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

Abstract

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 <i>S</i> = 0 to <i>S</i> = 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 (<i>T</i>_1/2) and the Tolman cone angle (θ). Complexes with <i>para</i>-substituted triaryl phosphoraminimato ligands (<i>p</i>-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