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 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.

The Effect of Ligand Design on Metal Ion Spin State—Lessons from Spin Crossover Complexes

“…A linear relationship between T ½ and the phosphoraminimato PR 3 group cone angle was found, with more bulky phosphoraminimate ligands stabilizing the high-spin forms of the complexes and reducing T ½ [42]. This trend was also reproduced by DF calculations [43].…”

“…Back-bonding is most important in complexes with 4- As well as the pyridyl-substituted complexes in Scheme 7, these arguments explain the behavior of [(PhB{MesIm} 3 )FeN=P(C 6 H 4 R) 3 ] ( Figure 5) whose C 6 H 4 R substituents cannot conjugate with the iron atom, being separated from it by a sp 3 -hybridized P atom. The spin state of the complex would then be governed by the δ-inductive properties of the R substituents, as observed in [42]. The positive linear free energy relationship for [Fe(saltrien R )] + ( Figure 5) also implies that iron/phenoxide π-bonding is important to those compounds.…”

“…Conversely, however, some other studies have observed the opposite trend. Most striking are three tetrahedral complexes [(PhB{MesIm}3)FeN=P(C6H4R)3], which afforded a negative free energy relationship between T½ and the aryl "R" substitutents in d 8 -thf solution ( Figure 5) [42]. That is, the low-spin state of these complexes is stabilized by electron-donating, not electron-withdrawing, substituents.…”

A Unified Treatment of the Relationship Between Ligand Substituents and Spin State in a Family of Iron(II) Complexes

“…A sterically crowded ligand sphere generally leads to high‐spin complexes 12. However, the effect of ligand electronic character on metal‐ion spin state is less clear‐cut, with electron‐withdrawing substituents being reported to stabilize either the low‐spin13, 14, 15, 16 or the high‐spin state17, 18 in different series of compounds. While the literature includes data from solution and the solid‐state, these effects are best quantified by solution measurements which determine a complex's spin state in the absence of crystal‐packing effects or any other influences from a rigid solid lattice 19.…”

“…Electron‐withdrawing substitutents indeed stabilize either the low‐spin13, 14, 15, 16 or the high‐spin state17, 18 of a complex, depending on their position in the molecule and on which types of substituent are considered. The relationship between ligand design and metal‐ion spin state is a fine balance between opposing M−L σ‐ and π‐bonding effects.…”