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

Four potential tetradentate ligands of formulae 1,2- bis-(3,5-di-tert-butyl-2-hydroxybenzamido)ethane (H4L1, 1), 1,2bis (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(L-3)Na-I(H2O)(2)] (5), [Co-III(L-n)Li-I(H2O)(2)] with n = 1 (6), 2 (7) and 3 (8) and [Co-II(L-4)Li-2(I)] (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 (L-3)(4-) anion build a slightly distorted square planar surrounding around the cobalt atom, the Co-N distances [1.858(3) and 1.861(3) A] being somewhat longer than the Co-O ones [1.798(3) and 1.801(3) A]. Magnetic and H-1 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.