Synthesis and characterization of a bis(imino)-N-heterocyclic carbene analogue to bis(imino)pyridine iron complexes

Article abstract of DOI:10.1021/om300885d

New bis(imino)-N-heterocyclic carbene ligands have been synthesized, and the corresponding iron complexes have been isolated and characterized. Whereas imidazole-derived complexes exhibited exclusively bidentate binding modes, 4,5,6-trihydropyrimidylidene-based ligands adopted a tridentate pincer conformation analogous to complexes of 2,6-bis(imino)pyridines. Bonding in the five-coordinate bis(imino)-N-heterocyclic carbene complex displayed considerably contracted iron-ligand bond distances in comparison to those in the analogous bis(imino)pyridine iron complex. Of particular note was an extraordinarily short iron-carbene carbon bond distance (1.812(2) A). In addition to demonstrating differences in bonding, the isoelectronic structures have different electrochemical properties that reflect the ability of the carbene to stabilize both oxidized and reduced forms of the parent organometallic species.

Full text of DOI:10.1021/om300885d

Article
pubs.acs.org/Organometallics
Synthesis and Characterization of a Bis(imino)‑N-heterocyclic
Carbene Analogue to Bis(imino)pyridine Iron Complexes
Hilan Z. Kaplan, Bo Li, and Jeffery A. Byers*
Eugene F. Merkert Chemistry Center, Department of Chemistry, Boston College, 2609 Beacon Street, Chestnut Hill, Massachusetts,
02467
S
* Supporting Information
ABSTRACT: New bis(imino)-N-heterocyclic carbene ligands
have been synthesized, and the corresponding iron complexes
have been isolated and characterized. Whereas imidazole-derived
complexes exhibited exclusively bidentate binding modes, 4,5,6-
trihydropyrimidylidene-based ligands adopted a tridentate pincer
conformation analogous to complexes of 2,6-bis(imino)pyridines.
Bonding in the five-coordinate bis(imino)-N-heterocyclic carbene
complex displayed considerably contracted iron−ligand bond
distances in comparison to those in the analogous bis(imino)pyridine iron complex. Of particular note was an extraordinarily
short iron−carbene carbon bond distance (1.812(2) Å). In addition to demonstrating differences in bonding, the isoelectronic
structures have different electrochemical properties that reflect the ability of the carbene to stabilize both oxidized and reduced
forms of the parent organometallic species.
Considering their utility in catalytic reactions, much effort
has been devoted to elucidating the structure−activity relation-
ships for iron and cobalt complexes containing such a privileged
class of ligand. Whereas most studies have focused on the easily
interchangeable imine moieties,4 relatively little has been done
to explore the steric and electronic implications of the central σ
donor.11 Except for a few examples involving para-substituted
pyridines,12 none of the reported variations resulted in five-
coordinate complexes containing a central heterocycle with
stronger σ-donating capabilities. Particularly absent from the
literature was a tridentate bis(imino)-N-heterocyclic carbene
analogue to the bis(imino)pyridine ligand. This finding may be
surprising, considering the extensive application of N-
heterocyclic carbenes (NHC) in transition-metal-catalyzed
processes.13 Nevertheless, NHC complexes of iron have
remained largely unexplored14 in comparison to the other
transition metals.
INTRODUCTION
■
Over the past 15 years, iron and cobalt complexes bearing 2,6-
bis(imino)pyridyl ligands have received considerable attention
due to their remarkable properties and noteworthy reactivity
across a broad range of transformations. In the late 1990s,
Gibson, Brookhart, and Bennett introduced the complexes as
olefin polymerization catalysts (e.g., 1; Figure 1) with
More recently, advances in NHC metalation methodology15
and a renewed interest in non-noble transition-metal catalysis
have led to an increase in the number of isolable iron-NHC
complexes. In concert with these developments, Lavoie and co-
workers reported the synthesis of the first 1,3-bis(imino)-
imidazol-2-ylidene ligands as potential N-heterocyclic carbene
analogues to the bis(imino)pyridine ligands of Brookhart and
Gibson (3; Figure 1).16 Rapid ligand decomposition upon
formation of the free carbene17 was circumvented by treatment
of the imidazolium carbene precursor with an in situ generated
copper amide reagent. Expanding upon a similar strategy
introduced by Danopoulos,15 the Lavoie group extended this
Figure 1. Bis(imino)pyridine and bis(imino)-N-heterocyclic carbene
complexes of iron.
efficiencies that rivaled those of the most potent early-
transition-metal metallocenes.1,2 Facile modifications to the
redox-active ligand3 enabled the selective production of olefin
dimers, oligomers, polymers,4 and most recently cyclotrimers5
when acetylene was used as the monomer feedstock. Reduction
of 1 leads to dinitrogen complexes such as 2 (Figure 1),6 which
have also been shown to be outstanding catalysts for the
hydrogenation7 and hydrosilyation8 of alkenes, as well as for
intermolecular [2 + 2] cycloaddition reactions of alkenes9 and
dienes.10
Received: September 14, 2012
Published: October 22, 2012
© 2012 American Chemical Society
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a
Scheme 1. Synthesis of Homologated 1,3-Bis(imino)imidazol-2-ylidene Iron Complexes
a
Reagents and conditions: (i) imidazole, DMF, 23 °C, 76%; (ii) N-(2,6-diisopropylphenyl)acetimidoyl chloride, PhMe, 23 °C, 97%; (iii) 5, CH3CN,
80 °C, 91%; (iv) 0.25 equiv of [Fe(HMDS)2]2, 0.5 equiv of FeCl2, THF, 23 °C (51% for 9, 35% for 10).
Figure 2. ORTEP plots of 9 (left) and 10 (right) at the 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity.
methodology to include the synthesis of iron, cobalt, and
chromium bis(imino)-N-heterocyclic carbene complexes.18
However, crystallographic, computational, and spectroscopic
techniques revealed bidentate ligand binding modes for these
structures rather than the expected tridentate chelates (e.g., 3;
Figure 1).
Concurrent with Lavoie’s 2012 report, our research group
had also isolated similar tetrahedral bis(imino)-N-heterocyclic
carbene complexes of iron. In order to explain why these
compounds adopted a four-coordinate geometry, we undertook
a systematic investigation of ligand analogues that were
sterically and electronically different from the 1,3-bis(imino)-
imidazol-2-ylidenes. Herein, we report the results of our efforts,
which include the synthesis and characterization of a bis-
(imino)-N-heterocyclic carbene ligand built upon a 4,5,6-
trihydropyrimidylidene framework. The iron complex reported
is the first example of a pentacoordinate iron structure bearing a
tridentate bis(imino)-N-heterocyclic carbene ligand (4; Figure
1), and to the best of our knowledge, it is also the first example
of an iron complex bearing the saturated 4,5,6-trihydropyr-
imidylidene N-heterocyclic carbene moiety.
would allow for tridentate coordination by relieving the angle
strain inherent to 3.
Divergent syntheses of the homologated ligands were
efficiently accomplished by iterative N-substitution of imidazole
(Scheme 1). Alkylation with α-chloroimine 5 and either
amidination or alkylation rapidly delivered the singly
homologated 7 or the symmetric, doubly homologated 8 in
high yields. Using a protocol similar to that reported by
Lavoie,18 premixing 0.5 equiv. of FeCl2 and 0.25 equiv. of the
20
dimeric [Fe(HMDS)2]2 in tetrahydrofuran (THF) followed
by treatment with the appropriate imidazolium chloride
afforded the corresponding NHC complexes 9 and 10 as pale
1
yellow solids. Unfortunately, H NMR spectra were compli-
cated by significant peak broadening, temperature- and
concentration-induced chemical shift changes, and missing
signals, all features typical of paramagnetic species.21 Along
these lines, magnetic moments of 5.0 and 5.1 μB for 9 and 10,
respectively, were measured by the Evans method.22 These
values are consistent with similar complexes and correspond to
S = 2 high-spin Fe(II) complexes. Crystals suitable for X-ray
crystallography were grown from vapor diffusion of pentane
into dichloromethane or THF solutions at −40 °C, which
revealed that both compounds were tetracoordinate in the solid
state (Figure 2).
We were surprised to find that both complexes 9 and 10
crystallized in distorted-tetrahedral geometries, irrespective of
ring expansion of one or both amidine fragments on the ligand.
Analogously to complex 3, complex 9 binds through the central
carbene as well as the amidine, notably preferring the five-
membered metallacycle over the possible six-membered
chelate. The bond metrics about the iron center are similar
to those in complex 3 (Table 1), which indicated that minimal
perturbation was imparted on the iron center as a function of
the unbound portion of the ligand.
RESULTS AND DISCUSSION
■
We hypothesized that the iron complexes containing 1,3-
bis(imino)imidazol-2-ylidene ligands such as 318 (Figure 1), are
four-coordinate complexes for one of two reasons. On the one
hand, a tridentate binding mode could be disfavored due to the
geometric constraints of a five-membered heterocyclic frame-
work, a hypothesis that has been put forward by both Gibson11a
and Mashima19 when exploring furan- and pyrrole-based
ligands. On the other hand, the added electron density from
the carbene donor to the iron center may discourage a higher
valency complex. The former hypothesis is supported by the
constrained Ccarbene−Nimidazole−Camidine angle observed within
the chelating ring of 3 in comparison to the analogous angle for
the unbound amidine (119.6° vs 125.0°, respectively).18 We
postulated that one-carbon homologation of the amidine arms
Akin to the bis(imino)-N-heterocyclic carbene compounds 3
and 9, 10 also adopts a distorted-tetrahedral geometry, due to
bidentate ligand coordination. The deviation from an ideal
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contain the 4,5,6-trihydropyrimidine heterocycle. Such com-
plexes would therefore provide fundamental structural and
electronic information pertinent to the growing area of iron N-
heterocyclic carbene chemistry.
Given the rapid and high-yielding nature of Lavoie’s
sequential substitution strategy,16 a similar sequence of
transformations was applied to the construction of 1,3-
bis(imino)trihydropyrimidinium chloride salt 12 (Scheme 2).
Table 1. Selected Bond Lengths (Å) and Angles (deg) for 3,
9, and 10
a
3
9
10
Fe(1)−C(1)
Fe(1)−Cl(1)
Fe(1)−Cl(2)
C(1)−N(1)
C(1)−N(2)
2.091(3)
2.240(1)
2.244(1)
1.369(4)
1.350(4)
1.256(4)
1.269(4)
2.145(3)
76.9(1)
2.085(2)
2.225(1)
2.243(1)
1.369(3)
1.342(3)
1.263(3)
1.274(3)
2.157(2)
77.1(1)
2.071(2)
2.252(1)
2.245(1)
1.362(3)
1.351(3)
1.247(4)
1.285(3)
2.131(2)
87.0(1)
b
N(4)−C(6)
C(4)−N(3)
Scheme 2. Synthesis of 1,3-Bis(imino)-4,5,6-
c
trihydropyrimidinium Chloride 12 and Pentacoordinate Iron
a
Fe(1)−N(3)
C(1)−Fe(1)−N(3)
Complex 4
c
N(1)−C(4)−N(3)
C(1)−N(1)−C(4)
C(1)−N(2)−C(6)
Cl(1)−Fe(1)−Cl(2)
Fe(1)−C(1)−N(1)
Fe(1)−C(1)−N(2)
114.9(3)
119.6(2)
125.0(3)
120.37(4)
111.7(2)
115.4(2)
119.9(2)
123.4(2)
115.13(3)
111.7(1)
144.6(2)
120.1(2)
124.2(2)
124.1(2)
121.36(3)
119.3(2)
137.1(2)
d
144.6(2)
b
a
Data taken from ref 18. The atom labeled C6 in complex 3
corresponds to that labeled C19 in complex 9 and C20 in complex 10.
c
The atom labeled C4 in complex 3 corresponds to that labeled C5 in
d
complex 10. The atom labeled C6 in complex 3 corresponds to that
labeled C18 in the homologated complex 9 and C19 in complex 10.
a
Reagents and conditions: (i) nBuLi, THF, −78 °C, then N-(2,6-
diisopropylphenyl)acetimidoyl chloride, −78 to 23 °C, 97%; (ii) N-
(2,6-diisopropylphenyl)acetimidoyl chloride, PhMe, 23 °C, 74%; (iii)
FeCl2, NaHMDS, THF, −40 to 50 °C, 54%.
tetrahedral geometry is clearly demonstrated by the C(1)−
Fe(1)−N(3) and Cl(1)−Fe(1)−Cl(2) bond angles (87.1(1)
and 121.36(3)°, respectively, for 10 compared to 77.1(1) and
115.13(3)° for 9). Specific to complex 10, however, is the
presence of two homologated imine arms, which necessitate
multidentate binding to afford at least one six-membered-ring
chelate. This feature inevitably leads to structural differences for
10 in comparison to 3 or 9. Whereas 10 displayed subtle
shortening of the Fe(1)−C(1) bond (2.071(2) Å for 10 vs
2.085(2) Å for 9) and modest elongation of the iron−chlorine
bonds (2.245(1) and 2.252(1) Å for 10 vs 2.225(1) and
2.2431(1) Å for 9), the most prominent consequence of the
expanded chelate was the bond metrics associated with the
metallacycle. As mentioned previously, an approximate 5°
compression of the C(1)−N(1)−C(4) junction was required
for the 1,3-bis(imino)imidazol-2-ylidene ligand to adopt a
bidentate binding mode in 3. While a similar compression was
observed for 9, the corresponding angles in 10 demonstrated
little angle strain associated with bidentate metal chelation
(C(1)−N(1)−C(4) and C(1)−N(2)−C(18) are 124.2(4) and
124.1(2)°, respectively).
The lack of strain associated with complex 10 suggested that
four-coordinate complexes of bis(imino)-N-heterocyclic car-
bene ligands could be due to the electronic disparities between
NHC’s and pyridines. We briefly undertook a study of electron-
deficient ligands analogous to 3 but found that the increased
electrophilicity of the imidazolium precursors led to cleavage of
the amidine arms under metalation reaction conditions. To
definitively rule out the steric contribution of the ligand on the
observed low-valent iron species, we undertook the synthesis of
a 1,3-bis(imino)-N-heterocyclic carbene ligand containing a six-
membered-ring heterocycle. We reasoned that a complex built
upon a 4,5,6-trihydropyrimidine N-heterocyclic carbene23 (e.g.,
4; Figure 1) would exhibit Ccarbene−Npyrimidine−Camidine angles
similar to those of the 2,6-bis(imino)pyridines,1 while still
maintaining the potent σ-donating ability of the NHC. A survey
of the literature revealed that few saturated N-heterocyclic
carbene iron complexes had been reported,24 none of which
Starting with commercially available 1,4,5,6-tetrahydropyrimi-
dine, deprotonation with n-butyllithium followed by amidina-
tion with N-(2,6-diisopropylphenyl)acetimidoyl chloride25
cleanly delivered intermediate 11.26 Subsequent N-substitution
in toluene and recrystallization from dichloromethane afforded
carbene precursor 12 in multigram quantities.
Unfortunately, attempts to metalate 12 using the previously
mentioned procedure resulted in incomplete conversion to the
iron−carbene complex. This observation was supported by the
isolation of a yellow crystalline solid, which was determined by
X-ray crystallography to be a [1,3-bis(imino)-4,5,6-
trihydropyrimidinium]FeCl3 “ate” species (13; see the
Supporting Information). Danopoulos and co-workers have
previously reported similar ion pairs during metalation
reactions of sterically encumbered imidazolium salts with
[Fe(HMDS)2]2, often as the major products when steric
constraints prevent the imidazolium precursors from coming
into close proximity to the metal center.15b On the basis of their
findings, the authors hypothesized that the “ate” complexes are
intermediates formed during the metalation process.
Consistent with these observations, we reasoned that ligation
of the apparently less reactive 1,3-bis(imino)-4,5,6-trihydropyr-
imidinium ligands 12 could be driven to the desired complex by
simply heating the reaction mixture . We were pleased to find
that gentle warming to 50 °C for 24 h converted all of the
starting material, including any traces of the “ate” complex, to
the new burgundy product 4. In addition to the color change,
infrared analysis of the solid product after crystallization yielded
amidine stretches at 1612 and 1560 cm−1 for 4, compared to
stretches at 1629 and 1587 cm−1 for 13. Interestingly,
independently prepared “ate” complex 13 could also be
converted to 4 by simple heating in the presence of
[Fe(HMDS)2]2. These observations are consistent with “ate”
complexes similar to 13 being intermediates en route to
carbene ligation. At this stage in our investigations, however, we
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cannot rule out the possibility that 13 forms as the kinetic
product, which is subsequently equilibrated through a pathway
different from the more thermodynamically stable 4 under the
reaction conditions.
Crystallization of 4 proved particularly challenging, but dark
red crystals suitable for X-ray diffraction could be obtained after
successive crystallizations from solvent mixtures containing
diethyl ether, THF, and dichloromethane. Much to our delight,
single-crystal diffraction confirmed the tetragonal structure to
be the desired pentacoordinate iron complex 4 (Figure 3). The
be 112.0(2). These values reflect geometries significantly more
acute than those in the aforementioned tetrahedral complexes
and are more compressed than the analogous angles in complex
1 (114.3(4) and 114.6(1)°). Combined, these structural
features result in a less open coordination environment for 4
in comparison to 1, the consequence of which is best reflected
by comparing the “wingspan” of the complexes. Defined as the
distance between the ipso carbons of the flanking aryl groups,
the opening contracts from 6.58 Å for 1 to 6.00 Å for 4.
Considering that catalytic processes involving pyridyl-diimine
iron catalysts are particularly sensitive to the steric environment
of the complex,3 it is anticipated that the structural parameters
of 4 will have significant implications for substrate binding and
overall reactivity.
In addition to elucidating the structural features of the
bis(imino)-N-heterocyclic carbene complex 4, the magnetic
and electrochemical properties were also evaluated for
comparison to those of the known bis(imino)pyridine complex
1. A magnetic moment (μeff) of 5.0 μB was measured for 4 using
Evans's method,22 which was consistent with the value
determined for 1 and suggests an S = 2 spin state for the
complex. Despite this similarity, cyclic voltammograms
obtained for the two complexes demonstrated notably different
redox properties (Figure 4, Table 2). Consistent with a more
Figure 3. ORTEP plot of 4 at the 50% probability level. Hydrogen
atoms and solvent molecules are omitted for clarity. Selected bond
lengths (Å) and angles (deg) for 4: Fe(1)−C(1), 1.812(2); Fe(1)−
N(4), 2.028(2); Fe(1)−N(3), 2.023(2); Fe(1)−Cl(1), 2.325(1);
Fe(1)−Cl(2), 2.28(1); C(5)−N(4), 1.306(2); C(19)−N(3),
1.305(3); C(1)−N(2)−C(5), 112.0(2); C(1)−N(1)−C(19),
112.0(2); Cl(1)−Fe(1)−Cl(2), 106.40(2).
distorted-square-planar geometry is very similar to that of the
bis(imino)pyridine complex 1.1 However, careful evaluation of
the data illuminates some notable differences between the
pyridine and the NHC containing molecules. One of the most
striking features of the new complex is the Fe(1)−C(1) bond
length, which at 1.812(2) Å is on par with the shortest iron−N-
heterocyclic carbene bonds ever reported.27 This value is in
contrast to the Fe(1)−C(1) bond lengths in 3 (2.091(3) Å)18
and 9 (2.085(2) Å), which are more typical for 16-electron
iron(II) complexes, as well as the iron−pyridine bond in 1
(2.091(4) Å).1 Additionally, the iron−amidine bond distances
(i.e., Fe(1)−N(3) and Fe(1)−N(4)) are also contracted in
relation to the analogous iron−imine bond distances of
bis(imino)pyridine complex 1 (2.023(2) and 2.028(2) Å for
4 vs 2.222(4) and 2.23(1) Å for 1). In fact, the iron−amidine
bonds in 4 are even shorter than the iron−imine bond lengths
in the higher oxidation state iron(III) bis(imino)pyridine
complexes (average Fe−N length 2.185 Å).28 Despite the
closer proximity of the 1,3-bis(imino)trihydropyrimidine ligand
to the iron center in 4, the Fe(1)−Cl(1) and Fe(1)−Cl(2)
distances are almost equivalent to those in complex 1 (2.280(2)
and 2.3250(6) Å for 4 vs 2.263(2) and 2.317(2) Å for 1). It is
also curious that the C(5)−N(4) and C(19)−N(3) bond
lengths in 4 (1.306(2) and 1.305(3) Å) are nearly identical with
the analogous bound Cimine−Nimine distances in bis(imino)-
pyridine 1 (1.30(1) and 1.30(1) Å).
Figure 4. Comparison of the cyclic voltammograms of 1 (red) and 4
(blue). Conditions: 0.01 M acetonitrile solution of analyte; 0.1 M
nBu4NPF6 as supporting electrolyte; platinum mesh as working
electrode; platinum wire as counter electrode; Ag/Ag+/I− as reference
electrode; scan rate of 100 mV s−1.
electron rich metal center, the E1/2 values for the transitions are
shifted to lower potential.29 For example, the central transition,
which we assign to a Fe(II)−Fe(III) transition,30 is shifted by
137 mV from 0.041 V for 1 to −0.096 V for 4 (vs ferrocene/
ferrocenium, Table 2). Interestingly, the next oxidation wave
demonstrates an even more significant cathodic shift of 244
mV. Aside from the Fe(II)−Fe(III) transition, we have not
differentiated any of the other redox events as being metal or
ligand centered. Nevertheless, the most striking features of the
voltammogram of 4 are the four seemingly reversible redox
events. This is in great contrast to the single reversible Fe(II)/
Fe(III) redox couple observed for 1. We believe that a
combination of the more σ-donating carbene and the low-lying
The reduced metal−ligand bond distances in 4 have a
profound effect on the critical C(1)−N(2)−C(5) and C(1)−
N(1)−C(19) amidine arm angles, which are both measured to
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Table 2. Redox Potentials for 1 and 4
a
E1/2
compd
b
b
c
c
1
4
−0.707 (ir)
−0.496 (ir)
−0.449 (r)
0.041 (r)
0.566 (qr)
−0.096 (r)
0.322 (r)
c
0.788 (r)
a
b
Values in V vs Fc/Fc+: r = reversible, ir = irreversible, qr = quasi-reversible. Reduction potential shown. Fe(II)/Fe(III) transition.
cloudy. The reaction was stirred for 1 h at 0 °C and then 3 h at 23 °C.
Water (20 mL) was carefully added to quench any remaining titanium
tetrachloride, and the mixture was filtered through a pad of Celite. The
filtrate was transferred into a separatory funnel containing water (300
mL), and the product was extracted with dichloromethane (3 × 100
mL). The combined organics were dried over magnesium sulfate,
filtered, and concentrated. The crude residue was purified by flash
column chromatography with 5% diethyl ether in pentane as eluent (Rf
= 0.47) to deliver the title compound 5 (8.79 g, 59%) as a yellow oil.
Spectroscopic data were in agreement with the reported literature
values.35
vacant p orbital may collectively stabilize both oxidized and
reduced forms of the parent complex, a feature which may
prove greatly beneficial to catalytic applications.
CONCLUSION
■
In conclusion, we have synthesized the first pentacoordinate
iron complex bearing a tridentate 1,3-bis(imino)-4,5,6-trihy-
dropyrimidine carbene ligand (4). Novel homologated ligands
based on bis(imino)imidazol-2-ylidenes were also developed,
but complexation to iron afforded distorted-tetrahedral
complexes similar to structures previously reported.18 In
contrast, tridentate chelation by a bis(imino)-4,5,6-trihydropyr-
imid-2-ylidene ligand (4) suggests that the steric environment
enforced by a central imidazol-2-ylidene is most likely
responsible for the observed binding modes to iron. Complex
4 contains one of the shortest iron−N-heterocyclic carbene
bonds reported to date, which combined with shortened iron−
amidine bond lengths leads to a more sterically congested
coordination environment relative to that of the parent
bis(imino)pyridine complex 1. Additionally, cyclic voltammo-
grams of complex 4 exhibit four reversible, cathode-shifted
redox events in comparison to one for complex 1. Explorations
into the origin of the unusual electronic and structural
properties of complex 4 as well as an evaluation of its catalytic
activity are currently underway.
Synthesis of N-1-[2-(2,6-Diisopropylphenylimino)propyl]-
imidazole (6). A solution of imidazole (1.15 g, 16.8 mmol, 5.00
equiv) in dimethylformamide (DMF, 5.00 mL) was cooled to 0 °C,
and a solution of α-chloroimine 5 (848 mg, 3.37 mmol, 1.00 equiv) in
DMF (1.70 mL) was added dropwise via cannula. The solution was
then gradually warmed to 23 °C and was stirred for 12 h. The mixture
was poured into saturated aqueous sodium bicarbonate (20 mL), and
the product was extracted with ethyl acetate (3 × 20 mL). The
combined organics were washed with water (30 mL) and brine (30
mL), and then dried over sodium sulfate, filtered, and concentrated.
The crude residue was purified by flash column chromatography with
1% triethyl amine and 5% methanol in dichloromethane as eluent (Rf
= 0.30) to deliver the title compound 6 as a white solid (752 mg,
1
76%). H NMR (CDCl3, 500 MHz): δ 7.60 (s, 1H), 7.14 (s, 1H),
7.11−7.03 (m, 3H), 7.00 (s, 1H), 4.87 (s, 2H), 2.66−2.57 (m, 2H),
1.59 (s, 3H), 1.12 (d, J = 6.6 Hz, 6H), 1.10 (d, J = 6.8 Hz, 6H). 13C
NMR (CDCl3, 125 MHz): δ 165.59, 144.88, 137.86, 135.91, 130.20,
124.13, 123.14, 119.54, 55.20, 28.22, 23.22, 22.93, 17.75. IR (neat):
2960, 2869, 1681, 1663, 1506, 1436, 1328, 1229, 784, 752 662 cm−1.
HRMS (ESI+): calcd for C18H26N3 [M + H]+ 284.2127, found
284.2113.
EXPERIMENTAL SECTION
■
Unless otherwise stated, all reactions were carried out in oven-dried
glassware in a nitrogen atmosphere glovebox or using standard
Schlenk line techniques.31 Solvents were used after passage through a
solvent purification system similar to that reported by Grubbs32 under
a blanket of argon and then degassed by briefly exposing the solvent to
vacuum. Nuclear magnetic resonance (NMR) spectra were recorded at
Synthesis of N-1-[1-(2,6-Diisopropylphenylimino)ethyl]-N-3-
[2-(2,6-diisopropylphenylimino)propyl]imidazolium Chloride
(7). In an inert-atmosphere glovebox, N-1-[2-(2,6-
diisopropylphenylimino)propyl]imidazole (6; 622 mg, 2.19 mmol,
1.00 equiv) was weighed into a vial equipped with a magnetic stirbar. A
solution of N-(2,6-diisopropylphenyl)acetimidoyl chloride25 (522 mg,
2.19 mmol, 1.00 equiv) in toluene (4.38 mL) was then added via pipet,
and the solution was stirred at 23 °C for 20 min. The solvent was
removed in vacuo, and the crude residue was washed with pentane (20
mL) to afford the title compound 7 as an off-white solid, which was
used without further purification (1.11 g, 97%). 1H NMR (CDCl3, 500
MHz): δ 12.32 (dd, J = 1.5, 1.5 Hz, 1H), 8.28 (dd, J = 2.0, 2.0 Hz,
1H), 7.54 (dd, J = 2.2, 1.5 Hz, 1H), 7.22−7.19 (m, 4H), 7.18−7.14
(m, 2H), 5.92 (s, 2H), 2.70−2.63 (m, 4H), 2.60 (s, 3H), 2.04 (s, 3H),
1.27 (d, J = 6.8 Hz, 3H), 1.23 (d, J = 7.1 Hz, 3H), 1.19 (d, J = 6.8 Hz,
3H), 1.12 (d, J = 6.8 Hz, 3H). 13C NMR (CDCl3, 125 MHz) δ 163.65,
148.40, 144.23, 140.67, 139.85, 136.60, 135.78, 125.78, 124.50, 124.22,
123.74, 123.34, 116.76, 55.72, 28.69, 28.43, 23.32, 23.14, 22.92, 22.66,
19.30, 16.90. IR (neat): 2962, 2925, 2868, 1696, 1675, 1577, 1539,
1440, 1377, 1336, 1188, 775, 753 cm−1. HRMS (ESI+): calcd for
C32H45N4 [M + H]+ 485.3644, found 485.3617.
1
ambient temperature on spectrometers operating at 500 MHz for H
NMR or 125 MHz for 13C NMR. Infrared (IR) spectra were recorded
on an ATR infrared spectrometer. Magnetic moments were
determined by the Evans method22 according to the procedure
published by Gibson and co-workers.33 Elemental analysis was carried
out by Robertson Microlit Laboratories (Ledgewood, NJ). High-
resolution mass spectra were obtained at the Boston College Mass
Spectrometry Facility. Cyclic voltammetry was carried out with 0.1 M
tetrabutylammonium hexafluorophosphate as the electrolyte in
acetonitrile. The sample solution was 0.01 M with respect to the
analyte. The working electrode was a platinum mesh, the counter
electrode was a platinum wire, and the reference electrode was a
nonaqueous Ag/Ag+/I− electrode. Cyclic voltammograms were
measured with scan rates of 100 mV s−1 and were referenced to Fc/
Fc+.34 2,6‑dippPDI)FeCl2 (1) was prepared according to the procedure
(
of Gibson and co-workers.2
Synthesis of 1-Chloro-2-(2,6-diisopropylphenylimino)-
propane (5). To a solution of chloroacetone (4.79 mL, 59.5 mmol,
1.00 equiv) in dichloromethane (107 mL) was added 2,6-
diisopropylaniline (11.8 mL, 62.5 mmol, 1.05 equiv), and the mixture
was cooled to 0 °C. A solution of titanium tetrachloride (3.30 mL, 29.8
mmol, 0.500 equiv) in dichloromethane (12.0 mL) was added
dropwise via cannula, immediately turning the reaction mixture a deep
yellow. Triethylamine (33.0 mL, 238 mmol, 4.00 equiv) was then
added dropwise, during which time the mixture turned orange-red and
Synthesis of N,N-1,3-Bis[2-(2,6-diisopropylphenylimino)-
propyl]imidazolium Chlori de (8). N-1 -[2-( 2, 6-
Diisopropylphenylimino)propyl]imidazole (6; 567 mg, 2.00 mmol,
1.00 equiv) was weighed into a 25 mL round-bottom flask equipped
with a magnetic stirbar and affixed to a 180° adaptor containing
ground glass joints and a Teflon stopcock. A solution of α-chloroimine
5 (504 mg, 2.00 mmol, 1.00 equiv) in acetonitrile (2.00 mL) was
added via cannula transfer, and the flask was sealed and heated to 80
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Organometallics
Article
°C for 19 h. The solvent was removed in vacuo, and the apparatus was
brought into an inert-atmosphere glovebox. The crude residue was
washed with pentane (10 mL) to afford the title compound 8 as an off-
white solid that was used without further purification (980 mg, 91%).
1H NMR (CDCl3, 500 MHz): δ 11.00 (s, 1H), 7.44−7.43 (m, 2H),
7.06−7.00 (m, 6H), 5.50 (s, 4H), 2.51−2.43 (m, 4H), 1.82 (s, 6H),
1.09 (d, J = 7.1 Hz, 6H), 0.96 (d, J = 6.8 Hz, 6H). 13C NMR (CDCl3,
125 MHz): δ 163.00, 144.24, 140.06, 135.76, 124.40, 123.17, 122.76,
55.56, 28.32, 23.21, 22.70, 18.92. IR (neat) 3063, 2962, 2870, 1679,
1432, 1359, 1186, 1112, 1045, 784, 771, 751, 643 cm−1. HRMS (ESI
+): calcd for C33H47N4 [M + H]+ 499.3801, found 499.3798.
combined organics were dried over sodium sulfate, filtered, and
concentrated to afford the title compound 11 as a white solid (677 mg,
97%) that was used without further purification. H NMR (CDCl3,
1
500 MHz): δ 7.87 (s, 1H), 7.11−7.08 (m, 2H), 7.02 (dd, J = 8.6, 7.1
Hz, 1H), 3.87 (t, J = 6.1 Hz, 2H), 3.49 (t, J = 4.9 Hz, 2H), 2.77−2.68
(m, 2H), 1.99−1.94 (m, 2H), 1.89 (s, 3H), 1.13 (d, J = 6.8 Hz, 3H),
1.12 (d, J = 6.8 Hz, 3H). 13C NMR (CDCl3, 125 MHz): δ 152.30,
144.56, 144.20, 137.80, 123.16, 123.09, 44.68, 41.29, 28.32, 23.34,
23.08, 21.27, 15.23. IR (neat): 2957, 2920, 2850, 1620, 1458, 1380,
749 cm−1. HRMS (ESI+): calcd for C18H28N3 [M + H]+ 286.2283,
found 286.2279.
Synthesis of [N-1-[1-(2,6-Diisopropylphenylimino)ethyl]-N-
3-[2-(2,6-diisopropylphenylimino)propyl]imidazol-2-ylidene]-
iron(II) Chloride (9). In an inert-atmosphere glovebox, anhydrous
iron(II) dichloride (60.8 mg, 0.480 mmol, 0.500 equiv) was weighed
into a vial equipped with a magnetic stirbar. A solution of
Synthesis of N,N-1,3-Bis[1-(2,6-diisopropylphenylimino)-
ethyl]-4,5,6-trihydropyrimidinium Chloride (12). In an inert-
atmosphere glovebox, 1-[1-(2,6-diisopropylphenylimino)ethyl]-4,5,6-
trihydropyrimidine (11; 819 mg, 2.87 mmol, 1.00 equiv) was dissolved
in toluene (5.00 mL). A solution of N-(2,6-diisopropylphenyl)-
acetimidoyl chloride25 (682 mg, 2.87 mmol, 1.00 equiv) in toluene
(4.60 mL) was then added via pipet, and the solution was stirred at 23
°C for 20 min, during which time a white precipitate formed. The
solvent was removed in vacuo, and the crude solid was recrystallized
from dichloromethane layered with pentane at −40 °C. The colorless
crystals were filtered, washed with pentane (3 × 10 mL), and dried in
vacuo to deliver the title compound 12 as a crystalline white solid
20
[Fe(HMDS)2]2 (181 mg, 0.240 mmol, 0.250 equiv) in THF (7.30
mL) was added to the solid iron(II) dichloride via pipet, and the
suspension was stirred at 23 °C for 20 min. During this time, the
reaction mixture turned light brown and homogeneous. The solution
was diluted with additional THF (20.0 mL), and then a suspension of
imidazolium chloride 7 (500 mg, 0.959 mmol, 1.00 equiv) in THF
(20.8 mL) was added dropwise. Immediately upon addition, the
imidazolium salt dissolved and the reaction mixture turned yellow. The
solution was stirred for 2 h, and then the solvent was removed in
vacuo. The crude yellow solid was dissolved in dichloromethane,
layered with pentane, and cooled to −40 °C. The yellow crystals were
filtered, washed with pentane (3 × 5 mL), and dried in vacuo to
deliver the title complex 9 (297 mg, 51%). Crystals suitable for X-ray
crystallography were grown by vapor diffusion of pentane into a
solution of 9 in dichloromethane at −40 °C. IR (neat): 2961, 2868,
1678, 1634, 1435, 1394, 1365, 1300, 1253, 1214, 810, 782, 754, 733
cm−1. μeff = 5.0 μB at 25 °C (Evans method). Anal. Calcd for
C32H44Cl2FeN4: C, 62.86; H, 7.25; N, 9.16. Found: C, 62.89; H, 7.35;
N, 8.98.
1
(1.188 g, 79%). H NMR (CDCl3, 500 MHz): δ 9.88 (s, 1H), 7.18−
7.11 (m, 6H), 4.36 (t, J = 5.9 Hz, 4H), 2.78−2.70 (m, 4H), 2.61 (s,
6H), 2.49 (p, J = 5.9 Hz, 2H), 1.21 (d, J = 6.8 Hz, 6H), 1.13 (d, J = 6.8
Hz, 6H). 13C NMR (CDCl3, 125 MHz): δ 153.86, 153.34, 141.97,
136.83, 125.07, 123.48, 42.11, 28.49, 23.52, 23.02, 18.79, 16.23. IR
(neat): 2960, 2868, 1620, 1586, 1234, 1196, 764, 751 cm−1. HRMS
(ESI+): calcd for C32H47N4 [M]+ 487.3812, found 487.3801.
Synthesis of [N,N-1,3-Bis[1-(2,6-diisopropylphenylimino)-
ethyl]-4,5,6-trihydropyrimid-2-ylidene]iron Chloride (4). In an
inert-atmosphere glovebox, anhydrous iron(II) dichloride (79.8 mg,
0.630 mmol, 1.05 equiv) was suspended in THF (6.0 mL) and cooled
to −40 °C. To the cold suspension was then added dropwise a
precooled (−40 °C) solution of sodium hexamethyldisilazane (116
mg, 0.630 mmol, 1.05 equiv) in THF (9.6 mL), and the mixture was
held at −40 °C and agitated occasionally for 12 h.37 The mixture was
then warmed to 23 °C for 20 min, during which time most of the
remaining solid dissolved. The orange-brown solution was recooled to
−40 °C, filtered through Celite, and added to a precooled (−40 °C)
suspension of 12 (312 mg, 0.600 mmol, 1.00 equiv) in THF (6.0 mL).
The suspension was gradually warmed to 23 °C, during which time it
turned deep orange. The reaction mixture was then transferred to a
Schlenk tube, removed from the drybox, and heated to 50 °C for 24 h,
during which time the solution turned a dark wine red. The volatiles
were removed in vacuo, and the crude solid was brought back into the
glovebox. The crude residue was dissolved in a mixture of
dichloromethane and THF (ca. 10/1 v/v), filtered through Celite,
and concentrated. Recrystallization from THF layered with pentane at
−40 °C delivered a burgundy solid (200 mg, 54%). Dark red, X-ray-
quality crystals were then grown by submitting the collected solid to
recrystallization from a mixture of Et2O, CH2Cl2, and THF (2/3/2 v/
v) at −40 °C. IR (neat): 2959, 2866, 1612, 1560, 1389, 1322, 1291,
1206, 794, 769 cm−1. μeff = 5.0 μB at 25 °C (Evans method). Anal.
Calcd for C32H46Cl2FeN4: C, 62.65; H, 7.56; N, 9.13. Found: C, 61.57;
H, 7.48; N, 8.46.36
Synthesis of [N,N-1,3-Bis[2-(2,6-diisopropylphenylimino)-
propyl]imidazol-2-ylidene]iron(II) Chloride (10). In an inert-
atmosphere glovebox, anhydrous iron(II) dichloride (56.1 mg, 0.466
mmol, 0.500 equiv) was weighed into a vial equipped with a magnetic
20
stirbar. A solution of [Fe(HMDS)2]2 (175 mg, 0.233 mmol, 0.250
equiv) in THF (7.90 mL) was added to the solid iron(II) dichloride
via pipet, and the suspension was stirred at 23 °C for 20 min. During
this time, the reaction mixture turned light brown and homogeneous.
The solution was diluted with additional THF (20.0 mL), and then a
suspension of imidazolium chloride 8 (500 mg, 0.932 mmol, 1.00
equiv) in THF (19.6 mL) was added dropwise. Immediately upon
addition, the imidazolium salt dissolved and the reaction mixture
turned a yellow-orange. The solution was stirred for 2 h, and then the
solvent was removed in vacuo. The crude yellow solid was dissolved in
THF, layered with pentane, and cooled to −40 °C. The yellow crystals
were filtered, washed with pentane (3 × 5 mL), and dried in vacuo to
deliver the title complex 10 (212 mg, 35%). Crystals suitable for X-ray
crystallography were grown by vapor diffusion of pentane into a
solution of 10 in THF at 23 °C. IR (neat): 2961, 2869, 1666, 1639,
1460, 1438, 1366, 809, 780, 747 cm−1. μeff = 5.1 μB at 25 °C (Evans
method). Anal. Calcd for C33H46Cl2FeN4: C, 63.37; H, 7.41; N, 8.96.
Found: C, 62.28; H, 7.17; N, 8.46.36
Synthesis of N-1-[1-(2,6-Diisopropylphenylimino)ethyl]-
4,5,6-trihydropyrimidine (11). A solution of 1,4,5,6-tetrahydropyr-
imidine (0.201 mL, 2.45 mmol, 1.00 equiv) in THF (20.0 mL) was
cooled to −78 °C, and n-butyllithium in hexanes (1.00 mL, 2.57 mmol,
2.57 M, 1.05 equiv) was added dropwise, during which time a white
precipitate formed. The precipitate dissolved upon completion of the
addition, and the mixture was stirred for 20 min at −78 °C. To the
reaction mixture was then added a solution of N-(2,6-
diisopropylphenyl)acetimidoyl chloride25 (582 mg, 2.45 mmol, 1.00
equiv) in THF (4.0 mL) via cannula transfer. The solution was stirred
for 1 h at −78 °C and was then warmed to 23 °C and stirred for
another 1 h. The mixture was poured into water (40 mL), and the
product was extracted with dichloromethane (3 × 30 mL). The
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Figures, tables, and CIF files giving H NMR spectra of 4, 9,
and 10 and crystallographic data for 4, 9, 10, and 13 (CCDC
reference numbers 900880−900883), including tables of crystal
data and structure refinement, bond lengths, angles, atomic
coordinates, equivalent isotropic displacement parameters, and
anisotropic displacement parameters. This material is available
free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
REFERENCES
■
■
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Corresponding Author
*Tel: 617-552-6725. E-mail: jeffery.byers@bc.edu.
Notes
The authors declare no competing financial interest.
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This article is based upon a poster presented by Hilan Z. Kaplan
(pictured here with Dr. Gladysz, left) at the first Organometallics
Symposium at the ACS meeting in Philadelphia, Pennsylvania on
August 21, 2012 (see Editor's Page in this issue). Mr. Kaplan received
his B.A. (Cum Laude) at the University of Pennsylvania in 2008 and
began his graduate studies in the Chemistry Department at Boston
College that same year. He is currently working under the direction of
Prof. Jeffery A. Byers, the corresponding author of this article. Mr.
Kaplan’s research interests encompass the development of iron
complexes bearing novel classes of N-heterocyclic carbene ligands
for polymerization and oxidation catalysis. After his doctoral work is
complete, he plans to pursue his enthusiasm for science and
technology in an industrial setting, developing processes and products
to better mankind. In his free time, Mr. Kaplan enjoys staying
productive as an avid painter and writer.
Whereas Prof. Jeffery A. Byers published his first paper in
Organometallics in 2008 as a graduate student working in the labs
of Prof. John E. Bercaw at the California Institute of Technology, this
paper represents his first contribution to the journal as a principal
investigator. After a postdoctoral fellowship at the Massachusetts
Institute of Technology under the direction of Prof. Timothy F.
Jamison, Prof. Byers began his professional career as an assistant
professor at Boston College in 2011. His group focuses on the use of
organometallic catalysis for synthetic, energetic, and materials
applications. In addition to his passion for chemistry, Jeff enjoys
classic literature and film, is an avid sports fan, and enjoys spending
time outdoors.
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■
We wish to thank Rui Liu and Professor Dunwei Wang for the
generous use of their potentiometer. We also thank Dr. John
Boylan and Dr. Thusitha Jayasundera of the Boston College
NMR Facility. This research was financially supported by The
Trustees of Boston College and the Boston College Research
Incentive Grant. Mass spectrometry instrumentation at Boston
College is supported by funding from the NSF (DBI-0619576).
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ABBREVIATIONS
■
NHC, N-heterocyclic carbene; THF, tetrahydrofuran;
NaHMDS, sodium bis(trimethylsilyl)amide
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addition of oxalyl chloride.
(26) The use of general bases such at triethylamine and imidazole
delivered an unindentified byproduct that was inseparable from the
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(36) Error in elemental analysis was systematically low due to a small
amount (2 wt %) of an unidentified inorganic impurity.
(37) It may be possible to form “Fe(HMDS)Cl” in situ at shorter
reaction times or higher temperatures. However, the progression of
the reaction (as monitored by color) occurs over the course of a few
hours at −40 °C. Attempts to make 4 using the same method as for 9
or 10 lead to less clean reaction mixtures. At this time, we have no
explanation for this phenomenon.
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