Comparative studies of praseodymium(III) selective sensors based on newly synthesized Schiff's bases

Praseodymium ion selective polyvinyl chloride (PVC) membrane sensors, based on two new Schiff's bases 1,3-diphenylpropane-1,3-diylidenebis(azan-1-ylidene)diphenol (M₁) and N,N′-bis(pyridoxylideneiminato) ethylene (M₂) have been developed and studied. The sensor having membrane composition of PVC: o-NPOE: ionophore (M₁): NaTPB (w/w; mg) of 150: 300: 8: 5 showed best performances in comparison to M₂ based membranes. The sensor based on (M₁) exhibits the working concentration range 1.0 × 10⁻⁸ to 1.0 × 10⁻² M with a detection limit of 5.0 × 10⁻⁹ M and a Nernstian slope 20.0 ± 0.3 mV decade⁻¹ of activity. It exhibited a quick response time as <8 s and its potential responses were pH independent across the range of 3.5-8.5.The influence of the membrane composition and possible interfering ions have also been investigated on the response properties of the electrode. The sensor has been found to work satisfactorily in partially non-aqueous media up to 15% (v/v) content of methanol, ethanol or acetonitrile and could be used for a period of 3 months. The selectivity coefficients determined by using fixed interference method (FIM) indicate high selectivity for praseodymium(III) ions over wide variety of other cations. To asses its analytical applicability the prepared sensor was successfully applied for determination of praseodymium(III) in spiked water samples.     

A novel strapped porphyrin receptor for molecular recognition

A novel strapped porphyrin receptor Zn1, in which two electron-rich bis(p-phenylene)-34-crown ether-10 units are incorporated, has been designed and synthesized from the newly developed intermediate 7 for investigating new chemistry of molecular recognition. ¹H NMR and UV-Vis studies revealed that Zn1 displays relatively weak binding abilities to neutral electron deficient naphthalene-1,8,4,5-tetracarboxydiimide (NDI) derivatives 13 (no simple complexing stoichiometry was observed), 19 (K_a=48(±5) M⁻¹) and 30 (K_a=46(±5) M⁻¹) in chloroform-d, strong binding ability to pyridine derivative 25, (K_a=1.5(±0.12)×10³ M⁻¹) in chloroform, moderately strong binding ability to tetracationic compound 35·4PF₆ (K_a=475(±50) M⁻¹) in acetone-d₆, and very strong binding affinity to compound 22 (K_a=6.5(±0.7)×10⁵ M⁻¹), which consists of one pyridine and two NDI units, in chloroform. Remarkable cooperative effect of the intermolecular metal-ligand coordination and donor-acceptor interactions in complex Zn1·22 was observed by comparing the complexing behaviors between Zn1 and the appropriately designed guests. Complex Zn1·22 possesses an unique three-dimensional tri-site binding feature. For comparison, the complexing affinity of 1 toward compounds 13, 19, and 30 in chloroform-d and 35·4PF₆ in acetone-d₆ has also been investigated and the binding patterns in different complexes were explored. The results demonstrate that strapped porphyrin derivatives are ideal precursors for constructing new generation of three-dimensional multi-site artificial receptors for molecular recognition and host-guest chemistry.     

Synthesis and photochromic properties of novel yellow developing photochromic compounds

Two 1-thiazolyl-2-thienylcyclopentene derivatives, 1a and 2a, and a 1-thiazolyl-2-vinylcyclopentene derivative 3a have been synthesized in an attempt to obtain photochromic compounds which change the color from colorless to yellow, and have low photocycloreversion quantum yields and high absorption coefficients of the colored isomers. All of these compounds underwent reversible photochromic reactions. Compounds 1a and 2a in toluene solutions changed the color upon 313 nm light irradiation from colorless to orange and pink, in which absorption maxima were observed at 494 nm (ε=10,000 M⁻¹ cm⁻¹) and 525 nm (ε=8500 M⁻¹ cm⁻¹), respectively. On the other hand, the colorless toluene solution of 3a turned yellow upon irradiation with 313 nm light, in which the absorption maximum was observed at 416 nm (ε=17,100 M⁻¹ cm⁻¹). The photocyclization/cycloreversion quantum yields of 3 were 0.19 and 0.0014, respectively. The conversion from the open- to the closed-ring isomer of 3 in the photostationary state under irradiation with 313 nm light was close to 100%.     

Macrocycles with two exclusive hydrogen-bonding modes

A series of large, 44-membered macrocycles 7a-e were synthesized and characterized, which display two different diagonal binding modes. The unsubstituted macrocycle 7a strongly binds naphthalene-2,6-dicarboxylate through hydrogen bonds with the association constant (K_a ± 15%) of 4500 M⁻¹ in 40% (v/v) CD₃CN/CDCl₃ at 23 ± 1 °C. Introduction of an electron-withdrawing substituent (Cl) at all four corners increases the binding affinity (22,000 M⁻¹ for 7b), while that of an electron-donating substituent (pyrrolidinyl) greatly decreases it (150 M⁻¹ for 7c). The same propensity has been observed with macrocycles 7d and 7e bearing different substituents at two diagonal corners, suggesting that the relative population of the binding modes would be modulated by controlling the electron density of the aromatic ring.     

Nickel pyrazolyl borate complexes: Synthesis, structure and analytical application in biological and environmental samples as anion selective sensors

A [{hydrotris(3-phenyl-5-methyl-1-pyrazolyl)borate}(3-phenyl-5-methyl-pyrazole) nickel chloride] [Tp^Ph,MeNi(Cl)Pz^Ph,MeH] (I) has been synthesized and explored as ionophores for the preparation of a poly (vinyl chloride) (PVC) membrane sensor for azide and thiocyanate anions. The compounds [Tp^Ph,MeNi(N₃)Pz^Ph,MeH] (II) and [Tp^Ph,MeNi(SCN)Pz^Ph,MeH] (III) were characterized by their crystal structures and proved to be bonded as monodentate through nitrogen atom of azide and thiocyanate anion. Potentiometric investigations also indicate high affinity of this receptor for thiocyanate and azide ions. PVC based membranes of I using as hexadecyltrimethylammonium bromide (HTAB) cation discriminator and o-nitrophenyloctyl ether (o-NPOE), dibutylphthalate (DBP), acetophenone (AP) and tributylphosphate (TBP) as plasticizing solvent mediators were prepared and investigated as SCN⁻ and N₃⁻ selective sensors. The best performance was shown by the membrane of thiocyanate with composition (w/w) of (I) (7%):PVC (31%):DBP (60%):HTAB (2%). This sensor works well over a wide concentration range 5.3 × 10⁻⁷ to 1.0 × 10⁻² M with Nernstian compliance (59.2 mV decade⁻¹ of activity) within pH range 2.5-9.0 with a response time of 11 s and showed good selectivity for thiocyanate ion over a number of anions. The sensor exhibits adequate life (3 months) and could be used successfully for the determination of thiocyanate content in human urine, saliva and river water samples. While the membrane of [Tp^Ph,MeNi(Cl)Pz^Ph,MeH] ionophore with composition (I) (6%):HTAB (4%):PVC (31%):TBP (59%) showed highest sensitivity and widest linear range for azide ion. These sensors exhibit the maximum working concentration range of 8.1 × 10⁻⁶ to 1.0 × 10⁻² M with Nernstian slope of 59.3 mV decade⁻¹ of activity. It can be applied for the monitoring of the azide ions concentration in aqueous black tea and orange juice samples.     

Manganese (II) selective PVC based membrane sensor using a Schiff base

N,N′,N″,N′′′-1,5,8,12-tetraazadodecane-bis(salicylaldiminato)(H₂L) has been used as ionophore for preparing Mn²⁺ selective sensor. Membranes of different composition with regard to ratio of H₂L:PVC:NPOE:NaTPB have been prepared and investigated. The best performance was obtained with the membrane of composition 10:150:150:10 (H₂L:PVC:NPOE:NaTPB) (w/w; mg). This membrane generated linear potential response in the concentration range of 5.0 × 10⁻⁶ to 1.0 × 10⁻¹ M with a Nernstian slope of 30.0 mV/decade of activity and fast response time (10 s). Hydrogen ion does not effect to the performance of sensor in the pH range 3.0-6.5. The sensor was found to be sufficient selective for Mn²⁺ over a number of alkali, alkaline and heavy metal ions and could therefore be used for the determination of manganese in various samples by direct potentiometry.     

Synthesis and characterization of phosphorous-bridged bisphenoxy titanium complexes and their application to ethylene polymerization

Phosphorous-bridged bisphenoxy titanium complexes were synthesized and their ethylene polymerization behavior was investigated. Bis[3-tert-butyl-5-methyl-2-phenoxy](phenyl)phosphine tetrahydrofuran titanium dichloride (4a) was obtained by treatment of 3 equiv of n-BuLi with bis[3-tert-butyl-2-hydroxy-5-methylphenyl](phenyl)phosphine hydrochloride salt (3a) followed by TiCl₄(THF)₂ in THF. THF-free complexes 5a-5d were synthesized more conveniently by the direct reaction of MOM-protected ligands (2a-2d) with TiCl₄ in toluene. X-ray analysis of 4a revealed that the ligand is bonded to the octahedral titanium (IV) center in a facial fashion and two chlorine atoms possess cis-geometry. Complexes 4a and 5a-5d were utilized as catalyst precursors for ethylene polymerization. Complex 5c gave high molecular weight polyethylene (M_w = 1,170,000, M_w/M_n = 2.0) upon activation with Al(ⁱBu)₃/[Ph₃C][B(C₆F₅)₄] (TB). Ethylene polymerization activity of 5d activated with Al(ⁱBu)₃/TB reached 49.0 × 10⁶ g mol (cat) ⁻¹ h⁻¹.     

New 2-aminoethylimidazole-based dicarboxylic acid receptor derived from cholestane

A new facial amphiphile cholestane-based receptor 1 containing a 2-imidazolylethylamino moiety at the 3α and 7α positions of cholestane was synthesized. Recognition selectivity of the new receptor 1 with various dicarboxylic acids was assessed by ¹H NMR titration. Maleic acid showed the highest binding constant among all the tested acids (K_a = 9.36 × 10⁴ M⁻¹).     

Dinitrogen extrusion from diazidophenylborane: Computational analysis of PhBNx (x = 6, 4, 2) isomers

The energies and structures of possible intermediates in the dinitrogen extrusion from diazidophenylborane 4a to give phenylborylene 11a were determined using density functional (B3LYP), multiconfigurational (CASSCF and MRMP2), and coupled cluster (CCSD(T)) computations in conjunction with basis sets of up to cc-pVTZ quality. Formation of 11a and nitrogen from 4a is an exothermic process (−21 kcal mol⁻¹). The triplet electronic ground state of azidophenylborylnitrene 5a (PhBN₄) is only 26 kcal mol⁻¹ higher in energy than 4a and the phenyl shift in 5a to yield N-azidophenyliminoborane 7a is highly exothermic.     

Metallo-tetraazaporphyrin based anion sensors: regulation of sensor characteristics through central metal ion coordination

The iron(III) and cobalt(III) complexes of 2,3,7,8,12,13,17,18-octakis(benzylthio)-5,10,15,20-tetraazaporphyrin, (OBTAP) were synthesized and incorporated into PVC matrix as ionophores to fabricate anion selective membrane electrodes that exhibit selective potentiometric response to azide and nitrite ions, respectively. The membrane of [Fe(OBTAP)]⁺ (III) with a composition of 6:190:200 (III:DBP:PVC) (w/w), and of [Co(OBTAP)]⁺ (IV) with a composition of 10:148:200 (IV:DOP:PVC) (w/w), i.e. 1a and 2b, respectively (where DBP:dibutylphthalate and DOP=dioctylphthalate) gave the best performance. The membrane 1a showed a slope of 29.2±0.2 mV per decade of activity for N₃⁻ in the working concentration range of 8.9×10⁻⁶ to 1.0×10⁻¹ M. The membrane 2b showed a slope of 30.0±0.2 mV per decade of activity for NO₂⁻ in the working concentration range of 1.1×10⁻⁵ to 1.0×10⁻¹ M. The membranes worked satisfactorily in the pH range of 4.3-10.5 (1a) and 2.8-6.4 (2b) and had fast response time of 12±2 and 13±2 s, respectively. Electrodes exhibited a high degree of selectivity for N₃⁻ and NO₂⁻, respectively, over several other monovalent and bivalent anions. Only SCN⁻ and S²⁻ (at >1.0×10⁻⁴ M) cause moderate interference for electrode 1a and Cl⁻ and S²⁻ (at >1.0×10⁻⁵ M) for electrode 2b. They gave reproducible results with the relative standard deviation in the observed values of potentials (σ) of 1.96 and 1.80 mV for electrodes 1a and 2b, respectively, from the least-squares fit line. The 90% confidence limit lies within ±0.2 mV per decade of activity. Reproducible results were obtained over a period of 5 months. Their performance in non-aqueous solvent mixtures having up to 50% (v/v) methanol, ethanol and acetone were evaluated and were found satisfactory. The proposed sensors are superior in terms of detection limit and response time in comparison to the reported ones.     

New water-soluble iminecalix[4]arene with a deep hydrophobic cavity

We report the synthesis of a new water-soluble iminecalix[4]arene host 4c with a deep hydrophobic cavity. The negatively charged four carboxylate functions on the top of the cavity play a major role in the recognition of charged molecular species. The ¹H NMR titration experiments revealed that host 4c binds with cationic (10-12) and neutral guests (6-9) in water with high binding constants in the order of 10⁴-10⁵ M⁻¹. Cationic guest 9 showed highest binding constant of 2.81 × 10⁵ M⁻¹ with host 4c amongst all tested guests. Selectivity over anionic guests (13-17) is established by the presence of negative charges at the top of the deep hydrophobic cavity, as guests 15 and 17 were not recognized by host 4c. Neutral pyridine derivatives with hydrophobic chains at para positions showed high binding constants of 6.02 × 10⁴-2.23 × 10⁵ M⁻¹. The data obtained for the recognition of the guests by host 4c revealed that the ionic as well as the hydrophobic-hydrophobic interactions are crucial in the molecular recognition in aqueous medium.     

Novel titanocene anti-cancer drugs derived from fulvenes and titanium dichloride

Starting from 6-(p−N,N-dimethylanilinyl)fulvene (1a) or 6-(pentamethylphenyl)fulvene (1b) [1,2-di(cyclopentadienyl)-1,2-di(p−N,N-dimethylaminophenyl)ethanediyl] titanium dichloride (2a) and [1,2-di(cyclopentadienyl)-1,2-bis(pentamethylphenyl)ethanediyl] titanium dichloride (2b) and their corresponding dithiocyanato complexes (3a, 3b) were synthesized. Titanocene 2b did not show a cytotoxic effect, but when 2a was tested against pig kidney carcinoma cells (LLC-PK) or human ovarian carcinoma cells (A2780/cp70) inhibitory concentrations (IC₅₀) of 2.7 × 10⁻⁴ and 1.9 ×  10⁻⁴ M, respectively, were observed.     

Synthesis,structure, and electrochemistry of pyridinecarboxamide cobalt(III) complexes; the effect of bridge substituents on the redox properties

Two series of complexes of the types trans-[Co^III(Mebpb)(amine)₂]ClO₄ {Mebpb²⁻ = N,N-bis(pyridine-2-carboxamido)-4-methylbenzene dianion, and amine = pyrrolidine (prldn) (1a), piperidine (pprdn) (2a), morpholine (mrpln) (3a), benzylamine (bzlan) (4a)}, and trans-[Co^III(cbpb)(amine)₂]X {cbpb²⁻ = N,N-bis(pyridine-2-carboxamido)-4-chlorobenzene dianion, and amine = pyrrolidine (prldn), X = PF₆ (1b), piperidine (pprdn), X = PF₆ (2b), morpholine (mrpln), X = ClO₄ (3b), benzylamine (bzlan), X = PF₆ (4b)} have been synthesized and characterized by elemental analyses, IR, UV–Vis, and ¹H NMR spectroscopy. The crystal structure of 1a has been determined by X-ray diffraction. The electrochemical behavior of these complexes, with the goal of evaluating the effect of axial ligation and equatorial substitution on the redox properties, is also reported. The reduction potential of Co^III, ranging from −0.53 V for (1a) to −0.31 V for (3a) and from −0.48 V for (1b) to −0.22 V for (3b) show a relatively good correlation with the σ-donor ability of the axial ligands. The methyl and chloro substituents of the equatorial ligand have a considerable effect on the redox potentials of the central cobalt ion and the ligand-centered redox processes.     

PVC based 5,10,15,20-tetrakis (4-methoxyphenyl) porphyrinatocobalt(II) membrane potentiometric sensor for arsenite

PVC based membranes of 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrinatocobalt(II) (TMOPP-Co) (I) as electroactive material with dibutyl butyl phosphonate (DBBP), dioctyl phthalate (DOP), 1-chloronaphthalene (CN), tri-n-butyl phosphate (TBP) and tris(2-ethylhexyl) phosphate (TEP) as plasticising solvent mediators have been prepared and tried for arsenite determination. The membrane having a composition 150:5:150 (PVC:I:DBBP) exhibited the best results with linear potential response in the concentration range of 7.9 × 10⁻⁵ to 1.0 × 10⁻¹ M of AsO₂⁻ with a slope of 28.8 mV/decade. The useful pH range of the sensor is 6.0-10.5. The membrane worked satisfactorily in non-aqueous medium up to 5% (v/v) non-aqueous content. The selectivity coefficient values for mono- and divalent anions indicate good selectivity for arsenite over a large number of anions.     

Control of molecular weight distribution in polycondensation polymers 2. Poly(ether ketone) synthesis

Synthesis of aromatic poly(ether ketone) (3) with a narrow molecular weight distribution (M_w/M_n) was investigated via polycondensation. M_ns of 3 could be controlled varying the feed ratio of monomer (1) and initiator (2) maintaining relatively narrow M_w/M_ns (<1.3). The kinetics of polycondensation obeyed a first-order relationship between polycondensation time and -(1/[2]₀) ln([1]/[1]₀), and the rate of polycondensation was estimated as 2.57 mol⁻¹ L h⁻¹. MALDI-TOF mass analysis of 3 indicated that polycondensation should proceed via chain growth manner to give 3 having an initiator unit in each chain end.     

Pyrenyl-appended imidazolium receptor for selective fluorescence sensing of oxalic acid

A new fluorescent imidazolium-based cholestane receptor 4 bearing a pyrene moiety was synthesized. The binding ability of 4 toward various dicarboxylic acids was examined by UV-vis and fluorescence spectroscopy. Receptor 4 showed the highest binding constant for oxalic acid among all the tested dicarboxylic acids (K_a = 5.06 × 10⁴ M⁻¹). Oxalic acid formed a complex with 4 with a 1:2 ratio in ethanol.     

Synthesis and electrochemical properties of new cobalt and manganese phthalocyanine complexes tetra-substituted with 3,4-(methylendioxy)-phenoxy

The synthesis and electrochemical properties of new cobalt and manganese phthalocyanine complexes, tetra-substituted with 3,4-(methylendioxy)-phenoxy at the peripheral (complexes 3 and 5) and non-peripheral (complexes 4 and 6) positions, are reported. Complexes 3 and 4 showed Q-band absorption, in DMF, at 668 and 686 nm, respectively while Q-band due to complexes 5 and 6 appeared at 732 and 760 nm, respectively in CHCl₃. All the complexes showed well resolved redox processes attributed to both metal and ring based processes. Complexes 3 and 4 showed four redox processes, labeled I, II, III and IV. For complex 3, process I (Co^IPc⁻²/Co^IPc⁻³) was observed at −1.45 V, II (Co^IIPc⁻²/Co^IPc⁻²) at −0.38 V, III (Co^IIIPc⁻²/Co^IIPc⁻²) at +0.49 V and IV (Co^IIIPc⁻¹/Co^IIIPc⁻²) at +0.97 V versus Ag|AgCl. Similar processes were observed for complex 4 at −1.36 V, −0.27 V, +0.56 V, +1.03 V versus Ag|AgCl, respectively. Complexes 5 and 6 showed two redox processes (I and II). For complex 5, these processes appeared at −0.79 V (Mn^IIPc⁻²/Mn^IIPc⁻³, I) and −0.07 V versus Ag|AgCl (Mn^IIIPc⁻²/Mn^IIPc⁻², II), while for complex 6, they were observed at −0.86 V and −0.04 V versus Ag|AgCl. Spectroelectrochemistry was used to probe and confirm the origin of these processes.     

Synthesis and properties of novel 5-(cyclohepta-2′,4′,6′-trienylidene)pyrimidine-2(1H),4(3H),6(5H)-triones: methodology for synthesizing cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborates

Novel condensation reaction of tropone with N-substituted and N,N′-disubstitued barbituric acids in Ac₂O afforded 5-(cyclohepta-2′,4′,6′-trienylidene)pyrimidine-2(1H),4(3H),6(5H)-trione derivatives (8a-f) in moderate to good yields. The ¹³C NMR spectral study of 8a-f revealed that the contribution of zwitterionic resonance structures is less important as compared with that of 8,8-dicyanoheptafulvene. The rotational barriers (ΔG^‡) around the exocyclic double bond of mono-substituted derivatives 8a-c were obtained to be 14.51-15.03 kcal mol⁻¹ by the variable temperature ¹H NMR measurements. The electrochemical properties of 8a-f were also studied by CV measurement. Upon treatment with DDQ, 8a-c underwent oxidative cyclization to give two products, 7 and 9-substituted cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborates (11a-c·BF₄⁻ and 12a-c·BF₄⁻) in various ratios, while that of disubstituted derivatives 8d-f afforded 7,9-disubstituted cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborate (11d-f·BF₄⁻) in good yields. Similarly, preparation of known 5-(1′-oxocycloheptatrien-2′-yl)-pyrimidine-2(1H),4(3H),6(5H)-trione derivatives (14a-d) and novel derivatives 14e,f was carried out. Treatment of 14a-c with aq. HBF₄/Ac₂O afforded two kinds of novel products 11a-c·BF₄⁻ and 12a,c·BF₄⁻ in various ratios, respectively, while that of 14d-f afforded 11d-f. The product ratios of 11a-c·BF₄⁻ and 12a-c·BF₄⁻ observed in two kinds of cyclization reactions were rationalized on the basis of MO calculations of model compounds 20a and 21a. The spectroscopic and electrochemical properties of 11a-f·BF₄⁻ and 12a-c·BF₄⁻ were studied, and structural characterization of 11c·BF₄⁻ based on the X-ray crystal analysis and MO calculation was also performed.     

Two near-UV and visible chromophores of CpCo(Dithiolene) complexes with pyridinium-dicyanomethylide group

Tetracyanoethylene oxide (TCNEO) reacted with [CpCo(dithiolene)] (Cp = η⁵-cyclopentadienyl) complexes having 4-pyridyl or 3-pyridyl group to undergo a dicyanomethylation to the nitrogen atom on the pyridyl group. The reaction of [CpCo(S₂C₂(⁴Py)₂)] (1) with TCNEO formed both the monodicyanomethylated [CpCo(S₂C₂(⁴Py)(⁴Py-C(CN)₂))] (1a) and bisdicyanomethylated [CpCo(S₂C₂(⁴Py-C(CN)₂)₂)] (1b). [CpCo(S₂C₂(²Py)(⁴Py))] (2) reacted with TCNEO to give [CpCo(S₂C₂(²Py)(⁴Py-C(CN)₂))] (2a) but no dicyanomethylation occurred on the 2-pyridyl group. 2 reacted with excess TCNEO to form the only dicyanomethylated acetylene derivative ²Py-CC-(⁴Py-C(CN)₂) (2c), followed by a dissociation of the CpCoS₂ fragment. The monodicyanomethylated [CpCo(S₂C₂(ⁿPy-C(CN)₂)(2-thienyl))] (n = 4 (4a) or 3 (5a)) complexes were also prepared from [CpCo(S₂C₂(ⁿPy)(2-thienyl))] (n = 4 (4) or 3 (5)) and TCNEO. 1b was structurally characterized by X-ray diffraction study. The all dicyanomethylated [CpCo(dithiolene)] complexes showed the dithiolene LMCT absorption in the range of 605-644 nm (ε = 7000-9200 M⁻¹ cm⁻¹) and very strong absorption due to their pyridinium-dicyanomethylide moieties in near-UV region (e.g. 1b: λ_max = 470 nm, ε = 43,400 M⁻¹ cm⁻¹). The CV of the all dicyanomethylated complexes exhibited two reduction waves. The first reduction is due to Co^III/Co^II and the second one is due to the reduction of the pyridinium-dicyanomethylide moiety. The reduced 1b⁻ is stable enough for several minutes according to the visible spectroelectrochemical measurement. The ESR spectrum of 1b⁻ indicated eight hyperfine splittings due only to the interaction with the nuclear spin of cobalt (I = 7/2).     

Some new DTPA-N,N″-bis(amides) functionalized by alkyl carboxylates: Synthesis,complexation and stability properties

A new series of DTPA-N,N″-bis(amide) ligands functionalized by alkyl carboxylates on the amide side-arms (1a–1l) and their Gd(III) complexes of the type [Gd(1)(H₂O)] · xH₂O (2a–2l) were synthesized and characterized by analytical and spectroscopic techniques. Potentiality of 2a–2l as contrast agent for magnetic resonance imaging (MRI CA) was investigated by measuring some relevant physicochemical properties such as (i) the protonation constants of 1a–1l, (ii) thermodynamic and conditional stability constants of 2a–2l, (iii) the selectivity (pGd) of 1a–1l for the Gd(III) ion over the endogenous metal ions such as Zn(II), Ca(II), and Cu(II), and (iv) the relaxivities (R₁ and R₂) of 2a–2l in aqueous and aqueous HP-β-CD solutions. Comparative studies reveal that most of new Gd(III) complexes show enhanced thermodynamic stability and selectivity as compared with those of [Gd(DTPA-BMA)(H₂O)] (DTPA-BMA = N,N″-di(methylcarbamoylmethyl)diethylenetriamine-N,N′,N″-triacetate). Also enhanced with 1a–1l (except 1f and 1h) is affinity for Gd(III) as compared with [DTPA-BMA]³⁻ under physiological conditions. The relaxivities (R₁ and R₂) of aqueous solutions of 2a–2l, on the other hand, drop significantly as compared with [Gd(DTPA-BMA)(H₂O)] although they increase dramatically (6–10 fold) in aqueous hydroxypropyl-β-cyclodextrin (HP-β-CD) solutions.