The crystallographic observation of molecular lithium oxide: Synthesis and solid-state structure of [Me2AlN(2-C5H4N)Ph]2(O)Li2 ·2THF

Article abstract of DOI:10.1039/b104388a

The reaction of Me₂Al[N(2-C₅H₄N)Ph], 1, with 1 equiv. (Me₃Si)₂NLi affords the lithium aluminate Me₂Al[N(SiMe₃)₂][N(2-C₅H₄ N)Ph]Li, 2, which exhibits solvent dependent reactivity towards oxygen. In toluene 2 appears to resist oxygenation but in the presence of THF it reacts to afford the new complex [Me₂AlN(2-C₅H₄N)Ph]₂(O)Li₂ ·2THF, 3. X-Ray diffraction data reveal that 3 has a butterfly-type Al₂Li₂ bimetallic core with two equivalents of reformed 1 stabilising discrete, molecular lithium oxide. The structures of 1-3 are probed theoretically.

Full text of DOI:10.1039/b104388a

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The crystallographic observation of molecular lithium oxide:
synthesis and solid-state structure of [Me2AlN(2-
C5H4N)Ph]2(O)Li2ؒ2THF†
David R. Armstrong,a Robert P. Davies,b David J. Linton,c Ronald Snaith (the late),
Paul Schoolerc and Andrew E. H. Wheatley*c
a Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, UK G1 1XL
b Department of Chemistry, Imperial College of Science, Technology and Medicine,
Exhibition Road, London, UK SW7 2AY
c Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, UK
CB2 1EW
Received 18th May 2001, Accepted 24th July 2001
First published as an Advance Article on the web 10th September 2001
The reaction of Me2Al[N(2-C5H4N)Ph], 1, with 1 equiv. (Me3Si)2NLi affords the lithium aluminate
Me2Al[N(SiMe3)2][N(2-C5H4N)Ph]Li, 2, which exhibits solvent dependent reactivity towards oxygen. In toluene 2
appears to resist oxygenation but in the presence of THF it reacts to afford the new complex [Me2AlN(2-C5H4N)-
Ph]2(O)Li2ؒ2THF, 3. X-Ray diffraction data reveal that 3 has a butterfly-type Al2Li2 bimetallic core with two
equivalents of reformed 1 stabilising discrete, molecular lithium oxide. The structures of 1–3 are probed theoretically.
the formation of formal Al–O bonds, the treatment of 2 with
Introduction
air (pre-dried over P2O5) results in a novel butterfly complex
Significant interest has been shown in the structure and reactiv-
(3) in which two equivalents of in situ reformed amidoalane, 1,
ity of heteropolymetallic species incorporating lithium. This is
stabilise a molecule of lithium oxide.
by virtue of their ability to induce chemical transformations
with selectivities that differ to those affected by homometallic
lithiated organics.1 Hence for example, sterically demanding tri-
Results and discussion
coordinate aluminium tris(2,6-diphenylphenoxide) has been
employed as
a selective activator of carbonyls towards
Reaction of 2-pyridylaniline with AlMe3 in toluene results
in the facile evolution of CH4 and formation of the simple
amidoalane 1. X-Ray crystallography (Fig. 1 and Table 1)
reveals that in the solid state 1 is a dimer based on an eight-
membered (NCNAl)2 chair [Al1–N1 = 1.8916(18) Å, Al1–N2 =
2.0033(18) Å].
Treatment of the amidoalane 1 with 1 equiv. of (Me3Si)2NLi
in toluene afforded a yellow solution from which colourless
blocks were deposited after storage at 45 ЊC. Initial NMR
spectroscopic studies indicated this to be the simple lithium
aluminate Me2Al[N(SiMe3)2][N(2-C5H4N)Ph]Li 2. While con-
organolithium-mediated α-alkylation.2 Lately, it has been noted
that in the presence either of the same Lewis acid3 or of methyl-
aluminium bis(2,6-di-tert-butyl-4-alkylphenoxide)4 organo-
lithium reagents add to α,β-unsaturated ketones at the 1,4-
rather than the 1,2-positions,5 lithium aluminate monomers
having been implicated in the process.6 Further studies into the
chemistry of mixed Li–Al systems have led to the report that
Me2AlN(2-C5H4N)Ph 1 reacts with ButLi to afford hydride-
containing clusters7 whose formation is critically dependent on
the use of organolithium precursors capable of β-eliminating
LiH.8 While this view was supported by the observation that the
treatment of 1 with MeLi affords [Me3AlN(2-C5H4N)Ph]Li,9
we wish to report here that the analogous treatment of 1 with
(Me3Si)2NLi affords a simple lithium aluminate 2. Moreover,
whereas previously reported oxygen scavenging by lithiated homo-
metallic10–18 and by lithium-containing heterobimetallic19–25
compounds have previously pointed to the encapsulation of
naked O2Ϫ, studies into the controlled oxygenation of lithium
aluminates have revealed remarkably different behaviour.26,27
Thus, for example, a selectively oxygenated heterobimetallic
mixed-anion ladder {[PhC(O)N(Me)Al(Me)(But)OMe]Liؒ
[PhC(O)N(Me)Al(Me)(OBut)OMe]Li}2 has lately been isolated
and fully characterised26 with oxo-insertion into Al–C bonds
observed.28 However, we also report here that the controlled
oxygenation of 2 follows a new path. Instead of affording a
heterobimetallic ladder in which oxo-capture has resulted in
† Electronic supplementary information (ESI) available: modelled
geometries of 4a–12 and tables of absolute and relative energies for 4a–
12 and calculated bond lengths and angles. See http://www.rsc.org/
suppdata/dt/b1/b104388a/
Fig. 1 Molecular structure of dimeric 1; hydrogen atoms omitted for
clarity.
2838
J. Chem. Soc., Dalton Trans., 2001, 2838–2843
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104388a

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Table
1
Selected bond lengths (Å) and angles (Њ) for dimeric
Me2Al[N(2-C5H4N)Ph] 1
Table 2 Selected bond lengths (Å) and angles (Њ) for polymeric
Me2Al[N(SiMe3)2][N(2-C5H4N)Ph]Li 2
Al1–N1
Al1–N2
Al1–C13
Al1–C12
1.8916(18)
N1–C1A
N1–C6
N2–C1
1.403(2)
1.406(2)
1.347(2)
Li1–N1
Li1–N3
Li2–N4
Li2–N6
Li1 ؒ ؒ ؒ C37
Li2 ؒ ؒ ؒ C19
Al1–N1
Al1–N2
Al2–N4
Al2–N5
Al1–C18
Al1–C19
2.03(1)
1.96(1)
2.03(1)
1.98(1)
2.36(1)
2.28(1)
1.931(2)
1.922(2)
1.946(2)
1.933(3)
1.973(3)
2.002(3)
Al2–C37
Al2–C38
Si1–N1
Si2–N1
Si3–N4
Si4–N4
N2–C7
N2–C12
N3–C7
N5–C26
N5–C31
N6–C26
1.999(3)
1.978(3)
1.757(2)
1.753(2)
1.749(2)
1.752(2)
1.368(4)
1.434(4)
1.355(4)
1.370(4)
1.447(4)
1.380(4)
2.0033(18)
1.958(2)
1.968(2)
N1–Al1–N2
Al1–N2–C1
N2–C1–N1A
103.59(8)
120.91(12)
116.67(15)
Al1–N1–C1A
Al1–N1–C6
116.32(12)
126.10(12)
Li1–N1–Al1
Li1–N3–C7
Li1–N1–Si1
Li1–N1–Si2
Al1–N2–C12
N1–Li1–N3
N1–Al1–N2
N2–Al1–C19
N2–C7–N3
C7–N2–C12
C7–N2–Al1
96.24(17)
127.2(2)
111.2(2)
99.69(18)
117.67(19)
114.8(2)
111.63(10)
108.05(12)
116.4(2)
Li2–N4–Al2
Li2–N6–C26
Li2–N4–Si3
Li2–N4–Si4
Al2–N5–C31
N4–Li2–N6
N4–Al2–N5
N5–Al2–C37
N5–C26–N6
C26–N5–C31
C26–N5–Al2
92.43(17)
116.7(3)
102.9(2)
109.3(2)
120.7(2)
116.0(3)
110.95(11)
104.03(12)
112.9(3)
112.8(3)
120.8(2)
116.5(2)
123.13(18)
Fig. 2 Molecular structure of the dimeric asymmetric unit of polymer
2; hydrogen atoms omitted for clarity.
firming this formulation, single crystal X-ray diffraction
revealed a polymeric structure containing two monomeric
fragments in the asymmetric unit (Fig. 2 and Table 2). It can be
seen that the straightforward 1 : 1 addition of [N(SiMe3)2]Ϫ to 1
has afforded the approximately tetrahedral aluminate fragment
{Me2Al[N(SiMe3)2][N(2-C5H4N)Ph]}Ϫ, the charge upon which
is offset by that on the Group 1 metal centre. The intra-
molecular coordination of Liϩ arises from its interaction with
both the hexamethyldisilazide and pyridyl N-centres [mean
Li–N = 2.03 and 1.97 Å, respectively]. The consequent form-
ation of six-membered NCNAlNLi heterocycles renders each
monomer in the asymmetric unit chiral at its hexamethyl-
disilazide N-centre (N1, N4) and means that solid-state 2 is
composed of racemic one dimensional chains comprising
monomeric units of alternating stereochemistry. The relatively
long Li–N(SiMe3)2 interaction implies that this N-centre adopts
a near sp2-type geometry29 with its unhybridized p-orbital
orientated towards Liϩ: a feature borne out by the bond angles
around nitrogen [mean Si–N–Si, Si–N–Al and Al–N–Li =
117.78, 114.65 and 94.34Њ, respectively]. Polymerisation results
from the intermolecular stabilisation of Liϩ by Al-bonded
methyl groups [mean Li ؒ ؒ ؒ C = 2.32 Å]. This process, similar to
that observed in the polymeric solid-state structures of
Me3AlN(SiMe3)2Li30 and Me3Al(TMP)Li (TMP = 2,2,6,6-
tetramethylpiperidide),31 presumably results from the electron-
rich nature of these Me groups. Several reports exist of species
which contain Al–N bonds and which incorporate external
Lewis bases in the solid state. Hence compounds with the core
systems (PhCH2)2N(H)ؒLi(µ-N)Al,32 (THF)3ؒLi(µ-N)Al,33
[(THF)nؒLi(µ-N)Al]2 (n = 1, 2),34,35 and LnؒLi(µ-N)2Al (n = 1, L =
THF, Et2O, C5H5N;36 n = 2, L = HNC4H8, THF35,37) are all
based on an Al–N–Li motif. However, the structure of 2 repre-
sents one of only a very limited number of lithium aluminates
that incorporate such a bonding pattern in the absence of solv-
ation by external Lewis base.9,30 It is noteworthy that deaggre-
gation of the solid-state structure of 2 is not induced by the
addition of Lewis base, and that polymeric 2 is thus the only
species which can be recrystallised from toluene–hexane to
which THF has been added after the reaction.
Scheme 1
solvent. Thus, the treatment of an in situ pre-formed solution of
2 in toluene with air (pre-dried over P2O5)26 resulted only in the
recrystallisation of unreacted 2. However, if the sample was
treated with THF immediately after dry air had been bubbled
through it, and subsequently stored at 5 ЊC, colourless blocks of
3 were deposited instead. X-Ray crystallography revealed the
second novel aspect to the treatment of 2 with oxygen; namely
that in the solid state, instead of oxo-insertion into Al–C inter-
actions akin to that observed elsewhere,26,27 oxygenation has
afforded the new trinuclear heterobimetallic adduct [Me2AlN(2-
C5H4N)Ph]2(O)Li2ؒ2THF (Fig. 3 and Table 3). This conversion
also occurs, with no detectable effect on the yield of 3, if the
order of THF addition and oxygenation is reversed. While
these observations imply that the introduced THF (or impur-
ities contained therein) might be directly responsible for the
formation of 3, such a possibility is inconsistent with the observ-
ation that the polymeric lithium aluminate 2 is isolable from
an anaerobic mixture of 1 and (SiMe3)2NLi in toluene–THF
(see above).
The solid-state structure of 3 has, at its core, an oxo-dianion
that is encapsulated in a heterobimetallic environment by virtue
of interactions both with Group 1 and Group 13 metal centres.
The two Liϩ ions in the complex bond to the O2Ϫ centre [Li1–
O1 and Li2–O1 = 1.91(2) and 1.88(2) Å, respectively] and the
Li1–O1–Li2 angle is, at 76.4(7)Њ, slightly contracted relative to
The behaviour of 2 on exposure to oxygen is unusual in two
respects. Firstly, there is a propensity for reaction with oxygen
which displays a significant dependence upon the choice of
J. Chem. Soc., Dalton Trans., 2001, 2838–2843
2839

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rings the small angles about Liϩ (mean N–Li–O = 91.0Њ) are
presumably enforced by strain in the six-membered ring itself
and by the inability of the lithium oxide bond angle to collapse
any further. Finally, the somewhat distorted tetrahedral
coordination spheres of each of the Liϩ centres are completed
by mono-THF solvation (mean Li–O = 1.907 Å).
Table 3 Selected bond lengths (Å) and angles (Њ) for [Me2AlN-
(2-C5H4N)Ph]2(O)Li2ؒ2THF 3
Li1–O1
Li1–O2
Li2–O1
Li2–O3
Li1–N2
Li1–N4
Li2–N2
Li2–N4
Al1–O1
Al2–O1
Al1–N1
1.91(2)
1.93(2)
1.88(2)
1.89(2)
2.28(2)
2.28(2)
2.21(2)
2.22(2)
1.746(5)
1.763(5)
1.939(6)
Al2–N3
Al1–C1
Al1–C2
Al2–C18
Al2–C19
N1–C3
N1–C9
N2–C9
N3–C20
N3–C26
N4–C26
1.916(6)
1.96(1)
1.99(1)
1.96(1)
1.98(1)
1.45(1)
1.38(1)
1.37(1)
1.44(1)
1.37(1)
1.38(1)
The crystal structure of 3 is rendered unusual in that it
demonstrates what is best viewed as the neutral amidoalane
entrapment of isolated Li2O. The solid-state structures of
several species have been reported which, it has been sug-
gested, incorporate Li2O as part of a higher order lithium
assembly. Hence, it has been suggested that (RLin)mؒLi2O frag-
ments are present in monolithiates {(2,6-OMe-C6H3Li)6ؒLi2O,10
[(c-C5H9)N(H)Li]12ؒLi2O,13 [BunC(NBut)2Li]4ؒLi2O14}, in the
mixed mono- and di-lithiate {[PhS(O)(NMe)C(H)(Me)Li]2ؒ
[(C6H4Li)S(O)(NMe)C(H)(Me)Li]2ؒLi2Oؒ3TMEDA}14 and in
geminal {[PhS(O)2C(SiMe3)Li2]6ؒLi2Oؒ10THF,11 [Pri2(Mes)Si-
PLi2]8ؒ Li2O,17 [Me2(PriMe2C)SiAsLi2]8ؒLi2O,17 [PhS(O)(NMe)-
C(H)(Ph)Li]2ؒLi2Oؒ6THF16} and vicinal {[(salen)Li2]3ؒLi2Oؒ
TMEDAؒH2O12 (H2salen = bis(salicylidene)ethylenediamine,
TMEDA = N,N,NЈ,NЈ-tetramethylethylenediamine)} dilithiates.
However, in all of the above cases, the oxo-dianion has been
encapsulated by an octahedral array of Liϩ centres, almost
invariably affording Oh symmetry clusters and precluding
emphatic identification of the Li2O unit. In the single lower-
symmetry example (C2 in solid-state [BunC(NBut)2Li]4ؒLi2O)14
the Li2O fragment is considered to be near-linear. Perhaps more
importantly, the isolation and solid-state characterisation of
both the oxo-encapsulating heterobimetallic cluster (TMP)4-
Mg2Li2(O)22,23 and of the 2 : 1 adduct [RuH(SiHPh2)(CO)X2]2ؒ
[Li2Ru4OCl8X4] (X = PBut2Me)19 have been reported.
Li1–O1–Li2
Li1–N2–Li2
Li1–N4–Li2
N2–Li1–N4
N2–Li2–N4
O1–Li1–N2
O1–Li1–N4
O1–Li2–N2
O1–Li2–N4
Li1–N4–C26
76.4(7)
63.0(6)
63.0(5)
92.8(6)
96.4(6)
90.5(6)
88.6(6)
93.5(6)
91.2(7)
130.9(6)
Li2–N4–C26
N4–C26–N3
C26–N3–Al2
N3–Al2–O1
Li1–N2–C9
Li2–N2–C9
N2–C9–N1
C9–N1–Al1
N1–Al1–O1
Al1–O1–Al2
104.3(6)
116.3(7)
127.1(5)
104.3(3)
93.1(6)
131.9(7)
115.0(7)
123.9(5)
100.2(3)
131.2(3)
The solid-state structure of lithium oxide itself40 reveals an
extended lattice in which each oxygen centre is in a cubic
coordination environment with three distinct types of Li–O–Li
bond angle (70.5, 109.5 and 180.0Њ). Only two molecular species
have been reported in which oxygen is bonded equivalently to
eight Li-centres: whereas the geometry at the chalcogen is
square antiprismatic in 2TMEDAؒLi8K2(ButO)8(O),41 a cubic
“lattice fragment” has been observed in Li8(R)6(O) (R =
4-azabenzimidazole derivative).42 The X-ray structure of 3
demonstrates the entrapment of a lithium oxide molecule
whose geometry at oxygen [76.4(7)Њ] is comparable with the
smallest set of angles in the extended oxide lattice.40 Lastly,
the continued observation of Al–C contacts in the stabilising
amidoalane ligands is also noteworthy given the well-known
propensity of oxygen for insertion into Al–C bonds.43
Fig. 3 Molecular structure of 3; hydrogen atoms and minor THF
disorder omitted for clarity.
that typically observed in (LiO)2 rings.6,38 Compound 3 can
therefore be regarded as the lithium salt of an oxo-bridged bis-
(aluminate). However, it can also be considered to represent a
co-complex incorporating a simple, non-linear molecule of lith-
ium oxide. Notably, this view is consistent with the stabilisation
of both Liϩ ions by the pyridyl N-centres of two equivalents of
in situ reformed 1 incurring the formation of a four-membered
(LiN)2 ring. That this heterocycle is contiguous with the two
concomitantly formed NLi2O rings (N2–Li1–O1–Li2 and N2–
Li1–O1–Li2) results in a tricyclic arrangement of approxi-
mately trigonal symmetry, wherein the dihedral angle between
the Li1–N2–Li2/Li1–N4–Li2 planes is 119.0Њ (while those
between the Li1–N2–Li2/Li1–O1–Li2 and Li1–N4–Li2/Li1–
O1–Li2 planes are 122.2 and 118.6Њ, respectively). The angles
within the (LiN)2 ring itself merit comment. While those about
the two metal centres are unremarkable (mean N–Li–N =
94.6Њ), those about the pyridyl N-centres are dramatically con-
stricted (mean Li–N–Li = 63.0Њ), reflecting the non-planarity of
this heterocycle and its correspondingly long Li–N(pyridyl)
interactions (mean 2.247 Å).39 The final cyclic arrangements
to stabilise the Li2O fragment involve the incorporation of
each Li–O bond into two six-membered NCNAlOLi rings (cf.
the generation of OCNAlOLi rings by selectively oxygenating
Al–C bonds)26,27 formed by virtue of coordination of the
approximately tetrahedral (mean N–Al–O = 102.3Њ) Al centres
to the oxo-dianion (mean Al–O = 1.755 Å). Thus, 1 acts not
only as a Lewis base through its pyridyl N-atoms, but also as a
Lewis acid through its Al-centres. Within the two six-membered
Theoretical calculations
The GAUSSIAN 9444 computer program was used to probe the
observed structures of 1–3 by both ab initio (with the 6-31G*
basis set45 at the RHF level) and DFT (6-31G* basis set45 with
B3LYP method46) calculations. Details of modelled mono- and
dimeric Me2AlN(2-C5H4N)Me (4a and 4b, respectively) and
Me2AlN(2-C5H4N)Ph (5a and 5b, respectively), monomeric
(Me3Si)2NLi 6a/b and Me2Al[N(SiMe3)2][N(2-C5H4N)R]Li
(R = Me, 7, R = Ph, 8) and of unsolvated and OMe2-solvated
[Me2AlN(2-C5H4N)R]2(O)Li2 (9 and 10a/b, respectively for R =
Me and 11 and 12, respectively for R = Ph) have been submitted
as ESI.†
The calculated structures of 4/5a and 4/5b reveal stabilisation
of the metal centre by both the deprotonated and pyridyl N-
centres. Whereas in a monomeric context this results in the
formation of a four-membered NCNAl ring, the dimer is based
on an eight-membered (NCNAl)2 ring core with intermolecular
Al–N(pyridyl) interactions akin to that observed for 1. Whereas
a small aggregation enthalpy of 3.1 kcal molϪ1 is noted for 4a/b
it is surprising to note that 5a is 1.7 kcal molϪ1 more stable than
the corresponding dimer (5b). However, the energy difference is
sufficiently small as to resist meaningful interpretation.
2840
J. Chem. Soc., Dalton Trans., 2001, 2838–2843

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The favourable formation of Me2Al[N(SiMe3)2][N(2-C5H4N)-
R]Li is pointed to by the theoretical finding of a 30.3 kcal molϪ1
(R = Me, 7) and a 30.5 kcal molϪ1 (R = Ph, 8) enthalpy of
formation for the lithium aluminate relative to the combined
energies of its components (4a ϩ 6a and 5a ϩ 6b, respectively).
Conclusions
Compound 3 is reproducibly obtained from the exposure of a
pre-formed solution of 2 to dry air (pre-dried over P2O5).
Although the presence of trace moisture cannot be entirely
ruled out, previous work with lithium aluminates,26,27 the isol-
ation and characterisation of 2 [inconsistent with LiOH
contamination of the (Me3Si)2NLi substrate], and the absence
of any isolable aluminoxane28,48 hydrolysis products, points to
molecular oxygen being responsible for the formation of Li2O.
NMR spectroscopic investigations into the formation of 3 are
ongoing and will focus particularly on the nature of any by-
products since the observation of (Me3Si)2NH would clearly
suggest the presence of moisture whereas the reaction of
O2 requires generation of the hydrazine [(Me3Si)2N]2.22 Also
of mechanistic interest is the observation that neither the
The resultant monomeric complexes both reveal
a six-
membered NCNAlNLi core of the type noted in polymeric 2, in
which the alkali metal centre is stabilised both by hexamethyl-
disilazide and by pyridyl N-centres. The approximately sp2
hybridization noted for the hexamethyldisilazide N-centre in
polymeric 2 is also replicated in the calculated structures of
monomeric 7 and 8. Moreover, calculations suggest that this
phenomenon is attributable to intramolecular agostic inter-
actions. Thus, theory reveals a single agostically bonding hexa-
methyldisilazide C-centre (Li ؒ ؒ ؒ C = 2.54 Å in 7 and 2.44 Å
in 8) which corresponds to the carbon atom which most closely
approaches the alkali metal centre in each of the crystal-
lographically independent molecules in the asymmetric unit
of 2. However, an interatomic distance of 2.937 Å (mean
for Li1 ؒ ؒ ؒ C6 and Li2 ؒ ؒ ؒ C20) suggests only very weak
intramolecular agostic bonding in the aggregated solid-state
structure.6,47
amidoalane43 component of 3 nor the lithium aluminate26,27
2
has demonstrated thermodynamic oxo-insertion into Al–C
bonds.28,43 The reasons why the oxygenation of 2 is solvent
dependent are also the subject of continued study, with calc-
ulations already suggesting that the way in which the alkali
metal centres in 3 are stabilised varies with the choice of
solvent.
The crystal structure of 3 reveals weak39 Li–N(pyridyl)
bonds (mean 2.247 Å; Table 3) and non-bonding [Li2 ؒ ؒ ؒ N1 =
2.281(17) Å; remaining Li ؒ ؒ ؒ N(AlMe2) > 2.5 Å] distances with
the distorted tetrahedral coordination spheres of each of the
Liϩ centres being completed by mono-THF solvation (see
above). Calculations on unsolvated and OMe2-solvated [Me2-
AlN(2-C5H4N)R]2(O)Li2 (9 and 10a/b, respectively for R =
Me and 11 and 12, respectively for R = Ph) point to these
solvent molecules being of structural importance with THF-
coordination evidently prevailing over the formation of Li–
N(AlMe2) interactions by buckling of the NCNAlOLi rings.
The calculated structure of 11 reveals the coordination of each
pyridyl N-centre to just a single alkali metal ion (Li–N = 2.042
Å, Li ؒ ؒ ؒ N = 2.841 Å). This is compensated for by the modifi-
cation of both six-membered NCNAlOLi rings, the result being
the formation of four-membered OAlNLi and six-membered
NCNLiOLi rings, which are edge-fused by virtue of both Li–O
and the new Li–NMe interactions (1.867 and 2.112 Å, respect-
ively). The introduction of two molecules of OMe2 results in
several structural modifications, which make the calculated
structure (10a with C2 symmetry imposed, 10b without) more
closely approach that of 3. The favourability of solvation is
revealed by formation enthalpies of Ϫ22.2 kcal molϪ1 for both
species (relative to 9 ϩ two free OMe2 molecules). Further, 10a
and 10b each show the interaction of pyridyl N-centres with
both Li centres [mean 2.283 Å (10a), mean 2.284 Å (10b)] and
the formation of six-membered NCNAlOLi rings of the type
noted in 4. Calculations on unsolvated (11) and OMe2-solvated
(12) [Me2AlN(2-C5H4N)Ph]2(O)Li2 reiterate the observations
noted for 9 and 10a/b. The modelled structure of 11 shows each
pyridyl N-centre bonding to one Liϩ ion (Li–N = 2.009 Å). The
lack of a second lithium–pyridyl nitrogen bond (Li ؒ ؒ ؒ N =
2.894 Å) is compensated for by stabilisation of the alkali metal
centres by edge-fused four-membered OAlNLi and six-
membered NCNLiOLi rings (Li–O and Li–NPh = 1.859 and
2.128 Å, respectively). The introduction of solvation by two
molecules of OMe2 results in the calculated structure (12)
closely resembling the observed structure of 3. A calculated
formation enthalpy of Ϫ25.8 kcal molϪ1 for 12 (relative to 11 ϩ
two free OMe2 molecules) points to the preference for solvation
by an etherate solvent. While modelled 12 reveals widely vari-
able behaviour of the pyridyl N-centres with respect to each
alkali metal ion [2.13–2.46 Å cf. 2.21(2)–2.28(2) Å in 3], bond
angles in the calculated (LiN)2 ring core nevertheless agree
excellently with observation. Finally, external solvation is
accompanied by dislocation of the Li–NPh bonds noted in 11,
and the emergence of NCNAlOLi rings of the type noted in 3,
albeit with slightly exaggerated non-bonding Li ؒ ؒ ؒ N distances.
Experimental
Methods and materials
All reactions and manipulations were carried out under an
inert atmosphere, using standard Schlenk techniques. Where
appropriate, the treatment of air sensitive reaction mixtures
with oxygen was achieved using air which had been pre-dried
over P2O5 (Lancaster). All other chemical reagents (Aldrich)
were used as received without further purification. Toluene,
THF and hexane were distilled off sodium or sodium–
potassium amalgam immediately prior to use.
NMR data were collected on either a Bruker DPX 400 (oper-
ating at 400.12 MHz for 1H) or a Bruker DRX 500 (operating at
1
500.05 MHz for H) FT NMR spectrometer. Chemical shifts
are quoted relative to TMS at δ 0.00.
Crystallographic data for complexes 1–3 were collected on a
Nonius Kappa CCD diffractometer equipped with an Oxford
Cryostream crystal cooling device.
Syntheses and characterisation
Me2Al[N(2-C5H4N)Ph], 1. Trimethylaluminium (0.5 ml, 2.0
M in hexane, 1.0 mmol) was added to 2-pyridylaniline (0.17 g,
1.0 mmol) in toluene (0.3 ml) at Ϫ78 ЊC under nitrogen and the
resultant slurry was warmed to room temperature. Gentle heat-
ing gave complete dissolution and the resultant yellow solution
was stirred for 1 h prior to filtration and storage at Ϫ30 ЊC.
After 1 week yellow blocks of 1 were deposited. Yield = 54%,
mp = 100–102 ЊC. Found: C, 70.19; H, 6.93; N, 11.11. Calc. for
1
C26H30Al2N4: C, 69.03; H, 6.64; N, 12.39%. H NMR spectro-
scopy (400 MHz, 27 ЊC, [2H6]benzene): δ 7.26–5.83 (m, 9H, Ar),
Ϫ0.12 (s, 6H, AlMe).
Me2Al[N(SiMe3)2][N(2-C5H4N)Ph]Li, 2. Method (i). Treat-
ment of the amidoalane 1 (0.11 g, 1 mmol) with 1 equiv.
(SiMe3)2NLi (0.08 g, 1 mmol) (both pre-weighed) in toluene
(2 ml) at Ϫ78 ЊC under nitrogen afforded a slurry which was
warmed to room temperature. Gentle heating gave complete
dissolution and the resultant yellow solution was stored at
45 ЊC for 1 day whereupon colourless blocks were deposited.
Yield = 20%, mp = 190–192 ЊC. Found: C, 55.42; H, 7.58; N,
11.10. Calc. for C18H33AlLiN3Si2: C, 56.69; H, 8.66; N, 11.02%.
1H NMR spectroscopy (400 MHz, 27 ЊC, CD3CN): δ 7.96–5.70
(m, 9H, Ar), 0.04 (s, 18H, SiMe3), Ϫ1.20, Ϫ1.27, Ϫ1.37 (m, 6H,
AlMe).
J. Chem. Soc., Dalton Trans., 2001, 2838–2843
2841

View Article Online
Table 4 Crystallographic data for complexes 1–3
1
2
3
Formula
Mr
Crystal system
Space group
a/Å
b/Å
c/Å
C26H30Al2N4
452.50
Triclinic
P1
8.198(4)
8.531(4)
9.462(6)
107.85(4)
106.44(5)
94.05(4)
595.1(6)
1
C19H33AlLiN3Si2
393.58
Orthorhombic
P21/n
13.8140(4)
17.6740(5)
19.7500(5)
90
99.557(2)
90
C34H46Al2Li2N4O3
626.59
Cubic
Pna21
20.16(1)
9.986(4)
18.129(4)
90
90
90
α/Њ
β/Њ
γ/Њ
U/Å3
4755.0(2)
8
3540(2)
4
Z
Radiation/Å
µ/mmϪ1
T /K
Mo-Kα, 0.71073
0.144
180(2)
3828
2095
0.0270
0.037, 0.040
0.1060, 0.1106
Mo-Kα, 0.71073
0.913
180(2)
16045
8324
0.0379
0.054, 0.079
0.148, 0.166
Mo-Kα, 0.71073
0.120
180(2)
4101
3440
0.0593
0.070, 0.145
0.123, 0.151
Measured refl.
Unique refl.
Rint
R1,a R1b
wR2,a wR2b
a Value of the residual considering only the reflections for which I > 2σ(I ). b Value of the residual for all reflections.
Method (ii). As for route (i) with the yellow solution
obtained at room temperature being treated with air (pre-dried
Acknowledgements
over P2O5)26 for 45 s. This afforded a dark brown suspension
which reverted to a yellow solution at reflux. Filtration, reduc-
tion of the solvent volume by one half and storage at room
temperature resulted in the deposition of crystalline 2 [by mp
and NMR spectroscopy; yield = 20% (first batch); while the
yield of material obtained, which had identical spectroscopic
properties, could be increased to 82% by removal of all the
solvent in vacuo after reflux, the mp of this material (at only
91–93 ЊC) suggested that polymerisation had not occurred].
We thank the UK EPSRC (D. J. L. & P. S.) and Gonville
& Caius College (A. E. H. W.) for financial support.
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Crystallographic structure solution and refinement
Crystallographic data for complexes 1–3 are given in Table 4.
The positions of some of the non-hydrogen atoms were deter-
mined by direct methods and refined against F 2 using
SHELXL-97.49 All of the non-hydrogen atoms were found by
successive iterations of least-squares refinement. Hydrogen
atoms were added in calculated positions and allowed to ride on
their parent atoms unless they appeared in difference maps. For
crystalline 1–3, all of the non-hydrogen atoms were refined
anisotropically. For crystalline 2, one of the two THF mole-
cules was disordered. This was modelled over two sites and the
atoms refined at half occupancy.
CCDC reference numbers 165070–165072.
See http://www.rsc.org/suppdata/dt/b1/b104388a/ for crystal-
lographic data in CIF or other electronic format.
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