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

Abstract

A metal-to-ligand charge transfer with mixed intraligand character is observed for the rhenium hexakisarylisocyanide complex [Re(CNAr)6]PF6 (CNAr = 2,6-dimethylphenylisocyanide, λmax = 300 nm). Upon oxidation to [Re(CNAr)6](PF6)2, the dominant low energy optical transition is a ligand-to-metal charge transfer (LMCT) mixed with intraligand transitions (λmax = 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 Oh 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 T1u 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.

Electronic Structure and Photophysics of Low Spin d5 Metallocenes

“…The vibrational fine structure known for this complex is not conveyed within its calculated absorption spectra as only electronic excitations were calculated. Interestingly, both low energy transitions exhibit 99% LMCT character, uncharacteristic of previously reported rhenium­(II) complexes where metal–ligand mixing is more prevalent. , Solvatochromism was similarly not observed for this complex across four solvents (dielectric constants ranging from 1.8–8.9), − further supporting the presence of highly symmetric LMCT excited states (Figures S5 and S6). …”

“…Interestingly, both low energy transitions exhibit 99% LMCT character, uncharacteristic of previously reported rhenium(II) complexes where metal−ligand mixing is more prevalent. 51,52 Solvatochromism was similarly not observed for this complex across four solvents (dielectric constants ranging from 1.8−8.9), 53−55 further supporting the presence of highly symmetric LMCT excited states (Figures S5 and S6). 52 The optical transitions that dominate the UV−vis absorption spectrum of MnCp* 2 are similar to those quantified for ReCp* 2 (Figure 5A).…”

Controlling the Photophysical Properties of a Series of Isostructural d6 Complexes Based on Cr0, MnI, and FeII

“…The increased energy is predominantly the result of the increased effective nuclear charge felt at the location of the relevant d-orbitals for manganese­(I) relative to that of chromium(0), resulting in a significant stabilization of the π­(d) orbitals (π­(d xy ), π­(d xz ), π­(d yz ) in Figure b; equivalent to t 2g in Figure a). The assignment of mixed IL/MLCT character of [Mn] + for the lowest-lying transition agrees with related studies on manganese­(I) hexakis­(arylisocyanide) complexes. ,,, Furthermore, prior computational studies on tungsten(0) and rhenium­(I) arylisocyanides have likewise shown that enhanced extinction coefficients can result from the lowest transition having mixed MLCT and π–π* (CN–C) character. , …”

“…36,47,49,55 Furthermore, prior computational studies on tungsten(0) and rhenium(I) arylisocyanides have likewise shown that enhanced extinction coefficients can result from the lowest transition having mixed MLCT and π−π* (C�N−C) character. 40,63 The UV−vis absorption spectra of [Cr], [Mn] + , and [Fe] 2+ (Figure 3) all display an intense band centered at 350 nm originating primarily from 1 π−π* transitions involving pyrene moieties, as also seen in the free L Pyr ligand (Table 2). At shorter wavelengths, 1 π−π* transitions from the m-terphenyl backbone of the isocyanide ligands appear.…”

A new era of LMCT: leveraging ligand-to-metal charge transfer excited states for photochemical reactions

“…It also further underscores the opportunity to leverage TD-DFT to help identify and visualize the ligand-based orbitals that participate in LMCT transitions, and gain a deeper understanding of the ligand properties needed to support low energy LMCT excited states. 22,47,55,63–65…”