Specific Chemistry of the Anions: [TaOF₅]^2–, [TaF₆]⁻, and [TaF₇]^2–

Abstract

The controlled crystallization of specific tantalum oxide-fluoride and tantalum fluoride anions ([TaOF 5 ] 2− , [TaF 6 ] − , and [TaF 7 ] 2− ) is demonstrated using organic reagents with varied corresponding pK a values in the presence of aqueous hydrofluoric acid. The identity of tantalum oxide-fluoride or fluoride anions of [TaOF 5 ] 2− , [TaF 6 ] − , and [TaF 7 ] 2− are shown to crystallize successively from solution to solid state by increasing the corresponding pK a of organic reagents, which lead to the subsequent increase of fluoride concentration in the hydrofluoric acid solution.With the use of this methodology, three new hybrid crystal structures were targeted: [H 2 (2,2′-bpy)]TaOF 5 (2,2′-bpy = 2,2′-bipyridyl) 1, [Hdpa]TaF 6 (dpa = 2,2′-dipyridylamine) 2, and [H 2 En]TaF 7 (En = ethylenediamine) 3, respectively. The applicability and comparison of this methodology for tantalum and previously prepared niobium compounds show that it can be broadly used to design new materials with specific functionalities for other transition metal oxide-fluorides. ■ INTRODUCTIONHigh second-order nonlinear optical (NLO) response in noncentrosymmetric (NCS) structures often are generated from asymmetric basic building units (BBUs), such as anionic groups with π-orbital systems or stereochemically active lone pairs [(BO 3 ) 3− , (B 3 O 6 ) 3− , (IO 3 ) − , (TeO x ) n− , [AsS 3 ] 3− , [SbS 3 ] 3− , etc.] 1−7 and second-order Jahn−Teller (SOJT) distorted d 0 transition metal centers (Ti 4+ , Nb 5+ , Ta 5+ , Mo 6+ , etc.). 8−12 Materials with potentially higher second-harmonic generation (SHG) efficiencies based on the concept of two combined acentric BBUs have been shown to crystallize in one NCS structure, either by the combination of acentric cations and acentric anions 13−15 or combination of two acentric anions. 8,16−19 Thus, to incorporate specific acentric BBUs into NCS structures, a rational synthetic approach to target the crystallization of specific anions is needed.Tantalum compounds with strong SHG responses have been recently reported. 16 Owing to the good solubility of tantalum oxide in hydrofluoric acid solutions, tantalum oxide-fluoride/ fluoride anions similar to other early transition metal oxidefluoride anions have been extensively examined. 20−23 Previous tantalum compounds with different tantalum oxide-fluoride/ fluoride anions (e.g., [ TaOF 5 ] 2− , 24,25 [Ta(O 2 )F 5 ] 2− , 26,27 [TaF 6 ] − , 28 [TaF 7 ] 2− , 29−33 [Ta 2 OF 10 ] 2− dimer 34 and the adamantine-like cage [Ta 4 O 6 F 12 ] 2− anions 35 ) have been reported for their diverse coordination chemistry and interesting properties. Solution-phase NMR, Raman, and IR spectroscopy at room temperature have shown that different tantalum oxide-fluoride/fluoride species exist in different fluoride concentration conditions. 36−41 However, the controlled crystallization of specific tantalum oxide-fluoride/ fluoride anions has not been studied so far. In this article, to better understand specific chemistry of tantalum and target potential acentric units of ...

CoMOF5(pyrazine)(H2O)2 (M = Nb, Ta): Two-Layered Cobalt Oxyfluoride Antiferromagnets with Spin Flop Transitions

“…The bond distance of four equatorial Nb–F 2 /O 2 is 1.910(3) Å and the two bridging Nb–F 1 ( trans ) bond lengths are both 1.925(4) Å. These bond lengths are all in agreement with other reported Co-based, Nb-based, and Ta-based oxyfluorides. ,− The pyrazine molecules are all parallelly aligned in the crystal structures of 1 and 2 , different with those in CoF 2 (H 2 O) 2 (pyrazine) and other reported pyrazine-contained hybrids. , The oxidation state of cobalt in +2, niobium in +5, tantalum in +5 are confirmed by bond valence sum (BVS) calculations using values reported by Brese and O’Keefe (Table ). …”

Ammonium tantalum oxide fluorides: new features of dynamic disorder

“…Examples include the series of A 2 TaF 7 (A = Ca, Sr, Ba, Pb), where TaF 7 2À is a CTP for A = Sr, Ba, Pb and it is a PB for A = Ca (Bunič et al, 2007). The CTP TaF 7 2polyhedron was identified in the crystal structures of hybrid compounds such as [(C 2 H 4 NH 3 ) 3 N](TaF 7 )F (Goreshnik et al, 2004), [H 4 tren](TaF 7 ) 2 ÁH 2 O (Saada et al, 2005b), (C 2 H 5 N 4 ) 2 -TaF 7 (Udovenko et al, 2015), and [C(NH 2 ) 3 ] 2 TaF 7 (Goreshnik, 2019), while the PB form was found in [H 2 N(C 2 H 4 ) 2 -NH 2 ](TaF 7 ) (Feng et al, 2010), [H 2 En](TaF 7 ) (Lu et al, 2014), and (C 2 H 6 N 4 ) 2 TaF 7 ÁH 2 O (Udovenko et al, 2015). Both CTP and PB polyhedra are separately present in the crystal structure of [H 4 tren](TaF 7 ) 2 (Saada et al, 2005b).…”

Two‐step Hydrothermal Syntheses and Structures of Three Tantalum Oxyfluoride Compounds with [M(phen)3]2+(M = Ru, Fe) Counter Ions

“…Tantalum readily forms a variety of fluorine compounds 5. Recently, the reported tantalum fluoride anions include the trigonal‐bipyramidal TaF 5 ,6 the octahedral [TaF 6 ] – anion,7 a series of pentagonal bipyramidal [TaF 7 ] 2– polyhedron anions,8 the tantalum oxyfluoride anions including the [TaOF 5 ] 2– and [Ta(O 2 )F 5 ] 2– anions,4,9 the linear [Ta 2 O 2 F 9 ] n – 10a,10b and [Ta 2 OF 10 ] 2– ,10c,10d the adamantane‐like [Ta 4 F 12 O 6 ] n – anion,11 and the [Ta 4 F 16 O 4 ] 2– , [Ta 8 F 24 O 12 ] 8– anions 12. The cations in above reported compounds are the proton, protonated amines, or organic ammoniums.…”