Stereodynamics in Eight-Coordination; A 2D NMR Spectroscopic and Computational Study of the Exchange Process in ThCl₄(Me₂NCH₂CH₂NMe₂)₂

Abstract

The (13)C{(1)H} NMR spectrum of eight-coordinate ThCl(4)(tmed)(2), where tmed = Me(2)NCH(2)CH(2)NMe(2), shows that two isomers are present at 219.8 K in a ratio of ≈8:1 and inversion of the five-membered Th-tmed ring is slow at this temperature in both isomers. The 2D (13)C{(1)H} exchange spectroscopy (EXSY) spectrum shows that each of the two inequivalent methyl groups of the major isomer does not exchange directly with each other but that they both exchange with both of the two inequivalent methyl groups found in the minor isomer. This implies that interconversion of the two enantiomers of the major isomer proceeds by a stepwise process that involves the minor isomer. The interconversion of the isomers involves a ring-inversion process that may proceed with or without Th-N bond breaking, and the NMR spectra cannot distinguish between these two processes nor can density functional theory (DFT) calculations (B3PW91 and M06 with consideration of dispersion effects and solvent) because these two possibilities proceed by way of transition states of similar energies in this case.

Eight coordinate 1,2-bis(dimethylarsino) and 1,2-bis(dimethylphosphino)-benzene complexes of uranium tetrachloride, UCl4[(1,2-Me2E)2C6H4]2 where E is As or P

“…The chemical and physical properties of the diars and diphos complexes of UCl 4 complete the destruction of the myth that the 5f-block metals thorium and uranium, and by implication some groups in the five-member chelate ring is rapid at 20ºC but slow at -50ºC, the exchange is still rapid at -80ºC in the UX 4 (dmpe) 2 complexes. 27…”

Synthesis and Characterization of Chelating Alkyl-borohydride Complexes of Thorium and Uranium: Th(CH2NMe2BH3)4 and U(CH2NMe2BH3)4

“…First, the metal character of the M–C bonds to the inner and outer positions of the D 2 d dodecahedron in 1 and 2 is not significantly different. Second, the U–C bonds in 2 have on average 2.4% more metal character than the Th–C bonds in 1 (Figure , Table S5); similar but slightly larger differences of 4–7% were predicted for the M–C bonds in [M­(C 6 Cl 5 ) 5 ] −8 and [M­(C 6 H 5 ) 6 ] − (M = Th, U). Third, the NLMOs corresponding to the M–C bonds have 7.3–10.3% and 9.0–12.0% more metal character than the NLMOs of each M–H–B bond in 1 and 2 , respectively.…”

“…In eight-coordinate compounds, the interconversion between dodecahedral, square antiprismatic, and bicapped trigonal prismatic geometries is typically almost barrierless . Accordingly, fluxional behavior is commonly observed in eight-coordinate compounds of thorium­(IV) and uranium­(IV). − Presumably, similar processes are responsible for the fluxionality of 1 and 2 , in which there is an exchange of ligated CH 2 and BH 3 groups between “inner” and “outer” positions of the D 2 d dodecahedron (Figure ).…”

The actinides

“…88 High-purity syntheses and structures are reported for (Ph 4 P) 2 [MCl 6 ] (M = Th, U, Np, Pu). 89 90 Low-temperature 13 C NMR spectroscopy of [ThCl 4 (tmed) 2 ] detects two isomers at 220 K. 91 The cation in [Th(OP(OCH 3 ) 3 ) 4 (O 2 P(OCH 3 ) 2 ) 2 ] 2 [ClO 4 ] 4 has tetragonal antiprismatic coordination of Th. 92…”