1. No category

Synthesis and characterization of a new nickel
supramolecular square
Paulo Torres 1, Cristian Cano 1, Álvaro Duarte 1, Eliseo Avella 1
[email protected], [email protected], [email protected], [email protected]
1
Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, sede Bogotá.
Abstract: The synthesis and characterization (IR, UV, 1H NMR, 31P, 19F) of the nickel(II) supramolecular square [7] was developed, through the synthesis of the complex [Ni(dppe)Cl 2]
[3] and its subsequent conversion into the complex [Ni (dppe) (OSO 2CF3)2] [5] by reaction with Ag-TOF. The complex [5] was necessary to perform a separation process by extraction
with CH2Cl2 using soxhlet extraction equipment type (a strategy not previously reported). Finally, the self-assembly between the complex [5] with the organic linker 4,4'-bipyridine was
developed to form [7].
Key words: Metal Organic Polyhedral (MOP), Building Blocks, Supramolecular Square
INTRODUCTION
Supramolecular macrocyclic architectures are synthesized from building blocks, through coordination bonds between transition metals and multidentate ligands, which direct their auto
assembly process. For this purpose, complementary building blocks with predefined geometries were designed to allow the modular synthesis of discrete supramolecular structures.
Such architectures are known as metal organic polyhedra (MOP), and are obtained by selfassembly processes between blocks of angular and linear construction, which allow for supplementary components with highly directional donor-acceptor links that will characterize
the geometry of the final self-assembly 1.
A synthesis approach developed mainly by M. Fujita and P. J. Stang, called directional bonding 2, 3, allows for the synthesis of complexes containing metal centers, which act as highly directional corners containing available sites that have the appropriate angles to form polyhedra
and polygons with the desired geometry. Using this approach, architectures in wide ranges of
size and shape (figure 1) can be synthesized by choosing the appropriate ligand and metal,
such as in the case of the synthesis of the supramolecular square, which uses directional complex transition metal centers with two available coordination sites located at 90° and connected with rigid linear binders 4. In this way, Stricklen et al synthesized the first square supramolecular in 1983 from self-assembly processes of angular complexes of Cr and W 5.
Separation by soxhlet extraction equipment
Figure 3. Step-wise synthesis of the nickel square supramolecular complex [7].
RESULTS AND ANALYSIS
The self-assembly reaction progress was followed by UV-Vis spectroscopy, because it was difficult to follow the process through a visually change in solution color. In Figure 4A, a gradual change in the spectrum of the solution could be observed during the reaction, passing from
two electronic transitions (π—π *) (336 nm, 308 nm) to a very broad band, which extends
through the UV region and which exhibits a maximum absorbance at 226 nm, corresponding
probably to the triflate anions that serve as counter ions in the square [7].
To confirm the synthesis of the macrocycle, several characterizations were preformed; some of
them are indicated in the figures 4B, 4C, 4D and 4E.
1
H-NMR spectrum presents a multiplet at 3.99 ppm corresponding to the aliphatic part of the
dppe ligand (-CH2-CH2-). Resulting signals between 7.10 ppm and 7.80 ppm indicate the presence of aromatic protons from dppe ligand; however, its splitting pattern is not clearly identified due to the low resolution of the spectrum. Towards 8.60 ppm the typical signal for 4,4'bipyridine ligand alpha hydrogens, which result as a doublet, is located (figure 4C). COSY 1H
-1H-NMR presents a cross peak section for the coupling of the aromatic hydrogens in the dppe
ligand (7.50 ppm) with the signal of the 4,4'-bipyridine alpha hydrogens (8.60 ppm); this coupling occurs probably due to the spatial proximity between these types of protons (figure 4B).
Figure 1. Directional bonding approach, used for the synthesis of polygons and polyhedra 4
A major achievement in the development of molecular architectures was the synthesis of molecular flasks (3D polyhedra), which have high symmetry cavities called nanoreactors and are
used for practical applications, such as host-guest interactions, catalysis, and molecular sensors. Figure 2 illustrates the ruthenium supramolecular flask constructed from the coordination of Ruthenium (II), using the complex [Ru([12]aneS 4) (H2O) (DMSO] (NO3)2 [A] as the
corners and 2,4,6-tri (4-pyridyl) -1,3,5-triazine [B] as the walls.6. The cavity of this flask has
proved very useful as a molecular sensor.
The 31P-NMR spectrum (figure 4D), presents one unique signal in 30.571 ppm because of two
reasons: the formation of a new species different from the signal of the phosphorus precursor
compound [5], and the presence of a single polyhedron (without polygonal equilibrium).
These new signals belong to the expected ligands contained in the supramolecular structure indicating small differences between free ligands and coordinated ligands. 19F-NMR spectrum
shows a band located at 78.876 ppm, which shows the shift toward low energy field due to the
presence of the triflate as a counterion in the square structure (figure 4E).
[A]
[C]
[B]
[D]
Figure 2. Molecular flask self-assembled from Ruthenium vertices 6
METHODOLOGY
Figure 3 describes the overall methodology for the synthesis of all complexes and the final self
-assembly 7. Complexes [1], [3] and [5], together with the square [7], were characterized by
spectroscopic and spectrometric techniques such as UV-Vis, FT-IR, TGA-DSC, 1H-NMR, 31PNMR and COSY 1H-1H-NMR. 19F and 13C-NMR were significantly hindered due to low solubility in virtually all deuterated solvents.
1. S. Chakraborty, S. Mondal, Q. Li, N. Das. Tetrahedron
Letters, 2013, 54, 1681–1685
2. T. R. Cook, Y. R. Zheng, P. J. Stang. Chem. Rev.
2013, 113, 734−777.
3. M. Fujita, J. Yazaki, K. Ogura. J. Am. Chem. Soc.
1990, 112, 5645-5647.
4. P. J. Stang, J. A. Whiteford, Mixed. J. Am. Chem. Soc.
1994, 94, 2313.
5. P. Stricklen, E. Volcko, J. Verkade. J. Am. Chem. Soc.
1983, 105, 2494.
6. K. Yamashita, M. Kawano, M. Fujita. Chem. Commun. 2007, 4102–4103
7. F. Fochi, P. Jacopozzi, E. Wegelius, K. Rissanen, P.
Cozzini, E. Marastoni, E. Fisicaro, P. Manini, R. Fokkens, E. Dalcanale. J. Am. Chem. Soc. 2001, 123, 75397552.
Acknowledgement: The authors gratefully acknowledge the National University of Colombia,
Faculty of Science, Department of Chemistry and NMR Laboratory
[E]
Figure 4. UV-Vis spectra, 1H-NMR, 31P-NMR, 19F-NMR, COSY 1H-1H-NMR and TGA-DSC for [7].
CONCLUSION
The synthesis of a nickel supramolecular square [7] was developed by the self-assembly of the
complex [Ni(CF3SO3)2(dppe)] [5] and the organic ligand 4,4'-bipyridine, and it was characterized by FT-IR, UV-VIS, TGA-DSC and NMR: 1H, 31P, COSY, and 19F .
It was determined that according to the geometry of the building blocks and the signals in the
NMR spectra, the most likely structure for this supramolecular entity is a square.