Heteroleptic Cu(I) bis-diimine complexes of 6,6′-dimesityl-2, 2′-bipyridine: A structural, theoretical and spectroscopic study

Article abstract of DOI:10.1021/ic302393p

A series of heteroleptic Cu(I) complexes containing 6,6′-dimesityl-2, 2′-bipyridine and phenanthroline-, bipyridine-, and biquinoline-based ligands is studied. The HETPHEN strategy is utilized to synthesize the heteroleptic complexes, which are stable in solution. The X-ray crystal structures of the complexes are presented; the solid-state four-coordinate Cu(I) geometries are quantified by using the τ₄ parameter. A feature of the crystal structures is the intramolecular π-stacking between the mesityl ring(s) and the diimine ligand; the phen-based complexes exhibit stacking between the phen ligand and one of the mesityl rings, creating a Pac-Man motif. On the other hand, the bpy-based complexes show different types of packing interaction, with both mesityl rings clamping down on the bpy based ligand to give π-stacking. Cyclic voltammetry is used to examine the redox chemistry of the complexes. The most positive potentials for the oxidation process are observed for the complexes with bulky substituents ortho to the coordination nitrogens atoms, i.e., 2,9-dimethyl-1,10-phenanthroline and 6,6′-dibromo-2,2′-bipyridine. The Cu(I) MLCT transitions of the complexes are investigated by resonance Raman spectroscopy in concert with TD-DFT calculations. The resonance Raman spectra of complexes containing substituted biquinolines are straightforward, in that vibrational bands of the biquinoline-based ligand are selectively enhanced over bpy(Mes)₂ bands. This is consistent with the purple color of the complexes, due to the lower energy of the biquinoline-based LUMO compared to the bpy(Mes)₂ LUMO. All the phen- and bpy-based complexes show enhancement of bpy(Mes)₂ bands.

Full text of DOI:10.1021/ic302393p

Article
pubs.acs.org/IC
Heteroleptic Cu(I) Bis-diimine Complexes of 6,6′-Dimesityl-2,2′-
bipyridine: A Structural, Theoretical and Spectroscopic Study
Michael G. Fraser, Holly van der Salm, Scott A. Cameron, Allan G. Blackman, and Keith C. Gordon*
MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Otago, Dunedin, New
Zealand
S
* Supporting Information
ABSTRACT: A series of heteroleptic Cu(I) complexes contain-
ing 6,6′-dimesityl-2,2′-bipyridine and phenanthroline-, bipyri-
dine-, and biquinoline-based ligands is studied. The HETPHEN
strategy is utilized to synthesize the heteroleptic complexes, which
are stable in solution. The X-ray crystal structures of the
complexes are presented; the solid-state four-coordinate Cu(I)
geometries are quantified by using the τ4 parameter. A feature of
the crystal structures is the intramolecular π-stacking between the
mesityl ring(s) and the diimine ligand; the phen-based complexes
exhibit stacking between the phen ligand and one of the mesityl
rings, creating a “Pac-Man” motif. On the other hand, the bpy-
based complexes show different types of packing interaction, with
both mesityl rings “clamping down” on the bpy based ligand to
give π-stacking. Cyclic voltammetry is used to examine the redox chemistry of the complexes. The most positive potentials for the
oxidation process are observed for the complexes with bulky substituents ortho to the coordination nitrogens atoms, i.e., 2,9-
dimethyl-1,10-phenanthroline and 6,6′-dibromo-2,2′-bipyridine. The Cu(I) MLCT transitions of the complexes are investigated
by resonance Raman spectroscopy in concert with TD-DFT calculations. The resonance Raman spectra of complexes containing
substituted biquinolines are straightforward, in that vibrational bands of the biquinoline-based ligand are selectively enhanced
over bpy(Mes)2 bands. This is consistent with the purple color of the complexes, due to the lower energy of the biquinoline-
based LUMO compared to the bpy(Mes)2 LUMO. All the phen- and bpy-based complexes show enhancement of bpy(Mes)2
bands.
phenanthroline, generally phenyl rings containing methyl or
methoxy groups at the 2- and 6-positions. It was found that the
steric bulk of the phenyl substituents prevent the formation of
the bis-homoleptic Cu(I) complex.
INTRODUCTION
■
Cu(I) complexes containing polypyridyl-based ligands are
interesting because of their photophysical properties and their
use in supramolecular assemblies.1 Certain Cu(I) polypyridyl
complexes have excited-state characteristics similar to those of
ruthenium complexes such as [Ru(2,2′-bipyridine)3]2+, in that
both have a long-lived charge-separated triplet excited state,
originating from an MLCT transition in the visible region of
the spectrum.2 These properties make Cu(I) polypyridyl
complexes potentially useful in applications such as dye-
sensitized solar cells (DSSCs).3−7 Ru(II) complexes containing
carboxylic acid-substituted polypyridyl ligands are currently the
most efficient TiO2 sensitizers in DSSCs.8,9 However, Cu(I)
polypyridyl complexes have recently been used as sensitizers in
DSSCs.6,10−12
One of the main drawbacks of Cu(I) complexes is their well-
known lability.13 Ligand exchange occurs rapidly in solution,
often preventing the isolation of stable heteroleptic [Cu(I)-
(diimine A) (diimine B)]+ complexes [where diimine A and
diimine B are two different ligands]. This, however, was
overcome by Schmittel and colleagues by using the so-called
“HETPHEN’” design strategy (Figure 1).14,15 This concept
requires bulky substituents at the 2- and 9-positions of
Subsequent addition of a second, less sterically bulky
phenanthroline ligand results in the desired heteroleptic
complexes. Schmittel and co-workers expanded this chemistry
to include heteroleptic phenanthroline complexes of the
analogous d10 metals, Ag(I) and Zn(II).16 It was also shown
that larger groups such as anthracene and naphthalene moieties
at the 2- and 9-positions also allow the formation of
heteroleptic Cu(I) species.17 This method of using sterically
bulky phenanthroline ligands has been extensively exploited in
the realm of supramolecular coordination chemistry.18−24 The
HETPHEN strategy can also potentially be applied to the
synthesis of heteroleptic Cu(I) complexes in which one diimine
ligand is an electron acceptor and the other is an electron donor
ligand, capable of sustaining photoinduced electron transfer.25
Such complexes would possess desirable photophysical proper-
ties for potential use in devices.
Received: November 1, 2012
© XXXX American Chemical Society
A
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Figure 1. HETPHEN strategy using bpy(Mes)2.
Figure 2. Ligands used to form heteroleptic [Cu(bpy(Mes)2)(NN)]+ complexes.
A recent report by Odobel and co-workers26 described a
series of heteroleptic Cu(I) complexes based on imidazole-
fused 2,9-mesityl-substituted phenanthroline ligands. The
heteroleptic complexes included, among others, a 3,6-dibutyl-
substituted dipyridophenazine ligand and 2,9-dimethyl-1,10-
phenanthroline (dmp) as the second diimine ligand. The
electronic properties of the complexes were characterized by
TD-DFT calculations. All but one of the heteroleptic
complexes displayed emission with nanosecond excited state
lifetimes in dichloromethane solution. The complex that did
not display emission was that with 1,10-phenanthroline (phen)
as the second diimine ligand. The absence of sterically bulky
groups at the 2- and 9-position of the phen ligand increases the
likelihood of forming a exciplex between the solvent and/or
counterion and the Cu(II) ion of the excited state.27 The
excited state properties resulting from MLCT excitation are
determined by the electronic and structural changes on going
from the ground-state Cu(I) to the excited state Cu(II). This
change in oxidation state results in the flattening of the
tetrahedral Cu(I) to a more “square-planar-like” geometry. In
the newly formed Cu(II) species a fifth coordination site is
made available, which can be bound to Lewis basic species such
as counterions and solvent molecules.2 McMillin and co-
workers27−31 studied the effect of varying solvent, counterion,
and the addition of external molecules on the emission intensity
and excited-state lifetime of [Cu(dmp)2]+ and other emissive
[Cu(NN)2]+ complexes. These investigations showed that any
factor limiting the formation of an exciplex increases the excited
state lifetime of the complex. Many groups have employed
ultrafast spectroscopic techniques to understand the structural
changes on going from the ground to the excited state of Cu(I)
polypyridyl complexes, namely, time-resolved absorption and
emission spectroscopy.32−35 The generally accepted mechanism
was proposed by Tahara and co-workers36,37 using femto-
second time-resolved emission spectroscopy. The experiments
showed that the dominant excited state pathway involves the
structural change (flattening distortion) occurring in the lowest
energy singlet state (S1), before intersystem crossing to the
lowest energy triplet state (T1).
This paper presents heteroleptic Cu(I) complexes of the
bipyridine-based ligand 6,6′-dimesityl-2,2′-bipyridine (bpy-
(Mes)2) with a series of diimine-based ligands as the second
nonmesityl-based ligand. This paper builds on our recent report
on heteroleptic Cu(I) complexes of sulfur-substituted dipyr-
idophenazine-based ligands.38 The bpy(Mes)2 ligand was first
synthesized by Schmittel et al.39 and meets the requirements
for the formation of heteroleptic complexes in the same way as
the more rigid 2,9-dimesitylphenanthroline ligands. The ligands
chosen to coordinate to the [Cu(bpy(Mes)2)]+ unit are shown
in Figure 2. These can be classified into three groups:
bipyridine-based ligands [2,2-bipyridine (bpy), 6,6′-dimethyl-
2,2′-bipyridine (Me2bpy), and 6,6′-dibromo-2,2′-bipyridine
(Br2bpy)], phenanthroline-based ligands [1,10-phenanthroline
(phen) and 2,9-dimethyl-1,10-phenanthroline (dmp)], and
biquinoline-based ligands [2,2′-biquinoline (biq), 4,4′-diphen-
yl-3,3′-dimethylene-2,2′-biquinoline (Phdbq), 4,4′-dimethyl-
3,3′-dimethylene-2,2′-biquinoline (Medbq), and 5,8-diphenyl-
dibenzo-1,10-phenanthroline (Phdbp)].
The ligands offer a range of structural and electronic features.
In this paper, we investigate the properties of these heteroleptic
Cu(I) complexes using a variety of techniques. The solid-state
structures of the compounds are investigated using X-ray
crystallography. The electrochemistry of the complexes are
investigated, and their electronic transitions are investigated via
TD-DFT calculations and resonance Raman spectroscopy.
B
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[Cu(bpy(Mes)2)(Medbq)]BF4·CH3OH. Recrystallized from methanol:
70 mg, yield = 81%; 1H NMR (400 MHz, CDCl3) δ 8.85 (2H, d, J = 8
Hz), 8.13 (2H, d, J = 8 Hz), 8.08 (2H, d, J = 8.4 Hz), 7.62−7.58 (4H,
m), 7.49 (2H, t, J = 8.4 Hz), 7.41 (2H, d, J = 7.6 Hz), 5.71 (4H, s),
3.20 (4H, s), 2.77 (6H, s), 1.67 (6H, s), 1.26 (12H, s); MS m/z (ESI
POS) 765.30 {Cu(bpy(Mes)2)(Medbq)}+. Anal. Calcd for
C50H46N4CuBF40.5·CH3OH: C, 69.77; H, 5.57; N, 6.45. Found: C,
69.72; H, 5.41; N, 6.37.
[Cu(bpy(Mes)2)(Phdbq)]BF4·CH3OH. Recrystallized from methanol:
69 mg, yield = 69%; 1H NMR (400 MHz, CDCl3) δ 8.96 (2H, d, J = 8
Hz), 8.38 (2H, t, J = 7.6 Hz), 7.70 (2H, d, J = 8.4 Hz), 7.63−7.44
(14H, m), 7.33 (4H, m), 5.86 (4H, s), 2.79 (4H, s), 1.78 (6H, s), 1.35
(12, s); MS m/z (ESI POS) 889.338 {(Cu(bpy(Mes)2)(Phdbq)}+.
Anal. Calcd for C60H50N4CuBF4·CH3OH: C, 72.68; H, 5.39; N, 5.55.
Found: C, 72.86; H, 5.27; N, 5.66.
EXPERIMENTAL SECTION
■
General. The ligands 1,10-phenanthroline, 6,6′-dimethyl-2,2′-
bipyridine, 6,6′-dibromo-2,2′-bipyridine, biquinoline, 2,9-dimethyl-
1,10-phenanthroline, and 2,2′-bipyridine were purchased from Aldrich
and used as received. [Cu(MeCN)4]BF4 and bpy(Mes)2 were
synthesized according to literature methods.39,40 The ligands Phdbq,
Medbq, and Phdbp were available from previous studies.41 The
syntheses of Cu(I) complexes were carried out in a mixture of dry
dichloromethane and acetonitrile, and crude products were purified by
either recrystallization from methanol or ether diffusion into a
concentrated dichloromethane solution of the complex. All syntheses
were carried out in a similar fashion, and that of [Cu(bpy(Mes)2)-
(phen)]BF4 is given in detail below as a typical example.
[Cu(bpy(Mes)2)(phen)]BF4. A 40 mg portion of bpy(Mes)2 (1.02 ×
10−4 mol) was dissolved in 20 mL of degassed CH2Cl2. To this
solution, [Cu(CH3CN)4]BF4 (32 mg, 1.02 × 10−4 mol) dissolved in a
minimum volume of CH3CN was then added at room temperature to
give a yellow solution. This was stirred for 5 min and then 1,10-
phenanthroline (18 mg, 1.02 × 10−4 mol) in a minimum volume of
CH2Cl2 was added, and the solution immediately turned dark red. This
was stirred at room temperature for 30 min, and the solvent was then
removed under reduced pressure to give a red oil. This was dissolved
in a minimum volume of CH2Cl2, and vapor diffusion of diethyl ether
into the solution over 24 h affords dark red crystals suitable for X-ray
[Cu(bpy(Mes)2)(Phdbp)]BF4. Recrystallized from methanol: 88 mg,
1
yield = 89%; H NMR (400 MHz, CDCl3) δ 9.02 (2H, d, J = 8 Hz),
8.40 (2H, t, J = 8 Hz), 7.98 (2H, d, J = 8.8 Hz), 7.80 (2H, d, J = 8.4
Hz), 7.74−7.59 (10H, m), 7.49−7.46 (6H, m), 7.36 (2H, s), 5.55 (4H,
s), 1.47 (6H, s), 1.29 (12H, s); MS m/z (ESI POS) 887.317
{Cu(bpy(Mes)2)(Phdbp)}+. Anal. Calcd for C60H48N4CuBF4: C,
73.88; H, 4.96; N, 5.75. Found: C, 73.52; H, 5.03; N, 5.69.
Physical Measurements. FT-Raman spectra were obtained using
a Bruker Equinox 55 interferometer coupled with a FRA-106 Raman
module and a D418T liquid-nitrogen-cooled germanium detector,
controlled by the Bruker OPUS v5.5 software package. A Nd:YAG
laser operating at 1064 nm and 50 mW of power was used. The
spectra were acquired with a resolution of 4 cm−1.
1
crystallography: 68 mg, yield = 92%; H NMR (400 MHz, CDCl3) δ
8.57 (2H, dd, J = 8.0, 0.8 Hz), 8.48 (2H, dd, J = 4.8, 1.6 Hz), 8.42 (2H,
dd, J = 8.0, 1.6 Hz), 8.25 (2H, t, J = 7.6 Hz), 7.86 (2H, s), 7.76 (2H,
dd, J = 8.0, 4.8 Hz), 7.47 (2H, dd, J = 7.6, 0.8 Hz), 5.77 (4H, s), 1.68
(12H, s), 1.41 (6H, s); MS m/z (ESI POS) 635.22 {Cu(bpy(Mes)2)-
(phen)}+. Anal. Calcd for C40H36N4CuBF4: C, 66.44; H, 5.02; N, 7.75.
Found: C, 66.09; H, 5.10; N, 7.83.
1H NMR spectra of CDCl3 solutions were recorded on a Varian 400
MHz spectrometer at room temperature. All spectra were referenced
to the residual CHCl3 peak at 7.26 ppm. Mass spectra were acquired
using a Micromass LCT instrument for electrospray measurement or a
Shimadzu QP8000 α with an electrospray ionization (ESI) probe.
Microanalyses were carried out at the Campbell Microanalysis
Laboratory at the University of Otago.
[Cu(bpy(Mes)2)(dmp)]BF4. Recrystallized from methanol: 70 mg,
1
yield = 92%; H NMR (400 MHz, CDCl3) δ 8.69 (2H, d, J = 8 Hz),
8.29 (2H, t, J = 8 Hz), 8.23 (2H, d, J = 8.4 Hz), 7.79 (2H, s), 7.51
(2H, d, J = 8.4 Hz), 7.45 (2H, d, J = 7.6 Hz), 5.99 (4H, s), 2.29 (6H,
s), 1.66 (6H, s), 1.55 (12H, s); MS m/z (ESI POS) 663.257
{Cu(bpy(Mes)(dmp)}+. Anal. Calcd for C42H40N4CuBF4: C, 67.15;
H, 5.37; N, 7.46. Found: C, 66.95; H, 5.40; N, 7.43.
Resonance Raman experiments were performed using a variety of
laser sources: for excitation at 350.7, 354.6, 406, and 413 nm, a
krypton ion laser (Innova I-302, Coherent Inc.) was used; for
excitation at 363.8, 457.9, 488, and 514.5 nm, an argon ion laser
(Innova Sabre, Coherent Inc.) was used; for 448.0 nm, a solid-state
diode laser (CrystaLaser) was used. Typically, laser intensity at the
sample was 25 mW with a beam diameter of approximately 300 μm.
The incident beam and the collection lens were arranged in a 135°
backscattering geometry to reduce Raman intensity loss by self-
absorption.42 An aperture matched lens was used to focus scattered
light through a narrow-band-pass filter (Ruggate), to remove the
Rayleigh scattering, and through a quartz wedge (Spex), to remove
polarization bias, and onto the 50 μm entrance slit of a spectrograph
(Acton Research SpectraPro 500i). The collected light was dispersed
in the horizontal plane by a 1200 grooves/mm ruled diffraction grating
(blaze wavelength 500 nm) and detected by a liquid nitrogen cooled,
back-illuminated Spec-10:100B CCD controlled by a ST-133
controller and WinSpec/32 (version 2.5.8.1) software (Roper
Scientific, Princeton Instruments).43−47 Wavenumber calibration was
performed using Raman bands from a 1:1 v/v toluene/acetonitrile
sample. Peak positions were reproducible to within 1−2 cm−1. Spectra
were obtained with a resolution of 5 cm−1.48,49 Freshly prepared
samples were held in a spinning NMR tube. Typically concentrations
were 1 mmol L−1 in CH2Cl2.
Argon purged, spectroscopic grade solvents were used for all
spectroscopic measurements. Spectral data were analyzed using
GRAMS/AI 8.00 (Thermo Electron Corp.) and OriginPro 7.5
(OriginLab Corp.).
Computational Methods. Calculations to determine geometry,
vibrational spectra, and electronic spectra were performed using
density functional theory with the B3LYP functional50,51 and the 6-
31G(d) basis set.52 A LANL2DZ core potential53 was used to model
the copper atoms.45,54,55 All calculations were carried out using the
Gaussian 09 software package.56 For theoretical Raman vibrational
energies a scale factor of approximately 0.97 was used; this was found
[Cu(bpy(Mes)2)(bpy)]BF4. Recrystallized from methanol: 78 mg,
1
yield = 81%; H NMR (400 MHz, CDCl3) δ 8.50 (2H, d, J = 8 Hz),
8.22 (2H, t, J = 8 Hz), 8.08 (2H, d, J = 4.8 Hz), 7.96−7.89 (4H, m),
7.49 (2H, d, J = 8 Hz), 7.37 (2H, m), 6.10 (4H, s), 1.83 (6H, s), 1.70
(12H, s); MS m/z (ESI POS) 612.258 {(Cu(bpy(Mes)2)(bpy)}+.
Anal. Calcd for C38H36N4CuBF4: C, 65.28; H, 5.19; N, 8.02. Found: C,
64.97; H, 5.31; N, 7.75.
[Cu(bpy(Mes)2)(Me2bpy)]BF4. Recrystallized by ether diffusion into
a dichloromethane solution of the complex: 63 mg, yield = 85%; H
1
NMR (400 MHz, CDCl3) δ 8.61 (2H, d, J = 8.0, 0.8 Hz), 8.26 (2H, t, J
= 7.6 Hz), 7.78 (4H, m), 7.49 (2H, d, J = 7.6 Hz), 7.16 (2H, t, J = 4.8
Hz), 6.28 (4H, s), 2.04 (6H, s), 1.87 (6H, s), 1.59 (12H, s); MS m/z
(ESI POS) 639.25 {(Cu(bpy(Mes)(Me2bpy)}+. Anal. Calcd for
C40H40N4CuBF4: C, 66.07; H, 5.55; N, 7.71. Found: C, 65.94; H,
5.75; N, 7.75.
[Cu(bpy(Mes)2)(Br2bpy)]BF4. Recrystallized from methanol: 72 mg,
1
yield = 83%; H NMR (400 MHz, CDCl3) δ 8.36 (2H, d, J = 8 Hz),
8.32 (2H, d, J = 8 Hz), 8.17 (2H, t, J = 8 Hz), 7.90 (2H, t, J = 8 Hz),
7.53 (2H, d, J = 7.6 Hz), 7.47 (2H, d, J = 8 Hz), 6.33 (4H, s), 1.89
(6H, s), 1.71 (12H, s); MS m/z (ESI POS) 769.042, {Cu(bpy-
(Mes)2)(Br2bpy)}+. Anal. Calcd for C38H34N4Br2CuBF4: C, 53.46; H,
4.00; N, 6.54. Found: C, 53.19; H, 4.34; N, 6.32.
[Cu(bpy(Mes)2)(biq)]BF4·0.5CH3OH. Recrystallized from methanol:
88 mg, yield = 87%; 1H NMR (400 MHz, CDCl3) δ 8.82 (2H, d, J = 8
Hz), 8.48 (2H, d, J = 8.8 Hz), 8.32 (2H, t, J = 7.6 Hz), 8.15 (2H, d, J =
8.8 Hz), 7.94 (2H, d, J = 8 Hz), 7.65−7.54 (6H, m), 7.44 (2H, d, J =
7.6 Hz), 5.69 (4H, s), 1.68 (6H, s), 1.24 (12H, s); MS m/z (ESI POS)
711. 25 {(Cu(bpy(Mes)2 )(biq)}+ . Anal. Calcd for
C46H40N4CuBF4·0.5CH3OH: C, 68.51; H, 5.19; N, 6.87. Found: C,
68.39; H, 5.19; N, 6.87.
C
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by minimizing mean absolute deviation (MAD) in the Raman shift
between calculated and experimental bands.57−59 Intensity correction
for calculated spectra was applied to convert the Raman activity given
by the Gaussian 09 program to Raman scattering cross sections.60−62
For the jth mode the differential Raman cross section (∂σj/∂Ω) is
directly proportional to the observed band intensity. This is related to
the Raman activity, Sj, given by the Gaussian 09 frequency calculation
(Gaussian Keyword: Freq=Raman) as follows:
angles of 109.5°), 0 for square planar (largest angles of 180°),
and 0.85 for a perfect trigonal pyramidal (largest angles of
120°), and intermediate geometries fall within the range of 0−
1.00. The τ4 index is used to quantify the effects of the different
ligands on the intramolecular π-stacking with the mesityl ring of
the bpy(Mes)2 ligand and also the distortion of the Cu(I) ion
from a perfect tetrahedral geometry. The τ4 value for typical
Cu(I) bisdiimine complexes such as [Cu(dmp)2]+ and
[Cu(6,6′-dimethylbipyridine)2]+ are around 0.75. N−Cu−N
angles in these complexes range from ≈125° to around 85°, the
latter being found in the five-membered chelate rings.
⎛
⎜
⎜
⎜
⎝
⎞
⎟
⎟
⎟
⎠
∂σ
∂Ω
(ν0 − ν)4
⎛
⎝
⎞
⎠
4
4
⎛
⎞
j
2 π
j
h
⎜
⎜
⎟
⎟
=
Sj
⎜
⎝
⎟
⎠
8π2cν
−hcν
⎡
⎤
j
45
j
1 − exp
⎢
⎣
⎥
⎦
kT
The crystal structures of [Cu(bpy(Mes)2)(phen)]BF4 and
[Cu(bpy(Mes)2)(dmp)]BF4 are shown in Figure 3. The
where ν0 is the laser excitation frequency and νj is the frequency of the
jth mode.
X-ray Crystallography. X-ray crystal structure measurements
were made with a Bruker diffractometer equipped with an Apex II
charge-coupled device (CCD) area detector using graphite-mono-
chromated Mo Kα (λ = 0.710 73 Å) radiation. The structures were
solved by direct methods and refined on F2 by use of all data in full-
matrix least-squares procedures. All non-hydrogen atoms were refined
anisotropically.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. All complexes were
synthesized using the same methodology. One equivalent of
[Cu(CH3CN)4]BF4 was added to a solution of bpy(Mes)2 in
CH2Cl2 at room temperature, and the solution subsequently
turned yellow due to the formation of [Cu(bpy(Mes)2)-
(CH3CN)2]+. The second diimine ligand was then added to the
mixture; the bpy- and phen-based ligands resulted in red
solutions, while the biquinoline-based ligands gave purple
solutions. These solutions were then reduced to dryness and
the impure solids subsequently recrystallized. Samples were
either purified by recrystallization from methanol or by ether
diffusion into a dichloromethane solution of the complexes; in
some cases, the solvate of the complexes was obtained. All of
the complexes presented are air-stable solids and appear to be
stable in solution for weeks.
The 1H NMR spectra of the complexes provides an effective
diagnostic peak due to the formation of the desired heteroleptic
complexes over the formation of any undesired homoleptic
complexes. This is the singlet due to the mesityl ring meta
aromatic proton that appears between 5.55 and 6.33 ppm in the
complexes reported. Upfield shifts of this nature, relative to the
free ligand (6.98 ppm), are unusual for aromatic protons and
are due to the shielding of aromatic protons by the π system of
the second diimine ligand.
Structural Studies. Solid-state structural studies by X-ray
crystallography were performed on all but one of the complexes
prepared. The crystallographic data and structural refinement
data for the complexes are shown in the Supporting
Information (Table S1). For many of the complexes, large
crystals with dimensions tens of millimeters long were able to
be grown, from either slow evaporation from MeOH or CHCl3
or ether diffusion into a concentrated dichloromethane solution
of the complexes. Features of some of the structures include
intramolecular π stacking between the diimine ligand and a
single mesityl ring and severe deviation from perfect tetrahedral
geometry. A quantitative method of characterizing four-
coordinate geometries is the τ4 geometry index proposed by
Houser.63 This uses the formula τ4 = {360° − (θ − φ)}/141°,
where θ and φ are the two largest angles in the four coordinate
geometry. The τ4 value of a perfect tetrahedron is 1.00 (largest
Figure 3. Space-filling diagrams (top) and ORTEP64 diagrams of
[Cu(bpy(Mes)2)(phen)]BF4, [Cu(bpy(Mes)2)dmp)]BF4. Ellipsoids
are drawn at the 50% probability level and hydrogen atoms are
omitted for clarity in the ORTEP diagrams.
tetrafluoroborate anions are not shown. Both complexes
crystallize in the P21/c space group and display intramolecular
π−π stacking interactions between the phen or dmp ligand and
one of the mesityl rings of the bpy(Mes)2 ligand. The distance
between the centroid of the π-stacking mesityl ring and the
plane of the phen ligand is 3.32 Å for both structures; these
distances are well within the ranges for π-stacking.65 This type
of intramolecular π-stacking has also been observed in similar
heteroleptic Cu(I) complexes incorporating dipyridophenazine
ligands.26,38
The similarities between these two structures are borne out
by the τ4 geometry parameter, as shown in Table 1. τ4 values of
0.667 and 0.656 for [Cu(bpy(Mes)2)(phen)]BF4 and [Cu-
(bpy(Mes)2)(dmp)]BF4, respectively, classify the solid-state
1
geometry as distorted trigonal pyramidal. The H NMR (vide
supra) and UV−vis absorption data (vide infra) are consistent
with the complexes adopting the expected tetrahedral Cu(I)
geometry in the solution phase, and presumably, the phen
ligand is therefore either located more centrally between the
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stacking between the bpy ligand and a single mesityl ring of the
bpy(Mes)2 ligand. This is particularly evident in both
[Cu(bpy(Mes)2)(Br2bpy)]BF4 and [Cu(bpy(Mes)2)(bpy)]-
BF4, where the bpy ligands sit more centrally between the
two mesityl rings, and the mesityl rings “clamp down” on the
bpy ligand.
Table 1. Largest N−Cu−N Bond Angles and τ4 Geometry
Parameter for Phenanthroline-Based Complexes
[Cu(bpy(Mes)2)(phen)]BF4 [Cu(bpy(Mes)2)(dmp)]BF4
largest angles N(2)−Cu(1)−N(4) =
127.11°
N(2)−Cu(1)−N(3) =
140.00°
N(2)−Cu(1)−N(3) =
138.81°
N(2)−Cu(1)−N(4) =
127.38°
A result of this is that [Cu(bpy(Mes)2)(Br2bpy)]BF4 and
[Cu(bpy(Mes)2)(bpy)]BF4 do not exhibit any Pac-Man-like
intramolecular interactions, as the bpy ligand sits more evenly
between the two mesityl rings. The methyl-substituted ligand
does show a slight Pac-Man effect. The clamping effect of the
mesityl rings is shown by the contracted N1−C1−C11 bond
angle of the sp2 hybridized carbon (Figure 5) of 113°−115° in
the bromo and bpy complexes, compared to angles of 118°−
120° in the phen complexes (Table 2). The differences in π-
interactions are probably due to the smaller π-system and
greater flexibility of the bipyridine ligands compared to the
phenanthroline moieties present in the phen-type complexes
described above. The differences in the bpy-type complex
structures are reflected in the variation in τ4 (ranging from
0.665 to 0.720); this is much greater than those observed in the
corresponding phen-type complexes. The τ4 of the bpy-type
complexes indicates severe distortion from a tetrahedral
geometry normally associated with four-coordinate Cu(I)
complexes. The solid-state geometry is better described as
distorted trigonal pyramidal.
τ4
0.667
0.656
mesityl rings or oscillates back and forth rapidly between each
mesityl ring. Presumably, if the π-stacking persisted in solution,
1
the H NMR spectra would display two sets of signals for the
mesityl protons. However, only one set of mesityl protons is
observed.
The two complexes are also very similar in terms of their
intermolecular stacking. The intramolecular π-stacking between
the phen ligands and the mesityl ring creates a “void” between
the phen ligand and the non-π-stacked mesityl ring, in effect a
“Pac-Man”-like semblance. This void is filled by an aromatic
portion of an adjacent cation, as shown below in Figure 4. In
The crystal structures of the biquinoline-based complexes are
shown in Figure 6. First, [Cu(bpy(Mes)2)(biq)]BF4, which
crystallizes in the orthorhombic space group P212121, with four
cations and four tetrafluoroborate anions in the unit cell. This
structure is similar to the bpy-based structures in the respect
that the mesityl rings clamp down onto the biq ligand via π-
interactions. The N1−C1−C11 and N2−C10−C20 bond
angles of the sp2-hybridized carbon are 116° and 115°,
respectively, slightly less contracted than the bpy compounds
(vide supra). The structure is twisted in such a way that one
mesityl ring (the “top” ring containing C11, as viewed in Figure
6) is closely associated with one quinoline half of the ligand
(N3 quinoline), while the other or “bottom” mesityl ring,
containing C20, is closely associated with the other quinoline
half (N4) of the biquinoline ligand. The biquinoline ligand is
twisted 19° around the C37−C38 bond (the bond that links
the two quinoline moieties, the N−C−C−N torsion angle),
and the bpy(Mes)2 ligand is twisted 10° around the C5−C6
bond to accommodate this pairing up of quinoline and mesityl
moieties. This can be contrasted with the [Cu(biq)2]BF4 crystal
structure, where the biquinoline ligands are orthogonal to each
other and each biquinoline ligand is almost planar with the
respective N−C−C−N dihedral angle less than 1°.75 The
extent of twisting in biquinoline ligand is also greater than
observed in the bpy type complexes, where the N−C−C−N
torsion angles range from 2° to 5°. Planes through the mesityl
rings are at a 7° angle to their respective quinoline half, with
centroid to centroid distances of 3.68 and 3.76 Å for the C11
mesityl to N4 pyridyl and C20 mesityl to N3 pyridyl rings,
respectively.
Figure 4. Intermolecular π-interactions of [Cu(bpy(Mes)2)(phen)]-
BF4, showing the shortest distance between carbon atoms in
angstroms.
addition, there are intermolecular interactions between the bpy
moiety of the adjacent bpy(Mes)2 ligand (labeled B in Figure
4) and the phen ligand and mesityl ring of the complex A. We
believe the Pac-Man-like structure is due to the creation of the
void between the phen ligand and mesityl ring, which is a result
of the intramolecular π-stacking. Consistent with this, the
crystal structure of [Cu(dpp)2]+ (dpp = 2,9-diphenyl-1,10-
phenanthroline) does not show this Pac-Man-like motif
because this structure does not possess the requisite intra-
molecular stacking to engineer the void space.66 This
interaction is seen in the [Cu(bpy(Mes)2)(phen)]BF4, [Cu-
(bpy(Mes)2)(dmp)]BF4, and also previously reported [Cu-
(bpy(Mes)2)(dipyridophenazine)]BF4 structure.38 In addition,
McMillin,66−68 White,69,70 and Karpishin71−74 describe Pac-
Man-like distortion of [Cu(NN)2]+ complexes as a rocking
distortion.
[Cu(bpy(Mes)2)(Phdbq)]BF4 crystallizes in the monoclinic
space group P21/c with four cations and four tetrafluoroborate
anions in each unit cell. The Phdbq ligand structure differs from
the biquinoline structure due to the 3,3′-dimethylene bridge
linking the two quinoline moieties, as well as the two phenyl
rings at the 4,4′-position. The Phdbq ligand is less twisted than
the biq ligand, with a N−C−C−N dihedral angle of 14°, the
The structures of the three bpy-based complexes [Cu(bpy-
(Mes)2)(bpy)]BF4, [Cu(bpy(Mes)2)(Me2bpy)]BF4, and [Cu-
(bpy(Mes)2)(Br2bpy)] are shown in Figure 5. One difference
in these structures compared to the phen-based complexes
discussed above is the less pronounced intramolecular π-
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Figure 5. Space-filling diagrams (top) and ORTEP64 diagrams with ellipsoids at the 50% probability level for [Cu(bpy(Mes)2)(bpy)]BF4,
[Cu(bpy(Mes)2)(Me2bpy)]BF4, and [Cu(bpy(Mes)2)(Br2bpy)]BF4, respectively.
Table 2. Largest N−Cu−N Bond Angles and τ4 Geometry Parameter for the Bipyridine-Based Complexes
[Cu(bpy(Mes)2)(bpy)]BF4
[Cu(bpy(Mes)2)(Me2bpy)]BF4
[Cu(bpy(Mes)2)(Br2bpy)]BF4
largest Angles
N(3)−Cu(1)−N(1) = 131.6°
N(2)−Cu(1)−N(4) = 129.88°
0.698
N(3)−Cu(1)−N(2) = 142.31°
N(3)−Cu(1)−N(1) = 124.27°
0.665
N(3)−Cu(1)−N(2) = 136.24°
N(3)−Cu(1)−N(1) = 127.13°
0.720
τ4
Figure 6. Space-filling diagrams (top) and ORTEP64 diagrams with ellipsoids at the 50% probability level for [Cu(bpy(Mes)2)(biq)]BF4,
[Cu(bpy(Mes)2)(Phdbq)]BF4, and [Cu(bpy(Mes)2)(Phdbp)]BF4, respectively.
dimethylene bridge restricting torsion of the ligand. The
mesityl rings interact through π-interactions with Phdbq ligand
and clamp onto the Phdbq ligand, as indicated with N−C−C
angles of 115°. The bpy(Mes)2 ligand is not as twisted as in the
case of the biquinoline structure and has a N−C−C−N
dihedral angle of 2°. However, the bpy(Mes)2 and Phdbq
ligands are tilted from an ideal orthogonal geometry, with an
acute angle between the two coordination geometries of 79°.
The [Cu(bpyMes)(Phdbp)]BF4 crystal structure reflects the
planar aromatic nature of the Phdbp ligand. There is still a
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Table 3. Largest N−Cu−N Bond Angles and τ4 Geometry Parameter for Biquinoline-Based Complexes
[Cu(bpy(Mes)2)(biq)]BF4
[Cu(bpy(Mes)2)(Phdbq)]BF4
[Cu(bpy(Mes)2)(Phdbp)]BF4
largest angles
N(4)−Cu(1)−N(2) = 133.58°
N(3)−Cu(1)−N(1) = 128.27°
0.696
N(4)−Cu(1)−N(2) = 134.61°
N(4)−Cu(1)−N(1) = 131.97°
0.663
N(3)−Cu(1)−N(1) = 133.64°
N(3)−Cu(1)−N(2) = 129.56°
0.687
τ4
slight twisting of the ligand though with the N−C−C−N
dihedral angle of 9°. This is much flatter than the Phdbq and
biq ligands, which have angles of 14° and 19°, respectively. The
mesityl rings clamp down on the Phdpb ligand with clamping
angles of 114° and 115°. The Phdbp and bpy(Mes)2 ligands are
not orthogonal to each other, with an acute angle between the
two ligand planes of 78° (Figure 6). This tilt results in the C11
mesityl ring lying over the center of the phenanthroline moiety,
while the C20 mesityl ring is over the N3 quinoline moieties.
The plane through the C11 mesityl rings is at 7° to the plane
through the atoms of the phenanthroline moiety, while the C20
mesityl ring is less parallel to the phen plane at 22°. The
distorted nature of the coordination geometry of the biquino-
line-based complexes is reflected in the τ4 values, given in Table
3. The coordination geometries in the solid state are best
described as distorted trigonal pyramidal.
Electrochemistry. The complexes were studied by cyclic
voltammetry in dry, nitrogen-purged dichloromethane; the half-
wave potentials of the oxidation and reduction (where
observed) processes are given in Table 4. Cyclic voltammo-
grams of [Cu(bpy(Mes)2)(bpy)]BF4, [Cu(bpy(Mes)2)-
(Me2bpy)]BF4, and [Cu(bpy(Mes)2)(Br2bpy)]BF4 are shown
in Figure 7.
Figure 7. Cyclic voltammograms of the first oxidation of [Cu(bpy-
(Mes)2)(bpy)]BF4, [Cu(bpy(Mes)2)(Me2bpy)]BF4, and [Cu(bpy-
(Mes)2)(Br2bpy)]BF4 with DMFc in solution. Solution concentrations
were typically about 10−3 M in CH2Cl2 with 0.1 M tetrabutylammo-
nium hexafluorophosphate (TBAPF6) added as a supporting electro-
lyte.
The dmp, Me2bpy, and Br2bpy complexes have the most
positive potentials for the oxidation process, owing to the
increased steric bulk around the Cu(I) center. The biquinoline-
based complexes all have similar oxidation potentials. This is
consistent with their similar Cu(I) coordination environments.
The E1/2 of the oxidation process of the phen- and bpy-based
complexes are comparable to those reported by Odobel and co-
workers26 in a study of the corresponding mesityl-substituted
phenanthroline complexes. This suggests that the relative ease
of forming square planar Cu(II) is not greatly altered by
substitution of the rigid phen(Mes)2 with the more flexible
bpy(Mes)2 ligand. This may be somewhat surprising consid-
ering the greater flexibility of the bpy(Mes)2 ligand compared
to the phenanthroline moiety.
For the phen, dmp, bpy, Me2bpy, and Br2bpy complexes, no
reduction was observed within the dichloromethane solvent
limit, meaning no ligand-based reduction [or Cu(0) formation]
is observed, consistent with previous work on the respective
homoleptic complexes.2 For the complexes where reduction
waves are observed, these are assigned as population of the
respective biquinoline-based ligand LUMO, consistent with
previous literature.76,77
a
Table 4. Electrochemical Data for the Complexes
b
c
E1/2/V vs SCE
E1/2/V vs SCE
[Cu(bpy(Mes)2)(phen)]BF4
[Cu(bpy(Mes)2)(dmp)]BF4
[Cu(bpyMes)2)(bpy)]BF4
[Cu(bpyMes)2)(Me2bpy)]BF4
[Cu(bpyMes)2)(Br2bpy)]BF4
[Cu(bpyMes)2)(biq)]BF4
0.78
0.92
0.73
0.91
1.06
0.90
0.84
0.91
0.91
−
−
−
−
−
−1.38
−1.49
−1.37
−1.29
[Cu(bpyMes)2)(Phdbq)]BF4
[Cu(bpyMes)2)(Medbq)]BF4
[Cu(bpyMes)2)(Phdbp)]BF4
E1/2 recorded relative to DMFc+/DMFc and converted vs SCE.
a
Solvent was CH2Cl2, and supporting electrolyte was TBAPF6.
b
c
Oxidation process. Reduction process.
All complexes exhibited a reversible oxidation wave, which is
assigned as the Cu(I) to Cu(II) process. The half-wave
oxidation potentials are dependent on the extent of the steric
bulk around the Cu(I) center. This is in accordance with the
change of preferred geometry as Cu(I) (tetrahedral) is oxidized
to Cu(II) (square-planar). The increased steric bulk around the
Cu(I) coordination sphere stabilizes the Cu(I) center to
oxidation.73 This is exemplified by comparing the oxidation
potentials of [Cu(phen)2]+ (0.50 V vs SCE) and [Cu(dmp)2]+
(0.83 V) in dichloromethane solution. The oxidation potentials
of the complexes presented here reflect this. The E1/2 values
observed for the oxidation of the complexes are lower (less
positive) when the ligand has no methyl groups or bromine
atoms ortho to the coordinating nitrogens, namely, the
[Cu(bpy(Mes)2)(phen)]BF4 and [Cu(bpy(Mes)2)(bpy)]BF4
complexes.
Electronic Absorption Spectra and TD-DFT Calcula-
tions. The electronic absorption spectra of the phen- and bpy-
based complexes are shown in Figure 8, and the corresponding
experimental and TD-DFT data are given in Table 5. All of the
phen- and bpy-based complexes are red in color. In the visible
region, the spectra are dominated by a band around 470 nm.
This is assigned as an MLCT transition from the Cu(I) metal
center to the bpy/phen ligands(s). Each complex has intense
bands in the UV region of the spectrum, due to ligand-based
π−π*-transitions.
The energies of the transitions are red-shifted from those of
the homoleptic Cu(I) complexes [Cu(dmp)2]+ (λmax, 455 nm),
[Cu(phen)2]+ (λmax, 435 nm), and [Cu(Me2bpy)2]+ (λmax, 455
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bpy-based complexes (Table 5) are similar, around 450 nm.
The calculation predicts the observed result that the Br2bpy
complex has a lower energy of absorption compared to the
other phen- and bpy-based complexes, observed at 482 nm
(predicted 476 nm). The nature of this transition appears
different from the other phen- and bpy-based complexes. This
is supported by the band shapes and extinction coefficients;
other than those of [Cu(bpy(Mes)2)(Br2bpy)]BF4, the band
shapes are similar to those of the respective homoleptic
complexes. The spectrum of [Cu(bpy(Mes)2)(Br2bpy)]BF4
shows the lowest energy maximum (482 nm), and the band
is much broader than those observed in the other spectra. This
is indicative of the electron-withdrawing nature of the bromine
atoms, which lowers the energy of the Br2bpy LUMO relative
to the bpy(Mes)2 ligand, and hence, the MLCT to the Br2bpy
ligand has greater oscillator strength.
For the other complexes with a bpy- or phen-based diimine
ligand, it can be assumed that the energies of the LUMO of the
bpy(Mes)2 and the bpy or phen diimine ligand are very similar.
This suggests that two MLCT transitions may be possible, each
populating orbitals on the two different diimine ligands.
However, for example, the TD-DFT calculations for the phen
complex suggest an MLCT transition made up of two
configurations, HOMO−1→LUMO and HOMO→LUMO+1
(Table 5), in which the LUMO is bpy-based and the LUMO+1
is phen/bpy-based; in essence, this is describing an MLCT
transition that is delocalized across both ligands. The existence
of delocalized MLCT excited states has been the subject of
intense research effort in the past.79−82 The weight of evidence
is that MLCT transitions in homoleptic Cu(I) complexes are
localized in solution, and this would certainly be expected for
heteroleptic systems.83−85
Figure 8. UV−vis absorption spectra of the phen- and bpy-based
[Cu(bpy(Mes)2)(NN)]+ complexes in CH2Cl2 solution.
nm). The lower energies for the MLCT transitions of the
heteroleptic [Cu(bpy(Mes)2)(NN)]+ complexes can be
attributed to intramolecular π−π-interactions between the
mesityl groups and the phen- and bpy-based ligand. Another
factor that determines the energy of the MLCT transition is the
steric bulk around the coordination sphere, as has been
observed previously.1,4,27 For example, the energies of the
MLCT transition of the phen (λmax, 476 nm) and bpy (λmax
,
472 nm) complexes are red-shifted from those of dmp (λmax
463 nm) and Me2bpy (λmax 465 nm) complexes. It should be
noted that [Cu(bpy(Mes)2)(Br2bpy)] does not fit this pattern;
this is because the Br2bpy ligand has a lower energy π* MO,
resulting in a red-shifted MLCT transition (482 nm). The
extinction coefficients of the complexes are typical for the
MLCT transition of Cu(I) polypyridyl complexes; the Br2bpy
complex has the smallest (3200 L mol−1 cm−1).
Time-dependent density functional theory (TD-DFT)
calculations were performed with a dichloromethane solvent
field on the geometry obtained from the in-vacuo optimization
calculation, using the crystal structure geometries. It is generally
found that the shifts in energies are well-predicted, but the
absolute values are offset; this is true for charge-transfer
transitions, as has been discussed by Dreuw and Head-
Gordon.78 With the exception of the Br2bpy complex, the
predicted energies for the MLCT transition of the phen- and
Each of the bpy- and phen-based complexes (except Br2bpy)
has calculated MLCT transitions that terminate on both of the
diimine ligands. Resonance Raman spectroscopy is used to gain
insight into the nature of these transitions (vide infra).
The UV−vis absorption spectra for the biquinoline-based
complexes are shown in Figure 9, with data given in Table 5.
The complexes all appear purple in color, with the absorption
maxima of the Medbq, Phdbq, and biq complexes at 533, 547,
and 541 nm, respectively. The Phdbp complex is dominated by
two bands of similar intensity in the visible region, with
absorption maxima at 561 and 451 nm.
Table 5. Experimental and Calculated Electronic Data for the Complexes
experimental
ε/L mol−1 cm−1
calculated
λ/nm
a
compound
λ/nm
f
MO configurations, major components (coefficient)
[Cu(bpy(Mes)2)(phen)]BF4
[Cu(bpy(Mes)2)(dmp)]BF4
476
463
6100
5000
450
452
0.13
0.06
H−1, L (0.50); H, L+1 (−0.49)
H−1, L (0.59); H−1, L+1 (−0.28)
[Cu(bpy(Mes)2)(bpy)]BF4
[Cu(bpy(Mes)2)(Me2bpy)]BF4
[Cu(bpy(Mes)2)(Br2bpy)]BF4
472
465
482
5400
4400
3200
452
451
476
0.11
0.06
0.05
H−1, L (0.36); H−1, L+1 (0.34)
H−1, L (0.21); H, L (0.61); H, L+1 (0.11)
H, L (0.88)
[Cu(bpy(Mes)2)(biq)]BF4
541
547
533
561
451
3600
4900
4400
5200
5000
507
492
497
520
449
0.07
0.08
0.08
0.11
0.08
H, L (0.80); H−1, L (0.11)
H, L (0.54); H−1, L (0.28)
H−1, L (0.23); H, L (0.64)
H, L (0.89)
[Cu(bpy(Mes)2)(Phdbq)]BF4
[Cu(bpy(Mes)2)(Medbq)]BF4
[Cu(bpy(Mes)2)(Phdbp)]BF4
H, L+1 (0.89)
a
H = HOMO, L = LUMO.
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Figure 9. UV−vis absorption spectra of the biquinoline-based
[Cu(bpy(Mes)2)(NN)]+ complexes in CH2Cl2 solution.
The energies of the predicted transitions for [Cu(bpy-
(Mes)2)(biq)]BF4 and [Cu(bpy(Mes)2)(Medbq)]BF4 are quite
similar, around 500 nm. The nature of the transitions are
MLCT from a Cu(I) dπ orbital to the biquinoline-based
LUMO. This is to be expected as the biquinoline-based LUMO
is much lower in energy than the bipyridine-based LUMO. This
presence of a second strong transition in the visible region for
[Cu(bpy(Mes)2)(Phdbp)]BF4 is predicted by the TD-DFT
calculation. The lowest energy band (observed 561 nm,
predicted 520 nm) is assigned as HOMO → LUMO, similar
to other biquinoline-based complexes. The second strong
transition is more accurately predicted at 449 nm (observed
451 nm) and is assigned as MLCT to Phdbp LUMO+1. The
LUMO+1 of Phdbp is low enough energy to be populated in
the visible region due to the extended aromatic system of the
ligand. The extended aromatic system of the Phdbp ligand
results in the low energy LUMO+1 being populated by an
MLCT transition in the visible region.
Normal and Resonance Raman Spectroscopy. The
frequencies and intensities of Raman bands can be calculated in
order to simulate a normal Raman spectrum, which can be
compared to the experimental data. The goodness-of-fit
between the experimental and calculated Raman data may be
parametrized by the mean-absolute deviation (MAD), in order
to determine the accuracy of calculations.86−89 Calculated
spectra can then be used to assign the bands, which is
important for identifying the enhanced vibrational models in
the resonance Raman spectra.
Figure 10. Comparison between the experimental and calculated FT-
Raman spectra of [Cu(bpy(Mes)2)(dmp)]BF4.
most prominent of these lie at approximately 1005, 1325, and
1560 cm−1.
Resonance Raman spectroscopy can be used to elucidate the
structural changes upon excitation into the initially populated
Franck−Condon state. This is possible as the enhancements
observed in resonance Raman spectra, compared to the normal
Raman spectra, are not random but specific to the modes of
vibration that mimic the structural distortion upon excita-
tion.90,91 Hence vibrational modes that are enhanced are
associated with the active chromophore. For the complexes
where the energy of the LUMOs on each ligand are close in
energy to each other and, hence, MLCT transition may
populate either/or both of the diimine ligands, the resonance
Raman spectra can elucidate which ligand is being populated in
the FC state. Of course, in addition to the structural distortion
(Δs) other factors can affect the resonance Raman scattering
cross section.45,55 Thus, it is possible that in these systems in
which there are heteroleptic ligands that one ligand may be a
much better Raman scatterer than the other and thus may
contribute more strongly to the observed spectrum.
As will be shown, the resonance Raman spectra and TD-DFT
results for the purple, biquinoline-based complexes are much
more straightforward to assign and interpret, as the dominant
transition is the MLCT which populates the LUMO of the
biquinoline-type ligand. The TD-DFT data for the complexes
predict the electronic transitions of the purple compounds as
populating the LUMO of the biquinoline-type ligand.
The resonance Raman spectra of [Cu(bpy(Mes)2)(dmp)]-
BF4 at a variety of excitation wavelengths are shown in Figure
11. The dmp complex shows enhancements of bands at 1374,
1464 1560, and 1593 cm−1 across the wavelengths 406−488
Figure 10 shows the calculated and experimental FT-Raman
spectra of [Cu(bpy(Mes)2)(dmp)]BF4; this has a MAD of 5
cm−1 on the basis of comparing the experimental values that are
greater than 20% of the maximum observed intensity. The
MADs for the other complexes range from 5 to 10 cm−1. This is
considered a good correlation.57 Although the calculated
vibrational spectra for the data result in satisfactory MADs,
for some of the complexes studied here an unambiguous
assignment of all vibrational modes is not always possible
because the transitions overlap. However, for each compound a
sufficient number of unambiguous assignments could be made
to characterize the Franck−Condon state.
For the normal Raman spectra of the complexes, a series of
bands at similar frequencies are apparent for all the complexes,
and these bands are assigned to the bpy(Mes)2 ligand. The
I
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nm. These bands are all modes located on the bpy(Mes)2
ligand.
bpy(Mes)2 ligand, which is consistent with the enhancement of
bpy bands over dmp bands. The resonance Raman spectra of
[Cu(bpy(Mes)2)(Br2bpy)]BF4 (Figure S1, Supporting Infor-
mation) across the same excitation wavelengths show similar
features to the dmp complex, that is, enhancement of bands
assigned as bpy(Mes)2 ligand vibrations.
The enhancement of bands due to the bpy(Mes)2 ligand in
preference to the Br2bpy ligand is somewhat surprising given
the energy and band-shape of the MLCT band in the
absorption spectrum of [Cu(bpy(Mes)2)(Br2bpy)]BF4, which
suggests that the chromophore is dominated by the Br2bpy
ligand. The TD-DFT calculation also predicts the dominant
MLCT transition being that involving the Br2bpy ligand.
The resonance Raman spectra of [Cu(bpy(Mes)2)-
(Me2bpy)]BF4 show two distinct patterns of enhancements
across the excitation wavelengths used (Figure 12). Excitation
Figure 11. Resonance Raman spectra (blue) of [Cu(bpy(Mes)2)-
(dmp)]BF4 in CH2Cl2 at a range of excitation wavelengths. The
solution concentration was approximately 10−3 mol L−1. FT-Raman
spectra (red) of powder sample acquired at 1064 nm.
Bands at 1418 and 1612 cm−1 are both vibrations based on
the dmp ligand and are not relatively enhanced across the same
wavelengths. Interestingly, the band around 1320 cm−1, which
is enhanced in the bpy-based compounds, is not enhanced in
the dmp spectra. The calculated vibrational mode for this
vibration predicts a delocalized vibration involving both ligands.
Excitation with 514 nm wavelength enhances the bpy-based
band at 1006 cm−1; this wavelength is probing the shoulder of
the MLCT band.
The shoulder of the MLCT band is assigned as a transition
with torsion between the two ligand planes, deviating from D2d
symmetry.33,92 The enhancement of this particular bpy-based
band at ca. 1006 cm−1 is observed not only for [Cu(bpy-
(Mes)2)(dmp)]BF4 (Figure 11), but also [Cu(bpy(Mes)2)-
(bpy)]BF4 (Figure S1, Supporting Information), and [Cu(bpy-
(Mes)2)(phen)]BF4 (Figure S2, Supporting Information). One
plausible explanation for this enhancement pattern is that the
1006 cm−1 band possesses a degree of torsional character that
selectively enhances it for the lower energy transition. This is
not inconsistent with the experimental data, but careful analysis
of the calculated mode does not provide unequivocal support
for this explanation. In the UV−vis spectra the lowest energy
transition (often referred to as band I) appears as a shoulder on
the main MLCT. In all cases in the DFT calculations this is a
HOMO→LUMO transition, thus having a similar orbital
parentage to the much more intense band II features (Table
5). In this sense, the DFT calculations do not appear to provide
decisive insight into this part of the optical spectrum.
Figure 12. Resonance Raman spectra (blue) of [Cu(bpy(Mes)2)-
(Me2bpy)]BF4 in CH2Cl2 at a range of excitation wavelengths. The
solution concentration was approximately 10−3 mol L−1. FT-Raman
spectra (red) of powder sample acquired at 1064 nm. Solvent bands
are marked (*).
wavelengths from 406 to 488 nm show bands enhanced at
1009, 1319, 1373, 1466, and 1559 cm−1. Excitation wavelengths
from 514 to 568 show enhancement of some of the same bands
that are enhanced across 406−488 nm (1009, 1319, and 1373
cm−1) but diminution of the 1466 and 1559 cm−1 bands.
The bands at 1009, 1373, and 1559 cm−1 are bpy bands of
the bpy(Mes)2 ligand, and the 1466 cm−1 band is a Me2bpy
band. Assignment of the 1319 cm−1 band is ambiguous, as two
calculated modes similar in calculated energy and intensity lie at
1303 and 1309 cm−1, but these are the same bpy-based
stretching modes, but localized on the bpy(Mes)2 and bpy
ligands, respectively.
The TD-DFT calculations predict the dominant config-
uration of the MLCT transition being HOMO−1 to LUMO
(59%), where the LUMO occupies the bpy moiety of the
J
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The same stretching modes on the two bpy-based ligands are
expected to have similar frequencies of vibration and, in this
complex, may in fact occur at the same energy. The
enhancement of bands due to both ligands indicates that
both ligands are involved in the MLCT chromophore. One
interpretation of the data is that the FC state of the molecule is
delocalized over both ligands. However, the FC state of
[Cu(I)(diimine)2]+ polypyridyls is known to be localized on
one of the diimine ligands. A more likely explanation is that, as
the energy levels of the MLCT transitions to each of the ligands
is very similar, in solution there exists a population of molecules
where the FC state is localized on the bpy(Mes)2 ligand and
also a population of molecules where the FC state is localized
on the Me2bpy ligand. TD-DFT calculations predict LUMOs of
similar energy. The band at 1612 cm−1 in the FT spectrum,
which is not enhanced with excitation of any of the wavelengths
used, is assigned as a stretching mode of the mesityl rings of the
bpy(Mes)2 ligand. This is consistent with the MLCT to the
bpy(Mes)2 ligand not having a lot of wave function amplitude
on the mesityl substituent.
The resonance Raman spectra of [Cu(bpy(Mes)2)(bpy)]BF4
and [Cu(bpy(Mes)2)(phen)]BF4 (Figures S1 and S2, Support-
ing Information) show very similar features to that of
[Cu(bpy(Mes)2)(Me2bpy)], with enhancement of bands due
to both ligands, indicating MLCT transitions that populate each
ligand. This is also consistent with the TD-DFT calculations
that predict the LUMO orbitals localized on each ligand to be
close in energy to each other and both involved in the
electronic transition (Table 5).
Figure 13. Resonance Raman spectra of [Cu(bpy(Mes)2)(Medbq)]-
BF4 in CH2Cl2 at a range of excitation wavelengths. The solution
concentration was approximately 10−3 mol L−1. FT-Raman spectra of
powder sample acquired at 1064 nm.
The resonance Raman spectra of the purple, biquinoline-
based complexes are much more straightforward than those of
the bpy- and phen-based complexes. The enhancement patterns
for all the purple complexes show selective enhancement of
vibrational modes of the biquinoline-based ligand over the
bpy(Mes)2 ligand. This is consistent with the lower energy of
the MLCT absorption in the visible spectra of the biquinoline
complexes. The resonance Raman spectra of [Cu(bpy(Mes)2)-
(Medbq)BF4 in Figure 13 is an example, and the spectra for the
other biquinoline-based complexes are shown in the Supporting
Information (Figures S4−S6). Three bands are drastically
enhanced over the wavelengths examined, those being at 1379,
1458, and 1550 cm−1. These are all vibrations of the Mebdq
ligand. Diminution of bands at 1010 and 1323 cm−1 compared
to the FT-spectrum, which are bpy-based bands, is also
consistent with the dominant chromophore being the Cu(I) to
Medbq MLCT transition.
To summarize, the resonance Raman spectra of the purple,
biquinoline-based complexes all show enhancement of bands
due to vibrational modes localized on the biquinoline-type
ligand. Therefore, the dominant chromophore is the MLCT
transition that populates the biquinoline-type ligand rather than
the bpy(Mes)2 ligand. The resonance Raman spectra [Cu(bpy-
(Mes)2)(Me2bpy)]BF4, [Cu(bpy(Mes)2)(bpy)]BF4, and [Cu-
(bpy(Mes)2)(phen)]BF4 show enhancement of both bpy-
(Mes)2 modes, and the second diimine ligand. This indicates
that the MLCT transitions to each ligand are close in energy to
each other and population of each localized FC state occurs.
Photophysical Properties. Many Cu(I) diimine com-
plexes are emissive from a long-lived triplet excited state in
solution. Cu(I) diimine complexes with bulky substituents
around the Cu center, which protect the excited state from
deactivation from solvent molecules, are typically emissive. For
this reason, one might expect the complexes with bulky groups
presented in this study, [Cu(bpy(Mes)2)(dmp)]BF4, [Cu(bpy-
(Mes)2)(Me2bpy)]BF4, and [Cu(bpy(Mes)2)(Br2bpy)]BF4, to
be emissive in solution. For these complexes we observe only
weak emission. Attempts to study the transient absorption of
the complexes were unsuccessful, as degassed solutions of the
complexes degraded immediately following irradiation with a
355 nm laser pulse, implying that any excited state is short-lived
(τ < 5 ns)
The weakly emissive nature of the heteroleptic complexes in
this study is in contrast to the findings of Odobel and co-
workers,26 where all but one of the complexes they studied
were found to be emissive in dichloromethane solution. The
complexes presented by Odobel and co-workers were found to
be emissive in acetonitrile, a solvent known to deactivate
excited states. However, other researchers have noted that bpy-
based ligands give short τ for Cu(I) complexes.93,94 The
oxidation potentials of the complexes presented here and those
of the complexes studied by Odobel and co-workers are
comparable, meaning the steric constraint around the Cu(I)
center is similar in both types of complexes. The increase of
steric bulk around the Cu(I) center has previously been used to
improve the emissive properties of the complexes. One possible
explanation for the nonemissive nature of the bpy(Mes)2-based
complexes is that, compared to the phenanthroline-based
complexes reported by Odobel and co-workers, the bpy
complexes are much more flexible than the phen-based systems.
This greater flexibility may result in photodisassociation of the
complexes, rather than sustaining the flattened triplet state
responsible for long-lived emission. In the case of homoleptic-
biquinoline-based Cu(I), weak emission has been observed at
low temperatures.77 However, strong emission is observed
when a benzoquinoline motif is used, creating a highly crowded
Cu(I) coordination sphere.95
K
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Article
(7) Breddels, P. A.; Blasse, G.; McMillin, D. R. Ber. Bunsenges. Phys.
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CONCLUSIONS
■
In this paper, we report the synthesis of nine heteroleptic Cu(I)
complexes, with bpy(Mes)2 and nine diimine ligands, using the
HETPEHN strategy. Eight of the complexes are characterized
by X-ray crystallography. The Cu(I) four-coordinate geometries
are quantified using the τ4 parameter. The phen-based
complexes exhibit stacking between the phen ligand and one
of the mesityl rings, creating a Pac-Man motif. On the other
hand, the bpy-based complexes show different types of packing
interaction, with both mesityl rings “clamping down” on the
bpy-based ligand and π-stacking. Cyclic voltammetry is used to
examine the oxidation process of the Cu(I) metal center, which
indicate the extent of steric bulk around the coordination
sphere. The Cu(I) MLCT transitions of the complexes are
investigated with resonance Raman spectroscopy in concert
with TD-DFT calculations. The resonance Raman spectra of
the purple complexes are straightforward, in that vibrational
bands of the biquinoline-based ligand are selectively enhanced
over bpy(Mes)2 bands. This is consistent with the purple color
of the complexes, due to the lower energy of the biquinoline-
based LUMO compared to the bpy(Mes)2 LUMO. All the
phen- and bpy-based complexes show enhancement of
bpy(Mes)2 bands. The resonance Raman spectra of [Cu(bpy-
(Mes)2)(bpy)]BF4, [Cu(bpy(Mes)2)(phen)]BF4, and [Cu-
(bpy(Mes)2)(Me2bpy)] show enhancement of bands due to
both ligand, suggesting that because the ligand LUMOs are
very close in energy, a population of the molecules with MLCT
to bpy/phen and a population of molecules with MLCT to
bpy(Mes)2 exist simultaneously.
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ASSOCIATED CONTENT
* Supporting Information
■
S
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X-ray crystallographic data for the Cu(I) complexes in CIF
format. Crystal structure refinement data, resonance Raman
spectra, Cartesian coordinates of all the optimized geometries,
and calculated frontier molecular orbitals of the complexes.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: kgordon@chemistry.otago.ac.nz.
■
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Support from the University of Otago, MacDiarmid Institute,
and New Zealand Ministry of Science and Innovation is
gratefully acknowledged.
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