An intermediate structure trapped in solid-state tautomerization process of (E)-4-[(4-chlorophenylimino)methyl]benzene-1,2,3-triol

Structural and aromatic aspects for tautomerism of (Z)-6-((4-bromophenylamino)methylene)-2,3-dihydroxycyclohexa-2,4-dienone

“…We have recently reported an intermediate structure [13] between canonical OH and NH form of a Schiff base compound, having very similar molecular skeleton to that in this study. We show that structural characterization of SA derivatives having regard to only the positions of their tautomeric proton may lead to overlook certain details related to molecular structures of them.…”

“…Although SA derivative reported here is in NH form, its geometric parameters are deviate from their relevant values. These differentiations in certain bond distances can be explained by the resonance effect between two canonical structures, the cis-keto tautomer and zwitterionic form [5,12,13]. Throughout solid-state tautomerization, the proton initially attached to O1 is transferred to N1 by forming N-HÁÁÁO type resonance-assisted H-bond (RAHB) whose details are given in Table 3, thereby introducing with appreciable keto character to the C13-O1 bond and partial double bond character to the C7-C8 due to the resonance effect mentioned before.…”

“…-H bond, because their molecular structures in the solid state can be regarded as a resonance hybrid of two canonical structures, cis-keto tautomer and zwitterionic form [5,12] (Chart 1a). The predominance of the zwitterionic over NH canonic form of Schiff bases can be discerned from details of their molecular geometries [13]. In this regard, scrutiny of crystal structure combined with quantum chemical studies supplies an efficient probe to be used in their structural classification.…”

“…Structural classification of their NH forms is intricate in solid state due to their intermediate bond distances [13]. As to this aspect, Ogawa and Harada [5,14] have proposed that NH form of SA derivatives is predominantly zwitterionic in crystals, while it is predominantly quinoidal (canonical NH) in gas-phase or solution and their stabilization is achieved by intermolecular H-bonds in molecular aggregate at low temperature.…”

The proton transfer process observed in the structure analysis and DFT calculations of (E)-2-ethoxy-6-[(2-methoxyphenylimino)methyl]phenol

“…Considering other tautomeric forms, it can be stated that OH and NH forms of o-hydroxy Schiff bases may coexist in the same crystal structure [15] or in the same molecular structure [16]. In general, o-hydroxy Schiff bases in NH form exist as resonance hybrid of zwitterionic and cis-quinoid [16] or OH and cisquinoid tautomers [17,18]. Electronic p-delocalization in the chelate ring facilitates to stabilize such molecules and determines the type of tautomer.…”