Oligosiloxanediols as building blocks for supra-molecular chemistry: Hydrogen-bonded adducts with amines form supramolecular structures in zero, one and two dimensions

Article abstract of DOI:10.1107/S0108768199013051

The structure of 1,1,3,3,5,5-hexaphenyltrisiloxane-1,5-diol-pyrazine (4/1), (C₃₆H₃₂O₄Si₃) ₄·C₄H₄N₂ (1), contains finite centrosymmetric aggregates; the diol units form dimers, by means of O-H...O hydrogen bonds, and pairs of such dimers are linked to the pyrazine by means of O-H...N hydrogen bonds. In 1,1,3,3,5,5-hexaphenyltrisiloxane-1,5-diol-pyridine (2/3), (C₃₆H₃₂O₄Si₃) ₂·(C₅H₅N)₃ (2), the diol units are linked into centrosymmetric pairs by means of disordered O-H...O hydrogen bonds: two of the three pyridine molecules are linked to the diol dimer by means of ordered O-H...N hydrogen bonds, while the third pyridine unit, which is disordered across a centre of inversion, links the diol dimers into a C₃³(9) chain by means of O-H...N and C-H...O hydrogen bonds. In 1,1,3,3-tetraphenyldisiloxane-1,3-diol-hexamethylenetetramine (1/1), (C₂₄H₂₂O₃Si₂)·C ₆H₁₂N₄ (3), the diol acts as a double donor and the hexamethylene-tetramine acts as a double acceptor in ordered O-H...N hydrogen bonds and the structure consists of C₂²(10) chains of alternating diol and amine units. In 1,1,3,3-tetraphenyldisiloxane-1,3-diol-2,2′-bipyridyl (1/1), C₂₄H₂₂O₃Si₂·C ₁₀H₈N₂ (4), there are two independent diol molecules, both lying across centres of inversion and therefore both containing linear Si-O-Si groups: each diol acts as a double donor of hydrogen bonds and the unique 2,2′-bipyridyl molecule acts as a double acceptor, thus forming C₂²(11) chains of alternating diol and amine units. The structural motif in 1,1,3,3-tetraphenyldisiloxane-1,3-diol-pyrazine (2/1), (C₂₄H₂₂O₃Si₂) ₂·-C₄H₄N₂ (5), is a chain-of-rings: pairs of diol molecules are linked by O-H...O hydrogen bonds into centrosymmetric R₂²(12) dimers and these dimers are linked into C₂²(13) chains by means of O-H...N hydrogen bonds to the pyrazine units. 1,1,3,3-Tetraphenyldisiloxane-1,3-diol-pyridine (1/1), C₂₄H₂₂-O₃Si₂·C ₅H₅N (6), and 1,1,3,3-tetraphenyldisiloxane-1,3-diol-pyrimidine (1/1), C₂₄H₂₂O₃Si₂·C ₄H₄N₂ (7), are isomorphous: in each compound the amine unit is disordered across a centre of inversion. The diol molecules form C(6) chains, by means of disordered O-H...O hydrogen bonds, and these chains are linked into two-dimensional nets built from R₆⁶(26) rings, by a combination of O-H...N and C-H...O hydrogen bonds.

Full text of DOI:10.1107/S0108768199013051

research papers
Acta Crystallographica Section B
Structural
Science
Oligosiloxanediols as building blocks for supra-
molecular chemistry: hydrogen-bonded adducts
with amines form supramolecular structures in
zero, one and two dimensions
ISSN 0108-7681
Brian O'Leary,a Trevor R.
Spalding,a George Fergusonb²
and Christopher Glidewellb*
The structure of 1,1,3,3,5,5-hexaphenyltrisiloxane-1,5-diol±
pyrazine (4/1), (C36H32O4Si3)4ÁC4H4N2 (1), contains ®nite
Received 5 July 1999
Accepted 13 October 1999
centrosymmetric aggregates; the diol units form dimers, by
means of OÐHÁ Á ÁO hydrogen bonds, and pairs of such dimers
are linked to the pyrazine by means of OÐHÁ Á ÁN hydrogen
bonds. In 1,1,3,3,5,5-hexaphenyltrisiloxane-1,5-diol±pyridine
(2/3), (C36H32O4Si3)2Á(C5H5N)3 (2), the diol units are linked
into centrosymmetric pairs by means of disordered OÐHÁ Á ÁO
hydrogen bonds: two of the three pyridine molecules are
linked to the diol dimer by means of ordered OÐHÁ Á ÁN
hydrogen bonds, while the third pyridine unit, which is
disordered across a centre of inversion, links the diol dimers
aChemistry Department, University College,
Cork, Ireland, and bSchool of Chemistry,
University of St Andrews, St Andrews, Fife KY16
9ST, Scotland
² On leave from: Department of Chemistry and
Biochemistry, University of Guelph, Guelph,
Ontario, Canada N1G 2W1.
3
into a C3 (9) chain by means of OÐHÁ Á ÁN and CÐHÁ Á ÁO
hydrogen bonds. In 1,1,3,3-tetraphenyldisiloxane-1,3-diol±
hexamethylenetetramine (1/1), (C24H22O3Si2)ÁC6H12N4 (3),
the diol acts as a double donor and the hexamethylene-
tetramine acts as a double acceptor in ordered OÐHÁ Á ÁN
Correspondence e-mail: cg@st-andrews.ac.uk
2
hydrogen bonds and the structure consists of C2 (10) chains of
alternating diol and amine units. In 1,1,3,3-tetraphenyldi-
siloxane-1,3-diol±2,20-bipyridyl (1/1), C24H22O3Si2ÁC10H8N2
(4), there are two independent diol molecules, both lying
across centres of inversion and therefore both containing
linear SiÐOÐSi groups: each diol acts as a double donor of
hydrogen bonds and the unique 2,20-bipyridyl molecule acts as
2
a double acceptor, thus forming C2 (11) chains of alternating
diol and amine units. The structural motif in 1,1,3,3-tetra-
phenyldisiloxane-1,3-diol±pyrazine (2/1), (C24H22O3Si2)2Á-
C4H4N2 (5), is a chain-of-rings: pairs of diol molecules are
linked by OÐHÁ Á ÁO hydrogen bonds into centrosymmetric
2
2
R2 (12) dimers and these dimers are linked into C2 (13) chains
by means of OÐHÁ Á ÁN hydrogen bonds to the pyrazine units.
1,1,3,3-Tetraphenyldisiloxane-1,3-diol±pyridine (1/1), C24H22-
O3Si2ÁC5H5N (6), and 1,1,3,3-tetraphenyldisiloxane-1,3-diol±
pyrimidine (1/1), C24H22O3Si2ÁC4H4N2 (7), are isomorphous:
in each compound the amine unit is disordered across a centre
of inversion. The diol molecules form C(6) chains, by means of
disordered OÐHÁ Á ÁO hydrogen bonds, and these chains are
6
linked into two-dimensional nets built from R6 (26) rings, by a
combination of OÐHÁ Á ÁN and CÐHÁ Á ÁO hydrogen bonds.
1. Introduction
The structures of tetraorganodisiloxane-1,3-diols, HOSiR2-
OSiR2OH, exhibit a wide range of hydrogen-bonding patterns
(Lickiss, 1995). The commonest pattern of molecular aggre-
gation, exhibited for example by both HOSiMe2OSiMe2OH
(Lickiss et al., 1993) and HOSiPh2OSiPh2OH (Hossain &
Hursthouse, 1988), consists of double chains of molecules in
which each hydroxyl group acts as both a hydrogen-bond
# 2000 International Union of Crystallography
Printed in Great Britain ± all rights reserved
ꢀ
Acta Cryst. (2000). B56, 273±286
O'Leary et al. Oligosiloxanediols 273

research papers
Table 1
Experimental details.
(1)
(2)
(3)
(4)
Crystal data
Chemical formula
4(C36H32O4Si3)ÁC4H4N2
2(C36H32O4Si3)Á-
3(C5H5N)
1463.07
Triclinic
P1
10.7764 (10)
12.8122 (12)
15.1733 (13)
91.999 (11)
94.851 (13)
109.063 (7)
1968.6 (3)
1
C24H22O3Si2ÁC6H12N4
C24H22O3Si2ÁC10H8N2
Chemical formula weight
Cell setting
Space group
2531.64
Triclinic
P1
554.79
Monoclinic
P21=c
16.1555 (9)
8.4622 (10)
21.8427 (16)
90
97.386 (5)
90
2961.4 (4)
4
1.244
570.78
Triclinic
P1
Ê
a (A)
14.2354 (10)
15.2960 (14)
18.7470 (14)
71.764 (6)
88.862 (6)
62.544 (7)
3402.7 (5)
1
8.9245 (8)
10.1900 (17)
17.3660 (19)
99.57 (2)
100.519 (13)
93.80 (3)
1523.5 (3)
2
Ê
b (A)
Ê
c (A)
ꢀ ꢁꢂ†
ꢁ ꢁꢂ†
ꢂ ꢁꢂ†
3
Ê
V (A )
Z
Dx (Mg m
3
)
Radiation type
1.235
1.234
1.244
Mo Kꢀ
0.71073
25
Mo Kꢀ
0.71073
25
Mo Kꢀ
0.71073
25
Mo Kꢀ
0.71073
25
Ê
Wavelength (A)
No. of re¯ections for cell
parameters
ꢃ range (ꢂ)
9.30±19.80
0.178
294 (1)
9.65±16.70
0.164
294 (1)
9.35±14.20
0.157
294 (1)
9.30±19.25
0.153
294 (1)
1
ꢄ (mm
)
Temperature (K)
Crystal form
Crystal size (mm)
Crystal colour
Block
0.36 Â 0.33 Â 0.29
Block
0.36 Â 0.34 Â 0.30
Block
0.34 Â 0.32 Â 0.20
Block
0.38 Â 0.36 Â 0.33
Colourless
Colourless
Colourless
Colourless
Data collection
Diffractometer
Data collection method
Absorption correction
No. of measured re¯ec-
tions
Enraf±Nonius CAD-4
ꢃ/2ꢃ scans
None
Enraf±Nonius CAD-4
Enraf±Nonius CAD-4
Enraf±Nonius CAD-4
ꢃ/2ꢃ scans
None
8519
ꢃ/2ꢃ scans
None
6651
ꢃ/2ꢃ scans
None
6633
14788
No. of independent
re¯ections
14788
8519
6436
6633
No. of observed re¯ec-
tions
10215
5535
3416
5141
Criterion for observed
re¯ections
Rint
ꢃmax (ꢂ)
I > 2ꢅ(I)
I > 2ꢅ(I)
I > 2ꢅ(I)
I > 2ꢅ(I)
0.000
26.92
0.000
26.89
0.012
26.90
0.000
26.93
Range of h, k, l
16 ! h ! 18
0 ! k ! 19
22 ! l ! 23
3
13 ! h ! 12
0 ! k ! 16
19 ! l ! 19
3
20 ! h ! 20
0 ! k ! 10
0 ! l ! 27
3
11 ! h ! 11
0 ! k ! 13
22 ! l ! 21
3
No. of standard re¯ections
Frequency of standard
re¯ections
Every 120 min
Every 120 min
Every 120 min
Every 120 min
Intensity decay (%)
0
3
6.9
2.8
Re®nement
Re®nement on
R‰F2>2ꢅꢁF2†Š
wRꢁF2†
F2
F2
F2
F2
0.0401
0.1203
1.077
14788
0.0434
0.1178
1.055
8519
0.0443
0.1156
0.946
6436
0.0365
0.1166
1.104
6633
S
No. of re¯ections used in
re®nement
No. of parameters used
H-atom treatment
Weighting scheme
807
472
409
376
H atoms constrained
w = 1/[ꢅ2(Fo2) +
(0.0698P)2 + 0.0963P],
where P=(Fo2 +2Fc2)/3
0.003
H atoms constrained
w = 1/[ꢅ2(Fo2) +
(0.0588P)2 + 0.1381P],
where P=(Fo2 +2Fc2)/3
0.000
H atoms constrained
w = 1/[ꢅ2(Fo2) +
(0.0585P)2], where P =
(Fo + 2Fc2)/3
0.048
0.259
0.238
SHELXL97 (Sheldrick,
1997)
0.0027 (6)
International Tables for
Crystallography (1992,
Vol. C, Tables 4.2.6.8
and 6.1.1.4)
H atoms constrained
w = 1/[ꢅ2(Fo2) +
(0.0593P)2 + 0.2883P],
where P=(Fo2 +2Fc2)/3
0.001
2
ꢁÁ=ꢅ†
max
3
Ê
Ê
Áꢆmax (e A
)
0.263
0.264
0.343
0.286
0.279
0.213
3
Áꢆmin (e A
Extinction method
)
SHELXL97 (Sheldrick,
1997)
0.0225 (9)
International Tables for
Crystallography (1992,
Vol. C, Tables 4.2.6.8
and 6.1.1.4)
SHELXL97 (Sheldrick,
1997)
0.0040 (7)
International Tables for
Crystallography (1992,
Vol. C, Tables 4.2.6.8
and 6.1.1.4)
SHELXL97 (Sheldrick,
1997)
0.0078 (14)
International Tables for
Crystallography (1992,
Vol. C, Tables 4.2.6.8
and 6.1.1.4)
Extinction coef®cient
Source of atomic scat-
tering factors
ꢀ
274 O'Leary et al. Oligosiloxanediols
Acta Cryst. (2000). B56, 273±286

research papers
Table 1 (continued)
(5)
(6)
(7)
Crystal data
Chemical formula
Chemical formula weight
Cell setting
2(C24H22O3Si2)ÁC4H4N2
909.28
Triclinic
C24H22O3Si2ÁC5H5N
493.70
Triclinic
P1
6.4469 (15)
10.2837 (15)
10.6088 (15)
74.645 (11)
81.968 (13)
85.511 (11)
671.0 (2)
1
C24H22O3Si2ÁC4H4N2
494.69
Triclinic
P1
6.4358 (5)
10.2497 (12)
10.4056 (10)
76.000 (7)
84.438 (9)
86.372 (12)
662.32 (11)
1
Space group
Ê
P1
a (A)
Ê
8.8365 (8)
10.8028 (10)
13.3231 (16)
108.740 (11)
89.774 (14)
94.792 (10)
1199.8 (2)
1
b (A)
Ê
c (A)
ꢀ ꢁꢂ†
ꢁ ꢁꢂ†
ꢂ ꢁꢂ†
3
Ê
V (A )
Z
Dx (Mg m
3
)
Radiation type
1.258
1.222
1.240
Mo Kꢀ
0.71073
25
Mo Kꢀ
0.71073
25
Mo Kꢀ
0.71073
25
Ê
Wavelength (A)
No. of re¯ections for cell para-
meters
ꢃ range (ꢂ)
14.50±20.50
0.175
294 (1)
10.0±19.0
0.162
294 (1)
9.50±17.0
0.165
294 (1)
1
ꢄ (mm
)
Temperature (K)
Crystal form
Crystal size (mm)
Crystal colour
Plate
0.39 Â 0.39 Â 0.10
Block
0.35 Â 0.35 Â 0.33
Block
0.38 Â 0.38 Â 0.36
Colourless
Colourless
Colourless
Data collection
Diffractometer
Enraf±Nonius CAD-4
Enraf±Nonius CAD-4
Enraf±Nonius CAD-4
Data collection method
Absorption correction
No. of measured re¯ections
No. of independent re¯ections
No. of observed re¯ections
Criterion for observed re¯ections
Rint
ꢃ/2ꢃ scans
None
5200
5200
3278
I > 2ꢅ(I)
0.000
26.89
ꢃ/2ꢃ scans
None
2899
2899
2449
I > 2ꢅ(I)
0.000
26.88
ꢃ/2ꢃ scans
None
2876
2876
2389
I > 2ꢅ(I)
0.000
26.91
ꢃmax (ꢂ)
Range of h, k, l
11 ! h ! 11
0 ! k ! 13
16 ! l ! 16
3
Every 120 min
0
8 ! h ! 8
0 ! k ! 13
12 ! l ! 13
3
Every 120 min
0
8 ! h ! 8
0 ! k ! 13
12 ! l ! 13
3
Every 60 min
0
No. of standard re¯ections
Frequency of standard re¯ections
Intensity decay (%)
Re®nement
Re®nement on
R‰F2 > 2ꢅꢁF2†Š
wRꢁF2†
F2
F2
F2
0.0411
0.1117
1.038
5200
0.0403
0.1094
1.069
2899
0.0423
0.1311
1.051
2876
S
No. of re¯ections used in re®ne-
ment
No. of parameters used
H-atom treatment
Weighting scheme
292
H atoms constrained
w = 1/[ꢅ2(Fo2) + (0.0577P)2],
163
H atoms constrained
162
H atoms constrained
w = 1/[ꢅ2(Fo2) + (0.0802P)2
+ 0.1328P], where P = (Fo
+ 2Fc2)/3
0.001
0.319
0.388
SHELXL97 (Sheldrick, 1997)
0.028 (6)
International Tables for Crystal-
lography (1992, Vol. C, Tables
4.2.6.8 and 6.1.1.4)
w = 1/[ꢅ2(Fo2) + (0.0522P)2
+ 0.1916P], where P = (Fo
2
2
2
where P = (Fo + 2Fc2)/3
+ 2Fc2)/3
0.000
0.295
0.258
SHELXL97 (Sheldrick, 1997)
0.034 (5)
International Tables for Crystal-
lography (1992, Vol. C, Tables
4.2.6.8 and 6.1.1.4)
ꢁÁ=ꢅ†
0.000
0.250
max
3
Ê
Ê
Áꢆmax (e A
)
3
Áꢆmin (e A
)
0.210
SHELXL97 (Sheldrick, 1997)
0.010 (2)
International Tables for Crystal-
lography (1992, Vol. C, Tables
4.2.6.8 and 6.1.1.4)
Extinction method
Extinction coef®cient
Source of atomic scattering
factors
donor and a hydrogen-bond acceptor. In HOSiMe2OSi-
Me2OH the double chain is generated by the action of a 21
2
screw axis and the hydrogen-bonding motif is of C2 (4) type
(Bernstein et al., 1995). In HOSiPh2OSiPh2OH, where there
are three independent molecules in the asymmetric unit, there
appears to be a similar pattern of hydrogen bonds, although
the hydroxyl H atoms were not unambiguously located
(Hossain & Hursthouse, 1988); the numerous close OÁ Á ÁO
ꢀ
Acta Cryst. (2000). B56, 273±286
O'Leary et al. Oligosiloxanediols 275

research papers
contacts across inversion centres indi-
cate that at least some of the hydroxyl H
atoms are disordered.
By contrast to these long-chain
structures, the related hexaphenyl-
trisiloxane-1,5-diol HOSiPh2OSiPh2O-
SiPh2OH (Behbehani et al., 1993)
consists of dimers: each molecule
contains an intramolecular OÐHÁ Á ÁO
hydrogen bond and these molecules are
linked by intermolecular OÐHÁ Á ÁO
hydrogen bonds into centrosymmetric
dimers. The hydrogen-bonding pattern
within the dimer unit can be assigned the
graph-set descriptors N1 = S(8)D, N2 =
4
R4 (8) (Bernstein et al., 1995), so that
this dimer is, in fact, isographic with the
dimer of the ferrocenediol Fe(C5H4C-
Ph2OH)2 (Ferguson et al., 1993a).
This ferrocenediol readily forms
hydrogen-bonded adducts with a wide
range of nitrogenous bases (Ferguson et
al., 1993b; Glidewell et al., 1994;
Ferguson et al., 1995): since, in general,
silanols are somewhat more acidic than
the corresponding carbinols, it may be
expected that both HOSiPh2OSiPh2OH
and HOSiPh2OSiPh2OSiPh2OH will
form hydrogen-bonded adducts with
nitrogenous bases even more readily
than Fe(C5H4CPh2OH)2. The structure
of one such adduct, a 3:2 complex of
HOSiPh2OSiPh2OH with pyridazine,
has in fact been reported (Ruud et al.,
1991): however, the poor quality of the
X-ray data [R = 0.060 with a data-to-
parameter ratio (n/p) of only 3.05;
Ê
ꢅ(SiÐC) 0.01±0.02 A] meant that no H-
atom sites could be identi®ed, although
the hydrogen-bond locations were
readily inferred from the non-bonded
distances.
In this paper we report the synthesis
and structural characterization of
hydrogen-bonded adducts formed by the
oligosiloxanediols HO(SiPh2O)nH (n = 2
and 3) with a range of ring and cage
amines.
1,1,3,3,5,5-Hexaphenyltrisil-
oxane-1,5-diol±pyrazine (4/1) (1) forms
a ®nite, zero-dimensional aggregate;
one-dimensional aggregates in the form
of chains are formed by 1,1,3,3,5,5-
hexaphenyltrisiloxane-1,5-diol±pyridine
(2/3) (2), 1,1,3,3-tetraphenyldisiloxane-
1,3-diol±hexamethylenetetramine (1/1)
Figure 1
The molecular components of (1), showing the atom-labelling scheme: (a) siloxanediol A, (b)
siloxanediol B and (c) pyrazine. Displacement ellipsoids are drawn at the 30% probability level.
(3)
and
1,1,3,3-tetraphenyldisil-
oxane-1,3-diol±2,20-bipyridyl (1/1) (4); a
ꢀ
276 O'Leary et al. Oligosiloxanediols
Acta Cryst. (2000). B56, 273±286

research papers
Table 2
Hydrogen-bond parameters (A, ).
tallization yielded (1), m.p. 376±378 K. Analysis: found C 70.4,
H 5.2, N 1.0%; C148H132N2O16Si12 requires C 70.2, H 5.3, N
1.1%.
Compound (2): Stoichiometric quantities of the trisilox-
anediol and pyridine were dissolved in ether, and after 4 h the
solution was concentrated until a white solid just began to
form: extraction into CH2Cl2/heptane (1/1 v/v) followed by
crystallization gave (2), m.p. 361±364 K. Analysis: found C
71.6, H 5.5, N 2.9%; C87H79N3O8Si6 requires C 71.6, H 5.3, N
2.9%.
ꢂ
Ê
Compound (1)
O1AÁ Á ÁN1
2.706 (2) H1AÁ Á ÁN1
1.96 O1AÐH1AÁ Á ÁN1
150
O1BÁ Á ÁO1A 2.673 (2) H1BÁ Á ÁO1A 1.95 O1BÐH1BÁ Á ÁO1A 146
O4AÁ Á ÁO1B 2.700 (2) H4AÁ Á ÁO1B 1.92 O4AÐH4AÁ Á ÁO1B 157
O4BÁ Á ÁO4A 2.794 (2) H4BÁ Á ÁO4A 1.88 O4BÐH4BÁ Á ÁO4A 174
Compound (2)
O4Á Á ÁN71
O1Á Á ÁO1i
O1Á Á ÁN81
C81Á Á ÁO1
2.722 (3) H4Á Á ÁN71
2.809 (2) H1Á Á ÁO1i
3.048 (4) H2Á Á ÁN81
3.048 (4) H81Á Á ÁO1
1.98 O4ÐH4Á Á ÁN71
2.01 O1ÐH1Á Á ÁO1i
2.23 O1ÐH2Á Á ÁN81
2.13 C81ÐH81Á Á ÁO1
150
164
173
167
Compound (3): Equimolar quantities of the disiloxanediol
and hexamethylenetetramine were dissolved in CH2Cl2, and
after 10 h the solution was concentrated to a viscous oil.
Extraction into CH2Cl2/toluene/cyclohexane (1/1/3 v/v)
followed by crystallization gave ®rstly unreacted HMTA and
then (3), m.p. 354±356 K. Analysis: found C 64.7, H 6.3, N
10.3%; C30H34N4O3Si2 requires C 64.9, H 6.2, N 10.1%.
Compound (4): Equimolar quantities of the disiloxanediol
and of 2,20-bipyridyl were dissolved in ether, and after 6 h the
solution was concentrated to a viscous oil. Addition of ice-cold
hexane yielded a white powder and recrystallization of this
from toluene/hexane (1/3 v/v) gave (4), m.p. 343±345 K.
Analysis: found C 71.2, H 5.4, N 4.9%; C34H30N2O3Si2 requires
C 71.5, H 5.3, N 4.9%.
Compound (5): Stoichiometric quantities of the disilox-
anediol and of pyrazine were dissolved in ether, and after 5 h
the solution was concentrated to give a sticky white solid.
Crystallization from toluene/hexane (1/2 v/v) yielded (5), m.p.
373±375 K. Analysis: found C 68.5, H 5.4, N 3.1%;
C52H48N2O6Si4 requires C 68.7, H 5.3, N 3.1%.
Compound (3)
O1Á Á ÁN1
2.804 (2) H1Á Á ÁN1
2.731 (2) H2Á Á ÁN2ii
2.10 O1ÐH1Á Á ÁN1
1.96 O2ÐH2Á Á ÁN2ii
144
156
O2Á Á ÁN2ii
Compound (4)
O1Á Á ÁN51
2.869 (2) H1Á Á ÁN51
2.846 (2) H2Á Á ÁN61
2.08 O1ÐH1Á Á ÁN51
2.04 O2ÐH2Á Á ÁN61
159
167
O2Á Á ÁN61
Compound (5)
O1Á Á ÁN51
2.720 (2) H1Á Á ÁN51
2.800 (2) H2Á Á ÁO1iii
1.94 O1ÐH1Á Á ÁN51
1.99 O2ÐH2Á Á ÁO1iii
159
167
O2Á Á ÁO1iii
Compound (6)
O2Á Á ÁO2iv
2.755 (2) H1Á Á ÁO2iv
2.995 (3) H2Á Á ÁN31
2.995 (3) H31Á Á ÁO2
1.94 O2ÐH1Á Á ÁO2iv
2.18 O2ÐH2Á Á ÁN31
2.14 C31ÐH31Á Á ÁO2
170
172
174
O2Á Á ÁN31
C31Á Á ÁO2
Compound (7)
O2Á Á ÁO2iv
2.736 (2) H1Á Á ÁO2iv
2.959 (4) H2Á Á ÁN31
2.959 (4) H31Á Á ÁO2
1.92 O2ÐH1Á Á ÁO2iv
2.17 O2ÐH2Á Á ÁN31
2.10 C31ÐH31Á Á ÁO2
170
163
174
O2Á Á ÁN31
C31Á Á ÁO2
Symmetry codes: (i)
(iv) x; 1 y; 1 z.
x; y; z; (ii) x; 1 ‡ y; z; (iii)
x; y; 1 z;
Compound (6): A stoichiometric quantity of pyridine was
added to a solution of the disiloxanediol in ether, and after 5 h
one-dimensional chain-of-rings is formed by 1,1,3,3-tetra-
phenyldisiloxane-1,3-diol±pyrazine (2/1) (5); and continuous
two-dimensional sheet structures are formed by the isomor-
phous 1,1,3,3-tetraphenyldisiloxane-1,3-diol±pyridine (1/1) (6)
and 1,1,3,3-tetraphenyldisiloxane-1,3-diol±pyrimidine (1/1)
(7).
2. Experimental
2.1. Syntheses
2.1.1. Diols. 1,1,3,3-Tetraphenyldisiloxane-1,3-diol was
prepared by hydrolysis of 1,3-dichloro-1,1,3,3,-tetra-
phenyldisiloxane using a stoichiometric quantity of water in
acetone, followed by crystallization from CH2Cl2/hexane (1/2
v/v): m.p. 385±387 K (lit.: 388 K; Harris, 1963). 1,1,3,3,5,5-
Hexaphenyltrisiloxane-1,5-diol was prepared according to the
published procedure (Selin, 1977): after crystallization from
ether/cyclohexane: (1/3 v/v) m.p. 384±386 K (lit.: 385±386 K;
Selin, 1977).
2.1.2. Adducts. Compound (1): Stoichiometric quantities of
the trisiloxanediol and pyrazine were dissolved in ether, and
after 3 h the solution was concentrated to a viscous oil.
Extraction into CH2Cl2/heptane (1/1 v/v) followed by crys-
Figure 2
The ®nite molecular aggregate in (1). For the sake of clarity, only the
Cn1A and Cn1B (n = 1±6) atoms of the phenyl rings are shown; the H
atoms of the pyrazine component are also omitted.
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research papers
the solution was concentrated to a viscous oil. This oil was
dissolved in toluene, and layering with hexane yielded crystals
of (6), m.p. 396±398 K (lit.: 396±398 K; Prescott & Selin, 1965).
Analysis: found C 70.8, H 5.7, N 3.0%; C29H27NO3Si2 requires
C 70.5, H 5.5, N 2.8%.
Compound (7): Equimolar quantities of the disiloxanediol
and pyrimidine were dissolved in ether, and after 5 h the
solution was concentrated to a viscous residue. Extraction into
toluene/hexane (1/2 v/v) followed by crystallization gave (7),
m.p. 353±355 K. Analysis: found C 68.1, H 5.3, N 5.7%;
C28H26N2O3Si2 requires C 68.0, H 5.3, N 5.7%.
atom bonded to O1 was disordered over two sites, labelled H1
and H2, with equal occupancy, and (c) that in each of (6) and
(7) the unique hydroxyl H atoms were similarly disordered, in
each case over two equally populated sites.
In the ®nal re®nement cycles, H atoms were treated as
riding atoms (CÐH 0.93 and 0.97, NÐH 0.89 and 0.90, OÐH
Ê
0.82 A). Table 2 contains details of the hydrogen bonding and
Table 3 presents selected molecular dimensions. Figs. 1, 3, 5, 7,
11 and 12 show the asymmetric units of (1)±(7), and Figs. 2, 4,
6, 8, 10 and 13 show aspects of the crystal structures.
3. Results and discussion
2.2. Data collection, structure solution and refinement
3.1. Supramolecular structures
Details of data collection, and structure solution and
re®nement are summarized in Table 1.1 For (3) the space
group P21/c was uniquely determined from the systematic
absences. Compounds (1), (2) and (4)±(7) are all triclinic: for
Å
each the space group P1 was chosen and con®rmed by the
successful structure solution and re®nement.
It is convenient to consider the adducts in terms of the
dimensionality of the supramolecular aggregates. Since the
trisiloxanediol is able, because of its molecular chain length, to
form intramolecular OÐHÁ Á ÁO hydrogen bonds, this opens
the possibility that the supramolecular aggregates formed by
this diol can be ®nite and zero-dimensional, in the sense that
there is no necessity to form extended structures (chains,
sheets or frameworks). The aggregates formed by the di-
siloxanediol, on the other hand, are all likely to be at least one-
dimensional, as the diol is unable to form intramolecular
hydrogen bonds. To this extent, the general pattern of beha-
viour of these two diols re¯ects the differences observed in the
structures of the pure substances (Hossain & Hursthouse,
1988; Behbehani et al., 1993).
In (2), one of the pyridine units (that containing N81) lies
across a centre of inversion and is therefore disordered:
similarly in (6) the unique pyridine lies across a centre of
inversion, while in (7) the pyrimidine not only lies across a
centre of inversion, but also exhibits rotational disorder. For
each of (2), (6) and (7) difference maps showed clearly that
the atom sites in the alternative orientations of the pyridine in
(2) and (6), and in the four orientations of the pyrimidine in
(7), were essentially coincident: in other words the centres of
inversion in each case lies at the centroid of the hetero-
aromatic ring. Therefore, the disordered pyridine molecule in
both (2) and (6) was modelled by a ring in which sites 1 and 4
were each occupied by (0.5 C + 0.5 N); similarly, the pyrimi-
dine in (7) was modelled by a ring in which sites 1 and 4 were
occupied by (0.5 C + 0.5 N), and sites 2, 3, 5 and 6 were
occupied by (0.75 C + 0.25 N). In each of the mixed-occupancy
sites in (2), (6) and (7), the alternative occupants, C or N, were
constrained to have identical atom coordinates and in addition
the two occupants of each site were constrained to have
identical anisotropic displacement parameters: in effect each
such site was occupied by a single atom having a composite
atomic scattering factor.
3.1.1. 1,1,3,3,5,5-Hexaphenyltrisiloxane-1,5-diol±pyra-
zine (4/1) (1). The centrosymmetric aggregate in (1)
contains four molecules of the diol and one molecule of
In (3) one of the phenyl rings (that containing C21±C26)
was found to exhibit orientational disorder, occupying two sets
of sites with equal occupancy, although there was no trace of
any disorder in the other phenyl rings in this compound. This
disorder was modelled by means of two independent rings
C2nA and C2nB (for n = 1±6), freely re®ned except that Si1
was restrained to lie in the plane of each ring, C2nA and C2nB.
For each of (1)±(7), careful inspection of difference maps in
the vicinity of the nitrogen and the hydroxylic O atoms
showed (a) that there was no evidence whatever for any
transfer of H atoms from O to N, (b) that in (2) the hydroxyl H
pyrazine (Fig. 1): the asymmetric unit thus consists of two diol
molecules and one half of a pyrazine molecule. The aggregate
is conveniently regarded as a pair of hydrogen-bonded diol
dimers linked by the pyrazine (Fig. 2). However, the
construction of the diol±dimer unit in (1) is signi®cantly
different from that of the centrosymmetric dimer found in the
pure diol [Behbehani et al., 1993; Scheme I(i)]. In the pure
1 Supplementary data for this paper are available from the IUCr electronic
archives (Reference: CF0008). Services for accessing these data are described
at the back of the journal.
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278 O'Leary et al. Oligosiloxanediols
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research papers
diol, each molecule forms an intramole-
cular OÐHÁ Á ÁO hydrogen bonds in an
S(8) motif (Bernstein et al., 1995), and
two such molecules are then linked by
paired intermolecular hydrogen bonds in
4
an R4 (8) motif, so that each hydroxylic
O atom acts as both donor and acceptor
of OÐHÁ Á ÁO hydrogen bonds. In (1), by
contrast, there are no intramolecular
OÐHÁ Á ÁO hydrogen bonds at all:
instead molecule B (Figs. 1 and 2) acts as
a double donor towards molecule A,
while molecule A acts a single donor to
molecule B, in an OÐHÁ Á ÁO hydrogen
bond, and as a single donor to pyrazine
in an OÐHÁ Á ÁN hydrogen bond. There is
2
thus an R2 (16) ring divided into two
2
R2 (10) sectors and having a single
hydrogen bond, O4AÁ Á ÁO1B, in common
[Scheme I(ii)]. Atom O4B acts only as a
donor of hydrogen bonds, but the other
three hydroxylic O atoms act as both
donors and acceptors, while N1 acts as
the fourth acceptor. The three OÐ
HÁ Á ÁO and one OÐHÁ Á ÁN hydrogen
bonds thus utilize all the donor capacity,
and hence there are no hydrogen-
bonding interactions between these
centrosymmetric ®ve-component aggre-
gates.
3.1.2. 1,1,3,3,5,5-Hexaphenyltrisil-
oxane-1,5-diol±pyridine (2/3) (2). The
asymmetric unit in (2) (Fig. 3) consists of
one molecule each of pyridine and the
diol, both lying in general positions,
together with one half of a pyridine
molecule disordered across a centre of
Å
inversion in space group P1, such that
the ring centroid is coincident with the
inversion centre. The two components in
general positions are linked by an O±
HÁ Á ÁN hydrogen bond: O4 acts as
hydrogen-bond donor to N71, and H4 is
fully ordered: however, the linking of the
diol units to the third pyridine molecule
gives rise to some complexity.
In analysing the supramolecular
structure (Fig. 4) it is convenient to
consider ®rst the interactions linking into
four-molecule aggregates the molecular
components which lie in general posi-
tions, and then to consider the effect of
the centrosymmetric and disordered
pyridine units in connecting together the
four-molecule aggregates. The ordered
pyridine and the diol are linked by an
OÐHÁ Á ÁN hydrogen bond, and a pair of
Figure 3
The molecular components of (2), showing the atom-labelling scheme: (a) the siloxanediol, (b) the
1
disordered pyridine and (c) the ordered pyridine. The site labelled X81 is occupied by (12C + N)
2
and the associated site H81 has occupancy. Only one orientation of the disordered pyridine is
1
2
shown. Atoms are depicted as in Fig. 1: the site labelled with a star (*) is at symmetry position
(
1
x; y; z).
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O'Leary et al. Oligosiloxanediols 279

research papers
there are two possible H-atom sites,
related by the centre of inversion.
However, at the local level, between
any such pair of O1 atoms only one of
the H-atom sites can be occupied at a
given instant: otherwise two hydroxylic
H atoms would be required to lie within
covalent bonding distance of one
another. Thus, the site occupied by the
hydroxylic H atom bonded to O1
effectively determines the location of
the corresponding H atom bonded to
O1i, assuming that hydrogen bonding is
always maximized at the local level,
and this in turn is correlated with the
orientation of the pyridine molecules
[see (II), where
R
represents
C5H5NÁ Á ÁHO(SiPh2O)2SiPh2Ð].
Where a hydroxylic O1 atom acts as
an acceptor in an OÐHÁ Á ÁO hydrogen
bond, it necessarily carries a free,
pendent H atom, and so an OÐHÁ Á ÁN
hydrogen bond can be formed with the
N81 of the disordered pyridine acting
as an acceptor. The neighbouring
symmetry-related O1i, acting as the
Figure 4
3
Part of the crystal structure of (2), showing one of the C3 (9) chains parallel to [100]. For the sake of
clarity, only the Cn1 (n = 1±6) atoms of the phenyl rings are shown; the ordered H atoms of the
pyridine components are also omitted. The sites labelled with a star (*) or hash (#) are at symmetry
donor in the OÐHÁ Á ÁO hydrogen bond
positions ( x; y; z) and (
1
x; y; z) respectively.
linking the two adjacent O1 sites,
carries no pendent H atom, but instead
acts as an acceptor in a CÐHÁ Á ÁO hydrogen bond, where the
donor is the para-carbon C81 in the alternative orientation of
the disordered pyridine. In any domain of the crystal, two such
arrangements are possible [Scheme (II)]: a combination of
these two static arrangements with equal probabilities yields
the centrosymmetric, disordered arrangement modelled from
the X-ray diffraction data. If, on the other hand, the pyridine
units were able to switch easily from one orientation to the
other, by rotation about the appropriate inertial axis, so
interchanging N81 with the corresponding C81, then this
would cause a large-scale shift of hydroxylic hydrogen bonds,
probably by means of concerted rotations about the terminal
SiÐO bonds (Aliev et al., 1995, 1998). Thus, at the local level
the hard (Braga et al., 1995) hydrogen bonds of OÐHÁ Á ÁN and
OÐHÁ Á ÁO types link the molecular building blocks into a
®nite aggregate, but if the soft CÐHÁ Á ÁO hydrogen bonds are
these units is linked by an OÐHÁ Á ÁO hydrogen bond across
the centre of inversion at the origin. The H atom bonded to O1
(Fig. 3, Table 2) is disordered over two sites with equal
occupancy, and hence between O1 and O1i [(i) x; y; z)]
3
included, these ®nite aggregates are linked into a C3 (9) chain
running parallel to the [100] direction (Fig. 4).
3.1.3. 1,1,3,3-Tetraphenyldisiloxane-1,3-diol±hexamethyl-
enetetramine (1/1) (3). In (3) both the diol and the hexa-
methylenetetramine (HMTA) units lie in general positions
(Fig. 5). The diol acts as a double donor and the HMTA as a
double acceptor, in the formation of two rather similar OÐ
HÁ Á ÁN hydrogen bonds. These hydrogen bonds serve to link
the molecular components into chains running parallel to the
[010] direction. Within the asymmetric unit O1 acts as a
hydrogen-bond donor to N1, while O2 at (x; y; z) acts as a
donor to N2 at (x; 1 ‡ y; z), thus generating by translation a
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280 O'Leary et al. Oligosiloxanediols
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2
Table 3
Selected geometric parameters (A, ).
C2 (10) chain (Fig. 6). The other
two N atoms, N3 and N4, play no
part in the hydrogen bonding; as
ꢂ
Ê
Compound (1)
Si1AÐO1A
Si1AÐO2A
Si2AÐO2A
Si2AÐO2A
Si2AÐO3A
Si3AÐO3A
Si3AÐO4A
N1ÐC1
1.629 (2)
Si1BÐO1B
Si1BÐO2B
Si2BÐO2B
Si2BÐO2B
Si2BÐO3B
Si3BÐO3B
Si3BÐO4B
N1ÐC2i
1.635 (2)
1.618 (2)
1.618 (2)
1.618 (2)
1.628 (2)
1.635 (2)
1.619 (2)
1.326 (3)
just
a
double acceptor of
1.611 (2)
1.626 (2)
1.626 (2)
1.613 (2)
1.616 (2)
1.631 (2)
1.321 (3)
1.372 (3)
hydrogen bonds, HMTA is
adopting its most common mode
of acceptor behaviour (Coupar et
2
al., 1997). Four of the C2 (10)
chains run through each unit cell,
but there are no contacts between
neighbouring chains shorter than
the sum of van der Waals radii.
3.1.4. 1,1,3,3-Tetraphenyldi-
siloxane-1,3-diol±2,20-bipyridyl
(1/1) (4). In the structure of (4)
there are two independent types
of diol molecule: these both lie
across centres of inversion, so that
each contains a strictly linear SiÐ
OÐSi bridge (Fig. 7). The unique
2,20-bipyridyl unit, by contrast, lies
in a general position. Each diol
C1ÐC2
Si1AÐO2AÐSi2A
Si2AÐO3AÐSi3A
N1ÐC1ÐC2
157.1 (1)
164.0 (1)
121.9 (2)
116.3 (2)
Si1BÐO2BÐSi2B
Si2BÐO3BÐSi3B
N1iÐC2ÐC1
157.9 (1)
141.2 (1)
121.9 (2)
C1ÐN1ÐC2i
O1AÐSi1AÐO2AÐSi2A
Si1AÐO2AÐSi2AÐO3A
O2AÐSi2AÐO3AÐSi3A
Si2AÐO3AÐSi3AÐO4A
Compound (2)
Si1ÐO1
Si1ÐO2
Si2ÐO2
5.7 (2)
8.9 (2)
129.5 (4)
78.0 (4)
O1BÐSi1BÐO2BÐSi2B
Si1BÐO2BÐSi2BÐO3B
O2BÐSi2BÐO3BÐSi3B
Si2BÐO3BÐSi3BÐO4B
60.6 (3)
105.7 (2)
49.6 (2)
44.7 (2)
1.625 (2)
1.616 (2)
1.612 (2)
Si2ÐO3
Si3ÐO3
Si3ÐO4
1.617 (2)
1.637 (2)
1.610 (2)
Si1ÐO2ÐSi2
163.8 (1)
Si2ÐO3ÐSi3
142.5 (1)
acts as
a double donor of
hydrogen bonds, forming NÐ
O1ÐSi1ÐO2ÐSi2
O2ÐSi2ÐO3ÐSi3
Compound (3)
Si1ÐO1
Si1ÐO3
N1ÐC1
N1ÐC2
N1ÐC5
N2ÐC1
N2ÐC3
N2ÐC4
6.6 (4)
51.0 (2)
Si1ÐO2ÐSi2ÐO3
Si2ÐO3ÐSi3ÐO4
172.2 (3)
16.7 (2)
HÁ Á ÁO bonds to two separate, but
1.607 (2)
1.618 (2)
1.465 (3)
1.477 (3)
1.483 (3)
1.470 (3)
1.476 (3)
1.467 (3)
Si2ÐO3
Si2ÐO2
N3ÐC2
N3ÐC3
N3ÐC6
N4ÐC4
N4ÐC5
N4ÐC6
1.618 (2)
1.611 (2)
1.461 (3)
1.456 (3)
1.461 (3)
1.464 (3)
1.454 (3)
1.459 (3)
symmetry-related,
bipyridyl
molecules: the bipyridyl molecule
acts as a double acceptor from two
different diol molecules. This 1:1
combination of a double donor
and a double acceptor generates a
2
simple C2 (11) chain motif. Within
Si1ÐO3ÐSi2
162.3 (2)
3.7 (4)
the asymmetric unit, atoms O1
and O2, which are components of
the diols centred at (0, 0, 0) and (12,
0, 12), respectively, act as donors to
N51 and N61, respectively: the
O1ÐSi1ÐO3ÐSi2
Compound (4)
Si1ÐO1
Si1ÐO3ÐSi2ÐO2
62.2 (4)
1.622 (2)
1.6108 (6)
Si2ÐO2
Si2ÐO21
1.620 (2)
1.6111 (5)
Si1ÐO11
Si1ÐO11ÐSi1ii
C52ÐN51ÐC56
180a
117.0 (2)
Si2ÐO21ÐSi2iii
C62ÐN61ÐC66
180a
117.1 (2)
symmetry-related
O1
at
( x; y; z) in the diol centred
at (0, 0, 0) acts as a donor to N51,
also at ( x; y; z), while N61 in
Inter plane angle
(N51ÐC56)^(N61ÐC66)
Compound (5)
Si1ÐO1
Si1ÐO3
N51ÐC52
C52ÐC53
36.5 (1)
the
same
bipyridyl
at
1.627 (2)
1.630 (2)
1.317 (3)
1.367 (4)
Si2ÐO2
Si2ÐO3
1.621 (2)
1.629 (2)
1.296 (3)
( x; y; z) accepts a hydrogen
bond from O2 at ( x; y; z), a
component of the diol centred at
N51ÐC53iv
12, 0, 12 ). Propagation of these
Si1ÐO3ÐSi2
N51ÐC52ÐC53
144.7 (1)
122.6 (2)
C52ÐN51ÐC53iv
N51ivÐC53ÐC52
115.1 (2)
122.3 (2)
(
interactions thus generates a chain
(Fig. 8) running parallel to the
[101] direction.
O1ÐSi1ÐO3ÐSi2
Compound (6)
Si1ÐO1
24.8 (2)
1.6067 (5)
180²
Si1ÐO3ÐSi2ÐO2
Si1ÐO2
12.8 (2)
1.621 (2)
3.1.5. 1,1,3,3-Tetraphenyldisi-
loxane-1,3-diol±pyrazine
(2/1)
Si1ÐO1ÐSi1v
Compound (7)
Si1ÐO1
(5). The supramolecular structure
in (5) forms a particularly elegant
chain-of-rings (Bernstein et al.,
1995) in which cyclic centrosym-
metric diol dimers alternate with
pyrazine molecules, also lying
1.6074 (5)
180²
Si1ÐO2
1.623 (2)
Si1ÐO1ÐSi1v
Symmetry
codes:
(i)
x; 1 y; z;
(ii)
x; y; z;
(iii)
1
x; y; 1 z;
(iv)
1
x; y; 1 z;
(v) 1 x; 1 y; 1 z. ² Siloxanediols lie across centres of inversion.
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Acta Cryst. (2000). B56, 273±286
O'Leary et al. Oligosiloxanediols 281

research papers
across centres of inversion. The asymmetric unit (Fig. 9)
consists of just one molecule of the diol, and one half of a
pyrazine molecule. Pairs of diol molecules are linked by paired
( x; y; 1 z), which is part of the pyrazine centred at (12, 0,
12). Thus, just two types of hydrogen bond, one OÐHÁ Á ÁO and
2
2
one OÐHÁ Á ÁN, generate a C2 (13)[R2 (12)] chain-of-rings
2
OÐHÁ Á ÁO hydrogen bonds into centrosymmetric R2 (12)
running parallel to the [100] direction (Fig. 10).
3.1.6.
dimers (Fig. 10): O2 at (x; y; z) acts as a donor to O1 at
( x; y; 1 z), leaving the two O1ÐH1 groups available to
act as donors to the pyrazine molecules. Within the asym-
metric unit O1 at (x; y; z) in the diol dimer centred at (0, 0, 12)
acts as a hydrogen-bond donor to N51 in the pyrazine centred
at ( 12, 0, 12): the symmetry-related O1 at ( x; y; 1 z) in the
same diol dimer in turn acts as a donor to N51, also at
1,1,3,3-Tetraphenyldisiloxane-1,3-diol±pyridine
(1/1) (6) and 1,1,3,3-tetraphenyldisiloxane-1,3-diol±pyrimi-
dine (1/1) (7). Although pyridine and pyrimidine might be
expected to act as single and double acceptors, respectively, of
hard hydrogen bonds, both these bases form adducts of 1:1
stoichiometry with 1,1,3,3-tetraphenyldisiloxane, and the
crystals of the resulting compounds (6) and (7) are isomor-
phous. Compound (7) contains identical numbers of hydroxyl
donors and nitrogen acceptors, while in (6) there is a twofold
excess of donors.
In both (6) and (7) (Figs. 11 and 12) the unit cell contains
just one molecule of the diol and one of the amine: the diol
components lie across centres of inversion and thus contain
linear SiÐOÐSi units. In (6) the pyridine molecules are
disordered across centres of inversion and hence appear in the
average structure as pseudo-pyrazine units with half-nitrogen
occupancy of two sites. The pyrimidine molecules in (7) are
not only disordered across centres of inversion, but they also
Figure 5
The molecular components of (3), showing the atom-labelling scheme:
(a) the siloxanediol and (b) the HMTA. Only one orientation of the
disordered phenyl ring C2m (m = 1±6) is shown. Atoms are depicted as in
Fig. 1.
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282 O'Leary et al. Oligosiloxanediols
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the diol molecules are linked into chains by means of OÐ
HÁ Á ÁO hydrogen bonds in which the H atoms are disordered.
1
1
Atom O2 at (x; y; z) in the diol centred at (12, , ) and the
2
2
symmetry-related atom O2 at ( x; 1 y; 1 z), which is a
1
1
component of the diol centred at ( 12, , ), are hydrogen
2
2
bonded across the centre of inversion at (0, 12, 21). Between each
pair of neighbouring O2 atoms, related by the inversion
centres at (n, 12, 21) (n = zero or integer), only one of the H-atom
sites can be occupied at any instant. Propagation of these
interactions generates a C(6) chain running parallel to the
[100] direction (Fig. 13).
The role of the disordered amine units, whether pyridine in
(6) or pyrimidine in (7), in linking these one-dimensional
aggregates, the C(6) chains, into a two-dimensional network is
entirely analogous to the role of pyridine in (2) in linking
®nite, zero-dimensional aggregates into a continuous one-
3
dimensional system, in that case a C3 (9) chain. In an entirely
similar manner to the action of pyridine in (2), the amine units
3
in (6) and (7) give rise to C3 (9) chains running parallel to the
[001] direction: the combination of the [100] and [001] chains
generates a continuous two-dimensional net parallel to (010)
6
and built from a single type of R6 (26) ring [Fig. 13 and
Scheme (III), where R represents ÐSiPh2OSiPh2Ð].
3.2. Hydrogen-bond dimensions
The hydrogen bonds of OÐHÁ Á ÁO and OÐHÁ Á ÁN types are
all reasonably short for such bonds involving only neutral
donors and acceptors. The OÁ Á ÁO distances (Table 2) range
Figure 6
Part of the crystal structure of (3), showing one of the C2 (10) chains
parallel to [010]. For the sake of clarity, H atoms bonded to C are omitted.
2
Ê
Ê
between 2.673 (2) A in (1) and 2.809 (2) A in (2), while the
OÐHÁ Á ÁO angles have an average value of 164ꢂ, close to the
most frequently observed value, 160ꢂ (Jeffrey & Saenger,
1991). The OÁ Á ÁN distances in OÐHÁ Á ÁN hydrogen bonds
involving fully ordered amine acceptors, i.e. those in (1) and
exhibit two-site rotational disorder: the net effect of these two
modes of disorder is that the site X31 (Fig. 12) is occupied by
(12C+12N) and sites X32 and X33 are each occupied by
(34C+14N).
In each of (6) and (7), the unique disiloxanediol was posi-
tioned so that it lies across the inversion centre at (12, 21, 21), and
Ê
Ê
(3)±(5), range from 2.706 (2) A in (1) to 2.869 (2) A in (2); the
mean value of the OÐHÁ Á ÁN angle in the fully ordered cases is
157ꢂ, again close to 160ꢂ (Jeffrey & Saenger, 1991).
In (2), (6) and (7) the DÁ Á ÁA distances
involving the disordered pyridine, in (2)
and (6), or pyrimidine in (7), represent a
compromise
between
the
ideal,
minimum-energy distance for OÐHÁ Á ÁN
and CÐHÁ Á ÁO hydrogen bonds. Hence,
the OÁ Á ÁN distances in these examples
are rather long for their type and the
CÁ Á ÁO distances are correspondingly
short.
3.3. Molecular geometries
3.3.1. Disiloxane components. The
most striking feature of the disilox-
anediol component in (3)±(7) is the
variation in the geometry of the SiÐOÐ
Si fragments. The disiloxanediol
components in (4), (6) and (7) contain
strictly linear SiÐOÐSi bridges, since
these molecules lie across centres of
Figure 7
The molecular components of (4), showing the atom-labelling scheme. Atoms are depicted as in
Fig. 1.
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O'Leary et al. Oligosiloxanediols 283

research papers
inversion (Figs. 7, 11 and 12); in (3) and (5), on the other hand,
the SiÐOÐSi angles are 162.3 (2) and 144.7 (1)ꢂ, respectively
(Table 3). In the pure diol (Hossain & Hursthouse, 1988),
where the asymmetric unit contains three molecules, the SiÐ
OÐSi angles range from 147.8 (3) to 162.5 (3)ꢂ, while in the
3:2 adduct formed by the diol with pyridazine (Ruud et al.,
1991) the structure contains both linear and bent (144.5ꢂ) SiÐ
OÐSi diol units.
These observations are consistent with the occurrence of an
extremely low bending force constant for linear SiÐOÐSi, so
that the geometry of this fragment is readily perturbed by
intermolecular forces. If the deviation from linearity of the
SiÐOÐSi fragment is denoted by ꢃ, the bending potential
function V(ꢃ) can be represented by an harmonic-quartic
function (Glidewell et al., 1972)
Vꢁꢃ† ˆ Aꢃ4 Bꢃ2:
ꢁ1†
This function exhibits a barrier of height (B2/4A) at ꢃ = 0
separating two minima at ꢃ = Æ(B/2A)1/2. When B is small
relative to A, the v = 0 vibrational level lies above the barrier
and this allows the system to bend in the range ꢃ = Æ(B/A)
with essentially no energy cost. The trisiloxanediol compo-
nents in (1) and (2) also exhibit a range of SiÐOÐSi angles
spanning over 20ꢂ, again consistent with a very low force
constant for the bending of the SiÐOÐSi unit.
In (1)±(3) and (5), where the SiÐOÐSi units are all non-
linear, there is no systematic difference between SiÐO
distances of SiÐOÐ(Si) and SiÐ
Figure 8
Part of the crystal structure of (4), showing one of the C2 (11) chains
parallel to [101]. For the sake of clarity, H atoms bonded to C are omitted.
2
OÐ(H) types. However, in (4), (6)
and (7), where the diol units lie
across centres of inversion with
consequent linearity of the SiÐ
OÐSi bridges, the SiÐO distances
to the bridging O are systematically
and signi®cantly shorter than the
terminal SiÐO distances. Similarly
short SiÐO distances, less than
Ê
1.62 A,
have
been
reported
previously in a number of molecular
systems containing linear SiÐOÐSi
bridges,
including
(Ph3Si)2O,
Liles,
Ê
1.616 (1) A (Glidewell
1978), [(PhCH2)3Si]2O, 1.613 (4) A
(Glidewell Liles, 1981),
&
Ê
&
t
Ê
[ Bu(OH)2Si]2O, 1.600 (1) A (Lickiss
et al., 1991), the pyridazine adduct
of 1,1,3,3-tetraphenyldisiloxane-1,3-
Ê
diol, 1.601 (4) A (Ruud et al., 1991),
and
[Ph2(MeNHNH)Si]2O,
Ê
1.6131 (13) A (He et al., 1994). The
SiÐC distances are all typical of
their type (Allen et al., 1987).
The conformations of the diol
components show marked devia-
tions from full staggering about the
skeletal SiÐO bonds (Table 3),
while this is a necessary consequence
Figure 9
The asymmetric unit of (5), showing the atom-labelling scheme. Atoms are depicted as in Fig. 1.
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284 O'Leary et al. Oligosiloxanediols
Acta Cryst. (2000). B56, 273±286

research papers
of the cyclic dimer formation in (1)
and (5), it is perhaps unexpected in
(2) and (3). The rotational barriers
about the SiÐO bonds may be lower
than those normally encountered for
single bonds in organic molecules
because of the larger radius of Si: in
any event, as with the SiÐOÐSi
angles, the effect of the inter-
molecular forces, speci®cally the
hydrogen bonds, is suf®cient to
perturb the diol components from
their energy minima.
3.3.2. Amine components. In (3),
there is a signi®cant difference
between those CÐN bond lengths in
the HMTA component which
involve the hydrogen-bond acceptor
and the remainder (Table 3): those
bonds involving N1 and N2 (Table 2)
span
the
Ê
range
1.465 (3)±
Ê
1.483 (3) A, with mean 1.473 A,
whereas those involving N3 and N4
span
the
Ê
range
1.454 (3)±
Ê
1.464 (3) A, with mean 1.459 A. This
is entirely consistent with the pattern
of behaviour previously observed in
hydrogen-bonded adducts of HMTA
(Coupar et al., 1997), although the
difference in mean CÐN distance is
much less than those observed in
protonated HMTA (Mak et al., 1983;
Chou et al., 1987; Tebbe & Nagel,
1995).
Figure 10
2
2
Part of the crystal structure of (5), showing a C2 (13) [R2 (12)] chain-of-rings parallel to [100]. For the
sake of clarity, only the Cn1 (n = 1±4) atoms of the phenyl rings are shown; the H atoms of the pyrazine
component are also omitted. The atoms labelled with a star (*) or hash (#) are at symmetry positions
(
x; y; 1 z) and (
1
x; y; 1 z), respectively.
In (1), (4) and (5), where the
heteroaromatic amines are fully
ordered, the CÐNÐC bond angles
are all signi®cantly less than 120ꢂ,
re¯ecting the greater electro-
negativity of N and mimicking the
effect of an electronegative substi-
tuent at this position (Domenicano
& Murray-Rust, 1979).
4. Concluding comments
In the supramolecular structures of
(1)±(7), the formation of OÐHÁ Á ÁN
hydrogen bonds between the silox-
anediols and the amines is always
accompanied by the formation of
OÐHÁ Á ÁO hydrogen bonds: these
are usually intermolecular. The
diversity both of the stoichiometries
of the adducts and of the dimen-
Figure 11
The molecular components of (6), showing the atom-labelling scheme. Atoms are depicted as in Fig. 1.
Atoms marked `a' and `b' are at symmetry positions (1 x; 1 y; 1 z) and ( x; 1 y; 2 z),
respectively. The site X31 in the disordered pyridine is occupied by (12C + 21N): the occupancies of the
1
2
H1, H2 and H31 sites are all (see text).
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O'Leary et al. Oligosiloxanediols 285

research papers
sionality of their structures indicate that structural prediction
in this series remains dif®cult. Similarly hard to predict is the
skeletal geometry of the siloxanediol component, rendered
dif®cult by the ease with which the SiÐOÐSi bridge angle is
perturbed by the intermolecular forces.
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Figure 12
The molecular components of (7), showing the atom-labelling scheme.
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Figure 13
Part of the crystal structure of (6) and (7), showing one of the R6 (26)
6
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and C21 atoms of the phenyl rings are shown; the H atoms of the amine
component are also omitted. Atoms marked with a star (*) or hash (#) are
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È
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Acta Cryst. (2000). B56, 273±286