A Zinc-Mediated Deprotective Annulation Approach to New Polycyclic Heterocycles

A straightforward approach to new polycyclic heterocycles, 1H-benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-ones, is presented. It is based on the ZnCl₂-promoted deprotective 6-endo-dig heterocyclization of N-Boc-2-alkynylbenzimidazoles under mild conditions (CH₂Cl₂, 40 °C for 3 h). The zinc center plays a dual role, as it promotes Boc deprotection (with formation of the tert-butyl carbocation, which can be trapped by substrates bearing a nucleophilic group) and activates the triple bond toward intramolecular nucleophilic attack by the carbamate group. The structure of representative products has been confirmed by X-ray diffraction analysis.     

Carbon Dioxide Capture and Utilization with Isomeric Forms of Bis(amino)-Tagged Zinc Bipyrazolate Metal–Organic Frameworks

Aiming at extending the tagged zinc bipyrazolate metal–organic frameworks (MOFs) family, the ligand 3,3’-diamino-4,4’-bipyrazole ( 3,3’-H₂L ) has been synthesized in good yield. The reaction with zinc(II) acetate hydrate led to the related MOF Zn(3,3’-L) . The compound is isostructural with its mono(amino) analogue Zn(BPZNH₂) and with Zn(3,5-L) , its isomeric parent built with 3,5-diamino-4,4’-bipyrazole. The textural analysis has unveiled its micro-/mesoporous nature, with a BET area of 463 m² g⁻¹. Its CO₂ adsorption capacity (17.4 wt. % CO₂ at p_CO2 = 1 bar and T = 298 K) and isosteric heat of adsorption (Q_st = 24.8 kJ mol⁻¹) are comparable to that of Zn(3,5-L) . Both Zn(3,3’-L) and Zn(3,5-L) have been tested as heterogeneous catalysts in the reaction of CO₂ with the epoxides epichlorohydrin and epibromohydrin to give the corresponding cyclic carbonates at T = 393 K and p_CO2 = 5 bar under solvent- and co-catalyst-free conditions. In general, the conversions recorded are higher than those found for Zn(BPZNH₂), proving that the insertion of an extra amino tag in the pores is beneficial for the epoxidation catalysis. The best catalytic match has been observed for the Zn(3,5-L) /epichlorohydrin couple, with 64 % conversion and a TOF of 5.3 mmol(carbonate) (mmol_Zn)⁻¹ h⁻¹. To gain better insights on the MOF-epoxide interaction, the crystal structure of the [epibromohydrin@ Zn(3,3’-L) ] adduct has been solved, confirming the existence of Br⋅⋅⋅(H)−N non-bonding interactions. To our knowledge, this study represents the first structural determination of a [epibromohydrin@MOF] adduct.     

trans Influence and Steric Effect on the Distortions in Planar NiS4, NiS2PN,and NiS2P2 Chromophores

The planar diamagnetic complexes [Ni(achdtc)₂] ( 1 ),[Ni(achdtc)(PPh₃)(NCS)] ( 2 ), and [Ni(achdtc)(PPh₃)₂]ClO₄ · 0.5EtOAc ( 3 ) (achdtc = allylcyclohexylcarbodithioate) were prepared, characterized by elemental analysis, electronic, IR, NMR (¹H, ¹³C, and ³¹P) spectroscopy and the crystal structures were determined by single‐crystal X‐ray crystallography. The characteristic thioureide bands occur at 1478, 1503, and 1507 cm^–1 for 1 , 2 , and 3 , respectively and the corresponding ¹³C chemical shifts are observed at 207.67, 204.16, and 202.31 ppm. ³¹P chemical shifts are observed at δ = 29.24 and 22.73 ppm for 2 and 3 , respectively, indicating a strong interaction. Electronic spectral bands are observed at 480, 483, and 475 nm for 1 , 2 , and 3 , corresponding to d → d_xy/d transitions. Ni–S distances are asymmetric. The trans influence of PPh₃ elongates the Ni–S bonds. The decrease in the S–Ni–S bite angle in 2 [78.80(16)°] and in 3 [78.36(2)°] compared to that observed in 1 [79.42(3)°] is due to the steric crowding of PPh₃ around the central metal atom. A comparison of the bond parameters of compounds 1 – 3 shows a change in the arrangement from a planar NiS₄ chromophore to distorted planar NiS₂P₂ chromophores. The observed distortion from planar to tetrahedral arrangement is influenced by the bulky triphenylphosphine ligand.     

Antibacterial Action of 4,4'-Bipyrazolyl-Based Silver(I) Coordination Polymers Embedded in PE Disks

Coupling the rigid spacer 4,4'-bipyrazole (H(2)BPZ), in its anionic or neutral form, to different silver(I) salts allowed isolation of the novel coordination polymers [Ag(2)(BPZ)] (1) and [Ag(H(2)BPZ)(X)] (X = NO(3), 2; ClO(4), 3; BF(4), 4; PF(6), 5; CH(3)SO(3), 6; CF(3)SO(3), 7), which were fully characterized by infrared and emission spectroscopies, thermal analysis, and X-ray powder diffraction. The crystal structure of 1 consists of 2-D layers containing 1-D chains of Ag(I) ions bridged by exo-tetradentate bipyrazolato moieties. The crystal structures of the [Ag(H(2)BPZ)(X)] species 2-7 feature 1-D chains of [Ag(H(2)BPZ)] stoichiometry, along which the metal centers are bridged by exo-bidentate bipyrazolyl spacers. Contacts among adjacent chains are mediated by the counterions through nonbonding interactions involving the Ag(I) ions and the pyrazolyl N-H groups. Thermogravimetric analyses disclosed the good thermal stability of these materials, decomposing in the range 200-300 °C. Under UV irradiation at room temperature, all the species showed a yellow-green emission centered in the range 520-522 nm. When embedded into polyethylene disks, 1, 2, and 4-7 demonstrated their activity as topical antibacterial agents against suspensions of E. coli, P. aeruginosa, and S. aureus: complete reduction of the three bacterial strains was achieved in 24 h, reduction of S. aureus reaching ca. 90% in only 2 h. Biocidal action was expressed also by contact susceptibility tests.     

Mechanochemical synthesis in copper(II) halide/pyridine systems: single crystal X-ray diffraction and IR spectroscopic studies

Whereas complexes of divalent metal halides (X = Cl, Br, I) with/from pyridine commonly crystallise as trans-[M(py)(4)X(2)]·2py, M on a site of 222 symmetry in space group Ccca, true for CuCl(2) and CuBr(2) in particular, the copper(II) iodide adduct is of the form [Cu(py)(4)I]I·2py, Cu on a site of mm2 symmetry in space group Cmcm, and five-coordinate (square-pyramidal), the same cationic species also being found in 2[Cu(py)(4)I](I(3))·[(py)(2)Cu(μ-I)(2)Cu(py)(2)] (structurally defined). Bromide or N-thiocyanate may be substituted for the unbound iodide ion in the solvated salt, resulting in complexes which crystallize in space group Ccca, but with both anions and the metal atom disordered. In [Cu(py)(4)(I(3))(2)], a pair of long Cu···I contacts approach a square-planar Cu(py)(4) array. Assignments of the ν(CuN) and ν(CuX) (X = Br, I, SCN) bands in the far-IR spectra are made, the latter with the aid of analogous assignments for [Cu(py)(2)X(2)] (X = Cl, Br), which show a dependence of ν(CuX) on the Cu-X bond length that is very similar to that determined previously for copper(i) halide complexes. The structure of the adventitious complex [(trans-)(H(2)O)(py)(4)CuClCu(py)(4)](I(3))(3)·H(2)O is also recorded, with six- and five-coordinate copper atoms; rational synthesis provides [{Cu(py)(4)}(2)(μ-Cl)](I(3))(3)·H(2)O with one water molecule less. In [{Cu(py)(4)Cl}((∞|∞))](I(3))·3py, square pyramidal [Cu(py)(4)Cl](+) cations, assisted by Cl···Cu interactions, stack to give rise to infinite polymeric strings. Several of these compounds were prepared mechanochemically, illustrating the applicability of this method to syntheses involving redox reactions as well as to complex syntheses involving up to five components. The totality of results demonstrates that the [Cu(II)(py)(4)] entity can be stabilized in an unexpectedly diverse range of mononuclear and multinuclear complexes through the presence of lattice pyridine molecules, the bulky triiodide ion, or a combination of both.     

Antibacterial activity of 15-deoxygoyazensolide isolated from the stems of Minasia alpestris (Asteraceae) against oral pathogens

This work reports the isolation of the sesquiterpene lactone 15-deoxygoyazensolide from the stems of Minasia alpestris and the evaluation of its antimicrobial activity against the following oral pathogens: Enterococcus faecalis, Streptococcus salivarius, Streptococcus sobrinus, Streptococcus mutans, Streptococcus mitis, Streptococcus sanguinis, and Lactobacillus casei. Despite the cytotoxicity and genotoxicity of other sesquiterpene lactones of the furanoheliangolide-type, our results revealed that this compound exhibits low antibacterial activity against the evaluated oral pathogens; however, an interesting selectivity against E. faecalis (minimum inhibitory concentration [MIC]=40 μg mL(-1)) and S. sobrinus (MIC=60 μg mL(-1)) was observed.     

Nanocomposite of montmorillonite with telechelic sulfonated poly(butylene terephthalate): Effect of ionic groups on clay dispersion, mechanical and thermal properties

Telechelic ionomeric poly(butylene terephthalate) nanocomposites with organically modified clays have been prepared by the melt intercalation technique both in Brabender mixer and in twin screw-extruder. The presence of ionic groups tethered at the end of the polymer chains permits electrostatic interaction between the polymer and the surface of an organically modified clays providing a thermodynamic driving force for the dispersion of the clay platelets in the polymer matrix. The improved dispersion has been verified by TEM and XRD analyses. Nanocomposites with telechelic polymers present therefore consistently higher thermo-mechanical properties and improved thermal and hydrolytic stability respect to nanocomposites with standard PBT. Nanocomposite obtained using PBT with 3% telechelic ionic groups and with 5% of clay present a heat deflection temperature that is 48 °C higher compared to that of the commercial material. The presence of the clay also slightly increases the thermal and hydrolytic stability respect to standard PBT.     

Evaluation of the antimicrobial activity of some chloro complexes of imidazole-2-carbaldehyde semicarbazone: X-ray crystal structure of cis-NiCl2(H2L)(H2O)

Some first row transition metal(II) complexes of imidazole-2-carbaldehyde semicarbazone (H₂L) have been synthesised and characterised. Single crystal X-ray diffraction studies have revealed the molecular structure of the neutral complex NiCl₂(H₂L)(H₂O). This latter complex shows a pseudo-octahedral geometry, with both chloride ions displaying a cis disposition. The asymmetry of this complex allows the formation of two enantiomers in the racemic crystal structure. The non-deprotonated semicarbazone ligand behaves as an N,N,O-donor, through the imidazole and imine N atoms and the O_keto atom. The coordinative behaviour of H₂L in CuCl₂(H₂L)(H₂O), ZnCl₂(H₂L)₂·0.5EtOH and CoCl₂(H₂L)₂·0.5H₂O is reported as only N,O-donor. The antimicrobial activity of the semicarbazone ligand and its metal complexes has been tested against some representative bacteria and fungi. A moderate inhibitory activity of the cobalt complex was detected towards the assayed phytopathogenic fungi Alternariatenuis and Sclerotinia minor (MIC 50 μg/mL).     

A nitrate bridged one-dimensional copper(II) coordination polymer with a tridentate (NNO) Schiff base: synthesis,X-ray structure and catalytic efficacy

Transition Metal Chemistry - A one-dimensional Cu(II) coordination polymer of [Cu(L)(μ-ONO2)]n (1) (HL = 4-methoxy-2-[1-(methylaminoethylimino)methyl]-phenol) with bidentate...