Synthesis, Structure, and Properties of a Series of Chiral Tweezer–Diamine Complexes Consisting of an Achiral Zinc(II) Bisporphyrin Host and Chiral Diamine Guest: Induction and Rationalization of Supramolecular Chirality

Abstract

We report here the synthesis, structure, and spectroscopic properties of a series of supramolecular chiral 1:1 tweezer-diamine complexes consisting of an achiral Zn(II) bisporphyrin (Zn2DPO) host and five different chiral diamine guests, namely, (R)-diaminopropane (DAP), (1S,2S)-diaminocyclohexane (CHDA), (S)-phenylpropane diamine (PPDA), (S)-phenyl ethylenediamine (PEDA), and (1R,2R)-diphenylethylene diamine (DPEA). The solid-state structures are preserved in solution, as reflected in their (1)H NMR spectra, which also revealed the remarkably large upfield shifts of the NH2 guest protons with the order Zn2DPO·DAP > Zn2DPO·CHDA > Zn2DPO·PPDA> Zn2DPO·PEDA ≫ Zn2DPO·DPEA, which happens to be the order of binding constants of the respective diamines with Zn2DPO. As the bulk of the substituent at the chiral center of the guest ligand increases, the Zn-Nax distance of the tweezer-diamine complex also increases, which eventually lowers the binding of the guest ligand toward the host. Also, the angle between the two porphyrin rings gradually increases with increasing bulk of the guest in order to accommodate the guest within the bisporphyrin cavity with minimal steric clash. The notably high amplitude bisignate CD signal response by Zn2DPO·DAP, Zn2DPO·CHDA, and Zn2DPO·PPDA can be ascribed to the complex's high stability and the formation of a unidirectional screw as observed in the X-ray structures of the complexes. A relatively lower value of CD amplitude shown by Zn2DPO·PEDA is due to the lower stability of the complex. The projection of the diamine binding sites of the chiral guest would make the two porphyrin macrocycles oriented in either a clockwise or anticlockwise direction in order to minimize host-guest steric clash. In sharp contrast, Zn2DPO·DPEA shows a very low amplitude bisignate CD signal due to the presence of both left- (dictated by the pre-existing chirality of (1R,2R)-DPEA) and right-handed screws (dictated by the steric differentiation at the chiral center) of the molecule, as evident from X-ray crystallography. The present work demonstrates a full and unambiguous rationalization of the observed chirality transfer processes from the chiral guest to the achiral host.

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

“…, by utilizing the chirogenesis that is the foundation of exciton coupled circular dichroism (ECCD). Parameters involved in this phenomenon have been extensively studied, and models to rationalize the effect have been proposed [3,4,5,6,7,8,9,10,11,12,13,14,15]. We found it tempting to test this effect by exposing the sterically flexible lysine methylester 23 , and the more rigid tryptophan methylester 24 (Figure 11) to tweezers 1 – 3 .…”

“…It occurred to us that detection of the induced circular dichroism ( i.e. , exciton coupled circular dichroism, ECCD) [3,4,5,6,7,8,9,10,11,12,13,14,15] that results from binding a chiral diamine guest might provide a simple experimental verification of the distortion modes available to our tweezers (Table 1). Tweezer flexibility and guest geometry determine the sign and amplitude of the detected ECCD, and suitable mnemonics for reliable prediction of the effect are still under development [8,9].…”

“…In the majority of these compounds, two porphyrin chromophores are attached by a single bond to a usually conformationally flexible spacer. They have been used extensively to determine the absolute configuration of guests with a single stereogenic center, or to distinguish enantiomers [3,4,5,6,7,8,9,10,11,12,13,14,15]. We recently have shown that bisporphyrin tweezers also can be utilized for determination of the relative stereochemistry in molecules with several stereocenters, employing a semi-rigid bisporphyrin tweezer 3 incorporating a glycoluril spacer [16,17].…”

“…However, bisporphyrins with flexible spacers are capable of strong binding to dinitrogen ligands. In typical examples, small aliphatic diamines were found to bind with K a = 10 3 –10 5 M −1 to a bisporphyrin with diphenylether spacer, with weaker binding for bulkier guests [5]. Much higher binding constants have been reported for the structurally more rigid DABCO (diazabicyclo[2.2.2]octane) with K a = 10 7 –10 9 M −1 [18,19].…”

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.…”