Electronic and NMR properties of meso-monosubstituted ferrocenylporphyrin: Evidence of π-conjugation between porphyrin and ferrocene

Article abstract of DOI:10.1016/S0277-5387(00)00481-2

The reaction of dipyrrylmethane with an equimolar mixture of ferrocenecarboxaldehyde and benzaldehyde in dry acetonitrile in the presence of a catalytic amount of trichloroacetic acid leads to the formation of 5-ferrocenyl-15-phenyl-2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin (1) with a yield of 27%. The metallation of 1 with Zn(OAc)₂·2H₂O and MnCl₂·4H₂O gives Zn·1 and MnCl·1, respectively. Cyclic voltammograms of 1, Zn·1 and MnCl·1 show reversible one-electron oxidation processes for the oxidation of ferrocene at 0.29, 0.22, and 0.37 V, respectively. Variable temperature ¹H NMR spectra of 1 in toluene-d₈ show two distinct singlets of equal intensities and line widths for NH protons. From the plot of ln (k/T) versus l/T according to the Eyring equation, gave straight line with ΔG(+) = 51 kJ mol^-1. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(00)00481-2

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 1961–1966
Electronic and NMR properties of meso-monosubstituted
ferrocenylporphyrin: evidence of p-conjugation between porphyrin
and ferrocene
Seog Woo Rhee b, Byung Bin Park a, Youngkyu Do b,*1, Jinkwon Kima,*2
a Department of Chemistry and RRC/NMR, Kongju National Uni6ersity, Kongju 314-701, South Korea
b Department of Chemistry, School of Molecular Science-BK21 and Center for Molecular Design and Synthesis,
Korea Ad6anced Institute of Science and Technology, Taejon 305-701, South Korea
Received 29 February 2000; accepted 26 June 2000
Abstract
The reaction of dipyrrylmethane with an equimolar mixture of ferrocenecarboxaldehyde and benzaldehyde in dry acetonitrile
in the presence of a catalytic amount of trichloroacetic acid leads to the formation of 5-ferrocenyl-15-phenyl-2,8,12,18-tetraethyl-
3,7,13,17-tetramethylporphyrin (1) with a yield of 27%. The metallation of 1 with Zn(OAc)2·2H2O and MnCl2·4H2O gives Zn·1
and MnCl·1, respectively. Cyclic voltammograms of 1, Zn·1 and MnCl·1 show reversible one-electron oxidation processes for the
1
oxidation of ferrocene at 0.29, 0.22, and 0.37 V, respectively. Variable temperature H NMR spectra of 1 in toluene-d8 show two
distinct singlets of equal intensities and line widths for NH protons. From the plot of ln (k/T) versus 1/T according to the Eyring
equation, gave straight line with DG‡=51 kJ mol−1. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Ferrocenylporphyrin; p-Conjugation; Electrochemical communication; Intramolecular hydrogen bonding; NH tautomerism
2. Experimental
1. Introduction
2.1. Materials
Many discrete molecules in which two photo- and
redox-active sites are linked by p-conjugation have been
Solvents were purified according to standard meth-
ods where applicable [8]. Tetra-n-butylammonium per-
chlorate (TBAP) was synthesized and purified by
procedure described in the literature [9]. 3,3%-Diethyl-
developed with great interest for applications in molec-
ular electronic devices [1,2]. In particular, porphyrins
[3,4] or ferrocenes [5] are the most frequently used
molecules for this purpose. However, examples of cova-
4,4%-dimethyl-2,2%-dipyrrylmethane was synthesized ac-
lently linked p-conjugates of these two units are quite
rare [6,7]. These facts and the recent demonstration on
the significant substituent effect on electronic properties
in porphyrins prompted us to explore new porphyrin–
ferrocene conjugates. Herein we describe the synthesis,
structure and conjugated feature of 5-ferrocenyl-15-
phenyl - 2,8,12,18 - tetraethyl - 3,7,13,17 - tetramethylpor-
phyrin (1).
cording to procedures described in the literature [10].
All other reagents were purchased from Aldrich Chem-
ical Co. or Sigma Chemical Co. and were used as
received.
2.2. Preparations
5-Ferrocenyl-15-phenyl-2,8,12,18-tetraethyl-3,7,13,17-
tetramethylporphyrin (1) was synthesized by a proce-
dure similar to that described by Osuka et al. for the
preparation of 5,15-diarylporphyrins [11]. The fer-
rocenecarboxaldehyde (0.79 g, 3.7 mmol) and benzalde-
hyde (0.39 g, 3.7 mmol) were dissolved in dry aceto-
nitrile (84 ml) containing trichloroacetic acid (0.56 g,
E-mail address: jkim@knu.kongju.ac.kr (J. Kim).
1 *Corresponding author
2 *Corresponding author. Tel.: +82-41-850-8496; fax: +82-41-
850-8479
0277-5387/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(00)00481-2

1962
S.W. Rhee et al. / Polyhedron 19 (2000) 1961–1966
3.4 mmol). The mixture was stirred for 30 min under
nitrogen. Then, the 3,3%-diethyl-4,4%-dimethyl-2,2%-
dipyrrylmethane (1.8 g, 7.4 mmol) in acetonitrile (20
ml) was added through cannula under positive nitrogen
pressure. After stirring for 60 h under nitrogen in the
dark, p-chloranil (2.93 g, 12 mmol) in THF (100 ml)
was added and stirred for 5 h under same conditions.
The solvent was evaporated to dryness, and the residual
solid was suspended on aqueous NaOH solution (10%,
100 ml) to dissolve the hydroquinone impurities. After
stirring for 2 h, filtered on a glass frit, washed with
water and then methanol. The crude products were
separated and purified by flash chromatography (SiO2,
3.8×25 cm, CHCl3). The first red band was 5,15-
diphenyl-2,8,12,18-tetraethyl-3,7,13,17-tetramethylpor-
phyrin (2), and the second olive green band was the
roform–methanol mixture gave purple crystals of Zn·1
(77%). MnCl·1 were obtained by treatment of refluxing
chloroform solution of 1 with saturated methanolic
MnCl2·4H2O solution and then by air-oxidation (58%).
2.3. Physical measurements
Elemental analyses (C, H, and N) were performed by
Fisons EA 1110 analyzer. 1H NMR spectra were
recorded on a Bruker Spectrospin 400 instrument with
chemical shifts being referenced against the signal of
the deuterated NMR solvents. Electronic spectra were
measured on a Shimadzu UV-3100S spectrophotome-
ter. Thermogravimetric analyses were performed under
nitrogen atmosphere at a heating rate of 10°C min−1
with DuPont TGA 2050 instrument. Electrochemical
measurements were carried out on a BAS-100W system.
Cyclic voltammetry was performed with 1×10−3 mol
dm−3 solution of sample in dry CH2Cl2 containing 0.1
mol dm−3 of (n-C4H9)4NClO4 as supporting elec-
trolyte. The potentials were determined with reference
to the Ag–AgCl electrode at room temperature. Under
these conditions ferrocene shows a reversible one-elec-
tron oxidation wave (E1/2=0.37 V).
desired
5-ferrocenyl-15-phenyl-2,8,12,18-tetraethyl-
3,7,13,17-tetramethylporphyrin (1). Recrystallization
from toluene–methanol mixture gave purple crystals of
1·0.5C6H5Me (0.70 g, 27% based on the amount used
dipyrrylmethane). Anal. Calc. for C48H50N4Fe·0.5C6-
H5Me (MW 784.87): C, 78.81; H, 6.93; N, 7.14. Found:
1
C, 77.58; H, 6.93; N, 7.19. H NMR (l ppm, CDCl3)
9.86 (s, 2H, meso-H), 8.25 (d, 1H, phenyl), 7.75 (m, 3H,
phenyl), 7.64 (d, 1H, phenyl), 7.25–7.16 (m, 2.5H,
phenyl of C6H5Me), 4.70 (t, 2H, ferrocenyl), 4.60 (t,
2H, ferrocenyl), 3.90 (m, 8H, CH2CH3), 3.72 (s, 5H,
ferrocenyl), 3.35 (s, 6H, CH3), 2.39 (s, 6H, CH3), 2.35
(s, 1.5H, C6H5Me), 1.72 (t, 6H, CH2CH3), 1.66 (t, 6H,
CH2CH3), −1.06 (s, 1H, NH), −1.32 (s, 1H, NH).
UV–Vis (C6H5Me) [umax, nm (log m, mol−1 dm3 cm−1)]
421 (5.19), 505 (4.05), 527 (4.01), 581 (3.94), 601 (3.95),
662 (3.53).
2.4. X-ray crystallography
A purple crystal of 1·0.5 C6H5Me with dimensions
0.5×0.4×0.25 mm, obtained from toluene–methanol
solution, was mounted on a thin glass fiber and inten-
sity data were collected on an Enraf–Nonius CAD4
diffractometer equipped with monochromated Mo Ka
,
(u=0.71073 A) radiation. Unit cell parameters were
Zn·1 was obtained by treatment of refluxing chloro-
determined from a least-squares fit of 25 accurately
centered reflections (22.45B2qB27.22). These dimen-
sions and other parameters, including conditions of
data collection, are summarized in Table 1. Data were
collected at 293 K in the ꢀ–2q scan mode. Three
intense reflections were monitored every 200 reflections
to check stability. Of the 9932 unique reflections mea-
sured, 6198 were considered observed (Fo\4|(Fo)) and
were used in subsequent structure analysis. Data were
corrected for Lorentz and polarization effects. Empiri-
cal absorption correction using the c scan technique
was applied. Maximum and minimum transmissions
were 99.83 and 96.21%, respectively. The SHELXS-86
program was utilized for the direct method. The struc-
ture refinements were performed with the SHELXL-93
program on F2 data. Anisotropic thermal parameters
for all non-hydrogen atoms were included in the refine-
ments. All hydrogen atoms bonded to carbon atoms
were included in calculated positions. This CꢀH bond
distance was fixed and U values were assigned based
approximately on the U value of the attached atom.
form solution of
Zn(OAc)2·2H2O solution. Recrystallization from chlo-
1
with saturated methanolic
Table 1
Details of the crystallographic data collection for 1·0.5C6H5Me
Chemical formula
Chemical formula weight
Space group
C48H50N4Fe·0.5C6H5Me
784.87
C2/c
50.557(5)
14.611(5)
23.593(3)
101.54(2)
17076(7)
16
1.255
0.395
0.71073
44
955
,
a (A)
,
b (A)
,
c (A)
i (°)
3
,
V (A )
Z
zcalc (g cm−3
)
v (mm−1
)
,
u (Mo Ka) (A)
2q max. (°)
Parameters refined
R(Fo)
0.0780
0.2032
1.070
wR(F2o) a
Goodness-of-fit
The other hydrogens (NꢀH) were included in located
a w=1/[|2(Fo2)+(0.1197P)2+37.5443P] where P=(Fo2+2F2c)/3.
2
,
positions with U=0.08 A . The toluene molecule is

S.W. Rhee et al. / Polyhedron 19 (2000) 1961–1966
1963
Fig. 1. Molecular structure of 1, monomeric (a), and dimeric (b) structure.
,
refined with the regular hexagon model (dCꢀC=1.39 A).
The X-ray study of 1 shows that an asymmetric cell
consists of one toluene solvent molecule and two ferro-
cenylporphyrins to form a dimer owing to a pꢀp inter-
action between the porphyrin planes. As shown in Fig.
1, the linked cyclopentadienyl (Cp) ring of the ferrocene
substituent and the porphyrin plane twisted with dihe-
dral angles of 47.1(3) and 49.3(3)°. The unusually small
dihedral angles caused by large steric hindrance of
b-pyrrolic methyl group imply that there might be a
chance for a weak p-overlap between the Cp and the
porphyrin rings. Furthermore, large steric hindrance of
the b-pyrrolic methyl groups not only prevents the
ferrocenyl moiety from rotating around the meso car-
bon but also results in distortion of the porphyrin ring.
A final difference Fourier map revealed several random
features (B0.627 eA−3). The full-matrix least-squares
,
refinement finally converged with R1=0.0780 (based on
F) and wR2=0.2032 (based on F2) for observed reflec-
tions (Fo\4|(Fo)).
3. Results and discussion
Compound 1 was synthesized by condensation of
dipyrrylmethane with an equimolar mixture of benz-
aldehyde and ferrocenecarboxaldehyde. Pure 1 was
separated from crude mixtures by column chromatog-
raphy, and 5,15-diphenyl-2,8,12,18-tetraethyl-3,7,13,17-
tetramethylporphyrin (2) was also obtained as a
side-product in 28% yield but 5,15-diferrocenyl-
2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin was
not formed, because of steric demand.
,
The CꢀC distance (1.50 A) between the Cp and meso
carbon of porphyrin is not significantly shorter than the
,
pure single bond distance (1.52 A) found in the 5,15-di-
ferrocenylporphodimethene [12]. However, It should be
considered that this bond distance of other ferrocenyl
porphyrin complexes characterized by our and other

1964
S.W. Rhee et al. / Polyhedron 19 (2000) 1961–1966
,
conjugated feature of 1 was further explored by spec-
troscopic and electrochemical means. Selected bond
distances and angles are listed in Table 2.
group was in the range of 1.477(5)ꢀ1.498(8) A which
,
are significantly shorter than 1.52(1) A. [6a,13] The
Table 2
Electrochemical properties of 1 and its metal com-
plexes Zn·1 and MnCl·1 in dichloromethane solution
are summarized in Table 3. The ferrocenyl groups of
compounds 1 and Zn·1 undergo a reversible one-elec-
tron transfer process at 0.29 and 0.22 V, respectively.
The values of half-wave oxidation potentials indicate
that the ferrocenyl groups in these compounds are more
susceptible for the oxidation than the free ferrocene.
The easier ferrocenyl oxidation might be considered as
a consequence of the great electron releasing ability of
the porphyrin ring. In the case of MnCl·1, the higher
oxidation state of Mn(III) is expected to reduce the
electron releasing tendency of the porphyrin ring and
thus make the ferrocene oxidation difficult. In fact, the
oxidation potential of MnCl·1 shifts to positive direc-
tion. The foregoing electrochemical analyses show that
the oxidation of the ferrocene subunit is strongly af-
fected by porphyrin ring as well as the central metal
through extended p-conjugation.
,
Selected bond distances (A) and angles (°) of 1·0.5C6H5Me
Fe1ꢀC21
Fe1ꢀC23
Fe1ꢀC25
Fe1ꢀC27
Fe1ꢀC29
C4ꢀC5
C5ꢀC21
C14ꢀC15
N1ꢀH1
2.072(6)
2.038(7)
2.013(7)
2.026(8)
2.02(1)
1.383(8)
1.505(9)
1.391(8)
0.948
Fe1ꢀC22
Fe1ꢀC24
Fe1ꢀC26
Fe1ꢀC28
Fe1ꢀC30
C5ꢀC6
C15ꢀC43
C15ꢀC16
N3ꢀH3
2.050(7)
2.009(7)
2.024(8)
2.03(1)
2.015(9)
1.427(8)
1.503(8)
1.397(8)
0.952
C4ꢀC5ꢀC6
C6ꢀC5ꢀC21
C16ꢀC15ꢀC43
122.1(6)
113.1(5)
117.8(5)
C4ꢀC5ꢀC21
C16ꢀC15ꢀC14
C14ꢀC15ꢀC43
124.1(6)
124.9(5)
117.1(5)
Hydrogen bonding parameters
N2…H1
N1…N2
N1…N4
2.021(5)
2.692(7)
3.160(7)
N4…H3
N3…N4
N2…N3
1.910(5)
2.718(7)
3.123(7)
Table 3
Half-wave potentials of porphyrin 1, Zn·1, MnCl·1
1H NMR data further support the strong electronic
interaction of the ferrocenyl group with the porphyrin
ring. In the case of 1 in chloroform, an electron with-
drawing effect of ferrocenyl group through p-conjuga-
tion with the porphyrin ring was suggested to account
for the large downfield shift of b-pyrrolic methyl pro-
tons adjacent to ferrocenyl group (3.35 ppm) compared
to those adjacent to phenyl group (2.39 ppm). Further-
more, the chemical shifts for meso CH (9.86 ppm) and
NH (−1.06 and −1.32 ppm) protons of 1 are quite
different from the chemical shift for meso CH (10.22
ppm) and NH (−2.41 ppm) protons of 2. This indi-
cates that the ring current in 1 might be partially
reduced due to the ring distortion [14]. Notably, the
large downfield shift of NH for 1 compared to 2 can be
interpreted by the NꢀH···N intramolecular hydrogen
bonding [15].
Compound
E1/2 (V) a
Ferrocene oxidation
Porphyrin oxidation
1
Zn·1
MnCl·1
Ferrocene
0.29
0.22
0.37
0.37
0.61
1.05
0.83
0.45
0.97 b
a Obtained in CH2Cl2 solution containing 0.1 mol dm−3 TBAP as
supporting electrolyte. Solutions were 1×10−3 mol dm−3 in com-
pound and potentials were determined with reference to a Ag–AgCl
electrode with 0.1 V s−1 scan rate.
b Two-electron process.
The NH tautomerism in porphyrin is an intramolecu-
lar proton transfer process coupled with migration of
double bonds. The two NH protons of the porphyrin 1
are situated on different chemical environments. Vari-
1
able temperature H NMR spectra of 1 in toluene-d8
show two distinct singlets of equal intensities and line
widths for NH protons as shown in Fig. 2. The chemi-
cal shifts are −0.47 and −0.72 ppm at 298 K. The
presence of ferrocenyl substituent at only C5 position
renders the two nitrogen-bonded protons magnetically
nonequivalent at room temperature. The coalescence of
the two NH peaks was unobserved up to the higher
temperature limit of 373 K and only line broadening
occurred, indicating that the exchange rate is very slow.
This observation can be explained by considering that
one kind of intramolecular NꢀH···N hydrogen bonding
is prevailing against the other due to a rectangular
Fig. 2. Variable temperature 1H NMR spectra for the nitrogen-
bonded inner protons of porphyrin 1 in dry toluene-d8. (a) observed
and (b) calculated.

S.W. Rhee et al. / Polyhedron 19 (2000) 1961–1966
1965
Scheme 1. Schematic representation of NH tautomerism.
elongation of porphyrin core. The X-ray analysis shows
,
that 1 has two sets of short (2.69 and 2.72 A) and long
,
(3.12 and 3.16 A) N···N distances and the short N···N
,
distances lie in the known range (2.71 A) observed for
the NꢀH···N hydrogen bonding units of porphyrin [15].
Thus, the tautomerisations between tautomer I and III
as well as between II and IV are not energetically
favorable (Scheme 1) [16]. Based on the assumption
that exchange is only responsible for the temperature-
dependent line broadening, the rate constants for ex-
change can be determined. From plot of ln (k/T) versus
1/T according to Eyring equation [17], a straight line
with DG‡=51 kJ mol−1 was obtained. This is similar
to values reported in the literatures [18]. The exchange
rate for NH protons depends on the solution condi-
tions. When anhydrous chloroform is employed as a
NMR solvent for 1, the coalescing of the two NH
peaks is not observed up to the higher temperature
limit of 328 K. This is similar to the result observed in
dry toluene as shown in Fig. 3. However, two NH
signals are coalesced below room temperature in chlo-
roform containing trace amount of residual water. It
implies that the NH tautomerism of 1 in wet chloro-
form may be mediated by the trace amount of water in
solvent. In dilute solution (0.04 mmol dm−3) of por-
phyrin, the signal of NH protons is a singlet at 298 K.
However, in concentrated solution (0.5 mmol dm−3),
the signals of NH protons split into two peaks because
the rate of exchange becomes slower. Such result sug-
gests that the rate of dynamic exchange of NH protons
is accelerated in proportion to the water concentration
of solution.
Fig. 3. 1H NMR spectra of 1 in various solvent conditions at 298 K.
(a) H21·0.5C6H5Me in anhydrous toluene-d8, (b) H21 in wet chloro-
form-d, and (c) H21 in anhydrous chloroform-d.
4. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 140998. 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).

1966
S.W. Rhee et al. / Polyhedron 19 (2000) 1961–1966
Commun. (1999) 637. (b) A.K. Burrell, W.M. Campbell, D.L.
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