Synthesis and properties of [M^II(L⁶)₂][ClO₄]₂(M = Fe, Co and Ni): structures of Co and Ni complexes and spin-state transition by Fe complex (L⁶= 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl]pyridine)

Abstract

A new non-planar tridentate ligand 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl]pyridine (L⁶) and its mononuclear bis-ligand complexes [M(L⁶)₂][ClO₄]₂ [M = Fe (1), Co (2) and Ni (3)] have been synthesized. In the dications of six-co-ordinate complexes of 2 and 3, each L⁶ is arranged in a meridional mode. Structural analyses reveal that the M-N_pyrazole bond length is appreciably shorter than the M-N_pyridine bond lengths. Due to the weak field nature of L⁶ (absorption spectral analyses) towards Fe(II), Co(II) and Ni(II) the complexes are uniformly high-spin at room-temperature. Temperature-dependent magnetic susceptibility measurements reveal that while Co(II) and Ni(II) complexes remain high-spin over the entire range investigated (6-300 K for 2 and 63-300 K for 3), the Fe(II) complex exhibits a temperature-induced (5.2-300 K) spin transition in going from the S = 2 state at 300 K to predominantly an S = 0 state below 80 K. From the linear ln K_eq [¹A₁ (ls) ↔ ⁵T₂ (hs)] vs. 1/T relationship (190-250 K) the derived thermodyanamic parameters are: ΔH = 9.7 ± 0.04 kJ mol^-1 and ΔS = 56 ± 0.18 J K^-1 mol^-1. While complex 1 displays in MeCN an irreversible M^III/M^II redox process [anodic peak potential, E_pa = 1.06 V vs. SCE], for complex 2 such a redox process is reversible [E_½ = 0.74 V vs. SCE and ΔE_p = 80 mV].