Synthesis, structure and characterization of three different dimension inorganic–organic hybrid vanadates: [Co 2 (mIM) 5 (H 2 O) 2 ]V 4 O 12 , [Ni 2 (mIM) 7 (H 2 O)]V 4 O 12 ·H 2 O and [Cd(eIM) 2 (H 2 O)]V 2 O 6