Structural characterization of iron azadiethylthiolate complexes: Insights from NMR, crystallography, and ion mobility-mass spectroscopy

Fe₃(CO)₁₂ reacted with azadiethylthiols afforded [Fe₂(μ-S(CH₂)₂NR(CH₂)₂S)(CO)₆]₂ (R = ⁿPr, 1A; ⁱPr, 2A). Structure analysis revealed that 1A features the anti-(a,e,a,e) configuration, while its iso-propyl 2A adopts the syn-(a,e,a,e) arrangement. NMR measurements at room temperature confirmed this structure, showing distinct signals for 2 NCH₂ and 2 SCH₂ methylene groups. Acid treatment of 1–2 yielded N-protonated species (1A–H, 2A–H), maintaining the anti-(a,e,a,e) configuration. Isolation of 1B–H revealed both the syn-(a,e,e,a) and anti-(a,e,e,a) configuration by X-ray single-crystallography. Protonation of 1A–H induced downfield shifts in methylene proton signals, with NH hydrogen recorded at 6.96 ppm. Furthermore, 1B–H displays similar NMR behavior, indicating the coexistence of two isomeric molecules of 1B–H, syn-(a,e,e,a) and anti-(a,e,e,a), which is consistent with the crystallographic results. Moreover, ion mobility-mass spectrometry (IM-MS) is harnessed for the swift identification and differentiation of these isomeric complexes in solution. By leveraging accurate mass measurements and MS/MS analyses, the molecular formula and constituents are validated. The sample mobility enables straightforward characterization of 1A, 1B, and 2A in relation to their stereo- and regio-configurations, obviating the need for extensive time or sample quantities. Integration of findings from X-ray crystallography, NMR, and IM-MS furnishes precise and comprehensive structural insights into the coordination complexes.