CHARACTERIZATION OF IRON SULFUR CUBANE CLUSTERS BY FAST-ATOM-BOMBARDMENT MASS-SPECTROMETRY - THE FORMATION OF IONIC [FE(M)S(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(m)S(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.