Syntheses, characterization, and luminescent properties of four alkaline earth metal–organic frameworks based on two similar N-containing carboxylates

Four coordination polymers {[Ca(3-pa)2(H2O)2]·3H2O} (1), {[Sr(3-pa)2(H2O)2]·3H2O} (2), {[Ca(4-pa)2(H2O)4]} (3) and {[Sr(4-pa)(H2O)6]·[4-pa]} (4), (3-pa = pyridine-3-carboxylate, 4-pa = pyridine-4-carboxylate) have been synthesized and structurally characterized. Lots of noncovalent interactions were found in the four compounds, aiding the formation of high-dimensional architectures from the low-dimensional structures. Complex 1 and 2 feature similar 1-D chain structures, which adopt face-to-face orientation to form an interesting 2-D network with pipe-like motif. Chain-like water clusters consisting of infinite tetragonal unit are trapped and stabled in the vacuums of the pipelines. The 2-D sheets are further connected into 3-D architecture by noncovalent interactions. Complex 3 has a discrete mononuclear structure, which is further connected into a 3-D framework by hydrogen bonds and π···π interactions. In complex 4, the zigzag-like chain structure units are parallel to each other and pack in ABAB mode to give rise to a 2-D network, which is stabled by π···π stacking and hydrogen bonds. The counteranions in 4 are hosted between the nets and act as “glue” linking the 2-D networks into a 3-D architecture. In addition, the photoluminescence properties of the compounds have been investigated.

Graphical abstractTwo alkaline earth metal and two similar N-containing carboxylates compared with each other to investigate the effect of metal ions and organic ligands on the structure. The result shows the different metal ions can result in same or different frameworks.Figure optionsDownload full-size imageDownload as PowerPoint slide