Effect of Boron-Doping on Gate-Opening CO2 Adsorption in Zinc-Benzimidazolate Coordination Networks.

Flexible metal-organic frameworks (MOFs) have attracted much attention as selective gas adsorption and storage. This report describes boron doping in zeolitic imidazolate framework-7 (B-ZIF-7), which exhibits reversible phase transition during CO2 adsorption/desorption. We have successfully prepared B-ZIF-7 coordination networks using boron-bridged benzimidazolate (B(bim)4-) as organic ligands. Powder X-ray diffraction (PXRD) measurements and infrared spectroscopy revealed that B-ZIF-7 has a crystal structure similar to that of ZIF-7 while containing boron bridging in the coordination network. Since B-ZIF-7 forms a cationic coordination network, the guest anions are encapsulated within the pore. CO2 adsorption/desorption measurements at 300 K showed that B-ZIF-7(NO3), which contains nitrate ions (NO3-) as guest anions in its pores, exhibits a S-shaped CO2 adsorption/desorption isotherm, which is characteristic of gate-opening type MOFs. Compared with ZIF-7, B-ZIF-7(NO3) has superior CO2 adsorption capacity in the low-pressure and superior CO2 storage capacity. The CO2 adsorption and desorption behavior of B-ZIF-7(NO3) was analyzed by in situ temperature-controlled PXRD measurements and thermogravimetric analysis under a CO2 atmosphere, and a reversible phase transition was observed. We have also successfully prepared B-ZIF-7(Cl) and B-ZIF-7(OTf) (OTf = CF3SO3-) with different guest anions. The CO2 adsorption/desorption behaviors of B-ZIF-7(Cl) and B-ZIF-7(OTf) were significantly different from those of B-ZIF-7(NO3) and ZIF-7. B-ZIF-7(Cl) showed gate opening at a higher pressure than ZIF-7, and B-ZIF-7(OTf) did not show S-shaped CO2 adsorption isotherm and showed adsorption behavior in micropores. These results indicate that the CO2 adsorption behavior of B-ZIF-7 depends on the interaction between the guest anions and CO2 molecules or the cationic framework and the bulkiness of the guest anions. Boron doping in a coordination network with boron-bridged imidazolate ligands is a promising strategy to increase the gas adsorption capability of porous materials.

Highly electron-deficient 1-propyl-3,5-dinitropyridinium: evaluation of electron-accepting ability and application as an oxidative quencher for metal complexes.

Impacts of the nitro groups on the electron-accepting and oxidizing abilities of N-propylpyridinium were evaluated quantitatively. A 3,5-dinitro derivative has efficiently quenched emission from photosensitizing Ru(ii) and Ir(iii) complexes owing to the thermodynamically-favored electron transfer to the pyridinium whose LUMO is greatly lowered by the presence of electron-withdrawing nitro groups.

General Synthesis of MOF Nanotubes via Hydrogen-Bonded Organic Frameworks toward Efficient Hydrogen Evolution Electrocatalysts.

The application scope of metal-organic frameworks (MOFs) can be extended by rationally designing the architecture and components of MOFs, which can be achieved via a metal-containing solid templated strategy. However, this strategy suffers from low efficiency and provides only one specific MOF from one template. Herein, we present a versatile templated strategy in which organic ligands are weaved into hydrogen-bonded organic frameworks (HOFs) for the controllable and scalable synthesis of MOF nanotubes. HOF nanowires assembled from benzene-1,3,5-tricarboxylic acid and melamine via a simple sonochemical approach serve as both the template and precursor to produce MOF nanotubes with varied metal compositions. Hybrid nanotubes containing nanometal crystals and N-doped graphene prepared through a carbonization process show that the optimized NiRuIr alloy@NG nanotube exhibits excellent electrocatalytic HER activity and durability in alkaline media, outperforming most reported catalysts. The strategy proposed here demonstrates a pioneering study of combination of HOF and MOF, which shows great potential in the design of other nanosized MOFs with various architectures and compositions for potential applications.

Tuning the Ground- and Excited-State Redox Potentials of Octahedral Hexanuclear Rhenium(III) Complexes by the Combination of Terminal Halide and N-Heteroaromatic Ligands.

The present study reports that the ground- and excited-state Re6(23e)/Re6(24e) redox potentials of an octahedral hexanuclear rhenium(III) complex can be controlled by systematically changing the number and type of the N-heteroaromatic ligand (L) and the number of chloride ions at the six terminal positions. Photoirradiation of [Re6(µ3-S)8Cl6]4- with an excess amount of L afforded a mono-L-substituted hexanuclear rhenium(III) complex, [Re6(µ3-S)8Cl5(L)]3- (L = 4-dimethylaminopyridine (dmap), 3,5-lutidine (lut), 4-methylpyridine (mpy), pyridine (py), 4,4'-bipyridine (bpy), 4-cyanopyridine (cpy), and pyrazine (pz)). The bis- and tris-lut-substituted complexes, trans- and cis-[Re6(µ3-S)8Cl4(lut)2]2- and mer-[Re6(µ3-S)8Cl3(lut)3]-, were synthesized by the reaction of [Re6(µ3-S)8Cl6]3- with an excess amount of lut in refluxed N,N-dimethylformamide. The mono-L-substituted complexes showed one-electron redox processes assignable to E 1/2[Re6(23e)/Re6(24e)] = 0.49-0.58 V versus Ag/AgCl. The ground-state oxidation potentials were linearly correlated with the pK a of the N-heteroaromatic ligand [pK a(L)], the 1H NMR chemical shift of the ortho proton on the coordinating ligand, and the Hammett constant (σ) of the pyridyl-ligand substituent. The series of [Re6(µ3-S)8X6-n (L) n ] n-4 complexes (n = 0, X = Cl, Br, I, or NCS; n = 1-3, X = Cl) showed a linear correlation with the sum of the Lever electrochemical parameters at the six terminal ligands (ΣE L). The cyclic voltammograms of the mono-L-substituted complexes (L = bpy, cpy, and pz) showed one-electron redox waves assignable to E 1/2(L0/L-) = -1.28 to -1.48 V versus Ag/AgCl. Two types of photoluminescences were observed for the complexes, originating from the cluster core-centered excited triplet state (3CC) for L = dmap, lut, mpy, and py and from the metal-to-ligand charge-transfer excited triplet state (3MLCT) for L = bpy, cpy, and pz. The complexes with the 3CC character exhibited emission features and photophysical properties similar to those of ordinary hexanuclear rhenium complexes. The emission maximum wavelength of the complexes with 3MLCT shifted to the longer wavelength in the order L = 4-phenylpyridine (ppy), bpy, pz, and cpy, which agreed with the difference between E 1/2[Re6(23e)/Re6(24e)] and E 1/2(L0/L-). The calculated oxidation potential of the excited hexanuclear rhenium complex with the 3CC character was linearly correlated with pK a(L), σ, and ΣE L. The ground- and excited-state oxidation potentials were finely tuned by the combination of halide and L ligands at the terminal positions.

Development of a synthetic equivalent of α,α-dicationic acetic acid leading to unnatural amino acid derivatives via tetrafunctionalized methanes.

Diethyl mesoxalate (DEMO) exhibits high electrophilicity and accepts the nucleophilic addition of a less nucleophilic acid amide to afford N,O-hemiacetal. However, our research showed that elimination of the amide moiety proceeded more easily than dehydration upon treatment with a base. This problem was overcome by reacting DEMO with an acid amide in the presence of acetic anhydride to efficiently obtain N,O-acetal. Acetic acid was eliminated leading to the formation of N-acylimine in situ upon treatment with the base. N-Acylimine is also electrophilic, accepting the second nucleophilic addition by pyrrole or indole to form α,α-disubstituted malonates. Subsequent hydrolysis followed by decarboxylation resulted in (α-indolyl-α-acylamino)acetic acid formation; homologs of tryptophan. Through this process, DEMO serves as a synthetic equivalent of α,α-dicationic acetic acid to facilitate nucleophilic introduction of the two substituents.

Synthesis and Self-Assembly Properties of Bola-Amphiphilic Glycosylated Lipopeptide-Type Supramolecular Hydrogels Showing Colour Changes Along with Gel-Sol Transition.

Supramolecular hydrogels formed by self-assembly of low-molecular-weight amphiphiles (hydrogelators) have attracted significant attention, as smart and soft materials. However, most of the observed stimuli-responsive behaviour of these supramolecular hydrogels are limited to gel-sol transitions. In this study, we present bola-amphiphilic glycosylated lipopeptide-type supramolecular hydrogelators that exhibit reversible thermochromism along with a gel-sol transition. The bola-amphiphiles have mono-, di-, tri- or tetra-phenylalanine (F) as a short peptide moiety. We investigate and discuss the effects of the number of F residues on the gelation ability and the morphology of the self-assembled nanostructures.

Multinuclear Ag Clusters Sandwiched by Pt Complex Units: Fluxional Behavior and Chiral-at-Cluster Photoluminescence.

Multinuclear Ag clusters sandwiched by Pt complex units were synthesized and characterized by single crystal X-ray diffraction and NMR studies. The sandwich-shaped multinuclear Ag complexes showed two different types of fluxional behavior in solution: rapid slippage of Pt complex units on the Ag3 core and a reversible demetalation-metalation reaction by the treatment with Cl anion and Ag ion, respectively. The Ag2 complex obtained by demetalation reaction from the Ag3 complex displayed U to Z isomerization. These multinuclear Ag complexes showed strong photoluminescence whose properties depended on the existence of Pt→Ag dative bonds. The Ag3 complex, identified to be "chiral-at-cluster", was optically resolved by the formation of a diastereomeric salt with a chiral anion. The enantiomers show circular dichroism (CD) and circularly polarized luminescence (CPL) properties which is unprecedented for compounds based on a chiral sandwich structure. Theoretical calculations allow to understand their structural features and photophysical properties.

A Heteropolynuclear Pt-Ag System Having Cycloplatinated Rollover Bipyridyl Units.

The synthesis and photophysical properties of the heteropolynuclear Pt-Ag complex having cyclometalated rollover bipyridine ligands (bpy*) and bridging pyrazolato ligands are reported. The Pt2Ag2 complex was synthesized by two step reactions from a Pt(II) complex precursor having the rollover bpy* ligand, [Pt(bpy*)(dmso)Cl], with 3,5-dimethylpyrazole (Me2pzH) and a subsequent replacement of NH protons in the Me2pzH moieties with the Ag(I) ion. The Pt2Ag2 complex exists as a mixture of U- and Z-shaped isomers in solution, whose structures were clearly determined by single-crystal X-ray structural analyses. NMR studies using the single crystals revealed rapid isomerization of the Pt2Ag2 complexes in solution, although the Pt2Ag2 structures were supported effectively by the multiple metal-metal interactions. Furthermore, the Pt2Ag2 framework can capture a Ag(I) ion during the U-Z isomerization to afford a Pt2Ag3 core with the formation of Pt → Ag dative bonds. The Pt2Ag3 complex showed further aggregation to form a dimer structure in the presence of coordinating solvent via the crystallization process. The formation of Pt → Ag dative bonds significantly changes the emission energy from the Pt2Ag2 complex, while the emission spectra of U- and Z-isomers of Pt2Ag2 complex almost coincide with each other and their emissive properties are very similar. The density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations revealed the effects of additional Ag(I) ion on the photophysical properties of the heteropolynuclear Pt-Ag complexes bearing the rollover bpy* ligands.

Bridging-arylene effects on spectroscopic and photophysical properties of arylborane-dipyrrinato zinc(ii) complexes.

Novel bis(dipyrrinato)zinc(ii) derivatives having 4-[bis(2,4,6-trimethylphenyl)boryl]phenyl (ZnBph) or 4-[bis(2,4,6-trimethylphenyl)boryl]-2,3,5,6-tetramethylphenyl groups (ZnBdu) at the 5-position of the dipyrrinato ligands were designed and synthesized. In ZnBph with the smaller dipyrrinate-arylene and arylene-dimesitylboryl dihedral angles, an intramolecular charge transfer arising from the presence of the vacant p orbital on the boron atom participates in the ππ* excited state in character in contrast to the pure ππ* excited state of ZnBdu. The synergistic ππ*/ILCT excited state was weakly fluorescent, and the fluorescence was enhanced upon binding of fluoride to the boron atom.

Synthesis and intramolecular ring transformation of N,N'-dialkylated 2,6,9-triazabicyclo[3.3.1]nonadienes.

The first and facile synthesis of N,N'-dialkylated 2,6,9-triazabicyclo[3.3.1]nonadienes was achieved by the [4 + 4] self-condensation of ß-formyl-ß-nitroenamine in the presence of ammonium acetate. The 2,6- and 2,9-dialkylated products were found to be interconvertible when dissolved in a solvent. This isomerization proceeds through intramolecular ring transformation via a common intermediate under equilibrium.

Structural diversification of bola-amphiphilic glycolipid-type supramolecular hydrogelators exhibiting colour changes along with the gel-sol transition.

We diversified the structures of bola-amphiphilic glycolipid-type supramolecular hydrogelators that exhibited reversible thermochromism along with a gel-sol transition. The hydrogelators were designed and synthesized to have homo- or hetero-saccharides on each end of their molecules. Herein, the effects of the saccharides' structure on the gelation ability are discussed.

Synthesis and Photoluminescence of Tetracyanidonitridorhenium(V) Complexes with Five-Membered N-Heteroaromatic Ligands and Photoluminescence-Intensity Change.

Novel tetracyanidonitridorhenium(V) complexes with five-membered N-heteroaromatic ligands, (PPh4)2[ReN(CN)4L] [L = imidazole (Him) (2), 1-methylimidazole (Mim) (3), and pyrazole (pyz) (4)] and (PPh4)2[ReN(CN)4L]·L [L = Him (5) and Mim (6)], were synthesized by the reactions of (PPh4)2[ReN(CN)4] (1) with Him, Mim, and pyz, and their structures were determined by single-crystal X-ray analysis. The complexes 2, 3, 4, and 6 showed intense photoluminescence, with the emission quantum yields (Φem) being 0.65-0.75 in the solid state at 296 K. In contrast, the Φem and τem values of 5 are significantly smaller and shorter, respectively, than the relevant values of 2. The interconversion reactions among 1, 2, and 5 accompanied by large photoluminescence-intensity changes were accomplished by solvent-free reactions and exposure of water. The mechanochemical reaction of 2 with 1 mol equiv of Him in the solid state gave 5. Complex 5 was also obtained by the mechanochemical reaction of 1 with 2 mol equivalents of Him in the solid state. By placing solid of 5 in water, the solid showed intense photoluminescence to give 2. Complex 1 was produced under vacuum at 185 °C from 2 or 5.

Controlling the Electronic Structures and Excited-State Characteristics of Dipyrrinatoiridium(III) Complexes by an Arylborane or an Arylamino Unit.

Novel dipyrrinatoiridium(III) complexes, in which a typical element substituent (i.e., arylborane or arylamino group) was introduced at the meso position of the 5-phenyldipyrrinato ligand, were designed and synthesized. The (dimesitylboryl)phenyl complex 1 showed fascinating photophysical properties arising from a synergistic interaction between metal-to-ligand charge transfer/ligand-to-ligand charge transfer and charge transfer from the π orbital of the aryl group to the vacant p orbital on the boron atom [π(aryl)-p(B) charge transfer]. On the other hand, the (N,N-diphenylamino)phenyl complex 2 showed the most intense and longest-lived emission from the 3ππ*-type excited state in the dipyrrinato moiety among the complexes in the present study. These characteristics of the complexes were evaluated in terms of the structures and spectroscopic and photophysical properties.

Electrochemical and Spectroscopic Behaviors of a Novel Ruthenium(II) Complex with a Six-Membered Chelate Structure.

A novel polypyridyl ruthenium(II) complex with a six-membered chelate ring ([Ru(dmb)2(8pyq)]2+) was designed and synthesized. The oxidation potential was shifted to the negative potential direction, and relatively intense metal-to-ligand charge transfer absorption in the longer-wavelength region was observed for [Ru(dmb)2(8pyq)]2+ compared with the reference complexes without any six-membered chelate rings. The electrochemical and spectroscopic properties of [Ru(dmb)2(8pyq)]2+ were discussed in terms of the chelate structure and coordination geometry with utilization of theoretical calculations.

Emission Tuning of Heteroleptic Arylborane-Ruthenium(II) Complexes by Ancillary Ligands: Observation of Strickler-Berg-Type Relation.

Novel heteroleptic arylborane-ruthenium(II) complexes having a series of ancillary ligands L' ([Ru(B2bpy)L'2]2+) in CH3CN showed low-energy/intense metal-to-ligand charge transfer (MLCT)-type absorption and intense/long-lived emission compared to the reference complexes. The spectroscopic and photophysical properties of [Ru(B2bpy)L'2]2+ were shown to be manipulated synthetically by the electron-donating ability of the ancillary ligand(s). The intense and long-lived emission observed for [Ru(B2bpy)L'2]2+ in CH3CN at 298 K is responsible for the accelerated radiative and decelerated nonradiative decay processes, which are controllable through the electronic structures of the ancillary ligand(s). On the basis of the present systematic study, furthermore, we succeeded in demonstrating the Strickler-Berg-type relation between the molar absorption coefficients of the MLCT bands and the radiative rate constants of the complexes.

U- to Z-shape isomerization in a Pt2Ag2 framework containing pyridyl-NHC ligands.

Synthesis and isomerization of a heteropolynuclear Pt2Ag2 complex containing pyridyl N-heterocyclic carbene (NHC) ligands are reported. The Pt2Ag2 complex could take two geometrical isomers possessing a twisted U-shaped structure and a Z-shaped structure. The rate of the isomerization reaction depended on the concentration of the solution, implying that the reaction took place through an intermolecular process.

Photophysical dynamics of the efficient emission and photosensitization of [Ir(pqi)2(NN)]+ complexes.

The photophysical dynamics of three complexes in the highly-emissive [Ir(pqi)2(NN)]+ series were investigated aiming at unique photophysical features and applications in light-emitting and singlet oxygen sensitizing research fields. Rational elucidation and Franck-Condon analyses of the observed emission spectra in nitrile solutions at 298 and 77 K reveal the true emissive nature of the lowest-lying triplet excited state (T1), consisting of a hybrid 3MLCT/LCIr(pqi)→pqi state. Emissive deactivations from T1 occur mainly by very intense, yellow-orange phosphorescence with high quantum yields and radiative rates. The emission nature experimentally verified is corroborated by theoretical calculations (TD-DFT), with T1 arising from a mixing of several transitions induced by the spin-orbit coupling, majorly ascribed to 3MLCT/LCIr(pqi)→pqi and increasing contributions of 3MLCT/LLCTIr(pqi)→NN. The microsecond-lived emission of T1 is rapidly quenched by molecular oxygen, with an efficient generation of singlet oxygen. Our findings show that the photophysics of [Ir(pqi)2(NN)][PF6] complexes is suitable for many applications, from the active layer of electroluminescent devices to photosensitizers for photodynamic therapy and theranostics.

Substrate switchable Suzuki-Miyaura coupling for benzyl ester vs. benzyl halide.

Two reaction conditions were developed to accomplish the substrate switchable Suzuki-Miyaura coupling of benzyl derivatives and arylboronic acid derivatives. Under conditions for esters, benzyl esters such as carbonates and acetates reacted with arylboronic acids to afford the corresponding diarylmethanes. However, the benzyl halides did not react under the same conditions. On the other hand, benzyl halides such as bromides and chlorides furnished diarylmethanes under conditions for halides, under which benzyl ester substrates did not react, in which water was found to play an important role. This switching system was tested using the intermolecular/intramolecular competitive reactions, during which the desired products could be synthesized by selecting the appropriate reaction conditions.

Zero-Magnetic-Field Splitting in the Excited Triplet States of Octahedral Hexanuclear Molybdenum(II) Clusters: [{Mo6X8}(n-C3F7COO)6]2- (X = Cl, Br, or I).

Temperature (T)-dependent emission from [{Mo6X8}(n-C3F7COO)6]2- (X = Cl (1), Br (2), and I (3)) in optically transparent polyethylene glycol dimethacrylate matrices were studied in 3 K < T < 300 K to elucidate the spectroscopic and photophysical properties of the clusters, in special reference to zero-magnetic-field splitting (zfs) in the lowest-energy excited triplet states (T1) of the clusters. The cluster complexes 1 and 2 showed the T-dependent emission characteristics similar to those of [{Mo6Cl8}Cl6]2-, while 3 exhibited emission properties different completely from those of 1 and 2. Such T-dependent emission characteristics of 1, 2, and 3 were explained successfully by the excited triplet state spin-sublevel (Φn, n = 1-4) model. The zfs energies between the lowest-energy (Φ1) and highest-energy (Φ4) spin sublevels, ΔE14, resulted by the first-order spin-orbit coupling, were evaluated to be 650, 720, and 1000 cm-1 for 1, 2, and 3, respectively. The emission spectra of 1, 2, and 3 in CH3CN at 298 K were reproduced very well by the ΔE14 values and the population percentages of Φn at 300 K. We also report that the ΔE14 values of the clusters correlate linearly with the fourth power of the atomic number (Z) of X: ΔE14 ∝ {Z(X)}4.

Low-Energy and Long-Lived Emission from Polypyridyl Ruthenium(II) Complexes Having A Stable-Radical Substituent.

Novel polypyridyl ruthenium(II) complexes having a 2,2'-bipyridine (bpy) derivative which possesses a 1,5-dimethyl-6-oxoverdazyl radical (OV) group as a stable-radical substituent were designed and synthesized. The radical-ruthenium(II) complexes showed low-energy/intense MLCT absorption and low-energy/long-lived MLCT emission, and these characteristics of the complexes were explained by the electron-withdrawing nature of the OV group. Furthermore, the radical-substituent effects were enhanced by the presence of the electron-donating methyl groups at the 4- and 4'-positions of bpy in the ancillary ligands. The detailed electrochemical, spectroscopic, and photophysical properties of the complexes were discussed in terms of the systematic modification of the second coordination sphere in the main and ancillary ligands.