An Expanded Porphycene with High NIR Absorptivity That Stabilizes Two Different Kinds of Metal Complexes

Article abstract of DOI:10.1002/anie.201711197

A new expanded porphycene with 26 π-electrons has been prepared by the McMurry coupling of 1,4-bis(3,4-diethyl-2-pyrryl)benzene dialdehyde. Expansion of the porphycene framework provides a ligand capable of stabilizing a bis(rhodium) and a monoruthenium complex. These new porphycene derivatives absorb strongly in the NIR spectral region, with appreciable absorptivity up to 1300 nm. On the basis of their ground- and excited-state spectroscopic features and structural parameters, both the free-base system and the bis(rhodium) complex are considered to be Hückel-type aromatic systems. This conclusion is supported by DFT calculations.

Full text of DOI:10.1002/anie.201711197

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Accepted Article
Title: An expanded porphycene with high NIR absorptivity that
stabilizes two different kinds of metal complexes
Authors: Gonzalo Anguera Pujadas, Won-Young Cha, Matthew Darren
Moore, James Thomas Brewster II, Michael Ying Zhao,
Vincent M Lynch, Dongho Kim, and Jonathan L. Sessler
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201711197
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10.1002/anie.201711197
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Accepted Article
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An expanded porphycene with high NIR absorptivity that stabilizes
two different kinds of metal complexes
Gonzalo Anguera, Won-Young Cha, Matthew D. Moore, James T. Brewster II, Michael Y. Zhao,
Vincent D. Lynch, Dongho Kim* and Jonathan L. Sessler*
Abstract: A new 26 π-electron expanded porphycene (5) has been
prepared via McMurry coupling of 1,4-bis(3,4-diethyl-2-
complex wherein the organic framework has undergone ring
contraction.
pyrryl)benzene dialdehyde. Expansion of the porphycene framework
provides a ligand capable of stabilizing a bis-rhodium (8) and a
ruthenium complex (9). These new porphycene derivatives absorb
strongly in the NIR spectral region with appreciable absorptivity up to
1300 nm. On the basis of their ground and excited state spectroscopic
features and structural parameters both the free-base system (5) and
the bis-rhodium complex (8) are considered to be Hückel-type
aromatic systems. This conclusion is supported by DFT calculations.
Porphycenes (1), reported in 1986 by Vogel and coworkers, were
the first constitutional isomers of porphyrins.[1] They consist of two
2,2’-bipyrrole subunits linked by two ethylene-bridges.[2,3] The
result is a planar, aromatic macrocycle formally known as
[18]porphyrin (2.0.2.0). As true for porphyrins, porphycenes
contain an 18 π-electron periphery. However, relative to
porphyrins, porphycenes generally display stronger absorbance
features in the far-red portion of the visible spectrum due to their
lower symmetry.[4] Porphycenes have been explored for a range
of applications, including as photosensitizers for photodynamic
therapy[5–7] and as ligands for catalysis.[8–10] Whereas expanded
versions of porphyrins have been widely studied,[11–13] expanded
porphycenes are all but unknown. Examples include bronzaphyrin
(2)[14] and an acetylene-cumulene porphycene derivative (3).[15,16]
Both 2 and 3 display red-shifted absorption features compared to
1, reflecting an increase in the electronic pathway, from 18 to 26
π-electrons. The annulated meso-dibenzoporphycene derivative
Scheme 1. Porphycene 1a, ethioporphycene 1b, bronzaphyrin 2, acetylene –
cumulene porphycene 3, dibenzoporphycene 4, and expanded porphycene 5.
In an effort to obtain a porphycene derivative with a larger
central cavity and with an extended π-electron periphery, we
sought to incorporate a benzene spacer between the two formal
halves of the porphycene core. With such considerations in mind,
the dipyrrolylbenzene precursor 6 reported by Setsune et al.[18,19]
was subject to formylation to produce 7 in 93% yield. An ensuing
McMurry reaction and oxidation with 2,3-dichloro-5,6-dicyano-p-
benzoquinone (DDQ) gave the extended porphycene 5 in 14%
yield (Schemes 1 & 2). The relatively low solubility of the product
allowed for simple recrystallization from CH2Cl2:hexanes.
Solutions of 5 in chlorinated solvents are dark-red; however, a
green metallic luster is seen in the solid state. An analysis of the
1H-NMR spectrum (CDCl3) revealed signals characteristic of an
aromatic Hückel type macrocycle.[20] For instance, ethylene
signals are seen at 9.16 ppm, along with benzene CH proton
resonances at 4.40 ppm (8H) (cf. Fig. S5). The presence of only
(4) also displays
a
red-shifted NIR infrared absorption
maximum.[17] However, neither 2 or 3 (nor 4 or any other
porphycene derivative of which we are aware) has been reported
as being effective in stabilizing a bis-metal complex. We thus
sought to prepare an expanded porphycene derivative that would
both display strong NIR absorption features and permit the
coordination of more than one metal center. As detailed below,
the 26 π-electron phenylene-bridged expanded porphycene (5)
fulfills both these design objectives. We have also found that this
system can be used to produce a sandwich-type organometallic
one such averaged CH signal is consistent with
a
[a]
Dr. G. Anguera, M. D. Moore, J. T. Brewster, M. Y. Zhao, Dr. V. D.
Lynch, Prof. J. L. Sessler.
Department of Chemistry, The University of Texas at Austin, 105
East 24th Street, Stop A5300, Austin, Texas, 78712 – 1224, United
States
E-mail: sessler@cm.utexas.edu
J. T. Brewster and Prof. J. L. Sessler.
Center for Supramolecular Chemistry and Catalysis, Shanghai
University, Shanghai 200444, China
Dr. W.-Y. Cha, Prof. D. Kim
conformationally mobile system, wherein the two benzene
subunits undergoing fast rotation on the NMR time scale. A
correlation spectroscopy (COSY) spectral study supports the
broad signal at 3.90 – 3.76 ppm (8H) being assigned to the -
CH2CH3 protons (cf. Fig. S6).
[b]
[c]
Spectroscopy Laboratory for Functional π-Electronic Systems and
Department of Chemistry, Yonsei University, Seoul 03722, Korea.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
Crystal data may be obtained from the Cambridge Crystallographic
Centre CCDC numbers 1580306, 1580307 and 1581856
Scheme 2: Synthesis of expanded porphycene 5 and complex 8.
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10.1002/anie.201711197
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the ring) is consistent with the proposed aromatic character. An
anisotropy of the current (induced) density (ACID) plot (Figure
2),[22] which directly displays the magnitude and direction of the
induced ring current under an external magnetic field applied to
the molecular plane, was also constructed. The π-conjugation
pathway of 5 has a substantial electron density in the planar
molecular framework, and a clear and definite clockwise current.
In addition, a harmonic oscillator model of aromaticity (HOMA)[23]
analysis of the π-conjugation pathways using the energy
minimized molecular structure and the X-ray diffraction data for 5
(vide infra) was carried out. The resulting HOMA values were
calculated to be 0.73 and 0.79 for these two inputs (cf. Fig. S31),
respectively. The large HOMA value for 5 and its planar structure
are again most easily rationalized in terms of this expanded
porphycene having [4n+2] Hückel aromatic character.
8
6
4
2
0
5
8
400 600 800 1000 1200 1400 1600
l/nm
Figure 1. Steady-state absorption spectra of the expanded porphycene 5
(solid line) and its bis-rhodium complex 8 (dashed line) in CHCl3.
An analysis of its UV-vis-NIR absorption spectrum provides
further support for the Hückel type aromaticity proposed for
compound 5 (Figure 1). In particular, in chloroform solution two
Soret-like bands at lmax (e) = 420 nm (7.0 x 104 M-1·cm-1) and 523
nm (6.7 x 104 M-1·cm-1), as well as a broad Q-band at 923 nm (5.9
x 104 M-1·cm-1), are seen. These bands are red-shifted relative to
those of porphycene 1b (cf. Table S1). Compared with other
porphycene derivatives, 5 displays higher absorption maxima and
larger extinction coefficients, and a low energy Q-band that
extends into the NIR spectral region (cf. Table S1). The extended
porphycene 4 also absorbs into the NIR (4, lmax = 1047 nm).
However, the extinction coefficients of its Q-bands are ≤ 5 x 103
M-1·cm-1 (vs. 5, lmax = 923 nm, 5.9 x 104 M-1·cm-1). Compound 2
displays higher extinction coefficients (i.e., up to 1 x 105 M-1·cm-
1). However, it absorbs at higher energies than either 4 or 5 (i.e.,
for 2, lmax = 889 nm). Compound 5 does not exhibit fluorescence
in the NIR region in CHCl3. However, in paraffin oil, a highly
viscous medium that may obstruct rotation of the bridging phenyl
groups, 5 gives rise to an observable fluorescence (cf. Fig. S21).
A small Stokes shift of 710 cm-1 is observed; this is taken as
evidence that the excited state is relatively rigid.
Figure 2. ACID plot of 5 (left) and 8 (right). Current density vectors indicate
clockwise current, when viewed from the front and facing the direction of the
external magnetic field vector (clockwise currents are diatropic). Isosurface
value is 0.05.
Single crystals suitable for X-ray diffraction analysis were
grown via slow evaporation of an initial CH2Cl2/hexanes (1:1, v/v)
solution. As can be seen from inspection of Figure 3, 5 shows a
largely planar macrocyclic ring system. Only the benzene
subunits are appreciably tilted out of the plane, presumably due
to CH --- HC steric interactions. The torsion angles between the
pyrroles and the phenylene subunits range from 21.03 to 15.04o.
Further analysis of the crystal structure reveals the presence of a
cavity that is noticeably larger than that present in porphycene
1b,[24] for which the N1-N2 (= N3-N4) distance is 2.732 and the
N1-N3 (= N2-N4) separation is 2.799 Å. In contrast, in 5 the
corresponding values are 7.06 Å for N1-N2 (= N3-N4) and 2.768
Å for N1-N3 (= N2-N4). The four nitrogen rectangle present in 5
defines an area of almost 20 Å2, while the internal are of typical
porphycenes (1b) is closer to 7 Å2.[1] The large cavity in 5 led us
to explore whether it could support the complexation of more than
one metal center.
In an effort to obtain insight into the intense NIR absorption
bands of compound 5, we performed molecular optimization and
TDDFT calculations at the B3LYP/6-31G(d,p) level. The energy
minimized molecular structure of 5 was found to be very similar to
the X-ray crystal structure. The simulated UV-vis spectrum of 5
was also found to coincide with the steady-state absorption
spectrum. On the basis of these calculations, the electronic
vertical transitions corresponding to the two Soret-like bands and
two Q-type bands observed in the steady-state absorption
spectrum of 5 mainly involve HOMO-1, HOMO, LUMO and
LUMO+1. These orbitals are all characterized by delocalized π-
electron densities through 26 π-electronic circuits (cf. Fig. S29).
Two different electronic vertical transitions, in the visible and NIR
regions, induce splitting in the Soret-like bands and Q-bands,
respectively (cf. Fig. S29).
In order to study the reactivity of 5, metal insertion studies
were carried out with salts of palladium, platinum, copper, nickel
and zinc; however, in no cases could a readily characterized
product obtained. Generally, starting material was recovered.
Likely the presence of the benzene subunit in the structure
prevents the complexation of these metal cations. Several
carbonyl reagents of rhodium and ruthenium have been used in
the literature for the preparation of bidentate complexes of
pyrroles derivatives, such as Rh2(CO)4Cl2 and Ru2(CO)6Cl4. For
instance, Setsune and coworkers prepared multinuclear rhodium
Nucleus independent chemical shift (NICS) calculations[21]
were carried out in an effort to gain insight into the magnetic
shielding effects present in compound 5 (cf. Fig. S30). The
negative nature of these values (e.g., -15.11 ppm at the center of
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10.1002/anie.201711197
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complexes of different expanded porphyrins by simply stirring the
rhodium precursor in degassed dichloromethane in the presence
of K2CO3 and the ligand.[18] The same methodology was applied
Table S14). The presence of the two rhodium di-carbonyl groups
on the same side of the macrocycle provides support for the 1H-
NMR spectral assignment made for the CH-phenylene signals,
to compound 5 (Scheme 2) and, after silica gel column purification, namely that free rotation of the phenylene bridges will be blocked
the bis-rhodium complex (8) was obtained in 50% yield. In stark
contrast with 5, compound 8 is highly soluble in chlorinated
solvents. This is attributed to the elimination of intermolecular π-
π interactions.
and that a greater number of CH signals should be observed in
the 1H-NMR spectrum of 8 than 5.
For complex 8, structural optimization and TDDFT
calculations were carried out at the B3LYP/LANL2DZp level to
account for the presence of the rhodium atoms. The energy
minimized molecular structure of 8 is similar to the solid-state
structure and the simulated absorption spectrum matches well
with the experimental spectrum as recorded in chloroform (cf. Fig.
S32). As true for compound 5, complex 8 is characterized by a
negative NICS value (-8.73 ppm) at the center of the complex (cf.
Fig. S33). An ACID plot (Figure 2) of 8 revealed a clear and
definite clockwise current within the [26]hexaphyrin-like
framework. A 0.66 HOMA value for 8, obtained by analysis of the
X-ray diffraction data, is consistent with the proposed Hückel
aromatic character (cf. Fig. S34).
Figure 3. X-ray crystal structures of 5 (a) and 8 (b) as viewed from the top, side
and bottom, respectively. Displacement ellipsoids are scaled to the 50%
probability level.
1H-NMR spectroscopic analysis of
8 reveals notable
differences with the free base. To resolve all expected proton
signals from 8, a variable temperature1H-NMR spectral study was
carried out using d8-toluene as the solvent (cf. Fig. S10). Below -
o
20 C, two signals can be observed at 9.98 ppm (b, Scheme 1,
4H) and -0.45 ppm (a, Scheme 1, 4H) that are not seen at room
temperature, both shifts being indicative of [4n + 2] π-electron
Hückel-type aromaticity. The best resolved 1H-NMR spectrum is
o
seen at -40 C (cf. Fig. S9). These resonances are assigned to
the inner- and outward pointing CH protons on the bridging
phenylene subunits, respectively. In contrast, in the 1H-NMR
spectrum of 5, the CH-benzene signal is not split even at low
temperature and appears as a time averaged signal at 4.40 ppm
(8H).
Figure 4. TA spectra of the (left) visible and (right) NIR spectral regions and
decay profiles (insets) of 5 (a) and 8 (b) in toluene.
Further support for the proposed Hückel aromaticity of
complex 8 came from an analysis of its UV-vis spectrum (Figure
1). Relative to the free-base 5, the larger Q-type band of 8 is
characterized by a ca. 90 nm bathochromic shift (lmax (5) = 923
nm; lmax (8) = 1014 nm). Even more than in the case of 5, the
absorption features of 8 tail into the NIR. The complex itself is light
red as a dilute chloroform solution. However, the maximum molar
absorptivity of 8 (e = 3.9 x 104 M-1·cm-1) is lower than that of 5 (see
above). In noted contrast to what is true for most porphyrin
derivatives, complex 8 is characterized by a higher extinction
coefficient for the Q-band than for the Soret band. Compound 8
does not exhibit appreciable luminescence.
To understand in greater detail the optical features of 5 and
8, their excited-state dynamics were probed using femtosecond
transient absorption (TA) spectroscopy (Figure 4). Both 5 and 8
exhibit prominent excited-state absorption (ESA) and ground-
state bleaching (GSB) signals that correspond to their steady-
state absorption features. Compound 5, in particular, gives rise to
intense GSB signals and a relatively weak ESA spectrum in the
NIR region. Such observations are typical of what is seen in the
case of aromatic expanded porphyrins.[25] As expected given its
non-fluorescent nature, the excited-state lifetime of 5 was
estimated to be 1.4 ps in toluene a value that is shortened to 1.0
ps in the case of 8. Presumably, this reduced lifetime is the result
of efficient intersystem crossing due to the heavy atom effect of
the coordinated rhodium metal cations. The observation of long
residual TA signals (up to 3 ns) is consistent with the population
of an excited-state triplet. Taken in concert, these spectroscopic
Single crystals suitable for X-ray diffraction analysis were
grown via the slow diffusion of methanol into a chloroform solution
of 8. From an inspection of Figure 3b it can be observed that both
rhodium centers are complexed on the same face of the
macrocycle. The torsion angle about C3-C4-C5-C6 is 161.41o (cf.
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G.A. thanks Fundación Ramón Areces (Madrid, Spain) for a
postdoctoral fellowship. This work was supported by the National
Science Foundation (grant CHE-1402004 to J.L.S.) and the
Robert A. Welch Foundation (F-0018 to J.L.S.). The work at
Yonsei University was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korea
government (MEST) (2016R1E1A1A01943379). J.T.B would like
to thank the NSF for an EAPSI fellowship.
features provide further evidence for the [4n+2] Hückel-type
aromaticity proposed for 5 and 8.
Conflict of Interest
The authors declare no conflict of interest
Keywords: porphycene • macrocycles • NIR • rhodium • McMurry
Figure 5. X-ray crystal structure of 9 as viewed from the top, side and bottom,
respectively. Displacement ellipsoids are scaled to the 50% probability level.
Substituents and hydrogens have been omitted for clarity
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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Author(s), Corresponding Author(s)*
A new expanded porphycene prepared
from the McMurry reaction of 1,4-bis(3,4-
diethyl-2-pyrryl)benzene dialdehyde has
Page No. – Page No.
been
prepared.
This
extended
Title
tetrapyrrolic macrocycle displays Hückel
type aromaticity (4n + 2 π electrons, n =
6), as inferred from NMR and UV-vis
spectroscopies, as well as CV, DFT
calculations, and TA studies. It was
found to support the formation of both
an aromatic bis-rhodium and non-
aromatic, ring contracted “sandwich-
type” mono-ruthenium complex.
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