A C3 Symmetric Nitrate Complex with a Thiophene-Based Tripodal Receptor

A thiophene-based tripodal receptor has been synthesized, and its complexes with nitrate and iodide have been determined by single-crystal X-ray analysis. In the nitrate complex, one nitrate is encapsulated in a selective orientation, forming a C-3 symmetric complex, which is bonded to three protonated secondary amines with six NH center dot center dot center dot O bonds. The anion is coordinated in a plane perpendicular to the principal rotation axis passing through the tertiary nitrogen of the receptor and the nitrogen of the encapsulated nitrate. High-level DFT calculations support the crystallographic results, demonstrating that an adduct with trigonal binding of three oxygen atoms is more stable than that of one oxygen atom of the encapsulate nitrate. On the other hand, in the structure of the iodide complex, all three iodides lie outside the cavity. H-1 NMR titration studies indicate that the receptor forms a 1:1 complex with nitrate yielding a binding constant of K = 315 M-1 in chloroform, showing a moderate selectivity over halides and perchlorate.