The central atom size effect on the structure of group 14 tetratolyls

Article abstract of DOI:10.1002/chem.200900360

The tetraphenyl derivatives of the Group 14 elements are of great potential interest as supramolecular constructs in extended porous networks. The tetratolyl Group 14 compounds were synthesized by using a general reaction scheme involving the nucleophilic addition of an organometallic reagent to the electrophilic center of the Group 14 element. The tetrachloride derivatives were used for the synthesis of the silane, germane and stannane compounds. Sn(Tol)₄ was found to have tunnel splittings much smaller than the minimum line width available on IN16, whereas the lighter analogues displayed a remarkable size effect of the central atom. The tunneling peaks were observed to persist up to around 30 K, before softening and extending into the limit of quasielastic diffusion in which the rotors undergo thermally excited rotation.

Full text of DOI:10.1002/chem.200900360

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DOI: 10.1002/chem.200900360
The Central Atom Size Effect on the Structure of Group 14 Tetratolyls
Maggie C. C. Ng,[a] Donald J. Craig,[a] Jason B. Harper,[a] Lambert van-Eijck,[b] and
John A. Stride*[a, c]
The tetraphenyl derivatives of the Group 14 elements are
of great potential interest as supramolecular constructs in
extended porous networks.[1] The tetratolyl derivatives are
particularly important precursors to usable constructs, cour-
tesy of the ease of functionalisation at the site of the para-
methyl group. The methyl groups constitute the outer con-
tact sphere of individual tetratolyl molecules, making them
particularly sensitive to the nature of the intermolecular in-
teractions. Upon variation of the central atom, the methyl
group dynamics allow a direct probe of these contact poten-
tials across the complete series of materials. Due to the simi-
lar electronic properties of the different central atoms, the
tolyl groups are chemically similar throughout the series
meaning that the dominant effect is simply that of molecular
size, which is dictated by the central atom. Methyl tunnel-
ling spectroscopy is an extremely sensitive probe of intermo-
lecular potentials and molecular interactions about torsional
rotor units such a methyl groups, with high-resolution inelas-
tic neutron scattering (hr-INS) spectroscopy being the most
direct method to measure these small energy splittings.[2]
This is most clearly manifested at low temperatures, with
the internal molecular vibrations restricted primarily to the
ground state. So-called methyl tunnelling transitions due to
quantum interactions, or quantum tunnelling, of the wave-
functions of ground state rotors thermally trapped in the tor-
sional potential energy minima and the equivalent positions
about the torsional axis, result in observable transitions in
hr-INS spectra.[2] Distinct peaks are obtained in the meV
spectral region for each distinct methyl environment. In this
respect, the tunnelling spectra can be related directly to the
crystal structure. The heavier analogues, having tin and lead
as the central atoms, are known to be isomorphic, in the
[3]
¯
space group I4, with the silicon and germanium tetratolyl
derivatives being reported under the Pc space group.[4] This
is in stark contrast to the tetraphenyl species that are iso-
morphic throughout the Group 14 elements,[5] indicating
that the methyl groups at the molecular periphery have pro-
found effects on the extent of molecular packing inter-
actions upon variation of the size of the central atom. The
implication of existing structural data is that a symmetry
break occurs at a central atom radius of about 1.3 ꢀ (lying
between the reported radii for germanium and tin atoms),[5b]
presumably due to steric hindrance in the methyl group
environments. However, the structure of the carbon ana-
logue, obtained herein for the first time, indicates that it is
of the same structure as the heavier tin and lead centred
materials.
The tetratolyl Group 14 compounds were synthesised by
using a general reaction scheme involving the nucleophilic
addition of an organometallic reagent to the electrophilic
centre of the Group 14 element. The tetrachloride deriva-
tives were used for the synthesis of the silane, germane and
stannane compounds (see Scheme 1 in the Supporting Infor-
mation). However the synthesis of the carbon analogue pro-
ceeded through the corresponding tetraphenyl derivative
(see Scheme 2 in the Supporting Information). Crystal data-
sets were collected for all of the compounds and were found
to be consistent with published data where available. The
[a] M. C. C. Ng, Dr. D. J. Craig, Dr. J. B. Harper, Dr. J. A. Stride
School of Chemistry, University of New South Wales
Sydney 2052 (Australia)
E-mail: j.stride@unsw.edu.au
[b] Dr. L. v. van-Eijck
Institute Laue-Langevin, 6 rue Jules Horowitz
BP156 38042 Grenoble Cedex 9, (France)
carbon analogue CACTHNUTRGNE(UNG Tol)4 was found to belong to the tetrago-
3
¯
nal space group I4 and with a cell volume of 1143.89 ꢀ ,
which should be compared to 1223.32 and 1215.58 ꢀ3 for
[c] Dr. J. A. Stride
Bragg Institute, Australian Nuclear Science and
Technology Organisation, PMB 1
Menai, NSW 2234 (Australia)
(Tol)4, respectively (Figure 1).[3,7] hr-INS
SnACTHNUTRGNENG(U Tol)4 and PbACHUTNGTRENNGUN
data for the carbon, silicon, germanium and tin analogues
were recorded on the IN16 backscattering spectrometer at
ILL, Grenoble, France over a range of temperatures, 2ꢀTꢀ
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900360.
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Figure 1. Structure of CACHTUNGTRENNUNG(Tol)4 viewed along the c-axis; all carbon atoms
Figure 2. Methyl tunnelling spectra across the X
to Sn(Tol)4. Measured data is shown as points with solid lines being fits
to the data according to the models detailed in the text. The dotted line
(in Sn(Tol)4 data) is the model fit to the scattering due to vanadium and
represents the intrinsic energy resolution.
ACHUTGTNERN(NGU Tol)4 series from CACHTUNGTRENNUNG(Tol)4
(grey) displayed as 50% ellipsoids with
spheres.
H atoms (white) as 0.3 ꢀ
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
65 K (Figure 2).[8]1 Sn
ACTHNUTRGNEN(UG Tol)4 was found to have tunnel split-
tings much smaller than the minimum line width available
on IN16 (0.8 meV), whereas the lighter analogues displayed
a remarkable size effect of the central atom (Figure 2). The
tunnelling peaks were observed to persist up to around
30 K, before softening and extending into the limit of qua-
sielastic diffusion in which the rotors undergo thermally ex-
cited rotation (Figure 3). The absence of observed tunnelling
frequencies in SnACHTUNGTRENNUNG(Tol)4 is due to the methyl groups being
highly hindered by intermolecular contacts, resulting in high
torsional barriers and hence very low tunnel splittings.[9] In
addition to the backscattering detectors, IN16 also has a rel-
atively low resolution bank of diffraction detectors that are
useful to verify the crystalline phase of the sample in situ of
hr-INS data collection (Figure 4).[8] The neutron diffraction
data indicate that none of the measured samples undergoes
a significant phase transition between room temperature
and 2 K, with C
metry powder pattern in stark contrast to Si
Ge(Tol)4, although naturally, there is significant cell contrac-
tion in all samples.
Both Si(Tol)4 and Ge
ACHTUNGTRENNUNG(Tol)4 and SnACHTNUGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
A
ACHTUNGRTEN(NUNG Tol)4 display a narrow dispersion of
environments, with resolution-limited peaks at 2.82(1) and
2.68(1) meV, respectively. Both excitation and de-excitation
Figure 3. Methyl tunnelling spectra for CACTHNUTRGNEUNG(Tol)4 at T=2, 30, 35, 45, and
1
The samples were loaded into flat aluminium cans such that the sample
55 K (bottom to top). Solid lines are fits to the data according to the
models detailed in the text. The data at T=30 K was found to be inter-
mediate between the two limiting models of tunnelling and QENS.
transmission was about 90% (0.3 mm thickness) and loaded into a
cryostat prior to irradiation. The instrument was configured for an inci-
dent wavelength of 6.27 ꢀ with SiACHTNUTRGNE(NUG 111) back scattering analysers. Data
were corrected for that of the empty can and were normalised with a
vanadium sample in order to correct for different detector efficiencies.
All data manipulation was performed by using the LAMP program at
ILL.[10]
from the thermally populated excited states were observed
at equal intensity due to detailed balance; even at T=2 K,
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Structure of Group 14 Tetratolyls
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fuse Lorentzian component, with severely restricted tunnel-
ling transitions believed to lie below the intrinsic resolution
limit of the instrument (IN16 has the highest available reso-
lution of all backscattering spectrometers to date). The cor-
relation time of the torsional diffusion is inversely propor-
tional to the FWHM of the Lorentzian function,[12] hence as
the tunnelling frequency depends upon the degree to which
the methyl diffusion is perturbed, a plot of the tunnel fre-
quency against the correlation time, the reciprocal of the
FWHM of the diffusive Lorentzian, should be linear
(Figure 5). This allows us to estimate the tunnel splitting in
Figure 4. Low resolution neutron diffraction data across the
XACTHNGUTRENU(NG Tol)4
series from SnACHTUNGTRENNUNG(Tol)4 to CACHTUGNTREN(NUGN Tol)4. Measured data is shown as points with
solid lines being idealised patterns according to room temperature crystal
structures. The absence of scattered intensity at around 2q=458 is due to
sample self-absorption.
Figure 5. Linear correlation (log-linear axes) between observed tunnel
splitting and QENS line width at T=2 K in the Group 14 tetratolyl
series. For highly hindered methyl groups the tunnel splitting falls within
the instrument resolution (shaded region).
excited states up to around 200 meV are fully populated. In
addition to the inelastic features, all of the compounds dis-
played a Lorentzian component, centred on the elastic line.
This is due to thermally induced residual disorder in the
methyl group environments, essentially torsional diffusion
about the three-fold axis of the methyl groups. The relative
ratio of the inelastic:elastic peaks was almost constant
across all samples, in accord with the assumption that the
spectra are dominated by proton-centred dynamics
(Figure 2). The fact that the tunnel splittings of the silicon
and germanium analogues were found to be very close in
energy, reflects the similarity in form, in both shape and
depth, of the intermolecular potential energy surfaces at the
methyl sites between the two species. The tunnelling spec-
SnACTHNUGTRENNG(U Tol)4 to be significantly below the instrumental resolu-
tion of 0.8 meV, given that only the Lorentzian is observable,
having a (FWHM)À1 =0.58 meVÀ1.
In addition to the tetratolyl derivatives, the dimeric hexa-
tolyl species Ge
for the first time. Dimeric Ge
much the same way as the monomeric GeAHCUTNGTRENNUNG
(Tol)6 was synthesised and is reported here
2A(Tol)6 was synthesised in
(Tol)4 analogue,
CTHUNGTRENNUNG
with changes in the temperature during synthesis defining
the nature of the final species. The room-temperature X-ray
crystal structure was obtained (Figure 6).[13] Dimeric phenyl
trum of CACHTUNGTRENNUNG(Tol)4 however, could only be accurately modelled
by three resolution-broadened inelastic features (FWHM=
2.12 meV) at 4.99(3), 6.87(31) and 8.52(24) meV of unequal
intensities, from low to high energy: 0.437(10), 0.056(8) and
0.033(9). The intensity of a single tunnelling transition is
proportional to the fraction of methyl groups present in the
sample occupying that precise environment.[11] This suggests
adducts of Group 14 elements such as X2ACTHUNGETRN(NUG C6H5)6 have been
reported and have been found to be largely analogous to
the monomeric forms.[14] In this case, the hr-INS spectrum of
Ge2ACHTUNGRTNE(NUNG Tol)6·C6H6 was found to be very similar to that of Ge-
AHCTUNGRTEG(NUNN Tol)4, with a tunnel splitting of 2.71(1) meV, indicative of a
general form for the intermolecular potential energy surface,
at the methyl group sites, in each of these materials.
that in CACHTUNGTRENNUNG(Tol)4 there are a range of sites; the three identified
sites having relative populations of 13:2:1, all of which are
broadened with respect to the available energy resolution,
indicating a lower absolute symmetry in the material. This
may be evidence of an intermolecular ordering of the
methyl groups at low temperature that we intend to revisit
with high-resolution neutron diffraction data as a function
The overall picture that emerges from the study of the
methyl group dynamics of the tetratolyl adducts of the
Group 14 elements is that the central atom size dominates
the fashion to which the molecules pack into the solid state.
In the relatively small CACTHNUTRGNE(UGN Tol)4 (molecular radius, defined
À
here as the mean X CMe distance, R=5.988 ꢀ), the closest
of temperature. The Sn
G
intermolecular methyl–methyl interactions are 4.150 ꢀ
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J. A. Stride et al.
result in relatively minor chemical modifications, the effects
upon the crystal packing geometry can be considerable.
Acknowledgements
This research was supported by UNSW ECR and ARC (DP0880199)
grants and an AMFRP access grant (#07/08-N-16). Author contributions:
J.A.S. conceived and designed the experiments and developed the inter-
pretation of results. M.C.C.N. synthesised all samples under the direction
of J.B.H. D.J.C. measured the X-ray data and solved the structures.
J.A.S., M.C.C.N. and L.v-E. measured the neutron data. All authors dis-
cussed the results and commented upon the manuscript.
Figure 6. Structure of Ge
2ACTHNUGTREN(NUNG Tol)6·C6H6 viewed along the b axis; all Ge2-
ACHTUNGTRENNUNG
and C6H6 shown in capped stick representation for clarity, (Ge in black,
C in grey and H in white).
Keywords: crystallography · Group 14 elements · molecular
dynamics · neutron scattering · spectroscopic methods
À
(HMe HMe), with the closest methyl–phenyl interactions of
À
4.935 ꢀ (HMe HAr). In this regard, the intermolecular con-
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tacts are relatively tenuous, reflected in the distribution of
methyl sites observed at low temperature. However, in the
larger species Sn
ACHTUNGTRENNUNG(Tol)4 and PbACHTUGNTREN(NUGN Tol)4, R=6.506 and 6.548 ꢀ,
respectively, the closest intermolecular methyl interactions
À
À
are around 2.2 ꢀ (Me Me) and 2.4 ꢀ (Me Ph). This re-
flects an increased molecular packing efficiency due to the
inter-digitation of methyl groups between phenyl groups of
neighbouring molecules and is manifested in the hr-INS
data as prohibitively narrow tunnel splittings. It is only in
the region around R=6.2 ꢀ, for the SiACHTNUGRTENNUG(Tol)4 and GeACHTUGNTREN(NUGN Tol)4
compounds, that the intermolecular methyl interactions
enter into a critical regime of around 3 ꢀ (this behaviour is
[7] Crystal structure data for CACHTNURTGNEUNG(Tol)4: C29H28, Mr =376.54, crystal size
0.25ꢊ0.20ꢊ0.15 mm3, tetragonal, space group, I4 (no. 82) a=
12.668(2), b=12.668(2), c=7.128(3) ꢀ, b=110.435(2)8, V=
1143.9(4) ꢀ3, T=294 K, Z=2, 1calcd =1.09 gcmÀ3, l=0.71073 ꢀ, 603
reflections collected, 387 unique (Rint =0.014), R(F)=0.049 and
wR2=0.067 using 387 reflections with I>2s(I). CCDC-712742 con-
tains the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif
[8] B. Frick, M. A. Gonzalez, Phys. B 2001, 301, 8–19.
¯
also reflected in the related dimeric material Ge2ACHTNUTRGNEN(UG Tol)6·C6H6
À
À
with Me Me and Me Ph interactions of around 4.8 and
2.7 ꢀ, respectively). In this scenario, the methyl groups
reside in well-defined intermolecular potential wells by
virtue of the relatively strong intermolecular interactions,
driving the whole crystal structure to a lower symmetry. Par-
adoxically, it is the lowering of the overall structural symme-
try in these materials, including distortion of the tetrahedral
molecular unit itself, which acts to augment the symmetry of
the potential energy surface at the methyl sites. This crystal
[9] S. Clough, A. Heidemann, N. J. Paley, J.-B. Suck, J. Phys. C 1980, 13,
6599–6606.
[10] LAMP, http://wwwold.ill.fr/data_treat/lamp/lamp.html.
[11] M. Prager, J. Combet, S. F. Parker, A. Desmedt, R. E. Lechner, J.
Phys. Condens. Matter 2002, 14, 10145–10157.
strain is relieved in SnACHTUNRTGENNUG(Tol)4 and PbACHTUTGNREN(NGUN Tol)4 as a result of the
[12] F. Volino, A. J. Dianoux, Mol. Phys. 1977, 34, 1263–1277.
[13] Crystal structure data for Ge2ACHTUNGRTNEG(UN Tol)6·C6H6: Ge2C48H48, Mr =784.1,
tendency of the tolyl groups to undergo inter-digitation in
light of the increased molecular radius, R. It is this interplay
of intermolecular interactions and inter-digitation that re-
stricts the unit cell volume change, from C
Pb(Tol)4, to only 20% of the change that would be expected
given molecular units based upon spheres of radius R. The
enhanced understanding of the intermolecular potential
energy surfaces across the tetratolyl species of the Group 14
elements facilitates the incorporation of large tetrahedral
supramolecular constructs such as the tetraphenyl deriva-
tives into macromolecular systems, including metal-organic
and covalent framework materials. It is apparent that whilst
the addition of methyl functionalities onto such groups may
crystal size 0.21ꢊ0.19ꢊ0.15 mm3, monoclinic, space group, P21/c
(no. 14) a=11.545(3), b=10.935(2), c=17.136(5) ꢀ, b=107.11(1)8,
V=2067.6(9) ꢀ3, T=294 K, Z=2, 1calcd =1.26 gcmÀ3, l=0.71073 ꢀ,
3822 reflections collected, 3631 unique (Rint =0.040), R(F)=0.056
and wR2=0.061 using 2269 reflections with I> 2s(I). CCDC-
712743 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif
ACHTUNGRTEN(NUNG Tol)4 through to
ACHTUNGTRENNUNG
[14] a) M. Drꢆger, Comments Inorg. Chem. 1986, 5, 201–214; b) C.
Schneider, M. Drꢆger, J. Organomet. Chem. 1991, 415, 349–362.
Received: February 9, 2009
Published online: June 2, 2009
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Chem. Eur. J. 2009, 15, 6569 – 6572