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[double bond, length as m-dash]N[double bond, length as m-dash]U bonding imposed by the supporting ligand. The U(4+)–U(4+) nitride complexes, [NBu(4)][{(((t)BuO)(3)SiO)(3)U}(2)(μ-N)], [NBu(4)]-1, and [NBu(4)][((Me(3)Si)(2)N)(3)U}(2)(μ-N)], 2, were synthesised by reacting NBu(4)N(3) with the U(3+) complexes, [U(OSi(O(t)Bu)(3))(2)(μ-OSi(O(t)Bu)(3))](2) and [U(N(SiMe(3))(2))(3)], respectively. Oxidation of 2 with AgBPh(4) gave the U(4+)–U(5+) analogue, [((Me(3)Si)(2)N)(3)U}(2)(μ-N)], 4. The previously reported methylene-bridged U(4+)–U(4+) nitride [Na(dme)(3)][((Me(3)Si)(2))(2)U(μ-N)(μ-κ(2)-C,N-CH(2)SiMe(2)NSiMe(3))U(N(SiMe(3))(2))(2)] (dme = 1,2-dimethoxyethane), [Na(dme)(3)]-3, provided a versatile precursor for the synthesis of the mixed-ligand U(4+)–U(4+) nitride complex, [Na(dme)(3)][((Me(3)Si)(2)N)(3)U(μ-N)U(N(SiMe(3))(2))(OSi(O(t)Bu)(3))], 5. The reactivity of the 1–5 complexes was assessed with CO(2), CO, and H(2). Complex [NBu(4)]-1 displays similar reactivity to the previously reported heterobimetallic complex, [Cs{(((t)BuO)(3)SiO)(3)U}(2)(μ-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 CO(2) but not H(2). The nitride complexes [NBu(4)]-1, 2, 4, and 5 along with their small molecule activation products were structurally characterized. Magnetic data measured for the all-siloxide complexes [NBu(4)]-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 χ versus T plot of 90 K and 55 K). Computational analysis indicates that the U(μ-N) bond order decreases with the introduction of oxygen-based ligands effectively increasing the nucleophilicity of the bridging nitride.