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

Abstract

Reaction products of titanium(IV) sulfate in HCl-acidic aqueous solution with the dimeric species linked through three intermolecular Ti-O-Ti bonds of the two tri-titanium(IV)-substituted α-Keggin polyoxometalate (POM) subunits are described. Two novel titanium(IV)-containing α-Keggin POMs were obtained under different conditions. One product was a dimeric species through two intermolecular Ti-O-Ti bonds of the two tetra-titanium(IV)-substituted α-Keggin POM subunits, i.e., [[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]26- (1). The other product was a monomeric α-Keggin species containing the tetra-titanium(IV) oxide cluster and two coordinated sulfate ions, i.e., [{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]3- (2). Molecular structures of 1 and 2 were also discussed based on host (lacunary site)-guest (titanium atom) chemistry.

Chemistry of Group IV Metal Ion‐Containing Polyoxometalates

“…The dimeric tri‐Ti IV ‐1,2,3‐substituted α‐Keggin polyoxometalate, [(α‐1,2,3‐PW 9 Ti 3 O 37 ) 2 O 3 ] 12– , has been considered as a very stable compound. Nevertheless, this compound reacted with Ti(SO 4 ) 2 · 4H 2 O under strongly acidic conditions to form two novel Ti IV ‐substituted POMs, i.e., [[{Ti(H 2 O) 3 } 2 (μ‐O)](α‐PW 9 Ti 2 O 38 )] 2 6– and [{Ti 4 (μ‐O) 3 (SO 4 ) 2 (H 2 O) 8 }(α‐PW 9 O 34 )] 3– 25. Dimeric tetra‐Ti IV ‐substituted POM, [[{Ti(H 2 O) 3 } 2 (μ‐O)](α‐PW 9 Ti 2 O 38 )] 2 6– , was obtained as potassium salt by a reaction of [(α‐1,2,3‐PW 9 Ti 3 O 37 ) 2 O 3 ] 12– with an excess of Ti(SO 4 ) 2 · 4H 2 O in an acidic aqueous solution (pH < 0.1), followed by stirring the solution at 60 °C for 10 min 25.…”

“…Nevertheless, this compound reacted with Ti(SO 4 ) 2 · 4H 2 O under strongly acidic conditions to form two novel Ti IV ‐substituted POMs, i.e., [[{Ti(H 2 O) 3 } 2 (μ‐O)](α‐PW 9 Ti 2 O 38 )] 2 6– and [{Ti 4 (μ‐O) 3 (SO 4 ) 2 (H 2 O) 8 }(α‐PW 9 O 34 )] 3– 25. Dimeric tetra‐Ti IV ‐substituted POM, [[{Ti(H 2 O) 3 } 2 (μ‐O)](α‐PW 9 Ti 2 O 38 )] 2 6– , was obtained as potassium salt by a reaction of [(α‐1,2,3‐PW 9 Ti 3 O 37 ) 2 O 3 ] 12– with an excess of Ti(SO 4 ) 2 · 4H 2 O in an acidic aqueous solution (pH < 0.1), followed by stirring the solution at 60 °C for 10 min 25. The molecular structure of [[{Ti(H 2 O) 3 } 2 (μ‐O)](α‐PW 9 Ti 2 O 38 )] 2 6– is composed of two “PW 9 Ti 4 O 45 ” Keggin POM halves linked via two Ti–O–Ti bonds between them.…”

“…Each half contains a Ti 4 center consisting of two Ti atoms of a one host (mono‐lacunary site)‐one guest (one Ti atom) coordination and two Ti atoms of a one host‐two guest coordination. In each Keggin POM half unit, six coordinated water molecules shown by bond valence sum (BVS) calculations occupy the six terminal positions of the two Ti atoms of the one host‐two guest coordination, which are shown as open circles in Figure 7 25…”

“…On the other hand, a three host‐four guest type monomeric POM, [{Ti 4 (μ‐O) 3 (SO 4 ) 2 (H 2 O) 8 }(α‐PW 9 O 34 )] 3– , was prepared by a 30 min reaction at 80 °C 25. The molecular structure is a monomeric POM composed of a tetra‐Ti cluster unit, [Ti 4 (μ‐O) 3 (SO 4 ) 2 (H 2 O) 8 ] 6+ , which is accompanied by two coordinated sulfate ions and eight coordinated water molecules constructed on a tri‐lacunary Keggin unit.…”

“…Although the relationship is not necessarily based on the non‐covalent interaction, the use of the term is convenient and useful on classifying and understanding the structures in the myriad Ti‐substituted POMs. As a matter of fact, in addition to the one host‐one guest relationship observed in most Ti‐substitutions of Keggin POMs, several examples due to one host‐ two guest, two host‐four guest and three host‐four guest relationships are seen (section 1.2) 23–25…”