Selective detection of DABCO using a supramolecular interconversion as fluorescence reporter

Article abstract of DOI:10.3762/bjoc.15.137

The quantitative double self-sorting between the three-component rectangle [Cu₄(1)₂(2)₂]⁴⁺ and the four-component sandwich complex [Cu₂(1)(2)(4)]²⁺ is triggered by inclusion and release of DABCO (4). The fully reversible and clean switching between two multicomponent supramolecular architectures can be monitored by fluorescence changes at the zinc porphyrin sites. The structural changes are accompanied by a huge spatial contraction/expansion of the zinc porphyrin-zinc porphyrin distances that change from 31.2/38.8 ? to 6.6 ? and back. The supramolecular interconversion was used for the highly selective detection of DABCO in a mixture of other similar compounds.

Full text of DOI:10.3762/bjoc.15.137

Selective detection of DABCO using a supramolecular
interconversion as fluorescence reporter
Indrajit Paul, Debabrata Samanta, Sudhakar Gaikwad and Michael Schmittel*
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 1371–1378.
Center of Micro and Nanochemistry and Engineering, Organische
Chemie I, Universität Siegen, Adolf-Reichwein-Str. 2, D-57068
Siegen, Germany
doi:10.3762/bjoc.15.137
Received: 28 February 2019
Accepted: 27 May 2019
Published: 21 June 2019
Email:
Michael Schmittel* - schmittel@chemie.uni–siegen.de
This article is part of the thematic issue "Novel macrocycles – and old
ones doing new tricks".
* Corresponding author
Keywords:
Guest Editor: W. Jiang
copper; detection; fluorescence; interconversion; macrocycles;
self-assembly; self-sorting; zinc porphyrin
© 2019 Paul et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The quantitative double self-sorting between the three-component rectangle [Cu4(1)2(2)2]4+ and the four-component sandwich
complex [Cu2(1)(2)(4)]2+ is triggered by inclusion and release of DABCO (4). The fully reversible and clean switching between
two multicomponent supramolecular architectures can be monitored by fluorescence changes at the zinc porphyrin sites. The struc-
tural changes are accompanied by a huge spatial contraction/expansion of the zinc porphyrin–zinc porphyrin distances that change
from 31.2/38.8 Å to 6.6 Å and back. The supramolecular interconversion was used for the highly selective detection of DABCO in
a mixture of other similar compounds.
Introduction
Since dynamic multicomponent supramolecular structures are functions, for instance by presenting three-state catalytic ma-
nowadays abundant [1,2], the weak intercomponent binding chinery with up and down regulation [21] or for networking cat-
[3-9] is often instrumentalized for supramolecular transformat- alytic machinery [22].
ions [10], but rarely exploited strategically for specific func-
tions. Elegant examples for the utility of dynamic interactions, In contrast, the actual contribution seeks to explore the utility of
in particular metal–ligand coordination, are thermally driven supramolecular rearrangements [23] for sensing and detection,
supramolecular devices, such as ball bearings [11,12], crank in particular with an emphasis on high selectivity. As a notable
engines [13], rotors [14-17] and oscillators [18]. Recent work example of the latter category, Nitschke recently reported the
from our group [19] has strived for combining metallosupramo- guest-induced transformation of porphyrin edge capsules to
lecular transformation(s) [20] with the creation of emergent cone-shaped inclusion complexes depending on the presence of
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C60/C70, however, a process that was not selective for one of shown in Scheme 1 is a dual-state supramolecular transformat-
the guests [24]. A spectacular case of guest sensing, but not ion driven by addition/removal of DABCO (4) and it requires a
guest-induced recognition, was demonstrated by Clever in a transition between completive vs incomplete self-sorting
supramolecular cage-to-cage conversion that allowed detection [27,28]. The fact that DABCO (4) exclusively drives the supra-
of the product by shape recognition [25]. Unfortunately, the molecular interconversion was further developed into a fluores-
cage-to-cage transformation proved to be rather slow. Schalley cent reporter system [29-31] with high selectivity.
used the addition of both, guests and hosts, to stimulate a
cascaded folding of cucurbit[7,8]uril pseudorotaxanes [26]. Results and Discussion
Neither of the above examples was demonstrated to be revers- To realize the aspired switching protocol, we have tested
ible after removing the guest. This compilation of remarkable metal-ion and guest-dependent completive and incomplete self-
results already indicates that guest-induced supramolecular sorting scenarios [32] by mixing ligands 4, 8, 9, 10 and
transformations are not yet explored to their full potential. [Cu(CH3CN)4]PF6 in a ratio of 1:1:1:2:1 (Scheme 2). Ligand 8
was equipped with a trimethoxyphenyl group to furnish a fourth
Herein, we will present the formation of self-assembled three to coordination to the copper(I) center and a sterically crowded
four-component supramolecules, such as rectangle 5 and sand- duryl group to prevent homoleptic complexation, while lutidine
wich 6 (Figure 1), as well as their responsiveness to a variety of 9 was selected to strengthen the HETPYP-I [33] (HETeroleptic
potential guests. Despite the topological simplicity of the PYridine and Phenanthroline complexation) coordination. In
assemblies involved, their multicomponent arrangements this setting, the binary complex 12 = [(4)(10)2] and the
require perfect heteroleptic control. Conceptually, the process heteroleptic metal complex 11 = [Cu(8)(9)]+ quantitatively
Figure 1: Molecular structures of ligands 1, 2, 3, and 4 and of the resulting products, i.e., rectangle 5, sandwich 6 and rhodium porphyrin dimer 7.
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Scheme 1: Guest addition/removal and reversible interconversion between supramolecular architectures.
Scheme 2: Completive and incomplete self-sorting in presence of copper(I) and rhodium complex 3.
formed side by side in a two-fold completive self-sorting. Luti- porting Information File 1, Figure S24) than towards zinc por-
dine 9 has a higher binding preference towards the copper phyrin 10 (log K(9)(10) = 1.82 0.21) [34] due to its bulky
phenanthroline [Cu(8)]+ (log K(9)·[Cu(8)]+ = 4.60 0.21, Sup- α-methyl groups. Therefore, in the self-sorting zinc porphyrin
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10 prefers to form the stable sandwich complex 12 with ascertained by the 1H DOSY NMR (Supporting Information
DABCO [35,36] at log β[(4)(10)2] = 7.20 0.15 (Supporting File 1, Figure S29) showing a single species with a diffusion
Information File 1, Figure S21) to satisfy maximum site occu- coefficient of 2.43 × 10−10 m2s−1. The thus derived molecular
pancy.
radius of 21.7 Å is in very good agreement with the computed
r = 21.5 Å (DFT, see Supporting Information File 1, Figure
Upon the addition of 2 equiv of rhodium porphyrin 3 [37], S31). Rectangle 5 was also characterized through the expected
DABCO was selectively removed from complex 12 [38] 1H NMR pattern in particular as the signal of proton h-H in
affording the sandwich complex 7 = [(3)2(4)] leaving complex ligand 1 is shifted diagnostically from 6.28 to 6.08 ppm due to
11 untouched and liberating two equiv of 10 (incomplete self- the shielding of the lutidine unit by the π-system of ligand 2
sorting, Scheme 2). This phenomenon is readily explained (Figure 2).
considering the stronger binding of rhodium porphyrin 3 to
DABCO (Δlog β = 2.40) compared with zinc porphyrin (Sup- Self-assembly in a similar manner using [Cu(CH3CN)4]PF6
porting Information File 1, Figure S22).
and ligands 1, 2 and 4 (2:1:1:1) afforded complex 6 =
[Cu2(1)(2)(4)]2+ as the exclusive product at room temperature.
With this ligand shuffling in mind, we wanted to probe the A single diffusion coefficient in the 1H DOSY NMR
guest-induced double self-sorting depicted in Scheme 1. There- (D = 4.40 × 10−10 m2s−1) as well as a single set of signals in the
fore, ligands 1 and 2 were synthesized by a palladium-cata- 1H NMR spectrum provided evidence of high purity. The ex-
lyzed Sonogashira coupling reaction (Supporting Information perimental radius of 12.0 Å reflects the computed radius of the
File 1). All compounds were fully characterized by 1H NMR, largely contracted aggregate (r = 12.3 Å). As seen in the
1H,1H-COSY, UV–vis, ESIMS and elemental analysis (Sup- 1H NMR, the lutidine unit of ligand 2 is split into two sets with
porting Information File 1).
proton e1’-H appearing at 6.56 ppm and e2’-H emerging at
7.32 ppm. The explicit upfield shift of proton e1’-H is due to
Subsequently, we prepared the supramolecular rectangle 5 shielding by the duryl group of the phenanthroline and very
and sandwich complex 6. At first, ligands 1, 2, and similar to the one experienced by the methyl protons (f2’-H)
[Cu(CH3CN)4]PF6 (1:1:2) were mixed in CD2Cl2 immediately that are diagnostically shifted upfield to −0.63 ppm. Proton h-H
giving rise to rectangle 5 at room temperature. The clear red is equally split into two sets reflecting the strong coordination
complex was characterized by ESIMS, 1H NMR, 1H,1H COSY, of one methoxy group at an unsymmetrically coordinated
UV–vis and by elemental analysis (Supporting Information copper(I) center. Similar to model system 12, sandwich com-
File 1). The ESIMS exhibited a single peak at m/z = 1534.5 plex 6 experiences a strong upfield shift of the DABCO protons
(Supporting Information File 1, Figure S19) representing 5 = but now these protons are split into two sets at −4.89 and
[Cu4(1)2(2)2]4+, constituting strong evidence that 5 is the sole −5.09 ppm which clearly supports the formation of a hetero
product of this particular reaction. This notion was further bisporphyrin sandwich (Figure 2). The ESIMS spectrum with
Figure 2: Comparison of partial 1H NMR spectra (400 MHz, CD2Cl2, 298 K) of (a) ligand 2; (b) ligand 1; (c) porphyrin 3; (d) complex 7 = [(3)2(4)];
(e) rectangle 5 = [Cu4(1)2(2)2]4+]; (f) sandwich complex 6 = [Cu2(1)(2)(4)]2+.
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its peak at m/z = 1589.4 is in line with the integrity of complex Equally, the emission wavelength changes by addition of
6. In sum, the clean formation of complexes 5 and 6 provides a DABCO. Figure 3c nicely illustrates the shift of the emission
reliable base for the elaboration of completive and incomplete band from λ = 602 → 618 nm (λexc = 557 nm) for the conver-
double self-sorted guest-induced structural rearrangements.
sion of complex 5 → 6 illustrating that DABCO inclusion into
the porphyrinic sandwich entails a shift of 16 nm. Finally, a
In order to verify the forward conversion shown in Scheme 1, single set of 1H DOSY confirms the successful rearrangement.
ligands 1 and 2 as well as [Cu(CH3CN)4]PF6 were mixed at a
1:1:2 ratio in CD2Cl2 to afford rectangle 5 (state I), as con- To probe the selectivity of the guest-induced transformation of
firmed by 1H NMR. The rectangle furthermore exhibits 5, the structure interconversion was tested with other potential
(Figure 3a,c) diagnostic absorption bands at λ = 550 and guests, using fluorescence and 1H NMR spectroscopy. In the
594 nm in dichloromethane (Q-band) and an emission at absence of any external ligand, the rectangle 5 shows its typical
λ = 602 nm (excited at λ = 557 nm; isosbestic point of conver- fluorescence at λ = 602 nm. Ligands, such as pyrazine,
sion 5 to 6). When 1 equiv of DABCO (4) was added at room 2-chloropyrazine, 1,4-dimethylpiperazine, anthracene, pyrene,
temperature, the deep red color of complex 5 immediately coronene, perylene, and perylene-3,4,9,10-tetracarboxylic dian-
changed to greenish, furnishing the sandwich complex 6 (state hydride were compared to DABCO. Only in presence of
II). As expected from the independently prepared sample, the DABCO the fluorescence maximum was shifted to λ = 618 nm
1H NMR shows two sets of DABCO protons at a 1:1 ratio and along with the color changing from red to green. As displayed
agreement with the full 1H NMR signature of 6. The guest-in- in Figure 3d, the results demonstrate that the emission doesn’t
duced conversion was further validated by a UV–vis titration. change in wavelength with any ligand except DABCO.
Upon the addition of DABCO the Q-band absorptions of 5 at
λ = 550 and 594 nm shifted to λ = 560 and 604 nm which is ex- The high selectivity of architecture 5 towards DABCO was at-
pected for the NDABCO → zinc porphyrin coordination [39,40]. tributed to factors such as binding strength of the ditopic
Figure 3: (a) UV–vis titration of rectangle 5 (2.98 μM) with DABCO (4); (b) several reversible cycles of interconversion monitored at λ = 594 and
604 nm; (c) emission spectra of states I and II (λexc = 557 nm); (d) emission spectra of 5 (λexc = 557 nm) after adding various potential guests.
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ligands and minimum steric repulsion. For instance, pyrazine (= 2 × [Cu2(1)(2)(4)]2+) was easily converted to state I by addi-
creates notable repulsive interactions of the α-H towards the tion of 4 equiv of 3 and state II was again regained by addition
zinc porphyrin ring in a sandwich complex. Apparently, of 2 equiv of DABCO. Multiple cycles of state I → state II
stability gains through π–π stacking in the sandwich with transformations were established through 1H NMR (Figure 4)
pyrene, coronene, etc. are not strong enough to compensate for and UV–vis spectra (Figure 3b).
the strain in [Cu2(1)(2)(guest)]2+. Encouraged by this finding,
we decided to probe the selectivity for DABCO in the presence DFT (B3LYP/6-31G(d)) calculations on rectangle 5 and sand-
of a mixture of all ligands (all ligands used at the same molar wich complex 6 allow the modeling of the supramolecular
amount). The emission is the same as that in Figure 3c for architecture and provide some structural insights. The DFT
DABCO alone demonstrating that DABCO is cleanly selected computations demonstrate that the sandwich complex is quite
even in such complex mixtures. Thus, DABCO is a highly strained and structurally distorted to a spiral shape (Supporting
selective trigger for the structural rearrangement of rectangle 5 Information File 1, Figures S31 and S32).
to sandwich complex 6.
Conclusion
Finally, we tested the reversibility of the system by addition and In conclusion we demonstrated three cycles of the fully
removal of DABCO using rhodium porphyrin 3 as scavenger of reversible DABCO-induced structural rearrangement between
DABCO. In line with the results of the model self-sorting multicomponent architectures 5 and 6. The multiple, clean and
scenarios in Scheme 2, the system turned out to be fully quantitative interconversion is the result of a delicate double
reversible without loss (Scheme 3). For example, state II self-sorted transformation requiring orthogonality of two
Scheme 3: The high selectivity for DABCO in the transformation.
Figure 4: Partial spectra (400 MHz, CD2Cl2, 298 K) showing the reversible switching between rectangle and sandwich complexes over 2.0 cycles;
(a) after mixing of [Cu(CH3CN)4]PF6, 1, and 2 in 2:1:1 ratio, furnishing rectangle 5 (state I); (b) after adding 2.0 equiv of DABCO, furnishing sandwich
complex 6 (state II); (c) after addition of 4.0 equiv of rhodium porphyrin 3, leading to formation of rectangle 5 and rhodium porphyrin dimer 7.
(d) Further addition of 2.0 equiv of DABCO furnishes state II; (e) finally, addition of 4.0 equiv of rhodium porphyrin 3 recovered rectangle 5 along with
4.0 equiv of complex 7.
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Supporting Information
Supporting Information File 1
Experimental details and characterization data.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-15-137-S1.pdf]
Acknowledgements
We are indebted to the Deutsche Forschungsgemeinschaft for
funding (Schm 647/20-2) and the University of Siegen for
continued support.
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Debabrata Samanta - https://orcid.org/0000-0002-4226-7147
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