Catalytic Dinitrogen Reduction to Ammonia at a Triamidoamine–Titanium Complex

Article abstract of DOI:10.1002/anie.201802576

Catalytic reduction of N₂ to NH₃ by a Ti complex has been achieved, thus now adding an early d-block metal to the small group of mid- and late-d-block metals (Mo, Fe, Ru, Os, Co) that catalytically produce NH₃ by N₂ reduction and protonolysis under homogeneous, abiological conditions. Reduction of [Ti^IV(Tren^TMS)X] (X=Cl, 1A; I, 1B; Tren^TMS=N(CH₂CH₂NSiMe₃)₃) with KC₈ affords [Ti^III(Tren^TMS)] (2). Addition of N₂ affords [{(Tren^TMS)Ti^III}₂(μ-η¹:η¹-N₂)] (3); further reduction with KC₈ gives [{(Tren^TMS)Ti^IV}₂(μ-η¹:η¹:η²:η²-N₂K₂)] (4). Addition of benzo-15-crown-5 ether (B15C5) to 4 affords [{(Tren^TMS)Ti^IV}₂(μ-η¹:η¹-N₂)][K(B15C5)₂]₂ (5). Complexes 3–5 treated under N₂ with KC₈ and [R₃PH][I], (the weakest H⁺ source yet used in N₂ reduction) produce up to 18 equiv of NH₃ with only trace N₂H₄. When only acid is present, N₂H₄ is the dominant product, suggesting successive protonation produces [{(Tren^TMS)Ti^IV}₂(μ-η¹:η¹-N₂H₄)][I]₂, and that extruded N₂H₄ reacts further with [R₃PH][I]/KC₈ to form NH₃.

Full text of DOI:10.1002/anie.201802576

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Catalytic Dinitrogen Reduction to Ammonia at a Triamidoamine-
Titanium Complex
Authors: Stephen Liddle, Laurence Doyle, Ashley Wooles, Lucy
Jenkins, floriana tuna, and Eric McInnes
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802576
Angew. Chem. 10.1002/ange.201802576
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http://dx.doi.org/10.1002/ange.201802576

10.1002/anie.201802576
Angewandte Chemie International Edition
COMMUNICATION
Catalytic Dinitrogen Reduction to Ammonia at a Triamidoamine-
Titanium Complex
Laurence R. Doyle,[a] Ashley J. Wooles,[a] Lucy C. Jenkins,[b] Floriana Tuna,[c] Eric J. L. McInnes,[c] and Stephen
T. Liddle*[a]
Abstract: We report unprecedented catalytic reduction of N2 to NH3 by a
molecular Ti complex, thus now adding an early d-block metal to the small
group of mid- and late-d-block metals (Mo, Fe, Ru, Os, Co) that are
capable of executing catalytic production of NH3 by N2 reduction and
protonolysis under homogeneous, abiological conditions. Under Ar,
been reported.[10] However, no abiological, early metal complex preceding
group 6 has ever been shown to catalytically convert N2 to NH3, and even
V-, Cr-, and W-triamidoamine complexes[11] do not facilitate N2
reduction/protonolysis to NH3 like Mo analogues.[8] Where Ti is concerned,
cleavage of N2 by molecular polyhydrides and low valent species and/or
stoichiometric protonolysis has been reported.[12] Recently, a heterogeneous
Ti-hydride was found to catalytically convert N2 and H2 to NH3 whilst TiO2
is known to photolytically convert N2 and H2O to NH3 and O2.[13] These
reports hint that Ti could hold significant promise in this arena. This is
appealing because Ti is the 9th most abundant element in the Earths crust,
2nd only to Fe for metals that can fix N2.
reduction of [TiIV(TrenTMS)X] (X
= =
Cl, 1A; I, 1B; TrenTMS
N(CH2CH2NSiMe3)3) with 1 equivalent of KC8 affords [TiIII(TrenTMS)] (2).
Addition of N2 to 2 affords [{(TrenTMS)TiIII}2(µ-η1:η1-N2)] (3); further
reduction with KC8 then gives [{(TrenTMS)TiIV}2(µ-η1:η1:η2:η2-N2K2)] (4).
Addition of 4 equivalents of benzo-15-crown-5 ether (B15C5) to 4 affords
[{(TrenTMS)TiIV}2(µ-η1:η1-N2)][K(B15C5)2]2 (5). Complexes 3-5, when
treated under N2 with excess KC8 and [R3PH][I], the latter utilising the
weakest H+-sources yet used in N2 reduction, produce up to 18 equivalents
of NH3 with only trace N2H4. When only acid is present N2H4 is the
dominant product, suggesting successive protonation eventually produces
[{(TrenTMS)TiIV}2(µ-η1:η1-N2H4)][I]2, with subsequent extrusion of N2H4,
that reacts further with [R3PH][I]/KC8 to form NH3, and then concomitant
reformation of 1B closing the catalytic cycle.
Here, we report the first abiological, homogeneous Ti complex that
is competent for catalytic reduction of N2 to NH3. We find that N2 binding
and partial activation occurs at TiIII supported by one of the simplest
triamidoamine ligands, priming the N2 for cooperative reduction by KC8.
Protonation to give NH3 in a catalytic cycle is facilitated by phosphonium
salts that are the weakest proton source used in any catalytic system to date.
Under argon, reduction of yellow [TiIV(TrenTMS)X] [X = Cl, 1A;[14] I,
1B; TrenTMS
=
N(CH2CH2NSiMe3)3] with KC8 yields green
[TiIII(TrenTMS)] (2).[15] Three broad 1H NMR resonances are observed (C6D6,
3.6, 1.0, and –22.3 ppm), whilst no 13C or 29Si NMR resonances could be
detected, and an EPR spectrum of 2 in frozen toluene-pentane exhibits g =
1.984, 1.943, and 1.891. This is consistent with the presence of TiIII,
unfortunately 2 has resisted exhaustive attempts to isolate it. However, slow
cooling of an N2-saturated pentane solution of 2 yields red crystals of
[{(TrenTMS)TiIII}2(µ-η1:η1-N2)] (3), Scheme 1.[15] In the solid state 3 has an
effective magnetic moment of 2.50 µB at 298 K, which is close to the spin-
only value for two s = ½ species (µeff = 2.5 βe for g = 2.0). The near
temperature independence of µeff indicates that any Ti···Ti interaction is
very weak.[15] EPR spectra of solid 3, give apparently axial g⊥ = 1.989 and
g|| = 1.940 at room temperature, however on cooling these diverge to
rhombic g = 1.993, 1.951, and 1.902 and are temperature independent
below 100 K suggesting that molecular motion at higher temperatures gives
higher effective symmetry. The magnetism and EPR spectra of 3 thus
suggest the presence of TiIII ions, and the structural and pectroscopic data,
Table 1, are consistent with modest activation of the N≡N triple bond.[16]
The formulation of 3 is corroborated by DFT calculations;[15] all
attempts to model a TiIV/TiIV/N22- combination were intractable or produced
the TiIII/TiIII/N2 formulation. Computed Ti-NN and N-N Mayer bond orders
of 0.56 and 2.38 are consistent with a weakly bound and activated N2. This
is also supported by computed Ti spin densities/charges of −0.55/0.99 and a
total net spin density/charge of −0.9/−0.56 on the N2-unit. The two SOMOs
are orthogonal Ti→N2-π* back-bonding interactions.
The conversion of dinitrogen, N2, into ammonia, NH3, is essential for
supplying N2 in a fixed form into the Earth’s biosphere,[1] and key to
providing NH3 to chemical industry on a vast scale.[2] However, the N≡N
triple bond, with a bond strength of 944 kJ mol–1, is one of the strongest
chemical bonds known, and with a high ionisation potential, negative
electron affinity, poor nucleophilicity and electrophilicity, large HOMO-
LUMO gap, and no permanent dipole, there are major kinetic and
thermodynamic barriers to activating N2, and thus to fixing it as NH3.[3,4]
Nature uses homogeneous nitrogenases based on V, Mo, and Fe to
execute multiple single-electron transfer and -protonation steps to convert
N2 to NH3.[5] In contrast, chemical industry uses N2 and H2 to produce NH3
over heterogeneous catalysts in Haber-Bosch processes.[6] However, each
process is energy intensive, reflecting the challenge of N2 activation, and so
there is interest in studying reactivity at molecular complexes to improve
our understanding of these elementary transformations.[7]
After the report that a Mo-triamidoamine complex catalytically
reduces N2 to NH3 in the presence of H+/e–,[8] a few Mo, Fe, Ru, Os, and Co
complexes have been shown to be catalytically competent in H+/e– mediated
N2
reduction
cycles,[9]
and
stoichiometric
reduction
and
protonation/hydrogenation of N2 and nitrides by a variety of metals have
[a]
Dr L. R. Doyle, Dr A. J. Wooles, Prof. S. T. Liddle
School of Chemistry
1
The University of Manchester
The H NMR spectrum of 3 (C6D6) reveals identical resonances to
Oxford Road, Manchester, M13 9PL, UK
E-mail: steve.liddle@manchester.ac.uk
Miss L. C. Jenkins
School of Chemistry
The University of Nottingham
University Park, Nottingham, NG7 2RD, UK
Dr F. Tuna, Prof. E. J. L. McInnes
School of Chemistry and Photon Science Institute
The University of Manchester
those of 2, consistent with N2 dissociation in solution. Complex 2 is similar
to [Ti(TrenDMBS)] [TrenDMBS = N(CH2CH2NSiMe2But)3],[17] and their optical
spectra exhibit broad absorptions at ~620 and ~640 nm, respectively,[15]
which is responsible for their green colours. In line with this, [Ti(TrenTMS)]
exhibits computed HOMO to LUMO+3/+4 (d-d) energy separations of
655/657 nm.
[b]
[c]
Oxford Road, Manchester, M13 9PL, UK
Supporting information for this article is given via a link at the end of the
document.
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10.1002/anie.201802576
Angewandte Chemie International Edition
COMMUNICATION
Scheme 1. Synthesis of 2-5 from 1A/B. Reagents and conditions: (i) Ar, 2 KC8, -2 KX, -2 C8; (ii) N2, cool; (iii) N2, warm; (iv) 2 KC8; (v) I2, -2
KI; (vi) 4 B15C5; (vii) N2, 4 KC8, -2 KX, -2 C8.
orders that are lower than in 4 at ~0.6 and 0.16, respectively. This is
Table 1. Key crystallographic bond lengths and Raman spectroscopic data for
consistent with computed Ti charges of 0.99 in 5 that are higher than those
3-5 and N2Hx benchmarks (x = 0, 2, 4).[15]
in 4, but the N2-unit is less charged at −0.86 overall.
Compound ν(N2), cm–1
exp(14N/15N);calc (Å)
d(N–N)
d(Ti–NN)
(Å)
Having established N2 reduction, attention turned to fixation, Table 2.
∠(Ti-N-
N) (°)
Treatment of 4 (10 mM, pentane) with ethereal 1M HCl (10 equivalents)
yielded 0.88 N2H4 and 0.13 NH3 equivalents (Entry 1). The near
stoichiometric yield of N2H4 confirms 4 as a hydrazido complex. In a
control, when 1A is identically quenched only 0.04 NH3 equivalents were
detected. This suggests that the small amount of NH3 produced is the result
of minor degradation of TrenTMS under the action of very strong acid.
To achieve catalytic turnover of N2, it was concluded that a milder
acid than ethereal HCl [pKa (Et2O·H)+ = –3.59] would be required. Also,
the synthesis of 4 requires strong K-based reductants, which react rapidly
with strong, soluble acids. Confirming this, using HCl/KC8 (30:30:4)
resulted in a sub-stoichiometric yield of N2H4 and NH3 (0.13 and 0.36
equivalents, respectively). It was anticipated that a weaker acid could be
effective for the protonation of activated N2 whilst minimising deleterious
side-reactions. However, commonly used N-based acids, such as lutidinium
[2,6-(CH3)2C5H3NH]+ and aryl/alkyl-ammonium salts were found to be
susceptible to reduction by KC8, and in some controls led to NH3
formation; as such, we anticipated false-positive results. Thus, more stable,
non-N-based acids were sought.
N2
2330
1583,1529
1076
1701/1644;1724
1201/1164;1247
1.098(1)
1.25
1.45
1.121(6)
1.315(3)
-
-
-
-
-
-
N2H2
N2H4
3
2.022(3)
1.814(2)
1.810(2)
1.712(4)
180
4
166.6(2)
172.9(2)
178.8(5)
5
1246/1203;1307
1.461(7)
Addition of two or four equivalents of KC8 to 3 or 1A, respectively,
under N2 affords red-brown [{(TrenTMS)TiIV}2(µ-η1:η1:η2:η2-N2K2)] (4),
Scheme 1.[15] The structure of 4, Table 1, shows the N2 ligand is bound in a
near-linear manner. The N-N bond distance is extended compared to 3 and
free-N2 and the Ti-NN bond distances are short, inferring some Ti-imido
character. These data along with Raman spectroscopy suggest strong
activation of N2. Multi-nuclear NMR spectra of C6D6 solutions of 4 and 4-
15N2 are consistent with a C3-symmetric TiIV/TiIV/N24– formulation.
Only
a closed-shell formulation for 4 gave a converged DFT
calculation,[15] where the Ti ions and N2-unit carry computed charges of
0.52 (av.) and −1.1 (total), respectively; this implies a covalent bonding
picture for the Ti-(µ-N2)-Ti unit that is supported by Ti-NN and N-N Mayer
bond orders of 1.25 and 1.44, respectively. For comparison, the Ti-Namide
and Ti-Namine Mayer bond orders are ~0.7 and 0.25, respectively. The
HOMO and HOMO−1 are two orthogonal, doubly-occupied Ti→N2-π*
back-bonding interactions. The KI ions clearly play a stabilising role, but
this is electrostatic, with K-N Mayer bond orders of <0.05.
Trialkylphosphonium salts, [R3PH][X], were examined due to their
mild acidity (pKa ~8-12) and tunable nature. Initial experiments were
conducted with [Cy3PH][I] (pKa = 9.7). Addition to 4 (10:1 ratio) in Et2O
produced 0.5 N2H4 and 0.05 NH3 equivalents (Entry 2). This is a lower
yield of N2H4 compared to using HCl, but [Cy3PH][I] is a weaker, less
soluble acid so it was expected to be less reactive. However, with excess
[Cy3PH][I]/KC8, 4 catalyses the production of up to 18 equivalents of NH3
per 4 (Entries 3-11). Using 4-15N2 under 15N2 confirmed the incorporation of
15N2 into the 15NH3/15NH4Cl product.
Addition of 4 equivalents of benzo-15-crown-5 (B15C5), Scheme 1,
gives red [{(TrenTMS)TiIV}2(µ-η1:η1-N2)][K(B15C5)2]2 (5).[15] The solid state
molecular structure of 5, Table 1, reveals that the Ti-N-N-Ti axis is closer
to linearity than in 4. The N-N and Ti-NN bond distances in 5 are longer
and shorter, respectively, suggesting strong N2 activation and significant Ti-
imido character. However, for 5 the ν(N2) Raman stretch, a better indicator
of N2 activation than bond distances, suggests reduced N2 activation
compared to 4, Table 1. UV/Vis spectra of 4 and 5 are essentially identical
in THF, but different in benzene, suggesting that the KI ions in 4 are labile
in polar donor solvent, but remain coordinated in non-polar solvents.
Only a closed-shell formulation for the dianion part of 5 gave a
converged calculation. The Ti-NN and N-N Mayer bond orders in 5 are
1.33 and 1.62, which are larger than the corresponding data for 4; the latter
is in-line with the Raman data, whilst the former suggests that more Ti-
imido character results from removal of KI ions. The fact that 5 contains a
charge-rich dianion is reflected by Ti-Namide and Ti-Namine Mayer bond
A sub-stoichiometric yield was obtained using [K2(C10H8)2(THF)] as
the reductant (Entry 12), likely due to its solubility in Et2O and thus greater
propensity to react with H+ in-situ, consuming acid and reductant for H2
production. Comparative runs in ethers (Et2O and THF), in which
intermediates may be solvated/stabilised, gave higher yields than those in
toluene or pentane (Entries 4 and 9-11). The acid anion was also varied:
–
[Cy3PH][Cl] (Cl–, higher coordinating ability) or [Cy3PH][BArF ] (BArF
=
4
4
[{3,5-(CF3)2C6H3}4B]−, non-coordinating) (Entries 4 and 13-14). For both
this resulted in lower yields of NH3, but supposed variation of a single
parameter will simultaneously affect several properties that all influence
catalytic turnover, preventing meaningful correlation. Similar catalytic
turnovers were obtained using [Bun PH][I] and [But3PH][I] (pKa = 8.4 and
3
11.4, respectively; Entries 15 and 16), or using a 4:3 ratio of [Cy3PH][I] to
KC8 (Entry 5), which accounts for an additional equivalent of acid being
consumed through protonation of NH3.
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10.1002/anie.201802576
Angewandte Chemie International Edition
COMMUNICATION
Table 2. Catalytic acidification experiments for the reaction of 4 with acid and reductant under N2 to produce NH3 and N2H4.†
Entrya
Solvent
Acid
Reductant
Acid
(eq.)
10
Reductant
(eq.)
-
NH3
(eq.)
0.13
0.05
N2H4
(eq.)
0.88
0.52
0.15
0.06
0.10
0.03
0.08
0
0.29
0.21
0
0.07
0.05
0
Fixed-Ng
(eq.)
1.89
1.09
6.71
12.03
11.01
17.83
17.86
1.53
6.40
4.31
8.95
0.50
2.55
4.77
11.91
7.97
2.00
Efficiencyh
(%)
-
-
17
12
11
9
9
-
6
4
9
0
3
5
12
8
-
1
2
3
4
5
6
pentane
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
pentane
toluene
THF
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
1M HCl
-
-
[Cy3PH][I]
[Cy3PH][I]
[Cy3PH][I]
[Cy3PH][I]
[Cy3PH][I]
[Cy3PH][I]
[Cy3PH][I]
[Cy3PH][I]
[Cy3PH][I]
[Cy3PH][I]
[Cy3PH][I]
[Cy3PH][Cl]
10
-
KC8
KC8
KC8
KC8
KC8
KC8
KC8
KC8
KC8
120
300
400
600
600
300
300
300
300
120
300
300
600
600
300
300
300
300
300
300
300
300
300
25
6.41
11.91
10.81
17.77/17.4f
17.70/17.6f
1.53
5.82
3.89
8.95
0.36
2.45
4.77
11.73
7.37
1.32
7b,c
8b,d
9
10
11
12
13
14
15
16
17e
K(Nap)(THF) 300
KC8
300
300
300
300
25
[Cy3PH][BArF ] KC8
4
[nBu3PH][I]
[tBu3PH][I]
[Cy3PH][I]
KC8
KC8
KC8
0.09
0.30
0.34
† Diazene (N2H2) was not analysed for due to its expected instability under these reaction conditions, however, complete disproportionation of N2H2 to N2H4
and N2 can only be expected to produce a maximum N2H4 yield of 50%. a All experiments were performed under N2 (unless otherwise noted) at –78 °C (2 h),
followed by gradual warming to 25°C and additional stirring for 15 h. b 4-15N2, c under 15N2; d under Ar; e[N2H5][I]; f yield calculated from 1H NMR; g Fixed-N (eq.)
= [NH3 (eq.)] + 2[N2H4 (eq.)]; h Efficiency = 100%[Red. (eq.)]/{3[NH3 (eq.)] + 4[N2H4 (eq.)]}.
With KC8 the major product is NH3, with little N2H4 (< 0.15 per 4)
detected. That 4 does not react further with KC8 in Et2O implicates an
initial protonation step rather than further reduction of 4 to form a TiIV-
nitride species, which could be responsible for NH3 production. A further
control demonstrated that [N2H5][I] is converted to NH3 in the absence of
4/4-15N2 (Entry 17), and with Entry 1, this suggests that the N2H4 to NH3
reduction/protonation may occur after dissociation from the active species.
We cannot rule out the presence of transient TiIV-nitrides,[18] but these
control experiments suggest that successive protonation may produce
“[{(TrenTMS)TiIV}2(µ-η1:η1-N2H4)][I]2”, with subsequent extrusion of N2H4,
that reacts with [Cy3PH][I]/KC8 to form NH3; concomitant formation of 1B
closes the catalytic cycle.
mechanism, where TiIV is reduced to TiIII, which binds and weakly activates
N2, priming it for cooperative K-mediated reduction[10e,23] to a strongly
activated state, which can then be protonated. Transient nitrides cannot be
ruled out at this point, but control experiments suggest that N2H4 is formed
and then converted to NH3 in the presence of acid and reductant. The
catalytic activity of one of the Ti complexes suggests that the end-on:end-
on bridging mode of N2 should possibly no longer be discounted as a
catalytically active coordination mode. These results add Ti to the small
number of previously exclusively mid- and late-d-block metal ions (Mo, Fe,
Ru, Os, Co) that can execute catalytic reduction of N2 to NH3.
Under Entry 4 conditions, 5 is catalytically competent producing 10
equivalents of NH3. Complex 3 produces 6 equivalents of NH3 under such
conditions, showing catalytic competence, but on dissolution a significant
proportion of 3 converts to 2, Scheme 1, retarding reactivity. In isolation, 3
reacts with acid to produce 0.03 and 0.1 equivalents of NH3 and N2H4,
respectively, underscoring the importance of KC8 activation of the
coordinated N2 in 3.
Acknowledgements
We gratefully acknowledge the UK EPSRC (grants EP/M027015/1 and
EP/P001386/1), ERC (grant CoG612724), Royal Society (grant UF110005),
and The University of Manchester for generous funding and support, and
the National EPSRC UK EPR Facility. Metrical parameters for the
structures of compounds 1A, 1B, and 3-5 are available free of charge from
the CCDC (1812642 to 1812646). All other data are available from the
authors on request
The reactivity of 5 is significant, because it is a clear-cut end-on:end-
on bridging N2 complex. To date, almost all N2-fixing catalysts contain
end-on terminal N2,[7] whereas those with end-on:end-on bridging N2 have
been proposed to dissociate to form end-on terminal complexes, or undergo
N2 cleavage to generate nitrides.[7,9b,d,18-20] Side-on:side-on bridging, which
usually results in strong N2 activation, has been reported for stoichiometric
N2 activation only. Conversion of end-on:end-on bridging to side-on:side-
on bridging in 5 seems unlikely on steric grounds, so the reactivity of 5
suggests that consideration might be given to recognising end-on:end-on
bridging as a catalytically competent N2 coordination mode. If this is the
case, it would provide an unusual symmetrical functionalisation of N2 to
N2H4 (in the absence of reductant to cleave the N-N bond) compared to the
traditional Chatt-type cycle,[21] since only one molecular Fe complex is
known to be selective for catalytic reduction of N2 to N2H4 rather than
NH3.[22]
Keywords: Dinitrogen reduction • Titanium • Ammonia • Nitrogen fixation
• Phosphonium salt
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complex that is competent for the homogeneous catalytic reduction of N2 to
NH3 under ambient conditions, and this chemistry is supported by the
simple TrenTMS ligand. The proton sources in this catalysis, [R3PH][I], are
the weakest yet used in N2 reduction to NH3, underscoring the importance
of balancing acid sources to strong reducing agents. We propose a plausible
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10.1002/anie.201802576
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Catalytic Dinitrogen Reduction to
Ammonia at a Triamidoamine-
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