Luminescent Cu(I) complex with bis(indazol-1-yl)phenylmethane as chelating ligand

Article abstract of DOI:10.1016/j.inoche.2020.107894

The cationic Cu(I) complex [Cu(N^N)₂]⁺, where N^N is bis(indazol-1-yl)phenylmethane, was synthesized as chloride or tetrafluoroborate salt by reacting CuCl or [Cu(NCCH₃)₄][BF₄] with bis(indazol-1-yl)p

Full text of DOI:10.1016/j.inoche.2020.107894

InorganicChemistryCommunications116(2020)107894
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Luminescent Cu(I) complex with bis(indazol-1-yl)phenylmethane as
chelating ligand
⁎
Valentina Ferraroa,b, , Marco Bortoluzzia,b, Jesús Castroc, Alberto Vomieroa,d, Shujie Youd
a Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari Venezia, Via Torino 155, I-30170 Mestre, VE, Italy
b Consorzio Interuniversitario Reattività Chimica e Catalisi (CIRCC), via Celso Ulpiani 27, 70126 Bari, Italy
c Departamento de Química Inorgánica, Universidade de Vigo, Facultade de Química, Edificio de Ciencias Experimentais, 36310 Vigo, Galicia, Spain
d Division of Materials Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology, SE-971 87 Luleå, Sweden
G R A P H I C A L A B S T R A C T
A R T I C L E I N F O
A B S T R A C T
Keywords:
Cu(I)
The cationic Cu(I) complex [Cu(N^N)2]+, where N^N is bis(indazol-1-yl)phenylmethane, was synthesized as
chloride or tetrafluoroborate salt by reacting CuCl or [Cu(NCCH3)4][BF4] with bis(indazol-1-yl)phenylmethane
under mild conditions. The structure of [Cu(N^N)2]Cl was ascertained by single-crystal X-ray diffraction. The
complex exhibited bright yellow emission upon excitation with near UV and violet light, attributed to triplet
LLCT/MLCT transitions on the basis of experimental data and computational outcomes.
Luminescence
N-donor ligands
Indazole
1. Communication
The Cu(I) metal centre has high affinity towards N-donor ligands.
Since the pioneering work of McMillin et al. [11–15], species based on
Photoactive complexes based on earth-abundant transition metals
are of huge interest for their application in advanced technology [1]. In
this context, a role of paramount importance is played by Cu(I)-based
coordination compounds, that can be successfully applied in the field of
electrical/light energy conversion. Depending upon the molecular
structure, both singlet and triplet excited states are potentially involved
in the emission from Cu(I) compounds [2–10].
the 1,10-phenathroline skeleton (phenR) have been widely used as
chelating ligands for the preparation of luminescent Cu(I) coordination
compounds, such as homoleptic species having general formula [Cu
(phenR)2]+ [16–18]. The chemical nature, the size and the position of
the substituents on the phenanthroline rings influence the ground and
excited state geometries, thus modulating the MLCT absorption and
emission properties of the corresponding complexes.
⁎ Corresponding author.
E-mail address: valentina.ferraro@unive.it (V. Ferraro).
https://doi.org/10.1016/j.inoche.2020.107894
Received 26 February 2020; Received in revised form 7 March 2020; Accepted 19 March 2020
Availableonline20March2020
1387-7003/©2020ElsevierB.V.Allrightsreserved.

V. Ferraro, et al.
InorganicChemistryCommunications116(2020)107894
Scheme 1. Synthesis of N^N and of [Cu(N^N)2]+
.
Fig. 1. 1H NMR spectra of [Cu(N^N)2][BF4] in DMSO-d6 at 298 and 338 K.
Focusing the attention on azoles, pyrazole-based ligands showed
complexes, despite the fact that most of the studies actually reported in
literature are focused on Cu(II) derivatives [22–31]. In particular,
homo- and heteroleptic compounds with indazolyl-imine ligands were
reported by Cabrera and co-workers [32,33]. It was highlighted that the
introduction of electro-withdrawing or unsaturated functional groups at
different positions in the indazole fragment allowed the fine-tuning of
interesting features, allowing for instance the isolation of luminescent
mono- and polynuclear Cu(I) halide complexes with coordination
numbers three and four [19–21]. N-donor ligands containing the
comparable indazole heterocycle recently showed to be viable alter-
natives to polypyridines for the preparation of luminescent Cu(I)
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V. Ferraro, et al.
InorganicChemistryCommunications116(2020)107894
extended it to the synthesis of bis(indazol-1-yl)phenylmethane. Because
of the interest of our research group towards the preparation of in-
expensive luminescent coordination compounds based on first row
transition elements [37–41], we investigated the reaction between Cu
(I) precursors and the ligand bis(indazol-1-yl)phenylmethane (N^N),
obtaining the photoluminescent homoleptic complex [Cu(N^N)2]+
.
Bis(indazol-1-yl)phenylmethane was synthesized in a two-steps
procedure, first reacting 1H-indazole with triphosgene under mild
conditions, to obtain 1H,1′H-1,1′-carbonyl-di-indazole. The preparation
of the same compound using phosgene as reactant was reported in 1924
[42], but the new synthetic procedure and the spectroscopic char-
acterization data are provided in ESI for completeness. 1H,1′H-1,1′-
carbonyl-di-indazole was then reacted with benzaldehyde under neat
conditions in the presence of CoCl2 to afford the final bis(indazol-1-yl)
phenylmethane ligand (see ESI for experimental details). It is worth
noting that the same compound was previously obtained from the re-
action of 1H-indazole with benzaldehyde in the presence of ZnCl2, or
with benzaldehyde dimethyl acetal in the presence of toluene-4-sulfonic
acid, or with benzylidene dichloride in the presence of NaOH [43].
Characterization data are summarized in ESI (see Fig. S1 for the HSQC
NMR spectrum).
Fig. 2. ORTEP view of the cation found in [Cu(N^N)2]Cl. Atoms are drawn at
30% probability level.
Electrochemical measurements on bis(indazol-1-yl)phenylmethane
showed an irreversible reduction peak around −2.6 V vs. Fc/Fc+ using
acetone as solvent and glassy carbon as working electrode. From the
onset of the peak, the energy of the LUMO was estimated around
−2.6 eV [44]. On the basis of DFT calculations, the LUMO and LUMO
+1 are π* molecular orbitals localized on the indazole fragments.
Bis(indazol-1-yl)phenylmethane (N^N) was reacted with the pre-
cursors CuCl or [Cu(NCCH3)4][BF4] under mild conditions, and in both
the cases the homoleptic complex [Cu(N^N)2]X (X = Cl, BF4) was iso-
lated. The reactions affording the ligand and the corresponding com-
plex are summarized in Scheme 1 for clarity.
Table 1
Selected bond lengths [Å] and angles [°] for [Cu(N^N)2]Cl.
Cu-N(12)
2.032(2)
2.026(2)
92.19(8)
116.99(8)
112.28(9)
Cu-N(22)
2.030(2)
2.033(2)
113.93(9)
130.87(8)
92.62(8)
Cu-N(42)
Cu-N(52)
N(12)-Cu-N(22)
N(12)-Cu-N(52)
N(22)-Cu-N(52)
N(12)-Cu-N(42)
N(22)-Cu-N(42)
N(42)-Cu-N(52)
Experimental details and characterization data are reported in ESI.
Elemental analyses are in agreement with the proposed formula and
conductivity measurements indicate that the complexes behave as 1:1
electrolytes. DSC-TGA measurements on [Cu(N^N)2]Cl showed that,
after melting at 155 °C, the compound starts decomposing, as evidenced
by the progressive weight loss (see Fig. S2).
The IR spectrum shows bands comparable to those of the free li-
gand, with the addition of the υBF4 stretching between 1180 and
1050 cm−1 in the case of X = BF4 [45]. The most diagnostic signals fall
between 1620 and 1450 cm−1 and are related to υC=N and υC=C
stretching vibrations. The 1H NMR spectra do not show any signal at-
tributable to coordinated acetonitrile, and the resonances of the co-
ordinated N^N ligands in DMSO-d6 solution are broad at 298 K, be-
coming sharp on heating the sample above 330 K and revealing that the
two coordinated ligands are equivalent on the NMR timescale (Fig. 1).
Diagnostic signals are the singlets related to the methine fragment at
9.23 ppm and to the indazole-CH at 8.18 ppm at 338 K. It is worth
noting that broad resonances were detected for [Cu(N^N)2]Cl also in
CDCl3 solution in the temperature range 213–318 K (Fig. S3). NMR data
therefore suggest fluxional behaviour of the complex in solution.
The unambiguous characterization of the complex was obtained
from single crystal X-ray structure determination of [Cu(N^N)2]Cl.
Fig. 2 shows an ORTEP [46] view of the cation found in the compound.
Selected distances and angles are set out in Table 1. The cation complex
consists of two bidentate neutral 1,1′-(phenylmethylene)bis(1H-in-
dazole) ligands coordinating Cu(I) in a distorted tetrahedral fashion
(see below). Cu-N bond distances are between 2.026(2) and 2.033(2) Å,
a narrow range consistent with the oxidation state for the copper atom
and similar to those found in other four coordinated compounds with
bis(pyrazol-1-yl) [47–51] or tris(pyrazol-1-yl) [52,53] ligands. Bond-
valence calculation [54] performed with PLATON [55,56] gives for the
Cu atom a value of 1.007, in agreement with the oxidation state 1+ for
it. The angle between chelating CuN12N22 and CuN42N52 planes
(expected 90° for perfect tetrahedron) is 81.75(84)°, somewhat far from
Fig. 3. Normalized absorption spectra of [Cu(N^N)2]+ in DMSO (light violet
line) and CH2Cl2 (violet line). DFT-optimized structure of [Cu(N^N)2]+
(ωB97X/def2-SVP calculation) with superimposed HOMO-1 (green) and LUMO
(yellow). Surface isovalue = 0.03 a.u. Hydrogen atoms are omitted for clarity.
(For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
the photoluminescence properties.
The coordination chemistry of bis(indazol-1-yl)methane and bis
(indazol-2-yl)methane towards Cu(I) was deeply investigated by
Pettinari, Álvarez and co-workers, but the complexes were not studied
from a photophysical point of view [34]. Comparable ligands were less
investigated, for instance bis(indazol-1-yl)phenylmethane was used
only for the preparation of a Rh(I) derivative [35].
We developed an improved synthetic method for the preparation of
bis(benzotriazol-1-yl)arylmethane ligands [36], and recently we
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V. Ferraro, et al.
InorganicChemistryCommunications116(2020)107894
Fig. 4. Normalized PLE (violet line, λemission = 600 nm) and PL spectra (λexcitation = 375 nm) of [Cu(N^N)2]X (X = Cl, yellow line; X = BF4, dark yellow line) and
CIE 1931 chromaticity diagram. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the value of 74.5° found in the related [CuI(H2CPz2)2][ClO4]
(H2CPz2 = bis(pyrazol-1-yl)methane) [47] compound. In other words,
[Cu(N^N)2]+ is less planar and closer to tetrahedron regularity than the
bis(pyrazol-yl)methane compound. Accordingly, the τ4-descriptor for 4-
coordination [57] is 0.70 (extreme forms: 0.00 for square planar, 1.00
for tetrahedron and 0.85 for trigonal pyramid), so the polyhedron
around copper atom is best described as a distorted seesaw with a N
(22)-Cu-N(42) angle of 130.87(8)° and, almost perpendicular, a N(12)-
Cu-N(52) angle of 116.99(8)°. Dihedral angle between these planes,
CuN22N42 and CuN12N52 ones, is 76.96(9)°. Fig. S4 shows the seesaw
geometry. On the other hand, the chelate angles are 92.19(8) and
92.62(8)°, only slightly lower than those found in the related bis(pyr-
azol-1-yl)methane compound, 94.1° [47]. The chelate rings adopt a
boat conformation with the methylene [0.437(2) and 0.431(3) Å] and
the copper atom [0.4161(10) and 0.3966(10) Å] above the best plane.
The phenyl rings are in axial position with respect to the plane, angles
of 11.37(13) and 10.98(16)° with the normal of each plane. Fig. S5
shows these features.
orbitals (HOMO, HOMO-1) and unoccupied π* orbitals localized on the
indazol-1-yl fragments (LUMO, LUMO+1, LUMO+12 and LUMO
+13), depicted for clarity in Fig. S8. Fig. 3 shows as example the su-
perposition of the HOMO-1 (green) and of the LUMO (yellow).
Solid [Cu(N^N)2]X complexes exhibit bright yellow emission upon
excitation with near-UV and violet light. The maximum of the emission
band (PL, Fig. 4) lies at 565 nm and the FWHM is 4600 cm−1. The
emission from [Cu(N^N)2]X can be obtained for excitation wavelengths
below 450 nm, as observable in the excitation (PLE) spectrum reported
in Fig. 4, with PLE maximum at 386 nm. The emission spectrum re-
mains constant on varying the excitation wavelength in the PLE range
and, as observable in Fig. 4, it is independent upon the nature of the
counterion. Powder and crystals of the compound show the same
photoluminescence features. The CIE 1931 chromaticity coordinates
are x = 0.437, y = 0.477, falling between the yellow and the yellowish
orange regions of the diagram in Fig. 4.
The time-resolved photoluminescence spectrum reported in Fig. 5
clearly shows the presence of two decay components, characterized by τ
values of 12 (63%) and 73 μs (37%), respectively. The average lifetime
is 35 μs. The tenths of microseconds range indicate that triplet excited
states are involved. Biexponential decay is quite common for lumines-
cent Cu(I) complexes, recent examples being oligophosphine-thiocya-
nate derivatives [58] and PNP coordination compounds [59]. In the
case of [Cu(TP)(PPh3)2]+, where TP = 2-(1H-tetrazol-5-yl)pyridine,
the two lifetimes were assigned as the triplet decays of MLCT and LLCT
excited states [60]. For what concerns [Cu(N^N)2]+, the TD-DFT opti-
mized geometries of the first (T1) and second (T2) triplet states agree
with 3LLCT/3MLCT transitions, being the electronic structures de-
scribed by the electron transfer from Cu- and indazole-centred mole-
cular orbitals (mainly HOMO, HOMO-1 and HOMO-2) to indazole-lo-
calized unoccupied orbitals (mainly LUMO). The orbitals most involved
are depicted in Fig. 5. T1 and T2 states have quite similar geometry, as
depicted in Fig. S9, and the poor rigidity of the molecule was indicated
by NMR studies. The energy difference between the two excited states
at T2 geometry is only 470 cm−1, therefore both T1 and T2 appear
reasonably involved in the emission from the complex.
As stated in the ESI, the position of the chloride anion resulted to be
disordered and therefore it could not be modelled, so the interactions
between cation and anion could not be studied. However, the cations
are connected by means of π,π-stacking interactions between the in-
dazol-1-yl moieties. Fig. S6 shows one of them, causing the growth of
the crystal along the a axis. This π,π-interaction (symm. op. x-0.5, y,
0.5-z and x+0.5, y, 0.5-z) leaves a distance between centroids of the
benzene rings of 3.772(2) Å and a slippage of 1.021 Å. The dihedral
angle between planes is 13.70(14)°. Other interesting interactions be-
tween planes (See Fig. S7) are those formed between one indazol-1-yl
ring and its symmetrical equivalent (symm. op. 0.5-x, 0.5-y, 0.5-z),
which generates dimeric entities. The distance between centroids is
4.139(2) Å and the ring slippage is 1.181 Å. The dihedral angle between
planes is 1.7 (2)°.
[Cu(N^N)2]+ absorbs light in CH2Cl2 and DMSO solution for wa-
velengths below 410 nm. As observable in Fig. 3, the use of a co-
ordinating solvent such as DMSO changes the absorption features, being
markedly less pronounced the band centred around 329 nm. This out-
come suggests that the lowest energy absorption could be related to
charge transfer between metal centre and coordinated ligands. TD-DFT
calculations clearly indicated that the lowest energy singlet transition
has MLCT character, involving metal-centred occupied molecular
The photoluminescence quantum yield (PLQY) measured at room
temperature for powder samples is 1.3%. The radiative (kr) and non-
radiative (knr) rate constants are estimated on the basis of the equation
PLQY = kr/(kr + knr) = τmkr [5] to be kr = 3.7·102 s−1 and
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V. Ferraro, et al.
InorganicChemistryCommunications116(2020)107894
Fig. 5. Semi-log plot of the time-resolved lumines-
cence decay of [Cu(N^N)2]Cl. TD-DFT-optimized
structures of T1 and T2 states of [Cu(N^N)2]+
(ωB97X/def2-SVP calculation). Superimposed orbi-
tals for T1: HOMO-2 and HOMO (green), LUMO
(yellow). Superimposed orbitals for T2: HOMO-1 and
HOMO (green), LUMO (yellow). Surface iso-
value = 0.03 a.u. Hydrogen atoms are omitted for
clarity. (For interpretation of the references to colour
in this figure legend, the reader is referred to the web
version of this article.)
Table 2
Absorption and photoluminescence data of [Cu(N^N)2]X.
X = Cl
X = BF4
UV–VIS (CH2Cl2, 298 K, nm) < 410, 288 (max), 329 (sh).
PL (solid, r.t., λexcitation = 375 nm, nm) 565 (FWHM = 4600 cm−1). CIE 1931: x = 0.437,
y = 0.477.
UV–VIS (DMSO, 298 K, nm) < 410, 291 (max), 298 (sh), 311 (sh)
PL (solid, r.t., λexcitation = 375 nm, nm) 565 (FWHM = 4600 cm−1). CIE
1931: x = 0.437, y = 0.477.
PLE (solid, λemission = 600 nm, nm) < 450, 389 (max).
PLE (solid, λemission = 600 nm, nm) < 450, 389 (max).
τ (solid, r.t., λexcitation = 317 nm, λemission = 635 nm, μs): 12 (63%), 73 (37%). PLQY: 1.3%.
kr = 3.7∙102 s−1. knr = 2.8∙104 s−1
.
knr = 2.8·104 s−1. The low quantum yield can be explained on con-
sidering the scarce rigidity of the molecule and its distortion towards
square planar geometry [9], evident for the T2 excited state, with τ4
parameter of 0.44 [57]. Both these factors favour non-radiative decay
routes. The photoluminescence data are summarized in Table 2.
In conclusion, in this communication we described the straightfor-
ward preparation and characterization of a new Cu(I) homoleptic
complex, representing the first example of luminescent species with a
ligand based on the bis(indazol-1-yl)methane skeleton. Lifetime mea-
surements indicate that the photophysical features are based on the
population of triplet states. The non-radiative decay appears related to
the distortion of the first coordination sphere, and it is therefore likely
to suppose that proper modifications of the bulk of the donor moieties
could improve the photoluminescence quantum yield.
Acknowledgments
Ca' Foscari University of Venice is gratefully acknowledged for fi-
nancial support (Bando SPIN 2018, D. R. 1065/2018 prot. 67416).
CACTI (University of Vigo) is gratefully acknowledged for X-ray data
collection. We sincerely thank Luca Pietrobon for TGA measurements.
A.V. and S.Y. acknowledge financial support from the Kempe
Foundation and the Knut & Alice Wallenberg Foundation.
Appendix A. Supplementary material
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.inoche.2020.107894.
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