Dimeric piperidino-alane and -gallane: Metal hydrides with a cyclic M(μ-N)2M core (M = Al or Ga)

Article abstract of DOI:10.1039/b209669m

The crystal structures of piperidino-alane and -gallane at 150 K have each been shown to consist of dimeric molecules [CH₂(CH₂)₄NMH₂]₂ centred on a planar, nearly square M(μ-N)₂M core (M = Al or Ga). The molecular structures thus contrast with the hydrogen-bridged units favoured by the sterically encumbered piperidino derivatives {[CMe₂(CH₂)₃CMe₂N] AlH₂}₃ and {[CMe₂(CH₂)₃CMe₂N]₂ AlH}₂ and are also compared with those of other amido derivatives of the Group 13 hydrides.

Full text of DOI:10.1039/b209669m

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Dimeric piperidino-alane and -gallane: metal hydrides with a cyclic
M(ꢀ-N)2M core (M ؍ Al or Ga)
Christina Y. Tang,a Anthony J. Downs,*a Tim M. Greeneb and Simon Parsons*c
a Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford,
UK OX1 3QR
b School of Chemistry, University of Exeter, Stocker Road, Exeter, UK EX4 4QD
c Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh,
UK EH9 3JJ
Received 2nd October 2002, Accepted 17th December 2002
First published as an Advance Article on the web 28th January 2003
The crystal structures of piperidino-alane and -gallane at 150 K have each been shown to consist of dimeric
molecules [CH2(CH2)4NMH2]2 centred on a planar, nearly square M(µ-N)2M core (M = Al or Ga). The molecular
structures thus contrast with the hydrogen-bridged units favoured by the sterically encumbered piperidino derivatives
{[CMe2(CH2)3CMe2N]AlH2}3 and {[CMe2(CH2)3CMe2N]2AlH}2 and are also compared with those of other amido
derivatives of the Group 13 hydrides.
Amido compounds of the types R2NMH2 and (R2N)2MH are
among the most robust derivatives of the Group 13 hydrides
MH3, where M = B, Al or Ga.1–3 Access to them is usually
gained via the adducts of the secondary amines R2(H)NؒMH3
and R2(H)NؒM(H)2(NR2). These eliminate dihydrogen to form
the corresponding amidometal derivatives [as in eqn. (1)], with
a facility that increases in the order B < Ga < Al.3 The deriv-
atives are noteworthy for the diversity of structures they adopt
and for their possible application as precursors to the binary
nitrides MN.2,4
(1)
been deliberately introduced into the 2 and 6 positions of the
The monomeric molecules R2NMH2 and (R2N)2MH nor-
piperidine ring with the aim of increasing the bulk of the amido
mally gain extra stability through augmenting the coordination
of the metal centre in cyclic oligomers. The metal atoms are
group in piperidinoalane derivatives.5 2,6-Dimethylpiperidino-
alane, CHMe(CH2)3CHMeNAlH2, is then reported to be a
dimer in the solid state but disorder has prevented a fuller struc-
tural characterisation. By contrast, 2,2,6,6-tetramethylpiper-
then bridged, usually by the amido but occasionally by
hydrido5 ligands. A sufficiently bulky amido group may yet
result in kinetic suppression of oligomerisation, but although
the homoleptic amides Al(NPri2)3 and Al[N(SiMe3)2]3 are
indeed monomeric,1 no examples of monomeric hydrido deriv-
atives that are long-lived under normal conditions have been
reported, so far as we are aware. Depending on the substituents
R, oligomerisation yields either dimeric or trimeric products,
usually with a cyclic M2N2 or M3N3 core, respectively. Increas-
ing the bulk of R favours the dimeric structure I, as exemplified
idinoalane, CMe2(CH2)3CMe2NAlH2 [(tmp)AlH2] appears on
the evidence of its IR and NMR spectra5 to be a trimer, but
with an Al3H3 cyclic skeleton akin to that found in [Me2AlH]3,14
rather than the normal Al3N3 unit. Such hydrogen-bridging has
been confirmed in the bis(piperidino)alane, (tmp)2AlH, the
crystal structure of which5 is composed of dimeric units with
a central Al(µ-H)2Al framework (III) similar to that in
[Me2AlH]2.14
6
by [Et2NGaH2]2 and [Me2N(H)NGaH2]2,7 or IV, as exempli-
fied by [HAl(NMe2)2]2,8 since these allow a greater separation
between the substituents bound to M and those bound to
nitrogen. When R is relatively compact, however, the trimeric
structure II is preferred, as in [Me2NAlH2]3 9 and [H2NGaH2]3,4
typically with a chairlike conformation for the 6-membered
M3N3 ring. That the balance between dimeric and trimeric
options may be a fine one is shown by dimethylamidogallane,
Me2NGaH2, which crystallises as a trimer10 yet vaporises at low
pressure as a dimer.11
Here we report the synthesis and X-ray structure analysis of
the simple piperidino derivatives CH2(CH2)4NMH2 for M = Al
and Ga which we show to be dimers with the piperidino ligands
fulfilling the bridging role to produce cyclic M(µ-N)2M
skeletons.
Experimental
Synthesis of piperidino-alane and -gallane
The amido function may be a heterocyclic unit. Thus, various
compounds of the type [CH2(CH2)xNMH2]n (x = 1–4; M = B, Al
or Ga) were reported by Storr et al. in 1972.12 Cryoscopic
measurements on benzene solutions indicated degrees of
association in the range n = 2–3. The gallium compound
Piperidinoalane, CH2(CH2)4NAlH2, 1, was prepared from
freshly recrystallised LiAlH4 (0.31 g, 8.1 mmol) and piper-
idinium chloride (0.78 g, 7.5 mmol) by the procedure used
for the synthesis of dimethylamidoalane.9 The cold solution
was filtered and the filtrate kept at 253 K over 3 weeks to give
colourless crystals of 1, which were stable in vacuo at temper-
atures up to ca. 273 K but decomposed rapidly at higher
CH2CH2NGaH2 has been shown to crystallise as a trimer based
on a cyclic chairlike Ga3N3 framework,13 but definitive struc-
tural information is otherwise quite sparse. Methyl groups have
D a l t o n T r a n s . , 2 0 0 3 , 5 4 0 – 5 4 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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Table 1 Crystal data, data collection and structure refinement for piperidinoalane, 1, and piperidinogallane, 2
Property
1
2
Space group
a/Å
b/Å
c/Å
P21/c
P21/c
8.7898(12)
6.1163(8)
12.9233(17)
92.009(2)
694.34(16)
2
8.7830(18)
6.1200(12)
12.930(3)
92.87(3)
694.1(2)
2
β/Њ
V/Å3
Z
Data/restraints/parameters
Conventional R [F > 4σ(F)]
Weighted R (F 2 and all data)
Largest difference peak and hole/e ÅϪ3
1419/1/70
R1 0.0412 (1189 data)
0.1202
1223/1/70
R1 0.0441 (1098 data)
0.1134
0.56, Ϫ0.24
1.08, Ϫ1.22
temperatures with the formation of dihydrogen and an
aluminium mirror.
The corresponding gallium compound, 2, was formed in
a similar manner from freshly prepared15 LiGaH4 (0.65 g,
8.1 mmol) and recrystallised piperidinium chloride (0.78 g,
7.5 mmol). Filtering of the solution and evaporation of the
solvent gave the adduct CH2(CH2)4(H)NؒGaH3 as a white solid;
heating this to 333 K in vacuo produced dihydrogen and colour-
less crystals of 2 which condensed on the walls of the reaction
vessel.
The identities and purities of the two products 1 and 2 were
checked by reference to their IR and Raman spectra and to the
1H NMR spectra of [2H8]toluene solutions and comparison
with previous reports.12 For X-ray analysis, crystals of 2
were selected and transferred under dry nitrogen at ambient
temperatures; crystals of 1 were selected from under cold
perfluoropolyether RS3000 oil.
X-Ray diffraction measurements
X-Ray data for 1 were collected on a Bruker SMART APEX
diffractometer with a CCD area detector, those for 2 on a
Stöe Stadi-4 four-circle diffractometer. In both cases, measure-
ments were made with graphite-monochromated Mo-Kα radi-
ation, the crystals each being held at 150 K. Details of the
data and data collection for crystals of 1 and 2 are given in
Table 1.
Absorption corrections were performed using the program
SADABS16 for 1 and ψ-scans for 2, and both the crystal struc-
tures were solved by direct methods (SHELXTL).17 Hydrogen
atoms attached to carbon were placed in calculated positions.
The hydrogens attached to the metal atoms were readily located
in a difference map in the case of the aluminium structure (see
Fig. 1, top).18 The corresponding region in the map calculated
for the gallium compound was much noisier (Fig. 1, bottom);
the positions we propose for the hydrides in 2 are based on this
map, but are evidently rather less certain than for the Al ana-
logue. The hydride positions in both structures were refined
with a similarity restraint placed on the M–H distances. All
non-hydrogen atoms were modelled with anisotropic displace-
ment parameters. The refinements proceeded by full-matrix
least squares against F 2 (SHELXTL).
Fig. 1 Slant plane difference maps illustrating hydride location in 1
(top) and 2 (bottom). Both were obtained using phases based on all
atoms except hydridic H1 and H2, and the final refined positions of
these atoms are superimposed. Contours are drawn at an interval of
0.16 e ÅϪ3 for both maps; positive and negative contours are shown as
solid and dashed lines, respectively.
CCDC reference numbers 194729 and 194730.
See http://www.rsc.org/suppdata/dt/b2/b209669m/ for crys-
tallographic data in CIF or other electronic format.
at an appreciable rate only at temperatures in excess of 323 K.
The elimination reaction appears not to be intramolecular but
to depend on association between the N–H bond of one adduct
molecule and the M–H bond of another, with non-classical
hydrogen bonding10,19 through the resulting M–H ؒ ؒ ؒ H–N
interactions giving the first sign of H2 formation. Such H ؒ ؒ ؒ H
interactions have been shown by both experiment and theory to
be stronger for M = Al than for M = Ga, mainly because of the
greater polarity of the Al–H bond.4,10,19
Results and discussion
The conditions of formation of piperidinoalane and piper-
idinogallane illustrate well the enhanced susceptibility of sec-
ondary amine complexes of alane, compared with their gallane
counterparts, to eliminate H2.3,10 Thus, CH2(CH2)4(H)NؒAlH3
eliminates H2 spontaneously at sub-ambient temperatures (ca.
250 K), whereas the corresponding gallane can be isolated and
is long-lived at ambient temperatures, H2 elimination setting in
D a l t o n T r a n s . , 2 0 0 3 , 5 4 0 – 5 4 3
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Table 2 Selected bond distances (Å) and angles (Њ) for crystalline piper-
idinoalane, 1, and piperidinogallane, 2, at 150 Ka
measurements suggest that both dimeric and trimeric forms of
the alane may coexist in benzene solutions.12 Between the
dimeric molecules of 1 and 2 there are no significantly short
contacts to suggest secondary interactions comparable with
those observed, for example, in crystalline cyclotrigallazane,
[H2NGaH2]3,4 and dimethylamine-alane, Me2(H)NؒH2Al-
(µ-H)2AlH2ؒN(H)Me2.10
Parameter
1 (M = Al)
2 (M = Ga)
M(1)–N(1)
M(1)–N(1)#1
M(1) ؒ ؒ ؒ M(1)#1
M(1)–H(1)
M(1)–H(2)
N(1)–C(2)
N(1)–C(6)
C(2)–C(3)
1.9449(14)
1.9410(14)
2.7599(10)
1.751(16)
1.745(16)
1.496(2)
1.489(2)
1.521(3)
1.516(3)
1.514(3)
2.011(3)
2.007(3)
2.8714(11)
1.55(3)
The dimeric molecular structures thus contrast with the tri-
meric structure assumed in the crystalline state by the aziridino-
1.56(3)
1.492(5)
1.478(5)
1.520(6)
1.531(7)
1.517(6)
1.524(5)
gallane, [CH2CH2NGaH2]3,13 the compound most closely
related to 1 and 2 for which definitive information is available.
Presumably the greater spatial requirements of the piperidino
compared with the relatively compact aziridino ligand are
responsible for this change. Interestingly, therefore, increasing
still further the spatial demands of the piperidino ligand by
replacing all the hydrogens at the 2 and 6 positions with methyl
groups is believed to result in a switch from amido- to
hydrogen-bridging in the alane [(tmp)AlH2]3.5
The dimensions of the central M(µ-N)2M cores of 1 and 2
conform to the pattern established for other amido-aluminium
and -gallium derivatives featuring structures of this type. Thus,
the cores appear in each case to be planar but slightly asym-
metric, or rhombic, with one pair of M–N bonds differing
in length by 0.003–0.004 Å from the other pair. In fact, with
N–M–N bond angles of 89.49(6)Њ for 1 and 88.79(13)Њ for 2,
both central units are almost square. The average M–N dis-
tances [1.943(2) and 2.009(3) Å for 1 and 2, respectively] fall
well within the ranges observed for other neutral amido com-
pounds containing M(µ-N)2M units, viz. 1.90–2.03 Å for Al and
1.96–2.10 Å for Ga.20 Shorter distances are found when elec-
tron-withdrawing substituents, e.g. halogens, are attached to the
metal; longer distances are typically the mark of bulky substi-
tuents at one or both centres. That the Ga–N bonds should also
be slightly longer (0.066 Å) than the Al–N bonds is also con-
sistent with the normal pattern, the average Al–N distance for
45 structures being 1.97(3) Å compared with an average Ga–N
distance for 12 structures of 2.04(4) Å. By contrast, Al–P and
Ga–P distances in analogous phosphido molecules containing
M(µ-P)2M cyclic frameworks are not significantly different
{2.43(3) Å for Al vs. 2.44(4) Å for Ga20}.
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
1.520(2)
N(1)–M(1)–N(1)#1
N(1)–M(1) ؒ ؒ ؒ M(1)#1
N(1)#1–M(1) ؒ ؒ ؒ M(1)#1
M(1)–N(1)–M(1)#1
H(1)–M(1)–H(2)
C(2)–N(1)–C(6)
C(6)–N(1)–M(1)#1
C(2)–N(1)–M(1)#1
C(6)–N(1)–M(1)
C(2)–N(1)–M(1)
N(1)–C(2)–C(3)
89.49(6)
44.69(4)
44.80(4)
90.51(6)
112.1(8)
109.13(13)
114.69(10)
115.33(11)
112.67(10)
113.61(10)
111.82(14)
111.24(16)
110.16(15)
110.58(15)
112.48(14)
88.79(13)
44.34(9)
44.45(9)
91.21(13)
122(2)
109.8(3)
114.5(2)
114.5(2)
112.5(2)
113.4(2)
111.7(3)
110.7(3)
110.1(3)
110.5(3)
112.1(3)
C(2)–C(3)–C(4)
C(3)–C(4)–C(5)
C(4)–C(5)–C(6)
N(1)–C(6)–C(5)
a Atom numbering scheme as shown in Fig. 2. Symmetry transform-
ations used to generate equivalent atoms: #1 Ϫx ϩ 1, Ϫy, Ϫz. For the
CH2 groups, the C–H bond distances were fixed at 0.99 Å.
The two crystalline piperidino derivatives 1 and 2 are iso-
morphous, with very similar unit cell parameters and molecular
dimensions. The molecular structure of 1 is illustrated in Fig. 2,
and salient bond distances and angles for both 1 and 2 are listed
Despite the decrease in the non-bonding interactions
between the substituents on adjacent metal and nitrogen atoms,
the M–N distances in the four-membered M2N2 ring are typi-
cally longer than those in the six-membered M3N3 ring. For
example, the Ga–N bond in [Me2NGaH2]2 is 0.046 Å longer
than that in [Me2NGaH2]3.10 It seems likely that cross-ring non-
bonded repulsions between the nitrogen atoms, and possibly
also between the metal atoms, play a significant part here. 1 and
2, wholly representative dimeric amides, feature N ؒ ؒ ؒ N dis-
tances of 2.736(3) and 2.811(6) Å and M ؒ ؒ ؒ M distances of
2.7599(10) and 2.8714(11) Å, respectively, i.e. significantly short
of the sums of the relevant van der Waals radii (namely, 3.10 Å
for N and ca. 3.8 Å for both Al and Ga21). The N ؒ ؒ ؒ N and
M ؒ ؒ ؒ M distances then contrast with those in the 6-membered
9
M3N3 rings of [Me2NAlH2]3 and [Me2NGaH2]3,10 which are
typical trimeric amides. Here we find N ؒ ؒ ؒ N distances aver-
aging to 3.147(3) Å (M = Al) and 3.209(3) Å (M = Ga), i.e.
slightly in excess of twice the van der Waals radius of N, and
M ؒ ؒ ؒ M distances averaging to 3.263(3) Å (M = Al) and
3.329(3) Å (M = Ga). Hence it seems that repulsive N ؒ ؒ ؒ N
interactions are the main factor counteracting the reduced
interactions between the substituents and opposing the
adoption of a dimeric rather than a trimeric structure.
Fig. 2 Structure of the dimeric [CH2(CH2)4NAlH2]2 molecule in
crystalline piperidinoalane, 1, at 150 K as determined by X-ray
diffraction.
in Table 2. Hence it is clear that the crystals are each composed
of dimeric molecules [CH2(CH2)4NMH2]2 (M = Al or Ga) in
which the central unit is a planar 4-membered M(µ-N)2M ring.
The piperidino functions provide symmetrical bridges between
the metal atoms to generate a H2M(µ-N)2MH2 skeleton with a
symmetry close to D2h (see Fig. 2). The dimeric structure of the
gallane is in keeping with that suggested by Storr et al.12 on the
evidence of solution studies. On the other hand, cryoscopic
The issue of potential non-bonded interactions has led us
also to consider the effect of replacing by methyl groups all the
hydrogen atoms at the 2 and 6 positions of the piperidino rings.
As things stand, the shortest intramolecular C(2,6)–H ؒ ؒ ؒ
H–M contacts measure 3.06 and 2.97 Å for crystalline 1 and 2,
respectively; these are therefore greater than twice the van der
Waals radius of H (2.40 Å21) and so give no hint of undue steric
D a l t o n T r a n s . , 2 0 0 3 , 5 4 0 – 5 4 3
542

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congestion. Replacing the hydrogens at the 2,6-carbon atoms by
four methyl groups which are assumed to take up a staggered
orientation about the newly established C–C bonds is then cal-
culated to give intramolecular M–H ؒ ؒ ؒ H(methyl) contacts of
1.89 Å and 1.94 Å for M = Al and Ga, respectively. Such short
distances clearly imply relatively strong repulsion between the
hydrogen atoms bound to the metal and those of the methyl
groups since there is little opportunity for H ؒ ؒ ؒ H bonding.19
It is perfectly understandable therefore that (tmp)AlH2 and
(tmp)2AlH should opt not for N-bridged but for H-bridged
oligomeric forms.5
The M–H distances determined from Fourier difference
maps also warrant some comment. The H atoms are not par-
ticularly obvious in difference maps of 2, but give best estimates
of the Ga–H distances of 1.55(3) Å. By contrast, the H atoms
are clearly defined in the corresponding maps of 1, giving a
mean Al–H distance of 1.75(2) Å. As illustrated by the results
of a search of the Cambridge Database20 (see Fig. 3), Al–H and
an appreciable negative charge on the (µ-N)2AlH2 units, charge
transfer undoubtedly being enhanced by the polar environment
of the molecules in the solid.19,22 Unfortunately the reactivity,
thermal instability and involatility of the compound have com-
bined to frustrate attempts to measure the IR or Raman spec-
trum of the crystalline solid. A resolution of this potentially
interesting feature must, it appears, await neutron diffraction
studies.
The geometries of the piperidino fragments in 1 and 2 relay
little additional information. There is no significant difference
between them, and the chair conformation and dimensions are,
within experimental error, similar to those in other bridging
piperidino derivatives,20 although the CN bonds [1.478(5)–
1.496(2) Å] are somewhat longer than in piperidine itself
[1.4608(8) Å].23
Acknowledgements
We thank the EPSRC (i) for financial support of the Oxford
and Edinburgh groups, and (ii) for an Advanced Fellowship
(to T. M. G.) and a studentship (to C. Y. T.).
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Fig.
3
Histograms of Al–H and Ga–H distances based on
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coordinate metal atom attached to a singly-coordinate hydrogen atom.
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18 Calculated with the MAPVIEW routine in WINGX, L. J. Farrugia,
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preparation.
Ga–H distances are still relatively poorly defined. Admittedly
some of the data carry large uncertainties, but the scale of the
problem is made clear by the finding that redetermination of
the same structure, e.g. that of [Me2NAlH2]3,9 has been known
to yield quite different results. According to the Database, the
average (unweighted) Al–H and Ga–H distances are 1.53(9)
and 1.51(11) Å, respectively. It follows that the results for the
gallane, 2, are quite unremarkable, but that the alane, 1, appears
to feature quite long Al–H bonds. Significantly, perhaps, the
H–M–H angles are also different, being 112(1)Њ for 1 and
122(2)Њ for 2. Unfortunately the high scatter observed for these
parameters in the Cambridge Database meant that no meaning-
ful systematic correlation could be derived from the structural
data on related molecules. However, the difference in the MH2
geometries observed here for 1 and 2 may imply reduced metal
ns character in the Al–H bonds of 1 and the accumulation of
D a l t o n T r a n s . , 2 0 0 3 , 5 4 0 – 5 4 3
543