A functionalized Ge3-compound with a dual character of the central germanium atom

Article abstract of DOI:10.1039/c3cc49635j

(cAAC)Ge(GeL)₂ (1) (cAAC = cyclic alkyl(amino) carbene; L = PhC(tBuN)₂), a functionalized Ge₃-compound was prepared. Quantum mechanical studies on 1 show a reciprocal relationship between the electronic state of the central tri-coordinated Ge atom and its reactivity towards protons, viz. tetravalent Ge(0) in terms of bonding and divalent Ge(0) in terms of reactivity. Thus the central Ge atom can be considered as having a hidden but highly reactive lone pair of electrons. However, the terminal Ge atoms can be considered as tri-coordinated divalent Ge(i) with an active lone pair of electrons. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc49635j

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A functionalized Ge3-compound with a dual
character of the central germanium atom†
Cite this: Chem. Commun., 2014,
50, 2986
Yan Li,ab Kartik Chandra Mondal,b Jens Lubben,b Hongping Zhu,*a Birger Dittrich,*b
¨
Indu Purushothaman,c Pattiyil Parameswaran*c and Herbert W. Roesky*b
Received 19th December 2013,
Accepted 14th January 2014
DOI: 10.1039/c3cc49635j
www.rsc.org/chemcomm
(cAAC)Ge(GeL)2 (1) (cAAC = cyclic alkyl(amino) carbene; L = PhC-
(tBuN)2), a functionalized Ge3-compound was prepared. Quantum
mechanical studies on 1 show a reciprocal relationship between the
electronic state of the central tri-coordinated Ge atom and its reactivity
towards protons, viz. tetravalent Ge(0) in terms of bonding and divalent
Ge(0) in terms of reactivity. Thus the central Ge atom can be considered
as having a hidden but highly reactive lone pair of electrons. However,
the terminal Ge atoms can be considered as tri-coordinated divalent
Ge(I) with an active lone pair of electrons.
The formation of multiple bonds between carbon and heavier group
14 elements (Si, Ge, and Sn) has been an attractive research topic for
decades.1 Stable germenes ({CQGez) can be accessed by protect-
ing both carbon and germanium centers using sterically demanding
Scheme 1 Representatives of the stable {CQGez species.
ligands (Scheme 1). The approaches to germenes A2 and B3 are (N-heterocyclic carbene).8 This finding enriches the carbene–Ge
assumed via transient carbenes, while B is best described as an chemistry, and a similar bonding situation is theoretically reported
adduct of cyclopropenylidene and germylene. Compound C4 is for the Si analogues.9 Although (cAAC)2Ge can be prepared easily by
prepared by treating Mes2Ge(F)–C(H)R2 with tBuLi, and D5 is using GeCl2(dioxane) as the Ge source,6c we are curious to study the
probably produced by the insertion of CS2 into a GeQGe double reaction behavior when a substituted germanium precursor is
bond. A germylone6a,b with composition (cAAC)2Ge(0) shows an employed. Herein, we report a novel Ge3-compound (1) which shows
interesting biradicaloid character.6c The NBO study demonstrates for the first time the direct formation of a GeQC bond using a
that the principal orbitals of (cAAC)2Ge are a lone pair on Ge and a stable cAAC. Moreover, theoretical studies predict an unusual dual
three-center C–Ge–C p-type orbital where 43% is at the Ge atom and character of the central Ge atom.
28.5% at each carbene carbon, different from the C - Ge donor–
The reaction of LGeCl (L = PhC(tBuN)2),10 cAAC and KC8 in a
acceptor bond present in the cyclic germylone.7 This is mainly molar ratio of 1:0.5:1.2 in THF at À78 1C for 1 h gives compound
attributed to the singlet spin ground state and the smaller HOMO– (cAAC)Ge(GeL)2 (1) isolated as red crystals in 35% yield (based on
LUMO energy gap of cAAC, when compared with that of NHC Ge) (Scheme 2). The molar ratio mentioned above is tested to be
necessary for the formation of 1. Interestingly, stirring the reaction
overnight at room temperature affords a mixture of 1, dark green
a State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry
plates of (cAAC)2Ge (2) and a small amount of colorless crystals of
and Chemical Engineering, Xiamen University, Xiamen, Fujian, 361005, China.
(cAACH)2O.11 2 is also slowly generated from the mother liquid of
E-mail: hpzhu@xmu.edu.cn
b
1 (Scheme 2). However, when NHC (1,3-bis(isopropyl)imidazol-2-
ylidene) is used instead of cAAC, only LGe–GeL10 is produced.
The solution of 1 is highly air sensitive, but it is stable in the
solid state, even when exposed to air for two days. 1 decomposes
above 183 1C. 1 is soluble in THF and toluene, but sparingly soluble
¨
¨
Institut fur Anorganische Cihemie, Georg-August-Universitat, Tammannstraße 4,
¨
37077-Gottingen, Germany. E-mail: hroesky@gwdg.de, bdittri@gwdg.de
c Department of Chemistry, National Institute of Technology Calicut,
NIT Campus P.O., Calicut-673 601, India. E-mail: param@nitc.ac.in
† Electronic supplementary information (ESI) available: Experimental proce-
dures, and X-ray crystallographic and computational information of 1. CCDC
967854 (1). For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3cc49635j
1
in n-hexane. The H NMR spectrum shows the resonances corre-
sponding to iPr-H (septet at 3.41 ppm) and >CH2 (singlet at 1.42 ppm)
2986 | Chem. Commun., 2014, 50, 2986--2989
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Notably, the Ge1–C1 bond distance (1.881(3) Å) is comparable
to that of the reported Ge–C double bond, while it is shorter
than that of 2 (ca. 1.94 Å)6c and the donating C - Ge (ca. 2.13 Å)
bond in the cAAC(GeCl2) adduct.6c The torsion angle of the
N1–C1–C2–Ge1 and Ge3–Ge1–Ge2–C1 planes is ca. 3.71, which
is smaller than those of A (av. 361)2 and C (av. 61),4 but close to
that of D (41).5 This flatness allows a p-bonding between the
carbene carbon (C1) and the central Ge1 atom.
To further understand the electronic structure and bonding of 1,
quantum chemical calculations were performed at the M06/def2-
TZVPP//BP86/def2-SVP level of theory.17a The calculated singlet
geometry (3) (Fig. S3, see ESI†) is in good agreement with the crystal
structure. The singlet state is more stable than the triplet state
by 24.6 kcal molÀ1. The calculated pyramidalization angle
(yp = 3601 À sum of the three angles around Ge atoms) indicates
the planar coordination around Ge1 (yp = 01) and pyramidal
coordination around Ge2 (yp = 75.01) and Ge3 (yp = 101.21) implying
the presence of a lone pair on Ge2 as well as on Ge3. The HOMO À 1
and HOMO À 2 (Fig. 2) represent the combinations of the lone pairs
on the terminal Ge atoms. It is to be noted that the C1–N1 bond
length (1.394 Å) is significantly elongated when compared with the
corresponding bond length (1.320 Å) in the free cAAC. This can be
understood from the HOMO which shows the back donation of the
lone pair on Ge1 to the C1–N1 p*–MO (Fig. 2).
The significant back donation from Ge1 to the C1–N1 p*–MO can
also be understood from the NBO charge and population analysis
(Table S2, see ESI†).17a The group charge of cAAC is À0.32 eÀ which
indicates a net charge flow from the Ge3L2 fragment to the carbene
ligand. This signifies that the p-back donation from Ge1 to the C1–N1
p*–MO of cAAC is much stronger than the s-donation from cAAC to
Ge1 of the Ge3L2 fragment. The Wiberg bond index of the Ge1–C1
bond (1.35) also indicates the partial double bond character.18 The
Wiberg bond index of the C1–N1 bond in 3 (1.12) is significantly
reduced as compared to that of free cAAC (1.50). Interestingly, the
NBO calculations suggest the presence of a p-type lone pair (100%
p-character) on Ge1 and a sp-hybrid type lone pair on Ge2 (81.5%
s-character and 18.5% p-character) and Ge3 (80.0% s-character and
20.0% p-character). Moreover, the occupancy of the lone pair on Ge1
is only 1.02 eÀ, whereas on Ge2 and Ge3 is 1.93 eÀ and 1.94 eÀ,
respectively. The molecular orbitals and the NBO charge and popula-
tion analysis suggest that Ge2 and Ge3 can be considered as divalent
Ge(I), where each Ge atom utilizes one electron for forming an
electron sharing bond with the central Ge1 atom and one electron
for the electron sharing bond with one N-atom of the amidinate
ligand. The second N-atom of the amidinate ligand donates two
Scheme 2 Synthesis of 1 and its transformation to germylone 2.
of cAAC. The amidinate ligand exhibits two tBu-H resonances (1.34
and 1.32 ppm), indicating the asymmetric structure of the molecule.
In the 13C NMR spectrum, the resonance for the carbene carbon
(C–Ge) is observed at 219.4 ppm which shifts upfield when com-
pared with that of the free cAAC (304.2 ppm)9 and 2 (232.6 ppm).6c
This resonance is significantly low field shifted relative to those
reported for the stable germaethene A (115 and 93 ppm) and the
conjugated Ar2GeQC(R)CRC(R)CQGeAr2 (Ar = 2,3,4-trimethyl-
6-tBu-phenyl; R = nBu and C6H5).12 The UV-visible spectrum of 1
recorded in C6D6 shows a strong absorption band at 490 nm.
The structure of 1 (Fig. 1) exhibits an asymmetric Ge3 backbone,
in which the central Ge1 atom adopts a three coordinate geometry
with one C and two Ge atoms. The geometry around the Ge1 atom is
nearly planar, which indicates the absence of a stereoactive lone pair
at Ge1. In contrast, the Ge2 and Ge3 atoms are both pyramidalized
due to the presence of a lone pair of electrons at the apex of a
distorted trigonal pyramid. The N2–Ge2–N3 and N4–Ge3–N5 planes
are nearly perpendicular (ca. 881). The Ge2–Ge1–Ge3 bond angle
is 107.65(2)1 which is comparable to those reported for the four-
membered zwitterionic ring L4Ge6,13 while it is smaller than that of
the bent trigermaallene (122.61(6)1).14 The Ge1–Ge3 (2.4929(6) Å)
and Ge1–Ge2 bond lengths (2.4746(7) Å) are close to that of the
Ge–Ge single bonds (2.43–2.47 Å),15 while shorter than that of
the Ge–Ge bond (2.5439(7) Å) in 2,6-Mes2C6H3Ge–GetBu3.16
Fig. 1 Molecular structure of 1. Selected bond lengths (Å) and angles (1):
Ge1–C1 1.881(3), Ge1–Ge2 2.4746(7), Ge1–Ge3 2.4929(6), C1–N1 1.375(5),
Ge2–N2 2.015(3), Ge2–N3 2.029(3); Ge2–Ge1–Ge3 107.65(2), C1–Ge1–
Ge2 128.95(11), C1–Ge1–Ge3 122.68(11), N1–C1–C2 107.3(3), N1–C1–Ge1
127.5(2), C2–C1–Ge1 124.9(3). The sum of angles around C1 and Ge1 are
ca. 359.7 and 359.31, respectively.
Fig. 2 Plot of important molecular orbitals of 3 showing the back donation
from Ge1 to the C1–N1 p*–MO (HOMO) and the lone pair combinations of
Ge2 and Ge3 (HOMO À 1 and HOMO À 2) at the M06/def2-TZVPP//BP86/
def2-SVP level of theory. The eigenvalues (eV) are given in parentheses.
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electrons to the Ge2/Ge3 atom. Thus two electrons of the terminal Ge
The proposed reaction mechanism for the formation of 1–2
atoms are retained as a lone pair. In contrast, the central Ge1 atom (Scheme 3) was theoretically studied (see the ESI† for detailed
utilizes two electrons for electron sharing bonds with the terminal Ge discussion).
atoms. The vacant in-plane s-orbital accepts a pair of electrons from
In summary, we have synthesized and characterized a novel
cAAC and the remaining two electrons on the p-orbital of Ge1 are Ge3-compound (cAAC)Ge(GeL)2 (1) in the singlet state. It is
back donated to the empty C1–N1 p*–MO of cAAC. Hence the tri- stable at room temperature in solid state under an inert atmo-
coordinated Ge1 atom can be considered as tetravalent Ge(0).17b,19
sphere. Compound 1 can slowly convert to germylone 2 in
The EDA-NOCV method was employed to deeply study the solution. Notably, cAAC exclusively favors the generation of 1,
nature of the interaction of cAAC with the Ge3L2 fragment.17b The while NHC does not. Compound 1 is the first example of direct
donor–acceptor interaction between the singlet cAAC and the formation of a GeQC bond by using a stable cAAC. Quantum
singlet Ge3L2 is found to be the most favorable bonding descrip- mechanical studies show a reciprocal relationship between the
tion and the corresponding EDA data are shown in Table S4 (see bonding and reactivity of the central Ge atom. The bonding
ESI†).17b The Ge1–C bond has a higher percentage of electrostatic pattern of the central Ge atom is substantially different as
interaction (54.0%) as compared to covalent interaction (46.0%). compared to the terminal Ge atoms. However, the reactivity
The two NOCV pairs of orbitals (CÀ1/C1 and CÀ2/C2) having a of all Ge atoms towards protonation is similar. The EDA-NOCV
major contribution to the total orbital interaction energy (Fig. S4, analysis suggests two bent bonds for the Ge1–CcAAC bond.
ESI†) and their corresponding deformation density indicate mix-
This work was supported by Deutsche Forschungs gemeinschaft
ing between the s-type and p-type fragment orbitals on the Ge3L2 (RO 224/60-1). We thank Anne Bretschneider for the measurement
and cAAC. The NOCV pairs of orbitals do not have an appropriate of the UV-visible spectrum. Y. L. thanks the China Scholarship
s- or p-symmetry.17b The corresponding deformation density plots Council (CSC) for a fellowship.
Dr1 (DE1 = À62.1 kcal molÀ1) and Dr2 (DE2 = À77.2 kcal molÀ1
)
do not show any significant variation of electron density along the
Ge1–CcAAC bond. However, significant accumulation and deple-
tion of electron density above and below the plane of the Ge1–C
bond are observed. Hence, the bonding situation in the Ge1–C
bond can be considered to be formed by two bent donor–acceptor
bonds.17b This is also reflected in the low bond dissociation
energy (De = 26.9 kcal molÀ1).
Notes and references
´
´
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´
´
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¨
¨
12 Although no accurate value is reported, it must be smaller than
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Scheme 3 Proposed mechanism for the formation of 1 and 2.
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