A Uranium Tri-Rhenium Triple Inverse Sandwich Compound

Salt metathesis between the anionic rhenium(I) compound, Na[Re(eta(5)-Cp)(BDI)] (BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-beta-diketiminate), and the uranium(III) salt, UI3 (1,4-dioxane)(1.5), generated the triple inverse sandwich complex, U[(mu-eta(5):eta(5)-Cp)Re(BDI)(3), which was isolated and structurally characterized as the Lewis base adducts, (L)U[(mu-eta(5):eta(5)-Cp)Re(BDI)](3) (1.L, L = THF, 1,4-dioxane, DMAP). The assignment as one uranium(III) and three rhenium(I) centers was supported by X-ray crystallography, NMR and EPR spectroscopies, and computational studies. An unusual shortening of the rhenium-Cp bond distances in 1.L relative to Na[Re(eta(5)-Cp)(BDI)] was observed in the solid-state and reproduced in calculated structures of 1-THF and the anionic fragment, [Re(eta(5)-Cp)(BDI)](-). Calculations suggest that the electropositive uranium center pulls electron density away from the electron-rich rhenium centers, reducing electron-electron repulsions in the rhenium-Cp moieties and thereby strengthening those interactions, while also making uranium-Cp bonding more favorable.