Tetra- and hexanuclear copper(I) iminothiolate complexes: synthesis, structures, and solid-state thermochromic dual emission in visible and near-infrared regions

Article abstract of DOI:10.1007/s11696-020-01251-w

Two new photoluminescent multinuclear Cu(I) cluster complexes supported by monoanionic bidentate ligand N-methylbenzimidazolethiolate (Me-bimt^–), [Cu_n(Me-bimt)_n] with n = 4 (1) and 6 (2), have been synthesized and structur

Full text of DOI:10.1007/s11696-020-01251-w

Chemical Papers
https://doi.org/10.1007/s11696-020-01251-w
ORIGINAL PAPER
Tetra‑ and hexanuclear copper(I) iminothiolate complexes: synthesis,
structures, and solid‑state thermochromic dual emission in visible
and near‑infrared regions
Yoshiki Ozawa1 · Marino Mori1 · Hidetoshi Kiyooka1 · Yuumi Sugata1 · Toshikazu Ono2 · Masaaki Abe1
Received: 13 February 2020 / Accepted: 13 June 2020
© Institute of Chemistry, Slovak Academy of Sciences 2020
Abstract
Two new photoluminescent multinuclear Cu(I) cluster complexes supported by monoanionic bidentate ligand N-methylbenzi-
midazolethiolate (Me-bimt–), [Cun(Me-bimt)n] with n=4 (1) and 6 (2), have been synthesized and structurally characterized
by single-crystal X-ray difraction analysis. For 1 and 2, the Cu(I) ions and the Me-bimt– ligands construct a cubane-type
{Cu4S4} and a hexagonal-prism {Cu6S6} frameworks, respectively. In the crystalline state, complexes 1 and 2 exhibit green
(λem =500 nm) and near-infrared (λem =876 nm) emission, respectively, under UV irradiation (λex =365 nm) at room tem-
perature. Both crystals reveal temperature-dependent dual emission below 200 K: complex 1 emits in the visible wavelength
region (λem =493 and 542 nm) and complex 2 in the visible to near-infrared wavelength region (λem =752 and 973 nm) which
are attributed to multiple photoexcited states at the cluster frameworks with distinct metal nuclearity.
Keywords Copper(I) · Cluster compounds · Crystal structure · Thermochromic dual emission · Near-infrared emission
Introduction
photo-excited states such as metal-to-ligand charge transfer
(MLCT), multi-metal cluster-centered (CC) charge transfer,
and inter-ligand (IL) transition (Yam and Lo 1999). In some
cases, radiative decay from two distinct photoexcited states
simultaneously occurs, which represents the “dual emission”
phenomenon (Ford et al. 1999).
Multinuclear transition-metal complexes with d10 elec-
tronic confguration, such as copper(I) and silver(I), show
rich photoluminescent behavior associated with multiple
Monoanionic iminothiolato (N=C–S–) bidentate ligands
such as pyridinethiolate and imidazolethiolate are able to
link two or more metal centers to form multinuclear cluster
complexes (Ford and Vogler 1993). Many literature stud-
ies are available for crystal structures of the iminothiolato
multicopper(I) complexes with a tetrameric {Cu4S4} core
and a hexameric, hexagonal pillar-type {Cu6S6} core, but
only a few studies have been reported so far on their photo-
physical properties (Kundu et al. 2014; Xie et al. 2005). Such
multicopper(I) complexes are typically formed via spontane-
ous self-assembly of the corresponding metallic sources and
ligands mixed together with the appropriate molar ratio in
solution, but the synthetic methodology to precisely control the
metal nuclearity (e.g., the number of metal centers in a single
molecule) of products remains yet, in general, to be estab-
lished for transition-metal coordination complexes. Herein
we describe the synthesis, molecular structures, and solid-
state photoluminescence properties of two new Cu(I) imi-
nothiolate clusters formulated [Cun(Me-bimt)n] with n=4 (1)
This work was presented at the International Conference on
Coordination and Bioinorganic Chemistry held in Smolenice
during June 2–7, 2019.
This paper is dedicated to Professor Milan Melník for his
outstanding contribution to coordination and bioinorganic
chemistry.
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11696-020-01251-w) contains
supplementary material, which is available to authorized users.
* Yoshiki Ozawa
ozawa@sci.u-hyogo.ac.jp
* Masaaki Abe
mabe@sci.u-hyogo.ac.jp
1
Graduate School of Material Science, University of Hyogo,
3-2-1 Kouto, Kamigori-Cho, Ako-Gun, Hyogo 789-1297,
Japan
2
Graduate School of Engineering, Kyushu University, 744
Moto-oka, Nishi-Ku, Fukuoka 819-0395, Japan
Vol.:(0123456789)
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Chemical Papers
and 6 (2), where Me-bimt– is a monoanionic bidentate ligand
N-methylbenzimidazolethiolate(1–). The synthetic conditions
employed gave initially a crude solid containing both tetramer
1 and hexamer 2 but the individual clusters were subsequently
transformed to either form by adopting appropriate solvent
combinations for recrystallization. Selective isolation of 1 and
2 has been unambiguously confrmed by single-crystal X-ray
difraction analyses. We show that both compounds exhibit
temperature-dependent dual emission in the solid state upon
UV excitation, wherein 1 exhibits thermochromic dual emis-
sion in the visible region and 2 from visible to near-infrared
(NIR) region.
Synthesis and characterization
Synthesis of (triphenylmethyl)sulfanylbenzimidazole
(Tr‑bimt).
This compound was prepared according to the literature
procedure (Doerge and Cooray 1991). A solution of benzi-
midazole-2-thiol (bimtH) (4.2 g, 28 mmol), trityl chloride
(8.0 g, 28 mmol), and Et3N (3.1 g, 28.5 mmol) in dry THF
(150 mL) was stirred for 8 h at 293 K. The resulting suspen-
sion was fltered to remove triethylammonium salt which
was washed thoroughly with THF. The combined fltrate was
concentrated by evaporation to dryness under reduced pres-
sure. The white solid of Tr-bimt was identifed by 1H NMR
spectroscopy as a mono-THF adduct. Yield: 10.1 g (92%).
The obtained product was used in the following synthesis
Experimental
Materials and methods
1
without further purifcation. H NMR (600 MHz, CDCl3,
Chemical reagents and solvents used in this study were pur-
chased from TCI and WAKO and used without further puri-
fcation unless otherwise stated. All of the synthetic reactions
in this study were performed under Ar atmosphere.
298 K) δ 7.70 (d, 1H, J=8.22 Hz, benzimidazole 7-H), 7.64
(s, 1H, NH), 7.47 (d, 6H, J=6.9 Hz, trityl group ortho H),
7.33–7.27 (m, 9H, trityl group meta and para H), 7.19–7.12
(m, 2H, benzimidazole 5,6-H), 6.99 (d, 1H, J=7.6 Hz, ben-
zimidazole 4-H), 3.75 (t, 4H, THF –O–CH2–CH2–), 1.85 (m,
4H, THF –O–CH2–CH2–).
Apparatus and equipment
IR measurements were performed on a JASCO FT/IR-4000
at 298 K by an attenuated total refectance (ATR) method. 1H
NMR spectroscopy was performed at 600 MHz on a JEOL
ECA-600 and a JEOL ECZ-600R/S1 spectrometers using
CDCl3 as solvents, and TMS was employed as an internal
standard to determine the chemical shift. Elemental analy-
sis was carried out by the Laboratory for Organic Elemen-
tal Microanalysis, Kyoto University. UV–Vis difuse refec-
tance spectra (DRS) were measured by a JASCO V-670
UV–Vis–NIR spectrometer with an ISN-723 integrating
sphere attachment. Luminescence spectra of the single crystals
were measured by a Hamamatsu PMA-11 (C7473-46) spec-
trometer (190–900 nm) and an Ocean Optics FLAME-S spec-
trometer (600–1100 nm). The crystalline samples were placed
in a Linkam THMS600 temperature-controlled (78–293 K)
microscope stage. A Panasonic UJ-20 UV-LED system was
used as an excitation light source (λex =365 nm). The emission
from the solid samples was introduced to the optical fbers of
the spectrometer through a microscope. Lifetimes and quan-
tum yields of solid-state photoluminescence were measured
on a Hamamatsu Quantaurus-Tau C11367-02 fuorescence
lifetime spectrometer and Quantaurus-QY C11347 absolute
PL yield spectrometer, respectively.
Synthesis of methyl‑2‑(triphenylmethyl)
sulfanylbenzimidazole (Tr–Me‑bimt).
This compound was obtained by a modifed method of the
literature (Doerge and Cooray 1991). Ground powder of
KOH (7.0 g, 125 mmol) was placed in a round-bottom fask
(500 mL) and dried under vacuum for 3 h. To this was added
a solution of Tr-bimt–THF adduct (10.0 g, 25.5 mmol) in dry
acetone (170 mL) followed by MeI (5.9 g, 41 mmol), and
the mixture was stirred for 1 h at 293 K. The resulting pale-
yellow solution was fltered to remove a suspended solid of
KOH. The fltrate was added to toluene (700 mL), and the
mixture was divided into two portions which were washed
frst with water (150 mL) and second with a saturated aque-
ous solution of NaCl (150 mL). After combining the indi-
vidual solutions, the solution was dried over anhydrous
Na2SO4 and was concentrated by evaporation to dryness
under reduced pressure. Yield: 7.1 g (68%). The yellow solid
of Tr–Me-bimt as a crude product was used in the following
synthesis without further purifcation. 1H NMR (600 MHz,
CDCl3, 298 K) δ 7.31–7.27 (m, 15H, trityl group), 7.23–7.12
(m, 4H, benzimidazole 4,7-H), 3.75 (s, 3H, CH3).
Synthesis of N‑methyl‑benzimidazolethiol (Me‑bimtH).
A solution of Tr–Me-bimt (7.1 g, 16.9 mmol) in acetic acid
(7.5 mL)–MeOH (142.5 mL) was heated under contentious
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stirring for 5 h at 358 K. The resulting yellow solution was
concentrated by evaporation to dryness under reduced pres-
sure. The yellow solid was dissolved in CH2Cl2 (150 mL)
and was washed with an aqueous solution of 10% sodium
bicarbonate (100 mL×3). The organic layer was dried over
anhydrous Na2SO4 and concentrated by evaporation to dry-
ness under reduced pressure. The light-yellow crude prod-
uct was collected by fltration and washed with toluene to
remove unreacted Tr–Me-bimt. The obtained white pow-
der was identifed by 1H NMR spectroscopy. Yield: 1.59 g
2015). For the refnement of 1, the structure was treated as
a two-component twin using TWIN and BASF commands
in the SHELXL program. It was found that the fnal R value
for 1 was lower at high temperature (290 K, R=0.091) than
low temperature (150 K, R=0.119), allowing us the high-
temperature data to use for structural discussion. In con-
trast, free from the twin problem, the results for structural
analysis of 2 were satisfactory at both temperatures (150 K,
R = 0.052; 290 K, R = 0.064). Under these circumstances,
the 290-K data were employed for both complexes for their
structural description and comparison.
1
(58%). H NMR (CDCl3, 600 MHz, 298 K) δ 10.68 (s,
br, 1H, SH), 7.27–7.20 (m, 1H, benzimidazole 7-H), 7.23
(ddd, 2H, J = 6.9, 6.5, and 1.4 Hz, benzimidazole 5,6-H),
7.17–7.15 (m, 1H, benzimidazole 4-position), 3.79 (s, 3H,
CH3).
Crystallographic data of 1 (T=290 K): C32H28Cu4N8S4,
Mr = 907.02, colorless prism, 0.34 × 0.34 × 0.28 mm3,
monoclinic, space group C2/c (No. 15), a = 20.307(2),
b = 15.107(1), c = 15.185(1) Å, β = 132.511(2)°,
V = 3434.0(4) Å3, µ(λ = 0.7107 Å) = 2.73 mm–1, Z = 4,
θmax = 30.0°, 20,220 reflections measured, 4976 unique
(Rint =0.07) which were used in all calculations. The fnal
R=0.091, wR(F2)=0.318, and GOF=1.09 (all data).
Crystallographic data of 2 (T=290 K): C48H42Cu6N12S6,
Mr =1360.53, red platelet, 0.20×0.15×0.07 mm3, trigonal,
space group R-3 (No. 142), a=13.479(2), c=24.061(3) Å,
V = 3786.0(10) Å3, µ(λ = 0.7107 Å) = 2.78 mm–1, Z = 3,
θmax = 30.0°, 15,063 reflections measured, 2445 unique
(Rint =0.073 which were used in all calculations. The fnal
R=0.064, wR(F2)=0.233 and GOF=1.17(all data).
CCDC reference numbers for 1 are 1,981,286 (290 K) and
1,981,287 (150 K) and those for 2 are 1,981,293 (290 K) and
1,981,294 (150 K).
Synthesis of N‑methyl‑benzimidazolethiolatocopper(I)
[Cun(Me‑bimt)n] (n= 4 and 6).
A solution of Me-bimtH (0.092 g, 0.6 mmol) with Et3N
(0.151 g, 1.5 mmol) in CH3CN (10 mL) was added dropwise
to a solution of [Cu(CH3CN)4]CF3SO3 (0.196 g, 0.5 mmol)
in CH3CN (10 mL) under continuous stirring at 293 K. The
resulting pale-yellow suspension was heated under continu-
ous stirring for 2 h at 323 K. A white solid formed was
separated by fltration, washed with CH3CN, and dried under
vacuum. This preparative procedure gave a mixture of two
species [Cun(Me-bimt)n] with n=4 and 6 on the basis of ele-
mental analysis and 1H NMR spectroscopy. Yield: 0.102 g
(87%). Anal. Calcd. for C8H7CuN2S: C, 42.37; H, 3.11; N,
12.35%. Found: C, 42.45; H, 3.01; N, 12.18%.
Density functional theory (DFT) calculations
Separated recrystallization and single‑crystal
X‑ray structure analysis for [Cu4(Me‑bimt)4] (1)
and [Cu6(Me‑bimt)6] (2).
DFT calculations were carried out with the ADF2019 suite
of programs (Baerends et al. 2019; te Velde et al. 2001).
The geometrical parameters for 1 and 2 were taken from the
crystal structures. Geometry optimizations were performed
with applying S4 and D3d molecular symmetry to 1 and 2,
respectively. Exchange correlation function (XC) of PW91
method was applied for calculations. The valence shell
atomic orbitals of Cu, S, N, C, and H atoms were described
by triple-zeta Slater-type basis sets with two polarization
functions (ADF database TZ2P).
Single crystals of 1 were grown at 293 K as colorless block-
shaped crystals from a toluene solution of the above-men-
tioned mixture by layering and slowly difusing Et2O. Single
crystals of 2 were obtained at 303 K as red platelet-shaped
crystals from a CH2Cl2 solution of the mixture by layering
and slowly difusing CH3CN. Single-crystal X-ray difrac-
tion measurements were performed at 290 and 150 K for
both crystals on a Rigaku Rapid image plate difractom-
eter using Mo Kα (λ=0.7107 Å) radiation. The crystal was
attached to a thin Nylon loop and kept under a cold N2 gas
stream from a He gas-expansion cryostat. All non-hydrogen
atoms were refned anisotropically, and hydrogen atoms
were placed at the calculated positions and treated according
to the riding model during refnements. All the calculations
for structure determination and refnements were carried out
using WinGX (Farrugia 2012) crystallographic software
suite with SHELXL-2016/6 refnement program (Sheldrick
Results and discussion
Synthesis and crystallization
We have synthesized and isolated single crystals of tetranu-
clear complex [Cu4(Me-bimt)4] (1) and hexanuclear complex
[Cu6(Me-bimt)6] (2), as shown in Fig. 1. The reaction of
[Cu(CH3CN)4]CF3SO3 with a slight excess of Me-bimtH
in CH3CN in the presence of Et3N (2 h, 323 K) gave an
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Fig. 1 Schematic of preparation and crystallization procedures of 1 and 2. Photographic images of the crystalline solid are also shown
of-white solid which was identifed as mixture of two spe-
cies [Cun(Me-bimt)n] with n=4 and 6 (87% yield). Selec-
tive isolation of either form was successfully achieved by
recrystallization of the mixture by the use of appropriate
solvent combination. Thus, complexes 1 and 2 were crys-
tallized separately at room temperature from toluene/Et2O
and CH2Cl2/CH3CN systems, respectively. Interestingly, the
color of the crystals is markedly diferent, e.g., colorless and
deep red for 1 and 2, respectively. It is noted that when lower
temperature conditions (such as 273 K) were applied com-
plex 2 was contaminated with 1, implying an equilibrium
between 1 and 2 in solution. The formation of the of-white
solid during the synthetic reaction may infer that complex 1
is a kinetic product.
Figure 2 shows 1H NMR spectrum of a CDCl3 solution pre-
pared by dissolution of 1 at 295 K, wherein two groups of
resonances were observed for the benzimidazole moiety with
labels (a–e) for the larger intensity group (defned as group
A) and (a’–e’) for the smaller (defned as group B). Here,
either group is tentatively assigned to 1 or 2. The chemical
shift data (CDCl3, 600 MHz, 295 K) and the assignment
are as follows. Group A: δ 7.32, 7.04, 6.93, 6.63 (s, 1H×4,
aromatic protons of the benzimidazole moiety), and 3.46 (s,
3H, CH3). Group B: δ 7.35, 6.96, 6.80, 6.38 (s, 1H×4, aro-
matic protons of the benzimidazole moiety), and 3.23 (s, 3H,
CH3). The observed B/A ratio in intensity was 0.4. Slightly
broadened resonances may infer the presence of exchange
between two species on the NMR timescale. Further study,
including variable-temperature NMR spectroscopy, is neces-
sary to describe in more detail the dynamics and kinetics of
these clusters in solution.
We have also found a reversible transformation between
1 and 2 upon recrystallization by appropriate solvent choice
mentioned above. Thus, recrystallization of red crystals 2
from toluene/Et2O gave colorless crystals 1 to be deposited
from the solution. Similarly, recrystallization of colorless
crystals 1 from CH2Cl2/CH3CN results in the deposition of
red crystals 2. The solvent combination thus leads to a com-
plete control over metal nuclearity of products as a crystal.
There are only a few previous reports for such a separation
of tetrameric and hexameric Cu(I) complexes in preparative
procedures (Chivers et al. 2001; Yue et al. 2009). In the cur-
rent study, we have shown not only a complete separation but
also a reversible conversion between tetramer and hexamer
through recrystallization.
Single‑crystal X‑ray difraction study
The molecular structures of 1 and 2 were unambiguously
determined by single-crystal X-ray difraction analysis.
The molecular structure and the {Cu4S4} framework of
1 at 290 K are depicted in Fig. 3. Selected interatomic
distances and bond angles are listed in Table 1. Complex
1 crystallizes in the monoclinic space group C2/c and has
a cuboidal {Cu4S4} framework, wherein four Cu(I) cent-
ers are bridged by N and S atoms from the bidentate Me-
bimt– ligands (Fig. 3a). In the crystalline lattice, a twofold
rotation axis parallel to the b-axis penetrates the molecule
and half of the molecule is crystallographically independ-
ent. The whole molecule has virtually an S4 symmetry.
A qualitative insight has been gained for solution equilib-
rium between 1 and 2 by 1H NMR spectroscopy. Dissolution
of either crystal, 1 or 2, in CDCl3 at room temperature was
found to give, within minutes, a light-yellow clear solution.
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Fig. 2 A 1H NMR spectrum
(600 MHz, 295 K) of a CDCl3
solution prepared upon dissolu-
tion of crystals of 1. Note that
two diferent species labeled as
(a–e) and (a’–e’) were readily
formed in the room-temperature
solution; these are described
in the main text as groups A
and B, respectively. Inset: the
proton labeling scheme for the
benzimidazolate moiety
Fig. 3 ORTEP diagrams of 1 at
290 K with thermal ellipsoids
at a 50% probability level.
Hydrogen atoms are omitted for
clarity. a The molecular struc-
ture. b The {Cu4S4} framework
projected along the c-axis and c
that along the b-axis. For b and
c, Cu•••Cu distances (Å) are
given. Symmetry code: (*) –x,
y, 1/2–z
Table 1 Selected interatomic distances (Å) and bond angles (°) of 1
The Cu(I) centers adopt a highly distorted, three-coordi-
nate geometry with an “NS2” donor set. Four Cu(I) and
four S atoms provided from Me-bimt– form a cuboidal
{Cu4S4} framework (Fig. 3b) with a tub-shaped, eight-
membered {Cu4S4} ring (Fig. 3c). Interatomic separations
between two Cu(I) centers perpendicular to the molecu-
lar S4 axis (Cu1•••Cu1* and Cu2•••Cu2*) are 3.158(2)
and 3.169(2) Å (Fig. 3b). In contrast, four other Cu•••Cu
contacts (Cu1•••Cu2, Cu1•••Cu2*, Cu1*•••Cu2, and
Cu1*•••Cu2*) are much shorter, 2.653(2) and 2.656(2) Å
(Fig. 3c) and these are also shorter than the summation
of van der Waals radii (2.8 Å). The observed Cu•••Cu
separations and tetragonal compression (along the b-axis)
of the {Cu4} core in 1 are typical of those found in
at 290 K
Interatomic distances
Cu1•••Cu2
Cu1•••Cu1*
Cu1—S1*
2.653(2) Cu1•••Cu2*
3.169(2) Cu2•••Cu2*
2.339(3) Cu2—S2*
2.234(3) Cu2—S1
1.989(7) Cu2—N3
2.656(2)
3.158(2)
2.349(3)
2.235(3)
1.993(6)
Cu1—S2*
Cu1—N1
Bond angles
S2*—Cu1—S1*
N1—Cu1—S2*
N1—Cu1—S1*
Cu2—S1—Cu1*
130.95(9)
S1—Cu2—S2*
N3—Cu2—S1
N3—Cu2—S2*
Cu1*—S2—Cu2*
130.65(9)
122.2(2)
106.8(2)
70.71(8)
122.4(2)
106.4(2)
70.95(8)
Symmetry code: (*) –x, y, 1/2–z
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Fig. 4 Stick-model representations for crystal packing of 1 at 290 K.
a Projected along the a*-axis (on the bc plane). b Projected along the
b-axis. Two sets of symmetry-equivalent ligand moieties are drawn
as pink and light-green sticks. Interatomic contacts within the sum of
the van der Waals radii are indicated as broken cyan lines
imidazolethiolate tetracopper(I) complexes reported ear-
lier (Raper et al. 1991; Venkatesh et al. 2014).
3.67(2) Å) are observed among molecules in the ac-plane
(Fig. 4b).
Crystal packing of 1 is illustrated in Fig. 4. Selected
intermolecular contacts are listed in Table S1 (Sup-
plementary data). The molecules are aligned along the
b-axis to form 1D columns via intermolecular van der
Waals contacts between benzene ring moieties in Me-
bimt– (C16•••C5′, 3.86(2) Å) (Fig. 4a). In addition, weak
π–π contacts (C6•••C6′, 3.31(3); C16•••C16, 3.33(2) Å)
and C–H•••S contacts (S1•••C3′, 3.69(1); C13•••S2′,
The molecular structure and {Cu6S6} framework of 2
at 290 K are depicted in Fig. 5. Selected interatomic dis-
tances and bond angles are listed in Table 2. Complex 2
crystallizes in the trigonal space group R-3. Two hexagonal
{Cu3S3} moieties are located mutually in an anti-parallel
fashion to form the {Cu6S6} framework, and six planar Me-
bimt– ligands support the distorted {Cu6} octahedron to
form a six-bladed paddle-like shape (Fig. 5a). The whole
Fig. 5 ORTEP diagrams of 2 at
290 K with thermal ellipsoids
at a 50% probability level.
Hydrogen atoms are omitted
for clarity. a The molecular
structure. b The {Cu6S6} frame-
work along the c-axis and c that
along the a-axis. For b and c,
Cu···Cu distances (Å) are given.
Symmetry codes: (i) 2/3+x–y,
1/3+x, 4/3–z; (ii) 1–y, 1+x–y,
z; (iii) 2/3–x, 4/3–y, 4/3–z;
(iv) –x+y, 1–x, z; (v) 1/3+y,
1/3–x+y, 4/3–z
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Table 2 Selected interatomic distances (Å) and bond angles (°) of 2
red crystals of 2 exhibit a low-energy, weaker shoulder
at 460 nm in addition to a high-energy, stronger band at
330 nm as shown in Fig. S2 (Supplementary data), the for-
mer of which causes the coloration of 2.
at 290 K
Interatomic distances
Cu1•••Cu1i
Cu1•••Cu1ii
3.137(2)
2.918(2)
Cu1—S1
Cu1—S1ii
Cu1—N1i
2.266(2)
2.259(2)
2.004(2)
To gain insights into absorption properties of 1 and 2,
especially to identify the nature of visible coloration in 2,
DFT calculations have been carried out. The results are
shown in Figs. S3 and S4 (Supplementary data). Irrespec-
tive of the metal nuclearity, the highest occupied molecular
orbital (HOMO) is assigned to an admixture of (Cu 3d, S
3p) orbitals in the {CunSn} cores (n=4 or 6) which are non-
bonding in character. In contrast, the nature of the lowest
unoccupied molecular orbital (LUMO) is diferent between
the two complexes. The LUMO for 1 is localized to the ben-
zimidazole moiety (ligand-π* orbital) (Fig. S3) whereas that
of 2 is ascribed to an admixture of (Cu 4 s/4p, S 4 s/4p)
orbitals in the {Cu6S6} core which is bonding in character
(Fig. S4). The LUMO level in 2 becomes lower than the
ligand-π* orbital (second LUMO) in energy. The decrease in
the LUMO level of 2 relative to 1 is ascribable to electronic
delocalization over the {Cu6S6} core (Ford et al. 1999; Sabin
et al. 1992; Yue et al. 2009). The high-energy bands in DRS
for both complexes (330 and 335 nm for 1 and 2, respec-
tively) are attributed to “{CnSn} cluster-to-ligand” charge-
transfer transitions. The low-energy band for 2, correspond-
ing to the narrower HOMO–LUMO gap, is attributable to
“{C6S6} cluster-centered” transitions in the {Cu6S6} core.
Bond angles
S1ii—Cu1—S1
Cu1iv—S1—Cu1
124.85(10)
80.31(8)
N1i—Cu1—S1
N1i—Cu1—S1ii
111.7(2)
116.3(2)
Symmetry codes: (i) 2/3+x–y, 1/3+x, 4/3–z; (ii) 1–y, 1+x–y, z; (iv)
–x+y, 1–x, z
molecule has a threefold rotoinversion axis with an exact
S6 symmetry, in which one Cu(I) and one Me-bimt– are
crystallographically independent. The Cu•••Cu distances
are 3.137(2) and 2.918(2) Å (Fig. 5b), indicating trigonal
elongation of the {Cu6} core (Fig. 5c). Each Cu(I) adopts
three-coordinate geometry with an “N2S” donor set.
Crystal packing of 2 is shown in Fig. 6. Selected intera-
tomic contacts are listed in Table S2 (Supplementary data).
The molecules are engaged each other like “gears” with
π•••π contacts between neighboring ligands (C6•••C7′,
3.40(1) Å) to form a 2D layer (Fig. 6a) which is perpendicu-
lar to the c-axis (Fig. 6b).
Difuse refectance spectra (DRS) and density
functional theory (DFT) calculations
Solid‑state photoluminescence
UV–Vis DRS of colorless crystals of 1 display a single band
centered at 335 nm and is transparent in the visible region,
as shown in Fig. S1 (Supplementary data). In contrast, deep
In the crystalline state, both compounds are emissive under
UV illumination (λex = 365 nm). The photoluminescence
Fig. 6 Stick- and space-fll model representations of the crystal packing of 2 at 290 K. a Projected along the c-axis. b Projected along the a-axis.
Intermolecular contacts within the sum of the van der Waals radii are indicated as broken cyan lines
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Chemical Papers
Fig. 7 Temperature-dependent
solid-state photoluminescence
spectra (λex =365 nm) of a
1 (T=78 to 298 K) and b 2
(T=78 to 293 K)
spectra of 1 and 2 at various temperatures are shown in
Fig. 7, which reveals a markedly contrast in luminescence
depending on the metal nuclearity and temperature.
an additional HE emission band begins to grow at ca.
720 nm. Further cooling leads to dual emission with LE
(NIR region) and HE (visible region). At 78 K, the LE
peak reaches 973 nm and the HE peak 752 nm. According
to DFT calculations for 2, the LE emission is tentatively
assigned to “{Cu6S6} cluster-centered” triplet excited state
and the HE emission to “{Cu6S6} cluster-to-ligand” charge-
transfer triplet excited state. In the literature, solid-state
(single) NIR emission is not unusual for transition-metal
complexes, including precedents for copper complexes like
[Cu6(mtc)6] (mtc– =di-n-propylmonothiocarbamate) (Sabin
et al. 1992), [Cu6(btt)6] (btt– =benzothiazolethiolate) (Yue
et al. 2009), [Cu6(Hmna)6] (H2mna=2-mercapto-nicotinic
acid) (Kundu et al. 2014), and [Cu4I4(SbR3)4] (R=isopropyl
and cyclohexyl) (Taylor et al. 2019, 2016). However, report
for dual-emission spanning to NIR, like 2, remains much
scarce (Xie et al. 2005).
At 293 K, complex 1 emits in greenish-blue with a broad
emission band centered at 502 nm (Fig. 7a). The emission
quantum yield was Φ = 0.16 at 293 K. The emission life-
time of τ=11 µs at 293 K suggests phosphorescence from
a triplet state. Intensity of this emission increased as the
temperature was decreased. Below 200 K, dual-emissive
character became apparent with the higher energy (HE)
peak at 493 nm and the lower energy (LE) peak at 542 nm.
Further decrease in temperature led to the decrease in the LE
intensity with concomitant growth of the HE band. At 78 K,
single emission, derived from the HE band, is observed with
a complete lack of the LE band. In the dual-emissive tem-
perature region, the energy of LE and HE bands remains
less afected but the intensity of both is highly temperature
dependent. On the basis of the emission lifetime and the
DFT analysis for 1 described above, we tentatively assign
the LE (λem =542 nm) and HE emissions (λem =493 nm) to
“{Cu4S4} cluster-to-ligand” charge-transfer and “{Cu4S4}
cluster-centered” excited states, respectively. We note that
the thermochromic dual-emissive character for iminothi-
olate tetracopper(I) clusters is reported herein for the frst
time, though it has been of extensive interest for cubane-
type [Cu4I4L4] clusters (Benito et al. 2015; De Angelis et al.
2006; Ford et al. 1999; Huitorel et al. 2018; Kitagawa et al.
2010; Nagaoka et al. 2018; Perruchas et al. 2010; Yang et al.
2016).
Conclusions
In this paper, we described the synthesis and successful
crystallization of two new Cu(I) complexes supported by
monoanionic iminothiolate ligands, N-methylbenzimida-
zolethiolate (Me-bimt–) and formulated [Cun(Me-bimt)n],
which comprises diferent metal nuclearity (n= 4 and 6).
The crystal structures for both clusters were unambiguously
determined by single-crystal X-ray diffraction analysis.
Interestingly, the color of the crystals was highly nuclearity-
dependent, colorless and deep red for 1 and 2, respectively,
which was rationalized by HOMO–LUMO gaps determined
by DFT calculations. Upon UV excitation at room tempera-
ture, complexes 1 and 2 gave solid-state photoluminescence
with lifetimes of microsecond order. Both complexes dis-
played temperature-dependent dual emission (78–298 K) in
the solid state. The thermochromic dual emission especially
found in the entire optical region from visible to NIR is unu-
sual for molecular compounds. The iminothiolate tetra- and
hexacopper(I) system reported herein is thus of considerable
interest in a new class of optical molecular thermometers
The thermochromic dual luminescence is also observed
for 2 but occurs in the NIR region (Fig. 7b). The emission
intensity of 2 is much weaker than that of 1. The temper-
ature-dependent spectral change for 2 appears more com-
plex than that in 1. At 293 K, complex 2 gives a single,
broad emission band centered at 876 nm. Similar to 1, the
emission lifetime of 2 is of microsecond order, τ = 1.7 µs
at 293 K, indicating phosphorescence from a triplet state.
The emission quantum yield is Φ = 0.01 at 293 K. Upon
cooling, the emission band shifts to a lower energy side
with maintaining the single emission feature. At 218 K,
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Chemical Papers
in the formation of heterometallic two- and three-dimensional
assemblies with brucite, pcu, and sql topologies. Cryst Growth
Des 14:4531–4544. https://doi.org/10.1021/cg500632d
Nagaoka S, Ozawa Y, Toriumi K, Abe M (2018) A Dual-emission
strategy for a wide-range phosphorescent color-tuning of a crys-
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zylpyridine). Chem Lett 47:1101–1104. https://doi.org/10.1246/
cl.180435
covering a wide range of wavelength. Many other applica-
tions may also be anticipated for this class of compounds
with a strong advantage for utilizing a naturally-abundant,
low-cost transition-metal, copper.
Acknowledgements This work was supported by JSPS KAKENHI
Grant no. JP16H06514 (Coordination Asymmetry) and the special
research grant from University of Hyogo (2016, 2017). Prof. Morifumi
Fujita (University of Hyogo) is acknowledged for useful discussion on
1H NMR spectroscopy.
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nescence of a copper iodide cluster. J Am Chem Soc 132:10967–
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Compliance with ethical standards
Conflict of interest The authors declare no competing fnancial inter-
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ties of hexanuclear copper(I) and silver(I) clusters. Inorg Chem
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Publisher’s Note Springer Nature remains neutral with regard to
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