MOF‐on‐MOF: Oriented Growth of Multiple Layered Thin Films of Metal–Organic Frameworks

The precise alignment of multiple layers of metal–organic framework (MOF) thin films, or MOF‐on‐MOF films, over macroscopic length scales is presented. The MOF‐on‐MOF films are fabricated by epitaxially matching the interface. The first MOF layer (Cu₂(BPDC)₂, BPDC=biphenyl‐4,4′‐dicarboxylate) is grown on an oriented Cu(OH)₂ film by a “one‐pot” approach. Aligned second (Cu₂(BDC)₂, BDC=benzene 1,4‐dicarboxylate, or Cu₂(BPYDC)₂, BPYDC=2,2′‐bipyridine‐5,5′‐dicarboxylate) MOF layers can be deposited using liquid‐phase epitaxy. The co‐orientation of the MOF films is confirmed by X‐ray diffraction. Importantly, our strategy allows for the synthesis of aligned MOF films, for example, Cu₂(BPYDC)₂, that cannot be grown on a Cu(OH)₂ surface. We show that aligned MOF films furnished with Ag nanoparticles show a unique anisotropic plasmon resonance. Our MOF‐on‐MOF approach expands the chemistry of heteroepitaxially oriented MOF films and provides a new toolbox for multifunctional porous coatings.     

Development of a novel approach for the production of dried genomic DNA for use as standards for qualitative PCR testing of food-borne pathogens

As part of a multi-centre European project, FOOD-PCR, the feasibility of a novel approach for production of dried bacterial DNA that could be used as certified reference materials (CRM) was assessed. Selected strains of Salmonella typhimurium, Listeria monocytogenes, Escherichia coli O157, Campylobacter jejuni and Yersinia enterocolitica were used to produce genomic DNA (gDNA). These preparations gave support to method development for qualitative polymerase chain reaction (PCR) detection methods for food-borne pathogens. Purified gDNA was transformed into stable and dry gDNA by using polypropylene vials as carrier and applying a vacuum-drying technique. The gDNA preparations were shown to be sufficiently stable under ambient transport conditions without cooling and proved to have long-term stability at 5°C of at least 22 months. The dried DNA was easily reconstituted by addition of distilled water then gentle shaking. These studies have shown that production of stable and dry bacterial gDNA material is feasible and could help satisfy the increasing need for certified reference DNA positive control samples in the field of PCR testing for detection and verification of food-borne microbial pathogens.

     

Total synthesis of (-)-microcarpalide, a novel microfilament disrupting metabolite

The stereoselective total synthesis of (-)-microcarpalide, a recently discovered 10-membered lactone of fungal origin displaying a remarkable disrupting action on actin microfilaments, was accomplished by using ring-closing metathesis (RCM) as the key step for the formation of the medium-sized ring. The diene ester required for the macrocyclization reaction was assembled via DCC-mediated esterification of two suitable partners, each bearing a terminal alkene group. The alcohol fragment was synthesized from n-bromohexane through a seven-step sequence entailing two consecutive stereoselective homologations of chiral boronic esters as strategic transformations for the sequential insertion of the two stereocentres with the final S absolute configuration, using (+)-pinanediol as the chiral director; final elaboration to the desired C(11) framework envisaged treatment with an allyl Grignard reagent and oxidative cleavage of the boronic scaffold. In contrast, the acidic fragment was prepared in ten steps from d-tartaric acid, whose C(4) backbone was elongated to the required C(7) skeleton by means of two distinct Swern-Wittig oxidation-homologation sequences.     

Assembling heterocycle-tethered C-glycosyl and alpha-amino acid residues via 1,3-dipolar cycloaddition reactions

The 1,3-dipolar cycloadditions of C-glycosyl nitrile oxides and acetylenes to an alkyne and an azide, respectively, bearing a masked glycinyl moiety furnished disubstituted isoxazoles and triazoles. Unveiling the glycinyl group in these cycloadducts afforded C-glycosyl alpha-amino acids in which the two bioactive entities were tethered through rigid five-membered heterocycles. Optimized entries to the same compounds involved the use of unmasked but protected alkyne- and azide-containing amino acids as the partners of 1,3-dipolar cycloadditions.     

Rotational dynamics of CO solvated in small He clusters: a quantum Monte Carlo study

The rotational dynamics of CO single molecules solvated in small He clusters (CO @ HeN) has been studied using reptation quantum Monte Carlo simulations for cluster sizes up to N = 30. Our results are in good agreement with the rotovibrational features of the infrared spectrum recently determined for this system and provide a deep insight into the relation between the structure of the cluster and its dynamics. Simulations for large N also provide a prediction of the effective moment of inertia of CO in the He nanodroplet regime, which has not been measured so far.     

Metal ion catalysis in the beta-elimination reactions of N-[2-(4-pyridyl)ethyl]quinuclidinium and N-[2-(2-pyridyl)ethyl]quinuclidinium in aqueous solution

Catalysis of the beta-elimination reaction of N-[2-(4-pyridyl)ethyl]quinuclidinium (1) and N-[2-(2-pyridyl)ethyl]quinuclidinium (2) by Zn(2+) and Cd(2+) in OH(-)/H(2)O (pH = 5.20-6.35, 50 degrees C, and mu = 1 M KCl) has been studied. In the presence of Zn(2+), the elimination reactions of both isomers occur from the Zn(2+)-complexed substrates (C). The equilibrium constants for the dissociation of the Zn(2+)-complexes are as follows: K(d) = 0.012 +/- 0.003 M (isomer 1) and K(d) = 0.065 +/- 0.020 M (isomer 2). The value of k(C)(H2O) for isomer 1 is 4.81 x 10(-6) s(-1). For isomer 2 both the rate constants for the "water" and OH(-)-induced reaction of the Zn(2+)-complexed substrate could be measured, despite the low concentration of OH(-) in the investigated reaction mixture [k(C)H2O)= 1.97 x 10(-6) s(-1) and k(C)(OH-)= 21.9 M(-1) s(-1), respectively]. The measured metal activating factor (MetAF), i.e., the reactivity ratio between the complexed and the uncomplexed substrate, is 8.1 x 10(4) for the OH(-)-induced elimination of 2. This high MetAF can be compared with the corresponding proton activating factor (Alunni, S.; Conti, A.; Palmizio Errico, R. J. Chem. Soc., Perkin Trans. 2 2000, 453), PAF = 1.5 x 10(6) and is in agreement with an E1cb irreversible mechanism (A(xh)D(E)* + D(N)) (Guthrie, R. D.; Jencks, W. P. Acc. Chem. Res. 1989, 22, 343). A value of k(C)(H2O)>or= 23 x 10(-7) s(-1) is estimated for the Cd(2+)-complexed isomer 2, while catalysis by Cd(2+) has not been observed for isomer 1.     

Relaxation processes of the liquid crystal ME6N in the isotropic phase studied by Raman scattering experiments

We have investigated the Raman profiles of the nu(C[Triple Bond]N) and nu(C=O) vibrational modes of the nematic liquid crystal ME6N (4-cyanophenyl-4(')-hexylbenzoate) in the isotropic phase at different temperatures and used them as probes of the dynamics and structural organization of this liquid. The vibrational time correlation functions of the nu(C[Triple Bond]N) mode, rather adequately interpreted within the assumption of exponential modulation function (the Kubo-Rothschild theory), indicate that the system experiences an intermediate dynamical regime that gets only slightly faster with increasing temperature. However, this theory fails in predicting the non-exponential behavior that the time correlation functions manifest in the long time range (t>3 ps). For this reason we have additionally approached the interpretation of vibrational correlation functions in terms of the theory formulated by Rothschild and co-workers for locally structured liquids. The application of this theory reveals that the molecular dynamics in this liquid crystal in the isotropic phase is that deriving from a distribution of differently sized clusters, which narrows as the temperature increases. Even at the highest temperature reached in this study (87 degrees C above the nematic-isotropic transition), the liquid has not yet achieved the structure of the simple liquid and the dynamics has not reached the limit of the single channel process. The vibrational and orientational relaxations occur in very different time scales. The temperature independence of the orientational dynamics in the whole range from 55 degrees C to 135 degrees C has been referred to the nonhydrodynamic behavior of the system, arising when local pseudonematic structures persist for times longer than the orientational relaxation. The occurrence of the process of resonant vibrational energy transfer between the C=O groups of adjacent molecules has been revealed in the isotropic phase by a slightly positive Raman noncoincidence effect in the band associated with the nu(C=O) mode. A qualitative interpretation is tentatively given in terms of partial cancellation of contributions deriving from structures having opposite orientations of their C=O groups.     

Luminescent ruthenium(II) bipyridine-calix[4]arene complexes as receptors for lanthanide cations

The synthesis and photophysical properties of novel luminescent ruthenium(II) bipyridyl complexes containing one, two, or six lower rim acid-amide-modified calix[4]arene moieties covalently linked to the bipyridine groups are reported which are designed to coordinate and sense luminescent lanthanide ions. All the Ru-calixarene complexes synthesized in this work are able to coordinate Nd(3+), Eu(3+), and Tb(3+) ions with formation of adducts of variable stoichiometry. The absorbance changes allow the evaluation of association constants whose magnitudes depend on the nature of the complexes as well as on the nature of the lanthanide cation. Lanthanide cation complex formation affects the ruthenium luminescence which is strongly quenched by Nd(3+) ion, moderately quenched by the Eu(3+) ion, and poorly or moderately increased by the Tb(3+) ion. In the case of Nd(3+), the excitation spectra show that (i) the quenching of the Ru luminescence occurs via energy transfer and (ii) the electronic energy of the excited calixarene is not transferred to the Ru(bpy)(3) but to the neodymium cation. In the case of Tb(3+), the adduct's formation leads to an increase of the emission intensities and lifetimes. The reason for this behavior was ascribed to the electric field created around the Ru calix[4]arene complexes by the Tb(3+) ions by comparison with the Gd(3+) ion, which behaves identically and can affect ruthenium luminescence only by its charge. However, especially for compounds 1 and 3, it cannot be excluded that some contribution comes from the decrease of vibrational motions (and nonradiative processes) due to the rigidification of the structure upon Tb(3+) complexation. In the case of Eu(3+), compounds 1, 2, and 4 were quenched by the lanthanide addition but the quenching of the ruthenium luminescence is not accompanied by europium-sensitized emission which suggests that an electron-transfer mechanism is responsible for the quenching. On the contrary, compound 3 exhibits enhanced emission upon addition of Eu(3+) (as nitrate salt); it is suggested that the lack of quenching in the [3.2Eu(3+)] adduct is due to kinetic reasons because the electron-transfer quenching process is thermodynamically allowed.     

Are ring currents still useful to rationalize the benzene proton magnetic shielding?

[structure: see text] The conventional interpretation of proton NMR chemical shifts is supported by large basis set ab initio quantum mechanical calculations. The benzene protons are predicted to lie within the deshielding zone defined in terms of the out-of-plane magnetic shielding domain. However, ring currents by themselves are not sufficient to account quantitatively for the observed benzene proton downfield chemical shift. sigma-Electron contributions must also be taken into account. The conventional explanation for the ethyne proton chemical shift is valid.