Layered, two-dimensional hydrogen bonding nets in the structure of the 1:1 encounter complex TMTTF-TCNB: Combined structural and spectroscopic study

Article abstract of DOI:10.1007/s10870-011-0021-y

The synthesis, crystal structure and spectroscopic properties as determined by infrared spectroscopy of the 1:1 encounter complex TMTTF-TCNB are reported. The complex crystallizes with each of the constituent molecules on an inversion center in the tricli

Full text of DOI:10.1007/s10870-011-0021-y

J Chem Crystallogr (2011) 41:936–943
DOI 10.1007/s10870-011-0021-y
ORIGINAL PAPER
Layered, Two-Dimensional Hydrogen Bonding Nets
in the Structure of the 1:1 Encounter Complex TMTTF–TCNB:
Combined Structural and Spectroscopic Study
•
Eric W. Reinheimer Maria Fernandez Ballesteros Rivas
•
•
Hanhua Zhao Kim R. Dunbar
Received: 28 December 2010 / Accepted: 1 February 2011 / Published online: 19 February 2011
ꢀ Springer Science+Business Media, LLC 2011
Abstract The synthesis, crystal structure and spectro-
scopic properties as determined by infrared spectroscopy of
the 1:1 encounter complex TMTTF–TCNB are reported.
The complex crystallizes with each of the constituent
molecules on an inversion center in the triclinic space
Keywords TMTTF ꢀ Tetracyanobenzene ꢀ TTF
Introduction
˚
group P - 1 with a = 6.7953(16) A, b = 7.9141(16) A,
˚
The field of organic materials that display charge transfer
properties has been a research topic of considerable interest
since the discovery of metallic conductivity in the charge
transfer salt TTF–TCNQ. In this material, partial chemical
oxidation of tetrathiafulvalene (TTF) by TCNQ (TCNQ =
7,7,8,8-tetracyanoquinodimethane) results in the formation
of a charge transfer salt with metallic conductivity that
reaches a maximum of 104 S cm-1 at 66 K prior to under-
going a metal-to-insulator transition [1–3]. Concomitant
with the partial charge transfer is the solid state organization
of the donor and acceptor molecules into segregated stacks.
This packing orientation maximizes molecular orbital
overlap among molecules of similar symmetry thus creating
a more facile pathway for the transfer of itinerant electron
density. Although the purely organic material TTF–TCNQ
is considered a seminal discovery in the realm of TTF-based
complexes, a considerably larger number of charge transfer
salts have been prepared electrochemically via slow galva-
nostatic oxidation at the surface of platinum electrodes in
the presence of inorganic anions of various charges and
geometries [4]. Central to this field of research is the prep-
aration of multifunctional hybrid materials exhibiting
physical properties once considered inimical in a continuous
crystalline lattice [5–14].
˚
c = 9.775(2) A, a = 98.63(3)ꢁ, b = 105.27(3)ꢁ, and
c = 94.49(3)ꢁ. The determination of this crystal structure
provided cyanide bond distances which when compared to
the reported literature values for free TCNB suggest the
presence of the neutral acceptor. The central C=C bond and
four ancillary C–S bonds in TMTTF provide structural
evidence that the donor, like the acceptor, also exists in its
neutral state. These crystallographic observations and
conclusions were confirmed by infrared spectroscopic
analysis. Of particular interest is a two dimensional
ꢀ
hydrogen bonding net which occurs within the (131) plane.
When stacked as repeat units along the a axis, this net is
reminiscent of stacked graphene layers in graphite.
E. W. Reinheimer (&) ꢀ M. F. B. Rivas ꢀ H. Zhao ꢀ
K. R. Dunbar
Department of Chemistry, Texas A&M University,
College Station, TX 77842-3012, USA
e-mail: ewreinheimer@csupomona.edu
Present Address:
E. W. Reinheimer
Department of Chemistry, California State Polytechnic
University, 3801 W. Temple Ave., Pomona, CA 91768, USA
Various novel materials have also been obtained using
substituted TTF derivatives. Among those derivatives is
the permethylated-TTF 2,3,6,7-tetramethyl-1,4,5,8-tetra-
thiafulvalene (TMTTF) which has seen widespread use
throughout the field of organic materials. For example, salts
have been prepared electrochemically between TMTTF and
Present Address:
M. F. B. Rivas
´
´
Departmento de Quımica, Universidad Nacional Autonoma
de Mexico, Circuito Exterior, Ciudad Universitaria,
04510 Coyoacan, DF, Mexico
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J Chem Crystallogr (2011) 41:936–943
937
2-
the Lindqvist polyoxometallate clusters W6O19
and
distances and stretching frequencies, suggest that the con-
stituent molecules co-crystallize in their neutral forms.
Mo6O192-, the hexanuclear niobium cluster Nb6Cl183-, the
dirhenium anion Re2Cl82- which contains a quadruple bond
between the rhenium atoms, Br- and PF6- [15–19]. Among
these salts, the latter two possessed the most interesting
transport properties and had a solid state packing which
mirrored those of the classic Bechgaard salts, known in the
solid state physics community to be the first examples of
organic superconductors [20–23]. Researchers in 1994
were successful in observing the entrance of (TMTTF)2[Br]
into the superconducting state under the application of
*26 kbar of pressure [18]. Later in 2000, (TMTTF)2[PF6]
was found also to become a superconductor after the
application of more than 50 kbar of external pressure [19].
The (TMTTF)2[X] family of salts, where ‘‘X’’ represents
any monovalent anion, were later christened the Fabre salts
after Jean-Marc Fabre, who launched the first systematic
study into the synthesis and properties of salts prepared
electrochemically with TMTTF [24].
Experimental Section
Preparation of Compounds
The donor TMTTF was prepared by the group of Prof.
´
Marc Fourmigue and used without additional purification.
The organic acceptor TCNB was purchased from Sigma–
Aldrich and used without additional purification. The sol-
vent acetonitrile, utilized in the preparation of the title
encounter complex, was not dried prior to its use. For the
complex’s preparation, crystals were grown by slow
evaporation of combined acetonitrile solutions of the donor
and acceptor. Single crystals of TCNB, used to prepare IR
samples for the cyanide stretching frequency analyses,
were prepared by slow evaporation of a clear, homoge-
neous solution of the acceptor dissolved in hot acetonitrile.
Salts containing TMTTF have also been successfully
prepared using chemical methods. Among this group are
those salts prepared with TCNQ and the quinone derivative
bromanil, both of which contain partially oxidized
TMTTF, a significant electronic feature necessary for the
observation of non-activated conductivity levels [25, 26].
A soluble salt with the formula (TMTTF)[BF4] was also
prepared by reacting equimolar amounts of TMTTF and
the strong oxidizer tris(p-bromophenyl)amminium tetra-
fluoroborate in CH2Cl2 [27]. This salt appears quite soluble
and could allow for hybrid materials to be prepared by
metathesis techniques.
TMTTF–TCNB
A CH3CN solution of TMTTF (0.026 g, 1.00 9 10-4 mol)
was combined with a solution of TCNB (0.024 g, 1.01 9
10-4 mol) dissolved in a minimum of warm CH3CN. The
solution was concentrated via slow evaporation in air over
the period of 1 month and placed in a freezer at -10 ꢁC.
This process resulted in the formation of black needle-like
crystals; yield 0.031 g (70%) (Fig. 1).
The organocyanide acceptor tetracyanobenzene (TCNB),
whose X-ray crystallographic structure was first reported in
1973, is known to form charge transfer complexes with
other organic donors and aromatic solvents [28–53].
Among some of the most fascinating discoveries with
TCNB include those with anthracene, naphthalene and
phenanthrene which show levels of orientational disorder
in the donor sublattice [37, 54]. Other donors studied for
complex formation with TCNB include benzanthracene,
p-phenylenediamine, pyrene, N,N-dimethyl-p-phenylene-
diamine, bis(2-methylbenzyl-cyanide), chiral 1,10-bis-2-
naphthol derivatives, TTF and o-Me2TTF (o-Me2TTF =
o-3,4-dimethyltetrathiafulvalene) [38, 48, 49, 55–59].
In order to expand the database of TMTTF complexes
with organocyanide acceptors, we report herein the 1:1
encounter complex which forms between TMTTF and
TCNB. An encounter complex can be described as a
complex which forms between a donor and acceptor in the
absence of dipolar interactions facilitated by charge
transfer. The resulting crystallographic data reveal inte-
grated stacks of TMTTF donors and TCNB acceptors,
which when combined with the analyses of critical bond
H3C
H3C
NC
NC
(a)
(b)
CH3
CH3
S
S
S
S
H
CN
CN
H
Fig. 1 TMTTF (a) and TCNB (b)
123

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J Chem Crystallogr (2011) 41:936–943
X-ray Crystallography
A black, needle-like crystal of dimensions 0.29 9 0.06 9
0.01 mm3 was secured to a cryoloop using Dow Corning
grease and placed into the liquid nitrogen stream of a Bruker
APEXII diffractometer where data was collected using
˚
Mo-Ka radiation (k = 0.71073 A) at 110 K. A hemisphere
of data was collected in multi-run mode using x as the
rotation axis. Data collection and initial indexing were
handled using SMART [60]. Frame integration, Lorentz-
polarization corrections, and final cell parameter calcula-
tions were carried out using SAINT [61]. Multi-scan
absorption corrections were performed using SADABS
[62]. The structure was solved using direct methods and
difference Fourier techniques. All hydrogen atoms were
attached via the riding model. The final structural refinement
included anisotropic temperature factors on all non-hydro-
gen atoms. Structure solution, refinement, graphics, and
creation of publication material were performed using
SHELXTL and XSEED [63–66]. The solid state structure
with anisotropic displacement ellipsoids at 50% probability
is shown in Fig. 2. Projections for TMTTF–TCNB in the ac
and bc planes are presented in Fig. 3a and b. Additional
information regarding refinement details and bond distances
for the 1:1 encounter complex are listed in Tables 1 and 2.
CCDC file 787194 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Results and Discussion
X-Ray Crystallographic Analysis
It has been well established by our group and others that
when organic chalcofulvalene donors form integrated
stacks with organic acceptors of non-equivalent redox
potentials, intermolecular forces such as p–p or SꢀꢀꢀS
interactions become the dominant feature in facilitating
Fig. 3 Projection of the crystal structure of TMTTF–TCNB in the ac
(a) and bc (b) planes illustrating the formation of integrated stacks of
donors and acceptors. The equivalent centroid-to-centroid distances
˚
of 3.398 A between the constituent molecules is highlighted (a)
Fig. 2 X-ray crystal structure
of TMTTF–TCNB with
anisotropic displacement
ellipsoids at 50% probability
123

J Chem Crystallogr (2011) 41:936–943
939
their adoption of a planar conformation [67–74]. In the
absence of these forces, neutral chalcofulvalene donors are
non-planar, often exhibiting significant bends of up to 30ꢁ
along dithiole or diseleno bridges [75]. Upon oxidation of
any tetrathiafulvalene donor, structural changes in the
molecule occur, including the adoption of a planar con-
formation followed by a lengthening of the central C=C
bond and a shortening of the C–S bonds in the central TTF
core [12, 13, 76–85]. These bond distances are the most
susceptible to the oxidation state of the donor and have
been used by Coppens and coworkers to develop an
empirical relationship that can be used to calculate the
overall oxidation state of the TTF derivative [86]. Upon
inspection of these bond distances and application of the
formula from Coppens, it can be calculated that the
TMTTF donor molecule in the title complex remains
neutral. The conclusion that TMTTF exists in its neutral
state is further supported by its central C=C bond occurring
between atom C1 and its symmetry generated congener C10
Table 1 X-ray crystallographic and refinement data for TMTTF–
TCNB
Compound
TMTTF–TCNB
CCDC code
Formula
787194
C20H14N4S4
438.59
Formula weight
Temp.
110(2)
Space group
P - 1
˚
a (A)
6.7953(14)
7.9141(16)
9.775(2)
98.63(3)
105.27(3)
94.49(3)
497.6(2)
1
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
Volume (A )
Z
Density (calculated) (mg/m3)
1.464
l (mm-1
)
0.491
˚
at a distance of 1.343(6) A. This bond distance is nearly
Scan
x scan
equivalent to analogous C=C bonds in other neutral chal-
cofulvalene derivatives and provides further evidence for
the donor’s neutrality [87]. Table 3 contains a summary of
the calculated charges for TMTTF in the title 1:1 complex
as well as other salts synthesized by both chemical and
electrochemical methods [15, 17, 19, 25–27, 88].
h range for data collection (ꢁ)
Reflections measured
Independent observed reflns.
Independent reflns. [I [ 2r]
Data/restraints/parameters
Rint
2.42–28.72
5592
2467
1792
2467/0/129
0.0425
The terminal cyanide ligands on the TCNB acceptor also
provide a qualitative indication of charge transfer. Much
like the scenario encountered with the organocyanide
acceptors TCNQ and TCNQF4 (TCNQF4 = 2,3,5,6-tetra-
fluoro-7,7,8,8-tetracyanoquinodiimine), any spontaneous
electron transfer from TMTTF to TCNB would be
accompanied by a lengthening of the cyanide bond dis-
tances as electrons are donated into their antibonding
p*-orbitals [89–92]. According to the X-ray crystallo-
graphic structure for TMTTF–TCNB, the bond distances
for the cyanide ligands of TCNB have values of 1.139(3)
Final R indices [I [ 2r]
R1 = 0.0437,
wR2 = 0.1069
R1 = 0.0594,
wR2 = 0.1147
0.986
R indices (all data)
Goodness-of-fit on F2
˚
Table 2 Bond distances for TMTTF–TCNB in A
S(1)–C(1)
S(1)–C(3)
S(2)–C(1)
S(2)–C(4)
N(1)–C(10)
N(2)–C(6)
C(1)–C(1a)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(6)–C(7)
C(7)–C(8)
C(7)–C(9)
C(8)–C(9)
C(9)–C(10)
1.749(2)
1.757(2)
1.748(2)
1.761(2)
1.139(3)
1.140(3)
1.343(4)
1.497(3)
1.324(3)
1.495(3)
1.439(3)
1.384(3)
1.397(3)
1.384(3)
1.439(3)
˚
and 1.140(3) A, values comparable to those reported in the
literature for free TCNB providing structural evidence that
the acceptor in the 1:1 encounter complex also appears to
be neutral [28].
Closer analysis of the structure for TMTTF–TCNB,
which crystallizes in the triclinic space group P - 1 with
halves of a TMTTF donor and TCNB acceptor forming the
ꢀ
contents of the asymmetric unit within the (131) plane,
reveals that the neutral donor and acceptor molecules stack
in an integrated fashion parallel to the a axis. Within these
stacks, uniform intramolecular separations with centroid-
˚
to-centroid distances of 3.398 A exist between TMTTF and
TCNB (Fig. 3a). Despite being neutral, TMTTF is planar,
showing no bends along either dithiole bridge. Such a
structural observation is reminiscent of that seen previously
123

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J Chem Crystallogr (2011) 41:936–943
Table 3 Estimated degree of ionicity for the TMTTF donor molecule in the title compound and its comparison to other calculated valences for
published TMTTF-containing materials
a
b
˚
A (A)
˚
B (A)
Salt
Molecule
Qc
Reference
[19]
(TMTTF)2Br
A
A
A
A
A
A
B
C
A
B
A
A
1.349(8)
1.349(8)
1.356(8)
1.368(8)
1.403(3)
1.392(8)
1.399(3)
1.357(5)
1.370(2)
1.390(2)
1.452(3)
1.343(4)
1.739(8)
1.739(8)
1.737(5)
1.739(3)
1.717(5)
1.728(4)
1.717(5)
1.750(3)
1.720(2)
1.720(2)
1.681(4)
1.749(3)
?0.26
?0.26
?0.37
?0.51
?1.22
?0.95
?1.17
?0.25
?0.75
?1.01
?2.32
?0.07
TMTTF–Bromanil
TMTTF–TCNQ
[26]
[26]
[27]
[17]
(TMTTF)BF4
(TMTTF)3[Re2Cl8]ꢀ2CH3CN
(TMTTF)2[W6O19
]
[15]
(TMTTF)[ClO4]2
[88]
TMTTF–TCNB
This work
a
Central C=C bond distance
b
c
Mean central C–S bond distance
Q = charge estimated with the formula Q = -17.92 ? 23.43 9 (A/B) from [86]
distance of 2.641(4) A. Concurrently, Csp2–HꢀꢀꢀN interac-
˚
in the 1:1 adduct o-Me2TTF–TCNB, where p–p interac-
tions between neutral donor and acceptor molecules at
ꢀ
tions were also observed within the (131) plane between
˚
distances \3.5 A resulted in donor planarity [59]. Within
the structure of the title compound, TMTTF molecules
TCNB moieties related to one another by translations along
˚
the b axis at a distance of 2.445(3) A forming infinite
ꢀ
within the (131) plane are related to one another through
ribbons of hydrogen bonding interactions parallel to this
axis. The unification of these two independent sets of
interactions defines a two dimensional net of hydrogen
glide planes parallel to the b axis. When one considers this
symmetry element, the closest interatomic distances
between neighboring sulfur atoms occur at distances near
ꢀ
bonding which lies within extensions of the (131) plane
˚
3.8 A. Despite being longer than the sum of the van der
and stack atop other nets along a in a manner akin to the
stacking of layers of graphene in graphite (Fig. 5). The
distances for the hydrogen bonds reported also provide
further structural evidence in support of the hypothesis that
Csp2–HꢀꢀꢀN interactions are stronger than their Csp3–HꢀꢀꢀN
counterparts as the distances for the former are shorter than
the latter [93].
Waals radii for two sulfur atoms in close proximity
˚
(3.6 A), these interactions form a novel repeat pattern in
the solid state; formed by the offset of TMTTF donors at an
˚
approximate distance of 6 A, a stair-step pattern within the
ꢀ
(131) plane results. Other sets of stairs repeat ad infinitum
through translations along the c axis (Fig. 4).
Along with this novel repeat pattern among the TMTTF
donors, a two-dimensional net of hydrogen bonding inter-
actions was found to exist encompassing interactions
between donor and acceptor moieties and among neigh-
boring acceptor molecules. Inspection of the solid state
structure revealed that two categories of hydrogen bonding
interactions occur: first, Csp3–HꢀꢀꢀN interactions exist
between TMTTF and TCNB, while a second hydrogen
bonding Csp2–HꢀꢀꢀN interaction occurs between TCNB
acceptors. Previously, in the structure for TTF–TCNB,
Bandoli and coworkers suggested that should interatomic
distances between nitrogen and hydrogen atoms be
IR Spectroscopy
The infrared (IR) spectra for TMTTF–TCNB and free
TCNB were measured as Nujol mulls placed between KBr
plates on a Nicolet 740 FT-IR spectrometer. Closer
inspection of the terminal cyanide stretches for these sys-
tems supports the conclusion obtained through crystal
structure determination: primarily, that the TMTTF donor
and TCNB acceptor co-crystallize as neutral molecules
without simultaneous charge transfer. Infrared spectra
obtained for TMTTF–TCNB and TCNB show cyanide
stretching frequencies around 2244 cm-1 suggesting that
no charge transfer has occurred between the donor and
acceptor. When one considers the large library of infrared
data available for the organocyanide family of acceptors,
electron donation into the antibonding p*-orbital of the
˚
\2.70 A, the sum of their corresponding van der Waals
radii, a hydrogen bonding interaction between the atoms
can exist [58]. Within the (131) plane, a Csp3–HꢀꢀꢀN
ꢀ
interaction between TMTTF and TCNB exists laterally
(parallel to the c axis) between atoms H2A and N2 at a
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J Chem Crystallogr (2011) 41:936–943
941
Fig. 4 Stair-step orientations of
the closest interatomic
interactions between
neighboring sulfur atoms
parallel to the b axis
Fig. 5 Two-dimensional
hydrogen bonding net within
ꢀ
projections of the (131) plane
containing Csp2–HꢀꢀꢀN
interactions between TCNB
acceptors (continuous line) and
Csp3–HꢀꢀꢀN interactions between
TMTTF and TCNB (dashed
line)
123

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J Chem Crystallogr (2011) 41:936–943
`
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ꢀ
axis forming a two dimensional net within the (131) plane.
The presence of black crystals in this system is cryptic,
suggesting the presence of charge transfer; however, no
charge transfer was observed as determined by X-ray
crystallography and infrared spectroscopy. Further analy-
ses, such as single crystal IR and Raman studies, will be
done on crystals of this adduct and the results will be
reported in due course.
Acknowledgments This work was supported by The Welch Foun-
dation (Grant A-1449) and the U.S. Department of Energy (Grant DE-
FG01-05ER05-01). We also acknowledge the National Science
Foundation (Grant 9807975) for the funds to purchase the X-ray
diffractometer. EWR would like to sincerely thank Nancy Erskine for
additional editorial assistance.
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