The spectroscopic properties and X-ray crystal structure of a discrete molecular alane, dimethylamidoalane, [Me2NAlH2]3

Polyhedron Vol. I I, No. Il. pp. 12954304, Printed in Great Britain

0277-5387/92 s5.oo+.oo Pergamon Press Ltd

1992

THE SPECTROSCOPIC PROPERTIES AND X-RAY CRYSTAL STRUCTURE OF A DISCRETE MOLECULAR ALANE, B~IMETHYLAMID~ALANJz, [Me,NAlH,], ANTHONY J. DOWNS,* DAWN DUCKWORTH, JONATHAN C. MACHELL and COLIN R. PULHAM Inorganic Chemistry Laboratory, University of Oxford, South Parks Road. Oxford OX1 3QR, U.K. (Received 18 December 1991; accepted 21 February 1992)

Abstract-The vibrational, ‘H and 13C NMR, and mass spectra of dimethylamidoalane (1) have been determined and analysed. The implications of these and other results have been confirmed by the X-ray crystal structure determination of 1, which reveals a discrete molecular species, [Me,NAlH,],, containing a six-membered [AlNJ3 ring in a chair conformation complying with C3” symmetry (1876 reflections). The mean values for salient interatomic distances and angles are : r(Al-N) 1.936(3), r(C-N) 1.505(5), r(Al-H) 1.55 A, LN-Al-N 108.8(l), LAI-N-Al 114.9(l) and LC-N-C 106.2(3)“. The properties of the alane are compared with those of related compounds.

Recent studies in this laboratory have shown that gallium hydrides like [GaH,],, I-3 [H,GaBH,],, ‘,4 HGa(BHJZ5 and [H2GaC1]2,6*7for all their thermal frailty, are typically made up of discrete molecular units which feature four- or, only very occasionally, five-fold coordination of the metal atom. By contrast, the corresponding aluminium hydrides tend, where they are known, to be polymeric and to feature coordination numbers at aluminium in excess of four. Such is the case, for example, with the involatile binary hydride, the Lx-form of which contains aluminium atoms octahedrally coordinated by six hydrogen atoms. 8However, there are exceptions and more tractable aluminium hydrides composed of more-or-less discrete molecular units including diorganoalanes of the type [R2AlHJ,,‘*” adducts of alane, e.g. Me,N 0A1H3,“,‘* and the mixed. hydride Al(BH4)3. ’ ‘sL3 Another relatively volatile derivative is dimethylamidoalane, a white crystalline solid formed by the reaction of lithium tetrahydroaluminate with dimethylammonium chloride in diethyl ether solution [eq. (l)] : I4 LiAlH, + [Me,NH,]+Cll

B

l/n[Me2NAlH2], + 2H2 + LiCl.

(1)

*Author to whom correspondence should he addressed.

The compound, which can be sublimed in vacua at near-ambient temperatures, is reported to be trimerit (i.e. n = 3) in benzene solution ;I5 a trimeric unit containing an [AlNj, ring is also evident from the structure determined for a single crystal. I6317 This contrasts with the dimeric forms assumed by the corresponding boron I8 and gallium I9 compounds. Otherwise, however, relatively little is known about dimethylamidoalane or related compounds. Here we describe the IR and Raman spectra of the solid compound, ‘H and 13C NMR spectra of [‘H,]toluene solutions, and the mass spectrum of the vapour. The structure of a single crystal has also been redetermined by X-ray studies to confirm the trimeric form of the molecular unit and assess more clearly its interaction with neighbouring units. The results are potentially significant on two counts. (1) Volatile compounds like dimethylamidoalane and the adducts Me3N*A1H3 and (Me3N)*A1H3 may function as precursors or intermediates in the deposition of aluminium films, e.g. for the growth of GaAlAs and the “metallization” of semiconductor devices.*’ (2) There are grounds for believing that dimethylamidoalane may be a useful source of other tractable alane derivatives, e.g. Me,NAl,H,.*r

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1296

A. J. DOWNS et al.

EXPERIMENTAL Synthesis

Isotopically natural or deuteriated samples of dimethylamidoalane were prepared from LiA1H4 (LiAlDJ and [Me,NH&l ([Me2ND2]C1) in accordance with the procedures described previously. I4 Manipulation involved, conventional vacuum-line techniques, but with rigorous measures to exclude the smallest traces of air, moisture or other impurities. I-4 After evaporation of the diethyl ether solvent, the crude product was purified by sublimation in uacuo at 308-3 13 K ; white acicular crystals of the alane collected in an all-glass U-trap cooled to 228 K. The trap, equipped with constrictions permitting its isolation and with a break-seal giving access to the isolated sample, had been preconditioned by heating under continuous pumping. LiAlH4, LiAlD4 and [Me,NHJCl (all from Aldrich) were purified beforehand by recrystallization ; diethyl ether (also from Aldrich) was purified and dried by standard methods. The salt [Me2ND2]Cl was prepared by dissolving [Me,NH,]Cl in a lOO-fold excess of D20, removing the solvent under vacuum and recrystallizing the product from dried [*H,]methanol. The identity and purity of a sample of the alane were checked by its m.p. (90°C1 ‘) and by elemental analysis (carried out by the Analytische Laboratorien, Elbach, Engelskirchen, F.R.G.). Found : C, 32.5; H, 10.8; N, 18.9; Al, 37.3. Calc. for Me,NAlH,: C, 32.9; H, 11.0; N, 19.2; Al, 36.9%.

Spectroscopic

measurements

IR spectra of solid samples of dimethylamidoalane were measured with a Perkin-Elmer Model 580A dispersive spectrophotometer (2004000 cm- ‘) or a Perkin-Elmer Model 1710 or a Mattson “Polaris” FTIR instrument (40&4000 cm- ‘). The Raman spectrum of the solid, measured with a Spex Ramalog 5 spectrophotometer interfaced with an SCADAS data acquisition system (Glen Creston), was excited at 1 = 514.5 nm using the output from a Spectra-Physics Model 165 Ar+ laser. Solid films of the alane were formed by condensation of the vapour on a CsI window (for IR studies) or a copper block (for Raman studies) sup-

* Lists of atomic coordinates, thermal parameters, bond lengths and angles have been deposited as Supplementary Material at the Cambridge Crystallographic Data Centre.

ported in an evacuated glass shroud and held at 77 K. After annealing, the condensate was retooled to 77 K before its spectrum was finally recorded. NMR spectra were recorded on a Bruker Model AM 250 instrument operating at 250 MHz for ‘H and 62.89 MHz for 13Cmeasurements. The mass spectrum of the sample vapour was measured using an A.E.I. MS902 spectrometer updated for data handling by Mass Spectrometer Services, the vapour being admitted from an ampoule connected directly to the stainless steel gas inlet system via a nozzle maintained at ambient temperature. Calibration was achieved by reference to the mass spectrum of heptacosafluorotributylamine.

Crystal structure analysis of dimethylamidoalane Crystal data. A white needle-like crystal with the approximate dimensions 0.4 x 0.4 x 0.7 mm was mounted in a glass capillary under dry nitrogen and transferred to an Enraf-Nonius CAD4 diffractometer. Measurements were made in the ~28 mode with graphite-monochromated Cu-K, radiation(1 = 1.54180A;scanwidth0.80+0.15tan8). Of the 2708 reflections so collected 1876 were considered to be unique and “observed” with Z > 3a(Z). The reflections were corrected for Lorentz and polarization effects and an empirical absorption correction was applied (min./max. correction 1.00/1.36). Decay in the intensities of the standard reflections (2,0,2 ; 0, - 1, - 6 ; 1,2,1) was taken to be 5%. C6H24N3A13, M = 219.2, primitive monoclinic, space group P2,/c, a = 6.527(l), b = 8.895(l), c = 25.729(3) A, a = 90, /I = 92.82(l), y = 90”, U = 1492 A3 (by least-squares refinement of diffractometer angles for 24 automatically centred reflections), 2 = 4, D, = 0.976 g cmW3, F(OO0) = 480, I = 20.82 cm-‘. The structure was solved by direct methods using SHELXS and MULTAN84” and full-matrix leastsquares refinement of all the non-hydrogen atoms. After all the atoms had been converted to have anisotropic thermal parameters, location of the hydrogen atoms was achieved from a difference map with the application of a Chebyshev weighting scheme (four polynomial coefficients 13.0243, -10.5185,10.5504and -3.99341).Themodelconverged at R = 5.45 and R, = 6.13%. Programs and sources of scattering factors are given in ref. 22.* Interatomic distances and angles are listed in Table 1, and average values for selected distances and angles in Table 2. Figures 1 and 2 give perspective views of the molecules and of the unit cell, respectively.

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X-ray crystal structure of a discrete molecular alane Table 1. Selected interatomic

distances (A) and angles (“) for ~me~ylamidoalane”

1.927(3) I .934(3) 1.941(3) 1.936(3) 1.930(3) 1.945(3) W)--cW Wlk--W) N(2>--e(31 Nw--cx41 N(3h-W) N(3W(6)

1.499(4) 1.506(5) LSOO(5) 1.514(4) 1.498(S) 1.512(4)

AWI--Wl) AW---A(2) AW---H~3~ Af(2I--W41 Ali3j-H{Sj Al(3)-H(6)

1.71 1.51 1.52 1.52 1.61 1.45

N(3I---AK3)_N(2)

108.9( 1) 108.1(l) 109.3(l)

Al(2~N(l~Al(l) Al(3j-N(2 )-Al(Z) Al(3 j-N(3)-Al( 1)

116.0(l) 114.7(l) 114.0(l)

W---NU)-_AW W--N(1>-A~~2) WW--NUt--AK11 CV$--N( 1)_A&21 C~3I---NW--AK2) C(3)-N(2)---Al(3) C(4)-N(2)-Al(2) ~~4I----NGQ-A~~3~ C(5)-N(3)--Al( 1) C(5t-N(3I--A1(3) C(6~N~3~AI(l) C(6)--N(3)-Al(3)

105.1(2) 104.6(2) 112.1(3) 112.4(3) I1 M(2) 112.6(2) 105.2(2) 105.2(2) 113.5(3) 112.4(2) 105.2(2) 103.9(2)

N(3I--AV)_N(0

WQ---AW-N(1

)

105.5(3) 106.4(3) 106.8(3) H(l~AI(ljN(l) W)-_AKl)-_N(3) H(2)-Al(1 jN( 1) H(2)-Alf 1)--N(3) H(3jA1(2)-N(1) H(3)-Al(2)-N(2) W44)-@2)-_N( 1) H(4~Ai(2~Nt2~ H(5I---AK3)_N(21 H(5%---A1(3)-_N(3) H(6~Al(3~N(2~ W6I---AK3k--N(3)

1.17.59(8) 103.1 l(8) 116.44(9) 108.~9) 113.06(9) 109.46(9) 108.13(9) 111.‘74(8) 112.95(8) 103.96(8) 105*91(P) 109.35(S)

DFor numbering of atoms see Fig. 1.

W----H(7) C(lI---W8~ WjH(9) C(2jWlO) C(2jI-W 1) C(2I----HG2) C(3I---Wl3) C(3k---W4) C(3I----Htl5) C(4I--wl6) C(4I---H(l7) C(4)-H( 18) C(S)-H(l9) W--W20) C(5)--H(2 1) c&j--H(22j C(6t-W23) C(6)--H(24)

1.03 1.07 0.93 1.02 1.03 0.97 1.08 1.09 0.99 1.04 0.97 0.93 1.10 1.20 0.99 1.01 1.12 1.04

H(7I---C(l)---N(l) H(8W(lk---NW H(9k---W)_N(l) H(1OjC(2jN(lI H(1 ljCt2jMlI H(l2jG2k--WI H(l3W(3jW2) H(l4~(3~N~2) H0 5jc(3jw2) H(l6)---c(4jN(2) H( 17)---C(4jNt2) Ht 18jC(4jN(2) Ht 19jC(5jW3) H(2OjC(5jW3) H(21)-C(5 j-N(3) H(22j~t6jN(3) H(23jC(6jW3) H(24jC16I---W3)

1lOS(2) 111.4(2) 117.4(2) 117.2(2) 108.0(2) 106.5(2) 112.7(2) 100.4(Z) 116.4(2) 107.0(2) 103.8(2) 108.4(2) 103.5(2) 110.9(2) 113.3(2) 104.0(2) 108.6(2) 107.4(2)

H(2jAVjW) H(4jAK2k--H(3) H(6jAK3>-H(5)

101.20 106.36 115.32

H(8jWI----H(7) H(9jCt 1I--+%71) H(9~(l~H~8) H(l1 jC(2jH(lO) H(l2jC(2 jH(l0) H(12 jC(2 jH(11) H(l4W~3)_-HU3) H(l5 jC(3 jH(l3) H(15 jC(3 jH(14) H(l~C(4jH(l6) H(lsjC(4 jH(l6f H(18 jC(4 jH(l7) H(20 jC(5 jH(l9) H(21 jC(5jH(l9) H(21 jC(5 jH(20) H(23 jC(6 jH(22) H(24w6 jH(22) H(24W(6jW23)

98.56 111.48 105.82 120.09 89.65 113.57 108.23 109.27 109.29 107.86 117.77 110.97 106.73 109.29 112.54 102.22 112.53 120.83

A. J. DOWNS et al.

1298

Table 2. Average bond lengths (A) and angles (“) for dimethylamidoalane r(M-N)

LN-M-N”

and related compounds L M-N-M“

Reference

Compound

Ring conformation

[Me2NAlH2], cis-[MeHNAlMe,], trans-[MeHNAlMe,],

[AIN], chair [AIN], chair [AIN], skew-boat (I) [AIN skew-boat (I) [A& skew-boat (I) [AIN] 3planar

1.936(3) 1.940(S) 1.901(10) 1.91(2) 1.935(5) 1.952(14)

108.8(l) 102.1(4) 100.9(6) 102.0(5) 101.4(2) 106.1(4)

114.9( 1) 122.3(4) 124.9(9) 119.9(5) 122.1(3) 133.9(5)

This work 24 24 25 26 26

KMe3SWJNH2L KMdWW2 KMdV3A~12

[AlN], [AIN], [AlN], [AIN],

1.958(7) 1.956(2) 1.966(2)* 1.970(2)

88.3(3) 86.9(I) 87.7(l) 88.1(l)

91.7(2) 93.1(l) 92.3(l) 91.9(l)

24 28 17 17

[Me,GaNH,h D-WaNCH2M3

[GaNj, skew-boat (I) [GaNJ 3 chair

1.98(2) 1.97(2)

98.6(8) 99.9(1.0)

122.3(9) 120.8(1.2)

27 45

[MeA@WHJ& [Me,AlNH 213 [Bu’,AlNH 2],, [Me,NAlMe,],

“M=AlorGa. ’ Exocyclic Al-N ‘Exocyclic Al-N

RESULTS

planar planar planar planar

distance 1.804(2) A. distance 1.814(2) A.

AND DISCUSSION

Crystal structure Earlier freezing-point studies of benzene tions had indicated that dimethylamidoalane

soluexists

under these conditions as a trimer, [Me,NAlH,],, which was presumed to be based on a cyclic sixmembered [AIN], skeleton.” The results of our single-crystal studies confirm those described previously ’ ‘3’ 7 in verifying that the crystalline compound is also composed of trimeric molecules, as illustrated in Figs 1 and 2, and with the bond lengths and angles given in Tables 1 and 2. The new results differ from the earlier ones only in being more specific about the location of hydrogen atoms ; in other respects there is no significant difference between the different sets of results. Scrutiny of the structure reveals no intermolecular Al * . . H distances shorter than 3.8 A, and so the solid consists of more-orless discrete trimeric [Me,NAlH,], units with no significant intermolecular contacts, in keeping with its comparatively high volatility. That the compound crystallizes as a trimer is noteworthy in that the corresponding gallium compound vaporizes as a dimer, [Me2NGaH212, and appears to retain this unit in the crystalline state. I9 This behaviour can hardly be ascribed to steric effects since the tetrahedral covalent radii of aluminium and gallium are very nearly equa12’ The alane also contrasts with the corresponding dimethylaluminium compound, the crystal structure of which reveals dimeric units, [Me2NAlMeJ2, each containing a four-membered

[AlNJ* ring.24 Here it is more reasonable to ascribe the difference to steric effects as the non-bonded distances between given substituents on adjacent aluminium and nitrogen atoms are greater in a dimeric than in a trimeric species. The chair conformation found for the [AIN], ring in crystalline dimethylamidoalane is similar to that exhibited by the related dimethylaluminium compound cis-[Me2A1NHMe]3.24 It is different from the skew-boat ring favoured not only by a second isomer, trans-[Me2A1NHMe],24 (see I), but also by the ethyleneimino derivative [Me2AlN(CH2)2]3,2S

Q

‘3) N(3) 1

13 H(5)

C(4)

Fig. 1. Molecular structure of dimethylamidoalane giving the crystallographic numbering scheme. The methyl hydrogen atoms are not shown but are numbered from H( 10) to H(24) inclusive, with H(10) to H( 12) bonded to C(l), H(13) to H(15) to C(2), etc.

X-ray crystal structure of a discrete molecular alane

Fig. 2. Unit cell of dimethylamidoalane.

the compound [Me,AlNH,], isomeric with dimethylamidoalane26,27 and its gallium analogue [Me2GaNH&.27 A third variation is to be found

in the planar ring in [(t-C4Hg)2A1NH2]s,26 perhaps reflecting the steric bulk of the substituents at aluminium. However, the energy difference between one [AlN], ring conformation and another is small, and so packing and non-bonded repulsion forces may well dictate the precise geometry.

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Of the trimeric amidoalanes studied to date, [Me2NAlH21J catches the eye for having endocyclic N-Al-N and Al-N-Al bond angles [108.8(l) and 114.9(l)“, respectively] approximating more closely than any other to the tetrahedral value. This suggests that the unusually small spatial demands of the two hydrogen substituents at aluminium may be a significant factor in permitting enlargement of the N-Al-N and narrowing of the Al-N-Al bond angle. At 1.936(3) A, the average Al-N distance differs little from those reported previously for compounds with a cyclic [AlN], skeleton (cf. cis-[Me,AlNH 1.90(5),24 Me], 1.940(5),24 trans-[Me,AlNHMe], [Me*AlN(CH,)d, 1.91(2),*’ [Me2A1NH& 1.935(5)26 and[(t-C,H,),AlNH,], 1.952(14)A;26seeTable2). The dimensions of the planar [A1Nj2 rings of the dimeric compounds [Me2A1NMe2]224 and [(Me3 Si)2AlNH2]2,28 with Al-N distances of 1.958(7) and 1.954(2) A, respectively, do not disclose any marked variation with ring size, and the cage-like molecules [PhA1NPh]4,2ga [HAlNPri]6,2gb [MeAlNMe]72gC and [HAINPI-“],*‘~ also feature Al-N distances in the range 1.9c1.96 A, only marginally longer than that in the binary compound AlN (1.893 A).2ge Al-N bonds in monoor polycyclic species are generally shorter than the ones found in acyclic systems containing a nitrogen base bound to a tetra-coordinated aluminium centre [e.g. Me3N*AlH3 2.063(8)‘* and Me,N*AlMe, 2.099(10) A30a]. Even here, however, the introduction of more electronegative substituents at aluminium leads to a shortening of the Al-N bonds [cf. Me,N - AlCl, 1.945(35)30band H,N.AlCl, 1.996(19) A3”‘j. Much shorter still, at 1.78(2) A, is the Al-N bond in the mononuclear tri-coordinated aluminium compound A1[N(SiMe3)2]3.3’ Overall, changes in the coordination number and in the charge distribution at the aluminium and nitrogen centres-with the implied changes in the relative energies of the aluminium-based LUMO and the nitrogen-based HOME-appear to be more significant influences than variations of geometry or supposed bond order. The average Al-H distance in [Me2NA1H213is, at 1.55 A, very close to those in lithium tetrahydroaluminate (1.547 A),32 Me,N.AlH, (1.560 A)” and [HAlNPi], (1.49 A) ;2gb by contrast, the corresponding distance in polymeric aluminium hydride is 1.72 A. 8The dimensions of the dimethylamido group are also in line with those of related molecules ; representative C-N bond lengths (in A) and C-N-C bond angles (in “) are as follows:[Me,NBClJ, 1.505, 108.5;33a[Me2NA1H2]3 1.505, 106.2; [Me?NAlMe,], 1.509. 107.6:24

A. J. DOWNS et al.

1300

Table 3. Vibrational spectra of dimethylamidoalane

(wave numbers in cm- ‘)

Annealed solid film at 77 K ~~o~~~~ IR spectrum”,b

[Me2NAlH& Raman spectrum”

[Me2NAlH& IR spectrum”

[Me,NAlD& IR spectrum”

v,lvn’

2977 w

2977 m

1.000

2928 m 2889 m 2839 w

2928 sh 2889 m 2840 w

1.ooo 1.ooo 1.ooo

2795 w 1847 m 1831 m 1809 m 1681 br 1479 m 1463 m 1452 m 1438 m

2801 m

0.998

1381 m

1.332

sym. v(Al-H)

1326 m

1.364

antisym. v(AI-H) ca 892 + ca 775

1460 m 1454 m

1.002 0.999

1238 w

1240m

1233 w

1.006

1170 VW 1121 VW 1106~ 1097 w 1079 VW 1039 m 1019 w

1168 w 1126m

1133 m 1119m

1.031 1.006

2981 m 2961 m 2928 s 2888 s 2844 m 2833 m 2797 m 1849 s 1828 s 1808 m 1682 br 1480 m

2920 mw 2870 mw

2800 w 1825 vs 1812 vs

1458 m 1453 sh 1437 m 1410 w 1397 w

1437 w

1265 w 1220 m 1165m 1120w 1103 ms 1070 VW 1038 w 1019 s 980 w

936 m 900 m 892 m 876 w 777 vw

880 vs 761 vs

742 m 748 vs

710 m 644m

e

Assignment

v(C-H)

+2 x 6(CH,)

(a, + e) (a, + e)

W-I,) 1

1098 m

1

1043 m 1018 m

1078 w 1043 m 1018 w

928 m 906 m 894 w 877 m 175 m

923 w 905 m 700 w 628 m 585 m

756 729 718 639 587

490 m 585 md

m s s m br

534 s 465 m

462 w

381 m 275 m

375 w 253 w

p(CH 3)+ antisym. v(N-C)

531 m

465 m 454 w 270 w

‘?w, weak ; m, medium ; s, strong; v, very ; br, broad ; sh, shoulder. b Ref. 34. ‘Calculated for corresponding bands in the IR spectra of the solids. dOverlapping of bands. e Region not studied.

1.ooo antisym. v(N-C) +p(CH,) 1.ooo 3 impurity? 1.005 sym. v(N-C)+p(CH,) 1.001 > 1.277 AIH, scissoring (a, + e) 1.396 1 AIHz waggizg (e) 1.325 + AlH, twisting (e) 1.363

AlH, rockini(a,+e) 1.304 > v(Al-N) (e) 1.003 v(Al-N) (a ,) 0.994 v(Al-N) (e) NC2 scissoring + NC* wagging

X-ray crystal structure of a discrete molecular alane 1.520, 111.8;33b [Me*NGaH& [Me2NAlC1& 1.463, 109.6;19 and [Me,NInMe,], 1.475, 109.0.33c Vibrational spectra of solid dimethylamidoalane

Details of the IR and Raman spectra of annealed solid films of dimethylamidoalane and of the IR spectra of a similar film of the AIDz isotopomer, and of a benzene solution of the isotopically natural compound (as reported previously34), are given in Table 3. The solid alane exhibits much the same IR spectrum as a benzene solution, although there are shifts of up to 30 cm- ’ in the energies of some of the bands. This tends to confirm that the trimeric structure is retained in benzene solution. I5 The spectra yield satisfactorily to analysis in terms of the group vibrations appropriate to the molecule [Me2NAlH,], (or [Me2NAlD213), which conforms to C3, symmetry. The stretching fundamentals of the [C2NAlH2]3 skeleton should therefore span the representations 5al + la2 + 6e and the remaining fundamentals (involving bending, rocking and the representations 6a1 + twisting modes) 4a2+ 10e ; only the a, and e modes are active in IR absorption and Raman scattering. Each of the assignments proposed in Table 3 is based on one or more of the following criteria : (1) analogy with the vibrational properties of other molecules containing an AlH2,” SiH2,35 GaH219 or NMe236 group or a cyclic [MNjn unit (M = Al or Ga) ;19,37(2) the selection rules and intensity patterns expected to govern the IR or Raman activity of the vibrational modes (with due allowance for the overlapping of near-degenerate modes) ; and (3) the effect of deuteriation at the AlH2 group on the wave number of a given IR absorption. Certain bands are readily identified with internal vibrations of the methyl groups forming part of the Me2N ligands : ’ 9,36 these occur in the following regions (cm- ‘) : 2800-3000, C-H stretching ; 1400-1480, CH3 deformation; and 1100-1300, CH3 rocking [admixed, at least partially, with antisymmetric v(N-C)] modes. There are no features obviously attributable to the CH3 torsional modes. The positions of the individual bands are virtually invariant, irrespective of the physical state of the compound and whether the aluminium atom is bound to hydrogen or deuterium. Hence, it is reasonable to assume that most of the remaining features in the spectra can be meaningfully interpreted in terms of the fundamentals of the C3” skeleton [C2NAlHJ3. The descriptions of the normal modes given in Table 3 appear at least to be self-consistent; how closely they represent the true picture it is impossible to say in the absence of a detailed normal coordinate analysis.

1301

AIHz groups. The four stretching vibrations of the terminal AlH2 groups (2al + 2e) are readily identified with the prominent bands at 180&1850 cm- ’ on the basis of their positions and their response to deuteriation (v&n - ,/?). That the modes occur at significantly lower energies than do the v(Ga-H) modes in the corresponding gallium compound [Me?NGaH& (187&1907 cm-‘)19 appears to be part of a general pattern [shared, for example, by the species MH4-,38a Me3N*MH338b and HMC1238C(M = Al or Ga)], in which the Al-H stretching force constant is consistently smaller than for Ga-H. Where direct comparisons can be made, terminal Al”‘-H bonds emerge as being somewhat longer (cf. Me,N. AlH, 1.560 A ;I2 Me,N * GaH, 1.497 A39) and weaker than their Ga-H counterparts, possibly because of their enhanced polarity ; by contrast, though, the univalent molecules AlH and GaH conform to the opposite pattern with w,(Al-H) > w,(Ga-H) and r,(Al-H) < r,(Ga-H). 4o The six active AlH2 deformation modes (involving scissoring, wagging, twisting or rocking motions, 2al +4e) appear, also on the evidence of their positions and response to deuteriation, to be confined to the region 60&900 cm-‘. NCzgroups. The stretching vibrations of the NC2 groups occur in the range 900-1100 cm-‘.36 The assignments proposed in Table 3 are consistent with the selection rules, with the minimal response to deuteriation at aluminium, and with the vibrational analyses carried out on other dimethylamido derivatives. 19*36 Bending modes of the NC2 group, expected at wave numbers < 400 cm- ‘, are less easy to identify but are probably responsible for IR and Raman bands in the region 20@400 cm- ‘. A13N3 ring. The A13N3 ring possesses four distinct stretching fundamentals, of which only three (a, + 2e) are active in IR absorption and Raman scattering. These we believe to be responsible for the bands between 450 and 600 cm-‘, and the assignments proposed in Table 3 are in keeping with the partial analysis described for the vibrational spectra of the cis and trans isomers of [R,AlNHMe], (R = Me or Et).4’ There are no obvious candidates for the various deformation modes of the A13N3 ring (2a’ + 2e), but these probably occur at wave numbers ~200 cm ‘, i.e. beyond the threshold of the present measurements. ‘H and 13C NMR spectra The ‘H NMR spectrum of a [2H,]toluene solution of dimethylamidoalane at 295 K showed a resonance near 6 2.2 due in part to the protons of the Me,N group and in part to residual methyl

A. J. DOWNS et al.

1302

Table 4. NMR properties of dimethylamidoalane and related compounds Compound

Solvent/T (K)

PbNAlH213 [Me,NGaH&

[*H,]toluene/203-295 C,H,/room temp.

PWW,

[‘H,]toluene/213

WW&Mn [Me,NAlMe&

[*H,]toluene/213 [*H ,]toluene/room temp. cyclohexane/room temp.

[HAl(NMe 2)J.

C,H,/room temp.

[MNMeJ312

C,H,/room temp.

[H,AlNC,H,oln Me,N*AlH, (Me,N),AlH,

C,H,/room temp. [‘H8]toluene/183 [‘H,]toluene/l83

Chemical shift (6,) Me,N protons” MH protonsD

Reference

ca 2.2 (br)b 2.3 (br)

This work e

-

-

2.38 (br) 2.33 (br) 2.81 (t) 2.49 (br) 2.77 (t) -

3.8 (t) 2.90 (br) 2.68 (br) 3.08 (br)

d

3.28 (br)

d

-

d

e

3.83 (t)

4.24 (t) 4.15 (t) 3.7 (t)

/ B 9

“M = Al or Ga; br = bridging Me,N or H unit; t = terminal Me,N or H unit. ‘Obscured partially by solvent resonance. ’ Ref. 46. dRef. 47. The two AlH resonances of [Me,AlH], are associated with different oligomers. ’ Ref. 48. /Ref. 49. gRef. 50. protons of the solvent. Only when the sample was cooled to 203 K did spin decoupling from the quadrupolar 27A1nucleus (I = 5/2)42 result in the development of a discernible broad resonance at 6 3.8, attributable to the AIHz protons of the [Me,N AlH& molecule. The NMR properties listed in Table 4 confirm that the chemical shifts tally with those of related molecules containing terminal Al-H bonds and/or Me,N groups bridging two metal centres. If the [Me2NAlH213 molecule retains its chair-like conformation in solution, the Al-H and N-CH3 protons form inequivalent sets according to whether the substituents occupy endo or exo sites. In the event, there was no hint of the doublet pattern to be expected in this case, but it was impossible to judge whether the pattern was simply masked by the broadness of the Al-H signal and overlapping of the N-CH3 signal, or whether there is rapid intramolecular exchange between the two sites, even at 203 K. The 13CNMR spectrum of the solution at 295 K included a relatively sharp resonance at 6c 42 clearly originating in the Me2N group,43 and under these conditions, at least, fast exchange appears to prevail.

Mass spectrum

The mass spectrum of the vapour over a sample of dimethylamidoalane heated to about 323 K was

dominated by three clusters of peaks with high m/z limits at 219, 146 and 73, corresponding to the molecular ions [Me2NAlHz]3f, [Me,NAlH&+ and Me2NA1Hz+, respectively. There can be little doubt, then, that the compound vaporizes as the trimer, but it is not possible to judge with certainty whether the peaks associated with the dimeric and monomeric species arise from the presence of the corresponding neutral molecules in the vapour or whether they represent fragmentation products of the trimer resulting from the ionization process. An earlier mass spectrometric study44 gave similar results, including circumstantial evidence that the family of peaks stemming from m/z = 146 is due, at least in part, to the presence in the vapour of some dimeric molecules [MezNAlH,],. It seems improbable, however, that there is an appreciable pressure of the monomer Me2NA1H2, featuring three-coordinate aluminium, at ambient temperatures and the ion Me2NAlH2+ is much more likely to be a fragmentation product. By contrast, the dimer [MezNGaH,], is the predominant species in the vapour of the corresponding gallane. I9It should be feasible to carry out matrix-isolation measurements to probe the composition of the alane vapour and assess the structures of the gaseous molecules. With the aid of the all-glass inlet system constructed for the Edinburgh electron-diffraction apparatus,3 we plan also to carry out electron-diffraction measurements on the vapour.

X-ray crystal structure of a discrete molecular alane Other features in the mass spectrum include rearrangement peaks typical of amine derivatives. These occur, for example, at m/z = 45 (due to Me,NH+) and 59 (due to MeNHAlH,+). In addition, there are prominent peaks at m/z = 27, 42,44,86,115 and 129 attributable to the molecular ions HCN+, CzH4N+, Me,N+, Me,NAlNH+, (MezN)zAl+ and Me3N2HzA1zf, respectively. The families of peaks near m/z = 219,146 and 73 imply that fragmentation occurs initially with loss of hydrogen bound to aluminium ; subsequent steps involve loss of methyl groups and of a metal atom. The related compounds [MezA1NH2]~26 and [MezGaNHz]327 display quite different fragmentation patterns, in which loss of methyl competes with loss of NH2 groups as a first step. Acknowledgements-We thank the S.E.R.C. for research grants and for the award of research studentships (to J.C.M. and C.R.P.).

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