Chemistry of aluminium(I)

Article abstract of DOI:10.1039/b505307b

In this article, we discuss the synthesis of tetrameric and monomeric aluminium(I) compounds. These compounds are prepared by reduction of the respective Al-X (X = halide) bond containing precursor. The tetrameric aluminium(I) compounds are synthesized by using sterically bulky ligands whereas a stable monomeric aluminium(I) compound is obtained using a monovalent chelating ligand. Theoretical studies are carried out on the monomer to understand the Lewis basicity. The presence of a lone pair of electrons plays an important role in the preparation of aluminium containing heterocyclic compounds, main group-main group and transition metal-main group compounds having donor-acceptor bonds by carrying out reactions with unsaturated compounds and Lewis acids. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b505307b

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FEATURE ARTICLE
www.rsc.org/chemcomm | ChemComm
Chemistry of aluminium(I)
Herbert W. Roesky* and S. Shravan Kumar
Received (in Cambridge, UK) 15th April 2005, Accepted 31st May 2005
First published as an Advance Article on the web 13th July 2005
DOI: 10.1039/b505307b
In this article, we discuss the synthesis of tetrameric and monomeric aluminium(I) compounds.
These compounds are prepared by reduction of the respective Al–X (X 5 halide) bond containing
precursor. The tetrameric aluminium(I) compounds are synthesized by using sterically bulky
ligands whereas a stable monomeric aluminium(I) compound is obtained using a monovalent
chelating ligand. Theoretical studies are carried out on the monomer to understand the Lewis
basicity. The presence of a lone pair of electrons plays an important role in the preparation of
aluminium containing heterocyclic compounds, main group–main group and transition
metal–main group compounds having donor–acceptor bonds by carrying out reactions with
unsaturated compounds and Lewis acids.
valent aluminium compounds has been observed in the past
Introduction
decade.3–5 In this feature article we report the development of
aluminium(I) chemistry in the last decade as well as the
paradigm shift in the synthesis of monomeric aluminium(I)
compounds from tetramers and its utility in the preparation of
aluminium heterocycles hitherto not possible in conventional
ways.
Aluminium is the third most abundant element in the earth’s
crust (7.4%). It never exists free in nature and it is usually
bound to oxygen (alumina) or to other elements like fluorine
(cryolite (Na3AlF6)). It is one of the major constituents of
naturally available minerals like feldspars, zeolites, and micas.
The majority of aluminium compounds known have a formal
oxidation state of +3 for the aluminium atom and this can be
attributed to the high stability of +3 oxidation state. The Hall–
He´roult process, which involves electrolysis of Al2O3 dissolved
in cryolite, is the widely employed process in the production of
aluminium metal.1,2 The area of low valent aluminium
chemistry although less explored is quite intriguing and
exciting from the perspective of synthetic methodology,
structure and theoretical implications. On account of these
factors a significant increase in the synthesis of these low
Chemistry of aluminium(I) compounds
The field of Al(I) chemistry has been developed to a much
lesser extent than that of Al(III) compounds owing to the low
stability of the +1 oxidation state of the aluminium atom but in
the last few years there has been an enhancement in the study
of aluminium(I) chemistry. Aluminium behaves like beryllium
and when subjected to anodic oxidation at high current
densities sub-valent aluminium ions are formed.2 There is
ample evidence for the existence of aluminium(I) compounds
like AlH, Al2O, Al2S, Al2Se, and AlX (X 5 F, Cl, Br, and I).6
AlH is formed when aluminium is subjected to heating with
hydrogen at 1500 uC and at atmospheric pressure. Al2O is
Institut fu¨r Anorganische Chemie der Georg-August-Universita¨t
Go¨ttingen, Tammannstrasse 4, D-37077, Go¨ttingen, Germany.
E-mail: hroesky@gwdg.de; Fax: +49 551/393373
Herbert W. Roesky was born in 1935 in Laukischken. He
obtained his Diplom and Dr. rer. nat. from the Georg-August-
Universita¨t, Go¨ttingen. He spent one year in postdoctoral work at
DuPont in Wilmington, DE, USA. In 1971 he became full
professor in Frankfurt and in 1980 he joined the faculty of
chemistry in Go¨ttingen. Since 2004 he has been working as an
Emeritus professor in Go¨ttingen and as Adjunct professor in
DeKalb, Illinois. Presently he is the president of the Go¨ttinger
Akademie der Wissenschaften. He has been a visiting professor
at Jawaharlal Nehru Centre for Advanced Scientific Research,
Bangalore, Tokyo Institute of Technology and Kyoto University
and also frontier lecturer at Texas A&M University at College
Station, the University of Texas at Austin, and the University of
Iowa at Iowa city. He is a member of the Academy of Sciences at
Go¨ttingen, the New York Academy of Sciences, the Indian
Academy of Sciences Bangalore, the Academy of Scientists
‘‘Leopoldina’’ in Halle, the Academia Europea of London,
Acade´mie des Sciences Paris, Third World Academy of Sciences,
and the Argentina Academy of Sciences. He has obtained several
awards such as the ACS Award for Creative Work in Fluorine
Chemistry, the Leibniz Award, and the Inorganic Chemistry
Award of the ACS. More than 950 research publications,
patents, and books document his activity in the field of inorganic
chemistry, catalysis, and material science. Moreover, he is well
known for his contributions to public understanding of chemistry.
Shravan Kumar Srisailam was born in 1979 in Hyderabad, India.
He did his Bachelor’s degree in physical sciences from Osmania
University, Hyderabad in 1999 and obtained his Master’s degree
in Chemistry from the Indian Institute of Technology Kanpur,
India in 2001. He received his doctorate in 2004 on the synthesis,
structural characterization and reactivity of homo- and hetero-
bimetallic imidoalanes and carbaalanes, aluminium hydrazide
and aluminium peroxide compounds at the Georg-August-
Universita¨t Go¨ttingen, Germany under the supervision of
Professor Herbert W. Roesky.
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obtained by heating a mixture of Al2O3 and silicon at 1800 uC
in a high vacuum. Both Al2S and Al2Se are prepared by
heating a mixture of aluminium shavings and Al2S3 and
Al2Se3, respectively, at 1300 uC. Aluminium(I) halides are
synthesized when metallic aluminium is heated with the
appropriate aluminium trihalide. If AlF3 and Al are heated
to 650–850 uC, which is below the sublimation temperature of
AlF3 (1291 uC), a white sublimate is obtained, which upon
X-ray examination revealed however only AlF3 and Al. AlF
can also be prepared by passing CF3H over molten aluminium
at 1000 uC.7 In 1948, Klemm and co-workers reported the
preparation of AlCl by the reaction of elemental chlorine with
aluminium at 1000 uC.8 AlCl is also obtained by reacting
aluminium and AlCl3 under reduced pressure and high
temperatures [eqn. (1)]. The formation of AlCl has been
thoroughly studied and its use in purifying aluminium has
been proposed.
used.17 Tetrameric halides ([Al4Br4?(NEt3)4] (6)18 and
[Al4I4?(PEt3)4] (7)19) of monovalent aluminium halides are
prepared by co-condensation of AlX (X 5 Br and I) with
strong donor ligands NEt3 and PEt3, respectively. Both the
compounds have a four-membered Al4 ring with terminally
arranged halide atoms and donor ligands. In the presence of
weak donor ligands like THF and THP (tetrahydropyran)
clusters of the type [Al22X20?12L] (L 5 THF, X 5 Br (8);20
L 5 THP, X 5 Cl (9)21) are obtained (Scheme 1).
A metastable solution of AlCl when reacted with SiCp*2
(Cp*
5
C5Me5) in diethyl ether gave compound
[Si(AlCl2?OEt2)4] (10),22 which contains four Si–Al bonds
and each AlCl2 unit is further coordinated by a diethyl ether
molecule. The most interesting reaction between AlCl and
dimethylacetylene MeCMCMe yields the dimeric 1,4-dichloro-
2,3,5,6-tetramethyl-1,4-dialumina-2,5-cyclohexadiene (11).23
The dimerization of 11 is due to the p-stacking of the two
monomers. A unique and fascinating compound [(Cp*Al)4]
(14) is obtained from the reaction of AlCl solution and
Cp*2Mg (Scheme 2).24–26 We followed a different strategy in
the synthesis of 14 by reduction of [Cp*AlX(m-X)2] (X 5 Cl,
Br, I) with a Na–K alloy or K.27 We observed that the lower
reaction temperature and the decrease in the bond energy of
the Al–X bond (AlCl (331 kJ mol21) . AlBr (256 kJ mol21) .
AlI (172 kJ mol21)) result in high yields of product 14
(Fig. 1).28 The starting material [Cp*AlX(m–X)2] is prepared
from Cp*SiMe3 and aluminium trihalide (Scheme 3).
Compound 14 decomposes around 205 uC and in the 27Al
NMR spectrum a singlet is observed at d 280.8 ppm.
Compound 14 is a tetramer with the aluminium atoms in
tetrahedral environment. The bond length between two
1200 0
C
?
2 AlðsÞzAlCl3ðgÞ
3 AlClðgÞ
(1)
/
600 0C
AlF and AlBr are also prepared using a similar strategy.
Tacke and Schno¨ckel followed a completely different approach
in the synthesis of AlCl.9 High yields of AlCl are obtained by
reacting aluminium and hydrogen chloride at low pressure
(less than 0.2 mbar) [eqn. (2)]. The IR spectrum shows a broad
absorption band with a maximum at 320 cm21 and a half-
width of about 100 cm21. The stability of AlCl in coordinating
solvents like ether is known but in weakly coordinating
solvents like toluene they are highly unstable.
˚
aluminium atoms is 2.769 A. Each aluminium atom is
927 0
C
AlðsÞzHClðgÞ DCCA AlClðgÞz1= H
(2)
2
2
coordinated to one Cp* (Cp* 5 C5Me5) ligand in an
5
g -fashion with Al–C bond length of 2.334 A. In addition to
˚
The reactivity of monovalent aluminium halides was
investigated by Schno¨ckel and co-workers by carrying out
matrix isolation experiments in solid argon.7 The matrix
induced reactions of AlCl with HBr and AlF with HCl and
HBr resulted in the formation of adducts AlCl?HBr, AlF?HCl,
and AlF?HBr, respectively. The formation of these species
gives insight into the mechanism of the formation of
aluminium(III) halides. Peroxo and bissuperoxo complexes of
aluminium XAlO2 and XAl(O2)2 are also reported when AlX
(X 5 F, Cl, and Br) is treated with dioxygen in an argon
matrix upon photoexcitation.10,11
[(Cp*Al)4] (14), only few neutral Al(I) organometallic com-
pounds [{(t-Bu)3SiAl}4] (15),29 [{(SiMe3)3CAl}4] (16) (Fig. 2),30
[Cp*3Al3AlN(SiMe3)2] (17),31 [{(t-BuCH2)Al}4] (18),32
[{(SiMe3)3SiAl}4] (19),33 and [2,6-i-Pr2C6H3N(SiMe3)Al]4
(20)34 are reported and this is primarily due to the tendency
of (RAl)4 to disproportionate into elemental aluminium and
AlR3. Compound 17 is obtained by the substitution of one of
the Cp* with N-based ligand. The reaction between Cp*Al and
LiN(SiMe3)2 at 60 uC leads to the formation of 17.
Due importance is given to the synthesis of aluminium
metalloid clusters12 as they show close similarity with the
topology of aluminium metal. These clusters contain a large
number of (naked) aluminium atoms having a minimum
number of donor ligands. The formation of aluminium
metalloid is directly dependent on the temperature. A rise in
temperature can increase the size of cluster and vice versa.
For example, AlCl solution when treated with LiN(SiMe3)2 at
27 uC, room temperature, and 60 uC yields the metalloids
[Al7{N(SiMe3)2}6]2 (1),13 [Al12{N(SiMe3)2}6]2 (2),14 and
[Al69{N(SiMe3)2}18]32 (3),15 respectively, whereas the reaction
of t-BuLi with AlCl solution followed by reduction with Na/K
alloy results in stable radical [t-BuAl]6 (4).16 However,
?
[Al77{N(SiMe3)2}20]22 (5) is obtained when an AlI solution is
Scheme 1 Reactions of AlX compounds.
4028 | Chem. Commun., 2005, 4027–4038
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Scheme 2 Reactions of AlX compounds.
Fig. 1 Molecular structure of [(Cp*Al)4] (14).
Fig. 2 Molecular structure of [{(SiMe3)3CAl}4] (16).
Triethylamine coordinated aluminium(I) iodide and bromide
solutions react with t-Bu3SiNa and [(SiMe3)3SiLi?3THF],
(Scheme 4). These compounds possess tetrahedral arrange-
ment in the solid-state.
respectively
to
yield
[{(t-Bu)3SiAl}4]
(15)
and
[{(SiMe3)3SiAl}4] (19) (Scheme 2). The reduction of
[(t-BuCH2)2AlCl] with potassium led to aluminium(I) tetramer
[{(t-BuCH2)Al}4] (18) and [(t-BuCH2)3Al] [eqn. (3)].
Compound [2,6-i-Pr2C6H3N(SiMe3)Al]4 (20) which is exclu-
sively nitrogen based is prepared by reacting [2,6-
i-Pr2C6H3N(SiMe3)AlI2]2 with Na/K alloy in hexane
Â
Ã
2 ðt-BuCH2Þ2AlCl z2 K?
Â
Ã
Â
Ã
(3)
(3)
1
=
fðt-BuCH2ÞAlg4 z ðt-BuCH2Þ3Al
18
4
Thus, steric and electronic factors of the R group coupled
with the easy synthesis of aluminium halides are the essential
requirements in the synthesis of (RAl)4 compounds. Synthesis
of few more tetramers from aluminium(I) compounds are
Scheme 3
Scheme 4
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Table 1 Characteristic features of aluminium(I) tetramers
˚
Al–Al distance/A
Compound
Mp/uC (decomp.)
Space group
[(Cp*Al)4] (14)a
[{(t-Bu)3SiAl}4] (15)
[{(SiMe3)3CAl}4] (16)
[Cp*3Al3AlN(SiMe3)2] (17)a
[{(t-BuCH2)Al}4] (18)
[{(SiMe3)3SiAl}4] (19)
[2,6-i-Pr2C6H3N(SiMe3)Al]4 (20)
205
—
282
.180
2.769
2.604
2.739
2.758b
¯
Triclinic, P1
Monoclinic, C2/c
Orthorhombic, P212121
¯
Triclinic, P1
.250
.270
Monoclinic, P21/c
¯
Triclinic, P1
2.607
2.635
a
b
Cp* 5 C5Me5. The distance between two aluminium atoms bound to Cp* ligands.
depicted in Scheme 2. Some characteristic features of tetra-
meric aluminium(I) compounds are described in Table 1.
the reactions with BBr3 and AlCl3 yield completely different
products. When the reaction is carried out with BBr3 exchange
of Cp* takes place to give an ionic compound [Cp*BBr][AlBr4]
(22)37 and formation of [Cp2*Al]+ takes place in the presence
of AlCl3. [Cp2*Al]+ has a sandwich-like structure isoelectronic
with Cp*2Mg.38 Cp*Al acts as an efficient ligand in transition-
metal complexes LnM–AlCp* (Ln 5 ligand).39 For instance,
[(Cp*Al)4] (14) in the presence of transition metal complexes
[NiCp2], [Co2(CO)8], [Cr(CO)5COT] (COT 5 cyclooctene),
[Fe(CO)5], and [(dcpe)Pt(H)(CH2t–Bu)] (dcpe 5 1,2-bis(dicy-
clohexylphosphino)-ethane) yields [(CpNi)2(m–AlCp*)2] (23),40
[{(Co(CO)3}2(m–AlCp*)2] (24),41 [(CO)5CrAlCp*] (25),42
[(CO)4FeAlCp*] (26),43 and [(dcpe)Pt(AlCp*)2] (27),44 respec-
Chemistry of [(Cp*Al)4] (14)
Tetramer [(Cp*Al)4] (14) undergoes dissociation in solution
and in the gas phase with formation of the monomeric Cp*Al
species.35 The monomeric Cp*Al has been thoroughly
investigated by 27Al NMR spectroscopy.31 The monomer
exists in singlet state with a lone pair of electrons on the
aluminium atom and its molecular structure was determined
using gas phase electron diffraction. Cp*Al is isolobal to
singlet carbene and its reactivity is demonstrated in the
presence of Lewis acids like B(C6F5)3 to give the Lewis acid–
base adduct of composition [Cp*AlB(C6F5)3] (21)36 whereas
˚
tively (Scheme 5). The Pt–Al bond distance (2.33 A) in 27 is
shorter in comparison to that in an alloy of platinum and
Scheme 5
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45
˚
aluminium (2.52 A). In the above-discussed reactions the
aluminium atom retains a formal oxidation state of +1.
The reactions of [(Cp*Al)4] (14) with main group element
compounds result in the formation of rings, cages, and clusters
and a change in oxidation state from +1 to +3 is also observed.
Treatment of 14 with (t-BuSb)4 gave
a
polyhedral
[(Cp*Al)3Sb2] (28)46 that has two fused four-membered rings.
Polyhedral compounds [(Cp*Al)3As2] (29)47 and [(Cp*Al)6P4]
(30)48 are also obtained when [(Cp*Al)4] (14) reacts with
arsenic and P4, respectively. Moreover, reactions are also
carried out using Group 16 elements like selenium and
tellurium. Colorless crystals of [(Cp*AlSe)4] (31)27 and pale
green shiny crystals of [(Cp*AlTe)4] (32)27 are obtained when a
solution of 14 in toluene is treated with an excess of selenium
and tellurium, respectively. Both compounds possess tetra-
meric geometry with a deviation in bond angles at Al, Se, and
Te centers (av Se–Al–Se 94.6u, Te–Al–Te 95.0u, Al–Se–Al
85.2u, Al–Te–Al 84.7u). The reaction of 14 with Ph2SiF2
resulted in the formation of cage compound 33 instead of
Cp*AlF2. Compound 33 has two Al–Si–Al units while the
bridging positions are occupied by the fluorine atoms.46 14
reacts with alkylsilylazides Me3SiN3 and (t-Bu)3SiN3 by an
oxidative addition to give imidoalanes [Cp*AlN{(Cp*)-
AlN(SiMe3)2)}]2 (34)49 and [Cp*AlNSi(t-Bu)3]2 (35),50 respec-
tively. In the synthesis of 35 silyl group migration and Cp*
rearrangement takes place simultaneously. Both compounds
34 and 35 have nearly planar four-membered Al2N2 rings.
Moreover, both aluminium and nitrogen atoms have coordi-
nation number three (Scheme 5).
Scheme 6
Fig. 3 Molecular structure of [Al{HC(CMeNAr)2}] (37).
solvents like toluene and benzene, respectively. The aluminium
atom in 37 is a part of the six-membered heterocycle. The
˚
Al–N bond length (1.957 A) is longer than that in the
corresponding Al(III) compounds [Me2Al{HC(CMeNAr)2}]
55
˚
˚
(av 1.922 A) and [(SeH)2Al{HC(CMeNAr)2}] (1.899 A).
Similarly, an acute bond angle is observed at the aluminium
center (N–Al–N 89.86(8)u). The long Al–N bond and small
N–Al–N bond angle lower the possibility of multiple Al–N
bond character. The fascinating aspect of this molecule is the
dual Lewis acid and Lewis base character. On the basis of
ab initio calculations,56 the stereochemically active lone pair of
electrons on the aluminium atom and the possibility of quasi-
trigonal-planar orientation are observed by analyzing the
Laplacian of electron density57 within the plane, which clearly
describes the Lewis basicity of compound 37. Moreover,
charge depletion close to the aluminium atom into the
semiplane of the six-membered ring was also observed
indicating the Lewis acidity of 37.
Chemistry of [Al{HC(CMeNAr)2}] (37)
Although a good number of tetrameric aluminium(I) com-
pounds of the type (RAl)4 are known, a monomeric species of
the composition of RAl has not been isolated. Spectroscopic
evidence for the transient existence of MeAl in the gas phase
has been provided by Schwarz et al.51 Though PhAl is not
isolated, it is considered as an intermediate in organic
transformations under photolytic conditions.52 Schno¨ckel
and co-workers made many attempts to synthesize CpAl but
were unsuccessful and this is possibly due to its high thermal
sensitivity and decomposition within seconds above 260 uC.53
However, they were able to characterize and determine the
structure of Cp*Al in the gas phase using electron diffrac-
tion.31,35 This species exists in equilibrium in solution with its
tetramer 14.
Main group heterocyclic compounds have found wide
application in pharmaceutical, agrochemical, and material
science.58 There is a tremendous increase in the synthesis of
Group 13 heterocycles in recent years. In the following
paragraphs, we describe our work in the synthesis of
aluminium containing heterocycles. Taking advantage of the
lone pair of electrons on the aluminium atom, compound 37 is
treated with Me3SiN3 in the synthesis of tetrazole
[{HC(CMeNAr)2}]Al[(NSiMe3)2N2] (38) (Fig. 4).59 The inter-
mediate [Me3SiNLAl(CMeNAr)2CH] which may have
formed in the course of the reaction undergoes [2 + 3]
cycloaddition reaction with Me3SiN3 to yield 38 (Scheme 7).
The formation of 38 precludes the dimerization of the
intermediate and it can be attributed to the presence of the
bulky chelating b-diketiminato ligand. Compound 38 has a
nearly planar AlN4 heterocycle and the geometries at the two
We followed different routes to synthesize more kinetically
stable aluminium(I) compounds. Considering the fact that
bulkiness of the ligand plays a crucial role in the synthesis of
aluminium(I) compounds we have chosen the b-diketiminato
ligand. Aluminium diiodides were prepared using the chelating
monovalent b-diketiminato ligand followed by reduction of
[I2Al{HC(CMeNAr)2}] (36) with potassium which resulted in
the formation of monomeric aluminium(I) carbenoid
[Al{HC(CMeNAr)2}] (37) (Scheme 6).54 This was the first
stable dicoordinate aluminium(I) compound to be prepared
and structurally characterized in the solid-state. Compound 37
(Fig. 3) is a thermally stable red crystalline solid, which has a
decomposition point above 150 uC. It is soluble in non-polar
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from the IR and NMR spectra the formation of the Al–H
bond is confirmed (v˜(Al–H) 1809 cm21 and d 4.8 ppm,
respectively).
1,3-Dimethyl-4,5-dimethylimidazol-2-ylidene
also reacts with 37 in similar fashion to give 40. Treatment
of 37 with two equivalents of diphenyldiazomethane Ph2CN2
under heating affords the first example of diiminylaluminium
compound [(Ph2CLN)2Al{HC(CMeNAr)2}] (41) in quantita-
tive yield (Scheme 8). The formation of 41 proceeds via the
oxidative addition of [Al{HC(CMeNAr)2}] (37) and the
initially generated Ph2CLN–NLCPh2. The X-ray crystal
structure shows a short Al–Niminyl bond length (1.774(4) and
˚
1.785(4) A) comparable to Al–N bond lengths of three-
coordinated aluminium complexes [(trip)2Al{N(H)dipp}]
˚
A),
˚
A),
(1.784(3)
[MeAl(Ndipp)]3
˚
(1.782(4)
and
Fig. 4 Molecular structure of [{HC(CMeNAr)2}]Al[(NSiMe3)2N2]
(38).
[Al{N(SiMe3)2}3] (1.78(2) A). C–N bond lengths of the iminyl
˚
groups (1.249(6) A) are also found to be shorter than ideal
CLN bond length. The short C–N bond may be due to the
electron delocalization over the phenyl rings.
Synthesis of cyclopropene derivatives is mainly centered on
Group 14 elements. They are prepared by the reaction of
carbenes R2C: or carbene analogues R2M: (M 5 Si, Ge, and
Sn) with alkynes. The chemistry of Group 13 cyclopropene
derivatives is mainly focussed on boron61 but due to the high
reactivity exhibited by the heavier main group cyclopropene
compounds62 we can see an upsurge in the synthesis of Al, Ga
and In based cyclopropene compounds. Like carbenes,
[Al{HC(CMeNAr)2}] (37) undergoes [1 + 2] cycloaddition
reaction with unsubstituted and substituted alkynes to give
three-membered aluminacyclopropene containing derivatives.
Compound 37 in the presence of excess acetylene yields an
enyne [{HC(CMeNAr)2}Al(CMCH)(CHLCH2)] (43) but
when the reaction temperature is kept below 250 uC
[{HC(CMeNAr)2}Al(g2–C2H2)] (42) is exclusively formed
(Scheme 9). The formation of 42 follows a [1 + 2] cycloaddition
reaction and further reaction, with acetylene results in the
formation of 43, which proceeds through a donor–acceptor
intermediate due to the Lewis acidic aluminium center in 42.63
The X-ray crystal structure of 42 reveals a three-membered
aluminacyclopropene Al(g2–C2H2) with short Al–C
Scheme 7
nitrogen atoms connected to the aluminium atom are both
trigonal planar. A similar five-membered AlN4 heterocycle is
formed when the reaction is carried out with Ph3SiN3.
It is indeed fascinating to study the reactivity of electron
rich aluminium(I) compound 37 in the presence of other
electron rich compounds like singlet N-heterocyclic carbenes
and diphenyldiazomethane,
a
precursor to :CPh2.60
The reaction between 37 and 1,3-diisopropyl-4,5-dimethylimi-
dazol-2-ylidene is carried out at 120 uC to yield adduct 39
(Scheme 8). During the course of the reaction a proton
migration from one of the methyl groups of the b-diketiminato
ligand to the aluminium atom takes place to generate an Al–H
bond. The exact mechanism of the migration is not clear but
˚
˚
(1.882(2) A) and C–C (1.358 A) bond lengths while 43 has
terminal acetylene and ethylene groups. Similarly, heterocyclic
three-membered ring containing compounds LAl[g2–
C2(SiMe3)2] (44), LAl[g2–C2Ph2] (45), and LAl[g2–
C2(SiMe3)Ph] (46) (L 5 b-diketiminato) are synthesized in
modest yield by carrying out a reductive coupling reaction of
[I2Al{HC(CMeNAr)2}] (36) with respective disubstituted
alkynes ((SiMe3)2C2, Ph2C2, and (SiMe3)(Ph)C2, respectively
(Scheme 10).64 A possible mechanism is suggested in the
formation of compounds 44, 45, and 46. It is assumed that
Scheme 9
Scheme 8
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Highly strained three-membered rings undergo facile ring
opening reactions. This is clearly evident in cyclopropene
containing compounds. In order to expand further the scope of
the ring opening chemistry, reactions are carried out on
aluminium containing three-membered cyclopropene com-
pounds 42 and 44 with unsaturated molecules. Treatment of
42 with organic azide N3Ar* (Ar* 5 2,6-Ar92C6H3, Ar9 5 2,4,6-
Me3C6H2) exhibits an unusual end-on azide insertion
resulting in
a four-membered aluminiumazacyclobutene
[{HC(CMeNAr)2}Al(CHLCH)(NNLNAr*)] (48).63 The reac-
tion of 44 with CO2 proceeds smoothly at room temperature to
generate aluminium bound a,b-unsaturated lactone
LAl[OC(O)C2(SiMe3)2}] (49) via C–C bond coupling. In the
IR spectrum a strong absorption band corresponding to the
CLO unit is observed at 1666 cm21. A similar reaction with
CS2 did not result in a five-membered heterocyclic ring; instead
an allene unit present in a seven-membered Al2C3S2 ring
containing compound 50 is obtained by carrying out the
reaction with 44 at 278 uC.66 During the course of the reaction
the color of the solution changes from red-black to yellow. The
formation of the allene unit in the final product is quite unique
and unexpected. It is possible that an intermediate analogous
to compound 49 may have formed in the reaction medium that
reacts further with the second molecule of 44 to give 50. The
bond angle of 122.57u for Al–S–Al is the widest among
dinuclear aluminium sulfides or thiolates. Benzophenone also
undergoes C–C bond coupling with 44 to give aluminadihy-
drofuran LAl[OC(Ph)2C2(SiMe3)2}] (51). The reaction is
carried out at room temperature in diethyl ether. This is a
first example of an aluminium bound dihydrofuran structure.
The reactivity of 44 with PhCN is monitored in diethyl ether at
room temperature to give aluminium containing pyrazole
structure LAl[NC(Ph)C2(SiMe3)2}] (52). Crystal structures of
51 and 52 show the tetrahedral geometry for the aluminium
Scheme 10
?
aluminium iodide radical (LAlI ), generated from compound
36 in the presence of potassium, couples with alkynes
followed by electron transfer to yield the desired products
(Scheme 11). To understand the mechanism, 36 is treated with
benzophenone (Ph2CO) in the presence of Na/K at room
temperature. The reaction yields aluminium pinacolate
{HC[(CMeNAr)2}Al[O2(CPh2)2}] (47) which is formed via
+
2 65 radical followed
?
the dimerization of the ketyl [K (OCPh2)
]
by the elimination of KI (Scheme 11). The variable tempera-
ture 1H NMR spectrum of 44 shows the strong bonding
between aluminium and CLC unit. In the 27Al NMR spectrum
of 44, aluminium atom resonates in the range d 90 ppm.
Compounds 44 and 45 have highly strained three-membered
rings (C–Al–C 42.57u (44) and 42.02u (45)) with a short Al–C
bond length (1.899 A (44) and 1.875 A (45)). Both the six-
membered rings and the three-membered rings are nearly
perpendicular to each other.
˚
atom. The Al–O bond length in 51 (1.727 A) is comparable to
˚
˚
that of 47 (1.730 A) whereas a bond length of 1.867 A is
observed for an Al–N bond in 52, which is shorter in
comparison to Al–N bond length in the aluminium iminato
˚
˚
67
˚
complex {(i-Bu)2Al[m–NLC(H)(C6H3-2,6-Me2)]}2 (1.946 A).
Aluminium bis(iminato) complex LAl[N2(Ct–Bu)2] (53) is
obtained via the replacement of the alkyne moiety when
compound 44 is treated with t-BuCN (Scheme 12).
It is often observed that synthesis of imidoalanes occurs at
elevated temperatures and involves the elimination of alkanes,
alkenes, or dihydrogen from amidoalanes. The elimination of
substituents results in a decrease in the steric crowding around
the imidoalane and thereby increases the possibility of
association leading to the formation of dimers, tetramers,
hexamers etc.68 On the other hand organoazides49,69 play a
significant role in the synthesis of unassociated rings having
Al–N bonds. The reaction between b-diketiminato stabilized
aluminium(I) compound 37 and N3-2,6-Dipp2C6H3 results in
the formation of two different isomers 54 and 55.70 They are
formed via a monomeric imidoalane (A) which exhibits latent
reactivity. The formation of the intermediate species A is
1
confirmed by H NMR kinetic study. Two different reaction
pathways are followed in the synthesis of 54 and 55. A [2 + 2]
cycloaddition reaction of the AlLN bond and a phenyl ring of
the Ar9 substituent on the nitrogen leads to 54 (path ‘‘a’’). The
Scheme 11
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Scheme 12
X-ray crystal structure reveals the formation of quasi-planar
˚
AlNC2 ring (D 5 0.0680 A) with an Al–N bond length of
˚
1.867(2) A. The phenyl ring after the cycloaddition reaction is
phosphorous (P4) and elemental sulfur (S8) in the successful
42
42
synthesis of P4
and S6
unit containing aluminium
phosphide [{HC(CMeNAr)2}Al]2P4 (56) (Fig. 5)76 and sulfide
[{HC(CMeNAr)2}Al]2S6 (57) (Fig. 6),77 respectively
(Scheme 14). During the reactions cleavage of two P–P and
four S–S bonds is observed. The presence of excess 37 could
not lead to the complete cleavage of the remaining P–P and
S–S bonds. The 31P NMR spectrum of 56 shows resonance at
d 78.6 ppm, which is downfield in comparison to the free P4
molecule (d 2519 ppm) and in the X-ray crystal structure
cleavage of two P–P bonds is noticed. The Al–P bond length of
nonplanar. Intramolecular C–H bond activation of the
isopropyl group of the Ar substituent generates 55 (path
‘‘b’’). In the IR spectrum an absorption band at 3298 cm21
assignable to the N–H stretching frequency is observed. The
crystal structure of 55 shows the formation of a new six-
membered AlN2C3 ring with aluminium in tetrahedral
geometry. Moreover, 54 can be transformed to 55 under
thermal conditions (Scheme 13).
˚
2.37 A is in the range of the Al–P bond length of [(Cp*Al)6P4]
˚
(30) (2.31–2.42 A). The charge distribution in the Al2P4 unit
Organophosphorus compounds and sulfur containing com-
pounds like metal sulfides, S8, P4S10, H2S, and organo
polysulfanes have attracted a great deal of attention owing
to their structural reactivity and due to their wide potential
application.71 Transition-metal sulfides are widely employed
not only in catalytic reactions like HDS (hydrodesulfurization)
of fossil fuels72 but also as intermediates in enzymatic
processes. Quite contrary to the availability of transition-
metal sulfides [Cp2TiS5]73 and [{(g5–MeC5H4)Ru(PPh3)(m–
S3)}2]74 and phosphides [Cp*2(CO)2Co2P4] (Cp* 5 C5Me5)75
Group 13 metal sulfides and phosphides are less explored. We
investigated the reactivity of 37 in the presence of white
was investigated using DFT calculations with RI-BP86/TZVP
within TURBO-MOLE. A net charge transfer from the
aluminium atoms to P4 is observed when calculated Mulliken
charges on the P atom (20.22) are taken into consideration,
which clearly demonstrates the formation of an ionic Al–P
bond. The bond order of zero shows the cleavage of the P–P
bonds whereas the binding energy of the order of 36.4 kcal
mol21 ((E(LM)2–P4 2 2*EL–M 2 EP4)/2) indicates a strong
interaction between aluminium atoms and P4. The synthesis of
pale yellow crystals of [(LAl)2S6] (57) is carried out at 278 uC.
4034 | Chem. Commun., 2005, 4027–4038
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Fig. 6 Molecular structure of [{HC(CMeNAr)2}Al]2S6 (57).
[L2Al2Sn] (L 5 ligand; n 5 2–8) species, which makes the
isolation of the species difficult.
Chemistry of heavier Group 13 carbenoids
It is interesting to know that Al(I) carbenoid was synthesized
prior to the discovery of the analogous Ga(I), In(I), and Tl(I)
carbenoids even though the stability of monovalent oxidation
state increases as Group 13 is descended. In general low valent
heavier Group 13 compounds containing a M–M bond
(M 5 Ga, In, and Tl) exist as tetramers or hexamers that
may dissociate to lower aggregates in the vapor or solution
phase. Although the pentamethylcyclopentadienyl derivative
of Ga(I) was structurally characterized as the monomer in the
gas phase by electron diffraction, it is highly unlikely to exist as
Scheme 13
The crystal structure of 57 shows that the aluminium atoms are
connected by two m–S3 chains to form an aluminium
hexasulfide Al2S6. The eight-membered Al2S6 ring has a
staggered conformation in contrast to the S8 molecule’s
˚
eclipsed conformation. The S–S bond length (av 2.08 A) is
˚
longer than that in the S8 molecule (av 2.05 A) whereas the
a
monomer in solid state. Thus, the Ga(I) compound
S–S–S bond angle (104.7u) is close to the bond angles found in
transition-metal sulfides [{(g5–MeC5H4)2Ti(m–S3)}2] (109.1u)
and [{(g5–MeC5H4)Ru(PPh3)(m–S3)}2] (105.2u). DFT calcula-
tions carried out on 57 using RI-BP86/TZVP within TURBO-
MOLE shows more than one stable conformation for the
Ga(Tpt-Bu2) (58) [Tpt-Bu 5 tris(3,5-di-tert-butylpyrazolyl)-
2
hydroborato] was structurally characterized for the first time
by Parkin and co-workers in the solid-state by carrying out the
reaction between in situ generated GaI and Na(Tpt-Bu2), which
also gave the adduct Ga(Tpt-Bu2) A GaI3 (59) (Scheme 15).78
Fig. 5 Molecular structure of [{HC(CMeNAr)2}Al]2P4 (56).
This journal is ß The Royal Society of Chemistry 2005
Scheme 14
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Scheme 15
Scheme 16
…
˚
The closest Ga Ga distance is ca. 10.4 A, which clearly
indicates the absence of any kind of interactions between the
monomers. The stability of this molecule is derived from the
presence of bulky tridentate ligand Tpt-Bu2. In continuing
efforts to synthesize monovalent gallium compounds Power
and co-workers reported b-diketiminato stabilized Ga(I)
monomer, [Ga{HC(CMeNAr)2}] (60), (Ar 5 2,6-i-Pr2C6H3)
(Scheme 16).79 It was obtained by the reaction of
[Li{HC(CMeNAr)2}] with GaI. During the course of the
reaction any Ga(III) compound, [I2Ga{HC(CMeNAr)2}]
formed was subsequently reduced with excess potassium to
generate the monovalent derivative. The Ga–N bond length
with the corresponding Al(I) and Ga(I) derivatives 37 and 60,
respectively. Although Cp* bound gallium(I) and indium(I)
compounds are extensively used in Lewis acid-Lewis base
adducts85 but the chemistry of b-diketiminato stabilized Ga(I),
In(I), and Tl(I) monomers 60–63 is still in its infancy. Power
and co-workers recently reported the synthesis of monomeric
imides [{HC(CMeNAr)2}GaN-2,6-Trip2C6H3] (64) by carry-
ing out the reaction between bulky azide N3-2,6-Trip2C6H3
and 60.86 The X-ray crystal structure shows a short Ga–Nimide
˚
bond (1.742(3) A) and a bent geometry for the imide nitrogen
˚
(2.054(4) A) in 60 is shorter than the corresponding bond
˚
length in 58 (2.230(5) A) and indicates the absence of multiple
(Ga–Nimide–C 134.6(3)u). The deviation from the linear
alignment is attributed due to the presence of bulky ligands
around the imide bond. A similar trans bent geometry is
observed at the nitrogen center in the compounds Ar9MNAr0
(Ar9 5 2,6-Dipp2C6H3 (Dipp 5 2,6-i-Pr2C6H3); Ar0 5 2,6(Xyl-
4-t-Bu2)C6H3; M 5 Ga (65) and In (66)), which are prepared
by the reaction of gallium and indium dimers (Ar9M)2
(M 5 Ga, In) with N3Ar0.87 When the present manuscript
was under preparation, Power et al. reported the synthesis of
Ga–N bond character which might have arisen from the
delocalization of p-electrons within the six-membered ring.
Recently, Schmidbaur and Jones have reported the anionic
Ga(I) heterocycle.80–82
Observing these exciting results Hitchcock and co-workers
gave impetus to the synthesis of monomeric indium and
thallium compounds in their +1 oxidation state. The In(I)
carbenoids [In{HC(C(R)NAr)2}]; (R 5 CF3 (61),83 Me(62)84),
stabilized by the bulky b-diketiminato ligand, having a two-
coordinate neutral In(I) singlet, is synthesized by carrying out a
‘one pot’ reaction of equimolar quantities of K[N(SiMe3)2],
readily available InI, and [H{HC(C(R)NAr)2}]; (R 5 CF3,
Me) in THF at 278 uC. A similar strategy is employed in the
preparation of Tl(I) carbenoid [Tl{HC(CMeNAr)2}] (63)
(Scheme 16).83 Compounds 61, 62, and 63 are isostructural
monomeric thallium(I) amide [TlN(Me)ArMes
]
2
(67)88
(Mes 5 2,4,6-Me3C6H2) by carrying out the reaction between
TlCl and bulky lithium amide [LiN(Me)ArMes2]2 but when the
diethyl ether adduct of lithiated aryl compound 2,6-
Trip2C6H3Li?Et2O is used 2,6-Trip2C6H3Tl (68) (Trip 5 2,4,6-
i-Pr3C6H2) is obtained.89 The characteristic features of
monovalent monomeric Group 13 compounds are shown in
Table 2.
Table 2 Characteristic features of monovalent monomeric Group 13 compounds
˚
M–N bond distance/A
Compound
Mp/uC
.150a
Space group
N–M–N bond angle (u)
[Al{HC(CMeNAr)2}] (37)
[Ga{HC(CMeNAr)2}] (60)
[In{HC(CMeNAr)2}] (61)
[In{HC(C(CF3)NAr)2}] (62)
[Tl{HC(CMeNAr)2}] (63)
Monoclinic, P21/n
Monoclinic, P21/n
Monoclinic, P21/n
1.957
2.054
2.268
2.357
2.428
89.86
87.53
81.12
78.23
76.67
202–204
203–205a
90a
¯
Triclinic, P1
Monoclinic, P21/n
104–105a
a
Decomposition.
4036 | Chem. Commun., 2005, 4027–4038
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16 C. Dohmeier, M. Mocker, H. Schno¨ckel, A. Lo¨tz, U. Schneider
and R. Ahlrichs, Angew. Chem., 1993, 105, 1491–1493, Angew.
Chem., Int. Ed. Engl., 1993, 32, 1428–1430.
17 A. Ecker, E. Weckert and H. Schno¨ckel, Nature, 1997, 387,
379–381.
18 M. Mocker, C. Robl and H. Schno¨ckel, Angew. Chem., 1994, 106,
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19 A. Ecker and H. Schno¨ckel, Z. Anorg. Allg. Chem., 1996, 622,
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20 C. Klemp, R. Ko¨ppe, E. Weckert and H. Schno¨ckel, Angew.
Chem., 1999, 111, 1851–1855, Angew. Chem., Int. Ed., 1999, 38,
1739–1743.
Conclusions
The present review focuses on the development of
aluminium(I) chemistry in the last decade.90,91 It also lays
emphasis on the synthesis of heavier analogous of aluminium
like Ga, In, and Tl. The preparation of monomeric and
tetrameric aluminium(I) compounds is possible only in the
presence of an aluminium–halogen bond. Even though several
tetrameric aluminium compounds are known, the ligands
present on them are not suitable enough in the synthesis of
monomeric species. The monovalent chelating b-diketiminato
21 C. Klemp, M. Bruns, J. Gauss, U. Ha¨ussermann, G. Sto¨sser, L. van
Wu¨hlen, M. Jansen and H. Schno¨ckel, J. Am. Chem. Soc., 2001,
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ligand plays
a
decisive role in the preparation of
22 A. Schnepf and H. Schno¨ckel, Adv. Organomet. Chem., 2001, 47,
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¨
23 C. Uffing, A. Ecker, R. Ko¨ppe, K. Merzweiler and H. Schno¨ckel,
Chem. Eur. J., 1998, 4, 2142–2147.
[Al{HC(CMeNAr)2}] (37). Thus, we have seen a gradual
transition in the synthetic methodology in the synthesis of
monomeric aluminium(I) compound 37 from tetrameric
aluminium(I) compounds by using a bulky ligand. Tetramer
[(Cp*Al)4] (14) reacts with several main group element
compounds and transition metal compounds and generates
cages, clusters, and rings whereas the [Al{HC(CMeNAr)2}]
(37) reacts with unsaturated compounds to afford aluminium
containing heterocyclic compounds. The reaction between
[(Cp*Al)4] (14) and Lewis acid B(C6F5)3 results in the
generation of a main group–main group donor–acceptor
bond. The chemistry of aluminium(I) is still in its infancy but
it has a great potential as it has already found wide application
in materials and agrochemicals.
24 C. Dohmeier, C. Roble, M. Tacke and H. Schno¨ckel, Angew.
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Acknowledgements
We are thankful to Christoph Schnitter, Stephan Schulz,
Chunming Cui, Hongping Zhu, and Ying Peng for their
outstanding experimental contributions. We are thankful to
Umesh Nehete for working on the manuscript. Financial support
by the Deutsche Forschungsgemeinschaft and the Go¨ttinger
Akademie der Wissenschaften is highly acknowledged.
¨
31 H. Sitzmann, M. F. Lappert, C. Dohmeier, C. Uffing and
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33 A. Purath and H. Schno¨ckel, J. Organomet. Chem., 1999, 579,
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34 M. Schiefer, N. D. Reddy, H. W. Roesky and D. Vidovic,
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36 J. D. Gorden, A. Voigt, C. L. B. Macdonald, J. S. Silverman and
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4038 | Chem. Commun., 2005, 4027–4038
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