Synthesis and reactivity of a phosphine-stabilized monogermanium analogue of alkynes

Article abstract of DOI:10.1021/ja2073577

The synthesis of the first isolable C-phosphino-Ge-aminogermyne, stabilized by a phosphine ligand, has been achieved. X-ray diffraction analysis indicates a quite long Ge-C bond whose length is between that of a single and a double bond but consistent with the theoretically predicted values for a germyne bearing π-donating substituents. The isomerization of this germyne derivative affords a new stable N-heterocyclic germylene through migration of the original phosphine ligand from germanium to the carbon center.

Full text of DOI:10.1021/ja2073577

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Synthesis and Reactivity of a Phosphine-Stabilized Monogermanium
Analogue of Alkynes
Juliette Berthe,† Juan Manuel Garcia,‡ Edgar Ocando,‡ Tsuyoshi Kato,*,† Nathalie Saffon-Merceron,||
Abel De Cꢀozar,§ Fernando P. Cossío,§ and Antoine Baceiredo*,†
†Universitꢀe de Toulouse, UPS and CNRS, LHFA UMR 5069, 118 route de Narbonne, F-31062 Toulouse Cedex 9, France
‡Laboratorio de Fisico-Quimica Organica, Centro de Quimica, Carretera Panamericana km 11, Caracas, Venezuela
§Facultad de Química, Universidad del País Vasco-Euskal Herriko Unibertsitatea, P. K. 1072, San Sebastian-Donostia, Spain
Universitꢀe de Toulouse, UPS, Institut de Chimie de Toulouse, ICT FR 2599, 118 route de Narbonne, F-31062 Toulouse, France
S Supporting Information
b
Scheme 1. Synthesis of Phosphinogermylene Diazo-
methanes 3 and Germynes 4
ABSTRACT: The synthesis of the first isolable C-phosphi-
no-Ge-aminogermyne, stabilized by a phosphine ligand, has
been achieved. X-ray diffraction analysis indicates a quite
long GeꢀC bond whose length is between that of a single
and a double bond but consistent with the theoretically
predicted values for a germyne bearing π-donating substit-
uents. The isomerization of this germyne derivative affords a
new stable N-heterocyclic germylene through migration of
the original phosphine ligand from germanium to the
carbon center.
arbonꢀcarbon triple bonds are ubiquitous in organic
C
chemistry.1 In marked contrast, the chemistry of heavier
analogues of alkynes has only recently emerged.2 Indeed, the
stable homonuclear alkyne analogues of the heavier group-14
elements, RꢀEEꢀR (I; E = SiꢀPb), have been isolated and
characterized during the past decade.3ꢀ6 Structural analysis of
these compounds disclosed a trans-bent rather than linear
geometry in which the bending angle increases in going from
silicon to lead. This trend indicates increasing lone-pair character
and decreasing EꢀE bond order (canonical structure II) upon
descending the group.3ꢀ6 The geometrical distortion of heavier
alkyne analogues relative to alkynes modifies their physical
properties, leading to their unique chemical behaviors.7
suggesting an increased contribution of canonical structure
IV.10a However, in spite of increased interest, the synthesis of
stable heteronuclear alkyne analogues remains very challenging,
and there have been only a few reports to date concerning the
synthesis of transient species as well as theoretical investiga-
tions.8ꢀ11 We very recently reported the synthesis of the first
isolable silyne A stabilized by a phosphine ligand.12 This molecule
shows a very short siliconꢀcarbon bond that can be regarded
as a silyneꢀbase adduct with a partial SiꢀC triple bond (type V).
The next challenge was to try to prepare the following and
unknown heavier heteronuclear alkyne analogue, the germyne.
Here we report the isolation and characterization of 4, the first
persistent phosphine-stabilized germynes, which have an elec-
tronic structure of type VI rather than type V, and their
transformation into the thermally more stable isomers of type
VII, representing new stable N-heterocyclic germylenes.
By analogy with the synthesis of silyne A,12 the germylene-
substituted (phosphino)diazomethanes 3 were chosen as pre-
cursors of germynes. Diazomethane derivatives 3 were readily
prepared by the reaction of the phosphine-stabilized chloro-
(amino)germylenes 1 with lithiated phosphinodiazomethane 213
(Scheme 1) and isolated as stable orange crystalline materials
[81% (3a), 65% (3b)]. 3a was obtained as a 60:40 mixture of
two diastereomers, as indicated by the two AX systems in the
In regard to the heteronuclear alkyne analogues in which one
carbon atom is replaced by a heavier group-14 element,
RꢀECꢀR0 (III and IV; E = SiꢀPb), similar geometrical and
electronic trends were theoretically predicted.8ꢀ11 Furthermore,
Kira’s group has experimentally demonstrated that a transiently
formed stannyne showed a singlet-carbene-like reactivity,
Received: August 10, 2011
Published: September 13, 2011
r
2011 American Chemical Society
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Figure 2. Possible structures for 4a and 4b. Relative energies (kcal/
mol) calculated at the BHandH/6-31G* level of theory are given in
parentheses. Structures 4a-I, 4b-I, and 4a-II were not found (n.f.).
Figure 1. Molecular structure of 4a. Thermal ellipsoids represent 30%
probability. H atoms have been omitted for clarity. Selected bond
lengths (Å) and angles (deg): CꢀGe, 1.887(5); CꢀP2, 1.549(5);
GeꢀN1, 2.005(3); GeꢀP1, 2.4895(12); P2ꢀN3, 1.657(4); P2ꢀN2,
1.670(4); CꢀGeꢀN1, 102.42(18); CꢀGeꢀP1, 113.57(14); N1ꢀGeꢀ
P1, 82.77(10); CꢀP2ꢀN3, 126.8(2); CꢀP2ꢀN2, 130.5(2);
N2ꢀP2ꢀN3, 102.4(2); GeꢀCꢀP2, 147.3(3). Torsion angles (deg):
N2ꢀP2ꢀCꢀGe, ꢀ73.6(6); N3ꢀP2ꢀCꢀGe, 113.9(5); N1ꢀGeꢀ
CꢀP2, ꢀ159.9(5). ∑θP2 = 359.70°, ∑θGe = 298.76°.
Figure 3. Most representative natural bond orbitals for 4a-III.
the theoretically predicted values of 1.862ꢀ1.887 Å for germynes
with electron-withdrawing and π-donating substituents (F or
OH).9d The trigonal-planar geometry around the P2 atom
(∑θP2α = 359.7°), and the extremely short P2ꢀC bond distance
(1.549 Å) clearly indicate a strong π interaction between the
phosphino substituent and the germyne fragment. Indeed, the
P2ꢀC bond length is in the range expected for phosphinocar-
benes15 or phosphaalkynes.17 Nevertheless, in spite of the P2ꢀC
multiple-bond character, the trigonal-planar phosphino fragment
is strongly twisted relative to the P2ꢀCꢀGe fragment, with a
N2ꢀP2ꢀCꢀGe torsion angle of ꢀ73.7°. This value is even
larger than that reported for methylenephosphonium salts
[(iPr2N)2PdC(TMS)2].18 These results clearly support the
biscarbenoid structure of type VI, which is completely different
from that of silicon analogue A, which shows a multiple-bonded
structure of type V.
DFT calculations on 4a and 4b indicated that the structures
4-I presenting GeꢀC triple bonds with a linear geometry around
the germyne carbon atom, similar to the case of the silyne
analogue,12 are not energy minima on the respective potential
energy surfaces (Figure 2). Instead, in the case of 4b, we found a
trans-bent structure (4b-II) as a local energy minimum, present-
ing a GeꢀC double bond with a lone pair on the central carbon
atom. However, the most stable isomer of 4b is similar to the
experimentally obtained structure with PꢀC double-bond char-
acter (4a-III). Isomer 4b-III is more stable than 4b-II by ca. 10
kcal/mol. In the case of 4a, only isomer 4a-III could be located
and characterized as a true energy minimum. This difference
might result from a distortion of the molecules for steric reasons.
Indeed, 4a with a more hindered phosphine ligand is more
strongly twisted than 4b (calculated N2ꢀP2ꢀCꢀGe twist
angles: 84.89° for 4a, 26.28° for 4b). However, in both com-
pounds 4a and 4b, the GeꢀC multiple-bonding character is not
favored because of the partial stabilization of the germyne by the
cyclic phosphines as well as strong π donation from the exocyclic
bis(diisopropylamino)phosphine moiety, thus resulting in a
stabilization of the CdP(NiPr2)2 unit. Indeed, natural bond
orbital (NBO) analysis of 4-III showed the presence of a lone
pair on the germanium atom (LPGe) and on the carbon atom
(LPC) as well as a PꢀC π bond (Figure 3). Interestingly, in
contrast to LPGe having enhanced s character [s(64.37%),
p(35.54%), d(0.09%)], LPC shows extremely high p character
31P NMR spectrum [major isomer, 64.1 and 80.0 ppm (3JPP
=
91.1 Hz); minor isomer, 64.0 and 80.9 ppm (3JPP = 54.6 Hz)].
An intense IR absorption band at 1995 cmꢀ1 is in agreement
with the presence of a diazo function. In the case of 3a, the
structure was unambiguously confirmed by X-ray diffraction
(XRD) analysis.14
Photolysis of diazomethane derivatives 3 (λ = 300 nm) at
ꢀ60 °C in tetrahydrofuran (THF) resulted in the clean forma-
tion of the phosphine-stabilized germynes 4. Monitoring of the
reaction by 31P NMR spectroscopy at ꢀ80 °C showed the com-
plete disappearance of the starting diazomethane derivative 3a
after 48 h of irradiation and the appearance of a new set of signals
for 4a [AX system: P(1) = 82 ppm, P(2) = ꢀ20 ppm (3JPP = 102.6
Hz)], in agreement with the formation of only one diastereomer.
The 31P NMR data for 4a appeared to be quite different from
those of the silicon analogue A [AX system: P(1) = 3 ppm, P(2)
=
46 ppm (3JPP = 47.3 Hz)]. Indeed, the P(2) signal appeared at
high field in the typical region for phosphinocarbenes,15 and the
chemical shift of P(1) remained almost unchanged relative to the
value for the starting diazo derivative 3a (80 ppm). The 13C
NMR signal of the central carbon atom [162.4 ppm (1JCP =43.4
Hz, 3JCP = 11.3 Hz)] was in the region expected for a phosphi-
nocarbene with a relatively small phosphorusꢀcarbon coupling
constant. The spectroscopic data for 4b were very similar to
those for 4a.14
Germyne derivative 4a was successfully isolated from diethyl
ether solution at ꢀ60 °C as orange crystals, and the molecular
structure was confirmed by XRD analysis (Figure 1).14 This
structure shows a twisted trans-bent arrangement with a
N1ꢀGeꢀCꢀP2 dihedral angle of 159.9° and coordination of
the phosphine ligand to the germanium center with a P1ꢀGe
distance of 2.490 Å. This P1ꢀGe distance is slightly elongated
relative to that in precursor 3a (2.443 Å). The GeꢀC bond
distance (1.887 Å) is quite long in comparison with the calculated
value for a GeꢀC triple bond in the parent compound (1.73 Å)9
and lies between the lengths of a single (1.98 Å)16 and a double
bond (1.80 Å).2 However, this bond distance is consistent with
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Scheme 2. Isomerization of 4a and 4b
ylide. These structural properties are similar to those previously
reported for a cyclic aminoꢀenamino germylene.20
In conclusion, we have successfully synthesized and fully
characterized 4, the first isolable germanium analogues of
alkynes, which are stabilized by coordination of a phosphine
ligand. In the case of 4a, XRD analysis indicated a quite long
GeꢀC bond length for a triple bond and an extremely short PꢀC
bond length. Germynes 4 rearrange at room temperature,
affording a phosphaalkene 5 (a typical reaction for singlet
phosphinocarbenes) and a new stable N-heterocyclic germylene
6.21 The results shown here demonstrate the significant differ-
ences not only in the structure of 4 but also in its chemical
behavior with respect to the silicon analogue A. Further studies of
the reactivity of this new species are under active investigation.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental, computational,
b
and crystallographic data, including CIF files. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
kato@chimie.ups-tlse.fr; baceired@chimie.ups-tlse.fr
’ ACKNOWLEDGMENT
Figure 4. Molecular structure of 6a. Thermal ellipsoids represent 30%
probability. H atoms have been omitted for clarity. Selected bond
lengths (Å) and angles (deg): GeꢀC1, 1.9023(15); GeꢀN1,
1.9149(13); C1ꢀP1, 1.7192(15); C1ꢀP2, 1.8382(15); P1ꢀC2,
1.7489(16); C2ꢀC3, 1.376(2); C3ꢀN1, 1.356(2); N1ꢀGeꢀC1,
103.18(6); GeꢀC1ꢀP1, 122.36(8); C1ꢀP1ꢀC2, 108.97(7);
P1ꢀC2ꢀC3, 128.25(12); C2ꢀC3ꢀN1, 128.06(14); C3ꢀN1ꢀGe,
125.60(10); P1ꢀC1ꢀP2, 113.85(8); GeꢀC1ꢀP2, 123.17(8).
We are grateful to the CNRS, ANR (NOPROBLEM), MICINN
(CSD2007-00006), and PCP (Nouveaux Ligands) for financial
support.
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