Tuning of the charge in octahedral ferric complexes based on pyridoxal-N-substituted thiosemicarbazone ligands

Abstract

Four novel mononuclear coordination compounds namely: [Fe(Hthpy)₂](SO₄)_1/2·3.5H₂O 1, [Fe(Hthpy)₂]NO₃·3H₂O 2, [Fe(H₂mthpy)₂](CH₃C₆H₄SO₃)₃·CH₃CH₂OH 3 and [Fe(Hethpy)(ethpy)]·8H₂O 4, (H₂thpy = pyridoxalthiosemicarbazone, H₂mthpy = pyridoxal-4-methylthiosemicarbazone, H₂ethpy = pyridoxal-4-ethylthiosemicarbazone), were synthesized in the absence or presence of organic base, Et₃N and NH₃. Compounds 1 and 2 are monocationic, and were prepared using the singly deprotonated form of pyridoxalthiosemicarbazone. Both compounds crystallise in the monoclinic system, C2/c and P2₁/c space group for 1 and 2, respectively. Complex 3 is tricationic, it is formed with neutral bis(ligand) complex and possesses an interesting 3D channel architecture, the unit cell is triclinic, P space group. For complex 4, the pH value plays an important role during its synthesis; 4 is neutral and crystallises with two inequivalent forms of the ligand: the singly and the doubly deprotonated chelate of H₂ethpy, the unit cell is monoclinic, C2/c space group. Notably, in 1 and 4, there is an attractive infinite three dimensional hydrogen bonding network in the crystal lattice. Magnetic measurements of 1 and 4 revealed that a rather steep spin transition from the low spin to high spin Fe(III) states occurs above 300 K in the first heating step. This transition is accompanied by the elimination of solvate molecules and thus, stabilizes the high spin form due to the breaking of hydrogen bonding networks; compared to 2 and 3, which keep their low spin state up to 400 K.