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Iodide and triiodide anion complexes involving anion–π interactions with a tetrazine-based receptor

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Abstract

Protonated forms of the tetrazine ligand L2 (3,6-bis(morpholin-4-ylethyl)-1,2,4,5-tetrazine) interact with iodide in aqueous solution forming relatively stable complexes (ΔG° = −11.6(4) kJ mol−1 for HL2+ + I = (HL2)I and ΔG° = −13.4(2) kJ mol−1 for H2L22+ + I = [(H2L2)I]+). When solutions of [(H2L2)I]+ are left in contact with air, crystals of the oxidation product (H2L2)2(I3)3I·4H2O are formed. Unfortunately, the low solubility of I3 complexes prevents the determination of their stability constants. The crystal structures of H2L2I2·H2O (1), H2L2(I3)2·2H2O (2) and (H2L2)2(I3)3I·4H2O (3) were determined by means of X-ray diffraction analyses. In all crystal structures, it was found that the interaction between I and I3 with H2L22+ is dominated by anion interactions with the π electron density of the receptor. Only in the case of 1, the iodide anions involved in close anion–π interactions with the ligand tetrazine ring form an additional H-bond with the protonated morpholine nitrogen of an adjacent ligand molecule. Conversely, in crystals of 2 and 3 there are alternate segregated planes which contain only protonated ligands hydrogen-bonded to cocrystallized water molecules or I3 and I forming infinite two-dimensional networks established through short interhalogen contacts, making these crystalline products good candidates to behave as solid conductors. In the solid complexes, the triiodide anion displays both end-on and side-on interaction modes with the tetrazine ring, in agreement with density functional theory calculations indicating a preference for the alignment of the I3 molecular axis with the molecular axis of the ligand. Further information about geometries and structures of triiodide anions in 2 and 3 was acquired by the analysis of their Raman spectra.

Graphical abstract: Iodide and triiodide anion complexes involving anion–π interactions with a tetrazine-based receptor

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Publication details

The article was received on 12 Jan 2017, accepted on 20 Feb 2017, published on 20 Feb 2017 and first published online on 20 Feb 2017


Article type: Paper
DOI: 10.1039/C7DT00134G
Citation: Dalton Trans., 2017, Advance Article
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    Iodide and triiodide anion complexes involving anion–π interactions with a tetrazine-based receptor

    M. Savastano, C. Bazzicalupi, C. García, C. Gellini, M. D. López de la Torre, P. Mariani, F. Pichierri, A. Bianchi and M. Melguizo, Dalton Trans., 2017, Advance Article , DOI: 10.1039/C7DT00134G

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