Cyclic Ti₉ Keggin Trimers with Tetrahedral (PO₄) or Octahedral (TiO₆) Capping Groups

Abstract

We have synthesized the cyclic Ti 9 Keggin trimers [(alpha-Ti 3PW 9O 38) 3(PO 4)] (18-) ( 1) and [(alpha-Ti 3SiW 9O 37OH) 3(TiO 3(OH 2) 3)] (17-) ( 2), which are both composed of three (Ti 3XW 9O 37) units (X = P or Si) linked via three Ti-O-Ti bridges and a capping group, which is either tetrahedral PO 4 ( 1) or octahedral TiO 6 ( 2). Polyanions 1 and 2 were fully characterized in the solid state (IR, X-ray diffraction, thermogravimetric and elemental analyses) and in solution ( (31)P or (183)W NMR).

Chemistry of Group IV Metal Ion‐Containing Polyoxometalates

“…Kortz et al synthesized a cyclic trimer, [(α‐Ti 3 PW 9 O 38 ) 3 (PO 4 )] 18– , which was composed of three tri‐Ti‐substituted α‐Keggin POM units linked via three Ti–O–Ti bonds and a capping group of the tetrahedral PO 4 3– ion (Figure 10). 47 This cyclic trimer was prepared by a reaction of [P 2 W 19 O 69 (H 2 O)] 14– with TiO(SO 4 ). Kortz et al also reported an analogous compound, [(α‐Ti 3 SiW 9 O 37 OH) 3 (TiO 3 (OH 2 ) 3 )] 17– (Figure 10), from a reaction of [{K(H 2 O) 2 }α‐Si 2 W 18 O 66 ] 15– and TiO(SO 4 ) in an aqueous solution 47.…”

Transformation of Tri-Titanium(IV)-Substituted α-Keggin Polyoxometalate (POM) into Tetra-Titanium(IV)-Substituted POMs : Reaction Products of Titanium(IV) Sulfate with the Dimeric Keggin POM Precursor under Acidic Conditions

“…We can reveal only the molecular structure of the POM, but not the crystal structure. These features are too common in the POM crystallography [9,10,11,12,13,14,15,16,17,18]. …”

“…Site-selective substitution of the W VI atoms in POMs with Ti IV atoms is particularly interesting, because of formation of the multicenter active sites with corner- or edge-sharing TiO 6 octahedra and, also, generation of oligomeric species through Ti-O-Ti bonds [9,10,11,12,13,14,15,16,17,18]. A number of catalytic reactions of titanium(IV)-containing POMs have also been reported so far [19,20,21].…”

“…, [{γ-SiTi 2 W 10 O 36 (OH) 2 } 2 O 2 ] 8- [14] and [{γ-GeTi 2 W 10 O 36 (OH) 2 } 2 O 2 ] 8- [15], have been also prepared as a dimeric species, while di-Ti IV -1,5-substituted β-Keggin POM has been isolated as a tetrameric species, [{β-Ti 2 SiW 10 O 39 } 4 ] 24- [16]. In addition, the cyclic tri-Ti IV -substituted Keggin trimers such as [(α-Ti 3 PW 9 O 38 ) 3 (PO 4 )] 18- [17], [(α-Ti 3 SiW 9 O 37 OH) 3 (TiO 3 (OH 2 ) 3 )] 17- [17], and {K[(Ge(OH)O 3 ) (GeW 9 Ti 3 O 38 H 2 ) 3 ]} 14- [18] have been recently reported.…”

Monomer and Dimer of Mono‐titanium(IV)‐Containing α‐Keggin Polyoxometalates: Synthesis, Molecular Structures, and pH‐Dependent Monomer–Dimer Interconversion in Solution

“…Furthermore, di‐titanium(IV)‐substituted γ‐Keggin silicotungstate and germanotungstate {i.e., [{γ‐SiTi 2 W 10 O 36 (OH) 2 } 2 O 2 ] 8–[2f] and [{γ‐GeTi 2 W 10 O 36 (OH) 2 } 2 O 2 ] 8– }2g have also been prepared as a dimeric species, whereas a di‐titanium(IV)‐1,5‐substituted β‐Keggin POM has been isolated as a tetrameric species, [{β‐Ti 2 SiW 10 O 39 } 4 ] 24– 2h. In addition, cyclic tri‐titanium(IV)‐substituted Keggin trimers such as [(α‐Ti 3 PW 9 O 38 ) 3 (PO 4 )] 18– ,2i [(α‐Ti 3 SiW 9 O 37 OH) 3 {TiO 3 (OH 2 ) 3 }] 17– ,2i and {K[{Ge(OH)O 3 }(GeW 9 Ti 3 O 38 H 2 ) 3 ]} 14–[2j] have recently been reported.…”