Metal hydroborates and hydroborato metalates, 21. Solvates of lithium (dimethylamino)trihydroborate

Abstract

Key Words: Borate lithium (dimethy1amino)trihydrido-/ TMEDA complex / Dioxane complex / 12-Crown-4 complex Li(Me2NBH3) (1) was prepared by deprotonation of Me2NH . BH3 with LiBu and its solubility determined in various solvents. Compound 1 is unstable in most ethereal solvents and decomposes in a reversible reaction into LiH and Li-(Me2N-BH,-NMe2-BH,). Five solvates of 1 were characterised by X-ray structure analysis. Li(Me,NH,) . TMEDA is a dimer in the solid state, and two units are connected to one another by B-H...Li bonds. The crystal structure of Li(Me2NBH,) . 0.5 dioxane is represented by layers consisting of tetrameric Li(Me,NBH,) units connected via dioxane molecules. Each Li atom is coordinated to one oxygen atom, one nitrogen atom, and two hydrogen atoms. Also Li-(Me2NBH3) . 0.5 02C3H6 forms an extended layer, however with pentacoordinated Li atoms which bear one oxygen, one nitrogen, and three hydrogen atoms. A similar layer structure was also found for Li(Me2NBH3) . 03C3H6 where each Li atom is bound to two oxygen atoms of different trioxane molecules, one nitrogen atom, and two hydrogen atoms. Finally, Li(Me2NBH3) . 12-crown-4 retains its molecular integrity in the solid state. Its Li atom is pentacoordinated by four oxygen atoms and one nitrogen atom.Although alkali metal (dia1kyJaniino)trihydroborates are known since 1961[2,31 they have received little attention until re~entlyr~,~'. This renewed interest is associated with their use as powerful but also selective reducing reagents in organic synthe~is[~,~! Nevertheless, our knowledge of their structure as well as our knowledge of their solution state is only marginal although they are well-defined by their "B-NMR data"]. In contrast to the structures of metal tetrahydroborates which contain p1-BH4. p2-BH4, and p3-BH4 groups['], the structure of metal aminotrihydroborates may be even more variable because one can expect the formation of B-N-M (M = metal atom) units besides B(H),M interactions. Li(Me2NBH1) seemed to be a suitable model compound to study this aspect. Synthesis and Physical Properties of Li(MeZNBH3)Li(Me2NBH3) (1) was readily synthesised by the known deprotonation of MezNH . BH3l61 with LiBu in h e~a n e [~] as depicted by eq. (1). However, small amounts of Bu2BNMe,, B(NMe&, and LiBBu4 are also formed as shown by "B-NMR spectroscopy. The formation of these species is formally described by eqs. (2j to (4). The solvates Li-(Me2NBH3) . 0.5 dioxane, Li(Me2NBH3) . 0.5 dioxolane, Li(Me2NBH3) . trioxane, and Li(Me2NBH3) . 12-crown-4 were obtained by adding the ether ligand to a solution of Me2NH . BH3 in toluene followed by deprotonation of the amine-borane with butyllithium. Lo] Part 20: Me,NH BH3 + LiBu --f Li(Me2NBH3) + BuH (1) (2) (3) (4) 1 1 + 2 LiBu --+ 3 LiH + Bu2BNMe2 3 Bu2BNMe2 -+ 2 BBu? + B(NMc2), BBu3 + LiBu + LiBBu4 Figure 1. Schematic rcpresentatioii of the solubility of LI-(Me2NBH,) (1) in various solvents and a1 different temperatures Solubility of Li[MezNMH31 in g/1 350.0 T m.0 t 250,O UI 0°C 0 Z O T e 40°C 200.0 150.0 100.0 ...

Synthesis, Separation and UV/Vis Spectroscopy of Pyrazino‐quinoxalino‐porphyrazine Macrocycles

“…The average BN bond length in the two molecules of 8 b (1.551 Å) is distinctly shorter than that in 1 (1.623(2) Å) in accordance with a stronger BN interaction, implying a greater contribution of resonance form shown in Scheme a than that seen for 2 – 7 . The geometry at lithium is that of a distorted tetragonal pyramid, and closely resembles that of ([12]crown‐4)Li(Me) 2 NBH 3 ( IX ; structure not shown),38 with four Li⋅⋅⋅O contacts varying in length between 1.960(12) and 2.361(12) Å. The average Li⋅⋅⋅N distance (2.048 Å) is slightly longer than in IX (2.002 Å).…”

The Synthesis, Structure and Reactivity of B(C6F5)3‐Stabilised Amide (MNH2) Complexes of the Group 4 Metals

“…The geometry at lithium is that of a distorted tetragonal pyramid, and closely resembles that of ( [12]crown-4)Li(Me) 2 NBH 3 (IX; structure not shown), [38] with four Li···O contacts varying in length between 1.960(12) and 2.361 (12) . The average Li···N distance (2.048 ) is slightly longer than in IX (2.002 ).…”

s-Block amidoboranes: syntheses, structures, reactivity and applications

“…77 Nöth and co-workers performed a structural study on the known reducing agent LiNMe 2 BH 3 (4) in combination with various solvents. 78 While the compound was found to be poorly soluble in aromatic solvents, addition of coordinating solvents led to a variety of different adducts. 4ÁTMEDA (TMEDA = N,N,N 0 ,N 0 -tetramethylethylenediamine) has a dimeric structure containing an 8-membered ring formed by Li + Á Á ÁH dÀ -B interactions ( Fig.…”