Experimental and theoretical study of novel luminescent Di-, Tri-, and tetranuclear copper triazole complexes

Article abstract of DOI:10.1021/om1011648

A number of novel luminescent di-, tri-, and tetranuclear copper complexes have been synthesized in good yields using a modular ligand system based on triazole-phosphines. This kind of ligand is easily accessible through an improved one-pot synthesis that

Full text of DOI:10.1021/om1011648

ARTICLE
pubs.acs.org/Organometallics
Experimental and Theoretical Study of Novel Luminescent Di-, Tri-,
and Tetranuclear Copper Triazole Complexes
Daniel M. Zink,† Tobias Grab,‡ Thomas Baumann,*,‡ Martin Nieger,§ Ericka C. Barnes,|| Wim Klopper,^ and
Stefan Br€ase*,†
†Institute of Organic Chemistry, Karlsruhe Institute of Technology, KIT Campus South, Fritz-Haber-Weg 6, D-76131 Karlsruhe,
Germany
‡cynora GmbH, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany
§Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55 (A.I. Virtasen aukio 1), FIN-00014
University of Helsinki, Finland
Institute of Nanotechnology, Karlsruhe Institute of Technology, KIT Campus North, Hermann-von-Helmholtz-Platz 1, D-76344
Eggenstein-Leopoldshafen, Germany
^Institute of Physical Chemistry, Karlsruhe Institute of Technology, KIT Campus South, Fritz-Haber-Weg 2, D-76131 Karlsruhe,
Germany
S Supporting Information
b
ABSTRACT: A number of novel luminescent di-, tri-, and tetranuclear copper
complexes have been synthesized in good yields using a modular ligand system
based on triazole-phosphines. This kind of ligand is easily accessible through an
improved one-pot synthesis that permits the facile introduction of various alkyl or
aromatic substituents at the 1-, 4-, or 5-position of the triazole ring. X-ray
crystallography revealed three different types of complexes: neutral di- and tetranuclear structures with Ci symmetry, and a charged
trinuclear structure posessing C3 symmetry. A spectroscopic evaluation showed promising photoluminescence properties in terms
of their potential use in organic light-emitting devices, with maxima in the range of 500ꢀ550 nm, depending on the substitution
pattern of the triazole ligand used for the complexation. Experimental observations were complemented by density functional theory
calculations of ground and excited states.
’ INTRODUCTION
due to their abundance and ability to form a wide range of
luminescent complexes.8 Among the examples mentioned above,
copper complexes represent the most prolific group of lumines-
cent metal compounds. Thus, complexes with chelating bisimine
ligands of the general composition [Cu(N∧N)2]þ, e.g. substi-
tuted bisphenanthroline 1 (Figure 1), are widely reported in the
literature. Other promising complexes possess the general composi-
tion [Cu(P∧P)2]þ, [Cu(N∧N)(P∧P)]þ, and/or [Cu(N∧N)
(P∧N)]þ, together with chelating phosphine ligand 2 and/or
heteroleptic ligand 38,9 (Figure 1).
Luminescent metal complexes have recently been the focus of
many scientific investigations, owing to their potential use as
organic light-emitting diodes,1 solar cells,2 and chemical
sensors,3,4 to name a few. Highlighting their application as
organic light-emitting diodes, a great number of these lumines-
cent complexes have been shown to significantly increase the
luminosity of the diode, such as in the case of triplet emitters. The
second- and third-row transition metals are commonly employed
as the central core of the complex, the most noteworthy of these
being platinum,5 ruthenium,6 osmium,6 and iridium.7 Coupled
with certain ligands, triplet emitters employing these transition
metals have been shown to possess extraordinarily high efficien-
cies. A downside to utilizing these metals, however, is their
inaccessibility; they are quite rare and expensive. For industrial
applications in mass markets, e.g., flat screens or other novel
lighting applications, the development of novel complexes that
not only possess characteristics similar to those of triplet emitters,
but also fulfill economic and environmental considerations,
becomes necessary. Therefore, in the past few years, the search
for more abundant and economical luminescent complexes has
intensified. The main focus of this research is on metals with d10
configurations, such as Cu(I), Ag(I), Au(I), Zn(II), and Cd(II),
Luminescent complexes of triazole ligands with ruthenium,10
iridium,10,11 platinum,12 osmium,13 and copper14 are also well-
known. In contrast to this, however, little is known about
luminescent complexes with the ClickPhos ligand 4 (Figure 1),
in spite of great potential owing to their simple and economical
preparation, good optical characteristics, and modal structure.
Most of the complexes described above are mononuclear, but
dinuclear and trinuclear metal complexes, such as those involving
platinum17 or rhenium,18 have recently gained attention as
potential novel optical materials. In this study, we report on
Received: December 10, 2010
Published: May 20, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/om1011648 Organometallics 2011, 30, 3275–3283
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Organometallics
ARTICLE
Scheme 2. Synthesis of the Copper Triazole Complexes
3aꢀo
Figure 1. Different N∧N, P∧P, and N∧P ligands.
Scheme 1. Synthesis of ClickPhos-like triazole ligands 219
Table 2. Synthesized Copper Triazole Complexes
ligand
metal salt
complex
stoichiometrya
color
2a
2a
2a
2a
2a
2b
2b
2b
2c
2d
2e
2g
2h
2i
CuI
3a
3b
3c
3d
3e
3f
L2Cu2I2
colorless
colorless
colorless
colorless
colorless
colorless
gray
CuBr
CuCl
CuI
[L3Cu3Br2][CuBr2]
[L3Cu3Cl2][CuCl2]
L2Cu4I4
CuI
[L3Cu3I2][CuI2]
[L3Cu3I2][CuI2]
[L3Cu3Br2][CuBr2]
[L3Cu3Cl2][CuCl2]
[L3Cu3I2][CuI2]
[L3Cu3I2][CuI2]
[L3Cu3I2][CuI2]
[L3Cu3I2][CuI2]
[L3Cu3I2][CuI2]
[L3Cu3I2][CuI2]
[L3Cu3I2][CuI2]
Table 1. ClickPhos-ligands Synthesized To Furnish Various
Copper Triazole Complexes
CuI
CuBr
CuCl
CuI
3g
3h
3i
compd
R1
R2
R3
gray
colorless
colorless
colorless
colorless
colorless
orange
2a
2b
2c
2d
2e
2f
Ph
Ph
Ph
Ph
Ph
Ph
CuI
3j
Bn
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
C6F5
CuI
3k
3l
hexyl
CuI
2-phenylethyl
CuI
3m
3n
3o
Ph
propyl
propyl
propyl
naphthyl
Ph
CuI
Bn
2j
CuI
colorless
2g
2h
2i
hexyl
a Based on mass spectroscopy and X-ray crystallography (for 3a,d,k).
Ph
4-nitrophenyl
at the 1- or 5-position. This was achieved by replacing an alkyl
substituent with a phenyl ring. In doing so, one can reveal the
extent at which the steric characteristics of the ligands affect the
complexation.
The synthesis of the complexes was carried out by reaction of
the triazole ligands with the corresponding amount of copper
halide in dichloromethane, with the complexation being com-
pleted after a few hours. The purification of the complexes was
accomplished by precipitation of the filtered reaction mixture in
cyclohexane. The complexes given in Table 2 were obtained in
high purity and moderate yields.
For the 4-(diphenylphosphino)-1,5-diphenyl-1H-1,2,3-tria-
zole (2a) and 1-benzyl-4-(diphenylphosphino)-5-phenyl-1H-1,
2,3-triazole (2b) ligands, copper(I) iodide as well as copper(I)
bromide and chloride, were used as metal salts. Members of the
homologous series of triazole copper halide complexes were
investigated, in order to gain further understanding of the trends
in structural and electronic effects. Only copper(I) iodide was
used as the metal salt in the preparation of all other trisubstituted
triazole copper complexes (Table 2), however, as the lumines-
cence of these complexes proved to be more intense than those
of the corresponding copper(I) bromide and chloride complexes.
The reaction of 4-(bis(perfluorophenyl)phosphino)-1,5-diphen-
yl-1H-1,2,3-triazole (2k) with copper(I) iodide showed no
complexation, most likely due to the electron-withdrawing effect
of the electronegative fluorine atoms that also serve to lower the
electron density of the phosphorus atom. Furthermore, the
reactions of different triazoles with copper(I) cyanide did not
result in the formation of a copper complex, further demonstrat-
ing the ability of halogen atoms to act as bridging ligands.
2j
Ph
Ph
3-chlorophenyl
Ph
2k
the synthesis and structure of new polynuclear and luminescent
complexes of various ClickPhos-type ligands with copper as the
central metal.
’ SYNTHESIS OF LIGANDS AND COMPLEXES
For the synthesis of the different di-, tri-, and tetranuclear
copper triazole complexes, we used a modular ligand system
based on ClickPhos-like 1,4,5-trisubstituted triazoles. Most of
these ligands are accessible not only according to the Sharpless or
Zhang reactions via a two-step procedure,15,16 but also, more
conveniently, by an easy-to-handle and straightforward one-pot
reaction.16 In our previous work, we not only improved on this
method (Scheme 1),19 but also synthesized all of the 1,4,5-
trisubstituted triazoles 2aꢀk summarized in Table 1 in moderate
to good yields.
The selection of ligands was based on two central considera-
tions: those of electronic effects and substitution pattern. First, a
set of triazole ligands with various electronic characteristics was
synthesized in order to determine electronic influences. This set
consisted of azides, alkynes, and phosphines with different
electron-withdrawing and -donating substituents. The substitu-
ents at two positions of the triazole were retained, while the third
substituent was varied in order to examine the electronic effects
of electron-poor and electron-rich triazole ligands on their
corresponding luminescence spectra. Second, trends in the
luminescence spectra were examined by varying the substituents
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Organometallics
ARTICLE
’ STRUCTURE OF COMPLEXES
sandwich-like structure. Figure 2 shows the crystal structure of
this complex. Selected bond lengths and angles are summarized
in Table 3.
The three-dimensional structures of the three di-, tri-, and
tetranuclear copper triazole complexes were determined by X-ray
structure analysis. Complex 3a, with 4-(diphenylphosphino)-1,5-
diphenyl-1H-1,2,3-triazole (2a) as the ligand and copper(I)
iodide as the metal salt, was obtained by recrystallization from
ethyl acetate. The X-ray structure analysis points to a dinuclear,
Ci-symmetric complex composed of two ligand molecules and
two molecules of copper(I) iodide. The copper(I) and the
μ2-iodide ions form a distorted square plane, in which the
copper(I) and the iodide ions each occupy opposite corners.
The two ligands coordinate only the copper(I) ions via their
respective phosphorus atoms and do not make use of any of the
triazole nitrogen atoms for further coordination to the metal
core, as might otherwise be expected. In addition, the copperꢀ
copper distance of 2.60 Å suggests that a direct metalꢀmetal
interaction exists between the two copper(I) ions, serving to
coordinatively saturate the metal. The influence of the bridging
iodine ions on the copperꢀcopper distance and possible
binding interactions in this kind of Cu2I2 core has been
extensively examined by Caulton et al.20 and, based on this
work, a metalꢀmetal interaction in complex 3a is possible. The
two phenyl substituents at the C-5 atom of the triazole are
nearly parallel to the planar copper(I) iodide square, forming a
The measured copperꢀcopper bond length of 2.60 Å is within
the range of the copperꢀcopper distance in the bulk metal
(2.56 Å) and those of other copper complexes with direct
metalꢀmetal interactions (2.55ꢀ2.72 Å),21 and is even shorter
than in complexes with the same kind of Cu2I2 core and assumed
copper interactions.20,22 The assumption of a direct metalꢀ
metal interaction is further supported by the fact that the copper
atom is coordinated to only three ligands and is therefore capable
of forming a metalꢀmetal bond.
The tetranuclear copper complex 3d was obtained under
slightly different conditions (i.e. diffusion of diethyl ether in
dichloromethane solution). Its structure was determined by
X-ray crystallography. In contrast to the previous complex, a
PꢀN motif was formed (Figure 3). In this tetranuclear, Ci-
symmetric complex consisting of two molecules of 4-
(diphenylphosphino)-1,5-diphenyl-1H-1,2,3-triazole and four
copper and four iodine ions, the four coppers and two of the
iodides form an octahedron, with the four copper(I) atoms
spanning the base and the two iodides occupying the remaining
axial positions. Each of the two remaining iodides acts as a
bridging ligand between two copper(I) atoms. The two 4-
(diphenylphosphino)-1,5-diphenyl-1H-1,2,3-triazole ligands co-
ordinate through their phosphorus atom and the nitrogen atom
at the 3-position of the triazole ring as chelating ligands between
two copper(I) atoms. Due to steric reasons, the two ligands and
the two iodides occupy positions opposite from each other. Each
Figure 2. Molecular structure of [(4-(diphenylphosphino)-1,5-diphen-
yl-1H-1,2,3-triazole)2Cu2I2] (3a) (hydrogen atoms omitted for clarity,
displacement parameters drawn at the 50% probability level).
Figure 3. Molecular structure of [(4-(diphenylphosphino)-1,5-diphen-
yl-1H-1,2,3-triazole)2Cu4I4] (3d) (hydrogen atoms omitted for clarity,
displacement parameters drawn at the 50% probability level).
Table 3. Selected Bond Lengths and Angles of Complex 3aa
Bond Lengths (Å)
I(1)ꢀCu(1)
I(1)ꢀCu(1A)#1
Cu(1)ꢀP(1)
2.5466(7) [2.636]
2.5965(6) [2.625]
2.2118(9) [2.268]
2.5965(6) [2.625]
2.6001(8) [2.586]
Cu(1)ꢀCu(1A)#1
Bond Angles (deg)
Cu(1)ꢀI(1)ꢀCu(1A)#1
P(1)ꢀCu(1)ꢀI(1)
60.73(2) [58.9]
P(1)ꢀCu(1)ꢀI(1A)#1
I(1)ꢀCu(1)ꢀI(1A)#1
115.86(3) [118.6]
119.27(2) [121.1]
122.43(3) [119.3]
a The corresponding values computed at the BP86/def2-SV(P) level are given in brackets. Symmetry transformations used to generate equivalent atoms:
(#1) ꢀx þ 1, ꢀy þ 1, ꢀz þ 1.
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Table 4. Selected Bond Lengths and Angles of Complex 3da
Bond Lengths (Å)
I(1)ꢀCu(1)
2.5885(9)
2.6358(9)
2.9679(10)
3.2472(9)
2.5881(9)
2.6005(9)
2.2001(16)
Cu(1)ꢀCu(2A)#1
Cu(1)ꢀCu(2)
Cu(2)ꢀN(1)
2.5398(10)
2.5881(9)
2.8818(11)
3.2472(9)
1.983(4)
I(1)ꢀCu(2)
I(1)ꢀCu(2A)#1
I(1)ꢀCu(1A)#1
I(2)ꢀCu(1A)#1
I(2)ꢀCu(2)
2.5398(10)
2.9679(10)
Cu(1)ꢀP(1)
Bond Angles (deg)
Cu(1)ꢀI(1)ꢀCu(2)
66.95(3)
I(2A)#1ꢀCu(1)ꢀI(1)
P(1)ꢀCu(1)ꢀI(1)#1
I(2A)#1ꢀCu(1)ꢀI(1A)#1
I(1)ꢀCu(1)ꢀI(1A)#1
N(1)ꢀCu(2)ꢀI(2)
111.29(3)
96.14(4)
Cu(1)ꢀI(1)ꢀCu(2A)#1
Cu(2)ꢀI(1)ꢀCu(2A)#1
Cu(1)ꢀI(1)ꢀCu(1A)#1
Cu(2)ꢀI(1)ꢀCu(1A)#1
Cu(2A)#1ꢀI(1)ꢀCu(1A)#1
Cu(1A)#1ꢀI(2)ꢀCu(2)
P(1)ꢀCu(1)ꢀCu(2A)#1
P(1)ꢀCu(1)ꢀI(2A)#1
P(1)ꢀCu(1)ꢀI(1)
53.88(2)
80.84(3)
86.59(3)
49.83(2)
55.03(2)
58.61(3)
147.11(5)
119.45(5)
127.58(5)
93.05(3)
93.41(3)
114.59(13)
124.21(13)
108.84(3)
105.75(13)
100.05(3)
99.16(3)
N(1)ꢀCu(2)ꢀI(1)
I(2)ꢀCu(2)ꢀI(1)
N(1)ꢀCu(2)ꢀI(1A)#1
I(2)ꢀCu(2)ꢀI(1A)#1
I(1)ꢀCu(2)ꢀI(1A)#1
1
a Symmetry transformations used to generate equivalent atoms: (#1) ꢀx þ /2, ꢀy þ /2, ꢀz þ 1.
3
copper(I) atom is tetrahedrally coordinated by three iodides and
either a nitrogen or a phosphorus atom. The rectangular base of
the octahedron composed of the four copper ions has two side
lengths measuring 2.53 and 2.88 Å (Table 4). The two shorter
bonds at 2.53 Å feature the iodides as μ2 ligands, while the
triazole ligands act as μ2 ligands in the case of the two longer
bonds. As mentioned above, such a bridging coordination of an
iodide might be an indication of a metalꢀmetal interaction. This
assumption is supported by the relatively short copperꢀcopper
distances. Of the CuꢀCu bond distances of 2.53 and 2.88 Å, the
latter may still be characterized as a direct interaction, based on
comparisons with the CuꢀCu bond length in bulk metal and
other similar complexes.
However, in this kind of complex, the copper(I) atoms are
coordinatively saturated, and a direct metalꢀmetal interaction is
therefore not very likely. Even though the bond lengths I(1)ꢀ
Cu(1A) and I(1A)ꢀCu(1) at 3.25 Å might seem too long for
strong interactions to be present, it is quite reasonable to assume
that weak interactions do exist to coordinatively saturate the copper
ions. This type of Cu4I4 core has been described in the literature.
Mezailles et al.23 and Fu et al.24 synthesized analogous complexes
with either 2-diphenylphosphino-3-methylphosphine or bis-
(dicyclohexylphosphino)methane as the bridging ligand, with the
resulting copperꢀiodide bond distances for the μ4-iodide within the
range of 2.68ꢀ2.88 Å. In contrast, a similar complex reported by
Camus et al.25 with bis(diphenylphosphino)methane shows only a
μ3-iodide, with the distance between this iodide and the noncoor-
dinated copper ion being about 3.34 Å. Therefore, under the
assumption of a μ4-coordinating iodide in the case of complex 3d,
the fourth coordination mode could only be a very weak interaction.
Single crystals of a complex from the reaction of 4-(di-
phenylphosphino)-1-phenyl-5-propyl-1H-1,2,3-triazole (2e) with
copper(I) iodide were obtained by slow diffusion of diethyl ether
in a saturated dichloromethane solution. The X-ray structure
analysis confirmed the presence of a charged, trinuclear metal
Figure 4. Structure of the cationic of complex 3k (anion, solvent, minor
disordered part and hydrogen atoms omitted for clarity, displacement
parameters drawn at the 50% probability level).
complex, [(4-(diphenylphosphino)-1-phenyl-5-propyl-1H-1,2,
3-triazole)3Cu3I2][CuI2] (3k), consisting of three ligands,
namely: 2e, four copper(I) ions, and four iodides. In the
positively charged ion [(4-(diphenylphosphino)-1-phenyl-5-
propyl-1H-1,2,3-triazole)3Cu3I2]þ, having C3 symmetry, three
copper(I) ions and two iodides form a trigonal bipyramid, in
which the three copper(I) ions form the plane, with the two
iodides occupying the remaining positions (Figure 4). Even
though the single crystals obtained were average in quality,
the structure of the complex was nevertheless unequivocally
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ARTICLE
Table 5. Selected Bond Lengths and Angles of Complex 3ka
Bond Lengths (Å)
I(1)ꢀCu(1)
I(1)ꢀCu(3)
I(1)ꢀCu(2)
I(2)ꢀCu(2)
I(2)ꢀCu(1)
I(2)ꢀCu(3)
Cu(1)ꢀN(53)
Cu(1)ꢀP(1)
2.7048(16) [2.746]
2.7114(15) [2.746]
2.7599(15) [2.746]
2.6870(15) [2.778]
2.6894(16) [2.778]
2.7329(15) [2.778]
2.021(9) [2.041]
Cu(1)ꢀCu(3)
Cu(1)ꢀCu(2)
Cu(2)ꢀN(1)
Cu(2)ꢀP(2)
Cu(2)ꢀCu(3)
Cu(3)ꢀN(27)
Cu(3)ꢀP(3)
2.6633(19) [2.636]
2.8158(18) [2.636]
2.002(9) [2.041]
2.210(3) [2.279]
2.8195(18) [2.636]
2.015(9) [2.041]
2.228(3) [2.279]
2.206(3) [2.279]
Bond Angles (deg)
Cu(1)ꢀI(1)ꢀCu(3)
Cu(1)ꢀI(1)ꢀCu(2)
Cu(3)ꢀI(1)ꢀCu(2)
Cu(2)ꢀI(2)ꢀCu(1)
Cu(2)ꢀI(2)ꢀCu(3)
Cu(1)ꢀI(2)ꢀCu(3)
N(53)ꢀCu(1)ꢀP(1)
N(53)ꢀCu(1)ꢀI(2)
P(1)ꢀCu(1)ꢀI(2)
Cu(3)ꢀCu(1)ꢀI(2)
N(53)ꢀCu(1)ꢀI(1)
P(1)ꢀCu(1)ꢀI(1)
I(2)ꢀCu(1)ꢀI(1)
58.91(4) [57.4]
62.02(4) [57.4]
62.03(4) [57.4]
63.17(4) [56.7]
62.69(4) [56.7]
58.83(4) [56.7]
120.3(3) [116.5]
101.7(3) [106.2]
109.31(9) [108.4]
61.40(4) [61.7]
115.1(3) [108.5]
101.37(9) [104.3]
108.86(5) [113.1]
N(1)ꢀCu(2)ꢀP(2)
N(1)ꢀCu(2)ꢀI(2)
P(2)ꢀCu(2)ꢀI(2)
N(1)ꢀCu(2)ꢀI(1)
P(2)ꢀCu(2)ꢀI(1)
I(2)ꢀCu(2)ꢀI(1)
N(27)ꢀCu(3)ꢀP(3)
N(27)ꢀCu(3)ꢀI(1)
P(3)ꢀCu(3)ꢀI(1)
N(27)ꢀCu(3)ꢀI(2)
P(3)ꢀCu(3)ꢀI(2)
I(1)ꢀCu(3)ꢀI(2)
125.4(3) [116.5]
103.6(3) [106.2]
112.06(8) [108.4]
106.8(3) [108.5]
100.55(9) [104.3]
107.32(5) [113.1]
118.1(3) [116.5]
105.6(3) [108.5]
113.43(9) [104.3]
105.8(3) [106.2]
105.79(9) [108.4]
107.40(5) [113.1]
a The corresponding values computed at the BP86/def2-SV(P) level are given in brackets.
determined: each copper(I) is coordinated to the two iodine
ions, a nitrogen, and a phosphorus atom, collectively forming a
flattened tetrahedral structure. Once again, the short cop-
perꢀcopper distance of 2.66ꢀ2.82 Å, which is within the range
of the sum of the van der Waals radii (2.80 Å),26 suggests the
existence of a metalꢀmetal interaction. The copperꢀiodine
distances in this kind of complex are almost equal, differing by
0.05 Å at most (Table 5). Each ligand acts as a chelating ligand
and coordinates to both copper ions via the phosphorus atom and
the nitrogen atom at the 3-position of the triazole ring. The
dinegative counterion [Cu2I4]2ꢀ is a planar rectangle consisting
of four iodides, with both copper(I) ions lying on one of the
diagonals of the rectangle. Consequently, each copper ion is
coordinated by three iodides and the other copper ion. Upon
examination of the literature, very few complexes with such a Cu3I2
core have been studied. To the best of our knowledge, the only two
examples have been described by Nardin et al.,27 with two bis-
(diphenylphosphino)methanes and one iodine as bridging ligands,
and Zhou et al.,26 with three bis(diphenylphosphino)methane
ligands. The bond lengths and angles of these complexes are in
good agreement with the values for complex 3k.
effects. All of the complexes have been synthesized and purified
as described above with a ligand to copper halide ratio of 3:2.
Single crystals were obtained either by recrystallization from
ethyl acetate (complex 3a) or through diffusion of diethyl ether
in dichloromethane (complexes 3d,k). Given the similar struc-
tural motif of complexes 3d,k, which differs significantly from
that of complex 3a, a solvent effect is thought to be responsible
for the formation of the different kinds of complexes. Complex 3a
is subject to harsh conditions during recrystallization in hot ethyl
acetate due to its low solubility. These conditions result in a
change in structure from a charged trinuclear basic structure such
as 3k to the uncharged, dinuclear structure that was revealed by
X-ray analysis. This rearrangement was proven by elemental
analysis before and X-ray structure analysis after the recrystalliza-
tion process. Taking this observation into account, a polar
solvent such as ethyl acetate seems to promote the formation
of a complex such as 3a, while in a less polar solvent combination
such as diethyl ether/dichloromethane, the formation of com-
plexes such as 3d,k might be favored. Theoretical calculations
that take solvent effects into account, for the purpose of
supporting this assumption, are difficult to interpret correctly
and have therefore not been performed; however, these remain
options for future work. The general structures of the complexes
3d,k are very similar: both complexes feature a plane composed
of four and three copper ions, respectively. Furthermore, each
edge of this plane is bridged either only by triazole ligands or by
both triazole ligands and iodide ions. The only true difference
between these two complexes is the substitution pattern at the
4-position of the triazole ring, with complex 3k possessing a
propyl residue, and with complex 3d featuring a phenyl ring
instead. In the case of complex 3k, the larger steric bulk of the
The analysis of the mass spectrometric data of all copper
triazole complexes indicates the exclusive formation of the
charged trinuclear complex. All mass spectra of the complexes
show very similar fragmentation patterns, in that several or all of
the following fragmentation signals occur: L3Cu3X2, L2Cu3X2,
LCu3X2, L2Cu2X, L2Cu, (LꢀN2)Cu. This observation confirms
that the form [(triazole)3Cu3X2][CuX2] is preferred by the
copper(I) triazole complexes (X = I, Br, Cl).
Since the complexes 3a,d,k feature an almost identical ligand
system, a rational explanation might be based on solvent or steric
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Figure 6. Photoluminescence spectra of complexes 3i,k,l,n.
Figure 5. Experimental photoluminescence (arbitrary units) as func-
tion of wavelength (nm).
propyl substituent compared to the phenyl group is most likely
the reason why this complex with three bridging triazole ligands
was formed.
’ DFT CALCULATIONS
Density functional theory (DFT) studies on complexes 3a,k
were performed with the BP86 functional,28 the def2-SV(P) basis
set of Weigend and Ahlrichs,29 and the Turbomole program
package.30 The resolution-of-identity (RI) approximation for the
Coulomb energy and operator was employed in all ground-state
energy calculations, using a self-consistent-field convergence
threshold of 10ꢀ9 hartree and geometry convergence thresholds
of 10ꢀ8 hartree and 10ꢀ5 hartree/bohr for the total energy and
the Cartesian gradient, respectively. The numerical quadrature
was performed on Turbomole’s grid m4. Geometries with Ci
point-group symmetry for 3a and C3 point-group symmetry for
3k were obtained. Note that the complex 3a was computed as a
neutral molecule, whereas 3k was computed as a monocation.
Analytic harmonic vibrational frequencies were computed to
ensure that the optimized geometries are true minima, that is, do
not show imaginary frequencies. Selected geometrical para-
meters of 3a,k are compared with experimental values in Tables 3
and 5, respectively.
The theoretical and experimental geometries for 3a are in fair
agreement (Table 3). The theoretical equilibrium geometry
appears to be slightly more symmetric than the experimental
structure, as shown by the smaller difference between the
calculated I(1)ꢀCu(1) and I(1)ꢀCu(1A) bond lengths.
Also, the calculated P(1)ꢀCu(1)ꢀI(1) and P(1)ꢀ
Cu(1)ꢀI(1A) bond angles are almost equal in magnitude,
whereas the experimental X-ray structure shows a difference of
almost 7ꢀ between these two angles.
The experimental X-ray structure of 3k is much less symmetric
than the gas-phase cation investigated in the DFT calculations, as
demonstrated by the existence of three distinct experimental
I(1)ꢀCu bond lengths and only one bond length in the DFT
calculations (Table 5).
It is tempting to speculate about cuprophilicity in view of the
relatively short computed copperꢀcopper distances of about
2.6 Å, but note that all equilibrium structures have been optimized
at the common gradient-corrected DFT level (BP86 functional).
At this level, dispersion-type Rꢀ6 terms are not accounted for,
and the BP86 functional unavoidably predicts repulsive behavior
Figure 7. Photoluminescence spectra of the complexes 3j,o.
between unsupported metallophilic fragments. Conversely, if a
common gradient-corrected DFT method is able to reproduce
an experimental structure, it should be clear that the interaction is
not of the metallophilic type.31
’ SPECTRAL INVESTIGATION
For further characterization of the metal complexes, photo-
luminescence spectra of some of the complexes were examined.
For this purpose, the individual complexes were dissolved in
dichloromethane in equal molarity and spin-coated on a glass
plate in order to guarantee a uniform layer thickness.
The photoluminescence of the selected complexes in Table 2
was measured as a function of the wavelength (Figure 5).
Complexes 3a,c, with 4-(diphenylphosphino)-1,5-diphenyl-
1H-1,2,3-triazole (2a) as the ligand and copper(I) chloride or
iodide as the metal salt, exhibit the largest intensities, as shown in
Figure 5. The luminescence of the other complexes is of lower
intensity, with their maxima slightly shifted to lower wavelength
in the range of 500ꢀ530 nm. Complexes 3a,c, with 4-(dip-
henylphosphino)-1,5-diphenyl-1H-1,2,3-triazole (2a) as ligand,
show luminescence peaks at approximately 530ꢀ540 nm, within
the green range of the spectrum, and differ only slightly in the
intensities and in the position of their maxima (Figure 5). The
copper(I) chloride complex 3c exhibits a somewhat higher
intensity than the copper(I) iodide complex 3a, which suggests
that the halogen atom has little influence on the type and the
intensity of the luminescence. When the phenyl substituent at the
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C-5 atom of the triazole is replaced by a propyl substituent, the
emission peak is shifted to 525 nm. Exchanging the phenyl
substituent at the nitrogen atom in the 1-position of the triazole
ring with a hexyl substituent or a 4-nitrophenyl substituent or,
alternatively, replacing both phenyl substituents in the 1- and
5-positions by propyl and hexyl substituents, respectively, shifts
the maximum into the short-wave range of the spectrum at
around 500 nm (Figure 6). These results indicate that these
substituents, by changing the electron density of the triazole unit
as a result of their electron-pushing or mesomeric characteristics,
account for an emission peak shifting to a higher energy region of
the spectrum.
Similar conclusions can be derived from the photolumines-
cence curves of complex 3j, with 4-(diphenylphosphino)-1-
phenethyl-5-phenyl-1H-1,2,3-triazole (2d) as the ligand, and
complex 3o, with 5-(3-chlorophenyl)-4-(diphenylphosphino)-
1-phenyl-1H-1,2,3-triazole (2j) as the igand. Both substituents
shift the maxima of the photoluminescence spectra into the
short-wave range to 520ꢀ525 nm (Figure 7).
These results strongly suggest that, by using electron-rich
substituents at all three positions of the triazole moiety,
the intensity can be increased and the maximum of the
luminescence can be shifted into the long-wave region of the
spectrum. With the help of the electron-rich triazole or the
electron-donating phosphine, the ligand can transfer a larger
electron density to the metal core. These changes can be
understood on the basis of HOMOꢀLUMO considerations.
Similar to the case for other copper complexes, the highest
occupied molecular orbital (HOMO) of the complex is mainly
composed of metal orbitals, and the lowest unoccupied molec-
ular orbital (LUMO) is located on the ligand sphere.32 An
electron-rich triazole increases the energy of the HOMO by
transfer of electron density, consequently making the HOMO
ꢀLUMO energy gap smaller and shifting the emission peak
into the long-wave range of the spectrum. Likewise, lowering
the HOMO energy by the use of electron-poor triazole ligands
increases the HOMOꢀLUMO energy gap and leads to a shift of
the maximum into the short-wave range of the spectrum. These
assumptions were confirmed by density functional computa-
tions. At the BP86/def2-SV(P) level, the HOMO energies of
3a,k are ꢀ4.1 and ꢀ6.0 eV, respectively. Emission is shifted by
about 10ꢀ15 nm toward shorter wavelengths from 3a to 3k. It
is evident from Figure 8 that the LUMO of the charged
trinuclear complex resides on the ligand sphere. The HOMO
consists mainly of one d orbital of each metal, as well as the lone
pair electrons of the phosphorus atom and the bonding orbital
Table 7. HOMOꢀLUMO Energy Gaps (in eV) Computed at
the BP86/def2-SV(P) and B3LYP/def2-SV(P) Levels for the
Complexes 3a,k at Their Ground-State Geometries and at the
B3LYP Optimized Geometries of the Excited States
Figure 8. Calculated HOMO (left) and LUMO (right) of complex 3k.
ground
state
lowest singlet
excited state
lowest triplet
excited state
functional
Complex 3a
BP86
1.79
3.57
B3LYP
2.97
2.96
3.39
Complex 3k
BP86
1.98
3.82
Figure 9. Calculated HOMO (left) and LUMO (right) of complex 3a.
B3LYP
3.39
Table 6. Vertical Absorptions and Emissions Computed at the B3LYP/def2-SV(P) TDDFT Level for the Complexes 3a,ka
absorption
emission
state
wavelength (nm)
character
wavelength (nm)
Complex 3a
character
1Ag
1Au
3Ag
3Au
406.2
406.8
408.5
409.0
97% HOMO f LUMOþ1
97% HOMO f LUMO
95% HOMO f LUMOþ1
95% HOMO f LUMO
510.2
508.4
515.5
513.0
99% HOMO f LUMO
99% HOMO f LUMOþ1
98% HOMO f LUMO
98% HOMO f LUMOþ1
Complex 3k
1A
1E
3A
3E
375.1
377.6
378.2
380.5
69% HOMO f LUMOþ1
83% HOMO f LUMO
86% HOMO f LUMOþ1
81% HOMO f LUMO
433.4
436.3
439.2
440.6
92% HOMO f LUMOþ1
80% HOMO f LUMO
100% HOMO f LUMOþ1
72% HOMO f LUMO, 21% HOMO f LUMOþ1
a The point-group symmetries were restricted to Ci and C3, respectively.
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of the carbonꢀnitrogen π bond. The LUMO is mainly com-
posed of antibonding orbitals of the triazole core and one
phenyl substituent.
with good quantum yields and tunable emission hues that
particularly fit into the current interest in OLEDs and related
subjects.
The HOMO and LUMO were also calculated for complex 3a.
Again, the HOMO was found to be on the metal center and the
LUMO on the triazole ligand (Figure 9). In contrast to the
case for the trinuclear complex 3k, the HOMO consists mainly of
the d orbitals of the metals and the nonbonding orbitals of the
bridging halides with an admixture of the lone pair electrons of
the phosphorus atoms of the two ligands. Similar to the case for
the previous complex 3k, the LUMO basically consists of
antibonding orbitals of the triazole moiety.
Because the HOMO and LUMO reside on the metal clusters
and ligand spheres, respectively, the electronic excitations dis-
cussed here are expected to be of metal-to-ligand charge transfer
character (MLCT). DFT calculations of such excitations are
known to be very difficult, if not impossible, especially with a
functional that lacks exact exchange such as the BP86 functional.
We have nevertheless attempted to compute absorption and
emission spectra for complexes 3a,k at the time-dependent DFT
level (TDDFT) using the B3LYP functional,33 which includes
certain amounts of HartreeꢀFock exchange, to obtain some
qualitative insight into these excitations. The TDDFT calcula-
tions were performed with the Turbomole program package,34
and the results are shown in Table 6.
In 3a, the first two LUMOs in the ground-state geometry have
ag (LUMO) and au (LUMOþ1) symmetry and are energetically
nearly degenerate. Hence, the excitation energies come in pairs.
In accord with the pronounced MLCT character, the singlet and
triplet excitations are also very similar in energy. Despite the
charge-transfer character, the longest computed emission wave-
length (515.5 nm) is in reasonable agreement with experiment
(ca. 530 nm). In 3k, the three ligand LUMOs transform
according to the irreducible representations a and e of the C3
point group and are almost degenerate. The corresponding
excitation energies are very similar and, as for 3a, the pronounced
MLCT character is reflected in the similar singlet and triplet
excitation energies. 3k displays much shorter wavelengths than
3a, in accord with experiment and with its lower lying HOMO
energy and larger HOMOꢀLUMO energy gap (Table 7).
Although the quantitative agreement between the computed
and measured emission wavelengths for 3k is rather poor
(which is not unexpected), the observed trends are captured
correctly by the TDDFT calculations, which hint at pronounced
MLCT-type excited states involving HOMOfLUMO(þ1)
orbital transitions.
’ CRYSTAL STRUCTURE STUDIES
Single-crystal X-ray diffraction studies were carried out on a Nonius
Kappa-CCD diffractometer at 123(2) K using Mo KR radiation (λ =
0.710 73 Å). Direct methods (3k,d) or Patterson methods (3a)
(SHELXS-97)35 were used for structure solution, non hydrogen atoms
were refined anisotropically (full-matrix least-squares refinement on F2
(SHELXL-97). H atoms were refined using a riding model. Semiempi-
rical absorption corrections were applied. In 3k one propyl group and
one CH2Cl2 solvent molecule were disordered.
Complex 3a: colorless crystals, C52H40Cu2I2N6P2, M = 1191.72,
crystal size 0.45 ꢁ 0.25 ꢁ 0.15 mm, triclinic, space group P1 (No. 2), a =
8.903(1) Å, b = 9.894(2) Å c = 15.726(3) Å, R = 74.45(2)ꢀ,
β = 89.19(2)ꢀ, γ = 65.38(2)ꢀ, V = 1205.7(4) Å3, Z = 1, F(calcd) =
1.641 Mg mꢀ3, F(000) = 588, μ = 2.271 mmꢀ1, 26 882 reflections
(2θmax = 55ꢀ), 5477 unique reflections (Rint = 0.049), 289 parameters,
R1 (I > 2σ(I)) = 0.032, wR2 (all data) = 0.088, GOF = 1.05, largest
difference peak and hole 1.440 and ꢀ1.236 e Åꢀ3
.
Complex 3d: yellow crystals, C52H40Cu4I4N6P2 C4H10O, M =
3
1646.72, crystal size 0.25 ꢁ 0.15 ꢁ 0.10 mm, monoclinic, space group
C2/c (No. 15), a = 24.312(3) Å, b = 9.928(1) Å, c = 25.220(3) Å, β =
108.98(1)ꢀ, V = 5756.4(11) Å3, Z = 4, F(calcd) = 1.900 Mg mꢀ3, F(000) =
3176, μ = 3.703 mmꢀ1, 35 230 reflections (2θmax = 55ꢀ), 6576 unique
reflections (Rint = 0.089), 330 parameters, R1 (I > 2σ(I)) = 0.057, wR2
(all data) = 0.151, GOF = 1.08, largest difference peak and hole 1.969
and ꢀ2.873 e Åꢀ3
.
Complex 3k: yellow crystals, [C69H66Cu3I2N9P3]þ 0.5[Cu2I4]ꢀ
3
3
2CH2Cl2, M = 2045.83, crystal size 0.24 ꢁ 0.18 ꢁ 0.06 mm, monoclinic,
space group P21/c (No. 14), a = 14.341(1) Å, b = 16.446(2) Å,
c = 32.859(4) Å, β = 93.29(1)ꢀ, V = 7737.1(14) Å3, Z = 4, F(calcd) =
1.756 Mg mꢀ3, F(000) = 4000, μ = 2.927 mmꢀ1, 52 479 reflections
(2θmax = 55ꢀ), 17 114 unique reflections (Rint = 0.054), 871 parameters,
1248 restraints, R1 (I > 2σ(I)) = 0.094, wR2 (all data) = 0.207, GOF =
1.28, largest difference peak and hole 2.528 and ꢀ1.914 e Åꢀ3. Due to the
bad quality of the data and the disorder, in addition to geometrical
restraints and constraints for the displacement parameters for the
disordered parts, general restraints for the displacement parameters were
applied. However, the structure was unequivocally determined.
Crystallographic data (excluding structure factors) for the structures
reported in this work have been deposited with the Cambridge Crystal-
lographic Data Centre as Supplementary Publication Nos. CCDC
801004 (3a), CCDC 801005 (3k), and CCDC 801006 (3d). Copies
of the data can be obtained free of charge on application to The
Director, CCDC, 12 Union Road, Cambridge DB2 1EZ, U.K. (fax, int.
codeþ(1223)336-033; e-mail, deposit@ccdc.cam.ac.uk).
’ CONCLUSION
In summary, phosphinotriazolyl ligands comprising a variety
of aryl and alkyl substituents have been synthesized. Treatment
of these potentially bidentate ligands with copper halides yielded
a series of new emissive metal complexes possessing three
different structural classes, mainly a charged trinuclear complex
(3k) and, probably due to solvent effects or steric reasons, a
dinuclear (3a) and a tetranuclear (3d) complex, respectively.
Both the absorption and the corresponding emission peak
positions of the di- and trinuclear complexes were measured,
for which a rational explanation is provided by the HOMO/
LUMO correlation via the assistance of computational ap-
proaches. We anticipate that such a design strategy might provide
new insight into the preparation of group 11 fluorescent dyes
’ ASSOCIATED CONTENT
S
Supporting Information. CIF files giving crystallo-
b
graphic data for compounds 3a (CCDC-801004), 3k (CCDC-
801005), and 3d (CCDC-801006) (CIF-format), text and
figures giving details of the syntheses and characterization data
for the compounds prepared in this paper, and tables giving
Cartesian coordinates (in Å) of the optimized BP86/def2-SV(P)
and B3LYP/def2-SV(P) ground-state equilibrium geometries of
the compounds 3a,k. This material is available free of charge via
the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
(17) Wang, K.-W.; Chen, J.-L.; Cheng, Y.-M.; Chung, M.-W.; Hsieh,
C.-C.; Lee, G.-H.; Chou, P.-T.; Chen, K.; Chi, Y. Inorg. Chem. 2010,
49, 1372.
(18) Mauro, M.; Procopio, E. Q.; Sun, Y.; Chien, C.-H.; Donghi, D.;
Panigati, M.; Mercandelli, P.; Mussini, P.; D’Alfonso, G.; De Cola, L.
Adv. Funct. Mater. 2009, 19, 2607.
Corresponding Author
*E-mail: braese@kit.edu (S.B.); baumann@cynora.de (T.B.).
(19) Zink, D. M.; Baumann, T.; Nieger, M.; Br€ase, S. Eur. J. Inorg.
Chem. 2011, 8, 1432–1437.
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