Versatility of 2,6-diacetylpyridine (dap) hydrazones in stabilizing uncommon coordination geometries of Mn(ii): synthesis, spectroscopic, magnetic and structural characterization

Abstract

Five seven- or eight-coordinate manganese complexes of hydrazone ligands have been prepared. Three seven-coordinate neutral Mn^II complexes: [Mn(dapA₂)]_n (1), [Mn(dapB₂)(H₂O)₂] (2), [Mn(dapS₂)(H₂O)₂] (3) have been synthesized from the bis-Schiff bases of 2,6-diacetylpyridine: dap(AH)₂, dap(BH)₂ and dap(SH)₂ (AH = anthraniloyl hydrazide, BH = benzoyl hydrazide, SH = salicyloyl hydrazide), respectively. Two eight-coordinate Mn^II complexes: [Mn(dapS)₂] (4) and [Mn(dapB)₂]·3H₂O (5) have been synthesized from the mono-Schiff bases dapBH and dapSH, respectively. The complexes have been characterized by elemental analyses and by IR, UV-Vis., FAB mass, EI mass and EPR spectroscopy. The molecular structures of 1, 3·DMF and 4·DMF have been determined by single-crystal X-ray diffraction. The mono-Schiff bases are monoanionic and the bis-Schiff bases are dianionic. The octa-coordinated mono-Schiff base complex 4 adopts a dodecahedral geometry, while the hepta-coordinated bis-Schiff base complex 1 forms a one-dimensional linear polymeric chain. A weak antiferromagnetic exchange interaction (J = −0.15 cm⁻¹) between the Mn(II) ions in 1 is attributed to weak Mn⋯Mn interaction through the PhNH₂ moiety of the ligand, as indicated by extended-Hückel molecular orbital calculations. A good simulation of the EPR spectrum of a frozen solution (DMSO at 4 K) of compound 1 was obtained with g = 2.0, D = 0.1 cm⁻¹, E = 0.01 cm⁻¹. The EPR spectrum of a powdered sample of compound 1 shows a large broadening of the signal, due in part, to the important zero-field splitting of the hepta-coordinated Mn(II) ion.