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 P(n)Pr3) 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.