Well-Defined Silica Grafted Molybdenum Bis(imido) Catalysts for Imine Metathesis Reactions

Article abstract of DOI:10.1021/acs.organomet.7b00115

Novel site-isolated tetracoordinated molybdenum complexes possessing bis(imido) ligands, [( - Si-O)₂Mo( - NR)₂] (R = t-Bu, 2,6-C₆H₃-i-Pr₂), were immobilized on partially dehydroxylated silica (SiO_2-200) by a rigorous surface organometallic chemistry protocol. The newly developed materials adorned with bis(imido) functional units, which were previously exploited mainly as spectator ligands on silica-supported olefin metathesis molybdenum catalysts, are found to be efficient heterogeneous catalytic systems for imine cross metathesis under mild conditions.

Full text of DOI:10.1021/acs.organomet.7b00115

Article
pubs.acs.org/Organometallics
Well-Defined Silica Grafted Molybdenum Bis(imido) Catalysts for
Imine Metathesis Reactions
Samir Barman,† Nicolas Merle,‡ Yury Minenkov,† Aimery De Mallmann,‡ Manoja K. Samantaray,†
Frederic Le Quemener,‡ Kai C. Szeto,‡ Edy Abou-Hamad,† Luigi Cavallo,† Mostafa Taoufik,*,‡
́
́
́
́
and Jean-Marie Basset*,†
†Physical Sciences and Engineering, KAUST Catalysis Center, King Abdullah University of Science and Technology (KAUST),
Thuwal 23955-6900, Saudi Arabia
‡Laboratoire de Chimie, Catalyse, Polymer
̀ ́ ́
es et Procedes, UMR 5265 CNRS/ESCPE-Lyon/UCBL, ESCPE Lyon, F-308-43,
Boulevard du 11 Novembre 1918, F-69616 Villeurbanne Cedex, France
S
* Supporting Information
ABSTRACT: Novel site-isolated tetracoordinated molybdenum complexes
possessing bis(imido) ligands, [(Si−O)2Mo(NR)2] (R = t-Bu, 2,6-C6H3-i-
Pr2), were immobilized on partially dehydroxylated silica (SiO2‑200) by a
rigorous surface organometallic chemistry protocol. The newly developed
materials adorned with bis(imido) functional units, which were previously
exploited mainly as spectator ligands on silica-supported olefin metathesis
molybdenum catalysts, are found to be efficient heterogeneous catalytic systems
for imine cross metathesis under mild conditions.
hexacoordinated molybdenum based bis(imido) complexes of
INTRODUCTION
■
the general formula Mo(NR)2Cl2dme4a,b toward imide/imine
cross metathesis under homogeneous catalytic conditions. In
our recent research endeavor to this end, we strived to explore
the potential of a tetracoordinated bis(imido) molybdenum
based system to serve as an intermediate in imido group (
NR) transfer reactions under heterogeneous catalytic con-
ditions. In particular, we sought to immobilize the tetracoordi-
nated bis(imido) molybdenum core structure on the surface of
silica by accurate surface organometallic chemistry (SOMC)
techniques and investigate their catalytic activities. SOMC has
emerged as an unique strategy for the isolation of catalytically
relevant organometallic fragments in well-defined sites on the
surface of oxide supports,8,9 thus allowing access to a myriad of
surface complexes that are active for a wide range of catalytic
reactions including alkane metathesis, alkene metathesis,
methane nonoxidative coupling, etc.8b,9 We recently commu-
nicated catalytic imine metathesis reactions using SOMC-
developed silica-supported imido zirconium complexes, which
represent the first heterogeneous catalysts active for imido
group (NR) transfer reactions.4j Here we disclose the SOMC
preparation of two Lewis base free well-defined tetracoordi-
nated bis(imido) molybdenum catalysts, [(Si−O−)2Mo(
NR)2] (R = t-Bu (2a), 2,6-C6H3-i-Pr2 (2b)), and their full
characterization using FT-IR and solid state NMR spectrosco-
py, elemental microanalysis, and X-ray absorption spectroscopy
Over the past decades researchers in the field of chemistry have
witnessed an unparalleled impetus from metal−alkylidene-
mediated olefin metathesis for the formation of new carbon−
carbon double bonds.1 The potential of the reaction has been
overwhelmingly exploited in organic synthesis1a,b,2 and the
synthesis of polymers.1c,d,3 Furthermore, efforts have also been
made, although to a lesser extent, toward the development of
analogous transition-metal-catalyzed methods for selectively
metathesizing carbon−nitrogen double bonds (eq 1).4 The
main advantage associated with the latter process in
comparison to the traditional approaches for carbon−
heteroatom bond formation, for instance acid-catalyzed imine
exchange,5 encompasses better selectivity, milder reaction
conditions, tolerance to the diverse functional groups, and
improved reaction rates.
Furthermore, the study of metal-mediated imine metathesis
still has a fundamental interest, as no definite mechanism has
yet been identified. In fact, in addition to the Chauvin-type [2 +
2] mechanism, a pathway involving an amide intermediate has
been proposed.4f,i,6,7 Finally, another fundamental benefit of
this study is its potential to obtain a better understanding of the
reactivity of transition-metal imido groups that have mainly
been used so far as spectator ligands in a number of catalytic
reactions, including olefin metathesis. The Meyer group in
particular has demonstrated the utility of a number of
Received: February 15, 2017
© XXXX American Chemical Society
A
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(XAS). 2a,b were both found to be efficient heterogeneous
catalysts for imine cross metathesis under mild conditions.
attributable to the remaining silanols, which are likely to be in
interactions with the surrounding ligands, extending from 3750
to 3200 cm−1 was observed. The presence of weak infrared
signals in the wavelength range of 3290−3250 cm−1 may hint at
a partial development of molybdenum amine species (Scheme
1) through a proton transfer reaction.
However, an elemental microanalysis study indicates Mo, C,
and N contents of 2.60, 2.88, and 0.66 wt % for 2a and of 1.55,
4.69, and 0.47 wt % for 2b, respectively, which corresponds to
C/Mo and N/Mo molar ratios of 8.8 and 1.74 for 2a
(theoretical 8.0 and 2.0); for material 2b these ratios are 24.2
and 2.07 (theoretical 24.0 and 2.0). On the basis of these
results it could be inferred that the materials 2a,b are likely to
contain mainly the bipodal tetracoordinated molybdenum
surface complexes with no significant residual neopentyl ligands
at the molybdenum center.
RESULTS AND DISCUSSION
■
The grafting of bis(imido) molybdenum precursor complexes,
[Mo(NR)2(CH2-t-Bu2)2] (1a, R = t-Bu; 1b, R = 2,6-C6H3-i-
Pr2) (see the Supporting Information for experimental details)
at room temperature on silica partially dehydroxylated at 200
°C (SiO2‑200), followed by degassing at 80 °C resulted in
materials 2a,b (Scheme 1). As we believe, reaction of 1a,b with
Scheme 1. Proposed Reaction Scheme for the Grafting of
Bis(imido) Molybdenum Precursor Complexes 1a,b on
SiO2‑200
To gain further insights into the aforementioned exper-
imental data, grafting of complexes 1a,b onto SiO2‑200 and their
possible transformation at elevated temperatures were inves-
tigated computationally using density functional theory (DFT)
a and cluster model of the silica support (see Figure S1 in the
Supporting Information). The proposed mechanism for
transformations of species 1a,b is given in Scheme 2. For
Scheme 2. Proposed Grafting Pathways for the Molecular
a
Complexes 1a,b onto a Silica Model (SiO2‑200
)
SiO2‑200 presumably occurs via a silanolysis reaction, thereby
resulting in a sharp depletion of a part of the isolated ν(SiO−
H) at 3747 cm−1 with a concomitant appearance of new bands
in the 2990−2870 cm−1 (for 2a) and 3070−2870 cm−1 ranges
(for 2b) as well as in the 1600−1360 cm−1 range (Figure 1).
These peaks are characteristic of aromatic and aliphatic ν(C−
H), ν(CC), and δ(C−H) vibrations of the ligands of surface-
grafted molybdenum complexes. Additionally, a broad band
a
The numerical values (red for 1a and blue for 1b) noted in
parentheses are free energies, in kcal mol−1.
clarity, we will use capital letters A−D to denote all the various
minimum-energy species. Transition states are described with
the notation exemplified by [A-B]⧧, referring to the transition
state indicating the transformation of A to B. When describing
the reaction profile, we concentrate on t-Bu complex 1a. The
main differences between the reaction profiles of species 1a and
1b are emphasized in the following section.
The grafting of complexes 1a onto SiO2‑200 support can be
initiated via two alternate mechanistic pathways. In the first
mechanism, the hydrogen transfer from one of the Si−O−H
groups to a CH2-t-Bu group bound to the Mo atom occurs via
transition state [1a-A]⧧. The reaction is highly exergonic by
33.9 kcal/mol of Gibbs free energy and results in the formation
of monopodal species A and liberation of a free CH3-t-Bu
molecule. In terms of the activation Gibbs free energy, such a
transformation requires 18.3 kcal/mol. As the reverse reaction
would require 52.2 kcal/mol of free energy, if it occurs, the
Figure 1. FT-IR spectra of SiO2‑200 (bottom), 2a (middle), and 2b
(top).
B
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transformation of 1a to A can be considered irreversible. The
monopodal species A can further undergo a few subsequent
transformations. First, another hydrogen transfer from the
vicinal silanol group on silica can occur to the CH2-t-Bu group
bonded to Mo via the transition state [A-B]⧧. This trans-
formation requires 22.4 kcal/mol of activation free energy and
leads to bipodal species B with release of a second CH3-t-Bu
molecule. This step is again found to be highly exergonic by
20.3 kcal/mol and essentially irreversible if it happens. The
second transformation the species A can undergo is the transfer
of hydrogen from surface silanol to the MoN double bond,
resulting in bipodal amido species D, in which Mo is
pentacoordinated. This reaction is exergonic by 3.1 kcal/mol
and requires only 11.1 kcal/mol of activation free energy. In
contrast to the processes 1a-A and A-B, the A-D transformation
is reversible, as process D-A would require only 14.2 kcal/mol.
The transformation of D to B via α-hydrogen transfer from the
NH group bound to Mo and liberation of a CH3-t-Bu molecule
would require 33.8 kcal/mol of Gibbs free activation energy.
This process is energetically favorable by 17.2 kcal/mol.
Finally, coming back to the initial species 1a, it can also
interact with the surface silanol groups via another mechanism.
Thus, hydrogen transfer can occur from the Si−O−H group to
the MoN bond, leading to monopodal amido species C in
which Mo is pentacoordinated. This transformation requires
only 13.8 kcal/mol of free energy and is slightly exergonic by
1.5 kcal/mol. In addition, this step is reversible and requires
only 15.3 kcal/mol of activation energy for the transformation
of C to 1a. Furthermore, C can undergo α-hydrogen transfer
from the NH group to the Mo-CH2-t-Bu moiety and form A
with liberation of one molecule of CH3-t-Bu. This process is
highly exergonic by 32.4 kcal/mol and requires 36.1 kcal/mol
of activation free energy; hence, it is irreversible if it occurs.
Finally, C can transform into the bipodal species D with release
of a CH3-t-Bu molecule via hydrogen transfer from the silica
OH group to one of the CH2-t-Bu groups bonded to the Mo
atom. Such a transformation is highly favorable by 35.5 kcal/
mol and requires 18.8 kcal/mol.
There are only two substantial differences between reaction
profiles for species 1a and 1b. First, the most important for the
species 1b, the transformation C-D requires ∼10 kcal/mol
higher activation free energy. We explain this by taking into
consideration the greater steric repulsion of the bulky 2,6-
diisopropylphenyl groups, which appears in the compact
transition state [C-D]⧧ derived from the 1b precursor in
comparison to the steric repulsion in its 1a-based counterpart.
Similarly, transition state [C-A]⧧ is higher by ∼10 kcal/mol for
the 1b derivative in comparison to its 1a counterpart. All other
differences between the reaction profiles for the 1a and 1b
precursors are miniscule and can be neglected.
Overall, the analysis of Scheme 2 clearly suggests species B to
be the thermodynamic product of the grafting of both 1a and
1b precursors. Identifying the kinetic product is more
complicated, however, as all the important reaction barriers
might be sensitive to both the silica model and particular DFT
functional chosen. However, as it is now, at 80 °C the species B
is most likely to be the major kinetic product as well at
reasonably long (of few days) reaction time. Otherwise, the
species D is a clear candidate to be the kinetic product as well
and perhaps a mixture of species D and B is formed at short
reaction time or with insufficient heating, as the difference [A-
B]⧧ − D is about 25.5−29.6 kcal/mol and can stabilize D. All
other species such as A and C are unlikely to be the kinetic
products, as they easily undergo chemical transformations
through relatively low free energy barriers.
The H MAS NMR spectrum of 2a (Figure 2a) displayed
1
only one resonance centered at 1.3 ppm that can be readily
Figure 2. NMR characterization: 1H MAS NMR spectra of (a) 2a and
(c) 2b; 13C CP MAS NMR spectra of (b) 2a and (d) 2b (11.75 T,
spinning rate 10 kHz).
assigned to the protons of tert-butyl groups. The 13C CP MAS
NMR spectrum (Figure 2b) showed signals at 30.0 and 69.0
ppm corresponding to NC(CH3)3 and NC(CH3)3 groups,
respectively. Two additional weak signals at 43.0 and 51.0 ppm
were observed. These peaks are presumably occurring due to
the minor formation of molybdenum amido species, as
indicated by infrared spectroscopy (Figure 1). On the other
hand, the solid-state NMR data (1H, 13C) of 2b (Figure 2c,d)
feature the expected signals of an anchored molybdenum imido
1
aryl species. The broad peak at 3.1 ppm arises from the H of
isopropyl − CH(CH3), and the peak centered at 1.0 ppm can
be assigned to the protons of the aliphatic −CH3 group. A weak
broad resonance centered at 6.9 ppm originates from the
aromatic protons. The intense resonances at 21.0 and 26.8 ppm
observed in the 13C spectrum are due to the aliphatic −CH3
groups. Aromatic carbons appear in the range of 120−152 ppm.
The structures of 2a,b were also studied by EXAFS
spectroscopy (Figures 3 and 4 and Tables 1 and 2). In both
cases the XANES part of the absorption spectra presented a
pre-edge structure with a maximum at 20004.0 eV (normalized
intensity of the peak of ca. 0.4), agreeing with geometries that
are not octahedral but probably tetrahedral for Mo(VI)
complexes. For material 2a, the results are consistent with
the following coordination sphere around Mo, ca. two imido
ligands at 1.725(7) Å and ca. two oxygens at 1.958(8) Å, which
can be assigned to siloxide ligands. The lengths found for Mo
N and Mo−O bonds are close to those observed by XRD for
C
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a
Table 2. EXAFS Parameters for 2b
type of neighbor
no. of neighbors distance (Å)
σ2 (Å2)
MoN-Ar
2
2
2
4
2
2
4
2
1.74(1)
1.955(8)
2.95(3)
0.0053(12)
0.0029(8)
0.0016(12)
0.017(6)
b
b
b
b
Mo−OSi
MoN−CAr<
MS3l MoN−CMe3<
MS4l MoN−CMe3<
Mo--O(Si)2
Mo- - -C
c
c
b
2.95
b
b
2.95
0.017
2.76(2)
3.48(5)
3.43(6)
0.0025(13)
0.0033(28)
0.008(6)
b
Mo- - -Si
a
Errors generated by the EXAFS fitting program “RoundMidnight” are
Figure 3. Molybdenum K-edge k3-weighted EXAFS (left) for 2a and
the corresponding Fourier transform (right; modulus and imaginary
part); (solid lines) experimental; (dashed lines) fit using the spherical
wave theory.
indicated in parentheses. Δk = [1.9−16.1 Å−1] − ΔR [0.5−3.5 Å]
2
([0.5−2.1 Å], when considering only the first coordination sphere). S0
= 0.85. ΔE0 = 9.0 1.1 eV (the same for all shells). Fit residue: ρ =
b
10.3%. Quality factor: (Δχ)2/ν = 2.79, with ν = 15/29. Shell
c
constrained to parameters above. MS3l, three-leg multiple scattering
path; MS4l, four-leg multiple scattering path.
improved by adding a layer of further backscatterers, ca. two
oxygen and two carbon atoms at 2.79(3) and 2.98(3) Å,
respectively, attributed to surface oxygen atoms from siloxane
bridges of the silica support and to the quaternary carbon of the
tert-butyl groups of the imido ligands. For this contribution two
multiple scattering pathways were added using the same
distance, since for this imido ligand Mo, N, and C atoms are
close to alignment. The consideration of silicon atoms from the
surface was not statistically validated, and associated parameters
were obtained within huge uncertainty margins and the
Debye−Waller (DW) factor is very important. This last
contribution is indicated in parentheses in the last row of
Table 1 but is not considered in the fit. This is in particular
probably due to a structural disorder due to the variety of the
bipodal sites on the surface of silica, which would lead in this
case to a variety of Mo- - -Si configurations,13 each site
presenting a slightly different EXAFS contribution in the
resulting spectrum.
For 2b, the study was conducted similarly and the fit of the
EXAFS spectrum agrees with a structure very close to that of
2a. The parameters, in particular distances and DW factors, are
reported in detail in Table 2. An extra level of carbon
backscaterrers at 3.48(5) Å had to be added to improve the fit,
which may be due to a contribution of the methyl groups of the
isopropyl fragments, and the Mo- - -Si contribution at 3.43(6)
Å could be observed. EXAFS results therefore fully agree with
the tetrahedral type structures [(Si−O−)2Mo(NR)2] (R =
t-Bu) and [(Si−O−)2Mo(NR)2] (R = 2,6-C6H3-i-Pr2) for
the silica-supported complexes 2a,b, respectively.
We investigated the utility of silica-supported tetracoordi-
nated molybdenum complexes 2a,b as heterogeneous catalysts
for catalytic imine metathesis using selected imine substrates
(Figure 5). In a typical catalytic experiment an equimolar
mixture of two imine substrates was combined with 4 mol % of
either catalyst 2a or 2b to react at 80 °C in toluene. After a
specific time the reactions were terminated (see Table 3 and
the Supporting Information for details) and the products of the
catalytic run were analyzed by gas chromatography−mass
spectrometry (GC-MS), which shows that the imine products
formed after each reaction are consistent with those expected
via a cross-metathesis reaction (eq 1). In addition, as expected,
the imide/imine metathesis products (see Schemes S2−S4 and
Table S1 in the Supporting Information for details) were also
found to be present in each of the reaction mixtures, as
Figure 4. Molybdenum K-edge k3-weighted EXAFS (left) for 2b and
the corresponding Fourier transform (right; modulus and imaginary
part): (solid lines) experimental; (dashed lines) fit using the spherical
wave theory.
a
Table 1. EXAFS Parameters for 2a
no. of
neighbors
type of neighbor
distance (Å)
σ2 (Å2)
MoN-t-Bu
1.9(2)
1.725(7)
1.958(8)
2.98(3)
0.0039(9)
0.0036(8)
0.0020(16)
0.025(4)
b
Mo−OSi
2.2(2)
b
MoN−CMe3
MS3l MoN−CMe3
MS4l MoN−CMe3
Mo- - -O(Si)2
(Mo- - -Si)
1.9
c
c
b
b
3.8
2.98
b
b
b
1.9
2.98
0.025
2(1)
2.79(3)
0.0051(32)
(0.29(28))
b
(2.2 )
(3.35(32))
a
Errors generated by the EXAFS fitting program “RoundMidnight” are
indicated in parentheses. Δk = [1.9−16.1 Å−1] − ΔR: [0.5−3.2 Å]
2
([0.5−2.1 Å], when considering only the first coordination sphere). S0
= 0.85. ΔE0 = 8.9 1.0 eV (the same for all shells). Fit residue: ρ =
7.7%. Quality factor: (Δχ)2/ν = 2.97, with ν = 14/26 ([(Δχ)2/ν]1 =
4.32 with ν = 10/16, considering only the first coordination sphere 
b
N and −O). Shell constrained to parameters above (in particular 2
NN + NO = 6, the same number of neighbors, the same distance, or the
c
same DW factor). MS3l, three-leg multiple scattering path; MS4l, four-
leg multiple scattering path.
molecular complexes such as [Mo(N-t-Bu)2(OB(mes)2)2]
(1.726(3) Å for MoN and 1.914(2) Å for Mo−O) and
[Mo(N-Ar)2(OB(mes)2)2] (1.707−1.753 Å for MoN and
1.86−1.91 Å for Mo−O),10 though the Mo−O distance is
longer but is in the usual range observed, e.g. 1.932−1.945 Å
for [Mo(N−Ar)(L)(CHCMe2Ph)],11 where L is a pyridine-
diolato ligand (1.745(3) Å for MoN) and up to 2.048(2) Å
for an Mo−O X-type ligand in [{Mo(NR)2Me(μ-OMe)}2]12
(1.754(2) Å for MoN). Similar parameter values were
obtained on fitting the k2·χ(k) spectrum. The fit could be
D
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(e.g., DME) free molybdenum complexes possessing bis-
(imido) functionalities, giving two unique well-defined bipodal
tetracoordinated molybdenum surface complexes. X-ray
absorption spectroscopic studies, complemented by the FT-
IR, SS NMR spectroscopy, and elemental microanalysis, were
undertaken to unequivocally elucidate the molecular level
structure of the surface-grafted molybdenum complexes.
Furthermore, the grafting of four-coordinated molybdenum
complexes onto the surface of silica and their probable
transformation at elevated temperatures were also investigated
computationally using density functional theory and a cluster
model of the silica support. Finally we explored the utility of
our newly developed well-defined molybdenum surface
complexes as heterogeneous catalytic media, which exhibited
efficient metal-mediated carbon−nitrogen double-bond meta-
thesis under mild conditions.
Figure 5. Imine substrates 3−6 which are utilized in this study.
Table 3. Catalytic Imine Metathesis Substrate Testing Using
Catalysts 2a,b
imine
combination
conversn
conversn
(%)
a
b
entry
cat.
time (h)
(%)
1
2
3
4
5
6
3 + 4
3 + 4
3 + 5
3 + 5
3 + 6
3 + 6
2a
2b
2a
2b
2a
2b
6/24/−
6/24/50
6/24/−
6/24/50
6/−/50
6/24/−
48/53/−
49/51/53
47/52/−
33/50/52
49/−/52
48/51/−
49/51/−
50/51/53
48/50/−
31/50/54
47/−/53
50/51/−
EXPERIMENTAL SECTION
■
Materials and General Procedures. All experiments were carried
out by using standard Schlenk techniques. Air- and moisture-sensitive
materials were stored and handled in a glovebox. Solvents were
purified and dried according to standard procedures. Mo(N-t-
Bu2)2Cl2(DME) and Mo(N-2,6-C6H3-i-Pr2)2Cl2(DME) were synthe-
sized following the literature procedures.14 Gas-phase analyses were
performed on a Hewlett-Packard 5890 series II gas chromatograph
equipped with a flame ionization detector and an Al2O3/KCl on fused
silica column (50 m × 0.32 mm). Elemental analyses were performed
at the Pascher Mikroanalytisches Labor at Remagen-Bandorf,
Germany. IR spectra were recorded on a Nicolet 6700 FT-IR
spectrometer in either transmission or DRIFT mode in an airtight cell
equipped with CaF2 windows. The samples were prepared under
argon within a glovebox. Typically, 64 scans were accumulated for each
spectrum (resolution 4 cm−1). For NMR characterization, solution
NMR spectra were recorded on an Avance-300 Bruker spectrometer.
All chemical shifts were measured relative to residual 1H or 13C
resonances in the deuterated solvent: C6D6, δ 7.16 ppm for 1H, 128.06
a
Determined by GC-FID after a specific reaction time with respect to
b
the first imine (3) given in column 2. With respect to the second
imine in column 2.
confirmed again by GC-MS analysis, thus suggesting a direct
involvement of the metal center in the metathesis reaction.4d
When these results were taken into consideration, a hypo-
thetical catalytic cycle (Scheme S1 in the Supporting
Information) for the imine metathesis by catalyst 2a or 2b,
possessing a tetracoordinated molybdenum center, could be
proposed. When imines 3 and 4 were combined with catalyst
2a or 2b, the metathesis products 7 and 8 (Scheme S2)
together with the imide/imine metathesis products 9 and 10
(for 2a) or 11 and 12 (for 2b) were formed. Interestingly, both
of these catalysts equilibrate imines 3 and 4 in just about 6 h
(Table 2) and at this stage the imines were present in
approximately a stoichiometric ratio. Similarly, a combination
of imines 3 and 5 in the presence of catalyst 2a or 2b also
undergoes metathetical exchange of the imido groups to
produce 13 and 14 (Scheme S3) and reaches equilibrium
approximately again after 6 h for 2a, although this takes longer
in the case of 2b. Furthermore, a mixture of imines 3 and 6 was
also found to undergo cross metathesis, at approximately a
similar reaction rate, in the presence of either catalyst 2a or 2b
to yield 7 and 17 as metathesis products (Scheme S4) and
reaches equilibrium within a time frame similar to that required
for other substrate combinations.
1
ppm for 13C. H and 13C solid-state NMR spectra were recorded on
Bruker Avance-500 and Bruker Avance-300 spectrometers with a
conventional double-resonance 4 mm CP MAS probe at the
́
Laboratoire de Chimie Organometallique de Surface. The samples
were introduced under argon into a zirconia rotor (4 mm), which was
then tightly closed. The rotation frequency was set to 10 kHz unless
otherwise specified. Chemical shifts were given with respect to TMS as
1
external reference for H and 13C NMR.
GC measurements were performed with an Agilent 7890A Series
(FID detection). Method for GC analyses: column HP-5, 30 m length
× 0.32 mm ID × 0.25 μm film thickness; flow rate, 1 mL/min (N2);
split ratio, 50/1; inlet temperature, 250 °C; detector temperature, 250
°C; temperature program, 40 °C (1 min), 40−250 °C (15 °C/min),
250 °C (1 min), 250−300 °C (10 °C/min), 300 °C (30 min). GC-MS
measurements were performed with an Agilent 7890A Series coupled
with Agilent 5975C Series instruments.
XAS. X-ray absorption spectra were acquired at the ESRF, using
beamline BM23, at room temperature at the molybdenum K-edge,
with a double crystal Si(111) monochromator detuned 70% to reduce
the higher harmonics of the beam. The spectra were recorded in the
transmission mode between 19.66 and 21.53 keV. Airtight sample
holders equipped with Kapton windows were filled with the Mo
samples within an argon-filled glovebox, and these sample holders
were placed within airtight containers also equipped with Kapton
windows in order to ensure a double air protection. Once they were
extracted from the glovebox, these containers were positioned on the
beamline in order to record the spectra. The spectrum of a Mo(0)
metal foil was recorded simultaneously in each case, and its E0 edge
value (annulation of the second derivative of the signal) was set at
19999.5 eV,15 in order to standardize the energy of the sample spectra.
Two or three spectra were averaged for each sample. The data analyses
were carried out using standard procedures (“Athena”16 and
From these above catalytic results it appears that the
activation of a given set of imine substrates, with the exception
of a combination of 3 and 5, is similar for both catalysts 2a and
2b: i.e., independent of the imido substituents bearing different
electronic and steric properties. This is in variance with the
previously reported results obtained under homogeneous
catalytic conditions for hexacoordinated molybdenum com-
plexes bearing a bis(imido) core structure,4d where the rate of
the reactions were hypothesized to significantly depend on the
steric properties of the imide nitrogen substituents.
CONCLUSIONS
■
In summary, a rigorous SOMC protocol was adopted to
decorate the surface of silica with novel types of Lewis base
E
DOI: 10.1021/acs.organomet.7b00115
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Organometallics
Article
“RoundMidnight”17 programs). The program FEFF8 was used to
calculate theoretical files for phases and amplitudes on the basis of
vacuum line. The reaction mixture was condensed by cooling with
liquid nitrogen, and the ampules were evacuated (<10−5 mbar). After
they were sealed, the ampules were placed in an oil bath (80 °C). The
reaction mixture was heated for a specific reaction time of 6, 24, or 50
h, and each time after reaction it was cooled to 22 °C. The product
solution was filtered to remove the catalyst, and the remaining liquid
product was analyzed by GC-FID and GC-MS.
Computational Details. Geometry Optimizations and Calcu-
lations of Thermochemical Corrections. All geometry optimizations
were performed using the PBE GGA22 functional as implemented in
the PRIRODA 13 DFT code.23 All electron basis sets (λ1)24
comparable in quality to the correlation consistent valence double-ζ
plus polarization (cc-PVDZ) basis sets of Dunning were used. All
stationary geometries were characterized by an analytically calculated
Hessian matrix. Possible relativistic effects (for molybdenum) were
taken into account via the Dyall Hamiltonian.25
The default, adaptively generated PRIRODA grid, corresponding to
an accuracy of the exchange-correlation energy per atom (1 × 10−8
hartree) was decreased by a factor of 100 for more accurate evaluation
of the exchange-correlation energy. Default values were used for the
self-consistent-field (SCF) convergence and the maximum gradient for
geometry optimization criterion (1 × 10−4 au), whereas the maximum
displacement geometry convergence criterion was decreased to 0.0018
au.
Translational, rotational, and vibrational partition functions for
thermal corrections to arrive at total Gibbs free energies were
computed within the ideal-gas, rigid-rotor, and harmonic oscillator
approximations. The temperature used in the calculations of
thermochemical corrections was set to 298.15 K in all the cases.
Single-Point (SP) Energy Evaluations. The energies were re-
evaluated at optimized geometries by means of the M0626 functional
as implemented in Gaussian 0927 code. The all-electron def2-tzvpp
basis set of Ahlrichs28 was used on the main elements. For the
molybdenum atom the Stuttgart ECP29 was used with the
corresponding valence def2-tzvpp basis set. The “Integral(grid =
ultrafine)” option was used for evaluation of the exchange-correlation
term. The default value for the SP SCF convergence was adopted.
Silica Model. A relatively large cluster model cut out from the β-
cristobalite-based SiO2 surface (model 001-4 in the paper published by
Rozanska et al.30) has been chosen to simulate a silica surface (see
Figure S1 in the Supporting Information).
model clusters of atoms.18 The S0 factor was evaluated using the
2
crystallized molecular complexes Mo(O)(CH2-t-Bu)3Cl and Mo(O)-
(CH2SiMe3)2(OCH2-t-Bu)2, also characterized by XRD. These were
studied diluted in BN and conditioned as wafers, and the average value
of S02 found from the fit of the EXAFS of these two reference samples,
S02 = 0.85, was chosen. The refinements were performed by fitting the
structural parameters Ni, Ri, and σi and the energy shift ΔE0 (the same
for all shells). The fit residue, ρ (%), was calculated by the formula
∑k [k3χexp(k) − k3χ (k)]2
cal
ρ =
× 100
∑k [k3χexp(k)]2
As recommended by the Standards and Criteria Committee of the
International XAFS Society,19 the quality factor, (Δχ)2/ν, where ν is
the number of degrees of freedom in the signal, was calculated and its
minimization considered in order to control the number of variable
parameters used in the fits.
Preparation of Mo(N-t-Bu2)2(CH2-t-Bu)2 (1a). 1a was synthe-
sized following a synthesis procedure identical with that reported by
Osborn et al.,20 with the exception that the precursor complex used in
this study was Mo(N-t-Bu2)2Cl2(DME). In brief, Mo(N-t-
Bu)2Cl2(DME) (0.20 g, 0.5009 mmol) was dissolved in pentane (ca.
10 mL) and added to a pentane solution of LiCH2-t-Bu (0.07823 g,
1.0019 mmol) at 23 °C and stirred for another 1.2 h at this
temperature. After the completion of the reaction the volatiles were
removed to leave a brown liquid product (yield 98%). The
spectroscopic data are consistent with those reported elsewhere.20
Preparation of Mo(N-2,6-C6H3-i-Pr2)2(CH2-t-Bu)2 (1b). For the
synthesis of 1b, a slightly modified procedure reported in the literature
was followed.21 Mo(N-2,6-C6H3-i-Pr2)2Cl2(DME) (1.0 g, 1.646
mmol) was dissolved in diethyl ether (ca. 15 mL) and slowly added
to a solution of LiCH2-t-Bu (0.257 g, 3.292 mmol) in 5 mL of diethyl
ether at −78 °C. Then the reaction mixture was warmed to 23 °C and
stirred for another 7.5 h. After this time the solvent content of the
reaction was removed under vacuum. The orange-red product
obtained was thoroughly washed with cold pentane and finally dried
under vacuum to give a bright orange solid product (yield 80%). The
spectroscopic data are consistent with those reported elsewhere.21
Grafting of Molecular Precursor 1a onto the Surface of
SiO2‑200 To Yield Material 2a. Mo(N-t-Bu2)2(CH2-t-Bu)2 (1a; 0.18
g, 0.473 mmol) was dissolved in 5 mL of dry pentane and was slowly
added to a slurry of SiO2‑200 (1.0 g, around ∼0.8 mmol of −OH) in
pentane (ca. 7 mL) and stirred at room temperature for 12 h. The
solvent was then removed by filtration, and the solid residue was
thoroughly washed with pentane (5 × 5 mL) followed by drying under
dynamic vacuum for 12 h. The light gray-brown solid obtained was
further degassed at 80 °C (under dynamic vacuum; <10−5 Torr) for
another 16 h to yield material 2a. Anal. Calcd for 2a: Mo, 2.60; C,
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00115.
■
S
Further details on materials and general procedures,
synthesis of precursor molybdenum complexes, grafting
of tetracoordinated molybdenum complexes onto the
silica surface, catalytic results, and computational details
(PDF)
1
2.88; N, 0.66. H MAS NMR (500 MHz): δ 1.3 ppm. 13C CP MAS
NMR (125 MHz): δ 69.0, 51.0, 43.0, and 30.0 ppm.
Grafting of Molecular Precursor 1b onto the Surface of
SiO2‑200 To Yield Material 2b. Mo(N-2,6-C6H3-i-Pr2)2(CH2-t-Bu)2
(1b; 0.148 g, 0.251 mmol) was dissolved in 5 mL of dry pentane and
was slowly added to a slurry of SiO2‑200 (1.0 g, around ∼0.8 mmol of
−OH) in pentane (ca. 7 mL) and stirred at room temperature for 12
h. The solvent was then removed by filtration, and the solid residue
was thoroughly washed with pentane (5 × 5 mL) followed by drying
under dynamic vacuum for 12 h. The light orange solid obtained was
further degassed at 80 °C (under dynamic vacuum; <10−5 Torr) for
another 16 h to yield material 2b. Anal. Calcd for 2b: Mo, 1.55; C,
Cartesian coordinates of the calculated structures (XYZ)
AUTHOR INFORMATION
Corresponding Authors
*E-mail for M.T.: mostafa.taoufik@univ-lyon1.fr.
*E-mail for J.-M.B.: jeanmarie.basset@kaust.edu.sa.
■
ORCID
4.69; N, 0.47. 1H MAS NMR (500 MHz): δ 6.9, 3.1, and 1.0 ppm. 13
C
Luigi Cavallo: 0000-0002-1398-338X
Notes
The authors declare no competing financial interest.
CP MAS NMR (125 MHz): δ 152.0, 141.0, 123.0, 120.0, 26.8, and
21.0 ppm.
General Procedure for Imine Metathesis. A clean oven-dried
ampule was charged, in a glovebox, sequentially with the catalyst 2a
(or 2b) (0.04 equiv), toluene (500 μL), 1.0 equiv of each of the imines
3 and 4 (or 3 and 5 or 3 and 6), and a Teflon-coated stir bar. The
ampule was then removed from the glovebox and placed on a high-
ACKNOWLEDGMENTS
The Competitive Research Grant Program, supported by the
KAUST Research Fund (project no. 2174 CGR3) is gratefully
■
F
DOI: 10.1021/acs.organomet.7b00115
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Organometallics
Article
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for his help during the recording of the XAS spectra on the
BM23 beamline at the ESRF (experiment code IN-986).
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DOI: 10.1021/acs.organomet.7b00115
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