DOI:10.1039/b411889h
Thisjournalis © TheRoyalSocietyofChemistry2004 3176 DaltonTrans .,2004,3176–3177
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COMMUNICATION
Metal-induced formation of a new chiral bidentate N,O ligand and
the synthesis of novel monomeric germylenes†
Matthias Driess,* Nicoleta Dona and Klaus Merz
Laboratory of Inorganic Chemistry I: Cluster and Coordination Chemistry,
Ruhr-University Bochum, Universitätsstrasse 150, D-44801, Bochum, Germany.
E-mail: matthias.driess@rub.de; Fax: +49 234-321-4378; Tel: +49 234-322-4158
Received2ndAugust2004,Accepted26thAugust2004
FirstpublishedasanAdvanceArticleontheweb6thSeptember2004
Lithiation of the N-benzyl substituted diketoamine ligand
PhCH
2
–N[CH
2
C(tBu)O]
2
with lithium diisopropylamide
(LDA)affordsanovelchiralanionicN,O-ligandL;LiLreacts
withGeCl
2
·dioxanetogivethecorrespondingnewmonomeric
germylenesLGeClandL
2
Ge.
Monomeric carbene-homologues MR
2
of the divalent Group 14
atoms M(II) (M = Ge, Sn, Pb) having small substituents R are
unstable toward oligomerisation. Pioneering work of Lappert and
co-workers has been shown that they can be stabilised kinetically
by using bulky substituents R (i.e., R = CH(SiMe
3
)
2
),
1
while
other research groups have been demonstrated that additional
intramolecular coordination X→M(II) (X = F, N, O) with
donor groups in the side chain of the organic ligand R enables a
larger variety of possibilities for thermodynamic stabilisation of
monomeric M(II) complexes.
2
Since monomeric M(II) complexes
with small ligands are still scarce, we currently investigate the
ability of different diketoamines RN[CH
2
C(tBu)O]
2
(R = H,
alkyl, aryl) as suitable di- and trianionic chelate ligands.
3
In fact,
the NH-derivative HN[CH
2
C(tBu)O]
2
1 is indeed suitable for
the synthesis of the unusual monomeric stannylene A which
results from the remarkable oxidation reaction of the initial
dimeric complex B with SnCl
2
(Scheme 1).
bidentate, anionic N,O-ligand in 3 is the first amino ketal which
represents a novel chiral anisotropy reagent for enantioselective
synthesis.
5
Although no intermediate could be detected, we
assume that 3 is formed via 3 as initial product. Due to the
higher nucleophilicity of the enolate oxygen atom, 3 undergoes
nucleophilic attack to the carbonyl function on the other side
chain of the molecule affording a new chiral anionic bidentate
ligand. Remarkably, the corresponding lithium ketoamine
enolate of 1 is, in contrast to 3, indefinitely stable in aprotic
solvents due to NH/O
−
vs. N
−
/OH tautomerism.
3
† Electronic supplementary information (ESI) available: synthesis and
characterisation of 2, 3, 4 and 5. See http://www.rsc.org/suppdata/dt/
b4/b411889h/
Scheme 1 Formation of the germylenes and stannylenes A–C by use
of the chelate ligand 1.
However, synthesis of the monomeric germylene-homolog C
from D, similar to the procedure for A, failed because of its high
sensitiveness even toward soft oxidation reagents.
3
These results
prompted us to investigate whether the N-benzyl substituted
diketoamine PhCH
2
N[CH
2
C(tBu)O]
2
2
4
is suitable for the
synthesis of a monomeric germylene. Thus we attempted at
first to generate the corresponding lithium amino enolate as
starting material. The reaction of 2 with one molar equiv.
LDA (lithium diisopropylamide) or BuLi does not lead to the
ketoamino-enolate 3 but furnishes a racemic mixture of the new
enamine-ketal monoanion in 3 (Scheme 2). The constitution
and connectivity of the rearranged skeleton of the anion in
3 is proven by its
1
H and
13
C NMR spectroscopic data ‡ and
subsequent transmetalation, affording 4 and 5 (see below). The
Scheme 2 Lithiation of 2 and formation of 3 from 3.
Up to now, there has been only one report on a photochemical
conversion of a diketone into a cyclic ketal.
6
The monoanionic
ketal in 3 can indeed serve as a bidentate N,O-ligand toward a
Ge(II) atom, affording the novel monomeric germylenes 4 and
5 (Scheme 3). Thus, reaction of 3 with GeCl
2
–dioxane in the
molar ratio of 1 : 1 in THF furnishes the chloro-substituted
germylene 4, which has been isolated in 85% yield in the form
of colorless crystals.
Scheme 3 Synthesis of the monomeric germylenes 4 and 5.
The composition and connectivity of 4 has been established
by NMR spectroscopy, mass spectrometry, elemental analysis
and additionally by an X-ray analysis.†§ Crystals of 4 suit-
able for an X-ray diffraction analysis have been obtained from
CH
2
Cl
2
solutions. The chiral molecules crystallize racemic
in the triclinic space-group P 1 and form infinite chains with
attractive ClH interactions (Fig. 1). Each molecule consists
of a three-coordinate, pyramidally surrounded Ge atom having
eight valence electrons due to the intramolecular N→Ge donor
coordination. The Ge–O and Ge–Cl distances of 183.2(3) and
234.7(2) pm are similar to values observed for related Ge(II)–O
2
and Ge(II)–Cl bonds,
7
respectively, but almost identical with those
in the recently reported germylene ClGe(OCH
2
CH
2
NMe
2
).
8
The
Ge–N distance of 211.3(4) pm reflects a strong dative N→Ge
bond similar to other previously studied amino-functionalized,
monomeric germylenes.
8,9
Noteworthy are also the different
C1–O1 (139.8(5) pm) and C1–O2 distances (145.6(6) pm) which
clearly represent their different partial charges at oxygen and the
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