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

Zr(IV), Hf(IV), Th(IV), and U(IV) bis(amide) dichloride complexes have been synthesized utilizing bulky amide ligands incorporating arene (Ar) frameworks. X-ray crystallography and computations suggest that Th(N H Ar iPr6 ) 2 Cl 2 and U(N H Ar iPr6 ) 2 Cl 2 both have an η 6 -Ar interaction, while Zr(N H Ar iPr6 ) 2 Cl 2 and Hf(N H Ar iPr6 ) 2 Cl 2 do not show appreciable M−Ar interactions. Additionally, solid-state X-ray structures and computations suggest that the compounds of the type M(N Dipp SiMe 2 Bn) 2 Cl 2 (THF) x (x=0 for M=Zr ( 2-Zr ), U ( 2-U ), x=1 for M=Th ( 2-Th ) have no considerable M−Ar interactions. Addition of two equiv. of the bulky terphenyl amide salt Na[N H Ar iPr6 ] (Ar iPr6 = 2,6-(2,4,6- i Pr 3 C 6 H 2 ) 2 C 6 H 3 ) to the tetravalent halides ZrCl 4 (THF) 2 , HfCl 4 , ThCl 4 (DME) 1.54 (DME = 1,2-dimethoxyethane), and UCl 4 (DME) 1.32 led to the formation of the desired M(N H Ar iPr6 ) 2 Cl 2 (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)–(η 6 -arene) interaction with a 5-coordinate geometry. The coordination of the arene ring is shown to be highly labile on the NMR timescale through 1 H NMR spectroscopy, evincing only weak interactions keeping it coordinated to the metal. Computational analysis suggests the η 6 -arene hapticity through employing various bond orders and an electron localization methodology. Alternatively, addition of two equiv. of the silyl amide salt Na[N Dipp SiMe 2 Bn] (Dipp = 2,6- i Pr 2 C 6 H 3 ; SiMe 2 Bn=Si(CH 3 ) 2 (CH 2 C 6 H 5 )) to ZrCl 4 (THF) 2 , ThCl 4 (DME) 1.54 , and UCl 4 led to the formation of the desired M(N Dipp SiMe 2 Bn) 2 Cl 2 (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)–(η 0 -arene) and emphasize the importance of using computations along with experimental bond lengths to assign hapticity.