Crystallizing Elusive Chromium Polycations

Metal-oxo clusters are commonly isolated from water with ligands for preventing the precipitation of metal hydroxides. However, unligated clusters are important for both mechanistic studies and applications. Isolating open-shell transition-metal clusters is especially difficult without ligation. Herein, we elucidate a cluster-isolation process that yields an unligated Zn2+-Al3+-Cr3+ polyoxocation. For decades, Cr3+-polyoxocations have proved elusive synthetic targets because of hydrolytic instability. We overcame the synthetic challenges by the following strategies: (1) pH-driven hydrolysis by oxidative dissolution of zinc; (2) metal nitrate concentrations 10x higher than conventional syntheses, suppressing cluster assembly; and (3) azeotropic evaporation of HNO3-H2O, driving cluster assembly and crystallization at the solution surface. Contrary to common cluster growth, the fully assembled cluster is never detected in the reaction solution. Because these reactive clusters do not persist in solution, uncontrolled precipitation of metal hydroxide is avoided. The proposed formation pathway opens opportunities to expand the composition space of metal-oxo clusters.