Solid-state NMR studies of metal ion and solvent influences upon the flexible metal-organic framework DUT-8

Within the present contribution, we describe solid-state NMR spectroscopic studies of the paddle wheel unit in the prototypic flexible MOF compound DUT-8(M) (M = Ni, Co, Zn). The ¹³C NMR chemical shift of these carboxylates shows a remarkable behavior. The pure 2,6-H₂ndc linker carboxylates as well as DUT-8(Zn) exhibit a¹³C chemical shift of only about 170 ppm. In contrast, much higher values are observed for DUT-8(Ni) and especially DUT-8(Co). In the open pore state, the shift strongly depends on the solvent polarity in these two latter cases. The present contribution elucidates the reason for this solvent influence. It is concluded that the solvent mainly modifies the isotropic Fermi contact coupling constant for the excited high-spin states in DUT-8(Ni) and DUT-8(Co).