Metal Complexes with Macrocyclic Ligands. Part XXVII. Cu2+-promoted hydrolysis of carboxylic- and phosphonic-acid esters in the side chain of tetraazamacrocycles

The X-ray structures of the Cu²⁺ complexes of 1,4,8,11-tetraazacyclotetradecane derivatives with an ethylpro-pionate and diethylphosphonate group, 5 and 6 , respectively, indicate that the metal ion is pentacoordinated by the four N-atoms of the macrocycle and one O-atom. In the case of 5 , it is the carbonyl O-atom of the carboxylate group, whereas for 6 it is the phosphonyl O-atom of the phosphonate group. The hydrolysis kinetics of the functional group in the Cu²⁺ complexes with the 1,4,8,11-tetraazacyclotetradecanes 3 – 6 have been measured by pH-stat and stopped-flow techniques. The rate law for the hydrolysis of the carboxylates 3 – 5 is proportional to the complex concentration and to [OH^?] up to pH 13, whereas that of the phosphonate 6 is proportional to [OH^?] up to pH 11.5, becoming independent of [OH^?] at pH > 11.5. The mechanisms of these two reactions are discussed, considering the possibility of an intra- or an intermolecular OH^? attack and the results of the X-ray structure analyses.     

Kinetics and mechanism of adenosine 5′-triphosphate hydrolysis catalyzed by the Cu2+ ion: The role of conformation and the catalytic effect of the OH− ion

The kinetics of 5′-ATP hydrolysis catalyzed by the Cu²⁺ ion has been investigated by HPLC in the pH range 5.6–7.8 at 25°C. Two series of experiments differing in the initial [Cu · ATP]₀ (1: 1) concentration have been carried out. The reaction was being conducted up to ≈40% ATP conversion. The (CuATP^2?)₂OH^??ub;DOH^??ub; complex, which consists of two monomeric Cy(CuATP^2?) molecules (in which the N7 atom and the γ-phosphate group are coordinated to Cu²⁺), is responsible for the formation of CuADP^? + P_i (P_i is an inorganic phosphate). The highest possible DOH^? concentration at a given pH is reached at the initial stage of hydrolysis. The pH value at which the highest initial rate of ADP formation is reached (pH_max (w _{0, ADP})) decreases as the D concentration increases. At pH > pH_max, the decrease in the ADP formation rate in the course of the processes is pH-independent and, once an ATP conversion of 20–26% is reached, hydrolysis proceeds in a steady-state regime such that ADP and AMP form from ATP by parallel reactions. The participation of the OH^? ion in the catalysis of the formation of hydrolysis intermediates is considered.     

Metal complexes with macrocyclic ligands. Part XXXIV. Geometrical and conformational changes during the on/off reaction of the side chain in the CuII complex of 11-(3-aminopropyl)-1,4,7,11-tetraazacyclotetradecane

Solution studies of the Cu²⁺ complex with 11-(3-aminopropyl)-1,4,7,11-tetraazacyclotetradecane(L) indicate that, depending on the pH and on the age of the solution, different species are present. Dissolving the solid [CuL](ClO₄)₂ in slightly acidic solution gives the protonated complex AH , characterized by an absorption maximum at 574 nm, by a relatively fast proton-induced dissociation kinetics and by the typical colour change in basic solution to give the deprolonated form A with coordinated side chain. AH slowly interconverts in acidic solution to a new species BH , which has an absorption maximum at 547 nm, and which is kineticaily more stable against acid dissociation and shows no coordination of the amino group of the side chain. In alkaline solution, however, the deprotonated form B deliver A in a base induced reaction. The X-ray diffraction studies of A and BH allow to determine the geometry of the metal ion and the configuration of the macrocycle. In A , the Cu²⁺ is pentacoordinated by the five N-atoms of the ligand and the macrocycle is in the RRSR configuration, whereas in BH the Cu²⁺ is octahedrally coordinated by the four N-atoms of the macrocycle and two axial perchlorate O-atoms with the macrocycle in the RRRS configuration. The amino group of the side chain is protonated and not coordinated. Thus, the on/off equilibrium of the side chain not only changes the geometry of the metal ion, as is generally found, but also alters the macrocyclic moiety.     

Metal Complexes with Macrocyclic Ligands. IX. Synthesis and properties of a new class of branched N4-macrocycles with an additional ligating group in the side chain

In the presence of Ni²⁺ the template reaction between 2,6-diacetylpyridine and 4-(2-dimethylaminoethyl)- or 4-(2-hydroxyethyl)-1, 7-diamino-4-azaheptane yields the complexes of either the open-chain ligand ( 3 and 11 ) or of the macrocycle ( 4 and 12 ). Reduction of the imino group in 4 and 12 with PtO₂/H₂ gives 5 and 13 , respectively. In the case of the dimethylamino derivative 5 a mixture of at least four isomers was obtained. These were partially separated by chromatography on Sephadex SP-25 cation exchanger. Through demetalation of the Ni²⁺ complexes by cyanide the new macrocycles 7 and 14 were isolated, from which the corresponding Zn²⁺ and Cu²⁺ complexes were prepared. The macrocyclic Ni²⁺-complexes 4, 12, 5 and 13 can exist in two forms depending on the pH of the solution. At low pH protonation of the dimethylamino or hydroxy group in the side chain occurs. The metal ion is then bound to the four nitrogen atoms of the macrocycle in a square planar ligand field. At higher pH, however, the dimethylamino or hydroxy group (the last one also in its deprotonated form) can coordinate to one of the axial positions, whereby pseudooctahedral coordination geometry is induced. This reaction can be quantitatively described by a reversible acid-base equilibrium, the pK_H of which greatly depends on the nature of the functional group, the degree of unsaturation of the macrocycle and the metal ion. The acid-base reaction and the concomitant structural change are a direct consequence of the unique combination of the rigid and kinetically stable structure of the macrocycle and of the flexible and kinetically labile functional group of the side chain.     

Effects of solvent on the reactions of coordination complexes: Part 17. Kinetics and mechanism of base hydrolysis of (αβS) (salicylato) (tetraethylenepentamine)cobalt (III) in methanol + water and dimethylsulphoxide + water media

The base hydrolysis of (αβS) (salicylato) (tetraethylenepentamine)cobalt(III) has been investigated in MeOH + water and DMSO + water media (0–70% (v/v) cosolvents) at 20.0 ? t°C ? 35.0 and I = 0.10 mol dm^?3 (ClO₄^?). The phenoxide species [(tetren)CoO₂CC₆H₄O]⁺ undergoes both OH^?-independent and OH^?-catalyzed hydrolysis via SN₁ICB and SN₁CB mechanism, respectively. The OH^?-independent hydrolysis of the phenoxide species is catalyzed by both DMSO + water and MeOH + water media, the former exerting a much stronger rate accelerating effect than the latter. The OH^?-catalyzed reaction is strongly accelerated by DMSO + water medium but insensitive to the composition of MeOH + water medium up to 40% (v/v) MeOH beyond which it was not detectable under the experimental conditions. Data analysis has been attempted on the basis of the solvent stabilizing and destabilizing effects on the initial state and transition state of the concerned reactions. The nonlinear variation of the activation parameters, ΔH^≠ and ΔS^≠, with solvent compositions presumably indicates that the solvent structural effects mediate the energetics of solvation of the initial state and transition state of the concerned reactions. The linearity in ΔH^≠ vs. ΔS^≠ plot accomodating all data for k₁ and k₂ paths in DMSO + water and MeOH + water further suggests that the solvent effects on these parameters are mutually compensatory.     

Metal Complexes with Macrocyclic Ligands. Part XXVI. Synthesis and spectral properties of Cu2+ complexes with mono-N-functionalized 1,4,8-trimethyl-1,4,8,11-tetraazacyclotetradecanes

A series of mono-N-functionalized 1,4,8-trimethyl-1,4,8,11-tetraazacyclotetradecanes 3 – 14 were synthesized by alkylating the secondary N-atom of the macrocycle 1 . The spectral properties of the Cu²⁺ complexes, studied under different pH conditions, are discussed in relation to the possibility of coordination of the donor group of the side chain to the axial position of the metal ion and to the effect of the length of the side chain.     

Simulation of the kinetics of adenosine 5′-triphosphate hydrolysis catalyzed by the Cu2+ ion: The role of conformation and the catalytic effect of the OH− ion

The hydrolysis kinetics of the dimeric complex (CuATP^2? · OH₂)₂ {D} up to ≈40% ATP conversion at 25°C, pH 5.7–7.8, and [Cu · ATP]₀ = (2.07 ± 0.03) × 10^?3 mol/l is analyzed by numerical simulation. CuADP^? + P_i (P_i is an inorganic phosphate) form from DOH^?, and the latter forms rapidly from D. The abstraction of H⁺ from the coordinated H₂O molecule is an irreversible reaction involving an OH^? ion from the medium. The maximum possible DOH^? concentration at a given pH is reached at the initial stage of hydrolysis (0.3–6.0 min after the initiation of hydrolysis). CuADP^? + P_i form from D via two consecutive irreversible steps. The ADP buildup rate in the process is determined by the reversible conformational transformation of DOH^? resulting in a pentacovalent intermediate (IntK). OH^? ions from the medium are involved both in IntK formation and in the reverse reaction and are a hydrolysis inhibitor. AMP forms from the intermediate IntK₃, which forms reversibly from DOH^?, OH^? ions from the medium being involved in the forward and reverse reactions. This is followed by irreversible (AMPH)^? formation involving H₃O⁺ ions from the medium. The rate and equilibrium constants are determined for the formation and decomposition of hydrolysis intermediates. The concentrations of the intermediates are plotted versus time for various pH values. The structures of the intermediates are suggested. The causes of a peak appearing in the initial ADP formation rate versus pH curve are analyzed.     

Synthesis and evaluation of benzothiazolyl and benzimidazolyl asparagines as amino acid based selective fluorimetric chemosensors for Cu

A series of emissive N-tert-butyloxycarbonyl benzyl ester asparagines bearing benzothiazole and benzimidazole units at the side chain, functionalised with electron donor or acceptor groups, were evaluated as novel amino acid based fluorimetric chemosensors for transition metal cations, such as Cu²⁺, Zn²⁺, Co²⁺ and Ni²⁺. Selective removal of the protecting groups at the N- and C-terminals was carried out in order to assess the influence of the presence of blocking groups on the overall coordination ability. The results indicate that there is a strong interaction through the donor N, O and S atoms at the side chain of the various asparagines, with high selectivity towards Cu²⁺ in a 1:1 complex stoichiometry. Association constants and detection limits for Cu²⁺ were calculated. The photophysical and metal ion sensing properties of these asparagines suggest that they can be suitable for incorporation into peptidic chemosensor frameworks.     

Metal Complexes with Macrocyclic Ligands. Part XLI. Nickel(II) and copper(II) complexes with mono-N-functionalized dithiadiazamacrocycles

Three N₂S₂ macrocycles ( 3, 10, 12 ) carrying an amino group as a pendant arm have been synthesized and their complexation properties towards Ni²⁺ and Cu²⁺ studied. The crystal structures of the Cu²⁺ complexes with 10-methyl-1,4-dithia-7,10-diazacyclododecane-7-ethanamine ( 3 ) and 11-methyl-1,4-dithia-8,11-diazacyclotetradecane-8-ethanamine ( 10 ) show that, in both cases, the Cu²⁺ is pentacoordinated by the four donor atoms of the macrocycle and the amino group of the side chain. In aqueous solution, however, two forms of the complexes with stoichiometries [MLH] and [ML] (M = Cu²⁺ or Ni²⁺) have been observed. In [MLH], the amino group is protonated and does not bind to the metal ion, whereas in [ML] the amino group is bound, and a pentacoordinated geometry results. The pK_a values for the equilibrium [ML] + H⁺?[MLH]⁺ decrease in the order 12 > 10 > 3 , indicating that the 2-aminoethyl side chain binds better to the Cu²⁺ than the 3-aminopropyl side chain. Cyclic voltammetry for the Cu²⁺/Cu⁺ pair shows that the 2-aminoethyl pendant arm stabilizes the Cu²⁺ oxidation state, when the metal ion is in the 14-membered ring ( 10 ), whereas it stabilizes Cu⁺ for the 12-membered macrocycle ( 3 ).     

Supramolecular Functions Related to the Redox Activity of Transition Metals

Abstract

The combination of the properties of different subunits in a multicomponent system may give rise to a function which is defined supramolecular. The presence of transition metals in one or more subunits may induce inter-component processes related to their redox and electron transfer (eT) properties, which trigger the supramolecular function (SF). The following examples are considered: (1) a receptor for transition metals is covalently linked to a fluorescent fragment; following recognition, a metal-to-fluorophore eT process quenches the fluorescence. SF: fluorosensing. (2) an azacyclam macrocycle, hosting the Ni^II/Ni^III redox couple, is covalently linked to a photoactive fragment: the Ni^III state quenches the neighboring fluorophore through an eT mechanism, the Ni^II state does not. SF: redox switching of a fluorescent signal. (3) a Cu^II ion is coordinated by two 2,2′-bipyridine molecules, each bearing a cyclam subunit containing a nickel centre; when nickel is in the divalent state, an inorganic anion X^? (N₃ ^?,NCO^?,NCS^?) is bound to Cu^II; on oxidation, X^? moves to the Ni^III centre. SF: electrochemically triggered translocation of X^? from copper to nickel and vice versa.     

Metal complexes with macrocyclic ligands. Part XXV. One-step synthesis of mono-N-substituted azamacrocycles with a carboxylic group in the side-chain and their complexes with Cu2+ and Ni2+

Mono-N-substituted azamacrocycles 2 – 7 , containing a carboxyalkyl or carboxyaryl side-chain, are obtained by reacting a five-fold excess of the macrocycle with 1 equiv. of a suitable halogenocarboxylic acid in alkaline aqueous EtOH. For halogenocarboxylic acids, which easily lactonize under alkaline conditions, a variant with the corresponding ester or nitrile as alkylating agent is also described. The salient point of this synthesis lies in the use of an excess of the macrocycle over the alkylating agent, thus reducing the amount of polyalkylation to a minimum, and in the easy separation of the excess of unreacted educt from the aminocarboxylic acid. These new ligands form Ni²⁺ and Cu²⁺ complexes, the spectral properties of which have been studied. In the case of the Cu²⁺ complexes with ligand 2 , 3 , and 6 , a pH-dependent color change is observed. This is explained with an equilibrium between a species, in which the carboxylate group is bound to the metal, and one, in which it is protonated and non-coordinated. In the case of the Ni²⁺ complexes with the same ligands, only the species with a coordinated carboxylate was observed. In the Cu²⁺ and Ni²⁺ complexes with ligands 4 and 5 , however, the carboxylate group does not coordinate at all, because of the length or the special structure of the chain.     

Recognition and catalytic hydrolysis of adenosine 5′-triphosphate by cadmium(II) and L-glutamic acid

Interactions among Cd²⁺, glutamic acid (Glu), and adenosine 5′-triphosphate (ATP) have been studied by potentiometric pH titration, IR, Raman, fluorescence, and NMR methods. In the Cd²⁺–ATP binary system, the main interaction sites are the α-, β-, and γ-phosphate groups, N-1, and/or N-7. Cd²⁺ binds to the N-1 site at relatively low pH and binds to the N-7 site of adenosine ring of ATP with increasing pH. In the Cd²⁺–Glu–ATP ternary system, ATP mainly binds to Cd²⁺ by the triphosphate chain. Oxygens of Glu coordinate with Cd²⁺ to form a complex to catalyze ATP hydrolysis. Hydrolysis of ATP catalyzed by the CdGlu complex was studied at pH 7.0 and 80°C by ³¹P-NMR spectrometry. Kinetics studies showed that the rate constant of ATP hydrolysis was 0.0199?min^?1 in the ternary system, which is 9.9-fold faster than that in the ATP solution (2.01?×?10^?3?min^?1). Hydrolysis occurs through an addition–elimination reaction mechanism with Cd²⁺ regulating the recognition and catalytic hydrolysis of ATP; water participates in the hydrolysis reaction of ATP at different steps with different functions in the ternary system.     

Study of the Alkaline Hydrolysis of the Azetidin-2-one Ring by ab initio Methods: Influence of the solvent

A comprehensive study of the alkaline hydrolysis of the β-lactam ring of azetidin-2-one was carried out using ab initio molecular-orbital calculations at the RHF/6-31 + G* level. The influence of the solvent on this reaction was investigated by using the reaction field method (SCRF); the solvent was found to suppress the interference of some gas-phase reactions and allow the presence of a transition state to be detected as the nucleophile approaches the β-lactam ring. The transition state corresponds to a structure where the OH^? group lies at a distance of 1.927 Å from the C?O group of the β-lactam ring and exhibits a potential barrier of 13.6 kcal/mol.     

A novel method for the simple and simultaneous preconcentration of Pb2+, Cu2+ and Zn2+ ions with aid of diethylenetriamine functionalized SBA-15 nanoporous silica compound

A rapid method for the extraction and monitoring of nanogram level of Pb²⁺, Cu²⁺ and Zn²⁺ ions using uniform silanized mesopor (SBA-15) functionalized with diethylenetriamine groups and flame atomic absorption spectrometry (FAAS) is presented. The preconcentration factor of the method is 100 and detection limit of the technique is 5.5?ng?mL^?1 and 1.4?ng?mL^?1 and 0.1?ng?mL^?1 for Pb²⁺, Cu²⁺ and Zn²⁺ respectively. The time and the optimum amount of the sorbent, pH and minimum amount of acid for stripping of ions from functionalized SBA-15 were tested. The maximum capacity the functionalized SBA-15 was found to be 183.0 (±1.9) µg, 156.0 (±1.5) µg and 80.0 (±1.6) µg of Pb²⁺, Cu²⁺ and Zn²⁺/mg functionalized SBA-15, respectively.     

Metal Complexes with Macrocyclic Ligands. Part XXIX.. Structural and kinetic studies of two isomeric Cu2+ complexes with 1,4-dimethyl-8-[2-(2-pyridyl)ethyl]-1,4,8,11-tetraazacyclotetradecane

The synthesis and complexation properties of 1,4-dimethyl-8-[2-(2-pyridyl)ethyl]-1,4,8,11-tetraazacyclotetra-decane ( 2 ) are described. This ligand forms with Cu²⁺ two complexes, one of which has been characterized by X-ray structure analysis. The structural, spectral, and kinetic studies indicate that the two Cu²⁺ complexes are isomers with the macrocycle in the trans-III and trans-I configuration. The rate of the interconversion of the trans-I isomer to the thermodynamically more stable trans-III species is proportional to [OH^?]. A mechanism for this reaction is proposed.     

Hg and Cu selective detection using a dual channel receptor based on thiopyrylium scaffoldings

2,4,6-Triphenylthiopyrylium functionalized with an aza-oxa-thia macrocycle is able to selectively recognize Hg²⁺ cation by a color change and Cu²⁺ cation by a remarkable significant emission enhancement.     

Metal complexes with macrocyclic ligands. XVI. Spectrophotometric and thermodynamic studies of solvents and unidentate ligands interaction with the pentacoordinate Co2+-, Ni2+- and Cu2+-complexes of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane

The interaction of solvents and of unidentate ligands such as N, SCN^?, OCN^? and OH^? with the Co²⁺-, Ni²⁺ and Cu²⁺-complexes of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (TMC) have been studied by Spectrophotometric and calorimetric techniques. The spectra in different solvents (Table 2) show that the Ni²⁺- and probably also the Cu²⁺-complex with TMC exist as square planar or pentacoordinate species or as a mixture of both, depending on the donor properties of the solvent. The [Co(TMC)]²⁺-complex is pentacoordinate in all the solvents studied. Ternary complexes [M(TMC)X]ⁿ⁺ are also formed by the unidentate ligands X = N, OCN^?, OH^?, F^? and NH₃ and their stability constants have been determined. Interesting is the high selectivity of [Ni(TMC)]²⁺ towards the addition of a further donor (Table 3). Only small ligands such as those listed above form stable adducts, whereas the larger ones such as imidazole or pyridine do not. This is a consequence of the special structure of the complex and of the trans-I-(RSRS)- conformation of the ligand in these complexes. Since the four methyl groups are all on the side of the macrocycle to which the additional unidentate ligand binds, steric interaction between the four methyl groups and the larger ligands prevents the formation of the adducts. The calorimetric measurements show that the stability of the complexes [M(TMC)X]ⁿ⁺ is due to both an enthalpic and entropic contribution which differ in their magnitude (Table 4). This indicates that several antagonistic factors are important in determining the overall stability.     

Reactivity of base catalysed hydrolysis of 2-pyridinylmethylene-8-quinolinyl-Schiff base iron(II) iodide complexes: solvent effects

Three ligands of 2-pyridinylmethylene-8-quinolinyl (L1), methyl-2-pyridinylmethylene-8-quinolinyl (L2), and phenyl-2-pyridinylmethylene-8-quinolinyl (L3), Schiff bases were synthesised by direct condensation of 8-aminoquinoline with 2-pyridinecarboxaldehyde, 2-acetylpyridine, or 2-benzoylpyridine. They coordinated to Fe(II) ion in a 1: 2 mole ratio followed by treatment with iodide ions affording complexes with a general formula [Fe(L)₂]I₂·2H₂O, (L = L1, L2, or L3). Spectrophotometric evaluation of the kinetics of base catalysed hydrolysis of these complex cations was carried out with an aqueous solution of NaOH in different ratios of water/methanol binary mixtures. Kinetics of the hydrolysis followed the rate law (k ₂[OH^?] + k ₃[OH^?]²)[complex]. Reactivity trends and their rate constants were compared and discussed in terms of ligand structure and solvation parameters. The methanol ratio affects the hydrolysis as a co-solvent which was analysed into initial and transition state components. The increase in the rate constant of the base hydrolysis of Fe(II) complexes, as the ratio of methanol increases, is predominantly caused by the strong effect of the organic co-solvent on the transition states.     

Metal Complexes with Macrocyclic Ligands Part XXX. Synthesis and structure of halocuprates of tetraprotonated 1,4,8,11-tetraazacyclotetradecane and its Cu2+ complex

By mixing acidic solutions of 1,4,8,11-tetraazacyclotetradecane (Cy) with CuX₂ (X = Cl^?, Br^?), either the hexahalocuprates of the tetraprotonated form of the macrocycle ([CyH₄] [CuX₆]) or the tetrahalocuprates of its Cu²⁺ complex ([CuCy] [CuX₄]) are obtained. The structures of the chloro derivatives are established by X-ray diffraction analysis. In [CyH₄] [CuCl₆], the Cu²⁺ is in a tetragonally distorted octahedral geometry with four short and two long Cu? Cl bonds. The tetraprotonated macrocycle is centrosymmetric, and its conformation is exodentate, so that the four ammonium groups are as far as possible from each other to minimize the electrostatic repulsion. In [CuCy] [CuCl₄], the Cu²⁺ ion complexed by the macrocycle is surrounded by four N-atoms in a square-planar arrangement. In addition, the axial positions are occupied by two Cl^? ions of two CuCl units, which act as bridges. The macrocycle is in the trans-III-configuration. The other Cu²⁺ ion is coordinated by four Cl^? ions in a distorted tetrahedral geometry. IR and VIS spectra of the chloro and bromo derivatives are used to discuss the structure of the bromo species.     

Metal Complexes with Macrocyclic Ligands. Part XXVIII.. Structures and stabilities of the Cu2+ complexes with 1,4,7-triazacyclononane-1-acetic acid

The potentiometric study of the complexation of 1,4,7-triazacyclononane-l-acetic acid ( 1 ) with Cu²⁺ (I = 0.5 (KNO₃), T = 25°) indicates the presence of the species [Cu( 1 )], [Cu( 1 )OH], [Cu( 1 )₂], and [(Cu( 1 ))₂OH], the stability constants of which are determined. The two complexes [Cu( 1 )]ClO₄ and [(Cu( 1 )₂)OH]ClO₄ were also characterized by X-ray structure analysis. In both cases, the Cu²⁺ ion is in a distorted square-pyramidal arrangement, penta-coordinated by the three N-atoms of the macrocycle, an O-atom of the carboxylate, and an additional O-atom either from a second carboxylate or from an OH^?, acting as a bridge between two metal centres.