Theoretical Analysis of the Three-Dimensional Structure of Tetrathiolato Iron Complexes

Abstract

The three-dimensional structures of a number of [M(SR)(4)](n-) complexes, where M is a 3d transition metal and R is an alkyl or aryl group, have been analyzed using density functional theory (DFT). Special attention is paid to the Fe(II)/Fe(III) mimics of rubredoxin. The Fe(II) model complex [Fe(SCH(3))(4)](2-) has an equilibrium conformation with D2d symmetry. The DFT energy has been decomposed into contributions for ligand-ligand and metal-ligand interactions. The latter contribution is analyzed with the angular overlap model (AOM) and constitutes the dominant stereospecific interaction in the Fe(II) complex. The sulfur lone-pair electrons exert anisotropic pi interactions on the 3d(6) shell of Fe(II), which are controlled by the torsion angles, omega(i), for the rotations of the S(i)-C(beta) bonds around the Fe-S(i) axes. In contrast, the pi interactions acting on the high-spin 3d(5) shell of Fe(III) are isotropic. As a consequence, the stereochemistry of the Fe(III) complexes is determined by the Coulomb repulsions between the ligands and has S(4) symmetry. The electrostatic repulsions between the lone pairs of the sulfurs are an essential component of the ligand-ligand interaction. The lone-pair repulsions distort the 90 degree angle SFeS' angles (delta + delta(t)) and give rise to a correlation between delta and omega, which is confirmed by crystallographic data. Both the Fe(II) and Fe(III) complexes exhibit structural bistability due to the presence of low-lying equilibrium conformations with S(4) symmetry in which the complex can be trapped by the crystalline host.

Magneto-Structural Correlations in a Series of Pseudotetrahedral [CoII(XR)4]2–Single Molecule Magnets: An ab Initio Ligand Field Study

“…67 A DFT and 4 ] 2− revealed the same two minima on the potential energy surface with respect to the torsion angles S−Fe−S−C. 68 The π anisotropy of the oxygen atom in (PPh 4 ) 2 [Co(OPh) 4 ] is small, which prevents crossing of the 4 E and 4 B terms ( 4 T 2 ) as a function of the ψ angle ( Figure 14, middle panel), so that the D value remains moderate in the range between −25 and −5 cm −1 .…”

High-frequency EPR study of the high-spin FeII complex Fe[(SPPh2)2N]2

“…Instead the torsion angles X-S-Fe-S 0 play a crucial role [34][35][36][37]31,38], as has also been observed for the corresponding high-spin Co II S 4 complexes [23,24]. In fact, an accurate estimation of spin-orbit coupling is required for the successful and unambiguous prediction of the spin-Hamiltonian parameters, which can only be achieved if the metal-ligand covalencies are properly described and low-symmetry effects are accurately taken care of [24].…”

On the accuracy of density functional theory for iron—sulfur clusters

“…between the last two optimization steps. Each optimization started from the same initial wave function and geometry, which eliminated potential conformational differences between optimized structures, which can affect the electronic structure 48, 49…”