Synthesis, optical spectroscopy, structural, and DFT studies on dimeric iodo-bridged Copper(I)complexes

Article abstract of DOI:10.1016/j.jorganchem.2019.04.017

Three new iodo-bridged copper(I)complexes of the type [CuI(PPh₃)L]₂, where L = Ar–≡–C₅H₄N, Ar = phenyl (C₁), biphenyl (C₂)and flourenyl (C₃)have been synthesized via coordination-driven self-assembly processes. Two of Cu(I)complexes, C₂ and C₃, have been characterized by single-crystal X-ray diffraction studies. The complexes have two molecules of the P-donor ligand and two molecules of the N-donor ligand in trans configurations, supporting the central Cu₂I₂ unit. Absorption properties of the complexes have been investigated. Extensive DFT calculation has been carried out to delineate the influence of aromatic spacers on the optical properties and the nature of excited states. The ease of synthesis of these Cu(I)dimers and the wide range of ethynylpyridine supporting ligands that can be incorporated highlights the potential for these materials to form polymers by linking through the ethylylpyridine ligands.

Full text of DOI:10.1016/j.jorganchem.2019.04.017

Accepted Manuscript
Synthesis, optical spectroscopy, structural, and DFT studies on dimeric iodo-bridged
Copper(I) complexes
Ashanul Haque, Rayya A. Al Balushi, Idris Juma Al-Busaidi, Rashid Ilmi, Nawal Al
Rasbi, Maharaja Jayapal, Muhammad S. Khan, Paul R. Raithby
PII:
S0022-328X(19)30074-9
DOI:
https://doi.org/10.1016/j.jorganchem.2019.04.017
JOM 20767
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 23 February 2019
Revised Date: 9 April 2019
Accepted Date: 18 April 2019
Please cite this article as: A. Haque, R.A. Al Balushi, I.J. Al-Busaidi, R. Ilmi, N. Al Rasbi, M. Jayapal,
M.S. Khan, P.R. Raithby, Synthesis, optical spectroscopy, structural, and DFT studies on dimeric iodo-
bridged Copper(I) complexes, Journal of Organometallic Chemistry (2019), doi: https://doi.org/10.1016/
j.jorganchem.2019.04.017.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis, Optical Spectroscopy, Structural, and DFT Studies on Dimeric Iodo-
bridged Copper(I) complexes
Ashanul Haque,a,b* Rayya A. Al Balushi,c Idris Juma Al-Busaidi,b, Rashid Ilmib, Nawal Al Rasbi,b Maharaja
Jayapal,b Muhammad. S. Khanb,* Paul R. Raithby d*
a Department of Chemistry, College of Science, University of Hail, Ha’il 81451, Kingdom of Saudi Arabia.
E-mail: a.haque@uoh.edu.sa
b Department of Chemistry, Sultan Qaboos University, P.O. Box 36, Al-Khod 123, Sultanate of Oman.
E-mail: msk@squ.edu.om
c
Department of Basic Sciences, College of Applied and Health Sciences, A’Sharqiyah University, P.O.
Box 42, Ibra 400, Sultanate of Oman.
d Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK.
E-mail: p.r.raithby@bath.ac.uk
Abstract:
Three new iodo-bridged copper(I) complexes of the type [CuI(PPh3)L]2, where L = Ar–≡–C5H4N, Ar =
phenyl (C1), biphenyl (C2) and flourenyl (C3) have been synthesized via coordination-driven self-assembly
processes. Two of Cu(I) complexes, C2 and C3, have been characterized by single-crystal X-ray
diffraction studies. The complexes have two molecules of the P-donor ligand and two molecules of the N-
donor ligand in trans configurations, supporting the central Cu2I2 unit. Absorption properties of the
complexes have been investigated. Extensive DFT calculation has been carried out to delineate the
influence of aromatic spacers on the optical properties and the nature of excited states. The ease of
synthesis of these Cu(I) dimers and the wide range of ethynylpyridine supporting ligands that can be
incorporated highlights the potential for these materials to form polymers by linking through the
ethylylpyridine ligands.
Keywords: Copper(I) complexes; Ethynylpyridine; DFT studies; Self-assembly; X-ray crystal structures
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1. Introduction
Transition metal complexes have attracted significant attention due to their diverse photophysical
properties and applications.[1-5] The last four decades has witnessed a great upsurge in the design and
development of d10 ion-based complexes such as those of copper(I) bearing homo- and heteroleptic
ligands.[6, 7] Among luminescent transition metal complexes, copper(I)-based systems offer easy
synthetic routes, a diverse range of structural arrangements, bright luminescence encompassing a broad
range of the visible and UV spectrum, high quantum yields, etc. It has been demonstrated that neutral
copper(I) complexes bearing N- and P-coordinating ligands surrounding a halogenido-bridged Cu2X2 (X =
halogen) core are good candidates for optoelectronic applications.[8] By simple variation of the
coordination sphere (i.e. the coordinating and bridging ligands) around the metal, both absorption and
emission can be manipulated. For instance, it has been demonstrated that metal to ligand charge transfer
(MLCT) or halide to ligand charge transfer (XLCT) emission bands can be significantly tuned by varying
the π* level of ligands.[8] Because of these features, several new copper(I) complexes have been
synthesized and tested as components for photovoltaics (PVs), organic-light emitting diodes (OLEDs),
sensors, etc.[7, 9] The low toxicity and the pseudo-tetrahedral geometry of the complexes has also
prompted researchers to assess their biological activity.[3]
Within the class of Cu2X2 complexes, synthetic manipulation can readily be achieved by altering
the nature of the supporting ligands, usually with donor N- or P-bridgehead atoms.[10-12] The pyridine
nitrogen donor atom has a strong tendency to interact with a variety of metals and, as a result, a number
of complexes bearing simple to functionalized pyridine (or higher pyridines) have been reported.[3] Not
only does the electron-deficient pyridine endow structural rigidity, but a judicious choice of
functionalization and macrocyclization through pyridine core can also impart helicity/chirality to the
resulting complexes.[13] Bearing these features in mind, we have investigated structural and
photophysical behavior of a number of acetylide-functionalized pyridinyl copper(I) complexes. Both
dimeric and tetrameric copper(I) complexes bearing ethynylpyridinyl-based coordinating ligands have
been investigated.[5, 14, 15] For example, we demonstrated that copper(I) complexes bearing pyridine
ligands connected to 4-ethynyl ferrocene show intriguing electrochemical properties. In such complexes,
the ferrocene units act as an electron reservoir/sink and was detrimental to the emission properties.[15]
Similarly, we recently demonstrated that dye sensitized solar cells (DSSC) based on Cu2X2 bridged
complexes with heteroarylethynyl attached to 4-position of pyridine (Chart 1) are highly emissive and
show up to 2% light to energy conversion efficiency.[5] The performance of the dye was found to depend
significantly on two factors: (i) nature of the aryl group attached to pyridine via π-linker; and (ii)
presence/absence of an anchoring group associated with the phosphine ligands.
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Chart 1:
Recently reported acetylide-functionalized pyridine-based Cu(I) complexes for PV
applications.
To examine the effect of replacing heterocyclic spacers by carbocyclic spacers (such as phenyl, biphenyl
and fluorene), we now report the synthesis, the structures and photophysical and DFT studies on three
new iodo-bridged copper(I) complexes supported by ethylylpyridine-based ligands. The complexes were
prepared via self-assembly in the solution phase under mild conditions.
2.
Experimental
2.1 General
All chemicals and reagents, except where stated otherwise, were obtained commercially and used
without further purification. Unless otherwise stated, syntheses were performed under a dry argon
atmosphere. Solvents were pre‐dried and distilled before use by standard procedures.[16] NMR spectra
1
were recorded on a Bruker AM‐400 spectrometer in CDCl3. H-NMR spectra were referenced to solvent
resonances and 31P{1H} NMR spectra were referenced to external phosphoric acid. IR spectra were
recorded directly on the sample as attenuated total reflectance (ATR) on Diamond using Cary 630 FT-IR
spectrometer. UV−vis spectra were recorded with an Agilent Cary 5000 UV−visible spectrophotometer
using a quartz cuvette with a 1 cm path length. Column chromatography was performed on either
Kieselgel 60 (230 – 400 mesh) or celite.
2.2 General protocol for the synthesis
2.2.1 Synthesis of ligands (L1-L3) and complexes (C1-C3)
The intermediates and final products were obtained via established cross-coupling reactions.[5, 15, 17]
Briefly, 4-trimethylsililyethynylpyridine was obtained by reacting an equimolar quantity of 4-iodopyridine
(2.0 mmol) with trimethylsilylethyne (TMSE, 2.0 mmol) in THF/iPrNH2 using Pd(II)/Cu(I) catalysts.
(Scheme 1) The crude product obtained was purified by silica column chromatography using
hexane/dichloromethane eluant systems. This was followed by a protodesilylation reaction under basic
condition (aq. KOH in MeOH/THF) and the 4-ethynylpyridine was separated by silica column
chromatography using pure dichloromethane as eluant. In the next phase of the reaction, respective aryl
halides (2.0 mmol) and 4-ethynylpyridine (2.0 mmol) were coupled via a Sonogashira cross-coupling
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reaction. Final products were purified using an alumina column and obtained as tan to light brown solid in
quantitative yields.
Phenylethynylpyridine (L1): IR (νmax) cm-1: 2199 (-C≡C-), 1598 (C-N), 1H‐NMR (700 MHz, CDCl3): δ(ppm)
8.45 (d, 2H, J = 6.4, H-py), 7.98 (d, 2H, J = 5.8, H-py), 7.35 (d, 2H, J = 6.2, H-Ph), 6.70-6.68 (m, 3H, H-
Ph). ESI-MS: m/z 179.9 (M+). C13H9N Anal. Calc.: C, 87.12; H 5.06; N, 7.82%. Found: C, 86.97; H, 4.94;
N, 7.77%.
1
Biphenylethynylpyridine (L2): IR (νmax) cm-1: 2208 (-C≡C-), 1605 (C-N), H‐NMR (700 MHz, CDCl3):
δ(ppm) 8.28 (d, 2H, J = 5.95 Hz, H-py), 7.68 (d, 2H, J = 4.55 Hz , H-py), 7.62-7.60 (m, 2H , H-biph), 7.46
(dd, 4H, J = 5.53, 13.51 Hz, H-biph), 7.38-7.35 (m, 3H,H-biph). ESI-MS: m/z 256.1 (M+). C19H13N Anal.
Calc.: C, 89.38; H, 5.13; N, 5.49 %. Found: C, 8.98; H, 4.83; N, 5.02%.
Fluorenylethynylpyridine (L3): IR (νmax) cm-1: 2205 (-C≡C-), 1608 (C-N), 1H‐NMR (700 MHz, CDCl3):
δ(ppm) 8.61 (d, 2H, J = 5.74 Hz, H-py), 8.27 (d , 2H, J = 5.95 Hz, H-py), 7.81-7.79 (m, 2H, H-fluorene),
7.78 (s, 1H, H-fluorene), 7.73 (m, 2H, H-fluorene), 7.59-7.57 (m, 2H, H-fluorene), 3.93 (s, 2H). ESI-MS:
m/z 268.1 (M+). C20H13N Anal. Calc.: C, 89.86; H, 4.90; N, 5.24%. Found: C, 88.26; H, 4.49; N, 4.94%.
Scheme 1: Synthesis of 4-arylethynylpyridine ligands (L1-L3).
2.2.2 Synthesis of Cu(I) complexes (C1-C3)
Cu(I) complexes were synthesized following the protocol reported in literature.[5, 15, 17] Complexes C1-
C3 were obtained by stirring a mixture of the appropriate arylethynylpyridine ligands (1.0 mmol), CuI (1.0
mmol) and triphenylphosphine (1.0 mmol) in an equimolar mixture in degassed dichloromethane.
(Scheme 2) After stirring for 24 h at room temperature, the mixture was filtered, and the filtrate was
concentrated under reduced pressure to produce neutral dimeric complexes C1-C3. All complexes are
stable to light and air and were fully characterized. Crystals suitable for X-ray analysis were obtained by
slow diffusion of dichloromethane (DCM) in hexane.
Complex [L1(CuI)2(PPh3)2] (C1): IR: 2206 cm‐1 ν(C≡C), 1488 cm‐1 ν(C=N). H‐NMR (700 MHz, CDCl3):
1
δ(ppm) 8.70 (d, J = 5.98 Hz, 4H, H-py), 7.91 (d, J = 6.3 Hz, 8 H, H-py and PPh3), 7.71–7.42 (m, 36 H, H-
Ph and PPh3). 31PNMR (121.53 MHz, CDCl3): δ = 29.18 (s, PPh3) ppm. ESI‐MS: m/z 1261.9 (M+).
C62H48Cu2I2N2P2 Anal. Calc.: C, 58.92; H, 3.83; N, 2.22%. Found: C, 59.55; H, 4.01; N, 2.17%
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1
Complex [L2(CuI)2(PPh3)2] (C2): IR: 2210 cm‐1 ν(C≡C), 1489 cm‐1 ν(C=N). H‐NMR (700 MHz, CDCl3):
δ(ppm) 8.65 (d, J = 4.48 Hz, 4 H, H‐py), 7.62 (d, J = 10.5 Hz, 12 H, H-py and PPh3), 7.55–7.34 (m, 40 H,
H-biph and PPh3). 31PNMR (121.53 MHz, CDCl3): δ = 29.22 (s, PPh3) ppm. ESI‐MS: m/z 1413.7 (M+).
C74H56Cu2I2N2P2: Anal. Calc.: C, 62.76; H, 3.99; N, 1.98%. Found: C, 64.88; H, 4.11; N, 2.01%.
Complex [L3(CuI)2(PPh3)2] (C3): IR: 2208 cm‐1 ν(C≡C), 1493 cm‐1 ν(C=N). H‐NMR (700 MHz, CDCl3):
1
δ(ppm) 8.64 (m, 4 H, H‐py), 7.81–7.79 (m, 4 H, H‐py), 7.74-7.57 (m, 30 H, H-flourene and PPh3), 7.52-
7.34 (m, 14 H, H-flourene and PPh3), 3.94 (s, 2H). 31PNMR (121.53 MHz, CDCl3): δ = 29.31 (s, PPh3)
ppm. ESI‐MS: m/z 1440.2 (M+). C76H56Cu2I2N2P2 Anal. Calc.: C, 63.38; H, 3.92; N, 1.95%. Found: C,
64.88; H, 4.19; N, 2.01 %.
Scheme 2: Synthesis of Cu(I) complexes (C1 C3).
2.3
X-ray Crystal Structure Determination
Single‐crystal X‐ray diffraction experiments were performed at 150(2) K on an Oxford Diffraction Gemini A
Ultra CCD diffractometer monochromatic Mo‐Kα radiation (λ = 0.71073 Å). The sample temperature was
controlled using an Oxford Diffraction Cryojet apparatus; CrysAlis Pro was used for the collection of
frames of data, indexing reflections and determining lattice parameters.[18] Structures were solved by
direct methods using SIR-92[19] and refined by full‐matric least‐squares, on F2, using SHELXL2014[20]
within the OLEX2 suite of programs.[21] A multi‐scan absorption correction was applied in all cases. The
hydrogen atoms were generated geometrically and refined with common isotropic thermal parameters.
Crystallographic data for the complexes is presented in Table 1. In the case of C3, the fluorene rings
showed significant disorder that could not be modelled effectively. As a last resort the unit was assigned
isotropic displacement parameters and the carbon atoms were assigned full occupancies. While the
refinement was stable with this model several significant residual electron density peaks in the vicinity of
the ligand remained. The cif files for the two structures have been deposited with the CCDC: CCDC
1898577-1898578
2.4
Computational studies
2.5 Computational details
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All DFT calculations were performed using the Gaussian 09[22] package with the Lee–Yang–Parr
correlation function (B3LYP).[23] For copper and iodide, the Hay and Wadt basis-set LANL2DZ
pseudopotential.[24, 25] was used. The 6-311+G(d).[26] basis-set was employed for N and P, while the
6-31G(d,p)[27] basis-set was operated for the atomic orbital of C, H to calculate all the optimized
structures.TD-DFT calculations were performed using the same functional and the polarizable continuum
model, CPCM with dichloromethane (ε = 8.93) as the surrounding solvent considering bulk solvent
effects.[28] TD-DFT-calculated electronic spectra were depicted by using GaussSum,[29] All the
molecular orbitals and spin-density diagrams were plotted using the Chemcraft Visualization program.[30]
3. Result & Discussion
3.1 Synthesis and Spectroscopic Characterization
The ligands L1 L3 and their corresponding complexes C1 C3 have been synthesized by adapting
previously reported procedure.[5] Briefly, arylhalides (4-Iodophenyl, 2-bromobiphenyl or 2-iodofluorenyl)
were cross-coupled with TMSA using Sonogashira coupling reaction followed by protodesilylation giving
the terminal alkynes (Scheme 1). The deprotected ligands obtained were then coupled with 4-
iodopyridine to produce ligands L1 L3 as yellow-brown powders in 60-78% yield. Analytical data (IR, NMR
and MS) confirmed the formulation of the ligands. For example, TMSA protection of 4-iodopyridine was
confirmed by the observed C≡C- stretching frequency (~ 2165 cm-1) while deprotection was confirmed
by the presence of the C≡H stretching frequency (~ 3223 cm-1) in addition to the C≡C band. Moreover,
the ν(C≡C) value of 4-ethynylpyridine was found to be lower than those of the 4-
(trimethylsilylethynyl)pyridine due to the fact that terminal alkynes (HC≡C─R) have lower νC≡C stretching
frequencies than their protected counterparts RC≡CR.[31] The complexes C1 C3 were obtained by swift
solution phase self-assembled coordination. The reaction of the ligand (L1 L3), with copper iodide and
triphenyl phosphine (1:1:1 ratio), in dry dichloromethane, under an argon atmosphere produced the
complexes C1 C3. In addition to the standard analytical techniques, single crystal X‐ray diffraction studies
provided the full three‐dimensional molecular and crystal structures of the three complexes.
3.2 Structural characterization
The crystal structures of the complexes: C2 and C3 were determined and are shown in Figures 1 and 2.
Crystal data is summarized in Table 1 and bond parameters are listed in Tables ST1-ST4
(supplementary information). Single crystals of the complexes were obtained by slow diffusion of hexane
into a dichloromethane solution of each complex.
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Figure 1.
The crystal structure of the dimeric complex C2. Hydrogen atoms are omitted for clarity.
The crystal structures of the dimeric complexes C2 and C3 have a similar planar, parallelepiped Cu2I2 core
and a trans arrangement of the two phosphine and the two L ligands. In each case the center of the Cu2I2
core sits on a crystallographic center of symmetry, so that the crystallographic asymmetric unit contains
half of a dimeric molecule. Each Cu(I) center is in a tetrahedral coordination environment consisting of
two bridging iodo ligands, one PPh3 and the bridgehead nitrogen atom of L2 and L3. The unique bond
distance for Cu-I, Cu-P and Cu-N are 2.6487(4), 2.2412(6) and 2.063(2) Å, respectively, in C2 and are
2.651(8), 2.235(2) and 2.059(6) Å, respectively, in C3. These bond distances are similar to the equivalent
bond lengths in a family of previously reported [(Py)(Ph3P)CuX]2 adducts.[32] The angles at the copper
center in the Cu2I2 core are obtuse, e.g. 111.45(1)° and 108.13(3)° in C2 and C3, respectively. While the
angle at the iodine center is acute, e.g. 68.54(1)° and 71.83(3)° in C2 and C3, respectively. The Cu···Cu
separations in the dimers: C2 (3.0196(5) Å) and C3 (3.1394(5) Å), are too long for a formal Cu-Cu bonding
interaction; however, it remains similar to reported analogues.
Figure 2.
The crystal structure of the dimeric complex C3. Hydrogen atoms are omitted for clarity.
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Table 1. Crystallographic data for C2-C3
C2
C3
Empirical formula
C74 H56 Cu2 I2 N2 P2
1416.02
150(2)
C76 H56 Cu2 I2 N2 P2
1440.04
150(2)
Formula weight
T(K)
λ ( Å)
0.71073
Monoclinic
P21/n
0.71073
Monoclinic
P21/n
Crystal system
Space group
a
b
c
9.43370(10)
22.5428(3)
14.2331(2)
90°.
9.3125(2)
22.1452(4)
15.2590(3)
90
α
98.562(1)
90
101.061(2)
90
β
γ
V (Å3)
2993.11(7)
2
3088.36(11)
2
Z
ρcalc. Mg m-3
1.571
1.549
µ (Mo-Kα) (mm-1)
1.842
1.787
F(000)
1416
1440
0.60 x 0.40 x 0.40
0.2 x 0.2 x 0.04
Crystal size (mm)
θ range (deg)
2.834 to 31.192
79295
2.971 to 29.689
40941
Reflections collected
Independent reflections [R(int)]
Completeness to theta = 25.242°
Max. and min. transmission
Data / restraints / parameters
Goodness-of-fit on F2
Final R1, wR2 indices [I>2σ(I)]
R indices (all data)
Largest diff. peak, hole (e.Å-3)
9128 [0.0375]
99.90%
7906 [0.0385]
99.80%
1.00 and 0.730
9128 / 0 / 370
1.133
1.000 and 0.9209
7906 / 0 / 302
1.034
0.0305, 0.0584
0.0454, 0.0647
1.101, -0.516
0.0719, 0.1870
0.0927, 0.2013
3.528, -1.267
3.3 Optical Spectroscopy
The electronic spectra of complexes C1 C3 were measured in dichloromethane at room temperature
(Figure 3a). The onset of absorption for all ethylpyridinyl-based ligands and their corresponding Cu
complexes were red-shifted compared to ligand precursor 4-(trimethylsilylethynyl)pyridine (λmax = 286 nm).
The complex incorporating the fluorenylethynylpyridine ligand (C3) showed red-shifted band compared to
phenyl and biphenylethnylpyridine Cu(I) complexes (C1 and C2). Complex C1 showed a strong absorption
band at 300 nm (λmax) whereas complexes C2 and C3 showed absorptions peaking at 310 and 334 nm,
respectively. This can be ascribed to the increasing π-conjugation length leading to a lower energy for
the complex and absorption at longer wavelengths. The absorption peaks in the UV spectrum (λmax < 400
nm) can be attributed to the π→π* transition associated with the arylethynylpyridine group. Figure 3b
shows the effect of attaching different aromatic spacers to 4-ethynylpyridine ligands on the absorption
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maxima. As expected, the λmax values move to the red (> 10 nm) upon the incorporation of heterocyclic
spacers.[5] Comparison of these data with our previous results [5] clearly indicate that the presence of a
heavy atom such as sulfur play important role in shifting the λmax values in dimeric iodo-bridged Cu(I)
complexes. As mentioned before, Cu(I) complexes are well known for their emissive properties in both the
solid and solution states.[8] In the past, we have demonstrated that the emission properties of
ethynylpyridine ligands-based Cu(I) complexes in DCM solution is mainly governed by the electron
withdrawing/donating nature of the spacer group. Moreover, there is only little impact of the alkynyl unit
(especially for mono-ynes) on the emission wavelength.[5] Therefore, a trend similar to absorption is
expected on the emission properties. We are currently investigating the solid-state as well as solution
luminescence, electrochemical, thermal and photovoltaic properties of carbocyclic and heterocyclic
spacer incorporated ethynylpyridine ligands-based Cu(I) complexes. The results will be communicated
separately.
Figure 3.
(a) Normalized UV-vis spectra of complexes C1-C3 in dichloromethane, and (b) the effect
of different carbocyclic and heterocyclic aromatic spacers (Ar = Phenyl, biphenyl,
fluorene, thiophene, bithiophene and terthiophene) attached to 4-ethynylpyridine ligands
in dimeric complexes of type [L(CuI)2(PPh3)2] (where L = Ar–≡–C6H4N) on the absorption
maxima. Data for the heterocyclic spacers taken from the ref.[5]
4. DFT Calculations
The structures of complexes C1-C3, bearing three different carbocylic groups (with varying conjugation
lengths) attached to the ethynylpyridine ligands were confirmed by DFT calculations employing the
B3LYP level of theory (see computational details in the experimental section). Geometry-optimization of
complexes C2 and C3 was performed using the X-ray crystallographic coordinates and the complex C1
model was optimized using the same hybrid level of theory. The minima were verified via computed
vibration frequencies. The DFT calculated bond-lengths and bond-angles of the optimized structures are
(Figure 4 and Table ST5, supplementary file) in good agreement with those obtained experimentally
(Table ST1-4, supplementary file). The time-dependent DFT (TD-DFT) calculations were executed on
these optimized geometries to analyze the photophysical properties (Figure 5). Other frontier molecular
orbitals involved in electronic transitions in complexes C1 C3, as obtained by TD-DFT calculation are
shown in Figure SF1-SF3 (supplementary information).
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Figure 4.
DFT optimized structures of (a) C1, (b) C2, and (c) C3.
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Figure 5.
TD-DFT-Calculated electronic spectra for the Cu(I) complexes (a) C1, (b) C2, and (c) C3
using the B3LYP level of theory. Each plot shows the simulated absorption profile
(green), with the bars marked by blue lines representing oscillator strength. Isosurface
plots of the highest-occupied and lowest-unoccupied molecular orbitals (HOMOs/LUMOs)
for each system are shown to the right of the simulated spectra.
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To obtain deeper insight of the electronic transitions for all three complexes, time-dependent (TD-DFT)
calculations have been performed. Here we semi-quantitatively explain the effect of extended conjugation
embedded within alkynyl group attached to the pyridyl ligands. The calculated and experimental UV-vis
spectra are depicted in Figure 6. To analyze the spectroscopic activity of these complexes, we have
mainly considered the transition in each case with maximum oscillator strengths, f, among all the
transitions with excitation wavelengths in the 300 400 nm region. From TD-DFT assignments it was
revealed that all transitions in this region are largely due to charge-transfer (CT) mainly from the iodide
ligands towards the pyridyl rings within the terminally conjugated alkynyl groups. Along with this, there is
a substantial contribution of MLCT involving the metal → substituted pyridyl rings. Tables 2 and ST6-ST7
(supplementary information) lists all excitation energies, wavelengths, oscillator strengths, and an
assignment of each state in terms of contributions from individual orbital transitions.
(a)
(b)
1
C
C1
C1
C2
C3
C1
1
C1
1.0
0.8
0.6
0.4
0.2
0.0
2
C2
2
C2
C2
3
C33
1.5
1.0
0.5
C
33
Bathochromic shift
ε
ε
ε
ε
300
400
500
600
700
300
400
500
600
700
λ
(nm)
λ
(nm)
Figure 6.
TD-DFT simulated (a) and absorption (b) electronic spectra of complexes C1 C3 in DCM
Table 2. TD-DFT calculated electronic transitions of C1 using B3LYP level of theory.
Excitation
energy (eV)
2.780
λ (nm)
445
f
Transition
Character
0.074 α/β-H[~58%L]→ α/β- Charge transfer (CT) mainly from iodide
L[~80%L](92%)
towards the pyridyl rings within the
terminally conjugated alkynyl groups. A
significant contribution of MLCT is also
observed involving metal → substituted
pyridyl rings.
3.756
330
0.310 α/β-H-3[~57%M]→
α/β-
Combination of MLCT involving metal →
substituted pyridyl rings with subsequent CT
from iodide towards pyridyl rings within the
terminally conjugated alkynyl groups
L+1[~89%L](86%)
α/β-H-5[~63%L]→
α/β-
L+1[~89%L](36%)
Charge transfer (CT) mainly from iodide
towards the pyridyl rings within the
terminally conjugated alkynyl groups. An
appreciable contribution of MLCT is also
observed
here
involving
metal
→
substituted pyridyl rings.
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3.8144
325
1.62
α/β-H-8[~50%L]→
α/β-
L+1[~89%L](28%)
Combination of MLCT involving metal →
substituted pyridyl rings with subsequent CT
from iodide towards the pyridyl rings within
three terminally conjugated alkynyl groups.
α/β-H-7[~53%M]→
α/β-L [~80%L](66%)
MLCT involving Metal →
substituted
pyridyl rings with terminally conjugated
alkynyl groups and CT in substituted pyridyl
rings with terminally conjugated alkynyl
groups.
4.013
308
0.295
α/β-H-10[~77%M]→
α/β-L[~80%L](34%)
MLCT involving metal →
substituted
pyridyl rings within the terminally conjugated
alkynyl groups.
α/β-H-8[~50%L]→
α
L+1[~890%L](46%)
Combination of MLCT involving metal →
substituted pyridyl rings with subsequent CT
from iodide towards the pyridyl rings within
the terminally conjugated alkynyl groups.
-
The calculated HOMO−LUMO gaps associated with low energy transitions (~ 440 450 nm) are also given
in the Table 3 though they are not clearly visible experimentally due to the broadening of absorption
bands. In complex C1, the brightest transition at nearly 300 nm (experimentally) could be assigned as
HOMO−5 to LUMO+1, HOMO−7 to LUMO, and HOMO−8 to LUMO+1 excitations, respectively, with
oscillator strengths, f = 1.62. We also found a considerably weaker transition between the HOMO and
LUMO orbitals at 445 nm (f = 0.074). A gradual red shift (Table 3) in the absorption edge (with maximum
f) of complex C2 (310 nm) and complex C3 (334 nm) has been observed experimentally, as we move
forward from complex C1 towards complex C3. This can be explained on the basis of their structural
configurations. Due to the incorporation of the more delocalized functionalities into the alkynyl group
attached to the pyridyl ligands in Cu(I) complexes, in going from C1 to C3 a noticeable bathochromic shift
is observed in the spectra. Here, the computational modeling also semi-qualitatively represents the key
inferences extracted from the optical spectroscopy. It summarized the importance of extended
delocalization of the ligands on the frontier orbitals of complex C1 and its analogues leading to red shift
across the metal centers and between the aromatic ligands.
Table 3. The calculated HOMO-LUMO gaps associated with low energy transitions, the experimental and
calculated λ values.
λ (nm)
Complex
∆(HOMO – LUMO)/ kcal/mol
Expt.
300
DFT calculated
C1
C2
C3
64.7838
74.0262
74.1203
324
353
375
310
334
5.
Conclusion
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We have synthesized a range of [CuI(PPh3)L]2 dimeric complexes, with a Cu2I2 core, where L is a series
of ethynyl-functionalized pyridine ligands (Ar-C≡C-C5H4N), to include the aryl groups phenyl (C1), biphenyl
(C2) and fluorenyl (C3) using established coordination-driven self-assembly synthetic strategies.
Crystallographic studies confirm the expected trans arrangement of the L ligands across the planar
parallelepiped Cu2I2 core. The UV/visible absorption spectra of the three ethylpyridinyl-based ligands and
their corresponding Cu complexes were red-shifted compared to ligand precursor 4-(trimethylsilyl
ethynyl)pyridine. In the complexes the magnitude of the red shift increases in the order C1 < C2 < C3
which can be ascribed to the increasing π-conjugation length of the arylethynylpyridine ligand. DFT
calculations confirm that the bands in the absorption spectra are a mixture of charge transfer from the
iodine ligands to the functionalized pyridine groups and MLCT from the copper centers also to the
functionalized pyridine ligands.
This study, coupled with our previous work, [5] indicates that these dinuclear copper (I) systems are
amenable to coordination with a wide range of functionalized Ar-C≡C-C5H4N ligands, and opens up the
possibility of synthesizing one-dimensional polymeric systems via further manipulation of the Ar groups to
form functionalized linkers through covalent bond formation.
Acknowledgements
AH acknowledges the Department of Chemistry, University of Hail, Kingdom of Saudi Arabia for providing
infrastructure support during the preparation of this manuscript. MSK thanks His Majesty’s Trust Fund for
Strategic Research (Grant No. SR/SQU/SCI/CHEM/16/02) for funding. PRR is grateful to the Engineering
and Physical Sciences Research Council (UK) for continued support (EP/K004956/1).
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Research Highlights
1. Three new ethynylpyridinyl-based Cu(I) complexes were synthesized and characterized.
2. Structures of two complexes were confirmed by single crystal X-ray diffraction.
3. Photo-physical and computational studies were performed.