Isocyanide Ligands Promote Ligand-to-Metal Charge Transfer Excited States in a Rhenium(II) Complex.

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.