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Title:High coordination number compounds of thorium borohydride, and of calcium and strontium N,N-dimethylaminodiboranates
Author(s):Dunbar, Andrew
Advisor(s):Girolami, Gregory S.
Contributor(s):Gewirth, Andrew A.; Nuzzo, Ralph G.; Rauchfuss, Thomas B.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):High Coordination Number
Thorium Borohydride
Abstract:We have determined the crystal structure of Th(BH4)4 and confirmed that it is isomorphous with the tetragonal form of its uranium analogue. Of the four BH4- groups per formula unit, two are terminal and are bound to thorium in a tridentate κ3 fashion, and the other two BH4- groups bridge between neighboring Th centers in a bis(bidentate) κ2,κ2 fashion. In this arrangement each Th center is bound to six BH4- groups which form a total of 14 Th-H bonds (2 × 3 + 4 × 2). If one ignores the hydrogen atoms, the six boron atoms about each Th center describe a distorted octahedral arrangement, in which the two terminal κ3-BH4- groups are mutually cis. The 14 hydrogen atoms surrounding the central thorium atom do not describe a simple regular coordination polyhedron, although it is possible to view the arrangement as a highly distorted bicapped hexagonal antiprism. The metal centers are linked into a three-dimensional polymer that consists of interconnected helical chains wound about fourfold screw axes. The most interesting result is that the Th and U complexes show some systematic differences in bond distances and angles, which is tentatively attributed either to the different f-electron configurations of these two ions (f0 vs f2) or to the lower energies of the f-orbitals on uranium. Few other examples of such electronic effects in actinide chemistry have been postulated. The reaction of ThCl4 with six equiv. of LiBH4 in diethyl ether produces the compound [Li(Et2O)2][Th(BH4)5], the thorium center in this molecule is surrounded by six borohydride ligands. Four of these six borohydrides are coordinated in a tridentate κ3 fashion, and the other two BH4- groups bridge between neighboring Th centers in a bis(bidentate) κ2,κ2 fashion forming a polymeric structure, as determined by single crystal X-ray diffraction. This is the first structurally characterized 16 coordinate compound of any kind. [Li(Et2O)2][Th(BH4)5] reacts with thf to form the mononuclear salt [Li(thf)4][Th(BH4)5(thf)]. The thorium atom is coordinated by five borohydride groups and one thf molecule. Investigation of the Th···B distances revealed that all five BH4- groups are coordinated in a tridentate fashion giving an overall coordination number of 16. [Li(Et2O)2][Th(BH4)5] can also be treated with 12-crown-4 to produce the [Li(12-crown-4)2]2[Th(BH4)6] salt. This result is interesting because, unlike the formation of [Li(thf)4][Th(BH4)5(thf)], which is the product of structural rearrangement, [Li(12-crown-4)2]2[Th(BH4)6] is the result of a disproportiation reacton. The enviorment about the 16-coordinate thorium atom is essentially the same as that in [Li(Et2O)2][Th(BH4)5] except the bidentate borohydride groups are not bridging. Treatment of ThCl4 with lithium tetrahydroborate in various ether solvents affords the complexes Th(BH4)4(Et2O)2, Th(BH4)4(thf)2, and Th(BH4)4(dme)2, where thf = tetrahydrofuran and dme = 1,2-dimethoxyethane. The complexes have been characterized by infrared spectroscopy, and by 1H and 11B NMR spectroscopy. The crystal structures of Th(BH4)4(Et2O)2 and Th(BH4)4(thf)2 have been characterized by single crystal X-ray diffraction; both adopt trans-octahedral geometries, if the BH4 groups are considered to occupy one coordination site. All four BH4 groups in both complexes are tridentate, making the thorium atoms 14 coordinate. The Th···B distances are 2.64-2.67 Å and the Th-O distances are 2.47-2.52 Å. All three ether adducts of Th(BH4)4 are volatile and readily sublime at 60 °C and 10-4 Torr; passage of the diethyl ether adduct Th(BH4)4(Et2O)2 over glass, Si(100), and aluminum substrates heated to 350 °C yields amorphous films of approximate stoichiometry ThB2. The thorium boride films deposited from the tetrahydrofuran adduct Th(BH4)4(thf)2 have stoichiometries closer to ThB2.5, and are contaminated with oxygen. The reaction of MBr2 (M = Ca or Sr) with two equiv. of sodium N,N-dimethylaminodiboranate, Na(H3BNMe2BH3) in Et2O at 0 ºC give a product which, after being dried in vacuum, proves to be the compound M(H3BNMe2BH3)2. Before they are dried in vacuum, the colorless crystals obtained from the above reaction consist as the diethyl ether adducts M(H3BNMe2BH3)2(Et2O)2. If the reaction of MBr2 with two equiv of Na(H3BNMe2BH3) is carried out in the more strongly coordinating solvent thf rather than diethyl ether, the thf solvate M(H3BNMe2BH3)2(thf)x, (x = 2 for Ca and 3 for Sr), is obtained. Treating the thf adducts with 1,2-dimethoxyethane (dme), bis(2-methoxyethyl) ether (diglyme), or N,N,N′,N′-tetramethylethylenediamine (tmeda) in thf affords the new compounds M(H3BNMe2BH3)2(dme)x, (x = 1 for Ca and 2 for Sr), M(H3BNMe2BH3)2(diglyme), and M(H3BNMe2BH3)2(tmeda), respectively, in greater than 60% yields. Treatment of the thf adducts with two equiv. of the crown ether 12-crown-4 in thf affords the charge-separated salts, [Ca(12-crown-4)2][H3BNMe2BH3]2, and [Sr(H3BNMe2BH3)(12-crown-4)2][H3BNMe2BH3]. All of the adducts, expect the two containing 12-crown-4, possess chelating κ2-BH3NMe2BH3-κ2 groups, in which two hydrogen atoms on each boron center are bound to the metal center. The 12-crown-4 compounds are unique within their respective series as the only ionic compound. The Ca atom is completely encapsulated by 12-crown-4 and the DMADB anions are charge-separated counterions. The Sr compound possesses an unusual κ1-BH3NMe2BH3 group because the Sr atom is almost completely encapsulated by two 12-crown-4 molecules; the other BH3NMe2BH3 anion is a charge-separated counterion within the unit cell. When heated, the dme, diglyme, and tmeda compounds of Ca melt without decomposition, and can be sublimed readily under reduced pressure (1 Torr) at 90 ºC, for dme, and 120 ºC, for diglyme and tmeda. When heated, the diglyme and tmeda compounds of Sr melt without decomposition, and can be sublimed readily under reduced pressure (1 Torr) at 120 ºC. The dme adduct of Ca is one of the most volatile calcium compounds known, and is a promising CVD precursor for the growth of calcium-containing thin films. The diglyme and tmeda adducts of Sr are some of the most volatile strontium compounds known, and are promising candidates as CVD precursors for the growth of strontium-containing thin films.
Issue Date:2012-02-01
Rights Information:Copyright 2011 Andrew Dunbar
Date Available in IDEALS:2014-02-01
Date Deposited:2011-12

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