Catalytic flow injection determination of vanadium by oxidation of N-(3-sulfopropyl)-3,3',5,5'-tetramethylbenzidine using bromate

A catalytic flow-injection (FI) method was developed for the determination of 10⁻⁹ mol l⁻¹ levels of vanadium(IV, V). The method is based on the catalytic effect of vanadium(V) on oxidation of N-(3-sulfopropyl)-3,3′,5,5′-tetramethylbenzidine (TMBZ·PS) using bromate as oxidant to form a yellow dye (λ_max=460 nm). The use of 5-sulfosalicylic acid (SSA) as an activator enhanced the sensitivity of the method. The calibration graphs with a working range 0.05–8.0 ng ml⁻¹ were obtained for vanadium(V). Vanadium(IV) was also determined, being oxidized by bromate. The detection limit (signal/noise, S/N=3) was 0.01 ng ml⁻¹ (ca. 2×10⁻¹⁰ mol l⁻¹) vanadium. The relative standard deviations (R.S.D.) for 15 determinations of 0.5 ng ml⁻¹ vanadium, and for ten determinations of 0.1 and 1.0 ng ml⁻¹ vanadium were 0.41, 2.6 and 0.25%, respectively, with a sampling rate of 15 samples h⁻¹. The proposed method was successfully applied to the determination of vanadium in natural waters.     

PVC membrane based potentiometric sensors for uranium determination

Two novel uranyl PVC matrix membrane sensors responsive to uranyl ion are described. The first sensor incorporates tris(2-ethylhexyl)phosphate (TEHP) as both electroactive material and plasticizer and sodium tetraphenylborate (NaTPB) as an ion discriminator. The sensor displays a rapid and linear response for UO₂²⁺ ions over the concentration range 1×10⁻¹–2×10⁻⁵ mol l⁻¹ UO₂²⁺ with a cationic slope of 25.0±0.2 mV decade⁻¹. The working pH range is 2.8–3.6 and the life span is 4 weeks. The second sensor contains O-(1,2-dihydro-2-oxo-1-pyridyl)-N,N,N′,N′-bis(tetra-methylene)uronium hexafluorophosphate (TPTU) as a sensing material, sodium tetraphenylborate as an ion discriminator and dioctyl phenylphosphonate (DOPP) as a plasticizer. Linear and stable response for 1×10⁻¹–5×10⁻⁵ mol l⁻¹ UO₂²⁺ with near-Nernstian slope of 27.5±0.2 mV decade⁻¹ are obtained. The working pH range is 2.5–3.5 and the life span of the sensor is 6 weeks. Interference from many inorganic cations is negligible for both sensors. However, interference caused by some ions (e.g. Th⁴⁺, Cu²⁺, Fe³⁺) is eliminated by a prior ion exchange or solvent extraction step. Direct potentiometric determination of as little as 5 μg ml⁻¹ uranium in aqueous solutions shows an average recovery of 97.2±1.3%. Application for the determination of uranium at levels of 0.01–1 wt.% in naturally occurring and certified ores gives results with good correlation with data obtained by X-ray fluorescence.     

Spectrophotometric method for determination of vanadium and its application to industrial, environmental, biological and soil samples

The very sensitive, fairly selective direct spectrophotometric method for the determination of trace amount of vanadium (V) with 1,5-diphenylcarbohydrazide (1,5-diphenylcarbazide) has been developed. 1,5-diphenylcarbohydrazide (DPCH) reacts in slightly acidic (0.0001–0.001 M H₂SO₄ or pH 4.0–5.5) 50% acetonic media with vanadium (V) to give a red–violet chelate which has an absorption maximum at 531 nm. The average molar absorption coefficient and Sandell’s sensitivity were found to be 4.23×10⁴ l mol⁻¹ cm⁻¹ and 10 ng cm⁻² of V^v, respectively. Linear calibration graph were obtained for 0.1–30 μg ml⁻¹ of V^v: the stoichiometric composition of the chelate is 1:3 (V: DPCH). The reaction is instantaneous and absorbance remain stable for 48 h. The interference from over 50 cations, anions and complexing agents has been studied at 1 μg ml⁻¹ of V^v. The method was successfully used in the determination of vanadium in several standard reference materials (alloys and steels), environmental waters (potable and polluted), biological samples (human blood and urine), soil samples, solution containing both vanadium (V) and vanadium (IV) and complex synthetic mixtures. The method has high precision and accuracy (s=±0.01 for 0.5 μg ml⁻¹).     

Synthesis and spectroscopic analysis of tetraphenylporphyrinatoantimony(V) complexes linked to boron-dipyrrin chromophore on axial ligands

Tetraphenylporphyrinatoantimony(V) complexes, linked to boron-dipyrrin chromophores on axial ligands, were synthesized. The fluorescence spectra of 1a, 1b and 1c (3-[4-(N,N′-difluorobornyl-5-dipyrrinyl)phenyl]propoxo(methoxo)antimony(V) tetraphenylporphyrin bromide (1a); 6-[4-(N,N′-difluorobornyl-5-dipyrrinyl)phenyl]hexyloxo(methoxo)antimony(V) tetraphenylporphyrin bromide (1b); bis{3-[4-(N,N′-difluorobornyl-5-dipyrrinyl)phenyl]propoxo}antimony(V) tetraphenylporphyrin bromide (1c)) were analyzed under the excitations of N,N′-difluorobornyl-5-dipyrrinylphenyl (Bdpy) and tetraphenylporphyrinatoantimony(V) (Sb(TPP)) chromophores. Under the irradiation of Bdpy chromophore, the excitation energy was transferred from Bdpy chromophore to the Sb(TPP) moiety at 0.13–0.40 of the quantum yields, even in a polar solvent. On the other hand, the emission of Sb(TPP) chromophores was quenched by Bdpy chromophores at rate constants of 10⁸–10⁹ s⁻¹, independent of on the solvent polarity. Under the excitation of the Bdpy chromophore of 1d (3-[4-(N,N′-difluorobornyl-5-dipyrrinyl)phenyl]propoxo(phenyloxo)antimony(V) tetraphenylporphyrin bromide) involving both the Bdpy and the phenoxy chromophores on the axial ligands, the excited singlet state of the Sb(TPP) chromophore generated by the energy transfer from the Bdpy chromophore was quenched by the phenoxy ligand via non-radiative processes involving electron transfer. However, rapid back electron-transfer may occur because no absorption of the anion radical of Sb(TPP) was observed by nanosecond laser photolysis.     

Synthesis, spectroscopic studies and nonlinear optical behavior of N,N′-bis(2-hydroxy-1-naphthylmethylidene)-1-methyl-1,2-diaminoethane-N,N′,O,O′-nickel(II)

A Schiff base complex N,N′-bis(2-hydroxy-1-naphthylmethylidene)-1-methyl-1,2- diaminoethane-N,N′,O,O′-nickel(II) has been synthesized. The title compound has been characterized by FT-IR and UV–vis spectroscopies. The UV–vis experiments indicate that the compound has solvatochromism in the UV region, implying non-zero molecular first hyperpolarizability. To investigate microscopic second-order nonlinear optical (NLO) behavior of the examined complex, the electric dipole moments (μ) and the first static hyperpolarizabilities (β) were computed using Finite Field second-order Møller Plesset (FF MP2) perturbation procedure. According to ab initio quantum mechanical calculations, the title complex exhibits non-zero β values, revealing microscopic second-order NLO behavior.     

Syntheses of mixed ligands complexes of Ru(II) with 4,4′-dicarboxy-2,2′-bipyridine and substituted pteridinedione and the use of these complexes in electrochemical photovoltaic cells

The synthesis, spectral and photoelectrochemical studies of mixed ligand complexes of [Ru(dcbpy)₂(LL)]Cl₂, where LL=2,4-(1,3-N,N′-dimethyl)pteridinedione (DMP), 6,7-dimethyl-2,4-(1,3-N,N′-dimethyl)pteridinedione (MDMP), 6,7-diphenyl-2,4-(1,3-N,N′-dimethyl)pteridinedione (PhDMP), dibenzo[h,j]-(1,3-N,N′-dimethyl)isoalloxazine (BIAlo), 6,7-bis(pyrid-2-yl)-2,4-(1,3-N,N′-dimethyl) pteridinedione (PyDMP) were carried out. These complexes were attached to sol–gel processed TiO₂ electrodes and the photocells fabricated were illuminated with polychromatic radiation in the presence of I₂/I₃⁻ as redox electrolyte. The incident photon to current conversion efficiency determined was found to be 20–48%.     

Cyano-bridged bimetallic complexes of copper(II) with tetracyanonickelate(II). Crystal structure of [Cu(dpt)Ni(CN)4]

Five new complexes of composition [Cu(dpt)Ni(CN)₄] (1) (dpt=dipropylenetriamine), [Cu(dien)Ni(CN)₄]·2H₂O (2) (dien=diethylenetriamine), [Cu(N,N′-dimeen)Ni(CN)₄]·H₂O (3) (N,N′-dimeen=N,N′-dimethylethylenediamine), [Cu(N,N-dimeen)Ni(CN)₄]·H₂O (4) (N,N-dimeen=N,N-dimethylethylenediamine) and [Cu(trimeen)Ni(CN)₄] (5) (trimeen=N,N,N′-trimethylethylenediamine) have been obtained by the reactions of the mixture of Cu(ClO₄)₂·6H₂O, appropriate amine and K₂[Ni(CN)₄] in water and have been characterized by IR and UV–Vis spectroscopies and magnetic measurements. The crystal structure of [Cu(dpt)Ni(CN)₄] (1) has been determined by single-crystal X-ray analysis. The structure of 1 consists of a one-dimensional polymeric chain ---Cu(dpt)---NC---Ni(CN)₂---CN---Cu(dpt)--- in which the Cu(II) and Ni(II) atoms are linked by CN groups. The nickel atom is four coordinate with four cyanide-carbon atoms (two cyano groups are terminal and two cyano groups (in cis fashion) are bridged) in a square-planar arrangement and the copper atom is five coordinate with two cyanide-nitrogen and three dpt-nitrogen atoms, in a distorted square-pyramidal arrangement. The temperature dependence of magnetic susceptibility (2–300 K) was measured for compound 1. The magnetic investigation showed the presence of a very weak antiferromagnetic interaction (J=−0.16 cm⁻¹) between the copper atoms within each chain through the diamagnetic Ni(CN)₄ ²⁻ ions.     

Determination and identification of malathion, ethion and dichlorovos using ion mobility spectrometry

Positive ion mobility spectra of different organophosphorus pesticides such as malathion (s-(1,2-dicarb-ethoxyethyl) o,o-dimethyl dithiophosphate), ethion (o,o,o′,o′-tetraethyl s,s′-methylene bis(phosphorodithioate)) and dichlorovos (2,2-dichlorovinyl dimethyl phosphate) have been studied in air at ambient pressure using ion mobility spectrometry method with ⁶³Ni ionization source. The limits of quantification (LOQs) were 1.0 × 10⁻⁹, 1.0 × 10⁻⁹ and 5.0 × 10⁻⁹ g for malathion, ethion and dichlorovos, respectively. The working range of these compounds was about three orders of magnitude and the relative standard deviation (R.S.D.) of repeatability at the 5 μg ml⁻¹ level were all below 15%. Furthermore, in this study, the influences of IMS cell temperature on the ion mobility spectra of these compounds were investigated.     

Ab initio study of 1,1-(methylphosphinylidene) bis(methanamine) and vibrational assignment of its N,N′-coordinated Pt(II) and Pd(II) chloro complexes

Infrared and Raman spectra of 1,1-(methylphosphinylidene) bis(methanamine) [mpbm, (CH₃)PO(CH₂NH₂)₂] and its N,N′-coordinated Pt(II) and Pd(II) have been studied in the 4000–200 cm⁻¹ frequency range. Ab initio calculations have been carried out for different conformations of the mpbm at HF/6-31G* level of the theory from which structural parameters, conformational stability and predicted infrared and Raman spectra have been obtained. A complete vibrational assignment of the lowest energy conformer, tttg⁻, as well as of its N,N′-coordinated Pt(II) and Pd(II) chloro-complexes was done on the basis of the calculated frequencies, relative infrared intensities, Raman activities and potential energy distribution (PED). The theoretical predictions are compared with the experimental results where appropriate.     

Simultaneous determination of vanadium and molybdenum as N-benzoyl-N-phenylhydroxamate complexes by combining solvent extraction and liquid chromatography

A normal-phase high-performance liquid chromatographic method for the selective simultaneous determination of vanadium and molybdenum with N-benzoyl-N-phenylhydroxylamine (BPHA) is described. The V(V)-BPHA and Mo(Vl)-BPHA complexes were preconcentrated by solvent extraction into chloroform and injected on to a nitrile-bonded column for chromatography. The mobile phase was a 5.9· 10⁻⁴ M solution of BPHA in chloroform (stabilized with amylene). The detection limits for vanadium and molybdenum were 2.1 and 3.3 ng ml⁻¹, respectively, for an aqueous to organic phase-volume ratio of 20:1. The procedure, applied to the analysis of a synthetic water, showed satisfactory accuracy and precision.     

Simultaneous determination of trace uranium(VI) and zinc(II) by adsorptive cathodic stripping voltammetry with aluminon ligand

Uranium(VI) complexed with aluminon (3-[bis(3-carboxy-4-hydroxy-phenyl)methylene]-6-oxo-1,4-cyclohexadiene-1-carboxylic acid triammonium salt) was determined by adsorptive cathodic stripping voltammetry (ACSV) using a hanging mercury drop electrode. Trace uranium(VI) and zinc(II) can be simultaneously determined in a single scan in the presence of aluminon and urea. Optimal conditions were found to be: accumulation time; 180–200 s, accumulation potential; 50 mV versus Ag/AgCl, scan rate; 40 mV s⁻¹, supporting electrolyte; 0.1 M sodium acetate buffer at pH 6.5–7.0, and concentration of aluminon; 1×10⁻⁶ M. The linear range of uranium(VI) and zinc(II) were observed over the concentration range 2–33 and 30–120 ng ml⁻¹, respectively. The detection limit (S/N=3) are 0.2 ng ml⁻¹ (uranium) and 30 ng ml⁻¹ (zinc). A good reproducibility shows RSDs of 2.5–4.0% (n=10). The procedure offers high selectivity, with the presence of urea masking some metal ions.     

Structural and spectral studies of N-2-(pyridyl)-, N-2-(3-, 4-, 5-, and 6-picolyl)- and N-2-(4,6-lutidyl)-N′-2-thiomethoxyphenylthioureas

Structures of the following compounds have been obtained: N-(2-pyridyl)-N′-2-thiomethoxyphenylthiourea, PyTu2SMe, monoclinic, P2₁/c, a=11.905(3), b=4.7660(8), c=23,532(6) Å, β=95.993(8)°, V=1327.9(5) ų and Z=4; N-2-(3-picolyl)-N′-2-thiomethoxyphenyl-thiourea, 3PicTu2SeMe, monoclinic, C2/c, a=22.870(5), b=7.564(1), c=16.941(4) Å, β=98.300(6)°, V=2899.9(9) ų and Z=8; N-2-(4-picolyl)-N′-2-thiomethoxyphenylthiourea, 4PicTu2SMe, monoclinic P2₁/a, a=9.44(5), b=18.18(7), c=8.376(12) Å, β=91.62(5)°, V=1437(1) ų and Z=4; N-2-(5-picolyl)-N′-2-thiomethoxyphenylthiourea, 5PicTu2SMe, monoclinic, C2/c, a=21.807(2), b=7.5940(9), c=17.500(2) Å, β=93.267(6)°, V=2893.3(5) ų and Z=8; N-2-(6-picolyl)-N′-2-thiomethoxyphenylthiourea, 6PicTu2SMe, monoclinic, P2₁/c, a=8.499(4), b=7.819(2), c=22.291(8) Å, β=90.73(3)°, V=1481.2(9) ų and Z=4 and N-2-(4,6-lutidyl)-N′-2-thiomethoxyphenyl-thiourea, 4,6LutTu2SMe, monoclinic, P2₁/c, a=11.621(1), b=9.324(1), c=14.604(1) Å, β=96.378(4)°, V=1572.4(2) ų and Z=4. Comparisons with other N-2-pyridyl-N′-arylthioureas having substituents in the 2-position of the aryl ring are included.     

Synthesis and efficiency of an epoxy-tannin chelating resin for preconcentrating and separating various rare elements

A new epoxy-tannin chelating resin was synthesized from epoxy resin and used for the preconcentration and separation of rare elements. The acidity, rate, reuse, capacity and interference on the adsorption of ions on the resin as well as the conditions of desorption of these ions from the resin were investigated by means of inductively coupled plasma atomic emission spectrometry (ICP-AES). The composition of the resin and mechanism of enrichment for some ions were discussed. The results show that the relative standard deviations for the determination of 50 ng ml⁻¹ Ga(III), In(III), Bi(III) and Sn(IV), 10 ng ml⁻¹ La(III), Y(III), Cr(III), Ti(IV) and V(V) and 1.0 ng ml⁻¹ Be(II) were in the range of 0.5–4.5%. The contents of these elements in a sample solution from a smelter determined by the new method were in agreement with those values obtained by Zeeman atomic absorption spectrometry with an average error <3.4%.     

Conformational and structural analysis of N-N′-bis(4-methoxybenzylidene)ethylenediamine

The Schiff base compound, N-N′-bis(4-methoxybenzylidene)ethylenediamine (C₁₈H₂₀N₂O₂) has been synthesized and its crystal structure has been investigated by X-ray analysis and PM3 method. The compound crystallizes in monoclinic space group P2₁/n with a=10.190(1), b=7.954(1), c=10.636(1) Å, β=111.68(1)°, V=801.1(1) ų, Z=2 and D_cal=1.229 Mgm⁻³. The title structure was solved by direct methods and refined to R=0.056 for 2414 reflections [I>3.0σ(I)] by full-matrix anisotropic least-squares methods. The energy profile of the compound was calculated by PM3 method as a function of θ[N1′–C9′–C9–N1]. The most stable molecular structure of the title compound is the anti conformation, which is different in energy by 5.0 and 1.0 kcal mol⁻¹ from the eclipsed conformation I and gauche conformations, (III and V), respectively.     

Ethylene oligomerization by diimine iron(II) complexes/EAO

The catalytic properties of a series of Fe(II) diimine complexes (diimine=N,N′-o-phenylenebis(salicylideneaminato), N,N′-ethylenebis(salicylideneaminato), N,N′-o-phenylenebisbenzal, N,N′-ethylenebisbenzal) in combination with ethylaluminoxane (EAO) for ethylene oligomerization have been investigated. Treatment of the iron(II) complexes with EAO in toluene generates active catalytic systems in situ that oligomerize ethylene to low-carbon olefins. The effects of reaction temperature, ratios of Al/Fe and reaction periods on catalytic activity and product distribution have been studied. The activity of complex FeCl₂(PhCH=o-NC₆H₄N=CHPh) with EAO at 200°C is 1.35×10⁵ g oligomers/mol Fe·h, and the selectivity of C_4–10 olefins is 84.8%.     

Kinetic-catalytic determination of traces of iron by the oxidative coupling reaction of 3-methyl-2-benzothiazolinone hydrazone with N,N-dimethylaniline

A new kinetic-catalytic method by the initial rate procedure for the determination of nanogram level of iron(III) is developed, which is based on its catalytic effect on the oxidative coupling of 3-methyl-2-benzothiazolinone hydrazone (MBTH) with N,N-dimethylaniline (DMA) to form an indamine dye (λ_max=590 nm) in the presence of hydrogen peroxide. Iron(II) is also determined, being oxidized to iron(III) by hydrogen peroxide. Calibration graphs obtained by the initial rate method are linear in the range 1–1000 ng ml⁻¹ Fe and as low as 10⁻⁸ M Fe(II, III) can easily be determined. The relative standard deviations are 6.6, 2.5 and 1.5% for ten determinations of 1, 20 and 60 ng ml⁻¹ of Fe(III), respectively. The method is applicable to the determination of iron in natural waters without preconcentration and separation.     

Flow-injection spectrophotometry of manganese by catalysis of the periodate oxidation of 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)

A sensitive flow-injection spectrophotometric procedure is proposed for the determination of manganese(II), based on its catalytic effect on the oxidation of 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) with periodate. By monitoring the change in absorbance of the oxidation product of ABTS at 415 nm, manganese(II) in the range 0.05–1.0 ng ml⁻¹ can be determined with a sampling frequency of 30 h⁻¹. A relative standard deviation (R.S.D.) (n=10) is 1.6% at the 0.5 ng ml⁻¹ level. The proposed method suffers from few interferences and has been successfully applied to the determination of manganese in river, lake and seashore water samples.     

Potentiometric Cd-selective electrode with a detection limit in the low ppt range

A recently introduced model describing the response curves of polymer membrane ion-selective electrodes (ISEs) is used here to optimize a Cd²⁺-ISE with a view to achieve very low detection limits. In a first step, the selectivity behavior is determined for two membranes based on the ionophore, N,N,N′,N′-tetradodecyl-3,6-dioxaoctanedithioamide (ETH 5435), but having different concentrations of the ionophore and ion-exchanger. Based on these data, the optimal response curves are calculated with a model that takes into account the ion fluxes induced by interfering ions. The experimental results with different ionic backgrounds correspond very well with the predicted effects. The best lower detection limit of 10⁻¹⁰ M or 11 ppt Cd²⁺ is achieved at pH 7 with an ionic background of 10⁻⁴ M NaNO₃.     

Far infrared spectra, conformational equilibria, vibrational assignments, ab initio calculations and structural parameters for 2-bromoethanol

The far infrared spectrum from 370 to 50 cm⁻¹ of gaseous 2-bromoethanol, BrCH₂CH₂OH, was recorded at a resolution of 0.10 cm⁻¹. The fundamental O–H torsion of the more stable gauche (Gg′) conformer, where the capital G refers to internal rotation around the C–C bond and the lower case g to the internal rotation around the C–O bond, was observed as a series of Q-branch transitions beginning at 340 cm⁻¹. The corresponding O–H torsional modes were observed for two of the other high energy conformers, Tg (285 cm⁻¹) and Tt (234 cm⁻¹). The heavy atom asymmetric torsion (rotation around C–C bond) for the Gg′ conformer has been observed at 140 cm⁻¹. Variable temperature (−63 to −100°C) studies of the infrared spectra (4000–400 cm⁻¹) of the sample dissolved in liquid xenon have been recorded. From these data the enthalpy differences have been determined to be 411±40 cm⁻¹ (4.92±0.48 kJ/mol) for the Gg′/Tt and 315±40 cm⁻¹ (3.76±0.48 kJ/mol) for the Gg′/Tg, with the Gg′ conformer the most stable form. Additionally, the infrared spectrum of the gas, and Raman spectrum of the liquid phase are reported. The structural parameters, conformational stabilities, barriers to internal rotation and fundamental frequencies have been obtained from ab initio calculations utilizing different basis sets at the restricted Hartree–Fock or with full electron correlation by the perturbation method to second order. The theoretical results are compared to the experimental results when appropriate. Combining the ab initio calculations with the microwave rotational constants, r₀ adjusted parameters have been obtained for the three 2-haloethanols (F, Cl and Br) for the Gg′ conformers.     

Constructions of a set of novel hydrogen-bonded supramolecules from reactions of cobalt(II) salt with bis(3,5-dimethylpyrazolyl)methane and different carboxylic acids

Reactions of CoX₂·6H₂O (X = Cl⁻, ClO₄⁻) with bis(3,5-dimethylpyrazolyl)methane (dmpzm) and formic acid, acetic acid, benzoic acid, salicylic acid, maleic acid, or fumaric acid under the presence of KOH solution produced a new family of Co(II)/dmpzm complexes, [Co(dmpzm)₂L]X·nH₂O (1: L = O₂CH, X = Cl, n = 2; 2: L = OAc, X = Cl, n = 3; 3: L = benzoate, X = ClO₄, n = 1/3; 4: L = salicylate, X = ClO₄, n = 1/3) and [Co₂(dmpzm)₄L](ClO₄)₂·nSolv (5: L = maleate, n = 3, Solv = H₂O; 6: L = fumarate, n = 2, Solv = MeOH). These compounds were structurally characterized by elemental analysis, IR spectroscopy, and single-crystal X-ray diffraction. Compounds 1–4 are mononuclear while 5–6 are binuclear. Each cobalt atom of 1–6 is hexacoordinate, with a distorted octahedral CoN₄O₂ coordination geometry incorporating two N,N′-bidentate dmpzm ligands and one O,O′-bidentate carboxylate ligand. There are rich intra- and intermolecular hydrogen bonds in the crystals of 1–6, thereby forming either 2D hydrogen-bonded networks (1 and 2) or 3D hydrogen-bonded networks (3–6). In addition, the thermal behaviors of 1–6 were also investigated.