Chemistry of a series of heterobimetallic complexes MnIII2(CaII/SrII)X2 (X = Cl-, Br-).

This manuscript describes the syntheses, structures and magnetism of MnIII-CaII/SrII complexes which are compositionally relevant in the context of the oxygen evolving complex (OEC) of photosystem II (PS II). A series of trimetallic tetraoxo complexes containing redox-inactive metal ions CaII or SrII were synthesized using a tetranucleating ligand framework. The structural characteristics of these complexes, with the oxido ligands bridging the redox-inactive metals and the manganese centres, make them particularly relevant to biological and heterogeneous metal-oxido clusters. Electrochemical studies of these compounds show that the reduction potentials are highly dependent upon the Lewis acidity of the redox-inactive metal, identifying the chemical basis for the observed differences in electrochemistry. This correlation provides insights into the role of the CaII/SrII ion in modulating the redox potential of the OEC and of other redox-inactive ions in tuning the redox potentials of other metal-oxide electrocatalysts. Temperature dependent magnetic measurements have also been performed for the complexes.

Experimental and Theoretical Exploration of ESIPT in a Systematically Constructed Series of Benzimidazole Based Schiff Base Probes: Application as Chemosensors.

Herein, we have utilized 2-(2-hydroxyphenyl)benzimidazole (HBI) to synthesize 3-(1H-benzoimidazol-2-yl)-2-hydroxy-5-methyl-benzaldehyde (HBIA) followed by three Schiff bases by using -ortho (H2 BIo), -meta (H3 BIdm) and -para (H2 BIp) substituted amino benzoic acids and studied their photophysical properties. We have successfully derived molecular structures of HBI, HBIA and H3 BIdm which reveals that in HBI and HBIA, the phenolic -OH is intramolecularly hydrogen bonded with sp2 N of benzimidazole group whereas in H3 BIdm, it is hydrogen bonded with imine C=N of Schiff base moiety, which is responsible for different solid state emission properties of the reported compounds. Extensive experimental and theoretical studies show that for all three Schiff bases, in solution due to activation of C=N isomerization, ESIPT operates through benzimidazole site and displays different emission from the solid state. Furthermore, H2 BIo, H3 BIdm and H2 BIp selectively sense Cu2+ in semi aqueous medium with nano-molar detection limit and in HuH-7 cells through the inhibition of ESIPT of process.

Turn on Fluorescence Sensing of Zn2+ Based on Fused Isoindole-Imidazole Scaffold.

Optical chemosensors caused a revolution in the field of sensing due to their high specificity, sensitivity, and fast detection features. Imidazole derivatives have offered promising features in the literature as they bear suitable donor/acceptor groups for the selective analytes in the skeleton. In this work, an isoindole-imidazole containing a Schiff base chemosensor (1-{3-[(2-Diethylamino-ethylimino)-methyl]-2-hydroxy-5-methyl-phenyl}-2H-imidazo[5,1-a]isoindole-3,5-dione) was designed and synthesized. The complete sensing phenomena have been investigated by means of UV-Vis, fluorescence, lifetime measurement, FT-IR, NMR and ESI-MS spectroscopic techniques. The optical properties of the synthesized ligand were investigated in 3:7 HEPES buffer:DMSO medium and found to be highly selective and sensitive toward Zn2+ ion through a fluorescence turn-on response with detection limit of 0.073 µm. Furthermore, this response is effective in gel form also. The competition studies reveal that the response of the probe for Zn2+ ion is unaffected by other relevant metal ions. The stoichiometric binding study was performed utilizing Job's method which indicated a 1:1 sensor-Zn2+ ensemble. Computational calculations were performed to pinpoint the mechanism of sensing.

A Quinoxaline-Naphthaldehyde Conjugate for Colorimetric Determination of Copper Ion.

This work facilitates detection of bivalent copper ion by a simple Schiff base probe QNH based on a quinoxaline-naphthaldehyde framework. The detailed study in absorption spectroscopy and theoretical aspects and crystal study of the probe and probe-copper complex has been discussed. The detection limit of the probe in the presence of Cu2+ is 0.45 µM in HEPES-buffer/acetonitrile (3/7, v/v) medium for absorption study. The reversibility of the probe-copper complex has been investigated by EDTA. The selective visual detection of copper has been established also in gel form.

Strategic Substitution of -OH/-NR2 (R=Et, Me) Imparts Colorimetric Switching between F- and Hg2+ by Salicyaldehyde/Benzaldehyde-Quinoxaline Conjugates.

We herein report two salicyaldehyde-quinoxaline (HQS and HQSN) conjugates and a benzaldehyde-quinoxaline (QBN) conjugate to fabricate selective chemosensors for F- and Hg2+ in the micromolar range. This work demonstrates how sensing outcomes are affected by modulating proton acidity by introducing an electron donating group, -NEt2 , in the probe backbone. Interestingly, the un-substituted probe HQS can selectively detect F- , whereas HQSN and QBN are selective for Hg2+ . In order to gain insights into the mechanism of sensing, geometry optimizations have been carried out on QS(-1) , QS(-1) ⋅⋅⋅HF, QSN(-1) and QSN(-1) ⋅⋅⋅HF and the experimental data are validated in terms of free energy and pKa values. Detailed DFT and TD-DFT analyses provide ample support towards the mechanism of sensing of the analytes.

Slow Magnetic Relaxation in a Co2 Dy Trimer and a Co2 Dy2 Tetramer.

The combination of Co(III) and Dy(III) with a compartmental Schiff base ligand (H3 L=3-[(2-Hydroxy-3-methoxy-benzylidene)-amino]-propane-1,2-diol), presenting three different coordinating pockets, has allowed the synthesis of two novel Co(III)-Dy(III) complexes: [Co2 Dy(HL)4 ]NO3 ⋅ 2CH3 CN (1), a rare example of trinuclear linear CoIII 2 DyIII complex (and the first with slow relaxation of magnetization in absence of a DC field) and [Co2 Dy2 (µ3 -OH)2 (HL)2 (OAc)6 ] ⋅ 4.6H2 O (2), the first tetranuclear CoIII 2 DyIII 2 cluster with a rhomb-like structure where the Co(III) ions are connected along the short diagonal of the rhomb. 1 presents two different relaxation processes: a fast relaxation dominated by Quantum tunnelling (QT) and a slow relaxation with an energy barrier of 40 K. 2 shows two close relaxation processes without applied DC fields that follow QT and Orbach mechanisms whereas for HDC =500 Oe, the QT is cancelled and a direct term appears. Here we present the synthesis, X-ray structure and magnetic characterization of these two Co(III)-Dy(III) single-ion/molecule magnets.

Slow magnetic relaxation and water oxidation activity of dinuclear CoIICoIII and unique triangular CoIICoIICoIII mixed-valence complexes.

Construction of efficient multifunctional materials is one of the greatest challenges of our time. We herein report the magnetic and catalytic characterization of dinuclear [CoIIICoII(HL1)2(EtOH)(H2O)]Cl·2H2O (1) and trinuclear [CoIIICoII2(HL2)2(L2)Cl2]·3H2O (2) mixed valence complexes. Relevant structural features of the complexes have been mentioned to correlate with their magnetic and catalytic properties. Unique structural features, especially in terms of significant distortions around the CoII centre(s), prompted us to test both spin-orbit coupling (SOC) and zero field splitting (ZFS) methodologies for the systems. The positive sign of D values has been established from X-band EPR spectra recorded in the 5-40 K temperature range and reaffirmed by CAS/NEVPT2 calculations. ZFS tensors are also extracted for the compounds along with CoIIGaIII and CoIIZnIICoIII model species. Interestingly, 1 shows slow relaxation of magnetization below 6.5 K in the presence of a 1000 Oe external dc field with two relaxation processes (Ueff = 37.0 K with τ0 = 1.57 × 10-8 s for the SR process and Ueff = 7 K with τ0 = 1.66 × 10-6 s for the FR process). As mixed valence cobalt complexes with various nuclearities are central to the quest for water oxidation catalysts, we were prompted to explore their features and to our surprise, water oxidation ability has been realized for both 1 and 2 with significant nuclearity control.

A quick accelerating microwave-assisted sustainable technique: permutated spiro-casing for imaging experiment.

A quick access tool for the one-pot, chromatography-free synthesis of the diversified dihydrospiro[indeno[1,2-b]pyridine-4,3'-indoline or acenaphthylene-1,4'-indeno[1,2-b]pyridine spiro-analogous via sustainable microwave condition in minimal 1:1 (v/v) aqueous ethanol without any metal catalyst is demonstrated here. This permutated spiro-casing was designed as fluorescence probe at physiological pH for selective detection of Zn2+, even in the presence of other competitive ions and showed a fluorescent enhancement with 1:1 metal/ligand complex. Moreover, this spiro sensor was successfully applied as an effective intracellular Zn2+ imaging agent in the biomedical study of human hepatocellular liver carcinoma cells (HepG2) due to its cell permeability property. A quick access technique for the permutated dihydrospiro-pyridine via chromatography-free sustainable microwave condition and its applications as organic fluorescence probe at physiological pH for selective detection of Zn2+ and effective intracellular Zn2+ imaging in HepG2 cells.

A quinoxaline-diaminomaleonitrile conjugate system for colorimetric detection of Cu2+ in 100% aqueous medium: observation of aldehyde to acid transformation.

In this work, we have strategically incorporated a quinoxaline derivative and a diaminomaleonitrile moiety to construct a chemosensor, 2-amino-3-[(quinoxalin-2-ylmethylene)-amino]-but-2-enedinitrile (H2qm). The notable feature of this strategy is to generate a highly conjugated Schiff base platform with interesting binding properties. Remarkably, H2qm exhibited a visual sensing ability towards Cu2+ in 100% aqueous medium. The effectiveness of the chemosensor has been demonstrated by utilizing it to determine the Cu2+ concentration in real samples. Interestingly, the reaction between H2qm and Cu(ClO4)2·6H2O in DMSO yielded a quinoxaline-2-carboxylic acid based compound and single crystal X-ray diffraction analysis unveiled the resulting structure as [(qa)2Cu(H2O)2] (Hqa = quinoxaline-2-carboxylic acid).

Aggregation-Induced Emission-Based Sensing Platform for Selective Detection of Zn2+ : Experimental and Theoretical Investigations.

Fluorescent chemosensors with aggregation induced emission enhancement (AIEE) emerge as promising tools in the field of sensing materials. Herein, we report the design, synthesis and applicability of a Schiff base chemosensor 1-(benzo[1,3]dioxol-4-ylmethylene-hydrazonomethyl)-naphthalen-2-ol (Hbdhn) of AIE characteristics that exhibits highly effective and selective response towards Zn2+ . The sensing effect of Hbdhn was evaluated by means of absorption/emission spectra and corresponding underlying photophysical mechanisms were proposed based on extensive quantum-chemical (TD)DFT calculations. The aggregated states in different DMSO/H2 O ratios and in a presence of Zn2+ were examined by fluorescence lifetime measurements, dynamic light scattering and scanning electron microscopy studies. The bioimaging abilities of Hbdhn were evaluated for Zn2+ in HepG2 cancer cells. The results demonstrate instant, stable in time and reproducible, colorimetric turn-on response with superb selectivity and sensitivity of Hbdhn towards Zn2+ , based on chelation enhanced fluorescence mechanism. AIEE improves further Hbdhn properties, leading to strong, long-lived fluorescence, with appearance of rod-like particles, in 90 % of water in DMSO and only 10 % of water in DMSO in the presence of Zn2+ . All these features combined with successful biomaging studies make Hbdhn one of the most promising candidate for practical applications among recently proposed related systems.

Visual detection of fluoride ion based on ICT mechanism.

A novel chemosensor (H3NTS) has been designed for a fast and accurate detection of F- in acetonitrile medium. The UV-Vis, 1H, 13C NMR experiments and computational calculations demonstrate that H3NTS probe offers a potential colorimetric sensor for discriminating F- among other relevant anions. The detection limit is as low as 6 µM and a reversible colorimetric switch could be devised by sequential addition of F-/Ca2+. The promising application of H3NTS is also confirmed in real examples where the concentration of F- is a critical parameter, e.g., commercial toothpaste, paper strip and gel form analysis of samples.

A versatile quinoxaline derivative serves as a colorimetric sensor for strongly acidic pH.

A facile and reliable method to monitor strongly acidic pH was developed. The sensing mechanism was found to involve the protonation-deprotonation equilibrium of the synthesized probe HQphy (1) (N-phenyl-N'-quinoxalin-2-ylmethylene-hydrazine) within a working range of pH 0.7-2.7. The eventual sensing of Fe3+ was the outcome of acidity imparted by [Fe(H2O)6]3+ ions in solution during the formation of [Fe(H2O)5(OH)]2+. The protonation-deprotonation phenomenon of HQphy was investigated using 1H NMR and single crystal X-ray diffraction experiments. The protonated probe, H2Qphy+, was crystallized with FeCl4-/ClO4- counter anions as [H2Qphy][FeCl4]·H2O (2)/[H2Qphy][ClO4]·H2O (3). A further complex containing the [H2Qphy] cation (4) was also formed. The complexes were characterized by SC-XRD experiments. Moreover, single crystal to single crystal transformation is observed between 1 and 3. In order to understand the sensing mechanism, various analytical studies, such as UV-Vis titration, ESI-MS spectrometry analysis and 1H NMR, were carried out in detail. A theoretical study correlates well with the experimental data, where the π(L) → π*(L) transition of the ligand is red shifted by 100 nm due to protonation of the quinoxaline moiety. The probe enables discrimination of trihalo acetic acid from its mono- and di-analogues.

A versatile chemosensor for the detection of Al3+ and picric acid (PA) in aqueous solution.

Developing chemosensors for efficient detection of Al3+ and picric acid in water is in high demand but challenging. In this paper, we have demonstrated the potential of a probe, 6,6'-(1E,1'E)-(2-hydroxypropane-1,3-diyl)bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(2-methoxyphenol) (H2Vm), as a "switch-on" Al3+ responsive fluorescence chemosensor in water. In addition to the ability of H2Vm to sense Al3+, the Al2-Vm2 complex offers selectivity towards picric acid (PA) in HEPES buffer (pH = 7.4) solution (detection limit (20.13 × 10-9 M) via "switch-off" mode. The reversible fluorescence response with low detection limit (11.34 × 10-9 M) in the pH range of 6.0-9.0 makes H2Vm suitable for tracking Al3+ in live HCT cells. The resulting Al2-Vm2 complex is also responsive towards PA in HCT cells. Thus, a versatile fluorescence spectroscopy-based chemosensor for Al3+ and picric acid has been developed with possible applications in human health and national security.

Dihydroindeno[1,2-b]pyrroles: new Al3+ selective off-on chemosensors for bio-imaging in living HepG2 cells.

In this study, a new molecular organic probe has been designed and synthesized by using recyclable, inexpensive and non-toxic polyethylene glycol (PEG-400) as a promoting reaction medium in water under environmentally benevolent conditions. The probe has been explored as a potential chemosensor to detect Al3+ ions using a HEPES buffer (pH = 7.4) solution. Investigations of the fluorescence behaviour of this sensor in DMSO/H2O (2 : 8, v/v) solution displayed a dramatic switch-on response only in the presence of Al3+, while other metal ions, like Li+, Na+, K+, Ag+, Ca2+, Mg2+, Mn2+, Ba2+, Cu2+, Ni2+, Co2+, Fe2+, Zn2+, Cd2+, Hg2+, Pb2+, Sr2+, Fe3+ or Cr3+, have almost no influence on the fluorescence behaviour. Various common anions, such as ClO4-, Cl-, or NO3- in the form of Al3+ salts [e.g. Al(ClO4)3, AlCl3 or Al(NO3)3], had no influence on the fluorescence behaviour of the sensors. The detection limit for Al3+ is in the order of 10-6 M in DMSO/H2O (2 : 8, v/v) HEPES buffer (pH = 7.4) solution. Notably, this is the first report of a dihydroindeno[1,2-b]pyrrole moiety acting as a sensor for the selective detection of Al3+ ions through an off-on fluorescence response. The potential of the probe was also confirmed by employing it for fluorescence bio-imaging with Al3+ on HepG2 cells.

I2 catalyzed access of spiro[indoline-3,4'-pyridine] appended amine dyad: new ON-OFF chemosensors for Cu2+ and imaging in living cells.

An easy access to an amine-appended spiro[indoline-3,4'-pyridine] ON-OFF chemosensor by a one-pot four-component reaction using commercially available and an environmentally benign catalytic amount of molecular I2 (10 mol%) in aqueous ethanol at ambient temperature is described. The generated system could be utilized for the selective detection of Cu2+ as it demonstrated a colorimetric naked eye change along with an ON-OFF fluorescence response towards Cu2+ at physiological pH. The sensors exhibited high selectivity for Cu2+ over other common cations with detection limit in the range of 10-7 (M). Notably, this is the first report of a spiro[indoline-3,4'-pyridine] moiety acting as a sensor for Cu2+via a on-off fluorescence response. In addition, the probe system was successfully applied for imaging Cu2+ in human hepatocellular liver carcinoma cells (HepG2), demonstrating a new avenue for molecular imaging and biomedical applications.

A Schiff base platform: structures, sensing of Zn(ii) and PPi in aqueous medium and anticancer activity.

A reaction of N1,N3-bis(3-methoxysalicylidene) diethylenetriamine (H2Vd) and Zn(NO3)2·6H2O, ZnBr2, ZnI2 and Cd(NO3)2·4H2O in a methanol solution led to zinc and cadmium complexes of different nuclearities, [Zn2(Vd·H)2(X)2]·CH3OH (X = NO3, Br, I) [1a, 1b and 1c] and Cd3(Vd)2(NO3)2 (2). In 1(a-c), two H2Vd ligands bridge the two metal centers whereas in 2, they provide sideways support to two terminal Cd2+ ions, providing an all-oxygen envelope to the central Cd2+ ion. All four compounds were characterized by elemental analysis, FT-IR spectroscopy and single crystal X-ray diffraction analysis. Complexes 1(a-c) exhibit dinuclear structures, whereas 2 exhibits a nearly linear trinuclear structure. The structural differences among these complexes are attributable to various coordination modes and flexible configurations of the H2Vd ligand. The ligand H2Vd is an excellent probe for sensing Zn2+ in solution, whereas complexes 1(a-c) are able to selectively detect pyrophosphate (PPi) in aqueous medium. The structure of the pyrophosphate (PPi) complex has been proposed using DFT calculations and the selectivity is due to the unique ability of this anion to simultaneously coordinate to both the Zn metal centers. The anticancer activity of complexes 1(a-c) was also explored.

CoII4, CoII7, and a Series of CoII2LnIII (LnIII = NdIII, SmIII, GdIII, TbIII, DyIII) Coordination Clusters: Search for Single Molecule Magnets.

We report herein the syntheses and investigation of the magnetic properties of a CoII4 compound, a series of trinuclear CoII2LnIII (LnIII = NdIII, SmIII, GdIII, TbIII, DyIII) complexes, and a CoII7 complex. The homometallic CoII4 core was obtained from the reaction of Ln(NO3)3·xH2O/Co(NO3)2·6H2O/H2vab/Et3N in a 0.5:0.5:1:2 ratio in methanol. Variation in synthetic conditions was necessary to get the desired CoII-LnIII complexes. The CoII-LnIII assembly was synthesized from Ln(NO3)3·xH2O/Co(OAc)2·4H2O/H2vab/NaOMe in a 0.4:0.5:1:1 ratio in methanol. The isostructural CoII2LnIII complexes have a core structure with the general formula [Co2Ln(Hvab)4(NO3)](NO3)2·MeOH·H2O, (where H2vab = 2-[(2-hydroxymethyl-phenylimino)-methyl]-6-methoxy-phenol) with simultaneous crystallization of CoII7 complex in each reaction. The magnetic investigation of these complexes reveals that both homometallic complexes and four CoII-LnIII complexes (except CoII-NdIII) display behavior characteristic of single molecule magnets.

Heterometallic Cu(II)-Dy(III) Clusters of Different Nuclearities with Slow Magnetic Relaxation.

The synthesis, structures, and magnetic properties of two heterometallic Cu(II)-Dy(III) clusters are reported. The first structural motif displays a pentanuclear Cu(II)4Dy(III) core, while the second one reveals a nonanuclear Cu(II)6Dy(III)3 core. We employed o-vanillin-based Schiff base ligands combining o-vanillin with 3-amino-1-propanol, H2vap, (2-[(3-hydroxy-propylimino)-methyl]-6-methoxy-phenol), and 2-aminoethanol, H2vae, (2-[(3-hydroxy-ethylimino)-methyl]-6-methoxy-phenol). The differing nuclearities of the two clusters stem from the choice of imino alcohol arm in the Schiff bases, H2vap and H2vae. This work is aimed at broadening the diversity of Cu(II)-Dy(III) clusters and to perceive the consequence of changing the length of the alcohol arm on the nuclearity of the cluster, providing valuable insight into promising future synthetic directions. The underlying topological entity of the pentanuclear Cu4Dy cluster is reported for the first time. The investigation of magnetic behaviors of 1 and 2 below 2 K reveals slow magnetic relaxation with a significant influence coming from the variation of the alcohol arm affecting the nature of magnetic interactions.

Pyridoxal Based Fluorescent Chemosensor for Detection of Copper(II) in Solution With Moderate Selectivity and Live Cell Imaging.

A pyridoxal-based fluorescent probe HL was synthesized for the detection of Cu(2+) in methanol with moderate selectivity. Upon addition of Cu(2+), to the solution of the probe in methanol exhibited a remarkable change in emission at 500 nm. With the limit of detection of 10 µM, the probe could well meet the recommended (less than 32 µM in drinking water) of the World Health Organization (WHO). The intracellular Cu(2+) imaging behaviour of HL was carried out on HeLa cells.

Doubly chloro bridged dimeric copper(II) complex: magneto-structural correlation and anticancer activity.

We have synthesized and structurally characterized a new doubly chloro bridged dimeric copper(II) complex, [Cu2(µ-Cl)2(HL)2Cl2] (1) based on a Schiff base ligand, 5-[(pyridin-2-ylmethylene)-amino]-pentan-1-ol). Single crystal X-ray diffraction shows the presence of dinuclear copper(II) centres in a square pyramidal geometry linked by obtuse double chloro bridge. The magnetic study illustrated that weak antiferromagnetic interactions (J = -0.47 cm(-1)) prevail in complex 1 which is well supported by magneto-structural correlation. This compound adds to the library of doubly chloro bridged copper(ii) complexes in the regime of spin state cross over. DFT calculations have been conducted within a broken-symmetry (BS) framework to investigate the exchange interaction further which depicts that the approximate spin projection technique yields the best corroboration of the experimental J value. Spin density plots show the presence of an ∼0.52e charge residing on the copper atom along with a substantial charge on bridging and peripheral chlorine atoms. The potential of complex1 to act as an anticancer agent is thoroughly examined on a series of liver cancer cell lines and screening shows the HepG2 cell line exhibits maximum cytotoxicity by phosphatidyl serine exposure in the outer cell membrane associated with ROS generation and mitochondrial depolarization with increasing time in the in vitro model system.