From Cluster to Polymer: Ligand Cone Angle Controlled Syntheses and Structures of Copper(I) Alkynyl Complexes

Article abstract of DOI:10.1002/anie.201604762

Copper(I) alkynyl complexes have attracted tremendous attention in structural studies, as luminescent materials, and in catalysis, and homoleptic complexes have been reported to form polymers or large clusters. Herein, six unprecedented structures of Cu^Ialkynyl complexes and a procedure to measure the cone angles of alkynyl ligands based on the crystal structures of these complexes are reported. An increase of the alkynyl cone angle in the complexes leads to a modulation of the structures from polymeric [((PhC≡CC≡C)Cu)₂(NH₃)]_∞, to a large cluster [(TripC≡CC≡C)Cu]₂₀(MeCN)₄, to a relatively small cluster [(TripC≡C)Cu]₈(Trip=2,4,6-iPr₃-C₆H₂). The complexes exhibit yellow-to-red phosphorescence at ambient temperature in the solid state and the luminescence behavior of the Cu₂₀cluster is sensitive to acetonitrile.

Full text of DOI:10.1002/anie.201604762

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Communications
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International Edition: DOI: 10.1002/anie.201604762
German Edition: DOI: 10.1002/ange.201604762
Alkynyl Complexes
From Cluster to Polymer: Ligand Cone Angle Controlled Syntheses
and Structures of Copper(I) Alkynyl Complexes
Xiao-Yong Chang, Kam-Hung Low, Juan-Yu Wang, Jie-Sheng Huang, and Chi-Ming Che*
Abstract: Copper(I) alkynyl complexes have attracted tremen-
dous attention in structural studies, as luminescent materials,
and in catalysis, and homoleptic complexes have been reported
to form polymers or large clusters. Herein, six unprecedented
structures of CuI alkynyl complexes and a procedure to
measure the cone angles of alkynyl ligands based on the
crystal structures of these complexes are reported. An increase
of the alkynyl cone angle in the complexes leads to a modu-
structural information poses challenges in probing the steric
effect of alkynyl ligands. Herein we introduce 2,4,6-triiso-
propylphenyl (Trip) and 2-tert-butylphenyl (tBuPh) for the
preparation of CuI alkynyl complexes with various nuclear-
ities (Figure 1). Crystal structures ranging from relatively
ꢀ
ꢀ
lation of the structures from polymeric [((PhC CC C)Cu)2-
ꢀ
ꢀ
(NH3)]1, to a large cluster [(TripC CC C)Cu]20(MeCN)4, to
ꢀ
a relatively small cluster [(TripC C)Cu]8 (Trip = 2,4,6-iPr3-
C6H2). The complexes exhibit yellow-to-red phosphorescence
at ambient temperature in the solid state and the luminescence
behavior of the Cu20 cluster is sensitive to acetonitrile.
O
ver the past decades, CuI alkynyl complexes have been
extensively studied for their structural diversity,[1] intriguing
photoluminescent properties,[2] and as key active species in
copper-catalyzed transformations of alkynes as well as in click
chemistry.[3] The widely utilized copper-catalyzed azide–
alkyne cycloaddition involves generation of CuI alkynyl
species of varying nuclearities,[3c,4] the formation of which is
driven by intermolecular CuI–CuI and CuI–ligand interac-
tions.[1] Apart from these intermolecular interactions, the
steric effect of ligand(s) plays an important role in the
assembly process of polynuclear CuI alkynyl complexes since
this can affect the metal–metal (M–M) contacts, M–L
coordination modes (L = ligand), and thus the stability and
crystal structures of the complexes. Introduction of sterically
bulky phosphine,[2a–d] s-hydrindacenyl,[5] or alkynyl ligands
can diminish intermolecular aggregation, consequently lead-
ing to the formation of small CuI alkynyl clusters. As a result
of the difficulty in obtaining good quality crystals suitable for
X-ray analysis, examples of structurally characterized homo-
leptic CuI alkynyl complexes are few. Reported examples
Figure 1. The synthesis of 1, 2, 3a, 3b, 4a, and 4b. The structures of
their Cu8, Cu10, Cu12, Cu16, Cu20, and Cu1 cores are shown.
small Cu8 (1) and Cu12 (2) clusters, to large Cu16 (3a) and Cu20
(3b) clusters, as well as to the Cu1 (4b) polymer, were
obtained by varying the steric hindrance of the ligand
employed. The steric hindrance is quantified herein by the
“cone angle” of the alkynyl ligand (see below), with bulky or
chelating ancillary ligands not being used in this work. These
CuI alkynyl complexes adopt metal core structures which, to
our knowledge, are unique among other reported metal
alkynyl complexes. Based on the structural information
obtained, a procedure to assess the cone angles of alkynyl
ligands is proposed.
[6]
[7]
include neutral Cu24 and Cu20 clusters, and cationic Cu18
[1d]
and Cu1ꢁ7
clusters obtained by using the alkynyl ligand
ꢀ
tBuC C bearing a bulky tert-butyl group. The rarity of
[*] Dr. X.-Y. Chang, Dr. K.-H. Low, J.-Y. Wang, Dr. J.-S. Huang,
Prof. Dr. C.-M. Che
State Key Laboratory of Synthetic Chemistry, Institute of Molecular
Functional Materials, HKU-CAS Joint Laboratory on New Materials
Department of Chemistry, The University of Hong Kong
Pokfulam Road, Hong Kong (China)
Although alkynyl ligands usually have little steric effect
around the donor carbon atom, remote bulky aromatic
substituent(s) could impose sizeable steric influence. In this
work, the bulky aryl group Trip was first chosen, which is
rarely employed in the alkynyl ligands of known metal
alkynyl complexes[8] and has not been incorporated in the
alkynyl ligands of reported homoleptic alkynyl complexes of
E-mail: cmche@hku.hk
Prof. Dr. C.-M. Che
HKU Shenzhen Institute of Research and Innovation
Shenzhen 518053 (China)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201604762.
coinage metals.[1c,9] The reaction of TripC CH with
ꢀ
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[Cu(MeCN)4]PF6 in CH2Cl2/MeCN in the presence of trie-
thylamine led to the formation of 1 (Figure 1), which crystal-
lized from CH2Cl2/MeCN in the monoclinic space group P21/
n.[19] As shown in Figure 2, eight copper atoms form a square
Figure 2. Crystal structure of 1.[19] Hydrogen atoms are omitted; alkynyl
ligands at the back of the molecule are represented as wireframe.
ꢁ
antiprism with Cu Cu distances in the range of 2.5000(7)–
ꢁ
ꢀ
2.8724(6) ꢀ. The eight TripC C ligands bound to the Cu8
core adopt four types of coordination mode: m2, h1,1; m2, h1,2
;
Figure 3. Crystal structures of a) 3a and b) 3b.[19] Inset: chemical
structure of the repeat unit in 3a and 3b. Hydrogen atoms are omitted
m3, h1,1,1; and m3, h1,1,2, with Cu C distances ranging from
ꢁ
1.890(2) ꢀ to 2.538(2) ꢀ. The Cu8 cluster can be viewed as an
ꢁ
for clarity. Cu Cu distances (ꢀ) in the Cu4 units: 2.462(2)–2.880(1)
ꢀ
assembly of two [(TripC C)Cu]4 units with Cu4 in a nearly
square arrangement.
ꢁ
(3a), 2.413(1)–2.890(1) (3b); Cu Cu distances (ꢀ) between Cu4 units:
2.563(2)–2.614(1) (3a), 2.473(1)–2.596(1) (3b).
ꢀ
As a less bulky alkyne, tBuPhC CH reacted with [Cu-
(MeCN)4]PF6 in the presence of triethylamine to afford 2 as
a red solid. Recrystallization of 2 in CH2Cl2/MeCN gave red
crystals from which diffraction data sets at low resolution
(2.0 ꢀ) were collected. The space group was determined to be
Figure S1a in the Supporting Information). There are four
THF molecules coordinating to terminal copper atoms with
Cu O distances of 2.04(2)–2.15(4) ꢀ.
ꢁ
cubic Fm3 m.[19] Attempts to obtain higher quality crystals of
For 3b, the X-ray crystal analysis showed that it has
a structure similar to that of 3a except for the presence of
a fifth Cu4 unit and four MeCN (instead of THF) molecules
(Figure 3b).[19] Presumably, the replacement of THF by
MeCN results in the extension of the structure from Cu16 to
Cu20. Recrystallization of 3b in CH2Cl2/THF was found to
give 3a as confirmed by X-ray crystal analysis, indicative of
cluster core transformation in solution. The similarity of the
metal alkynyl frameworks observed in 3a and 3b indicates
that both THF and MeCN function as ancillary ligands to
stabilize the cluster but have minor effects on the assembly
mode of the clusters. Also, the MeCN coordinated at the end
of 3b is away from the steric repulsion between adjacent Trip
ꢀ
2 were unsuccessful. Complex 2 has a cuboctahedral Cu12 core
similar to that in [Cu12S8]4ꢁ,[10a] [Cu12S6(dpppt)4],[10b] [Cu12-
(NPh)8]4ꢁ [11] [Cu12(NPEt3)8]4+,[12] and [Cu12(tBu3SiP)6].[13]
,
As the steric bulkiness of alkynyl ligands could be
decreased by coupling a second C C unit to the terminal
sp carbon atom, we prepared a Trip-capped diynyl ligand
ꢀ
ꢀ
ꢀ
TripC CC CH. Reaction of this ligand with [Cu-
(MeCN)4]PF6 in the presence of triethylamine afforded an
orange solid which gave yellow bipyramidal crystals 3a and
orange crystals 3b upon recrystallization in CH2Cl2/THF/
MeOH and CH2Cl2/MeCN, respectively.
X-ray crystal analysis of 3a reveals a cluster structure
bearing a rod-like Cu16 core.[19] As shown in Figure 3a, the
structure can be considered as packing of four Cu4 tetrahe-
ꢀ
units, therefore it is conceivable that the homoleptic [TripC
CC CCu] complex has a similar nuclearity and configuration
ꢀ
ꢁ
ꢀ
drons supported by Cu Cu (2.462(2)–2.880(1) ꢀ) and p C
to 3b.
ꢁ
C–Cu (Cu C: 1.920(8)–2.452(9) ꢀ) interactions. The Cu4R4
The formation of Cu16/Cu20 clusters 3a/3b, Cu12 cluster 2,
and Cu8 cluster 1 from reactions of [Cu(MeCN)4]PF6 with
unit has a pseudo D2 symmetry (Figure 3, inset). Each diynyl
ligand s-coordinates to two Cu atoms of the Cu4 tetrahedron
and p-coordinates to another copper atom of a nearby Cu4
unit. Upon viewing the structure along the chain direction, the
ligands are approximately oriented in eight different direc-
tions because of the staggered packing of Cu4 units (see
ꢀ
ꢀ
ꢀ
ꢀ
TripC CC CH, tBuPhC CH, and TripC CH, respectively,
revealed that enhanced steric effect of the alkynyl ligand
allows formation of homoleptic CuI alkynyl complexes with
decreased nuclearity of down to Cu8, in contrast to the Cu24,[6]
Cu20,[7] and Cu18/Cu17 analogues reported in literature.
[1d]
2
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We further explored the structures of CuI alkynyl
complexes using a ligand having minimal steric hindrance.
ꢀ
ꢀ
PhC CC CH was employed because of its compact config-
uration and ease of preparation. To obtain diffraction-quality
crystals for structure determination, the following method
was developed. A mixture of acetonitrile and ammonium
hydroxide (1:1 v/v; ammonium hydroxide concentration 28%
NH3 in H2O), as a buffer layer, was carefully dropped onto
a solution of CuI in ammonium hydroxide prior to adding
ꢀ
ꢀ
a dilute solution of PhC CC CH in acetonitrile. All solutions
were degassed and the reaction was protected from oxygen.
One week later, yellow needle-shaped crystals of 4a were
obtained as the major product with orange needle-shaped
crystals 4b obtained as the minor product.
Figure 5. Crystal structure of 4b.[19] a) Side view with hydrogen atoms
omitted for clarity; b) packing diagram of Cu10 cores in the crystal
lattice.
As shown in Figure 4, the structure of 4a reveals two
crystallographically non-equivalent copper atoms, Cu1 and
ꢁ
between the Trip-capped alkynyl ligands. Some of the Cu Cu
distances (2.4258(6)–2.7692(9)) are less than the sum of
metallic radii (2.56 ꢀ) and/or the van der Waals radii of two
ꢀ
copper atoms (2.80 ꢀ). In addition, p C C–Cu interactions
ꢀ
between secondary C C units (the one far away from the
metal core) of the diynyl ligands and copper atoms have not
been detected in the crystal structures of 3a, 3b, 4a and 4b.
The self-assembly of homoleptic CuI alkynyl complexes
involves competitive intermolecular metal–metal and metal–
ligand interactions as well as steric interactions of alkynyl
ligands. Bulky ligand substituents facilitate the generation of
clusters, whereas relatively small groups have a tendency
towards the formation of polymeric structures. In this work,
we introduce the use of the term ligand cone angle q to
describe the steric bulkiness of alkynyl ligands. Ligand cone
angle is commonly used to reflect the size of phosphine
ligands.[15] Based on the Tolmanꢁs cone angle concept, Mingos
developed a method to assess the cluster cone angle.[16] The
cone angles of other ligands including isocyanides have also
been reported.[17]
Figure 4. Polymeric chain structure of 4a.[19] Hydrogen atoms are
omitted.
ꢁ
ꢀ
ꢀ
Cu2, two PhC CC C ligands, and one ammonia ligand in
each polymer repeat unit.[19] The polymeric structure could be
viewed as a spiral staircase: the central column was formed by
In view of the multinuclear nature of CuI alkynyl clusters,
the bridging mode of alkynyl ligands, and the difference in
geometries between phosphine and alkynyl ligands, there is
no universally applicable method for the cone angle deter-
mination of CuI alkynyl systems. Analogous to Mingosꢁ
cluster cone angle definition, we estimated the cone angles by
setting the apex of the cone in the center of the metal core
polyhedron. For complexes without a regular polyhedron
metal arrangement (3a/3b and 4a), the metal core can be
treated as a combination of small polyhedra and the center of
the sub-polyhedron is set as the apex. Based on the crystal
structural data, a general model with a distance of 2.68 ꢀ
from the apex to the terminal sp carbon atom is proposed. To
better cover the phenyl- and alkyl-substituted alkynyl ligands,
a trapezoid–circle equivalent cone angle method is used. The
ꢁ
bonding of Cu1 atoms (Cu1 Cu1 2.4441(8) ꢀ) and the stairs
ꢁ
were composed of Cu1 Cu2 bonds (2.5350(8) ꢀ). The diynyl
ligands are coordinated to copper atoms in m2, h1,1 and m4,
h1,1,1,2 coordination modes, with Cu C distances of 1.986(5)–
ꢁ
2.632(4) ꢀ. No intermolecular interaction is observed
between the polymeric chains. Thermogravimetric analysis
(TGA) measurements (Figure S3) revealed that 4a under-
went explosive decomposition with a recoil effect.
Complex 4b crystallized in the monoclinic space group
P21/n and was characterized as a decanuclear cluster.[19] As
ꢀ
shown in Figure 5, the structure of 4b consists of ten PhC
ꢁ
ꢀ
CC C ligands, six ammonia ligands, and a unique cyclic
“wrinkled” Cu10 cluster core consisting of two zigzag Cu5
ꢁ
units. The two Cu5 units in 4b (Cu Cu 2.4258(6)–2.5518(7) ꢀ)
ꢁ
are connected in head-to-tail manner by two Cu Cu bonds
geometrical definition of q is shown in Figure 6a, using
ꢁ
ꢁ
ꢀ
(2.5518(7) ꢀ), unlike the discrete zigzag Cu4 unit (Cu Cu
TripC C as an example. Firstly a trapezoid (green area in
ꢀ
ꢀ
2.450–2.693 ꢀ) in [(PhC CCu)4(PMe3)4] containing phos-
Figure 6a) perpendicular to the C C unit is employed to
phine ligands.[14]
reflect the bulkiest part of the ligand, and the bases (a and b)
and height (h) of the trapezoid are determined by using the
thickness and width of ligand (space-filling model), respec-
tively. A circle (red area in Figure 6a) with the same area as
the trapezoid is defined for obtaining the radius (r) of the
ꢁ
The Cu Cu distances observed in complexes supported by
Trip-capped alkynyl ligands, such as 1, 3a, and 3b, are
significantly longer than those of 4a and 4b having less bulky
alkynyl ligands, probably due to larger steric interactions
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Figure 6. a) Schematic diagram showing the cone angle q in CuI
Figure 7. Emission spectra of 3b in the solid state before (black line)
and after (blue line) vacuum drying. Inset: photo of solid 3b (a) and
vacuum-dried 3b (b) under irradiation with UV light (l=365 nm).
ꢁ
ꢀ
alkynyl complexes (the TripC C ligand is used as an example, in this
case a=b). b) Chart showing the change of nuclearity of CuI alkynyl
complexes with changes in the cone angle of the alkynyl ligands.
vacuum (Figure 7). This effect is probably as a result of the
loss of coordinated MeCN molecules,[18] as supported by fact
that nitrogen was not detected in the elemental analysis of
a vacuum-dried sample. Additionally, the yellow emission was
restored after the vacuum-dried 3b was treated with MeCN or
exposed to MeCN vapor. All of the emission lifetimes of the
CuI alkynyl complexes are in the microsecond range revealing
that the emissive excited states are triplet in origin.
circular cone [Eq. (1)], and subsequently the equivalent cone
angle for the alkynyl ligand is estimated by using Equa-
tion (2):
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ða þ bÞh
2 p
ð1Þ
ð2Þ
r ¼
r
q ¼ 2tanꢁ1
In summary, six CuI alkynyl complexes with various
nuclearities have been prepared and structurally character-
ized by single-crystal X-ray analysis. The steric effect of the
alkynyl ligands plays a major role in the formation of the
complexes. The use of the term cone angle is introduced to
evaluate the steric hindrance of alkynyl ligands and the cone
angles correlate reasonably well with the nuclearities of the
complexes formed.
l
where l denotes the distance between the apex and trapezoid.
As this is a semiquantitative method, crystal packing effects
and ligand meshing are not taken into account in the model.
The steric bulkiness of the alkynyl ligands are found to fall in
ꢁ
ꢁ
ꢁ
ꢀ
ꢀ
ꢀ
ꢀ
the
order:
TripC C > tBuPhC C > TripC CC C >
ꢁ
ꢁ
ꢁ
ꢀ
ꢀ
ꢀ
ꢀ
tBuC C > PhC C > PhC CC C . This order is based on
their q values, which were estimated to be 728, 668, 548, 518,
458, and 338, respectively. Correspondingly, clusters with
ꢀ
nuclearities of 8, 12, 16(20), 20(24), and polymers ([PhC
Acknowledgements
CCu]1 and 4a) have been observed (Figure 6b). In view of
the similarities in the repeating units, metal core arrangement,
and ligand orientations between 3b and polymer 4a (Fig-
ure S1), 3b (an interesting intermediate structure between
a small cluster and a one-dimensional polymer) can be viewed
as a depolymerized fragment resulting from ligand repulsion
We gratefully acknowledge support from the National Key
Basic Research Program of China (No. 2013CB834802), the
University Grants Committee (Area of Excellence Program
AoE/P-03/08), and the Strategic Research Theme of HKU on
New Material.
in a polymeric chain. It is reasonable to use the cone angle of
ꢁ
ꢀ
ꢀ
TripC CC C (548, ligand in 3b) as an approximate lower
limit value of aryl alkynyl ligands for the formation of CuI
alkynyl cluster structures: upon increasing the cone angle of
the alkynyl ligand, a cluster with smaller nuclearity can be
prepared. Similarly, by decreasing the cone angle of the
alkynyl ligand, a polymerized product can be anticipated.
The luminescence behavior of the CuI alkynyl complexes
was investigated (except for 4a and 4b owing to their
instability in air); the photophysical and spectroscopic data
are summarized in Table S2. Homoleptic complex 1 exhibited
an orange emission in the solid state (lem = 599 nm, quantum
yield F = 0.21) and in solution (lem = 582 nm, F = 0.18 in
CH2Cl2), while 2 of higher nuclearity showed a weak red
emission in the solid state. Excitation of 3a and 3b in the solid
state at lexc = 450 nm gave rise to yellow emission bands
(lem = 571 and 564 nm for 3a and 3b, respectively). Notably,
a bathochromic shift of the yellow emission to weak red
emission was observed upon drying the sample of 3b under
Keywords: alkyne ligands · cluster compounds ·
coordination polymer · copper · luminescence
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[19] CCDC 1473525 (1),
1474744 (2),
1473526 (3a·4THF),
1473528 (4a), and
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Cambridge Crystallographic Data Centre.
Received: May 16, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Alkynyl Complexes
Knowing all the angles: By varying the
steric bulk of alkynyl ligands, a CuI alkynyl
polymer (Cu1) and clusters with various
nuclearities (Cu4n, n=2–5) were formed
and structurally characterized. Based on
the crystal structures, a method to mea-
sure the cone angle of alkynyl ligands for
evaluating their steric properties was
proposed. The nuclearity of the complex
was found to increase with decreasing
alkynyl ligand cone angle.
X.-Y. Chang, K.-H. Low, J.-Y. Wang,
J.-S. Huang, C.-M. Che*
&&&& — &&&&
From Cluster to Polymer: Ligand Cone
Angle Controlled Syntheses and
Structures of Copper(I) Alkynyl
Complexes
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!