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 light-emitting diode (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 infrared (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.