Stereodynamics in Eight-Coordination;
A 2D NMR Spectroscopic
and Computational Study of
the Exchange Process in ThCl<sub>4</sub>(Me<sub>2</sub>NCH<sub>2</sub>CH<sub>2</sub>NMe<sub>2</sub>)<sub>2</sub>
- Publication date
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Abstract
The <sup>13</sup>C{<sup>1</sup>H} NMR spectrum of eight-coordinate
ThCl<sub>4</sub>(tmed)<sub>2</sub>, where tmed = Me<sub>2</sub>NCH<sub>2</sub>CH<sub>2</sub>NMe<sub>2</sub>, 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 <sup>13</sup>C{<sup>1</sup>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
