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

Uranium(IV) metallocene complexes (Cp-iPr4)(2)U(N-3)(2) (1-N-3), (Cp-iPr)(2)U(NCO)(2) (1-NCO), and (Cp-iPr4)(2)U(OTf)(2) (1 -OTf) containing the bulky Cp-iPr4 ligand (Cp-iPr4 = tetra(isopropyl)-cyclopentadienyl) were prepared directly from reactions between (Cp-iPr4)(2)UI2 or (Cp-iPr4)(2) UI and corresponding pseudohalide salts. The mixed-ligand complex (Cp-iPr4)(2)U(N-3)(OTf) (1-N-3-OTf) was isolated after heating a 1:1 mixture of 1-N-3 and 1-OTf. The coordination of 1 equiv B(C6F5)(3) to 1-N-3 produced the borane-capped azide (Cp-iPr4)(2)U(N-3)[(mu-eta(1):eta(1)-N-3)B(C6F5)(3)] (2-N-3), while the reaction of 1 equiv B(C6F5)(3) with 1-NCO yielded (Cp-iPr4)(2)U(NCO)[(mu-eta(1):eta(1)-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)(2)UI and NaOCN led to the isolation of the uranium(III) cyanate-bridged molecular square [(Cp-iPr4)(2)U(mu-eta(1):eta(1)-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-3 versus the cyanate in 2-NCO. Reactivity studies comparing 3-OCN to the azide analogue [(Cp-iPr4)(2)U(mu-eta(1):eta(1)-N-3)](4) (3-N-3) demonstrated significant differences in the chemistry of cyanates and azides with trivalent uranium. A computational analysis of 1-NCO, 1-N-3, 2-NCO, and 2-N-3 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.