Investigation of Host-Guest Interactions in 2-Ureido-4-ferrocenylpyrimidine Derivatives

In the present study, synthesis, conformational behavior, host-guest complex formation, and electrochemical properties of novel 6-substituted-2-ureido-4-ferrocenylpyrimidines were explored. A comprehensive NMR spectroscopic investigation was carried out to confirm the structure and conformational equilibrium of the ureidopyrimidines through studying the temperature- and concentration dependence of NMR spectra. Low-temperature NMR measurements were used to clarify structural changes inflicted by a 2,6-diaminopyridine guest. Association constant (K_assoc) values of host-guest complexes were calculated based on low-temperature titrations. It was shown that the introduction of a pyridin-2-yl substituent in the pyrimidine ring in host 10 induced a considerable change not only in the conformational equilibrium of the host itself but also in that of the host-guest complex. Geometries and relative stabilities of the conformers of host 10 as well as its host-guest complexes were determined by quantum chemical calculations. Electrochemical behavior of ureidopyrimidine hosts and host-guest complexes was investigated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements. Two ureidopyrimidine derivatives were immobilized on the surface of spectral graphite electrodes, and their electrochemical response on the addition of 2,6-diaminopyridine was compared. These results also supported the importance of the pyridin-2-yl substituent in the efficient sensing of the guest.