Protonation, coordination chemistry, cyanometallate “supercomplex” formation and fluorescence chemosensing properties of a bis(2,2′-bipyridino)cyclophane receptor

Abstract

A new polyazamacrocycle (L) containing two 2,2′-bipyridino (bpy) units, where the heteroaromatic nitrogen atoms point outwards from the macrocyclic cavity, was synthesized and characterized by elemental analysis, ESI-MS, ¹H and ¹³C NMR, FTIR and TGA. Five protonation constants involving aliphatic nitrogens with log K in the range 9.39–3.07 were determined by potentiometry and NMR and a sixth protonation (log K = 2.2) involving a bipyridine moiety could be detected by UV-Vis and NMR titrations. The interaction of L with the cyanometallate anions [Pt(CN)₄]²⁻ and [Co(CN)₆]³⁻ was studied by potentiometry yielding respectively log K values in the ranges 4.0–6.4 and 5.2–10.5, covering protonation degrees from 1 to 5. Studies on L with [Ru(CN)₆]⁴⁻ led to early precipitation preventing potentiometry, but crystals of [H₄L][Ru(CN)₆]·10H₂O and also of [H₂L](ClO₄)₂·4H₂O suitable for X-ray diffraction could be obtained and the crystalline structures are described. The interaction of L with Zn²⁺ was investigated by potentiometry and UV-Vis showing the formation of mono- and dinuclear complexes involving the bipyridines. The macrocycle was used as a bridging ligand for the successful synthesis of a new dinuclear [(Ru(bpy)₂)₂(L)](PF₆)₄·4HPF₆ complex that was fully characterized. The protonation (log K in the range 9.9–1.6) and coordination features of this compound towards Cu²⁺ and Zn²⁺ were explored by UV-Vis absorption and emission spectroscopies.