Formation of ammonia from an iron nitrido complex

Full text of DOI:10.1002/anie.200900381

Communications
DOI: 10.1002/anie.200900381
Iron Nitrido Complexes
Formation of Ammonia from an Iron Nitrido Complex **
Jeremiah J. Scepaniak, Jessica A. Young, Ranko P. Bontchev, and Jeremy M. Smith*
Iron nitrido complexes have attracted interest due to their
possible involvement in the catalytic cycle of the nitrogenase
[
1]
enzyme. Iron complexes with terminal nitrido ligands have
[
2]
[3,4]
been spectroscopically characterized in the + 4, + 5,
and
[
4,5]
+
6
oxidation states, although it is only recently that
isolable iron(IV) nitrido complexes have been discovered.
[
6,7]
Our group recently reported the electronic structure and
preliminary reactivity of an electrophilic tris(carbene)borate
(
nꢀ1)
Scheme 1. Thermodynamic cycle for the formation of [M
]=NH
n
[7]
from [M ]ꢁN. Concerted HAT follows the diagonal pathway, while
stepwise proton and electron transfers follow the horizontal and
vertical pathways. [M] represents a metal center and its ancillary
ligands.
iron(IV) nitrido complex.
Nitrido complexes are also proposed as intermediates in
[8]
cycles for the reduction of N to NH (e.g. the Chatt cycle).
2
3
Catalytic formation of ammonia by a nitrido complex requires
that ammonia production be coupled to regeneration of the
nitrido ligand. Therefore, in addition to the protons required
to create the NꢀH bonds, electrons are also needed to reduce
Herein we report the formation of ammonia from a
terminal iron(IV) nitrido complex by reaction with a hydro-
gen-atom donor. Mechanistic investigations suggest that at
least one HAT step is involved in the reaction. A related
reaction between the iron nitrido complex and an organic
radical results in the reductive formation of a CꢀN bond.
the metal center for further N cleavage and nitrido ligand
2
regeneration. Achieving catalysis requires reaction conditions
that avoid undesired side reactions, particularly the reduction
of protons by the electron source. In the catalytic formation of
ammonia by bulky molybdenum tris(amido)amine com-
plexes, this aim is achieved by using a proton source that
has limited solubility in the reaction medium as well as by
Synthesis of the iron(IV) nitrido complex [PhB-
ꢀ
(MesIm) FeꢁN] (1), where PhB(MesIm) is a bulky tris-
3
3
(carbene)borate ligand, was achieved in a similar manner to
other recently reported iron(IV) nitrido complexes
[9]
slowly delivering the reducing agent to the reaction mixture.
[
6,7]
From a thermodynamic perspective, the addition of a
single proton and a single electron to a metal nitrido complex
(Scheme 2).
Reaction of the iron chloride complex [PhB-
[13]
(MesIm) FeCl] with excess NaN yields the crystallograph-
3
3
is equivalent to the transfer of
a
hydrogen atom
ically characterized azido complex [PhB(MesIm) FeꢀN=N=
3
[
10]
ꢀ1
(
Scheme 1). With this in mind, we reasoned that reaction
N] (n
=
N
=
N
= 2078 cm ), which is then photolyzed to form 1 in
N
with a hydrogen-atom donor could result in the formation of
NH3 from a metal nitrido complex in solution. However,
hydrogen-atom transfer reactions involving metal nitrido
high yield. The X-ray crystal structure of 1 shows similar
structural features to our related [PhB(tBuIm) FeꢁN] com-
3
[
7]
plex (Scheme 2, inset). Thus, the FeꢀN (1.499(5) ꢀ) and Feꢀ
C bond lengths (1.885(6)–1.921(6) ꢀ) as well as the N-Fe-C
bond angles (120.7(3)–123.3(3)8) are similar to those in
[
11]
complexes in solution appear to be without precedent,
despite being well-known for other metal–ligand multiple
bonds (e.g. Fe=O and Fe=NR).[12]
[PhB(tBuIm) FeꢁN]. Both the H and N NMR spectra are
1
15
3
consistent with the solid-state structure.
[
*] J. J. Scepaniak, J. A. Young, Prof. J. M. Smith
Department of Chemistry and Biochemistry
New Mexico State University
Las Cruces NM 88011 (USA)
Fax: (+1)575-646-2649
E-mail: jesmith@nmsu.edu
Dr. R. P. Bontchev
Cabot Corporation
5401 Venice Ave. N.E., Albuquerque, NM 87113 (USA)
[
**] Funding by New Mexico State University, an Arts and Science
minigrant, and the Department of Energy, Office of Basic Energy
Sciences (DE-FG02-08ER15996) is gratefully acknowledged. The
Bruker X8 X-ray diffractometer was purchased via an NSF CRIF:MU
award to The University of New Mexico, CHE-0443580. We thank
Eileen Duesler for X-ray data collection.
Supporting information for this article, including full experimental
details, is available on the WWW under http://dx.doi.org/10.1002/
anie.200900381.
Scheme 2. Synthesis of [PhB(MesIm) FeꢁN] (1; Mes=2,4,6-trimethyl-
3
phenyl). Inset: X-ray crystal structure of 1; solvent molecules and
hydrogen atoms are omitted for clarity.
3
158
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Chemie
[
18]
Reaction of 1 with excess TEMPO-H (TEMPO-H = 1-
hydroxy-2,2,6,6-tetramethylpiperidine) results in quantitative
DMSO).
Therefore proton transfer from TEMPO-H
[
19]
(pK = 31.0 in DMSO)
uphill, with DGPT > 18 kcalmol .
The free energy for hydrogen-atom transfer to 1 was
estimated in a similar manner. While 1 reacts with TEMPO-H
to 1 is also thermodynamically
a
ꢀ1
formation of the yellow iron(II) complex [PhB(MesIm) Fe-
3
[
14]
(
(
tempo)]. The other products of this reaction are ammonia
up to 74% based on Fe) and TEMPO (greater than 95%),
ꢀ
1 [19]
which was characterized by EPR spectroscopy (Scheme 3).
(OꢀH bond dissociation energy (BDE) = 69.7 kcalmol )
to produce ammonia, no reaction is observed between 1 and
ꢀ1 [20]
9
,10-dihydroanthracene (CꢀH BDE = 78 kcalmol )
or
ꢀ1
[20]
xanthene (CꢀH BDE = 76 kcalmol ). This result provides
an upper limit for the NꢀH BDE of the parent imido complex
III
[
PhB(MesIm) Fe ꢁNH], and therefore for hydrogen-atom
3
ꢀ1
transfer from TEMPO-H to 1, DG
> ꢀ7 kcalmol .
HAT
Kinetic investigations were used to provide further insight
into the reaction mechanism. The rate of reaction between 1
and TEMPO-H in THF, measured under pseudo-first-order
conditions by UV/Vis spectroscopy, was found to be first
order in both 1 and TEMPO-H, consistent with the rate law
rate = k [Fe][TEMPO-H]. The second-order rate constant at
H
ꢀ3
ꢀ1 ꢀ1
2
98 K is k = 8.1(3) ꢁ 10
m
s . The kinetic isotope effect
H
(
k /k = 3.1) determined from the rate of reaction between 1
H
D
Scheme 3. Reaction of 1 with TEMPO-H. Inset: Spectral evolution of
the reaction in THF at 299 K. [Fe]=0.354 mm, [TEMPO-H]=21.3 mm.
Traces are shown at 230 s intervals.
and TEMPO-D is consistent with either initial proton or
hydrogen-atom transfer. Analysis of the temperature depend-
ence of the rate constant gives activation enthalpy and
°
ꢀ1
°
entropy values of DH = (11.1 ꢃ 0.3) kcalmol and DS =
The reaction of 1 with TEMPO-H is notable for its
selectivity and mildness, producing ammonia while simulta-
neously reducing the iron center. As mentioned above, the
formation of ammonia from a nitrido complex coupled with
reduction of the metal center is typically achieved using
ꢀ(37.4ꢃ0.8) e.u. (288–318 K).
°
The free energy of activation at 298 K is DG = (22.2 ꢃ
ꢀ1
°
0.3) kcalmol . Since DG > DG , the combined thermody-
PT
namic and kinetic data do not clearly distinguish between
mechanisms involving initial proton or hydrogen-atom trans-
fer from TEMPO-H to 1. However, three additional obser-
vations lead us to favor a mechanism involving initial HAT:
1) 9,10-dihydroanthracene (pK = 30.1 in DMSO)
xanthene (pK = 30.0 in DMSO) have similar acidities to
TEMPO-H, and therefore our finding that these reagents do
not react with 1 argues against initial proton transfer from
TEMPO-H to 1. 2) We find that the rate of reaction between
1 and TEMPO-H in the lower polarity solvent C H is similar
to that observed in THF.
unaffected by the ionic strength of the solution. The latter
[2a,15]
separate proton and electron sources.
Formation of ammonia from [PhB(MesIm) FeꢁN] and
3
[20]
TEMPO-H requires multiple steps. At least three possible
mechanisms for the first step of the reaction between 1 and
TEMPO-H are possible. The iron(IV) nitrido complex could
react by electron transfer (ET) to form an anionic iron(III)
and
a
[
20]
a
III
ꢀ
+
nitrido complex [PhB(MesIm) Fe ꢁN] and TEMPO-H . A
3
second possibility is initial proton transfer (PT) to 1 to yield
6
6
IV
+
[21]
the iron(IV) imido complex [PhB(MesIm) Fe ꢁNH] and
3) The rate of reaction is
3
ꢀ
[21]
TEMPO . Finally, HAT from TEMPO-H leads to the
formation of the iron(III) imido complex [PhB(MesIm) Feꢁ two observations suggest that charged intermediates are not
3
NH] and TEMPO.
involved in the rate-determining step (see Scheme 1), arguing
against initial proton transfer from TEMPO-H to 1.
Thermodynamic and kinetic investigations were under-
taken to determine the mechanism by which 1 reacts with
TEMPO-H. The thermodynamics of electron transfer to 1
were determined by cyclic voltammetry. No reduction waves
are observed at potentials greater than ꢀ2.5 V (vs. ferrocene/
Our experimental data therefore favor a reaction mech-
anism in which the initial NꢀH bond-forming step occurs by
HAT [Eq. (1)]:
IV
III
+
ð1Þ
½
Fe ꢄꢁN þ TEMPO-H ! ½Fe ꢄꢁNꢀH þ TEMPOꢅ
ferrocenium (Fc/Fc ) in MeCN) in the cyclic voltammogram
of 1. Consistent with this result, no reaction was observed
+
between 1 and [Cp* Co] (E = ꢀ1.91 V vs. Fc/Fc in MeCN;
Although initial proton transfer cannot be definitively
2
1/2
[16]
Cp* = C Me ). Since TEMPO-H is not strongly reducing
excluded at present, it is significant that a hydrogen-atom
donor provides both protons and electrons required for
formation of NꢀH bonds coupled to reduction of the metal
5
5
+
[17]
(
E1/2 ꢂ 0.71 V vs. Fc/Fc in MeCN), initial electron transfer
ꢀ1
to 1 is thermodynamically uphill (DG > 74 kcalmol ).
ET
Direct measurement of the free energy for proton transfer
to 1 is complicated by the fact that the nitrido complex is
decomposed by acids (e.g. HOAc). The upper limit for the
center. This strategy can be extended to other hydrogen-atom
donors such as metal hydrides. For example, reaction of 1 with
the weakly acidic metal hydride [Co(dppe) H] (CoꢀH BDE =
2
IV
+
ꢀ1
[22]
pK for the conjugate acid of 1, [PhB(MesIm) Fe ꢁNH] ,
64 kcalmol , dppe = 1,2-bis(diphenylphosphanyl)ethane)
a
3
was determined by evaluating the reactivity of 1 towards a
also leads to the formation of NH (22%, not optimized).
3
series of acids. Complex 1 was found to react with PhCH SH
The feasibility of one-electron chemistry is further
illustrated by the reaction of a carbon-centered radical with
2
[18]
(
pK = 15.4 in DMSO) but not with phenol (pK = 18.0 in
a
a
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3159

Communications
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CPh ] (Scheme 4). This complex has been crystallographically
3
[
[
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N-C angle (177.7(2)8).
The complex has a paramagneti-
1
cally shifted H NMR spectrum that is consistent with the
solid-state structure and a solution magnetic moment
(
2.5(3) m ) indicative of low-spin iron(III) (S = 1/2). Thus,
B
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formation of ammonia. Although not conclusive, mechanistic
investigations suggest a mechanism involving HAT from
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Keywords: iron · nitrides · radicals · reaction mechanisms ·
tripodal ligands
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Angew. Chem. Int. Ed. 2009, 48, 3158 –3160