Experimental evidence for the solid-state nitrite-ligand photoisomerization mechanism in nickel(II) square planar complexes

Two square-planar nitrite nickel(II) photoswitches were designed and synthesized, referred to as Ni-4d and the related oxime Ni-4d’. In the ground-state single crystals of both compounds, the nitro form (Ni−N(O)2) is the dominant one. For the two systems it was possible to generate and detect both endo-nitrito and exo-nitrito linkage isomers using 530 nm LED light at 100 K. Over time the endo-nitrito form takes over and almost 100% conversion can be achieved, as confirmed by photocrystallographic and IR spectroscopic experiments. The stability of this isomer is similar for both systems, which is confirmed by the experimentally determined decay temperature (𝑇d) values. Furthermore, when 660 nm LED light is applied at 90 K the exo-nitrito form can be generated up to about 25% population with no admixture of the endo-nitrito isomer for Ni-4d. Based on the kinetic parameters the exo-nitrito isomer is slightly less stable for Ni-4d’. In this work we report for the first time the experimental evidence of the earlier theoretically-predicted nitro-to-endo-nitrito mechanism via the exo-nitrito form for square-planar nickel(II) nitrite coordination compounds in the solid state.