Reversible photoswitching between nitrito-N and nitrito-O isomers in trans-[Ru(py)4(NO2)2]

Abstract

Nitro-nitrito photoisomerisation is investigated in solid samples and solutions of trans-[Ru(py)₄(NO₂)₂]. Using light of wavelength 325 nm 50% of the N-bound Ru–NO₂ ligands can be switched to the O-bound Ru-ONO configuration (nitrito-N to nitrito-O isomerisation) at temperatures below T = 250 K in solids. The population of the isomeric configurations is determined with infrared spectroscopy from the decrease of the area of the ν(NO) stretching and δ(NO) deformation modes. In a frozen methanol–ethanol solution nearly 100% can be converted to the nitrito-O configuration. Upon heating above T = 250 K the Ru–NO₂ configuration is restored. The nitrito-O Ru−ONO configuration can be partially transferred back to the nitrito-N configuration by irradiation with light in the spectral range 405–442 nm. Using absorption spectroscopy on a frozen methanol–ethanol solution, two new bands at 447 and 380 nm are observed in the nitrito-O configuration compared to one at 334 nm of the nitrito-N ground state configuration. The photoconversion is initiated by the metal-to ligand charge transfer transition Ru(d) → π*(NO₂,py) as shown by the calculated partial density of states using Density Functional Theory. The calculations yield also the structure of the nitrito-N and nitrito-O isomer as well as the corresponding vibrational densities. The experimental structure of the ground state is determined using powder diffraction.