Heteroleptic Cu(I) complexes containing phenanthroline-type and 1,1′-bis(diphenylphosphino)ferrocene ligands: Structure and electronic properties

Article abstract of DOI:10.1016/j.ica.2006.07.085

Three new heteroleptic Cu(I) complexes containing one phenanthroline and one diphosphine type ligand ([Cu(N-N)(P-P)]⁺) have been prepared. In particular, one ligand is constituted by 1,10-phenanthroline (1), 2,9-dimethyl-1,10-phenanthroline (2) and 2,9-diphenethyl-1,10-phenanthroline (3) and the other ligand is in all cases 1,1′-bis(diphenylphosphino)ferrocene (dppf). Therefore, copper and iron metal centres are quite close one another, as evidenced by X-ray crystal diffraction. The structure together with the electrochemical and photophysical properties of these complexes have been compared to that of the corresponding complexes where dppf has been replaced by bis[2-(diphenylphosphino)-phenyl]ether (POP). Cyclic voltammetric experiments evidenced that the first oxidation process is located on the ferrocene moiety and that oxidation of Cu(I) is moved to more positive potential values and a chemical reaction is coupled to the electron transfer process. The absorption spectra show a metal-to-ligand charge transfer (MLCT) band, typical of Cu(I) phenanthroline complexes, at a higher energy compared to the homoleptic [Cu(N-N)₂]⁺ species. No emission at either room temperature or 77 K has been observed for compounds 2 and 3, contrary to the high luminescence observed for the corresponding POP complexes. This result is consistent with a photoinduced energy transfer from the Cu(I) complex to the ferrocene moiety.

Full text of DOI:10.1016/j.ica.2006.07.085

Inorganica Chimica Acta 360 (2007) 1032–1042
www.elsevier.com/locate/ica
Heteroleptic Cu(I) complexes containing phenanthroline-type and
1,10-bis(diphenylphosphino)ferrocene ligands: Structure and
electronic properties
a,
a
b
b,
*
Nicola Armaroli *, Gianluca Accorsi , Giacomo Bergamini , Paola Ceroni
,
c
d
d
d
´
Michel Holler , Omar Moudam , Carine Duhayon , Beatrice Delavaux-Nicot ,
d,
*
Jean-Francois Nierengarten
a
`
Istituto per la Sintesi Organica e la Fotoreattivita, Laboratorio di Fotochimica, Consiglio Nazionale della Ricerche, via Gobetti 101, 40129 Bologna, Italy
b
Dipartimento di Chimica ‘‘G. Ciamician’’, Universita` di Bologna, via Selmi 2, I-40126 Bologna, Italy
Laboratoire de Physico-Chimie Bioinorganique, Universite´ Louis Pasteur et CNRS (UMR 7509), Ecole Europe´enne de Chimie,
`
´
Polymeres et Materiaux (ECPM), 25 rue Becquerel, 67087 Strasbourg Cedex 2, France
c
d
Laboratoire de Chimie de Coordination du CNRS, 205 Route de Narbonne, 31077 Toulouse Cedex 4, France
Received 3 July 2006; accepted 28 July 2006
Available online 9 August 2006
Dedicated to Professor Vincenzo Balzani on occasion of his 70th birthday.
Abstract
Three new heteroleptic Cu(I) complexes containing one phenanthroline and one diphosphine type ligand ([Cu(N–N)(P–P)]+) have
been prepared. In particular, one ligand is constituted by 1,10-phenanthroline (1), 2,9-dimethyl-1,10-phenanthroline (2) and 2,9-diphen-
ethyl-1,10-phenanthroline (3) and the other ligand is in all cases 1,10-bis(diphenylphosphino)ferrocene (dppf). Therefore, copper and iron
metal centres are quite close one another, as evidenced by X-ray crystal diffraction. The structure together with the electrochemical and
photophysical properties of these complexes have been compared to that of the corresponding complexes where dppf has been replaced
by bis[2-(diphenylphosphino)-phenyl]ether (POP). Cyclic voltammetric experiments evidenced that the first oxidation process is located
on the ferrocene moiety and that oxidation of Cu(I) is moved to more positive potential values and a chemical reaction is coupled to the
electron transfer process. The absorption spectra show a metal-to-ligand charge transfer (MLCT) band, typical of Cu(I) phenanthroline
complexes, at a higher energy compared to the homoleptic [Cu(N–N)2]+ species. No emission at either room temperature or 77 K has
been observed for compounds 2 and 3, contrary to the high luminescence observed for the corresponding POP complexes. This result is
consistent with a photoinduced energy transfer from the Cu(I) complex to the ferrocene moiety.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Cu(I) complex; Ferrocene; Luminescence; Electrochemistry
1. Introduction
The photophysics of Cu(I)-bisphenanthroline com-
plexes ([Cu(N–N)2]+) has been the object of intensive
investigations for over two decades [1–3]. These com-
*
Corresponding authors. Tel.: +39 051 2099535; fax: +39 051 2099456.
E-mail addresses: armaroli@isof.cnr.it (N. Armaroli), paola.ceroni@
pounds exhibit metal-to-ligand charge transfer (MLCT)
absorption bands in the visible spectral region and, in
unibo.it (P. Ceroni), mholler@chimie.u-strasbg.fr (M. Holler), jfnieren-
garten@lcc-toulouse.fr (J.-F. Nierengarten).
0020-1693/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.07.085

N. Armaroli et al. / Inorganica Chimica Acta 360 (2007) 1032–1042
1033
poorly donor solvents, they may exhibit MLCT lumines-
2.2. General instrumentation and procedures
cence [3]. Molecular rearrangements and exciplex quench-
ing occur in these tetrahedral compounds upon light
excitation, which have been elucidated via bimolecular
quenching studies [4], ultrafast transient absorption [5],
and time resolved X-ray spectroscopy [6,7]. A number
of sophisticated architectures having [Cu(N–N)2]+ as key
building blocks have been prepared, including catenanes
[8], rotaxanes [9], knots [10], helicates [11], dendrimers
[12], racks [13], grids [14], boxes [14], and macrocycles
[15]. Some of these systems exhibit interesting light-
induced processes, also able to trigger motions at the
molecular level [16].
All reactions were performed in standard glassware
under an argon atmosphere. Evaporation and concentra-
tion were performed at water aspirator pressure, and dry-
ing was in vacuo at 10ꢀ2 Torr. Elemental analyses were
carried out on a Perkin–Elmer 2400 B analyzer at the
L.C.C. Microanalytical Laboratory in Toulouse. Mass
spectra were obtained at the Service Commun de Spec-
´
´
trometrie de Masse de l’Universite Paul Sabatier et du
CNRS de Toulouse (fast atom bombardment
(FAB > 0)) spectra were recorded on a Nermag R 10-
10H spectrometer. A 9 kV xenon atom beam was used
to desorb samples from the 3-nitrobenzyl alcohol matrix.
1H and 13C NMR spectral analyses have been performed
Despite the fact that strongly luminescent [Cu(N–N)2]+
complexes are now available in a rigid matrix at 77 K
[17,18], the emission quantum yield of these compounds
at room temperature remains rather low (60.002).
Recently, however, new classes of Cu(I) compounds have
been proposed which show a strongly enhanced lumines-
cence performance [19–21], such as heteroleptic mononu-
clear complexes containing one phenanthroline and one
diphosphine type ligand ([Cu(N–N)(P–P)]+). Taking N–
N = 2,9-diphenethyl-1,10-phenanthroline and P–P = bis-
[2-(diphenylphosphino)-phenyl]ether (POP), we have been
able to obtain crystals for X-ray characterization and,
through a slight modification of the phenanthroline ligand,
we have recently made two [Cu(N–N)(P–P)]+ complexes
with luminescent quantum yields as high as 0.28 in an oxy-
gen-free dichloromethane solution [22]. One of these com-
pounds, where N–N = 2,9-di-n-butyl-1,10-phenanthroline
[19], has been also successfully employed as an active mate-
rial in a light emitting electrochemical cell exhibiting an
efficacy comparable to similar devices made with [Ru-
(bpy)3]2+ [21].
at 293 K on a Bruker DPX 300 spectrometer. H and 13C
1
NMR
spectra
are
referenced
to
external
tetramethylsilane.
2.3. X-ray crystallographic study
Data were collected at a low temperature on an IPDS
STOE diffractometer using a graphite-monochromated
˚
Mo Ka radiation (k = 0.71073 A) and equipped with an
Oxford Cryosystem Cryostream Cooler Device. The final
unit cell parameters have been obtained by means of a
least-square refinement performed on a set of 8000 well
measured reflections, and crystal decay has been moni-
tored during the data collections, no significant fluctua-
tions of intensities have been observed. The structures
were solved by direct methods using SIR92 [24], and
refined by means of least-squares procedures on F2 using
the programs of the PC version of CRYSTALS [25]. Atomic
scattering factors were taken from the International tables
for X-ray Crystallography [26]. All non-hydrogen atoms
excluding some solvent molecules were refined anisotrop-
ically. Hydrogen atoms were introduced in calculated
positions in the last refinements and only one overall iso-
tropic displacement parameter was refined. The drawings
of molecules are performed with Cameron [27] with 30%
probability displacement ellipsoids for non-hydrogen
atoms.
In this paper we present three novel [Cu(N–N)(P–P)]+
complexes, 1–3, in which the diphenylphospine-type ligand
POP has been replaced by 1,10-bis(diphenylphosphino)fer-
rocene (dppf). Compounds 2 and 3 have been investigated
in parallel with the corresponding POP-type complexes 4
and 5 in order to elucidate the dramatic effect on the elec-
trochemical and photophysical properties induced by the
presence of the strongly reducing ferrocene unit within
the P–P ligand.
2.4. Syntheses of compounds 1–3
2. Experimental
In a typical procedure, a CH2Cl2 solution (10 mL) of
[Cu(CH3CN)4][BF4] and dppf in a 1:1 stoichiometry
(1.8 · 10ꢀ4 mol) was added to a stirred solution of the
appropriate phenanthroline derivative (1 equiv.) in CH2Cl2
(10 mL) at room temperature. After 1 h, the mixture was
concentrated to a volume of ca. 8 mL, and the product pre-
cipitated with 20 mL ether. Compounds 1–3 were further
purified by recrystallization from CH2Cl2/Et2O, and
obtained as yellowish crystals in 71–98% yield.
2.1. Materials
CH2Cl2 (for analysis, Acros) and Et2O (rectapur
Prolabo) were simply degassed and used without further
purification. 1,10-Phenanthroline (99%+), and 2,9-
dimethyl-1,10-phenanthroline Æ 0.5H2O (99%+) were from
Acros Organics. 2,9-diphenylethyl-1,10-phenanthroline
was synthesized according to a published procedure [23].
1,10-Bis(diphenylphosphino)ferrocene (dppf) (99%) was
from Strem. Complexes 4 and 5 have been prepared as pre-
viously described [22].
Compound 1: M.p. 250 ꢁC. 1H NMR (300 MHz,
CDCl3): d 4.38 (br s, 4H), 4.52 (br s, 4H), (7.19, 7.35) (each
m, 20H), 7.78 (dd, J = 8 Hz, J = 5 Hz, 2H), 8.09 (s, 2H),

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N. Armaroli et al. / Inorganica Chimica Acta 360 (2007) 1032–1042
8.57 (br d, J = Hz, 2H), 8.77 (br d, J = 5 Hz, 2H). 13C {1H}
NMR (75 MHz, CDCl3): d 72.73, m; 74.53, m; 74.77, m;
125.18, s; 127.48, s; 128.74, m; 129.73, s; 130.28, s;
132.89, m; 133.20, s; 137.68, s; 143.14, m; 150.10, s. Anal.
Calc. for C46H36N2P2FeCuBF4 (1) (884.95): C, 62.43; H,
4.10; N, 3.17. Found: C, 62.37; H, 4.12; N, 3.00%.
The experimental error on the potential values was esti-
mated to be 10 mV.
Spectroelectrochemical experiments were performed in
an optically transparent thin layer electrode (OTTLE)
cell constituted by two quartz windows, a three electrode
set melt sealed in a polyethylene spacer. The three elec-
trodes are: two Pt minigrids (32 wires/cm) as working
and counter electrodes and a silver wire as quasi-refer-
ence electrode, positioned very close to the working elec-
trode to minimize Ohmic resistance. The cell has a
sandwich type arrangement. The light is passed through
the central part of the working electrode minigrid (ca.
80% transmittance) and the light path is of the order
of 100–200 lm. The diffusion of electrogenerated species
between the electrodes is negligible on the experimental
time scale (10–50 s). The solution used for spectroelectro-
chemical experiments was prepared as previously
described for cyclic voltammetry. In order to localize
the right potential window corresponding to the redox
process of interest, a thin layer cyclic voltammetry
(TLCV) was recorded at 5 mV/s before the spectroelect-
rochemistry. After refilling the OTTLE cell with new
solution, the electrolysis potential was varied with
20–50 mV steps and UV–Vis–NIR absorption spectra
were recorded with an Agilent Technologies 8543 diode
array spectrophotometer.
Compound 2: M.p. 265 ꢁC. 1H NMR (300 MHz,
CDCl3): d 2.35 (s, 6H), 4.64 (br s, 4H), 4.75 (br s, 4H),
(7.12, 7.26) (each m, 20H), 7.56 (d, J = 8 Hz, 2H), 8.10
(s, 2H), 8.54 (d, J = 8 Hz, 2H). 13C {1H} NMR (CDCl3,
75 MHz): d 27.61, s; 72.73, m; 74.43, m; 74.83, m; 125.69,
s; 126.69, s; 128.21, s; 128.72, m; 130.07, s; 132.21, m;
133.83, m; 138.55, s; 142.93, m; 159.36, s. Anal. Calc. for
C48H40N2P2FeCuBF4 (2) (913.66): C, 63.15; H, 4.42; N;
3.07. Found: C, 62.92; H, 4.45; N, 2.89%.
Compound 3: M.p. 200 ꢁC. 1H NMR (300 MHz,
CDCl3): d (2.35, 2.68) (each m, each 4H), 4.29 (br s, 4H),
4.46 (br s, 4H), 6.49 (m, 4H), 6.88 (m, 16H), 7.18 (m,
10H), 7.63 (d, J = 8 Hz, 2H), 8.14 (s, 2H), 8.63 (d,
J = 8 Hz, 2H). 13C {1H} NMR (75 MHz, CDCl3): d
33.30, s; 42.61, s; 72.40, m; 73.84, m; 74.80, m; 123.04, s;
126.52, s; 127.14, s; 128.44, s; 128.63, s; 128.88, m;
132.02, s; 132.08, m; 134.04, m; 139.32 s; 139.62, s;
142.74, br s; 161.72, s. 13C {1H} {31P} NMR (75 MHz,
CDCl3): d 33.29, 42.60, 72.33, 73.70, 73.90, 74.78, 123.0,
126.49, 127.15, 128.42, 128.63, 128.79, 128.87, 130.02,
132.08, 134.02, 139.33, 139.63, 142.74, 161.68, each s. Anal.
Calc. for C62H52FeN2P2CuBF4 (3) (1093.23): C, 68.12; H,
4.79; N, 2.56. Found: C, 67.79; H, 4.87; N, 2.56%. FAB-
MS: 1005.2 (15% [MꢀBF4]+ calc. for C62H52FeN2P2Cu),
617.0 (35% [dppfCu]+, calc. for C34H28FeP2Cu), 554.1
(10% [dppf]+, calc. for C34H28FeP2), 451.1 (100% [RCu]+,
calc. for C28H24N2Cu).
2.6. Photophysical measurements
The absorption spectra were recorded with a Perkin–
Elmer k4 spectrophotometer. Emission spectra were
obtained with an Edinburgh FLS920 spectrometer (con-
tinuous 450 W Xe lamp), equipped with a Peltier-cooled
Hamamatsu R928 photomultiplier tube (185–900 nm) or
a Hamamatsu R5509-72 supercooled photomultiplier
tube (193 K, 400–1700 nm range). The corrected spectra
were obtained via a calibration curve supplied by the
instrument manufacturer. The emission quantum yields
were determined according to the approach described
by Demas and Crosby [28] using air-equilibrated
[Ru(bpy)3Cl2] in water solution rem = 0.028 [29] as stan-
dard. O2 removal from CH2Cl2 solutions was accom-
plished by bubbling argon for 10 min. The emission
lifetimes were determined with the time-correlated single
photon counting technique using an Edinburgh FLS920
spectrometer equipped with a laser diode head as excita-
tion source (1 MHz repetition rate, kexc = 407 or 637 nm,
300 ps time resolution upon deconvolution) and an Ham-
amatsu R928 PMT as detector. For 77 K luminescence
measurements the samples were put in glass tubes
(2 mm diameter) and inserted in a special quartz dewar
filled up with liquid nitrogen. For solid state (as powder)
luminescence measurements the samples were put on a
quartz surface and the emitted light was collected at an
angle of 90ꢁ relative to excitation. All measurements were
carried out in spectroscopy grade solvents, used without
further purification.
2.5. Electrochemical measurements
Electrochemical experiments were carried out in argon-
purged CH2Cl2 (Romil Hi-Dryꢂ) solutions at room tem-
perature or at 213 K (by employing an electrochemical
cell in which the temperature can be controlled by an
external thermostat) with an EcoChemie Autolab 30 mul-
tipurpose instrument interfaced to a personal computer.
The working electrode was a glassy carbon electrode
(0.08 cm2, Amel); the counter electrode was a Pt spiral
and a silver wire was employed as a quasi-reference elec-
trode (QRE). The potentials reported are referred to SCE
by measuring the AgQRE potential with respect to ferro-
cene (+0.46 V versus SCE). The concentration of the
compounds examined was of the order of 1 · 10ꢀ3 M;
0.1 M
tetrabutylammonium
hexafluorophosphate
(TBAPF6) was added as the supporting electrolyte. Cyclic
voltammograms were obtained with scan rates in the
range 0.05–5 V/s. The number of electrons exchanged in
each process was estimated by comparing the current
intensity of the corresponding CV wave with that
observed for the monoelectronic oxidation of ferrocene,
after correction for differences in the diffusion coefficients.

N. Armaroli et al. / Inorganica Chimica Acta 360 (2007) 1032–1042
1035
3. Results and discussion
3.1. Synthesis
Complexes 1–3 (Scheme 1) were obtained in good yield
by reaction of an equimolar CH2Cl2 solution of
[CuCH3CN][BF4] and ddpf with a stirred CH2Cl2 solution
of 1 equiv. of the appropriate phenanthroline derivative R,
where R = 1,10-phenanthroline for 1, 2,9-dimethyl-1,10-
phenanthroline for 2, and 2,9-diphenylethyl-1,10-phenan-
1
throline for 3. The compounds were characterized by H
and 13C NMR spectroscopy and their purity was verified
by elemental analysis (see Section 2).
Crystals of 1 and 2 suitable for X-ray analysis were
obtained by slow recrystallization from a CH2Cl2/Et2O
mixture. The molecular structure of the cations of com-
pounds 1 and 2 is shown in Figs. 1 and 2, respectively.
Their crystallographic data are provided in Table 1, and
selected bond lengths and angles are listed in Table 2.
In these compounds, the copper atom is in an N2P2 dis-
torted tetrahedral environment in which both the phenan-
throline and the dppf moieties are chelating ligands. They
are rather symmetrically coordinated to the copper atom
through their N and P atoms, respectively. The Cu–N
and Cu–P distances are within the normal range [20,30].
The distorsion mainly arises from the restricted bite angle
of the phenanthroline ligand N(1)–Cu–N(2), 80.2(2)ꢁ and
80.6(2)ꢁ, for 1 and 2, respectively, whereas the correspond-
ing diphosphine bite angle P(1)–Cu–P(2) is 111.88(4)ꢁ and
113.18(8)ꢁ, respectively. The dihedral angle between the
planes Cu–P(1)–P(2) and Cu–N(1)–N(2) is 74ꢁ and 86ꢁ,
respectively. For 1 and 2, the distance between the iron
Fig. 1. Cameron representation of the structure of the cation in
[(phen)Cu(dppf)][BF4] (1). Thermal ellipsoids are drawn at the 30%
probability level.
Fig. 2. Cameron representation of the structure of the cation
[(dmp)Cu(dppf)]+ present in 2. Thermal ellipsoids are drawn at the 30%
probability level.
˚
and the copper atoms of 4.032(1) and 4.101(1) A, respec-
tively, is close to that of another (dppf)FeCu compound,
in which the copper atom exhibits a tetrahedral geometry
[31]. In the dppf moiety, the Cp rings are staggered by
20ꢁ, and 34ꢁ around the Cpꢁ ꢁ ꢁCp0 axis. The coplanarity
of the phosphorus atoms with the Cp ring is essentially
maintained. The bond lengths within the ferrocenyl and
phenanthroline moieties compare well with the values
reported in the literature [20,30,32].
Scheme 1.

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N. Armaroli et al. / Inorganica Chimica Acta 360 (2007) 1032–1042
Table 1
tion induces: (i) a lengthening of the Cu(1)–P(2) bond when
compared to the Cu(1)–P(1) bond, and (ii) a decrease of the
P(2)–Cu(1)–N(2) angle 97.13(10)ꢁ concomitant with an
increase of the P(1)–Cu(1)–N(2) angle 134.12(11)ꢁ relative
to the other N–P–Cu angles (see Table 2). Moreover, the
phenanthroline ligands of a pair of neighbouring cations
partially overlay one another (see Fig. 3). The components
of this pair are related to one another through a centre of
inversion that is located between the phenanthroline rings.
In this case, for the p–p stacking, the average value of the
Crystallographic and structure refinement data for compounds 1 and 2
Compound
1
2
Chemical formula
C46H36BN2F4P2FeCu C48H40BN2F4P2FeCu,
CH2Cl2 Æ 0.5H2O
1006.94
triclinic
P1
10.650(1)
13.916(1)
15.899(1)
79.43(1)
82.53(1)
78.28(1)
2257.5(4)
2
Formula weight
Crystal system
Space group
884.95.
monoclinic
P21/n
11.450(1)
16.575(1)
20.606(1)
90
93.58(1)
90
3903.0(5)
4
ꢀ
˚
a (A)
˚
b (A)
˚
c (A)
a (ꢁ)
B (ꢁ)
c (ꢁ)
˚
distance between the two phenanthroline ligands is 3.4 A.
In contrast, for compound 2, as illustrated by Fig. 2,
both the PPh2 ligands diverge from the phenanthroline
plane. In addition, there is no p–p intermolecular interac-
tion. This is the result of the insertion of solvent molecules
between two neighbouring phenanthroline ligands belong-
ing to different cations, as shown by Fig. 4. The shortest
existing distance between the phenanthroline plane and a
3
˚
V (A )
Z
Dcalc (Mg/m3)
l (Mo Ka) (mmꢀ1
F(000)
1.51
1.055
1808
180
1.48
1.037
1028
180
)
T (K)
h Range for data
collection (ꢁ)
Index ranges
3.00–26.00
3.00–26.00
˚
solvent molecule (Cl of CH2Cl2) is 3.0 A.
(ꢀ14, ꢀ20, ꢀ25) to
(14, 20, 25)
38718
(ꢀ13, ꢀ17, ꢀ19) to
(13, 17, 19)
22565
The X-ray analysis of 1 revealed another interesting
feature. Actually, two C–H/p intermolecular interactions
are clearly established: the first interaction involves one
hydrogen atom of the ferrocene moiety (H161) and one
ring of the phenanthroline ligand belonging to an adjacent
molecule (D3), the second interaction occurs between two
phosphine groups, involving a proton of one phenyl ring
of a phosphine group (H451) and one of the phenyl ring
of a neighbouring molecule (D4) (see Fig. 5). These hydro-
Reflections collected
Independent reflections
7618 (0.08)
8253 (0.05)
(Rint
)
Goodness-of-fit on F2
1.073
5069/0/490
1.098
5296/0/521
Data/restraints/
parameters
R
wR
0.0553 [I > 0.7r(I)]
0.0657 [I > 0.7r(I)]
1.01 and ꢀ0.91
0.0904 [I > 1.4r(I)]
0.1066 [I > 1.4r(I)]
2.55 and ꢀ2.24
Largest difference peak
and hole (e A
ꢀ3
˚
˚
gen atoms are located at 2.8 and 2.6 A from the corre-
)
sponding centre of the ring. These observed distances
are within the range of the typical ones of C–H/p interac-
tions [33]. Both the C–H bonds point to the corresponding
rings with a CHꢁ ꢁ ꢁcentroid plane angle of 161ꢁ, and 156ꢁ,
respectively.
Table 2
˚
Selected bond lengths (A) and bond angles (ꢁ) for compounds 1 and 2 with
esd’s in parentheses
Thus, the molecular network of 1 could be described as
a stacking of planes defined by six interactions. The BF4
anions are located between these planes constituted by
the cationic interacting moieties. The shortest distance
between a fluorine atom and a proton of the phenanthro-
˚
Bond lengths (A)
Bond angles (ꢁ)
Compound 1
Cu(1)–P(1)
Cu(1)–P(2)
Cu(1)–N(1)
Cu(1)–N(2)
2.250(2)
2.299(1)
2.094(4)
2.089(4)
P(1)–Cu(1)–P(2)
P(1)–Cu(1)–N(1)
P(2)–Cu(1)–N(1)
P(1)–Cu(1)–N(2)
P(2)–Cu(1)–N(2)
N(1)–Cu(1)–N(2)
111.88(4)
114.52(11)
115.82(11)
134.12(11)
97.13(10)
80.20(14)
˚
line, phenyl, and ferrocene unit is 2.3, 2.4 and 2.3 A,
respectively.
Compound 2
Cu(1)–P(1)
Cu(1)–P(2)
Cu(1)–N(1)
Cu(1)–N(2)
3.2. Electrochemical properties
2.267(2)
2.279(2)
2.121(6)
2.100(6)
P(1)–Cu(1)–P(2)
P(1)–Cu(1)–N(1)
P(2)–Cu(1)–N(1)
P(1)–Cu(1)–N(2)
P(2)–Cu(1)–N(2)
N(1)–Cu(1)–N(2)
113.18(8)
115.09(17)
109.21(18)
115.63(18)
118.89(17)
80.6(2)
The electrochemical characterization of compounds 2–5,
together with that of dppf model compound, has been per-
formed by cyclic voltammetry in dichloromethane/
TBAPF6 solutions. Table 3 reports the corresponding
half-wave potentials (E1/2 in V, versus SCE) measured at
room temperature together with those of the model
compounds.
POP complexes, 4 and 5, present a one-electron transfer
process both in the cathodic and in the anodic region
(Table 3). These processes are electrochemically reversible
(fast heterogeneous electron transfer rates) and attain
chemical reversibility at scan rates higher than 2 V/s. dppf
complexes, 2 and 3, present a one-electron transfer process
The main structural different feature between com-
pounds 1 and 2 concerns the existence of p–p interactions
in compound 1. Actually, for this latter compound, we note
that, in the solid state, one of the phenyl rings of the PPh2
ligand is almost parallel with the phenanthroline moiety
(angle between planes: 13ꢁ). The average value of the dis-
˚
tance between the two planes is around 3.5 A. This interac-

N. Armaroli et al. / Inorganica Chimica Acta 360 (2007) 1032–1042
1037
Fig. 3. Cameron representation of the structure of the cation [(phen)Cu(dppf)]+ present in 1, showing the pairwise stacking of phenanthroline ligands
between two neighbouring cations.
Fig. 4. Cameron representation showing the presence of solvent molecules between two neighbouring cations [(dmp)Cu(dppf)]+ of 2.
in the cathodic region at potential values practically identi-
cal to those of POP complexes (Table 3) and two processes
in the anodic region (see, e.g. Fig. 6): the first is completely
reversible, while the second one is chemically irreversible
also at 5 V/s and 213 K and presents an anodic peak much
higher than the first one. Therefore, a chemical reaction is

1038
N. Armaroli et al. / Inorganica Chimica Acta 360 (2007) 1032–1042
Fig. 5. Compound 1: Cameron view (cationic part) showing the p–p intramolecular and intermolecular interactions, and the labelled CH–p intermolecular
interactions (dashed lines) that promote the self-assembly of the supramolecular architecture of a 2D network; (b,c) plane.
Table 3
Half-wave potentials (E1/2 in V vs. SCE) of the investigated compounds in
dichloromethane/TBAPF6 solution at room temperature
phen/phenꢀ
Fc+/Fc
Cu(II)/Cu(I)
2
ꢀ1.75
ꢀ1.74
ꢀ1.75
ꢀ1.75
ꢀ1.72a
+0.87
+0.87
+1.68d
+1.69d
+1.36
3
4
5
+1.41
[Cu(dmp)2]+
[Cu(dpep)2]+
dppf
+0.75b
+1.08c
+0.64
+0.46
Ferrocene
a
Data reported in Ref. [34].
Data reported in Ref. [35].
Data reported in Ref. [36].
b
c
d
Chemically irreversible process; Epa value at 0.2 V/s.
Fig. 6. Cyclic voltammetry of 2 in dichloromethane/TBAPF6 solution at
room temperature. Working electrode: glassy carbon; scan rate: (a) 2 V/s
and (b) 0.2 V/s.
associated with the second oxidation process and the prod-
uct of this reaction is easier to oxidize than the reagent spe-
cies. By comparison of the half-wave potentials of the
investigated compounds with that of ferrocene and of the
homoleptic [Cu(N–N)2]+ compounds, namely [Cu(dmp)2]+
(dmp = 2,9-dimethyl-1,10-phenanthroline), [Cu(dpep)2]+
(dpep = 2,9-diphenethyl-1,10-phenanthroline) (Fig. 7, Table
3), we can assign the observed electron transfer processes
either to the metal centre or to one of the two ligands,

N. Armaroli et al. / Inorganica Chimica Acta 360 (2007) 1032–1042
1039
Fig. 7. E1/2 values of the investigated compounds compared to those of
model compounds: [Cu(dmp)2]+, [Cu(dpep)2]+ and ferrocene. Open circles
denotes Epa values at 0.2 V/s for chemically irreversible processes.
Fig. 8. Absorption spectra (solid lines) of 2 in a dichloromethane/
TBAPF6 solution recorded in an OTTLE cell during a spectroelectro-
chemical experiment corresponding to monooxidation of the starting
complex. For comparison purposes the absorption spectra of dppf (dotted
line) and [Cu(dmp)2]+ (dashed line) have been reported.
i.e. phenanthroline-type or phosphine one. In particular,
in the cathodic region the reduction process occurs at prac-
tically identical potential values for all the investigated
Cu(I) complexes and it involves the phenanthroline ligand,
as confirmed by the very close E1/2 value reported for
[Cu(dmp)2]+ [34].
ical irreversibility of this electron transfer process reflects
the instability of a Cu(II) complex with an oxidized dppf
ligand that is likely to undergo fast dissociation.
A spectroelectrochemical study was performed on 2 in
dichloromethane/TBAPF6 solution by an OTTLE cell in
order to spectroscopically characterize the mono-oxidized
species and verify if any new band due to interactions
between oxidized ferrocene and Cu(I) complex arises.
Fig. 8 shows that, upon oxidation, the initial band at
380 nm decreases and a new shoulder at about 460 nm
increases. These results are not consistent with a decrease
of dppf absorption at 440 nm (Fig. 8). Moreover, the new
shoulder in the absorption spectrum cannot be attributed
to the oxidized dppf.1 The observed spectroscopic changes
are not reversible upon re-reduction, demonstrating that
on the spectroelectrochemical experiment time-scale
(approximately 1 order of magnitude longer than the cyclic
voltammetric one) a chemical reaction is coupled to ferro-
cene oxidation. It is likely that the Cu(I) complex with a
dppf oxidized ligand is not stable towards ligand exchange,
so that partial formation of [Cu(dmp)2]+ occurs. This
hypothesis is supported by the new absorption feature at
460 nm, consistent with an MLCT band of the homoleptic
complex (see Fig. 8). No other absorption band in the NIR
region has been observed, ruling out the possibility to
observe electronic transitions due to metal (iron and cop-
per) centre interactions.
For POP complexes, the oxidation process can be
assigned to Cu(II)/Cu(I) redox couple in substantial agree-
ment with data reported by McMillin et al. on complex 4
[19]. Oxidation of the Cu(I) centre is more difficult in the case
of 5 compared to 4 (DE = 50 mV) in agreement with the cor-
responding positive shift reported in the literature on going
from [Cu(dmp)2]+ [35] to [Cu(dpep)2]+ [36] (Table 3).
Indeed, bulkier phenyl substituents on the phenanthroline
ligand hinders the formation of a more flattened structure
appropriate for the Cu(II) oxidation state. Moreover, POP
complexes present Cu(I) oxidation at more positive poten-
tials compared to the homoleptic phenanthroline complexes.
Indeed, the phosphine ligands are able to better stabilize
Cu(I) rather than the Cu(II) oxidation state because of more
diffuse donor orbitals. Furthermore, in POP complexes the
N–Cu–N bite angle is larger; thus interaction with nitrogen
lone pairs is less favourable and the Cu(II) oxidation state is
less stabilized than in the homoleptic phenanthroline com-
plexes. Correspondingly, MLCT excited state is shifted to
a higher energy (vide infra).
In the case of dppf complexes, the first oxidation process
involves the ferrocene moiety. In particular, the free dppf
ligand shows an oxidation process shifted to more positive
potentials than ferrocene because of electron-withdrawing
phosphorous atoms. When a Cu(I) complex is present,
dppf oxidation occurs at more positive potentials because
of electrostatic and inductive effects. The second oxidation
process observed for 2 and 3 involves a Cu(II)/Cu(I) redox
couple and it is shifted toward more positive potentials
compared to POP complexes because of electrostatic effect
due to the very close oxidized ferrocene moiety. The chem-
3.3. Photophysical properties
The electronic absorption spectra of 2–5 and dppf (as
reference compound for the ligand containing ferrocene)
1
A control spectroelectrochemical experiment on dppf solution did not
evidence the formation of a band at 460 nm upon oxidation.

1040
N. Armaroli et al. / Inorganica Chimica Acta 360 (2007) 1032–1042
Fig. 10. The emission spectra of 4 (left) and 5 (right) in CH2Cl2 solution
(dotted line), 77 K rigid matrix (dashed line) and solid state matrix (solid
Fig. 9. The electronic absorption spectra of 2 (dashed-dotted line), 3
(dashed-dotted-dotted line), 4 (dashed line), 5 (solid line) and dppf (dotted
line) in dichloromethane solution at room temperature. The 350–550 nm
spectral window has been multiplied by a factor of 20.
line). kexc = 380 nm in all cases, OD = 0.3 in CH2Cl2.
excluded in the present case, which would also contribute
to increase the MLCT absorption on the low-energy side.
This hypothesis is also corroborated, in the case of 3, by
the detection of a weak emission band centred at 630 nm,
which corresponds to that of a related homoleptic complex
reported earlier [3,18]. 4–5 Photoluminescence spectra in
three different media have been recorded and are depicted
in Fig. 10.
The emission bands detected in the range between 490
and 550 nm are ascribable to deactivations from the
MLCT states [19,37]; the related luminescence and lifetime
data are included in Table 4. For CH2Cl2 solutions of 4 and
5, when oxygen is removed by bubbling argon, the emission
intensity and lifetime values are dramatically increased
compared to aerated solutions. A similar trend has been
observed on passing from RT to 77 K or solid state matri-
ces. Under these conditions, a spectral blue shift is also
found (Fig. 10) along with the lifetime prolongation (up
to two orders of magnitude, see Table 4). By the increased
lifetime values, which is known to be strictly correlated
with the emission intensity, a dramatic enhancement of
luminescence quantum yield can be expected which, unfor-
tunately, is very hard to measure in rigid media with an
acceptable accuracy. The PL features of 4 and 5 highlight
the fruitful combination of POP type coordinating ligands
have been recorded in dichloromethane solution. The typ-
ical spectral profiles [22] for mixed [Cu(N–N)(P–P)]+ com-
plexes are observed (Fig. 9). Ligand-centred (LC) spectral
features are predominant in the UV spectral region, while
the bands observed in the VIS window are attributed to
MLCT transitions involving the phenanthroline ligand
[19,20,37]. A discussion concerning the MLCT energy lev-
els of heteroleptic [Cu(N–N)(P–P)]+ complexes relative to
that of homoleptic [Cu(N–N)2]+ ones has been reported
elsewhere [22].
The MLCT absorption profiles of 2 and 3 exhibit a small
red shift and intensity increase relative to that of complexes
lacking the ferrocene unit (4–5). Probably, due to its elec-
tron donating character, the ferrocene moiety tends to
facilitate the Cu(I) ! Cu(II) oxidation, leading to a
decrease of the MLCT energy level(s). In addition, the dppf
ligand also contributes to the absorption intensity in the
380–530 nm spectral window (Fig. 9).
It is known that Cu(I) heteroleptic complexes with N–N
and P–P ligands tend to dissociate and form the corre-
sponding [Cu(N–N)2]+ complexes when dissolved in
organic solvents [19]. Therefore, the formation of some
amount of Cu(I)-bisphenanthroline complexes cannot be
Table 4
Luminescence and lifetime data in dichloromethane solution (298 and 77 K) and the solid state matrixa
298 K
77 K
Solid state
b
b
b
kmax(em) (nm)
Uem
s (ls)
kmax(em) (nm)
s (ls)
kmax(em) (nm)
s (ls)
4
5
544
0.0069
0.09c
0.32
514
25.0
526
12.0
4.62c
550
4.5 · 10ꢀ3
0.24
12.90c
494
33.0
536
13.4
0.19c
a
b
c
Complexes 2 and 3 have not been reported since they are not emitting.
Emission maxima from uncorrected spectra.
Degassed solution.

N. Armaroli et al. / Inorganica Chimica Acta 360 (2007) 1032–1042
1041
with 1,10-phenanthrolines in order to achieve a substantial
enhancement of the emission performance and, in perspec-
tive, some degree of tunability of the energy level of the
emitting state, by placing suitable substituents on the P–P
ligands.
can be modified by appending different luminescent and
redox-active units to the phosphine ligand in order to
obtain useful functions, such as sensitized emission.
Acknowledgments
Contrary to 4 and 5, 2 and 3 do not show luminescence
under any conditions. Most likely, the potentially lumines-
cent MLCT level (see above) is quenched by an intramolec-
ular energy transfer to the ferrocene unit. The substantial
energy gap (ꢂ1.5 eV) between the MLCT state (ꢂ
2.65 eV, calculated from 77 K emission spectra) and the
ferrocene lowest triplet level (1.16 eV) [38] is consistent,
in terms of energy, with this hypothesis. An alternative
quenching mechanism could be an electron transfer and,
from the electrochemical data (Table 3), one can estimate
the energy of an hypothetical ferrocene+–phenꢀ charge
separated state at about 2.60 eV, i.e. nearly isoenergetic
to the MLCT level. Due to the poor, if any, driving force
for electron transfer, also occurring in a medium of a rela-
tively low polarity where electron transfer is disfavoured
[38], we tend to exclude this mechanism, which should com-
pete with a strongly favoured energy transfer process (see
above).
G.A. and N.A. acknowledge CNR (commessa PM-P04-
ISTM-C1-ISOF-M5, Componenti Molecolari e Supramol-
`
ecolari o Macromolecolari con Proprieta Fotoniche ed
Optoelettroniche) and MIUR (Contract FIRB
RBNE019H9K, Molecular Manipulation for Nanometric
Machines). G.B. and P.C. acknowledge FIRB (Manipola-
zione molecolare per macchine nanometriche and LaTE-
MAR: Laboratorio di Tecnologie Elettrobiochimiche
Miniaturizzate per l’Analisi e la Ricerca).
Appendix A. Supplementary data
CCDC No. 612886 and CCDC No. 612887 contain the
supplementary crystallographic data for 1 and 2, respec-
tively. These data can be obtained free of charge from
the Cambridge Crystallographic Data Centre, 12, Union
Road, Cambridge CB2 1EZ, UK (fax: (internat.) + 44
1223 336 033; e-mail: deposit@ccdc.cam.ac.uk). Supple-
mentary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2006.07.085.
4. Conclusions
The investigated heteroleptic Cu(I) complexes contain
one dppf and one phenanthroline type ligand. As evi-
denced by the crystal structure, the copper atom is in a
distorted tetrahedral environment in which two nitrogen
atoms of the phenanthroline and two phosphorous atom
of the dppf ligands are coordinated. The copper and iron
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