Acyclic germylones: Congeners of allenes with a central germanium atom

Article abstract of DOI:10.1021/ja406112u

The cyclic alkyl(amino) carbene (cAAC:)-stabilized acyclic germylones (Me₂-cAAC:)₂Ge (1) and (Cy₂-cAAC:) ₂Ge (2) were prepared utilizing a one-pot synthesis of GeCl ₂(dioxane), cAAC:, and KC₈ in a 1:2:2.1 molar ratio. Dark green crystals of compounds 1 and 2 were produced in 75 and 70% yields, respectively. The reported methods for the preparation of the corresponding silicon compounds turned out to be not applicable in the case of germanium. The single-crystal X-ray structures of 1 and 2 feature the C-Ge-C bent backbone, which possesses a three-center two-electron π-bond system. Compounds 1 and 2 are the first acyclic germylones containing each one germanium atom and two cAAC: molecules. EPR measurements on compounds 1 and 2 confirmed the singlet spin ground state. DFT calculations on 1/2 revealed that the singlet ground state is more stable by ～16 to 18 kcal mol^-1 than that of the triplet state. First and second proton affinity values were theoretically calculated to be of 265.8 (1)/267.1 (2) and 180.4 (1)/183.8 (2) kcal mol ^-1, respectively. Further calculations, which were performed at different levels suggest a singlet diradicaloid character of 1 and 2. The TD-DFT calculations exhibit an absorption band at ～655 nm in n-hexane solution that originates from the diradicaloid character of germylones 1 and 2.

Full text of DOI:10.1021/ja406112u

Subscriber access provided by Old Dominion University Libraries & VIVA (Virtual Library of Virginia)
Article
Acyclic Germylones: Congeners of Allenes with a Central Germanium Atom
Yan Li, Kartik Chandra Mondal, Herbert W Roesky, Hongping Zhu, Peter
Stollberg, Regine Herbst-Irmer, Dietmar Stalke, and Diego Marcelo Andrada
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja406112u • Publication Date (Web): 19 Jul 2013
Downloaded from http://pubs.acs.org on July 25, 2013
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 8
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Acyclic Germylones: Congeners of Allenes with a Central
Germanium Atom
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Yan Li,a,b Kartik Chandra Mondal,b Herbert W. Roesky,*b Hongping Zhu,*a Peter Stollberg,b Regine
HerbstꢀIrmer,b Dietmar Stalke,*b Diego M. Andrada,*c,d
a
State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols Ethers–Esters, Colꢀ
lege of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian, 361005, China
b
Institut für Anorganische Chemie, GeorgꢀAugustꢀUniversität, Tammannstraβe 4, 37077ꢀGöttingen, Germany.
c
Institute of Physical Chemistry, GeorgꢀAugustꢀUniversität, Tammannstraβe 6, 37077ꢀGöttingen, Germany.
d Instituto de Investigaciones en Fisicoquímica de Córdoba, Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba,
Ciudad Universitaria 5016, Córdoba, Argentina.
KEYWORDS : cyclic alkyl(amino) carbene, acyclic germylone, DFT calculation
ABSTRACT: The cyclic alkyl(amino) carbene (cAAC:) stabilized acyclic germylones (Me2ꢀcAAC:)2Ge (1) and (Cy2ꢀcAAC:)2Ge
(2) were prepared utilizing a oneꢀpot synthesis of GeCl2(dioxane), cAAC:, and KC8 in a 1:2:2.1 molar ratio. Dark green crystals of
compounds 1 and 2 were produced in 75% and 70% yields, respectively. The reported methods for the preparation of the correꢀ
sponding silicon compounds turned out to be not applicable in the case of germanium. The singleꢀcrystal Xꢀray structures of 1 and
2 feature the CꢀGeꢀC bent backbone which possesses a threeꢀcenter twoꢀelectron π bond system. Compounds 1 and 2 are the first
acyclic germylones containing each one germanium atom and two cAAC: molecules. EPR measurements on compounds 1 and 2
confirmed the singlet spin ground state. DFT calculation on 1/2 revealed that the singlet ground state is more stable by ~16 to 18
kcal molꢀ1 than that of the triplet state. First and second proton affinity values were theoretically calculated to be of 265.8 (1)/267.1
(2) and 180.4 (1)/183.8 (2) kcal molꢀ1, respectively. Further calculations, which were performed at different levels suggest a singlet
diradicaloid character of 1 and 2. The calculation exhibit an absorption band at ~655 nm in nꢀhexane solution that originates from
the diradicaloid character of germylones 1 and 2.
its precursor chlorogermyliumylidene cation by sodium naphꢀ
INTRODUCTION
thalenide in moderate yield (Scheme 1). The HOMO of B
consists of the πꢀtype orbital including Ge−C πꢀbonding interꢀ
Allenes are the compounds with a fascinating R2C=C=CR2
action (backꢀbonding) and a σꢀloneꢀpair orbital at the Ge cenꢀ
backbone and have attracted organic chemists for several decꢀ
ter. The calculated values of proton affinities (PA) support the
ades due to their exciting structures and reactivities.1 Based on
germylone character of B. Subsequently corresponding cyclic
DFT calculations2 Frenking et al. proposed that the allenes
siladicarbene was published by the same group.5b
with a heavier central element of group 14 form a ylidone
NHCs play a crucial role in the stabilization of compounds
of E with formal oxidation state of zero.6 Since the first synꢀ
structure with the L:→E←:L (A) arrangement (Scheme 1)
featuring a bent configuration with the formal oxidation state
of zero of the central atom E. According to their theoretical
calculations the bonding situation can be considered as a doꢀ
norꢀacceptor interaction between two donor ligands L: and the
central acceptor E, while the two lone pairs at the E atom are
retained.
thesis of cyclic alkyl(amino) carbene (cAAC:) by Bertrand et
al.,7 activation of small molecules such as NH3 has been
achieved which was not successful with NHCs.
Scheme 1. Representation of Ylidone
The pioneering work on the homoꢀ and heteroꢀallenes has
been done by Kira et al. and Escudié et al.3 The chemistry of
this class of allenes has been intensively explored by them and
others.3 However, so far the related heavier allenes with two
terminal carbons were rarely documented, although some of
the bent allenes (carbodicarbene) have been prepared.4 The
heavier analogues of carbene were utilized for the syntheses of
such interesting compounds.3 Very recently, Driess et al. reꢀ
ported5a a bisꢀNꢀheterocyclic carbene (bNHC) stabilized cyclic
germadicarbene (B) through the reductive dehalogenation of
A
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 8
Theoretical calculations demonstrate that cAAC: possesses
in the presence of 10 mol% of [Na4(Et2O)4(IPy)2].9b The yield
of the reaction remains almost unaltered (47ꢀ50%) even
though 100 mol% of [Na4(Et2O)4(IPy)2] are employed. Comꢀ
pound 3 can also be synthesized in 51% yield by utilizing a
1
2
3
4
5
6
7
8
a singlet spin ground state, and in contrast to NHC:, a smaller
energy gap between the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital
(LUMO).7cꢀd Due to the electronic disparity of nitrogen (σꢀ
electronꢀwithdrawing and πꢀdonating) and carbon (σꢀelectronꢀ
donating and non πꢀdonating), the difference of neighboring
atoms at the carbene carbon leads to a remarkable change in
the reactivities of cAAC:. In our previous work we reported on
a unique diradicaloid siladicarbene (cAAC:)2Si (C),8a which is
easily accessible through the reduction of (cAAC·)2SiCl2 (D)8b
with two equivalents of KC8.8 Compound C is the first report
of a silylone (Scheme 2) which was confirmed by theoretical
calculations. The singlet spin ground state of C was assigned
by EPR spectroscopy and the diradicaloid character was
shown by theoretical calculation, which originated from the
small HOMOꢀLUMO energy gap. This breakthrough opens a
new field of main group chemistry. Inspired by the facile acꢀ
cess8 of (cAAC:)2Si we extended our work to germanium.
Herein, we report on the preparation, characterization and
detailed theoretical investigation of the first acyclic germyloꢀ
nes (Me2ꢀcAAC:)2Ge (1) and (CyꢀcAAC:)2Ge (2).
catalytic amount
(3 mol%) of commercially available
lithium diisopropylamide (LDA) (Scheme 3). This result sugꢀ
gests that the formation of Me2ꢀcAAC:→GeCl2 (3) is possibly
triggered by a cation such as Li+ or Na+. Compound 3 is colorꢀ
less and soluble in THF and toluene. It crystallizes in two
shapes, colorless plates and sticks. The single crystal Xꢀray
diffraction on both types of crystals showed similar structures
but different space groups (Supporting Information). The 13C
NMR spectrum of 3 exhibits a resonance of the carbene carꢀ
bon atom (245.2 ppm), which is significantly upfield shifted
(304.2 ppm), when compared with that of Me2ꢀcAAC:, 7a while
downfield shifted when compared with that of the reported
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
aNHC:→GeCl2 (155.8 ppm, aNHC:
=
1,3ꢀbis(2,6ꢀ
diisopropylphenyl)ꢀ2,4ꢀdiphenylimidazolꢀ5ꢀylidene).11
Scheme 3. Synthetic Strategy for Preparing Compound 3
Li-NiPr2
Scheme 2. The Synthetic Route for Silylone C8a,c
(3 mol%)
+
GeCl2(dioxane)
GeCl2
toluene, r.t., 4 h
N
N
Dip
Dip
3
Me2-cAAC:
The reduction of Me2ꢀcAAC:→GeCl2 (3) with two equivaꢀ
lents of KC8 in the presence of an additional equivalent of
Me2ꢀcAAC: was not successful for the preparation of comꢀ
pound 1. This result reveals that Me2ꢀcAAC:→GeCl2 (3) is not
the right precursor for the germanium analogue of (Me2ꢀ
cAAC:)2Si.
RESULT AND DISCUSSION
Recently, we have found that the dark blue colored diradical
(cAAC·)2SiCl2 is formed in high yield when cAAC: is reacted
with NHC:→SiCl2 in a 3:1 molar ratio. This reaction is exoꢀ
thermic and always easily produces (cAAC·)2SiCl2. The bond
between each carbene carbon and silicon in the diradical
(cAACꢁ)2SiCl2 is found to be an electron sharing single bond
rather than a usual coordinate bond. The corresponding reacꢀ
tion of NHC:→GeCl26b,10 with cAAC: does not proceed, indeꢀ
pendent of the molar ratio or temperature (up to 60 °C). The
formation of (cAAC·)2GeCl2 is not favored due to the lower
GeꢀC bond energy of 80 kJ molꢀ1, when compared with that of
the SiꢀC bond.
Scheme 4. Preparation of Compounds 1 and 2
When the GeCl2(dioxane) adduct was treated with cAAC:
in a 1:1 molar ratio in THF or toluene or diethylether, the
cAAC:H+ cation was exclusively obtained. This was concludꢀ
ed from NMR studies. The analogous NHC:H+ is also formed
Finally, when a 1:2:2.1 molar ratio of GeCl2(dioxane),
Me2ꢀcAAC: and KC8 was reacted in THF at ꢀ78 °C, the resultꢀ
ant suspension was slowly warmed up to room temperature. It
afforded a dark blue solution. The resultant solution was dried
and extracted with nꢀhexane. The dark greenish plates of
(Me2ꢀcAAC:)2Ge (1) were formed in 75% yield (Scheme 4)
from the filtrate which was stored at 0 °C in a refrigerator.
Moreover a small amount (about 3%) of (Me2ꢀcAACH)2O (4)
was also isolated as colorless plates which were characterized
both by NMR and single crystal Xꢀray diffraction (Supporting
Information). The source of H2O in (Me2ꢀcAACH)2O is possiꢀ
bly THF. Nevertheless, the formation of (Me2ꢀcAACH)2O was
(NHC:H+GeCl3 ,
NHC:
=
1,3ꢀbis(2,6ꢀ
ꢀ
diisopropylphenyl)imidazolꢀ2ꢀylidene) in 2% yield when
GeCl2(dioxane) was treated with NHC: while NHC:→GeCl2 is
produced in 98% in this reaction.6b,10 Since cAAC: is more
nucleophilic7a,9a than that of NHC:, it abstracts a proton possiꢀ
bly from the coordinated dioxane ligand of GeCl2(dioxane).
The protons of the CH2 groups of dioxane in GeCl2(dioxane)
are as expected more acidic than those of uncoordinated dioxꢀ
ane. However, we observed that Me2ꢀcAAC:→GeCl2 (3) can
be successfully prepared (Scheme 3) in 50% yield when
GeCl2(dioxane) is treated with Me2ꢀcAAC: in a 1:1 molar ratio
B
ACS Paragon Plus Environment

Page 3 of 8
Journal of the American Chemical Society
less than 0.5% when the reaction was carried out in dioxane at which are in situ coordinated and stabilized by cAAC: (pathꢀ
1
2
3
4
5
6
7
8
0 °C for one hour. The yield of compound 1 remains almost
the same. A oneꢀpot reaction utilizing potassium metal failed
to produce 1 instead (Me2ꢀcAACH)2O was obtained in 40%
yield (Supporting Information). A similar synthetic strategy
was also applied in the successful preparation of 2ꢀ
germadisilaallene.3c Compound 2 was synthesized following
the similar procedure as given in Scheme 4. The yields of
compounds 1 and 2 are higher than that of the reported cyclic
germylone B (45% yield).5a This is surprising because cyclic
species are in general more easily formed than the correspondꢀ
ing acyclic ones. Thus it can be argued that cAAC: is superior
when compared with that of NHC: toward the stabilization of
germylones, since cAAC: is more nucleophilic and electroꢀ
philic than NHC:.7
way b). Under this condition the formation of elemental gerꢀ
manium is not observed, however (cAAC:)2Ge is formed inꢀ
stead.
Compounds 1 and 2 are EPR silent confirming that their
singlet ground state is like that of C.8b,c Compounds 1 and 2
exhibit UVꢀvisible absorption bands at 310 (values of 2, 320),
410 (414), 495 (503), and 653 (657) nm. These values are
comparable to those found for the red colored cyclic germyloꢀ
ne B5a (286, 420, and 564 nm), 2ꢀgermadisilaallene3c (409 and
599 nm), and trigermaallene3e (280, 380, 435, 496, and 630
nm). B does not show any absorption band above 600 nm.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CRYSTAL STRUCTURE DESCRIPTION
Compounds 1 and 2 are less stable than the corresponding
silicon molecule (cAAC:)2Si. A slow decomposition to an
unknown black powder is observed during the crystallization.
The green crystals of 1 and 2 are stable in an inert atmosphere
at room temperature for at least two weeks. However, 1 and 2
can be stored in air for several hours and then slowly oxidize
to Me2ꢀcAAC=O and CyꢀcAAC=O, respectively. When N2O
gas was passed through the THF solution of 1 and 2 for one
hour, the latter compounds were obtained in 90ꢀ92% yield.8c
The carbene carbon atoms of compounds 1 and 2 show a upꢀ
field shift (232.6 (1) and 232.8 ppm (2)) relative to that of
compound 3 (245.2 ppm) in the 13C NMR spectra. The carbene
carbon resonances of 1 and 2 appear downfield shifted when
compared with those of C (210.9 ppm)8b,c and cyclic germyloꢀ
ne B (196 ppm)5a.
Compounds 1 and 2 crystallize from nꢀhexane solutions in the
triclinic space group P
1 and are isostructural to the analogous
silicon compounds. Both the compounds have similar (Figure
1 and 2) molecular structures containing one germanium coorꢀ
dinated by two cAAC: molecules. The asymmetric unit of 1
contains two molecules of (Me2ꢀcAAC:)2Ge and one lattice
solvent molecule (nꢀhexane), whereas the asymmetric unit of 2
contains one molecule of (Cy2ꢀcAAC:)2Ge and half of a lattice
solvent molecule (nꢀhexane).
Scheme 5. Plausible Mechanism for the Formation of 1
Figure 1. Molecular structure of compound 1 with the anisoꢀ
tropic displacement parameters drawn at the 50 % probability
level. The hydrogen atoms and isopropyl groups are omitted
for clarity.
Selected bond lengths and angles of compounds 1, 2 and B
are shown in Table 1. The GeꢀCcarbene bond distances of 1
(1.9386(16), 1.9417(15) Å) and 2 (1.954(2), 1.9386(18) Å) are
slightly shorter than those found in germylone B (1.965(3),
1.961(3) Å).5a It is important to note that both GeꢀC bond disꢀ
tances differ significantly in 2, while they are almost identical
in 1. This is first observed in the analogous silicon compounds.
A purpleꢀcolored {C6H3ꢀ2,6ꢀMes2}2Ge with two coordinate
germanium has been reported with GeꢀC bond length of
2.033(4) Å which is slightly longer than those of 1 and 2.13
A proposed mechanism for the formation of compound 1
is given in Scheme 5. The reaction of GeCl2(dioxane), Me2ꢀ
cAAC: and KC8 in 1:1:1 molar ratio resulted in a brown mixꢀ
ture. The identification of both intermediate species of
cAAC:→Ge(·)Cl and the final product of (cAAC:)2Ge was not
successful (pathway a). The cAAC:→Ge(·)Cl radical might be
unstable and decompose rapidly after its formation. As menꢀ
tioned above, the direct reaction of GeCl2(dioxane) and Me2ꢀ
cAAC: did not result in compound 3. Moreover, in the oneꢀpot
reaction the cAAC:H+ cation was not found, which was conꢀ
firmed by NMR. Obviously the interaction between
GeCl2(dioxane) and Me2ꢀcAAC: at the early stage of the reacꢀ
tion is not preferred, when THF or dioxane is used as a donor
solvent. However, the use of nonꢀcoordinating solvent such as
toluene in preparation of 1 led to the decomposition of 1. It is
rational that in the environment of donor solvent, GeCl2 first
reacts with two equivalents of KC8 to generate Ge(0) atoms
The DFT calculation on B revealed that the HOMO conꢀ
sists of the πꢀtype orbital at the germanium center including
GeꢀC πꢀbonding and the short GeꢀC bond was explained by πꢀ
backꢀbonding like that in C. This could be explained by the
fact that
C
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 8
central atom and the carbene is an electron sharing bond. With
1
2
3
4
an angular sum close to 360° (Table 1) we anticipate the carꢀ
benes in both compounds 1 and 2 to be in the singlet state. Full
tables of bond lengths and bond angles of compounds 1ꢀ4 are
given in the Supporting Information.
5
6
COMPUTATIONAL STUDIES
7
8
9
In order to gain further insight into the electronic structure of
the studied compounds, theoretical calculations have been
carried out. Specifically, we have performed geometry optimiꢀ
zations on compounds 1 and 2 at the M05ꢀ2X/def2ꢀSVP13
level of theory considering three electronic states, namely,
singlet closedꢀshell, singlet brokenꢀsymmetry and triplet state.
For a better visualization, we have prepared superposition
plots of the Xꢀray structures with the optimized ones (Figures
S3ꢀS10, Supporting Information). Our theoretical geometries
show that both singlet states are in a better agreement with the
Xꢀray structures than the triplet state. The latter structure has a
good representation of the side ligands but presents a signifiꢀ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Molecular structure of compound 2 with the anisoꢀ
tropic displacement parameters drawn at the 50 % probability
level. The hydrogen atoms and isopropyl groups are omitted
for clarity.
Table 1. Selected bond lengths and angles of compounds 1, 2, and
B.
cant deviation on the bridge CcarbeneꢀGeꢀCcarbene
.
An improvement of the electronic energies of each species
has been done at several level of theory, namely, M05ꢀ
2X/def2ꢀTZVPP, B3LYP/def2ꢀTZVPP,14 PBE/def2ꢀTZVPP15
and BP86/def2ꢀTZVPP.16 All these methods predict that both
the singlet states have almost the same energy while the triplet
form is 16.6 kcal molꢀ1 (M05ꢀ2X), 18.6 kcal molꢀ1 (B3LYP),
22.0 kcal molꢀ1 (BP86) and 17.4 kcal molꢀ1 (PBE) higher in
energy for compound 1. Likewise, the computed singleꢀtriplet
electronic energy differences for compound 2 are 16.7 kcal
molꢀ1 (M05ꢀ2X), 18.3 kcal molꢀ1 (B3LYP), 21.5 kcal molꢀ1
(BP86) and 17.1 kcal molꢀ1 (PBE). These results support the
experimental EPR outcome of a singlet state. However, it is
important to stress that among all the functionals used M05ꢀ
2X was only one which recognized a singlet broken symmetry
state, which would fall in agreement with the observation of a
diradical species (see Tables S3 and S4).
Compounds
1
2
B5a
1.9386(16)ꢀ
1.9417(15)
1.367(19)
1.954(2)ꢀ
1.9386(18)
1.365(2)
1.965(3)ꢀ
1.961(3)
1.378(4)
GeꢀCcarbene
N1ꢀC1 [Å]
N2ꢀC21/C24 [Å]
1.3666(19)
1.369(3)
1.377(4)
107.22(12)ꢀ
107.07(12)
114.71(6) ꢀ
115.27(6)
113.55(10)ꢀ
113.89(11)
136.53(11)ꢀ
136.26(11)
107.56(16)ꢀ
107.15(15)
102.9(2)ꢀ
103.3(1)
NꢀCcarbeneꢀC/N [°]
CꢀSi/GeꢀC [°]
117.24(8)
86.5(1)
128.6(2)
128.1(2)
Nꢀ Ccarbene –Si/Ge
[°]
112.94(13)ꢀ
113.74(13)
135.80(14)ꢀ
136.78(14)
N/Me2Cꢀ Ccarbene
ꢀ
Si/Ge [°]
Sum of angles at
carbene [°]
Sum of angles at
N [°]
357.3ꢀ357.36
359.83ꢀ359.9
356.3ꢀ357.67 359.6ꢀ360.0
359.92ꢀ359.9
C
ꢀ
In Figure 3 we have depicted the shape of the molecular
orbitals HOMOꢀ1, HOMO and LUMO of the singlet state
form for compound 1. Additionally, in the supporting inforꢀ
mation the frontier molecular orbitals are displayed for the
broken symmetry state (Figure S11).
the cAAC: is a much better πꢀacceptor7 than the bNHC:. of
B.5a The CꢀGeꢀC bond angles of compounds 1 and 2 are
114.71(6)ꢀ115.27(6)° and 117.24(8)°, respectively. In comparꢀ
ison the CꢀSiꢀC angles are 117.18(8)ꢀ117.70(8)/ 118.16(6)° for
C.8a,c In contrast, the CꢀGeꢀC bond angle (86.5(1)°) in B is
sharper5a by 28.5°/30.7° (Table 1) when compared with those
of 1 and 2 indicating a stronger πꢀbond in the CꢀGeꢀC part of
1/2 like that of C, than that of B. Thus the CꢀGeꢀC backbone
of 1 and 2 is significantly less bent than that of cyclic B. The
reported {C6H3ꢀ2,6ꢀMes2}2Ge compound has a similar CꢀGeꢀC
bond angle (114.4(2)°),12 while larger bent angles are observed
in compounds 2ꢀgermadisilaallene3c (132.38(2)°) and trigerꢀ
maallene3e (122.61(6)°), respectively. Unlike the configuration
of the heavy analogues of trisilaallene,3aꢀf the central Ge atoms
in 1 and 2 are fixed without dynamic disorders detected. The
CcarbeneꢀN bond lengths of 1 and 2 are 1.367(19)ꢀ1.3666(19)
and 1.365(2)ꢀ1.369(3) Å which are similar to those values
found in B and C.6,7 As we could show recently the geometry
at the nitrogen atoms in the cAAC is an indicator for the bondꢀ
(a) HOMOꢀ1
(b) HOMO
(c) LUMO
Figure 3. Frontier KS molecular orbitals (isocontour 0.045 au)
of compounds 1 (M05ꢀ2X/def2ꢀTZVPP). Hydrogen atoms are
omitted for clarity.
The shapes of the orbitals fall into the typical features of
the carbone(0) and silylone(0) compounds giving hints into
their nature.2 The HOMOꢀ1 is a σ-type lone pair orbital loꢀ
cated on Ge. On the other hand, the HOMO is a πꢀtype orbital
with the largest contribution on Ge which is normal for backꢀ
donation. Interestingly, the LUMO orbital has a great contriꢀ
bution of the πꢀantibonding orbital NꢀCcarbene of each carbene
ing situation in (cAACꢁ)2X, X = SiCl2, Si, BH, species.8c
A
deviation from the planar geometry at the nitrogen atom like in
the free carbene should be observed if the bond between the
D
ACS Paragon Plus Environment

Page 5 of 8
Journal of the American Chemical Society
moiety and a smaller one on the Ge atom. Additionally, the
where c02 is the weight of the closedꢀshell configuration in the
CI wavefunction and cd is weight of the double excitation.
According with our values the diradicaloid character is 54 %
for 1 and 2.
2
1
2
3
4
5
6
7
8
frontier degenerate singly occupied molecular orbitals of the
singlet diradicaloid systems are shown in Figure S11. Strikingꢀ
ly, the HOMO is split up in two different spatial orbitals which
mainly involve a πꢀtype GeꢀCcarbene orbital, each one with a
cAAC: ligand.
Finally, to help in interpreting the UVꢀvisible spectra band
assignment, we have carried out TDꢀCAMꢀB3LYP/def2ꢀ
SVP22 calculations with the both singlet configurations in the
gas phase and in the solvent nꢀhexane (PCM model).23 The
results are gathered in the Tables S3 and S4. The results for
the closedꢀshell configuration yield three main excitations,
namely, for compound 1 at 565.3 nm (568.6 nm; results for
compound 2), 325.6 nm (328.2 nm) and 280.6 nm (282.9 nm)
with the oscillator strengths of 0.280 (0.258), 0.026 (0.025)
and 0.166 (0.157), respectively. They mainly correspond to the
electron promotion HOMO→LUMO, HOMO→LUMO+5 and
HOMOꢀ1→LUMO+1, in that order (Supporting inforꢀ
mation, Figure S12). In nꢀhexane, the absorption bands are
slightly shifted and the oscillator strength is slightly stronger.
In addition, when the TDꢀDFT calculation was performed on
compound 1 and 2 in their broken symmetry state a band apꢀ
peared at 618.1 nm (628.8 nm in nꢀhexane) and 628.2 (637.6
nm in nꢀhexane) for compounds 1 and 2, respectively. This is
in good agreement with the experimental UVꢀvisible spectra
that give a diradicaloid band at 653 nm for 1 and 657 nm for 2.
The assignation of these bands are the promotion of one elecꢀ
tron SHOMOꢀα(β) into LUMOꢀα(β) (Figure S13). Based on
the theoretical evidences, the resonance structures of 1 are
given in Scheme 6.
The Natural Bond Orbital (NBO)17 analysis on compounds
1 and 2 as closedꢀshell species has been carried out including a
threeꢀcenter bond search. The results pointed out that the prinꢀ
cipal orbitals are a lone pair on Ge with occupancy of 1.770
electrons (1.760 electrons for compound 2) and a threeꢀcenter
CꢀGeꢀC πꢀtype orbital where 43 % is at Ge atom and 28.5 % at
each of the carbene carbon with occupancy of 1.935 electrons
(1.933 electrons). Strikingly, the NBO analysis also indicated
the presence of two threeꢀcenter πꢀantibonding orbitals at the
CꢀGeꢀC bridge, the first one with an occupancy of 0.628 elecꢀ
trons and located on the two carbene carbon atoms; the second
one is a πꢀtype orbital placed on Ge (57 %) and Ccarbene
(21.5 %) which has an occupancy of 0.200 electrons. Indeed,
application of the secondꢀorder perturbation theory of the
NBO method revealed the occurrence of the strong stabilizing
twoꢀelectron donorꢀacceptor interactions from the lone pair of
Ge (LP(Ge)) into the three center antibonding orbitals (π*(Cꢀ
GeꢀC)), and also from the lone pair of the N atoms (LP(N)), at
the cAAC: moieties, into the (π*(CꢀGeꢀC)). For instance, the
computed energies are 42.3 kcal molꢀ1 for LP(Ge)→π*(CꢀGeꢀ
C) donorꢀacceptor interaction and 26 kcal molꢀ1 for the two
LP(N)→π*(CꢀGeꢀC) donorꢀacceptor interactions which indiꢀ
cate a high stabilization effect by delocalization. The Wiberg
bond order18 of the GeꢀCcarbene bonds are 1.146 and 1.148 a.u.
for compounds 1 and 2 (in that order) explaining the short
distances observed in the Xꢀray structures. Furthermore, the
computed NPA charge on the Ge is 0.36 a.u. for 1 and 0.37 a.u.
for 2 supporting the nature of these compounds as Ge(0)
germylones.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 6. Resonance Structures of 1. The Schematic Repreꢀ
sentation Drawn in the Right Formula Shows the Results of a
Proposed threeꢀcenter twoꢀelectron π Bond.8b,c
To confirm the classification of compounds 1 and 2 we
have computed their proton affinity at the BP86/def2ꢀSVP
level of theory. The results suggest a high first proton affinity,
namely, 265.8 kcal molꢀ1 and 267.1 kcal molꢀ1 for 1 and 2,
respectively. Moreover, the second proton affinity is lower but
still high, i.e., 180.4 and 183.8 kcal molꢀ1 for 1 and 2, respecꢀ
tively. These values of the proton affinities are in the normal
range for germylones.2bꢀc,5a
To have a better understanding of the electronic ground
state CASSCF(2,2)/def2ꢀSVP//M05ꢀ2X/def2ꢀSVP19 calculaꢀ
tions were performed. These multireference calculations led to
a CI vector having coefficients of 0.96 (1 and 2) for the
closedꢀshell 2,0 configuration, 0.0 (1 and 2) for the 1,1 configꢀ
uration, and ꢀ0.28 (1 and 2) for the 0,2 configuration. Thereꢀ
fore, compounds 1 and 2 are closedꢀshell singlet species havꢀ
ing a contribution of double excited state which gives a singlet
diradicaloid character. In this regard, the singlet diradical inꢀ
dex, proposed by Neese and coꢀworkers,20ꢀ21 can be calculated,
for an ab initio CI calculation with the canonical MOs, by the
following equation:
CONCLUSION
We have developed novel methodologies for the synthesis of
acyclic germylones (Me2ꢀcAAC:)2Ge (1) and (CyꢀcAAC:)2Ge
(2) and germylenedichloride Me2ꢀcAAC:→GeCl2 (3). We have
experienced that the germanium analogue of the diradical
(Me2ꢀcAACꢁ)2SiCl2 is not formed due to the weaker GeꢀC
single bond energy. An adduct between the Me2ꢀcAAC: and
GeCl2, Me2ꢀcAAC:→GeCl2 (3), was obtained. However, the
reduction of 3 with KC8 in the presence of Me2ꢀcAAC: did not
give the desired germylone (Me2ꢀcAAC:)2Ge. This suggests
that 3 is not the right precursor for the synthesis of (Me2ꢀ
cAAC:)2Ge (1). Hence in the search for an alternative method
we have demonstrated that
a
oneꢀpot reaction of
GeCl2(dioxane), Me2ꢀcAAC: and KC8 led to the germylones 1
and 2 yielding 75% and 70%, respectively. The dark greenish
crystals of 1 and 2 are stable in an inert atmosphere at room
temperature for at least two weeks and can be stored in air for
several hours. DFT calculation showed that single ground state
of 1/2 is more stable by ~16 to 18 kcal molꢀ1 than that of triplet
c02cd2
d = 200
c02 + cd2
E
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 8
623. (j) Takagi, N.; Shimizu, T.; Frenking, G. Chem. Eur. J. 2009, 15,
3448ꢀ3456.
state. Two proton affinity values were theoretically calculated
to be 265.8 (1)/267.1 (2) and 180.4 (1)/183.8 (2) kcal molꢀ1,
respectively. Importantly, the computed NPA charges on the
Ge are 0.36 a.u. for 1 and 0.37 a.u. for 2, which suggest that
the formal oxidation state of germanium atom in 1/2 is zero.
The theoretical results pointed out that the principal orbitals
are a lone pair on Ge with occupancy of 1.770 electrons (1.760
electrons for 2) and a threeꢀcenter CꢀGeꢀC πꢀtype orbital
where 43 % is at Ge atom and 28.5 % at each of the carbene
carbon with occupancy of 1.935 electrons (1.933 electrons for
2). Further calculations demonstrated that these compounds
are germylones in nature and possess 54 % of a diradicaloid
character which is due to the stronger π–accepting ability of
cAAC: carbenes. The absorption band at 653 nm for 1 and 657
nm for 2 originated due to diradicaloid character of 1 and 2.
Studies of the chemistry of the new compounds (Me2ꢀ
cAAC:)2Ge (1) and (Cy2ꢀcAAC:)2Ge (2) are currently in proꢀ
gress.
1
2
3
4
5
6
7
8
(3) For reviews on heteroallenes, see: (a) Ishida, S.; Iwamoto, T.;
Kabuto, C.; Kira, M. Nature 2003, 421, 725ꢀ727. (b) Kira, M.; Iwaꢀ
moto, T.; Ishida, S.; Masuda, H. J. Am. Chem. Soc. 2009, 131, 17135–
17144. (c) Iwamoto, T.; Abe, T.; Kabuto, C.; Kira, M. Chem. Comꢀ
mun. 2005, 5190–5192. (d) Kira, M. Chem. Comunm. 2010, 46,
2893– 2903. (e) Iwamoto, T.; Masuda, H.; Kabuto, C.; Kira, M. Orꢀ
ganometallics 2005, 24, 197ꢀ199. (f) Escudié, J.; Ranaivonjatovo, H.;
Rigon, L. Chem. Rev. 2000, 100, 3639ꢀ3696. (g) Ghereg, D.; Saffon,
N.; Escudié, J.; Miqueu, K.; Sotiropoulos, J.ꢀM. J. Am. Chem. Soc.
2011, 133, 2366–2369.
(4) (a) Weber, E.; Seichter, W.; Hess, B.; Will, G.; Dasting, H. J. J.
Phys. Org. Chem. 1995, 8, 94ꢀ96. (b) Dyker, C. A.; Lavallo, V.; Donꢀ
nadieu, B.; Bertrand, G. Angew. Chem. Int. Ed. 2008, 47, 3206–3209;
Angew. Chem. 2008, 120, 3250–3253. (c) Hofmann, M. A.;
Bergstrasser, U.; Reiss, G. J.; Nyulaszi, L.; Regitz, M. Angew. Chem.
Int. Ed. 2000, 39, 1261ꢀ1263; Angew. Chem. 2000, 112, 1318ꢀ1320.
(d) Chen,W.ꢀC.; Hsu, Y.ꢀC.; Lee, C.ꢀY.; Yap, G. P. A.; Ong, T.ꢀG.
Organometallics 2013, 32, 2435−2442
(5) (a) Xiong, Y.; Yao, S.; Tan, G.; Inoue, S.; Driess, M. J. Am.
Chem. Soc. 2013, 135, 5004−5007. (6) Xiong, Y.; Yao, S.; Inoue, S.;
Epping, J. D.; Driess, M. Angew. Chem. Int. Ed. 2013, 52, 1ꢀ5; Angew.
Chem. 2013, 125, 7287ꢀ7291
(6) (a) Wang, Y.; Xie, Y.; Wei, P.; King, R. B.; Schaefer III, H. F.;
Schleyer, P. v. R.; Robinson, G. H. Science, 2008, 321, 1069ꢀ1071. (b)
Sidiropoulos, A.; Jones, C.; Stasch, A.; Klein, S.; Frenking G. Angew.
Chem. Int. Ed. 2009, 48, 9701ꢀ9704; Angew. Chem. 2009, 121, 9881ꢀ
9884. (c) Wang, Y.; Robinson, G. H. Chem. Commun. 2009, 5201–
5213 and the references therein.
(7) (a) Lavallo, V.; Canac, Y.; Präsang, C.; Donnadieu, B.; Bertrand,
G. Angew. Chem. Int. Ed. 2005, 44, 5705–5709; Angew. Chem. 2005,
117, 5851ꢀ5855. (b) Martin, C. D.; Soleilhavoup, M.; Bertrand, G.
Chem. Sci., 2013, 4, 3020ꢀ3030 ; (c) Martin, D.; Melaimi, M.; Soleilꢀ
havoup, M.; Bertrand, G. Organometallics, 2011, 30, 5304–5313. (d)
Back, O.; HenryꢀEllinger, M.; Martin, C. D.; Martin, D.; Bertrand, G.
Angew. Chem. Int. Ed. 2013, 52, 2939ꢀ2943; Angew. Chem. 2013,
125, 3011–3015.
(8) (a) Mondal, K. C.; Roesky, H. W.; Schwarzer, M. C.; Frenking,
G.; Niepötter, B.; Wolf, H.; HerbstꢀIrmer, R.; Stalke, D. Angew.
Chem., Int. Ed. 2013, 52, 2363ꢀ2367; Angew. Chem. 2013, 125, 3036ꢀ
3040. (b) Mondal, K. C.; Roesky, H. W.; Schwarzer, M. C.; Frenking,
G.; Tkach, I.; Wolf, H.; Kratzert, D.; HerbstꢀIrmer, R.; Niepötter, B.;
Stalke, D. Angew. Chem., Int. Ed. 2013, 52, 1801ꢀ1805; Angew.
Chem. 2013, 125, 1845ꢀ1850. (c) Mondal, K. C.; Samuel, P. P.; Treꢀ
tiakov, M.; Singh, A. P.; Roesky, H. W.; Stückl, A. C.; Niepötter, B.;
Carl, E.; Wolf, H.; HerbstꢀIrmer, R.; Stalke, D. Inorg. Chem. 2013,
52, 4736–4743.
(9) (a) Martin, D.; Soleilhavoup, M.; Bertrand, G. Chem. Sci. 2011,
2, 389–399. (b) Nayek, H. P.; Arleth, N.; Trapp, I.; Löble, M.; Onaꢀ
Burgos, P.; Kuzdrowska, M.; Lan, Y.; Powell, A. K.; Breher, F.;
Roesky, P. W. Chem. Eur. J. 2011, 17, 10814–10819.
(10)Thimer, K. C.; AlꢀRafia, S. M. I.; Ferguson, M. J.; McDonald, R.;
Rivard, E. Chem. Commun., 2009, 7119–7121.
(11) Singh, A. P.; Ghadwal, R. S.; Roesky, H. W.; Holstein, J. J.;
Dittrich, B.; Demers, JꢀP; Chevelkovb, V.; Langeb, A. Chem. Comꢀ
mun., 2012, 48, 7574–7576.
(12) Simons, R. S.; Pu, L.; Olmstead, M. M.; Power, P. P. Organomeꢀ
tallics 1997, 16, 1920ꢀ1925.
(13) (a) Zhao, Y.; Schultz, N. E.; Truhlar, D. G. J. Chem. Theory
Comput., 2006, 2, 364ꢀ382. (b) Zhao, Y.; Schultz, N. E.; Truhlar, D.
G. J. Chem. Phys. 2005, 123, 161103ꢀ161106. (c) Weigend, F.; Ahlꢀ
richs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297ꢀ3305.
(14) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648ꢀ5652. (b) Lee,
C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785ꢀ789.
(15) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996,
77, 3865ꢀ3868. (b) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110,
6158ꢀ6170.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ASSOCIATED CONTENT
Supporting Information
Experimental details for the synthesis, crystallographic data, taꢀ
bles of bond lengths and angles, UVꢀvisible spectra and theoretiꢀ
cal calculation details are available free of charge via the Internet
at http://pubs.acs.org.
Corresponding Author
* Prof. Dr. H. W. Roesky; Eꢀmail: hroesky@gwdg.de
* Prof. Dr. H. Zhu; Eꢀmail: hpzhu@xmu.edu.cn
* Prof. Dr. D. Stalke; Eꢀmail: dstalke@chemie.uniꢀgoettingen.de
* Dr. D. M. Andrada; Eꢀmail: die@mail.fcq.unc.edu.ar
ACKNOWLEDGMENT
This manuscript is dedicated to Professor Helmut Schwarz on the
occasion of his 70th birthday. This work was funded by the
Deutsche Forschungsgemeinschaft (RO 224/60ꢀ1, HWR) and the
Danish National Research Foundation (DNRF93) funded Center
for Materials Crystallography (DS). We thank very much Prof.
Dr. Ricardo A. Mata for his valuable comments and guidance to
carry out the theoretical calculation. We thank Prof. F. Meyer and
N. Bruckner for UVꢀvisible and Dr. A. C. Stückl for EPR measꢀ
urements. Y. Li thanks the Chinese Scholarship Council for a
fellowship.
REFERENCES
(1) Simth, M. B.; March, J.; March´s Advanced Organic Chemistry:
Reactions, Mechanisms, and Structure (6th ed), New York, 2007,
WileyꢀInterscience.
(2) (a) Tonner, R.; Frenking, G. Angew. Chem. Int. Ed. 2007, 46,
8695–8698. Angew. Chem. 2007, 119, 8850ꢀ8853. (b) Takagi, N.;
Shimizu, T.; Frenking, G. Chem. Eur. J. 2009, 15, 8593ꢀ8604. (c)
Takagi, N.; Shimizu, T.; Frenking, G. Chem. Eur. J. 2012, 18, 1772ꢀ
1780. (d) Tonner, R.; Öxler, F.; Neumüller, B.; Petz, W.; Frenking, G.
Angew. Chem., Int. Ed. 2006, 45, 8038ꢀ8042; Angew. Chem. 2006, 47,
8206–8211. (e) Frenking, G.; Neumüller, B.; Petz, W.; Tonner, R.;
Öxler, F. Angew. Chem, Int. Ed. 2007, 46, 2986ꢀ2987; Angew. Chem.
2007, 17, 3044ꢀ3045. (f) Frenking, G.; Tonner, R. Pure Appl. Chem.
2009, 81, 597ꢀ614. (g) Klein, S.; Tonner, R.; Frenking, G. Chem. Eur.
J. 2010, 16, 10160ꢀ10170. (h) Frenking, G.; Tonner, R. Wiley Interꢀ
disciplinary Reviews: Computational Molecular Science 2011, 1, 869ꢀ
878. (i) Takagi, N.; Frenking, G. Theor. Chem. Acc. 2011, 129, 615ꢀ
F
ACS Paragon Plus Environment

Page 7 of 8
Journal of the American Chemical Society
(16) (a) Perdew, J. P. Phys. Rev. B 1986, 33, 8822ꢀ8824. (b) Becke,
A. D. Phys. Rev. A 1988, 38, 3098ꢀ3100.
1
(17) (a) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988,
88, 899ꢀ926. (b) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J.
Chem. Phys. 1985, 83, 735ꢀ746. (c) Reed, A. E.; Weinhold, F. J.
Chem. Phys. 1985, 83, 1736ꢀ1740. (d) Foster, J. P.; Weinhold, F. J.
Am. Chem. Soc. 1980, 102, 7211ꢀ7218.
2
3
4
5
(18) Wiberg, K. B. Tetrahedron 1968, 24, 1083ꢀ1096.
(19) (a) Hegarty, D.; Robb, M. A. Mol. Phys. 1979, 38, 1795ꢀ1812.
(b) Eade, R. H. A.; Robb, M. A. Chem. Phys. Lett. 1981, 83, 362ꢀ368.
(c) Yamamoto, N.; Vreven, T.; Robb, M. A.; Frisch, M. J.; Schlegel,
H. B. Chem. Phys. Lett. 1996, 250, 373ꢀ378. (d) Frisch, M.; Ragazos,
I. N.; Robb, M. A.; Bernhard Schlegel, H. Chem. Phys. Lett. 1992,
189, 524ꢀ528.
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(20) Bachler, V.; Olbrich, G.; Neese, F.; Wieghardt, K. Inorg. Chem.
2002, 41, 4179ꢀ4193.
(21) (a) Herebian, D.; Bothe, E.; Neese, F.; Weyhermuller, T.;
Wieghardt, K. J. Am. Chem. Soc. 2003, 125, 9116ꢀ9128. (b) Herebian,
D.; Wieghardt, K. E.; Neese, F. J. Am. Chem. Soc. 2003, 125, 10997ꢀ
11005. (c) Neese, F. J. Phys. Chem. Solids 2004, 65, 781ꢀ785.
(22) Yanai, T.; Tew, D. P.; Handy, N. C. Chem. Phys. Lett. 2004, 393,
51ꢀ57.
(23) Miertš, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117ꢀ
129.
TOC
G
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 8
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
8
ACS Paragon Plus Environment