Identification of molecular species in solution by theoretical calculations: how far can we trust the models?

[M"(R,R'-acac)2] (M=Mn,Fe,Co,Ni,Cu,Zn) complexes are classic coordination compounds 1 applied as building blocks, catalysts or catalysts precursors for organometallic synthesis and materials preparation. Due to its low aqueous solubility, molecular structure determination relies on solid state data, particularly X-ray diffraction and infrared. Under anhydrous condition they precipitate as distorted octahedral oligomers2 while they present themselves in the monomeric form in diluted solutions. The assumed preference for distorted octahedral geometry is assured by solvent molecules. If the synthesis uses hydrated materials two waters are assumed to be present. The known crystal structures are almost always the trans isomers, as seen in trans-[Fe11(acach(H20)2].3 Water is green chemistry favorite solvent and much effort has been applied to develop active catalysts in water. The determination of the molecular identities present in solution is fundamental to understand the mechanisms of action of the catalyst and to identify the relevant forms. Solution conditions were simulated for [Fe11(acac)2], [Fe11(acach(H20)] and [Fe11(acach(H20)2] isomers, with OFT (gas phase and implicit solvent SMD model), Molecular Dynamics under PBC (with iron parameterized Force Field) and our newly developed explicit OFT method (Cocoon). They show several possible structures for five- and six-coordinate compounds. BSSE corrected Gibbs energies with different basis sets do not support trans isomer as the energy minimum, same as the kinetic calculations from the trigonal bipyramidal intermediate.