Preparation of new 2‐amino‐ and 2,3‐diamino‐pyridine trifluoroacetyl enamine derivatives and their application to the synthesis of trifluoromethyl‐containing 3H‐pyrido[2,3‐b][1,4] diazepinols

The synthesis of a novel series of the intermediates N²(N³)‐[1‐alkyl(aryl/heteroaryl)‐3‐oxo‐4,4,4‐trifluoroalk‐1‐en‐1‐yl]‐2‐aminopyridines [F₃CC(O)CH?CR¹(2? NH?C₅H₃N)] and 2,3‐diaminopyridines [F₃CC(O)CH?CR¹(2‐NH₂‐3‐NH? C₅H₃N)], where R¹ = H, Me, C₆H₅, 4‐FC₆H₄, 4‐CIC₆H₄, 4‐BrC₆H₄, 4‐CH₃C₆H₄, 4‐OCH₃C₆H₄, 4,4′‐biphenyl, 1‐naphthyl, 2‐thienyl, 2‐furyl, is reported. The corresponding series of 2‐aryl(heteroaryl)‐4‐trifluoromethyl‐3H‐pyrido[2,3‐b][1,4]diazepin‐4‐ols obtained from intramolecular cyclization reaction of the respective trifluoroacetyl enamines or from the direct cyclocondensation reaction of 4‐methoxy‐1,1,1‐trifluoroalk‐3‐en‐2‐ones with 2,3‐diaminopyridine, under mild conditions, is also reported.     

Resonant Raman spectroscopy of graphene grown on copper substrates

A study of resonant Raman spectroscopy of the as-grown graphene on copper foils is presented. Different laser energies have been used to excite the sample, in order to obtain the dependence of the Raman features (intensities, frequencies and line widths) on the laser energy. We show that the normalised spectra acquired using green laser lines are more intense, with a maximum around 2.3 eV. Moreover, the results show a broader 2D (or G′) band when a UV laser is used to excite the sample, which is explained by the manifestation of the trigonal warping effect in the dispersion of electrons and phonons around the Dirac point.     

An additional structural and electrical study of polymeric haloplumbates(II) with heterocyclic diprotonated amines

The crystal structures of three polymeric bidimensional piperazinium, N,N'-dimethylpiperazinium, and N-benzylpiperazinium hydrate haloplumbates(II) and one polymeric monodimensional N,N'-dimethylpiperazinium hydrate haloplumbate(II) were determined by means of X-ray analysis. The (pipzH2)[PbCl4] salt is monoclinic, space group P2(1)/c, with a = 5.778(2) A, b = 22.612(26) A, c = 9.061(4) A, beta = 95.37(6) degrees, Z = 4; (me2pipzH2)[PbBr4] crystallizes in the monoclinic P2(1) space group with a = 6.101(3) A, b = 18.822(12) A, c = 6.229(2) A, beta = 98.62(4) degrees, Z = 2; the crystals of (me2pipzH2)2[Pb3I10].4H2O are monoclinic, space group P2(1)/c, a = 19.054(4) A, b = 12.239(3) A, c = 18.273(4) A, beta = 93.42(12) degrees, Z = 4; (benzpipzH2)3[Pb2Br10].2H2O crystallizes in the monoclinic P2(1)/c space group with a = 22.380(22) A, b = 9.304(15) A, c = 24.577(25) A, beta = 94.28(11) degrees, Z = 2. Different model type structures, such as one-dimensional linear chain, ribbonlike, and perovskite-like, were observed, and factors governing these structural arrangements are pointed out. The compounds were also investigated by means of thermal and electrical measurements, and correlations between electrical properties and crystal structures were noted.     

Stereoselective synthesis of (+)-1-deoxyaltronojirimycin

A stereocontrolled, facile and high-yield approach for producing (+)-altroDNJ, has been developed starting from the inexpensive commercial cis 2-butene-1,4-diol. Sharpless epoxidation and a subsequent dihydroxylation were used for the introduction of all stereocentres; finally, the ring closure under basic conditions afforded the piperidine heterocycle.     

Pressure induced friction collapse of rare gas boundary layers sliding over metal surfaces

In this Letter we show that friction of anticorrugating systems can be dramatically decreased by applying an external load. The counterintuitive behavior that deviates from the macroscopic Amonton law is dictated by quantum mechanical effects that induce a transformation from anticorrugation to corrugation in the near-surface region. We describe the load-driven modifications occurring in the potential energy surface of different rare gas-metal adsorbate systems, namely, Ar, Kr, Xe on Cu(111), and Xe on Ag(111), and we calculate the consequent friction drop for the commensurate Xe/Cu system by means of combined ab initio and classical molecular dynamics simulations.     

Steric distortion of planar NiP<Subscript>2</Subscript>N<Subscript>2</Subscript> chromophores: a spectral,cyclic voltammetric and single crystal X-ray structural study

The complexes trans-[Ni(4-MP)₂(NCS)₂]·MeCN (1) and trans-[Ni(3-MP)₂(NCS)₂] (2) (4-MP = tri(4-methylphenyl)phosphine, 3-MP = tri(3-methylphenyl)phosphine) were prepared and characterized by IR, UV–visible, NMR spectra, CV, TGA and single crystal X-ray crystallography. Both the complexes have planar geometry and are diamagnetic. The Ni–P distances in both complexes are relatively short as a result of strong back donation from nickel to phosphorus. The phenyl rings in the 3-MP analogue (2) show increased pitching with reference to the plane formed by the ipso carbons due to increased steric effects. For complex (2), the N–Ni–N and P–Ni–P angles are significantly lower than the almost linear N–Ni–N and N–Ni–P angles observed for both complex (1) and trans-[Ni(PPh₃)₂(NCS)₂]. This observation indicates that the 3-methylphosphine ligand forces complex (2) to distort towards a tetrahedral geometry. IR spectra of both complexes show strong bands around 2,090 cm⁻¹ due to N-coordinated thiocyanate, while the electronic spectra contain d–d transitions around 452 nm. Cyclic voltammograms show that the irreversible one-electron reduction potentials increase in the following order: trans- [Ni(PPh₃)₂(NCS)₂] < trans- [Ni(3-MP)₂(NCS)₂] < trans-[Ni(4-MP)₂(NCS)₂], revealing the electron releasing effect of the methyl groups. The planar complexes exhibit interallogony in coordinating solvents.     

Single‐Molecule Magnetism,Enhanced Magnetocaloric Effect,and Toroidal Magnetic Moments in a Family of Ln4 Squares

Three cationic [Ln₄] squares (Ln=lanthanide) were isolated as single crystals and their structures solved as [Dy₄(μ₄‐OH)(HL)(H₂L)₃(H₂O)₄]Cl₂?(CH₃OH)₄?(H₂O)₈ ( 1 ), [Tb₄(μ₄‐OH)(HL)(H₂L)₃(MeOH)₄]Cl₂?(CH₃OH)₄?(H₂O)₄ ( 2 ) and [Gd₄(μ₄‐OH)(HL)(H₂L)₃(H₂O)₂(MeOH)₂]Br₂?(CH₃OH)₄?(H₂O)₃ ( 3 ). The structures are described as hydroxo‐centered squares of lanthanide ions, with each edge of the square bridged by a doubly deprotonated H₂L^2? ligand. Alternating current magnetic susceptibility measurements show frequency‐dependent out‐of‐phase signals with two different thermally assisted relaxation processes for 1 , whereas no maxima in χ_M“ appears above 2.0 K for complex 2 . For 1 , the estimated effective energy barrier for these two relaxation processes is 29 and 100 K. Detailed ab initio studies reveal that complex 1 possesses a toroidal magnetic moment. The ab initio calculated anisotropies of the metal ions in complex 1 were employed to simulate the magnetic susceptibility by using the Lines model (POLY_ANISO) and this procedure yields J₁=+0.01 and J₂=?0.01 cm^?1 for 1 as the two distinct exchange interactions between the Dy^III ions. Similar parameters are also obtained for complex 1 (and 2 ) from specific heat measurements. A very weak antiferromagnetic super‐exchange interaction (J₁=?0.043 cm^?1 and g=1.99) is observed between the metal centers in 3 . The magnetocaloric effect (MCE) was estimated by using field‐dependent magnetization and temperature‐dependent heat‐capacity measurements. An excellent agreement is found for the ?ΔS_m values extracted from these two measurements for all three complexes. As expected, 3 shows the largest ?ΔS_m variation (23 J Kg^?1 K^?1) among the three complexes. The negligible magnetic anisotropy of Gd indeed ensures near degeneracy in the (2S+1) ground state microstates, and the weak super‐exchange interaction facilitates dense population of low‐lying excited states, all of which are likely to contribute to the MCE, making complex 3 an attractive candidate for cryogenic refrigeration.     

Trans influence of triphenylphosphines and pseudohalogens on Ni-S bonds: Synthesis, spectral and single crystal X-ray structural studies on NiS2PN and NiS2PC chromophores

Trans influence of triphenylphosphines and pseudohalogens on Ni-S bonds of NiS₂PN and NiS₂PC chromophores has been investigated by synthesizing and characterizing them. The complexes show the characteristic thioureide IR band at ∼ 1530 cm⁻¹. Electronic spectrum of the cyanide analogue shows a strong blue shift relative to others. X-ray structures of [Ni(pipdtc)(4-MP)(NCS)] (1), [Ni(pipdtc)(PPh₃)(NCS)] (2) and [Ni(pipdtc)(PPh₃)(CN)] (3) (pipdtc = piperidinecarbodithioate anion, 4-MP = tri(4-methylphenyl)phopshine) are reported. Ni-S bond distance trans to 4-MP(1) is longer than the distances in (2) and (3) and Ni-S bond distances trans to Ni-NCS/CN decrease as follows: (3) > (2) > (1). Particularly, 4-MP shows a highly significant trans influence than triphenylphosphine on Ni-S bond. Similarly, CN⁻ exerts a marginally significant trans influence compared to NCS-. Thioureide C-N distances are relatively very short due to the drift of electron density towards the metal. The Ni-N-C angle (163.5(2)°) observed in (2) indicates deviation from linearity to a larger extent compared to that in (1) (176.3(3)°) due to the steric effect of the 4-methyl group. The reduction potentials (CV) for the mixed ligand complexes are much less compared to that of the parent NiS₄ chromophore due to the π-acidic phosphines.      

Electrospinning of Gelatin/PEO Blends: Influence of Process Parameters in the Nanofiber Properties

Summary: Results of electrospinning of gelatin/PEO blends from aqueous solutions are presented. The effects of applied electric field (15–25 kV), flow rate (0.25–0.75 mL/h) and gelatin concentration in the final fiber diameter were studied. It was observed that the resulting fiber system presented high polydispersity, where fiber diameters ranged from 150 nm to 1.3 µm. In some cases an adequate fibrous system were not obtained. It was observed that the average diameter decreased mainly when the flow rate and gelatin concentration decreased.     

Direct, regioselective, and chemoselective preparation of novel boronated tryptophans by Friedel-Crafts alkylation

A facile synthetic approach to the direct preparation of various novel unnatural boronated protected tryptophans using a regio- and chemoselective electrophilic substitution of 4- and 5-boronated indoles with N-protected dehydroalanine is described. The gram-scale synthesis of two free tryptophan boronic acids is also reported.