Comparative Analysis of Photoswitching Properties of Analogous Nickel(II) and Palladium(II) Nitrite Complexes in the Solid State

Two analogous photoswitchable square-planar nitrite coordination compounds of Ni^II and Pd^II with the (N,N,O)-donor (1-phenyl-3-(2'-picolylimino)prop-2-en-1-one) ancillary ligand, Ni-1a and Pd-1a, are reported. The samples were thoroughly characterized (photo)crystallographically, spectroscopically, and computationally. Ni-1a crystallizes in the P1̅ space group with one molecule comprising the asymmetric unit, whereas Pd-1a tends to form polymorphic (Pd-1a and Pd-1a′) and solvatomorphic (with dichloromethane: Pd-1a-DCM and with chloroform: Pd-1a-CHCl₃) crystal structures. In the Ni-1a crystal structure, the nitro binding mode is dominant; however, some traces of the endo-nitrito isomer are detected, whereas for the palladium derivative, purely the nitro form is observed for all the crystal forms in the 100–290 K temperature range. Ni-1a, Pd-1a, and Pd-1a-DCM are efficient photoswitches working between 100 and 200 K, with conversions as high as 80–100% upon 470–530 nm light-emitting diode irradiation. At 200 K and above, the decrease in population of the photogenerated product is observed. Under continuous light irradiation, some notable population of the endo-nitrito isomer can be generated and crystallograhically detected also at higher temperatures (up to around 240 K). In the case of Ni-1a, the endo-nitrito form is generated faster, and its decay is slower at 200 K and at higher temperatures than for Pd-1a. It should also be noted that for Pd-1a, only one symmetry-independent molecule is photoswitchable. In turn, the Pd-1a′ and Pd-1a-CHCl₃ systems appeared nonphotoactive. Such behavior can be explained by the energetic factors, nitrite group orientation, and the shape of the reaction cavity. Interestingly, a strong linear correlation between the M···M distance in the crystal structures of the examined photoswitches and the population of the endo-nitrito form is noted.