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 generalformula UVI(O)(CC-C6H4-R)[N(SiMe3)2]3, with variation of the parasubstituent (R = NMe2, OMe, Me, Ph, H, Cl) on the aryl(acetylide) ring, wasprepared. These compounds were analyzed by 13C NMR spectroscopy, whichshowed that the acetylide carbon bound to the uranium(VI) center, U-CC-Ar, was shifted strongly downfield, withδ(13C) values ranging from 392.1to 409.7 ppm for Cl and NMe2 substituted complexes, respectively. Theseextreme high-frequency13C resonances are attributed to large negativeparamagnetic (σpara) and relativistic spin−orbit (σSO) shielding contributions,associated with extensive U(5f) and C(2s) orbital contributions to the U−Cbonding in title complexes. The trend in the 13C chemicalshift of the terminalacetylide carbon is opposite that observed in the series of parent(aryl)acetylenes, due to shielding effects of the para substituent. The 13Cchemical shifts of the acetylide carbon instead correlate with DFT computedU−C bond lengths and corresponding QTAIM delocalization indices or Wiberg bond orders. SQUID magnetic susceptibilitymeasurements 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 (thermallyinaccessible) paramagnetic excited states (involved also in the perturbation-theoretical treatment of the unusually largeparamagnetic and SO contributions to the 13C shifts). Thus, together with reported data, we demonstrate that the sensitive 13CNMR shifts serve as a direct, simple, and accessible measure of uranium(VI)-carbon bond covalency.