Confinement of a Au-N-heterocyclic carbene in a Pd6L12metal-organic cage

Article abstract of DOI:10.1039/d0ra07509d

A Au(i)-N-heterocyclic-carbene (NHC)-edged Pd6L12 molecular metal-organic cage is assembled from a Au(i)-NHC-based bipyridyl bent ligand and Pd2+. The octahedral cage structure is unambiguously established by NMR, electrospray ionization-mass spectrometry and single crystal X-ray crystallography. The electrochemical behaviour was analyzed by cyclic voltammetry. The octahedral cage has a central cavity for guest binding, and is capable of encapsulating PF6- and BF4- anions within the cavity. This journal is

Full text of DOI:10.1039/d0ra07509d

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Confinement of a Au–N-heterocyclic carbene in
a Pd6L12 metal–organic cage†
Cite this: RSC Adv., 2020, 10, 39323
Lihua Zeng,‡ Shujian Sun,‡ Zhang-Wen Wei,
Yu Xin, Liping Liu
*
and Jianyong Zhang
A Au(I)–N-heterocyclic-carbene (NHC)-edged Pd6L12 molecular metal–organic cage is assembled from
a Au(I)–NHC-based bipyridyl bent ligand and Pd2+. The octahedral cage structure is unambiguously
established by NMR, electrospray ionization-mass spectrometry and single crystal X-ray crystallography.
The electrochemical behaviour was analyzed by cyclic voltammetry. The octahedral cage has a central
cavity for guest binding, and is capable of encapsulating PF6ꢀ and BF4ꢀ anions within the cavity.
Received 1st September 2020
Accepted 19th October 2020
DOI: 10.1039/d0ra07509d
rsc.li/rsc-advances
chlorotrimethylsilane. Then Au(I)–NHC compound c25 was
synthesized by the reaction of imidazolium salt b with
Introduction
The self-assembly of metal–organic cages (MOCs)1–3 has been
investigated for diverse potential applications, such as catal-
ysis,4 sensing,5,6 molecular storage and sequestration,7 drug
delivery,8 and so on. For MOCs the shape, cavity size, and ligand
can inuence their guest binding and transformation capa-
bility.9–11 N-heterocyclic-carbenes (NHCs) are a class of electron-
donating ligands,12 and have been applied in catalyzing various
organic transformations.13,14 Discrete assemblies based on NHC
ligands including molecular metallocycles and cages have
received increasing attention15–18 because they may induce
reactivity change, selectivity and product distribution varia-
tion.19,20 However, few molecular cages with well-dened cavi-
ties are reported in the literature.21–23 Remarkably, Nitschke
et al. reported M4L6 (M ¼ Zn(II), Cd(II)) cages with Au(I)–NHC-
based ligand.23 In our effort to build NHC-based cages with
large cavity, we wish to report herein that a Pd6L12 metal–
organic cage containing twelve Au(I)–NHC centres is assembled
from a rigid, bent N-heterocyclic-carbene-based bispyridyl
ligand and palladium(II) ions.
HAuCl4$4H2O and 3-chloropyridine in the presence of base
(Na2CO3). Subsequently, L was synthesized by Suzuki–Miyaura
cross-coupling reaction between c and 3-pyridylboronic acid
1
(Fig. S1–S10†). H NMR shows that the imidazolium C–H reso-
nance appeared at d 10.34 for b and disappeared for L. 13C NMR
spectra show the resonance of the imidazolium C–H carbon at
139.60 ppm for b and the resonance of the metallated carbene-
carbon atoms at 185.55 ppm. The coordination cage Pd6L12 was
successfully assembled by heating a 2 : 1 mixture of L and
Pd(NO3)2$2H2O in DMSO at 70 ꢁC for 6 h. 1H NMRꢀindicates that
the quantitative formation of Pd6L12 (12 NO3 anions are
omitted for clarity, the same hereinaer) (Fig. 2 and S11†). The
cage is highly symmetric. Compared with the free ligand L the
original sharp signals of pyridine moieties turn into broad
peaks and shi downeld, for example the signals at 9.05, 8.64
and 8.26 ppm for the Hi, Hh and Hc pyridyl protons are
downeld-shied to 9.74, 9.09 and 8.62 ppm, respectively,
which is ascribed to the coordination of palladium(II) ions to the
ligand.
Gratifyingly, single crystals of Pd6L12 (NO3ꢀ salt) suitable for
X-ray diffraction analysis were obtained over one month by slow
diffusion of ethyl acetate vapour into a solution of Pd6L12 in
DMSO (Table S1†). The crystals crystallize in the trigonal space
Results and discussion
For the self-assembly of Pd6L12 cage, bis(pyridyl)-functionalised
Au(I)–NHC ligand L was designed and synthesized (Fig. 1a).
First diiodo-functionalised imidazolium chloride (b) was
synthesized from bis-Schiff's compound a24 and para-
formaldehyde via a ring-closing step in the presence of
ꢀ
group R3. The cubic symmetric unit is composed of six Pd(II)
metal centres linked by twelve L ligands, forming the octahe-
dron edges (Fig. 1b). Nevertheless, nitrate ions and solvent
molecules could not be reasonably located in this highly
disordered structure. The bend angle between pyridine rings
and central Au–NHC ring is 174.1ꢁ. Two pyridyl donors on the
same ligands adopt syn-conformation. The bond angles
MOE Laboratory of Polymeric Composite and Functional Materials, School of
Materials Science and Engineering, School of Chemistry, Sun Yat-Sen University,
Guangzhou 510275, China. E-mail: zhjyong@mail.sysu.edu.cn
C
carbene–Au–Itrans range from 176 to 179ꢁ are in fact close to
linearity (C18–Au1–I1 176.1(11)ꢁ) and the Au1–C18 bond length
† Electronic supplementary information (ESI) available: Additional experimental
details, spectra and structure data. CCDC 2013514. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/d0ra07509d
‡ These authors contributed equally to the work.
ꢁ
of 1.96(3) A, which is close to those of the reported (NHC)gold(I)
complexes.26 Each Pd(II) has a square planar geometry with Pd–
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Fig. 1 (a) Synthetic route of L, single crystal X-ray structures of Pd6L12, (b) sticks, (c) ball-and-stick model, (d) space-filling model. Anions, solvent
molecules and hydrogen atoms are omitted for clarity. Color coding: C: gray; N: red; I: purple; Au: orange; Pd: blue.
1
Further evidence for Pd6L12 was provided by H–1H homo-
nuclear correlation spectroscopy (COSY) and 1H–1H nuclear
overhauser effect spectroscopy (NOESY), which both reveal
important cross peaks between the two observed sets of NMR
signals (e.g. Hc–Hb and Hc–Hh) (Fig. S12 and S13†). In addition,
diffusion-ordered NMR spectroscopy (DOSY) shows the selec-
tive formation of a single species (Fig. S14†). The same diffusion
coefficient at D ¼ 5.75 ꢂ 10ꢀ11 m2 sꢀ1 corresponds to the
ꢁ
dynamic radius of 19.0 A according to the Stokes–Einstein
equation.28 Further structural evidence was given by electro-
spray ionization-mass sꢀpectrometꢀry (ESI-MS) (Fig. 3). Aer
anion exchange of NO3 for PF6 ions in DMSO solution,
a series of prominent peaks with continuous charge states of
[M–(PF6ꢀ)n]n+ (n ¼ 4–8) were detected for Pd6L12. The isotopic
Fig. 2 1H NMR spectra of (a) L and (b) Pd6L12 in DMSO-d6 (400 MHz,
298 K).
distribution patterns of each peak agreed well with the simu-
lated patterns.
The chemical composition of L and Pd6L12 were character-
ized by Fourier transform infrared spectroscopy (FT-IR)
(Fig. S15†). The absorption of Pd6L12 at 1632 cmꢀ1 corre-
sponding to the C]N in-ring stretching in the pyridine rings
shis to lower energy compare with L at 1638 cmꢀ1, which is
attributed to the coordination of L to Pd2+ via the nitrogen
atom. Both L and Pd6L12 exhibit characteristic C–Ncarbene bands
ꢁ
N bond distances of 1.98(2)–2.03(2) A and the angle between the
two 3-pyridyl coordinating motifs in L is 88–93ꢁ, within the
normal range for those reported analogous pyridine-based
ligand assembled Pd6L12 complexes.27,28 The cavity size is
ꢁ3
approximately 20.2 ꢂ 20.4 ꢂ 28.7 A , which is dened by the six
typically at 1467 and 1443 cmꢀ1
.
29 The strong absorpꢀtion band
ꢁ
Pd(II) ions. The opposing Pd(II)–Pd(II) distance is 28.7 A and
at approximately 1384 cmꢀ1 is attributed to the NO3 ions for
Pd6L12.
ꢁ
adjacent Pd–Pd distances are 20.2–20.4 A. One of the two dii-
sopropyl groups on each ligand points to the cavity, and the C–C
ꢁ
To investigate the electrochemical behaviour, cyclic voltam-
metry analyses were carried out in the potential range from ꢀ2.0
distance of diisopropyl groups on opposite ligands is 16.3 A.
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current for Pd6L12 were acquired at scan rates of 50–200 mV sꢀ1
,
showing that two reduction waves are located below the scan
rates of 100 mV sꢀ1 (Fig. S16†). With the increase in the sweep
rate, the two waves merge into one broad peak, suggesting that
the redox process takes place under simple diffusion-control for
the cage.36
The resulting cage structure of Pd6L12 was further investi-
gated to explore its encapsulation of anions. The original nitrate
anions are difficult to detect by NMR, so anion exchange of
nitrate with other anions were performed. Various anions (20
equiv., excess) were added to a DMSO solution of Pd6L12
(0.75 mmol Lꢀ1) and the mixture was allowed to react for 4 h at
room temperature, then diethyl ether was added to precipitate
out pale yellow solid prior to the acquisition of NMR spectros-
copy. Aer the introduction of NaPF6 31P NMR spectroscopy
shows that phosphorus resonances from PF6ꢀ in DMSO appear
expected sharp septet, with the main peak at d ꢀ144.19 ppm
ꢀ
with coupling constant of JPF ¼ 711.2 Hz,37 while bound PF6
anions display the multiplets centred on d ꢀ142.70 ppm, shied
by Dd ¼ 1.49 ppm with JPF ¼ 719.3 Hz to lower eld (Fig. 5a and
S17†). Moreover, 19F NMR shows that one new set of peaks at
ꢀ
ꢀ
Fig. 3 (a) ESI-TOF-MS spectrum of Pd6L12 (M) (PF6 as counterion)
ꢀ65.19 ppm (JFP ¼ 725.7 Hz) are downshied with free PF6
and (b) measured (top) and calculated (bottom) isotope patterns for
(ꢀ70.15 ppm, JFP ¼ 710.6 Hz) (Fig. 5b and S18†). Additionally for
1H NMR Hi proton resonance inside the central cavity is upeld
shied by 0.70 ppm (Fig. S19†), while other proton resonances
are essentially unaffected. These results demonstrate that two
ꢀ
various charge states observed for [Pd6L12] .
12+$12PF6
to +2.0 V at a scan rate of 50 mV sꢀ1 in a solution of tetrabuty-
lammonium hexauorophosphate (TBAPF6) in dry DMSO
(0.10 mol Lꢀ1) as a supporting electrolyte on glassy carbon
electrode (3 mm in diameter) (Fig. 4). L gave a reduction wave at
a potential of ꢀ1.068 V, indicating the reduction of Au(I) to
metallic Au(0) corresponding to previously reported Au(I)–NHC
analogues.30,31 The analogous process was signicantly shied
to a more anodic potential at ꢀ0.890 V for Pd6L12. In addition to
the reduction processes, a single irreversible oxidation peak was
also observed at 0.812 V for Pd6L12, positively shied by 0.287 V
from L (+0.525 V), which is consistent with the oxidation
processes of Iꢀ to I2 on clean glassy carbon surface.32,33 It is
ꢀ
types of PF6 anions are present in the solution, some are
worth noting that
a new irreversible cathodic potential
appeared at ꢀ0.732 V, attributed to the reduction process of
ꢀ
NO3ꢀ to NO2 within the cage,34 perhaps owing to the change of
ionic status in the cavity aer coordination.35 The reduction
Fig. 4 Cyclic voltammograms of (a) L and (b) Pd6L12 in DMSO solution Fig. 5 (a) 31P NMR (162 MHz, 298 K), and (b) 19F NMR (376 MHz, 298 K)
at a scan rate of 50 mV sꢀ1, results are reported versus Ag wire.
spectra of PF6ꢀ@Pd6L12 in DMSO-d6.
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encapsulated within the cavity, and others are free in the
solution.
Synthesis of Pd6L12
L (174.0 mg, 0.2 mmol) and Pd(NO3)2$2H2O (27.0 mg, 0.1 mmol)
were dissolved in DMSO (10.0 mL) were mixed. Each time 1 mL
of the mixture was taken and stirred at 70 ꢁC for 6 h. The
quantitative formation of Pd6L12 was conrmed by 1H NMR and
1H DOSY NMR. The addition of diethyl ether to the yellow
homogeneous solution precipitated the product, and it was
collected by ltration, washed with diethyl ether, and dried in
BF4ꢀ anions can also be encapsulated inside the cavity. With
the addition of KBF4 to Pd6L12,
19F NMR shows two signals, one
strong signal at ꢀ148.27 ppm corresponding well with the peak
of free BF4ꢀ in DMSO-d6 (ꢀ148.3 ppm), and the other new signal
at ꢀ144.1 ppm (Fig. S20 and S21†). 11B NMR show that a new
signal at 2.79 ppm is also assigned to bound BF4ꢀ in the cavity
(Fig. S22 and S23†).38,39 Simultaneously 1H NMR reveals that the
proton resonance of Hi inside the central cavity is shied
upeld (Dd ¼ 0.30 ppm) compared to Pd6L12 (Fig. S19†). These
1
vacuo to give a pale yellow solid (13.8 mg, 70%). H NMR (500
MHz, DMSO-d6): d 9.74 (s, 2H), 9.08 (s, 2H), 8.60 (s, 2H), 7.88 (d,
J ¼ 39.1 Hz, 8H), 2.55 (s, 4H), 1.35 (s, 24H). 1H DOSY NMR (400
MHz, DMSO-d6): D ¼ 7.16 ꢂ 10ꢀ11 m2 sꢀ1. Pd6L12 (PF6ꢀ salt) was
prepared by adding excess NaPF6 to the DMSO solution of
Pd6L12. Electrospray ionization mass spectra were recorded for
Pd6L12 (PF6ꢀ salt). m/z calcd for C444H504N48Au12I12P8F48Pd6 [M–
ꢀ
results suggest that some BF4 anions are bound within the
cavity of Pd6L12. However, addition of OTfꢀ to Pd6L12 causes
neither new signals nor notable shis, and 19F NMR gives only
one sharp signal corresponding to free OTfꢀ, suggesting that
the nitrate ions in the cage was unaffected by OTfꢀ (Fig. S19 and
S24†).
4(PF6)]4+ 3049.4240, found 3049.4123; m/z calcd for C444H504
N48Au12I12P7F42Pd6 [M–5(PF6)]5+ 2410.5462, found 2410.5425;
m/z calcd for
444H504N48Au12I12P6F36Pd6 [M–6(PF6)]6+
1984.6278, found 1984.6254; m/z calcd for C444H504N48Au12I12
P5F30Pd6 [M–7(PF6)]7+ 1680.4003, found 1680.3982; m/z calcd for
-
C
Conclusions
-
In summary, we have successfully synthesized Au(I)–NHC bis-
pyridine ligand (L) and Au(I)–NHC-edged Pd6L12 coordination
cage has been assembled from L and Pd2+. The cage Pd6L12 has
been fully characterized by NMR, diffusion-ordered NMR
spectroscopy, FT-IR, mass spectrometry and cyclic voltammo-
gram. The single crystal X-ray diffraction conrms unequivo-
cally the octahedral structure of Pd6L12. The octahedral cage has
central cavity for guest binding, and haꢀs been shown to be
C
444H504N48Au12I12P4F24Pd6 [M–8(PF6)]8+ 1452.2296, found
1452.2276; m/z calcd for C74H84N8Au2I [2L–I]+ 1605.5190, found
1605.5175.
X-ray crystallography for Pd6L12
Quality single crystals were obtained by the slow diffusion of
ethyl acetate vapour into a DMSO solution of Pd6L12 (NO3ꢀ salt)
to give colourless crystals of Pd6L12 over one month. A crystal
was picked (0.2 ꢂ 0.2 ꢂ 0.2 mm3) and coated in paratone oil,
attached to a glass silk which was inserted in a stainless steel
stick, then quickly transferred to the Agilent Technologies
SuperNova X-ray diffractometer with the Enhance X-ray Source
ꢀ
capable of encapsulating PF6 and BF4 anions within the
cavity. It paves a way to build NHC-based metal–organic
container molecules with large cavity for subsequent guest
binding and transformation.
ꢁ
of Cu Ka radiation (l ¼ 1.54184 A) using the u–f scan tech-
Experimental
Synthesis of L
nique. Data collection were measured at 150.00 (10) K. The unit
cell parameters were solved by direct methods and the unit cell
parameters rened against all data by anisotropic full-matrix
least-squares methods on F2 with the SHELXL program.40
Hydrogen atoms were calculated in ideal positions (riding
model). All nitrate ions and solvent molecules could not be
reasonably located because of highly disordered structures, and
were removed by PLATON/SQUEEZE routine.41
A mixture of c (689.1 mg, 0.715 mmol), 3-pyridylboronic acid
(357.5 mg, 2.86 mmol), anhydrous potassium carbonate
(988.1 mg, 7.15 mmol), tetrakistriphenylphosphine palla-
dium(0) (0.00715 mmol, 8.3 mg) in 18 mL degassed DMF–H2O
ꢁ
(v/v ¼ 5 : 1) was stirred at 100 C for 24 h under argon atmo-
sphere, the progress of the reaction was monitored by TLC. Aer
completion of the reaction, the solvent was removed and the
residue was dispersed in water and extracted with CH2Cl2. The
CH2Cl2 solution was washed with water and brine, dried over
anhydrous MgSO4, ltered and concentrated in vacuo. Puri-
cation by column chromatography on silica gel (ethyl aceta-
te : hexane 1 : 1) to give the product as white powder which was
further puried by washing with hexane (464 mg, 75%). 1H
NMR (400 MHz, DMSO-d6): d 9.05 (s, 2H), 8.64 (s, 2H), 8.27–8.22
(m, 2H), 8.08 (s, 2H), 7.76 (s, 4H), 7.57–7.53 (m, 2H), 2.59 (dt, J ¼
14.0, 6.9 Hz, 4H), 1.36 (d, J ¼ 6.8 Hz, 12H), 1.30 (d, J ¼ 6.9 Hz,
12H). 13C NMR (101 MHz, CDCl3): d 185.55, 149.02, 148.43,
146.49, 140.41, 136.25, 134.79, 133.79, 123.65, 123.29, 123.14,
29.04, 24.48, 24.06. Q-TOF-MS(ESI+) (m/z): calcd. for [M + H]+
[C37H43N4AuI + H]+ 867.2198, found 867.2206 and calcd for [M–
AuI + H]+ [C41H44N2O2 + H]+ 543.3488, found 543.3471.
Crystallographic data for Pd6L12: C74H84Au2I2N8Pd FW ¼
ꢁ
ꢁ
ꢀ
1839.62, trigonal, R3, a ¼ 29.8768 (16) A, b ¼ 29.8768 (16) A, c ¼
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ3
74.341 (3) A, a ¼ 90 , b ¼ 90 , g ¼ 120 , V ¼ 57 468 (7) A , Z ¼ 18,
T ¼ 150.00 (10) K, l ¼ 1.54184 A, rcalcd ¼ 0.957 mg mꢀ3, m ¼
ꢁ
9.349 mmꢀ1, 13 205 reections were collected (7693 were
unique) for 6.856 < 2q < 79.938, R(int) ¼ 0.0372, R1 ¼ 0.0888, wR2
¼ 0.2668 [I $ 2s(I)], R1 ¼ 0.1033, wR2 ¼ 0.2821 (all data) for 725
parameters and 825 restraints, GOF ¼ 1.059. Selected bond
lengths and bond angles are presented in Table S1.† The crystal
structure was submitted to the Cambridge Structural Database
under the CCDC number 2013514.
Conflicts of interest
There are no conicts to declare.
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¨
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