Tetranuclear copper(I) clusters: Impact of bridging carboxylate ligands on solid state structure and photoluminescence

Article abstract of DOI:10.1039/b707957e

The X-ray crystallographic characterization and solid state photoluminescence (PL) study of three new tetranuclear copper(i) clusters, [Cu₄(O₂CR)₄], R = (3-F)C₆H ₄ (1), (2,3,4-F)₃C₆H₂ (2), and CF₃/C₆F₅ (3), revealed a dependence of PL on the structural type. The Royal Society of Chemistry.

Full text of DOI:10.1039/b707957e

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Tetranuclear copper(I) clusters: impact of bridging carboxylate ligands
on solid state structure and photoluminescence{
Yulia Sevryugina, Oleksandr Hietsoi and Marina A. Petrukhina*
Received (in Austin, TX, USA) 25th May 2007, Accepted 28th June 2007
First published as an Advance Article on the web 18th July 2007
DOI: 10.1039/b707957e
used to manipulate supramolecular assembly processes in the
solid state.
The X-ray crystallographic characterization and solid state
photoluminescence (PL) study of three new tetranuclear
copper(I) clusters, [Cu4(O2CR)4], R = (3-F)C6H4 (1), (2,3,4-
F)3C6H2 (2), and CF3/C6F5 (3), revealed a dependence of PL on
the structural type.
The first report on fluorescence of aliphatic copper(I) carboxyl-
ates appeared in the literature in 1981,9 but there have been no
follow-up studies on the origin of photoluminescence or structure–
property relationships for this class of compounds. The emission
maxima of copper(I) formate, acetate, propionate, butyrate,
valerate, hexanoate, and heptanoate were found to vary in the
broad range of 535–660 nm at room temperature (lex = 305–
325 nm), but those were not correlated with the structures of the
complexes. The photoluminescence of copper(I) pyridylacrylate,
[Cu(3-PYA)]‘, and the water adduct, [Cu(2-PYA)(H2O)]‘, both
displaying 2D layered structures, was observed at the same
wavelength, 580 nm (lex = 250 nm), in the solid state.10 The
emission maximum for the recently synthesized copper(I) 2,6-
Polynuclear complexes of copper(I), silver(I), and gold(I) attract
considerable interest due to their remarkable photophysical
properties1 and are broadly studied as potential optoelectronic
materials.2 The attractive interactions between closed d10 shells in
various clusters are subjects of special attention3 since the
arrangement of metal centers and, in particular, the number of
those in close proximity affects the photophysical behavior of the
resulting polynuclear systems. Therefore, the controlled formation
of small clusters of the above metals having defined nuclearities
and the study of their structure–property correlations are of great
importance in designing functional inorganic materials.
bis(trifluoromethyl)benzoate5 was measured at 558 nm (lex
=
350 nm), close to that for analogous copper(I) acetate that emits at
560 nm at room temperature.9
In this regard, polynuclear copper(I) carboxylates exhibiting
rich luminescent properties and a remarkable structural diversity
have recently attracted our attention. Only a handful of copper(I)
carboxylate complexes have been crystallographically character-
ized to date, all showing unique polynuclear structural
arrangements in the solid state. Copper(I) acetate4 and 2,6-
bis(trifluoromethyl)benzoate5 exhibit 1D polymeric structures
based on dicopper units that are further linked by intermolecular
copper–oxygen interactions. Copper(I) pivalate has recently been
found to exhibit an infinite double-helical chain structure held
together by cuprophilicity.6 In contrast, copper(I) benzoate7 and
trifluoroacetate8 are composed of tetrameric molecules, in which
four copper(I) centers are bridged by carboxylate groups
alternately above and below the Cu4-plane. However, while the
former was reported7 to have discrete tetracopper clusters in the
solid state (Scheme 1a), in the latter the tetranuclear units form a
polymeric zigzag ribbon (Scheme 1b). This stems from the great
electron-withdrawing properties of trifluoroacetate groups that
enhance the electrophilicity of the copper(I) centers and enforce
additional intermolecular copper–oxygen interactions between
the Cu4-units. Thus, fluorination of carboxylate ligands
holding a specific polynuclear copper(I) core together can be an
important structure-controlling factor that allows switching on and
off of the intermolecular forces between clusters. This effect can be
Since the reactivity and properties of copper(I) clusters are
governed by their molecular structures, in this work we aim to
expand the copper(I) carboxylate family and examine the influence
of electrophilic substituents of bridging benzoates on the solid state
structure of copper(I) complexes. Specifically, we are interested in
revealing the effects of a structural type on photophysical
properties of the new copper(I) systems, an issue that has not
been addressed yet due to the limited number of crystallographi-
cally characterized copper(I) carboxylates.
Herein, to access the corresponding copper(I) complexes, we
used a homologous set of carboxylate ligands of varied electro-
philicity, namely 3-fluorobenzoate, 2,3,4-trifluorobenzoate, and
pentafluorobenzoate. The syntheses of new copper(I) complexes
were performed by ligand exchange procedures11,12 based on
refluxing copper(I) trifluoroacetate with excess carboxylic acid in
benzene (see ESI for experimental procedures{). While ligand
Department of Chemistry, University at Albany, 1400 Washington Ave.,
Albany, NY, 12222-0100, USA. E-mail: marina@albany.edu;
Fax: +1 518-442-3462; Tel: +1 518-442-4406
{ Electronic supplementary information (ESI) available: Synthesis,
characterization, X-ray details, and additional plots of the structures.
See DOI: 10.1039/b707957e
Scheme 1 Tetranuclear copper(I) benzoate (a) and trifluoroacetate (b).
Chem. Commun., 2007, 3853–3855 | 3853
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substitution was completed in the first two cases to afford
[Cu(O2C(3-F)C6H4)] (1) and [Cu(O2C(2,3,4-F)3C6H2)] (2), for the
latter we initially isolated an intermediate product of the general
formula [Cu(O2CCF3)1/2(O2CC6F5)1/2] (3) that has both the parent
trifluoroacetate and the new pentafluorobenzoate groups. We have
to stress that growing crystals of electrophilic copper(I) complexes
from solution is a great challenge due to their avidity for additional
coordination8 and the ease of disproportionation reactions.13 We
overcame these problems by using the micro-scale gas phase
deposition technique,14 which we have already proved to be a very
efficient way to avoid interfering solvent effects and to exclude the
presence of exogenous ligands during crystallization. Pure crystal-
line products in the form of colorless blocks have been isolated by
sublimation–deposition procedures under reduced pressure in the
220–240 uC range for 1, 160–220 uC for 2, and at ca. 150 uC for 3.
Complexes 1–3 were fully characterized in the solid state using
single crystal X-ray diffraction{ and spectroscopic techniques (see
ESI for more details{).
Fig. 2 A fragment of a polymeric chain in [Cu4(O2CCF3)2(O2CC6F5)2]‘
(3). The F atoms are omitted for clarity.
The structural characterizations of 1–3 revealed that all
complexes contain the planar Cu4-core with carboxylate bridges
positioned above and below the plane (Fig. 1). In compound 3,
which incorporates two different carboxylate ligands, pentafluoro-
benzoate groups bridge the neighboring Cu(1)–Cu(2) and Cu(2)–
Cu(3) pairs and trifluoroacetate groups link the neighboring
Cu(3)–Cu(4) and Cu(4)–Cu(1) pairs. Importantly, the solid state
structures of 1 and 2 are based on the discrete tetracopper units,
similar to that of copper(I) benzoate, [Cu4(O2CC6H5)4] (4). At the
same time, the mixed carboxylate compound containing very
electrophilic trifluoroacetate ligands and fully fluorinated benzoate
groups, [Cu4(O2CCF3)2(O2CC6F5)2] (3), is found to be structurally
similar to copper(I) trifluoroacetate, [Cu4(O2CCF3)4] (5). In
contrast to 1 and 2, complex 3 exhibits a polymeric structure
built on intermolecular copper–oxygen interactions in the solid
state (Fig. 2).
category of metallophilic interactions similar to those in Au(I)
3
…
compounds. The Cu Cu distances vary to some extent within
the [Cu4(O2CR)4] series and are longer for complexes containing
trifluoroacetate ligands and having extended structures. Thus, the
…
longest Cu Cu separation between the carboxylate-bridged
copper(I) atoms is observed in 3 and 5 (2.832(1) and 2.833(1) s,
respectively). The former compound also has the longest and the
…
shortest Cu Cu diagonals between the non-bridged metal centers
(averaged to 4.822(1) and 2.787(1) s for two crystallographically
unique tetramers). Consequently, the interior Cu–Cu–Cu angles in
3, averaged to 59.95(3) and 119.67(4)u, are the smallest and the
largest angles in this series. Interestingly, the copper(I) atoms in 2
…
compose a unique rhombic core with equal Cu Cu distances
…
(2.6895(6) s), which are the shortest carboxylate-bridged Cu Cu
contacts in the whole family.
A close comparison of the newly prepared complexes 1–3 with
the previously reported copper(I) benzoate7 (4) and trifluoroace-
tate8 (5) reveals several noteworthy structural features (ESI{). In
The average intramolecular Cu–Ocarb distances are similar in 1
and 2 (1.857(4) and 1.853(2) s, respectively) and are slightly longer
than those in 4 (1.840(15) s).7 In 3, however, the average Cu–Ocarb
bond length of 1.873(4) s is longer than that in 1 or 2, but similar
to that in 5 (averaged to 1.870(5) s).8 The intermolecular copper–
oxygen contacts in 3 in the range of 2.405(4)–2.745(4) s are
comparable to those of 2.621(6) s in 5.
…
general, the Cu Cu distances in 1–3 are close to the sum of the
van der Waals radii (rvdW(Cu) = 1.40 s)15 and fall into the
Complexes 1–3 expand the family of Cu4-based carboxylates,
which now allows for the first analysis of their structure–property
relationships. All copper(I) carboxylates 1–3 exhibit yellow-to-
green photoluminescence (PL) upon exposure to UV radiation in
the solid state. The PL measurements (lex = 350 nm) carried out
at room temperature on crystalline samples in the range of 250–
750 nm revealed broad emission bands centered at 502 nm for 1
and 507 nm for 2, while lmax for 3 is red-shifted to 583 nm. Thus,
the emission wavelengths are very close for the structurally similar
complexes [Cu4(O2C(3-F)C6H4)4] (1) and [Cu4(O2C(2,3,4-
F)3C6H2)4] (2), having discrete tetranuclear structures. In
contrast, the tetramers [Cu4(O2CCF3)2(O2CC6F5)2]‘ (3) and
[Cu4(O2CCF3)4]‘ (5), having analogous polymeric structures in
the solid state, both display yellow emission centered at the same
wavelength (Fig. 3).
Fig. 1 The molecular structures of [Cu4(O2C(3-F)C6H4)4] molecule in 1,
[Cu4(O2C(2,3,4-F)3C6H2)4] in 2 and [Cu4(O2CCF3)2(O2CC6F5)2] in 3, with
the Cu4-cores shown by dashed lines.
In this work, we have also prepared the fully substituted
copper(I) pentafluorobenzoate complex in the single crystalline
3854 | Chem. Commun., 2007, 3853–3855
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Our future studies will be aimed at further expansion of the
copper(I) carboxylate family and its comprehensive analysis to
shed light on how to control the nuclearities, shapes and solid state
structures of copper(I) clusters as well as their photoluminescence
behavior by selecting a specific metal–ligand combination.
This work was supported by the National Science Foundation
Career Award (NSF-0546954) and the Donors of the American
Chemical Society Petroleum Research Fund (ACS PRF-42910-
AC3).
Notes and references
{ Crystal data for 1: C28H16Cu4F4O8, M = 810.57, monoclinic, C2/c, a =
17.1591(18), b = 16.0078(16), c = 12.3894(13) s, b = 129.578(1)u, V =
2623.0(5) s3, Z = 4, T = 173(2) K, m(Mo-Ka) = 3.281 mm21, 9452
reflections measured, 2302 unique, full-matrix least-squares refinement on
F2 converged at R1 = 0.0538 and wR2 = 0.1162 for 199 parameters and
1535 reflections with I . 2s(I) (R1 = 0.0933, wR2 = 0.1321 for all data) and
a GOF of 1.061. For 2: C28H8Cu4F12O8, M = 954.50, orthorhombic, Fddd,
a = 15.7443(19), b = 16.574(2), c = 21.682(3) s, V = 5658.1(12) s3, Z = 8,
T = 173(2) K, m(Mo-Ka) = 3.100 mm21, 11 397 reflections measured, 1706
unique, full-matrix least-squares refinement on F2 converged at R1 =
0.0390 and wR2 = 0.0967 for 127 parameters and 1313 reflections with I .
2s(I) (R1 = 0.0542, wR2 = 0.1074 for all data) and a GOF of 1.015. For 3:
Fig. 3 Photoluminescence spectra of solid samples of 1 (#), 2 (m), 3
(%), and 5 ($).
form. However, the crystals grew as extremely thin needles and our
numerous attempts to find one suitable for an X-ray diffraction
analysis were unsuccessful. Interestingly, the crystalline sample of
copper(I) pentafluorobenzoate shows emission centered at ca.
590 nm (lex = 350 nm), close to that of 3 and 5. We may now
¯
C18Cu4F16O8, M = 902.34, triclinic, P1, a = 11.4301(8), b = 14.4443(11), c =
speculate that this fact suggests that copper(I) pentafluorobenzoate
…
16.7589(12) s, a = 71.969(1), b = 74.374(1), c = 70.565(1)u, V =
2438.0(3) s3, Z = 4, T = 173(2) K, m(Mo-Ka) = 3.611 mm21, 21 451
reflections measured, 10 984 unique, full-matrix least-squares refinement on
F2 converged at R1 = 0.0538 and wR2 = 0.1062 for 865 parameters, 72
restraints, and 6917 reflections with I . 2s(I) (R1 = 0.0992, wR2 = 0.1241
for all data) and a GOF of 0.961 CCDC 648824–648826. For crystal-
lographic data in CIF or other electronic format, see DOI: 10.1039/
b707957e
also has the Cu4-based extended structure built on Cu
O
intermolecular interactions, similar to those of 3 and 5.
Thus, the current study resulted in the preparation and
structural analysis of three new tetranuclear [Cu4(O2CR)4]
complexes, of which compounds 1 (R = (3-F)C6H4) and 2 (R =
(2,3,4-F)3C6H2) have discrete molecular structures, while 3 (R =
…
CF3/C6F5) is a polymer based on axial Cu O interactions
analogous to copper(I) trifluoroacetate. The latter two extended
structures are good illustrations of the enhancement of inter-
molecular interactions between isolated copper clusters when
ligands with greater electron-withdrawing abilities are used.
Control of such individually weak but collectively strong
intermolecular interactions can be critical to rational molecular
design in supramolecular chemistry.
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Importantly, the consideration of photoluminescence properties
along with the structural features for the tetranuclear copper(I)
clusters revealed that the emission wavelength exhibits
a
dependence on the structural type (discrete clusters vs. extended
motifs) for this series. This is the first such observation for the
copper(I) carboxylate family, which became possible only after
new members of similar structural types were synthesized and
crystallographically characterized. To verify the observed trend, we
prepared single crystals of copper(I) benzoate, for which the
tetranuclear core structure was reported back in 1977.7 The PL
measurements for [Cu4(O2CC6H5)4] revealed an emission red-
shifted to ca. 600 nm (lex = 350 nm). Intrigued by this fact, we re-
collected the X-ray diffraction experiment and, when analysing the
solid state structure of 4, found its essential difference with that of
1 and 2. In 4, there is a close alignment of two crystallographically
independent tetramers (ESI, Fig. 4{) with intermolecular copper–
copper contacts at 3.239(2) s (Cu–Cu–Cu angle is 162.35(7)u).
These types of interaction were overlooked in the past and are
absent in the solid state structures of 1 and 2.
14 M. A. Petrukhina, Coord. Chem. Rev., 2007, 251, 1690.
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