Cation-Bonding and Protonation of the Fe4-Square Cluster

Abstract

The synthesis and characterization of discrete, molecular iron-oxo clusters is pursued in the interest of molecular magnets, bioinspired materials and models for the geochemical aqueous-mineral interface. Iron-oxo clusters are challenging to synthesize in water, due to the extremely acidic and reactive nature of dissolved iron species, and thus require chelating ligands to passivate and neutralize the cluster surface. The 2-hydroxy-1,3-N,N,N′,N′-diamino-propanetetraacetic acid (HPDTA) ligand has been used to isolate several Al and Fe cluster geometries, including the square clusters Fe4(HPDTA)2 and Al4(HPDTA)2. While prior reports on the Fe4(HPDTA)2 cluster have focused on the magnetic properties, no solution characterization has been carried out. Using electrospray ionization mass-spectrometry (ESI-MS) we show this anionic Fe4(HPDTA)2 cluster can be dissolved intact in water, and recrystallized with virtually any metal as a countercation. The bonding of the metal cation to the square face of the cluster trends with ionic radii of the cations, as shown by structural characterization of Fe4(HPDTA)2 with Li+, Na+, Cs+, Mg2+, Ba2+, La3+, Eu3+, and Zn2+. This trend is similar to that observed for association of cations on metal oxide surfaces in the environment. Furthermore, protonation of the bridging oxo ligands of this series of Fe4(HPDTA)2 clusters is variable (0, 1, or 2 protons), and structures as a function of protonation is discussed. This paper, largely structural in nature, sets the foundation for future aqueous phase studies of iron-oxo molecular clusters as models for the oxide-water interface in the natural aqueous environment.