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Imidazole is a fundamental building unit possessing a conjugated five-membered ring system with two N-donor coordination sites. The parent imidazole nucleus can be derivatized to three types of compounds at the 1-, 2-, or 4-positions, generating 1, 2, or 4-imidazole-containing ligands, or can be endowed with other functional groups to form multi-functional ligands. Imidazole and its derivatives have been widely employed as excellent candidates for targeted metal–organic frameworks (MOFs) in the domain of coordination chemistry. Moreover, recent reports show a boom in exploratory synthesis using imidazole derivatives to construct novel MOFs due to their distinct characteristics. In consideration of the rapidly growing research on imidazole ligands, herein, we would like to highlight the recent advances in the rational design of MOFs based on imidazole derivatives from our and other research groups; we would also like to provide new insights into the rational design of organic tectons and the construction of these advanced crystalline materials with desirable properties and functionalities.

The solvent free reaction of Zn(OAc)2 with an imidazole ligand resulted in a 1-dimensional (1D) chain structure with the formula [Zn(Im)(HIm)2(OAc)] (Im = imidazolate, HIm = imidazole, OAc = carboxylate) (1). The results by kinetic experiments suggest that (1) likely evolves from an unidentified metastable phase. This new 1D chain precursor (1) is a quaternary system comprising Zn, Im, OAc and HIm. We have used this 1D compound as a single precursor to prepare 3-dimensional (3D) Zn(Im)2 frameworks. Specifically, two Zn(Im)2 frameworks with known zni and coi topologies were prepared via thermal-mediated transformation when heated in an open atmosphere and in a closed vessel, respectively. In an open atmosphere, the kinetic experiments confirmed the rapid phase transformation from (1) to zni without formation of an intermediate phase. In contrast, in the formation process of coi in a closed vessel, phase transformation from (1) to zni and immediately from zni to coi was observed. Thus, in a closed vessel, the released molecules, HOAc and HIm, were occluded in the framework, which may together exert a cooperative structure-directing effect on the formation of the coi framework. Thus, a single precursor approach was established for the synthesis of ZIFs. The present work represents the first example of transforming a 1D coordination polymer into 3D ZIF framework structures.

A series of four- and six-connected entangled structures, namely [Cd(oba)(bbi)]·0.5H2O (1), [Ni(oba)(bbi)]2·H2O (2), [Co(bpea)(bbi)]·H2O (3), [Co2(bpea)2(bbi)2]·1.5H2O (4), [Cd(oba)(1,4-bix)] (5), and [Ni2(oba)2(1,4-bix)(H2O)2] (6), where oba = 4,4′-oxybis(benzoate), bpea = biphenylethene-4,4′-dicarboxylate, 1,4-bix = 1,4-bis(imidazol-1-ylmethyl)benzene, and bbi = 1,1′-(1,4-butanediyl)bis(imidazole), have been successfully synthesized through a hydrothermal process. The structures of 1 and 2 both consist of two interpenetrating CdSO4 nets. In 1, the two nets are related by symmetry. By comparison, the individual nets are, unusually, crystallographically distinct in 2. The structures of 3 and 4 both contain interpenetrating diamond networks. In 3, there are 6 interpenetrating nets, and they show the usual mode of interpenetration. For 4, however, there are 7 nets, and a rare ‘abnormal’ [4 + 3] interpenetration mode is observed. In 5, the oba and 1,4-bix ligands linked the Cd(II) atoms into a deeply corrugated 2D sheet. The corrugated 2D sheets polycatenate each other in a parallel manner with DOC (degree of catenation) = 2, yielding a rare 2D → 3D parallel polycatenation net. In 6, the dinuclear Ni(II) units are bridged by the oba ligands to generate a 2D (4,4) square-grid with a rhombic window. It is interesting that the two identical (4,4) square-grids show 2D → 2D parallel interpenetration but are then further crosslinked by the 1,4-bix ligands to yield an unusual 3D self-penetrating net with an unique 6-connected 44.611 topology. The importance of bbi conformations and metal sources in the framework formation of complexes 1–6 was unraveled. Their infrared spectra, powder X-ray diffraction (XRD) and photoluminescent properties were also investigated in detail.

Two types of bis(imidazole) ligands – 1,4-bis(imidazol-1-ylmethyl)benzene (bix) and 1,4-bis(imidazolyl)-2-butyne (bib) – have been used to bridge octahedral metal centres and form simple cubic networks. Crystal structures of Cd(bix)3(ClO4)2, Cd(bix)3(NO3)2, Zn(bib)3(BF4)2 and Co(bib)3(NO3)2 reveal that all possess the α-polonium topology and are triply interpenetrating.

Seven new coordination polymers, namely, [Co2(mfda)2(bip)(H2O)2]·2H2O (1), [Zn2(mfda)2(bip)]·H2O (2), [Co2(mfda)2(bib)(H2O)] (3), [Zn2(mfda)2(bib)2]·(DMF)3·H2O (4), [Zn2(mfda)2(bib)] (5), [Zn2(mfda)2(bite)] (6) and [Ni4(mfda)4(bite)3(μ2-H2O)2]·4H2O (7) (H2mfda = 9,9-dimethylfluorene-2,7-dicarboxylic acid, bip = 1,3-bis(imidazole)propane, bib = 1,4-bis(imidazole)butane and bite =1,4-bis(imidazole)butene), have been synthesized and structurally characterized. Compounds 1 and 3 feature a two-fold interpenetrating framework of pcu topology. Compound 2 exhibits a three-fold interpenetrating framework of pcu topology. Compound 4 displays a four-fold interpenetrating framework of dia topology. Compounds 5 and 6 exhibit a uninodal six-connected network with unusual acs topology. Compound 7 shows a uninodal eight-connected framework with CsCl topology. The luminescence properties of compounds 2, 4, 5 and 6 were also measured.

Seven new metal–organic frameworks, namely, [Zn5(μ3-OH)2(bdc)2(bimf)2]·4H2O (1), [Zn(bdc)(bimf)]·2H2O (2), [Cd(bdc)(bimf)(H2O)]·2H2O (3), [Cd4(cdc)4(bimf)2(H2O)2]·2H2O (4), [Cd(cdc)(bimf)] (5), [Zn2(Hbtc)2(bimf)2]·H2O (6) and [Cd3(btc)2(bimf)3(H2O)]·2H2O (7) have been synthesized and structurally characterized (H2bdc = 1,4-benzenedicarboxylic acid, H2cdc = 1,4-cyclohexanedicarboxylic acid, H3btc = 1,3,5-benzenetricarboxylic acid and bimf = 9,9-dimethyl-2,7-bisimidazolylfluorene). Compound 1 displays a uninodal 12-connected fcu-framework based on pentanuclear zinc clusters. Compound 2 exhibits a 4-fold interpenetrating framework of SrAl2-topology. Compound 3 presents a 2-fold interpenetration of (4,4) net. Compound 4 shows a uninodal 8-connected framework of hex-topology. Compound 5 features a binodal framework of PtS-topology. Complex 6 features a binodal framework with an unusual (66)·(64·82) topology. Complex 7 defines a pentanodal framework containing 3, 4 and 8-connecting nodes. Their photochemical properties were measured.

Two Cd(II)-containing coordination polymers based on a new tri(4-imidazolylphenyl)amine (Tipa) ligand and carboxylates, namely, [Cd(Tipa)(L1)]·2H2O (1) and [Cd3(Tipa)2(L2)2Cl2]·4H2O (2), where H2L1 = 5-OH-1,3-benzenedicarboxylic acid and H2L2 = 1,4-benzendicarboxylic acid, have been synthesized by varying the carboxylates under hydrothermal conditions. In 1, adjacent Cd(II) centers are linked by L1 anions to generate Cd-carboxylate chains, which are further extended by Tipa ligands to form a puckered double sheet. The most peculiar feature of 1 is the entangled fashion between the [Cd2(Tipa)2] loop and the L1 ligand rod. Every [Cd2(Tipa)2] loop of each layer is threaded by one L1 ligand rod of an adjacent layer. In this entangled mode, the 2D → 3D polyrotaxane framework was obtained, which represents an unusual 2D → 3D metal–organic polyrotaxane framework entangled in a highly rare parallel fashion. In 2, two identical 3D self-penetrating frameworks interpenetrate each other in a 2-fold mode. If we regard each Cd2(Tipa)2 window as a loop and each L2 ligand as a single rod, undoubtedly, the structure of 2 also can be described as a 3D metal–organic polyrotaxane framework. As far as we know, compound 2 is the first example of a 2-fold interpenetrating 3D framework with polyrotaxane features. The luminescent properties for the two compounds were also investigated.

Four new coordination polymers, constructed from 4,4′-(hexafluoroisopropylidene)diphthalic acid (H4L) and 1,1′-(1,4-butanediyl)bis(imidazole) (biim-4) ligand, have been synthesized: [Cd(HL)(Hbiim-4)(biim-4)0.5(H2O)]·2H2O (1), [Cd2(L)(biim-4)0.5(H2O)2]·0.5H2O (2), [Mn2(L)(biim-4)0.5(H2O)2]·0.5H2O (3), and [Zn2(L)(biim-4)1.5]·(CH3CH2OH) (4), where H4L = 4,4′-(hexafluoroisopropylidene)diphthalic acid, and biim-4 = 1,1′-(1,4-butanediyl)bis(imidazole). Compound 1 displays a 1D chain structure, which is extended by intermolecular hydrogen-bonding interactions to give a novel trinodal (4,5)-connected (4·64·8)(42·63·8)(43·64·83) topology. Compounds 2 and 3 are nearly isostructural with the same uncommon (5,6)-connected frameworks. Compound 4 shows an unprecedented self-penetrating (4,5)-connected net with Schlafli symbol of (42·6·72·8)(62·72·82)(42·63·73·82). The structural and topological differences of these four compounds indicate that the pH values and the central metals have significant effects on producing novel structures and topologies of the coordination complexes. The thermogravimetric and photoluminescent properties were also investigated for the compounds.

Five novel metal–organic frameworks (MOFs) have been hydrothermally synthesized, namely, [Co(bimb)(2,4′-bpdc)(H2O)2]·H2O (1), [Co2(bimb)3(2,4′-bpdc)2(H2O)2]·H2O (2), [Co(bix)(2,4′-bpdc)] (3), [Co(bix)0.5(2,4′-bpdc)(H2O)0.25] (4) and [Co(bimh)(2,4′-bpdc)] (5), based on mixed ligands, 2,4′-biphenyldicarboxylic acid (2,4′-H2bpdc) as organic linkers and different bis(imidazole) ligands as co-ligands: (1,4-bis(imidazol-1-yl)-butane (bimb), 1,4-bis(imidazol-1-yl-methylene)-benzene (bix) and 1,6-bis(imidazol-1-yl)-hexane (bimh)). The coordination polymers were characterized by single-crystal X-ray diffraction, thermogravimetric analysis (TGA) and element analysis (EA). Reactions between CoCl2·6H2O, 2,4′-H2bpdc and neutral bimb ligand afforded two crystallographically different coordination polymers (1 and 2) through tuning the ligand/metal ratio meticulously. Compound 1 crystallizes in a noncentrosymmetric Pna21 space group with 65·8 CdSO4-type topology, while compound 2 is isolated with P21/c space group and displays a 4-connected 66 network. By substituting bix for bimb ligand, alteration of the metal/ligand ratio in the initial reaction system induced a transition from 2D layer (compound 3) to 3D architecture (compound 4). Notably, the structure of 4 is constructed from [Co(2,4′-bpdc)] layers, which are further pillared by bix ligands into a 3D 3,4-connected (63)·(63·103) network. In 5, the left- and right-handed helical chains constructed by Co(II) and bimh ligands are bridged by 2,4′-bpdc2− anions to form a layer-like structure.