Gold and silver complexes with the ferrocenyl-pyrazolate ligand FcCH 2pz

Article abstract of DOI:10.1016/j.ica.2005.02.017

We have synthesized a series of gold(I), gold(III) or silver(I) derivatives with ferrocenylmethylpyrazole (pzCH₂Fc) as ligand. X-ray structure determination for [Au(C₆F₅)(pzCH₂Fc)] reveals a linear geometry for the gold centre and association of the molecules in pairs through intermolecular gold(I)-gold(I) interactions of 3.1204(6) ?. These molecules are further assembled into chains and the chains into a 3D structure through several hydrogen bonds. [Ag(OTf)(pzCH₂Fc)] (OTf = triflate) is a dimer with a distorted tetrahedral geometry at the silver(I) centre and the triflate acting as bridging ligand.

Full text of DOI:10.1016/j.ica.2005.02.017

Inorganica Chimica Acta 358 (2005) 4177–4182
www.elsevier.com/locate/ica
Gold and silver complexes with the ferrocenyl-pyrazolate ligand
FcCH2pz
*
Eva M. Barranco, M. Concepcion Gimeno, Antonio Laguna , M. Dolores Villacampa
´
´ ´ ´
Departamento de Quımica Inorganica, Instituto de Ciencia de Materiales de Aragon, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
Received 19 January 2005; accepted 13 February 2005
Available online 23 March 2005
Dedicated to Professor Hubert Schmidbaur.
Abstract
We have synthesized a series of gold(I), gold(III) or silver(I) derivatives with ferrocenylmethylpyrazole (pzCH2Fc) as ligand.
X-ray structure determination for [Au(C6F5)(pzCH2Fc)] reveals a linear geometry for the gold centre and association of the mol-
˚
ecules in pairs through intermolecular gold(I)–gold(I) interactions of 3.1204(6) A. These molecules are further assembled into chains
and the chains into a 3D structure through several hydrogen bonds. [Ag(OTf)(pzCH2Fc)] (OTf = triflate) is a dimer with a distorted
tetrahedral geometry at the silver(I) centre and the triflate acting as bridging ligand.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Gold; Silver; Ferrocene; Pyrazole
1. Introduction
to produce polymetallic species owing to their versatile
coordination possibilities [13–18].
Ferrocene has been extensively used as starting prod-
uct in the synthesis of new derivatives. It is a versatile
molecule with important properties such as high electron
density, aromaticity and reversible redox characteristics.
These properties together with the ease of preparation of
species in which one or two cyclopentadienyl rings are
substituted with a great variety of organic fragments
that may contain O, N, S, P, etc. as donor atoms make
of ferrocene a very suitable building block in many fields
of research [1–3]. Studies of these ligands are important
because the complexes thus obtained may be interesting
in many research areas looking for special properties,
such as redox or non linear optical properties, charge
transport, liquid crystals, electrochemical recognition
or catalysis [1–12]. Ferrocenylpyrazole is one of the
above systems and this ligand has recently been used
In this paper, we report the synthesis of ferro-
cenylmethylpyrazole and its reactivity with gold or silver
derivatives. The single crystal X-ray analysis of the new
complexes shows the formation of dimers for the gold(I)
or silver(I) derivatives, due to aurophilic interactions or
the presence of triflate bridges, respectively. Also, the
presence of hydrogen bonding leads to the formation
of a supramolecular array for the gold compound.
2. Results and discussion
The ferrocenylmethylpyrazole (pzCH2Fc) ligand (1)
has been synthesized by reaction of N,N-dimethylami-
nomethylferrocene with pyrazole in acetic acid. The
yellow solid is air- and moisture-stable and the
LSIMS + mass spectrum shows the molecular peak at
1
*
m/z = 266 (100%). The H NMR spectrum shows the
resonances of the ferrocenyl unit with a singlet for the
Corresponding author. Tel.: + 34 976 761185; fax: + 34 976 761187.
E-mail address: alaguna@unizar.es (A. Laguna).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.02.017

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E.M. Barranco et al. / Inorganica Chimica Acta 358 (2005) 4177–4182
Table 2
unsubstituted cyclopentadienyl ring and two multiplets
for the a and b protons of the substituted cyclopentadie-
nyl unit. The methylene protons appear as a singlet and
for the pyrazole protons two doublets and one doublet
of doublets are present. The 1H NMR data are collected
in Table 1. The reaction of 1 with [AuX(tht)] (tht = tet-
rahydrothiophene) gives the complexes [AuX(pzCH2Fc)]
(X = Cl (2) or C6F5 (3)) (Scheme 1), which are isolated
as yellow air- and moisture-stable solids. IR spectra
show the presence of Au–Cl [2, 325(m) cmÀ1], or
Au(I)–C6F5 bonds [3, 1505(vs), 956(s) and 808(m)
cmÀ1]. The 19F NMR spectra show the presence of
one type of pentafluorophenylgold(I). The LSIMS +
mass spectrum of 3 shows as the most intense peak the
molecular peak at m/z = 630.
˚
Selected bond lengths (A) and angles (ꢁ) for complex 3
Au–C(21)
Au–N(2)
2.023(7)
2.072(5)
3.1204(6)
1.351(8)
1.369(7)
N(1)–C(1)
N(2)–C(2)
C(2)–C(3)
C(3)–C(4)
1.474(8)
1.332(8)
1.390(9)
1.355(10)
Au–Au#1
N(1)–C(4)
N(1)–N(2)
C(21)–Au–N(2)
C(21)–Au–Au#1
N(2)–Au–Au#1
C(4)–N(1)–N(2)
C(4)–N(1)–C(1)
N(2)–N(1)–C(1)
C(2)–N(2)–N(1)
178.9(2)
C(2)–N(2)–Au
N(1)–N(2)–Au
N(1)–C(1)–C(11)
N(2)–C(2)–C(3)
C(4)–C(3)–C(2)
N(1)–C(4)–C(3)
129.3(4)
87.89(17)
91.28(14)
108.6(5)
128.6(5)
122.5(5)
106.8(5)
123.9(4)
112.1(5)
110.1(6)
105.5(6)
109.1(6)
Symmetry transformations used to generate equivalent atoms: #1
Àx,y,Àz + 1/2.
The crystal structure of complex 3 has been con-
firmed by an X-ray diffraction study; selected bond
lengths and angles are collected in Table 2. The gold
centre has a linear geometry with a N(2)–Au–C(21) an-
gle of 178.9(2). In the lattice, the molecules are associ-
ated into pairs via an intermolecular AuÁ Á ÁAu
˚
[Au(C6F5)(FcCH2NHpyMe)] (2.073(4) and 1.988(4) A)
˚
[19], [Au(C6F5)(3-Fcpy)] (2.124(15) and 2.00(2) A) [20]
or [Au(C6F5)(Ph2C@NAN@CPh2)] (2.069(5) and
˚
1.992(6) A) [21]. The cyclopentadienyl rings deviate
slightly from an eclipsed geometry [torsion angle
C(11)–Cp–Cp–C(18) 3.4ꢁ].
˚
interaction of 3.1204(6) A (Fig. 1) and the pairs form
chains held together by HÁ Á ÁAu and HÁ Á ÁF interactions
The reaction of 1 with [M(OTf)PPh3] (OTf = triflate,
M = Au, Ag), 1:1 molar ratio, leads to the cationic com-
plexes [M(pzCH2Fc)(PPh3)]OTf [M = Au (4) or Ag (5)].
These complexes are air- and moisture-stable yellow sol-
ids at room temperature and they behave as electrolyte
1:1 in acetone solutions [150 (4) and 107 (5)
ohmÀ1 cm2 molÀ1]. They were readily characterized by
elemental analysis, IR and NMR spectroscopy. A sin-
glet is observed in the phosphorus spectra at room tem-
perature [30.3 (4), 14.9 (5) ppm]. At -55 ꢁC complex 5
shows two doublets because of the coupling with both
silver nuclei, [J(P109Ag), 745.6; J(P107), 657.7 Hz]. IR
spectra show the presence of the triflate group at
1265(vs, br), 1223(s), 1152(s) and 1029(s) cmÀ1 or
1253(vs, br), 1223(s), 1152(s) and 1032(s) cmÀ1, for 4
and 5, respectively. The LSIMS + mass spectrum shows
the cationic molecular peak at m/z = 725 (30%, 4) or 636
(55%, 5).
˚
(Fig. 2). The shortest H1BÁ Á ÁAu distance is 2.932 A with
˚
a C1Á Á ÁAu distance of 3.568 A and a C1–H1BÁ Á ÁAu an-
˚
gle of 123ꢁ. The bond distance H4Á Á ÁF1 is 2.59 A with a
˚
C4Á Á ÁF1 distance of 3.387 A and a C4–H4Á Á ÁF1 angle of
141ꢁ. The chains are further linked to form a 3D
structure through additional C–HÁ Á ÁF hydrogen bonds
(Table 3).
The Au–N and Au–C bond lengths, 2.072(5) and
˚
2.023(7) A, compare well with the values reported in
complexes with the N–Au–C6F5 moiety as
Table 1
1H NMR data for complexes 1–6
Complex d(C5H4) d(C5H5) d(CH2) d(pz)
1
2
3
4
5
6
4.20(m) 4.19(s)
4.28(m)
5.10(s) 6.24(dd), J(HH) 2.1, 1.4 Hz
7.36(d)
7.52(d)
The structure determination of complex 5 has been
determined and reveals the presence of dimers in which
triflate anions are acting as bridging ligands as depicted
in Fig. 3. The complex crystallizes with one molecule of
dichloroethane. Selected bonds and angles are collected
in Table 4. The silver centre displays a highly distorted
tetrahedral geometry; the angles around Ag range from
84.63(13)ꢁ [O(2)–Ag–N(2)] to 160.66(11)ꢁ [N(2)–Ag–
4.13(m) 4.13(m) 5.02(m) 6.21(m)
4.24(m)
7.32(m)
7.46(m)
4.22(m) 4.20(s)
4.45(m)
5.45(s) 6.37(dd), J(HH) 2.6, 1.7 Hz
7.56(d)
7.64(d)
4.25(m) 4.08(m) 5.36(s) 6.44(m)
7.71(m)
8.02(m)
˚
P(1)]. The silver centre lies only 0.23 A out of the plane
formed by the atoms O(2), P(1) and N(2). The Ag–O
˚
distances, 2.544(4) and 2.612(4) A, longer than other
values reported for tetracoordinate silver complexes
[22–25], are consistent with weak bonds as has been
found in other silver–triflate complexes. The longer
4.05(m) 4.09(m) 5.10(m) 6.23(m)
4.22(m)
7.54(m)
7.62(m)
4.10(m) 4.26(m) 5.27(m) 6.33(m)
4.26(m)
˚
7.32(m)
7.55(m)
2.612(4) A Ag–O3#2 indicates a weaker bond to the sec-
ond triflate unit.

E.M. Barranco et al. / Inorganica Chimica Acta 358 (2005) 4177–4182
4179
Fig. 1. Molecular structure of the dimeric unit of 3 showing the atom numbering scheme. Radii are arbitrary.
Fig. 2. Association of dimers in complex 3 through C–HÁ Á ÁAu and C–HÁ Á ÁF interactions.
˚
Table 3
Hydrogen bonds for complex 3 (A and ꢁ)
Me2pz)3}(PMePh2)] (from 2.316(6) to 2.336(5) A) [27]
or the complexes {Fe[C5H4CH(pz)2]2AgX}n (X = BF4,
˚
˚
PF6, SO3CF3 and SbF6) (from 2.230(3) to 2.420(4) A)
[28].
˚
The Ag–P bond distance, 2.3670(14) A, is similar to
that reported for the complex [Ag{HB(3,5-Me2pz)3}
D–HÁ Á ÁA
d(D–H) d(HÁ Á ÁA)
d(DÁ Á ÁA)
Æ(DHA)
C(4)–H(4)Á Á ÁF(1)#2
0.95
0.95
2.59
2.56
2.65
2.93
3.387(8)
3.199(8)
3.334(8)
3.568(6)
141.5
124.4
129.4
122.9
C(4)–H(4)Á Á ÁF(2)#3
C(14)–H(14)Á Á ÁF(4)#4 0.95
C(1)–H(1B)Á Á ÁAu#2
0.99
˚
(PMePh2)] (2.336(2) A) [27] but shorter than those
Symmetry transformations used to generate equivalent atoms: #2
Àx,Ày + 1,Àz; #3 x,y À 1,z; #4 Àx + 1/2,Ày + 3/2,Àz + 1.
˚
found in [Ag2(pz)2(PPh3)3] (2.461(1) and 2.484(1) A)
[26], or other tetrahedral silver complexes such as
˚
[Ag(dppe)2]NO3 [29] (range 2.488(3)–2.527(3) A), [Ag
(phen){(PPh2)2C2B10H10}]ClO4 [30] (2.463(2), 2.479(2)
˚
A) or [Ag{C5(CO2Me)5}(PPh3)2] [22] (2.428(2),
˚
2.414(2) A). The cyclopentadienyl rings deviate slightly
˚
The Ag–N distance, 2.196(4) A, is shorter than those
found in some four-coordinate pyrazole–silver com-
˚
plexes as [Ag2(pz)2(PPh3)3] (2.295(2) and 2.323(2) A for
the tetracoordinated silver atom) [26], [Ag{HB(3,5-
from an eclipsed geometry [torsion angle C(1)–Cp–Cp–
C(6) 11.5ꢁ]. Additionally, complex 5 displays some

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E.M. Barranco et al. / Inorganica Chimica Acta 358 (2005) 4177–4182
Table 5
˚
Hydrogen bonds for complex 5 (A and ꢁ)
D–HÁ Á ÁA
d(D–H) d(HÁ Á ÁA) d(DÁ Á ÁA) Æ(DHA)
C(12)–H(12)Á Á ÁO(1)
0.93
2.44
2.54
2.57
2.40
3.357(6)
3.322(6)
3.413(6)
3.245(6)
170.3
138.1
150.6
151.1
C(41)–H(41A)Á Á ÁO(2)#1 0.97
C(9)–H(9)Á Á ÁO(3)#2
0.93
0.93
C(36)–H(36)Á Á ÁO(3)#2
Symmetry transformations used to generate equivalent atoms: #1
Àx + 1,Ày + 2,Àz; #2 Àx + 1,Ày + 1,Àz.
H2
C
H2
C
N
N
N
N
i
Au
Fe
Fe
X
ii
iii
X = Cl (2), C6F5 (3)
Fig. 3. Molecular structure of complex 5. Radii are arbitrary. H atoms
are omitted for clarity.
H2
C
H2
C
N
N
N
N
OTf
M
Fe
Fe
C6F5 Au C6F5
C6F5
PPh3
Table 4
˚
Selected bond lengths (A) and angles (ꢁ) for compound 5
M = Au (4), Ag (5)
6
Ag–N(2)
Ag–P(1)
Ag–O(2)
2.196(4)
2.3670(14)
2.544(4)
2.612(4)
1.498(7)
1.449(6)
N(1)–C(44)
N(1)–N(2)
1.345(6)
1.358(5)
1.327(6)
1.393(7)
1.374(7)
Scheme 1. (i) [AuX(tht)] (X = Cl, C6F5); (ii) [M(OTf)PPh3] (M = Au,
Ag); (iii) [Au(C6 F5)3(OEt2)].
N(2)–C(42)
C(42)–C(43)
C(43)–C(44)
Ag–O3#2
C(10)–C(41)
C(41)–N(1)
3. Experimental
N(2)–Ag–P(1)
N(2)–Ag–O(2)
P(1)–Ag–O(2)
160.66(11)
84.63(13)
109.46(9)
113.3(4)
111.3(4)
128.3(4)
120.1(4)
105.3(4)
127.9(3)
126.8(3)
111.2(5)
105.0(5)
107.2(5)
3.1. General procedures
All the reactions were carried out under an argon
atmosphere at room temperature. IR spectra were re-
corded on a Perkin–Elmer 883 spectrophotometer, over
the range 4000–200 cmÀ1, by using Nujol mulls between
polyethylene sheets. Conductivities were measured in
about 5 · 10À4 M solutions with a Philips 9509 conduc-
timeter. Mass spectra were recorded on a VG Autospec
by the LSIMS technique with nitrobenzil alcohol as ma-
N(1)–C(41)–C(10)
C(44)–N(1)–N(2)
C(44)–N(1)–C(41)
N(2)–N(1)–C(41)
C(42)–N(2)–N(1)
C(42)–N(2)–Ag
N(1)–N(2)–Ag
N(2)–C(42)–C(43)
C(44)–C(43)–C(42)
N(1)–C(44)–C(43)
1
trix. H, 19F and 31P{1H} NMR spectra were recorded
Symmetry transformations used to generate equivalent atoms: #2
1Àx,1 À y,Àz.
on a Bruker ARX300 or GEMINI 2000 apparatus;
chemical shifts are quoted relative to SiMe4 (external,
1H), CFCl3 (external, 19F) and 85% H3PO4 (external,
31P). C, H and N analyses were performed with a
Perkin–Elmer 2400 microanalyzer. The starting materi-
als [AuX(tht)] (X = Cl, C6F5) [31], [Ag(OTf)(PPh3)]
(M = Au, Ag) [32] and [Au(C6F5)3(OEt2)2] [33] were pre-
pared by published procedures.
intra- or intermolecular C–HÁ Á ÁO(triflate) hydrogen
bonds that are collected in Table 5.
The ligand 1 also reacts with [Au(C6F5)3(OEt2)] (1:1
molar ratio) to give the gold(III) complex
[Au(C6F5)3(pzCH2Fc)] (6) as a yellow air- and mois-
ture-stable solid. The LSIMS + mass spectrum shows
the molecular peak at m/z = 964 as the most intense
peak. The signals due to three C6F5 groups are present
in the IR spectrum. The 19F NMR spectrum shows
two different types of pentafluorophenyl groups in a ra-
tio 1:2, typical of the presence of a ‘‘Au(C6F5)3’’ frag-
ment (see Scheme 1).
3.2. Preparation of [pzCH2Fc] (1)
To a 30 ml acetic acid solution of pyrazole (14 mmol)
was added N,N-dimethylaminomethylferrocene (7
mmol). The orange solution was heated at 80 ꢁC for 3 h
and the solution was neutralized with a saturated aque-
ous solution of NaHCO3. The product was extracted

E.M. Barranco et al. / Inorganica Chimica Acta 358 (2005) 4177–4182
4181
with 3 · 10 ml of dichloromethane and dried with anhy-
3.4. Preparation of [M(pzCH2Fc)(PPh3)]OTf
(M = Au (4) or Ag (5))
drous magnesium sulfate. The dichloromethane solution
was concentrated to ca. 3 ml and further addition of
diethyl ether afforded complex 1 as a brown solid. It
was purified by chromatography with alumina and a mix-
ture of diethyl ether/hexane (80/20). Yield: 30%.
C14H14FeN2 requires: C, 63.2; H, 5.3; N, 10.5. Found:
C, 62.95; H, 5.1; N, 10.35%.
To a dichloromethane solution (20 ml) of 1 (26.6 mg,
0.1 mmol) was added a dichloromethane solution (10
ml) of [Au(OTf)PPh3] (0.1 mmol, prepared in situ with
[AuCl(PPh3)] and AgOTf) or [Ag(OTf)PPh3] (0.1 mmol,
51.9 mg). The mixture was stirred for about 1 h and then
filtered through celite. Concentration to ca. 2 ml and
addition of hexane (20 ml) afforded complexes 4 or 5
as yellow solids. Yield of 4: 65%. C33H29AuF3Fe-
N2O3PS requires: C, 45.35; H, 3.35; N, 3.2; S, 3.65.
Found: C, 44.97; H, 3.2; N, 3.45; S, 3.55%. Yield of 5:
75%. C33H29AgF3FeN2O3PS requires: C, 50.45; H, 3.7;
N, 3.55; S, 4.05. Found: C, 50.4; H, 3.2; N, 3.5; S, 3.95%.
3.3. Preparation of [AuX(pzCH2Fc)] (X = Cl (2) or
C6F5 (3))
A dichloromethane solution (20 ml) of 1 (26.6 mg, 0.1
mmol) and [AuX(tht)] (0.1 mmol, X = Cl, 32 mg; C6F5,
45.2 mg) was stirred for about 30 min and then filtered
through celite. Concentration to ca. 2 ml and addition of
hexane (20 ml) afforded complexes 2 or 3 as yellow sol-
ids. Yield of 2: 45%. C14H14AuClFeN2 requires: C,
33.75; H, 2.85; N, 5.6. Found: C, 33.4; H, 2.9; N,
5.55%. Yield of 3: 75%. C20H14AuF5FeN2 requires: C,
38.2; H, 2.25; N, 4.45. Found: C, 38.05; H, 2.0; N,
4.5%. 19F NMR: À112.1 (m, o-F), À155.2 (t, p-F, J(F–
F) 20.1 Hz), À158.7 (m, m-F).
3.5. Preparation of [Au(C6F5)3(pzCH2Fc)] (6)
A dichloromethane solution (20 ml) of 1 (26.6 mg, 0.1
mmol) and [Au(C6F5)3(OEt2)] (77.2 mg, 0.1 mmol) was
stirred at room temperature for about 1 h and then fil-
tered through celite. Concentration to ca. 2 ml and addi-
tion of hexane (20 ml) afforded complex 6 as a yellow
Table 6
Details of crystal data and structure refinement for complexes 3 and 5
Compound
3
5 Æ C2H4Cl2
Empirical formula
Formula weight
Temperature (K)
C20H14AuF5FeN2
630.15
173(2)
C35H33AgCl2F3FeN2O3PS
884.28
173(2)
˚
Wavelength (A)
0.71073
monoclinic
C2/c
0.71073
monoclinic
P2(1)/n
Crystal system
Space group
Unit cell dimensions
˚
a (A)
25.013(5)
12.344(2)
15.786(2)
90
14.994(5)
9.961(5)
24.530(7)
90
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
127.341(9)
90
106.22(2)
90
3
˚
Volume (A )
3875.1(11)
8
2.160
3518(2)
4
1.670
Z
Density (calculated) (Mg/m3)
Absorption coefficient (mmÀ1
F(000)
)
8.366
2384
1.281
1784
Crystal habit
Crystal size (mm3)
orange prism
0.70 · 0.45 · 0.40 mm3
2.58–25.00
orange prism
0.45 · 0.23 · 0.15
2.22–25.09
Theta range for data collection (ꢁ)
Index ranges
Reflections collected
À1 6 h 6 29, À14 6 k 6 5, À18 6 l 6 15
5268
0 6 h 6 17, À11 6 k 6 1, À29 6 l 6 28
6760
Independent reflections
Completeness to theta = 25.09ꢁ
Absorption correction
3403 [R(int) = 0.0395]
99.6%
6201 [R(int) = 0.0438]
99.1%
W-scans
W-scans
Maximum and minimum transmission
Refinement method
0.1347 and 0.0674
full-matrix least-squares on F2
3403/0/262
0.894 and 0.807
full-matrix least-squares on F2
6201/0/442
Data/restraints/parameters
Goodness-of-fit on F2
1.034
1.004
Final R indices [I > 2r(I)]
R Indices (all data)
Largest difference between peak and hole (e A
R1 = 0.0400, wR2 = 0.0952
R1 = 0.0446, wR2 = 0.0971
2.422 and À2.809
R1 = 0.0455, wR2 = 0.0834
R1 = 0.0872, wR2 = 0.0954
0.690 and À0.630
À3
˚
)

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E.M. Barranco et al. / Inorganica Chimica Acta 358 (2005) 4177–4182
[5] I. Manners, Adv. Organomet. Chem. 37 (1995) 132.
[6] I. Manners, Chem. Commun. (1999) 857.
[7] Y. Zhu, O. Clot, M.O. Wolf, G.P.A. Yap, J. Am. Chem. Soc. 120
(1998) 1812.
solid. Yield: 40%. C32H14AuF15FeN2 requires: C, 39.85;
H, 1.45; N, 2.9. Found: C, 39.8; H, 1.6; N, 2.55%. 19F
NMR: À122.7 (m, 2F, o-F), À124.0 (m, 4F, o-F),
À156.9 (t, 2F, p-F, J(F–F) 20.0 Hz), À157.4 (t, 1F,
p-F, J(F–F) 19.9 Hz), À160.6 (m, 4F, m-F) and
À161.6 (m, 2F, m-F).
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Data were recorded with a Siemens P4 (3) or a Stoe-
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The structures were refined using the program SHELXL-
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Complete X-ray data (excluding structure factors) for
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bridge Crystallographic Data Centre as supplementary
publication nos. 260995 and 260996. These data can
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conts/retrieving.htlm [or from the Cambridge Crystallo-
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1EZ, UK; fax: (internat.) +44 1223 336033; e-mail:
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Acknowledgement
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We thank the Ministerio de Educacion y Ciencia –
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FEDER (project CTQ2004-05495-C02-01/BQU) for
financial support.
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