A new, cheap, easy-to-synthesize and air-stable photoswitchable nickel(II) complex, QTNiNO2, is reported. The metal centre in QTNiNO2 is coordinated by a nitro group and a [2-methyl-8-aminoquinoline]-1-tetralone ligand. The compound crystallizes in the tetragonal space group I41/a with one complex molecule comprising the asymmetric unit, and the crystals are stable under ambient conditions. Irradiation of the solid-state form of QTNiNO2 with 530–660 nm LED light at 160 K converts the ambidentate nitro moiety fully to the nitrito linkage isomer which is stable up to around 230 K, as indicated by IR spectroscopy measurements. The structures of all species present in the examined crystals and their thermal stability were confirmed via X-ray multi-temperature and photocrystallographic experiments. The impact of temperature on the (photo)isomerization reaction taking place in a single crystal was additionally investigated. The experimental results are supported by computational analyses of crystal packing and intermolecular interactions that influence the isomerization process studied.

A new photoswitchable half-sandwich nitro Ni II complex with the N-heterocyclic carbene ligand was synthesized and successfully crystallized. The compound constitutes an analogue of a known nitrate nickel(II) compound, [Ni(η 5 -Cp)(IMes)(NO 3 )] (Cp = cyclopentadienyl, C 5 H 5 ; IMes = 1,3-bis(2,4,6trimethylphenyl)imidazol-2-ylidene), and crystallizes with two symmetry-independent molecules in the asymmetric unit. When irradiated with 470 nm lightemitting diode light, the complex molecules undergo a photoisomerization reaction in the solid state. Formation of the respective NO 2 group linkage isomers was studied photocrystallographically at different temperatures. After irradiation of the single-crystal sample for ca. 3 h at 100 K approximately 20% conversion of the η 1nitro (Ni−NO 2 ) ligand to its exo-nitrito (Ni−ONO) form was observed. At ∼130 K the exo-nitrito binding mode transforms further to the more stable endo-nitrito conformation, whereas at temperatures higher than 175 K the crystal converts back to the ground state. It appears that the photoisomerization reaction can be to some extent triggered, or additionally stimulated, thermally. Consequently, the highest conversion level (ca. 90%) was achieved after sample irradiation at 145 K. Despite similar molecular energies and intermolecular interactions, the two symmetry-independent molecules behave somewhat differently upon irradiation and temperature changes. The photocrystallographic findings and molecular aspects were supplemented by theoretical computations, including the quantum-mechanics/molecular-mechanics approach.

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.

The first transition-metal nitro complex that exhibits optically reversible room-temperature photoswitchability with notable nitro-to-nitrito conversion of ca. 55% is reported.

Photo-induced NO rotation from N-bound (blue) to O-bound (red) with blue light at 100 K in the compound [Ru(NO)2(PCy3)2Cl]BF4.