Challenges in modelling and optimisation of stability constants in the study of Cu-(TAPS)x-(OH)y system by polarography

This work describes the application of polarography, a technique scarcely used for modelling and optimisation of stability constants, in the study of copper complexes with [(2-hydroxy-1,1-bis(hydroxymethyl)ethyl)amino]-1-propanesulfonic acid (TAPS). Direct current polarography (DCP), using low total copper ion and large total ligand to total copper concentration, enabled the full characterization of Cu-(TAPS)_x-(OH)_y system, whose complexation occurs in the pH range of copper hydrolysis and Cu(OH)₂ precipitation. Cu-(TAPS)_x-(OH)_y system was studied by DCP and glass electrode potentiometry (GEP) in aqueous solution at fixed total ligand to total metal concentrations ratios and varied pH values (25.0 °C; I = 0.1 M, KNO₃). The predicted model, as well as the overall stability constants values, are (as log β): CuL⁺ = 4.2, CuL₂ = 7.8, CuL₂(OH)⁻ = 13.9 and CuL₂(OH)₂²⁻ = 18.94. GEP only allowed confirming the stability constants for CuL⁺ and CuL₂ and was used to determine the pKa of TAPS, 8.342.Finally, a briefly comparative analysis between TAPS and other structural related buffers was done. Evaluation based on log β_CuL versus pKa revealed that TES, TRIS, TAPS and AMPSO coordinated via amino and hydroxymethylgroups forming a five-membered chelate ring. For BIS-TRIS and TAPSO, and possibly DIPSO, one or more five-membered chelate rings involving additional hydroxyl groups are also likely formed.     

Modelling and Optimization of Stability Constants of Cadmium or Zinc with Biological Buffers (DIPSO or TAPS) in Aqueous Solutions by Electrochemical Techniques

The complexation behavior of four systems involving cadmium(II) or zinc(II) in aqueous solutions with the biological buffers 3-[N,N-bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO), and [(2-hydroxy-1,1-bis(hydroxymethyl)ethyl)amino]-1-propanesulfonic acid (TAPS) was studied by direct current polarography (DCP) and glass electrode potentiometry (GEP), at 25.0 ± 0.1 °C and ionic strength 0.1 mol·dm^?3 KNO₃. Except for the Cd–TAPS system, for which full characterization of the system was possible either by DCP or GEP, full characterization of the other metal-buffer systems (Zn–DIPSO, Zn–TAPS and Cd–DIPSO) was only possible using DCP. For Zn-buffers systems, ZnL⁺ and $ {\text{ZnL(OH)}}_{2}^{ - } $ ZnL(OH) 2 ? (where L stands for buffer) were identified. For the Zn–DIPSO system, the overall stability constant values (as log₁₀ β) are 2.1 ± 0.2 and 13.4 ± 0.2, respectively. For the Zn–TAPS system, the overall stability constants values (as log₁₀ β) are 2.4 ± 0.1 and 12.9 ± 0.3, respectively. For the Cd–DIPSO system, the overall stability constants values (as log₁₀ β) of CdL⁺ and CdL(OH) are 2.9 ± 0.1 and 6.9 ± 0.3, respectively. For the Cd–TAPS system, only the species CdL⁺ was identified with log₁₀ β = 2.5 ± 0.1.     

Challenges in modelling and optimisation of stability constants in the study of metal complexes with monoprotonated ligands: Part I. A glass electrode potentiometric and polarographic study of a Cu-TAPSO-OH system

The behaviour of solutions containing 3-(N-tris[hydroxymethyl]methylamine)-2-hydroxypropanesulfonic acid (TAPSO) and copper(II) was studied by two analytical techniques, direct current polarography (DCP) and glass electrode potentiometry (GEP), at fixed total TAPSO to total copper(II) concentration ratios and various pH values, at 25 °C and ionic strength 0.1 M KNO₃. DCP and GEP, when used independently, were not able to provide a final metal-ligand model. Combined interpretation of data from DCP and GEP indicated the formation of six main species, CuL⁺, CuL(OH), CuL(OH)₂⁻, CuL₂, CuL₂(OH)⁻ and CuL₂(OH)₂²⁻ for which stability constants (as log β) were found to be 4.41, 11.43, 17.55, 8.08, 14.3 and 20.3, respectively. Five of these complexes, CuL(OH), CuL(OH)₂⁻, CuL₂, CuL₂(OH)⁻ and CuL₂ (OH)₂²⁻ are reported for the first time.     

New lead(II) complexes with N,S-ligands,including a lead pyrazolonate with unusual packing flexibility

The reaction of lead(II) acetate in methanol with thiosemicarbazones derived from β-keto esters and β-keto amides (HTSCs) afforded two lead(II) thiosemicarbazonates and numerous homoleptic ([PbL₂]) and/or heteroleptic ([Pb(OAc)L]) complexes containing deprotonated pyrazolones L⁻ formed by metal-induced cyclization of the starting HTSC ligands. All the complexes isolated were characterized by IR spectroscopy in the solid state and by ¹H and ¹³C NMR spectroscopy in DMSO solution; in addition, crystals containing [Pb(L⁶)₂] and [Pb(L⁷)₂] were examined by X-ray crystallography. [Pb(L⁶)₂] · 0.5H₂O · 0.3MeOH (HL⁶ = 4-ethyl-2,5-dihydro-3-methyl-5-oxo-1H-pyrazole-1-carbothiamide) showed three types of molecule with significant structural differences that appear to be determined by packing interactions. In all three molecules the Pb?Pb distances are very short [3.6096(8)–3.7562(8) Å], but density-functional-theoretic calculations at the B3LYP level do not support the existence of Pb–Pb bonds. In [Pb(L⁷)₂] (HL⁷ = N-ethyl-2,5-dihydro-3-methyl-5-oxo-1H-pyrazole-1-carbothiamide) all the molecules are of a single type, and they are linked in a three-dimensional network by weak intermolecular Pb?O bonds.     

Crystal structure, thermal expansion and conductivity of anisotropic La1−xSrxGa1−2xMg2xO3−y (x=0.05, 0.1) single crystals

Crystal structure and anisotropy of the thermal expansion of single crystals of La_1−xSr_xGa_1−2xMg_2xO_3−y (x=0.05 and 0.1) were measured in the temperature range 300-1270 K. High-resolution X-ray powder diffraction data obtained by synchrotron experiments have been used to determine the crystal structure and thermal expansion. The room temperature structure of the crystal with x=0.05 was found to be orthorhombic (Imma, Z=4, a=7.79423(3) Å, b=5.49896(2) Å, c=5.53806(2) Å), whereas the symmetry of the x=0.1 crystal is monoclinic (I2/a, Z=4, a=7.82129(5) Å, b=5.54361(3) Å, c=5.51654(4) Å, β=90.040(1)°). The conductivity in two orthogonal directions of the crystals has been studied. Both, the conductivity and the structural data indicate three phase transitions in La_0.95Sr_0.05Ga_0.9Mg_0.1O_2.92 at 520-570 K (Imma-I2/a), 770 K (I2/a-R3c) and at 870 K (R3c-R-3c), respectively. Two transitions at 770 K (I2/a-R3c) and in the range 870-970 K (R3c-R-3c) occur in La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85.     

Separation and quantification of [RhCln(H2O)6−n] (n = 0-6) complexes, including stereoisomers, by means of ion-pair HPLC-ICP-MS

A hyphenated ion-pair (tetrabutylammonium chloride—TBACl) reversed phase (C₁₈) HPLC-ICP-MS method (High Performance Liquid Chromatography Inductively Coupled Plasma Mass Spectroscopy) for anionic Rh(III) aqua chlorido-complexes present in an HCl matrix has been developed. Under optimum chromatographic conditions it was possible to separate and quantify cationic Rh(III) complexes (eluted as a single band), [RhCl₃(H₂O)₃], cis-[RhCl₄(H₂O)₂]⁻, trans-[RhCl₄(H₂O)₂]⁻ and [RhCl_n(H₂O)_6−n]³⁻ⁿ (n = 5, 6) species. The [RhCl_n(H₂O)_6−n]³⁻ⁿ (n = 5, 6) complex anions eluted as a single band due to the relatively fast aquation of [RhCl₆]³⁻ in a 0.1 mol L⁻¹ TBACl ionic strength mobile phase matrix. Moreover, the calculated t_1/2 of 1.3 min for [RhCl₆]³⁻ aquation at 0.1 mol kg⁻¹ HCl ionic strength is significantly lower than the reported t_1/2 of 6.3 min at 4.0 mol kg⁻¹ HClO₄ ionic strength. Ionic strength or the activity of water in this context is a key parameter that determines whether [RhCl_n(H₂O)_6−n]³⁻ⁿ (n = 5, 6) species can be chromatographically separated. In addition, aquation/anation rate constants were determined for [RhCl_n(H₂O)_6−n]³⁻ⁿ (n = 3-6) complexes at low ionic strength (0.1 mol kg⁻¹ HCl) by means of spectrophotometry and independently with the developed ion-pair HPLC-ICP-MS technique for species assignment validation. The Rh(III) samples that was equilibrated in differing HCl concentrations for 2.8 years at 298 K was analyzed with the ion-pair HPLC method. This analysis yielded a partial Rh(III) aqua chlorido-complex species distribution diagram as a function of HCl concentration. For the first time the distribution of the cis- and trans-[RhCl₄(H₂O)₂]⁻ stereoisomers have been obtained. Furthermore, it was found that relatively large amounts of ‘highly’ aquated [RhCl_n(H₂O)_6−n]³⁻ⁿ (n = 0-4) species persist in up to 2.8 mol L⁻¹ HCl and in 1.0 mol L⁻¹ HCl the abundance of the [RhCl₅(H₂O)]²⁻ species is only 8-10% of the total, far from the 70-80% as previously proposed. A 95% abundance of the [RhCl₆]³⁻ complex anion occurs only when the HCl concentration is above 6 mol L⁻¹. The detection limit for a Rh(III) species eluted from the column is below 0.147 mg L⁻¹.     

Determination of cadmium, chromium and lead in marine sediment slurry samples by electrothermal atomic absorption spectrometry using permanent modifiers

A procedure for the determination of cadmium, chromium, and lead in marine sediment slurries by electrothermal atomic absorption spectrometry is proposed. Slurry was prepared by mixing 10 mg of ground sample with particle size smaller than 50 μm completed to the weight of 1.0 g with a 3% nitric acid and 10% hydrogen peroxide solution. The slurry was maintained homogeneous with an aquarium air pump. For cadmium, the best results were obtained using iridium permanent with optimum pyrolysis and atomization temperatures of 400 and 1300 °C, respectively, a characteristic mass, m_o (1% absorption), of 2.3 pg (recommended 1 pg). Without modifier use, zirconium, ruthenium, and rhodium m_o were 3.4, 4.1, 4.6, and 4.8 pg, respectively. For chromium, the most sensitive condition was obtained with zirconium permanent with optimum pyrolysis and atomization temperatures of 1500 and 2500 °C, m_o of 6.6 pg (recommended 5.5 pg); and without modifier use, rhodium, iridium, and ruthenium m_o were 5.3, 8.8, 8.8, and 8.9 pg, respectively. For lead, the best modifier was also zirconium, m_o of 8.3 pg for the optimum pyrolysis and atomization temperatures of 600 and 1400 °C, respectively, (recommended m_o of 9.0 pg). For iridium, ruthenium, without modifier, and rhodium, m_o were 14.7, 15.5, 16.5, and 16.5 pg, respectively. For all the modifiers selected in each case, the peaks were symmetrical with r² higher than 0.99. Being analyzed (n = 10), two marine sediment reference materials (PACS-2 and MESS-2 from NRCC), the determined values, μg l⁻¹, and certified values in brackets, were 2.17 ± 0.05 (2.11 ± 0.15) and 0.25 ± 0.03 (0.24 ± 0.01) for cadmium in PACS-2 and MESS-2, respectively. For chromium in PACS-2 and MESS-2 the values were 94.7 ± 5.6 (90.7 ± 4.6) and 102.3 ± 10.7 (106 ± 8), respectively. Finally, for lead in PACS-2 and MESS-2, the results obtained were 184 ± 7 (183 ± 8) and of 25.2 ± 0.40 (21.9 ± 1.2), respectively. For cadmium and lead in both samples and chromium in PACS-2, calibration was accomplished with aqueous calibration curves. For chromium in MESS-2, only with the standard addition technique results were in agreement with the certified ones. The limits of detection (k = 3, n = 10) obtained with the diluents were 0.1, 3.4, and 3.6 μg l⁻¹ for cadmium, chromium, and lead, respectively.     

Preparation and pyrolysis of poly(allyl iminoalane-co-ethyl iminoalane)s [HAlN(allyl)]m[HAlNEt]n

Poly(allyl iminoalane-co-ethyl iminoalane)s {[HAlN(allyl)]_m[HAlNEt]_n; Allyl/Et-alanes}, have been prepared by reactions of lithium hydridoaluminate (LiAlH₄) with a mixture of allylamine hydrochloride (CH₂CHCH₂NH₂ · HCl; allylNH₂ · HCl) and ethylamine hydrochloride (CH₃CH₂NH₂ · HCl; EtNH₂ · HCl) with various allyl/Et ratios. Spectroscopic analyses indicate that Allyl/Et-alane(1/3) (allyl/Et = 1/3) contains octamers possessing Al-H and C-H groups as well as CC, Al-N, and C-N bonds. The loss of aluminum during pyrolysis of Allyl/Et-alane(1/3) at 1600 °C under an Ar atmosphere is 15%, which is less than the value reported for the pyrolysis of poly(ethyliminoalane) (36%). The suppression can be ascribed to cross-linking reactions involving allyl groups (hydroalumination and polymerization of the allyl groups), judging from infrared (IR) and solid-state nuclear magnetic resonance (NMR) spectroscopy.     

A computational study of SbnF5n (n = 1-4): Implications for the fluoride ion affinity of nSbF5

This contributions shows with a series of ab initio MP2 and DFT (BP86 and B3-LYP) computations with large basis sets up to cc-pVQZ quality that the literature value of the standard enthalpy of depolymerization of Sb₄F_20(g) to give SbF_5(g) (+18.5 kJ mol⁻¹) [J. Fawcett, J.H. Holloway, R.D. Peacock, D.R. Russell, J. Fluorine Chem. 20 (1982) 9] is by about 50 kJ mol⁻¹ in error and that the correct value of (Sb₄F_20(g)) is +68 ± 10 kJ mol⁻¹. We assign , , and values for Sb_nF_5n with n = 2-4 and compare the results to available experimental gas phase data. Especially the MP2/TZVPP values obtained in an indirect procedure that rely on isodesmic reactions or the highly accurate compound methods G2 and CBS-Q are in excellent agreement with the experimental data, and reproduce also the fine experimental details at temperatures of 423 and 498 K. With these data and the additional calculation of [Sb_nF_5n+1]⁻ (n = 1-4), we then assessed the fluoride ion affinities (FIAs) of Sb_nF_5n(g), nSbF_5(g), nSbF_5(l) and the standard enthalpies of formation of Sb_nF_5n(g) and [Sb_nF_5n+1]⁻_(g): FIA(Sb_nF_5n(g)) = 514 (n = 1), 559 (n = 2), 572 (n = 3) and 580 (n = 4) kJ mol⁻¹; FIA(nSbF_5(g)) = 667 (n = 2), 767 (n = 3) and 855 (n = 4) kJ mol⁻¹; FIA(nSbF_5(l)) = 434 (n = 1), 506 (n = 2), 528 (n = 3) and 534 (n = 4) kJ mol⁻¹. Error bars are approximately ±10 kJ mol⁻¹. Also the related Gibbs energies were derived. Δ_fH°([Sb_nF_5n+1]⁻_(g)) = −2064 ± 18 (n = 1), −3516 ± 25 (n = 2), −4919 ± 31 (n = 3) and −6305 ± 36 (n = 4) kJ mol⁻¹.     

Spectrofluorimetric determination of iridium(IV) traces using 4-pyridone derivatives

A new spectrofluorimetric determination of iridium(IV) with 3-hydroxy-2-methyl-1-phenyl-4-pyridone (HX) or 3-hydroxy-2-methyl-1-(4-tolyl)-4-pyridone (HY) is reported. Iridium(IV) react with HX or HY and chelates were extracted into chloroform or dichloromethane. The organic phase showed fluorescence. The fluorescence measurements to quantify iridium were carried out in its fluorescent band centred at λ_ex=373 nm and λ_em=480 nm. Under optimal conditions, the calibration graphs were linear over the concentration range of 0.1-7.6 μg ml⁻¹ of iridium for Ir(IV)-HX and 0.1-5.8 μg ml⁻¹ for Ir(IV)-HY with a correlation coefficients of 0.999 and 0.992 and relative standard deviation of ±1.1%.The method is free from interference by Rh(III) and Pt(IV), which normally interfere with other methods. Iridium can be determined in the presence of 300-fold excess of rhodium(III) and 10-fold excess of platinum(IV).The method was applied successfully to the determination of iridium in some synthetic mixtures and mineral sample gave satisfactory results.     

Syntheses,characterization and luminescent properties of two lead(II) fumarate metal-organic frameworks

It is of interest that the hydrous 3D metal-organic framework (MOF) {[Pb₂(fum)₂(H₂O)₄] · 2H₂O}_n (1) has been synthesized by the reaction of the fum dianion with the lead(II) ion (fum = fumarate) in the presence of pyrazole, while the anhydrous 3D MOF [Pb(fum)]_n (2) is obtained by the reaction of the fum dianion with the lead(II) ion in the presence of pyrazine. These complexes were further characterized by FT-IR spectroscopy, thermogravimetric analysis (TG), X-ray analysis and solid state photoluminescence spectra. The arrangement of the ligands displays a coordination gap around the Pb atom, occupied possibly by a stereoactive lone pair of electrons on lead(II), with the coordination around both the eight-coordinated lead atom in 1 and six-coordinated lead atom in 2 exhibiting a hemidirected geometry. The fum ligand shows different ligation behavior toward the lead(II) ions in these complexes. These compounds exhibit photoluminescence with the maximum emission located in the UV region.     

Solvent effects on the copolymerization kinetics of ionic (AMPS) and non-ionic (HEAm) acrylamide derivatives

The free-radical copolymerization of two N-substituted acrylamide monomers, the ionic AMPS (2-acrylamido-2-methyl-1-propanesulfonic acid) and the non-ionic HEAm (2-hydroxyethylacrylamide) is presented. Despite bearing similar polymerizable functionalities, HEAm is more reactive toward free-radical addition than AMPS in water. In a mixed aqueous solvent containing salt, (0.5 M LiNO₃, 50 wt%) and ethanol (50 wt%), the reactivity ratio was found to be r_AMPS = 0.53 and r_HEAm = 1.06 indicating that copolymers with a nearly random distribution of sulfonic and hydroxy functionalities can be prepared.     

Heat capacity and thermodynamic properties of crystalline ornidazole (C7H10ClN3O3)

The molar heat capacities of 1-(2-hydroxy-3-chloropropyl)-2-methyl-5-nitroimidazole (Ornidazole) (C₇H₁₀ClN₃O₃) with purity of 99.72 mol% were measured with an adiabatic calorimeter in the temperature range between 79 and 380 K. The melting-point temperature, molar enthalpy, Δ_fusH_m, and entropy, Δ_fusS_m, of fusion of this compound were determined to be 358.59±0.04 K, 21.38±0.02 kJ mol⁻¹ and 59.61±0.05 J K⁻¹ mol⁻¹, respectively, from fractional melting experiments. The thermodynamic function data relative to the reference temperature (298.15 K) were calculated based on the heat capacities measurements in the temperature range from 80 to 380 K. The thermal stability of the compound was further investigated by DSC and TG. From the DSC curve an intensive exothermic peak assigned to the thermal decomposition of the compound was observed in the range of 445-590 K with the peak temperature of 505 K. Subsequently, a slow exothermic effect appears when the temperature is higher than 590 K, which is probably due to the further decomposition of the compound. The TG curve indicates the mass loss of the sample starts at about 440 K, which corresponds to the decomposition of the sample.     

A novel tripodal ligand with organosulfur alligator clips for deposition of tetrairon(III) single-molecule magnets on gold

A propeller-like tetrairon(III) complex functionalized with two 1,2-dithiolan-3-yl groups was synthesized and magnetically characterized. The compound has formula [Fe₄(thioctic)₂(dpm)₆] and was specifically designed to be grafted on gold surfaces. It was prepared by reacting [Fe₄(OMe)₆(dpm)₆] (Hdpm = dipivaloylmethane) with a new tripodal ligand, H₃thioctic, obtained by esterification of 2,2-bis(hydroxymethyl)-propane-1,3-diol with (±)-α-lipoic acid (also known as thioctic acid). Direct current and alternating current magnetic measurements revealed single-molecule magnet behaviour with an effective anisotropy barrier of 14.0(1) K resulting from a high spin (S = 5) ground state and an easy-axis anisotropy.     

Crystal structure and the standard molar enthalpy of formation of a coordination polymer [Cu(nip)(phen)]n

A coordination polymer [Cu(nip)(phen)]_n was hydrothermally synthesized by the reaction of Cu(NO₃)₂ with 5-nitroisophthalic acid and phen. Single-crystal structure analysis showed that the complex crystallized in the monoclinic space group P2₁/c; a = 10.6566(13); b = 12.5931(15); c = 13.0514(16) Å; β = 95.474(2)°, V = 1743.5(4) Å³; Z = 4. The standard molar enthalpy of formation of the complex was determined to be −554 ± 11 kJ mol⁻¹.     

Preparation and cis-to-trans transformation study of square-planar [Pt(Ln)2Cl2] complexes bearing cytokinins derived from 6-benzylaminopurine (Ln) by view of NMR spectroscopy and X-ray crystallography

Mononuclear, square-planar platinum(II) complexes involving derivatives of aromatic cytokinins as the ligands, and having the general formula cis-[Pt(L_n)₂Cl₂] (1–3) and trans-[Pt(L_n)₂Cl₂] (4–6), where n = 1–3, L₁ = 2-chloro-6-(benzylamino)-9-isopropylpurine, L₂ = 2-chloro-6-[(4-methoxybenzyl)amino]-9-isopropylpurine and L₃ = 2-chloro-6-[(2-methoxybenzyl)-amino]-9-isopropylpurine, have been synthesized and characterized by elemental analysis, MALDI-TOF mass, FT IR, ¹H, ¹³C, ¹⁵N and ¹⁹⁵Pt NMR spectral measurements. Dynamic cis-to-trans isomerization process of complex 1 in N,N′-dimethylformamide (DMF) has been investigated by means of multinuclear NMR spectroscopy. The solid-state structures of 1, 4 · (DMF)₂, and 5 have been determined by single crystal X-ray analysis. X-ray structures revealed that the heterocyclic ligands are coordinated to platinum via nitrogen atom N(7) in all the complexes studied. In vitro cytotoxicity of the prepared complexes against MCF7, G361, K562, and HOS has been evaluated. Owing to low solubility of the complexes in water, the cytotoxicity has been only tested up to 5 μM concentration. Unfortunately, all complexes have been found to be non-cytotoxic in the accessible concentration range.     

cis-trans Germanium chains in the intermetallic compounds ALi1-xInxGe2 and A2(Li1-xInx)2Ge3 (A=Sr, Ba, Eu)—experimental and theoreticalstudies

Two types of strontium-, barium- and europium-containing germanides have been synthesized using high temperature reactions and characterized by single-crystal X-ray diffraction. All reported compounds also contain mixed-occupied Li and In atoms, resulting in quaternary phases with narrow homogeneity ranges. The first type comprises EuLi_0.91(1)In_0.09Ge₂, SrLi_0.95(1)In_0.05Ge₂ and BaLi_0.99(1)In_0.01Ge₂, which crystallize in the orthorhombic space group Pnma (BaLi_0.9Mg_0.1Si₂ structure type, Pearson code oP16). The lattice parameters are a=7.129(4)-7.405(4) Å; b=4.426(3)-4.638(2) Å; and c=11.462(7)-11.872(6) Å. The second type includes Eu₂Li_1.36(1)In_0.64Ge₃ and Sr₂Li_1.45(1)In_0.55Ge₃, which adopt the orthorhombic space group Cmcm (Ce₂Li₂Ge₃ structure type, Pearson code oC28) with lattice parameters a=4.534(2)-4.618(2) Å; b=19.347(8)-19.685(9) Å; and c=7.164(3)-7.260(3) Å. The polyanionic sub-structures in both cases feature one-dimensional Ge chains with alternating Ge-Ge bonds in cis- and trans-conformation. Theoretical studies using the tight-binding linear muffin-tin orbital (LMTO) method provide the rationale for optimizing the overall bonding by diminishing the π-p delocalization along the Ge chains, accounting for the experimentally confirmed substitution of Li forIn.     

The structure of N-meso-(6,8,8,14,16,16-hexamethyl-1,5,9,13-tetraazacyclohexadeca-5,13-diene)nickel(II) chloro(isothiocyanato)zincate(II)

A yellow compound which was crystallised from a solution of (6,8,8,14,16,16-hexamethyl-1,5,9,13-tetraazacyclohexadeca-5,13-diene)bis(isothiocyanato)nickel(II) in aqueous zinc(II) chloride has cations with singlet ground state nickel(II) in square-planar coordination by the nitrogen atoms of the macrocycle. The asymmetric unit has two similar cations. The N₄ group of one cation is near coplanar (r.m.s. displacements ±0.009(1) Å, with Ni displaced by 0.048(1) Å from this plane) while the other cation has significant tetrahedral twisting of the N₄ group (r.m.s. displacements of N atoms ±0.126(2) Å, with Ni displaced by 0.027(2) Å from this plane). The mean Ni–N distances are Ni–N_amine = 1.950(6) and Ni–N_imine = 1.897(6) Å. Both cations have N-meso configurations with saddle conformations, with the substituted chelate rings in boat conformations tilted to one side of the NiN₄ ‘plane’ and the unsubstituted chelate rings tilted to the other side, one in a boat conformation and the other with the central methylene group disordered, the components forming boat {s.o.f. 0.70(1) and 0.74(1) for the two cations} and chair conformation chelate rings. The counter-ions have tetrahedrally coordinated zinc(II) ions, one as [ZnCl₂(NCS)₂]²⁻ ions and the other with one ligand site with disordered Cl⁻ {s.o.f. 0.78(1)} and NCS⁻ ligands, i.e. with disordered [ZnCl₂(NCS)₂]²⁻ and [ZnCl(NCS)₃]²⁻ ions, with an overall composition of [Ni(trans-Me₆[16]diene)][ZnCl_1.9(NCS)_2.1].     

Determination of pKa value, kinetic studies of decomposition and oxygen transfer for chromium(VI) peroxide

The determination of pK_a value for the unstable chromium(VI) peroxide, CrO(O₂)₂(H₂O) in aqueous solution is presented. The pK_a value is found to be (1.55 ± 0.03). The kinetic decomposition of chromium(VI) peroxide is dependent on the concentration of hydrogen peroxide in the pH range between 2.5 and 4.0. We have proposed the possible explanation for the formation of triperoxo chromium complex of hydrogen peroxide which is dependent on decomposition. Activation of coordinate peroxide in chromium(VI) peroxide observed in the kinetic studies is by reduction of thiolato-cobalt(III) complex. The rate constant (M⁻¹ s⁻¹, 15 °C) for the oxygen atom transfer reaction from CrO(O₂)₂(OH)⁻ to (en)₂Co(SCH₂CH₂NH₂)²⁺ is found to be (25.0 ± 1.3).     

Substitution kinetics of W(CO)5(η-bis(trimethylsilyl)ethyne) with triphenylbismuthine

The labile complex W(CO)₅(η²-btmse) undergoes replacement of bis(trimethylsilyl)ethyne, btmse, by triphenylbismuthine in cyclohexane solution at an observable rate in the temperature range of 35-50 °C yielding almost solely W(CO)₅(BiPh₃) as the final product. The kinetics of this substitution reaction was studied in cyclohexane solution by quantitative FT-IR spectroscopy. The substitution reaction obeys a pseudo-first-order kinetics with respect to the concentration of the starting complex. The observed rate constant, k_obs, was determined at four different temperatures and three different concentrations of the entering ligand BiPh₃ in the range 16.8-65.4 mM. From the evaluation of kinetic data a possible reaction mechanism was proposed in which the rate determining step is the cleavage of metal-alkyne bond in the complex W(CO)₅(η²-btmse). A rate law was derived from the proposed mechanism. From the dependence of k_obs on the entering ligand concentration, the rate constant k₁ for the rate determining step was estimated at all temperatures. The activation enthalpy (106 ± 2 kJ mol⁻¹) and the activation entropy (111 ± 6 J K⁻¹ mol⁻¹) were determined for this rate determining step from the evaluation of k₁ values at different temperatures. The large positive value of the activation entropy is consistent with the dissociative nature of reaction. The large value of the activation enthalpy, close to the calculated tungsten-alkyne bond dissociation energy, also supports this dissociative rate-determining step of the substitution reaction.