Exceptional Manganese(II) Stability and Manganese(II)/Zinc(II) Selectivity with Rigid Polydentate Ligands

While MnII complexes meet increasing interest in biomedical applications, ligands are lacking that enable high MnII complex stability and selectivity vs. ZnII , the most relevant biological competitor. We report here two new bispidine derivatives, which provide rigid and large coordination cavities that perfectly match the size of MnII , yielding eight-coordinate MnII complexes with record stabilities. In contrast, the smaller ZnII ion cannot accommodate all ligand donors, resulting in highly strained and less stable six-coordinate complexes. Combined theoretical and experimental data (X-ray crystallography, potentiometry, relaxometry and 1 H NMR spectroscopy) demonstrate unprecedented selectivity for MnII vs. ZnII (KMnL /KZnL of 108 -1010 ), in sharp contrast to the usual Irving-Williams behavior, and record MnII complex stabilities and inertness with logKMnL close to 25.

Cationic tri(ferrocenecarbonitrile)silver(I)

The synthesis of the tri-coordinated ferrocenecarbonitrile silver(I) complex [Ag(N≡CFc)3]OTf (3) is reported. Its electrochemical behavior shows that the three ferrocenyl units are oxidized in a very close potential range. In addition, the molecular structure of 3 in the solid state is discussed, showing that silver(I) is exclusively coordinated by three ferrocenecarbonitrile molecules.

Bispidines for radiopharmaceuticals

Radiometal based radiopharmaceuticals for imaging and therapy require selective ligands (bifunctional chelators, BFCs) that form metal complexes, which are inert against trans-chelation under physiological conditions, linked to a biological vector, directing them to the targeted tissue. Bispidine ligands with a very rigid backbone and widely variable donor sets are reviewed as an ideal class of BFCs, and recent applications are discussed.

Oxidation of Cobalt(II) Bispidine Complexes with Dioxygen

Bispidine (3,7-diazabicyclo[3.3.1]nonane) ligands, derivatives of diazaadamantane, possess a very rigid backbone and have a high degree of pre-organization for cis-octahedral coordination geometries. Despite their rigidity, they exert a flexible coordination sphere, resulting in stable complexes with a variety of metal ions in various oxidation states. Due to the known high III/II redox potentials of their cobalt complexes, the CoII bispidine complexes are generally resistant to oxidation by dioxygen. Discussed in the present study are various CoII bispidine complexes with tetra- and pentadentate bispidines, with one of these complexes shown to be unstable under aerobic conditions. The decay process has been identified as an oxidative elimination of the 2-methylene pyridine substituent at one of the tertiary amine donors, resulting in picolinate, which is found coordinated to a CoIII product, where the dealkylated N-donor remains unprotonated. The mechanism of this interesting reaction has been studied, and details of the resulting structure of the product complex are discussed.

1,2-Dicyanoferrocene: synthesis and characterization

A convenient synthesis of 1,2-dicyanoferrocene (3) was developed by using 2,4-dinitrofluorobenzene as a dehydrating agent for the treatment of 1,2-ferrocenedicarbaldehyde dioxime (2) to give 3 in yields exceeding 80%. Compounds 2 and 3 have been characterized by IR and NMR (1H, 13C{1H}) spectroscopy and by electrochemistry (cyclic voltammetry and square-wave voltammetry). Furthermore, the molecular structures of 2 and 3 in the solid state have been determined by single-crystal X-ray crystallographic studies.

An efficient ultrasonic-mediated one-pot synthesis of 2,3,6,7,9-pentaazabicyclo[3.3.1]nonanes via a N,N-dimethylformamide dimethylacetal catalyzed Mannich-like reaction

A novel and straightforward method for one-pot synthesis of functionalized 2,3,6,7,9-pentaazabicyclo[3.3.1]nonane derivatives has been developed.

Comparative analysis of the performance of commonly available density functionals in the determination of geometrical parameters for copper complexes

In this study, a set of 50 transition-metal complexes of Cu(I) and Cu(II), were used in the evaluation of 18 density functionals in geometry determination. In addition, 14 different basis sets were considered, including four commonly used Pople's all-electron basis sets; four basis sets including popular types of effective-core potentials: Los Alamos, Steven-Basch-Krauss, and Stuttgart-Dresden; and six triple-ζ basis sets. The results illustrate the performance of different methodological alternatives for the treatment of geometrical properties in relevant copper complexes, pointing out Double-Hybrid (DH) and Long-range Correction (LC) Generalized Gradient Approximation (GGA) methods as better descriptors of the geometry of the evaluated systems. These however, are associated with a computational cost several times higher than some of the other methods employed, such as the M06 functional, which has also demonstrated a comparable performance. Regarding the basis sets, 6-31+G(d) and 6-31+G(d,p) were the best performing approaches. In addition, the results show that the use of effective-core potentials has a limited impact, in terms of the accuracy in the determination of metal-ligand bond-lengths and angles in our dataset of copper complexes. Hence, these could become a good alternative for the geometrical description of these systems, particularly CEP-121G and SDD basis sets, if one is considering larger copper complexes where the computational cost could be an issue.

Hybridization vs. bond stretching isomerism in Ru(II) cyclometalated complexes of 2-phenylpyridine

The phenomenon of formation of two isomers, yellow and orange, of the cyclometalated Ru(II) complex, [Ru(o-C₆H₄-py)(MeCN)₄]⁺, was investigated by EELS spectroscopy and theoretical calculations. Both forms show very similar structures and spectroscopic properties, but slight differences in X-ray data and absorption between them were noted. No double minimum on the potential energy surface was found and thus these two forms cannot be considered as bond stretching isomers. However, the DFT study revealed the change in the hybridization of the carbon in trans-position of one of acetonitrile ligands. This effect can be responsible for the difference in colour. The results of the theoretical modelling coincide well with the experimental EELS data.

Mono- and Dinuclear Copper(II) and Iron(II) Complexes of a Tetradentate Bispidine-diacetate Ligand

The synthesis of a new tetradentate bispidine ligand (LH2 = 2,2′-(1,5-dimethyl-3,7-diazabicyclo[3.3.1]nonan-3,7-diyl)diacetic acid), containing two tertiary amine and two carboxylic groups, is reported along with the preparation and characterization of the corresponding Cu(ii) and Fe(iii) complexes. The mononuclear [LCu(OH2)]·4H2O (1) complex contains a five-coordinate Cu(ii) centre, which adopt a square pyramidal geometry with the four donor atoms of the ligand (N2O2) occupying the equatorial plane and a water molecule occupying the axial position. An axial electron paramagnetic resonance (EPR) signature is observed for 1 (gx = 2.054, gy = 2.050, gz = 2.234; Ax = 18 × 10–4 cm–1, Ay = 20 × 10–4 cm–1, Az = 188 × 10–4 cm–1) in frozen methanolic solution (0.1 mM). Dimerization of 1 in concentrated solution (10 mM) was observed by EPR spectroscopy (g∥ = 2.24, g⊥ = 2.07, A∥ = 195 × 10–4 cm–1, and A⊥ = 12 × 10–4 cm–1 for each Cu centre). The structure of the dimeric species [LCu(OH2)]2 (1b) was determined by a combination of molecular mechanics with the simulation of the EPR spectrum (MM-EPR). The dimer has each Cu(ii) centre coordinated by the two amines and one carboxylate of one ligand (L), while the other carboxylate bridges to the second Cu(ii) centre; each coordination sphere is completed by an axial water ligand, with the Cu···Cu distance 5.5 Å (relative orientation from EPR simulation: α = 60°, β = 0°, γ = 25°). The aqueous reaction between the tetradentate ligand (L) and Fe(ii) leads to the formation of an oxo-bridged diiron(iii) complex, [LFe-(μ-O)-FeL] (2), with a Fe–O–Fe angle of 180° (dFe···Fe = 3.516 Å), as revealed by X-ray crystallography. The Mössbauer spectrum of 2 consists of one quadrupole doublet with an isomer shift (δ) of 0.37 mm s–1 and a quadrupole splitting (ΔEQ) of 0.73 mm s–1, which is consistent with S = 5/2 Fe(iii) centres. Variable-temperature magnetic susceptibility measurements show the presence of intramolecular antiferromagnetic interactions between the two Fe(iii) centres, with an exchange coupling constant J of –91(3) cm–1 (H = –2JS1·S2).