High-field EPR study and crystal and molecular structure of trans-RSSR-[CrCl2(cyclam)]nX (X = ZnCl42−, Cl− and Cl−·4H2O·0.5HCl)

Abstract

For the first time, HF-EPR (94.5 GHz) spectroscopy has been used to determine crystal field parameters in chromium(III) coordination compounds. The large zero-field splitting parameters of the dark-green photochromic trans-RSSR-[CrCl₂(cyclam)]₂ZnCl₄, 1, the red–purple trans-RSSR-[CrCl₂(cyclam)]Cl, 2 and the red–purple trans-RSSR-[CrCl₂(cyclam)]Cl·4H₂O·0.5HCl, 3, where cyclam = 1,4,8,11-tetraazacyclotetradecane, have been obtained. A full analysis of EPR spectra at 94.5 GHz of diluted complexes 1, 2 and 3 at 300 K revealed that they are extremely sensitive to D and E values. The rhombic distortion was precisely determined for each compound. For 1, g = 2.01, D = −0.305 cm⁻¹, E = 0.041 cm⁻¹ and λ = |E/D| = 0.1396; for 2, g = 2.01; D = −0.348 cm⁻¹, E = 0.042 cm⁻¹ and λ = |E/D| = 0.1206 and for 3, g = 1.99, D = −0.320 cm⁻¹, E = 0.041 cm⁻¹ and λ = |E/D| = 0.1281. The EPR study at 94.5 GHz at 10 K allowed us to confirm the sign of the D value for all compounds. These data indicate that at room temperature the crystal field is mainly rhombic and as the temperature decreases, the rhombicity of the D tensor increases slightly. These found differences between 1, 2 and 3 allowed us to establish the importance of the intermolecular interactions in the solid state due to different hydrogen bonding networks in their crystalline arrangement.