Molecule to Supramolecule: Chirality Induction, Inversion, and Amplification in a Mg(II)porphyrin Dimer Templated by Chiral Diols

Abstract

A clear and unambiguous rationalization of chirality induction, amplification, and subsequent inversion processes has been demonstrated using an achiral Mg(II)porphyrin dimer (host) and a series of chiral diols (guests) upon stepwise formation of a 1:1 host−guest polymer and 1:2 host−guest monomer via intermolecular assembling and disassembling processes. Crystallographic characterizations are reported here for both the polymer and the monomeric complexes, which enable us to completely scrutinize the structural and geometrical changes systematically in rationalizing their optical properties. The sign of the CD couplets for both the polymer and monomer are just opposite between R and S guests, which suggests that the chirality is dictated solely by the stereogenic projection of the chiral centers. Stronger intra-and intermolecular coupling in the polymeric complexes is responsible for the highly enhanced CD couplets as compared to the monomer and have only intramolecular coupling as also observed in their X-ray structures. DFT studies clearly support the experimental observations. ■ INTRODUCTIONControlling supramolecular and macromolecular chirality has become a fascinating research area in the past two decades, not only for understanding the asymmetry of various biological systems but also due to its potential applications to materials for chiral sensing, chiral separation, asymmetric catalysis, etc. 1−3 Of particular interest is the precise regulation of chirality, which has been so far achieved by employing host− guest stoichiometry, metal coordination, solvents, anions, temperature, etc. 4,5 Porphyrins are of special interest for detailed investigations of the supramolecular chirality induction process owing to their interesting photophysical properties, versatile modification, great biological importance, and wide applicability. 5−7 Stoichiometry mediated chirality induction and inversion have been known to occur via an intramolecular 1:1 and 1:2 host−guest complexation mechanism. 6,7 There are only a few reports available in which the chirality induction and inversion processes have been rationalized with X-ray structures using the same host−guest system. 6c,d In the present work, we report stepwise chirality induction, amplification, and inversion processes occurring via intermolecular assembling and disassembling between the Mg(II)porphyrin dimer host and chiral diol guest by varying their stoichiometry. At a lower guest concentration, the 1D polymer of the 1:1 host−guest complex was formed, which upon great addition of the guest converted into the monomeric 1:2 host−guest complex. A clear rationalization of chirality induction, amplification, and inversion phenomena has been reported here, which are fully supported by single crystal X-ray structure determination and DFT calculations. ■ RESULTS AND DISCUSSIONA urea-bridged Ni(II)porphyrin dimer has been synthesized using the reported procedure. 8 After demetalation using concentrated sulfuric acid, magnesium was inserted into the free base porphyrin di...

Hg···Hg···Hg Interaction Stabilizes Unusual Trinuclear Double Sandwich Structure of Mercury(II) Porphyrins

“…The X-ray structure of such complexes with Hg­(II) ion was successfully determined using a highly flexible porphyrin dimer, which revealed very unusual geometrical features and properties and displayed strong mercurophilic interactions. Highly flexible urea-bridge of the porphyrin dimer facilitates the stabilization of such “double sandwich” arrangement having a linear Hg 3 core and, thereby, modulate possible intermacrocyclic and mercurophilic interactions.…”

“…Urea-bridged porphyrin dimer is highly flexible and can easily interconvert between three conformations, i.e., cis–cis , trans–cis , and trans–trans , based on the solvent polarity (Scheme S2). Two porphyrin rings should be in a cofacial arrangement but slipped, which is the geometric requirement for strong π–π interaction. , Although the porphyrin dimer is highly flexible, two strong mercurophilic interactions only bring two porphyrin macrocycles on the exact top of each other with fully eclipsed geometry which are, otherwise, highly unstable.…”

Hydrogen-Bonding Interactions Trigger Induction of Chirality via Formation of a Cyclic Dimer

“…17 Upon complexation with the substrates, the conformation of the host changes from cis-cis to cis-trans, clearly reflected in the generation of two sets of signals for the nonequivalent protons. 15,17 Figure 5 represents the 1 H NMR spectra upon complexation of L 5 with 1 Zn as an illustrative example. Trace C shows the spectrum of 1 Zn , while trace B displays the 1 H NMR spectrum of a crystalline sample of (1 Zn •L 5 ) 2 in CDCl 3 .…”

“…Interestingly, upfield shifts of the substrate protons were much less for similar complexations of diols with 1 Mg which result in a polymeric structure. 15 1 H− 1 H COSY was recorded (Figure S15) at 295 K; however, it was not very informative due to the broad nature of the resonances. The 1 H NMR spectrum of (1 Zn •L 5 ) 2 has also been recorded at 233 K (Figure 5B and Figure S16); all of the peaks are relatively sharp and greatly shifted upfield.…”

“…16b,16 In contrast, the 1 H NMR spectrum of 1 Mg displays a cis-trans orientation in CDCl 3 at 295 K, clearly reflected in the two signals at 6.55 and 8.90 ppm for the nonequivalent −NH protons. 15 The polymeric complex (1 Mg •butanediol (2R,3R) ) n , formed upon complexation of 1 Mg with (2R,3R)-butanediol, displays two signals for the −NH protons at 8.97 and 6.60 ppm, and the bound −OH peak of the substrate gets shifted upfield to −3.01 ppm upon exposure to the strong ring current. 15 In addition, three signals were observed for the meso protons at 10.08, 9.96, and 9.85 ppm.…”