A new macrocyclic cryptand with squaramide moieties: an overstructured Cu(II) complex that selectively binds halides: synthesis, acid/base- and ligational behavior, and crystal structures

The synthesis and characterization of the novel 24,29-dimethyl-6,7,15,16-tetraoxotetracyclo-[19.5.5.0(5,8).0(14,17)]-1,4,9,13,18,21,24,29-octaazaenatriaconta-Delta(5,8),Delta(14,17)-diene (L) are reported. Molecule L incorporates two squaramide functions in a overstructured chain connecting two opposite nitrogen atoms of the Me(2)[12]aneN(4) polyaza macrocyclic base to obtain a cage topology. The basicity and binding properties of L towards Cu(II) were determined by means of potentiometric measurements in aqueous solution (298.1+/-0.1 K, I=0.15 mol dm(-3)). Molecule L behaves as a diprotic base under the experimental conditions employed and forms only mononuclear Cu(II) complexes in which the squaramide moieties are not involved in the stabilization of the metal ion that is stabilized by the amine functions of the polyaza base inside the three-dimensional cavity. The [CuL](2+) species was tested as a host for the series of halide anions. UV-visible spectrophotometric experiments permitted the determination of the addition constants of halides to the Cu(II)-complexed species. The [CuL](2+) species binds the anions F(-), Cl(-), and Br(-) by forming the [CuLX](+) species, but does not bind the biggest I(-) anion. A trend of selectivity as a function of the hydrogen-bonding capability as well as the dimensions of the anion were established; the maximum value of selectivity was for addition of the F(-) anion (log K=4.8). This selectivity is due to the presence of the overstructured chain containing the squaramide groups up to the Me(2)[12]aneN(4) macrocyclic base. The squaramide groups, by providing hydrogen-bond contacts, permit the [CuL](2+) species to selectively bind these anions through the formation of a hydrogen-bond network with F(-) and Cl(-). The crystal structures of the [CuLF](+) and [CuLCl](+) cations support the results obtained in aqueous solution.     

New bis-cresol-bridged bis(1,4,7-triazacyclononane) ligand as receptor for metal cations and phosphate anions

The synthesis and characterization of a new bis([9]aneN3) ligand (H2L) containing two [9]aneN3 macrocyclic moieties separated by a 2,2'-methylene-bis-cresol (cresol = 4-methyl-phenol) unit is reported. A potentiometric and (1)H NMR study in aqueous solution reveals that H2L is in a zwitterionic form, and protonation of the cresolate oxygens occurs only with the formation of the highly charged (H5L)(3+) and (H6L)(4+) species at acidic pH values. The coordination properties of H2L toward Cu(II), Zn(II), Cd(II), and Pb(II) were studied by means of potentiometric and UV spectrophotometric measurements. The ligand gives both mono- and binuclear complexes in aqueous solution. At acidic pH values the ligand forms stable binuclear [M2H2L](4+) complexes, where each metal is coordinated by two amine groups of [9]aneN3 and the deprotonated oxygen of the adjacent cresol unit; the remaining amine group is protonated. Deprotonation of the [M2H2L](4+) species at alkaline pH values affords [M2L](2+) complexes, where all amine groups of the [9]aneN3 moieties are involved in metal coordination. Binding of mono-, di- and triphosphate, and adenosine triphosphate (ATP) was studied by means of potentiometric, (1)H and (31)P NMR measurements and by molecular dynamics simulations. The receptor forms stable 1:1 adducts with di-, triphosphate, and ATP, while the interaction with monophosphate is too low to be detected. In the complexes both the [9]aneN3 moieties act cooperatively in the substrate binding process. The stability of the adducts increases in the order diphosphate < triphosphate < ATP. This trend is explained in terms of increasing number of charge-charge interactions between the phosphate chains and the protonated [9]aneN3 subunits and, in the case of ATP, of stacking interactions between the adenine and cresol units.     

Encapsulation of metal cations and anions within the cavity of bis(1,4,7-triazacyclononane) receptors

The synthesis and characterisation of a new bis([9]aneN3) ligand (L4) containing two [9]aneN3 macrocyclic moieties separated by a 2,6-dimethylenepyridine unit is reported. A potentiometric and 1H NMR study in aqueous solution reveals that ligand protonation occurs on the secondary amine groups and does not involve the pyridine nitrogen. The coordination properties toward Cu(II), Zn(II), Cd(II) and Pb(II) were studied by means of potentiometric and UV spectrophotometric measurements. The ligand can form mono- and binuclear complexes in aqueous solution. In the 1 : 1 complexes, the metal is sandwiched between the two [9]aneN3 moieties and the pyridine N-donor is coordinated to the metal, as actually shown by the crystal structure of the compound [ZnL4](NO3)2.CH3NO2. L4 shows a higher binding ability for Cd(II) with respect to Zn(II), probably due to a better fitting of Cd(II) ion inside the cavity generated by the two facing [9]aneN3 units. The formation of binuclear complexes is accompanied by the assembly of OH-bridged M2(OH)x (x = 1-3) clusters inside the cavity defined by the two facing [9]aneN3 units, and pyridine is not involved in metal coordination. A potentiometric and (1)H NMR study on the coordination of halogenide anions by L4 and its structural analogous L3 in which the two [9]aneN3 units are separated by a shorter quinoxaline linkage, shows that bromide is selectively recognised by L4, while chloride is selectively bound by L3. Such a behaviour is discussed in terms of dimensional matching between the spherical anions and the cavities generated by the two [9]aneN3 units of the receptors.     

Zinc(II) and PPi selective fluorescence OFF-ON-OFF functionality of a chemosensor in physiological conditions

A fluorescent chemosensor based on a quinoline derivative, L(2) (OFF state), selectively senses Zn(2+) by effective chelate-enhanced fluorescence (ON state), which further shows selectivity toward PPi over competing anions like Pi, AMP, and ATP via fluorescence quenching (OFF state) in a 100% aqueous HEPES buffer (pH 7.4). A plausible mode for the selective binding of PPi to 1 has been demonstrated by quantum mechanical density functional theory calculations and high-resolution mass spectrometry analysis.     

Pentacoordinate nickel(II) complexes double bridged by phosphate ester or phosphinate ligands: spectroscopic, structural, kinetic, and magnetic studies

The bis(phosphatediester)-bridged complexes [[Ni([12]aneN(3))(mu-O(2)P(OR)(2))](2)](PF(6))(2) [[12]aneN(3)=Me(3)[12]aneN(3), 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene; R=Me (1), Bu (2), Ph (3), Ph-4-NO(2) (4); [12]aneN(3)=Me(4)[12]aneN(3), 2,4,4,9-tetramethyl-1,5,9-triazacyclododec-1-ene; R=Me (5), Bu (6), Ph (7), Ph-4-NO(2) (8)] were prepared by hydrolysis of the phosphate triester with the hydroxo complex [[Ni([12]aneN(3))(mu-OH)](2)](PF(6))(2) or by acid-base reaction of the dialkyl or diaryl phosphoric acid and the above hydroxo complex. The acid-base reaction was also used to synthesise the phosphinate-bridged complexes [[Ni([12]aneN(3))(mu-O(2)PR(2))](2)](PF(6))(2) [[12]aneN(3)=Me(3)[12]aneN(3), R=Me (9), Ph (10); [12]aneN(3)=Me(4)[12]aneN(3), R=Me (11), Ph (12)]. The molecular structures of complexes 2, 3 and 12 were established by single crystal X-ray diffraction studies. The eight-membered rings defined by the nickel atoms and the bridging ligands show distorted twist-boat, chair and boat-boat conformations in 2, 3 and 12, respectively. The experimental susceptibility data for compounds 2, 3 and 12 were fitted by least-squares methods to the analytical expression given by Ginsberg. The best fit was obtained with values of J=-0.11 cm(-1), D=-9.5 cm(-1) and g=2.20 for 2; J=-0.97 cm(-1), D=-9.3 cm(-1) and g=2.21 for 3; and J=-0.14 cm(-1), D=-11.9 cm(-1) and g=2.195 for 12. The magnetic-exchange pathways must involve the phosphate/phosphinate bridges, because these favour antiferromagnetic interactions. The observation of a higher exchange parameter for compound 3 is a consequence of a favourable disposition of the O-P-O bridges. The kinetics for the hydrolysis of TNP (tris(4-nitrophenyl)phosphate) with the dinuclear nickel(II) hydroxo complex [[Ni(Me(3)[12]aneN(3))(mu-OH)](2)](PF(6))(2) was studied by UV-visible spectroscopy. The proposed mechanism for TNP-promoted hydrolysis can be described as one-substrate/two-product, and can be fitted to a Michaelis-Menten equation.     

Zn(II) and Cd(II)-based complexes for probing the enzymatic hydrolysis of Na4P2O7 by alkaline phosphatase in physiological conditions

Newly synthesized Zn(II) and Cd(II)-based complexes show unique selectivity towards inorganic pyrophosphate (PPi) in 100% aqueous medium at pH = 7.4 and act as a "turn-on" and "turn-off" real-time assay, respectively, for the evaluation of the enzymatic activity of alkaline phosphatase.     

Coordination properties of new bis(1,4,7-triazacyclononane) ligands: a highly active dizinc complex in phosphate diester hydrolysis

The synthesis and characterization of three new bis([9]aneN(3)) ligands, containing respectively 2,2'-bipyridine (L(1)), 1,10-phenanthroline (L(2)), and quinoxaline (L(3)) moieties linking the two macrocyclic units, are reported. Proton binding and Cu(II), Zn(II), Cd(II), and Pb(II) coordination with L(1)-L(3) have been studied by potentiometric titrations and, for L(1) and L(2), by spectrophotometric UV-vis measurements in aqueous solutions. All ligands can give stable mono- and dinuclear complexes. In the case of L(1), trinuclear Cu(II) complexes are also formed. The stability constants and structural features of the formed complexes are strongly affected by the different architecture and binding properties of the spacers bridging the two [9]aneN(3) units. In the case of the L(1) and L(2) mononuclear complexes, the metal is coordinated by the three donors of one [9]aneN(3) moiety; in the [ML(2)](2+) complexes, however, the phenanthroline nitrogens are also involved in metal binding. Finally, in the [ML(3)](2+) complexes both macrocyclic units, at a short distance from each other, can be involved in metal coordination, giving rise to sandwich complexes. In the binuclear complexes each metal ion is generally coordinated by one [9]aneN(3) unit. In L(1), however, the dipyridine nitrogens can also act as a potential binding site for metals. The dinuclear complexes show a marked tendency to form mono-, di-, and, in some cases, trihydroxo species in aqueous solutions. The resulting M-OH functions may behave as nucleophiles in hydrolytic reactions. The hydrolysis rate of bis(p-nitrophenyl)phosphate (BNPP) was measured in aqueous solution at 308.1 K in the presence of the L(2) and L(3) dinuclear Zn(II) complexes. Both the L(2) complexes [Zn(2)L(2)(OH)(2)](2+) and [Zn(2)L(2)(OH)(3)](+) and the L(3) complex [Zn(2)L(3)(OH)(3)](+) promote BNPP hydrolysis. The [Zn(2)L(3)(OH)(3)](+) complex is ca. 2 orders of magnitude more active than the L(2) complexes, due both to the short distance between the metal centers in [Zn(2)L(3)(OH)(3)](+), which could allow a bridging interaction of the phosphate ester, and to the simultaneous presence of single-metal bound nucleophilic Zn-OH functions. These structural features are substantially corroborated by semiempirical PM3 calculations carried out on the mono-, di-, and trihydroxo species of the L(3) dizinc complex.     

Discovery and evaluation of highly active imidotitanium ethylene polymerisation catalysts using high throughput catalyst screening

A family of ca. 50 imidotitanium precatalysts [Ti(NR)(Me(3)[9]aneN(3))Cl(2)](R = alkyl or aryl; Me(3)[9]aneN(3)= 1,4,7-trimethyltriazacyclononane) were prepared in good yields using semi-automated procedures; high-throughput screening techniques identified seven highly active ethylene polymerisation precatalysts with activities in the range ca. 3 400 to 10 000 kg(PE) mol(-1) h(-1) bar(-1).     

Probing the effects of ligand structure on activity and selectivity of Cr(III) complexes for ethylene oligomerisation and polymerisation

A series of distorted octahedral Cr(III) complexes containing tridentate S-, S/O- or N-donor ligands comprised of three distinct architectures: facultative {S(CH(2)CH(2)SC(10)H(21))(2) (L(1)) and O(CH(2)CH(2)SC(10)H(21))(2) (L(2))}, tripodal {MeC(CH(2)S(n)C(4)H(9))(3) (L(3)), MeC(CH(2)SC(10)H(21))(3) (L(4))} and macrocyclic {(C(10)H(21))[9]aneN(3) (L(5)), (C(10)H(21))(3)[9]aneN(3) (L(6)), with [9]aneN(3)=1,4,7-triazacyclononane} are reported and characterised spectroscopically. Activation of [CrCl(3)(L)] with MMAO produces very active ethylene trimerisation, oligomerisation and polymerisation catalysts, with significant dependence of the product distribution upon the ligand type present. The properties of the parent [CrCl(3)(L)] complexes are probed by cyclic voltammetry, UV-visible, EPR, EXAFS and XANES measurements, and the effects upon activation with Me(3)Al investigated similarly. Treatment with excess Me(3)Al leads to substitution of Cl ligands by Me groups, generation of an EPR silent Cr species (consistent with a change in the oxidation state of the Cr to either Cr(II) or Cr(IV)) and substantial dissociation of the neutral S and S/O-donor ligands.     

AlMe3 and ZnMe2 adducts of a titanium imido methyl cation: a combined crystallographic, spectroscopic, and DFT study

A combined experimental and DFT study of the reactions of the titanium imido methyl cation [Ti(NtBu)(Me3[9]aneN3)Me]+ (4+) with AlMe3 and ZnMe2 is described. Reaction of 4+ with AlMe3 gave [Ti(NtBu)(Me3[9]aneN3)(mu-Me)2AlMe2]+ (7+), the first structurally characterized AlMe3 adduct of a transition metal alkyl cation and a model for the presumed resting state in MAO-activated olefin polymerizations. Reaction of 4+ with ZnMe2 also gave a methyl-bridged heterobinuclear species, namely [Ti(mu-NtBu)(Me3[9]aneN3)(mu-Me)2ZnMe]+ (8+), the first directly observed ZnMe2 adduct of a transition metal alkyl cation. At room temperature, all three metal-bound methyls of 8+ underwent rapid exchange with those of free ZnMe2, whereas at 233 K only the terminal Zn-Me group exchanged significantly. Addition of AlMe3 to 8+ quantitatively formed 7+ and ZnMe2. Reaction of 4+ with Cp2ZrMe2 gave [Ti(NtBu){Me2(mu-CH2)[9]aneN3}(mu-CH2)ZrCp2]+ (10+) via a highly selective double C-H bond activation reaction in which both alkyl groups of Cp2ZrMe2 were lost. DFT calculations on models of 7+ confirmed the approximately square-based pyramidal geometries for the bridging methyl groups. Calculations on 8+ found that the formation of the Ti(mu-Me)2Zn moiety is assisted by an Nimide-->Zn dative bond. DFT calculations for the sterically less encumbered methyl cation [Ti(NMe)(H3[9]aneN3)Me]+ found strong thermodynamic preferences for adducts featuring Nimide-->M (M = Al or Zn) interactions. This offers insight into recently observed structure-productivity trends in MAO-activated imido-based polymerization catalysts. Calculations on the metallocenium adducts [Cp2Ti(mu-Me)2AlMe2]+ and [Cp2Ti(mu-Me)2ZnMe]+ are described, each showing alpha-agostic interactions for the bridging methyl groups. For these systems and the imido ones, the coordination of AlMe3 to the corresponding monomethyl cation is ca. 30 kJ mol-1 more favorable than for ZnMe2.     

Selective binding and fluorescence sensing of diphosphate in H2O via Zn(2+)-induced allosteric regulation of the receptor structure

A terpyridine-based receptor featuring two [9]aneN(3) units is able to selectively bind and sense diphosphate over mono- and triphosphate in aqueous solution at pH 7, thanks to the conformational change of its structure induced by Zn(2+) coordination to the polypyridyl moiety.     

Nano‐Conjugate Fluorescence Probe for the Discrimination of Phosphate and Pyrophosphate

We describe a pyrophosphate (PPi) probe that is based on a fluorescent dicarboxylate‐substituted poly(para‐phenyleneethynylene) (PPE) and 10 nm cobalt–iron spinel nanoparticles (NPs) in aqueous media. The spinel NPs efficiently quench the fluorescence of the PPE at a concentration of 20–30 pmol. Addition of phosphate anions to the PPE–NP construct displaces the quenched PPE to give rise to a fluorescent response; we found that PPi and phosphate (Pi) have significantly different binding affinities for the self‐assembled materials. We can discern >40 nM PPi in the presence of 0.1 mM Pi at pH 7, which suggests that these assemblies may be useful in bio‐analytical applications. This displacement assay was used to effectively determine the ability of pyrophosphatase to hydrolyze PPi to Pi.     

Highly selective colorimetric sensing pyrophosphate in water by a NBD-phenoxo-bridged dinuclear Zn(II) complex

A novel NBD-phenoxo-bridged dinuclear Zn(II) complex is found to be an effective colorimetric sensor for pyrophosphate (PPi) in pure aqueous solution over a wide pH range. This sensor shows high binding affinity (K(a)≈ 3 × 10(8) M(-1)) and high selectivity for PPi, and can be also used to assay the activity of pyrophosphatase in real time.     

A Zn2+-specific turn-on fluorescent probe for ratiometric sensing of pyrophosphate in both water and blood serum

A novel fluorescent sensor composed of a naphthalene functionalized tetraazamacrocycle ligand 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3-methyl naphthalene (1) and Zn(2+) has been designed and prepared, which can be utilized for selective and ratiometric sensing of pyrophosphate (PPi) over other phosphate-containing anions in aqueous solution at physiological pH. Notably, the water soluble 1 itself also exhibits a selective enhanced fluorescent response to Zn(2+), and the complex 1-Zn(2+) thus formed eventually fulfils the synergic Zn(2+) coordination-altered strategy with PPi. Furthermore, the ratiometric sensing of 1-Zn(2+) towards PPi performed well even in blood serum milieu. Finally, the sensor 1-Zn(2+) was successfully employed to monitor a real-time assay of inorganic pyrophosphatase (PPase) by means of ratiometric fluorescent measurements for the first time.     

Triisopropyltriazacyclononane copper(II): an efficient phosphodiester hydrolysis catalyst and DNA cleavage agent

A 6000-fold rate enhancement has been observed for the hydrolysis of bis(p-nitrophenyl)phosphate (BNPP) in the presence of 0.2 mM Cu(i-Pr(3)[9]aneN(3))(2+) at pH 9.2 and 50 degrees C. In a direct comparison, the rate of hydrolysis of BNPP is accelerated at least 60-fold over the previously reported catalyst Cu([9]aneN(3))(2+). As observed for Cu([9]aneN(3))(2+), hydrolysis is selective for diesters over monoesters. Hydrolysis of BNPP by Cu(i-Pr(3)[9]aneN(3))(2+) is catalytic, exhibiting both rate enhancement and turnover. The reaction is inhibited by both p-nitrophenyl phosphate and inorganic phosphate. The reaction is first-order in substrate and half-order in metal complex, with a k(1.5) of 0.060 +/- 0.004 M(-1/2) s(-1) at 50 degrees C. The temperature dependence of the rate constant results in a calculated activation enthalpy (Delta H(++) of 51 +/- 2 kJ mol(-1) and activation entropy (Delta S(++)) of -110 +/- 6 J mol(-1) K(-1). The kinetic pK(a) of 7.8 +/- 0.2 is close to the thermodynamic pK(a) of 7.9 +/- 0.2, consistent with deprotonation of a coordinated water molecule in the active form of the catalyst. The active catalyst [Cu(i-Pr(3)[9]aneN(3))(OH)(OH(2))](+) is in equilibrium with an inactive dimer, and the formation constant for this dimer is between 216 and 1394 M(-1) at pH 9.2 and 50 degrees C. Temperature dependence of the dimer formation constant K(f) indicates an endothermic enthalpy of formation for the dimer of 27 +/- 3 kJ mol(-1). The time course of anaerobic DNA cleavage by Cu(i-Pr(3)[9]aneN(3))(2+) is presented over a wide range of concentrations at pH 7.8 at 50 degrees C. The concentration dependence of DNA cleavage by Cu([9]aneN(3))(2+) and Cu(i-Pr(3)[9]aneN(3))(2+) reveals a maximum cleavage efficiency at sub-micromolar concentrations of cleavage agent. DNA cleavage by Cu(i-Pr(3)[9]aneN(3))(2+) is twice as efficient at pH 7.8 as at pH 7.2.     

1,4,7,10-tetraazacyclododecane metal complexes as potent promoters of phosphodiester hydrolysis under physiological conditions

Previously reported mono- and dinuclear Zn(II), Cu(II), and Ni(II) complexes of 1,4,7,10-tetrazacyclododecane ([12]aneN4 or cyclen) with different heterocyclic spacers (triazine, pyridine) of various lengths (bi- and tripyridine) or an azacrown-pendant have been tested for the hydrolysis of bis(4-nitrophenyl)phosphate (BNPP) under physiological conditions (pH 7-9, 25 degrees C). All Zn(II) complexes promote the hydrolysis of BNPP under physiological conditions, while those of Cu(II) and Ni(II) do not have a significant effect on the hydrolysis reaction. The hydrolysis kinetics in buffered solutions (0.05 M Bis/Tris, TRIS, HEPES, or CHES, I=0.1 M, NaCl) at 25 degrees C were determined by the initial slope method (product conversion<5%). Comparison of the second-order pH-independent rate constants (kBNPP, M(-1) s(-1)) for the mononuclear complexes ZnL1, ZnL3, and ZnL6, which are 6.1x10 (-5), 5.1x10(-5), and 5.7x10(-5), respectively, indicate that the heterocyclic moiety improves the rate of hydrolysis up to six times over the parent Zn([12]aneN4) complex (kBNPP=1.1x10(-5) M(-1) s(-1)). The reactive species is the Zn(II)-OH- complex, in which the Zn(II)-bound OH- acts as a nucleophile. For dinuclear complexes Zn2L2, Zn2L4, and Zn2L5, the rate of reaction is defined by the degree of cooperation between the metal centers, which is determined by the spacer length. Zn2L2 and Zn2L4 possessing shorter spacers are able to hydrolyze BNPP 1 to 2 orders of magnitudes faster than Zn2L5. The second-order rate constants k of Zn2L4 and Zn2L2 at pH 7, 8, and 9 are significantly higher than those of previously reported related complexes. The high BNPP hydrolytic activity may be related to pi-stacking and hydrophobic interactions between the aromatic spacer moieties and the substrate. Complexes Zn2L4 and Zn2L2 show hydrolytic activity at pH 7 and 8, which allows for the hydrolysis of activated phosphate esters under physiological conditions.     

1,4,7,10-tetraazacyclododecane metal complexes as potent promoters of carboxyester hydrolysis under physiological conditions

New 1,4,7,10-tetrazacyclododecane ([12]aneN4 or cyclen) ligands with different heterocyclic spacers (triazine and pyridine) of various lengths (bi- and tripyridine) or an azacrown pendant and their mono- and dinuclear Zn(II), Cu(II), and Ni(II) complexes have been synthesized and characterized. The pKa values of water molecules coordinated to the complexed metal ions were determined by potentiometric pH titrations and vary from 7.7 to 11.2, depending on the metal-ion and ligand properties. The X-ray structure of [Zn2L2]mu-OH(ClO4)3.CH3CN.H2O shows each Zn(II) ion in a tetrahedral geometry, binding to three N atoms of cyclen (the average distance of Zn-N = 2.1 A) and having a mu-OH bridge at the apical site linking the two metal ions (the average distance of Zn-O- = 1.9 A). The distance between the Zn(II) ion and the fourth N atom is 2.6 A. All Zn(II) complexes promote the hydrolysis of 4-nitrophenyl acetate (NA) under physiological conditions, while those of Cu(II) and Ni(II) do not have a significant effect on the hydrolysis reaction. The kinetic studies in buffered solutions (0.05 M Tris, HEPES, or CHES, I = 0.1 M, NaCl) at 25 degrees C in the pH range of 6-11 under pseudo-first-order reaction conditions (excess of the metal complex) were analyzed by applying the method of initial rates. Comparison of the second-order pH-independent rate constants (kNA, M-1 s-1) for the mononuclear complexes ZnL1, ZnL3, and ZnL8, which are 0.39, 0.27, and 0.38, respectively, indicates that the heterocyclic moiety improves the rate of hydrolysis up to 4 times over the parent Zn([12]aneN4) complex (kNA = 0.09 M-1 s-1). The reactive species is the Zn(II)-OH- complex, in which the Zn(II)-bound OH- acts as a nucleophile, which attacks intermolecularly the carbonyl group of the acetate ester. For dinuclear complexes Zn2L2, Zn2L4, Zn2L5, Zn2L6, and Zn2L7, the mechanism of the reaction is defined by the degree of cooperation between the metal centers, determined by the spacer length. For Zn2L7, having the longest triaryl spacer, the two metal centers act independently in the hydrolysis; therefore, the reaction rate is twice as high as the rate of the mononuclear analogue (kNA = 0.78 M-1 s-1). The complexes with a monoaryl spacer show saturation kinetics with the formation of a Michaelis-Menten adduct. Their hydrolysis rates are 40 times higher than that of the Zn[12]aneN4 system (kNA approximately 4 M-1 s-1). Zn2L6 is a hybrid between these two mechanisms; a clear saturation curve is not visible nor are the metal cores completely independent from one another. Some of the Zn(II) complexes show a higher hydrolytic activity under physiological conditions compared to other previously reported complexes of this type.     

Zn(II) complex of terpyridine for the highly selective fluorescent recognition of pyrophosphate

Pyrophosphate ion (PPi) is crucial in varieties of biological processes and industrial applications, and thus it is very important how to recognize it with high selectivity. In this contribution, one terpyridine (tpy)-based fluorescent molecule, 4-(methylphenyl)-2,2':6',2'-terpyridine (mptpy), has been reported to display a highly selective recognition for PPi in the presence of Zn(II). After exposure toward the Zn(II) ion, the characteristic emission of mptpy at 376 nm red-shifted to 406 nm with a strong enhancement upon an excitation at 280 nm, and then blue-shifted to 388 nm with the further addition of PPi. Absorption and fluorescence measurements showed that other phosphates including phosphate (Pi) as well as nucleotide triphosphates could not induce the spectral changes similar to PPi, demonstrating the unique binding effect between mptpy-Zn(II) and PPi. This process could also discriminate PPi from other inorganic anions. Therefore, a tpy-based fluorescence method for the highly selective recognition of PPi could be developed.     

Binding of inorganic oxoanions to macrocyclic ligands: effect of the degree of protonation on supramolecular assemblies formed by phosphate and [18]aneN(6)

Five macrocycle-oxoanion adducts have been isolated from aqueous solutions containing 1,4,7,10,13,16-hexaazacyclooctadecane ([18]aneN(6), L) and phosphoric acid whose pH had been adjusted to selected values in the 1-8 range. Four products, (H(6)L)(H(2)PO(4))(6).2H(3)PO(4) (1), (H(6)L)(H(2)PO(4))(6) (2), (H(4)L)(H(2)PO(4))(4).2H(2)O (4), and (H(4)L)(HPO(4))(2).7H(2)O (5) crystallized from aqueous solutions at pH 1, 3, 6, and 8, respectively, while (H(4)L)(H(2)PO(4))(4) (3) crystallized on diffusion of EtOH into an aqueous reaction mixture at pH 6. Single-crystal X-ray structure determinations enabled an examination of supramolecular interactions between protonated forms of [18]aneN(6), phosphoric acid and its conjugate bases, and water of solvation. The macrocycle adopts a variety of conformations in order to accommodate the supramolecular constructs formed by the oxoanions and solvent molecules as the relative proportions of interacting species are altered. At pH 1 and 3, the fully protonated macrocycle, [LH(6)](6+), is found with six H(2)PO(4)(-) anions. At pH 6 and 8, the tetraprotonated macrocycle, [LH(4)](4+), crystallizes with four H(2)PO(4)(-) and two HPO(4)(2)(-), respectively. Variations in the solute of crystallization are evident, with phosphoric acid being present at the lowest pH and water at pH 6 and 8. In 5, the seven unique water molecules form a string-of-pearls motif within which a new heptameric isomer, consisting of a water pentamer that uses a single water to interact with the other two unique water molecules, is found. Structures 1, 2, 4, and 5 exhibit eta-3 H-bonding of ammonium protons to a single oxygen of the guest phosphates located above and below the macrocyclic ring. In 3, two phosphate oxygens of the cavity anion interact with the macrocycle, one of which participates in eta-2 H-bonding with ammonium groups.     

Tetraphenylethene based zinc complexes as fluorescent chemosensors for pyrophosphate sensing

We described a serious of zinc complexes that exhibit characteristic fluorescence responses toward pyrophosphate(PPi) and adenosine triphosphate(ATP) in aqueous media. These novel probes exploited tetraphenylethene(TPE) as fluorophore and macrocycle-polyamine(including 1,4,7,10-tetraazacyclododecane and 1,4,7-triazacyclononane) Zn(II) complexes as binding group. These ‘‘OFF–ON' type probes exhibited promising selectivity and sensitivity to PPi and ATP via a restriction of intramolecular rotation(RIR) mechanism. The detection limit for PPi was found within nmol/L range.