FULL PAPER
DOI: 10.1002/chem.201202105
Central-Atom Size Effects on the Methyl Torsions of Group XIV Tetratolyls
Maggie C. C. Ng,[a] Jason B. Harper,[a] Anton P. J. Stampfl,[b] Gordon J. Kearley,[b]
Stꢀphane Rols,[c] and John A. Stride*[a, b]
Abstract: The Group XIV tetratolyl
series X(C6H4-CH3)4 (X=C, Si, Ge, Sn,
ics, reinforced with the findings of mo-
lecular dynamics and contact surface
calculations, based upon the solid-state
structures. The torsional modes in the
lightest analogue were found to be pre-
dominantly intramolecular: the Si and
Ge analogues have a high degree of in-
termolecular methyl–methyl group in-
teractions, whilst the heaviest ana-
logues (Sn and Pb) showed pronounced
intermolecular methyl interactions with
the whole phonon bath of the lattice
modes.
ACHTUNGTRENNUNG
Pb) were studied by using inelastic
neutron scattering to measure the low-
energy phonon spectra to directly
access the methyl-group torsional
modes. The effect of increased molecu-
lar radius as a function of the size of
the central atom was shown to have
direct influence on the methyl dynam-
Keywords: crystal engineering
·
density functional calculations · in-
termolecular interactions · molecu-
lar dynamics · neutron scattering
Introduction
symmetry space group Pc[6] (Figure 1; CCDC references for
all species are: CTol4: 712743, SiTol4: SUCZIZ, GeTol4:
SUCZOF, SnTol4: PTOLSN, PbTol4: SOMMEM). The ra-
tionale behind this rather bizarre behavior is believed to lie
in the contact interactions of the methyl groups on neigh-
boring molecules. Thus, in CTol4, the relatively small molec-
ular radius results in methyl group dynamics independent of
those on neighboring molecules. Meanwhile, in SnTol4 and
PbTol4, the methyl groups interdigitate with the phenyl
groups of nearest neighbor molecules effectively coupling
them to the lattice modes. In SiTol4 and GeTol4, the molecu-
lar radii are such that the methyl groups on neighboring
molecules are potentially in direct contact, resulting in the
lowering of symmetry. Whilst the tunneling spectra gave a
direct indication of this behavior, we have now sought to di-
rectly observe the methyl dynamics in these materials to
confirm this simple, yet effective rationale. Because pure
torsional modes are inactive in both infrared and Raman
spectroscopies due to the absence of a modulation in the
dipole moment or polarizability tensor, this requires inelas-
tic scattering of thermally moderated neutrons, for which
there are no vibrational selection rules.[7]
The tetraphenyl derivatives of Group XIV elements have
great promise as supramolecular constructs in extended
porous networks.[1–3] The tetratolyl Group XIV derivatives
are important precursors toward readily usable constructs
courtesy of the ease of functionalization at the site of the
para-methyl group, yet they make a fascinating molecular
series in their own right. The methyl groups constitute the
very outer contact sphere of individual molecules and so in-
termolecular interactions potentially dominate the methyl-
group dynamics. We have previously shown that the methyl-
tunneling spectra of this series, recorded at high-resolution
inelastic neutron scattering, vary dramatically across the
series.[4] Because the tolyl groups are chemically similar
throughout the series, the dominant effect is simply that of
molecular size, which is dictated by the central atom. The
ꢀ
smallest analogue, tetratolylmethane, CACHTUNRTGNE(UNG C6H4 CH3)4 (CTol4)
is, in fact, isostructural with the largest two of the series,[4,5]
the stannane and plumbane analogues (all in space group
¯
I4), with the silane and germane species lying in the lower-
[a] Dr. M. C. C. Ng, Dr. J. B. Harper, Prof. J. A. Stride
School of Chemistry, University of New South Wales
Sydney 2052 (Australia)
Experimental Section
Fax : (+61)2-9385-6141
[b] Dr. A. P. J. Stampfl, Prof. G. J. Kearley, Prof. J. A. Stride
Bragg Institute, Australian Nuclear Science and
Technology Organisation, PMB 1
General: The silane, germane, and stannane compounds were synthesized
by using a general reaction Scheme involving the nucleophilic addition of
an organometallic reagent to the appropriate tetrachloride (Scheme 1 in
the Supporting Information). The synthesis of the carbon analogue pro-
ceeded through the corresponding tetraphenyl derivative (Scheme 2 in
the Supporting Information) and the plumbane through lead(II) chloride.
Single-crystal X-ray datasets were collected for all of the compounds and
were found to be consistent with published data, with no phase transi-
tions observed down to T=150 K; X-ray powder-diffraction profiles were
Menai, NSW 2234 (Australia)
[c] Dr. S. Rols
Institute Laue-Langevin, 6 rue Jules Horowitz
BP156, 38042 Grenoble Cedex 9 (France)
Supporting information for this article is available on the WWW
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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