SOMO-HOMO level conversion in triarylmethyl-cored N-heterocyclic carbene-Au(I) complexes triggered by selecting coordination halogens

Conventionally, organic radicals adhere to the Aufbau principle, the energy level of the singly occupied molecular orbital (SOMO) is not below the highest occupied molecular orbital (HOMO), but somewhat abnormal phenomena have appeared recently. In this study, we introduce a novel strategy by incorporating unique NHC-Au-X units into a tris(2,4,6-trichlorophenyl)methyl (TTM) system to create metal-involved open-shell complexes, denoted as TTM-NHC-Au-X (X = I, Br, or Cl). Density-functional theory calculations were used to predict an inversion in the energy of the SOMO and highest doubly occupied molecular orbital (HOMO) of TTM-NHC-Au-I, which is supported by experimental results. Organometallic radicals TTM-NHC-Au-X demonstrated distinct properties with different coordinated halides. The radical behaviors have been investigated by EPR, UV-vis spectroscopy and cyclic voltammetry, additional structural information provided by structurally comparing related the precursor complexes given by X-ray crystallography. TTM-NHC-Au-I with SOMO-HOMO conversion (SHC) features a highly thermal decomposition temperature up to 305 °C. Furthermore, the photostability of TTM-NHC-Au-I was found to be 75 and 23 times greater than that of TTM-NHC-Au-Br and TTM-NHC-Au-Cl, respectively. These findings provide valuable insights into the structural and electronic design principles governing the occurrence of SOMO-HOMO conversion in open-shell systems.