Spectroscopic and electrochemical properties of heteroleptic cationic copper complexes bis-(diphenylphosphino)alkane-(2,2′-biquinoline)copper(I) . Crystal structure of bis(diphenylphosphino)ethane-(2,2′-biquinoline) copper(I) perchlorate

Article abstract of DOI:10.1016/j.ica.2010.07.032

A series of [Cu^(I)(2,2′-biquinoline)(L)](ClO₄) complexes (L = bis(diphenylphosphino)methane (bppm), 1,2-bis(diphenylphosphino) ethane (bppe), 1,4-bis(diphenylphosphino)butane (bppb)) have been synthesized and characterized by elemental analysis, conductivity, ESI-mass, NMR and UV-Vis spectroscopies, cyclic voltammetry, X-ray diffraction ([Cu^(I)(2, 2′-biquinoline)(bppe)](ClO₄)) and DFT calculations. These compounds are monometallic species in a distorted tetrahedral arrangement, in contrast with related compounds found as dinuclear according to diffraction studies. The spectroscopic properties are not directly correlated with the length of alkyl chain bridge between the bis-diphenylphosphine groups. In this way, the chemical shift of some 2,2′-biquinoline protons and the metal to ligand charge transfer (Cu to 2,2′-biquinoline) follows the order [Cu(2,2′-biquinoline)(bppm)](ClO₄), [Cu(2,2′-biquinoline) (bppb)](ClO₄), [Cu(2,2′-biquinoline)(bppe)](ClO₄). The same dependence is followed by the potentials to Cu(II)/Cu(I) couple. These results are discussed in terms of inter-phosphorus alkane chain length and tetrahedral distortions on copper.

Full text of DOI:10.1016/j.ica.2010.07.032

Inorganica Chimica Acta 363 (2010) 3809–3816
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Inorganica Chimica Acta
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Spectroscopic and electrochemical properties of heteroleptic cationic copper
complexes bis-(diphenylphosphino)alkane-(2,20-biquinoline)copper(I). Crystal
structure of bis(diphenylphosphino)ethane-(2,20-biquinoline)copper(I) perchlorate
a
a
a
a
b
Juan Guerrero a, , Luis Cortez , Luis Lemus , Liliana Farías , Juan Costamagna , Claudio Pettinari ,
*
Miriam Rossi c, Francesco Caruso d
a Faculty of Chemistry and Biology, Universidad de Santiago de Chile, Santiago 33, Chile
b Dipartimento di Scienze Chimiche, Università di Camerino, via S. Agostino 1, 62032 Camerino, MC, Italy
c Vassar College, Department of Chemistry, Poughkeepsie, NY 12604-0484, USA
d Instituto di Chimica Biomolecolare (ICB-CNR), Università di Roma ‘‘La Sapienza”, Istituto Chimico, Ple Aldo Moro 5, 00185 Rome, Italy
a r t i c l e i n f o
a b s t r a c t
A series of [Cu(I)(2,20-biquinoline)(L)](ClO4) complexes (L = bis(diphenylphosphino)methane (bppm), 1,2-
bis(diphenylphosphino)ethane (bppe), 1,4-bis(diphenylphosphino)butane (bppb)) have been synthe-
sized and characterized by elemental analysis, conductivity, ESI-mass, NMR and UV–Vis spectroscopies,
cyclic voltammetry, X-ray diffraction ([Cu(I)(2,20-biquinoline)(bppe)](ClO4)) and DFT calculations. These
compounds are monometallic species in a distorted tetrahedral arrangement, in contrast with related
compounds found as dinuclear according to diffraction studies. The spectroscopic properties are not
directly correlated with the length of alkyl chain bridge between the bis-diphenylphosphine groups. In
this way, the chemical shift of some 2,20-biquinoline protons and the metal to ligand charge transfer
(Cu to 2,20-biquinoline) follows the order [Cu(2,20-biquinoline)(bppm)](ClO4), [Cu(2,20-biquino-
line)(bppb)](ClO4), [Cu(2,20-biquinoline)(bppe)](ClO4). The same dependence is followed by the poten-
tials to Cu(II)/Cu(I) couple. These results are discussed in terms of inter-phosphorus alkane chain
length and tetrahedral distortions on copper.
Article history:
Received 25 April 2010
Received in revised form 30 June 2010
Accepted 12 July 2010
Available online 21 July 2010
Keywords:
Heteroleptic copper (I) complexes
Biquinoline and bis-
diphenylphosphinealkane ligands
Spectroscopic properties
X-ray crystal structure
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
in homoleptic copper(I) complexes with phenanthroline ligands,
the geometry varies from D2d to D2 by loss of symmetry, modifying
The interest in studying the behavior of coordination com-
pounds of copper(I) with polypyridine ligands arises from their
application either in catalysis [1], bioinorganic systems [2], and
photochemical research [3] among other areas [4]. Several studies
have shown that in four-coordinated copper(I) complexes, either
their electronic properties as well as their reactivity (as a conse-
quence of the former), are affected by dynamic behavior related
to the conformational flexibility of these complexes in solution.
In this way, distortions of the ideal tetrahedral coordination geom-
etry toward an extreme square-planar geometry, drive to a diver-
sity of processes, such as: modification of redox properties,
decoordination of ligands and in some cases equilibria between dif-
ferent copper(I) species [5].
the energy of the HOMO, as shown through the shift of metal to
ligand charge transfer band in their electronic spectra [3e,6].
The geometry of copper(I) complexes may also be modulated by
ligands allowing a less restricted way of coordination around the
metal center and potential distortion of the tetrahedral arrange-
ment [3,7].
Considering a conformation distant from the ideal tetrahedron,
it is expected that a more distorted geometry would reduce the
necessary energy to reach a square-planar geometry, which is opti-
mum for Cu(II) complexes, modifying favorably the oxidation
potentials when the variable factor is the flattening conformation
in a series of copper(I) complexes [7,8]. For example, photo-phys-
ical studies show that the flattening distortion in polypyridine
copper(I) complexes reduces their emissive properties, which can
be improved by the use of more rigid ligands or with substituents
that avoid distortions from the tetrahedral geometry [6].
On the other hand, heteroleptic copper(I) complexes show
interesting photo-physical properties, when chelating diphos-
phines are used together to polypyridine ligands [3c,9]. Also, ter-
tiary phosphines have been selected as ligands to play a major
The effect of coordination geometry over properties of copper(I)
complexes can be understood through the modification of the
molecular orbitals as the ligands induce distortions of the ideal
tetrahedral structure required by the copper(I) ion. For example,
* Corresponding author. Tel.: +56 2 7181057.
E-mail address: juan.guerrero@usach.cl (J. Guerrero).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.07.032

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J. Guerrero et al. / Inorganica Chimica Acta 363 (2010) 3809–3816
role in the stabilization of low oxidation states in transition met-
als [5b].
compartment cell, provided with a glass carbon working electrode,
a Pt auxiliary electrode and a AgCl/Ag reference electrode. The CV
measurements were performed at room temperature and N2 atmo-
sphere, using solutions 1 ꢁ 10ꢀ3 mol/L of the complexes and
0.10 mol/L of tetraethylammonium perchlorate as supporting elec-
To evaluate the role of ligand flexibility in the conformation
of copper(I) complexes, we have studied three heteroleptic com-
plexes, where the ligands are the high electron-delocalized
2,20-biquinoline (biq), and a series of three chelating alkyl diphos-
phines with inter-phosphorus bridge of variable length: bis-(diphen-
ylphosphine)methane (bppm); 1,2-bis(diphenylphosphine)ethane
(bppe); 1,4-bis(diphenylphosphine)butane (bppb), see Fig. 1.
The effect of structural factors such as distortions of the tetrahe-
dral environment of copper(I) and the length of the inter-phosphorus
alkane chain, among others are presented and discussed to explain
the changes observed in NMR, UV–Vis spectra and cyclic voltam-
mograms for this series of copper(I) complexes.
trolyte in CH3CN as solvent, and scan rates of 20 and 100 mV sꢀ1
.
The potentials were informed as E1/2 versus Ag/AgCl.
Red crystals of [Cu(I)(biq)(bppe)](ClO4) were grown by slow
diethyl ether diffusion into a solution of the complex in dichloro-
methane, under N2 atmosphere. A Bruker Apex-2 diffractometer
was used with Mo K
a graphite-monochromatized radiation to
collect data on a suitable crystal at 125 K. Diffracted beams were
measured using a CCD detector. Crystal and data collection de-
tails are given in Table 1. The structure was solved by the hea-
vy-atom method using CAOS [10] and refined by full-matrix
least-squares procedures on F. Hydrogen atoms were located by
model according to C–H = 0.96 Å and refined riding on their at-
tached atoms.
2. Experimental
The theoretical study involved calculations using software pro-
grams from ACCELRYS [11]. Density functional theory (DFT) code
DMol3 was applied to calculate energies and optimize geometries;
graphical displays were generated with Materials Studio Visualizer
[12]. We employed Double Numerical Polarized (DNP) basis set
that includes all the occupied atomic orbitals plus a second set of
valence atomic orbitals plus polarized d-valence orbitals [11]. Cor-
relation generalized gradient approximation (GGA) was applied in
the manner suggested by Perdew–Burke–Ernzerhof (PBE) [13].
Spin unrestricted approach was exploited with all electrons being
considered explicitly. The real space cutoff of 4 Å was imposed
for numerical integration of the Hamiltonian matrix elements.
The self-consistent-field convergence criterion was set to the
root-mean square change in the electronic density to be less than
10ꢀ6 e Åꢀ3. The convergence criteria applied during geometry opti-
mization were 2.72 ꢁ 10ꢀ4 eV for energy and 0.054 eV Åꢀ1 for
force. Calculations were performed using X-ray coordinates of
the dppe compound as starting data. The minimum obtained was
later modified to establish a methylene or a tetramethylene bridge
for the dppm and dppb complexes, respectively. In the latter case,
the bridge was widely varied to avoid being trapped in minima of
higher energy. The minimum nature of these converged structures
was checked by calculating the corresponding frequencies and so
data shown in Table 3 do not have any associated imaginary
frequency.
2.1. Instrumental
Analysis of C, H and N were performed using a Fisons element
analyzer, model EA-1108. Molar conductivities were obtained from
10ꢀ3 mol/L solutions in acetone at 25 °C using a Cole-Palmer con-
ductivimeter, model 01481. The positive and negative electrospray
mass spectra were obtained with a Series 1100 MSI detector HP
spectrometer, using an acetonitrile mobile phase. Solutions
(3 mg/mL) for electrospray ionization mass spectrometry (ESI-
MS) were prepared using reagent-grade acetonitrile.
Electronic spectra were measured on an Analytik Jena Specord
S-100 spectrophotometer at room temperature, from 1.5 ꢁ 10ꢀ5
mol/L solutions in acetone, tetrahydrofurane and chloroform.
1H, 31P{1H} NMR, 1H–1H 2D-COSY and 1H–1H 2D-NOESY spectra
were recorded on
a Bruker Avance 400 MHz spectrometer
(400.133 MHz for 1H, 160.984 MHz for 31P) equipped with a
5 mm multinuclear broad-band dual probehead, incorporating a
z-gradient coil. All the measurement were done in CDCl3 at
300 K. Chemical shifts (in ppm) for 1H, were calibrated respect to
the residual protonated signal of the solvent (CHCl3, 7.26 ppm)
and reported relative to Me4Si. 31P{1H} spectra were calibrated re-
spect to the external pattern H3PO4 10%.
Cyclic voltammetry measurements were carried out using a
Voltalab PGZ-100 equipment, supplied with the analyzer software
Voltamaster-4. The electrolytic cell used was a conventional three-
Fig. 1. Structure and numbering for protons of the ligands using in NMR assignments.

J. Guerrero et al. / Inorganica Chimica Acta 363 (2010) 3809–3816
3811
Table 1
taining 0.60 mmol of the corresponding bis(diphenylphos-
phine)alkane in dry dichloromethane under stirring and N2
atmosphere at room temperature. After 1 h, 0.038 g of 2,20-biquin-
oline (0.15 mmol) dissolved in 20 mL of dichloromethane were
added dropwise. After the addition was completed (30 min
approximately), the mixture was stirred for 4 h at room tempera-
ture. The orange solution was quickly filtered over powder cellu-
lose and concentrated under reduced pressure to approximately
2 mL, then cooled to 4 °C followed by addition of cold diethyl ether
to give the [Cu(I)(biq)(bis(diphosphine)alkane)](ClO4) complexes as
a microcrystalline precipitate. Ether in excess was removed under
reduced pressure.
X-ray diffraction data of [Cu(I)(2,20-biquinoline)(1,2-bis(diphenylphosphino)ethane)]-
(ClO4).
Crystal description
Cell setting
Space group
Formula weight
Chemical formula
a (Å)
prism
monoclinic
P21/n
817.69
C44H36ClCuN2O4P2
14.8285(5)
16.4564(6)
15.6263(5)
92.486(4)
3809.6(4)
4
b (Å)
c (Å)
b (°)
V (Å3)
Z
Crystal size (mm)
0.36 ꢁ 0.27 ꢁ 0.21
d (g/cm3)
1.426
2.3.1. [Cu(I)(biq)(bppm)](ClO4)
l
(mmꢀ1
)
0.774
T (K)
125
Yellow crystals, yield: 60%. Molar conductivity (acetone):
103.0 S cm2 molꢀ1. M.p. 127 °C (dec.). Anal. Calc. for (CuC43H34-
N2O4P2Cl): C, 64.26; N, 3.49; H, 4.26. Found: C, 64.47; N, 3.42; H,
4.67%. Mass spectra: m/z = 703.1. 31P NMR (CDCl3): ꢀ16.71 ppm.
F(0 0 0)
h Range (°)
h, k, l
Reflections refined I > 2
Parameters
1688
1.80–30.25
0 > h > 20, 0 > k > 23, ꢀ21 > l > 21
r(I)
5996
487
R1, wR2 (all data)
0.035, 0.062
0.030, ꢀ0.120
0
2.3.2. [Cu(I)(biq)(bppe)](ClO4)
Residuals (e ÅAꢀ3
)
Red crystals, yield: 75%. Molar conductivity (acetone):
102.2 S cm2 molꢀ1. M.p. 183 °C (dec.). Anal. Calc. for (CuC44H36-
N2O4P2Cl): C, 64.63; N, 3.43; H, 4.44. Found: C, 64.98; N, 3.37; H,
4.77%. Mass spectra: m/z = 717.2. 31P NMR (CDCl3): ꢀ10.10 ppm.
Table 2
Selected bond distances (Å) and angles (°) of [Cu(I)(2,20-
biquinoline)(1,2-bis(diphenylphosphino)ethane)](ClO4).
2.3.3. [Cu(I)(biq)(bppb)](ClO4)
Distances
Cu–N1
Cu–N2
Cu–P1
Cu–P2
C35–C36
2.028(2)
2.027(3)
2.246(1)
2.235(1)
1.473(5)
Orange crystals, yield: 80%. Molar conductivity (acetone):
105.7 S cm2 molꢀ1. M.p. 144 °C (dec.). Anal. Calc. for (CuC46H40-
N2O4P2Cl): C, 65.32; N, 3.31; H, 4.77. Found: C, 65.25; N, 3.31; H,
4.88%. Mass spectra: m/z = 745.2. 31P NMR (CDCl3): ꢀ12.50 ppm.
Angles
N1–Cu–N2
P1–Cu–P2
N1–Cu–P1
N1–Cu–P2
N2–Cu–P1
N2–Cu–P2
81.0(1)
3. Results and discussion
92.92(3)
119.09(6)
126.17(7)
112.22(7)
127.85(7)
The reaction of one equivalent of [Cu(CH3CN)4](ClO4) with two
equivalents of the respective bis(diphenylphosphino)alkane ligand
(Fig. 1), afforded the [Cu(I)(bis(diphenylphosphine)alkane)2](ClO4)
complexes in colorless solutions. The addition of one equivalent
of 2,20-biquinoline dissolved in dichloromethane yielded colored
solutions, from which the complexes were obtained as microcrys-
talline powder by precipitation with cold diethyl ether.
Conductivity measurements show a 1:1 ionic ratio for all the
complexes, which is in agreement with mass results (see above)
for mononuclear species. The complexes are air stable both in solid
and in solution phase in most of the commonly used solvents, do
not experiencing chemical changes in solution during the time re-
quired for each experiment, as it has been reported for related sys-
tems [5,15].
Table 3
Selected angles (°) and bond lengths (Å) of [Cu(I)(2,20-biquinoline)(bis(diphenylphos-
phino)alkane)](ClO4) obtained with DFT or X-ray methods.
Alkyl method
Methyl DFT
Ethyl X-ray
Ethyl DFT
Butyl DFT
DHA
81.4
76.0
81.0
2.337
2.333
2.060
2.060
5.6
85.6
84.7
91.7
80.5
2.288
2.290
2.070
2.072
6.9
81.9
109.3
79.5
2.270
2.299
2.090
2.114
12.8
P–Cu–P
N–Cu–N
Cu–P1
Cu–P2
Cu–N1
Cu–N2
IQDH
92.92(3)
81.0(1)
2.246(1)
2.235(1)
2.028(2)
2.027(3)
10.4
3.1. Description of the crystal structure
2.2. Synthesis and characterization
The crystal structure shows a discrete mononuclear [Cu(biq)-
(bppe)]+ cation complex, with the perchlorate ion located between
the aromatic rings of 2,20-biquinoline and 1,2-bis-diphosphine
ligands. The cation coordination geometry corresponds to a dis-
torted CuN2P2 tetrahedron. A side view of the molecule showing
anisotropic ellipsoids and labelling is depicted in Fig. 2.
Selected bond angles and bond distances are given in Table 2.
The average Cu–N and Cu–P distances are 2.028(2) and
2.240(1) Å, respectively, and are within the observed range for sim-
ilar heteroleptic copper(I) species containing substituted phen-
anthrolines instead of biquinolines [9,16], whose variation is in
the Cu–N range from 2.02 to 2.07 Å [17], while in [Cu(L)2]+
complexes, L = 3,30-substituted-2,20-biquinoline, the Cu–N bond
[Cu(CH3CN)4]ClO4 was prepared according to the literature pro-
cedure [14]. All the solvents were dried and purified by standard
methods. 2,20-Biquinoline and alkyl-diphenylphosphines (Aldrich)
reagents were used as received. The acetonitrile used for electro-
chemical studies was dried with CaH2 and distilled prior to use.
2.3. General synthesis procedure for the
[Cu(biq)(bis(diphenylphosphine) alkane)](ClO4) complexes
About 0.050 g of [CuI(CH3CN)4](ClO4) (0.15 mmol) dissolved in
dichloromethane were added to a 100 mL three-neck reactor con-

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J. Guerrero et al. / Inorganica Chimica Acta 363 (2010) 3809–3816
lengths vary according to the ligand substituent. The Cu–P
distances in [Cu(biq)(bppe)]+ are slightly different to each other
(Cu–P1 = 2.235(1) and Cu–P2 = 2.246(1) Å), but they fall in the
range of reported data for related bis-diphosphine copper com-
plexes. For example, in the closely related cationic complex
(1,3-bis(diphenylphosphino)propane)-(2,9-dimethyl-1,10-phenan-
throline)-copper(I) hexafluorophosphate, they are 2.261(1) and
2.227(1) Å, confirming the difference between Cu–P bond lengths.
Interestingly, in the less symmetric complex [Cu(pymtH)(dppp)Cl]
3.2. DFT calculations
The DFT optimized geometry of the [Cu(biq)(bppe)]+ complex
shows differences with its X-ray crystal structure, such as wider
dihedral angle (DHA) and shorter Cu–P and Cu–N bond lengths
than in the crystal structure, see Table 3. These may be due to a
constrained structure in the crystal because of packing. An addi-
tional source of difference may be the electrostatic influence from
the perchlorate environment, which is missing in the isolated
cationic species from DFT calculation.
(pymtH = pyrimidine-2-thione,
dppp = bis(diphenylphosphine)-
propane) both Cu–P bond lengths are equal, 2.276(1) Å [18], and
so a more symmetric complex displays different Cu–P bonds
whereas for a more asymmetric complex there is Cu–P bond length
equivalence.
The bite angle N1–Cu–N2, 81.0(1)°, is comparable to that ob-
served in the heteroleptic complex [Cu(dmp)(PPh3)2]+, 80.5° (9)
(dmp = 2,9-dimethyl-1,10-phenanthroline). The dihedral angle
(DHA) of the complex, defined between the planes N1-Cu–N2 and
P1-Cu–P2, is 85.6°, indicating deviation from the ideal tetrahedral
value of 90°, expected for a copper(I) geometry. The interquinoline
dihedral angle between planes of both quinoline moieties (IQDHA)
is 10.4°. Table 3 indicates distortion from planarity for this
ligand.
In addition, the torsion angle N1–C35–C36–N2 is 6.1°. This value
is lower than those in the literature observed for several complexes;
for example 8.3° in RuCl2(NBD)(biq) (NBD = Norbornadiene), 17.7°
in [Re(CO)3(biq)(py)](CF3SO3) (py = pyridine) and 12.1° and 17.4°
for both isomers of [Re(CO)3(biq)(Bzpy)](CF3SO3) (Bzpy = benzyl-
pyridine) [19].
On the other hand, tension on the ethyl bridge can be observed
through the angles P1–C25–C26 = 112.2° and C25–C26–
P2 = 113.0°, as they are wider than the tetrahedral value. This
seems to be confirmed by the torsion angle P1–C25–C26–
P2 = 46.0° when compared with those of the two dppp complexes
mentioned earlier that have two torsion angles larger than 70° and
so the ethyl complex seems to have a more flattened alkyl link than
the n-propyl ligand.
In the X-ray structure it is also noticeable a greater planarity
distortion in the quinoline moieties of the [Cu(I)(biq)(bppe)](ClO4)
complex, (IQDHA of 10.4°) also observed in other complexes
[19,20]. This is also probably due to crystal packing that is missing
in the isolated DFT structure (IQDHA of 6.1°).
Despite the differences between DFT and X-ray structures of
[Cu(I)(biq)(bppe)]+, useful information can be deduced when the
optimized geometries of the three complexes are compared. First,
no correlation is observed between the calculated dihedral angles,
DHA, with the length of the inter-phosphorus chain (Table 3), hav-
ing the bppm and bppb complexes similar DHA values.
In addition, the tension on the aliphatic inter-phosphorus
bridge can be understood in terms of the angles in hetero-cycles
formed by Cu–P2–Cn atoms as shown in Fig. 3. For example, devia-
tion from the ideal 109.5° value for the carbon atom are in the
order: bppm (P–C–P = 100.53°) > bppb (P–(C)4–P = 114–118°) >
bppe (P–(C)2–P = 112.64°) (Fig. 3). The tension on the aliphatic
bridge could be responsible of the rearrangement process imposed
on the complexes by the phosphine ligand, where the bppm
complex should be the more restricted. This trend is also observed
in solution phase, both spectroscopically (NMR and UV–Vis) and
by cyclic voltammetry (see below).
3.3. NMR spectroscopy
Proton signal assignments were carried out by the concerted
use of 1D and 2D NMR experiments that included 2D-COSY and
Fig. 2. X-ray crystal structure of the complex for [Cu(I)(2,20-biquinoline)(1,2-bis(diphenylphosphino)ethane)](ClO4) showing anisotropic ellipsoids and H atoms omitted for
clarity.

J. Guerrero et al. / Inorganica Chimica Acta 363 (2010) 3809–3816
3813
Fig. 3. DFT results for phosphine ligands, including P-Cu–P, in [Cu(I)(2,20-biquinoline)(bis(diphenylphosphino)alkane)](ClO4) complexes.
2D-NOESY spectra. The chemical shift (d in ppm) and
D
d (dcom-
siderably shifted to lower field, c.a. 3.00 ppm compared to the free
plex ꢀ dbiq) data for 1H, of the free ligands and their corresponding
complexes are summarized in Table 4.
ligands, in agreement with the chemical shift observed in other
Cu(I) complexes containing phosphine ligands. For example, the
ꢀ10.10 ppm chemical shift observed for [Cu(I)(biq)(bppe)]+ is com-
parable to the ꢀ10.56 ppm chemical shift previously reported for a
related Cu(I) azine complex with the same bppe ligand [9].
The aromatic proton signals of the phosphine ligands are poorly
differentiated compared to those in biquinoline ligand. However, it
1H NMR signals pattern for biquinoline ligands are well known
[19–22], corresponding to six aromatic resonances distributed in
two doublets for AX spin system formed by H3 and H4 protons,
and in a spin system formed by the remaining protons which are
observed as two doublets for H5 and H8, and two double-doublet
(observed as a pseudo triplet) for H6, H7, protons (Table 4). How-
ever, the chemical shift for each proton in 2,20-biquinoline is very
sensitive to the distribution of the rest of the ligands in a complex.
For this reason it was required 2D NMR experiments to obtain an
unambiguous signal assignment.
is possible to observe NOESY correlations between H and Ha in
a
two of the three complexes which can be used for structural
assignment in the complex.
Contrary to the deshielding effect over the 2,20-biquinoline pro-
tons expected by the metal complexation, some protons are high
Protons assignment of the coordinated biquinoline was per-
formed as follows: H4 and H5 were identified through dipolar cou-
pling provided by 2D-NOESY spectra. The AX system formed by H3
and H4 was then verified by scalar correlation in 2D-COSY spectra,
which also provides the connectivity for assignments of H5–H6–
H7–H8 protons.
The protons in the inter-phosphorus chain of the bis-biphos-
phine-alkane ligands appear at higher field, with a correct relative
integration for a mononuclear heteroleptic stoichiometry. How-
ever, these signals are broad and without fine structure (Fig. 4).
Similarly, the only signal observed in 31P{1H} NMR spectra of
bis-biphosphine-alkane ligands in the complexes, is also a broad
line that suggest a slow conformational motion of the alkane chain
on the NMR timescale. The 31P frequency in the complexes is con-
field shifted (see Dd in Table 4). H7 and H8 are the most shielded
protons because they are the closest ones to the copper atom.
The high sensitivity of these protons respect to the structure in
biquinoline complexes has been well established in a previous
work [19–22].
This behavior could be explained by the spatial arrangement of
both ligands in the complexes, where H7 and H8 protons would lie
in the shielded region of some aromatic rings of bis-biphosphines-
alkane ligand.
Based on the X-ray crystal structure of [Cu(I)(biq)(bppe)]+ and
DFT calculations, the biquinoline ligand has only a slight deviation
from coplanarity of their quinoline halves.
The X-ray structure suggests that the biphosphine is the
ligand experiencing most of the conformational rearrangement in
Table 4
1H NMR chemical shifts for ligands and complexes.
Compound
Chemical shifts/ppm (multiplicity)
Aromatic protons
Aliphatic inter-phosphorus bridge
H3
H4
H5
H6
H7
H8
H
a
Hb
H
c
Ha
Hb
2,20-Biquinoline
[Cu(biq)(bppm)](ClO4)
(bppm)
8.85 (d)
8.85 (d)
8.30 (d)
8.33 (d)
7.86 (d)
8.23 (d)
7.58 (t)
7.76 (t)
7.75 (t)
7.58 (t)
8.25 (d)
7.89 (d)
7.09 (m)
7.43 (m)
6.95 (t)
7.29 (m)
7.09 (m)
7.29 (m)
3.21 (s)
2.81 (s)
D
da
0
0.03
8.79 (d)
0.37
7.98 (d)
0.18
7.49 (t)
ꢀ0.17
ꢀ0.36
[Cu(biq)(bppe)](ClO4)
(bppe)
8.97 (d)
6.96 (t)
7.43 (d)
7.20 (d)
7.34 (m)
7.07 (t)
7.34 (m)
7.31 (t)
7.34 (m)
2.47 (dd)
2.10 (t)
D
d
0.12
8.74 (d)
0.49
8.61 (d)
0.12
7.87 (d)
ꢀ0.09
ꢀ0.79
ꢀ0.82
[Cu(biq)(bppb)](ClO4)
(bppb)
7.51 (t)
7.30 (t)
7.81 (d)
7.02 (d)
7.37 (m)
6.90 (t)
7.33 (m)
7.06 (t)
7.33 (m)
2.79 (t)
2.02 (t)
2.34 (t)
1.56 (m)
D
d
ꢀ0.11
0.31
0.01
ꢀ0.07
ꢀ0.45
ꢀ0.44
d are informed relative to Si(CH3)4 at 300 K in CDCl3.
a
D
d = dcomplex ꢀ dbiquinoline
.

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J. Guerrero et al. / Inorganica Chimica Acta 363 (2010) 3809–3816
2,20-biquinoline, due to their higher
p delocalization in comparison
with the phenyl groups of the biphosphine ligands [23]. In
addition, the Cu(I) to bis-biphosphine-alkane charge transfer are
expected to appears overlapped by the more intense intraligand
biquinoline absorption bands (about 350 nm).
The MLCT band of the heteroleptic complexes are shifted
50–100 nm toward short wavelengths compared to the homoleptic
[Cu(biq)2]+ complex (kmax TCML is 550 nm), in accordance to the
greater p-acceptor nature of phosphorus in the biphosphine ligand
respect to biquinoline [23a].
Furthermore Fig. 5 and Table 5, show a notorious shifting of me-
tal to ligand charge transfer band (MLCT) at longer wavelength in
the order [Cu(I)(biq)(bppm)](ClO4) < [Cu(I)(biq)(bppb)](ClO4) <
[Cu(I)(biq)(bppe)](ClO4).
Fig. 4. 1H NMR spectra for [Cu(I)(2,20-biquinoline)(bis(diphenylphosphino)alkane)]-
(ClO4) complexes recorded at 300 K in CDCl3.
Considering the MLCT as a process occurring between copper
and the biquinoline ligand, it can be seen that distortions imposed
by the bis-biphosphine-alkane ligand modify the energy level dis-
tribution in the complexes. Then, the red-shifting of MLCT must be
due to the HOMO destabilization which reduces the necessary en-
ergy to reach the excited states.
In addition, the tension imposed by the aliphatic bridge in the
diphosphine could restrict the conformational changes. Conse-
quently, it has not been observed a linear correlation between
lengths of the alkane chain and both the MLCT energy and 1H
NMR results (see above).
[Cu(I)(biq)(bppe)]+ complex in response to the coordination geom-
etry requirements of the copper. Furthermore, in the crystal struc-
ture Fig. 2, it can be observed that H7 and H8 are between two
phenyl rings of the bppe ligand, where the magnetic currents
shield both protons.
The
Dd values of H7 and H8 protons along the series of com-
plexes (see Table 4), show no direct correlation with the inter-
phosphorus chain length. In effect, the shielding for these protons
increases in the following order [Cu(I)(biq)(bppm)](ClO4) <
[Cu(I)(biq)(bppb)](ClO4) < [Cu(I)(biq)(bppe)](ClO4).
The chemical shifts of the H7 and H8 protons are in direct rela-
tion with their orientation toward shielding currents of phenyl
ring, which can be determined by effect of the alkyl bridge length
over the complex geometry. Consequently, the decreasing in the
3.5. Cyclic voltammetry
Electrochemical properties were determined by cyclic voltam-
metry in acetonitrile solution, under nitrogen at room temperature
frequency difference (Dd between H7 and H8 protons can be indic-
ative of a tetrahedral geometry distortion, which favor the collapse
of phenyl ring toward quinoline fragments in the order observed
(see above) for this series of complexes.
3.4. UV–Vis spectroscopy
Electronic spectra data of the complexes and ligands in acetone,
tetrahydrofurane, and chloroform are shown in Table 5.
The spectral region below 400 nm is dominated by intense
intraligand transition bands, which are similar to the spectrum
p
to
*
p
of the free biquinoline ligand [19–22]. These bands undergo a
red-shift upon complexation to copper(I). A lower energy metal
to ligand charge transfer band (MLCT) is observed in the range
400–500 nm for the complexes (Fig. 4), corresponding to a transi-
tion from the HOMO orbital centered in Cu atom (d10 full shell),
*
toward a
p -biquinoline orbital [23]. This assignment is supported
by the sensitivity observed for this band to the polarity of the
solvents and by their position in the spectra, in agreement
Fig. 5. UV–Vis spectra of the complexes in acetone, in the region of the metal to
ligand charge transfer band.
*
with the fact that the LUMO is a
p antibonding orbital of the
Table 5
UV–Vis data for the [Cu(I)(biq)L](ClO4) complexes in solution.
Ligand (L)
bppm
Solvent
kabs/nm (10ꢀ2 e/L molꢀ1 cmꢀ1
)
*
Cu(I) ? biq
(p
?
p
)
(CH3)2CO
THF
CHCl3
(CH3)2CO
THF
CHCl3
(CH3)2CO
THF
411 (7.82)
431 (sh)
430 (sh)
360 (82.06), 344 (66.51), 330 (sh)
360 (53.29), 340 (68.13), 326 (71.74)
361 (87.93), 346 (73.14)
291 (95.48), 253 (21.326)
270 (200.07)
bppe
486 (17.53)
484 (19.64)
476 (23.68)
450 (23.49)
456 (19.54)
451 (21.54)
361 (104.62), 343 (99.76), 332 (sh)
362 (117.99), 345 (11.037), 332 (sh)
362 (136.72), 346 (12.209), 333 (sh)
360 (147.98), 344 (12.678)
361 (136.27), 345 (11.451)
361 (149.74), 346 (12.589)
295 (148.21), 253 (15.532)
273 (23.724), 262 (sh)
bppb
292 (12.103), 253 (18.576)
271 (23.016)
CHCl3
sh: Shoulder.

J. Guerrero et al. / Inorganica Chimica Acta 363 (2010) 3809–3816
3815
Table 6
Cyclic voltammetry results of [Cu(I)(2,20-biquinoline)(bis(diphenylphosphino)alkane)]-
(ClO4) complexesa.
b
Complexes
E1/2 (V)
Reductionc
Oxidationd
[Cu(I)(biq)(bppm)](ClO4)
[Cu(I)(biq)(bppe)](ClO4)
[Cu(I)(biq)(bppb)](ClO4)
ꢀ1.57
ꢀ1.64
ꢀ1.55
ꢀ1.16
ꢀ1.15
ꢀ1.19
1.23
1.10
1.14
a
In acetonitrile at room temperature.
Potential measurements are referred to the Ag/AgCl, in 10ꢀ1 mol/L tetra-
b
ethylammoniunperchlorate/acetonitrile solution at room temperature.
c
E1/2 = ½(Epc ꢀ Epa) to scan rate of 20 mV sꢀ1
.
d
E1/2 = ½(Epc ꢀ Epa) to scan rate of 100 mV sꢀ1
.
in ꢀ2.00 to +2.00 V versus SCE potential range (Table 6). Fig. 6
shows a representative cyclic voltammogram for [Cu(I)(biq)(bppb)]
(ClO4).
The complexes show a single anodic peak for the Cu(I) oxidation
process around 1.20 V, the scan rate being 20 mV sꢀ1 [7,24]. Scan-
ning anodically from 0.00 to 2.00 V, only a cathodic peak is ob-
served for the Cu(II)/Cu(I) redox process. Scan rate dependence
suggest that the copper complexes are oxidized in an irreversible
diffusion-controlled step process [25]. These oxidation potentials
compare well to those reported for similar systems [24,25].
The potentials values observed for this process shows the fol-
lowing trend: [Cu(I)(biq)(bppe)](ClO4), +1.10 V; [Cu(I)(biq)(bppb)]
(ClO4), +1.14 V; [Cu(I)(biq)(bppm)](ClO4), +1.23 V. The trend in
the positive potential values is coincident with the spectroscopic
result previously discussed (UV–Vis and NMR).
Fig. 7.
DE1/2 vs. absorption maxima to MLCT.
observed between the energy of the MLCT band and
E1/2 = E1/2(oxidation) ꢀ E1/2(first reduction)) (Fig. 7).
It is expected that the first reduction potential be dependent on
D
E1/2, where
D
the coplanarity of the quinoline moieties, since greater interquino-
line dihedral angle (more distortion) drives to the loss of electronic
conjugation on the ligand and consequently more negative poten-
tial values [22,23]. Thus, the biq/biqÅꢀ couple is indicative of the
coplanarity degree of both quinolines halves. Therefore, the small
differences in the reduction potentials through the series is an-
other evidence that biquinoline is not the ligand that experience
the most important structural changes, while the conformational
rearrangement of the diphosphine ligands seems to be the main
responsible of variations in the HOMO energy along the complexes
series.
Optimized geometries support the fact that the bppm and bppe
complexes have similar IQDHA (5.6° and 6.9° respectively) and also
similar first reduction potential values (ꢀ1.16 and ꢀ1.15 V respec-
tively), while the wider calculated IQDHA of the bppb complex
(12.8°) could justify the more negative potential value.
Furthermore, shifting to more positive potential has been usu-
ally associated to an essentially tetrahedral surrounding around
the metal center [7,8].
The complexes show two successive reduction processes in
the 0.0 – ꢀ2.0 V range, which are centered in E1/2 = ꢀ1.17 V and
E1/2 = ꢀ1.60 V potential values. The first reduction couple shows
D
Ep values concordant with a quasi-reversible process, which cor-
*
responds to the addition of one electron to a
p
orbital of the coor-
dinated biquinoline ligand (biq/biqÅꢀ) in agreement with the
potential values reported for several biquinoline complexes. Also,
this fact is in agreement with the highest
p delocalization of
The more negative reduction process could be tentatively
assigned to the addition of a second electron to biquinoline
(biqÅꢀ/biqÅ2ꢀ), over other possible processes, such as phosphine
ligand or metal-centered Cu(I)/Cu(0) reductions. The difference
about 500 mV between the first and the second wave has been
used to support this assignation [19,20,22,23,26]. On the other
hand, the voltammograms do not show characteristic peaks which
could be assigned to a demetalation, Cu(I) to Cu(0), process.
biquinoline respect to phosphine ligands, supporting experimen-
*
p orbital of the biquinoline ligand. Further
support for this assignment comes from the good linear correlation
tally that LUMO is the
4. Conclusions
The experimental results obtained by cyclic voltammetry and
UV–Vis, processes involving charge transfer, either electrochemi-
cally or by light absorption, could be tentatively associated to the
ability of the phosphine to determine the global geometry of com-
plexes. This influence is corroborated by NMR results which con-
firmed the importance of these structural considerations.
The geometry of the complexes arises from two combined ef-
fects, the requirements imposed by the cation for a tetrahedral
coordination and the tension produced by length of the alkane
chain.
In summary, the experimental results are product of inter-phos-
phorus chain that induce several effects, that include bond dis-
Fig. 6. Cyclic voltammogram for [Cu(I)(2,20-biquinoline)(1,4-bis(diphenylphos-
tances, bond angles, modification in
p backbonding among
phine)butane](ClO4) in acetonitrile. Speed sweep, 20 mV sꢀ1
.
others, that influences the HOMO an LUMO orbitals, which are

3816
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Acknowledgments
We are grateful for the provided support by DICYT USACH,
FONDECYT No. 1101029 and MECESUP-0007 for NMR spectrometer.
US National Science Foundation through Grant 0521237, for X-ray
diffractometer.
Appendix A. Supplementary material
CCDC 775226 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.ica.2010.07.032.
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