Synthesis, crystal structure, spectroscopic, and photoreactive properties of a ruthenium(II)-mononitrosyl complex

The sizeable differences observed in the switching capabilities of two closely related ruthenium-nitrosyl complexes are investigated computationally by DFT, and are found to arise from the very different UV–visible spectra in the ruthenium-isonitrosyl metastable state. [Display omitted] A compound of formula [RuIICl(NO)(Cl-py)4](PF6)2, in which Cl-py is the 4-chloropyridine, has been synthesized in four steps and fully characterized. It crystallizes in the P1¯ triclinic space group as [RuIICl(NO)(Cl-py)4](PF6)2·1.25H2O. Upon irradiation at λ=473nm in the solid state, the N-bounded nitrosyl ligand (ground state GS: [RuII(NO)]) turns into O-bounded nitrosyl metastable state 1 (MS1: [RuII(ON)]). The population of the long-lived metastable RuII(ON) isomer is equal to 27% on powder samples, therefore 3 times less than that of the parent [RuIICl(NO)(py)4](PF6)2 derivative. Spectroscopy and TD-DFT studies are proposed to find a rational for this difference at the molecular level, which is tentatively related to different UV–visible spectra in the metastable RuII(ON) isomer. Surprisingly, and while the switching efficiency of [RuIICl(NO)(Cl-py)4](PF6)2 appears relatively modest, its capability for releasing the biologically active nitric oxide (NO) radical under irradiation in solution is find to be about 100 times that of the [RuIICl(NO)(py)4](PF6)2 derivative.