Chelate Chemistry. III. Chelates of High Coordination Number

Not So Similar: Different Ways of Nb(V) and Ta(V) Catecholate Complexation

The reactions between catechol (H₂cat) and niobium(V) or tantalum(V) precursors in basic aqueous solutions lead to the formation of catecholate complexes of different natures. The following complexes were isolated and characterized by single-crystal X-ray diffraction (SCXRD): (1) (NH₄)₃[NbO(cat)₃]∙4H₂O; (2) K₂[Nb(cat)₃(Hcat)]·2H₂cat·2H₂O; (3) Cs₃[NbO(cat)₃]·H₂O; (4) (NH₄)₄[Ta₂O(cat)₆]·3H₂O; (5) Cs₂[Ta(cat)₃(Hcat)]·H₂cat; (6) Cs₄[Ta₂O(cat)₆]·7H₂O. The isolated crystalline products were characterized by elemental analysis, X-ray powder diffraction (XRPD), FTIR, and TGA. The structural features of these complexes, such as {Ta₂O} unit geometry, Cs-π interactions, and crystal packing effects, are discussed.

Tantalum, easy as Pi: understanding differences in metal–imido bonding towards improving Ta/Nb separations

The separation and purification of niobium and tantalum, which co-occur in natural sources, is difficult due to their similar physical and chemical properties. The current industrial method for separating Ta/Nb mixtures uses an energy-intensive process with caustic and toxic conditions. It is of interest to develop alternative, fundamental methodologies for the purification of these technologically important metals that improve upon their environmental impact. Herein, we introduce new Ta/Nb imido compounds: M(^tBuN)(TriNOx) (1-M) bound by the TriNOx³⁻ ligand and demonstrate a fundamental, proof-of-concept Ta/Nb separation based on differences in the imido reactivities. Despite the nearly identical structures of 1-M, density functional theory (DFT)-computed electronic structures of 1-M indicate enhanced basic character of the imido group in 1-Ta as compared to 1-Nb. Accordingly, the rate of CO₂ insertion into the M Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N_imido bond of 1-Ta to form a carbamate complex (2-Ta) was selective compared to the analogous, unobserved reaction with 1-Nb. Differences in solubility between the imido and carbamate complexes allowed for separation of the carbamate complex, and led to an efficient Ta/Nb separation (S_Ta/Nb = 404 ± 150) dependent on the kinetic differences in nucleophilicities between the imido moieties in 1-Ta and 1-Nb.

A selective separation of the critical metals tantalum and niobium was accomplished from π-bonding-based reactivity differences of imido complexes. New insights into Ta/Nb separations were gained through detailed kinetic and computational studies.

Labeling of Hinokitiol with 90Y for Potential Radionuclide Therapy of Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC), the most common form of primary liver tumors, is the fifth cancer in the world in terms of incidence, and third in terms of mortality. Despite significant advances in the treatment of HCC, its prognosis remains bleak. Transarterial radioembolization with radiolabeled microspheres and Lipiodol has demonstrated significant effectiveness. Here we present a new, simple radiolabeling of Lipiodol with Yttrium-90, for the potential treatment of HCC.

Cerium Tetrakis(tropolonate) and Cerium Tetrakis(acetylacetonate) Are Not Diamagnetic but Temperature-Independent Paramagnets

A new synthesis of cerium tetrakis(tropolonate), Ce(trop)₄, where trop is deprotonated 2-hydroxy-2,4,6-cycloheptatrienone) or Ce(O₂C₇H₅)₄, is developed that results in dark-purple crystals whose X-ray crystal structure shows that the geometry of the eight-coordinate compound closely resembles a D_{2 d} dodecahedron, based on shape parameters. The magnetic susceptibility as a function of the temperature (4-300 K) shows that it is a temperature-independent paramagnet, χ = 1.2(3) × 10^-4 emu/mol, and the L_III-edge X-ray absorption near-edge structure spectrum shows that the molecule is multiconfigurational, comprised of a f¹:f⁰ configuration mixture in a 50:50 ratio. Ce(acac)₄ and Ce(tmtaa)₂ (where acac is acetylacetonate and tmtaaH₂ is tetramethyldibenzotetraaza[14]annulene) have similar physical properties, as does the solid-state compound CeO₂. The concept is advanced that trop^-, acac^-, tmtaa^2-, cot^2-, and O^2- are redox-active ligands that function as electron donors, rendering the classification of these compounds according to their oxidation numbers misleading because their magnetic susceptibilities, χ, are positive and their effective magnetic moments, μ_eff, lie in the range of 0.1-0.7 μ_B at 300 K.

Crystal structure of trans-bis((E)-7-oxo-4-(phenyldiazenyl)cyclohepta-1,3,5-trien-1-olato)-κ2 O,O′)-bis(pyridine-κN)cobalt(II), C36H28CoN6O4

C36H28Co1N6O4, triclinic, P1̄ (no. 2), a = 6.3166(4) Å, b = 8.5454(5) Å, c = 14.8075(9) Å, α = 105.157(2)°, β = 94.494(3)°, γ = 101.890(2)°, V = 747.5(2) Å3, Z = 1, R gt(F) = 0.0332, wR ref(F 2) = 0.0790, T = 100(2) K.

Synthesis and Structural Properties of Lanthanide Complexes Formed with Tropolonate Ligands

We have previously reported the unique luminescence properties of ML4 complexes formed between tropolonate ligands and a series of lanthanide cations, several of them emitting in the near-infrared domain. The synthesis and composition of ML4 lanthanide tropolonate complexes have been previously described in the literature, but no structural information has been available so far. In this work, the crystal structures of several lanthanide tropolonate complexes (Ln3+ = Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+) have been isolated and systematically analyzed by X-ray diffraction and compared by using different criteria including the Kepert formalism. Such comparative work is rare in lanthanide coordination chemistry. The analysis of the structures in the solid state reveals that although the packing of the ML4 complexes depends on the nature of the metal ion, the coordination geometries around the different lanthanides is virtually similar for all the cations that have been analyzed; an indication that lanthanide-centered f orbitals play a role in controlling this coordination geometry. Analysis of the solution's behavior by stability constant determination reveals the formation of complexes with similar ML4 stoichiometries as those observed in the solid state. Nevertheless, analysis of the luminescence lifetimes indicates that the coordination environment around the lanthanide cations are different in the solid state and in solution, with the presence of one molecule of water bound to the lanthanide cation in solution. The presence of such a water molecule is a significant source of nonradiative deactivation of the excited states of the lanthanide cations, an unfavorable condition that leads to significant loss in fluorescence intensity of these lanthanide complexes. This exemplifies that such comparative analysis between the solid state and solution is important for the rationalization of the luminescence properties of the complexes. This analysis will aid us in optimizing ligand design for improved photophysical properties of the complex.

Synthesis, Structure, and Reactivity of O-Donor Ir(III) Complexes: C−H Activation Studies with Benzene

Various new thermally air- and water-stable alkyl and aryl analogues of (acac-O,O)2Ir(R)(L), R-Ir-L (acac-O,O = kappa2-O,O-acetylacetonate, -Ir- is the trans-(acac-O,O)2Ir(III) motif, R = CH3, C2H5, Ph, PhCH2CH2, L = Py) have been synthesized using the dinuclear complex [Ir(mu-acac-O,O,C3)-(acac-O,O)(acac-C3)]2, [acac-C-Ir]2, or acac-C-Ir-H2O. The dinuclear Ir (III) complexes, [Ir(mu-acac-O,O,C3)-(acac-O,O)(R)]2 (R = alkyl), show fluxional behavior with a five-coordinate, 16 electron complex by a dissociative pathway. The pyridine adducts, R-Ir-Py, undergo degenerate Py exchange via a dissociative mechanism with activation parameters for Ph-Ir-Py (deltaH++ = 22.8 +/- 0.5 kcal/mol; deltaS++ = 8.4 +/- 1.6 eu; deltaG++298 K) = 20.3 +/- 1.0 kcal/mol) and CH3-Ir-Py (deltaH++ = 19.9 +/- 1.4 kcal/mol; deltaS++ = 4.4 +/- 5.5 eu; deltaG++298 K) = 18.6 +/- 0.5 kcal/mol). The trans complex, Ph-Ir-Py, undergoes quantitatively trans-cis isomerization to generate cis-Ph-Ir-Py on heating. All the R-Ir-Py complexes undergo quantitative, intermolecular CH activation reactions with benzene to generate Ph-Ir-Py and RH. The activation parameters (deltaS++ =11.5 +/- 3.0 eu; deltaH++ = 41.1 +/- 1.1 kcal/mol; deltaG++298 K = 37.7 +/- 1.0 kcal/mol) for CH activation were obtained using CH3-Ir-Py as starting material at a constant ratio of [Py]/[C6D6] = 0.045. Overall the CH activation reaction with R-Ir-Py has been shown to proceed via four key steps: (A) pre-equilibrium loss of pyridine that generates a trans-five-coordinate, square pyramidal intermediate; (B) unimolecular, isomerization of the trans-five-coordinate to generate a cis-five-coordinate intermediate, cis-R-Ir- square; (C) rate-determining coordination of this species to benzene to generate a discrete benzene complex, cis-R-Ir-PhH; and (D) rapid C-H cleavage. Kinetic isotope effects on the CH activation with mixtures of C6H6/C6D6 (KIE = 1) and with 1,3,5-C6H3D3 (KIE approximately 3.2 at 110 degrees C) are consistent with this reaction mechanism.

An Example of O2 Binding in a Cobalt(II) Corrole System and High-Valent Cobalt−Cyano and Cobalt−Alkynyl Complexes

The novel cobalt corrolazine (Cz) complexes (TBP)(8)CzCoCN (1) and (TBP)(8)CzCo(CCSiPh(3)) (2) have been synthesized and examined in light of the recent intense interest regarding the role of corrole ligands in stabilizing high oxidation states. In the case of 2, the molecular structure has been determined by X-ray crystallography, revealing a short Co[bond]C distance of 1.831(4) A and an intermolecular pi-stacking interaction between Cz ring planes, and this structure has been analyzed in regards to the electronic configuration. By a combination of spectroscopic techniques it has been shown that 1 is best described as a cobalt(III)[bond]pi-cation-radical complex, whereas 2 is likely best represented as the resonance hybrid (Cz)Co(IV)(CCSiPh(3)) <--> (Cz+*)Co(III)(CCSiPh(3)). The reduced cobalt(II) complex, [(TBP)(8)CzCo(II)(py)](-), has been generated in situ and shown to bind dioxygen at low temperature to give [(TBP)(8)CzCo(III)(py)(O(2))](-). For the reduced complex [(TBP)(8)CzCo(II)(py)](-), the EPR spectrum in frozen solution is indicative of a low-spin cobalt(II) complex with a d(z)2 ground state. Exposure of [(TBP)(8)CzCo(II)(py)](-) to O(2) leads to the reversible formation of the cobalt(III)-superoxo complex [(TBP)(8)CzCo(III)(py)(O(2))](-), which has been characterized by EPR spectroscopy. VT-EPR measurements show that the dioxygen adduct is stable up to T approximately 240 K. This work is the first observation, to our knowledge, of O(2) binding to a cobalt(II) corrole.