A Cu(ii) metallocycle for the reversible self-assembly of coordination-driven polyrotaxane-like architectures

Article abstract of DOI:10.1039/C8DT02693A

We report the design and synthesis of a Cu(ii) metallocycle (1) and use the possibility to expand the Cu(ii) coordination sphere to self-assemble mechanically interlocked species via interpenetration. Metallocycle 1 can be used as a platform to reversibly

Full text of DOI:10.1039/C8DT02693A

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A Cu(II) metallocycle for the reversible self-
assembly of coordination-driven polyrotaxane-like
Cite this: DOI: 10.1039/c8dt02693a
architectures†
Giada Truccolo,a Zeno Tessari,a Jacopo Tessarolo,b Silvio Quici,c Lidia Armelao*a,b
b
and Marzio Rancan
*
We report the design and synthesis of a Cu(II) metallocycle (1) and use the possibility to expand the Cu(II)
coordination sphere to self-assemble mechanically interlocked species via interpenetration. Metallocycle
1 can be used as a platform to reversibly assemble a 1D + 1D → 1D coordination-driven polyrotaxane (3),
where 1 acts as the hosting ring as well as the stopper, and 4,4’-bipyridine is the guest-axle. A coordinat-
ing solvent can substitute the 4,4’-bipyridine axle to disassemble the polyrotaxane (3 → 2) that is easily
restored by further adding 4,4’-bipyridine (2 → 3). Other polyrotaxanes can be isolated by reacting 1 with
pyridine (4) and phenylpyridine (5). Interconversion among the presented species is demonstrated and
ensured by the open position of each copper center in platform 1.
Received 2nd July 2018,
Accepted 10th July 2018
DOI: 10.1039/c8dt02693a
rsc.li/dalton
cal assembly of these platforms. This picture is confirmed by
experimental evidence where MOFs have been prepared starting
Introduction
Mechanically interlocked molecular architectures, i.e. molecules from discrete metal organic polyhedra (MOP) or clusters4 inter-
that are entangled as a consequence of their topology, have linked to give a framework due to ditopic ligands. This step-by-
attracted increasing attention.1 The rapid growth of coordi- step bottom–up approach can also be extended to the prepa-
nation polymers and MOFs chemistry offered a great opportu- ration of polymeric interlocked species, provided that suitable
nity to explore structural interlocking, interpenetration and molecular platforms are used. In this context, coordination-
entanglement.2 Many interlocked topologies of coordination driven cycles, boxes and capsules endowed with peculiar host–
polymers are based on rotaxane- and catenane-like motifs.2 guest properties are ideal building blocks to develop new inter-
However, entanglement deriving from metal ion and ligand locked architectures. In 1994, Fujita firstly reported a [2]catena-
self-assembly has been found in coordination polymers, essen- ne5a that quantitatively self-assembled from two preformed
tially by serendipity. The only way to introduce by design rotax- Pd(II)-based metallocycles and, in 1999, the first example of an
ane architectures into 1D, 2D and 3D networks is to use ligands interpenetrated metallo-capsule.5b Since then, the possibility to
that can thread an organic ring leading to the so called MORFs obtain interlocked species starting from metallo-cycles or
(metal–organic rotaxane frameworks), as firstly pioneered by -capsules has been reported by several authors.6 On the other
Kim3a and Loeb to develop molecular machine-like components hand, the self-assembly of infinite interlocked chains such as
in an ordered array.3b,c Many MOFs can be structurally polycatenanes7 based on discrete coordination-driven boxes
described as interlinked polyhedra and clusters via a hierarchi- and cages is not very common, and rotaxane oligomers,8
poly-pseudo-rotaxanes7d or polyrotaxanes9 are even more rare.
Winpenny and collaborators demonstrated how to elegantly syn-
aDepartment of Chemical Sciences, University of Padova, via Marzolo 1,
thesize main-chain [n]rotaxane oligomers,8a,b a daisy chain8c
35131 Padova, Italy. E-mail: lidia.armelao@unipd.it
bInstitute of Condensed Matter Chemistry and Technologies for Energy (ICMATE),
and a 1D polyrotaxane9a based on ion-paired hybrid inorganic–
organic structures where dialkylammonium axles thread the
National Research Council (CNR), c/o Department of Chemical Sciences,
monoanionic [Cr7NiF8(O2CtBu)16]− ring. Lu et al.7d firstly
University of Padova, via Marzolo 1, 35131 Padova, Italy.
E-mail: marzio.rancan@unipd.it, marzio.rancan@cnr.it
reported on a 2D poly-pseudo-rotaxane based on a discrete stool-
like metallocycle. Lee et al.9b described a 1D polyrotaxane con-
structed starting from a discrete dicopper metallocycle and a
ditopic pyridyl ligand, where the two building blocks react to
form both a 1D coordination polymer (the axle) and a discrete
rectangular box (the ring) threaded by an infinite chain.
cInstitute of Molecular Science and Technologies (ISTM), National Research Council
(CNR), Via C. Golgi 19, 20133 Milano, Italy
†Electronic supplementary information (ESI) available: Synthetic procedures
and characterization (NMR, SCXRD, and PXRD). CCDC 1844080–1844085. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8dt02693a
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The ligand L has been prepared using a two step-procedure
and fully characterized as described in the ESI.† The obtained
green powder of 1 is soluble in highly coordinating solvents
such as DMA, DMF and pyridine and is slightly soluble in
THF. Dark green single crystals of {[Cu2(L)2]·2(CHCl3)},
1·2CHCl3, were obtained by pentane vapor diffusion in a
chloroform/THF solution or using the reactant diffusion tech-
nique of the two building blocks. Metallocycle 1 hosts two
chloroform molecules that have hydrogen interactions with
the oxygen atoms of L (O⋯HCCl3, ca. 2.5 Å, Fig. 2a). The
metallocycle involves two Cu2+ ions and two ligands that bind
in a distorted square planar coordination to the metal centres.
The two metal ions are located at a distance of 12.5 Å, while
the two central carbons of the ligand spacer have a distance of
11.1 Å. Thanks to its flat shape, platform 1 packs on layers
stabilized by stacking interactions (Fig. S9, ESI†). By dissolving
1 in DMA, an emerald green solution was obtained suggesting
a change in Cu2+ coordination geometry. The vapor diffusion
of diethyl ether or diisopropyl ether in a 1 DMA solution led to
the quantitative crystallization of the compound {[Cu2(L)2]
(DMA)2} (2, Fig. 2b), where a DMA molecule coordinates each
copper center in the apical position of a distorted square
pyramid. These results confirm that 1 has the desired features
to act as a platform for the formation of polyrotaxanes via
coordination and supramolecular tools: (i) a pocket with
hosting properties and (ii) the ability to coordinate other
species. To explore this possibility, we decided to use 4,4′-
bipyridine (bipy) for the following features: (i) it has σ-donor
atoms to coordinate 1, (ii) it is a ditopic ligand that can lead to
the formation of a chain of “1-bipy” alternating units, and
finally (iii) its dimensions are compatible with the metallocycle
pocket size.
Fig. 1 Building blocks (a) for the Cu(II) platform self-assembly (b). Grey
arrows: ligand coordination vectors. (c) Coordination expansion of the
metal centre.
Here, we describe a Cu(II) metallocycle ([Cu2(L)2], 1) that is
used to reversibly assemble an interlocked 1D + 1D → 1D poly-
rotaxane ({[Cu2(L)2]bipy}n, 3) via coordination chemistry and
supramolecular tools. Metallocycle 1 is large enough to host
molecules in its pocket and to be threaded with an axle. In 3,
metallocycle 1 acts as the hosting ring as well as the stopper,
and 4,4′-bipyridine (bipy) is the guest axle. The process is fully
reversible since a coordinating solvent (dimethylacetamide,
DMA) easily disassembles the polyrotaxane 3 forming the com-
pound {[Cu2(L)2](DMA)2} (2). The same metallocycle also reacts
with pyridine (py) and phenyl-pyridine (phpy) leading to the
discrete assemblies 4 {[Cu2(L)2](py)2} and 5 {[Cu2(L)2](DMA)
(phpy)} that in the solid state adopt other polyrotaxane-like
motifs. Moreover, interconversion among the different species
here presented is enabled by the coordination chemistry of the
open position at each copper site. In fact, the platform is a
[Cu2(L)2] metallocycle self-assembled starting from a Cu(II)
source and a bis-β-diketone (L) ligand with a two-fold symmetry.
Lindoy and coworkers have previously shown how metallocycles
bearing transition metals that can expand their coordination act
as platforms to hierarchically assemble extended metallo-supra-
molecular assemblies.10 According to L coordination vectors
and Cu(II)-β-diketonate coordination geometry, each Cu site is
expected to adopt an approximate square planar geometry with
the possibility of expanding its coordination by reacting with
molecules bearing σ-donor atoms (Fig. 1). As we have previously
demonstrated, this strategy can be used to select a triangular
metallo-supramolecular host from a constitutional dynamic
library through specific interactions with σ-donor guests.11
Conceptually, the preparation of the presented polyrotaxanes
involves a two step-process. In the first step, we prepared a metal-
locycle 1 with hosting properties and metal ions that can expand
their coordination sphere. In the second step, a σ-donor mole-
cule is coordinated at the platform through the metal open sites
and, simultaneously, is used to thread a second metallocycle.
A chloroform solution of L and bipy was layered with a di-
chloromethane buffer and then with a methanol solution of
Cu(CH3COO)2. The slow reactant diffusion led to the for-
Results and discussion
Metallocycle 1 was prepared in quantitative yield combining a
chloroform solution of L‡ and copper acetate in methanol.
Fig. 2 (a) Structure of 1, {[Cu2(L)2]·2CHCl3}. (b) Structure of 2,
{[Cu2(L)2]2(DMA)2}. Colour code: C grey, O red, Cl green, Cu purple; H is
omitted for clarity, except in CHCl3 molecules (white).
‡L: 1-(4-{[4-(3-oxo-3-phenylpropanoyl)phenyl]methyl}phenyl)-3-phenylpropane-
1,3-dione.
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mation of two different types of crystals: long dark green
needles, identified as 1, and tiny light green needles, identi-
fied as {[Cu2(L)2](bipy)}n, 3. Compound 3 can be prepared as a
pure compound and in quantitative yield by mixing a DMA
solution of 1, hence having in solution the species 2, with
bipy, to give a light green microcrystalline powder as con-
firmed by comparison of the experimental PXRD pattern with
the simulated one (Fig. S6b†). In 3, the copper centers coordi-
nate a bipy molecule in the apical position of a distorted
square pyramidal geometry. This leads to the development of a
stair-like polymeric 1D chain (Fig. 3a). However, the structure
of 3 is much more complex, with the two 1D chains being
interlocked in parallel (Fig. 3b), resulting in a system that can
be classified as a 1D + 1D → 1D coordination-driven polyrotax-
ane (Fig. 3c).2b The Cu(II) platform acts as the hosting ring as
well as the stopper, and 4,4′-bipyridine is the guest-axle of the
rotaxane-like unit. In fact, a bipy molecule coordinated two
metallocycles 1 to form a 1D chain and it is at the same time
hosted by a third metallocycle of a second chain. Small vari-
ations in the metallocycle dimension are observed if compared
to 1 and 2 with a Cu⋯Cu distance of 12.0 Å, while the two
central carbon atoms of the ligand spacer have a distance of
11.8 Å. The distance Cu-bipy-Cu is 11.5 Å. The bipy unit inside
the 1 pocket has hydrogen interactions with oxygen atoms of L
(ESI,† O⋯HC, ca. 2.6 Å). Only four examples of 1D + 1D → 1D
polyrotaxanes based on metallocycle rings have been reported
so far.12 However, in these cases the metallocycle does not
exist in solution as a discrete unit. In contrast, in the case here
described, the polyrotaxane can be self-assembled starting
from the single building blocks (Cu2+, L, and bipy) in a one-
pot reaction as well as starting from the metallocycle platform
1 coupled to bipy (Fig. 4a). This allowed us also to easily
reverse the polyrotaxane formation. In fact, in the presence of
a coordinating solvent, such as DMA, polyrotaxane 3 is dis-
assembled since DMA substitutes the bipy axle coordinating
the copper ions and leading to compound 2. UV-Vis analysis
showed that a DMA solution of the Cu(II) platform and the dis-
solved polyrotaxane in DMA have the same spectrum. Once
Fig. 4 (a) Self-assembly of polyrotaxane 3 via a one-pot reaction or
through the Cu(II) platform (1 → 2 → 3). (b) Scheme of the reversible
polyrotaxane formation. UV-Vis of the metallocycle 1 and of the dis-
solved polyrotaxane 3 (inset: DMA solution of 3), and a single crystal of 3.
disassembled, the entangled system is easily and quantitatively
restored by adding more bipy and isolated as a polycrystalline
powder (Fig. 4b). The microcrystalline powder obtained has
been characterized via PXRD. The resulting diffractogram is in
very good agreement with the calculated one (Fig. S6b†) as
observed from the single crystal data of 3, confirming the
nature and purity of the compound. Determining the self-
assembly pathway of a coordination polyrotaxane is not an
easy task. The ESI-MS of a diluted 1·THF solution (10−5 M)
Fig. 3 Polyrotaxane 3, {[Cu2(L)2](bipy)}n. (a) A single polymeric chain of “1-bipy” alternating units. (b) Two interlocked chains giving the polyrotaxane.
(c) Schematic representation of the 1D + 1D → 1D coordination-driven polyrotaxane and of the rotaxane-like motif.
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plus a stoichiometric amount of bipy gave a low intensity different molecules, i.e. DMA and phpy. Both the molecules
signal at m/z = 1199.4, compatible with the species {[Cu2(L)2] take the apical position of a distorted square pyramid. At var-
(bipy) + H}+. To gain more information on polyrotaxane for- iance to the findings for the other compounds, the two mole-
mation we used pyridine (py) and phenylpyridine (phpy) as cules lie on the same side of the metallocycle. The two copper
σ-donor molecules. These two molecules have very similar ions are located at a distance of 12.4 Å, while the two central
coordination behaviours compared to bipy, but not being carbon atoms of the ligand spacer have a distance of 10.9 Å;
divergent ditopic ligands they should lead to discrete systems. both values are very similar to those found in 1, 2 and 3. Also
Platform 1 promptly dissolves in py to form an emerald green 5 shows interpenetration, with the DMA molecule of one ring
solution, and single crystals of {[Cu2(L)2](py)2} (4, Fig. 5a) were threading a second one to give a 1D poly-pseudo-rotaxane
isolated from a DMA/py solution after partial evaporation. The (Fig. 5d).
distances between the two metal ions and between the two
Finally, as it is possible to reversibly assemble polyro-
central carbon atoms of the ligand spacer deviate from the taxane 3 using the open coordination sites in platform 1, we
data found in 1, 2 and 3 with values of 13.5 Å (Cu⋯Cu) and demonstrated that similar interconversion can be exploited
9.9 Å (C⋯C). At variance to the other compounds, the σ-donor among all the compounds here presented, as depicted in
atom does not take the longer apical positions, but it lies on Fig. 6.
an equatorial site of the distorted square pyramids, and the
On the basis of the coordination and host–guest properties
apical positions are occupied by the oxygen atoms of of platform 1 coupled to its tendency to form polyrotaxane-like
L. Considering the short CH⋯CH contacts (2.6 Å) between two motifs, as evidenced by compounds 4 and 5, the pathway
different phenyl rings in the apical position (Fig. 5b, dashed depicted in Fig. 7 for the formation of the 1D + 1D → 1D
lines), a supramolecular 1D chain is formed in the solid state. coordination-driven polyrotaxane 3 appears plausible. Starting
Moreover, these terminal phenyl rings slightly penetrate other from two building blocks (1 and bipy, stage a), the pathway
rings. Pairs of these 1D chains entangle, with each metallo- proceeds via the formation of {[Cu2(L)2](bipy)} discrete units
cycle of one chain threaded by a Ph⋯Ph axle of the other (stage b), as supported by ESI-MS analysis. This step may be
chain, establishing a 1D + 1D → 1D supramolecular polyrotax- followed by the formation of daisy chain-like motifs (stage c).
ane with the same intertwining of the coordination-driven Although we did not observe any evidence on the formation of
polyrotaxane 3. When dissolving 1 in DMA and adding an these units, we reasoned that this should be the key step to
excess of phpy, single crystals of {[Cu2(L)2](DMA)(phpy)} (5, form the interlocked architecture. The growth of the daisy
Fig. 5c) were obtained by vapor diffusion of diethyl ether or chain (stage n) leads to the final 1D + 1D → 1D intertwined
diisopropyl ether. In this case, platform 1 coordinates two system.
Fig. 6 Interconversion paths among the compounds here presented.
Fig. 5 (a) Structure of 4, {[Cu2(L)2](py)2}, and (b) its 1D + 1D → 1D poly-
rotaxane intertwining supported by CH⋯CH interactions (dashed lines).
(c) Structure of 5, {[Cu2(L)2]2(phpy)(DMA)}, and (d) its 1D poly-pseudo-
rotaxane motif (lateral view) with the DMA molecules threading the
metallocycles.
Fig. 7 (a) Proposed pathway for the formation of the 1D + 1D → 1D
coordination-driven polyrotaxane 3.
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C 69.91%, N 1.21%, H 4.95% (exp.); C 69.93%, N 1.16%,
H 5.03% (calc.).
Conclusions
In conclusion, we have demonstrated how a discrete metallo-
{[Cu2(L)2](bipy)}n (3). Method A. A methanol solution (4 ml)
cycle can be used as a versatile platform to self-assemble poly- of Cu(CH3COO)2·H2O (5.00 mg, 0.025 mmol) was quickly
rotaxane architectures. The proposed strategy to access these added to a chloroform solution (4 ml) of L (11.50 mg,
coordination-driven polymeric intertwined structures is to 0.025 mmol) and bipy (3.90 mg, 0.025 mmol). The instan-
design and synthesize a metallocycle with two key features: taneous precipitation of a clear green solid was observed, and
(i) host–guest properties and (ii) metal centers prone to expand it was recovered by filtration and washed with chloroform.
their coordination sphere. In a second step, such features are Yield: 95%. Elemental analysis for 3, C72H52Cu2N2O8:
exploited by reacting the platform with suitable σ-donor mole- C 72.11%, H 4.30%, N 2.35% (exp.); C 72.05%, H 4.37%,
cules to obtain coordination-driven polyrotaxanes. Hence, the N 2.33% (calc.).
1D + 1D → 1D polyrotaxane 3 was straightforwardly obtained
Method B. 4.1 mg (0.004 mmol) of 1 were dissolved in a
by adding the ditopic bipy ligand to 1. The formation of the minimum volume of DMA (ca. 1 ml) to give an emerald green
polyrotaxane architecture is accompanied by a phase change solution. A bipy (0.005 mmol) solution in THF was added to
from solution to a crystalline solid that can be easily dis- give a clear green solid in quantitative yield. Elemental analysis
assembled in the presence of a coordinating solvent (3 ⇄ 2). for 3, C72H52Cu2N2O8: C 71.95%, H 4.41%, N 2.37% (exp.);
The existence of 1 in solution as a discrete species is of para- C 72.05%, H 4.37%, N 2.33% (calc.).
mount importance for the reversible process. In the same way,
Method C (for single crystals suitable for SCXRD). A chloro-
the coupling of platform 1 with py and phpy led to two discrete form solution of L (11.85 mg), bipy (3.93 mg) and triethyl-
metallocycles (4 and 5). Upon crystallization, and due to the amine (7.5 μl) was layered with dichloromethane and with a
interpenetration of metallocycle pockets, we obtained a further methanol solution of Cu(CH3COO)2·H2O (5.00 mg). The test
1D + 1D → 1D polyrotaxane supported by CH⋯CH interactions tube was stoppered and the solutions were let to diffuse slowly.
(4) and a poly-pseudo-rotaxane (5). A judicious use of the After a few days, two types of crystals were obtained: long dark
coordination chemistry of the metal open coordination sites green needles (yield: 50%) and tiny light green needles (yield:
also allowed the easy interconversion of each compound.
25%). The first kind has been identified as 1 and the second
one as 3 by single crystal XRD.
{[Cu2(L)2](py)2} (4). 10 mg (0.01 mmol) of 1 were dissolved
in a minimal amount of a DMA/py solution, mixed and heated
for 1 hour and left undisturbed for slow evaporation. The
DMA/py ratios used were: 9 : 1, 4 : 1, 7 : 3, 3 : 2, and 1 : 1. After a
Experimental
Synthesis
Ligand L. Ligand L was prepared using a two step-procedure, month, single crystals of 4 were obtained from the solutions
(i) Friedel Crafts acylation and (ii) Claisen condensation, and with DMA/py ratios of 1 : 1, 2 : 3 and 3 : 7 (yield: 23%, 53% and
characterized as described in the ESI.†
72%, respectively). Elemental analysis for 4, C72H54Cu2N2O8:
[Cu2(L)2] (1). Method A. A methanol solution (4 ml) of C 72.02%, H 4.58%, N 2.31% (exp.); C 71.93%, H 4.53%,
Cu(CH3COO)2·H2O (5.00 mg, 0.025 mmol) was quickly added N 2.33% (calc.).
to a chloroform solution (4 ml) of L (11.50 mg, 0.025 mmol).
{[Cu2(L)2](DMA)(phpy)} (5). 10 mg (0.01 mmol) of 1 were dis-
The instantaneous precipitation of an olive green solid was solved in a minimal amount of a DMA solution. To this solu-
observed, and it was recovered by filtration and was washed tion an excess of phpy was added. The mixture was stirred and
with chloroform. Yield: 90%. Crystals of 1·2CHCl3 were iso- heated for 1 hour. A slow diffusion of diethyl ether or diiso-
lated by pentane vapor diffusion in a 1·chloroform/THF solu- propyl ether vapor was carried out. After two days, plate-shaped
tion. Elemental analysis for 1, C62H44Cu2O8: C 71.40%, light green crystals were quantitatively isolated and identified
H 4.33% (exp.); C 71.32%, H 4.25% (calc.).
as 5. Elemental analysis for 5, C77H60Cu2N2O8: C 72.75%,
Method B. Single crystals of 1·2CHCl3 were obtained using a H 4.68%, N 2.27% (exp.); C 72.91%, H 4.77%, N 2.21% (calc.).
reactant diffusion technique. A chloroform solution (4 ml) of L
Interconversion experiments
(11.50 mg, 0.025 mmol) was layered with dichloromethane
(ca. 2.5 ml) and with
a
methanol solution (4 ml) of
Compound 3 from 2, 4 and 5. Compound 2, or 4 or 5 was
Cu(CH3COO)2·H2O (5.00 mg, 0.025 mmol). The test tube was dissolved in a minimum amount of THF. To this mixture a
stoppered and the solutions were let to diffuse slowly. After a bipy solution (THF) was added. A green microcrystalline pre-
few days, long dark green needles of 1 were obtained.
cipitate was formed. The nature and purity of the compound
{[Cu2(L)2](DMA)2} (2). 4.1 mg (0.004 mmol) of 1 were dis- were confirmed by PXRD (Fig. 5b).
solved in the minimum volume of DMA (ca. 1 ml) to give an
Compound 4 from 2, 3 and 5. Compound 2, or 3 or 5 was
emerald green solution. The solution was stirred, heated at dissolved in a minimum amount of a 1 : 1 DMA/py solution.
50 °C for 1 hour and filtered. A slow diffusion of diethyl ether Slow evaporation led to single crystals of 4. The nature and
or diisopropyl ether vapor was carried out. After three days, purity of the compound were confirmed by PXRD (Fig. 5c).
needle-shaped light green crystals were quantitatively isolated
Compound 5 from 2, 3 and 4. Compound 2, or 3 or 4 was
and identified as 2. Elemental analysis for 2, C70H60Cu2NO10: dissolved in a minimum amount of a 1 M phpy solution in
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DMA. Diethyl ether vapour diffusion led to the quantitative
isolation of compound 5. The nature and purity of the com-
pound were confirmed by PXRD (Fig. 5d).
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X-ray diffraction
Powder X-ray diffraction (PXRD). PXRD patterns were
obtained with a Bruker D8 Advance diffractometer, in Bragg–
Brentano geometry, equipped with a Göbel mirror using
Cu Kα. The patterns were acquired in the 5–40° 2θ range
(0.03° per step and 10 s per step).
Single crystal X-ray diffraction (SCXRD). The details of
SCXRD data collections, crystal structures solution and refine-
ment, and crystallographic tables for each compound are
reported in the ESI.†
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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This work was supported by the University of Padova
(grant: P-DISC #CARL-SID17 BIRD2017-UNIPD), project
CHIRoN. J. T. thanks ICMATE-CNR for a postdoctoral fellow-
ship. Dr R. Seraglia (ICMATE-CNR) is acknowledged for
ESI-MS analysis.
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