Brightly phosphorescent trinuclear copper(I) complexes of pyrazolates: Substituent effects on the supramolecular structure and photophysics

Article abstract of DOI:10.1021/ja0427146

Synthetic details, solid-state structures, and photophysical properties of a group of trimeric copper(I) complexes containing pyrazolate ligands are described. The reaction of copper(I) oxide and the fluorinated pyrazoles [3-(CF₃)Pz]H, [3-(CF₃),5-(Me)Pz]H, and [3-(CF ₃),5-(Ph)Pz]H leads to the corresponding trinuclear copper(I) pyrazolates, {[3-(CF₃)Pz]Cu}₃, {[3-(CF₃),5-(Me) Pz]Cu}₃, and {[3-(CF₃),5-(Ph)Pz]Cu}₃, respectively, in high yield. The {[3,5-(i-Pr)₂Pz]Cu}₃ compound was obtained by a reaction between [Cu(CH₃CN) ₄][BF₄], [3,5-(i-Pr)₂Pz]H, and NEt₃. These compounds as well as {[3,5-(Me)₂Pz]Cu}₃ and {[3,5-(CF₃)₂Pz]Cu}₃ adopt trimeric structures with nine-membered Cu₃N₆ metallacycles. There are varying degrees and types of intertrimer Cu·Cu interactions. These contacts give rise to zigzag chains in the fluorinated complexes, {[3-(CF₃)Pz]Cu} ₃, {[3-(CF₃),5-(Me)Pz]Cu}₃, {[3-(CF ₃),5-(Ph)Pz]Cu}₃, and {[3,5-(CF₃) ₂Pz]Cu}₃, whereas the nonfluorinated complexes, {[3,5-(Me)₂Pz]Cu}₃ and {[3,5-(i-Pr)₂Pz]Cu} ₃ form dimers of trimers. Out of all the compounds examined in this study, {[3-(CF₃),5-(Ph)Pz]Cu}₃ has the longest (3.848 A) and {[3,5-(Me)₂Pz]Cu}₃ has the shortest (2.946 A) next-neighbor intertrimer Cu·Cu distance. The Cu·Cu separations within the trimer units do not vary significantly (typically 3.20-3.26 A). All of these trinuclear copper(I) pyrazolates show bright luminescence upon exposure to UV radiation. The luminescence bands are hugely red-shifted from the corresponding lowest-energy excitations, rather broad, and unstructured even at low temperatures, suggesting metal-centered emissions owing to intertrimer Cu·Cu interactions that are strengthened in the phosphorescent state. The {[3-(CF₃),5-(Ph)Pz]Cu}₃ compound exhibits an additional highly structured phosphorescence with a vibronic structure corresponding to the pyrazolyl (Pz) ring. The luminescence properties of solids and solutions of the trimeric compounds in this study show fascinating trends with dramatic sensitivities to temperature, solvent, concentration, and excitation wavelengths.

Full text of DOI:10.1021/ja0427146

Published on Web 04/29/2005
Brightly Phosphorescent Trinuclear Copper(I) Complexes of
Pyrazolates: Substituent Effects on the Supramolecular
Structure and Photophysics
H. V. Rasika Dias,*,† Himashinie V. K. Diyabalanage,† Maha G. Eldabaja,‡
Oussama Elbjeirami,‡ Manal A. Rawashdeh-Omary,‡ and Mohammad A. Omary*,‡
Contribution from the Department of Chemistry and Biochemistry, The UniVersity of Texas at
Arlington, Arlington, Texas 76019, and Department of Chemistry, UniVersity of North Texas,
Denton, Texas 76203
Received December 3, 2004; E-mail: dias@uta.edu; omary@unt.edu
Abstract: Synthetic details, solid-state structures, and photophysical properties of a group of trimeric copper-
(I) complexes containing pyrazolate ligands are described. The reaction of copper(I) oxide and the fluorinated
pyrazoles [3-(CF3)Pz]H, [3-(CF3),5-(Me)Pz]H, and [3-(CF3),5-(Ph)Pz]H leads to the corresponding trinuclear
copper(I) pyrazolates, {[3-(CF3)Pz]Cu}3, {[3-(CF3),5-(Me)Pz]Cu}3, and {[3-(CF3),5-(Ph)Pz]Cu}3, respectively,
in high yield. The {[3,5-(i-Pr)2Pz]Cu}3 compound was obtained by a reaction between [Cu(CH3CN)4][BF4],
[3,5-(i-Pr)2Pz]H, and NEt3. These compounds as well as {[3,5-(Me)2Pz]Cu}3 and {[3,5-(CF3)2Pz]Cu}3 adopt
trimeric structures with nine-membered Cu3N6 metallacycles. There are varying degrees and types of
intertrimer Cu‚‚‚Cu interactions. These contacts give rise to zigzag chains in the fluorinated complexes,
{[3-(CF3)Pz]Cu}3, {[3-(CF3),5-(Me)Pz]Cu}3, {[3-(CF3),5-(Ph)Pz]Cu}3, and {[3,5-(CF3)2Pz]Cu}3, whereas the
nonfluorinated complexes, {[3,5-(Me)2Pz]Cu}3 and {[3,5-(i-Pr)2Pz]Cu}3 form dimers of trimers. Out of all
the compounds examined in this study, {[3-(CF3),5-(Ph)Pz]Cu}3 has the longest (3.848 Å) and {[3,5-(Me)2Pz]-
Cu}3 has the shortest (2.946 Å) next-neighbor intertrimer Cu‚‚‚Cu distance. The Cu‚‚‚Cu separations within
the trimer units do not vary significantly (typically 3.20-3.26 Å). All of these trinuclear copper(I) pyrazolates
show bright luminescence upon exposure to UV radiation. The luminescence bands are hugely red-shifted
from the corresponding lowest-energy excitations, rather broad, and unstructured even at low temperatures,
suggesting metal-centered emissions owing to intertrimer Cu‚‚‚Cu interactions that are strengthened in
the phosphorescent state. The {[3-(CF3),5-(Ph)Pz]Cu}3 compound exhibits an additional highly structured
phosphorescence with a vibronic structure corresponding to the pyrazolyl (Pz) ring. The luminescence
properties of solids and solutions of the trimeric compounds in this study show fascinating trends with
dramatic sensitivities to temperature, solvent, concentration, and excitation wavelengths.
Introduction
is polymeric and exists in two distinct crystalline phases,
R-[Cu(Pz)]n and â-[Cu(Pz)]n, which differ in the interchain
The structure and properties of copper pyrazolates are of
significant interest.1 These compounds have been found to
exhibit a variety of structures ranging from polymers to trimers
with exo-bidentate coordination of the pyrazolate ligand to the
copper centers.1-15 For instance, [Cu(Pz)]n (1) (Pz ) pyrazolate)
Cu‚‚‚Cu contacts.11 Structurally characterized trinuclear copper-
(I) pyrazolates containing nine-membered Cu3N6 metallacycles
include {[3,5-(Me)2Pz]Cu}3,3 {[3,4,5-(Me)3Pz]Cu}3,10 {[3,5-
(Me)2,4-(NO2)Pz]Cu}3 (2),8 {[3,5-(i-Pr)2Pz]Cu}3,12 and {[3,5-
(CF3)2Pz]Cu}3.5 The trimeric compound, {[2-(3-Pz)Py]Cu}3, has
a pyridyl sidearm.16,17 Larger substituents in the 3-, 4-, and
5-positions at the pyrazole ring lead to tetrameric structures such
as in {[3,5-(t-Bu)2Pz]Cu}4 (3),12,14 {[3-(i-Pr),5-(t-Bu)Pz]Cu}4,12
{[3,5-(carbo-sec-butoxy)2Pz]Cu}4,14 and {[3,5-(i-Pr)2,4-(Br)Pz]-
Cu}4.18 A tetramer and a trimer derived from the same
† The University of Texas at Arlington.
‡ University of North Texas.
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9
10.1021/ja0427146 CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 7489-7501
7489

A R T I C L E S
Chart 1
Dias et al.
pyrazolate ligand are also known (e.g., {[3,5-(Ph)2Pz]Cu}4 and
{[3,5-(Ph)2Pz]Cu}3).6,7 Apart from the interests related to
structural diversity, some of these copper pyrazolates have been
utilized for examining interactions between closed-shell d10
copper systems,16,17 catalytic applications,6,14 and as luminescent
materials.15,19 Copper(I) pyrazolates also serve as excellent
precursors to obtain various mixed ligand complexes of copper,
for example, {[3,5-(CF3)2Pz]Cu(2,4,6-collidine)}2 and {[3,5-
(Me)2,4-(NO2)Pz]Cu(CNCy)2}2.8,20
An area of research focus in our laboratories concerns
structural and spectroscopic studies of copper complexes
featuring fluorinated pyrazolates.19,20 In contrast to the nonflu-
orinated analogues, copper complexes of fluorinated pyrazolates
are rare.1,2 Dias et al. have described the synthesis of a copper-
(I) complex containing a heavily fluorinated pyrazolate, {[3,5-
(CF3)2Pz]Cu}3 (4).5 We have recently communicated that solids
and glassy solutions of this molecule exhibit bright luminescence
that can be fine- and coarse-tuned to multiple bright visible
colors by varying the solvent, concentration, temperature, and
excitation wavelength.19 These types of neutral metal complexes
containing fluorinated ligands are of particular interest as
emitting materials for molecular light-emitting devices (MOLEDs)
because fluorination facilitates thin-film fabrication and because
the presence of a closed-shell transition metal should enhance
the phosphorescence.21-25 Fluorinated ligands also endow other
beneficial properties to metal adducts, such as improved thermal
and oxidative stability and reduced concentration quenching.23-27
An interesting study of the luminescence properties of copper-
(I) pyrazolates was reported recently by Aida et al.15 Although
these molecules do not contain fluorinated ligands, they
demonstrated that poly(benzyl ether)-substituted pyrazolate
adducts of copper(I) can be employed to form brightly lumi-
nescent superhelical fibers. It is also noteworthy that in contrast
to the AuI adducts, photophysical studies of such trimetallic
species involving the lighter coinage metal family members,
CuI and AgI, have not received much attention.28-30 In this
paper, we describe the synthesis of several trimeric copper(I)
adducts of fluorinated pyrazolates, their solid-state structures,
and photophysical properties. To investigate the possible effects
of the electron-withdrawing and -donating substituents on the
luminescence and structure of these materials, we also include
in this study the nonfluorinated compounds, {[3,5-(Me)2Pz]Cu}3
and {[3,5-(i-Pr)2Pz]Cu}3.3,12
Experimental Section
General Procedures. All manipulations were carried out under an
atmosphere of purified nitrogen using standard Schlenk techniques.
Solvents were purchased from commercial sources, distilled from
conventional drying agents, and degassed by the freeze-pump-thaw
method twice prior to use. The glassware was oven-dried at 150 °C
overnight. NMR spectra were recorded at 25 °C on a JEOL Eclipse
500 spectrometer (1H, 500.16 MHz; 13C, 125.78 MHz; 19F, 470.62
MHz). Proton and carbon chemical shifts are reported in parts per
million versus Me4Si. 19F NMR chemical shifts were referenced relative
to external CFCl3. Infrared spectra were recorded on a JASCO FT-IR
410 spectrometer. Melting points were obtained on a Mel-Temp II
(18) Dias, H. V. R.; Diyabalanage, H. V. K. Unpublished results.
(19) Dias, H. V. R.; Diyabalanage, H. V. K.; Rawashdeh-Omary, M. A.;
Franzman, M. A.; Omary, M. A. J. Am. Chem. Soc. 2003, 125, 12072-
12073.
(25) Zhang, J.; Kan, S.; Ma, Y.; Shen, J.; Chan, W.; Che, C. Synth. Metals
2001, 121, 1723-1724 and references therein.
(26) Dias, H. V. R.; Lu, H.-L.; Kim, H.-J.; Polach, S. A.; Goh, T. K. H. H.;
Browning, R. G.; Lovely, C. J. Organometallics 2002, 21, 1466-1473 and
references therein.
(20) Omary, M. A.; Rawashdeh-Omary, M. A.; Diyabalanage, H. V. K.; Dias,
H. V. R. Inorg. Chem. 2003, 42, 8612-8614.
(21) Haneline, M. R.; Tsunoda, M.; Gabbaie, F. P. J. Am. Chem. Soc. 2002,
124, 3737-3742.
(27) Dias, H. V. R.; Kim, H.-J.; Lu, H.-L.; Rajeshwar, K.; de Tacconi, N. R.;
Derecskei-Kovacs, A.; Marynick, D. S. Organometallics 1996, 15, 2994-
3003 and references therein.
(22) Omary, M. A.; Kassab, R. M.; Haneline, M. R.; Elbjeirami, O.; Gabbaie,
F. P. Inorg. Chem. 2003, 42, 2176-2178.
(23) Adachi, C.; Baldo, M. A.; Forrest, S. R. J. Appl. Phys. 2000, 87, 8049-
(28) Yam, V. W.-W.; Lo, K. K. W. Mol. Supramol. Photochem. 1999, 4, 31-
8055.
112.
(24) Grushin, V. V.; Herron, N.; LeCloux, D. D.; Marshall, W. J.; Petrov, V.
A.; Wang, Y. Chem. Commun. 2001, 1494-1495.
(29) Yam, V. W.-W.; Lo, K. K.-W. Chem. Soc. ReV. 1999, 28, 323-334.
(30) Ford, P. C.; Cariati, E.; Bourassa, J. Chem. ReV. 1999, 99, 3625-3648.
9
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Phosphorescent Trinuclear Copper(I) Complexes of Pyrazolates
A R T I C L E S
apparatus and were not corrected. Elemental analyses were performed
using a Perkin-Elmer Model 2400 CHN analyzer. Cu2O was purchased
from commercial sources. [3-(CF3)Pz]H,31 [3-(CF3),5-(Me)Pz]H,32 and
[3-(CF3),5-(Ph)Pz]H33 were prepared by published methods. {[3,5-
(CF3)2Pz]Cu}3 and {[3,5-(Me)2Pz]Cu}3 were isolated using slightly
modified literature procedures as described below. Tetrakis(acetonitrile)-
copper(I) tetrafluoroborate was prepared by following a procedure
analogous to that reported for [Cu(CH3CN)4](PF6) but employing
aqueous HBF4 instead of HPF6.34
reaction medium (see the procedure for {[3-(CF3)Pz]Cu}3). {[3,5-(CF3)2-
Pz]Cu}3 can be purified either by recrystallizing using hexane or by
vacuum sublimation at 80 °C/3.0 mmHg. Mp: 188-190 °C.
{[3,5-(Me)2Pz]Cu}3: This was prepared using the published pro-
cedure.3 It produces a mixture of colorless trimers, {[3,5-(Me)2Pz]Cu}3,
and a brown-red, polymeric solid with a composition {[3,5-
(Me)2Pz]2Cu}n. Due to the high insolubility, it is difficult to separate
the desired product from the crude mixture. Usually, the trimer
separation has been achieved by manually separating the crystals.
However, we found that the trimers could be obtained very conveniently
in pure form by the vacuum sublimation of the crude mixture at 245
°C/3 mmHg. Mp: 325-327 °C.
{[3,5-(i-Pr)2Pz]Cu}3: A 100 mL, two-necked, round-bottomed flask
equipped with a magnetic stirrer was connected to a nitrogen line. The
flask was purged thoroughly with nitrogen and charged with 3,5-
diisopropylpyrazole (0.50 g, 3.30 mmol) and acetone (20.0 mL). [Cu-
(CH3CN)4][BF4] (0.52 g, 1.65 mmol) was added while stirring. After
the solution became clear, degassed triethylamine (1.0 mL) was added
to the mixture over a period of 1 min. The product precipitated as a
white solid. The resulting suspension was stirred for 30 min and then
filtered. The product was washed with acetone (15.0 mL) and hexane
(5.0 mL) and dried under reduced pressure. X-ray quality crystals were
obtained from hexane at 5 °C. Yield: 88% Mp: 158-160 °C. 1H NMR
(CDCl3): δ 1.34 (d, 36H, J ) 6.5 Hz, CH3), 3.08 (septet, 6H, CH),
5.91 (s, 3H, H4). 13C{1H} NMR (CDCl3): δ 23.6 (CH3), 28.7 (CH),
96.3 (s, Pz-C4), 160.7 (C-iPr). IR (KBr, cm-1): 3608, 3583, 2925, 2855,
2723, 1996, 1734, 1717, 1699, 1685, 1653, 1558, 1540, 1522, 1459,
1377, 1300, 1174, 1104, 1051, 796, 779. Anal. Calcd for C27H45N6-
Cu3: C, 50.33; H, 7.04; N, 13.04. Found: C, 50.43; H, 7.30; N, 13.12.
5
3
{[3-(CF3)Pz]Cu}3: Cu2O (0.29 g, 1.90 mmol), 3-trifluorometh-
ylpyrazole (0.50 g, 3.68 mmol), and acetonitrile (0.25 mL) were mixed
in about 40.0 mL of degassed benzene. The resulting mixture was heated
at 50-60 °C overnight under nitrogen. After cooling, the solution was
filtered through a bed of Celite to remove some insoluble material.
The filtrate was collected, and the solvent was removed under reduced
pressure to obtain crude {[3-(CF3)Pz]Cu}3 as a colorless solid. X-ray
quality crystals were grown from hexane at 5 °C. Yield: 85%. Mp:
156-157 °C. 1H NMR (CDCl3): δ 6.65 (br s, 3H, H4), 7.67 (br s, 3H,
1
H5). 13C{1H} NMR (CDCl3): δ 104.3 (Pz-C4), 122.0 (CF3, q, JCF
)
2
268 Hz), 140.2 (Pz-C5), 141.1, (CCF3, q, JCF ) 36.0 Hz). 19F NMR
(CDCl3): δ -61.4. IR (KBr, cm-1): 3853, 3745, 3159, 2962, 1735,
1653, 1521, 1457, 1380, 1350, 1308, 1240, 1171, 1154, 1121, 1083,
1003, 957, 879, 782, 743, 719, 656, 550. Anal. Calcd for C12H6F9N6-
Cu3: C, 24.19; H, 1.01; N, 14.10. Found: C, 24.32; H, 1.31; N, 14.20.
{[3-(CF3),5-(Me)Pz]Cu}3: Cu2O (0.15 g, 1.11 mmol) and 3-triflu-
oromethyl-5-methylpyrazole (0.50 g, 3.33 mmol) were mixed in
benzene (40.0 mL). Acetonitrile (1.00 mL) was added, and the resulting
mixture was refluxed for 12 h. After cooling, the solution was filtered
through a bed of Celite to remove some insoluble material. The filtrate
was collected, and the solvent was removed under reduced pressure to
obtain crude {[3-(CF3),5-(Me)Pz]Cu}3 as a colorless solid. X-ray quality
crystals were obtained from dichloromethane at 5 °C. Yield: 92%.
X-ray Structure Determination. A suitable crystal covered with a
layer of hydrocarbon oil was selected and mounted with paratone-N
oil on a cryo-loop and immediately placed in the low-temperature
nitrogen stream. The X-ray intensity data were measured at 100(2) K
on a Bruker SMART APEX CCD area detector system equipped with
an Oxford Cryosystems 700 series Cryostream cooler, a graphite
monochromator, and a Mo KR fine-focus sealed tube (λ ) 0.710 73
Å). The detector was placed at a distance of 5.995 cm from the crystal.
The data frames were integrated with the Bruker SAINT-Plus software
package. The data were corrected for absorption effects using the
multiscan technique (SADABS).
1
Mp: 214-215 °C. H NMR (CDCl3): δ 2.28 (s, 6H, CH3), 2.32 (s,
3H, CH3), 6.37 (s, 3H, H4). 13C{1H} NMR (CDCl3): δ 13.0 (s, CH3),
103.5 (Pz-C4), 121.1 (CF3, q, 1JCF ) 276 Hz), 143.2 (CCF3, q, 2JCF
)
39 Hz), 150.6 (s, CCH3). 19F NMR (CDCl3): δ -59.8 (br s, CF3, 6F),
-60.4 (br s, CF3, 3F). IR (KBr, cm-1): 3608, 3132, 2925, 1597, 1539,
1350, 1244, 1169, 1128, 1083, 1010, 798, 761, 719, 685, 664. Anal.
Calcd for C15H12F9N6Cu3: C, 28.24; H, 1.90; N, 13.17. Found: C,
28.20; H, 1.74; N, 13.33.
{[3-(CF3),5-(Ph)Pz]Cu}3: Cu2O (0.14 g, 4.72 mmol) and 3-triflu-
oromethyl-5-phenylpyrazole (1.00 g, 4.72 mmol) were mixed in about
40.0 mL of benzene. Acetonitrile (0.25 mL) was added, and the resulting
mixture was refluxed for 12 h. After cooling, the solution was filtered
through a bed of Celite to remove some insoluble material. The filtrate
was collected, and the solvent was removed under reduced pressure to
obtain crude {[3-(CF3),5-(Ph)Pz]Cu}3 as a colorless solid. X-ray quality
crystals were obtained from hexane at 5 °C. Yield: 90%. Mp: 123-
125 °C. 1H NMR (CDCl3): δ 6.69 (s, 3H, H4), 7.42 (m, 9H, Ph), 7.54
(m, 6H, Ph). 13C{1H} NMR (CDCl3): δ 102.1 (Pz-C4), 121.0 (CF3, q,
Compounds {[3-(CF3),5-(Me)Pz]Cu}3, {[3-(CF3),5-(Ph)Pz]Cu}3, {[3,5-
(CF3)2Pz]Cu}3, and {[3,5-(i-Pr)2Pz]Cu}3 form good quality crystals for
crystallography. Data collection for these compounds and the structure
solution and refinement proceeded smoothly. {[3-(CF3)Pz]Cu}3 forms
small, very thin needles. Nevertheless, we were able to collect sufficient
data for {[3-(CF3)Pz]Cu}3 and solve the structure, as well. It crystallizes
with two {[3-(CF3)Pz]Cu}3 moieties in the asymmetric unit. The struc-
tures of compounds described in this manuscript were solved and refined
using the Bruker SHELXTL (version 6.14) software package. All the
non-hydrogen atoms were refined anisotropically. The hydrogen atoms
were included at calculated positions during the refinement. Further
details of the data collection and refinements are given in Table 1.
2
1JCF ) 250 Hz), 126.2, 129.3, 129.6, 144.5 (CCF3, q, JCF ) 37 Hz),
145.6, 149.2. 19F NMR (CDCl3): δ -61.2. IR (KBr, cm-1): 3120,
2956, 1734, 1438, 1346, 1279, 1254, 1154, 1065, 1020, 985, 913, 841,
809, 760, 719, 690, 507. Anal. Calcd for C30H18F9N6Cu3: C, 43.72; H,
2.20; N, 10.20. Found: C, 44.10; H, 2.05; N, 9.98.
{[3,5-(CF3)2Pz]Cu}3: This compound could be obtained from a
reaction between [3,5-(CF3)2Pz]H and copper(I) oxide in benzene, as
reported earlier.5 However, for some reason, certain runs gave fairly
low yields of the expected trimer. We found that this problem could
be solved by using benzene containing a few drops of CH3CN as the
Photophysical Measurements. Steady-state luminescence spectra
were acquired with a PTI QuantaMaster Model QM-4 scanning
spectrofluorometer. The excitation and emission spectra were corrected
for the wavelength-dependent lamp intensity and detector response,
respectively. Lifetime data were acquired using fluorescence and
phosphorescence subsystem add-ons to the PTI instrument. The pulsed
excitation source was generated using the 337.1 nm line of the N2 laser
pumping a freshly prepared 1 × 10-2 M solution of the continuum
laser dye, Coumarin-540A, in ethanol, the output of which was
appropriately tuned and frequency doubled to attain the excitation
wavelengths needed based on the luminescence excitation spectra for
each compound. Cooling in temperature-dependent measurements for
the crystals was achieved using an Oxford optical cryostat, model
Optistat CF ST, interfaced with a liquid nitrogen or liquid helium tank.
Absorption spectra were acquired with a Perkin-Elmer Lambda 900
(31) Gerus, I. I.; Gorbunova, M. G.; Vdovenko, S. I.; Yagupol’skii, Y. L.;
Kukhar, V. P. Zh. Org. Khim. 1990, 26, 1877-1883.
(32) Atwood, L. J.; Dixon, K. R.; Eadie, D. T.; Stobart, S. R.; Zaworotko, M.
1
J. Inorg. Chem. 1983, 22, 774-779. (Mp: 80-82 °C. H NMR (CDCl3):
δ 2.33 (s, 3H, CH3), 6.30 (s, 1H, H4), 12.4 (br s, NH). 19F NMR (CDCl3):
δ -61.7.)
(33) Dias, H. V. R.; Goh, T. K. H. H. Polyhedron 2004, 23, 273-282, and
references therein.
(34) Kubas, G. J. Inorg. Synth. 1979, 19, 90-92.
9
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A R T I C L E S
Dias et al.
Table 1. X-ray Crystallographic Data for {[3-(CF3)Pz]Cu}3, {[3-(CF3),5-(Me)Pz]Cu}3, {[3-(CF3),5-(Ph)Pz]Cu}3, {[3,5-(CF3)2Pz]Cu}3, and
{[3,5-(i-Pr)2Pz]Cu}3
{[3-(CF3)Pz]Cu
}
{[3-(CF3),5-(Me)Pz]Cu
}
{
[3-(CF3),5-(Ph)Pz]Cu
}
{[3,5-(CF3)2Pz]Cu
}
{[3,5-(i-Pr)2Pz]Cu}
3
3
3
3
3
empirical formula
C12H6Cu3F9N6
595.85
100(2) K
0.71073 Å
monoclinic
P21/c
C15H12Cu3F9N6
637.93
100(2) K
0.71073 Å
monoclinic
Cc
C30H18Cu3F9N6
824.12
100(2) K
0.71073 Å
monoclinic
P21/c
C15H3Cu3F18N6
799.85
100(2) K
0.71073 Å
triclinic
C27H45Cu3N6
644.31
100(2) K
0.71073 Å
monoclinic
P21/c
formula weight
temperature
wavelength
crystal system
space group
unit cell
P1h
dimensions
a
15.0852(7) Å
18.0948(9) Å
13.1160(6) Å
90°
12.7722(5) Å
22.3549(9) Å
7.4500(3) Å
90°
11.7498(4) Å
19.5123(7) Å
13.1595(5) Å
90°
9.1553(4) Å
11.3906(5) Å
12.2324(5) Å
66.4070(10)°
74.4530(10)°
78.5470(10)°
1120.21(8) Å3
2
14.3989(14) Å
13.4806(13) Å
17.2240(16) Å
90°
b
c
R
â
100.7710(10)°
90°
107.7660(10)°
90°
90.6880(10)°
90°
112.324(2)°
90°
γ
volume
3517.1(3) Å3
8
2025.69(14) Å3
4
3016.80(19) Å3
4
3092.7(5) Å3
4
Z
density (calcd)
absorption
coefficient
F(000)
2.251 Mg/m3
3.699 mm-1
2.092 Mg/m3
3.219 mm-1
1.814 Mg/m3
2.185 mm-1
2.371 Mg/m3
2.992 mm-1
1.384 Mg/m3
2.069 mm-1
2304
1248
1632
768
1344
R1, wR2 [I>2σ(I)]
R1, wR2 (all data)
0.0598, 0.1467
0.0949, 0.1699
0.0220, 0.0583
0.0226, 0.0585
0.0310, 0.0789
0.0348, 0.0809
0.0356, 0.0939
0.0383, 0.0968
0.0248, 0.0657
0.0279, 0.0672
double-beam UV-vis-NIR spectrophotometer for solutions of crystal-
line samples prepared in HPLC-grade acetonitrile using standard 1 cm
quartz cuvettes. Luminescence and lifetime studies for frozen solutions
were conducted for selected samples by placing a 5 mm Suprasil quartz
cylindrical tube containing the appropriate solution in a liquid nitrogen-
filled dewar flask with a Suprasil quartz cold finger and then inserting
this setup in the sample compartment of the PTI instrument.
{[3-(CF3)Pz]Cu}3, {[3-(CF3),5-(Me)Pz]Cu}3, {[3-(CF3),5-
(Ph)Pz]Cu}3, {[3,5-(CF3)2Pz]Cu}3, and {[3,5-(i-Pr)2Pz]Cu}3 are
soluble in most common organic solvents, such as benzene,
toluene, THF, hexane, and CH2Cl2. Compound {[3,5-(Me)2Pz]-
Cu}3, as indicated above, is insoluble in organic solvents. It is
an air-stable solid. The relative air stability of the other copper
adducts also merits comment. On the basis of our observations,
{[3-(CF3),5-(Me)Pz]Cu}3 and {[3-(CF3),5-(Ph)Pz]Cu}3 are quite
air stable in the solid state as well as in solution. They survive
in solution without decomposition for several weeks. {[3-
(CF3),5-(Me)Pz]Cu}3 is the more stable compound of the two.
Trimeric {[3-(CF3)Pz]Cu}3 is the least air-stable solid of all the
fluorinated copper(I) complexes described in this work. It
decomposes easily in solution as well as in the solid state. Even
though nonfluorinated, {[3,5-(i-Pr)2Pz]Cu}3 is relatively air
stable in the solid state (for 2-3 days); once dissolved in organic
solvents, it quickly decomposes, forming a green solution.
The 19F NMR spectra of {[3-(CF3)Pz]Cu}3 and {[3-(CF3),5-
(Ph)Pz]Cu}3 show only one signal each, with a chemical shift
that is very close to that of the respective free ligand. Similarly,
apart from the disappearance of the peak corresponding to the
pyrazole N-H, other proton resonances in the 1H NMR spectra
do not show a notable shift going from the free pyrazoles to
the respective copper adducts, {[3-(CF3)Pz]Cu}3 and {[3-
(CF3),5-(Ph)Pz]Cu}3. Interestingly, {[3-(CF3),5-(Me)Pz]Cu}3
displays two broad peaks in the 19F NMR spectrum. The methyl
groups also appear as two signals close to one another in the
1H NMR spectrum.
Results and Discussion
Synthesis of Copper(I) Pyrazolates. The reaction of copper-
(I) oxide and fluorinated pyrazoles, [3-(CF3)Pz]H, [3-(CF3),5-
(Me)Pz]H, and [3-(CF3),5-(Ph)Pz]H, in benzene containing a
few drops of acetonitrile leads to the corresponding copper(I)
pyrazolates, {[3-(CF3)Pz]Cu}3, {[3-(CF3),5-(Me)Pz]Cu}3, and
{[3-(CF3),5-(Ph)Pz]Cu}3, respectively, in high yield (eq 1). We
have reported the synthesis of {[3,5-(CF3)2Pz]Cu}3 following
a similar method using just benzene as the solvent.5 However,
the addition of a few drops of acetonitrile improves the
reproducibility and leads to consistently high yields. The {[3,5-
(i-Pr)2Pz]Cu}3 was obtained in 88% yield by a reaction between
[Cu(CH3CN)4][BF4] and {[3,5-(i-Pr)2Pz]H in the presence of
NEt3 (eq 2). While this work was underway, the synthesis of
{[3,5-(i-Pr)2Pz]Cu}3 via a different route (i.e., using [3,5-(i-
Pr)2Pz]Na and CuCl; yield ) 37%) was reported by Fujisawa
et al.12 The {[3,5-(Me)2Pz]Cu}3 was prepared by the published
method,3 but the purification was achieved by a more convenient
and efficient vacuum sublimation process, rather than by
manually separating the crystals from the crude mixture as
described previously.
X-ray Crystal Structures. Single-crystal X-ray analysis of
{[3-(CF3)Pz]Cu}3 reveals a trinuclear structure (Figure 1) that
features an asymmetrically oriented pyrazolyl group; the sub-
stituents at the pyrazolyl group’s 3- and 5-positions (H and CF3)
do not show a regular, alternating pattern. The pyrazolyl-bridged
copper ions feature a linear geometry. The nine-membered
Cu3N6 metallacycle is essentially planar. There are long inter-
triangle contacts involving the copper atoms forming extended
chains (Figure 1). These Cu‚‚‚Cu distances range from 3.100
to 3.482 Å. The intratrimer Cu‚‚‚Cu separations within the
copper triangles (“[Cu3]” units) range from 3.214 to 3.264 Å.
These distances are much longer than the estimated sum of the
3[Pz′]H + 1.5 Cu2O f {[Pz′]Cu}3 + 1.5 H2O
(1)
where
Pz′ ) [3-(CF3)Pz], [3-(CF3),5-(Me)Pz],
[3-(CF3),5-(Ph)Pz], or [3,5-(CF3)2Pz]
3[Cu(CH3CN)4][BF4] + 3[3,5-(i-Pr)2Pz]H + 3 Et3N f
{[3,5-(i-Pr)2Pz]Cu}3 + 3 [Et3NH][BF4] (2)
9
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Phosphorescent Trinuclear Copper(I) Complexes of Pyrazolates
A R T I C L E S
Figure 1. X-ray crystal structure of {[3-(CF3)Pz]Cu}3 showing the labeling
scheme of a molecular unit (left) and the packing of the Cu3N6 metallacycles
(right).
Figure 3. X-ray crystal structure of {[3-(CF3),5-(Ph)Pz]Cu}3 showing the
labeling scheme of a molecular unit (left) and the packing of the Cu3N6
metallacycles (right).
Figure 4. X-ray crystal structure of {[3,5-(CF3)2Pz]Cu}3 showing the
labeling scheme of a molecular unit (left) and the packing of the Cu3N6
metallacycles (right).
Figure 2. X-ray crystal structure of {[3-(CF3),5-(Me)Pz]Cu}3 showing the
labeling scheme of a molecular unit (left) and the packing of the Cu3N6
metallacycles (right).
van der Waals radii of two copper atoms (2.80 Å) or the Cu-
Cu distance in the open-shell metallic copper (2.556 Å).35
The X-ray crystal structure of {[3-(CF3),5-(Me)Pz]Cu}3 is
illustrated in Figure 2. This molecule also exhibits asymmetri-
cally oriented CF3 groups and does not show alternating (CF3,
Me, CF3, Me, ...) pattern in the periphery. The copper ions
feature a linear geometry. The nine-membered Cu3N6 metalla-
cycle is essentially planar. The [Cu3] units form extended chains
(Figure 2). The intertrimer Cu‚‚‚Cu separations range from 3.704
to 3.915 Å. The Cu‚‚‚Cu separations within the [Cu3] units range
from 3.201 to 3.245 Å.
The solid-state structure of {[3-(CF3),5-(Ph)Pz]Cu}3 (Figure
3) reveals a typical triangular arrangement of copper atoms with
pyrazolyl moieties occupying the edge positions. Just as in
{[3-(CF3)Pz]Cu}3 and {[3-(CF3),5-(Me)Pz]Cu}3, the arrange-
ment of the pyrazolyl groups in {[3-(CF3),5-(Ph)Pz]Cu}3 is not
symmetrical. The copper atoms are linearly coordinated by the
two nitrogens of the [3-(CF3),5-(Ph)Pz]- ligands. The nine-
membered Cu3N6 metallacycle shows significant deviation from
planarity, perhaps as a result of the adverse steric effects of
having the CF3 and Ph substituents on the neighboring pyrazolyl
ligands. The {[3,5-(Ph)2Pz]Cu}3 compound also exhibits a
distorted Cu3N6 metallacycle. The intratrimer Cu‚‚‚Cu separa-
tions within the [Cu3] units of {[3-(CF3),5-(Ph)Pz]Cu}3 range
from 3.147 to 3.258 Å. The inter-triangle contacts between
copper atoms are rather long (3.848 and 4.636 Å, alternating
with one another in the infinite zigzag chains of trimers; see
Figure 5. Packing of the Cu3N6 metallacycles in {[3,5-(Me)2Pz]Cu}3 (left)
and {[3,5-(i-Pr)2Pz]Cu}3 (right).
Figure 3). The extended packing of the [Cu3] units of {[3-
(CF3),5-(Ph)Pz]Cu}3 is similar to that observed in {[3,5-(CF3)2-
Pz]Cu}3. However, {[3,5-(CF3)2Pz]Cu}3 features a planar nine-
membered metallacycle (Figure 4) and somewhat shorter
intertrimer Cu‚‚‚Cu separations (3.813 and 3.987 Å, based on
the data collected at 100 K).
The X-ray crystal structure of the nonfluorinated adduct,
{[3,5-(Me)2Pz]Cu}3, was reported earlier.3 To make sure that
we use well-defined material with known solid-state structures
for the luminescence work, we re-collected the data for the
{[3,5-(Me)2Pz]Cu}3 crystals obtained by the new sublimation
method. The solid-state structure of the isopropyl analogue,
{[3,5-(i-Pr)2Pz]Cu}3, was also determined at 100 K. The unit
cell parameters are similar to those observed at room temperature
for {[3,5-(Me)2Pz]Cu}3 and at 208 K for {[3,5-(i-Pr)2Pz]Cu}3.3,12
Both of these copper adducts show significant Cu‚‚‚Cu interac-
tions between the [Cu3] units, leading to the formation of isolated
dimers of trimers (Figure 5). The shortest intertrimer Cu‚‚‚Cu
distances in {[3,5-(Me)2Pz]Cu}3 and {[3,5-(i-Pr)2Pz]Cu}3 are
2.946 and 2.989 Å, respectively. The intertrimer Cu‚‚‚Cu
(35) (a) Bondi, A. J. Phys. Chem. 1964, 68, 441-451. (b) Winter, M. http://
www.webelements.com/, 2004.
9
J. AM. CHEM. SOC. VOL. 127, NO. 20, 2005 7493

A R T I C L E S
Dias et al.
Table 2. Bond Distances (Å) and Bond Angles (deg) for Selected Pyrazolatocopper(I) Trimers
{[3-(CF3)Pz]Cu}3
1.845(8)-1.876(7)
3.214-3.264
{[3-(CF3),5-(Me)Pz]Cu}3
1.852(3)-1.863(3)
3.201-3.245
{[3-(CF3),5-(Ph)Pz]Cu}3
1.8604(19)-1.8653(18)
3.147-3.258
{[3,5-(CF3)2Pz]Cu}3
1.855(2)-1.863(2)
3.218-3.247
{[3,5-(Me)2Pz]Cu}3
1.845(4)-1.858(4)
3.195-3.257
2.946
173.4(2)-175.3(2)
ref 3
Cu-N
Cu‚‚‚Cu (intra)
Cu‚‚‚Cu (inter)
N-Cu-N
ref
3.100-3.482
3.704-3.915
3.848-4.636
3.813-3.987
174.5(3)-178.2(3)
this work
178.07(12)-178.97(12)
this work
174.17(8)-175.83(8)
this work
178.39(10)-178.58(10)
this work
{[3,4,5-(Me)3Pz]Cu}3
1.848(4)-1.855(4)
3.212
{[3,5-(i-Pr)2Pz]Cu}3
1.8448(10)-1.8628(10)
3.195-3.235
{[3,5-(Ph)2Pz]Cu}3
2.041(7)-2.105(7)
3.280-3.406
5.400
169.2(3)-178.6(3)
ref 7
{[2-(3-Pz)Py]Cu}3
1.864(7)-1.895(7)
3.52
Cu-N
Cu‚‚‚Cu (intra)
Cu‚‚‚Cu (inter)
N-Cu-N
ref
3.069
2.989
2.905
173.9(2)-174.6(2)
ref 10
169.84(4)-176.89(5)
this work
166.5(3)-170.8(3)
ref 16
contacts of {[3,5-(Me)2Pz]Cu}3 are nearly orthogonal to the
[Cu3] planes. In contrast, the two [Cu3] units of {[3,5-(i-Pr)2Pz]-
Cu}3 adopt a more flattened chair (i.e., a “lazy boy” recliner)
arrangement with intratrimer Cu‚‚‚Cu separations ranging from
3.195 to 3.235 Å.
Overall, {[3-(CF3)Pz]Cu}3, {[3-(CF3),5-(Me)Pz]Cu}3, and
{[3-(CF3),5-(Ph)Pz]Cu}3 represent rare copper(I) pyrazolates
featuring fluorinated substituents. Only a few other trimetallic
copper(I) pyrazolates have been reported. These include [3,5-
(Me)2,4-(X)Pz]Cu}3 (where X ) H, Cl, Br, I, Me),3,4,10 {[2-
(3-Pz)Py]Cu}3,16 [3,5-(Ph)2Pz]Cu}3,7 [3,5-(p-Me-Ph)2Pz]Cu}3,1
[3,5-(p-Cl-Ph)2Pz]Cu}3,1 {[3,5-(Me)2,4-(NO2)Pz]Cu}3,8 {[3,5-
(i-Pr)2Pz]Cu}3,12 and the highly fluorinated complex, {[3,5-
(CF3)2Pz]Cu}3.5 The number of such complexes that have been
characterized using X-ray crystallography is even fewer among
the aforementioned compounds.
Selected bond distances and angles of trimeric copper(I)
pyrazolates are listed in Table 2. In general, despite the weakly
donating nature of fluorinated pyrazoles, there is essentially no
difference between the Cu-N bond distances of the fluorinated
versus nonfluorinated systems. In the absence of adverse steric
interactions, the inter-triangle Cu‚‚‚Cu contacts appear to be
shorter in the nonfluorinated systems. For example, {[3,5-(i-
Pr)2Pz]Cu}3 and {[3,5-(Me)2Pz]Cu}3 show Cu‚‚‚Cu separations
between [Cu3] units at 2.9-3.0 Å. {[3,4,5-(Me)3Pz]Cu}3 also
has short Cu‚‚‚Cu separations (see Table 2). The intertrimer
Cu‚‚‚Cu contacts in these compounds result in the formation
of dimers of [Cu3] units. The 3,5-diphenyl-substituted analogue,
{[3,5-(Ph)2Pz]Cu}3, represents an exception.7 It features severely
distorted, fairly well-separated [Cu3] units, perhaps as a result
of the adverse steric effects of having two bulky Ph substituents
on neighboring pyrazolyl ligands.
whereas the fluorinated analogue features significantly longer
Cu‚‚‚Cu separations and chains of [Cu3] units. The pyrazoles
in the two compounds, {[3,4,5-(Me)3Pz]Cu}3 and {[3,5-(Me)2,4-
(NO2)Pz]Cu}3, are also close sterically but somewhat different
electronically. They show a similar trend; that is, {[3,4,5-
(Me)3Pz]Cu}3 forms dimers of [Cu3] units, while {[3,5-(Me)2,4-
(NO2)Pz]Cu}3 forms chains of [Cu3] with longer Cu‚‚‚Cu
distance. These data seem to indicate that electron-withdrawing
groups weaken the intertrimer Cu‚‚‚Cu contacts and favor long-
range interactions in infinite chains of trimers instead of dimers
of trimers.
The intratrimer and intertrimer CuI‚‚‚CuI separations observed
for the trinuclear species herein are to be placed in the proper
perspective as to what extent do they constitute bonding
interactions. This is important because the topic of Cu-Cu
bonding in Cu(I) complexes has elicited a great deal of
discussion and controversy.38 Work by Cotton and co-workers
has established the absence of a CuI-CuI bond39 in the covalent
sense known for complexes in which the transition metal has
partially occupied d-orbitals, for which M-M bonds with a bond
order of up to four are known.40 This absence of “significant
covalent bonding” has been concluded to be the case even when
CuI‚‚‚CuI separations are extremely short--as short as 2.35
Å!39 A combination of strong CuI-ligand bonding and very
small bite distances (e.g., 2.2 Å) of the bridging ligands leads
to the short CuI‚‚‚CuI contacts in such cases. Plentiful examples
exist for short CuI‚‚‚CuI separations that are largely dictated
either by ligand assistance or electrostatic interactions (see refs
39 and 41 and relevant citations therein). Nevertheless, there
are some studies that suggest genuine intramolecular cupro-
philicity based on spectroscopic evidence that includes reso-
42
nance Raman bands attributed to υCu-Cu
.
Thus, we conclude
The fluorinated systems form chains of [Cu3] units with
closest intertrimer Cu‚‚‚Cu contacts ranging from 3.100 Å (as
in {[3-(CF3)Pz]Cu}3) to 3.848 Å (for {[3-(CF3),5-(Ph)Pz]Cu}3).
The nonfluorinated system, {[3,5-(Me)2,4-(NO2)Pz]Cu}3, also
packs as chains of [Cu3] with intertrimer Cu‚‚‚Cu separations
of 3.261 Å. Although this compound does not have fluorinated
substituents such as CF3, on the pyrazole rings, it has nitro
groups that are even more electron-withdrawing. We note that
{[3,5-(i-Pr)2Pz]Cu}3 and {[3,5-(CF3)2Pz]Cu}3 contain pyrazolyl
ligands bearing substituents that are somewhat similar steri-
cally36,37 but rather different electronically.33 Therefore, the
structures of these two compounds may provide some clues as
to how ligand electronic effects influence the packing of [Cu3]
units. As mentioned above, the nonfluorinated {[3,5-(i-Pr)2Pz]-
Cu}3 exhibits shorter Cu‚‚‚Cu contacts and dimers of [Cu3] units,
that bridging ligands and electrostatic interactions may obscure
any genuine cuprophilicity in such situations. The intratrimer
CuI‚‚‚CuI contacts in the trinuclear {[3-(R),5-(R′)Pz]Cu}3
complexes herein fall into this category because the bridging
pyrazolate ligands may provide significant assistance that could
obscure any significant CuI‚‚‚CuI interactions. On the other hand,
there are cases where relatively short CuI‚‚‚CuI separations are
(38) Carvajal, M. A.; Alvarez, S.; Novoa, J. J. Chem.sEur. J. 2004, 10, 2117-
2132.
(39) (a) Cotton, F. A.; Feng, X.; Matusz, M.; Poli, R. J. Am. Chem. Soc. 1988,
110, 7077-7083. (b) Cotton, F. A.; Feng, X.; Timmons, D. J. Inorg. Chem.
1998, 37, 4066-4069. (c) Clerac, R.; Cotton, F. A.; Daniels, L. M.; Gu,
J.; Murillo, C. A.; Zhou, H. C. Inorg. Chem. 2000, 39, 4488-4493.
(40) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms, 2nd
ed.; Oxford University Press: Oxford, U.K., 1992.
(41) Poblet, J.-M.; Benard, M. Chem. Commun. 1998, 11, 1179-1180.
(42) See, for example: (a) Che, C. M.; Mao, Z.; Miskowski, V. M.; Tse, M.
C.; Chan, C. K.; Cheung, K. K.; Phillips, D. L.; Leung, K. H. Angew.
Chem., Int. Ed. 2000, 39, 4084-4088. (b) Fu, W. F.; Gan, X.; Che, C. M.;
Cao, Q. Y.; Zhou, Z. Y.; Zhu, N. N. Y. Chem.sEur. J. 2004, 10, 2228-
2236.
(36) Smart, B. E. ACS Monograph 1995, 187, 979-1010.
(37) O’Hagan, D.; Rzepa, H. S. Chem. Commun. 1997, 645-652.
9
7494 J. AM. CHEM. SOC. VOL. 127, NO. 20, 2005

Phosphorescent Trinuclear Copper(I) Complexes of Pyrazolates
A R T I C L E S
Figure 6. Luminescence spectra for single crystals of trinuclear {[3-(R),5-(R′)Pz]Cu}3 complexes. The compounds are identified by the R and R′ substituents.
The spectra were acquired using the optical cryostat setup and are normalized and offset for clarity purposes. Photographs are shown for crystals packed in
Suprasil quartz tubes while being exposed to UV light with the appropriate wavelength at room temperature (RT), immediately after removal from a liquid
nitrogen bath (77 K) and at an intermediate temperature where relevant while the sample is warming but before reaching RT.
due to a genuine cuprophilicity that is unassisted by bridging
ligands or electrostatics; the shortest such separations are in the
range of 2.9-3.0 Å.16,17,43 This cuprophilic bonding, however,
is not covalent bonding with a bond order of one; otherwise,
these distances would be much shorter since the covalent radius
of Cu(I) has been estimated to be only ca. 1.13 Å,44 plus there
would be some chemical sense from an electronic structure
standpoint to justify the formation of a covalent single bond
between two d10 atoms. Instead, this bonding falls into the
general category of metallophilic bonding, which has been
reviewed by Pyykko¨45 and is most well-known for Au(I)
compounds, largely owing to the work of Schmidbaur, who first
coined the term “aurophilic attraction” to describe this type of
bonding.46 Metallophilic bonding has been attributed to cor-
relation effects that are strengthened by relativistic effects.45
Rigorous theoretical treatments reported to date estimate that
pure cuprophilic bonding is in the range of ca. 3.5-4 kcal/mol
for the dimeric models studied.47,48 The intertrimer CuI‚‚‚CuI
interactions in the trinuclear {[3-(R),5-(R′)Pz]Cu}3 complexes
herein fall into this category of genuine unassisted cuprophilic
bonding. One way this bonding manifests itself is through the
unassisted dimerization of [Cu3] units in crystals of the
nonfluorinated complexes, {[3,5-(i-Pr)2Pz]Cu}3 and {[3,5-
(Me)2Pz]Cu}3. In addition to the aforementioned short inter-
trimer CuI‚‚‚CuI distances, further insights can be gained from
the geometry in these dimers of trimers. A close inspection of
Figure 5 (as well as appropriate manipulations of the crystal-
lographic cif files) reveals that the two Cu atoms involved in
intertrimer cuprophilic interactions in each trimer are “attracted”
to the other two Cu atoms in the adjacent trimer, leading to
deviation from planarity of the two Cu3N6 metallacycles. A
similar situation has been encountered for {[2-(3-Pz)Py]Cu}3,
which also exhibits a dimer of trimer structure with a chair
conformation.16 Further crystallographic arguments that dem-
onstrate genuine cuprophilicity in such dimer of trimer structures
are found in ref 16, which we support. In a theoretical study,
Poblet and Benard argued for the significance and genuine
nature of cuprophilic interactions in such dimer of trimer
structures relative to other cases in which cuprophilicity was
claimed; they estimated conservatively a total stabilization of
ca. 6 kcal/mol owing to cuprophilicity in each dimer of
trimers.41,49 The theoretical arguments in refs 41 and 47 suggest
that unassisted cuprophilic bonding should be accounted for even
at rather long distances, such as 4.5 Å. This brings us to the
second way by which genuine unassisted cuprophilic bonding
manifests itself, that is, through the formation of infinite linear
chains of [Cu3] units in crystals of the fluorinated com-
plexes herein. Despite the rather long crystallographic intertrimer
CuI‚‚‚CuI distances, the long-range and cooperative nature of
such interactions in the crystalline solid state lends significant
overall stabilization given the aforementioned theoretical argu-
ments, even if such interactions may be rather weak on a per-
unit basis. As the following section illustrates, the significance
of even the weakest cases of genuine cuprophilic interactions
is multiplied in the low-lying electronic excited states that give
rise to fascinating luminescence behavior.
(43) Kohn, R. D.; Seifert, G.; Pan, Z.; Mahon, M. F.; Kociok-Kohn, G. Angew.
Chem., Int. Ed. 2003, 42, 793-796.
(44) Bayler, A.; Schier, A.; Bowmaker, G. A.; Schmidbaur, H. J. Am. Chem.
Soc. 1996, 118, 7006-7007.
(45) Pyykko¨, P. Chem. ReV. 1997, 97, 599-636.
(46) (a) Schmidbaur, H. Gold Bull. 1990, 23, 11-21. (b) Schmidbaur, H. Chem.
Soc. ReV. 1995, 391-400. (c) Grohmann, A.; Schmidbaur, H. In Compre-
hensiVe Organometallic Chemistry II; Abel, E. W., Stone, F. G. A.,
Wilkinson, G., Eds.; Pergamon: Oxford, 1995; Vol. 3, Chapter 1. (d)
Schmidbaur, H. Gold: Progress in Chemistry, Biochemistry and Technol-
ogy; Wiley: New York, 1999.
(49) We deem the estimation in ref 41 to be “conservative” because: (a) it
only considers intertrimer unassisted cuprophilic interactions, (b) it considers
only 50% of the MP2 stabilization, and (c) it assumes no cooperativity,
despite evidence to the contrary in several other studies of metallophilic
bonding.
(47) Pyykko¨, P.; Runeberg, N.; Mendizabal, F. Chem.sEur. J. 1997, 3, 1451-
1465.
(48) Hermann, H. L.; Boche, G.; Schwerdtfeger, P. Chem.sEur. J. 2001, 7,
5333-5342.
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A R T I C L E S
Dias et al.
Table 3. Summary of the Photophysical Parameters for Crystals
of Pyrazolatocopper(I) Trimers Studied
max
λem
λexc
τ
SS
fwhm
1
b
1
c
compound
temp
(nm) (nm)a
(
µ
s)
(cm-
)
(cm-
)
{[3-(CF3),5-(Me)Pz]Cu}3 RT
77 K 659 308 62.2 ( 1.9 17300 1940
RT 645 306 52.6 ( 0.8 17200 2880
77 K 663 302 64.4 ( 1.0 18000 1760
RT 659 306 51.0 ( 3.8 17500 2690
77 K 663 297 53.2 ( 3.2 18600 2000
RT
656 304 50.1 ( 2.9 17700 4220d
77 K 577 299 68.5 ( 1.5 16100 2440
RT 662 309 28.0 ( 1.1 17300 2850
634 315 49.5 ( 1.0 16000 2260
{[3,5-(CF3)2Pz]Cu}3
{[3-(CF3)Pz]Cu}3
{[3,5-(Me)2Pz]Cu}3
{[3,5-(i-Pr)2Pz]Cu}3
77 K 562 292 57.8 ( 0.7 16500 2250
{[3-(CF3),5-(Ph)Pz]Cu}3 77 Ke 565 306 107 ( 4 15000 2120
77 Kf 447 294 5080 ( 88 11600
a Major lowest-energy feature. b Apparent Stokes’ shift.50 c Full-width
at half-maximum for unstructured emissions. d Total broadening for two
strongly overlapping bands fit to two Gaussians with fwhm of 3310 and
2810 cm-1; see text. e Unstructured emission. f Structured emission.
Figure 8. Representative emission spectra for frozen solutions (77 K) of
{[3,5-(CF3)2Pz]Cu}3 versus solvent and concentration. Lifetimes at λmax
(right-to-left, respectively) are 80 ( 3, 62 ( 1, 52 ( 2, 98 ( 2, 84 ( 1,
and 21 ( 1 µs. The photograph shows selected CH2Cl2, toluene, and CH3-
CN frozen solutions in Suprasil quartz tubes exposed to UV light
immediately after removal from a liquid nitrogen bath. The temperature
increase on removing the sample from the bath changes the emission color
of the 34 mg/mL CH2Cl2 solution from orange to red. Luminescence colors
of frozen solutions in other solvents that are not shown include red
(cyclohexane or pentane), orange (methanol or ethanol), and blue (acetone).
involved are distorted rather hugely! A logical explanation for
such a huge distortion is a dramatic compression of Cu‚‚‚Cu
separations in the phosphorescent state from the rather long
separations (vide supra) in the ground state because the lowest-
energy excited states in aggregates of d10 complexes involve
transitions from σ*nd antibonding filled orbitals to vacant
bonding molecular orbitals arising from the (n + 1)s/p subshells.
The presence of Cu‚‚‚Cu interactions should decrease the triplet
energy of Cu(I) from the 22 649 cm-1 value for the free ion,51
especially in the phosphorescent excited state. Hence, the
unstructured emissions may be assigned to intratrimer or
intertrimer Cu‚‚‚Cu interactions because the compounds herein
exhibit both (Table 2). An intertrimer distortion is expected to
be more facile than an intratrimer distortion because the former
involves lattice vibrations (phonons), which are much lower in
energy than molecular vibrations. Representative frozen solu-
tions data are illustrated in Figure 8, showing unstructured
emissions whose energies (and colors) are solvent-dependent
and which exhibit red shifts upon increasing the concentration
in some solvents to approach the solid-state behavior. These
results are consistent with relating the unstructured emissions
to intertrimer interactions. Hence, we assign the emitting states
responsible for the unstructured emissions to be triplet Cu-Cu
bonded *[Cu3]2 excimers. Enhanced M-M bonding in the
excited state has been reported for Cu(I) and other d10
luminescent materials,52 leading to the formation of excimers
Figure 7. A comparison between the luminescence excitation spectrum
for crystals with the absorption spectrum for solutions of {[3-(CF3)Pz]Cu}3.
Photophysical Properties. Figure 6 shows the solid-state
luminescence spectra for single crystals of the compounds in
this study. All of these copper(I) pyrazolates exhibit bright
luminescence upon exposure to UV radiation. The observed
lifetimes in the microsecond regime (Table 3) suggest phos-
phorescence. The consistent increase in the lifetime values upon
cooling from room temperature to 77 K suggests a reduction in
the nonradiative decay rate, as expected. The luminescence
bands are hugely red-shifted from the corresponding lowest-
energy excitations, rather broad, and unstructured (with one
exception noted below) even at low temperatures for single
crystals (conditions that favor resolution). These observations
are consistent with an assignment to a metal-centered phospho-
rescence for the unstructured emissions. The excitation peaks
shown likely represent spin-forbidden transitions because they
are significantly red-shifted from the major absorption bands
seen in solution, as illustrated in the representative example
shown in Figure 7 for the {[3-(CF3)Pz]Cu}3 compound. Hence,
the large energy separation between the lowest-energy lumi-
nescence excitation feature and the unstructured emission band
for each compound seems to represent a bona fide Stokes’
(50) Nevertheless, we will refer to the Stokes’ shifts in this paper as “apparent”
ones (Table 3) because it is hard to establish the exact states involved in
the absorption and emission processes. An ongoing theoretical study
addresses the exact states involved in the absorption and emission
transitions.
3
(51) Weighted average for the three D3,2,1 spin-orbit states. See: Moore, C.
E. Atomic Energy LeVels. Nat. Bur. Stand: Washington, 1958; Circ. 467,
Vol. III.
(52) See, for example: (a) Hollingsworth, G.; Barrie, J. D.; Dunn, B.; Zink, J.
I. J. Am. Chem. Soc. 1988, 110, 6569-6570. (b) Vitale, M.; Ryu, C. K.;
Palke, W. E.; Ford, P. C. Inorg. Chem. 1994, 33, 561-566. (c) Rawashdeh-
Omary, M. A.; Omary, M. A.; Patterson, H. H.; Fackler, J. P., Jr. J. Am.
Chem. Soc. 2001, 123, 11237-11247. (d) Omary, M. A.; Patterson, H. H.
J. Am. Chem. Soc. 1998, 120, 7696-7705. (e) Omary, M. A.; Hall, D. R.;
Shankle, G. E.; Siemiarczuck, A.; Patterson, H. H. J. Phys. Chem. B 1999,
103, 3845-3853.
shift.50 The fact that rather broad (fwhm ) ca. 2-3 × 103 cm-1
)
unstructured emission bands that are separated by 15.0-18.6
× 103 cm-1 from the corresponding lowest-energy excitation
bands are observed for crystals of the trinuclear complexes
herein (Table 3) suggests that the phosphorescent excited states
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Phosphorescent Trinuclear Copper(I) Complexes of Pyrazolates
A R T I C L E S
Chart 2
band can be resolved from the more dominant lime green band
at temperatures below ∼100 K. The relative intensity of the
green band decreases gradually upon heating between 4 and
100 K, and the band disappears at higher temperatures. The
lime green emission becomes essentially the only major emission
between 100 and 230 K, but a further temperature increase
results in a gradual generation of the orange emission. The
combination of the lime green and orange emission bands
between 230 and 250 K gives the yellow emission color, while
further heating toward room temperature results in the domi-
nance of the orange emission. The lifetime data indicate that
all of the three aforementioned bands are due to phosphores-
cence with lifetime values that increase for each band upon
cooling; this increase is especially substantial in the coldest
temperature range, 4 < T (K) < 30, at which a very sharp slope
is seen due to the annihilation of the nonradiative decay rate at
such temperatures. Extrapolation of this slope to 0 K results in
an estimated value for the radiative decay constant (krad) of ∼3.3
× 103 s-1 for the green band at ∼510 nm and 1.7 × 103 s-1 for
the lime green band at ∼560 nm. The differences in krad, τ values
at different temperatures, and luminescence excitation profiles
suggest that these two bands are poorly coupled with one
another, a situation similar to that encountered by the studies
of Ford et al. for the two emission bands responsible for the
luminescence thermochromism of Cu4L4X4 (L ) aromatic
amine; X ) halide) clusters.30,52b In contrast, the 660 nm orange
band seems to be coupled with the 560 nm lime green band
since the two bands have essentially indistinguishable excitation
profiles and lifetime values (Figure 9), so these two bands are
likely in a thermal equilibrium. Analysis of the changes in the
relative intensities of these two bands in the 225-270 K
temperature range (relative peak areas from a two Gaussian fit
at each temperature) gives an activation energy of 4850 cm-1
(13.9 kcal/mol) for the internal conversion from the triplet state
responsible for the lime green emission to the triplet state
responsible for the orange emission. Other samples besides
{[3,5-(i-Pr)2Pz]Cu}3 also show luminescence thermochromism,
principally due to changes in the relative intensities of two
emission bands. Note in Figure 6 that {[3-(CF3),5-(Me)Pz]Cu}3,
{[3,5-(CF3)2Pz]Cu}3 and {[3-(CF3)Pz]Cu}3 exhibit shoulders in
the yellow region that become more well-defined at 77 K in
addition to the dominant peak in the red region. For {[3,5-(CF3)2-
Pz]Cu}3, the orange emission color at 77 K is due to the
combination of these two peaks, but heating toward room
temperature leads to the disappearance of the yellow shoulder,
leaving only the red emission at intermediate temperatures in
the range of ∼100-240 K; band broadening at higher temper-
atures, including room temperature, leads to an orange emission
color. Similarly, the emission color changes for {[3,5-(CH3)2-
Pz]Cu}3 between lime green to yellow are due to a change in
the relative intensity of two peaks with the higher-energy peak
becoming more dominant upon cooling. The very broad
emission of {[3,5-(CH3)2Pz]Cu}3 at room temperature (fwhm
) 4220 cm-1) gave an excellent fit (R2 ) 0.9985; ø2 ) 1.942
× 10-4) to two Gaussian peaks with maxima at 17 049 (∼587
nm) and 15 320 cm-1 (∼653 nm) and fwhm values of 3310
and 2810 cm-1, respectively.
and exciplexes that are analogous to those in organic com-
pounds.53 M-M-bonded excimers and exciplexes are now
becoming commonly recognized for closed-shell transition-metal
species.52,54 The assignment of the unstructured luminescence
bands of the trimeric complexes herein to intertrimer Cu‚‚‚Cu
interactions explains the lack of an intuitive trend upon varying
the substituents. If the luminescence bands were due to
transitions within an individual molecular unit of the trimeric
complex, one would expect a much more pronounced substituent
effects due to the expected influence on the electronic structure
of fluorinated versus nonfluorinated R groups. The pyrazolyl
ring substituents can alter the electron density on copper, as
evident from the υCO data for the related tris(pyrazolyl)-
boratocopper(I) carbonyl complexes (see 5 for several ex-
amples).33 The central role of intertrimer interactions on the
luminescence bands is consistent with the assignments given
by Raptis et al.55 and Fackler et al.56 for trinuclear Au(I)
pyrazolate and carbeniate complexes, respectively. Another
striking feature in the solid-state luminescence data in Figure 6
is “luminescence thermochromism”, the extent of which varies
among the different compounds. The most dramatic changes
are for {[3,5-(i-Pr)2Pz]Cu}3, in which cooling from room
temperature to 77 K changes the emission color from orange
(λmax ) 662 nm) to green (λmax ) 562 nm), while at intermediate
temperatures, the sample’s emission appears yellow. This
compound was, hence, selected for a detailed temperature-
dependent luminescence study, the results of which are il-
lustrated in Figure 9. These data show that there are actually
three emission bands for {[3,5-(i-Pr)2Pz]Cu}3 with maxima near
510 (green), 560 (lime green), and 660 nm (orange). The relative
intensities of these bands change with temperature. The green
(53) (a) The Exciplex; Gordon, M., Ware, W. R., Eds; Academic Press: New
York, 1975. (b) Turro, N. J. Modern Molecular Photochemistry; Benjamin/
Cummings: Menlo Park, CA, 1978; pp 135-146. (c) Lamola, A. A. In
Energy Transfer and Organic Photochemistry; Lamola, A. A., Turro, N.
J., Eds.; Wiley-Interscience: New York, 1969; pp 54-60. (d) Lowry, T.
H.; Schuller-Richardson, K. Mechanism and Theory in Organic Chemistry;
Harper & Row: New York, 1981; pp 919-925. (e) Kopecky, J. Organic
Photochemistry: A Visual Approach; VCH: New York, 1991; pp 38-40.
(f) Michl, J.; Bonacic-Koutecky, V. Electronic Aspects of Organic
Photochemistry; Wiley: New York, 1990; pp 274-286.
(54) For reviews about M-M-bonded excimers and exciplexes, see: (a) Nagle,
J. K. Spectrum 1999, 12, 15. (b) Horva´th, A.; Stevenson, K. L. Coord.
Chem. ReV. 1996, 153, 57-82. (c) Omary, M. A.; Patterson, H. H.
Electronic Spectroscopy: Luminescence Theory. Encyclopedia of Spec-
troscopy & Spectrometry; Academic Press: London, U.K., 2000; pp 1186-
1207. (d) Callear, A. B. Chem. ReV. 1987, 87, 335-355.
(55) Yang, G.; Raptis, R. G. Inorg. Chem. 2003, 42, 261-263.
(56) Rawashdeh-Omary, M. A.; Omary, M. A.; Fackler, J. P., Jr.; Galassi, R.;
Pietroni, B. R.; Burini, A. J. Am. Chem. Soc. 2001, 123, 9689-9691.
The above results indicate that the luminescence thermo-
chromism in {[3-(R),5-(R′)Pz]Cu}3 complexes occurs as a result
of changes in the relative intensities between multiple emission
bands. Hence, we assign these changes to be a result of thermal
9
J. AM. CHEM. SOC. VOL. 127, NO. 20, 2005 7497

A R T I C L E S
Dias et al.
Figure 9. Temperature-dependent luminescence data for crystals of {[3,5-(i-Pr)2Pz]Cu}3. The spectra were acquired using the optical cryostat setup and are
normalized and offset for clarity purposes. The relative intensities are accurate for the traces at the same temperature. All lifetime data were generated with
285 nm laser excitation and have errors similar to those in Table 3 (1-8%).
relaxation between different emitting states, as opposed to being
due to factors such as thermal compression of M‚‚‚M distances,
which are known for other closed-shell coordination compounds
that show M‚‚‚M packing in the solid state.57 Although similar
compressions (i.e., leading to the formation of more closely
linked dimers of trimers) do occur in the compounds herein
upon cooling (e.g., the shortest intertrimer Cu‚‚‚Cu distances
for {[3,5-(CF3)2Pz]Cu}3 and {[3,5-(Me)2Pz]Cu}3 at RT and 100
K are 3.879 and 3.813 Å, and 2.946 Å and 2.874 Å,
respectively), the direct impact of such slight compressions is
usually57 a gradual red shift in the emission maximum of one
band upon cooling, while the above data clearly show that this
is not the cause of the major luminescence thermochromism
changes observed. There is a consistent trend in the above data
to suggest that the higher-energy emitting states at lower
temperatures are depopulated upon heating, while the lower-
energy emitting states responsible for the lower-energy emis-
sions at higher temperatures are populated. Figure 10 illustrates
the photophysical model that we propose for the trinuclear
compounds herein according to the above data and discussion.
The occurrence of multiple emitting states is not a very common
phenomenon in luminescent molecular compounds owing to the
so-called Kasha’s rule,58 but it is certainly not unprecedented.
One relevant example of a precedent is represented by Cu4L4X4
clusters, whose luminescence thermochromism was attributed
by Ford et al. to changes in the relative intensities between two
(57) See, for example: (a) Gliemann, G.; Yersin, H. Struct. Bonding 1985, 62,
87-153. (b) Connick, W. B.; Henling, L. M.; Marsh, R. E.; Gray, H. B.
Inorg. Chem. 1996, 35, 6261-6265. (c) Nagasundaram, N.; Roper, G.;
Biscoe, J.; Chai, J. W.; Patterson, H. H.; Blom, N.; Ludi, A. Inorg. Chem.
1986, 25, 2947-2951. (d) Burini, A.; Bravi, R.; Fackler, J. P., Jr.; Galassi,
R.; Grant, T. A.; Omary, M. A.; Pietroni, B. R.; Staples, R. J. Inorg. Chem.
2000, 39, 3158-3165.
(58) Kasha, M. Discuss. Faraday Soc. 1950, 9, 14-19.
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Phosphorescent Trinuclear Copper(I) Complexes of Pyrazolates
A R T I C L E S
disappears when the solution warms to room temperature. There
is a consistent shift to a lower emission energy that we have
seen on heating frozen solutions to higher temperatures to
approach the fluid state (see Figure 8 caption for an illustration).
This illustrates another case of “luminescence rigidochromism”,
which was first noted by Wrighton and Morse for ClRe(CO)3-
(diimine) complexes60 and is now known for several classes of
coordination compounds.30,61,62 The solvent dependence of the
emission color of glassy solutions (Figure 8) illustrates a
dramatic case of “luminescence solvatochromism”, which is
related to a few fascinating observations for several classes of
d10 and d8 complexes that have been reported to interact with
solvent molecules to change their emission colors.63 What is
unusual about the results here is not only the selectivity even
for similar solvents (e.g., toluene vs benzene) and the versatility
of solvents in which {[3,5-(CF3)2Pz]Cu}3 shows luminescence
solvatochromism but also the qualitative changes in the visible
emission colors and spectra when the concentration is varied
in the same solvent. The most striking changes were seen in
dichloromethane, in which the luminescence was tuned to
essentially all visible colors between blue and red by varying
the concentration. This “concentration luminochromism” in
which multiple Visible colors are emitted by controlling the
luminophore concentration is an unprecedented optical phe-
nomenon, to our knowledge. The closest analogy is to the
changes from monomer to excimer emission in polynuclear
aromatic organic molecules53 and to the changes from smaller
to larger *[Ag(CN)2-]n or *[Au(CN)2-]n oligomers52c-e upon
increasing the concentration, but these cases were not as
dramatic in covering multiple emissions in the visible region
for the same compound as the case shown in Figure 8. The
emission color changes with solvent, temperature, concentration,
and/or excitation wavelength were not limited to {[3,5-(CF3)2-
Pz]Cu}3. Figure 11 illustrates some of these changes for other
compounds. We attribute the changes in the luminescence
energies with concentration to *[Cu3]n excimers and exciplexes
that vary in n and/or the number of intertrimer Cu‚‚‚Cu
interactions between adjacent trimers. A red shift in the emission
energy upon increasing the concentration would take place, for
example, due to the formation of a *[Cu3]3 trimer exciplex
instead of a *[Cu3]2 excimer, or due to the formation of a *[Cu3]2
excimer with two intertrimer Cu-Cu bonds instead of a *[Cu3]2
Figure 10. Proposed photophysical model for the trinuclear Cu(I) pyra-
zolates. The transitions shown depict: (a) solution absorption bands (e.g.,
Figure 7, left); (b) solid-state excitation bands (e.g., Figure 7, right); (c)
higher-energy emission bands (e.g., the green emission of {[3,5-(i-Pr)2Pz]-
Cu}3 in Figure 6); (d) internal conversion to a lower triplet; and (e) lower-
energy emission bands (e.g., the orange emission of {[3,5-(i-Pr)2Pz]Cu}3
in Figure 6).
emitting states of different origins, a cluster-centered lower-
energy band and an interligand charge-transfer higher-energy
band.30,52b Another example is represented by molecular emis-
sions in the mercury vapor, for which multiple emission bands
have been attributed to various bound states of the *Hg2 excimer
and various bound states of the *Hg3 trimer exciplex.54d,59 The
model in Figure 10 is suggested for {[3-(R),5-(R′)Pz]Cu}3
complexes that exhibit two unstructured emissions, but the same
principle is valid for {[3,5-(i-Pr)2Pz]Cu}3 with slight modifica-
tions. Because the compound exhibits three unstructured emis-
sions, a third bound state is suggested for {[3,5-(i-Pr)2Pz]Cu}3.
Also, the multiple excitation and emission maxima do not
necessarily correlate with one another energy wise for this
compound. Figure 9 shows that the higher-energy green
emission peaks at 4 and 77 K are associated with lower-energy
excitation peaks compared to those for the lime green lumi-
nescence peaks. This anomaly has been noticed earlier in species
that exhibit similar multiple luminescence bands due to excited-
-
54d,59
state M-M bonding, such as *[Ag(CN)2
]
n
52d and *Hgn
(60) Wrighton, M.; Morse, D. L. J. Am. Chem. Soc. 1974, 96, 998-1003.
(61) For more examples, see: (a) Vogler, A.; Kunkely, H. J. Am. Chem. Soc.
1986, 108, 7211-7212. (b) McKiernan, J.; Pouxviel, J. C.; Dunn, B.; Zink,
J. I. J. Phys. Chem. 1989, 93, 2129-2133. (c) Franville, A.-C.; Dunn, B.;
Zink, J. I. J. Phys. Chem. B 2001, 105, 10335-10339. (d) Watts, R. J.;
Missimer, D. J. Am. Chem. Soc. 1978, 100, 5350-5357. (e) Zuleta, J. A.;
Bevilacqua, J. M.; Rehm, J. M.; Eisenberg, R. Inorg. Chem. 1992, 31,
1332-1337. (f) Colombo, M. G.; Hauser, A.; Gudel, H. U. Top. Curr.
Chem. 1994, 171, 143-171. (g) Rawlins, K. A.; Lees, A. J.; Fuerniss, S.
J.; Papathomas, K. I. Chem. Mater. 1996, 8, 1540-1544. (h) Clark, I. P.;
George, M. W.; Johnson, F. P. A.; Turner, J. J. Chem. Commun. 1996, 13,
1587-1588. (i) Turner, J. J.; George, M. W.; Clark, I. P.; Virrels, I. G.
Laser Chem. 1999, 19, 245-251. (j) Seneviratne, J.; Cox, J. A. Talanta
2000, 52, 801-806. (k) Forster, L. S.; Rund, J. V. Inorg. Chem. Commun.
2003, 6, 78-81. (l) Fernandez, S.; Fornies, J.; Gil, B.; Gomez, J.; Lalinde,
E. Dalton Trans. 2003, 5, 822-830. (m) Itokazu, M. K.; Polo, A. S.; Iha,
N. Y. M. J. Photochem. Photobiol. A 2003, 160, 27-32.
excimers and exciplexes, and also for Cu4L4X4 clusters.30,52b
The remarkable luminescence behavior of the trinuclear
complexes herein and the sensitivity of the emission to various
factors warrant further comments. The data are reminiscent of
several novel optical phenomena that have been reported
individually for other metal complexes. These phenomena are
observed collectiVely, together with yet another new phenom-
enon even in a single compound, as we illustrate for a compound
that we have studied in some detail, {[3,5-(CF3)2Pz]Cu}3. The
“luminescence thermochromism” phenomenon is seen rather
dramatically for various solids (Figures 6 and 9), including {[3,5-
(CF3)2Pz]Cu}3. Solutions of {[3,5-(CF3)2Pz]Cu}3 show bright
phosphorescence when cooled to liquid nitrogen temperature.
When they are removed from the liquid nitrogen bath, the
emission color often changes as the sample starts to warm, but
then the emission intensity decreases dramatically and essentially
(62) For a review, see: Lees, A. J. Comments Inorg. Chem. 1995, 17, 319-
346.
(63) (a) White-Morris, R. L.; Olmstead, M. M.; Jiang, F.; Tinti, D. S.; Balch,
A. L. J. Am. Chem. Soc. 2002, 124, 2327-2336. (b) Mansour, M. A.;
Connick, W. B.; Lachicotte, R. J.; Gysling, H. J.; Eisenberg, R. J. Am.
Chem. Soc. 1998, 120, 1329-1330. (c) Cariati, E.; Bu, X.; Ford, P. C.
Chem. Mater. 2000, 12, 3385-3391. (d) Fernandez, E. J.; Lopez-de-
Luzuriaga, J. M.; Monge, M.; Olmos, M. E.; Perez, J.; Laguna, A.;
Mohamed, A. A.; Fackler, J. P., Jr. J. Am. Chem. Soc. 2003, 125, 2022-
2023. (e) Kunugi, Y.; Mann, K. R.; Miller, L. L.; Exstrom, C. L. J. Am.
Chem. Soc. 1998, 120, 589-590.
(59) Koperski, J.; Atkinson, J. B.; Krause, L. J. Mol. Spectrosc. 1998, 187, 181-
192.
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A R T I C L E S
Dias et al.
significantly longer, by an order of magnitude, for the structured
emission (Table 3). The spacing between the structured peaks
is essentially uniform with an average of 1400 cm-1. Several
pyrazolate ring vibrations exist in that region (see the infrared
data above). Hence, the structured band likely corresponds to a
vibronic progression in a normal mode that is centered on the
Pz ring. These results are consistent with an assignment to a
Pz ligand-centered phosphorescence that is sensitized via an
internal heavy atom effect for the structured emission of {[3-
(CF3),5-(Ph)Pz]Cu}3. We have recently communicated20 that the
mononuclear complexes [H2B(3,5-(CF3)2Pz)2]M(2,4,6-collidine)
with M ) Cu (6) and Ag exhibit such a Pz ligand-centered
phosphorescence, the energy and profile of which showed
indifference to the identity of the metal and agreed well with
the phosphorescence bands reported65 for phenyl-substituted
pyrazoles. The dinuclear complex, {[3,5-(CF3)2Pz]Cu(2,4,6-
collidine)}2 (7), exhibits a very similar phosphorescence band
that is slightly red-shifted from that of the mononuclear
complexes, [H2B(3,5-(CF3)2Pz)2]M(2,4,6-collidine).20 We have
Figure 11. Illustration of the sensitivity of the emission spectra for frozen
solutions of: (a) (top) {[3-(CF3)Pz]Cu}3 to solvent and concentration, and
(b) (bottom) {[3,5-(i-Pr)2Pz]Cu}3 to excitation wavelength. The curve colors
are representative of the visible luminescence colors for the samples.
excimer with one intertrimer Cu-Cu bond. This assignment is
3
suggested that the π-π* emissive state is slightly perturbed
similar to the one given by Patterson and co-workers for the
-
-
by interaction with copper. A similar argument may be valid
for the structured emission band of {[3-(CF3),5-(Ph)Pz]Cu}3
such that a triplet ligand to metal-metal charge transfer state,
3LMMCT, in the dinuclear and trinuclear complexes is possible.
If the structured emission of {[3-(CF3),5-(Ph)Pz]Cu}3 were to
be simply due to a 3π-π* transition, one would expect a
stronger overlap between the excitation and emission profiles
and a smaller Stokes’ shift, as is the case typically for
phosphorescence in organic molecules.66 Finally, it is interesting
to note that the excitation profile corresponding to the green
emission of {[3,5-(i-Pr)2Pz]Cu}3 has resolved vibronic structure
at low temperatures (Figure 9). A spacing of ∼1000 cm-1 is
observed, significantly lower than the ∼1400 cm-1 spacing
observed in the structured emission bands of {[3-(CF3),5-(Ph)-
Pz]Cu}3. The vibronic structure in the excitation bands must
be related to an excited-state normal mode that is centered on
the Pz ring instead of being due to excited-state Cu-Cu
vibrations. This is because M-M vibrations, even for fully
bonded excimers with excited-state M-M single bonds, usually
excited-state oligomerization of Ag(CN)2 and Au(CN)2
species in solutions and doped solids.52c-e The solvatochromic
changes of the luminescence spectra can thus be related to
different extents of excited-state association of the [Cu3] units,
so that the emitting species in solution are *[Cu3]n oligomeric
excimers and exciplexes. However, one notices in Figure 8 that
some frozen solution emission bands are red-shifted beyond the
solid-state emission in Figure 6. Two reasons may be responsible
for this result. First, it is possible that the intertrimer interactions
in the glassy solutions in question lead to a lower-energy
emitting state than the one in the solid state. Enhanced
intermolecular interactions between d10 complexes in solution
are not uncommon. For example, Balch and co-workers have
reported that Au(I) carbene complexes that are both nonlumi-
nescent and do not exhibit intermolecular Au-Au interactions
in the solid state become brightly luminescent in frozen solutions
due to enhanced aggregation in solution.63a Eisenberg and co-
workers have also reported the formation of luminescent species
with enhanced Au-Au interactions upon sorption of solvent
molecules.63b Second, formation of an exciplex between an
excited-state *[Cu3]n oligomer and a ground-state solvent
molecule may lead to a situation in which the resulting *{[Cu3]n‚
solvent} exciplex is a lower-energy emitter than the *[Cu3]n
species alone. Che and co-workers have suggested that interac-
tions between *[Au]2 excimers and solvent molecules result in
a red shift in the emission energies for dimeric Au(I) phosphine
complexes.64 The known high reactivity53,54 of excimers and
exciplexes coupled with the low coordination number in linear
monovalent coinage metal complexes makes the formation of
*[Mn‚solvent] exciplexes a reasonable contention. In conclusion,
the dramatic changes in the luminescence properties for frozen
solutions of the trimeric Cu(I) complexes compared to those of
the solid state may be attributed to both the alteration of the
intertrimer interactions between adjacent [Cu3] units as well as
possible interactions with solvent molecules.
have much lower wavenumbers of ca. 100-150 cm-1 52c,54,64
.
Thus, the lower wavenumber spacing in the excitation spectra
is attributed to significant weakening of the π bonds within the
Pz moieties upon photoexcitation. Alternatively, the progression
seen in the excitation spectra of {[3,5-(i-Pr)2Pz]Cu}3 may be
due to a different Pz ring normal mode from that seen in the
emission spectra of {[3-(CF3),5-(Ph)Pz]Cu}3. The IR data for
the two compounds show vibrations in the regions of both
progressions. Unfortunately, we have not obtained structured
emission and structured excitation profiles for the same com-
pound to make a more direct comparison. Nevertheless, the IR
data do not show significant changes in the Pz vibrations
between different Cu3Pz′3 compounds that differ only in
substituents. Clearly, further studies are warranted for this family
of compounds in order to understand the vibrational changes
associated with the electronic transitions.
Crystals of {[3-(CF3),5-(Ph)Pz]Cu}3 exhibit a highly struc-
tured blue emission in addition to the lower-energy unstructured
metal-centered emission. The phosphorescence lifetime is
(65) (a) Swaminathan, M.; Dogra, S. K. Spectrochim. Acta 1983, 39A, 973-
977. (b) Catalan, J.; Fabero, F.; Claramunt, R. M.; Santa Maria, M. D.;
Foces-Foces, M. C.; Cano, F. H.; Martinez-Ripoll, M.; Elguero, J.; Sastre,
R. J. Am. Chem. Soc. 1992, 114, 5039-5048.
(66) McGlynn, S. P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy of the
Triplet State; Prentice Hall: Englewood Cliffs, NJ, 1969.
(64) (a) Fu, W. F.; Chan, K. C.; Miskowski, V. M.; Che, C. M. Angew. Chem.,
Int. Ed. Engl. 1999, 38, 2783-2785. (b) Zhang, H. X.; Che, C. M. Chem.s
Eur. J. 2001, 7, 4887-4893.
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7500 J. AM. CHEM. SOC. VOL. 127, NO. 20, 2005

Phosphorescent Trinuclear Copper(I) Complexes of Pyrazolates
A R T I C L E S
Chart 3
studies determined the ground-state distance to be 3.63-3.71
Å,54d,68 which shrinks in the excited-state responsible for the
well-known 335 nm excimer emission to 2.5 ( 0.1 Å.68
Besides the scientific significance of the above results, the
observation of bright phosphorescence at room temperature for
crystals and sublimed thin films is prompting us to pursue using
these types of multinuclear d10 pyrazolate complexes as emitting
materials for MOLEDs. Indeed, preliminary experiments have
led to a successful construction of a device based on {[3,5-
(CF3)2Pz]Cu}3, for which the electroluminescence spectrum
showed the same orange emission band as that exhibited in the
photoluminescence spectrum of the single crystals at room
temperature.69 Work is underway in an independent detailed
electroluminescence study to optimize the device heterostructure
and to test other compounds in this family as MOLED emitters.
Finally, the dramatic luminescence color changes for the
materials herein upon temperature and solvent variations are
promising for sensor applications.
Concluding Remarks
This work describes details of efficient syntheses, structures
including solid-state packing, and photophysical properties of
a series of fluorinated trinuclear copper(I) pyrazolates and related
nonfluorinated analogues. Reaction between copper(I) oxide and
readily available fluorinated pyrazoles affords the corresponding
copper(I) pyrazolates in excellent yields. X-ray data show that
the fluorinated compounds, {[3-(CF3)Pz]Cu}3, {[3-(CF3),5-
(Me)Pz]Cu}3, {[3-(CF3),5-(Ph)Pz]Cu}3, and {[3,5-(CF3)2Pz]-
Cu}3, form trimers that pack in zigzag chains. In contrast, the
nonfluorinated compounds, {[3,5-(Me)2Pz]Cu}3 and {[3,5-(i-
Pr)2Pz]Cu}3, form dimers of trimers with much shorter
intertrimer Cu‚‚‚Cu separations than those in the fluorinated
analogues. This study illustrates the significant impact of
packing via Cu‚‚‚Cu intertrimer interactions on the photophysi-
cal properties of trinuclear Cu(I) pyrazolate complexes. Despite
the inherent weakness of such closed-shell cuprophilic interac-
tions in the electronic ground state, their strong enhancement
in the emissive excited states is responsible for the bright
luminescence bands that can be tuned across the visible region.
It is amazing that fascinating luminescence properties owing
to intertrimer interactions occur in compounds in which such
interactions are so weak in the electronic ground state (e.g.,
3.813 and 3.987 Å in {[3,5-(CF3)2Pz]Cu}3; 3.848 and 4.636 Å
in {[3-(CF3),5-(Ph)Pz]Cu}3) such that one would be tempted
to naively ignore such interactions when assigning the lumi-
nescence bands. The strong Cu-Cu bonding in many low-lying
excimer-type excited states should bring down these distances
to the bonding range in a manner akin to that encountered in
organic excimers, for which the ground-state crystallographic
requirement is that aromatic molecules must be stacked in
columns or arranged in pairs with interplanar distances of ∼3.5
Å (at which obviously there is no ground-state bonding) in order
to exhibit excimer-type emissions in the solid state,67 and in
the *Hg2 mercury excimer for which several experimental
Acknowledgment. This work is supported by the National
Science Foundation (CHE-0314666 to H.V.R.D.; CAREER
Award CHE-0349313 to M.A.O.) and the Robert A. Welch
Foundation (Grant Y-1289 to H.V.R.D.; Grant B-1542 to
M.A.O.).
Supporting Information Available: X-ray crystallographic
data at 100 K for {[3-(CF3)Pz]Cu}3, {[3-(CF3),5-(Me)Pz]Cu}3,
{[3-(CF3),5-(Ph)Pz]Cu}3, {[3,5-(i-Pr)2Pz]Cu}3, and {[3,5-(CF3)2-
Pz]Cu}3 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA0427146
(67) For example, see: (a) Stevens, B. Spectrochim. Acta 1962, 18, 439-448.
(b) Ferguson, J. J. Chem. Phys. 1958, 28, 765-768.
(68) (a) van Zee, R. D.; Blankespoor, S. C.; Zwier, T. S. J. Chem. Phys. 1988,
88, 4650-4654. (b) Ceccherini, S.; Moraldi, M. Chem. Phys. Lett. 2001,
337, 386-390.
(69) Williams, C. D.; Gnade, B. E.; Omary, M. A.; Dias, H. V. R. Unpublished
results.
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