FT-ICR STUDIES OF SPUTTERED METAL CLUSTER IONS .5. THE CHEMISTRY OF IRON CLUSTER CATIONS WITH AMMONIA AND HYDRAZINE

In an FT-ICR mass spectrometer, bare Fe(n)+ clusters in the size range n = 2-13 are produced by sputtering, thermalized, and reacted with ammonia and hydrazine. Their reactivity toward ammonia reveals a pronounced size effect. Whereas Fe2+ and Fe3+ behaved completely inert, Fe5+ through Fe13+ ions merely showed adsorption of intact ammonia molecules. In contrast, Fe4+ ions proved highly reactive, dehydrogenating the ammonia. Intermediates Fe4(NH)m+ with m = 1, 2 were formed which suggest the existence of two binding sites in the Fe4+ cluster. With hydrazine, a totally different behavior was observed. Here, reactivity rises from Fe2+ to Fe8+ in a rather smooth function of cluster size to stay nearly constant from there on. In general, Fe(n)+ cluster ions exhibit a variety of reactions without distinct size specificity. Fe2+ ions bind hydrazine under asymmetric or under symmetric dissociation to produce Fe2(NH)+ or Fe2(NH2)+. Fe3+ ions are the first clusters to add under dehydrogenation a hydrazine fragment with an intact N2 unit, thereby yielding Fe3(N2H2)+. From Fe4+ through Fe13+, hydrazine is adsorbed under elimination of two hydrogen molecules to yield adduct clusters of the type Fe(n)N2+. CID studies of Fe6N2+ allow the conclusion that the N2 unit is neither a physisorbed nitrogen molecule nor is it totally dissociated on the cluster surface.