An iron ketimide single-molecule magnet [Fe4(N[double bond, length as m-dash]CPh2)6] with suppressed through-barrier relaxation.

Reaction of FeBr2 with 1.5 equiv. of LiN[double bond, length as m-dash]CPh2 and 2 equiv. of Zn, in THF, results in the formation of the tetrametallic iron ketimide cluster [Fe4(N[double bond, length as m-dash]CPh2)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.