Crystalline TCNQ and TCNE adducts of the diborane(4) compounds B2(1,2-E2C6H4)2 (E = O or S)

Article abstract of DOI:10.1039/a901169b

Crystal structures of 1:1 adducts of diborane(4) compounds and the electron acceptors TCNQ and TCNE, namely B₂(1,2-E₂C₆H₄) ₂·TCNQ (E = O or S) and B₂(1,2-E₂C₆H₄) ₂·TCNE (E = O or S), have been found to show predominantly two-dimensional heteromolecular packing motifs with a variety of interlayer packings.

Full text of DOI:10.1039/a901169b

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Crystalline TCNQ and TCNE adducts of the diborane(4)
compounds B2(1,2-E2C6H4)2 (E ؍ O or S)
Todd B. Marder,a Nicholas C. Norman,*b A. Guy Orpen,*b Michael J. Quayle†b and
Craig R. Riceb
a The University of Durham, Department of Chemistry, Science Laboratories, South Road,
Durham, UK DH1 3LE
b The University of Bristol, School of Chemistry, Cantock’s Close, Bristol, UK BS8 1TS
Received 11th February 1999, Accepted 12th May 1999
Crystal structures of 1:1 adducts of diborane(4) compounds and the electron acceptors TCNQ and TCNE,
namely B2(1,2-E2C6H4)2ؒTCNQ (E = O or S) and B2(1,2-E2C6H4)2ؒTCNE (E = O or S), have been found to
show predominantly two-dimensional heteromolecular packing motifs with a variety of interlayer packings.
Charge transfer complexes of electron rich alkenes, such as
tetrathiafulvalene [2-(1,3-dithiol-2-ylidene)-1,3-dithiole] (TTF)
and tetramethyltetraselenafulvalene (TMTSF), and electron
All five crystal structures are notably similar, having the tri-
clinic space group P1 with Z = 1. The component molecules
¯
each lie over an inversion centre and the unit cell volumes are
576.3, 582.0, 516.8, 470.5 and 419.2 Å3 for 6ؒ1a, 6ؒ1b, 7ؒ1, 6ؒ2
and 7ؒ2, respectively. The crystal structure of 3ؒ1 which has
acceptors, such as TCNQ
1 (7,7,8,8-tetracyano-p-quino-
dimethane) and TCNE 2 (tetracyanoethene), constitute an
important and much studied class of crystalline organic
material with important properties including metallic conduct-
ivity and superconductivity.1 Another electron rich alkene to
have been studied in this regard, although one which affords
materials with significantly less satisfactory electronic proper-
ties,1 is dibenzotetrathiafulvalene 3, together with its selenium
and tellurium analogues 4 and 5. The structure of the adduct
3ؒ1 has been established by X-ray crystallography.2 In view of
the close structural relationship between compound 3 and the
diborane(4) compound B2(1,2-S2C6H4)2 6 (they differ by only
two electrons), and with the related oxo-derivative B2(1,2-
O2C6H4)2 7,3 we sought to investigate whether 6 and 7 would
form similar crystalline adducts with TCNQ and TCNE and to
explore the nature of the intermolecular interactions.
been previously characterised2 also crystallises in the space
3
¯
group P1 with Z = 1 and a unit cell volume of 574.9 Å .
Selected crystallographic details for all structures are given in
Table 1.
All six crystal structures (i.e. the five described here and 3ؒ1)
show a predominant two-dimensional packing in which the
layers consist of the component species in a 1:1 ratio. In the
third dimension, the crystal structures contain an ABCABC
packing arrangement in which the layers are stacked parallel
above each other along the crystallographic c axis direction.
Despite the overall similarity of the structures, however, the
two-dimensional arrangements of the component molecules
do differ between each structure, utilising a range of inter-
molecular interactions to pack efficiently. The two-dimensional
crystal structures for 6ؒ1a, 3ؒ1, 7ؒ1, 6ؒ2 and 7ؒ2 are shown in
Figs. 1–5 respectively. A discussion for each is given first
followed by a description of the three-dimensional packing.
Simplified representations of the two-dimensional structures
are shown in Fig. 6.
NC
CN
NC
CN
NC
CN
NC
CN
1
2
Common to all six crystal structures are parallel homo-
molecular ribbons in the crystallographic a direction linked
by heteromolecular interactions. Intermolecular interactions
were recorded with cut-off limits S ؒ ؒ ؒ S 3.80, S ؒ ؒ ؒ H 3.25 and
N ؒ ؒ ؒ H 2.95 Å. As shown in Fig. 7 three angles have been
employed to describe the orientations of the molecules within
the layers. The angle θ describes misalignment of the molecular
axes relative to each other, i.e. the direction of propagation in
the heteromolecular ribbon. This has been calculated as the
angle between the molecular axes of each species [through the
E
E
E
E
E
E
E
C
C
B
B
E
3, E = S; 4, E = Se; 5, E = Te
6, E = S; 7, E = O
Results and discussion
A solution of equimolar quantities of compounds 6 and 1 in
CH2Cl2 showed no colour change (6 is colourless, 1 is pale
yellow) and 1H and 11B NMR analysis provided no evidence for
any adduct formation in solution.4,‡ However, cooling to Ϫ30 ЊC
afforded dark red crystals of the 1:1 adduct 6ؒ1a. Red crystals
of the 1:1 adducts 6ؒ2, 7ؒ1 and 7ؒ2 were prepared similarly.
In addition, by employing 1,2-dichloroethane as a solvent, a
second polymorphic form of 6ؒ1a (6ؒ1b) was also crystallised.
Full experimental details and analytical data are provided in the
Experimental section.
B–B bond for 6 and 7, through the C᎐C(CN ) unit for 1 and
᎐
2
through the C᎐C bond for 2]. The angles ω and φ (φЈ is used
᎐
for 2) describe the angle of propagation of the homomolecular
ribbons. These are calculated as the angle between the molecu-
lar axis of one molecule and the line relating an atom in one
molecule to the corresponding atom in the next molecule in the
homomolecular ribbon, i.e. the a axis. Table 2 records these
angles for each crystal structure.
The two-dimensional structures of adducts 6ؒ1a (Fig. 1) and
6ؒ1b (not shown) are essentially the same. The homomolecular
ribbons of 6 are apparently driven primarily by S ؒ ؒ ؒ S and
S ؒ ؒ ؒ H contacts, there being two S ؒ ؒ ؒ S interactions and eight
S ؒ ؒ ؒ H contacts per molecule including a three-centred inter-
action between S(2), S(1a) and H(6a). The symmetry independ-
† Present address: Department of Chemical Engineering, UMIST,
PO Box 88, Manchester, UK M60 1QD.
‡ The proton and boron solution chemical shifts of compounds 6 and 7
shift markedly upfield when the boron centres are complexed by nitro-
gen (and phosphorus) donor ligands as discussed in ref. 4.
J. Chem. Soc., Dalton Trans., 1999, 2127–2132
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Table 1 Selected crystallographic details for the complexes 6ؒ1a, 6ؒ1b, 7ؒ1, 6ؒ2, 7ؒ2 and 3ؒ1
6ؒ1a
6ؒ1b
7ؒ1
6ؒ2
7ؒ2
3ؒ1a
Empirical formula
Formula weight
C24H12B2N4S4
506.24
C24H12B2N4S4
506.24
C24H12B2N4O4
442.00
C18H8B2N4S4
430.14
C18H8B2N4O4
365.90
C26H12N4S4
508.65
Crystal system
Space group
Triclinic
P1
Triclinic
P1
Triclinic
P1
Triclinic
P1
Triclinic
P1
Triclinic
P1
¯
¯
¯
¯
¯
¯
a/Å
b/Å
c/Å
α/Њ
7.803(3)
7.855(4)
9.867(4)
73.85(5)
85.71(3)
83.18(3)
576.3(4)
173(2)
7.862(2)
8.386(2)
9.745(2)
71.97(3)
72.71(2)
88.28(2)
582.0(2)
173(2)
7.1996(11)
8.560(3)
9.414(3)
67.00(2)
83.16(2)
75.47(2)
516.8(3)
173(2)
6.861(3)
7.172(3)
10.862(3)
80.80(2)
79.61(3)
64.00(2)
470.5(3)
173(2)
5.962(3)
6.024(4)
12.686(6)
97.88(2)
97.66(3)
108.91(3)
419.2(4)
173(2)
9.215(3)
10.644(4)
7.734(2)
113.32(3)
122.28(2)
67.64(3)
574.9(3)
Room
β/Њ
γ/Њ
V/Å3
T/K
temperature
1
Z
1
1
1
1
1
µ/mmϪ1
0.434
0.430
0.098
0.517
0.103
Total reflections
Independent reflections
Rint
4904
1963
0.0488
0.0470 (1324)
6120
2637
0.0337
0.0365 (1867)
4293
1749
0.0427
0.0547 (1164)
4889
2123
0.0398
0.0470 (1523)
2677
1847
0.0416
0.0576 (1508)
R1 [I > 2σ(I)] (data)
a Details taken from ref. 2.
Table 2 Crystal packing parameters
Interlayer
separation/Å
Adduct
θ/Њ
7.3
4.3
9.2
52.3
8.9
56.9
ω/Њ
φ/Њ
6ؒ1a
6ؒ1b
3ؒ1
54.9
54.1
55.5
56.6
77.2
54.4
59.5
58.4
57.8
Ϫ71.1
Ϫ65.6a
68.6a
3.59
3.43
3.44
3.44
3.60
3.39
7ؒ1
6ؒ2
7ؒ2
a Values given are for φЈ (see Fig. 7).
Fig. 2 A view of the two-dimensional packing arrangement of
molecules of 3 (red) and 1 (green) in adduct 3ؒ1. Details as in Fig. 1.
Fig. 1 A view of the two-dimensional packing arrangement of
molecules of 6 (red) and 1 (green) in adduct 6ؒ1a showing the atom
numbering scheme. Atoms are drawn as spheres of arbitrary radius.
Fig. 3 A view of the two-dimensional packing arrangement of
molecules of 7 (red) and 1 (green) in adduct 7ؒ1. Details as in Fig. 1.
ent intermolecular interactions for all crystal structures are
listed in Table 3 and are categorised according to the ribbon
type they form. The angle of propagation ω in the homo-
molecular direction of 6 in 6ؒ1a is 54.9Њ [54.1Њ in 6ؒ1b] and for 1
planes through 6 and 1 being 11.9, 10.9 and 3.6Њ in 6ؒ1a, 6ؒ1b
and 3ؒ1 respectively.
in 6ؒ1a φ is 59.5Њ [58.4Њ in 6ؒ1b]. These ribbons of 1 have eight
In adduct 7ؒ1 (Fig. 3) the arrangement of molecules of 7 is
similar to those of 6 in 6ؒ1, with an angle of propagation ω of
56.6Њ [cf. 54.9Њ in 6ؒ1a]. Thus there are two O ؒ ؒ ؒ O and eight
O ؒ ؒ ؒ H–C contacts per molecule and the three-centred H ؒ ؒ ؒ E
interaction is also present. Molecules of 1 propagate at similar
᎐
C᎐N ؒ ؒ ؒ H contacts per molecule. The two-dimensional layers in
᎐
3ؒ1 (Fig. 2) are similar to those in 6ؒ1a and 6ؒ1b, with homo-
molecular ribbons propagating at angles of ω = 55.5Њ and
φ = 57.8Њ. These ribbons also contain the same intermolecular
distances, with a similar length for the chalcogen-containing
molecular ribbons and slightly longer (by ca. 0.2 Å) lengths for
the ribbons of 1. The structures of 6ؒ1a, 6ؒ1b and 3ؒ1 show near
linear heteromolecular ribbons [the molecular axes of 6 and 1
magnitudes of φ (Ϫ71.1Њ, cf. 59.5Њ in 6ؒ1a), but rotated in the
᎐
opposite sense. As a result there are only four short C᎐N ؒ ؒ ؒ H
᎐
contacts per molecule rather than eight. The lateral displace-
ment of molecules of 1 in the direction orthogonal to the
homomolecular axis precludes the interaction N(2) ؒ ؒ ؒ H(11A)
(present in 6ؒ1a). The main difference in the two crystal struc-
tures lies in the angle of propagation along the heteromolecular
chains, which are less linear in 7ؒ1 (θ = 52.3Њ) [cf. 7.3Њ in 6ؒ1a].
Thus, the heteromolecular ribbons are better termed zigzag,
are canted at angles θ of only 7.3, 4.3 and 9.2Њ in 6ؒ1a, 6ؒ1b
᎐
and 3ؒ1 respectively] which contain six C᎐N ؒ ؒ ؒ H (<2.95 Å)
᎐
contacts per molecule of 1. Thus, it appears that the homo-
molecular interactions are at least numerically dominant in
these crystal structures as indicated by the greater number of
interactions formed in the homomolecular directions. The
layers are slightly ruffled with the angle between the mean
although as in 6ؒ1a and 6ؒ1b they extend along the crystallo-
᎐
graphic b axis and contain six C᎐N ؒ ؒ ؒ H–C contacts per
᎐
2128
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Table 3 Selected intermolecular distances for adducts 6ؒ1a, 6ؒ1b, 3ؒ1, 7ؒ1, 6ؒ2 and 7ؒ2
6ؒ1a
6ؒ1b
3ؒ1
Aa
A
A
S1 ؒ ؒ ؒ S1i
S2 ؒ ؒ ؒ H6ii
S1 ؒ ؒ ؒ H6ii
3.582(2)
3.177(4)
3.245(4)
S1 ؒ ؒ ؒ S1i
S2 ؒ ؒ ؒ H6ii
S1 ؒ ؒ ؒ H6ii
3.613(1)
3.236(2)
3.193(2)
S2 ؒ ؒ ؒ S2i
S1 ؒ ؒ ؒ H3ii
S2 ؒ ؒ ؒ H3i
3.716
2.904
3.277
B
B
N1 ؒ ؒ ؒ H11iii
N1 ؒ ؒ ؒ H12iii
2.653(4)
2.860(5)
N1 ؒ ؒ ؒ H11iii
N1 ؒ ؒ ؒ H12iii
2.682(3)
2.731(3)
N1 ؒ ؒ ؒ H8iii
N1 ؒ ؒ ؒ H9iii
2.913
2.874
C
C
C
N1 ؒ ؒ ؒ H4
N2 ؒ ؒ ؒ H5
N2 ؒ ؒ ؒ H3iv
2.906(5)
2.708(5)
2.679(5)
N1 ؒ ؒ ؒ H4
N2 ؒ ؒ ؒ H5
N2 ؒ ؒ ؒ H3iv
2.926(3)
2.751(3)
2.625(3)
N1 ؒ ؒ ؒ H5iv
N2 ؒ ؒ ؒ H4iv
N2 ؒ ؒ ؒ H6v
2.997
2.592
2.770
7ؒ1
6ؒ2
7ؒ2
A
A
A
O1 ؒ ؒ ؒ H3i
O2 ؒ ؒ ؒ H3ii
O2 ؒ ؒ ؒ O2ii
2.995(3)
3.047(4)
3.805(3)
S1 ؒ ؒ ؒ S1i b
S1 ؒ ؒ ؒ S2ii
S2 ؒ ؒ ؒ H6iii
3.578(2)
4.064(2)
3.366(3)
O1 ؒ ؒ ؒ H3i
O2 ؒ ؒ ؒ H3ii
O1 ؒ ؒ ؒ O1i
2.685(3)
2.702(3)
3.491(4)
B
N2 ؒ ؒ ؒ H12iii
2.572(3)
CN ؒ ؒ ؒ CNc
3.231
CN ؒ ؒ ؒ CNc
3.239
C
C
C
C
N1 ؒ ؒ ؒ H4iv
N2 ؒ ؒ ؒ H5v
N1 ؒ ؒ ؒ H6v
2.737(4)
2.684(4)
2.833(4)
N8 ؒ ؒ ؒ H4iv
N8 ؒ ؒ ؒ H5v
N9 ؒ ؒ ؒ H3vi
N9 ؒ ؒ ؒ H5vi
2.624(4)
2.831(4)
2.922(4)
2.658(4)
N8 ؒ ؒ ؒ H5iii
N9 ؒ ؒ ؒ H4
N9 ؒ ؒ ؒ H6iv
2.603(3)
2.787(3)
2.763(3)
a Letter types refer to homomolecular interactions (A) [for 3, 6 and 7] and (B) [for 1 and 2] or heteromolecular interactions (C). Symmetry codes:
6ؒ1a, i 1 Ϫ x, 2 Ϫ y, Ϫz, ii Ϫ1 Ϫ x, y, z, iii 1 Ϫ x, Ϫ1 Ϫ y, 1 Ϫ z, iv 1 ϩ x, y, z; 6ؒ1b, i Ϫx, Ϫy, 2 Ϫ z, ii 1 ϩ x, y, z, iii 1 Ϫ x, Ϫ1 Ϫ y, 1 Ϫ z, iv Ϫ1 ϩ x,
y, z; 3ؒ1, i x, y, Ϫ1 ϩ z, ii Ϫx, Ϫy, Ϫ1 ϩ z, iii 1 Ϫ x, Ϫy, Ϫ1 Ϫ z, iv 1 Ϫ x, 1 Ϫ y, Ϫz, v 1 Ϫ x, 1 Ϫ y, 1 Ϫ z; 7ؒ1, i Ϫ1 Ϫ x, y, z, ii Ϫx, 2 Ϫ y, 1 Ϫ z, iii
1 ϩ x, y, z, iv Ϫx, Ϫy, 1 Ϫ z, v 1 Ϫ x, Ϫy, 1 Ϫ z; 6ؒ2, i 2 Ϫ x, Ϫy, Ϫz, ii Ϫ1 ϩ x, y, z, iii Ϫ1 ϩ x, 1 ϩ y, z, iv 1 Ϫ x, Ϫ1 Ϫ y, 1 Ϫ z, v 2 Ϫ x, Ϫ1 Ϫ y,
1 Ϫ z, vi x, 1 ϩ y, z, vii x, Ϫ1 ϩ y, z; 7ؒ2, i Ϫx, 1 Ϫ y, 1 Ϫ z, ii 1 ϩ x, 1 ϩ y, z, iii 2 Ϫ x, Ϫy, 2 Ϫ z, iv Ϫ1 Ϫ x, Ϫ1 ϩ y, z. b Interlayer contact.
c
᎐
Calculated as the distance between the midpoint of each C᎐N group.
᎐
Fig. 5 A view of the two-dimensional packing arrangement of
molecules of 7 (red) and 2 (green) in adduct 7ؒ2. Details as in Fig. 1.
Fig. 4 A view of the two-dimensional packing arrangement of
molecules of 6 (red) and 2 (green) in adduct 6ؒ2. Details as in Fig. 1.
molecule. The layers remain planar, however, as in 6ؒ1a with the
angle between the two mean planes of 7 and 1 being only 6.8Њ.
Homomolecular ribbons are also present in adduct 6ؒ2 (Fig.
4) and the two-dimensional layer is similar to that observed in
6ؒ1a, although the angle of propagation for molecules of 6, ω,
is somewhat more inclined (77.2Њ, cf. 54.9Њ in 6ؒ1a). This more
canted structure for the ribbon results from the translation of
molecules of 6 in the crystallographic b direction and gives rise
to six S ؒ ؒ ؒ S and four S ؒ ؒ ؒ H–C interactions per molecule,
as opposed to two S ؒ ؒ ؒ S and eight S ؒ ؒ ؒ H–C contacts per
molecule in 6ؒ1a. The three-centred interaction observed in the
crystal structures containing 1 is absent. Whilst the relative
strengths of these interactions are not known with certainty (all
are near the sum of the van der Waals radii in length), it is
noted, however, that the same number is present in each case.
In 6ؒ2 molecules of 2 propagate at an angle of φ = Ϫ65.6Њ
(6.1a), (6.1b) and (3.1)
(7.1)
(7.2)
(6.2)
Fig. 6 Simplified representations of the two-dimensional structures
6ؒ1a, 6ؒ1b, 3ؒ1, 7ؒ1, 6ؒ2 and 7ؒ2. Molecules of 3, 6 and 7 in red,
molecules of 1 and 2 in green.
᎐
᎐
resulting in antiparallel C᎐N ؒ ؒ ؒ C᎐N interactions, which are
᎐
᎐
notably absent in the crystal structures of 6ؒ1 and 7ؒ1. In 6ؒ2 θ
interatomic distances are shorter (by ca. 0.3 Å, see Table 3).
Molecules of 2 in 7ؒ2 are arranged in a different orientation
from that in 6ؒ2, rotated by ca. 135Њ (φ = Ϫ68.6 and 65.6Њ for 7ؒ2
is 8.9Њ leading to a near linear heteromolecular ribbon, contain-
᎐
ing eight C᎐N ؒ ؒ ؒ H–C contacts per molecule of 2, more than
᎐
᎐
᎐
are present in 6ؒ1 and 7ؒ1.
and 6ؒ2 respectively). The ribbons of 2 show C᎐N ؒ ؒ ؒ C᎐N
᎐
᎐
In adduct 7ؒ2 (Fig. 5) the homomolecular ribbons of 7 con-
tain the same interactions as in 7ؒ1, and the propagation angle
ω is 54.4Њ (cf. 56.6Њ in 7ؒ1), although the shortest of the
interactions as is the case in 6ؒ2. However, as in 7ؒ1, the hetero-
molecular ribbons are not linear, the misalignment angle θ
being 56.9Њ, cf. 52.3Њ in 7ؒ1. The heteromolecular contacts
J. Chem. Soc., Dalton Trans., 1999, 2127–2132
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NC
NC
E
E
E
E
CN
CN
B
B
θ
E
E
E
NC
NC
ω
ϕ
B
B
CN
CN
E
B
CN
CN
ϕ; ϕ' = 90° - ϕ
E
E
E
CN
B
Fig. 10 A view of three of the two-dimensional layers in adduct 3ؒ1.
Details as in Fig. 8.
CN
E
CN
CN
θ
E
E
ω
CN
B
B
CN
E
E
Fig. 7 Parameters ω, θ, φ, φЈ used to define molecular packing in the
structures of adducts 6ؒ1a, 6ؒ1b, 3ؒ1, 7ؒ1, 6ؒ2 and 7ؒ2.
Fig. 8 A view of three of the two-dimensional layers in adduct 6ؒ1a
showing the three-dimensional structure (black above magenta above
cyan).
Fig. 11 A view of three of the two-dimensional layers in adduct 7ؒ1.
Details as in Fig. 8.
difference between the two polymorphs is that in 6ؒ1b there is
only slight overlap of the five-membered rings of 6 and none
involving their six-membered rings. Since the layers in the poly-
morphs are similar, 6ؒ1b can be described as being derived from
6ؒ1a by the slipping of sheets. There is a striking difference,
however, in the comparison of the three-dimensional structures
of both polymorphs of 6ؒ1 with that of 3ؒ1. It is apparent that
the heteromolecular ribbons in 3ؒ1 (Fig. 10) align almost dir-
ectly over each other, these stacks containing a multitude of
heteromolecular π–π interactions with molecules of 1 sand-
wiched between the aromatic groups of molecules of 3 in
adjacent layers.
In the structures of adducts 7ؒ1, 6ؒ2 and 7ؒ2 some degree of π
stacking is apparent, but not as much as in 3ؒ1. In 7ؒ1 (Fig. 11)
there are some heteromolecular π stacks formed through the
partial eclipsing of 7 with 1 in adjacent layers. There is no sig-
nificant overlap of molecules of 6. Heteromolecular stacking is
more evident in 6ؒ2 (Fig. 12) where molecules of 2 are sand-
wiched between molecules of 7. Finally, homomolecular inter-
layer packing is prominent in 7ؒ2 (Fig. 13) with the aromatic
rings of molecules of 7 overlapping.
Fig. 9 A view of three of the two-dimensional layers in adduct 6ؒ1b.
Details as in Fig. 8.
᎐
include three short and one long C᎐N ؒ ؒ ؒ H–C interaction per
᎐
molecule of 2.
The layer packings in the structures of adducts 6ؒ1a, 6ؒ1b,
3ؒ1, 7ؒ1, 6ؒ2 and 7ؒ2 show considerable variation (see Figs. 8–
13), but all have an ABCABC type packing arrangement. As
noted above, the two-dimensional structures in 6ؒ1a, 3ؒ1 and
6ؒ1b are essentially identical. It is in the third dimension that
these crystal structures differ, accompanied by changes in the
unit-cell volume [V = 576.3(4) and 582.0(2) Å3 for 6ؒ1a and
6ؒ1b, respectively]. In 6ؒ1a (Fig. 8) a degree of arene ring stack-
ing is evident with the extremes of the arene groups of 6 being
Having established that compounds 6 and 7 formed co-
crystals with 1 and 2, as is the case for 3, it was of interest to
determine the extent of any charge transfer. In 3ؒ1 and other
related crystal structures the degree of charge transfer, z, from
the donor to the acceptor molecules, has been assessed by spec-
troscopic methods (IR, Raman and UV-vis) and by variations
in molecular geometries.5 In this latter method the charge
transferred from the HOMO of the donor molecule (i.e. 3 or
eclipsed in adjacent layers. Arene ring overlap for molecules of
᎐
1 seems unimportant, with antiparallel C᎐N orientations prom-
᎐
inent. In polymorph 6ؒ1b (Fig. 9) molecules of 1 again have
little arene ring overlap between layers. The most notable
2130
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Table 4 Selected bond lengths (Å) for crystals containing compound 1
Table 6 Bond lengths (Å) for crystals containing compound 6
1a
C᎐C (ring) 1.346(5) 1.339(5)
6ؒ1a
6ؒ1b
3ؒ1
7ؒ1
6a
6ؒ1a
6ؒ1b
6ؒ2
1.343(2)
1.346(6)
1.337(3)
1.450(3)b
1.364(3)
B–B
B–S
S–C
1.675(5)
1.792(2)
1.756(2)
1.681(8)
1.791(4)b
1.754(4)b
1.676(4)
1.791(3)b
1.758(2)b
1.682(6)
1.789(3)b
1.755(3)b
᎐
C–C (ring) 1.448(4) 1.448(5)b 1.442(3)b 1.442(6)
C᎐C
1.374(5) 1.383(5)
1.375(3)
1.384(4)
᎐
C–C
᎐
C᎐N
᎐
1.441(5) 1.437(5)b 1.436(3)b 1.438(4)b 1.441(3)b
a See ref. 3(b). b Average of two values.
1.140(5) 1.158(4)b 1.143(2)b 1.138(5)b 1.150(3)b
a See ref. 6. b Average of two values.
Table 7 Bond lengths (Å) for crystals containing compound 7
Table 5 Bond lengths (Å) for crystals containing compound 2
7a
7ؒ1
7ؒ2
2a
2b
6ؒ2
7ؒ2
B–B
B–O
O–C
1.678(5)
1.388(2)
1.387(2)
1.677(6)
1.388(3)b
1.396(3)b
1.684(5)
1.389(3)b
1.384(3)b
C᎐C
1.344(3)
1.437(2)
1.135(2)
1.348(2)
1.434(2)c
1.146(2)c
1.356(6)
1.438(4)c
1.143(4)c
1.356(5)
1.439(3)c
1.139(3)c
᎐
C–C
a See ref. 3(b). b Average of two values.
᎐
C᎐N
᎐
a See ref. 7; cubic form. b Ref. 8; monoclinic form. c Average of two
values.
Table 8 Lattice energies (kcal molϪ1) for experimental structures
Adduct
Lattice energy
6ؒ1a
6ؒ1b
3ؒ1
Ϫ73.92
Ϫ66.80
Ϫ63.96
Ϫ70.43
Ϫ63.31
Ϫ50.95
7ؒ1
6ؒ2
7ؒ2
Tables 6 and 7 and also show negligible changes. Thus, despite
the red colour associated with the co-crystals of 6/7 with 1/2
described here, there is no structural evidence for any appre-
ciable charge transfer although this is not unexpected due to the
relatively poor electron donating ability of benzenoid deriv-
atives such as 3 as noted in the Introduction; preliminary
electrochemical studies on 6 and 79 together with PES, EHMO
and UV-vis results3a also confirm that they are poor electron
donors.§ Nevertheless, the fact that the crystalline adducts
described here do form indicates that boron analogues of TTF
and TMTSF are worthy synthetic targets that are likely to
afford crystalline adducts with 1/2 where the degree of charge
transfer (and hence conductivity) may be much greater.
Fig. 12 A view of three of the two-dimensional layers in adduct 6ؒ2.
Details as in Fig. 8.
As a final aspect of this study, lattice energy calculations10a,11
have been performed to evaluate and compare the relative
stability of each crystal structure, particularly for the poly-
morphic pair 6ؒ1a and 6ؒ1b, and to assess the importance of
the intermolecular interactions. The procedure followed the
methods used by Buttar et al.12 The results are presented in
Table 8 and suggest that the packing in polymorph 6ؒ1a is 7.12
kcal molϪ1 more stable than that in 6ؒ1b (as might be expected
given its higher density and lower unit cell volume). Lattice
energy calculations using the experimental structure have been
asserted to reproduce experimental values successfully.10 The
experimental lattice energy results for the polymorphs 6ؒ1a and
6ؒ1b appear to be consistent with current theories on differences
in lattice energies between polymorphs being less than ca. 10%
of the total lattice energy.10b,12 Since the two-dimensional layers
of 6ؒ1a, 6ؒ1b and 3ؒ1 are essentially indentical, one might
expect that the relative differences in lattice energies be associ-
ated with layer stacking and particularly to arise from favour-
able π–π interactions. If this were the case, 3ؒ1 would be the
most stable structure type since, as noted above, the ribbons
align above each other in a more organised fashion than in 6ؒ1a
and 6ؒ1b. However, the lattice energy of 3ؒ1 is less than that of
either 6ؒ1a or 6ؒ1b suggesting that arene stacking is of little
Fig. 13 A view of three of the two-dimensional layers in adduct 7ؒ2.
Details as in Fig. 8.
analogue) to the LUMO of the acceptor molecule (i.e. 1
or analogue) can be estimated by contrasting the relevant bond
lengths in the crystal structures of the pure acceptor (and
donor) with those in the co-crystallised materials. Such con-
trasts, which are supported by experiment, recognise that on
population of the LUMO of TCNQ (1) the structure becomes
more benzenoid and less quinoid in character. Similar prin-
ciples hold for structures of TCNE (2) and its adducts. The
degree of charge transfer can be evaluated in terms of the ratios
of bond distances. However, as shown in Tables 4 and 5,
changes in the molecular structures of 1 and 2 from the native
molecule to that in the crystalline adducts reported here are
negligible; selected bond lengths for the molecular stuctures of
6 and 7 in the parent structures and in the adducts are noted in
§ Measurement of the electrical conductivity of the crystalline adducts
reported here has been hampered by the small size of the crystals.
J. Chem. Soc., Dalton Trans., 1999, 2127–2132
2131

View Article Online
importance here (at least insofar as the force fields are able to
reproduce such interactions).
a hydrocarbon oil mounted on a glass fibre under argon were
made with graphite-monochromated Mo-Kα X-radiation
–
The apparent thermodynamic preference for the formation
of co-crystals in all these cases is striking and presumably
favoured by formation of heteromolecular contacts. There may,
however, be (kinetic) crystallisation effects at work, since the
co-crystals are presumably less soluble than the individual
component species.
(λ = 0.71073 Å) using a Siemens SMART area diffractometer.
CCDC reference number 186/1460.
See http://www.rsc.org/suppdata/dt/1999/2127/ for crystallo-
graphic files in .cif format.
Lattice energy calculations
In view of the dominance of the two-dimensional layers and
the results of the lattice energy calculations, it becomes appar-
ent that interlayer π stacking is not important in the formation
of these co-crystals. Indeed, as evidenced by the existence of
polymorphs 6ؒ1a and 6ؒ1b the layers can slip at very little
energy penalty. Attempted structure optimisations led to sig-
nificant layer slippings, without affecting the sequence of lattice
energies, and with very small effects on the intralayer structures.
This may be taken as support for the empirical observations
above, that it is the intralayer structure that is the robust motif
in these structures.
Atomic point charges for each component molecule were
assigned individually using GAUSSIAN13 at the 6-31G level.
Lattice energy calculations were performed on the experimental
geometry using the Crystal Packer module in Cerius 214 and
with unrestricted geometry optimisation with the Dreiding15
force field.
Acknowledgements
T. B. M. thanks Natural Sciences and Engineering Research
Council of Canada (NSERC), N. C. N. thanks the EPSRC,
Laporte plc and The Royal Society and A. G. O. thanks EPSRC
for research support and for a studentship (to M. J. Q.). T. B. M.
and N. C. N. also thank NSERC and The Royal Society for
supporting this collaboration via a series of Bilateral Exchange
Awards.
Experimental
General procedures
All reactions were performed using standard Schlenk tech-
niques under an atmosphere of dry, oxygen-free dinitrogen. All
solvents were distilled from appropriate drying agents immedi-
ately prior to use (sodium for Et2O and hexanes and CaH2 or
3 Å molecular sieves for chlorocarbons). Microanalytical data
were obtained at the University of Bristol.
References
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Ferraris, D. O. Cowan and T. J. Kistenmacher, Mol. Cryst. Liq.
Cryst., 1982, 87, 137.
3 (a) F. J. Lawlor, N. C. Norman, N. L. Pickett, E. G. Robins,
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10 (a) R. Docherty and W. Jones, Organic Molecular Solids, Properties
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Compounds 6 and 7 were prepared by the literature
methods,3a 1 and 2 were procured commercially.
Preparations
In a typical preparation, a pale yellow solution of compound 2
(0.027 g; 0.21 mmol) in CH2Cl2 (2 cm3) was added to a colour-
less solution of 6 (0.063 g; 0.21 mmol) in CH2Cl2 (2 cm3) result-
ing in no noticeable colour change. This reaction solution was
then cooled to Ϫ30Њ C and maintained at this temperature for
24 h. After this time a crop of small dark red crystals had
formed. The remaining solution was then removed by syringe
and the resulting crystals of 6ؒ2 washed with Et2O (1 cm3) and
hexane (2 × 2 cm3) and dried under vacuum (0.051 g, 56%). One
of these was used for X-ray crystallography (C9H4BN2S2
requires C, 50.3; H, 1.9; N, 13.0. Found: C, 50.4; H, 1.8; N,
13.3%). Crystals of 6ؒ2 with the same unit cell dimensions were
also obtained from chlorobenzene and 1,2-dichloroethane.
All other compounds were prepared similarly. Crystals of
adduct 6ؒ1a were obtained from CH2Cl2 solution (60%) (C12H6-
BN2S2 requires C, 56.9; H, 2.4; N, 11.1. Found: C, 58.7; H, 2.5;
N, 13.0%), of 6ؒ1b from 1,2-dichloroethane solution (54%)
(Found: C, 57.8; H, 2.7; N, 12.5%), of 7ؒ1 from CH2Cl2 solution
(53%) (C12H6BN2O2 requires C, 65.2; H, 2.7; N, 12.7. Found: C,
64.1; H, 2.9; N, 9.6%) and of 7ؒ2 from CH2Cl2 solution (39%)
(C9H4BN2O2 requires C, 59.1; H, 2.2; N, 15.3. Found: C, 57.0;
H, 2.4; N, 14.3%). Crystals of 7ؒ2 with the same unit cell as
those grown from CH2Cl2 were also obtained from 1,2-dichloro-
ethane.
11 A. Gavezotti, J. Am Chem. Soc., 1991, 113, 4622.
12 D. Buttar, M. H. Charlton, R. Docherty and J. Starbuck, J. Chem.
Soc., Perkin Trans. 2, 1998, 763.
13 GaussView v. 1.01, Gaussian Inc., Semichem, Pittsburgh, 1997.
14 Cerius 2 v. 3.5, Molecular Simulations Inc., Cambridge, 1997.
15 S. L. Mayo, B. D. Olafson and W. A. Goddard (III), J. Phys. Chem.,
1990, 94, 8897.
X-Ray crystallography
Many of the details of the structure analyses are listed in Table
1. X-Ray diffraction measurements on single crystals coated in
Paper 9/01169B
2132
J. Chem. Soc., Dalton Trans., 1999, 2127–2132