Teaching old compounds new tricks: Efficient N2 fixation by simple Fe(N2)(diphosphine)2 complexes

Article abstract of DOI:10.1039/c6dt00884d

The Fe(0) species Fe(N₂)(dmpe)₂ exists in equilibrium with the previously unreported dimer, [Fe(dmpe₂)₂(μ-N₂)]. For the first time these complexes, alongside Fe(N₂)(depe)₂, are shown unambiguously to produce N₂H₄ and/or NH₃ upon addition of triflic acid; for Fe(N₂)(depe)₂ this represents one of the highest electron conversion efficiencies for Fe complexes to date.

Full text of DOI:10.1039/c6dt00884d

View Article Online
View Journal
Dalton
Transactions
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: L. R. Doyle, P. J.
Hill, G. G. Wildgoose and A. E. Ashley, Dalton Trans., 2016, DOI: 10.1039/C6DT00884D.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/dalton

Page 1 of 6
Ple aDs ea l dt oo nn To rt a and sj ua sc tt imo na sr gins
View Article Online
DOI: 10.1039/C6DT00884D
Dalton Transactions
COMMUNICATION
Teaching old compounds new tricks: efficient N fixation by
2
simple Fe(N )(diphosphine) complexes
2
2
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
a
Laurence R. Doyle, Peter J. Hill, Gregory G. Wildgoose and Andrew E. Ashley*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The Fe(0) species Fe(N )(dmpe) exists in equilibrium with the experiments performed on related phosphine complexes -
2
2
+
8
previously unreported dimer, [Fe(dmpe ) (μ-N )]. For the first [trans-Fe(H)(N )(depe) ]
time these complexes, alongside Fe(N )(depe) , are shown [cis-Fe(H)(N ){E(CH CH PPh ) }] (E = N, P),
unambiguously to produce N H and/or NH upon addition of Fe(H)(N )(DMeOPrPE) ]
triflic acid; for Fe(N )(depe) this represents one of the highest [(MeOCH CH CH ) PCH ] ) - have also been shown to
(depe
=
Et PCH CH PEt ),
2
2
2
2
2
2
2
2
2
+
8,9
and [trans-
2
2
2
2
2
2 3
+
(DMeOPrPE
=
2
4
3
2
2
1
0
2
2
2
2
2 2
2 2
electron conversion efficiencies for Fe complexes to date.
generate similar yields of NH and/or N H . However, in all of
3 2 4
these experiments the Fe(0) species were not isolated; in the
case of the archetypal dmpe system, was reported to be
unstable with respect to dissociation of N in vacuo, leading to
Homogeneous catalysts capable of fixing N to NH under mild
2
3
1
1
conditions have been researched for over 50 years. Fe, which
catalyses the industrial Haber-Bosch process (as Fe metal), is
also considered to perform a crucial role in biological N2
fixation, performed at the Fe-Mo cofactor of the most
abundant nitrogenase enzyme and mediated by successive
2
5,8
its decomposition.
2
proton-coupled electron transfers. Whilst the active site for
N binding and reduction at the Fe-Mo cofactor is contested,
2
less common nitrogenases with closely related Fe-V and Fe-
3
only cofactors implicate the importance of Fe, and a
4
mechanism for Fe-mediated N fixation has been proposed.
2
The first major breakthrough in N2 fixation by a
homogeneous Fe complex was reported in 1991 by Leigh et al.,
utilising chelating Me PCH CH PMe (dmpe) as an ancillary
Fig. 1 Synthesis and acidification of
et al.; NaNap = sodium naphthalenide, highlighting the disparate results for NH
production.
2 performed by (i) Leigh et al. and (ii) Komiya
3
2
2
2
2
5
ligand. In the eponymous reaction cycle, the Fe(II) complex
trans-Fe(H)(N )(dmpe) ][BPh ] was reductively deprotonated
In contrast with these findings, Komiya et al. successfully
[
11
2
2
4
synthesised pure Fe(N )(depe) (
2
)
using an alternative route
2
2
to form the Fe(0) intermediate Fe(N )(dmpe) (1) which, upon
2
2
and discovered that only N and H were produced upon
2
2
in situ acidification of the reaction mixture using various strong
treatment with HCl; this result cast uncertainty on the
candidacy of Fe(N )L (L = 2 electron donor) complexes being
proton sources, was documented to produce NH (isolated as
3
2
4
+
NH via a base distillation onto fresh acid and quantified using
the spectrophotometric indophenol method); the highest
4
the active NH producing species in Leigh-type experiments
3
6
(
Fig. 1). Furthermore, Field et al. recently showed that the
5,7,8
yields were obtained using HCl.
Since Fe was recovered as
+
positive detection of NH3 (as NH ) using the indophenol
4
Fe(II), the yields of NH (up to 20%) were calculated based on
3
method can arise from interference caused by free phosphine
ligands, which may contaminate the analyte during the base
each Fe providing a maximum of 2 electrons (out of a total of
6
) to reduce N ; accordingly Fe(0) must be consumed as the
12
2
distillation step; this was corroborated by the absence of
sacrificial reductant. Analogous deprotonation/reprotonation
+
1
14
1
resonances for NH4 in the H and N{ H} NMR spectra of the
+
analyte from the Leigh reaction of [trans-Fe(H)(N )(dmpe) ] .
2
2
a.
Clearly, the isolation of pure samples of such species, and their
Department of Chemistry, Imperial College London, Exhibition Road, South
Kensington, London SW7 2AZ, UK. E-mail: a.ashley@imperial.ac.uk Tel: +44 0(20) subsequent reaction with acids to assess their capability of
b.75945810.
producing reduced forms of N , is crucial to clarifying this long-
standing conundrum.
2
School of Chemistry, University of East Anglia, Norwich Research Park, Norwich,
NR4 7TJ, UK
†
Electronic Supplementary Information (ESI) available: Full experimental
procedures and characterization data. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Ple aDs ea l dt oo nn To rt a and sj ua sc tt imo na sr gins
Page 2 of 6
COMMUNICATION
Journal Name
View Article Online
DOI: 10.1039/C6DT00884D
Scheme 1 Synthesis of
1 and 3.
Recently we reported convenient multi-gram syntheses of
13
dmpe and depe, and sought to reinvestigate the historically
curious N -fixation chemistry mediated by their Fe(N )
2
2
complexes. Herein we report the synthesis and
characterisation of the Fe(0) species, [Fe(dmpe) ] (μ-N ) ( ),
, these
3
2
2
2
‡
which reacts with N cleanly to produce 1. Alongside 2
2
Fig. 3 Solid-state structure of
3
; H atoms omitted for clarity; ellipsoids shown at
isolated compounds react with TfOH (CF SO H) to produce 30% probability. Selected bond lengths (Å) and angles (°): Fe1-N1 1.854(2);
3
3
Fe2-N2 1.850(2); N1-N2 1.144(3); N2-N1-Fe1 178.7(2); N1-N2-Fe2 179.1(2).
N H and/or NH , thus unambiguously confirming that these
2
4
3
complexes are active for the fixation of N , for the first time.
2
The solid-state structure shows the independent [Fe(dmpe) N]
2
fragments in
3 both adopt near ideal trigonal bipyramidal
coordination, with the two equatorial Fe(1)P(4)P(14) and
Fe(2)P(24)P(34) best planes bisecting one another almost
perpendicularly [82.08(4)°]. The bridging N2 ligand is
approximately linear, exhibiting typical bond lengths for both
the single Fe–N and triple N–N bonds; the latter is comparable
1
5
with the previously reported structure of
2 [1.139(13) Å] and
indicates weak activation of the N unit in both complexes. The
2
bond lengths and angles seen in
3 are in close agreement with
the geometry optimised structure reported by Tyler et al. in
their theoretical study of N2 fixation mediated by various
Fe(dmpe) intermediates, in which dimerisation of
1
(with
was calculated to be
unfavourable by 20 kcal mol . Furthermore, a low energy
2
concomitant loss of N ) to form
3
2
–
1 16
–
1
3
1
1
15
1
barrier of only 6 kcal mol was calculated for the dissociation
of to and [Fe(dmpe) ]. Despite this, it has been possible to
prepare
details): after generating a crude solution of
solvent was mostly removed in vacuo until a slurry of solid
mixture of and ) in a small volume of solvent remained,
after which this suspension was stirred for several days under
Ar. Using this protocol, less soluble selectively crystallises as
N is slowly depleted upon condensation of , and residual
are subsequently removed by rinsing with additional
Fig. 2 (i) P{ H} and (ii) N{ H} NMR spectra of the reduction of trans-
1
5
2 2 2 8
Fe(Cl) (dmpe) under N with KC (4 eq.) in hexane.
3
1
3
2
14
on a multi-gram scale (using N ; see ESI for further
2
1
5
KC reduction of trans-Fe(Cl) (dmpe) under a N atmosphere
in hexane (Scheme 1), as previously described by Field et al.,
generates solutions of
3.3 ppm (s, fwhm = 6 Hz); N{ H} NMR: 
8
2
2
2
1
, the hexane
14
15
31
1
1
- N in situ [ P{ H} NMR:

(ppm) =
(ppm) = –48.8 (d), –
= 5.9 Hz] along with a trace amount of the
2
5
(
1
3
1
1
6
4
1
7.0 (d), J
Nα-Nβ
3
known decomposition product [Fe(dmpe) ] (μ-dmpe) (4)
P{ H} NMR:
However, another broader singlet was also observed
downfield in the P{ H} NMR spectrum [
2
2
1
1
2
31
1
[
(ppm) = 61.4 ppm (d), 8.2 ppm (m)]; see Fig. 2.
and
4
cold hexane. The resulting sample was then rapidly
31
1
(ppm) = 66.0 ppm,
recrystallised (redissolved in hexane, filtered and cooled to –
fwhm = 14 Hz], in addition to an upfield singlet (–54.9 ppm) in
31
3
5°C) yielding a microcrystalline solid of ≥ 98% purity ( P NMR
15
1
the N{ H} NMR spectrum of this solution. To assess the
reported instability of in the absence of N , the solvent was
removed in vacuo and the remaining oily solid dried for several
spectroscopy) that provided satisfactory elemental (CHN)
analysis. Crystalline is thermally unstable and is best stored
under Ar at ≤ –30°C; under these conditions decomposition (to
a mixture of , and Fe metal) appears to be minimal after
several months.
Solutions of
1
2
3
–
3
hours at ca. 10 mbar pressure. Unexpectedly, subsequent
dissolution of this solid in hexane under Ar revealed to still
be present by P{ H} NMR spectroscopy, albeit in a lower ratio
relative to the unassigned resonances. Curiously, the amount
remained almost unchanged. Gratifyingly, slow
evaporation of the solvent (Ar atmosphere) yielded large, deep
red crystals whose solution-phase P{ H} and N{ H} NMR
spectra corresponded to the aforementioned unidentified
resonances, and which were solved by single crystal X-ray
1, 4
1
31
1
3
prepared under an Ar atmosphere
17
7
decompose to
4 and Fe metal; this occurs relatively slowly in
of
4
non-polar alkane solvents (pentane, t½ = 13 d) yet more
readily in donor solvents (THF, t½ ≈ 1.5 d; ε = 7.52).
r
31
1
15
1
Dissolution
of
3
in
N -saturated
2
solvents
quantitatively generates
1
, which proceeds more slowly in
aliphatics than ethereal donor solvents (THF, 0.25 M, 2 d),
whilst in the highly polar non-donor organic solvent 1,2-
diffraction as the new compound [Fe(dmpe) ] (μ-N ) (3, Fig. 3).
2
2
2
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 6
Ple aDs ea l dt oo nn To rt a and sj ua sc tt imo na sr gins
Journal Name
COMMUNICATION
1
8
difluorobenzene (εr = 13.8), conversion to
1 is almost detected only trace amounts (< 0.5 % per Fe) of NH and no
View Article Online
3
DOI: 10.1039/C6DT00884D
instantaneous. Thus, it would appear that a large solvent N H , including when
polarity facilitates dissociation, rather than the donor ability of Fe(H)(N )(PP) ] (PP = dmpe, depe) using the original method
1
was prepared in situ from [trans-
2
4
+
2
2
the solvent.
The Raman active ν(N–N) stretch of solid
indicates a significant increase in the activation of the N ligand confirms that, under this acidification protocol (whether
of Leigh et al.; the latter results corroborate those of Field et
(1933 cm ) al. Identical results were also obtained for pure 2 and 3 which
–
1
5
3
2
compared to the monomeric complex
1
[IR(KBr): ν(N–N) = formed under Leigh-type conditions or using isolated pure
–
1 5
1
975 cm ). In fact, neutral 3 has one of the lowest ν(N–N) samples), neither of these dmpe/depe complexes can produce
stretches recorded for a low-spin Fe system, which is the yields of NH previously reported.
3
comparable with those found in the anionic complexes
i
[
(P E)Fe(N )][Na(12-crown-4) ] (P = 2-P Pr C H ; E = B, Si; Table 1 Selected yields of N
2
H
4
and NH
3
from the acidification of
1
-3, and
3
2
2
2 6 4
–
1 19
IR(THF): ν(N–N) = 1918, 1920 cm ) reported by Peters et al. related complexes, with acid.
(P B)Fe(N )][Na(12-crown-4) ] is notable for being the first
synthetic Fe complex able to catalyse the fixation of N to NH3
from proton and electron equivalents, demonstrating the
feasibility of a single Fe site to perform this fundamental
transformation; here, a very strong reductant (KC ) and a
[
–
3
2
2
e
N H NH3 N-atom
2 4
2
Entry Compound
Solvent
a
a
yield Ref
(%) (%) yield (%)
b
(%)
d
d
d
d
d
d
d
d
d
d
d
1
2
3
4
5
6
7
8
9
1
1
1
1
2
2
2
2
2
3
3
THF
Et O
0
0
0
0
0
9.1
9.1
3.8
0
2
0
8
2
9.1
9.1
3.8
18.2
18.2
7.7
powerful acid [H(OEt ) (BArF ); BArF = B(3,5-(CF ) C H ) ]
pentane
pentane
THF
2
2
24
24
3 2 6 3 4
c
were used in excess. In contrast, for Leigh-type chemistry,
electron equivalents for the N2 reduction are ultimately
supplied by Fe(0) species, generated via reductive
deprotonation of a Fe–H bond in the Fe(II) precursor. Thus, to
assess the reducing power of such Fe(0)N -phosphine
complexes, cyclic voltammetry measurements were performed
0
3.6
2.6
6.2
10.5
7.8
4.5
1.5
0
4.9
11.1
31.5
28.3
53.5
54.8
10.8
4.1
Et
Et
2
O
O
11.2
6.3
14.2
11.5
24.8
26.3
5.0
c
2
pentane 20.9
pentane
pentane
pentane
2
c
24.0
4.3
2.0
e
1
0
on
[
1 (generated from 3/N2), 2, and 3 (2 mM in Et O;
2
c,e
n
+/0
11
2.0
Bu N][BArF ] electrolyte; Cp Fe
reference). For both
and 3 a reversible one-electron oxidation was
observed at various scan rates which can be assigned to the
Fe(0)/Fe(I)] redox couple (centred at –2.03 V and –2.23 V, for
] respectively; see ESI). Conversely, the
revealed a single irreversible
oxidation at ca. –2.0 V [Fe(0) → Fe(I)], and three smaller
unassigned reduction processes between ca. –2.0 and –2.4 V. Fe; yield assuming each Fe supplies a max. of two electrons; performed at –78 °C;
is unstable under these this work;
conditions, and the additional reduction processes may involve
highly reactive [Fe(dmpe) ] (via N dissociation from [
an Et O adduct, or solvent-activation product(s) thereof.
4
24
2
f
2
10
1
2
FeN
2
(DMeOPrPE)
Et
2
O/THF
2
NR
0.3
0
15
0
9.5
26.5
0
compounds
2
12
13 1, prepared in situ hexane
0
0.4
0
[
2
g
d
1
1
4
5
2
Et
2
O
O
0.2
0
0.9
0
+
+
↔ [
2
] and
3
↔ [
3
h
d
2
Et
2
cyclic voltammogram of
1
a
All reactions performed at 25°C using TfOH, unless stated otherwise. Yields per mol
b
c
d
+
e
f
Accordingly it appears that [
1
]
performed under Ar;
from deprotonation of [trans-
+
g
h
Fe(H)(N
2
)(DMeOPrPE)
2
] ;
[H(OEt
2
)
2
][BArF24
]
used;
2,6-dimethylpyridinium
+
+
(lutidinium) triflate used. NR = not reported. Yields are averaged over all runs (see ESI
for more details).
1
] ), or
2
2
2
Nonetheless, the neutral Fe(0) compounds
powerful reducing agents, and considerably stronger than the upon
commonly employed CoCp and CoCp* (–1.33 and –1.84 V vs. Fe(N )(DMeOPrPE) complex with the following acids: HCl (4%),
1
-
3
are notably Tyler et al. reported a marked difference in the yields of NH3
acidification of their Leigh-type prepared
2
2
2
2
+
/0
21
Cp Fe in 1,2-dimethoxyethane), which have been used as HBF (7%), and TfOH (up to 15%); in the latter case they
2
4
external reductants in catalytic N2 fixation by Mo showed, using a phenanthroline spectrophotometric test, that
2
2,23
complexes.
In the knowledge that
after acidification all Fe species are present as Fe(II), thus
1
-
3
are potent reductants, we verifying the hypothesis that each Fe(0) can only supply a
+
sought to establish conclusively whether they are able to maximum of two electrons for the reduction of N (or H to
2
convert N to the reduced forms N H and NH in the presence H2). These yields were suggested to reflect increasing
2
2
4
3
of protons, and furthermore in the absence of any potential favourability
of
NH3
formation
with
decreasing
contaminants (synthetic by-products/decomposites) from coordination/ion-pairing of the anion of the acid. It should be
Leigh-type deprotonation reactions. Our protocol (see ESI) for noted that whilst NH was quantified either by NMR
3
10
25
the quantitative assay of NH used the relative integration of spectroscopy or the indophenol test, the DMeOPrPE ligand
3
+
1
‡‡
the NH4 resonance in the H spectrum against a calibrated is expected to be far less volatile than dmpe/depe and thus
insert. Quantitative analysis of N2H4 employed
unlikely to interfere with the latter method. To our delight,
spectrophotometric method which relies on reaction with when using TfOH to acidify
, we were able to detect
significant amounts of N H and/or NH , which showed a
a
1-3
24
acidic para-dimethylaminobenzaldehyde indicator solution;
2
4
3
by performing thorough control experiments we found, marked dependence on solvent and/or temperature; these
crucially, that neither NH , nor dmpe, nor depe interfered with data are reported in Table 1, alongside other reported Fe(N )L4
3
2
§
the results. Using HCl to acidify pristine solutions of
1
, we Leigh-type experiments for comparison. Historically, yields of
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Ple aDs ea l dt oo nn To rt a and sj ua sc tt imo na sr gins
Page 4 of 6
COMMUNICATION
Journal Name
–
NH from Leigh-type experiments are quoted per Fe centre, were privileged in their activity in comparison with either Cl or
3
View Article Online
–
23
DOI: 10.1039/C6DT00884D
In our study, it is possible that the intermediate
have also included two other measures in order to resolve the coordinating ability of TfO strikes the best balance of lability
efficiency of the ability of Fe(N )L species to produce these properties to facilitate proton-coupled electron transfer events
however since we have mixtures of N H /NH products we [BArF24]
.
2
4
3
–
2
4
azanes: (1) a combined fixed-N electron yield was calculated during N fixation mediated at the Fe centre. We have also
2
on the basis that reduction of N2 to N H /NH requires probed the use of the weaker acid 2,6-dimethylpyridinium
2
4
3
3
1
four/three electrons (per mol of product), which takes into (lutidinium) triflate [pK (H O) = –6.77] with our most efficient
account that each Fe provides a maximum of two electrons;
a
2
5,25
compound 2; in this case no azanes were produced, and
(
2) a fixed-N atom yield, calculated by the fraction of N atoms instead protonation at the metal centre resulted in clean
+
8
from the starting material that end up as N H or NH . Clearly conversion to the Fe(II) compound trans-[(H)Fe(N )(depe) ] ,
the yields for these reactions may be interpreted in several as ascertained by P{ H} NMR ( = 81.4 ppm) and H NMR
ways, and all may be worth considering in the absence of (hydride signal at
2
4
3
2
2
31
1
1
2
 = –18.20 ppm; J = 49 Hz) spectroscopy.
HP
greater mechanistic understanding of these rapid, and Thus it appears that if too weak an acid source is used,
complex, transformations.
formation of the thermodynamic Fe–H product is strongly
Using TfOH, the highest electron yields were obtained for
2
3
favoured.
(
entries 5-9; ≤ 55%), followed by
1
(entries 1-4; ≤ 18%) and
The increased yields of N fixation products for
2
relative to
2
(entries 10-11; ≤ 11%); these yields, in particular for
2
, are
1 may be attributed to the augmented steric bulk around the
amongst the highest reported for complexes of Fe, and Fe centre conferred by the depe ligand, which also protects
+
highlight the delicate dependence on the acidification the metal centre from non-productive direct H attack. Despite
2
0,23,26
conditions, which is typical for N fixation chemistry.
In a greater degree of N2 activation and a more negative
2
these reactions initial protonation of the N ligand is a critical reduction potential for , the conversion yields are lower than
3
2
+
step, thereby triggering subsequent electron transfer; the for
1. However, since the reduction of H to H competes with
2
efficacy of this process will presumably depend primarily on N fixation, the more potently reducing
the strength of the H source employed. Previous calculations discrimination between the processes, translating to lower
3
may lead to poorer
2
+
have shown that protonation at Fe is more thermodynamically yields of N H and NH vs. H formation.
2
4
3
2
favourable than at the terminal N atom in Fe(N )(dmpe) by
some 40 kcal mol , and it is expected that the latter process Fe (N )(dmpe/depe) complexes, previously synthesised in situ
In conclusion, we have finally verified that simple

2
2
–
1
0
2
2
would result from kinetic factors, such as the use of a strong from Leigh-type deprotonations, are capable of producing
1
6
and sterically bulky acid source. The effect of solvent on the appreciable amounts of N H and NH using TfOH as the acid
yields obtained for
2
4
3
0
2 is conspicuous, which generally decrease source. In the case of the Fe (N )(depe) the reaction is
2 2
in the order: pentane > Et O > THF. Whilst TfOH is insoluble in particularly efficient based on the number of electrons
2
pentane and mass transfer effects may explain the high yields available, and represents one of the highest conversions (55 %)
obtained from this medium, in both Et O and THF [pK (H O) = to date. The significant proportion of N H produced in these
2
a
2
2 4
27
–
3.59 and –2.08, respectively] it is expected that acidity of reactions suggests that NH formation may proceed via N H
TfOH [pK (H O) ≈ –12], will be levelled to the donor solvent, intermediates; further reduction may occur on Fe and/or via
3
2
4
28
32
a
2
hence the protonating power of TfOH in the solvents used is an outer sphere pathway. Mechanistic investigations into
expected to follow the same order, correlating with a greater understanding this reactivity are currently underway.
+
efficiency of H attack on N . Another factor may be the
We wish to thank the EPSRC for PhD studentship funding
(LRD and PJH), the ERC (Starting Grant no. 307061, PiHOMER;
2
29
aggregation of TfOH due to strong intermolecular H-bonding,
with a bulkier proton source favouring protonation at the GGW) and the Royal Society for University Research
exposed N ligand over the Fe centre.
Fellowships (AA and GGW).
2
Curiously, when H(OEt ) (BArF ) in Et O is employed as the
2
2
24
2
acid source, only trace amounts of N H and NH are observed;
2
4
3
Notes and references
since TfOH and HCl [pK (H O) = –8] are expected to be levelled
a
2
3
1
to protonated Et O, taken together these experiments provide ‡ We do not see any solution-phase spectroscopic evidence ( P,
2
15
1
a situation where the solution pH can be viewed as
approximately constant, and hence the effect of the anion on
these reactions can be resolved. It is envisaged that strongly
coordinating anions may bind/ion-pair more favourably to
N, H NMR spectroscopy) for the formation of the depe
analogue of 3, and we believe that the increased steric impact of
replacing Me with Et in the ligand backbone is sufficient to
preclude the formation of a dimeric species [Fe(depe) ] ( -N2).
‡ NH : H NMR (DMSO-d : δ ≈ 7.3 ppm, t(1:1:1), J = 51 Hz);

2
2
+
4
1
1
‡
6
NH
3
3
1
protonated intermediates along the N -fixation pathway, 2,5-dimethylfuran insert (vinylic proton resonance): H NMR
2
which could sequester their reactivity and hence inhibit the (DMSO-d
: δ = 5.83 ppm).
6
§
This methodology was validated by acidification experiments
on authentic samples of NH Cl or N H ·2HCl, in the presence of
formation of N H or NH ; is it is therefore surprising that both
2
4
3
4
2 4
HCl and H(OEt ) (BArF ) are ineffective, since the coordinating
2
2
24
trans-FeCl (PP) (PP = dmpe, depe).
–
–
2
2
ability of the counteranions follows the order Cl >> TfO >
1
N. Hazari, Chem. Soc. Rev., 2010, 39, 4044; K. C. MacLeod, P.
L. Holland, Nat. Chem., 2013, , 559; B. A. MacKay, M. D.
Fryzuk, Chem. Rev., 2004, 104, 385.
–
30
[
BArF ] . This trend has been previously observed in the
24
5
catalytic reduction of N to NH by Mo PNP-pincer complexes,
2
3
–
where proton sources incorporating TfO as the counteranion
4
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 6
Ple aDs ea l dt oo nn To rt a and sj ua sc tt imo na sr gins
Journal Name
COMMUNICATION
2
B. K. Burgess, D. J. Lowe, Chem. Rev., 1996, 96, 2983; L. C.
Seefeldt, B. M. Hoffman, D. R. Dean, Annu. Rev. Biochem,
32 NH formation from Fe(N H ) complexes: J. L. Crossland, L. N.
3 2 4
View Article Online
DOI: 10.1039/C6DT00884D
Zakharov, D. R. Tyler, Inorg. Chem., 2007, 46, 10476 and ref
2
009, 78, 701; B. M. Hoffman, D. R. Dean, L. C. Seefeldt, Acc.
14.
Chem. Res., 2009, 42, 609.
33 S. W. Gerritz, A. M. Sefler, J. Comb. Chem., 2000, 2, 39.
3
4
Y. Hu, M. W. Ribbe, J. Biol. Inorg. Chem., 2015, 20, 435.
B. M. Hoffman, D. Lukoyanov, Z.-Y. Yang, D. R. Dean, L. C.
Seefeldt, Chem. Rev., 2014, 114, 4041.
5
J. G. Leigh, M. Jimenez-Tenorio, J. Am. Chem. Soc., 1991, 113,
5
862.
6
7
M. W. Weatherburn, Anal. Chem., 1967, 39, 971.
A. Hills, D. L. Hughes, M. Jimenez-Tenorio, G. J. Leigh, A. T.
Rowley, J. Chem. Soc., Dalton Trans., 1993, 3041.
D. A. Hall, G. J. Leigh, J. Chem. Soc., Dalton Trans., 1996,
8
9
1
1
3
539.
T. A. George, D. J. Rose, Y. D. Chang, Q. Chen, J. Zubieta,
Inorg. Chem., 1995, 34, 1295.
0 J. D. Gilbertson, N. K. Szymczak, D. R. Tyler, J. Am. Chem.
Soc., 2005, 127, 10184.
1 S. Komiya, M. Akita, A. Yoza, N. Kasuga, A. Fukuoka, Y. Kai, J.
Chem. Soc., Chem. Commun., 1993, 787.
1
1
2 L. D. Field, N. Hazari, H. L. Li, Inorg. Chem., 2015, 54, 4768.
3 L. R. Doyle, A. Heath, C. H. Low, A. E. Ashley, Adv. Synth.
Catal., 2014, 356, 603.
1
1
1
4 L. D. Field, H. L. Li, A. M. Magill, Inorg. Chem., 2009, 48, 5.
5 C. Perthuisot, W. D. Jones, New J. Chem., 1994, 18, 621.
6 R. B. Yelle, J. L. Crossland, N. K. Szymczak, D. R. Tyler, Inorg.
Chem., 2009, 48, 861.
1
7 C. A. Tolman, S. D. Ittel, A. D. English, J. P. Jesson, J. Am.
Chem. Soc., 1978, 100, 4080.
1
1
8 A. Mansingh, D. B. McLay, J. Chem. Phys., 1971, 54, 3322.
9 M.-E. Moret, J. C. Peters, Angew. Chem. Int. Ed., 2011, 50
,
2
5
063; Y. Lee, N. P. Mankad, J. C. Peters, Nat. Chem., 2010,
58.
2,
2
0 Examples of catalytic N to NH fixation mediated by Fe
2 3
complexes: J. S. Anderson, J. Rittle, J. C. Peters, Nature, 2013,
5
1
01, 84; S. E. Creutz, J. C. Peters, J. Am. Chem. Soc. 2014,
36, 1105.
2
1 J. R. Aranzaes, M.-C. Daniel, D. Astruc, Can. J. Chem., 2006,
, 288.
8
4
2
2
2 D. V. Yandulov, R. R. Schrock, Science, 2003, 301, 76
3 K. Arashiba, Y. Miyake, Y. Nishibayashi, Nat. Chem., 2011, 3,
1
20.
2
2
4 G. W. Watt, J. D. Chrisp, Anal. Chem., 1952, 24, 2006.
5 C. G. Balesdent, J. L. Crossland, D. T. Regan, C. T. López, D. R.
Tyler, Inorg. Chem., 2013, 52, 14178.
2
2
2
6 H.-P. Jia, E. A. Quadrelli, Chem. Soc. Rev., 2014, 43, 547.
7 E. M. Arnett, C. Y. Wu, J. Am. Chem. Soc., 1960, 82, 4999.
8 E. Raamat, K. Kaupmees, G. Ovsjannikov, A. Trummal, A.
Kütt, J. Saame, I. Koppel, I. Kaljurand, L. Lipping, T. Rodima,
V. Pihl, I. A. Koppel, I. Leito, J. Phys. Org. Chem., 2013, 26
62.
9 C. M. Burba, N. M. Rocher, R. Frech, J. Phys. Chem. B, 2009,
13, 11453.
0 I. Krossing, I. Raabe, Angew. Chem. Int. Ed., 2004, 43, 2066;
S. H. Strauss, Chem. Rev., 1993, 93, 927.
,
1
2
3
3
1
1 K. Clarke, K. Rothwell, J. Chem. Soc., 1960, 1885.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Dalton Transactions
Page 6 of 6
View Article Online
DOI: 10.1039/C6DT00884D
8
1x71mm (300 x 300 DPI)