An iron ketimide single-molecule magnet [Fe4(NCPh2)6] with suppressed through-barrier relaxation

Abstract

Reaction of FeBr₂ with 1.5 equiv. of LiNCPh₂ and 2 equiv. of Zn, in THF, results in the formation of the tetrametallic iron ketimide cluster [Fe₄(NCPh₂)₆] (1) in moderate yield. Formally, two Fe centers in 1 are Fe(I) and two are Fe(II); however, Mössbauer spectroscopy and SQUID magnetometry suggests that the [Fe₄]⁶⁺ core of 1 exhibits complete valence electron delocalization, with a thermally-persistent spin ground state of S = 7. AC and DC SQUID magnetometry reveals the presence of slow magnetic relaxation in 1, indicative of single-molecule magnetic (SMM) behaviour with a relaxation barrier of U_eff = 29 cm⁻¹. Remarkably, very little quantum tunnelling or Raman relaxation is observed down to 1.8 K, which leads to an open hysteresis loop and long relaxation times (up to 34 s at 1.8 K and zero field and 440 s at 1.67 kOe). These results suggest that transition metal ketimide clusters represent a promising avenue to create long-lifetime single molecule magnets.