Fr Francium

Two multiligand coordination compounds, [Mn(AZT)₄(H₂O)₂](PA)₂·4H₂O (1) and [Co(AZT)₂(H₂O)₄](PA)₂ (2), were synthesized with 3-azido-1,2,4-triazole (AZT) as a ligand and picrate (PA) as a counteranion and characterized by elemental analysis and FTIR spectroscopy. The crystal structures were determined by single-crystal X-ray diffraction. The results show that the crystals of 1 and 2 have a triclinic space group (P) and orthorhombic space group (Pbca), respectively. Moreover, 1 and 2 have distorted octahedral structures. Their thermal decomposition mechanisms were investigated by differential scanning calorimetry and thermogravimetric analysis. The experimental data showed that the energies of combustion were approximately equal to the energies of combustion of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and 1,3,5,7-tetranitro-1,3,5,7-tetraazocane (HMX). The nonisothermal kinetic parameters were studied by applying the methods of Kissinger and Ozawa. The sensitivity properties showed that 1 and 2 had a higher flame sensitivity than 2,4,6-trinitrotoluene, RDX, and HMX.

The structure of [Co(AZT)₂(H₂O)₄](PA)₂ (AZT = 3-azido-1,2,4-triazole, PA = picrate) shows that the Co^II ion is six-coordinate in a slightly distorted octahedral geometry. The energy of combustion and enthalpy of formation are 8.83 MJ kg^–1 and –3419.53 kJ mol^–1, respectively. Nonisothermal kinetic analysis indicated that the Arrhenius equation can be expressed as ln k = 15.23 – 153.3 × 10³/(RT).

When working with chemical solution deposition techniques, one of the main issues for optimal performance of CeO₂ buffer layers in coated conductors is the insufficient chemical stability of the CeO₂ layer during YBa₂Cu₃O₇ (YBCO) thermal processing. This work focusses on the morphology and nanostructure in thin CeO₂ films prepared by means of a novel aqueous synthesis route and incorporated into a Ni–W/La₂Zr₂O₇/CeO₂/YBa₂Cu₃O₇-coated conductor. Optimization of precursor chemistry and thermal processing led to a reduction in barium cerate formation. In a new precursor design, iminodiacetic acid was used as a stabilizing ligand, which resulted in an improved morphology of the buffer layer. A shelf life of more than 6 months was established by using a metal-to-ligand ratio of 1 to 5. During thermal processing, a combination of a slow calcination ramp with a high sintering ramp, short sintering dwell time and a low oxygen partial pressure during the synthesis resulted in a root mean square roughness below 3 nm for AFM analysis, a [111] to [002] ratio of 1 to 90 in X-ray diffraction and well-defined patterns in reflection high-energy electron diffraction (RHEED) analysis of the CeO₂ surface. Trifluoroacetate-YBCO was deposited on top of the CeO₂ buffer layer. Cross-section analysis with a focussed ion beam allowed us to correlate the morphology and nanostructure of the CeO₂ buffer layer with the formation of BaCeO₃ and the appearance of voids and secondary phases throughout the YBa₂Cu₃O₇ layer.

Smooth, well-textured CeO₂ buffer layers for coated conductors were synthesized by aqueous precursor formulations. The texture and morphology were characterized. Deposition of YBa₂Cu₃O₇ (YBCO) by metal–organic deposition with trifluoroacetate (TFA-MOD) and focussed ion beam analysis allowed the characterization of YBCO and the growth of secondary phases as a function of properties of the buffer layer.

A series of one-dimensional heterotrimetallic assemblies, [Cu₂Ln(L)₂(H₂O)₄][M(CN)₆]·nH₂O [Ln = Gd, M = Co (1), Fe (2), Cr (3), and Ln = La, M = Co (4), Fe (5), Cr (6)], were prepared by the reaction of a Cu₂Ln precursor complex, [Cu₂Ln(L)₂(NO₃)₃], with K₃[M(CN)₆] in water. All of the assemblies were isomorphous and formed a 1D zigzag chain, in which the [Cu₂Ln(L)₂(H₂O)₄]³⁺ and [M(CN)₆]^3– units were alternately positioned and were linked in a Cu–NC–M–CN–Cu manner. Compound 1 showed magnetic behaviour similar to that of the discrete precursor complex, [Cu₂Gd(L)₂(NO₃)₃], owing to the diamagnetic nature of the [Co^III(CN)₆]^3– unit. In the case of 2, a simple summation of the magnetic behaviour of the [Cu₂Gd(L)₂(H₂O)₄]³⁺ and [Fe(CN)₆]^3– units was observed, whereas ferromagnetic interactions were found to be operative between the Cu²⁺ and Cr³⁺ ions in compound 3. The same magnetic interactions between Cu²⁺ and M³⁺ were confirmed in compounds 4 to 6, which included the diamagnetic La³⁺ ion. The differences in the magnetic behaviours of 2 and 3 can be explained by the overlap of the d(Cu)–pπ(CN) orbitals in the bent Cu–N≡C linkage and the spin-density on the cyanide nitrogen of the [Cr(CN)₆]^3– unit.

Novel cyanide-bridged assemblies that have ordered alternate arrays of three types of paramagnetic metal centres were obtained from the reaction of preorganized heterobimetallic trinuclear complexes, [Cu₂Ln(L)₂(NO₃)₃] (Ln^III = Gd, La) and K₃[M(CN)₆] (M^III = Co, Fe, Cr). These form a 1D zigzag chain that is extended by the M^III–CN–Cu^II linkages and exhibit different magnetic behaviour, which depends on the combination of the metal ions.

Visible-light-responsive photonic structures have been prepared in alcohol solvents by using silica-modified PbS colloidal nanocrystal clusters (CNCs) as building blocks. Further modification of the PbS CNCs with a coating of silica allowed the dispersion of the particles into nonaqueous solutions. Repulsive electrostatic and solvation forces contribute to the self-assembly of the PbS@SiO₂ spheres. The core–shell particles have optical properties similar to those of CNCs, and they can also be assembled into close-packing films through simple drop-casting on silicon substrates. Embedding droplets of such a PbS@SiO₂ colloidal solution in a polymer matrix produced solid composite materials with visible-light-responsive optical properties with potential applications as sensors and optical switches.

Visible-light-responsive photonic structures have been prepared in alcohol solvents by using silica-modified PbS colloidal nanocrystal clusters as building blocks. Repulsive electrostatic and solvation forces contribute to the self-assembly of the PbS@SiO₂ spheres. Solid polymer composite films with similar light-responsive optical properties have also been produced in polymer matrices.

The new compound [Co(tren)(trenH₂)]₂[{Co(tren)}₂V₁₅Ge₆O₄₂S₆(H₂O)]·9H₂O [1, tren = tris(2-aminoethyl)amine] has been obtained under solvothermal conditions and features the unique thiogermanatovanadatopolyoxoanion[V^IV₁₅Ge^IV₆O₄₂S₆(H₂O)]^12– as the main structural motif. Compound 1 crystallizes in the monoclinic space group P2₁/c with a = 15.4711(2), b = 26.4031(4), c = 26.7213(4) Å, V = 10874.2(3) ų, and Z = 4. The [V₁₅Ge₆O₄₂S₆(H₂O)]^12– cluster anion displays the spin topology reported for the molecular magnets [V^IV₁₅As^III₆O₄₂(H₂O)]^6– and [V^IV₁₅Sb^III₆O₄₂]^6– and therefore represents a new member of the {V₁₅E₆} family, which has allowed us to study the magnetic exchange interactions between the vanadyl (d¹) groups of the geometrically frustrated central V₃ triangle, which is sandwiched between strongly antiferromagnetically coupled V₆ hexagons. The cluster shell is expanded by two {Co(tren)}²⁺-based complexes through Co–S bonds, which reduce the high negative charge of the anion. The Co²⁺ ions in the [Co(tren)(trenH₂)]⁴⁺ countercations are coordinated to one tetradentate tren and a monodentate, doubly protonated tren ligand.

The new polyoxovanadate [{Co(tren)}₂V₁₅Ge₆O₄₂S₆(H₂O)]^8– was directly synthesized from vanadate and elemental Ge and S. The cluster core [V₁₅Ge₆O₄₂S₆(H₂O)]^12– is the first V–Ge–O–S cluster and features the spin topology of the seminal molecular magnet [V₁₅As₆O₄₂(H₂O)]^6–. The cluster anion is expanded by sulfur-bound, pentacoordinate Co²⁺ complexes.

Zinc dichloride and dimethylzinc were treated with several acyclic and bicyclic guanidines. Three different bicyclic guanidines and their potassium guanidinate salts were treated with ZnCl₂. The reaction with the neutral guanidines afforded mononuclear complexes, stabilized by intramolecular hydrogen bonding. The reaction with the potassium guanidinate salts led to trinuclear complexes, which were extremely water sensitive. The reaction in the presence of added water furnished a tetranuclear complex with a central OZn₄ unit in good yield, to which six guanidinate ligands were bound. The reaction of dimethylzinc with 2-[N,N′-diisopropylguanidino]pyridine and 2-[N,N′-diisopropylguanidino]quinoline afforded di- and tetranuclear Zn methyl complexes with mono- and dianionic guanidinate ligands. Dinuclear complexes of the monoanionic guanidinate ligands were formed at room temperature. At higher temperatures (75 °C), complete deprotonation of the guanidino groups and formation of tetranuclear Zn alkyl complexes was observed, which feature low-coordinate zinc sites.

Bicyclic and acyclic guanidinate ligands stabilize several new oligonuclear zinc complexes.

The interaction of the VO²⁺ ion with pyrone derivatives and tropolone, which form very effective antidiabetic compounds, is critically re-examined. The binary systems with ethylmaltol (Hema) and tropolone (Htrop) were studied in aqueous solution and in the solid state through the combined application of spectroscopic (EPR, UV/Vis and IR) and pH-potentiometric techniques. The results were compared with those of the systems with maltol (Hma) and kojic acid (Hkoj) and rationalized on the basis of DFT simulations. All the ligands L^– form [VOL]⁺, cis-[VOL₂(H₂O)] and cis-[VOL₂(OH)]^– species in aqueous solutions and a square-pyramidal [VOL₂] complex in the solid state, which transforms into cis-[VOL₂(solvent)] when it is dissolved in water or in a coordinating solvent. The coordinating properties of the ligands studied were compared with those of pyridinone [3-hydroxy-1,2-dimethyl-4(1H)pyridinone (Hdhp), and 1,2-diethyl-3-hydroxy-4(1H)pyridinone (Hdepp)] derivatives and catechol (H₂cat), and were explained by postulating that from pyrones to pyridinones and to catechol the donor set changes progressively from (CO, O^–) to (O^–, O^–). DFT calculations allowed us to determine the relative stability of the four possible structures (square pyramidal, trans- and two cis-octahedral) of the bis-chelated species and the aromaticity of the protonated, neutral and deprotonated form of the ligands through the calculation of the HOMA (harmonic oscillator model of aromaticity) index. The relationship between the electric charge on the oxygen donors, the mean distances and the difference between the lengths of C–O_ket and C–O_phen bonds with (i) the pK_a of the ligands, (ii) the pK of deprotonation of the equatorially coordinated water molecule in cis-[VOL₂(H₂O)], and (iii) the ⁵¹V hyperfine coupling constant (A_z) of [VOL₂], cis-[VOL₂(H₂O)] and cis-[VOL₂(OH)]^– was also found and discussed.

The chelating and spectroscopic properties of pyrone and pyridinone derivatives, tropolone and catechol (which form very effective antidiabetic compounds) towards the VO²⁺ ion are explained in terms of aromaticity of the fully deprotonated form of the ligands, and of total electric charge on the oxygen donors.

Paramagnetic gadolinium(III) complexes are widely used to increase contrast in magnetic resonance (MR) images. Contrast enhancement depends on the concentration of the gadolinium complex and on its relaxivity, an inherent property of the complex. Increased relaxivity results in greater image contrast or the ability to detect the contrast agent at a lower concentration. An increase in the relaxivity enables the imaging of abundant molecular targets. Relaxivity depends on the structure of the complex, kinetics of inner-sphere and second-sphere water exchange, and on the rotational dynamics of the molecule. The latter, and in some cases the former, properties of the complex change when it is bound to its target. All of these properties can be rationally tuned to enhance relaxivity. In this Microreview, we summarize our efforts in understanding and optimizing the relaxivity of contrast agents targeted to serum albumin and to fibrin.

The effect of changing a single donor atom in a gadolinium(III) complex targeting human serum albumin can be seen in the graphic. The single donor group modification alters inner-sphere water exchange by three orders of magnitude and changes relaxivity by a factor of five. Water exchange can be rationally tuned to optimize relaxivity.

Gas-phase addition of a basic ligand to dipositive uranyl coordination complexes comprising diacetone alcohol (DAA) results in water-elimination, which indicates aldol dehydration of DAA to produce mesityl oxide. A novel attribute of the observed gas-phase chemistry is that a ligand exothermically associates to a coordination complex to provide the excitation required to induce chemistry in other ligands, with the added “spectator ligand” remaining intact in the product. Dehydration of DAA was observed for addition of tetrahydrofuran, acetone, and 2-propanol to uranyl complexes [UO₂(DAA)₂]²⁺ and [UO₂(DAA)(acetone)₂]²⁺. In contrast, [UO₂(DAA)₂(acetone)]²⁺ did not exhibit ligand-addition chemistry, which is attributed to a high degree of coordinative saturation at the uranium metal center.

Gas-phase addition of a basic ligand to dipositive uranyl coordination complexes comprising diacetone alcohol results in dehydration to produce mesityl oxide. A novel attribute of this process is that a ligandexothermically associates to a coordination complex, providing excitation to induce chemistry in other ligands; the added “spectator ligand” remains intact.

The second polyhydridoniobocene complex that was characterized by X-ray diffraction is reported. On the basis of H–H distances and H–Nb–H angles, [Nb(η⁵-C₅H₄SiMe₃)₂(H)₃] (1) is classified as a “compressed hydride”. Compound 1 acts as an efficient single-component initiator for the ring-opening polymerization of ϵ-caprolactone and δ-valerolactone. ϵ-Caprolactone and δ-valerolactone are both polymerized within a few hours to yield high-to-medium-molecular-weight polymers with medium to broad polydispersities. Polymer end-group analysis showed that the polymerization proceeds through a coordination–insertion mechanism based on the cleavage of the ring between the oxygen atom and the acyl carbon atom.

The structure of [Nb(η⁵-C₅H₄SiMe₃)₂(H)₃] (1) justifies its classification as a “nonclassical” transition metal hydride. Complex 1 is an initiator for the ROP of lactones. A mechanism consisting of an interaction between the lactone carbonyl group and the metal, insertion of the lactone into the Nb–H bond, and the generation of a metal alkoxide–aldehyde propagating species has been proposed.