Andrea Bencini

PROTON COORDINATION BY POLYAMINE COMPOUNDS IN AQUEOUS SOLUTION

Abstract The present article is concerned with proton transfer reactions in aqueous solution of open-chain, macrocyclic and macropolycyclic or cage compounds having nitrogen atoms as protonation sites in the molecular framework, although several compounds with additional different donors will be considered. The main purpose of this review is to collect some significant examples of proton transfer processes in order to show how the electronic properties and molecular topology of polyamines affect the thermodynamic parameters of their protonation equilibria.

Affinity and nuclease activity of macrocyclic polyamines and their CuII complexes.

The stability constants of Cu(II) complexes that consist of either an oxaaza macrocycle with two triamine moieties linked by dioxa chains, or two macrocyclic ligands with a polyamine chain which are connecting the 2 and 9 positions of phenanthroline, have been determined by means of potentiometric measurements. The results are compared to those reported for other ligands with a similar molecular architecture. Of the complexes that contain phenanthroline in their macrocycle, the Cu(II) ion of the complex with the smallest and most rigid macrocycle (L3) has an unsaturated coordination sphere, while in the complex with the largest macrocycle (L5) the Cu(II) ion is coordinatively almost saturated. These results are corroborated by the crystal structure of the [CuL5](ClO4)2 complex. The affinity of the ligands and the complexes towards nucleic acids was studied by measuring the changes in the melting temperature, which showed that the affinity of the macrocyclic ligands towards double-stranded DNA or RNA is generally smaller than that of their linear analogues that bear a similar charge, with a strong preference for polyA-polyU, a model for RNA. However, the complexes of two of the changed macrocyclic ligands which contain a phenanthroline unit (L4, L5) showed a distinctly larger increase in their melting temperature deltaTm with DNA (polydA-polydT), which is reversed again in favor of RNA upon metallation to the dinuclear copper complex with L5. Experiments with supercoiled plasmid DNA showed a particularly effective cleavage with a mononuclear Cu(II) complex that contains a phenanthroline unit (L6). Related ligands showed less activity towards DNA, but not so towards the biocidic bis(p-nitrophenyl)phosphate (BNPP). In both cases (with DNA and BNPP) the activity seemed to increase with decrease of coordinative saturation of the Cu(II) ion, with the exception of one particular ligand (L6). Experiments with radical scavengers in the DNA experiments showed some decrease in cleavage, which indicates the participation of redox processes.

Thermodynamic and structural aspects of transition metal compounds. Polynuclear complexes of aza-macrocycles

A. Introduction 51 B. Macrocycles and macrobicycles able to incorporate several metal ions 52 C. Bis(macrocycles) 71 D. Bridged macrocyclic metal complexes 73 Acknowledgements 83 References 83

Tuning the Activity of Zn(II) Complexes in DNA Cleavage : Clues for Design of New Efficient Metallo-Hydrolases

The hydrolytic ability toward plasmid DNA of a mononuclear and a binuclear Zn(II) complex with two macrocyclic ligands, containing respectively a phenanthroline (L1) and a dipyridine moiety (L2), was analyzed at different pH values and compared with their activity in bis( p-nitrophenyl)phosphate (BNPP) cleavage. Only the most nucleophilic species [ZnL1(OH)]+ and [Zn2L2(OH)2]2+, present in solution at alkaline pH values, are active in BNPP cleavage, and the dinuclear L2 complex is remarkably more active than the mononuclear L1 one. Circular dichroism and unwinding experiments show that both complexes interact with DNA in a nonintercalative mode. Experiments with supercoiled plasmid DNA show that both complexes can cleave DNA at neutral pH, where the L1 and L2 complexes display a similar reactivity. Conversely, the pH-dependence of their cleavage ability is remarkably different. The reactivity of the mononuclear complex, in fact, decreases with pH while that of the dinuclear one is enhanced at alkaline pH values. The efficiency of the two complexes in DNA cleavage at different pH values was elucidated by means of a quantum mechanics/molecular mechanics (QM/MM) study on the adducts between DNA and the different complexed species present in solution.

Exploring the Binding Ability of Phenanthroline-Based Polyammonium Receptors for Anions: Hints for Design of Selective Chemosensors for Nucleotides

The synthesis of receptor 2,6,10,14,18-pentaaza[20]-21,34-phenanthrolinophane (L1), containing a pentaamine chain linking the 2,9 positions of a phenanthroline unit, is reported. The protonation features of L1 and of receptor 2,6,10,14,18,22-hexaaza[23]-24,37-phenanthrolinophane (L2) have been studied by means of potentiometric, (1)H NMR, and spectrofluorimetric measurements; this study points out that the fluorescent emission of both receptors depends on the protonation state of the polyamine chain. In fact, the receptors are emissive only at neutral or acidic pH values, where all the aliphatic amine groups are protonated. Potentiometric titrations show that L2 is able to bind selectively ATP over TTP, CTP, and GTP. This selectivity is lost in the case of L1. (1)H and (31)P NMR measurements and molecular mechanics calculations show that the phosphate chains of nucleotides give strong electrostatic and hydrogen-bonding interactions with the ammonium groups of the protonated receptors, while the nucleobases interact either via pi-stacking with phenanthroline or via hydrogen bonding with the ammonium groups. Of note, MM calculations suggest that all nucleotides interact in an inclusive fashion. In fact, in all adducts the phosphate chain is enclosed within the receptor cavities. This structural feature is confirmed by the crystal structure of the [(H(6)L2)(2)(TTP)(2)(H(2)O)(2)](4+) adduct. Fluorescence emission measurements at different pH values show that L2 is also able to ratiometrically sense ATP in a narrow pH range, thanks to emission quenching due to a photoinduced electron transfer (PET) process from an amine group of the receptor to the excited phenanthroline.

Anion Binding by Protonated Forms of the Tripodal Ligand Tren

The interaction of the protonated forms of tris(2-aminoethyl)amine (tren) with NO(3)(-), SO(4)(2-), TsO(-), PO(4)(3-), P(2)O(7)(4-), and P(3)O(10)(5-) was studied by means of potentiometric and microcalorimetric measurements in a 0.10 M NMe(4)Cl aqueous solution at 298.1 +/- 0.1 K, affording stability constants and the relevant energetic terms DeltaH degrees and TDeltaS degrees of complexation. Thermodynamic data show that these anion complexation processes are mainly controlled by electrostatic forces, although hydrogen-bond interactions and solvation effects also contribute to complex stability, leading, in some cases, to special DeltaH degrees and TDeltaS degrees contributions. The crystal structures of [H(3)L][NO(3)](3) and [H(3)L][TsO](3) evidence a preferred tridentate coordination mode of the triprotonated ligands in the solid state. Accordingly, the H(3)L(3+) receptor binds a single oxygen atom of both NO(3)(-) and TsO(-) by means of its three protonated fingers, although in the crystal structure of [H(3)L][TsO](3), one conformer displaying bidentate coordination was also found. Modeling studies performed on the [H(3)L(NO(3))](2+) complex suggested that the tridentate binding mode is the preferred one in aqueous solution, while in the gas phase, a different complex conformation in which the receptor interacts with all three oxygen atoms of NO(3)(-) is more stable.

Potential ATPase mimics by polyammonium macrocycles: Criteria for catalytic activity

Abstract A series of polyammonium macrocycles, ranging in size from the 18-membered ring [18]aneN6 to the 36-membered [36]aneN12 were examined as potential ATPase mimics. The rates of hydrolysis of ATP were followed at pH 3.0 and 7.0 using 31P NMR and HPLC techniques. Stability constants as a function of degree of protonation, distribution curves for the ligands as a function of pH, and distribution curves for the mixed species of nucleotides, inorganic phosphate, and macrocycle were also determined. All of the macrocycles catalyzed the hydrolysis of ATP to some extent compared to noncatalyzed hydrolysis. A critical dependence on macrocyclic ring size was observed, with [21]aneN7 being the best catalyst at both pHs. Stability constants of the complexes formed between the phosphate species and macrocycle increase with increasing degree of protonation and decreasing ring size. The trend in stability constants for phosphate species was found to be PO43− > P2O74− > ATP4− > ADP3− > AMP2− for a given degree of protonation. The crystal structure of tetraprotonated [21]aneN7 was determined. The compound N7C14Cl4H41O crystallizes in the monoclinic space group P21 (#4) with unit cell dimensions a = 7.472(1), b = 19.480(2), c = 8.3638(9) A , β = 100.38(1)o, and V = 1197.4(3) A 3 . The structure was solved by direct methods and refined using full-matrix least-squares techniques to give a final R = 0.041 and Rw = 0.055.

Redox chemosensors: coordination chemistry towards CuII, ZnII, CdII, HgII, and PbII of 1-aza-4,10-dithia-7-oxacyclododecane ([12]aneNS2O) and its N-ferrocenylmethyl derivativeElectronic supplementary information (ESI) available: synthetic details including analytical and spectroscopic data for the isolated complexes. Ortep views of the coordination sphere around the metal centres in 1, 2 and 5. See http://www.rsc.org/suppdata/dt/b2/b210806m/

The coordination chemistry of the mixed donor 12-membered macrocyclic ligand 1-aza-4,10-dithia-7-oxacyclododecane ([12]aneNS2O) with CuII, ZnII, CdII, HgII, and PbII has been investigated both in water solution and in the solid state. The protonation constant for [12]aneNS2O and stability constants with the aforementioned metal ions have been determined potentiometrically and compared with those reported for other mixed N/S/O-donor tetradentate 12-membered macrocycles. The measured values are consistent with trends observed previously for aza macrocycles as secondary N-donors are replaced by O- and S-donors. In particular our results show that HgII in water has the highest affinity for [12]aneNS2O followed by CuII, CdII, PbII, and ZnII. For each considered metal ion, 1 ∶ 1 complexes of [12]aneNS2O have been isolated in the solid state; those of CuII, HgII, and CdII have also been characterised by X-ray crystallography. In the cases of copper(II) and cadmium(II) complexes the ligand adopts a folded [2424] conformation, whereas a more planar [3333] conformation is observed in the case of the mercury(II) complex. The macrocycle [12]aneNS2O and its structural analogue [12]aneNS3 have then been used as receptor units in the design and synthesis of the new ferrocene-containing redox-active ionophores N-ferrocenylmethyl 1-aza-4,10-dithia-7-oxacyclododecane (L1) and N-ferrocenylmethyl 1-aza-4,10,7-trithiacyclododecane (L2). Electrochemical studies carried out in MeCN in the presence of increasing amounts of CuII, ZnII, CdII, HgII, and PbII showed that the wave corresponding to the Fc/Fc+ couple of the uncomplexed ionophores L1 and L2 is gradually replaced by a new reversible wave at more positive potentials and corresponding to the Fc/Fc+ couple of the complexed ionophores. The maximum shift of the ferrocene oxidation wave was found for L1 in the presence of ZnII (230 mV) and PbII (220 mV), whereas for L2 a selective sensing response for CuII over the other guest metal cations was observed with an oxidation peak shift of 230 mV.

Selective recognition of carboxylate anions by polyammonium receptors in aqueous solution. Criteria for selectivity in molecular recognition

The interaction in aqueous solution of the polyammonium receptor 1,4,7,10,13,16,19-heptaazacyclohenicosane (8, [21]aneN7) with the carboxylic species, 1,2-benzenedicarboxylic (1), 1,3-benzenedicarboxylic (2), 1,2,3-benzenetricarboxylic (3), 1,3,5-benzenetricarboxylic (4), cis,cis-1,3,5-trimethyl-1,3,5-cyclohexanetricarboxylic (5), cis,trans-1,3,5-trimethyl-1,3,5-cyclohexanetricarboxylic (6), and citric (7) acids has been followed by potentiometry and cyclic voltammetry at 298.1 K in 0.15 mol dm–3 NaClO4, as well as by NMR spectroscopy. The analysis of the potentiometric and electrochemical data show the formation of 1:1 adducts. A novel method based on both potentiometry and cyclic voltammetry to establish selectivity patterns in systems presenting different protonation possibilities is introduced. Preorganization of the substrates in a shape complementary to the receptor yields interesting features such as the selective recognition of 5 over its epimer 6 and recognition of the dicarboxylic species 1 and 2 over the tricarboxylic ones 6 and 7.

Colorimetric response to anions by a "robust" copper(II) complex of a [9]aneN3 pendant arm derivative: CN- and I- selective sensing.

The 1 : 1 complex [Cu(L)](BF(4))(2)·MeCN (1) of the tetradentate ligand 1-(2-quinolinylmethyl)-1,4,7-triazacyclononane (L) selectively changes its colour in the presence of CN(-) in H(2)O and MeCN (without undergoing decomplexation from the macrocyclic ligand). The same complex in MeCN assumes different colours in the presence of CN(-) or I(-).