U^III–CN versus U^IV–NC Coordination in Tris(silylamide) Complexes

Treatment of the metallacycle [UN*₂(<i>N</i>,<i>C</i>)] [N* = N­(SiMe₃)₂; <i>N</i>,<i>C</i> = CH₂SiMe₂N­(SiMe₃)] with [HNEt₃]­[BPh₄], [HNEt₃]­Cl, and [pyH]­[OTf] (OTf = OSO₂CF₃) gave the cationic compound [UN*₃]­[BPh₄] (<b>1</b>) and the neutral complexes [UN*₃X] [X = Cl (<b>3</b>), OTf (<b>4</b>)], respectively. The dinuclear complex [{UN*­(μ-<i>N</i>,<i>C</i>)­(μ-OTf)}₂] (<b>5</b>) and its tetrahydrofuran (THF) adduct [{UN*­(<i>N</i>,<i>C</i>)­(THF)­(μ-OTf)}₂] (<b>6</b>) were obtained by thermal decomposition of <b>4</b>. The successive addition of NEt₄CN or KCN to <b>1</b> led to the formation of the cyanido-bridged dinuclear compound [(UN*₃)₂(μ-CN)]­[BPh₄] (<b>7</b>) and the mononuclear mono- and bis­(cyanide) complexes [UN*₃(CN)] (<b>2</b>) and [M]­[UN*₃(CN)₂] [M = NEt₄ (<b>8</b>), K­(THF)₄ (<b>9</b>)], while crystals of [K­(18-crown-6)]­[UN*₃(CN)₂] (<b>10</b>) were obtained by the oxidation of [K­(18-crown-6)]­[UN*₃(CN)] with pyridine <i>N</i>-oxide. The THF adduct of <b>1</b>, [UN*₃(THF)]­[BPh₄], and complexes <b>2</b>–<b>7</b>, <b>9</b> and <b>10</b> were characterized by their X-ray crystal structure. In contrast to their U^III 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 <b>2</b> and <b>9</b> exhibit the isocyanide U–NC coordination mode of the cyanide ligand. This U^III/U^IV 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