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

Abstract

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.

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

“…0.75 V, allowing for the selective reduction and leaching of the Nb complex, resulting in a S Ta/Nb value of 6 ± 2. 34 This study demonstrated a proof-of-concept Ta/Nb separation procedure based on fundamental differences in bonding interactions in Ta versus Nb complexes.…”

“…4-250. [19][20][21][22][23][24][25]34 However, it should be understood that this system is not yet industrially viable due to the air/moisture sensitivity of the compounds, the moderate to low yields of 1-M, and the high cost of the starting materials. Instead, this result should be viewed as a fundamental proof-of-concept separation demonstrating the viability of this novel separation strategy.…”

Solvent-Impregnated Sorbents for Tantalum from Niobium Separation Using a Fixed-Bed Column

“…The dedication of our method to obtaining ultrapure reagents on a small scale also justifies the use of small amounts of HF, not recommended for safety reasons, whose presence provides such an excellent separation effect. It should be mentioned that in the recent literature, fluoride systems for the separation of Nb and Ta are still eagerly used [40,45], although gradually one can find attempts to eliminate the use of fluoride systems for the separation of Nb and Ta in oxalate [46,47] or phosphate [48] or electrochemical [49] systems, which is certainly a very good direction for large-scale industrial separation of these elements.…”

Syntheses of Group 5 Amide Amidinates and Their Reactions with Water: Different Reactivities of Nb(V) and Ta(V) Complexes

“…There have been reports of different reactivities of Nb­(V) and Ta­(V) compounds. − Our search of the literature, including the 1904–2013 annual reviews on niobium and tantalum chemistry by the Royal Society of Chemistry, shows that the reduction of Nb­(V) complexes to Nb­(IV) species is more likely than their Ta­(V) analogues, − which is a major difference between the Nb­(V) and Ta­(V) chemistries. Bradley and Thomas have reported several reactions between MCl 5 [M = Nb­(V); Ta­(V)] and LiNR 2 (R = Me, Et, n Pr, n Bu). , As the alkyl chains increase, the extent of Nb­(V) to Nb­(IV) reduction increases in the products, while tantalum complexes remains at Ta­(V).…”

“…Upon heating Ta­(O 2 CNEt 2 ) 5 , in contrast, it remains in the V oxidation state. Studies of [Na­(CH 3 CN) 3 (Et 2 O)]­[M­(( S )-BINOLate) 3 ] [M = Nb, Ta; BINOL = ( S )-(−)-1,1′-bis­(2-naphthol)] have shown selective Nb­(V) reduction, while the Ta­(V) analogue exhibits no such reduction, as Furigay, Schatz, Schelter, and co-workers have recently reported . There are relatively few reports of different reactivities of Nb­(V) and Ta­(V) compounds while preserving the V oxidation state. , Muetterties and Wright have reported that isostructural, eight-coordinate Nb and Ta complexes [M­(tropolonate) 4 ] + Cl – show significant differences to hydrolysis .…”