Slow Magnetic Relaxation in a Rare, Neutral, Formally Divalent Terbium Bis(amide) Complex.

Neutral organometallic complexes containing a formally divalent terbium ion are especially scarce, owing to the highly negative TbIII/TbII reduction potential and the thermodynamic stability of the TbIII ion. In fact, there are only two crystallographically characterized neutral terbium(II) complexes known to date. Here, we present the synthesis of the unprecedented heteroleptic Tb(III) bis(amide) chloride complex, (NHAr*)2TbCl (1) (where Ar* = 2,6-(Ar')2C6H3, Ar' = 2,4,6-(iPr)3C6H2), which was chemically reduced using the strong reducing agent, KC8, to yield a new member of this small family of highly reactive TbII compounds, namely, the homoleptic terbium bis(amide) complex, (NHAr*)2Tb (2). Notably, the spectroscopic and magnetic characterization of 2 revealed that the compound contains a formally divalent terbium ion since the additional electron was exclusively found to reside in the primarily arene ligand-based π* orbitals, as corroborated by ab initio calculations. Furthermore, 2 exhibits slow magnetic relaxation between 1.8 and 16 K under a 1250 Oe applied dc field. Ab initio calculations uncovered that magnetic relaxation occurs through the first excited spin-orbit state. Study of 2 via SQUID magnetometry hints at considerably weaker 4f-5d magnetic coupling relative to other TbII complexes, rendering its electronic structure unique relative to that of other neutral organometallic TbII complexes.