Poly[guanidinium [tri-μ-formato-κ6 O:O′-formato-κ2 O,O′-yttrium(III)]]

In the title coordination polymer, {(CH6N3)[Y(CHO2)4]}n, the yttrium(III) ion is coordinated by one O,O-bidentate formate ion and six μ2 bridging formate ions, generating a square-antiprismatic YO8 coordination polyhedron. The bridging formate ions connect the metal ions into an anionic, three-dimensional network. Charge compensation is provided by guanidinium ions, which interact with the framework by way of N—H⋯O hydrogen bonds. The guanidine molecules reside in porous channels of 3.612 by 8.189 Å, when considering the van der Waals radii of the nearest atoms (looking down the a-axis).

In the title coordination polymer, {(CH 6 N 3 )[Y(CHO 2 ) 4 ]} n , the yttrium(III) ion is coordinated by one O,O-bidentate formate ion and six 2 bridging formate ions, generating a squareantiprismatic YO 8 coordination polyhedron. The bridging formate ions connect the metal ions into an anionic, threedimensional network. Charge compensation is provided by guanidinium ions, which interact with the framework by way of N-HÁ Á ÁO hydrogen bonds. The guanidine molecules reside in porous channels of 3.612 by 8.189 Å , when considering the van der Waals radii of the nearest atoms (looking down the aaxis). Liu et al. (2011) have published the erbium (Er) analog of the title compound, catena-(tris(-formato)-formato-erbium diaminomethaniminium , with nearly identical cell parameters and unit-cell volume. They also document a similar Er-based structure that employs a different solvent (1H-imidazol-3ium). The presence of formic acid in the reaction is likely a result of the DMF hydrolysis as it is known to be a common impurity in DMF (IUPAC, 1977). The diaminomethaniminium ion was generated in situ, through 2-amino-4,6-dihydroxypyrimidine ring cleavage (Calza, et al., 2004). In regard to the observed chirality of the title compound, it has been previously documented that there is a great propensity for virtually any metal-organic framework (MOF) to crystallize in a chiral space group (Lin, 2007

Comment
The title compound (I) C 4 H 4 O 8 Y . (CH 6 N 3 ) is comprised of a yttrium (Y) atom fully coordinated by formate molecules, thereby creating a three-dimensional network structure of linked Y formate nodes. Figure 1 shows the coordination of the formate groups about Y atom. The figure shows three monodentate (bridging) formate molecules designated here by their associated C atoms (C2, C3, C4) along with a bidentate formate (C1) molecule exclusively bound to the metal center. The monodentate formate molecules bridge to neighboring metal clusters to generate the three-dimensional network.
Considering the summation of charge around the Y +3 atom, one obtains a full -1 charge from the bidentate molecule along with a total of six monodentate formate molecules, each contributing -0.5 worth of charge for a total of -4 charge for each Y-formate cluster. This net -1 charge on the cluster is balanced by the presence of one diaminomethaniminium molecule for every cluster present. Figure 2 shows the three-dimensional network of the structure as viewed down the a axis. The plot shows Y-formate clusters as node polyhedra that are linked by formate ligands. For clarity purposes, hydrogen atoms on formate molecules as well as the diaminomethaniminium solvent molecules have been removed. The void space between the linked nodes are filled by the diaminomethaniminium ions which pack along the a axis direction. Two of the Y-formate polyhedra in Figure 2 are labeled as A and B clusters and are located near the center of the image. This has been done for reference purposes. Figure 3 shows a smaller region of the three-dimensional network to better discuss connectivity. This figure, also viewed down the a axis, shows the location of the diaminomethaniminium molecules within the pores of the framework.
Noting the polyhedra labeled A and B in Figure 3, one can compare this area back to Figure 2 and how it relates to the larger three-dimensional array. Considering the lattice shown in Figure 3, one can see that the A-B polyhedra pair (or bicluster) are linked by the C3 formate ligand, which exclusively bridges the A and B nodes. This A-B polyhedral bicluster is then bridged to neighboring A-B bi-clusters via the C4 formate ligand along the b-c plane (i.e. parallel to the plane of the image). The C1 formate is the bidentate ligand and does not link to neighboring Y-formate nodes, but instead truncates within the void space. Note that the formate ligands are plotted in Figure 3 without bound H atoms for purposes of clarity, while the diaminomethaniminium molecules are shown with H atoms present. The C2 formate ligand is not visible in this image, but links the Y-formate nodes along the a axis direction. It is worth noting in regard to the C2 formate molecule that when one considers the location of the C2 and C2A formate molecules in Figure 1; it is clear that whereas, the C4 formate alternates orientation along the b axis direction in a similar zigzag fashion, as can be assessed by careful evaluation of Figure 3.
In regard to the observed chirality of (I), it has been previously documented that there is a great propensity for virtually any Metal-Organic Framework (MOF) to crystallize in a chiral space group (Lin, 2007). This is thought to be inherent to the topological variety of these materials, as there are a multitude of coordination capabilities between the metal nodes and organic ligands.

Figure 1
The molecular structure of Y metal center for compound (I), with atom labels and 50% probability displacement ellipsoids for non-H atoms.