Formation and stability of Zr(IV), Hf(IV), Th(IV), and U(IV) bis(amide) dichloride complexes using bulky amide ligands: Insights into metal-arene interactions and hapticity

Addition of two equiv. of the bulky terphenyl amide salt Na[N^HAr^iPr6] (Ar^iPr6 = 2,6-(2,4,6-ⁱPr₃C₆H₂)₂C₆H₃) to the tetravalent halides ZrCl₄(THF)₂, HfCl₄, ThCl₄(DME)_1.54 (DME = 1,2-dimethoxyethane), and UCl₄(DME)_1.32 led to the formation of the desired M(N^HAr^iPr6)₂Cl₂ (M = Zr (1-Zr), Hf (1-Hf), Th (1-Th), U (1-U)) in good to moderate yields. The crystal structures of 1-Th and 1-U are particularly interesting as they exhibit an uncommon example of an actinide(IV)–(η⁶-arene) interaction with a 5-coordinate geometry. The coordination of the arene ring is shown to be highly labile on the NMR timescale through ¹H NMR spectroscopy, evincing only weak interactions keeping it coordinated to the metal. Computational analysis suggests the η⁶-arene hapticity through employing various bond orders and an electron localization methodology. Alternatively, addition of two equiv. of the silyl amide salt Na[N^DippSiMe₂Bn] (Dipp = 2,6-ⁱPr₂C₆H₃; SiMe₂Bn=Si(CH₃)₂(CH₂C₆H₅)) to ZrCl₄(THF)₂, ThCl₄(DME)_1.54, and UCl₄ led to the formation of the desired M(N^DippSiMe₂Bn)₂Cl₂(THF)_x (x = 0 for M = Zr (2-Zr), U (2-U), x = 1 for M = Th (2-Th) in moderate yields. The crystal structures of 2-Zr, 2-Th, and 2-U hint at possible metal–arene interactions due to relatively short M−C_ipso distances. However, low bond orders and meager metal–arene charge transfer signify these complexes are most accurately described as M(IV)–(η⁰-arene) and emphasize the importance of using computations along with experimental bond lengths to assign hapticity.