A molecular conductor based on axially CN-substituted cobalt tetrabenzoporphyrin

Article abstract of DOI:10.1246/cl.2011.1257

A molecular conductor based on axially CN-substituted cobalt tetrabenzoporphyrin is reported. The crystal structure is isomorphous with a phthalocyanine-based conductor, however, estimation of effectiveness of π-π overlap and electrical transport measurement reveal that a slight change in molecular structure of macrocyclic π-conjugated ligands has significant influence on intermolecular interaction and electrical properties.

Full text of DOI:10.1246/cl.2011.1257

doi:10.1246/cl.2011.1257
Published on the web October 15, 2011
1257
A Molecular Conductor Based on Axially CN-Substituted Cobalt Tetrabenzoporphyrin
Masaki Matsuda,*1,2 Hiroko Ohishi,1 Madoka Tofuku,2 Naho Muramoto,2 and Jun-ichi Yamaura3
1Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555
2Faculty of Science, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555
3Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581
(Received August 29, 2011; CL-110722; E-mail: masaki@sci.kumamoto-u.ac.jp)
A molecular conductor based on axially CN-substituted
cobalt tetrabenzoporphyrin is reported. The crystal structure is
isomorphous with a phthalocyanine-based conductor, however,
estimation of effectiveness of ³-³ overlap and electrical
transport measurement reveal that a slight change in molecular
structure of macrocyclic ³-conjugated ligands has significant
influence on intermolecular interaction and electrical properties.
ligand. However, no report is available on molecular conductors
composed of axially CN-substituted [M(tbp)] complex because
of difficult and complicated preparation procedures.
In this study, first [Zn(tbp)] was synthesized by the template
cyclization of isoindoline-3-acetic acid with zinc acetate
reported by Edwards et al.6 Second H2(tbp) was obtained by
demetalation of [Zn(tbp)] in concentrated sulfuric acid, and
[Co(tbp)] was prepared from H2(tbp) and bis(acetylacetonato)-
cobalt(II) in refluxing 1-chloronaphthalene. The obtained
[Co(tbp)] was stirred with NaCN in ethanol at room temperature,
leading to Na[Co(tbp)(CN)2], and the cation exchange was
carried out by metathesis using tetraphenylphosphonium (TPP)
iodide. The molecular conductor TPP[Co(tbp)(CN)2]2 was fabri-
cated by electrochemical oxidation of obtained TPP[Co(tbp)-
(CN)2] in acetonitrile under the constant current of 1 ¯A.
TPP[Co(tbp)(CN)2]2 could be also obtained by electrolysis of
[Co(tbp)] with tetraphenylphosphonium iodide in propionitrile,
as in the case of the preparation of TPP[Co(Pc)(CN)2]2.3
The crystal structure of TPP[Co(tbp)(CN)2]2 determined
by X-ray diffraction is shown in Figure 1a.7 As expected,
TPP[Co(tbp)(CN)2]2 is isostructural with TPP[Co(Pc)(CN)2]2,8
and the 1:2 ratio of cation:[Co(tbp)(CN)2] units gives an
effective charge of ¹0.5 for one [Co(tbp)(CN)2] unit; each tbp
ring is formally oxidized by 0.5e from the initial closed-shell
tbp2¹. Along the c axis, the [Co(tbp)(CN)2] units uniformly
stack with the ³-³ overlapping of peripheral benzene rings,
meaning that HOMOs of [Co(tbp)(CN)2] units are expected to
form a one-dimensional 3/4-filled band.
The regular stacking of [Co(tbp)(CN)2] units along the
c axis, induced by special symmetry of TPP cation,8 is the
same as that of [Co(Pc)(CN)2] units in TPP[Co(Pc)(CN)2]2,
however, there are intermolecular N£H contacts within the sum
of van der Waals radii between CN ligands and CH groups
at the meso positions of adjacent two [Co(tbp)(CN)2] units
(Figure 1b). As seen in Figure 2, the molecular structure of the
[Co(tbp)(CN)2] unit is quite similar to that of [Co(Pc)(CN)2] unit
except for the meso positions. While nitrogen atoms at the meso
positions of Pc repulse CN ligands of adjacent [Co(Pc)(CN)2]
units, the less electronegative methine groups make it possible
to form the intermolecular N£H contacts with CN ligands of
adjacent [Co(tbp)(CN)2] units. As a result, CN ligands of the
[Co(tbp)(CN)2] unit become more linear.
Phthalocyanine (Pc), a well-known macrocyclic ³-conju-
gated ligand, is widely used in molecular electronics as a donor
molecule or a p-type semiconductor,1 and axially CN-substituted
¹
metallophthalocyanine complexes of [MIII(Pc)(CN)2] (M = Co
or Fe) are known as versatile building blocks for molecular
conductors showing various crystal structure, electronic struc-
ture, and fascinating phenomena such as giant negative
magnetoresistance.2 Since to control and understand inter-
molecular interaction is a most important subject in the study
of molecular conductors, there are several reports on using other
axial ligands of Cl or Br as well as other macrocyclic
³-conjugated ligands of 1,2- or 2,3-naphthalocyanines. Such
substitution with bulky ligands can surely modulate ³-³
overlapping and electrical and magnetic properties.3-5
Herein, we present a molecular conductor of TPP[Co(tbp)-
(CN)2]2 (TPP: tetraphenylphosphonium), based on a macro-
cyclic ³-conjugated ligand of tetrabenzoporphyrin (tbp)
(Chart 1). The difference in molecular structure of tbp and Pc
is only atoms at the meso positions bridging four pyrrole rings;
substitution of the bridging nitrogen atoms of Pc with less
electronegative methine groups forms tbp. In addition, the
extended Hückel calculation shows that the distribution of
HOMO coefficients of tbp and Pc are similar to each other.
These features suggest that the crystal structure of the molecular
conductor based on tbp is isostructural with that based on Pc,
and comparison of the ³-³ overlap and electrical transport
properties of the tbp-based molecular conductor with those of
the Pc-based one is meaningful because we can understand
influence on electrical transport properties caused by the slight
change in the molecular structure of macrocyclic ³-conjugated
N
The discrepancy of the intermolecular contacts strongly
suggests changes in intermolecular interaction, therefore, we
calculated angles between the c axis and the normal vector of
the least-squares plane of 24 non-hydrogen atoms composing
porphin for TPP[Co(tbp)(CN)2]2 and tetraazaporphyrin for
TPP[Co(Pc)(CN)2]2, respectively. The angle in TPP[Co(tbp)-
(CN)2]2 is 26.42°, which is about 3% larger than 25.59° in
TPP[Co(Pc)(CN)2]2. The difference is reflected in interplanar
N
N
N
N
N
N
N
N
N
Co
N
Co
N
Chart 1.
Chem. Lett. 2011, 40, 1257-1259
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1258
Co
1.932(2)
1.934(9)
C
N
1.124(2)
1.15(1)
121.1(2)
122.3(9)
121.5(2)
120.3(9)
118.2(2)
116.4(8)
117.6(2)
117.6(8)
121.5(2)
121.3(9)
120.7(2)
122.2(9)
133.0(1)
131.4(8)
132.4(1)
132.0(8)
120.8(1)
121.4(8)
121.3(1)
120.9(9)
123.8(1)
122.3(8)
123.8(1)
122.4(8)
117.2(1)
116.1(9)
117.9(2)
116.5(9)
106.2(1)
107.0(7)
106.3(1)
107.1(7)
110.0(1)
109.9(7)
125.0(1)
121.4(7)
121.2(1)
121.1(1)
122.4(8)
122.4(8)
106.6(1)
106.8(7)
110.5(1)
109.0(7)
125.7(1)
128.6(8)
126.2(1)
127.7(8)
106.5(1)
105.8(7)
132.2(1)
130.8(8)
132.4(1)
131.9(8)
106.6(1)
108.3(7)
107.1(1)
107.5(7)
126.7(1)
125.4(6)
126.2(1)
126.8(6)
110.1(1)
108.9(7)
110.0(1)
109.8(7)
124.0(1)
122.9(8)
124.3(1)
121.6(8)
126.5(1)
125.9(6)
126.6(1)
125.5(6)
125.9(1)
128.0(8)
125.7(1)
128.6(8)
90.01(5)
89.9(3)
125.3(1)
121.6(7)
88.30(6)
87.5(3)
91.53(6)
92.4(3)
Co
178.2(1)
174.6(9)
C
N
Figure 2. Bond lengths (¡) and angles (degree) of [Co(tbp)-
(CN)2] and [Co(Pc)(CN)2] units in TPP[Co(tbp)(CN)2]2 and
TPP[Co(Pc)(CN)2]2. The values for TPP[Co(Pc)(CN)2]2 are
cited from ref 8 and underlined.
Figure 1. (a) Crystal structure of TPP[Co(tbp)(CN)2]2 and
(b) molecular arrangement along the c axis. Dashed lines depict
intermolecular N£H contacts.
distances of peripheral benzene rings; 3.44 and 3.50 ¡ for
TPP[Co(tbp)(CN)2]2, and 3.40 and 3.46 ¡ for TPP[Co(Pc)-
(CN)2]2.8 As a result, effectiveness of the ³-³ overlapping in
TPP[Co(tbp)(CN)2]2 becomes reduced; the overlap integral
between tbp rings estimated by extended Hückel calculation is
7.5 © 10¹3, which is 12% smaller than 8.5 © 10¹3 of the overlap
integral between Pc rings of TPP[Co(Pc)(CN)2]2.8 The reduction
of the ³-³ overlapping caused by the substitution of the
bridging nitrogen atoms of Pc with less electronegative methine
is more significant than that by steric repulsion observed in
bulky axial ligand systems of TPP[Co(Pc)X2]2 (X = Cl or Br).3
Temperature dependence of electrical resistivity of
TPP[Co(tbp)(CN)2]2, measured by using a four-probe method,
is shown in Figure 3, together with that of TPP[Co(Pc)(CN)2]2.
Figure 3. Temperature dependence of electrical resistivity
along the c axis of TPP[Co(tbp)(CN)2]2 and TPP[Co(Pc)(CN)2]2.
¹2
The resistivity at room temperature of 7.8 © 10 ³ cm is ten
¹3
times larger than that of TPP[Co(Pc)(CN)2]2 (8.2 © 10 ³ cm).8
In spite of the expected 3/4-filled band, the temperature
dependence of resistivity of both conductors shows semi-
conducting behavior with small activation energies of 0.0185
and 0.01 eV for TPP[Co(tbp)(CN)2]2 and TPP[Co(Pc)(CN)2]2,
respectively. In one-dimensional quarter-filled system, it is
suggested that strong electron correlation should cause charge
disproportionation, leading to an insulating ground state.9 For
TPP[Co(Pc)(CN)2]2, the charge disproportionation is certainly
reported;10 therefore, it seems that the ground state of
TPP[Co(tbp)(CN)2]2 is also the charge-disproportionate state
and that the electronic system becomes more sensitive to
electron correlation owing to the reduction of bandwidth caused
by the change in the intermolecular contacts.
In conclusion, by using a macrocyclic ³-conjugated ligand
of tbp, we have succeeded in fabricating a molecular conductor
of TPP[Co(tbp)(CN)2]2. Although the molecular structure of tbp
is similar to that of Pc and crystal structure of the obtained
TPP[Co(tbp)(CN)2]2 was isostructural with TPP[Co(Pc)(CN)2]2,
substitution of nitrogen atoms at meso positions with CH groups
significantly affected ³-³ overlapping and electrical conduc-
tivity, suggesting that a slight change in molecular structure of
macrocyclic ³-conjugated ligands can effectively control inter-
Chem. Lett. 2011, 40, 1257-1259
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1259
molecular interaction. These results lead to new developments of
molecular functional materials.
6
7
L. Edwards, M. Gouterman, C. B. Rose, J. Am. Chem. Soc.
1976, 98, 7638.
Crystal data for TPP[Co(tbp)(CN)2]2: C100H60N12PCo2, fw
1578.49, tetragonal, P42/n, a = 21.687(3), c = 7.5462(2) ¡,
V = 3547.11 ¡3, Dcalcd = 1.48 g cm¹3. The intensity data
were collected on a Bruker SMART APEX difractometer
with graphite monochromated Mo K¡ radiation (- =
0.7107 ¡), ®(Mo K¡) = 0.556 cm¹1, 3429 reflections with
I > 2.0·(I), R = 0.0555, Rw = 0.1690 (262 variables).
CCDC-843883.
H. Hasegawa, T. Naito, T. Inabe, T. Akutagawa, T.
Nakamura, J. Mater. Chem. 1998, 8, 1567.
H. Seo, C. Hotta, H. Fukuyama, Chem. Rev. 2004, 104,
5005.
This work was supported in part by a Grant-in-Aid for
Scientific Research on Innovation Area of Molecular Degrees of
Freedom (No. 23110720) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan.
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Chem. Lett. 2011, 40, 1257-1259
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/