A R T I C L E S
Chizmeshya et al.
to a H3Ge-SiH2-GeH2-GeH3 molecular structure in which a
central SiH2GeH2 core is bonded with two terminal GeH3
moieties. In particular, the sextets at 3.30 and 3.03 ppm can be
associated, respectively, with the SiH2 and GeH2 protons of the
central -SiH2GeH2- while the triplets at 3.23, 3.13 ppm
correspond to terminal GeH3 protons. The SiH2 and GeH2 NMR
frequencies in this case correlate well with those in the
previously reported (H3Ge)2SiH2 and GeH3GeH2SiH3 com-
pounds.7 The assignment of the sextet at 3.30 ppm to the SiH2
Si-Ge (or Ge-Si-Ge-Ge) skeletal plane, as well as the
isobutane-like positional isomer i-Si(SiGeGe) (Figure 1). These
isomers are completely analogous to those previously reported
in tetrasilane and related molecules, and in subsequent discussion
we simply refer to them as n-GeSiSiGe, g-GeSiSiGe, n-
GeSiGeGe and g-GeSiGeGe, and i-Si(SiGeGe), as shown in
Figure 1. Our results indicate that the observed spectra of both
the synthesized compounds are completely accounted for by a
admixture of the n-, g-, and i-type conformations.
1
protons was further corroborated by a H-29Si HMQC NMR
Initial basis set convergence tests showed that a full resolution
of differences in the calculated properties of the conformational/
positional isomers requires an even more stringent treatment
than that used in the prior studies on (H3Ge)xSiH4-x hydrides,
particularly with regards to calculated vibrational properties.
Accordingly all of the calculations described in the present work
were carried out at the B3LYP/6-311G++(3df,2pd) level as
implemented in the Gaussian03 code.11
spectrum, which indicates a direct coupling to a single 29Si
1
resonance found at -98.2 ppm. In addition, a 2D H COSY
NMR spectrum was used to unequivocally establish the specific
ordering of the Ge-Si-Ge-Ge backbone constituents. Cross-
peaks correlate an Si-H and three Ge-H resonances at 3.30,
3.23, 3.13, and 3.03 ppm. These correspond, respectively, to
SiH2, GeH3 (terminal) connected to GeH2, Ge-H3 (terminal)
connected to SiH2, and GeH2. The mass spectra showed an
isotopic envelope at 256-238 amu as the highest mass peak
corresponding to the parent ion (SiGe3Hx+) and a fragmentation
pattern consistent with the molecular structure.
The results of the structural optimizations, obtained using
“tight” convergence criteria, are presented in Table 1. The table
lists the bond length, bond angle, and bond torsion data for
n-GeSiSiGe, g-GeSiSiGe, n-GeSiGeGe, g-GeSiGeGe, and i-Si-
(SiGeGe). In all molecules, the skeletal structure exhibits typical
Si-Si and Si-Ge bonds with the gauche conformations
exhibiting slightly (0.002 Å) longer/shorter Si-Si/Si-Ge bonds
than their linear counterparts. In the isobutane-like isomer i-Si-
(SiGeGe) the Si-Si bond lengths (2.351 Å) are found to be
intermediate to those in the other isomers, while the Si-Ge
bond is dilated to a value of 2.400 Å. All the Si-Si values,
2.350-2.352 Å, correspond closely (∼0.1%) to that found in
(C4F9SO3)2(SiH2)2 (4): Compound 4 is isolated at 95% yield
as a colorless, pyrophoric solid with a melting point of 68 °C.
It is readily soluble in ethers, CHCl3, and CH2Cl2 and slightly
1
soluble in toluene. The H NMR spectrum showed a singlet
centered at 5.08 ppm (δ Si-H) due to SiH2 and 29Si satellite
peaks. These exhibit a triplet of triplets with a one-bond Si-H
coupling of 272 Hz and a H-H three-bond coupling of 3.6 Hz.
This indicates the presence of a second SiH2 group consistent
with the -SiH2SiH2- core. Furthermore, a proton decoupled
1H-29Si HMQC showed that the proton resonance at 5.08 ppm
is directly attached to a Si atom at -30.1 ppm. These values
are consistent with those observed for the triflate analogue (SO3-
CF3)2(SiH2)2 (5).10 Compound 4 was further characterized by
FTIR, mass spectrometry, and C, H, F elemental analysis, and
the data are fully consistent with the proposed structure (see
Experimental Section).
x
bulk silicon a 3/4 ) 2.352 Å where a (5.431 Å) is the lattice
constant of crystalline Si in the diamond structure. The Ge-
Ge value found in n-GeSiGeGe and g-GeSiGeGe, 2.446 Å, is
also close to that of bulk germanium (2.449 Å). The hetero-
nuclear skeletal Si-Ge bond lengths exhibit a distribution in
the range 2.396-2.400 Å and represent an average of the Si-
Si and Ge-Ge values (2.399 Å).
The Si-H and Ge-H bond lengths occur exclusively as
central SiH2 moieties and terminal GeH3 groups in the n- and
g-GeSiSiGe isomers, with values of 1.486 Å and 1.539 Å,
respectively. In the corresponding GeSiGeGe isomers the Si-H
bonds associated with the SiH2 moiety has essentially the same
value, 1.485 Å, while the GeH2 moiety exhibits a slightly longer
Ge-H bond (1.542-1.548 Å) than that in a GeH3 terminal
group, as expected. Due to its unique structure the i-Si(SiGeGe)
isomer possesses both terminal SiH3 and GeH3 groups and a
single SiH moiety. Here the Ge-H bond lengths associated with
the terminal GeH3 groups have the same value (1.539 Å) as
that in GeH3 groups of other isomers, while the corresponding
terminal group Si-H3 bond lengths are slightly contracted
(1.483 Å). The longest Si-H bond length is associated with
the central SiH moiety and has a value of 1.488 Å. Collectively,
these bond length trends are consistent with those found
previously in the heaviest members of the (H3Ge)xSiH4-x (x )
3, 4) family of molecules.
Ab Initio Simulations of Molecular Properties
Structural and Thermochemical Properties: To corroborate
the identification of (GeH3)2(SiH2)2, (GeH3)2SiH(SiH3), and
(GeH3)2(SiH2GeH2) described above we simulated the structural,
energetic, and vibrational trends of these compounds. In prior
studies7 we demonstrated that B3LYP hybrid density functional
theory (DFT) simulations provide an excellent account of the
ground-state structural, thermochemical, and vibrational proper-
ties of the (H3Ge)xSiH4-x family of molecular hydrides. In
particular use of the 6-311G++(2d,2p) basis set yielded small
typical bond length and frequency discrepancies (on the order
of 0.4% and 1.4%, respectively). Accordingly, our first attempt
at reconciling the observed IR spectra with those calculated was
based on the same procedure and on the assumption that the
fundamental structures are n-butane-like. Although this approach
provided a close match to most of the main spectral features
observed, several strong low-frequency features remained unac-
counted for in the calculated IR pattern. In view of the prior
success of the B3LYP approach we concluded that n-butane-
like molecular structures alone could not account for the
spectrum of the synthesized compounds. We therefore expanded
our calculations to include the gauche conformational isomers
in which a terminal GeH3 group is rotated out of the Ge-Si-
The data presented in Table 1 also shows that the inter-
metallic skeletal bond angles, as well as those involving
hydrogen-metal bonds, are essentially unchanged between all
isomers studied. The only exception is found in the H-Ge-Si
(11) Frisch, M. J. et al. Gaussian 03, revision B.04; Gaussin Inc.; 2003.
9
6922 J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006