O N = L (1) 2 LiAlH 4 + H 2 SO 4 Et 2 O –80 °C [2AlH 3 •L] 2 (2) 2 LiAlH 4 + H 2 SO 4 Et 2 O –80 °C [1.5AlH 3 •L] 3 L•HCl Al(2) C(5) C(6) C(11) N(1) C(3) C(2) O(4) C(12) Al(1) a sin γ c sin α Self-assembling polymeric mixed donor (O/N) N-ethylmorpholine adducts of alane (AlH 3 ) Philip C. Andrews, a Colin L. Raston,* a Brian W. Skelton b and Allan H. White b a Department of Chemistry, Monash University, Clayton, Victoria 3168, Australia b Department of Chemistry, The University of Western Australia, Nedlands, W. A., Australia 6907 The 2 :3 and 1 :2 stoichiometric reactions of N-ethyl morpholine with the Et 2 O adduct of AlH 3 , prepared from LiAlH 4 and H 2 SO 4 in Et 2 O at 280 °C, afford alane-rich hydride-bridged polymers exhibiting a range of bonding modes of Lewis bases to AlH 3 including unprecedented mono Al···H hydride bridging. Lewis-base adducts of alane (AlH 3 ) are important precursors in CVD production of thin films. 1 The stability of such compounds and hence ease of decomposition is often determined by the nature of the Lewis bases present. Attempts at rationalising the stability of such complexes has led to a great deal of solid-state structural information on N-amine and P-phosphine stabilised adducts, for example AlH 3 ·2quinuclidine, AlH 3 ·2NMe 3 and AlH 3 ·PBu t 3 which are monomeric five- or four-coordinate species, dimeric [AlH 3 ·NMe 2 (CH 2 Ph)] 2 with weak hydride bridging and thus five-coordinate metal centres, and polymeric [AlH 3 ·tmen] H and [AlH 3 ·Pr i 2 PCH 2 CH 2 PPr i 2 ] H which have polydentate ligands associated with five-coordinate metal centres. 2 In contrast the only structurally authenticated alane derivatives involving Lewis bases donating through an O atom are AlH 3 ·2thf, [AlH 2 (m-H·thf] 2 3 and the mixed-donor complex [AlH 3 ·N-methylmorpholine] H 1. 4 Such complexes are im- portant since they give insight into the possible decomposition processes for alane and gallane (GaH 3 ), Lewis-base adducts in the presence of air and/or moisture where simple O-complexa- tion at the metal centre is likely to be the primary process for decomposition, as well as how such adducts bind to oxidised silica surfaces. 5 Herein, we report the synthesis and crystal structures of two alane adducts of N-ethylmorpholine (L): namely [2AlH 3 ·L] H 2 and [1.5AlH 3 ·L] H 3. The compounds spontaneously self- assemble into one- or two-dimensional polymeric structures with head-to-head hydride bridging and/or two O-donating groups to the same metal centre. This has implications for building even more complex systems for polydentate donor ligands in general. In addition, a new structural type for alane has been established, and overall both structures taken together summarise the structural variety found for all other charac- terised Lewis base adducts of alane and gallane. Complexes 2 and 3 were prepared as shown in reactions (1) and (2).† In reaction (1) the N-ethylmorpholine is added immediately on cooling to 280 °C an Et 2 O solution of alane preformed from sulfuric acid (98%) and an Et 2 O solution of LiAlH 4 . In reaction (2) the hydrochloride salt of the morpholine is present together with the LiAlH 4 in Et 2 O prior to the addition of the sulfuric acid. The use of the hydrochloride salt in reaction (2) is important in finely controlling the stoichiometry of the reaction. Therefore an L : AlH 3 ·xEt 2 O ratio of 1 : 2 for 2 and 1 : 1.5 for 3 is ensured. Colourless crystals of both 2 and 3 are obtained at 230 °C from an Et 2 O solution, although the colourless rhomboidal crystals of 3 were also obtained from in vacuo sublimation. Both sets of crystals are stable at room temperature with 2 melting with gas evolution in vacuo at 66–67 °C and finally decomposing to metal at ca. 137 °C, and 3 decomposing > 150 °C. Due to the quadrupolar 27 Al nucleus the only meaningful information from the NMR studies was the relative chemical shifts of the AlH 3 protons, giving a relatively broad singlet, and the protons and 13 C in the complexed N-ethylmorpholine. The solid-state structures of 2 and 3 are shown in Figs. 1 and 2 respectively.‡ Both structures are polymeric crystallising in the space group P1 – , albeit showing significant differences: 2 forms a polymeric chain due to the linking of discrete N and O bound AlH 3 units via hydride bridging, while this is also a feature of 3 there are several other important additional features which give rise to a two-dimensional net-like structure. The repeating unit in 3 consists of four L moieties and two lots of three different types of AlH 3 units giving in total three different trigonal-bipyramidal Al environments. One Al centre behaves in a similar fashion to that as seen in 2. However the two other Fig. 1 Crystal structure of 2 showing 20% ellipsoids; selected distances (Å) (bridging hydride distances in italics) and angles (°): Al(1)–N(1) 2.133(3), Al(2)–O(4) 1.993(3), 3[Al(1)–H] 1.56(3), 1.48(2), 1.47(2), 3[Al(2)–H] 1.54(3), 1.49(3), 1.56(3), Al(1)···H(01a) 2.11(3), Al(2)···H(02c) 1.93(3); N(1)–Al(1)–Al(1A) 138.95(8), O(4)–Al(2)–Al(2B) 136.46(9), Al(1)–N(1)– C(11) 107.4(2), Al(1)–N(1)–C(6) 110.3(2), 3[N(1)–Al(1)–H] 95(1), 94(1), 101(1); 3[H–Al(1)–H], 122(2), 113(1), 120(1). 3[O(4)–Al(2)–H], 94(1), 94(1), 94(1); 3[H–Al(2)–H] 128(2), 116(2), 114(2) Chem. Commun., 1997 245 Published on 01 January 1997. Downloaded on 24/10/2014 01:16:35. View Article Online / Journal Homepage / Table of Contents for this issue