Approaches to the Molecular Recognition of Lanthanides Using Azulenyl-, Phenyl-, Beta-Naphtyl- and Vinyl-Malonate Derivatives

Molecular recognition remains a very active field in the supramolecular chemistry research [1, 2] as well as the elaboration of novel improved ligands. The main focus of our group was on the azulene compounds characterized by an electron rich five-atoms cyclic moiety and a electron-poor seven-atoms cyclic moiety. This "push-pull" double structure where the azulenyl is the electron donating group and the substituent is the electron withdrawing group offers some interesting properties to the molecule. Due to this particular structure the azulenic derivatives could be involved in both oxidation and reduction processes while the nature of the substituent and its’ orientation greatly influences the electrochemical properties (number of waves and the oxydo-reduction potential). The azulenic derivatives are also polyvalent starting materials and the formation of electrode-deposed polymers with electro-activity has been widely reported.

In this regard certain new derivatives containing azulenyl and a vinyl-ester group have been synthesized and then physically and chemically characterized [3]. Their ability to interact with lanthanide trivalent cations have been studied by means of electrochemical and UV-Vis procedures. Those interactions are based on the high polarizability of azulenic ligands which can form strong dipole-dipole coordination bonds with the metallic cations. This polarisation is favored by the stabilisation of the polar structure as tropylium ion (Figure 1).

Starting from the positive results obtained towards some lanthanide cations (Sm³⁺, Eu³⁺, Yb³⁺, Tb³⁺) in solution, with this new class of azulenyl-malonates and –malonimides [4, 5], we also considered for this purpose their more simple phenyl- and β-naphtyl-vynilmalonate derivatives.

The compounds were characterized by electrochemical methods (cyclic voltammetry and differential pulse voltammetry) and the polymeric film formation was also tested by potential scanning or controlled potential electrochemistry. The titration of the ligand solution with several lanthanides offered a view on their complexing properties. UV-Vis spectroscopy and ¹D-NMR further indicated the formation of architectures with various stoichiometries and the existence of multiple binding possibilities, supporting a pseudo-cooperative binding. X-ray diffraction couldn’t offer more relevant information as the crystals obtained were of poor quality. This is also confirmed by the observation that the structure in the assumed complexing region greatly influences the ligand behaviour, the complexing ability increasing in the order: Nf-CH = C(COOEt)₂ > Ph-CH = CH-CH = C(COOEt)₂ > Ph-CH = C (COOEt)₂> Nf, Ph-CH = C (CN) COOEt.

Acknowledgments: POSDRU/137390 project for the financial support in realising this work.

References

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(2)     C. Coluccini, D. Pasini, P. Righetti, D. A. Vander Griend, Tetrahedron, 2009, 65, 10436–10440.

(3)     A. C. Razus, S. Nica, L. Cristian, M. Raicopol, L. Birzan, A. E. Dragu, Dyes Pigm., 2011, 91, 55-61.

(4)     C.-A. Amarandei, G. O. Buica, G. A. Inel, L. Birzan, E. M. Ungureanu, Acta Chimica Slovenica, 2014, 61, 894-899.

(5)     E.-M. Ungureanu, G.-O. Buica, A. C. Razus, L. Birzan, R. Weisz, M.-R. Bujduveanu, Rev. Chim.(Buc), 2012, 63(1), 27-33.