Photo- and Thermoswitchable Half-Sandwich Nickel(II) Complex: [Ni(η5-C5H5)(IMes)(η1-NO2)]

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(eta(5)-Cp)(IMes)(NO3)] (Cp = cyclopentadienyl, C5H5; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), and crystallizes with two symmetry-independent molecules in the asymmetric unit. When irradiated with 470 nm light-emitting diode light, the complex molecules undergo a photoisomerization reaction in the solid state. Formation of the respective NO2 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 eta(1)-nitro (Ni-NO2) ligand to its exo-nitrito (Ni-ONO) form was observed. At similar to 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.