Tetranuclear Zn/4f coordination clusters as highly efficient catalysts for Friedel-Crafts alkylation

Article abstract of DOI:10.1039/c6cc03608b

A series of custom-designed, high yield, isoskeletal tetranuclear Zn/4f coordination clusters showing high efficiency as catalysts with low catalytic loadings in Friedel-Crafts alkylation are described for the first time. The possibility of altering the 4f centers in these catalysts without altering the core topology allows us to further confirm their stability via EPR and NMR, as well to gain insights into the plausible reaction mechanism, showcasing the usefulness of these bimetallic systems as catalysts.

Full text of DOI:10.1039/c6cc03608b

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DOI: 10.1039/C6CC03608B
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ARTICLE
Tetranuclear 3d/4f coordination clusters as highly efficient
catalysts for Friedel Crafts alkylation
Kieran Griffiths,a Prashant Kumar, a Geoffrey R. Akien,b Nicholas F. Chilton,c Alaa Abdul-Sada,a
Received 00th January 20xx,
Accepted 00th January 20xx
Graham J. Tizzard,d Simon J. Coles,d George E. Kostakis*a
DOI: 10.1039/x0xx00000x
A series of custom-designed high yield isoskeletal tetranuclear 3d/4f coordination clusters showing high efficiency as
catalysts with low catalytic loadings in Friedel Crafts alkylation are described for the first time. The possibility to alter the 3d
and the 4f centers in these catalysts without altering the core topology allows to further confirm their stability via EPR and
NMR, as well to gain insights regarding the plausible reaction mechanism, showcasing their usefulness of these bimetallic
systems as catalysts
www.rsc.org/
Polynuclear Coordination Clusters (CCs), assembled by organic anticipated to be seen when Co or Ni or Cu or Zn are used. We
ligands, transition metal (3d) and/or lanthanide (4f) elements, recently committed to studying the catalytic properties of
represent a rapidly expanding class of materials.1–4 However, tetranuclear 3d/4f CCs possessing a rigid defect dicubane
only a few and recent examples showed the use of these topology assembled solely by the Schiff base organic ligand (E)-
materials as catalysts.5–8 Friedel–Crafts (FC) reactions9 consist 2-(2-hydroxy-3-methoxybenzylidene-amino)phenol)
(H2L)
one of the most important carbon – carbon bond forming (Scheme S1). We showed that a series of isoskeletal24 MII LnIII
2
2
reactions in organic and medicinal chemistry. In recent years, (M = Co or Ni, Ln = Y, Nd, Eu, Gd, Tb, Dy) CCs, effectively catalyse
catalysts based on zinc or lanthanide elements have been
domino reaction at room temperature.8,25 These
a
extensively employed in FC reactions and sizeable development heterometallic species remain intact in organic solvents and this
has taken place.10,11 In particular, the indole ring system precise topology brings the 3d and the 4f centres very close
represents one of the most abundant and important (approximately 3.3Å), allowing both metal centres to
heterocycles in nature exhibiting wide-ranging biological coordinate to the substrates and promote the coupling reaction
activity.12 The nucleophilic addition of aldehydes or ketones to in high yield, highlighting the catalytic utility of this tetranuclear
indoles, in the presence of a Lewis acid, is a facile route towards bimetallic motif.
the synthesis of bisindolylmethane (BIMs) derivatives with only
one equivalent of water generated as a side product. BIMs have
recently been shown to be useful in the treatment of
fibromyalia13, antibacterial agents14 and even in the prevention
of cancer.14 Various catalytic systems and metal salts have been
reported for the synthesis of BIMS, however these methods
suffer from a number of disadvantages such as expense; toxicity
of reagents; high temperatures, high catalytic loading
(stoichiometric-10%) and photosensitivity (silver salts).15–18
In 3d/4f chemistry, 4f centres may be replaced by YIII or GdIII
without altering the core topology,19 therefore permitting 89Y
NMR20 or EPR21 for characterization of the solution species, as
well as allowing study of the catalytic reaction in situ to gain
mechanistic insights. Also, the possibility to alter the 3d centres
while retaining the topology22,23 may provide more insights
regarding the reaction mechanism, due to the different
coordination behaviour to substrates (heteroatoms N, O or s),
Being motivated from our previous results, and having in mind
26
that Ln(OTf)3 and Zn(ClO)4/Schiff base27 compounds have been
employed as catalysts in the reaction of indole derivatives with
aldehydes and ketones with a loading of 10 mol%, we decided to
synthesize and characterize a series of isoskeletal ZnII LnIII CCs and
2
2
study, for the first time, their application towards FC reactions.
The combination under aerobic conditions of H2L with freshly
prepared Ln(NO3)3 xH2O and Zn(NO3)2·6H2O in the presence of Et3N
in EtOH afforded, in very good yields, precipitates subsequently
crystallized by vapour diffusion of Et2O in N,N’-DMF solutions,
affording the isoskeletal air-stable tetranuclear defect dicubane
compounds with the general formula [ZnII LnIII L4(NO3)2(DMF)2]
2
2
where Ln is Y (1Y), Sm (1Sm), Eu (1Eu) Gd (1Gd), Dy (1Dy), Tb (1Tb)
and Yb (1Yb) (Figure 1). These compounds were characterized in full
by X-Ray crystallography (Tables S1-S2), IR, ESI-MS, TGA and
elemental analysis (see ESI).
The ZnII ion adopts a slightly distorted octahedral geometry
with an O5N donor set, while the Ln ion has a very distorted square
antiprismatic geometry, by virtue of chelating nitrate anions, with an
O7N donor set. Five out of six (Zn) and six out of eight (Ln)
coordination sites are occupied by donors from the organic ligands.
The remaining coordination sites are occupied by one DMF molecule
(Zn) and one nitrate anion (Ln, chelated mode). For 1Y, the Y⋯Zn
a.Department of Chemistry, School of Life sciences, University of Sussex, Brighton
BN1 9QJ.UK.
b.Department of Chemistry, Lancaster University, LancasterLA1 4YB, UK.
c. School of Chemistry, The University of Manchester, Manchester M13 9PL, UK.
d.UK National Crystallography Service, Chemistry, University of Southampton,
SO171BJ, UK
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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distances are 3.34882(7) and 3.5272(8) Å and the Zn⋯Zn distance is the catalytic action is homogeneous. We then performed a series of
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DOI: 10.1039/C6CC03608B
3.2949 (8) Å; the other 1Ln species are similar.
reactions using a 2.5% catalyst loading and varying the H2O/EtOH
ratio (Table S4) and quantitative yields (entries 5-6) were obtained at
3/2 and 1/1 EtOH/H2O ratio. After selecting the optimum solvent
system, we conducted reactions without 1Dy catalyst (Table 1, Entry
1) and no product was obtained. Next, due to the heteronuclear
(Zn/Ln) character of our molecules, Lanthanide and zinc triflates
were utilized at 10 mol% loading (Table 1, entries 2-5), in order to
identify their influence toward the bisindolylalkane product.
Yttrium triflate (Table 1, entry 3) was found to give quantitative
yields and dysprosium triflate (Table 1, entry 2) produced only
slightly lower yields, while zinc triflate (Table 1, entry 5) showed poor
catalytic performance. Compound 1Dy afforded almost quantitative
yields of the desired product after 2h at 10 mol% loading. By use of
1Sm, 1Eu, 1Tb, 1Gd, 1Yb comparable performance to 1Dy was
obtained, however the use of 1Y at 2.5 mol% loading afforded the
desired material in quantitative yield (Table 1, Entry 10). Promisingly,
the catalyst loading for 1Y could be decreased to 1 mol% with only a
slight decrease in the yields (Table 1, entry 11), being far lower than
other reported systems16,17,27. Importantly, reducing the catalyst
loading for yttrium triflate to 2.5 mol% (Table 1, entry 4) resulted in
a severe decrease in yield, compared to the ability of 1Dy and 1Y to
maintain high yields at only a few mol%.
Figure 1. Molecular structure of 1Ln. Colour code: ZnII: grey; LnIII: light blue; O: red; N:
blue; C: yellow. Hydrogen atoms are omitted for clarity.
The identity of 1Ln in solution was confirmed by electron spray
ionization mass (ESI-MS) spectrometry studies; in all cases, we
observed two peaks in the MS (positive-ion mode) at m/z which
corresponds perfectly to the monocationic [ZnII LnIII L4(NO3)]+ and
2
2
the dicationic [ZnII LnIII2L4]2+ fragments, respectively (see Figures S2-
2
S19).
Table 1. Comparison of catalytic activity for Ln(OTf)3 and (1-3)-Ln catalysts
For further confirmation of the solution stability of 1Ln, Q-band
EPR studies of 1Gd in both solid and solution (80% DMF, 20% Et2O)
phase were performed, Figure 2. Simulations with PHI28 confirm that
the spectra owe to S = 7/2 GdIII ions with rhombic zero-field splitting
(ZFS) (see Figure S1). The highly sensitive, finger-print-like ZFS of the
S = 7/2 state directly indicates that the coordination environment of
the GdIII ion is unchanged in solution.
Entrya
Catalyst
Loading/mol%b
Time/ h
Yield/ %c
1
2
None
Dy(OTf)3
Y(OTf)3
Y(OTf)3
Zn(OTf)3
1Dy
N/A
10
10
2.5
10
10
5
12
12
12
12
12
2
0
95
3
Quantitative
4
55
5
18
6
Quantitative
7
1Dy
2
96
8
1Dy
2.5
1
12
12
12
12
12
12
12
12
12
92
9
1Dy
87
10
11
12
13
14
15
16
1Y
2.5
1
Quantitative
1Y
96
87
53
80
75
94
1Sm
2.5
2.5
2.5
2.5
2.5
1Eu
Figure 2. Experimental EPR spectra at Q-band (34.0865 GHz) of 1Gd, recorded at
7K in solid (green line) and solution phase (red line).
1Tb
1Gd
For benchmarking studies, indole and benzaldehyde were
selected as reactants with 1Dy as catalyst at 10% loading, and
reaction parameters were subsequently optimized. The first set of
reactions were performed in order to identify the ideal solvent
system; after screening several solvents (Table S3), we identified that
the ethanol/water (2/1) solvent system (Table S3, Entry 9) provided
the best catalytic yield. The use of DMF, acetonitrile or EtOH as
solvent (Table S3, Entries 4 – 6), resulted in the same product but in
lower yields, indicating that the presence of H2O is crucial to obtain
high yields. No conversion to product was observed in low polarity
solvents in which 1Dy is insoluble (Table S3, entries 1-3), suggesting
1Yb
a
Reaction conditions: Indole, 1 mmol; benzaldehyde, 0.5 mmol; catalyst; 10mL
EtOH/H2O (2:1); room temperature. b Catalyst loading calculated per equivalent of LnIII.
c Determined by 1H NMR spectroscopy.
We then explored the scope of the reaction by employing a
variety of aldehydes and substituted indoles (Table 2). The reaction
proceeds smoothly with very good to excellent yields. Products 7a,
8c and 8f were characterized via single crystal X-Ray diffraction
(Figure S20). The next step was to involve ketones in place of
2 | J. Name., 2012, 00, 1-3
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aldehydes. Long reaction times, even with a slight increase of the
ARTICLE
To gain further information regarding the plausible mechanism
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DOI: 10.1039/C6CC03608B
temperature (50 °C) did not yield any product with both aliphatic and to identify the limitations of the present catalysts, we decided to
(acetone, cyclohexanone) and aromatic ketones (acetophenone) and perform the following set of reactions. First, we employed an
no unexpected side products were observed. This opposite aliphatic aldehyde (cyclohexanecarbaldehyde) instead of an
behaviour recalls the selective reduction of ketones in presence of aromatic aldehyde in reaction with indole, that gave very low yields
aldehydes (Luche reaction).29 In this reaction, aldehydes bond to Ln of the expected product 9 (Scheme S1) after 72 hours. Second, we
centres via hemiketal form, whilst ketone remains unprotected, thus examined substituted indoles such as 2-methyl-indole, 2-
reduced by NaBH4. Efforts to monitor the formation of the hemiketal (trifluoromethyl)-indole, 3-methyl indole, indole-3-acetic acid and N-
form by NMR were not successful. To gain information regarding the methyl indole in reaction with benzaldehyde. The use of 2-methyl-
reaction mechanism we performed UV-Vis binding studies of 2- indole gave the expected product 10 (Scheme S1) in quantitative
napthaldehyde with Zn(OTf)2, Dy(OTf)3 and 1Dy (Figure S21). 1Dy is yield in only 2 hours, indicating that substitution in position 2
found to have almost double the rate of 2-napthaldehyde quenching promotes the reaction. Compound 10 was characterized via single
compared to Dy(OTf)3, and Zn(OTf)2 is significantly slower than both. crystal X-Ray diffraction (Figure S22). However, the use of the
This observation indicates that 2-napthaldehyde prefers electron withdrawing group -CF3 in place of -CH3 [reaction with 2-
coordination to Dy over Zn.
(trifluoromethyl)-indole] only gave a yield of 16%, product 11
indicating a substantial influence on the catalytic activity. The use of
indole-3-acetic acid did not yield any product. A logical explanation
for deterioration of catalytic activity is that indole-3-acetic acid is in
competition with the benzaldehyde to coordinate to the Ln centre,
leading to a poisoning of the catalyst, however the use of 3-methyl-
indole did not result in the formation of the BIM. The latter indicates
that if the most active site (C-3) of the indole group is blocked the
reaction does not proceed. Finally, the reaction of N-methyl-indole
with benzaldehyde did not yield in any product, showcasing that
coordination of the nitrogen atom to Zn(II) is crucial.
Table 2. 1Y catalysed FC alkylation of indole and aldehydes.
R
Zn
Y
Y
HN
NH
n
Z
Zn
O
H
N
Y
H
H
R
O
Y
Zn
Zn
N
-H2O
STEP 7
O
H
O
O
H
Zn
N
R
H
R
R
Y
N
Y
H
STEP 1
H
H
STEP 6
2H
Zn
Zn
Y
Y
STEP 2
-H
O
R
H
HN
N
Y
n
Z
H
Y
Y
STEP 5
Zn
N
Zn
Zn
O
Zn
R
Y
Y
Y
N
H
H
Zn
N
O
O
H
R
H
STEP 3
STEP 4
R
H
H
N
H
Scheme 1. A plausible mechanism for the FC alkylation.
Despite the diamagnetic nature of 1Y, characterizing it in the
solution state by NMR proved difficult due to its asymmetry, dynamic
behaviour, and its relatively low solubility. However, the peak areas
for the imine, aromatic, and methoxy protons were all consistent
with the structure as determined by X-ray crystallography, Figure 1.
Gradually warming to 75 °C in 10 °C steps caused gradual broadening
of the peaks, which then recoalesced on cooling back to room
temperature, without any apparent decomposition of the complex.
Using multiple solvent suppression, it was possible to monitor the
smooth conversion of benzaldehyde and indole to the product in
protonated 2:1 EtOH/H2O. With Y(OTf)3, the concentration of
intermediates appeared to be very low, but with prolonged
acquisition it was possible to identify a minor species using 15N-
HMBC. Its chemical shift of 113 ppm is distinctly different from indole
(130 ppm) and the product (125 ppm), but the proton chemical shifts
of 7.33 (127.9 ppm in 13C-HSQC) and 4.99 ppm (too close to the
a Reaction conditions; aldehyde, 0.5 mmol; substituted indole, 1 mmol; EtOH/H2O, (2:1)
10mL; room temperature; 12h. 2.5% Loading of 1Y.
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L. Pecoraro, Angew. Chem. Int. Ed., 2011, 50, 9660–9664.
solvent peak to identify 13C-correlations) suggest that it may be the
benzaldehyde-indole hemiaminal. In the absence of catalyst, the
product forms very slowly (ca. 10% in 1 month), but under the same
spectroscopic conditions it was not possible to detect the same
correlations. This raises the prospect that it may well be a key
intermediate, but more favourable conditions may be required to
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Given knowledge of the structure of the catalyst and the data
given above, we propose the following mechanism for the FC
reaction (Scheme 1). The first step of the reaction involves the
coordination of the Nindole atom to the ZnII and the Ocarbonyl atom of
the aldehyde to the Ln part of the catalyst (Step 1). In the presence
of H2O the aldehyde may coordinate to the Ln centre via its hydrated
form. Then, deprotonation of the coordinated indole leads to the
formation of negative centre at C-3 of the indole moiety (Step 2). The
two organic moieties are very close [Y⋯Zn distances are 3.34882(7)
and 3.5272(8) Å] to favour the formation of benzaldehyde-indole
hemiaminal, as suggested by the NMR studies, (Steps 3-4) followed
by alkylation of one more indole moiety (Step 5). Finally, the catalytic
cycle ends by a proton exchange with an additional indole moiety
(Step 6) and release of the bis-adduct product and water and reform
the active catalyst species (Step 7).
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Conclusions
Overall, the results presented herein confirm for the first time, that
these tetranuclear CCs may indeed be useful as FC catalysts, with low
catalyst loading. Our philosophy to alter the 3d or the 4f elements in
these solution-stable bimetallic 3d/4f species allow us to confirm
their stability via EPR and gain useful insights regarding the plausible
reaction mechanism via NMR. Moreover, the possibility to tune the
organic periphery of these catalysts,8,25 achieving immobilization,
showcases that the present catalysts are highly desirable and the
appropriate vehicle to open new avenues in the fields of 3d/4f
chemistry and catalysis. Further work is necessary to expand the
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Acknowledgements
We thank the EPSRC (UK) for funding (grant number EP/M023834/1),
the University of Sussex for offering a PhD position to K.G., the EPSRC
UK National Crystallography Service at the University of
Southampton30 for the collection of the crystallographic data for
compounds 7a, 8c, 8f and 10, and the EPSRC UK National Electron
Paramagnetic Resonance Service at The University of Manchester.
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Notes and references
‡Catalytic Protocol. To a mixed solution of EtOH/H2O (2:1 in 10mL),
aldehyde (0.5mmol) and Indole (1mmol) were added followed by
catalyst (2.5% with respect to aldehyde). The resultant solution was
stirred for 12H, upon which time product had precipitated. The
cloudy solution was filtered and precipitate washed with hexanes
(3x10mL) and water (3x10mL). For products which did not
precipitate, the clear solution was concentrated under reduced
pressure and extracted in ethyl acetate (30mL) from water (20mL).
The ethyl acetate layer was dried with MgSO4 and concentrated
under reduced pressure. The resulting oil was washed with Hexanes
(60mL) resulting in a powdered solid which was further washed with
water (3x10mL). Full Experimental Details are given in the Supporting
Information. § CCDC 1452416-1452423
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Murray, J. Comput. Chem., 2013, 34, 1164–1175.
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