Multimetallic clusters serve as critical reaction centers in biological and industrial systems, and efforts to understand these systems have resulted in the development of molecular analogues for which atomically resolved insight is facile. However, elucidating the effect of metal-metal and metal-ligand bonding is complicated by changes in geometry and nuclearity during reaction courses. Using a templating ligand, we have developed a series of [Fe3], [Fe2Zn], and [FeZn2] clusters which vary the number of "active" ferrous ions through substitution with “spectator” zinc ions. From these precursors, we have prepared homologous series of μ3-nitrido and μ3-hydrido clusters, accessed via photolytic cleavage of azide precursors, or by the direct addition of H–, respectively.
In the case of the nitrido clusters, structural and spectroscopic evidence indicates that the addition of zinc ions to the [M3] core results in a substantial increase in Fe—N multiple-bonding, which is accompanied by a destabilization of the resulting mixed-metal [M3(μ3-N)] clusters. The reactions of the nitrides with nucleophiles and electrophiles (namely PMe2Ph and MeI), was also investigated and correlated with the metal composition of the trimetallic core. Complementary studies of the hydrido clusters reveals that each species adopts a high-spin electron configuration, representing unusual examples of structurally and spectroscopically characterized paramagnetic iron hydrides. The nature of the iron-iron interactions depend strongly on the core composition, and strong ferromagnetic coupling in the [Fe3(μ3-H)] case gives rise to single-molecule magnetism. For both families of clusters, the impact of iron substitution toward the overall electronic structure was studied by cyclic voltammetry and SQUID magnetometry. These studies clarify the role of metal-metal cooperativity and metal-ligand bonding in the behavior of polynuclear reaction centers.