Charge-Transfer Spectroscopy of Bisaxially Coordinated Iron(II) Phthalocyanines through the Prism of the Lever’s E_L Parameters Scale, MCD Spectroscopy, and TDDFT Calculations

Abstract

The position of the experimentally observed (in the UV–vis and magnetic circular dichroism (MCD) spectra) low-energy metal-to-ligand charge-transfer (MLCT) band in low-spin iron­(II) phthalocyanine complexes of general formula PcFeL2, PcFeL′L″, and [PcFeX2]2– (L, L′, or L″ are neutral and X– is an anionic axial ligand) was correlated with the Lever’s electrochemical E L scale values for the axial ligands. The time-dependent density functional theory (TDDFT)-predicted UV–vis spectra are in very good agreement with the experimental data for all complexes. In the majority of compounds, TDDFT predicts that the first degenerate MLCT band that correlates with the MCD A-term observed between 360 and 480 nm is dominated by an e g (Fe, dπ) → b 1u (Pc, π*) single-electron excitation (in traditional D4h point group notation) and agrees well with the previous assignment discussed by Stillman and co-workersInorg. Chem.199433573583 The TDDFT calculations also suggest a small energy gap for b 1u /b 2u (Pc, π*) orbital splitting and closeness of the MLCT1 e g (Fe, dπ) → b 1u (Pc, π*) and MLCT2 e g (Fe, dπ) → b 2u (Pc, π*) transitions. In the case of the PcFeL2 complexes with phosphines as the axial ligands, additional degenerate charge-transfer transitions were observed between 450 and 500 nm. These transitions are dominated by a 2u (Pc + L, π) → e g (Pc, π*) single-electron excitations and are unique for the PcFe­(PR3)2 complexes. The energy of the phthalocyanine-based a 2u orbital has large axial ligand dependency and is the reason for a large energy deviation for B1 a 2u (Pc + L, π) → e g (Pc, π*) transition. The energies of the axial ligand-to-iron, axial ligand-to-phthalocyanine, iron-to-axial ligand, and phthalocyanine-to-axial ligand charge-transfer transitions were discussed on the basis of TDDFT calculations.

Resolving a Half-Century-Long Controversy between (Magneto)optical and EPR Spectra of Single-Electron-Reduced [PcFe]−, [PcFeL]−, and [PcFeX]2– Complexes: Story of a Double Flip

“…The second pair of transitions was predicted at 515 nm and has the highest intensity in this spectral envelope. This band is dominated by 1e g (Pc)/d π (β) → 1b 2u *­(Pc) single-electron excitations and resembles the MLCT 2 band observed in the Pc(2−)­Fe II L 2 and [Pc(2−)­Fe II X 2 ] 2– complexes . The pair of degenerate transitions that are dominated by the 1e g (Pc)/d π (β) → 1b 1u *­(Pc) single-electron transitions (band that resembles MLCT 1 transition in Pc(2−)­Fe II L 2 and [Pc(2−)­Fe II X 2 ] 2– complexes) was predicted to have zero intensity (Table ).…”

“…The UV–vis spectrum of the tetracoordinate [PcFe] − anion has a significantly more intense band at 512 nm and a less intense ∼690 nm transition compared to the pentacoordinate species. In excellent agreement with Lever and Wilshire’s data, the molar extinction coefficients for all absorption bands in the 400–850 nm spectral envelope are significantly reduced (by at least 4 times) compared to the intensity of the Q-band in the parent PcFeL 2 and [PcFeX 2 ] 2– complexes and a similar trend was observed for all reduced ring-substituted iron phthalocyanines reported to date. In contrast, only a slight red shift of the Q-band was observed upon reduction of PcCo (Figure ).…”

“…All reduced [PcFeL] − and [PcFeX] 2– complexes can be reoxidized back to the initial PcFe II L 2 and [PcFe II X 2 ] 2– compounds with 80–90% yield under spectroelectrochemical conditions (Figures S7–S17). It is interesting to note that the PPh 3 ligand in [PcFe­(PPh 3 )] − gets exchanged with DMSO molecules during the reoxidation reaction as the final spectrum is indistinguishable from the PcFe­(DMSO) 2 complex but the resultant spectrum is very different from the UV–vis spectra of PcFe­(PR 3 ) 2 compounds …”

“…First, we are interested in generating and isolating reduced PcM species using chemical reducing agents. − Thus, in continuation of our studies on the isolated single-electron-reduced phthalocyanine species, we have studied the reduction and isolation of iron­(II) phthalocyanine derivatives. Second, we recently discussed the b 1u *–b 2u * energy gap in PcFeL 2 , PcFeL′L″, or [PcFeX 2 ] 2– complexes (Figure ), which, based on magnetic circular dichroism (MCD) and time dependent density functional theory (TDDFT) data, should be around 0.10–0.15 eV . Similar to main-group phthalocyanines of the general formula [Pc(3−)­M II ] − (M = Mg or Zn), − we speculated that this gap can be directly measured using MCD spectra of [Pc(3−)­Fe II L n ] − ( n = 1 or 2) complexes as 7e g * → 1b 1u */1b 2u * transitions should be observable in the 500–600 nm region, assuming that the first reduction of the PcFe, PcFeL 2 , and [PcFeX 2 ] 2– species is phthalocyanine-centered.…”

“…However, if one would assume that the experimentally observed MCD pseudo A-term in the ∼650− 690 nm area belongs to the MLCT 1 band and another pseudo Aterm observed around 515 nm belongs to the MLCT 2 band, then the 1b 1u *−1b 2u * energy gap should be significantly larger than that predicted by TDDFT calculations. One important fact we would like to mention is that the MLCT 1 and MLCT 2 bands in Pc(2−)Fe II L 2 and [Pc(2−)Fe II X 2 ] 2− complexes are heavily dominated by the 1e g /d π → 1b 1u * and 1e g /d π → 1b 2u * singleelectron contributions,98 while much stronger configurational interactions were predicted for the same transitions in the [PcFe] − , [PcFe(DMF)] − , and [PcFe(NCS)] 2− complexes (Table8).Overall, our DFT and TDDFT calculations explain well the presence of the axial s = 1/2 EPR spectra as well as Pc…”