Complexation of Chiral Zinc-Porphyrin Tweezer with Achiral Diamines: Induction and Two-Step Inversion of Interporphyrin Helicity Monitored by ECD

Abstract

We report here the synthesis of a new chiral Zn(II) bisporphyrin tweezer in which two achiral Zn(II) porphyrin moieties are covalently linked by (1R,2R)-diphenylethylenediamine, which produces a strong chiral field around the porphyrin moieties. The chiral tweezer exhibits not only intensity modulation in UV-vis and CD exciton couplets but also a dramatic change, namely, the inversion in the sign of the interporphyrin helicity upon binding of achiral diamines of varying lengths. The stoichiometry-controlled formation of a 1:1 sandwich complex followed by a 1:2 open complex was realized with smaller achiral diamines (n: 2-5) at their low and high concentration regions, respectively, leading to two-step inversion of chirality. With longer achiral diamines (n: 6-8), however, only 1:1 sandwich complexes are formed with no change of sign in the CD couplet. As compared to a 1:2 open complex, a 1:1 sandwich complex shows an enhanced CD response as two porphyrin units come closer in space. Structural insights of the host-guest complexes have been obtained spectroscopically along with molecular mechanics minimizations with the newly implemented OPLS-3 force field followed by geometry optimization using density functional theory of the most stable conformer. The amide bridge in the Zn(II) bisporphyrin has a low rotational barrier, which provides conformational flexibility to change interporphyrin helicity between 1:1 and 1:2 binding depending on the size of the achiral guests in order to minimize host-guest steric interactions.

Metalloporphyrin Dimers Bridged by a Peptoid Helix: Host-Guest Interaction and Chiral Recognition

“…Owing to the opposite preference of peptoid backbone and guest amino acid for the orientation of co-facial porphyrins, a diminished ECCD signal is observed ( Figure 6 b). In addition, the M-shape ECCD signal of 2 with l -Lys-OMe (equivalence higher than 1) suggests the possible equilibrium of two (or more) conformers giving different exciton coupling signals [ 24 ]. With the highest equivalents of l - and d -Lys-OMe, 2 did not exhibit the mirror image of ECCD ( Figure 7 b), unlike achiral 1 that exhibited the mirror image shown in Figure 2 b.…”

“…Chiral co-facial porphyrins are interesting molecular tweezers by virtue of their enantioselective binding with chiral guests. By connecting two porphyrins with chiral linkers, such as 1,1′-bi-2-naphthol (BINOL) [ 23 ], chiral diamine [ 24 ], or benzyl ester [ 25 ], a pre-organized chiral interporphyrin twist can be maintained through intramolecular chirality induction. The chiral porphyrin tweezer recognizes and differentiates chiral guests such as amino acids [ 26 ] or diamines [ 27 ], and this leads to enantioselective host–guest binding.…”

Сhiral and Racemic Fields Concept for Understanding of the Homochirality Origin, Asymmetric Catalysis, Chiral Superstructure Formation from Achiral Molecules, and B-Z DNA Conformational Transition

“…A strong chiral field was shown to exist between two flat aromatic "pincers" (tweezers) [196,197] separated by some kind of a more or less rigid "teter" with a chiral moiety as a chirality inductor. Porphyrin or metalloporphyrin groups at the chain ends with the chiral inductor in the middle of this host molecule, for example 50, can be used as tweezers [198][199][200][201]. The length of the chain the inductor and porphyrin group separating can exceed 13 single and double bonds 173 .…”

Heterocomponent ternary supramolecular complexes of porphyrins: A review

“…In general, porphyrin-tweezers have two covalently linked porphyrin units which are used to bind a guest molecule between the porphyrin moieties [33]. For example, a chiral linker may generate asymmetry in the whole structure of tweezers, hence allowing for building of chiral supramolecular complexes with various external guests such as achiral diamines [34,35]. However, in the case of ternary complexes, a third component is needed to stabilize the tweezer conformation ( Fig.…”