Magnetic phase transition in valence tautomers as polymorphs of 3-iodolawsone: Single crystal X-ray structure, DSC and EPR studies

Three polymorphs of 3-iodolawsone (I–III) have been isolated and characterized by single crystal X-ray diffraction, electron paramagnetic resonance (EPR) spectroscopy and differential scanning calorimetry (DSC) techniques. Polymorphs I and II were crystallized from methanol in two different valence tautomeric forms: hydroxynaphthoquinone (HNQ) and hydroxynaphthosemiquinone (HNSQ) as needles and thick plates, respectively. Polymorph III was crystallized from ethanol in HNQ form, as thin plates. Chiral crystals of I belongs to the orthorhombic P212121 space group; II and III belong to the monoclinic non-centrosymmetric space group Cc. The molecules are packed via OH⋯O bonding and π⋯π stacking interactions. The study reveals that relatively stronger intermolecular H-bonding (2.02 Å) and most favoured π⋯π stacking (∼3.49 Å) interactions lead II to HNSQ radicals, presence of which was confirmed by EPR (g = 2.0052) spectroscopy. Interestingly, these materials exhibit crystal-to-crystal magnetic phase transition from polymorph I (diamagnetic) to polymorph II (paramagnetic structure) at 171.1 °C as identified by DSC and X-ray crystallographic studies.