Bis- and tris-(methylphosphonic) acid derivatives of a 14-membered tetraazamacrocycle containing pyridine: synthesis, protonation and complexation studies

Complexation of C-Functionalized Cyclams with Copper(II) and Zinc(II): Similarities and Changes When Compared to Parent Cyclam Analogues

Herein, we report a comprehensive coordination study of the previously reported ligands cyclam, CB-cyclam, TMC, DMC, and CB-DMC and of their C-functional analogues, cyclam-E, CB-cyclam-E, TMC-E, DMC-E, and CB-DMC-E. This group of ligands includes cyclam, cross-bridged cyclams, their di- or tetramethylated derivatives, and the analogues bearing an additional hydroxyethyl group on one β-N position of the ring. The Cu(II) and Zn(II) complexes of these macrocycles have been highlighted previously for the biological interest, but the details of their structures in the solid state and in solution remained largely unexplored. In particular, we analyzed the impact that adding noncoordinating N-methyl and C-hydroxyethyl functionalities has in the structures of the complexes. All the Cu(II) and Zn(II) complexes were synthesized and investigated using single crystal X-ray diffraction and NMR, electronic absorption, and EPR spectroscopies, along with DFT studies. Dissociation kinetics experiments in acidic conditions and electrochemical studies were also performed. Special attention was paid to analyze the different configurations present in solution and in the solid state, as well as the impact of the C-appended hydroxyethyl group on the coordination behavior. Various ratios of the trans-I, trans-III, and cis-V configurations have been observed depending on the degree of N-methylation and the presence of the ethylene cross-bridge.

Aromatic and heteroaromatic azacrown compounds: advantages and disadvantages of rigid macrocyclic ligands

Current approaches to the synthesis of aromatic and heteroaromatic azamacrocycles and their derivatives are summarized and systematized. The relationship between the structure of azacrown compounds and their complexation behaviour towards metal cations is analyzed. The diversity of practical applications of azamacrocyclic derivatives in medicine, biology and analytical and organic chemistry, as well as for the design of molecular devices is demonstrated.

The bibliography includes 307 references.

Phosphonated chelates for nuclear imaging

A series of bis-, tris- and tetra-phosphonated pyridine ligands is presented. In view of their potential use as chelates for radiopharmaceutical applications, the physico-chemical properties of the ligands and of their Co(II), Ni(II), Cu(II), and Zn(II) complexes were studied by means of potentiometry and UV-Vis absorption spectroscopy. The pKa values of the ligands and of the complexes, as well as the stability constants for the formation of the complexes, are presented. The kinetic aspects of the formation of Cu(II) complexes and of their dissociation in acidic media were studied by means of stopped flow experiments, and the stability of the Cu(II) complex toward reduction to Cu(I) was investigated by cyclic voltammetry and by titration with different reducing agents. The different thermodynamic and kinetic aspects of the polyphosphonated ligands were compared with regard to the impact of the number of phosphonic acid functions. Considering the very promising properties for complexation, preliminary SPECT/CT imaging experiments were carried out on mice with (99m)Tc using the bis- and tetra-phosphonated ligands L(2) and L(1). Finally, a bifunctional version of chelate L(1), L*, was used to label MTn12, a rat monoclonal antibody with both specificity and relatively high affinity for murine tenascin-C. The labeling was monitored by MALDI/MS spectrometry and the affinity of the labeled antibody was checked by immunostaining experiments. After chelation with (99m)Tc, the (99m)Tc-L*-MTn12 antibody was injected into a transgenic mouse with breast cancer and the biodistribution of the labeled antibody was followed by SPECT/CT imaging.

Oxygen atom transfer mediated by an iron(IV)/iron(II) macrocyclic complex containing pyridine and tertiary amine donors

A new non-heme iron model complex containing a high-spin iron(II) complexed with N-methylated pyridine-containing macrocycle was synthesized and crystallographically characterized. The complex generates peroxo- and high-valent iron-oxo intermediates in reactions with tert-butylhydroperoxide and isopropyl 2-iodoxybenzoate, respectively, allowing to gain insight into the formation and reactivity of enzyme-like intermediates related to biological oxygen activation. The formation and reactivity of these intermediate species were investigated by the stopped-flow methodology. The mechanism of oxygen transfer to organic substrates involving reaction of oxoiron(IV) intermediate was elucidated on the basis of spectroscopic and kinetic data. Incorporation of a pyridine ring into the macrocycle increased the reactivity of the Fe(IV)=O intermediates in comparison with polyamine tetraaza macrocyclic complexes: ferryl (Fe(IV)=O) species derived from 3 demonstrated electrophilic reactivity in transferring an oxygen atom to substituted triarylphosphines and to olefins (such as cyclooctene). However, iron(III) alkylperoxo intermediate was unreactive with cyclooctene.

Synthesis, Cu(II) complexation, 64Cu-labeling and biological evaluation of cross-bridged cyclam chelators with phosphonate pendant arms

A new class of cross-bridged cyclam-based macrocycles featuring phosphonate pendant groups has been developed. 1,4,8,11-tetraazacyclotetradecane-1,8-di(methanephosphonic acid) (CB-TE2P, 1) and 1,4,8,11-tetraazacyclotetradecane-1-(methanephosphonic acid)-8-(methanecarboxylic acid) (CB-TE1A1P, 2) have been synthesized and have been shown to readily form neutral copper(II) complexes at room temperature as the corresponding dianions. Both complexes showed high kinetic inertness to demetallation and crystal structures confirmed complete encapsulation of copper(II) ion within each macrocycle's cleft-like structure. Unprecedented for cross-bridged cyclam derivatives, both CB-TE2P (1) and CB-TE1A1P (2) can be radiolabeled with (64)Cu at room temperature in less than 1 h with specific activities >1 mCi μg(-1). The in vivo behavior of both (64)Cu-CB-TE2P and (64)Cu-CB-TE1A1P were investigated through biodistribution studies using healthy male Lewis rats. Both new compounds showed rapid clearance with similar or lower accumulation in non-target organs/tissues when compared to other copper chelators including CB-TE2A, NOTA and Diamsar.

Highly stable acyclic bifunctional chelator for 64Cu PET imaging

Ligand L1, based on a pyridine scaffold, functionalized by two bis(methane phosphonate)aminomethyl groups, was shown to display a very high affinity towards Cu(II) (log K CuL=22.7) and selectivity over Ni(II), Co(II), Zn(II) and Ga(III) (Δ log K ML>4) as shown by the values of the stability constants obtained from potentiometric measurements. Insights into the coordination mode of the ligand around Cu(II) cation were obtained by UV-Vis absorption and EPR spectroscopies as well as density functional theory (DFT) calculations (B3LYP model) performed in aqueous solution. The results point to a pentacoordination pattern of the metal ion in the fully deprotonated [CuL1]6− species. Considering the beneficial thermodynamic parameters of this ligand, kinetic experiments were run to follow the formation of the copper(II) complexes, indicating a very rapid formation of the complex, appropriat e for 64Cu complexation. As L1 represents a particularly interesting target within the frame of 64Cu PET imaging, a synthetic protocol was developed to introduce a labeling function on the pyridyl moiety of L1, thereby affording L2, a potential bifunctional chelator (BFC) for PET imaging.

Cyclam derivatives containing three acetate pendant arms: synthesis, acid-base, metal complexation and structural studies

The ligands 1,4,8,11-tetraazacyclotetradecane-1,4,8-triacetic-11-methylphosphonic acid (H5te3a1p) and 1,4,8,11-tetraazacyclotetradecane-1,4,8-triacetic acid (H3te3a) were synthesized, the former one for the first time. The syntheses of these ligands were achieved from reactions on 1,4,8,11-tetraazacyclotetradecane-1,4,8-tris(carbamoylmethyl) hydroiodide (te3am x HI), and compounds (Hte3am)+, 1, and (H7te3a1p)2+, 4, were characterized by X-ray diffraction. Structures of two other compounds resulting from side-reactions, (H2te2lac)2+, 2, and (H4te2a2pOEt2)2+, 4, were also determined by X-ray diffraction. Potentiometric titrations of H5te3a1p and H3te3a were performed at 298.2 K and ionic strength 0.10 mol dm(-3) in NMe4NO3 to determine their protonation constants. 1H and 31P NMR titrations of H5te3a1p were carried out in order to determine the very high first protonation constant of this ligand and to elucidate the sequence of protonation. Potentiometric studies of the two ligands with Ca2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Pb2+ metal ions performed in the same experimental conditions showed that the complexes of H5te3a1p present very high thermodynamic stability while complexes of H3te3a, particularly Co2+ and Zn2+, are even more stable. 31P NMR spectra of the cadmium(II) complex of H5te3a1p showed that the phosphonate moiety was coordinated to the metal ion. The UV-vis-NIR spectroscopic data and magnetic moment values of Co2+ and Ni2+ complexes of H5te3a1p and H3te3a together with the EPR of the corresponding Cu2+ complexes indicated that all these complexes adopt distorted octahedral coordination geometries in solution. This was confirmed by the single crystal structure of [Cu2(Hte3a)(H2O)3Cl]Cl0.5(ClO4)0.5 x 2H2O that showed two distorted octahedral copper centres bridged by a N-acetate pendant arm with a CuCu distance of 4.890(1) A. The first one is encapsulated into the macrocyclic cavity surrounded by four nitrogen and two oxygen donors from the macrocycle, whereas the second one is on the periphery of the macrocycle and is coordinated to two oxygen atoms of one acetate pendant arm in chelating fashion, one chloride and three water molecules.

Properties of a new 4-imidazolyl derivative of a 14-membered tetraazamacrocyclic chelating agent

A new bis-N,N'-(5-methylimidazol-4-ylmethyl) derivative of a 14-membered tetraazamacrocycle, L1, has been synthesized. The protonation constants of this compound and the stability constants of its complexes with divalent first-row transition metal ions and Fe3+ were determined at 298.2 K in aqueous 0.10 mol dm(-3) KNO3. Compound L1 exhibits high overall basicity, which is mainly conferred by the imidazolyl groups. The complexes of the divalent first row-transition metal ions of L1 follow the Irving-Williams order of stability with the maximum for Cu2+ as expected, but a steep fall of constants is verified for the Mn2+, Fe2+ and Co2+, in one side, and for the Zn2+ complexes, in the other side. Additionally, L1 shows a large affinity for Fe3+, and the relative stability constants for its Cd2+ and Pb2+ complexes indicate that L1 may be useful for the complexometric determination of these two toxic metal ions in solutions containing both metal ions. These studies together with NMR, UV-vis and EPR spectroscopic data indicated the presence of mononuclear complexes, which adopt distorted pyramidal or octahedral geometries depending on the metal centre. The X-ray crystal structure of [Cu(HL1)](PF6)2(NO3).H2O showed that the coordination sphere of the copper centre can be described as a distorted square pyramid with the basal plane defined by three nitrogen donors of the macrocycle backbone and one nitrogen atom from one imidazolyl pendant arm. The apical position is occupied by the nitrogen atom of the macrocycle trans to the pyridine ring. To achieve this coordination environment, the macrocycle is folded along the axis defined by the two N atoms contiguous to the pyridine ring. The free methylimidazolyl arm points away from the metal centre leading to an intramolecular Cu[dot dot dot]N distance of 5.155(1) A.

Thermodynamic study of lanthanide(iii) complexes with bifunctional monophosphinic acid analogues of H4dota and comparative kinetic study of yttrium(iii) complexes

New bifunctional H(4)dota-like ligands with three acetic acid and one phosphinic acid pendant arms and propionate (H(5)do3ap(PrA)) or 4-aminobenzyl (H(4)do3ap(ABn)) reactive groups bound to the phosphorus atom were investigated. Potentiometric studies showed that the ligands have a similar basicity to the parent H(4)dota and the stability constants of their complexes with sodium(i) and selected lanthanide(III) ions are also similar. Formation and acid-assisted decomplexation kinetics of yttrium(III) complexes with a series of H(4)dota-like ligands (H(4)dota and its phosphinic/phosphonic acid analogues) were studied and the reactions are sensitive to a slight modification of the ligand structure. The (2-carboxyethyl)phosphinic acid derivative H(5)do3ap(PrA) and the phosphonic acid ligand H(5)do3ap form complexes faster than H(4)dota. The most kinetically inert complex is that with H(4)do3ap(ABn). Rates of complexation and decomplexation can depend on the ability to transfer proton(s) outside/inside the complex cavity and, therefore, on the hydrophobicity of the ligands. The results demonstrate that the new bifunctional ligands are suitable for labelling biomolecules with yttrium(iii) radioisotopes for utilization in nuclear medicine.

Metal Phosphonates Based on Bis(benzimidazol-2-ylmethyl)imino Methylenephosphonate: From Discrete Dimer to Two-Dimensional Network Containing Metallomacrocycles

Hydrothermal reactions of bis(benzimidazol-2-ylmethyl)imino(methylenephosphonic acid) {[(C(7)H(5)N(2))CH(2)]2NCH(2)PO(3)H(2), bbimpH2} with metal salts result in four new compounds, namely, Mn2{[(C(7)H(5)N(2))CH2]2NCH(2)PO(3)}2(H2O)2.2H2O (1), Cd2{[(C(7)H(5)N(2))CH2]2NCH(2)PO(3)}2.2H2O (2), Fe2{[(C(7)H(5)N(2))CH2]2NCH(2)PO(3)}2.H2O (3), and CuI(2){[(C(7)H(5)N(2))CH2]2NCH(2)P(OH)O2}2 (4). Compounds 1 and 2 have dinuclear structures in which two {MN(3)O(3)} octahedra are linked through edge sharing. In compound 3, a chain structure is observed where the {FeN(3)O(2)} trigonal bipyramids are linked by {CPO(3)} tetrahedra through corner-sharing. The structure of compound 4 is unique. The monovalent Cu(I) ions are connected by the imidazole nitrogen atoms from the bbimp(2-) ligands forming a 16-member metallomacrocycle. These metallomacrocycles are further connected by the phosphonate oxygen atoms, leading to a two-dimensional net containing 16- and 32-member rings. Magnetic studies of 1 and 3 reveal that weak ferromagnetic interactions are mediated between magnetic centers in compound 1, while antiferromagnetic interactions were observed in compound 3.

Thermodynamic, kinetic and solid-state study of divalent metal complexes of 1,4,8,11-tetraazacyclotetradecane (cyclam) bearing two trans (1,8-)methylphosphonic acid pendant arms

Divalent metal complexes of macrocyclic ligand 1,4,8,11-tetraazacyclotetradecane-1,8-bis(methylphosphonic acid)) (1,8-H4te2p, H4L) were investigated in solution and in the solid state. The majority of transition-metal ions form thermodynamically very stable complexes as a consequence of high affinity for the nitrogen atoms of the ring. On the other hand, complexes with Mn2+, Pb2+ and alkaline earth ions interacting mainly with phosphonate oxygen atoms are much weaker than those of transition-metal ions and are formed only at higher pH. The same tendency is seen in the solid state. Zinc(II) ion in the octahedral trans-O,O-[Zn(H2L)] complex is fully encapsulated within the macrocycle (N4O2 coordination mode with protonated phosphonate oxygen atoms). The polymeric {[Pb(H2L)(H2O)2].6H2O}n complex has double-protonated secondary amino groups and the central atom is bound only to the phosphonate oxygen atoms. The phosphonate moieties bridge lead atoms creating a 3D-polymeric network. The [{(H2O)5Mn}2(micro-H2L)](H2L).21H2O complex contains two pentaaquamanganese(II) moieties bridged by a ligand molecule protonated on two nitrogen atoms. In the complex cation, oxygen atoms of the phosphonate groups on the opposite sites of the ring occupy one coordination site of each metal ion. The second ligand molecule is diprotonated and balances the positive charge of the complex cation. Complexation of zinc(II) and cadmium(II) by the ligand shows large differences in reactivity of differently protonated ligand species similarly to other cyclam-like complexes. Acid-assisted dissociations of metal(II) complexes occur predominantly through triprotonated species [M(H3L)]+ and take place at pH < 5 (Zn2+) and pH < 6 (Cd2+).

Bis- and tris-(3-aminopropyl) derivatives of 14-membered tetraazamacrocycles containing pyridine: synthesis, protonation and complexation studies

New N-(3-aminopropyl) (L1, L2) and (2-cyanoethyl) (L3, L4) derivatives of a 14-membered tetraazamacrocycle containing pyridine have been synthesized. The protonation constants of L1 and L2 and the stability constants of their complexes with Ni2+, Cu2+, Zn2+ and Cd2+ metal ions were determined in aqueous solutions by potentiometry, at 298.2 K and ionic strength 0.10 mol dm(-3) in KNO3. Both compounds have high overall basicity due to the presence of the aminopropyl arms. Their copper(II) complexes exhibit very high stability constants, which sharply decrease for the complexes of the other studied metal ions, as usually happens with polyamine ligands. Mono- and dinuclear complexes are formed with L2 as well as with L1, but the latter exhibits mononuclear complexes with slightly higher K(ML) values while the dinuclear complexes of L2 are thermodynamically more stable. The presence of these species in solution was supported by UV-VIS-NIR and EPR spectroscopic data. The single crystal structures of [Cu(H2L2)(ClO4)]3+ and [CoL3Cl]+ revealed that the metal centres are surrounded by the four nitrogen atoms of the macrocycle and one monodentate ligand, adopting distorted square pyramidal geometries. In the [CoL3Cl]+ complex, the macrocycle adopts a folded arrangement with the nitrogen atom opposite to the pyridine at the axial position while in the [Cu(H2L2)(ClO4)]3+ complex, the macrocycle adopts a planar conformation with the three aminopropyl arms located at the same side of the macrocyclic plane.

Cyclam (1,4,8,11-tetraazacyclotetradecane) with one methylphosphonate pendant arm: a new ligand for selective copper(ii) binding

The new ligand, [(1,4,8,11-tetraazacyclotetradecan-1-yl)methyl]phosphonic acid (H(2)te1P, H(2)L), was synthesized and its complexing properties towards selected metal ions were studied potentiometrically. The ligand forms a very stable complex with copper(ii)(logbeta(CuL)= 27.34), with a high selectivity over binding of other metal ions (i.e. logbeta(ZnL)= 21.03). The crystal structures of the free ligand (in its protonated form with bromide as counter-ion) and two copper(ii) complexes (obtained by crystallization at various pH) were determined. The free ligand adopts the common conformation for such macrocycles with the protonated nitrogen atoms in the corners of a virtual rectangle. In the trans-Br,O-[Cu(Br)(Hte1P)].H(2)O species, the central metal ion is surrounded by four in plane nitrogen atoms, one oxygen atom of the pendant moiety in the apical position and a bromide anion positioned trans to the oxygen atom, forming a distorted octahedral coordination sphere. In the compound [Cu(H(2)te1P)][Cu(Hte1P)]Br(3).6H(2)O, obtained from a highly acidic solution, the bromide anions are placed further away from the copper(ii) ion and the coordination environment (N(4)O) is thus square-pyramidal. In both structures, the protons are associated with non-coordinated phosphonate oxygen atoms.

Thermodynamic and Kinetic Studies of Lanthanide(III) Complexes with H5do3ap (1,4,7,10-Tetraazacyclododecane-1,4,7-triacetic-10-(methylphosphonic Acid)), a Monophosphonate Analogue of H4dota

Solution properties of complexes of a new H4dota-like ligand containing three acetate and one methylphosphonate pendant arms (H5do3ap, H5L) were studied. The ligand exhibits a high last dissociation constant (pKA = 13.83) as a consequence of the presence of phosphonate moiety. In solution, successive attachment of protons leads to several reorganizations of protonation sites and the neutral zwitterionic species H5do3ap has the same solution structure as in the solid state, where the nitrogen atom binding methylphosphonate and the opposite nitrogen atoms are protonated. Stability constants with Na+ and trivalent lanthanide ions (La3+, Ce3+, Eu3+, Gd3+, Lu3+) and Y3+ have been determined. The constants are comparable or higher than those of H4dota due to the higher overall basicity of H5do3ap. Formation of the stable protonated complexes, as well as complexes with the L:M = 1:2 stoichiometry, was proved. Formation and decomplexation kinetics of the Ce3+ and Gd3+ complexes were investigated. The mechanism of formation of the H5do3ap complexes is similar to that observed for H4dota complexes and the complex species with mono- or diprotonated ligand on the cyclen ring are considered as the reaction intermediates. Acid-assisted decomplexation of H5do3ap complexes is faster in comparison with those of H4dota. This is caused by higher basicity of the phosphonate pendant arm and the ring nitrogen atoms, which facilitates the proton transfer from the bulk solution to the nitrogen atoms of cyclen ring.