Catalytic reduction of N2 to NH3 by an Fe-N 2 complex featuring a C-atom anchor

Article abstract of DOI:10.1021/ja4114962

While recent spectroscopic studies have established the presence of an interstitial carbon atom at the center of the iron-molybdenum cofactor (FeMoco) of MoFe-nitrogenase, its role is unknown. We have pursued Fe-N₂ model chemistry to explore a

Full text of DOI:10.1021/ja4114962

Article
pubs.acs.org/JACS
Catalytic Reduction of N2 to NH3 by an Fe−N2 Complex Featuring a
C‑Atom Anchor
Sidney E. Creutz and Jonas C. Peters*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
ABSTRACT: While recent spectroscopic studies have estab-
lished the presence of an interstitial carbon atom at the center
of the iron−molybdenum cofactor (FeMoco) of MoFe-
nitrogenase, its role is unknown. We have pursued Fe−N2
model chemistry to explore a hypothesis whereby this C-atom
(previously denoted as a light X-atom) may provide a flexible
trans interaction with an Fe center to expose an Fe−N2
binding site. In this context, we now report on Fe complexes
of a new tris(phosphino)alkyl (CPiPr3) ligand featuring an axial carbon donor. It is established that the iron center in this scaffold
binds dinitrogen trans to the Calkyl-atom anchor in three distinct and structurally characterized oxidation states. Fe−Calkyl
lengthening is observed upon reduction, reflective of significant ionic character in the Fe−Calkyl interaction. The anionic
(CPiPr3)FeN2− species can be functionalized by a silyl electrophile to generate (CPiPr3)Fe−N2SiR3. (CPiPr3)FeN2− also functions
as a modest catalyst for the reduction of N2 to NH3 when supplied with electrons and protons at −78 °C under 1 atm N2 (4.6
equiv NH3/Fe).
INTRODUCTION
■
The biological reduction of atmospheric N2 to NH3 is a
fascinating yet poorly understood transformation that is
essential to life.1 The iron−molybdenum cofactor (FeMoco)
of MoFe nitrogenase catalyzes N2 reduction and has been
extensively studied.2 This cofactor has attracted the attention of
inorganic and organometallic chemists for decades who have
sought inspiration to explore the ability of synthetic iron and
molybdenum complexes to bind and reduce dinitrogen.3−6
Advances in the past decade have included two molybdenum
systems that facilitate catalytic turnover of N2 to NH3 in the
presence of inorganic acid and reductant sources,7−9 and iron
complexes that support a range of NxHy ligands relevant to
nitrogen fixation,10−13 effect reductive N2 cleavage,14,15 and
facilitate N2 functionalization.16−18
Figure 1. (Top) Structure of the FeMo cofactor of nitrogenase,
The presence of an interstitial light atom in the MoFe
showing a putative site for dinitrogen binding and highlighting the
nitrogenase cofactor was established in 2002,19 and structural,
trigonal bipyramidal coordination environment at Fe. Possible sites of
spectroscopic, and biochemical data have more recently
established its identity as a C-atom.20 The role of the C-atom
is unknown. This state of affairs offers an opportunity for
organometallic chemists to undertake model studies that can
illuminate plausible roles for this interstitial C-atom, and hence
critical aspects of the mechanism of N2 reduction catalysis. In
particular, Fe-alkyl complexes that are more ionic in nature than
a prototypical transition metal−alkyl may be relevant to
modeling the Fe−Cinterstitial interaction of the possible N2
binding site in the cofactor (Figure 1).
We have suggested that a possible role played by the
interstitial C-atom is to provide a flexible Fe−Cinterstitial
interaction that exposes an Fe−N2 binding site on a belt iron
atom trans to the Fe−C linkage (Figure 1).3,15,21−23
H-atoms on cofactor prior to N2 binding not shown. (Bottom)
Possible role of Lewis acidic (LA) or aryl substituents in stabilizing
ionic character in the N2−Fe−Calkyl interaction.
Subsequent modulation of the Fe−C interaction and hence
the local Fe geometry as a function of the N2 reduction state
might enable the Fe center to stabilize the various NxHy
intermediates sampled along a pathway to NH3.
To test the chemical feasibility of this hypothesis for Fe-
mediated N2 reduction, our group has previously employed
phosphine-supported Fe complexes in approximately trigonal
Received: November 11, 2013
© XXXX American Chemical Society
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geometries (pseudotetrahedral, trigonal pyramidal, or trigonal
bipyramidal) to bind and functionalize dinitrogen. Tripodal
trisphosphine ligands featuring an axial donor (X = N, Si, B)
and aryl backbones have been used to canvass the ability of low-
valent iron in such geometries to bind and activate dinitrogen
(Figure 2).23−25 The (TPiPrB)Fe-system (TPRB = tris(o-
regard to considering the role an Fe−Cinterstitial interaction
might play in facilitating N2 binding and reduction within the
cofactor. To this end, we embarked on the synthesis of the new
ligand (CPiPr3)H and the development of its Fe−N2 chemistry.
RESULTS AND DISCUSSION
■
Ligand Synthesis. Whereas the ligands (SiPiPr3)H and
TPiPrB are straightforward to synthesize by the addition of
lithiated o-phosphinophenyl precursors to HSiCl3 and
BCl3,24,26 the preparation of (CPiPr3)H via an analogous
method by addition of phosphinoaryllithium moieties to a C1
source (e.g., triple addition to dimethylcarbonate followed by
deoxygenation of the resultant triarylmethanol product) has
proven ineffective in our hands. However, an orthogonal
synthetic approach based on elaboration of an initially formed
triarylmethane scaffold afforded a viable approach to the
preparation of (CPiPr3)H on a multigram scale and in
reasonable yields. This synthesis of (CPiPr3)H follows an
approach inspired by a previously reported synthesis of Ph2P(o-
C6H4CH2C6H4-o)PPh2,27 and hinges on the sequential
formation and cleavage of two diaryliodonium ions to give
the tris(2-halophenyl)methane precursor (5) (Scheme 1).
Scheme 1. Synthesis of (CPiPr3)H (1)
Figure 2. Select trigonal bipyramidal scaffolds previously studied by
−
our lab, and the present (CPiPr3)FeN2 system.
phosphinoaryl)borane) has proven rich in this context, and
has most recently been shown to be a modestly effective
catalyst for the reduction of N2 to NH3 in the presence of
proton and electron sources at low temperature and 1 atm
N2.21 An important feature of the (TPiPrB)Fe-system is the
presence of a flexible Fe−B interaction.15,25 This flexibility may
facilitate the formation of intermediates featuring Fe−Nx π-
bonding (e.g., FeNNH2, FeN, FeNH) during catalysis.
Whether the aforementioned hypothesis concerning a hemi-
labile role for the interstitial C-atom of FeMoco is correct or
not, these inorganic model studies lend credibility to the idea so
far as the principles of coordination chemistry are concerned.
To extend our studies to systems that place a C-atom in a
position trans to an Fe−N2 binding site, we have sought related
ligand scaffolds that feature a C-atom anchor. In designing
these scaffolds, we have hypothesized that the proposed
flexibility of the Fe−C linkage in the FeMo cofactor may be
facilitated by the ability of the environment around the
interstitial carbidefive additional electropositive Fe atoms
to stabilize developing negative charge on the carbon. With this
in mind, we have previously reported iron complexes of a
tris(phosphino)alkyl ligand whose axial carbon binding site is
flanked by three electropositive silyl groups (Figure 2) which
play a role in stabilizing the substantial ionic character of this
Fe−Calkyl bond (Figure 1).22
The synthesis of o-iodotriphenylmethane has been previously
reported28 and is readily effected in three steps from
commercially available 2-nitrobenzaldehyde on a 20-g scale.
Cyclization of this species to the diaryliodonium bromide salt
(2) is accomplished by a previously reported technique.29 Slow
but clean ring-opening of 2 by CuBr and [TBA][Br] in
acetonitrile gives 2-bromo-2′-iodotriphenylmethane (3). The 2-
bromo-2′-iodotriphenylmethane species was targeted rather
than 2,2′-diiodotriphenylmethane in order to mitigate the
possibility of complications from excessive oxidation in the next
step.
Herein we report a new tris(phosphino)alkyl ligand, (CPiPr3),
featuring aryl linkers bound to the axial carbon. We reasoned
that possible delocalization of negative charge buildup into the
aryl π-system would allow for increased flexibility in the Fe−C
bond; this flexibility is expected to facilitate possible catalytic N2
functionalization and reduction, as discussed above. Addition-
ally, as this ligand is closely structurally related to the SiP3, TPB,
and NP3 ligands whose iron coordination chemistry we have
Formation of a second diaryliodonium cation as its iodide
salt follows via an analogous procedure to regioselectively
generate 4, which can be straightforwardly decomposed to 2-
bromo-2′,2″-diiodotriphenylmethane (5) by heating to 200 °C
iPr
extensively explored, Fe complexes of CP3 are of obvious
comparative interest and would be particularly beneficial with
B
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for 15 min under an inert atmosphere. Each step in the
synthesis of 5 from o-iodotriphenylmethane can be accom-
plished in 75% yield or more (overall yield: 38% over five
steps).
structure (Figure 3) shows a tetrahedral environment at the
iron center and a bidentate binding mode for the ligand. One-
Lithiation of 5 with 6 equiv of tert-butyllithium at −78 °C
followed by treatment with 3 equiv of diisopropylphosphine
chloride gives the desired tris(o-diisopropylphosphinophenyl)-
methane, (CPiPr3)H (1) in 67% yield (Scheme 1). The
protonated form of the ligand, 1, is characterized by a single
1
peak in its phosphorus NMR spectrum at −9.1 ppm. The H
NMR spectrum, while indicative of 3-fold symmetry, also shows
features suggestive of a rigid ligand scaffold where rotation
about the phosphine-carbon bonds is hindered; in particular,
four magnetically inequivalent sets of resonances are observed
for the isopropyl methyl hydrogens. Additionally, the central
C−H methine proton is shifted markedly downfield (8.15
ppm) and manifests as a quartet due to through-space coupling
to the three phosphorus atoms. Similar NMR properties were
observed for the central methine proton in a related
trisphosphine ligand based on a tris(indolyl)methane scaffold.30
Metalation at Iron and Precursor Complexes. We
initially hoped to effect metalation of 1 by first deprotonating it
to give an alkali metal complex followed by transmetalation
with an iron(II) halide or other transition metal precursor. To
our frustration, 1 proved unexpectedly difficult to deprotonate
even with very strong bases such as benzyl potassium and
Schlosser’s base,31 perhaps due in part to the steric protection
of the methine proton; additionally, the acidity of this proton is
likely not as high as for bare triphenylmethane since the ligand
bulk limits the extent to which the aryl rings can approach a
coplanar configuration to afford resonance stabilization of a
resulting carbanion.32 Furthermore, the strategy used for
metalation of the (SiPiPr3)H ligand on ironusing methyl
Grignard with FeCl2 to generate a methyl iron complex which
then eliminates methane with concomitant formation of the
iron−silicon bond24was not effective for (CPiPr3)H. It
appeared to instead result in reduction of iron without the
formation of the desired iron−carbon bond. Thus, it was
necessary to develop a different protocol for the formation of a
(CPiPr3)Fe-complex featuring an iron−carbon bond.
Figure 3. Crystal structures of {(CPiPr3)H}FeI2 (6, top left),
{(CPiPr3)H}FeBr (8, top right), and (CPiPr3)Fe(H)(N2) (9, bottom).
Ellipsoids shown at 50% probability; hydrogen atoms (except the
triarylmethine C−H and Fe−H hydride) and solvent molecules
omitted for clarity.
electron reduction of 6 in benzene or toluene using a range of
reagents including sodium amalgam, potassium graphite, or
alkylmagnesium/lithium reagents, results in the formation of
the deep brick-red four-coordinate iron(I) complex {(CPiPr3)-
H}FeI (7). The bromide congener, {(CPiPr3)H}FeBr (8), is
analogously prepared and has been crystallographically
characterized (Figure 3); its most notable feature is the endo
orientation of the unactivated methine C−H. This proton is
located within the ligand cage pointed nearly linearly toward
the iron center. Both 7 and 8 are unstable with respect to
disproportionation to Fe(0), (CPiPr3)H, and {(CPiPr3)H}FeX2
(X = I, Br), especially in coordinating solvents. However, if
appropriate conditions are employed, 7 is sufficiently long-lived
to be generated and used without further purification for
subsequent reactions.
Combining 1 and iron(II) iodide in toluene cleanly affords
the tetracoordinate, κ2-bisphosphine diiodide high-spin iron(II)
complex (6) as a yellow powder (Scheme 2). Its solid-state
Scheme 2. Synthesis of Iron Complexes of (CPiPr3)H
Further reduction of 7 with sodium metal in a 5:1 mixture of
Et2O and DME at −78 °C causes formal insertion of the Fe
center into the C−H bond of the (CPiPr3)H ligand and uptake
of atmospheric N2 to give yellow, diamagnetic (CPiPr3)Fe(H)-
(N2) (9). The position of the iron hydride is identifiable in the
XRD difference map of 9, as is the presence of an Fe−C bond
at 2.155(2) Å (Figure 3). IR data for 9 show a strong N−N
vibration at 2046 cm−1 and an Fe−H vibration at 1920 cm−1.
The properties of 9 can be compared to the isostructural
(SiPiPr3)Fe(H)(N2) and [(NPiPr3)Fe(H)(N2)]+ complexes23,33
and other closely related species such as {[P(CH2CH2PiPr2)3]-
Fe(H)(N2)}+;34 the vibrational and metrical properties of the
N2 ligand suggest a more activated dinitrogen moiety in 9
relative to its congeners.
Deprotonation of 9 to afford (CPiPr3)FeN2 was canvassed
−
but proved unsuccessful. A more circuitous but ultimately
−
effective route to (CPiPr3)FeN2 proceeded via treatment of 9
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with anhydrous HCl in Et2O to afford dark red-orange
(CPiPr3)FeCl (10) in good yield (Scheme 2). The crystal
structure of 10 was not reliably determined due to its
propensity to crystallize in a cubic space group with extensive
whole molecule disorder. Complex 10 is paramagnetic and its
room temperature solution magnetic moment of 4.9 μB is
suggestive of a high-spin, S = 2 ground state. A lower spin state
might have been reasonably anticipated to arise from a
presumably strong-field ligand set composed of three
diisopropylarylphosphines and an alkyl group. For comparison,
(SiPiPr)3FeCl exhibits an intermediate S = 1 ground state.24 The
Calkyl anchor in 10 thereby appears to be a weaker-field donor
than the silyl anchor in (SiPiPr)3FeCl.
small and as such are difficult to reliably interpret. But given the
fact that (CPiPr3) appears to have a weaker-field donor set than
(SiPiPr3) according to the observed ground spin states of
(CPiPr3)FeCl (S = 2) and (SiPiPr3)FeCl (S = 1), one might have
reasonably anticipated (SiPiPr3)FeN2 to have a lower υ(NN)
than (CPiPr3)FeN2.
Both a one-electron oxidation and a one-electron reduction
of 11 are accessible (Figure 5). The Fe(II/I) couple appears at
Synthesis and Characterization of the {(CPiPr3)FeN2}n
(n = 0, −1, +1) Series. Reduction of the chloride precursor 10
affords entry into the desired series of trigonal bipyramidal iron
dinitrogen complexes. Stirring 10 over sodium metal in THF
produces the neutral low-spin Fe(I) complex (CPiPr3)FeN2
(11) (υ(NN) = 1992 cm−1) (Scheme 3). Complex 11 is low-
Scheme 3. Synthesis of the Dinitrogen Adduct Series
(CPiPr3)FeN2 (11), (CPiPr3)FeN2 (12), and (CPiPr3)FeN2
−
+
(13)
Figure 5. Cyclic voltammogram of 11; scan rate 0.5 V/s.
−1.20 V (vs Fc/Fc+) and is quasi-reversible; the current in the
cathodic wave is diminished and an irreversible reduction wave
appears at −1.65 V. This is very similar electrochemical
behavior to what has been documented for (SiPiPr3)FeN2 and
suggests that the same phenomenon is responsible for the
observations in this system17that is, N2 coordinates reversibly
to the {(CPiPr3)Fe}+ complex; partial loss of N2 upon oxidation
of (CPiPr3)FeN2 is likely responsible for the quasi-reversibility
of the (II/I) couple, and the reduction at −1.65 V is most
reasonably attributed to the cationic species {(CPiPr3)Fe(L)}+
(where L may be THF, or may be a vacant site), which then
takes up N2 upon reduction. The Fe(I/0) couple is fully
reversible, consistent with the formation of a stable (CPiPr3)-
spin and paramagnetic (S = 1/2); it has been crystallo-
graphically characterized (Figure 4) and shows a distortion
from trigonal symmetry with one widened P−Fe−P angle
(132.5°), as expected due to the Jahn−Teller active ground
state. The N2 vibrational frequency and N−N bond length
(1.134(4) Å) show that the dinitrogen ligand in this complex is
somewhat more activated than that in the isoelectronic
(SiPiPr3)FeN2 complex (υ(NN) = 2003 cm−1, N−N =
1.1245(2) Å) or in the neutral Fe(0) complex (TPiPrB)FeN2
(υ(NN) = 2011 cm−1).17,25 These differences are relatively
−
FeN2 anion. This reduction occurs at an unusually negative
Figure 4. Crystal structures of (CPiPr3)FeN2 (11, left), (CPiPr3)FeN2− (12[K(Et2O)3], center, ethyl groups of coordinated Et2O molecules omitted),
+
and (CPiPr3)FeN2 (13, right). Ellipsoids are shown at 50% probability and hydrogen atoms are omitted for clarity.
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potential (−2.55 V vs Fc/Fc+). For comparison, the reduction
Notably, the overall change in the bond length is greater in
of (SiPiPr3)FeN2 to (SiPiPr3)FeN2 occurs at −2.2 V.17
the CPiPr case (0.084 Å from 13 to 12) than for the more
−
3
The Fe−N2 adduct triad {(CPiPr3)FeN2}n (n = 0 (11), −1
(12), +1 (13)) proved synthetically accessible. Treatment of 10
with an excess of potassium graphite (KC8) in Et2O results in
covalent SiPiPr system, where the overall change is only 0.045
3
Å despite the longer total bond length. This suggests a greater
degree of flexibility in the Fe−Calkyl interaction. A similar
conclusion was drawn for the {(CSiPPh3)Fe(CO)}n (n = +1, 0,
−1) series, where an even more pronounced Fe−C lengthening
was observed upon reduction.22
immediate reduction to the very dark brown-blue CPiPr3FeN2
−
anion (12). The IR spectrum of a thin film deposited from
diethyl ether solution shows a υ(NN) vibration at 1870 cm−1,
suggestive of a close ion pair with the potassium ion capping
the N2 moiety. Accordingly, treatment of the potassium
complex with two equivalents of 12-crown-4 results in the
formation of [(CPiPr3)FeN2][K(12-crown-4)2] (12[K(12-
crown-4)2]) with a shift of the υ(NN) vibration to 1905
cm−1. The anion has been crystallographically characterized
(Figure 4) as its K(Et2O)3 salt, [(CPiPr3)FeN2][K(Et2O)3]
(12[K(Et2O)3]; the bulk material after drying is solvated by 0.5
molecules of Et2O per anion, 12[K(Et2O)0.5]).
In the case of the (TPiPrB)Fe system, a highly flexible Fe−B
interaction has been observed as a function of the ligand
positioned trans to the B-atom that may be important to its
success in activating N2 in both stoichiometric and catalytic
reactions.15,21,35 However, an analogous series of N2 complexes
has not been characterized to allow for direct comparison.
Whereas the anion [(TPiPrB)]FeN2]− has been studied by X-
ray crystallography (Fe−B = 2.311(2) Å), the [(TPiPrB)Fe]+
cation does not coordinate N2 at atmospheric pressure, and
attempts to obtain the crystal structure of neutral (TPiPrB)FeN2
have been unsuccessful.25,35 Nonetheless, our chemical
intuition is that the Fe−B linkage in (TPiPrB)Fe will be
appreciably more flexible than the Fe−C linkage in (CPiPr3)Fe.
Oxidation of 11 with 1 equiv of [Cp*2Fe][BArF ] (ArF = 3,5-
4
trifluoromethylphenyl; Cp* = pentamethylcyclopentadienide)
in Et2O gives rise to [(CPiPr3)FeN2][BArF ] (13) as an orange
4
crystalline solid, which has also been structurally characterized
(Figure 4). The dinitrogen ligand in 13 (υ(NN) = 2128 cm−1),
is labile and in solution under an N2 atmosphere appears to be
in equilibrium with a solvated or vacant cation [(CPiPr3)Fe-
(L)]+; in addition to the electrochemical properties discussed
above, evidence from UV−vis spectroscopy is consistent with
the loss of coordinated N2 under vacuum (see Supporting
Information, SI).
−
The Calkyl-Fe interactions in both (CPiPr3)FeN2 (12) and
(CSiPPh3)FeN2 reflect a higher degree of ionic character than
−
in a prototypical Fe−Calkyl bond, with (CSiPPh3)FeN2 being
−
most striking in this context.22 Comparative DFT studies of
(CSiPPh3)FeN2 and (CPiPr3)FeN2 including NBO analyses,
support this view,22,36 predicting strong polarization of the σ-
bond pair toward the C-atom (23% Fe/77% C in (CSiPPh3)-
−
−
FeN2 ; 27% Fe/73% C in (CPiPr3)FeN2 ) (Figure 6). As
−
−
Whereas a related series was accessible for the silyl-anchored
{(SiPiPr3)FeN2−}n system (n = 0, +1, −1),17 only the anion
(CSiPPh3)FeN2 proved accessible for the previously reported
C
alkyl-anchored system.22 Hence, the present {(CPiPr3)FeN2}n
series allows for a direct comparison of how the anchoring
atom (Si vs C) responds across three redox states when
positioned trans to an N2 ligand of an isostructural trigonal
bipyramidal framework.
In the case of the {(SiPiPr3)FeN2}n series, the Fe−Si bond
distance decreases upon reduction from 2.298(7) Å in the
+
−
(SiPiPr3)FeN2 cation to 2.2526(9) Å in the (SiPiPr3)FeN2
anion (Table 1). In direct contrast, the Fe−C bond distance in
Table 1. Select Characterization Data for the Fe−N2 Adducts
{(CPiPr)3FeN2}n and {(SiPiPr3)FeN2}n (n = −1, 0, 1)
a
b
X = C, Si
[X−Fe−N2]−
X−Fe−N2
[X−Fe−N2]+
Fe−C (Å)
Fe−Si (Å)
2.1646(17)
2.2526(9)
1.7397(16)
1.763(3)
1870
2.152(3)
2.2713(6)
1.797(2)
1.8191(1)
1992
2.081(3)
2.298(7)
1.864(7)
1.914(2)
2128
Figure 6. (A) Isocontour plot of the Fe−Calkyl σ bond of
12[K(Et2O)3] located from NBO analyses. (B) Contour plot of one
of the Caryl π* orbitals which accepts delocalized electron density from
the Fe−Calkyl σ bond.
Fe−NX=C (Å)
Fe−NX=Si (Å)
ν(N2)X=C (cm−1
)
ν(N2)X=Si (cm−1
)
1891
2003
2143
spin state
S = 0
S = 1/2
S = 1
expected, the Fe−C bond in 12 is slightly more covalent than
a
b
that in (CSiPPh3)FeN2 , where the axial carbon is flanked by
−
All data tabulated for X = Si is taken from ref 17. For X = C, data
provided is for the [K(Et2O)3]+ salt (Figure 4). For X = Si, data
provided is for the [Na(THF)3]+ salt.
electropositive silicon atoms. Comparative NBO analyses for
(CSiPPh3)FeN2 , (SiPiPr3)FeN2 , and simplified model systems
−
−
were discussed at greater length in a previous report.22
{(CPiPr3)FeN2}n increases upon reduction, from 2.081(3) Å in
13 to 2.152(3) Å in 11 to 2.1646(17) Å in 12. The different
responses manifest in these two systems may be due to the
electropositive silicon atom binding more strongly to the more
electron-rich iron, whereas the more electronegative Calkyl binds
more strongly to the higher-valent, more electron-deficient iron
center.
Second-order perturbation analysis from an NBO calculation
indicates the presence of stabilizing donor−acceptor inter-
actions between filled and virtual orbitals, representing
deviations from a simple Lewis structure description due to
electronic delocalization.36 In the case of 12, significant
interactions between the filled Fe−Calkyl σ bond and π*
orbitals of the aryl rings (Cipso−Cortho) are evident (Figure 6).
E
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Three primary donor−acceptor interactions (one to each ring)
are located, representing stabilizations of 6.70 kcal/mol, 5.99
kcal/mol, and 5.95 kcal/mol. This result suggests that
stabilization of the negative charge on carbon by delocalization
onto the aryl rings is at least partially responsible for the
observed ionic character of the Fe−C bond, and hence for its
increased flexibility. We suggest that a similar stabilization of
ionic character at an N2−Fe−Cinterstitial site of the cofactor may
facilitate N2 binding.
Treatment of 12[K(Et2O)0.5] with 10 equiv of [H(Et2O)2]-
[BArF ] in the absence of added reductant generates negligible
4
ammonia (<0.05 equiv), implying that both acid and reductant
are necessary for the production of substantial amounts of NH3.
In order to examine possible reasons for the limited turnover
for ammonia production with this system, we sought to
determine the fate of the precatalyst over the course of the
experiment. An analysis of the iron-containing products of a
reaction mixture using 10 equiv of [H(Et2O)2][BArF ] and 12
4
Reactivity Studies. To compare the reactivity of (CPiPr3)-
equiv of KC8 (Figure 7) identified the major iron-containing
FeN2− at the bound N2 ligand with (SiPiPr3)FeN2 , (CSiPPh3)-
−
−
FeN2 , and (TPiPrB)FeN2 , treatment of 12 with TMSCl at
−
−78 °C was examined and afforded the diamagnetic diazenido
complex (CPiPr3)FeN2SiMe3 (14) (υ(NN) = 1736 cm−1). This
product, although not structurally characterized, is spectro-
scopically similar to complexes of the structurally related Si-
and B-anchored systems.15,17
More interesting is the comparative behavior of (CPiPr3)-
−
FeN2 on treatment with proton/electron equivalents at low
temperature. Numerous studies have explored the possibility of
Fe−N2 protonation/reduction to release ammonia,3−6,37 which
in all but one case21 afforded low chemical yields of NH3 (ca. ≤
10% per Fe in one step; 35% per Fe overall in two independent
synthetic steps14). The previously described C-anchored system
(CSiPPh3)FeN2 (Figure 2) followed a similar trend, affording
−
negligible NH3 on treatment at low temperature with
[H(Et2O)2][BArF ] and KC8. The Si-anchored system
4
(SiPiPr3)FeN2 also afforded substoichiometric NH3 yields
−
(35% per Fe) when similarly treated, and instead produced
some N2H4 (∼45% per Fe) when H(Et2O)BF4 and CrCl2 were
employed.24
By contrast, cooling a solution of 12[K(Et2O)0.5] in Et2O at
−78 °C followed by the addition of 40 equiv KC8 and then 38
equiv [H(Et2O)2][BArF ] leads to the formation of 4.6 0.8
4
equiv NH3 (230% per Fe; average of 8 runs; eq 1), a yield that
establishes a modest degree of N2 reduction catalysis at low
temperature. No N2H4 is observed. With 12[K(12-crown-4)2]
as the catalyst, the NH3 yield is slightly lower at 3.5 0.3 equiv.
NH3 quantification was carried out by UV−vis using the
indophenol protocol38 as recently described in detail for the
21
−
(TPiPrB)FeN2 catalyst system. The total NH3 product yield
is lower for (CPiPr3)FeN2 than that which was obtained for
−
Figure 7. Spectroscopic analyses of reaction mixtures following the
catalytic production of NH3 using 12[K(Et2O)0.5] as a catalyst.
Symbols indicate characteristic resonances attributed to 9, 10, and 11.
(TPiPrB)FeN2 when acid was added prior to the reductant.
−
The significance of these modest differences is unclear,
especially given the extreme air-sensitivity of the catalysts and
the low turnover numbers. The order of addition of reagents
has a minor effect; reversing the order and adding first acid,
then reductant to 12[K(Et2O)0.5] decreases the yield to 3.8
0.6 equiv NH3 per Fe. In side-by-side comparisons using the
1
(A),(C) H NMR and IR spectra of a postcatalytic reaction mixture
using 10 equiv of [H(Et2O)2][BArF ] and 12 equiv of KC8. (B),(D)
4
1H NMR and IR spectra of a postcatalytic reaction mixture using 38
equiv of [H(Et2O)2][BArF ] and 40 equiv of KC8.
4
same batches of reagents (KC8 and [H(Et2O)2][BArF ]) and
4
product as (CPiPr3)FeN2 (11), which is readily reduced by KC8
even at low temperature to reform the precatalyst 12. However,
a significant amount of (CPiPr3)Fe(N2)(H) (9) is also present;
9 is not catalytically competent, generating no detectable
ammonia when subjected to the catalytic conditions, and its
formation is likely an important limiting factor in the catalyst
performance. Another identifiable species by 1H NMR
spectroscopy is (CPiPr3)FeCl (10). Despite our efforts to
the same order of addition (reductant added first), 12[K-
(Et2O)0.5] afforded 4.4 0.2 equiv NH3 per Fe, as compared to
−
−
5.0 1.1 for (TPiPrB)FeN2 and 0.8 0.4 for (SiPiPr3)FeN2 .
N2 (1 atm)
+
xs KC8
⎯⎯⎯⎯⎯⎯1⎯⎯2⎯⎯⎯⎯→ NH3 (4.6 equiv)
remove all Cl− in the preparation of [H(Et2O)2][BArF ], the
−78 °C,Et2O
4
large excess of acid employed in this experiment likely ensures a
non-negligible Cl− impurity that may also attenuate catalyst
activity. The identity of another diamagnetic hydride-bearing
+
xs HBArF ·2 (Et2O)
(1)
4
F
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Journal of the American Chemical Society
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1
species apparent in the H NMR spectrum is not currently
known.
ligand trans to an Fe−N2 binding site, and that this flexibility is
enhanced by the ability of the carbon to accommodate a
significant ionic charge. It seems plausible to us that the
inorganic carbide ligand in FeMoco could be similarly, and
perhaps more, able to stabilize substantial ionic character in an
Fe−Cinterstitial bond (Figure 1), resulting in a flexible interaction
exposing an N2 binding site that can be further modulated as a
function of the NxHy reduction state.
At this early stage, reliable conclusions concerning the
influence of the carbon atom on the intimate stepwise
mechanism of nitrogen reduction are premature. Even within
our synthetic series, it may be that different catalysts follow
different mechanistic pathways (distal vs alternating, or some
hybrid path);21,41 for instance the most flexible system,
(TPiPrB)Fe, may be better suited to facilitate a distal pathway
that samples strongly π-bonded intermediates, while (CPiPr3)-
Fe, which we presume is less flexible, could instead be
dominated by an alternating or hybrid pathway. Whether these
structurally related iron systems mediate nitrogen reduction by
a common or different mechanism will be challenging to
determine but is a fascinating question. The work presented
here adds to the context needed for further mechanistic studies
on both synthetic and biological iron systems for catalytic
nitrogen fixation.
Further product analysis using the full catalytic conditions
(38 equiv of [H(Et2O)2][BArF ] and 40 equiv of KC8 with
4
respect to the catalyst), showed that increasing amounts of
(CPiPr3)Fe(N2)(H) (9) are formed as the system turns over,
corroborating the idea that this species serves as a catalytically
inactive sink which builds up throughout the reaction.
Integration of the NMR spectrum of such a reaction mixture
against an internal standard suggests that approximately 70% of
the catalyst has been converted to 9 (see SI); even at this point,
however, some active catalyst remains in the form of 11 (Figure
7). The unknown hydride species present in the aforemen-
tioned reaction mixture, derived from fewer equivalents of acid
and reductant, is no longer observed.
Notably, in neither of these experiments is any free ligand 1
(nor any ligand decomposition product) detected; it appears
that all of the iron present remains ligated by the CPiPr3 ligand.
This lack of degradation is promising, and suggests that
improvements to the N2 reduction catalysis, in terms of
turnover number, may yet prove possible if the formation of
terminal hydride 9 can be limited by modification of either the
ligand scaffold and/or the catalytic conditions. Indeed, it may
be that biological nitrogenases are designed to avoid catalyti-
cally inactive hydride sinks by evolving hydrogen.39 An iron-
sulfur cluster might be a particularly good design in this
context.40
EXPERIMENTAL METHODS
■
General. All manipulations were carried out using standard Schlenk
or glovebox techniques under an N2 atmosphere. Unless otherwise
noted, solvents were deoxygenated and dried by thoroughly sparging
with N2 followed by passage through an activated alumina column in a
solvent purification system by SG Water, USA LLC. Nonhalogenated
solvents were tested with a standard purple solution of sodium
benzophenone ketyl in tetrahydrofuran in order to confirm effective
moisture removal. O-Iodotriphenylmethane,28 H(OEt2)2[B(3,5-
(CF3)2−C6H3)4],42 KC8,43 [(TPB)FeN2][Na(12-crown-4)2],25
[(SiPiPr3)FeN2][Na(12-crown-4)2]17 and [(CSiPPh3)FeN2][K(18-
crown-6)2]22 were prepared according to literature procedures.
[Decamethylferrocenium][B(3,5-(CF3)2-C6H3)4] was prepared by
treating [ferrocenium][B(3,5-(CF3)2−C6H3)4]44 with decamethylfer-
rocene and used without purification. FeI2(THF)2 was prepared by
treating Fe powder with I2 in THF,45 and was dried to FeI2 by heating
under vacuum at 80 °C for 6 h. All other reagents were purchased
from commercial vendors and used without further purification unless
otherwise stated.
CONCLUSIONS
■
To conclude, we have synthetically introduced the tripodal
(CPiPr3)H ligand and have prepared and structurally compared
its {(CPiPr3)FeN2}n complexes (n = 0, −1, +1) with those of the
isostructural series {(SiPiPr3)FeN2}n. The {(CPiPr3)FeN2}n
complexes feature an axial N2 ligand bound trans to an axial
C-atom in a trigonal bipyramidal geometry, a design meant to
crudely model one plausible geometry for a single Fe−N2
binding site in the iron−molybdenum cofactor (FeMoco). The
Calkyl−Fe interaction in the (CPiPr3)Fe system exhibits a
substantially higher degree of ionic character, and is more
flexible, than for the related Sisilyl−Fe interaction in the
isostructural and isoelectronic (SiPiPr3)Fe system.17 We suggest
that this type of Fe−C flexibility crudely models the flexibility
one can intuit for an N2−Fe−Cinterstitial interaction within
Physical Methods. Elemental analyses were performed by
Robinson Microlit Laboratories (Ledgewood, NJ). Deuterated
solvents were purchased from Cambridge Isotope Laboratories, Inc.,
FeMoco. Whereas the N2 anion (SiPiPr3)FeN2 does not
effectively facilitate the delivery of H-atoms to N2 to produce
−
1
degassed, and dried over active 3-Å molecular sieves prior to use. H
NH3 via proton/reductant equivalents, an Et2O solution of
and 13C chemical shifts are reported in ppm relative to
tetramethylsilane, using residual proton and 13C resonances from
solvent as internal standards. 31P and 19F chemical shifts are reported
in ppm relative to 85% aqueous H3PO4 and CFCl3, respectively.
Solution phase magnetic measurements were performed by the
method of Evans.46 Optical spectroscopy measurements were taken on
a Cary 50 UV−vis spectrophotometer using a 1-cm two-window
quartz cell. Electrochemical measurements were carried out in a
glovebox under a dinitrogen atmosphere in a one compartment cell
using a CH Instruments 600B electrochemical analyzer. A glassy
carbon electrode was used as the working electrode and platinum wire
was used as the auxiliary electrode. The reference electrode was Ag/
AgNO3 in THF. The ferrocene couple Fc+/Fc was used as an internal
reference. Solutions (THF) of electrolyte (0.2 M tetra-n-butylammo-
nium hexafluorophosphate) and analyte were also prepared under an
inert atmosphere.
isoelectronic and isostructural (CPiPr3)FeN2 under 1 atm of
−
N2 releases ca. 4.6 equiv NH3 relative to Fe. The modest
−
catalytic N2 reduction behavior of (CPiPr3)FeN2 at −78 °C is
21
comparable to (TPiPrB)FeN2 .
−
It is noteworthy that among the isostructural SiPiPr3, TPiPrB,
and CPiPr3 series, the system with the most flexible axial linkage,
(TPiPrB)Fe, gives the greatest catalytic yield under a common
set of reaction conditions, while the least flexible, (SiPiPr3)Fe,
gives only substoichiometric yields of ammonia; the (CPiPr3)Fe
system falls in between the two both in terms of flexibility and
catalytic competence. While we emphasize caution in
interpreting these differences given the low overall turnover
numbers, they are consistent with the previously advanced
hypothesis that a flexible Fe−Cinterstitial interaction might
facilitate N2 binding and reduction at a single Fe site. Our
structural and DFT studies22 demonstrate that, in the right
environment, a carbon atom can serve as a modestly flexible
X-ray Crystallography. XRD studies were carried out at the
Beckman Institute Crystallography Facility on a Bruker Kappa Apex II
diffractometer (Mo Kα radiation). Structures were solved using
G
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Article
SHELXS and refined against F2 on all data by full-matrix least-squares
with SHELXL.47 The crystals were mounted on a wire loop. Methyl
group hydrogen atoms not involved in disorder were placed at
calculated positions starting from the point of maximum electron
density. All other hydrogen atoms, except where otherwise noted, were
placed at geometrically calculated positions and refined using a riding
model. The isotropic displacement parameters of the hydrogen atoms
were fixed at 1.2 (1.5 for methyl groups) times the Ueq of the atoms to
which they are bonded. Further details for each structure can be found
in the SI.
Computations. A single-point calculation and Natural Bond
Orbital (NBO) analysis was carried out on [(CPiPr3)FeN2][K(Et2O)3]
(12) using the crystallographically determined atomic coordinates at
the B3LYP/6-31++G(d,p) level of theory using the Gaussion03 suite
of programs.48 NBO analysis located a polarized σ interaction between
Fe and the C-atom anchor (C01). Further details of the computational
results can be found in the SI.
reaction mixture was allowed to warm to room temperature and stirred
for 30 min, and then the solvent was removed in vacuo. The solid
material was suspended in 200 mL of diethyl ether and 200 mL of
water, and then solid potassium iodide (15 g, 0.090 mol) was added
and the mixture was shaken vigorously for 5 min, during which time a
fine yellow precipitate developed. The precipitate was collected atop a
sintered glass frit and washed copiously with water and diethyl ether
(9.95 g, 0.0173 mol, 95%). 1H NMR ((CD3)2SO, 300 MHz, 298 K,
δ): 8.20 (dd, J = 8 Hz, 1 Hz, 2H), 7.83 (dd, J = 8 Hz, 1 Hz, 2H), 7.72
(dd, J = 8 Hz, 1 Hz, 1H), 7.60 (td, J = 8 Hz, 1 Hz, 2H), 7.47−7.39 (m,
3H), 7.33 (td, J = 8 Hz, 1 Hz, 1H), 7.23 (dd, J = 8 Hz, 1 Hz, 1H), 6.02
(s, 1H) ppm. 13C NMR ((CD3)2SO, 75.4 MHz, 298 K, δ): 138.9
(s), 135.4 (s), 135.1 (s), 135.0 (s), 133.4 (s), 132.8 (s), 131.7 (s),
130.7 (s), 130.0 (s), 128.0 (s), 117.2 (s), 110.0 (s), 58.8 (s) ppm. ESI-
MS (positive ion, amu): Calcd. 446.9, 448.9; Found 446.9, 448.9.
2-Bromo-2′,2″-diiodotriphenylmethane (5). Solid 10-(2-bro-
mophenyl)-10H-dibenzo[b,e]iodininium iodide (4.54 g, 7.88 mmol)
was sealed inside a Schlenk tube under N2 and heated to 200 °C for 15
min, and then cooled to room temperature. The resulting dark violet
residue was taken up in dichloromethane (50 mL) and washed with
saturated aqueous sodium thiosulfate (50 mL) and then water (30
mL) and saturated aqueous sodium chloride (30 mL), then dried over
magnesium sulfate, filtered, and concentrated to dryness in vacuo. The
resulting off-white residue was recrystallized from methanol to give the
desired product as a fine white powder, which was collected atop a
sintered glass frit and washed with cold methanol (3.4 g, 5.90 mmol,
75%). 1H NMR (CDCl3, 300 MHz, 298 K, δ): 7.93 (d, J = 8 Hz, 2H),
7.64 (d, J = 8 Hz, 1H), 7.30−7.16 (m, 4H), 7.00 (t, J = 8 Hz, 2H), 6.72
(d, J = 8 Hz, 3H), 6.04 (s, 1H) ppm. 13C NMR (CDCl3, 75.4 MHz,
298 K, δ): 144.1 (s), 141.1 (s), 140.2 (s), 133.3 (s), 131.1 (s), 130.7
(s), 128.6 (s), 128.5 (s), 127.3 (s), 126.7 (s), 103.6 (s), 65.4 (s) ppm.
MS (amu): Calcd. 573.8, 575.8; Found 446.9, 448.9 ([M−I]+), 368.1
([M−I−Br]+), 320.1, 322.1 ([M−2I]+).
10-Phenyl-10H-dibenzo[b,e]iodininium Bromide (2). The
procedure for the generation of 2 and 4 (below) was adapted from
a reported method for the generation of diaryliodonium salts.29 3-
Chloroperoxybenzoic acid (9.0 g, ∼70% by mass, ∼0.037 mol) was
dissolved in dichloromethane (150 mL) and cooled to 0 °C. 2-
Iodotriphenylmethane (11.7 g, 0.0316 mol) was added as a solid in
portions over the course of 10 min, during which time there was no
observable change to the reaction mixture. This mixture was stirred at
0 °C for 10 min and then neat trifluoromethanesulfonic acid (8.74 mL,
0.0990 mol) was added via syringe over the course of 5 min. The
reaction mixture turned dark brown. After an additional 20 min, the
reaction mixture was allowed to warm to room temperature and stirred
for 1 h, and then the solvent was removed in vacuo. The solid material
was suspended in 200 mL of diethyl ether and 200 mL of water, and
then solid sodium bromide (14 g, 0.136 mol) was added, and the
mixture was shaken vigorously for 5 min, during which time a fine off-
white precipitate developed. The precipitate was collected atop a
sintered glass frit and washed copiously with water and diethyl ether
Tris(2-(diisopropylphosphino)phenyl)methane (“(CiPrP3)H”)
(1). 2-Bromo-2′,2″-diiodotriphenylmethane (2.00 g, 3.48 mmol) was
dissolved in diethyl ether (100 mL) and cooled to −78 °C while
stirring. Solid t-butyllithium (1.36 g, 21.23 mmol) was added in
portions over the course of 10 min, and the reaction mixture was
stirred at low temperature for 3 h. Then chlorodiisopropylphosphine
(1.96 g, 12.8 mmol) was dissolved in 10 mL of diethyl ether and added
to the reaction mixture. The reaction mixture was allowed to warm
slowly to room temperature overnight, resulting in the precipitation of
a fine white solid. The reaction mixture was filtered through silica and
the pale yellow-orange filtrate was concentrated to a sticky yellow solid
which was triturated with acetonitrile to give an off-white powder. The
solid was washed copiously with acetonitrile and then dried under
1
(14.2 g, 0.0316 mol, quant). H NMR ((CD3)2SO, 300 MHz, 298
K, δ): 8.27 (dd, J = 8 Hz, 1 Hz, 2H), 7.68 (td, J = 8 Hz, 1 Hz, 2H),
7.46 (td, J = 8 Hz, 1 Hz), 7.27 (m, 3H), 6.78 (dm, J = 8 Hz, 2H), 6.09
(s, 1H) ppm. 13C NMR ((CD3)2SO, 75.4 MHz, 298 K, δ): 140.3
(s), 138.3 (s), 135.0 (s), 132.7 (s), 131.7 (s), 129.6 (s), 128.9 (s),
127.9 (s), 127.4 (s), 117.4 (s), 57.7 (s) ppm. ESI-MS (positive ion,
amu): Calcd. 370.0; Found 370.0.
2-Bromo-2′-iodotriphenylmethane (3). 10-Phenyl-10H-
dibenzo[b,e]iodininium bromide (16.11 g, 0.0358 mol) was suspended
in dry, degassed acetonitrile (250 mL), and solid tetrabutylammonium
bromide (25 g, 0.078 mol) and copper(I) bromide (8 g, 0.06 mol)
were added. The mixture was heated to a vigorous reflux and stirred at
reflux for five days. The dark brown reaction mixture was then
concentrated to dryness in vacuo, extracted with toluene, and filtered
through a silica plug. The pale yellow filtrate was concentrated to
dryness, and the resulting material was recrystallized from methanol to
give the desired product as an off-white powder which was collected
atop a sintered glass frit and washed with cold methanol (12.7 g,
0.0282 mol, 79%). 1H NMR (CDCl3, 300 MHz, 298 K, δ): 7.90 (dd, J
= 8 Hz, 1 Hz, 1H), 7.60 (dd, J = 8 Hz, 1 Hz, 1H), 7.34−7.18 (m, 5H),
7.13 (td, J = 8 Hz, 1 Hz, 1H), 7.03 (dd, J = 8 Hz, 1 Hz, 2H), 6.95 (td, J
= 8 Hz, 1 Hz, 1H), 6.79 (dd, J = 8 Hz, 1 Hz, 2H), 6.02 (s, 1H) ppm.
13C NMR (CDCl3, 75.4 MHz, 298 K, δ): 145.2 (s), 142.2 (s), 141.1
(s), 140.1 (s), 133.1 (s), 131.2 (s), 130.7 (s), 130.0 (s), 128.5 (s),
128.3 (s), 128.2 (s), 128.0 (s), 127.2 (s), 126.7 (s), 126.3 (s), 102.9
(s), 60.8 (s) ppm. MS (amu): Calcd. 449.9, 447.9; Found 449.9, 447.9.
10-(2-Bromophenyl)-10H-dibenzo[b,e]iodininium Iodide (4).
3-Chloroperoxybenzoic acid (5 g, ∼70% by mass, ∼0.0203 mol) was
dissolved in dichloromethane (200 mL) and cooled to 0 °C. 2-Bromo-
2′-iodotriphenylmethane (8.2 g, 0.0182 mol) was added as a solid in
portions over the course of 10 min, during which time there was no
observable change in the reaction mixture. This mixture was stirred at
0 °C for 10 min and then neat trifluoromethanesulfonic acid (5.04 mL,
0.0571 mol) was added via syringe over the course of 5 min. The
reaction mixture turned dark brown. After an additional 30 min, the
1
vacuum, giving 1.4 g (2.36 mmol, 68%) of the desired product. H
NMR (C6D6, 300 MHz, 298 K, δ): 8.15 (q, J = 6 Hz, 1H), 7.44 (d, J =
7 Hz, 3H), 7.06 (td, J = 7 Hz, 2 Hz, 3H), 7.00−6.93 (m, 6H), 2.27
(septet of doublets, J = 4 Hz, 7 Hz, 3H), 1.73 (septet of doublets, J = 3
Hz, 7 Hz, 3H), 1.40 (dd, J = 7 Hz, 13 Hz, 9H), 1.32 (dd, J = 7 Hz, 12
Hz, 9H), 0.88 (dd, J = 7 Hz, 13 Hz, 9H), 0.44 (dd, J = 7 Hz, 12 Hz,
9H) ppm. 13C NMR (C6D6, 75.4 MHz, 298 K, δ): 159.0 (d, J = 29
Hz), 144.8 (d, J = 17 Hz), 140.0 (s), 139.3 (s), 132.4 (s), 59.1 (m),
32.7 (m), 30.0 (m), 29.4 (s), 27.3 (m), 21.0 (s) ppm. 31P NMR (C6D6,
121.4 MHz, 298 K, δ): −9.1 ppm. Anal. Calcd. for C37H55P3: C, 74.97;
H, 9.35. Found: C, 74.73; H, 9.49.
{(CPiPr3)H}FeI2 (6). (CPiPr3)H (500 mg, 0.843 mmol) was added to
FeI2 (350 mg, 1.13 mmol) in 15 mL of toluene and stirred at 60 °C for
2 h, at which point the reaction mixture was filtered through
diatomaceous earth and the yellow filtrate was concentrated to give a
yellow powder (761 mg, 0.843 mmol, quant). Crystals suitable for X-
ray diffraction were grown by layering of pentane over a saturated
toluene solution. 1H NMR (C6D6, 300 MHz, 298 K, δ): 179.69, 26.00,
18.60, 14.92, 14.28, 13.62, 12.74, 9.96, 9.00, 8.29, 6.76, 6.16, 5.72, 5.48,
4.97, 4.28, 3.78, 0.30, 0.13, −0.48, −0.91, −2.02, −3.68, −5.09, −9.45
ppm. μeff (C6D6, Evans’ method, 298 K): 4.85 μB.
(CPiPr3)Fe(N2)H (9). (CPiPr3)HFeI2 (370 mg, 0.410 mmol) was
suspended in benzene (10 mL) and stirred vigorously over an excess
of 0.7% sodium/mercury amalgam (25 mg Na, 1.1 mmol) for 2 h. The
H
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initially yellow suspension turned a deep brick red color during this
time due to the formation of {(CPiPr3)H}FeI (7). The reaction
mixture was filtered through diatomaceous earth and concentrated to
dryness in vacuo. The deep red residue was then suspended in diethyl
ether (15 mL) at −78 °C and 3 mL of dimethoxyethane was added;
this solution was vigorously stirred over excess sodium mirror for 4 h
at −78 °C, during which time the color lightened to orange. The
reaction mixture was then filtered through diatomaceous earth and
concentrated to dryness. The residue was extracted into pentane and
again filtered through diatomaceous earth, giving a lighter yellow-
orange filtrate which was concentrated to dryness again. This residue
could be recrystallized from diethyl ether by slow evaporation to give
yellow crystalline solids. These solids were washed with hexamethyldi-
siloxane and minimal cold diethyl ether, and then dried in vacuo to
give 155 mg (0.229 mmol, 56%) of the desired product. Crystals
suitable for X-ray diffraction were grown by evaporation of a
concentrated pentane solution into hexamethyldisiloxane. 1H NMR
(C6D6, 300 MHz, 298 K, δ): 7.57 (t, J = 6 Hz, 1H), 7.34 (m, 1H), 7.08
(m, 2H), 6.96 (m, 2H), 6.83−6.75 (m, 4H), 6.65 (m, 1H), 6.50 (m,
1H), 2.94 (septet, J = 8 Hz, 1H), 2.75 (m, 2H), 2.36 (septet, J = 6 Hz,
1H), 2.05 (septet, J = 7 Hz, 1H), 1.75−1.17 (m, 25H), 1.02 (dd, J = 7
Hz, 11 Hz, 3H), 0.65 (dd, J = 7 Hz, 15 Hz, 3H), 0.56 (dd, J = 7 Hz, 10
Hz, 3H), 0.27 (dd, J = 8 Hz, 13 Hz, 3H), −10.2 (ddd, J = 38 Hz, 53
Hz, 50 Hz) ppm. 31P NMR (C6D6, 121.4 MHz, 298 K, δ): 90.1 (dt, J =
100 Hz, 17 Hz, 1P), 67.0 (m, 1P), 63.4 (dt, J = 100 Hz, 17 Hz, 1P)
ppm. IR (thin film; cm−1): 2046 (N−N), 1920 (Fe−H). Anal. Calcd.
for C37H55FeP3N2: C, 65.68; H, 8.19; N, 4.14. Found: C, 65.91; H,
7.89; N, 3.94.
[(CPiPr3)FeN2][K(Et2O)0.5] (12[K(Et2O)0.5]). (CPiPr3)FeCl (40 mg,
0.0586 mmol) was dissolved in diethyl ether(5 mL) at room
temperature and an excess of potassium graphite (KC8, 25 mg) was
added. The reaction mixture was stirred for 10 min and then filtered
through diatomaceous earth. The dark brown solution was
concentrated to about 2 mL and then pentane was layered over the
ether solution, and it was allowed to stand overnight during which
time dark bluish-brown crystals formed. The supernatant was
decanted, and the crystals were washed thoroughly with pentane
and thoroughly dried under vacuum, giving 26 mg of the desired
product (0.0277 mmol, 47%). NMR analysis indicates the presence of
0.5 ether solvent molecules per anion. Crystals suitable for X-ray
diffraction were grown by vapor diffusion of pentane into a diethyl
ether solution; in these crystals the potassium cation is solvated by
1
three diethyl ether molecules. H NMR (d8-THF, 300 MHz, 298 K,
δ): 7.04 (s, 3H), 6.67 (s, 3H), 6.47 (s, 6H), 3.38 (q, J = 7 Hz, 2H,
diethyl ether (CH3CH2)2O), 2.99 (br s, 3H), 2.14 (br s, 3H), 1.42 (d,
J = 6 Hz, 9H), 1.36 (d, J = 5 Hz, 9H), 1.12 (t, J = 7 Hz, 3H, diethyl
ether (CH3CH2)2O), 1.01 (d, J = 5 Hz, 9H), 0.12 (d, 9H) ppm. 31P
NMR (5:1 C6D6/d8-THF, 121.4 MHz, 298 K, δ): 68.1 ppm. IR (thin
film deposited from Et2O; cm−1): 1870 (N−N).
[(CPiPr3)FeN2][K(12-c-4)2] (12[K(12-c-4)2]). A sample of 12 (15
mg, 0.020 mmol) was dissolved in diethyl ether (1 mL) and 12-crown-
4 (8.8 mg, 0.050 mmol) was added as a solution in diethyl ether (1
mL). The resulting solution was layered with pentane and allowed to
stand overnight, resulting in the crystallization of 12[K(12-crown-4)2]
as a very dark blue solid. The crystals were washed with pentane and
1
dried under vacuum, giving 10 mg of material (53% yield). H NMR
(d8-THF, 300 MHz, 298 K, δ) 6.86 (br s, 6H), 6.47 (s, 6H), 3.62 (s,
36H, 12-crown-4), 1.43 (s, 9H), 1.30 (s, 9H), 0.91 (s, 9H), 0.16 (s,
9H) ppm. 31P (C6D6, 121.4 MHz, 298 K, δ): 66 ppm. IR (thin film;
cm−1) 1905 (N−N).
{(CPiPr3)H}FeBr (8). {(CPiPr3)H}FeBr2 (5.0 mg, 0.0070 mmol,
generated by treating (CPiPr3)H with anhydrous FeBr2 in toluene) was
dissolved in toluene, cooled to −78 °C, and treated with isopropyl
magnesium chloride (3.5 μL, 2.0 M in Et2O). The reaction mixture
rapidly turned dark brick-red. It was stirred at low temperature for 1 h
and then allowed to warm to room temperature for 30 min before
being filtered and concentrated. The dark red powder was not purified,
but was analyzed by NMR in C6D6, and X-ray quality crystals were
grown by layering pentane over a filtered benzene solution.
[(CPiPr3)FeN2][B(3,5-(CF3)2−C6H3)4] (13). (CPiPr3)FeN2 (7.3 mg)
was dissolved in diethyl ether (1 mL) and a solution of [Fe(C5Me5)2]-
[B(3,5-(CF3)2−C6H3)4] in diethyl ether (1 mL) was added dropwise
while stirring at room temperature. The reaction mixture was then
concentrated to give an orange solid which was washed with benzene
and then dried in vacuo. Crystals suitable for X-ray diffraction were
grown by slow evaporation of a diethyl ether solution into
hexamethyldisiloxane. 1H NMR (4:1 C6D6/THF-d8 under N2, 300
MHz, 298 K, δ): 16.65, 14.48, 8.15, 7.60, 2.71 ppm. (Note: the exact
position of the paramagnetically shifted NMR peaks varies with the
composition of the solvent due to the likely exchange of the N2 ligand
with THF). μeff (d8-THF, Evans’ method, 298 K): 4.3 μB. IR (thin film;
cm−1): 2128 (N−N). Satisfactory elemental analysis could not be
obtained due to the lability of the coordinated N2 ligand.
(CPiPr3)FeCl (10). (CPiPr3)Fe(N2)H (61 mg, 0.0901 mmol) was
dissolved in diethyl ether (8 mL) and cooled to −78 °C. HCl in
diethyl ether (1.0 M, 108 μL, 0.108 mmol) was added to the solution
in one portion. The reaction mixture was stirred at low temperature
for 1 h and then warmed to room temperature and stirred overnight.
The color darkened to deep red-orange, and the reaction mixture was
filtered through diatomaceous earth and concentrated to dryness. The
red residue was recrystallized by evaporation of a pentane solution into
hexamethyldisiloxane and the resulting dark red crystals were washed
sparingly with cold pentane and dried in vacuo, giving 46 mg (0.0673
mmol, 75%) of (CPiPr3)FeCl. Crystals suitable for X-ray diffraction
were grown by evaporation of a concentrated pentane solution into
(CPiPr3)FeN2SiMe3 (14). [(CPiPr3)FeN2][K(Et2O)0.5] (35 mg,
0.0465 mmol) was dissolved in diethyl ether (2 mL) and cooled to
−78 °C. Trimethylsilyl chloride (6 μL, 0.0473 mmol) was dissolved in
diethyl ether (1 mL) and added dropwise to the stirring reaction
mixture. The reaction was stirred at low temperature for one hour and
then warmed to room temperature for 1 h, concentrated to dryness,
taken up in pentane, filtered through diatomaceous earth, and
concentrated. The red-orange residue was recrystallized by slow
evaporation of a pentane solution into hexamethyldisiloxane, and the
resulting red solids were washed with cold hexamethyldisiloxane and
dried in vacuo to give 21 mg (0.0280 mmol, 60%) of solid material,
which was contaminated with a small amount of CP3FeN2 (11) which
we were unable to remove by repeated recrystallization. 14
decomposes slowly to 11 over time. 1H NMR (C6D6, 300 MHz,
298 K, δ) 7.33 (br m, 3H), 6.80 (t, J = 4 Hz, 6H), 6.63 (m, 3H), 2.67
(septet, J = 7 Hz, 3H), 1.97 (septet, J = 7 Hz, 3H), 1.45 (m, 18H),
0.96 (q, J = 7 Hz, 9H), 0.72 (q, J = 7 Hz, 9H), 0.12 (s, 3H) ppm. 31P
(C6D6, 121.4 MHz, 298 K, δ): 80.1 ppm. IR (thin film; cm−1) 1736
(N−N).
1
hexamethyldisiloxane. H NMR (C6D6, 300 MHz, 298 K, δ): 179.93,
26.47, 23.05, 17.44, 17.22, 15.03, 11.66, 1.52, −10.27, −13.36, −16.82
ppm. μeff (C6D6, Evans’ method, 298 K): 4.92 μB. Anal. Calcd. for
C37H54FeP3Cl: C, 65.06; H, 7.97. Found: C, 64.96; H, 8.01.
(CPiPr3)FeN2 (11). (CPiPr3)FeCl (82 mg, 0.120 mmol) was
dissolved in THF (2 mL) and stirred over sodium mirror for 20
min, or until NMR analysis showed complete consumption of the
starting material, and then filtered and concentrated. The residue was
extracted with pentane and filtered through diatomaceous earth, and
concentrated to a brownish-orange residue which was recrystallized by
evaporation of a pentane solution into hexamethyldisiloxane. The dark
brown-orange crystals were washed with hexamethyldisiloxane and
cold pentane and dried in vacuo to give 39 mg (0.0581 mmol, 48%) of
(CPiPr3)FeN2. Crystals suitable for X-ray diffraction were grown by
evaporation of a concentrated pentane solution into hexamethyldisi-
loxane. 1H NMR (C6D6, 300 MHz, 298 K, δ): 19.3 (very broad), 10.4,
6.8, 3.0, 2.0, 0.6, −1.4 ppm. μeff (C6D6, Evans’ method, 298 K): 1.75
μB. IR (thin film; cm−1): 1992 (N−N). Anal. Calcd. for
C37H54FeP3N2: C, 65.78; H, 8.06; N, 4.15. Found: C, 66.03; H,
8.01; N, 3.86.
Ammonia Quantification. A Schlenk tube was charged with HCl
(4 mL of a 1.0 M solution in Et2O, 4 mmol). Reaction mixtures were
vacuum transferred into this collection flask. Residual solid in the
reaction vessel was treated with a solution of [Na][O-t-Bu] (40 mg,
0.4 mmol) in 1,2-dimethoxyethane (1 mL) and sealed. The resulting
I
dx.doi.org/10.1021/ja4114962 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
suspension was allowed to stir for 10 min before all volatiles were
again vacuum transferred into the collection flask. After completion of
the vacuum transfer, the flask was sealed and warmed to room
temperature. Solvent was removed in vacuo, and the remaining residue
was dissolved in H2O (1 mL). An aliquot of this solution (20 μL) was
then analyzed for the presence of NH3 (trapped as [NH4][Cl]) via the
indophenol method.38 Quantification was performed with UV−vis
spectroscopy by analyzing absorbance at 635 nm.
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Standard Catalytic Procedure with [(CPiPr3)FeN2][K(Et2O)0.5
]
(12). [(CPiPr3)FeN2][K(Et2O)0.5] (1.9 mg, 0.0025 mmol) was
dissolved in Et2O (0.5 mL) in a small Schlenk tube equipped with a
stir bar. This solution was cooled to −78 °C in a cold well inside of the
glovebox. A suspension of KC8 (14 mg, 0.100 mmol) in Et2O (0.75
mL) was cooled to −78 °C and added to the reaction mixture with
stirring. After five minutes, a similarly cooled solution of HBArF • 2
4
Et2O (93 mg, 0.092 mmol) in Et2O (1.0 mL) was added to the
suspension in one portion with rapid stirring. Any remaining acid was
dissolved in cold Et2O (0.25 mL) and added subsequently, and the
Schlenk tube was sealed. The reaction was allowed to stir for 60 min at
−78 °C before being warmed to room temperature and stirred for 15
min.
For further details of catalytic runs with other precatalysts and/or
modified conditions, and complete tables of results, see SI.
ASSOCIATED CONTENT
* Supporting Information
■
S
Spectroscopic data for new compounds, further experimental
and computational details, and additional data on catalytic runs.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(21) Anderson, J. S.; Rittle, J.; Peters, J. C. Nature 2013, 501, 84.
(22) Rittle, J.; Peters, J. C. Proc. Natl. Acad. Sci. U.S.A. 2013, 110,
15898.
(23) Macbeth, C. E.; Harkins, S. B.; Peters, J. C. Can. J. Chem. 2005,
83, 332.
AUTHOR INFORMATION
■
(24) Mankad, N. P.; Whited, M. T.; Peters, J. C. Angew. Chem., Int.
Ed. 2007, 46, 5768.
(25) Moret, M.-E.; Peters, J. C. Angew. Chem., Int. Ed. 2011, 50, 2063.
(26) Bontemps, S.; Bouhadir, G.; Dyer, P. W.; Miqueu, K.; Bourissou,
D. Inorg. Chem. 2007, 46, 5149.
Corresponding Author
jpeters@caltech.edu
Notes
The authors declare no competing financial interest.
(27) Lesueur, W.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.
Inorg. Chem. 1997, 36, 3354.
ACKNOWLEDGMENTS
(28) Bickelhaupt, F.; Jongsma, C.; de Koe, P.; Lourens, R.; Mast, N.
R.; van Mourik, G. L.; Vermeer, H.; Weustink, R. J. M. Tetrahedron
1976, 32, 1921.
(29) Bielawski, M.; Olofsson, B. Chem. Commun. 2007, 2521.
(30) Ciclosi, M.; Lloret, J.; Estevan, F.; Lahuerta, P.; Sanau, M.;
Perez-Prieto, J. Angew. Chem., Int. Ed. 2006, 45, 6741.
(31) Schlosser, M. Pure Appl. Chem. 1988, 60, 1627.
(32) Hoffmann, R.; Bissel, R.; Farnum, D. G. J. Phys. Chem. 1969, 73,
1789.
(33) Lee, Y.; Kinney, R. A.; Hoffman, B. M.; Peters, J. C. J. Am. Chem.
Soc. 2011, 133, 16366.
(34) Field, L. D.; Guest, R. W.; Vuong, K. Q.; Dalgarno, S. J.; Jensen,
P. Inorg. Chem. 2009, 48, 2246.
(35) Anderson, J. S.; Moret, M.-E.; Peters, J. C. J. Am. Chem. Soc.
■
This work was supported by the NIH (GM 070757) and the
Gordon and Betty Moore Foundation, and through the NSF via
a GRFP award to S.E.C. Larry Henling and Dr. Tzu-Pin Lin are
thanked for their assistance with X-ray crystallography. Jon
Rittle is thanked for testing the catalytic activity of [CSiPPh3]-
−
FeN2 .
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