Sk Asif Ikbal

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.

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 complexs 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.

A Nonempirical Approach for Direct Determination of the Absolute Configuration of 1,2-Diols and Amino Alcohols Using Mg(II)bisporphyrin

We report here a simple, facile, and direct nonempirical protocol for determining the absolute stereochemistry of a variety of chiral 1,2-diols and amino alcohols at room temperature with no chemical derivatization using Mg(II)bisporphyrin as a host. Addition of excess substrates resulted in the formation of a 1:2 host-guest complex in which two substrates bind in an unusual endo-endo fashion because of interligand H-bonding within the bisporphyrin cavity leading to the formation of a unidirectional screw in the bisporphyrin moiety that allowed us an accurate absolute stereochemical determination of the chiral substrate via exciton-coupled circular dichroism (ECCD). The sign of the CD couplet has also been found to be inverted when the stereogenic center is moved by one C atom simply from the bound to an unbound functionality and thus able to discriminate between them successfully. Strong complexation of the alcoholic oxygen with Mg(II)bisporphyrin rigidifies the host-guest complex, which eventually enhances its ability to stereochemically differentiate the asymmetric center. The ECCD sign of a large number of substrates has followed consistent and predictable trends; thus, the system is widely applicable. Moreover, computational calculations clearly support the experimental observations along with the absolute stereochemistry of the chiral substrate.

Self-assembly of cobalt(II) and zinc(II) tetranitrooctaethylporphyrin via bidentate axial ligands: synthesis, structure, surface morphology and effect of axial coordination

A series of remarkably stable supramolecular architectures of cobalt(II) and zinc(II) tetranitrooctaethylporphyrins (tn-OEP) mediated by long and conjugated bipyridyl axial ligands (L) have been synthesized in one pot in excellent yields and structurally characterized. Linear 1D polymeric chains were observed in the X-ray structures of the six-coordinated Co(II) complexes in which all the porphyrin units are aligned parallel to each other to have so-called “shish kebab” like architectures in order to maintain offset-stacked overlap. In contrast, ZnII(tn-OEP) forms five-coordinate porphyrin dimers [ZnII(tn-OEP)]2·L with “wheel-and-axle” like architectures which are then self-aggregated in a perpendicular manner to fill space more effectively. The surface patterns of the polymeric molecules on HOPG surface are found similar as observed in the X-ray structures. Large downfield shift and the peak broadening of the axial ligand resonances in solution are discerned in the 1H NMR of [CoII(tn-OEP)]·L which is due to the contiguity to the paramagnetic Co(II) center. In contrast, upfield shift of the 1H NMR resonances of the axial ligand L are observed for [ZnII(tn-OEP)]2·L due to the shielding effect of porphyrin ring current which confirms the retention of the solid state structures in solution. However, the axial ligation with monodentate pyridine to CoII(tn-OEP) produces six-coordinate complex [CoIII(tn-OEP)(py)2]+ in which Co(II) spontaneously oxidizes to Co(III) in air. The first 1e-reductive response in cyclic voltammetry of [CoIII(tn-OEP)(py)2]+, which is assigned as CoIII/CoII, has been shifted to −0.42 V which explains why addition of pyridine to CoII(tn-OEP) in air results in spontaneous oxidation of Co(II) to Co(III). Weaker ligand field strength of bidentate bipyridyl axial ligand L compared to its smaller monodentate counterpart pyridine has also been demonstrated here.

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

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.