Density functional theory investigations on binding and spectral features of complexes of ferrocenyl derivatives with cucurbit [7]uril

Abstract Host–guest interactions between ferrocene (Fc) and its derivatives containing electron donating and electron withdrawing substituents viz., 2-ferrocenyl benzoxazole (FcOz) or 2-ferrocenyl benzothiazole (FcTz) and cucurbit [7]uril (CB[7]) host have been analyzed employing density functional theory incorporating various exchange correlation functionals. Electronic structures of these complexes reveal the encapsulation of ferrocene ring within the CB[7] container with the benzoxazole or benzothiazole substituent interacting with host portals via C = O ⋯ H interactions. Further the M06-2X based calculations show stronger host–guest binding compared to those predicted from wB97x, PBE0 and B3LYP functionals. The contributions of electrostatic and other dispersive interactions toward such binding have been assessed. The 1 H NMR chemical shifts from the gauge independent atomic orbital (GIAO) theory using different functionals demonstrated that subtle changes in δ H signals of cyclopentadienyl protons and those encapsulated in the CB[7] cavity in water are better reproduced by the wB97x functional. Calculated infrared spectra revealed that the complexation is accompanied by frequency shift of the C–N stretching in benzoxazole or benzothiazole ring, and the characteristic methine and carbonyl vibrations of CB[7]. The direction of such frequency shifts has been explained from the natural bond orbital analyses and ‘Quantum Theory of Atoms in Molecules’ approach. Time dependent density functional theory calculations further demonstrated that substitution on the cyclopentadienyl ring brings a bathochromic shift of the ~ 200 nm band in the electronic spectra for the unsubstituted Fc-CB[7] complex. Furthermore, FcOz@CB[7] or FcTz@CB[7] complexes exhibit low energy absorption band at ~ 480 nm in its electronic spectra as a result of intramolecular charge transfer from 3d orbitals of Fe to π* orbitals of benzothiazole or benzoxazole, unlike that in the Fc@CB[7] complex.