Non-heme iron, vanadium, and copper complexes bearing hemicryptophane cavities were evaluated in the oxidation of methane in water by hydrogen peroxide. According to 1 H nuclear magnetic resonance studies, a hydrophobic hemicryptophane cage accommodates a methane molecule in the proximity of the oxidizing site, leading to an improvement in the efficiency and selectivity for CH 3 OH and CH 3 OOH compared to those of the analogous complexes devoid of a hemicryptophane cage. While copper complexes showed low catalytic efficiency, their vanadium and iron counterparts exhibited higher turnover numbers, ≤13.2 and ≤9.2, respectively, providing target primary oxidation products (CH 3 OH and CH 3 OOH) as well as over-oxidation products (HCHO and HCOOH). In the case of caged vanadium complexes, the confinement effect was found to improve either the selectivity for CH 3 OH and CH 3 OOH (≤15%) or the catalytic efficiency. The confined space of the hydrophobic pocket of iron-based supramolecular complexes plays a significant role in the improvement of both the selectivity (≤27% for CH 3 OH and CH 3 OOH) and the turnover number of methane oxidation. These results indicate that the supramolecular approach is a promising strategy for the development of efficient and selective bioinspired catalysts for the mild oxidation of methane to methanol.

Two new supramolecular complexes consisting of an achiral bisporphyrin host and a chiral diamine guest are reported. One shows a remarkably high amplitude bisignate CD signal while the other one shows a very low value. X-ray structure and other spectroscopic investigations of the tweezer complexes clearly rationalize the origin of the optical activity that has so far remained an unresolved issue.

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.

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.

A series of supramolecular architectures of magnesium tetranitrooctaethylporphyrins mediated by several bidentate axial ligands have been synthesized in excellent yields and structurally characterized. Six conjugated axial ligand with increasing chain lengths have been utilized in the present investigations in which the Mg···Mg nonbonding distance between successive ions also increases from 0.73 to 2.70 nm in the series. To the best of our knowledge, this is the first report where stable metallo-porphyrin polymers with such long spacers have been synthesized in one pot so easily. Linear one-dimensional (1D) polymeric chains were observed in the X-ray structure of the six-coordinated complexes in which porphyrin units are aligned parallel to each other to have so-called "shish kebab" like architectures to maintain offset-stacked overlap. However, after an optimum Mg···Mg nonbonding distance, these 1D chain do not continue, rather they form five-coordinated porphyrin dimers with "wheel-and-axle" like architectures which are then self-aggregated by π-π interactions in a perpendicular manner to fill space created by large bridging ligands more effectively which consequently results in spherical structures. The structures of the molecules in solution and their surface patterns on highly ordered pyrolytic graphite (HOPG) have also been investigated.

A clear rationalization of the origin of chirality transfer from an optically active diol guest to an achiral Mg(ii)bisporphyrin host in a series of 1 : 2 host-guest supramolecular complexes has been reported here that has so far remained the most outstanding issue for the chirogenic process.

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.

Four new chiral tweezer-diamine complexes, consisting of an achiral ferrocene-bridged Zn(II)bisporhyrin host (1) and two small diamines (1R,2R)-1,2-diphenylethylene diamine {(1R,2R)-DPEA} and (1S,2S)-1,2-cyclohexane diamine {(1S,2S)-CHDA} and two extended diamines (1R,2R)-N,N'-bis-(isonicotinoyl)-1,2-diphenylethylene diamine {(1R,2R)-DPEApy} and (1S,2S)-N,N'-bis-(isonicotinoyl)-1,2-cyclohexane diamine {(1S,2S)-CHDApy} chiral guests, are reported. Additions of (1R,2R)-DPEA and (1S,2S)-CHDA separately to 1 in dichloromethane result in the formation of 1:1 sandwich complexes 1·DPEA(R,R) and 1·CHDA(S,S), respectively, at low guest concentration and 1:2 anti complexes 1·(DPEA(R,R))2 and 1·(CHDA(S,S))2, respectively, at higher guest concentration. In contrast, separate additions of (1R,2R)-DPEApy and (1S,2S)-CHDApy to 1 produce only 1:1 sandwich complexes of 1·DPEApy(R,R) and 1·CHDApy(S,S), respectively. The binding constants at 295 K between 1 and (1R,2R)-DPEA are observed to be (4.7 ± 0.2) × 10(4) M(-1) and (4.3 ± 0.3) × 10(3) M(-1) for 1:1 sandwich and 1:2 anti form, respectively, while the respective values with (1S,2S)-CHDA are (1.5 ± 0.2) × 10(5) M(-1) and (5.9 ± 0.3) × 10(3) M(-1). However, much larger values of (2.5 ± 0.3) × 10(5) M(-1) and (1.3 ± 0.3) × 10(6) M(-1) have been observed with DPEApy(R,R) and CHDApy(S,S), respectively, to produce the corresponding 1:1 sandwich complexes. 1·DPEApy(R,R) (Acal, -1759 cm(-1) M(-1)) (Acal = Δε1 - Δε2, representing the total amplitude of the calculated circular dichroism (CD) couplets) shows ∼10-fold increase in CD amplitude compared to the values observed for 1·DPEA(R,R) (Acal, +187 cm(-1) M(-1)), while 1·CHDApy(S,S) (Acal, +1886 cm(-1) M(-1)) shows nearly 3-fold increase of the CD amplitude compared to the value observed for 1·CHDA(S,S) (Acal, -785 cm(-1) M(-1)) at 295 K. The Acal values of -1759 cm(-1) M(-1) and +1886 cm(-1) M(-1) observed for the 1·DPEApy(R,R) and 1·CHDApy(S,S), respectively, are extremely high. To the best of our knowledge, these are some of the largest values reported for a chirality induction process involving bisporphyrin tweezer receptors. The large enhancement in the CD signal intensity is due to the well complementarity size between Zn(II)bisporphyrin host and the extended chiral diamines guest, which results large unidirectional twisting of two porphyrin units to accommodate the guests having preorganized binding sites with minimum host-guest steric interactions. It is interesting to note that 1·DPEA(R,R) and 1·DPEApy(R,R) show CD signal opposite in sign to each other, which happens to be the case between 1·CHDA(S,S) and 1·CHDApy(S,S) also.