Isopolymolybdates Heteropolytungstates Formation and Structure

Examples V 10 O Mo 7 O Mo 8 O 26 4-Ta 6 O Al 13 O 4 (OH) 24 (H 2 O) or AlO 4 Al 12 (OH) 24 (H 2 O) PW 12 O P 2 W 18 O Co 4 P 4 W 30 O 112 (H 2 O) or Co 4 (H 2 O) 2 (P 2 W 15 O 56 ) As 12 Ln 16 (H 2 O) 36 W 148 O

? ? ? How do we make sense of these?

Chromate - Dichromate CrO H + → Cr 2 O H 2 O yellow orange

Chromate - Dichromate CrO H + → Cr 2 O H 2 O Think of CrO 4 2- as being composed of 4 O 2- with a central Cr 6+ One O 2- is neutralized by the acid. O H + → H 2 O

Chromate - Dichromate The Cr 2 O 7 2- ion has each Cr at the center of a tetrahedron with the two tetrahedrons sharing a corner.

Chromate ion Dichromate ion

Molybdates Molybdates form polyions by edge sharing rather than corner sharing. The Mo 6+ ions are at the center of an octahedron.

Polymolybdates Unlike chromates, molybdates are more complicated. As oxides are neutralized, larger structures are formed. MoO 4 2- → Mo 7 O → Mo 8 O → Mo 36 O

Molybdates and Tungstates What three structures can be built from three octahedrons by edge sharing? No corner sharing. No face sharing. In the following structures there is an oxide at each corner and a metal ion in the center of each octahedron.

1 1

2 2

3 3

3 From Opposite Side 3

Electrostatic Repulsions In the linear structure, the center metal ion has little space to move to be farther away from the other two metal ions. In the 90° structure, there is more possibility of movement away from each other but still not as much as the 60° structure.

Electrostatic Repulsions So, 60° structures are preferred over 90° structures, which are preferred over 180° structures. The best structures will maximize 60° and minimize 180°. The building block is the 60° structure.

What’s in the Literature? Lots of errors in the older literature. 1. Systems not at equilibrium when measured, especially tungsten systems. 2. Bad theory. Assumed that diffusion in solution was only related to molecular weight (like gases).

Isopolyoxometalates Maximize 60°; minimize others: details of each follows.

Decavanadate Ion, V 10 O 28 6-

Heptamolybdate Ion, Mo 7 O 24 6-

Octamolybdate Ion, Mo 8 O 26 4-

Hexamolybdate Ion, Mo 6 O 19 2-

Heteropolyoxometalates Some examples PW 12 O CoW 12 O CoW 12 O P 2 W 18 O PW 6 O 24 7-

Keggin Structure When structures get large, must have some corner sharing.

A Heteropolytungstate: XW 12 O 40 n-

Another View

Synthesis Product depends on: 1. stoichiometry of reactants 2. pH 3. temperature A variety of ions can be put into the cavity. P(V), Si(IV), Co(II), Co(III), C(IV),...

Synthesis Can also have a heteroion replace one of the tungstens. A P(V) in the center and a Co(II) replacing a W. A Co(II) in the center and a Co(II) replacing a W.

Synthesis Co 2+ + WO 4 2- → CoW 11 Co(H 2 O)O precipitate potassium salt CoW 11 Co(H 2 O)O H + → CoW 12 O (aq) this contains Co 2+ in the center Oxidize to Co 3+ → CoW 12 O precipitate as potassium salt Result: tetrahedral Co(III) in the center

Lacunary Structures Can have structures with a piece of the regular structure missing. Remove a W 3 unit from the Keggin structure and have a W 9 structure remaining. PW 11 O Remove one W unit from the Keggin structure and have a W 11 structure remaining.

Put two W9 units together and get Dawson Structure P 2 W 18 O 62 6-

Include some transition metal ions Co 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 16-

M(H 2 O)P 5 W 30 O The Lemon Green: metal ionBlue: water Red: phosphorus

As 12 Ln 16 (H 2 O) 36 W 148 O

Uses CoW 12 O with cobalt(III) Oxidizing agent in synthesis of pharmiceuticals

Uses Oxygen-evolving catalyst J. Am. Chem. Soc. 2008, 130,

Uses PMo 12 O Developing Agent for TLC acts as oxidizing agent some molybdenums reduced from +6 to +5 forming a blue color (“molybdenum blue”) as a visible spot.

Uses Catalyst