Heterobimetallic lithium alkyltriimido aluminate cages containing the [R′Al(NR)3]4- tetraanion (R′ - Bu n, Et; R = 2-OMeC6H4)

Article abstract of DOI:10.1039/b307589c

Attempted metallation of a triamidoaluminane gives a complex containing the [R′Al(NR)₃]^4- anion whose formal tetranegative charge is the highest charge observed crystallographically for a simple mononuclear imido main group anion system.

Full text of DOI:10.1039/b307589c

Heterobimetallic lithium alkyltriimido aluminate cages containing the
[RAAl(NR)3]42 tetraanion (RA = Bun, Et; R = 2-OMeC6H4)†
May C. Copsey, John C. Jeffery, Christopher A. Russell,* John M. Slattery and Jennifer A. Straughan
School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK BS8 1TS.
E-mail: chris.russell@bris.ac.uk; Fax: +44 (0)117 9290509; Tel: +44 (0)117 9287599
Received (in Cambridge, UK) 3rd July 2003, Accepted 1st August 2003
First published as an Advance Article on the web 12th August 2003
Attempted metallation of a triamidoaluminane gives a
complex containing the [RAAl(NR)3]42 anion whose formal
tetranegative charge is the highest charge observed crystallo-
graphically for a simple mononuclear imido main group
anion system.
place
of
EtAlCl2
which
gives
the
dimeric
[Li2PhB(NR)2]2·3THF {R = (2-OMe)C6H4, 4} as the only
crystalline product.
Complexes 1–4 were characterised by elemental analysis, 1H
NMR and IR spectroscopies. Additionally, single crystal X-ray
diffraction studies were carried out on 2, 3 and 4.‡The structures
of 2 and 3 showed them to be ion-separated and dimeric,
Imido p-block anion chemistry is a field that has blossomed in
the last decade.1,2 Most attention has focused on complexes
where the central atom is from groups 15 and 16 of the periodic
table, with imido analogues of the phosphite,3 phosphate,4,5
sulfite6 and sulfate7 anions being among the analogues of more
common oxoanions that have been identified. Related imido
group 13 anions have received somewhat less attention.
Metallation studies on triamidoboranes, B{N(H)R}3, in our
own group8 and in the groups of Chivers9 and Nöth10 have
shown competition between alkyllithium reagents acting as a
base giving imidoborates, and the alkyllithium acting as a
nucleophile to give organoimido borates. Imido anions of the
heavier group 13 elements are limited to three reports, despite
the extensive number and fascinating structural variety in
neutral imido group 13 complexes that are known.11 Two
reports from the group of Chivers describe reactions of the
triamido group 13 dimers, [M{N(H)But}3]2 (M = Al, Ga), with
organolithium reagents. This results in partial lithiation to
produce imido group 13 anions that display the unusual
consisting of two [Li(THF)4]+ cations and
a [Li6{RA-
Al(NR)3}2]22 dianion {RA = Bun (2), Et (3)}. There are
additional molecules of THF that crystallise in the lattice that
are removed upon isolation of the material and do not feature in
the spectroscopic data. Complexes 2 and 3 are isostructural save
for the alkyl chain attached to the aluminium centre and the
number of molecules of lattice THF (1 per unit cell of 2, 3 per
unit cell of 3). Structural data are only presented for 3 (see Fig.
1).
The 14 membered Al2N6Li6 core of the dianion of 3 adopts a
rhombododecahedral arrangement that has been commonly
property of trapping unreacted organolithium reagent.12,13
A
further report from Henderson et al. describes the hetero-
adamantanyl imidoalane anion, [(HAl)4(NPh)6{Li(OEt2)}3]2,
produced from reaction of the primary amidolithium LiN(H)Ph
with the alane adduct H3Al·N(Me)C5H8.14 In this report, we
describe the syntheses of lithium organotriimidoaluminate
complexes using two alternative routes and contrast this with
the corresponding imido borate chemistry.
Reaction of AlCl3 with 3 equivalents of LiN(H)R {R =
(2-OMe)C6H4} in THF results in the formation of the
triamidoaluminane [Al{N(H)R}3]n (1). We have been unable to
unequivocally assign the degree of oligomerisation, but the 1H
NMR spectrum of 1 in d8-toluene shows several overlapping
singlets for the methoxy groups which we have interpreted in
terms of a dimeric Al2N6 structure exhibiting cis and trans
isomers, similar to that reported for [Al{N(H)But}3]2 by
Chivers et al.12 and in agreement with the observations of
dimeric tris(primary amido)gallanes by Cowley and Jones.15
Treatment of 1 with LiBun (3 equiv. per monomer unit of 1)
leads to metallation of all three N–H protons and nucleophilic
addition of the n-butyl moiety to give the lithium organo-
triimidoaluminate, [Li(thf)4]2[Li6{BunAl(NR)3}2] (2). A sim-
ilar compound, [Li(THF)4]2[Li6{EtAl(NR)3}2] (3), was pre-
pared by reaction of ethylaluminium dichloride with three
equivalents of LiN(H)R {R = (2-OMe)C6H4} followed by
metallation of the remaining N–H protons with three equiva-
lents of LiBun. These are the first reported examples of
trimetallated products of triamidoaluminanes. An interesting
contrast between the reactivity of amidoaluminanes and
amidoboranes is seen in the equivalent reaction using PhBCl2 in
Fig. 1 Crystal structure of 3. Hydrogen atoms and the thf molecules of
crystallisation have been omitted for clarity. Selected bonds (Å) and angles
(°): Al(1)–N(3) 1.882(1), Al(1)–N(1) 1.886(1), Al(1)–N(2) 1.887(1), Al(1)–
C(51) 2.010(2), Al(2)–N(5) 1.878(1), Al(2)–N(4) 1.878(1), Al(2)–N(6)
1.887(1), Al(2)–C(27) 2.004(2), Li(1)–N(1) 2.048(3), Li(1)–N(2) 2.054(3),
Li(1)–N(4) 2.095(3), Li(5)–N(4) 2.020(3), Li(5)–N(5) 2.059(3), Li(5)–N(2)
2.110(3), Li(4)–N(6) 2.050(3), Li(4)–N(5) 2.054(3), Li(4)–N(3) 2.117(3),
Li(2)–N(3) 2.034(3), Li(2)–N(1) 2.092(3), Li(2)–N(6) 2.112(3), Li(6)–N(6)
2.038(3), Li(6)–N(4) 2.074(3), Li(6)–N(1) 2.115(3), Li(3)–N(2) 2.036(3),
Li(3)–N(3) 2.037(3), Li(3)–N(5) 2.108(3), av. N–Al(1)–N 101.0, av. N–
Al(2)–N 101.1, av. N–Al(1)–C 117.0, av. N–Al(2)–C 116.9, av. N–Li–N
90.3 (when both N atoms are bound to the same Al centre), av. N–Li–N
107.9 (when the N atoms are bound to different Al centres), av. Li–N–Li
117.2 (when both Li atoms are associated with the same Al centre), av. Li–
N–Li 69.7 (when both Li atoms are associated with different Al centres).
† Electronic supplementary information (ESI) available: syntheses of 1–4.
See http://www.rsc.org/suppdata/cc/b3/b307589c/
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CHEM. COMMUN., 2003, 2356–2357
This journal is © The Royal Society of Chemistry 2003

observed for related neutral E2N6Li6 imido group 14 and 15
complexes.1,2 It is interesting that the dianion adopts an Al/N/Li
motif commonly seen in imido p-block anion chemistry with
imidoaluminium units linked by Li cations, rather than forming
an Al/N cage anion with separate Li cations as may be
anticipated from the rich structural chemistry observed in
imidoalanes.
composed solely of 4 which forms a dimeric cage in the solid
state with four lithium centres sandwiched between two
[PhB(NR)2]22 dianions. This possesses the same components
as have been seen in related studies by Chivers et al.16 The
differences in the observed products from similar boron and
aluminium chemistry can be accounted for by the fact that the
boron atoms in 4 adopt planar geometries whereas the
aluminium centres in 3 readily adopt tetrahedral geometries.
This may be explained by the relatively strong p-bonding found
for the second period elements B and N. Thus, reaction stops
before nucleophilic addition of an extra formal Li2NR fragment
can occur, which is conceptually what occurs to give the
corresponding aluminium product. However, the observation of
tetrahedral borate anions in the reaction solution illustrates the
very fine thermodynamic balance present in the boron system
between the planar and tetrahedral forms.
The [Li6{RAAl(NR)3}2]22 dianion unit can be thought of as
being built from two [RAAl(NR)3]42 tetraanions which sand-
wich six Li+ cations. The formal tetranegative charge on the
RAAl(NR)3 unit is to our knowledge the highest charge observed
crystallographically for a simple mononuclear imido main
group anion system. The two tetrahedral aluminium centres
each have a terminal ethyl chain and use the three imido arms to
bind the Li centres that hold the dimeric cage together. Oxygen
atoms from the anisyl moiety additionally solvate each Li centre
in the cage such that each Li has distorted tetrahedral geometry.
It is interesting to note that all eight lithium centres are not
incorporated into a single dimeric unit. There may be several
reasons for this. Firstly, the 14 membered dianion of 3 contains
a hexagonal prismatic Li6N6 framework similar to those
commonly seen in lithium amide chemistry. The high thermo-
dynamic stability of this unit may be of critical importance in
stabilising this heterobimetallic core. Secondly, steric con-
straints may preclude incorporation of all the Li centres into a
single cage structure. Lastly, there remains a question as to
whether a hypothetical 16 membered Li8Al2(NR)6 cage geome-
try is possible that allows sufficient favourable Li+–N2 contacts
to outweigh the accompanying repulsive Li+–Li+ and N2–N2
contacts.
The reaction to form 2 provides an interesting contrast to the
observations of Chivers et al. on metallation studies of
triamidoaluminanes and gallanes, [E{N(H)But}3]2.12,13 This
group found that only partial lithiation occurs to produce mixed
amido/imido group 13 anions that trap excess organolithium
reagent. In forming 2 full deprotonation of the imido groups has
occurred followed by nucleophilic addition of the organoli-
thium reagent. There are many possible reasons for this
difference in reactivity, notably the fact that arylamido groups
are more readily deprotonated than alkylamido groups and that
the methoxy sidearms of 2 help stabilise the Li+ counter cations.
In addition, the differences in steric bulk between the two
systems is likely to be a significant feature.
Another intriguing aspect of the system is revealed from a
comparison of related aluminium and boron systems. Hence,
whereas reaction of EtAlCl2 with three equivalents of 2-me-
thoxyanilido lithium and subsequent metallation with LiBun (3
equiv.) produces [Li(THF)4]2[Li6{EtAl(NR)3}2] (3) containing
the tetraanionic [EtAl(NR)3]42 unit, the corresponding reaction
of PhBCl2 with 2-methoxyanilido lithium (3 equiv.) and LiBun
(3 equiv.) gives [Li2PhB(NR)2]2·3THF (4) containing the
dianionic [PhB(NR)2]22 fragment. It is interesting to note that
the 11B NMR spectrum of the reaction mixture used to form 4
shows a broad peak at d = 33.5 ppm corresponding to 4 and
another sharp peak at d = 2.2 ppm that corresponds to a
tetrahedral boron environment that is probably due to tetra-
hedral borate anions present in solution where an anion
{probably either Bun2 or free N(H)R2} complexes the boron
centre. However, the only crystalline material isolated is
In conclusion, we have found two routes towards alkyl-
triimido aluminium tetraanions and have found an interesting
contrast to the related boron chemistry.
We gratefully acknowledge The Royal Society (University
Research Fellowship for CAR), the University of Bristol (JMS)
and the EPSRC (MCC) for financial support.
Notes and references
‡ CCDC 214484. See http://www.rsc.org/suppdata/cc/b3/b307589c/ for
crystallographic data in .cif or other electronic format.
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