A computational study of a high-spin iron(I) complex for possible dinitrogen reduction to ammonia

Abstract

A series of high-spin, low coordinate, paramagnetic iron complexes bearing a phenyltris((tert- butylthio)methyl)borate ligand were computationally modeled with density functional theory (DFT) and complete active space self-consistent field theory (CASSCF). The iron complexes examined in this research were inspired by nitrogenase, a naturally occurring, dinitrogen- fixating, iron-containing metalloenzyme. DFT and CASSCF offer a convenient way to explore reactions, complexes, and molecular orbitals without an immediate need to perform synthetic experiments. Our computational work can be used to guide synthetic efforts as well as urge future theoretical work in related research. DFT was utilized to compute two different thermodynamic properties: bond dissociation free energy (BDFE N-H) and Gibb's free energy. The conductor-like polarizable continuum model (CPCM) was applied to examine the solution phase of the system, and all BDFE and DeltaG values found were endothermic in tetrahydrofuran (THF). The methods, BP86 and BP86 ZORA, examined the gas phase of the system. The BDFE and DeltaG values calculated when using those two methods were largely inconsistent, which lead to the conclusion that the solution phase model is the most appropriate method for computing values of the dinitrogen complex ([Fe] 2(Mu-N 2)) and its related complexes. An N 2 vibrational mode was found (1915.30 cm -1) for [Fe] 2(Mu-N 2), which reflects a strongly coordinated dinitrogen bridge (Fe-N identical to N-Fe). Broken symmetry DFT (BSDFT) was used to examine the exchange coupling, which was found to have positive values (JAB =82.51 cm -1, 61.88 cm -1, 81.36 cm -1), and implied that [Fe] 2(Mu-N 2) is ferromagnetically coupled. Lastly, CASSCF and DFT were applied to plot and characterize certain molecular orbitals of [Fe] 2(Mu-N 2). The plotted and characterized molecular orbitals reflected moderate (DFT) to strong (CASSCF) covalent bonding between iron and dinitrogen. All this data reflected the synthetic plausibility of dinitrogen coordination to the bridged, Fe(I) complex ([Fe] 2(Mu-N 2)) that can be reduced through the dinitrogen cleavage mechanism.