Ammonia and Hydrazine from Coordinated Dinitrogen by Complexes of Iron(0)

Article abstract of DOI:10.1002/ejic.201900058

The iron(0) dinitrogen complexes [Fe(N₂)(PP₃^R)] (PP₃^R = P(CH₂CH₂PR₂)₃, R = Ph, iPr, Cy) were synthesized by reduction of the precursor chloro complexes with potassium graphite. On reaction with triflic acid, [Fe(N₂)(PP₃^R)] complexes afforded ammonia and hydrazine in yields of up to 23 and 16 % respectively. The complex [Fe(N₂)(PP₃^Ph)] which has only been previously synthesized in situ, has now been isolated and fully characterized by ¹⁵N NMR spectroscopy and by X-ray crystallography.

Full text of DOI:10.1002/ejic.201900058

A Journal of
Accepted Article
Title: Reduction of Dinitrogen to Ammonia and Hydrazine by
Complexes of Iron
Authors: Leslie David Field, Hsiu Lin Li, Scott Dalgarno, and Ruaraidh
McIntosh
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201900058
Link to VoR: http://dx.doi.org/10.1002/ejic.201900058

10.1002/ejic.201900058
European Journal of Inorganic Chemistry
FULL PAPER
Ammonia and Hydrazine from coordinated Dinitrogen by
Complexes of Iron(0)
Leslie D. Field,*[a] Hsiu L. Li,[a] Scott J. Dalgarno[b] and Ruaraidh D.
McIntosh[b]
R
Abstract: The iron(0) dinitrogen complexes [Fe(N2)(PP3R)] (PP3
=
Recently, we revisited the chemistry of Fe(0) dinitrogen
complexes containing bidentate phosphine ligands [Fe(N2)(PP)2]
i
P(CH2CH2PR2)3, R = Ph, Pr, Cy) were synthesized by reduction of
the precursor chloro complexes with potassium graphite. On
reaction with triflic acid, [Fe(N2)(PP3R)] complexes afforded ammonia
and hydrazine in yields of up to 23 and 16% respectively. The
complex [Fe(N2)(PP3Ph)] which has only been previously synthesized
in situ, has now been isolated and fully characterized by 15N NMR
spectroscopy and by X-ray crystallography.
(PP
=
1,2-bis(diethylphosphino)ethane
(depe),
1,2-
bis(dimethylphosphino)ethane (dmpe)) with acids. In our hands,
little or no reaction at the coordinated dinitrogen ligand was
observed with the main outcome being protonation at the metal
center to afford metal hydride complexes. However, treatment
of [Fe(N2)(dmpe)2] with trimethylsilyl triflate (TMSOTf) and then
+
triflic acid afforded NH4 indicating reaction at coordinated
dinitrogen.[11] Tyler et al. have reported treatment of a related
system
[Fe(N2)(DMeOPrPE)2]
(DMeOPrPE
=
1,2-
Introduction
bis(di(methoxypropyl)phosphino)ethane) with acids such as triflic
acid (TfOH), HBF4 and HCl to give ammonium in yields of up to
17% with respect to Fe.[12] Recently, Ashley et al. have reported
treatment of [Fe(N2)(dmpe)2] with TfOH to give hydrazine in
yields of up to 9.1% while similar treatment of [Fe(N2)(depe)2]
afforded hydrazine (up to 24%) and ammonia (up to 10.5%).
Treatment of the dinitrogen-bridged complex [Fe(dmpe)2]2(µ-N2)
with TfOH afforded only low yields of hydrazine and ammonia (in
4.3 and 1.5% yield respectively).[13]
In this paper we report dinitrogen complexes of iron
containing the bulky tetraphosphine ligands [Fe(N2)(PP3 )] (PP3
= P(CH2CH2PR2)3 (R = Ph 1, Pr 2, Cy 3) and demonstrate that
these complexes produce ammonia and hydrazine on treatment
with triflic acid (Scheme 1) in single batch experiments.
Nitrogen fixation, where inert dinitrogen is converted to more
reactive forms such as ammonia or hydrazine, is a process of
fundamental importance, as the nitrogen necessary for essential
biomolecules such as proteins and nucleic acids is ultimately
derived from this process.[1] While nitrogen can be fixed
industrially by the Haber-Bosch process or biologically by
anaerobic microorganisms such as rhizobia, there is currently
interest in the development of new catalysts to convert
dinitrogen to ammonia (and hydrazine) under relatively mild
conditions.[2] In particular, iron-containing catalysts are of
interest given that iron is present in both the Haber-Bosch
catalyst and in the enzyme nitrogenase utilized by most
R
R
i
biological systems.[3]
Recent research by Peters,[4]
Nishibayashi,[5] Mézailles,[6] Shi and Deng[7] in this area have
demonstrated iron-mediated catalysis on a laboratory scale.
In this paper we report the chemistry of N2 bound to Fe(0)
complexes containing the bulky tripodal tetraphosphine PP3
ligands (PP3 = P(CH2CH2PR2)3 where R = aryl, alkyl). Many of
the metal complexes of the PP3 ligands are known to be
PR2
Fe
R2
TfOH
P
+
+
N
N
+
NH4
N2H5
P
PR2
=
1
2
3
R
R
R
Ph
iPr
Cy
catalytically active in
a
range of different organic
=
=
transformations.[8] The PP3 ligands constrain the geometry of
octahedral complexes such that the remaining non-PP3 donor
atoms must be in adjacent sites in the octahedral coordination
sphere.[9] Our interest lies in the iron PP3 complexes containing
dinitrogen as a ligand where little has reported on the reactivity
of the dinitrogen ligand itself.[10]
Scheme 1.
For most Ru(0) and Fe(0) phosphines containing a N2
ligand, there are a number of competing reaction pathways
(including the relatively facile substitution of N2, protonation at
the metal centre and reaction of the coordinated N2). The
rationale for testing PP3-type ligands with bulky substituents on
the terminal phosphorus donors was that this type of ligand
partially encapsulates the metal and provides steric protection to
shut down or slow down reactions at the metal centre and to
direct reaction to the coordinated dinitrogen. We have recently
reported the reaction of analogous Ru(0) PP3 dinitrogen
complexes with acid to give mixtures of ammonia and
hydrazine.[14]
[a]
Prof. Dr. L. D. Field, Dr. H. L. Li
School of Chemistry
University of New South Wales
NSW 2052, Australia
E-mail: l.field@unsw.edu.au
http://www.chemistry.unsw.edu.au/staff/les-field
Dr. S.J. Dalgarno, Dr. R. D. McIntosh
School of EPS-Chemistry
[b]
Heriot-Watt University
Edinburgh, Scotland UK EH14 4AS
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/ejic.201900058
European Journal of Inorganic Chemistry
FULL PAPER
Results and Discussion
atmosphere of 15N2 gas over
a
degassed solution of
Synthesis of dinitrogen complexes. [Fe(N2)(PP3Ph)] 1. The
iron complex [Fe(N2)(PP3Ph)] 1 was synthesized by direct
[Fe(N2)(PP3Ph)] 1 in THF-d8 and then allowing exchange to take
place over six days. Additional coupling was evident in the
31P{1H} NMR spectrum due to coupling with the 15N-labeled
reduction of [FeCl(PP3Ph)]+BPh4 in benzene or toluene with KC8
-
and the complex was characterized by the usual spectroscopic
dinitrogen ligand. The metal bound nitrogen, Nα, (δ -31.5)
2
methods (Scheme 2). Complex 1 has been reported previously,
exhibits larger 15N-31P coupling (2JP(trans)-Nα = 9 Hz, JP(cis)-Nα
=
without isolation, by deprotonation of [FeH(N2)(PP3Ph)]+BPh4
5 Hz) compared to coupling to the terminal nitrogen, Nβ, (δ -3.5)
(2JP(trans)-Nβ = 2 Hz). There is an additional 15N-15N coupling
(1JNα-Nβ) of 5 Hz. The 15N chemical shifts for both Nβ, and Nα
(Figure 2), and are within the range reported for other Fe(0)
dinitrogen complexes (δ 18.1 to -49.1).[10a, 10b, 11, 19]
-
with KOtBu in tetrahydrofuran (THF).[15]
Crystals of 1 suitable for X-ray crystallography were grown
from a benzene / hexane solution (Figure 1). The geometry
about the Fe center is trigonal bipyramidal with the three arms of
Ph
the PP3 ligand being equivalent by symmetry, similar to the
[Fe(N2)(PP3iPr)] 2.
The iron(0) dinitrogen complex
structure of [Fe(N2)(PP3iPr)] and [Ru(N2)(PP3iPr)].[10a]
[Fe(N2)(PP3iPr)] 2 was synthesized by KC8 reduction of the chloro
complex [FeCl(PP3iPr)]+BPh4 in THF under an atmosphere of
-
nitrogen.[10a] The dihydrido complex [FeH2(PP3iPr)] 4 was a
persistent minor byproduct in the synthesis of 2.
-
+
BPh4
PPh2
Fe
PPh2
Fe
Ph2
Ph2
P
KC8
N2
P
Cl
N
N
P
P
PPh2
PPh2
1
Scheme 2.
Figure 2. 15N NMR spectrum of [Fe(15N2)(PP3Ph)] 1-15N (THF-d8, 41 MHz).
The N-N bond length of 1.131(11) Å is similar to the N-N bond
lengths in [Fe(N2)(PP3iPr)][10a] and [Fe(N2)(PP3Cy)][10c] (1.1279(16)
and 1.134(3) Å respectively) and falls within the range for other
Fe complexes containing end-on bound dinitrogen (0.966-
1.149 Å)[6, 16] indicating a slight degree of activation compared to
free dinitrogen (1.10 Å).[17]
An authentic sample of the dihydrido complex 4 was
synthesized independently and the complex was characterized
spectroscopically (see Supplementary Material). While we have
not studied the mechanism of formation of the metal dihdride, it
probably arises from the reaction of the reduced iron species
with H2 formed by reaction of KC8 with adventitious water[20] or
with THF.[21]
[Fe(N2)(PP3Cy)]. [Fe(N2)(PP3Cy)] 3 was synthesized by reduction
of the chloro complex [FeCl(PP3Cy)]+BPh4 with KC8. The
-
synthesis of [Fe(N2)(PP3Cy)] by a similar method has been
previously reported, however the dihydrido complex
[FeH2(PP3Cy)] 5 was formed concurrently as a byproduct in
variable amounts and the dinitrogen compound was isolated by
fractional crystallization.[10c] By employing short reaction times
(40 mins) and using THF as the solvent or a solvent mixture of
pentane and THF (4:1), the dihydrido complex was formed only
as a minor byproduct (Scheme 3).
+
Figure 1. ORTEP plot of [Fe(N2)(PP3Ph)] 1 50% ellipsoid probability, and
hydrogen atoms excluded for clarity. Selected bond lengths (Å) and angles
PR2
PR2
Fe
R2
PR2
Fe
R2
R2
P
KC8
N2
P
(deg):
Fe1-N1 1.826(8), Fe1-P1 2.175(2), Fe1-P2 2.2162(12), N1-N2
H
P
+
N
Fe
N
1.131(11), N1-Fe1-P1 180, N1-Fe1-P2 95.84(4), P1-Fe1-P2 84.16(4), P2-Fe1-
P2i / P2ii 118.978(15), N2-N1-Fe1 180.
Cl
P
H
P
P
PR2
PR2
PR2
The infrared stretching frequence, ν(N≡N), is 2020 cm-1 which
matches that previously reported for the complex.[15] The 31P{1H}
NMR spectrum of 1 exhibits a quartet (2JPP 42 Hz) at 177.8 ppm
for the central phosphorus atom and a doublet at 85.9 ppm for
the terminal phosphorus atoms.[18] The 15N-labeled analogue
[Fe(15N2)(PP3Ph)] 1-15N was synthesized by placing an
minor
byproduct
iPr
=
2
R
4
5
R
R
iPr
=
=
=
Cy
3
R
Cy
Scheme 3.
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10.1002/ejic.201900058
European Journal of Inorganic Chemistry
FULL PAPER
+ [13]
Formation of ammonia and hydrazine from dinitrogen
complexes of Fe.
as N2H5 .
“% N2 converted” is a raw measure of the amount
of coordinated N2 that is converted to reduced products. We
note that this measure of % N2 converted takes no account of
the source of electrons required to fuel the production of NH3
and N2H4 products. In batch reactions (with no added reducing
agents), all of the required electrons must derive from Fe(0) in
the starting complexes. If all of the starting Fe(0)-N2 complex
was converted to NH3, and all Fe(0) was oxidized to Fe(II) then 3
equivalents of the Fe(0) complex would be required to supply
the electrons required to completely reduce N2 and the
maximum possible yield would be 33%. In the reaction of
complex 1 with TfOH in pentane as the solvent, the % N2
converted was 12%.
Reaction of [Fe(N2)(PP3Ph)] 1 with Acids. Treatment of
+
[Fe(N2)(PP3Ph)] 1 with TfOH in pentane afforded NH4+ and N2H5
in 23 and 1% yield respectively (Table 1). Our method of choice
for quantification of both NH3 and N2H4 is GC-MS. NH3 can be
detected directly by GCMS and N2H4 can be analyzed by GCMS
after derivatization with acetone to form acetone azine
(Me2C=N-N=CMe2;
2--propanone).[22]
2-(1-methylethylidene)hydrazone
Table 1. Yields of ammonia and hydrazine from reactions of [Fe(N2)(PP3R)] (R
= Ph (1), iPr (2), Cy (3) with acids.
Lower yields of NH4+ were obtained when the reactions were
carried out in benzene (8%) and also only a low yield of
N2H5 (1%) giving rise to a % N2 converted of 5% (Table 1). The
ability of complex 1 to mediate the formation of NH4 and N2H5
Complex
Solvent, Conditions
% yield
% yield
% N2
+
+
of NH4
of N2H5
converted [b]
+
[a]
[a]
+
+
is interesting as it contrasts directly with the results from the
1
1
1
Pentane, TfOH
Benzene, TfOH
23
8
1
1
1
12
5
+
analogous Ru complex [Ru(N2)(PP3Ph)] 6, where no NH4 or
N2H5 was formed at all.[14] There is a significant difference in
+
the IR stretching frequency ν(N≡N) for compound 1 (2020 cm-1)
compared with its Ru analogue 6 (2080 cm-1),[23] and the greater
activation of the N2 ligand in the Fe complex may contribute to
the better conversion of coordinated N2 to reduced products on
treatment with acid.
Pentane, TfOH,
Cp*2Co
12
7
1
1
Benzene / Et2O, HCl
0
<1
1
<1
8
Benzene,
[H(OEt2)2][B(C6F5)4]
14
+
No NH4 was observed when HCl was used as the acid
+
although a trace amount of N2H5 was detected. Use of the
1
1
1
Et2O,
[H(OEt2)2][B(C6F5)4]
5
<1
0
0
0
0
3
<1
0
oxonium acid containing a BArF anion [H(OEt2)2][B(C6F5)4]
+
afforded NH4 in 14% yield when the reaction was carried out in
Pentane,
[H(OEt2)2][B(C6F5)4]
benzene, although only 5% when in diethyl ether and very little
NH4+ was formed when the reaction was carried out in pentane.
The analogous oxonium acid containing a different BArF
Benzene,
[H(OEt2)2][B(C6H3-3,5-
(CF3)2)4]
+
anion [H(OEt2)2][B(C6H3-3,5-(CF3)2)4] did not afford NH4 in
+
either benzene or diethyl ether and very little N2H5 . In all cases
of the reaction of complex 1 with acids, only a trace amount of
N2H5+ was formed (<1%).
1
Et2O,
[H(OEt2)2][B(C6H3-3,5-
(CF3)2)4]
0
<1
<1
For complex 1, triflic acid was the most effective acid in
producing reduced products from coordinated dinitrogen,
[H(OEt2)2][B(C6F5)4] triggered nitrogen reduction but was less
effective than triflic acid under the reaction conditions. We
observed only traces of reduction products when
[H(OEt2)2][B(C6H3-3,5-(CF3)2)4] was used as the acid however
this may reflect, the fact that the oxonium acids are less soluble
than triflic acid in the reaction solvents. Weaker acids were
ineffective in producing ammonia or hydrazine from complex 1.
There was no increase in % N2 converted on addition of 6
equivalents of a strong external reductant (Cp*2Co) (Table 1)
however this probably reflects the fact that Cp*2Co would react
rapidly with the strong acid in the reaction mixture.
2 [c]
2 [c]
2 [c]
Pentane, TfOH
Benzene, TfOH
17
13
28
16
7
24
13
17
Pentane, TfOH,
Cp*2Co
3
3 [d]
3 [d]
3 [d]
Pentane, TfOH
Benzene, TfOH
14
6
4
3
3
11
6
Pentane, TfOH,
Cp*2Co
19
12
Reaction of [Fe(N2)(PP3iPr)] with TfOH. Treatment of the iron
[a] Yield calculation based on 1:1 ratio of metal complex:ammonium or
hydrazinium as quantified by GC. [b] Percentage of complexed dinitrogen
+
complex [Fe(N2)(PP3iPr)] 2 with TfOH in pentane afforded NH4
+
+
+
from the starting material which ends up as NH4 or N2H5 (calculated as
and N2H5 in 17 and 16% yield respectively (Table 1). Yields of
NH4 and N2H5 were slightly lower when the reaction was
conducted in benzene as the solvent.
half % yield NH4+ + % yield of N2H5+). [c] Contained up to 14% [FeH2(PP3 Pr)].
i
+
+
[d] Contained up to 8% [FeH2(PP3Cy)].
+
+
The formation of ammonia and hydrazine in this reaction
contrasts strongly with the previously reported work on
[Fe(N2)(dmpe)2] where no ammonium was formed on treatment
The combined yields of NH4 and N2H5 are best expressed
as “% N2 converted” which reflects the percentage of complexed
dinitrogen from the starting material which ends up as NH4 or
+
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10.1002/ejic.201900058
European Journal of Inorganic Chemistry
FULL PAPER
with various acids.[11] It is, however, more in line with reports by
Tyler and Ashley where they observed N2 reduction with
[Fe(N2)(DMeOPrPE)2] or [Fe(N2)(depe)2] with TfOH and this may
be due to the greater steric bulk of the various phosphine
ligands compared to dmpe. [12-13]
products and this appears to reflect the fact that it is a strong
acid with a weakly coordinating counterion. Other strong acids
including [H(OEt2)2][B(C6F5)4] also afforded ammonia and
hydrazine but in lesser quanties and weaker acids afforded no
(or negligible) amounts of hydrazine or ammonia. Stronger
acids are probably required to protonate a weakly activated
dinitrogen ligand and trigger the transfer of electrons from the
metal to nitrogen.
When [Fe(N2)(PP3iPr)] 2 was reacted with TfOH in pentane
+
with an added 6 equivalents of Cp*2Co, the proportion of NH4
+
formed was increased and the amount of N2H5 decreased
however there was no increase in the overall % N2 converted.
While we have not studied it in detail, there is clearly a
strong solvent dependence of the reaction. In other work with
ruthenium complexes of dinitrogen,[14] the spent (oxidized) form
of the metal has been isolated as a complex with the solvent and
we have postulated that there may be an important role for the
solvent in stabilizing the oxidized state of the metal and hence
providing an additional driving force for the reduction of
dinitrogen.
The reactions we have described in this work are all batch
reactions where the only source of electrons for the reduction of
nitrogen is the low oxidation state of the starting metal
complexes. Under the experimental conditions employed in this
work, the addition of an external reductant (Cp*2Co) was
ineffective as a source of additional reducing power for the
reduction and this probably reflects the fact that the strong
reductant would have only a short lifetime in the strongly acidic
reaction mixture.
Reaction of [Fe(N2)(PP3Cy)] 3 with TfOH. Treatment of
[Fe(N2)(PP3Cy)] 3 with TfOH in pentane afforded NH4+ and N2H5
+
in 14 and 4% yields respectively (Table 1). When the reaction
+
+
was carried out with benzene as the solvent, NH4 and N2H5
were obtained in 6 and 3% yields respectively.
As with the analogous complexes 1 and 2, Cp*2Co did not
significantly increase the % N2 converted for the reaction of
complex 3 with TfOH.
For all three Fe complexes 1, 2 and 3, reaction with triflic acid
afforded the protonated phosphine ligands at the end of the
reaction, where the ligand was protonated either at the central or
terminal positions or at both positions depending on the amount
of acid present in the reaction mixture. For complex 1, the main
phosphorus-containing products were [HP(CH2CH2PPh2)3]+
and/or [HP(CH2CH2PPh2H)3]4+. For complex 2, the main end
product was P(CH2CH2PiPr2H)3]3+ where only the terminal
phosphorus atoms are protonated. Complex 3 affords a mixture
of both [P(CH2CH2PCy2H)3]3+ and/or [HP(CH2CH2PCy2H)3]4+.
The formation of protonated phosphine ligand indicates that the
structural integrity of the complexes is lost under the reaction
conditions and presumably, Fe(II) is liberated into the reaction
medium.
While we observed a significant solvent effect on the yield of
the reduced products, we have not explored this in depth. The
role of solvent may be important in the reduction process
particularly in stabilizing the oxidized metal species following
reaction of the metal dinitrogen complexes with acid. With the
analogous Ru complexes [Ru(N2)(PP3iPr)] and [Ru(N2)(PP3Cy)],
stable triflate and solvent complexes were isolated[14] as the
oxidized metal species after reduction of the nitrogen ligand.
Experimental Section
General experimental procedures are contained in the supplementary
information as well as the procedure for quantifying ammonia and
hydrazine using GC-MS. Tris[2-(diisopropylphosphino)ethyl]phosphine
-
(PP3iPr), [FeCl(PP3iPr)]+BPh4 and [Fe(N2)(PP3iPr)] 2 were prepared by
literature methods or slight modifications thereof.[10a] The complexes
- [24]
- [25]
[FeCl(PP3Ph)]+BPh4 , [FeCl(PP3Cy)]+ BPh4
and [Fe(N2)(PP3Cy)] 3 [10c]
were prepared using literature methods or slight modifications thereof.
Preparation of [Fe(N2)(PP3Ph)] 1.
Potassium graphite (0.109 g,
suspension of
0.809 mmol, 4.3 equiv) was added to
a
-
[FeCl(PP3Ph)]+BPh4 (0.202 g, 0.187 mmol) in toluene (7 mL) under an
atmosphere of nitrogen and the reaction mixture was stirred at room
temperature for
diatomaceous earth to afford a dark orange-red solution. The solvent
was removed under reduced pressure to give red solid.
Recrystallization from mixture of benzene and hexane afforded
4
d.
The reaction mixture was filtered through
Conclusions
a
a
R
i
R
A series of [Fe(N2)(PP3 )] complexes [R = Ph, Pr, Cy; PP3
=
[Fe(N2)(PP3Ph)] 1 as a red crystalline solid (71 mg, 94 µmol, 50% yield).
Anal. Calcd for C42H42FeN2P4 (754.55): C, 66.86; H, 5.61; N, 3.71.
Found: C, 67.11; H, 5.75; N, 3.23. 1H NMR (THF-d8, 400 MHz): δ 7.10
P(CH2CH2PR2)3] were synthesized and treatment of these
complexes with triflic acid afforded ammonia in yields up to 23%
with the highest yield from [Fe(N2)(PP3Ph)] 1 in pentane as the
solvent. In all cases, hydrazine was also obtained as a product
in yields up to 16% with the highest yield from [Fe(N2)(PP3iPr)] 2
in pentane as the solvent. This result confirms that Fe(0) can
provide sufficient reducing power to effect the reduction of
coordinated N2. The bulky PP3 ligands employed in this work
were designed to partially encapsulate the metal center and to
slow or inhibit the competing reaction of the metal center with
acid.
(m, 12H, o-H), 7.02 (m, 6H, p-H), 6.93 (m, 12H, m-H), 2.28 (m, 6H, CH2),
2
1.86 (m, 6H, CH2). 31P{1H} NMR (THF-d8, 162 MHz): δ 177.8 (q, JPP
=
41 Hz, 1P, PC), 85.9 (d, 3P, PT). IR: 3067w, 3040w, 2020s ν(N≡N),
1583m, 1570w, 1478s, 1431s, 1407m, 1330w, 1302w, 1272w, 1225w,
1185w, 1156w, 1082m, 1070m, 1055w, 1026m, 998w, 964w, 902w,
879m, 856m, 824m, 794s, 751m, 735s, 691s, 673s, 654w, 619w cm-1.
CCDC 1570094 contains the crystallographic data for this compound.
The 15N-labeled analogue [Fe(15N2)(PP3Ph)] was prepared by degassing a
solution of [Fe(N2)(PP3Ph)] in THF-d8 by three freeze-pump-thaw cycles,
placing an atmosphere of 15N2 gas in the headspace, then allowing the
solution to stand for six days to allow exchange to occur (approximately
While we examined a range of different acids, triflic acid
provided the highest conversion of coordinated N2 to reduced
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10.1002/ejic.201900058
European Journal of Inorganic Chemistry
FULL PAPER
2
88% enrichment). 31P{1H} NMR (THF-d8, 162 MHz): δ 177.8 (dq, JPP
=
=
3 days in which time the color changed from red to purple. The volatiles
were removed under reduced pressure. The residue was dissolved in
DMSO-d6 or DMSO and analyzed for NH3 and N2H4.
2
2
3
41 Hz, JNP = 9 Hz, 1P, PC), 85.9 (ddd, 2JPP = 41 Hz, JNP = 5 Hz, JNP
2 Hz, 3P, PT). 15N{1H} NMR (THF-d8, 41 MHz): δ -3.5 (m, 1N, Nβ), -31.5
(m, 1N, Nα).
(iii) In a typical reaction, [H(OEt2)2][B(C6F5)4] (0.193 g, 0.233 mmol) was
added to a suspension of [Fe(N2)(PP3Ph)] 1 (22 mg, 29 µmol) in pentane
(3 mL) under nitrogen. The reaction mixture was stirred at room
temperature overnight and the volatiles were removed under reduced
pressure. The residue was dissolved in DMSO-d6 or DMSO and
analyzed for NH3 and N2H4.
Reaction of [Fe(N2)(PP3Ph)] 1 with TfOH.
(i) In a typical reaction, TfOH (43 mg, 0.29 mmol) was added to the red
suspension of [Fe(N2)(PP3Ph)] (23 mg, 30 µmol) in pentane (2 mL) under
nitrogen. The reaction mixture was stirred at room temperature overnight
during which time the solution decolorized and purple and white solids
precipitated from the solution. The volatiles were removed under
reduced pressure. The residue was dissolved in DMSO-d6 or DMSO and
analyzed for NH3 and N2H4 using GC-MS. The main phosphorus-
Reaction of [Fe(N2)(PP3Ph)] 1 with [H(OEt2)2][B(C6H3-3,5-(CF3)2)4].
(i)
In a typical reaction, [H(OEt2)2][B(C6H3-3,5-(CF3)2)4] (0.281 g,
Ph
0.278 mmol) and [Fe(N2)(PP3Ph)] 1 (26 mg, 34 µmol) in benzene (5 mL)
were stirred at room temperature under nitrogen overnight. The volatiles
were removed under reduced pressure. The residue was dissolved in
DMSO-d6 or DMSO and analyzed for NH3 and N2H4.
containing product at the end of the reaction was protonated PP3
[HP(CH2CH2PPh2H)3]4+ 31P{1H} NMR (DMSO-d6, 243 MHz): δ 55.6 (br,
.
1P, HPC+), 34.0 (br, 3P, HPT).
(ii) In a typical reaction, TfOH (45 mg, 0.30 mmol) was added to the red
suspension of [Fe(N2)(PP3Ph)] (22 mg, 29 µmol) in benzene (2 mL) under
nitrogen. The purple suspension was stirred at room temperature
overnight and the volatiles removed under reduced pressure. The
(ii) [H(OEt2)2][B(C6H3-3,5-(CF3)2)4] (0.250 g, 0.247 mmol) was added to a
suspension of [Fe(N2)(PP3Ph)] 1 (23 mg, 30 µmol) in diethyl ether (5 mL)
under nitrogen and the reaction mixture stirred at room temperature
overnight. The volatiles were removed under reduced pressure. The
residue was dissolved in DMSO-d6 or DMSO and analyzed for NH3 and
N2H4.
residue was dissolved in DMSO-d6 or DMSO and analyzed for NH4+ and
+
N2H5
.
The main phosphorus-containing product at the end of the
Ph
reaction was protonated PP3 [HP(CH2CH2PPh2H)3]4+
.
31P{1H} NMR
(DMSO-d6, 243 MHz): δ 55.5 (br, 1P, HPC+), 33.8 (br, 3P, HPT,
(HPP3Ph)+).
Reaction of [Fe(N2)(PP3iPr)] 2 with TfOH.
An authentic sample of [HP(CH2CH2PPh2H)3]4+ was prepared for
spectroscopic comparison by reaction of P(CH2CH2PPh2)3 with TfOH
(see Supplementary Material).
(i) In a typical reaction, TfOH (47 mg, 0.31 mmol) was added to a
solution of [Fe(N2)(PP3iPr)] 2 (19 mg, 35 µmol) in pentane (2 mL) under
nitrogen. The orange solution turned colorless and a purple brown solid
precipitate formed. After stirring at room temperature for 1.5 h, the
volatiles were removed under reduced pressure. The residue was
Reaction of [Fe(N2)(PP3Ph)] 1 with TfOH and CoCp*2. In a typical
reaction, TfOH (53 mg, 0.35 mmol) in pentane (0.5 mL) was added to a
suspension of [Fe(N2)(PP3Ph)] (30 mg, 40 µmol) and CoCp*2 (79 mg,
0.24 mmol) in pentane (2 mL) under nitrogen. The reaction mixture was
stirred at room temperature for 3 days and the volatiles were removed
under reduced pressure. The residue was dissolved in DMSO-d6 or
DMSO and analyzed for NH3 and N2H4.
+
dissolved in DMSO-d6 or DMSO and analyzed for NH4 and N2H5+. The
main phosphorus-containing product at the end of the reaction was
iPr
protonated PP3
[P(CH2CH2PiPr2H)3]3+
.
31P{1H} NMR (DMSO-d6,
122 MHz): δ 38.0 (d, 3JPP = 34 Hz, 3P, HPT+), -9.1 (br s, 1P, PC).
(ii) In a typical reaction, TfOH (45 mg, 0.30 mmol) was added to a
solution of [Fe(N2)(PP3iPr)]
2 (24 mg, 37 µmol, contained 14%
Reaction of [Fe(N2)(PP3Ph)] 1 with HCl.
[FeH2(PP3iPr)]) in benzene (2 mL) under nitrogen. The orange solution
turned colorless and a whitish precipitate formed. The reaction mixture
was stirred at room temperature overnight and the volatiles removed
under reduced pressure. The residue was dissolved in DMSO-d6 or
A solution of HCl (2 M in diethyl ether, 0.11 mL, 0.23 mmol) was syringed
into a red suspension of ([Fe(N2)(PP3Ph)] (22 mg, 29 µmol) in benzene
(2 mL) under nitrogen. The color of the reaction mixture went through a
series of changes from red to yellow then to pale pink after stirring at
room temperature overnight. The volatiles were removed under reduced
pressure. The residue was dissolved in DMSO-d6 or DMSO and
analyzed for NH3 and N2H4.
+
+
DMSO and analyzed for NH4 and N2H5
.
The main phosphorus-
iPr
containing product at the end of the reaction was protonated PP3
[P(CH2CH2PiPr2H)3]3+
.
31P{1H} NMR (DMSO-d6, 162 MHz): δ 37.0 (d,
3JPP = 36 Hz, 3P, HPT+), -9.9 (q, 1P, PC).
An authentic sample of [P(CH2CH2PiPr2H)3]3+ was prepared for
spectroscopic comparison by reaction of [P(CH2CH2PiPr2H)3]3+ with TfOH
(see Supplementary Material).
Reaction of [Fe(N2)(PP3Ph)] 1 with [H(OEt2)2][B(C6F5)4].
(i) In a typical reaction, [H(OEt2)2][B(C6F5)4] (0.237 g, 0.286 mmol) was
added to a suspension of [Fe(N2)(PP3Ph)] 1 (27 mg, 36 µmol) in benzene
(2 mL) under nitrogen. The reaction mixture was stirred at room
temperature for 3 days in which time the color changed from red to
purple. The volatiles were removed under reduced pressure. The
residue was dissolved in DMSO-d6 or DMSO and analyzed for NH3 and
N2H4.
Reaction of [Fe(N2)(PP3iPr)] 2 with TfOH and CoCp*2. In a typical
reaction, TfOH (61 mg, 0.41 mmol) in pentane (0.5 mL) was added to a
solution of [Fe(N2)(PP3iPr)]
2 (32 mg, 50 µmol, contained 14%
[FeH2(PP3iPr)]) and CoCp*2 (0.10 g, 0.30 mmol) in pentane (2 mL) under
nitrogen. The dark brown solution turned into a brown suspension. The
reaction was stirred at room temperature overnight and the volatiles
removed under reduced pressure. The residue was dissolved in
DMSO-d6 or DMSO and analyzed for NH3 and N2H4.
(ii)
[H(OEt2)2][B(C6F5)4] (0.210 g, 0.254 mmol) was added to a
suspension of [Fe(N2)(PP3Ph)] 1 (24 mg, 32 µmol) in diethyl ether (2 mL)
under nitrogen. The reaction mixture was stirred at room temperature for
Reaction of [Fe(N2)(PP3Cy)] 3 with TfOH.
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10.1002/ejic.201900058
European Journal of Inorganic Chemistry
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(i) In a typical reaction, TfOH (25 mg, 0.17 mmol) in pentane (0.5 mL)
was added to a solution of [Fe(N2)(PP3Cy)] 3 (16 mg, 18 µmol, contained
8% [FeH2(PP3Cy)]) 4 in pentane (2 mL) under nitrogen. The orange
suspension was stirred at room temperature overnight and the volatiles
were removed under reduced pressure. The residue was dissolved in
DMSO-d6 or DMSO and analyzed for NH3 and N2H4. The main
phosphorus-containing products at the end of the reaction were
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[P(CH2CH2PCy2H)3]3+ and [HP(CH2CH2PCy2H)3]4+
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DMSO and analyzed for NH3 and N2H4. The main phosphorus-
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.
3
31P{1H} NMR (DMSO-d6, 162 MHz): δ 29.0 (d, JPP = 35 Hz, 3P,
HPT+), -10.0 (q, 1P, PC).
An authentic sample of [P(CH2CH2PCy2H)3]3+ was prepared for
spectroscopic comparison by reaction of [P(CH2CH2PiPr2H)3]3+ with TfOH
(see supplementary material).
[6]
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Reaction of [Fe(N2)(PP3Cy)] 3 with TfOH and Cp*2Co. TfOH (91 mg,
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29 µmol, contained 7% [FeH2(PP3Cy)]) 4 and Cp*2Co (63 mg, 0.19 mmol)
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room temperature overnight and the volatiles were removed under
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NH3 and for N2H4.
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Acknowledgements
We gratefully acknowledge UNSW for funding. We also thank
Dr. Martin Bucknall (Bioanalytical Mass Spectrometry Facility,
Mark Wainwright Analytical Centre, University of New South
Wales) and Dr. Samantha Furfari (School of Chemistry,
University of New South Wales) for their assistance with GC-MS
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Conflict of interest
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The authors declare no conflicts of interest.
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/chem.2019xxxxx.
Keywords: ammonia • catalysis • nitrogen fixation • iron •
hydrazine
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10.1002/ejic.201900058
European Journal of Inorganic Chemistry
FULL PAPER
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Entry for the Table of Contents
FULL PAPER
In the presence of strong acids,
coordinated N2 in [Fe(N2)(PP3R)]
(PP3R = P(CH2CH2PR2)3; R = Ph, iPr,
Cy) is reduced to a mixture of
ammonia and hydrazine. The bulky
PP3 ligands were designed to
partially encapsulate the metal
center and to slow or inhibit the
competing reaction of the metal
center with acid.
Nitrogen reduction to ammonia
Leslie D. Field,* Hsiu L. Li, Scott J.
Dalgarno and Ruaraidh D. McIntosh
PR2
Fe
R2
TfOH
P
+
+
N
N
+
NH4
N2H5
P
PR2
Page No. – Page No.
=
Ammonia and Hydrazine from
coordinated Dinitrogen by Complexes
of Iron(0)
R
Ph, iPr,
cyclohexyl
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