Gas-phase characterization by photoelectron spectroscopy of unhindered, low-coordinate germanium compounds: Germaimines, germylenes, and germaisonitriles

Article abstract of DOI:10.1021/om990236f

Low-coordinate, unhindered germanium compounds, germaimines, germylenes, and germaisonitriles, have been generated and characterized by the combination of flash vacuum thermolysis (FVT) of appropriately substituted germacyclopentenes and ultraviolet photo

Full text of DOI:10.1021/om990236f

5322
Organometallics 1999, 18, 5322-5329
Ga s-P h a se Ch a r a cter iza tion by P h otoelectr on
Sp ectr oscop y of Un h in d er ed , Low -Coor d in a te
Ger m a n iu m Com p ou n d s: Ger m a im in es, Ger m ylen es, a n d
Ger m a ison itr iles1
Se´verine Foucat,† Thierry Pigot,† Genevie`ve Pfister-Guillouzo,*,†
He´le`ne Lavayssie`re,‡ and Ste´phane Mazie`res‡
Laboratoire de Physico-Chimie Mole´culaire UMR CNRS 5624, Avenue de l’Universite´,
64000 Pau, France, and Laboratoire d’He´te´rochimie Fondamentale et Applique´e, 118,
Route de Narbonne, F-31062 Toulouse, France
Received April 5, 1999
Low-coordinate, unhindered germanium compounds, germaimines, germylenes, and
germaisonitriles, have been generated and characterized by the combination of flash vacuum
thermolysis (FVT) of appropriately substituted germacyclopentenes and ultraviolet photo-
electron spectroscopy (UV-PES). These products have characteristic ionization potentials
(IPs) at low energy consistent with their great kinetic instability and their high reactivity.
The ionization potentials associated with the ejection of an electron from the πGedN orbital
for germaimines and the nGe lone pair for germylenes are useful fingerprints of these
compounds. At higher temperature these transient species lead to the formation of
germaisonitriles that show two characteristic IPs associated with the πGedN and the nGe orbital
ionizations. Characterization of these three unhindered species is supported by ab initio
calculations of the geometrical and electronic structures by using the B3LYP density
functional hybrid with LANL2DZ(d) and 6-311G(d) basis sets.
Group 14 low-coordinate element (Si, Ge, Sn) chem-
electron spectroscopy with flash vacuum thermolysis
were found to be particularly well-adapted to the study
of low-coordinate, unhindered silicon (>SidP-, >Sid
N-)12-14 species. Only few low-coordinate, unhindered
germanium species have been characterized in the gas
phase. For example, to our knowledge, only three gas-
phase characterizations of germanium-nitrogen double
bond (or triple bond) containing compounds15-17 have
been reported. We present here the results of our
investigation of the thermal decomposition of appropri-
ately substituted germacyclopentenes as a route to three
low-coordinate germanium compound types: germaimines
(>GedN-), germylenes (>Ge:), and germaisonitriles
(:GetN-). Scheme 1 shows the different decomposition
processes involved. The course of these decompositions
istry has been extensively studied for the last 30 years.2
Contrary to early beliefs, these compounds have an
intrinsic thermodynamic stability despite their high
reactivity.3-5 This stability has allowed their indirect
characterization by chemical trapping. In recent years
two different approaches to the study of such molecules
became possible. In the early 1980s it was discovered
that such reactive species could be kinetically stabilized
by appropriate substitution with bulky groups, which
permitted the study of their chemical reactivity and
determination of their structure by standard methods.
The other approach involves the use of well-established
techniques for the study of short-lived species: matrix
isolation and IR and/or UV spectroscopic characteriza-
tion,6-8 gas-phase generation by flash vacuum ther-
molysis (FVT), or vacuum gas-solid reactions (VGSR)
coupled with mass spectrometry or UV-PES.9-11
Our laboratory has wide experience in the latter
approach, and recent studies of the coupling of photo-
(9) Lacombe, S.; Gonbeau, D.; Cabioch, J . L.; Pellerin, B.; Denis, J .
M.; Pfister-Guillouzo, G. J . Am. Chem. Soc. 1988, 110, 6964.
(10) Bock, H.; Solouki, B. Angew. Chem., Int. Ed. Engl. 1981, 20,
427.
*Corresponding author. E-mail: genevieve.pfister@univ-pau.fr.
Fax: +33 (0)5.59.80.83.44.
(11) (a) Dyke, J . M.; J orland, G. D.; Lewis, R. A.; Morris, A. J . Chem.
Phys. 1982, 86, 2913. (b) Minsek, D. W.; Chen, P. J . Chem. Phys. 1990,
94, 8399.
(12) Metail, V.; J oanteguy, S.; Chrostowska-Senio, A.; Pfister-
Guillouzo, G.; Systermann, A.; Ripoll, J . L. Inorg. Chem. 1997, 36, 1482.
(13) Metail, V.; J oanteguy, S.; Chrostowska-Senio, A.; Pfister-
Guillouzo, G.; Systermann, A.; Ripoll, J . L. Main Group Chem. 1997,
2, 97.
(14) Lefevre, V.; Ripoll, J . L.; Dat, Y.; J oanteguy, S.; Metail, V.;
Chrostowska-Senio, A.; Pfister-Guillouzo, G. Organometallics 1997, 16,
1635.
(15) Foucat, S.; Pigot, T.; Pfister-Guillouzo, G.; Mazieres, S.; La-
vayssiere, H. Eur. J . Inorg. Chem., in press.
(1) Application of photoelectron spectroscopy to molecular properties.
Part 56. Part 55: Miqueu, K.; Sotiropoulos, J . M.; Pfister-Guillouzo,
G.; Romanenko, V. Eur. J . Inorg. Chem., in press.
(2) Neumann, W. P. Chem. Rev. 1991, 91, 311.
(3) Barrau, J .; Escudie´, J .; Satge´, J . Chem. Rev. 1990, 90, 283.
(4) Satge´, J . Adv. Organomet. Chem. 1982, 21, 241.
(5) Satge´, J .; Massol, M.; Rivie`re, P. J . Organomet. Chem. 1973, 56,
1.
(6) Visser, P.; Zuhse, R.; Wong, M. W.; Wentrup, C. J . Am. Chem.
Soc. 1996, 118, 12598.
(7) Radziszewski, J . Y.; Littman, D.; Balaji, G.; Fabry, L.; Gross.
G.; Michl, J . Organometallics 1991, 12, 4816.
(8) Maier, G.; Glattahar, Angew. Chem., Int. Ed. Engl. 1994, 33,
473.
(16) Guimon, C.; Pfister-Guillouzo, G. Organometallics 1987, 6,
1387.
(17) Ang, H.; Lee, F. K. J . Chem. Soc., Chem. Commun. 1989, 310.
10.1021/om990236f CCC: $18.00 © 1999 American Chemical Society
Publication on Web 11/16/1999

Germaimines, Germylenes, and Germaisonitriles
Organometallics, Vol. 18, No. 25, 1999 5323
Sch em e 1
depends on the substituents on the germanium (F or
Cl) and on the nitrogen (t-Bu or Me3Si).
At 753 K, this PE spectrum is different and indicates
that 1a is completely transformed (Figure 1b). During
internal thermolysis, where the oven is inside the
spectrometer, a new band appears at 7.3 eV. Moreover,
Me3SiF is the only volatile and stable compound gener-
ated during a separate product study of the pyrolysis.
(In this case, the oven is external of the spectrometer
and only long-lived molecules are detected; see Experi-
mental Section for a detailed description of the ther-
molysis experiments.) The presence of Me3SiF was
demonstrated by its characteristic ionization at 10.9 eV
(Figure 1b). Thus, according to Scheme 1, the spectrum
in Figure 1b corresponds to that of the germaimine 2a
if the Me3SiF bands are subtracted.
Stu d y of N-Tr im eth ylsilyl Sp ecies. Syn th esis of
th e P r ecu r sor s 1a a n d 1b. The monohalo-3,4-dimeth-
ylgermacyclopent-3-enes 1a and 1b were synthesized
by reaction of the corresponding lithium amide with the
respective dihalo-3,4-dimethylgermacyclopent-3-ene (eq
1).
On further heating, the PE spectrum changes up to
973 K. The band at 7.3 eV disappears while character-
istic ionizations of 2,3-dimethylbuta-1,3-diene (DMB)
(8.7, 10.3, and 11.4 eV) are observed in addition to those
of Me3SiF (Figure 1c). The formation of these products
of the thermal decomposition is confirmed by a separate
product study. Moreover, in a pyrolysis experiment in
which the products were identified by NMR spectro-
scopy DMB and Me3SiF were present in 1:1 ratio. The
PE spectrum at 973 K (Figure 1d) obtained after
subtraction of the PE spectra of Me3SiF and DMB
displays a first band at 9.0 eV followed by two broader
signals around 10.7 and 13.5 eV. These could be due to
the germaisonitrile 4a generated by loss of DMB and
Me3SiF in the thermolysis of 1a (Scheme 1).
(1)
Th er m olysis of Ger m a cyclop en ten es 1a a n d 1b.
Figure 1 shows the PE spectra of the germacyclopent-
3-ene 1a at 293, 753, and 973 K. The PE spectrum
(Figure 1a) of 1a shows, first, a set of three bands at
8.7, 10.4, and 11.5 eV. The next band is broader and
appears at higher ionization energy, around 13.5 eV.
This spectrum can be assigned on the basis of known
experimental data of the sila- and germacyclopentene
series18 and of HN(SiMe3)2.19,20 The latter displays a
first band at 8.8 eV characteristic of the ionization of a
nitrogen lone pair electron. This clearly indicates that
the first band of 1a , at 8.7 eV, corresponds to the
ejection of an electron from the nN lone pair and from
the πCdC orbital of the germacyclopentenic ring. The
second band shows a shoulder at 10.0 eV, which is due
to ionization of electrons in σGeC orbitals. The second
and third bands at 10.4 and 11.5 eV arise from the
ionizations of σSiC and σGeN electrons, respectively.
The PE spectra of the germacyclopentene 1b at 293,
713, and 1023 K are depicted in Figure 2. The PE
spectrum of 1b (Figure 2a) is characterized by only one
well-defined band at 8.5 eV followed by broad signals
at 10.1, 11.0, 12.0, and 13.0 eV. The first band is
assigned to the ionizations of the nitrogen atom lone
pair and the πCdC double bond electrons of 1b in line
with the PES spectrum of 1a . The ionizations of
electrons in the exo σSiC and cyclic σGeC orbitals are seen
around 10.1 eV. Ionization energies between 11.0 and
12.0 eV arise from ionizations of electrons mainly
localized on the chlorine atom.
(18) Guimon, C.; Pfister-Guillouzo, G.; Manuel, G.; Mazerolles, P.
J . Organomet. Chem. 1978, 149, 149.
(19) The Chemistry of Organic Silicon Compounds; Pata¨ı, S., Rap-
poport, Z., Eds.; Wiley: New York, 1989; p 629.
(20) Starzewski, K. A. O.; Dieck, H. T.; Bock, H. J . Organomet.
Chem. 1974, 65, 311.
Up to 713 K, the following changes are observed: the
band at 10.1 eV disappears and a new one at 9.2 eV
appears (Figure 2b). In contrast to the thermolysis of

5324 Organometallics, Vol. 18, No. 25, 1999
Foucat et al.
F igu r e 1. PE spectra of 1a at (a) 293 K, (b) 753 K, and (c) 973 K and (d) 1a at 973 K after subtraction of Me3SiF and
DMB spectra.
1a at 753 K, trimethylchlorosilane is not formed; DMB
is the only volatile product, as confirmed by a separate
product study. Furthermore, a lower ionization energy
at 7.3 eV characteristic of the nitrogen atom lone pair
of the germaimine 2a is not observed. Thus, the PE
spectrum (Figure 2d) corresponds to that of the ger-
mylene 3b, which is generated by DMB loss in the
thermal decomposition of 1b (Scheme 1).
than those of 2a , 3b, and 4a .
A second decomposition takes place at higher tem-
peratures (around 1023 K). The first band at 9.2 eV
disappears, and three new bands at 8.7, 10.3, and 11.4
eV, characteristic of the DMB ionizations are observed
(Figure 2c). Under these experimental conditions, only
DMB and Me3SiCl are detected in a separate product
study. Subtraction of the PE spectra of DMB and Me3-
SiCl from that observed at 1023 K (Figure 2e) gave a
new PE spectrum, which is quite similar to that in
Figure 1d, displaying a first band at 9.0 eV followed by
two broader bands centered at 10.7 and 13.5 eV. It is
likely that these bands correspond to the ionizations of
the germaisonitrile 4a (Scheme 1).
The calculated lengths of the GedN double bond and
the N-Si single bond (respectively 1.683 and 1.704 Å)
in 2a ′ are in good agreement with experimental data
previously obtained by single-crystal X-ray diffraction21-24
(RGedN ) 1.688-1.704 Å and RSi-N ) 1.648-1.673 Å).
Moreover, the evaluated GeNSi angle is around 136.0°,
a value that Ando observed for an N-silylated dimesi-
tylgermaimine.24 The N-silylated germaimines remain
Assign m en t of t h e Th er m olysis Sp ect r a a n d
Discu ssion . The scheme of decomposition presented in
Scheme 1 is backed up by ab initio calculations of the
first ionization energies of germaimine 2a ′, germylene
3b′, and germaisonitrile 4a ′ with less bulky substituents
(21) Veith, M.; Becker, S.; Huch, V. Angew. Chem., Int. Ed. Engl.
1990, 29, 216.
(22) Veith, M.; Detemple, A.; Huch, V. Chem. Ber. 1991, 124, 1135.
(23) Veith, M.; Rammo, A. Z. Anorg. Allg. Chem. 1997, 623, 861.
(24) Ando, W.; Ohtaki, T.; Kabe, Y. Organometallics 1994, 13, 434.

Germaimines, Germylenes, and Germaisonitriles
Organometallics, Vol. 18, No. 25, 1999 5325
F igu r e 2. PE spectra of 1b at (a) 293 K, (b) 713 K, and (c) 1023 K, (d) 1b at 713 K after subtraction of DMB, and (e) 1b
at 1023 K after subtraction of DMB and Me3SiCl spectra.
2
Ta ble 1. F ir st Ion ic Sta te En er gies (eV) of
Ger m a im in es (Me2>GedN-SiH3,
Me2>GedNSiHMe2, 2a ′, 2c′)
band at 7.3 eV of Figure 1b corresponds to the A′ ionic
state (nN), which is estimated at 7.78 eV, considering
the effect of a -SiHMe2 group (about 0.3 eV) obtained
by calculations on two less substituted N-silylated
germaimines (Table 1). We suppose that the effects of
-SiHMe2 and -SiMe3 groups are rather similar. The
ionic state
2A′(nN)
2A′′(πCdC
)
2A′′(πGedN
)
Me2GedNSiH3
Me2GedNSiHMe2
8.36
8.06
8.08
7.71
8.71
8.31
8.67
7.78
2
2a ′
2c′
8.29
7.93
second band, at 8.5 eV, arises from the ionic state A′′
associated with the π electron ionizations of the GedN
bond (calculated at 8.27 eV with the -SiHMe2 group
effect) and of the CdC double bond (evaluated at 8.29
eV). In accord with the higher energy of σGeC and σSiC
electron ionizations, we assign the 9.9 and 10.3 eV
bands to these orbitals, respectively.
bent in contrast to N-silylated silaimines, which are
nearly linear.13,25
The evaluation of the first ionic state energies of 2a ′
presented in Table 1 confirms the germaimine 2a
characterization and correlates the experimental bands
with the corresponding ionic state energies. The first
In contrast to the results of Heinemann,26 we char-
(25) Wiberg, N.; Schurz, K.; Reber, G.; Muller, G. J . Chem. Soc.,
Chem. Commun. 1986, 591.
(26) Heinemann, C.; Herrmann, W. A.; Thiel, W. J . Organomet.
Chem. 1994, 475, 73.

5326 Organometallics, Vol. 18, No. 25, 1999
Foucat et al.
Ta ble 2. F ir st Ion ic Sta te En er gies (eV) of
The syntheses of fluoro-3,4-dimethylgermacyclopent-3-
ene 1c and cyclodigermazane 5c have been reported
previously15 (eq 2). We have thermolyzed separately
Ger m ylen es (3b′, :GeClN(SiH3)2)
ionic state
3b′
:GeClN(SiH3)2
2A′′(nN - nCl
)
2A′(nGe
)
9.10
9.78
9.73
Ta ble 3. F ir st Ion ic Sta te En er gies (eV) of
Ger m a ison itr iles (4a ′, 4d ) a n d Ger m a n itr ile HGeN
ionic state
2Π(ΠGedN
)
2Σ(nGe
)
4a ′
4d
HGeN
9.14
9.93
9.94
10.48
11.69
11.4
(2)
acterize only one minimum on the potential energy
surface of the germylene 3b′, which corresponds to a
coplanar structure. The GeN bond length calculated
(1.857 Å) is slightly shorter than those in :Ge(N(SiMe3))2
(RGe-N ) 1.89 Å).29 Conversely, the GeCl bond is longer
(2.28 Å) than those in the symmetrical compound :GeCl2
(RGe-Cl ) 2.183 Å).27,28
these compounds and analyzed if the same species were
generated from these two different precursors.
The PE spectra of 1c at 293, 673, and 1073 K are
reported in Figure 3.
The ionization energies evaluation is presented in
Table 2. According to the number of heavy atoms in 3b′
and the large basis set used, we have only evaluated
the first 2A′′ ionic state energy. It corresponds to the
ejection of an electron from the nitrogen and chlorine
As for the precursor 1a , we interpret the spectrum of
1c from the germacyclopentene18 and HN(t-Bu)SiMe3
amine19 PES studies. The latter shows a first band at
8.4 eV associated with the ionization of a nitrogen lone
pair electron. According to these results, we attribute
the first band of the Figure 3a PE spectrum at 8.4 eV
to the ejection of an electron from the nN lone pair and
the πCdC germacyclopentenic ring. The next signals at
9.7 and 10.3 eV result, respectively, from the ionizations
of the antibonding combinations of σGeC and σSiC elec-
trons. The shoulder at 11.6 eV corresponds to the
ejection of an electron from the σGeN orbitals.
On heating to 673 K, this spectrum is completely
different. A new band is observed at 7.0 eV, followed
by a second intense one at 8.7 eV with a shoulder at
8.1 eV. The next bands at 10.9, 11.4, and 13.0 eV are
poorly resolved. The ionization at 10.9 eV is character-
istic of Me3SiF, whose formation is confirmed by a
separate product study and NMR analysis of the ther-
molysis products. Then, the Figure 3b spectrum corre-
sponds to the germaimine 2c.
Up to 1073 K, the PE spectrum changes. The previous
band at 7.0 eV disappears, while the second one at 8.7
eV widens (Figure 3c). In a separate product study we
detect DMB, isobutene, and Me3SiF by their character-
istic ionizations at 8.7, 10.3, and 11.4 eV for the first,
9.4 and 11.8 eV for the second, and 10.9 eV for the third.
The loss of isobutene (â-elimination) is often observed
when thermolyzing compounds with tert-butyl groups.
Subtraction of the PE spectra of DMB and Me3SiF from
that observed at 1073 K gives a new spectrum (Figure
3d). We have not undertaken a third subtraction, that
of isobutene, in order to minimize the loss of resolution
in the PE spectrum. The ionizations of this stable
product are mentioned in Figure 3d. Then, we suppose
that the bands at 9.9 and 11.4 eV correspond to those
of the germaisonitrile 4d ionizations.
In Figure 4 are depicted the 5c spectra at 293, 673,
and 1173 K. The spectrum of Figure 4a is very similar
to that of the cyclodigermazane [(CH3)2GeN-t-Bu]2
previously studied in our laboratory (two bands at 7.4
and 7.7 eV in the low-energy area associated with the
ionizations of the antibonding and bonding combinations
of the nitrogen atom lone pair electrons, respectively,
followed by three bands at 9.3, 9.7, and 10.1 eV). In 5c,
2
lone pair antibonding combination. The A′ ionic state
is associated with the nGe orbital ionization and is
calculated for the :GeClN(SiH3)2 germylene. This last
value was evaluated at 9.73 eV; in fact, it is intermedi-
ate between the experimental ionization energies as-
sociated with the nGe orbital of :GeCl2 (10.55 eV)30 and
:Ge(N(SiMe3))2 (8.68 eV).31 This theoretical study con-
firms the characterization of germylene 3b. The first
band at 9.2 eV in Figure 2b is attributed to the ejection
of an electron from the nitrogen and chlorine atom lone
pair antibonding combination, while ionization of the
germanium atom lone pair electron corresponds to the
band at 10.0 eV. The higher energy bands around 10.7
eV are associated with the ionizations of chlorine atom
lone pair electrons. The ionizations of electrons in the
σSiC orbitals also occur within this energy range.
The optimized GeN bond length of the germaisonitrile
4a ′ (1.671 Å) is slightly shorter than that of a GedN
double bond (about 1.69-1.70 Å21-24), in agreement with
the formation of a GetN triple bond.
By the calculations of the first ionic state energies
(Table 3) and the decomposition pathway, we could
associate the band at 9.0 eV (Figures 1d and 2e) with
the ionizations of the electrons of the two πGedN orbitals
of 4a . The broad signal centered at 10.7 eV is due to
ionization of the germanium atom lone pair electron.
The σSi-C and σSi-N ionizations are also situated within
this band.
Stu d y of N-ter t-Bu tyl Sp ecies. Th er m olysis of
Ger m a cyclop en ten e 1c a n d Cyclod iger m a za n e 5c.
(27) Schultz, G.; Tremmel, J .; Hargittai, I.; Berecz I.; Bohatka, S.;
Kagramanov, N. D.; Maltsev, A. K.; Nefedov, O. M. J . Mol. Struct. 1979,
55, 207.
(28) Vajda, E.; Hargittai, I.; Kolonits, M.; Ujszarzy, K.; Tamas, J .;
Maltsev, A. K.; Mikaelian, R. G.; Nefedov, O. M. J . Organomet. Chem.
1976, 105, 33.
(29) Fjelberg, T.; Hope, H.; Lappert, M. F.; Power, P. P.; Thorne, A.
J . J . Chem. Soc., Chem. Commun. 1983, 639.
(30) J onkers, G.; Van der Kerk, S. M.; De Lange, C. A. Chem., Phys.
1982, 70, 69.
(31) Harris, D. H.; Lappert, M. F.; Pedley, J . B.; Sharp, G. J . J .
Chem. Soc., Dalton Trans. 1976, 945.

Germaimines, Germylenes, and Germaisonitriles
Organometallics, Vol. 18, No. 25, 1999 5327
F igu r e 3. PE spectra of 1c at (a) 293 K, (b) 673 K, and (c) 1073 K and (d) 1c at 1073 K after subtraction of DMB and
Me3SiF spectra.
the germacyclopentene rings are perpendicular to the
molecular plane formed by the Ge2N2 ring. That could
induce different interactions and especially the desta-
bilization of the bonding combination of the nitrogen
atom lone pairs. So, the band at 7.4 eV is associated
with the two ionizations of the antibonding and bonding
combinations of the nitrogen atom lone pair electrons,
while the ejection of an electron from the antibonding
and bonding combinations of the πCdC orbitals of ger-
macyclopentenic rings is observed at 8.2 eV. The next
bands, above 9.0 eV, correspond to the ejection of an
electron from the asymmetric and symmetric combina-
tions of GeC and GeN bonds.
(Figure 4d), is identical to that of 1c at 1073 K without
the DMB, Me3SiF, and isobutene ionizations. In fact, it
shows at 9.9 and 11.4 eV two bands, followed by another
broad one at 13.5 eV. So, we attribute these experimen-
tal ionization energies to the germaisonitrile 4d , identi-
cally synthesized by flash thermolysis of 1c and 5c.
Assign m en t of th e Sp ectr a of th e Th er m olysis
P r od u cts a n d Discu ssion . Ab initio calculations on
germaimine 2c′ and germaisonitrile 4d confirm the
previous assignment.
The lengths of GedN and N-C bonds of 2c′, calcu-
lated at 1.696 and 1.460 Å, respectively, are close to the
experimental values obtained by single-crystal X-ray
From 673 to 873 K this spectrum changes and shows
a band at 7.0 eV (Figure 4b), followed by a second one,
more intense, at 8.7 eV. The other ionizations are found
at 9.7, 10.3, and 11.4 eV. The last broader signal is
centered around 13.0 eV. We notice that this spectrum
is identical to that of 1c at 673 K without Me3SiF
ionizations. Thus we confirm the formation of ger-
maimine 2c generated by thermal decomposition of 1c
by loss of Me3SiF and of 5c by symmetrical cleavage of
the cyclodigermazane.
At 1173 K, the band at 7.0 eV characteristic of
germaimine 2c disappears, while we observe the DMB
(8.7, 10.3, 11.4 eV) and isobutene (9.4, 11.8 eV) ioniza-
tions (Figure 4c). The loss of these stable and volatile
compounds was confirmed by a separate product study
coupled with PES. The new spectrum, obtained after
the subtraction of the DMB and isobutene spectra
diffraction of cyclic germaimine N-arylated (RGedN
)
1.691-1.703 Å and RN-C ) 1.399-1.411 Å).32 The GeNC
angle is slightly smaller than that of the N-methylated
silanimine (SiNC 114° for 2c′, 137° for (CH3)2SidN-
CH3).
The evaluation of the first ionic state energies of
germaimine 2c′ (Table 1) confirms the germaimine 2c
characterization. Following our recent study on N-
alkylated silaimines,12 we estimate the destabilizing
effect of a tert-butyl group on the nitrogen atom com-
pared to that of the methyl group as 0.5 eV for the nN
orbital and 0.9 eV for πSidN. Assuming that substituent
effects on the GedN and SidN patterns are close, we
(32) Meller, A.; Ossig, G.; Maringgele, W.; Stalke, D.; Herbst-Irmer,
R.; Freitag, S.; Sheldrick, M. J . Chem. Soc., Chem. Commun. 1991,
1123.

5328 Organometallics, Vol. 18, No. 25, 1999
Foucat et al.
F igu r e 4. PE spectra of 5c at (a) 293 K, (b) 673 K, and (c) 1173 K and (d) 5c at 1173 K after subtraction of DMB and
Me3SiF spectra.
attribute the first broad band at 7.0 eV (Figures 3b and
4b) to the ionizations of the πGedN and nN electrons. The
signal at 8.1 eV is associated with the ejection of an
electron from the πCdC orbital. The ionizations of the
combinations of asymmetric and symmetric σGeC elec-
trons are located at higher energy, 9.7 and 10.3 eV,
respectively. The intense band at 8.7 eV is characteristic
of the first ionization of DMB. It seems that the second
decomposition process, corresponding to the loss of a
DMB molecule, has begun at this temperature during
internal thermolysis.
The confirmation of the germaisonitrile 4d formation
by thermal decomposition of two different precursors is
backed up by an ab initio study of the first ionization
energies of 4d (Table 3). This linear molecule contains,
like germaisonitrile 4a ′, a triple GeN bond (about 1.653
Å), shorter than a double bond (about 1.71 Å). We have
also evaluated the first ionic state energies of the
germaisonitrile structural isomer, germanitrile, HGeN
(Table 3).
The analysis of the electronic structure of HGeN
indicates that this species posseses a “real” triple bond
(σ bond and two π bonds), while germaisonitrile should
be considered a nitrene in the triplet state in interaction
with a nonhybridized germanium atom.
The energies of the first two ionic states of the two
isomers are very close (Table 3). However, as for the
silicon analogues silaisonitriles,33,34 we find that the
germaisonitrile is much more stable than the germani-
trile (70.7 kcal/mol). So, given the decomposition path-
way and the high isomerization barrier (83.7 kcal/mol),
we assign the PE spectrum in Figure 4d to germaisoni-
trile 4d . The band at 9.9 eV is assigned to the two Π
ionic states corresponding to ionization of the πGedN
electrons. The next band at 11.4 eV corresponds to the
2Σ+ ionic state associated with the ionization of a
germanium lone pair electron.
2
Con clu sion
This study shows the potential of PES to characterize
low-coordinate germanium compounds. Germacyclo-
pentenes are a useful source of these reactive species.
This work constitutes a simultaneous characterization
in the gas phase of three classes of low-coordinate
germanium compounds: germaimines, germylenes, and
germaisonitriles. Each species exhibits characteristic
low-energy ionizations which allow their unambiguous
identification. The observed ionization potentials allow
some general observations: (1) Ionization potentials
arising from the germaimine or germaisonitrile πGedN
orbital are strongly destabilized when we compare them
to those of the analogous carbon compounds. This result
explains the high kinetic instability of the germanium
species. (2) We note the ability of the germanium
analogue of carbenes to be electronically stabilized by
adjacent heteroatom lone pairs.
(33) Nyulaszi, L.; Karpati, T.; Veszpre´mi, T. J . Am. Chem. Soc. 1994,
116, 7239.
(34) Apeloig, Y.; Albrecht, K. J . Am. Chem. Soc. 1995, 117, 7263.

Germaimines, Germylenes, and Germaisonitriles
Organometallics, Vol. 18, No. 25, 1999 5329
(CH2); 129.94 (Cd). Anal. Calcd for C12H28NFSi2Ge: C, 43.16;
H, 8.39. Found: C, 42.89; H, 8.59.
Exp er im en ta l Section
F VT-P ES Cou p lin gs. Photoelectron spectra were recorded
on an Helectros 0078 spectrometer, monitored by a microcom-
puter system supplemented by a digital analogic converter
(DAC). The spectra were constructed with 2000 points and
were accurate within 0.1 eV. They were recorded with 21.21
eV HeI radiation as the photon source and calibrated on well-
known helium ionization at 4.98 eV and those of nitrogen at
15.59 and 16.98 eV.
Two types of pyrolysis experiments, coupled with photo-
electron spectroscopy, were performed. First, a flash pyrolysis
was effected inside the ionization chamber of a UV-PES
spectrometer equipped with an internal heating device. The
pressure of the ionization chamber was 10-5 mbar without
sample. This first experiment, called “internal thermolysis”,
allowed the analysis of both the species with short lifetime
and their decomposition products. The second system was an
apparatus external to the spectrometer which allowed a
separate product study. Thermolysis was performed in a
vacuum device (pressure in the oven: 10-3 mbar without
sample) connected to the spectrometer. In this case, only the
stable and volatile species were characterized after cryogenic
distillation. Unstable compounds decomposed and/or poly-
merized because of the long distance between the oven and
the ionization head.
Syn th esis of 1b, C12H28NClSi2Ge, 1-Bis(tr im eth ylsilyl-
a m in o)-1-ch lor o-3,4-d im eth yl-1-ger m a cyclop en t-3-en e. In
a 100 mL flask 2 g of 1,1-dichloro-3,4-dimethyl-1-germacycle-
pent-3-ene35 was added to 25 mL of anhydrous THF. Then 8.9
mL of a 1 M (Me3Si)2NLi solution was added dropwise. The
mixture was stirred for 2 h. Solvents were evaporated, and
40 mL of hexane was added. The LiCl precipitate was filtered
and the hexane evaporated. Vacuum distillation provided 1.96
g of a colorless liquid. Bp: 60 °C/0.01 mmHg. Yield: 1.96 g,
63%. MS [EI,m/z] (%): 351 [M+•,4]; 269 (8); 254 (100); 146(22).
1H NMR (C6D6) δ (ppm): 0.26 (s, 18H); 1.56 (s, 6H), 1.98 (m,
4H). 13C NMR (C6D6) δ (ppm): 4.43 (CH3Si); 16.66 (CH3); 36.64
(CH2); 129.14(Cd). Anal. Calcd for C12H28NClSi2Ge: C, 41.13;
H, 8.00. Found: C, 40.78; H, 8.24.
The syntheses of 1c and 5c have been described15 previously.
Ca lcu la tion Meth od s. The theoretical evaluations were
performed with an ab initio molecular orbital calculation
method contained in the Gaussian 94 program package.36 The
geometrical parameters of neutral species were optimized with
the density functional theory using the B3LYP hybrid analyti-
cal function and LANL2DZ(d) (D95 on H, C, N,37 Los Alamos
ECP plus DZ on Si, Ge38) basis set for 2a ′ and 2c′, the 6-311G-
(d) basis set for 3b′ and 4a ′, and the 6-311G(d,p) basis set for
4d . Ionization energies (IPs) were evaluated at ∆SCF (IPs )
Eneutral - Eion) level.
During external thermolysis, the pyrolysate could also be
condensed and analyzed by 1H NMR spectroscopy. The spectra
were recorded on Bruker 400 MHz or on Hitachi R-1200 60
MHz spectrometers.
Ack n ow led gm en t. We thank the Institut du De´-
veloppement des Ressources Scientifiques (IDRIS-Or-
say, France) administrated by the CNRS for the calcu-
lation facilities for this research.
Syn th esis of 1a , C12H28NF Si2Ge, 1-Bis(tr im eth ylsilyl)-
a m in o-1-flu or o-3,4-d im eth yl-1-ger m a cyclop en t-3-en e. In
a 250 mL flask, a solution of (Me3Si)2NLi (30.6 mmol, 4.9 g,
19 mL of n-BuLi 1.6 M in hexane) was added dropwise to a
solution of 1,1-difluoro-3,4-dimethyl-1-germacyclopent-3-ene35
(5.85 g, 30.4 mmol) in 25 mL of dry benzene at room
temperature. After the addition, the mixture was heated at
reflux for 2 h. Lithium fluoride was removed by centrifugation
and washed twice with benzene. Solvents were evaporated,
and the residue was distilled under vacuum. Bp: 76 °C/0.07
mmHg. Yield: 3.95 g, 39%. MS [EI,m/z] (%): 335 [M+•],(9); 316
OM990236F
(36) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Gill, P. M.;
J ohnson, B. G.; Robb, M. A.; Cheeseman, J . R.; Keith, T. A.; Petersson,
G. A.; Montgomery, J . A.; Raghavachari, K.; Al-Lahan, M. A.; Zakreze-
wski, V. G.; Ortiz, J . V.; Foresman, J . B.; Ciolowski, J .; Stefanov, B.
B.; Nanayahkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen,
M.; Wong, W.; Andres, J . L.; Replogle, E. S.; Gomperts, R.; Martin, R.
L.; Fox, D. J .; Binkley, J . S.; Defrees, D. J .; Baker, J .; Stewart, J . P.;
Head-Gordon, M.; Gonzalez, C.; Pople, J . A. Gaussian 94 (Revision D.1);
Gaussian Inc.: Pittsburgh, PA, 1995.
1
(5); 253 (4); 238 (100). H NMR (C6D6) δ (ppm): 0.24 (s, 18H);
1.53 (s, 6H), 1.80 (m, 4H). 19F ΝΜR (C6D6) δ (ppm): -88.2 (m).
13C NMR (C6D6) δ (ppm): 4.32 (CH3Si); 18.94 (CH3); 29.91
(37) Dunning, T. H.; Hay, P. J . Modern Theoretical Chemistry;
Schaefer, H. F., Ed.; Plenum: New York, 1976.
(38) Hay, P. J .; Wadt, W. R. J . Chem., Phys. 1985, 82, 270. Hay, P.
J .; Wadt, W. R. J . Chem., Phys. 1985, 82, 284. Hay, P. J .; Wadt, W. R.
J . Chem. Phys. 1985, 82, 299.
(35) Manuel, G. Ph.D. Dissertation, Paul Sabatier University,
Toulouse, 1970.