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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
Author(s) -
Deeth Robert J.,
Halcrow Malcolm A.,
Kershaw Cook Laurence J.,
Raithby Paul R.
Publication year - 2018
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201704558
Subject(s) - chemistry , ab initio , steric effects , spin crossover , spectrochemical series , ligand (biochemistry) , pyridine , force field (fiction) , ligand field theory , crystallography , computational chemistry , protonation , spin states , stereochemistry , physics , quantum mechanics , inorganic chemistry , ion , biochemistry , receptor , organic chemistry , medicinal chemistry
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 [H 3 bpp] 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 parameter 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 [BF 4 ] − or [PF 6 ] − 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, T 1/2 , 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 T 1/2 . One parameter set for all substituted bpp ligands is insufficient and further LFMM development will be required.