U(III)-CN versus U(IV)-NC coordination in tris(silylamide) complexes.

Treatment of the metallacycle [UN2(N,C)] [N = N(SiMe3)2; N,C = CH2SiMe2N(SiMe3)] with [HNEt3][BPh4], [HNEt3]Cl, and [pyH][OTf] (OTf = OSO2CF3) gave the cationic compound [UN3][BPh4] (1) and the neutral complexes [UN3X] [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 [(UN3)2(μ-CN)][BPh4] (7) and the mononuclear mono- and bis(cyanide) complexes [UN3(CN)] (2) and [M][UN3(CN)2] [M = NEt4 (8), K(THF)4 (9)], while crystals of [K(18-crown-6)][UN3(CN)2] (10) were obtained by the oxidation of [K(18-crown-6)][UN3(CN)] with pyridine N-oxide. The THF adduct of 1, [UN3(THF)][BPh4], and complexes 2-7, 9 and 10 were characterized by their X-ray crystal structure. In contrast to their U(III) analogues [NMe4][UN3(CN)] and [K(18-crown-6)]2[UN3(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 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.