Synthesis of Ln(III) chloride tetraphenylporphyrin complexes

Article abstract of DOI:10.1021/ic015612e

The isolation and identification of the first examples of anhydrous lanthanide chloride tetraphenylporphyrin complexes have been described. The purple complexes were generated by the reaction of dilithiotetraphenylporphyrin bis(dimethoxyethane) with lanth

Full text of DOI:10.1021/ic015612e

Inorg. Chem. 2002, 41, 1704−1706
Synthesis of Ln(III) Chloride Tetraphenylporphyrin Complexes
Timothy J. Foley, Khalil A. Abboud, and James M. Boncella*
Department of Chemistry and Center for Catalysis, UniVersity of Florida,
GainesVille, Florida 32611-7200
Received November 9, 2001
The isolation and identification of the first examples of anhydrous
lanthanide chloride tetraphenylporphyrin complexes have been
described. The purple complexes were generated by the reaction
of dilithiotetraphenylporphyrin bis(dimethoxyethane) with lanthanide
trichloride tris(tetrahydrofuran) salts to yield the products in up to
85% yield. The crystal structures for the holmium and ytterbium
complexes are also presented.
complexes absorb strongly at the desired energy but they
have been more difficult to synthesize and lack the structural
diversity of the diketones. Herein we report a simple, high-
yielding, general synthesis for anhydrous LnTPP complexes.
Porphyrin complexes of the early transition metals have
been studied extensively.4 There are far fewer studies of
porphyrin complexes of the lanthanide metals, with the
majority of work focusing on the diamagnetic members.5 The
first reported synthesis of a LnTPP complex entailed distil-
lation of acetylacetone from a Ln(acac)3 in the presence of
the TPPH2 giving LnTPP(acac).6 While the preparation
reported a high crude yield as determined by UV-visible
data, isolated yields are low (10-30%) due to the need for
extensive column chromatography to isolate the product.
More recently LnTPP complexes have been synthesized by
amine elimination reactions between lanthanide trisamide
complexes and TPPH2.7
Recently, we have been interested in the use of lanthanide
tetraphenylporphyrin (LnTPP) complexes as the luminescent
centers in near-infrared (NIR) polymer electroluminescent
devices (PLED).1 These devices possess an active layer that
is a blend of a LnTPP complex and a poly-p-phenylene
(PPP). Excited states on the PPP, generated by electron-
hole recombination, are transferred to an acetylacetonoate
(acac) tetraphenylporphyrin complex of ytterbium or erbium,
which then emit at 980 or 1550 nm, respectively. Perfor-
mance of the devices should be governed by the Fo¨rster
energy transfer equation, which indicates that the efficiency
of energy transfer is directly proportional to the spectral
overlap of the donor molecule’s (PPP) fluorescence and the
molar absorbtivity of the acceptor molecule (LnTPP) at those
wavelengths.2 Prior to this work many groups reported on
NIR emitting PLEDs that incorporated lanthanide trisdiketone
phenathroline complexes. Although these complexes are easy
to synthesize, they have relatively low molar absorbtivities
at typical polymer emission wavelengths and therefore are
expected to act as poor Fo¨rster energy acceptors when
coupled with light-emitting polymers.3 In contrast, LnTPP
We have focused our efforts on the salt metathesis reaction
of LnCl3‚3(THF) with dilithiotetraphenylporphyrin bis-
dimethoxyethane (eq 1).8 Reaction of the two components
in refluxing toluene gives complete conversion to the
lanthanide tetraphenylporphyrin chloride dimethoxyethane
complex (LnTPPCl(DME)). Progress of the reaction is
* Author to whom correspondence should be addressed. E-mail: boncella@
chem.ufl.edu.
(1) Harrison, B. S.; Foley, T. J.; Shim, J.; Bouguettaya, M.; Padmanaban,
G.; Boncella, J. M.; Holloway, P. H.; Ramakrishnan, S.; Reynolds, J.
R.; Schanze, K. S. Appl. Phys. Lett. 2001, 79, 3770.
(2) Guillet, J. Polymer Photophysics and Photochemistry, 1st ed.: Cam-
bridge University Press: Cambridge, 1985.
(3) (a) Sun, R. G.; Wang, Y. Z.; Zheng, Q. B.; Zhang, H. J.; Epstein, A.
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S. R. J. Appl. Phys. 2000, 87, 8049. (c) Yu, G.; Liu, Y. Q.; Wu, X.;
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Kawamura, Y.; Wada, Y.; Yanagida, S. Jpn. J. Appl. Phys. 1 2001,
40, 350.
(4) Kadish, K. M.; Smith, K. M.; Guilard, R. The Porphyrin Handbook;
Academic Press: San Diego, CA, 2000; Vol. 3, pp 1-40.
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Schaverien, C. J. J. Chem. Soc., Chem. Commun. 1991, 458. (c)
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(8) Arnold, J.; Dawson, D. Y.; Hoffman, C. G. J. Am. Chem. Soc. 1993,
115, 2707.
1704 Inorganic Chemistry, Vol. 41, No. 7, 2002
10.1021/ic015612e CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/12/2002

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Figure 1. ORTEP drawing of YbClTPP‚DME with thermal ellipsoids at
50% probability. Distances (Å): Yb-Cl 2.605, Yb-O1 2.438, Yb-O2
2.395, Yb-N1 2.334, Yb-N2 2.314, Yb-N3 2.310, Yb -N4 2.339.
conveniently monitored by following the appearance of the
Soret band of the metalloporphyrin at 422 nm and by the
increase of the Q-band absorbance at 551 nm.9 The resulting
mixture is cloudy with lithium chloride, which is separated
from the solution by hot filtration. The solution is then
Figure 2. ORTEP drawing of HoClTPP‚DME with thermal ellipsoids at
50% probability. Distances (Å): Ho-Cl 2.603, Ho-O1 2.473, Ho-O2
2.459, Ho-N1 2.335, Ho-N2 2.358, Ho-N3 2.359, Ho-N4 2.376.
1
reduced to /3 its original volume and cooled to 0 °C,
leads to dihedral angles of 21.36(16)° and 16.71(18)° for
the N1-N4 and N2-N3 planes. In contrast, the N1-N2 and
N3-N4 dihedral angles are 4.0(2)° and 6.5(2)°. The seven-
coordinate Ho atom lies 1.154(3) Å above the centroid of
the pyrrole nitrogen mean plane and has an average Ho-N
distance of 2.357(1) Å, as is consistent with its slightly larger
ionic radius. The angle between the line perpendicular to
the N4 mean plane passing through the N4 mean plane
centroid and the line going through the metal and the centroid
is 0.7°, placing the holmium above the center of the ring.
The porphyrin ring adopts a domed conformation with the
mean planes defined by the pyrrole rings having dihedral
angles of N1-N2 13.02(21)°, N2-N3 14.95(13)°, N3-N4
8.97(17)°, and N4-N1 15.09(16)°.
Dilithioporphyrins have been used as the starting material
for various other metal porphyrin complexes.11 Most notably
dilithiooctaethylporphyrin reacts with 1 equiv of scandium
trichloride to produce the scandium chloride octaethylpor-
phyrin complex in a good yield.12 The complex lacks any
coordinating solvents such as that observed in our system.
This is in all likelihood due to the much smaller size of
scandium relative to lanthanide metals. In addition, the metal
is much closer to the plane of the porphyrin ring, being only
displaced from the centroid of the 4 N mean plane by 0.68
Å.13 It can therefore be expected that the porphyrin ring also
provides some amount of steric hindrance to the coordination
environment around the metal center. More recently, a seven-
coordinate ytterbium tetra(p-tolyl)porphyrin complex has
been generated which has a metal to porphyrin centroid
displacement of 1.090°.7a
whereupon the LnTPPCl(DME) precipitates in approximately
75-85% yield. This synthetic approach has been applied to
ytterbium, thulium, erbium, and holmium, though we expect
that it will be applicable to the larger lanthanide metals
(though it is possible that the larger size of the early Ln
metals may result in the formation of dimers or complexes
with higher coordination numbers). The formation of lithium
chloride ate complexes, which has been observed for other
lanthanide salt metathesis reactions, is presumably suppressed
due to the low polarity of toluene.10 Attempts to repeat this
reaction in ethereal solvents produced less than satisfactory
results (low yields with the material contaminated with
lithium chloride). Once the complexes have been isolated,
there is no change in their spectral properties on standing
for several days under an inert atmosphere or when exposed
to alcohols. Exposure to 1 M HCl in diethyl ether produces
the 416 nm Soret band of the free tetraphenylporphyrin,
suggesting that the complex has been demetalated. The
stability of these complexes to ambient conditions is not
known, but exposure to water should result in the displace-
ment of the DME and the subsequent coordination of
two or more water molecules to the metal center of the
LnTPP.7
Thermal ellipsoid plots of the solid-state structures of the
ytterbium and holmium complexes, in conjunction with
selected atom labels and bond distances, are presented in
Figures 1 and 2, respectively. The ytterbium ion sits 1.105-
(1) Å above the centroid of the least squares mean plane
defined by the four pyrrole nitrogens and has an average
Yb-N bond distance of 2.324(2) Å. The angle between the
line perpendicular to the N4 mean plane passing through the
N4 mean plane centroid and the line passing through the
metal and the centroid is 0.8°, placing the ytterbium directly
above the center of the porphyrin cavity. To accommodate
the relatively large metal, the ring loses planarity and adopts
a saddle shape. Defining mean planes using the pyrrole rings
The remaining chloride on the complexes can be easily
replaced by a second salt metathesis reaction. Addition of
potassium acetylacetonoate to the Yb complex in dimethoxy-
ethane at room temperature leads to the YbTPP acac complex
in 91% isolated yield, giving an overall yield of 72% based
on LnCl3‚3(THF).14 This yield is a substantial improvement
(11) (a) Arnold, J.; Hoffman, C. G.; Dawson, D. Y.; Hollander, F. J.
Organometallics 1993, 12, 3645. (b) Dawson, D. Y.; Brand, H.;
Arnold, J. J. Am. Chem. Soc. 1994, 116, 9797. (c) Kane, K. M.; Lemke,
F. R.; Petersen, J. L. Inorg. Chem. 1995, 34, 4085.
(12) Arnold, J.; Hoffman, C. G. J. Am. Chem. Soc. 1990, 112, 8620.
(13) Sewchok, M. G.; Haushalter, R. C.; Merola, J. S. Inorg. Chim. Acta
1988, 144, 47.
(9) UV-visible spectra were taken under an inert, anhydrous atmosphere.
(10) (a) Wong, W. K.; Zhang L. L.; Xue, F.; Mak, T. C. W. Polyhedron
1997, 16, 2013. (b) Evans, W. J.; Anwander, R.; Ziller, J. W.; Khan,
S. I. Inorg. Chem. 1995, 34, 5927. (c) Guan, J.; Songchun, J.; Shen,
Q. Organometallics 1992, 11, 248. (d) Campazzi, E.; Solari, E.;
Scopelliti, R.; Floriani, C. Inorg. Chem. 1999, 38, 6240.
Inorganic Chemistry, Vol. 41, No. 7, 2002 1705

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on the current method used to synthesize the LnTPP acac
compounds.6 We are currently exploring this methodology
for the synthesis of a variety of LnTPP(X) complexes, where
the anionic ligand X is chosen to optimize the performance
of NIR PLEDs.
Acknowledgment. This work was supported by the
Defense Advanced Research Projects Agency. (Grant No.
DAAD 19-00-1-0002).
Supporting Information Available: This material is available
free of charge via the Internet at http://pubs.acs.org.
(14) Identity of the compound was established via 1H NMR and was in
agreement the published spectrum.6
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1706 Inorganic Chemistry, Vol. 41, No. 7, 2002