Induction of supramolecular chirality in di-zinc(ii) bisporphyrin via tweezer formation: synthesis, structure and rationalization of chirality

Abstract

Two new supramolecular complexes consisting of an achiral bisporphyrin host and a chiral diamine guest are reported. One shows a remarkably high amplitude bisignate CD signal while the other one shows a very low value. X-ray structure and other spectroscopic investigations of the tweezer complexes clearly rationalize the origin of the optical activity that has so far remained an unresolved issue.

Synthesis and Properties of Bis-Porphyrin Molecular Tweezers: Effects of Spacer Flexibility on Binding and Supramolecular Chirogenesis

“…The enediyne spacer, on the other hand, shows no twisting in the absence of a guest, and might bind a chiral guest without preference for porphyrin twisting in a particular direction, which would result in cancellation of the CD signal. Such a situation has been described by Rath and co-workers, based on X-ray crystallography [26]. However, there are also examples where tryptophan methylester 24 bound to bisporphyrin tweezers fails to give a CD signal at all [9].…”

“…Binding studies with tweezers 1 – 3 and a series of dinitrogen guests (Scheme 4), with both variable and fixed N−N distances, were performed to probe the impact of conformational flexibility on binding affinity. If we compare their maximal N−N distances with those obtained from the calculated Zn−Zn distances of their complexes with tweezer 3 , subtraction of twice the assumed Zn−N bond length (2.2 Å) [26] gives the N−N distance of bound guest, which is 2.62 Å ( 19 ), 6.76 Å ( 20 ), 7.84 Å ( 21 ) and 8.12 Å ( 22 ), respectively, indicating coiling of the flexible guests 21 and, in particular, 22 . As shown by pronounced isosbestic points (Figure 6 and Figure 7), accompanied by red shifts of the Soret- and Q-bands, formation of a single, well defined complex is indicated (Table 3) [10,45,46] which, according to NMR data ( vide infra ), is a 1:1 complex (see Supplementary Materials).…”

“…The effect of rigidity variation on CD performance has been addressed for chiral bisporphyrin tweezers without bound guests [25], whereas we here investigate achiral bisporphyrin tweezers. Recently, Rath and co-workers have shown that chirogenesis in a bisporphyrin tweezers requires porphyrin twisting [26].…”

Unusual crystal structure and chirality of uridine 5′-monophosphate coordination polymer

“…From previous studies, the coordination ratio of the auxiliary ligand going from binary to ternary in the solution could be studied by UV titration. [25][26][27][28] In order to study the interactions of bpda/bpe with Cu-UMP in an aqueous medium, UV titrations were carried out by adding an auxiliary ligand to the solution of Cu-UMP. UV absorbance was measured as a function of concentration of bpda and bpe ( Fig.…”

Synthesis of a Series of Zinc Porphyrins and Spectroscopic Changes upon Coordination Reaction with Imidazole Derivatives

“…Porphyrins are often used as model compounds for some important biological systems due to their unique roles in biology and sensing systems, such as in imitating plants and the reaction center of bacterial photosynthesis, or the active center of cell pigment P‐450 . The axial coordination reaction between a porphyrin and a small molecule containing a nitrogen atom has been widely used as a reaction model of hemoglobin, myoglobin, cytochrome‐ c , and other biological molecules.…”