The compound of claim 1 wherein:R is C1 to C4 alkyl fully substituted by halogen atoms which are the same or different; R3 and R5 are each a hydrogen atom; R4 is selected from a halogen atom, CF3, OCF3, SCF3, SOCF3, SO2 CF3, OCHF2 or C1 to C4 alkyl; and R6 is a halogen atom. 3.