Tuning the structure, reactivity and magnetic communication of nitride-bridged uranium complexes with the ancillary ligands

Molecular uranium nitride complexes were prepared to relate their small molecule reactivity to the nature of the U & xe001;N & xe001;U bonding imposed by the supporting ligand. The U4+-U4+ nitride complexes, [NBu4][{(((BuO)-Bu-t)(3)SiO)(3)U}(2)(mu-N)], [NBu4]-1, and [NBu4][((Me3Si)(2)N)(3)U}(2)(mu-N)], 2, were synthesised by reacting NBu4N3 with the U3+ complexes, U(OSi((OBu)-Bu-t)(3))(2)(mu-OSi((OBu)-Bu-t)(3)) and [U(N(SiMe3)(2))(3)], respectively. Oxidation of 2 with AgBPh4 gave the U4+-U5+ analogue, [((Me3Si)(2)N)(3)U}(2)(mu-N)], 4. The previously reported methylene-bridged U4+-U4+ nitride [Na(dme)(3)][((Me3Si)(2))(2)U(mu-N)(mu-kappa(2)-C,N-CH2SiMe2NSiMe3)U(N(SiMe3)(2))(2)] (dme = 1,2-dimethoxyethane), [Na(dme)(3)]-3, provided a versatile precursor for the synthesis of the mixed-ligand U4+-U4+ nitride complex, [Na(dme)(3)][((Me3Si)(2)N)(3)U(mu-N)U(N(SiMe3)(2))(OSi((OBu)-Bu-t)(3))], 5. The reactivity of the 1-5 complexes was assessed with CO2, CO, and H-2. Complex [NBu4]-1 displays similar reactivity to the previously reported heterobimetallic complex, [Cs{(((BuO)-Bu-t)(3)SiO)(3)U}(2)(mu-N)], [Cs]-1, whereas the amide complexes 2 and 4 are unreactive with these substrates. The mixed-ligand complexes 3 and 5 react with CO and CO2 but not H-2. The nitride complexes [NBu4]-1, 2, 4, and 5 along with their small molecule activation products were structurally characterized. Magnetic data measured for the all-siloxide complexes [NBu4]-1 and [Cs]-1 show uncoupled uranium centers, while strong antiferromagnetic coupling was found in complexes containing amide ligands, namely 2 and 5 (with maxima in the chi versus T plot of 90 K and 55 K). Computational analysis indicates that the U(mu-N) bond order decreases with the introduction of oxygen-based ligands effectively increasing the nucleophilicity of the bridging nitride.