A coordination compound of Ge0 stabilized by a diiminopyridine ligand

Article abstract of DOI:10.1002/anie.201309421

Reduction of the cationic Ge^II complex [dimpyrGeCl][GeCl ₃] (dimpyr=2,6-(ArN=CMe)₂NC₅H_3, Ar=2,6-iPr₂C₆H₃) with potassium graphite in benzene affords an air sensitive,

Full text of DOI:10.1002/anie.201309421

Angewandte
Chemie
DOI: 10.1002/anie.201309421
Germanium(0) Compounds
A Coordination Compound of Ge0 Stabilized by a Diiminopyridine
Ligand**
Terry Chu, Lee Belding, Art van der Est, Travis Dudding, Ilia Korobkov, and
Georgii I. Nikonov*
Dedicated to Professor Nikolai A. Ustynyuk on the occasion of his 70th birthday
Abstract: Reduction of the cationic GeII complex
[dimpyrGeCl][GeCl3] (dimpyr= 2,6-(ArN = CMe)2NC5H3,
Ar = 2,6-iPr2C6H3) with potassium graphite in benzene affords
an air sensitive, dark green compound of Ge0, [dimpyrGe],
which is stabilized by a bis(imino)pyridine platform. This
compound is the first example of a complex of a zero-valent
Group 14 element that does not contain a carbene or carbenoid
ligand. This species has a singlet ground state. DFT studies
revealed partial delocalization of one of the Ge lone pairs over
=
the p*(C N) orbitals of the imines. This delocalization results
in a partial multiple-bond character between the Ge atom and
imine nitrogen atoms, a fact supported by the X-ray crystallog-
raphy and IR spectroscopy data.
T
he chemistry of the Group 14 elements in low oxidation
Scheme 1. Examples of Si0 and Ge0 compounds.
states has recently received renewed attention.[1–3] In partic-
ular, the unique zero oxidation state, E(0), has recently been
realised. Following the pioneering report by Kira et al.[4] on
trisilaallene (1) and its heavier congeners,[5–7] a series of E(0)
compounds were prepared, all showing the same prominent
feature: coordination of the Group 14 centre in the zero
oxidation state to a carbene or silylene ligand. These
compounds can be placed into two major categories: the
patterns. Herein, we report the first Ge0 coordination com-
pound that is stabilized by a trinitrogen platform and free
from soft carbene or carbenoid ligands. During the revision of
this paper, Fischer and Flock et al. disclosed a closely related
Sn0 compound supported by a diiminopyridine ligand.[15]
Chirik et al. reported a series of diiminopyridine com-
plexes of iron in the formal zero oxidation state.[16] The unique
ability of diiminopyridine (dimpyr) ligands to stabilize low
oxidation states was related to its non-innocent redox-active
nature, which allows for partial delocalization of electron
density from an electron rich metal to the ligand. This redox
activity of dimpyr ligands makes them promising for the
stabilization of the low oxidation states of main-group-
element centers.[17] Roesky et al. recently reported that the
bulky dimpyr compound 2,6-(ArN = CMe)2NC6H3 (5, Ar=
2,6-iPr2C6H3) reacts with two equivalents of GeCl2·diox
(diox = 1,4-dioxane) to give a GeII cation [ClGe(dimpyr)]+
(6; Scheme 2).[18] We independently observed a similar reac-
!
double-bonded species L!E = E L (E = Si,[8] Ge,[9] Sn;[10]
L = NHC carbene; 2 in Scheme 1)[11] and a wider class of so-
!
called ylidones, L!E L, in which the central atom is bound
to two carbenoid ligands by means of donor–acceptor
bonds.[7,12] The valence electrons of the central atom of
ylidones are retained as two lone pairs. Recently, the research
groups of Roesky and Driess reported the carbene-stabilized
E(0) compounds 3[13] and 4[14] (Scheme 1). The extension of
the E(0) chemistry to other types of ligand environments, and
in particularly to those based on hard N and O donors, is of
interest because it can provide new structural and reactivity
[*] T. Chu, L. Belding, Prof. Dr. A. van der Est, Prof. Dr. T. Dudding,
Dr. G. I. Nikonov
Chemistry Department, Brock University
500 Glenridge Ave., St. Catharines, ON, L2S 3A1 (Canada)
E-mail: gnikonov@brocku.ca
Dr. I. Korobkov
X-ray Core Facility, Faculty of Science, University of Ottawa
150 Louis Pasteur, Ottawa, ON, K1N 6N5 (Canada)
[**] G.I.N. thanks the Petroleum Research Fund for support and CFI/
OIT for an equipment grant.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309421.
Scheme 2. Preparation of cation 6.
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tion. The cation 6 can also be cleanly generated by reacting
the diiminopyridine 5 with one equivalent of GeCl2·diox in
the presence of one equivalent of BCl3 or Et2O·BF3 to give the
salts [ClGe(dimpyr)]+[BCl4]ꢀ and [ClGe(dimpyr)]+[BClF3]ꢀ.
These products were characterized by 1H, 13C, 11B, and
19F NMR spectroscopy, IR spectroscopy, and by comparison
to the related data for [ClGe(dimpyr)]+[GeCl3]ꢀ. The molec-
ular structure of 6[GeCl3] was established by X-ray analy-
sis.[19] It differs from that reported by Roesky et al. by the
presence of a solvent molecule but is otherwise very similar.
The reduction of compound 6[GeCl3] by two equivalents
of potassium graphite in benzene affords dark green com-
pound 7, which can be classified as a complex of Ge0 with the
ligand 5 (Scheme 3). Complex 7 is extremely air and moisture
VT 1H NMR spectra did not show any significant temperature
dependence, which must occur in the case of temperature
accessible excitation to the triplet state. Finally, we discovered
that mixing a sample of purified 7 with the starting complex
6[GeCl3] results in the appearance of the same EPR signal but
with a roughly 20-fold increase in intensity (see Figure S18 in
the Supporting Information). Thus, we tentatively assign the
EPR-active species to a radical formed by the reduction of 6
by 7.[21]
The molecular structure of 7 is shown in Figure 1. The
molecule is bisected by a crystallographically imposed mirror
plane so that the left side of the molecule is a reflection of the
right side. The coordination sphere of germanium in 7 is
comprised of three nitrogen atoms: one from the central
Scheme 3. Preparation of compound 7.
1
sensitive. It was characterized by H and 13C NMR spectros-
copy and IR spectroscopy, and its structure was confirmed by
X-ray analysis. Solution NMR spectra of 7 in C6D6 are
consistent with an NMR-averaged C2v geometry. In particular,
the iPr groups of the ligand are equivalent and give rise to two
doublets at d = 1.08 ppm and 1.16 ppm, both coupled to
a septet at d = 2.66 ppm (3JH-H = 6.9 Hz) integrated as four
protons. The iminoacyl signal was observed as a singlet at d =
Figure 1. Molecular structure of complex 7 (thermal ellipsoids are set
at 50% probability; hydrogen atoms and thermal ellipsoids for the
ligand carbon atoms are omitted for clarity). The left-hand part of the
molecule is related to the right-hand part by a crystallographically
imposed mirror plane that runs through C3 and N1. The Ge1 atom is
disordered but only one position is shown for clarity.
= ꢀ
2.04 ppm (N C CH3) integrated as six protons. The NMR-
averaged C2v symmetry of 7 is persistent in [D8]toluene in the
temperature range ꢀ70–+258C. In the IR spectrum of 7, the
ꢀ1
=
N C(imine) stretch at 1575 cm is bathochromically shifted
pyridine core and two from the imine groups. The imine
nitrogen atoms, N2 and N2a, coordinate to the Ge centre from
opposite sides. Refinement of the Ge atom in the special
position results in significant elongation of its thermal
ellipsoid in parallel with the N2-N2a vector, thus suggesting
disorder. This disorder was successfully modeled by shifting
the Ge atom in the plane of the pyridine fragment towards
relative to the free ligand (1642 cmꢀ1) and the cation 6
ꢀ1
=
(1655 cm ), thus suggesting decreased C N bond order.
An interesting feature of the 1H NMR spectrum of 7 is the
upfield shift of the para-CH resonance of the pyridine ligand
3
upon formation of the complex: d = 6.35 ppm (t, JH-H
=
7.6 Hz) in 7 versus d = 7.29 ppm in the free ligand. An
analogous feature is observed for the related dimpyr complex
of Sn0, which shows increased shielding of the p-CH(pyr)
signal at d = 6.23 ppm,[15] and for the isoelectronic dimpyr
ꢀ
one of the imine nitrogen atoms. The resulting Ge1 N2 bond
ꢀ
of 2.047(7) ꢀ is noticeably shorter than the Ge1 N2a distance
ꢀ
of 2.306(7) ꢀ. The latter value is still comparable to the Ge
I
+
=
complex of P , [P(FcN CH)2NC6H3] (Fc = ferrocenyl): d =
7.12 ppm versus d = 7.84 ppm in the free ligand (in
D3CCN).[17] Our initial interpretation of this unusual shift
was that it might be related to the availability of a low-lying
triplet state with increased hyperfine coupling to the p-CH.
Indeed, EPR spectra taken of the crude reaction mixture and
of recrystallized complex 7 showed a weak signal, the
simulation of which was consistent with significant spin
localization in the para position of the ring.[20] However,
multiple recrystallizations of 7 caused a marked decrease in
the intensity of the EPR signal and DFT calculations (see
below) showed that the lowest triplet state of 7 lies
17.7 kcalmolꢀ1 above the singlet ground state. Moreover,
imine bonds in 6[GeCl3] (2.2757(17) and 2.2623(17) ꢀ),
however. The Ge1 atom is located 1.8988(18) ꢀ away from
the pyridine nitrogen N1, a distance shorter than the
corresponding distance (2.0739(18) ꢀ) in the cation 6[GeCl3].
0
ꢀ
Neither of these distances is longer than the N Fe bond in
the complex [dimpyrFe(N2)2] (1.8362(14) ꢀ),[16a] although the
covalent radius of iron (1.32(3) ꢀ) is larger than the covalent
radius of germanium (1.20(4) ꢀ).[22] This feature likely
reflects the higher effective oxidation state of iron in the
dimpyr complex[23] and the hybridization of the germanium
ꢀ
atom in 7. That is, the N Ge bonds have increased Ge p-
character so that the orbitals hosting the lone pairs of
germanium have increased s-character.[24] The N2–Ge1–N2a
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bond angle is more open in 7 than in 6[GeCl3] (152.76(8)8
versus 146.50(7)8), thus reflecting the closer proximity of the
ꢀ
Ge atom to the pyridine ring in the former. The N2 C4 bond
of the imine fragment of 7 is elongated to 1.320(2) ꢀ, thus
indicating some decrease in multiple-bond character. A
similar situation is observed in the Fe0 complex [dimpyr-
Fe(N2)2], which has a N C(imine) bond of 1.332(2) ꢀ,[16a] and
=
in the related Sn0 dimpyr compound (1.308(3) and
1.321(3) ꢀ).[15] For comparison, the N C(imine) bonds in
=
ꢀ
6[GeCl3] are normal at 1.278(3) and 1.274(3) ꢀ. The C1 C2
ꢀ
and C2 C3 bond lengths in the pyridine ring of 7 (1.389(2)
and 1.378(2) ꢀ, respectively) are very close to the corre-
ꢀ
sponding C C distances in 6[GeCl3] (range 1.377–
ꢀ
1.393(3) ꢀ), whereas the pyridine N1 C1 distance of
1.4051(19) ꢀ is elongated compared to 1.343(3) and
1.345(3) ꢀ in 6[GeCl3], thus suggesting partial charge transfer
to the ring.
From a qualitative perspective, the spectral and X-ray
crystallography data for 7 suggest that its singlet ground state
can be represented as a resonance of canonical forms A and B
(Scheme 4). Form B is a singlet biradical[25] with unpaired
Figure 2. Calculated structure and orbital compositions of key frontier
orbitals of the singlet state of 7, and the HOMO of the triplet state.
a slight yet measureable (1.28 ꢀ vs. 1.36 ꢀ) difference
=
between the two N C imine bond lengths, both of which
are elongated when compared with those computed for the
free ligand (1.27 ꢀ). Consistent with our experimental
ꢀ1
=
observations, both N C(imine) stretches in 7 (1726 cm
and 1401 cmꢀ1) were calculated to be of lower frequency
than those of the free ligand (1772 cmꢀ1 and 1777 cmꢀ1). Also
consistent with the experimental data is an upfield shift of the
para-CH resonance of the pyridine ligand in the computed
NMR spectrum of 7 (d = 6.75 ppm) relative to the free ligand
(d = 7.96 ppm).
Scheme 4. Valence-bond representation of 7.
electron density localized on the imine carbon atoms. It can
be viewed as being the result of charge transfer from the Ge0
=
center to the antibonding p*(C N) orbital, which is primarily
For comparison, the computed triplet state of complex 7
(Figure 2) is symmetric and 17.7 kcalmolꢀ1 higher in energy
than the singlet state. Analysis of the spin density revealed
that the largest component of the spin resides on Ge, with
significant spin density on the pyridine backbone.[20] Structur-
ally, the two imine bonds are equivalent in length (1.34 ꢀ) and
localized on the carbon atoms. The contribution of B accounts
for the elongation of the N C(imine) bond in 7 and the red
shift of the N C stretch in the IR spectrum of 7 in comparison
=
=
with the free ligand 5.
To gain additional insight into the electronic structure of
7, DFT calculations at the [wB97XD/6-311G(d,p)] level were
performed. The optimized singlet ground-state structure of 7
ꢀ
ꢀ
both the N2 Ge and the N2a Ge bond lengths are 2.07 ꢀ.
Some additional features of the triplet include distortion of
ꢀ
ꢀ
is shown in Figure 2. The calculated N1 Ge distance of 1.92 ꢀ
is close to the value determined by X-ray crystallography
the Ge atom from the plane of the ligand, an elongated N1
Ge distance (1.96 ꢀ), and slight twisting of the N-aryl ring
systems with respect to the pyridine backbone, as opposed to
the perpendicular relationship observed in the singlet state.
Molecular orbital (MO) analysis provided further insight
into the structure of 7. The HOMO of the singlet corresponds
to a diffuse p-type orbital centered at the Ge0 atom. This
orbital spans the two bridging imine nitrogen atoms and
includes what appears to be a back-bonding interaction
between a Ge0 4p orbital and a 2p orbital from the pyridine
nitrogen (Figure 2). Another tell-tale feature of the HOMO is
the presence of an antibonding interaction across the
(1.899 ꢀ). Interestingly, the DFT calculations confirm the Cs
symmetry of 7 by showing two different Ge N bond lengths
ꢀ
to the imine nitrogen atoms N2 and N2a: 1.93 ꢀ and 2.55 ꢀ,
respectively, which are in good agreement with the values of
(2.047(7) ꢀ and 2.306(7) ꢀ) determined by X-ray crystallog-
raphy. This difference represents about 14% displacement of
the Ge atom from the central position. This asymmetry is
indicative of a short-lived stationary point on a shallow
double-well potential energy surface (derived from a fluxional
exchange of the Ge atom between the two imine nitrogen
atoms),[26] and indeed the VT NMR data discussed above
suggest that exchange between two degenerate sites occurs
very easily in solution even at very low temperature.
Associated with the asymmetric nature of computed 7 is
=
iminoacyl N C(imine) bond, which accounts for the elonga-
tion of this metric observed by X-ray crystallography and the
red shift of the imine stretch. Essentially, the HOMO of 7 can
be viewed as being the result of delocalization of a Ge-centred
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stirred for 2 h at room temperature. The dark green mixture was
filtered and the residue was washed with benzene (3 ꢁ 5 mL). Volatile
compounds were removed from the combined filtrate to give the
product as a green solid (0.196 g, 0.354 mmol, 64%). 1H NMR
(300 MHz, C6D6): d = 7.18 (m, 8H, C6H3, m-H Py), 6.35 (t, 1H, p-H
Py, 3JH-H = 7.6 Hz), 2.66 (hept, 4H, CH(CH3)2, 3JH-H = 6.9 Hz), 2.04 (s,
6H, NCCH3), 1.16 (d, 6H, CH(CH3)2, 3JH-H = 6.8 Hz), 1.08 ppm(d,
lone pair over the p-system of the ligand, which is comprised
of the imine and pyridinyl nitrogen atoms. We can therefore
conclude that bonding of the Ge atom to the imine groups has
a partial multiple-bond character, which allows us to account
ꢀ
for the shorter Ge N(imine) bonds in 7 relative to 6[GeCl3]
observed in the X-ray studies (see above). On the other hand,
the HOMO–1 corresponds to a Ge0-centered s-type lone pair,
while the HOMO–2 is a bonding component of the 3c–4e
interaction between an in-plane Ge 4p orbital and the two
lone pairs of the imine nitrogen atoms N2 and N2a. The
LUMO of 7 lies 0.21 eV above the HOMO and resides
entirely on the ligand as is apparent from the lack of any
orbital density at Ge. The HOMO of the triplet state closely
resembles the LUMO of the singlet state in accordance with
the Frank-Condon principle. Based on the computed energy
difference, the triplet state is not thermally accessible.
3
6H, CH(CH3)2, JH-H = 7.0 Hz). 13C{1H} (75 MHz, CDCl3): d = 146.1
(NCCH3), 141.7 (ipso-C Ph), 141.3 (o-C Ph), 137.4 (o-C Py), 125.9 (p-
C Ph), 124.1 (m-C Py), 123.3 (m-C Ph), 113.9 (p-C Py), 28.4
(CH(CH3)2), 25.9, 23.4 (CH(CH3)2), 15.0 ppm (NCCH3). IR (nujol):
ꢀ1
=
~
n ¼1575 cm (N C). Elemental analysis Calcd for C33H43GeN3: C,
71.50; H, 7.82; N, 7.58. Found: C, 70.89; H, 7.96; N, 7.15.
CCDC 968783 (6[GeCl3]) and 968782 (7) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Received: October 29, 2013
Revised: December 4, 2013
Published online: February 2, 2014
Interestingly, NBO analysis also revealed the nonintuitive
presence of four Ge-centred lone pairs. One lone pair sits in
a s orbital with an occupancy of 1.77e, which can be seen in
the HOMO–1 and corresponds to a classic Lewis representa-
tion of a lone pair. The second lone pair, corresponding to the
HOMO–2, has a low occupancy (0.27e) as a result of the
above-mentioned delocalization into the p-system of the
ligand. The third and fourth lone pairs also have small
occupancies (0.55e and 0.50e, respectively), and are perhaps
an artifact of the 3c–4e bond (i.e., a nonclassical Lewis
representation). This is supported by strong donation of the
imine nitrogen lone pairs into the orbitals assigned by NBO as
Keywords: DFT calculations · EPR spectroscopy · germanium ·
.
low oxidation states · pyridine
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=
0.11 kcalmolꢀ1, N(2)LP!GeLP(3) = 42.3 kcalmolꢀ1, N(2a)LP
!
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=
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All manipulations were performed using standard inert atmosphere
(N2 gas) glove-box and Schlenk techniques. Dichloromethane,
hexanes, and benzene were dried by using a Grubbs-type solvent
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!
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=
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Angew. Chem. Int. Ed. 2014, 53, 2711 –2715
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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