Conversion of dinitrogen to tris(trimethylsilyl)amine catalyzed by titanium triamido-amine complexes

Article abstract of DOI:10.1039/c8cc09742a

By using a triaryl-Tren ligated titanium dinitrogen complex, K₂[{(Xy-N₃N)Ti}₂(μ₂-N₂)] (3), prepared by two-electron reduction of [TiCl(Xy-N₃N)] (1-Cl) under N₂ atmosphere, catalytic fixation of molecular nitrogen to form tris(trimethylsilyl)amine was achieved under ambient conditions with a turnover number (TON) of up to 16.5 per titanium atom.

Full text of DOI:10.1039/c8cc09742a

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Conversion of dinitrogen to tris(trimethylsilyl)amine catalyzed by
titanium triamido-amine complexes
Priyabrata Ghana,a Franziska D. van Krüchten,a Thomas P. Spaniol,a Jan van Leusen,a Paul Kögerler,a
Received 00th January 20xx,
Accepted 00th January 20xx
and Jun Okudaa,
DOI: 10.1039/x0xx00000x
www.rsc.org/
Abstract: By using a triaryl-Tren ligated titanium dinitrogen were developed within the last decades with TONs of up to 226
complex, K2[(Xy-N3N)Ti2(µ2-N2)] (3), prepared by two-electron for the molybdenum complex trans-[Mo(N2)2(depf)2] (depf =
reduction of [TiCl(Xy-N3N)] (1-Cl) under N2 atmosphere, catalytic 1,1’-bis(diethylphosphino)ferrocene).[8b] Except an early report
fixation of molecular nitrogen to form tris(trimethylsilyl)amine was by Mori et al. on TiCl3/TiCl4 catalyzed incorporation of molecular
achieved under ambient conditions with a turnover number (TON) nitrogen into organic molecules through N(SiMe3)3,[11] no well-
of up to16.5 per titanium atom.
defined, homogeneous titanium-based catalyst for the
conversion of N2 to N(SiMe3)3 is known. We envisaged the aryl-
substituted triamino-amine ligand H3(Xy-N3N) (Xy-N3N = {(3,5-
Me2C6H3)NCH2CH2}3N3–) to be a more robust ligand over the
trimethylsilyl-substituted analogue due to the weak N-Si bond
strength and a general tendency of C-H activation within a
SiMe3 group of the later one.[12] We present here a systematic
study on a series of triaryl-Tren ligated titanium complexes,
those catalyze the conversion of dinitrogen to N(SiMe3)3.
Fixation of atmospheric dinitrogen into value-added products such
as ammonia, hydrazine or any organoamines is a challenging goal
due to the inertness of the N2 molecule with its extremely strong

1
NN bond (BDE = 944 kJ mol ), zero dipole moment and large
HOMO-LUMO gap.[1] Nature realizes nitrogen fixation smoothly
under ambient conditions using the enzyme nitrogenase that
contains Fe and Mo within its active sites.[1a, 1c] In contrast, the
energy-intensive industrial Haber-Bosch process based on a Fe-
The starting material [TiCl(Xy-N3N)] (1-Cl) was obtained in two
steps from H3(Xy-N3N) and Ti(NMe2)4 via the procedure used for
the preparation of [MoCl(Xy-N3N)] from H3(Xy-N3N) and
Mo(NMe2)4.[12b] After heating a toluene solution of H3(Xy-N3N)
and Ti(NMe2)4 to 80 °C for 20 h, [Ti(NMe2)(Xy-N3N)] (1-NMe2)
was obtained as a bright red solid in 99% yield. Treating this
compound with [NEt3H]Cl in CH2Cl2 at ambient temperature
afforded the titanium(IV) chlorido complex 1-Cl as a dark brown
solid in 82% yield (Scheme 1). Notably, this complex could not
be prepared by reacting Li3(Xy-N3N) with [TiCl4(THF)2] following
the procedure used to prepare [TiCl(TMS-N3N)].[13] The Ti(IV)
based heterogeneous catalyst requires extremely high pressure
1c]
and temperature.[1a,
Recent research has focused on
developing mid-transition metal based catalysts for the fixation
of N2.[2] Catalysts based on early transition metals that can
transform N2 into value-added products are still rare.[3] Very
recently, a trimethylsilyl-substituted triamido-amine ligated
titanium(III)
{(Me3Si)NCH2CH2}3N3–) was reported by Liddle et al. to activate
molecular N2. The two-electron-reduced species K2[ (TMS-
N3N)Ti 2(µ2-N2)] was further shown to catalyze the conversion
complex
[Ti(TMS-N3N)]
(TMS-N3N
=


of N2 to NH3 with a maximum turnover number (TON) of up to
18.[4] An alternative method for N2 fixation is to convert it into
higher value tris(trimethylsilyl)amine, N(SiMe3)3 by using
Me3SiX (X = Cl, Br and I) as a source of electrophile. Despite the
early success with the conversion of N2 to N(SiMe3)3,[5] only few
homogeneous catalysts based on V,[6] Cr,[7] Mo,[8] Fe[9] or Co[10]
a. Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52056
Aachen, Germany. E-mail: jun.okuda@ac.rwth-aachen.de.
†Electronic Supplementary Information (ESI) available: NMR and IR spectroscopic
data, and X-Ray crystallographic data. CCDC 1883119 (2) and 1883120 (3).
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Scheme 1. Synthesis of titanium(IV) complexes; Ar = C6H3-3,5-Me2.
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complexes were characterized by multinuclear NMR
spectroscopy and elemental analysis (see Electronic Supporting
Information (ESI), section 2.1 and 2.2).
View Article Online
DOI: 10.1039/C8CC09742A
Liddle et al. have recently reported that [TiCl(TMS-N3N)] can be
reduced to the Ti(III) species [Ti(TMS-N3N)] that can bind
dinitrogen.[4] Reduction of 1-Cl with one equivalent of
potassium metal in benzene under ambient conditions liberated
small amount of H3(Xy-N3N) along with a red solid, which was
characterized by single-crystal X-ray diffraction as a Ti(III) dimer
[Ti(Xy-N3N)]2 (2). The red solid is insoluble in all common organic
solvents and cannot be purified from KCl contaminations. To
avoid the formation of an inorganic salt, hydrogenolysis of Ti(IV)
alkyl complexes was performed.[13] Reacting 1-Cl with MeMgCl
or with LiCH2SiMe3 gave the titanium alkyls [Ti(Me)(Xy-N3N)] (1-
Me) or [Ti(CH2SiMe3)(Xy-N3N)] (1-CH2SiMe3), isolated as red and
brown solids in 91 and 71% yield, respectively (Scheme 1), and
fully characterized (see ESI, section 2.3 and 2.4). Apart from the
diagnostic resonances for the Xy-N3N3– ligand, the 1H NMR
spectrum of 1-Me in benzene-d6 displays one singlet for the Ti-
Figure 1. Molecular structure of
2
in the solid state with thermal ellipsoids at 50%
probability level. Selected bond lengths and angles: Ti1····Ti1’ 3.4288(5), Ti1-N1
2.249(1), Ti1-N2 2.038(1), Ti1-N3 2.027(1), Ti1-N4 2.222(1), Ti1-N1’ 2.212(1) Å;
Ti1-N1-Ti1’ 100.44(4)°.
indicating either no or only a very weak interaction between the
two d1 metal centers.
Temperature- and field-dependent magnetic susceptibility data of
compound 2 (Figure S10) were corrected for very small levels of ferri-
or ferromagnetic impurities[15] and reach
Me group at
 1.27 ppm, while 1-CH2SiMe3 shows two singlets
at 2.36 (CH2SiMe3) and –0.19 ppm (CH2SiMe3) for the Ti-

CH2SiMe3 group.
a χmT value of
Heating a solution of 1-Me or 1-CH2SiMe3 in benzene with H2 or
with PhSiH3 at 60 – 65 °C afforded compound 2 (Scheme 2),
which was isolated after work-up as an analytically pure red
solid in moderate yield (see ESI, section 2.5). During the
synthesis of 2, no corresponding Ti(IV) hydride, [Ti(H)(Xy-N3N)],
was observed by 1H NMR spectroscopy, indicating fast
conversion into the Ti(III) dimer by H2 elimination. In general,
Ti(IV) hydrides are known to be unstable and decompose under
ambient conditions.[13-14]
0.746 cm3 K mol–1 at room temperature, fully in line with the
expected range of 0.680–0.802 cm3 K mol–1 for two non-interacting
Ti(III) centers.[16] With decreasing temperature, χmT decreases almost
linearly down to 50 K, reaching 0.690 cm3 K mol–1 and then displays
a drop-off characteristic for antiferromagnetic exchange interactions
as well as the Zeeman effect of the external field. The low-
temperature (2.0 K) field-dependent molar magnetization (Figure S
10, inset) is small, only reaching 0.6 NA μB at 5.0 T – still well below
the saturation magnetization Mm,sat ≈ 2 NA μB that corresponds to the
excited Stotal = 1 state. This finding also reflects the intramolecular
antiferromagnetic coupling, resulting in a Stotal = 0 ground state. A fit
of the combined data sets, utilizing the computational framework
CONDON,[15d] yields the exchange energy J = –2.6 ± 0.1 cm–1 (using
the ‘–2J S1⋅S2’ convention) for Ti(III) centers in an approximately C4v-
symmetric ligand field (see ESI for details).
A suspension of 2 in THF did not react with N2 in contrast to the Ti(III)
complex [Ti(TMS-N3N)] which gave [(TMS-N3N)Ti2(µ2-N2)] at low
Scheme 2. Synthesis of Ti(III) complex 2; Ar = C6H3-3,5-Me2.
The solid-state structure of 2 was determined by single-crystal temperature.[4] However,
a
dinitrogen compound K2[(Xy-
X-ray diffraction and confirmed by elemental analysis; the N3N)Ti2(µ2-N2)] (3) was observed after reduction of 2 with two
electronic structure was established by SQUID magnetometry equivalents of potassium naphthalenide under an N2 atmosphere.
(see ESI, section 2.5). In contrast to the corresponding Whereas this reaction was not selective and gave large amounts of
tris(anilide) Ti(III) complex [Ti{NtBu(3,5-Me2C6H3)}3],[15] which is unidentified side products, compound 3 was obtained directly by
monomeric both in solution and in the solid state, structural treating a solution of 1-Cl in THF over K-mirror under N2 gas in 80%
analysis of 2 reveals a dimeric structure with an inversion yield by NMR spectroscopy (Scheme 3). Extraction of the crude
center. Each titanium center resides in a square-pyramidal N5 reaction mixture with benzene or toluene separated the impurity
environment of one amine and four amide groups. A noticeable from compound 3, isolated as an analytically pure, red-brown solid
feature is the perfectly planar four-membered Ti2N2 core (Ti1- in 61% yield. Reduction of 1-Cl with potassium naphthalenide under
N1-Ti1’-N1’ torsion angle: 0.00(4)°) formed by the Ti centers and N2 atmosphere selectively afforded compound 3 in a higher yield
an amide arm of each Tren ligand. The distance between both (86%) (Scheme 3). For comparison, the 15N-enriched complex
Ti centers of 3.4288(5) Å significantly exceeds the sum of the K2[(Xy-N3N)Ti2(µ2-15N2)] (3-15N2) was also prepared by reduction of
1-Cl with potassium naphthalenide under 15N2 atmosphere.
single bond covalent radii of two titanium centers (2.72 Å),
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C8CC09742A
Scheme 3. Synthesis of the dinitrogen complex 3; Ar = C6H3-3,5-Me2.
Compound 3 was fully characterized by multinuclear NMR and IR
spectroscopy, single-crystal X-ray diffraction and elemental
analysis. In the solid state, 3 adopts an approximately, but not
crystallographically imposed centrosymmetric structure with the
inversion center at the center of the N2 ligand (Figure 2). The
molecular structure of 3 resembles the reported structure for the
trimethylsilyl-substituted complex K2[(TMS-N3N)Ti
2(µ2-N2)].[4] It
reveals a nearly linear Ti-N-N-Ti core with an expected distorted Figure 2.Molecular structure of
3
in the solid state with thermal ellipsoids at 50%
probability level. Selected bond lengths and angles:N1-N2 1.305(6), Ti1-N1
trigonal bipyramidal geometry around the Ti centers. Linear 1.795(6), Ti2-N2 1.796(6), (K-N2)ave 3.097, Ti1-N3 2.027(5), Ti1-N4 2.098(5), Ti1-
N5 2.136(6), Ti1-N6 2.252(6), Ti2-N7 2.095(5), Ti2-N8 2.142(6), Ti2-N9 2.027(5),
coordination of the N2 ligand is evident by the Ti1-N1-N2 and Ti2- Ti2-N10 2.241(6), K1-N5 3.0426, K2-N8 3.0446(6) Å; Ti1-N1-N2 175.6(4), Ti2-N2-
N1 177.2(4)°.
N2-N1 bond angles of 175.6(4)° and 177.2(4)°, respectively. The
N–N bond distance of 1.305(6) Å is significantly longer than the
distance in free N2 (1.098(1) Å), but compares well to that in
K2[(TMS-N3N)Ti
2(µ2-N2) (d(N-N) = 1.315(3) Å).[4] The Ti–NN bond
distances (d(Ti-NN)avg = 1.7955 Å) are remarkably shorter than the
corresponding Ti–Namide or Ti–Namine single bond distances (d(Ti-
with a TiIV/TiIV/N2 formulation, as confirmed by quantum
4–
chemical calculations for K2[
(TMS-N3N)Ti
2(µ2-N2)].[4]
Liddle et al. recently reported that the trimethylsilyl-substituted
complex K2[

(TMS-N3N)Ti
2(µ2-N2)] can catalytically transform N2
[4]
into NH3. We mainly focused on the catalytic formation of
trimethylsilylamine, N(SiMe3)3, by using Me3Si+ as an electrophile
instead of H+. Initial attempts to silylate 3 by reaction with Me3SiCl
Namide avg
compare well to that in K2[
1.812 Å)[4] or to that in the titanium hydrazido complex
[Cp2Ti(py)(=NNPh2)] (d(Ti-NN)
1.757(2) Å),[17] indicating
)
= 2.0875 Å; d(Ti-Namine
)
= 2.2465 Å). The values
avg

(TMS-N3N)Ti2(µ2-N2)] (d(Ti-NN)avg =
or with a 1:1 mixture of Me3SiCl and Na[B{C6H3-3,5-(CF3)24] did
=
not work, but treatment of 3 with Me3SiCl in the presence of a
strong reducing agent under N2 atmosphere catalytically
produced N(SiMe3)3. Catalytic results for the conversion of N2 into
N(SiMe3)3 by using 3 as a catalyst under different conditions are
summarized in Tables 1 and S2. In addition, catalytic activities of
1-Cl, 1-Me and 2 were also tested under similar conditions and
the outcomes are summarized in Tables 1 and S2. Stirring a
suspension of an alkali metal or an alkali metal derivative with
Me3SiCl in the presence of a catalyst in THF-d8 under a slightly
elevated N2 pressure (ca. 1.2 bar) at ambient temperature
afforded N(SiMe3)3. In addition to N(SiMe3)3 some silane and
disilane side products such as Me3SiH and Me3SiSiMe3 also form
in the reaction as confirmed by 1H, 13C and 29Si NMR spectroscopy.
significant multiple bond character of the Ti–NN bond in 3. The
potassium cations in 3 are encapsulated in the pockets of the

[(Tren)Ti-N2-Ti(Tren)]2 core mainly via

2-coordination to the
N=N -bond, and 
6-coordination to one of the xylyl group of each
Tren-ligand (distance between the potassium cation and the
centroid of one of the xylyl groups: d(K1···arenecentroid) = 2.825(2)
Å; d(K2···arenecentroid
potassium cations with the xylyl groups, which is reminiscence of
the well-known cation- interaction between alkali metal ions
) = 2.850(2) Å). Coordination of the

and arene system,[18] as well as coordination to a THF molecule
weakens the potassium N2-ligand interaction. This is apparent
from the elongation of the K–N distances of 3 (d(K-NN)avg = 3.097
Å) compared to that found in K2[
(TMS-N3N)Ti2(µ2-N2)] (d(K-
NN)avg = 2.757 Å).[4]
Table 1: Catalytic silylation of N2 in THF-d8 at r.t.
Further structural information for 3 and 3-15N2 was obtained from
the IR, Raman and multinuclear NMR spectroscopic studies. The
Raman spectrum of compound 3 and 3-15N2 shows a sharp band
e yield
(%)
Ent.
Cat.
Reductant
Me3SiCl/
reductant (equiv.)
N(SiMe3)3/Ti
(equiv.)
at 1267 and 1216 cm for the N=N stretching vibration,
1
1
2
3
4
5
6
7
8
1-Cl
1-Me
K
K
K
Li
Na
K
KC8
K + C10H8
(cat.)
200/200
150/150
2
2
3.9
7
3,5
16,5
~0,5
9,5
6
8
1,6
2.8
1.4
6.6
-
respectively (Figure S15). The band positions are at slightly higher
wavenumbers compared to that observed for K2[(TMS-
2
3
3
3
3
3
1500/1500
1500/1500
1500/1500
1500/1500
1500/1500
1500/1500


1
1
N3N)Ti2(µ2-N2)] ( 
(14N2) = 1201 cm ; (15N2) = 1164 cm ),
indicating stronger N-N bonds in 3 and 3-15N2 compared to the
later. The H and 13C NMR spectra of 3 and 3-15N2 indicate an
1
overall C3-symmetric structure in solution. The most notable
3.8
spectroscopic feature is the chemical shift of the singlet at
 393.9
ppm in the 15N NMR spectrum of 3-15N2, comparable to the value
Since catalytic functionalization of N2 is well known to strongly
depend on the reductant, we tested several reducing agents using
3 as a catalyst. We found that silylation works best with potassium
found in K2[(TMS-N3N)Ti  390.1 ppm, referenced vs.
2(µ2-15N2)] (
NH3).[4] All structural and spectroscopic features are consistent
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K. Myers, A. E. Ashley, J. Am. Chem. Soc. 2016, 138, 13521;
metal, which gives a TON up to 16.5 per titanium atom after 7
days (Table 1, entry 6). In contrast, catalysis under the same
conditions using KC8 as reducing agent gives TON of up to only 0.5
(Table 1, entry 7). Catalysis with 1-Cl, 1-Me and 2 as catalysts also
produces N(SiMe3)3 (Table 1), but the TON’s are always lower
than that observed with 3 under same conditions. Notably, the
catalytic reactions are very slow and the overall conversion after
7 days is below 50% in all cases, except for the catalytic run with
potassium metal. Stirring the reaction mixture further increases
the TON of the cycle. The catalyst is still active after this time, as
indicated by the increased TON up to 17 after 16 days when
lithium is used (see ESI, Table S1). The 1H and 29Si NMR spectra of
all mixtures confirm the presence of a Ti-containing diamagnetic
species, which currently cannot be assigned unambiguously.
Results from 1D and 2D NMR spectroscopic experiments suggest
this species to be [Ti{N(SiMe3)2}(Xy‐N3N)], as corroborated by the
integrals in the 1H NMR spectrum as well as a 3-fold symmetry of
the triamido-amine ligand. Several attempts to isolate this
titanium amide complex [Ti{N(SiMe3)2}(Xy-N3N)] in pure form
from the catalytic reaction mixture failed due to its extremely
high solubility even in n-pentane and at low temperatures.
In conclusion, we have reported that a series of well-defined Ti-
based homogeneous catalysts convert dinitrogen into
tris(trimethylsilyl)amine under ambient conditions with a TON up
to 16.5 per titanium atom after 7 days. In addition, we
characterized a dimeric Ti(III) complex 2. Unlike [Ti(TMS-N3N)],[4]
this Ti(III) dimer does not bind N2 but can also activate N2 under
ambient conditions when treated with a strong reducing agent.
The detailed mechanism for the conversion of N2 into N(SiMe3)3
at the fragment [Ti(Xy-N3N)]n are currently being studied.
View Article Online
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft is
gratefully acknowledged. We also thank Dr. Jürgen Tirrée, Florian
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DOI: 10.1039/C8CC09742A
Table of Content (TOC)
Conversion of dinitrogen to tris(trimethylsilyl)amine catalyzed by titanium triamido-amine
complexes
Priyabrata Ghana,a Franziska D. van Krüchten,a Thomas P. Spaniol,a Jan van Leusen,a Paul Kögerler,a and Jun
Okudaa,
Catalytic fixation of molecular dinitrogen into tris(trimethylsilyl)amine with TON up to 33 under ambient
conditions was achieved using a well-defined titanium complex with a triamidoamine ligand.