Synthesis, structural characterization, and properties of aluminum (III) meso-tetraphenylporphyrin complexes axially bonded to phosphinate anions

Article abstract of DOI:10.1021/ic0617801

Aluminum (III) meso-tetraphenylporphyrins axially bonded to phosphinate anions have been synthesized and characterized by NMR and UV-visible spectroscopy, single-crystal X-ray diffraction, and FAB⁺ mass spectrometry. According to the solvent an

Full text of DOI:10.1021/ic0617801

Inorg. Chem. 2006, 45, 10049−10051
Synthesis, Structural Characterization, and Properties of Aluminum (III)
meso-Tetraphenylporphyrin Complexes Axially Bonded to Phosphinate
Anions
Se´bastien Richeter,*,† Julien Thion,† Arie van der Lee,‡ and Dominique Leclercq†
Laboratoire de Chimie Mole´culaire et Organisation du Solide, UMR 5637, UniVersite´ Montpellier
II, CC 007, Place Euge`ne Bataillon, 34095 Montpellier Cedex 05, France, and Institut Europe´en
des Membranes, UMR 5635, UniVersite´ Montpellier II, CC 047, Place Euge`ne Bataillon,
34095 Montpellier Cedex 05, France
Received September 20, 2006
Aluminum (III) meso-tetraphenylporphyrins axially bonded to
phosphinate anions have been synthesized and characterized by
NMR and UV−visible spectroscopy, single-crystal X-ray diffraction,
and FAB+ mass spectrometry. According to the solvent and the
size of the anion, these compounds are able to self-assemble in
two different manners.
Figure 1. Synthesis of complexes 1 and 2.
catalytic properties and their rich photo- and electrochem-
istry,6 their use as molecular building blocks for the design
of porphyrin arrays remains limited. The synthesis of por-
phyrin arrays including aluminum(III) porphyrins is mostly
limited to the use of phenolates7 and carboxylates8 as the
axial ligand. Surprisingly, there is no example of alumi-
num(III) porphyrins axially bonded to phosphinate anions.
Here we report the synthesis of aluminum(III) meso-
tetraphenylporphyrin (AlIIITPP) complexes axially bonded
to phosphinate anions and present their properties as su-
pramolecular building blocks. Complexes 1 and 2 were
obtained by a “one-pot” strategy using the meso-tetraphen-
ylporphyrin free base (H2TPP) as starting material (Fig-
ure 1). H2TPP was treated with AlMe3, and the phosphinic
acid derivative was added to the reactive intermediate
A self-assembly process can be described as a spontaneous
association of molecules under equilibrium into stable
aggregates held together by noncoValent bonds.1 Such
phenomena are frequent in biological systems, and an
outstanding example is provided by bacteriochlorophyll
molecules.2 Metalloporphyrins are attractive compounds for
the design of self-assembling chromophores.3 The axial
coordination on the metal center is a powerful synthetic way
to synthesize large multiporphyrinic systems.4 Actually, much
attention have been paid to porphyrin containing zinc(II),
magnesium(II), tin(IV), or rhodium(III) as metal centers.5
Although aluminum(III) porphyrins are well known for their
* To whom correspondence should be addressed. E-mail: sricheter@
univ-montp2.fr.
† Laboratoire de Chimie Mole´culaire et Organisation du Solide.
(4) (a) Michelsen, U.; Hunter, C. A. Angew. Chem., Int. Ed. 2000, 39,
764-767. (b) Haycock, R. A.; Hunter, C. A.; James, D. A.; Michelsen,
U.; Sutton, L. R. Org. Lett. 2000, 2, 2435-2438. (c) Tsuda, A.;
Nakamura, T.; Sakamoto, S.; Yamaguchi, K.; Osuka, A. Angew.
Chem., Int. Ed. 2002, 41, 2817-2821. (d) Twyman, L. J.; King, A.
S. H. Chem. Commun. 2002, 910-911. (e) Furutsu, D.; Satake, A.;
Kobuke, Y. Inorg. Chem. 2005, 44, 4460-4462. (f) Takahashi, R.;
Kobuke, Y. J. Org. Chem. 2005, 70, 2745-2753.
‡ Institut Europe´en des Membranes.
(1) Lehn, J.-M. Supramolecular Chemistry, Concepts and PerspectiVes;
VCH: Weinheim, 1995.
(2) (a) Bystrova, M. I.; Mal’gosheva, I. N.; Krasnovskii, A. A. Mol. Biol.
1979, 13, 582-594. (b) Balaban, T. S.; Holzwarth, A. R.; Schaffner,
K.; Boender, G.-J.; de Groot, J. J. M. Biochemistry 1995, 34, 15259-
15266.
(3) (a) For examples, see: Chambron, J.-C.; Heitz, V.; Sauvage, J. -P. In
The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guillard, R.,
Eds.; Academic Press: Orlando, 2000; Vol. 6, Chapter 40, pp 1-42.
(b) Goldberg, I. Cryst. Eng. Comm. 2002, 4, 109-116. (c) Drain, C.
M.; Russell, K. C.; Lehn, J. -M. Chem. Commun. 1996, 337-338. (d)
Drain, C. M.; Nifiatis, F.; Vasenko, A.; Batteas, J. D. Angew. Chem.,
Int. Ed. 1998, 37, 2344-2347. (e) Wang, Z.; Medforth, C. J.; Shelnutt,
J. A. J. Am. Chem. Soc. 2001, 126, 15954-15955. (f) Balaban, T. S.;
Goddard, R.; Linke-Schaetzel, M.; Lehn, J.-M. J. Am. Chem. Soc.
2003, 125, 4233-4239. (g) Balaban, T. S.; Bhise, A. D.; Fischer, M.;
Linke-Schaetzel, M.; Roussel, C.; Vanthuyne, N. Angew. Chem., Int.
Ed. 2003, 42, 2140-2144. (h) Balaban, T. S.; Linke-Schaetzel, M.;
Bhise, A.D.; Vanthuyne, N.; Roussel, C. Eur. J. Org. Chem. 2004,
3919-3930.
(5) (a) For examples, see: Sanders, J. K. M.; Bampos, N.; Clyde-Watson,
Z.; Darling, S. L.; Hawley, J. C.; Kim, H. -J.; Mak, C. C.; Webb, S.
J. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guillard,
R., Eds.; Academic Press: New York, 2000; Vol. 3, pp 1-48. (b)
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.
(6) (a) Sugimoto, H.; Kimura, T.; Inoue, S. J. Am. Chem. Soc. 1999, 121,
2325-2326. (b) Konishi, K.; Aida, T.; Inoue, S. J. Org. Chem. 1990,
55, 816-820. (c) Hirai, Y.; Aida, T.; Inoue, S. J. Am. Chem. Soc.
1989, 111, 3062-3063.
(7) Kumar, P. P.; Maiya, B. G. New J. Chem. 2003, 27, 619-625.
(8) Davidson, G. J. E.; Tong, L. H.; Raithby, P. R.; Sanders, J. K. M.
Chem. Commun. 2006, 3087-3089.
10.1021/ic0617801 CCC: $33.50
Published on Web 11/09/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 25, 2006 10049

COMMUNICATION
Figure 3. Two different views in CPK of one [1‚EtOH]∞ supramolecular
chain. Light gray, C porphyrin; dark gray, C ethanol; dark green, C Ph on
P(3). (Ph meso and hydrogen atoms, except (O5)H, omitted for clarity).
Figure 2. X-ray structure of [1‚EtOH]∞ supramolecular chains. Light gray,
C porphyrin; dark gray, C ethanol; dark green, C Ph on P(3). Hydrogen
atoms, except (O5)H, are omitted for clarity.
Me-AlIIITPP. The desired complexes 1 and 2 were isolated
by addition of pentane to the reaction mixture and obtained
in high yield (85-87%).
Crystals of the complex 1‚EtOH were obtained from slow
diffusion of pentane into a solution of 1 in a mixture of
CHCl3/EtOH in a 98:2 ratio (no crystals were obtained
without EtOH). This is the first X-ray structure of a
AlIIITPP complex axially bonded to a phosphinate anion.9
Crystallographic analysis showed that complex 1 and ethanol
molecules self-assemble to form polymeric 1-D supramo-
lecular chains, [1‚EtOH]∞, where subunits are linked through
coordination and hydrogen bonds (Figure 2). The porphyrin
is not flat but has a slight saddle shape (the C5-Al-C15
and C10-Al-C20 angles are close to 172°). The coor-
dination geometry around the AlIII is slightly distorded
octahedral and is coordinated in the equatorial positions by
the four nitrogen donors (dAl-N ) 2.00 Å). One axial
position of the AlIII is occupied by the oxygen of the
phosphinate ligand which forms a bent interaction with the
metal (Al-O(2)-P(3) ) 161.5°). The second axial position
of the AlIII is occupied by the oxygen of one ethanol
molecule. The Al-O(2)phosphinate distance is shorter than the
Al-O(5)ethanol distance (1.86 compared to 1.98 Å). These
distances are close to those found in the literature for AlIII
Schiff base phosphinate complexes.10 The O(2)-Al-O(5)
linkage is almost linear (∼179°). The ethanol molecules
link the building blocks 1 together through hydrogen bonds
with the PdO functions (O(4)-O(5) ) 2.50 Å) and
coordination bonds with the AlIII (O(5)-Al). The distance
between two AlIII atoms is 7.75 Å. Two views of one chain
are represented in the Figure 3. All the ethyl groups of the
ethanol molecules (dark gray) are situated on the same side
of the chain.
Figure 4. 1H NMR spectra of 1 (200 MHz, 300 K, 20 mM) (a) in C6D6
+ 10% CD3OD and (b) in C6D6.
the phosphorus atom, respectively, Hpara, Hmeta, and Hortho
strongly shielded by the porphyrinic ring current. These
chemical shifts are close to those of a benzoate bound to a
six-coordinate AlIIITPP.8 In our case, the second axial ligand
is a molecule of CD3OD.
Significant broad signals are observed by 1H NMR of the
complex 1 in benzene-d6 (without CD3OD) at 25 °C (Figure
4b). The protons resonate at higher fields, showing the close
proximity between the porphyrins and suggesting the forma-
tion of self-assembled species containing six-coordinate
AlIIITPP complexes aggregated into chains through bridging
phosphinates.10 When the sample is heated at 75 °C, sharp
signals are displayed corresponding to complex 1 with a five-
coordinate AlIII (the Hortho resonate at 5.20 ppm, see the
Supporting Information). The addition of a coordinating
solvent such as CD3OD to the NMR sample leads to sharper
signals corresponding to the monomeric complex 1‚CD3OD
(Figure 4a). It demonstrates that PdO-Al interactions are
involved in the self-assembled structures (π-π stacking
interactions are not responsible for the self-assembly pro-
cess).11 This process is reversible: when solvents are
evaporated and fresh benzene-d6 is added, broad signals are
displayed again. Sanders and co-workers showed that a six-
coordinate AlIII-porphyrin complex was never detected with
benzoates as the axial ligand (except if a coordinating
(9) Crystal data for 1.EtOH (C58H44N4O3PAl): Mw ) 902.97, monoclinic,
space group P121/c1 (No. 14), a ) 12.8820(4) Å, b ) 26.0619(7) Å,
c ) 15.0590(4) Å, â ) 111.15(2)°, V ) 4715.19(70) Å3, Z ) 4, dcalcd
) 1.268 g‚cm3, T ) 173 K, λ(Mo KR) ) 0.71073 Å, µ(Mo KR) )
0.128 mm-1, 87 791 reflns collected, 15 579 unique reflns, R1obsd for
[I > 2σ(I)]) 0.1037, wR2all(F2) ) 0.1017 for all data, GOF on F2 )
1.301, trans. coeff. ) 0.980-0.990.
(10) Mitra, A.; Parkin, S.; Atwood, D. A. Inorg. Chem. 2006, 45, 3970-
3975.
(11) (a) Gardner, M.; Guerin, A. J.; Hunter, C. A.; Michelsen, U.; Rotger,
C. New J. Chem. 1999, 309-316. (b) Ma, C. T. L.; MacLachlan, M.
J. Angew. Chem., Int. Ed. 2005, 44, 4178-4182.
Complex 1 is air and moisture stable, and no dissociation
of the P-OsAl bond is observed in solution even if a
1
large excess of a coordinating solvent is added. The H
NMR spectrum of the complex 1‚CD3OD in a mixture of
C6D6/CD3OD in a 9:1 ratio shows the axially bound
phosphinate anion (Figure 4a). The characteristic signals of
the TPP ligand are displayed. The three signals at 6.71, 6.54,
and 4.88 ppm are due to the protons of the phenyl rings on
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COMMUNICATION
Figure 5. UV-visible absorption spectra of 1 in CHCl3 (a), CHCl3/
CH3OH 9:1 (b), and n-hexane/CHCl3 98:2 (c). Picture inset: quartz cells
with self-assembled complex 1 in n-hexane/CHCl3 98:2 (left) and not self-
assembled complex 1 in CHCl3 (right) at 24 µM (quartz cells of 1 cm path).
Figure 6. FAB+ mass spectrum of the complex 2. The different fragments
observed are schematically represented.
oligomeric species more than the dimer. It was possible to
observe by FAB+ mass spectrometry a weak signal at m/z
) 1715 corresponding to the dimer [1]2.14 The exact
association constant Kassoc for this system could not be
extracted, but Kassoc for the PdO-Al interaction in n-hexane/
CHCl3 98:2 must be greater than 105 M-1.15 Bulky groups
on the porphyrin and/or the phosphorus atom prevent the
formation of stable PdO-Al interactions. Thus, more stable
self-assembled structures are obtained when the size of these
groups is decreased. Complex 2 has only one phenyl ring
on the phosphorus atom. This structural change dramatically
decreases its solubility. This compound is only soluble in
the presence of a coordinating molecule. It can be ex-
plained by the formation of stable oligomeric structures with
PdO-Al interactions which are disassembled by methanol.
FAB+ mass spectrometry provided evidence for oligomeric
structures (Figure 6). Signals up to the tetramer fragment
(m/z ) 2984.2) are detected, showing the stability of the
-[Al-O-PdO]- backbone toward FAB+ mass spectrom-
etry conditions.
In summary, aluminum (III) meso-tetraphenylporphyrin
complexes axially bonded to phosphinate anions have been
prepared and characterized. These compounds are new
building blocks to obtain 1-D self-assembled structures.
According to the solvent and the size of the anion, two self-
assembly pathways are possible to give supramolecular
chains like [1‚EtOH]∞ or self-assembled structures with
PdO-Al interactions.
molecule is added).8 With phosphinates as the axial ligand,
six-coordinate AlIIITPP are observed in nonpolar solvents.
The reciprocal recognition between the donor site PdO and
the acceptor site AlIII leads to self-assembled structures with
PdO-Al interactions.
The self-assembly process can also be observed by UV-
visible spectroscopy. A solvatochromic behavior induced by
self-assembly of the complex 1 is observed. The UV-visible
absorption spectrum of 1 in chloroform (Figure 5a) shows
the typical absorption bands of AlIIITPP derivatives. Ac-
cording to previous studies,12 the AlIII is square pyramidal
(4NTPP + 1Ophosphinate). No self-assembly occurs at 2.4 × 10-5
M in chloroform, and the solution is pink (Figure 5, inset).
The sharp Soret band and one intense Q-band appear,
respectively, at 415 and 546 nm. When an AlIII-ligating entity
such as methanol is added, the Soret band and the intense
Q-band are bathochromically shifted and appear, respectively,
at 422 and 558 nm (Figure 5b). A second intense Q-band
appears at 597 nm. This spectral evolution shows that the
AlIII is octahedral (4NTPP + 1Ophosphinate + 1OCH3OH). The
recognition of the complementary polar functional parts
PdO and AlIII is effective in nonpolar solvents. When a
concentrated solution of 1 in chloroform is diluted by adding
n-hexane to reach 2.4 × 10-5 M, a change of color from
pink to brown is observed (Figure 5, inset). Dramatic changes
in the UV-visible absorption spectrum result from the self-
assembly process (Figure 5c). The Q bands are red-shifted
and show that the AlIII is octahedral (two intense Q-bands
at 564 and 604 nm). The Soret band is less intense and
appears split and broad. This spectral evolution arises from
the excitonic coupling between the neighboring chromo-
phores within the self-assembled structures.13 As expected,
the addition of a coordinating solvent such as methanol to a
solution of the complex 1 in n-hexane leads to complete
disassembly.11
Acknowledgment. We are grateful to the Centre National
de la Recherche Scientifique and the Ministe`re de l’Education
Nationale de France for financial support.
Supporting Information Available: General experimental
details, full synthetic procedure and characterization of compounds
1 and 2, estimation of the Kassoc for 1, and crystallographic data for
[1‚EtOH]∞. This material is available free of charge via the Internet
at http://pubs.acs.org.
Spectroscopic analyses tend to show that complex 1 is able
to self-assemble alone in solution by PdO-Al interactions,
but there is no spectroscopic evidence of the formation of
IC0617801
(14) MALDI-TOF mass spectrometry was not so efficient to detect
oligomeric fragments because disassembly processes should occur
during the laser photoexcitation of the porphyrin. See ref 3f.
(15) Kassoc was estimated on the assumption that the concentration of the
free complex 1 is <20% of the total concentration 24 µM. See the
Supporting Information.
(12) Kaisu, Y.; Misu, N.; Tsuji, K.; Kaneko, Y.; Kobayashi, H. Bull. Chem.
Soc. Jpn. 1985, 58, 103-108.
(13) Gouterman, M.; Holten, D.; Lieberman, E. Chem. Phys. 1977, 25,
139-153.
Inorganic Chemistry, Vol. 45, No. 25, 2006 10051