IMes is an abbreviation for an organic compound that is a common ligand in organometallic chemistry.  It is an N-heterocyclic carbene (NHC).  The compound, a white solid, is often not isolated but instead is generated upon attachment to the metal centre.
First prepared by Arduengo, the heterocycle is synthesized by condensation of 2,4,6-trimethylaniline and glyoxal to give the diimine.  In the presence of acid, the resulting glyoxal-bis(mesitylimine) condenses with formaldehyde to give the dimesitylimidazolium cation.  This cation is the conjugate acid of the NHC.