Crystal and Molecular Structure of Manganese(II) Lapacholate, a Novel Polymeric Species Undergoing Temperature-Reversible Metal to Ligand Electron Transfer

Lapachol (2-hydroxy-3-(3-methyl-2-butenyl)-1,4-naphtoquinone) (HLap, C15H14O3) reacts with Mn2+ producing a novel polymeric complex with formula: [Mn(Lap)2]n. Two ligands chelate the metal through their ortho oxygen (O1, O2) moiety while two para oxygens, from other Lap ligands, complete the octahedral coordination sphere. Thus far, all reported Lap metal complexes are mononuclear, lack the metal-trans-quinonic (para) oxygen binding and have Lap as a bidentate ligand. Synthesis, X-ray diffraction, IR, and UV-visible spectroscopic properties, thermogravimetric analysis, and differential thermal analysis of this complex are reported along with a density functional theory study describing electron transfer from the Mn to the Lap ligand at low temperature. X-ray structure determinations at 125, 197, and 300 K describe the progressive trend of a Mn contribution to the Mn-O1 bond length as a function of T. The Mn-O1 bond distance increases with temperature and may be therefore associated with a semiquinonate action at low T by the carbonyl O1 donor (and corresponding to MnIII). It transforms to a more classical coordinative bond at room T and stabilizes a MnII species; this is a reversible phenomenon involving MnII-MnIII oxidation states.