Hydrothermal synthesis of three new 3-D framework rare-earth mellitates

Hydrothermal reaction of rare-earth ions and mellitic acid [MEL-H6=C6(COOH)6] has been carried out to determine the effects of metal size and ancillary aquation on polymer architecture. Three new 3-D framework rare-earth mellitate polymer types are formed with reduced aquation levels compared to ambient phases. Larger ions Ln=La to Er form 9-coordinate [Ln2(MEL)(H2O)6] 1 (orthorhombic, Pnnm, for La a=13.642(2), b=6.769(1), c=10.321(2) Å, V=953.1 Å3) and smaller ions Ln=Tm to Lu form the 7-coordinate [Ln2(MEL)(H2O)4]H2O 2 (tetragonal, P4/ncc, for Tm a=18.443(3), c=10.374(2) Å, V=3529 Å3). In the case of Ln=Er, Tm a third type with μ-aqua groups is found [Ln2(MEL)(μ-H2O)2(H2O)2] 3 (monoclinic P21/c, for Tm a=5.781(1), b=14.840(1), c=8.920(1) Å, β=93.46(1)°, V=763.8 Å3). This 8-coordinate phase is favored over 1 at higher temperature (200°C) for Er and favored over 2 at lower temperatures (160°C) for Tm.