Reactivity Modes of an Iron Bis(alkoxide) Complex with Aryl Azides: Catalytic Nitrene Coupling vs Formation of Iron(III) Imido Dimers

Article abstract of DOI:10.1021/acs.organomet.5b00231

The iron bis(alkoxide) complex Fe(OR)₂(THF)₂ (R = C^tBu₂Ph), 1, was found to have strikingly different reactivity with various aryl azides, ArN₃. Azides with methyl or ethyl groups in the ortho positions of the phenyl ring react catalytically via nitrene coupling to give azoarenes, ArNNAr. Catalyst loading as low as 1 mol % yields clean, quantitative conversion of aryl azides to azoarenes at room temperature in as little as 4 h. A combination of two different aryl azides leads to the catalytic formation of all three possible azoarenes, including the asymmetric one. In contrast, reactions with aryl azides lacking ortho substituents yield stable dimeric iron imido complexes of the form (RO)(THF)Fe(μ-NAr)₂Fe(THF)(OR) (Ar = 4-(trifluoromethyl)phenyl, 5; Ar = phenyl, 6; Ar = 3,5-dimethylphenyl, 7), which do not undergo catalytic nitrene coupling. The isocyanide adduct Fe(OR)₂(CNR)₂ (4, R = 2,6-dimethylphenyl) was obtained from the reaction of Fe(OR)₂(THF)₂ with two equivalents of isocyanide. No C-N bond formation was observed in the reaction of compound 4 with azides or in the reaction of compounds 5-7 with isocyanides. (Figure Presented).

Full text of DOI:10.1021/acs.organomet.5b00231

Article
pubs.acs.org/Organometallics
Reactivity Modes of an Iron Bis(alkoxide) Complex with Aryl Azides:
Catalytic Nitrene Coupling vs Formation of Iron(III) Imido Dimers
James A. Bellow,† Maryam Yousif,† Alyssa C. Cabelof,‡ Richard L. Lord,‡ and Stanislav Groysman*,†
†Department of Chemistry, Wayne State University, 5101 Cass Avenue, Detroit, Michigan 48202, United States
‡Department of Chemistry, Grand Valley State University, Allendale, Michigan 49401, United States
S
* Supporting Information
ABSTRACT: The iron bis(alkoxide) complex Fe(OR)2(THF)2 (R =
CtBu2Ph), 1, was found to have strikingly different reactivity with various
aryl azides, ArN3. Azides with methyl or ethyl groups in the ortho positions of
the phenyl ring react catalytically via nitrene coupling to give azoarenes,
ArNNAr. Catalyst loading as low as 1 mol % yields clean, quantitative
conversion of aryl azides to azoarenes at room temperature in as little as 4 h. A
combination of two different aryl azides leads to the catalytic formation of all
three possible azoarenes, including the asymmetric one. In contrast, reactions
with aryl azides lacking ortho substituents yield stable dimeric iron imido
complexes of the form (RO)(THF)Fe(μ-NAr)2Fe(THF)(OR) (Ar = 4-
(trifluoromethyl)phenyl, 5; Ar = phenyl, 6; Ar = 3,5-dimethylphenyl, 7), which do not undergo catalytic nitrene coupling. The
isocyanide adduct Fe(OR)2(CNR)2 (4, R = 2,6-dimethylphenyl) was obtained from the reaction of Fe(OR)2(THF)2 with two
equivalents of isocyanide. No C−N bond formation was observed in the reaction of compound 4 with azides or in the reaction of
compounds 5−7 with isocyanides.
INTRODUCTION
■
Azoarenes are highly colored photoresponsive compounds that
have found various industrial uses as food and textile dyes, as
well as chemical indicators.1,2 In addition, azoarenes have been
investigated for their utilization in optical storage devices,3
liquid crystal displays,4 molecular switches,5 and drug delivery.6
Common routes to the formation of azoarenes include azo
coupling via a diazonium intermediate, as well as the Mills and
Wallach reactions.7 Catalytic transition metal-mediated for-
mation of azoarenes from azides may be beneficial to the
existing stoichiometric methods, as they lead to the desired
product in one step, with dinitrogen as the sole byproduct.
Middle to late transition metal complexes are particularly useful
vehicles for the formation of nitrene from azides and its
subsequent transfer.8 Transition metal-catalyzed nitrene trans-
fer reactions often result in the amination of alkanes and
aziridination of olefins.9,10 However, while stoichiometric
nitrene coupling has been reported by several groups,11
catalytic formation of azoarenes via transition metal catalysts
is rare.12
We are investigating the reactivity of earth-abundant 3d
metal centers in bis(alkoxide) ligand environments. We have
recently reported the synthesis of an iron(II) complex in a
bis(alkoxide) ligand environment, Fe(OR)2(THF)2 (R =
CtBu2Ph) (1).13 We have also demonstrated that compound
1 is a highly reactive species capable of reductively coupling
adamantyl azide to form the bridging hexazene complex
(RO)2Fe(μ-κ2:κ2-AdN6Ad)Fe(OR)2 (Figure 1).13b Such reac-
tivity has been previously observed only with strongly reducing
Fe(I), Mg(I), or Zn(I) systems.14 The unusual reactivity of 1
Figure 1. Reaction of 1 with adamantyl azide forming the bridging
hexazene complex (RO)2Fe(μ-κ2:κ2-AdN6Ad)Fe(OR)2.13b
with an alkyl azide prompted us to next investigate its reactivity
with aryl azides. Herein we report that 1 is capable of forming
both symmetric and asymmetric azoarenes catalytically and
selectively from aryl azides featuring two ortho substituents. We
also isolate three bridging Fe(III)-imide dimers, as well as the
reaction byproduct Fe(OR)3, and propose a possible
mechanism that leads to these products.
RESULTS AND DISCUSSION
■
The addition of one equivalent of mesityl azide to 1 in toluene
led to immediate gas evolution and a gradual color change to
dark orange (Figure 2). Upon reaction workup, two different
orange products cocrystallized. The first product, as observed
by X-ray diffractometry, was the iron(III) tris(alkoxide)
complex Fe(OR)3, 2, a rare example of iron in a trigonal
planar coordination environment.15 The second product, also
identified by X-ray crystallography, was azomesitylene,
Received: March 19, 2015
© XXXX American Chemical Society
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other new peaks are visible in the NMR, but the isocyanide
peaks disappear completely, presumably due to isocyanide
coordination to the metal. To examine this hypothesis, we have
independently synthesized the isocyanide adduct Fe-
(OR)2(CNR′)2 (4). Green Fe(OR)2(CNR′)2 was obtained
by the addition of two equivalents of the isocyanide to the
solution of complex 1 in toluene and characterized by X-ray
diffraction (Figure 2), 1H NMR spectroscopy, and IR
spectroscopy. We next treated a C6D6 solution of 4 with
mesityl azide or phenyl azide. 1H NMR spectroscopy
demonstrates that no reaction between 4 and mesityl/phenyl
azide takes place.
Nitrene coupling was also found to be catalytic (Scheme 1).
Using 1−10% catalyst loading, mesityl azide was cleanly
Scheme 1. Catalytic Formation of Azoarenes from Aryl
Azides Using Fe(OR)2(THF)2 (1)
Figure 2. (Top) Stoichiometric reaction of Fe(OR)2(THF)2 (1) with
mesityl azide yielding Fe(OR)3 (2) and azomesitylene (3);
independent synthesis of Fe(OR)3; synthesis of Fe(OR)2(CNR′)2
(4). (Bottom) X-ray crystal structures of the obtained products 2−4
(40% probability ellipsoids). Selected bond distances (Å) and angles
(deg) for 2: Fe−O, 1.816(1); O−Fe−O, 119.99(2). Selected bond
distances (Å) and angles (deg) for 4: Fe−O, 1.833(3); Fe−C,
2.085(4); O−Fe−O, 125.5(2).
converted to azomesitylene (see Table S1). NMR spectroscopy
in the presence of an internal standard demonstrates that the
conversion is nearly quantitative; pure azomesitylene was
isolated in 95% yield using 1 mol % catalyst. Similar catalytic
conversion of 2,6-diethylphenyl azide to the corresponding
azoarene is also possible. NMR monitoring of the reaction
mixture demonstrates full conversion within 24 h. Pure azo(2,6-
diethylbenzene) was isolated in 93% yield at 5 mol % catalyst.
In addition to the symmetric coupling products, we also
demonstrate that asymmetric coupling is possible with our
system. Using an equal number of equivalents of mesityl azide
and 2,6-diethylphenyl azide with 10% catalyst loading led to
catalytic formation of all three possible azoarene products in
similar quantities, as confirmed by NMR yield (35% of
azomesitylene, 30% of the mixed azoarene, and 25% of
azo(2,6-diethylphenylbenzene). All symmetric and asymmetric
azoarenes were identified by NMR spectroscopy and mass
spectrometry. Control experiments indicate that Fe-
(OR)2(THF)2 is essential for the catalytic or stoichiometric
nitrene formation. No azide consumption or azoarene
formation was observed when Fe(OR)2(THF)2 was replaced
by FeCl2 or FeCl3 or in the absence of any iron catalyst.
Surprisingly, reducing the steric bulk of the aryl azide to
para-tolyl or ortho-tolyl groups yielded no azoarene formation,
even at elevated temperatures, as evidenced by NMR. However,
a color change to red-brown and gas evolution were observed
in both cases, implying formation of metal-imido species.
Similarly, no azoarene formation was observed in the reaction
with the more electron-rich aryl azide azidoanisole or for the
MesNNMes, 3. Running the same reaction in C6D6 and taking
NMR of the resulting dark orange solution (in the presence of
an internal standard) revealed quantitative consumption of the
starting azide, as well as three new peaks corresponding to a
new product featuring a mesityl group, consistent with the
formation of 3. Thus, both X-ray crystallography and NMR
spectroscopy indicate that instead of reductive coupling of the
aryl azide to form hexazene, nitrene formation occurs followed
1
by the subsequent N−N coupling to form an azoarene. H
NMR spectroscopy indicates that azoarene forms quantita-
tively. We were not able to determine the yield of complex 2
(Fe(OR)3) due to its nearly featureless NMR and UV−vis
spectra. Complex 2 can be independently synthesized from the
reaction of an Fe(III) source, FeCl3, with three equivalents of
LiOR (Figure 2).
We have investigated the reactivity of the transient nitrene
functionality toward activation of a weak C−H bond (of
cyclohexadiene) and N−C bond formation with an isocyanide
(2,6-dimethylphenyl isocyanide). The reaction appears to be
highly preferential toward nitrene coupling versus C−H bond
activation: on running the reaction in cyclohexadiene as a
solvent, only the azoarene product is observed by NMR, with
no evidence of C−H activation. To test for the possible C−N
bond formation, we have studied the reactivity of the
Fe(OR)2(THF)2/azide mixture in the presence of an
isocyanide. Addition of a 1:1 mixture of isocyanide and mesityl
azide to a solution of 1 in C6D6 leads to the formation of a
green solution. No azoarene formation is detected, and no
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electron-poor 4-(trifluoromethyl)phenyl azide. Thus, we
suggest that the azoarene formation in our system is governed
by a steric effect: the presence of the two R groups in the ortho
positions is required for catalysis. To determine the product
formed in the reaction with the sterically unhindered azides, a
stoichiometric reaction of 1 with 4-(trifluoromethyl)phenyl
azide was attempted. Crystallization led to the isolation of black
crystals confirmed to be the iron mono(alkoxide) dimer
above). Unfortunately, we were not able to detect 2 in the
reaction mixtures due to its relatively featureless UV−vis and
NMR spectra, as well as its overall instability.
Complexes 5−7 appear to be stable molecules that are
unreactive toward additional equivalents of azide. After treating
compound 7 with an excess of mesityl azide and stirring for 24
h, no color change was observed, and no new peaks appeared in
the 1H NMR spectrum. The reaction of compound 6 with 2,6-
dimethylphenyl isocyanide gave similar results. Additionally,
heating a solution of 7 to 50 °C for 4 h in an attempt to force
formation of the azoarene led to no reaction.
(RO)(THF)Fe(μ-NAr)2 Fe(OR)(THF) (Ar
= 4-
(trifluoromethyl)phenyl), 5, possessing two bridging imido
moieties (Figure 3). The Fe2(μ-NAr)2 diamond core motif
The mechanism to rationalize formation of the compounds
described in the paper is presented in Figure 4. Initial
Figure 4. Proposed “comproportionation” reaction explaining
formation of the iron-imido dimers and Fe(OR)3.
Figure 3. (Top) Reaction of Fe(OR)2(THF)2 (1) with aryl azides
yielding (RO)(THF)Fe(μ-NAr)2Fe(OR)(THF) (5−7). (Bottom) X-
ray crystal structures of compounds 5 (left) and 7 (right) (40%
probability ellipsoids). Selected bond distances (Å) and angles (deg)
for 5: Fe−N, 1.881(3)/1.897(3); Fe−O1, 1.803(2); Fe−O2, 2.075(2);
N−Fe−N, 96.03(6); O1−Fe−O2, 102.58(2). Selected bond distances
(Å) and angles (deg) for 7: Fe−N, 1.875(1)/1.902(1); Fe−O1,
1.833(1); Fe−O2, 2.085(1); N−Fe−N, 96.03(6); O1−Fe−O2,
102.58(2).
coordination of the azide to the metal followed by dinitrogen
extrusion leads to the formation of the reactive iron imido
intermediate Fe(OR)2(NAr). We were not able to directly
observe Fe(OR)2(NAr); however, we shed light on its
electronic structure by DFT calculations (see below). We
also note that a compound similar to Fe(OR)2(NAr) was
proposed in other systems.17,18 Specifically, an iron(IV) imido
complex was postulated as an intermediate on the route to
intramolecular C−H activation using adamantyl azide in the
similar bis(aryloxide) system Fe(OC6H2-2,6-Ad-4-R)2 (R = Me,
iPr).19 The postulated intermediate Fe(OR)2(NAr) is strongly
oxidizing and reacts with one equivalent of the reducing Fe(II)
bis(alkoxide) complex 1 in a comproportionating fashion to
yield two iron(III) complexes. One of them is the iron(III)
tris(alkoxide) complex 2, and the other is the iron(III)
mono(alkoxide) imide dimer (RO)(THF)Fe(μ-NAr)2Fe(OR)-
(THF). It is possible that the formation of the dimer
(RO)(THF)Fe(μ-NAr)2Fe(OR)(THF) is preceded by the
formation of the monomer Fe(OR)(NAr)(THF); however,
we were not able to obtain any evidence to this end. Both
Fe(OR)2(NAr) and (RO)(THF)Fe(μ-NAr)2Fe(OR)(THF)
can be responsible for the observed catalytic nitrene coupling,
via either a mononuclear (as previously described by Hillhouse
and proposed in the reaction mechanism by Peters)10f,12b or
dinuclear mechanism (coupling of bridging imidos). Kinetic
experiments demonstrate that the reaction appears to be of
second order in the azide (see Figures S31−35); this finding
may be consistent with both mechanisms. To elucidate the
reaction order in Fe, we attempted the initial rate method, in
which we changed the concentration of the iron precursor
while following the decrease in the concentration of the azide.
However, several different measurements did not lead to a
definitive integer rate. It is possible that several mechanisms are
responsible for the nitrene coupling in our system, and the
depicted in the crystal structure of 5 is rare in diiron species,
but cluster compounds containing several iron centers with
bridging aryl imidos are more numerous.16 Analogous bis-
(imido) products were isolated using the smaller phenyl azide
(6) and 3,5-dimethylphenyl azide (7). Crystals suitable for X-
ray diffraction could not be isolated for complex 6, but
elemental analysis confirms the identity of the bulk product.
Crystals of the bis(imido) complex 7 were also obtained
(Figure 3). We note that the structures of 5 and 7 are
consistent with the Fe(III) oxidation state, as indicated by the
Fe−OR bond distances and the presence of the Fe2(μ-NAr)2
core (observed only for Fe(III) in previously synthesized
complexes).15b,16d−g The structures of both 5 and 7 are
centrosymmetric, containing only half of the complex in the
asymmetric unit. Two distinct, albeit very close, Fe−N bonds
are observed (see Figure 3 for details). The observed Fe−Fe
distances of 2.543(1) Å (5) and 2.5268(4) Å (7) are closely
related to the Fe−Fe distances in the previously reported
(NHC)(X)Fe(μ-NAr)2Fe(NHC)(X) complexes, which were in
the 2.424(4)−2.527(1) Å range (NHC = N-heterocyclic
carbene, X = Cl, F).16g We postulate that the formation of
5−7, containing iron mono(alkoxide) centers, is accompanied
by the formation of the iron tris(alkoxide) complex 2.
Formation of compound 2 as a byproduct was observed in
the stoichiometric and catalytic coupling of azoarenes (see
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prevalence of the given mechanism is determined by the
concentration of the iron species.
future work will focus on the catalytic formation of additional
diazenes with a particular focus on the diazenes that could be
used as dyes and on the further investigation of the reaction
mechanism.
To better understand the electronic structure of the putative
intermediate Fe(OR)2(NAr), it was explored using DFT at the
B3LYP/6-311G(d) level of theory.20 Spin states ranging from
singlet to septet were optimized, and the high-spin quintet state
was found to be lowest in energy. B3LYP is documented to
overstabilize high-spin states due to its parametric dependence
on the amount of Hartree−Fock exchange,21 so single-point
energies at the optimized B3LYP structures were performed
with OPBE,22 a functional known to produce accurate spin-
state splittings.23 The quintet was still the lowest energy spin
state with OPBE (Table S7). In principle, this quintet species
should be high-spin FeIV with two OR− and one NAr2− ligands
and therefore should only exhibit α spin density at the metal.
The computed spin density is shown in Figure 5. In addition to
EXPERIMENTAL SECTION
■
General Methods and Procedures. All reactions involving air-
sensitive materials were executed in a nitrogen-filled glovebox. The
syntheses of the lithium salt of the ligand and the iron bis(alkoxide)
complex 1 were reported previously.13 Thallium(I) hexafluorophos-
phate was purchased from Strem. Mesityl azide,24 2,6-diethylphenyl
azide,25 and 3,5-dimethylphenyl azide26 were synthesized according to
literature procedures. All other materials were used as purchased from
Aldrich. All solvents were purchased from Fisher Scientific and were of
HPLC grade. The solvents were purified using an MBraun solvent
purification system and stored over 3 Å molecular sieves. The
complexes were characterized using NMR and IR spectroscopies, mass
spectrometry, X-ray crystallography, elemental analysis, and the Evans
method. NMR spectra were recorded at the Lumigen Instrument
Center (Wayne State University) on a Varian Mercury 400 MHz
NMR spectrometer in C6D6 at room temperature. IR spectra of
powdered samples were recorded on a Shimadzu IR Affinity-1 FT-IR
spectrometer outfitted with a MIRacle10 attenuated total reflectance
accessory with a monolithic diamond crystal stage and pressure clamp.
UV−visible spectra were obtained on a Shimadzu UV-1800
spectrometer. Low-resolution mass spectra were obtained on a
Shimadzu LCMS 2020 using a 1:1 mixture of acetonitrile and water
as the eluent, with 0.1% formic acid added. Elemental analyses were
performed by Midwest Microlab LLC and Galbraith Laboratories Inc.
Synthesis and Characterization of Metal Complexes.
Stoichiometric Reaction of Fe(OR)2(THF)2 (1) with Mesityl Azide. A
5.0 mL toluene solution of mesityl azide (7.9 mg, 0.0488 mmol) was
added dropwise to a 5.0 mL toluene solution of 1 (31.2 mg, 0.0488
mmol), leading to a gradual color change from yellow to a dark red-
orange. Gas evolution was also observed. The reaction was stirred for 1
h, upon which the volatiles were removed in vacuo to give a red-
orange residue. This residue was dissolved in a minimum amount of
hexane and placed in the freezer. After 24 h, two different types of
orange crystals had formed. The products (characterized by X-ray
crystallography) were determined to be the iron tris(alkoxide), 2, and
the azoarene MesNNMes, 3. An analogous experiment was run in
C6D6 in the presence of 1,3,5-trimethoxybenzene as an internal
standard. After stirring the reaction for 4 h, NMR revealed total
consumption of the starting azide, with 3 as the only NMR-active
product, formed in near-quantitative (96%) yield.
Figure 5. Spin density isosurface plot (iso = 0.002 au) for quintet
Fe(OR)2(NAr).
α spin density at the Fe center, there is significant β spin
density on the NAr ligand. A corresponding orbital analysis
(Table S4) showed five α electrons at Fe with weak overlap
between a d−π orbital (Sαβ ∼0.53) and a NAr π orbital, which
suggests this species is best described as a high-spin FeIII
antiferromagnetically coupled to NAr−•. This description is
consistent for OPBE, which does not have explicit exchange
and therefore should not overstabilize an electronic config-
uration with maximal exchange (d5) at the metal center. It is
worth noting, however, that all of the states considered have
spin contamination and must be interpreted with caution. Our
formulation of the electronic structure of Fe(OR)2(NAr) is
consistent with previous calculations by Betley and co-workers
on high-spin Fe-imido species, which found Fe to be Fe(III)
AF-coupled with an imido radical.10h However, we note that
while DFT describes our Fe(OR)2(NAr) species as an imido
radical, it did not activate a CH bond, unlike Betley’s system.
Preparation of Fe(OR)3 (2) Using an Fe(III) Precursor. A 5.0 mL
THF solution of the lithium salt of the ligand LiOR (62.1 mg, 0.275
mmol) was prepared, along with a 5.0 mL THF solution of FeCl3
(14.9 mg, 0.0915 mmol). The ligand salt was added to the metal
chloride with stirring, upon which the solution color changed from
yellow to orange. After stirring for 15 min, a THF solution of TlPF6
(63.9 mg, 0.275 mmol) was added dropwise, leading to immediate
formation of a white precipitate. The reaction was stirred for an hour,
upon which the precipitate was filtered off, and the volatiles from the
remaining filtrate were removed in vacuo to give an orange residue.
This residue was redissolved in a minimum amount of hexane and
placed in the freezer to afford orange crystals (52.9 mg, 63%). IR
(cm−1): 2974 (m), 2873 (m), 1558 (m), 1485 (w), 1386 (m), 1155
(w), 1053 (s), 1018 (m), 901 (w), 769 (m), 745 (w). μeff = 6.3 μB
(calcd 5.9). λmax (εM): 317 (10 823), 519 (656). The compound was
also characterized by X-ray crystallography. Anal. Calcd for
C45H69O3Fe: C, 75.7; H, 9.7. Found: C, 73.8; H, 9.6. The compound
is unstable at room temperature, even under an inert atmosphere.
After 24 h at room temperature in the solid state, orange crystals of
compound 2 deform into an oily, dark green-brown residue. 1H NMR
of this green-brown residue reveals the presence of decomposed
ligand. Even submitting a single crystal of this complex for elemental
analysis under dry ice did not yield satisfactory elemental analysis
results.
SUMMARY
■
We have discovered a new catalyst capable of performing the
rare nitrene coupling to form azoarenes. To the best of our
knowledge, this is the first example of a nitrene-coupling
precatalyst that contains an Fe(II) center, as previous examples
included Fe(I) or Ru(I) complexes. We demonstrated that the
iron bis(alkoxide) 1 selectively converts aryl azides with steric
bulk in the ortho positions catalytically to yield clean azoarenes
in high yield. For nonbulky aryl azides, formation of the
unreactive bridging imido species 5−7 was observed. Our
D
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Fe(OR)2(CNAr)2 (4). To a stirred toluene solution of the iron
bis(alkoxide) 1 (60.3 mg, 0.09 mmol) was added a toluene solution of
2,6-dimethylphenyl isocyanide (24.8 mg, 0.19 mmol) in one portion.
Immediately the solution color changed to green. The reaction was
stirred for 1 h, upon which the volatiles were removed in vacuo. The
resulting green residue was dissolved in a minimum amount of hexane,
filtered, and placed in the freezer to afford green X-ray quality crystals
(42.0 mg, 59%). IR (cm−1): 2978 (m), 2955 (m), 2878 (m), 2129 (s),
1489 (w), 1381 (m), 1358 (w), 1096 (s), 1072 (s), 1022 (m), 887
(m), 772 (s), 745 (s), 702 (s), 648 (m). μeff = 4.9(2) μB (calcd 4.9).
Anal. Calcd for C48H64N2O2Fe: C, 76.2; H, 8.5; N, 3.7. Found: C,
76.1; H, 8.3; N, 3.8.
(RO)Fe(THF)(μ-NAr)2Fe(THF)(OR) (Ar = 4-(trifluoromethyl)phenyl)
(5). A 5.0 mL THF solution of 4-(trifluoromethyl)phenyl azide (14.1
mg, 0.075 mmol) was added dropwise to a 5.0 mL THF solution of 1
(48.1 mg, 0.075 mmol). Immediately gas evolution was observed,
along with a solution color change to dark green. Over the course of a
half hour the solution color gradually changed to black. The reaction
was stirred for 4 h, upon which the volatiles were removed in vacuo to
yield a black residue. Recrystallization from hexane at −35 °C afforded
the product as black crystals (9.2 mg, 48%). IR (cm−1): 2963 (w),
2941 (w), 2880 (w), 1607 (w), 1489 (w), 1389 (w), 1319 (s), 1161
was massed to obtain a yield (see Table S1). The nature of the product
was verified using mass spectrometry and H NMR spectroscopy.
1
Procedure for the Synthesis of Asymmetric Azoarenes. The same
procedure as above (general procedure for the synthesis of azoarenes)
was used, but an equal number of equivalents of both azides were first
combined prior to dropwise addition to the catalyst. The reaction was
stirred for the appropriate time, upon which NMR of an aliquot was
taken. Mass spectrometry confirmed the presence of the asymmetric
product for the reaction of mesityl azide and 2,6-diethylphenyl azide.
Control Experiments. Four separate control experiments were
conducted. First, mesityl azide was stirred in the presence of iron(II)
chloride and the internal standard for 24 h. NMR revealed that no
reaction had taken place. Next, mesityl azide was stirred in the
presence of iron(III) chloride and the internal standard for 24 h. NMR
revealed that no reaction had taken place. Next, mesityl azide was
stirred in the presence of the lithium salt of the ligand, LiOR, and the
internal standard for 24 h. NMR revealed that no reaction had taken
place. Finally, mesityl azide alone was stirred in the presence of the
internal standard for 24 h. NMR revealed that no reaction had taken
place.
Kinetics Experiments. 2,6-Diethylphenyl azide (628.2 mg, 3.59
mmol) and the internal standard 1,3,5-trimethoxybenzene (142.4 mg,
0.85 mmol) were dissolved in C6D6. This solution was then added in
one portion to a stirred solution of 1 mol % of 1 (22.9 mg, 0.036
mmol). NMR spectra of an aliquot of the solution were taken at
regular intervals over a period of 7.5 h. The integration ratio with the
standard was used to calculate the concentration of the azide, and its
decrease was monitored. A plot of 1/[azide] vs time revealed a linear
progression, suggesting that the catalysis is second order in azide (see
Figures S31 and S32). Plots of the additional kinetics experiments
using different concentrations of 1 and 2,6-diethylphenyl azide are
given in Figures S33−35.
X-ray Crystallographic Details. The structures of 2, 3, 4, 5, and 7
were confirmed by X-ray analysis. The structure of 3 has been
previously published.27 The crystals were mounted on a Bruker
APEXII/Kappa three-circle goniometer platform diffractometer
equipped with an APEX-2 detector. A graphic monochromator was
employed for wavelength selection of the Mo Kα radiation (λ =
0.710 73 Å). The data were processed, and the structure was solved
using the APEX-2 software supplied by Bruker-AXS. The structure was
refined by standard difference Fourier techniques with SHELXL (6.10
v., Sheldrick G. M., and Siemens Industrial Automation, 2000).
Hydrogen atoms were placed in calculated positions using a standard
riding model and refined isotropically; all other atoms were refined
anisotropically.
Computational Details. Electronic structure calculations were
carried out using DFT28 as implemented in Gaussian09.29 Geometry
optimizations were performed at the B3LYP20 level of theory using the
6-311G(d) basis set based on inadequacies we observed with double-ζ
basis sets in a previous study.30 No symmetry constraints were
imposed during geometry optimizations. All optimized structures were
confirmed to have stable wave functions31 and to be local minima by
analyzing the harmonic frequencies.32 Cartesian coordinates, frequen-
cies, and thermodynamics for all species may be found in Tables S5,
S6, and S7.
(m), 1107 (m), 1065 (s), 1015 (m), 839 (m), 745 (m), 706 (m). μeff
=
3.81 μB (calcd 4.00). The compound was also characterized by X-ray
crystallography. Anal. Calcd for C50H72O4N2F6Fe2: C, 61.7; H, 7.0; N,
2.8. Found: C, 61.3; H, 6.9; N, 3.0.
(RO)Fe(THF)(μ-NAr)2Fe(THF)(OR) (Ar = phenyl) (6). A 5.0 mL THF
solution of phenyl azide (7.7 mg, 0.065 mmol) was added dropwise to
a 5.0 mL THF solution of 1 (41.3 mg, 0.065 mmol). Immediately gas
evolution was observed, along with a solution color change to dark
green. Over the course of a half hour the solution color gradually
changed to a dark maroon. The reaction was stirred for 4 h, upon
which the volatiles were removed in vacuo to yield a brown residue.
Precipitation from hexane at −35 °C afforded the product as a red-
brown solid (7.4 mg, 52%). IR (cm−1): 2920 (w), 2851 (w), 1520 (m),
1489 (w), 1462 (m), 1387 (m), 1119 (m), 1049 (s), 1016 (s), 968
(m), 845 (w), 721 (m), 710 (m). μeff = 4.51 μB (calcd 4.00). Anal.
Calcd for C52H70O4N2Fe2: C, 68.5; H, 8.3; N, 3.2. Found: C, 68.4; H,
8.1; N, 3.5.
(RO)Fe(THF)(μ-NAr)2Fe(THF)(OR) (Ar = 3,5-dimethylphenyl) (7). A
5.0 mL THF solution of 3,5-dimethylphenyl azide (10.4 mg, 0.0707
mmol) was added dropwise to a 5.0 mL THF solution of 1 (45.3 mg,
0.0709 mmol). Immediately gas evolution was observed, along with a
solution color change to dark green. Over the course of a half hour the
solution color gradually changed to a dark maroon. The reaction was
stirred for 4 h, upon which the volatiles were removed in vacuo to
yield a brown residue. Recrystallization from pentane at −35 °C
afforded the product as sticky brown crystals (7.3 mg, 44%). IR
(cm−1): 3003 (w), 2974 (w), 2938 (m), 2880 (m), 2833 (w), 1647
(w), 1593 (m), 1541 (w), 1506 (w), 1476 (s), 1435 (m), 1206 (m),
1140 (w), 1096 (m), 1049 (w), 1018 (s), 746 (s), 708 (w). μeff = 4.80
μB (calcd 4.00). The compound was also characterized by X-ray
crystallography. Anal. Calcd for C54H80O4N2Fe2: C, 69.5; H, 8.6; N,
3.0. Found: C, 68.0; H, 8.6; N, 1.9. The product was sticky and
difficult to transfer in appreciable amounts. As such, satisfactory
elemental analysis could not be obtained, even after repeated attempts.
Catalytic Conversion of Aryl Azides to Azoarenes. General
Procedure for the Synthesis of Azoarenes. Fe(OR)2(THF)2 (25.0
mg, 0.039 mmol) (1) was dissolved in either THF or C6D6 in a
scintillation vial. The aryl azide (∼120−140 mg for 5 mol % of 1) and
the internal integration standard 1,3,5-trimethoxybenzene (65.0 mg,
0.39 mmol) were dissolved in the same solvent in a separate vial. The
azide solution was then added dropwise to the solution of compound
1, upon which gas evolution was observed, along with a gradual
solution color change to red-orange. The reaction was stirred for the
appropriate time (see Table S1), upon which NMR spectra were
obtained for an aliquot of the solution, confirming complete
consumption of the starting azide. The solution was then passed
through a plug of silica to obtain the purified azoarene product, which
ASSOCIATED CONTENT
■
S
* Supporting Information
General methods and procedures, synthesis of all complexes,
catalysis procedures, NMR, IR, UV−vis, and mass spectra,
Evans data, X-ray crystallography, detailed catalytic data,
detailed kinetics data, cif files for 2, 3, 5, and 7, and DFT
calculation details. The Supporting Information is available free
of charge on the ACS Publications website at DOI: 10.1021/
acs.organomet.5b00231.
E
DOI: 10.1021/acs.organomet.5b00231
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(14) (a) Cowley, R. E.; Elhaïk, J.; Eckert, N. A.; Brennessel, W. W.;
Bill, E.; Holland, P. L. J. Am. Chem. Soc. 2008, 130, 6074−6075.
(b) Bonyhady, S. J.; Green, S. P.; Jones, C.; Nembenna, S.; Stasch, A.
Angew. Chem., Int. Ed. 2009, 48, 2973−2977. (c) Bonyhady, S. J.;
Jones, C.; Nembenna, S.; Stasch, A.; Edwards, A. J.; McIntyre, G. J.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: groysman@chem.wayne.edu.
Notes
Chem.Eur. J. 2010, 16, 938−955. (d) Gondzik, S.; Schulz, S.; Blaser,
̈
The authors declare no competing financial interest.
D.; Wolper, C.; Haack, R.; Jansen, G. Chem. Commun. 2014, 50, 927−
̈
929. (e) Fohlmeister, L.; Jones, C. Aust. J. Chem. 2014, 67, 1011−
1016.
ACKNOWLEDGMENTS
■
We thank WSU and ACS PRF for the support of this work. We
also thank Shervyn Karthanal and Katlyn Holt for technical
assistance and Profs. Neal Mankad (UIC) and Stephen
Matchett (GVSU) for helpful discussions. J.A.B. thanks WSU
Graduate School for the Wilfred Heller Research Fellowship.
Computational resources were provided by NSF-MRI award
CHE-1039925 through the Midwest Undergraduate Computa-
tional Chemistry Consortium.
(15) (a) Cantalupo, S. A.; Lum, J. S.; Buzzeo, M. C.; Moore, C.;
DiPasquale, A. G.; Rheingold, A. L.; Doerrer, L. H. Dalton Trans.
2010, 2, 374−383. (b) Chambers, M. B.; Groysman, S.; Villagran
́
, D.;
Nocera, D. G. Inorg. Chem. 2013, 52, 3159−3169.
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