Preparation and molecular and crystal structures of uranium(IV) borohydride-dimethyl ether and uranium(IV) borohydride-diethyl ether

Abstract

Uranium(IV) borohydride reacts with dimethyl ether and diethyl ether to form U(BH/sub 4/)/sub 4/.O(CH/sub 3/)/sub 2/ and U(BH/sub 4/)/sub 4/.O(C/sub 2/H/sub 5/)/sub 2/, respectively. The yields are greater than 90 percent. The latter compound can also be prepared by reacting UF/sub 4/ and LiBH/sub 4/ in diethyl ether for 1 week. Both compounds are green, crystalline solids that can be sublimed in bulk at 50/sup 0/C and 10/sup -5/ mm. Single-crystal x-ray diffractometry has shown both of these materials to be monether complexes in the solid state. U(BH/sub 4/)/sub 4/.O(CH/sub 3/)/sub 2/ is orthorhombic, Pnma, with a = 11.423 (5) A, b = 10.120 (4) A, c = 9.915 (4) A, and Z = 4. U(BH/sub 4/)/sub 4/.O(C/sub 2/H/sub 5/)/sub 2/ is monoclinic, P2/sub 1/, with a = 7.95 (1) A, b = 15.24 (1) A, c = 5.74 (1) A, ..beta.. = 106.0 (1)/sup 0/, and Z = 2. The structure of each complex consists of infinite linear chains of alternating uranium and boron atoms joined by double-hydrogen-bridge bonds. The remaining borohydrides are attached to the uranium atom by triple-hydrogen-bridge bonds. The ether moieties are associated to the uranium by the oxygen atoms. In the dimethyl ether adduct successivemore » ether molecules along the chain are trans, whereas in the diethyl ether compound they are all cis. The total coordination about the uranium atom is 14, i.e., 1 oxygen atom and 13 hydrogen atoms. The average U--B distances for the triple-bridge and double-bridge bonds are 2.53 (2) and 2.89 (1) A, respectively. The average U--O bond length is 2.46 (3) A. A refinement of the anomalous dispersion term f'' for uranium in the ethyl compound shows that the crystal was twinned, containing unequal fractions of the enantiomeric configurations. 5 tables, 4 figures.« less