Synthesis, molecular and crystal structure of the zwitterionic Re(V)-oxo complex ReOCl3(L)(PPh3) with L = 2-(diethylaminomethyl)-4-methylphenol

Article abstract of DOI:10.1016/S0020-1693(99)00384-9

Ligand exchange between 2-(diethylaminomethyl)-4-methylphenol (N^∩OH) and ReOCl₃(PPh₃)₂ yields green crystals of the zwitterionic complex ReOCl₃(O^∩NH)(PPh₃). The X-ray diffraction study reveals that the structure consists of monomeric units in which the distorted octahedral coordination around Re is provided by the oxo group, the three Cl atoms in a mer arrangement, the PPh₃ ligand and the phenolate oxygen trans to the oxo group. The amino-phenol is monodentate via the phenolate oxygen, whereas the amine in the side-arm has been quaternized by migration of the phenol proton. The resulting ammonium group forms an intramolecular N-H...Cl hydrogen bond with an adjacent Cl ligand, making the complex highly resistant to deprotonation. The ¹H NMR spectra indicate that the N-H proton is not labile in CDCl₃. Electron delocalisation along the O=Re-O-C(phenolate) moiety via the sp-hybridised oxygen of the end-on coordinated phenolate would be consistent with the short Re-O(phenolate) distance of 1.915(3) A observed.

Full text of DOI:10.1016/S0020-1693(99)00384-9

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 295 (1999) 209–213
Synthesis, molecular and crystal structure of the zwitterionic
Re(V)–oxo complex ReOCl3(L)(PPh3) with
L=2-(diethylaminomethyl)-4-methylphenol
Fabienne Connac a, Yolande Lucchese a, Miche`le Dartiguenave a,*, Yvon Coulais b,
Andre´ L. Beauchamp c
a Laboratoire de Chimie Inorganique, Uni6ersite´ P. Sabatier, 118 route de Narbonne, 31062 Toulouse, France
b Laboratoire d’Imagerie Morphologique et Fonctionnelle, Uni6ersite´ P. Sabatier, Faculte´ de Me´decine Toulouse-Purpan, 133 route de Narbonne,
31062 Toulouse, France
c De´partement de Chimie, Uni6ersite´ de Montre´al, CP 6128, Succ. Centre-6ille, Montre´al, Que´bec, Canada H3C 3J7
Received 10 June 1999; accepted 6 July 1999
Abstract
Ligand exchange between 2-(diethylaminomethyl)-4-methylphenol (NSOH) and ReOCl3(PPh3)2 yields green crystals of the
zwitterionic complex ReOCl3(OSNH)(PPh3). The X-ray diffraction study reveals that the structure consists of monomeric units
in which the distorted octahedral coordination around Re is provided by the oxo group, the three Cl atoms in a mer arrangement,
the PPh3 ligand and the phenolate oxygen trans to the oxo group. The amino–phenol is monodentate via the phenolate oxygen,
whereas the amine in the side-arm has been quaternized by migration of the phenol proton. The resulting ammonium group forms
an intramolecular NꢀH···Cl hydrogen bond with an adjacent Cl ligand, making the complex highly resistant to deprotonation. The
1H NMR spectra indicate that the NꢀH proton is not labile in CDCl3. Electron delocalisation along the OꢁReꢀOꢀC(phenolate)
moiety via the sp-hybridised oxygen of the end-on coordinated phenolate would be consistent with the short ReꢀO(phenolate)
,
distance of 1.915(3) A observed. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Rhenium–oxo system; Amino–phenol; Hydrogen bond; Crystal structures
the fact that they are known to coordinate to Cu(II)
1. Introduction
[2], Zn(II) [2,3], Ni(II) [4], W(VI) [5] and lan-
thanide(III) [6] centres.
Schiff base complexes, particularly those obtained
by condensation of salicylaldehyde with various
amines, represent an important class of compounds in
the coordination chemistry of Re(V)–oxo systems.
This series of complexes are usually hexacoordinate,
neutral or anionic, with a ligand composition and dis-
tribution depending on the reaction conditions [1]. On
the other hand, (aminomethyl)phenol ligands, where
the lack of unsaturated CꢁN bonds precludes electron
delocalisation over a planar metallacycle, have not yet
been reported to form Re–oxo complexes, in spite of
Our research group is currently investigating the
chemistry of the ReꢁO complexes with various P,O-
donor ligands (PSOH) [7–9], whose importance
as models for radiopharmaceuticals is well known.
This study is now being extended to 2-(dialkylamino-
methyl)phenols. Here we wish to report on the
reaction of 2 - (diethylaminomethyl) - 4 - methylphenol
(NSOH) with ReOCl3(PPh3)2, which gives a quantita-
tive yield of ReOCl3(OSNH)(PPh3), a complex in
which the ligand is coordinated in a monodentate
fashion via oxygen, with the proton being shifted onto
the amino nitrogen atom. This species of an unprece-
dented type in rhenium chemistry was characterised
spectroscopically and its structure was determined by
X-ray diffraction.
* Corresponding author. Tel.: +33-5-6155 6121; fax: +33-5-6155
6118.
E-mail address: dartigue@iris.ups-tlse.fr (M. Dartiguenave)
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00384-9

210
F. Connac et al. / Inorganica Chimica Acta 295 (1999) 209–213
2. Experimental
NERMAG R10-10 mass spectrometer using the FAB
(MNBA) technique.
2.2. Preparation of ReOCl3(OSNH)(PPh3)
2.1. Materials and methods
2 - (Diethylaminomethyl) - 4 - methylphenol - (2 - Et2-
NCH2-4-Me-C6H4OH (NSOH) [10] and ReOCl3(PPh3)2
[11] were synthesised by procedures described in the
literature.
A mixture of 0.50 g (0.60 mmol) of ReOCl3(PPh3)2
and 0.23 g (1.24 mmol) of 2-(diethylaminomethyl)-4-
methylphenol in toluene (40 ml) was heated at 65°C for
30 min. Triethylamine (0.2 ml) was then added and the
mixture was kept at 65°C for a further 30 min. The
green precipitate was filtered, washed with ethanol and
ether, and finally dried in vacuo. Yield: 53% (0.24 g).
Anal. Calc. for C30H34Cl3NO2PRe: C, 47.15; H, 4.48;
N, 1.83; P, 4.05; Re, 24.37. Found: C, 47.34; H, 4.31;
N, 1.74; P, 3.95; Re, 24.08%. IR (cm−1, KBr): 982,
w(ReꢁO); 1286, w(CꢀORe). Mass spectrum (FAB+):
[M]+ 763; [M−Cl]+ 728; [M−L−Cl]+ 535. 31P{1H}
NMR (ppm, CDCl3, 161.98 MHz): −12.0 (s). 1H
NMR (ppm, CDCl3, 400 MHz) (see atom labelling in
Scheme 1): 1.30 (t, 6H, N(CH2CH3)2); 2.06 (s, 3H,
IR spectra (4000–400 cm−1) were recorded as KBr
1
pellets on a Perkin–Elmer 1600 spectrophotometer. H
and 31P{1H} NMR spectra were recorded in CDCl3 at
room temperature (r.t.) on a Bruker ARX 400 appara-
tus. For the 1H spectra, the residual solvent signal
(l=7.27 ppm) was used as internal standard. For
31P{1H} NMR, the external standard was H3PO4 (85%/
D2O, l=0.00 ppm). Mass spectra were obtained with a
2
CH3Ph); 2.87 (h, 2H, N(CHaHbCH3)2, J(Ha–Hb)ꢀ14
3
3
Hz, J(Ha–HN)ꢀ7 Hz, J(Ha–HMe)ꢀ7 Hz); 3.32 (m,
3
2H, N(CHaHbCH3)2); 3.64 (d, 2H, PhꢀCH2ꢀN, J(H–
HN)=6.3 Hz); 6.50 (s, 1H, H3); 6.32, 6.58 (d, 1H each,
H5, H6, J=8.4 Hz,); 7.23–7.80 (m, 15H, PPh3); 8.15
3
(s, br, 1H, HN).
2.3. X-ray diffraction
Scheme 1.
Single crystals were obtained by slow crystallisation
in CH2Cl2–ether at 20°C. The experiment was carried
out with an Enraf–Nonius CAD-4 diffractometer using
graphite-monochromatised Cu Ka radiation (u=
Table 1
Crystallographic data and experimental details
Formula
C30H34Cl3NO2PRe
764.14
monoclinic
P21/c
18.503(3)
9.172(2)
20.030(5)
117.81(2)
3006.7(11)
4
,
1.54056 A). The reduced cell was deduced from 25
Formula weight
Crystal system
Space group
reflections detected on a rotation photograph. The Nig-
gli parameters ruled out non-primitive lattices. The
monoclinic Laue symmetry and systematic absences
checked on the full data set unambiguously defined
space group P21/c. Crystallographic data are sum-
marised in Table 1.
,
a (A)
,
b (A)
,
c (A)
i (°)
3
,
V (A )
Z
The intensities were collected by the ꢀ/2q scan tech-
nique. Orientation and intensity were checked every
hour with five standards. A whole sphere of data
(20 637 reflections) was collected. Intensity fluctuation
remained within 92.1% during data collection. The
data were corrected for absorption (Gaussian integra-
tion, NCRVAX [12], transmission range 0.13–0.64).
Equivalent reflections were averaged and the correc-
tions for the effects of Lorentz and polarisation were
applied. The final data set consisted of 5693 indepen-
dent reflections, of which 4420 with I\2.0|(I) were
retained for structure determination. The structure was
solved by the direct methods of SHELXS-86 [13] and DF
syntheses. Refinement was carried out on F2 with
SHELXL-93 [14]. All non-hydrogen atoms were refined
anisotropically. Phenyl and methylene hydrogens were
Temperature (K)
220(2)
1.688
0.23×0.17×0.04
green plate
3.50
Dcalc (g cm−3
)
Crystal size (mm)
Crystal shape and color
v (mm−1
)
2q Range (°)
Unique data
0–140
5693
4420
Observed data [I\2|(I)]
Weighting scheme (w−1
)
[|2(Fo2)+(0.0176P)2] a
0.0295
b
R [I\2|(I)], R1
wR2
R (all data), R1
0.0664
0.0434
0.0694
0.969
b
wR2
b
Goodness-of-fit (S
)
a P=(Fo2+2Fc2)/3.
b R=ꢀ(ꢁFoꢂ−ꢂFcꢁ)/ꢀ(ꢂFoꢂ), wR2=[ꢀw(Fo2−Fc2)2/ꢀ(wFo2)2]1/2, S=
[ꢀw(Fo2−Fc2)2/(no. of reflns.−no. of params.)]1/2
.

F. Connac et al. / Inorganica Chimica Acta 295 (1999) 209–213
211
introduced at idealised positions. The methyl group
orientations and N–H proton position were determined
from a DF map. All hydrogen atoms were allowed to
ride on their respective non-hydrogen atom, and their
isotropic temperature factor was adjusted to 50%
(methyl) or 20% (others) above the Uiso factor of this
atom. Refinement converged to a conventional R factor
of 0.0295. The general background in the final DF map
was within 90.40 e A−3, whereas the highest peak and
,
−3
,
deepest valley (near Re) were 1.05 and −0.77 e A
,
respectively.
3. Results and discussion
3.1. Reacti6ity
Fig. 1. ORTEP drawing of the ReOCl3(OSNH)(PPh3) molecule. Ellip-
soids correspond to 40% probability. Hydrogens are omitted for
clarity, except for that of the ammonium group, which forms a
hydrogen bond (thin line) with Cl(1).
2 - (Diethylaminomethyl) - 4 - methylphenol (NSOH)
and ReOCl3(PPh3)2 were heated for 30 min in toluene
at 65°C, whereupon ligand exchange took place accord-
ing to Eq. (1).
intramolecular NꢀH···Cl hydrogen bond involving the
ammonium group.
ReOCl3(PPh3)2+NSOH
“ReOCl3(OSNH)(PPh3) (1)+PPh3
To our knowledge, crystallographic results are avail-
able for only four other oxoꢀRe(V) compounds of the
ReO2Cl3P type. In all cases, the arrangement of the
donor atoms is the same as observed here. Three are
neutral and contain a carbonyl-like donor: ReOCl3-
(PPh3)(DMF) [16], ReOCl3(PhEt2P)(PhEt2PꢁO) [17]
and ReOCl3(Ph2PꢀCH2ꢀP(ꢁO)Ph2) [18,19]. The fourth
compound, [ReOCl3{(OC6H4)PPh2}]−, is the most
closely related to the present system, since it is anionic
and its coordination sphere includes a phosphine and
phenolate oxygen [20].
Departure from idealised octahedral angles in com-
pound 1 originates mainly from the widespread ten-
dency of the ReꢁO group to repel the adjacent
metalꢀligand bonds. However, in the ReO2Cl3P struc-
tures known, only the ReꢀCl bonds are affected. The
effect remains small in 1 (OꢀReꢀCl=93.3–95.4°) com-
pared with the above compounds, where at least one
(1)
The green precipitate is readily soluble in CH2Cl2 and
CHCl3, but nearly insoluble in Et2O, EtOH, toluene,
acetonitrile and hydrocarbons. It is stable as a solid as
well as in solution, and deprotonation is never ob-
served, even when NEt3 or the very strong bases NaH
or KOBut are added during the synthesis.
Elemental analysis and fast atom bombardment mass
spectroscopy (FAB+) agree with the formula ReOCl3-
(OSNH)(PPh3). The molecular ion peak is observed at
763.
3.2. X-ray diffraction study
A crystallographic study was undertaken to deter-
mine the ligand binding mode and the stereochemistry
around the metal. The ORTEP drawing of the molecule
is shown in Fig. 1. Selected bond distances and angles
are listed in Table 2.
Table 2
,
Selected distances (A) and angles (°)
The crystal contains monomeric molecules in which
the octahedral coordination around the metal includes
the oxo group, the PPh3 ligand and three Cl atoms
adopting a mer arrangement. The phenolate oxygen
occupies the sixth coordination site, trans to the oxo
group, in agreement with the marked preference of
oxoꢀrhenium(V) compounds with oxygen donor ligands
for a trans-OꢁReꢀO configuration [15]. The amino–
phenol does not act as a bidentate ligand as previously
observed [2–5], but as a monodentate phenolate in
which the ortho side-arm plays the role of proton
quencher. This zwitterionic species is stabilised by an
ReꢀO(1)
ReꢀCl(1)
ReꢀCl(3)
O(1)ꢀC(1)
1.915(3)
2.449(1)
2.396(1)
1.353(5)
ReꢀO(2)
ReꢀCl(2)
ReꢀP
1.669(3)
2.421(1)
2.471(1)
O(1)ꢀReꢀO(2)
O(1)ꢀReꢀCl(1)
O(1)ꢀReꢀCl(2)
O(1)ꢀReꢀCl(3)
O(1)ꢀReꢀP
O(2)ꢀReꢀCl(1)
O(2)ꢀReꢀCl(2)
O(2)ꢀReꢀCl(3)
178.2(2)
86.1(1)
85.7(1)
87.5(1)
89.8(1)
95.4(1)
93.3(1)
93.6(1)
O(2)ꢀReꢀP
88.6(1)
87.61(4)
87.30(5)
174.57(4)
171.78(4)
88.50(4)
96.12(4)
166.0(3)
Cl(1)ꢀReꢀCl(2)
Cl(1)ꢀReꢀCl(3)
Cl(1)ꢀReꢀP
Cl(2)ꢀReꢀCl(3)
Cl(2)ꢀReꢀP
Cl(3)ꢀReꢀP
ReꢀO(1)ꢀC(1)

212
F. Connac et al. / Inorganica Chimica Acta 295 (1999) 209–213
angle is greater than 99.9° (OꢁReꢀCl=92.7–106.5°,
mean=98.8°). In contrast, the ReꢀP bond is displaced
slightly towards the ReꢁO bond: the O(2)ꢀReꢀP angle is
88.6(1)° in 1 and in the 86.7ꢀ87.5° range for the remain-
ing compounds. This pattern seems to be typical of
phosphines, independent of whether the phosphine is
monodentate or chelating.
bond is approximately eclipsed with the ReꢀCl(3) bond
(Cl(3)ꢀReꢀPꢀC(21)= −5.7(2)°). The phenyl ring of the
phenolate unit is roughly coplanar with the ReOCl3
plane (dihedral angle of 19°), thereby preventing short
contacts with the phenyl rings of PPh3.
3.3. Spectroscopic results
The most remarkable feature of 1 concerns the pheno-
late ligand: it is approximately end-on coordinated
(ReꢀOꢀC=166.0(3)°) and it forms a ReꢀO bond
The IR spectrum displays a strong absorption at 982
cm−1 assigned to w(ReꢁO). Three medium bands in the
same region (932, 946 and 970 cm−1) correspond to
w(NꢀC) vibrations [24]. The w(CꢀORe) frequency of the
coordinated phenolate group is detected as a strong
band at 1286 cm−1, shifted by +27 cm−1 from the free
ligand position. These frequencies correspond with those
observed in the Ni and Cu complexes [2,4]. The w(NꢀH)
vibration is likely responsible for a medium, relatively
sharp, band at 3062 cm−1, including various weaker
CꢀH features in the low wavenumber wing.
,
(1.915(3) A) much shorter than those observed for
[ReOCl3{(OC6H4)PPh2}]− (2.016(6) A) [20] and other
,
compounds with the same PSO− ligand [8,20,21]. Con-
sidering that a ReꢀOꢀC angle of ꢀ127° is imposed by
the chelate ring in the latter case, it would be tempting
to ascribe this large increase in bond length to a change
of the hybridisation state of oxygen from sp to approx-
imately sp2. However, the structure of the ReO(OPh)2-
(pz4B) complex, where pz4B− is a tridentate tetrakis-
(pyrazolyl)borate ligand [22], suggests that this effect
should not be so large, since ReꢀO distances of 1.946(5)
The solid-state structure is retained in solution, as
evidenced from the 1H NMR data. The spectrum of 1 at
r.t. exhibits a clear signal at 8.15 ppm for the NꢀH
proton. The aromatic ring gives, as expected, a singlet at
6.50 ppm for H3 (Scheme 1), doublets at 6.32 and 6.58
ppm for the coupled H5 and H6 protons, and a singlet
at 2.06 ppm for the 4-methyl group. The usual multiplet
is observed at 7.2–7.8 ppm for PPh3. The remaining
resonances were assigned by comparison with those of
related Zn compounds [2,3]. Even though the a-CH2
protons are inequivalent under static conditions in the
crystal, they give an averaged signal at 3.64 ppm because
of fast motion in solution. It should be noted, however,
that instead of the singlet expected, a doublet is obtained
due to coupling with the NꢀH proton (3J=6.3 Hz),
which indicates that the latter proton is not labile on the
NMR time scale. The two ethyl groups are also averaged
in solution. The ethyl CH2 protons are inequivalent and
give individual resonances, being connected to a N atom
bearing three different substituents. The multiplet at 2.87
ppm (Ha) actually contains seven components as a result
of couplings with the geminal Hb proton (2Jꢀ14 Hz),
the three methyl protons HMe (3Jꢀ7 Hz) and the HN
proton (3Jꢀ7 Hz). A multiplet is found for the second
ethyl CH2 proton at 3.32 ppm, but it is not so well
resolved. As mentioned previously for Zn complexes [3],
the high-field signal likely corresponds to the proton
closer to the coordinated Cl atoms. The terminal methyl
groups appear as a pseudo-triplet, indicating equal
coupling constants with the two adjacent CH2 protons.
,
and 1.963(5) A are found for phenolate ligands, which
make ReꢀOꢀC angles of 125.9(4) and 127.5(4)°, respec-
tively. Therefore, hybridisation may play a role, but
other constraints introduced by the chelate ring, includ-
ing intramolecular non-bonded contacts and distortion
in the coordination polyhedron (for instance, the
OꢁReꢀO angle is 163.6(3)°), also undoubtedly contribute
to making the ReꢀO bond longer in the chelates.
The remaining bonds to rhenium are more conven-
,
tional. The ReꢁO distance (1.669(3) A) is shorter than
for ReO(OR)Cl2(PPh3)2 and related compounds [25], but
apparently normal for a ReO2Cl3P complex of this type,
since the same value was obtained for [ReOCl3-
{(OC6H4)PPh2}]− [20]. The ReꢀP bond in 1 (2.471(1) A)
,
,
is longer than in the latter compound (2.422(2) A), but
such a difference is normal for this pair of ligands. The
chelate compound shows ReꢀCl bonds systematically
shorter than in 1. For instance, the mutually trans
,
ReꢀCl(2) and ReꢀCl(3) bonds (2.421(1) and 2.396(1) A)
in 1, although in the normal range for mono-oxo com-
pounds [25], are significantly longer than those of
[ReOCl3{(OC6H4)PPh2}]− (2.398(3) and 2.374(3) A).
,
Probably because the small angle (76.1(2)°) in the chelate
ring leaves more room around the metal for the remain-
ing ligands in the latter case. The ReꢀCl(1) bond in 1,
,
trans to P, is particularly long (2.449(1) A versus 2.369(2)
,
A in the chelate): this is consistent with the strong trans
influence of phosphines. The NꢀH···Cl hydrogen bond
does not deviate much from linearity (NꢀHꢀCl=158°),
,
but the NꢀCl separation (3.318(5) A) is somewhat
,
greater than the typical value of 3.21 A proposed for this
4. Conclusion
type of bond [23].
The phosphine adopts the common propeller confor-
mation (ReꢀPꢀCn1ꢀCortho angles=45.6(4), 61.6(4) and
51.9(4)° for n=1, 2 and 3, respectively) and its PꢀC(21)
In contrast with the related PSOH ligands, which
form chelates with the Re(V) oxo core, the present
NSOH molecule binds in a monodentate manner via the

F. Connac et al. / Inorganica Chimica Acta 295 (1999) 209–213
213
deprotonated phenolate group, the amino group acting
as proton quencher. This difference observed between
the two types of ligands originates from the relative
affinities of the protons and Re center for amine and
phosphine donors, the amino group being more reactive
towards the electrophilic proton than toward the
rhenium centre, whereas the opposite trend applies to
phosphorus.
The complex described in this paper results from
selective ligand exchange leading to the substitution of
one PPh3 by a phenolate ligand. Under similar reaction
conditions, [ReOCl4]− gave several reactive species,
which could not be separated and isolated. On the
other hand, no reaction was observed to take place with
ReNCl2(PPh3)2.
and to the Natural Sciences and Engineering Research
Council of Canada, for financial support.
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The orientation of the phenolate group in the
complex raises interesting questions about the
electronic structure. In a linear OꢁReꢀOꢀR unit, the
alkoxide oxygen possesses p orbitals available to
interact with the rhenium d orbitals and generate a
delocalised p-system. Generally, this factor does not
seem to introduce huge stabilisation in simple
ReO(OR)Cl2L2 molecules (R=Me, Et, i-Pr; L=PPh3,
N-heterocycle) [25], since the ReꢀOꢀR angle is usually
near 145°, although angles \168° were found in two
cases. The effect may be more important in the present
case, since delocalisation could extend over the phenyl
ring. Partial multiple character in the ReꢀO(2) bond
,
would be consistent with this bond being ꢀ0.04 A
shorter than in ReO(OPh)2(pz4B) [22], where con-
jugation is impossible since the PhO− ligand is cis to
the ReꢁO bond. However, the remaining ligands being
very different in these two compounds, a larger sample
of structures is needed to reliably assess the role of
p-bonding. It should be noted that the amino–phenol
ligand used in this study could be a valuable tool for
investigating this point, since its protonated side-arm
helps freezing in the end-on configuration, a phenolate
ligand that could otherwise coordinate with a bent
orientation.
[13] G.M. Sheldrick, SHELXS-86, A crystallographic computation sys-
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Chemie, Universita¨t Go¨ttingen, Germany, 1986.
[14] G.M. Sheldrick, SHELXL-93, Program for the refinement of crystal
structures, Institut fu¨r Anorganische Chemie, Universita¨t Go¨ttin-
gen, Germany, 1993.
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New York, 1988.
[16] V.S. Sergienko, Zh. Neorg. Khim. 39 (1994) 1985.
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(1993) 23.
[20] C. Bolzati, F. Tisato, F. Refosco, G. Bandoli, A. Dolmella, Inorg.
Chem. 35 (1996) 6221.
[21] H. Luo, I. Setyawati, S.J. Rettig, C. Orvig, Inorg. Chem. 34 (1995)
2287.
5. Supplementary material
[22] A. Paulo, A. Domingos, J. Marcalo, A.P. de Matos, I. Santos,
Inorg. Chem. 34 (1995) 2113.
[23] G.H. Stout, L.H. Jensen, X-ray Structure Determination, a
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(1968) 786.
Lists of atomic coordinates and thermal parameters,
distances, bond angles, torsion angles and least-squares
planes for the crystal structure are available upon
request.
[25] (a) R. Graziani, U. Casellato, R. Rossi, A. Marchi, J. Crystallogr.
Spectrosc. Res. 15 (1985) 573. (b) C.J.L. Lock, G. Turner, Can.
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Acknowledgements
´
duca-
We are grateful to the French Ministe`re de l’E
tion Nationale, de la Recherche et de la Technologie,