Uranium Metallocene Azides, Isocyanates, and Their Borane-Capped Lewis Adducts

Uranium(IV) metallocene complexes (Cp^iPr4)₂U(N₃)₂ (1-N), (Cp^iPr)₂U(NCO)₂ (1-NCO), and (Cp^iPr4)₂U(OTf)₂ (1-OTf) containing the bulky Cp^iPr4 ligand (Cp^iPr4 = tetra(isopropyl)cyclopentadienyl) were prepared directly from reactions between (Cp^iPr4)₂UI₂ or (Cp^iPr4)₂UI and corresponding pseudohalide salts. The mixed-ligand complex (Cp^iPr4)₂U(N₃)(OTf) (1-N-OTf) was isolated after heating a 1:1 mixture of 1-N and 1-OTf. The coordination of 1 equiv B(C₆F₅)₃ to 1-N produced the borane-capped azide (Cp^iPr4)₂U(N₃)[(μ-η¹:η¹-N₃)B(C₆F₅)₃] (2-N), while the reaction of 1 equiv B(C₆F₅)₃ with 1-NCO yielded (Cp^iPr4)₂U(NCO)[(μ-η¹:η¹-OCN)B(C₆F₅)₃] (2-NCO) in which the borane-capped cyanate ligand had rearranged to become O-bound to uranium. The reaction of (Cp^iPr4)₂UI and NaOCN led to the isolation of the uranium(III) cyanate-bridged "molecular square" [(Cp^iPr4)₂U(μ-η¹:η¹-OCN)]₄ (3-OCN). Cyclic voltammetry and UV-vis spectroscopy revealed small differences in the electronic properties between azide and isocyanate complexes, while X-ray crystallography showed nearly identical solid-state structures, with the most notable difference being the geometry of borane coordination to the azide in 2-N versus the cyanate in 2-NCO. Reactivity studies comparing 3-OCN to the azide analogue [(Cp^iPr4)₂U(μ-η¹:η¹-N₃)]₄ (3-N) demonstrated significant differences in the chemistry of cyanates and azides with trivalent uranium. A computational analysis of 1-NCO, 1-N, 2-NCO, and 2-N has provided a basis for understanding the energetic preference for specific linkage isomers and the effect of the B(C₆F₅)₃ coordination on the bonding between uranium, azide, and isocyanate ligands.