13C 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 UVI(O)(C≡C-C6H4-R)[N(SiMe3)2]3, with variation of the para substituent (R = NMe2, OMe, Me, Ph, H, Cl) on the aryl(acetylide) ring, was prepared. These compounds were analyzed by 13C NMR spectroscopy, which showed that the acetylide carbon bound to the uranium(VI) center, U- C≡C-Ar, was shifted strongly downfield, with δ(13C) values ranging from 392.1 to 409.7 ppm for Cl and NMe2 substituted complexes, respectively. These extreme high-frequency 13C resonances are attributed to large negative paramagnetic (σpara) and relativistic spin-orbit (σSO) shielding contributions, associated with extensive U(5f) and C(2s) orbital contributions to the U-C bonding in title complexes. The trend in the 13C 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 13C 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 (f0) 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 13C shifts). Thus, together with reported data, we demonstrate that the sensitive 13C NMR shifts serve as a direct, simple, and accessible measure of uranium(VI)-carbon bond covalency.