Metal-Metal-Bonded Fe 4 Clusters with Slow Magnetic Relaxation.

Reaction of FeBr 2 with Li(N═C t Bu 2 ) (0.5 equiv) and Zn 0 (2 equiv) results in the formation of the formally mixed-valent cluster [Fe 4 Br 2 (N═C t Bu 2 ) 4 ] ( 1 ) in moderate yield. The subsequent reaction of 1 with Na(N═C t Bu 2 ) results in formation of [Fe 4 Br(N═C t Bu 2 ) 5 ] ( 2 ), also in moderate yield. Both 1 and 2 were characterized by zero-field 57 Fe Mössbauer spectroscopy, X-ray crystallography, and superconducting quantum interference device magnetometry. Their tetrahedral [Fe 4 ] 6+ cores feature short Fe-Fe interactions (ca. 2.50 Å). Additionally, both 1 and 2 display S = 7 ground states at room temperature and slow magnetic relaxation with zero-field relaxation barriers of U eff = 14.7(4) and 15.6(7) cm -1 , respectively. Moreover, AC magnetic susceptibility measurements were well modeled by assuming an Orbach relaxation process.