Reactivity of digermylenes toward potassium graphite: Synthesis and characterization of germylidenide anions

Article abstract of DOI:10.1021/ic202138t

The synthesis and characterization of the digermylenes [LGe-GeL] [L = L¹ (3A), L² (3B)] supported by the 2,6-diiminophenyl (L¹) and 2-imino-5,6-methylenedioxylphenyl (L²) ligands are described. Their reactivitie

Full text of DOI:10.1021/ic202138t

Article
pubs.acs.org/IC
Reactivity of Digermylenes toward Potassium Graphite: Synthesis
and Characterization of Germylidenide Anions
Siew-Peng Chia,† Hui-Xian Yeong,† and Cheuk-Wai So*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
21 Nanyang Link, 637371 Singapore, Singapore
S
* Supporting Information
ABSTRACT: The synthesis and characterization of the digermylenes [LGe−GeL] [L = L1 (3A), L2 (3B)] supported by the
2,6-diiminophenyl (L1) and 2-imino-5,6-methylenedioxylphenyl (L2) ligands are described. Their reactivities toward potassium
graphite are also reported. The reaction of [LGeCl] [L = L1 (2A), L2 (2B)] with KC8 in tetrahydrofuran (THF) at room
temperature afforded the digermylenes [LGe−GeL] [L = L1 (3A), L2 (3B)], which are the first examples of diaryldigermylenes
stabilized by o-imino donor(s). The treatment of 3A with 2 equiv of KC8 in Et2O, followed by the addition of excess
tetramethylethylenediamine (TMEDA), results in cleavage of the GeI−GeI bond to afford the germylidenide anion
[L1GeK·TMEDA] (4A). Similarly, the reaction of 3B with excess KC8 in THF afforded the germylidenide anion [L2GeK] (4B).
The molecular structures of compounds 4A and 4B as determined by single-crystal X-ray diffraction analysis show that the K
atoms are η1-coordinated with the low-valent Ge atoms. Moreover, the negative charges at the Ge atoms in compounds 4A and
4B are stabilized by electron delocalization in the germanium heterocycles.
(Chart 1).5 Although their energy difference is very small
(ca. 5 kcal/mol), [Ar′SnSnAr′] has a multiple-bonded structure M
INTRODUCTION
■
Stable heavier group 14 alkyne analogues of composition REER
(R = bulky terphenyl or silyl ligand; E = Si, Ge, Sn, Pb) have
attracted much attention in the past decades.1 These complexes
can be synthesized by incorporating sterically hindered sub-
stituents at the heavier group 14 elements.2 The X-ray
structures of heavier group 14 alkyne analogues show that
they have a trans-bent and planar geometry in which the R−E−
E angle decreases from silicon to lead. It was also demonstrated
that the heavier alkyne analogues can undergo one- and two-
electron reduction to give the radical anions [REER]•− and the
doubly reduced species [REER]2−.2g,3 A comparison of their
structural data with those of the heavier alkyne analogues can
provide insight into the E−E bonding.4,5 For example, the one-
electron reduction of the distannyne [Ar′SnSnAr′] [Ar′ = C6H3-
2,6-(C6H3-2,6-Pri2)2] with potassium afforded [K(THF)6]-
[Ar′SnSnAr′] (THF = tetrahydrofuran), in which the Sn−Sn
bond is lengthened significantly from 2.6675(4) to 2.8081(9) Å
and the Ar′−Sn−Sn angle is narrowed appreciably from
125.24(7) to 97.91(16)°.2g When two electrons are added to
[Ar′SnSnAr′], the doubly reduced species [Ar′SnSnAr′]2− has a
shorter Sn−Sn bond [2.7754(3) Å] than that in [K(THF)6]-
[Ar′SnSnAr′]. Theoretical studies show that [Ar′SnSnAr′] has
Chart 1. Multiple-Bonded M and Single-Bonded S Structures
in solution as in the solid-state structure. Orbital analysis shows that
the lowest occupied molecular orbitals of M and S are different, in
which M and S have small antibonding (slipped πin*) and large
bonding (πout) character, respectively. In view of experimental data
and theoretical studies, the reduction of [Ar′SnSnAr′] can be
explained by the fact that the one-electron reduction leads to the
formation of the S− anion instead of the M− anion. The addition of
one more electron to the S− anion shortens the Sn−Sn bond
because the bonding πout orbital is doubly occupied.
either a multiple-bonded structure [RE
bonded structure [RE−ER] (S) on the potential energy surface
ER] (M) or single-
Received: October 3, 2011
Published: December 21, 2011
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© 2011 American Chemical Society
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Recently, a series of novel base-stabilized group 14 element(I)
at δ 5.18 and 5.27 ppm in the spectrum. The signal for the
MeCN protons (δ 1.68 ppm) shows an upfield shift
compared with that of [L2Li] (δ 1.90 ppm).9
Compounds 2A and 2B have been characterized by X-ray
crystallography. The molecular structures of 2A and 2B are
shown in Figures 1 and 2, respectively. In the molecular
6
̈
̈
dimers [RE−ER] (E = Si, Ge, Sn) were synthesized. They are
̈
̈
comprised of a E−E single bond and a lone pair of electrons at
each E atom. Their structures resemble the single-bonded
structure S of [Ar′SnSnAr′]. Thus, they are considered as base-
stabilized heavier alkyne analogues. The fact that heavier alkyne
analogues supported by bulky terphenyl and silyl ligands can be
singly or doubly reduced prompted our interest in investigating
the reduction of intramolecularly base-stabilized group 14
element(I) dimers. To the best of our knowledge, the reduction
of intramolecularly base-stabilized group 14 element(I) dimers is
not reported.
Herein, we report the syntheses of digermylenes stabilized
by 2,6-diiminophenyl (L1; Scheme 1) and 2-imino-5,6-methyl-
Scheme 1. 2,6-Diiminophenyl (L1) and 2-Imino-5,6-
methylenedioxylphenyl (L2) Ligands
Figure 1. Molecular structure of compound 2A (50% thermal
ellipsoids). H atoms, the disordered NAr substituent at the C20 atom,
and the disordered Ge1A and Cl1A atoms are omitted for clarity.
enedioxylphenyl (L2) ligands. They are comprised of a Ge−Ge
single bond and a lone pair of electrons at each Ge atom.
Their structures resemble the single-bonded structure S of
[Ar′SnSnAr′]. We also describe the reduction of the digermy-
lenes with potassium graphite to afford the germylidenide anions.
RESULTS AND DISCUSSION
■
Synthesis of [LGeCl]. The reaction of [L1Br]7 with BunLi
in THF at −78 °C,8 followed by treatment with GeCl2·dioxane,
afforded [L1GeCl] (2A; Scheme 2). Similarly, [L2GeCl] (2B)
Scheme 2. Syntheses of 2−4
Figure 2. Molecular structure of compound 2B (50% thermal
ellipsoids). In the asymmetric unit of 2B, there are two independent
molecules with slightly different bond lengths and angles. Only one of
them is shown for clarity. H atoms are omitted for clarity.
structure of 2A, the NAr substituent at the C20 atom and the
Ge−Cl moiety are disordered. The disordered substituents are
omitted for clarity in Figure 1. In the asymmetric unit of 2B,
there are two independent molecules with slightly different
bond lengths and angles (Table 1). Only one of them is
discussed here for clarity. The 2,6-diiminophenyl and 2-imino-
5,6-methylenedioxylphenyl ligands are bonded in a C,N-chelate
fashion to the Ge1 atoms in 2A and 2B, respectively. The Ge
atoms adopt a distorted trigonal-pyramidal geometry. The sum
of the bond angles at the Ge1 atoms (2A, 267.40°; 2B,
268.76°) is comparable with that of the three-coordinated
chlorogermylene [(Mamx)GeCl] (270.25°) supported by the
2,4-di-tert-butyl-6-[(N,N-dimethylamino)methyl]phenyl ligand
(Mamx).10 The geometries at the Ge1 atoms in 2A and 2B
indicate that they are almost nonhybridized and possess lone-
pair electrons with high s character. In compound 2A, the
Ge1−C19 [2.028(3) Å] and Ge1−N1 [2.247(3) Å] bonds
was synthesized by the reaction of [L2Li]9 with GeCl2·dioxane
in Et2O. Compounds 2A and 2B were isolated as highly air-
and moisture-sensitive orange and yellow crystalline solids,
respectively. They are soluble in hydrocarbon solvents and have
been characterized by NMR spectroscopy. The 1H NMR
spectrum of 2A displays one set of signals due to the 2,6-
diiminophenyl ligand. In the spectrum, there are two singlets at
δ 8.06 and 8.08 ppm, which correspond to two nonequivalent
HCN protons. The results indicate that compound 2A
retains its solid-state structure in solution. The 1H NMR
spectrum of 2B displays one set of signals due to the 2-imino-
5,6-methylenedioxylphenyl ligand. It is worth noting that the
−OCH2O− protons are nonequivalent and show two singlets
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Table 1. Selected Bond Lengths (Å) and Angles (deg) of Compounds 2−4
2A
3B
Ge1−C19
Ge1−Cl1
Ge1−N1
N1−C13
C13−C14
2.028(3)
2.3288(8)
2.247(3)
1.286(4)
1.453(5)
C14−C19
C18−C19
C18−C20
N2−C21
1.393(5)
1.387(4)
1.461(5)
1.424(9)
Ge1−C1
C1−C7
C7−C8
C8−N1
1.968(4)
1.436(5)
1.437(5)
1.310(5)
C22−C28
C28−C29
C29−N2
1.434(5)
1.447(5)
1.321(4)
C1−Ge1−N1
C1−Ge1−Ge2
N1−Ge1−Ge2
Ge1−C1−C7
C1−C7−C8
80.95(14)
101.50(10)
96.99(8)
113.2(3)
114.1(3)
115.5(3)
115.5(2)
C22−Ge2−N2
C22−Ge2−Ge1
N2−Ge2−Ge1
Ge2−C22−C28
C22−C28−C29
C28−C29−N2
C29−N2−Ge2
80.87(14)
98.22(11)
99.20(9)
114.3(3)
113.3(3)
114.9(3)
115.5(2)
C19−Ge1−Cl1
C19−Ge1−N1
N1−Ge1−Cl1
Ge1−N1−C13
100.61(8)
76.97(12)
89.82(7)
111.5(2)
N1−C13−C14
C13−C14−C19
C14−C19−Ge1
118.2(3)
115.6(3)
116.0(2)
2B
C7−C8−N1
C8−N1−Ge1
Ge1−Cl1
Ge1−C1
Ge1−N1
2.3024(8)
1.992(3)
2.054(2)
C1−C7
C7−C8
N1−C8
1.423(4)
4A
1.460(4)
1.301(3)
K1−N2
Ge1−C1
Ge1−K1
Ge1−N1
N1−C7
3.179(3)
1.909(4)
3.3560(9)
1.925(3)
1.361(4)
C6−C7
C1−C6
C1−C2
C2−C20
N2−C20
1.396(5)
1.450(5)
1.423(5)
1.460(5)
1.281(4)
C1−Ge1−N1
C1−Ge1−Cl1
N1−Ge1−Cl1
Ge1−C1−C7
80.40(11)
93.98(8)
94.38(7)
113.6(2)
C1−C7−C8
C7−C8−N1
C8−N1−Ge1
114.3(2)
115.7(3)
115.50(18)
3A
C1−Ge1−K1
C1−Ge1−N1
K1−Ge1−N1
Ge1−N1−C7
N1−C7−C6
C7−C6−C1
126.84(11)
83.43(13)
148.44(9)
115.5(2)
114.5(3)
114.0(3)
4B
C6−C1−Ge1
Ge1−C1−C2
C1−C2−C20
C2−C20−N2
C20−N2−K1
N2−K1−Ge1
112.6(2)
130.2(3)
121.4(3)
124.2(3)
160.8(2)
55.45(6)
Ge1−Ge2
Ge1−C33
C33−C38
C38−C39
C39−N3
Ge1−N3
2.5059(5)
1.967(3)
1.418(5)
1.440(5)
1.300(4)
1.986(3)
Ge2−C1
C1−C6
C6−C7
C7−N1
Ge2−N1
1.990(3)
1.413(5)
1.438(4)
1.305(4)
2.036(3)
Ge1−K1
Ge1−C1
C1−C7
C7−C8
C8−N1
3.5252(17) Ge1−N1
1.916(5)
2.991(5)
2.865(5)
2.723(4)
3.5357(19)
N3−Ge1−C33
N3−Ge1−Ge2
C33−Ge1−Ge2
Ge1−C33−C38
C33−C38−C39
C38−C39−N3
C39−N3−Ge1
83.21(13)
102.31(8)
116.11(10)
110.4(3)
114.9(3)
116.7(3)
113.4(2)
N1−Ge2−C1
C1−Ge2−Ge1
N1−Ge2−Ge1
Ge2−C1−C6
C1−C6−C7
81.92(12)
89.10(9)
112.82(8)
112.0(2)
114.6(3)
117.8(3)
112.6(2)
1.919(6)
1.464(9)
1.375(9)
1.377(8)
O1−K1
O4−K1
O6−K1
Ge2−K1
C6−C7−N1
C7−N1−Ge2
N1−Ge1−K1 126.69(15) C1−C7−C8
C1−Ge1−K1 68.54(18) C7−C8−N1
114.1(6)
113.7(6)
3B
N1−Ge1−C1 82.4(2)
Ge1−C1−C7 112.9(5)
C8−N1−Ge1 116.9(4)
O1−K1−Ge1 64.64(9)
Ge1−Ge2
Ge1−N1
2.5685(5)
2.022(3)
Ge2−N2
Ge2−C22
2.017(3)
1.954(4)
are comparable with those in [{2,6-(CH2NEt2)2C6H3}GeCl]
[Ge−C, 1.941(11) Å; Ge−N, 2.337(11) Å], which comprises
an aryl ligand with two o-amino donors.11 The Ge1···N2 dis-
tance [2.62(1) Å] in 2A is significantly longer than the N−Ge
dative bonds in N-donor-stabilized chlorogermylenes such as
[{C5H4N-2-C(SiMe3)2}GeCl] [2.082(4) and 2.075(4) Å]12
and [(Mamx)GeCl] [2.0936(13) Å],10 but it is shorter than the
sum of the van der Waals radii (ca. 3.55 Å). The results indicate
that the interaction between the Ge1 and N2 atoms is weak.
In compound 2B, the Ge1−C1 [1.992(3) Å] and Ge1−N1
[2.054(2) Å] bonds are comparable with those in [(Mamx)-
GeCl] [Ge−C, 2.0156(14) Å; Ge−N, 2.0936(13) Å].10
Synthesis of [LGe−GeL]. The reaction of [LGeCl] with
KC8 in THF at room temperature afforded the digermylenes
[LGe−GeL] [L = L1 (3A), L2 (3B)], which are the first
examples of diaryldigermylenes stabilized by o-imino donor(s).
Other examples of digermylenes stabilized by amidinate ligands
were reported by research groups of Roesky and Jones.6b,d,i
Recently, the GeI radical [HC{C(But)NAr}2Ge•] was synthe-
sized by the reduction of the corresponding chlorogermylene
with Na(C10H8) in THF.13 The results imply that digermylenes
are formed via the dimerization of GeI radical intermediates.
Compounds 3A and 3B were isolated as highly air- and
moisture-sensitive purple and red crystalline solids, respectively.
They are soluble in hydrocarbon solvents and have been
1
characterized by NMR spectroscopy. The H NMR spectrum
of 3A at room temperature displays one doublet at δ 0.87 ppm
and one broad singlet at δ 2.74 ppm for the Pri substituents.
The results are not consistent with the solid-state structure,
suggesting that the imino substituents are fluxional in solution
at room temperature. In this regard, the 1H NMR spectra of 3A
was acquired at −100 °C, whereupon a multiplet at δ 0.28−
1.40 ppm for the CHMe2 protons and seven broad singlets at δ
2.04−3.74 ppm for the CHMe2 protons were resolved. The IR
spectrum of 3A also shows two NC stretching modes of the
nonequivalent imino substituents at ν 1587 and 1624 cm−1.
1
The H NMR spectrum of 3B shows one set of signals due to
the 2-imino-5,6-methylenedioxylphenyl ligand. The signal for
the MeCN protons (δ 1.97 ppm) shows a downfield shift
compared with that of 2B. The UV−vis spectrum of 3A in
toluene shows three absorption bands at 438, 586, and 702 nm
in the visible-light region, which shows a bathochromic shift
compared with that of the tin congener (425, 457, and 561 nm)
reported by Roesky and co-workers.6j The shift is comparable with
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that observed in the electronic spectra of the amidinate-stabilized
on each Ge atom. The Ge−Ge bonds [3A, 2.5059(5) Å; 3B,
2.5685(5) Å] are comparable with that in [PhC(NBut)2Ge]2
[2.569(5) Å] and [ButC(NAr)2Ge]2 [2.6380(8) Å].6b,d The
results imply that the Ge−Ge bonds do not have any multiple-
bond character. The Ge−C and Ge−N bonds in compounds
3A and 3B are slightly shorter than those in 2A and 2B,
respectively.
Reaction of [LGe−GeL] with KC8. The reaction of the
digermylene 3A with 2 equiv of KC8 in Et2O, followed by the
addition of excess tetramethylethylenediamine (TMEDA), results
in cleavage of the GeI−GeI bond to afford the germylidenide
anion [L1GeK·TMEDA] (4A).14 Similarly, the reaction of 3B
with excess KC8 (2.7 equiv) in THF for 2 days afforded the
germylidenide anion [L2GeK] (4B). The results are in contrast
with the reduction of the heavier alkyne analogue [Ar′SnSnAr′]
with potassium, by which the doubly reduced species
K2[Ar′SnSnAr′] was formed.2g Moreover, the reduction of the
amidinate-stabilized digermylene [ButC(NAr)2Ge]2 leads to
decomposition to give elemental germanium and [ButC-
(NAr)2K].6b
1
1
t
̈
group 14 element(I) dimers [{R C(NAr)2}E]2 [R = C6H4-4-Bu ;
Ar = C6H3-2,6-Pri2; E = Si (629 nm), Ge (502 nm), Sn
(388 nm)].6i Moreover, this is opposite to the electronic spectra of
multiple-bonded heavier group 14 alkyne analogues, in which
there is a hypsochromic shift upon descending the group.2j These
imply that the Ge−Ge bond in 3A has little π character. Similarly,
the UV−vis spectrum of 3B in THF shows three absorption bands
at 475, 506, and 659 nm in the visible-light region.
Compounds 3A and 3B have been characterized by X-ray
crystallography. Their molecular structures are shown in
Figures 3 and 4, respectively. They are comprised of gauche-
Power et al. showed that the radical anions [REER]•− and
the doubly reduced species [REER]2− (E = Ge, Sn; R =
terphenyl substituent) were synthesized by the reaction of the
corresponding heavier chlorocarbene analogues [RECl] with
excess alkali metal.15 In this regard, the reaction of compound 2A
with 2 equiv of KC8 in Et2O was performed to afford compound
4A. The reaction appears to proceed through the formation of
compound 3A, which then reacts with two molecules of KC8 to
form 4A. Similarly, the reaction of compound 2B with excess
KC8 (3.5 equiv) afforded compound 4B. The X-ray crystal
structures of compounds 4A and 4B show that the K atoms are
η1-coordinated with the low-valent Ge atoms (see below). Their
crystallographic data also suggest that the negative charges at the
Ge atoms are stabilized by electron delocalization in the GeCCCN
five-membered rings. Recently, research groups of Driess and
Jones reported the synthesis of the N-heterocyclic germylidenide
complexes I and II by the reaction of [HC{C(R)N(Ar)}GeCl]
(R = Me, But) with 2 equiv of alkali metal (Scheme 3).16 It is
Figure 3. Molecular structure of compound 3A (50% thermal
ellipsoids). H atoms and Pri substituents are omitted for clarity.
Scheme 3. Germylidenides I and II
worth noting that the mechanism for the formation of I and II is
suggested to proceed through several reductive processes and ring
contraction, which is different from that for 4A and 4B.
Figure 4. Molecular structure of compound 3B (50% thermal
Compound 4A was isolated as a highly air- and moisture-
sensitive green crystalline solid, which is stable in solution
and the solid state at room temperature in an inert atmosphere.
ellipsoids). H atoms and Pri substituents are omitted for clarity.
bent structures similar to the amidinate-stabilized digermylene
[PhC(NBut)2Ge]2.6d In contrast, their structures are different
from the more sterically hindered digermylene [ButC-
(NAr)2Ge]2,6b which shows a trans-bent structure. The Ge
atoms in 3A and 3B adopt a distorted trigonal-pyramidal
geometry, which indicates that there is a lone pair of electrons
1
The H NMR spectrum of 4A displays one doublet at δ
1.18 ppm and one septet at δ 3.23 ppm for the Pri substituents.
The results indicate that compound 4A has C2v symmetry in
solution and the imino substituents may be equivalently
1
coordinated to the Ge atom. The H NMR signal for the
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HCN proton (δ 8.15 ppm) shows a downfield shift
compared with that of 2A, suggesting that the negative charge
at the Ge atom is stabilized by electron delocalization in the
GeCCCN five-membered ring. The electronic delocalization
is also supported by the bond lengths of the GeCCCN five-
membered ring (see below).
Compound 4B was isolated as a highly air- and moisture-
sensitive red crystalline solid. Compound 4B shows different
color compared with green 4A and pale-yellow I, which implies
the noninnocence of the ligands.16a Moreover, the molecular
structures of compounds 4A and 4B are different from that of I,
which comprises a η5-coordinated K atom.
Compound 4B is stable in ethereal solvents and the solid
state. However, it decomposes in toluene and benzene, which
was confirmed by NMR spectroscopy. The identification of the
decomposed compound is still in progress. Thus, spectroscopic
analysis of 4B can only be performed in THF-d8. The 1H NMR
spectrum shows one set of signals due to the 2-imino-5,6-
methylenedioxylphenyl ligand. The signal for the MeCN
protons (δ 2.12 ppm) shows a downfield shift compared with
that of 2B, indicating that there is electron delocalization in the
GeCCCN five-membered ring.
Compounds 4A and 4B have been characterized by X-ray
crystallography. Compound 4A has a polymeric structure by
the interaction of the K1 atoms with the C4 atoms of the adjacent
germylidenide molecules (Figure 5b). In the monomeric unit of
compound 4A (Figure 5a), the K1 atom is η1-coordinated with
the Ge1 atom of the GeCCCN ring, which is different from I
and II comprising a η5-coordinated alkali-metal center.16 The K1
atom is also coordinated with the N2−4 atoms of the imino
substituent and TMEDA (average K−N bond length: 2.961 Å).
The coordination sphere on the K1 atom is further supplemented
by an interaction with the ipso-C atom of a Ar substituent
[K1···C21, 3.314(4) Å]. The GeCCCN five-membered ring in
4A is planar, whereas those in 2A and 3A are puckered. The
Ge1−K1 bond [3.3560(9) Å] is comparable with the tris-
(trimethylgermyl)germanide complex [(Me3Ge)3GeK(18-crown-
6)] [3.4213(11) Å]17 and the germylidenide complex I [3.449(1)
and 3.573(1) Å].16a In addition, the Ge1−N1 [1.925(3) Å],
N1−C7 [1.361(4) Å], C6−C7 [1.396(5) Å], C1−C6 [1.450(5) Å],
and Ge1−C1 [1.909(4) Å] bond lengths suggest that there
is an appreciable electron delocalization in the GeCCCN five-
membered ring compared with those in compounds 2A and
I [Ge−N, 1.944(2) Å; N−C, 1.382(3) Å; C−C, 1.371(3) and
1.411(3) Å; Ge−C, 1.887(2) Å].16a
In the monomeric form of compound 4B (Figure 6a), the K1
atom is η1-coordinated with the Ge1 atom of the GeCCCN
ring. The coordination sphere on the K1 atom is further
supplemented by an η2-interaction of the phenyl ring, the O1
atom of the −OCH2O− substituent, and the O3 atom of the
disordered Et2O molecule. This leads to the K1 atom displacing
from the GeCCCN plane by 2.358 Å. The Ge1−K1 bond
[3.5252(17) Å] and the bond lengths of the GeCCCN ring
[Ge1−N1, 1.916(5) Å; C8−N1, 1.377(8) Å; C7−C8, 1.375(9) Å;
C1−C7, 1.464(9) Å; Ge1−C1, 1.919(6) Å] are comparable
with those in 4A. In addition, the electrostatic interaction
of the K1 atom with the O4 and O6 atoms and the Ge2 atom
of the adjacent germylidenide molecules leads to a polymeric
form (Figure 6b). In the asymmetric unit (Figure S1 in the
Supporting Information), there is solvent disordering at the K
atoms. The K2 and K4 atoms are coordinated with either a THF
or a Et2O molecule.18
Figure 5. Molecular structure of compound 4A (20% thermal
ellipsoids): (a) perspective view of the molecule; (b) its polymeric
form. H atoms (a and b) and Pri substituents (b) are omitted for clarity.
In conclusion, the digermylenes [LGe−GeL] [L = L1 (3A),
L2 (3B)], which are supported by 2,6-diiminophenyl (L1) and
2-imino-5,6-methylenedioxylphenyl ligands (L2), were synthe-
sized by the reaction of the chlorogermylenes [LGeCl] [L = L1
(2A), L2 (2B)] with 1 equiv of KC8. The germylidenide anion
4A was synthesized by the reaction of 3A with 2 equiv of KC8
and excess TMEDA. Similarly, the germylidenide anion [L2GeK]
(4B) was synthesized by the reaction of 3B with excess KC8. The
crystallographic and spectroscopic data of compounds 4A and
4B show that the K atoms are η1-coordinated with the low-valent
Ge atoms. Moreover, the negative charges at the Ge atoms in
compounds 4A and 4B are stabilized by electron delocalization
in the germanium heterocycles.
EXPERIMENTAL SECTION
■
General Procedure. All manipulations were carried out under an
inert atmosphere of argon gas using standard Schlenk techniques.
Solvents were dried over and distilled over Na/K alloy prior to use.
L1Br and L2Li were prepared as described in the literature.7,9 The
1H and 13C NMR spectra were recorded on a JEOL ECA 400
1006
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CH(CH3)2), 7.00−7.05 (m, 2H, Ph), 7.14−7.18 (m, 7H, Ph), 8.06 (s,
1H, CHN), 8.08 (s, 1H, CHN). 13C{1H} NMR (100.5 MHz,
C6D6, 25 °C): δ 24.44 (CH(CH3)2), 24.56 (CH(CH3)2), 24.99
(CH(CH3)2), 25.16 (CH(CH3)2), 28.63 (CH(CH3)2), 28.69 (CH-
(CH3)2), 123.96, 126.61, 128.93, 132.29 (PhGe), 140.41, 140.56,
145.06, 145.24 165.48, 165.59 (NAr), 171.69, 171.93 (CNAr). UV−
vis (toluene): λmax (ε) 218 (15 538), 245 (18 501), 336 (1786),
407 nm (932 dm3 mol−1 cm−1). IR (Nujol, cm−1): 2955s, 2924s,
2853s, 1628w, 1609w, 1551w, 1458m, 1377m, 1364w, 1323w, 1260w,
1175w, 1167w, 1096w, 1049w, 799m, 750w, 721w.
[L2GeCl] (2B). A solution of L2Li (0.68 g, 2.06 mmol) in Et2O
(57 mL) was added dropwise to a stirred suspension of GeCl2·dioxane
(0.48 g, 2.06 mmol) in Et2O (4.7 mL) at 0 °C. The reaction mixture
was raised to ambient temperature and stirred additionally for 15 h.
The precipitate was filtered, and the filtrate was concentrated to afford
yellow crystals of 2B. Yield: 0.50 g (56.4%). Mp: 207 °C. Elem anal.
Calcd for C21H24ClGeNO2: C, 58.59; H, 5.62; N, 3.25. Found: C,
58.21; H, 5.43; N, 3.19. 1H NMR (395.9 MHz, C6D6, 23.3 °C): δ 0.84
3
3
(d, JHH = 6.8 Hz, 3H, CH(CH3)2), 0.91 (d, JHH = 7.2 Hz, 3H,
CH(CH3)2), 1.15 (d, 3JHH = 6.8 Hz, 3H, CH(CH3)2), 1.43 (d, 3JHH
=
6.3 Hz, 3H, CH(CH3)2), 1.68 (s, 3H, CH3), 2.64 (sept, 3JHH = 6.8 Hz,
3
1H, CH(CH3)2), 3.21 (sept, JHH = 6.8 Hz, 1H, CH(CH3)2), 5.18 (s,
1H, OCH2O), 5.27 (s, 1H, OCH2O), 6.53 (d, 3JHH = 8.2 Hz, 1H, Ph),
3
6.83 (d, JHH = 8.2 Hz, 1H, Ph), 6.99−7.01 (m, 1H, Ph), 7.10−7.13
(m, 2H, Ph). 13C{1H} NMR (100.6 MHz, C6D6, 24.0 °C): δ 17.18,
24.08, 24.22, 25.23 (CH(CH3)2), 28.24, 29.15 (CH(CH3)2), 101.33,
107.48, 124.03, 124.81, 125.23, 135.36, 138.08, 141.12, 143.15, 145.22
(Ar/Ph), 150.73 (CH3), 151.11 (OCH2O), 178.88 (CNAr). UV−vis
(THF): λmax (ε) 217 (7326), 254 (2297), 260 (1809), 348 nm (2409
dm3 mol−1 cm−1). IR (Nujol, cm−1): 2953s, 2922s, 2853s, 1549m,
1445m, 1435m, 1377w, 1362w, 1316m, 1256m, 1152w, 1128w, 1096w,
1043m, 935w, 887w, 802w, 777w.
[L1Ge−GeL1] (3A). THF (20 mL) was added to a mixture of 2A
(1.12 g, 2.00 mmol) and KC8 (0.28 g, 2.07 mmol) at room temp-
erature. The resulting blue mixture was stirred for 1 day. The insoluble
precipitate was then filtered off, and volatiles were removed under
vacuum. The residue was extracted with hexane and then filtered. The
blue filtrate was concentrated to afford 3A as purple crystals. Yield:
0.56 g (53.4%). Mp: 250 °C. Elem anal. Calcd for C64H78Ge2N4: C,
1
73.25; H, 7.44; N, 5.34. Found: C, 73.11; H, 7.25; N, 5.21. H NMR
(399.5 MHz, THF-d8, 25 °C): δ 0.87 (br d, 48H, CH(CH3)2), 2.74
(br s, 8H, CH(CH3)2), 6.93 (br s, 2H, Ph), 7.00−7.12 (m, 13H, Ph),
7.75−7.77 (br m, 3H, Ph), 8.03 (br s, 4H, CHN). 1H NMR (399.5
MHz, THF-d8, −100 °C): δ 0.28−1.40 (m, 48H, CH(CH3)2), 2.04
(br s, 1H, CH(CH3)2), 2.14 (br s, 2H, CH(CH3)2), 2.56 (br s, 1H,
CH(CH3)2), 2.78 (br s, 1H, CH(CH3)2), 3.05 (br s, 1H, CH(CH3)2),
3.20 (br s, 1H, CH(CH3)2), 3.74 (br s, 1H, CH(CH3)2), 6.52−7.37
(m, 15H, Ph), 7.51 (s, 1H, CHN), 7.53 (s, 1H, CHN), 8.03−8.14
(m, 2H, Ph), 8.39 (s, 1H, Ph), 8.61 (s, 1H, CHN), 8.70 (s, 1H,
CHN). 13C{1H} NMR (100.5 MHz, C6D6, 25 °C): δ 24.78 (br s,
CH(CH3)2), 28.47 (CH(CH3)2), 123.43, 124.55, 125.88, 138.48
(PhGe), 140.60 (br, NAr), 146.96 (br, NAr), 161.25 (br, NAr), 179.19
(br, CNAr). UV−vis (toluene): λmax (ε) 284 (12 453), 438 (1373),
586 (1693), 702 nm (6516 dm3 mol−1 cm−1). IR (Nujol, cm−1): 2955s,
2924s, 2853s, 1624m, 1587w, 1549w, 1489w, 1460m, 1381m, 1360w,
1337w, 1304m, 1260w, 1211w, 1175w, 1159w, 1090w, 1057w, 1020w,
995w, 924w, 887w, 880w, 799w, 758w, 716w.
Figure 6. Molecular structure of compound 4B (20% thermal
ellipsoids): (a) perspective view of the molecule; (b) its polymeric
form. H atoms and disordered solvent molecule(s) are omitted for
clarity in parts a and b. Ar substituents are also omitted for clarity in
part b.
1
spectrometer. The chemical shifts δ are relative to SiMe4 for H and
13C NMR. Elemental analyses were performed by the Division of
Chemistry and Biological Chemistry, Nanyang Technological
University. Melting points were measured in sealed glass tubes and
were not corrected.
[L1GeCl] (2A). BunLi (2.0 M in cyclohexane, 3.0 mL, 6.00 mmol)
was added dropwise to a THF solution (50 mL) of L1Br (2.66 g,
5.00 mmol) at −78 °C, and the reaction mixture was stirred for 1 h.
It was warmed to −40 °C and stirred for another 2 h. A THF solution
(10 mL) of GeCl2·dioxane (1.27 g, 5.50 mmol) was then added to the
reaction mixture at −78 °C. The resulting red solution was warmed to
room temperature gradually and stirred for 12 h. Solvent was removed
under vacuum, and the residue was extracted with toluene. LiCl was
then filtered off, and the red filtrate was concentrated to afford 2A as
orange crystals. Yield: 2.24 g (80.0%). Mp: 245 °C. Elem anal. Calcd
for C32H39ClGeN2: C, 68.64; H, 7.03; N, 5.01. Found: C, 68.31; H,
6.91; N, 4.85. 1H NMR (395.9 MHz, C6D6, 25 °C): δ 1.14−1.26 (m of
overlapping d, JH−H = 6.37 Hz, 24H, CH(CH3)2), 3.37 (br s, 4H,
[L2Ge−GeL2] (3B). THF (14.2 mL) was added to a mixture of 2B
(0.91 g, 2.12 mmol) and KC8 (0.32 g, 2.37 mmol) at ambient temp-
erature. The resulting green reaction mixture was stirred overnight.
The solvent was then removed in vacuo, and the residue was extracted
with diethyl ether (50 mL). The insoluble precipitate was filtered
off, and the green filtrate was concentrated to yield red crystals
of 3B. Yield: 0.59 g (70.5%). Mp: 243 °C. Elem anal. Calcd for
C42H48Ge2N2O4: C, 63.83; H, 6.13; N, 3.55. Found: C, 63.67; H, 5.92;
1
N, 3.31. H NMR (395.9 MHz, C6D6, 23.3 °C): δ 0.89 (br s, 6H,
CH(CH3)2), 1.12 (br s, 6H, CH(CH3)2), 1.22 (br s, 6H, CH(CH3)2),
1.47 (br s, 6H, CH(CH3)2), 1.97 (s, 6H, CH3), 2.60 (br s, 2H,
CH(CH3)2), 3.41 (br s, 2H, CH(CH3)2), 5.17 (br s, 2H, OCH2O),
5.28 (br s, 2H, OCH2O), 6.67 (d, 3JHH = 8.2 Hz, 2H, Ph), 7.09 (br s,
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Table 2. Crystallographic Data for Compounds 2−4
2A
2B
3A
formula
M
C32H39ClGeN2
C21H24ClGeNO2
430.45
C64H78Ge2N4
1048.48
purple
559.69
color
cryst syst
space group
a/Å
orange
yellow
monoclinic
P2(1)/c
monoclinic
P2(1)/c
monoclinic
P2(1)/c
13.8962(6)
20.0041(9)
23.8699(10)
90
9.0808(2)
26.5379(5)
14.0952(2)
90
14.9469(3)
15.6737(3)
18.6069(4)
90
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
Z
118.933(1)
90
112.2490(10)
90
120.555(3)
90
2972.78(10)
4
4034.55(14)
8
5714.0(4)
4
d
calcd/g cm−3
1.251
1.417
1.219
μ/mm−1
1.142
1.665
1.094
F(000)
1176
1776
2216
cryst size/mm
index range
0.20 × 0.20 × 0.06
−12 ≤ h ≤ 12
−37 ≤ k ≤ 37
−20 ≤ l ≤ 20
73 916
0.12 × 0.10 × 0.04
−20 ≤ h ≤ 20
−21 ≤ k ≤ 21
−25 ≤ l ≤ 26
57 560
0.16 × 0.08 × 0.06
−17 ≤ h ≤ 17
−25 ≤ k ≤ 25
−29 ≤ l ≤ 29
79 103
no. of rflns collected
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
0.0572, 0.1605
0.0952, 0.1840
1.087
0.0432, 0.1042
0.0836, 0.1305
1.078
0.0440, 0.1032
0.0835, 0.1309
1.031
GOF, F2
no. of data/restraints/param
largest diff peak, hole/e Å−3
9068/380/474
1.651, −0.926
3B
11 670/0/479
1.552, −0.800
4A
11 671/38/668
0.499, −0.594
4B
formula
M
C42H48Ge2N2O4
790.00
C38H55GeKN4
679.55
C100H134.45Ge4K4N4O12
2031.33
color
cryst syst
space group
a/Å
red
green
red
monoclinic
P2(1)/c
monoclinic
P2(1)/c
monoclinic
P2(1)/n
12.1121(4)
15.1482(6)
22.2158(9)
90
12.0702(6)
19.1886(10)
16.1095(7)
90
15.8408(6)
26.3835(8)
25.1989(9)
90
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
Z
110.586(2)
90
95.285(3)
90
95.867(2)
90
3815.8(3)
4
3715.3(3)
4
10476.4(6)
4
d
calcd/g cm−3
1.375
1.215
1.288
μ/mm−1
1.619
0.967
1.352
F(000)
1640
1448
4250
cryst size/mm
index range
0.26 × 0.14 × 0.12
−14 ≤ h ≤ 15
−18 ≤ k ≤ 18
−27 ≤ l ≤ 27
48 879
0.40 × 0.20 × 0.12
−18 ≤ h ≤ 17
0 ≤ k ≤ 28
0 ≤ l ≤ 23
12 818
0.40 × 0.04 × 0.04
−19 ≤ h ≤ 19
−31 ≤ k ≤ 31
−29 ≤ l ≤ 30
129 898
no. of rflns collected
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
0.0417, 0.1024
0.0751, 0.1283
1.103
0.0749, 0.1558
0.1812, 0.1866
0.916
0.0585, 0.1374
0.1615, 0.1975
1.036
GOF, F2
no. of data/restraints/param
largest diff peak, hole/e Å−3
7786/0/461
0.507, −0.944
12 818/0/409
0.622, −0.798
19 228/630/1333
0.758, −0.698
3H, Ph), 7.18−7.21 (m, 5H, Ph). 13C{1H} NMR (100.5 MHz, C6D6,
23.0 °C): δ 17.55, 23.90, 25.14, 25.48 (CH(CH3)2), 28.31 (CH-
(CH3)2), 100.26, 105.47, 123.75, 124.06, 134.09, 140.97, 142.96,
144.61 (Ar/Ph), 148.95 (CH3), 160.00 (OCH2O), 166.90 (CNAr).
UV−vis (THF): λmax (ε) 217 (14 210), 253 (6816), 259 (6681), 261
(6370), 295 (5136), 347 (4280), 475 (477), 506 (756), 659 nm (4357
dm3 mol−1 cm−1). IR (Nujol, cm−1): 2953s, 2922s, 2853s, 1607w,
1493w, 1456s, 1377s, 1337w, 1319w, 1304w, 1258m, 1192w, 1177w,
1142w, 1113m, 1094m, 1043m, 1018m, 986w, 935w, 874w, 849w,
800m, 777w, 737w, 721w, 704w, 642w.
[L1GeK·TMEDA] (4A). Method A. Et2O (20 mL) was added to
a mixture of 3A (0.54 g, 0.52 mmol) and KC8 (0.14 g, 1.03 mmol) at
room temperature. The resulting green mixture was stirred for 1 day.
The insoluble precipitate was then filtered off, and TMEDA (0.90 mL,
1008
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Inorganic Chemistry
Article
6.04 mmol) was added at 0 °C. The resulting green solution was
stirred at room temperature for 3 h. The solution was filtered and
concentrated to afford 4A as green crystals. Yield: 0.53 g (75.7%).
Method B. Et2O (20 mL) was added to a mixture of 2A (1.12 g,
2.00 mmol) and KC8 (0.56 g, 4.15 mmol) at room temperature. The
resulting green mixture was stirred for 1 day. The insoluble precipitate
was then filtered off, and TMEDA (0.90 mL, 6.04 mmol) was added at
0 °C. The resulting green solution was stirred at room temperature for
3 h. The solution was filtered and concentrated to afford 4A as green
crystals. Yield: 0.76 g (55.9%).
ASSOCIATED CONTENT
* Supporting Information
CIF files giving X-ray data for 2−4 and the asymmetric unit of
compound 4B. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: CWSo@ntu.edu.sg.
■
Mp: 155 °C. Elem anal. Calcd for C38H55GeKN4: C, 67.14; H, 8.16;
N, 8.25. Found: C, 66.21; H, 7.13; N, 7.51. Attempts to obtain
acceptable elemental analysis data for compound 4A failed because of
its extreme air sensitivity. 1H NMR (399.5 MHz, C6D6, 25 °C): δ 1.18
(d, 3JH−H = 6.87 Hz, 24H, CH(CH3)2), 1.87 (s, 12H, NCH3), 2.00 (s,
Author Contributions
†Equal contribution to the manuscript.
ACKNOWLEDGMENTS
■
3
4H, NCH2), 3.23 (sept, JH−H = 6.87 Hz, 4H, CH(CH3)2), 6.85 (t,
This work was supported by the Academic Research Fund
Tier 1 (RG 47/08). Authors thank Dr. Y. Li for X-ray
crystallography.
3
3JH−H = 7.31 Hz, 1H, Ph), 7.09−7.18 (m, 6H, Ph), 7.36 (d, JH−H
=
7.31 Hz, 2H, Ph), 8.15 (s, 2H, CHN). 13C{1H} NMR (100.5 MHz,
C6D6, 25 °C): δ 25.27 (CH(CH3)2) 28.05 (CH(CH3)2), 45.45
(NCH3), 57.59 (NCH2), 118.01, 123.30, 125.20, 128.88 (PhGe),
137.27, 141.87, 149.37, 149.96 (NAr), 167.43 (CNAr). UV−vis
(THF): λmax (ε) 224 (6987), 236 (6218), 241 (6092), 246 (6059), 252
(5950), 258 (5875), 267 (6000), 278 (6107), 292 (6168), 395 (2170),
433 (2020), 703 nm (739 dm3 mol−1 cm−1). IR (Nujol cm−1): 2955s,
2922s, 2853s, 1601m, 1584w, 1547w, 1504w, 1462m, 1433m, 1414w,
1391w, 1377w, 1358w, 1335w, 1315m, 1290w, 1258w, 1190w, 1175w,
1155w, 1136w, 1098w, 1080w, 1034w, 1011w, 962w, 949w, 932w,
845w, 802w, 789w, 779w, 756w, 682w.
REFERENCES
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[L2GeK] (4B). Method A. THF (20 mL) was added to a mixture
of 3B (0.59 g, 0.75 mmol) and excess KC8 (0.27 g, 2.02 mmol) at
ambient temperature. The resulting red reaction mixture was stirred
for 2 days. The insoluble precipitate was filtered off. Volatiles of the
filtrate were removed in vacuo. The residue was extracted with Et2O
(30 mL). After filtration and concentration of the filtrate, 4B was
afforded as red crystals. Yield: 0.16 g (21.0%).
Method B. THF (31 mL) was added to a mixture of 2B (1.30 g,
3.03 mmol) and excess KC8 (1.43 g, 10.6 mmol) at ambient tem-
perature. The resulting red reaction mixture was stirred overnight.
The insoluble precipitate was filtered off. Volatiles of the filtrate were
removed in vacuo. The residue was extracted with Et2O (50 mL).
After filtration and concentration of the filtrate, 4B was afforded as red
crystals. Yield: 0.67 g (43.5%).
(3) (a) Kinjo, R.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2007,
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Mp: 380 °C. Elem anal. Calcd for C21H24GeKNO2: C, 58.08; H,
5.57; N, 3.23. Found: C, 56.86; H, 4.13; N, 2.52. Attempts to obtain
acceptable elemental analysis data for compound 4B failed because of
its extreme air sensitivity. 1H NMR (400.1 MHz, THF-d8, 23.3 °C): δ
3
3
(5) Takagi, N.; Nagase, S. Organometallics 2007, 26, 469.
1.03 (d, JHH = 6.8 Hz, 6H, CH(CH3)2), 1.12 (d, JHH = 7.0 Hz, 6H,
3
(6) (a) Asay, M.; Jones, C.; Driess, M. Chem. Rev. 2011, 111, 354.
(b) Green, S. P.; Jones, C.; Junk, P. C.; Lippert, K.-A.; Stasch, A. Chem.
Commun. 2006, 3978. (c) Leung, W.-P.; Chiu, W.-K.; Chong, K.-H.;
Mak, T. C. W. Chem. Commun. 2009, 6822. (d) Nagendran, S.; Sen, S.
CH(CH3)2), 2.12 (s, 3H, CH3), 2.65 (sept, JHH = 6.8 Hz, 2H,
CH(CH3)2), 5.72 (s, 2H, OCH2O), 6.31 (d, JHH = 8.7 Hz, 1H, Ph),
3
7.04−7.07 (m, 4H, Ph). 13C{1H} NMR (100.6 MHz, THF-d8,
23.3 °C): δ 15.71, 23.83 (CH(CH3)2), 27.05, 27.86 (CH(CH3)2),
98.18, 104.07, 119.73, 122.50, 125.25, 128.24, 128.55, 130.18, 145.41
(Ph/Ar), 147.49 (CH3), 148.06 (OCH2O), 155.93 (CNAr). UV−
vis (THF): λmax (ε) 220 (2591), 252 (1945), 254 (1955), 261 (1800),
268 (1663), 297 (1227), 328 (1174), 505 (120), 662 nm (510 dm3
mol−1 cm−1). IR (Nujol, cm−1): 2940s, 2909s, 2855s, 1609w, 1460s,
1377m, 1339w, 1246w, 1196w, 1096w, 1026w, 926w, 777w, 721w.
X-ray Data Collection and Structural Refinement. Intensity
data for compounds 2−4 were collected using a Bruker APEX II
diffractometer. The crystals of 2−4 were measured at 103(2) K. The
structures were solved by direct phase determination (SHELXS-97)
and refined for all data by full-matrix least-squares methods on F2.19 All
non-H atoms were subjected to anisotropic refinement. The H atoms
were generated geometrically and allowed to ride in their respective
parents atoms; they were assigned appropriate isotopic thermal
parameters and included in the structure-factor calculations. Selected
X-ray crystallography data of 2−4 are summarized in Table 2.
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Inorganic Chemistry
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(14) The reaction of compound 3A with 1 or 2 equiv of KC8 in
toluene afforded [L1GeK], which was isolated as a green crystalline
solid. However, X-ray-quality crystals cannot be obtained.
(15) (a) Olmstead, M. M.; Simons, R. S.; Power, P. P. J. Am. Chem.
Soc. 1997, 119, 11705. (b) Pu, L.; Senge, M. O.; Olmstead, M. M.;
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(18) Compound 4B can only be synthesized in THF. The volatiles of
1
the reaction mixture were removed in vacuo. The H NMR spectrum
of the crude product shows the presence of two THF molecules, which
could coordinate to the K atom. Compound 4B can only be
crystallized in Et2O. As a result, THF and Et2O molecules compete the
coordination sphere of K atoms, which lead to the solvent disordering
in the asymmetric unit.
(19) Sheldrick, G. M. SHELXL-97; Universitat Gottingen: Gottingen,
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Germany, 1997.
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