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

Reaction of the azidoborate salt N(n-Bu)(4)(C(6)F(5))(3)B(N(3)) (N(n-Bu)(4)1) with the uranium(III) tris(anilide) complex (THF)U(Nt-BuAr)(3) (2; THF = tetrahydrofuran; Ar = 3,5-Me(2)C(6)H(3)) results in formation of the paramagnetic uranium(V) nitridoborate complex N(n-Bu)(4)(C(6)F(5))(3)BNU(N[t-BuAr)(3)] (N(n-Bu)(4)3). Chemical oxidation of N(n-Bu)(4)3 is facile and provides the diamagnetic uranium(VI) nitridoborate complex (C(6)F(5))(3)BNU(Nt-BuAr)(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 UN(nitrido) distances (1.916(4) A and 1.880(4) A, respectively). Density functional theory was used to calculate the optimized structure of the truncated model (C(6)F(5))(3)BNU(NMePh)(3); the procedure was carried out similarly for several other relevant complexes featuring UN multiple bonds. Bond multiplicities based on Nalewajski-Mrozek valence indices were calculated, the results of which suggest that the UN(nitrido) interaction in 3 is close to a full triple bond.