UIII–CN versus UIV–NC Coordination in Tris(silylamide) Complexes

Treatment of the metallacycle [UN*2(N,C)] [N* = N­(SiMe3)2; N,C = CH2SiMe2N­(SiMe3)] with [HNEt3]­[BPh4], [HNEt3]­Cl, and [pyH]­[OTf] (OTf = OSO2CF3) gave the cationic compound [UN*3]­[BPh4] (1) and the neutral complexes [UN*3X] [X = Cl (3), OTf (4)], respectively. The dinuclear complex [{UN*­(μ-N,C)­(μ-OTf)}2] (5) and its tetrahydrofuran (THF) adduct [{UN*­(N,C)­(THF)­(μ-OTf)}2] (6) were obtained by thermal decomposition of 4. The successive addition of NEt4CN or KCN to 1 led to the formation of the cyanido-bridged dinuclear compound [(UN*3)2(μ-CN)]­[BPh4] (7) and the mononuclear mono- and bis­(cyanide) complexes [UN*3(CN)] (2) and [M]­[UN*3(CN)2] [M = NEt4 (8), K­(THF)4 (9)], while crystals of [K­(18-crown-6)]­[UN*3(CN)2] (10) were obtained by the oxidation of [K­(18-crown-6)]­[UN*3(CN)] with pyridine N-oxide. The THF adduct of 1, [UN*3(THF)]­[BPh4], and complexes 2–7, 9 and 10 were characterized by their X-ray crystal structure. In contrast to their UIII analogues [NMe4]­[UN*3(CN)] and [K­(18-crown-6)]2[UN*3(CN)2] in which the CN anions are coordinated to the metal center via the C atom, complexes 2 and 9 exhibit the isocyanide U–NC coordination mode of the cyanide ligand. This UIII/UIV differentiation has been analyzed using density functional theory calculations. The observed preferential coordinations are well explained considering the electronic structures of the different species and metal–ligand bonding energies. A comparison of the different quantum descriptors, i.e., bond orders, NPA/QTAIM data, and energy decomposition analysis, has allowed highlighting of the subtle balance between covalent, ionic, and steric factors that govern the U–CN/NC bonding.