Novel neutral octanuclear copper(i) complexes stabilized by pyridine linked bis(pyrazolate) ligands

Article abstract of DOI:10.1039/b712411b

Two novel, neutral, octanuclear copper(i) complexes displaying twisted-boat Cu₈ conformations and short Cu-Cu interactions have been synthesized from hydrothermal reactions; the complexes show unusual multiple band emissions. This journal is Th

Full text of DOI:10.1039/b712411b

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Novel neutral octanuclear copper(I) complexes stabilized by pyridine linked
bis(pyrazolate) ligands†‡§
Yongbo Zhou and Wanzhi Chen*
Received 13th August 2007, Accepted 28th September 2007
First published as an Advance Article on the web 10th October 2007
DOI: 10.1039/b712411b
Two novel, neutral, octanuclear copper(I) complexes display-
ing twisted-boat Cu8 conformations and short Cu–Cu inter-
actions have been synthesized from hydrothermal reactions;
the complexes show unusual multiple band emissions.
The attractive interaction that exists between closed-shell late
transition metals has received extensive attention.1 Multinuclear
complexes or coordination polymers having such direct metal–
metal interactions are of great interest since the conductivity,
magnetic properties and electrochemical behavior of these metal
aggregates have proved to be potential applications in the field of
nano-scale or molecular scale electronics.2
Pyrazolate ligands can form various complexes with coinage
metal ions with diverse structural motifs.3 They are able to bind
two or more closed-shell d10 metal centers together in distances
close to the sum of the covalent radii of two metals, thus pyra-
zolate ligands are suitable linkages for the construction of metal
aggregates having metal–metal interactions. Several monovalent
Group 11 metal–pyrazolate clusters involving trinuclear,2d,3a–c
tetranuclear,3d,e and dimers of trinuclear3f ,g centers have been
Scheme 1
reported. However, the higher nuclearity pyrazolate clusters are
not known, and the coinage metal complexes of bis(pyrazolate)
ligands have been scarcely reported.4
four dianionic ligands. The structure is shown in Fig. 1. Each of
the four deprotonated 2,6-bis(5-methyl-1H-pyrazol-3-yl)pyridine
ligands links four different copper ions in an exo-tetradentate
mode. The two pyrazolyl groups in each L1 are different in
their coordination mode, one pyrazolyl ring is bound to two
As a continuation of our research on the metallophilic inter-
actions between closed-shell metals,5 herein we report the syn-
thesis, solid-state structures, ESI-MS spectra and photophysical
properties of two fascinating octanuclear complexes [Cu8(L)4] (1,
H2L1 = 2,6-bis(5-methyl-1H-pyrazol-3-yl)pyridine; 2, H2L2 = 2,6-
bis(5-phenyl-1H-pyrazol-3-yl)pyridine) stabilized by the pyridine
linked bis(pyrazolate) ligands (see ESI§).
The bis(pyrazolyl) ligands were prepared from the condensation
reactions of the corresponding 1,3-diketones with hydrazine
hydrate. The Cu8 complexes were obtained by the hydrothermal
reaction of H2L with cuprous oxide at 220 ◦C (Scheme 1). 1 and
2 were isolated as stable yellow crystalline solids.¶ The absence
of downfield resonance signals in their 1H NMR spectra at about
13 ppm due to NH and the disappearance of N–H strectching
bands at ca. 3200 cm−1 in the IR spectra of 1 and 2 show that the
ligands are deprotonated.
X-Ray diffraction analysis shows that the octanuclear com-
plexes are neutral and eight copper(I) ions are held together by
Department of Chemistry, Zhejiang University, Xixi campus, Hangzhou,
310028, China. E-mail: chenwzz@zju.edu.cn
† The HTML version of this article has been enhanced with colour images.
‡ CCDC reference numbers 656595 and 656596. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b712411b
§ Electronic supplementary information (ESI) available: Full experimental
1
and characterization details for 1 and 2. See DOI: 10.1039/b712411b
Fig. 1 Molecular structure of Cu8L4 (1) with H atoms omitted.
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neighboring Cu atoms, while the other links two discrete Cu atoms.
Because of deprotonation and complexation, one pyrazole ring in
each L1 is twisted severely from the pyridine ring with dihedral
angles ranging from 21.1 to 32.7◦, which are distinctly reflected
1
by its H NMR spectrum, where two sets of resonance signals
for the pyrazole protons are observed. The central pyridine rings
are not coordinated as indicated by the rather long Cu–Npyridine
˚
distances ranging from 2.547(6) to 2.808(6) A. Each copper atom
is coordinated by two pyrazolate nitrogen atoms from different L1
moieties in bent geometries with N–Cu–N angles in the range
155.6(3)–167.1(2)◦. The Cu–Npyrazolate bond distances are observed
˚
to be 1.850(6)–1.883(6) A, which are consistent with the known
values of other Cu(I)–pyrazolate complexes.3b,d
Interestingly, the eight copper ions form an eight-membered
ring with a twisted boat conformation because of the cuprophilic
interaction. The copper ring is stabilized by eleven Cu–Cu bonds,
which can be divided into three classes: four pyrazolate-bridged,
four ligand-bridged, and three “unbridged” Cu–Cu bonds are
Fig. 2 Metallacycles connected by Cu8N20C16 showing the twisted boat
conformation of the copper(I) atoms of 1.
ionic compounds. So far, no neutral octanuclear copper(I) clusters
supported by N donating ligands have been reported. In addition,
most of the reported octanuclear copper(I) clusters are of cubic
arrangement, compounds 1 and 2 represent the first examples
displaying a twisted-boat conformation of Cu8 clusters.
˚
˚
within the ranges 2.901(1)–3.054(1) A, 2.824(1)–2.897(1) A, and
˚
2.616(1)–3.037(1) A, respectively. The shortest Cu–Cu sepa-
ration is observed for the “ligand-unsupported” Cu(1)–Cu(2)
˚
˚
(2.616(1) A), whereas the longest one is Cu(6)–Cu(8) (3.054(1) A).
These Cu–Cu separations are approximately similar to the sum of
One surprise is the scarcity of reports of the ESI-MS studies
on pyrazolate closed-shell d10 coinage metal complexes.3g In the
ESI-MS spectra of 1 in acetonitrile, the molecular peak at 1457.90
amu was observed corresponding to [Cu8(L1)4]+ (calcd 1455.84
amu), whereas the most intense peak was seen at 665.04 amu
assignable to [Cu3(L1)2]+ (calcd 665.16 amu). Interestingly, the
peaks at 806.74, 947.22, and 1394.93 amu were also observed
ascribed to [Cu5(L1)2(H2O)]+ (calcd 806.86), [Cu3(L1)3(CH3CN)]+
6
˚
the van der Waals radii of 2.80 A.
Although 1 and 2 crystallize in quite different space groups, the
two complexes have similar Cu8 cores (see ESI§). The central Cu8
core is surrounded by four L2 ligands. The eleven Cu–Cu contacts
˚
fall in the range 2.588(4)–3.019(4) A. Again the shortest one is
found to be the “ligand-unbridged” Cu–Cu distance, contrary to
what one might suppose, and is comparable to the Cu–Cu distance
of 2.556 A found in the metal.
(calcd 946.49 amu), and [Cu7L1 ]+ (calcd 1393.86 amu), re-
˚
4
There has been much discussion of cuprophilicity, and relatively
short Cu–Cu contacts in bi-, oligo-, and polynuclear copper(I)
complexes, which may or may not involve some kind of bonding, in
recent years.2c,3b,7 Although cuprophilic interaction is much weaker
than those between other coinage metals, it often determines
the self-assembly of copper(I) aggregates and plays an important
role in the luminescent properties of the multinuclear copper(I)
complexes containing N- or P-donating ligands.8 The copper–
spectively. Peaks larger than the molecular weight, at 1522.07
and 1839.71 amu, can also be observed and are assigned to
[Cu9(L1)4]+ (calcd 1520.77 amu) and [Cu10(L1)5]+ (calcd 1839.77
amu), respectively. The ESI-MS spectra of 2 in DMF displays
a molecular peak at 1954.72 amu corresponding to [Cu8(L2)4]+
(calcd 1954.96 amu). Under the ESI-MS conditions, compound 2
also undergoes extensive fragmentation through loss of ligand or
copper atoms. The peaks corresponding to [Cu7(L2)4], [Cu6(L2)4],
[Cu6(L2)3], [Cu4(L2)2], [Cu3(L2)2], and [Cu2(L2)] are all found.
The observation of the molecular peaks for both 1 and 2
suggests that the octanuclear copper frameworks are maintained in
solution.
The electronic absorption spectrum of 1 in acetonitrile shows
notable absorption bands centered at 224, 246, 273, and 306 nm.
The high-energy wavelength bands below 300 nm are quite
similar to the p–p band absorptions for H2L1, whereas the
intense absorption band at 306 nm which is not observed for
the ligand may be assigned to the metal–metal 3d–4p transition.
The absorption band greater than 300 nm for 2 is concealed
by the tail of the strong absorption at 269 nm. A similar
absorption at 310 nm has also been seen in [Cu2(dcpm)2]2+
(dcpm = bis(dicyclohexylphosphino)methane) complexes due to
3d–4p transitions.8
˚
copper distances are found to cover a wide range from 2.5 A to
˚
˚
2.9 A, and even longer distances of 3.3 A have been attributed
to cuprophilicity to some extent. Although there is some dispute
over the existence of the cuprophilic interaction, more and more
theoretical and experimental evidence has been accumulated over
the past decade to support Cu–Cu interaction rather than a simple
approach of two metals because of the steric constraints of the
bridging ligands.
Also interestingly, the complexes contain several neutral met-
allacycles such as Cu2N2, Cu3N2, Cu2N3C4, etc., which are linked
together by eight copper, twenty nitrogen and sixteen carbon
atoms (Fig. 2). Most of the reported pyrazolate metallacycles are
of nine membered M3N6 metallacycles,2d,3a–c the neutral Cu4N12
and Ag4N12 metallacycles are also known.3d Coordination com-
pounds having geometrically shaped metallacycles show promise
in various applications such as selective sensors, gas storage,
sorters, catalysis, and luminescent materials.
The solid-state emissive spectra for the crystals of the copper
complexes are shown in Fig. 3. The ligand precursors and the
copper clusters are intensely emissive in their solid states. The
solid state emission spectra of H2L1 and H2L2 show similar single
sharp bands at 366 nm upon excitation at 338 nm. The emission
It is noted that octanuclear copper(I) clusters are relatively
rare and are often stabilized by functional ligands incorporating
heteroatoms such as S, P, Se and halides etc.,9 and most of them are
5124 | Dalton Trans., 2007, 5123–5125
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solid-state structures and luminescence of the Cu(I) aggregates
will be studied further.
This work is supported by the NSF of China (Grant
20572096) and the Zhejiang Provincial Natural Science Founda-
tion (R405066).
Notes and references
¯
˚
¶ Crsytal data for 1: C55H50Cu8N20O, triclinic, P1, a = 11.901(2) A, b =
◦
˚
˚
14.426(2) A, c = 18.410(2) A, a = 80.163(2), b = 73.035(2), c = 70.652(2) ,
V = 2842.8(7) A , Z = 2, Dcalc = 1.770 Mg m−3, 14775 reflections collected,
3
˚
9836 reflections independent [Rint = 0.0285], goodness-of-fit on F2 1.021,
R [I > 2rI]: 0.0484, 0.1055. For 2: C92H60Cu8N20, orthorhombic, Fdd2,
3
˚
˚
˚
˚
a = 23.916(15) A, b = 39.80(3) A, c = 16.666(10) A, V = 15864(18) A ,
Z = 8, Dcalc = 1.636 Mg m−3, 18203 reflections collected, 3664 reflections
independent [Rint = 0.1065], goodness-of-fit on F2 0.906, R [I > 2rI]:
0.0769, 0.1827.
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Fig. 3 The solid-state emission (right) and excitation (left) spectra of 1
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454, 470, 484, 497 and 511 nm for 1 and 409 sh, 443, 455, 473,
486, 498 and 517 nm for 2) with kmax = 470 nm and 517 nm upon
excitation at 220 nm, respectively. Vibronically structured bands
with a spacing between the local maxima of the emission bands of
about 1446 cm−1 for 1 and about 1432 cm−1 for 2, characteristic
3e
=
=
of the m(C N) or m(N N) stretch, were also observed. The
appearance of this structure is suggestive of involvement of the
pyrazolate ligands in the emission process.
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have been fully investigated. However, little attention has so
far been focused on the photophysical properties of copper(I)
complexes with a larger cluster core structure. Because emission
bands greater than 400 nm were not observed for the ligands,
the low-energy emission bands centered at ca. 470 nm for the
complexes may tentatively be assigned to Cu–Cu bonded excited
states. The Cu8 complexes exhibit emission bands at almost the
same positions, suggest that these emissions probably originate
from the same electronic states assignable to ligand-to-ligand
charge transfer and MLCT (d–p) transitions. The low-energy
bands around 511 nm for 1 and 517 for 2 may be attributed to
metal-centered charge transfer processes. In CH2Cl2, complexes
1 and 2 are also luminescent. The spectra show broad bands
centered at ca. 465 nm with not well-resolved vibronic features
upon excitation at 227 nm. The large Stokes shifts and the spectral
features again demonstrate that the low-energy emissions of the
complexes originate from the metal-centered excitation states.
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and emission properties of two Cu8 clusters featuring eleven Cu–
Cu bonds and twisted-boat Cu8 cores representing the first oc-
tanuclear copper(I) complexes supported by N-donating ligands.
They are stable in the solid state as well as in solution. The
emission spectra of these compounds are interesting and worthy of
further investigation. Variation of the pyrazolyl ring substituents
and bridges between the two pyrazolyl rings may lead to novel
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The Royal Society of Chemistry 2007
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©