Syntheses of the uranium complexes [U{N(SiMe3)2}2{N(SiMe3)(SiMe2CH2B(C6F5)3)}] and [U{C(Ph)(NSiMe3)2}2{μ3-BH4}2].: Determination of hydrogen positions by single-crystal x-ray and neutron diffraction

The complex [U{N(SiMe3)(2)}(2){N(SiMe3)(SiMe2CH2B(C6F5)(3))}] (1) is formed in the reaction between the hydride complex [U{N(SiMe3)(2)}(3)(H)] and B(C6F5)(3), and H-2 is evolved. The X-ray [C36H53BF15N3Si6U . 3.5C(6)D(6), triclinic, space group P (1) over bar, Z = 2, 90 K, a = 14.065(1) Angstrom, b = 14.496(1) Angstrom, c = 18.759(1) Angstrom, alpha = 82.898(1)degrees, beta = 74.415(1)degrees, gamma = 62.919(1)degrees] and neutron structure [C36H53BF15N3Si6U . 3.5C(6)D(6), triclinic, space group P (1) over bar, Z = 2, 20 K, a = 13.993(1) Angstrom, b = 14.383(1) Angstrom, c = 18.720(1) Angstrom, alpha = 82.810(1)degrees, beta = 74.200(1)degrees, gamma = 63.053(1)E] of compound 1, which crystallizes with 3.5 molecules of C6D6 per asymmetric unit, show the electron deficiency of the uranium atom to be effectively compensated by the formation of multicenter bonds between U and three Si-CH2 units of the amido ligands. The reaction of the uranium complex [U{C(Ph)(NSiMe3)(2)}(2)(Cl)(2)] with [Na(BH4)] gives the complex [U{C(Ph)(NSiMe3)(2)}(2){mu(3)-BH4}(2)] (2). The X-ray structure of 2 [C26H54B2N4Si4U, monoclinic, space group C2/c, Z = 4, 90 K, a = 21.613(1) Angstrom, b = 9.233(1) Angstrom, c = 18.132(1) Angstrom, beta = 98.803(1)degrees] proves unequivocally the mu(3) coordination of the BH4 moieties. In both single-crystal X-ray structure determinations, all hydrogen and deuterium atoms could be located and isotropically refined, including those which are directly coordinated to the uranium. The reliability of the refined hydrogen and deuterium positions for compound 1 is confirmed by comparison of the X-ray and neutron structure determinations. The ability to locate the hydrogen and deuterium positions in these uranium compounds by single-crystal X-ray diffraction is due to good crystal quality, the measurement of data at low temperature, and the use of image plate technology for data collection.