Yasuhide Inokuma

X-ray analysis on the nanogram to microgram scale using porous complexes

X-ray single-crystal diffraction (SCD) analysis has the intrinsic limitation that the target molecules must be obtained as single crystals. Here we report a protocol for SCD analysis that does not require the crystallization of the sample. In our method, tiny crystals of porous complexes are soaked in a solution of the target, such that the complexes can absorb the target molecules. Crystallographic analysis clearly determines the absorbed guest structures along with the host frameworks. Because the SCD analysis is carried out on only one tiny crystal of the complex, the required sample mass is of the nanogram–microgram order. We demonstrate that as little as about 80 nanograms of a sample is enough for the SCD analysis. In combination with high-performance liquid chromatography, our protocol allows the direct characterization of multiple fractions, establishing a prototypical means of liquid chromatography SCD analysis. Furthermore, we unambiguously determined the structure of a scarce marine natural product using only 5 micrograms of the compound.

Unambiguous Identification of Möbius Aromaticity for meso-Aryl-Substituted [28]Hexaphyrins(1.1.1.1.1.1)

meso-Aryl-substituted [28]hexaphyrins(1.1.1.1.1.1) have been examined by (1)H, (13)C, and (19)F NMR spectroscopies, UV-vis absorption spectroscopy, magnetic circular dichroism spectroscopy, and single-crystal X-ray diffraction analysis. All of these data consistently indicate that [28]hexaphyrins(1.1.1.1.1.1) in solution at 25 degrees C exist largely as an equilibrium among several rapidly interconverting twisted Möbius conformations with distinct aromaticities, with a small contribution from a planar rectangular conformation with antiaromatic character at slightly higher energy. In the solid state, [28]hexaphyrins(1.1.1.1.1.1) take either planar or Möbius-twisted conformations, depending upon the meso-aryl substituents and crystallization conditions, indicating a small energy difference between the two conformers. Importantly, when the temperature is decreased to -100 degrees C in THF, these rapid interconversions among Möbius conformations are frozen, allowing the detection of a single [28]hexaphyrin(1.1.1.1.1.1) species having a Möbius conformation. Detailed analyses of the solid-state Möbius structures of compounds 2b, 2c, and 2f showed that singly twisted structures are achieved without serious strain and that cyclic pi-conjugation is well-preserved, as needed for exhibiting strong diatropic ring currents. Actually, the harmonic-oscillator model for aromaticity (HOMA) values of these structures are significantly large (0.85, 0.69, and 0.71, respectively), confirming the first demonstration of stable Möbius aromatic systems consisting of free-base expanded porphyrins without the assistance of metal coordination.

Radical CH Functionalization of Heteroarenes under Electrochemical Control

Electrochemical reactions are shown to be effective for the C-H functionalization of a number of heterocyclic substrates that are recalcitrant to conventional peroxide radical initiation conditions. Monitoring reaction progress under electrochemical conditions provides mechanistic insight into the C-H functionalization of a series of heterocycles of interest in medicinal chemistry.

A Porous Coordination Network Catalyzes an Olefin Isomerization Reaction in the Pore

All-trans retinal efficiently diffused into the pore of a porous coordination network consisting of ZnI(2) and an electron-deficient triazine-cored ligand. Enclathrated retinal was isomerized into the 13-cis form and easily replaced with all-trans retinal in solution, thus revealing the catalysis of retinal isomerization by the porous network.

meso-(4-(N,N-dialkylamino)phenyl)-substituted subporphyrins: remarkably perturbed absorption spectra and enhanced fluorescence by intramolecular charge transfer interactions.

A series of meso-(4-(N,N-dibenzylamino)phenyl)-substituted subporphyrins was synthesized by means of Buchwald-Hartwig amination protocol. Substitution of the amino group at the 4-position of the meso-phenyl substituent resulted in a remarkable red shift in the absorption spectra and drastic enhancement of fluorescence intensity probably as a consequence of intramolecular CT interaction. These characteristics have been utilized to construct a cation-sensing system by appending a 1-aza-15-crown-5 unit to subporphyrin that displays large spectral changes upon cation binding.

The crystalline sponge method updated

The protocols of the crystalline sponge method, particularly those in the soaking, data collection and refinement processes, are considerably improved to give reliable structural information.

Where is the Oxygen? Structural Analysis of α‐Humulene Oxidation Products by the Crystalline Sponge Method

Crystal structures of α-humulene, a cyclic sesquiterpene, and its oxidized subproducts, were analyzed by the crystalline sponge method. Regio- and stereochemistry, including absolute configuration when a chiral oxidant was applied, and the stable conformations of all the scaffold-related compounds were successfully determined for samples on a 5-50 μg scale.

Corrigendum: X-ray analysis on the nanogram to microgram scale using porous complexes

This corrects the article DOI: 10.1038/nature11990

The Catalytic Z to E Isomerization of Stilbenes in a Photosensitizing Porous Coordination Network

The tris(4-pyridyl)triazine ligand (1) is an important organic building block for self-assembled coordination cages and networks. Typically employed because of its rigid planarity, triangular shape, and commercial availability, ligand 1 is extremely electron-deficient and, upon coordination of the pyridyl arms, 1 can become electroand photochemically active. Guest interactions with the low-lying lowest unoccupied molecular orbital (LUMO) of 1 in triazine-based hosts regularly generate host– guest charge-transfer complexes, and photoirradiation can induce efficient energy transfer or, in some cases, photoinduced electron transfer. We hypothesized that network 2, which is generated from 1 and ZnI2, could display similar photochemistry in the solid state; thus we examined the photoinduced isomerization of stilbene within coordination network 2 (Scheme 1a). Enclathrated within the pores of 2, (Z)-stilbene selectively isomerized to (E)-stilbene under visible light irradiation (Scheme 1b); the Z/E equilibrium ratio typical for the photostationary state (Z/E = 92:8 at lex = 313 nm) was not obtained. As guest molecules can freely diffuse from the pores of 2 into the solution, crystals of 2 efficiently catalyzed the one-way Z!E isomerization of stilbene in cyclohexane. The porous network complex [{(ZnI2)3(1)2}·x(C6H5NO2)]n (2, x 5.5) employed in this work was prepared according to the reported procedure. When the as-synthesized network 2 was soaked in a solution of (Z)-stilbene (3a) in cyclohexane, the crystals immediately turned from pale to bright yellow. Elemental analysis showed the inclusion of approximately one molecule of (Z)-3 a per unit with a formula of [{(ZnI2)3(1)2}·x((Z)-3a)·y(cyclohexane)]n (2’, x 1.1, y 1.0). The diffuse reflectance UV/Vis spectrum showed a new, broad charge-transfer (CT) band at approximately 450 nm. Since this CT band was not observed in a solution of ligand 1 and (Z)-3 a in toluene, coordination of 1 to zinc(II) ions and proximity of the guest with 1 in the network pore play a crucial role in inducing effective CT interactions. Crystals of 2’, which were suspended in a solution of (Z)3a in cyclohexane, were photoirradiated with a Xe lamp (lex = 400–500 nm) for 83 h. This procedure resulted in greater than 98% conversion into (E)-3 a in both the crystal and in the supernatant, as determined by H NMR spectroscopy. To analyze the stilbene contained in network 2’, the crystals were decomposed with hydrochloric acid and extracted with CHCl3. No other photo-by-products, for example, dihydrophenanthrene or photooxidized products, were detected. Finally, X-ray diffraction analysis of the photoirradiated network 2’ provided convincing evidence of the formation of (E)-3a in the pores of network 2’ (Figure 1 and the Supporting Information). Enclathrated molecules of (E)-3a exhibited only minor disorder and are distributed over three non-equivalent positions, one of which interacts with a nearby triazine moiety 1 by aromatic–aromatic interactions (interplanar distance ca. 3.4 ; Figure 1b). Based on the following observations, we believe the selective photoisomerization of (Z)-3a to (E)-3 a only occurs within the pores of 2’: 1) In the absence of network 2, photoisomerization did not occur and even the individual network component(s) (ligand 1 and/or ZnI2) were insufficient to catalyze the conversion. 2) When crystals of 2’ were removed during photoirradiation, isomerization stopped. 3) Photoisomerization was dramatically retarded when pyrene, which is strongly bound by 2’ and inhibits guest exchange, was added to the reaction mixture. 4) The E/Z ratio of stilbene increases faster within the crystals of 1 than in the supernatant (Figure 2). These results demonstrate that Scheme 1. a) Preparation of porous coordination networks 2 and 2’. b) Z!E photoisomerization of stilbenes within network 2’.

X-ray snapshot observation of palladium-mediated aromatic bromination in a porous complex.

Pd-mediated aromatic bromination is intriguing to synthetic and organometallic chemists due to both its synthetic utility and, more importantly, a proposed mechanism involving an uncommon Pd(IV)/Pd(II) catalytic cycle. Here, we report an X-ray snapshot observation of a Pd reaction center during a Pd-mediated aromatic bromination in a single crystal of a porous coordination network crystalline scaffold. Upon treatment of a single crystal with N-bromosuccinimide, sequential X-ray snapshots revealed that the aryl-Pd(II)-L species embedded in the network pores was converted to the brominated aryl product through a transient aryl-Pd(II)-Br species, which is normally unobservable because of its rapid dimerization into insoluble Pd2(μ-Br)2 species. Though the reaction pathway may be biased by the crystalline state, the new X-ray snapshot method relies on crystalline flasks to provide important mechanistic insight.