Structural, electronic and computational studies of heteroleptic Cu(I) complexes of 6,6′-dimesityl-2,2′-bipyridine with sulfur-substituted dipyridophenazine ligands

Article abstract of DOI:10.1016/j.poly.2012.08.001

A series of stable heteroleptic Cu(I) complexes of 6,6′-dimesityl-2, 2′-bipyridine with three sulfur-substituted dipyridophenazine ligands, and unsubstituted dppz are synthesized and studied. The X-ray crystal structures of the complexes display coordinative flexibility of the Cu(I) metal ion and display strong intramolecular π-π stacking interactions. The coordination geometries of the Cu(I) metal centers are characterized by the τ₄ parameter, and range from 0.626 for [Cu(bpy(Mes)₂)(dppz(Sdec) ₂]BF₄ to 0.757 for [Cu(bpy(Mes)₂)(dppzS ₂CS)]BF₄. Cyclic voltammetry reveals Cu(I)/Cu(II) oxidation potentials around 0.83 V for all the complexes studied, consistent with a similar coordination environment of the complexes in solution. The ligand-based reduction potentials are consistent with the electronic nature of the sulfur substituent. Resonance Raman spectroscopy and TD-DFT calculations reveal the presence of an MLCT → bipyridine transition with 488 and 514.5 nm excitation, and dppz-type chromophores ranging from UV (351 nm) to the visible (457 nm) excitation wavelengths. For [Cu(bpy(Mes)₂) (dppzS₂CS)]BF₄ and [Cu(bpy(Mes)₂)(dppz(Sdec) ₂]BF₄ the most intense optical transition at 434 and 442 nm respectively are assigned as intraligand (IL)/MLCT transitions.

Full text of DOI:10.1016/j.poly.2012.08.001

Polyhedron 52 (2013) 623–633
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Structural, electronic and computational studies of heteroleptic Cu(I) complexes of
6,60-dimesityl-2,20-bipyridine with sulfur-substituted dipyridophenazine ligands
⇑
Michael G. Fraser, Holly van der Salm, Scott A. Cameron, Jonathan E. Barnsley, Keith C. Gordon
MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Otago, Dunedin, New Zealand
a r t i c l e i n f o
a b s t r a c t
A series of stable heteroleptic Cu(I) complexes of 6,60-dimesityl-2,20-bipyridine with three sulfur-substi-
tuted dipyridophenazine ligands, and unsubstituted dppz are synthesized and studied. The X-ray crystal
structures of the complexes display coordinative flexibility of the Cu(I) metal ion and display strong intra-
Article history:
Available online 8 August 2012
Dedicated to Alfred Werner on the 100th
Anniversary of his Nobel prize in Chemistry
in 1913.
molecular p–p stacking interactions. The coordination geometries of the Cu(I) metal centers are charac-
terized by the s4 parameter, and range from 0.626 for [Cu(bpy(Mes)2)(dppz(Sdec)2]BF4 to 0.757 for
[Cu(bpy(Mes)2)(dppzS2CS)]BF4. Cyclic voltammetry reveals Cu(I)/Cu(II) oxidation potentials around
0.83 V for all the complexes studied, consistent with a similar coordination environment of the com-
plexes in solution. The ligand-based reduction potentials are consistent with the electronic nature of
the sulfur substituent. Resonance Raman spectroscopy and TD-DFT calculations reveal the presence of
an MLCT ? bipyridine transition with 488 and 514.5 nm excitation, and dppz-type chromophores rang-
ing from UV (351 nm) to the visible (457 nm) excitation wavelengths. For [Cu(bpy(Mes)2)(dppzS2CS)]BF4
and [Cu(bpy(Mes)2)(dppz(Sdec)2]BF4 the most intense optical transition at 434 and 442 nm respectively
are assigned as intraligand (IL)/MLCT transitions.
Keywords:
Copper(I)
Heteroleptic
Dipyridophenazine
Crystal structure
DFT
Resonance Raman spectroscopy
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
in the visible region of the spectra [9]. A distinct advantage of Ru(II)
over Cu(I) is that, due to its inert nature, it offers greater versatility
Alfred Werner’s pioneering work on coordination compounds
originated in determining the octahedral geometry of a series of
Co(III) complexes with ligands such as ammonia and chloride. Dis-
tinguishing features of these complexes are their various colors
depending on the nature of the ligands bound to the metal center.
For example, [Co(NH3)5Cl]Cl2 is purple and [Co(NH3)6]Cl3 is yellow.
The origins of the different colors are of course due to weak d–d
transitions of different energies arising from the variation of ligand
field strength. Much stronger electronic transitions, including me-
tal-to-ligand charge transfer (MLCT) transitions, give coordination
compounds fascinating properties that make them suitable for
emerging technologies such as molecular-based LED screens
[1,2], and photovoltaics, such as dye sensitized solar cells (DSSCs)
[3,4]. Ruthenium(II) coordination compounds with polypyridyl li-
gands have been intensely studied as dyes for DSSCs, and are cur-
rently the best performing [5]. However, recently there have been
attempts to replace Ru(II) with the more abundant metal copper, in
the form of copper(I) [6–8]. Certain Cu(I) polypyridyls have attrac-
tive excited-state characteristics, similar to Ru(II) complexes
such as [Ru(2,20-bipyridine)3]2+; both have a long-lived charge-
separated triplet excited state, originating from an MLCT transition
in building heteroleptic complexes. The ability to construct donor–
acceptor Ru(II) complexes for DSSCs is one such application [10].
The lability of Cu(I) means that ligand exchange occurs very rapidly
in solution preventing the isolation of stable [Cu(I)(diimine1)
(diimine2)]+ heteroleptic complexes (where (diimine1) and
(diimine2) are two different ligands) [11]. This has been overcome
by Schmittel and colleagues by using the so-called ‘HETPHEN’ de-
sign strategy [12–16]. The first report of stable heteroleptic Cu(I)
complexes was achieved by the use of 2,9-disubstituted phen-
anthrolines, where the 2,9 positions are appended with bulky
groups such as mesityl moieties [12]. It was found that the steric
bulk of the mesityl substituents prevents the formation of the
bis-homoleptic Cu(I) complex. Subsequent addition of a second,
less sterically bulky phenanthroline ligand, results in the desired
heteroleptic complexes. This method has mainly been used phe-
nanthroline-type ligands and has been exploited in the realm of
supramolecular coordination chemistry [14–23].
Recently Odobel and co-workers [24,25] realized the potential to
form heteroleptic diimine Cu(I) complexes that are capable of sus-
taining photoinduced electron transfer, where (diimine1) acts as an
electron donor and (diimine2) as an acceptor. The authors investi-
gated a series of imidazole-fused phenanthroline ligands with
mesityl rings substituted at the 2,9 position, and their heteroleptic
Cu(I) complexes with phenanthroline-based ligands including
⇑
Corresponding author.
E-mail address: kgordon@chemistry.otago.ac.nz (K.C. Gordon).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.08.001

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M.G. Fraser et al. / Polyhedron 52 (2013) 623–633
3,6-dibutyl substituted dipyridophenazine (dppz). These types of
complexes open up the opportunity to tune photophysical proper-
ties, with the intention for use in electronic devices, such as DSSCs.
The authors presented structural, electronic, photophysical and
theoretical studies of the complexes. A recent report from Constable
and co-workers investigated a series of heteroleptic Cu(I) com-
plexes as dyes for DSSCs [26]. The authors used a new strategy,
where the heteroleptic Cu(I) complex assembles on the TiO2 semi-
conductor surface [26]. Initially, (diimine1) which possesses
anchoring groups, binds onto the TiO2 surface, and this is followed
by introduction of a complex, [Cu(diimine2)2]+, which undergoes li-
gand exchange to give a surface-bound heteroleptic [Cu(diimine1)
(diimine2)2]+ complex.
This paper presents heteroleptic Cu(I) complexes of the bipyri-
dine-based ligand, 6,60-dimesityl-2,20-bipyridine (bpy(Mes)2) with
a series of sulfur-substituted dppz ligands (Fig. 1). The complex
with unsubstituted dppz, [Cu(bpy(Mes)2)(dppz)]BF4 is also
presented. The bpy(Mes)2 ligand was initially synthesized by
Schmittel et al. [14], however, only one heteroleptic Cu(I) complex
utilizing the bpy(Mes)2 ligand was reported. The flexible
bpy(Mes)2 ligand meets the requirements to form heteroleptic
complexes similar to its rigid phenanthroline counterparts.
Metal complexes of dppz-based ligands have been extensively
studied due to their interesting photophysical properties. For
example, [Ru(bpy)2(dppz)]2+ is non-emissive in aqueous solution,
however, upon addition of double-helical DNA, the solution
becomes strongly emissive [27]. A series of spectroscopic and the-
oretical investigations revealed the interesting photophysical
properties of dppz-type complexes are due to energetically similar
unoccupied molecular orbitals, which occupy different areas of the
ligand [28,29]. In dppz-type complexes, the lowest lying MOs may
have phenanthroline character, b1(phen) (LUMO+1 in dppz) in
which the electron density is predominantly across the A, B and
C rings, or phenazine character, b1(phz) (LUMO in dppz), in which
amplitude is on the B, D and E rings [30] (Fig. 2). On the basis of
these close lying MOs, a number of excited states for a complex
such as [Ru(bpy)2(dppz)]2+ may be formed. For example, the tran-
Fig. 2. Dppz ligand structure with ring labels and numbering, and phenazine (phz)
LUMO and phenanthroline (phen) LUMO+1.
not affect its binding affinity to transition metal cations. The pres-
ence of the bright MLCT phen-based state is a consequence of the
favorable radiative decay from this state. Variable temperature
studies [35] have revealed that the MLCT dppz phen-based state
for [Ru(phen)2(dppz)]2+ is entropically favoured whereas the MLCT
phz is the enthalpically favoured state. The MLCT phz state has a
low radiative decay rate due to the poor metal dp/phz overlap,
and a high non-radiative decay rate because of the greater extent
of reorganization due to the larger charge separation in this state.
In a previous study we reported three dppz ligands, with sulfur-
based moieties at the 11- and 12-position, along with their
{Re(CO)3Cl} complexes [36]. Substitution of dppz with thiocyanate
(SCN) groups results in typical behavior of an electron-withdrawing
group. However, substitution of dppz with the electron-donating
trithiocarbonate (S2CS) or deca-alkylthioether groups (Sdec), pro-
duces a new electronic transition, atypical of dppz-type systems,
attributed to intra-ligand charge transfer (ILCT) from the sulfur ad-
duct to the phenazine-based lowest unoccupied molecular orbital
(LUMO). Upon complexation of the substituted dppz ligand to
{Re(CO)3Cl} this ILCT red-shifts and increases in intensity.
sitions Ru dp ? phen and Ru dp ? phz occur and these are termed
MLCT phen and MLCT phz, respectively. The MLCT to phz is a non-
emissive ‘dark state’ and the MLCT to phen is an emissive ‘bright
state’.
Substitution of the dppz backbone has been used to alter the
relative energies of the LUMO and LUMO+1 (phen and phz MOs),
in an attempt to tune the photophysical properties. For example,
introduction of electron-withdrawing moieties such as bromine
substituents at the 2- and 7-positions (phen end) stabilizes the
‘bright state’ (MLCT phen), to an extent that no further relaxation
to the dark MLCT phz is possible [31]. Substitution at the 11- and
12-positions result in greater alteration of the energy of the
phenazine-based LUMO compared to that of the phen-based LUMO
[32–34]. Addition of groups at the phz end of the molecule does
Using resonance Raman spectroscopy in concert with TD-DFT
calculations, it is revealed that these new transitions are a mixture
of MLCT and S ? phz ILCT in nature. In effect, negative charge is
being donated to the phz-based LUMO from two directions; the
Fig. 1. General structure of [Cu(bpy(Mes)2)(dppz-R)]BF4 complexes presented in this paper.

M.G. Fraser et al. / Polyhedron 52 (2013) 623–633
625
metal d
p
and the S units. This assignment is possible because the
C, 67.80; H, 6.60; N, 6.86%. M/z (ESI POS, Calc., expt.) 1081.5025,
nature of the electronic transitions may be probed using resonance
Raman spectroscopy [37–43]. Within the resonance condition, in
which the excitation laser wavelength is coincident with an elec-
tronic absorption of the molecule of interest, an enhancement of
the Raman scattering signal is observed [44]. This band enhance-
ment is not random, but shows specific mode enhancements
depending on the nature of the resonant electronic transition
[45]. In heteroleptic complexes the nature of the absorbing chro-
mophores may be discerned through resonance Raman experi-
ments as the ligand bearing the acceptor orbital shows
enhancement of vibrational modes from that ligand, while the
vibrational signature of the other ligands present remains unen-
hanced and thus only shows a weak signal [42,43]. By examining
the resonance Raman spectra of the complexes reported herein
an assignment of the nature of absorbing chromophores may be
made [46]. Such an analysis may be greatly strengthened by using
the frequency calculations for the molecule of interest, which al-
lows the assignment of enhanced bands to specific modes and thus
ligands; and the TD-DFT calculations that provides insight into the
energy ordering and oscillator strength of the electronic transitions
[47,48].
In this paper we explore the effects of binding the
{Cu(bpy(Mes)2}+ moiety to the dppz ligands. The solid-state struc-
ture of the four complexes are studied by X-ray crystallography,
each complex displays unique features including Cu(I) geometry
and packing interactions. The oxidation potentials of the Cu(I)
complexes reflect the extent of steric bulk around the Cu(I) center.
The ligand-based reduction potentials display the electron-
donating/withdrawing effect of the sulfur-based substituent on
the phz LUMO. We have shown that binding of a {Cu(bpy(Mes)2}+
moiety to the dppzS2CS and dppz(Sdec)2 ligands results in a red-
shift and increase in intensity of the intraligand charge transfer,
comparable to binding of a {Re(CO)3Cl} fragment. These transitions
are analyzed using TD-DFT calculations and resonance Raman
spectroscopy.
1081.5064 {Cu(bpy(Mes)2)(dppz(Sdec)2)}+.
2.1.2. [Cu(bpy(Mes)2)(dppz(SCN)2)]BF4ꢀCH2Cl2
Bulk sample recrystallized by diethyl ether diffusion into a
dichloromethane solution of the complex. Crystals suitable for X-
ray crystallography were grown by slow diffusion of ether into
dichloromethane solution. Yield = 85 mg (81%). 1H NMR
(400 MHz, (CD3)2SO) d 9.62 (2H, d, J = 8 Hz), 9.01 (2H, s), 8.82
(4H, m), 8.38 (2H, t, J = 8 Hz), 8.09 (2H, dd, J = 4.8, 8 Hz), 7.70
(2H, d, J = 8 Hz), 5.86 (4H, s), 1.70 (12H, s), 1.09 (6H, s). Microanal-
ysis expected for C48H36N8S2CuBF4ꢀCH2Cl2: C, 57.45; H, 3.74; N,
10.94. Found: C, 57.69; H, 3.90; N, 11.14%. M/z (ESI POS, Calc.,
expt.) 851.1800, 851.1856 {Cu(bpy(Mes)2)(dppz(SCN)2)}+.
2.1.3. [Cu(bpyMes)(dppzS2CS)]BF4ꢀ0.5CH2Cl2
Bulk sample recrystallized by diethyl ether diffusion into a
dichloromethane solution of the complex. Crystals suitable for X-
ray crystallography were grown by slow evaporation of a methanol
solution. Yield = 90 mg (86%). 1H NMR (400 MHz, CDCl3) d 9.61
(2H, d, J = 8 Hz), 8.63 (4H, m), 8.52 (2H, s), 8.28 (2H, t, J = 8 Hz),
7.95 (2H, dd, J = 8 Hz, J = 5.2 Hz), 7.47 (2H, d, J = 8 Hz), 5.90 (4H,
s), 1.74 (12H, s), 1.22 (6H, s). Microanalysis expected for C47H36
N6S3CuBF4ꢀ0.5CH2Cl2: C, 58.58; H, 3.83; N, 8.63. Found: C, 58.93;
H, 3.69; N, 8.91%. M/z (ESI POS, Calc., expt.) 843.1459, 843.1556
{Cu(bpy(Mes)2)(dppzS2CS)}+.
2.1.4. [Cu(bpy(Mes)2)(dppz)]BF4ꢀCH2Cl2
Recrystallized by diethyl ether diffusion into a dichloromethane
solution of the complex. Yield = 74 mg (89%). 1H NMR (400 MHz,
CDCl3) d 9.68 (2H, d, J = 8 Hz) 8.66 (2H, d J = 8 Hz), 8.45 (2H,
J = 6.4, 3.2 Hz), 8.29 (2H, dd, J = 7.6, 0.8 Hz), 5.89 (4H, s), 1.74
(12H, s), 1.19 (6H, s). Microanalysis calculated for C46H38N6CuBF4-
ꢀCH2Cl2: C, 62.02; H, 4.43; N, 9.24. Found: C, 61.68; H, 4.46; N,
9.31%. M/z (ESI POS, Calc., expt.) 737.2454, 737.2445
{Cu(bpy(Mes)2)(dppz)}+.
2.2. Physical measurements
2. Experimental
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 ac-
2.1. Materials and reagents
[Cu(CH3CN)4]BF4 was synthesized by the literature method
from commercially available Cu(BF4)2ꢀxH2O [49]. Sulfur-substi-
tuted dppz ligands were synthesized by published methods [36].
The four Cu(I) complexes were synthesized in a similar manner,
the details are given for [Cu(bpy(Mes)2)(dppz(Sdec)2)]BF4 as an
example.
quired with a resolution of 4 cmꢁ1
.
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 exci-
tation 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
2.1.1. [Cu(bpy(Mes)2)(dppz(Sdec)2)]BF4
Forty milligrams (0.1 mmol) of bpy(Mes)2 was dissolved in
20 mL of degassed CH2Cl2. To this solution, 32 mg (0.1 mmol) of
[Cu(CH3CN)4]BF4 (in 5 mL CH3CN), was then added at room tem-
perature and the solution turned yellow. This was stirred for
5 min then 64 mg (one equivalent) of dppz(Sdec)2 (in 15 mL CH2-
Cl2) was added and the solution immediately turned dark red. This
was stirred at room temperature for 30 min then the solvent was
removed under reduced pressure. The resulting red powder was
recrystallized from methanol. Yield = 106 mg (89%). Crystals suit-
able for X-ray crystallography were grown by slow evaporation
of an ethanol solution. 1H NMR (400 MHz, CDCl3) d 9.61 (2H, d,
J = 8 Hz), 8.66 (2H, d, J = 8 Hz), 8.56 (2H, d, J = 5.2 Hz), 8.28 (2H, t,
J = 8 Hz), 8.04 (2H, s), 7.89 (2H, dd, J = 4.8, 8 Hz), 5.88 (4H, s),
3.28 (4H, t, J = 8 Hz), 1.94 (4H, t, J = 8 Hz), 1.73 (12H, s), 1.62 (4H,
m), 1.30 (24H, m), 1.20 (6H, s), 0.89 (6H, t, J = 5.6 Hz). Microanaly-
sis expected for C66H78N6S2CuBF4: C, 67.76; H, 6.72; N, 7.19. Found:
was 25 mW with a beam diameter of approximately 300 lm. The
incident beam and the collection lens were arranged in a 135°
backscattering geometry to reduce Raman intensity loss by self-
absorption [50]. An aperture matched lens was used to focus scat-
tered light through a narrow-bandpass filter (Ruggate), to remove
the Rayleigh scattering, and a quartz wedge (Spex), to remove
polarization bias, and onto the 50 lm entrance slit of a spectro-
graph (Acton Research SpectraPro 500i). The collected light was dis-
persed in the horizontal plane by a 1200 grooves/mm ruled
diffraction grating (blaze wavelength 500 nm) and detected by a li-
quid nitrogen cooled, back-illuminated Spec-10:100B CCD con-
trolled by a ST-133 controller and WinSpec/32 (version 2.5.8.1)
software (Roper Scientific, Princeton Instruments) [39,41,51,52].
Wavenumber calibration was performed using Raman bands
from a 1:1 v/v toluene/acetonitrile sample. Peak positions were

626
M.G. Fraser et al. / Polyhedron 52 (2013) 623–633
reproducible to within 1–2 cmꢁ1. Spectra were obtained with a
resolution of 5 cmꢁ1 [53,54]. Freshly prepared samples were held
in a spinning NMR tube. Typically concentrations were 1 mmol Lꢁ1
in CH2Cl2.
methods and refined on F2 by use of all data in full-matrix least
squares procedures. All non-hydrogen atoms were refined
anisotropically.
Argon purged, spectroscopic grade solvents were used for all
spectroscopic measurements. Spectral data was analyzed using
GRAMS/AI 8.00 (Thermo Electron Corporation) and OriginPro 7.5
(OriginLab Corporation).
3. Results and discussion
3.1. General
The heteroleptic Cu(I) complexes are synthesized by the HET-
PHEN method developed by Schmittel [12]. Stoichiometric
amounts of the Cu(I) precursor, [Cu(CH3CN)4)]BF4 and bpy(Mes)2
ligand are mixed in dichloromethane solution. The steric bulk of
the mesityl groups prevents the formation of the homoleptic
Cu(I) complex of the bpy(Mes)2 ligand. Subsequent addition of
the dppz ligand results in the desired heteroleptic complex. The
complexes are air-stable solids and dichloromethane solutions of
complexes are stable for at least a week. The 1H NMR spectra of
the complexes provide an effective diagnostic peak due to the for-
mation 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 around 5.8 ppm.
Upfield shifts upon coordination relative to the free ligand
(6.98 ppm), are unusual for aromatic protons, and are due to the
2.3. Computational methods
Calculations to determine geometry, vibrational spectra and
electronic spectra were performed using density functional theory
with the B3LYP functional and the 6-31G(d) basis set. A LANL2DZ
core potential was used to model the copper atoms. All calculations
were carried out using the GAUSSIAN 09 software package [55]. For
theoretical Raman vibrational energies a scale factor of approxi-
mately 0.97 was used, this was found by minimizing mean abso-
lute deviation (MAD) between calculated and experimental
bands [56]. Intensity correction for calculated spectra was applied
to convert the Raman activity given by the GAUSSIAN 09 program to
Raman scattering cross sections [57–59]. For the jth mode the dif-
ferential Raman cross section (@rj/@X) is directly proportional to
the observed band intensity. This is related to the Raman activity,
Sj, given by the GAUSSIAN 09 frequency calculation (Gaussian Key-
word: Freq = Raman) as follows:
shielding of the aromatic protons by the
gand [19].
p system of the dppz li-
0
1
!
3.2. Structural studies
ꢀ
ꢁ
4
@
rj
X
24p4
45
ð
m0
ꢁ
mj
Þ
h
@
A
h
i
¼
Sj
ð1Þ
@
8p
2cmj
ꢁhcmj
Crystals of suitable quality were able to be grown for each of the
complexes studied. The crystallographic details are displayed in
Table S1 in the supporting information. Perspective views of each
of the complexes are shown in Fig. 3. Each structure shows unique
features regarding the coordination geometry and intra- and inter-
1 ꢁ exp
kT
where
m
0 is the laser excitation frequency and
m
j is the frequency of
the jth mode.
2.4. X-ray crystallography
molecular p interactions. Features include severe distortion of the
Cu coordination sphere from an ideal tetrahedron geometry. A
quantitative method of characterizing four-coordinate geometries
is the s4 geometry index proposed by Houser [60]. This uses the
X-ray crystal structure measurements were made with a Bruker
diffractometer equipped with an Apex II charge-coupled device
(CCD) area detector using graphite monochromated Mo
(k = 0.71073 Å) radiation. The structures were solved by direct
K
a
formula
s
4 = {360° ꢁ (h +
u)}/141° where h and u are the two larg-
est angles in the four coordinate geometry. The s4 values range
Fig. 3. Coordinative flexibility of Cu(I) complexes. Perspective views of (a) [Cu(bpy(Mes)2)(dppz(Sdec)2)]BF4, (b) [Cu(bpy(Mes)2)(dppzS2CS)]BF4, (c) [Cu(bpy(Mes)2)
(dppz(SCN)2)]BF4, and (d) [Cu(bpy(Mes)2)(dppz)]BF4. Hydrogen atoms, solvent and anions removed for clarity.

M.G. Fraser et al. / Polyhedron 52 (2013) 623–633
627
Table 1
s4 parameters and longest N–Cu–N bond angles for the complexes.
[Cu(bpy(Mes)2(dppz)]BF4
[Cu(bpy(Mes)2(dppz(SCN)2)]BF4
[Cu(bpy(Mes)2(dppz(S2CS)]BF4
[Cu(bpy(Mes)2(dppz(Sdec)2)]BF4
Largest angles
N(2)–Cu–N(3) = 131.17°
N(2)–Cu–N(4) = 134.55°
N(2)–Cu–N(4) = 144.42°
N(4)–Cu–N(1) = 125.19°
N(3)–Cu–N(1) = 127.59°
N(2)–Cu–N(4) = 125.57°
N(2)–Cu–N(4) = 146.45°
N(1)–Cu–N(4) = 125.23°
s4
0.669
0.641
0.757
0.626
from 1 for a perfect tetrahedral (largest angles of 109.5°), 0 for
square planar (largest angles of 180°) and 0.85 for a perfect trigonal
pyramid (largest angles of 120°), intermediate geometries fall
within the range of 0–1. The s4 value for typical Cu(I) bisdiimine
complexes such as [Cu(DMP)2]+ and [Cu(6,6-dimethylbipyri-
dine)2]+ are around 0.75. The largest angles in these complexes
are ꢂ125°. The smallest angles, around 85°, are the N–Cu–N angle
of the five-membered chelate ring.
orthogonal, with the angle between the two coordination planes
of 72°. A major influence on how this complex packs is of course
the deca-alkyl chains, with the complex molecules packing in such
a way that the alkyl chains aggregate together (Fig. S2 supplemen-
tary information).
[Cu(bpy(Mes)2)(dppz(SCN)2)]BF4 crystallized with one dichlo-
romethane solvent molecule per cation. Interestingly, examination
of the BF4 anion revealed a similar issue of residual electron den-
sity around the boron atom as encountered in the [Cu(bpy(Mes)2)
(dppz)]BF4 structure. This was modeled as a 25% occupancy
chloride anion. In the [Cu(bpy(Mes)2)(dppz(SCN)2)]BF4 structure,
the BF4 anion is modeled at 80% occupancy with the residual elec-
tron density modeled as a 20% chloride anion. Similar to the
[Cu(bpy(Mes)2)(dppz(Sdec)2)]BF4 structure, intramolecular stack-
ing between the dppz ligand and a mesityl ligand is accompanied
with twisting of ligands away from orthogonal D2d symmetry.
The angle between the coordination planes is 71.25°. The two thio-
cyanate substituents are near co-planar to the dppz ligand, with
torsion angles out of the E-ring plane of 9° and 6°. In the free ligand
one SCN group is near co-planar with a torsion angle of 2°, the
other SCN group is near orthogonal to the dppz ligand with a tor-
sion angle of 85°. The distinguishing feature of the intermolecular
packing interactions in the [Cu(bpy(Mes)2)(dppz(SCN)2)]BF4 struc-
ture is the offset head-to-tail p-stacking between two dppz(SCN)2
ligand moieties (Fig. S3 supplementary information). The distance
between the pyrazine ring centroid and the plane through phz
atoms of the adjacent cation is 3.40 Å. The two p-stacked dppz li-
gands are offset by 1.48 Å. Similar dimers between dppz complexes
have been observed in other dppz complexes [61].
The [Cu(bpy(Mes)2)(dppzS2CS]BF4 structure is unique in that
the dppz ligand does not interact solely with one mesityl ring,
but both of the mesityl rings, which ‘clamp down’ on the dppz li-
gand. The clamping effect of the mesityl rings is shown by the con-
tracted N–C–C bond angles around the sp2 hybridized carbon of
113° and 114°. In comparison the [Cu(bpy(Mes)2)(dppz)]BF4 struc-
ture, where the dppz ligand interacts with one mesityl ring only,
the corresponding angles are 118° and 119°. The dppz ligand lying
evenly between the two mesityl rings confers a coordination
geometry similar to that of [Cu(DMP)2]+, resulting in a s4 value of
0.757.
The s4 values for the complexes in this study are displayed in
Table 1. All four of the complexes’ s4 values are indicative of dis-
torted trigonal–pyramidal geometry. This is primarily due to intra-
molecular
p-interactions between the dppz ligand and one or both
of the mesityl rings. The variation of the
s4 values indicates the dif-
ferences in these intramolecular interactions. Each complex also
has different intermolecular packing interactions as a result of
the nature of the substituents. The 1H NMR (vide supra), UV–Vis
absorption and oxidation potentials (vide infra) are consistent with
the complexes adopting the expected tetrahedral Cu(I) geometry in
the solution phase, and presumably, therefore, the dppz ligand is
either located more centrally between the mesityl rings, or shifts
back and forth rapidly between each mesityl ring.
[Cu(bpy(Mes)2)(dppz)]BF4 crystallized in the monoclinic space
group P21/n with one disordered dichloromethane solvent mole-
cule per cation, which was modeled with a 85/15 occupancy over
two sites. The fluorine atoms of the BF4 anion modeled well. How-
ever, the temperature factor for the boron atom indicated a signif-
icant amount of residual electron density. This was modeled as a
25% occupancy chloride anion, and the BF4 75%, which rectified
the issue with the boron atom. Presumably the adventitious chlo-
ride ions originated from impurities in the Cu(BF4)2 starting mate-
rial or solvents used in the synthesis or recrystallization steps. The
solid state packing of [Cu(bpy(Mes)2)(dppz)]BF4 displays intramo-
lecular
of the mesityl rings. The distance between the centroid of the
stacking mesityl ring and the plane of the ‘phen’ atoms of the dppz
ligand is 3.41 Å, well within range for stacking. The dppz li-
gand is near parallel to the -stacked mesityl ring, with an angle
of 4.05° between their respective planes. The intimate intramolec-
ular -stacking between the dppz ligand and the mesityl ring cre-
ates a ‘void’ between the dppz ligand and the non -stacked
p–p-stacking interactions between the dppz ligand and one
p
-
p–p
p
p
p
mesityl ring, in effect a ‘pac-man’-like assembly. This ‘void’ is filled
by an adjacent cation, as shown in Fig. S1 (supplementary informa-
tion), with close intramolecular interactions between the bpy moi-
ety of the neighboring cation. Another feature of this structure is
the unusual twisting of the aromatic backbone of the dppz ligand.
The outer phenyl ring is twisted 13° relative to a plane through the
‘phen’ atoms of the ligand.
The complexes form dimers via complementary intermolecular
-interactions between a mesityl ring of complex A and an adja-
p
cent dppzS2CS ligand (B), with a centroid to plane distance of
3.6 Å. The mesityl moiety of the second complex B binds to the
dppzS2CS ligand of complex A (Fig. S4 supplementary information).
To summarize, each of the complexes display intramolecular
interactions between the dppz ligand and mesityl ring(s). The
Cu(I) coordination geometries are distorted from an ideal tetrahe-
dral geometry as a result of these interactions. The flexible
bpy(Mes)2 ligand twists and results in a departure from orthogonal
arrangement between the two diimine ligands, as shown by the s4
values. Each complex displays unique intermolecular interactions,
in part determined by the sulfur substituents.
[Cu(bpy(Mes)2)(dppz(Sdec)2)]BF4 was crystallized by slow
ꢀ
evaporation of an ethanol solution in the triclinic space group P1.
A disordered BF4 anion was modeled over two sites with 70/30
occupancy. A 70% occupancy ethanol solvent molecule is also dis-
ordered over two sites (50/20). [Cu(bpy(Mes)2)(dppz(Sdec)2)]BF4
does display intramolecular interaction between one mesityl ring
and the dppz ligand. The distance between the centroid of the
p
-
The electronic structure of the ligands also has a role to play in
the crystal packing. For [Cu(bpy(Mes)2)(dppz)]BF4 the ligand
stacks with one of the mesityl rings, this is also observed when
electron withdrawing groups are present, as in [Cu(bpy(Mes)2)
stacking mesityl ring and the plane of the ‘phen’ atoms of the dppz
ligand is 3.38 Å. However, compared to the [Cu(bpy(Mes)2)
(dppz)]BF4 complex, the {Cu(bpy(Mes)2}+ moiety is twisted from

628
M.G. Fraser et al. / Polyhedron 52 (2013) 623–633
(dppz(SCN)2)]BF4. With the introduction of electron-donation sub-
stituents to the dppz framework, two types of structures are ob-
served. In the case of [Cu(bpy(Mes)2)(dppzS2CS)]BF4 the dppz is
more electron rich and the ligand balances between the two mesi-
tyl groups – furthermore the mesityl groups lies over the S2CS
group of the neighboring complex. This is not observed in the crys-
tal structure of Cu(bpy(Mes)2)(dppz(Sdec)2)]BF4 which may be be-
cause of the extensive interactions in this structure between the
alkyl chains of neighboring complexes.
3.4. Electronic absorption spectra and TD-DFT calculations
In a previous study we reported the electronic properties of the
dppz-based ligands studied in this work, as well as their respective
{Re(I)(CO)3Cl} complexes. The electronic absorption spectra of the
dppzS2CS and dppz(Sdec)2 ligands showed their lowest energy
absorption (427 and 434 nm respectively) significantly red-shifted
from unsubstituted dppz (378 nm). Using TD-DFT calculations and
resonance Raman spectroscopy we assigned these transitions as
charge transfer transitions from the sulfur moieties to the
phenazine LUMO. However, the dppz(SCN)2 ligand shows an
absorption spectrum of a typical dppz ligand that is appended with
electron-withdrawing groups. The {Re(I)(CO)3Cl} complex of
dppz(SCN)2 is typical of a Re-dppz complex, with the Re(I)-based
MLCT transition manifested as a weak tail to the red of the dppz-
based ligand-centered transition. The {Re(I)(CO)3Cl} complexes of
dppzS2CS and dppz(Sdec)2 show absorption spectra atypical for
Re-dppz complexes. The low energy band, assigned as a charger-
transfer from the sulfur moiety to the phenazine LUMO, further
red-shifts and grows in intensity upon complexation of the rhe-
nium tricarbonyl moiety. Analysis of the resonance Raman band
enhancements and the calculated molecular orbitals indicated
the nature of the unique transitions is a mixture of MLCT and
S ? phenazine charge transfer.
The absorption spectra of the complexes presented in this work
are shown in Fig. 4. [Cu(bpy(Mes)2)(dppz(SCN)2)]+ is characterized
by the Cu(I) MLCT transition at 471 nm. The ligand centered bands
are only slightly red-shifted and of similar intensity to that of the
free ligand. The lowest energy transition of the {Cu(I)(bpy(Mes)2)}
complexes of dppzS2CS and dppz(Sdec)2 show a substantial in-
crease in intensity from that of the free ligand. The Cu(I) MLCT
transition manifests as a shoulder on the intense transition. For
[Cu(bpy(Mes)2)(dppz(Sdec)2)]+ a much broader band exists from
300 to 350 nm compared to that of the free ligand (narrow band
at 290 nm).
3.3. Electrochemistry
The complexes were studied by cyclic voltammetry in dry,
nitrogen-purged dichloromethane; the half potentials of the oxida-
tion and reduction processes are given in Table 2.
The half-wave oxidation potentials are dependent on the extent
of steric bulk around the Cu(I) center. This is in accordance with
the change of preferred geometry from Cu(I) (tetrahedral) oxidiz-
ing to Cu(II) (square-planar). The increased steric bulk around
the Cu(I) coordination sphere stabilizes the Cu(I) center to oxida-
tion [62]. This is exemplified by comparing the oxidizing potentials
of [Cu(1,10-phenanthroline)2]+ (0.5 V versus SCE) and [Cu(2,9-di-
methyl-1,10-phenanthroline)2]+ (0.83 V versus SCE) in dichloro-
methane solution. The coordination geometry around the Cu(I)
center is the same in each of these complexes (in solution), one
would expect similar oxidation values between the complexes, as
is observed. These values are comparable to complexes with
similar coordination environments published by Odobel and co-
workers [24].
The reduction potentials are assigned as ligand based reduction
of the phz MO. It is well established that the change in reduction
potential of dppz-type systems is a good indicator of alteration of
the phz LUMO energy level by the substituent [33,34,63]. The order
of Ered in the complexes reflects the electron donating or with-
drawing nature of the substituents as established in our previous
study concerning a series of sulfur-substituted dppz ligands and
their Re(I)(CO)3Cl complexes [36]. It is interesting to note that
the Sdec and S2CS substituents, although electron donating, extend
the conjugation of the phz-like MO, resulting in a lowering of
reduction potentials for the ligands and their requisite complexes.
Indeed, in the case of S2CS the stabilizing effect of the increased
conjugation is almost as great as substitution with the electron-
withdrawing SCN substituents. In the case of dppz(SDec)2 the
extension of conjugation is much more limited because it can only
be provided by the sulfur atoms. Furthermore it has been previ-
ously noted that metal oxidation potentials are mute to substituent
effects at the phz portion of dppz-type ligands because the metal
communicates only with the phen-based MO that is less affected
by such substituents [33,34,36,64].
It appears that for the dppzS2CS and dppz(Sdec)2 ligands bind-
ing of a {Cu(I)bpy(Mes)2}+ moiety results in similar changes (red-
shift and increase of intensity) to the absorption bands as binding
a {Re(I)(CO)3Cl} moiety.
3.5. Resonance Raman Spectroscopy – TD-DFT calculations
The electronic spectra of metal complexes with dppz ligands are
dominated by two types of transitions. For dppz and ligands with
electron-withdrawing groups there are a group of intense ligand-
centered transitions that may be tuned by the substituent from
364 to 410 nm [34,64]. In addition at lower energy there are MLCT
transitions that may present as distinct bands, in the case of ruthe-
nium(II) complexes [28,65] or as red edge shoulders on the ligand-
centered transitions in the case of complexes with {Cu(PPh3)2}+ or
{Re(CO)3Cl} moieties [34]. The MLCT transitions terminate on the
dppz(phen) MO although in the case of electron-withdrawing sub-
stituents on the E-ring some phz character is apparent in the elec-
tronic excitation as evidenced by the resonance enhancement of
modes associated with the electron-withdrawing substituents
[32,38].
Table 2
Oxidation and reduction potentials.
Eox
Ered
Free ligand
reduction
D
Rb
The electronic spectra of dppz ligands with sulfur substitution
at the E-ring show intense low energy bands that are associated
with a bi-directional charge transfer transition of MLCT and
S ? phz character [36]. Therefore, in the systems studied herein
where there are two different ligands present, the bpy mesityl
and dppz, there are quite a number of possible transitions because
of the electronic structure of the substituted dppz ligand.
The electronic absorption spectra of the complexes show the
presence of a number of states. In the case of [Cu(bpy(Mes)2)
[Cu(bpy(Mes)2)(dppz(SCN)2)]BF4
[Cu(bpy(Mes)2)(dppzS2CS)]BF4
[Cu(bpy(Mes)2)(dppz(Sdec)2)]BF4 0.81
0.84a ꢁ0.70 ꢁ0.85
0.15
0.14
0.13
0.05
0.83
ꢁ0.84 ꢁ0.98
ꢁ1.10 ꢁ1.23
ꢁ1.23 ꢁ1.28
ꢁ1.25 ꢁ1.28
[Cu(bpy(Mes)2)(dppz)]BF4
0.83
c
[Cu(PPh3)2(dppz)]BF4
Potentials recorded against DMFc+/DMFc and converted to vs. SCE. Solvent
CH2Cl2, supporting electrolyte TBAPF6.
The decrease in reduction potential upon coordination; Ered(complex)–Ered(free
ligand).
a
b
c
Previously studied, included for comparison.
(dppz)]+ there are strong dppz
p ?
p⁄ transitions at 377 and

M.G. Fraser et al. / Polyhedron 52 (2013) 623–633
629
Fig. 4. Electronic absorption spectra of the Cu(I) complexes recorded in CH2Cl2.
360 nm and a broad, weaker transition at 471 nm that may be as-
signed as the Cu ? bpy mesityl MLCT. This MLCT transition is
weaker than that observed in [Cu(dmp)2]+ (where dmp = 2,9-di-
methyl-1,10-phenanthroline) because of the effects of the 2- and
9-substituents which shorten the dipole length and weaken that
transition [66]. A consequence of this is a diminution of the reso-
nance Raman effect as the effect is dependent on the extinction
coefficient of the resonant transition [67,68]. The electronic transi-
tion assignments are in agreement with the TD-DFT calculations
although an additional Cu ? dppz MLCT transition is predicted
which is difficult to see experimentally because it is convoluted
with the dominant dppz
p ?
p⁄ bands [34,64].
In order to effectively use the resonance Raman data one can
calculate the frequencies and normal Raman spectra of the com-
plexes and compare these to the experimental data [48]; in addi-
tion to assisting in the band assignments this is also a potent test
of the efficacy of the calculation. The goodness-of-fit between the
experimental and calculated Raman data may be parameterized
by the mean absolute deviation (MAD) [69–72].
Fig. 5 shows this comparison of [Cu(bpy(Mes)2)(dppz)]BF4; this
has a MAD of 8.5 cmꢁ1 based on comparison of the experimental
values that are greater than 20% of the maximum observed
intensity, to a calculated spectrum. The MADs obtained for the
other complexes are 8.2, 4.2 and 9.7 cmꢁ1 for [Cu(bpy(Mes)2)
(dppz(SCN)2)]BF4,
[Cu(bpy(Mes)2)(dppzS2CS)]BF4
and
[Cu
(bpy(Mes)2)(dppz(Sdec)2)]BF4 respectively, the experimental and
calculated spectra are provided in the supplementary information
(Figs. S5–S7). These are sufficiently small to suggest that the calcu-
lations are providing a good model of the complexes.
Fig. 5. Comparison between the experimental and calculated FT-Raman spectra of
The resonance Raman spectral data for selected bands with
their respective assignments are presented in Table 4; the reso-
nance Raman spectra for [Cu(bpy(Mes)2)(dppzS2CS)]+ are
shown in Fig. 6, and [Cu(bpy(Mes)2)(dppz)]+, [Cu(bpy(Mes)2)
(dppz(SCN)2)]+ and [Cu(bpy(Mes)2)(dppz(Sdec)2)]+ are provided
in the supporting information Figs. S8–S10. For all of the com-
plexes bands are predicted and observed at approximately 1015
and 1315 cmꢁ1, which are assigned as bpy modes. In the case of
[Cu(bpy(Mes)2)(dppz)]BF4.
ence of the Cu ? bpy MLCT transition as being the lowest-energy
electronic transition for all of the complexes. In addition dppz
modes at 618 and 765 cmꢁ1 for [Cu(bpy(Mes)2)(dppz)]+ show
strong preferential enhancement at higher energies (kexc = 351
through to 413 nm); these are assigned to dppz modes and are
[Cu(bpy(Mes)2)(dppz)]+,
[Cu(bpy(Mes)2)(dppz(SCN)2)]+
and
characteristic of dppz p ?
p⁄ ligand-centered excitation [64].
[Cu(bpy(Mes)2)(dppzS2CS)]+ these show preferential enhancement
with red excitation (kexc = 488 and 514.5 nm). These findings are
consistent with the TD-DFT calculations, which indicate the pres-
The [Cu(bpy(Mes)2)(dppzS2CS)]+ and [Cu(bpy(Mes)2)(dppz(S-
dec)2)]+ show similar band enhancements to those observed for
[Cu(bpy(Mes)2)(dppz)]+; but they have additional features that

630
M.G. Fraser et al. / Polyhedron 52 (2013) 623–633
the molecular orbital nature of the resonant electronic transitions.
The enhancement of mode 155 (1080 cmꢁ1) over mode 157
(1099 cmꢁ1) in the UV wavelengths was also observed in the
[Re(CO)3Cl(dppzS2CS)] complex. The enhancement pattern indi-
cates different orbital parentage between the transitions probed,
even though both involve the S2CS substituent. The mode en-
hanced in the UV (1080 cmꢁ1) is in resonance with the electronic
transition calculated at 314 nm (experimental 309 nm – Table 3),
and the acceptor MO configuration is the LUMO+4. The mode en-
hanced with 448 and 457 nm excitation is in resonance with the
electronic transition calculated at 408 nm, and the major orbital
configuration is Hꢁ3 ? LUMO. If one assumes a C2v microsymme-
try for the dppzS2CS ligand in the complex then a transition involv-
ing Hꢁ3 and LUMO is symmetry allowed as the both donor and
acceptor orbitals have b1 symmetry (Fig. 7).
These acceptor orbitals are akin to the calculated MOs responsi-
ble for the same observation in the Re(CO)3Cl complex. Mode 155
(1080 cmꢁ1) is a symmetric stretch of the S2CS moiety and as such
is consistent with the population of L+4 (L+3 in the rhenium com-
plex) [36], as this MO is antibonding across all S2CS bonds. Mode
157 (1099 cmꢁ1) is an asymmetric stretch, consistent with the
phz LUMO, which has non-bonding character around the C–S but
antibonding across the C@S. The same argument is presented for
the {Re(CO)3Cl} complex in our previous publication.
For [Cu(bpy(Mes)2)(dppzS2CS)]+, two strong transitions are pre-
dicted at 410 and 408 nm respectively. These are assigned to the
strong feature in the experimental UV–Vis spectra at 434 nm.
The dominant configurations are, Hꢁ1 ? L+3 (410 nm) and
Hꢁ3 ? LUMO (408 nm). The occupied MOs in this transition differ;
Fig. 6. Resonance Raman spectra of [Cu(bpy(Mes)2)(dppzS2CS)]BF4 in CH2Cl2 at a
range of excitation wavelengths. FT-Raman spectra of powder sample acquired at
1064 nm.
Hꢁ3 is a ligand based MO with high sulfur character, Hꢁ1 is a d
p
metal orbital. The LUMO is phz-based and L+3 is a dppz delocalized
MO. These calculations imply that the strong band centered at
434 nm in the experimental UV–Vis spectrum is due to a combina-
tion of S ? phz ILCT and Cu ? dppz MLCT transitions (Fig. 8).
To the red of the intense band at 434 nm, the Cu ? bpy(Mes)2
MLCT is present as a shoulder in the UV–Vis spectrum, and this
assignment is confirmed by the enhancement of a bpy band at
1318 cmꢁ1, exclusively in the low energy wavelength spectra
(488 and 514 nm). This is accounted for in the TD-DFT calculation,
the lowest calculated transition with substantial oscillator strength
are associated with the strong electronic transition observed at
about 435 nm. In the case of [Cu(bpy(Mes)2)(dppzS2CS)]+ this is
quite striking in the band pair at 1080 and 1099 cmꢁ1 correspond-
ing to modes 155 and 157 respectively. Mode 155 is only enhanced
in the UV, with 351 and 356 nm excitation; it has no discernible
signal with redder excitation. Conversely mode 157 is only
strongly enhanced in the red. These two modes are calculated to
be associated with the S2CS moiety and they provide insight into
Table 3
Experimental and calculated electronic data for the complexes.
Complex
Experimental
k (nm)a
482
Calculated
b
e
(Mꢁ1 cmꢁ1
8300
)
k (nm)c
fd
Configuration (%)e
Assignment
[Cu(bpy(Mes)2)(dppzS2CS]BF4
451
0.265
Hꢁ1 ? L+1 (20)
H ? L+1 (41)
dp
? bpy(Mes)2
434
309
47000
53000
410
408
314
0.10
0.517
0.35
Hꢁ1 ? L+3 (37)
Hꢁ3 ? LUMO (85)
Hꢁ9 ? L+4 (30)
Hꢁ7 ? L+4 (12)
d
p
? dppz delocalized
S ? phz
dppz ? dppz,S
p
[Cu(bpy(Mes)2)(dppz(SCN)2)]BF4
[Cu(bpy(Mes)2)(dppz(Sdec)2)]BF4
471
401
7600
455
408
0.156
0.10
Hꢁ1 ? L+2 (30)
HOMO ? L+1 (67)
Hꢁ6 ? LUMO
HOMO ? L+1 (66)
HOMO ? LUMO (39)
Hꢁ1 ? LUMO (55)
Hꢁ3 ? L+3
d
d
p
p
? bpy(Mes)2
? phen
19000
dppz
? bpy(Mes)2
IL/MLCT
(d ,S ? phz)
dppz
S ? phen
? bpy(Mes)2
dppz
p ?
p⁄
482
442
12000
39500
503
459
426
341
0.002
0.133
0.232
0.647
dp
p
309
53500
p
?
p⁄
[Cu(bpy(Mes)2)(dppz)]BF4
471
377
5470
10100
448
338
0.169
0.217
HOMO ? L+1 (53)
Hꢁ11 ? LUMO (77)
dp
p
?
p⁄
a
b
c
Experimental absorption maxima.
Experimental extinction coefficient (L molꢁ1 cmꢁ1).
Calculated absorption maxima.
d
e
Calculated oscillator strength.
Molecular orbital coefficients, major components (% contribution).

M.G. Fraser et al. / Polyhedron 52 (2013) 623–633
631
Table 4
Experimental and calculated (in parenthesis) FT-Raman data for selected bands, including vibrational modes and assignments.
mexpt (cmꢁ1) (mcalc (cmꢁ1), mode number)
Assignment
CuDppz
CuDppzSCN
CuDppzS2CS
CuDppzSdec
639 (622, 94)
621 (616, 89)
Dppz phz
Dppz phen
Dppz phz
Bpy
Dppz phen
CS3
708 (702, 88)
784 (778,113)
782 (777, 107)
1011 (1002, 138)
1011 (1001, 129)
1013 (1009, 145)
1047 (1057, 160)
1080 (1074, 155)
1099 (1092, 157)
CS3
1181 (1175, 169)
1183 (1182, 171)
1205 (1192, 172)
1239 (1225, 175)
Bpy
Bpy
Dppz phen & phz
Bpy mes
Dppz phen & phz
Bpy
1239 (1254, 163)
1315 (1329, 178)
1253 (1260, 177)
1282 (1265, 178)
Dppz phen
Bpy
1316 (1309, 190)
1326 (1321, 191)
1318 (1317, 186)
1317 (1316,192)
Dppz phen
Dppz phz
Dppz phz
Bpy mes
Dppz phz
Dppz phen
Dppz E-ring
Delocalized
Bpy mes
Delocalized
Dppz phen
Dppz phz
Bpy
1340 (1344, 180)
1360 (1353, 181)
1381 (1381, 182)
1401 (1401, 183)
1442 (1434, 194)
1399 (1384, 195)
1446 (1440, 208)
1475 (1473, 213)
1486 (1478, 215)
1403 (1418, 204)
1446 (1454, 211)
1448 (1450, 204)
1465 (1447, 196)
1478 (1480, 214 & 215)
1531 (1544, 236)
1590 (1595, 241)
1558 (1573, 217)
1590 (1595, 223)
1576 (1577, 225)
Dppz phen
Asymmetric E-ring
Fig. 7. Acceptor orbitals involved in S2CS band enhancement.
Fig. 8. Molecular orbitals involved in IL/MLCT transition of [Cu(bpy(Mes)2)(dppzS2CS)]BF4.
at 451 nm (f = 0.265) is dominated by Cu d
figurations, (Hꢁ1 ? L+1, HOMO ? L+1).
p
? bpy(Mes)2 p⁄ con-
The enhancement of the 1099 cmꢁ1 S2CS mode with the red
excitation may be a consequence of the fact that the IL/MLCT

632
M.G. Fraser et al. / Polyhedron 52 (2013) 623–633
Fig. 9. Molecular orbitals involved in IL/MLCT transition of [Cu(bpy(Mes)2)(dppz(Sdec)2)]BF4.
transition is very strong (
e
> 40000 mol Lꢁ1 cmꢁ1) and thus excita-
acid substituted bpy(Mes)2)Åꢁ in nature, and thus would afford
tion at 514.5 nm still shows preresonance that is competitive in
terms of intensity enhancement with the resonance of the Cu to
good injection kinetics.
mesityl MLCT (
e
ꢃ 5000 mol Lꢁ1 cmꢁ1). However, the enhance-
4. Conclusions
ment of the S2CS modes in the red excitation, may be due to the
Cu ? bpy(Mes)2 MLCT having some sulfur character, which is not
inconsistent with the TD-DFT results.
In this paper we report the synthesis of four stable heteroleptic
Cu(I) complexes, with bpy(Mes)2 and four dppz ligands, using the
HETPHEN strategy. The complexes are characterized by X-ray crys-
tallography, each with unique intra- and inter-molecular packing
interactions and Cu(I) coordination geometry as established by
the s4 index. The Cu(I/II) oxidation potentials reflect the similarity
between the complexes of their coordination environment in solu-
tion. The complexes with electron-donating substituents appended
to the dppz ligand [Cu(bpy(Mes)2)(dppz(Sdec)2)]BF4 and
[Cu(bpy(Mes)2)(dppzS2CS)]BF4 display interesting optical proper-
ties, as previously observed in Re(I) complexes. These transitions
were further investigated with resonance Raman spectroscopy
and TD-DFT calculations. These techniques show a series of elec-
tronic transitions some of which, the most intense in the visible re-
gion, being the IL/MLCT type.
The resonance Raman spectra and the FT-Raman spectrum for
[Cu(bpy(Mes)2)(dppz(Sdec)2)]+ are presented in the supplementary
information (Figs. S6 and S10). Examination of the spectra with
488 and 514.5 nm excitation show similar features to
[Cu(bpy(Mes)2)(dppzS2CS)]+, with enhancement of a bpy mode at
1317 cmꢁ1 (mode 192), consistent with the shoulder at 482 nm
in the experimental UV–Vis spectrum being assigned as
Cu ? bpy(Mes)2 MLCT. This is accounted for in the TD-DFT calcula-
tion, (HOMO ? L+1, 503 nm).
The resonance Raman spectra of [Cu(bpy(Mes)2)(dppz(Sdec)2)]+
with 457 and 448 nm excitation results in enhancement of modes
associated with the E-ring of the dppz ligand (1446 cmꢁ1) and also
the phz portion of the molecule (1220 cmꢁ1). Excitation with 457
and 448 nm wavelengths is in resonance with the strong band cen-
tered at 442 nm in the experimental UV–Vis spectrum. Two calcu-
lated transitions with strong oscillator strength are associated with
this band, (459 nm and 426 nm), the dominant configurations
being Hꢁ2 ? LUMO and HOMO ? LUMO. Inspection of these orbi-
tals (Fig. 9) reveals the nature of the strong transition to be IL/
MLCT, in that electron density is being donated from both the sul-
fur portion of the ligand, and the metal ion, to the phz based LUMO.
Enhancements of E-ring and phz modes along with calculated elec-
tronic transitions indicating IL/MLCT transitions was also observed
in the corresponding {Re(CO)3Cl} complex.
In essence, these systems offer the possibility of forming dyes in
which the oxidation potential is decoupled from the optical transi-
tion. The redox potential is determined by the distortion around
the copper center and possible effects of the phen-based MO,
whereas the optical properties are dominated by the interaction
of the S donor and phz MO. It may be possible to utilize these
compounds in dye-sensitized solar cells by modifying the
bpy(Mes)2 ligand to incorporate carboxylate groups. The effect of
this would be to dramatically lower the energy of the bpy-type
MO but the complex would still retain the optical transitions from
the dppzS-donor ligand. This would have a potential advantage of
creating a lowest energy excited state that was Cu(II)(carboxylic
Appendix A. Supplementary data
CCDC 876957–876960 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: de-
posit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2012.08.001.
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