Unusual formation of a N-heterocyclic germylene via homolytic cleavage of a C-C bond

Article abstract of DOI:10.1039/c4cc00251b

Reaction of the monoanionic radical salt IP^-K⁺ (IP = (Py)CH(=NR); Py = C₅H₄N, R = 2,6-iPr₂C ₆H₃; α-iminopyridine) with GeCl₂(dioxane) afforded compound (IPGeCl)₂ (1) which produced red blocks of IPGe: (2), when treated with KC₈ in toluene. 1 is a digermylene formed via C-C coupling between two carbon-centered radicals. 2 can be considered as an analogue of a N-heterocyclic carbene, which exhibits a five-membered GeC ₂N₂ ring with one CC double bond. 2 is formed by two-electron reduction of 1 with cleavage of the two Ge-Cl bonds and the central C-C single bond. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c4cc00251b

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Unusual formation of a N-heterocyclic germylene
via homolytic cleavage of a C–C bond†
Cite this: Chem. Commun., 2014,
50, 3356
Yan Li,ab Kartik Chandra Mondal,b Peter Stollberg,b Hongping Zhu,*a
Herbert W. Roesky,*b Regine Herbst-Irmer,b Dietmar Stalke*b and Heike Fliegl*c
Received 11th January 2014,
Accepted 1st February 2014
DOI: 10.1039/c4cc00251b
www.rsc.org/chemcomm
Reaction of the monoanionic radical salt IPꢀꢁK+ (IP = (Py)CH(=NR);
Py = C5H4N, R = 2,6-iPr2C6H3; a-iminopyridine) with GeCl2(dioxane)
afforded compound (IPGeCl)2 (1) which produced red blocks of
IPGe: (2), when treated with KC8 in toluene. 1 is a digermylene
formed via C–C coupling between two carbon-centered radicals. 2
can be considered as an analogue of a N-heterocyclic carbene,
Q
which exhibits a five-membered GeC2N2 ring with one C C double
bond. 2 is formed by two-electron reduction of 1 with cleavage of
the two Ge–Cl bonds and the central C–C single bond.
The N-heterocyclic germylene (NHGe) with two-coordinate germa-
nium is known as a congener of the corresponding carbene (NHC),
which has been of great interest to chemists for decades.1 The
formation of the heterocycles increases the stability of the NHGe
compounds (Fig. 1). The strategy for synthesizing germylenes A–D
mainly involves metathesis reactions with the elimination of
small molecules.2 In these reactions the ring system functions
as an ‘innocent’ ligand. However, molecules containing Ge and
non-innocent ligands3 have been rarely reported.
Fig. 1 Representatives of NHGe with a two-coordinate germanium.
a-Iminopyridine (IP, Scheme 1) as a non-innocent ligand has
attracted much attention in radical chemistry. It easily can be
converted to the corresponding monoanionic p-radical or to the
dianions when treated with alkali or alkaline earth metals.4 Although
Scheme 1 Syntheses of compounds (IPGeCl)2 (1) and IPGe: (2).
the corresponding chemistry of transition metals and groups 1, 2, was prepared by the reaction of IPꢀ Mg(THF) with SiCl4 in the
2
and 13 elements has been well documented,4b,5 related chemistry of presence of Mg metal.4b The corresponding Ge compound has not
group 14 elements is yet to be explored. Recently, compound IP2Si(IV) been reported to date. Very recently, reports on autoionized group
14 compounds were published.6a,b The latter reference describes
a State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry
and Chemical Engineering, Xiamen University, Xiamen, Fujian, 361005, China.
(LMCl)+(MCl3)ꢁ (L = 2-[C(Me) = N(2,6-iPr2-C6H3)]-6-(MeO)C6H3N;
M = Ge(II), Sn(II)) by using a modified IP ligand, which shows
spontaneous dissociation of MCl2 (M = Ge(II) and Sn(II)) towards
the formation of the cationic GeCl+ and SnCl+ species, respectively.7
Herein, by employing the IP ligand, we present the synthesis of a
NHGe compound via an unusual homolytic cleavage of a C–C bond
by reduction of its germanium chloride precursor (IPGeCl)2 (1).
Compound 1 is isolated as yellow crystals in 60% yield by
the reaction of GeCl2(dioxane) with IPꢀꢁK+ in a 1 : 1 molar ratio
(Scheme 1). IPꢀꢁK+ is prepared in situ by treating IP with K metal
in THF. The 1H NMR spectrum of 1 clearly shows two septets
E-mail: hpzhu@xmu.edu.cn
b
¨
¨
Institut fur Anorganische Chemie, Georg-August-Universitat,
¨
Tammannstraße 4, 37077-Gottingen, Germany. E-mail: hroesky@gwdg.de,
dstalke@chemie.uni-goettingen.de
c Department of Chemistry, Centre for Theoretical and Computational Chemistry,
University of Oslo, Postboks 1033 Blindern, 0315 Oslo, Norway.
E-mail: heike.fliegl@kjemi.uio.no
† Electronic supplementary information (ESI) available: Experimental section,
details of crystal refinement and high resolution ICP-MS spectrometry, and
computational details. CCDC 953613–953616 (1–4). For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c4cc00251b
3356 | Chem. Commun., 2014, 50, 3356--3358
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Surprisingly, the reaction of 1 with 2.5 equivalents of KC8 in
toluene at ꢁ78 1C resulted in a novel NHGe species which was
isolated as red blocks in 48% yield (Scheme 1). The utilization of
toluene as a solvent is crucial and advantageous over THF and n-hexane
since the reduction in the latter two solvents was not successful.
The C(24) resonance of 2 (120.3 ppm) in the 13C NMR spectrum
is remarkably downfield shifted when compared with that of 1
(76.1 ppm), which exhibits the sp2 orbital hybridization of C(24). The
UV-visible spectrum shows a strong absorption band at 425 nm
which is comparable to those found for D (266, 360, and 460 nm).12
2 is highly soluble in organic solvents such as THF, toluene, and
n-hexane as well as sensitive to oxygen and moisture. The one-pot
reaction of the IP ligand, GeCl2(dioxane), and KC8 in a molar ratio of
Fig. 2 Molecular structure of 1. Anisotropic displacement parameters
depicted at the 50% probability level. Hydrogen atoms and iPr groups are
omitted for clarity. Selected bond lengths [Å] and [1] (only the values for the left 1 : 1 : 2 resulted in a brown oily mixture. We also tried the reaction of
backbone are listed): Ge(1)–Cl(1) 2.3604(11), Ge(1)–N(1) 2.0405(14), Ge(1)–N(2)
the dianionic salt (IP)2Na4(Et2O)4 (ref. 4a) with GeCl2(dioxane), but
1.8987(14), N(2)–C(6) 1.4607(19), C(6)–C(24) 1.580(2), C(5)–C(6) 1.512(2),
only 1 was obtained instead of the expected 2.
N(1)–C(5) 1.348(2); N(1)–Ge(1)–N(2) 80.65(6), N(1)–Ge(1)–Cl(1) 90.43(5),
C(6)–N(2)–Ge(1) 119.23(10), N(2)–C(6)–C(5) 107.45(12), C(5)–C(6)–C(24)
The X-ray single crystal structure13 of 2 is shown in Fig. 3, which
shows a five-membered GeC2N2 ring with a two-coordinate Ge atom.
The Ge atom is perfectly located in the N(1)–C(23)–C(24)–N(2) plane to
which the 2,6-substituted aryl ring is perpendicularly arranged. The
114.80(13), N(2)–C(6)–C(24) 114.43(12), N(1)–C(5)–C(6) 115.42(14).
(4.40 and 2.66 ppm) and four doublets (1.48, 1.10, 1.05 and 0.62 ppm) Ge(1)–N(1) (1.907(2) Å) and Ge(1)–N(2) (1.860(2) Å) bond lengths are
for four iPr groups of the aryl rings, indicating their asymmetric shorter than those of 1 (2.0405(14) Å and 1.8987(14) Å) while compar-
configuration. The X-ray single crystal structure analysis demonstrated able to those of D (1.8658(17) Å and 1.8650(18) Å) (see ESI†). The
1 to be a digermylene with a C–C single bond in the center N(2)–C(24) distance in 2 is 1.372(3) Å, which is much shorter than that
(C(6)–C(24)) (Fig. 2). The distorted tetrahedrally coordinated C(6) in 1 (1.4607(19) Å). Moreover, the C(23)–C(24) distance (1.369(3) Å) in 2
and C(24) show all features of sp3 hybridized carbon atoms. The is shorter by ca. 0.14 Å when compared with that in 1 (1.512(2) Å), but
C(5)–C(6) bond length (1.512(2) Å) indicates a single bond, while the still longer than a common C–C double bond (1.34 Å). In addition, the
pyridine rings retain the aromatic character. The C(6)–C(24) bond pyridinyl ring in 1 is more uniform than that of 2. All these changes
length (1.580(2) Å) is slightly longer than that of a standard C(sp3)– show that the bond lengths of 2 are averaged, which indicates a
C(sp3) (1.54 Å) bond. 1 is possibly formed via C–C coupling of two conjugated system. The altering bond lengths of compounds 1, 2, and
radical intermediates IPꢀGeCl. Such dimerization of p-radicals is also heterofulvene-like D are shown in Scheme S3 (see ESI†).
observed in the Al, Ga, Zn, and lanthanide metal (Yb and Sm)
To obtain a better view of the bicyclic heteroatom containing
derivatives containing redox-active ligands.5b,g,8 However, the synthesis unsaturated ring, theoretical calculations are carried out. The calcu-
of the silicon congener led to a mixture of products (IPSiCl3)2 (3) and lated current strength susceptibilities for selected bonds of compound
(IPH)(SiCl3) (4) when IPꢀꢁK+ was treated with SiCl4 (see ESI†).
2 are reported together with the obtained current pathways as shown
It is worth mentioning that in 2004, Fedushkin et al. reported in Fig. S8 (see ESI†). Diatropic currents are assumed to circle clock-
on (dpp-BIANꢀ)GeCl (dpp-BIAN = 1,2-bis{(2,6-diisopropylphenyl)- wise, while paratropic ones circle anti-clockwise. For the five-
imino}acenaphthene),9 a radical germylene compound, which membered Ge containing GeC2N2 ring, the current strength lies at
showed for the first time the stabilization of the germanium(II) 10.8 nA Tꢁ1, which is only 1 nA Tꢁ1 weaker than the one obtained for
monochloride with a radical-anionic non-innocent ligand. In
2008, our group reported on the reaction of [PhC(NtBu)2]Ge–Cl
with potassium leading to the formation of the Ge(I)–Ge(I) com-
pound [PhC(NtBu)2]2Ge2, which may be generated due to the
coupling of two [PhC(NtBu)2]Geꢀ intermediates.10 Such dimeriza-
tion is prevented by increasing the conjugation and the size of
the ring from four-membered CN2Ge to six-membered C3N2Ge.
A stable radical {[N(Dip)C(tBu)]2CH}Geꢀ (Dip = 2,6-iPr2C6H3) was
prepared by reduction of {[N(Dip)C(tBu)]2CH}Ge–Cl with sodium
naphthalenide or {[N(Mes)C(Me)2]CH}2Mg2 (Mes = mesityl), as
reported by Jones et al.11 It is worth noting that the ligands
employed in these compounds show good stabilization properties
toward low-valent Ge atoms, involving the metal–ligand electron
transfer process. The five-membered C2N2Ge ring of 1 is
dearomatized due to a lack of conjugation at C(6)–C(24). It is
Fig. 3 Molecular structure of 2. Anisotropic displacement parameters
depicted at the 50% probability level. Hydrogen atoms are omitted for
clarity. Selected bond lengths [Å] and [1]: Ge(1)–N(1) 1.907(2), Ge(1)–N(2)
1.860(2), N(2)–C(25) 1.436(3), N(2)–C(24) 1.372(3), C(23)–C(24) 1.369(3),
intriguing to study the formation of the final product when 1
was reduced with KC8.
N(1)–C(23) 1.393(3); N(2)–Ge(1)–N(1) 82.93(9), C(24)–N(2)–Ge(1)
115.11(16), C(23)–C(24)–N(2) 115.2(2) and C(24)–C(23)–N(1) 112.8(2).
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Centre for Theoretical and Computational Chemistry (CTCC), Grant
No. 179568/V30. Y.L. thanks the China Scholarship Council for a
¨
fellowship, and Prof. Dr G. Worner and Dr K. Simon for the high
resolution ICP-MS measurement. The Norwegian Supercomputing
Program (NOTUR) is highly acknowledged (Grant No. NN4654K).
Notes and references
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Scheme 2 Proposed mechanism for the formation of 2 (the pink arrows
demonstrate the switch of the lone pair on the Npyridinyl atom).
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The elongation of the central C–C bond of 1 implies a relatively
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dissociation energy of 1 to 2 was calculated to be 107.5 kJ molꢁ1, which
is much weaker than the binding energy of ethane (382.8 kJ molꢁ1) at
the same level of theory. However, the investigation of the C(6)–C(24)
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for the formation of 2 is still unclear. A two-electron transfer process of
1 may initially occur by the treatment with KC8. The two Ge–Cl bonds
and the central C–C single bond are homolytically cleaved (Scheme 2).
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are spontaneously triggered. Two equivalents of 2 are finally generated
as a stable product in this reaction. IPSn:, the analogue of 2 is
hitherto unknown, while a coupled species (Py-CHNSiRR0)2Sn
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In summary, we have shown an unusual formation of a NHGe
derived from its precursor (IPGeCl)2 (1). The implemented
radical properties of IP in its Ge derivative cannot be retained,
instead radical coupling and cleavage of bonds accompanied
with electron transfer occurred. This finding provides a new view
of the chemistry of carbene analogues.
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This work was supported by the Deutsche Forschungsge- 13 Some unusual residual electron density can be observed around Ge on
one of the molecules in the lattice, which may be due to the influence of
meinschaft (RO 224/60-1, H.W.R.) and the Danish National Research
some unknown impurity. However, supported by various methods of
Foundation (DNRF93) funded by Centre for Materials Crystallography
characterization including the high resolution ICP-MS spectrometry, the
(D.S.). H.F. thanks the Norwegian Research Council through the CoE
existence of compound 2 is definitely confirmed (see ESI†).
3358 | Chem. Commun., 2014, 50, 3356--3358
This journal is ©The Royal Society of Chemistry 2014