Siamese-twin porphyrin: A pyrazole-based expanded porphyrin providing a bimetallic cavity

Article abstract of DOI:10.1002/anie.201005780

Porphyrins joined at the hip: A long-sought expanded porphyrin has been synthesized that contains two porphyrin-like {N₄} binding sites linked by two bridging pyrazole units. Structural and spectroscopic evidence indicates the absence of macroc

Full text of DOI:10.1002/anie.201005780

Communications
DOI: 10.1002/anie.201005780
Expanded Porphyrins
Siamese-Twin Porphyrin: A Pyrazole-Based Expanded Porphyrin
Providing a Bimetallic Cavity**
Lina K. Frensch, Kevin Prꢀpper, Michael John, Serhiy Demeshko, Christian Brꢁckner,* and
Franc Meyer*
Dedicated to Professor Herbert W. Roesky on the occasion of his 75th birthday
Expanded porphyrins are synthetic analogues of porphyrins
consisting of at least 17 atoms in a cyclic conjugated frame-
work that contains at least three pyrrole or pyrrole-like
heterocyclic subunits.[1] These macrocyclic systems have
aroused great interest as near-IR-absorbing or -emitting
chromophores and as two-photon dyes; their cavities are
large enough to function as anion-recognition systems, and
they may exhibit intriguing conformations.[2] Hexaphyrin 1 is
a representative expanded porphyrin that exists in a number
of conformations and oxidation states, capable of forming a
variety of mono- and bimetallic complexes, including the
formation of Mꢀbius aromatic systems.[2d,3]
incorporating a pyrazole unit, carbaporphyrin 2, introduced
in 2008 by Lash et al.[5]
We report here the synthesis of
a pyrazole-based
expanded porphyrin consisting of four pyrrole and two
pyrazole units (Figure 1). This arrangement can be inter-
Figure 1. Framework of the Siamese-twin porphyrin highlighting the
two fused constituent porphyrin-like macrocycles.
preted as the fusion of two {N4} porphyrin-like coordination
sites. Thus, we like to dub it “Siamese-twin porphyrin”. This
naming is descriptive of the framework structure but does not
imply the presence of porphyrinic conjugation pathways
inherent to this system. We have previously shown pyrazoles
with chelating side arms to be versatile bridging ligands in di-
and multinuclear transition-metal complexes.[6] Consequently,
one of the goals of the work was to bind two metals in close
proximity in these highly preorganized binding pockets.[7]
We are not the first to conceive the Siamese-twin
porphyrin structure. Lind, in a 1987 dissertation, describes
many innovative, but apparently unsuccessful, approaches
toward this expanded porphyrin.[8] Recently we reported a
bis(pyrrolylmethylene)pyrazole building block that was
designed for the 3+3-type synthesis of the target macrocycle.
Alas, this could not be achieved, though other interesting
macrocycles were synthesized.[9] Three major problems
became evident in our, as well as Lindꢁs, work:[8,10] While
MS evidence indicated that a macrocyclization of the
appropriate building blocks was likely, the macrocycles
could not be isolated. Moreover, the tremendous conforma-
tional flexibility and the large number of stereoisomers
formed made it difficult to identify these intermediates by
NMR spectroscopy. Lastly, our inability to reliably oxidize
any of the methylene groups attached to the pyrazoles
stymied progress.[11]
Naturally, the majority of expanded porphyrins employ
pyrrolic building blocks but macrocycles incorporating, for
instance, furan or thiophene building blocks are also known.[4]
There are no expanded porphyrins containing pyrazole units.
In fact, we are aware of only one porphyrin analogue
[*] Dipl.-Chem. L. K. Frensch, Dipl.-Chem. K. Prꢀpper, Dr. M. John,
Dr. S. Demeshko, Prof. Dr. F. Meyer
Institut fꢁr Anorganische Chemie
Georg-August-Universitꢂt Gꢀttingen
Tammannstrasse 4, 37077 Gꢀttingen (Germany)
Fax: (+49)551-39-3063
E-mail: franc.meyer@chemie.uni-goettingen.de
Homepage: http://www.meyer.chemie.uni-goettingen.de
Prof. Dr. C. Brꢁckner
Department of Chemistry, Unit 3060
University of Connecticut
Storrs, CT 06269-3060 (USA)
Fax: (+1)860-486-2743
E-mail: c.bruckner@uconn.edu
Homepage: http://bruckner.chem.uconn.edu
We thus decided to prepare the pyrrole–pyrazole hybrid 3
which is phenyl substituted in the meso positions as well as on
the pyrazole (Scheme 1). Together with the b-ethyl substitu-
ents of the pyrrole, this was thought to enforce a conforma-
tion that places all substituents on the outside of the
macrocyclic, nonconjugated precursor to the target structure.
[**] Financial support from the Fonds der chemischen Industrie, the
Studienstiftung des deutschen Volkes, the DAAD (to L.K.F.), and the
NSF (CHE-0517782 to C.B.) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005780.
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Figure 2. UV/Vis absorption spectra of 8 (a), 9·Cu2 (d) in
CH2Cl2, 9H4 (g) in CH2Cl2 +1% NEt3 and 9H4·2H+ (c) in
CH2Cl2 +1%TFA.
tion, and are distinctly nonporphyrinic (extinction coefficients
at least one order of magnitude less than porphyrins, e398 nm
=
15060mÀ1 cmÀ1, no Soret band, absence of multiple Q-bands;
Figure 2).[13,14]
The 1H and 13C NMR signals of 9H4 are broadened
beyond interpretation. The NMR spectra of the diprotonated
form 9H4·2H+·2(CF3CO2 ) are much simpler than those of
À
the precursor 8 (or the free-base form), but still display some
line broadening because of its conformational flexibility.
Spectra recorded at À258C show a maximum sharpening of
the signals (Figures S4 and S5). The presence of four distinct
ethyl groups carrying diastereotopic CH2 protons points
toward a lower overall symmetry for 9H4·2H+ than antici-
pated for the target structure. A grossly twisted nonplanar
geometry of 9H4·2H+ is also predicted by DFT calculations
Scheme 1. Synthesis of the Siamese-twin porphyrin 9H4 (see the
Supporting Information for details). All stereocenters are marked with
an asterisk.
We also reasoned that any oxidation in this electron-rich
system ought to be facilitated. As we will demonstrate here,
this strategy was crowned with success.
1
(BP86 functional, Figure S16). Two broad H resonances at
11.4 and 13.3 ppm (2H each) were assigned to the pyrrole
groups by 1H,15N HSQC spectroscopy, whereas a third broad
peak at 15.4 ppm (2H) represents the protons on the pyrazole
moieties (Figure S9). As expected, these protons undergo
readily deuterium exchange with [D4]methanol (Figure S7).
The low-field-shifted pyrrole/pyrazole NH signals show
the absence of any diatropic ring current that would be
indicative of a large aromatic p system (cf. the NH resonance
around À2 ppm for a typical porphyrin). Alternatively, the
positions of the NH signals may be attributable to the
presentation of the NH groups toward the outside of the
(aromatic) molecule. However, the computations show that
the bulky phenyl and ethyl substituents prevent a complete
inversion of the pyrrole/pyrazole units. In fact, NOESY
spectra show a correlation of the NH signals, proving that
they face each other. Thus, ligand 9H4·2H+ incorporates the
nonaromatic p system shown, rather than the 26 p electron
system that would be analogous to that of the hexaphyrins.[14]
Complexation of ligand 9H4 with CuII resulted in the
formation of a nonpolar (neutral), blue-green bimetallic
complex with the composition C92H80N8Cu2 (for [M+]),
displaying a hyperchromic but otherwise only moderately
The preparation of the key building block 3 was made
possible by our recent synthesis of 3,5-dibenzoyl-4-phenyl-
1H-pyrazole (4),[12] and followed the general route (reduction
to 5, halogenation to 6, followed by substitution with 3,4-
diethylpyrrole (7)) as the synthesis of its non-phenyl-substi-
tuted analogue.[9] The modified pyrrole–pyrazole hybrid 3 was
isolated as a mixture of stereoisomers.
The reaction of 3 with 1 equivalent of benzaldehyde and
1 equivalent of trifluoroacetic acid (TFA) in CH2Cl2 provided
the “Siamese-twin porphyrinogen” 8. The reaction time, the
type of acid, and the concentrations of all components
critically determine the outcome of this condensation reac-
tion. The HRESI(+) mass spectrum of 8 indicates the
formation of this macrocycle (of composition C92H93N8 for
[MH+]; Figures S2 and S3 in the Supporting Information). Its
1H NMR spectrum is complex because of the presence of
multiple stereoisomers (Figure S1). Light-yellow porphyrino-
gen 8 (e390 nm = 1250mÀ1 cmÀ1) is converted to a deep-green
product using dichlorodicyano-p-quinone (DDQ; Figure 2).
The HRESI(+) MS of this product indicates the compo-
sition C92H85N8 (for [MH+]). Both the mono- and diproto-
nated species (C92H86N8 for [MH22+]) are detectable (Fig-
ure S11). The UV/Vis spectrum of product 9H4 is reminiscent
of the spectra of bilindiones and related tetrapyrrolic pig-
ments, including their minimal changes upon (de)protona-
shifted
spectrum
(e391 nm = 45000mÀ1 cmÀ1
;
Scheme 2,
Figure 2).
The molecular structure of 9·Cu2 was determined by
single-crystal X-ray diffractometry (Figure 3).[15] The struc-
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Scheme 2. Synthesis of the bimetallic Siamese-twin porphyrin copper
complex 9·Cu2, highlighting the presence of two independent conjuga-
tion pathways (in bold).
ture confirms the spectroscopically determined Siamese-twin
porphyrin connectivity of the macrocycle and the presence of
the bimetallic copper complex. Immediately noticeable are
the strongly saddled individual copper-binding pockets and
the overall helical twist of the macrocycle (D2 symmetry).
The absence of counterions to the CuII2 complex confirms
the dianionic nature of each copper-binding pocket. Each CuII
ion coordinates in a severely distorted square-planar fashion
(the N1-Cu1-N3 angle is 1678; the N1-Cu1-N2 and N3-Cu1-
N4 planes are 498 off coplanarity). The Cu···N distances range
from 1.96 to 2.01 ꢂ, with the longer bonds to the pyrazole N3
and N4 atoms. Overall, each binding pocket is more distorted
than in, for instance, [{ß-octaethyl-meso-tetraphenylporphyr-
inato}CuII].[16] The Cu···Cu distance of 3.88 ꢂ is well within
the range of metal–metal separations in pyrazolate-bridged
binuclear systems (3.5–4.0 ꢂ).[6,17]
À
In (aromatic) metalloporphyrins, the neighboring Ca
N
À
and Ca Cmeso bonds are equal in length. The corresponding
bond lengths in 9·Cu2 differ, however. The bonds show an
alternating long–short pattern, characteristic of a conjugated,
but not macrocyclic-aromatic p system (Figure S15).[14] This
finding is consistent with the optical properties of the
macrocycle.
Magnetic susceptibility measurements were carried out
for 9·Cu2 (Figure 4). At room temperature, the effective
magnetic moment of 2.73 mB is close to the spin-only value of
two uncoupled S = 1/2 CuII ions (2.66 mB for g = 2.17). When
the temperature is lowered, meff gradually increases to reach a
maximum of 3.06 mB at 8 K. Experimental data were modeled
to the isotropic Heisenberg–Dirac–van Vleck spin Hamilto-
nian for exchange coupling and Zeeman splitting,[18,19] re-
vealing a sizeable ferromagnetic coupling constant of J =
+ 16.3 cmÀ1 (g = 2.17) between the CuII centers.
Figure 3. ORTEP plot (thermal ellipsoids set at 50% probability) of the
molecular structure of 9·Cu2. A) Top view and numbering scheme
used. B) Side view illustrating the near-orthogonal twist between the
two (idealized) square-planar {N4Cu} planes. C) View along the Cu1–
Cu1’ axis. For clarity, all hydrogen atoms and solvent molecules (four
toluene molecules per unit cell) are omitted. In addition, phenyl and
ethyl groups are omitted for clarity in (B) and (C). For further details,
see the Supporting Information.
The possibility of ferromagnetic coupling in [CuII(m-N,N’-
pyrazolato)2CuII] complexes was predicted to exist,[20a] but all
complexes reported hitherto exhibit only antiferromagnetic
exchange (typically in the range À245 ꢀ J ꢀ À70 cmÀ1).[20,21]
The helical twist of the macrocycle 9·Cu2 can be quantified by
considering the two Cu1-Npz-Npz-Cu1’ torsion angles t of 78.78
and 80.08; that is, the two magnetic dx2Ày2 metal orbitals are
nearly orthogonal to each other.[22] The quasi-orthogonality
results in a minimal overlap integral through the bridging
ligand. Hence, a vanishing antiferromagnetic exchange con-
sign of the metal–metal interaction from antiferro- to
ferromagnetic was observed for several bridging ligands,[23–26]
the realization of ferromagnetic coupling is unprecedented
for bridging pyrazolate units.
In conclusion, we have synthesized a pyrazole-based
expanded porphyrin 9H4 and its homobimetallic CuII com-
plex 9·Cu2. This Siamese-twin porphyrin merges the archi-
tecture of two porphyrinoid {N4} coordination sites, yet
neither of the individual coordination sites, nor the entire
macrocycle show porphyrinoid aromaticity. The formal
replacement of a pyrrolic building block in a porphyrin by a
pyrazole was previously shown to inhibit the aromatic
conjugation pathways.[5a] This finding is mirrored in the
tribution (Jantiferro) to the overall coupling term (J = Jferro
+
Jantiferro) is observed, rendering the system ferromagnetically
coupled. Although the torsion-angle-dependent change of the
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Brinas, C. Brꢃckner, F. Meyer, Chem. Eur. J. 2008, 14, 4823 –
4835.
[10] S. Katsiaouni, PhD Thesis, Georg-August-Universitꢄt Gꢀttin-
gen, Germany, 2007.
[11] Similar observations were also made by Lash and co-workers.[5a]
[12] A. Sachse, L. Penkova, G. Noel, S. Dechert, O. A. Varzatskii,
I. O. Fritsky, F. Meyer, Synthesis 2008, 5, 800 – 806.
[13] H. Falk, The Chemistry of Linear Oligopyrroles and Bile
Pigments, Springer, New York, 1989.
[14] M. Stepien, L. Latos-Grazynski, Top. Heterocycl. Chem. 2009,
19, 83 – 153.
[15] CCDC 792555 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
[16] L. D. Sparks, C. J. Medforth, M. S. Park, J. R. Chamberlain,
M. R. Ondrias, M. O. Senge, K. M. Smith, J. A. Shelnutt, J. Am.
Chem. Soc. 1993, 115, 581 – 592.
[17] A. L. Gavrilova, B. Bosnich, Chem. Rev. 2004, 104, 349 – 383.
[18] O. Kahn, Molecular Magnetism, Wiley-VCH, Weinheim, 1993.
[19] Simulation of the experimental magnetic data with a full-matrix
diagonalization of exchange coupling and Zeeman splitting was
performed with the julX program (E. Bill: Max-Planck Institute
for Bioinorganic Chemistry, Mꢃlheim/Ruhr, Germany).
[20] a) D. Ajꢅ, A. Bencini, F. Mani, Inorg. Chem. 1988, 27, 2437 –
2444; b) V. P. Hanot, T. D. Robert, J. Kolnaar, J. P. Haasnoot, J.
Reedijk, H. Kooijman, A. L. Spek, J. Chem. Soc. Dalton Trans.
1996, 4275 – 4281; c) H. Matsushima, H. Hamada, K. Watanabe,
M. Koikawa, T. Tokii, J. Chem. Soc. Dalton Trans. 1999, 971 –
977, and references therein; d) T.-L. Hu, J.-R. Li, C.-S. Liu, X.-S.
Shi, J.-N. Zhou, X.-H. Bu, J. Ribas, Inorg. Chem. 2006, 45, 162 –
173.
Figure 4. Plot of meff versus T for 9·Cu2 (magnetic field of 0.5 T from
*
295 to 2.0 K); experimental data ( ) and calculated curve fit (c).
Inset: Illustration of the Cu-Npz-Npz-Cu dihedral angle.
pyrazole-based macrocycle 9H4. The large twist inherent to
the macrocycle allowed the first preparation of a ferromag-
netically coupled doubly pyrazolato-bridged dicopper(II)
complex.
Received: September 15, 2010
Published online: January 14, 2011
Keywords: bimetallic complexes · copper · magnetic properties ·
.
porphyrinoids · pyrazole
[1] a) J. L. Sessler, D. Seidel, Angew. Chem. 2003, 115, 5292 – 5333;
Angew. Chem. Int. Ed. 2003, 42, 5134 – 5175; b) R. Misra, T. K.
Chandrashekar, Acc. Chem. Res. 2008, 41, 265 – 279.
[2] See, for example: a) J. L. Sessler, A. Gebauer, S. J. Weghorn in
The Porphyrin Handbook, Vol. 2 (Eds.: K. M. Kadish, K. M.
Smith, R. Guilard), Academic Press, San Diego, 2000, pp. 55;
b) Z. S. Yoon, A. Osuka, D. Kim, Nat. Chem. 2009, 1, 113 – 122;
c) Z. S. Yoon, D.-G. Cho, K. S. Kim, J. L. Sessler, D. Kim, J. Am.
Chem. Soc. 2008, 130, 6930 – 6931; d) T. Higashino, J. M. Lim, T.
Miura, S. Saito, J.-Y. Shin, D. Kim, A. Osuka, Angew. Chem. Int.
[21] a) P. King, R. Clꢆrac, C. E. Anson, A. K. Powell, Dalton Trans.
2004, 852 – 861; b) C. Miranda, F. Escartꢇ, L. Lamarque, E.
Garsꢇa-Espaꢈa, P. Navarro, J. Latorre, F. Lloret, H. R. Jimꢆnez,
M. J. R. Yunta, Eur. J. Inorg. Chem. 2005, 189 – 208; c) S. Pal,
A. K. Barik, S. Gupta, A. Hazra, S. K. Kar, S.-M. Peng, G.-H.
Lee, R. J. Butcher, M. S. El Fallah, J. Ribas, Inorg. Chem. 2005,
44, 3880 – 3889; d) S. Tanase, I. A. Koval, E. Bouwman, R.
de Gelder, J. Reedijk, Inorg. Chem. 2005, 44, 7860 – 7865; e) J.
Teichgrꢄber, G. Leibeling, S. Dechert, F. Meyer, Z. Anorg. Allg.
Chem. 2005, 631, 2613 – 2618; f) V. Mishra, F. Lloret, R.
Mukherjee, Eur. J. Inorg. Chem. 2007, 2161 – 2170; g) D. J.
de Geest, A. Noble, B. Moubaraki, K. S. Murray, D. S. Larsena,
S. Brooker, Dalton Trans. 2007, 465 – 475; h) Q. F. Mokuolu, D.
Foguet-Albiol, L. F. Jones, J. Wolowska, R. M. Kowalczyk, C. A.
Kilner, G. Christou, P. C. McGowana, M. A. Halcrow, Dalton
Trans. 2007, 1392 – 1399; i) L. Penkova, S. Demeshko, M.
Haukka, V. A. Pavlenko, F. Meyer I. O. Fritsky, Z. Anorg.
Allg. Chem. 2008, 634, 2428 – 2436.
´
Ed. 2010, 49, 4950 – 4954; e) M. Ste˛pien, B. Szyszko, L. Latos-
´
Graz˙ynski, J. Am. Chem. Soc. 2010, 132, 3140 – 3152; f) T. Koide,
K. Furukawa, H. Shinokubo, J.-Y. Shin, K. S. Kim, D. Kim, A.
Osuka, J. Am. Chem. Soc. 2010, 132, 7246 – 7247; g) H. Fliegl, D.
Sundholm, S. Taubert, F. Pichierri, J. Phys. Chem. A 2010, 114,
7153 – 7161.
[3] For recent examples: a) S. Mori, A. Osuka, J. Am. Chem. Soc.
2005, 127, 8030 – 8031; b) S. Mori, A. Osuka, Inorg. Chem. 2008,
47, 3937 – 3939; c) T. Koide, G. Kashiwazaki, K. Furukawa, A.
Osuka, Inorg. Chem. 2009, 48, 4595 – 4595.
[4] a) Y. Matano, H. Imahori, Acc. Chem. Res. 2009, 42, 1193 – 1204;
b) T. K. Chandrashekar, S. Venkatraman, Acc. Chem. Res. 2003,
36, 676 – 691; c) A. Asat, D. Dolphin, Chem. Rev. 1997, 97, 2267 –
2340.
[5] a) T. D. Lash, A. M. Young, A. L. Von Ruden, G. M. Ferrence,
Chem. Commun. 2008, 6309 – 6311; b) A. M. Young, T. D. Lash
in Abstracts of Papers, 239th ACS National Meeting, San
Francisco, CA, USA, March 21 – 25, 2010, ORGN-234.
[6] J. Klingele, S. Dechert, F. Meyer, Coord. Chem. Rev. 2009, 253,
2698 – 2741.
[7] S. Shimizu, A. Osuka, Eur. J. Inorg. Chem. 2006, 1319 – 1335.
[8] a) E. J. Lind, M.S. Thesis, Michigan State University, East
Lansing, MI, USA, 1984; b) E. J. Lind, PhD Thesis, Michigan
State University, East Lansing, MI, USA, 1987. Note that in
these theses the target macrocycle was referred to as octaaza-
[1.5.1.5]platyrin.
[22] Searching the Cambridge Structural Database reveals that the
torsion angles are usually smaller than 328 with the exception of
one complex featuring t = 41.58 (for which magnetic properties
are not reported): X. Liu, J. A. McAllister, M. P. de Miranda,
E. J. L. McInnes, C. A. Kilner, M. A. Halcrow, Chem. Eur. J.
2004, 10, 1827 – 1837.
[23] Ni-NNN-Ni torsion angles in E,E-azido-bridged nickel(II)
complexes, see for example: a) J. Ribas, A. Escuer, M. Monfort,
R. Vicente, R. Cortꢆs, L. Lezama, T. Rojo, Coord. Chem. Rev.
1999, 193–195, 1027 – 1068, and references therein; b) G. Leibel-
ing, S. Demeshko, S. Dechert, F. Meyer, Angew. Chem. 2005, 117,
7273 – 7276; Angew. Chem. Int. Ed. 2005, 44, 7111 – 7114; c) S.
Demeshko, G. Leibeling, S. Dechert, F. Meyer, Dalton Trans.
2006, 3458 – 3465.
[24] Mn-N-O-Mn torsion angles in triangular {(MnIII)3O} substruc-
tures, see for example: R. Inglis, L. F. Jones, C. J. Milios, S. Datta,
[9] a) S. Katsiaouni, S. Dechert, C. Brꢃckner, F. Meyer, Chem.
Commun. 2007, 951 – 953; b) S. Katsiaouni, S. Dechert, R. P.
Angew. Chem. Int. Ed. 2011, 50, 1420 –1424
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A. Collins, S. Parsons, W. Wernsdorfer, S. Hill, S. P. Perlepes, S.
[26] M-O-N-C2py torsion angles in copper(II) and nickel(II) com-
plexes with nitroxide and related radicals, see for example: A.
Okazawa, Y. Nagaichi, T. Nogami, T. Ishida, Inorg. Chem. 2008,
47, 8859 – 8868. (Note that the torsion angle close to 08 implies
the orthogonal arrangement between metal ds orbital and
radical p* orbital.)
Piligkos, E. K. Brechin, Dalton Trans. 2009, 3403 – 3412.
[25] Cu-N-C-C torsion angles in dicopper(II) complexes with a m-
phenylenediamine bridge, see for example: C. Yuste, J. Fer-
rando-Soria, D. Cangussu, O. Fabelo, C. Ruiz-Pꢆrez, N. Marino,
G. De Munno, S.-E. Stiriba, R. Ruiz-Garcꢇa, J. Cano , F. Lloret,
M. Julve, Inorg. Chim. Acta 2010, 363, 1984 – 1994.
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