Nitroso-oxime tautomerism in nitroso pyrroles: Revealing spectral properties and crystalline structure

• Evidence of nitroso-oxime tautomerism supported by crystal structure analysis. • Variable temperature NMR reveals equilibrium between nitroso and oxime forms. • First crystal structures of oxime tautomers in alpha -nitroso thienylpyrroles. • Resonance assisted H-bonding stabilizes oxime tautomers in the solid state. Tautomerism plays a fundamental role in chemical reactivity, molecular recognition, and hydrogen bonding networks, influencing the stability and properties of many functional molecules. Among the various tautomeric equilibria, the nitroso-oxime interconversion has been poorly studied. In this work, a series of nine α-nitroso pyrroles containing thienyl moieties have been synthesized, characterized and studied by spectroscopic and crystallographic techniques. Tautomerism was evidenced by variable-low temperature 1 H NMR experiments. In addition, the crystallographic data of seven of them and four precursor pyrroles, allowed a comparative analysis of the supramolecular interactions in these heterocycles. Analysis of pyrroles revealed that N-H···S, S···π, C-H···π, and Br···Br interactions were responsible of crystal structure stability, whereas the α-nitroso pyrroles crystallize in centrosymmetric space groups with different arrangements. Notably, five of the nitroso derivatives revealed a centrosymmetric N-H···O dimeric pattern leading to an R 2 2 (10) dimeric synthon. Interestingly, two of the nitroso pyrroles crystallized in their oxime tautomeric form, as evidenced by a significantly shorter C ipso –N nitroso (1.294 Å) bond and an elongated N–O bond (1.367 Å), in contrast to the nitroso tautomeric forms (C–N: 1.336 Å, N–O: 1.277 Å). Cooperative strong O-H···N hydrogen bonds give rise to an intriguing R 4 4 (20) tetrameric synthon in the asymmetric unit for the oxime tautomeric forms. These hydrogen bonds are classified as resonance-assisted hydrogen bonds (RAHBs) and contribute significantly to the stabilization of the oxime form in the solid state. This study underscores how supramolecular synthons and RAHBs influence tautomerism in heterocyclic systems, offering valuable insights into the cooperative nature of hydrogen bonding. These results redefine the structural landscape of nitroso-pyrroles and open new perspectives for their reactivity and potential applications in organic synthesis and materials science.