Solid state and solution investigations of derivatives of Group 11 metal ions with 1-benzyl-2-imidazolyldiphenylphosphine (L). Electrochemical behavior of [M2L3]2+ (M = CuI; AgI) and [AuL2]+ complexes

Article abstract of DOI:10.1016/S0020-1693(01)00582-5

The molecular crystal structure of (Bzim)Ph₂P = L = 1-benzyl-2-imidazolyldiphenylphosphine (monoclinic; space group P2₁/c #14; a = 9.054(2), b = 9.394(3), c = 21.677(8) ?; β = 94.52(2)°; Z = 4) is reported here. This new P,N ligand reacts with Cu^I or Ag^I metal ions to give [M₂L₃]²⁺ dinuclear complexes where the bihapto ligand always bridges the metal centers. When L was added to [Au₂L₂]²⁺ complex the dinuclear gold cycle underwent a cleavage and the mononuclear species [AuL₂]⁺ was obtained. The solution behavior of the complexes was investigated by ¹H and ³¹P NMR at variable temperature, showing neither dissociative nor equilibrium reactions. The strong coordinating ability of the (Bzim)Ph₂P ligand was also indicated by cyclic voltammetry investigation and the electrogenerated Ag(0) neutral complex, namely [Ag₂(μ-(Bzim)Ph₂P)₃], was detected as transient compound. The molecular crystal structure of [Cu₂(μ-(Bzim)Ph₂P)₃][BF₄] ₂ is also reported (monoclinic; space group P2₁/n #14; a = 19.413(9), b = 19.81(1), c = 21.678(7) ?; β = 115.51(2)°; Z = 4).

Full text of DOI:10.1016/S0020-1693(01)00582-5

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Inorganica Chimica Acta 323 (2001) 45–54
Solid state and solution investigations of derivatives of Group 11
metal ions with 1-benzyl-2-imidazolyldiphenylphosphine (L).
Electrochemical behavior of [M2L3]2+ (M=CuI; AgI) and
[AuL2]+ complexes
Fiorella Bachechi a, Alfredo Burini b,*, Marco Fontani c, Rossana Galassi b,
Alceo Macchioni d, Bianca Rosa Pietroni b, Piero Zanello c, Cristiano Zuccaccia d
a Istituto di Strutturistica Chimica ‘G. Giacomello’ CNR, I-00016 Rome, Italy
b Dipartimento di Scienze Chimiche Uni6ersita` di Camerino, 6ia S. Agostino 1, I-62032 Camerino, Italy
c Dipartimento di Chimica, Uni6ersita` di Siena, 6ia Aldo Moro, I-53100 Siena, Italy
d Dipartimento di Chimica, Uni6ersita` di Perugia, 6ia Elce di Sotto 8, I-06123 Perugia, Italy
Received 2 April 2001; accepted 23 July 2001
Abstract
The molecular crystal structure of (Bzim)Ph2P=L=1-benzyl-2-imidazolyldiphenylphosphine (monoclinic; space group P21/c
I
,
c14; a=9.054(2), b=9.394(3), c=21.677(8) A; i=94.52(2)°; Z=4) is reported here. This new P,N ligand reacts with Cu or
AgI metal ions to give [M2L3]2+ dinuclear complexes where the bihapto ligand always bridges the metal centers. When L was
added to [Au2L2]2+ complex the dinuclear gold cycle underwent a cleavage and the mononuclear species [AuL2]+ was obtained.
The solution behavior of the complexes was investigated by 1H and 31P NMR at variable temperature, showing neither
dissociative nor equilibrium reactions. The strong coordinating ability of the (Bzim)Ph2P ligand was also indicated by cyclic
voltammetry investigation and the electrogenerated Ag(0) neutral complex, namely [Ag2(m-(Bzim)Ph2P)3], was detected as transient
compound. The molecular crystal structure of [Cu2(m-(Bzim)Ph2P)3][BF4]2 is also reported (monoclinic; space group P21/n c14;
,
a=19.413(9), b=19.81(1), c=21.678(7) A; i=115.51(2)°; Z=4). © 2001 Published by Elsevier Science B.V.
Keywords: Copper; Silver; Gold; Dinuclear complexes; Phosphine; P,N ligand; Structures; NMR spectroscopy; Cyclic voltammetry
(Bzim)Ph2P [27], similar to 2-(diphenyl)phosphino-
pyridine (dppy) used in several transition metal deriva-
1. Introduction
tives [28]. The two heterocycles in the phosphine lig-
Group 11 metal ions play an important role in
ands differ in terms of a softer character of the
modern inorganic chemistry research. They are in-
imidazole compared to the pyridine, thus (Bzim)Ph2P
volved, for example, in: supramolecular chemistry [1–
allows a better stabilization of complexes with soft
8], inorganic polymers [9,10], bioinorganic and
metals such as those of Group 11 in low oxidation
medicinal chemistry [11–15] catalytic processes [16–18],
states.
advanced inorganic materials [4,5,19,20], and coordina-
Our goal is to investigate the ability of (Bzim)Ph2P to
tion chemistry [21]. Moreover, they are important for
theoretical aspects [22–26].
coordinate late transition metal ions and to compare
the results with those obtained with dppy and other
analogous P,N ligands [28].
As an example, we found that intermolecular closed
shell gold–gold interactions are present in the complex
[(Bzim)Ph2PAuCl] [27], while they are absent in related
complexes bearing dppy or Ph3P as ancillary ligands
[29,30].
Recently we reported the synthesis of
a new
P,N ligand, 1-benzyl-2-imidazolyldiphenylphosphine,
* Corresponding author. Tel.: +39-0737-402 243; fax: +39-0737-
637 345.
E-mail address: burini@camserv.unicam.it (A. Burini).
0020-1693/01/$ - see front matter © 2001 Published by Elsevier Science B.V.
PII: S0020-1693(01)00582-5

46
F. Bachechi et al. / Inorganica Chimica Acta 323 (2001) 45–54
1
Dinuclear cyclic compounds with a [M(m-(Bzim)-
the CH2-benzylic protons that can be used as an H
NMR spectroscopic probe useful for the characteriza-
tion of its derivatives.
Ph2P)]22+ unit (M=Ag(I); Au(I)) have already been
studied and it was found that the structure of the silver
complexes, the coordination numbers and the geometry
of the metal centers are strongly affected by the nature
of counterion used [31]. In addition dinuclear iridium
complexes of (Bzim)Ph2P give reversible CꢀH bond
activation reactions to yield ortho-metalation in the
phenyl ring of the P,N ligand [32].
(1)
Here we report the synthesis, characterization and
the electrochemistry of dinuclear [M2(m-(Bzim)-
Ph2P)3]2+ units (M=Cu(I); Ag(I)) and of mononuclear
[Au((Bzim)Ph2P)2][BF4] complex.
The six CH2-benzylic protons of 1 resonate at ap-
proximately 4.8 ppm where a singlet and two doublets
(integral ratio 1:1:1) are present. The occurrence of two
doublets is attributed to the H–H geminal coupling
(Jgem=16 Hz) of the CH2-protons of the two head-to-
head P,N ligands. This coupling is indicative that the
rotation around the CꢀN bond is partially hindered
probably for the steric hindrance between the Bzim
group and the phenyl rings of the ligands. The singlet is
due to the CH2-protons belonging to the third P,N
bridging ligand, where the chemical shifts for the CH2-
protons in the possible conformations are easily aver-
aged by a slow rate motion.
Dinuclear phosphine complexes with a 3:2 ligand/
metal ratio have been reported for all M(I) Group 11
metal ions, e.g. [Ag2(m-dppm)3][O3SCF3]2 [33] (dppm=
bis(diphenylphosphino)methane),
[34] (dmpm=bis(dimethylphosphino)methane), [Cu2-
(m-dppy)3MeCN][BF4]2 [35], [Ag2(m-dppy)(m-Cl)2-
[Ag2(m-dmpm)3]2+
(dppy)2] [36] and [Ag2(dppy)(m-dppy)2][ClO4]2 [37]. In
the latter two dinuclear silver complexes, the
pyridylphosphines act in the same manner as bi- and
monohapto coordinating ligands.
2. Results and discussion
The formation of [Cu2(m-(Bzim)Ph2P)3]2+ unit was
definitely confirmed by solving the X-ray crystal struc-
ture of [Cu2(m-(Bzim)Ph2P)3][BF4]2 (2). Crystals of com-
plex 2 were obtained in low yields from Cu(BF4)2 using
(Bzim)Ph2P as the reducing agent. In compound 2,
solvent molecules coordinated to the metal were not
detected while, in compound 1, CH3CN molecules were
found by IR spectroscopy to coordinate the copper
centers. In fact weak signals at 2292, 2261 and 2248
cm−1 were recorded and attributed to the stretching
modes of coordinated CH3CN. However, in compound
1, the coordination of CH3CN in CDCl3 solution is
ambiguous because in the 1H NMR spectrum, the
signal at l 1.89 ppm does not undergo substantial shift
after the addition of different amounts of CH3CN.
In a previous study [31] regarding [Ag2L2]2+ cyclic
units, in addition to the expected signals, the 31P NMR
spectra showed a pattern of signals that could be
assigned to [Ag2L3]2+ species present as impurity (less
than 1%). Thus, we attempted to synthesize the [Ag2(m-
(Bzim)Ph2P)3]2+ complexes in agreement with the reac-
tion strategies reported in Scheme 1. The compounds
3–5 were isolated in good yields.
The reaction of (Bzim)Ph2P with [Cu(CH3CN)4][PF6]
in CH3CN solution under dinitrogen atmosphere gave a
white microcrystalline compound having the dinuclear
[Cu2(m-(Bzim)Ph2P)3]2+ core 1. This species was also
isolated when a 1:1 metal/ligand molar ratio was used
in the unsuccessful attempt to obtain the [Cu2(m-
(Bzim)Ph2P)2]2+ complex. In contrast, the complexes
[Cu2(m-dppy)2(CH3CN)n][BF4]2 (n=4 or 2) were re-
ported in literature [35].
31P NMR spectrum of 1 recorded at room tempera-
ture in CDCl3 solution shows two broad signals at l
−11.41 and −16.35 ppm indicative of two non-equiv-
alent phosphorous nuclei. In agreement with the pro-
posed frame reported in Sketch 1 the two NMR signals
are in a 2:1 ratio and the signal at −11.41 ppm has to
be attributed to two equivalent head-to-head phospho-
rous nuclei of the bridging ligands, while the signal at
−16.35 ppm, which looks like an unresolved pseudo-
triplet, is due to the phosphorous nuclei of the third
bridging ligand. A peculiarity of (Bzim)Ph2P ligand is
The NMR spectra of compounds 3–5 did not show
the presence of equilibrium reactions between the
[Ag2(m-(Bzim)Ph2P)3]2+ and [Ag2(m-(Bzim)Ph2P)2]2+
species. Moreover, it is interesting to emphasize that
using Ag(CF3SO3) as silver salt, independently from the
metal/ligand molar ratio used (1:1 or 2:3, respectively),
only the species [Ag2(m-(Bzim)Ph2P)3]2+ was obtained.
Scheme 1.

F. Bachechi et al. / Inorganica Chimica Acta 323 (2001) 45–54
47
to the presence of the Ag isotopomers. As in the case of
complexes 1 and 2 the resonance that falls at higher
frequency is due to the two head-to-head phosphorous
nuclei. The values of the 31J1
coupling constants
107/9
Ag
are, as expected [39], much lPess than those relative to
[Ag2L2]2+ analogous complexes [31] and are little de-
pendent on the nature of the counterion. This little
dependence is the same observed in previously reported
studies [31]: by decreasing the coordinating capability
of the counterion both the 31P chemical shifts and the
phosphorous–silver coupling constant (at room tem-
perature) increase from −4.1 to −2.6 ppm and from
463 to 497 Hz, respectively.
By applying the strategy reported in Scheme 1(b) to
the complex [Au2(m-(Bzim)Ph2P)2][BF4]2, mononuclear
[Au((Bzim)Ph2P)2][BF4] (6) and unreacted starting ma-
terial were isolated. Compound 6 was then obtained in
good yields using the appropriate stoichiometry. Even
if tricoordinated gold(I) compounds have been reported
in literature and the crystal structure of [Au2(m-
dmpm)3]2+ dicationic complex [34] has been solved, in
our case the synthesis of [Au2L3]2+ species was not
successful. Elemental analysis of compound 6 is indica-
tive of two ligands coordinated to a gold(I) ion. More-
over, 31P NMR spectrum of 6 shows a singlet at l 23.82
ppm due to equivalent phosphorous atoms coordinated
to the metal as reported in Sketch 2.
Fig. 1. Dependence of the 31P NMR spectrum of complex 3 on
temperature.
This behavior highlights that even though the coordina-
tion mode of NO3− and CF3SO3− anions are similar,
the electronic influence of the counterions plays an
important role in the synthesis of these silver
derivatives.
(2)
1
The H NMR spectra of complexes 3–5 appear to be
simpler than that predictable basing on the structure of
complex 1. This is due to a dynamical process that
equilibrates the magnetic environments of the protons
belonging to the three P,N ligands. As an example, only
one resonance is observed for the protons in position 5
of the imidazole rings, integrating for three protons.
The resonances become broader by decreasing the tem-
perature and at 193 K the two predicted resonances, in
a 2:1 ratio, become distinguishable even though badly
resolved. The 31P NMR spectra at room temperature
appear as apparent broad triplets while by decreasing
the temperature they transform in two broad doublets
of intensity 2:1 (see Fig. 1). The description of such
spectra can be done by considering that at room tem-
perature the dynamical process that averages the two
Considering the fact that two nitrogen atoms of the
imidazole ring are available to coordinate another
metal ion, the complex 6 could be an interesting build-
ing block for the synthesis of cyclic heteronuclear
complexes.
2.1. X-ray crystal structure analyses of 2 and L
X-ray diffraction effects of single crystals of the
complex and the free ligand were collected on a Rigaku
AFC5R diffractometer using graphite monochromated
Cu Ka radiation and a 12 kV rotating anode generator.
Crystals of 2 were unstable unless completely immersed
in the mother liquor. Several different strategies of
crystal mountings and data collections were used to
environments has
a
rate that afford 107/9Ag107/9
Ag%31P31P% spin system [38]. In fact, at low temperature
2
the two 31P environments are distinguishable but the
dynamic behavior does not allow solving the split due

48
F. Bachechi et al. / Inorganica Chimica Acta 323 (2001) 45–54
obtain a complete set of diffraction effects. Finally, the
best data were obtained by rapid scanning from just
one crystal coated by glue when still wet. Intensity data
were collected by ꢀ–2q technique to a maximum 2q
value of about 120° for 2 and 124° for L. The structures
were solved by the Patterson method and subsequent
Fourier syntheses, in the case of 2, and by direct
methods [40], in the case of L, and refined by full-ma-
trix least-squares method with anisotropic thermal
parameters for all non-hydrogen atoms for L. For 2 the
two BF4− groups and the carbon atoms of the phenyl
groups together with the hydrogen atoms were refined
isotropically as rigid groups. The other hydrogen atoms
of 2 and the all hydrogen atoms of L were included at
calculated positions and not refined. Crystal parameters
and structure determination data for 2 and L are given
in Table 1. Atomic scattering factors and anomalous
dispersion terms were taken from the International
Tables [40]. All calculations were performed using the
TEXSAN crystallographic software package [41].
2.2. Molecular structure of [Cu2(v-(Bzim)Ph2P)3][BF4]2
(2)
The crystal structure of [Cu2(m-(Bzim)Ph2P)3][BF4]2
(2) consists of a dinuclear copper complex where three
(Bzim)Ph2P ligands bridge the metal atoms. Fig. 2
shows the ORTEP plot of compound 2 and Table 2 lists
the selection of bond distances and angles.
Cu(1) and Cu(2) show a distorted trigonal pyramidal
geometry with a different environment around the cop-
per atoms. In fact, Cu(1) is coordinated through the
nitrogen atoms N(3), N(5) of the imidazole groups to
two different phosphine ligands while the third P,N
ligand bonds the Cu(1) metal atom through the phos-
phorous atom P(1). Cu(2) has the opposite situation
with two phosphorous P(2), P(3) atoms and one nitro-
gen N(1) atom coordinated to the metal. The average
values of CuꢀP and CuꢀN bond distances are 2.238 and
Table 1
Crystal data and structure refinement parameters for 2 and L
2
L
Empirical formula
Formula weight
C
66H57N6B2F8P3Cu2 C22H19N2P
1327.83
342.38
,
Wavelength Cu Ka (A)
Crystal system
Space group
1.54178
1.54178
monoclinic
P21/n (c 14)
monoclinic
P21/c (c 14)
,
1.96 A, respectively. These distances are in the range of,
Unit cell dimensions
but shorter than, those found in the related compound
,
a (A)
19.413(9)
19.81(1)
21.678(7)
115.51(2)
7523(6)
9.054(2)
9.394(3)
21.677(8)
94.52(2)
1838(1)
[Cu2(m-dppy)3(MeCN)][BF4]2 [35]. In particular the
,
b (A)
,
CuꢀN average distance in 2 is 0.12 A shorter than the
,
c (A)
i (°)
V (A )
average CuꢀN distance found in [Cu2(m-dppy)3-
(MeCN)][BF4]2. It is noteworthy that in [Cu2-
(BPIA)2]2+ unit [42] as well (BPIA=bis(2-pyridyl)-
methyl((1-methylimidazol-2-yl)methylamine), where the
pyridyl and imidazolyl rings coexist in the BPIA ligand,
the CuꢀN distance is shorter when N is the basic
nitrogen atom belonging to the imidazoles rather than
to pyridyne rings. The geometry of the trigonal planar
coordination of the copper atoms is affected by the
steric hindrance of the ligand, which is greater at the P
than at the N donor atoms.
3
,
Z
4
4
Dcalc (g cm−3
)
1.173
18.07
2720
1.237
13.43
720
v (Cu Ka) (cm−1
F(000)
)
Crystal size (mm)
Diffractometer
Scan type
0.20×0.20×0.25
Rigaku AFC5R
ꢀ–2q
16.00
1.42+0.30 tan q
120.20
0.10×0.25×0.50
Rigaku AFC5R
ꢀ–2q
32.00 (3 rescans)
0.94+0.30 tan q
124.20
Scan rate (° min−1
Scan width (°)
)
2q Maximum (°)
Number of measured reflections
Total
12 086
3314
Thus the PCuN2 unit at Cu(1) presents a NꢀCuꢀN
angle smaller (114(1)°) and the two PꢀCuꢀN angles
greater than the ideal value of 120° (average 123°). The
reverse pattern occurs in the P2CuN unit of Cu(2). The
PꢀCuꢀP angle is greater (127.1(3)°) and the two
PꢀCuꢀN angles smaller than the ideal value of 120°
(average 116°). As a consequence of this quite regular
inverse symmetry, the dihedral angles formed by the
planes through the bridges, Cu(1)ꢀP(1)ꢀC(1)ꢀ
N(1)ꢀCu(2), P1, Cu(1)ꢀN(3)ꢀC(1A)ꢀP(2)ꢀCu(2), P2 and
Cu(1)ꢀN(5)ꢀC(1B)ꢀP(3)ꢀCu(2), P3, do not differ very
much: P1ꢀP2 is 120°; P1ꢀP3 117° and P2ꢀP3 123°.
Also, the dppy complex [35] has similar features, but
this ligand seems more flexible, since it can deviate from
planarity in the formation of the bridges, and it is
capable of adjusting to the greater bulk at the P atoms
with greater distortions.
Unique
10 092 [Rint=0.21] 3101
[Rint=0.044]
Corrections
Lorentz
Lorentz
–polarization
0.93–1.00
–polarization
0.86–1.00
Full-matrix
least-squares
ꢀw(ꢁFoꢁ−ꢁFcꢁ)2
4F2o/|2(Fo)2
0.03
Abs.: trans. factors
Refinement method
Full-matrix
least-squares
ꢀw(ꢁFoꢁ−ꢁFcꢁ)2
4F2o/|2(Fo)2
0.03
Function minimized
Least-squares weights
p Factor
No. of observations (I\3(I)) 3011
No. of variables
Reflection/parameter ratio
Residuals: R; Rw
Goodness-of-fit indicator
Max. shift/error
Largest difference peak and 1.01 and −2.32
hole (e A
1436
195
226
15.44
6.35
0.133; 0.200
0.10
0.068; 0.085
1.99
0.59
0.02
0.37 and −2.06
−3
,
)

F. Bachechi et al. / Inorganica Chimica Acta 323 (2001) 45–54
49
Fig. 2. ORTEP plot of 2 with the numbering scheme of the atoms. Thermal ellipsoids are shown at 40% probability level.
Differently from the dppy derivative, where only one
copper center shows a short distance with the BF4−
(see Section 1) and of the coordinative constraint of this
rigid bridging ligand.
,
anion (CuꢀF=2.755(6) A), both copper atoms of com-
pound 2 complete their coordination sphere with the
fluorine atoms of the BF4− anions, having Cu(1)ꢀF(1)
and Cu(2)ꢀF(5) bond distances of 2.73(2) and 2.81(2)
2.3. Molecular structure of (Bzim)Ph2P (L)
The crystal structure of (Bzim)Ph2P consists of dis-
crete molecules in the unit cell. In Fig. 3 an ORTEP plot
of the ligand L is reported and selected bond distances
and angles are listed in Table 3. The phosphorous atom
bonded to two phenyl rings and one imidazole group
shows a trigonal pyramidal geometry, expected for
triorganophosphines, with the CꢀPꢀC angles ranging
from 100.7(3) to 103.6(3)°. After coordination to metal
ions [27,31] these angles become larger and a maximum
deviation of 5–6° is observed for the angles CPhꢀPꢀCPh.
The imidazolyl and phenyl rings are planar and show
normal bond distances and angles expected for these
aromatic cycles. If bond distances and angles of the
imidazolyl ring are compared with the same structural
parameters after coordination through N(1) to the cop-
per center, significant variations indicative either of
electron density transfer from ligand to metal or vice
versa have not been observed.
,
A, respectively. In contrast to {[m-(Bzim)Ph2PAg-
(BF4)]2}ꢀ [31], The BF4− anions in 2 do not form
bridges between the dinuclear units and discrete
molecule are observed in the unit cell. The
,
Cu(1)···Cu(2) distance of 2.691(5) A, shorter than the
sum of the van der Waals radii (2.86 A), is indicative of
,
a weak metalꢀmetal bonding interaction but the cop-
perꢀcopper short distance could be the effect of the
bridges formed by the rigid (Bzim)Ph2P ligands that
force the metal atoms close to each other. However,
this distance is even shorter than those found in other
three-bridged dinuclear copper complexes: [Cu2(m-
,
dppy)3(MeCN)][BF4]2, (2.721(3) A) [35], [Cu2(m-
,
dppm)2(m-Cl)Cl(dmcn)], (3.293(1) A) [43], (dppm=
bis(diphenylphosphino)methane; dmcn=dimethyl-
cianamide) and [Cu2(m-dppm)2(m-O2CCH3)][BF4],
,
(2.7883(11) A) [44]. Thus, the occurrence of the CuꢀCu
short distance we found in 2 could be the result of the
ability of (Bzim)Ph2P to activate the MꢀM interactions
Upon coordination the ligand undergoes three main
changes. The conformation around the P(1)ꢀC(1) bond

50
F. Bachechi et al. / Inorganica Chimica Acta 323 (2001) 45–54
Table 3
varies to allow coordination by both donor atoms, the
CꢀPꢀC angles, as already mentioned, become larger
and the external angles at the imidazolyl C(1) modify.
Thus, angle PꢀC(1)ꢀN(2) becomes larger, about 128°,
instead of 121° in the free ligand and angle
,
Selected bond lengths (A) and bond angles (°) of the ligand L
Bond lengths
P(1)ꢀC(1)
P(1)ꢀC(11)
P(1)ꢀC(17)
N(1)ꢀC(1)
N(1)ꢀC(2)
N(2)ꢀC(1)
N(2)ꢀC(3)
N(2)ꢀC(4)
C(2)ꢀC(3)
C(4)ꢀC(5)
1.807(6)
1.832(8)
1.836(6)
1.332(7)
1.371(8)
1.367(7)
1.353(8)
1.469(8)
1.349(9)
1.509(9)
N(1)ꢀC(1)ꢀN(2)
N(1)ꢀC(2)ꢀC(3)
C(1)ꢀN(1)ꢀC(2)
C(1)ꢀN(2)ꢀC(3)
C(1)ꢀN(2)ꢀC(4)
C(3)ꢀN(2)ꢀC(4)
N(2)ꢀC(3)ꢀC(2)
N(2)ꢀC(4)ꢀC(5)
C(4)ꢀC(5)ꢀC(6)
C(4)ꢀC(5)ꢀC(10)
109.8(6)
110.1(7)
105.7(5)
107.7(5)
127.0(6)
125.4(6)
106.7(6)
115.1(6)
123.5(6)
118.1(6)
Table 2
,
Selected bond lengths (A) and bond angles (°) of the complex 2
Bond lengths
Cu(1)···Cu(2)
Cu(1)ꢀP(1)
Cu(1)ꢀN(3)
Cu(1)ꢀN(5)
Cu(1)ꢀF(1)
Cu(2)ꢀP(2)
Cu(2)ꢀP(3)
Cu(2)ꢀN(1)
Cu(2)ꢀF(5)
P(1)ꢀC(1)
2.691(5)
2.215(8)
1.96(3)
1.94(3)
2.73(2)
2.26(1)
2.241(9)
1.97(2)
2.81(2)
1.78(3)
1.82(2)
P(1)ꢀC(17)
P(2)ꢀC(1A)
P(2)ꢀC(11A)
P(2)ꢀC(17A)
N(3)ꢀC(1A)
N(3)ꢀC(2A)
P(3)ꢀC(1B)
P(3)ꢀC(11B)
P(3)ꢀC(17B)
N(5)ꢀC(1B)
N(5)ꢀC(2B)
1.79(2)
1.77(3)
1.82(3)
1.81(2)
1.32(3)
1.41(3)
1.82(3)
1.81(3)
1.81(2)
1.34(5)
1.36(3)
Bond angles
C(1)ꢀP(1)ꢀC(11)
C(1)ꢀP(1)ꢀC(17)
C(11)ꢀP(1)ꢀC(17)
P(1)ꢀC(1)ꢀN(1)
P(1)ꢀC(1)ꢀN(2)
100.7(3)
C(6)ꢀC(5)ꢀC(10)
P(1)ꢀC(11)ꢀC(12)
P(1)ꢀC(11)ꢀC(16)
P(1)ꢀC(17)ꢀC(18)
P(1)ꢀC(17)ꢀC(22)
118.4(7)
117.7(6)
124.3(5)
116.9(6)
126.6(5)
103.6(3)
102.2(3)
128.6(5)
121.5(5)
P(1)ꢀC(11)
Bond angles
PꢀC(1)ꢀN(1) becomes smaller, about 123°, instead of
129°, to adjust to the bihapto coordination. In addition,
bridging the copper atoms, the plane of the imidazolyl
ring includes the donor atoms and the metal centers.
Cu(2)···Cu(1)ꢀP(1)
Cu(2)···Cu(1)ꢀN(3)
Cu(2)···Cu(1)ꢀN(5)
Cu(2)···Cu(1)ꢀF(1)
P(1)ꢀCu(1)ꢀN(3)
P(1)ꢀCu(1)ꢀN(5)
P(1)ꢀCu(1)ꢀF(1)
N(3)ꢀCu(1)ꢀN(5)
N(3)ꢀCu(1)ꢀF(1)
N(1)ꢀCu(2)ꢀF(5)
87.8(2)
92.8(6)
92.2(7)
174.4(6)
124.4(7)
121.2(9)
86.6(9)
114(1)
N(5)ꢀCu(1)ꢀF(1)
Cu(1)···Cu(2)ꢀP(2)
Cu(1)···Cu(2)ꢀP(3)
Cu(1)···Cu(2)ꢀN(1)
Cu(1)···Cu(2)ꢀF(5)
P(2)ꢀCu(2)ꢀN(1)
P(2)ꢀCu(2)ꢀP(3)
P(3)ꢀCu(2)ꢀN(1)
P(2)ꢀCu(2)ꢀF(5)
P(3)ꢀCu(2)ꢀF(5)
91(1)
86.9(2)
88.6(2)
95.6(6)
176.0(6)
116.5(8)
127.1(3)
116.4(8)
89.1(8)
94.1(8)
2.4. Electrochemistry
91(1)
86(1)
Fig. 4, which compares the cyclic voltammetric be-
havior of complex 3, [Ag2(m-(Bzim)Ph2P)3][CF3SO3]2,
with that of Ag(CF3SO3) in acetonitrile solution, de-
picts well the strong coordinating ability of the 1-ben-
zyl-2-imidazolyldiphenylphosphine ligand.
As a matter of fact it is immediately seen that not
only 3 undergoes a reduction process at a peak poten-
Fig. 4. Cyclic voltammograms recorded with a platinum electrode in
MeCN solutions containing [NEt4][PF6] (0.1 M) and: (a) Ag(CF3SO3)
Fig. 3. ORTEP plot of L with the numbering scheme of the atoms.
Thermal ellipsoids are shown at 40% probability level.
(1.8×10−3 M); (b) 3 (1.8×10−3 M). Scan rate 0.1 V s−1
.

F. Bachechi et al. / Inorganica Chimica Acta 323 (2001) 45–54
51
tial value more negative by 0.3 V than that of free Ag+
ion, but the cathodic peak is also accompanied by a
directly associated reoxidation peak in the backscan,
which precludes chemical reversibility for the redox
change; nevertheless, the concomitant appearance of
the anodic stripping peak, typical of the oxidation of
electrodeposited metals, testifies that the reversibility is
only partial. Controlled potential coulometric measure-
ments on solutions of 3 (Ew= −0.3 V) prove the
two-electron nature of the cathodic step. In confirma-
tion that the electrogenerated Ag(0) neutral congener
[Ag2(m-(Bzim)Ph2P)3] possesses a transient chemical sta-
bility in the cyclic voltammetric timescale, analysis [45]
of the cyclic voltammograms with scan rate shows that
the ipa/ipc current ratio, which is equal to 0.7 at 0.02
V s−1, progressively increases up to 0.9 at 0.2 V s−1
,
Fig. 6. Cyclic voltammograms recorded with a platinum electrode in
a MeCN solution of 1 (0.7×10−3 M). [NEt4][PF6] supporting elec-
allowing us to evaluate roughly the lifetime of the
congener around 10 s. It is noteworthy, as Fig. 5
illustrates, that further increase in the scan rate leads to
a slight splitting of the cathodic wave in two almost
overlapping steps, prompting us to assume that the
involved electrode mechanism might be effectively de-
picted as an ErevEirrC process.
The significant slowing down of the height of the
reoxidation stripping peak with the scan rate further
supports that the chemical complication C (namely, the
decomplexation of the Ag(0) centers) following the two
one-electron additions is rather slow, within the inter-
vention times of cyclic voltammetry.
Even if partial, the chemical reversibility of the
Ag(I)/Ag(0) reduction in metal complexes is rather
unusual, in that it commonly appears as quite an
irreversible process [46].
We have pointed out the occurrence of such slight
wave-splitting in that, as illustrated in Fig. 6, the Cu(I)
analog 1, [Cu2(m-(Bzim)Ph2P)3](PF6)2, also undergoes
two slightly separated one-electron reductions (as well
trolyte (0.1 M). Scan rate: (a) 0.2 V s−1
.
an irreversible multi-electron, ligand centered oxidation
at Ep= +1.13 V).
As seen, in this case also, the first reduction step
possesses features of chemical reversibility, whereas the
second electron addition causes fast copper metal elec-
trodeposition as revealed by the relevant anodic strip-
ping peak.
The appearance of separated reduction waves for the
Ag(I) and Cu(I) complexes suggests that, even if very
weak, a minor electronic interaction between the two
metallic centers likely within the two frameworks exists.
By way of speculation we note that in both cases the
electrochemical quasi-reversibility of the first steps (at
0.5 V s−1, DEp=200 mV for the silver complex and
530 mV for the copper complex) preludes that high
geometrical
strains
accompany
the
[M2(m-
(Bzim)Ph2P)3]2+/+ reduction.
To conclude we recall that complex 4, [Ag2(m-
(Bzim)Ph2P)3](BF4)2, affords electrochemical behavior
quite similar to that of 3, whereas the monogold com-
plex 6, [Au(m-(Bzim)Ph2P)2](BF4), gives rise to an irre-
versible oxidation (Ep= +0.8 V) in dichloromethane
solution.
Table 4 summarizes the most significant electrochem-
ical features for the dinuclear Ag(I) and Cu(I) com-
plexes under discussion.
3. Experimental
The reactions were performed under nitrogen atmo-
sphere using Schlenk techniques and solvents were
purified with standard methods. Silver and copper salts
have been purchased by Aldrich, (Bzim)Ph2P was syn-
thesized according to the literature method [27].
Elemental analyses were carried out by Carlo Erba
1106 elemental microanalyzer. Infrared spectra were
recorded in the range 4000–200 cm−1 on a Perkin–
Fig. 5. Cyclic voltammograms recorded with a platinum electrode in
a MeCN solution of 3 (0.8×10−3 M). [NEt4][PF6] supporting elec-
trolyte (0.1 M). Scan rates: (---) 0.5 V s−1; (—) 1.00 V s−1
.

52
F. Bachechi et al. / Inorganica Chimica Acta 323 (2001) 45–54
Elmer 1310 spectrophotometer using Nujol mulls in CsI
windows.
of white crystals suitable for X-ray structure determina-
tion was recovered after two months.
1H, 19F, and 31P NMR spectra were measured on
Bruker DPX 200 and DRX 400 spectrometers. Refer-
encing is relative to TMS (1H), CCl3F (19F), and 85%
H3PO4 (31P). NMR samples were prepared by dissolv-
ing about 20 mg of compound in 0.5 ml of deuterated
solvent.
The crystals gave a satisfactory elemental analysis,
1H NMR (except for the absence of CH3CN signal) and
31P{1H} NMR spectra showed the same signals as
reported for compound 1.
3.3. Synthesis of [Ag2(v-(Bzim)Ph2P)3][CF3SO3]2 (3)
Materials and apparatus for electrochemistry have
been described elsewhere [47]. Either a BAS 100 A or a
BAS 100 W were used as polarizing units. Potential
values are referred to the saturated calomel electrode.
Under the present experimental conditions, the one-
electron oxidation of ferrocene occurs at E°%= +0.38
V.
To a THF solution (6 ml) of (Bzim)Ph2P (0.1 g; 0.29
mmol), AgCF3SO3 (0.05 g; 0.195 mmol) in THF solu-
tion (6 ml) was added. A white precipitate is readily
formed and the suspension was left stirring for 2 h at
r.t. and in darkness. After centrifugation, the white
solid was dissolved in CH2Cl2 (5 ml) and by adding few
drops of hexane the analytical sample was obtained.
Yield 95%.
3.1. Synthesis of [Cu2(v-(Bzim)Ph2P)3(CH3CN)2][PF6]2
(1)
Elemental analysis for C68H57N6P3Ag2F6S2O6. Anal.
Found: C, 52.62; H, 3.43; N, 5.18; S. 3.94. Calc. for
C68H57N6P3Ag2F6S2O6: C, 53.00; H, 3.73; N, 5.45; S,
4.16%.
To a CH3CN solution (8 ml) of [Cu(CH3CN)4]PF6
(0.0362 g; 0.10 mmol), solid (Bzim)Ph2P (0.05 g, 0.15
mmol) was added. The cloudy solution was warmed up
to reflux for 24 h and then it was cooled down to room
temperature (r.t.). After filtration over a celite bed (2
cm) the solution was concentrated to 2 ml, and 12 ml of
diethyl ether was layered. A microcrystalline white
product was obtained after two months in low yield
(35%). Faster crystallizations were also attempted, but
the formation of oil was always observed.
1H NMR (CD2Cl2, 285 K): l 7.80 (d, 3H, 3JHH=1.3,
3
4
5 Hz), 7.41 (tt, 6H, JHH=7.4, JHH=1.3 Hz, m-HA),
7.29 (t, 12H, JHH=8.0 Hz, m-HB), 7.24–7.14 (br m,
3
24H, H-1, o-HB, p-HA, p-HB), 6.44 (d, 6H, JHH=7.3,
3
o-HA), 4.75 (s, 6H, CH2). 19F{1H} NMR (CD2Cl2, 285
K): l −79.7 (s). 31P{1H} NMR (CD2Cl2, 285 K): l
−4.1 (br Harris t [38], ꢁ JPAg+3JPAgꢁ=463 Hz).
1
Elemental analysis for C70H63N8P5Cu2F12. Anal.
Found: C, 54.66; H, 4.41; N, 7.68. Calc. for
C70H63N8P5Cu2F12: C, 55.09; H, 4.16; N, 7.34%.
1H NMR (CD3CN, 253 K): l 7.60 (s), 7.53 (s),
7.45–6.80 (m), 6.22 (d), 6.10 (d), 4.87 (s), 4.77 (dd).
31P{1H} NMR (CDCl3, 298 K): −11.41 (m), −16.35
(t).
3.4. Synthesis of [Ag2(v-(Bzim)Ph2P)3][BF4]2 (4)
3.4.1. Method A
To a THF solution (8 ml) of (Bzim)Ph2P (0.05 g; 0.15
mmol), AgBF4 (0.01895 g; 0.1 mmol) in CH3OH (4 ml)
solution was added. The colorless reaction mixture was
stirred for 2 h and then pumped down to dryness. The
residue was dissolved in CH2Cl2 (3 ml) and filtered
through a celite bed (2 cm). Hexane (3 ml) was layered
and 0.044 g of a white microcrystalline compound was
collected after leaving the compound overnight at 4 °C.
Yield 65%.
3.2. Synthesis of [Cu2(v-(Bzim)Ph2P)3][BF4]2 (2)
To a CH3CN solution (10 ml) of (Bzim)Ph2P (0.05 g;
0.15 mmol), solid Cu(BF4)2 (0.014 g, 0.058 mmol) was
added. The colorless solution turned immediately green
and during 3.5 h of magnetic stirring it turned colorless
again. The solution was filtered off and concentrated to
2 ml. By layering 15 ml of diethyl ether, a small amount
3.4.2. Method B
To a suspension of [Ag2(m-(Bzim)Ph2P)2][BF4]2 (0.016
g; 0.15 mmol) in THF (10 ml), solid (Bzim)Ph2P (0.051
Table 4
Electrochemical characteristics for the reduction processes of [M2(m-(Bzim)Ph2P)3]2+ (M=Cu(I), Ag(I)) in acetonitrile solution
a
Complex
E°% (2+/0) a (V)
−0.08
DEp (mV)
E°% (2+/+) (V)
DEp (mV)
Ep (+/0) (V)
[Ag2(m-(Bzim)Ph2P)3]2+
[Cu2(m-(Bzim)Ph2P)3]2+
Ag+
160
−0.06 b
−0.44 c
200 b
480 c
−0.24 b
−0.80 c
+0.12 c
a Overall two-electron reduction measured at 0.1 V s−1
b Measured at 0.5 V s−1, sec text.
.
c Measured at 0.2 V s−1
.

F. Bachechi et al. / Inorganica Chimica Acta 323 (2001) 45–54
53
g; 0.015 mmol) was added. The formation of a very thin
Acknowledgements
precipitate was readily observed and the suspension was
stirred for 2 h and then centrifuged to yield the white
analytical sample. Yield 60%.
This work was supported by grants from The Univer-
sity of Camerino, University of Perugia and the minis-
Elemental analysis for C66H57N6P3Ag2B2F8. Anal.
Found: C, 55.81; H, 4.06; N, 6.06. Calc. for
C66H57N6P3Ag2B2F8: C, 55.96; H, 4.06; N, 5.93%.
1H NMR (CD2Cl2, 285 K): l 7.70 (br, 3H, 5),
7.03–7.33 (m, 42H), 6.46 (br, 6H, o-HA), 4.73 (br, 6H,
CH2). 19F{1H} NMR (CD2Cl2, 217 K): l −150.89 (b,
10BF4−), −150.95 (b, 11BF4−). 31P{1H} NMR (CD2Cl2,
tero dell’Universita` e della Ricerca Scientifica e
Tecnologica (MURST, Rome, Italy), Programme di
Rilevante Interesse Nazionale, Cofinanziamento 2000–
2001.
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285 K): l -3.5 (br Harris t [38] ꢁ JPAg+3JPAgꢁ=470
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