Vanadium Chemistry (3): The oxalato complexes

Oxalate

 V(V) forms a surprising complex: cis-[VO2(C2O4)2]3- whose ammonium salt (NH4)3[VO2(C2O4)2].2H2O is bright yellow and forms large, maybe even single crystals, but solution tends to be unsaturated till the end.

Preparation of this is quite hard to describe: Mixing same molar NH4VO3, H2C2O4 and (NH4)2C2O4 together in solution is OK, but then it darkens soon even if you are very precise. You may use a few milliliter H2O2 to oxidize it back it turns red, and then in a few days it darkens and repeat H2O2 addition... finally it does not become dark but yellow, then go to vaporize at RT. Seems to be stable both as solid and solution, but wet reduced  dark blue parts of the solid turns into red insoluble substance in a few months. If you managed to get some good seeds and saturated solution, you may grow very large crystals as usual. Solubility is so high that visible growth is observed every few hours, and extremely large crystals can be obtained in only a few days. Crystals tend to be well-formed and extremely clear except on the surface of seed, due to unavoidable dissolution when starting growth, so using small seeds is recommended.

V(IV) forms a complex: [VO(H2O)(C2O4)2]2- whose ammonium salt (NH4)2[VO(H2O)(C2O4)2].H2O can hardly be crystallized without ethanol.

However, a DMAP(4-dimethylaminopyridinium) salt can be made. In this salt, oxo and aquo are trans. It is sparingly soluble. Follow the steps to prevent toxic DMAP vapor.

Steps:
1. Roast 1x ammonium metavanadate till it becomes black, then red

2. Add 1.5x oxalic acid and water, boil till no CO2 evolves

3. Cool down, then add 1x oxalic acid and 2x 4-dimethylaminopyridine(dmap), the mixture suddenly turn into a pasty stuff, probably DMAP oxalate?

4. Add some more water

5. Heat and stir, and solid gradually turns purple.

6. Cool down, filter and collect the solid. No large crystals can be growed, but the powder is very beautiful.

WIP;;;