Facile access to a Ge(II) dication stabilized by isocyanides

Article abstract of DOI:10.1039/c6cc03789e

Herein, we introduce isocyanide as a ligand in main group chemistry and describe the facile isolation of a Ge(ii) dication. The reaction of 2,6-dimethylphenylisocyanide with GeCl₂ leads to the formation of a Ge(ii) dication with two [GeCl₃]^- molecules as counter anions. The dicationic Ge(ii) center is bound to four isocyanide ligands and also holds a lone pair of electrons. DFT calculations reveal that the dication is stabilized only by σ-donation from the four isocyanide ligands. Natural population analysis gives a charge of +0.74 on the Ge(ii) center, indicating that the positive charge is shared by the isocyanide substituents.

Full text of DOI:10.1039/c6cc03789e

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Facile access to a Ge(II) dication stabilized by isocyanides†
V. S. V. S. N. Swamy,a Sandeep Yadav,a Shiv Pal,c Tamal Das,b Kumar Vanka*b and Sakya S. Sen*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Herein, we introduce isocyanide as a ligand in main group
chemistry and describe the facile isolation of a Ge(II) dication. The
reaction of 2,6‐dimethylphenylisocyanide with GeCl2 leads to the
formation of a Ge(II) dication with two [GeCl3]‐ molecules as
counter anions. The dicationic Ge(II) center is bound to four
isocyanide ligands and also holds a lone pair of electrons. DFT
calculations reveal that the dication is stabilized only by σ‐
donation from the four isocyanide ligands. Natural population
analysis gives a charge of +0.74 on the Ge(II) center, indicating
that the positive charge is shared by the isocyanide substituents.
Bonding in Ge(II) dications is very intriguing. This is because
such dications hold a lone pair of electrons and three empty p
orbitals in the valence shell. Unlike neutral germylenes1 and
Ge(II) monocations2, where the valence electron counts are six
and four respectively, the valence electron count in the Ge(II)
dication is only 2. Such a bonding description indicates the
Scheme 1. Selected examples of Ge(II) dications, A‐G.
Moreover, isocyanides are isoelectronic to NHCs, and
therefore the isocyanide metal bond can be characterized as a
carbene‐type bond. There are few reports on the reactions of
isocyanides with low valent main group compounds such as
[:Ge(ArMe6)2] [ArMe6=2,6‐(2,4,6‐(CH3)3‐C6H2)2‐C6H3],10 or the
Mg(I) dimer11, with regard to an exploration of their reactivity.
However, isocyanides have never been explored as ligands for
realizing a compound with a low valent main group element.
Braunschweig and coworkers recently reported the reactions
of terminal borylene complexes with isocyanides to access a
base‐stabilized boraketenimine and a borylenediisocyanide
that manifested the multiple complexations of CO and
isocyanides to B; a bonding feature previously not known for
main group elements.12,13 We present herein the reaction of
2,6‐dimethylphenylisocyanide with GeCl2 leading to a Ge(II)
dication with two molecules of [GeCl3]‐ as counter anions. To
the best of our knowledge, this is the first example of a main
group compound/cation in a low oxidation state stabilized by
isocyanide ligands.
synthetic
challenge
to
isolating
such
dications.
Understandably, therefore, the number of Ge(II) dications that
have been isolated so far is very limited (Scheme 1). There are
mainly two synthetic strategies that have been employed for
isolating Ge(II) dications: (a) the use of a N‐heterocyclic
4
carbene (NHC)3 and bis‐carbene
(
A
and
B
)
and (b)
encapsulation of Ge(II) dications in macrocycles like cryptand
(C
), crown ethers ( and ), and azamacrocycles ( and
D
E
F
G
).5‐8 It
was only recently that Braunschweig et al. reported a third
approach, which relies upon the use of an electron rich
coordinatively unsaturated transition metal fragment
[Pt(PCy3)2] to stabilize Sn(II) and Pb(II) dications.9
In order to search for new neutral ligands for isolating
Ge(II) dications, we turned our attention towards isocyanides,
which are stronger σ‐donors and weaker π‐acceptors than CO.
a. Inorganic Chemistry and Catalysis Division, CSIR‐National Chemical Laboratory,
Dr. Homi Bhabha Road, Pashan, Pune 411008, India. E‐mail: ss.sen@ncl.res.in
b. Physical and Material Chemistry Division, CSIR‐National Chemical Laboratory, Dr.
Homi Bhabha Road, Pashan, Pune 411008, India
The reaction of 2,6‐dimethylphenylisocyanide with
GeCl2∙dioxane at ambient temperature led to a color change
from colorless to brown‐red and resulted in the Ge(II) dication
c. Indian Institute of Science Education and Research, Pune, Dr. Homi Bhabha Road,
Pashan, Pune 411008, India. E‐mail: k.vanka@ncl.res.in
‐
[(RNC)4Ge:]2+[:GeCl3]2 (
1) (Scheme 2). Compound
1
is soluble
in toluene, THF, and dichloromethane, but insoluble in n‐
†This paper is dedicated to Professor Holger Braunschweig on the occasion of his
55th birthday
Electronic Supplementary Information (ESI) available: Synthesis, X‐ray
hexane. In the 1H NMR spectrum of
1
, a sharp singlet
crystallographic data (CCDC 1473203) and theoretical details for compound
1.
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resonance appeared at δ 1.92 ppm for the 24 methyl protons, 2.277 Å and 2.278 Å, while the average Cl−Ge−ClVibewonArdticalenOgnllienes
DOI: 10.1039/C6CC03789E
which were shifted significantly upfield in comparison to those are 96.10° and 95.55°, respectively. These bond lengths and
of 2,6‐dimethylphenylisocyanide (δ 2.06 ppm).
angles are in good agreement with the previously reported
[:GeCl3]− counter anions.15,18
Scheme 2. Synthesis of Ge(II) dication, 1.
Pale yellow crystals were grown from a saturated toluene
solution of
in the monoclinic space group P21/c.14 The asymmetric unit of
contains a four‐coordinate Ge(II) dication with two [GeCl3]‐
1 at –35 °C in a freezer. The compound crystallized
1
Figure 1. Molecular structure of 1. Anisotropic displacement parameters are depicted
at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond
distances (Å) and bond angles (deg): Ge1–C1 2.033(4), Ge1–C2 2.044(4), Ge1–C3
2.044(4), Ge1–C4 2.065(5); N1–C1–Ge1 177.3(4), N2–C2–Ge1 176.7(3), N3–C3–Ge1
170.3(3), N4–C4–Ge1 178.1(4), C1–Ge1–C2 111.51(15), C1–Ge1–C3 108.29(15), C2–
Ge1–C3 116.73(16), C1–Ge1–C4 112.16(16), C2–Ge1–C4 104.10(15), C3–Ge1–C4
103.78(16), C1–N1–C10 178.5(4), C2–N2–C11 177.4(4), C3–N3–C20 174.9(4), C4–N4–
C29 178.2(4).
units as counter anions balancing the charge (Fig. 1). The two
[GeCl3]− ions in the asymmetric unit exhibit no significant
bonding interaction with the Ge(II) dication. The closest
approach of the chloride atoms of one of the [GeCl3]‐ units
from a methyl hydrogen is 2.941 Å. The Ge(II) dication is bound
to four isocyanide ligands and assumes a distorted tetrahedral
geometry, as all the bond angles around the Ge atom range
from 104.10(15)° to 116.73(16)°. Therefore, the coordination
The formation and bonding of
1 was investigated
environment of the Ge(II) cation in
in , where the Ge(II) atom exhibits pyramidal geometry.
1 is very different from that
computationally using DFT. All the calculations were done at
the PBE/TZVP level of theory [for more details, please see the
Supporting Information (SI) file]. We have done the geometry
A
Roesky and coworkers reported the isolation of a four‐
coordinate Ge(II) cation [{(LB)GeCl}+{GeCl3}−] from the reaction
of a substituted Schiff base 2,6‐diacetylpyridinebis‐(2,6‐
diisopropylanil) with GeCl2. However, the compound exhibited
optimization of compound
1 for the ground state singlet and
triplet geometries. The singlet geometry is seen to be more
stable than the triplet by 48.6 kcal/mol. The optimized
geometry (shown in Figure SI1) and the geometrical
parameters have been given in the SI. As the structure
indicates, the central germanium atom is tri‐coordinated with
a square based pyramidal geometry.15 Therefore,
1 is a very
unusual example of a Ge(II) atom exhibiting a distorted
tetrahedral geometry as well as holding a stereochemically
inactive lone pair of electrons. Despite the rather bulky
‐
three isocyanide ligands, with the two GeCl3 units coming
close to the Ge2+ center. This is slightly different from the X‐ray
structure where the four isocyanide ligands were seen to be
equally coordinated to the central germanium. So as to
examine the possibility of displacement of one isocyanide
ligand, variable temperature (–70 to 70 °C) 1H NMR
character of the isocyanide ligand, the Ge–C bond lengths in
1
[2.033(4), 2.044(4), 2.044(4), and 2.065(5) Å] fall within the
average Ge–C single bond range (1.90–2.05 Å)16 and are
marginally shorter than the Ge–C bond length in
A (2.070(6)
Å)3 and in [tBuNC→{:Ge(ArMe6)2}] (2.075(3) Å).10 For the four
isocyanide ligands, the N≡C triple‐bond lengths lie in a very
narrow interval from 1.138(5) Å to 1.142 (5) Å and are about
the same as in most free isocyanides (1.14 to 1.16 Å).17 Three
of the Ge‐C‐N angles are nearly linear, ranging from 176.7(3)°
to 178.1(4)°. However, the remaining Ge‐C‐N angle (170.3(3)°)
is slightly bent. The isocyanide ligands are almost linear about
the nitrogen atoms, with the C‐N‐C angles in the range
174.9(4)°‐178.5 (4)°. It should be noted that the non‐increase
in the C–N bond lengths, and the virtually linear C‐N‐C angles
indicate nearly unperturbed isocyanide molecules and negate
the possibility of any backbonding from Ge to C. In each of the
spectroscopic analysis of
1 was carried out. However, the VT
analysis did not give any evidence of displacement of the
isocyanide ligand.
The HOMO and LUMO of compound
1 were found to be
localized on the phenyl group of the isocyanide ligand that had
the weaker coordination to the central germanium (Fig. 2(a)).
Also, the structure of the germanium dication (12+) without
counter anions was optimized in order to determine the
nature of the MOs on the dicationic Ge(II) center. It was seen
that the HOMO of the germanium dication had the maximum
lone pair contribution (Fig. 2(b)). This result, in conjunction
with the HOMO and LUMO obtained for 1, indicates that the
nature of the frontier orbitals is significantly changed when the
ion pair complex is considered, rather than just the dication:
[:GeCl3]− anions of
germanium atom, resulting in a trigonal pyramidal shape of
1, three chlorine atoms are bound to the
the anion. The average Ge−Cl bond lengths for the anions are
2 | J. Name., 2012, 00, 1‐3
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there is delocalization away from the Ge2+ center when the ion
pair is formed. This is likely due to inter ionic interactions and
may be of relevance only for the salt in the solid state, as the
ions are likely to be separated to a greater extent in solution.
The NBO charge analysis was also done for 1, and the results
indicate that the total positive charge is not localized at the Ge
center. The charge on Ge was found to be +0.74 and the
charge on the RNC groups bonded with the germanium were
seen to carry +0.27 charge on average, indicating that the
positive charge was delocalized to some extent in the complex.
Notes and references
1
View Article Online
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2
For reviews in germylene monocation, see: (a) V. Y. Lee and
A. Sekiguchi, Organometallic Compounds of Low‐coordinate
Si, Ge, Sn and Pb: From Phantom atom to Stable Compounds,
Wiley, 2010, Chapter 1; (b) V. S. V. S. N. Swamy, S. Pal, S.
Khan and S. S. Sen, Dalton Trans., 2015, 44, 12903–12923; (c)
T. A. Engesser, M. R. Lichtenthaler, M. Schleep, and I.
Krossing, Chem. Soc. Rev., 2016, 45, 789–899.
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8
(a)
(b)
Figure 2. (a) HOMO of 1 and (b) HOMO of 12+ (contour value ±0.03). Hydrogen atoms
have been omitted for clarity.
The NBO analysis was also employed to understand the
bonding in the complex (see the SI). For 1, it was seen that out
of the four RNC groups, two had stronger interactions with the
Ge(II) center. For these two, the two most important orbitals
corresponded to the relevant σ and π interactions between
the central germanium and the isocyanide ligand, with the σ
and π energy contributions for the two Ge–C bonds being
118.0 and 48.0, and 21.0 and 2.0 kcal/mol respectively. Also,
the % contributions of the σ interaction for the two Ge–C
bonds were seen to be 86% and 96%. Therefore, the data
indicates that σ bonding is dominant in the interaction
between germanium and the RNC groups (more information is
provided in the ESI).
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In 2008, Baines et al. introduced macrocyclic ligands in
order to realize the Ge(II) dication.5 Research in this area
underwent remarkable progress in the following years.6‐8 In
the current work, we have introduced isocyanide as a ligand in
main group chemistry and demonstrated that 2,6‐
dimethylphenylisocyanide reacts with GeCl2 to give a Ge(II)
12 H. Braunschweig, R. D. Dewhurst, F. Hupp, M. Nutz,
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dication (1) in a single step. The Ge(II) atom in the dicationic
fragment is bound to four isocyanide ligands, possesses a
stereochemically inactive lone pair of electrons and exhibits a
distorted tetrahedral geometry. The Ge(II) atoms in the
[:GeCl3]− units assume a pyramidal geometry. The ease of
accessing the Ge(II) dication through this route will open up a
synthetic avenue for obtaining non‐metallic cations and
dications.
This work was supported by CSIR‐NCL (MLP029926) and
SERB, India (SB/S1/IC‐10/2014). VSVSN, SY, and TD thank CSIR,
India and SP thanks UGC, India for their research fellowships.
KV and TD acknowledge “MSM” (CSC0129) and DST
(EMR/2014/000013) for funding. We thank the reviewers for
their valuable insights that has led to the improvement of the
manuscript.
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