Provided is a compound having formula (I): wherein R2 is selected from —C1, —Br and —CN; R1 and R4 are independently selected from H and —F; R631, R632, R641 and R642 are independently selected from —H, —F and substituted or unsubstituted C1-C3 alkyl groups; and R651 and R652 are independently selected from H and substituted or unsubstituted C1-C3 alkyl groups and substituted or unsubstituted phenyl groups; and wherein at least one of R631, R632, R641, R642 and R652 is not —H, or wherein when all of R631, R632, R641, R642 and R652 are —H, R651 is not Me or Et.