5T2⇌1A1 and 6A1⇌2T2 spin transitions in iron(II) and iron(III) complexes of 2,2′-bi-2-imidazoline and related ligands

Abstract

A series of six-co-ordinate iron(II) complexes [Fe^IIL₃]X₂(X = ClO₄ or BPh₄) of the bidentate ligands 2,2′-bi-2-imidazoline (L³), 2,2′-bi-1,4,5,6-tetrahydropyrimidine (L⁴), 2,2′-bi-2-oxazoline (L⁵), and 5,5′-dimethyl-2,2′-bi-2-oxazoline (L⁶) have been prepared and characterised by their physical properties. In all cases co-ordination is via the α-di-imine group. Magnetic susceptibility and Mössbauer effect measurements as a function of temperature show that the complexes of L⁵ and L⁶ are fully high-spin (S= 2) over the temperature range 90–360 K, while for complexes of L³(X = ClO₄ but not BPh₄) and L⁴(X = ClO₄ and BPh₄) a ⁵T₂⇌¹A₁ spin transition is observed in this temperature range. Electronic spectra of the corresponding nickel(II) complexes indicate a correlation between Dq(Ni) and the spin ground state of the iron(II) complexes. The iron(III) complex [Fe^IIIL³₃][ClO₄]₃ is high-spin (S= 5/2), while [FeL⁴₃][ClO₄]₃ has a ²T₂ ground state but with a thermally accessible ⁶A₁ excited state. Iron(III) complexes of L⁵ and L⁶ are unstable and could not be isolated. Reaction of the iron(II) complexes of L³ and L⁴ with dioxygen gave the iron(III) complexes [Fe^IIIL₂(L – H)][ClO₄]₂ is good yield [(L – H)= monodeprotonated ligand] which may be reversibly converted to the [Fe^IIIL₃]³⁺ species on treatment with one equivalent of HClO₄. The temperature-dependent magnetic moments of the complexes [FeL₂(L – H)][ClO₄]₂ are interpreted in terms of ²T₂⇌⁶A₁ spin equilibria.