A Trigonal Bipyramidal Uranyl Amido Complex: Synthesis and Structural Characterization of [Na(THF)2][UO2(N(SiMe3)2)3]
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posted on 2000-11-02, 00:00 authored by Carol J. Burns, David L. Clark, Robert J. Donohoe, Paul B. Duval, Brian L. Scott, C. Drew TaitThe 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], whereas 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 UO and U−N bond distances of 1.796(5) and 2.310(4) Å, respectively. One of the
oxo ligands is also coordinated to the sodium counterion. 1H 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 in 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 UO bonding as
a function of increased electron density donated from the equatorial region. Raman spectroscopic measurements
of the ν1 symmetric OUO 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 Pna21, a = 22.945(1) Å,
b = 15.2830(7) Å, c = 12.6787(6) Å, z = 4, R1 = 0.0309, wR2 = 0.0524.
