Diaminocarbene and phosphonium ylide ligands: a systematic comparison of their donor character

The coordinating properties of the diaminocarbene (A) and phosphonium ylide (B) ligand types have been investigated systematically through a test family of C,C-chelating ligands containing two moieties of either kind. The overall character of o-C6H4A(a)B(b) ligands (a + b = 2) has been analyzed from the IR CO stretching frequencies of isostructural complexes [(eta(2)-C6H4A(a)B(b))Rh(CO)2][TfO]. The test moieties A = NC2H2N(+)(Me)C(-) and B = Ph2P(+)CH2(-) were first considered. While the ligands bearing at least one diaminocarbene end (AA, a = 2 and AB, a = 1) could be generated (and trapped by complexation), the bis-ylide case BB (a = 0) proved to be awkward: treatment of the dication C6H4(P(+)Ph2Me)2 with n-BuLi indeed lead to the Schmidbaur's carbodiphosphorane Ph3PCPPh2Me, through an unprecendented ylido-pentacoordinated phosphorane which could be fully characterized by NMR techniques. The bis-ylide ligand type C6H4B2 could however be generated by bridging the phosphonium methyl groups by a methylene link (B2 = (P(+)Ph2CH(-))2CH2), preventing the formation of the analogous highly strained carbodiphosphorane. The three complexes [(eta(2)-C6H4A(a)B(b))Rh(CO)2][TfO] were fully characterized, including by X-ray diffraction analysis and (103)Rh NMR spectroscopy. Comparison of their IR spectra indicated that the A2 type bis-NHC ligand is less donating than the hybrid AB type, which is itself less donating than the B2 type bis-ylide ligand. The excellent linear variation of the nu(CO) frequencies vs a (= 0, 1, 2) shows that the coordinating moieties act in a pseudoindependent way. This was confirmed by DFT calculations at the B3PW91/6-31G**/LANL2DZ*(Rh) level. It is therefore demonstrated that a phosphonium ylide ligand is a stronger donor than a diaminocarbene ligand.     

A new chiral probe for sulfate anion: UV,CD, fluorescence,and NMR spectral studies of 1:1 and 2:1 complex formation and structure of chiral guanidinium-p-dimethylaminobenzoate conjugate with sulfate anion

A new chiral chromophoric host 1, possessing a 4-(N,N-dimethylamino)benzoate (DMAB) group tethered to a chiral bicyclic guanidinium subunit, was synthesized and applied to the probe for sulfate anion. Host 1 showed typical successive 1:1 and 2:1 host:guest complexation behavior toward the divalent sulfate anion, as revealed by UV-vis, CD, fluorescence, and 1H NMR spectroscopic studies. The DMAB chromophore was shown to be a sensitive CD spectral probe for assessing not only the complexation behavior but also complex stoichiometry and structure. The stepwise 1:1 and 2:1 complexation constants (K1 and K2) were determined as 1.53 x 10(6) and 4.84 x 10(4) M(-1), respectively, by NMR titration in CD3CN. The CD exciton chirality method allowed us to determine the chiral sense (spatial arrangement) of the two DMAB moieties in the 2:1 complex as negative (counterclockwise). The dual fluorescence behavior of DMAB was employed for elucidating the role of the countercation upon complexation of host 1 with sulfates possessing lipophilic countercation(s) such as tetrabutylammonium.     

Synthesis, characterization and cation-induced dimerization of new aza-crown ether-appended metalloporphyrins

New metalloporphyrins bearing one or two aryl-aza-crown ether moieties at meso-positions have been synthesized using a palladium catalyzed amination reaction and fully characterized by spectral techniques. X-Ray structural data have been presented for the zinc and copper complexes of mono-substituted aza-crown ether appended metalloporphyrins. UV-Vis and (1)H NMR spectroscopic studies showed that addition of K(+) cations to a solution of monomeric aza-crowned porphyrins in CHCl(3)/MeOH led to cation-induced dimerization of these porphyrins, whereas addition of Na(+) cations yielded a monomeric complex. Axial coordination of the exobidentate ligand (DABCO) to zinc complexes of aza-crowned porphyrins and following binding metal ions led to formation of sandwich complexes with high stability constants.     

Synthesis, spectral characterisation of 2-(5-methyl-1H-benzimidazol-2-yl)-4-bromo/nitro-phenols and their complexes with zinc(II) ion, and solvent effect on complexation

2-(5-Methyl-1H-benzimidazol-2-yl)-4-bromo/nitro-phenols (HLBr and HLNO2) and their Zn(II) complexes with ZnX2 (X = Cl, I, NO3) were synthesized and characterized by elemental analysis, molar conductivity, IR, 1H and 13C NMR spectra. The OH proton appears near the NH protons in the 1H NMR spectra of the ligands because of the strong intramolecular hydrogen bonding between the OH hydrogen and the C=N nitrogen atoms. The complexation is investigated in ethanol and isopropanol and it is observed that isopropanol is a better solvent than ethanol for the complex forming. HLBr gives harder complexation reaction with Zn(II) according to HLNO2 because of the stronger intramolecular hydrogen bonding in HLBr, and the both ligands react easier with Zn(NO3)2 than ZnCl2 and ZnI2. The Zn(II) complexes of HLBr have 1:1 M:L ratio and ionic character, however, HLNO2 give a non-ionic complex that has 1:2 M:L ratio. In the complexes the phenolic hydrogen is eliminated and a chelate structure is formed.     

Calix[4]arene bisphosphite ligands bearing two distal 2,2′-biphenyldioxy or 2,2′-binaphthyldioxy moieties: conformational flexibility and allyl-palladium complexes

Achiral and chiral calix[4]arene bisphosphite ligands (2 and 3) bearing two distal 2,2′-biphenyldioxyphosphinoxy and 2,2′-binaphthyldioxyphosphinoxy moieties, respectively, have been synthesized. Each of these ligands exists in two pairs of interconverting conformations in solution. The partial cone conformer (A) of the (bis)biphenyldioxyphosphinoxy ligand 2 has been separated by fractional crystallization and its structure established by X-ray crystallography. The mechanism of interconversion of the pairs of conformers (A/B and C/D) has been probed by two-dimensional NMR spectroscopy. The ¹H and ³¹P NMR evidence strongly supports a similar kind of exchange mechanism for ligand 3. Freezing of the cone conformer from the interconverting C/D pair of conformers of ligand 2 has been achieved by complexation with (allyl)palladium moieties. The methyl-allyl complex (2d) is moderately effective for catalytic regioselective allylic alkylation of crotyl acetate.     

(Thio)ureido anion receptors based on a 1,3-alternate oxacalix[2]arene[2]pyrimidine scaffold

In pursuit of highly preorganized macrocyclic host molecules for the complexation of anions, a series of oxacalix[2]arene[2]pyrimidine-based bis(thio)ureido receptors were synthesized and fully characterized. The pincer-like 1,3-alternate conformation of the oxacalix[4]arene scaffold, essential for an efficient host-guest interaction, was visualized by single-crystal X-ray analysis and supported by variable-temperature NMR studies. The anion binding properties of the receptors were evaluated via (1)H NMR titration experiments, showing intermolecular interactions with H(2)PO(4)(-), AcO(-), BzO(-), and Cl(-) ions. The host molecule bearing 4-nitrophenyl substituents on the bisurea binding pocket showed association constants in the range of 200-400 M(-1) in the strongly competitive solvent mixture of DMSO/0.5% H(2)O.     

Potentiometric and multinuclear NMR studies on the interaction of aluminum with ascorbic acid in acidic aqueous solution

The complex-formation equilibria between aluminum(III) ion and L-(+)-ascorbic acid (AA) in 0.1 M KCl ionic medium at 25 degrees C and 0.15 M NaCl ionic medium at 37 degrees C were studied by glass electrode pH-metric measurements. The obtained experimental results were explained by the formation of the following complexation species: a weak mononuclear 1:1 species AlL(2+) together with two trinuclear mixed-hydroxo species Al(3)H(-5)L(4) and Al(3)H(-5)L(3+) in acidic aqueous solutions. Meanwhile, the formation of the complexes and structures of Al with AA were proved by multinuclear (1H, 13C, 27Al) NMR spectra in the pH range 2.0-5.0. It is supposed that Al directly coordinates with AA at O-3 moiety; also, Al can coordinate with the O-1 and O-2 moieties of ascorbate ion through the weakly binding and the intramolecular hydrogen bonding in acidic aqueous solutions.     

Synthesis and structure of lower rim C-linked N-tosyl peptidocalix[4]arenes

Chiral p-tert-butylcalix[4]arenes functionalized at the lower rim with amino acid residues have been prepared. The (1)H and (13)C NMR spectra indicate that the macrocycles preferably adopt a cone conformation. Calix[4]arenes bearing amino acid moieties were prepared as a class of receptors selective for anions that are bound through hydrogen bonding with the NH group. The association constants are dependent on the nature of the substituents at the lower rim. Derivative 9 shows the strongest complexation and the largest selectivity for N-tosyl-(L)-alaninate. Finally, a preliminary X-ray crystal study of the difunctionalized receptor 6f shows the "flattened cone" conformation in the solid state.     

Light-driven fluorescence enhancement of phenylazo indazole-terminated polystyrene

The well-defined phenylazo indazole-terminated polystyrene (PS) was successfully prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization of styrene (St) mediated by a novel chain transfer agent (CTA) bearing phenylazo indazole moieties in the Z group, benzyl-5-(4-(dimethylamino)phenylazo) indazole-1-carbodithioate (BPCD). The fluorescence emission of BPCD and the phenylazo indazole-terminated PS in chloroform (CHCl₃) before and after the 365 nm ultraviolet irradiation was investigated. Interestingly, the fluorescence intensities of BPCD and the phenylazo indazole-terminated PS in CHCl₃ were both sensitive to the 365 nm ultraviolet irradiation. The fluorescence intensities of these solutions increased with the irradiation time and reached maximum at 110 min. The light-driven fluorescence enhancement of BPCD and the phenylazo indazole-terminated PS were both attributed to the formation of spherical aggregate originated from the trans-cis isomerization of azobenzene moieties in BPCD and PS chain, which was confirmed by transmission electron microscopy (TEM), ¹H NMR, UV and dynamic light scattering (DLS) spectra.     

Novel small stable gold nanoparticles bearing fluorescent cysteine-coumarin probes as new metal-modulated chemosensors

Emissive molecular probes based on amino acid moieties are very appealing because of their application as new building blocks in peptide synthesis. Two new bioinspired coumarin probes (L1 and L2) were synthesized and fully characterized by elemental analysis, infrared, (1)H NMR, (13)C NMR, UV-vis absorption and emission spectroscopy, matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF-MS), lifetime measurements, and X-ray crystal diffraction. Their sensing ability toward alkaline earth, transition, and post-transition metal ions (Ca(2+), Zn(2+), Cd(2+), Cu(2+), Ni(2+), Hg(2+), Ag(+), and Al(3+)) and their acid-base behavior (H(+), OH(-)) were explored in absolute ethanol by absorption and fluorescence spectroscopy. Compound L1 shows a strong complexation constant with the soft metal ions Zn(2+), Cd(2+), and Ag(+). Compound L2 shows a high fluorescence quantum yield, and it could be used as a non-pH-dependent fluorescent biological probe. Very small gold nanoparticles (AuNPs) using compounds L1 and L2 as stabilizers were obtained by using a reductive method and were characterized by UV-vis, light scattering, and transmission electron microscopy (TEM). Dynamic light scattering and TEM studies show that the formation of small nanoparticles is around 4.27 ± 0.64 nm for L1 and around 2.69 ± 0.96 nm for L2. The new stable Cou@AuNPs behaved as supramolecular chemosensors, which have been selective for the heavy element Hg(2+), with a concomitant change of color from pink to dark red/brown and an increase of size up to 100-fold.     

Planar-chiral (2E,7Z)- and (2Z,7E)-cyclonona-2,7-dien-1-yl carbamates by asymmetric,bis-allyic,alpha, alpha'-cycloalkylation--studies on their conformational stability

Enantiomerically enriched (>80 % ee) (M,1R,2Z,7E)- and (M,1R,2E,7Z)-cyclonona-2,7-dienyl carbamates have been synthesized by an intramolecular cycloalkylation of the corresponding 1-lithio-9-chloronona-2,7-dienyl carbamates. The enantioenriched precursors were generated by asymmetric deprotonation of the dienyl carbamates by means of n-BuLi/(-)-sparteine. The primarily obtained cyclononadienes, each bearing one element of planar and centre chirality, were formed by an , coupling of both allylic moieties. These are the thermodynamically less favoured epimers, which arise from those allyllithiums bearing the carbamoyloxy residue in a 1-endo position. Both (M,R)-cyclononadienes epimerize slowly (t(1/2) = 209 min and 328 min at 308 K) with inversion of the planar chirality to the corresponding, more stable (P)-epimers (ratios 3:97 and 30:70). The kinetics were measured by 1H NMR spectroscopy, the activation energies EA were found to be 26.4 and 27.5 kcal mol(-1). From quantum-chemical calculations with the B3LYP density functional, the epimerization process was shown to consist of two coupled major conformational changes with high energy barriers. The calculated values match well with the observed ones.     

Photostabilization of oxolinic acid in hydroxypropyl-β-cyclodextrins; implications for the effect of molecular self-assembly phenomena

Oxolinic acid (OXA) is a first-generation quinolone antibacterial agent, known to cause drug induced photosensitivity. In the present work its photoinduced degradation was monitored under simulated solar irradiation. The effect of photoprotecting agents on OXA stability was also assessed by drug complexation with hydroxypropyl-β-cyclodextrin (HPβCD). The complex was studied by UV-Vis and ¹H (2D) NMR Spectroscopy. A photostability indicating chromatographic method was developed and validated. Because OXA is insoluble in acidic solutions, and because an acidic solvent is necessary for successful chromatographic separation, a procedure was developed to pre-treat the sample. This method is suitable for the separation of degradation products from OXA and from each other. The method was also evaluated in the presence of HPβCD, in order to ensure that inclusion complexation did not generate inaccuracies. Investigation of OXA photodegradation profiles confirms first order kinetics and acceleration at higher initial sample concentrations. A 94% photostabilization upon complexation with HPβCD was achieved. Furthermore, molecular self association phenomena were determined by self titration experiments, using ¹H NMR Spectroscopy and suggestions were made for the photostabilization mechanism of cyclodextrins.     

Novel photoswitchable receptors: synthesis and cation-induced self-assembly into dimeric complexes leading to stereospecific [2+2]-photocycloaddition of styryl dyes containing a 15-crown-5 ether unit

Styryl dyes 4a-e containing a 15-crown-5 ether unit and a quinoline residue with a sulfonatoalkyl or sulfonatobenzyl N-substituent were synthesized. The relationship between the photochemical behavior of these dyes and their aggregates derived from complexation with Mg(2+) in MeCN was studied using (1)H NMR and absorption spectroscopy. The E-isomers of 4a-e were shown to form highly stable dimeric (2:2) complexes with Mg(2+). Upon irradiation with visible light, the dimeric complexes undergo two competing photoreactions, viz., geometric E --> Z isomerization, resulting in an anion-capped 1:1 complex of the Z-isomer with Mg(2+) and stereospecific syn-head-to-tail [2+2]-cycloaddition, affording a single isomer of bis-crown-containing cyclobutane. The N-substituent in the dye has a dramatic effect on the photochemical behavior of the dimeric complex. Molecular dynamics and semiempirical quantum-chemical calculations were carried out to interpret the observed photocycloaddition in the dimer. Conformational equilibria for the dimer of (E)-4b were analyzed using (1)H NMR spectroscopy.     

Photoreversible Supramolecular Polymerisation and Hierarchical Organization of Hydrogen‐Bonded Supramolecular Co‐polymers Composed of Diarylethenes and Oligothiophenes

Diarylethene 1 equipped with two monotopic melamine hydrogen‐bonding sites and oligothiophene‐functionalized ditopic cyanurate (OTCA) were mixed in a nonpolar solvent to form AA‐BB‐type supramolecular co‐polymers (SCPs) bearing photoswitchable moieties in their main chains and extended π systems as side chains. UV/Vis, fluorescence, dynamic light scattering (DLS), TEM, and AFM studies revealed that the two functional co‐monomers formed flexible quasi‐one‐dimensional SCPs in solution that hierarchically self‐organized into helical nanofibers through H‐aggregation of the oligothiophene side chains. Upon irradiating the SCPs with UV light, a transition occurred from the H‐aggregated state to non‐aggregated monomeric oligothiophene side chains, as shown by spectroscopic studies, which indicates the formation of small oligomeric species held together only by hydrogen‐bonding interactions. TEM and AFM visualized unfolded fibrils corresponding to elongated single SCP chains formed upon removal of solvent. The helical nanofibers were regenerated upon irradiating the UVirradiated solution with visible light. These results demonstrated that the supramolecular polymerisation followed by hierarchical organization can be effectively controlled by proper supramolecular designs using diarylethenes and π‐conjugated oligomers.     

Synthesis of cyclic bis- and trismelamine derivatives and their complexation properties with barbiturates

Cyclic bis- and trismelamine derivatives were prepared from cyanuric chloride by stepwise substitutions with appropriate amines. The complexation abilities of these melamine derivatives with barbituric acid derivatives were evaluated by UV-vis spectroscopy and (1)H NMR. The structure was also confirmed by X-ray crystallography. Both the acyclic and the cyclic bismelamine derivatives formed a 1 : 1 complex via six hydrogen bonds with barbituric acid derivatives. van't Hoff analyses on the complexation of the bismelamines with the barbituric acid derivative revealed that the complexation of the cyclic bismelamine was entropically favored and enthalpically less favored process than those of the acyclic bismelamine. X-Ray crystallographic analysis and (1)H NMR studies revealed that the cyclic trismelamine bound one barbituric acid derivative into the cavity via six hydrogen bonds by two melamine moieties and another barbituric acid via three hydrogen bonds by the residual melamine moiety.     

Light-Triggered Transformation of Molecular Baskets into Organic Nanoparticles

Discovering novel and functional photoresponsive materials is of interest for improving controlled release of molecules and scavenging toxic compounds for cleaning our environment or designing chemosensors. In this study, we report on the photoinduced decarboxylation of basket 1 ⁶⁻, containing three glutamic acids at its rim. This concave compound is, in an aqueous environment (30 mm phosphate buffer at pH 7.0), monomeric (¹H NMR DOSY, DLS) with glutamic acid residues randomly oriented about its rim (¹H NMR and MM-OPLS3). The irradiation (300 nm) of 1 ⁶⁻ leads to the exclusive removal of its α-carboxylates to give amphiphilic 2 ³⁻ possessing γ-carboxylates. The photochemical transformation is a consecutive reaction with mono- and bis-decarboxylated products observed with ¹H NMR spectroscopy and ESI mass spectrometry. Amphiphilic 2 ³⁻ is a preorganized molecule (MM-OPLS3) that, in water, aggregates into organic nanoparticles (ca. 50–200 nm in diameter; DLS, TEM and cryo-TEM) having a critical aggregation concentration of 12 μm (UV/Vis). As the transition of monomeric 1 ⁶⁻ into nanoparticulate 2 ³⁻ is triggered with light, we reasoned that stimuli-responsive formation of the soft material lends itself to nanotechnology applications such as controlled release or scavenging of targeted compounds.     

Multistimuli‐Responsive Supramolecular Assembly of Cucurbituril/Cyclodextrin Pairs with an Azobenzene‐Containing Bispyridinium Guest

A linear supramolecular architecture was successfully constructed by the inclusion complexation of α‐cyclodextrin with azobenzene and the host‐stabilized charge‐transfer interaction of naphthalene and a bispyridinium guest with cucurbit[8]uril in water, which was comprehensively characterized by ¹H NMR spectroscopy, UV/Vis absorption, fluorescence, circular dichroism spectroscopy, dynamic laser scattering, and microscopic observations. Significantly, because it benefits from the photoinduced isomerization of the azophenyl group and the chemical reduction of bispyridinium moiety with noncovalent connections, the assembly/disassembly process of this supramolecular nanostructure can be efficiently modulated by external stimuli, including temperature, UV and visible‐light irradiation, and chemical redox.     

Synthesis and characterization of dendrons and dendrimers skeleton-constructed with azobenzene moiety

A series of dendrons and dendrimers skeleton-constructed with azobenzene moiety based upon 4-carboxy-4′-(1,2-propanediolether)-azobenzene as an AB₂ monomer, via a convergent approach, proceeding in a repeated stepwise growth manner starting from 4-carboxy-4′-(n-butylether)-azobenzene as a peripheral monomer, were synthesized, and characterized by NMR, FTIR, and MALDI-TOF-MS analysis. Their regular molecular architecture and thus monodispersed molecular weights were confirmed by GPC. The UV-Vis absorbance and ¹H NMR spectrum study indicated that the azobenzene moieties in CHCl₃ solution took fully trans-cis isomerization under UV irradiation, and reversely isomerization back to the trans by visible light irradiation or by heat.     

Evaluation of microstructural features of a new polymeric organic stationary phase grafted on silica surface: A paradigm of characterization of HPLC-stationary phases by a combination of suspension-state H NMR and solid-state C-CP/MAS-NMR

Silica-supported poly(octadecylacrylate) (Sil-ODA_n), polymeric octadecylsilyl silica (polymeric ODS), and monomeric octadecylsilyl silica (monomeric ODS) were studied by a combination of suspension-state ¹H NMR and solid-state ¹³C CP/MAS-NMR to probe the mechanisms underlying their functions as stationary phases for RP-HPLC. Sil-ODA_n, with a strong temperature dependent separation behaviour showed correspondent temperature dependent manifestations in both suspension-state ¹H NMR and solid-state ¹³C CP/MAS-NMR experiments. With a gradual increase in temperature, intensity of proton signals (¹H NMR) of octadecyl moieties (mainly methylene groups) rose dramatically. This dramatic rise was at the same temperature of an endothermic peak detectable in its DSC thermogram indicating a relatively complete solid to liquid phase transition. In addition temperature dependencies of the ratio of trans to gauche conformed well to temperature dependencies of the separation factor between naphthacene and triphenylene (as a simple indicator of shape selectivity). Therefore NMR spectra of Sil-ODA_n were used as a reference for ascertaining percentage of octadecyl moieties of liquid type mobility in the two other stationary phases. Using this method we determined percentage of liquid phase in polymeric ODS and monomeric ODS at various temperatures. We suggest a combination of suspension-state ¹H NMR and solid-state ¹³C CP/MAS-NMR for structure-dynamic characterization of various kinds of hydrocarbon chains grafted onto the silica particles.     

基于原子转移自由基聚合技术的偶氮苯星形液晶聚合物的合成与表征

利用原子转移自由基聚合(ATRP)技术合成了含不同端基取代基的偶氮苯三臂星形侧链液晶聚合物. 均苯三酚与2-溴异丁酰溴通过酯化反应制备三官能团引发剂, 引发偶氮苯单体6-[4-(4-甲氧基苯基偶氮)酚氧基]己基甲基丙烯酸酯(MMAzo)或6-[4-(4-乙氧基苯基偶氮)酚氧基]己基甲基丙烯酸酯(EMAzo)的ATRP反应. 利用核磁共振氢谱(¹H NMR)、凝胶色谱(GPC)、差示扫描量热法(DSC)和偏光显微镜(POM)等手段对星形聚合物进行表征. 星形聚合物的液晶性与相应均聚物相似, 但偶氮苯端基取代基的不同导致星形聚合物的液晶性差别显著. 在紫外/可见光照射下星形聚合物呈现明显的异构化转变.