Phosphine oxide functionalised with two bipyridine subunits: A novel ligand for the engineering of sterically hindered complexes

Article abstract of DOI:10.1039/b102520c

Ligand L, containing two 6-methyl-6′-methyl-yl-2,2′-bipyridine moieties linked by a "PhP=O" spacer, has been prepared. Subsequent complexation with [Cu([Cu(CH₃CN)₄](BF₄) led to a unique complex of formula [CuL](BF₄

Full text of DOI:10.1039/b102520c

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Phosphine oxide functionalised with two bipyridine subunits: a novel
ligand for the engineering of sterically hindered complexes
Laurent Douce,a Lo•c Charbonniere,a Michele Cesariob and Raymond Ziessel*a
a L aboratoire de Chimie, dÏElectronique et de Photonique Moleculaires (CNRS UPRESA 7008),
ECPM 25, rue Becquerel, F-67087 Strasbourg cedex 2, France.
E-mail: ziessel=chimie.u-strasbg.fr
b Institut de Chimie des Substances Naturelles, CNRS, F-91128 Gif-sur-Y vette, France
Received (in Montpellier, France) 19th March 2000, Accepted 24th April 2001
First published as an Advance Article on the web 10th July 2001
Ligand L, containing two 6-methyl-6@-methyl-yl-2,2@-bipyridine moieties linked by a ““PhP2OÏÏ spacer, has been
prepared. Subsequent complexation with [Cu(CH CN) ](BF ) led to a unique complex of formula [CuL](BF ).
3
4
4
4
The molecular structure has been determined by X-ray crystallography in the solid state and NMR studies in
solution. The complex displays an intense absorption band in the visible wavelength range with maxima at 418 and
509 nm, and is oxidised reversibly at ]0.66 V (*E \ 80 mV) to the corresponding Cu(II) species, while two
p
successive reductions of the coordinated bipyridine fragments occur at [1.60 (*E \ 66) and [1.84 V (*E \ 70
p
p
mV) vs. SCE. Direct spectrophotometric titrations with global analysis allowed us to elicit stability constants for
the emergent complex and to highlight the formation of a dinuclear complex in the presence of excess copper.
Research to synthesise fascinating molecular structures, often
involving interlocking of complementary molecular com-
ponents, has blossomed during the past decade.1 Numerous
examples have been engineered with the prospect of forming
symmetrical and esthetic structures. In several cases cationic
metal centres provide the impetus for self-organisation into
This provides the key element by which to include this struc-
ordered structures, some of them exhibiting useful catalytic,2
mesomorphic3 or electronic properties.4 Many of the new
complexes generated over the last few years rely on the coor-
dination of transition metal cations to polypyridine fragments
incorporated into oligomeric ribbons or macroscopic
loops.5h8 By virtue of forming relatively stable and well-
deÐned complexes with metal cations, Lehn and co-workers
have led the Ðeld by designing linear polybipyridyl ligands
that self-organise in the presence of copper(I) cations to form
double-stranded helicates.9 Furthermore, related structures
have been constructed using various kinds of oligopyridines
and di†erent transition metals such as Cu(II), Co(II), Cd(II) and
Zn(II).10 SpeciÐcally, such elaborated molecular architectures
are made possible by incorporating several polypyridine units
into oligomeric or macrocyclic multitopic ligands. Some
attempts have been made to identify the driving forces for the
formation of one structure vs. another by systematic variation
of the metal and ligand component, but very little is known
about the mechanism of formation of these complexes.11
ture into elaborated complexes and the impetus for construc-
ting macroscopic sensors.
Experimental
General methods
The 200.1, 400.1 (1H), 50.3 (13C) and 162 MHz (31P) NMR
spectra were recorded at room temperature, unless otherwise
speciÐed, using perdeuterated solvent as an internal standard:
d(H) relative to residual protiated solvent in CDCl (7.26);
3
d(C) relative to the solvent in CDCl (77.0). Melting points
3
were obtained on a Buchi 535 capillary melting point appar-
atus in open-ended capillaries and are uncorrected. FT-IR
spectra were measured from KBr pellets with a Nicolet 210
spectrometer. Fast-atom bombardement (FAB, positive mode)
mass spectra were obtained using m-nitrobenzyl alcohol (m-
NBA) as the matrix. UV-Vis absorption spectra and spectro-
photometric titrations were performed on a Uvikon 933 spec-
trophotometer using degassed acetonitrile as solvent
containing 10~3 M tetrabutylammonium hexaÑuorophos-
phate as an inert salt. Titrations were performed according to
literature procedures14 with typical concentrations of
It is worthwhile pointing out that only a few metal-induced
self-assembled structures have been produced with bipyridine-
grafted phosphanes.12 Hybrid ligands, in which
a
phosphorous(V) is used as a connecting atom, are available13
but genuine bipyridineÈP(V) ligands are very scarce. The aim
of the present work is to partially Ðll this gap and to describe
the full characterisation of ligand L, its related mononuclear
copper(I) complex and to determine its stability constant in
solution. The molecular structure of the complex in the solid
state was deduced from X-ray crystallographic studies.
5 ] 10~5
M
for the ligand and 5 ] 10~4
M
for
[Cu(CH CN) ](BF ).
3
4
4
Electrochemical studies employed cyclic voltammetry with
a conventional 3-electrode system using a BAS CV-50W volt-
ammetric analyser equipped with a Pt microdisk working
electrode and a Ag wire counter-electrode. Ferrocene was
used as an internal standard and was calibrated against a
saturated calomel reference electrode (SCE) separated from
the electrolysis cell by a glass frit presoaked with electrolyte
solution. Solutions contained the electrode-active substrate
One clear advantage of this new ligand is that it can be used
in the complexation of a variety of di†erent transition metals
and also with lanthanide cations and could readily be func-
tionalised either at the phenyl ring or at the methyl positions.
1024
New J. Chem., 2001, 25, 1024È1030
DOI: 10.1039/b102520c
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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(ca. 2 ] 10~4 M) in deoxygenated and anhydrous acetonitrile
containing tetra-n-butylammonium tetraÑuoroborate (0.2 M)
as supporting electrolyte. The quoted half-wave potentials
were reproducible within 20 mV.
Crystallography
Crystal data for [CuL](BF ):
C
H
CuN POBF ,
4
30 27
4
4
M \ 640.88g, triclinic, space group P1, a \ 7.430(3),
b \ 14.002(5), c \ 15.206(6) A, a \ 114.78(2), b \ 80.67(3),
Materials
c \ 98.49(3)¡, U \ 1411.7(9) A
Ž
3, Z \ 2, D \ 1.508 g cm~3,
c
j \ 0.710 73 A, T \ 293(2) K, k (Mo Ka) \ 0.889 mm~1,
F(000) \ 656. A prismatic crystal of 0.12 ] 0.15 ] 0.25 mm
was mounted on an EnrafÈNonius Kappa-CCD di†ractome-
ter with graphite-monochromated Mo-Ka radiation. The data
were collected at room temperature. A full sphere of data was
collected by / axis rotation in 2.0¡ increments over 360¡, with
130 s exposures per frame. Dezingering was accomplished by
measuring each frame twice. The crystal-to-detector distance
was 35 mm. Data were analysed using Kappa-CCD soft-
ware.17 Cell dimensions were reÐned with HKL scalepack.18
Data reduction was performed with Denzo.18 Of a total of
18 102 reÑections collected, 4975 were independent and 3244
unique reÑections were larger than 2p(I). The structure was
solved by direct methods (SHELXS-86)19 and reÐned on F2
for all reÑections by least-squares methods using SHELXL-
93.20 Hydrogen atoms, located by di†erence syntheses, were
reÐned as riding models (CÈH \ 0.93 A, CÈH \ 0.96 A)
PhPCl and NaIO are commercially available. 6,6@-
2
4
Dimethyl-2,2@-bipyridine15 and [Cu(CH CN) ](BF )16 were
3
4
4
prepared and puriÐed according to literature procedures. Di-
isopropylamine and tetrahydrofuran were dried over suitable
reagents and freshly distilled under argon before use. All reac-
tions were carried out under dry argon by using Schlenk-tube
and vacuum-line techniques.
Synthesis of ligand L. A solution of n-butyllithium in hexane
(3.85 mL, 1.5 M, 5.75 mmol) was slowly added to a solution of
diisopropylamine (1 mL, 7.13 mmol) in THF (50 mL) at
[78 ¡C. After 0.5 h, solid 6,6@-dimethyl-2,2@-bipyridine (1.059
g, 5.75 mmol) was carefully added to the stirred solution,
which immediately turned deep blueÈgreen. The temperature
was allowed to rise to room temperature and kept at this tem-
perature for 0.5 h and then cooled to [78 ¡C for an addi-
tional hour. Then, phenyldichlorophosphine (0.39 mL, 2.875
mmol) was slowly added to the solution. Stirring was main-
tained for 0.25 h and the solution was allowed to warm to
room temperature. The resultant yellow solution was evapo-
rated under vacuum and the yellow residue was dissolved in a
mixture of dichloromethane and water containing sodium
periodate. After stirring for 1 h, the remaining aqueous phase
was extracted with dicholoromethane (3 ] 50 mL) and the
ar
Me
and assigned an isotropic thermal parameter of 1.2 that of the
bonded atoms. The asymmetric unit consists of one complex
cation and one (BF ) counterion. This last is a†ected by dis-
4
order, two di†erent sites being located by di†erence Fourier
syntheses. Better convergence was obtained with occupancy
factors of 0.8 and 0.2, respectively. ReÐnement was pursued
with restraints and the minor position was reÐned iso-
tropically. The Ðnal conventional R factor was 0.068 for 3244
data, 20 restraints and 398 parameters, and 0.11 for all data,
combined organic extracts dried over MgSO and evaporated
4
to dryness. The solid was puriÐed by column chromatography
wR(F2) \ 0.19, w \ 1/[p2(F 2) ] (0.1126 P)2 ] 0.27 P] where
on alumina (CH Cl Èhexane: 50 : 50 as eluant) to give 0.75 g
0
2
2
P \ (F 2 ] 2F 2)/3. The largest di†erence peak and hole are
of L (53%). C
H N PO (M \ 490.549) Anal. found: C,
0
c
30 27
4
r
respectively 0.59 and [0.045 eAŽ 3.
73.22; H, 5.29; N, 11.28; calc.: C, 73.46; H, 5.55; N, 11.42%.
CCDC reference number 154547. See http://www.rsc.org/
suppdata/nj/b1/b102520c/ for crystallographic data in CIF or
other electronic format.
1H NMR (200.1 MHz, CDCl , 25 ¡C): d 8.26 (d, 2H, J \ 8.7),
3
7.91 (d, 2H, J \ 8.0), 7.63 (m, 6H), 7.31 (m, 5H), 7.12 (d, 2H,
J \ 7.3), 3.86 [(AB) X, A \ B \ H, X \ P, 4H, J \ J
\
2
AB
AX
J
\ 14.5 Hz], 2.60 (s, 6H, CH ). 13CM1HN NMR (100.61
BX
3
MHz, CDCl , 25 ¡C): d 158.15 (Cq), 156.42 (d, Cq, J \ 2),
3
PC
155.72 (Cq), 152.55 (d, Cq, J \ 8), 137.80 (d, CH, J \ 2),
PC PC
137.24 (CH), 131.94 (d, Cq, J \ 97), 131.90 (d, CH, J \ 3),
PC PC
131.55 (d, CH, J \ 9), 128.50 (d, CH, J \ 12), 125.13 (d,
PC PC
CH, J \ 4), 123.58 (CH), 119.45 (d, CH, J \ 3), 118.50
PC PC
Results and discussion
Ligand L was prepared by lithiation of 6,6@-dimethyl-2,2@-bi-
pyridine with lithium diisopropylamine (LDA) at low tem-
perature, followed by reaction with dichlorophenylphosphine.
Subsequent oxidation with sodium periodate under phase
transfer conditions provide the target ligand L in fair yield.
The synthesis of L is summarised in Scheme 1.
(CH), 40.92 (d, CH , J \ 62), 25.01 (CH ). 31PM1HN NMR
2
PC
3
(162 MHz, CDCl , 25 ¡C): d 36.5 (s). FAB-MS: 491.2
3
[M ] H]`, 475.2 [M [ O ] H]`. FT-IR (KBr, l/cm~1):
1577 (s), 1439 (vs), 1185 (m).
The analytical and spectroscopic characteristics are consis-
tent with the proposed structure. The P2O function was evi-
Synthesis of copper(I) complex. To a solution of L (250 mg,
0.51 mmol) in 15 mL CH Cl under argon was added
2
2
[Cu(CH CN) ](BF ) (160 mg, 0.51 mmol) in 15 mL CH CN.
3
4
4
3
After stirring for 2 h, the deep red solution was evaporated to
dryness. Recrystallisation from acetoneÈhexane yielded red
crystals (310 mg, 95%). C
Anal. found: C, 56.15; H, 4.13; N, 8.69; calc.: C, 56.22; H,
H
N POCuBF (M \ 640.900)
30 27
4
4
r
4.25; N, 8.74%. 1H NMR (400.13 MHz, CDCl , 25 ¡C): d 8.52
3
(d, 1H, H14 bipy), 8.50 (d, 1H, H15 bipy), 8.30 (d, 1H, H5
bipy), 8.21 (d, 1H, H4 bipy), 8.18 (t, 1H, H6 bipy), 8.12 (t, 1H,
H16 bipy), 8.02 (t, 1H, H3 bipy), 7.86 (t, 1H, H13 bipy), 7.78 (d,
1H, H7 bipy), 7.60 (t, 1H, 3J \ 8, H21 phenyl), 7.44 (br m,
HH
4H, H2 ] H17 bipy ] 2 H20 phenyl), 7.23 (dd, 2H, 3J
\
PH
11.5, 3J \ 8, 4J \ 1.5, 2 H19 phenyl), 6.71 (dd, 1H,
HH
HH
3J \ 8, 4J \ 2, H12 bipy), 4.12 and 3.40 (2H, J \ 14,
AB
2J \ 2J \ 18.5, H10 ] H11 resp., CH ), 4.35 and 3.66
AP
BP
2
(2H, J
\ 14, 2J \ 18.5, 2J \ 8 Hz, H9 and H8 resp.,
A{B{
A{P
B{P
CH ), 1.92 (s, 3H CH ), 1.89 (s, 3H, CH ). 31PM1HN NMR
2
3
3
(81.01 MHz, CDCl , 25 ¡C): d 37.38 (s). FT-IR (KBr, l/cm~1):
3
1596 (s), 1565 (s) 1463 (s), 1439 (s), 1053 (vs br). FAB-MS: m/z
553.3 [M [ BF ]`.
Scheme1
4
New J. Chem., 2001, 25, 1024È1030
1025

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denced by IR spectroscopy at l
\ 1185 cm~1 and the
(P/O)
presence of a phosphorus(V) atom was conÐrmed by a singlet
at 36.5 ppm in the 31PM1HN NMR spectrum and a doublet for
the geminal carbon at 40.9 ppm with 1J \ 62 Hz in the
PC
13CM1HN NMR spectrum. The 1H NMR spectrum of L dis-
plays, as expected, an (AB) X (A \ B \ H, X \ P) system at
2
3.86 ppm (J \ J \ J \ 14.5 Hz) for the two CH
AB
AX
BX
2
protons.
Reaction of
[Cu(CH CN) ](BF ) in a mixture of acetonitrile and dichloro-
L
with an equimolar amount of
3
4
4
methane, under anaerobic conditions, led to the formation of
a deep-red mononuclear complex. Single crystals, suitable for
X-ray di†raction, were obtained after multiple recrystal-
lisations based on slow di†usion of pentane in an acetone
solution of the complex. The crystal structure of the
[CuL](BF ) complex has been resolved and shows the molec-
4
ular structure to consist of discrete entities, evidencing a 1 : 1
ligand-to-metal stoichiometry. An ORTEP view of the
complex is given in Fig. 1 and selected bond lengths and
angles are given in Table 1.
The two bipyridine subunits are wrapped around a single
Cu(I) cation. Each copper(I) cation is coordinated to four
nitrogen atoms provided by the pyridine rings to give a dis-
torted tetrahedral arrangement with a small bite angle of ca.
81¡. Two sets of CuÈN bonds are found: [2.013(4), 2.028(4) A]
for the pyridine rings close to the P2O function and [2.045(4),
2.048(4) A] for the pyridine rings bearing the methyl groups.
The two bipyridine chelates are almost planar with a some-
what more pronounced dihedral angle between the two pyri-
dine rings in one bipyridine [the angle between the pyridine
rings containing N1A and N1B is 1.8(3)¡ while the other two
containing N1C and N1D are tilted by 8.8(2)¡]. The tilt angles
of the mean plane belonging to the phenyl ring and the adja-
cent pyridine rings are 56.6(2)¡ for N1A, 58.4(4)¡ for N1B,
32.7(1)¡ for N1C and 41.5(1)¡ for N1D. The shortest intermo-
lecular distance of 4.4 A is found between the oxygen atoms of
the P2O functions for the two centrosymmetrical subunits
Fig. 2 Centrosymmetrical arrangement of two complexes within the
crystallographic cell: (i) [x, y, z] and (ii) [[x, 1 [ y, 1 [ z]. OiÉ É ÉOii
distance 4.398 (7) A.
lying at [x, y, z] and [[x, 1 [ y, 1 [ z], as depicted in Fig. 2.
The packing of the molecules in the centrosymmetrical cell is
shown in Fig. 3 the individual molecules are aligned along the
a axis. The shortest intermolecular distance within a single
chain is of the order of 3.4 A between the mean plane of two
adjacent pyridine rings belonging to two neighbouring com-
plexes. In view of the absence of any symmetry element in the
individual complex, each complex is chiral, but the bulk of the
material is a racemic mixture of both isomers.
The 1H NMR spectrum of this complex in deuteriated chlo-
roform displays a complicated pattern for the CH protons
2
attributed to an AA@BB@X spin system (A \ B \ H, X \ P;
Fig. 4). The non-equivalence of each proton in both CH
groups indicates the absence of any symmetry element in the
2
complex, in perfect agreement with the solid state structure.
Fig. 1 ORTEP view of the [CuL] cation. Displacement ellipsoids
are shown at the 50% probability level.
Table 1 Selected bond lengths (A) and angles (¡) of the [CuL](BF )
4
complex
CuÈN1A
2.013(4)
2.028(4)
2.045(4)
2.048(4)
121.1(2)
81.5(2)
132.8(2)
139.2(2)
80.0(2)
CuÈN1C
CuÈN1B
CuÈN1D
N1AÈCuÈN1C
N1AÈCuÈN1B
N1CÈCuÈN1B
N1AÈCuÈN1D
N1CÈCuÈN1D
N1BÈCuÈN1D
Fig. 3 Packing diagram over three crystallographic cells along the a
axis.
109.3(2)
1026
New J. Chem., 2001, 25, 1024È1030

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Fig. 4 (Top) Section of the 1H NMR spectrum of the [CuL](BF )
4
complex in CDCl showing the AA@BB@X spin system (A \ B \ H,
3
Fig. 5 (A) Section of the 1H NMR spectrum of the [CuL](BF )
4
complex in CDCl showing the aromatic part of the spectrum
3
X \ P). (Bottom) 2D COSY NMR spectrum corresponding to the
together with the labelling of the protons. (B) Molecular view of the
complex with numbering of each proton with NOESY and COSY
interaction pathways.
AA@BB@X spin system shown in the top half.
These data point to a di†erentiation of the two bipyridine
moieties upon complexation with copper(I) and to the disap-
pearance of the symmetry plane present in the free ligand.
The chemical shifts and some relevant coupling constants
for this spin system are gathered in Table 2. From the 2D
COSY NMR spectrum, a weak and long range coupling
H21 at 7.60 ppm (3J \ 8 Hz). For the protons of the bipyri-
HH
dine arms, the assignment was achieved in two steps. In the
Ðrst step, two families of 9 protons were separated. Starting
from one of the singlets of the methyl substituent (the one at
1.89 ppm, for example) scalar correlations allowed for assign-
ment of the protons on the adjacent pyridine ring (doublet at
7.44 ppm, triplet at 8.02 ppm and doublet at 8.21 ppm, respec-
tively, for the protons in positions 5@, 4@ and 3@ of the pyridine
ring of our example). A dipolar coupling between the protons
in positions 3@ and 3 of this bipyridine gave the link between
the two pyridine systems and the scalar couplings permitted
the further assignment of the two remaining protons on this
pyridine (doublet at 8.30 ppm, triplet at 8.18 ppm and doublet
at 7.78 ppm, respectively, for the protons in positions 3, 4 and
5 in our example). Finally, the scalar correlations between the
proton in the 5 position of the pyridine and the complicated
spin system of the methylene bridges gave the resonances due
to H and H (4.35 and 3.66 ppm, respectively, in our
between the A parts of the AB systems (4J \ 1 Hz) can be
AA{
seen (Fig. 4). The lack of any symmetry element in the
complex is also evidenced by the presence of two singlets for
the methyl groups at 1.89 and 1.92 ppm. These results unam-
biguously point to a slow interconversion process on the
NMR timescale, with no chemical exchange between the two
chiral forms of the complex.
From the chemical shifts, and 2D-COSY and NOESY
experiments, the assignment of the protons of the ligand was
possible (see Fig. 5 and Table 2). A Ðrst set of three signals
corresponds to the Ðve protons of the phenyl ring and were
assigned in the following way: H19 at 7.23 ppm (3J \ 11.5,
PH
3J \ 8, 4J \ 1.5 Hz), H20 at 7.44 ppm (3J \ 8 Hz) and
HH
HH
HH
A
B
example). The same procedure applied to the second bipyri-
dine arm allowed the assignment of the 9 remaining signals
(Fig. 5). The second step involved the exact assignment of
these two families of protons. This was achieved thanks to the
observation of a considerable upÐeld shift of the doublet at
6.71 ppm, which was attributed to a proton at the 5 position
of one of the bipyridine moieties (H7 or H12). On the basis of
the X-ray crystal structure determination previously discussed,
it appears that one of these protons [H12 in Fig. 5(B)] is
located in the shielding region of the phenyl ring, highlighting
the inÑuence of the ring current, a phenomenon that has
already been observed in numerous extended aromatic
systems.21 Assignment of H12 combined with the Ðrst step-by-
step assignment of the protons of the bipyridine moieties gave
the total assignment. From these results it appears that the
weak 4J coupling observed between H8 and H11 in the
AA@BB@X spin system is perfectly in accord with the W-shaped
Table 2 Chemical shifts and relevant coupling constants for the 1H
NMR spectrum of [CuL](BF ) in CDCl ; for atom labelling see Fig.
4
3
5(B)
Phenyl ring H19 7.23
H20 7.44
H21 7.60
Bipyridine
moities
H1
H2
H3
H4
H5
H6
H7
H8
1.89
7.42
8.02
8.21
8.30
8.18
7.78
H18 1.92
H17 7.44
H16 8.12
H15 8.50
H14 8.52
H13 7.86
H12 6.71
3.66 (2J \ 14,
H11 3.40 (2J \ 14,
HH
HH
2J \ 8 Hz)
2J \ 18.5 Hz)
PH
PH
H9
4.35 (2J \ 14,
H10 4.12 (2J \ 14,
HH
HH
2J \ 18.5 Hz)
2J \ 18.5 Hz)
PH
PH
New J. Chem., 2001, 25, 1024È1030
1027

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arrangement of the H8ÈCÈPÈCÈH11 frame. This shape is
observed in the solid state structure and is highly favourable
for the observation of such long-range coupling.22
Finally, an important aspect of the NOESY experiment is
the observation of exchange peaks between H7 and H12, H6
and H13, H5 and H14, H4 and H15 and H3 and H16. These
exchange processes are observed as a result of a transfer of
saturation between the interchanging sites.22,23 From this
observation, it can be estimated that the exchange process
occurs at a rate at least equal to the inverse of the longitudinal
relaxation time (T ) of the aromatic protons, that is, of the
1
order of a second.22,24 Very interestingly, the 400 MHz 1H
NMR spectrum showed the two methyl signals to be separat-
ed by only 0.03 ppm. The separation of these peaks is only
observable when the following requirement is fulÐlled:
q ] *l [ 2p, where q is the time required for the observation
of the distinct signals and *l the di†erence in frequency
between the two observed signals. From this relation, one can
conclude that the exchange process is shorter than 1/q, which
gives an upper limit of 75 s~1. We can thus estimate that the
rate of the interconversion process is situated in a window
ranging from a few to 75 s~1. Finally, the 31PM1HN NMR
Fig. 6 Evolution of the UV-Vis absorption spectrum of a solution of
L upon addition of increasing amounts of Cu` ([Cu`]/[L] \ 0 to 3
in CH CN with 10~3 M TBAPF ). Inset: UV part of the absorption
3
6
spectrum obtained with increasing amounts of Cu` from 0 to 1 equiv.
vs. L.
tion of two distinct new absorbing species and an evolving
factor analysis suggested a concentration maximum for the
Ðrst species at an M/L ratio of 1, the second species appearing
only for larger values. The titration was then Ðtted to the fol-
lowing model using the SpectÐt software.27
spectrum shows a singlet at 37.4 ppm in CHCl , in keeping
3
with the observation that only the bipyridine moities of the
ligand are complexed to the metal centre.
Cu` ] L H [CuL]`b \ [CuL]/([Cu`][L])
ML
The electrochemical behaviour of [CuL](BF ) was studied
4
2Cu` ] L H [Cu L]2`b \ [Cu L]/([Cu`]2[L])
by cyclic voltammetry in argon degassed acetonitrile solution
2
M2L
2
at room temperature. Scanning anodically from 0.00 to ]1.60
V vs. SCE, a single oxidation peak at ]0.66 V is observed
with a peak potential di†erence *E \ E [ E equal to 80
Least squares Ðtting of the data to this model converges to
values of 7.5 ^ 0.5 and 13.1 ^ 0.7 for log b and log b
,
ML
M2L
respectively. The evolution of the concentrations of the species
as a function of the M/L ratio (Fig. 7) conÐrmed the presence
of the 1 : 1 complex as the major species (85%) at M/L B 1.
p
pa
pc
mV, which is independent of the scan rate between 10 and
1000 mV s~1. The peak current increase linearly with u1@2,
which conÐrms an electron transfer mechanism without any
chemical complication on the cyclic voltammetry time scale.
This behaviour shows that the mononuclear copper(I)
complex is oxidised in a reversible, monoelectronic di†usion-
controlled step. Scanning cathodically from 0.00 to [2.00 V
shows two successive reduction peaks at [1.60 and [1.84 V
with a peak potential di†erence *E \ E [ E equal to 66
p
pc
pa
and 70 mV. Owing to the monoelectronic nature and reversi-
bility of these two redox processes it is likely that they corre-
spond to the successive reduction of the bipyridine ligands
coordinated to the copper centre. It is expected that a metal-
centred reduction providing Cu(0) should lead to an irrevers-
ible process accompanied by metallic copper deposition on
the working electrode. It is surmised that ligand L provides
excellent shielding of the cuprous centre and that the P(O)Ph
Fig. 7 Calculated evolution of the concentration of the species
formed in solution as a function of the [Cu`]/[L] ratio. The total
ligand concentration is 7.0 ] 10~5 M.
subunit displays
a signiÐcant withdrawing e†ect, which
It is interesting to note that the second species formed still
displays an absorption band in the visible region. This point
indicates that the coordination of the second Cu` cation does
not change the coordination of the two diimines around the
Ðrst Cu` centre, as would be expected if the two metal atoms
were each coordinated to one bipyridine arm (form A). In this
last case, disappearance of the MLCT transition in the visible
region would certainly be expected.28 A possible explanation
is the coordination of the second metal to the oxygen atom of
the phosphine oxide moiety (form B), which is consistent with
the pronounced perturbations in the UV domain associated
with variations in the p ] p* transitions of the phenyl group.
favours the successive reduction of each bipyridine sub-
unit. This is further conÐrmed by the fact that
[Cu(neocuproin) ](BF )25 (neocuproin \ 2,9-dimethyl-1,10-
2
4
phenanthroline) is substantially more difficult to reduce (by
120 mV) when studied under the same experimental condi-
tions.
In order to gain a deeper insight into the mechanism of
formation of this complex and to provide some information
about stability constants, spectrophotometric titrations have
been carried out in acetonitrile solutions containing 10~3 M
of [(n-Bu) N](PF ) as inert salt. Upon addition of the Ðrst
4
6
aliquots of Cu`, the colourless solution rapidly turned red as
a result of a fast complexation process. For M/L ratios
increasing from 0 to 1 (Fig. 6), the p ] p* transition centred at
291 nm for the free ligand is gradually shifted to lower energy
with concomitant appearance of an absorption band in the
visible region (j \ 420 nm), typical of an MLCT transition
max
in Cu`Èbis(diimine) complexes.26 At higher M/L ratio values,
further changes are observed in the p ] p* region, but less
pronounced than the previous one. Interestingly, the absorp-
tion band in the visible region remained essentially
unchanged. Factorial analysis of the data revealed the forma-
1028
New J. Chem., 2001, 25, 1024È1030

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A set of additional experiments by phosphorous NMR in
solution and FT-IR in the solid state support the possible
coordination of a second cation to the mononuclear complex
in the presence of a large excess of the metal precursor. Spe-
ciÐcally, addition of 2 equiv. of [Cu(CH CN) ]` to a solution
emerge from this study is that a mononuclear complex is pref-
erentially formed. An interesting aspect of this work, however,
concerns the relatively high association constant determined
by spectrophotometric titrations but also the discovery that in
the presence of excess metal precursor a dinuclear complex is
formed. It is surmised that within this complex the P2O frag-
ment coordinates to the second copper(I) centre, a situation
that could be favoured by the p-acidic character of the bipyri-
dine fragments.
3
4
containing [CuL](BF ) in CD CN under argon shows a sig-
4
3
niÐcant downÐeld shift of the original singlet of 3.4 ppm with
an enlargement of the peak (full width at half height \ 1900
Hz) due to a dynamic process. Unfortunately, it was not pos-
sible to follow the evolution of the P2O stretching vibration in
the IR spectrum, as a consequence of the superimposition of
Acknowledgements
this band with the intense absorption due to the BF ~
4
counteranion at 1053 cm~1.29 All these results indicate that at
This work was supported by the Centre National de la
Recherche ScientiÐque and by the Engineering School of
Chemistry (ECPM), We thank Dr Arlette Cavallo-Solladie for
computational simulations and Dr Michel Schmitt for the
COSY and NOESY NMR measurements.
least in acetonitrile and in the presence of an excess of
copper(I), additional copper coordination to the appended
P2O function is possible. Further attempts to isolate and to
structurally characterise this dinuclear complex have failed up
to now.
Finally, it is worth pointing out that the copper-induced
coordination of the bipyridine subunits does not result in a
helicoidal arrangement as previously observed with more Ñex-
ible frameworks. Molecular simulations using MM2 or PM3
programs seem to conÐrm that the dimers (helicate or meso-
helicate) are less stable than the monomeric complexes.
Stereoelectronic e†ects could be postulated to play a major
role in the formation of the mononuclear vs. the dimeric
complex. Similar observations were recently made with
ferrocene-bridged bis(bipyridine) compounds.30
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