Supramolecular Chirogenesis in Zinc Porphyrins:  Interaction with Bidentate Ligands, Formation of Tweezer Structures, and the Origin of Enhanced Optical Activity

Abstract

The complexation behavior, binding properties, and spectral parameters of supramolecular chirality induction in the achiral host molecule, syn (face-to-face conformation) ethane-bridged bis(zinc porphyrin), upon interaction with chiral bidentate guests (diamines and amino alcohols) have been studied by means of UV-vis, CD, fluorescence, (1)H NMR, and ESI MS techniques. It was found that the guest structure plays a decisive role in the chirogenesis pathway. The majority of bidentate ligands (except those geometrically unsuitable) exhibit two major equilibria steps: the first guest ligation leading to formation of the 1:1 host-guest tweezer structure (K(1)) and the second guest molecule ligation (K(2)) forming the anti bis-ligated species (1:2). The second ligation is much weaker (K(1) >> K(2)) due to the optimal geometry and stability of the 1:1 tweezer complex. The enhanced conformational stability of the tweezer complex ensures an efficient chirality transfer from the chiral guest to the achiral host, consequently inducing a remarkably high optical activity in the bis-porphyrin.

Syn–anti conformational switching: Synthesis and X-ray structures of tweezer and anti form in a zinc porphyrin dimer induced by axial ligands

“…However, the changes in the porphyrin Q-band region (500-700 nm) are less pronounced. The spectral pattern is associated with 1:1 host-guest tweezer-2 Á ODAB by comparing with the similar spectrum reported previously for tweezer-2 Á DACH (DACH: 1,2-diaminocyclohexane) [18,[20][21][22][23] and was confirmed by the ESI mass spectral analysis that reveals a signal at m/z 1326.6558 assigned for tweezer- Table 1 Crystal data and data collection parameters for the complexes.…”

“…8), because of ligand's close proximity to the two porphyrin subunits and hence strong ring current effect. Similar shifts of the guest's protons clamped between the porphyrin subunits have been also observed in tweezer-2 Á DACH reported earlier [22]. On the other hand, protons of the porphyrin subunits (-CH 2 (b) and 10,20-meso-H) are significantly downfield shifted due to conformational change upon tweezer formation, which result in moving the two porphyrin rings further apart from each other (compared to 2) and subsequently decreasing the shielding effect of the neighboring porphyrin.…”

“…Two porphyrin planes in 2 are found to be in a face-to-face conformation in nonpolar and non-coordinating solvent [20][21][22][23]. This spatial arrangement is extremely stable with no observable changes in the conformation even upon heating up to 110°C.…”

Absolute Configurational Assignments of Amino Acid Esters by a CD‐Sensitive Malonamide‐Linked Zinc Bisporphyrinate Host

“…[6a] In other cases, when a bidentate ligand is used, remarkable CD spectral changes have been observed at high ligand molar excess. [15] Such spectral changes were not observed in this study (see Figures S5-S14). So all these suggest amino acid esters function as monodentate ligands.…”

Supramolecular Chirality in Porphyrin Chemistry

“…The more rigidly ethane-linked bis-porphyrin, 105 (Figure 28), was able to form the corresponding supramolecular chiral structures upon interaction, as with bidentate and with monodentate guests [5,15,[78][79][80][81][82][83][84][85][86]. In the case of enantiopure monoamines, the induced CD amplitude was in the range of 7.2-129.7 cm…”