In case you don't have space to take a backup only try this method. Otherwise dont try this. (Especially if the database you are refering is production data)
1. Try to restart the server. By doing this you make the tempdb recreated. You may gain some space to take a log backup.
2. If the above method too doesn't give enough space to backup. You need to change the recovery model of the database into SImple. Truncate the transaction log using Shrink-> File option. Change the recovery model to the original setting and take a full database backup immediately after. Remember, thsi has its own risks.
Cheers,
Preethiviraj Kulasingham
MCITP:DBA