Zeolitic Boron Imidazolate Frameworks

Low-connectivity (4- or 3-connected) of the basic structural building block is closely associated with the open architecture and porosity in 3-D framework materials. The synthetic development of low-connectivity frameworks with new composition and topology continues to attract much attention because applications of such materials depend on the unique composition or topology of each material.[1–6] Currently, the synthesis of metal-organic frameworks (MOFs) is an active research area.[1, 7] The vast majority of new microporous materials synthesized in the past decade belong to this family.[8–13] Among numerous known MOFs, a family of materials that closely resemble zeolite topologies are those based on divalent metal (M2+) imidazolates (im−) in which M2+ and im− replace Si4+ (Al3+) and O2−, respectively, resulting in the general framework composition M(im)2, just like SiO2.[8–11] The success in generating new zeolitic topologies is attributed to structure-directing effects of imidazolate ligands and other conditions such as solvents. Here we demonstrate a versatile synthetic method capable of generating a large family of low-connectivity framework materials. This method is based on the crosslinking of various pre-synthesized boron imidazolate complexes by monovalent cations (e.g., Li+ and Cu+) into extended frameworks. One advantage of this method is that it allows the use of ultra-light chemical elements (i.e., Li and B) as framework vertices. Furthermore, unlike the Zn-im system in which it is difficult to pre-synthesize 3-connected Zn(im)3 − unit, both 4-connected B(im)4 − and 3-connected HB(im)3 − can be readily synthesized (Scheme 1) prior to solvothermal synthesis, which further increases the diversity of materials accessible through this method. For the creation of 4-connected topologies, our strategy is reminiscent of the strategy that led to the discovery of microporous AlPO4 by analogy with porous silica.[2] Just as Al3+ and P5+ can replace two Si4+ sites in a porous silicalite, Li+ and B3+ can replace two Zn2+ sites in a Zn(im)2 type framework. Scheme 1 The pre-synthesized 4- and 3-connected B-centred ligands. A total of eleven boron imidazolate framework materials have been made (Table 1). In addition to 4-connected topologies, 3-connected and mixed (3,4)-connected 3-D framework topologies have also been realized. These materials exhibit eight distinct topological features (Table 1). Three particularly interesting materials are the unprecedented 4-connected framework materials (BIF-1-Li, BIF-2-Li and BIF-3-Li) based on the lightest possible tetrahedral nodes in the Periodic Table: Li and B (excluding Be which is not studied in this work because of its toxicity). Table 1 A Summary of Crystal Data and Refinement Results.[a] BIF-1-Li exhibits a 3D tetrahedral framework in which each Li+ or B3+ center is connected to four imidazoly (denoted as im) ligands to create a SiO4-like tetrahedron (i.e., Li(im)4 and B(im)4) (Figure 1a). Each im− linker bridges one Li+ center and one B3+ center with the Li…B distance ranging from 5.585 to 5.683 A. The Li(im)4 and B(im)4 tetrahedra alternate through corner-sharing into a 4-connected 3-D zni-type framework containing 4-, 6-, and 12-membered rings (Figure 1b, Figure S2). Figure 1 View of the coordination environment in BIF-1-Li (a) and BIF-3-Cu (c), respectively, showing the connectivity between the Li+ and B3+ tetrahedra or the Cu+ and B3+ tetrahedra; and the zni net of BIF-1-Li (b) and the sodalite net of BIF-3-Cu (d). BIF-2-Li, another 3-D 4-connected Li-B imidazolate framework, was synthesized by using 2-methylane-imidazolate (mim) as the crosslinking ligand and it possesses the non-interpenetrating diamond (dia) topology (Figure S3). To demonstrate the general applicability of our synthetic strategy, we have also explored other M+/M3+ combinations. We have found that the same framework topology present in BIF-1-Li and BIF-2-Li can also be made by using Cu+ in place of Li+ (Table 1) Prior to this work, some polymeric materials based on [B(im)4]− and divalent metals (e.g., Pb, Mg, Ca, Sr) are known.[14] However, it was observed that such combinations between [B(im)4]− and M2+ have a tendency to produce layered materials. There is one known Li-B-im structure, however, half of the lithium sites in this phase contain terminal H2O and CH3OH ligands.[15] Two of the most interesting materials reported here are isostructural LiB(mim)4 (BIF-3-Li) and CuB(mim)4 (BIF-3-Cu) with neutral sodalite (SOD) topology containing 4- and 6-rings formed by alternating corner-sharing Li(mim)4 (or Cu(mim)4) and B(mim)4 tetrahedra (Figure 1c and 1d). The synthetic method described here can also lead to the synthesis of framework materials with the mixed (3,4) connectivity. The solvothermal reaction of CuI and sodium tetrakis(benzimidazolyl)borate (NaB(bim)4) at 85 °C or 120 °C led to the crystallization of BIF-4 and BIF-5 with (3,4)-connected nets, respectively. In the asymmetric unit of BIF-4, two independent tetrahedral Cu+ ions show 4- and 3-connectivity due to one terminal acetonitrile ligand at the Cu1 site, and two independent B(bim)4 − ligands show μ4- and μ3-coordination modes, respectively (Figure 2a). BIF-4 features a unique (3,4)-connected framework with sqc1436 topology containing 4- and 8-membered rings (Figure 2b). The uncoordinated bim ligand in μ3-B(bim)4 − is located in the 8-membered ring. Figure 2 (a) The coordination environment in BIF-4; (b) view of the 4-ring and 8-ring in BIF-4, the terminal bim groups in the B(bim)4 − ligands are highlighted in the 8-ring; (c) the coordination environment in BIF-5; (d) view of the 4-ring and 8-ring ... In BIF-5, the B(bim)4 − ligand is 4-connected to four Cu+ ions. However, two independent Cu+ ions show two different coordination environments: CuIN3 tetrahedron and linear CuN2 (Figure 2c) (iodide anion in CuIN3 is a terminal ligand). The 3-connected Cu1 sites are linked by the B(bim)4 − ligands into a layer with 4- and 8-rings (Figure 2d). The linear Cu2 centres further connect these layers through the additional coordination of the B(bim)4 − ligands, generating a (4.6.8)(4.62.83) net by considering the B(bim)4 − ligands as the 4-connected nodes and the Cu1 sites as the 3-connected nodes (Figure S4). By employing 3-connected boron-imidazolate ligands, 3-connected frameworks (BIF-6 to BIF-8) have also been prepared. In BIF-6 with the formula CuIBH(im)3, tripodal BH(im)3 − ligands are linked through planar 3-coordinate Cu+ ions into a 2D 4.82 layer (Figure 3a and Figure S5). In BIF-7, the planar 3-connected Cu+ ions are bridged by the tripodal BH(mim)3 − ligands into a 3D 2-fold interpenetrating framework with the (10,3)-b topology (also called the ths net) (Figure 3b and Figure S6). In comparison, BIF-8 features a 3-connected 3-D 2-fold interpenetrating framework with the chiral (10,3)-a topology (also called the srs net) (Figure 3e). Figure 3 View of the tripodal ligands in BIF-6 (a), BIF-7 (b) and BIF-8 (c and d). In BIF-8, the racemic BH(eim)3 − ligands (c and d) act as 3-connected building blocks in two-fold interpenetrating srs net of opposite handedness (e). In BIF-8, one prominent feature is the spontaneous resolution of the racemic BH(eim)3 − ligands into two interpenetrating 3-connected chiral sub-nets with opposite handedness. The BH(eim)3 − ligand is chiral because of the conformation of the ethyl groups (Figure 3c and 3d). It is worth noting that there are two sources of chirality here: chiral BH(eim)3 − ligands (R- and S-configurations) and chiral srs nets (denoted +γ* and −γ* for two enantiomeric forms). Because each intrinsically chiral srs net selects only one enantiomer of BH(eim)3 − ligands, the spontaneous resolution of racemic BH(eim)3 − ligands into two independent but interpenetrating frameworks is observed (Figure 3e). To determine zeolitic properties of such materials, BIF-3-Cu and BIF-3-Li with the sodalite topology were studied by gas adsorption measurements performed on Micromeritics ASAP 2010 surface area and pore size analyzer. The permanent porosity was confirmed by N2 adsorption measurements. The samples were degassed at 200°C prior to the measurement. They exhibit type I isotherm, indicating the microporous nature. The Langmuir surface areas are 182.3 and 726.5 m2/g for BIF-3-Cu and BIF-3-Li, respectively (Figure 4, Figure S13). The CO2 adsorption isotherms of BIF-3-Cu and BIF-3-Li at 273 K were also evaluated. As shown in Figure 5, the maximum adsorptions at ~1 atm are 21.9 and 34.5 cm3·g−1 for BIF-3-Cu and BIF-3-Li, respectively. Figure 4 N2 gas sorption isotherm of BIF-3-Li at 77K. P/P0 is the ratio of gas pressure (P) to saturation pressure (P0), with P0=769 torr (adsorption: dotted line; desorption: circles). Figure 5 CO2 adsorption isotherms of BIF-3-Li (dotted line) and BIF-3-Cu (circles) at 273 K. In conclusion, we have synthesized a family of boron imidazolate framework materials by crosslinking 3- or 4-connected boron imidazolate complexes (e.g., B(im)4 − or HB(im)3−) with 3- or 4-connected Li+ or Cu+ centers. These materials possess various 4-connected, mixed (3,4)-connected, and 3-connected 3-D open frameworks with topologies ranging from zeolitic sodalite type to chiral (10,3)-a type. The permanent microporosity has been demonstrated for BIF-3-Cu and BIF-3-Li with the sodalite topology. There materials represent a unique family of materials that border between MOFs and covalent frameworks because of the co-existence of covalent (B–N) and coordination bonds (Li-N and Cu-N).