Time-resolved Infrared Characterization on the Photolysis of Roussin's Red Phenyl Ester in Different Solvents

Abstract

Roussin's red esters (RRE) are regarded as a precursor to the release of a nitrogen monoxide upon irradiation. The 355-nm nanosecond pulsed laser was employed to initiate the photolysis of Roussin's red benzyl ester, Fe2(μ-SPh)2(NO)4, in different solvent compositions monitored with time-resolved step-scan Fourier-transform infrared spectroscopy. Density functional theory assisted in predicting the energetics and assignments of vibrational bands of the photolytic intermediates. The photolysis in cyclohexane (cHex) gives rise to two downward bands at 1785 and 1760 cm–1 owing to parent depletion coupled with an upward band at 1805 cm–1 that accounts for the symmetric NO stretch of Fe2(μ-SPh)2(NO)3. In a binary solvent of cHex/THF = 2/1, a quick coordination of THF to Fe2(μ-SPh)2(NO)3 forms Fe2(μ-SPh)2(NO)3(THF), which contributes to the two upward NO-associated vibrations at 1738 and 1717 cm–1. Similarly, the two upward features at 1740 and 1718 cm–1 attributed to Fe2(μ-SPh)2(NO)3(DX) were observed in a mixed solvent of cHex/1,4-dioxane(DX) = 2/1. In this work, we deliver direct spectroscopic evidence that cyclohexane serves as a weakly interactive medium to solvate the transient species, whereas THF and DX are capable of coordinating to the vacant sites of the photolytic intermediates via lone pair electrons on oxygen, leading to different transient species and cascading products. The combination of theoretical and time-resolved IR spectroscopic approaches successfully conquers the difficulties in the prediction of photolysis of RRE, suggesting further applicability in tracking the photochemistry of iron-containing species at the molecular level. Moreover, the distinct photolytic products characterised in the steady-state spectra ascertain the solvent-controlled selectivity in the organic reactions.