An Unusually Flexible Expanded Hexaamine Cage and Its CuII Complexes: Variable Coordination Modes and Incomplete Encapsulation

Heterobimetallic 3d-4f complexes supported by a Schiff-base tripodal ligand

Complexes featuring multiple metal centres are of growing interest regarding metal-metal cooperation and its tuneability. Here the synthesis and characterisation of heterobimetallic complexes of a 3d metal (4: Mn, 5: Co) and lanthanum supported by a (1,1,1-tris[(3-methoxysalicylideneamino)methyl]ethane) ligand is reported, as well as discussion of their electronic structure via electron paramagnetic resonance (EPR) spectroscopy, electrochemical experiments and computational studies. Competitive binding experiments of the ligand and various metal salts unequivocally demonstrate that in these heterobimetallic complexes the 3d metal (Mn, Co) selectively occupies the κ6-N3O3 binding site of the ligand, whilst La occupies the κ6-O6 metal binding site in line with their relative oxophilicities. EPR spectroscopy supported by density functional theory analysis indicates that the 3d metal is high spin in both cases (S = 5/2 (Mn), 3/2 (Co)). Cyclic voltammetry studies on the Mn/La and Co/La bimetallic complexes revealed a quasi-reversible Mn2+/3+ redox process and poorly-defined irreversible oxidation events respectively.

N,N-Alkylation Clarifies the Role of N- and O-Protonated Intermediates in Cyclen-Based 64Cu Radiopharmaceuticals

Radioisotopes of Cu, such as ⁶⁴Cu and ⁶⁷Cu, are alluring targets for imaging (e.g., positron emission tomography, PET) and radiotherapeutic applications. Cyclen-based macrocyclic polyaminocarboxylates are one of the most frequently examined bifunctional chelators in vitro and in vivo, including the FDA-approved ⁶⁴Cu radiopharmaceutical, Cu(DOTATATE) (Detectnet); however, connections between the structure of plausible reactive intermediates and their stability under physiologically relevant conditions remain to be established. In this study, we share the synthesis of a cyclen-based, N,N-alkylated spirocyclic chelate, H₂DO3A^C4H8, which serves as a model for N-protonation. Our combined experimental (in vitro and in vivo) and computational studies unravel complex pH-dependent speciation and enable side-by-side comparison of N- and O-protonated species of relevant ⁶⁴Cu radiopharmaceuticals. Our studies suggest that N-protonated species are not inherently unstable species under physiological conditions and demonstrate the potential of N,N-alkylation as a tool for the rational design of future radiopharmaceuticals.

Ligand Control of 59Co Nuclear Spin Relaxation Thermometry

Studying the correlation between temperature-driven molecular structure and nuclear spin dynamics is essential to understanding fundamental design principles for thermometric nuclear magnetic resonance spin-based probes. Herein, we study the impact of progressively encapsulating ligands on temperature-dependent ⁵⁹Co T ₁ (spin-lattice) and T ₂ (spin-spin) relaxation times in a set of Co(III) complexes: K₃[Co(CN)₆] (1); [Co(NH₃)₆]Cl₃ (2); [Co(en)₃]Cl₃ (3), en = ethylenediamine); [Co(tn)₃]Cl₃ (4), tn = trimethylenediamine); [Co(tame)₂]Cl₃ (5), tame = triaminomethylethane); and [Co(dinosar)]Cl₃ (6), dinosar = dinitrosarcophagine). Measurements indicate that ⁵⁹Co T ₁ and T ₂ increase with temperature for 1-6 between 10 and 60 °C, with the greatest ΔT ₁/ΔT and ΔT ₂/ΔT temperature sensitivities found for 4 and 3, 5.3(3)%T ₁/°C and 6(1)%T ₂/°C, respectively. Temperature-dependent T ₂* (dephasing time) analyses were also made, revealing the highest ΔT ₂*/ΔT sensitivities in structures of greatest encapsulation, as high as 4.64%T ₂*/°C for 6. Calculations of the temperature-dependent quadrupolar coupling parameter, Δe ² qQ/ΔT, enable insight into the origins of the relative ΔT ₁/ΔT values. These results suggest tunable quadrupolar coupling interactions as novel design principles for enhancing temperature sensitivity in nuclear spin-based probes.

Efficient and selective separation of aqueous sulfate through recognition and precipitation

Sulfate anions are selectively separated from aqueous solution in the form of precipitates by a mono-protonated organic receptor, constructed in situ through anion-templated chemical synthesis.

Trigonal prismatic metal complexes: a not so rare coordination geometry?

The solid state structures of two metal complexes of a hexaamine macrobicyclic ligand, in which the metal ion has an exact trigonal prismatic geometry, have been determined. Theoretical calculations showed this is the most stable geometry for d(0), d(10) and high spin d(5) metal complexes of the ligand with M-N bond distances >∼2.35 Å.

The X-ray absorption spectroscopic model of the copper(II) imidazole complex ion in liquid aqueous solution: a strongly solvated square pyramid

Cu K-edge extended X-ray absorption fine structure (EXAFS) and Minuit X-ray absorption near-edge structure (MXAN) analyses were combined to evaluate the structure of the copper(II) imidazole complex ion in liquid aqueous solution. Both methods converged to the same square-pyramidal inner coordination sphere [Cu(Im)(4)L(ax)](2+) (L(ax) indeterminate) with four equatorial nitrogen atoms at EXAFS, 2.02 ± 0.01 Å, and MXAN, 1.99 ± 0.03 Å. A short-axial N/O scatterer (L(ax)) was found at 2.12 ± 0.02 Å (EXAFS) or 2.14 ± 0.06 Å (MXAN). A second but very weak axial Cu-N/O interaction was found at 2.9 ± 0.1 Å (EXAFS) or 3.0 ± 0.1 Å (MXAN). In the MXAN fits, only a square-pyramidal structural model successfully reproduced the doubled maximum of the rising K-edge X-ray absorption spectrum, specifically excluding an octahedral model. Both EXAFS and MXAN also found eight outlying oxygen scatterers at 4.2 ± 0.3 Å that contributed significant intensity over the entire spectral energy range. Two prominent rising K-edge shoulders at 8987.1 and 8990.5 eV were found to reflect multiple scattering from the 3.0 Å axial scatterer and the imidazole rings, respectively. In the MXAN fits, the imidazole rings took in-plane rotationally staggered positions about copper. The combined (EXAFS and MXAN) model for the unconstrained cupric imidazole complex ion in liquid aqueous solution is an axially elongated square-pyramidal core, with a weak nonbonded interaction at the second axial coordination position and a solvation shell of eight nearest-neighbor water molecules. This core square-pyramidal motif has persisted through [Cu(H(2)O)(5)](2+), [Cu(NH(3))(4)(NH(3),H(2)O)](2+), (1, 2) and now [Cu(Im)(4)L(ax))](2+) and appears to be the geometry preferred by unconstrained aqueous-phase copper(II) complex ions.