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Cation radicals of organogermatranes.

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Full Paper
Received: 29 April 2010
Revised: 7 July 2010
Accepted: 7 July 2010
Published online in Wiley Online Library: 4 October 2010
(wileyonlinelibrary.com) DOI 10.1002/aoc.1710
Cation radicals of organogermatranes†
Saida Soualmia , Luba Ignatovich b and Viatcheslav Jouikovc∗
Cation radicals (CRs) of 1-substituted germatranes were generated by one-electron electrochemical oxidation and studied by
real-time EPR spectroscopy for the first time. In contrast to CRs of trialkylamines, the CRs of germatranes are much more stable
and can be observed in acetonitrile solution at 253 K. As follows from g-factors and hfc constants, spin density in the CRs of aryl
germatranes is localized mainly on the axially directed pz orbital of N atom, pointed inside the atrane cage. EPR spectroscopy
and Fermi contact coupling from DFT B3LYP/LANL2DZ calculations reveal an interesting feature: no contribution from the Ge
atom was found in the spin delocalization in these species. Contrary to the neutral germatranes and to what was expected
from the qualitative estimations, the nitrogen atom in the CR does not adopt an exo-configuration and is practically planar.
Experimental hfc constants and DFT B3LYP/LANL2DZ calculations support the observed feature: the geometry around the N
atom in CR is close to planar whereas the Ge atom undergoes remarkable reorganization, approaching a trigonal bipyramid
in benzyl germatranes and tetrahedral configuration in aryl germatranes. 1-Substitution of germatranes with the groups with
low own ionization potential (R2 NC6 H4 -, where R = Me, Et) inverts the roles of the fragments – upon electrooxidation, these
c 2010 John Wiley & Sons, Ltd.
compounds behave as corresponding germatranyl-substituted organic derivatives. Copyright Keywords: germatranes; cation radicals; electrooxidation; EPR; spectroelectrochemistry
Introduction
Appl. Organometal. Chem. 2010, 24, 865–871
∗
Correspondence to: Viatcheslav Jouikov, UMR 6510 CPM, University of Rennes
1, 35042 Rennes, France. E-mail: vjouikov@univ-rennes1.fr
†
This article is published in Applied Organometallic Chemistry as a special
issue on In Memoriam Professor Edmunds Lukevics, edited by Luba Ignatovich,
Latvian Institute of Organic Synthesis, Aizkraukles 21, Riga, LV-1006, Latvia, and
Vladimir Gevorgyan, University of Illinois at Chicago, Department of Chemistry,
Chicago, United States.
a University of Setif, 19000 Setif, Algeria
b Latvian Institute of Organic Synthesis, LV-1006 Riga, Latvia
c UMR 6510 CPM, University of Rennes 1, 35042 Rennes, France
c 2010 John Wiley & Sons, Ltd.
Copyright 865
Since their first synthesis in 1965 by Mehrotra and Chandra,[1]
the different aspects of germanium derivatives of the large family
of metallatranes, germatranes, have been extensively studied.
One of the most intriguing features of these compounds is the
intramolecular coordination existing between N and Ge atoms and
determining their specific properties in many contexts. Structural
characteristics,[2] mass spectroscopy,[3] NMR, UV, dipole moments
and other physico-chemical properties as well as synthesis and
reactions[4] of germatranes have been studied and reviewed.[5]
A great deal of reported work attests that the most important
practical properties of germatranes are certainly those related to
their versatile and high biological activity.[6] Nowadays numerous
organogermanium and coordinative germanium compounds
possessing neurotropic, analgesic, hypotensive, fungistatic,
bactericidal, antiviral, antimalarial, antitumour, radioprotective,
interferon-inducing and immunomodulating properties are
synthesized.[6,7]
Phenylsiloxygermatranes are effective for memory enhancement: 80% inhibition of retrograde amnesia was demonstrated by
triphenylsiloxygermatrane.[8] Benzylgermatrane and its 3-Br and
4-Br derivatives also improve memory processes and completely
prevent retrograde amnesia in mice caused by electroshock.[9]
In addition, bromobenzyl germatranes have high anesthetic and
anti-Corazol activity. Germatranes with cysteamine or methylcysteamine substituents were reported to have remarkable radioprotective properties, which are important for the applications in the
field of chemical methods for radiation protection.[10] Membraneprotective and antioxidant activities of 1-(isopropoxy)germatrane
account for the wound healing effect shown on rats with acetateinduced stomach ulcers.[11] 1-(Chloromethyl)germatrane[12]
and 1-hydroxygermatrane carboxylic salts[13] have pronounced
antiviral activity. Also, immunomodulating activity was shown
by germatranes: they stimulate spontaneous IgG synthesis and
enhance cooperative B cell response; at the same time, they inhibit
the proliferative response to T-cell mitogen and to allogenic
lymphocytes in mixed cultures.[14]
The interest in germatranes is enhanced by the fact that
their 1-organyl and 1-heteryl derivatives are usually substantially
less toxic than corresponding silicon analogs, with the LD50 of
germatranes exceeding 1000 mg kg−1 .[6] Even the most toxic
of all germatranes, 2-thienylgermatrane, is 55 times less toxic
than 2-thienylsilatrane, and 1-phenylgermatrane is 100 times
less toxic compared with its silicon analog.[15] Biological activity
of germatranes is governed by the nature of the substituent on
the germanium atom, e.g. 2-thienylgermatrane is a very toxic
material (mean lethal dose in white mice LD50 = 16.5 mg kg−1 )
whereas its furan analog, 2-furylgermatrane, is virtually nontoxic
(LD50 = 2050 mg kg−1 [6a] ). Introduction of a second thiophene
ring into 2-thienylgermatrane lowers the acute toxicity by about
27 times (LD50 of 2,2 -bithienylgermatrane is 447 mg kg−1 [2b] ).
The acute toxicity of p-N-diethyl- and p-N-dimethylaminophenyl
germatranes for white mice is even lower, the LD50 being 3250
and 3680 mg kg−1 , respectively.[6a]
On the other hand, redox properties of the substrates are
often in relation with their biological activity, so the study of
redox behavior of germatranes would be very useful for their
structure–activity correlations. However of all metallatranes, even
S. Soualmi, L. Ignatovich and V. Jouikov
with a standard rectangular cavity. The modulation frequency
was 100 kHz; g-factors were corrected using DPPH as standard
(g = 2.0036 ± 0.0003). Microwave power was adjusted to 2–8
mW to maintain the EPR signal intensity below power saturation.
The EPR capillary spectroelectrochemical cell was a three-electrode
version of the cell described previously.[20] Post-processing and
simulation of the EPR spectra were carried out using WINEPR
System and WINEPR SimFonia software provided by Bruker.[21]
Structure optimization of neutral germatranes and their
cation radicals and frequency analysis in order to check the
absence of negative frequencies were performed by DFT
B3LYP/LANL2DZ//HF/6-311G calculations using GAUSSIAN 03
software.[22]
Scheme 1. 1-Organyl germatranes studied.
silatranes – probably the most explored representatives – have
been very little studied by electrochemistry;[16,17] therefore even
less is known on the electrochemical behavior of germatranes. The
first paper providing the potentials of electrochemical oxidation
of three germatranes was published in 1973 by E. Lukevics and
coworkers.[16] More recently, Lukevics and colleagues reported
anodic oxidation of benzyl-substituted germatranes by cyclic
voltammetry supported by DFT calculations.[18]
So far, electron paramagnetic resonance (EPR, or ESR) spectroscopy had not been used for studying germatranes, although
this method could provide very valuable information on where the
spin is localized (N, Ge, atrane cage atoms) and on intramolecular
electronic interactions.[19] The values of hfc constants determined
through EPR spectra of paramagnetic species are directly related
to the spin density on the corresponding atom and to the relative
contributions of s- and p-type orbitals in its hybridization. Thus the
geometry around such a spin-interacting center can be accurately
estimated by EPR spectroscopy in solution.
For studying redox properties, the nature of primary products
of electron transfer and the geometry of spin-carrying centers
in germatranes, real-time EPR spectroelectrochemistry seems to
be a very suitable method permitting controlled oxidation of
germatranes with simultaneous detection and study of the cation
radical or radical species thus formed.
In the present communication we report electrochemical oxidation of 1-substituted germatranes 1–6 resulting in germatrane
cation radicals that were detected by cyclic voltammetry and
studied by real-time CW EPR-spectroelectrochemistry (Scheme 1).
Reagents and Solutions
The synthesis of germatranes was described previously.[9,23]
Analytical grade CH3 CN (SDS) was twice distilled under inert
gas (Ar) over CaH2 , the solvent distilled the second time was
collected over activated 4 Å molecular sieves. Water content in the
solvent was checked by Karl Fischer titration to be not higher
than 18–20 ppm. Electrochemical-grade tetrabutylammonium
salts (Fluka) were dried and kept in a desiccator over P2 O5 and
used without further purification.
Results and Discussion
Voltammetry
Cyclic voltammetry of 1-organo germatranes 1–6 was studied at a
glassy carbon (GC) and Pt disk electrodes in CH3 CN–0.1 M Bu4 NBF4
solution. Voltammograms of aliphatic amines are often distorted
by adsorptional interactions of nitrogen with the electrode surface, especially metallic, but the oxidation of all germatranes of
this reaction series shows a well-shaped peak at both GC and Pt
electrodes (Fig. 1), though the reproducibility at GC electrode was
somewhat better. For benzyl germatranes, there is also a second
Experimental
Instrumentation
866
All electrochemical experiments (cyclic voltammetry and two-step
chronoamperometry) were performed using a PAR 2273 scanning
potentiostat piloted with PowerSuite software. Glassy carbon
disk working electrodes (0.7 and 3 mm diameter) or a 0.5 mm
Pt disk electrode were used, carefully polished and rinsed with
acetonitrile and Et2 O before each run. As counter-electrode, a
glassy carbon rod (2.5 × 75 mm) was used, and the potentials
measured were referred to a Ag–0.1 M AgNO3 in CH3 CN reference
electrode, connected with the analyte through Luggin capillary
filled with CH3 CN–0.1 M Bu4 NPF6 . Bu4 NPF6 or Bu4 NBF4 (as 0.1
M solutions in acetonitrile) were used as supporting electrolytes.
The IR-compensation facility of the potentiostat was always used
for correcting ohmic drops in the solution. The temperature was
maintained at 20 ◦ C unless otherwise specified in the text.
EPR spectra were registered on a Bruker EMX (X-band) spectrometer with a Gunn diode at the working frequency 9.46 GHz, coupled
wileyonlinelibrary.com/journal/aoc
Figure 1. Scan rate normalized cyclic voltammograms of (a) benzyl
germatrane 1 (v = 32 V s−1 ); (b) p-Br-benzyl germatrane 2 (v = 1 V s−1 );
(c) o-bromo-phenyl germatrane 4 (v = 40 V s−1 ). Glassy carbon disk
electrode. Solvent: CH3 CN–0.1 M Bu4 NBF4 .
c 2010 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2010, 24, 865–871
Cation radicals of organogermatranes
Table 1. Parameters of anodic oxidation of germatranes 1–6 at a GC
disk electrode
Compound
Ep (V)a
Ep − Ep/2
(mV)
Ep /lg(v)
(mV)
1
2
3
4
5
6
0.872
0.899
1.239
0.956
0.429
0.423
59
83
95
72
73
77
37
33
41
a
25
28
n
1.1
1.1
1.0
0.9
1.1
1.0
Peak potential at v = 1 V s−1 vs Ag–0.1 M AgNO3 in CH3 CN.
Appl. Organometal. Chem. 2010, 24, 865–871
EPR Spectroelectrochemistry
Cation radicals of o-bromo-phenyl germatrane 4 were generated
in the spectroelectrochemical EPR cell when applying potential
E = 0.92 V to its Pt micro spiral working electrode. The end-to-end
width of the spectrum of 4+• formed by a group of nine rays is
quite large for an organic radical, = 165 G (Fig. 2). The isotropic
g-factor of the spectrum is g = 2.00446 which is far from those of
known Ge-centered radicals[28] and is typical for N-centered radical
species.[19] The nitrogen hfc constant a(N) = 18.62 G is close to but
does not exceed ∼20 G, the value above which the configuration
around the N atom starts showing non-planarity. Therefore the
nitrogen atom in the CR of this germatrane lies practically in the
plane formed by three adjacent CH2 carbons. A very remarkable
feature of this cation radical is that two α-methylene protons
differ by more than 40 times in the interaction with the spin, as is
seen from the corresponding hfc constants: a(α-Haxial ) = 37.09 G
and a(α-Hlateral ) = 0.88 G. It characterizes 4+• as a rather rigid
structure with the orientation of corresponding lateral σ (C–H)
bonds practically in the N(CH2 -)3 plane and being orthogonal to
the spin carrying n (pz ) orbital of N atom whereas axial σ (C–H)
bonds, almost parallel to the N–Ge–C axis of the cation radical,
show much stronger spin-orbital coupling. β-Methylene protons
are averaged in their interactions with the unpaired electron and
all six of them show the same hfc constant, a(β-H) = 1.185 G.
c 2010 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
867
oxidation peak at more positive potentials (about Ep ≈ 1.8–2.2 V,
Fig. 1b). Although the voltammograms of these compounds do
not reveal strong heterogeneous influence, their first peak width
(E = Ep − Ep/2 ≈ 60–100 mV) is larger than 58 mV, the theoretical value for an electrochemically reversible electron transfer. Also,
the Ep of 1–4 increases by 33–45 mV per decade of the scan rate
(Table 1), which is slightly more than the 30 mV expected for an EC
process with the CRs issued from reversible electron transfer and
undergoing fast first-order reaction. The Ep −lg(C) behavior of 1–4
in the interval of concentrations 10−4 ≤ C ≤ 2 × 10−3 mol L−1 did
not show any visible dependence of Ep on C and ruled out bimolecular self-reactions of putative electrogenerated cation radicals. In
addition, at reasonably high scan rates all the compounds studied
show a reverse, cathodic, peak (Fig. 1), even at room temperature.
Therefore, given the above, electron withdrawal from these compounds can be described as both electrochemically and chemically
reversible. A slight coherent increase in the values of characteristic
potential-related parameters of oxidation of the considered germatranes [Ep /lg(v), Ep −Ep/2 and Ep a −Ep c ] therefore seems not
to be an issue of heterogeneous interactions with the electrode
surface but is related to non-negligible reorganization energy
accompanying the formation of CRs from neutral molecules.
Peak currents ip of the first step of oxidation of 1–6 are linear
with concentration of the substrate and also with the square root
of the scan rate (ip /v 1/2 = const.), attesting diffusional control
of the limiting currents at this potential. The absolute number
of electrons transferred at the first oxidation step, n (Table 1),
was determined by combining the parameter ip /v 1/2 from cyclic
voltammetry at different scan rates with the Cottrell coefficient
of i –t curves obtained from double-step chronoamperometry
at the same electrode and the same solution.[24] The n-value
determination is in agreement with the rate-determining
deprotonation of the CRs. In fact, although the basicity of
germatranes is attenuated by N → Ge coordination, the CH acidity
of electrogenerated CRs is high enough to protonate starting
germatrane stoichiometrically, thus decreasing the apparent
current of its oxidation by two (n ≈ 0.5 at v < 0.1 V s−1 [18] ). When
this auto-protonation of the substrate is overridden by increasing
the scan rates above v ≥ 0.5 V s−1 , the n-value becomes close to
unity for all germatranes of this reaction series (Table 1).
At higher scan rates, a reverse peak appears on the voltammograms of p-Br-benzyl germatrane 2 and, to a greater extent, of nonsubstituted benzyl germatrane1 (Fig. 1a, b). The oxidation peak
width of 1 is smallest in the reaction series (Ep −Ep/2 in Table 1), suggesting it is closer to pure Nernstian system, but the cathodic signal
which appears for 1 at v ≈ 30 V s−1 does not grow much upon
further increasing the scan rate. Instead, the Ep /lg(v) slope,
approaching 30 mV at low scan rates, increases at v ≥ 30 V s−1 as
electron transfer kinetics becomes the limiting step of the process.
Voltammograms of o-bromo-phenyl germatrane 4 under
similar conditions (some THF was added to overcome low
solubility of this compound, which caused increased ohmic
drops) and different scan rates are shown in Fig. 1c. The peak
current ip in the cathodic half-scan is higher at the same scan rate
(v = 1 V s−1 ) than for benzyl germatranes and it rapidly increases
to form a perfectly reversible system at about 25 V s−1 . Although
the voltammograms look more as arising from a quasi-reversible
oxidation, the CRs of this germatrane are definitely more stable
than those of 1 and 2. Very similar behavior was observed for
o-F-phenyl germatrane (Table 1).
Thus one-electron oxidation of germatranes 1–4 results in corresponding germatrane cation radicals even though the kinetics of
electron transfer is somewhat slow. Potential-determining deprotonation of these CRs can be limited at relatively slow scan rates,
which creates favorable conditions for stabilizing these species at
low temperatures and studying them by EPR spectroscopy.
Anodic oxidation of 1-[4-(dialkylamino)phenyl]-germatranes 5
and 6 also shows the oxidation peak whose cathodic counterpart
appears at v = 1 V s−1 , attesting the formation of primary
cation radicals. However the electrochemical behavior of these
germatranes has several peculiarities compared with 1–4. First,
their oxidation potentials are unusually low, Ep = 0.423 V and
0.429 V vs Ag–AgNO3 for 5 and 6, respectively, falling rather
into the range of oxidation potentials of substituted anilines.[25]
Also, their peak widths are much smaller than those for Br- and
F-substituted phenyl germatranes, Ep − Ep/2 = 73 and 77 mV for
5 and 6, respectively (Table 1).
The fact that the oxidation potentials of germatranes are higher
compared with Ep s of silatranes[16] and, to an even greater extent,
of trialkylamines,[18,26] indicates stronger involvement of the lone
pair of N in N→Ge relative to N→Si dative interaction, which agrees
with the relative strength of these intramolecular bonds.[5c,27]
As a consequence, nitrogen in germatranes is less available for
oxidation and protonation.
S. Soualmi, L. Ignatovich and V. Jouikov
a
b
Figure 2. EPR spectrum of the cation radical of o-bromo-phenyl germatrane, 4+• , electrogenerated in CH3 CN–0.1 mol L−1 Bu4 NPF6 at E = 0.92 V;
modulation amplitude a = 0.4 G; T = 253 K.
868
Another remarkable feature of 4+• is that the contribution of
the aromatic moiety into spin delocalization is not seen in this
CR, or if there is any, the hfc constants from the aromatic protons
are below the resolution of the spectrum. This means that N pz
orbital (the lone pair of nitrogen), providing main electron density
to the HOMO of germatranes, and hence losing one electron
upon oxidation, is still axially oriented inside the silatrane cage
when becoming SOMO – the orbital carrying unpaired electron.
The conjugation of this orbital with the lateral fragments (αand β-CH2 groups), especially with the spatially nearest orbitals
of three axial σ (C–H) bonds, stabilizes the CR. Thus the space
occupied by unpaired electron is hidden inside the atrane cage
and protected from intermolecular radical interactions by lateral
aliphatic branches. Besides this, there must be another effect
providing an additional stabilization to this species because 4+•
has a substantially longer lifetime than the CR of parent Et3 N
(both CRs undergo first-order deprotonation[18,26] and have similar
carbon environment around N). Indeed, in Et3 N+• free rotation
about N–C and C–C bonds is possible and therefore the same spin
stabilization is enabled as in 4+• [EPR spectrum of Et3 N+• shows
different pattern: a(N) = 20.8 G and 6 × a(α-H) = 19 G[29] ], but this
CR is not detectable by EPR under these conditions.
A very similar spectrum was observed for the CR of o-F-phenyl
germatrane 3+• , electrogenerated at E = 1.22 V under the same
conditions. Its g-factor is slightly smaller, g = 2.0042, probably
because of the electron-acceptor effect of F in the aromatic ring.
The general pattern (nine rays) is similar to and the spectral width
is also practically the same as for 4+• , = 160 G. Nitrogen and
proton hfc constants are quasi-identical in both spectra, therefore
showing a similar feature of interactions of the unpaired electron
with α- and β-CH2 protons of the atrane cages in 3+• and 4+• .
Similarly, no visible contribution of the aromatic ring to the spin
delocalization is seen for 3+• .
Less stable than the CRs of aryl germatranes, the cation radical
of benzyl germatrane 1+• needed slightly lower temperature for
its stabilization and detection by EPR spectroscopy (Fig. 3). The
first remarkable feature of the EPR spectrum of electrogenerated
CR of this germatrane is that it is substantially narrower than
those of o-Br- and o-F-phenyl germatranes (end-to-end width is
≈ 27–28 G only) and it does not show large hfc constants
characteristic for the aryl substituted CRs. The spectrum is a
little noisy but its Fourier transform filtering and reconstruction
using Bruker SimFonia software allowed extraction of the essential
wileyonlinelibrary.com/journal/aoc
3350
3370
3390
H, Gauss
Figure 3. EPR spectrum of the cation radical of benzyl germatrane 1+•
generated in CH3 CN–0.1 mol L−1 Bu4 NPF6 at E = 0.85 V: (a) experimental
and (b) simulated using the parameters in the text. Modulation amplitude
a = 0.4 G; T = 233 K.
parameters. The spectrum is centered at g = 2.0035, indicating
that it is not a Ge-localized radical.[28] Nitrogen hfc constant
is rather small now, a(N) = 4.337 G only, but in contrast to
the CRs 3+• and 4+• , a large contribution from the benzyl
fragment in spin-orbital interactions is seen: a(p-H) = 8.426 G,
2 × a(o-H) = 2.907 G, 2 × a(m-H) = 0.972 G for aromatic ring
protons and 2 × a(H) = 0.31 G for the protons of benzylic
methylene group. This feature outlines a quite different spin
delocalization, mostly on the aromatic ring with only very little of
the unpaired electron density remaining on the atrane nitrogen.
The benzylic methylene group in this system plays the key role
of an electronic bridge. Its σ (Csp3 -Ge) orbital, affected by threecenter N–Ge–C bonding and whose orientation is favorable for
conjugation with the π pz electrons of the aromatic system, allows
the spin density to be transmitted from N atom (where free electron
appears upon withdrawal of one electron from the doublet of N) to
the phenyl group so as to form the delocalized thermodynamically
more stable spin configuration. In 1-aryl substituted germatranes
3 and 4 such a ‘bridge’ is absent since σ (Csp2 -Ge) orbital is
orthogonal to the orbitals of aromatic π -system which therefore
cannot accommodate spin density.
This mechanism involves non-negligible reorganization energy
(both internal/structural and external/solvent related) accompanying the formation of this CR from the neutral molecule; for this
reason, electron transfer during the oxidation of benzyl germatranes has rather quasi-reversible character, in good agreement
with the results of cyclic voltammetry.
The EPR spectrum of the cation radical 5+• , electrogenerated at
E = 0.35 V in the CH3 CN/0.1 M Bu4 NPF6 solution, is quite different
from those observed for the CRs of aryl germatranes 3 and 4.
For the CR of 1-[4-(dialkylamino)phenyl]-germatrane, the isotropic
g-factor of 2.003 is still close to those of the CRs of 1–4; also,
the characteristic pattern from one N atom is present. However
in contrast to the germatranes of similar structures 3 and 4, the
EPR spectral signature of 5+• also shows spin coupling with the
c 2010 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2010, 24, 865–871
Cation radicals of organogermatranes
aromatic ring (ao−H = 7.309 G and am−H = 0.65 G) and with
two N-alkyl groups (hfc for ethyl group protons: aCH2 = 5.718 G
and aCH3 = 1.89 G). The hfc constant from the nitrogen triplet
(aN = 11.73 G) is smaller than in the CRs of 3 and 4 and
corresponds to those in CRs of dialkylamino benzenes.[19] Neither
is a contribution from Ge atom from the atrane cage nitrogen
seen in the EPR pattern. Similar electron distribution follows
from the EPR spectrum of 6+• . Thus the oxidation of 1-[4(dialkylamino)phenyl]-germatranes follows the scheme typical for
the substituted anilines, with germatranyl moiety only acting as a
weak electron-donor substituent.
DFT Calculations
Geometry of germatranes 1–4 and of their cation radicals was
optimized following a combined procedure, first at the HF/6-311G
level, well reproducing general geometric parameters, and then
(geometry/NBO) at the DFT B3LYP/LANL2DZ level, which better
accounts for long-term and delocalizing electronic interactions.
Final optimized structures were checked by frequency analysis at
the same level for the absence of imaginary vibrations.
In neutral molecules of 1–4 the HOMO, mostly built from
n-electrons of nitrogen, is much higher than the next lowerlying occupied orbital, HOMO-1, which is built with the important
contribution of the orbitals of ArCH2 (1, 2) or Ar (3, 4) fragments.
Therefore in the first place it is the nN orbital that is affected
by electrooxidation providing the electron to be removed. Its
involvement in the nN → σ ∗ (Ge–C) interaction does not change
the situation compared with tertiary amines. However the Fermi
contact term and NBO analysis from B3LYP/LANL2DZ calculations
on 1+• revealed an interesting feature that nitrogen atom in this
CR does not carry any remarkable part neither of spin nor of
the charge density (Fig. 4). The unpaired electron in this species is
almost entirely accommodated by benzyl moiety whereas positive
charge is mostly localized on Ge atom (Fig. 4, Table 2). Compared
with the neutral molecule, trans-annular N → Ge interaction in 1+•
is enhanced because of this charge, which provokes shortening
of the N–Ge distance upon electron withdrawal (Table 2). Also,
the configuration of Ge atom in 1+• is much closer to a trigonal
bipyramid (all three ∠O–Ge–C angles are approaching 90◦ and
the difference between Ge–N and Ge–C distances is smaller).
Finally, the configuration of the methylene carbon connecting
the germatrane fragment with the Ph ring is closer to the nondistorted Csp3 geometry, arising from charge-induced contraction
of the structure of 1 when passing to 1+• .
Figure 4. Top: spin distribution in 1+• and flattening of Ge atom (by DFT
B3LYP/LANL2DZ//HF/6-311G calculations); the involvement of O atoms in
SOMO stabilization is also seen. Bottom: SOMO of the CRs of o-fluorophenyl germatrane 3+• showing pyramidalization of Ge and planarity of N
atom as well as the axial Npz spin-carrying orbital and α-CH bonds.
Such redistribution of the electron density accompanying the
formation of the CR of benzyl germatrane from the neutral
molecule engages at least two important effects. First, the nuclear
skeleton follows electron density redistribution (electrochemical
ET is intrinsically adiabatic), thus differentiating the geometry
of neutral and oxidized forms, and second, direct and reverse
electron transfers (i.e. oxidation of 1 to 1+• and reduction of 1+•
back to 1) affect different orbitals. Both phenomena impose quasireversible character of oxidation of 1 and 2 in fact observed by
cyclic voltammetry.
For aryl-substituted germatranes 3 and 4 and their CRs,
surprising trends in the geometry modification are found. The
atrane nitrogen flattens, yet remaining on the side of endoconfiguration, but the N–Ge distance in 3+• undergoes elongation
by about 28%. The Ge atom adopts the configuration with
Ge puckering out of the original Ge(-O)3 plane, approaching
tetrahedral geometry – the angles formed by Ge with three O
atoms are remarkably increased (∠C–Ge-O ≈ 113◦ ) while the
angles between oxygens are smaller than but still closer to 108◦ .
The NBO electron configuration of Ge in 3+• is 4S(0.35) 4p(0.48),
which also reflects this unexpected pyramidalization. No spin
Table 2. Selected geometrical parameters for germatranes and their CRs (distances in Å, angles in deg), Fermi contact terms (FCT, MHz) and NBO
charges from DFT B3LYP/LANL2DZ calculations
N
Species
1
1+•
2
2+•
3
3+•
7a
7+•
l(Ge–C)
2.425
2.184
2.516
2.148
2.441
3.129
2.476
2.211
1.992
2.102
1.923
2.072
2.896
2.924
2.881
2.856
1.954
1.907
1.946
1.990
Ar )
∠C–Ge–O
∠O–Ge–O
∠CAr –C–Ge
100.92
95.19
104.30
95.98
101.50
113.13
101.94
98.39
116.48
119.20
114.73
118.96
116.13
105.61
115.92
117.81
112.76
109.60
113.91
108.84
Ge
FCT
q(NBO)
FCT
q(NBO)
0.3217
−0.3274
−0.0001
1.2001
0.2816
−0.2251
0.0000
1.1907
0.9364
−0.1940
0.0000
1.1661
0.8697
−0.3231
0.0000
1.1689
Phenyl germatrane (7).
Appl. Organometal. Chem. 2010, 24, 865–871
869
a
l(Ge–N)
l(Ge–C1
c 2010 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
S. Soualmi, L. Ignatovich and V. Jouikov
density is present on the aromatic ring. Fermi contact terms for N
and Ge atoms in 3+• (Table 2) are in good agreement with the EPR
data: most of the free electron density is localized at N atom and
no coupling with Ge atom.
Surprisingly, the DFT B3LYP/LANL2DZ calculations on the
CR of non-substituted phenyl germatrane 7+• , considered for
comparison, does not show similar trend in geometry modification
when going from the neutral molecule to the CR (Table 2): the
N–Ge distance shortens and the ∠C–Ge–O angles flatten. The
only difference between 3 and 7 is the fluoro substitution at
the most perturbing ortho-position of the aromatic ring in 3; so
most probably the electron-acceptor character of F and its backbonding lateral orbital interactions with O atoms (relative NBO
charge on Ge is higher in 3+• than in 7+• , Table 2) account for this
effect.
The absence of spin-orbital coupling from Ge atom in EPR
spectra of 1–4 is consistent with the orbital structure and spin
density distribution in their CRs from B3LYP/LANL2DZ calculations:
the Fermi contact term (Table 2) is near zero at Ge atom.
Acknowledgments
Rennes Metropole is gratefully acknowledged for its support of
the works on 14A group elements electrochemistry; S.S. thanks
the Algerian Ministry of Education for the travel grant.
Conclusions
870
Anodic oxidation of 1-substituted germatranes occurs via electrochemically reversible electron transfer resulting in corresponding
cation radicals detected by CV and real-time EPR spectroelectrochemistry. The involvement of n(pz ) electrons of the atrane
nitrogen into intramolecular N → Ge dative coordination substantially reduces the ease of electron withdrawal during the oxidation
of germatranes (by about 500–600 mV) compared with parent tertiary amines and by about 70–100 mV compared with silatranes
studied so far by cyclic voltammetry. Aryl germatranes with the
substituents in the aromatic ring whose own oxidation potential is
lower than that of the germatranyl moiety – p-(N,N-dialkylaniline)substituted germatranes – follow the oxidation pattern and form
the CRs of corresponding N,N-dialkylaniline derivatives substituted
with germatranyl group.
A very remarkable feature of the cation radicals of aryl
germatranes is that the atrane nitrogen is practically planar:
the values of nitrogen hfc constants are aN = 18–19 G, typical
for planar N-centered radicals. Axial and equatorial α-methylene
protons in the cation radicals show about 40 times difference in
their response to the magnetic field. Neither is any contribution
from Ge from the aromatic ring seen in the spin delocalization.
While the N atom remains flat, the Ge atom in the CR of aryl
germatranes goes out of the Ge(-O)3 plane.
For the CRs of benzyl germatranes, quite different spin
distribution was found: the aromatic system together with an
‘electron bridge’ σ (C–Ge) orbital accommodates the major part
of the spin density while only a small part of it is resides on the N
atom. The results of both EPR spectroscopy and DFT calculations
converge in that the atrane nitrogen in the CR does not adopt an
exo-configuration. Instead, a remarkable molecular motion around
the Ge atom occurs which, contrary to what was found for the CRs
of aryl germatranes, renders it much flatter and closer to trigonal
bipyramid geometry. The interactions of Ge with its environment
and with all three O atoms are very important; the contraction of
the 3c–4e bond N–Ge–Csp2 upon oxidation and favored metal
pentacoordination in the cation radicals of germatranes is the
main driving force determining the geometry of these species.
When the outer to N part of the N–Ge–C 3c–4e bond
(Ge–C fragment) has no possibility for lateral overlapping by
wileyonlinelibrary.com/journal/aoc
virtue of its conjugative preferences (the conjugation being cut
off by orthogonal orientation of the susceptible to conjugative
interactions orbitals of the substituent), a distonic CR is formed,
with unpaired electron remaining at the atrane nitrogen. In the
case when this N–(Ge–C) center is open for further conjugation,
spin delocalization finds better stabilized configuration with spin
density shifted to the aromatic system and very small amount of
it still seeing the atrane N atom. Thus the formation of distonic
cation radicals observed for aryl germatranes might reflect a typical
feature for the germatranes substituted with the groups ‘ending’
electron transmission by the absence of further conjugation:
aliphatic, vinyl and ethynyl substituents. If so, electrochemistry
will provide a new insight into the electronic interactions in
these compounds and open possibilities for designing molecular
electronic devices with specific electronic, electromechanical and
optical properties. Further studies on these interesting species are
in progress.
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