Synthesis, Thermochemistry, Bonding, and ¹³C NMR Chemical Shift Analysis of a Phosphorano-Stabilized Carbene of Thorium

Reaction of the Th­(IV) metallacycle [Th­(CH₂SiMe₂NSiMe₃)­(NR₂)₂] (1; R = SiMe₃), with Ph₃PCH₂ affords the Th­(IV) carbene [Th­(CHPPh₃)­(NR₂)₃] (2) in good yield. In solution, complex 2 exists in equilibrium with complex 1 and free ylide, Ph₃PCH₂. The thermodynamic parameters of this equilibrium were probed using variable-temperature NMR spectroscopy, and these results are compared to those collected for the isostructural U­(IV) complex [U­(CHPPh₃)­(NR₂)₃]. X-ray diffraction studies, together with NMR spectroscopic data and DFT calculations, provide clear evidence for actinide–carbon multiple bonding in the title complex 2, which features the shortest Th–C distance measured thus far. This interaction is best characterized as a strongly polarized σ­(Th–C) bond augmented by a three-center two-electron π­(Th–C–P)-type interaction. In addition, ¹³C NMR chemical shifts of carbon atoms bonded to the thorium center were identified as quantitative measures of the An–C bond covalency for a series of structurally related Th carbenes.