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 Tb^III/Tb^II reduction potential and the thermodynamic stability of the Tb^III 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 Tb^II 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 Tb^II complexes, rendering its electronic structure unique relative to that of other neutral organometallic Tb^II complexes.