“Anhydrous” means different things to different people. – Seth Herzon
I’m sure you’ve seen it written, probably somewhere on this website, that ruthenium olefin metathesis catalysts are stable to air and water. But stability is not a black and white issue. It’s all grey area. So exactly how grey are Grubbs catalysts?
For a typical 1st or 2nd Generation Grubbs catalyst, you should store it in a cool place under an inert atmosphere (N2 or Ar). If you do, it should stay good for years. The reason for this precaution isn’t so much about water. The catalysts are much more stable to water than they are to oxygen, which can react with (and kill) the catalyst, but this is usually pretty slow in the solid state. Practically speaking, if you plan to go through a bottle in a reasonable amount of time, it’s okay to keep it on your bench and just scoop it out as needed.1
Once you dissolve your (pre)catalyst into solution, though, things get trickier. Dissolution allows for ligand dissociation, which gives a more reactive 14-electron Ru species (the “active catalyst”). Given a chance, this can coordinate oxygen and begin down the road to decomposition. As you’d expect, the rate of this oxidative decomposition is related to the rate of ligand dissociation, so it depends on both the catalyst structure and the solvent. As a rule of thumb, the quicker the catalyst initiates, the quicker it decomposes. And the more polar the solvent, the faster you get into trouble. Generally speaking, if you plan to make a stock solution of catalyst, you should use it relatively quickly (the same day).
1 It’s best to blow a stream of an inert gas into the reagent bottle prior to resealing it to get as much oxygen out as possible. But be careful not to open your gas line too much and blow your expensive catalyst all over the bench…