Conversion of Dinitrogen to Nitriles at a Multinuclear Titanium Framework

Full text of DOI:10.1002/anie.201607426

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International Edition: DOI: 10.1002/anie.201607426
German Edition: DOI: 10.1002/ange.201607426
Nitrogen Fixation
Hot Paper
Conversion of Dinitrogen to Nitriles at a Multinuclear Titanium
Framework
Murali Mohan Guru, Takanori Shima, and Zhaomin Hou*
At present, ammonia (NH3), produced by the energy-intensive
Haber–Bosch process, is the only nitrogen source for the
industrial preparation of nitrogen-containing organic products.
Although the activation and functionalization of dinitrogen
(N2) under milder conditions have received much recent
interest, studies on the direct use of N2 as a feedstock for
organic synthesis are still in their infancy. Herein we report the
synthesis of nitriles using titanium-activated nitrogen species as
a nitrogen source. We have found that a mixed diimide/
dinitride tetranuclear titanium complex generated by N2
activation serves as a unique platform for the synthesis of
nitriles through reaction with acid chlorides. This protocol
features simple reaction condition (608C in benzene), no
requirement for extra reagents, and unprecedented functional
In order to develop alternative routes without using NH3
for the synthesis of nitrogen-containing chemicals, much
effort has been devoted to the activation of N2 using
molecular organometallic complexes in the past decades. It
has been reported that the reduction and cleavage of N2 could
be achieved under mild conditions by use of a combination of
transition metal complexes with strongly reducing metal
reagents such as KC8, Na/Hg, and Mg[5] or with H2.[6] The
nitrogen species split in this way were evaluated in a number
[7]
À
of E N (E = H, C, Si, B) bond formation reactions.
Regarding nitrile synthesis via the direct use of metal-
activated dinitrogen species, Cummins and co-workers
reported the conversion of molybdenum and niobium nitride
species to nitriles through reaction with acid chlorides
(Scheme 1a).[8] Very recently, Schneider and co-workers
reported the synthesis of acetonitrile through the reaction
of a rhenium nitride species with ethyl triflate (Scheme 1b).[9]
Despite these advances, the development of new molecular
systems for the synthesis of nitriles using an N2-based,
functional group compatible route remained an important
and attractive research subject.
À
group tolerance (compatible with aromatic C X (X = Cl, Br, I)
bonds, nitro group, and ammonia-sensitive aldehyde and
chloromethyl moieties).
Activation and functionalization of dinitrogen (N2) is
a long-standing important research subject, as N2 is an
abundant and easily accessible resource but it is chemically
inert under ordinary conditions. The only commercially
successful process using N2 gas as a reagent is the Haber–
Bosch reaction, in which N2 is completely reduced to
ammonia (NH3) with H2 at high temperature (350–5508C)
and high pressure (150–350 atm) on solid catalysts.[1] To date,
NH3 (the fully hydrogenated form of N2) is the only nitrogen
source for the industrial preparation of nitrogen-containing
organic products, while the direct use of N2 as a feedstock for
organic synthesis has remained a challenge.
We recently reported the formation of a tetranuclear
titanium diimide/tetrahydride complex [(Cp’Ti)4(m3-NH)2(m2-
H)4] (1, Cp’ = C5Me4SiMe3) by hydrogenolysis of a half-
sandwich titanium trialkyl complex [Cp’Ti(CH2SiMe3)3] with
H2 in the presence of N2.[6a] This complex represents the first
example of a tetranuclear metal complex derived from N2
activation. In this paper, we report the conversion of
ꢀ
Organic nitriles (RC N) are versatile intermediates for
production of pharmaceuticals, agrochemicals, dyes, pig-
ments, polyamides, and various fine chemicals.[2] Numerous
ways are known for the preparation of nitriles, most of which
are based on NH3.[3] Cyanation reactions catalyzed or
mediated by copper and palladium compounds were also
reported for the synthesis of nitriles,[4] but this approach
required the use of a toxic cyanide source whose synthesis
again required NH3.
[*] Dr. M. M. Guru, Dr. T. Shima, Prof. Dr. Z. Hou
Advanced Catalysis Research Group, RIKEN
Center for Sustainable Resource Science
2-1 Hirosawa, Wako, Saitama 351-0198 (Japan)
E-mail: houz@riken.jp
Dr. T. Shima, Prof. Dr. Z. Hou
Organometallic Chemistry Laboratory, RIKEN
2-1 Hirosawa, Wako, Saitama 351-0198 (Japan)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201607426.
Scheme 1. Nitrile synthesis through conversion of metal-activated
dinitrogen species.
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complex 1 to a novel diimide/dinitride complex [(Cp’Ti)4(m3-
NH)2(m3-N)2] (2) by reaction with N2 and the use of the
diimide/dinitride complex 2 as a nitrogen source for the
synthesis of nitriles. We found that the reaction of the mixed
diimide/dinitride complex 2 with various acid chlorides
straightforwardly afforded the corresponding nitriles under
simple conditions without requiring any extra reagents
(Scheme 1c). This transformation was compatible with
a broad range of acid chloride substrates, including those
with p-toluoyl chloride 4a was investigated. When the mixed
diimide/dinitride complex 2 was heated with four equiv of 4a
in benzene at 608C for 12 h, p-tolunitrile 5a was isolated in
85% yield (Table 1).[11] In contrast, no reaction was observed
between the tetranitride complexes 3 and 4a under the same
conditions.[12] These results suggest that the imide titanium
species should be much more reactive than the nitrides.
The reaction of 2 with various acid chlorides was then
examined in benzene at 608C. As shown in Table 1, aromatic
acid chlorides bearing either electron-donating or electron-
withdrawing groups were suitable substrates for this reaction.
À
with functional groups such as aromatic C X (X = Cl, Br, I)
bonds, nitro group, and ammonia-sensitive aldehyde and
chloromethyl moieties. 15N-isotope-labeled nitriles can also
be efficiently prepared by using the 15N-enriched analog 2-15N
derived from 15N2 gas.
Table 1: Synthesis of nitriles by reaction of a mixed imide/nitride
titanium complex 2 with acid chlorides 4.[a]
When the purple-brown titanium diimide tetrahydride
complex 1 was heated in solid state under N2 (1 atm) at 1808C
for two days, a dark-blue solid was obtained (Scheme 2).
Scheme 2. Dinitrogen activation by a tetranuclear titanium hydride/
imide complex.
1H NMR analysis suggests that the product may contain
a mixed valence TiIII/TiIV diimide/dinitride complex [(Cp’Ti)4-
(m3-NH)2(m3-N)2] (2) as a major component (ca. 95%)
together with a small amount of paramagnetic species (ca.
5%). Attempts to separate 2 from the paramagnetic species
by repeated recrystallization in various solvents did not give
a pure product. Nevertheless, micro elemental analysis was in
agreement with the formula of 2 (see the Supporting
Information). The unique Ti4N2(NH)2 cubane framework in
2 was confirmed by X-ray crystallographic studies (see the
Supporting Information). These results suggest that both 2
and the unidentified species should have the similar crystal-
linity and similar composition.[10] In an analogous fashion, the
reaction of [(Cp’Ti)4(m3-15NH)2(m-H)4] (1-15N), which was
obtained by reaction of [Cp’Ti(CH2SiMe3)3] with 15N2 and
H2,[6a] with 15N2 afforded the 15N-enriched diimide/dinitride
analog 2-15N.
When 2 or 2-15N was allowed to react with 1.5 molar equiv
of p-benzoquinone at room temperature, the corresponding
dehydrogenated tetranitride complex 3 or 3-15N was obtained,
respectively, as dark green solid in about 80% yield
(Scheme 2). The 15N NMR spectrum of 3-15N showed
a sharp singlet at dN 500.8 (using MeNO2 as standard
reference), in contrast to that of 2-15N which showed
a broad singlet at dN 379.0 for the m3-15N species and
a broad singlet at dN À101.5 for the m3-15NH unit. The solid
structure of 3 was confirmed by X-ray crystallographic studies
(see the Supporting Information).
[a] Reaction conditions: 2 or 2-15N (0.02 mmol, 1 equiv), 4 (0.08 mmol,
4 equiv), benzene (2 mL), 12 h, isolated yield, unless otherwise noted.
[b] 2 (0.01 mmol, 1 equiv), 4m (0.05 mmol, 5 equiv), C6D6 (0.4 mL),
808C, 1 day, NMR yield. [c] 2 (0.01 mmol, 1 equiv), 4n (0.06 mmol,
6 equiv), C6D6 (0.4 mL), 1008C, 2 days, NMR yield.
In order to see if the imide and nitride species in
a tetranuclear titanium framework could be incorporated
into an organic substrate, the reaction of complexes 2 and 3
2
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The reaction of para tert-butyl (4b)- and methoxy (4c)-
substituted benzoyl chlorides gave the desired nitrile products
5b and 5c in 78% and 76% isolated yields, respectively.
and dN À176.3, which are assignable to the bridging nitride (m-
15N) and amidate (15NHC(O)R) units, respectively. The
13C NMR spectrum of 6-15N showed a doublet at dC 181.6
with J15NC = 12.5 Hz for the amidate carbon atom. An X-ray
crystallographic diffraction analysis of 6 confirmed the
molecular framework (see the Supporting Information), but
further refinement was difficult because of disorder problems.
When 6 alone was heated in benzene at 608C for 12 h,
only a trace amount of cinnamyl nitrile 5k was observed.
However, heating 6 with an equimolar amount of cinnamoyl
chloride 4k gave 5k in 80% yield, together with an oxo-
bridged binuclear titanium chloride complex 7 (42% yield)
and some uncharacterized paramagnetic titanium species
(Scheme 3). The solid structure of 7 was confirmed by X-ray
diffraction study (see the Supporting Information).
À
Aromatic C X (X = Cl (4d), Br (4e), I (4 f)) bonds were
compatible with the reaction conditions, leading to formation
of the corresponding halogenated nitrile products 5d–f in
good yields. The reaction of 2 with p-nitrobenzoyl chloride 4g
selectively afforded the NO2-substituted benzonitrile product
5g. More remarkably, ammonia-sensitive functional groups
such as aldehyde (4h) and chloromethyl (4i) also survived the
present reaction conditions. 2-Methylbenzoyl chloride (4j)
was easily transformed to o-tolunitrile 5j, showing that
a substituent at the ortho position of the acyl chloride group
does not obstruct the reaction. a,b-Unsaturated acid chloride
such as cinnamoyl chloride 4k selectively afforded the
corresponding a,b-unsaturated nitrile product 5k. The reac-
tion of phenylacetyl chloride 4l gave the desired product
phenylacetonitrile 5l in high yield. Representative aliphatic
nitriles such as acetonitrile (5m) and pivalonitrile (5n) were
easily prepared by the corresponding reactions of acetyl
chloride 4m and pivaloyl chloride 4n with 2.
On the basis of the above experimental observations,
a proposed reaction mechanism is shown in Scheme 4.
The 15N-enriched nitriles 5-15N were efficiently obtained
from the reaction of 2-15N with acid chlorides in an analogous
fashion (Table 1). As a typical example, the C 15N unit in 4-
ꢀ
bromobenzonitrile-15N (5e-15N) showed a singlet at dN À115.0
in the 15N NMR spectrum and a doublet at dC 118.3 (J15NC
=
16.3 Hz) in the 13C NMR spectrum. The C 15N bond in 5e-15N
showed a strong signal at 2198 cmÀ1 in the IR spectrum, in
contrast to that of 5e (2222 cmÀ1).
ꢀ
The present conversion of the diimide/dinitride titanium
complex 2 to nitriles did not require any extra additives
ꢀ
(either reducing agent or base) for the formation of a C N
group, which stands in contrast with what was observed
previously in the case of molybdenum[8] and rhenium[9] nitride
species. To gain information on the reaction mechanism of the
present transformation, we tried to isolate and characterize
some reaction intermediates. In the reaction of 2 with
cinnamoyl chloride 4k, a binuclear titanium complex 6 with
a bridging m-amidate unit, a m-nitride unit and a terminal
chloride ligand bonding to each Ti atom was isolated in 30%
yield, after the reaction mixture was heated at 608C for 3 h
(Scheme 3). The reaction of 2-15N with 4k under the same
conditions gave the 15N-enriched analog 6-15N in 25% isolated
yield. The 1H NMR spectrum of 6-15N showed a doublet at dH
6.41 (J15NH = 67.0 Hz, 1H) for the amidate NH unit. The 15N
NMR spectrum of 6-15N gave two sharp singlets at dN 433.6
Scheme 4. A plausible mechanism for nitrile formation.
Nucleophilic addition of the imide units in 2 to the CO
group of acid chloride 4 could give B via a possible transition
state A. Chloride migration to titanium would lead to
formation of C, which is analogous (albeit not identical) to
the amidate/nitride intermediate 6 isolated in the reaction of 2
with 4k.[13] The reaction of C with two molecules of acid
chloride 4 would release the nitrile product 5 and give D,
possibly through proton transfer from the amidate units to the
À
nitride ligands and C O bond cleavage in C followed by
addition of the resulting imide species to the acid chloride 4.
Chloride migration to the metal center could give E, which
upon dehydroxylation of the amidate units would release the
nitrile product 5 and generate a mixed oxo/hydroxyl/chloride
complex F. Ligand redistribution in F could yield the binu-
clear titanium oxo/chloride complex 7, which was experimen-
tally confirmed, together with uncharacterized paramagnetic
TiIII species such as G. The reaction of 6 with 4k to give 5k and
7 observed experimentally (Scheme 3) might take place
through a process similar to the reaction of C with 4 to give
5, 7, and G via D, E, and F.[13]
Scheme 3. Isolation and transformation of a binuclear titanium ami-
date/nitride complex 6.
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To demonstrate the recyclability of the titanium com-
plexes in the present N2 activation and functionalization,
a crude reaction mixture of 2 and an acid chloride was treated
with 1.0m HCl in Et2O. The titanium trichloride complex was
isolated in 83% yield. As reported previously,[6a] the titanium
trichloride complex [Cp’TiCl3] could be easily transformed to
the trialkyl complex [Cp’Ti(CH2SiMe3)3], which was able to
activate N2 with H2. Thus, a synthetic cycle for the trans-
formation of N2 and acid chlorides to nitriles mediated by
titanium could be realized as shown in Scheme 5.
(grant number 21429201) from National Science Foundation
of China (NSFC).
Keywords: dinitrogen activation · hydrides · nitriles ·
organometallics · titanium
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Scheme 5. A synthetic cycle for the transformation of N2 and acid
chlorides to nitriles mediated by titanium complexes.
In summary, we have demonstrated that a mixed diimide/
dinitride tetranuclear titanium complex 2 derived from the
activation of N2 can be easily converted to nitriles through
reaction with acid chlorides. The reaction takes place
efficiently under mild conditions without requirement for
any extra reagents. Functional groups such as aromatic C-X
(X = Cl, Br, I) bonds, nitro group, and ammonia-sensitive
aldehyde and chloromethyl moieties are compatible. 15N-
isotope-labeled nitriles can be efficiently prepared by using
the 15N-enriched analog 2-15N derived from 15N2 gas. The
titanium complexes are recyclable in the present conversion
of N2 to nitriles. Moreover, the difference in behaviors for the
nitrile formation observed among the diimide/dinitride com-
plex 2, its tetranitride analog 3, and other metal nitride
species reported in the literature demonstrates that an
appropriate metal/ligand combination is highly important
for the transformation of metal-activated nitrogen species.
Studies on the activation and functionalization of dinitrogen
by titanium complexes with different supporting ligands are
currently in progress.
CCDC-1493845 (2), CCDC-1493846 (3), CCDC-1493848
(6), and CCDC-1493847 (7) contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge
Crystallographic Data Centre.
Acknowledgements
This work was supported in part by Grant-in-Aids for
Scientific Research (S) 26220802 and (C) 26410082 from
Japan Society for Promotion of Science (JSPS) and a grant
4
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[11] Other solvents such as toluene and THF were also suitable for
this reaction, albeit giving a slightly lower yield of 5a (ca. 70%).
[12] When the tetranitride complex 3 and 4a was heated at 1208C
overnight, 5a was obtained in about 20% yield.
[13] The dimeric titanium amidate/nitride complex 6 could possibly
be formed through isomerization (or rearrangement) of a reac-
tion intermediate like C during the isolation process. For
simplicity, the reaction mechanism is proposed on the basis of
the original tetranuclear framework rather than the isolated
dimeric species 6.
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[10] The unidentified paramagnetic species might possibly be a fur-
ther dehydrogenated trinitride monoimide species like [(Cp’Ti)4-
(m3-N)3(m3-NH)]. For more details, see the Supporting Informa-
Received: August 1, 2016
Published online: && &&, &&&&
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Communications
Nitrogen Fixation
M. M. Guru, T. Shima,
Z. Hou*
&&&& — &&&&
Conversion of Dinitrogen to Nitriles at
a Multinuclear Titanium Framework
À
Activation and functionalization of N2: A
mixed diimide/dinitride tetranuclear tita-
nium complex formed by activation of
dinitrogen served as a unique platform
for the synthesis of nitriles (see picture).
Functional groups such as aromatic C X
(X=Cl, Br, I) bonds, nitro groups, and
ammonia-sensitive aldehyde and chloro-
methyl moieties were compatible with the
synthetic method.
6
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