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

Abstract

Uranium(IV) metallocene complexes (CpiPr4)2U(N3)2 (1-N3), (CpiPr)2U(NCO)2 (1-NCO), and (CpiPr4)2U(OTf)2 (1-OTf) containing the bulky CpiPr4 ligand (CpiPr4 = tetra(isopropyl)cyclopentadienyl) were prepared directly from reactions between (CpiPr4)2UI2 or (CpiPr4)2UI and corresponding pseudohalide salts. The mixed-ligand complex (CpiPr4)2U(N3)(OTf) (1-N3-OTf) was isolated after heating a 1:1 mixture of 1-N3 and 1-OTf. The coordination of 1 equiv B(C6F5)3 to 1-N3 produced the borane-capped azide (CpiPr4)2U(N3)[(μ-η1:η1-N3)B(C6F5)3] (2-N3), while the reaction of 1 equiv B(C6F5)3 with 1-NCO yielded (CpiPr4)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 (CpiPr4)2UI and NaOCN led to the isolation of the uranium(III) cyanate-bridged "molecular square" [(CpiPr4)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-N3 versus the cyanate in 2-NCO. Reactivity studies comparing 3-OCN to the azide analogue [(CpiPr4)2U(μ-η1:η1-N3)]4 (3-N3) demonstrated significant differences in the chemistry of cyanates and azides with trivalent uranium. A computational analysis of 1-NCO, 1-N3, 2-NCO, and 2-N3 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.