Evidence for Silica Surface Three- And Five-Membered Metallacycle Intermediates in the Catalytic Cycle of Hydroaminoalkylation of Olefins Using Single-Ti-Metal Catalysts

Article abstract of DOI:10.1021/acs.organomet.0c00246

The single-site silica-supported group IV metal amido complex [Ti(NMe2)4] gives the tris(amido)-supported fragment [(Si-O-)Ti(-NMe2)3], which transforms into a three-membered metallacycle (called a metallaaziridine) by an α-H transfer between two amido ligands. When the three-membered metallacycle reacts with 1-octene, it gives a five-membered metallacycle by insertion of the double bond into the M-C bond of the metallaziridine. These two metallacycles, key intermediates in the catalytic cycle of the hydroaminoalkylation of terminal olefins, were isolated and fully characterized following the surface organometallic chemistry (SOMC) concept and procedures. This paper shows that surface organometallic chemistry can be used to identify and fully characterize three- and five-membered metallacycles of Ti in the hydroaminoalkylation of olefins.

Full text of DOI:10.1021/acs.organomet.0c00246

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Article
Evidence for Silica Surface Three- and Five-Membered Metallacycle
Intermediates in the Catalytic Cycle of Hydroaminoalkylation of
Olefins Using Single-Ti-Metal Catalysts
Layal F. Yaacoub, Maha A. Aljuhani, Abdesslem Jedidi, Manal S. Al-Harbi, Walid Al Maksoud,
́
́
Wiebke Wackerow, Edy Abou-Hamad, Jeremie D. A. Pelletier, Mohamad El Eter,* Luigi Cavallo,*
and Jean-Marie Basset*
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ABSTRACT: The single-site silica-supported group IV metal
amido complex [Ti(NMe2)4] gives the tris(amido)-supported
fragment [(Si−O−)Ti(−NMe2)3], which transforms into a
three-membered metallacycle (called a metallaaziridine) by an α-
H transfer between two amido ligands. When the three-membered
metallacycle reacts with 1-octene, it gives a five-membered
metallacycle by insertion of the double bond into the M−C bond
of the metallaziridine. These two metallacycles, key intermediates in
the catalytic cycle of the hydroaminoalkylation of terminal olefins,
were isolated and fully characterized following the surface
organometallic chemistry (SOMC) concept and procedures. This paper shows that surface organometallic chemistry can be used
to identify and fully characterize three- and five-membered metallacycles of Ti in the hydroaminoalkylation of olefins.
ydroaminoalkylation, reported first by Maspero1 and
2
Hthen by Nugent, is a reaction described by the addition
Figure 1. General reaction of hydroaminoalkylation according to
Nugent.2
Figure 3. Expected surface three- and five-membered metallacycle key
intermediates for hydroaminoalkylation.
Figure 2. Catalytic mechanism of homogeneous hydroaminoalkyla-
tion.3,4,7−15
Received: April 7, 2020
of an α-amino C−H moiety across an alkene double bond
(Figure 1).2−4
This reaction is a new path for functionalizing simple
amines, which are essential in the pharmaceutical, agro-
chemical, and fine chemical industries.5 In homogeneous
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Interestingly, the isolated species was a three-membered
cycle (called a metallaziridine39−47) obtained at room
temperature under vacuum (dynamic vacuum, 10−5 mbar for
3 h) (Figure 5) and this three-membered metallacycle was very
well characterized by FTIR, elemental analysis, SS NMR, and
DNP SENS.45,48
THREE-MEMBERED-RING FRAGMENT:
PREPARATION AND CHARACTERIZATION
FTIR and elemental analysis have been carried out for the
titanium tris(amido) complex. The obtained results were
■
Figure 4. Titanium tris(amido) fragment immobilized on SiO2‑700
.
Figure 6. FTIR spectra of SiO2‑700 (black), and the isolated
metallacycles (1) [(Si−O−)Ti(η2-MeNCH2)(η1-NMe2)(η1-
HNMe2)] (red) and (1′) [(Si−O−)Ti(HNMe2)(η2-NMeCH2CH-
(C6H13)CH2)(−NMe2)] (blue).
Figure 5. Titanaaziridine (1) formation at room temperature and
under dynamic vacuum (10−5 mbar).
Table 1. Elemental Analysis for the Tris(amido) Complex 1
ratio
N/C
N/M
C/M
catalysis, the reaction mechanism was proposed and the five-
membered-ring formation was demonstrated under exper-
imental conditions (X-ray)6 to occur by insertion of terminal
alkene into the M−C bond of a metallaaziridine (Figure
2).3,4,7−16
complex 1 ( 5%)
predicted for tris(amido)
0.5
0.5
3.1
3
5.7
6
consistent with the proposed structure of the surface complex
[(Si−O−)Ti(NMe2)3]. The FTIR spectrum of the starting
SiO2‑700 shows an intense ν(O−H) peak located at 3745 cm−1.
This peak disappears after grafting of [Ti(NMe2)4]. A new
range of signals appears for C−H (stretching and bending).
Intense bands are observed at 2970−2776 cm−1 (ν(CH)) and
weak bands at 1500−1400 cm−1 (δ(CH)). In addition, no
band was observed which could be ascribed to a Si−NMe2
moiety. This absence of Si−NMe2 means that there is no
formation of doubly or triply bridged Ti as would occur by
opening of a Si−O−Si bond (Figure 6, spectrum 1).
In addition, an elemental analysis of the tentative structure
of the surface complex [(Si−O−)Ti(−NMe2)3] shows
ratios ( 5% error) consistent with the predicted values for
the tris(amido) complex [(Si−O−)Ti(−NMe2)3] (Table
1).
As is well-known, the objectives of surface organometallic
chemistry (SOMC) are predicting (by rational approach) new
reactions, improving existing ones, and understanding the
mechanism, using the single-site concept (Figure 3).17−28
Therefore, in the present study, we have explored the
possibility of grafting the homoleptic tetrakis(dimethylamido)
complexes [Ti(NMe2)4] on highly dehydroxylated silica
(SiO2‑700), with the intent of drawing a full image of what
was done previously in our group with the homoleptic
zirconuim complex.29−31 The objective was to isolate, using
SOMC concepts and tools,32−35 all or some of the elementary
steps of the catalytic cycle of heterogeneous hydroaminoalky-
lation previously proposed in homogeneous catalysis.36−39
The reason to choose SOMC is as follows. By immobiliza-
tion of surface organometallic fragments on surfaces, (1) it
prevents bimolecular reactions and (2) it can isolate and fully
characterize reaction intermediates.
We should notie here that the five-membered metallacycle
was suggested in the case ofhydroaminoallylation, but it was in
the case of Zr and with propene as the olefin. Obviously as
frequently occurs with group IV metals, there is a strong
similarity of mechanisms among all of these metals.
Consequently, with this objective in mind, an isolation of the
surface [(Si−O−)Ti(−NMe2)3] has been tested by reacting
the homoleptic [Ti(NMe2)4] complex with SiO2‑700 (highly
dehydroxylated silica treated at 700 °C) (Figure 4).
Nonetheless, the 13C NMR and the 1H NMR spectra are not
consistent with the tentative structure [(Si−O−)Ti-
(−NMe2)3]. Accordingly, the 1H NMR spectrum of the
compound obtained (Figure S4 in the Supporting Informa-
tion) reveals a peak located at 2.5 ppm, assigned to the proton
present in the dimethylamido ligands and a second peak at 1.6
ppm. The presence of these peaks is not consistent with the
expected tris(amido) complex [(Si−O−)Ti(−-NMe2)3]
that should show three equivalent dimethylamido ligands
(one peak). Therefore, a different structure has to be well
thought out: a metallaaziridine fragment, in which methylene
B
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Figure 7. 15N MAS DNP SENS spectra (left) and 2D 1H−13C HETCOR (heteronuclear correlation) NMR spectra (right) of [(Si−O−)Ti(η2-
MeNCH2)(η1-NMe2)(η1-HNMe2)] (1).
was directly coordinated to the titanium atom. Additionally,
the 13C NMR spectrum of the supported metal complex
reveals three clearly identified signals: at 43 ppm (br), at 38
ppm (low intensity), and a signal of much lower intensity at 82
ppm. Therefore, further advanced SS NMR experiments were
carried out to confirm this hypothesis.
1
The HETCOR spectrum reveals the H−13C correlations
(Figure 7). Clear correlations are seen between the methyl
protons at 2.5 ppm and the carbons at 38 and 43 ppm. Another
more interesting correlation is observed between the signal at
82 ppm39 and the proton at 4.6 ppm. Two-dimensional proton
double- and triple-quantum (DQ, TQ) correlation experiments
(Figure S4 in the Supporting Information) were performed to
determine the number of protons attached to the same carbon
and to differentiate between CH2 and CH3 groups. A strong
autocorrelation peak observed for the proton at 2.5 ppm in
both DQ and TQ spectra is compatible with the assignment of
this resonance to methyl protons. The proton resonance at 4.6
ppm does not appear in DQ and TQ spectra because of the
low intensity of this peak.
Figure 8. Five-membered metallacycle (1′) formation by insertion of
1-octene into surface titanaaziridine (1).
Table 2. Elemental Analysis for the Five-Membered Ring 1′
ratio
N/C
N/M
C/M
The natural-abundance 15N DNP spectrum obtained for 1
(Figure 7) displays three resonances at 6, 15, and 41 ppm.
Considering their relative abundance, we assign the intense
signal shifted downfield at 41 ppm to a σ-bonded −NMe2
ligand. The second peak at 15 ppm is attributed to the N
present in the aziridine fragment, and the weak upfield-shifted
signal at 6 ppm is compatible with a π-bonded NH(CH3)2
(ligand type L coordinated group).
complex 1′ ( 5%)
predicted for the five-membered metallacycle
0.2
0.21
2.8
3
13.2
14
octene in the titanaaziridine fragment (see Figure 6, spectrum
1′). The elemental analysis (Table 2) of complex 1′ strongly
confirms that one molecule of 1-octene has been incorporated
in the metallaaziridine complex 1 very likely by simple 1,2-
insertion in which the long alkyl fragment is away from the
metal for obvious steric reasons.
FIVE-MEMBERED-RING FRAGMENT:
PREPARATION AND CHARACTERIZATION
The 1H SSNMR spectrum of the complex 1′ exhibits a peak
at 2.9 ppm assigned to the CH2 of the five-membered ring next
to the metal center. This CH2 peak overlaps with another CH2
signal at 2.4 ppm in the five-membered ring. Two major
signals, the first at 1.15 ppm for CH2 and the second at 0.7
ppm for CH3, are present in the 1-octene tail. The CH2 and
CH3 assignments were validated by the DQ and TQ
correlations (Figure 9).
■
To continue our investigation on the mechanism of hydro-
aminoalkylation, we isolated the five-membered-ring (key
intermediate) complex [(SiO−)Ti(η1-HNMe2)(η2-
NMeCH2CH(C6H13)CH2)(−NMe2)] (1′) formed by inser-
tion of 1-octene in the M−C bond of the metallaaziridine.
The titanaaziridine was treated with neat liquid 1-octene
(1.5 equiv, 24 h, and 165 °C); the obtained product was
washed with pentane and dried under vacuum (10−5 mbar) for
2−3 h (Figure 8). An FTIR spectrum of the obtained complex
1′ shows an increase in the intensity of the ν(C−H) bands
(2985−2776 cm−1) that is expected by the insertion of the 1-
The 1.9 ppm peak assigned to the CH group in the five-
membered ring represents the more complex hydrocarbon
motif around the titanium center; it appears as a very low
intensity peak in the 1H spectra logically without any
autocorrelation in DQ or TQ frequencies. The 13C SSNMR
C
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Figure 11. 15N MAS DNP SENS spectrum of [(Si−O−)Ti(η1-
HNMe2)(η2-NMeCH2CH(C6H13)CH2)(−NMe2)] (1′).
Figure 12. Stack plot of the spectra for the three-membered (1) and
five-membered complexes (1′).
Figure 9. 1H−1H DQ spectra (top) and 1H−1H TQ spectra
(bottom) for [(Si−O−)Ti-(η1-HNMe2)[η2-NMeCH2(C6H13)-
CH2](−NMe2)] (1′).
ppm and the proton peak at 2.4 ppm. The signal at 22 ppm for
1
13C correlating with the 1.15 ppm signal for H is consistent
with the CH2 fragments of the 1-octene queue.
A strong correlation is observed between the proton peak at
1.9 ppm and the carbon peak at 31 ppm; this signal is
attributed to the CH group.
The natural-abundance 15N SSNMR spectroscopy experi-
ments were performed for 1′ using the DNP-SENS technique
(Figure 11). The obtained spectrum exhibits three peaks,
centered at 7, 40, and 66 ppm (Figure 12). The signals at 7 and
40 ppm are tentatively related to π-bonded NH(CH3)2 and σ-
bonded −NMe2, respectively. The new peak at 66 ppm in the
spectrum of 1′ Figure 12 is assigned to the N atom involved in
the five-membered ring.
CATALYTIC TESTS
Hydroaminoalkylation reaction studies were conducted with a
mixture of 1-octene and a selected dialkylamine in 1 mL of
■
Table 3. Hydroaminoalkylation Catalytic Tests with
Titanaazizidine (1) as a Catalyst
Figure 10. 2D 1H−13C HETCOR NMR spectrum of the five-
membered-ring intermediate 1′.
a
entry
amine
combined conversion (%)
spectrum of this complex shows six major peaks at 12, 22, 29,
31, 36, and 43 ppm. The resonance at 36 ppm is significantly
intense in comparison to the others; this peak is assigned to
the CH2 of the five-membered ring next to the titanium center.
This interpretation is compatible with the multiple-quanta
experimental spectra that reveal autocorrelations in double-
quantum (DQ) and triple-quantum (TQ) frequencies for the
signals at 2.4 ppm.
1
2
3
4
5
6
7
N-methylaniline
diethylamine
dibutylamine
N-ethylaniline
diphenylamine
dimethylamine
ethylmethylamine
9.5
23
15
0
0
0
0
1
In addition, the 2D H−13C HETCOR NMR spectrum
a
Conditions: 6 mol % of catalyst complex 1, 165 °C, 1.5 equiv of 1-
octene, 1 mL of toluene, 48 h.
(Figure 10) shows a correlation between the carbon peak at 36
D
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DENSITY FUNCTIONAL THEORY (DFT)
CALCULATIONS
■
All calculations were carried out with the Gaussian 09 suite of
programs. Density functional theory (DFT) via the PBE1PBE
functional (PBE0) was used for geometry optimizations. The
SVP basis set was employed for the Si, C, N, O, and H atoms,
and the SDD basis set was employed for Ti metal. Grimme D3
corrections were included in the energy evaluation. To obtain
more accurate values, the reported energies have been obtained
via single-point calculations using again the PBE0-D3 func-
tional, by improving the main-group-atom basis set to the
triple-ζ TZVP basis set.
Calculations have been performed using Ti as the metal and
propene as the olefin. For the sake of simplicity, in the
presence of HNMe2 we consider the tris(amido) amino
complex 1 as the reference structure at 0 kcal/mol in energy.
With 1 as the starting point, a fourth HNMe2 molecule
coordinates to 1, leading to 2. The first reactive step is a β-H
transfer between two amido ligands, leading to the metal-
laaziridine species 3, via transition state 2-3 and an energy
barrier of 32.9 kcal/mol. According to the calculations, 3 is
located 14.1 kcal/mol above 1; therefore, the aziridine
fragment is formed after the tris(amido) fragment is exposed
to a high vacuum (10−5 mbar) for 24 h. The next step is the
endergonic displacement of a dimethylamino ligand by
propene, leading to 4, 21.8 kcal/mol above 1, followed by
the rapid reaction of propene with the metallaaziridine moiety.
Propene addition occurs via transition state 4-5, at 34.1 kcal/
mol, and leads to intermediate 4, only 4.4 kcal/mol above 1.
We calculated that 1,2-insertion of propene into the Ti−N
bond, leading to 5, is the favored regiochemistry, as it is
favored by 4.4 kcal/mol over the transition state for 2,1-
insertion.
Figure 13. 3D representation of the aziridine complex 3 and the rate-
determining transition state 4-5 intermediate complex 2′.
toluene and the titanaaziridine complex catalyst 1 that was
heated at 165 °C for 48 h in a vacuum-sealed ampule.
The screening was conducted using selected substrates such
as dialkylamines (diethylamine and dibutylamine) and
arylalkylamines (N-methylaniline and N-ethylaniline) (Figures
S5−S7 in the Supporting Information). All the runs and blanks
were carried out under similar conditions. Branched and linear
alkylation products detected as expected.
Catalysis with N-methylaniline as the substrate (Table 3,
entry 1) reached a combined conversion of 9.5%. For entries 2
and 3 with dialkylamine substrates there were higher combined
conversions of 23% and 15%, respectively. However, in entries
4−7 there was no conversion observed, because the substrates
are sterically hindered (Table 3).
Figure 14. Energy profile for hydroaminoalkylation promoted by the Ti amido complex 1.
E
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Authors
Layal F. Yaacoub − KAUST Catalysis Center, Division of
The reaction evolves via H transfer from an amido ligand to
the metallacycle moiety via transition state 5-6, at 21.3 kcal/
mol, opening the metallacycle and leading to intermediate 5,
−12.1 kcal/mol below 1. Coordination of an HNMe2
molecule, followed by H transfer from the coordinated
dimethylamino to the N atom of the alkylated amido ligand,
via transition state 7-8 at 3.3 kcal/mol, leads to intermediate 8.
The catalytic cycle is closed by displacement of the
hydroaminoalkylated product by a dimethylamino ligand.
The overall thermochemistry of the reaction is calculated to
be −8.3 kcal/mol with the adopted computational protocol.
The overall energy span from 1 to 4-5, 34.4 kcal/mol, is
consistent with the experimentally required long reaction times
and high temperatures. The structures of the metallaaziridine
complex 3 and the rate-determining transition state 4-5 are
shown in Figure 13. In complex 3 the Ti assumes a distorted-
trigonal-bipyramidal geometry, with the Ti−O, the Ti−amido,
and the Ti−aziridine bonds in the equatorial plane and the two
amino ligands along the axis. This disposition is preserved in
the rate-determining transition state 4-5, with propene
replacing one of the amino ligands. The energy profile diagram
is illustrated in Figure 14.
Physical Sciences and Engineering, King Abdullah University of
Science and Technology (KAUST), Thuwal 23955-6900,
Kingdom of Saudi Arabia; Lebanese University, Faculty of
sciences section III, Tripoli, Lebanon
Maha A. Aljuhani − KAUST Catalysis Center, Division of
Physical Sciences and Engineering, King Abdullah University of
Science and Technology (KAUST), Thuwal 23955-6900,
Kingdom of Saudi Arabia
Abdesslem Jedidi − Department of Chemistry, Faculty of
Science, King Abdulaziz University, Jeddah 21589, Kingdom of
Saudi Arabia; orcid.org/0000-0003-4070-3299
Manal S. Al-Harbi − Department of Chemistry, Faculty of
Science, King Abdulaziz University, Jeddah 21589, Kingdom of
Saudi Arabia
Walid Al Maksoud − KAUST Catalysis Center, Division of
Physical Sciences and Engineering, King Abdullah University of
Science and Technology (KAUST), Thuwal 23955-6900,
Kingdom of Saudi Arabia
Wiebke Wackerow − KAUST Catalysis Center, Division of
Physical Sciences and Engineering, King Abdullah University of
Science and Technology (KAUST), Thuwal 23955-6900,
Kingdom of Saudi Arabia
CONCLUSION
■
Edy Abou-Hamad − King Abdullah University of Science and
Technology (KAUST), Thuwal 23955-6900, Kingdom of
Saudi Arabia
To summarize, we succeeded in isolating two metallacycles
involved in the hydroaminoalkylation of olefins: a titanaazir-
idine obtained by α-H transfer between two adjacent σ-bonded
−N(CH3)2 groups and the five-membered metallacycle
obtained after insertion of 1-octene or any terminal alkene in
the metal−carbon (M−C) bond of our well-characterized
metallaaziridine fragment. It should be emphasized here that
SOMC is unique in stabilizing such a metallacycle, especially
the five-membered rings. Therefore, the reason for such a
difference is very likely that there are no bimolecular reactions
with surfaces, whereas in solution such bimolecular reactions
easily occur.
́
́
Jeremie D. A. Pelletier − KAUST Catalysis Center, Division of
Physical Sciences and Engineering, King Abdullah University of
Science and Technology (KAUST), Thuwal 23955-6900,
Kingdom of Saudi Arabia
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00246
Author Contributions
All authors have given approval to the final version of the
manuscript.
ASSOCIATED CONTENT
■
Notes
sı
* Supporting Information
The authors declare no competing financial interest.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00246.
ACKNOWLEDGMENTS
■
FTIR, elemental analysis, NMR characterization, sub-
strate analysis, and blank tests (PDF)
The King Abdullah University of Science and Technology
(KAUST) supported the research. The authors acknowledge
core lab at KAUST for their assistance. L.C. and A.J. are
grateful to the KAUST Supercomputing Laboratory for the
resources. A.J. thank High‐Performance Computing Center
(AZIZ supercomputer) for the support.
Cartesian coordinates for the calculate structures (XYZ)
AUTHOR INFORMATION
■
Corresponding Authors
Mohamad El Eter − Lebanese University, Faculty of sciences
section III, Tripoli, Lebanon; Email: Mohamad_eleter@
hotmail.com
Luigi Cavallo − KAUST Catalysis Center, Division of Physical
Sciences and Engineering, King Abdullah University of Science
and Technology (KAUST), Thuwal 23955-6900, Kingdom of
Saudi Arabia; orcid.org/0000-0002-1398-338X;
Email: Luigi.cavallo@kaust.edu.sa
Jean-Marie Basset − KAUST Catalysis Center, Division of
Physical Sciences and Engineering, King Abdullah University of
Science and Technology (KAUST), Thuwal 23955-6900,
Kingdom of Saudi Arabia; orcid.org/0000-0003-3166-
8882; Email: jeanmarie.basset@kaust.edu.sa
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