Study of β-cyclodextrin–pyromellitic diimide complexation. Conformational analysis of binary and ternary complex structures by induced circular dichroism and 2D NMR spectroscopies

Abstract

Complexation of N-alkyl derivatives of PMDI with β-CD is probed using a variety of techniques. Although MALDI-TOF and CV experiments suggested complex formation, it is very evident from UV–vis and NMR experiments that these complexes are different from regular inclusion complexes. A clear understanding of the structure of the binary complex PMDI@β-CD could be obtained using ICD and NMR ROESY experiments. ICD signals were negative which suggest that the PMDI moiety is placed outside of the cavity. ROESY experiments provide support for this contention. When the alkyl group is t-butyl or 2-propyl, the CH₃ protons exist very close to the inner protons of β-CD, but the aromatic proton of PMDI is clearly outside the β-CD cavity. Based on these results we proposed a structure for PMDI@β-CD with the PMDI moiety placed at the narrow rim of β-CD and the N-alkyl group projecting into the cavity and designated these as “rim-binding” complexes. Additional experiments showed that β-CD can accommodate a PMDI moiety at the narrow rim and an adamantane moiety in its cavity simultaneously, resulting in the formation of ternary complexes PMDI⊃β-CD⊂ADA. Structure of the ternary complex was also probed by ROESY. The ternary complex formation can be utilized for the design of higher order functional materials such as CD-based hydrogels.