Poly[tris(μ-2-aminobenzene-1,4-dicarboxylato)tetrakis(N,N-dimethylformamide)diyttrium(III)]

The asymmetric unit of the title coordination polymer, [Y2(C8H5NO4)3(C3H7NO)4]n, contains one Y3+ ion, three half-molecules of the 2-aminobenzene-1,4-dicarboxylate (abz) dianion and two O-bonded N,N-dimethylformamide (DMF) molecules. Each abz half-molecule is completed by crystallographic inversion symmetry and its –NH2 group is disordered in each case [relative occupancies within the asymmetric unit = 0.462 (18):0.538 (18), 0.93 (2):0.07 (2) and 0.828 (16):0.172 (16)]. The combination of disorder and crystal symmetry means that each of the four C—H atoms of the benzene ring of each of the dianions bears a statistical fraction of an –NH2 group. The coordination geometry of the yttrium ion is a fairly regular YO8 square antiprism arising from its coordination by two DMF molecules, four monodentate abz dianions and one O,O-bidentate abz dianion. The polymeric building unit is a dimeric paddle-wheel with two metal ions linked by four bridging abz dianions. Further bridging linkages connect the dimers into a three-dimensional framework containing voids in which highly disordered DMF molecules are presumed to reside.

The asymmetric unit of the title coordination polymer, [Y 2 (C 8 H 5 NO 4 ) 3 (C 3 H 7 NO) 4 ] n , contains one Y 3+ ion, three half-molecules of the 2-aminobenzene-1,4-dicarboxylate (abz) dianion and two O-bonded N,N-dimethylformamide (DMF) molecules. Each abz half-molecule is completed by crystallographic inversion symmetry and its -NH 2 group is disordered in each case [relative occupancies within the asymmetric unit = 0.462 (18):0.538 (18), 0.93 (2):0.07 (2) and 0.828 (16):0.172 (16)]. The combination of disorder and crystal symmetry means that each of the four C-H atoms of the benzene ring of each of the dianions bears a statistical fraction of an -NH 2 group. The coordination geometry of the yttrium ion is a fairly regular YO 8 square antiprism arising from its coordination by two DMF molecules, four monodentate abz dianions and one O,O-bidentate abz dianion. The polymeric building unit is a dimeric paddle-wheel with two metal ions linked by four bridging abz dianions. Further bridging linkages connect the dimers into a three-dimensional framework containing voids in which highly disordered DMF molecules are presumed to reside.

Related literature
For a related structure containing a similar paddle-wheel motif, see: Braun et al. (2001).

Comment
The title compound (I) is a metal-organic framework (MOF) which is a weak scatterer of X-rays and data were collected from a very small crystal. These were sufficient for refinement of the framework structure but not the disordered solvent.
The asymmetric unit ( Fig. 1) of the title compound consists of an yttrium atom coordinated by 8 oxygen atoms, 6 from 2-amino-1,4-benzenedicarboxylic acid (BDC-NH 2 ) moieties and 2 from coordinated dimethylformamide solvent. The inorganic cornerstone of the MOF is a paddle wheel type unit comprising two yttrium atoms linked by four bridging Y-BDC-NH 2 molecules (Fig. 2). Such units are well known in transition metal MOF structures (Braun, 2001 The BDC-NH 2 linkers at the ends of the paddle wheel are offset in a trans-type conformation and link the units in chains which, when the structure is viewed along the a-axis (Fig. 3), bisect the angle of the b and c axes. When we view the structure along the axis of the paddle wheel unit (Fig. 4) we see that the bridging BDC-NH 2 molecules also link to further units in chains parallel to the a and b-axes producing a three-dinemsional network. The amino groups are disordered over all four possible positions of the benzene rings of the BDC-NH 2 linker molecules, except in the case of the ring described by C10, C11 and C12, in which the NH 2 was found to be localized on C12. All of the disordered C-N bonds were restrained to have the same bond distance.
1 molecule of DMF solvent per ASU was located in the void space of the MOF using difference Fourier maps but the resulting model gave a poor refinement. The program Squeeze from the PLATON suite (Spek, 2009 ) was used to remove residual electron density from the solvent accessible voids giving a chemically sensible structure and acceptable refinemnt statistics. The squeeze calculation suggests voids containing 90 electrons or 2.25 DMF molecules in each unit cell. This is in reasonable agreement with the disordered solvent observed in the difference Fourier map.

Experimental
The Title complex Y-BDC-NH 2 was prepared by dissolving Y(NO 3 ) 3 .6H 2 O (0.383 g, 1 mmol) and 2-amino 1,4-benzenedicarboxylic acid (H 2 N-H 2 BDC) (0.181 g, 1 mmol) in N,N`-dimethylformamide (DMF) (20 ml) at room temperature in a test tube. The mixture thus obtained was placed in a pre-heated oven at 80°C for 24 hrs. Colourless plates of (I) were selected directly from the mother liquor as prepared and mounted on cryoloops.

Refinement
Most of the non hydrogen atoms positions were obtained from the direct methods solution and the remainder (mainly carbon atoms) were located using difference Fourier maps during refinement. Hydrogen atoms were placed in ideal positions and refined with a riding model. Fig. 1. ORTEP view of the asymmetric unit of (I) with thermal ellipsoids at 50% probability. Hydrogen atoms are omitted for clarity.