Synthesis and a high field NMR study of a 2,2′-bipyridyl substituted β-cyclodextrin and its luminescent ReI metal complex

Abstract

The synthesis and an NMR spectroscopic study of the substituted β-cyclodextrin derivative β-CD(per-OMe)bpy 3, and the metal complex β-CD(per-OMe)bpyRe(CO)₃Br 4 are reported. The ligand 3 was prepared by condensation of monohydroxypermethylated β-cyclodextrin 1 with 6-bromomethyl-2,2′-bipyridine 2 under standard Williamson reaction conditions. The neutral complex, β-CD(per-OMe)bpyRe(CO)₃Br 4, was obtained by treatment of the ligand 3 with Re(CO)₅Br. 600 MHz two-dimensional NMR experiments (DQF-COSY, HOHAHA, ROESY) were used to establish the purity and structure of the ligand 3 and complex 4 in CDCl₃. The two diastereotopic bipyridyl methylene protons (H7a, H7b) in ligand 3 appear as a singlet at 4.82 ppm. The protons H3′, H4′, H5′, and H6a′, 6b′ on the adjacent ring R2 are shifted downfield compared to the protons on the other six sugar rings. In the complex 4, the bipyridyl methylene protons (H7a, H7b) appear as 2 AX systems, R2H6a′ appears as a doublet of doublets and the aromatic protons appear as complex multiplets, consistent with the formation of two diastereomeric complexes 4a and 4b. While circular dichroism measurements suggest that the metal is directed over the cavity, NOE connectivities between the bipyridyl protons and the surface of the cyclodextrin cavity were not detected in ROESY experiments. Preorganized assemblies based on 4, in which the metal is directed over the cavity, provide ideal systems in which to study photochemical communication between the photoactive unit and a guest molecule encapsulated in the cyclodextrin cavity.