Carboxylate Groups in a 90° Arrangement: 1,3-Allenedicarboxylic Acid and Its Alkali Metal and Thallium(I) Coordination Compounds

With its two carboxylate functions arranged in an 90 degrees angle, 1,3-allenedicarboxylic acid (point group C-1) possesses a symmetry distinguished from all other dicarboxylic acids which are generally utilized as linker compounds in coordination polymers or MOFs. Crystal structures of coordination compounds with the 1,3-allenedicarboxylate dianion are reported herein for the first time. The diacid was converted with metal carbonates M2CO3 to furnish the crystalline lithium, sodium, rubidium, cesium and thallium(I) compounds. In the solid state, their structures differ significantly. Common motif is the dianion, which is linked by the cations to columns, layers or strands. The sodium compound exhibits an unusual structural feature: One of the three crystallographically different sodium ions coordinates to four instead of six oxygen atoms with seesaw geometry, i.e. two equatorial positions of an octahedral coordination are unoccupied. Thus, an almost linear (179 degrees) O-Na-O unit holds two ancillary oxygen atoms with perpendicular geometry (angles 94 degrees and 95 degrees).