Iron(III) and iron(II) complexes of 1-thia-4,7-diazacyclononane ([9]aneN2S) and 1,4-dithia-7-azacyclononane ([9]aneNS2). X-Ray structural analyses, magnetic susceptibility, Mössbauer, EPR and electronic spectroscopy†

Abstract

The complexes [Fe([9]aneN₂S)₂][ClO₄]₂, [Fe([9]aneN₂S)₂][ClO₄]₃ and [Fe([9]aneNS₂)₂][ClO₄]₂ ([9]aneN₂S = 1-thia-4,7-diazacyclononane and [9]aneNS₂ = 1,4-dithia-7-azacyclononane) have been prepared and the latter two characterised by X-ray crystallography. The Mössbauer spectra (isomer shift/mm s^–1, quadrupole splitting/mm s^–1, 4.2 K) for [Fe([9]aneN₂S)₂][ClO₄]₂ (0.52, 0.57), [Fe([9]aneN₂S)₂][ClO₄]₃ (0.25, 2.72) and [Fe([9]aneNS₂)₂][ClO₄]₂ (0.43, 0.28) are typical for iron(II) and iron(III) complexes. Variable-temperature susceptibility measurements for [Fe([9]aneN₂S)₂][ClO₄]₂ (2–300 K) revealed temperature-dependent behaviour in both the solid state [2.95 µ_B (300 K)–0.5 µ_B (4.2 K)] and solution (ΔH ⁰ 20–22 kJ mol^–1, ΔS ⁰ 53–60 J mol^–1 K^–1). For [Fe([9]aneN₂S)₂][ClO₄]₃ in the solid state [2.3 µ_B (300 K)–1.9 µ_B (4.2 K)] the magnetic data were fit to a simple model (H = –λL·S + µL_z) to give the spin–orbit coupling constant (λ) of –260 ± 10 cm^–1. The solid-state X-band EPR spectrum of [Fe([9]aneN₂S)₂][ClO₄]₃ revealed axial symmetry (g_⊥ = 2.607, g_|| = 1.599). Resolution of g_⊥ into two components at Q-band frequencies indicated a rhombic distortion. The low-temperature single-crystal absorption spectra of [Fe([9]aneN₂S)₂][ClO₄]₂ and [Fe([9]aneNS₂)₂][ClO₄]₂ exhibited additional bands which resembled pseudo-tetragonal low-symmetry splitting of the parent octahedral ¹A_1g → ¹T_2g and ¹A_1g → ¹T_1g transitions. However, the magnitude of these splittings was too large, requiring 10Dq for the thioether donors to be significantly larger than for the amine donors. Instead, these bands were tentatively assigned to weak, low-energy S → Fe^II charge-transfer transitions. Above 200 K, thermal occupation of the high-spin ⁵T_2g ground state resulted in observation of the ⁵T_2g → ⁵E_g transition in the crystal spectrum of [Fe([9]aneN₂S)₂][ClO₄]₂. From a temperature-dependence study, the separation of the low-spin ¹A_1g and high-spin ⁵T_2g ground states was approximately 1700 cm^–1. The spectrum of the iron(III) complex [Fe([9]aneN₂S)₂][ClO₄]₃ is consistent with a low-spin d⁵ configuration.