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

Abstract

Molecular uranium nitride complexes were prepared to relate their small molecule reactivity to the nature of the UNU bonding imposed by the supporting ligand. The U4+-U4+ nitride complexes, [NBu4][{((tBuO)3SiO)3U}2(μ-N)], [NBu4]-1, and [NBu4][((Me3Si)2N)3U}2(μ-N)], 2, were synthesised by reacting NBu4N3 with the U3+ complexes, [U(OSi(OtBu)3)2(μ-OSi(OtBu)3)]2 and [U(N(SiMe3)2)3], respectively. Oxidation of 2 with AgBPh4 gave the U4+-U5+ analogue, [((Me3Si)2N)3U}2(μ-N)], 4. The previously reported methylene-bridged U4+-U4+ nitride [Na(dme)3][((Me3Si)2)2U(μ-N)(μ-κ2-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)2N)3U(μ-N)U(N(SiMe3)2)(OSi(OtBu)3)], 5. The reactivity of the 1-5 complexes was assessed with CO2, CO, and H2. Complex [NBu4]-1 displays similar reactivity to the previously reported heterobimetallic complex, [Cs{((tBuO)3SiO)3U}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 CO2 but not H2. 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 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.