Acidic NH Protons Research Articles

Makromolekulare Hydrazon Hauptkettenphotoschalter

AbstractHydrazone – bestehend aus einer dynamischen Iminbindung und einem sauren NH‐Proton – haben sich in letzter Zeit aufgrund ihrer Fähigkeit zur Bildung thermisch bistabiler (Z) bzw. (E) Isomere als vielseitige Potoschalter erwiesen. In diesem Artikel stellen wir zwei lichtsensitive Homopolymere vor, die strukturell unterschiedliche Hydrazone als sich wiederholende Einheiten in der Hauptkette enthalten und durch „Kopf‐Schwanz“‐Polymerisation mittels azyklischer Dien‐METathese (ADMET) synthetisiert wurden. Der Hauptunterschied der beiden Polymere liegt dabei im Design des Hydrazon‐Photoschalters, insbesondere in der Lage des aliphatischen Arms, der den Rotor mit dem aliphatischen Polymerrückgrat verbindet. Es zeigt sich, dass sich das hydrodynamische Volumen der Polymere in verdünnter Lösung während der Photoisomerisierung (λ=410 nm) in entgegengesetzter Weise ändert. Darüber hinaus zeigt sich nach der Photoisomerisierung eine Veränderung der Glasübergangs‐temperatur (Tg) um fast 10 °C bei beiden Polymeren, unabhängig vom Design des jeweiligen Hydrazon‐Monomers. Die hier vorgestellte Designstrategie erlaubt es, makromolekulare Eigenschaften durch Strukturänderungen photochemisch zu kontrollieren.

Main-chain Macromolecular Hydrazone Photoswitches.

Hydrazones-consisting of a dynamic imine bond and an acidic NH proton-have recently emerged as versatile photoswitches underpinned by their ability to form thermally bistable isomers, (Z) and (E), respectively. Herein, we introduce two photoresponsive homopolymers containing structurally different hydrazones as main-chain repeating units, synthesized via head-to-tail Acyclic Diene METathesis (ADMET) polymerization. Their key difference lies in the hydrazone design, specifically the location of the aliphatic arm connecting the rotor of the hydrazone photoswitch to the aliphatic polymer backbone. Critically, we demonstrate that their main photoresponsive property, i.e., their hydrodynamic volume, changes in opposite directions upon photoisomerization (λ=410 nm) in dilute solution. Further, the polymers-independent of the design of the individual hydrazone monomer-feature a photoswitchable glass transition temperature (Tg ) by close to 10 °C. The herein established design strategy allows to photochemically manipulate macromolecular properties by simple structural changes.

Anion and Temperature Responsive Molecular Switches Based on Trimetallic Complexes of Ru(II) and Os(II) That Demonstrate Advanced Boolean and Fuzzy Logic Functions.

Anion- and temperature-induced alteration of the photoredox behaviors of our recently reported trimetallic complexes was carried out to fabricate potential molecular switches. The triads [(phen)2Ru(d-HIm-t)M(t-HIm-d)Ru(phen)2]6+ (phen = 1,10-phenanthroline, d-HIm-t = heterotopic bipyridine-terpyridne type bridging ligand, and M = RuII and OsII) possess two acidic imidazole NH protons that upon interaction with anions give rise to considerable changes in their photophysical and electrochemical behaviors. Red-shifts of the absorption and emission maximum and a decrease in the M3+/M2+ potential occur when the triads are treated with basic anions. In fact, the triads can function as triple-channel sensors for F-, AcO-, CN-, OH-, and H2PO4- in acetonitrile and as selective probes for CN- and SCN- in water. The equilibrium constants for the receptor-anion interaction are on the order of 106 M-1, while the limit of detection was on the order of 10-9 M. Temperature plays a key role in the luminescence properties of the triads by adjusting the energy barrier between the emitting triplet metal-to-ligand charge transfer and non-emitting triplet metal-centered levels. A decrease in temperature leads to increases in the emission intensity and lifetime of the triads displaying the "on state", whereas an increase in temperature leads to a decrease in their emission characteristics and thus indicates the "off state" and that the process is fully reversible. In essence, the triads could function as potential switches on the basis of the reversible change in their spectral and redox behaviors under the influence of anion, acid, and temperature. The most important outcome of this study is mimicking several advanced Boolean and fuzzy logic functions by utilizing the spectral response of the triads upon the action of said external stimuli.

Anion- and solvent induced modulation of photophysical properties of a luminescent bimetallic Ru(II) complex: Experimental and TD-DFT study

Anion- and solvent induced modulation of phtophysical behavior of a binuclear Ru(II) complex of the type [(bpy)2Ru(H2L)Ru(bpy)2]4+, where H2L = 2,5-bis[1,10]phenanthroline[4,5-f]-imidazol-2-yl)thiophene} and bpy = 2,2′-bipyridine was thoroughly investigated in this work. The complex shows strong room temperature emission with excited state lifetime varying between 122 and 1089 ns, depending upon the nature of the solvent. The complex possesses two imidazole NH protons in its second coordination sphere. These two acidic NH protons were subjected to successive deprotonation upon increasing the pH of the solution and pKa values were determined in both water and MeCN. Taking advantage of these acidic NH protons, anion sensing behavior of the complex was thoroughly investigated in both acetonitrile and water through multiple optical channels. The complex behaves as sensor for F−, CN−, AcO-and H2PO4- in acetonitrile without much selectivity, while selectivity dramatically increases in water where it exclusively sense CN−. In presence of excess anions, deprotonation of the imidazole polarized N–H fragments takes place which is also signaled by its change of color from yellow to orange visible to the naked eye. Absorption andemission titration data were used to calculate the binding/equilibrium constants of the interaction between the receptor and the anions as well as the detection limit of the receptor towards selected anions. Interestingly, substantial modulation of the lifetime also makes the complex a lifetime-based sensor for selective anions.In conjunction with the experimental investigations, detailed density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were also performed and a good correlation between the experimental and the TD-DFT calculated results allowed us to provide a detailed assignment of the main absorption and emission spectral features of the complex.

β-carboline-based turn-on fluorescence chemosensor for quantitative detection of fluoride at PPB level.

A novel β-carboline-based chemosensor, having an acidic NH proton that leads to fluoride-induced deprotonation involving a vivid color change from colorless to yellow is described. The absorption spectrum of the chemosensor in acetonitrile has a peak at 375 nm, which changes to 428 nm with the gradual addition of only fluoride in the solution with a clear isosbestic points at 357 nm and 392 nm. More interestingly, the chemosensor gives a turn-on type of fluorescence at 554 nm in the presence of fluoride. Further it was found that the sensor is highly selective towards fluoride over other anions including chloride, bromide, iodide, nitrate, borate, perchlorate and can quantitatively detect fluoride at ppb level with a limit of detection of 0.02 mg/ L or 20 ppb. The chemosensor was successfully demonstrated to assess the fluoride concentration in the tap water.

Amidophosphine‐stabilized palladium complexes catalyse Suzuki–Miyaura cross‐couplings in aqueous media

We report a simple and efficient procedure for Suzuki–Miyaura reactions in aqueous media catalysed by amidophosphine‐stabilized palladium complexes trans‐{L3PPh2}2PdCl2 (3), trans‐{L3PPhtBu}2PdCl2 (4), [Pd(η3‐C3H5)(L3PPh2)Cl] (5) and {Pd[2‐(Me2NCH2)C6H4](L3PPh2)Cl} (6). The acidity of the NH proton in complexes 3, 4, 5, 6 plays an important role in their catalytic activity. In addition, the palladium complexes cis‐{L1PPh2}PdCl2 (1) and trans‐{L2PPh2}2PdCl2 (2) stabilized by phosphines containing Y,C,Y‐chelating ligands L1,2 have also been found to be useful catalysts for Suzuki–Miyaura reactions in aqueous media. The method can be effectively applied to both activated and deactivated aryl bromides yielding high or moderate conversions. The catalytic activity of couplings performed in pure water increases when utilizing a Pd complex with more acidic NH protons. A decrease of palladium concentration from 1.0 to 0.5 mol% does not lead to a substantial loss of conversion. In addition, Pd complex 1 can be efficiently recovered using two‐phase system extraction. Copyright © 2016 John Wiley & Sons, Ltd.

Synthesis and properties of N-(alkenylidene)trifluoromethanesulfonamides

N-Alkenylidenetrifluoromethanesulfonamides TfN=CH–CR=C(Me)R′ (R, R′ = H, Me) have been synthesized by reaction of N-sulfinyltrifluoromethanesulfonamide TfNSO with (E)-but-2-enal, (E)-2-methylbut- 2-enal, and 3-methylbut-2-enal. Despite greater stability of N-alkenylidenetrifluoromethanesulfonamides relative to their propargyl isomers TfNHCH2C≡CR, no rearrangement of the latter into the former occurs due to the presence of an acidic NH proton, which hampers formation of carbon-centered anion.

Cyclic Voltammetry Studies of H-Bond Complex of a p-Phenylenediamine-Based Urea with 1,8-Naphthyridine As the Proton-Coupled Electron Transfer Guest with Platinum and Glassy Carbon As Working Electrodes

Reactions in which both electron and proton are transferred, called proton-coupled electron transfer (PCET) reactions, are essential to many of the fundamental chemical processes of life. While PCET reactions have been well studied, it is only relatively recently that the role of H-bonding intermediates has been examined. The role that H-bonding can play in PCET was demonstrated by studies in the Smith group with U(H)H, a p-phenylenediamine-based urea which undergoes a reversible oxidation in methylene chloride.1 The overall oxidation of U(H)H is a 1 e- transfer with a surprising 2 e-/ 1 H+ mechanism where removal of the first electron produces a radical cation with an acidic NH proton that is then removed by the dimethylamino group from another U(H)H. This leads to removal of a second electron and creates the final products: the protonated electroinactive species, HU(H)H+, and the doubly oxidized form, U(H)+. Since this corresponds to 2 e- per 2 ureas, the process appears to be a net 1 e-. The interesting observation is that on the return scan two distinct pathways are observed. At the more negative potential, one pathway proceeds via the reduction of the U(H)+. This path dominates at low concentrations and fast scan rates. We hypothesize that the other path, which occurs at less negative potential, involves reduction of a H-bond complex formed between HU(H)H+ and U(H)+, leading to an overall electrochemically reversible process. In this study, cyclic voltammetry of U(H)H in the presence of 1,8-naphthyridine has been examined. The addition of naphthyridine results in an increase in current height and the appearance of a second oxidation wave at a more positive potentials . The increase in current is due to competition for the acidic proton between naphthyridine and the dimethylamino group of another U(H)H. NMR titration analysis show that naphthyridine H-bonds to the reduced U(H)H meaning naphthyridine blocks the H-bond sites from another U(H)H. Interestingly, the shape and behavior of the more positive oxidation wave alters depending on the electrode material. We hypothesize that the change in the CV wave is due to electrode fouling on the platinum (Pt) and poorly polished glassy carbon (GC) electrodes. With a carefully polished GC electrode, which minimizes the effect of electrode fouling, the observed second oxidation wave shifts slightly to more negative potentials and appears to merge with the first oxidation wave as the concentration of naphthyridine increases. This behavior reflects a mechanism that is similar to that of U(H)H by itself except the dimethylamino group of the other urea is replaced with naphthyridine. Therefore, the appearance of the second oxidation wave is due to oxidation of the H-bonding complex between the radical cation urea, U(H)H+, and naphthyridine, which is accompanied by proton transfer to naphthyridine. 1. Clare, L. A.; Pham A. T.; Magdaleno, F.; Acosta, J.; Woods, J. E.; Cooksy, A. L.; Smith, D. K. J. Am. Chem. Soc., 2013, 135 (50), 18930–18941.

Corrole-BODIPY Dyads: Synthesis, Structure, and Electrochemical and Photophysical Properties

A free-base and its Cu(III) derivative of bichromophoric meso-β linked corrole-BODIPY dyad were synthesized and structurally characterized by single crystal X-ray diffraction (XRD). Both corrole and BODIPY fragments maintained respective ground state electronic isolation despite their connection through a single bond due to a tilted orientation as observed by XRD. This was further supported by UV-vis and cyclic voltammetric studies. The Cu(III)-metalated dyad exhibits temperature-dependent paramagnetic behavior as observed in the variable temperature (1)H NMR due to the presence of a Cu(II)-corrole-π-cation radical. Importantly, the free-base exhibits complete fluorescence quenching probably due to photoinduced electron transfer to a low lying charge separated state. Interestingly, emission was regained upon addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) due to the deprotonation of corrole. The "turn on" fluorescence behavior and the presence of acidic NH protons were further exploited toward basic anion sensing utility.

Stepwise microhydration of aromatic amide cations: formation of water solvation network revealed by infrared spectra of formanilide(+)-(H2O)(n) clusters (n ≤ 5).

Hydration of peptides and proteins has a strong impact on their structure and function. Infrared photodissociation spectra (IRPD) of size-selected clusters of the formanilide cation, FA(+)-(H2O)n (n = 1-5), are analyzed by density functional theory calculations at the ωB97X-D/aug-cc-pVTZ level to determine the sequential microhydration of this prototypical aromatic amide cation. IRPD spectra are recorded in the hydride stretch and fingerprint ranges to probe the preferred interaction motifs and the cluster growth. IRPD spectra of cold Ar-tagged clusters, FA(+)-(H2O)n-Ar, reveal the important effects of temperature and entropy on the observed hydration motifs. At low temperature, the energetically most stable isomers are prominent, while at higher temperature less stable but more flexible isomers become increasingly populated because of entropy. In the most stable structures, the H2O ligands form a hydrogen-bonded solvent network attached to the acidic NH proton of the amide, which is stabilized by large cooperative effects arising from the excess positive charge. In larger clusters, hydration bridges the gap between the NH and CO groups (n ≥ 4) solvating the amide group rather than the more positively charged phenyl ring. Comparison with neutral FA-(H2O)n clusters reveals the strong impact of ionization on the acidity of the NH proton, the strength and topology of the interaction potential, and the structure of the hydration shell.

Recognition of fractional non-innocent feature of osmium coordinated 2,2'-biimidazole or 2,2'-bis(4,5-dimethylimidazole) and their interactions with anions.

Mononuclear complexes [Os(II)(bpy)2(H2L1)](ClO4)2 ([](ClO4)2) and [Os(II)(bpy)2(H2L2)](ClO4)2 ([](ClO4)2) incorporating two free NH protons at the back face of the coordinated H2L, and deprotonated L(2-) bridged symmetric dinuclear complexes [(bpy)2Os(II)(μ-L1(2-))Os(II)(bpy)2)](ClO4)2 ([](ClO4)2) and [(bpy)2Os(II)(μ-L2(2-))Os(II)(bpy)2)](ClO4)2 ([](ClO4)2) (bpy = 2,2'-bipyridine, H2L1 = 2,2'-biimidazole and H2L2 = 2,2'-bis(4,5-dimethylimidazole)) have been characterised. Crystal structures of [](ClO4)2 and the meso (ΔΛ) diastereomeric form of [](ClO4)2 have been determined. The crystal structure of [](ClO4)2 also reveals the hydrogen bonding interactions between its free acidic NH protons and the oxygen atoms of the perchlorate anion in the nearby asymmetric unit. Experimental and DFT/TD-DFT calculations have divulged the non-innocent feature of the doubly deprotonated L(2-) in (3+) and (3+), leading to the resonating formulation of {Os(II)(μ-L(2-))Os(III)} ↔ {Os(II)(μ-L˙(-))Os(II)}, instead of a simple mixed valent situation {Os(II)(μ-L(2-))Os(III)}. The dinuclear complexes (3+) and (3+) display one broad and moderately intense near-IR transition near 1000 nm corresponding to a mixed Os(dπ)/L(π) → Os(dπ)/L(π*) MLLMCT (metal/ligand to ligand/metal charge transfer) transition. Different experimental studies have also established the interaction of (2+) and (2+) with the selective anions.

Microhydrated aromatic cluster cations: binding motifs of 4-aminobenzonitrile-(H₂O)n cluster cations with n ≤ 4.

Infrared photodissociation (IRPD) spectra of mass-selected 4-aminobenzonitrile-(water)n cluster cations, ABN(+)-(H2O)n with n ≤ 4, recorded in the N-H and O-H stretch ranges are analyzed by quantum chemical calculations at the M06-2X/aug-cc-pVTZ level to determine the evolution of the initial microhydration process of this bifunctional aromatic cation in its ground electronic state. IRPD spectra of cold clusters tagged with Ar and N2 display higher resolution and allow for a clear-cut structural assignment. The clusters are generated in an electron impact source, which generates predominantly the most stable isomers. The IRPD spectra are assigned to single isomers for n = 1-3. The preferred cluster growth begins with sequential hydration of the two acidic NH protons of the amino group (n = 1-2), which is followed by attachment of secondary H2O ligands hydrogen-bonded to the first-shell ligands (n = 3-4). These symmetric and branched structures are more stable than those with a cyclic H-bonded solvent network. Moreover, in the size range n ≤ 4 the formation of a solvent network stabilized by strong cooperative effects is favored over interior ion hydration which is destabilized by noncooperative effects. The potential of the ABN(+)-H2O dimer is characterized in detail and supports the cluster growth derived from the IRPD spectra. Although the N-H bonds are destabilized by stepwise microhydration, which is accompanied by increasing charge transfer from ABN(+) to the solvent cluster, no proton transfer to the solvent is observed for n ≤ 4.

A new colorimetric and fluorescent chemosensor based on thiacalix[4]arene for fluoride ions

A new thiacalix[4]arene based fluorescent chemosensor thiacalix[4]arene-N-(quinolin-8-yl)acetamide (TCAN8QA) has been synthesized. TCAN8QA has been found to exhibit highly selective behavior for F− ions among all other anions, that is, Cl−, Br−, I−, PO4−3, OH−, H2PO4−, and CH3COO− in the absorption spectra as well as in the emission spectra. Red shift and quenching in emission spectra constituting the signature for fluoride detection are due to photoinduced charge transfer (PCT) which can be attributed to deprotonation of acidic NH proton in the presence of fluoride ions.

Analyte interactions with a new ditopic dansylamide-nitrobenzoxadiazole dyad: a combined photophysical, NMR, and theoretical (DFT) study.

We report herein the synthesis and photophysical studies on a new multicomponent chemosensor dyad comprising two fluorescing units, dansylamide (DANS) and nitrobenzoxadiazole (NBD). The system has been developed to investigate receptor-analyte binding interactions in the presence of both cations and anions in a single molecular system. A dimethyl amino (in the DANS unit) group is used as a receptor for cations, and acidic hydrogens of sulfonamide and the NBD group are used as receptors for anions. The system is characterized by conventional analytical techniques. The photophysical properties of this supramolecular system in the absence and presence of various metal ions and nonmetal ions as additives are investigated in an acetonitrile medium. Utility of this system in an aqueous medium has also been demonstrated. The absorption and fluorescence spectrum of the molecular system consists of a broad band typical of an intramolecular charge-transfer (ICT) transition. A low quantum yield and lifetime of the NBD moiety in the present dyad indicates photoinduced electron transfer (PET) between DANS and the NBD moiety. The fluorescence intensity of the system is found to decrease in the presence of fluoride and acetate anions; however, the quenching is found to be much higher for fluoride. This quenching behavior is attributed to the enhanced PET from the anion receptor to the fluorophore moiety. The mechanistic aspect of the fluoride ion signaling behavior has also been studied by infrared (IR) and (1)H NMR experiments. The hydrogen bonding interaction between the acidic NH protons of the DPN moiety and F(-) is found to be primarily responsible for the fluoride selective signaling behavior. While investigating the cation signaling behavior, contrary to anions, significant fluorescence enhancement has been observed only in the presence of transition-metal ions. This behavior is rationalized by considering the disruption of PET communication between DANS and the NBD moiety due to transition-metal ion binding. Theoretical (density functional theory) studies are also performed for the better understanding of the receptor-analyte interaction. Interestingly, negative cooperativity in binding is observed when the interaction of this system is studied in the presence of both Zn(2+) and F(-). Fluorescence microscopy studies also revealed that the newly developed fluorescent sensor system can be employed as an imaging probe in live cells.

Transition Metal Chlorides Are Lewis Acids toward Terminal Chloride Attached to Late Transition Metals

Two different neutral tridentate imine-donor pincer ligands interact with excess MCl2 (M = Co or Cu) to form compounds of the same stoichiometry, (LMCl2)2·MCl2, where the assembling force is the electron richness of the terminal chlorides on the LMCl2 unit. Finite aggregation occurs for M = Co, but for M = Cu, an infinite polymeric structure is adopted, all because MCl2 is bifunctional, which thus bridges multiple MCl units. The bis-pyrazolylpyridine ligand has two acidic NH protons, and both of these are involved in intramolecular hydrogen bonds. The generality of this Lewis acid aggregation is discussed.

Heterolytic Cleavage of Dihydrogen by an Iron(II) PNP Pincer Complex via Metal-Ligand Cooperation.

The bis-carbonyl Fe(II) complex trans-[Fe(PNP-iPr)(CO)2Cl]+ reacts with Zn as reducing agent under a dihydrogen atmosphere to give the Fe(II) hydride complex cis-[Fe(PNP-iPr)(CO)2H]+ in 97% isolated yield. A crucial step in this reaction seems to be the reduction of the acidic NH protons of the PNP-iPr ligand to afford H2 and the coordinatively unsaturated intermediate [Fe(PNPH-iPr)(CO)2]+ bearing a dearomatized pyridine moiety. This species is able to bind and heterolytically cleave H2 to give cis-[Fe(PNP-iPr)(CO)2H]+. The mechanism of this reaction has been studied by DFT calculations. The proposed mechanism was supported by deuterium labeling experiments using D2 and the N-deuterated isotopologue of trans-[Fe(PNP-iPr)(CO)2Cl]+. While in the first case deuterium was partially incorporated into both N and Fe sites, in the latter case no reaction took place. In addition, the N-methylated complex trans-[Fe(PNPMe-iPr)(CO)2Cl]+ was prepared, showing no reactions with Zn and H2 under the same reaction conditions. An alternative synthesis of cis-[Fe(PNP-iPr)(CO)2H]+ was developed utilizing the Fe(0) complex [Fe(PNP-iPr)(CO)2]. This compound is obtained in high yield by treatment of either trans-[Fe(PNP-iPr)(CO)2Cl]+ or [Fe(PNP-iPr)Cl2] with an excess of NaHg or a stoichiometric amount of KC8 in the presence of carbon monoxide. Protonation of [Fe(PNP-iPr)(CO)2] with HBF4 gave the hydride complex cis-[Fe(PNP-iPr)(CO)2H]+. X-ray structures of both cis-[Fe(PNP-iPr)(CO)2H]+ and [Fe(PNP-iPr)(CO)2] are presented.

Strong NH⋯S hydrogen bonds in molybdoenzyme models containing anilide moieties

Monooxomolybdenum(IV) and dioxomolybdenum(VI) benzenedithiolate derivatives containing anilide moieties were synthesized and characterized by (1)H NMR, UV-visible, IR, and Raman spectroscopies. These complexes exhibit very strong intramolecular NH···S hydrogen bonds formed by the acidic anilide NH proton and the desired coplanar structure, resulting in significantly positive redox potential and remarkable acceleration in the reduction of Me3NO in DMF.

A simple naphthalimide-based receptor for selective recognition of fluoride anion

Synthesis of a simple fluorescent naphthalimide based receptor 4-aminobenzyl-N-methyl-1,8naphthalimide 3 was carried out as a selective fluoride ion sensor. In acetonitrile, interaction of 3 with different anions such as, AcO – , F – , Cl – , Br – , I – , and SCN – revealed significant fluorescence quenching only with the F – anion. In the presence of the fluoride anion, the color of the solution changed from a fluorescent green to violet and this was visible to the naked eye. The probable mode of sensing mechanism by photo-induced electron transfer (PET) reaction is attributed to deprotonation of acidic NH proton in the presence of fluoride which was confirmed by change in optical properties intramolecular charge transfer (ICT) and 1 H NMR spectral data analysis. The Job’s plot analysis displayed a 1:1 stoichiometry for interaction between 3 and F – . The extent of fluorescence quenching was estimated by Stern-Volmer plot.

Ion Interactions with a New Ditopic Naphthalimide‐Based Receptor: A Photophysical, NMR and Theoretical (DFT) Study

A new multi-component chemosensor system comprising a naphthalimide moiety as fluorophore is designed and developed to investigate receptor-analyte binding interactions in the presence of metal and non-metal ions. A dimethylamino moiety is utilized as receptor for metal ions and a thiourea receptor, having acidic protons, for binding anions. The system is characterized by conventional analytical methods. The absorption and fluorescence spectra of the system consist of a broad band typical for an intramolecular charge transfer (ICT). The effects of various metal-ion additives on the spectral behavior of the present sensor system are examined in acetonitrile. It is found that among the metal ions studied, alkali/alkaline earth-metal ions and transition-metal ions modulate the absorption and fluorescence spectra of the system. As an additional feature, the anion signaling behavior of the system in acetonitrile is studied. A decrease in fluorescence efficiency of the system is observed upon addition of fluoride and acetate anions. Fluorescence quenching is most effective in the case of fluoride ions. This is attributed to the enhancement of the photoinduced electron transfer from the anion receptor to the fluorophore moiety. Hydrogen-bond interactions between the acidic NH protons of the thiourea moiety and the F(-) anions are primarily attributed to the fluoride-selective signaling behavior. Interestingly, a negative cooperativity for the binding event is observed when the interactions of the system are studied in the presence of both Zn(2+) and F(-) ions. NMR spectroscopy and theoretical calculations are also carried out to better understand the receptor-analyte binding.

Synthesis, characterization and cytotoxicity studies of platinum(II) complexes with amino acid ligands in various coordination modes

Reactions of [Pt(CO3)(PPh3)2]·CH2Cl2 (1) with non-substituted and alkyl substituted amino acids, NH(R)CH(R′)CO2H (R/R′=H/Me, L1; H/iPr, L2; H/CH2CHMe2, L3; Me/H, L4; Et/H, L5), in the presence of Tl[PF6] in methanol afforded with liberation of CO2 the formation of platinum(II) complexes of the type [Pt(PPh3)2{NHR–CHR′–C(O)O-κN,κO}][PF6] (R/R′=H/Me, 2; H/iPr, 3; H/CH2CHMe2, 4; Me/H, 5; Et/H, 6). Single-crystal X-ray diffraction analysis of complex 4 exhibited a square-planar coordination of the platinum atom having coordinated two triphenylphosphane ligands and a deprotonated κN,κO-coordinated leucine ligand (L3−H). On varying the pKa value of the amino group, platinum(II) complexes with different coordination modes of amino acid ligands were obtained. Thus, treatment of complex 1 with N-acetyl l-alanine (L6), possessing a comparatively highly acidic NH proton, in 1:1 ratio in methanol resulted in the formation of [Pt(PPh3)2{N(COMe)–CHMe–C(O)O-κN,κO}] (7), while reacting N-phenyl glycine (L7) having a moderately acidic NH proton with complex 1 afforded a mixture of complexes [Pt(PPh3)2{NPh–CH2–C(O)O-κN,κO}] (8) and [Pt(PPh3)2{NHPh–CH2–C(O)O-κO}2] (10). Treatment of complex 1 with two equivalents of L6/L7 in dichloromethane resulted in the formation of [Pt(PPh3)2{NHR–CHR′–C(O)O-κO}2] (R/R′=COMe/Me, 9; Ph/H, 10). An analogous reactivity was observed for l-lactic acid on treating with complex 1 in 1:1 and 2:1 ratio resulting in [Pt(PPh3)2{O–CHMe–C(O)O-κO,κO′}] (11) and [Pt(PPh3)2{HO–CHMe–C(O)O-κO}2] (12). The identities of all complexes have been proven by NMR (1H, 13C, 31P) spectroscopic and high-resolution ESI mass-spectrometric investigations. In vitro cytotoxicity studies against human tumor cell lines (8505C, A2780, HeLa, SW480, and MCF-7) showed the highest activities for the neutral complex 7. Furthermore, complexes 7 and 9 against the A2780 cell line induced an apoptotic mode of cell death, which was further supported by morphological investigation and DNA laddering. Cell cycle perturbation studies showed that both complexes induced faster cell death than cisplatin.