Kinetic spectrophotometric method for the determination of selenium(IV) by its catalytic effect on the reduction of spadns by sulphide in micellar media

A simple kinetic spectrophotometric method was developed for the determination of ultratrace amounts of Se(IV). The method is based on the reduction of spadns by sulphide in micellar media. The reaction was monitored spectrophotometrically by measuring the decrease in the absorbance of spadns at 515 nm with a fixed-time method. The decrease in the absorbance of spadns is proportional to the concentration of Se(IV) in the range 0.5–100 ng/mL with a fixed time of 2.5–7.0 min from the initiation of the reaction. The limit of detection is 0.3 ng/mL Se(IV). The relative standard deviation for the determination of 0.02 and 0.10 μg/mL Se(IV) was 2.10 and 1.95%, respectively. The method was applied to the determination of Se(IV) in water. The text was submitted by the authors in English.     

Micro Determination of Ammonia by a Kinetic Spectrophotometric Method

 A simple and sensitive kinetic spectrophotometric method is described for the determination of micro amounts of ammonia. The method is based on the inhibiting effect of ammonia on the oxidation rate of aqua ethylenediamine-tetraacetate chromium(III) [Cr(HEDTA)(H₂O)] with N-bromo succinimide (NBS) to yield chromium(VI). The reaction was followed spectrophotometrically by measuring the rate of change in absorbance with time at 365 nm, which is the maximum absorption wavelength of chromium(VI). Using the recommended procedure, ammonia can be determined up to 153 μg ml⁻¹ with a linear calibration graph and a detection limit of 0.09 μg ml⁻¹. The method was successfully applied to the determination of ammonia in waste water samples. Recovery experiments for the analyzed samples were 97–100% with relative standard deviation, Sr (%) of ≤ 2.9% indicating high accuracy and precision of the proposed method. The interference of various cations and anions in the determination of ammonia was investigated. Received March 30, 2001 Revision September 3, 2001     

Selective extraction and preconcentration of cerium(IV) in water samples by cloud point extraction and determination by inductively coupled plasma optical emission spectrometry

Cloud point extraction (CPE) was used for the selective extraction and separation of cerium(IV) from aqueous solutions. The method is based on the formation of cerium(IV)-n-p-tolylbenzohydroxamic acid (n-TBHA) complex that is extracted into the micellar phase (Triton X-114) at a temperature above the cloud point temperature (CPT). After phase separation, the surfactant rich phase was diluted up to 1.0 mL and determined by inductively coupled plasma-optical emission spectrometry (ICP-OES). Under the optimum extraction conditions and instrument parameters, by preconcentration of only 10.0 mL of sample in the presence of 0.09% Triton X-114, an enhancement factor of 13.8 was obtained. The analytical curve was linear in the range of 1.5–1200 μg L⁻¹ and the detection limit was 0.4 μg L⁻¹. The method was applied to the determination of Ce(IV) in water samples with satisfactory results.     

Determination of trace mercury in geological samples by direct slurry sampling cold vapor generation atomic absorption spectrometry

A new method based on ultrasonic slurry sampling atomic absorption spectrometry (AAS) was established for the determination of trace mercury in geological samples by use of recently developed mercury cold vapor generation using formic acid under ultraviolet (UV) irradiation. The generated mercury cold vapor was rapidly separated from the matrix and swept into a T-tube for the measurement of atomic absorbance. Under the optimal experimental conditions, up to 1000-fold of Cu(II), Co(II), Ni(II), Cr(VI), Mn(II), Fe(III), and Zn(II) caused no significant interference with the determination of 50 μg L⁻¹ Hg. The limit of detection at sub-ppb level was obtained for mercury. The method was applied to the determination of mercury in geological samples with satisfactory results. Correspondence: Xiandeng Hou, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China     

Direct and Derivative Spectrophotometric Determination of Zinc with 2,4-Dihydroxybenzaldehyde Isonicotinoyl Hydrazone in Potable Water and Pharmaceutical Samples

A simple and sensitive spectrophotometric method is developed for the determination of zinc in aqueous solutions. The metal ion forms a greenish-yellow colored complex with 2,4-dihydroxybenzaldehyde isonicotinoyl hydrazone (2,4-DHBINH) in the pH range of 4–10. The complex shows maximum absorbance at 390 nm and in the pH range of 6–8. Beer's law is obeyed in the range 0.10–1.50 μg/mL of Zn(II). The molar absorptivity and Sandell's sensitivity are 3.55 × 10⁴ mol⁻¹ cm⁻¹ and 0.0016 μg/cm², respectively. The composition of the complex is 1 : 1. The effect of interfering ions has been studied, and the method was applied to the determination of zinc(II) in potable water and pharmaceutical samples. A fast derivative spectrophotometry method is also proposed for the determination of zinc in the range 0.06–1.60 μg/mL, which is more sensitive than the zero order method. __________ Translated from Zhurnal Analiticheskoi Khimii, Vol. 60, No. 9, 2005, pp. 933–937. Original Russian Text Copyright ? 2005 by Sivaramaiah, Raveendra Reddy. The text was submitted by the authors in English.     

Fluorometric determination of DNA using nano-SiO2 particles as an effective dispersant and stabilizer for acridine orange

A sensitive fluorometric method for the determination of ctDNA (calf thymus DNA) is presented. It has good selectivity and sensitivity and uses nano-SiO₂ particles as an effective dispersant and stabilizer for acridine orange (AO). Compared to resonance light scattering (RLS) and the conventional method that uses organic dyes as fluorescence probe, the new method is more tolerant towards coexisting foreign substances and also more stable. With 20 mg nano-SiO₂ particles, 10 μmol L⁻¹ AO, at pH 8.01 and an ionic strength of 0.02 mol L⁻¹, the interaction of AO with nano-SiO₂ and ctDNA results in fluorescent signal enhancement. The extent of enhancement was in good proportion to the concentration of ctDNA at excitation/emission wavelengths of 490/523 nm, respectively. The calibration curve was linear over 0.66–55.60 μg mL⁻¹. The determination limit (3σ) was 15 μg mL⁻¹. The method was applied to the determination of ctDNA in synthetic samples with satisfactory results.     

Development and Validation of Selective Spectrophotometric Methods for the Determination of Pregabalin in Pharmaceutical Preparation

Three simple, quick and sensitive methods are described for the spectrophotometric determination of pregabalin (Pgb) in pharmaceutical preparations. Among them, the first two methods are based on the reaction of Pgb as n-electron donors with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) as π-acceptors to give highly colored complex species. The colored products were quantitated spectrophotometrically at 494 and 841 nm for DDQ and TCNQ, respectively. Optimization of the different experimental conditions was conducted. Beer’s law was obeyed in the concentration ranges 2.0—30.0 and 1.5—10 µg•mL－1 for DDQ and TCNQ methods, respectively. The third method is based on the interaction of ninhydrin (NN) with primary amine present in the pregabaline. This reaction produces a blue coloured product in N,N-dimethylformamide (DMF) medium, which absorbs maximally at 573 nm. Beer’s law was found in the concentration range 40.0—180.0 μg•mL－1. The methods were applied successfully to the determination of this drug in pharmaceutical dosage forms.     

Study of Solid Phase Extraction Prior to Spectrophotometric Determination of Platinum with N-(3,5-Dimethylphenyl)-N′-(4-Aminobenzenesulfonate)-Thiourea

A highly sensitive, selective and rapid method for the determination of platinum based on the rapid reaction of platinum(IV) with N-(3,5-dimethylphenyl)-N′-(4-aminobenzenesulfonate)-thiourea (DMMPT) and the solid phase extraction of the Pt(IV)-DMMPT complex with C₁₈ membrane disks was developed. In the presence of pH = 3.8 buffer solution and cetyl trimethylammonium bromide (CTMAB) medium, DMMPT reacts with platinum to form a violet complex of a molar ratio of 1:3 (platinum to DMMPT). This complex was enriched by solid phase extraction with C₁₈ membrane disks, and an enrichment factor of 200 was obtained. The molar absorptivity of the complex is 9.51 × 10⁴ L · mol⁻¹ · cm⁻¹ at 755 nm, and Beer’s law is obeyed in the range of 0.01–3.0 μg mL⁻¹ in the measured solution. The relative standard deviation for eleven replicate samples of 0.01 μg mL⁻¹ level is 1.79%. The detection limit reaches 0.02 μg L⁻¹ in the original samples. This method was applied to the determination of platinum in water and soil samples. The relative standard deviations are 2.9–3.4%. The recoveries are 94–105%. The values of determination obtained agree with those of the ICP-MS method. The results are satisfactory.     

Spectrofluorimetric Determination of Cu2＋ Using New Fluorogenic Reagent

A new synthesized fluorogenic reagent,8-[(2-pyridine)methylideneamino] quinoline (PMAQ), was utilized for spectrofluorimetric determination of Cu(II) at trace levels. PMAQ, a good fluorogenic reagent, though insoluble in water, but is soluble in ethanol and 20% ethanol-water. The excitation and the fluorescence wavelengths of PMAQ were 310 and 434 nm respectively. When the reagent was complexed with Cu(II), the fluorescence intensity decreased proportionally with the concentration of Cu(II) at pH 4.5 by a static quenching effect. The highest sensitivity to Cu^2＋ determination was shown to be at PMAQ concentration of 1.0×10^－5mol•L^－1. In order to enhance the quenching effect, the Cu(II)-PMAQ complex solution was kept at 22 ℃ for 20 min. Though the interferences by Co(II) and Fe(III) were very serious, they were however, completely eliminated by being masked with oxalate and ascorbate ions respectively. The linear dynamic range for Cu(II) determination was between 25—441 µg•L^－1 with the detection limit of 18 µg•L^－1 (RSD＝3.7%, n＝6). The proposed method was successfully applied to the determination of Cu(II) in real samples including human blood serum, commercial tea and wheat flour.     

Determination of trace heavy metals in waters by flame atomic absorption spectrometry after preconcentration with 2,4-dinitrophenyldiazoaminoazobenzene on Amberlite XAD-2

A new column, solid-phase extraction (SPE), preconcentration method was developed for determination of Cd, Co, and Cu ions in natural water samples by flame atomic absorption spectrometry. The procedure is based on the retention of analytes in the form of 2,4-dinitrophenyldiazoaminoazobenzene (DNDAA) complex on a mini column of DNDAA-XAD-2 resin. The effects of pH, eluent type, eluent concentration, eluent volume, resin quantity, sample volume, sample flow rate, and matrix ions (Na, Ca, and Mg) were investigated on the recovery of the metals using model solutions. The detection limit for Cd, Co, and Cu was 0.062, 0.084, and 0.057 μg L⁻¹ and the quantification limit was 0.17, 0.24, and 0.12 μg L⁻¹ respectively. The method was validated by the analysis of a certified reference material with the results being in agreement with those quoted by manufactures. The developed method was applied to the determination of trace metal ions in tap water, river water samples with satisfactory results.     

Assembly of azido- or cyano-bridged binuclear complexes containing the bulky [Mn(phen)2]2+ building block: syntheses, crystal structures, and magnetic properties

Two new cyano-bridged heterobinuclear complexes, [Mn(II)(phen)2Cl][Fe(III)(bpb)(CN)2] x 0.5CH3CH2OH x 1.5H2O (1) and [Mn(II)(phen)2Cl][Cr(III)(bpb)(CN)2] x 2H2O (2) [phen = 1,10-phenanthroline; bpb(2-) = 1,2-bis(pyridine-2-carboxamido)benzenate], and four novel azido-bridged Mn(II) dimeric complexes, [Mn2(phen)4(mu(1,1)-N3)2][M(III)(bpb)(CN)2]2 x H2O [M = Fe (3), Cr (4), Co (5)] and [Mn2(phen)4(mu(1,3)-N3)(N3)2]BPh4 x 0.5H2O (6), have been synthesized and characterized by single-crystal X-ray diffraction analysis and magnetic studies. Complexes 1 and 2 comprise [Mn(phen)2Cl]+ and [M(bpb)(CN)2]- units connected by one cyano ligand of [M(bpb)(CN)2]-. Complexes 3-5 are doubly end-on (EO) azido-bridged Mn(II) binuclear complexes with two [M(bpb)(CN)2]- molecules acting as charge-compensating anions. However, the Mn(II) ions in complex 6 are linked by a single end-to-end (EE) azido bridging ligand with one large free BPh4(-) group as the charge-balancing anion. The magnetic coupling between Mn(II) and Fe(III) or Cr(III) in complexes 1 and 2 was found to be antiferromagnetic with J(MnFe) = -2.68(3) cm(-1) and J(MnCr) = -4.55(1) cm(-1) on the basis of the Hamiltonian H = -JS(Mn)S(M) (M = Fe or Cr). The magnetic interactions between two Mn(II) ions in 3-5 are ferromagnetic in nature with the magnetic coupling constants of 1.15(3), 1.05(2), and 1.27(2) cm(-1) (H = -JS(Mn1)S(Mn2)), respectively. The single EE azido-bridged dimeric complex 6 manifests antiferromagnetic interaction with J = -2.29(4) cm(-1) (H = -JS(Mn1)S(Mn2)). Magneto-structural correlationship on the EO azido-bridged Mn(II) dimers has been investigated.     

Stoichiometric and catalytic secondary O-atom transfer by Fe(III)-NO2 complexes derived from a planar tetradentate non-heme ligand: reminiscence of heme chemistry

An Fe(III) nitro complex [(bpb)Fe(NO2)(py)] (2) of the tetradentate ligand 1,2-bis(pyridine-2-carboxamido)benzene (H2bpb, H is the dissociable amide proton) has been synthesized via addition of NaNO2 to [(bpb)Fe(py)2](ClO4) (1) in MeCN or DMF. This structurally characterized Fe(III) nitro complex exhibits its nuNO2 at 1384 cm(-1). The reaction of 1 with 2 equiv of Et4NX (X = Cl-, Br-) affords the high-spin complexes (Et4N)[(bpb)Fe(Cl2)] (3) and (Et4N)[(bpb)Fe(Br)2] (4), respectively. The structure of 4 has been determined. The addition of an equimolar amount of Et4NCl, Et4NBr, or Et4NCN to a solution of 2 affords the mixed-ligand complexes (Et4N)[(bpb)Fe(NO2)(Cl)] (5), (Et4N)[(bpb)Fe(NO2)(Br)] (6), and (Et4N)[(bpb)Fe(NO2)(CN)] (7), respectively. These complexes are all low spin with isotropic g values of 2.15. Under anaerobic conditions, the reactions of 5-7 with Ph3P in MeCN afford the five-coordinate {Fe-NO}7 nitrosyl [(bpb)Fe(NO)] (and Ph3PO) via secondary oxygen-atom (O-atom) transfer. The O-atom transfer to Ph3P by 5-7 becomes catalytic in the presence of dioxygen with transfer rates in the range of 1.70-13.59 x 10-3 min(-1). The O-atom transfer rates and turnover numbers (5 > 6 > 7) are reflective of the strength of the axial donors (Cl- > Br- > CN-). The catalytic efficiencies of complexes 5-7 are limited due to formation of the thermodynamic end products [(bpb)Fe(X)2]- (where X = Cl- for 5, Br- for 6, and CN- for 7).     

Syntheses, crystal structures and properties of Co(II) complexes with N,N′-bis(3-pyridylmethyl)-1,4-benzenedimethyleneimine (bpb), and Cd(II), Hg(II) complexes with reduced bpb

Five novel coordination polymers, [Co(bpb)₂Cl₂] (1), [Co(bpb)₂(SCN)₂] (2), [Cd(H₄bpb)_0.5(dmf)(NO₃)₂] (3), [Cd₂(H₄bpb)Br₄] (4), and [Hg₂(H₄bpb)I₄] (5) [bpb=N,N′-bis(3-pyridylmethyl)-1,4-benzenedimethyleneimine, H₄bpb=N,N′-bis(3-pyridylmethyl)-1,4-benzenedimethylamine], were synthesized and their structures were determined by X-ray crystallography. In the solid state, complex 1 is a 1D hinged chain, while 2 has 2D network structure with the ligand bpb serving as a bridging ligand using its two pyridyl N atoms. The imine N atoms keep free of coordination and bpb acts as a bidentate ligand in both 1 and 2. Complexes 3, 4, and 5 with reduced bpb ligand, i.e. H₄bpb, show similar 2D network structure, in which ligand H₄bpb serves as a tetradentate ligand. Thermogravimetric analyses for complexes 1-5 were carried out and found that they have high thermal stability. The magnetic susceptibilities of compounds 1, 2 were measured over a temperature range of 75-300 K.     

1,2-bis(pyridine-2-carboxamido)benzenate(2-), (bpb)2-: a noninnocent ligand. Syntheses, structures, and mechanisms of formation of [(n-Bu)4N][FeIV2(mu-N)(bpb)2(X)2] (X = CN-, N3-) and the electronic structures of [MIII(bpbox1)(CN)2] (M = Co, Fe)

The well-known tetradentate ligand 1,2-bis(pyridine-2-carboxamido)benzenate(2-), (bpb)2-, and its 4,5-dichloro analogue, (bpc)2-, are shown to be "noninnocent" ligands in the sense that in coordination compounds they can exist in their radical one- and diamagnetic two-electron-oxidized forms (bpbox1)- and (bpbox2)0 (and (bpcox1)- and (bpcox2)0), respectively. Photolysis of high-spin [(n-Bu)4N][FeIII(bpb)(N3)2] and its (bpc)2- analogue in acetone solution at room temperature generates the diamagnetic dinuclear complex [(n-Bu)4N][FeIV2(mu-N)(bpb)2(N3)2] and its (bpc)2- analogue; the corresponding cyano complex [(n-Bu)4N][FeIV2(mu-N)(bpb)2(CN)2] has been prepared via N3- substitution by CN-. Photolysis in frozen acetonitrile solution produces a low-spin ferric species (S = 1/2) which presumably is [FeIII(bpbox2)(N)(N3)]-, as has been established by EPR and M?ssbauer spectroscopy. The mononuclear complexes [(n-Bu)4N][FeIII(bpb)(CN2)] (low spin), [Et4N][CoIII(bpb)(CN)2] and Na[CoIII(bpc)-(CN)2].3CH3OH can be electrochemically or chemically one-electron-oxidized to give [FeIII(bpbox1)(CN)2]0 (S = 0), [CoIII(bpbox1)(CN)2]0 (S = 1/2), and [CoIII(bpcox1)(CN)2]0 (S = 1/2). All complexes have been characterized by UV-vis, EPR, and M?ssbauer spectroscopy, and their electro- and magnetochemistries have been studied. The crystal structures of [(n-Bu)4N][FeIII(bpb)(N3)2].1/2C6H6CH3, Na[FeIII(bpb)(CN)2], Na[CoIII(bpc)(CN)2].3CH3OH, [(n-Bu)4N][FeIV2(mu-N)(bpb)2(CN)2], and [(n-Bu)4N][FeIV2(mu-N)(bpb)(N3)2] have been determined by single-crystal X-ray diffraction.     

Lu(Et2dtc)3(phen)的生成反应焓变、摩尔热容和恒容燃烧能测定

A ternary solid complex Lu(Et₂dtc)₃(phen) has been obtained from the reaction of hydrated lutetium chloride with sodium diethyldithiocarbamate (NaEt₂dtc), and 1,10-phenanthroline (o-phen·H₂O) in absolute ethanol. IR spectrum of the complex indicates that Lu³⁺ binds with sulfur atom in the Na(Et₂dtc)₃ and nitrogen atom in the o-phen. The enthalpy change of liquid-phase reaction of formation of the complex, Δ_rH_m^Ө (l), was determined to be (-32.821 ± 0.147 ) kJ·mol^-1 at 298.15 K by an RD-496 Ⅲ type heat conduction microcalormeter. The enthalpy change of the solid-phase reaction of formation of the complex, Δ_rH_m^Ө (s), was calculated to be (104.160 ± 0.168) kJ · mol^-1 on the basis of an appropriate thermochemistry cycle. The thermodynamics of liquid-phase reaction of formation of the complex was investigated by changing the temperature of liquid-phase reaction. Fundamental parameters, such as the activation enthalpy (ΔH_≠^Ө), the activation entropy (ΔS_≠^Ө), the activation free energy (ΔG_≠^Ө), the apparent reaction rate constant (k), the apparent activation energy (E), the pre-exponential constant (A) and the reaction order (n), were obtained by combination the reaction thermodynamic and kinetic equations with the data of thermokinetic experiments. The molar heat capacity of the complex, c_m, was determined to be (82.23 ± 1.47) J·mol^-1·K^-1 by the same microcalormeter. The constant-volume combustion energy of the complex, Δ_cU, was determined as (-17 898.228 ± 8.59) kJ·mol^-1 by an RBC-Ⅱtype rotating-bomb calorimeter at 298.15 K. Its standard enthalpy of combustion, Δ_cH_m^Ө, and standard enthalpy of formation, Δ_fH_m^Ө, were calculated to be (-17 917.43 ± 8.11) kJ·mol^-1 and (-859.95 ±10.12) kJ·mol^-1, respectively.     

Structure and Magnetic Property of an Oxygenand Carboxamide-bridged One-dimensional Iron（Ⅲ） Complex [Fe2（μ-O）（bpb）2]n

The structure of the title complex [Fe2(μ-O)(bpb)2]n (bpb 2=1,2-bis(pyridine-2carboxamido) benzenate) has been characterized by single-crystal X-ray diffraction analysis.The complex (C36 H24 Fe2N8O5,Mr=760.33) crystallizes in the monoclinic space group P2 1/c with a=8.4615(17),b=33.704(7),c=11.173(2),β=91.12(3)o,V=3185.7(11)3,Z=4,D c=1.585 g/cm 3,μ(MoK)=0.970 mm-1,F(000)=1552,R int=0.0381,the final R=0.0454 and wR=0.1114.The magnetic susceptibilities of the complex have been examined in the temperature range of 2-300 K.The magnetic data were fitted to give the parameters of J d=-77.7(9) and J c=-11.5(6) cm-1.     

Dihydrogen binding to isostructural s = (1)/(2) and s = 0 cobalt complexes

Two isostructural, nonclassical Co(H(2)) complexes are prepared from their Co(N(2)) precursors using tris(phosphino)silyl and tris(phosphino)borane ancillary ligands. Comproportionation of CoBr(2) and Co metal in the presence of TPB (tris-(o-diisopropylphophinophenyl)borane) gives (TPB)CoBr (4). One-electron reduction of 4 triggers N(2) binding to give (TPB)Co(N(2)) (2-N(2)) which is isostructural to previously reported [SiP(3)]Co(N(2)) (1-N(2)) ([SiP(3)] = tris-(o-diisopropylphosphinophenyl)silyl). Both 1-N(2) and 2-N(2) react with 1 atm H(2) to generate thermally stable H(2) complexes 1-H(2) and 2-H(2), respectively. Both complexes are characterized by a suite of spectroscopic techniques in solution and by X-ray crystallography. The H(2) and N(2) ligands in 2-H(2) and 2-N(2) are labile under ambient conditions and the binding equilibria are observable by temperature-dependent UV/vis. A van't Hoff analysis allows for the ligand binding energetics to be determined (H(2): ΔH(o) = -12.5(3) kcal mol(-1) and ΔS(o) = -26(3) cal K(-1) mol(-1); N(2): ΔH(o) = -13.9(7) kcal mol(-1) and ΔS(o) = -32(5) cal K(-1) mol(-1)).     

Fe(bpb)(CN)2]- as a versatile building block for the design of novel low-dimensional heterobimetallic systems: synthesis, crystal structures, and magnetic properties of cyano-bridged Fe(III)-Ni(II) complexes [(bpb)(2-) = 1,2-bis(pyridine-2-carboxamido)benzenate

A dicyano-containing [Fe(bpb)(CN)2]- building block has been employed for the synthesis of cyano-bridged heterometallic Ni(II)-Fe(III) complexes. The presence of steric bpb(2-) ligand around the iron ion results in the formation of low-dimensional species: five are neutral NiFe2 trimers and three are one-dimensional (1D). The structure of the 1D complexes consists of alternating [NiL]2+ and [Fe(bpb)(CN)2]- generating a cyano-bridged cationic polymeric chain and the perchlorate as the counteranion. In all complexes, the coordination geometry of the nickel ions is approximately octahedral with the cyano nitrogen atoms at the trans positions. Magnetic studies of seven complexes show the presence of ferromagnetic interaction between the metal ions through the cyano bridges. Variable temperature magnetic susceptibility investigations of the trimeric complexes yield the following J(NiFe) values (based on the spin exchange Hamiltonian H = -2J(NiFe) S(Ni) (S(Fe(1)) + S(Fe(2))): J(NiFe) = 6.40(5), 7.8(1), 8.9(2), and 6.03(4) cm(-1), respectively. The study of the magneto-structural correlation reveals that the cyanide-bridging bond angle is related to the strength of magnetic exchange coupling: the larger the Ni-N[triple bond]C bond angle, the stronger the Ni- - -Fe magnetic interaction. One 1D complex exhibits long-range antiferromagnetic ordering with T(N) = 3.5 K. Below T(N) (1.82 K), a metamagnetic behavior was observed with the critical field of approximately 6 kOe. The present research shows that the [Fe(bpb)(CN)2]- building block is a good candidate for the construction of low-dimensional magnetic materials.     

Assembling metals (Co II and Mn II) with pyridylcarboxylates in the presence of azide: synthesis, structural aspects and magnetic behavior of three coordination polymers

The reaction between Co(NO3)2.6H2O and substituted pyridylcarboxylic acid [nicotinic acid (Hnic) or trans-3-pyridylacrylic acid (Htpa)] in the presence of NaN3 under hydrothermal conditions yielded [Co(1.5)(nic)2 (Hnic)(N3)]n (1) and [Co(1.5)(tpa)2 (N3)(H2O)]n (2), respectively. Single crystal structure analyses reveal that both complexes are 3D complicated coordination polymers. The basic repeating units in both of the complexes are Co(3) trinuclear clusters containing syn-syn bridging carboxylate and end-on azido linker. A similar reaction using MnCl2.4H2O in presence of equimolar amounts of Htpa and NaN3 yielded a 2D corrugated sheet [Mn(tpa)2]n (3) containing no azide. Complex 3 can also be synthesized under hydrothermal conditions using Natpa in the absence of NaN3. Surprisingly, the same reaction at room temperature yielded a known mononuclear complex [Mn(tpa)2(H2O)4]. Variable temperature magnetic studies down to 2 K revealed the dominant antiferromagnetic nature of the first two complexes with a ferrimagnetic type of behavior despite the facts that they are homometallic and homospin systems. The susceptibility data in both cases were analyzed by a Co3 trinuclear model as well as considering inter-trimer interactions. Complex 3 is weakly antiferromagnetic in nature with an exchange parameter of J = -2 cm(-1) through the syn-anti bridging carboxylate pathway.     

Observation of inductive effects that cause a change in the rate-determining step for the conversion of rhenium azides to imido complexes

The cationic oxorhenium(V) complex [Re(O)(hoz)(2)(CH(3)CN)][B(C(6)F(5))(4)] [1; Hhoz = 2-(2'-hydroxyphenyl)-2-oxazoline] reacts with aryl azides (N(3)Ar) to give cationic cis-rhenium(VII) oxoimido complexes of the general formula [Re(O)(NAr)(hoz)(2)][B(C(6)F(5))(4)] [2a-2f; Ar = 4-methoxyphenyl, 4-methylphenyl, phenyl, 3-methoxyphenyl, 4-chlorophenyl, and 4-(trifluoromethyl)phenyl]. The kinetics of formation of 2 in CH(3)CN are first-order in both azide (N(3)Ar) and oxorhenium(V) complex 1, with second-order rate constants ranging from 3.5 × 10(-2) to 1.7 × 10(-1) M(-1) s(-1). A strong inductive effect is observed for electron-withdrawing substituents, leading to a negative Hammett reaction constant ρ = -1.3. However, electron-donating substituents on phenyl azide deviate significantly from this trend. Enthalpic barriers (ΔH(?)) determined by the Eyring-Polanyi equation are in the range 14-19 kcal mol(-1) for all aryl azides studied. However, electron-donating 4-methoxyphenyl azide exhibits a large negative entropy of activation, ΔS(?) = -21 cal mol(-1) K(-1), which is in sharp contrast to the near zero ΔS(?) observed for phenyl azide and 4-(trifluoromethyl)phenyl azide. The Hammett linear free-energy relationship and the activation parameters support a change in the mechanism between electron-withdrawing and electron-donating aryl azides. Density functional theory predicts that the aryl azides coordinate via N(α) and extrude N(2) directly. For the electron-withdrawing substituents, N(2) extrusion is rate-determining, while for the electron-donating substituents, the rate-determining step becomes the initial attack of the azide. The barriers for these two steps are inverted in their order with respect to the Hammett σ values; thus, the Hammett plot appears with a break in its slope.