Pyrene-Connected Tetraimidazolylidene Complexes of Iridium and Rhodium. Structural Features and Catalytic Applications

Article abstract of DOI:10.1021/acs.organomet.8b00633

A pyrene-connected tetra-imidazolium salt has been prepared starting from commercially available 1,3,6,8-tetrabromopyrene, and used as tetra-NHC precursor in the preparation of tetranuclear Rh(I) and Ir(I) complexes. The tetra-NHC ligand displays axial ch

Full text of DOI:10.1021/acs.organomet.8b00633

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Pyrene-Connected Tetraimidazolylidene Complexes of Iridium and
Rhodium. Structural Features and Catalytic Applications
́
Ana Gutierrez-Blanco, Eduardo Peris, and Macarena Poyatos*
́
Institute of Advanced Materials (INAM), Universitat Jaume I, Av. Vicente Sos Baynat s/n, Castellon, E-12071, Spain
S
* Supporting Information
ABSTRACT: A pyrene-connected tetra-imidazolium salt has been prepared starting from
commercially available 1,3,6,8-tetrabromopyrene, and used as tetra-NHC precursor in the
preparation of tetranuclear Rh(I) and Ir(I) complexes. The tetra-NHC ligand displays axial
chirality upon coordination to the MCl(cod) (M = Rh and Ir) fragments, giving rise to right-
and left-handed helix conformations. The catalytic activity of the resulting complexes was
studied in two relevant reactions that lead to the formation of five- and six-membered oxygen-
containing heterocycles, namely, the cyclization of acetylenic carboxylic acid and the coupling
of diphenylcyclopropenone with substituted phenylacetylenes.
rhodium,6 iridium,7 and gold8 complexes, we further explored
the importance of π-stacking interactions in homogeneously
catalyzed reactions. The inherent capability of pyrene to afford
INTRODUCTION
■
The widespread use of N-heterocyclic carbene (NHC) ligands
arises from their extraordinary stereoelectronic versatility and
π-stacking interactions with graphitic surfaces also allowed to
their capability to incorporate a wide variety of additional
support pyrene-tagged complexes,6,7,9 such as complex B in
functional groups.1 The presence of these additional functions
Chart 1,7 onto reduced graphene oxide and to study the
makes the resulting complexes good candidates for undergoing
activity and recyclability properties of the resulting hetero-
supramolecular interactions since they may establish reversible
genized catalysts. Additionally, and taking advantage of the
noncovalent interactions with the other reaction partners
fluorescent properties of pyrene, we prepared NHC-based
(substrate, additive, and counterion). The introduction of
these interactions by design is an important tool for modifying
the properties of a metal-based catalyst, and constitutes the
complexes with interesting photophysical properties.10
More recently, we prepared a tetra-Au(I) complex
basis of supramolecular catalysis.2 In addition, over the past
connected by 1,3,6,8-tetraethynylpyrene, which turned out to
be one of the most efficient Au-based fluorescence emitters in
years, discrete supramolecular complexes held together by M−
solution reported to date (C, Chart 1).11
CNHC bonds have become of interest, and this required the
All of these findings illustrate how pyrene-adorned NHC-
metal complexes constitute an interesting family of materials
preparation of poly-NHC ligands with suitable topologies that
offer the possibility of forming self-assembled structures.3
with unusual photophysical and catalytic properties. In this
new work, we report the preparation of a pyrene-connected
tetra-imidazolium salt and its use as tetra-NHC precursor in
the preparation of rhodium and iridium complexes. The
catalytic properties of the rhodium and iridium complexes were
studied in the cyclization of acetylenic carboxylic acids and in
the cycloaddition of diphenylcyclopropenone and alkynes.
Our group has been particularly interested in studying the
catalytic behavior of NHC-based complexes bearing extended
polyaromatic systems as additional functions. We demon-
strated that, due to the ability of polyaromatic groups to afford
π-stacking noncovalent interactions, their catalytic properties
clearly differ from those shown by analogues lacking these
polyaromatic systems.4 In line with this, a series of NHC-based
complexes containing pyrene in their structure were prepared.
Employing pyrene-containing palladium5 (A, Chart 1), nickel,5
RESULTS AND DISCUSSION
■
The pyrene-tetra-imidazolium salt 2 was prepared by a three-
step procedure starting from commercially available 1,3,6,8-
tetrabromopyrene, as displayed in Scheme 1. Treatment of
1,3,6,8-tetrabromopyrene with 4 equiv of imidazole in the
presence of CuI and K2CO3 in refluxing DMF resulted in the
formation of neutral tetraimidazolyl-pyrene compound 1.
Chart 1
Received: August 30, 2018
© XXXX American Chemical Society
A
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Article
indicates that the symmetry of the complex in solution is C2,
rather than that expected for a 1,3,6,8-(symmetrically)-
tetrasubstituted pyrene (C2v). As a result of this loss of
symmetry, complexes 3 and 4 feature two different sets of
metalated carbon atoms, thus showing two carbene carbon
resonances in their 13C NMR spectra [185.5 and 183.6 ppm
(1JRh‑C = 51 Hz) for 3, and 182.4 and 181.2 ppm for 4]. We did
Scheme 1. Synthesis of Tetra-imidazolium Salt 2
1
not observe changes in the H NMR spectra of complexes 3
and 4 upon performing VT-NMR experiments in CDCl3.
In order to estimate the electron-donating character of the
tetra-NHC ligand, we transformed tetra-iridium complex 4
into its carbonyl derivative 5 (Scheme 2) by bubbling carbon
monoxide through a solution of the complex in dichloro-
methane at 0 °C. Complex 5 displays only one carbene-carbon
resonance at 180.7 ppm in the 13C NMR spectrum, indicating
that the rotation about the Ir−Ccarbene bond is no longer
restricted, and thus suggesting a pseudo-C2v symmetry for the
complex in solution. Complex 5 displays two strong CO
stretching bands at 1977 and 2064 cm−1, which allowed us to
calculate the Tolman Electronic Parameter (TEP) as 2047
cm−1, by using well-known correlations.13 For comparative
purposes, we prepared the complex [1-n-butyl-3-phenyl-
imidazol-2-ylidene]dicarbonylchlororhodium (7, Scheme 3),
Compound 1 was isolated as a highly insoluble dark green
solid, in almost quantitative yield.
The N-quaternization of the four imidazole rings of
compound 1 with n-BuI and subsequent anion metathesis
using [NH4](PF6) in methanol afforded the hexafluorophos-
phate tetra-imidazolium salt 2. Compound 2 was isolated as a
dark brown solid, in a 65% overall yield.
The 1H NMR spectrum in acetone-d6 of the pyrene-
containing tetra-imidazolium salt 2 shows the resonance due to
the four equivalent acidic protons of the NCHN groups at 9.7
ppm. The 13C NMR spectrum shows the resonance due to the
carbon atoms of the NCHN groups at 139.1 ppm. The ESI-MS
spectrum (positive ions) of 2 exhibits an intense peak at m/z
279.9, which was assigned to [M − 3(PF6)]3+.
We initially considered that compound 2 could be used as
the precursor of a bis-CNHCCCNHC-pincer ligand, in which the
two potential pincer arms are linearly opposed and connected
by an aromatic skeleton. However, all our attempts to achieve
this coordination mode to Rh(III) and Ir(III) by the
Scheme 3. Synthesis of Monometallic Complexes 6 and 7
12
metalation/transmetalation strategy using Zr(NMe2)4 were
unsuccessful. Instead, the reaction of 2 with [MCl(cod)]2 (M
= Rh or Ir) afforded the tetrametallic complexes 3 and 4
(Scheme 2). Complexes 3 and 4 were characterized by NMR
which can be regarded as the mono-rhodium counterpart of 5.
For this complex, we found a TEP value of 2045 cm−1, thus
indicating that the electron-donating strength of the tetra-
NHC ligand in 5 is similar to that shown by 1-n-butyl-3-
phenylimidazol-2-ylidene (in 7).
Scheme 2. Synthesis of Tetrametallic Complexes 3−5
The X-ray diffraction studies confirmed the molecular
structure of complex 4 and revealed the presence of two
independent molecules in the asymmetric unit. Each molecule
consists of four n-butyl-imidazolylidene-Rh(I)Cl(cod) units
connected by the pyrene core. The two molecules are
enantiomers as, upon coordination, the tetra-NHC ligand
displays axial chirality. The top view of one of the enantiomers
is given in Figure 1 (top), and shows a helical (or screw-
shaped) geometry, giving rise to a left-handed helix
conformation.
As can be observed from the side view of the molecular
structure of 4 (Figure 1, bottom), the two metal units at the
pyrene 1- and 6-positions are above the plane formed by the
pyrene central unit, placing the chloride ligands in close
proximity to the protons at the 2- and 7-positions, at an
average distance of 2.84 Å. In a similar manner, the two metal
units at the pyrene 3- and 8-positions are below the pyrene
plane, placing the chloride atoms in close proximity to the
protons at the 4- and 9-positions, at an average distance of 2.90
Å. This is in agreement with our observations in the 1H NMR
of complexes 3 and 4, and indicate the presence of weak
hydrogen-bonding interactions, which are also evident in the
solid state. The imidazolylidene rings deviate from the plane of
the pyrene core by an average angle of 64°. The Supporting
Information includes different perspective views of the other
enantiomer of 4.
spectroscopy, electrospray mass spectrometry, and elemental
1
analysis. Both the H and 13C NMR spectra are in agreement
with the two-fold symmetry of the complexes, and with the
presence of only one of the possible rotational isomers. The
observed tetra-NHC ligand/cod ratio obtained from 1H NMR
integration along with the positive ion ESI-MS signals at 802.9
and 981.7 m/z, assigned to [M − 2Cl]2+, confirmed the
formation of the tetranuclear complexes 3 and 4, respectively.
Whereas the presence of a singlet at 8.49 ppm indicates the
equivalency of four of the protons of the pyrene core of the
tetra-imidazolium salt 2, these protons are no longer equivalent
in complexes 3 and 4, due to the restricted rotation about the
Cpyr−Nimid and the M−Ccarbene bonds, upon coordination of
the MCl(cod) fragment. These resonances appear as two
doublets at 9.85 and 8.11 ppm (complex 3) and at 9.35 and
7.97 ppm (complex 4). One of the two doublets is downfield
shifted, thus strongly suggesting the presence of weak
hydrogen-bonding interactions with the chloride ligands, as
illustrates the drawing of 3 and 4 in Scheme 2. This situation
B
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Table 1. Catalytic Cyclization of Acetylenic Carboxylic
a
Acids
cat. loading
(mol %)
time
(h)
conversion
b
entry
substrate
cat.
(%)
1
2
3
4
5
6
7
8
10
11
12
13
14
15
16
17
4-pentynoic acid
4-pentynoic acid
4-pentynoic acid
4-pentynoic acid
4-pentynoic acid
4-pentynoic acid
4-pentynoic acid
4-pentynoic acid
4-pentynoic acid
4-pentynoic acid
4-pentynoic acid
5-hexynoic acid
5-hexynoic acid
5-hexynoic acid
5-hexynoic acid
5-hexynoic acid
5-hexynoic acid
3
3
3
3
4
4
4
4
6
6
6
3
3
4
4
6
6
1
0.1
0.01
0.001
1
2.5
21
21
21
2.5
4
21
21
21
21
>99
>99
35
16
>99
>99
>99
9
>99
>99
12
83
0
79
21
0.1
0.01
0.001
0.4
0.04
0.004
0.1
0.01
0.1
0.01
0.4
21
192
216
192
192
192
216
31
0
18
0.04
a
Reaction conditions: in an NMR tube, 0.5 mmol acetylenic acid,
b
Figure 1. Top and side views of one of the enantiomers of 4. Solvent
(CHCl3 and CH2Cl2) and hydrogen atoms have been removed for
clarity. n-Butyl substituents and 1,5-cylooctadiene ligands are shown
in the wireframe form. Selected distances (Å): Ir1−C1 2.07(4), Ir2−
C2 1.84(4), Ir3−C3 2.07(5), Ir4−C4 1.88(4).
catalyst 3, 4, or 6, CD3CN (0.75 mL), 80 °C. Conversions were
determined by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene
as internal standard.
resulted much less efficient; complexes 3 and 4 required 8 days
of reaction to afford 80% conversion (entries 13 and 15).
In order to study if the use of tetrametallic complexes
produced an improvement in the catalytic efficiencies
compared to those provided by their monometallic analogues,
we prepared the mono-rhodium complex [1-n-butyl-3-phenyl-
imidazol-2-ylidene](1,5-cyclooctadiene)chlororhodium, 6
(Scheme 3). Unfortunately, our attempts to isolate a pure
sample of the mono-iridium complex were unsuccessful.
In the cyclization of 4-pentynoic acid, the mono-rhodium
complex 6 performed better than its tetrametallic counterpart
(complex 3) when using catalyst loadings of 0.4 and 0.04 mol
% based on the concentration of the metal. Nevertheless, using
a catalyst loading of 0.004 mol %, the two catalysts led to
similar outcomes (compare entries 4 and 12). In order to
explain this behavior, we studied the time-dependent reaction
profiles of the cyclization of 4-pentynoic acid using different
concentrations of 3, and we determined the reaction order
with respect to the catalyst, by plotting the concentration of
the product against a normalized time scale t[cat]n (being n the
order of the catalyst).17 The power value gives the right
reaction order with respect to the catalyst when the corrected
conversion curves overlay. Four catalyst loadings of 1, 0.1, 0.01,
and 0.001 mol % were employed to determine the reaction
order in catalyst.
In order to evaluate the catalytic activity of the isolated
complexes, we selected organic transformations typically
catalyzed by Rh(I), namely, the cyclization of acetylenic
carboxylic acids14 and the synthesis of cyclopentadienones.15
These two reactions are of practical relevance to the
pharmaceutical industry because they allow the formation of
five- and six-membered oxygen-containing heterocycles. We
believe that these two reactions constitute two excellent
models for the comparison of the activity of our new
complexes with the related mononuclear ones, in order to
evaluate if the presence of the four metal units in the catalyst,
or the presence of the pyrene core, have any relevant role in
the activity of our systems.
Complexes 3 and 4 were first tested in the intramolecular
cyclization of 4-pentynoic and 5-hexynoic acid, which leads to
the formation of five- and six-membered rings, respectively.
The reactions were performed in an NMR tube containing
0.75 mL of CD3CN with different catalyst loadings (10−3−1
mol %), at 80 °C.
The results shown in Table 1, indicate that the iridium
complex outperforms the rhodium one, both in the cyclization
of 4-pentynoic and 5-hexynoic acids. Complexes 3 and 4
achieved full conversion to γ-methylene-γ-butyrolactone after
2.5 h, using a catalyst loading of 1 mol %. With a catalyst
loading of 0.1 mol % (0.4 mol % based on the concentration of
metal), full conversion was achieved by complex 4 in 4 h,
whereas a longer reaction time (21 h) was required for
complex 3 (compare entries 2 and 6). At a catalyst loading of
0.01 mol % (0.04 mol % based on metal), the iridium complex
achieved full conversion after 21 h (entry 7). In agreement
with previous reports,14c,d,16 the cyclization of 5-hexynoic acid
Visual analysis of the reaction profiles depicted in Figure 2
indicates that the order in catalyst is 1 (Figure 2b), although
the curve for the higher concentration (1 mol %) is clearly out
of the fit. This result is explained as a consequence of the low
solubility of catalyst 3 in acetonitrile (the same applies to the
iridium catalyst 4; see the Supporting Information for details),
and therefore, once the saturation concentration has been
C
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cycloaddition reaction of cyclopropenones and alkynes (Table
2). This reaction, first reported by Wenders and co-workers,
Table 2. [3 + 2] Cycloaddition Reactions of
Diphenylcyclopropenone with Substituted
a
Phenylacetylenes
b
entry
R
cat. cat. loading (mol %) time (h) conversion (%)
1
2
3
4
5
6
7
8
9
CO2Et
CO2Et
CO2Et
CH3
3
3
3
3
3
4
4
4
4
4
1
24
24
48
48
48
24
24
24
24
24
72
60
71
64
30
95
75
92
44
30
0.5
0.1
0.1
0.1
1
0.5
0.1
0.1
0.1
H
CO2Et
CO2Et
CO2Et
CH3
10
H
a
Reaction conditions: 0.225 mmol of diphenylcyclopropenone, 0.15
mmol of alkyne, catalyst 3 or 4, toluene-d8 (0.3 M), 110 °C.
b
Conversions were determined by 1H NMR spectroscopy using 1,3,5-
trimethoxybenzene as internal standard.
constitutes a selective and high-yielding route to valuable
cyclopentadienones (CPDs).15a Using 1 mol % of Rh(I)
complex 3, the [3 + 2] cycloaddition of diphenylcyclo-
propenone and ethylpropionate gave the corresponding CPD
in 72% after 24 h at 110 °C (entry 1). The catalytic activity of
3 compares well with those given by other Rh(I) complexes
described in the literature, bearing NHC15b and cyclic(amino)-
(aryl)carbene ligands.15c
In all cases, the tetra-iridium complex 4 showed higher
catalytic activity than 3 in the reaction of diphenylcyclo-
propenone with ethylphenylpropiolate (compare entries 1−3
and 6−8), and constitutes one of the best catalysts described
to date for this type of transformation. As well as 3, 4 showed
moderate catalytic activity in the reactions using 1-phenyl-1-
propyne and phenylacetylene.
Figure 2. (a) Time-dependent reaction profile of the cyclization
reaction of 4-pentynoic acid using catalyst 3. (b) Reaction profile with
normalized time scale assuming a catalyst order of 1/2. (c) Reaction
profile with normalized time scale assuming a catalyst order of 1.
Reaction conditions are the same as those shown in Table 1. In all
three graphics, the evolution is shown as consumption of 4-pentynoic
acid. Lines are only to guide the eye.
CONCLUSIONS
■
In summary, we described a new pyrene-tetra-imidazolium salt
that we used for the preparation of a series of Rh(I) and Ir(I)
pyrene-tetra-NHC complexes. The coordination of MCl(cod)
units results in the formation of helix-type structures
dominated by the restricted rotation about the Cpyr−Nimid
and M−CNHC bonds, which yield hydrogen-bonding inter-
actions between the chloride ligands and the protons of the
pyrene connector. The two complexes were tested in the
cyclization of acetylenic carboxylic acids, and in the reaction of
diphenylcyclopropenone with substituted phenylacetylenes to
form cyclopentadienones. In the first of these two reactions,
the activity of the tetrametallic catalysts is similar to the activity
shown by a monometallic analogue, only at low concentrations.
At higher concentrations, the monometallic complex outper-
forms the tetrametallic one. This behavior is explained as a
consequence of the low solubility of the tetrametallic catalyst,
which leads to a maximum concentration of the catalyst when
the saturation of the solution has been reached. In the coupling
of diphenylcyclopropenone with substituted phenylacetylenes,
reached, increasing the amount of catalyst added to the
reaction media does not produce any improvement in the
reaction rate. This result clearly contrasts with our previous
findings using other pyrene-containing ligands, for which
fractional reaction orders were observed, as a consequence of
the formation of nonactive dimers by π-stacking self-
association of the catalyst.7,8 The different solubility of 3 and
6 explains why, at higher concentrations, the monometallic
complex outperforms the tetrametallic one, while, at lower
concentrations, both catalysts display similar activity, as
expected for two complexes with quasi-identical stereo-
electronic properties. Furthermore, the first-order reaction
found with respect to the concentration of 3 (and 4) suggests
that the cooperativity between the metal units in both
tetrametallic catalysts is negligible.
In order to widen the study on the catalytic activities of
complexes 3 and 4, we decided to test them in the [3 + 2]
D
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isolated as a highly insoluble dark green solid. Yield: 452.0 mg
(>99%). 1H NMR (500 MHz, DMSO-d6): δ 8.43 (br s, 2H, CHpyrene),
8.27 (br s, 4H, NCHN), 8.08 (br s, 4H, CHpyrene), 7.85 (br s, 2H,
CHimid), 7.36 (br s, 2H, CHimid). Electrospray MS (cone 20 V) (m/z,
fragment): 467.2 [M + H]+ (calc. for [M + H]+: 467.2).
the tetra-iridium complex (4) shows higher activity than the
tetra-rhodium one (3). Although there is clearly room for
improvement, to the best of our knowledge, complex 4 is the
first iridium complex tested in this reaction.
Synthesis of the Tetra-imidazolium Salt 2. Under aerobic
conditions, a mixture of compound 1 (517 mg, 1.11 mmol, 1
equiv) and n-BuI (16 mL, 89 mmol, 80 equiv) was heated in a thick-
walled Schlenk tube fitted with a Teflon cap at 100 °C for 72 h. After
cooling at room temperature, the excess of n-BuI was distilled under
vacuum. The resulting black solid residue was washed several times
with diethyl ether and ethyl acetate, and collected by filtration. The
iodide salt (500 mg, 0.42 mmol, 1 equiv) was dissolved in MeOH (15
mL) and treated with [NH4](PF6) (342.3 mg, 2.10 mmol, 5 equiv).
The suspension was heated at 40 °C overnight. Compound 2 was
EXPERIMENTAL SECTION
■
General Considerations. Anhydrous solvents were dried using a
solvent purification system (SPS M BRAUN) or purchased and
degassed prior to use by purging them with dry nitrogen. All the
reagents and solvents were used as received from commercial
suppliers. Column chromatography was performed on silica gel
Merck 60, 62−200 mm unless otherwise stated, using mixtures of
solvents. NMR spectra were recorded on a Varian Innova 500 MHz or
on a Bruker 400/300 MHz, using DMSO-d6, acetone-d6, or CDCl3 as
solvent. Elemental analyses were carried out on a TruSpec Micro
Series. Infrared spectra (FT-IR) were performed on a Bruker
EQUINOX 55 spectrometer with a spectral window of 4000−600
cm−1. Electrospray Mass Spectra (ESI-MS) were recorded on a
Micromass Quatro LC instrument. MeOH, CH3CN, or CH2Cl2 was
used as mobile phase, and nitrogen was employed as the drying and
nebulizing gas.
1
collected by filtration as a brown solid. Yield: 885 mg (71%). H
NMR (400 MHz, acetone-d6): δ 9.72 (s, 4H, NCHN), 9.06 (s, 2H,
CHpyr), 8.49 (s, 4H, CHpyr), 8.31 (s, 4H, CHimid), 8.26 (s, 4H,
3
CHimid), 4.62 (t, JH‑H = 8 Hz, 8H, NCH2CH2CH2CH3), 2.18−2.10
(m, 8H, NCH2CH2CH2CH3), 1.58−1.49 (m, 8H, NCH2CH2CH2-
3
CH3), 1.02 (t, JH‑H = 8 Hz, 12H, NCH2CH2CH2CH3). 19F{1H}
NMR (376 MHz, CDCl3): δ −72.3 (d). 31P{1H} NMR (162 MHz,
CDCl3): δ −144.4 (m). 13C{1H} NMR (100 MHz, acetone-d6): δ
139.1 (NCHN), 131.2 (Cq pyr), 128.8 (Cq pyr), 126.9 (CHpyr), 126.4
(CHpyr), 125.9 (CHimid), 125.2 (Cq pyr), 124.8 (CHimid), 51.3
(NCH2CH2CH2CH3), 32.5 (NCH2CH2CH2CH3), 20.2 (NCH2-
CH2CH2CH3), 13.7 (NCH2CH2CH2CH3). Electrospray MS (cone
20 V) (m/z, fragment): 173.7 [M − 4(PF6)]4+, 279.9 [M −
3(PF6)]3+, 492.3 [M − 2(PF6))]2+ (calcd. for [M − 4(PF6)]4+: 173.6,
[M − 3(PF6)]3+: 279.8, [M − 2(PF6))]2+: 492.2). Anal. Calcd for
C44H54N8P4F24·2CH2Cl2 (1444.68): C, 38.24; H, 4.05; N, 7.76.
Found: C, 38.43; H, 3.92; N, 7.82.
Synthesis of the Rh(I) and Ir(I) Complexes. Synthesis of 1-n-
Butyl-3-phenylimidazolium Hexafluorophosphate. Imidazole (1.3
g, 19.6 mmol, 4 equiv), CuSO4 (196 mg, 1.23 mmol, 0.25 equiv), and
K2CO3 (2 g, 14.7 mmol, 3 equiv) were placed together in a high
pressure Schlenk tube fitted with a Teflon cap. The tube was
evacuated and filled with nitrogen three times. Iodobenzene (0.56
mL, 4.9 mmol, 1 equiv) was added, and the resulting suspension was
heated at 150 °C for 24 h. After this time, the resulting solid was
washed with water (3 × 40 mL) and filtrated using a Buchner. The
̈
solid was extracted with MeOH (3 × 20 mL). 1-Phenylimidazole,
which was isolated as an oil after removal of the volatiles, was
employed in the next step without further purification. Yield: 400 mg
(57%). A mixture of 1-phenylimidazole (400 mg, 2.8 mmol, 1 equiv)
and n-BuI (0.3 mL, 2.8 mL, 1 equiv) in dry THF (20 mL) was heated
at 110 °C in a high pressure Schlenk tube fitted with a Teflon cap,
during 12 h. After removal of the volatiles, 1-n-butyl-3-phenyl-
imidazolium iodide was isolated as a brown oil. A mixture of 1-n-
butyl-3-phenylimidazolium iodide (885 mg, 2.7 mmol, 1 equiv) and
[NH4](PF6) (550 mg, 3.4 mmol, 1.25 equiv) in MeOH (15 mL) was
heated at 40 °C overnight. After removal of the volatiles, the crude
was washed with dichloromethane and the insoluble salts were
separated by filtration. The desired product was isolated as an off-
white solid after precipitation in a mixture dichloromethane/diethyl
ether. Yield: 900 mg (93%). 1H NMR (400 MHz, CDCl3): δ 9.08 (s,
1H, NCHN), 7.61−7.48 (m, 7H; 5H, CHphenyl and 2H, CHimid), 4.36
Synthesis of the Tetrametallic Complexes 3−5. General
Procedure for Complexes 3 and 4. Compound 2 (1 equiv) was
placed in a Schlenk tube. The tube was evacuated and filled with
nitrogen three times. The solid was suspended in dry CH3CN and
NEt3 (50 equiv) was added to the suspension. The resulting solution
was heated at 40 °C for 45 min. The corresponding metal precursor
(2.2 equiv of [RhCl(cod)]2 or [IrCl(cod)]2) was placed in a second
Schlenk tube. The tube was evacuated and filled with nitrogen three
times. The solid was then suspended in dry CH3CN and subsequently
cannulated over the first Schlenk. The resulting mixture was heated
under reflux overnight. Once at room temperature, the resulting
yellow precipitate was collected by filtration. The crude product was
purified by column chromatography. Elution with a 9:1 CH2Cl2/
acetone mixture afforded a bright red band that contained the desired
complex.
3
Synthesis of 3. A mixture of [RhCl(cod)]2 (127.8 mg, 0.260
mmol) in dry CH3CN (10 mL) was cannulated to a suspension of 2
(150 mg, 0.118 mmol) and NEt3 (0.82 mL, 5.9 mmol) in dry CH3CN
(30 mL). After the general workup, complex 3 was isolated as a bright
yellow solid. Yield: 107.1 mg (54%). 1H NMR (300 MHz, CDCl3): δ
9.86 (d, 3JH‑H = 7 Hz, 2H, CHpyr), 8.98 (s, 2H, CHpyr), 8.15 (d, 3JH‑H
(t, JH‑H = 8 Hz, 2H, NCH2CH2CH2CH3), 1.98−1.91 (m, 2H,
NCH2CH2CH2CH3), 1.46−1.40 (m, 2H, NCH2CH2CH2CH3), 0.98
(t, 3JH‑H = 8 Hz, 3H, NCH2CH2CH2CH3). 19F{1H} NMR (376 MHz,
CDCl3): δ −72.3 (d). 31P{1H} NMR (162 MHz, CDCl3): δ −144.32
(m). 13C{1H} NMR (100 MHz, CDCl3): δ 134.4 (NCHN), 134.3
(Cq phenyl), 130.7 (CHphenyl), 127.8 (CHphenyl), 123.1 (CHimid), 122.3
(CHphenyl), 121.5 (CHimid), 50.6 (NCH2CH2CH2CH3), 31.9 (NCH2-
CH2CH2CH3), 19.5 (NCH2CH2CH2CH3), 13.4 (NCH2CH2CH2-
CH3). Electrospray MS (cone 20 V) (m/z, fragment): 201.2 [M −
PF6]+ (calcd. for [M − PF6]+: 201.3). Anal. Calcd for C13H17N2PF6·
CH2Cl2 (431.18): C, 39.00; H, 4.44; N, 6.50. Found: C, 39.42; H,
4.18; N, 6.99.
3
= 1.5 Hz, 2H, CHimid), 8.11 (d, JH‑H = 7 Hz, 2H, CHpyr), 7.40 (d,
3JH‑H = 1.5 Hz, 2H, CHimid), 7.21 (d, 3JH‑H = 1.5 Hz, 2H, CHimid), 7.20
3
(d, JH‑H = 1.5 Hz, d, 2H, CHimid), 5.27−5.21 (m, 4H, NCH2-
CH2CH2CH3), 4.88−4.78 (m, 8H, CHcod), 4.71−4.61 (m, 8H,
CHcod), 4.50−4.43 (m, 4H, NCH2CH2CH2CH3), 3.35 (q, 3H,
CH2 cod), 3.29 (q, 3H, CH2 cod). The rest of the signals corresponding
to the n-butyl and cod ligands are displayed in the region between
2.50 and 0.30 ppm. 13C{1H} NMR (75 MHz, CDCl3): δ 185.5 (d,
Synthesis of the Neutral Precursor 1. A mixture of 1,3,6,8-
tetrabromopyrene (500 mg, 0.97 mmol, 1 equiv), imidazole (265.6
mg, 3.86 mmol, 4 equiv), K2CO3 (1079 mg, 7.73 mmol, 8 equiv), and
CuI (73.6 mg, 0.39 mmol, 0.4 equiv) were placed together in a high
pressure Schlenk tube fitted with a Teflon cap. The tube was
evacuated and filled with nitrogen three times. The solids were
suspended in anhydrous DMF (12 mL), and the resulting solution
was heated at 160 °C for 72 h. Then, the reaction mixture was allowed
to reach room temperature. Distilled water (75 mL) was added, and
the suspension was stirred for 2 h. The resulting solid was collected by
1
1
Rh-Ccarbene, JRh‑C = 51 Hz), 183.6 (d, Rh-Ccarbene, JRh‑C = 51 Hz),
134.4 (Cq pyr), 134.4 (Cq pyr), 128.1 (CHimid), 127.6 (CHimid), 127.3
(Cq pyr), 127.0 (Cq pyr), 125.5 (CHimid), 125.1 (Cq pyr), 123.9 (CHimid),
122.1 (CHpyr), 121.8 (CHpyr), 121.7 (CHpyr), 98.3 (d, Rh-CHcod
,
1
1JRh‑C = 6.75 Hz), 98.1 (d, Rh-CHcod, JRh‑C = 6.75 Hz), 97.5 (d, Rh-
CHcod, 1JRh‑C = 7.5 Hz), 96.9 (d, Rh-CHcod, 1JRh‑C = 7.5 Hz), 69.7 (d,
Rh-CHcod, 1JRh‑C = 14.25 Hz), 69.4 (d, Rh-CHcod, 1JRh‑C = 14.25 Hz),
68.7 (d, Rh-CHcod, 1JRh‑C = 13.5 Hz), 66.6 (d, Rh-CHcod, 1JRh‑C = 13.5
Hz), 52.0 (NCH2CH2CH2CH3), 51.1 (NCH2CH2CH2CH3), 35.06
filtration using a Buchner and washed with water. Compound 1 was
̈
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DOI: 10.1021/acs.organomet.8b00633
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(CH2 cod), 33.1 (NCH2CH2CH2CH3), 33.1 (NCH2CH2CH2CH3),
31.5 (CH2 cod), 31.0 (CH2 cod), 30.5 (CH2 cod), 29.7 (CH2 cod), 28.6
(CH2 cod), 28.4 (CH2 cod), 27.1 (CH2 cod), 20.3 (NCH2CH2CH2CH3),
20.3 (NCH2CH2CH2CH3), 14.1 (NCH2CH2CH2CH3), 14.0 (NCH2-
CH2CH2CH3). Electrospray MS (cone 20 V) (m/z, fragment): 802.9
[M − 2Cl]2+ (calcd. for [M − 2Cl]2+: 802.2). Anal. Calcd for
C76H98N8Rh4Cl4·CH2Cl2 (1762.03): C, 52.49; H, 5.72; N, 6.36.
Found: C, 51.95; H, 5.86; N, 6.97.
5.11 (m, 1H, CHcod), 4.87−4.77 (m, 1H, NCH2CH2CH2CH3), 4.44−
4.34 (m, 1H, NCH2CH2CH2CH3), 3.42−3.37 (m, 1H, CHcod), 2.78−
2.73 (m, 1H, CHcod), 2.32−2.22 (m, 2H, CH2 cod), 2.11−1.99 (m, 2H,
CH2 cod), 1.96−1.71 (m, 4H, CH2 cod and 2H, NCH2CH2CH2CH3),
3
1.57−1.46 (m, 2H, NCH2CH2CH2CH3), 1.06 (t, JH‑H = 12 Hz, 3H,
NCH2CH2CH2CH3). 13C{1H} NMR (100 MHz, CDCl3): δ 182.2 (d,
Rh-Ccarbene
(CHPh), 124.7 (CHPh), 121.9 (CHimid), 120.9 (CHimid), 95.7 (d, Rh-
,
1JRh‑C = 49 Hz), 140.5 (Cq Ph), 128.8 (CHPh), 127.9
1
1
Synthesis of 4. A mixture of [IrCl(cod)]2 (137.6 mg, 0.173 mmol)
in dry CH3CN (8 mL) was cannulated to a suspension of 2 (100 mg,
0.079 mmol) and NEt3 (0.55 mL, 3.925 mmol) in dry CH3CN (20
mL). After the general workup, complex 4 was isolated as a bright
CHcod, JRh‑C = 7 Hz), 72.1 (d, Rh-CHcod, JRh‑C = 14 Hz), 71.3 (d,
Rh-CHcod
,
1JRh‑C = 14 Hz), 52.0 (NCH2CH2CH2CH3), 33.0
(CH2 cod), 32.4 (NCH2CH2CH2CH3), 31.2 (CH2 cod), 29.8 (CH2 cod),
29.2 (CH2 cod), 20.3 (NCH2CH2CH2CH3), 14.1 (NCH2CH2CH2-
CH3). Electrospray MS (cone 20 V) (m/z, fragment): 411.3 [M −
Cl]+ (calcd. for [M − Cl]+: 411.1). Anal. Calcd for C21H28N2RhCl·
2CH2Cl2 (616.68): C, 44.80; H, 5.23; N, 4.54. Found: C, 44.67; H,
5.39; N, 4.83.
1
orange solid. Yield: 87.7 mg (54%). H NMR (300 MHz, CDCl3): δ
3
9.36 (d, JH‑H = 9.5 Hz, 2H, CHpyr), 8.82 (s, 2H, CHpyr), 7.98 (d,
3JH‑H = 9.5 Hz, 2H, CHpyr), 7.92 (d, 3JH‑H = 1.5 Hz, 2H, CHimid), 7.41
(d, 3JH‑H = 1.5 Hz, 2H, CHimid), 7.23 (d, 3JH‑H = 1.5 Hz, 2H, CHimid),
3
7.20 (d, JH‑H = 1.5 Hz, 2H, CHimid), 5.01−4.94 (m, 4H,
Synthesis of 7. CO gas was passed through a solution of complex 6
(33 mg, 0.074 mmol) in CH2Cl2 (10 mL) at 0 °C, for 30 min. After
this time, the solvent was removed under vacuum. The corresponding
carbonyl derivative was obtained as a yellow solid in quantitative
yield. Complex 7 was found unstable in solution as well as in the solid
NCH2CH2CH2CH3), 4.55−4.44 (m, 8H, CHcod and 4H, NCH2-
CH2CH2CH3), 4.32−4.27 (m, 8H, CHcod), 3.03−3.00 (m, 3H,
CH2 cod), 2.92−2.88 (m, 3H, CH2 cod). The rest of the signals
corresponding to the n-butyl and cod ligands are displayed in the
region between 2.15 and 0.10 ppm. 13C{1H} NMR (100 MHz,
CDCl3): δ 182.4 (Ir-Ccarbene), 181.2 (Ir-Ccarbene), 133.7 (Cq pyr), 133.5
(Cq pyr), 128.4 (CHimid), 127.7 (CHimid), 127.4 (Cq pyr), 126.8 (Cq pyr),
124.8 (CHimid), 124.7 (Cq pyr), 123.4 (CHimid), 121.4 (CHpyr), 121.4
(CHpyr), 121.1 (CHpyr), 84.3 (CHcod), 84.2 (CHcod), 84.0 (CHcod),
82.5 (CHcod), 53.4 (CHcod), 53.1 (CHcod), 52.6 (CHcod), 51.7
(NCH2CH2CH2CH3), 50.8 (NCH2CH2CH2CH3), 50.4 (CHcod),
35.8 (CH2 cod), 33.6 (CH2 cod), 33.1 (NCH2CH2CH2CH3), 32.9
(NCH2CH2CH2CH3), 32.3 (CH2 cod), 31.2 (CH2 cod), 30.5 (CH2 cod),
29.4 (CH2 cod), 29.0 (CH2 cod), 27.6 (CH2 cod), 20.2 (NCH2CH2-
CH2CH3), 20.2 (NCH2CH2CH2CH3), 14.0 (NCH2CH2CH2CH3),
14.0 (NCH2CH2CH2CH3). Electrospray MS (cone 20 V) (m/z,
fragment): 981.7 [M − 2Cl]2+ (calcd. for [M − 2Cl]2+: 981.3). Anal.
Calcd for C76H98N8Ir4Cl4 (2034.34): C, 44.87; H, 4.86; N, 5.51.
Found: C, 45.16; H, 5.62; N, 5.13.
1
state, which prevented a correct elemental analysis. H NMR (300
MHz, CDCl3): δ 7.73−7.71 (m, 2H, CHPh), 7.51−7.44 (m, 3H,
CHPh), 7.28−7.27 (d, 3JH‑H = 2 Hz, 1H, CHimid), 7.17−7.16 (d, 3JH‑H
= 2 Hz, 1H, CHimid), 4.61−4.52 (m, 1H, NCH2CH2CH2CH3), 4.19−
4.10 (m, 1H, NCH2CH2CH2CH3), 1.98−1.88 (m, 2H, NCH2-
CH2CH2CH3), 1.46−1.39 (m, 2H, NCH2CH2CH2CH3), 1.00 (t,
3JH‑H = 7.3 Hz, 3H, NCH2CH2CH2CH3). 13C{1H} NMR (75 MHz,
CDCl3): δ 187.2 (d, Rh-Ccarbene, 1JRh‑C = 53.25 Hz), 181.2 (d, Rh-CO,
1
1JRh‑C = 77.25 Hz), 172.66 (d, Rh-CO, JRh‑C = 41.25 Hz), 139.8
(Cq Ph), 129.3 (CHPh), 128.9 (CHPh), 125.5 (CHPh), 122.9 (CHimid),
122.3 (CHimid), 52.1 (NCH2CH2CH2CH3), 32.3 (NCH2CH2CH2-
CH3), 19.9 (NCH2CH2CH2CH3), 13.9 (NCH2CH2CH2CH3). IR
(KBr): 2067 (νCO), 1997 (νCO) cm−1.
Synthesis of 5. CO gas was passed through a solution of complex 4
(30 mg, 0.015 mmol) in CH2Cl2 (10 mL) at 0 °C, for 30 min. After
this time, the solution was concentrated under reduced pressure and
precipitated with hexane. The corresponding carbonyl derivative
precipitated as a pale yellow solid in quantitative yield. 1H NMR (400
MHz, CDCl3): δ 8.36 (br, 2H, CHpyr), 8.03 (br, 4H, CHpyr), 7.49 (s,
4H, CHimid), 7.33 (s, 4H, CHimid), 4.57 (br, 4H, NCH2CH2CH2CH3),
4.46 (br, 4H, NCH2CH2CH2CH3), 2.07−2.02 (m, 8H, NCH2-
CH2CH2CH3), 1.54−1.51 (m, 8H, NCH2CH2CH2CH3), 1.06 (t,
3JH‑H = 9.5 Hz, 12H, NCH2CH2CH2CH3). 13C{1H} NMR (100
MHz, CDCl3): δ 180.7 (Ir-Ccarbene), 175.6 (Ir-CO), 168.1 (Ir-CO),
133.3 (Cq pyr), 129.0 (Cq pyr), 125.4 (CHpyr), 125.0 (CHpyr), 125.0
(CHimid), 124.8 (Cq pyr), 122.0 (CHimid), 51.6 (NCH2CH2CH2CH3),
32.9 (NCH2CH2CH2CH3), 20.0 (NCH2CH2CH2CH3), 13.9 (NCH2-
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00633.
NMR spectra of 1-n-butyl-3-phenylimidazolium hexa-
fluorophosphate, compounds 1 and 2, and complexes
3−7, X-ray crystallographic data of complex 4, and the
time-dependent reaction profile of the cyclization
reaction of 4-pentynoic acid using catalyst 4 (PDF)
CH2CH2CH3). IR (KBr): 2064 (νCO), 1977 (νCO) cm−1
.
Accession Codes
Electrospray MS (cone 20 V) (m/z, fragment): 1791.3 [M − Cl]+
(calcd. for [M − Cl]+: 1789.1). Anal. Calcd for C52H50N8O8Ir4Cl4·
CH2Cl2 (1910.62): C, 33.32; H, 2.74; N, 5.86. Found: C, 33.25; H,
2.96; N, 6.21.
CCDC 1864607 contains the supplementary crystallographic
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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Synthesis of the Monometallic Complexes 6 and 7. Synthesis of
6. 1-n-Butyl-3-phenylimidazolium hexafluorophosphate (50 mg, 0.144
mmol, 1 equiv), [RhCl(cod)]2 (35.5 mg, 0.072 mmol, 0.5 equiv), and
K2CO3 (60 mg, 0.432 mmol, 3 equiv) were placed together in a high
pressure Schlenk. The resulting mixture was dissolved in acetone (3
mL) and stirred for 20 h at 60 °C. After this time, the mixture was
cooled to room temperature. The solution was then filtered and the
volatiles were removed under vacuum. The resulting solid was
dissolved in CH2Cl2, filtered through a pad of Celite, and the solvent
was removed under vacuum. The crude solid was purified by column
chromatography. Elution with CH2Cl2 afforded a bright orange band
that contained the desired complex. Yield: 53.5 mg (83.5%). 1H NMR
(400 MHz, CDCl3): δ 8.22−8.20 (m, 2H, CHPh), 7.55−7.50 (m, 2H,
CHPh), 7.45−7.40 (m, 1H, CHPh), 7.17 (d, 3JH‑H = 2 Hz, 1H, CHimid),
7.02 (d, 3JH‑H = 2 Hz, 1H, CHimid), 5.26−5.22 (m, 1H, CHcod), 5.18−
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: poyatosd@uji.es.
ORCID
Eduardo Peris: 0000-0001-9022-2392
Macarena Poyatos: 0000-0003-2000-5231
Notes
The authors declare no competing financial interest.
F
DOI: 10.1021/acs.organomet.8b00633
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the
Universitat Jaume I (UJI-B2017-07 and P11B2015-24). We
are grateful to the Serveis Centrals d’Instrumentacio Cientifica
(SCIC-UJI) for providing spectroscopic facilities. We also
thank Dr. Louise N. Dawe (Wilfrid Laurier University) for her
valuable advice on the refinement of the structure of complex
4.
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DOI: 10.1021/acs.organomet.8b00633
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