Diaqua-β-octaferrocenyltetraphenylporphyrin: A multiredox-active and air-stable 16π non-aromatic species

Article abstract of DOI:10.1039/c8dt04135k

Herein the synthesis and properties of the first β-octaferrocenyltetraphenylporphyrin, {TPPFc₈(H₂O)₂}, in its extraordinary stable and non-aromatic 16π form are reported, showing seven separate reversible redox events. As oxidation progresses, the neighbouring ferrocenyls of the pyrrole subunits display moderate electronic coupling, while electron transfer along the 16π porphyrin cycle was, due to its non-aromatic nature, not observed.

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Diaqua-β-octaferrocenyltetraphenylporphyrin:
a multiredox-active and air-stable 16π
non-aromatic species†‡
Cite this: DOI: 10.1039/c8dt04135k
Received 16th October 2018,
Accepted 12th December 2018
Rasha K. Al-Shewiki,a Marcus Korb, a Alexander Hildebrandt, a Stefan Zahn,
b
a
Sergej Naumov,b Roy Buschbeck,a Tobias Rüffer *a and Heinrich Lang
DOI: 10.1039/c8dt04135k
rsc.li/dalton
Herein
the
synthesis
and
properties
of
the
first porphyrins of type C exhibit a rich redox chemistry and redox-
β-octaferrocenyltetraphenylporphyrin, {TPPFc8(H2O)2}, in its extra- dependent spectroscopic versatility9 and hence allow the study
ordinary stable and non-aromatic 16π form are reported, showing of electron transfer in the respective mixed-valence species.11
seven separate reversible redox events. As oxidation progresses, This allowed their use as components in diverse applications,
the neighbouring ferrocenyls of the pyrrole subunits display mod- including molecule-based electronic devices,12 optical limit-
erate electronic coupling, while electron transfer along the 16π ers,13 molecular electrogenic sensors14 and multi-electron
porphyrin cycle was, due to its non-aromatic nature, not observed.
redox catalysts.15
Effective electronic communication between Fc groups,
substituents and the (metallo)porphyrinoid core requires a
direct covalent linkage. Any spacer between them weakens
electron transfer in the appropriate mixed-valence species.16,17
The first Fc-substituted porphyrin featuring direct σ-bonded Fc
functionalities is meso-5,10,15,20-tetraferrocenyl porphyrin (C,
Fig. 1) which was synthesized as early as 1977.9a Since then
plenty of further meso-substituted representatives have been
described.18 In contrast, β-Fc-substituted representatives have
not been reported so far, which is astonishing, since the Fc
termini would be in direct conjugation with the aromatic
C20N4 porphyrin core. However, several differently substituted
and air-sensitive β,β′-fused bis(metallo-porphyrinato)metallo-
cenes were discussed of which, depending on the substitution
pattern, significant electronic interactions upon oxidation
1 Introduction
Multiferrocenyl-substituted aromatic and heteroaromatic com-
pounds are one of the most fascinating families of molecules
due to their intriguing electronic properties and, for example,
rich variety of molecular structures.1 To date, numerous repre-
sentatives of multiferrocenyl compounds have been reported
including ferrocenyl (Fc = Fe(η5-C5H4)(η5-C5H5)) end-grafted
dendrimers2,3 or super-crowded Fc-based compounds like
Fc6C6, (FcCuC)6C6, cC4EFc4 (E = NMe, NPh, O, S) and
(cC4SFc3)2 in which steric encumbrance directly influences
chemical
conjugation
and
hence
electrochemical
interactions.4,5
In order to combine the excellent electrochemical pro-
perties and thermal stability of ferrocenes/ferrocenyls with
those of (metallo)porphyrinoids, much effort has been devoted
to synthesize, for example, β-substituted phthalocyanines6
(type A molecules, Fig. 1), corroles,7 tetraazaporphyrins8
(B, Fig. 1) and especially meso-functionalized porphyrins9,10
(C Fig. 1). It was shown that multiferrocenyl-functionalized
aChemnitz University of Technology, Inorganic Chemistry, Strasse der Nationen 62,
09111 Chemnitz, Germany. E-mail: tobias.rueffer@chemie.tu-chemnitz.de;
https://www.tu-chemnitz.de/chemie/anorg/mitarbeiter
bLeibniz-Institut für Oberflächenmodifizierung e.V., Permoserstrasse 15,
04318 Leipzig, Germany
†Dedicated to Prof. Dr D. Stalke on the occasion of his 60th birthday.
‡Electronic supplementary information (ESI) available. CCDC 1864072 and
1864073. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c8dt04135k
Fig. 1 Multiferrocenylated phthalocyanines (A), tetraazaporphyrins (B)
and meso-porphyrins (C).
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between the metallocenyl moieties and metalloporphyrin frag-
ments were noticed.19
In our studies using multiferrocenyl-functionalized 5-mem-
bered heterocycles as model compounds for electron transfer
studies, we recently described the synthesis and characteriz-
ation of 3,4-Fc2-cC4H2E (E = H, N, …) pyrroles of which it
appeared that 3,4-Fc2-cC4H2NH rapidly decomposed.20,21
Therefore, this did not allow us to prepare Fc-functionalized 18π
porphyrins by applying an Adler-type synthesis approach.22
In this work we describe the synthetic procedure to achieve
an
β-octaferrocenyltetraphenylporphyrin
via
copper(II)-
β-octabromotetraphenylporphyrin as its precursor of choice.
The title compound was characterised using spectroscopic
data (ESI-TOF, IR, UV/Vis) and its molecular structure was
determined by a single crystal X-ray crystallographic study,
allowing the assignment of the 16π non-aromatic nature.
Quantum chemical calculations were used to reinforce this
assignment. Furthermore, electrochemical (CV and SWV) and
spectroelectrochemical measurements were performed to
allow the assignment of seven individual and reversible oxi-
dations (six one-electron and one two-electron redox pro-
cesses). However, no electron transfer interaction throughout
the entire C20N4 core could be observed.
Scheme 1 Synthesis of {TPPFc8(H2O)2} (i) N-bromosuccinimide, CHCl3,
reflux, 40 h. (ii) 1st ferrocene, KOtBu, thf, tBuLi, −78 °C, 1 h; 2nd
[ZnCl2·2thf], 0–50 °C, 1 h, 25 °C, 1 h; 3rd [[P(tBu)2CMe2CH2Pd(μ-Cl)]2],
95 °C, 48 h.
version [M + H]+, while the basic peak is [M − 2H2O + H]+. In
addition, the IR spectrum confirms the presence of water and
the absence of N–H vibrations at 3310–3326 cm−1, giving
further evidence for a 16π porphyrin. As the solubility of
{TPPFc8(H2O)2} in NMR solvents, even in CD2Cl2, is poor, no
meaningful 1H NMR or 13C NMR spectra could be recorded.
The UV/Vis spectrum of {TPPFc8(H2O)2} did neither show a
strong Soret (B) band in the region of 400–470 nm, as present
in H2TPP, CuTPP and CuTPPBr8·CHCl3, nor significant Q
bands between 500 and 700 nm (Fig. 2), but is consistent with
those reported for 16π porphyrins.24a,26 For {TPPFc8(H2O)2}
two distinctive absorptions at 232 and 362 nm were observed,
while in the range of 500–700 nm a general and comparative
strong absorption with maxima at 529 and 597 nm is charac-
teristic (Fig. 2).24a
2 Results and discussion
Herein, we report a synthetic methodology for the synthesis of
β-octa-ferrocenyl-5,10,15,20-tetraphenylporphyrin denoted as
{TPPFc8(H2O)2}, which was obtained in its 16π state and as a di-
aqua adduct by using a post-Fc functionalization of CuTPPBr8
under Negishi C,C cross-coupling reaction conditions.
In this respect, copper(II)-5,10,15,20-tetraphenylporphyrin,
CuTPP, was treated with
a
ca. 62-fold excess of
N-bromosuccinimide23 to give CuTPPBr8·CHCl3 in good yields
(Scheme 1).24a In order to prepare 18π CuTPPFc8,
CuTPPBr8·CHCl3 was treated with a 34-fold excess of FcZnCl
under typical Negishi C,C cross-coupling reaction conditions
25
in the presence of [P(tBu)2CMe2CH2Pd(μ-Cl)]2 as a catalyst
under solvothermal conditions (95 °C, 48 h). After appropriate
work-up, the obtained crude residue was purified by column
chromatography to give a green solid material. ESI-MS studies
confirmed the residue to consist of a mixture of β-H2TPPFc7
and β-TPPFc8. The title compound could be separated by silica
gel preparative thin-layer chromatography as {TPPFc8(H2O)2}
in reasonable yield.24a,b Based on the spectroscopic and struc-
tural data it was identified as a representative of rarely
described 16π porphyrins.26
Porphyrin {TPPFc8(H2O)2} is stable at ambient temperature
in the solid state and in solution under aerobic conditions,24a
which is a surprise since previously reported 16π porphyrins
are of low stability. As {TPPFc8(H2O)2} is insoluble in almost
all common organic solvents except for dichloromethane its
spectroscopic characterization is limited.
Fig. 2 UV/Vis spectra (in CH2Cl2, 220–800 nm) of H2TPP (c = 2.1147 ×
10−6 M, black), CuTPP (c = 3.1053 × 10−6 M, orange), CuTPPBr8·CHCl3
(c = 8.7959 × 10−6 M, blue) and {TPPFc8(H2O)2} (c = 4.9644 × 10−6 M,
ESI-TOF mass-spectroscopic studies of {TPPFc8(H2O)2}
showed the expected molecular ion peak in the protonated green). The inset displays the enlarged spectral parts from 500–800 nm.
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Cyclic voltammetry (CV) and square wave voltammetry potential range and are not resolvable by CV or SWV.
5+–8+
(SWV) measurements showed that the consecutive oxidation of Furthermore, the cations TPPFc8
are poorly soluble in the
8+
the TPPFc8 part of {TPPFc8(H2O)2} to TPPFc8 in seven revers- electrolyte solution, which causes a deposition on the elec-
ible oxidation steps is possible by using [N(nBu)4][B(C6F5)4] as trode surface during oxidation. This deposition results in a
the weakly coordinating electrolyte,27 while reduction to lower observed current for the last redox step as well as the
4−
7+/5+
5+/4+
TPPFc8 occurs in two irreversible 2-electron steps (Fig. 3 and strong cathodic currents of the TPPFc8
and TPPFc8
5, Table 1, Fig. S15 in the ESI‡).24a The redox processes from reduction processes (Fig. 3) accompanied by ipa/ipc values of
TPPFc8 to TPPFc85+/6+ and TPPFc86+/7+ take place in a very close 0.62 and 0.76, respectively (Table 1).
The oxidation process of {TPPFc8(H2O)2} was monitored by
spectroelectrochemical UV/Vis/NIR measurements revealing
strong charge transfer transitions in the NIR range of the spec-
trum upon oxidation to TPPFc84+. The assignment of these
transitions was achieved by spectral deconvolution using three
Gaussian shaped bands: (i) a ligand to metal charge transfer
(LMCT νmax = 12 100 cm−1, εmax = 6250 L mol cm−1, Δν =
1
2
2338 cm−1, orange, Fig. 4), (ii) an inter-valence charge transfer
(IVCT νmax = 9100 cm−1, εmax = 5550 L mol cm−1, Δν
=
1
2
5300 cm−1, green, Fig. 4), and (iii) one band representing the
continuation of the spectra into the UV region (see the dotted
line, Fig. 4). As IVCT transitions are expected to occur in due
course of oxidations and disappear upon full oxidation, and as
the transition marked in green in Fig. 4 exhibits these features
its assignment became obvious. On the other hand, LMCT
transitions persist in due course of oxidations and change
their position only marginally. This helped to assign the
orange labelled band in Fig. 4 as a LMCT transition.
The relatively high extinction of the IVCT transition can be
explained by the multitude of groups in which the transition
4+
occurs; therefore TPPFc8 behaves like individual 3,4-diferro-
cenyl pyrroles with a fourfold concentration. In fact the physi-
cal parameter of the IVCT band resembles those of diferroce-
nyl ethylenes,28 diferrocenyl pyrroles21 and diferrocenyl male-
imides29 in shape, energy and 1/4th of their extinction. From
the spectroelectrochemical data it can be concluded that the
two ferrocenyls which are C–C bonded to an individual pyrrole
subunit are moderately coupled to each other (Robin and Day
Fig. 3 Top: CV of {TPPFc8(H2O)2} in the range of −500 to 1000 mV.
Bottom: SWV of {TPPFc8(H2O)2} in the range of −500 to 1000 mV with
process deconvolution showing seven events with an integral ratio of
1 : 1 : 1 : 1 : 1 : 1.9 : 0.8.
Measurement
conditions:
Concentration
0.50 mmol L−1 (for {TPPFc8(H2O)2}), 25 °C, reference decamethyl ferro-
cene, electrolyte: a 0.1 mol L−1 solution of [N(nBu)4][B(C6F5)4] in CH2Cl2.
Table 1 CV data of {TPPFc8(H2O)2}. Measurement conditions:
Concentration 0.50 mmol L−1 (for {TPPFc8(H2O)2}), 25 °C, reference
decamethyl ferrocene, electrolyte: a 0.1 mol L−1 solution of [N(nBu)4][B
(C6F5)4] in CH2Cl2
Wave
E°′a (mV)
ΔEpb (mV)
ipa/ipc
−2320c
−1810c
−139
−42
—
—
—
—
4−/2−
TPPFc82−/0
TPPFc80/+
TPPFc8+/2+
TPPFc82+/3+
TPPFc83+/4+
TPPFc84+/5+
TPPFc85+/7+
TPPFc87+/8+
TPPFc8
72
63
75
72
100
100
64
0.98
0.97
0.96
0.98
0.62
0.76
0.96
67
147
420
520
690
Fig. 4 Left: UV/Vis/NIR spectra of {TPPFc8(H2O)2} at rising potentials
(0–675 mV vs. Ag/AgCl, bottom; 675–1300 mV Ag/AgCl, top); arrows
indicate increasing or decreasing absorptions. Spectra of
4+
8+
{TPPFc8(H2O)2} (black, 0 mV), TPPFc8 (blue, 675 mV) and TPPFc8
a Eo′ = formal potential. b ΔEp = difference between the cathodic and
anodic peak potentials. c Cathodic peak potential Epc of the irreversible
process.
(red, 1300 mV) are highlighted. Right: Deconvolution of the experi-
mental spectrum of TPPFc8 (blue) with two main contributing tran-
sitions (green, IVCT; orange, LMCT).
4+
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class II regime).30 However, an electron transfer interaction
throughout the entire C20N4 core is not observed.
The two reduction processes of the TPPFc8 part of
{TPPFc8(H2O)2} were monitored by UV/Vis spectroscopy. The
2−
first process reduces TPPFc8 to TPPFc8 at −1.8 V vs. Fc/Fc+
showing the observation of the typical Q-band (ca. 540 nm,
Fig. 5) characteristic of 18π TPP systems. This confirms the
aforementioned assumption that TPPFc8 is a 16π porphyrin.26
Applying a more cathodic potential (−2.3 V) led to a 2nd, irre-
4−
versible two electron reduction and thus the 20π TPPFc8
Fig. 6 Ball-and-stick models of the molecular structures of CuTPPBr8
(left, two different perspective views) and {TPPFc8(H2O)2} (right). All
carbon-bonded hydrogen atoms are omitted for clarity.
macrocycle is formed. This reduction is accompanied by a
decrease of the intensity and disappearance of the Q-band of
the in situ generated 16π species (Fig. 5).
Electron poor31 or geometry distorted32 porphyrins have
attracted considerable attention as synthetic models of natu-
rally occurring porphyrinoids and β-2,3,7,8,12,13,17,18-octa-
MTArylBr8 porphyrins led to a ruffle-type distortion, as typical
of NiTCF3Br8.39
bromo-meso-5,10,15,20-tetraphenyl(metallo)
porphyrins
In the case of {TPPFc8(H2O)2} the crystallographic charac-
terization revealed, besides dichloromethane molecules as
packing solvents, the presence of two H2O molecules, which
interact with the 16π porphyrin core by hydrogen bonds
(Fig. 6).24a,34 A related situation was observed for {OiPTPP
(H2TArylBr8/MTArylBr8) as prominent representatives. Beyond
this we demonstrated in an earlier study the applicability of
CuTPPBr8 for a surface-confined 2D polymerization on Au(111)
to create a close-packed covalently bonded network.33 For
further optimization of such experiments and for comparison
with the data of {TPPFc8(H2O)2} the geometry parameters of
CuTPPBr8 are needed. Hence, we structurally characterized
CuTPPBr8 by single crystal X-ray determination.24a,34 Like its
metal-free H2TPPBr8,35 CuTPPBr8 exhibits crystallographically
imposed S4 symmetry in the solid state (Fig. 5). As observed
for H2TArylBr8/MTArylBr8 type porphyrins,24a CuTPPBr8 is also
saddle-shape distorted.32 The deviations of all atoms forming
the C20N4 core including the Cu atom from a calculated mean
plane are shown in Fig. 6 together with the RMS and MD para-
meter.36 With respect to the MD parameter it is astonishing to
notice that among all other H2TArylBr8/MTArylBr8 type por-
phyrins only Fe(NO)TPPBr8 (MD = 0.629) is more distorted.37
For H2TPPBr8,35 for example, the MD parameter amounts to
(H2O)2} (OiPTPP
=
octaisopropyltetraphenylporphyrin),26e
although this 16π porphyrin is reported as being water-sensi-
tive and to decompose with time in aqueous organic solvents.
Such
a reactivity was not observed for {TPPFc8(H2O)2}
(vide infra).24 That the title compound represents a 16π por-
phyrin can also be derived from its solid state structure. First,
the C20N4 core of {TPPFc8(H2O)2} possesses alternating single
and double bonds, although their different bond lengths do
not obey the 3σ criteria.24,40 Very large deviations from a mean
plane of the atoms defining the C20N4 core are noticed (Fig. 7),
as typically observed for all other 16π porphyrins.26e
With respect to the RMS value the distortion observed for
{TPPFc8(H2O)2} is exceeded by 16π {OiPTPP(H2O)2} (TMS =
1.078) only.26e However, the degree of distortion can be
expressed as well from the torsion angles formed between the
2,5-dimethylene-pyrole units. The atoms of {TPPFc8(H2O)2}
which are involved in these torsions are connected by green
0.616.24 A significantly larger degree of the saddle distortion
has been observed for double-protonated H4TPPBr8
2+ 38
,
while
a replacement of the four aryl substituents in H2TArylBr8/
Fig. 5 Left: CV of {TPPFc8(H2O)2} in the range of −1200 to −2700 mV
Fig. 7 Deviation of each atom from a calculated mean plane defined by
the 24 core atoms (in units of 0.001 A) in the molecular structure of
CuTPPBr8 (left, including the copper atom) and {TPPFc8(H2O)2} (right),
along with RMS and MD values.36 For atoms involved in green marked
bonds the absolute values of the torsion angles (in °) are given.
(2nd wave is shown). Right: UV/Vis spectra (400–800 nm) of
2−
{TPPFc8(H2O)2} (black, −400 mV vs. Ag/AgCl), TPPFc8
(blue,
4−
−1600 mV vs. Ag/AgCl) and TPPFc8 (red, −2500 mV vs. Ag/AgCl);
arrows indicate increasing or decreasing absorptions.
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marked bonds in Fig. 6. The absolute values of the torsion of {TPPFc8(H2O)2} are much larger and hence its astonishingly
angles range from 79.8(15) to 86.3(14)° with an average of high stability against air, moisture, wet solvents and even
84.0(29)°. This indicates a nearly perpendicular alignment of water is thoroughly understood. As a consequence thereof,
these units to each other (Fig. 6). Compared to the related data {TPPFc8(H2O)2} is regarded as a non-aromatic species.
of {OiPTPP(H2O)2} (range: 65.1 to 70.5°; average 67.8°)26e the
distortion of {TPPFc8(H2O)2} is thus substantially larger.
While several different methods allowed the identification
3 Conclusion
of {TPPFc8(H2O)2} as a 16π porphyrin (vide supra), the question
It was shown that the rational synthesis of a porphyrin with
remains whether it represents a non-aromatic or an anti-aro-
matic species.26 Schleyer et al. proposed that the calculation of
the nucleus-independent chemical shifts (NICS) should be
used to make this decision.41 Thanks to its simplicity and
efficiency this method has found broad application.42 For
{TPPFc8(H2O)2} a slight aromatic nature is indicated by NICS
at the center of the four N atoms (−2.7 ppm) and at the center
of the heterocyclic units (−1.3 ppm).24a However,
{TPPFc8(H2O)2} possesses four aromatic phenyl and eight aro-
matic Fc groups which might be responsible for the calculated
overall weak aromaticity. As transition metals are furthermore
known to induce ring currents without an aromatic behav-
iour,43,44 we determined the bond order in {TPPFc8(H2O)2} by
calculating the Wiberg bond index.45 The results are displayed
in Fig. 8 and allow the conclusion that especially the pyrrole
rings are characterized by the alternation of single and double
bonds. While one N–Ch bond possesses a bond order of 1.60,
eight β-bonded ferrocenyl groups under typical Negishi C,C
cross-coupling conditions is possible and not limited by steric
requirements. However, as
a non-aromatic 16π system
{TPPFc8(H2O)2} was obtained without any evidence of the for-
mation of the related 18π H2TPPFc8 porphyrin, the crucial
point of its synthesis remains unexplored.24a Formally, the for-
mation of 16π species requires the deprotonation and oxi-
dation of the related 18π species. Nevertheless, the outstand-
ing high stability of {TPPFc8(H2O)2}, attributed to its geometry
distortion, demonstrates that 16π porphyrins do not necess-
arily represent lab curiosities but might find applicability in
all the fields where their 18π counterparts are already used.
Conflicts of interest
The authors declare no conflict of interest.
the neighboured one has
a bond order of only 1.10.
Additionally, the Ch–Cm bond orders differ strongly and
represent the sequence of a typical single and double bond
(Fig. 8). Thus, the calculated NICS and the alternating bond
pattern confirm the non-aromatic nature of {TPPFc8(H2O)2}.
Further evidence for the assignment of 16π {TPPFc8(H2O)2}
as a stable and non-aromatic species comes from the interplay
between geometry distortion, non- or anti-aromatic nature and
stability.26 It is already learned that strong geometry distor-
Acknowledgements
This work has been supported by the Deutsche
Forschungsgemeinschaft through project FOR 1154 “Towards
Molecular Spintronics”.
tions support a high stability of an individual 16π species.26d
A
Notes and references
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reported 16π porphyrins is their sensitivity to water and/or wet
solvents.26 Compared to {OiPTPP(H2O)2}26e the torsion angles
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Fig. 8 Atom labels and calculated Wiberg bond orders45 for one
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