P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry , University of Pennsylvania , 231 South 34th Street , Philadelphia , Pennsylvania 19104 , United States.
Department of Inorganic Chemistry, Faculty of Natural Sciences , Comenius University , SK-84215 Bratislava , Slovakia.
Inorg Chem. 2019 Apr 1;58(7):4152-4163. doi: 10.1021/acs.inorgchem.8b03175. Epub 2019 Mar 8.
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.
一系列铀(VI)-炔基配合物的通式为 U(O)(C≡C-CH-R)[N(SiMe)3]，其中 R 是芳基(炔基)环上的取代基，变化为 NMe、OMe、Me、Ph、H 和 Cl。这些化合物通过 C NMR 光谱分析，结果表明与铀(VI)中心结合的炔基碳，U-C≡C-Ar，强烈地向高场移动，δ(C)值分别为 Cl 和 NMe 取代的配合物的 392.1 到 409.7 ppm。这些极端高频的 C 共振归因于大的负磁(σ)和相对论自旋轨道(σ)屏蔽贡献，与标题配合物中铀(5f)和 C(2s)轨道对 U-C 键的广泛贡献有关。终端炔基碳的 C 化学位移的趋势与母体(芳基)乙炔系列相反，这是由于取代基的屏蔽效应。炔基碳的 C 化学位移反而与 DFT 计算的 U-C 键长和相应的 QTAIM 离域指数或 Wiberg 键序相关。SQUID 磁化率测量表明铀(VI)配合物的范维克温度独立顺磁(TIP)，这表明磁场诱导了 U(VI)离子的单重基态(f)与低能(热不可及)顺磁激发态之间的混合(这也涉及到对 C 位移的异常大顺磁和 SO 贡献的微扰理论处理)。因此，结合已报道的数据，我们证明了敏感的 C NMR 位移是铀(VI)-碳键共价性的直接、简单和可访问的度量。
