Toward a Porphyrin-Style NHC: A 16-Atom Ringed Dianionic Tetra-NHC Macrocycle and Its Fe(II) and Fe(III) Complexes

Article abstract of DOI:10.1021/acs.organomet.8b00923

N-heterocyclic carbenes (NHCs), and macrocyclic NHCs in particular, have been applied successfully to the stabilization of high oxidation states on transition metal complexes. This access to high oxidation states has enabled their application in oxidative catalysis including aziridination and epoxidation. However, the number of macrocyclic tetra-NHC ligands is still limited, especially those featuring anionic charge, which is beneficial in this regard. In this manuscript, we report a facile and high yielding synthesis for only the second example of such a dianionic, macrocyclic tetra-NHC ligand. This 16-atom macrocycle has the ring size and charge of a porphyrin but with the increased electron donation of NHCs. Its Fe(II) and Fe(III) complexes are reported, and their reactivities for ligand addition and oxidation were tested. Multiple oxidation catalysis reactions were tested with both the Fe(II) and Fe(III) complexes with reagents such as trimethylamine-N-oxide, oxygen (from air), diazodiphenylmethane, and P-tert-butyl-dibenzo-7³phosphanorbornadiene (^tBuPA) to explore the possibilities for single site oxidation reactions.

Full text of DOI:10.1021/acs.organomet.8b00923

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Toward a Porphyrin-Style NHC: A 16-Atom Ringed Dianionic Tetra-
NHC Macrocycle and Its Fe(II) and Fe(III) Complexes
Markus R. Anneser, Gaya R. Elpitiya, Xian B. Powers, and David M. Jenkins*
Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States
*
S Supporting Information
ABSTRACT: N-heterocyclic carbenes (NHCs), and macro-
cyclic NHCs in particular, have been applied successfully to
the stabilization of high oxidation states on transition metal
complexes. This access to high oxidation states has enabled
their application in oxidative catalysis including aziridination
and epoxidation. However, the number of macrocyclic tetra-
NHC ligands is still limited, especially those featuring anionic
charge, which is beneficial in this regard. In this manuscript,
we report a facile and high yielding synthesis for only the second example of such a dianionic, macrocyclic tetra-NHC ligand.
This 16-atom macrocycle has the ring size and charge of a porphyrin but with the increased electron donation of NHCs. Its
Fe(II) and Fe(III) complexes are reported, and their reactivities for ligand addition and oxidation were tested. Multiple
oxidation catalysis reactions were tested with both the Fe(II) and Fe(III) complexes with reagents such as trimethylamine-N-
3
t
oxide, oxygen (from air), diazodiphenylmethane, and P-tert-butyl-dibenzo-7λ phosphanorbornadiene ( BuPA) to explore the
possibilities for single site oxidation reactions.
INTRODUCTION
■
Macrocyclic N-heterocyclic carbenes (NHCs) have gained
considerable attention in organometallic chemistry for a wide
variety of applications, ranging from catalysis to fluores-
1
−12
cence.
occurring porphyrins, a main emphasis for research has been
Since their structure inherently resembles naturally
1
,4,13,14
on their heme-analogue Fe complexes.
Indeed, several
of those Fe-NHC complexes are more effective catalysts than
heme analogues for oxidation reactions like aziridination and
1
5−17
epoxidation.
Combining those traits with other known
advantages of iron, such as very low toxicity, natural
abundance, and low cost, the current effort put into the
investigation of this type of organometallic systems is highly
1
0
rational.
Our group recently reported the synthesis of an 18-atom
tetra-NHC macrocycle with dianionic borates in the macro-
Figure 1. Selected examples of 16- and 18-membered macrocyclic
BMe
2
,Et
H
16
cycle (Figure 1, (
TC ), bottom right). Overcoming the
imidazolium precursors that form tetra-NHCs with transition metals
1
,4,14,16
neutrality of previous NHC ligands (Figure 1, top row) for the
first time has a number of beneficial effects: improved
(R = H, Ph).
The example in the bottom left is newly reported
herein.
1
8
18
solubility, increased electron donation, and the first
18−20
stabilization of main group metals with this class of ligand.
The iron complex of (
2
BMe ,Et
H
In this paper, we showcase the synthesis of a 16-atom
dianionic tetra-NHC, which is the most structurally similar to a
porphyrin to date. This tetra-NHC is ligated to iron(II) and
iron(III) to prepare square planar and square pyramidal
complexes, respectively. Additionally, we report the reactivity
of these complexes and explore some initial forays into
catalytic applications. In particular, we investigated the ability
TC ) was highly effective as an
16
aziridination catalyst with alkyl azides. Nonetheless, we are
cognizant that a small change in ring size from 18- to 16-
membered macrocycles can have a surprisingly severe effect on
the reactivity of the coordinated metal center, as seen for metal
porphyrins, and has also been demonstrated for macrocyclic
4
,13,14,21
NHCs by the groups of Jenkins, Meyer, and Kuhn.
̈
Thus, we desired to prepare a new dianionic, 16-membered
tetra-NHC ligand that would be the most “porphyrin-like” for
any macrocyclic NHC to date (Figure 1).
Received: December 18, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00923
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Organometallics
Article
for these complexes to perform single site oxidation catalysis
iron complexes could be significantly improved by precise
with O, CR , and PR transfer reagents.
2
control of temperature and stoichiometry (Scheme 2).
RESULTS AND DISCUSSION
BMe ,Me
H
2
■
Scheme 2. Synthesis of [(
[(
Imidazolium Precursor (
TC )Fe] (2) (top) and
BMe
2
,Me
H
The preparation of the 16-membered, dianionic, tetra-NHC
TC )FeBr] (3) from the 16-Membered
BMe
2
,Me
H
BMe ,Me
H
precursor (
TC )Br2 (1) can be easily achieved in
2
TC )Br (1)
2
excellent yields in two steps, beginning from commercial
imidazole. Addition of bromodimethylborane to 1,1′-methyl-
enediimidazole (which can be prepared from imidazole and
1
methylene bromide) in benzonitrile at 80 °C gives 1 at 95%
yield as a white powder that is collected via filtration (Scheme
1
). To remove residual benzonitrile, which is critical for the
Scheme 1. Synthesis of the 16-Membered Macrocyclic
BMe
2
,Me
H
Imidazolium (
TC )Br (1)
2
n
Mixing a suspension of 1 in THF with BuLi at −80 °C does
not result in complete deprotonation. Consequently, warming
the reaction to 0 °C for at least 1 h is necessary to remove all of
the imidazolium protons, which generates a clear yellow
solution of the free tetra-NHC. All attempts to isolate this
species failed, which was not unanticipated given its apparent
instability. The transparent yellow carbene solution is then
cooled back down to −80 °C and mixed with the iron salts,
either FeCl or FeBr , yielding the respective complexes,
n
22
subsequent deprotonation steps with BuLi, we found that
the best method was to repeatedly wash the powder with
anhydrous THF and thoroughly dry the product under high
vacuum conditions. Compound 1 was confirmed by DART
1
13
MS, H and C NMR data, and single crystal X-ray diffraction
(Figures S1, S2, and S37).
2
3
BMe
2
,Me
H
BMe
2
,Me
H
Compound 1 is air stable as a solid, but it is not stable in
[(
TC )Fe] (2) and [(
TC )FeBr] (3), in
aqueous solution after extended exposure. Despite its
instability in water, it is possible to exchange the counteranion
in aqueous conditions. Dissolving 1 in a 1:2 mixture of
MeOH/H O followed by addition of aqueous KPF solution
moderate to good yields (Scheme 2). While 3 is stable even
in wet and oxygenated solvent for prolonged periods of time, 2
is highly sensitive to air and moisture. This instability can be
attributed to its highly unsaturated square planar coordination
and its extremely low oxidation potential (vide infra).
A further difficulty lies in the purification of the iron
complexes, since the separation of the Li-salts formed as
reaction byproducts (LiBr) is not trivial. It is important to
thoroughly dry the residual solids between each of the three
extractions with benzene and filter over dried Celite to ensure
a Li-salt-free product.
2
6
(
NH PF does not work) causes immediate precipitation that
4 6
replaces the bromide counterions with PF , which improves its
6
solubility in organic solvents.
The synthesis of the iron complexes based on 1 was a
significant challenge, since the dianionic nature of the tetra-
NHC ligand impedes the deprotonation of the imidazolium
4
,20
precursor considerably.
The imidazolium protons’ lower
1
acidity is also reflected in the H NMR shift of the imidazolium
precursor 1 in DMSO-d to 8.97 ppm (8.60 ppm for 1-PF ),
The physical characterization of 2 and 3 via cyclic
6
6
voltammetry show similarities to their cationic counterparts
4
which is considerably lower than that for comparable neutral
compounds but higher compared to its 18-membered analogue
reported previously by Ku
̈
hn but also the critical influence of
the overall charge to the redox potentials of the Fe(II)/III)
steps in these systems (+0.15 vs −0.71 V for 4, vide infra). In
accordance with its observed stability, 3 shows an irreversible
oxidation at +360 mV and an irreversible reduction at −1250
mV in acetonitrile solution. Significantly, 3 has a lowered
Fe(II/III) reduction potential by almost 300 mV compared to
1
6,18
(
8.41 ppm).
These values also indicate substantial C−H···
19
Br hydrogen bonding, as is reported for similar compounds.
This close interaction between Br and H is clearly seen in the
single crystal X-ray of 1 (Figure S37).
Nitrogenous bases, such as LiHMDS or LDA, were
ineffective, since 1 could not be deprotonated completely.
BMe
2
,Et
H
its 18-membered analogue ([(
TC )FeBr]), and thus, this
Likewise, an internal base approach with Fe(HMDS) ·THF,
smaller ligand seems to make higher oxidation states more
2
16
which has been successfully applied for other cationic Fe tetra-
NHC complexes, did not yield any isolable products, even
readily available for the Fe-center. Since the structures of
both 2 and 3 strongly resemble Fe-porphyrins, it is reasonable
that they have the same spin states as isostructural complexes.
4
,14
under elevated temperature conditions.
In light of these
n
failures, we turned to an even stronger base approach, BuLi,
which was effective for the 18-membered borate macrocycle.
Complex 2 shows an intermediate spin of S = 1 and 3 a spin of
16
20,23,24
S = 1/2 (both determined by the Evans method).
For 3,
Fortunately, we were able to improve upon the yields and
reproducibility of this reaction, since they were previously very
the spin is independent of the solvent, but 2 will react with
strong donor solvents and switch into a low-spin configuration
(S = 0) (vide infra, complex 4).
1
6,20
low for similar systems (8−25%).
The key discovery for
the improvement of the synthesis was determining that even
BuLi does not readily achieve complete deprotonation of 1
The X-ray crystal structure of 2 reveals a square planar
n
25
coordinated Fe(II) center (τ = 0.02) (Figure 2) which is
4
BMe
2
,Et
H
16
and that the stability of the free tetra-NHC is modest at room
temperature. Therefore, the yield and reproducibility of the
isostructural to our previously reported [(
TC )Fe].
The Fe−C bonds are on average 1.940(4) Å long, which is
B
DOI: 10.1021/acs.organomet.8b00923
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Article
BMe
2
,Me
H
Scheme 3. Synthesis of [(
TC )Fe(NCMe) ] (4),
2
BMe
BMe
2
,Me
,Me
H
[
[
(
(
TC )Fe(PPh )] (5), and
TC )Fe(CN Bu) ] (6) from [(
3
2
H
t
BMe
2
,Me
H
TC )Fe] (2)
2
to dramatic color changes of the resulting complexes, turning
BMe
2
,Me
H
the purple solution of 2 into red [(
TC )Fe(NCMe) ]
2
BMe
2
,Me
H
(
[
4), yellow [(
(
TC )Fe(PPh )] (5), or green
TC )Fe(CN Bu) ] (6), respectively. The binding of
3
BMe
2
,Me
H
t
2
the axial acetonitrile ligands in 4 is a fully reversible process
and is accompanied by the change of the spin state for the
Fe(II) center (S = 1 to S = 0), which was also observed for 5
and 6. The acetonitrile ligands can be readily removed by
drying in a vacuum for a short time. While the S = 0 ground
state is generally observed for similar acetonitrile complexes
BMe
2
,Me
H
Figure 2. X-ray crystal structures of [(
TC )Fe] (2) (top) and
BMe
2
,Me
H
[
(
TC )FeBr] (3) (bottom). Green, purple, blue, gray, and
with NHCs, they tend not to lose their axial acetonitrile ligands
olive ellipsoids (50% probability) represent Fe, Br, N, C, and B atoms,
respectively. Solvent molecules and H atoms are omitted for clarity.
Selected interatomic bond distances (Å) and angles (deg) are as
follows: (2) Fe1−C1 = 1.943(3); Fe1−C2 = 1.941(3); Fe1−C3 =
.934(3); Fe1−C4 = 1.941(3); C1−Fe1−C3 = 179.2(1); C2−Fe1−
C4 = 178.3(1); (3) Fe1−C1 = 1.989(4); Fe1−C2 = 1.983(4); Fe1−
1,4,14
this easily.
Cyclic voltammetry measurements in MeCN
for 4 show a reversible Fe(II/III) redox event at −710 mV
(
versus Fc), which is 860 mV less than for the dicationic
Me,Me
H
2+
4
analogue [(
ously reported [(
TC )Fe(NCCH ) ] complex. The previ-
3 2
2
1
BMe
,Et
H
TC )Fe] complex notably has a S = 1
Br1 = 2.4493(9).
ground state but does not bind to axial acetonitrile ligands at
13
all, which is surprising given their structural similarity. This
result is interesting, since it implies that 2 may have a distinct
electronic structure from either of the other reported
macrocyclic tetra-NHC Fe(II) complexes which would
potentially lead to different reactivity (vide infra).
slightly longer than in its dicationic, octahedral analogue,
Me,Me
H
2+
4
[(
TC )Fe] (1.908(6) Å), but shorter than for its 18-
BMe
2
,Et
H
membered neutral counterpart, [(
TC )Fe] (1.989(24)
1
6
Å). In solution, the average symmetry of 2 is D , reflected in
the paramagnetic shifted H NMR signals in benzene-d at
2.9, 7.7, and −20.0 ppm (Figure S4). The structure of 3
shows a square pyramidal coordinated Fe(III) center with an
axial bromide. The Fe−C distances are slightly longer
1.986(4) Å) compared to 2 but again shorter than in the
analogous 18-membered Fe(III) complex, [(
2.037(16)). The Fe(III) center in 3 is located 0.42 Å above
the plane of the four carbene carbons, and the Fe−Br distance
is 2.4493(9) Å. Despite being paramagnetic, 3 shows a
characteristic set of NMR signals at room temperature in
CD CN. The signals at +20.0, −0.2, −10.1, −24.7, and −43.8
ppm are relatively sharp and integrate to 2, 8 (2 + 6), 6, 4, and
H’s, respectively, which demonstrates the axial asymmetry
i.e., CH and BMe give two peaks each) (Figure S5).
Complexes 5 and 6 show distinct differences in reactivity
and symmetry as befits their different coordination numbers
and level of saturation. Phosphine adduct 5 shows the same
high sensitivity to oxygen and moisture as its parent compound
2 and exhibits diamagnetic behavior in both benzene and
2
h
1
6
1
1
(
acetonitrile. H NMR data from complex 5 show a C
2
v
BMe
2
,Et
H
TC )FeBr]
symmetry (Figure S8), indicated by the two doublets arising
1
6
(
at 5.58/4.50 ppm and the separated signals for the CH groups
3
3
1
bound to the boron atoms (0.20/−0.14 ppm). Since the
P
NMR signal at 78.7 ppm does not change in either benzene or
acetonitrile, we conclude that 5 remains a square pyramidal
coordination in both cases (Figure S10). Finally, there is also
no evidence for the formation of a bis-phosphine complex even
when a large excess of phosphine is added to 2 (10 equiv).
3
4
t
(
In a contrary manner, the smaller steric bulk of CN Bu
2
2
To achieve a better understanding of the electronic
allows for 2 equiv to bind readily to 2, forming the symmetric,
bis-substituted complex 6. The H and C NMR data clearly
1
13
properties of the new ligand 1, complex 2 was combined
t
with the commonly used ligands MeCN, PPh , and CN Bu
indicate an average D symmetry on an NMR time scale
3
2h
1
3
(
Scheme 3) which allow for unique spectroscopic handles. All
(Figures S11 and S12). C NMR shows the shifts of the C
NHC
reactions took place immediately at room temperature and led
signals at 191.9 ppm, which is similar to other reported Fe(II)
C
DOI: 10.1021/acs.organomet.8b00923
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Article
26,27
NHC complexes bearing isocyanide ligation.
The carbon
signals of the CCN atoms show up at 180.2 ppm and are
1
3
14
28
broadened with no observable C− N coupling. The IR
−
1
band of the isocyanide is observed at 2017 cm which is
comparable to the structurally similar complex reported by
−
1
29
Song (2013 cm ). Since 6 is coordinatively saturated with a
strongly bound ligand, it is not surprising that it has improved
stability to air and water in solution.
Reactivity of 2 with Oxidative Transfer Reagents.
Since similar complexes either have been reported to form
Fe(IV) oxo complexes or are highly effective for epoxidation
1
4,17
catalysis,
we investigated the reactivity of 2 toward
trimethyl N-oxide and molecular oxygen or air (Scheme 4).
Scheme 4. Synthesis of the [((BMe TC )Fe) O] (7) from 2
2
,Me
H
2
by Trimethyl N-Oxide or Oxygen from Air
Figure 3. X-ray crystal structure of [((BMe TC )Fe) O] (7). Green,
2
,Me
H
2
red, blue, gray, and olive ellipsoids (50% probability) represent Fe, O,
N, C, and B atoms, respectively. Solvent molecules and H atoms are
omitted for clarity. Selected interatomic bond distances (Å) and
angles (deg) are as follows: (7) Fe1−C1 = 1.966(2); Fe1−C2 =
1
1
.963(2); Fe2−C3 = 1.969(2); Fe2−C4 = 1.970(2); Fe1−O1 =
.731(2); Fe2−O1 = 1.730(2); Fe1−O1−Fe2 = 180.00.
t
Scheme 5. Reactivity of 2 with N CPh and BuPA
2
2
BMe
2
,Me
H
Addition of Me NO to 2 gave [((
TC )Fe) O] (7) in
2
3
good yield and no intermediate formation of an Fe(IV) oxo
species could be observed, despite it being likely the initial
14
product of the reaction of 2 with trimethyl N-oxide. Complex
is also formed if a solution of 2 is exposed to air. Not
surprisingly, this makes 7 the primary impurity observed in 2,
, and 5, formed from even trace amounts of water or oxygen
7
4
impurities. However, yield and purity are usually significantly
diminished, compared to the pathway utilizing trimethyl N-
oxide. Complex 7 is completely stable toward air and moisture
30
and does not even readily act as an oxidizing agent to PPh3,
Table 1. Test Reactions with N CPh and 2, 3, and 7
yielding only 15% conversion to 5 after 4 days (Figure S15).
The structure and physical characterization of 7 are in line
with similar bridging oxos and tetracarbene complexes.
Complex 7 represents a strongly antiferromagnetic coupled
2
2
a
b
c
entry
cat. no.
load (%)/time (h)
conv. (%)
1
2
3
4
5
6
2
3
7
25/1
100
<5
<5
∼15
100
100
25/24
25/24
-/100
5/16
1
13
system which yields diamagnetic H and C NMR (Figures
S13 and S14). X-ray crystallography shows that the Fe1−O−
BMe
2
,Me
H
Fe2 angle is 180° and the “[(
TC )Fe]” fragments are
2
2
twisted by 78.6° along this axis (Figure 3). The Fe−C
distances are 1.967(3) Å on average and compare closely to
those in 3 and other reported Fe(III) tetra-NHC complexes.
1/72
a
9
General reaction conditions: Reactions were performed in CD
CN
3
or C D : 30 mg (0.15 mmol) of N CPh was used in each run,
6
6
2
2
Since 2 was not effective for catalytic oxygen transfer
reactions, we explored viable alternatives for oxidation
catalysis. Several tetra-NHC Fe(II) complexes are reported
dissolved 5 mL of the respective solvent at room temperature, and the
b
solid catalyst was added to start the reaction. Time is given after full
c
1
conversion of the substrate. Conversions were determined by H
NMR.
16,17,205
to be potent single site catalysts,
but none of those
species are reported to show any reactivity toward potential
carbene or phosphine transfer. Consequently, the reactivity of
BMe
2
,Me
H
[
(
TC )Fe] with “CR ” and “PR” could be highly novel.
acetonitrile and the initial red solution turns yellow within 1 h
at room temperature. Analysis of the products by H and C
2
1
13
For these reactions, we chose diazodiphenylmethane
3
(
(
N CPh ) and P-tert-butyl-dibenzo-7λ phosphanorbornadiene
NMR revealed the products are a mixture of three compounds:
1,2-bis(diphenylmethylene)hydrazine (DPH, 8), tetraphenyle-
thene (TPE, 9), and another unidentified compound (“X”)
forming in a 50/35/15 molecular ratio. Both products 8 and 9
are known side products in the decomposition of N CPh but
2
2
t
BuPA) (Scheme 5). While diazoalkanes have been studied
31,32
extensively for group transfer catalysis,
there are very few
However, stoichiometric
transfers of “PR” groups reported mediated by Cr or W
t
33−35
reports of reactions with BuPA.
2
2
36,37
complexes are known.
are usually only observed after prolonged heating in the
38,39
Test reactions with our new complexes showed catalytic
absence of a catalyst.
Control reactions showed no
reactivity toward the aforementioned products and also
preceded at a much slower rate (Table 1, entry 4). Lower
2
2
D
DOI: 10.1021/acs.organomet.8b00923
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Article
catalyst loadings of 5% or even 1% showed similar results,
although they naturally required prolonged reaction times to
achieve complete conversion (Table 1, entries 5 and 6; Figures
S16−S24). No reactions were observed when complex 3 or 7
was substituted for 2 (Table 1, entries 2 and 3). These
combined findings clearly support a catalytic activity of 2 with
differences between the 16- and 18-atom macrocycles, we
believe that a plethora of novel chemistry can be developed
with this new tetra-NHC ligand.
EXPERIMENTAL SECTION
■
BMe ,Me
2
H
Synthesis of (
TC )Br2 (1). 1,1′-Methylenediimidazole
N CPH but with low selectivity toward the products.
2
2
(3.85 g, 26.0 mmol) was dissolved in 150 mL of benzonitrile in a
For phosphinidene transfer (PR), Cummins has developed
BuPA and related compounds. BuPA reacts with 2 at a
2
50 mL round-bottom flask. Bromodimethylborane (6.30 g, 52.0
t
40
t
mmol) was pipetted into the benzonitrile solution, and the mixture
catalyst loading of 5 mol % and achieves full conversion of the
substrate after only 2 h at room temperature (Table 2, entry
was stirred for 15 min. A second portion of 1,1′-methylenediimidazole
(
4.45 g, 26.0 mmol) was then added into the flask. The resulting
solution was brought out of the glovebox, and the reflux condenser
t
was connected under a steady stream of N . The reaction mixture was
2
Table 2. Test Reactions of 2, 3, and 7 with BuPA
heated to 80 °C and stirred for 24 h. After 24 h, the solution was
cooled to 0 °C and filtered over a fine sintered-glass frit. The off-white
precipitate was then washed with acetonitrile (2 × 20 mL), THF (3 ×
a
b
c
entry
cat. no.
load (%)/time (h)
conv. (%)
1
2
3
4
5
6
2
3
7
10/2
10/24
10/24
-/8
10/8
1/20
100
<1
<1
<5
<5
3
0 mL), and diethyl ether (2 × 100 mL) and dried under reduced
pressure, leaving the white product (13.2 g, 95% yield). Crystals for X-
ray diffraction were grown from a solution in methanol by vapor
d
1
diffusion with diethyl ether. H NMR (DMSO-d , 499.74 MHz): δ
6
e
e
2
8.87 (s, 4H), 7.87 (s, 4H), 7.47 (s, 4H), 6.49 (s, 4H), 0.23 (s, 12H).
1
3
100
C NMR (DMSO-d , 125.66 MHz): δ 137.5, 123.3, 121.6, 57.2, 8.6.
6
a
IR: 3106, 3056, 3010, 2933, 1550, 1536, 1304, 1262, 1366, 1304,
1252, 1136, 1108, 1087, 1049, 1020 970, 950, 880, 799, 759, 712,
General reaction conditions: Reactions were performed in C D : 10
mg (0.04 mmol) of BuPA was used in each run, dissolved 1 mL of
C D at room temperature, and the catalyst was added to start the
6
6
t
−
1
2+
6
89, 665, 658 cm . DART HR MS (m/z): [M] Found: 189.1340.
6
6
b
c
Calcd: 189.1306. Anal. Calcd for C H B N Br : C, 40.20; H, 5.25;
reaction. Time is given after full conversion of the substrate. Yields
18 28
2
8
2
1
d
N, 20.80. Found: C, 39.92; H, 5.04; N, 20.64.
2 2
were determined by H NMR. Control run at 80 °C. A similar
BMe
,Me
H
BMe
,Me
H
e
BMe ,Et
H
Synthesis of [(
.56 mmol) was suspended in 150 mL of THF in a 250 mL Schlenk
flask. In a second 250 mL Schlenk flask, anhydrous FeCl (710 mg,
TC )Fe] (2). (
TC )Br (1) (3.00 g,
2
2
reaction at rt showed no conversion. [(
TC )Fe] was used as
5
catalyst. Reaction also heated to 80 °C.
2
5
.60 mmol) was suspended in 25 mL of THF, and finally, a small
n
Straus tube (25 mL) was filled with 9.1 mL of 2.5 M BuLi solution in
hexanes (22.8 mmol). All three flasks were taken out of the glovebox
and connected to a Schlenk line. The ligand suspension and the BuLi
1
). The major product of the reaction is identified as cyclo-
t
(
P Bu) (10) which is formed with very high selectivity and
3
n
1
13
31
was unambiguously identified by means of H, C, and
P
41
solution were cooled to −80 °C in an acetone/dry ice bath for 15
NMR (Figures S25−S30). As with N CPh , no reactivity was
observed between BuPA and either 3 or 7 (Table 2, entries 2
and 3). BuPA is quite stable in the absence of 2, and only
n
2
2
min. Under vigorous stirring of the solution of 1, the BuLi solution
t
was added to the ligand suspension and mixed at −80 °C for at least 1
h. The now yellow suspension was allowed to warm up to 0 °C
(cooled with an ice bath) and vigorously stirred for another hour or
until a clear yellow solution was obtained. The clear solution was then
cooled back down to −80 °C and added to the flask containing the
t
minor decomposition was found during a control reaction
(
<5%, Table 2, entry 4) after 8 h at 80 °C. Low catalyst
t
loadings of 1 mol % of 2 showed clean conversion of BuPA in
FeCl /THF suspension, which resulted in the mixture immediately
less than 1 day at room temperature (Table 2, entry 6).
2
BMe
2
,Et
H
changing its color to brownish red. The mixture was allowed to slowly
warm up to room temperature and was stirred overnight. The dark
solution was evaporated in a 30 °C water bath (vacuum <100 mTorr)
for several hours to yield a brown solid. The solid was transferred into
the glovebox and extracted with benzene (3 × 50 mL) and filtered
over a fine frit until the extract was no longer colored. The brownish-
red benzene solution was evaporated, thoroughly dried, and again
extracted with benzene at least two more times. The benzene solution
was filtered over Celite over a fine frit. The solution was concentrated
and layered with pentane. After several days, purple crystals were
collected, washed with pentane, and dried under a vacuum. The
combined yield of crystalline product was 1.55 g (3.61 mmol, 65%
Notably, the previously reported [(
TC )Fe] does not
t
show any activity in the decomposition of BuPA, not even at
8
0 °C (Table 2, entry 5), again demonstrating the distinct
differences in reactivity between two highly similar complexes.
CONCLUSION
■
In conclusion, we have synthesized a third generation of
macrocyclic tetra-NHC ligands that moves ever closer to the
structure and charge of a porphyrin but with the increased
electron donation of NHCs to the transition metal. We have
prepared iron complexes with this new 16-atom macrocyclic
ligand in modest to good yield and demonstrated that the
addition of new ligands is distinct from its previously reported
1
yield). H NMR (C D , 499.74 MHz): δ 12.8 (4H), 7.7 (16H),
6
6
−
20.0 (4H). IR: 3132, 2925, 1607, 1541, 1464, 1412, 1354, 1277,
−
1
1164, 1108, 1060, 1034, 1032, 953, 815, 797, 720, 676 cm . DART
BMe
2
,Et
H
+
1
8-atom counterpart, [(
TC )Fe]. The iron(III) species,
HR MS (m/z): [M] 430.1682 (found), 430.1659 (calcd).
BMe
2
,Me
H
BMe
2
,Me
H
[(
TC )FeBr], is highly stable and unreactive with
Synthesis of [(
TC )FeBr] (3). The synthesis of 3 was
2
BMe ,Me
H
oxidants, while the iron(II) species, [(
TC )Fe], is
performed the same way as the synthesis of 2 using anhydrous FeBr3
(
548 mg, 1.86 mmol), 1 (1.00 g, 1.86 mmol), and 3.1 mL of 2.5 M
highly reactive across a variety of group transfer reagents. In
the case of oxygen transfer, a stable bridging oxo complex is
n
BuLi solution (7.81 mmol) in hexanes. The dark green, clear solution
was evaporated in a 30 °C water bath (vacuum <100 mTorr) for
several hours to yield a dark solid. The solid was transferred into the
glovebox, extracted with benzene, and filtered over a fine frit until the
extract was no longer colored (ca. 100 mL). The greenish benzene
solution was concentrated under reduced pressure to about 10 mL.
The dark blue supernatant solution was separated from the yellow
powder formed during the evaporation of the benzene, and the
formed from either Me NO or air. On the other hand,
3
BMe
2
,Me
H
t
reactions of [(
TC )Fe] with N CPh and BuPA lead to
2 2
degradative catalysis of these reagents at room temperature.
Diazodiphenylmethane yields 1,2-bis(diphenylmethylene)-hy-
drazine and tetraphenylethene as catalytic products, while
t
t
BuPA yields exclusively cyclo-(P Bu) . Given the reactivity
3
E
DOI: 10.1021/acs.organomet.8b00923
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
residue was washed with a little benzene twice. The remaining yellow
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00923.
■
powder was redissolved in benzene and filtered over Celite. After
*
S
1
drying, the yield was 280 mg (0.55 mmol, 29%). H NMR (MeCN-d ,
3
4
99.74 MHz): δ 20.0 (2H), −0.2 (8H), −10.1 (6H), −24.7 (4H),
−
43.8 (4H). IR: 3164, 3136, 2918, 1625, 1546, 1465, 1416, 1331,
−
1
1
285, 1171, 1152, 1115, 1062, 1042, 952, 817, 797, 735, 682 cm .
+
DART HR MS (m/z): [M−CH ] 494.0605 (found), 494.0603
Additional experimental details and selected NMRs and
HR MSs (PDF)
3
(
calcd).
Synthesis of [(BMe2,MeTC )Fe(NCCH ) ] (4). Addition of 5 mL of
H
3
2
acetonitrile to 50 mg of 2 led to an immediate color change from
purple to red. Removal of the solvent in a vacuum led to the recovery
of the starting material, 2. In situ characterization clearly indicated the
Accession Codes
CCDC 1885234−1885237 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
coordination of two acetonitrile molecules in solution. H NMR
(
0
1
(
MeCN-d , 499.74 MHz): δ 7.29 (s, 4H), 7.23 (s, 4H), 6.05 (s, 4H),
3
13
.05 (s, 12H). C NMR (MeCN-d , 125.66 MHz): δ 193.4, 122.4,
3
+
19.4, 61.0, 13.4. DART HR MS (m/z): [M−NCCH ] 471.1918
3
found), 471.1925 (calcd).
Synthesis of [(BMe2,MeTC )Fe(PPh )] (5). Complex 2 (100 mg,
H
3
AUTHOR INFORMATION
Corresponding Author
E-mail: jenkins@ion.chem.utk.edu.
ORCID
David M. Jenkins: 0000-0003-2683-9157
Notes
The authors declare no competing financial interest.
■
*
0
.23 mmol) was dissolved in 5 mL of acetonitrile in a 20 mL vial. To
this purple solution, triphenyl phosphine (60. mg, 0.23 mmol) was
added, which resulted in an immediate color change to yellowish
brown. The solution was stirred for 15 min and was extracted three
times with 5 mL of pentane to remove excess triphenylphosphine.
The resulting solution was evaporated under a vacuum, and the brown
solid was extracted with benzene (3 × 3 mL). The resulting solution
was evaporated, yielding 90 mg of beige solid (0.13 mmol, 65% yield).
1
H NMR (MeCN-d , 499.74 MHz): δ 7.19 (s, 4H), 7.06 (t, J = 7.2
3
ACKNOWLEDGMENTS
Hz, 3H), 7.00 (s, 4H), 6.92 (t, J = 7.2 Hz, 6H), 6.72 (t, J = 7.2 Hz,
■
6
6
H), 5.58 (d, J = 12.3 Hz, 2H), 4.50 (d, J = 12.3 Hz, 2H), 0.20 (s,
The authors gratefully acknowledge the National Science
Foundation (NSF-CAREER/CHE-1254536) for financial
support of this work. The University of Tennessee also
provided additional financial support for this work via the X-
ray facility.
13
H), −0.14 (s, 6H). C NMR (MeCN-d , 125.66 MHz): δ 196.0 (d,
3
J = 22.5 Hz), 133.5 (d, J = 21.0 Hz), 131.5 (d, J = 8.5 Hz), 127.5,
3
1
1
2
1
6
4
27.3(d, J = 7.6 Hz), 123.5, 119.6, 60.5, 25.3. P NMR (MeCN-d3,
02.40 MHz): δ 78.7. IR: 3130, 3054, 2926, 1585, 1463, 1433, 1415,
339, 1279, 1267, 1168, 1137, 1108, 1059, 1028, 950, 815, 800, 740,
−
1
+
92 cm . DART HR MS (m/z): [M−PPh ] 430.1649 (found),
3
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■
30.1659 (calcd) [PPh +H] 260.0993 (found), 263.0990 (calcd).
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BMe
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F
DOI: 10.1021/acs.organomet.8b00923
Organometallics XXXX, XXX, XXX−XXX

Organometallics
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DOI: 10.1021/acs.organomet.8b00923
Organometallics XXXX, XXX, XXX−XXX