Catalytic studies of cyclometalated gold(III) complexes and their related UiO-67 MOF

Article abstract of DOI:10.1016/j.mcat.2020.111009

Cyclometalated gold(III) complexes Au(L)(OAc^F)₂ (L = phenylpyridine dicarboxylic diester (ppyde) or phenylpyridine dicarboxylic acid (ppydc)) have been prepared reacting Au(OAc)₃ with corresponding phenyl pyridines (ppyde or ppydc) in trifluoroacetic acid (HOAc^F) under microwave heating. Further treatment of Au(L)(OAc^F)₂ with aqua regia resulted in dichloro complexes Au(L)Cl₂. Au-functionalized UiO-67 MOF has been synthesized by exchanging linkers of UiO-67 with Au(ppydc)Cl₂, furnishing the MOF with (N^C)-cyclometalated Au(III) centers. The catalytic activities of the molecular cyclometalated complexes and the Au-incorporated MOF were studied in gold-catalyzed propargyl ester cyclopropanations. Almost all complexes and the MOF showed catalytic activity to the cyclopropanation product (up to 97% conversion), with a preference for the trans diastereoisomer (up to 14:86 d.r.). The recyclability of the most active molecular complex has also been investigated.

Full text of DOI:10.1016/j.mcat.2020.111009

Molecular Catalysis 492 (2020) 111009
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Catalytic studies of cyclometalated gold(III) complexes and their related
UiO-67 MOF
a
b
a
a
Volodymyr A. Levchenko , Huey-San Melanie Siah , Sigurd Øien-Ødegaard , Gurpreet Kaur ,
b
a,
Anne Fiksdahl , Mats Tilset *
Department of Chemistry and Center for Materials Science and Nanotechnology (SMN), University of Oslo, P.O. Box 1126 Blindern, N-0318 Oslo, Norway
a
b
Department of Chemistry, Norwegian University of Science and Technology, Høgskoleringen 5, N-7491 Trondheim, Norway
A R T I C L E I N F O
A B S T R A C T
Keywords:
Gold
Catalysis
Propargyl ester
MOF
Aqua regia
UiO-67
Cyclometalation
Cyclometalated gold(III) complexes Au(L)(OAcF)
(L = phenylpyridine dicarboxylic diester (ppyde) or phe-
2
nylpyridine dicarboxylic acid (ppydc)) have been prepared reacting Au(OAc) with corresponding phenyl pyr-
3
F
idines (ppyde or ppydc) in trifluoroacetic acid (HOAc ) under microwave heating. Further treatment of Au(L)
F
(
OAc )
2
with aqua regia resulted in dichloro complexes Au(L)Cl
2
. Au-functionalized UiO-67 MOF has been
synthesized by exchanging linkers of UiO-67 with Au(ppydc)Cl
2
, furnishing the MOF with (N^C)-cyclometalated
Au(III) centers. The catalytic activities of the molecular cyclometalated complexes and the Au-incorporated MOF
were studied in gold-catalyzed propargyl ester cyclopropanations. Almost all complexes and the MOF showed
catalytic activity to the cyclopropanation product (up to 97% conversion), with a preference for the trans dia-
stereoisomer (up to 14:86 d.r.). The recyclability of the most active molecular complex has also been in-
vestigated.
1. Introduction
The ppy motif, as a ligand in cyclometalated metal complexes, has
potential uses in the MOF community, especially in heterogeneous
Gold catalysis comprises one of the most effective methods for the
catalysis [21]. One family of MOFs that has attracted widespread at-
tention is the UiO-series – UiO-66, UiO-67 and UiO-68 [22]. Due to
their high thermal and chemical stability and ease of modification, they
have been utilized for incorporation of various metal-functionalized
linkers. However, the incorporation of the ppy ligand into UiO-67 is yet
to be explored with gold, although reports with Ir, Ru and Rh exist
(Fig. 1) [21,23–25].
The introduction of a gold linker with a covalent Au–C bond is ex-
pected to increase the robustness and recyclability of the catalytic
material and give better catalytic performance. Existing reports on
covalently-bonded Au-MOFs are limited. While this work was in pre-
paration, Toste and co-workers reported a stabilization of (C^C)Au(III)
complexes by inclusion into MOFs [26]. The catalytic results indicated
an enhanced stability of the supported Au complex compared to its
molecular analogue. The attempted inclusion of (salen)–Au(III) com-
plex into a IRMOF-3 framework has been reported, but was discovered
to contain mostly Au(0) species [27]. Later, a proline-functionalized
IRMOF-3 with Au active centers was reported, but analysis revealed
inclusion of both Au(III) and Au(0) species [28]. Other than one Schiff
base Au(III)-functionalized IRMOF-3 and the recent work by Toste
activation of C–C double and triple bonds leading to broad diversity of
follow-up chemical transformations, such as heterocycles and natural
compounds syntheses [1–5]. In the field of gold catalysis, the devel-
opment of Au(I) catalysts has dominated over Au(III) catalysts, due to
the superior stability of Au(I) salts compared to Au(III) salts. However,
stable Au(III)-ligated complexes have been developed and studied for
their bioactivity, such as anticancer and inhibition of DNA/RNA
synthesis [6,7]. Au(III) complexes that have been used in catalysis have
been primarily less robust and ligand-free Au(III) species, with some
exceptions such as pincer (C^C^N)Au and (C^C)Au complexes [8–12]. In
the past decade, Au(III) complexes as catalysts have gained popularity
due, in part, to the development of robust synthetic procedures [13].
The Tilset group has reported the syntheses of various cyclometalated
Au(III) complexes using microwave heating [14–16]. (N^C)Au com-
plexes bearing a phenyl pyridine ligand (ppy) are extensively studied
complexes with various substituents on aromatic rings. This class of
cyclometalated complexes has proven to be active in the catalysis of a
large range of reactions, including aromatic addition to vinyl ketones
[
17], AAA-coupling reaction [18,19] and oxazole synthesis [20].
⁎
Corresponding author.
E-mail address: mats.tilset@kjemi.uio.no (M. Tilset).
https://doi.org/10.1016/j.mcat.2020.111009
Received 26 February 2020; Received in revised form 4 May 2020; Accepted 9 May 2020
2
468-8231/ © 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license
(
http://creativecommons.org/licenses/BY/4.0/).

V.A. Levchenko, et al.
Molecular Catalysis 492 (2020) 111009
Fig. 1. Reported cyclometalated metal complexes, from left to right: Linker for the UiO-67 MOF, cyclometalated iridium complexes [23,25], cyclometalated ru-
thenium complexes [24] and cyclometalated gold complexes described in this work.
[
26,29], no other stable covalently-bonded Au(III) complexes have
NMR spectrum of 2a (δ 10.02) and 2b (δ 10.06) compared with that of
the free ppydc and ppyde ligands (δ 9.18 in both ligands) indicated a
successful cyclometalation. A similar deshielding effect was observed
for protons α to chelating C atom in 2a (δ 8.33) and 2b (δ 8.39)
compared to those in ppydc (δ 8.27) and ppyde (δ 8.29) (see ESI). The
molecular ions with expected isotope distribution were observed in the
been incorporated into a MOF. The goal of this investigation was to
incorporate the Au(ppydc) unit into the UiO-67 MOF framework and
study the catalytic activity of the Au(III)-MOF in a gold-catalyzed re-
action. The Au(ppydc) unit contains a stable Au-metallacycle comprised
of covalent Au–C and coordinative Au–N bonds and is expected to give
a robust, heterogeneous catalyst, capable of carrying out a range of
gold-catalyzed reactions [30].
This work demonstrates the successful incorporation of an Au(III)-
functionalized linker into the framework of the UiO-67 MOF. The re-
sulting MOF was tested in a cyclopropanation reaction and compared to
its molecular cyclometalated Au(III) analogs in solution.
35
35/37
high-resolution mass-spectra of the complexes – two Cl, one
Cl
and two 37Cl ions for 2a,b confirming the presence of two halogen
atoms in the complex (see ESI).
Crystals suitable for structure determination by X-ray diffraction
analysis were obtained by vapor diffusion of dichloromethane into the
trifluoroacetic acid solution for 1a, pentane into dichloromethane so-
lution for 1b and dichloromethane into DMSO solution for 2a. The
ORTEP figures and the selected bond distances as well as angles are
summarized and depicted in Fig. 2.
2
. Results and discussion
The solid-state structures of 1a and 1b reveal the expected square-
planar geometry around the Au(III)-center. In both complexes the Au–O
bond which is trans to carbon atom is elongated compared to the one
trans to nitrogen due to the stronger trans influence of aryl-C vs pyr-N
2
.1. Synthesis and characterization of cyclometalated Au(III) complexes
in HOAcF
3
The cyclometalation of ppydc and ppyde with Au(OAc)
under microwave heating resulted in formation of complexes 1a and 1b
in 46% and 65% yields, respectively (Scheme 1).
(
2.064(3) vs 2.047(3) Å in 1b and 2.088(3) vs 2.006(2) Å in 1a). The
1
Au–C distances are similar, 1.993(3) and 1.991(3) Å in 1b and 1a re-
spectively. The bond angles around the central Au atom are also similar
in 1a and 1b with no differences greater than 3° for both complexes.
The carbonyl groups in 1a and 1b were found to be planar with respect
to aromatic system. The Au–Cl bond distances in 2a of 2.2713(6) (trans
to N) and 2.3613(6) (trans to C) Å are typical for (N^C)AuCl complexes,
2
which span the range 2.262–2.282 Å for trans to N and 2.361–2.372 Å
for trans to C. [33–35]
The H NMR spectra of the complexes 1a and 1b were compared to
those of uncomplexed ligands ppydc and ppyde and revealed one
proton less in the cyclometalated complexes. The loss of a proton at the
chelating C and appearance of a singlet to its neighbor H at δ 7.50 and
7
formation of the Au–C covalent bond. Also, the F NMR spectrum re-
vealed two signals corresponding to OAc groups.
1
.51 in the H NMR spectra of 1a and 1b, respectively, indicated the
1
9
F
F
Initial attempts to substitute OAc groups by Cl in treatment with
F
aqueous NaCl or HCl resulted in full exchange of OAc groups and led to
1
9
formation of 2a and 2b. The reaction was followed by F NMR spec-
troscopy and the disappearance of the two fluorine signals confirmed
complete Cl-ion exchange. One major drawback of this method of
preparation is that the reaction results in 2b and 2a which appears as a
grey material presumably due to contamination with NMR-silent in-
organic gold species. The inorganic gold species can be removed by
washing 1a with aqua regia, in a protocol that was previously employed
in our group [31,32]. In addition to the dissolution of inorganic Au
2.2. Synthesis and characterization of MOF
Our initial attempts to synthesize a Au(III)-functionalized UiO-67
MOF focused on direct assembly from the starting building blocks
(ZrCl , H bpdc and complex 1a or 2a) in DMF at 120 °C in the presence
4 2
of benzoic acid as a modulator. Despite the success of the direct as-
sembly for other metal-functionalized MOFs [25], this synthesis method
resulted in UiO-67 with no incorporation of Au as evidenced by single-
crystal X-ray diffraction analysis. The absence of Au in the isolated
material may be attributed to the reduction of Au(III) with amines that
were generated during the synthesis. Such reduction during the
synthesis was also observed for other types of Au-functionalized MOFs
[27].
F
species, aqua regia substituted OAc groups with Cl, furnishing pure 2a
and 2b in 87% and 80% yields, respectively, after two steps.
The absence of signals in the 19F NMR spectra of the complexes 2a
F
and 2b indicated the successful exchange of the OAc groups with ha-
lides. A strong deshielding for the proton α to the chelating N in the H
1
Scheme 1. Synthesis of complexes 1a and 1b
followed by ligand exchange in aqua regia
furnishing complexes 2a and 2b.
2

V.A. Levchenko, et al.
Molecular Catalysis 492 (2020) 111009
Fig. 2. ORTEP representation of crystal struc-
tures of 1a, b and 2a with atoms drawn using
5
0% probability ellipsoids. Selected bond dis-
tances (Å) and angles (deg), for 1a: Au1-N1
.008(2), Au1-C1 1.991(3), Au1-O3 2.006(2),
2
Au1-O1 2.088(3), N1-Au1-C1 81.9(1), C1-Au1-
O3 93.7(1), O3-Au1-O1 92.51(9), O1-Au1-N1
9
1
1.8(1). For 1b: Au1-N1 2.002(3), Au1-C7
.993(3), Au1-O1 2.064(3), Au1-O3 2.047(3);
N1-Au1-C7 81.7(1), C7-Au1-O3 94.8(1), O3-
Au1-O1 88.5(1), O1-Au1-N1 95.0(1). For 2a:
Au1-N1 2.041(2), Au1-C1 2.025(2), Au1-Cl1
2
8
9
.3613(6), Au1-Cl2 2.2713(6); N1-Au1-C1
1.63(7), C1-Au1-Cl2 93.67(6), Cl2-Au1-Cl1
0.43(2), Cl1-Au1-N1 94.29(5).
The destructive reduction of the Au-linker has been overcome by
employing a post-synthetic linker exchange (PSLE) [36] protocol to
substitute the biphenyl linker in UiO-67 with a gold(III) ppydc unit
2.3. Catalytic studies of gold complexes
The catalytic activity of diacids 1a and 2a, diesters 1b and 2b, and
UiO-67-[Au]Cl was studied in the cyclopropanation reaction between
propargyl ester 3 and styrene (4) [37] in DCM at room temperature
using 10 mol% of gold catalyst (Scheme 2b). The reaction progress was
monitored by 1H NMR spectroscopy (Fig. 5 and Table 1). The pre-
viously published NMR study of this reaction with butyl-bisoxazoline-
Au(III) (BOX) complexes showed high activity with gold(III) complexes
and an interesting cis–trans isomerization (Scheme 2c) [37].
Complexes 1b, 2a and 2b showed catalytic activity over 24 h
(45–97% conversion, Fig. 5 and Table 1, entries 2–4) with 2b as the
most efficient catalyst (conversion > 95% after 4 h), giving quantitative
yield after isolation (Fig. 5 and Table 1, entry 4). Changing the solvent
(
Scheme 3). The PSLE was carried out by heating UiO-67 with 1a or 2a
in DMSO at 85 °C in the microwave oven for 6 h. This method was
successful for complex 2a to give MOF UiO-67-[Au]Cl, but 1a was
thermally unstable under these conditions and incorporation of 1a into
the MOF failed.
The obtained UiO-67-[Au]Cl was isolated as a pale-yellow powder,
after washing with hot DMSO and acetone followed by activation under
dynamic vacuum. The crystallinity of UiO-67 was retained after in-
corporation of 2a, as evidenced by powder X-ray diffraction analysis
and SEM (Figs. 3a and c). Energy-dispersed X-ray spectroscopy (EDS)
was used to study the spatial distribution of gold in MOF. EDS elements
mapping on UiO-67-[Au]Cl showed a homogeneous distribution of Au
within the MOF (Fig. 3f). Activated UiO-67-[Au]Cl exhibited a Brau-
from CH
2 2 6 3
Cl to DMSO-d or MeCN-d completely retarded the catalytic
activity of 2b.
2
nauer–Emmett–Teller (BET) surface area of 867 m /g, which is less
The bis(trifluoroacetate) analogue 1b resulted in lower conversion
(77%, 24 h, Table 1, entry 3), which is presumably due to the decom-
position of the catalyst. After only 4 h, a deep purple color had evolved,
indicating the possible reduction of the catalyst to gold nanoparticles.
Examination of the 1H NMR data also revealed a reduction in the
concentration of complex 1b. In comparison, complex 2b remained
stable through the whole catalytic run, with no signs of degradation.
Generally, the complexes 1b and 2b are much more active than 1a
and 2a (Fig. 5 and Table 1, entries 3 and 4 vs 1 and 2), where the
2
than half the BET of the original UiO-67 (2113 m /g), and their ni-
trogen absorption isotherms are depicted in Fig. 3d. The decrease in
surface area can be attributed to the smaller pore size due to the pre-
sence of the Au-functionalized linkers in the pores. The Au-functiona-
lization had surprisingly a little impact on MOF’s thermal stability,
which is stable up to 400 °C and is comparable to the unfunctionalized
UiO-67 MOF (Fig. 3b), according to TGA measurements.
The presence of the Au(ppy) units in UiO-67-[Au]Cl was confirmed
by 1H NMR spectroscopy of the digested material in 1 M D
PO
in
differences appeared to correspond to catalyst solubility in DCM-d
2
3
4
DMSO-d
6
. The degree of functionalization was calculated by integration
(soluble 1−2b and insoluble 1−2a). Despite the insolubility of 2a, the
complex showed moderate activity (45% conversion, 24 h, Table 1,
entry 2). The catalytic nature of the apparently insoluble 2a was in-
1
of the proton resonances for bpdc and ppydc in H NMR spectrum,
which showed 16% Au-substitution. This corresponds to the replace-
ment of one out of six bpdc linkers with a Au linker (see ESI for cal-
culations). Linker exchange in single crystals of UiO-67 was carried out
under the same conditions used for the bulk samples. Single crystal X-
ray structure determination of UiO-67-[Au]Cl showed a successful in-
corporation of the Au(ppydc) unit into the UiO-67 framework (Fig. 4).
One of the linkers shows thermal vibrations of the aryl groups of the
linker, and the position and thermal vibrations of the partially occupied
Au atom. The occupancy of the Au atom is slightly below 1%, and it is
disordered by symmetry over four equivalent sites. Other ligands of the
Au atom are missing from the structure due to disorder and their poor
scattering power.
2
vestigated by stirring 2a in DCM-d for 24 h, filtering off the solids and
mixing the filtrate with the propargyl ester 3 and styrene (4). NMR
analysis of the mixture over 24 h showed no conversion, indicating that
catalysis occurs in a heterogeneous manner for complex 2a. An addi-
tional experiment was performed by stirring 2a with propargyl acetate
3 and styrene (4) for 1 h followed by filtration of the suspension and
subjecting the solute to the 1H NMR. Absence of conversion of pro-
pargyl acetate 3 supported the heterogeneous nature of the catalysis.
The reactions were conducted several times and, in all cases, catalytic
activity was observed, although results appeared to be sensitive to the
quality of the substrate, styrene or catalyst.
Recent literature on the catalytic activity of cationic BOX-Au(III)
3

V.A. Levchenko, et al.
Molecular Catalysis 492 (2020) 111009
Fig. 3. (a) PXRD patterns of UiO-67 and UiO-67-[Au]Cl. (b) TGA plots of UiO-67 and UiO-67-[Au]Cl. (c) SEM image of UiO-67-[Au]Cl. (d) Nitrogen sorption isotherm
1
of UiO-67-[Au]Cl. (e) H NMR (400 MHz, DMSO-d
6
) spectra of 2a and UiO-67-[Au]Cl. (f) Au mapping of UiO-67-[Au]Cl by EDX.
3
complexes and neutral Au(III) complexes (AuCl and pyr-menthone-
AuCl
2
) report that cationic Au(III) complexes show higher catalytic
activity than the neutral complexes (Scheme 2c) [37]. Ligand exchange
of complex 2b was therefore attempted with AgOTf, NaPF
6
and NaBArF
prior to mixture with the reagents.
The cyclopropanation reaction catalyzed by complex 2b/AgOTf
resulted in immediate formation of Au nanoparticles, but also full
conversion within the first few minutes of the reaction with no further
changes over the next 24 h (Table 1, entry 5). Reactions using 2b/
NaPF
6
or NaBArF resulted in immediate decomposition of the gold
complex and/or propargyl ester 3 to unknown products (Table 1,
footnote d).
UiO-67-[Au]Cl was tested at 10% catalyst loading and gave mod-
erate conversion (56%, 24 h, NMR scale, Table 1, entry 6). Notably,
after 8 h, a pink discoloration was observed on the walls of the NMR
tube. The pink material is suspected to be Au nanoparticles that origi-
nate from decomposition of the UiO-67-[Au]Cl catalyst. When the ex-
periment was repeated at larger scale with stirring, high conversion was
obtained (80%, 24 h, Table 1, entry 7). The heterogeneous nature of the
catalyst was investigated by filtering the catalyst from the reaction
mixture after stirring for 1 h. NMR monitoring of the filtrate showed
that conversion of propargyl ester 3 continued the solid UiO-67-[Au]Cl
was removed, indicating leaching of gold species into the solution,
which gave moderate conversion (52%, 24 h). Reference reactions with
Fig. 4. Structure of UiO-67[Au]Cl showing a partial unit cell.
4

V.A. Levchenko, et al.
Molecular Catalysis 492 (2020) 111009
Scheme 2. (a) Cyclometalated complexes and Au(III)-MOF utilized in the current study; (b) propargyl ester cyclopropanation test reaction used to study the catalytic
activity of the Au(III) species in the current study; (c) complexes studied before in the catalytic cyclopropanation reaction.
UiO-67 and UiO-67 with AuNPs gave no conversion (entries 8–9).
2.4. Cis and trans selectivity in NMR studies
In published reports, the cis isomer of the cyclopropanation product
is usually formed, although subsequent cis–trans isomerization is ob-
served with different rates dependent on the gold ligand and the sub-
strate [37]. In contrast, all experiments with our neutral gold complexes
gave high trans selectivity (up to 14:86 dr), and no further change in
cis:trans ratio. The results indicate that our substituted ppy-ligands give
a different stereoselectivity than other gold(III) complexes and are not
active for cis:trans isomerization within the time span the reactions
were carried out in (24 h). The cationic 2b/AgOTf complex decomposed
during the reaction and gave equal amounts of cis and trans isomers,
supporting our theory that a different gold species is the catalytic
species in this reaction.
2.5. Effect of reaction conditions on conversion. Recyclability of complex
2b
Fig. 5. Comparison of the catalytic activity of complexes 1a, 1b, 2a, and 2b.
Monitoring of the integrity of the catalysts was possible during the
1
catalytic runs by means of H NMR and complex 2b was observed to be
intact after a reaction time of 24 h (see ESI). This observation prompted
an investigation of the catalyst’s recyclability. The experiments were
scaled up and conversions evaluated with respect to modifications in
the reaction conditions, and recovery of the catalyst was carried out
after completion of the reaction. First, the experiment was investigated
with respect to temperature and heating method (conventional or mi-
crowave) (Table 2). When the reaction was repeated at larger scale
(
108 mg propargyl ester 3, 30 mg complex 2b), the conversion was
Scheme 3. Post synthetic linker exchange in reaction between UiO-67 MOF and 2a under microwave heating in DMSO.
5

V.A. Levchenko, et al.
Molecular Catalysis 492 (2020) 111009
Table 1
a
Results from NMR studies of the gold-catalyzed cyclopropanation reaction between propargyl ester 3 and styrene (4). .
Entry
Catalyst
Conversion of 3 (1 h) [%]
Cis:trans (1 h)
Conversion of 3 (24 h) [%] (isolated yield in parenthesis)
Cis:trans (24 h)
1
2
3
4
5
6
7
8
9
1a
2a
1b
2b
0
17
33
43
–
0
45
–
32:68
22:78
10:90
50:50
20:80
20:80
–
23:77
22:78
16:84
50:50
14:86
18:82
–
b
77 (80 )
97 (> 99)
> 99
56
80
2b/AgOTf
UiO-67-[Au]Cl
UiO-67-[Au]Cl
UiO-67
> 99c,d
48
e
49
0
0
0
0
UiO-67@AuNPs [38]
–
–
c
Approx. 30% of species of unknown nature also generated within 1 h.
d
F
Immediate decomposition observed with similar observations when NaBAr and NaPF
6
were attempted.
a
Reagents and conditions: 5 mg of 3 with 11 μL of styrene with 10 mol% Au catalyst with 0.6 mL of CD
2
Cl
2
in NMR tube at 21 °C. Conversions and cis/trans ratios
1
were determined by H NMR.
b
Dark precipitate in dark solution after 4 h observed.
Double scale compared to the NMR scale and stirred.
e
Table 2
Conversion dependence on reaction conditions for the scaled-up reaction between propargyl ester 3 and styrene (4) catalyzed by gold(III) complex 2b (10 mol%).
Entry
Heating source
Time
Temperature, °C
Conversion of 3,%
cis:trans
1
2
3
4
5
6
7
–
MW
MW
MW
24 h
25
50
79
84
85
90
> 95
20:80
20:80
20:80
20:80
20:80
20:80
20:80
20:80
30 min
45 min
1 h 30 min
1 h 30 min
9 h
100
100
85
100
55
a
MW
oil bath
oil bath
oil bath
b
5 h
5 h
65
65
> 95
c
> 95d
8
a
Decomposition of catalyst occurred.
Isolated yield 73%, cis/trans 26/74.
Recovered catalyst from entry 7.
b
c
d
Yield of cyclopropanated product 5 = 89% and was measured by 1H NMR with dimethyl sulfone as internal standard.
Table 3
Recyclability of 2b.
of the recyclability of Au(III) complexes are scarce and the catalytic
ability and recyclability of complex 2b is an exciting discovery [19,39].
Recyclability of complex 2b in situ was also studied by conducting
an experiment under the best conditions found, depicted in Table 2,
Cycle
Conversion of 3, %
Stability of 2b
1
2
3
4
> 95
yes
> 95
yes
> 95
yes
> 95
decomposed
1
entry 7. After 5 h, the substrate conversion was determined by H NMR
analysis and an additional 1 equivalent of propargyl ester 3 and 1
equivalent of styrene (4) was introduced to the reaction mixture. The
reaction was repeated with an additional 3 cycles and the results are
given in Table 3.
The cyclopropanation reaction catalyzed by 2b could be repeatedly
used for 3 cycles with no degradation of 2b, which was observed only at
the 4th cycle.
slower at r.t. and conversion was moderate after 24 h (50%, Table 2,
entry 1). Utilization of microwave heating resulted in high conversions
of the propargyl ester over short periods of time, but the reaction did
not go to completion (79–85%, Table 2, entries 2–4). An increase in
temperature and reaction time resulted in eventual decomposition of
the gold catalyst, although high conversion of the substrate was ob-
tained (90%, Table 2, entry 5). Further studies showed that heating in
an oil bath at 65 °C for 5 h gave full conversion of propargyl ester 3 and
generated the product 5 in high yield (Table 2, entry 7).
3. Conclusions
1
As anticipated, complex 2b was stable ( H NMR) under the selected
A convenient and robust method of synthesizing stable cyclometa-
lated gold(III) complexes has been developed. Their structures have
been confirmed through NMR and X-ray crystal analysis. The first Au
(ppy)-functionalized linker has successfully been incorporated into the
framework of MOF UiO-67. The resulting MOF has been analyzed by X-
ray spectroscopy analysis, 1H NMR, TGA and nitrogen adsorption
analysis. The results indicate the integrity of the Au(ppy) unit in the
framework of UiO-67 MOF. NMR studies on the catalytic activity of the
gold(III) complexes and Au–MOF were carried out in a known cyclo-
propanation reaction. Most of the catalysts showed moderate to
conditions (Table 2, entry 7) and after full conversion of 3, the isolation
of complex 2b was carried out by precipitating the complex from the
reaction mixture with pentane. The recovery degree was found to be
9
3% (by mass) and the identity of complex 2b was confirmed by 1
H
NMR (see ESI). The recovered complex 2b was subjected to a second
round of catalyzing the cyclopropanation reaction and demonstrated
1
the same level of activity giving high yield (89%, calculated by H NMR
using an internal standard, Table 2, entry 8). To our knowledge, reports
6

V.A. Levchenko, et al.
Molecular Catalysis 492 (2020) 111009
excellent conversions on the NMR scale. When the reaction was scaled
up with complex 2b, the catalyst was found to be recoverable and re-
cyclable in new reactions.
9.03 (s, 1 H), 8.92 (dd, J = 8.4, 1.9 Hz, 1 H), 8.66 (d, J =8.5 Hz, 1 H),
8.22 (d, J =8.1 Hz, 1 H), 8.11 (dd, J = 8.1, 1.5 Hz, 1 H), 7.51 (s, 1 H),
4.42 (q, J =7.2 Hz, 2 H), 4.35 (q, J =7.0 Hz, 2 H), 1.34 (dt, J = 13.6,
1
7
(
.1 Hz, 6 H).13C{ H} NMR (151 MHz, CD
2
Cl
2
C
): δ 166.6 (C8), 164.3
C1′), 161.8 (C1), 161.3 (CO, J =37.8 Hz), 160.9 (CO, J =39.3 Hz)
4. Experimental
1
1
1
6
49.7 (CH-5), 145.6 (CH-3), 144.5 (C2), 141.6 (C8′), 134.8 (CH-2′),
31.9 (CH-3′), 129.4 (CH-4′), 129.0 (CH-5′), 127.1 (C8′), 122.7 (CH-4),
18.1 (CF3, J = 289.9 Hz), 116.1 (CF3, J = 288.4 Hz), 63.9 (-CH2-(6)),
19
2.7 (-CH2-(6′)), 14.3 (CH-3(7)), 14.2 (CH-3(7′)). F NMR (376 MHz,
+
CD
2
Cl
2
): δ
F 6 6 3
-77.05, -75.98. (ref. C F ). MS (ESI , CH OH): m/z 580.100
F
+
(
[M-OAc -Et+H] , 100%). Anal. calcd. for C21H16AuF6NO8: C,
(
1)
3
4.97; H, 2.24; N, 1.94. Found: C, 33.77; H, 2.17; N, 1.87%.
.3. Synthesis of 2a
4.1. Synthesis of 1a
4
The teflon liner for the microwave oven synthesis reactor was filled
The Teflon liner for the microwave oven synthesis reactor was filled
with gold(III) acetate (68 mg, 0.182 mmol, 1.1 equiv.), 6-(4-carbox-
yphenyl)nicotinic acid (40 mg, 0.164 mmol, 1.0 equiv.) and tri-
fluoroacetic acid (15 mL). The reaction mixture was heated in the mi-
crowave oven at 130 °C for 1 h. Still warm, the suspension was
centrifuged and clear solution was decanted. The flask with solution
was kept overnight in a dark place at room temperature until the
crystalline product started to precipitate. The crystals were filtered and
washed with water (until pH of the filtrate was 7), diethyl ester (10 mL)
and dried under a stream of air giving the 50 mg of the complex 1a as
white solid (46% yield). X-ray-quality crystals were obtained by cooling
the warm reaction mixture to room temperature.
with 6-(4-carboxyphenyl)nicotinic acid (60 mg, 0.248 mmol, 1.0
equiv.), Au(OAc) (102 mg, 0.273 mmol, 1.1 equiv.) and trifluoroacetic
acid (15 mL). The mixture was heated in the microwave oven at 130 °C
for 1 h. After cooling to room temperature, the solvent was removed
under reduced pressure. After the solvent removal, the solid crude was
stirred with aqua regia solution (9 mL HCl and 3 mL HNO ) for 30 min.
3
The pale yellow precipitate was filtered through a fine frit, washed with
water and dried under a stream of air for 30 min. The product was
3
obtained in 87% yield (110 mg).
1
H NMR (600 MHz, DMSO-d
6
H
): δ 10.02 (d, J =1.9 Hz, 1H, H-5),
8
.78 (dd, J = 8.4, 1.9 Hz, 1H, H-3), 8.57 (d, J =8.5 Hz, 1H, H-4), 8.33
Notes: material for NMR spectra was prepared by removing tri-
fluoroacetic acid under reduced pressure immediately after the micro-
wave synthesis and decomposes in solution over short period of time
making spectra acquisition troublesome. Complex 1a that precipitates
(
d, J =1.5 Hz, 1H, H-5′), 8.15 (d, J =8.1 Hz, 1H, H-4′), 7.98 (dd,
13
J = 8.0, 1.6 Hz, 1H, H-3′). C NMR (151 MHz, DMSO-d
(
1
1
6
): δ
C
165.9
C1′), 165.5 (C8), 163.8 (C1), 151.1 (C2′), 149.1 (CH-5), 145.9 (C8′),
43.9 (CH-3), 133.3 (C9), 130.3 (CH-5′), 129.9 (CH-3′), 128.1 (C2),
out of the TFA solution is insoluble in DMSO-d
6
, DMF-d
7
and CD
3
CN.
+
27.6 (CH-4′), 122.9 (CH-4). MS (ESI , CH
3
OH): m/z 531.939
1
H
NMR (400 MHz, DMSO-d
6
): δ
H
9.07–8.95 (m, 1H, H-5),
35
37
+
35/37
+
+
(
[
M + Na] , 100%), 533.936 ([
M + Na] , 63.7%), 535.933
OH): m/z 531.9386 (calcu-
8
4
.93–8.81 (m, 1H, H-3), 8.71–8.58 (m, 1H, H-4), 8.28–8.14 (m, 1H, H-
+
(
[
M + Na] , 10.6%). HRMS (ESI , CH
3
′), 8.13–8.00 (m, 1H, H-3′), 7.50 (s, 1H, H-5′). 1H NMR (400 MHz,
8 2 4
lated for C13H AuCl NNaO 531.9388 (+0.5 ppm)). Anal. calcd. for
C13H8AuCl2NO4: C, 30.61; H, 1.58; N, 2.75. Found: C, 30.12; H, 1.63;
N, 2.71%.
CD
3
CN): δ
H
9.10 (s, 1H, H-5), 8.81 (dd, J = 8.4, 1.8 Hz, 1H, H-3), 8.28
(
d, J =8.5 Hz, 1H, H-4), 8.13 (d, J =8.0 Hz, 1H, H-4′), 7.90 (d, J
1
3
1
=
8.0 Hz, 1H, H-3′), 7.62 (s, 1H, H-5′). C{ H} NMR (151 MHz, DMSO-
): δ 165.2, 164.7, 163.2, 158.4 (q, CO, J = 37.9, 36.9 Hz), 148.1,
45.1, 145, 140.3, 133.4, 130.9, 128.4, 127.9, 127.2, 123.4, 115.7 (q,
d
6
C
4
.4. Synthesis of 2b
1
1
3
1
CF
1
1
3
, J = 292.2, 290.3 Hz). C{ H} NMR (151 MHz, CD
65.5, 163.3, 158.9 (q, CO, J =39.5 Hz), 150, 146.3, 146.1, 141.6,
34.2, 132.4, 129.3, 129.2, 128.4, 124.2 (signals of CF group over-
OH): m/z 697.658
OH): m/z 697.6582
12AuF NO 697.0080
3 C
CN): δ 166.8,
The teflon liner for the microwave oven synthesis reactor was filled
with gold(III) acetate (90 mg, 0.24 mmol, 1.2 equiv.), ethyl 6-(4-
ethoxycarbonyl)phenyl)nicotinate (60 mg, 0.20 mmol, 1.0 equiv.) and
3
(
+
lapped with s+olvent signal). MS (ESI , CH
3
trifluoroacetic acid (15 mL). The reaction mixture was heated in the
microwave oven at 130 °C for 1 h. The volatiles were removed under
reduced pressure and afforded Au(ppyde)(OAc )
which was used without further purification. The crude was then taken
up with aqua regia solution (9 mL HCl and 3 mL HNO ) for 30 min. The
+
(
[
(
[M + CH
AuF
+0.65 ppm)).
3
OH] , 7%). HRMS (ESI , CH
3
C
17
H
8
6
NO +CH
8
3
OH]+ (calculated for C14
H
6
9
F
2
as a pale-yellow solid
3
4.2. Synthesis of 1b
pale yellow precipitate was filtered through a fine frit, washed with
water and dried under a stream of air for 30 min. The product was
The teflon liner for the microwave oven synthesis reactor was filled
with gold(III) acetate (45 mg, 0.120 mmol, 1.2 equiv.), ethyl 6-
4(ethoxycarbonyl)phenyl)nicotinate (30 mg, 0.1 mmol, 1.0 equiv.) and
trifluoroacetic acid (20 mL). The reaction mixture was heated in the
microwave oven at 130 °C for 1 h. After cooling down to room tem-
perature, the volatiles were removed in vacuum and solid crude was
obtained in 80% yield (90 mg).
1
H NMR (600 MHz, DMSO-d
6
H
): δ 10.06 (d, J =1.9 Hz, 1H, H-5),
(
8.84 (dd, J = 8.4, 2.0 Hz, 1H, H-3), 8.63 (d, J =8.4 Hz, 1H, H-4), 8.39
(d, J =1.6 Hz, 1H, H-5′), 8.23 (d, J =8.1 Hz, 1H, H-4′), 8.04 (dd,
J = 8.0, 1.6 Hz, 1H, H-3′), 4.44 (q, J =7.1 Hz, 2H, H-6), 4.37 (q, J
=7.1 Hz, 2H, H-6′), 1.36 (dt, J = 14.3, 7.1 Hz, 6H, H-7 and 7′). 1
H
extracted with warm (40−45 °C) CH
2
Cl
2
(2 × 15 mL). Combined frac-
NMR (600 MHz, CDCl
J = 8.4, 1.8 Hz, 1 H), 8.67 (d, J =1.4 Hz, 1 H), 8.12 (dd, J = 8.1,
1.5 Hz, 1 H), 8.07 (d, J =8.4 Hz, 1 H), 7.72 (d, J =8.1 Hz, 1 H), 4.52 (q,
3
): δ
H
10.41 (d, J =1.8 Hz, 1 H), 8.78 (dd,
tions were filtrated through a folded filter and volume of the filtrate
was reduced to 10 mL under vacuum. Product was precipitated as a
white solid upon addition of the pentane (20 mL). The filtration re-
J =7.1 Hz, 2 H), 4.44 (q, J =7.1 Hz, 2 H), 1.45 (dt, J = 19.8, 7.1 Hz,
7 H). 13C{ H} NMR (151 MHz, DMSO-d
1
sulted in 47 mg of 1b as a white powder in 65% yield.
6
C
): δ 165.7 (C1), 164.4 (C1′),
1
H NMR (600 MHz, CD
2
Cl
2
): δ
H
9.23 (dd, J = 1.8, 0.6 Hz, 1H, H-5),
162.3 (C8), 151.0 (C9), 148.8 (CH-5), 146.2 (C8′), 143.8 (CH-3), 132.3
(C2′), 129.9 (CH-5′), 129.8 (CH-3′), 127.9 (CH-4′), 127.1 (C2), 123.1
8
8
.84 (dd, J = 8.4, 1.8 Hz, 1H, H-3), 8.16 (dd, J = 8.0, 1.4 Hz, 1H, H-3′),
.11 (d, J =8.4 Hz, 1H, H-4), 7.76 – 7.71 (m, 2H, H-4′ and H-5′), 4.47
(CH-4), 62.4 (CH
2
(6)), 61.6 (CH
2 3 3
(6′)), 14.14 (CH (7)), 14.06 (CH (7′)).
+
+
(
q, J =7.2 Hz, 2H, CH2(6)), 4.38 (q, J =7.1 Hz, 2H, CH2(6′)), 1.41 (dt,
MS (ESI , CH OH): m/z 588.002 ([M+Na ], 7%), 584.051 ([M
3
1
+
+
J = 19.0, 7.2 Hz, 6H, CH3(7 and 7′)). H NMR (400 MHz, DMSO-d
6
): δ
H
3 3
+H O ], 100%). HRMS (ESI , CH OH): calculated for
7

V.A. Levchenko, et al.
Molecular Catalysis 492 (2020) 111009
C
17
O
4
H
16AuNCl
2
Na+ [M+Na+] 588.0014, found 588.0016 (Δ
analysis, Visualization, Writing - original draft, Writing - review &
editing. Sigurd Øien-Ødegaard: Investigation, Formal analysis,
Validation, Visualization. Gurpreet Kaur: Investigation, Formal ana-
lysis, Validation, Visualization. Anne Fiksdahl: Supervision, Resources,
Methodology, Conceptualization, Writing - review & editing. Mats
Tilset: Supervision, Resources, Methodology, Conceptualization,
Writing - review & editing.
-
0.2 ppm). Anal. calcd. for C17H16AuCl2NO4: C, 36.06; H, 2.85; N,
2.47. Found: C, 35.78; H, 2.90; N, 2.43%.
4.5. Synthesis of 3
To a stirred ethynylmagnesium bromide solution (38 mL of 0.5 M
solution in THF, 19 mmol) on ice bath, a solution of p-anisaldehyde
2 g, 14.69 mmol) in THF (5 mL) was added under nitrogen atmosphere.
Solution was stirred for 15 min on ice bath and overnight at room
temperature. Water (50 mL) with Et O (40 mL) were added and the
organic phase was separated. The aqueous phase was extracted with
Et O (2 × 40 mL) and the combined phases were dried with Na SO
Filtration followed by solvent removal under reduced pressure resulted
in 1-phenylprop-2-yn-1-ol as a viscous, yellow-brown oil which was
used for next step without further purification. Mixture of 1-phenyl-
prop-2-yn-1-ol (2.17 g, 13.38 mmol) acetic anhydride (3.6 mL) and
trimethylamine (5.3 mL) in DCM (20 mL) was stirred at room tem-
perature overnight. The solution was poured into 1 M NaOH until
pH = 14 and mixed with DCM (25 mL). After the organic layer was
separated, the compound was extracted with DCM (2 × 25 mL). The
(
Acknowledgements
2
This work has been supported by the Research Council of Norway
through grant no. 250795 (stipend to V.L.). The Research Council of
Norway has also supported us through the Norwegian NMR Platform,
NNP (226244/F50). The NV-faculty at NTNU is thanked for its support
(PhD stipend to H.S.M.S.). Gurpreet Kaur and Christopher Affolter are
acknowledged for supplying UiO-67 MOF, and carrying out TGA and
PXRD measurements. Erlend Aunan carried out BET measurements.
2
2
4
.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.111009.
combined organic layer was dried over Na
ified by column chromatography using ethyl acetate/hexane mixture
1:10) as an eluent. The obtained pale-yellow oil solidified in the
2 4
SO , filtered off, and pur-
(
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9