Variable Lithium Coordination Modes in Two- and Three-Dimensional Lithium Boron Imidazolate Frameworks

Despite years of efforts, known metal−organic frameworks rely predominantly on the use of group 12, d- and f-block elements. Recently, there has been a surge of interest in the use of s- and p-block elements (e.g., Mg and Al) as framework polyhedral nodes. Still, there are much fewer framework materials constructed from lightweight second-row elements (e.g., Li and B). Here we report three framework structures constructed by using lithium and boron as 3- or 4-connected nodes. It is shown that structural features of the poly(azolyl)borate anion can be used to induce 3-connected lithium sites, as compared to much more common 4-connected lithium sites. The creation of 3-connected lithium is of significance because it may offer open metal sites for enhanced guest binding. BIF-10 is a 3-connected three-dimensional lithium boron imidazolate built from the tripodal [BH(mim)3] anion (mim = 2-methylimidazolyl). One unique feature of BIF-10 is that lithium sites in two interpenetrating sublattices have different coordination numbers (tetrahedral with a terminating DMF molecule or trigonal planar). BIF-11 built from the tetrahedral [B(2,4-dmim)4] anion (2,4-dmim = 2,4-dimethylimidazolyl) possesses the sodalite-type framework and represents an ideal example that demonstrates the competitive structure-directing effect of two methyl substituents. BIF-12 built from tetrahedral [B(bim)4] anion (bim = benzimidazolyl) has a 3-connected layer structure with a dangling bim group, revealing another way in which the structure of poly(azolyl)borate anions can induce the 3-connected lithium site.