Exploring Photoswitchable Properties of Two Nitro Nickel(II) Complexes with (N,N,O)-Donor Ligands and Their Copper(II) Analogues

Abstract

Two photoswitchable nickel­(II) nitro coordination compounds and their copper­(II) analogues are reported. In all these systems, the metal center is chelated by (N,N,O)-donor ligands containing either 2-picolylamine or 8-aminoquinoline fragments. The studied compounds were thoroughly investigated using crystallographic and spectroscopic techniques supplemented by computational analysis. They are easy to synthesize and stable, and all compounds undergo the nitro group isomerization reaction. Nevertheless, there are significant differences between the copper and nickel systems regarding their structural and switchable properties. According to the solid-state IR spectroscopy results, 400–660 nm light irradiation of the ground-state (η2-O,O′)-κ-nitrito copper­(II) complexes at 10 K induces a rather moderate conversion to a metastable linkage isomer, which is visible only up to approximately 60–80 K. In turn, upon visible light irradiation (ca. 530 nm excitation wavelength), the ground-state nitro isomers of the examined nickel­(II) complexes transform into the endo-nitrito forms. It was possible to achieve about 35% conversion for both nickel­(II) systems and to determine the resulting crystal structures at 160 K in the case of single crystals after 30–45 min of exposure to LED light (crystals decayed with longer irradiation), and roughly 95% conversion was achieved for thin-film samples as indicated by the IR spectroscopy results. Traces of the endo-nitrito linkage isomers remained up to 200–220 K, and the isomerization reaction was proven to be fully reversible.

An optically reversible room-temperature solid-state cobalt(iii) photoswitch based on nitro-to-nitrito linkage isomerism

“…[19][20][21] Furthermore, Mochida's group has recently reported new efficient platinum(II) nitro coordination compounds 22,23 and we have presented some nickel(II)-based systems of this kind. [24][25][26] Nevertheless, up to now PLIs have been usually noted and/or generated in crystal structures at relatively low temperatures (below 240 K). There are only a few exceptions, mostly platinum-group compounds, such as the 100%photoswitching Pd II complex with PLIs detected in the solid state up to 260 K under continuous illumination, 20 or the Pt II nitro complex 21 for which PLI was observed at 300 K, though with a rather moderate conversion level (30%).…”

“…It reaches almost 30 Å 3 , which is comparable to the literature-reported values for transition-metal complexes undergoing similar transformations. 7,19,[24][25][26]35 Furthermore, it appears that this moiety is less involved in the intermolecular interactions than the other two (ESI ‡). The O5a-N3a-O6a group is engaged in only one notable weak interaction, i.e.…”

Photogeneration of Several Linkage Isomers and Investigation of Forward and Backward Nitro-Nitrito Isomerization Processes in a Palladium Complex

“…The directionality of the photoisomerization depends then not only on water presence/absence but on the temperature at which light excitation occurs setting up the photochemical equilibrium between all isomers and allowing the formation of the less stable exo-isomers PLI3, 4. The reaction route also might be tuned by the closest environment of the photoswitchable ligand, which is variable in function of temperature 3,8,17,39 and product content; 40−43 however, in the current case no significant differences are observed between GS structures measured at 10 and 150 K (Tables 1, S2).…”

“…For example, studies by Sauvage, Stoddart, and Feringa on molecular machines were honored by a Nobel Prize in 2016. , There exist various families of switchable ligands in coordination compounds, among which the nitro-ligand NO 2 – can undergo photoinduced isomerization M-NO 2 → M-ONO (M = metal) . Moreover, depending on the nature of the complex and the experimental conditions, the nitrito isomer ONO can be stabilized in several configurations, such as endo -ONO, exo-ONO, or κ 2 -O,O-(ONO). − …”

“…The light excitation leads to a decrease of the intensity of the ν(NO 2 ) bands at 1441 (ν as ), 1424 sh (ν as ), 1338 (ν s ), 1309 (ν s ), and 1255 (ν s ) cm −1 and to the appearance of new bands at 1467 (ν as ), 1097 (ν s ), and 1053 (ν s ) cm −1 characteristic of the ν(ONO) modes. 8,12,17,26,46,47 Importantly, the intensities of the bands at 1053 and 1097 cm −1 are different and can be used for the estimation of the relative population of PLIs. Based on the populations of PLIs determined by XRD, the bands are assigned to the PLI1 and PLI2, respectively.…”