Modeling ligand electrochemical parameters by repulsion‐corrected eigenvalues

Abstract

Ligand electrochemical parameters, E L , more commonly known as Lever parameters, have played a major research role in understanding redox processes involved in inorganic electrochemistry, enzymatic reactions, catalysis, solar cells, biochemistry, and materials science. Despite their broad usefulness, Lever parameters are not well understood at a first-principles level. Using density functional theory, we demonstrate in this contribution that a ligand's Lever parameter is fundamentally related to the ligand's ability to alter the eigenvalue of the electroactive spin-orbital in an octahedral transition metal complex. Our analysis furthers a first-principles understanding of the nature of Lever parameters.

Application of Lever’s EL Parameter Scale toward Fe(II)/Fe(III) versus Pc(2-)/Pc(1-) Oxidation Process Crossover Point in Axially Coordinated Iron(II) Phthalocyanine Complexes

“…The spectral changes associated with the oxidation of two anionic complexes ([PcFe II (NCO) 2 ] 2– and [PcFe II (NCS) 2 ] 2– ) are rather interesting and correlate well with the oxidation of the [PcFeCl 2 ] 2– complex (prepared in the DMA/0.1 M LiCl system) reported by Lever and Wilshire in 1978 . Both of these complexes have low (negative) first oxidation potentials and possess almost pure σ-donor axial ligands, ,, and thus, the iron-centered occupied d-orbitals should be significantly energetically destabilized as compared to the phthalocyanine-centered a 1 u molecular orbital. Also, based on the UV–vis spectra discussed below, as well as mass spectrometry data, the axial ligands in the iron­(III) derivatives of these compounds are not as strongly bound to the iron center as compared to all other complexes discussed in this work.…”

“…However, when a freshly prepared solid PcFe II Py 2 was dissolved in a DMF/0.3 M TBAP or was in situ-generated using a DCM/Py mixture (consisting of only 2 mol equiv of pyridine)/0.1 M TBAP, the spectral changes associated with electrooxidation were accompanied by the appearance of 850, 691, 565, and 527 nm bands (Figure 7d) for the latter system (i.e., DCM with 2 equiv of Py), while a similar picture was observed in DMF/0.3 M TBAP for the solid PcFe II Py 2 sample. More importantly, the intensity of the band at ∼690 nm as well as the presence of the transition at 565 nm could well be indicative of the formation of a [PcFe III Py 2 ] + species; however, the reduction of the [PcFe III Py 2 ] + species in neat DCM was not fully reversible 75 Both of these complexes have low (negative) first oxidation potentials and possess almost pure σ-donor axial ligands, 2,123,124 and thus, the iron-centered occupied d-orbitals should be significantly energetically destabilized as compared to the phthalocyaninecentered a 1u molecular orbital. Also, based on the UV−vis spectra discussed below, as well as mass spectrometry data, the axial ligands in the iron(III) derivatives of these compounds are not as strongly bound to the iron center as compared to all other complexes discussed in this work.…”