X-ray powder diffraction characterization of the elusive tetraphosphine
Si„CH2PPh2…4 silane
Norberto Masciocchi and Simona Galli
Dipartimento di Scienze Chimiche e Ambientali, Università dell’Insubria, via Valleggio 11,
22100 Como, Italy
Mona Bogza and Janet Blümel
Department of Organic Chemistry, University of Heidelberg, Im Neuenheimer Feld 270,
D-69120 Heidelberg, Germany
共Received 12 September 2006; accepted 1 December 2006兲
X-ray powder diffraction data for the tetraphosphinic Si共CH2PPh2兲4 silane are reported. Its crystal
and molecular structures were determined by simulated annealing and full-profile Rietveld
refinement methods. Si共CH2PPh2兲4 was found to have tetragonal symmetry with P-421c space
group. The lattice parameters were determined to be a = 17.211共2兲 Å, c = 7.553共1兲 Å, V
= 2237.5共5兲 Å3. The crystal structure was found to contain isolated Si共CH2PPh2兲4 molecules. In
each Si共CH2PPh2兲4 molecule, the central Si atom was fixed at the −4 symmetric position bearing
four CH2PPh2 branches. This environment was confirmed by 31P CP/MAS NMR measurements.
Thermo-diffractometric measurements in the 20– 120 ° C range were also used to estimate the linear
and volumetric thermal expansion coefficients 共⳵ ln V / ⳵T = 1.8⫻ 10−4 K−1兲, typical for very “soft”
materials. © 2007 International Centre for Diffraction Data. 关DOI: 10.1154/1.2424476兴
Key words: powder diffraction, silane, phosphine, Rietveld refinement
I. INTRODUCTION
Chelating phosphines incorporating ethoxysilane groups
are used successfully as linkers for immobilizing catalysts on
oxide supports in order to improve their recyclability and
lifetime. During the preparation of several phosphines of the
type 共EtO兲xSi共CH2PPh2兲4−x 关x = 1 – 3, Ph= C6H5 共Bogza
et al., 2005兲兴, we also synthesized the tetraphosphine
Si共CH2PPh2兲4 in high yields from cheap starting materials.
Si共CH2PPh2兲4 has the potential to act as a multidentate
bridging ligand for different metals, allowing the preparation
of organometallic polymers or dendrimers incorporating
metal complexes, as shown for the analogue
Si共CH2CH2PPh2兲4 共Hendriksen et al., 1989; Ropartz et al.,
2002兲. While the ethoxy derivatives could eventually be prepared as single crystals and were characterized by conventional X-ray diffraction structure methods 共Bogza et al.,
2005兲, Si共CH2PPh2兲4 was only obtained as a colorless polycrystalline material. In this paper, Si共CH2PPh2兲4 was characterized by ab initio X-ray powder diffraction 共XRPD兲 methods 共Masciocchi et al., 2005兲 and multinuclear NMR
spectroscopy in solution 共1H, 13C, 29Si, and 31P兲, 31P CP/
MAS NMR in the solid state, and high-resolution 共FAB兲
mass spectrometry.
II. EXPERIMENTAL
Ph2PCH2Li·共TMEDA兲 共1.76 g, 5.44 mmol兲 was dissolved in a mixture of pentane 共10 ml兲 and THF 共5 ml兲, and
then treated with SiCl4 共0.23 g, 1.36 mmol兲. Stirring the reaction mixture for 2 h at RT caused the precipitation of a
colorless material, which was filtered, washed with pentane,
and dried in vacuo. Yield: 0.74 g, 0.89 mmol, 64.5%. Relevant spectral data can be found in Bogza et al. 共2005兲.
55
Powder Diffraction 22 共1兲, March 2007
Diffraction data were recorded on a Bruker AXS D8
Advance diffractometer operating in the ␪ : ␪ mode, equipped
with a secondary beam graphite monochromator, a Na共Tl兲I
scintillation counter, and pulse-height amplifier discrimination. CuK␣ radiation 共␭ = 1.5418 Å兲 was used. The X-ray
generator and diffractometer settings were 40 kV, 40 mA,
DS 0.5°, AS 0.5°, and RS 0.1 mm. Carefully ground powders
were deposited on a quartz zero-background plate. Experimental conditions were step scan mode, with 5 ⬍ 2␪ ⬍ 105°,
⌬2␪ = 0.02°, and t = 30 s step−1. Silicon NBS 640b was used
as an external standard. The recorded pattern 共shown in Figure 1兲 could not be matched with any of the phases reported
in the Powder Diffraction File 共ICDD, 2005兲.
Peak search methods and indexing of the first 16 lines
共2␪ ⬍ 27° 兲 by TOPAS-R 共Bruker AXS, 2005兲 allowed the
determination of a primitive tetragonal cell with approximate
lattice parameters a = 17.20, c = 7.54 Å, V = 2231 Å3, and
GoF= 53.1. A survey of the 2006 version of the Cambridge
Structural Database showed no match of any compound with
these lattice metrics 共or one derived therefrom by standard
cell-reduction programs兲. We therefore decided to solve the
structure by ab initio XRPD methods using the experimental
data shown above and the assigned hkl indices are reported
in Table I.
Systematic absences, density, and geometrical considerations indicated Z = 2 and 共among others兲 space group
P-421c, later confirmed by successful structure solution and
refinement. The structural model employed in the final
whole-pattern Rietveld refinement was determined ab initio
by the simulated annealing technique implemented in
TOPAS-R; the silicon atom was fixed at the cell origin and
共partially flexible兲 rigid bodies for the phosphine branches
were used, helping convergence and stability, with three refinable conformational parameters: the C-P-C-Si and two
C-C-P-C torsional angles. The background contribution was
0885-7156/2007/22共1兲/55/4/$23.00
© 2007 JCPDS-ICDD
55
Figure 1. Raw diffraction data for Si共CH2PPh2兲4 in the 5 ⬍ 2␪ ⬍ 60° range.
modelled by a polynomial function, no preferred orientation
correction was found to be necessary, and a single isotropic
B M value was refined. Peak shapes were described by the
fundamental parameters approach with anisotropic peak
broadening. The final Rietveld refinement plot and a sketch
of the crystal and molecular structures are shown in Figures
2 and 3, respectively. Table II contains the relevant crystal
data and data analysis parameters, and Table III contains the
final fractional atomic coordinates.
TABLE I. X-ray powder diffraction data for Si共CH2PPh2兲4.
2␪obs 共deg兲
I/Io
h
k
l
dobs 共Å兲
dcal 共Å兲
7.263
10.286
12.827
14.559
16.452
18.600
20.126
20.660
21.316
22.037
23.138
23.809
24.379
24.699
25.862
26.042
26.388
27.837
28.803
30.204
31.995
32.511
100.0
1.3
1.1
5.8
62.0
1.0
82.0
4.1
1.8
28.0
3.2
4.5
18.0
5.4
5.3
4.1
3.4
4.2
3.5
6.0
4.7
3.7
0
0
1
0
1
0
1
0
0
1
0
1
1
2
2
1
0
2
2
2
2
1
0
2
1
1
2
3
1
0
1
12.1544
8.5945
6.9093
6.0772
5.3850
4.7674
4.4105
4.2972
4.1689
4.0304
3.8436
3.7342
3.6491
3.6033
3.4547
3.4249
3.3710
3.2054
3.0994
2.9585
2.7974
2.7458
4.7
3.6
3.5
4.6
2.1
2.2
1.2
1.5
1
2
1
2
1
2
1
4
1
2
2
4
1
1
2
2
1
2
1
2
1
3
2
2
1
4
1
1
3
4
7
12.1615
8.5930
6.8959
6.0793
5.3837
4.7666
4.4086
4.2957
4.1650
4.0304
3.8410
3.7342
3.6482
3.6017
3.4423
3.4189
3.3749
3.2024
3.0971
2.9566
2.7951
2.7518
33.303
33.880
35.455
35.681
36.089
37.008
37.711
38.580
1
0
0
2
2
3
3
0
4
3
4
0
4
1
0
4
5
2
3
3
4
5
6
4
6
5
5
0
6
6
0
2.6882
2.6438
2.5298
2.5143
2.4868
2.4271
2.3835
2.3318
2.6925
2.6464
2.5292
2.5138
2.4888
2.4263
2.3837
2.3350
56
Powder Diffr., Vol. 22, No. 1, March 2007
Thermodiffractometric experiments were performed in
air in the 10– 120 ° C range using a custom-made sample
heater with nominal ±0.1 ° C stability, assembled by Officina
Elettrotecnica di Tenno, Ponte Arche, Italy.
III. DISCUSSION
The crystals of Si共CH2PPh2兲4 contain isolated molecules, and each Si共CH2PPh2兲4 molecule has one central Si
atom located at the −4 symmetric position, the origin of the
unit cell of the acentric P-421c space group 共see Figure 3兲.
This feature is also shared by the Si共CH2X兲4 关X = Cl 共Daiss et
al., 2004兲 and Br 共Ilg et al., 2006兲兴 and SiAr4 species 关Ar
= Phenyl 共Claborn et al., 2002兲, Pyrrol-l-yl 共Frenzel et al.,
1995兲, 2-Furan-2-yl 共Neugebauer et al., 2000兲兴. At variance,
Si共CH2OH兲4 共Ilg et al., 2006兲 and C共CH2OH兲4 共Semmingsen, 1988兲, but not C共CH2Br兲4, P21 / c 共Klaeboe et al., 1986兲,
crystallize in different tetragonal space groups, although
maintaining crystallographically imposed S4 symmetry. Consistently, all four CH2PPh2 branches share identical environments, so that in the 31P CP/MAS NMR spectrum only one
sharp singlet appears at ␦ − 29.6 ppm 共versus 85% H3PO4兲,
i.e., at slightly higher field than the 共EtO兲xSi共CH2PPh2兲4−x
共x = 1 , 2兲 derivatives 共Bogza et al., 2005兲. Despite the typical
low accuracy suffered by the individual geometrical parameters 共bond distances and angles兲 derived from XRPD data,
the observed coordination at the central atom is rather well
defined 关Si-C共1兲 1.848共3兲 Å, C共1兲-Si-C共1兲 108.0共2兲–
110.2共1兲°兴, matching single crystal data results of the tetrahedral species cited above.
As for other nearly globular species, the crystal packing,
dictated by weak共er兲, mostly H¯H intermolecular contacts,
reflects archetypic structures, which in this case, can be retraced to the body-centered cubic one. Worthy of note, the
crystal structure of the title compound, determined by the
relative arrangement of the different molecules and by the
ordered disposition of their four branches, is acentric, but
neither polar nor chiral 共and reflects the S4 symmetry of the
isolated molecule兲.
On measuring the powder pattern of Si共CH2PPh2兲4 at
different temperatures 共10– 120 ° C兲 with 10 ° C steps, we
did not observe any discontinuity in the cell volume 共nor in
the a and c cell parameters兲, which would be necessary if a
dehydration process were active; thus, there is no manifestation of water in the lattice cavities located near 0 , 0 , 41 关which
Masciocchi et al.
56
Figure 2. Final Rietveld refinement plot with peak markers and difference plot at the bottom.
PLATON 共Spek, 2005兲 estimated as 4 ⫻ 15 Å3, with shortest
interactions of 3.45 Å—H atoms excluded兴. A nearly ideal
linear behavior for this temperature dependence is shown in
Figure 4. From the cell parameters thus obtained, the linear
and volumetric thermal expansion coefficients could be derived 共⳵ ln a / ⳵T = 4.3⫻ 10−5 K−1, ⳵ ln c / ⳵T = 9.9⫻ 10−5 K−1,
and ⳵ ln V / ⳵T = 1.8⫻ 10−4 K−1兲, showing values typical for
very “soft” materials. For comparison, near RT, ⳵ ln V / ⳵T
= 1.8⫻ 10−4 K−1 for 共liquid兲 mercury, 2.2⫻ 10−4 K−1 for
共solid兲 sulphur, and 4.8⫻ 10−4 K−1 for glycerine 共Perry et al.,
1998兲.
IV. CONCLUSIONS
In the absence of single crystals, the structure of
Si共CH2PPh2兲4, a recently published member of the polysubstituted Si共CH2X兲4 class, was determined by employing
uniquely conventional laboratory X-ray powder diffraction
data and state-of-the-art ab initio techniques. The final model
was eventually refined, in P-421c, by the Rietveld method,
using rigid groups with flexible conformational freedom 共one
C-P-C-Si and two C-C-P-C torsion angles兲. This work
completes the structural characterization of the
共EtO兲xSi共CH2PPh2兲4−x series and allows a detailed comparison with simpler tetrasubstituted E共CH2X兲4 共E = C, Si兲 derivatives.
Moreover, with the aid of a custom made sample heater,
the temperature dependence of the lattice parameters was
also determined, which indicated a 2% increase in volume
upon heating from 10 ° C up to 120 ° C. This large value,
typical for nonrigid molecular solids, is likely related to a
dynamic process involving large variations of the intra- and
intermolecular contacts, eventually leading to reorientation
of the globular molecules well before melting. Work can be
anticipated in the direction of studying the thermal behavior
of Si共CH2PPh2兲4 at higher temperatures, coupling TG and
DSC observations to HT diffraction data.
TABLE II. Crystal data and data analysis parameters for Si共CH2PPh2兲4.
Figure 3. Schematic drawing of Si共CH2PPh2兲4, viewed down 关001兴. In each
molecule, the Si atom is the unique central atom bearing four CH2PPh2
branches, and P atoms are those connected to a methylene and two phenyl
groups. Hydrogen atoms are shown as white spheres. Top: the molecular
structure 共left, ball and sticks; right, space filling models兲; bottom: the crystal packing. Relevant geometrical parameters: Si–C共1兲 1.848共3兲 Å, C共1兲-Si–
C共1兲 108.0共2兲–110.2共1兲°, Si–C共1兲-P–C共2兲 179.4共5兲, Si–C共1兲-P–C共8兲
67.4共4兲°.
57
Powder Diffr., Vol. 22, No. 1, March 2007
Formula
fw, g mol−1
System
Space group
a, Å
c, Å
V, Å3
Z
␳, g cm−3
F共000兲
2␪ range
R p, Rwp
RBragg
GoF
C52H48P4Si
824.87
Tetragonal
P − 42lc
17.211共2兲
7.553共1兲
2237.5共5兲
2
1.224
868
5–105
0.114, 0.146
0.056
3.06
X-ray powder diffraction characterization of the elusive tetraphosphine ...
57
TABLE III. Fractional atomic coordinates for Si共CH2PPh2兲4.
Atom
Si
P
C 共1兲
C 共2兲
C 共3兲
C 共4兲
C 共5兲
C 共6兲
C 共7兲
C 共8兲
C 共9兲
C 共10兲
C 共11兲
C 共12兲
C 共13兲
H 共1A兲
H 共1B兲
H 共3兲
H 共4兲
H 共5兲
H 共6兲
H 共7兲
H 共9兲
H 共10兲
H 共11兲
H 共12兲
H 共13兲
ACKNOWLEDGMENTS
x/a
y/b
z/c
0
0.102 06
0.019 40
0.090 51
0.130 69
0.120 75
0.069 72
0.028 08
0.038 02
0.191 94
0.197 83
0.264 77
0.325 36
0.320 51
0.254 44
0.009 38
−0.025 38
0.160 83
0.147 51
0.067 02
−0.007 21
0.010 77
0.155 93
0.266 93
0.372 70
0.366 66
0.245 90
0
0.093 23
0.084 71
0.188 63
0.212 74
0.287 60
0.337 36
0.313 65
0.239 19
0.100 18
0.155 09
0.161 01
0.112 23
0.056 98
0.051 35
0.132 35
0.073 59
0.180 64
0.298 29
0.387 46
0.349 14
0.221 21
0.190 51
0.198 14
0.116 93
0.022 63
0.009 84
0
0.293 64
0.143 76
0.396 33
0.544 71
0.608 92
0.528 34
0.384 78
0.317 57
0.168 23
0.037 47
−0.062 57
−0.035 75
0.092 06
0.195 79
0.082 56
0.211 00
0.590 29
0.724 52
0.572 07
0.331 18
0.204 54
0.018 02
−0.135 36
−0.098 59
0.111 41
0.282 23
Figure 4. Temperature dependence of the unit cell volume of Si共CH2PPh2兲4,
in the 10– 120 ° C range.
58
Powder Diffr., Vol. 22, No. 1, March 2007
The authors are indebted to the Fondazione Provinciale
Comasca and the Deutsche Forschungsgemeinschaft 共SFB
623兲 for financial support. We also thank Professor Angelo
Sironi 共University of Milan兲 for helpful discussions.
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