KCS

Photoreactivity in the MOFs has a great impact on solid state chemistry.^[1] In this work, we identify the significant correlation of solvents and coordination environments. Through solvothermal reaction of Zn(NO₃)₂·6H₂O, 1,4-bis[2-(4’-pyridyl)ethenyl]benzene (bpeb), and 1-adamantanecarboxylic acid (Hadc), two types of Zn(II) based MOFs are prepared under different solvent conditions. A 1D railroad MOF, [Zn₂(bpeb)₂(adc)₄] (1) is synthesized from dimethylacetamide (DMA) which is yellow block-shaped crystals. The dinuclear repeating unit consists of two Zn(II), two bpeb ligands and four adc ligands. Zn atoms are hexagonally six-coordinated by two nitrogen atoms of bpeb ligands and four oxygen atoms of adc ligands. On the other hand, a 1D railroad MOF, [Zn₂(bpeb)₂(adc)₃(fa)] (2) is synthesized from dimethylformamide (DMF) which is colorless yellow block-shaped crystals. In particular, formate ligands which are originate from partial pyrolysis of DMF participate in coordination bonds and provide electron pairs to Zn(II). The dinuclear repeating unit consists of two Zn(II), four bpe ligands and four adc ligands. Zn(II) is six-coordinated by two nitrogen atoms of bpeb ligands and four oxygen atoms of adc ligands. Especially, the double bonds of bpeb ligands in 2 are parallelly aligned to each other at a distance of 3.8 Å. According to Schmidt’s Criteria,^[2] the distance between olefins within the range of 3.6-4.1 Å is photoreactive. Hence, the C=C bonds of bpeb ligands in 2 undergo [2+2] cycloaddition reaction. When UV is irradiated to 2, double bonds are converted into cyclobutane rings. Especially, bpeb ligands have two sites of double bond but only single dimerization is observed which is adjacent to Zn(II). The photo-dimerized MOF [Zn₂(dimer)₂(adc)₃(fa)] (3, dimer = (E)-4,4'-((1R,2S,3R,4S)-3,4-diphenylcyclobutane-1,2-diyl)bis(cinnamylbenzene)) was confirmed by ¹H nuclear magnetic resonance and single crystal X-ray diffraction analyses.

References

[1] G. K. Kole, J. J. Vittal, Solid-state reactivity and structural transformations involving coordination polymers, Chem. Soc. Rev., 2013, 42, 1755-1775.

[2] G. M. J. Schmidt, Photodimerization in the solid state, Pure Appl. Chem., 1971, 27, 647-678.