Construction of olefinic coordination polymer single crystal platforms: precise organic synthesis, in situ exploration of reaction mechanisms and beyond

Olefin [2+2] photocycloaddition reactions based on coordination-bond templates provide numerous advantages for the selective synthesis of cyclobutane compounds. This review outlines the recent advances in the design and construction of single crystal platforms of olefinic coordination polymers for precise organic synthesis, in situ exploration of reaction mechanisms, and possible developments as comprehensively as possible. Numerous examples are presented to illustrate how the arrangements of the olefin pairs influence the solid-state photoreactivity and examine the types of cyclobutane products. Furthermore, the photocycloaddition reaction mechanisms are investigated by combining advanced techniques such as single crystal X-ray diffraction, powder X-ray diffraction, nuclear magnetic resonance, infrared spectroscopy, fluorescence spectroscopy, laser scanning confocal microscopy and theoretical calculations. Finally, potential applications resulting from promising physicochemical properties before and after photoreactions are discussed, and existing challenges and possible solutions are also proposed.

Regulation of Crystal Structures and Solid-State Photoreactivity of Diolefin Coordination Polymers by Carboxylate Ligands

Olefinic coordination polymers (CPs) have recently drawn more attention, owing to the many possibilities in conformational conversions and photochemical reactivity that olefin molecules offer. In the presence of different carboxylic acids, we utilize one diolefin ligand 4,4'-((1E,1'E)-(2,5-dimethoxyl-1,4-phenylene)bis(ethene-2,1-diyl))dipyridine (OCH3-bpeb) and Cd(II) to assemble six different crystalline CPs (1-6). By fine-tuning the substituent size, carboxyl group number, and geometrical configuration of carboxylate ligands, these diolefin CPs show quite different crystal architecture models, from one-dimensional intersecting stacking to one-dimensional parallel stacking to three-dimensional interpenetrated structure. Of these, four kinds of CPs (1, 2, 5, and 6) are demonstrated to be photoreactive for [2 + 2] cycloaddition reactions, as confirmed by proton nuclear magnetic resonance and single-crystal X-ray diffraction. Both 2 and 5 can be dimerized into different cyclobutane products in a single-crystal-to-single-crystal manner under visible light, and remarkably, the photocycloaddition reaction of 5 involves a rare phase transition with structural symmetry enhancement from P1̅ to P2/n. This work demonstrates the power of carboxylate ligands in tuning single crystal structures and photocycloaddition reactions of CPs, which provides important references for the further exploration of other physicochemical properties of functionalized olefin-containing complexes.

Zinc(II)-Based Ring-and-Rod Coordination Layer as an Excitation-Wavelength-dependent Dual-Emissive Chemosensor for Discriminating Fe3+, Cr3+, and Al3+ in Water

The reactions of Zn(NO3)2, 3,6-bis(pyridin-3-yl)-9H-carbazole (bpycz), and 2,5-dihydroxyterephthalic acid (H4dhbdc) or 2-bromoterephthalic acid (Br-1,4-H2bdc) under hydro(solvo)thermal conditions yielded corresponding coordination polymers (CPs) {[Zn(H2dhbdc)(bpycz)]•0.5H2O}n (1) and [Zn(Br-1,4-bdc)(bpycz)]•2DMAc•H2O (2), respectively, with high thermal stability approaching 350 °C. CP 1 adopts a ring-and-rod layer structure, which is topologically described as a 4-connected net with the point symbol of 2•65. Two layers are interpenetrated in parallel interlocking mode to form a double 2D → 2D polyrotaxane entanglement with extra-framework void space of 19.6%. CP 2 has a non-interpenetrating ring-and-rod layer structure of 4-connected 2•65 net topology, with extra-framework void space of 16.6%. Thermally activated 1 and 2 revealed CO2 uptakes of 101.1 and 98.6 cm3 g-1, respectively, at P/P0 = 1 and 195 K. X-ray powder diffraction (XRPD) patterns confirmed that 1 and 2 both possessed high chemical stability in H2O, CH3OH, acetone, and DMF, and framework stability during gas adsorption-desorption. The H2O suspension of 1 displayed excitation-dependent dual-emissive properties, appearing at 432 nm upon excitation at 300 nm and at 528 nm upon excitation at 365 nm. Of note, 1 was capable of detection of Fe3+, Cr3+, and Al3+ ions in H2O, showing good anti-interference ability, excellent selectivity, and high sensitivity. More interesting, the dual-emissive properties make 1 to be an excellent luminescence chemosensor to screen Fe3+, Cr3+, and Al3+ from a pool of metal ions in H2O upon excitation at 300 nm via luminescence quenching effect and then discriminate Fe3+, Cr3+, and Al3+ upon excitation at 365 nm via luminescence quenching, unaltered, and enhancement responses, respectively. On the other hand, the H2O suspension of 2 demonstrated an excitation-independent emission appearing at around 430 nm, which could be utilized to sensitively detect Fe3+ and Cr3+ ions with good anti-interference ability and excellent selectivity via luminescence quenching effect. Further, 1 and 2 were recyclability and possessed cycling stability. The plausible sensing mechanisms for 1 and 2 toward Fe3+, Cr3+, and Al3+ were also explored in detail.

New structurally diverse photoactive cadmium coordination polymers

Four structurally diverse coordination polymers 1-4 (CP1-CP4) were designed and constructed from Cd(II) ions and various carboxyl ligands (H₂oba, 4,4'-oxydibenzoic acid; H₂bpa, (E)-4,4'-(ethene-1,2-diyl)dibenzoic acid; H₂pbda, 4,4'-((1,3-phenylenebis(methylene))bis(oxy))dibenzoic acid) and the alkene containing ligand (CH₃-bpeb, 4,4'-((1E,1'E)-(2,5-dimethyl-1,4-phenylene)bis(ethene-2,1-diyl))dipyridine). CP1-CP4 possess Cd₂ binuclear secondary building units (SBUs). The geometry of the dicarboxylate ligands and the reaction conditions determined the final structure with a variety of motifs. CP1 possesses an interdigitated 2D structure, while CP2 consists of a 1D channel-like motif with isolated CH₃-bpeb molecules embedded in the channels. The solid-state structure of CP3 consists of two unique layers interpenetrated to form a 2D + 2D → 2D polycatenated backbone, while a 1D channel-like motif filled by isolated CH₃-bpeb molecules was observed for CP4. In all four coordination polymers pairs of CH₃-bpeb molecules were bound or encapsulated by the Cd₂ secondary building units at an appropriate distance and orientation for solid-state [2 + 2] photodimerization of one pair of CC bonds. Desolvation of CP3 with heat resulted in a decrease in solid-state fluorescence and a slowing of the rate of solid-state photodimerization.

A single-crystal to single-crystal transition from a 7-fold interpenetrated coordination polymer to a non-interpenetrated one by photochemical [2 + 2] polymerization and their sensing properties

Two complexes, namely [Zn(bpeb)(sda)] (1) and [Zn(poly-bpeb)(sda)] (2), were synthesized by an organic ligand with an extensively conjugated system, bpeb = 1,4-bis[2-(3-pyridyl) vinyl]-benzene, H₂sda = sulfonyldibenzoic acid and d¹⁰ metal centers Zn²⁺. Structural analysis revealed that compound 1 was nonporous and possessed 7-fold interpenetrated three-dimensional (3D) frameworks constructed from one-dimensional (1D) Zn-bpeb and Zn-sda chains. Interestingly, due to the short distance between the vinyl groups from two neighboring bpeb ligands, compound 1 could undergo a photochemical [2 + 2] polymerization reaction to generate 2 in a single-crystal to single-crystal (SCSC) manner under the irritation of UV. Moreover, the organic polymer in 2 could be depolymerized by heating to realize the reversible transformation from 2 to 1. Furthermore, both compounds 1 and 2 could be used as fluorescent sensors for 2,4,6-trinitrophenol (TNP) with high selectivity and sensitivity.

Modulating the regioselectivity of solid-state photodimerization in coordination polymer crystals

Coordination polymers [Cd(1,4-bpeb)(L1)] (1), [Zn2(1,4-bpeb)2(L2)2(SO42-)2] (2) and [Cd(1,4-bpeb)(L3)] (H2O) (3) (H2L1, 3-[2-(3-hydroxy-phenoxymethyl)-benzyloxy]-benzoic acid; HL2, 1H-Indazole-3-carboxylic acid; H3L3, benzene-1,2,3-tricarboxylic acid; 1,4-bpeb, 1,4-bis[2-(4-pyridyl)vinyl]benzene have been synthesized under solvothermal conditions. Complexes 1-3 underwent photodimerization in the solid-state to give quantitative yields of single isomeric products. The choice of carboxyl ligands L and metal center determined the arrangement of 1,4-bpeb ligands, which in turn directed the regiochemistry of the final photoproducts. The solid-state network structures of cadmium based 1 and 3 had 1,4-bpeb pairs aligned face-to-face with both C[double bond, length as m-dash]C centres in each ligand at an appropriate distance and alignment for photodimerization to give the corresponding para-[2.2]cyclophane (pCP) exclusively. By contrast, compound 2 possessed dinuclear (ZnSO4)2 metallocycles that positioned the 1,4-bpeb "arms" face-to-face, but with C[double bond, length as m-dash]C centres offset at an appropriate distance for only one pair to undergo [2 + 2] cycloaddition to yield a single stereoisomer of the monocyclobutane photo-product bpbpvpcb. This work highlights crystal engineering design principles that can be used to facilitate regio- and stereospecificity in solid-state transformations.

Zn(ii)/Cd(ii) based mixed ligand coordination polymers as fluorosensors for aqueous phase detection of hazardous pollutants

The recent developments and prospects of fluorosensors with a handful of recent examples based on mixed ligand Zn(ii)/Cd(ii) coordination polymers for aqueous phase detection of organic as well as inorganic pollutants have been discussed.

A water-stable zinc(ii)-organic framework as a multiresponsive luminescent sensor for toxic heavy metal cations, oxyanions and organochlorine pesticides in aqueous solution

A novel metal-organic framework with the formula [Zn3(DDB)(DPE)]·H2O (1) (H5DDB = 3,5-di(2',4'-dicarboxylphenyl)benzoic acid and DPE = 1,2-di(4-pyridyl)ethylene) has been solvothermally synthesized by employing a rigid carboxylate ligand H5DDB to assemble with Zn(ii) ions in the presence of a flexible bis(pyridyl) linker DPE. The Zn-MOF is a 3D framework with six-nuclear clusters and possesses remarkable water stability and pH stability. Interestingly, complex 1 can sensitively and selectively sense Fe(iii), Cr(iii), Cr(vi), Mn(vii) and the pesticide 2,6-Dich-4-NA with low detection limits in aqueous solution. Moreover, complex 1 also exhibits selectivity for 2,6-Dich-4-NA detection in real samples including carrot, grape and nectarine extracts, and its detection ability is almost unchanged in the presence of the surfactant sodium dodecyl sulfate (SDS). The possible mechanisms of luminescence quenching have been explained by the weak affinity of nitrogen atoms, resonance energy transfer, and photoinduced electron transfer. To our knowledge, this is the first example of a MOF-based multiresponsive fluorescent probe for the simultaneous detection of Fe(iii), Cr(iii/vi), Mn(vii) and the pesticide 2,6-Dich-4-NA in aqueous solution.