Pincer-porphyrin hybrids

Article abstract of DOI:10.1021/ol049034s

(Chemical Equation Presented) Starting from tetrakis(3,5-bis(bromomethyl) phenyl)porphyrin, pincer-porphyrin hybrid molecules (tetrakis(ECE-pincer) porphyrin; E = N, P, S) based on a tetraphenylporphyrin skeleton have been prepared in high yields. These m

Full text of DOI:10.1021/ol049034s

ORGANIC
LETTERS
2004
Vol. 6, No. 18
3023-3026
Pincer-Porphyrin Hybrids
Bart M. J. M. Suijkerbuijk,† Martin Lutz,‡ Anthony L. Spek,‡
Gerard van Koten,† and Robertus J. M. Klein Gebbink*
,†
Debye Institute, Department of Metal-Mediated Synthesis, and BijVoet Center for
Biomolecular Research, Department of Crystal and Structural Chemistry,
Utrecht UniVersity, Padualaan 8, 3584 CH Utrecht, The Netherlands
r.j.m.kleingebbink@chem.uu.nl
Received May 25, 2004
ABSTRACT
Starting from tetrakis(3,5-bis(bromomethyl)phenyl)porphyrin, pincer-porphyrin hybrid molecules (tetrakis(ECE-pincer)porphyrin; E ) N, P, S)
based on a tetraphenylporphyrin skeleton have been prepared in high yields. These multi-ligand site compounds could be selectively metalated
at their peripheries, which was shown by X-ray crystallography.
Inspired by Nature’s use of macrocyclic, tetrapyrrolic
molecules in processes such as oxygen transport and the
conversion of light energy into chemical potential, porphyrins
are widely used in chemical research.1 These photoactive
molecules have been employed in systems ranging from
artificial photosynthetic model systems2 and hemoglobin and
myoglobin mimics3 to building blocks in supramolecular
multicomponent systems.4 In addition, porphyrins containing
a covalently or noncovalently attached extra-annular metal
center have been shown to be interesting for, e.g., molecular
electronic devices5 and supramolecular architectures. Several
porphyrin systems containing peripheral ligand sites have
been constructed to date,6,4a,7-9 and the corresponding metal
complexes were extensively studied. However, with the
exception of metallocene derivatives,5,10 few examples have
(5) Boyd, P. D. W.; Burrell, A. K.; Campbell, W. M.; Cocks, P. A.;
Gordon, K. C.; Jameson, G. B.; Officer, D. L.; Zhao, Z. Chem. Commun.
1999, 637-638.
(6) (a) Flamigni, L.; Barigelletti, F.; Armaroli, N.; Collin, J.-P.; Sauvage,
J.-P.; Williams, J. A. G. Chem. Eur. J. 1998, 4, 1744-1754. (b) Uyeda, H.
T.; Zhao, Y.; Wostyn, K.; Asselberghs, I.; Clays, K.; Persoons, A.; Therien,
M. J. J. Am. Chem. Soc. 2002, 125, 13806-13813.
(7) (a) Prodi, A.; Indelli, M. T.; Kleverlaan, C. J.; Alessio, E.; Scandola,
F. Coord. Chem. ReV. 2002, 229, 51-58. (b) Iengo, E.; Zangrando, E.;
Alessio, E. Eur. J. Inorg. Chem. 2003, 13, 2371-2384.
(8) Richeter, S.; Jeandon, C.; Gisselbrecht, J.-P.; Ruppert, R.; Callot, H.
J. J. Am. Chem. Soc. 2002, 124, 6168-6179 and references therein.
(9) (a) Ma¨rkl, G.; Reiss, M.; Kreitmeier, P.; No¨th, H. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 2230-2233. (b) Darling, S. L.; Stulz, E.; Feeder,
N.; Bampos, N.; Sanders, J. K. M. New J. Chem. 2000, 24, 261-264. (c)
Stulz, E.; Scott, S. M.; Ng, Y.-F.; Bond, A. D.; Teat, S. J.; Darling, S. L.;
Feeder, N.; Sanders, J. K. M. Inorg. Chem. 2003, 42, 6564-6574.
† Debye Institute.
‡ Bijvoet Center for Biomolecular Research.
(1) (a) The Porphyrins; Dolphin, D., Ed.; Academic Press: New York,
1978. (b) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,
R., Eds.; Academic Press: San Diego, 2000.
(2) (a) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461. (b) Gust,
D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26, 198-205. (c)
Kurreck, H.; Huber, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 849-866.
(d) Harriman, A.; Sauvage, J. P. Chem. Soc. ReV. 1996, 41-48. (e) Gust,
D.; Moore, T. A.; Moore, A. L. Pure Appl. Chem. 1998, 70, 2189-2200.
(f) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40-
48.
(3) Collman, J. P.; Fu, L. Acc. Chem. Res. 1999, 32, 455-463.
(4) (a) Imamura, T.; Fukushima, K. Coord. Chem. ReV. 2000, 198, 133-
156. (b) Wojaczynski, J.; Latos-Grazynski, L. Coord. Chem. ReV. 2000,
204, 113-171.
10.1021/ol049034s CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/05/2004

been reported concerning metal atoms directly attached to
the (tetraphenyl)porphyrin skeleton via a metal-to-carbon σ
bond. Notable examples include direct meso-metalation11 and
ortho-metalation of a pyridylporphyrin.12 In search of novel
peripherally metalated porphyrins, we anticipated that a
robust metal-ligand system would be highly desirable as
the extra-annular metal site. If this metal-ligand system, in
addition, were to have other interesting properties, this would
only add to the impetus of connecting such a system to a
porphyrin. Hence, the metallo-ECE-pincer system (with ECE
pincer being the monoanionic, potentially tridentate ligand
[2,6-(ECH2)2C6H3]- (E ) NR2, SR, PR2)) was selected
because it is known to be chemically robust and because of
its catalytic and physicochemical properties.13,14 Linear,
Hammett-type relations are known between substituents at
the para position with respect to the metal in metallopincers
and both the electron density at the metal and the catalytic
activity of the complexes.15 Since the electronic properties
of a porphyrin can be fine-tuned by the introduction of a
metal, metallopincer-metalloporphyrin hybrid systems might
provide an interesting and novel entry into catalyst modula-
tion. Taking these facts into account, we sought for a way
to combine both (metallo)porphyrin chemistry and metallo-
pincer chemistry and, hence, to construct a molecular hybrid
of both. Because these materials can, in theory, incorporate
a plethora of different metals in both coordination moieties,
we anticipated that their synthesis would provide access to
interesting homo- and heteromultimetallic systems.
Scheme 1a
a (a) pyrrole, propionic acid, reflux; (b) HBr/HOAc, CH2Cl2, rt;
(c) HNMe2, CH2Cl2, rt; (d) LiPPh2-BH3, THF, -40° C; (e)
thiophenol or 4-tert-butylthiophenol, K2CO3, 18-C-6, THF, rt; (f)
Et2NH, THF, reflux.
Since several pincer ligand systems are known (i.e., SCS,
NCN, and PCP), we were interested in synthesizing a
common hybrid ligand precursor. Most of the synthetic routes
to these types of ligands involve nucleophilic substitution
of a benzylic bromine atom by the desired heteroatom-
containing nucleophile. Hence, a tetraphenylporphyrin de-
rivative containing benzyl bromide moieties at all of its 3,5-
phenyl positions would be highly desirable. Thus, starting
from 3,5-dimethylaniline, 3,5-bis(methoxymethyl)benzalde-
hyde 1 was obtained in a 29% overall yield (4 steps) and
subsequently used in an Adler-type condensation reaction
with pyrrole to yield 12% of 5,10,15,20-tetrakis(3,5-bis-
(methoxymethyl)phenyl) porphyrin 2 (see Scheme 1).
1
The structure of porphyrin 2 was confirmed by H NMR
and 13C NMR spectroscopy, mass spectrometry, elemental
analysis, and X-ray crystallography (see Supporting Informa-
tion, Figure S1). Following a modified literature procedure,16
treatment of 2 with HBr/AcOH in CH2Cl2 gave 5,10,15,20-
tetrakis(3,5-bis(bromomethyl)phenyl) porphyrin 3 in 67%
yield.
This compound proved to be an excellent general ligand
precursor for the syntheses of tetrakis(ECE-pincer)porphyrins
4, 5, 6a, and 6b (E ) N, P, and S, respectively; see Scheme
1). Nucleophilic replacements of the bromine atoms by the
appropriate nucleophiles proceeded smoothly under standard
conditions under exclusion of light and molecular oxygen.
Tetrakis(NCN)porphyrin 4 was obtained after treatment of
3 with an excess of dimethylamine in CH2Cl2 as a purple,
crystalline solid in 96% yield. Its structural composition was
confirmed by X-ray crystallography; however, the disorder
in the crystal was too high to warrant publication. Notably,
in contrast to most porphyrins, 4 dissolves in solvents ranging
from polar (MeCN, MeOH) to nonpolar (pentane, hexane)
at room temperature.
(10) For selected examples, see: (a) Wang, H. J. H.; Jaquinod, L.; Nurco,
D. J.; Vicente, M. G. H.; Smith, K. M. Chem. Commun. 2001, 2646-2647.
(b) Gryko, D. T.; Zhao, F.; Yasseri, A. A.; Roth, K. M.; Bocian, D. F.;
Kuhr, W.; Lindsey, J. S. J. Org. Chem. 2000, 65, 7356-7362. (c) Gogan,
N. J.; Siddiqui, Z. U. Can. J. Chem. 1972, 50, 720-725.
(11) (a) Arnold, D. P.; Sakata, Y.; Sugiura, K.; Worthington, E. I. Chem.
Commun. 1998, 2331-2332; Arnold, D. P.; Healy, P. C.; Hodgson, M. J.;
Williams, M. L. J. Organomet. Chem. 2000, 607, 41-50. (c) Hodgson, M.
J.; Healy, P. C.; Williams, M. L.; Arnold, D. P. J. Chem. Soc., Dalton
Trans. 2002, 4497-4504. (d) Hartnell, R. D.; Arnold, D. P. Organometallics
2004, 23, 391-399. (e) Hartnell, R. D.; Arnold, D. P. Eur. J. Inorg. Chem.
2004, 6, 1262-1269.
(12) Darling, S. L.; Goh, P. K. Y.; Bampos, N.; Feeder, N.; Montalti,
M.; Prodi, L.; Johnson, B. F. G.; Sanders, J. K. M. Chem. Commun. 1998,
2031-2032.
Using LiPPh2-BH3 in THF as a nucleophile, borane-
protected tetrakis(PCP)porphyrin 5BH3 was obtained in 99%
yield. Deprotection with Et2NH quantitatively yielded tet-
rakis(PCP)porphyrin 5. Fourfold platination of 5 by a
transcyclometalation (TCM) reaction17 with NCN-PtCl was
(13) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2000, 40, 3750-
3781.
(14) Singleton, J. T. Tetrahedron 2003, 59, 1837-1857.
(15) (a) van de Kuil, L. A.; Luitjes, H.; Grove, D. M.; Zwikker, J. W.;
van der Linden, J. G. M.; Roelofsen, A. M.; Jenneskens, L. W.; Drenth,
W.; van Koten, G. Organometallics 1994, 13, 468-477. (b) Slagt, M. Q.;
Rodr´ıguez, G.; Grutters, M. P.; Klein Gebbink, R. J. M.; Klopper, W.;
Jenneskens, L. W.; Lutz, M.; Spek, A. L.; van Koten, G. Chem. Eur. J.
2004, 10, 1331-1344.
(16) Jux, N. Org. Lett. 2000, 2, 2129-2132.
Org. Lett., Vol. 6, No. 18, 2004
3024

palladium coordination environments, 7b was obtained in
90% yield (see Scheme 2). Palladation was spectroscopically
confirmed by the disappearance of the H NMR resonance
Scheme 2a
1
belonging to the hydrogen atom connected to Cipso, which
was previously situated at δ ) 7.68 ppm. In addition, a
downfield shift of the AB pattern of the tert-butylphenyl-
sulfido protons from δ ) 7.37 and 7.33 ppm to δ ) 7.95
and 7.49 ppm, respectively, was observed in combination
with an upfield shift of the hydrogens positioned meta with
respect to Cipso
.
5,10,15,20-Tetrakis(3,5-bis((4-tert-butylphenylsulfido) meth-
yl)-4-chloropalladio(II)phenyl)porphyrin (7b) was fully char-
acterized (1H, 13C NMR, UV-vis, mass spectrometry, and
elemental analysis), and all analyses consistently showed a
quantitative and selective introduction of palladium at the
extra-annular SCS sites, without affecting the inner tetra-
dentate N,N′,N′′,N′′′ coordination site. Similar selective
palladation was also observed for a related SCS-pincer-
substituted porphyrin by Reinhoudt and co-workers.19
Selective palladation was corroborated by X-ray crystal-
lography. The molecular geometry of 7b in the solid state
shows a molecule with C2V symmetry (see Figure 1). A
a (a) NCN-PtCl, toluene, reflux; (b) (1) [Pd(NCMe)4](BF4)2,
CH2Cl2/MeCN, rt, (2) LiCl, MeCN.
then performed in toluene (see Scheme 2). Addition of a
clear, colorless solution of NCN-PtCl in toluene to a clear,
dark-red solution of 5 in toluene gave a voluminous purple
precipitate and a colorless supernatant within minutes. This
precipitate was not further analyzed but most probably
consisted of a coordination compound in which the phos-
phorus atoms of 5 had replaced the nitrogen atoms of NCN-
PtCl in coordinating to platinum.17b Heating this suspension
for 72 h at 110 °C did not visibly change the reaction
mixture, which still consisted of a purple precipitate in a
colorless supernatant. However, when analyzing the super-
natant, it consisted solely of a solution of the free NCN ligand
in toluene. The purple precipitate was collected and washed
with toluene, pentane, and finally CH2Cl2. The compound
is not soluble in pentane, toluene, or benzene and is only
sparingly soluble in solvents such as THF, CH2Cl2, and
CHCl3. The resulting product 8 was fully characterized (1H
and 31P NMR, UV-vis, MALDI-TOF, and elemental
analysis), and all analyses showed that we succeeded in
selectively metalating the porphyrin at its extra-annular pincer
sites.
Finally, the introduction of four SCS pincer ligands around
a mutual porphyrin core was undertaken. Allowing 3 to react
with 8 equiv of thiophenol in THF in the presence of 18-
crown-6 and K2CO3 furnished tetrakis(SCS)porphyrin 6a in
88% yield. Unfortunately, 4-fold palladation of this com-
pound using [Pd(MeCN)4](BF4)2 in a mixture of CH2Cl2 and
acetonitrile18 led to a product that was insufficiently soluble
for proper characterization. Hence, tert-butyl derivative 6b
was synthesized similarly in 93% yield using commercially
available 4-tert-butylthiophenol as the nucleophile. Com-
pound 6b was palladated according to the procedure used
for 6a, and after exchange of acetonitrile for chloride in the
Figure 1. Crystal structure of 7b. Hydrogen atoms and co-
crystallized molecules of toluene have been omitted for clarity. Only
the major disorder component of the tert-butyl groups is shown.
Selected distances [Å] and dihedral angles [deg]: Cl(1)-Cl(1a)
24.2734(19), Pd(1)-Cl(1) 2.3925(12), Pd(1)-S(1) 2.2916(14),
Pd(1)-S(2) 2.2804(14), Pd(1)-Cipso 1.988(4); porphyrin-pincer-
(Pd(1)) 56.5(7), porphyrin-pincer(Pd(2)) 67.7(7).
(17) (a) Dani, P.; Karlen, T.; Gossage, R. A.; Smeets, W. J. J.; Spek, A.
L.; van Koten, G. J. Am. Chem. Soc. 1997, 119, 11317-11318. (b) Albrecht,
M.; Dani, P.; Lutz, M.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc.
2000, 122, 11822-11833. (c) Dijkstra, H. P.; Albrecht, M.; van Koten, G.
Chem. Commun. 2002, 126-127.
(18) Dijkstra, H. P.; Meijer, M. D.; Patel, J.; Kreiter, R.; van Klink, G.
P. M.; Lutz, M.; Spek, A. L.; Canty, A. J.; van Koten, G. Organometallics
2001, 20, 3159-3168.
slightly distorted porphyrin core is surrounded by four
diorganosulfide moieties (dihedral angles between porphyrin
(19) Huck, W. T. S.; Rohrer, A.; Anikumar, A. T.; Fokkens, R. H.;
Nibbering, N. M. M.; van Veggel, F. C. J. M.; Reinhoudt, D. N. New J.
Chem. 1998, 165-168
Org. Lett., Vol. 6, No. 18, 2004
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and meso-phenyl groups are 56.5(7)° and 67.7(7)°, each of
which is cyclopalladated at the position between the
CH2SPhtBu groups. This affords square planar Pd(II) centers
with a ligand environment that comprises tridentate S,C,S′
coordination by the organic moiety with a chloride ligand
trans to the metal-bonded aromatic carbon atom (Cipso). Two
identical pincer groups opposite to each other (around
palladium atoms Pd(2) and Pd(2a), Figure 1) show a distorted
ligand environment, wherein the two tert-butylphenyl groups
do not point away from each other in an anti way but rather
adopt a quasi syn geometry.
18 nm for 7b and 8, respectively. This observation is not
uncommon for peripherally metalated porphyrins but none-
theless emphasizes that there is electronic communication
between the metallo-pincer and porphyrin parts of the
molecule in the excited state.20 Moreover, fluorescence
measurements (λexc ) 516 nm) of 6b and 7b revealed a 74%
quenching of the fluorescence of the latter with respect to
the former, which further supports an intramolecular com-
munication in the excited state. We believe that the fluores-
cence quenching is caused by a singlet to triplet intersystem
crossing, which can be mediated by heavy metal atoms.
In conclusion, a new class of pincer ligand-functionalized
porphyrins has been developed. Starting from a general
ligand precursor, 5,10,15,20-tetrakis(3,5-bis(bromomethyl)-
phenyl)porphyrin (3), tetrakis-NCN-, -PCP-, and -SCS-
porphyrins were synthesized in high yields. The tetrakis-
SCS- and PCP-porphyrins were selectively metalated at their
peripheral ligand sites, as judged from NMR spectrometry
and X-ray crystallography. Thus, we have shown that
organometallic moieties comprising an exceedingly stable
metal-to-carbon σ bond can be fitted onto a porphyrin core,
yielding multiligand systems. In addition, UV-vis and
fluorescence spectroscopy provide evidence for an ap-
preciable amount of electronic communication between the
subunits of the hybrid in the excited state. We are currently
focusing on the metalation chemistry of the reported tetrakis-
(ECE)porphyrins, i.e., the selectivity of metal incorporation
in- or outside the porphyrin core, and on potential catalytic
and physicochemical applications.
The introduction of metal atoms in the periphery of the
porphyrin affects the electronic spectra of the hybrid
molecules, which are completely dominated by the porphyrin
part (see Figure 2).
Acknowledgment. This work was supported by the
Council for Chemical Sciences of The Netherlands Organi-
zation for Scientific Research (CW/NWO) and The Neth-
erlands Research School Combination Catalysis (NRSC-C).
We thank Dr. C. A. van Walree for help with the fluores-
cence measurements.
Figure 2. Soret region of the electronic spectra of pincer-porphyrin
hybrids 5-8. The Soret peaks are normalized.
Supporting Information Available: Synthetic procedures
and spectroscopic characterization for compounds 1-8 and
crystallographic data for compounds 2 and 7b in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
The B- and Q-bands of free ligands 5 (420, 514, 550, 588,
and 643 nm) and 6b (420, 516, 549, 591, and 645 nm) shift
bathochromically to 426, 522, 557, 596, and 650 nm,
respectively, for 7b and to 431, 522, 561, 600, and 654 nm
for 8 upon peripheral metalation (for the Q-band region, see
Supporting Information Figure S2). In addition, the Soret
bands broaden upon peripheral metalation from a full width
at half-maximum (fwhm) of 12 nm for 5 and 6b to 14 and
OL049034S
(20) Dixon, I. M.; Collin, J.-P. J. Porphyrins Phthalocyanines 2001, 5,
600-607.
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