Characterization of Iron-Sulfur Cubane Clusters by Fast Atom Bombardment Mass Spectrometry: The Formation of Ionic [Fe_mS_n] Clusters through Gas-Phase Unimolecular Reduction Processes and Their Solution Parallels

Fast atom bombardment mass spectrometry (FAB-MS) has been used to analyze a series of iron-sulfur clusters (A)₂Fe₄S₄X₄, where A = R₄N or Ph₄P and X = Cl, Br, SEt, SPh. A cluster with mixed Cl, SPh ligands was also studied. The usefulness of the FAB technique in characterizing these and related biologically-relevant complexes is evaluated. The best FAB-MS results for these clusters were obtained with 3-nitrobenzyl alcohol (NBA) and 2-nitrophenyl octyl ether (NPOE) as matrices. The most unique feature of the negative ion FAB mass spectra is the identification of the intact ionic core [Fe₄S₄X₄]^-, preformed anions [(A)Fe₄S₄X₄]^-, and a series of cluster fragment ions. A mechanism is proposed to explain the formation of small [Fe_mS_n] clusters through unimolecular reduction processes that involve only +2 and +3 oxidation states for the Fe atoms. This work demonstrates that FAB-MS can be employed as a valid method for rapid molecular weight determination as well as structural elucidation of [Fe₄S₄] cluster-containing complexes.