Ab Initio Ligand Field Molecular Mechanics and the Nature of Metal-Ligand π-Bonding in Fe(II) 2,6-di(pyrazol-1-yl)pyridine Spin Crossover Complexes

A ligand field molecular mechanics (LFMM) force field has been constructed for the spin states of [Fe(bpp)₂]²+ (bpp = 2,6-di(pyrazol-1-yl)pyridine) and related complexes. A new charge scheme is employed which interpolates between partial charges for neutral bpp and protonated [H₃bpp]³+ to achieve a target metal charge. The LFMM angular overlap model (AOM) parameters are fitted to fully ab initio d orbital energies. However, several AOM parameters sets are possible. The ambiguity is resolved by calculating the Jahn-Teller distortion mode for high spin which indicates that in [Fe(bpp)₂]²+pyridine is a π acceptor and pyrazole a weak π donor. The alternative fit, assumed previously, where both ligands act as π donors leads to an inconsistent distortion. LFMM optimisations in the presence of [BF₄]- or [PF₆]- anions are in good agreement with experiment and the model also correctly predicts the spin state energetics for 3-pyrazolyl substituents where the interactions are mainly steric. However, for 4-pyridyl or 4-pyrazolyl substituents, LFMM only treats the electrostatic contribution which, for the pyridyl substituents, generates a fair correlation with the spin crossover transition temperatures, T₁/₂, but in the reverse sense to the dominant electronic effect. Thus, LFMM generates its smallest spin state energy difference for the substituent with the highest T₁/₂. One parameter set for all substituted bpp ligands is insufficient and further LFMM development will be required.