Preparation and characterization of lithium imidoalanate complexes Li 2[(RN)4(AlH2)6] (R = Me or tBu): First examples of aluminum-rich imidoalanes with an adamantoid structure

Article abstract of DOI:10.1021/ic7006827

Whereas methylammonium chloride, [MeNH₃]Cl, reacts with LiGaH₄ in an ether solution to give, according to the conditions, either the adduct MeH₂N·GaH₃ or the cationic derivative [(MeH₂N)₂Ga

Full text of DOI:10.1021/ic7006827

Inorg. Chem. 2007, 46, 5439−5446
Preparation and Characterization of Lithium Imidoalanate Complexes
t
Li2[(RN)4(AlH2)6] (R
)
Me or Bu): First Examples of Aluminum-Rich
Imidoalanes with an Adamantoid Structure
Christina Y. Tang,† Andrew R. Cowley,† Anthony J. Downs,*,† and Simon Parsons*,‡
Inorganic Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford OX1 3QR, U.K.,
and School of Chemistry, UniVersity of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, U.K.
Received April 10, 2007
Whereas methylammonium chloride, [MeNH3]Cl, reacts with LiGaH4 in an ether solution to give, according to the
conditions, either the adduct MeH2N·GaH3 or the cationic derivative [(MeH2N)2GaH2]+Cl-, the corresponding reaction
of [MeNH3]Cl or [tBuNH3]Cl with LiAlH4 proceeds mainly, with H2 elimination, to the imidoalane Li2[(RN)4(AlH2)6]
t
(R
)
Me, 1, or Bu, 2). The crystal structure of 1·2Et2O reveals, for the first time, anionic units with an adamantane-
-
like Al6N4 skeleton. The Li cations exist at two distinct sites, each linked via Li(
µ
-H)Al bridges to two [(MeN)4(AlH2)6]2
t
cages. Despite disordering of the Bu groups, the crystal structure of 2 evidently includes analogous anionic units.
By contrast, the main product of the reaction between [iPrNH3]Cl and LiAlH4 under similar conditions is the known
neutral, hexameric imidoalane [iPrNAlH]6, 3, the crystal structure of which has been redetermined.
∆
Introduction
AlX3 + RNH3Cl -HX8 1/n[RHNAlX2]n -HX8 1/m[RNAlX]m
(1)
Imidoalanes with the generic empirical formula RNAlX
(where R ) an organic group and X ) H, an organic group,
or a halogen) can be prepared in various ways, but an
appropriate primary amine and aluminum(III) compound are
the usual precursors, as exemplified by eqs 1 and 2.1-9 An
alternative synthesis recently reported10 involves the reaction
LiAlH4 + RNH3Cl f 1/m[RNAlH]m + 3H2 + LiCl
(2)
between the organoaluminum(I) compound [Cp*Al]4 (Cp* )
C5Me5) and a trialkylsilyl azide RR′2SiN3, which proceeds
in accordance with eq 3 with N2 elimination and formation
* To whom correspondence should be addressed. E-mail:
tony.downs@chem.ox.ac.uk (A.J.D.), s.parsons@ed.ac.uk (S.P.). Tel: 0044-
(0)1865-272697. Fax: 0044-(0)1865-272690.
1/4[Cp*Al]4 + RR′2SiN3 f 1/m[Cp*AlNSiRR′2]m + N2
† University of Oxford.
(3)
‡ University of Edinburgh.
(1) Lappert, M. F.; Power, P. P.; Sanger, A. R.; Srivastava, R. C. Metal
and Metalloid Amides; Ellis Horwood: Chichester, U.K., 1980.
(2) Cesari, M.; Cucinella, S. In The Chemistry of Inorganic Homo- and
Heterocycles; Haiduc, I., Sowerby, D. B., Eds.; Academic Press:
London, 1987; Vol. 1, p 167.
(3) Rings, Clusters and Polymers of Main Group and Transition Elements;
Roesky, H. W., Ed.; Elsevier: Amsterdam, The Netherlands, 1989.
(4) Veith, M. Chem. ReV. 1990, 90, 3.
(5) Taylor, M. J.; Brothers, P. J. In Chemistry of Aluminium, Gallium,
Indium and Thallium; Downs, A. J., Ed.; Blackie Academic and
Professional: Glasgow, U.K., 1993; p 111.
(6) Chang, C.-C.; Ameerunisha, M. S. Coord. Chem. ReV. 1999, 189, 199.
(7) Aldridge, S.; Downs, A. J. Chem. ReV. 2001, 101, 3305.
(8) Uhl, W.; Roesky, H. W. Heteropolyalanes, -gallanes, -indanes and
-thallanes. In Molecular Clusters of the Main Group Elementss
Extending Borders from Molecules to Materials; Driess, M., No¨th,
H., Eds.; Wiley-VCH: Weinheim, Germany, 2004; p 357.
(9) Robinson, G. H. In ComprehensiVe Coordination Chemistry II; Parkin,
G. F. R., Ed.; Elsevier Pergamon: Oxford, U.K., 2004; Vol. 3, p 347.
(10) Schulz, S.; Thomas, F.; Priesmann, W. M.; Nieger, M. Organometallics
2006, 25, 1392.
of the corresponding imidoalane [Cp*AlNSiRR′2]m. Imi-
doalanes are noteworthy on two counts.
First, with the exception of HC(MeCDippN)2AlNC6H3-
2,6-Trip2 (Dipp ) C6H3-2,6-iPr2 and Trip ) C6H2-2,4,
6-iPr3), where a monomeric structure may well be enforced
by the uncommon bulk of the substituents R and X,11 the
compounds display considerable diversity of nuclearity and
structure, as illustrated in Figure 1, in the cluster forms they
adopt. The frameworks range from the heterocubane Al4N4
of [RNAlX]4 (I),4,8,12 through the hexagonal prismatic Al6N6
(11) Hardman, N. J.; Cui, C.; Roesky, H. W.; Fink, W. H.; Power, P. P.
Angew. Chem., Int. Ed. 2001, 40, 2172.
(12) Waggoner, K. M.; Power, P. P. J. Am. Chem. Soc. 1991, 113, 3385.
Harlan, C. J.; Bott, S. G.; Barron, A. R. J. Chem. Soc., Dalton Trans.
1997, 637.
10.1021/ic7006827 CCC: $37.00
Published on Web 06/01/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 13, 2007 5439

Tang et al.
of aluminum nitride, one of the best electrical insulators for
microelectronic applications.20
We have reported recently on how the reaction of an amine
hydrochloride, [RNH3]Cl, with LiGaH4 in an ether solution
can be made to deliver gallane adducts of the type RH2N‚
t
GaH3 (R ) Me or Bu);21 these are labile, decomposing at
or near ambient temperatures with the elimination of H2 and
formation of the corresponding amido derivative [RHNGaH2]n.
The corresponding aluminum derivatives are much more
labile, with dehydrogenation typically proceeding in ac-
cordance with eq 2 to produce the imide [RNAlH]m under
comparable conditions.2,4,7,22,23 Somewhat unexpectedly, there-
fore, we find that the reaction in an ether solution of 1 mol
of LiAlH4 with about 1.5 mol of [RNH3]Cl (R ) Me, 1, or
tBu, 2) indeed results in H2 elimination but yields as the
major aluminum product a new lithium imidoalanate with
the composition Li2Al6N4R4. This has been authenticated by
elemental analysis and by its vibrational, solid-state 27Al
NMR and mass spectra, while single-crystal X-ray diffraction
of 1‚2Et2O and 2 at 150 K establishes the formulations Li-
[Li(OEt2)2][(MeN)4(AlH2)6] and Li2[(tBuN)4(AlH2)6], respec-
tively, incorporating adamantane-like [(RN)4(AlH2)6]2- an-
ionic cages (R ) Me and tBu). In the case of 1‚2Et2O, these
anions are linked through Al-H‚‚‚Li bridges to Li+ cations
of two quite distinct types. By contrast, the main product of
the reaction between [iPrNH3]Cl and LiAlH4 carried out
under similar conditions is the known hexameric isopropy-
limidoalane, [iPrNAlH]6,24 3, the crystal structure of which
has been redetermined.
Figure 1. Cages formed by known imidoalanes.4
of [RNAlX]6 (II),4,8,13 to the more complex Al7N7 and Al8N8
frameworks of [RNAlX]7 (III) and [RNAlX]8 (IV), respec-
tively.4,8 The last two may be regarded as being formed by
the fusion of either two pseudocubic fragments or of a cubic
and hexagonal prismatic fragment. The nature of the product
is a sensitive function of the substituents R and X, the
temperature, and the solvent used. That there is but a fine
balance in the choice of product is demonstrated by the
finding that iPrNAlH may be either a tetramer or a hexamer
and that nPrNAlH occurs as either a hexamer or an octamer.4,8
Coordination by acidic or basic functions results in rupture
of some of the bonds in the cage and distortion of the regular
geometry, as in [Me2N(CH2)3NAlH]6‚2LiH14 and [iPrNAlH]6‚
AlH3.15 In addition, mixed imides add further variety to the
cage structures. For example, the cores of the compounds
(ClAl)4(NMe)2(NMe2)416 and (ClAl)2(OAl)(MeNAl)(NMe2)617
are Al4N6 and Al4N5O cages, respectively, each with an
adamantane-like structure (V); the structure of (MeNAlMe)6-
(Me2AlNHMe)2 can be derived from a hexagonal prism with
two edges being broken to insert two Me2AlNHMe bridges
(VI);18 and (iPrNAlH)2(iPrHNAlH2)3 presents an Al5N5 cage
with the structure VII.19
Experimental Section
The vacuum-line methods used for the preparation of the new
lithium imidoalanates 1 and 2 have been described elsewhere.25
Diethyl ether and other solvents were dried by standard methods
and distilled before use. The source materials (LiAlH4, [MeNH3]Cl,
[iPrNH3]Cl, [sBuNH3]Cl (sBu ) sec-butyl), and [tBuNH3]Cl), from
Aldrich Chemicals, were recrystallized before use.
IR measurements on a KBr disc were made in transmission using
a Nicolet Magna-IR 560 FTIR spectrometer; detection was with a
liquid N2-cooled MCTB detector covering the range 400-
4000 cm-1, typically at a resolution of 0.5 cm-1. Raman spectra,
excited at λ ) 514.5 nm with an Ar+ laser, were recorded with a
Dilor Labram 300 spectrometer having a CCD detector. 27Al magic-
angle spinning (MAS) NMR spectra were measured for solid
powder samples at 104.2 MHz (9.4 T) on a Varian/Chemagnetics
Infinity spectrometer using a 4 mm double-resonance probe and a
sample rotation rate of 15 kHz. To obtain quantitative spectra, a
Second, imidoalanes share with amidoalanes an (AlN)n
cagelike or cyclic skeleton that gives them the potential to
act as precursors for the chemical vapor or thermal deposition
(20) See, for example: Haussonne, J.-M. Mater. Manuf. Processes 1995,
10, 717. Bertalet, D. C.; Liu, H.; Rogers, J. W. J. Appl. Phys. 1994,
75, 5385. Rockensu¨ss, W.; Roesky, H. W. AdV. Mater. 1993, 5, 443.
(21) Marchant, S.; Tang, C. Y.; Downs, A. J.; Greene, T. M.; Himmel,
H.-J.; Parsons, S. Dalton Trans. 2005, 3281.
(22) Greenwood, N. N. In New Pathways in Inorganic Chemistry; Ebsworth,
E. A. V., Maddock, A. G., Sharpe, A. G., Eds.; Cambridge University
Press: Cambridge, U.K., 1968; p 37.
(13) Reddy, N. D.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-G. Inorg.
Chem. 2002, 41, 2374. Styron, E. K.; Lake, C. H.; Powell, D. H.;
Krannich, L. K.; Watkins, C. L. J. Organomet. Chem. 2002, 649, 78.
Reddy, N. D.; Kumar, S. S.; Roesky, H. W.; Vidovic, D.; Magull, J.;
Noltemeyer, M.; Schmidt, H.-G. Eur. J. Inorg. Chem. 2003, 442.
(14) Perego, G.; Dozzi, G. J. Organomet. Chem. 1981, 205, 21.
(15) Perego, G.; Cesari, M.; Del Piero, G.; Balducci, A.; Cernia, E. J.
Organomet. Chem. 1975, 87, 33.
(16) Thewalt, U.; Kawada, I. Chem. Ber. 1970, 103, 2754.
(17) Leggett, G.; Motevalli, M.; Sullivan, A. C. J. Organomet. Chem. 2000,
598, 36.
(18) Amirkhalili, S.; Hitchcock, P. B.; Smith, J. D. J. Chem. Soc., Dalton
Trans. 1979, 1206.
(23) Wiberg, E.; Amberger, E. Hydrides of the Elements of Main Groups
I-IV; Elsevier: Amsterdam, The Netherlands, 1971.
(24) Cucinella, S.; Salvatori, T.; Busetto, C.; Perego, G,; Mazzei, A. J.
Organomet. Chem. 1974, 78, 185. Cesari, M.; Perego, G.; Del Piero,
G.; Cucinella, S.; Cernia, E. J. Organomet. Chem. 1974, 78, 203.
(25) Pulham, C. R.; Downs, A. J.; Goode, M. J.; Rankin, D. W. H.;
Robertson, H. E. J. Am. Chem. Soc. 1991, 113, 5149. Downs, A. J.;
Pulham, C. R. AdV. Inorg. Chem. 1994, 41, 171.
(19) Perego, G.; Del Piero, G.; Cesari, M.; Zazzetta, A.; Dozzi, G. J.
Organomet. Chem. 1975, 87, 53.
5440 Inorganic Chemistry, Vol. 46, No. 13, 2007

Lithium Imidoalanate Complexes
Table 1. Crystallographic Data for Compounds 1‚2Et2O, 2, and 3
param
empirical formula
1‚2Et2O
2
3
C12H44Al6Li2N4O2
452.28
C16H48Al6Li2N4
472.36
C18H48Al6N6
510.51
fw
cryst dimens (mm)
cryst system
space group
unit cell dimens
a (Å)
0.10 × 0.12 × 0.24
monoclinic
P21/c
0.20 × 0.20 × 0.20
0.20 × 0.40 × 0.50
cubic
triclinic
Fd3hm
P 1h
13.0826(2)
12.1616(2)
18.1906(4)
90
94.7891(8)
90
2884.1
4
1.042
0.234
27.5
38 223
6851 (0.060)
286
18.3521(9)
18.3521(9)
18.3521(9)
90
90
90
6181.0
8
1.015
0.216
24.962
850
10.5708(2)
16.5470(4)
17.0406(4)
92.1463(9)
93.8153(9)
91.3870(11)
2970.90
4
1.141
0.233
27.536
23 915
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
d(calcd) (Mg m-3
)
abs coeff (mm-1
max (deg)
reflns measd
)
θ
unique reflns (Rint
)
299 (0.063)
25
13 578 (0.1078)
589
no. of params
conventional R [R > 3σ(F)]
weighted R (F2 and all data)
GOF on F2 (S)
0.0494
0.0584
1.0630
+0.68/-0.51
0.0878
0.0802
2.6310
+0.42/-0.26
0.0405
0.0449
1.0872
+0.31/-0.32
largest difference peak/hole (e Å-3
)
single pulse excitation was applied using a short pulse length
(0.83 µs). Each spectrum resulted from 600 scans separated by a 5
s delay. The 27Al chemical shifts were referenced to an aqueous
solution of Al(NO3)3 (0 ppm). Mass spectrometric measurements
were made with a Micromass GC-TOF instrument with field
ionization, using a temperature-programmed solid probe inlet. C,
H, and N analyses were carried out by the Elemental Analysis
Service at London Metropolitan University.
956 mw, 841 w, 695 mw, 607 mw, 464 w, 368 s, 261 mw, 185 w.
27Al MAS NMR spectrum of the solid powder: δAl +112. Field
ionization MS (m/z): 429 [Al9(NMe)6H12]+, 369 [Al9N4(NMe)2H12]+,
355 [Al9N3(NMe)2H12]+, 295 [Al7N3(NMe)2H6]+, 281 [Al7N2-
(NMe)2H6]+, 221 [Al6N4H3]+, 207 [Al6N3H3]+, 147 [Al3(NMe)2H8]+,
73 [Al2NH5]+, 57 [MeNAlH]+.
Similar experiments were carried out with LiAlH4 and each of
the hydrochlorides of iPrNH2, sBuNH2, and tBuNH2. However, only
[tBuNH3]Cl gave in the white solid Li2[(tBuN)4(AlH2)6], 2, a major
product analogous to 1; yields of the pure crystals were typically
about 45% based on the amount of LiAlH4 taken. Anal. Calcd for
C16H48N4Al6Li2: C, 40.68; H, 10.24; N, 11.86. Found: C, 40.52;
H, 10.25; N, 11.61. IR spectrum of a KBr disc (wavenumbers in
cm-1): 2974 m, 2957 s, 2900 m, 2865 m, 1837/1805/1732 s,vbr
[ν(Al-H)], 1469 m, 1392 m, 1364 ms, 1228 m, 1186 ms, 1094 w,
1070 m, 1033 w, 942 ms, 890 mw, 788 s, 738 m, 693 s, 639 w,
487 w. Raman spectrum of the solid (wavenumbers in cm-1):
2986 m, 2964 m, 2936 m, 2908 m, 1850 s,br [ν(Al-H)], 1482 m,
1455 m, 1233 m, 919 m, 843 mw, 796 mw, 774 mw, 518 m,
465 s, 415 w, 390 w, 356 mw, 237 mw, 225 s. 27Al MAS NMR
spectrum of the solid powder: δAl +105.
Synthesis and Authentication. Lithium tetrakis(methylimido)-
hexakis(aluminate), Li2[(MeN)4(AlH2)6], 1, was synthesized by a
procedure similar to that used for methylamine-gallane, MeH2N‚
GaH3,21 and involving the reaction of LiAlH4 (in place of LiGaH4)
with the freshly recrystallized amine hydrochloride [MeNH3]Cl in
dry Et2O solution at room temperature. In a typical experiment,
LiAlH4 (20 mmol) was treated with a 50% mol excess of [MeNH3]-
Cl (31 mmol). After filtering and concentrating by evaporating the
solvent under vacuum, the solution was left for 4-6 weeks at room
temperature; white crystals (identified by X-ray analysis of a single
crystal at 150 K as the dietherate of 1) were then observed to
separate, typically in yields of about 58% based on the amount of
LiAlH4 taken. Similar results were achieved irrespective of whether
the reaction occurred exclusively at room temperature or the
temperature of the mixture was gradually raised from 195 K to
ambient. However, the first procedure was preferred in this and
subsequent experiments, since the second was apt to lead to poorer
yields and less pure products, presumably through the survival of
one or more reaction intermediates. The crystals release 2 mol of
Et2O relatively easily but are otherwise lastingly stable under
moisture-free conditions at ambient temperatures. Indeed, there was
no sign of significant decomposition at temperatures up to ca.
160 °C, whereupon the solid began to turn yellow and dihydrogen
was evolved. The compound was insufficiently soluble in any
organic solvent with which it did not react to permit satisfactory
solution NMR measurements. Anal. Calcd for ether-free solid
C4H24N4Al6Li2: C, 15.80; H, 7.96; N, 18.43. Found: C, 15.90; H,
7.85; N, 18.56. IR spectrum of a KBr disc (wavenumbers in cm-1):
2961 ms, 2893 m, 2817 m, 1844 s [ν(Al-H)], 1473 m, 1262 s,
1099 ms, 1020 s, 956 s, 804 s, 706 s, 648 ms. Raman spectrum of
the solid (wavenumbers in cm-1): 2969 sh, 2932 s, 2899 m,
2881 sh, 2820 s, 1852 s [ν(Al-H)], 1467/1435 mw, 987 mw,
The reaction with [sBuNH3]Cl gave no crystals at room tem-
perature and only a liquid mixture of inseparable products on
removal of the solvent. That with [iPrNH3]Cl gave, by contrast,
mainly the known24 neutral hexamer of isopropylimidoalane,
[iPrNAlH]6, 3. Yields of the pure crystals were typically about 30%.
Anal. Calcd for C18H48Al6N6: C, 42.35; H, 9.47; N, 16.47. Found:
C, 42.23; H, 9.56; N, 16.38. IR spectrum of a KBr disc (wave-
numbers in cm-1): 2974 s, 2870 ms, 2726 w, 2625 m, 1850/1788/
1700 s [ν(Al-H)], 1467 m, 1385 m, 1364 m, 1327 mw, 1261 mw,
1164 ms, 1131 s, 1094 ms, 954 s, ca. 800 s,vbr, 510/490/462 mw.
Raman spectrum of the solid (wavenumbers in cm-1): 2967 ms,
2933 ms, 2902 m, 2864 ms, 2716 w, 2629 w, 1864 s [ν(Al-H)],
1462 m, 1440 mw, 1382 w, 1358 mw, 1330 mw, 1164 w, 1133
ms, 992 w, 960 w, 937 m, 847 ms, 808 m, 763 m, 695 mw, 658 m,
524 m, 495 w, 446 mw, 419 vs, 331 mw, 310 w, 288 m, 254 ms.
X-ray Crystallography. Table 1 gives crystal data and other
information relating to the structure determination and refinement
for single crystals of 1‚2Et2O, 2, and 3. Two independent sets of
measurements were made on single crystals of 1‚2Et2O, each
Inorganic Chemistry, Vol. 46, No. 13, 2007 5441

Tang et al.
tBu,21 x ) 2, R ) Me30) have been characterized, being
relatively long-lived at or near ambient temperatures despite
their innate susceptibility to decomposition through elimina-
tion of dihydrogen and formation of the corresponding
amidogallane, [RxH2-xNGaH2]n. They have been prepared
by the interaction of the appropriate amine hydrochloride
with LiGaH4 in Et2O solution. However, the reaction
represented by eq 4 is by no means the only one taking place
in such a mixture and, depending on the conditions and
mounted under perfluoropolyether oil on a glass fiber. X-ray
diffraction data (λ (Mo KR) ) 0.710 73 Å) were then collected on
a Bruker Smart Apex diffractometer in one case and on an Enraf-
Nonius Kappa CCD diffractometer in the second. Single crystals
of 2 and 3, grown by slow crystallization from Et2O solution aided
by petroleum infusion, were each similarly mounted under per-
fluoropolyether oil, and X-ray data (λ(Mo KR ) 0.710 73 Å) were
collected on an Enraf-Nonius Kappa CCD diffractometer. In every
case, the temperature of the crystal was maintained at 150 K by an
Oxford Cryosystems Cryostream device.26 Semiempirical absorption
corrections were made from equivalent reflections.27 The intensity
data were processed using the DENZO-SMN package.27
Examination of the systematic absences showed the space group
of the monoclinic crystals of 1‚2Et2O to be P21/c. With the direct
methods program SIR92,28 the structure was solved, the Al, O, N,
and methyl C atoms of the anion being located. The Li and
remaining C atoms were located in a Fourier difference map.
Subsequent full-matrix least-squares refinement was carried out with
the CRYSTALS program suite.29 Coordinates and anisotropic thermal
parameters of all non-hydrogen atoms were refined. The AlH2
hydrogen atoms were located in a Fourier difference map and their
coordinates and isotropic thermal parameters subsequently refined.
Other hydrogen atoms were positioned geometrically after each
cycle of refinement. A three-term Chebychev polynomial weighting
scheme was applied. Refinement converged satisfactorily to give
R ) 0.0494, Rw ) 0.0584.
particularly the reacting proportions, other products may be
isolated, including, for example, the cationic gallane deriva-
tives [H2Ga(NH2R)2]+Cl- (R ) Me, iPr, sBu, or tBu)31,32 and
i
i
[H2Ga(NH2 Pr)NH(iPr)GaH2NH(iPr)GaH2(NH2 Pr)]+Cl-.32
2[RNH3]Cl + LiGaH4 f [(RH2N)2GaH2]+Cl- +
LiCl + 2H2 (5)
2[(RH2N)2GaH2]+Cl- + LiGaH4 f
[H2Ga(NH2R)NH(R)GaH2NH(R)GaH2(NH2R)]+Cl- +
LiCl + 2H2 (6)
The crystal structure of 2 was modeled in Fd3hm, with the
[(tBuN)4(AlH2)6]2- anion located about a site of 4h3m (Td) symmetry.
The pivot carbon atom (C1) of the tBu group sits on a site of 0.3m
(C3V) symmetry, but the methyl groups (C2) do not lie in the
associated mirror planes and are therefore disordered over two
orientations. The anisotropic displacement parameters of the carbon
atoms are large, and it is likely that the description presented here
is an approximate, average structure; though detailed analysis of
the geometry would be inappropriate, this structure does establish
the atomic connectivity in 2. Attempts to model the structure in
Fd3h or with the methyl groups represented as a diffuse ring of
density were no more successful, yielding no improvement in the
data-fitting. The C1-C2 distance was restrained to 1.50(1) Å during
refinement. The Li and the H atoms attached to Al were located in
a difference map, and it was even possible to refine the hydride
positional and isotropic displacement parameters freely. The H
Primary and secondary amine adducts of alane are much
more labile than their gallane counterparts, decomposing
rapidly with the elimination of H2 at ambient temperatures.
By carrying out the reaction between [Me2NH2]+Cl- and
LiAlH4 in an ether solution at ca. 248 K and causing
crystallization to occur at 203 K, however, it has been
possible to isolate the dimethylamine adduct Me2(H)N‚
AlH3.31 Adducts that are thermally robust at room temper-
ature can be formed, though only with sterically demanding
secondary amines such as 2,2,6,6-tetramethylpiperidine.33
Spurred by the success of recent experiments aimed at
isolating primary amine adducts of gallane, we have carried
out similar experiments to see whether, by working at
appropriately low temperatures, analogous alane derivatives
can be intercepted before they start to decompose. In no case
has this yet proved feasible, but we have found that the
reaction of LiAlH4 with about 1.5 mol of [RNH3]Cl (R )
t
atoms of the Bu group were placed in calculated positions. All
non-H atoms were modeled with anisotropic displacement param-
eters. At 8%, the final R factor is high by modern standards, but
the value reflects the difficulties of modeling the disorder.
There were no problems of disordering with the triclinic crystal
of the hexamer 3. The structure determined for the crystal at 150
K proved not to be significantly different from that determined
previously when the crystal was presumably at room temperature.24
t
Me or Bu) in an ether solution results in dihydrogen
elimination even at 195 K and that the filtered solution yields
on standing at or near room-temperature white crystals of a
new, major product shown by its elemental analysis,
spectroscopic properties, and single-crystal X-ray analysis
to be the dilithium salt of the anion [(RN)4(AlH2)6]2- (R )
Results and Discussion
Selected primary and secondary amine adducts of gallane
with the general formula RxH3-xN‚GaH3 (x ) 1, R ) Me or
t
Me, 1, or Bu, 2).
(26) Cosier, J.; Glazer, A. M. J. Appl. Crystallogr. 1986, 19, 105.
(27) Otwinowski, Z.; Minor, W. Processing of X-ray Diffraction Data
Collected in Oscillation Mode. In Methods in Enzymology; Carter, C.
W., Sweet, R. M., Eds.; Academic Press: New York, 1997; Vol. 276.
(28) Altomare, A.; Cascarano, G.; Giacovazzo, G.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435.
(29) Watkin, D. J.; Prout, C. K.; Carruthers, J. R.; Betteridge, P. W.; Cooper,
R. I. CRYSTALS, issue 11; Chemical Crystallography Laboratory:
Oxford, U.K., 2001.
(30) Tang, C. Y.; Coxall, R. A.; Downs, A. J.; Greene, T. M.; Parsons, S.
J. Chem. Soc., Dalton Trans. 2001, 2141.
(31) Tang, C. Y.; Downs, A. J.; Greene, T. M.; Marchant, S.; Parsons, S.
Inorg. Chem. 2005, 44, 7143.
(32) Tang, C. Y.; Cowley, A. R.; Downs, A. J., unpublished results.
(33) Atwood, J. L.; Koutsantonis, G. A.; Lee, F.-C.; Raston, C. L. J. Chem.
Soc., Chem. Commun. 1994, 91.
5442 Inorganic Chemistry, Vol. 46, No. 13, 2007

Lithium Imidoalanate Complexes
Similar experiments involving the hydrochlorides of other
primary amines, viz., [iPrNH3]Cl and [sBuNH3]Cl, were
likewise unable to forestall dihydrogen elimination but have
failed so far to deliver any products analogous to 1 and 2.
Instead, [iPrNH3]Cl gives under these conditions mainly the
neutral isopropylimidoalane known previously24 to occur as
a crystalline hexamer [iPrNAlH]6. Evaporation of the solvent
from the products of the reaction with [sBuNH3]Cl yielded
only a viscous liquid mixture from which no pure product
could be isolated.
X-ray analysis of the crystals formed at 273 K and then
held at 150 K indicated that 1 was first isolated as a diethyl
ether solvate. Warming to room temperature, however,
resulted in the slow loss of the ether with breakup of the
individual crystals, and subsequent elemental and spectro-
scopic analysis then found the solid to be more-or-less ether-
free. Thus, C, H, and N elemental analyses were consistent
with the empirical formula R2H6N2Al3Li for 1 (R ) Me)
and 2 (R ) tBu), respectively. The vibrational spectra gave
no hint of significant features in the range of 3100-
3500 cm-1 suggestive of the presence of N-H bonds.34 On
the other hand, they did include distinctive strong absorption
or scattering near 1850 cm-1 that was clearly indicative of
the presence of Al-H bonds,7,23 as in [Me2NAlH2]3, for
example,35 as well as a pattern of bands at 2800-3000,
1400-1500, and 900-1200 cm-1 that was consistent with
the presence of an NMe or NtBu moiety.21,34
The mass spectrum of 1 gave no hint of molecular
fragments exhibiting the isotopic patterns characteristic of
Li, while the lack of a second naturally abundant isotope of
Al or N prevented any unambiguous interpretation of the
results. However, the pattern of the mass peaks suggested
as a recurring feature the Al3N2 unit in monomeric, dimeric,
or trimeric guises, with varying numbers of H and Me
substituents attached. Thus, the strongest peak (at m/z ) 221)
appeared to correspond to [Al6N4H3]+, and the different
progressions implied that fragmentation of clusters was
occurring with the progressive loss of H, Me, N, and Al
constituents.
With solution NMR measurements being precluded by the
low solubility of 1 and 2 in organic solvents with which they
did not react, the 27Al MAS NMR spectra of each of the
solid powders at room-temperature revealed a single reso-
nance with δAl ) +112 (1) or +105 (2), a shift characteristic
of a tetrahedrally coordinated aluminum center.36
Two independent structural analyses were carried out on
single crystals of 1‚2Et2O isolated from an ether solution at
ambient temperatures, mainly to check the reproducibility
of the method of synthesis, with results that were identical
within the limits of experimental uncertainty. The most
striking feature of the structure is the [(MeN)4(AlH2)6]2-
anion, which occupies a site with no crystallographic
Figure 2. Structure and atom labeling of the [(MeN)4(AlH2)6]2- anion in
1‚2Et2O.
symmetry. This is accompanied by one [Li(OEt2)2]+ cation,
also on a site with no crystallographic symmetry, and two
unsolvated Li+ cations on crystallographic centers of inver-
sion. As illustrated in Figure 2, the anion has an Al6N4
adamantoid framework subject to only minor deviations from
ideal Td symmetry. Salient bond lengths and angles are listed
in Table 2. The Al-N distances, ranging from 1.890(3) to
1.912(3) Å, average 1.900 Å, that is, well within the range
normally associated with AlN cluster compounds.37 Despite
the negative charge, the distances are quite short by the
standards of heterocubane imides of the type [RNAlX]4
(1.89-1.948 Å)4,8,12 but neither more nor less than average
length by the standards of the hexagonal faces of hexagonal
prismatic imides of the type [RNAlX]6 (1.873-1.926 Å).4,8,13
The Al-N-Al angles vary between 109.9(2)° and 115.5-
(2)° and the N-Al-N angles between 103.9(2)° and 105.7-
(2)°. There is no hint of Al‚‚‚Al interaction within the Al6N4
cage, with Al‚‚‚Al distances at 3.11-3.22 Å appreciably
longer than those in either aluminum metal (2.86 Å) or a
heterocubane imidoalane of the type [RNAlX]4 (ca.
2.7 Å).4,8,12 With Al-H and N-C bonds measuring on
average 1.52 and 1.514 Å, respectively, and H-Al-H angles
near 112°, the bonding of the substituents is plainly not out
of the ordinary.37 Although the charge and the spatial and
electronic demands of the substituents attached to the Al and
N atoms undoubtedly play their part in determining the Al-N
distances in an AlN cage such as this, mutual Al‚‚‚Al and
N‚‚‚N repulsions are likely also to be a factor, so that Al-N
distances tend to shorten as the Al-N-Al and N-Al-N
angles are able to widen. Neither of the compounds (ClAl)4-
(NMe)2(NMe2)416 and (ClAl)2(OAl)(MeNAl)(NMe2)6,17 pre-
viously described as featuring adamantoid frameworks, is
(34) Socrates, G. Infrared and Raman Characteristic Group Frequencies;
3rd ed.; Wiley: Chichester, U.K., 2001.
(35) Downs, A. J.; Duckworth, D.; Machell, J. C.; Pulham, C. R.
Polyhedron 1992, 11, 1295.
(36) See Downs, A. J. In Chemistry of Aluminium, Gallium, Indium and
Thallium; Downs, A. J., Ed.; Blackie Academic and Professional:
Glasgow, U.K., 1993; p 25.
(37) Allen, F. H. Acta Crystallogr., Sect. B 2002, 58, 380. Bruno, I. J.;
Cole, J. C.; Edgington, P. R.; Kessler, M.; Macrae, C. F.; McCabe,
P.; Pearson, J.; Taylor, R. Acta Crystallogr., Sect. B 2002, 58, 389.
Inorganic Chemistry, Vol. 46, No. 13, 2007 5443

Tang et al.
in the packing diagram of Figure 3. The ether-free Li+ cations
occupy roughly octahedral sites, forming tridentate Al(µ-
H)3Li bridges to each of two neighboring anions, and thus
establishing a network of infinite chains. The Li‚‚‚H distances
cover the range of 1.98(4)-2.21(4) Å, with a mean of
2.09 Å. The [Li(OEt2)2]+ ions, with Li-O coordinate links
measuring 1.961(7) Å, are each linked to a single
[(MeN)4(AlH2)6]2- anion via a bidentate Al(µ-H)2Li bridge
with Li‚‚‚H distances of 1.92(3) and 2.03(4) Å. The
impression of a distorted tetrahedral coordination geometry
is, however, qualified by the perception of a further,
monodentate Al(µ-H)Li bridge to a second anion, this time
with a significantly longer Li‚‚‚H distance of 2.56(4) Å. At
1.961(7) Å, the Li-O coordinate links are slightly longer
than those in [LiCl(OEt2)]4 and [LiI(OEt2)]4 (1.91-1.92 Å)41
and considerably longer than those in [LiBr(OEt2)]4
(1.84 Å).42 The ethyl groups of the ether molecules appeared
to display a distorted geometry with abnormally large thermal
parameters, suggesting that they are disordered. Attempts to
resolve this disorder did not lead to any significant improve-
ment in agreement between the calculated and observed
intensities. In view of the relatively loose binding of the ether
molecules, however, such disorder in the crystal would come
as no surprise.
Table 2. Selected Distances (Å) and Interbond Angles (deg) for the
Compound 1‚2Et2O
Distancesa
Al(1)-N(1)
Al(1)-N(2)
Al(1)-H(1)
Al(1)-H(2)
Al(2)-N(1)
Al(2)-N(3)
Al(2)-H(3)
Al(2)-H(4)
Al(3)-N(2)
Al(3)-N(3)
Al(3)-H(5)
Al(3)-H(6)
Al(4)-N(1)
Al(4)-N(4)
Al(4)-H(7)
Al(4)-H(8)
Al(5)-N(2)
Al(5)-N(4)
Al(5)-H(9)
Al(5)-H(10)
Al(6)-N(3)
Al(6)-N(4)
Al(6)-H(11)
Al(6)-H(12)
1.890(3)
1.912(3)
1.46(4)
1.57(3)
1.904(3)
1.909(3)
1.47(5)
1.50(3)
1.907(3)
1.897(3)
1.60(4)
1.48(4)
1.896(3)
1.903(2)
1.54(4)
1.51(3)
1.898(3)
1.889(2)
1.58(4)
1.50(3)
1.898(3)
1.893(3)
1.48(3)
1.56(4)
N(1)-C(1)
N(2)-C(2)
N(3)-C(3)
N(4)-C(4)
1.519(5)
1.512(4)
1.515(5)
1.510(4)
Li(1)-H(4)
Li(1)-H(8)
Li(1)-H(12)
Li(2)-H(5)
Li(2)-H(10)
Li(2)-H(11)
Li(3)-O(1)
Li(3)-O(2)
Li(3)-H(2)
Li(3)-H(7)
Li(3)-H(9)
2.06(3)
2.00(4)
2.18(4)
1.98(4)
2.21(4)
2.13(3)
1.961(6)
1.961(7)
1.92(3)
2.56(4)
2.03(4)
O(1)-C(5)
O(1)-C(7)
O(2)-C(9)
O(2)-C(11)
1.430(4)
1.455(5)
1.410(6)
1.436(6)
Anglesa
105.67(12) N(1)-Al(4)-N(4)
N(1)-Al(1)-N(2)
N(1)-Al(1)-H(1)
N(2)-Al(1)-H(1)
N(1)-Al(1)-H(2)
N(2)-Al(1)-H(2)
H(1)-Al(1)-H(2)
N(1)-Al(2)-N(3)
N(1)-Al(2)-H(3)
N(3)-Al(2)-H(3)
N(1)-Al(2)-H(4)
N(3)-Al(2)-H(4)
H(3)-Al(2)-H(4)
N(2)-Al(3)-N(3)
N(2)-Al(3)-H(5)
N(3)-Al(3)-H(5)
N(2)-Al(3)-H(6)
N(3)-Al(3)-H(6)
H(5)-Al(3)-H(6)
105.36(11)
109.8(14)
109.6(13)
107.5(13)
108.1(13)
116.0(19)
105.40(11)
109.4(13)
108.3(13)
109.7(14)
107.5(14)
116.0(18)
105.36(12)
110.3(13)
110.0(13)
108.9(14)
113.1(14)
116.5(17)
110.9(17)
107.3(12)
106.2(12)
109.6(21)
N(1)-Al(4)-H(7)
N(4)-Al(4)-H(7)
N(1)-Al(4)-H(8)
N(4)-Al(4)-H(8)
H(7)-Al(4)-H(8)
Although disorder permitted the determination of only a
rather imprecise crystal structure for 2, the results are wholly
consistent with the presence of an anion [(tBuN)4(AlH2)6]2-
analogous to that in 1‚2Et2O. The Al6N4 framework has
crystallographic Td symmetry, and the refinements revealed
a tBu-substituted anion having mean Al-N, C-N, and Al-H
distances estimated to be 1.899(3), 1.532(13), and 1.42(6)
Å, respectively, and therefore entirely in keeping with the
precedents set by the Me-substituted anion of 1‚2Et2O. With
the location of the Li atoms in the crystal, it was apparent
that the structure consists of Al6N4 clusters linked through
hydride bridges to six-coordinate Li atoms to build up a
three-dimensional framework (see Figure 4).
103.85(12) N(2)-Al(5)-N(4)
113.4(18)
113.9(17)
108.4(13)
105.9(13)
110.9(22)
N(2)-Al(5)-H(9)
N(4)-Al(5)-H(9)
N(2)-Al(5)-H(10)
N(4)-Al(5)-H(10)
H(9)-Al(5)-H(10)
104.23(12) N(3)-Al(6)-N(4)
106.6(13)
107.0(14)
112.3(17)
115.4(17)
110.8(22)
N(3)-Al(6)-H(11)
N(4)-Al(6)-H(11)
N(3)-Al(6)-H(12)
N(4)-Al(6)-H(12)
H(11)-Al(6)-H(12) 109.2(19)
Al(1)-N(1)-Al(2) 112.92(15) Al(2)-N(3)-Al(3)
Al(1)-N(1)-Al(4) 110.65(14) Al(2)-N(3)-Al(6)
Al(2)-N(1)-Al(4) 111.39(14) Al(3)-N(3)-Al(6)
115.49(16)
110.43(14)
110.10(15)
106.2(3)
107.7(2)
106.4(3)
The structure determined for the triclinic crystals of the
neutral alane, [iPrNAlH]6, 3, did not differ significantly from
that reported previously by Cesari et al. in 1974.24 Hence
the individual molecules consist of hexagonal prismatic
Al6N6 cages (II) built up from two six-membered, planar
(AlN)3 rings joined together by transverse Al-N bonds.
Within the rings, the Al-N distances span the range 1.889-
(3)-1.912(3) Å, averaging 1.900 Å, whereas the transverse
Al-N distances, spanning the range of 1.948(3)-1.963(3)
Å and averaging 1.956 Å, are somewhat longer. This
difference can be explained by the justifiable assumption of
Al(1)-N(1)-C(1)
Al(2)-N(1)-C(1)
Al(4)-N(1)-C(1)
107.5(2)
108.0(2)
106.1(2)
Al(2)-N(3)-C(3)
Al(3)-N(3)-C(3)
Al(6)-N(3)-C(3)
Al(1)-N(2)-Al(3) 113.36(14) Al(4)-N(4)-Al(5)
Al(1)-N(2)-Al(5) 110.85(13) Al(4)-N(4)-Al(6)
Al(3)-N(2)-Al(5) 109.90(15) Al(5)-N(4)-Al(6)
111.56(12)
110.90(13)
111.75(12)
106.49(19)
108.56(19)
107.3(2)
Al(1)-N(2)-C(2)
Al(3)-N(2)-C(2)
Al(5)-N(2)-C(2)
107.5(2)
108.0(2)
106.9(2)
Al(4)-N(4)-C(4)
Al(5)-N(4)-C(4)
Al(6)-N(4)-C(4)
a See Figure 2 for atom labeling.
strictly comparable with the [(MeN)4(AlH2)6]2- anion. Not
only is there a reversal of the roles of metal and nonmetal
atoms but the cages are also made less regular by the
inclusion of more than one type of metal or nonmetal site.
A similar reversal of roles in an adamantoid framework is
also experienced when N is traded for other nonmetal atoms,
as in (Et2Al)4[(PH)4(SiiPr2)2].38
(39) Hauback, B. C.; Brinks, H. W.; Fjellvåg, H. J. Alloys Compd. 2002,
346, 184. Kang, J. K.; Lee, J. Y.; Muller, R. P.; Goddard, W. A., III
J. Chem. Phys. 2004, 121, 10623.
(40) Andrianarison, M. M.; Avent, A. G.; Ellerby, M. C.; Gorrell, I. B.;
Hitchcock, P. B.; Smith, J. D.; Stanley, D. R. J. Chem. Soc., Dalton
Trans. 1998, 249. Gardiner, M. G.; Lawrence, S. M.; Raston, C. L.
Inorg. Chem. 1999, 38, 4467.
(41) Mitzel, N. W.; Lustig, C. Z. Naturforsch., B: Chem. Sci. 2001, 56,
443. Brym, M.; Jones, C.; Junk, P. C.; Kloth, M. Z. Anorg. Allg. Chem.
2006, 632, 1402.
39
As in the case of solid LiAlH4 and related lithium
derivatives,40 the lithium cations fulfill a vital role in directing
the packing of the [(MeN)4(AlH2)6]2- anions, as indicated
(42) Naumann, F.; Hampel, F.; Schleyer, P. v. R. Inorg. Chem. 1995, 34,
6553.
(38) von Ha¨nisch, C. Eur. J. Inorg. Chem. 2003, 2955.
5444 Inorganic Chemistry, Vol. 46, No. 13, 2007

Lithium Imidoalanate Complexes
Figure 3. Chain formation via interactions with Li+ ions in the crystal structure of 1‚2Et2O. Ellipsoids are drawn at the 30% probability level, and H atoms
are shown as circles of arbitrary radius.
Scheme 1
91.2°. The mean extracage distances N-C and Al-H
average 1.512 and 1.50 Å, respectively. The packing of the
molecules is essentially determined by the methyl‚‚‚methyl
contacts.
With a reaction mixture that offers a variety of possible
chemical pathways, it is not easy to judge how the adaman-
t
toid anion [(RN)4(AlH2)6]2- (R ) Me or Bu) comes to be
formed. Perhaps the most obvious starting point is the alane
adduct RH2N‚AlH3. In contrast to its gallane counterpart,21
this may be expected to suffer H2 elimination even at
subambient temperatures, very likely to form the correspond-
ing alkylimidoalane, which may well be a tetramer, [RNAlH]4,
with a heterocubane framework analogous to that of one form
of the corresponding isopropylimido derivative.4,8 This may
then react with 2 mol of AlH4- (see Scheme 1) with retention
of the tetrahedral (NR)4 fragment but expansion of the Al4
tetrahedron to an Al6 octahedron. Alternatively, the tetramer
[RNAlH]4 may react with 2 mol of RH2N‚AlH3 in a similar
Figure 4. (a) Crystal structure of 2 viewed along b and showing how the
Li+ ions link the [(tBuN)4(AlH2)6]2- anions into a three-dimensional array.
manner but with H2 elimination to form the neutral hexamer
The methyl groups have been omitted for clarity. Ellipsoids are drawn at
[RNAlH]6. Considerable circumstantial evidence suggests
the 30% probability level. (b) Alternative ball-and-stick representation.
that AlN cages are susceptible to quite facile changes of
nuclearity and form,4,8,12,13 with the faces of the cages open
greater s character in the bonds of the (AlN)3 faces. Within
these faces, the Al-N-Al and N-Al-N bond angles
to nucleophilic attack.43
average 123.2° and 116.5°, respectively, while the corre-
sponding angles made by the transverse bonds are 88.7° and
However, the reactions of primary amine hydrochlorides
with LiGaH4 are also instructive, since intermediate gallane
Inorganic Chemistry, Vol. 46, No. 13, 2007 5445

Tang et al.
derivatives are less reactive and so are more amenable to
isolation and characterization. With an excess of the hydro-
chloride, the reactions have been shown to afford cationic
gallane derivatives of the type [(RH2N)2GaH2]+Cl-, where
dimerization with deprotonation at the bridging amido
functions (before, during, or after the event) and expulsion
of the coordinated primary amine molecules would provide
a feasible route to [(RN)4(AlH2)6]2-. Such a mechanism
seems more in keeping with the conditions favored in our
experiments (with an excess of the hydrochloride) than the
earlier postulate involving tetrameric [RNAlH]4.
R ) Me, Pr, Bu, or Bu, as in eq 5.30,31 Cationic alane
derivatives have also been prepared. For example, treatment
of Me3N‚AlH3 with a tridentate or tetradentate nitrogen base
L yields products of the type [H2AlL]+[AlH4]-,44 in which
the cationic centers are stabilized by penta- or hexacoordi-
nation of the metal atom (cf. tetracoordination of the gallane
i
s
t
With the discovery of anions of the type [(RN)4(AlH2)6]2-
t
(R ) Me or Bu) with an adamantoid framework, we have
thus been able to add a new aluminum-rich example to the
remarkable range of cage structures open to imidoalanes.4,8
In the process we have revealed a hitherto unknown
excursion taken by the interaction of a primary amine with
an aluminum(III) hydride. Our results thus add to the
experience of previous studies4,8,12,13 in demonstrating how
critical to the outcome the reaction conditions can be. That
this outcome is also markedly dependent on the nature of
the alkyl substituent at nitrogen is underlined, moreover, by
the different responses of the amine hydrochlorides [RNH3]-
Cl to LiAlH4, even under a given set of conditions, according
i
cations). In the case where R ) Pr, a further reaction of
2 mol of [(RH2N)2GaH2]Cl with LiGaH4 results, ac-
cording to eq 6, in the elimination of LiCl and 2 mol
of H2 with the formation of the trigallane derivative
[H2Ga(NH2R)NH(R)GaH2NH(R)GaH2(NH2R)]+Cl-, in which
the cation has the structure 4.32 There is no reason to think
that a product analogous to this cannot be formed when
i
s
t
to whether R ) Me, Pr, Bu, or Bu.
Acknowledgment. The Engineering and Physical Sci-
ences Research Council (U.K.) is thanked for financial
support of the Oxford and Edinburgh research groups and
for funding a research studentship and, later, a postdoctoral
assistantship (for C.Y.T.). We are grateful to Dr. N. Rees
for measuring the 27Al MAS spectra.
[RNH3]Cl reacts with LiAlH4, with the difference that the
more acidic and coordinatively unsaturated Al atoms would
be expected to induce further, facile reactions. For example,
Supporting Information Available: Crystallographic data in
CIF format for the compounds 1‚2Et2O, 2, and 3. This material is
available free of charge via the Internet at http://pubs.acs.org.
(43) Perhaps the clearest evidence of this is seen in the structure of the
loosely bound complex PriNdNPri‚[PriNAlH]4. See Uhl, W.; Molter,
J.; Neumu¨ller, B. Chem.sEur. J. 2001, 7, 1512.
(44) Atwood, J. L.; Robinson, K. D.; Jones, C.; Raston, C. L. J. Chem.
Soc., Chem. Commun. 1991, 1697.
IC7006827
5446 Inorganic Chemistry, Vol. 46, No. 13, 2007