(1,4-phenylene)bis(methylene) phosphonic acid esters and (1,4-phenylene)bis(methylene) triphenyl phosphonium salt compounds

1. A process for preparing a diphosphonic acid ester of formula [VII] or [VII′] or a diphosphonium salt of formula [VIII] or [VIII′], which process comprising reacting a halogenated aryl compound of formula [XV] or [XV′] with a trialkyl phosphite of formula [XVI] or triphenylphosphine (PPh 3 ):

wherein R 77 , R 78 , R 79 and R 80 may be the same or different provided that at least three thereof are cyano groups or that at least two thereof are nitro groups and the others independently represent a hydrogen atom, a cyano group, a nitro group or a halogen atom, R 77′ , R 78′ , R 79′ and R 80′ may be the same or different and independently represent a group selected from a hydrogen atom and a halogen atom provided that at least three thereof are fluorine atoms, and X represents a halogen atom;

P(OR 81 ) 3 or P(OR 82 ) 3   formula [XVII];

wherein R 81 and R 82 , respectively, may be the same or different and independently represent a hydrocarbon group; and

wherein R 75 R 76 , R 75′ and R 76′ may be the same or different and independently represent an alkyl or alkenyl group having from 1 to 4 carbon atoms, a cyclohexyl group or a phenyl group, and R 77 , R 78 , R 79 , R 80 , R 77′ , R 78′ , R 79′ , R 80′ and X, respectively, have the same meaning as defined above.

2. A process according to claim 1 , wherein R 75 and R 76 , and R 75′ and R 76′ , respectively, represent a saturated hydrocarbon group having from 1 to 4 carbon atoms.

3. A process according to claim 1 , wherein said diphosphonic acid ester or said diphosphonium salt is of formula (19′), or (20′), respectively: