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

Abstract

Uranium(IV) metallocene complexes (Cp^iPr4)₂U(N₃)₂ (<b>1-N</b>_<b>3</b>), (Cp^iPr)₂U(NCO)₂ (<b>1-NCO</b>), and (Cp^iPr4)₂U(OTf)₂ (<b>1-OTf</b>) 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) (<b>1-N</b>_<b>3</b><b>-OTf</b>) was isolated after heating a 1:1 mixture of <b>1-N</b>_<b>3</b> and <b>1-OTf</b>. The coordination of 1 equiv B(C₆F₅)₃ to <b>1-N</b>_<b>3</b> produced the borane-capped azide (Cp^iPr4)₂U(N₃)[(μ-η¹:η¹-N₃)B(C₆F₅)₃] (<b>2-N</b>_<b>3</b>), while the reaction of 1 equiv B(C₆F₅)₃ with <b>1-NCO</b> yielded (Cp^iPr4)₂U(NCO)[(μ-η¹:η¹-OCN)B(C₆F₅)₃] (<b>2-NCO</b>) 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 (<b>3-OCN</b>). 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 <b>2-N</b>_<b>3</b> versus the cyanate in <b>2-NCO</b>. Reactivity studies comparing <b>3-OCN</b> to the azide analogue [(Cp^iPr4)₂U(μ-η¹:η¹-N₃)]₄ (<b>3-N</b>_<b>3</b>) demonstrated significant differences in the chemistry of cyanates and azides with trivalent uranium. A computational analysis of <b>1-NCO</b>, <b>1-N</b>_<b>3</b>, <b>2-NCO</b>, and <b>2-N</b>_<b>3</b> 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.