2-Dimensional molecular wiring based on toroidal delocalization of hexaarylbenzene

Article abstract of DOI:10.1039/c0cc00128g

A series of hexaarylbenzenes having two peripheral thienyliron substituents has been prepared, and it is revealed that the two metal centres communicate with each other through toroidal delocalization among the peripheral aromatic groups.

Full text of DOI:10.1039/c0cc00128g

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2-Dimensional molecular wiring based on toroidal delocalization
of hexaarylbenzenew
Yuya Tanaka, Takashi Koike and Munetaka Akita*
Received 26th February 2010, Accepted 28th April 2010
First published as an Advance Article on the web 18th May 2010
DOI: 10.1039/c0cc00128g
A series of hexaarylbenzenes having two peripheral thienyliron
substituents has been prepared, and it is revealed that the two
metal centres communicate with each other through toroidal
delocalization among the peripheral aromatic groups.
Electron or hole delocalization over 3-dimensional (3D)
organic framework has attracted increasing attention not only
from the viewpoint of understanding the mechanism of the
intriguing phenomenon but also from the viewpoint of
application to molecular devices.1 Hexaarylbenzene (HAB)2–5
has been regarded as one of the promising 3D topologies and
is expected to exhibit unique electronic features, because HAB
Scheme 1 Toroidal delocalization over hexaarylbenzene.
adopts a propeller-like conformation, which enables p–p
interaction between the peripheral aromatic rings facing each
and Th–Fe bonds occurring at a rate faster than the NMR
time scale. Despite the very long carbon linkage (C10) separating
the two iron centres (through-bond separation = 16.69 A),
complex 1 showed substantial communication between the
two iron centres through the hexaarylbenzene core, as
characterized by the Vab value (5.0 ꢀ 102 cmꢁ1) estimated
by Hush theory for the mixed-valent 1+ cation. The perfor-
mance of the HAB complex 1 turned out to be comparable
to that of (m-p-CRC–C6H4–CRC){Fe(Z5-C5Me5)(dppe)}2
(Vab = 590/2000 cmꢁ1)9z taking into account the different
chain lengths (C8 vs. C10 (1)) and Z5-C5R5 ligands.
other, and sextuple accumulation of the p–p interactions
would lead to so-called ‘‘toroidal delocalization’’ (Scheme 1).
HABs bearing redox- or photo-active organic fragments have
been studied to prove the toroidal delocalization.3–5
In our laboratory development of organometallic molecular
devices has been one of our research activities, and the study
on molecular wire6 has been recently extended to higher order
systems (junctions) essential for wiring.7 Compared to 1D
molecular wire, the 2D system has been left undeveloped
mainly because of a lack of appropriate 2D and 3D organic
p-conjugated systems besides ethylene and benzene skeletons.
Herein we wish to report that HAB system can serve as a
2D-wiring device by virtue of the toroidal delocalization of the
HAB moiety.
We prepared the HAB complex having two peripheral
thienyliron substituents at the ortho-positions, o-C6Ph4(2,5-
C4H2S-Fe)2
1 [Fe = =
Fe(Z5-C5H5)(dppe); 2,5-C4H2S
2,5-thienylene] (Scheme 1).z The molecular structure of 1
was determined by X-ray crystallography (Fig. 1),y which
revealed (1) the separation between the ipso-carbon atoms of
the thiophene rings (2.897 A) comparable to those of organic
HAB derivatives,8 (2) the propeller-like conformation of the
peripheral aromatic groups with the dihedral angles between
the central benzene core (Bz) and the peripheral aromatic rings
(Ar) being 54–661,8 and (3) the anti-configuration of the two
thiophene rings. A single set of rather sharp 1H-NMR signals
for 1 indicated rotation of the substituents around the Bz–Ar
Chemical Resources Laboratory, Tokyo Institute of Technology,
R1-27, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
E-mail: makita@res.titech.ac.jp; Fax: +81-45-924-5230
w Electronic supplementary information (ESI) available: Experimental
details for the synthesis of 1–5; electrochemical, UV-vis and NIR data
for 1–5; details of X-ray crystallographic analyses of 1, 2 and 5. CCDC
767557–767558. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0cc00128g
Scheme 2 Hexaarylbenzene-diiron complexes and their references.
Chem. Commun., 2010, 46, 4529–4531 | 4529
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Fig. 1 ORTEP drawings of 1, 2 and 5 drawn with thermal ellipsoids at the 30% probability level.
The communication observed for 1 could result from the
toroidal delocalization among the peripheral aromatic rings
(Scheme 1) but p-conjugation through the central benzene ring
cannot be excluded a priori because of free rotation of the
peripheral aryl rings, suggested by the NMR behavior
mentioned above. Thus it is essential to understand the nature
of the metal–metal interaction, i.e. to separate contribution of
the three feasible communication modes between the two iron
centres: (A) p–p interaction between the two thiophene rings,
(B) accumulated p–p interactions among the outer benzene
rings, and (C) p-conjugation through the central benzene ring
(Scheme 3). A combination of the two types of p–p inter-
actions A and B constitutes toroidal delocalization. To
separate the three factors A–C the reference compounds 2–5
(Scheme 2) were prepared, and their communication perfor-
mance was characterized.
thiophene rings. It was thus concluded that the rotation
around the Bz–Th bonds in 2 was completely frozen at room
temperature.
Relevant para-substituted derivatives 4 and 5 were also
prepared to examine the positional effect of the thienyliron
fragments. Complex 4 is the para-isomer of 2 and two
pathways D and E are possible as in the case of 1, whereas,
for 5, the p-conjugated route E is the only viable pathway
(Scheme 3). NMR analysis of 4 and 5 showed free rotation of
the Bz–Th junctions. For 5, the dihedral angles between
the Bz and Th planes (73.71) and the Feꢃ ꢃ ꢃFe separations
(through-bond: 19.46 A, through-space: 14.49 A) were
determined by X-ray crystallography (Fig. 1).y
Communication performance between the two iron centres
in the diiron complexes 1–5 was characterized on the basis of
electrochemical analysis (KC) and near IR measurement
(electronic coupling derived from IVCT band; Vab) (Table 1).11
CV and DPV charts of the obtained complexes (CH2Cl2
solutions; see Electronic Supplementary Informationw) contained
reversible two electron redox waves in the range of ꢁ400–800 mV
(vs. Fc/Fc+) attributed to the Fe(II)/Fe(III) processes. The
ortho-derivatives 1–3 showed two separated redox processes,
while the para-derivatives 4 and 5 showed single broadened
redox waves, which were analyzed by simulation. Compro-
portionation constants KC indicating the thermodynamic
stability of the 1e-oxidized, mixed valence species were
calculated from the DE values (E1/21–E1/22), as summarized
in Table 1. The ortho-substituted HAB complex 1 turned out
to show the KC value of 248 being significantly larger than
those of the reference complexes, and the ortho-derivatives
showed performance better than the para-derivatives.
The concepts of molecular design of the reference
compounds (Scheme 2) are as follows.z In the case of complex
2, methylation at the 3-position of the thiophene rings would
retard free rotation of the substituents around the central
benzene (Bz) core-thiophene (Th) bond10 so as to eliminate
pathway C, whereas, for complex 3 without the peripheral
phenyl substituents, pathway B is not viable. Actually, for 2,
the Bz–Th rotation was completely frozen as confirmed by the
two inequivalent 31P NMR signals of the dppe ligands coupled
with each other [dP (C6D6) 105.6, 107.2 (d ꢀ 2, JPP = 19.5 Hz)].
The molecular structure of 2 determined by X-ray crystallo-
graphy (Fig. 1)y revealed (1) the dihedral angles between the
Bz and Th planes (681 and 681; cf. 1: 541 and 621) becoming
closer to the right angle leading to an orthogonal arrangement,
in addition to the features common to 1: (2) the ipso-carbon
atom separation (2.90(1) A), (3) the through-bond Fe–Fe
distance (16.70 A), (4) the propeller-like conformation of the
aromatic rings, and (5) the anti-conformation of the two
For definitive discussion on communication between the
two iron centres, electronic coupling (Vab) was determined on
the basis of the IVCT bands of monocationic species appearing
in near IR region (see Electronic Supplementary Informationw).
The monocationic species 1+–5+ were generated in situ via
comproportionation of the neutral species 1–5 and the
dicationic species 12+–52+, and the data and Vab values are
summarized in Table 1. The half-height widths of the IVCT
bands obtained by deconvolution of the near IR spectra
(n1/2exp) are comparable to the calculated half-height widths
(n1/2calcd), indicating that the diiron complexes are Robin-Day
Class II compounds.11 As can be seen from Table 1, the order
of the Vab values is in accord with that of KC, and the
Vab value for complex 1 is the largest among the derivatives
examined.
Scheme 3 Possible pathways for communication between the two
iron centres.
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Table 1 Electrochemical and near IR data for 1–5
Complex
DE/mVa
KC
l
max/nmb
e/Mꢁ1cmꢁ1
n1/2exp/cmꢁ1
n1/2calcd/cmꢁ1c
Vab/cmꢁ1d
Pathwayse
1
2
3
4
5
142
102
103
80
248
53
55
23
2500
1923
2235
1681
—
3900
800
2500
1200
—
3250
3400
3310
3003
—
3040
3466
3215
3707
—
5.0 ꢀ 102
3.6 ꢀ 102
4.3 ꢀ 102
2.1 ꢀ 102
0
A, B, C
A, B
A, C
D, E
E
60
10
a
b
c
d
[Complex] = B2.0 ꢀ 10ꢁ3 Mꢁ1, [Bu4NꢃPF6] = 0.1 Mꢁ1
.
Observed in CH2Cl2. n1/2calcd = (2310ꢃ nmax
)
1/2 (in cmꢁ1). Feꢃ ꢃ ꢃFe distances were
e
obtained from the X-ray results (Fig. 1). See Scheme 3.
Taking into account the rotational behavior of the peri-
pheral aromatic rings mentioned above, the possible communi-
cation pathways for each derivative are summarized as shown
in the last column of Table 1. If additivity is assumed for the
Vab values, the Vab values for the ortho-series compounds 1–3
can be tentatively expressed as shown in the following equa-
y Crystal data for 1ꢃ3CH2Cl2 (at ꢁ60 1C): C103H88Cl6Fe2P4S2, fw =
ꢀ
1838.26, triclinic, space group P1, a = 17.548(3) A, b = 17.970(2) A,
c = 18.225(3) A, a = 104.363(8)1, b = 105.373(6)1, g = 115.240(6)1,
V = 4562(1) A3, Z = 1, dc = 1.338 g cmꢁ3, R1 = 0.0887 (refined on
F2) for 10679 data (I > 2s(I)) and 1009 parameters; Crystal data for 2ꢃ
2MeOHꢃCH2Cl2 (at ꢁ60 1C):C105H86Cl2Fe2O2P4S2, fw = 1750.34,
monoclinic, space group P21/n, a = 17.354(3) A, b = 25.1295(17) A,
c = 20.621(3) A, b = 99.251(2)1, V = 8876(2) A3, Z = 4,
dc = 1.310 g cmꢁ3, R1 = 0. 0794 (refined on F2) for 6721 data
(I > 2s(I)) and 1020 parameters; Crystal data for 5ꢃ2CH2Cl2
tions: Vab(1) = Vab(A) + Vab(B) + Vab(C) = 5.0 ꢀ 102 cmꢁ1
;
V
V
ab(2) = Vab(A) + Vab(B) = 3.6 ꢀ 102 cmꢁ1; Vab(3) =
ab(A) + Vab(C) = 4.3 ꢀ 102 cmꢁ1. By solving the equations,
(at –60 1C): C82H74Cl4Fe2P4S2, fw = 1500.91, triclinic, space group
ꢀ
P1,
a = 9.048(3) A, b = 11.720(6) A, c = 17.902(8) A,
the values for the three pathways A–C are estimated to be as
a = 86.243(14)1, b = 80.025(19)1, g = 73.990(18)1, V = 1796.9(13) A3,
Z = 1, dc = 1.387 g cmꢁ3, R1 = 0.0616 (refined on F2) for 4066 data
(I > 2s(I)) and 425 parameters. For details of spectroscopic and
crystallographic data, see ESI. CCDC767557 (1), CCDC767558 (2),
CCDC767559 (5)).
follows: Vab(A) = 2.9 ꢀ 102 cmꢁ1, Vab(B) = 7.0 ꢀ 10 cmꢁ1
,
and Vab(C) = 1.4 ꢀ 102 cmꢁ1. The order of the magnitude of
the Vab values is Vab(A) > Vab(C) > Vab(B). It is notable that
(1) the p–p interaction (A) is more efficient than the through-
bond p-conjugation (C) and (2) the toroidal delocalization
(A+B) is the major communication pathway of the ortho-HAB
derivative 1 (3.6 ꢀ 102 cmꢁ1 of 5.0 ꢀ 102 cmꢁ1).
z The Vab values of 590 and 2000 cmꢁ1 were obtained when the
complex was considered to be a Class II and Class III compound,
respectively.11
1 A. J. Berresheim, M. Muller and K. Mullen, Chem. Rev., 1999, 99,
¨
¨
Similarly, we can separate the Vab values of the para-series
compounds 4 and 5 into the contributions from the through-space
(toroidal) interaction (D: 208 cmꢁ1) and the through-bond
p-conjugation (E: 0 cmꢁ1). Thus substantial toroidal inter-
action between the distant para-positioned metal centres in 4 is
observed, while the through-bond interaction (E) in 5 is
negligible as clearly indicated by the lack of an IVCT band.
These results are regarded as definite evidence for the toroidal
delocalization of the HAB system.
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and Reactions, Wiley-VCH, Weinheim, 2005; H. Hopf, Tetra-
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In conclusion, the present study has revealed that (1) the
HAB system can serve as a promising 2D wiring device, in
addition to the classical ethylene and benzene skeleton, and (2)
toroidal interaction turns out to be the dominant communi-
cation pathway for the HAB complexes 1 and 4. Multiply
metallated HAB complexes are now being studied to clarify
communication among more than two metal centers.
7 M. Akita and T. Koike, Dalton Trans., 2008, 3523; T. Ozawa and
M. Akita, Chem. Lett., 2004, 33, 1180; M. Akita, Y. Tanaka,
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The financial supports from the Japanese government
(Grants-in-Aid for Scientific Research: Nos. 18065009 and
20044007; M. A.) and the Japan Society for the Promotion
of Science (Y.T.) are gratefully acknowledged.
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C. Lapinte, J. Am. Chem. Soc., 2006, 128, 2463.
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Notes and references
z The HAB derivatives were prepared by lithiation of the corresponding
dibromo precursors (Fe - Br) with BuLi followed by treatment with
I–Fe(Z5-C5H5)(CO)2 and photochemical ligand replacement with
dppe in a toluene–MeCN mixed solvent. See Electronic Supple-
mentary Informationw.
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 4529–4531 | 4531