Comparative Analysis\nof Photoswitching Properties\nof Analogous Nickel(II) and Palladium(II) Nitrite Complexes in the\nSolid State

Two analogous photoswitchable square-planar nitrite coordination\ncompounds of Ni<sup>II</sup> and Pd<sup>II</sup> with the (<i>N</i>,<i>N</i>,<i>O</i>)-donor (1-phenyl-3-(2'-picolylimino)prop-2-en-1-one)\n ancillary ligand, <b>Ni-1a</b> and <b>Pd-1a</b>, are\nreported. The samples were thoroughly characterized (photo)crystallographically,\nspectroscopically, and computationally. <b>Ni-1a</b> crystallizes\nin the <i>P</i>1̅ space group with one molecule comprising\nthe asymmetric unit, whereas <b>Pd-1a</b> tends to form polymorphic\n(<b>Pd-1a</b> and <b>Pd-1a′</b>) and solvatomorphic\n(with dichloromethane: <b>Pd-1a-DCM</b> and with chloroform: <b>Pd-1a-CHCl</b><sub><b>3</b></sub>) crystal structures. In\nthe <b>Ni-1a</b> crystal structure, the nitro binding mode is\ndominant; however, some traces of the <i>end</i>o-nitrito\nisomer are detected, whereas for the palladium derivative, purely\nthe nitro form is observed for all the crystal forms in the 100–290\nK temperature range. <b>Ni-1a</b>, <b>Pd-1a</b>, and <b>Pd-1a-DCM</b> are efficient photoswitches working between 100\nand 200 K, with conversions as high as 80–100% upon 470–530\nnm light-emitting diode irradiation. At 200 K and above, the decrease\nin population of the photogenerated product is observed. Under continuous\nlight irradiation, some notable population of the <i>endo</i>-nitrito isomer can be generated and crystallograhically detected\nalso at higher temperatures (up to around 240 K). In the case of <b>Ni-1a</b>, the <i>endo</i>-nitrito form is generated\nfaster, and its decay is slower at 200 K and at higher temperatures\nthan for <b>Pd-1a</b>. It should also be noted that for <b>Pd-1a</b>, only one symmetry-independent molecule is photoswitchable.\nIn turn, the <b>Pd-1a′</b> and <b>Pd-1a-CHCl</b><sub><b>3</b></sub> systems appeared nonphotoactive. Such behavior\ncan be explained by the energetic factors, nitrite group orientation,\nand the shape of the reaction cavity. Interestingly, a strong linear\ncorrelation between the M···M distance in the crystal\nstructures of the examined photoswitches and the population of the <i>endo</i>-nitrito form is noted.