NMR Titration Studies Research Articles

Spectroscopic and TDDFT investigation on highly selective fluorogenic chemosensor and construction of molecular logic gates

1,5-Bis(2-fluorene)thiocarbohydrazone (FBTC) was designed and synthesized for selective sensing of fluoride and copper ions. The binding constants of FBTC towards fluoride and copper ions have been calculated using the Benesi-Hildebrand equation, and FBTC has more binding affinity towards copper ion than fluoride ion. The 1H NMR and 13C NMR titration studies strongly support the deprotonation was taken from the N–H protons followed by the formation of hydrogen bond via N–H…F. To understand the fluoride ion sensing mechanism, theoretical investigation had been carried out using the density functional theory and time-dependent density functional theory. The theoretical data well reproduced the experimental results. The deprotonation process has a moderate transition barrier (481.55kcal/mol). The calculated ΔE and ΔG values (−253.92 and −192.41kcal/mol respectively) suggest the feasibility of sensing process. The potential energy curves give the optimized structures of FBTC-F complex in the ground state and excited state, which states the proton transition occurs at the excited state. The excited state proton transition mechanism was further confirmed with natural bond orbital analysis. The reversibility of the sensor was monitored by the alternate addition of F− and Cu2+ ions, which was explained with “Read-Erase-Write-Read” behaviour. The multi-ion detection of sensor used to construct the molecular logic gate, such as AND, OR, NOR and INHIBITION logic gates.

Selective Sensing of Phosphates by a New Bis-heteroleptic RuII Complex through Halogen Bonding: A Superior Sensor over Its Hydrogen-Bonding Analogue.

The selective phosphate-sensing property of a bis-heteroleptic RuII complex, 1[PF6 ]2 , which has a halogen-bonding iodotriazole unit, is demonstrated and is shown to be superior to its hydrogen-bonding analogue, 2[PF6 ]2 . Complex 1[PF6 ]2 , exploiting halogen-bonding interactions, shows enhanced phosphate recognition in both acetonitrile and aqueous acetonitrile compared with its hydrogen-bonding analogue, owing to considerable amplification of the RuII -center-based metal-to-ligand charge transfer (MLCT) emission response and luminescence lifetime. Detailed solution-state studies reveal a higher association constant, lower limit of detection, and greater change in lifetime for complex 1 in the presence of phosphates compared with its hydrogen-bonding analogue, complex 2. The 1 H NMR titration study with H2 PO4- ascertains that the binding of H2 PO4- occurs exclusively through halogen-bonding or hydrogen-bonding interactions in complexes 1[PF6 ]2 and 2[PF6 ]2 , respectively. Importantly, the single-crystal X-ray structure confirms the first ever report on metal-assisted second-sphere recognition of H2 PO4- and H2 P2 O72- with 1 through a solitary C-I⋅⋅⋅anion halogen-bonding interaction.

ChemInform Abstract: Hydrogen‐Bonding Catalysis of Tetraalkylammonium Salts in an Aza‐Diels—Alder Reaction

Abstractof imines and Danishefsky diene (I)

Experimental and theoretical studies of novel hydroxynaphthalene based chemosensor, and construction of molecular logic gates

2-Hydroxynaphthalenyl-N(4)-cyclohexyl thiosemicarbazone (CHNT) was synthesized and characterized for selective sensing of fluoride, cyanide and copper ions. The interaction between CHNT with fluoride, cyanide and copper ions have been investigated through their absorption, fluorescence and electrochemical behavior, and binding constants of corresponding ions have been calculated, which is in order of Cu2+<CN−<F−. The 1H NMR titration study strongly support the deprotonation was takes place from the N2–H8 proton which is near to the naphthalene group. The fluoride and copper sensing mechanism of CHNT has been investigated by DFT and TDDFT methods The theoretical results indicate that the proton of the N2−H in thiosemicarbazone group is captured by the added fluoride ion and then deprotonated in excited state. The excited state proton transition mechanism was further confirmed with NBO and PES analysis. The CHNT found good reversibility character with the alternative addition of Ca2+ and F−. The multi ion detection of CHNT was used to construct the NOR, XNOR and NAND molecular logic gates.

P-tert-Butylcalix[4]arene core based ferrocenyl dendrimers: Novel sensor for toxic Hg2+ ion even in presence of Zn2+, Cu2+ and Ag+ ions

A new class of p-tert-butylcalix[4]arene cored triazolyl ferrocene conjugates 1–3 has been synthesized in good yield using copper(I) catalyzed alkyne azide cycloaddition (CuAAC). In UV–vis spectrum the absorption intensity increases as the dendrimer generation increases though λmax remains almost constant for all the synthesized dendrimers. All the synthesized calixarene conjugated ferrocenyl dendrimers 1, 2 and 3 sense toxic Hg2+ ion at the lowest concentration of 2×10−5M even in the presence of Zn2+, Cu2+, Ag+ as revealed by Cyclic Voltammetric and through NMR titration studies.

Endocyclic and Endo–Exocyclic Silver(I) Complexes of Thiaoxaaza Macrocycles: Crystallographic and NMR Studies

A comparative investigation of the coordination behaviour of a 19-membered NO2S2-macrocycle (L1) and a 20-membered NO2S3-macrocycle (L2) is reported. On silver(i) complexation, L1 yields a discrete endocyclic mononuclear complex, whereas L2 affords a one-dimensional coordination polymer in which the endo-coordinated macrocyclic complex units are linked by silver(i) ions outside the cavity via endo–exo-coordination. The larger ring cavity along with the flexible nature of L2 may induce the weaker endocyclic complexation and contribute to exocyclic coordination as well. In NMR titration studies for the corresponding complex system, however, no evidence of exocyclic coordination was observed. Instead, L1 and L2 form a stable endocyclic 1 : 1 silver(i) nitrate complex, with a higher affinity for the former ligand.

Anion Recognition by Aliphatic Helical Oligoureas.

Anion binding properties of neutral helical foldamers consisting of urea type units in their backbone have been investigated. 1 H NMR titration studies in various organic solvents including DMSO suggest that the interaction between aliphatic oligoureas and anions (CH3 COO- , H2 PO4- , Cl- ) is site-specific, as it largely involves the urea NHs located at the terminal end of the helix (positive pole of the helix), which do not participate to the helical intramolecular hydrogen-bonding network. This mode of binding parallels that found in proteins in which anion-binding sites are frequently found at the N-terminus of an α-helix. 1 H NMR studies suggest that the helix of oligoureas remains largely folded upon anion binding, even in the presence of a large excess of the anion. This study points to potentially useful applications of oligourea helices for the selective recognition of small guest molecules.

Molecular and Nanoaggregation in Cyclometalated Iridium(III) Complexes through Structural Modification

New terpyridyl ligands TP1, TP2 and cyclometalated iridium(III) complexes 1 and 2 based on these ligands have been synthesized. The ligands and complexes have been characterized by elemental analysis and spectroscopic studies (ESI‐MS, 1H and 13C NMR, UV/Vis, fluorescence). The molecular structure of 1 has been verified by X‐ray single‐crystal analysis. It has been unambiguously established that variation of the substituents on 1 and 2 leads to molecular aggregation in 1, while 2 remains nonaggregated. Furthermore, complexes 1 and 2 have been successfully utilized as capping agents for the stabilization of gold nanoparticles (AuNPs). It is of note that 1 forms discretely, while 2 aggregates AuNPs through the assemblage of ultrasmall nanoparticles. It has been affirmed by 1H NMR titration studies that –NH groups from 1 and 2 are involved in the capping of AuNPs. The role of simple structural variations in directing molecular and nanoaggregation has been clearly established for the first time by spectroscopic (UV/Vis, fluorescence, 1H NMR titration) and morphological studies [SEM, TEM, EDX (energy‐dispersive X‐ray), DLS (dynamic light scattering)].

Colorimetric Naphthalene‐Based Thiosemicarbazide Anion Chemosensors with an Internal Charge Transfer Mechanism

A series of thiosemicarbazide anion chemosensors substituted with naphthalene and 4‐nitrophenyl or phenyl units were synthesized. The molecules were characterized by using 1H,13C DEPT and 15N NMR spectroscopy. The anion binding properties of compounds 1–4 were investigated by UV/Vis absorption and fluorescence spectroscopy in DMSO, DMSO/H2O (9:1 v/v), and acetonitrile with hydroxide, fluoride, acetate, hypophosphate, and chloride anions. Striking color changes were observed with nitro‐containing chemosensors 2–4 with all anions, with the exception of chloride, which was attributed to an internal charge‐transfer mechanism. 1H/13C/15N NMR titration studies and DFT and charge distribution calculations support a mechanism involving deprotonation of the central N–H bond. Compound 4 with a methylene spacer between the naphthalene and thiosemicarbazide moieties was deliberately designed as a fluorescent photoinduced electron‐transfer anion (PET) sensor. However, the methylene spacer did not prevent delocalization of the HOMO orbital between the planar aromatic ring and the thiosemicarbazide unit as expected on the basis of the “fluorophore–spacer–receptor” PET model.

Structurally simple azo based chromogenic R1 for the selective sensing of cyanide ion in aqueous medium

Structurally simple azo linked Schiff base R1 was synthesized and characterized using IR, 1H NMR and mass spectroscopic techniques. The selective sensing of cyanide anion over other anions such as fluoride, chloride, bromine, iodide, acetate, nitrate, hydrogen sulphate and dihydrogenphosphate was revealed by naked-eye color change and UV–vis spectra. Addition of CN− ions to the R1 showed a dramatic change in color from pale yellow to intense yellow along with the formation of a new band at 415nm in the absorption spectrum even in 100% aqueous medium. The interaction between R1 and cyanide was also investigated using 1H NMR titration study. The R1 showed limit of detection (LOD) up to micromolar level at 2.72×10−6M. Job's plot reveals the existing binding mode of R1 and CN− ion in molar ratio of 1:1 (R1:CN−).

Hydrogen-Bonding Catalysis of Tetraalkylammonium Salts in an Aza-Diels-Alder Reaction.

A piperidine-derived tetraalkylammonium salt with a non-coordinating counteranion worked as an effective hydrogen-bonding catalyst in an aza-Diels-Alder reaction of imines and a Danishefsky diene. The hydrogen-bonding interaction between the ammonium salt and an imine was observed as part of a (1) H NMR titration study.

Design and synthesis of a new organic receptor and evaluation of colorimetric anion sensing ability in organo-aqueous medium

A new organic receptor has been designed and synthesized by the combination of aromatic dialdehyde with nitro-substituted aminophenol resulting in a Schiff base compound. The receptor exhibited a colorimetric response for F− and AcO− ion with a distinct color change from pale yellow to red and pink respectively in dry DMSO solvent and yellow to pale greenish yellow in DMSO:H2O (9:1, v/v). UV–Vis titration studies displayed a significant shift in absorption maxima in comparison with the free receptor. The shift could be attributed to the hydrogen bonding interactions between the active anions and the hydroxyl functionality aided by the electron withdrawing nitro substituent on the receptor. 1H NMR titration and density functionality studies have been performed to understand the nature of interaction of receptor and anions. The lower detection limit of 1.12ppm was obtained in organic media for F− ion confirming the real time application of the receptor.

Pd3 cluster catalysis: Compelling evidence from in operando spectroscopic, kinetic, and density functional theory studies

Identification of metal cluster catalysis is a topic that is being investigated since a long time. Here, we report a Pd3 metal cluster catalytic reaction investigated by means of operando studies. We discovered that atomically defined tri-nuclear palladium (Pd3) is a surprisingly active catalyst for the cycloisomerization of 2-phenylethynylaniline. Operando 1H NMR spectroscopy and X-ray extended absorption fine structure (EXAFS) measurements have indicated that the structural integrity of such a catalyst remains intact throughout the reaction, which has also been confirmed by an ex situ X-ray photoelectron spectroscopy (XPS) study and catalyst recycling experiments. Kinetic data derived from operando IR spectroscopy measurements have shown that Pd3 is the active catalytic species. Density functional theory calculations have revealed a reaction pathway consistent with the kinetic data, further supported by NMR titration and X-ray crystal structure studies. Overall, the present study presents a clear example of metal cluster catalysis.

Reinvestigation of the photoreaction of 1,4-bis(2,4,6-triphenylpyridinio)benzene: Synthesis of a diazonia derivative of hexabenzoperylene by multiple photocyclization

Abstract The photo-induced oxidative cyclization of the 1,1′-p-phenylene-bis(2,4,6-triphenylpyridinium) was reinvestigated. While mono-cyclization was claimed to be the photoproduct in a previous report, it was found that the diazonia derivative of hexabenzoperylene can be formed as the final photoproduct by four-fold cyclization. The photoreaction affords one of the two possible stereoisomers (twisted and anti forms), and the stereostructure of the product is assigned to the twisted form by 1H NMR titration study with a chiral complexing reagent and by analysis of the absorption spectrum using TD-DFT calculations. The diazonia derivative of hexabenzoperylene exhibits yellow and red luminescence in the solution and solid states respectively and possesses well-separated four-reversible one-electron reduction potentials.

Off–On–Off fluorescence behavior of an intramolecular charge transfer probe toward anions and CO2

The photophysical behavior of a newly developed fluorescent probe, tricyanoethylphenyl phenanthroimidazole (TCPPI) has been studied. Upon interaction of different class of anions TCPPI displayed naked–eye sensitive fluorescence “turn–on” response to detect selectively F− (0.98μM, 18.62ppb) and CN− (1.12μM, 29.12ppb) anions in acetonitrile (MeCN). Job's plot analysis revealed a 1:1 binding stoichiometry between probe and anions. The spectral data analysis and 1H NMR titration studies suggested about the affinity of F− and CN− anions with moderately acidic −NH fragment of imidazolyl unit of probe through deprotonation and H-bonding interaction. Moreover, the anion activated probe upon interaction with CO2 revived photophysical properties of probe, “On-Off-On” type fluorescence and enabled anion-induced CO2 sensing in the medium.

Spectroscopic and TD-DFT studies on the turn-off fluorescent chemosensor based on anthraldehyde N(4) cyclohexyl thiosemicarbazone for the selective recognition of fluoride and copper ions

The copper and fluoride ions sensing mechanism of a chemosensor based on anthraldehyde N(4) cyclohexyl thiosemicarbazone (AntCy) was investigated via colorimetric, fluorescence, electrochemical and NMR titration studies. Detailed investigations on their sensing mechanism was done using DFT and TD-DFT studies. 1H NMR titration shows deprotonation of NH protons by fluoride ion is a prominent step in naked eye recognition. The Gibbs free energy of overall sensing reaction has moderate transition barrier with 18.19kcalmol−1. Using the vibrational frequency analysis, all the local minima of ground state and excited state were confirmed. Due to the small size and strong electronegativity of fluoride, an intramolecular hydrogen bonding interaction with N(3)–H, which is closer to anthracene moiety was found to be preferentially formed. The excited state proton transfer mechanism was further confirmed with donor–acceptor interactions using Natural Bond Orbital (NBO) analysis and Potential Energy Surface (PES) analysis. The ICT mechanism for copper ion sensing was also confirmed with TD-DFT calculations.

Regulation of Mammalian Dynein Intermediate Chain

Cytoplasmic dynein is a protein complex responsible for transporting cellular cargos on microtubules. Dynein's activities are regulated by other proteins including dynactin which mediates dynein-cargo interactions and increases dynein's processivity. The intermediate chain (IC) of dynein binds to dynactin p150Glued subunit, an interaction central to dynein regulation. The N-terminal domain of IC, N-IC is partially disordered, and contains binding site for a coiled-coil domain of p150Glued and a serine-rich region known to undergo phosphorylation in vivo. There are conflicting results to the effect of phosphorylation on dynactin binding. Using different techniques, one group showed phosphorylated S84 and a phosphomimetic mutant S84D abolished p150Glued binding in vitro while another group demonstrated S84D was still able to bind. We use mutagenesis, constructs of varying length, isothermal titration calorimetry (ITC) and NMR spectroscopy to identify the effect of phosphomimetic S84D on structure of IC and its ability to bind p150Glued. Backbone assignments of IC2C1-96 and secondary chemical shifts show residues 4-38 (helix 1) and 52-66 (helix 2) adopt helical structures and the rest of the protein is disordered. NMR and ITC titration studies identify the first 44 residues as sufficient for binding. The phosphomimetic mutant IC2CS84D abolishes p150Glued binding, without causing major changes in structure and dynamics as detected from NMR measurements. Chemical shift comparison between WT and S84D shows long-range effects on residues in the linker between helix 1 and 2, suggesting conformational changes in the linker. Mutations in the linker region restore p150Glued binding. Together, these results demonstrate for the first time phosphorylation in a disordered region affects binding at a distant site in the sequence, and is fine-tuned by changes in a disordered linker separating two structured domains. This regulatory control of dynein-dynactin interactions is key to cargo- and temporal-specific alterations in dynein transport.

Synthesis and spectroscopic studies of axially bound tetra(phenothiazinyl)/tetra(bis(4′-tert-butylbiphenyl-4-yl)aniline)-zinc(II)porphyrin-fullero[C60 & C70]pyrrolidine donor–acceptor triads

Two zinc porphyrin based dyads, 5, 10, 15, 20-tetra-4-(2-(10H-phenothiazin-10-yl)ethoxy)phenyl porphyrinato zinc (II) and 5,10,15,20-tetra-4-(bis(4′-tert-butylbiphenyl-4-yl)amino)phenyl porphyrinato zinc(II) containing four phenothiazine or bis(4′-tert-butylbiphenyl-4-yl)aniline moieties tethered to porphyrin framework were synthesized, and systematically characterized using 1H NMR, mass, UV–visible and steady state fluorescence techniques. Excitation of 5,10,15,20-tetra-4-(bis(4′-tert-butylbiphenyl-4-yl)amino)phenyl porphyrinato zinc(II) at 335nm, resulted in quenched emission of bis(4′-tert-butylbiphenyl-4-yl)aniline moiety, when compared to pristine bis(4′-tert-butylbiphenyl-4-yl)aniline, and appearance of porphyrin emission at 618nm, due to intramolecular energy transfer from bis(4′-tert-butylbiphenyl-4-yl)aniline moiety to porphyrin. The porphyrin dyads were titrated with phenylimidazole appended fullerenes, N-Methyl-4-(1H-imidazol-1-yl)phenylfullero[C60]pyrrolidine or N-Methyl-4-(1H-imidazol-1-yl)phenylfullero[C70]pyrrolidine and supramolecular triads of the type, (PTZ or BBA)4-ZnP:(C60Im or C70Im) were formed in which, zinc porphyrin and fullerenes act as primary electron donor and acceptors, while phenothiazine or bis(4′-tert-butylbiphenyl-4-yl)aniline moieties behave as secondary electron donors in photo-induced electron transfer events. Optical absorption and 1H NMR titration studies confirmed the formation of supramolecular triads and the steady state fluorescence experiments revealed the quenching of porphyrin emission upon addition of increasing amounts of N-Methyl-4-(1H-imidazol-1-yl)phenylfullero[C60]pyrrolidine or N-Methyl-4-(1H-imidazol-1-yl)phenylfullero[C70]pyrrolidine indicating the occurrence of photo-induced electron transfer from 1ZnP⁎ to co-ordinated fullerene moieties.

Conjugated, trans‐Spanning Ligands as Models for Multivalent p‐Phenyleneethynylenes

AbstractA conjugated, pyridine‐containing, phenylethynyl ligand that forms complexes with AgI and PdII has been developed. NMR titration studies with PdII reveal a stoichiometric binding of the ligand to the metal atom, while similar studies with AgI reveal a binding that is dynamic on the NMR timescale. Analysis of the NMR spectroscopic data by Job's plot analysis and non‐linear curve fitting of a titration curve reveals a 1:1 binding ratio of ligand/silver cation and an association constant of Ka = 53 M–1. X‐ray crystal structures of the ligand–metal complexes suggest ample room for the nearly barrierless rotation of the unsubstituted central benzene ring of the para‐phenylethynyl chain. Subtle electronic differences in substituted systems provide some evidence of impaired rotation.

A Cyanuric Acid Platform Based Tripodal Bis-heteroleptic Ru(II) Complex of Click Generated Ligand for Selective Sensing of Phosphates via C-H···Anion Interaction.

A new bis-heteroleptic trinuclear Ru(II) complex (1[PF6]6) has been synthesized from electron deficient cyanuric acid platform based copper-catalyzed azide-alkyne cycloaddition, i.e., CuAAC click generated ligand, 1,3,5-tris [(2-aminoethyl-1H-1,2,3-triazol-4-yl)-pyridine]-1,3,5-triazinane-2,4,6-trione (L1). Complex 1[PF6]6 displays weak luminescence (ϕf = 0.002) at room temperature with a short lifetime of ∼5 ns in acetonitrile. It shows selective sensing of hydrogen pyrophosphate (HP2O7(3-)) through 20-fold enhanced emission intensity (ϕf = 0.039) with a 15 nm red shift in emission maxima even in the presence of a large excess of various competitive anions like F(-), Cl(-), AcO(-), BzO(-), NO3(-), HCO3(-), HSO4(-), HO(-), and H2PO4(-) in acetonitrile. Selective change in the decay profile as well as in the lifetime of 1[PF6]6 in the presence of HP2O7(3-) (108 ns) further supports its selectivity toward HP2O7(3-). UV-vis and photoluminescence titration profiles and corresponding Job's plot analyses suggest 1:3 host-guest stoichiometric binding between 1[PF6]6 and HP2O7(3-). High emission enhancement of 1[PF6]6 in the presence of HP2O7(3-) has resulted in the detection limit of the anion being as low as 0.02 μM. However, 1[PF6]6 shows selectivity toward higher analogues of phosphates (e.g., ATP, ADP, and AMP) over HP2O7(3-)/H2PO4(-) in 10% Tris HCl buffer (10 mM)/acetonitrile medium. Downfield shifting of the triazole C-H in a (1)H NMR titration study confirms that the binding of HP2O7(3-)/H2PO4(-) is occurring via C-H···anion interaction. The single crystal X-ray structure of complex 1 having NO3(-) counteranion, 1[NO3]6 shows binding of NO3(-) with complex 1 via C-H···NO3(-) interactions.