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 <b>Ni-4d</b> and the related oxime <b>Ni-4d'</b>. 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 <i>endo</i>-nitrito and <i>exo</i>-nitrito linkage isomers using 530 nm light-emitting diode (LED) light at 100 K. Over time, the <i>endo</i>-nitrito form takes over, and almost 100% conversion can be achieved, as confirmed by photocrystallographic and infrared (IR) spectroscopic experiments. The stability of this isomer is similar for both systems, which is confirmed by the experimentally determined decay temperature (<i>T</i> _d) values. Furthermore, when 660 nm LED light is applied at 90 K, the <i>exo</i>-nitrito form can be generated up to about 25% population with no admixture of the <i>endo</i>-nitrito isomer for <b>Ni-4d</b>. Based on the kinetic parameters, the <i>exo</i>-nitrito isomer is slightly less stable for <b>Ni-4d'</b>. In this work, we report for the first time the experimental evidence of the earlier theoretically predicted nitro-to-<i>endo</i>-nitrito mechanism via the <i>exo</i>-nitrito form for square-planar nickel-(II) nitrite coordination compounds in the solid state.