Isolation of the Elusive Heptavanadate Anion with Trisalkoxide Ligands

Abstract

The unprecedented heptavanadate cluster has been isolated from reactions between trisalkoxide ligands and vanadate in water at pH = 2 as a series of alkylammonium [H x V 7 O 18 (H 2 O)((OCH 2 ) 3 CR)] (4–x)- salts ( 1 – 3 , R = CH 2 OH; 4 , R = CH 3 ). Their structures have been determined and the partial stability of 4 in water assessed by a combination of multinuclear NMR spectroscopy and ESI-MS. The heptavanadate unit reported herein could represent an intermediate species in the formation of decavanadate that is blocked by attachment of tripodal ligands.

The Smallest Polyoxotungstate Retained by TRIS-Stabilization

“…To further examine the solution behavior of W 2 , 13 C and 183 W NMR spectroscopic studies were performed in D 2 O at pH 7.5 (Figures S6 and S7). The 183 W NMR spectrum of Na 2 W 2 (0.058 M) in D 2 O shows one intense signal with a chemical shift at −3.4 ppm and one minor signal at −91.9 ppm (Figure S6A).…”

“…1 , 7 Discrete binuclear complexes of pentavalent tungsten [W V 2 O 4 (Y)] 2− (Y 4− = hexadentate ligand, e.g., ethylenediaminetetraacetate) in octahedral coordination are known; 11 however, they have not been obtained by acidification of orthotungstate. The strategy of using organic ligands to inhibit the formation of large clusters has previously been applied to POMos 12 and POVs 13 while it has not been reported for POTs. Tris(hydroxymethyl)aminomethane ((HOCH 2 ) 3 CNH 2 , TRIS) has been recently utilized to stabilize and isolate elusive heptavanadate.…”

“…The W 2 with the symmetrical attachment of two additional –CH 2 OH (green in Figure 3 ) gives signals at 792.1, 814.1, 832.1, and 854.1 m / z . The attachment of TRIS ligands through three functional groups to the trimeric face of W VI ions is a known strategy in POM chemistry for elusive anion stabilization 13 and may be the reason that these compounds are detected in solution. The ESI-MS spectrum of Na 2 W 2 recorded in the negative mode ( Figure S4 ) demonstrates the absence of signals from any IPOTs.…”

“…In acidified solutions of WO 4 2– , both in the presence or absence of heteroions, anions with less than six W are underrepresented. − The smallest discrete isopolytungstates (IPOT) verified to exist both in the solid state and in solutions are the Lindqvist hexatungstate [W 6 O 19 ] 2– in nonaqueous media and heptatungstate [W 7 O 24 ] 6– in water. , Discrete binuclear complexes of pentavalent tungsten [W V 2 O 4 (Y)] 2− (Y 4− = hexadentate ligand, e.g., ethylenediaminetetraacetate) in octahedral coordination are known; however, they have not been obtained by acidification of orthotungstate. The strategy of using organic ligands to inhibit the formation of large clusters has previously been applied to POMos and POVs while it has not been reported for POTs. Tris­(hydroxymethyl)­aminomethane ((HOCH 2 ) 3 CNH 2 , TRIS) has been recently utilized to stabilize and isolate elusive heptavanadate .…”

“…The strategy of using organic ligands to inhibit the formation of large clusters has previously been applied to POMos and POVs while it has not been reported for POTs. Tris­(hydroxymethyl)­aminomethane ((HOCH 2 ) 3 CNH 2 , TRIS) has been recently utilized to stabilize and isolate elusive heptavanadate . While in biochemistry, TRIS (p K a 8.06) is used as pH buffer between 7.0 and 9.2, in POM chemistry, TRIS is usually used for covalent organic functionalization via alkoxo-groups −CH 2 OH attachment to the POM, − and −NH 2 plays the key role for postfunctionalization through amide bond formation. − Recently, we expanded the role of TRIS by showing that TRIS, as part of a buffer solution that is normally considered unimportant, plays a defining role in the formation of a new member of the Keggin family .…”