N-N bond cleavage in diazoalkanes by a Bis(imino)pyridine iron complex

Full text of DOI:10.1021/ja8075132

Published on Web 12/10/2008
N-N Bond Cleavage in Diazoalkanes by a Bis(imino)pyridine Iron Complex
Sarah K. Russell, Emil Lobkovsky, and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca, New York 14853
Received September 22, 2008; E-mail: pc92@cornell.edu
1
The reactivity of diazoalkanes with first row transition metals is
of long-standing interest due to the utility of carbene transfer in
organic synthesis1 and the intermediacy of MdCR2 species in olefin
metathesis.2 Diazoalkanes have also been used as mimics of
dinitrogen to understand N2 coordination and functionalization.3
Motivated by the economic and environmental advantages of finding
base metal alternatives to established precious metal catalysts4
coupled with the role of iron in nitrogen-fixing bacteria,5 the
coordination chemistry of diazoalkanes with reduced iron complexes
is of fundamental and potentially practical interest.
The diazoalkane chemistry of iron has been dominated by porphyrin
and related macrocyclic complexes and catalytic cyclopropanation
methods have been developed.6 Our laboratory has recently reported
the unusual chemistry of the bis(imino)pyridine iron diazoalkane
complex, (iPrPDI)FeN2CHSiMe3 (1-N2CHSiMe3; iPrPDI ) 2,6-(2,6-
iPr2-C6H3NdCMe2)2C5H3N), which undergoes hydrogenative N-N
and N-C bond cleavage to yield the corresponding iron ammonia
complex, 1-NH3, and SiMe4.7 Inspired by these observations, we
sought to explore the scope of bis(imino)pyridine diazoalkane chem-
istry. Here we describe a rare example of the cleavage of the N-N
bond in a series of monosubstituted diazoalkanes that occurs in the
absence of H2 under mild conditions in solution.
ments was accomplished by a H, 13C HSQC NMR experiment
and isotopic labeling with D2 gas. Continued hydrogenation of
1-NHCHPh for one week at 23 °C in benzene-d6 yielded the iron
amine complex, 1-NH2CH2Ph.
Green 1-NHCHPh exhibits signature doublets centered at 3.74
and 17.95 ppm (3JH-H ) 21.6 Hz) for the C-H and N-H of the
coordinated imine. Characterization by X-ray diffraction (Figure
2) revealed chelate distortions consistent with two electron reduc-
tion.8 The X-ray data was of sufficient quality such that all of the
hydrogens were located in the difference map and established
formation of a square planar iron complex with the trans isomer
of the imine, consistent with the large 3J coupling constants
1
observed by solution H NMR spectroscopy.
Addition of one equivalent of N2CHPh to a benzene-d6 solution of
the bis(imino)pyridine iron dinitrogen complex, 1-(N2)2, resulted in
immediate and quantitative conversion to two new iron products. A
combination of 1H NMR, 13C NMR, infrared spectroscopy and
combustion analysis identified these compounds as the iron benzonitrile
and benzaldimine compounds, 1-NCPh and 1-NHCHPh, respectively,
arising from N-N bond cleavage of the diazoalkane (Figure 1).
Figure 2. Molecular structure of 1-NHCHPh at 30% probability ellipsoids.
Hydrogen atoms, except those on the imine, omitted for clarity.
The scope of the N-N bond cleavage reaction was assayed with
a series of monosubstituted diazoalkanes. In each case, rapid and
quantitative conversion to the corresponding bis(imino)pyridine iron
nitrile and imine complexes was observed (Figure 1). All of the
bis(imino)pyridine iron imine complexes exhibit 1H NMR doublets
diagnostic for the NH and CH peaks, similar to those observed for
1-NHCHPh (see Supporting Information). One notable case is the
reaction of 1-(N2)2 with N2CHtBu. In addition to the expected
1-NCtBu and 1-NHCHtBu compounds, a third product, identified
as the iron diazoalkane complex, 1-N2CHtBu, was observed in
approximately 60% yield. Importantly, 1-N2CHtBu does not convert
to 1-NCtBu and 1-NHCHtBu over time or upon addition of excess
diazoalkane, thereby ruling out its intermediacy on the N-N bond
cleavage pathway.
A series of experiments was conducted to gain additional insights
into the mechanism of iron-promoted N-N bond cleavage. Addition
of 0.5 equiv. of benzalazine, Ph(H)CdN-NdC(H)Ph, to a benzene-
d6 solution of 1-(N2)2 resulted in immediate formation of 1-NCPh
and 1-NHCHPh. While symmetric azine cleavage to form two
M-NdCR2 species is well-documented,10,11 so-called “non-
symmetrical” N-N bond breaking to yield metal nitrile and imine
complexes remains rare. A seminal example was reported by
Milstein and co-workers whereby a pincer-ligated rhodium(I)
dinitrogen species cleaves benzalazine to yield the corresponding
benzonitrile and imine complexes.12 In toluene solution at room
Figure 1. Scope of diazoalkane N-N bond cleavage with 1-(N2)2.
1
Both 1-NCPh and 1-NHCHPh exhibit H NMR spectroscopic
features consistent with temperature independent paramagnetism.8
For example, the imine methyl groups are shifted upfield to -0.12
and -2.82 ppm in benzene-d6 and the m-pyridine downfield to 11.32
and 11.16 ppm for the nitrile and imine complexes, respectively.
These peaks do not shift dramatically upon cooling or warming.
1-NHCHPh was independently synthesized from addition of 1 atm
of H2 to isolated 1-NCPh, a rare example of iron-promoted nitrile
hydrogenation.9 Confirmation of the N-H and C-H peak assign-
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36 J. AM. CHEM. SOC. 2009, 131, 36–37
10.1021/ja8075132 CCC: $40.75
2009 American Chemical Society

C O M M U N I C A T I O N S
temperature, the latter complex decomposed to unidentified prod-
ucts. No such decomposition was observed for any of the iron imine
complexes described in this work.
Because [1dCHPh] was proposed as an intermediate, several
attempts were made to observe or trap it. Slow or inverse addition
of diazoalkane to 1-(N2)2 resulted in rapid N-N cleavage with no
evidence for any intermediates. Performing the reaction under dilute
conditions or at low temperature also had no effect. Likewise,
addition of N2CHPh in the presence of norbornene, isoprene,
styrene, acetophenone, pyridine, or 4 atm of H2 also produced no
change in the product distribution. Attempts were also made to trap
a bis(imino)pyridine iron alkylidene by intramolecular Lewis base
coordination. Ortho-alkoxy substituted diazoalkanes, popularized
by Hoveyda and co-workers,14 also yielded the expected N-N bond
cleavage products with no evidence for alkylidene intermediates
(Figure 1). Despite the inability to observe the putative iron
alkylidene, we favor the pathway in Figure 3 as the observed
diazoalkane complex, 1-N2CHtBu, did not yield N-N bond
cleavage products upon treatment with excess diazoalkane.
In summary, the reduced bis(imino)pyridine iron dinitrogen
complex, 1-(N2)2 promotes rapid N-N bond cleavage in mono-
substituted diazoalkanes in solution under mild conditions, high-
lighting the utility of electrons stored in the bis(imino)pyridine
chelate to affect cleavage of strong bonds.
Performing the reaction of 1-(N2)2 with 10 equiv. of N2CHPh
yielded the expected cleavage products, 1-NCPh and 1-NHCHPh,
along with benzalazine, establishing catalytic conversion of diaz-
oalkane to azine. Treatment of this mixture with additional
diazoalkane produced more azine, demonstrating the catalytic
competency of the products. A crossover experiment was conducted
whereby 5 equivalents of each of N2CHPh and N2CHTol were
added to 1-(N2)2. In addition to the four expected iron products, a
statistical mixture of the three possible azines was detected by GC-
1
MS and H NMR spectroscopy (see Supporting Information).
Finally, an isotopic labeling experiment was conducted. Addition
of N2CDPh to 1-(N2)2 yielded 1-NCPh and 1-NDCDPh with no
evidence for isotopic incorporation into the bis(imino)pyridine
1
2
chelate of either product. Monitoring the reaction by H and H
NMR spectroscopy at 23 °C over the course of minutes allowed
observation of a diamagnetic, Cs symmetric intermediate (t1/2
≈
15 min) with a 2H NMR resonance centered at 5.20 ppm in benzene.
This species was not detected when the reaction was performed
with natural abundance N2CHPh. Importantly, this compound is
indeed an intermediate as it cleanly converts (confirmed by
integration versus an internal standard) to 1-NCPh and 1-NDCDPh
over the course of minutes. Based on these limited data, this
intermediate is either the iron alkylidene, [1dCHPh], or the
metallacycle, 2. Because of the isotopic sensitivity and observed
diamagnetism, we tentatively favor 2. In either case, the observation
of an intermediate upon isotopic labeling but not with the natural
abundance diazoalkane establishes a C-H(D) bond breaking event
as the rate determining step during N-N bond cleavage.
Acknowledgment. We thank the Packard Foundation (Fellowship
in Science and Engineering to P.J.C.) and the U.S. National Science
Foundation and Deutsche Forschungsgemeinschaft for a Cooperative
Activities in Chemistry between U.S. and German Investigators grant.
We also thank Prof. Suzanne Bart for conducting preliminary
experiments.
Supporting Information Available: Complete experimental proce-
dures, selected NMR spectra, and crystallographic data for 1-NHCHPh.
This material is available free of charge via the Internet at http://
pubs.acs.org.
A proposed mechanism for N-N bond cleavage in monosub-
stituted diazoalkanes is presented in Figure 3 using N2CHPh as a
representative substrate. The sequence begins with formation of a
putative iron alkylidene complex, [1dCHPh], arising from nu-
cleophilic attack of 1-(N2)2 on the diazoalkane carbon. Collecting
the gas generated during N-N bond cleavage with a Toepler pump
provided 97% of the expected N2 gas (in addition to that arising
from 1-(N2)2) consistent with this pathway. [4π + 2π] cycloaddition
of an additional equivalent of diazoalkane with transient [1dCHPh]
generates the iron azine complex, 213 which was observed for the
deuterated isotopologue. Rate-determining 1,3-hydrogen migration
followed by retrocyclization, nitrile and imine linkage isomerization
and capture by additional free bis(imino)pyridine iron complexes,
either in the form of 1-(N2)2 or 2, yields the observed products.
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Figure 3. Proposed mechanism for N-N bond cleavage in diazoalkanes
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