Isocyanide Ligands Promote Ligand-to-Metal Charge Transfer Excited States in a Rhenium(II) Complex
Tayliz M. Rodriguez, Mawuli Deegbey, Chun‐Hsing Chen, Elena Jakubı́ková, Jillian L. Dempsey
Abstract
A metal-to-ligand charge transfer with mixed intraligand character is observed for the rhenium hexakisarylisocyanide complex [Re(CNAr) 6 ]PF 6 (CNAr = 2,6-dimethylphenylisocyanide, λ max = 300 nm). Upon oxidation to [Re(CNAr) 6 ](PF 6 ) 2, the dominant low energy optical transition is a ligand-to-metal charge transfer (LMCT) mixed with intraligand transitions (λ max = 650 nm). TD-DFT was used to identify the participating ligand-based orbitals in the LMCT transition, revealing that the majority of the donor orbital is based on the aryl ring (85%) as opposed to the CN bond (14%). For both [Re(CNAr) 6 ] + and [Re(CNAr) 6 ] 2+, structural characterization by X-ray diffraction reveals deviations from O h geometry at the central Re ion, with larger reduction in symmetry observed for Re(II). For [Re(CNAr) 6 ] +, these structural changes lead to a broadening of the strong ν(C≡N) stretch (2065 cm –1 ), as the degeneracy of the T 1u IR-active mode is broken. Furthermore, a shoulder is observed for this ν(C≡N) stretch, resulting from deviation of the C–N–Ar bond from linearity. By contrast, [Re(CNAr) 6 ] 2+ has two weak bands in the ν(C≡N) region (2065 and 2121 cm –1 ). DFT calculations indicate that reduction of symmetry at the central rhenium ion manifests in the decrease in intensity and the observed split of the ν(C≡N) band. Stability of both complexes are limited by light-induced decomposition where Re(I) dissociates a isocyanide ligand upon irradiation and Re(II) absorbance decays under ambient light. These data provide new insights to the electronic structure of [Re(CNAr) 6 ] 2+, enhancing our understanding of LMCT excited states and the versatility of isocyanide ligands.