Metal-Ligand Redox Cooperativity in Cerium Amine-/Amido-Phenolate-Type Complexes.

The synthesis and characterization of two cerium complexes of redox-active amine/amido-phenolate-type ligands are reported. A tripodal framework comprising the tris(2-(3',5'-di-tert-butyl-2'-hydroxyphenyl)amino-phenyl) amine (H6Clamp) proligand was synthesized for comparison of its cerium complex with a potassium-cerium heterobimetallic complex of the 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)imino]quinone (dippap) proligand. Structural studies indicate differences in the cerium(III) cation coordination spheres, where CeIII(CH3CN)1.5(H3Clamp) (1-Ce(H3Clamp)) exhibits shorter Ce-O distances and longer Ce-N bond distances compared to the analogous distances in K3(THF)3CeIII(dippap)3 (2-Ce(ap)), due to the gross structural differences between the systems. Differences are also evident in the temperature-dependent magnetic properties, where smaller χT products were observed for 2-Ce(ap) compared to 1-Ce(H3Clamp). Solution electrochemical studies for the complexes were interpreted based on ligand- and metal-based oxidation events, and the cerium(III) oxidation of 2-Ce(ap) was observed to be more facile than that of 1-Ce(H3Clamp), behavior that was cautiously attributed to the rigidity of the encrypted 1-Ce(H3Clamp) complex compared to the heterobimetallic framework of 2-Ce(ap). These results contribute to the understanding of how ligand designs can promote facile redox cycling for cerium complexes of redox-active ligands, given the large contraction of cerium-ligand bonds upon oxidation.

Observing Similarities and Differences in the Properties of Isostructural Niobium(V)/Tantalum(V) Coordination Compounds with Strong Pi-Donor Ligands.

While niobium and tantalum are found together in their mineral ores, their respective applications in technology require chemical separation. Nb/Ta separations are challenging due to the similar reactivities displayed by these metals in the solution phase. Coordination complexes of these metals have been studied in the contexts of catalysis, small-molecule activation, and functional group insertion reactivity; relatively few studies exist directly comparing the properties of isostructural Nb/Ta complexes. Such comparisons advance the development of Nb/Ta separation chemistry through the potential for differential reactivity. Here, we explore fundamental physicochemical properties in extensively characterized Nb/Ta coordination complexes [Na(DME)3][MClamp], (Clamp6- = tris-(2-(3',5'-di-tert-butyl-2'-oxyphenyl)amidophenyl)amine; M = Nb, Ta) to advance the understanding of the different electronic, optical, and excited-state properties that these metals exhibit in pi-loaded coordination complexes.

Selective Reduction of Niobium(V) Species to Promote Molecular Niobium/Tantalum Separation.

The critical metals niobium (Nb) and tantalum (Ta) coexist in mineral sources, requiring a separation step to purify the elements from one another. The industrial separation process by solvent extraction uses stoichiometric hydrofluoric acid to manifest differences in the speciation of these otherwise chemically similar elements. The identification of alternative methods to separate Nb/Ta is desirable for fluoride waste reduction. In pursuit of this goal, the novel complexes [Na(CH3CN)3(Et2O)][M((S)-BINOLate)3] [M = Nb (1-Nb), Ta (1-Ta)] were synthesized and characterized. In electrochemical studies, a reduction event at the potential -2.04 V versus ferrocene/ferrocenium was observed for 1-Nb, whereas 1-Ta exhibited no metal-based waves in the electrochemical window. In addition to the inherent 4d/5d orbital energy differences between Nb/Ta, density functional theory calculations suggest a larger degree of π donation from the ligands to the metal cation in 1-Ta compared to 1-Nb, destabilizing the lowest unoccupied molecular orbital. This phenomenon contributes to a calculated reduction potential difference of ca. 0.75 V, allowing for the selective reduction of 1-Nb and separation of the reduction product through leaching with diethyl ether for a separation factor of 6 ± 2.

Separation of Aldehydes and Reactive Ketones from Mixtures Using a Bisulfite Extraction Protocol.

The purification of organic compounds is an essential component of routine synthetic operations. The ability to remove contaminants into an aqueous layer by generating a charged structure provides an opportunity to use extraction as a simple purification technique. By combining the use of a miscible organic solvent with saturated sodium bisulfite, aldehydes and reactive ketones can be successfully transformed into charged bisulfite adducts that can then be separated from other organic components of a mixture by the introduction of an immiscible organic layer. Here, we describe a simple protocol for the removal of aldehydes, including sterically-hindered neopentyl aldehydes and some ketones, from chemical mixtures. Ketones can be separated if they are sterically unhindered cyclic or methyl ketones. For aliphatic aldehydes and ketones, dimethylformamide is used as the miscible solvent to improve removal rates. The bisulfite addition reaction can be reversed by basification of the aqueous layer, allowing for the re-isolation of the reactive carbonyl component of a mixture.