A theoretical study of zero-field splitting in Fe(IV)S₆ (S = 1) and Fe(III)S₆ (S = 1/2) core complexes, [Fe^IV(Et₂dtc)_3−n(mnt)_n]⁽ⁿ⁻¹⁾⁻ and [Fe^III(Et₂dtc)_3−n(mnt)_n]ⁿ⁻ (n = 0, 1, 2, 3): The origin of the magnetic anisotropy

Multireference ab initio calculations and ligand field analysis of a series of complexes with Fe(IV)S6 (S = 1) [FeIV(Et2dtc)3−n(mnt)n](n−1)− and Fe(III)S6 (S = 1/2) [FeIII(Et2dtc)3−n(mnt)n]n− cores ((mnt)2− = maleonitriledithiolate (2−), (Et2dtc)1− = diethyldithiocarbamato (1−) ligands, n = 0, 1, 2, 3) are reported and used to understand their magnetic and spectroscopic (ESR) properties. These systems feature large and variable values of D for the S = 1 complexes of Fe(IV) and strongly anisotropic g-tensors for the S = 1/2 complexes of Fe(III). The calculations are in good to excellent agreement with experiment. We utilize a historic concept put forward by Orgel as early as 1961 [39] in order to analyze the computational data. The non-additive contributions to ligand field due to the π-conjugated systems of the chelate ligands mnt2− and Et2dtc− are responsible for the large magnetic anisotropy. These contributions are even more important than geometric distortions imposed by the rigid ligand cores. The correlations have been demonstrated and quantified using an extended ligand field (LF) model with parameters adjusted to complete active space self-consistent field (CASSCF) calculations corrected for dynamic correlation with the second order N-electron valence perturbation theory (NEVPT2). According to this analysis, the topology of the intrinsic π-electron system of the mnt2− and Et2dtc− ligands causes a splitting of the octahedral t2g orbitals of different sign for mnt2− (e > a1, in-phase coupling) and Et2dtc1− (a1 > e, out-of-phase coupling). When combined with the π-donor ability of the mnt2− and Et2dtc− shown by theory and experiment to be much stronger in mnt2− compared to Et2dtc1− this leads to large orbital contributions to the magnetic moment and to a negative D for [Fe(mnt)(dtc)2] with an easy axis of magnetization bisecting the SFeS(mnt) bite angle. Using this ab initio based renewed concept, field dependent isothermal magnetizations reported previously (Milsmann et al., 2010 [25]) have been re-interpreted. We show that the orthorhombic anisotropy for [FeIV(Et2dtc)(mnt)2]1− (2ox) and [FeIV(Et2dtc)2(mnt)]0 (3ox), that has never been discussed before, leads to large zero-field splitting parameter E. At the same time it is pointed out, that the D and E spin-Hamiltonian parameters cannot be uniquely extracted from a fit to the magnetic susceptibility data, unless combined with other sophisticated spectroscopic experiments. Applying the same anisotropic π-bonding model, orbital contributions leading to strongly anisotropic g-tensors reported from simulation of ESR data of the Fe(III)S6 (S = 1/2) cores in complexes [FeIII(Et2dtc)3−n(mnt)n]n− (n = 0, 1, 2, 3) have been rationalized

Dealing with Complexity in Open-Shell Transition Metal Chemistry from a Theoretical Perspective: Reaction Pathways, Bonding, Spectroscopy, And Magnetic Properties

This chapter illustrate the challenges that are met in theoretical transition metal chemistry: (1) reactivity of high-valent iron-oxo sites and the challenge of multiple spin-state channels; (2) The treatment of magnetic spectroscopic observables in the case of (near) orbital degeneracy; (3) The experimentally validated description of transition metal complexes with coordinated ligand radicals; (4) The calculation of the magnetic properties of oligonuclear transition metal clusters with applications to Photosystem II. The subjects treated in the chapter are related to the fact that open-shell transition metals display a high degree of electronic complexity. This shows up in their reaction pathway that will frequently show multistate reactivity. Likewise, the magnetic and electronic properties of open-shell transition metals can be very complicated, as the case of Jahn Teller systems, and special techniques need to be employed to successfully model them. The intricate bonding situations that are created by exchange coupling (in essence, nothing but a very weak chemical bond) in metal radical systems and oligonuclear metal clusters are another area that is highly challenging to theory

DFT-Based Explanation of the Effect of Simple Anionic Ligands on the Regioselectivity of the Heck Arylation of Acrolein Acetals

The Heck arylation of acrolein acetal has been studied computationally and compared to the corresponding reaction with allyl ethers. The reaction can be controlled to give either cinnamaldehydes or arylpropanoic esters by addition of different coordinating anions, acetate, or chloride. The computational study reveals that coordinating acetate raises the energy of an intermediate sufficiently to block the access to an otherwise favorable β-hydride elimination. The reaction path is also compared to that of allyl ethers, which always give significant amounts of cinnamyl ether products under all reaction conditions. The difference between the two substrate classes could be rationalized in terms of relative hydride donating power of the two substrates. © 2009 American Chemical Society