Tetranuclear copper(I) iodide complexes of chelating bis(1 -benzyl-1 H-1,2,3triazole) ligands: Structural characterization and solid state photoluminescence

Article abstract of DOI:10.1021/ic902236n

A series of tetranuclear Cu₄l₄(Ln)₂ clusters (1-3) supported by the chelating 4,4'-(4,5-diX-1,2-phenylene)bls(1 -benzyl-1 H-1,2,3-triazole) ligands (L1, X = H; L2, X = CH₃; L3, X = F) have been prepared. Crystal

Full text of DOI:10.1021/ic902236n

2834 Inorg. Chem. 2010, 49, 2834–2843
DOI: 10.1021/ic902236n
Tetranuclear Copper(I) Iodide Complexes of Chelating Bis(1-benzyl-1H-1,2,3-
triazole) Ligands: Structural Characterization and Solid State Photoluminescence
Gerald F. Manbeck, William W. Brennessel, Christopher M. Evans, and Richard Eisenberg*
Department of Chemistry, University of Rochester, Rochester, New York 14627
Received November 12, 2009
A series of tetranuclear Cu4I4(Ln)2 clusters (1-3) supported by the chelating 4,40-(4,5-diX-1,2-phenylene)bis-
(1-benzyl-1H-1,2,3-triazole) ligands (L1, X = H; L2, X = CH3; L3, X = F) have been prepared. Crystal structure
determinations have shown that the clusters adopt a distorted “step-type” geometry in which the triazole ligands exhibit both
chelating and bridging coordination modes to Cu(I) ions. When L3 was employed in a 3:2 CuI/L3 ratio, a crystalline product,
4, was obtained. Product 4 is actually a 1:1 cocrystallization of the tetranuclear cluster 3 (denoted 4a) and the dinuclear
Cu2I2(L3)2 complex (4b). X-ray structures of 1, 2, and 4 show the presence of close Cu Cu contacts in the tetranuclear
3 3 3
and dinuclear forms. 1H NMR experiments indicate that rapid exchange occurs in these systems. All clusters are brightly
luminescent in the solid state at 77 K and at room temperature with λmax = 495-524 nm. Emission intensity decays fit a
single exponential with lifetimes on the order of 10 to 35 microseconds at room temperature and 90 to 140 microseconds at
77 K. The origin of emission is tentatively assigned as a metal-to-ligand phenylene π* charge transfer.
Introduction
than those employing organic dyes. In electroluminescent
devices, only 25% of the excitons generated are of singlet
multiplicity and 75% are triplets. While organic emitters utilize
the singlet excitons for light emission, they are unable to use the
triplet excitons in a similar way, thereby restricting their overall
quantum efficiencies. On the other hand, metal complex lumi-
nophores exhibit emission from triplet excitons, and are also
able to promote rapid intersystem crossing to convert singlet
excitons into emissive triplet excited states. This means that the
potential overall efficiency of metal complex luminophores is
substantially higher than that of organic emitters.
Recent production of marketable flat panel displays based on
organic light-emitting diodes (OLED0s) and electrolumine-
scence1,2 has led to a surge in research directed toward impro-
vement of device efficiency, durability, and flexibility.3-12
Typically, the emission in these devices arises from a lumino-
phore doped in a polymer matrix. As a consequence of
spin statistics,13,14 quantum efficiencies of devices containing
transition metal complexes as the luminophores are often better
*To whom correspondence should be addressed. E-mail: eisenberg@
chem.rochester.edu.
Of metal complex luminophores that are triplet emitters,
iridium(III) tris-cyclometalated systems have enjoyed parti-
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Published on Web 02/16/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2835
Unfortunately, iridium, as well as platinum which also forms
luminescent cyclometalated complexes, are relatively rare and
costly, so that an impetus exists to find suitable emitters based
on less expensive metals. In this context, the luminescent
properties of copper(I) complexes have driven recent interest,
and a limited number of OLED devices containing such
emitters have been studied.28-38 For the preparation of devices
by chemical vapor deposition (CVD), other factors regarding
potential dopant emitters must also be considered, most nota-
bly, their thermal stability and sublimability. While many Cu(I)
complexes are photoluminescent, they generally do not satisfy
the last criterion for CVD device preparation because they are
cationic or highly aggregated, making them unsublimable.
In 2005, Harkins and Peters reported a neutral dinuclear
Cu(I) complex that exhibited an impressive phosphorescence
quantum yield and excited state lifetime.39 The complex was
also found to be sublimable and thermally stable, leading to its
incorporation in an OLED device with an external quantum
efficiency of 16%.40 Related mononuclear complexes show
similar photophysical properties.41 On the basis of these results,
the synthesis and study of new Cu(I) complexes that possess
charge neutrality for sublimability, thermal stability for CVD,
and bright photoluminescence become even more compelling.
While the majority of emissive Cu(I) complexes studied con-
tain nitrogen-heterocycle ligands,31,42-47 one type of hetero-
cycle which has not been investigated thoroughly as a ligand
is the 1,4-disubstituted-1,2,3-triazole. In fact, the literature is
nearly void of isolated copper(I) complexes of the 1,2,3-tria-
zole, although Fokin and others have shown that chelating
polytriazoles are viable ligands for accelerating by increasing
the yield and decreasing the reaction time in triazole synthesis,
presumably by stabilizing intermediates of the active Cu(I)
catalyst.48 The closest example of an isolated complex is a
tetranuclear L4Cu4Br4 complex where L = 1,2,3-triazole-1,5-
R-quinoline.49 The ligand was obtained by the Cu(II)-pro-
moted oxidation of quinoline-2-carbaldehyde hydrazone, and
the copper(I) complex was isolated in situ as a tetranuclear step-
type cluster that exhibited solid state emission with λmax=400 nm.
While an impressive number of papers have dealt with
triazole synthesis,50 their use in constructing complicated
architectures,51,52 and triazoles as links between molecular
components such as porphyrins and fullerenes,53 there are a
limited numberof reported transition metal complexes of 1,4-
disubstituted triazoles. The virtues of making 1,4-disubsti-
tuted-1,2,3-triazoles by Sharpless “click” chemistry (eq 1)54
and their relative stability once formed provide a strong case
for increased study of triazole coordination chemistry. To
date, palladium(II) and platinum(II) complexes have been
prepared with monodentate 1,2,3-triazoles55 and chelating
phosphine-triazole ligands.56 Ruthenium(II) complexes with
2-triazolo-pyridine ligands have been prepared for compar-
ison of the electronic properties of a chelating triazole to
2,20-bipyridyl.57,58 The complex [ReCl(CO)3(Bn-pyta)]
(Bn-pyta = 1-benzyl-4-(2-pyridyl)-1,2,3-triazole) exhibits a
blue shift in its electronic spectrum relative to that of
[ReCl(CO)3(bpy)],59 consistent with a higher lowest unoccu-
pied molecular orbital (LUMO) energy for the pyta
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2836 Inorganic Chemistry, Vol. 49, No. 6, 2010
Manbeck et al.
Scheme 1
filled metal d-orbitals, while having the rigidity to suppress
excited state distortions that commonly enhance non-radia-
tive relaxation in Cu(I) complexes.42,47,60-62
added to redissolve the precipitate. Upon standing for 1-2
days, block crystals formed that were identified by X-ray
crystallography and microanalysis as the tetranuclear cluster,
Cu4I4(L1)2 (1). With the 1:1 metal-to-ligand ratio, a 1:1 Cu/
L1 dinuclear complex with halide bridges and a planar
rhombic core had been anticipated with each Cu(I) ion
tetrahedrally coordinated by the two bridging halides and
the chelating bis(triazole) ligand. This type of coordination
had been obtained before with chelating ligands such as
diphosphines35,63-67 and N-heterocycles such as pyridine
derivatives68-70 or phenanthroline,71,72 and has been seen
for related complexes containing monodentate phosphines
and N-heteroaromatic ligands as well.73 However, despite
these precedents and the 1:1 Cu/L1 stoichiometry used in
these reactions, only the tetranuclear cluster 1 was obtained.
Subsequently, 2and 3were prepared without the use of ex-
cess ligand, that is, CuI/L ratios of 2:1 were employed. In the
synthesis of 3 using the 2:1 metal-to-ligand ratio, two diffe-
rent crystalline products were obtained. The first was the
analogous tetranuclear cluster 3, which was the major
product, but a second crystalline form, identified as 4, was
also present in small amounts. This form was found to be a
1:1 cocrystallization of the tetranuclear cluster 3 (deno-
ted as 4a) and the initially anticipated 1:1 dinuclear Cu2I2-
(L2)2 compound (4b).
Results and Discussion
Synthesis. Ligands L1-L3 were synthesized via the
“click” reaction of 4,5-disubstituted-1,2-bis(trimethylsilyl-
ethynyl)arenes that were in turn obtained from Sonogoshira
coupling of trimethylsilylacetylene and the corresponding
o-dihaloarenes (Scheme 1). For R = F, the commonly used
catalyst system of PdCl2(PPh3)2 and CuI was sufficiently
active to work successfully with the 1,2-dibromoarene but for
R=HorCH3, it was necessary to use the corresponding 1,2-
diiodoarenes for the coupling chemistry. For triazole synth-
esis via click chemistry, the procedure reported by Fletcher58
in which alkyne deprotection, alkyl azide formation, and
cycloaddition are carried out in a single reaction vessel was
preferred, although isolation of the free alkynes prior to cycli-
zation worked with equal success. Ligands were obtained in
satisfactory yields and purified without employing column
chromatography.
For initial exploration of the coordination chemistry of the
ligands, 1 equiv of L1 was stirred in acetonitrile with CuI.
After 3 h, addition of diethyl ether resulted in precipitation of
a fine white powder, to which a few drops of acetonitrile were
as 3 is a green emitter and 4 exhibits bright blue emission.
The crystals were large enough to separate 3 and 4
manually by pipet. In 4, the empirical ratio of CuI to L3
is 3:2, but the exclusive synthesis of this crystalline form
using this particular metal-to-ligand ratio resulted only in
mixtures of 3 and 4. Ultimately, crystals of 4 were obtained
The relative amounts of crystalline 3 and 4 are discerni-
ble under UV irradiation with a hand-held lamp (365 nm)
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Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2837
Table 1. Summary of Crystallographic Data
1
L3
3
4 2Et2O
3
formula
fw
T (K)
C48H40Cu4I4N12
1546.68
100.0(1)
triclinic
P1
9.4633(7)
11.0674(8)
12.2561(9)
90.175(2)
108.660(1)
100.074(1)
1195.0(2)
1
2.149
4.389
26582
10825
0.0395
7836
307
0.973
C24H18F2N6
428.44
100.0(1)
monoclinic
P21/c
5.4883(7)
18.914(2)
19.122(2)
90
91.535(2)
90
1984.3(4)
4
1.434
0.102
31774
8603
0.0565
5700
361
1.031
0.0524
0.1278
C48H36Cu4F4I4N12
1618.65
100.0(1)
triclinic
P1
9.1964(6)
11.7435(8)
12.9597(8)
114.113(1)
91.823(1)
101.446(1)
1242.1(1)
1
2.164
4.239
28875
11823
0.0472
8428
325
0.956
C104H92Cu6F8I6N24O2
3004.66
100.0(1)
triclinic
P1
14.0558(7)
14.4281(7)
15.4527(8)
70.249(1)
88.674(1)
65.815(1)
2666.2(2)
1
1.871
2.988
62064
25335
0.0596
16179
696
0.992
cryst syst
space group
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V(A )
Z
F
calcd (g cm-3
)
μ (mm-1
)
no. reflns collected
no. unique reflns
Rint
no. of obsd reflns
no. of params
GOF on F2
R1 [I > 2σ(I)]
wR2
0.0388
0.0821
0.0382
0.0711
0.0449
0.0813
Table 2. Selected Bond Lengths (Angstroms) and Angles (Degrees)
1
3
4a
4b
Cu1-Cu2A
2.6311(5)
2.7465(8)
2.7711(4)
2.5564(2)
2.6723(4)
2.7148(5)
2.5883(5)
2.040(3)
2.046(2)
2.083(2)
90.2(1)
59.84(1)
75.37(1)
61.30(2)
57.31(1)
76.45(2)
120.89(7)
2.6443(5)
2.7403(7)
2.7286(4)
2.5746(4)
2.6449(4)
2.6929(4)
2.5891(4)
2.037(2)
2.070(2)
2.079(2)
90.94(9)
59.47(1)
75.37(1)
61.77(1)
58.38(1)
75.18(2)
120.95(6)
2.7034(5)
2.6857(8)
2.6131(4)
2.5968(5)
2.6331(5)
2.7520(5)
2.6112(5)
2.028(3)
2.057(3)
2.083(3)
92.2(1)
58.99(1)
80.73(1)
59.78(2)
60.10(1)
78.71(2)
125.03(7)
2.5303(7) [Cu3-Cu3A]
Cu2-Cu2A
Cu1-I1
2.6272(4) [Cu3-I3]
2.6091(4) [Cu3-I3A]
Cu1-I2A
Cu2-I1
Cu2-I1A
Cu2-I2
Cu1-N1 (chelate)
Cu1-N4 (chelate)
Cu2-N5 (bridge)
N1-Cu1-N4
I2A-Cu1-Cu2A
Cu2-I1-Cu1
Cu2-I1-Cu2A
Cu1-I1-Cu2A
Cu2A-Cu2-Cu1A
N5-Cu2-I2
2.053(2) [Cu3-N7]
2.042(3) [Cu3-N10]
92.1(1) [N7-Cu3-N10]
57.79(1)[Cu3-I3-Cu3A]
107.50(7) [N7-Cu3-I3A)]
as the sole product by seeding an ether/acetonitrile
solution of CuI and L3 in a 3:2 ratio with a few crystals
of 4 which had been collected previously. It was not
possible to isolate the binuclear compound 4b as a
separate entity despite several attempts to do so, in-
cluding the use of excess ligand. A reaction carried out
with a 1:5 CuI to L3 ratio yielded only mixtures of 3 and
4. In addition, if during the reactions of L with CuI, a
small volume of acetonitrile was employed, a white
precipitate formed after 1-2 h regardless of the CuI/
L ratio used. This precipitate was found to exhibit blue
emission, but its identity could not be established by X-
ray crystallography or NMR spectroscopy. The 1H
NMR spectrum of this precipitate showed broadened
resonances suggesting the presence of mixtures or co-
ordination polymers. Dissolution of the precipitate in
acetonitrile followed by addition of ether until satura-
tion resulted in crystallization of the tetranuclear com-
pounds 1-3.
successfully as rapid oxidation to Cu(II) was observed
even under an inert atmosphere.
In an attempt to prepare mixed ligand complexes of the
formulation CuI(L3)L0, 1 equiv of PPh3 was added to 3. In a
separate experiment, CuI, L3, and PPh3 were combined in
equimolar amounts in MeCN to achieve the same product.
However, both experiments resulted in the formation of
[PPh3CuI]4. No mixed species such as [Cu4I4(L2)(PPh3)2] or
[CuI(L3)(PPh3)] were obtained. Likewise, mixed ligand spe-
cies incorporating 2,9-dimethylphenanthroline could not be
obtained. Upon addition of this ligand to an acetonitrile
solution of 3, rapid precipitation of [CuI(2,9-Me2-phen)]2
occurred.
X-ray Crystallography. Crystals of 1-4 were grown
from saturated acetonitrile/diethyl ether solutions (∼1:5
v/v) that were covered and left to stand for 1-2 days at
room temperature. Unit cell, data collection and structure
refinement parameters for all structures reported here are
provided in Table 1 (see Supporting Information for the
cif file containing all crystal and refinement information
as well as all metrical parameters). Selected bond lengths
and angles are given in Table 2. Complexes 1, 3, 4a, and 4b
All complexes are stable to air for extended periods
of time in the solid state, and for several days in solut-
ion. Analogues with CuBr or CuCl were not prepared

2838 Inorganic Chemistry, Vol. 49, No. 6, 2010
Manbeck et al.
Figure 1. ORTEP drawings of 4 with 4a and 4b shown separately. Thermal ellipsoids are drawn at 50% probability level. Hydrogen atoms are omitted for
clarity.
˚
˚
each lie on a crystallographic inversion center with sym-
metry related atoms in Table 2 and the structural figures
denoted by “A”.
atoms (2.6311(5)-2.7034(5) A and 2.6857(8)-2.7465(8) A,
respectively) in the structures of 1, 3, and 4a, indicate some
positive or attractive interaction between these closed shell
d10 metal ions. As imposed crystallographically, atoms
defining the central step, Cu2, Cu2A, I1, and I1A, all reside
in the same plane, while the outer Cu2I2 units are folded
along the Cu1-Cu2A and Cu1A-Cu2 axes. An extended
network connected by π-stacking is observed in the struc-
ture of 4 (Figure 2). The phenylene rings of adjacent 4a units
Complexes 1, 3, and4a adopt the same distorted step-type
structure with minor differences in bond lengths and angles.
The structures of the two complexes in 4 are shown in
Figure 1. In the tetranuclear complexes 1, 3, and 4a, there
are two types of Cu(I) ions present. In each Cu4I4 cluster,
Cu1 ions are coordinated by N1 and N4 of the triazole rings
of a chelating triazole ligand and by two bridging iodide
ligands, whereas the Cu2 ions are coordinated by three μ2-I
ligands and N5 of a bridging triazole moiety. To accommo-
date the Cu1 ions of the cluster, the triazole rings of each
ligand are significantly twisted out of plane of the phenylene
backbone. The structure contains two different types of
iodide ligands: two that are doubly bridging (μ2) and two
that are triply bridging (μ3). The short copper-copper
distances between Cu1 and Cu2 and between the two Cu2
˚
overlap with an interplanar distance of 3.390 A, and a
second π-stacking arrangement exists between the triazole
˚
rings of 4b and the benzyl rings of 4a at 3.464 A.
The dinuclear compound 4b contains the planar Cu2I2
rhombic core that was originally expected and is very
similar in geometry to the central Cu2I2 portion of 1, 3,
˚
and 4a. The Cu Cu distance of 2.5303(7) A (compared
3 3 3
˚
to 2.6857(8) A in 4a) is among the shortest in planar
dinuclear Cu2(μ2-I) compounds such as those supported

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2839
Figure 3. Typical step-type cluster arrangement for complexes formu-
lated as (CuIL)4 or Cu4I4L2 where L = monodentate ligand.
ligands. The latter case is limited to a few examples, most
of which contain NSN bis(pyridyl) thioether ligands. The
structures of 1, 3, and 4a are rare examples of Cu4I4 step
clusters with tridentate ligands. Uniquely in this series of
compounds, adjacent atoms of the triazole rings (N4 and
N5) form part of the donor set. Close metal-metal dis-
tances are accompanied by a folding of the outer Cu2I2
units of the clusters. The Cu Cu distances in 1, 3, and
3 3 3
85,86,88,89
4a are comparable to those reported in other distorted
˚
step clusters (2.655-2.778 A),
although exam-
ples with two unique close metal-metal contacts are
more rare.83,87,90 Generally, metal-metal distances in
step-type clusters are greater than the sum of of the van
˚
Figure 2. Packing diagram for 4 indicating π-stacking interactions
between adjacent phenylene rings of 4b and between 4b benzyl rings
and 4a triazole rings.
der Waals radii (2.8 A).
NMR Studies. The 1H NMR spectra of 1-4 in CD3CN
at 25 °C are characterized by resonances similar to those of
the respective free ligands. For 3 the H NMR spectrum
74
by bulky pyridine ligands (2.557 A), benzimidazoles
1
˚
75
(2.546 A), 2-benzoyl pyridines (2.587 A), imidazolyl
70
˚
˚
exhibits a sharp benzyl CH2 singlet, multiplets from the
benzyl aromatic protons, an A2B2 pattern characteristic of
the phenylene backbone protons, and a singlet from the CH
protons of the triazole rings. In light of the absence of ligand
symmetry in the crystal structure of 3, the observation of
only a single set of ligand resonances in the 1H NMR spectra
of these compounds was unexpected and suggestive of
fluxional behavior or rapid exchange between the free ligand
and the complex. Disregarding slight changes in chemical
shift upon coordination, the only observable difference in
proton signals was a slight broadening of the triazole CH
resonance in the clusters (2.5 Hz fwhm vs 1.5 Hz). Similar
observations were also made for 1 and 2.
76
pyridines (2.592 A), phenanthroline (2.609 A), and
71
˚
˚
others.77-79 Separations shorter than 2.530 A in planar
˚
Cu2I2 are observed only for dinuclear compounds with
˚
bridging ligands such as diphosphino carbenes (2.356 A).
80
The step-type arrangement (Figure 3) in cuprous iodide
cluster chemistry is often obtained in discrete complexes
with monodentate,81,82 bidentate,83,84 or tridentate85-91
(74) Healy, P. C.; Kildea, J. D.; Skelton, B. W.; White, A. H. Aust. J.
Chem. 1989, 42, 115.
(75) Toth, A.; Floriani, C.; Chiesivilla, A.; Guastini, C. Inorg. Chem.
1987, 26, 3897.
(76) Oshio, H.; Watanabe, T.; Ohto, A.; Ito, T.; Masuda, H. Inorg. Chem.
1996, 35, 472.
Variable temperature proton NMR spectra of 3 in
CD3CN from -35 °C to þ 65 °C are shown in Figure 4
and Supporting Information, Figure S1 (the former has
spectra at three representative temperatures while the
latter contains the full experiment). The sample was first
cooled in 10 °C increments, then warmed to 65 °C, and
finally cooled back to room temperature. Throughout the
entire range of temperatures, only a single set of reso-
nances was observed, with the most notable change in the
spectra being the chemical shift of the triazole signal vary-
ing from 7.70 ppm at -35 °C to 7.53 ppm at 65 °C. A
slight decrease in line width was also observed at -35 °C.
Upon returning to room temperature after heating, the
spectrum was identical to that prior to heating.
(77) Ramos, J.; Yartsev, V. M.; Golhen, S.; Ouahab, L.; Delhaes, P.
J. Mater. Chem. 1997, 7, 1313.
(78) Engelhardt, L. M.; Healy, P. C.; Skelton, B. W.; White, A. H. Aust. J.
Chem. 1988, 41, 839.
(79) Belsky, V. K.; Ishchenko, V. M.; Bulychev, B. M.; Soloveichik, G. L.
Polyhedron 1984, 3, 749.
(80) Gischig, S.; Togni, A. Organometallics 2005, 24, 203.
(81) Healy, P. C.; Pakawatchai, C.; Raston, C. L.; Skelton, B. W.; White,
A. H. J. Chem. Soc., Dalton Trans. 1983, 1905.
(82) Cariati, E.; Roberto, D.; Ugo, R.; Ford, P. C.; Galli, S.; Sironi, A.
Inorg. Chem. 2005, 44, 4077.
(83) Lee, C. S.; Wu, C. Y.; Hwang, W. S.; Dinda, J. Polyhedron 2006, 25,
1791.
(84) Mochida, T.; Okazawa, K.; Horikoshi, R. Dalton Trans. 2006, 693.
(85) Su, C. Y.; Kang, B. S.; Sun, J. Chem. Lett. 1997, 821.
(86) Ma, Z. C.; Xian, H. S. J. Chem. Crystallogr. 2006, 36, 129.
(87) Diez, J.; Gamasa, M. P.; Panera, M. Inorg. Chem. 2006, 45, 10043.
(88) Song, R. F.; Xie, Y. B.; Li, J. R.; Bu, X. H. CrystEngComm 2005, 7,
249.
(89) Samanamu, C. R.; Lococo, P. M.; Woodul, W. D.; Richards, A. F.
Polyhedron 2008, 27, 1463.
(90) Xie, Y. B.; Ma, Z. C.; Wang, D. J. Mol. Struct. 2006, 784, 93.
(91) Peng, R.; Li, D.; Wu, T.; Zhou, X. P.; Ng, S. W. Inorg. Chem. 2006,
45, 4035.
To probe further the possibility of dynamic solution
behavior, 1 equiv of L3 was added to a CD3CN solution
of complex 3, and the mixture was cooled to -35 °C. 1H
NMR spectra of the solutions containing L3 þ 3, L3, and
3 at room temperature and -35 °C are shown in Figure 5.

2840 Inorganic Chemistry, Vol. 49, No. 6, 2010
Manbeck et al.
Figure 5. 1H NMR spectra of 3 þ 1 equiv of L3 in CD3CN from -35 to
þ25 °C. (a) Phenylene C-H, (b) triazole C-H, (c) benzyl C-H.
The combined results of these variable temperature NMR
studies indicate that exchange occurs at a rate rapid on the
NMR time scale. In the first experiment, only a single set of
resonances is present at low temperature. This suggests that
in solution rapid exchange occurs even at -35 °C so that the
inequivalent triazoles of each ligand in the solid state
structure are completely averaged. A possible mechanism
of exchange to rationalize the observed equilibration is loss
of N5 bridging coordination to leave Cu2 trigonally co-
ordinated by three bridging iodides in solution (eq 2). In this
manner, triazole equivalence is achieved yielding a single set
of resonances on the NMR time scale. In the second
Figure 4. Variable temperature1H NMRofCu4I4(L3)2 inCD3CN from
-35 to þ 65 °C. (a) phenylene C-H, (b) triazole C-H (c) aromatic benzyl
C-H.
The complete set of spectra is presented in the Supporting
Information, Figure S2. The triazole CH resonance of
free L3 (spectrum 5) is sharper than that of the complex
(spectrum 4) at a lower chemical shift. Resonances for the
mixture of L3 and 3 (spectrum 2) are nearly identical to
those of complex 3. Upon cooling of the sample to -35 °C,
only one set of resonances was observed (spectrum 1).
The only significant change is the shift in triazole CH
resonance to higher frequency as was observed upon
cooling 3 without added L3. Addition of a second
equivalent of L3 to the mixture at room temperature
(spectrum 3) resulted in additional line broadening com-
pared to spectrum 2 with a slight upfield shift in the triazole
CH resonance.
1
experiment, in which H NMR spectra were measured of
3 þ L3, the absence of free L3 resonances further supports
the notion of intermolecular exchange occurring rapidly in
this series of compounds. As additional evidence for rapid
interconversion of several species, the 1H NMR spectrum of
4 is nearly identical to that of 3. Only one set of resonances is
observed, and 4a and 4b are not seen separately. Thus, once
dissolved, the tetranuclear clusters likely exist as a mixture
of Cu4I4L2, Cu2I2L2, and free L that are undergoing rapid
exchange. Rapid exchange has been examined in other
tetranuclear Cu4I4 clusters as well.87
Emission Spectroscopy. Solid state photophysical data
for 1-4 and solution emission maxima for L1-L3 are
summarized in Table 3. Complexes 1-4 are not emissive
in solution, but their solid state luminescence appears
bright. The emission spectra for 1-4 at room temperature
(Figure 6) are independent of excitation wavelength.
Emission maxima at 298 K range from 495 (2 and 4) to
524 (1). At 77 K, there is a slight red shift (5-20 nm) in
emission (Supporting Information, Figure S3) and for
1-3, the same order of emission energy is observed.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2841
Table 3. Photophysical Data for Complexes 1-4 and Ligands L1-L3
orbitals can be higher in energy than π* orbitals of the
ligands.93 Clusters 1-4 are the first examples of distorted
step-type Cu4I4 clusters with tridentate ligands exhibiting
bright photoluminescence. In tetranuclear (CuIL)4 cu-
max
max
solution
λemmax/nm
solid λem
/
solid λem
/
τ/μs,
τ/μs
nm (298 K)c
nm (77 K)c
298 Kd,e 77 Kd,e
L1 370a,b
bane complexes with short Cu Cu contacts, dual emis-
L2 342a,b
3 3 3
sion is observed from 3CC and 3XLCT states.94 For
dinuclear complexes in which the emission has been
assigned to an MLCT excited state, a slight blue shift is
often observed at low temperatures.85,87 The temperature
dependence of emission in 1-3 most closely matches the
behavior of the low energy 3CC band in Cu4I4(pyr)4
which red shifts from 580 nm at 298 K to 619 nm at 77 K
with an increase in lifetime from 11 μs to 25.5 μs.94 With
these similarities in mind, it is tempting to assign the
L3 355a,b
1
2
3
4
524
495
514
495
545
513
528
503
33
12
43
26
110
93
140
140
a Spectra obtained in 1.0 ꢀ 10-5 M MeCN at room temperature.
b λex = 260 nm. c λex = 365 nm. d λex = 335 nm. e Luminescence decay
data are fit to a single exponential decay y = y0 þ Ae(-t/τ)
.
excited state of 1-3 as 3(CC) modified by close Cu Cu
3 3 3
contacts; however, the ligand substitution effects (Me vs
F vs H) are not consistent with this assignment. Twisting
of the triazoles out of plane with the phenylene ring
prevents influence of the 4,5-phenylene substitutions on
electron density at the coordinated triazole via resonance
effects. Also, the trends in emission are not consistent
with an inductive field effect. Congruity between ligand
solution emission trends and solid state cluster trends
suggests involvement of the ligand π* orbitals in the com-
plex excited state, most likely as a component of the
lowest unoccupied molecular orbital (LUMO). A metal-
based or halogen-based highest occupied molecular orbi-
tal (HOMO) cannot be assigned definitively in the
absence of stable CuBr or CuCl analogues since emission
energies in dinuclear complexes can correlate well with
the ligand field strength of the halogen.35
Figure 6. Normalized solid state emission spectra of 1-4 at 298 K
(λexc = 365 nm) and solution emission of L1-L3 (1 ꢀ 10-5 M in aceto-
nitrile, λexc = 260 nm).
The problem becomes more difficult when comparing
the emissions of 3 and 4. Metal-metal distances in 3 and
4a are similar, but shorter in 4b. Since 4b could not be
isolated, its individual emission properties are unknown.
A possible explanation for the blue shift from 3 to 4 is the
Emission bands at room temperature and 77 K are broad
and featureless with large Stokes shifts of 5,900-9,200
cm-1 from the excitation maxima of 350-400 nm. Time-
resolved emission traces can be fit to single exponential
decay with lifetimes at 298 and 77 K of about 30 and 120
μs, respectively, suggestive of triplet state emissions.
The order of emission energy of the complexes in the
solid state (2 > 3 > 1) follows that of the free ligands in
solution (Figure 6). This trend does not correlate with any
inductive or resonance parameters92 that describe the
effects of methyl or fluoro substitution of the ligand
phenylene backbone. While ligand emission likely arises
from a π-π* excited state, the nature of the excited state
in 1-4 is more difficult to assign. Possibilities include
ligand-based 3(π-π*), metal-to-ligand charge transfer
combined effects of different Cu Cu distances and
3 3 3
aromatic intra-complex π-stacking in 4. Assuming the
LUMO of the complex is largely ligand-based, this π-
stacking interaction can modify the energy level of the
LUMO.
In summary, the following factors must be considered
in assigning the nature of emission. There is a red shift and
increase in lifetimes upon cooling which are similar to
3(CC) emissions in tetranuclear cubane complexes. Li-
gand and complex emission follow the same trend in
energy. A blue shift in emission occurs from 3 to 4 in
which the phenylene ring of 4a is involved in intra-
complex π-stacking. Thus, without evidence for contri-
bution of empty cluster-centered orbitals to the LUMO,
the excited state in 1-4 is assigned as metal-to-ligand
phenylene π* charge transfer.
3
3(MLCT), halogen-to-ligand charge transfer (XLCT),
cluster-centered 3(CC), or an admixture of more than one
of the above.
In several reported examples, the emissive properties of
multinuclear Cu(I) complexes with short Cu Cu dis-
3 3 3
tances are affected by metal-metal interactions;91,93,94
however, short metal-metal distances do not necessarily
influence the emission as the unoccupied Cu 4s and 4p
Conclusions
New 4,40-(4,5-diX-1,2-phenylene)bis(1-benzyl-1H-1,2,3-
triazole) ligands L1-L3 have been prepared and their
Cu4I4L2 complexes 1-3 have been characterized. These are
the first Cu(I) iodide complexes of chelating 1,2,3-triazoles.
X-ray crystallographic studies have shown that the complexes
adopt distorted “step-type” geometries in which the triazole
ligands chelate through the N3 atom of the triazoles and bridge
Cu(I) centers through coordination of both N2 and N3 of a
(92) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
(93) Araki, H.; Tsuge, K.; Sasaki, Y.; Ishizaka, S.; Kitamura, N. Inorg.
Chem. 2007, 46, 10032.
(94) (a) Kyle, K. R.; Ryu, C. K.; Dibenedetto, J. A.; Ford, P. C. J. Am.
Chem. Soc. 1991, 113, 2954. (b) Ford, P. C.; Cariati, E.; Bourassa, J. Chem. Rev.
1999, 99 (12), 3625.

2842 Inorganic Chemistry, Vol. 49, No. 6, 2010
Manbeck et al.
given triazole. This N2 triazole coordination has not been
observed before in 1,2,3-triazole coordination chemistry. In
approximately 4000 strong reflections from the actual data collec-
tion after integration.96
Structures were solved using SIR9798 and refined using
SHELXL-97.99 Space groups were determined based on sys-
tematic absences, intensity statistics, and Cambridge Structural
Database frequencies.100 Direct-methods solutions were calcu-
lated which provided most non-hydrogen atoms from the
difference Fourier map. Full-matrix least-squares (on F2)/dif-
ference Fourier cycles were performed which located the re-
maining non-hydrogen atoms. Non-hydrogen atoms were
refined with anisotropic displacement parameters, and hydro-
gen atoms were placed in ideal positions and refined as riding
atoms with relative isotropic thermal parameters.
conjunction with the distorted step-geometry, short Cu Cu
3 3 3
contacts are present. Under certain synthetic conditions, crys-
tals are obtained containing a 1:1 cocrystallization of tetra-
nuclear Cu4I4(L3)2 and binuclear Cu2I2(L3)2. 1H NMR
experiments have been carried out to probe the solution
dynamics of the complexes in view of the fact that ligand
resonances are simpler than the solid state structures would
predict. It is found that exchange occurs extremely rapidly,
even at low temperatures. To our knowledge, these are the first
examples of distorted Cu4I4 step-type clusters with tridentate
ligands that exhibit solid state luminescence. All complexes are
brightly emissive with trends in emission energy following those
of the free ligands. Low temperature emission and lifetime data
have been measured, and the data support an assignment of the
excited state as MLCT Cu d to triazole π*.
Synthesis of 1,2-Diethynylbenzene. To a solution of o-diiodo-
benzene (2.74 g, 8.3 mmol) in N2 purged diisopropylamine (50 mL)
was added PdCl2(PPh3)2 (0.29 g, 5%) and CuI (0.28 g, 17.5%) in
one portion. The vessel was sealed with a septum, and trimethylsi-
lylacetylene (2.87 mL, 2.5 equiv) was added dropwise via syringe.
The reaction darkened with heavy precipitates and was stirred at
25 °C for 1 h and then at 80 °C for 20 h. Upon cooling, the amine
salts were removed by filtration, and the solvent was evaporated.
The dark residue was taken up in hexanes and filtered through a frit
packed with a 3 in. plug of silica gel. The silica was washed with 300
mL of hexanes. Removal of the solvent using a rotary evaporator
provided a yellow oil which was dissolved in 80 mL of degassed
MeOH. KOH (20 mg in 2 mL H2O) was added. After stirring for
2 h, the solvent was evaporated, and the residue was partitioned
between ether (50 mL) and saturated NaCl (20 mL). The organic
layer was dried (MgSO4), filtered, and evaporated to provide the title
compound. NMR data were in agreement with literature values.101
Synthesis of 1,2-Dimethyl-4,5-bis(trimethylsilyl)ethynyl ben-
zene. To a solution of 1,2-diiodo-4,5-dimethyl benzene (6.00 g,
18 mmol) in N2 purged diisopropylamine (125 mL) was added
PdCl2(PPh3)2 (0.63 g, 5%) and CuI (0.51 g, 15%) as solids in one
portion. Trimethylsilylacetylene (5.47 mL, 2.5 equiv) was added
via syringe. The mixture was sealed with a septum and stirred for
16 h at 60 °C, cooled, and filtered. After evaporation of the
solvent, the residue was filtered through a plug of silica gel and
eluted with 10:1 hexanes/ethylacetate. The solvent was evapo-
rated to provide 4.27 g (80%) of the title compound. NMR data
are in agreement with the literature.102
Experimental Section
General Procedures. NMR spectra were obtained using Bru-
ker Avance 400 and 500 MHz spectrometers with 1H NMR
chemical shifts referenced to residual solvent peaks in CDCl3 or
CD3CN. Mass spectra of organic compounds were measured on
a Shimadzu 2010 series liquid chromatography mass spectro-
meter (LCMS) under atmospheric pressure chemical ionization
(APCI). Steady-state excitation and emission spectra were
measured using a Spex Fluoromax-P fluorometer with excita-
tion and emission slits set for a 2 nm band-pass. Ligand emission
spectra in acetonitrile were obtained at a concentration of
1 ꢀ 10-5 M. Solid state emission samples were prepared by
packing a capillary tube with the emissive complex. For low
temperature measurements, the capillary was held in a vacuum
dewar flask filled with liquid nitrogen. Lifetime measurements
were performed using a model LN203C nitrogen laser (Laser
Photonics Inc.) with λ = 337.1 nm and a 0.1 nm spectral width
pulsed at 1 pulse/second. Signals were detected by a silicon
photodiode and photomultiplier tube (1P28 type) with a 2 ns rise
time and viewed with a Hewlett-Packard 54510-B oscilloscope
connected to a computer via a GPIB interface.
Synthesis of 1,2-Diethynyl-4,5-difluorobenzene. The same
procedure used to prepare 1,2-diethynylbenzene was followed
using 1,2-dibromo-4,5-difluorobenzene (1.00 g, 3.67 mmol),
PdCl2(PPh3)2 (0.13 g, 5%), and CuI (0.12 g, 17.5%). A yield
of 67% yield was obtained after workup. Spectral data are in
agreement with literature values.103
4,40-(1,2-Phenylene)bis(1-benzyl-1H-1,2,3-triazole) (L1). To a
stirred solution of 1,2-diethynylbenzene (1.13 g, 8.95 mmol) in
1:1 t-BuOH/H2O (60 mL) were sequentially added NaN3 (1.28
g, 19.69 mmol), benzyl bromide (2.12 mL, 17.9 mmol), sodium
ascorbate (0.53 g, 2.68 mmol), and CuSO4 (2.7 mL of a 0.5 M
solution, 1.34 mmol). The reaction was stirred overnight at
room temperature whereupon heavy precipitates formed. Water
(50 mL) was added, and the mixture was heated until complete
dissolution. Tan microcrystals (1.81 g, 52% yield) formed upon
cooling. 1H NMR (CDCl3 25 °C): δ 7.72-7.70 (m, 2H, Ar-H),
7.43-7.41 (m, 2H, Ar-H), 7.36-7.35 (m, 6H, Ar-H), 7.21
Sonogashira reactions and complex syntheses were carried
out under standard Schlenk conditions. Diisopropyl amine and
acetonitrile (HPLC grade) were degassed via sparging with N2
for 20 min prior to use. Trimethylsilylacetylene, 1,2-diiodoben-
zene, and 1,2-dibromo-4,5-difluorobenzene were purchased
from Oakwood Chemicals. 1,2-Diiodo-4,5-dimethylbenzene
was prepared by literature methods from o-xylene.95 Benzyl
bromide and sodium azide were purchased from Aldrich Che-
mical Co. and used as received.
Crystal Structure Determinations. Each crystal was placed onto
the tip of a glass fiber using Paratone and mounted on a Bruker
SMART Platform diffractometer equipped with an APEX II CCD
area detector. All data were collected at 100.0(1) K using MoKR
radiation (graphite monochromator).96 For each sample, a set of
preliminary cell constants and an orientation matrix were deter-
mined from reflections harvested from three orthogonal wedges of
reciprocal space. Full data collections were carried out with frame
exposure times of 45-60 s at detector distances of 4 cm. Randomly
oriented regions of reciprocal space were surveyed for each sample:
four major sections of frames were collected with 0.50° steps in ω at
four different j settings and a detector position of -38° in 2θ.
The intensity data were corrected for absorption,97 and
final cell constants were calculated from the xyz centroids of
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Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2843
(s, 2H, triazole), 7.22-7.19 (m, 4H, Ar-H), δ 5.40 (s, 4H,
-CH2-) CD3CN: δ 7.70-7.68 (m, 2H, Ar-H), 7.47-7.45,
(m, 4H, Ar-H, triazole), 7.39-7.33 (m, 6H, Ar-H),
7.26-7.25 (m, 4H, Ar-H), 5.45 (s, 4H, -CH2-). MS (APCI
positive) Calcd m/z 392.17, Found 393.15 (MþH)+.
began to form, after which a few drops of MeCN were added to
prepare a saturated solution. After standing 12-24 h, the
products were collected as block crystals.
Complex 1. The general procedure was followed using L1 (38
mg, 97 μ-mol) and CuI (37 mg, 190 μ-mol). 1 crystallized as
white blocks (62 mg, 83% yield). 1H NMR (CD3CN, 25 °C): δ
7.67-7.65 (m, 4H, Ar-H), 7.52 (s, 4H, triazole), 7.48-7.46, (m,
4H, Ar-H), 7.38-7.35 (m, 12H, Ar-H), 7.27-7.26 (m, 8H,
Ar-H), 5.47 (s, 8H, -CH2-). Anal. Calcd for C48H40Cu4I4N12:
C, 37.27; H, 2.61; N, 10.87. Found: C, 37.20; H, 2.27; N,
10.71.
4,40-(4,5-Dimethyl-1,2-phenylene)bis(1-benzyl-1H-1,2,3-tria-
zole) (L2). A mixture of 1,2-dimethyl-4,5-bis(trimethylsilyl)-
ethynyl benzene (0.40 g, 1.3 mmol) and K3CO3 (0.55 g 3 equiv.)
was stirred in 10 mL 1:1 t-BuOH/H2O for 15 min. Sequentially,
sodium azide (0.19 g (2.2 equiv.), benzyl bromide (0.32 mL, 2.6
mmol), sodium ascorbate (0.005 g, 20%), and CuSO4 (0.26 mL
of a 0.5 M solution in water, 10%) were added. The mixture was
stirred at 60 °C for 24 h, cooled, then diluted with water and a
few drops of NH4OH. The brown precipitate was collected,
dissolved in hot EtOH, and filtered through a pad of Celite.
Upon standing at -10 °C, L2 crystallized as white needles (0.19
g, 34%) 1H NMR (CDCl3 25 °C): δ 7.49 (s, 2H, Ar-H), 7.36-
7.34 (m, 6H, Ar-H), 7.20-7.18 (m, 6H, Ar-H, triazole), 5.38
(s, 4H, -CH2-), 2.30 (s, 6H, -CH3). CD3CN: δ 7.37 (s, 2H,
Ar-H), 7.37 (s, 2H, triazole), 7.36-7.35 (m, 6H, Ar-H),
7.33-7.23 (m, 4H, Ar-H), 5.44 (s, 4H, -CH2-), 2.31 (s, 6H,
-CH3). MS (APCI positive) Calcd m/z 420.20, Found 421.20
(MþH)þ.
Complex 2. The general procedure was followed using L2 (38
mg, 90 μ-mol) and CuI (70 mg, 180 μ-mol). Block crystals were
1
obtained (70 mg, 97%). H NMR (CD3CN, 25 °C): δ 7.64 (s,
4H, triazole), 7.41-7.29 (m, 24H, Ar-H), 5.49 (s, 8H, -CH2-),
2.36 (s, 12H, -CH3). Anal. Calcd for C52H48Cu4I4N12: C, 38.97;
H, 3.02; N, 10.49. Found: C, 38.98; H, 2.78; N, 10.36.
Complex 3. The general procedure was followed with L3 (37
mg, 86 μ-mol) and CuI (33 mg, 173 μ-mol). Two subsequent
crops of block crystals were collected for a total of 60 mg (86%).
In a separate experiment, about 5% of the resultant crystals
1
were identified as 4. H NMR (CD3CN, 25 °C): δ 7.60 (t, 4H,
Ar-H, J = 12.35), 7.53 (s, 4H, triazole), 7.39-7.35 (m, 12H,
Ar-H), 7.27-7.26 (m, 8H, Ar-H), 5.47 (s, 8H, -CH2-). Anal.
Calcd for C48H36Cu4F4I4N12: C, 35.62; H, 2.24; N, 10.38.
Found: C, 35.57; H, 1.95; N, 10.27.
4,40-(4,5-Difluoro-1,2-phenylene)bis(1-benzyl-1H-1,2,3-tria-
zole) (L3). To a stirred solution of 1,2-diethynyl-4,5-difluoro-
benzene (0.228 g, 1.4 mmol) in 1:1 t-BuOH/H2O (8 mL) were
sequentially added NaN3 (0.20 g, 3.1 mmol), benzyl bromide
(0.33 mL, 2.8 mmol), sodium ascorbate (0.056 g, 10%), and
CuSO4 (0.28 mL of a 0.5 M solution, 5%). The reaction was
stirred 12 h at 60 °C. The mixture was cooled and triturated with
dilute aqueous NH4OH. After decanting the solution phase, the
solid was dissolved in CH2Cl2 and filtered through a plug of
silica gel eluting with a 1:1 mixture of hexanes/ethylacetate (300
mL). The volume was reduced to about 10 mL, and ether (25
mL) was added. After 30 min, a crop of white crystals formed.
The crystals were collected by vacuum filtration and washed
with cold ether to provide 0.15 g of L3 in 42% yield. 1H NMR
(CDCl3 25 °C): δ 7.55 (t, 2H, Ar-H J = 9.53), 7.37-7.36 (m,
6H, Ar-H), 7.21-7.20 (m, 6H, Ar-H, triazole), 5.39 (s, 4H,
-CH2-). CD3CN: δ 7.62 (t, 2H, Ar-H J = 9.93), 7.46 (s, 2H,
triazole), 7.38-7.35 (m, 6H, Ar-H), 7.26-7.25 (m, 4H, Ar-H),
5.47 (s, 4H, -CH2-). MS (APCI positive) Calcd m/z 428.16,
Found 429.20 (MþH)þ.
Complex 4. The general procedure was used with L3 (20 mg,
47 μ-mol) and CuI (13 mg, 70 μ-mol). The saturated solution
was seeded with about 1 mg of 4 and left to stand at room
temperature. After 2 days, additional compound 4 was collected
as white microcrystals (22 mg, 66%). 1H NMR (CD3CN,
25 °C): δ7.60 (t, 8H, Ar-H, J = 12.4), 7.50 (s, 8H, triazole)
7.39-7.35 (m, 24H, Ar-H), 7.28-7.25 (m, 16H, Ar-H), 5.47
(s, 16H, -CH2-). Anal. Calcd for C96H72Cu6F8I6N24-
2Et2O: C, 41.47; H, 3.09; N, 11.19. Found: C, 41.27; H, 2.84;
N, 11.00.
Acknowledgment. We thank the National Science Founda-
tion GOALI program (Grant CHE-0616782) for support of this
research. G.F.M. is grateful for a GAANN fellowship.
1
Supporting Information Available: Variable temperature H
NMR spectra, ligand and complex absorption spectra, solid
state emission spectra at 77 K, luminescence decay traces at 298
and 77 K, crystallographic data in cif format. This material is
available free of charge via the Internet at http://pubs.acs.org.
General Procedure for Cluster Synthesis. A mixture of L and
CuI (1:2 for 1-3 and 2:3 for 4) was dissolved in acetonitrile and
stirred for 3 h. Diethyl ether was added until a white precipitate