We have reported here a series of ligands containing pentadentate 6,6'-(azanediylbis(methylene))dipicolinic acid units that differ in the substituent present at the amine nitrogen atom (acetate: HDPAAA; phenyl: HDPAPhA; dodecyl: HDPAC12A; 4-hexylphenyl: HDPAC6PhA). The protonation constants of the hexadentate DPAAA and pentadentate DPAPhA ligands and the stability constants of their Mn complexes were determined using pH-potentiometry (25 °C, 0.15 M NaCl). The mono-hydrated [Mn(DPAAA)] complex (log K = 13.19(5)) was found to be considerably more stable than the bis-hydrated [Mn(DPAPhA)] analogue (log K = 9.55(1)). A detailed H andO NMR relaxometric study was carried out to determine the parameters that govern the proton relaxivities of these complexes. The [Mn(DPAC12A)] complex, which contains a dodecyl lipophilic chain, forms micelles in solution characterized by a critical micellar concentration (cmc) of 96(9) μM. The lipophilic [Mn(DPAC6PhA)] and [Mn(DPAC12A)] derivatives form rather strong adducts with Human Serum Albumin (HSA) with association constants of 7.1 ± 0.1 × 10 and 1.3 ± 0.4 × 10 M, respectively. The X-ray structure of the complex {K(HO)}{[Mn(DPAAA)(HO)]} shows that the Mn ion in [Mn(DPAAA)] is coordinated to the six donor atoms of the ligand, a coordinated water molecule completing the pentagonal bipyramidal coordination environment.

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Extremely inert paramagnetic nickel(ii) complexes based on a cross-bridged cyclam platform present responses at the1H (CEST) and19F frequencies.

We report a detailed study of the structure of the [Bi(DO3A)] complex both in the solid state and in solution. The X-ray crystal structure of [Bi(DO3A)] evidences the octa-coordination of the Bi(iii) ion, which is directly coordinated to the four nitrogen atoms of the cyclen unit and three oxygen atoms of the carboxylate groups. The octa-coordination is completed by an oxygen atom of a neighboring carboxylate group, which bridges Bi(iii) ions thanks to μ2-η1:η1 coordination, resulting in the formation of a coordination polymer. The Bi(iii) ion presents a twisted-square antiprismatic (TSAP) coordination geometry associated with the Δ(δδδδ)/Λ(λλλλ) enantiomeric pair. A computational DFT study indicates that the 6s2 lone pair of Bi(iii) is stereochemically active in [Bi(DO3A)] and most of the cyclen-based complexes reported in the literature. Depending on the spatial arrangement of the donor atoms of the ligand the lone pair points in different directions, so that Bi(iii) can easily accommodate different coordination environments. In solution the [Bi(DO3A)] complex exists as a monomeric complex, as demonstrated by DOSY measurements. The stability constant of the [Bi(DO3A)] complex, determined by using batch spectrophotometric titrations, was found to be rather high (log K = 26.85(5)). The complex presents characteristic absorption in the UV spectrum at 299 nm (ε = 8770 M-1 cm-1) that was attributed to the Bi(iii)-centered 6p ← 6s band on the basis of TDDFT calculations. Spectrophotometric titrations reveal weak binding of different anions to the [Bi(DO3A)] complex, with association constants of K11 = 3.55(8), 3.09(7), 6.2(1) and 2.19(5) for Cl-, Br-, I- and N3-, respectively.