toms bound to a boron atom where at least one of R1 to R4 is other than a fluorine atom. [0030] It is preferred that at least one of R1 to R4 is a group represented by the general formula: CnF2n+1, where n is an integer of 1 to 4, or a group represented by the general formula: CmF2m+1SO2, where m is an integer of 1 to 4.