Luminescent η5-pentamethylcyclopentadienyl tantalum(V) complexes: Synthesis, characterization, and emission spectroscopy

Article abstract of DOI:10.1016/S0020-1693(02)01276-8

A series of η⁵-pentamethylcyclopentadienyl tantalum(V) complexes have been prepared and their electronic and emission spectra examined. Compounds of the form Cp*TaX₄ (where X=Cl, Br, OC₆F₅) and Cp*TaCl₂X₂ (where X=OC₆Cl₅, 2,6-OC₆H₃Cl₂), as well as CpTaX₄, have been found to luminesce at 77 K in frozen glasses (only Cp*TaCl₄, reported previously, luminesces at ambient temperature in both solid and fluid solution). The shifts in luminescence maxima with ligand variation lead to the conclusion that the emissive state is a ligand-to-metal charge transfer in which the highest occupied orbital is based on the donor atom of one of the monodentate ligands and the lowest unoccupied orbital is Ta-based.

Full text of DOI:10.1016/S0020-1693(02)01276-8

Inorganica Chimica Acta 345 (2003) 340Á344
/
www.elsevier.com/locate/ica
Luminescent h5-pentamethylcyclopentadienyl tantalum(V)
complexes: synthesis, characterization, and emission spectroscopy
Zachary J. Tonzetich, Richard Eisenberg *
Department of Chemistry, University of Rochester, Rochester, NY 14627, USA
Received 6 August 2002; accepted 24 September 2002
This paper is dedicated to Dick Schrock in recognition of his friendship over the years and his outstanding research which has transformed the way
organometallic chemists view metalÃcarbon bonds and what they can do
/
Abstract
A series of h5-pentamethylcyclopentadienyl tantalum(V) complexes have been prepared and their electronic and emission spectra
examined. Compounds of the form Cp*TaX4 (where XꢀCl, Br, OC6F5) and Cp*TaCl2X2 (where XꢀOC6Cl5, 2,6-OC6H3Cl2), as
/
/
well as CpTaX4, have been found to luminesce at 77 K in frozen glasses (only Cp*TaCl4, reported previously, luminesces at ambient
temperature in both solid and fluid solution). The shifts in luminescence maxima with ligand variation lead to the conclusion that
the emissive state is a ligand-to-metal charge transfer in which the highest occupied orbital is based on the donor atom of one of the
monodentate ligands and the lowest unoccupied orbital is Ta-based.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Luminescence; Tantalum complexes; Cyclopentadienyl ligands; Emission spectroscopy
1. Introduction
(LMCT) luminescent states. For the Zr complexes
studied by Yam et al. [2], the shift in emission energy
with ligand substitution led to the notion, supported by
calculations, that the HOMO in these complexes is
primarily a chloride or thiolate-based p-orbital with an
empty Zr d-orbital as the LUMO. On the other hand, in
studies of the Cp*TaX4 complexes by Sullivan and co-
workers [1], it was proposed that the HOMO for the
LMCT excited state was mainly cyclopentadienyl-based
rather than X-based.
In an examination of different heavy metal emitters
for possible application in display technology utilizing
OLED’s, we were impressed by the intensity of the
yellow emission from Cp*TaCl4 in the solid state at
ambient temperature. In the earlier study by Sullivan, a
series of luminescent derivatives was created by sub-
stitution of a single chloride ligand with a chelating
carboxylate anion [1]. In the present study, we have
endeavored to replace all of the chlorides of Cp*TaCl4
by other monodentate anionic ligands. The choice of X-
type ligand was made based on both its electronic nature
and its structure. Since Cp*TaCl4 hydrolyzes very
readily under ambient conditions [6], the use of bulkier
While the luminescence of late transition metal
complexes has received a great deal of attention, the
corresponding behavior of early metal systems is much
more limited and has remained distinctly less explored
[1Á
/
4]. Only a few examples of luminescent metal
complexes having d0 metal ion configurations have
ꢁ
been reported including Cp2ZrCl2, Cp2 ZrCl2 and their
bis thiolate analogs [2], and Cp*TaCl4 and its mono-
substituted derivatives containing either a triflate or a
carboxylate in place of one chloride [1]. Other lumines-
cent d0 metal complexes contain ligands that form
multiple bonds with the metal center such as the imido
complexes Ta(Å
/
NR)X3L2 [3Á5]. In contrast with emis-
/
sive late transition metal complexes that generally have
metal-to-ligand charge transfer (MLCT) excited states,
the early metal cyclopentadienyl derivatives have been
assigned as having ligand-to-metal charge transfer
* Corresponding author. Tel.: ꢂ
3596
E-mail address: eisenberg@chem.rochester.edu (R. Eisenberg).
/
1-716-275 5573; fax: ꢂ1-716-273
/
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 2 7 6 - 8

Z.J. Tonzetich, R. Eisenberg / Inorganica Chimica Acta 345 (2003) 340Á
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341
ligands was considered to have the possibility of
imparting some stability to the Cp*TaX4 complexes.
The ligands used in this study are phenoxides, and the
resultant complexes shown in Scheme 1 have led to
modification of the LMCT assignment made by Sullivan
and co-workers [1].
Table 1
Absorption and emission energies (cmꢃ1) for Cp*Ta(V) complexes
a
b
Complex
Eabs (Mꢃ1 cmꢃ1
)
Eem
Cp*TaCl4 (1)
28,800 (1870)
23,100 (670)
28,400 (2010)
21,100 (250)
30,300 (1250)
30,000 (4010)
27,100 (4010)
28,100 (3090)
30,200 (2680)
18,300
17,200
Cp*TaBr4 (2)
CpTaCl4 (3)
18,300
19,000
18,300
Cp*Ta(OC6F5)4 (4)
Cp*Ta(OC6Cl5)2Cl2 (5)
Cp*Ta(OC6H3Cl2)2Cl2 (6)
Cp*Ta(OPh)4
2. Results and discussion
Complexes 1Á6 (see Scheme 1) were prepared and
/
characterized by different spectroscopies and by ele-
mental analyses (C, H). While all of the compounds
1
appear to be pure by H NMR spectroscopy, elemental
a
Absorption spectra at 298 K.
Emission spectra at 77 K in toluene glass.
b
cmꢃ1 (o Â
the visible region of the spectrum between 20,600 and
23,000 cmꢃ1 (o Â500 Mꢃ1 cmꢃ1). On the other hand,
the absorption spectrum of compound 3 shows a broad
absorption between 20,600 and 30,600 cmꢃ1. The
/
2000 Mꢃ1 cmꢃ1) and a weaker absorption in
analyses were only satisfactory for 4 and 6 and just
slightly outside the acceptable limit for 5 (the crystalline
sample of 5 may have had occluded solvent, thus
explaining the slightly higher C result). For both 2 and
3, however, multiple analyses from different recrystal-
lized or reprecipitated samples yielded results that were
unacceptably low in carbon. Difficulties of this type
have been observed previously in analyzing other
cyclopentadienyl complexes. The absorption spectrum
and 77 K frozen glass emission spectrum were obtained
for each complex with spectroscopic results provided in
Table 1. In these measurements, the absorption spectro-
scopy was obtained in toluene, while the emission
measurements were made in a toluene glass. Each of
the complexes, except for 1, displayed no room tem-
perature luminescence in the solid state or fluid solution.
Fig. 1 shows the emission of 2, 4 and 5 in toulene glasses.
Compounds 2 and 3 were examined first because of
their very similar electronic nature to 1. Substitution of
bromide for chloride in 2 and Cp for Cp* in 3 provided
a simple means of gauging the effect of ligand substitu-
tion at each of these positions. The absorption spectrum
for 1 and 2 shows two distinguishable features. Each
compound exhibits a strong absorption around 28,300
/
emission spectra of 1Á3 at 77 K in frozen toluene each
/
consist one band with that of compound 2 shifted to
lower energy from the bands of 1 and 3 by approxi-
mately 1100 cmꢃ1. While the substitution of Cp for Cp*
does not affect the emission energy significantly, the
substitution of bromide for chloride clearly does. These
initial results thus suggest that the emission in the
Cp(*)TaX4 complexes may involve charge transfer
from the X ligand to the metal rather than from Cp*,
as proposed previously [1].
Compounds 4Á6 were synthesized to explore further
/
ligand effects on the excited state of the formally d0
Ta(V) Cp*TaX4 system and to attempt to overcome
hydrolysis tendencies arising from the oxophilicity of
tantalum [7,8]. The complexes were prepared from
compound 1 by reaction with the desired phenol in the
presence of triethylamine. The pentafluoro- and penta-
chlorophenoxide ligands were chosen because of their
lack of hydrogen atoms. In addition to 4Á
/6, other
Cp*TaX4 phenoxide complexes with XꢀOPh, p-
/
OC6H4OMe, p-OC6H4-i-Pr were synthesized, but these
complexes were found not to be photo-emissive. The
lack of luminescence from any of the Cp*TaX4 systems
containing CÃ
/
H bonds may relate to the involvement of
these bonds in facile non-radiative decay pathways. For
complex 4, the fluorine atoms provided a convenient
NMR handle in the characterization of the complex, but
for complex 5, a similar ease of NMR characterization
was missing. Previously, it had been reported that steric
hindrance from groups located in the 2 and 6 positions
of the phenol ring interfered with the formation of a
tetra-substituted complex [8], and therefore, it was not
evident whether complete substitution of pentachloro-
phenoxide had occurred. To examine this question more
1
closely, compound 6 was synthesized and found by H
NMR spectroscopy to contain two phenoxide and two
Scheme 1.

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Fig. 1. Emission spectra of Cp*TaBr4 (2), Cp*Ta(OC6F5)4 (4) and Cp*Ta(OC6Cl5)2Cl2 (5) in a toluene frozen glass at 77 K. Excitation energies are
22,200 cmꢃ1 (450 nm) for 2, 26,700 cmꢃ1 (375 nm) for 4 and 24,100 cmꢃ1 (415 nm) for 5.
chloride ligands. The two pairs of phenoxide ligands in 5
and 6 are presumably in a trans relationship due to
steric reasons. Additionally, each compound showed
only one Cp* resonance in the NMR spectrum indicat-
ing that only a single isomer of each Cp*TaCl2(OAr)2
complex (5 and 6) was present.
The absorption spectrum of compound 4 displays an
intense band around 30,000 cmꢃ1, similar to those of
compounds 1 and 2, but lacks the lower energy band
found for those complexes. The absorption spectrum of
Cp*Ta(OPh)4 is similar to that of 4 with only one band
around 30,000 cmꢃ1. The absence of the lower energy
absorption band accounts for the colorless nature of 4
and Cp*Ta(OPh)4 in solution, whereas both of the
chloride-containing complexes 5 and 6 are yellow in
Fig. 2. Emission and excitation spectra of Cp*Ta(OC6Cl5)2Cl2 (5) in a
toluene frozen glass at 77 K. Excitation was at 24,100 cmꢃ1 (415 nm)
and emission was monitored at 18,700 cmꢃ1 (535 nm).
solution. The results from the electronic spectra of the
different Cp*Ta complexes suggest that absorption in
the visible region of the spectrum involves a chloride
ligand and associated ligand-to-metal charge transfer.
For 5 and 6, the absorption spectra each show only one
orbital (HOMO). This assignment agrees with that made
by Yam for the Cp2ZrX2 systems, but differs from that
made by Sullivan in which the Cp ligand was proposed
to be involved in the transition as the origin of the
HOMO. While low temperature lifetime measurements
were not performed for the complexes in the present
study, the previous assignment of a triplet excited state
to the Cp*TaCl4 complex [1] gives credence to the
notion that in these Ta complexes, the excited state is in
fact a triplet.
very broad band with a maximum around 28,000 cmꢃ1
,
unlike the spectra for 1 and 2 that exhibit two distinct
absorptions, but the single bands are broad enough to
encompass the lower energy chloride-based transitions.
From Table 1, it is seen that the three emissive
compounds that contain chloride as a ligand, including
the dichloride complex 5, have approximately the same
energy (18,300 cmꢃ1) whereas for the bromide complex
2, the emission shifts to lower energy and for the
pentafluorophenoxide complex 4 the emission is blue-
shifted, as seen in Fig. 1. (In Fig. 2, the emission and
excitation spectra of 5 are shown.) The results are
consistent with an LMCT excited state in which an X
ligand orbital acts as the highest occupied molecular
3. Conclusion
Derivatives of Cp*TaCl4 have been synthesized and
examined by emission spectroscopy. The phenoxide

Z.J. Tonzetich, R. Eisenberg / Inorganica Chimica Acta 345 (2003) 340Á
/344
343
derivatives appear to be sensitive to steric hindrance in
the 2, 6 positions of the aromatic ring, but appear
somewhat more robust than the Cp*TaX4 complexes.
Based on 77 K emission measurements, the lumines-
cence from the Cp*TaX4 derivatives appears to arise
from a ligand-to-metal charge transfer excited state
(LMCT) involving orbitals based on the X-type ligands,
in contrast with an earlier assignment for related Cp*Ta
complexes.
TMSCl. The reaction was stirred at r.t. for 12 h, and
then filtered through celite. The solvent was removed in
1
vacuo giving 4.45 g (97% yield) of a pale yellow oil. H
NMR (CD2Cl2): d 1.75 (s, 15H); d ꢃ0.15 (d, 9H) [9].
/
4.5. h5-Pentamethylcyclopentadienyl tantalum
tetrachloride, Cp*TaCl4
The previously reported complex [10] was prepared
via a slightly modified procedure reported here. A slurry
of 7.64 g (0.021 mol) of TaCl5 in 50 ml of C6H5CH3 was
4. Experimental
chilled at ꢃ
(0.021 mol) of TMSCp* was dissolved in 15 ml of
C6H5CH3 and the solution chilled to ꢃ30 8C. The cold
/
30 8C for 30 min. In a separate vial, 4.45 g
4.1. General procedures
/
TMSCp* solution was added drop-wise to the slurry of
TaCl5, and the resulting red reaction mixture stirred for
All reactions were carried out under an inert atmo-
sphere in a Vacuum Atmospheres glove-box or using
standard Schlenck line techniques. Solvents were de-
gassed and passed through activated alumina prior to
use. NMR solvents were dried over Linde molecular
sieves and referenced to the residual solvent peak in the
12 h at r.t. While stirring, a bright orangeÁyellow
/
precipitate formed in the reaction flask. The precipitate
was collected by filtration and dried in vacuo, yielding
7.79 g (80% yield) of an orangeÁ
yellow powder. 1H
/
NMR (CD2Cl2): d 2.71 (s). 13C NMR (CD2Cl2): d 133.8
(C5Me5); d 14.5 (C5Me5) [10].
1
case of H and 13C NMR spectra or to trifluorotoluene
in the case of 19F NMR spectra. Cp*H, trimethylsilyl
chloride (TMSCl), pentachlorophenol, 2,6-dichlorophe-
nol and pentafluorophenol were purchased from Al-
drich and used as received. TaCl5 and TaBr5 were
purchased from Strem and TMSCl was purchased
from Acros and used as received.
4.6. h5-Pentamethylcyclopentadienyl tantalum
tetrabromide, Cp*TaBr4
The bromide complex was prepared by an analogous
route as the chloride starting from 1.99 g (0.0034 mol) of
TaBr5. The complex was isolated as 1.71 g (80% yield) of
an orange powder. 1H NMR (CD2Cl2): d 2.92 (s).
Elemental analysis results were unsatisfactory, suggest-
ing the possible presence of unreacted TaBr5 but the
compound appeared pure by NMR spectroscopy. Anal.
Calc. for C5H15Br4Ta: C, 18.89; H, 2.38. Found: C, 7.98;
H, 1.70%.
4.2. Measurements
Absorption spectra were recorded in C6H5CH3 in
resealable quartz cuvettes. Measurements were taken on
a Hitachi U2000 vis/uv spectrophotometer. Emission
spectra were recorded in C6H5CH3 glasses at 77 K in
sealed EPR tubes. The measurements were taken on a
SPEX Fluorolog-3 flourometer with a low temperature
Dewar apparatus.
4.7. h5-Cyclopentadienyl tantalum tetrachloride,
CpTaCl4
4.3. Potassium pentamethylcyclopentadienide, KCp*
The Cp analog of Cp*TaCl4 was prepared according
to a slightly modified literature procedure beginning
with 2.15 g (0.0060 mol) of TaCl5 and 1.0 ml (0.0060
mol) of commercial TMSCp [11]. The reaction produced
1.55 g (67% crude yield) of a brown powder that was
recrystallized from CH2Cl2 giving pink crystalline nee-
A slurry of 2.56 g (0.064 mol) of potassium hydride in
40 ml of THF was chilled at ꢃ30 8C for 1 h. To this
/
slurry was added drop-wise 10 ml (0.064 mole) of cold
Cp*H. The mixture was allowed to warm to room
temperature (r.t.) and then heated at 70 8 for 2 h during
which time vigorous gas evolution occurred. After
heating, 20 ml of hexanes was added and the reaction
mixture filtered. The filtered product was dried in vacuo,
yielding 9.94 g (89% yield) of a white powder.
1
dles. H NMR (CD2Cl2): d 7.07 (s).
4.8. h5-Pentamethylcyclopentadienyl tantalum
tetrapentafluorophenoxide, Cp*Ta(OC6F5)4
Cp*TaCl4 (0.250 g, 0.546 mmol) and pentafluorophe-
nol (0.412g, 2.24 mmol) were combined in a reaction
vessel and 45 ml of C6H5CH3 was transferred in vacuo
at 77 K. The reaction was allowed to warm to 196 K and
Et3N (approximately 1 ml) was transferred in vacuo at
which point the yellow reaction mixture became an
4.4. (Trimethylsilyl)pentamethylcyclopentadiene,
TMSCp*
To a slurry of 3.50 g (0.020 mol) of KCp* in 50 ml of
THF at 0 8 was added, drop-wise, 2.8 ml (0.022 mol) of

344
Z.J. Tonzetich, R. Eisenberg / Inorganica Chimica Acta 345 (2003) 340Á344
/
orange solution. The reaction was then allowed to warm
to r.t. and stir for 16 h during which time the orange
solution became pale yellow and a precipitate formed.
The reaction was filtered through celite and the volatiles
removed in vacuo giving 0.220 g (38% yield) of a white
powder which was recrystallized from Et2O. H NMR
(CD2Cl2): d 2.49 (s). 13C NMR (CD2Cl2); d 137.4 (m,
OC6F5); d 130.2 (C5Me5); d 8.7 (C5Me5). 19F NMR
4.11. h5-Pentamethylcyclopentadienyl tantalum
tetraphenoxide, Cp*Ta(OPh)4
This previously reported complex [12] was prepared
by an alternate procedure analogous to Cp*Ta(OC6F5)4.
A white powder was collected (0.103 g) in 33% yield
starting from 0.200 g (0.437 mmol) of Cp*TaCl4 and
0.186 g (1.98 mmol) of phenol. The compound was
1
1
(C6D6): d ꢃ
/
97.03 (d, 8F) (JFÃF
ꢀ
/
18.8 Hz); d ꢃ
/
102.12
recrystallized from Et2O. H NMR (C6D6): d 7.06 (dd,
8H); d 6.83 (d, 8H); d 6.70 (t, 4H); d 2.11 (s, 15H).
(t, 8F) (JFÃF
Hz). Anal. Calc. for C34H15F20O4Ta: C, 38.95; H, 1.44.
ꢀ
/
20.7 Hz); d ꢃ104.16 (t, 8F) (JFÃF
/
ꢀ/22.5
Found: C, 38.80; H, 1.29.
Acknowledgements
4.9. h5-Pentamethylcyclopentadienyl tantalum
bis(pentachlorophenoxide) dichloride,
Cp*Ta(OC6Cl5)2Cl2
This work was carried out with financial support from
the Eastman Kodak Company, for which we are grate-
ful. Z.J.T. would also like to acknowledgement Thad
Wadas and James McGarrah for assistance with spec-
troscopic measurements.
This compound was prepared in an analogous fashion
to Cp*Ta(OC6F5)4 starting from 0.207 g of Cp*TaCl4
(0.452 mmol) and 0.484 g of pentachlorophenol (1.82
mmol). The resulting yellow residue was washed with
Et2O and filtered to give 0.322 g (78% yield) of a yellow
powder that was recrystallized from C6H5CH3/C5H12.
1H NMR (CD2Cl2): d 2.65 (s). Anal. Calc. for
C22H15Cl12O2Ta: C, 28.79; H, 1.65. Found: C, 30.30;
H, 1.60%.
References
[1] S. Paulson, B.P. Sullivan, J.V. Caspar, J. Am. Chem. Soc. 114
(1992) 6905.
[2] V.W.W. Yam, G.Z. Qi, K.K. Cheung, J. Chem. Soc., Dalton
Trans. (1998) 1819.
[3] K.S. Heinselman, M.D. Hopkins, J. Am. Chem. Soc. 117 (1995)
12340.
[4] K.S. Heinselman, V.M. Miskowski, S.J. Geib, L.C. Wang, M.D.
Hopkins, Inorg. Chem. 36 (1997) 5530.
4.10. h5-Pentamethylcyclopentadienyl tantalum bis(2,6-
dichlorophenoxide) dichloride, Cp*Ta(OC6H3Cl2)2Cl2
[5] D.S. Williams, A.V. Korolev, Inorg. Chem. 37 (1998) 3809.
[6] P. Jernakoff, C. de Merc de Belleton, G.L. Geoffroy, Organo-
metallics 6 (1987) 1362.
[7] A. van Asselt, B.J. Burger, V.C. Gibson, J.E. Bercaw, J. Am.
Chem. Soc. 108 (1986) 5347.
This compound was prepared in an analogous fashion
to Cp*Ta(OC6Cl5)2Cl2 starting from 0.110
g of
[8] V.C. Gibson, J.E. Bercaw, W.J. Bruton, Jr., R.D. Sanner,
Organometallics 5 (1986) 976.
Cp*TaCl4 (0.240 mmol) and 0.613 g of 2,6-dichloro-
phenol (1.00 mmol). The resulting yellow residue was
washed with C5H12 and filtered to give 0.095 g (56%
[9] G.H. Llinas, M. Mena, F. Palacios, P. Royo, R. Serrano, J.
Organomet. Chem. 340 (1988) 37.
1
[10] R.D. Sanner, S.T. Carter, Jr., W.J. Bruton, J. Organomet. Chem.
240 (1982) 157.
yield) of a yellow powder. H NMR (THF-d8): d 7.22
(d, 4H) (JHÃH
d 2.63 (s, 15H). Anal. Calc. for C22H21Cl6O2Ta: C,
ꢀ
/
6.5 Hz); d 6.78 (t, 2H) (JHÃH
ꢀ6.5 Hz);
/
[11] A.M. Cardoso, R.J.H. Clark, S. Moorhouse, J. Chem. Soc.,
Dalton Trans (1980) 1156.
[12] V.C. Gibson, T.P. Kee, J. Organomet. Chem. 444 (1993) 91.
37.16; H, 2.98. Found: C, 37.47 H, 3.14%.