Heterometallic 3d–4f Coordination Polymers Based on 1,4,7-Triazacyclononane-1,4,7-triyl-tris(methylenephosphonate)

Transition metal complexes of the (2,2,2-trifluoroethyl)phosphinate NOTA analogue as potential contrast agents for 19F magnetic resonance imaging

A new hexadentate 1,4,7-triazacyclononane-based ligand bearing three coordinating methylene-(2,2,2-trifluoroethyl)phosphinate pendant arms was synthesized and its coordination behaviour towards selected divalent (Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) and trivalent (Cr3+, Fe3+, Co3+) transition metal ions was studied. The ligand forms stable complexes with late divalent transition metal ions (from Co2+ to Zn2+) and the complexes of these metal ions are formed above pH ∼3. A number of complexes with divalent metal ions were structurally characterized by means of single-crystal X-ray diffraction. The complex of the larger Mn2+ ion adopts a twisted trigonally antiprismatic geometry with a larger coordination cavity and smaller torsion of the pendant arms, whereas the smaller ions Ni2+, Cu2+ and Zn2+ form octahedral species with a smaller cavity and larger pendant arm torsion. In the case of the Co2+ complexes, both coordination arrangements were observed. The complexes with paramagnetic metal ions were studied from the point of view of potential utilization in 19F magnetic resonance imaging. A significant shortening of the 19F NMR longitudinal relaxation times was observed: a sub-millisecond range for complexes of Cr3+, Mn2+ and Fe3+ with symmetric electronic states (t2g3 and HS-d5), the millisecond range for the Ni2+ and Cu2+ complexes and tens of milliseconds for the Co2+ complex. Such short relaxation times are consistent with a short distance between the paramagnetic metal ion and the fluorine atoms (∼5.5-6.5 Å). Among the redox-active complexes (Mn3+/Mn2+, Fe3+/Fe2+, Co3+/Co2+, Cu2+/Cu+), the cobalt complexes show sufficient stability and a paramagnetic-diamagnetic changeover with the redox potential lying in a physiologically relevant range. Thus, the Co3+/Co2+ complex pair can be potentially used as a smart redox-responsive contrast agent for 19F MRI.

Hydrated Metal Ions as Weak Bronsted Acids Show the Promoting Effects in Proton Conduction

It is well-known that the hydrated metal ions can act as Bronsted acids, which tend to donate protons increasing the acidic proton concentration in materials, as well as the proton...

Comparison of phosphonate, hydroxypropyl and carboxylate pendants in Fe(III) macrocyclic complexes as MRI contrast agents

Fe(III) macrocyclic complexes containing a macrocycle and three pendant groups including phosphonate (NOTP =1,4,7-triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid), carboxylate (NOTA = 1,4,7 - triazacyclononane - N,N',N″ - triacetate) or hydroxypropyl (NOHP =(2S,2'S,2"S)-1,1',1″-(1,4,7-triazonane-1,4,7-triyl)tris(propan-2-ol)) were studied in order to compare the effect of these donor groups on solution chemistry and water proton relaxivity. All three complexes, Fe(NOTP), Fe(NOHP) and Fe(NOTA), display a large degree of kinetic inertness to dissociation in the presence of phosphate and carbonate, under acidic conditions of 100 mM HCl or 1 M HCl or to trans-metalation with Zn(II). The r1 proton relaxivity of the complexes at 1.4 T, 33 °C is compared over the pH range of 1 to 10. At pH 7.4, 33 °C, 1.4 T, Fe(NOHP) has the largest relaxivity (1.5 mM-1 s-1), Fe(NOTP) is second at 1.0 mM-1 s-1, whereas Fe(NOTA) is the lowest at 0.61 mM-1 s-1. Fe(NOTP), Fe(NOHP) and Fe(NOTA) all show an increase in relaxivity at very acidic pH values (< 3) that is consistent with an acid-catalyzed process. Variable temperature 17O NMR studies at near neutral pH are consistent with the absence of an inner-sphere water molecule for Fe(NOTP) and Fe(NOHP), supporting second-sphere or outer-sphere water contributions to proton relaxation. Fe(NOTP) shows contrast enhancement in T1 weighted MRI studies in mice and clears through a renal pathway.

Polar layered coordination polymers incorporating triazacyclononane-triphosphonate metalloligands

Reactions of metalloligands M^III(notpH₃) (M = Fe, Co and notpH₆ = 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid)) with Zn(OAc)₂ under hydrothermal conditions resulted in new metal phosphonates Zn₂Fe(notp)Cl(H₂O) (1) and ZnCo(notpH)(H₂O)·2H₂O (2). They crystallize in polar space groups P6₃ (for 1) and Pca2₁ (for 2), respectively, and exhibit layer structures in which the inorganic layers are separated by the organic groups of the notp ligands. However, the layer topologies of the two compounds are quite different. In 1, the layer contains 6-membered rings composed of one {FeN₃O₃} octahedron, one {Zn1O₃Cl}, one {Zn2O₄} and three {PO₃C} tetrahedra via corner-sharing connections, while in 2, the layer contains 10-membered rings composed of two {CoO₃N₃} octahedra, three {ZnO₄} and five {PO₃C} tetrahedra via vertex-sharing connections. Dielectric measurements on single crystals of 2 confirmed the presence of high dielectric anisotropy. Proton conductivity measurements revealed that the proton conduction is more favourable in 2 due to the presence of a continuous hydrogen bond network in this compound.

Metal phosphonates incorporating metalloligands: assembly, structures and properties

This feature article summarizes the current status of metal–metalloligand phosphonates including the synthetic strategies, crystal structures and properties. Future challenges in this field are discussed.

Coordination polymer-derived nano-sized zinc ferrite with excellent performance in nitro-explosive detection

Herein, a mixed metal coordination polymer, {(H₂pip)[Zn_1/3Fe_2/3(pydc-2,5)₂(H₂O)]·2H₂O} 1 {where H₂pip = piperazinediium and pydc-2,5 = pyridine-2,5-dicarboxylate}, was successfully synthesized using a hydrothermal technique. To confirm the structure and phase purity of 1, single crystals of an isomorphous pure Fe compound, {(H₂pip)[Fe(pydc-2,5)₂(H₂O)]·2H₂O} 1a, were synthesized based on similar synthetic conditions. Single crystal X-ray data of 1a confirmed the one-dimensional anionic metal-organic coordination polymer hydrogen bonded with protonated piprazine (piperazinediium) and lattice water molecules. The phase purity of 1 and 1a were confirmed via powder X-ray diffraction. Compound 1 was systematically characterized using IR, TGA, SEM, and EDX elemental mapping analysis. Compound 1 was used as a single source precursor for the preparation of nano-sized ZnFe₂O₄via thermal decomposition. The as-obtained ZnFe₂O₄ was fully characterized using PXRD, SEM, TEM, and EDX elemental mapping analysis. It was found that ZnFe₂O₄ was formed in its pure form with particle size in the nano-dimension. The aqueous dispersion of nano-sized ZnFe₂O₄ exhibits a strong emission at 402 nm upon excitation at 310 nm. This emissive property was employed for luminescence-based detection of nitroaromatic explosives in an aqueous medium through luminescence quenching for the first time. Importantly, selective detections have been observed for phenolic nitroaromatics based on differential luminescence quenching behaviour along with a detection limit of 57 ppb for 2,4,6-trinitrophenol (TNP) in water.

Ionothermal synthesis, crystal structure, topology and catalytic properties of heterometallic coordination polymers constructed from N-(phosphonomethyl) iminodiacetic acid

Four FeLn heterometallic coordination polymers with the H₄pmida ligand were ionothermally synthesized and characterized. Their catalytic properties in the Knöevenagel condensation reaction were investigated.

First tetrazole-bridged d–f heterometallic MOFs with a large magnetic entropy change

The first example of tetrazole-bridged 3d–4f heterometallic MOFs {(H₃O)₃[Gd₃Mn₂(Trz)₄]·12H₂O}_n was obtained, exhibiting a large magnetic entropy change.

Two dimensional porous 3d–4f heterometallic coordination polymers constructed by pyridine-2,3-dicarboxylic acid

Four porous Co^IIILn^III 3d–4f heterometallic coordination polymers constructed by the pydcH₂ ligand were hydrothermally synthesized and characterized. The adsorption and catalytic properties of compound Co^IIIDy^III were investigated.