Computational Study of Iron Bis(dithiolene) Complexes: Redox Non-Innocent Ligands and Antiferromagnetic Coupling

Abstract

The molecular and electronic structure of monomeric ([Fe(S2C2H2)2](z), [Fe(S2C2(C6H4-p-OCH3)2)2](z)) and dimeric ([{Fe(S2C2H2)2}2](z)) iron bis(dithiolene) complexes, and of their phosphine adducts ([(PH3)Fe(S2C2H2)2](z), [(P(C6H5)3)Fe(S2C2H2)2](z), [(PH3)Fe(S2C2(C6H4-p-OCH3)2)2](z)), carrying various charges (z = 0, 1-, 2-), have been investigated by density functional theory (DFT). Net total spin polarization values S of zero, two, and four have been considered for all neutral model compounds and their dianions, whereas all monoanions have been examined with net total spin polarization values S of one, three, and five. The DFT calculations utilized the pure functional BP86, as well as the hybrid functionals B3LYP and B3LYP*. For the monomers, the calculations reveal the presence of redox non-innocent dithiolene ligands and antiferromagnetic coupling between the ligands and the metal center. For the dimers, complexes with antiferromagnetically coupled iron centers have been found to represent structures of low energy, if not lowest energy structures. The spin-coupling constant of [{Fe(S2C2H2)2}2](2-) is calculated as J = -230 cm(-1). On the basis of the computational results, a model for reversible, electrochemically controlled binding and release of phosphine ligands to iron bis(dithiolene) complexes is proposed. Only BP86 and B3LYP* results, but not those of B3LYP calculations, are in qualitative agreement with experimental findings. BP86 calculations provide the best quantitative match in comparison with the experiment.

Spin‐State‐Dependent Properties of an Iron(III) Hydrogenase Mimic

“…The removal of the axial ligand (complex 2 ) gave a square‐planar [Fe III (mnt) 2 ] – complex with an S = 3/2 ground spin state for all functionals investigated. This was also found by Jacobsen et al for a related complex with hydrogen atoms instead of cyanide groups 29. We observed some quantitative changes in the relative energies and the ordering of higher spin states compared to those of 1 .…”

“…The B3LYP functional decreases the Gibbs energy difference to the high‐spin state and gives an S = 5/2 state that is +6.6 kcal/mol above the S = 3/2 state followed by the S = 1/2 state at +8.2 kcal/mol. The energy difference of 3.7 kcal/mol (BP86) between the S = 3/2 and 1/2 states is larger than the value obtained by Jacobsen et al29…”

“…Calculations on an iron(III) square‐planar complex with four equatorially bound sulfur atoms (i.e., with a vacant axial binding site) gave even larger deviations 29. For the S = 3/2 state, the calculated equatorial iron–sulfur bond lengths deviated by 0.05 Å from the those of the X‐ray structure.…”

“…For example, Chang et al,28,45 described a square‐pyramidal iron(III) complex with the iron(III) ion surrounded by two equatorially bound sulfur atoms and two nitrogen atoms as well as an axial chloride ligand (Figure 7B). Another square‐planar iron(III) complex (Figure 7C)29 was also found to exhibit a quartet ground state. In their investigation of redox‐noninnocent S,S′‐coordinated benzene‐1,2‐dithiolate ligands, Ray et al46 described a pentacoordinate iron(III) complex with an intermediate ground state (Figure 7D).…”

“… Iron(III) complexes found to exhibit a quartet ground state include (A) 1 , (B) chloro[ N , N′ ‐ethylenebis(mercaptosalicylideneiminato)]iron(III),28,45 (C) an iron bis(dithiolene) model complex29 and (D) bis(benzene‐1,2‐dithiolato)iron(III)‐4‐ tert ‐butylpyridine 46…”