Steric and Electronic Control of the Spin State in Three-Fold Symmetric, Four-Coordinate Iron(II) Complexes

The three-fold symmetric, four-coordinate iron­(II) phosphoraminimato complexes PhB­(MesIm)3Fe–NPRR′R″ (PRR′R″ = PMePh2, PMe2Ph, PMe3, and PnPr3) undergo a thermally induced S = 0 to S = 2 spin-crossover in fluid solution. Smaller phosphoraminimato ligands stabilize the low-spin state, and an excellent correlation is observed between the characteristic temperature of the spin-crossover (T1/2) and the Tolman cone angle (θ). Complexes with para-substituted triaryl phosphoraminimato ligands (p-XC6H4)3PN– (X = H, Me and OMe) also undergo spin-crossover in solution. These isosteric phosphoraminimato ligands reveal that the low-spin state is stabilized by more strongly donating ligands. This control over the spin state provides important insights for modulating the magnetic properties of four-coordinate iron­(II) complexes.