Direct Synthesis of Silylamine from N2and a Silane: Mediated by a Tridentate Phosphine Molybdenum Fragment

Article abstract of DOI:10.1002/anie.201604812

A homogeneous system which is able to yield silylamine from N₂and bis(silane) in one pot is reported. Mechanistically a {(triphosphine)molybdenum(I)} fragment, generated in situ, splits N₂into the corresponding nitrido complex at room temperature. Then, functionalization of the molybdenum nitrido is achieved by double Si?H addition under mild reaction conditions. Moreover, the bis(silyl)amine product is decoordinated from the metal center.

Full text of DOI:10.1002/anie.201604812

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201604812
German Edition: DOI: 10.1002/ange.201604812
Nitrogen Fixation
Direct Synthesis of Silylamine from N and a Silane: Mediated by
2
a Tridentate Phosphine Molybdenum Fragment
Qian Liao, Anthony Cavaillꢀ, Nathalie Saffon-Merceron, and Nicolas Mꢀzailles*
In memory of Pascal Le Floch
Abstract: A homogeneous system which is able to yield
silylamine from N and bis(silane) in one pot is reported.
2
Mechanistically a {(triphosphine)molybdenum(I)} fragment,
generated in situ, splits N2 into the corresponding nitrido
complex at room temperature. Then, functionalization of the
molybdenum nitrido is achieved by double SiÀH addition
under mild reaction conditions. Moreover, the bis(silyl)amine
product is decoordinated from the metal center.
N
fixation at ambient conditions has undergone tremen-
2
dous advances in the past years, yet it still remains a chal-
[1]
lenge. In the first of the two common strategies for N2
reduction, N , coordinated at metal a center(s), reacts with an
2
+
+
electrophile (H , R , or SiMe Cl) and external electrons. In
3
[2]
nature, this process is achieved by nitrogenase enzymes.
Besides that, several carefully designed molecular catalysts
have been reported for this transformation, thus affording
[
3]
[4]
NH3 or the silylamine N(SiMe3)3 (Scheme 1a). It is to be
noted that in these cases, the electrophile also consumes the
electrons directly in an efficient competitive process which
inevitably results in large amounts of unwanted side products
(
H or SiÀSi compounds). In the second strategy, N , also
2
2
coordinated and/or activated by a metal(s), reacts with the
neutral compound EÀH (E = H or SiH R; Scheme 1b). The
2
Scheme 1. Approaches to N reduction and functionalization.
2
Haber–Bosch process, requiring high pressures and temper-
atures (350–5508C, 150–350 atm), is a representative example
of this strategy employing heterogeneous catalysts. In this
[
5]
[8]
nitrogen product. In most cases, a strong acid such as HCl
case, N is split at the surface of the catalyst into a nitrido
derivative. Functionalization of the nitrido into ammonia
has to be employed to cleave the MÀN bond, thus preventing
2
further development of catalytic systems. To date, the
5
then requires activation of H at the catalyst surface followed
{[(h -C Me H) Zr] (m-N )} complex is the only example that
2
5
4
2
2
2
by sequential hydrogen transfers to terminal nitrogen moi-
eties. It allows the synthesis of about 100 million tons/year of
can be hydrogenated to yield substoichiometric amounts of
[6]
[9]
NH (10–15%). Herein we describe the first molybdenum-
3
ammonia from N2 and H . For a homogeneous system,
(I)-based homogeneous system which is able to a) reduce and
2
although early-transition-metal N complexes (group IV and
cleave N into the corresponding nitrido complex, as well as
2
2
[7]
V) display rich reactivity towards EÀH bonds (E = H, Si, B),
the strength of MÀN bonds prevents the release of the
b) enables the double HÀSi addition across the MoꢀN bond
to afford bis(silyl)amine species (Scheme 1c). Finally, the
molybdenum(I) fragment reacts preferentially with N in the
2
presence of the bis(silane), and thus opens the way for the
development of a catalytic process.
[
*] Dr. Q. Liao, A. Cavaillꢀ, Dr. N. Mꢀzailles
Laboratoire Hꢀtꢀrochimie Fondamentale et Appliquꢀe
Universitꢀ Paul Sabatier, CNRS
Our strategy for N2 functionalization relied on the
splitting of N in a first step, and this process has only been
2
118 Route de Narbonne, 31062 Toulouse (France)
[10]
[11]
reported to occur at a few molybdenum, rhenium, and
iron centers (thermally or photochemically). We recently
reported that the [(P P2 )Mo(N ) (m-N )Mo(N ) (P P )]
E-mail: mezailles@chimie.ups-tlse.fr
[
12]
Dr. N. Saffon-Merceron
Institut de Chimie de Toulouse ICT-FR2599, Universitꢀ Paul Sabatier
Ph Cy
Ph Cy
2
2
2
2
2
2
molybdenum(0) dimer complex did not result in N splitting,
31062 Toulouse Cedex (France)
2
although it could be used as catalyst for the reduction of N
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
2
[13]
into N(SiMe ) (Scheme 1a). The corresponding stoichio-
3
3
1002/anie.201604812.
metric N functionalization study allowed us to isolate the
2
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1
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molybdenum(IV) nitrido complex. We postulated that the
temperature (Scheme 2b). It was isolated in 70% yield, and
characterized by multinuclear NMR spectroscopy (see the
Supporting Information). Formation of the NÀSi bond was
same complex could be obtained by the direct N splitting
2
(
six-electron reduction) by two unsaturated molybdenum(I)
2
9
1
31
fragments, which are generated in situ under N . Reduction of
shown by a singlet at d = À44.6 ppm in the Si{ H, P} NMR
2
Ph Cy
the [(P P2 )MoCl ] complex 1 with two equivalents of Na/
spectrum (Dd = 16 ppm vs. PhSiH ). Further evidence for 1,2-
3
3
Hg in the presence of NaI was thus attempted under an N2
atmosphere (1 atm) at room temperature (Scheme 2a). A
addition of the SiÀH bond across the MoꢀN bond was given
1
by a hydride resonance in the HNMR spectrum at d =
2
2
À5.97 ppm [dt, J(P,H) = 60 Hz, J(P,H) =
5
1 Hz, 1H, Mo-H]. Final proof was given
by X-ray structure analysis (Figure 1, left).
In 4, the MoÀN bond distance of 1.787-
(4) ꢀ is much longer than in the starting 3
[
1.656(2) ꢀ] and is indicative of a decrease
in bond order as expected upon function-
alization.
Having achieved a first hydrosilylation
of the molybdenum nitrido, we postulated
that an intramolecular approach might
favor a second SiÀH addition, and the
reaction between the bis(silane) HSiMe -
2
(
CH ) SiMe H and 3 was performed.
2 2 2
Upon heating at 508C, an equilibrium
mixture of the starting nitrido complex
and a new diamagnetic complex, 5, was
formed. This result is readily explained by
the lower reactivity of R SiH versus
3
PhSiH , and in turn results in a thermody-
3
namically less favorable process. A twenty-
fold excess of the bis(silane) was required
to allow full displacement of the equilib-
Scheme 2. N splitting and hydrosilylation on molybdenum. a) Two alternative synthetic
2
strategies for N splitting. b) Reactivity of the nitrido complex 3 toward silane and bis(silane). rium toward 5 (see Figures S8–S10). Iso-
2
Ph Cy
single diamagnetic complex, the known [(P P )Mo(N)(I)]
2
[13]
31
complex 3, was observed by P NMR spectroscopy (two
singlets at d = 69 and 121 ppm; see Figure S2 in the Support-
ing Information), and crystallized in 60% yield.
0
It is to be noted that no [Mo (N )] complex, resulting from
2
over reduction, was observed in this reaction (either by NMR
and IR). Mechanistically, the formation of 3 implies the
intermediacy of a molybdenum(I) dimer featuring a bridging
N ligand, for example, the complex 2 (Scheme 2a). Alter-
2
natively, NaHBEt (two equiv) can be used as a reducing
3
agent, but it results in a lower yield of 3 (ca. 30%). When
1
5
15
labelled N was used to prepare [ N]3, the signal of the
2
Ph Cy
1
5
Figure 1. X-ray structures of [(P P
2
)Mo(H)(I)(NSiH
Ph)] (4) and
2
nitrido ligand was found at d = 830 ppm ( N NMR spectros-
Ph Cy
[20]
[
(P P )Mo(PMe )(H)(I)] (8). Thermal ellipsoids shown at 50%
2
3
copy).
probability, except for the carbon atoms on silicon and the carbon
atoms in the Cy and Ph groups. Hydrogen atoms, except for the
hydrides, have been omitted for clarity.
[
14]
Addition of an EÀH bond to a terminal MꢀN
and
has been reported in rare instances. Moreover,
none of these complexes were prepared from N , except
[
15]
M=NR
2
[
16]
a single recent example of the addition to Fe=NÀNR .
A
2
metal fragment that can achieve N splitting, followed by
lation of this complex in pure form was not possible because
of the favorable reverse reaction to the nitrido. However, the
spectra recorded from the crude mixture showed a signal at
2
nitride and imide functionalization by EÀH bonds all the way
[
17]
to the free amine has never been reported.
Thus the
2
reactivity of MoꢀN with either HÀH or HÀSiR was studied.
d = À5.94 ppm [q, J(P,H) = 56 Hz, 1H, Mo-H] in the
3
1
No reaction between 3 and H was observed but the same
H NMR spectrum, along with two signals at d = À2.5 and
2
2
9
1
31
reaction with a stoichiometric amount of PhSiH resulted in
the formation of a single diamagnetic complex [4; 79% yield
À10.2 ppm in the Si{ H, P} spectrum for the NSi and SiH
moieties, respectively. Together, these data confirmed the 1,2-
addition of one SiÀH bond of the bis(silane) to the MoꢀN
3
(
NMR) vs. internal standard] within 30 minutes at room
2
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bond. In a second stage, the crude mixture was heated at 808C
to promote the second intramolecular SiÀH insertion.
of the second SiÀH on the imido complex, followed by
elimination of the amine from the coordination sphere.
Finally, the development of a proper catalytic process
requires that each intermediate reacts selectively with one of
the reagents. So far, this has never been achieved. For
2
9
1
The Si{ H} NMR spectrum showed the formation of the
bis(silyl)amine 6 (d = 11.7 ppm, compared to authentic
sample) in 77% yield (based on 3) as determined by GC-
MS analysis of the crude reaction mixture (Scheme 2b). The
formation of 6 implies a reduction process at the molybdenum
center to give molybdenum(II). Isolation of the complex 7 has
+
example, the classical use of the strongly oxidizing Me to
[18]
functionalize the nitrido intermediate
to give nitrile/
isonitrile derivatives is incompatible with the reaction con-
not been achieved so far, but we propose it to be an N -free,
ditions required for N reduction. We have thus performed
2
2
unsaturated complex, because of the lack of a MoÀN IR
the one-pot reaction between 1, 2 equivalents of Na/Hg, and
2
stretch. We could however prove the reduction to
molybdenum(II) when the reaction was carried out in the
20 equivalents of the bis(silane) under N (see Figure S18).
2
Most satisfyingly, 6 was produced in 57%, which is higher
presence of PMe . The diamagnetic complex 8 was isolated in
than the yields of the two isolated steps, that is, 46% (60% for
3
[19]
5
0% yield from the crude reaction mixture (Figure 1, right).
N2 splitting and 77% for amine formation).
When the
To definitely prove the functionalization of the nitrido N, the
reaction was carried out under argon, two diamagnetic
complexes featuring hydride ligands were observed.
Although they have not been characterized so far, these
complexes obviously result from oxidative addition in the
SiÀH bonds. Most importantly for our purposes, these two
1
4
15
same reaction was carried out under N with [ N]3. This
2
1
5
15
reaction yielded the N-labeled silylamine [ N]6 as the only
2
9
amine as proved unambiguously by GC-MS and Si NMR
spectra (Figure 2). Not only did this experiment prove the
origin of the N atom of the 6, but it also proved the non-
reversibility of the splitting of 2 into 3. Heating 5 therefore
resulted in the unprecedented overall apparent 1,1-addition
reactions prove that in situ generated molybdenum(I) species
react much faster with N to form the nitrido complex rather
2
than with the silane, and this preference is a key for further
developments.
In conclusion, we show for the first time that a homoge-
Ph Cy
I
neous metal fragment, {(P P2 )Mo }, is both able to split N
2
and allow subsequent functionalization of the generated
nitrido complex by SiÀH bonds all the way to the free amine
under very mild reaction conditions (atm. pressure of N , mild
2
heating). In the first step, each metal fragment acts as a three-
electron reducing agent for N , while the SiÀH moieties
2
provide the electrons needed for the two functionalizing steps.
These findings pave the way for the development of a catalytic
process as proposed in Scheme 3, and will require the
1
5
15
Figure 2. Reactivity of N-labeled complex [ N]3 with bis(silane)
Scheme 3. Potential four-step homogeneous catalytic cycle for the
1
4
29
1
15
under N . A,B) Si{ H} NMR spectra of compound 6 and [ N]6.
C,D) Mass spectra of 6 and [ N]6.
functionalization of N using bis(silane) built on the three stoichiomet-
2
2
1
5
ric steps presented herein.
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Angewandte
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reduction of the molybdenum(II) into molybdenum(I)
dotted arrow in Scheme 3). Current work is dedicated to
this goal.
Kuriyama, K. Arashiba, K. Nakajima, H. Tanaka, N. Kamaru,
K. Yoshizawa, Y. Nishibayashi, J. Am. Chem. Soc. 2014, 136,
(
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Experimental Section
1
5
Ph Cy
Synthesis of [ N]3: [Mo(P P2 )Cl ]·(THF)2 (1; 36.0 mg,
3
0
3
.0400 mmol) and NaI (12.0 mg, 0.0800 mmol) were dissolved in
mL of THF in a schlenk tube. Then 182 mg of freshly prepared
2015, 6, 3940.
sodium amalgam (1.0 wt% Na) was put in another small test tube
inside the schlenk tube. The solution was degassed by four freeze-
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pump-thaw cycles and then backfilled with N2 ( ꢁ 1 atm). After
stirring for 15 min, the solution and the sodium amalgam were mixed
by inverting the schlenk tube. The mixture was stirred for 4 hours,
thus affording a light-brown solution. After the solvent was removed
under vacuum, 5 mL of toluene were added to extract the residue.
Insoluble residues were separated by centrifugation and the solution
containing the product was dried under vacuum. Green crystals of
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[
Mo(P P2 )( n)(I)] ([ N]3) were grown by vapor diffusion of n-
pentane into its THF solution. Then the mother liquor was filtered,
and the crystals were collected, rinsed with minimum of n-pentane,
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Synthesis of 8: Complex 3, [Mo(P P2 )(N)(I)] (31.9 mg,
0
.0400 mmol) and HMe SiCH CH SiMe H (0.80 mmol) were mixed
2 2 2 2
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Acknowledgments
Financial support from CNRS and Universitꢂ de Toulouse is
acknowledged. N.M. thanks the “Rꢂgion Midi-Pyrꢂnꢂes” as
well as the IDEX of the Universitꢂ Fꢂdꢂrale Toulouse Midi-
Pyrꢂnꢂes for generous funding, including a post-doctoral
fellowship for Q.L. A.C. thanks the Ecole Polytechnique for
a Ph.D. fellowship.
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Keywords: molybdenum · nitrogen fixation · P ligands ·
reduction · silanes
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Angewandte
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[19] Alternative pathways resulting in the formation of the silylamine
Ph Cy
6 were envisaged from the reactions between (P P )MoCl3,
2
Ph Cy
0
Ph Cy
(P P )Mo (N ) or (P P )Mo=N-N(SiMe -CH -CH -SiMe )
2
2
x
2
2
2
2
2
with excess of bis(silane) (Ref. [13]). None of them led to
formation of the desired product.
2
015, 137, 10140.
[20] CCDC 1455338 (4) and 1455339 (8) contain the supplementary
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Centre.
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16] D. L. M. Suess, J. C. Peters, J. Am. Chem. Soc. 2013, 135, 4938.
17] Protonation or alkylation of molybdenum and rhenium terminal
nitrido complexes has been reported (Refs [3a,11] and [18a]).
Protonation of iron nitrido: K. C. MacLeod, S. F. McWilliams,
B. Q. Mercado, P. L. Holland, Chem. Sci. 2016, DOI: 10.1039/
C6SC00423G.
[
18] a) J. J. Curley, E. L. Sceats, C. C. Cummins, J. Am. Chem. Soc.
Received: May 17, 2016
2006, 128, 14036; b) J. J. Curley, A. F. Cozzolino, C. C. Cummins,
Revised: June 24, 2016
Dalton Trans. 2011, 40, 2429; c) I. Klopsch, M. Kinauer, M.
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Nitrogen Fixation
Split up: Reaction of N with an in situ
2
generated molybdenum(I) fragment sta-
bilized by a tridentate phosphine ligand,
Q. Liao, A. Cavaillꢁ, N. Saffon-Merceron,
N. Mꢁzailles*
&&&& — &&&&
results in direct N splitting and forma-
2
tion of the nitrido complex. Functionali-
zation of the nitrido all the way to the
formation of bis(silyl)amine can be ach-
ieved under mild reaction conditions with
bis(silane).
Direct Synthesis of Silylamine from N2
and a Silane: Mediated by a Tridentate
Phosphine Molybdenum Fragment
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!