Pyrogallol-based dipodal optical probe as new smart analytical tool for sustainable detection of cobalt in biosystem.

The present research focuses on the micro-level detection of cobalt ions in biological and environmental samples using a new probe. The probe is a multifunctional symmetrical dipodal molecule with two pyrogallol binding units attached to the malonate scaffold through a propylene spacer. It was synthesized and characterized by 1H NMR, 13C NMR, IR, electronic spectroscopy, and mass spectrometry. The molecule's binding, thermodynamic, and photophysical properties are also described. The designed probe demonstrates an excellent sensing ability for Co(II) based on the ESIPT "OFF-ON" fluorescence mechanism. The experiments explore the high selectivity of the ligand for cobalt sensing over a wide range of metal ions of biological and environmental importance. The fluorescence intensity shows a linear response to Co(II) in 5-100 µM concentration with a detection limit of 8.75 x 10-5 and a 2.65-fold enhancement in the intensity. These results establish its potential application as a fluorescence sensor. The probe is also employed as a colorimetric sensor for the qualitative determination of cobalt ions in DMSO solution. The interesting behavior of the probe motivated us further to study its coordination properties with divalent cobalt in solution. The pre-organized assembly with an appropriate cavity size favors the ligand for an efficient Co(II) encapsulation by coordinating through imine-Ns and aromatic ring-Os donors, giving high formation constants.

Schiff Bases: A Versatile Fluorescence Probe in Sensing Cations.

Metal cations such as Zn2+, Al3+, Hg2+, Cd2+, Sn2+, Fe2+, Fe3+ and Cu2+ play important roles in biology, medicine, and the environment. However, when these are not maintained in proper concentration, they can be lethal to life. Therefore, selective sensing of metal cations is of great importance in understanding various metabolic processes, disease diagnosis, checking the purity of environmental samples, and detecting toxic analytes. Schiff base probes have been largely used in designing fluorescent sensors for sensing metal ions because of their easy processing, availability, fast response time, and low detection limit. Herein, an in-depth report on metal ions recognition by some Schiff base fluorescent sensors, their sensing mechanism, their practical applicability in cell imaging, building logic gates, and analysis of real-life samples has been presented. The metal ions having biological, industrial, and environmental significance are targeted. The compiled information is expected to prove beneficial in designing and synthesis of the related Schiff base fluorescent sensors.

Photophysical Studies of a Catechol Based Polyfunctional Dipodal Chelator: Application for Optical Probe for Selective Detection of Fe(III).

A novel catechol based dipodal fluorescent chelator N,N'-bis[3-[(E)-(2,3-dihydroxyphenyl)methyleneamino]propyl]propanediamide(MPC), has been developed and its photophysical behaviour was studied by experimental (UV-VIS and fluorescence) and DFT method. The design of the molecule has been inspired from the naturally occurring siderophore enterobactin, a catechol based chelator with amide linkage, that shows an excellent binding efficiency towards Fe(III). The dipodal molecule (MPC) presented here, carries two catechol pendant binding moieties linked to the malonate central unit through propylene spacers by amide linkage. MPC showed good selectivity for Fe(III) at 10-4 M concentration in aqueous medium amongst the biologically and environmentally important metal ions chosen viz., Na(I), K(I), Al(III), Cr(III), Fe(III), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II), by demonstrating a remarkable quenching in the fluorescent emission from 262 a.u. to 55 a.u. at λmax = 477 nm. Also, the pre-organized assembled ligand favored an efficient Fe (III) encapsulation through coordination by imine nitrogen and catecholate oxygen donors. High formation constant (log ß = 31.3) for 1:1 metal-ligand complex evaluated by both potentiometric and spectrophotometric methods, established the strong binding efficiency of the ligand for Fe(III) metal ion. The binding stoichiometry in the complex was also confirmed from Stern -Volmer and Hill Plot analysis. Further investigation on the emission behavior of MPC in a completely DMSO system explored its suitability for extensive applications in the areas such as, metallurgy, material science, iron contamination remedial in the materials etc.. DFT studies suggest that the ligand displays a U-shaped geometry with a parallel π-stacking and the hydrogen bond between two arms. The experimental infrared, electronic, fluorescence, 1H nmr, 13C nmr spectra were correlated with the theoretical results. The nature of electronic transitions were identified from the TDDFT calculation. The ligand forms a hexa-coordinated complex with six Fe-O bonds extending an orthorhombic geometry due distortion from a regular octahedron.

Photoluminescence and Coordination Behaviour of Lanthanide Complexes of Tris (Aminomethyl)Ethane-5-Oxine in Aqueous Solution.

Photophysical properties of a multidentate tripodal ligand, 5,5'-(2-(((8-hydroxyquinolin-5-yl) methylamino)methyl)-2-methylpropane-1,3-diyl) bis (azanediyl)bis (methylene)diquinolin-8-ol, (TAME5OX), with La3+ and Er3+ ions have been examined for photonics applications. The change in behavior in electronic spectra of these complexes reveals the use of TAME5OX as a sensitive optical pH based sensor to detect Ln3+ ions whereas indication of strong green fluorescence allows simultaneous sensing within the visible region in competitive medium. The intense fluorescence intermittently gets quenched under acidic and basic conditions due to photoinduced intramolecular electron transfer from the excited 8-hydroxyquinoline (8-HQ) moiety to the metal ion. This renders these compounds the OFF-ON-OFF type of pH-dependent fluorescent sensor. The thermodynamic stability and coordination behaviour of the chelator with the said lanthanide ions have also been probed by potentiometric, UV - visible and fluorescence spectrophotometric method. TAME5OX forms protonated complex [Ln (H4L)]4+ below pH ~4.0 which sequentially deprotonates through one proton process with increase of pH. The stability constants of neutral complexes have been determined to be in the range log ß110 = 32-34 and pLn in the range of 14-20, indicating TAME5OX is a good synthetic lanthanide chelator. Theoretical spectra were also calculated by ZINDO/s methodology at single excitations (CIS) level on PM7 as sparkle energy-minimized geometries.

Spectroscopic, photophysical, solution thermodynamics and computational study of europium and terbium complexes with a flexible quinolinol-based symmetric tripodal chelator.

Coordination behaviour and photonic properties of a flexible tripodal multidentate ligand, 5,5',5''-(Cyclohexane-1,3,5-triyltris(oxy))tris(methylene)triquinolin-8-ol (CYTOM5OX), with Eu and Tb ions were explored through potentiometric, UV-visible and fluorescence spectrophotometric methods. The interaction of tripod towards the lanthanides has been examined for thermodynamic equilibrium, photophysical behavior, and time-resolved luminescence measurements. Besides, a theoretically empirical method followed by quantum mechanical treatment of these chelates has also been summarized. A noticeable change in the electronic spectra of these complexes disclose CYTOM5OX as a pH-sensitive optical sensor for lanthanides detection; also, a strong green fluorescence allows simultaneous sensing in the visible region in a competitive medium. The intense fluorescence intermittently gets quenched under acidic and basic conditions due to photoinduced intramolecular electron transfer from the excited 8-hydroxyquinoline (8-HQ) moiety to the metal ion. The thermodynamically stable Ln3+ complexes in aqueous solution show stability constants in the range log ß110 = 33-35 and pLn3+ in the range of 21-22. The hydration number that was predicted by the coordination scan was substantiated by lifetime measurements reveal presence of coordinated water molecules in Eu(CYTOM5OX) and Tb(CYTOM5OX) complexes. Theoretical spectra were also calculated by ZINDO/s methodology at single excitations (CIS) level on PM7 as sparkle energy-minimized geometries.

Spectroscopic, potentiometric and theoretical studies on the binding properties of a novel tripodal polycatechol-imine ligand towards iron(III).

A novel multidentate tripodal ligand, cis,cis-1,3,5-tris[(2,3-dihydroxybenzylidene)aminomethyl]cyclohexane (TDBAC, L) containing one catechol unit in each arms of a tripodal amine, cis,cis-1,3,5-tris(aminomethyl)cyclohexane was investigated as a chelator for iron(III) through potentiometric and spectrophotometric methods in an aqueous medium of 0.1N ionic strength and 25+/-1 degrees C as well as in ethanol by continuous variation method. From pH metric in water, three protonation constants characterized for the three-hydroxyl groups of the catechol units at ortho were used as input data to evaluate the stability constants of the complexes. Formation of monomeric complexes FeLH3, FeLH2, FeLH and FeL were depicted. In ethanol, formation of complexes FeL, Fe2L and Fe3L were characterized. Structures of the complexes were explained by using the experimental evidences and predicted through molecular modeling calculations. The ligand showed potential to coordinate iron(III) through three imine nitrogens and three catecholic oxygens at ortho to form a tris(iminocatecholate) type complex.

Potentiometric and spectrophotometric study of a new dipodal ligand N,N'-bis{2-[(2-hydroxybenzylidine)amino]ethyl}malonamide with Co(II), Ni(II), Cu(II) and Zn(II).

A new dipodal ligand, N,N'-bis{2-[(2-hydroxybenzylidine)amino]ethyl}malonamide (BHAEM) was synthesized by Schiff base condensation of N,N'-bis(2-aminoethyl)malonamide with two equivalent of salicylaldehyde and characterized on the basis of elemental analyses and various spectral (UV-vis, IR, (1)H NMR and (13)C NMR) data. The complexation reaction of the ligand with H(+), Co(II), Ni(II), Cu(II) and Zn(II) in solution was investigated by spectrophotometric and potentiometric method. Two protonation constants of BHAEM assigned for two hydroxyl groups of aromatic ring were determined and its hydrolysis mechanism was proposed through potentiometric result. In presence of metal ions, BHAEM shows different coordination properties. All metal ions form ML type complex where the ligand coordinates to the metal ion through two N-amine and two O-phenolate groups. In addition, Ni(II) and Cu(II) form additional complex species of the type MLH(-1) and MLH(-2), respectively due to ionization of amide protons. The molecular geometry of BHAEM was examined theoretically using the molecular mechanics MM3 force field followed by semi-empirical PM3 method and various spectral data UV-vis, IR and (1)H NMR were calculated from the energy-minimized structure applying semi-empirical ZINDO, PM3 and TNDO/2 method, respectively and compared with the experimental data. The probable structure of metal complexes in solution were proposed through calculated minimum strain energy by applying molecular mechanics MM(+) force field coupled with molecular dynamics simulation. Further the proposed structure of Cu(BHAEM) was refined through semi-empirical AM1/d method.

Development of a C3-symmetric benzohydroxamate tripod: Trimetallic complexation with Fe(III), Cr(III) and Al(III).

The design, synthesis and physicochemical characterization of a C3-symmetry Benzene-1,3,5-tricarbonylhydroxamate tripod, noted here as BTHA, are described. The chelator was built from a benzene as an anchor, symmetrically extended by three hydroxamate as ligating moieties, each bearing O, O donor sites. A combination of absorption spectrophotometry, potentiometry and theoretical investigations are used to explore the complexation behavior of the ligand with some trivalent metal ions: Fe(III), Cr(III), and Al(III). Three protonation constants were calculated for the ligand in a pH range of 2-11 in a highly aqueous medium (9:1 H2O: DMSO). A high rigidity in the molecular structure restricts the formation of 1:1 (M/L) metal encapsulation but shows a high binding efficiency for a 3:1 metal ligand stoichiometry giving formation constant (in ß unit) 28.73, 26.13 and 19.69 for [M3L]; M=Fe(III), Al(III) and Cr(III) respectively, and may be considered as an efficient Fe-carrier. The spectrophotometric study reveals of interesting electronic transitions occurred during the complexation. BTHA exhibits a peak at 238 nm in acidic pH and with the increase of pH, a new peak appeared at 270 nm. A substantial shifting in both of the peaks in presence of the metal ions implicates a s coordination between ligand and metal ions. Moreover, complexation of BTHA with iron shows three distinct colors, violet, reddish orange and yellow in different pH, enables the ligand to be considered for the use as colorimetric sensor.

TAME5OX, abiotic siderophore analogue to enterobactin involving 8-hydroxyquinoline subunits: thermodynamic and photophysical studies.

The synthesis, thermodynamic and photophysical properties of trivalent metal complexes of biomimetic nonadentate analogue, 5,5'-(2-(((8-hydroxyquinolin-5-yl)methylamino)methyl)-2-methylpropane-1,3-diyl)bis(azanediyl)bis(methylene)diquinolin-8-ol (TAME5OX), have been described. Combination of absorption and emission spectrophotometry, potentiometry, electrospray mass spectrometry, IR, and theoretical investigation were used to fully characterize metal (Fe(+3), Al(+3) and Cr(+3)) chelates of TAME5OX. In solution, TAME5OX forms protonated complexes [M(H3L)](3+) below pH 3.4, which consecutively deprotonates through one to three-proton processes with rise of pH. The formation constants (Logß11n) of neutral complexes formed at or above physiological pH, have been determined to be 30.18, 23.27 and 22.02 with pM values of 31.16, 18.07 and 18.12 for Fe(+3), Al(+3) and Cr(+3) ions, respectively, calculated at pH 7.4, indicating TAME5OX is a powerful among synthetic metal chelator. The results clearly demonstrate that the ligand in a tripodal orchestration firmly binds these ions over wide pH range and forms distorted octahedral complexes. The binding and the coordination event could be monitored from absorption and fluorescence spectroscopy. The high thermodynamic stability in water at physiological pH of ferric complex of TAME5OX indicates that these complexes are resistant to hydrolysis and therefore are well suited for the development of device for applications as probes. The ligand displays high sensitive fluorescence enhancement to Al(3+) at pH 7.4, in water. Moreover, TAME5OX can distinguish Al(3+) from Fe(3+) and Cr(3+) via two different sensing mechanisms: photoinduced electron transfer (PET) for Al(3+) and internal charge transfer (ICT) for Fe(3+) and Cr(3+). Density functional theory was employed for optimization and evaluation of vibrational modes, NBO analysis, excitation and emission properties of the different species of metal complexes observed by solution studies.

Spectroscopic, potentiometric and theoretical studies of novel imino-phenolate chelators for Fe(III).

The present study was targeted to explore the binding properties of two strong chelators for Fe(III) based on tripodal-iminophenolate moiety. Complexation behavior of the tripodal systems cis-cis cyclohexane-1,3,5-tricarboxylic acid tris-({2-[(2-hydroxy-benzylidene)-amino]-ethyl}-amide (CYCOENSAL, L(1)) and cis-cis cyclohexane-1,3,5-tricarboxylic acid tris-({3-[(2-hydroxy-benzylidene)-amino]-propyl}-amide (CYCOPNSAL, L(2)) is described. Three protonation constants obtained are assigned for three hydroxyl groups of aromatic ring were employed for the evaluation of the formation constants of the metal complexes. Both ligands liberate three protons each forming monomeric complexes of type FeLH3, FeLH2, FeLH and FeL (L=L(1) and L(2)). The first species FeLH3 depicted at low pH, where the ligands were coordinated through three imine nitrogen and other species form subsequently from FeLH3 in steps upon deprotonation and coordination of the phenolic oxygen giving encapsulated tris(phenolate) complexes. The probable structures of the metal complexes formed in solution were proposed through molecular modeling calculations. L(2) was observed to be highly selective towards Fe(III) as compared to L(1).

Synthesis of Imine-Naphthol Tripodal Ligand and Study of Its Coordination Behaviour towards Fe(III), Al(III), and Cr(III) Metal Ions.

A hexadentate Schiff base tripodal ligand is synthesized by the condensation of tris (2-aminoethyl) amine with 2-hydroxy-1-naphthaldehyde and characterized by various spectroscopic techniques like UV-VIS, IR, NMR, MASS, and elemental analysis. The solution studies by potentiometric and spectrophotometric methods are done at 25 ± 1°C, µ = 0.1 M KCl, to calculate the protonation constants of the ligand and formation constants of metal complexes formed by the ligand with Fe(III), Al(III), and Cr(III) metal ions. The affinity of the ligand towards Fe(III) is compared with deferiprone (a drug applied for iron intoxication) and transferrin (the main Fe(III) binding protein in plasma). Structural analysis of the ligand and the metal complexes was done using semiempirical PM6 method. Electronic and IR spectra are calculated by semiempirical methods and compared with experimental one.

Experimental and theoretical approach of photophysical properties of lanthanum(III) and erbium(III) complexes of tris(methoxymethyl)-5-oxine podant.

With the aim of evaluating the coordination behavior of a novel polydentate tripodal ligand, 5-[[3-[(8-hydroxy-5-quinolyl)methoxy]-2-[(8-hydroxy-5-quinolyl)methoxymethyl]-2-methyl propoxy]methyl]quinolin-8-ol (TMOM5OX), towards La(III) and Er(III) metal ions, the detailed investigations of photophysical properties by theoritical and experimental (potentiometric, UV-visible and fluorescence spectrophotometry) methods were carried out. TMOM5OX has been found to form protonated complex [Ln(H4L)](4+) (Ln=La or Er) below pH 3.8, which consecutively deprotonates through one-proton processes with rise of pH. The formation constants (logß) of neutral complexes have been determined to be 36.42 (LaL) and 35.76, 37.62 (for ErL and ErL2, respectively) and the pLn (pLn=-log[Ln(3+)]) values of 24.6 and 27.1 for La(III) and Er(III) ions, respectively, calculated at pH 7.4, indicating TMOM5OX is a good lanthanide synthetic chelator. The absorption spectroscopy of these complexes show marked spectral variations due to characteristic lanthanide transitions, which support the use of TMOM5OX as a sensitive optical pH based sensor to detect Ln(III) metal ions in biological systems. In addition, these complexes have also been shown to exhibit strong green fluorescence allowing simultaneous sensing within the visible region under physiological pH in competitive medium for both La(III) and Er(III) ions. The intense fluorescence from these compounds were revealed to intermittently get quenched under acidic and basic conditions due to the photoinduced intramolecular electron transfer from excited 8-hydroxyquinoline (8-HQ) moiety to metal ion, just an opposite process. This renders these compounds the OFF-ON-OFF type of pH-dependent fluorescent sensors. The complexes coordination geometries were optimized using the sparkle/PM6 model and the theoretical spectrophotometric studies were carried out in order to validate the experimental findings, based on ZINDO/S methodology at configuration interaction with single excitations (CIS) level. These results clearly attest for the efficacy of the theoretical models employed in all calculations and create new interesting possibilities for the design in-silico of novel and highly efficient lanthanide-organic frameworks.

Spectroscopic and pH-metric studies on the complexation of a novel tripodal amine-phenol ligand towards Al(III), Ga(III) and In(III).

The aqueous coordination chemistry of a newly synthesized tripodal amine-phenol ligand, cis,cis-1,3,5-tris{(2-hydroxybenzyl)aminomethyl}cyclohexane (THAC) towards H+, Al3+, Ga3+ and In3+ ions have been investigated in aqueous medium of 0.1 M KCl ionic strength and 25±1 °C by potentiometric and spectrophotometric methods. Three protonation constants assigned to phenolic hydroxyl groups were determined and were used as input data to evaluate the formation constants of the metal complexes. The ligand formed various monomeric metal complex species MLH3, MLH2, MLH, ML and MLH(-1) with Ga(III) and In(III); MLH3, ML and MLH(-1) with Al(III). The calculated stability constants followed the order Ga(III)>In(III)>Al(III), and their pM values at physiological condition were found to be higher than the corresponding transferrin complexes.

Synthesis, spectroscopic and theoretical studies of two novel tripodal imine-phenol ligands and their complexation with Fe(III).

Two novel tripodal imine-phenol ligands, cis,cis-1,3,5-tris{(2-hydroxybenzilidene)aminomethyl}cyclohexane (TMACHSAL, L(1)) and of cis,cis-1,3,5-tris{[(2-hydroxyphenyl)ethylidene]aminomethyl}cyclohexane (Me(3)-TMACHSAL, L(2)) have been synthesized and characterized by elemental analyses and various spectral (UV-vis, IR and (1)H and (13)C NMR) data. The complexation reactions of the ligands with H(+) and Fe(III) were investigated by potentiometric and spectrophotometric methods at an ionic strength of 0.1M KCl and 25 +/- 1 degrees C in aqueous medium. Three protonation constants each for ligands L(1) and L(2) were determined and were used as input data to evaluate the formation constants of the metal complexes. Formations of metal complexes of the types FeLH(3), FeLH(2), FeLH, FeL and FeLH(-1) were depicted in solution. Experimental evidences suggested for a formation of tris(iminophenolate) type metal complex by the ligands. The ligand L(1) showed higher affinity towards iron(III) than L(2). The pFe value related to L(1) (pFe = 20.14) is approximately four units higher than L(2) (pFe = 16.41) at pH = 7.4. The structures of the metal complexes were proposed through the molecular mechanics calculation using MM3 force field followed by semi-empirical PM3 method.

Tripodal amine catechol ligands: a fascinating class of chelators for aluminium(III).

Complexation constants based on potentiometric titrations and spectrophotometric measurements in an aqueous medium of 0.1M KCl at 25+/-1 degrees C for the complexes of Al(III) with multidentate tripodal polycatechol-amine ligands, cis,cis-1,3,5-tris[(2,3-dihydroxybenzylamino)aminomethyl]cyclohexane (TMACHCAT, L(1)) and N(1),N(3),N(5)-tris(2-(2,3-dihydroxybenzylamino)ethyl)cyclohexane-1,3,5-tricarboxamide (CYCOENCAT, L(2)) have been summarized in this paper. Both the ligands released six protons to form various monomeric complexes of the types AlLH(3), AlLH(2), AlLH and AlL (L=L(1) and L(2)). The first species AlLH(3) depicted at low pH for which a monocapped type geometry was suggested, where the ligands were coordinated through three catecholic oxygens at ortho. Other species are formed subsequently from the species AlLH(3) in steps upon deprotonation and coordination of the catecholic oxygens at meta to give encapsulated tris(catechol) type complexes. The probable structures of the metal complexes formed in solution were proposed through molecular modeling calculations. The pAl values calculated for AlL(1) and AlL(2) are appreciably higher than transferrin. The ligand L(2) showed higher affinity towards Al(III) than L(1) and desferrioxamine (DFO), the only approved drug for the treatment of aluminium intoxication.