Computational investigations on the electronic structure and reactivity of thiourea dioxide: sulfoxylate formation, tautomerism and dioxygen liberation

Thiourea dioxide, (NH2)2CSO2 (TDO), is known to decompose in alkaline media to yield the strong reducing agent sulfoxylate, SO2H−, with interesting applications in (bio)catalysis. The electronic structure of TDO has been debated recently, with two different descriptions put forth based on crystal structures and electronic structure calculations, all gravitating around the unusually long C‒S bond in TDO: a zwitterionic structure and a carbene one. Here, computational methods (density functional and Hartree–Fock) are used to reconcile the two descriptions. The geometrical parameters as well as the molecular orbitals and population analyses are all consistent with TDO being an adduct of SO2 with the carbene (NH2)2C. The reactivity of TDO towards tautomerization, sulfoxylate formation and dioxygen liberation is also examined; all of the data points toward its tautomer, formamidine sulfinic acid (FSA), being the stable tautomer in solution, with TDO stabilized by supramolecular interactions in the solid state.