Reaction of the uranyl(VI) ion (UO22+) with a triamidoamine ligand: Preparation and structural characterization of a mixed-valent uranium(V/VI) oxo-imido dimer

The synthesis and structural characterization of a mixed-valent uranium(V/V I) oxo-imido complex are reported. Reaction of the uranyl chloride complex [K(18-crown-6)](2)[UO2Cl4] (1) with the triamidoamine ligand Li-3 [N((CH2CH 2NSiBuMe2)-Me-t)(3)] yields oxo-imido [K(18-crown-6)(Et2O)][UO(mu (2)-NCH2C H2N((CH2CH2NSiBuMe2)-Me-t)(2))](2) (2) as the major isolated uranium produc t in moderate yield. The reaction that forms 2 involves activation of both the triamidoamine ligand and the uranyl dioxo unit of 1. An X-ray crystal s tructure determination of 2 reveals a dimeric complex in which the coordina tion geometry at each uranium center is that of a capped trigonal bipyramid . The multidentate triamidoamine ligand coordinates to uranium through the capping amine and two of the three pendant amido ligands, while the third p endant amido donor has been activated to generate a bridging imido ligand b y loss of the silyl substituent. One of the uranyl oxo groups is retained a s a terminal ligand to complete the coordination sphere for each uranium ce nter. The oxo and imido nitrogen may be regarded as the axial ligands of th e trigonal bipyramid, while the two amido ligands and the other imido donor occupy equatorial coordination sites. The central amine of the tripodal se t serves as the capping ligand. Distortion of the axial O-U-N angle from 18 0 degrees emanates from the proximity of the capping amine and the bridging interaction to the other uranium center. The structure and bonding in 2 ar e assessed in the context of metal-ligand multiple bonding in high-valent a ctinide complexes. The possibility of valence averaging [5.5/5.5 vs 5.0/6.0 ] via delocalization or rapid intramolecular electron-transfer dynamics of the unpaired electron is also discussed in the context of crystallographic, spectroscopic (NMR, IR, Raman, and EPR), and electrochemical data. Crystal data for 2: triclinic space group P (1) over bar, a = 12.1144(6) Angstrom, b = 12.6084(6) Angstrom, c = 14.5072(7) Angstrom, alpha = 101.374(1)degree s, beta = 103.757(1)degrees, gamma = 109.340(1)degrees, z = 1, R1 = 0.0523, wR2 = 0.1359.