Bimetallic Cooperative Cleavage of Dinitrogen to Nitride and Tandem Frustrated Lewis Pair Hydrogenation to Ammonia

Article abstract of DOI:10.1002/anie.201902195

Although reductive cleavage of dinitrogen (N₂) to nitride (N^3?) and hydrogenation with dihydrogen (H₂) to yield ammonia (NH₃) is accomplished in heterogeneous Haber–Bosch industrial processes on a vast scale, sequentially coupling these elementary reactions together with a single metal complex remains a major challenge for homogeneous molecular complexes. Herein, we report that the reaction of a chloro titanium triamidoamine complex with magnesium effects complete reductive cleavage of N₂ to give a dinitride dititanium dimagnesium ditriamidoamine complex. Tandem H₂ splitting by a phosphine–borane frustrated Lewis pair (FLP) shuttles H atoms to the N^3?, evolving NH₃. Isotope labelling experiments confirmed N₂ and H₂ fixation. Though not yet catalytic, these results give unprecedented insight into coupling N₂ and H₂ cleavage and N?H bond formation steps together, highlight the importance of heterobimetallic cooperativity in N₂ activation, and establish FLPs in NH₃ synthesis.

Full text of DOI:10.1002/anie.201902195

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Accepted Article
Title: Bimetallic Cooperative Cleavage of Dinitrogen to Nitride and
Tandem Frustrated Lewis Pair Hydrogenation to Ammonia
Authors: Stephen Liddle, Laurence Doyle, and Ashley Wooles
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902195
Angew. Chem. 10.1002/ange.201902195
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http://dx.doi.org/10.1002/ange.201902195

Angewandte Chemie International Edition
10.1002/anie.201902195
COMMUNICATION
Bimetallic Cooperative Cleavage of Dinitrogen to Nitride and
Tandem Frustrated Lewis Pair Hydrogenation to Ammonia
Laurence R. Doyle, Ashley J. Wooles, and Stephen T. Liddle*
3
-
[6]
Abstract: Although reductive cleavage of dinitrogen (N
2
) to nitride
that spontaneously cleave N
resulting metal-nitrides react with H
react with H , to make a metal-amide, M-NH
derive from azides rather than N
bind N and react further with H
3
coordinated N after exposure to
2
to N are rare, and none of the
; other metal-nitrides can
, or NH , but they
-splitting. Very few complexes
, giving mono-protonation of
-
(
N
3
) and hydrogenation with dihydrogen (H
2
) to yield ammonia (NH
3
)
2
is accomplished in heterogeneous Haber Bosch industrial processes
on a vast scale, sequentially coupling these elementary reactions
together with a single metal complex remains a major challenge for
homogeneous molecular complexes. Here, we report that reaction of
a chloro titanium triamidoamine complex with magnesium effects
2
2
3
[
7]
2
2
2
[
8]
[9]
2
,
NH
Weakly acidic M-H
complexes to give M-NNH
3
in low yield, or NH
+
[10]
H .
M-N
formed.
cleaving H
alternative approach that side-steps the H
transition metal hydrides can activate N and cleave it to N ,
but no subsequent hydrogenation occurs,
2
complexes can effect protonation of
[
11]
complete reductive cleavage of N
dimagnesium ditriamidoamine complex. Tandem H
phosphine-borane Frustrated Lewis Pair (FLP) shuttles H-atoms to
2
to give a dinitride dititanium
2
2
,
and in one case NH
Thus, in contrast to HB NH synthesis, activating and
appears to be the impediment to progress. In an
-activation issue,
3
was
[
11b]
2
splitting by a
3
2
3
-
the N , evolving NH
-fixation. Though not yet catalytic, these results give
unprecedented insight into coupling N - and H -cleavage and N-H
bond formation steps together, highlight the importance of
heterobimetallic cooperativity in N -activation, and establish FLPs in
NH synthesis.
3
. Isotopic labelling experiments confirm N
2
- and
2
3
- [12]
H
2
2
[
12a,b]
2
2
or reactions stop
It has been proposed that
Frustrated Lewis Pairs (FLPs), that are capable of activating and
splitting H , could be combined with metal-based reductive
cleavage of N
FLP-mediated hydrogenations of organic substrates,
combining this concept with N activation to make NH has
remained an unmet goal, but mono-/di-protonation of metal-N
units, borylation/silylation of M-N -B FLPs, and a model for FLP-
[
9,12c,d,13]
2
at µ-N(H) or µ-NH stages.
2
3
2
[
14]
2
.
However, despite the burgeoning nature of
[
15]
Ammonia, NH
vast array of organo-nitrogen compounds. Nitrogenase
enzymes produce NH from dinitrogen, at ambient
3
, is a key chemical feedstock for the production of
2
3
[
1]
a
[16]
2
3
2
N ,
2
+
temperature and pressure, but this requires eight protons, H ,
[17]
N
2
activation have now been realised.
Here, we report homogeneous reductive cleavage of N
by a Ti-Mg complex supported by a simple triamidoamine
–
eight electrons, e , and hydrolysis of sixteen ATP molecules per
2
to
[
2]
N
2
.
In contrast, chemical industry produces NH
Haber Bosch, HB, process where N and H combine to give
NH over heterogeneous metal catalysts. However, high
temperatures and pressures are required in HB NH synthesis,
reflecting the challenge of cleaving the N triple bond.
Following the inception of HB NH synthesis, considerable
efforts have been made to gain insight into the chemical and
3
using the
3-
N
2
2
3-
ligand, and conversion of N to NH
split by a FLP. Heterobimetallic cooperativity facilitates reductive
cleavage of N , and by using a FLP the barrier to activating and
cleaving H is overcome. The bridging nature of the nitride
renders it amenable to full protonation to give NH that can be
removed by vacuum distillation without the need for acidification,
thereby introducing FLPs to NH -synthesis.
3 2
using hydrogen from H
[
2]
3
3
2
2
2
3
3
[
3]
physical processes involved. The rate-limiting step is N
chemisorption on the metal catalyst and N-N bond cleavage to
2
3
[5j]
TMS
TMS
Previously, we reported that [Ti(Tren )Cl] [1, Tren
=
3
-
give surface-bound nitrides, N , which then combine with
adsorbed H-atoms, produced by essentially barrier-less H
dissociation on the metal surface, to give NH . Because the
3–
{
N(CH
2
CH
2
NSiMe
3
)
3
} ] can be reduced by K (K mirror, KC
8
, or
to give the end-
2
)] and further reduction
2
TMS
KC10
H
8
) to give [Ti(Tren )]; this can bind to N
2
3
TMS
on:end-on N
with K gives [{Ti(Tren )}
where the N-N bond is retained, Scheme 1. Nevertheless, 2
produces up to 18 equivalents of NH when treated with excess
2
-complex [{Ti(Tren )}
2
(N
reaction is heterogeneous it is difficult to study in atomic detail;
however, there is evidence that multi-metallic sites are integral
TMS
4-
2
(N
2
K
2
)] (2) that formally contains N
2
to N
2
activation, and that the presence of various Group 1 and 2
3
metal oxides enhances catalyst reactivity by direct interaction
+
– [5j]
H /e . Since it is clear that K (E° = –2.92) plays a cooperative,
directing role in that reduction chemistry we investigated what
[
3]
with N
interest in studying homogeneous complexes as surrogates for
heterogeneous HB NH synthesis as they can provide detailed
information on reaction intermediates and mechanisms.
Ti, V, Mo, Fe, Ru, Os, and Co complexes have been
2
or as “electronic” promoters. Thus, there is great
effect replacing it with the milder reducing agent Mg (E° = –2.37)
3
[18]
would have.
2
Thus, 1 was treated with excess Mg in N -
[
1-3]
saturated 1,2-dimethoxyethane (DME) solvent, in the presence
of ten equivalents of 1,4-dioxane, Scheme 1. Brief sonication is
required to initiate the reaction - we assume due to a kinetic
delay from surface passivation of the Mg - as demonstrated by a
colour change from orange to green. After a 3 day stir, a white
precipitate forms with a purple-brown supernatant, which was
concentrated and removed by filtration. The remaining solids
were extracted with THF and filtered from the excess Mg and
+
– [4,5]
shown to catalytically reduce N
catalytic cycles are based on sequential biomimetic-type
protonation/reduction of M-N linkages, contrasting to direct
cleavage of N /H in HB chemistry. Indeed, molecular species
2
to NH
3
with H /e .
These
2
2
2
[
*]
Dr L. R. Doyle, Dr A. J. Wooles, Prof. S. T. Liddle
School of Chemistry
insoluble [MgCl
2 ∞
(1,4-dioxane)] by-product. Removal of the THF
The University of Manchester
and recrystallisation of the resulting solid from toluene gives
Oxford Road, Manchester, M13 9PL, UK
E-mail: steve.liddle@manchester.ac.uk
TMS
2 3 2 2
colourless crystals of diamagnetic [{Mg(Tren )} {(µ -N) (Ti) }]
[
19]
(
3), Scheme 1, in a ~20% crystalline yield.
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
10.1002/anie.201902195
COMMUNICATION
Scheme 1. The Ti complex 1 gives different N
2
-reduction products depending on the nature of the reducing agent, producing the hydrazido complex 2 when
8
reduced with KC (reference 5j), or the dinitride complex 3 when reduced with elemental Mg (this work).
The solid-state structure of 3, Figure 1, consists of two Ti
2
single molecule of N , with concomitant rearrangement of the
TMS
–
TMS 3–
IV
II
centres, each coordinated by two [Mg(Tren )] fragments (both
(Tren
)
ligand to support both Ti and Mg centres that in
1
1
3–
κ -N:κ -N') and two µ
and a Mg centre each. The resulting Ti(µ-N)
with each pyramidalised nitride (sum of angles = 280.91(14)°)
residing 1.04 Å from the center of each Ti Mg plane. Multi-
solutions of 3
exhibit sharp resonances within normal diamagnetic ppm ranges
3
-N ligands, which bridge both Ti centres
turn stabilise the two nitride ligands formed.
1
5
2
Ti core is planar,
In order to verify the formulations of 3 and 3- N and probe
their reactivity, we performed acidification experiments assaying
+
1
2
NH
spectroscopy and the indophenol spectrophotometric method,
Scheme 2. NH was isolated from the reaction mixture as NH Cl
via a standard base-distillation procedure employing a HCl trap.
3
as its conjugate acid NH
4
by H quantified-NMR (qNMR)
1
13
29
6 6
nuclear H, C, and Si NMR spectra of C D
3
4
IV
IV
consistent with a closed-shell, diamagnetic Ti /Ti complex, and
1
5
are congruent with the solid-state structure in which the usual 3-
Acidification of 3 or 3- N in DME with ten equivalents of HCl (1
TMS 3–
aq
+
fold symmetry of (Tren
)
is lost and the two distinct SiMe
3
M in Et
97%) or NH
quantitative yields. H NMR spectra of the HCl-trapped volatile
fractions in DMSO revealed diagnostic triplet (7.29 ppm, JNH
1 Hz) and doublet (7.20 ppm, JNH = 71 Hz) resonances for
2
O, pK
a
(Et
2 4
O·H) = –3.59) yielded NH Cl (av. 1.94 eq.,
1
5
environments are retained (2:1 ratio).
4
Cl (av. 1.84 eq., 92%), respectively, in near
1
=
5
1
4
+
15
+
15
NH
the latter reveals a quintet resonance at −357.0 ppm (JNH = 71
Hz). Notably, treating 3 with the weaker acid [Cy PH][I] (10 eq.;
also gave a good yield of
4 4
and NH , respectively, and the N NMR spectrum for
3
aq
THF
[20]
pK
a
= 9.7, H
Cl (av. 1.44 eq., 72%).
Encouraged by the reactivity of 3 and 3- N towards H we
turned our attention towards effecting reactivity with H . Complex
is unreactive towards H (1-4 bar) in various hydrocarbon,
2
O; pK
α
= 9.7)
NH
4
1
5
+
2
3
2
aromatic, or ethereal solvents. However, since the strongly basic
nitride ligands in 3 were readily protonated by the relatively weak
[
5j]
phosphonium acid [Cy
reacted with in tandem via the H
2
3
PH][I], we surmised that 3 might be
-activated complex
), generated from the known
H
2
t
[21]
[
Bu
t
3
PH][HBCF] (BCF = B(C
P Bu /BCF FLP system using H
treatment of 3 with [ Bu
0.97 eq., 49%), isolated from the reaction by distillation onto
frozen DCl (200 eq.; 2 M in Et O) to form NH Cl (NH DCl
6 5 3
F )
[19]
3
2
in toluene.
Gratifyingly,
t
3
PH][HBCF] (6 eq.) in THF yielded NH
3
TMS
Figure 1. Molecular structure of [{Mg(Tren )}
2
{(µ
3
-N)
2
(Ti)
2
}] (3) at 100 K with
(
5
0% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected
2
x
D
y
3
distances (Å) and angles (°) are: Ti1-N2, 2.0147(19); Ti1-N3A, 2.0400(19);
+
expected), Scheme 2. The D trapping acid was employed in
Ti1-N5, 1.8524(19); Ti1-N5A, 1.8532(19); Mg1-N1, 1.996(2); Mg1-N2,
order to unequivocally differentiate chemically this step from the
2
.518(2); Mg1-N3, 2.432(2); Mg1-N4, 2.1544(19); Mg1-N5, 2.1330(19); Ti1-
t
preceding
NH
3
-forming
reaction
with
[ Bu
3
PH][HBCF].
PD][DBCF]
(N = N/ N, 0.76-1.25 eq.,
38-63%), trapped with HCl to form NH D Cl (NHD Cl expected).
N5-Ti1A, 92.62(9); Ti1-N5-Mg1, 94.88(8); Ti1A-N5-Mg1, 93.41(8).
15
t
Analogously, reactions between 3/3- N and [ Bu
3
1
4
15
(
2 3
synthesized from D ) generated ND
The nitride assignment and origin from incorporation of
x
y
3
gaseous N
2
into 3 was confirmed by repeating the synthesis
The ammonium salts were redissolved in dry DMSO-d6 and
1
5
15
1
under an atmosphere of
N
2
; this reproducibly gives the
N
2
-
analysed by H qNMR, then subsequently reanalysed after
TMS
15
15
isotopologue [{Mg(Tren )}
2
{(µ
3
- N)
2
(Ti)
2
1
}] (3- N) that exhibits
conversion to NH Cl by hydrolysis with HCl to effect complete
4
aq
5
1
a singlet resonance at 405.4 ppm in its N NMR spectrum in
H/D exchange. This enabled a comparison of the relative
H
1
5
6 6
C D . The FTIR and Raman spectra of 3 and 3- N do not exhibit
integrals for the NH D Cl resonances before (x + y = 4) and after
x
y
1
4
15
any N/ N isotopically shifted absorptions in the range of N-N
single, double, or triple bond stretches, whilst a much weaker
(x = 4) hydrolysis, from which the values of both x and y could
+
be determined (see SI). Thus, incorporation of either H from
–
1
–1
15
t
+
t
Raman band at 754 cm (3) and 730 cm (3- N) is likely to be
a breathing mode; the N5···N5A distance of 2.560(2) Å in the
crystal structure further rules out the presence of any N-N bond
3 3
[ Bu PH][HBCF] or D from [ Bu PD][DBCF] into the ammonium
product is unequivocally demonstrated, although the quantified
values are lower than expected due to some H/D exchange.
This H/D exchange occurs prior to the hydrolysis reaction, which
cannot be avoided by variation of the experimental procedure,
1
5
in 3 and 3- N. Thus, the reduction of 1 to 3 involves the binding,
activation, and complete six-electron reductive cleavage of a
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Angewandte Chemie International Edition
10.1002/anie.201902195
COMMUNICATION
1
5
t
Scheme 2. The reaction of 3- N with HCl produces up to 1.94 equivalents of NH
3
, pre-prepared [ Bu PD][DB(C
3
6
F
5
)
3
] in THF produces 1.25 equivalents of
1
5
15
t
15
3 3 6 5 3 2 3
ND , whereas in situ treatment of 3- N with P Bu , B(C F ) , and D produces 0.29 equivalents of ND .
but the extent of H-D and D-H exchange essentially mirror each
dominated by polymetallic activation and heterobimetallic
[
2a,3,10a,10b]
other internally validating this hypothesis. Further, one possible
cooperative effects.
with Ti-Mg-mediated cleavage of
heterogeneous hydrogenation of N
partially replicated, adding NH -synthesis, prominent by its
absence, to the family of FLP hydrogenation reactions.
In conclusion, by harnessing a cooperative combination of
Ti and Mg we have cleaved N to two nitrides in a six-electron
reduction. The resulting nitrides are basic and liberate NH when
treated with acids. By employing a FLP to cleave H we have
sidestepped an apparent barrier to H -activation, enabling the
resulting phosphonium-borohydride to fully protonate the nitrides
to give NH , thus establishing FLPs in NH synthesis from N
activation and hydrogenation chemistry. The polymetallic nature
and reactivity of this N -fixation chemistry, which uniquely
By combining FLP-splitting of H
to produce NH the
in HB chemistry has been
2
t
cause is thought to be the co-distillation of NH
3
with the P Bu
3
N
2
3
by-product, which was chemically trapped with the HCl or DCl
2
t
t
31
1
acid as [ Bu
DMSO-d ): 51.68 ppm, s; and 50.88 ppm, t, JPD = 70 Hz,
respectively). Thus, partial H /D exchange may occur within an
3
PH][Cl] and [ Bu
3
PD][Cl], respectively ( P{ H} NMR
3
[
14,16]
(
6
+
+
+
equilibrium mixture of NH
3
/NH
= 11.4, H
Having established tandem N and H
conversion to NH , a preliminary in situ reaction suggests that
FLP-chemistry has potential to emerge as an effective vehicle in
4
(pK
a
=
9.3,
H
2
O) and
2
t
t
+
t
+
THF
[20]
Bu
3
P/ Bu
3
PH ([ Bu
3
PH] , pK
a
2
O; pK
α
= 10.7).
3
2
2
activation prior to
2
3
2
1
5
this area. Hydrogenation of 3- N performed in situ using
3
3
2
-
t
P Bu
~2 bar) and volatile-trapping with frozen HCl yielded NH
0.29(±0.05) eq., 15%], Scheme 2. At that level D-incorporation
3
/BCF in 1,2-difluorobenzene (DFB) under a D
2
atmosphere
(
4
Cl
2
[
combines key heterogeneous HB steps together at a single
homogeneous metal complex, when most complexes can only
execute an individual reaction step, provides a simple molecular
could not be unequivocally confirmed due to relatively increased
H/D exchange occurring under these conditions. The lower yield
for an in situ tandem reaction rather than stepwise suggests that
yet to be determined competitive side-reactions are operating.
[
23]
system amenable to further atomic-level study.
However, it is encouraging that NH
3
could be isolated by
vacuum distillation from this preliminary, unoptimised experiment.
The observed colour change of orange to green then
purple-brown when 1 is reduced with Mg suggests that, as is the
Acknowledgements
We gratefully acknowledge the UK EPSRC (grants
EP/M027015/1 and EP/P001286/1), ERC (grant CoG612724),
Royal Society (grant UF110005), and The University of
Manchester for generous funding and support.
[
5j]
case for K-mediated reduction that gives 2, 1 is reduced to
TMS
[
[
Ti(Tren )], which is green, and reversibly binds N
2
to form
TMS
{Ti(Tren )}
2
(N
2
)]
transiently.
Further
reduction with
magnesium then gives 3, in which complete reductive cleavage
3
-
of N
afford 2; this may be accounted for by the more polarizing nature
2
to N has occurred, in contrast to partial reduction by K to
Keywords: Dinitrogen reduction • Frustrated Lewis pairs • Titanium •
2
+
+
Ammonia • Nitrogen fixation
of Mg compared to K , since the former has a high charge to
radius ratio, possibly resulting in additional polarization of the N
molecule promoting its complete rupture. The fact that K-
2
[
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reduction of 1 gives N
2
with retention of a N-N bond but milder
3
-
magnesium gives complete cleavage to N underscores how
2
018, 1, 490-500; b) R. J. Burford, M. D. Fryzuk, Nat. Rev. Chem. 2017,
, 0026.
[
10a]
cooperative effects are key to N
2
-activation.
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1
valent transition metal nitrides are often unreactive towards H
2
,
[3]
[4]
[5]
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to give quantitative liberation of NH
3
demonstrating complete N-
N dissociation and two highly basic nitrides in 3. Thus, when 3 is
t
t
treated with [ Bu
successive protonation steps occur from N to NH
which can be isolated from the reaction by vacuum distillation,
3 3
PH][HBCF] (or [ Bu PD][DBCF]), all three
3
-
3
3
(or ND ),
[
22]
circumventing the need for additional acid to be employed.
These points can be related to HB chemistry, where the ability to
3
-
2
reductively cleave N to N and any subsequent reactivity is
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Angewandte Chemie International Edition
10.1002/anie.201902195
COMMUNICATION
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Angewandte Chemie International Edition
10.1002/anie.201902195
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Laurence R. Doyle, Ashley J. Wooles,
and Stephen T. Liddle*
Page No. – Page No.
Bimetallic Cooperative Cleavage of
Dinitrogen to Nitride and Tandem
Frustrated Lewis Pair Hydrogenation
to Ammonia
Text for Table of Contents
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