A tethered porphyrin dimer with π overlap of a single pyrrole ring

Article abstract of DOI:10.1002/(SICI)1521-3773(19980918)37:17<2368::AID-ANIE2368>3.0.CO;2-K

The special pair of the bacterial photosystem has been modeled with a porphyrin dimer (the partial structure is shown). As with the natural system, only one pyrrole ring from each monomer subunit participates in π overlap.

Full text of DOI:10.1002/(SICI)1521-3773(19980918)37:17<2368::AID-ANIE2368>3.0.CO;2-K

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For the zinc(ii) porphyrin/bacteriochlorin dimer system, this
reduces to attaching a zinc-ligating atom along an axis through
the midpoint of the C2 ± C3 bond (see 2), with its lone pair of
electrons positioned several angstroms outside of and above
the p perimeter to interact with the zinc atom of the partner
chromophore. These requirements are met by a 7-azabi-
cyclo[2.2.1]heptadiene framework fused at C2 ± C3 of the
porphyrin ring.
A Tethered Porphyrin Dimer with p Overlap of
a Single Pyrrole Ring
Spencer Knapp,* Jayasree Vasudevan,
Thomas J. Emge, Byron H. Arison,
Joseph A. Potenza,* and Harvey J. Schugar*
A ªspecial pairº of p-interacting bacteriochlorophyll chro-
mophores serves as the central electronic feature of the
bacterial photosystem.[1] It is unique among its accompanying
antenna and accessory pigments in that full p overlap is
limited to the pyrrole 1-rings.[2] We report here the synthesis
and spectroscopic characterization of 1, a model system of the
special pair (Figure 1). This amine-tethered zinc(ii) porphyrin
dimer exhibits mutual p overlap that is likewise limited to a
single pyrrole ring.
Nitration[8] of [5,10,15,20-tetrakis(3,5-difluorophenyl)por-
[3, 9]
phyrinato]nickel(ii)
(81% yield, Scheme 1) gave the
Figure 1. Top view of dimer 1 along with a side view of the partial
structure. The inter-subunit contacts, as indicated by the 2D-ROESY NMR
experiment for a solution of 1 in CDCl3, are marked with dashed arrows.
Arˆ 3,5-difluorophenyl, R ˆ 2-phenylethoxycarbonyl.
Dimers of zinc(ii) porphyrin[3, 4] and bacteriochlorin[5] have
been assembled by attaching a zinc-ligating heterocyclic
substituent such as 2-pyridyl or 2-imidazolyl to a peripheral
carbon atom of the chromophore. The natural tendency of
porphyrinoids to p stack in parallel planes about 3.2 Š apart,[6]
as well as the length, orientation, and point of attachment of
the ligating tether, serve to position the p systems of the two
rings in a stable dimeric arrangement that can be studied in
solution. Overlapping component pigments in the bacterial
photosystem possess spectroscopic and electronic character-
istics critical for photosynthesis, such as red-shifted Qy bands
and perhaps unobserved features hidden by intense higher
energy absorbances due to auxillary pigments.[7] There is
therefore an impetus to construct and evaluate model dimers
with similar p overlap of a single pyrrole ring.
Scheme 1. Synthesis of 1. Arˆ 3,5-difluorophenyl. a) CNCH2CO2CH3, 1,8-
diazabicyclo[5.4.0]-7-undecene (DBU), THF, tBuOH, 15 min, reflux;
b) 1. 1% H2SO4 in TFA, room temperature (RT), 1 h; 2. Zn(OAc)2,
MeOH, CH2Cl2, RT, 10 min; 3. LiCl, DMSO, 3 h, reflux; 4. (BOC)2O,
DMAP, CH2Cl2, 5 min; c) 1. bis(2-phenylethyl) acetylenedicarboxylate,
toluene, 15 min, reflux; 2. 10% TFA in CH2Cl2, 08C, 4 h; 3. Zn(OAc)2,
MeOH, CH2Cl2, RT, 15 min; d) spontaneous dimerization in solution.
mononitro derivative 2, which was subjected to a Barton ±
Zard procedure[10] to deliver the pyrroloporphyrin 3. Replace-
ment[11] of NiII with ZnII and subsequent decarboxylation[12]
led to 4, which was further converted into the N-BOC
derivative 5 (N-Boc ˆ tert-butoxycarbonyl). Thereby activat-
ed,[13] this diene reacted with bis(2-phenylethyl) acetylenedi-
carboxylate[14] to give the Diels ± Alder adduct 6 (the phenyl-
ethyl groups were chosen to improve the solubility of the
dimer). Removal of the BOC group with trifluoroacetic acid
(TFA) resulted in loss of ZnII as well, but the metal was
subsequently reintroduced to provide the required amino-
tethered porphyrin 7.
[*] Prof. Dr. S. Knapp, Prof. Dr. J. A. Potenza, Prof. Dr. H. J. Schugar,
Dr. J. Vasudevan, Dr. T. J. Emge
Department of Chemistry
RutgersÐThe State University of New Jersey
610 Taylor Road, Piscataway, NJ 08854-8087 (USA)
Fax: (‡1)732-445-5312
E-mail: knapp@rutchem.rutgers.edu
Dr. B. H. Arison
Merck Research Laboratories, Rahway, NJ (USA)
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Several lines of evidence indicate that 7 dimerizes in
solution to afford the overlapped structure 1. Osmometric
molecular weight determination of a solution in CHCl3
(13.13 mg in 3 mL) gave Mr ˆ 2304 (calcd for
In the region of the Q bands the spectra of 7 ´ DMAP and
the dimer 1 are qualitatively similar.[17, 18] More concentrated
solutions of 1 (5.1 ± 12.9 mgmL ) show increased intensities
1
for all absorbances according to Beerꢁs Law, whereas stepwise
C132H78F16N10O8Zn2:
2367).
The
UV/Vis
spectrum
dilution of 1 to 0.026 mgmL 1 caused the emergence of blue-
1
1
(5.1 mgmL , Figure 2) shows a Q-band region that matches
shifted Q bands at 19200, 18200, and 17200 cm , which are
diagnostic for the four-coordinate zinc(ii) porphyrin monomer
7.[19]
Attempts to grow crystals of 1 with a wide variety of
solvents and solvent mixtures were unsuccessful. However,
the cyclic zinc(ii) porphyrin hexamer 8 (Figure 3) was
Figure 2. UV/Vis spectra (CHCl3, 258C): a) 7 ´ DMAP (reference mono-
mer; [7] ˆ 5  10 4 m, [DMAP] ˆ 5  10 3 m, path length 0.016 cm, offset
A‡1.3); b) 1 (4.3  10 3 m, path length 0.0027 cm) from the dimerization of
7; c) Q-band absorptions of 1 as in b), enlarged ten times.
that of typical five-coordinate zinc(ii) poryphyrins, but also a
strongly split Soret band, which is indicative of close p
interaction between porphyrin units.[15] The 400-MHz 1H
NMR spectrum of a solution of 1 in CDCl3 (2 mg in 0.5 mL) at
238C shows signals for a single species. Included are several
resonances for strongly shielded centers, which are assignable
to protons in the vicinity of the amino nitrogen atom: The
signal for the amino protons appears at d ˆ 2.01, that for the
bridgehead H atoms at 0.05, and that for the ester a-
methylene protons at 3.40. Upon addition of five equivalents
of 4-(N,N-dimethylamino)pyridine (DMAP) to the solution in
CDCl3 (DMAP disrupts zinc(ii) porphyrin/amino ligation[3]),
the corresponding resonances appear at positions expected
for the DMAP-coordinated monomer (e.g., bridgehead pro-
tons at d ˆ 4.94, a-methylene protons at d ˆ 4.22).
Figure 3. Ball-and-stick view of the hexameric units in crystals of 8 along
the threefold inversion axis. Hydrogen atoms, ester groups, difluorophenyl
groups, and solvate molecules have been omitted for clarity.
obtained as red rhomboids by slow evaporation of a solution
of dimer 1 in p-xylene/CHCl3 (1/1).[20] Each zinc atom of the
hexamer is apically coordinated to the secondary amino
nitrogen atom of an adjoining monomer unit 7 to afford a
square-pyramidal geometry similar to that in self-coordinat-
ing zinc(ii) porphyrin and bacteriochlorin dimers.[4, 5] In
contrast to the dimers, in which the porphyrin planes are
approximately parallel and in p contact, the hexamer 8
features adjoining porphyrin ring p systems that are approx-
imately orthogonal (Figure 4). This geometry is reminiscent of
that in a polymeric oligomer of [10,15,20-triphenyl-5-(4-
pyridyl)porphyrinato]zinc(ii)[22] and in other metalloporphyr-
A 2D-ROESY NMR experiment[4, 16] performed at 108C
allowed the unambiguous assignment of all resonances. The
nature of the overlap of the subunits in 1 is indicated by the
pattern of shielding in which signals for protons closer to the
coordinating amine appear at significantly higher field than
those for otherwise similar protons. Most conclusively, inter-
subunit crosspeaks (Figure 1) indicate the through-space
proximity of the ester a-methylene protons of one subunit
to the (otherwise) remote pyrrole protons and endo-ortho-
protons of the aryl rings of the other subunit, and also the
proximity in the dimer of the remaining pyrrole protons to the
para-protons of the overlying aryl substituents of the partner
subunit.
Gaussian deconvolution[17] of the electronic spectrum for a
solution of 1 in CHCl3 (Figure 2) reveals that the prominent
Soret absorption seen for the reference monomer 7 ´ DMAP
1
1
at 22900 cm is split (23600, 22700 cm ). The observed
1
splitting of about 900 cm is comparable to the range (890 ±
1040 cm ) previously observed for self-coordinating porphyr-
1
in dimers whose overlapped region comprises an entire edge
(the b-carbon atoms of two pyrrole rings and their connecting
meso-carbon atom).[4]
Figure 4. Ball-and-stick view of two adjoining porphyrin units in the
hexameric crown in crystals of 8 approximately normal to one of the
porphyrin planes.
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ins.[23] Such orthogonal porphyrin systems do not show
splitting of the Soret absorptions.[22, 23]
[15] a) M. A. Thompson, M. C. Zerner, J. Fajer, J. Phys. Chem. 1991, 95,
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99, 6374 ± 6386.
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Chem. Soc. 1987, 109, 607 ± 609.
Dimer 1 and hexamer 8 also differ fundamentally in that the
former features zinc coordination by the amino lone pair syn
to the porphyrin ring (the N ± H bond is anti), whereas the
lone pair in the latter is coordinated in an anti position (the
N ± H bond is syn). As a consequence, hexamer 8 cannot arise
directly from trimerization of dimer 1, but instead both
structures must come from monomer 7. Crystals of 8 dissolve
in CDCl3 to reform 1, according to the NMR spectrum. The
degree of aggregation of 7 is thus phase-, concentration-, and
additive-dependent.
Structural variations in partially p-overlapped porphyrin
dimers now include the modes pyrrole-over-pyrrole (1), edge-
over-edge,[3, 4] and pyrrole-over-edge.[4] The consequences for
the electronic spectra for the first two modes are similar: split
Soret absorptions and relatively unaffected Q bands. In
contrast, the pyrrole-over-edge dimer[4] exhibits unusually
broad Soret absorptions and a slightly split low-energy Q
band. A detailed understanding of the origin of these
structure-dependent effects is needed, but lacking at present.
The structural dependences of other photophysical properties
(e.g., fluorescence polarization, resonance Raman effects) of
these dimers and their radical cations also remain to be
determined.
[17] Gaussian-deconvoluted UV/Vis spectra: 7 ´ DMAP: lmax [cm 1] (e Â
10 3 molcmL 1) ˆ 23800 (60), 22900 (400), 18800 (3), 17900 (22),
16500 (7); 1 (e calcd based on the concentration of ZnII): 24880 (46),
23600 (130), 22700 (180), 18700 (5), 17600 (17), 16400 (7).
[18] Less splitting of the relatively weak Q bands is predicted by the
excitonic coupling model: M. Kasha, H. R. Rawls, M. A. El-Bayoumi,
Pure Appl. Chem. 1965, 11, 371 ± 392.
[19] K. M. Kadish, L. R. Shiue, Inorg. Chem. 1982, 21, 3623 ± 3630.
[20] Crystals of 8 were fragile and unstable in the absence of mother liquor.
A crystal (0.64 Â 0.64 Â 0.18 mm) was mounted in a glass capillary
along with mother liquor (separated from the crystal). Data were
collected at 297 K (CAD-4 diffractometer, rotating Cu anode, CuKa
radiation, l ˆ 1.5418 Š). Attempts to collect data at low temperature
were unsuccessful owing to crystal fracture. Crystal data:
C66H39N5O4F8Zn ´ 2.5C8H10 ´ 0.2CHCl3; Mr ˆ 1472.67, trigonal, space
3
Å
group R3 (no. 148), a ˆ 42.183(4), c ˆ 21.849(2) Š, Vˆ 33670(5) Š ,
Z ˆ 18, 1calcd ˆ 1.307 gcm 3; of 10108 reflections (2 < q < 428), 5120
were unique and 3211 with I > 2s(I). Data were corrected for Lorentz,
polarization, and absorption (numerical, SHELX-76[21a]) effects. The
structure was solved by the heavy-atom method SHELXS-86[21b] and
refined on F2 with all unique data. Disorder in the difluorophenyl and
ester groups and the presence of loosely bound solvates necessitated
use of distance and displacement parameter restraints. The restraints
used do not appear severe, and the central zinc(ii) porphyrin portion of
the structure does not appear to be disordered. Refinement
(SHELXL-97,[21c] 5120 data, 973 parameters, 820 restraints) gave
RF ˆ 0.062 [I > 2s(I)], wRF2 ˆ 0.150 (all data), GOF ˆ 0.94. A final
difference map showed excursions of 0.3 to 0.3 eŠ 3. Crystallo-
graphic data (excluding structure factors) for the structure reported in
this paper have been deposited with the Cambridge Crystallographic
Data Center as supplementary publication no. CCDC-100975. Copies
of the data can be obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB21EZ, UK (fax: (‡44)1223-336-033;
e-mail: deposit@ccdc.cam.ac.uk).
Received: January 5, 1998
Revised version: April 20, 1998 [Z11327IE]
German version: Angew. Chem. 1998, 110, 2537 ± 2540
Keywords: N ligands ´ NMR spectroscopy ´ photosynthesis
´ porphyrinoids ´ UV/Vis spectroscopy
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2370
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