Abstract
We prepared a new C2h-symmetric bridging ligand, 3,3′-(pyrazine-2,5-diyl)dibenzoic acid (3,3′-PDBA), through a Suzuki coupling reaction. 3,3′-PDBA contains a central pyrazine ring instead of the phenyl ring of 3,3′-terphenyldicarboxylic acid (3,3′-TPDC). Despite the geometrical similarity of the two bridging ligands, the reaction between Zn(NO3)2·6H2O and 3,3′-PDBA in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) yielded a structurally different Zn-based metal-organic framework (Zn-MOF). The Zn-MOF, [Zn2(3,3′-PDBA)2(DABCO)1.5]·2DMF·H2O, had two-dimensional (2D) layers, and the interlocked 2D layers formed a porous 3D framework. Interestingly, one of the two available N atoms of DABCO remained intact. The uncoordinated N atoms of the dangling DABCO ligand and the pyrazinyl N atoms of the 3,3′-PDBA bridging ligand were fully exposed toward the 1D channels. Therefore, the 1D channels represented a highly nitrogen-rich environment. Gas sorption analysis indicated that the Zn-MOF was selective for adsorption of CO2 at 196 K over N2 (77 K) and H2 (77 K). The exceptionally high zero surface coverage heat of CO2 adsorption (Qst = 79.5 kJ mol−1) was attributable to the openly accessible multiple Lewis basic sites in the nitrogen-rich 1D channels. Zn-MOF also showed good Lewis base catalytic activities in three model aldol-type reactions with good recyclability due to chemically accessible 3° amine sites.
Highlights
The incorporation of chemically accessible Lewis basic sites into the channels of metal-organic frameworks (MOFs) is still challenging but significantly important to develop new functional crystalline porous materials for diverse applications, such as selective CO2 capture and heterogeneous catalysis[1,2,3,4,5,6]
Solvent-free Zn-based metal-organic framework (Zn-MOF) 1 with openly accessible 3° amine sites is both a good CO2 sorbent with high selectivity over N2 and H2 and a substrate-selective heterogeneous catalytic system for aldol-type C–C bond forming reactions with good recyclability[13,14]. Stimulated from these interesting results, we attempted to modify the structure of the 3,3′-TPDC linker in Zn-MOF 1 to tune the cage-like 1D channels to contain more Lewis basic sites
After careful examination of the channel structure of Zn-MOF 1 (Fig. S1), it can be seen, once the central phenyl ring of the 3,3′-TPDC linker is replaced by a pyrazinyl moiety using a 3,3′-PDBA (3,3′-(pyrazine-2,5-diyl)dibenzoic acid) linker, the 1D channels become more nitrogen-rich
Summary
The incorporation of chemically accessible Lewis basic sites into the channels of metal-organic frameworks (MOFs) is still challenging but significantly important to develop new functional crystalline porous materials for diverse applications, such as selective CO2 capture and heterogeneous catalysis[1,2,3,4,5,6]. Solvent-free Zn-MOF 1 with openly accessible 3° amine sites is both a good CO2 sorbent with high selectivity over N2 and H2 and a substrate-selective heterogeneous catalytic system for aldol-type C–C bond forming reactions with good recyclability[13,14] Stimulated from these interesting results, we attempted to modify the structure of the 3,3′-TPDC linker in Zn-MOF 1 to tune the cage-like 1D channels to contain more Lewis basic sites. After careful examination of the channel structure of Zn-MOF 1 (Fig. S1), it can be seen, once the central phenyl ring of the 3,3′-TPDC linker is replaced by a pyrazinyl moiety using a 3,3′-PDBA (3,3′-(pyrazine-2,5-diyl)dibenzoic acid) linker, the 1D channels become more nitrogen-rich The gas sorption, adsorptive iodine and CS2 encapsulation properties and heterogeneous catalytic activities of newly prepared Zn-MOF 2 are investigated
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