Synthesis and characterization of N-confused porphyrinatoantimony(V): Toward a low energy gap molecular wire

Article abstract of DOI:10.1016/S0022-328X(00)00405-8

N-Confused tetraarylporphyrinatoantimony(V) dimethoxides were synthesized and their X-ray crystallographic structure, absorption spectra and voltammetric spectra were studied. X-ray crystallographic structure revealed neutral molecules with no counter anion. From the absorption spectra and voltammetric studies we estimated their energy gaps to be about 0.2 eV less than the corresponding porphyrinatoantimony(V). The axial ligands could easily be exchanged in solvent alcohol by acid promotion. These characteristics of N-confused porphyrinatoantimony(V) indicate that they are good candidates for the molecular wire component.

Full text of DOI:10.1016/S0022-328X(00)00405-8

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 611 (2000) 551–557
Synthesis and characterization of N-confused
porphyrinatoantimony(V): toward a low energy gap molecular
wire
Takuji Ogawa a,*, Hiroyuki Furuta b, Minako Takahashi a, Ayako Morino a,
Hidemitsu Uno c
a Department of Chemistry, Faculty of Science, Ehime Uni6ersity, Matsuyama 790-8577, Japan
b Department of Chemistry, Graduate School of Science, Kyoto Uni6ersity, Kyoto 606-8502, Japan
c Ad6anced Instrumentation Center for Chemical Analysis, Ehime Uni6ersity, Matsuyama 790-8577, Japan
Received 18 April 2000; accepted 22 April 2000
Abstract
N-Confused tetraarylporphyrinatoantimony(V) dimethoxides were synthesized and their X-ray crystallographic structure,
absorption spectra and voltammetric spectra were studied. X-ray crystallographic structure revealed neutral molecules with no
counter anion. From the absorption spectra and voltammetric studies we estimated their energy gaps to be about 0.2 eV less than
the corresponding porphyrinatoantimony(V). The axial ligands could easily be exchanged in solvent alcohol by acid promotion.
These characteristics of N-confused porphyrinatoantimony(V) indicate that they are good candidates for the molecular wire
component. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: N-Confused porphyrin; Antimony; Molecular wire
solubility of the arrays was high even for long wires,
one disappointment is that the electronic resonance
between the porphyrin p-systems was not efficient be-
cause the porphyrin planes in the arrays are almost
perpendicular to each other, and the energy gap did not
decrease when the arrays were elongated [4]. Another
possible method to make long porphyrin wires while
keeping both the good resonance and high solubility is
to connect them in an axial direction [5]. The semi-em-
pirical molecular orbital calculation for the system in
Fig. 1 showed that the resonance between the por-
phyrin moieties is feasible, and the Eg of the system
steadily decreases when the arrays become longer, and
as the central element became heavier, the Eg of the
molecule became smaller.
Although the arrays of the Group 15 element com-
plexes of porphyrins are promising as a molecular wire
in this sense, counter anions present in these molecules
will disturb the precise analysis of the electronic proper-
ties. In the Group 15 element porphyrins, the central
elements are formally cationic, since porphyrins act as
dianionic ligands when complexing. Thus, the position
1. Introduction
The term ‘molecular wire’ is used for molecules that
are electronically conductive with a highly preferred
axis for conduction. The mechanism of conduction
through a single molecule is quite different from that of
the bulk organic conductive materials. In the latter the
charges are carried by soliton in most cases. In con-
trast, conduction through single molecules generally
involves tunneling or a resonant tunneling mechanism
[1]. The resistance through a molecule of length L with
an energy gap Eg is roughly estimated as Rꢀ(12.9
,
kV) exp (0.7245)ꢀEg(eV)L(A) [2]. According to this
equation, the tunneling resistance will be less when the
Eg of the molecule is smaller. Porphyrin wires are
potentially highly conductive since they have a low Eg,
as is easily predicted from their deep color. We have
synthesized meso–meso coupled porphyrin arrays as
good candidates for a molecular wire [3]. Although the
* Corresponding author. Fax: +81-89-9279590.
E-mail address: ogawat@dpc.ehime-u.ac.jp (T. Ogawa).
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00405-8

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T. Ogawa et al. / Journal of Organometallic Chemistry 611 (2000) 551–557
of the counter ion in the molecule, which will affect the
electronic properties considerably, remains ambiguous.
An ‘N-confused porphyrin’ (NCP; e.g. 1 and 2) is a
porphyrin isomer wherein one of the four pyrrole rings
is inverted [6]. Recently, both theoretical [7] and exper-
imental [8] studies have been reported on such exotic
porphyrins that possess three nitrogens and one carbon
as chelating donor atoms. The NCPs can behave both
as a dianionic ligand (P2−) and as a trianionic ligand
(P3−) when complexing with metal ions. If NCP works
as P2− for a Group 15 element, the central element of
the product will be cationic, and if it acts as P3− the
central element will be electronically neutral. We have
synthesized antimony complexes of N-confused te-
traarylporphyrins as an initiatory step, and studied
their structure and properties to elucidate their poten-
tial as components of highly conductive molecular wires
(Chart 1).
2. Results and discussion
2.1. Reaction of N-confused porphyrins with SbBr3
Semi-empirical molecular orbital calculations (PM3)
performed in the system depicted in Chart 2 predicted
that the cationic structure (3) would be about 13 kcal
mol−1 more stable than the neutral one (4+HBr).
Chart 2.
However, in reality the reaction of N-confused te-
traphenylporphyrin (1) with antimony(III) bromide in
dry pyridine, followed by column chromatography us-
ing methanol–dichloromethane as the eluents afforded
dimethoxy-coordinated N-confused tetraphenylpor-
phyrinatoantimony(V) (5) (Scheme 1). The reaction of
N-confused tetrakis(4-methylphenyl)porphyrin (2) with
antimony(III) bromide in dry pyridine also afforded the
corresponding product 6. 1H-NMR spectra of these
products are consistent with the neutral structures 5
and 6. If the N-confused porphyrin works as P2−, the
outer CꢀH of the inverted pyrrole ring should be a
doublet because of the spin coupling with the outer
NꢀH. Actually, in N-confused porphyrinatopalladiu-
m(II) the outer CꢀH of the inverted pyrrole appeared as
a doublet at l 8.44 with a coupling constant of 2.9 Hz.
However, in these cases the outer CꢀH were singlets (5,
l 9.83 and 6, l 9.84), indicating that there are no
adjacent NꢀH protons in these molecules. The neutral
structure was further confirmed by a single-crystal X-
ray crystallographic analysis of 6 showing that no
counter anion was present in the crystal (Fig. 2). Crys-
tallographic data are shown in Table 1.
Chart 1.
2.2. Comparison of [Sb(NCTTP)(OCH3)2] (6) with
[Sb(TTP) (OCH3)2] (7)
It is interesting to compare the physical properties of
6 with 7 in which only the charge of the central element
is different while the structure remains almost identical.
1
As abbreviated in Table 2, in the H-NMR spectra
Fig. 1. PM3 calculated HOMO–LUMO energy gap of oligomeric
porphyrinato pnictogenides(V). ‘P monomer’ indicates the molecule
of HSC6H4Sꢀporphyrinato phosphine(V)ꢀSC6H4SH (will be abbrevi-
ated as HSC6H4SꢀPorꢀSC6H4SH) in which the sulfur atoms coordi-
nate to the central phosphine. ‘P dimer’ indicates the molecule
HSC6H4SꢀPorꢀSC6H4SꢀPorꢀSC6H4SH, and ‘P trimer’ means
HSC6H4SꢀPorꢀSC6H4SꢀPorꢀSC6H4SꢀPorꢀSC6H4SH. ‘As monomer’
is the same as ‘P monomer’ except that the central atom is arsine.
Other abbreviations were named in the same way.
the axial methoxy group appeared at l −1.58 for 6
and −2.05 for 7, and the b-proton of the pyrrole ring
exhibited at l 8.83–9.15 for 6 and 9.58 for 7. These
data show that the ring current generated by the aro-
matic p-electronic system is more effective in 7 than in
6. In the non-symmetrical structure of 6, the aromatic-
ity seems to be decreased, probably because of the less

T. Ogawa et al. / Journal of Organometallic Chemistry 611 (2000) 551–557
553
The lower energy gap of 6 compared with 7 was also
observed in the absorption spectrum shown in Fig. 4.
The umax of the longest wavelength band of 6 is 683 nm
(1.82 eV), while that of 7 is 610 nm (2.03 eV); the
energy difference (0.21 eV) obtained from the absorp-
tion spectrum was consistent with the value obtained by
voltammetric methods shown above (0.16 eV).
2.3. Axial group exchange of [Sb(NCTTP)(OCH3)2] (6)
When [Sb(NCTTP)(OCH3)2] (6) was refluxed in etha-
nol for 2 days, the axial group OCH3 was exchanged
Table 1
Crystallographic Data for [Sb(NCTTP)(OCH3)2] (6)
Empirical formula
Formula weight
Space group
C50H42N4O2Sb
852.66
P21/n (no. 14)
,
a (A)
14.72(4)
9.795(4)
15.008(6)
90
90.87(4)
90
2163(6)
2
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
3
,
V (A )
Z
Dcalc (g cm−3
)
1.315
6.80
v(Mo–Ka) (cm−1
)
,
u (A)
0.71069
Total: 5124, unique: 4163
No. of reflections measured
[Rint=0.041]
Direct methods
Full-matrix least-squares
on F2
Fig. 2. X-ray crystallographic structure of 6. Hydrogen atoms were
removed for clarification. Because of small difference of electron
density of nitrogen and carbon atoms, or disorder of the position of
the inverted pyrrole ring, the position of the inverted nitrogen could
not be determined by the X-ray crystallographic analysis. The bond
Structure solution
Refinement
Function minimized
Least squares weights
No. of observations (I\1.00|(I))
No. of variables
ꢁw(F2o−F2c)2
w=1/(|2(F2o)
3115
,
,
length of OꢀSb was 1.961(6) A, which is ca. 0.02–0.03 A longer than
that of [Sb(TTP)(OCH(CH3)2)2]Cl [12].
259
Reflection/parameter ratio
Residuals: R; Rw
R1
12.03
0.115; 0.174
0.058
effective phase matching of the p-electronic system.
Such weak aromaticity of the p-electronic system af-
fects the chemical properties of N-confused porphyrins,
and the ‘inverted’ pyrrole ring could be attacked easily
by nucleophiles and/or electrophiles [8j,k].
Goodness-of-fit indicator
1.28
1.53
Maximum peak in final difference
−3
,
map (e A
)
Minimum peak in final difference
map (°)
−0.74
In Fig. 3, cyclic and differential pulse voltammo-
graphs of compounds 6 and 7 are depicted, and the
representative data are shown in Table 2. The first
oxidation potential of [Sb(NCTTP)(OCH3)2] (6) (+690
mV vs. Ag/AgNO3) was 850 mV lower than that of
[Sb(TTP) (OCH3)2] (7) (+1540 mV vs. Ag/AgNO3),
and the first reduction potential of 6 (−1400 mV vs.
Ag/AgNO3) was 690 mV higher than that of 7 (−710
mV vs. Ag/AgNO3). This is a reasonable consequence
of the fact that the central element of 6 is neutral and
that of 7 is cationic, and a cationic species is hard to
oxidize and easy to reduce. An important result is that
the D(Eox−Ered) is 160 mV smaller in 6 (2090 mV) than
in 7 (2250 mV), that is, the energy gap of 6 is less than
7.
Table 2
Comparison of [Sb(NCTTP)(OCH3)2] (6) with [Sb(TTP) (OCH3)2] (7)
6
7
1H-NMR
Axial ꢀOCH3
b proton of pyrrole
−1.58
8.83ꢀ9.15 9.58
+690
−1400
−2.05
Electrochemical vs. Eox1 (mV)
+1540
potential
Ag/AgNO3
Lowest
absorption
band
Ered1 (mV)
−710
2250
610
D(Eox1−Ered1) (mV) 2090
umax (nm)
683
umax (eV)
1.82
2.03

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T. Ogawa et al. / Journal of Organometallic Chemistry 611 (2000) 551–557
creased slowly at room temperature (open circles in Fig.
5). In another run, CF3COOH was added to the solu-
tion about 20 min after the sample was dissolved in the
solvent. Before addition of the acid, the exchange reac-
tion was slow, however, after the addition of the acid
the reaction was accelerated, and 10 min after the
addition more than 80% of the starting material 6 was
converted into the deuterated compound. A total of 30
min after the addition the exchange of CH3Oꢀ into
CD3Oꢀ was complete, and only a trace amount of the
starting 6 could be observed (closed circle in Fig. 5)
(Scheme 1).
Such a facile exchange of the axial groups was not
observed for 7. Scheme 2 shows a probable mechanism
to explain the difference in the reactivity between por-
phyrinatoantimony and N-confused porphyrinatoanti-
mony. The acid promotion of the exchange reaction
indicates that this is an ‘SN1-like’ reaction and that the
cationic species A must be involved as the key interme-
diate. In the case of 6, the starting molecule is electron-
ically neutral, and the cationic intermediate A can be
stabilized by resonance with the N-confused porphyrin
p-electronic system. In contrast, in the case of 7, the
starting molecule itself is cationic and the doubly posi-
tive charge at the antimony atom in an intermediate
like A will destabilize the system greatly, and the ex-
change reaction must be retarded.
Such an easy ligand exchange will be useful in mak-
ing long molecular wires; mixing 6 with the linking diol
or dithiol molecules will make long molecules con-
nected in axial directions, as illustrated in Fig. 1. A
preliminary reaction of 6 with 1,2-ethylenediol was
carried out to give the corresponding dimer and trimer.
These results will be reported elsewhere.
Fig. 3. Voltammographs of 6 and 7. Solid lines show the cyclic
voltammograph and broken lines indicate the differential pulse
voltammograph. Working electrode, Pt disk; counter electrode, Pt
wire; reference electrode, Ag/AgNO3 in CH3CN; solvent CH2Cl2;
electrolyte, tetrabutylammonium perchlorate. Cyclic voltammograms
were measured with a scan rate of 100 mV s−1, and differential pulse
voltammograms were obtained with the following conditions: Scan
rate, 20 mV s−1; pulse amplitude, 50 mV; sample width, 17 ms; pulse
width, 50 ms; pulse period, 200 ms; quiet time, 2 s.
Fig. 4. Absorption spectra of 6 and 7 in CH2Cl2.
with OCH2CH3 to give [Sb(NCTTP)(OCH2CH3)2] (8)
quantitatively. Fig. 5 illustrates the time course of the
exchange reaction probed by 1H-NMR spectroscopy.
Compound 6 was dissolved in 10% CDCl3–CD3OD
and monitored by H-NMR spectroscopy. As the axial
CH3Oꢀ group was exchanged with the CD3Oꢀ of the
solvent, the intensity of the CH3Oꢀ group peak de-
Fig. 5. Time course of the axial exchange reaction of 6 with CD3Oꢀ
group. Open circles represent the reaction without acid, and closed
circles are the acid-promoted reaction. In the latter run, the acid was
added ca. 20 min after the compound was dissolved in the solvent.
The reactions were monitored by 1H-NMR in ca. 10% CDCl3–
CD3OD. The consumption of the starting material 6 was determined
by the ratios of the integral areas of axial CH3Oꢀ groups and methyl
of the meso-tolyl groups. The probe temperature was 27°C.
1

T. Ogawa et al. / Journal of Organometallic Chemistry 611 (2000) 551–557
555
Scheme 1.
Scheme 2.
Here we have found that (1) N-confused porphyrina-
toantimony(V) is a neutral molecule having no counter
ion; (2) the energy gap of the molecule is about 0.2 eV
less than the corresponding porphyrinatoantimony(V),
and (3) the linkage of the molecule in the axial direc-
tions will be facile because of the easy axial ligand
exchange. These characteristics of N-confused por-
phyrinatoantimony(V) indicate that they are good can-
didates for the molecular wire component.
and reference electrodes were, respectively, a platinum
disk, a platinum wire and an Ag/Ag+ 0.010 M elec-
trode. Mass spectra were measured on a JEOL JMS-
LX2000. Melting points were measured with a Yanako
BY-1. The elemental analyses were performed at the
Microanalysis Division of the Advanced Instrumenta-
tion Center for Chemical Analysis, Ehime University.
Antimony(III) bromide, bromine, hydrogen bromide
solution, methanol, dichloromethane were used as re-
ceived. Pyridine was distilled under nitrogen from cal-
cium hydride. N-Confused tetraarylporphyrins were
prepared according to methods in the literature [6,9].
3. Experimental
1H-NMR (270 MHz) NMR spectra were recorded on
a JEOL JNM-GSX270 and a JEOL JNM-EX. 1H
chemical shifts are referenced to internal tetramethylsi-
lane (0 ppm). UV–vis spectra were recorded with
JASCO V-570. Voltammetric measurements were per-
formed with BAS CV-50WS. The working, auxiliary
3.1. Synthesis of N-confused tetraphenylporphyrinatoan-
timony(V) dimethoxide (5)
A solution of N-confused tetraphenylporphyrin (61
mg, 0.1 mmol) and antimony(III) bromide (420 mg, 1.2
mmol) in dry pyridine (5 cm3) was stirred under nitro-

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T. Ogawa et al. / Journal of Organometallic Chemistry 611 (2000) 551–557
gen for 24 h. The solvent was removed under vacuum,
and the solute was purified by passing through an
alumina chromatography column, followed by a silica-
gel chromatography column using dichloromethane–
methanol as the eluents. The product was recrystallized
from dichloromethane–methanol to give purple crystals
of 5 (27 mg, 34% isolated yield): m.p.\270°C. 1H-
NMR: l −1.59 (s, 6H, OCH3), 7.77–8.23 (m, 20H,
phenyl), 8.84–9.16 (m, 7H), 9.84 (s, 1H, CHꢁN of the
inverted pyrrol). EIMS (70eV) m/e 795 [M+, 4], 732
[100, M+−(CH3O)2]. UV–vis (CH2Cl2) umax 444, 552,
592, 680 nm.
−3, respectively. All calculations were performed us-
,
A
ing the TEXSAN crystallographic software of the Molec-
ular Structure Corporation. The crystal data and
analytical conditions and final atomic coordinates and
isotropic temperature factors are listed in Table 1.
3.4. Axial ligand exchange of N-confused
tetraphenylporphyrinatoantimony(V) dimethoxide (5)
N-Confused
tetraphenylporphyrinatoantimony(V)
dimethoxide (5) (82 mg, 0.1 mmol) was refluxed in
ethanol (10 cm3) for 2 days. After the solvent was
removed in vacuo the product was purified by silica-gel
column chromatography to give N-confused tetra-
phenylporphyrinatoantimony(V) diethoxide (8) in
quantitative yield; 1H-NMR (CDCl3): l −2.17 (t, J=7
Hz, 6H), −1.57 (q, J=7 Hz, 4H), 7.77–8.22 (m, 20H,
phenyl), 8.81–9.12 (m, 6H), 9.83 (s, 1H, CHꢁN of the
inverted pyrrol). FABMS (3-nitrobenzylalcohol) m/z
3.2. Synthesis of N-confused
tetrakis(4-methylphenyl)porphyrinatoantimony(V)
dimethoxide (6)
A solution of N-confused tetrakis(4-methylphenyl)-
porphyrin (67 mg, 0.1 mmol) and antimony(III) bro-
mide (420 mg, 1.2 mmol) in dry pyridine (5 cm3) was
stirred under nitrogen for 24 h. The solvent was re-
moved under vacuum, and the solute was purified by
passing through an alumina chromatography column,
followed by a silica-gel chromatography column using
dichloromethane–methanol as the eluents. The product
was recrystallized from dichloromethane–methanol to
give purple crystals of 6 (38 mg, 42% isolated yield):
777
2(CH3CH2O)ꢀH, 55%].
[M+−CH3CH2OH,
16%],
733
[M+−
3.5. Axial ligand exchange of N-confused
tetrakis(4-methylphenyl)porphyrinato-antimony(V)
dimethoxide (6)
N-Confused tetrakis(4-methylphenyl)porphyrinato-
antimony(V) dimethoxide (6) (85 mg, 0.1 mmol) was
refluxed in ethanol (10 cm3) for 2 days. After the
solvent was removed in vacuo the product was purified
by silica-gel column chromatography to give N-con-
1
m.p.\270°C. H-NMR: l −1.58 (s, 6H, OCH3), 2.66
(s, 3H, CH3), 2.69 (s, 3H, CH3), 2.70 (s, 6H, 2CH3),
7.57–7.60 (m, 8H, aryl), 8.07–8.15 (m, 8H, aryl), 8.84–
8.90 (m, 4H), 9.12–9.17 (m, 2H), 9.84 (s, 1H, CHꢁN of
the inverted pyrrol). FABMS (m-nitrobenzyl alcohol)
m/e 851 [M+], 819 [M+−CH3OH], 789 [M+−
2(CH3O)ꢀH]. UV–vis (CH2Cl2) umax 445, 553, 596, 683
nm.
fused
tetrakis(4-methylphenyl)porphyrinato-antimo-
ny(V) diethoxide (9) in quantitative yield; 1H-NMR
(CDCl3): l −2.17 (t, J=7 Hz, 6H), −1.57 (q, J=7
Hz, 4H), 2.66 (s, 3H, CH3), 2.69 (s, 3H, CH3), 2.70 (s,
6H, 2CH3), 7.57–7.60 (m, 8H, aryl), 8.07–8.15 (m, 8H,
aryl), 8.84–8.90 (m, 4H), 9.12–9.17 (m, 2H), 9.84 (s,
1H, CHꢁN of the inverted pyrrol).
3.3. X-ray crystal structure analysis of 6
Single crystals were obtained by recrystallization
from MeOH–CH2Cl2. Intensity data were collected on
a Rigaku AFC7S diffractometer using a ꢀ−2q scan
technique to a maximum 2q value of 45.0°. Of the 5124
reflections that were collected, 4163 were unique. The
structure was solved by direct methods [10] and ex-
panded using Fourier analysis [11]. The non-hydrogen
atoms were refined anisotropically. Hydrogen atoms
were included but not refined. The final cycle of full-
matrix least-squares refinement on F2 was based on
3115 observed reflections (I\1.00|(I)) and 259 vari-
able parameters and converged (the largest parameter
shift was 12.03 times its estimated S.D.) with
unweighted and weighted agreement factors of:
R=ꢁ(F2o−F2c)/ꢁFo2 =0.115, Rw=[ꢁw(Fo2 −F2c)2/ꢁw-
(F2o)2]1/2=0.174, and R1=ꢁꢂFoꢃ−ꢃFcꢂ/ꢁꢃFoꢃ=0.058.
The maximum and minimum peaks on the final differ-
ence Fourier map corresponded to 1.53 and −0.74 e
4. Supplementary material
Complete lists of bond lengths and angles, hydrogen
atom coordinates and anisotropic thermal parameters
have been deposited at the Cambridge Crystallographic
Data Centre (No. CCDC-143383). Copies of this infor-
mation may be obtained free of charge from: The
Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax: +44-1223-336033; e-mail: deposit@
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
This work was supported by a Grant-in-Aid for
Scientific Research (No. 11120240) from the Ministry

T. Ogawa et al. / Journal of Organometallic Chemistry 611 (2000) 551–557
557
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