Insights into the charge-transfer character of electronic transitions in RCp2Ti(C2Fc)2 complexes using solvatochromism, resonance Raman spectroscopy, and TDDFT. Issue 6 (27th January 2021)

• Record Type: Journal Article
• Authors: Carlton, Elizabeth S.; Sutton, Joshua J.; Gale, Ariel G.; Shields, George C.; Gordon, Keith C.; Wagenknecht, Paul S.
• Is Part Of: Dalton transactions. Volume 50:Issue 6(2021)
• Page Start: 2233
• Page End: 2242
• Publication Date: 2021-01-27
• Subjects: Chemistry, Inorganic -- Periodicals; Chemistry, Physical and theoretical -- Periodicals; Chemistry, Inorganic -- Periodicals; 546.05
• DOI: 10.1039/d0dt04282j
• Languages: English
• ISSNs: 1477-9226

Abstract

Abstract : Augmented with TDDFT, solvatochromism and resonance Raman spectroscopy of R Cp2 Ti(C2 Fc)2 complexes are consistent with the characterization of the lowest-energy electronic transition as a delocalized ethynylferrocene to Ti IV charge transfer. Abstract : A series of complexes with low-energy Fe II to Ti IV metal-to-metal charge-transfer (MMCT) transitions, Cp2 Ti(C2 Fc)2, Cp*2 Ti(C2 Fc)2, and MeOOC Cp2 Ti(C2 Fc)2, was investigated using solvatochromism and resonance Raman spectroscopy (RRS) augmented with time-dependent density functional theory (TDDFT) calculations in order to interrogate the nature of the CT transitions. Computational models were benchmarked against the experimental UV-Vis spectra and B3LYP/6-31G(d) was found to most faithfully represent the spectra. The energy of the MMCT transition was measured in 15 different solvents and a multivariate fit to the Catalán solvent parameters – solvent polarizability (SP), solvent dipolarity (SdP), solvent basicity (SB), and solvent acidity (SA) – was performed. The effect of SP indicates a greater degree of electron delocalization in the excited state (ES) than the ground state (GS). The small negative solvatochromism with respect to SdP indicates a smaller dipole moment in the ES than the GS. The effect of SB is consistent with charge-transfer to Ti. Upon excitation into the MMCT absorption band, the RRS data show enhancement of the alkyne stretching modes and of the out-of-plane bending modes of the cyclopentadienyl ring connected to Fe and the alkyne bridge. This is consistent with changes in the oxidation states of Ti and Fe, respectively. The higher-energy transitions (350–450 nm) show enhancement of vibrational modes consistent with ethnylcyclopentadienyl to Ti ligand-to-metal charge transfer (LMCT). The RRS data is consistent with the TDDFT predicted character of these transitions. TDDFT suggests that the lowest-energy transition in Cp2 Ti(C2 Fc)2 CuI, where CuI is coordinated between the alkynes, retains its Fe II to Ti IV MMCT character, in agreement with the RRS data, but that the lowest-energy transitions have significant CuI to Ti character. For Cp2 Ti(C2 Fc)2 CuI, excitation into the low-energy MMCT absorption band results in selective enhancement of the symmetric alkynyl stretching mode.