Synthesis and characterization of iron(II) and iron(III) complexes with a tridentate O,N,O′-ligand

• The coordination chemistry of the pyrazol-5-ol ligand precursor 1 was studied.
• 1 forms with Fe(OAc)2 and DMAP the complexes 2 [Fe(II)(1-2H)(DMAP)3].
• 1 forms with Fe(OAc)2 and PPh3 the complexes 3 [Fe(III)(1-2H)(OPPh3)(OEt)]2.
• The ligand coordinates in a O,N,O′-tridentate fashion and stabilizes different oxidation states.

The coordination chemistry of the ligand precursor 1-benzoyl-4,5-dihydro-3,5-bis(trifluoromethyl)-1H-pyrazol-5-ol (1a) to iron(II) acetate was studied. In dependence of the added co-ligand different complex geometries were observed. In case of 4-dimethylaminopyridine (DMAP) as co-ligand an octahedral iron(II) complex was found with an O,N,O′-coordination of the tridentate ligand (1a-2H), in which the ligand is planar, the oxygen donors are trans to each other, and the nitrogen donor is in a cis position. The other coordination sites on the iron center are occupied by DMAP ligands. In contrast to that, with triphenylphosphane as the co-ligand an oxidation process took place, which revealed an octahedral iron(III) complex with a comparable geometry for the tridentate ligand (O,N,O′-coordination, 1a-2H) demonstrating the usefulness of 1a-2H to stabilize different oxidation states. The additional coordination sites are occupied by one triphenylphosphane oxide and ethoxide ligands. Interestingly, the ethoxide ligands act as bridging ligands to form a bimetallic complex.

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