Effect of N-methylation of macrocyclic amine ligands on the spin state of iron(III): a tale of two fluoro complexes.

Syntheses and characterization of [(cyclamacetate)FeF]PF6 (1) and the corresponding N-methylated complex [(trimethylcyclamacetate)FeF]PF6 (3) are presented. Compound 1 is prepared in good yields from the analogous chloro complex, whereas 3 is prepared by hydrolysis of the oxo-bridged diiron compound (Me3cyclam-acetate)Fe-O-FeCl3 (2) in the presence of PF(6) anions. Magnetic susceptibility and spectroscopic data including electron paramagnetic resonance and Mössbauer spectra indicate that 1 contains low-spin Fe(III) (S = 1/2), while 3 is high spin (S = 5/2). Both octahedral fluoro complexes were investigated theoretically by density functional theory in order to determine why the spin states of the two molecules are different. Energies calculated using the B3LYP functional correctly predict 1 to have a low-spin S = 1/2 ground state and 3 to be high spin, regardless of whether a solvation model is included. The difference between 1 and 3 is most likely a combination of steric effects caused by the N-methyl groups, which compel the Fe-N bond distances to be longer in 3 than they ordinarily would be, and also electronic effects, which cause the N-methylated ligand to be a weaker sigma donor than its nonmethylated counterpart.