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

Abstract

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 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 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.

Reduction of Carbonyl Groups by Uranium(III) and Formation of a Stable Amide Radical Anion

“…The average U−N amide bond length was 2.2616(16) Å. Compared with U IV (OTf)[N(SiMe 3 ) 2 ] 3 , a compound previously reported by Ephritikhine and co‐workers, the U−O and U−N amide bond lengths of 6 were longer, likely due to the added steric demand of the N , N ‐dimethylbenzamide ligand present in the inner coordination sphere . Elongation of the N , N ‐dimethylbenzamide‐uranium bond upon oxidation tends to support the ligand‐radical character observed for 2 .…”

A spatially separated [KBr6]5− anion in the cyanido-bridged uranium(IV) compound [U2(CN)3(NH3)14]5+[KBr6]5−·NH3

“…While a multitude of cyanide complexes of the transition metals are known [1,2], only a few cyanido complexes of uranium(IV) have been reported [3][4][5][6][7][8][9][10]. The first actinoid compound containing cyanide anions was K 2 [UO 2 (CN) 4 ] described by Aloy, followed by K 2 [UO 2 (CN) 2 (NO 3 ) 2 ] [11,12].…”

“…However, the existence of both compounds was doubted and literature suggests to regard them as nonexistent [13,14]. Besides (18-crown-6), N* = N(SiMe 3 ) 2 ), of which the crystal structures have been determined [8,9]. Also, some CN −bridged uranium complexes with sterically demanding ligands are known [6,7,9,[17][18][19].…”

Cerium(III) and Uranium(IV) Complexes of the 2-Fluorophenyl Trimethylsilyl Amide Ligand: C–F → Ln/An Interactions that Modulate the Coordination Spheres of f-Block Elements

“…The average U−N bond length found at 2.278 Å was slightly longer than the U−N bond length at 2.239(1) Å found in UCl[N(SiMe 3 ) 2 ] 3 . 66 The lengthening of U−N bonds to adapt for N−C−C−F chelate rings at a U 4+ cation was noted previously for U[N(C 6 F 5 ) 2 ] 4 . 29 Complex 2-Cl was observed to be C 3 symmetric in solution by 1 H and 19 F NMR spectroscopies.…”