C NMR Shifts as an Indicator of U-C Bond Covalency in Uranium(VI) Acetylide Complexes: An Experimental and Computational Study.

A series of uranium(VI)-acetylide complexes of the general formula U(O)(C≡C-CH-R)[N(SiMe)], with variation of the para substituent (R = NMe, OMe, Me, Ph, H, Cl) on the aryl(acetylide) ring, was prepared. These compounds were analyzed by C NMR spectroscopy, which showed that the acetylide carbon bound to the uranium(VI) center, U- C≡C-Ar, was shifted strongly downfield, with δ(C) values ranging from 392.1 to 409.7 ppm for Cl and NMe substituted complexes, respectively. These extreme high-frequency C resonances are attributed to large negative paramagnetic (σ) and relativistic spin-orbit (σ) shielding contributions, associated with extensive U(5f) and C(2s) orbital contributions to the U-C bonding in title complexes. The trend in the C chemical shift of the terminal acetylide carbon is opposite that observed in the series of parent (aryl)acetylenes, due to shielding effects of the para substituent. The C chemical shifts of the acetylide carbon instead correlate with DFT computed U-C bond lengths and corresponding QTAIM delocalization indices or Wiberg bond orders. SQUID magnetic susceptibility measurements were indicative of the Van Vleck temperature independent paramagnetism (TIP) of the uranium(VI) complexes, suggesting a magnetic field-induced mixing of the singlet ground-state (f) of the U(VI) ion with low-lying (thermally inaccessible) paramagnetic excited states (involved also in the perturbation-theoretical treatment of the unusually large paramagnetic and SO contributions to the C shifts). Thus, together with reported data, we demonstrate that the sensitive C NMR shifts serve as a direct, simple, and accessible measure of uranium(VI)-carbon bond covalency.