Dinuclear Cu(I) Halides with Terphenyl Phosphines: Synthesis, Photophysical Studies, and Catalytic Applications in CuAAC Reactions

Article abstract of DOI:10.1021/acs.inorgchem.0c01397

Several dinuclear terphenyl phosphine copper(I) halide complexes of composition [CuX(PR2Ar′)]2 (X = Cl, Br, I; R = hydrocarbyl, Ar′ = 2,6-diarylterphenyl radical), 1-5, have been isolated from the reaction of CuX with 1 equiv of the phosphine ligand. Most

Full text of DOI:10.1021/acs.inorgchem.0c01397

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Article
Dinuclear Cu(I) Halides with Terphenyl Phosphines: Synthesis,
Photophysical Studies, and Catalytic Applications in CuAAC
Reactions
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́
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Alvaro Beltran, Inmaculada Gata, Celia Maya, Joao Avo, Joao Carlos Lima, Cesar A. T. Laia,
̃
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Riccardo Peloso,* Mani Outis,* and M. Carmen Nicasio*
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ABSTRACT: Several dinuclear terphenyl phosphine copper(I)
halide complexes of composition [CuX(PR2Ar′)]2 (X = Cl, Br, I;
R = hydrocarbyl, Ar′ = 2,6-diarylterphenyl radical), 1−5, have been
isolated from the reaction of CuX with 1 equiv of the phosphine
ligand. Most of them have been characterized by X-ray diffraction
studies in the solid state, thus allowing comparative discussions of
different structural parameters, namely, Cu···Cu and Cu···Aryl
separations, conformations adopted by coordinated phosphines, and
planarity of the Cu2X2 cores. Centrosymmetric complexes [CuI-
(PMe2ArXyl2)]2, 1c, and [CuI(PEt2ArMes2)]2, 3c, despite their similar
structures, show very distinct photoluminescence (PL) in powder
form at room temperature. The photophysical behavior of these
compounds in liquid solution, solid−solid Zeonex solution and
powder samples at room temperature and 77 K have been investigated and supported by DFT calculation. Identification of vibronic
coupling modes, done by group theory calculations and the technique of projection operators, shows that the manifestation of these
modes is conditioned by crystal packing. Complexes [CuI(PMe2ArXyl2)]2, 1c, and [CuI(PEt2ArMes2)]2, 3c, display remarkable activity
in copper-catalyzed azide−alkyne cycloaddition reactions involving preformed and in situ-made azides. Reactions are performed in
H2O, under aerobic conditions, with low catalyst loadings and tolerate the use of iodoalkynes as substrates.
spin−orbit coupling (SOC) constant of I,18 a remarkable
INTRODUCTION
■
advantage of these compounds can be thought of as conferring
Cu(I) halides with phosphines display a variety of coordination
heaviness to a light metal based core. First row (3d) transition
numbers and geometries primarily dictated by the size of the
phosphine and, to a lesser degree, by the nature of the halide.1
metals are not heavy enough for an efficient SOC, but a high
SOC is required for rapid intersystem crossing (ISC) in order
to achieve phosphorescent metal complexes. Third row (5d)
transition metal based complexes like those of Ir(III) and
Pt(II) exhibit the desirable fast ISC as a high SOC is induced
by the heavy metal center itself. These phosphorescent
complexes have been the focus of many investigations,
especially for organic light emitting diodes (OLEDs).19,20
However, these are very expensive metals, and their sources are
increasingly scarce. Thus, luminescent Cu(I) halide aggregates
represent an attractive sustainable alternative to those based on
heavy precious metals.
Thus, for a 1:1 stoichiometry, tetranuclear [CuX(PR3)]4 with a
cubane-like structure is frequently encountered for less bulky
phosphines such as PEt3,2 PMe2Ph,3 and PPh3.4 Furthermore,
for the larger halides (Br and I),5 less common stepped cubane
structures have also been found. As the size of phosphine
increases, dinuclear [CuX(PR3)]2 species with three-coordi-
nate Cu(I) centers are stabilized, as noted for PCy3,6 P(o-
8
Tol)3,7 and PBz3 (Bz = benzyl) among others.9 Mononuclear
two-coordinate [CuX(PR3)] is obtained for the highly
10
sterically demanding phosphines PMes3 (Mes = mesityl)
and P(tmp)3 (tmp = 2,4,6-trimethoxyphenyl).11
Due to their flexible coordination properties, Cu(I) halide
aggregates have been incorporated, as building blocks, in
coordination polymers,1,12 which exhibit a range of photo-
physical properties.13 In this context, discrete {CunIn}clusters
display interesting properties such as polymorphism, thermo-
chromism, and thermally activated delayed fluorescence
(TADF).14−17 Concerning the relativistic nephelauxetic effect
through the covalent character of Cu−I, bonds, and large
Received: May 12, 2020
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Additionally, Cu(I) complexes enjoy a rich catalytic
chemistry, including cross-coupling reactions,21 oxidations,22
borylation reactions,23 C−H functionalization,24 and 1,3-
dipolar cycloadditions, in particular, the Cu-catalyzed azide−
alkyne cycloaddition (CuAAC).25 The latter is the most
prominent example of click chemistry.26 Nitrogen-containing
ligands and N-heterocyclic carbenes, albeit to a lesser extent,
have commonly been used as supporting ligands in CuAAC
reactions.25,27 However, Cu(I) complexes stabilized by
phosphine ligands have scarcely been applied.28 Recently,
heptane, or petroleum ether. 31P{1H} NMR spectra of
complexes 1−5 revealed that the broad resonances of the P
nuclei are slightly or moderately shifted to higher frequencies
(Δδ ca. 0.5 to 15 ppm) upon coordination to the metal center,
analogously to what was observed for related Ni,31 Pt, Rh, Ir,30
and Au32 compounds. The 1H NMR and 13C{1H} NMR
spectra of CuX(PR2Ar) species in CDCl3 do not show any
relevant differences compared to the free ligands, which is
consistent with a monodentate coordination mode of the P
ligand in conjunction with fast rotations around the P−Cu
and/or P−Caryl bonds. Nevertheless, the previously observed
κ1-P,η1-C coordination of the terphenyl phosphine30−32 cannot
be ruled out in solution, given that a fast interchange of the
two flanking aryl rings of the terphenyl group in the
coordination environment of the metal would also account
for the observed NMR properties. If only mononuclear and
dinuclear complexes are considered, four possible structures in
solution can be envisaged (Figure 1).
́
́
Diez-Gonzalez and co-workers employed mononuclear and
tetranuclear Cu(I) halides bearing phosphorus(III) ligands as
catalysts in efficient CuAAC reactions performed in water
under mild conditions.29
We have developed a family of heteroleptic phosphine
ligands bearing a bulky terphenyl substituent, PR2Ar′ (Ar′ =
terphenyl radical; Chart 1), and we have evaluated their
Chart 1. Terphenyl Phosphines Used in This Work
Figure 1. Possible coordination modes of PR2Ar′ ligands and
geometries around the Cu center for mononuclear and dinuclear
CuX(PR2Ar′) species in solution based on NMR data. Arrows
indicate fast rotations on the NMR time scale. (a) κ1-P monodentate
coordination, dinuclear, trigonal planar geometry at Cu; (b) κ1-P,η1-C
bidentate coordination, dinuclear, tetrahedral geometry at Cu; (c) κ2-
P,C bidentate coordination, mononuclear, trigonal planar geometry at
Cu; (d) κ1-P monodentate coordination, mononuclear, linear
geometry at Cu.
electronic and steric parameters.30,31 Herein, we report on the
synthesis and structures of a series of dinuclear [CuX-
(PR2Ar′)]2 complexes. Two new phosphorescent complexes
containing the Cu2I2 core have been identified and their
excited state dynamics where low-lying electronic transitions
are mainly (M+X)LCT are discussed. In addition, the catalytic
activity of these complexes in CuAAC reactions is also
examined.
DOSY 1H NMR experiments of CDCl3 solutions of
complexes 1c, 2a, 3c, and 5b were performed in order to get
more information about the nuclearity of the CuX(PR2Ar′)
species in solution (see Figure S1 in the SI). In all cases, only
one species was detected with calculated diffusion coefficients
of ca. (6−7) × 10−10 m2·s−1, in accord with previously reported
data for complexes of similar size.33 Moreover, the diffusion
coefficients were used to calculate the corresponding hydro-
dynamic radii of the molecules with the Stokes−Einstein
equation giving values of ca. 6−7 Å in good agreement with the
equivalent radii estimated from the X-ray molecular structures
of the dimers.
X-ray diffraction analyses were conducted on single crystals
of complexes 1, 2c, 3b, 3c, 4c, and 5c, revealing that their
molecular structures in the solid consist of bimetallic dihalo-
bridged neutral complexes, [Cu(μ-X)(PR2Ar′)]2, with the
metal centers lying in a distorted trigonal planar geometry.
RESULTS AND DISCUSSION
■
Synthesis and Structural Studies of Cu(I) Complexes.
Dinuclear complexes 1−5, of the general formula CuX-
(PR2Ar′) (PR2Ar′ = L1, X = Cl, Br, I: 1a, 1b, 1c; PR2Ar′ =
L2, X = Cl, Br, I: 2a, 2b, 2c; PR2Ar′ = L3; X = Cl, Br, I: 3a, 3b,
3c; PR2Ar′ = L4, X = I: 4c; PR2Ar′ = L5 X = Cl, Br, I: 5a, 5b,
5c) were synthesized by mixing equimolar amounts of the
corresponding copper(I) halide, CuX, and the dialkyl
terphenyl phosphine, PR2Ar′, in dichloromethane at room
temperature. The transformation of the metal precursor into
the product occurred with concomitant disappearance of the
colorless copper halide, which was almost insoluble in the
reaction medium. Reaction times of about 12 h were required
to achieve high yields.
Compounds 1−5 were isolated as air-stable colorless solids.
They are highly soluble in chlorinated and aromatic solvents
and poorly soluble in saturated hydrocarbons such as pentane,
B
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This type of complex has numerous precedents in the
literature.6−9 ORTEP diagrams for the molecular structures
of complexes 1c and 3c are depicted in Figure 2 as
representative examples.
separations (ca. 2.7 Å). On the basis of the comparison of the
Cu···Caryl distances reported in Table 1 with the sum of the
covalent radii of C(sp2) and Cu (ca. 2.0 Å),34 the existence of
“classical” copper−carbon covalent bonding is ruled out
clearly. For the sake of comparison, the extent of copper−
carbon interactions in complexes 1−5 appears much smaller
than in related unsaturated platinum complexes [PtMe(OH2)-
(PMe2 ArD i p p 2 )]+ ,3 0 b [PtMe2 (PMe2 ArD i p p 2 )],3 0 b
[PtCl2(PMe2ArDipp2)],30a and [Pt(tBuCHCH2)-
(PMe2ArDipp2)],30c in which shorter Pt···Caryl metal−carbon
separations of ca. 2.2−2.3 Å are observed. Another point of
interest for the structural properties of complexes 1−5 is the
existence, in some cases, of typical d10−d10 interactions,35
resulting in Cu−Cu separations of approximately 2.6−2.7 Å.
For the iodide complexes 1c and 2c, which contain the
dimethyl phosphines PMe2ArXyl2 (L1) and PMe2ArDipp2 (L2),
the Cu−Cu distance matches almost perfectly the double of
the covalent radius of copper (1.32 Å),34 thus indicating a clear
Cu(I)−Cu(I) bond. The correlation between the metal−metal
separation and the CuXCu bond angle for complexes 1−5 is
plotted in Figure S8 (see Supporting Information) and
discloses a fair linear dependence, particularly for acute angles.
In general, R substituents in PR2Ar′ bulkier than CH3 seem to
impede the formation of a metal−metal bond, reasonably for
steric reasons, with copper−copper separations larger than 3 Å.
The solid state structures of complexes 1−5 deserve some
additional comments on their molecular symmetry and the
conformation of the dicopper [Cu2X2] cores and the
phosphine ligands. Although most of the [Cu(μ-X)(PR2Ar′)]2
species studied in this work show Ci symmetry (C1 in three
cases), by slight rotations of the aryl rings around the C−C and
P−C bonds and slight translations of the X ligands for
nonplanar [Cu2X2] cores, a C2h symmetry would be easily
reached. Figure 3 provides a schematic representation of the
C2h-like molecular structure and the symmetry elements
showed by the majority of [Cu(μ-X)(PR2Ar′)]2 dimers in an
idealized geometry.
Figure 2. ORTEP views for the molecular structures of complexes 1c
(top) and 3c (bottom). Selected bond distances (Å) and angles (deg)
for 1c: Cu1−P1 2.2171(9), Cu1−I1 2.5853(5), Cu1−I1′ 2.5886(5),
Cu1−Cu′ 2.6175(6), Cu1−I1−Cu1′ 60.78(1), P1−Cu1−I1
115.03(3), P1−Cu1−I1′ 117.55(3), I1−Cu1−I1′ 119.22(2); for 3c:
Cu1−P1 2.217(1), Cu1−I1 2.6125(5), Cu1−I1′ 2.5795(6), Cu1−
I1−Cu1′ 80.45(2), P1−Cu1−I1 126.60(3), P1−Cu1−I1′ 126.97(3),
I1−Cu1−I1′ 99.55(2).
The conformations adopted by the terphenyl phosphines
used in this work have recently been analyzed by some of us.31
Three possible conformations have been identified (Figure 4),
type C being the most common by far in the dicopper
complexes of this work. This observation could reasonably be
explained by the existence of some Cu···C interactions.
Anyway, taking into account that the NMR spectra of all
complexes are consistent with fast rotations around the P−Cu
and/or P−C bonds, the energy separations between the
different conformations in solution are likely small. The neutral
[Cu2X2] cores are planar or quasi-planar (deviations from the
Interestingly, for all structures, the slightly pyramidalized
copper atoms lie at ca. 0.3−0.4 Å above the plane defined by
the two halides and the P atom and point toward one of the
lateral rings of the terphenyl group. This observation may
suggest the existence of weak bonding interactions, reasonably
of an electrostatic nature, between the copper center and one
or two carbon atoms of the aromatic rings, especially for
complexes 1a and 5c, which exhibit the shortest Cu···Caryl
Table 1. Selected Structural Parameters for [Cu(μ-X)(PR2Ar′)]2 Complexes 1−5 in the Solid State
Cu−Cu
Cu−X−Cu
[Cu2X2]
planarity
Cu−Ar (shortest
Cu−C)
sum of bond angles
at Cu
point
conformation of
b
complex
X
R
Ar′
(Å)
(deg)
group
PR2Ar′
a
1a
1b
1c
2c
3b
3c
4c
5c
Cl
Br
I
I
Br
I
Me
Me
Me
Me
Et
Et
iPr
Cyp
ArXyl2
ArXyl2
ArXyl2
ArDipp2
ArMes2
ArMes2
ArXyl2
ArXyl2
3.04
2.88
2.62
2.69
3.15
3.35
3.12
3.90
82
no
no
yes
no
yes
yes
yes
yes
2.76
2.86
356
352
352
C1
A and C
a
a
73
Cs (C1)
C2h(Ci)
Cs (C1)
C2h(Ci)
C2h(Ci)
C2h(Ci)
C2h(Ci)
C
C
B
C
C
C
C
60.7
2.96
3.20
a
a
a
63
354
80.4
80.5
73.7
90.0
2.83
2.90
3.82
2.69
353
352
354
352
I
I
a
b
Mean. Symmetry of the [CuX(PR2Ar′)]2 complexes reached after slight rotation of the aryl groups around the P−Cipso and C−C axis and/or
translations of the X ligands (point group found in the X-ray structure). See Discussion.
C
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with TD-DFT calculations (see below). Intraligand electronic
transitions are observed at lower wavelengths in the complexes,
matching the observed transition in the UV−vis spectra of the
respective free ligands.
The photoluminescence (PL) of 1c and 3c is negligible in
solution at room temperature, even in degassed conditions.
Thus, to allow the characterization of their optical properties,
the PL of these complexes must be studied in the solid state
(powder, dispersion in polymer matrix and in frozen solution
at 77 K).
Both complexes are emissive in the powder solid phase,
however they display different features at different temper-
atures. At 77 K, the excitation and emission spectra of both
complexes are very similar (Figure 6). At room temperature,
Figure 3. C2h-like molecular structure and the symmetry elements in
most of the [Cu(μ-X)(PR2Ar′)]2 dimers described in this work.
Figure 4. Conformations observed in the solid state by the Cu-
coordinated terphenyl phosphines PR2Ar′ used in this work.
Figure 6. Diffuse reflectance spectra of the complexes under
Kubelka−Munk (KM) transformation function (f(r∞)bold lines)
overlapped with the excitation spectra at 77 K collected at 515 nm for
1c and at 525 nm for 3c (pointed lines). Emission spectra of powder
samples excited at 350 nm at 77 and 300 K.
middle plane less than 0.13 Å) with no apparent relation with
other metric parameters.
Photophysical Studies. The nature of transitions within a
sole isolated molecule can be assigned through UV−vis spectra
of complexes 1c and 3c as well as the spectra of their
corresponding free ligands shown in Figure 5. The complexes
display an additional electronic transition at longer wave-
lengths (above 300 nm) assigned to metal and halide to ligand
charge transfer band ((M+X)LCT), which has been confirmed
the emission band of 3c becomes slightly broader without
significant change of its emission maximum with ΦPL of 19%,
while 1c has only very residual emission. The diffuse
reflectance of powder samples with the Kubelka−Munk
transformation function f(r∞) are shown in Figure 6, which
exhibits barely resolved absorption bands at 285 and 335 nm
coincident with the excitation and UV−vis bands in solution.
This same position confirms the localization of the excitation
and thus emission from isolated molecules.36
As isolated molecules can indeed be emissive in the solid
state, we did the characterization also in Zeonex solid−solid
solution at room temperature. In this rigid matrix, at room
temperature, 1c and 3c display almost identical spectral
behavior (Figure 7) as those found for powder samples at 77
K. This behavior discards the possibility of aggregation
induced emission. Room temperature decay time measurement
performed on films placed in vacuum exhibit a monoexpo-
nential decay with lifetime of 56 μs for 1c and 82 μs for 3c
(Figure S9 in SI). In an air saturated environment, one can
clearly notice the quenching effect of O2. The long lifetime and
the sensitivity to oxygen strongly suggest that luminescence
arises from excited triplet states.
The narrowing of the emission band at 77 K without a
noticeable shift of observed emission (Figure 6 top) and the
large Stokes shift (ca. 1.42 eV) is compatible with the
Figure 5. UV−vis spectra of complexes 1c and 3c (bold line) and
their corresponding free ligands (dashed line) in cyclohexane.
D
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Table 2. Electronic Configuration of First, Second Singlet
Excited State and the Closest Triplet State in Energy As
Well As the Main Involved Molecular Orbitals (f and E Are
the Calculated Oscillator Strength and Excitation Energy,
Respectively)
complex
state
f
E (nm)
MO (%)
1c
S1
S2
T8
S1
S2
T8
0.0008
339
338
340
335
334
339
HOMO→LUMO (80%)
HOMO→LUMO+1(79%)
HOMO→LUMO+1(20%)
HOMO→LUMO (92%)
HOMO→LUMO+1(92%)
HOMO→LUMO+1(80%)
0.0078
a
-
3c
0.0000
0.0080
a
-
a
N/A as calculations were done without SOC inclusion.
Figure 7. Excitation and emission spectra of complexes 1c and 3c in a
solid film dispersion in a vacuum and in the presence of air.
involve different types of orbitals, thus the ISC between them
would be weak via vibronic spin−orbit coupling.37 However, it
is expectable that ISC from S2 to the nearly isoenergetic T8
state could be performed due to the internal heavy atom effect
assignment of the observed emission to phosphorescence from
a lower triplet state. For a better understanding of the emission
properties, a deeper analysis with theoretical calculations using
TD-DFT were carried out.
of iodine acting on the unpaired electron of the HOMO
38
̂
centered on the Cu2I2 moiety so that ⟨S2|HSO|T8⟩ ≠ 0.
The density of the triplet manifold in the sense of spatial
character can be discussed as well. In the case of complex 3c,
80% of the closest triplet state to S2 is (HOMO, LUMO+1) in
the configuration interaction. In the case of the complex 1c,
however, the (HOMO, LUMO+1) contribution is just 20%,
but there are other Tn (n < 8) values very close in energy to
each other which also have some contribution of (HOMO,
LUMO+1) in their configuration interaction set (Table S3 in
SI).
TD-DFT Calculations. Calculated excitation energies for
optimized geometries in cyclohexane are in good agreement
with the experimental UV−vis spectra (Figure S10 in the SI).
TD-DFT calculations reveal an essentially (M+X)LCT
character of low-lying electronic transitions with some
contribution of intraligand charge transfer (ILCT) through
participation of P atoms. In both 1c and 3c, which are
centrosymmetric complexes, the S1 state would be achieved by
a mainly HOMO → LUMO transition, forbidden by the
Laporte symmetry selection rule (u → u). However, the S2
state is reached by a mainly HOMO → LUMO+1, which is a
symmetry allowed transition (u → g). DFT calculations show
also that LUMO and LUMO+1 are degenerate molecular
orbitals (Figure 8).
Quantum mechanical properties of these systems like the
participation of atomic p orbitals of I in the lowest charge
transfer state and small ΔEST explain the ISC and the
experimentally observed phosphorescence. Computational
results show that ISC from the initially Franck−Condon
(FC) region after excitation is possible without considerable
structural change of the excited molecule. Given the spatial
character of nearly degenerate charge transfer states, the ISC
rate constant is expected not to be large enough to be
dominant over other possible competitive pathways,38a namely,
ISC
IC
S2 ⎯⎯→ T8 versus S2 → S1. Although it is quite logical to accept
that ISC from S2 to triplet manifold is favored in rigid media as
in this kind of environment, vibronic couplings responsible for
IC from S2 to S1 would be more hindered; this type of analysis
of the configuration of the excited states does not explain why
powder samples of 1c are not emissive at room temperature.
Our DFT calculations provide further useful information
regarding molecular orbital energy levels. Accordingly, LUMO
and LUMO+1 are quite close in energy, the energetic
difference between them being 0.008 eV for 1c and 0.001 eV
for 3c, close enough to be considered degenerate in both cases.
Despite the degeneracy, we have treated LUMO and LUMO
+1 individually and not as a pair because by symmetry
operations of the C2h point group they cannot be transformed
into each other or any linear combination of them. Moreover,
LUMO and LUMO+1 have different parities (Figure 8) so,
although they are automatically orthogonal molecular orbitals
of the same kind, they do not have the same symmetry
(⟨πu*|πg*⟩ = 0) and cannot transform as an E type irreducible
representation (irrep).39 Almost simultaneous transition to two
potential energy surfaces S1 and S2 (Table 2) means vertical
transition corresponds to a molecular geometry point where
Figure 8. Frontier orbitals, their respective parity, and energies.
LUMO and LUMO+1 are degenerated molecular orbitals.
These theoretical results confirm that once the molecule
achieves the excited state, any internal conversion (IC) from S2
to S1 would lead to a nonradiative decay path, as S1 to S0 is a
forbidden transition. So the population of the triplet manifold
should be done upon ISC from S2. In both cases, the closest
triplet state in energy is T8, which is of mainly (HOMO,
LUMO+1) configuration, like with S2 (Table 2). The same
configuration of S2 and T8 has important consequences: (i) a
very small energy gap between the two charge transfer states
due to decreased exchange integral between the unpaired
electrons and (ii) the same electronic configuration does not
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these states are nearly degenerate. So a conical intersection
(CI) between S1 and S2 within the FC region can be expected.
1
1
S1 and S2 have Bg and Au symmetries respectively so the
degeneracy we are looking for would be between states which
belong to different irreps of the same Abelian point group. In
this case, the CI is not required by symmetry, as in the Jahn−
Teller case, but rather occurs due to the nature of the two
electronic states involved and shall be considered an accidental
intersection case.40 Unlike homoleptic [Cu(N^N)2]+ com-
plexes which exhibit flattening Jahn−Teller distortion,41 in
these three-coordinate systems, the pseudo Jahn−Teller effect
cannot be expected. The excited state of the heteroleptic
dinuclear complexes 1c and 3c is formed by promotion of an
electron from HOMO, which mainly corresponds to the Cu2I2
moiety (Figure 8). Due to the highly covalent character of
Cu−I bonds on the one hand and great participation of iodine
atoms in HOMO on the other, it is not plausible to assume a
pure d9 configuration for any Cu centers of the [Cu2I2]•+ core.
However, in three coordinated Cu(I) complexes, the
possibility of other types of distortion in the excited state
cannot be ruled out.42
Figure 9. Four vibrational modes of bu symmetry: asymmetric P−Cu
stretching and P^(Cu−Cu) bending (top); Cu2I2 in plane ring
deformation and out of plane ring puckering (bottom).
present in 1c (Figure S11 in the SI), and thus individual
molecules at 300 K have enough space and flexibility to
undergo Cu2I2 ring deformation, breaking down the geometry
at the FC zone, where the molecule needs to undergo ISC and
emit light. The packing force is strong enough to block the
deactivating vibrational modes significantly, as in the Raman
spectra one can notice a characteristic resolved band at 126
cm−1 observable in 1c but not clear in the spectrum of 3c. The
126 cm−1 wavenumber is consistent with the Cu−I stretching
in Raman spectroscopy,45 and on the other hand this band is
not observable in the free ligands, so we can assign it to some
Cu−I stretching in 1c. Although it can correspond only to an
even stretching mode (ag or bg), experimental evidence of the
flexibility of the Cu2I2 core conditioned by solid state packing
is presented by Raman spectroscopy. (Figure 10).
The existence of a CI between the optically allowed S2(1Au)
and the dark S1(1Bg) in the vicinity of the FC zone helps us to
postulate a more accurate mechanism of luminescence. We
already saw that the ground state geometry of both 1c and 3c
belongs to the C2h abelian point group and does not contain
degenerate irreps, but upon vertical excitation, a degenerate
electronic state is imposed. As mixing between electronic states
along nuclear displacements become significant at near
degeneracy critical geometries, the excited molecules in the
FC zone (from which ISC would occur) can undergo
nonadiabatic transition from S2 to S1 through vibronic
coupling.43a,b More importantly, since the geometry corre-
sponding to the CI is not expected to be quite different from
that at the FC point, electronic coupling around the CI point
can be approximated by low-order expansion around the FC
geometry.43b,c In this way, nuclear dynamics occurring within
the S1−S2 can be approached as pairwise interacting states by
the well-established linear vibronic coupling (LVC) model. In
the LVC model, the symmetry of the vibrational coordinates
that couple electronic states of an Abelian point group is
determined by the direct product of the irreps of the electronic
states.40a,43a In our case, the interstate vibronic coupling
between S1 and S2 will not vanish when the coupling modes are
of bu symmetry as Bg ⊗ Au = Bu.
Taking the FC point as reference geometry is a very useful
approximation, making it possible to figure out how these
coupling modes would look. To do so, let us focus on the
dimeric coordination sphere with six atoms, [Cu2I2(P)2], for
the sake of simplicity. By group theory calculation, it is known
that among 12 (3N − 6) internal coordinates in the C2h point
group, four of them will be bu nontotally symmetric odd
normal modes. Using the technique of projection operators
and being aware of the presence of the Cu2I2 four-membered
ring,44 one can determine how these symmetry adapted
vibrational normal modes look (Figure 9).
These group theory results can explain why, unlike other
rigid media, in the room temperature powder solid phase the
PL behavior of 1c and 3c is so different. Complexes 1c and 3c
interact distinctly with adjacent molecules through short
contacts. In 3c, there are short contacts established through
iodine atoms, so the Cu2I2 core is somehow stuck within the
crystalline network while short contacts through iodine are not
Figure 10. Raman spectra of 1c and 3c compared to their
corresponding free ligands L1 and L3 in order to confirm the
assignment of the resolved band at 126 cm−1 to Cu−I stretching
mode.
On the basis of spectroscopic observations supported by
theoretical calculations, we suggest a mechanism of lumines-
cence (Figure 11). According to this scheme the FC geometry
1
of the (M+X)LCT excited molecule should be maintained in
3
order to populate the bright (M+X)LCT state. Otherwise
distortions, namely, Cu2I2 ring deformation, act as a non-
radiative path at the conical intersection point, which
corresponds to a photochemical funnel between S2 and dark
S1.
The suggested mechanism can explain the photophysical
behavior in deaerated solution where emission quenching is
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effect of the nature of the halide was observed. Finally, less
than 20% yield of triazole was obtained in blank experiments
carried out with copper halide salts CuX (X = Cl, Br, and I) as
catalysts in the absence of any phosphine ligand (entries 10−
12). It is important to note that no sign of oxidation or
disproportionation of any of the Cu(I) complexes was
observed during the catalyst screening, even though the
reaction was performed in aqueous media and under aerobic
conditions. This observation underlines the effective protection
imparted by terphenyl phosphine ligands to the Cu(I) center
during the catalysis.
To study the scope of the reaction, complexes 1c and 3c
were selected as the catalysts, since the these two Cu(I) iodide
derivatives exhibited the best long-term air stability of each
series. As shown in Table 4, a variety of azides and alkynes
Figure 11. Radiative transitions (red arrows) and nonradiative
transitions (curvy arrows) involved in the suggested mechanism of
phosphorescence.
Table 4. Scope of Cycloaddition Reactions of Azides and
a b
,
Alkynes Catalyzed by Complexes 1c and 3c
independent of oxygen concentration. While at room temper-
ature none of the two complexes is emissive, in frozen solution
at 77 K the spectral behavior of both 1c and 3c is almost
identical to those of other rigid media (Figure S12 in SI). So
the quenching of phosphorescence in these complexes can be
attributed to their geometrical flexibility in solution rather than
specific interactions with solvent molecules.
Catalytic Studies. A survey on the catalytic properties of
complexes 1−5 in the CuAAC reaction between benzyl azide
and phenyl acetylene, the model reaction, was accomplished.
The trials were conducted using the reaction conditions
29
́
optimized by Diez and co-workers for the catalyst CuBr-
(PPh3)3, namely, using water as the solvent and 0.5 mol %
[Cu] catalyst loading under air. As shown in Table 3, all
catalysts tested proved to be active in the model system under
these conditions. However, those with the less bulky
phosphine ligands (L1 and L3) displayed the best activities
(entries 1−3 and 6−8) in the shortest reaction times.
Furthermore, within each series of catalysts no significant
Table 3. Catalytic Performance of Complexes 1−5 in the [3
a
+ 2] Cycloaddition of Benzyl Azide and Phenyl Acetylene
b
entry
[Cu]
time (h)
yield (%)
1
2
3
4
5
6
7
8
1a
1b
1c
2a
2b
3a
3b
3c
5a
6
6
6
20
20
6
6
6
20
6
6
92
96
96
58
55
92
97
95
62
20
16
16
a
Reaction conditions: azide (1 mmol), 1-alkyne (1 mmol), Cu
b
catalyst (0.5 mol % Cu), H2O (3 mL) under air. Isolated yield base
on azide (average of two runs)
were smoothly converted into the corresponding triazoles in
excellent yields under the reaction conditions. Benzyl,
cynnamyl, and phenyl azides reacted in short reaction times
(5−6 h), whereas the alkyl-substituted n-octyl azide required a
longer reaction time (12 h) to reach completion (14o−14r).
Conversely, the outcome was largely unaffected by the
substituents on the terminal alkynes. Thus, aryl, alkyl,
heteroaryl (pyridine), carboxylate, and hydroxyl groups were
equally well tolerated (see for example 14a−14g).
9
10
11
12
CuCl
CuBr
CuI
6
a
Reaction conditions: benzyl azide (1 mmol), phenyl acetylene (1
mmol), Cu catalyst (0.005 mmol of Cu), H2O (3 mL) under air.
b
Isolated yields (average of two runs).
G
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Organic azides, particularly those of low molecular weight or
with high nitrogen content, are potentially explosive.46
Therefore, various protocols that make use of in situ generated
azides for the preparation of triazoles have been reported in the
literature.47 Given that most of the azides employed in this
work were prepared from the reaction of alkyl halides with
NaN3, the activities of catalysts 1c and 3c were examined in
the three-component reaction of benzyl bromide, NaN3, and
phenyl acetylene using the same reaction conditions as those
described above for the reactions with the preformed azides.
Under these conditions, only catalyst 3c provided the triazole
in quantitative yield, although in extended reaction time when
compared with the reaction carried out with the isolated benzyl
azide. The versatility of catalyst 3c for the synthesis of triazoles
by the three-component reaction of alkyl halides, NaN3, and 1-
alkynes is illustrated in Table 5. In all instances, the triazoles
Table 6. Screening of Reaction Conditions for the
Cycloaddition between Benzyl Azide and
Iodoethynylbenzene Catalyzed by Complexes 1c and 3c
a
cat. loading (mol %
Cu)
yield
b
entry catalyst
solvent
additive
(%)
1
2
3
4
5
6
7
8
9
1a
3c
1a
3c
3c
1a
3c
1a
3c
0.5
0.5
0.5
0.5
5
1
1
1
1
H2O
H2O
THF
THF
THF
H2O
H2O
neat
trace
trace
trace
trace
20%
>95
2,6-lutidine
2,6-lutidine
2,6-lutidine
2,6-lutidine
c
90
>95
Table 5. Scope of Three-Component Reactions of Aryl
c
a b
,
neat
90
Halides, NaN3 and 1-Alkynes Catalyzed by Complex 3c
a
Reaction conditions: benzyl azide (0.5 mmol), iodoethynylbenzene
b
(0.5 mmol), solvent (3 mL), 24 h. Determined by 1H NMR with an
internal standard. 5−10% of 5H-triazole was formed.
c
observed when the reactions were carried out under neat
conditions (entries 8 and 9).
The optimized reaction conditions were then applied to
different azides and 1-iodoalkynes giving the products 1,4-
disubstituted 5-iodo-1,2,3-triazoles with the yields in the range
of 45−95% (Table 7). n-Octyl azide was the less reactive;
Table 7. Scope of Cycloaddition Reactions of Azides and 1-
a b
,
Iodoalkynes Catalyzed by 1c
a
Reaction conditions: alkyl halide (1 mmol), NaN3 (1.3 mmol), 1-
b
alkyne (1 mmol), catalyst 3c (0.5 mol % Cu), H2O (3 mL). Isolated
yields (average of two runs). With MeI
c
were obtained in good to high yields under otherwise unaltered
reaction conditions. It is interesting to note that only very few
Cu-based catalyst systems accomplished the one-pot synthesis
of triazoles with such a low catalyst loading.29c,48
5-iodo-1,2,3-Triazoles are versatile intermediates for further
functionalization via the halogen substituent.49 However, there
are still a limited number of efficient Cu-based catalyst systems
for the CuAAC reactions between organic azides and 1-iodo
alkynes.29c,48,50 This prompted us to investigate the behavior of
complexes 1c and 3c in this challenging transformation. Benzyl
azide and iodoethynylbenzene were chosen as the model
system. All reactions were conducted in water at room
temperature under air and with 0.5 mol % [Cu]. Neither of the
two Cu(I) catalysts was active under these conditions (Table
6, entries 1 and 2). Changing the solvent to THF or increasing
the catalyst loading up to 5 mol % did not produced any
significant improvement in the reaction yield (entries 3 and 5).
The addition of nitrogen bases has been reported to be
beneficial in these reactions.48,50 Thus, by performing the
cycloaddition in the presence of 4 mol % of 2,6-lutidine, high
yields of the 5-iodotriazole were obtained with both catalyst
(entries 6 and 7). However, catalyst 3c was less selective since
small amounts of dehalogenated 5-H-triazol were also
observed. No change either in yield or in selectivity was
a
Reaction conditions: azide (0.5 mmol), iodoethynylbenzene (0.5
b
mmol), catalyst 1c (1 mol % Cu), H2O (3 mL), 24 h. Isolated yields
(average of two runs). With 2 mol % of Cu. Neat. Trace amounts
of 5-H-triazole detected by H NMR.
c
d
e
1
consequently, reactions involving this azide were performed
under neat conditions to obtain the products in moderate to
good yields (15c and 15f). Only in the case of the
cycloaddition of benzyl azide and iodoethynylcyclopropane
were trace amounts of the dehalogenated triazol detected
(15d).
CONCLUSIONS
■
A family of Cu(I) halide complexes stabilized by dialkyl
terphenyl phosphines, PR2Ar′, has been prepared and
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density filters (D 0.6). DFT calculations have been carried out with
the program Gaussian 09 using the B3LYP functional, 6-31G** basis
set for C and H, and a standard double-ζ polarized basis set, namely,
the LANL2DZ set for Cu, I, and P augmented with polarization
functions on P (a d orbital with exponent 0.364 for P, upon EMSL
basis set exchange database). For Cu, P, and I, we also applied
effective core potentials. Geometry optimization and TD-DFT
calculations were performed in cyclohexane with the PCM solvation
method. Molecular orbital visualization for all the complexes was
performed using Avogadro 1.2.0.
characterized. X-ray diffraction studies disclosed a bimetallic
dihalo-bridged structure for compounds [CuX(PR2Ar′)]2 with
a distorted trigonal planar geometry at the metal center and, in
most cases, a C2h-like symmetry. The presence of a Cu−Cu
bond was detected for two iodide derivatives with an
intermetallic distance of ca. 2.7 Å. Compounds [CuI-
(PMe2ArXyl2)]2 (1c) and [CuI(PEt2ArMes2)]2 (3c) behave in
the same way in degassed liquid solution at 300 K (none of the
two complexes is emissive), in solid−solid solution, i.e.,
Zeonex films, in the solid state at 77 K and in frozen
cyclohexane solution (both complexes are emissive). In
microcrystalline powder samples with sufficient rigid structure
and order, however, the radiative relaxation of excited states is
highly conditioned by interactions with adjacent molecules at
room temperature. In such a way, just by blocking very specific
vibrational modes at dimeric core site, it is possible to “turn
on” the emission at room temperature. In this study, the
centrosymmetric structure of complexes allowed the identi-
fication of such nonradiative deactivation vibrational modes
and showed how structural knowledge can influence the design
of novel luminescent Cu(I) complexes in the future. Among
the dinuclear Cu(I) complexes prepared, [CuI(PMe2ArXyl2)]2
1c and [CuI(PEt2ArMes2)]2 3c are efficient catalysts for [3 + 2]
cycloaddition of azides and terminal alkynes. Catalytic
cycloadditions are conducted in water under air using low
catalyst loadings. Remarkably, these conditions are compatible
with the use of 1-iodoalkynes, a substrate scarcely used in
CuAAC reactions.
Synthesis of [CuCl(PMe2ArXyl2)], 1a. A mixture of the ligand
PMe2ArXyl, L1 (0.080 g, 0.231 mmol), and CuCl (0.023 g, 0.231
mmol) in CH2Cl2 (5 mL) was stirred at room temperature for 20 h.
After filtration, the resulting solution was evaporated to dryness,
1
affording 1a as a colorless solid. Yield: 0.098 g, 98%. H NMR (300
MHz, CDCl3): δ 7.57 (t, 3JHH = 7.6 Hz, 1H, p-C6H3), 7.30 (t, 2H, p-
Xyl), 7.18 (d, 3JHH = 7.6 Hz, 4H, m-Xyl), 7.07 (dd, 3JHH = 7.6 Hz, 4JHP
2
∼ 1 Hz, 2H, m-C6H3), 2.05 (s, 12 H, CH3-Xyl), 1.14 (d, JHP = 6.8
Hz, 6H, P−CH3). 13C NMR (75 MHz, CDCl3): δ 147.1 (br s, o-
C6H3), 140.2 (br s, ipso-Xyl), 135.4 (o-Xyl), 131.8 (p-C6H3), 130.5
(d, 3JCP = 6 Hz, m-C6H3), ipso-C6H3 masked by more intense signals,
128.7 (m-Xyl), 128.6 (p-Xyl), 21.5 (CH3-Xyl), 14.0 (d, 1JCP = 23 Hz,
P-CH3). 31P{1H} NMR (121 MHz, CDCl3): δ 34.3 (br s). Elemental
analysis calculated (found) for C48H54Cu2Cl2P2: C, 64.71 (64.5); H,
6.11 (6.2).
General Catalytic Procedure for the [3 + 2] Cycloaddition of
Azides and Terminal Alkynes: Procedure A. A vial fitted with a
screw cap was loaded with the alkyne (1 mmol), the azide (1 mmol),
the copper complex (0.5 mol % Cu), and water (3 mL) under air. The
reaction mixture was stirred at room temperature for 5−12 h. The
product was extracted with ethyl acetate. The combined organic layers
were washed with brine and dried over anhydrous MgSO4, and the
solvent was removed under a vacuum. When required, the product
was purified by flash column chromatography.
General Catalytic Procedure for the [3 + 2] Cycloaddition of
in Situ Generated Azides and Terminal Alkynes: Procedure B.
A vial fitted with a screw cap was loaded with the alkyne (1 mmol),
the aryl halide (1 mmol), sodium azide (1.3 mmol), the copper
complex (0.5 mol % Cu), and water (3 mL) under air. The reaction
mixture was stirred at room temperature for 5−12 h. The product was
extracted with ethyl acetate. The combined organic layers were
washed with brine and dried over anhydrous MgSO4, and the solvent
was removed under a vacuum. When required, the product was
purified by flash column chromatography.
General Procedure for the [3 + 2] Cycloaddition of Azides
and Iodoalkynes. In a vial fitted with a screw cap, the catalyst 1c
(1−2 mol %) was added to a mixture of iodoalkyne (1 mmol), 2,6-
lutidine (4 mol %), and azide (1 mmol) in water (3 mL) under air.
The reaction mixture was stirred at room temperature for 24 h. Then,
the reaction was quenched by adding aqueous NH4OH (1 mL, 10%
solution). The volatile components were removed by evaporation,
and the crude residue was extracted with diethyl ether and washed
with water. The solvent was removed under a vacuum, and the
product was purified by flash column chromatography.
EXPERIMENTAL SECTION
■
All preparations and manipulations were carried out under oxygen-
free nitrogen, using conventional Schlenk techniques. Solvents were
rigorously dried and degassed before use. Ligands L1−L5,30a,31a
organic azides,51 and iodoalkynes50a were synthesized following
previously reported procedures. Reagents were purchased from
commercial suppliers and used without further purification. NMR
spectra were recorded on Bruker Avance DPX-300, Avance DRX-400,
Avance DRX-500, and 400 Ascend/R spectrometers. The 1H and 13
C
resonances of the solvent were used as the internal standard, and the
chemical shifts are reported relative to TMS while 31P was referenced
to external H3PO4. Elemental analyses were performed by the Servicio
́
́
de Microanalisis of the Instituto de Investigaciones Quimicas (IIQ).
X-ray diffraction studies were accomplished at Centro de Inves-
tigacion Tecnologia e Innovacion, CITIUS (Universidad de Sevilla),
́
́
́
́
́
and Centro de Investigacion en Quimica Sostenible, CIQSO
(Universidad de Huelva). Diffuse reflectance spectra were acquired
in a Shimadzu UV-2501PC equipped with an integrating sphere. The
powdered samples were smashed between two quartz lamellae,
accommodated on a BaSO4 filled support, and the spectra run using
an identical BaSO4 filled support as a blank. The remission function,
F(R), was calculated using the Kubelka−Munk equation for optically
thick samples. UV−vis absorbance spectra were acquired on a UV−
vis−NIR Varian Cary 5000 spectrophotometer, and fluorescence
spectra were recorded on a SPEX Fluorolog-3 Model FL3−22
spectrofluorimeter. Time-resolved emission spectra were acquired in
the same apparatus, using a pulsed xenon lamp with a full-width of 3
μs. Spectra and decays were collected with a minimum 50 μs delay to
remove any interference from the lamp. Luminescence quantum
efficiencies were measured by the absolute method with an integrated
sphere. Films for optical characterization were prepared in a Zeonex
(10% in toluene) matrix by drop-casting onto a quartz substrate with
an emitter concentration of 1% (m/m). The Raman analysis was
carried out using a Labram 300 Jobin Yvon spectrometer, equipped
with a HeNe laser 17 mW operating at 633 nm. The laser beam was
focused with a 50× Olympus objective lens. The laser power at the
surface of the samples was varied with the aid of a set of neutral
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01397.
Experimental details, analytical, spectroscopic and crystal
data, figures and tables (PDF)
Accession Codes
CCDC 2001229−2001231, 2001348, 2001349, 2001251,
2001352 and 2003447 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free for
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif or by contacting The
I
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Inorganic Chemistry
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Article
́
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.
Serrano and Ms. Maria M. Alcaide for performing and
discussing DOSY experiments. Thanks are also due to Centro
́
́
́
de Investigacion, Tecnologia e Innovacion de la Universidad
de Sevilla (CITIUS). BioISI- Biosystems & Integrative
Sciences Institute for computer time and Dr. Vanessa Otero
are acknowledged for assistance with Raman spectra. J.A.
̂
̃
acknowledges Fundaca̧ o para a Ciencia e a Tecnologia (FCT,
I.P.) for funding under grant SFRH/BPD/120599/2016 and
project PTDC/QUI-QFI/32007/2017.
AUTHOR INFORMATION
■
Corresponding Authors
́
Riccardo Peloso − Instituto de Investigaciones Quimicas (IIQ),
́
́
Departamento de Quimica Inorganica and Centro de
́
́
Innovacion en Quimica Avanzada (ORFEO−CINQA),
́
Consejo Superior de Investigaciones Cientificas (CSIC) and
Universidad de Sevilla, 41092 Sevilla, Spain; orcid.org/
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Cesar A. T. Laia − LAQV-REQUIMTE, Departamento de
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orcid.org/0000-0001-6410-6072
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01397
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank MINECO (Grants CTQ2014-52769-C3-3-R,
CTQ2017-82893-C2-2-R, and CTQ2016-75193-P) for finan-
cial support. This work was also supported by the Associate
Laboratory for Green Chemistry-LAQV, which is financed by
national funds from FCT/MCTES (UIDB/50006/2020).
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financed by FCT (the Portuguese national funding agency for
science, research, and technology). We are grateful to Mr.
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