as by-product; e aldehyde as by-product; f Yield of 6,7-oxyrane product; and yield of
g
×
1
-oct-2-ketone product.
Table 4. Olefin epoxidation of cis-cyclooctene (cy8), styrene (sty), cis-hex-3-en-1-ol (cis), trans-hex-3-
en-1-ol (trans), geraniol (ger), 1-octene (1-oct), and S-(+)-limonene (S-lim) catalyzed by materials
MCM-1, MCM-2, and MCM-3.
Entry
Reaction a
Catalyst
Conv. % b
100/79/60 100/100/100
Select. % c
Yield %
1
2
3
MCM-1
MCM-2
MCM-3
100
100
100
100
100
100
100
33/7 d
36/9 d
4
5
MCM-1
MCM-2
40
45
82
80
6
7
MCM-3
MCM-1
85
73
12
10/75
73
100
8
9
MCM-2
MCM-3
76
53
100
100
76
53
1
0
1
MCM-1
MCM-2
75
80
0
0
0 e
0 e
1
1
2
3
MCM-3
MCM-1
75
56
0
0 e
23/33 f
1
59
28/38 f
1
1
4
5
MCM-2
MCM-3
66
23
58
50
12/11 f
13/22 h
19/23 h
5/9 h
16
17
18
MCM-1
MCM-2
MCM-3
35
42
14
38
45
34
1
2
2
9
0
1
MCM-1
MCM-2
MCM-3
83
87
89
23
24
37
3/16/30/34 g
4/17/30/36 g
7/26/28/28 g
a
All reactions were carried in dichloromethane in the presence of 2 eq. of oxidant (tbhp) and 175 mg of catalyst
b
c
100%; d benzaldehyde as
at 328 K; calculated after 24 h; calculated as “Yield of epoxide”/“Conversion”
×
by-product; e aldehyde as by-product; f yield of 6,7-oxyrane product; yield of Z-epoxide, E-epoxide, Z-limonene
g
h
alcohol, and E-limonene alcohol products; and yield of 1-oct-2-ketone product.

Molecules 2019, 24, 578
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The immobilization of the three complexes in MCM did not improve their capability as catalysts.
This result was surprising, since in a previous work dealing with another Mo(II) catalyst precursor
with the same ligand ([MoX (CO) (IMP)]), activity increased when the metal complex was supported
2
3
in MCM [26]. However, the allylic Mo(II) system described in the present work was usually a better
catalyst [27].
Epoxidation of cy8 remained very efficient when any of the three materials acted as catalysts
(entries 1–3), but conversions of cis into its epoxide became lower, even though the selectivity was
kept (entries 4–6). The conversions of ger (entries 13–15) decreased from 98% to 56, 66, and 23%
for MCM-1, MCM-2, and MCM-3, respectively, and the relative amount of the 2,3-oxyrane also was
tendentially lower for the two more active catalysts. The oxidation of trans was interesting, in the sense
that no epoxide was formed and the reaction was 100% selective for the aldehyde (entries 10–12), with
conversions slightly higher than in the homogeneous catalysts
1 and 2, and significantly higher for
3
. The heterogeneous catalysts, except MCM-2, were less active than the homogeneous ones in the
oxidation of 1-oct, and the selectivity decreased to very modest values (less than half, entries 16–18).
On the other hand, higher conversions took place for the three catalysts with sty, and the selectivity
toward the epoxide increased from ~2:1 to ~3:1(entries 4–6), with MCM-3 behaving in a different way
than the others (much less epoxide formed). Another substrate, S-lim, was tested with the materials as
catalysts. The conversions were high, in the range of 83–89% (entries 19–21), but the yield of epoxide
was very low, even considering both Z- and E-epoxides. The E- and Z-alcohols were obtained in much
higher proportion. The selectivity toward the epoxide was extremely low.
The analogous complex with 8-aminoquinoline [27], for instance, displayed a much lower
conversion in the oxidation of cis upon immobilization (14 compared with 69%), but the opposite
was observed with trans (56 and 75%), with the selectivity always at 100%. The lower activity of the
heterogeneous catalysts in general may be explained by the steric hindrance of the side chain of the
Y-IMP complexes, which seemed to be more effective in the confined pores of the material.
The catalytic performance of the heterogeneous systems was also assessed in terms of leaching
and recycling. To study the leaching, a catalytic experiment, using cy8 and MCM-1, was started and
the catalyst was filtered from the mixture after 2 h. The conversion at this point was 82% and after
2
4 h only 86% (Figure S6), while it reached 100% in the reference experiment with the catalyst present
the entire time. This showed that the reaction practically stopped when the catalyst was removed,
indicating a small amount of leaching to the homogeneous phase. Therefore, the material acted as
a heterogeneous catalyst. The recycling experiments were performed for the same system (cy8 and
MCM-1). After one 24 h cycle, the solid catalyst was separated; another load of substrate, solvent,
and oxidant was added; and the reaction was followed for another 24 h. The conversions were 100%,
7
9%, and 60% in the first, second, and third cycle, respectively, reflecting a loss of activity of ~20%
between cycles.
The Mo(II) complexes and the Mo(II)-containing materials were catalyst precursors that had to be
oxidized to Mo(VI) by tbhp in the first step to afford the catalysis active species. In this process, the allyl
and the carbonyl ligands were lost and the Mo=O and Mo-O-Mo bonds were formed, with the bidentate
ligand—here Y-IMP—remaining bound to the metal [27]. The complexes
as reflected in their NMR spectra, and, more surprisingly, the two main isomers even coexisted in
the solid for . This suggested that several isomers of the active species (see Scheme 3) were more
likely to exist in the solution than with sterically crowded ligands, such as 2,9-R -1,10-phenanthroline
1–3 were fluxional in solution,
1
2
(R = Me, tBu) [27]. This could be a possible explanation for the lack of selectivity observed in general.
The immobilization procedure may have also ended up with several supported species, which would
then be oxidized.
2
.4. Computational Studies
Density functional theory (DFT) calculations [28] were performed using the Amsterdam Density
Functional (ADF) program [29] (more details in Experimental) in order to explore the energy difference

Molecules 2019, 24, 578
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between the axial and the equatorial isomers. As described above, for the IMP ligand, the two isomers
could coexist not only in solution but also in the solid state, which was an unprecedented finding.
Indeed, the Me-IMP complex was observed in the solid state as the axial isomers. It was found that the
axial isomer was the most stable for the three complexes by small amounts, namely 1.20 for
1
, 1.38
. The meaning of these values is that the two isomers had essentially
the same energy and the crystal packing had a determining effect on the species present in the solid
state. A view of relevant packing motifs for and can be seen in Figure S7 (SI). A slight change to the
methodology, for instance by optimizing without considering the solvent, led to the equatorial isomer
−
1
for
2
, and 1.13 kcal mol for 3
·
1
2
−
1
of
1
being more stable by only 0.13 kcal
·
mol . A close look at the molecular structures (Figure S7
in SI) suggested that the addition of the Me or Ph substituent would influence the solid-state structure.
3
. Materials and Methods
All reagents were obtained from Aldrich and used as received. All the work involving
sensitive compounds was carried out using standard Schlenk techniques. Commercial-grade
solvents were dried and deoxygenated by standard procedures (i.e., THF, toluene, and benzene over
Na/benzophenone ketyl, CH Cl over CaH ), distilled under nitrogen, and kept over 4 Å molecular
2
2
2
3
sieves. The organometallic complex [Mo(
η -C H )Br(CO) (CH CN) ] [30,31] and the ligands L1–L3
3 5 2 3 2
were prepared as previously described [32]. Ligands C H NCY=N(CH ) CH (Y = H, L2 Y = CH ,
5
4
2 2
3
3
L3 Y = Ph, L4) and C H NCY=N(CH ) Si(OEt ) (Y = H, L2Si Y = CH , L3Si) were prepared as
5
4
2 3
3
3
reported [32]. MCM-41 and post synthetic derivatized materials MCM-IMP-Si, MCM-Me-IMP-Si, and
MCM-Ph-IMP-Si were synthesized as previously described, using [(C H )N(CH ) ]Br (CTAB) as the
16
33
3 3
templating agent [32]. Before the grafting experiment, physisorbed water was removed from calcined
◦ ◦ −2
540 C for 6 h under air) MCM-41 by heating at 180 C in vacuum (10 Pa) for 2 h. Moreover, hybrid
(
materials were prepared using ligands 3-chloropropyltriethoxysilane, L2Si, and L3Si as reported,
with an organic material load of ≈3% [11,33,34].
−
1
−1
FTIR spectra were obtained on a Nicolet 6700 in the 400–4000 cm range with 4 cm resolution,
using KBr pellets for complexes and DRIFT for materials. Powder XRD measurements were recorded
with a Philips Analytical PW 3050/60 X’Pert PRO (
θ/2θ) equipped with X’Celerator detector and with
automatic data acquisition (X’Pert Data Collector (v2.0b) software, Philips, Eindhoven, Netherlands),
using a monochromatic Cu K
α
radiation as the incident beam, operating at 40 kV–30 mA. XRD
◦
diffraction patterns were obtained by continuous scanning in a 2
θ
-range of 2 to 10 with 2
θ
-step size
◦
of 0.017 and a scan step time of 99.695 s. A Bruker Avance 400 spectrometer with frequencies of
4
1
13
1
13
00.13 MHz for H and 100.61 MHz for C was used to obtain H and C solution NMR spectra.
The solid-state NMR spectra were obtained on a (9.4 T) Bruker Avance 400P spectrometer at the
29
University of Aveiro by Dr. Paula Ferreira (Bruker, Billerica, MA, USA). The Si spectra were recorded
2
9
◦
with a frequency of 79.49 MHz. Si MAS NMR spectra were recorded with 40 pulses, spinning rates
of 5.0–5.5 kHz, and 60 s recycle delays and 29Si CP MAS NMR spectra with 5.5 µs 1H 90 pulse, 2 ms
contact time, a spinning rate of 8 kHz, and 4 s recycle delays. Chemical shifts are given in ppm with
tetramethylsilane (TMS) as reference. 13C solid-state NMR spectra were recorded at 125.76 MHz on a
◦
Bruker Avance 500 spectrometer (Bruker, Billerica, MA, USA). The N sorption was measured in an
2
automatic apparatus (ASAP 2010; Micrometrics). BET specific surface areas (SBET, p/p from 0.03 to
0
0
p
2
.13) and specific total pore volume, V , were estimated from N adsorption isotherms measured at
7
7 K. The PSD were calculated by the BJH method using the modified Kelvin equation, with correction
for the statistical film thickness on the pore walls [24]. The statistical film thickness was calculated
using the Harkins-Jura equation in the p/p range from 0.1 to 0.95. Microanalyses (C, N, S, H) and
0
atomic absorption for determination of molybdenum and tungsten were performed at the Institute of
Chemical and Biological Technology (Instituto de Tecnologia Química e Biológica, or ITQB) and the
University of Vigo.

Molecules 2019, 24, 578
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3
3
.1. Synthesis of [MoBr(η -C H )(CO) C H NCH=N(CH ) CH ] (Complex 1)
3 5 2 5 4 2 2 3
A solution of [MoBr( 3
added to a solution of the ligand C H NCH=N(CH ) CH (1.2 mmol, 0.177 g), which was also
η -C H )(CO) (CH CN) ] (1 mmol) in dichloromethane (10 mL) was
3 5 2 3 2
5
4
2 2
3
in dichloromethane (10 mL). The resulting mixture was allowed to stir for 6 h at room temperature
and under a nitrogen atmosphere. The solution was then filtered and concentrated by evaporation.
After the addition of pentane, a purple crystalline powder precipitated. The product was filtered and
dried under a vacuum.
Yield (η): 88 % (0.36 g).
Elemental analysis MoBrC H N O (421.14)%: Calc C 39.93, N 6.65, H 4.07; found C 37.21,
14
17
2
2
N 6.29, H 4.05.
IR (KBr pellets,
vs), 1654 (m), 1647 (m), 1599 (m), 1470 (m), 1306 (w), 1262 (s), 1251 (w), 1152 (w), 1103 (s), 1024 (s), 978
ν
cm 1): 3075 (w), 3044 (m), 2997 (m), 2967 (m), 2936 (w), 2870 (m), 1942 (s), 1845
−
(
(
m), 803 (s), 795 (s), 668 (w), 628 (w), 572 (m).
1
H-NMR (400.13 MHz, (CD ) (CO), room temp., ppm): 1.09 (t, 3H, H ), 1.25, 1.44 (Hanti), 2.31 (t,
3
2
1
2
H, H ), 2.95–4.00 (Hsyn), 4.24 (t, 2H, H ), 4.46 (Hmeso), 7.48 (t, 1H, H ), 7.69 (d, 1H, H ), 7.86 (t, 1H,
2 3 8 6
H ), 8.24 (s, 1H, H ), 8.48 (d, 1H, H ).
7
4
9
13
C{1H} NMR (100.61 MHz, (CD ) (CO), room temp., ppm): 11.6 (C1), 23.5 (C2), 55.1-68.4 (Callyl),
3
2
7
2.7 (C3), 126.3 (C6), 127.6 (C8), 137.7 (C7), 150.2 (C9), 152.7 (C5), 160.2 (C4).
3
3
.2. Synthesis of [MoBr(η -C H )(CO) C H NCCH =N(CH ) CH ] (Complex 2)
3
5
2
5
4
3
2 2
3
A solution of [MoBr( 3
added to a solution of the ligand C H NCCH =N(CH ) CH (1.2 mmol, 0.194 g), which was also
η -C H )(CO) (CH CN) ] (0.5 mmol) in dichloromethane (10 mL) was
3 5 2 3 2
5
4
3
2 2
3
in dichloromethane (10 mL). The resulting mixture was allowed to stir for 6 h at room temperature
and under a nitrogen atmosphere. The solution was then filtered and concentrated by evaporation,
yielding a pink crystalline powder. The product was filtered and recrystallized with hexane, and dried
under a vacuum.
Yield (η): 90 % (0.39 g).
Elemental analysis MoBrC H N O (435.17)%: Calc C 41.4, N 6.44, H 4.4; found C 41.15, N 6.27,
15
19
2
2
H 3.95.
IR (KBr pellets,
vs), 1593 (m), 1562 (m), 1435 (m), 1375 (w), 1323 (m), 1302 (s), 1254 (w), 1051 (w), 1026 (s), 928 (m),
ν
cm 1): 3072 (m), 3047 (w), 3021 (w), 2968 (m), 2933 (m), 2873 (w), 1936 (s), 1853
−
(
7
93 (s), 754 (s), 658 (w), 633 (w), 560 (m).
1
H-NMR (400.13 MHz, CDCl , room temp., ppm): 1.24 (t, 3H, H ), 1.49, 1.61 (Hanti), 2.25 (m, 2H,
3
1
H ), 2.58 (s, 3H, H ), 3.88 (t, 2H, H ), 4.11, 4.26 (Hsyn), 4.41, 4.80 (Hmeso), 7.53 (t, 1H, H ), 7.87 (d, 1H,
2
10
3
8
H ), 7.94 (t, 1H, H ), 8.47 (d, 1H, H ).
6
7
9
13
C{1H} NMR (100.61 MHz, CDCl , room temp., ppm): 11.9 (C1), 16.2 (C2), 22.9 (C10), 55.2, 61.5
3
(Callyl-terminal), 64.0 (Callyl-center), 72.8 (C3), 125.6 (C7), 126.2 (C6), 137.6 (C8), 150.1 (C9), 154.6 (C5),
1
65.7 (C4).
3
3
.3. Synthesis of [MoBr(η -C H )(CO) C H NCPh=N(CH ) CH ] (Complex 3)
3 5 2 5 4 2 2 3
A solution of [MoBr( 3
added to a solution of the ligand C H NCCH =N(CH ) CH (1.2 mmol, 0.269 g), which was also
η -C H )(CO) (CH CN) ] (0.5 mmol) in dichloromethane (10 mL) was
3 5 2 3 2
5
4
3
2 2
3
in dichloromethane (10 mL). The resulting mixture was allowed to stir for 6 h at room temperature
and under a nitrogen atmosphere. The solution was then filtered and concentrated by evaporation,
yielding a violet crystalline powder. The product was filtered and recrystallized with hexane, and
dried under a vacuum.
Yield (η): 85 % (0.41 g).
Elemental analysis MoBrC H N O (485.23)%: Calc C 47.03, N 5.77, H 4.36 found C 46.72,
19
21
2
2
N 5.49, H 4.01.

Molecules 2019, 24, 578
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IR (KBr pellets,
cm 1) 3106 (w), 3058 (m), 2977 (m), 2929 (m), 2874 (m), 1924 (vs), 1848 (vs), 1586
−
ν
(
s), 1554 (m), 1468 (s), 1443 (s), 1325 (s), 1257 (m), 1164 (w), 1017 (s), 953 (w), 801 (s), 756 (s), 705 (s).
1
H-NMR (400.13 MHz, (CD ) (CO), room temp., ppm): 0.73, 0.97 (2t, 6H, H ), 1.16, 1.25, 1.34, 1.48
3
2
1
(
Hanti), 2.29, 2.45 (2m, 4H, H ), 3.14, 3.28 (Hsyn), 3.58, 3.76, 3.95 (Hmeso), 4.08 (m, 2H, H ), 7.20 (t, 1H,
2
3
H ), 7.20 (d, 1H, H ), 7.52 (m, 2H, H H ), 7.65 (t, 1H, H ), 7.72 (t, 1H, H ), 8.01 (t, 1H, H ), 8.74 (d,
12
14
11 15
8
13
7
1
H, H ), 8.91 (d, 1H, H ).
6 9
13
C{1H} NMR (100.61 MHz, (CD ) (CO), room temp., ppm): 11.0 (C1), 23.2 (C2), 52.7, 53.2, 63.5
3
2
(Canti), 52.9, 53.0, 59.6 (Csyn), 62.6, 73.3 (Cmeso), 72.0 (C3), 126.6 (C11), 127.7 (C12), 128.9 (C14 C15),
1
29.0 (C8), 130.9 (C13), 138.2 (C7), 150.3 (C6), 152.2 (C9), 155.3 (C4).
3
.4. Synthesis of Materials MCM-IMP-Si, MCM-Me-IMP-Si, and MCM-Ph-IMP-Si
MCM-Ph-IMP-Si
A suspension of MCM (0.8 g) in toluene (10 mL) was added to a solution of pycaPhSi (1.13 mmol,
.437 g) in toluene (10 mL) and was heated at 375 K for 9 h with stirring. The resulting yellowish solid
was filtered, washed four times with dichloromethane (4 15 mL), and dried in a vacuum for 3 h at
23 K.
IV (KBr,
446 (w), 1398 (m), 1309 (m), 1221 (s), 1051 (vs), 953 (s), 793 (m), 696 (m), 559 (w), 463 (m).
0
×
3
ν
cm 1) 3402 (vs), 3213 (s), 2920 (s), 2006 (w), 1626 (s), 1618 (s), 1541 (m), 1466 (w),
−
1
3
.5. Synthesis of Materials MCM-1, MCM-2, and MCM-3
A solution of [MoBr( 3
-C H )(CO) (CH CN) ] (0.65 mmol) in dry toluene (10 mL) was added to
a suspension of 1 g of the material MCM-IMP-Si, MCM-Me-IMP-Si, or MCM-Ph-IMP-Si, in dry toluene
20 mL). The reaction mixture was stirred under a N atmosphere at reflux temperature overnight.
η
3
5
2
3
2
(
2
The resulting material was then filtered off, washed with CH Cl , and dried under a vacuum for 3 h.
2
2
3
MCM-[MoBr(η -C H )(CO) (IMP)] (MCM-1) (pink powder). Elemental analysis (%) found C
3
5
2
6
.01, N 1.64, H 1.92, Mo 4.9.
cm 1) 2983 (w), 2947 (w), 2902 (w;
−
ν
N-H), 2031, 1934 (m;
ν
), 1618 (s; νC=N), 1446
IR (KBr
ν
C≡O
(
m), 1396 (m), 1234 (vs), 1061 (vs), 957 (vs), 796 (s).
13
C CPMAS NMR (
δ
ppm) 9.0 (SiCH ), 17.45 (SiOCH CH ), 21.33 (CH CH CH ), 42.97 (CH N),
2
2
3
2
2
2
2
5
8.12 (SiOCH CH )
2
3
29
2
67.49 (T3),
102.34 (Q3),
109.80 (Q4) 29Si MAS NMR
Si CPMAS NMR (
δ
ppm) -58.92 (T ),
−
−
−
2
3
3
4
(δ ppm) −58.13 (T ), −68.84 (T ), −103.35 (Q ), −108.81 (Q ).
3
MCM-[MoBr(η -C H )(CO) (Me-IMP)]
(MCM-2) Elemental analysis (%) found C 5.08, N 2.03,
3
5
2
H 1.90, Mo 4.79.
cm 1) 3066 (w), 3032 (w), 2976 (m), 2924 (w) 1938, 1833, (s;
−
ν
), 1653, 1618 (vs; νC=N),
IR (KBr
ν
C≡O
1
5
495 (m), 1448 (m), 1388 (m), 1240 (vs), 1105 (vs), 798 (vs), 735 (s).
13
C CPMAS NMR (
δ
ppm) 9.53 (SiCH ), 17.64 (SiOCH CH ), 20.94 (CH CH CH ), 42.25 (CH N),
2
2
3
2
2
2
2
8.26 (SiOCH CH ).
2
3
29
1
58.62 (T2),
66.30 (T3),
102.25 (Q3),
109.85 (Q4) 29Si
Si CPMAS NMR (
δ
ppm)
−
49.83 (T ),
−
−
−
−
2
3
3
4
MAS NMR (δ ppm) −55.44 (T ), −65.51 (T ), −102.41 (Q ), −108.04 (Q ).
3
MCM-[MoBr(η -C H )(CO) (Ph-IMP)]
(MCM-3) Elemental analysis (%) found C 14.16, N 2.68,
3
5
2
H 2.26, Mo 3.45.
IV (KBr,
cm 1) 3415 (vs), 2960 (s), 2927 (s), 1936 (m), 1844 (m), 1621 (m), 1493 (vw), 1321 (w),
229 (vs), 1061 (vs), 942 (m), 791 (m), 691 (w), 550 (w), 449 (s).
Single crystal X-ray diffraction data of and were collected using a Bruker SMART
APEX II diffractometer (Bruker, Billerica, MA, USA) equipped with a CCD area detector and
graphite-monochromated Mo-K radiation ( = 0.71073 Å). Data were corrected for absorption, as
−
ν
1
1
2
α
λ
well as for Lorentz and polarization effects. The structures were solved by intrinsic phasing with
the SHELXT program [35] and refined by full-matrix least-squares using the SHELXL program [36].

Molecules 2019, 24, 578
16 of 19
All non-hydrogen atoms were refined with anisotropic thermal displacements. The hydrogen atoms
were positioned in geometrically idealized positions and refined with isotropic temperature factors
depending on the parent atom using a ridging model. The exception was the positions of the terminal
hydrogen atoms (two CH groups) of the allyl ligand, which were obtained from last difference Fourier
2
maps calculated for each complex, and subsequently refined with individual isotropic temperature
factors. Crystal data and selected refinement details are summarized in Table S2. Solid-state structures
were drawn with the Mercury software [37]. Crystal structures were deposited with Cambridge
Crystallographic Data Centre (CCDC) under the following CCDC numbers: 1888029 (
2), and can be obtained free of charge via http://www.ccdc.cam.ac.uk/datarequest/cif.
DFT calculations [38] were performed using the ADF program package [38]. The geometries were
optimized with gradient correction, using the Local Density Approximation of the correlation energy
Vosko-Wilk-Nusair) [38], and the Generalized Gradient Approximation (Becke’s [39 40] exchange
1) and 1888030
(
(
,
and Perdew’s [41] correlation functionals), without symmetry constraints. Relativistic effects were
accounted with the ZORA approximation [42]. The core orbitals were frozen for O, C, and N (1s) and S
(
1s, 2s, 2p). Triple
ζ
Slater-type orbitals (STO) were used to describe the valence shells O, C, and N
, 3d, 4f). Triple
(2s and 2p). A set of two polarization functions was added to O, C, and N, (single
ζ
ζ
STO were used to describe the valence shells of H (1s) augmented with two polarization functions
single 2s, 2p). The structures were modeled after those of compounds and described above.
Three-dimensional structures were obtained with Chemcraft [43].
(
ζ
1
2
4
. Conclusions
The structure of the new complex [Mo( 3
PrN=C(H)C H N=IMP = N-isopropyl 2-iminomethylpyridine), was particularly interesting since
i
η
-C H )Br(CO) { PrN=C(H)C H N}], where
3 5 2 5 4
i
5
4
the two isomers of this seven-coordinate and pseudo-octahedral complex were for the first
time found [44] in the crystal, and there was no particular disorder. The Me analogue,
3
i
[
Mo(
η -C H )Br(CO) { PrN=C(Me)C H N}] crystallized as the less common axial isomer, essentially
3 5 2 5 4
3
observed in Mo(
η
-C H )X(CO) complexes of bis(4-Y-phenyl)-acenaphthenequinone diimine (X = Cl,
3 5 2
Br, SCN; Y can be a variety of substituents), owing to their II-acceptor capability. In all cases analyzed,
the DFT calculated energy difference was very small and the packing forces determined the outcome.
The complexes were immobilized in MCM-41, and their activity as olefin epoxidation catalysts
remained or decreased slightly when going from the homogeneous to the heterogeneous catalyst.
Supplementary Materials: The following are available online, Figure S1: NMR spectra of
1
: 1H (bottom) and
(top), and C-NMR spectrum of
, Figure S3: Selection of the upfield region of the bidimensional COSY spectrum of complex , Figure
S4: Selection of the upfield region of the HMQC spectrum of complex , Figure S5: Selection of the downfield
region of HMQC spectrum of complex , Figure S6: Kinetic profiles for cis-cyclooctene oxidation in the leaching
experiment using material MCM-1. Figure S7: Self-assembly in the solid state of complexes (view a) and (view
b) by C-H···O bonding contacts between carbonyl groups and C-H protons from allyl ( ) or IMP ( ) ligands. Figure
S8: DFT calculated structures of the axial and equatorial isomers of complexes and , with selected distances
1
3
1
13
C (top), Figure S2: H-NMR spectra of ligand Me-IMP (center), complex
complex
2
2
3
3
3
1
2
1
2
1
,
2
3
1
13
(
Å), Table S1: Characteristic H and C-NMR resonances (
δ
/ppm) for IMP, Me-IMP, Ph-IMP, and complexes
1,
2
3
3
and 3, Table S2: Crystal data of [Mo(η -C H )Br(CO) (IMP)] (1) and [Mo(η -C H )Br(CO) (Me-IMP)] (2).
3
5
2
3
5
2
Author Contributions: Conceptualization, M.J.C.; methodology, M.V.-D.; validation, M.V.-D.; C.D.N.; and V.F.;
investigation, M.V.-D.; J.M.; R.S.; V.F.; and P.B.; writing—original draft preparation, M.J.C.; C.D.N.; and V.F.;
supervision, M.J.C.
Funding:
This research was funded by Fundação para a Ciência e a Tecnologia, Portugal,
(
UID/MULTI/00612/2013). The crystallographic studies were supported by CICECO—Aveiro Institute
of Materials (UID/CTM/50011/2019), financed by National Funds through the FCT/MEC, and co-financed by
QREN-FEDER through COMPETE under the PT2020 Partnership Agreement.
Acknowledgments: We wish to acknowledge the elemental analysis service at Instituto de Tecnologia Química
e Biológica, Universidade Nova de Lisboa, Oeiras, Portugal. M. Vasconcellos-Dias thanks FCT for a PhD grant
(
SFRH/BD/37690/2007). We acknowledge M. S. Saraiva and P. Ferreira for help with some catalytic reactions and
solid-state NMR, respectively.

Molecules 2019, 24, 578
17 of 19
Conflicts of Interest: The authors declare no conflict of interest.
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2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
CC BY) license (http://creativecommons.org/licenses/by/4.0/).
(