The synthesis, structure and properties of copper(ii) complexes of asymmetrically functionalized derivatives of 1,4,7-triazacyclononane

endo- versus exo-Cyclic coordination in copper complexes with methylthiazolylcarboxylate tacn derivatives

Three tacn (1,4,7-triazacyclononane)-based ligands substituted by methylthiazolylcarboxylate (tha) and/or methylthiazolyl (th) arms have been examined for copper complexation with the aim to study the impact of carboxylate groups on the complexation of Cu(ii), which can present an endo- or exo-cyclic coordination. Two new ligands have been synthesised: H₃no3tha, tacn bearing three methylthiazolylcarboxylate arms, and H₂no1th2tha, tacn with one methylthiazolyl and two methylthiazolylcarboxylate arms, while Hno2th1tha had already been described. Their complexation behaviour with 1 or 1.5 equivalents of metal was studied on the basis of preliminary results showing the tendency of tha arms to form exocyclic polynuclear species. The solid state studies of the Cu(ii) and Zn(ii) complexes were investigated and some of their structures were characterised by X-ray diffraction. The physicochemical properties of the complexes in solution were also investigated by means of potentiometric measurements, UV-vis spectroscopy, EPR and computational studies, NMR characterisation of the corresponding Zn(ii) complexes and redox behaviour by electrochemistry. Mono- and tri-nuclear complexes ML and M₃L₂ were formed and isolated, highlighting the tendency of methylthiazolylcarboxylate arms, when carried by a tacn platform, to form exo-cyclic and polynuclear complexes. However, this exhaustive study evidences that the "out of cage" and "in cage" present different behaviour in terms of stability.

A Practical Route for the Preparation of 1,4,7-Triazacyclononanyl Diacetates with a Hydroxypyridinonate Pendant Arm

The preparation of triazamacrocyclic hydroxypyridinonate (HOPO-TACN) derivatives as potential chelators for metals in biomedical applications was reported. The synthesis is based on a convergent synthetic approach, in which the key intermediate di-tert-butyl-2,2′-(1,4,7-triazonane-1,4-diyl) diacetate was coupled with a hydroxypyridinonate pendant arm. The method is suitable for rapid syntheses of metal chelator HOPO-TACNs of biomedical interest.

Macrocyclic metal complexes for metalloenzyme mimicry and sensor development

Examples of proteins that incorporate one or more metal ions within their structure are found within a broad range of classes, including oxidases, oxidoreductases, reductases, proteases, proton transport proteins, electron transfer/transport proteins, storage proteins, lyases, rusticyanins, metallochaperones, sporulation proteins, hydrolases, endopeptidases, luminescent proteins, iron transport proteins, oxygen storage/transport proteins, calcium binding proteins, and monooxygenases. The metal coordination environment therein is often generated from residues inherent to the protein, small exogenous molecules (e.g., aqua ligands) and/or macrocyclic porphyrin units found, for example, in hemoglobin, myoglobin, cytochrome C, cytochrome C oxidase, and vitamin B12. Thus, there continues to be considerable interest in employing macrocyclic metal complexes to construct low-molecular weight models for metallobiosites that mirror essential features of the coordination environment of a bound metal ion without inclusion of the surrounding protein framework. Herein, we review and appraise our research exploring the application of the metal complexes formed by two macrocyclic ligands, 1,4,7-triazacyclononane (tacn) and 1,4,7,10-tetraazacyclododecane (cyclen), and their derivatives in biological inorganic chemistry. Taking advantage of the kinetic inertness and thermodynamic stability of their metal complexes, these macrocyclic scaffolds have been employed in the development of models that aid the understanding of metal ion-binding natural systems, and complexes with potential applications in biomolecule sensing, diagnosis, and therapy. In particular, the focus has been on "coordinatively unsaturated" metal complexes that incorporate a kinetically inert and stable metal-ligand moiety, but which also contain one or more weakly bound ligands, allowing for the reversible binding of guest molecules via the formation and dissociation of coordinate bonds. With regards to mimicking metallobiosites, examples are presented from our work on tacn-based complexes developed as simplified structural models for multimetallic enzyme sites. In particular, structural comparisons are made between multinuclear copper(II) complexes formed by such ligands and multicopper enzymes featuring type-2 and type-3 copper centers, such as ascorbate oxidase (AO) and laccase (Lc). Likewise, with the aid of relevant examples, we highlight the importance of cooperativity between either multiple metal centers or a metal center and a proximal auxiliary unit appended to the macrocyclic ligand in achieving efficient phosphate ester cleavage. Finally, the critical importance of the Zn(II)-imido and Zn(II)-phosphate interactions in Zn-cyclen-based systems for delivering highly sensitive electrochemical and fluorescent chemosensors is also showcased. The Account additionally highlights some of the factors that limit the performance of these synthetic nucleases and the practical application of the biosensors, and then identifies some avenues for the development of more effective macrocyclic constructs in the future.

The influence of the combination of carboxylate and phosphinate pendant arms in 1,4,7-triazacyclononane-based chelators on their 68Ga labelling properties

In order to compare the coordination properties of 1,4,7-triazacyclononane (tacn) derivatives bearing varying numbers of phosphinic/carboxylic acid pendant groups towards 68Ga, 1,4,7-triazacyclononane-7-acetic-1,4-bis(methylenephosphinic) acid (NOPA) and 1,4,7- triazacyclononane-4,7-diacetic-1-[methylene(2-carboxyethyl)phosphinic] acid (NO2AP) were synthesized using Mannich reactions with trivalent or pentavalent forms of H-phosphinic acids as phosphorus components. Stepwise protonation constants logK1-3 12.06, 3.90 and 1.95, and stability constants with GaIII and CuII, logKGaL 24.01 and logKCuL 16.66, were potentiometrically determined for NOPA. Both ligands were labelled with 68Ga and compared with NOTA (tacn-N,N',N″-triacetic acid) and NOPO, a TRAP-type [tacn-N,N',N″- tris(methylenephosphinic acid)] chelator. At pH 3, NOPO and NOPA showed higher labelling efficiency (binding with lower ligand excess) at both room temperature and 95 °C, compared to NO2AP and NOTA. Labelling efficiency at pH = 0-3 correlated with a number of phosphinic acid pendants: NOPO >> NOPA > NO2AP >> NOTA; however, it was more apparent at 95 °C than at room temperature. By contrast, NOTA was found to be labelled more efficiently at pH > 4 compared to the ligands with phosphinic acids. Overall, replacement of a single phosphinate donor with a carboxylate does not challenge 68Ga labelling of TRAP-type chelators. However, the presence of carboxylates facilitates labelling at neutral or weakly acidic pH.

Synthesis of new microbial pesticide metal complexes derived from coumarin-imine ligand

A series of metal complexes of zinc(II), cadmium(II), copper(II), nickel(II) and palladium(II) have been synthesized from coumarin-imine ligand, 8-[(1E)-1-(2-aminophenyliminio)ethyl]-2-oxo-2H-chromen-7-olate, [HL]. The structures of the complexes were proposed in the light of their spectroscopic, molar conductance, magnetic and thermal studies. The ligand coordinated in a tridentate manner through the azomethine nitrogen, the phenolic oxygen and the amine nitrogen and all complexes were non-electrolytes with different geometrical arrangements around the central metal ion. Photoluminescence data unambiguously showed remarkable fluorescence enhancement to Zn(2+) over other cations. The antimicrobial screening tests revealed that copper(II) complex exhibited the highest potency and its minimum inhibitory concentration on the enzymatic activities of the tested microbial species was determined. No toxin productivity was detected for all tested toxigenic species upon the exposure of copper complex.

Iron complexes of new pentadentate ligands containing the 1,4,7-triazacyclononane-1,4-diacetate motif. spectroscopic, electro-, and photochemical studies

Three new pentadentate, pendent arm macrocycles containing the 1,4,7-triazacyclononane-1,4-diacetate motif have been synthesized, and their coordination chemistry with Fe(III) has been investigated. Eight new octahedral Fe(III) complexes containing chloro, azido, or mu-oxo ligands have been synthesized, five of which have been characterized by X-ray crystallography. Spectroscopic characterization of these octahedral Fe(III) complexes by UV-vis, IR, electrochemistry, EPR, magnetic susceptibility, and zero-field Mössbauer measurements firmly establishes the high-spin state of the iron in all complexes. Electrochemistry studies of the azido-Fe(III) complexes show that they can be reversibly oxidized to the corresponding Fe(IV) species at -20 degrees C, and Fe(II), Fe(III), and Fe(IV) species show characteristic IR and UV-vis profiles. Photolysis of one of the azido complexes was studied as a function of temperature (room temperature vs 77 K) and wavelength (480, 419, and 330 nm). Photoreduction to a high-spin Fe(II) species occurs under all conditions, which is proposed to be the dominant photochemical pathway generally available to high-spin ferric azido complexes.