Synthesis of unique extended π structures by Pt-mediated benzannulation of nickel(II) tetraalkynylporphyrins

Article abstract of DOI:10.1002/chem.201101741

A piece of the π In the presence of PtCl₂, Ni^II tetraalkynylporphyrin complex undergoes a six-endo-dig-type cyclization to form two unusual structural isomers of Ni^II bisphenanthroporphyrins. Under the same conditions, Ni<

Full text of DOI:10.1002/chem.201101741

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DOI: 10.1002/chem.201101741
Synthesis of Unique Extended p Structures by Pt-Mediated Benzannulation
of Nickel(II) Tetraalkynylporphyrins
Leigh J. K. Boerner,[a] Mahendra Nath,[a] Maren Pink,[b] and Jeffrey M. Zaleski*[a]
The facile modification of macrocycles to produce p-ex-
tended porphyrins has long been studied due to their poten-
tial applications as optoelectronic devices,[1] photovoltaic
materials,[2] and photoinduced energy or charge transport in
dye-sensitized solar energy cells.[3] Fused-ring porphyrins
have extremely large molar extinctions[4] and absorb well
into the red and infrared regions of the electromagnetic
spectrum due to their small HOMO–LUMO gaps.[5] These
properties enable some systems to exhibit large two-photon
absorption cross sections,[6] which have become relevant for
applications such as in vivo imaging.[7]
The synthesis of fused, benzannulated units by radical cas-
cade reactions of polyacetylenes has been shown to be an
effective and accessible pathway towards new aromatic mol-
ecules.[8] Radical cyclization reactions of this type can be ap-
plied to and coupled with the flexible electronic nature of a
porphyrin backbone through installation of alkynes at the b
pyrrole periphery of the macrocycle. Subsequently, the al-
kynes can be thermally or photochemically activated to pro-
terparts.[10] However, due to their increased alkyne function-
alization, these constructs can potentially produce a more p-
extended, fused porphyrin, and thus still serve as an impor-
tant precursor for a cyclization-based strategy to larger ring
systems, albeit by an alternative synthetic method.
To this end, organometallic approaches to cyclizing small
enediyne units are now well established.[11] Early on, Buch-
wald et al. demonstrated a remarkably facile cyclization of
the acyclic ligand 1,2-bis(diphenylphosphinoethynyl)ben-
zene[12] by s coordination of PdCl2 or PtCl2 to the phosphine
functionality at the alkyne termini position. Analogously,
OꢀConnor et al. have also shown ambient temperature ene-
diyne activation through p complexation of the alkynes with
RuII or FeII.[13] Although chemically facile, both methods ne-
cessitate either specific alkyne functionalization or unfavora-
ble conditions to remove the activating metal fragment, nei-
ther of which are congruent with the efficient synthesis of
extended porphyrin constructs.
An alternate strategy for cyclization of alkynes, enynes,
and enediynes has been developed that employs catalytic
PtCl2 acting initially by p complexation to the unsaturated
organic framework.[14] With respect to the enediyne motifs
specifically, the proposed cyclization mechanism involves a
duce
a series of extended structure picenoporphryins
through a Bergman cyclization (Scheme 1) with variable
success.[9] Owing to convoluted side reactions and invoked
steric effects, the tetraalkynylporphyrins have higher activa-
tion barriers and are less isolable than their dialkynyl coun-
À
6-endo-dig pathway, resulting in C C bond formation at the
1- or 4-positions on the newly
formed benzene ring.[15] This ef-
ficient cyclization requires nei-
ther the extreme conditions of
the traditional thermal or pho-
tochemical reaction pathways,
nor the additional functionali-
zation or special workup to
generate the p-extended fused-
Scheme 1. Bergman cyclization of porphyrinic enediynes both thermally and photochemically forms picenopor-
phyrin structures.
ring porphyrins. Within this
theme, herein is described the
sub-stoichiometric PtCl2-medi-
ated cyclization of NiII tetraalkynylporphyrins to produce, in
high yields, unique NiII bisphenanthro- and -picenophenan-
throporphyrins with markedly redshifted electronic features.
In the presence of PtCl2, the thermal reaction of
(2,3,12,13-tetraethynyl-5,10,15,20-tetraphenylporphyrin)nick-
el(II) (2) in toluene (908C for 8 h) produces a mixture of ex-
tended structure NiII bisphenanthroporphyrin structural iso-
mers 4 and 5 in 50% yield (Scheme 2). The PtII-catalyzed
cyclization mechanism has not been examined in detail, but
it differs from the well-documented Bergman cyclization
[a] L. J. K. Boerner, Dr. M. Nath, Prof. J. M. Zaleski
Department of Chemistry, Indiana University
Bloomington, IN 47405 (USA)
Fax : (+1)812-855-8300
E-mail: Zaleski@indiana.edu
[b] Dr. M. Pink
Molecular Structure Center, Indiana University
Bloomington, IN 47405 (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101741.
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only doublet resonances in the
same chemical shift region. Two
of these, d=9.57 (d, J=4.8 Hz,
2H) and 8.92 (d, J=4.8 Hz,
2H), can be assigned to the b
pyrrole protons of the main
porphyrin core due to their typ-
ically small coupling constants
(Jꢀ4.0–5.0 Hz) and suggest the
structure is that of 5. A more
precise structural assignment of
both NiII bisphenanthropor-
phyrin isomers can be made by
employing 2D COSY (correla-
tion through bonds) NMR anal-
ysis of the b pyrrole chemical
shift region of
(Figure 1). In the top spectrum,
4
and
5
Scheme 2. PtII-catalyzed 6-endo-dig cyclization of NiII tetraalkynylporphrins (top). Thermal Bergman cycliza-
tion followed by PtII-mediated 6-endo-dig cyclization of NiII tetraalkynylporphyrins (bottom).
pathway[3c,16] in that the radical character on the newly
formed ring reacts by electrophilic aromatic substitution
with only one adjacent meso-phenyl group to form a p-ex-
tended phenanthrene unit fused to the porphyrin backbone.
Liu et al.[14e] have proposed a route involving the direct co-
ordination of PtCl2 at C1 or C4 of the intermediate 1,4-
benzyl diradical for single enediyne motifs, which leads to
À
asymmetric C C bond formation within the larger tetraphe-
nylporphyrin skeleton. Indeed, the presence of the known
product
(2,3-diethynyl-5,20-diphenylpiceno[10,11,12,13,-
ACHTUNGTRENNUNG14,15-fghij]porphyrin)nickel(II) (3) is not detected in the re-
action mixture, demonstrating that products 4 and 5 are not
a direct result of traditional Bergman cyclization and that
these unique p-extended structures are formed through an
alternative route.
A thermal Bergman cyclization can also be employed in
tandem with the PtCl2-mediated cyclization to form more
extended, asymmetric porphyrin structures that are unat-
tainable by previous methods. Thermolysis of 2 at 1158C for
1.5 h forms picenoporphrin 3 in 10% yield, which can then
be further reacted with one equivalent of PtCl2 for 8 h at
908C to form the highly aromatic NiII picenophenanthropor-
phyrin 6 in 40% yield. This is the most expanded construct
obtained to date from a multiple alkyne porphyrin precur-
sor. Compared to the thermal Bergman cyclization reactivi-
ty,[10] the yield of the isolated pure compound is quite high,
and results in a uniquely fused aromatic porphyrin without
an established synthetic route.
Analysis of the reaction of 2 with PtCl2 to form NiII bi-
sphenanthroporphyrins 4 and 5 reveals two products identi-
cal in solubility, mass, UV/Vis, and IR spectroscopies, each
1
formed in 25% yield. The H NMR spectra of the reaction
reveals two distinct spectral signatures; the first contains a
set of singlets at d=8.86 and 9.94 ppm, and indicate a highly
symmetrical product, which can be assigned as bisphenan-
throporphyrin 4. The second spectral component exhibits
Figure 1. 2D COSY NMR spectra (selected regions) for (5,10-
diphenylbisACTHNUTRGNEpNUG henanthro[20,1,2,3-abt-12,13,14,15-lmn]porphyrin)nickel(II)
(4, top) and (5,15-diphenylbisphenanthro[20,1,2,3-abt-10,11,12,13-jkl]por-
phyrin)nickel(II) (5, bottom).
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Synthesis of Unique Extended p Structures
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no cross peaks are shown for the b pyrrole resonances of 4
at d=8.86 and 9.94 ppm, signifying that these are isolated
protons with no through space meso-phenyl ring interac-
tions, and are antipodal to the ring fused side. In the bottom
spectrum (Figure 1), the correlation peaks of the resonances
at d=8.92 and 9.57 ppm show that these protons reside on
carbons adjacent to a fused meso-phenyl ring, and are there-
fore indicative of structure 5 (for full characterization, see
the Supporting Information).
doublet at d=8.85 ppm (J=4.6 Hz) in 6 is directly compara-
ble to the two protons in 5 at d=8.92 ppm (J=4.8 Hz), and
therefore can be assigned to the only b pyrrole proton that
is diametrically opposite of the phenanthro-quadrant and
non-adjacent to the fused piceno-unit. The neighboring b
pyrrole resonance was then found by searching COSY and
NOESY cross peaks, which were in turn correlated to the
adjacent piceno-group through NOESY cross peaks, and so
on until all the 1H resonances were confidently assigned
(see the Supporting Information).
X-ray crystal analysis of the NiII bisphenanthroporphyrin
4 (Figure 2)[17] reveals a structure where the porphyrin back-
bone is significantly distorted from the normal plane of the
macrocycle due to the classic NiII-induced mixture of ruffle
The electronic absorption spectra of these compounds il-
lustrate a stepwise redshift of both Soret and Q-bands as a
function of additional p delocalization (Figure 3). From the
tetraalkynyl starting material
2 (lmax =440, 557, and
Figure 3. Comparison of normalized electronic spectra of 2 (c), 3
(a), 4 (g), and 6 (b) in CH2Cl2.
609 nm), the Soret and low energy Q-band shift bathochro-
matically by approximately 26–28 nm upon formation of the
partially cyclized picenoporphyrin 3 (lmax =472, 587, and
635 nm).[10] Even this significant redshift in the Q-region is
overshadowed by the structure of 4 (lmax =470, 642, and
683 nm), which arises from the PtII-mediated cyclization/rad-
ical additions that extend the conjugation, as well as accen-
tuate the disparity in symmetry within the x,y plane, leading
to an additional contribution of the bathochromism. Similar-
ly for 6 (lmax =477, 639, 671, and 717 nm), formation of the
further p-extended piceno-phenathroporphyrin hybrid back-
bone generates the lowest energy electronic features of this
set of porphyrinoid structures as evidenced by the strong Q-
band above 700 nm. Moreover, reduction in the symmetry
of the porphyrin core lifts the degeneracy and shifts the en-
ergies of the traditional Gouterman frontier orbitals result-
ing in an increasing Q-band:Soret ratio with redshift (3.91
for 6).[5a]
Figure 2. X-ray crystal structure of 5,10-diphenylbisphenanthro[20,1,2,3-
abt-12,13,14,15-lmn]porphyrin nickel(II) (4).
and saddle deformations.[18] The formation of the two conju-
gated phenanthrene systems exaggerates this deviation from
planarity, because the porphyrin core is forced to expand in
the x direction slightly to accommodate the new fused-ring
À
system. As evidence, the Ni N3 distance (1.95 ꢂ) is some-
À
À
À
what elongated compared to the Ni N1, Ni N2, and Ni N4
(1.91, 1.88, and 1.89 ꢂ, respectively). As a result, the phen-
À
À
anthrene adjacent angle C24 C25 C39 (119.28) contracts
In addition to creating accessibility to unique extended
À
À
relative to the opposite C37 C40 C1 angle (121.48). The
overall effect is a highly twisted porphyrin core, which can
be clearly seen in the side view of 4 in Figure 2.
porphyrin constructs in good yields, the PtII-mediated cycli-
zation reaction can also activate (2,3,12,13-tetrakis
methylsilylethynyl)-5,10,15,20-tetraphenylporphyrin)nick-
el(II) 1, which is stable in solution for 7 days at 2008C, and
ACHTUNGTRENNUNG(tri-
AHCTUNGTRENNUNG
With solid NMR and structural characterizations of 4 and
5, structural analysis of the hybrid NiII picenophenanthro-
porphyrin 6 followed directly. The highest upfield b pyrrole
ACHTUNGTRENNUNG
shows a solid state cyclization temperature by differential
scanning calorimetry (DSC) of 3878C. Reaction of 1 with
Chem. Eur. J. 2011, 17, 9311 – 9315
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signature remarkably similar to that of 5 (see the Supporting
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À
SiCH3 resonances suggest the formation of more than one
isomer of 7, but the simplicity of the spectrum indicates that
only two or three isomers may be present in solution. Also
detected (by mass spectrometry and TLC) are the trace
À
yields of cyclized products with one, three, and four TMS
groups. Although not an optimized synthetic protocol, the
reaction does demonstrate the ability to deprotect a stable
species and cyclize with the same reagent.
In summary, this work demonstrates the first facile syn-
thesis of novel compounds NiII bisphenanthroporphyrins 4
and 5 and NiII picenophenanthroporphyrin 6 through a
PtCl2-mediated 6-endo-dig cyclization pathway. The reaction
products are generated in good yields and at modest reac-
tion temperatures, demonstrating that this synthetic method-
ology can be used to circumvent the need for extensive
alkyne functionalization or stringent reaction conditions in
order to produce unique, p-fused exocyclic porphyrins. The
PtII-mediated cyclization strategy can even yield formerly in-
accessible cyclized products from protected enediyne motifs
possessing high thermal barriers. This shows that the PtCl2-
mediated benzannulation of NiII tetraalkynylporphyrins can
be an important tool in the production of otherwise unat-
tainable, p-extended porphyrins that may find utility in op-
toelectronic, photovoltaic, or imaging applications.
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The generous support of the National Science Foundation (NSF CHE-
0956447) is gratefully acknowledged.
Keywords: cyclization
· fused-ring systems · nickel ·
platinum · porphyrinoids
[17] The structure of compound 4 was confirmed by X-ray crystallo-
graphic analysis (see the Supporting Information). CCDC-826271
(4) contains the supplementary crystallographic data for this paper.
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Chem. Eur. J. 2011, 17, 9311 – 9315

Synthesis of Unique Extended p Structures
COMMUNICATION
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
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Received: June 7, 2011
Published online: July 15, 2011
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www.chemeurj.org
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