New anionic cobalt complexes using highly hindered bis-amides with varying donor abilities as ligands

Abstract

Four potential tetradentate ligands of formulae 1,2-bis-(3,5-di-tert-butyl-2-hydroxybenzamido)ethane (H₄L¹, 1), 1,2-bis-(3,5-di-tert-butyl-2-hydroxybenzamido)propane (H₄L², 2), 1,2-bis-(3,5-di-tert-butyl-2-hydroxybenzamido)benzene (H₄L³, 3) and 1,8-bis-(3,5-di-tert-butyl-2-hydroxybenzamido)naphthalene (H₄L⁴, 4) have been prepared and the crystal structures of three of them (1, 3 and 4) determined by single crystal X-ray diffraction. The investigation of their complexing ability toward Co(II) afforded the compounds of formulae [Co^III(L³)Na^I(H₂O)₂] (5), [Co^III(Lⁿ)Li^I(H₂O)₂] with n = 1 (6), 2 (7) and 3 (8) and [Co^II(L⁴)Li^I₂] (9). Complexes 5–8 are square planar Co(III) species, as corroborated by the crystal structure of 5. In this compound, two amide-nitrogen and two phenolate-oxygen atoms of a fully deprotonated (L³)⁴⁻ anion build a slightly distorted square planar surrounding around the cobalt atom, the Co–N distances [1.858(3) and 1.861(3) Å] being somewhat longer than the Co–O ones [1.798(3) and 1.801(3) Å]. Magnetic and ¹H NMR data at room temperature for 6–8 support the occurrence of an intermediate S = 1 low-lying state for the Co(III) center which is stabilized by the strong donating ability of the fully deprotonated bis-amidate ligands. In the case of the compound with the naphthalene derivative (9), the analytical and spectroscopic data suggest the occurrence of a low spin Co(II) complex. The weakening of the ligand field strength of the tetradentate bis-amidate ligand in the naphthalene derivative (5–6–5 ring-membered fused chelate) when compared to the situation in complexes 5–8 (5–5–5 ring-membered fused chelate) would account for this feature.