A trigonal bipyramidal uranyl amido complex:: Synthesis and structural characterization of [Na(THF)2] [UO2(N(SiMe3)2)3]

The synthesis and structural characterization of a rare example of a uranyl complex possessing three equatorial ligands, [M(THF)(2)][UO2(N(SiMe3)(2))(3)] (3a, M = Na; 3b, M = K), are described. The sodium salt 3a is prepared by protonolysis of [Na(THF)(2)](2)[UO2(N(SiMe3)(2))(4)], wherea's the potassium salt 3b is obtained via a metathesis reaction of uranyl chloride UO2Cl2(THF)(2) (4) with 3 equiv of K[N(SiMe3)(2)]. A single-crystal X-ray diffraction study of 3a revealed a trigonal-bipyramidal geometry about uranium, formed by two axial oxo and three equatorial amido ligands, with average U=O and U-N bond distances of 1.79675) and 2.310(4) Angstrom, respectively. One of the oxo ligands is also coordinated to the sodium counterion. H-1 NMR spectroscopic studies indicate that THF adds reversibly as a ligand to 3 to expand the trigonal bipyramidal geometry. The degree to which the coordination sphere in,3 is electronically satisfied;with only three amido donors:is suggested by (1) the reversible THF coordination, (2) a modest elongation in the bond distances for a five-coordinate U(VI) complex, and (3) the basicity of the oxo ligands as evidenced ih the contact to Na. The vibrational spectra of the series of uranyl amido complexes [UO2(N(SiMe3)(2))(n)](2-n) (n = 2-4) are compared, to evaluate the effects on the axial U=O bonding as a function of increased electron density donated from the equatorial region. Raman spectroscopic measurements of the nu (1) symmetric O=U=O stretch show progressive axial bond weakening as the number of amido donors is increased. Crystal data for [Na(THF)(2)][UO2(N(SiMe3)(2))(3)]: orthorhombic space group Pna2(1), a = 22.945(1) Angstrom, b = 15.2830(7) Angstrom, c = 12.6787(6) Angstrom, z = 4, R1 = 0.0309, wR2 = 0.0524.