Just dissolve the Schiff base in MeOH (concentrated sol.) and left standing the solution via slow evaporation of solvent at room temperature. Pls read the paper: Z. Naturforsch. 2007, 62b, 807 – 817.
Did you try with MeOH via slow evaporation of solvent? I never found DMF/MeOH mixture for Schiff base crystallizatio, rather try with n-pentane/MeOH or simply dissolve in n-penatne and left for slow evaporation. I think, it'll work. Good luck.
Dear , DMF removal will difficult, if you go toward water workup for its removal then you will loose your product yield. , you can also try DCM, otherwise stick around methanol.