Symmetry Breaking in a Triferrous Extended Metal Atom Chain

Semilocal and random phase approximation (RPA) density
functional
theory (DFT) and complete active space (CASSCF + NEVPT2) methodologies
were applied to investigate a new class of extended metal atom chain
(EMAC) complexes. A novel triferrous complex has been synthesized
recently that does not utilize the usual 2,2′-dipyridylamine
(dpa) ligand framework, which essentially always results in a tetragonal
coordination environment and general formula M3(dpa)4X2, where X is an anion. Instead, the triferrous
complex utilizes a dianionic, 2,6-bis(trimethylsilylamido)pyridine
ligand (L2–) resulting in the formation of trigonal
complexes with general formula Fe3L3. To better understand the
electronic structure of this complex, calculations were utilized to
explore the experimentally isolated Fe3L3, and a smaller theoretical
complex, in order to compare and contrast with the traditional dpa-based
EMACs. Due to the absence of anionic, axial ligands, the sigma nonbonding
orbitals formed from the metal d orbitals are lower in energy than
in the dpa complexes, and compete with the pi bonding orbitals for
occupation in the Fe3L3 complex. While the idealized geometry of
these complexes is D3h, a helical distortion of the ligands and subsequent electronic symmetry
breaking due to Jahn–Teller distortions are predicted utilizing
both semilocal and RPA DFT methods, ending in a C2 structure that closely matches the reported crystal
structure. Predicted Mössbauer isomer shifts and ultraviolet/visible
(UV/vis) spectra also agree with the experimental data available in
the literature. Magnetic coupling constants also indicate ferromagnetic
coupling between nearest neighbor irons. Two-dimensional (2D) potential
energy surfaces were calculated for a range of fixed Fe–Fe
bond lengths, revealing a flat potential energy surface over a wide
range of Fe–Fe bond lengths and verifying the ability of RPA
to act as a higher-level check on semilocal DFT results. In order
to verify the predicted high-spin ground state, CASSCF + NEVPT2 was
applied to selected molecular configurations and confirmed the predictions
made by DFT. These calculations shed light on the physical ground
state electron configuration of Fe3L3 and correlate this
electronic configuration with the available experimental data.