Thioether-ligated nickel. Synthesis, x-ray crystal structure and redox behaviour of complexes of hexadentate ligands incorporating thioether and triazene-1-oxide functions

Abstract

The hexadentate ligands RN(O)NNHC₆H₄S(CH₂)₃SC₆H₄NHN(O)(NR (H₂L¹: R = Me; H₂L²: R = Prⁿ; general abbreviation H₂L) and their nickel(II) complexes, [NiL¹] and [NiL²], have been synthesized. The X-ray structure of [NiL¹] has revealed the presence of a severely distorted NiN₂O₂S₂ coordination sphere with longer than normal Ni---S distances : 2.494(4) and 2.526(3) Å. The low symmetry of the ligand field is reflected in large splittings of octahedral v₁ and v₂ bands. In dichloromethane solution the nickel(III)-nickel(II) redox couple is observable cyclic voltammetrically and the E_1/2 values are: [NiL¹] = 0.74 and [NiL² = 0.75 V (vs SCE). Frozen (77 K) solutions of the electrogenerated nickel(III) species display rhombic EPR spectra: g₁ = 2.195, g₂ = 2.145, g₃ = 2.038 for [NiL¹⁺; g₁ = 2.199, g₂ = 2.138, g₃ = 2.035 for [NiL²]⁺. The g₁ and g₂ parameters for each complex can be considered as split components of g_⊥ of the idealized geometry affording the inequality g_⊥ > g_|, which corresponds to an effective axial elongation and (d_z2)¹ ground state.