Uranium-Nitrogen Multiple Bonding: The Case of a Four-Coordinate Uranium(VI) Nitridoborate Complex

Reaction of the azidoborate salt [N(n-Bu)(4)][(C6F5)3B(N-3)] ([N(n-Bu)(4)][1]) with the uranium(III) tris(anilide) complex (THF)U(N[t-Bu]Ar)(3) (2; THF = tetrahydrofuran; Ar = 3,5-Me2C6H3) results in formation of the paramagnetic uranium (V) nitridoborate complex [N(n-Bu)(4)][(C6F5)(3)B N U(N[t-Bu]Ar)(3)] ([N(n-Bu)(4)][3]). Chemical oxidation of [N(n-Bu)(4)][3] is facile and provides the diamagnetic uranium(VI) nitridoborate complex (C6F5)(3)B N U(N[t-Bu]Ar)(3) (3). [N(n-Bu)(4)][3] and 3 are the first nitridoborate complexes of uranium and were characterized by multinuclear NMR spectroscopy, single crystal X-ray diffraction methods, and elemental analysis. The X-ray crystal structures of [N(n-Bu)(4)][3] and 3 reveal extremely short U N-nitrido distances (1.916(4) angstrom and 1.880(4) angstrom, respectively). Density functional theory was used to calculate the optimized structure of the truncated model (C6F5)(3)B N U(N[Me]Ph)(3); the procedure was carried out similarly for several other relevant complexes featuring U N multiple bonds. Bond multiplicities based on Nalewajski-Mrozek valence indices were calculated, the results of which suggest that the U N-nitrido interaction in 3 is close to a full triple bond.