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/VI) 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((CH2CH2NSiBuMe2)-Me-t)(3)] yields oxo-imido [K(18-crown-6)(Et2O)][UO(mu (2)-NCH2CH2N((CH2CH2NSiBuMe2)-Me-t)(2))](2) (2) as the major isolated uranium product 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 structure determination of 2 reveals a dimeric complex in which the coordination 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 pendant amido donor has been activated to generate a bridging imido ligand by loss of the silyl substituent. One of the uranyl oxo groups is retained as a terminal ligand to complete the coordination sphere for each uranium center. The oxo and imido nitrogen may be regarded as the axial ligands of the trigonal bipyramid, while the two amido ligands and the other imido donor occupy equatorial coordination sites. The central amine of the tripodal set serves as the capping ligand. Distortion of the axial O-U-N angle from 180 degrees emanates from the proximity of the capping amine and the bridging interaction to the other uranium center. The structure and bonding in 2 are assessed in the context of metal-ligand multiple bonding in high-valent actinide 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)degrees, beta = 103.757(1)degrees, gamma = 109.340(1)degrees, z = 1, R1 = 0.0523, wR2 = 0.1359.