Catalytic Imido-Transfer Reactions of Well-Defined Silica-Supported Titanium Imido Complexes Prepared via Surface Organometallic Chemistry

Article abstract of DOI:10.1021/acs.organomet.9b00779

We expand the series of well-defined silica-supported titanium imido complexes of general formula (SiO)Ti(═NtBu)X(py)_n prepared via surface organometallic chemistry in order to analyze the effect of the X ligand on their catalytic properties. Two new surface complexes, (SiO)Ti(═NtBu)Cl(py)₂ (2s) and (SiO)Ti(═NtBu)Cp(py) (3s), have been prepared and characterized with physicochemical techniques, and their performance in oxo/imido heterometathesis and other catalytic imido-transfer reactions has been studied and compared to that of the previously reported catalyst (SiO)Ti(═NtBu)(Me₂Pyr)(py)₂ (1s; Me₂Pyr = 2,5-dimethylpyrrolyl).

Full text of DOI:10.1021/acs.organomet.9b00779

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Article
Catalytic Imido-Transfer Reactions of Well-Defined Silica-Supported
Titanium Imido Complexes Prepared via Surface Organometallic
Chemistry
Pavel A. Zhizhko,* Andrey V. Pichugov, Nikolai S. Bushkov, Andrey V. Rumyantsev, Kamil I. Utegenov,
Valeria N. Talanova, Tatyana V. Strelkova, Dmitry Lebedev, Deni Mance, and Dmitry N. Zarubin*
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ABSTRACT: We expand the series of well-defined silica-supported titanium
imido complexes of general formula (SiO)Ti(NtBu)X(py)n prepared via
surface organometallic chemistry in order to analyze the effect of the X igand
on their catalytic properties. Two new surface complexes, (SiO)Ti(
NtBu)Cl(py)2 (2s) and (SiO)Ti(NtBu)Cp(py) (3s), have been
prepared and characterized with physicochemical techniques, and their
performance in oxo/imido heterometathesis and other catalytic imido-
transfer reactions has been studied and compared to that of the previously
reported catalyst (SiO)Ti(NtBu)(Me2Pyr)(py)2 (1s; Me2Pyr = 2,5-
dimethylpyrrolyl).
steps constitutes a catalytic cycle, resulting in the imidation of
various oxo substrates (Scheme 1). On the basis of the
INTRODUCTION
■
Surface organometallic chemistry (SOMC)1 is a growing
interdisciplinary field that utilizes coordination and organo-
metallic molecular precursors for the preparation of well-defined
metal sites covalently attached (grafted) to the surface of a solid
support in order to gain insight into the structures of the active
species and reaction mechanisms on the surfaces of classical
heterogeneous catalysts, as well as to discover and develop new
catalytic reactions. The SOMC approach is based on a rigorous
control of the interaction of well-defined molecular complexes
with a carefully prepared support having a desired density of
surface functionalities regarded as ligands entering the
coordination sphere of the metal, followed by detailed
characterization of the resulting surface species with phys-
icochemical techniques, study of the ligand effects, and building
of the structure−activity relationships similarly to what is
typically done in molecular chemistry and catalysis. Briefly
speaking, SOMC considers the surface as an additional
dimension for the application of the basic principles of
coordination and organometallic chemistry.
Scheme 1. Oxo/Imido Heterometathesis Catalytic Cycle
stoichiometric reactivity of transition-metal imido complexes4
it was clear that the most active catalysts should be found in the
left part of the d block of the Periodic Table, in particular among
group 4 metals; however, the instability of the terminal oxo
derivatives of these metals (the formation of oxo-bridged
oligomers, which in this case means deactivation) prevented the
development of this chemistry.
One of the important features of SOMC is the possibility to
design and stabilize the supported species that would readily
decompose in solution via bimolecular pathways. Remarkable
examples include silica-supported early-transition-metal hydri-
des2a and low-coordinate isolated single sites.2b Our group has
been developing catalytic oxo/imido heterometathesis reac-
tions3 where the interconversion of the transition-metal oxo and
imido components via [2 + 2] cycloaddition/cycloreversion
Special Issue: Organometallic Chemistry at Various
Length Scales
Received: November 14, 2019
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SOMC provided an elegant solution to this problem: if a
properly designed monomeric imido complex is grafted onto the
surface, the metal site will remain monomeric and the
oligomerization of the transient oxo intermediate will be
avoided.5 We have recently reported the preparation of the
well-defined silica-supported Ti imido complex6 [(SiO)Ti-
(NtBu)(Me2Pyr)(py)2] (1s) (Chart 1; Me2Pyr = 2,5-
point in SOMC. In our work we have defined the following
criteria for the design of the molecular precursors. (i) The
complexes should be monomeric and contain a terminal imido
ligand as required for catalysis; however, excessive steric bulk
that can suppress the reactivity is not desirable. This criterion
can be problematic for early-transition-metal imides that often
tend to lose the supporting L ligands (especially when X ligands
are sufficiently electron donating), leading to the dimerization of
the resulting coordinatively unsaturated species8 (3-coordinated
in the case of group 4). (ii) The imido moiety bound to early
transition metals is highly reactive and can be easily protonated
by the surface silanol groups during grafting, leading to the
supported amido species. Hence, the molecular precursor
should also bear a more reactive, easily cleavable X ligand that
will play the role of an anchoring site. An alkyl group would be
the ideal choice; however, for the reasons mentioned above, so
far we have not been able to prepare monomeric Ti imido alkyls
analogous to the respective group 5 systems.5,9 Previous studies
have shown that the 2,5-dimethylpyrrolyl ligand10 is an excellent
leaving group for grafting early-transition-metal alkylidene11 and
imido6 complexes on silica. It is sufficiently reactive to be easily
and selectively cleaved with the surface silanols, while the
releasing pyrrole (Me2PyrH) serves as a convenient probe for
quantification of the grafting reaction by analysis of the liquid
phase. In addition, its variable hapticity and moderate steric bulk
may be advantageous to favor the monomeric structures of the
complexes. (iii) A tert-butyl substituent at the imido group is
chosen because its quaternary carbon is very sensitive to the
state of the imido ligand and provides a convenient tool for its
characterization by solid-state NMR. Protonation typically leads
to a 5−10 ppm upfield shift in 13C and the corresponding
resonances for Ti imido, amido, and amino species can be
expected at roughly 70, 60, and 50 ppm, respectively.12 In
addition, the starting material [Ti(NtBu)Cl2(py)3] is readily
available via Mountford’s protocol.13 (iv) To enable unambig-
uous assignments in solid-state NMR, it is also desirable that the
ligands be simple and symmetrical and give distinctive NMR
signatures. These requirements lead to the following general
structure [Ti(NtBu)(Me2Pyr)(X)(L)n]. We report here the
synthesis of the monopyrrolyl monochloride complex 2 (X = Cl,
L = py, n = 2) and show that it can be further used as a precursor
for introducing other X ligands via substitution of the chloride.
Treatment of [Ti(NtBu)Cl2(py)3] with 1 equiv of
NaMe2Pyr or LiMe2Pyr in THF leads to a selective substitution
Chart 1. Grafted and Molecular Complexes Studied in This
Work
dimethylpyrrolyl) that displayed unprecedented activity in the
heterometathetical imidation of carbonyl compounds with N-
sulfinylamines.6,7 In this report we continue to explore the
chemistry of silica-supported Ti imides and begin a more
detailed study of the electronic effects of the ligands on the
catalytic activity. We describe here two new well-defined silica-
supported Ti imido complexes where the ancillary X ligand
(Me2Pyr) is replaced with a very poorly electron donating
chloride or a strongly donating Cp ligand, [(SiO)Ti(
NtBu)Cl(py)2] (2s) and [(SiO)Ti(NtBu)Cp(py)] (3s),
prepared from the corresponding molecular 2,5-dimethylpyr-
rolyl derivatives 2 and 3 (Chart 1), as well as a study of the
performance of the surface complexes 1s−3s in oxo/imido
heterometathesis and other catalytic imido-transfer reactions.
RESULTS AND DISCUSSION
Synthesis of the Molecular Precursors. The proper
choice of a suitable molecular precursor for grafting is a crucial
■
Scheme 2. Preparation of Molecular and Grafted Complexes
B
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of one chloride ligand and isolation of 2 in high yield as a yellow
powder that can be recrystallized from Et2O/pentane to give
XRD-suitable orange crystals. Complex 2 reacts with 1 equiv of
CpNa to give 3, which crystallizes from toluene as red crystals
(Scheme 2). Single-crystal X-ray diffraction studies show that
both complexes are monomeric, the geometries being shown in
Figure 1. The structure of 2 is very similar to that of the
Figure 2. IR spectra of SiO2‑700 and grafted materials.
Figure 1. Thermal ellipsoid plots of (a) [Ti(NtBu)Cl(Me2Pyr)-
(py)2] (2) (only one of the two independent molecules is shown) and
(b) [Ti(NtBu)Cp(Me2Pyr)(py)] (3) at the 50% probability level.
Hydrogen atoms are omitted.
and meta CH carbons of pyridine at ca. 151, 138, and 123 ppm,
respectively, and a signal of the CH3 groups of the NtBu ligand at
ca. 30 ppm. The Cp ring of 3s appears at 109 ppm. Most
importantly, the characteristic quaternary carbon of the NtBu
group is clearly observed in both spectra (at ca. 71 and 68 ppm
for 2s and 3s, respectively). All resonances correlate well with
those in the liquid-state spectra of the molecular precursors (see
the Supporting Information). The high resolution of the spectra
allows for tentative identification of the minor byproduct surface
species. The peaks at 115 and 16 ppm in the spectrum of 3s can
be assigned to the Cp and Me2Pyr ligands of the protonated
species 3s′ (Scheme 2), the quaternary carbon of NHtBu being
visible as a tiny peak at 59 ppm (see the Supporting Information
for the enlarged picture). The signals at 106 and 16 ppm in the
spectrum of 2s correlate well with those of the surface complex
1s that has been characterized before6 and could arise from the
substitution of the chloride instead of the Me2Pyr ligand in 2
during grafting. On the other hand, these signals could be also
attributed to the protonated complex 2s′ (Scheme 2). The peak
at 48 ppm in this case may correspond to NHtBu of 2s′;
however, such a resonance should be expected at slightly lower
field (as in 3s′) and we would rather ascribe the signal at 48 ppm
to a trace amount of tBuNH2 (the same chemical shift in liquid-
state NMR) that is sometimes observed in the spectra of
different grafted NtBu complexes, likely due to adventitious
moisture during the measurement, even though neither option
can be excluded. The trace signals corresponding to the
previously characterized bis-pyrrolyl complex 16 and can be
described as a trigonal bipyramid with two pyridines in the apical
positions. The molecule of the Cp derivative 3 has a typical
piano-stool geometry. The TiN bond lengths in complexes
1−3 increase in the order 2 < 1 < 3 and are 1.686(2), 1.695(2),
and 1.705(1) Å, respectively, which correlates with the donating
ability of the ancillary ligands (see below). Me2Pyr ligands in all
three complexes are flat and coordinated in an η1 fashion.
Similarly to 1, Ti−NMe Pyr bond lengths in 2 and 3 are relatively
2
long (ca. 2.03−2.09 Å), consistent with the weak π-donating
character of the pyrrolyl ligand.14 However, the angles ∠Ti−
NMe Pyr−Me2Pyrcentroid are only slightly distorted from 180° (ca.
2
158° for 1, 162° for 2, and 178° for 3) and thus some extent of π-
donation is not excluded.10a,15 The pyrrolyl ring in 2 lies in the
equatorial plane. Two sets of resonances for the methyl and CH
1
groups of the Me2Pyr ligand are observed in the H and 13C
NMR of 2 (analogously to 1), and only one is observed for the
Cp derivative 3, indicating a lower rotation barrier around the
Ti−N bond in the latter case.
Grafting. Complexes 2 and 3 were grafted onto the surface of
silica partially dehydroxylated at 700 °C (SiO2−700) in a standard
mannerby slow stirring of a solution of the complex in
benzene with a suspension of silica overnight, subsequent
decanting of the solution, and drying the solid material under
high vacuum. In both cases 0.8−0.9 equiv of Me2PyrH per Ti is
released during grafting according to NMR quantification of the
combined washings/volatiles, suggesting the predominant
formation of surface complexes 2s and 3s (Scheme 2; the
scheme also shows the possible expected byproducts, see
below). Elemental analysis of the grafted materials reveals 1.1 wt
% of Ti in both cases (corresponding to 0.23 mmol of Ti g−1)
and gives a Ti/N/C/Cl ratio of 1/3.4/14.2/1.0 for 2s and a Ti/
N/C ratio of 1/2.7/14.8 for 3s, close to the expected values of 1/
3/14/1 and 1/2/14, respectively. IR spectroscopy confirms
almost quantitative consumption of the surface silanol groups
(the band at 3747 cm−1) and the appearance of νCH and δCH
vibrations of the ligands (Figure 2).
1
remaining Me2Pyr ligand are also detectable in the H MAS
NMR of 2s and 3s (see the Supporting Information). On the
basis of the intensities of the peaks together with the elemental
analysis data and the amount of Me2PyrH released during
grafting, the abundance of these minor surface forms can be
estimated as no more than 10%.
Catalytic Studies. The materials 1s−3s were investigated in
a set of catalytic test reactions that are typically associated with
imido complexes. Since the main focus of our research is
catalytic oxo/imido heterometathesis, we have first tested the
new catalysts in the imidation of carbonyl compounds with N-
sulfinylamines. The reaction of benzophenone with N-sulfinyl-p-
toluidine (TolNSO) in boiling heptane (98 °C) at 1 mol %
catalyst loading was used as a standard test, and TOF after the
first 15 min of the reaction and half-conversion time were used
to evaluate the catalyst performance, as has been established
before.6 The results are shown in Table 1 and Figure 4a. From
these data we can see that (i) the nature of the ancillary ligands
Solid-state NMR is particularly helpful in elucidating the
structures of surface complexes (Figure 3). 13C CP MAS spectra
of both materials show distinct resonances of the ortho, para,
C
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Figure 3. 13C CP MAS solid-state NMR spectra of 2s and 3s.
catalyst 1s is surprising, especially considering the fact that the
preference for the electron-rich metal center in this reaction
could be rationalized as a result of the imido nitrogen lone pair
being in that case less involved in bonding with Ti and more
localized on the nitrogen atom (this is also reflected in longer
Table 1. Activity of 1s−3s in Imidation of Benzophenone
with TolNSO
a
b
catalyst
TOF15 min
,
h−1
τ1/2
d
TiN, see above), which in turn makes the imido group more
prone toward the nucleophilic attack on the ketone.4c
[(SiO)Ti(NtBu)(Me2Pyr)(py)2] (1s)
[(SiO)Ti(NtBu)Cl(py)2] (2s)
[(SiO)Ti(NtBu)Cp(py)] (3s)
385(18)
53(9)
258(11)
5 min
1−2 h
10 min
The difference between 1s and 3s may arise from different
origins. First, it should be noted that, unlike 1s and 2s which are
both 5-coordinated, 3s is formally a 6-coordinated complex, and
in addition the formal electron count gives 14e for 1s and 2s,
while 3s can be regarded as having 16e (the imido ligand is
counted as an X2L ligand). Thus, a direct comparison between
1s and 3s may be ambiguous. Another factor that can play a role
in catalysis is the potentially flexible hapticity of the pyrrolyl ring.
However, a more likely explanation consists of the Me2Pyr
ligand being transformed under the reaction conditions. Indeed,
the amide ligand bound to the oxophilic Ti atom is very reactive
and susceptible to protonolysis and insertion reactions (which is
also the reason for clean and selective grafting via this ligand),
and therefore it is reasonable to expect that it may not stay intact
in the presence of the reactive organic substrates. This is also in
agreement with the preliminary DFT calculation results which
show that when Me2Pyr and NtBu ligands are simultaneously
present in the coordination sphere of Ti the HOMO is actually
located on the pyrrolyl while the imido orbitals involved in
catalysis lie lower in energy. Thus, it may appear that 1s is only a
precatalyst and the Me2Pyr ligand cannot be directly placed on
a
TOF averaged for the first 15 min of the reaction (standard
b
deviations are given in parentheses). Half-conversion time.
on Ti has a very strong influence on the catalytic performance
and (ii) electron-donating ligands generally favor the activity.
Me2Pyr and Cp complexes 1s and 3s are both very efficient
catalysts, while 2s bearing a chloride, despite being an
isostructural analogue of 1s, displays almost 1 order of
magnitude lower activity. It is worth noting, however, that Cp
derivative 3s is somewhat less active than 1s, the difference being
even more pronounced when the more sterically demanding
MesNSO (Mes = 2,4,6-Me3C6H2) is used instead of TolNSO. In
this case catalysts 1s and 3s give TOF15 min values of ca. 80 vs 30
h−1 and half-conversion times of 45 min vs 2 h, respectively.
While chloride is a very poorly donating ligand and η5-Cp is a
strong σ-/π-donor, η1-Me2Pyr is known to be a good σ-donor
but a weak π-donor, and thus the overall electron-donating
ability of these ligands can be ranked as follows: Cl < Me2Pyr <
Cp.14,16 In this context the superior activity of the pyrrolyl
Figure 4. Catalytic performance of supported catalysts 1s−3s in (a) imidation of benzophenone with TolNSO, (b) self-condensation of TolNCO, (c)
self-condensation of TolNSO, and (d) guanylation reaction between DCC and TolNH2. Lines are added to guide the eye.
D
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the same scale as the more stable Cp and Cl, and its role during
catalysis, as well as the origin of the high activity of 1s, requires a
more careful analysis. It should also be mentioned that the
competition between cycloaddition reactions of the TiNR
linkage and insertions into the reactive Ti−X bonds in related
systems has been discussed in detail by Mountford.17
of benzophenone is observed: 1s > 3s > 2s (Table 3); however,
in this case 1s displays significantly better performance than 3s.
Table 3. Activity of 1s−3s in Condensation of TolNCO
We were interested to evaluate the synthetic potential of using
isocyanates instead of N-sulfinylamines under the same
conditions. Isocyanates are well-known imidating agents for
oxo complexes18 (in this case the coproduct of the reaction is
CO2, while SO2 is released in the reactions with N-sulfinyl-
amines), and they have also been used in the catalytic imidation
of carbonyl compounds.3e,19 Surprisingly, we have found that
when TolNCO is used under conditions identical with those
employed for TolNSO only trace amounts of benzophenone
imine product (1−2%) are detected after 1 h of reaction, and
only 5−10% after 24 h, with any of the catalysts 1s−3s (Table
2). It is worth noting that the same order of activity as in the case
a
b
catalyst
TOF30 min
,
h−1
τ1/2
[(SiO)Ti(NtBu)(Me2Pyr)(py)2] (1s)
[(SiO)Ti(NtBu)Cl(py)2] (2s)
[(SiO)Ti(NtBu)Cp(py)] (3s)
144(17)
22(13)
44(17)
<30 min
>4 h
2−4 h
a
TOF averaged for the first 30 min of the reaction (standard
deviations are given in parentheses). Half-conversion time.
b
We have also tested the catalyst 1s under conditions similar to
those reported by Sundermeyer for the homogeneous
phtalocyanine-supported Ti oxo complex [(Pc)Ti(O)],
which fully converts TolNCO into carbodiimide at 190 °C
and 0.3 mol % catalyst loading within 6 days.3g The reaction with
0.3 mol % of 1s is complete after reflux overnight at 174 °C (in
fact, ca. 80% conversion is achieved already after 1.5 h), and thus
silica-supported catalyst 1s displays even better performance
than the homogeneous catalyst. It should be noted, however,
that all the reported transition-metal-based systems are
dramatically inferior to the well-known phospholene oxide
catalysts,23 and thus, despite being intriguing, this reaction at the
moment is mainly interesting from a mechanistic point of view.
In this respect, important for us is the correlation between the
difficulty of self-condensation of isocyanates and their poor
reactivity as imidating agents for ketones and other oxo
substrates.
Table 2. Activity of 1s−3s in Imidation of Benzophenone
with TolNCO
a
conversion after 24 h, %
catalyst
Ti, mol %
imine
carbodiimide
1s
1s
2s
3s
5
1
1
1
70
9
4
15
16
2
5
6
a
Conversions into imine and carbodiimide products. These tests were
A similar transformation can be observed for N-sulfinylamines
in the absence of other oxo substrate: i.e., their self-condensation
into sulfurdiimines with liberation of SO2 (Table 4), as has been
performed only once, and thus no standard deviations are given. Note
that the accuracy of GC data at low conversions is rather low.
of N-sulfinylamines is observed (1s > 3s > 2s). Increasing the
catalyst loading to 5 mol % gives 70% conversion into imine after
24 h with the most active catalyst 1s. Thus, N-sulfinylamines are
strikingly more reactive imidating agents in this transformation
in comparison to the corresponding isocyanates, which implies
that at least in the case of isocyanates the limiting step of the
catalytic cycle (Scheme 1) is the imidation of MO rather than
an imido-transfer to ketone. This result correlates with the data
on the stoichiometric imidation of transition-metal oxo
complexes using N-sulfinylamines and isocyanates,18,20 the
latter typically requiring higher temperatures and longer
reaction times.
Table 4. Activity of 1s−3s in Condensation of TolNSO
a
b
catalyst
TOF30 min
,
h−1
τ1/2
[(SiO)Ti(NtBu)(Me2Pyr)(py)2] (1s)
[(SiO)Ti(NtBu)Cl(py)2] (2s)
[(SiO)Ti(NtBu)Cp(py)] (3s)
49(4)
22(5)
44(8)
4 h
4−8 h
4 h
a
TOF averaged for the first 30 min of the reaction (standard
b
deviations are given in parentheses). Half-conversion time.
In the course of the reaction we have also detected the
formation of small amounts of the carbodiimide TolNCNTol
(15% after 24 h with 5 mol % of 1s, Table 2). This product likely
results from a concurrent oxo/imido heterometathesis process
where 1 equiv of RNCO acts as an imidating agent and another 1
equiv as an oxo component (although alternative mechanisms
have also been proposed21). This reaction was reported for
several transition-metal oxo and imido complexes3g,h,22 and
typically requires harsh reaction conditionsheating at temper-
atures as high as 140−190 °C (sometimes in neat isocyanate).
We have tested the catalysts 1s−3s in the self-condensation of
TolNCO. The reaction in refluxing toluene (110 °C) is very
slow and reaches ca. 90% conversion within 4 days when 1 mol %
of 1s is used as a catalyst. Increasing the temperature to 174 °C
(reflux in n-decane) results in much faster reaction
quantitative conversion is reached within 1 h with catalyst 1s
(Figure 4b). Again the same order of activity as in the imidation
demonstrated before for Mo- and Ta-based catalysts.3c,5
Similarly to the imidation of benzophenone, self-condensation
proceeds much more easily in the case of N-sulfinylamines (the
reactions with 1 mol % of Ti are relatively fast already at 98 °C).
The results for 1s−3s are shown in Table 4 and Figure 4c. The
difference between the catalysts is not strongly pronounced in
this case; however, the same order of activity is observed, with
catalysts 1s and 3s being slightly more active than 2s.
Cycloaddition reactions between early-transition-metal imido
complexes and unsaturated compounds serve as entry points
into many other catalytic cycles besides oxo/imido heterometa-
thesis. In particular, cycloaddition with alkynes is a key
elementary step in alkyne hydroamination,14a,24 carboamina-
tion,25 and iminoamination,26 as well as in pyrrole synthesis via
[2 + 2 + 1] coupling of alkynes with azo compounds27 and
organic azides.28 Ti-catalyzed hydroamination of alkynes
E
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(Scheme 3) has been extensively studied by many groups,
especially by Odom,14a,26 who has also recently investigated the
Table 5. Activity of 1s−3s in Catalytic Guanylation
Scheme 3. Alkyne and Carbodiimide Hydroamination
Catalytic Cycles
a
b
catalyst
TOF30 min
,
h−1
τ1/2
[(SiO)Ti(NtBu)(Me2Pyr)(py)2] (1s)
[(SiO)Ti(NtBu)Cl(py)2] (2s)
[(SiO)Ti(NtBu)Cp(py)] (3s)
46(5)
48(7)
182(8)
2−4 h
2−4 h
<15 min
a
TOF averaged for the first 30 min of the reaction (standard
b
deviations are given in parentheses). Half-conversion time.
CONCLUSION
■
We have prepared and characterized two new well-defined silica-
supported Ti imido complexes with the Ti atom in different
ligand environments. Although we have so far not been able to
find a catalyst that would exceed the performance of the most
efficient previously reported oxo/imido heterometathesis
catalyst 1s, we have identified the important trends in activity
that will guide further catalyst development. In particular we
have shown that (i) the performance of structurally similar
catalysts may vary drastically depending on the nature of the
ancillary ligands, (ii) an electron-donating environment tends to
generally favor the activity, and (iii) the same order of activity is
observed for different oxo/imido heterometathesis reactions (1s
> 3s > 2s), which gives an additional proof that all of these
reactions follow the same general mechanism. We have also
shown that silica-supported Ti imido complexes are active in
other catalytic imido-transfer reactions typical for molecular
imido complexes, in particular the hydroamination of alkynes
and carbodiimides; a more detailed study of these reactions is
underway.
silica-supported systems.29 We have preliminarily tested the
grafted catalysts 1s−3s in the hydroamination of internal
alkynes (which are known to be significantly more challenging
substrates than the terminal alkynes) with p-toluidine (TolNH2)
at 5 mol % Ti loading in boiling toluene (110 °C). So far we have
not been able to observe activity with 4-octyne; however,
moderate activity is found in the hydroamination of
diphenylacetylene. According to the preliminary data the
catalyst 3s displays the best performance (ca. 90% conversion
in 24 h), while 1s is less active (ca. 70% after 24 h) and 2s is
almost inactive under these conditions. This result is in line with
Bergman’s finding30 that [Ti(NAr)Cp(NHAr)(py)] (Ar =
2,6-Me2C6H4), which is structurally very similar to the surface
complex 3s, efficiently catalyzes the hydroamination of
diphenylacetylene, although the activity of 3s is significantly
lower. On the other hand, the only reported heterogeneous Ti
system operates under much harsher conditions,29 and thus this
result is worth studying in more detail.
Hydroamination of carbodiimides (also referred to as
“guanylation of amines with carbodiimides”) catalyzed by Ti
imido complexes was first reported by Richeson in 200331 and
proposed to follow a mechanism similar to that of hydro-
amination of alkynes (Scheme 3). Many catalytic systems have
been tested since that time,32 including one report of a silica-
supported Ti catalyst by Odom.29a Using the reaction between
equimolar amounts of dicyclohexylcarbodiimide (DCC) and
TolNH2 in boiling toluene (110 °C) at 1 mol % catalyst loading,
we have found that heterogeneous catalysts 1s−3s display good
activities in this transformation (Table 5 and Figure 4d).
Especially noteworthy is the catalyst 3s, which reaches almost
quantitative conversion within 30 min. Interestingly, the
performances of the catalysts 1s and 2s are nearly identical.
This result could be explained on the basis of the fact that both
pyrrolyl and chloride ligands could be expected to be substituted
in the presence of excess aniline, leading to formally the same
catalytic species and hence close activities. In contrast, the Cp
ligand of 3s is more likely to stay intact during the reactionthis
is another typical example in organometallic chemistry when the
ubiquitous cyclopentadienyl ligand provides a suitable frame-
work for productive and stable catalysis.
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise stated, all manipu-
lations were carried out under an argon atmosphere using standard
Schlenk or glovebox techniques. All glassware was dried in an oven at
130 °C for at least 2 h prior to use. Solvents were distilled from Na/
benzophenone (THF, Et2O, toluene, C6H6, C6D6, pentane, heptane,
decane) or CaH2 (CH2Cl2, CDCl3), degassed by three freeze−pump−
thaw cycles, and stored over activated 3 Å MS. TolNH2 and
diphenylacetylene were sublimed under vacuum. DCC was dissolved
in dry CH2Cl2, filtered, evaporated, and distilled under vacuum.
TolNCO and 4-octyne were used as received. TolNSO was prepared by
refluxing TolNH2 in toluene with 1.1 equiv of SOCl2 (Merck, “for
synthesis” grade) overnight (or until the solid that initially formed fully
dissolved), with subsequent evaporation of the solvent and vacuum
distillation. 1H NMR (CDCl3, 400 MHz): δ 7.79 (d, 3J 8.2, 2H), 7.22
(d, 2, 2H), 2.39 (s, 3H). Benzophenone was dried either by sublimation
under vacuum (cooling the cold finger with tap water) or by treating its
toluene solution with 3 Å MS with subsequent crystallization at low
temperature (both procedures give close results in catalysis). Silica
(Degussa Aerosil, 200 m2 g−1) was compacted with distilled water,
calcined at 500 °C under air overnight, and then treated at 700 °C
under high vacuum (ca. 10−5 mbar) for 24 h (support referred to as
SiO2‑700).1c The solution NMR spectra were recorded using Bruker
Avance 400, Avance II 300, DRX 300, and DRX 500 spectrometers.
Chemical shifts (δ) are given in ppm and coupling constants (J) in Hz.
1H chemical shifts were referenced relative to the (residual) solvent
peak: CDCl3 (1H, 7.26; 13C, 77.16), C6D6 (1H, 7.16; 13C, 128.06).
Solid-state NMR spectra were recorded on a Bruker Avance III 400
spectrometer, using a 4 mm double-resonance probe. Chemical shifts
were referenced externally relative to solid adamantane (1H, 1.91; 13C,
38.5 (CH2)). Samples were loaded in 4 mm zirconia rotors inside the
glovebox and tightly closed. Cross-polarization magic angle spinning
F
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(CP MAS) type experiments were employed in order to acquire 13C
washed several times with fresh benzene. The resulting solid was dried
under high vacuum (ca. 10−5 mbar) for 2 h to afford 720 mg of yellow
powder. Filtrate solutions and volatiles trapped upon drying were
collected and analyzed by 1H NMR spectroscopy in C6D6 using
ferrocene as an internal standard, indicating that 0.15 mmol of
Me2PyrH was released upon grafting (0.82 equiv/Ti). Elemental
analysis (wt %): Ti 1.1% (0.23 mmol Ti g−1), C 3.93%, H 0.75%, N
1.11%, Cl 0.85%; corresponding to 14.2 C/Ti, 32.6 H/Ti, 3.4 N/Ti,
and 1.0 Cl/Ti (expected Ti/N/C/H/Cl = 1/3/14/19/1).
1
spectra at 10 kHz MAS, and H decoupling was performed using the
SPINAL64 sequence.33 IR spectra of silica and grafted materials were
recorded using a Bruker spectrometer placed in the glovebox, equipped
with OPUS software. Samples were pressed into pellets inside the
glovebox, and transmission spectra were measured in the region of
4000−400 cm−1. GC/FID was performed using a Chromatec Crystal
5000.2 gas chromatograph equipped with a Restek RTX-35 column.
Elemental analyses were performed in the Laboratories of Micro-
[(SiO)Ti(NtBu)Cp(py)] (3s). A red benzene solution of Ti(
NtBu)Cp(Me2Pyr)(py) (3; 78 mg, 0.22 mmol) was added to a
suspension of SiO2‑700 (846 mg) in benzene at room temperature. The
suspension was slowly stirred overnight while silica became dark red
and the solution colorless. The solid was separated by decantation and
washed several times with fresh benzene. The resulting solid was dried
under high vacuum (ca. 10−5 mbar) for 2 h to afford 877 mg of an
orange powder. Filtrate solutions and volatiles trapped upon drying
were collected and analyzed by 1H NMR spectroscopy in C6D6 using
ferrocene as an internal standard, indicating that 0.20 mmol of
Me2PyrH was released upon grafting (0.92 equiv/Ti). Elemental
analysis (wt %): Ti 1.1% (0.23 mmol Ti g−1), C 4.08%, H 0.57%, N
0.86%; corresponding to 14.8 C/Ti, 24.8 H/Ti, and 2.7 N/Ti (expected
Ti/N/C/H = 1/2/14/19).
analysis of INEOS RAS and ETH Zurich. X-ray diffraction data were
̈
taken using an Oxford Diffraction Xcalibur S κ geometry diffractometer
(Mo Kα radiation, graphite monochromator, λ = 0.71073 Å). To
prevent evaporation of cocrystallized solvent molecules, the crystals
were coated with Paratone oil in the glovebox and the data were
collected at 100 K. The cell parameters were obtained with intensities
detected on three batches of five frames. The crystal−detector distance
was 4.5 cm, and the narrow data were collected using 1° increments.
Unique intensities detected on all frames using the Oxford Diffraction
Red program were used to refine the values of the cell parameters. The
structure was solved by direct methods using Olex2 1.2 with the
SHELXTL-2012 package, and for all structures all atoms, including
hydrogen atoms, were found by difference Fourier syntheses. All non-
hydrogen atoms were anisotropically refined on F.
Catalytic Studies. Stock solutions of the substrates (ca. 0.15 M)
and C6Me6 (internal standard) in dry solvents were prepared and stored
under argon. A catalyst (typically 20−30 mg) was placed in a double-
neck Schlenk flask equipped with a backflow condenser at one neck and
a septum for taking aliquots at the second neck. The required volume of
the solution was added to the catalyst, the flask was immediately
immersed in a preheated oil bath, and the reaction was carried out
under reflux. Aliquots of the reaction mixture were taken over the
course of the reaction and analyzed with GC. The conversions of the
reagents were determined from their consumption with respect to the
internal standard; the conversions into products were calculated using
calibrations of the isolated products versus internal standard and usually
matched well with the reagent conversions (no calibration was
performed for alkyne hydroamination, and only consumption of the
reagents was monitored in this case). The TON and TOF were
calculated with respect to the total amount of the metal in the catalyst
determined by elemental analysis. Each test was repeated several times
(TOFs reported in Tables 1−5 are the average values, half-conversion
times are given as ranges). For more details see ref 6. Ph2CNTol and
TolNSNTol were prepared in our previous work.5 TolNC
NTol34 was isolated as a pale yellow liquid in 83% yield after distillation
under vacuum from the experiment with 0.3 mol % catalyst 1s (reflux in
n-decane overnight). 1H NMR (CDCl3, 400 MHz): δ 7.13 (d, 3J 8.2,
4H), 7.08 (d, 4H), 2.34 (s, 6H). Guanidine (NHCy)2C(NTol)35
was isolated as a white powder by crystallization from toluene in 79%
yield from one of the catalytic runs with 1 mol % catalyst 3s: mp 153 °C
(lit. mp 157−158 °C). 1H NMR (CDCl3, 400 MHz): δ 7.04 (d, 3J 8.0,
2H), 6.74 (d, 2H), 3.63 (br s, 2H), 3.40 (br s, 2H), 2.27 (s, 3H), 2.01−
1.96 (m, 4H), 1.70−1.57 (m, 6H), 1.39−1.30 (m, 4H), 1.17−1.05 (m,
6H). The identity of the diphenylacetylene hydroamination product
was confirmed using ESI-MS (MH+ calcd 286.1590, found 286.1588).
[Ti(NtBu)Cl(Me2Pyr)(py)2] (2). To a solution of Ti(NtBu)-
Cl2(py)313 (966 mg, 2.26 mmol) in THF (20 mL) was added a solution
of NaMe2Pyr (265 mg, 2.26 mmol) in THF (5 mL) at −35 °C, and the
reaction mixture was stirred overnight at room temperature
(alternatively, the starting materials can be mixed as solids and
dissolved in cold THF; LiMe2Pyr gives the same yields but note
however that it is insoluble in THF). The solution turned from red to
yellow-orange. The solvent was evaporated under vacuum, and the
residue was extracted with toluene to give an orange solution. Toluene
was evaporated, and the orange oily residue was triturated in heptane to
give a yellow powder that was filtered, washed with fresh heptane, and
dried under vacuum to give 746 mg (81%) of analytically pure product.
The complex is insoluble in heptane, moderately soluble in Et2O, and
soluble in toluene and benzene. Large orange XRD-suitable crystals
were obtained by diluting an Et2O solution of the product with pentane
(to ca. 1/1 Et2O/pentane ratio) and cooling the solution to −35 °C
overnight. 1H NMR (C6D6, 400 MHz): δ 8.88 (d, 3J 4.9, 4H, o-CHpy),
6.74 (t, 3J 7.6, 2H, p-CHpy), 6.50 (t, 3J 6.6, 4H, m-CHpy), 6.45 (s, 1H,
Me2Pyr), 6.13 (s, 1H, Me2Pyr), 3.12 (s, 3H, Me2Pyr), 2.03 (s, 3H,
Me2Pyr), 1.06 (s, 9H, NtBu). 13C NMR (C6D6, 75 MHz): δ 151.8 (o-
CHpy), 138.1 (p-CHpy), 133.8 (quat CMe2Pyr), 129.6 (quat CMe2Pyr),
124.2 (m-CHpy), 107.2 (CHMe2Pyr), 106.8 (CHMe2Pyr), 72.4 (NCMe3),
31.3 (NCMe3), 19.2 (Me2Pyr), 14.8 (Me2Pyr). Anal. Found (calcd for
C20H27N4ClTi): C, 59.21 (59.05); H, 6.79 (6.69); N, 13.63 (13.77).
CCDC 1960613 contains the crystallographic data.
[Ti(NtBu)Cp(Me2Pyr)(py)] (3). To a solution of Ti(NtBu)-
Cl(Me2Pyr)(py)2 (2; 412 mg, 1.01 mmol) in THF (10 mL) was added
a solution of CpNa (90 mg, 1.0 mmol) in THF (5 mL) at −35 °C, and
the reaction mixture was stirred overnight at room temperature. The
solution turned from orange to red. The solvent was evaporated under
vacuum, and the residue was extracted with toluene to give a dark red
solution. The solution was concentrated and cooled to −35 °C to give
red crystals that were decanted, washed with pentane, and dried under
vacuum. Yield: 221 mg (61%). 1H NMR (C6D6, 300 MHz): δ 8.39 (d,
3J 4.9, 2H, o-CHpy), 6.64 (t, 3J 7.6, 1H, p-CHpy), 6.41 (s, 2H, Me2Pyr),
6.27 (t+s, 2H m-CHpy, + 5H C5H5), 2.34 (s, 6H, Me2Pyr), 1.02 (s, 9H,
NtBu). 13C NMR (C6D6, 125 MHz): δ 151.1 (o-CHpy), 134.5 (quat
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00779.
C
Me2Pyr), 134.3 (p-CHpy), 124.1 (m-CHpy), 110.5 (C5H5), 107.5
IR and NMR spectra (PDF)
(CHMe2Pyr), 68.3 (NCMe3), 32.0 (NCMe3), 19.2 (Me2Pyr) (chemical
shift of quat CMe2Pyr was extracted from HMBC spectrum). Anal. Found
(calcd for C20H27N3Ti): C, 67.35 (67.23); H, 7.58 (7.62); N, 11.65
(11.76). CCDC 1960614 contains the crystallographic data.
[(SiO)Ti(NtBu)Cl(py)2] (2s). An orange benzene solution of
Ti(NtBu)Cl(Me2Pyr)(py)2 (2; 73 mg, 0.18 mmol) was added to a
suspension of SiO2‑700 (694 mg) in benzene at room temperature. The
suspension was slowly stirred overnight while silica became orange and
the solution colorless. The solid was separated by decantation and
Accession Codes
CCDC 1960613−1960614 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
G
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Organometallics XXXX, XXX, XXX−XXX

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pubs.acs.org/Organometallics
Article
Supported Organometallics and Single-Site Catalysis. Catal. Lett. 2017,
AUTHOR INFORMATION
Corresponding Authors
■
́
147, 2247−2259. (c) Coperet, C.; Comas-Vives, A.; Conley, M. P.;
́
Estes, D. P.; Fedorov, A.; Mougel, V.; Nagae, H.; Nunez-Zarur, F.;
̃
Pavel A. Zhizhko − A. N. Nesmeyanov Institute of Organoelement
Compounds, Russian Academy of Sciences, Moscow 119991,
Russia; orcid.org/0000-0001-5026-8150;
Email: zhizhko@ineos.ac.ru
Dmitry N. Zarubin − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
Moscow 119991, Russia; orcid.org/0000-0003-2626-8024;
Email: zaroubine@ineos.ac.ru
Zhizhko, P. A. Surface Organometallic and Coordination Chemistry
toward Single-Site Heterogeneous Catalysts: Strategies, Methods,
Structures, and Activities. Chem. Rev. 2016, 116, 323−421. (d) Pelletier,
J. D. A.; Basset, J.-M. Catalysis by Design: Well-Defined Single-Site
Heterogeneous Catalysts. Acc. Chem. Res. 2016, 49, 664−677.
(e) Stalzer, M. M.; Delferro, M.; Marks, T. J. Supported Single-Site
Organometallic Catalysts for the Synthesis of High-Performance
́
Polyolefins. Catal. Lett. 2015, 145, 3−14. (f) Conley, M. P.; Coperet,
C. State of the Art and Perspectives in the “Molecular Approach”
towards Well-Defined Heterogeneous Catalysts. Top. Catal. 2014, 57,
843−851. (g) Serna, P.; Gates, B. C. Molecular Metal Catalysts on
Supports: Organometallic Chemistry Meets Surface Science. Acc.
Chem. Res. 2014, 47, 2612−2620. (h) Liang, Y.; Anwander, R.
Nanostructured Catalysts via Metal Amide-Promoted Smart Grafting.
Dalton Trans. 2013, 42, 12521−12545. (i) Wegener, S. L.; Marks, T. J.;
Stair, P. C. Design Strategies for the Molecular Level Synthesis of
Supported Catalysts. Acc. Chem. Res. 2012, 45, 206−214. (j) Basset, J.
Authors
Andrey V. Pichugov − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
Moscow 119991, Russia; Higher Chemical College, D. Mendeleev
University of Chemical Technology of Russia, Moscow 125047,
Russia; orcid.org/0000-0001-9584-4684
Nikolai S. Bushkov − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
Moscow 119991, Russia; Department of Chemistry, Moscow
State University, Moscow 119991, Russia; orcid.org/0000-
0001-7803-6300
Andrey V. Rumyantsev − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
Moscow 119991, Russia; Department of Chemistry, Moscow
State University, Moscow 119991, Russia
Kamil I. Utegenov − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
Moscow 119991, Russia
Valeria N. Talanova − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
Moscow 119991, Russia
́
M.; Candy, J. P.; Coperet, C. In Comprehensive Organometallic
Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier:
Oxford, 2007; pp 499−553. (k) Anwander, R. SOMC@PMS. Surface
Organometallic Chemistry at Periodic Mesoporous Silica. Chem. Mater.
2001, 13, 4419−4438. (l) Marks, T. J. Surface-Bound Metal
Hydrocarbyls. Organometallic Connections between Heterogeneous
and Homogeneous Catalysis. Acc. Chem. Res. 1992, 25, 57−65.
(m) Iwasawa, Y. Chemical Design Surfaces for Active Solid Catalysts.
Adv. Catal. 1987, 35, 187−264. (n) Burwell Jr, R. L. Organometallic
Complexes on Alumina. J. Catal. 1984, 86, 301−314. (o) Yermakov, Y.
I. Organometallic Compounds in the Preparation of Supported
Catalysts. J. Mol. Catal. 1983, 21, 35−55. (p) Yermakov, Y. I.
Supported Catalysts Obtained by Interaction of Organometallic
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Supported Organometallic Catalysts. Adv. Chem. Ser. 1974, 132, 212−
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Tatyana V. Strelkova − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
Moscow 119991, Russia
́
(2) (a) Coperet, C.; Estes, D. P.; Larmier, K.; Searles, K. Isolated
Surface Hydrides: Formation, Structure, and Reactivity. Chem. Rev.
Dmitry Lebedev − Department of Chemistry and Applied
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2016, 116, 8463−8505. (b) Coperet, C. Single-Sites and Nanoparticles
Biosciences, ETH Zurich, Zurich CH-8093, Switzerland;
̈ ̈
at Tailored Interfaces Prepared via Surface Organometallic Chemistry
from Thermolytic Molecular Precursors. Acc. Chem. Res. 2019, 52,
1697−1708.
orcid.org/0000-0002-1866-9234
Deni Mance − Department of Chemistry and Applied Biosciences,
̈ ̈
ETH Zurich, Zurich CH-8093, Switzerland
(3) (a) Zhizhko, P. A.; Zhizhin, A. A.; Zarubin, D. N.; Ustynyuk, N. A.
Oxo/Imido Heterometathesis between N-Sulfinylamines and Ketones
Catalyzed by a Silica-Supported Molybdenum Imido Complex.
Mendeleev Commun. 2012, 22, 64−66. (b) Zhizhko, P. A.; Zhizhin, A.
A.; Zarubin, D. N.; Ustynyuk, N. A.; Lemenovskii, D. A.; Shelimov, B.
N.; Kustov, L. M.; Tkachenko, O. P.; Kirakosyan, G. A. Oxo/Imido
Heterometathesis of N-Sulfinylamines and Carbonyl Compounds
Catalyzed by Silica-Supported Vanadium Oxochloride. J. Catal. 2011,
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Molybdenum-Mediated Imido-Transfer Reaction of N-Sulfinylamines
with Dimethylformamide. Mendeleev Commun. 2009, 19, 165−166.
(d) Zhizhin, A. A.; Zarubin, D. N.; Ustynyuk, N. A. An Imido-Transfer
Reaction of Aldehydes with N-Sulfinylamines Using Vanadium and
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Organometallics 1999, 18, 5170−5175. (f) Jain, S. L.; Sharma, V. B.;
Sain, B. Ruthenium Catalyzed Imido-Transfer Reactions of Aldehydes:
An Easy Access to N-Sulfonyl Aldimines under Mild Reaction
Conditions. J. Mol. Catal. A: Chem. 2005, 239, 92−95. (g) Darwish,
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.9b00779
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the Russian Science
Foundation (Grant No. 19-73-10163). Characterization of the
molecular compounds was performed with the financial support
from the Ministry of Science and Higher Education of the
Russian Federation using the equipment of the Center for
Molecular Composition Studies of INEOS RAS. P.A.Z. thanks
́
Prof. C. Coperet and his group at ETH Zu
experiments were performed.
̈
rich, where certain
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