Vanadium-catalysed aerobic cleavage of C-C bonds in substituted cyclohexanones to afford carboxylic acids: Two model complexes with tetrahedral geometry around vanadium(V)

Article abstract of DOI:10.1002/ejic.200390120

Two monosilanol/disilanol derivatives were synthesised by treatment of Ph₃SiOH or Ph₂Si(OH)₂ with [nBu₄N]VO₃ (3) in ethanol solution. The mononuclear anionic dioxo vanadium(v) species [{Ph₃SiO}₂VO₂]^- (4) and the dinuclear complex anion [{Ph₂SiO₂VO₂}₂]^2- (5) thus obtained are unusual and novel model compounds with ≡Si-O-V(O)₂O linkages characterised by single-crystal X-ray diffraction. The catalytic activities of these tert-butylammonium complexes, together with those of the simple vanadium derivatives 3 and 5, of the vanadyl precursors VOSO₄·5H₂O (1) and [VO(acac)₂] (2), and also of "H₆[PMo₉V₃O₄₀] ·aq", were tested for aerobic C-C bond cleavage in 2-hydroxycyclohexanone and 2-methylcyclohexanone. Monomeric vanadium species show the same chemistry as the polyoxometalate but with lower yields of acids or keto acids, and so the redox and acid properties of HPA-n (n = 3, 4) are better tuned. Mechanisms for these oxidation reaction, including electron transfer from a substrate coordinated to vanadium, are proposed. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/ejic.200390120

FULL PAPER
Vanadium-Catalysed Aerobic Cleavage of C؊C Bonds in Substituted
Cyclohexanones To Afford Carboxylic Acids: Two Model Complexes with
Tetrahedral Geometry around Vanadium(
V
)
[a]
[b]
[a]
[c]
Maxence Vennat, Patrick Herson, Jean-Marie Br e´ geault,* and Georgiy B. Shul’pin
Keywords: Catalysis / Oxidation / Dioxygen / Oxo ligands / Vanadium
Two monosilanol/disilanol derivatives were synthesised by
treatment of Ph SiOH or Ph Si(OH) with [nBu N]VO (3) in
[VO(acac)
2 6 9 3
] (2), and also of ‘‘H [PMo V O40]·aq’’, were tested
3
2
2
4
3
for aerobic C−C bond cleavage in 2-hydroxycyclohexanone
ethanol solution. The mononuclear anionic dioxo vana- and 2-methylcyclohexanone. Monomeric vanadium species
−
dium(
plex anion [{Ph
and novel model compounds with ϵSi−O−V(O)
v
) species [{Ph
3
SiO}
2
VO
2
]
(4) and the dinuclear com- show the same chemistry as the polyoxometalate but with
2
−
2
SiO
2
VO
2
}
2
]
(5) thus obtained are unusual
O linkages
characterised by single-crystal X-ray diffraction. The cata-
lytic activities of these tert-butylammonium complexes, to-
gether with those of the simple vanadium derivatives 3 and
lower yields of acids or keto acids, and so the redox and acid
properties of HPA-n (n = 3, 4) are better tuned. Mechanisms
for these oxidation reaction, including electron transfer from
a substrate coordinated to vanadium, are proposed.
2
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
5,
of the vanadyl precursors VOSO
4
·5H
2
O
(1) and Germany, 2003)
Introduction
Vanadium() oxoalkoxides such as [VO(OEt) ] or hetero-
3
polyacids containing vanadium() ions with common for-
mula H3ϩn[PMo12ϪnV O ]·xH O (denoted HPA-n, where
n
40
2
n ϭ 1, 2, 3 etc.) have been proposed as precursors for the
[
1Ϫ4]
catalytic oxidation of α-ketols and ketones by dioxygen.
It has also been demonstrated that aqueous solutions of
HPA-n are multicomponent systems: the higher the value
of n, the more complex the system. These solutions contain
a great number of polyanions, positional isomers of these,
as well as monomeric metallo species such as cis-dioxo
ϩ
[
VO2] (pervanadyl moieties) in equilibrium with lacunary
[
5]
31
heteropolyanion. The P NMR spectrum of ‘‘aqueous
H [PMo V O ]’’, for example, attests to the existence of
6
9
3
40
several species containing P, Mo and V (Figure 1).
31
40]’’ (1 mol·dmϪ3;
Figure 1. P NMR spectrum of ‘‘H [PMo V O
6 9 3
Preparations of HPA-n obtained by diethyl ether extrac- pH 0.8) prepared by treatment of MoO .xH O, V O and H PO
3
2
2
5
3
4
tion[ or by an oxo-peroxo route give EPR signals attrib-
6]
[7]
with H O at room temperature (the identification is supported by
2 2
[5,7,47]
solutions and solids prepared with 2 р n р 10 Ϫ see ref.
[
[
[
a]
b]
c]
Syst e` mes Interfaciaux a` lЈEchelle Nanom e´ trique, Universit e´
Pierre et Marie Curie/CNRS,
ϩ
utable to [VO]2 species, as do catalytic systems with vana-
dium() oxoalkoxides. At low pH (Ͻ2), heteropolyanion so-
case 196, 4 place Jussieu, 75252 Paris cedex 05, France
Fax: (internat.) ϩ 331/44275536
E-mail: bregeaul@ccr.jussieu.fr
ϩ
lutions contain a solvated cis-dioxo cation [VO (H O) ] ,
Chimie Inorganique et Mat e´ riaux Mol e´ culaires, Universit e´
Pierre et Marie Curie/CNRS,
2
2
4
ϩ [8,9]
usually written as [VO ] .
This is considered to be one
2
case 42, 4 place Jussieu, 75252 Paris cedex 05, France
Fax: (internat.) ϩ 331/44273841
[5]
of the catalytically active species. Such complexity makes
mechanistic studies very difficult. Moreover, in aerobic ke-
tone oxidations, HPA-n species are active not only as redox
systems but also play a very important role as Brønsted
E-mail: herson@ccr.jussieu.fr
Semenov Institute of Chemical Physics, Russian Academy of
Sciences, ul. Kosygina, dom 4, Moscow 119991, Russia
Fax: (internat.) ϩ 7-095/1376130
[
3,10]
E-mail: shulpin@chph.ras.ru
acids for the enolisation process.
To the best of our
908
 2003 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 1434Ϫ1948/03/0305Ϫ0908 $ 20.00ϩ.50/0 Eur. J. Inorg. Chem. 2003, 908Ϫ917

Vanadium-Catalysed Aerobic Cleavage of CϪC Bonds in Substituted Cyclohexanones
FULL PAPER
knowledge there is no iono-covalent crystal structure con- oxo species [nBu N][{Ph SiO} VO ] (4a) or a dinuclear
4
3
2
2
ϩ
taining the pervanadyl cation [VO (H O) ] .
complex [nBu N] [{Ph SiO VO } ] (5a), respectively, in fair
2
2
4
4 2 2 2 2 2
In this work we have prepared and characterised two to nearly quantitative yields. Formulae of the anions of
novel dioxovanadium() complexes with two organosilanol- both complexes are presented in Scheme 1.
ato ligands. As other catalyst precursors we have also used
simple vanadium() and vanadium() derivatives, mainly
mononuclear compounds, which Ϫ unlike HPA-n Ϫ are not
strong acids. Naturally, it was very interesting to study the
influence of acids on the oxidation reactions. We hoped that
combinations of simple vanadium complexes with added
acids would provide models of catalytically active systems
based on various V-containing HPA-n species. Work on the
use of these complexes in homogeneous catalytic systems
(oxidation of 2-hydroxycyclohexanone and 2-methylcyclo-
hexanone) with other precursors and comparison of conver-
sions and selectivities with those obtained with HPA-n has
made us refine our previous mechanism.[ The novel four-
coordinate complexes should open perspectives for incorp-
oration of oxovanadium catalysts into polymeric mat-
3]
[
11]
erials.
Organosilanols model the hydroxy groups that lie
[
12]
on silica surfaces, and the complexes synthesised can be
regarded as models for metal oxides supported on silica sur-
[
13]
faces.
Scheme 1
Results and Discussion
The new vanadium-siloxane complexes were fully charac-
As catalyst precursors we used simple derivatives of vana- terised by elemental analysis, vibrational spectroscopy and
dium(), such as VOSO ·5H O (1) and [VO(acac) ] (2), as room-temperature single-crystal X-ray diffraction studies.
4
2
2
well as vanadium() derivatives in the form of tetrabu- Crystals of [PPh
][{Ph
SiO} VO ] (4b; the nBu N cation
3 2 2 4
4
tylammonium salts: [nBu N]VO (3) and two pervanadyl was replaced by PPh
to prepare crystals for X-ray analysis)
4
3
4
complexes, [nBu N][{Ph SiO} VO ] (4a) and [nBu N] - and of 5a present novel anionic structures, shown in Fig-
4
3
2
2
4
2
[
{Ph SiO VO } ] (5a).
ures 2 and 3. Selected bond lengths and angles are listed in
2
2
2 2
Table 1 and 2.
Synthesis of New Vanadium Pre-Catalysts
Solid-State Structure of the Mononuclear Complex
Only a few examples of mono- and dioxovanadium()
[
PPh ][{Ph SiO} VO ] (4b)
4 3 2 2
complexes derived from silanols or disilanols have been
13,14]
described
Ph SiO) VO
in
the
literature.[
Compounds
Compound 4b consists of tetraphenylphosphonium cat-
have been ions and monomeric vanadosiloxane anions. There is one
[
15Ϫ19]
[20]
(
and [(tBu) Si(O) VOCl]
3
3
2 2 3
structurally characterised. The latter contains a twelve- independent anionic species in the asymmetric unit, loosely
membered ring, {VOSiO} , in which the vanadium and sil- connected through a series of phenyl ring interactions. The
3
icon atoms exist in distorted tetrahedral coordinations. Tris- crystal packing is determined by van der Waals forces, the
˚
ilanols such as R Si O (OH) [R ϭ cyclopentyl, cyclohexyl closest C···C approach being 3.6 A, greater than the sum of
7
7
9
3
(
cy), phenyl, etc.] are known to react with VOCl , VO(O- the van der Waals radii. The geometry at vanadium() is a
3
nPr) or VO(CH SiMe ) to afford mixtures of the mono- tetrahedral coordination unusual for mononuclear com-
3
2
3 3
meric and dimeric vanadium() silsesquioxanes [(cy) - plexes (Figure 2), with the oxygen atoms of the Ph SiO li-
7
3
[13,18,21]
Si O VO] and [(cy) Si O VO] .
An anionic dioxo gands occupying two sites and two terminal oxygen atoms
7
12
7
7
12
2
complex (H NEt ) [{Ph SiO VO } ] was prepared in low occupying the remaining sites. The VO arrangement is no
2
2 2
2
2
2 2
2
[
22]
0
yield (29%) from diphenylsilanediol and vanadium() di- doubt favoured (as in some other d systems) because this
alkylamides, VO(NEt ) . allows better Opπ Ǟ Mdπ bonding than a linear arrange-
2
3
We focused our initial efforts on synthesising oxo-peroxo ment would. The characteristic lengths in 4b are similar to
species and in this paper cis-dioxoanionic complexes by un- those observed in 5a (vide infra); the VO bond lengths to
˚
demanding reactions, with the goal of using them to gener- the terminal oxygens are 1.621(3) and 1.607(3) A to O(1)
ϩ
ate catalytic intermediates with the [VO2] group for and O(2), respectively; this suggests a bond order ap-
carbonϪcarbon bond cleavage in α-hydroxy ketones, cyclo- proaching two for the VϭO linkages in the VO group. The
2
˚
alkanones, etc. with dioxygen. Treatment of Ph SiOH or average VϪO(Si) bond length is 1.8145 A. The SiϪO bond
3
Ph Si(OH) with compound 3, dissolved in ROH (R ϭ Me, lengths are similar to those found in the parent siloxane
2
2
[
23Ϫ26]
Et), resulted in the formation of a mononuclear anionic di- rings.
Eur. J. Inorg. Chem. 2003, 908Ϫ917
909

M. Vennat, P. Herson, J.-M. Br e´ geault, G. B. Shul’pin
FULL PAPER
Figure 2. CAMERON representation[ (ellipsoids at 30% probability level) of [{Ph
61]
Ϫ
3
2 2
SiO} VO ] , showing the atom-labelling scheme (H
atoms omitted for clarity)
[61]
2 2 2 2
SiO VO }
]2Ϫ, showing the atom-labelling scheme
Figure 3. CAMERON representation (ellipsoids at 30% probability level) of [{Ph
H atoms omitted for clarity)
(
910
Eur. J. Inorg. Chem. 2003, 908Ϫ917

Vanadium-Catalysed Aerobic Cleavage of CϪC Bonds in Substituted Cyclohexanones
FULL PAPER
˚
tetraphenyl phosphonium cations are well behaved and have
the expected dimensions.
Table 1. Selected bond lengths [A] and angles [°] for 4b
Bond length
Bond angle
Solid-State Structure of the Dinuclear Complex
V(1)ϪO(1)
V(1)ϪO(2)
1.621(3)
1.607(3)
1.815(3)
1.814(3)
1.612(3)
1.853(5)
1.869(5)
1.865(5)
1.601(3)
1.864(5)
1.865(5)
1.862(5)
O(1)ϪV(1)ϪO(2)
O(1)ϪV(1)ϪO(11)
O(2)ϪV(1)ϪO(11)
O(1)ϪV(1)ϪO(12)
O(2)ϪV(1)ϪO(12)
O(11)ϪV(1)ϪO(12)
108.4(2)
109.5(2)
110.8(2)
109.7(2)
110.6(2)
107.8(2)
[
nBu N] [{Ph SiO VO } ] (5a)
4 2 2 2 2 2
V(1)ϪO(11)
V(1)ϪO(12)
Si(1)ϪO(11)
Si(1)ϪC(101)
Si(1)ϪC(107)
Si(1)ϪC(113)
Si(2)ϪO(12)
Si(2)ϪC(201)
Si(2)ϪC(207)
Si(2)ϪC(213)
Compound 5a consists of discrete dimeric anions and
tetrabutylammonium cations. The complex possesses crys-
¯
tallographic site symmetry 1, and there is one half of the
complex anion per crystallographically independent unit.
The anionic moiety has two vanadium() atoms bridged by
two diphenylsiloxy ligands to form an eight-membered ring,
‘
‘{VOSiO} ’’, in an extended chair conformation. In addi-
2
tion, each vanadium is bonded to two terminal oxygen
atoms. For a side view of the anion showing the conforma-
tion of the ring, see Figure 3. The O(1) and O(2) bond
˚
lengths to V(1) are 1.613(3) and 1.619(3) A, respectively;
˚
these correspond to VϭO linkages in compound 4b. Similar
VϭO bond lengths are found for the trigonal-bipyramidal
Table 2. Selected bond lengths [A] and angles [°] for 5a
Bond length
Bond angle
environment of [VO
2
(pic)(HMPA)], with 1.600(5) and
˚
[37]
1
2
.606(5) A, and for [nBu ][VO (pca) ] (pcaH ϭ pyrazine-
-carboxylic acid), for which the vanadium is found to have
4
2
2
V(1)ϪO(1)
V(1)ϪO(2)
1.613(3)
1.619(3)
1.817(3)
1.807(3)
1.613(4)
1.618(3)
1.872(5)
1.872(5)
O(1)ϪV(1)ϪO(2)
O(1)ϪV(1)ϪO(11)
O(2)ϪV(1)ϪO(11)
O(1)ϪV(1)ϪO(12)
O(2)ϪV(1)ϪO(12)
O(11)ϪV(1)ϪO(12)
109.0(2)
108.9(2)
111.5(2)
108.2(2)
an octahedral coordination geometry with two terminal oxo
V(1)ϪO(11)
V(1)ϪO(12)
Si(1)ϪO(11)
Si(1)ϪO(12Ј)
Si(1)ϪC(101)
Si(1)ϪC(107)
ligands in the cis positions [d(VϭO) ഠ 1.620(1) and
˚
[38]
109.3(2) 1.634(1) A].
109.9(2)
The VϪO(Si) bond lengths of 1.817(3) and of 1.807(3) A˚
are short in comparison with the chelate ligand VϪO(C)
˚
bond lengths of 1.990(1) and 1.996(1) A found in the pyraz-
ine-2-carboxylato complex.
[
38]
We consider that the
VϪO(Si) distances in 4b and in 5a indicate very strong
bonds. The SiϪO and SiϪC bond lengths are in good
The Si(1)O(11)V(1)O(12)Si(2) moiety in 4b is almost
agreement
cyclotetrasiloxanes
with
values
and in 4b (vide supra).
found
in
other
planar, the largest deviation from the least-squares plane
[
23Ϫ26]
˚
being 0.17 A for V(1). The angle between the O(1)V(1)O(2)
The tetrahedral geometry around vanadium is evidenced
by an average OϪVϪO bond angle of 109.5°. Again, four-
coordination is not very common for inorganic anionic di-
and Si(1)V(1)Si(2) planes is 88.7°. In the siloxane (Ph SiO)4
2
the {SiϪO} ring is somewhat closer to planarity, the max-
4
imum deviation from the root-mean square plane being
V
nuclear V complexes; another typical dioxovanadium()
˚
[24]
0
1
.1 A.
salt that has been found to have a tetrahedral structure with
In 4b, the VϪOϪSi bond angles are 151.4(2)° and
[
39]
chains is KVO3. The average VϪOϪSi bond angle in 5a
Ϫ of 144.1(2)° Ϫ is smaller than those found in 4b and
similar to the values of 145° found for the SiϪOϪSi angle
46.7(2)°. The formation of the tetrahedral VO species was
2
recently evaluated by a theoretical study of the extended
Hückel (EH) type, this species being excluded in the cases
[
40]
in [(Me SiO) ], but significantly smaller than those found
[27]
2
4
of SiO , TiO and γ-Al O .
To the best of our know-
2
2
2
3
for the previously reported tetrasiloxane compounds (ca.
ledge, no monomeric tetrahedral vanadium() species con-
taining oxo organic ligands has yet been observed in aque-
ous solution, and such tetrahedral units are also not com-
mon for dioxovanadium() moieties in the solid state: only
[
23Ϫ26]
1
59.5°),
also with a puckered eight-membered ring,
4Ϫ
[35]
and for VϪOϪV in [V O ] [158.2(2)°]. The OϪSiϪO
4
12
angle of 111.5(2)°, the CϪSiϪC of 108.1(2)° and the
OϪSiϪC angles (mean ca. 109.3°) are close to the expected
values commonly found for tetrahedral geometry in cyclic
[
28]
[29]
distorted square-pyramidal, trigonal-bipyramidal
octahedral
and
[
30]
geometries have recently been characterised
[
23Ϫ26]
siloxanes;
this may indicate that no strain is involved
with cis-dioxo groups,[28Ϫ30] but tetrahedral species exist in
˚
in bridging the nearly 4.4 A V···V distance. The closest
C···C approach being 3.7 A, the crystal packing must be
some chains of inorganic vanadates. With large metal cat-
˚
2
ϩ
ions such as Ba , the structures adopted by dinegative me-
tavanadates, MV O , contains chains of vertex-shared VO
4
determined by van der Waals forces .
2
6
[
31,32]
tetrahedra.
This structural motif is typically found in
Main Features of Vibrational Studies of Synthesised
Complexes
[33]
[34]
orthovanadates such as LiMnVO
and LnVO4.
The
4
4
Ϫ
[
V O ] ion has a cyclic tetrametaphosphate-like structure
4
12
[35]
with an Ot-V-Ot angle of only 102.3(1)°, the three others
The infrared spectra of the complexes are consistent with
3
Ϫ
being 111°, while [V O ] is pentanuclear with OϪVϪO the deprotonation of the silanol functionalities in the com-
5
14
angles ranging from 107.6(4) to 112.7(3)°.[ In 4b, the plexes. They show features similar to those observed for
36]
Eur. J. Inorg. Chem. 2003, 908Ϫ917
911

M. Vennat, P. Herson, J.-M. Br e´ geault, G. B. Shul’pin
FULL PAPER
other mono- or dinuclear pervanadyl species;[ in addition Table 3. Oxidation of 2-hydroxycyclohexanone (6) to adipic acid
41]
Ϫ1
a band at 1588 cm may be assigned as ν(CϪC) of the (Scheme 2)
[
42,43]
phenyl ring,
1
and a very sharp band at 1113 for 4a and
Entry[a] Catalyst precursor T (°C) Conversion (%)[b] Yield (%)[c]
Ϫ1
117 cm for 5a is due to an in-plane deformation of the
phenyl ring δ(CϪH), with some ν(SiϪC) character. Con-
sistently with the structural data (vide supra) and the exist-
ence of short VϭO bonds, there are strong ν(VϭO) bands
1
2
3
4
5
6
7
8
9
VOSO
4
·5H
2
O (1) 20
46
95
42
96
40
77
77
100
72
100
100
26
56
43
51
39
30
61
60
66
71
90
60
[VO(acac) ] (2)
2
20
60
20
60
20
60
20
60
60
Ϫ1
at ca. 939 and 927 cm (sh.) for 4a, and at 941 and 925
Ϫ1
[nBu N]VO (3)
4
3
cm for 5a; peaks observed in the two above regions for
ϩ
previous [VO ] complexes are assigned to ν and ν modes
2
as
s
4a
5a
of the VO group, respectively. A maximum at 925 cmϪ1
[44]
2
for 5a and a shoulder (4a) are tentatively attributed to
Ϫ1
10
SiϪO(V) stretching. SiϪC stretches appear at ca. 743 cm
11
HPA-3
for both compounds. In the Raman spectra, comparison of
Ϫ1
the ν(VϭO) at 943 cm for solid 4a with the maximum at
[a]
Conditions: vanadium complex, 0.076 mmol; 2-hydroxycy-
Ϫ1
9
35 cm (4a in methanol), together with other unattrib- clohexanone, 7.6 mmol; methanol, 5 mL; dioxygen, 0.1 MPa; reac-
[b]
uted maxima, suggests that the overall structure of the per- tion time 24 h.
Conversion is percentage of the substrate con-
sumed. [ Yield (%) is (mmol of dimethyl adipate per mmol of the
substrate)ϫ100 (determined after addition of ethereal diazometh-
ane).
c]
vanadyl complex 4a is conserved in methanol at room tem-
perature, as would be expected for a compound originally
prepared in methanol or ethanol.
Catalytic Oxidative Cleavage of α-Ketols (α-Hydroxy
Ketones)
For the oxidation of 2-hydroxycyclohexanone (6) we used
simple vanadium derivatives 1, 2 and 3, the synthesised
compounds 4a and 5a, and also ‘‘H [PMo V O ]·11H O’’
6
9
3
40
2
(
HPA-3). Methanol was employed as solvent and the reac-
tions were carried out at two temperatures, 20 and 60 °C
Table 3). The main product was adipic acid (Scheme 2),
(
and the concentration of this compound was measured
after esterification by addition of ethereal diazomethane.
The course of the reaction was followed by dioxygen con-
sumption, monitored by a gas burette system, and by GC
analysis. Dioxygen uptake was observed in the reactions
catalysed by mononuclear complexes, with moderate con-
version even at room temperature (Table 3, entries 1, 3, 5). Scheme 2
An induction period was only found in the case of
[
VO(acac) ]. During this period the active catalyst precursor
2
probably changes; this transformation may be de-coordina- lective formation of the esters of 3-acetyl-2,2-dimethylcyclo-
tion of one of the acac ligands strongly bound to vanadium. butaneacetic acid (methyl esters of the cis-pinonic acids)
Higher degrees of conversion were obtained at 60 °C was observed, with 100% conversion and high diastereose-
(
entries 2, 4, 6). It can be seen that mono-oxovanadium() lectivity (99:1 for 1S,3S:1S,3R and 1R,3R:1R,3S, respect-
[
4]
complexes and polyoxovanadium() species are moderately ively). As found in this work, compound 4a does not cata-
active but the selectivity is rather low. Thus, oxidation cata- lyse the oxidative transformations of 7 at all at room tem-
lysed by VO3 results in the formation of undesired prod- perature. We assume that steric hindrance is a crucial factor
Ϫ
ucts (cyclohexanedione in a yield Յ 20%, dimethyl pentane- governing the activity of complex 4a; the bulkiness of the
dioate, butanedioate etc.). Catalysis by 4a or 5a, and espe- three C H groups on Si(1) and Si(2) (Figure 2) and the
6
5
cially by HPA-3, favours the formation of dimethyl adipate. methyl group of the 2-hydroxypinan-3-one may prevent the
It should be noted that the oxidative cleavage of 2-hydroxy- interaction of the VO group with the α-hydroxy group of
2
[
45]
cyclohexanone to produce adipic acid, as well as of vari- 7. The inactivity of 4a in reaction with 7 is in agreement
ous substituted α-hydroxy ketones,[ does not require the with the assumption that only relatively small mononuclear
presence of a strong Brønsted acid and occurs with both cis-dioxo species are responsible for the catalytic activity. It
4]
mononuclear vanadium() and () species; [VOCl ]/ seems that the HPA-3 anion is also too bulky and that dir-
3
[46]
MeCN/O systems give only α-diketones.
ect coordination of this anion with 7 is sterically restricted.
2
Recently the ‘‘HPA-3/O /MeOH’’ system has been suc- However, the dissociation of HPA-3 to produce, inter alia,
2
ϩ
cessfully used in the oxidative cleavage at room temperature a small reactive species such as [VO2] would explain the
of
(1S,2S,5S)-2-hydroxypinan-3-one
(7a)
and
its high efficiency of HPA-3 in the oxidative transformation of
(1R,2R,5R) enantiomer 7b (see Scheme 2). The diastereose- sterically hindered 7.
912
Eur. J. Inorg. Chem. 2003, 908Ϫ917

Vanadium-Catalysed Aerobic Cleavage of CϪC Bonds in Substituted Cyclohexanones
FULL PAPER
Catalytic Oxidative Cleavage of 2-Methylcyclohexanone (8) gen consumption has an induction period during which the
acac ligand is replaced by other ligands. Addition of 10
equivalents of PTSA increases the yield only in the case of
Unlike 2-hydroxycyclohexanone, 2-methylcyclohexanone
is not oxidised in methanol by the V -based catalytic sys-
tems without an induction period if there is no Brønsted
acid present in the reaction solution (Table 4, entries 6 and
V
2
(entry 4). Replacement of PTSA by trifluoromethanesul-
fonic acid gives the highest yield (entry 5). The addition of
either PTSA or CF SO H (entries 4 and 5) results in O
2
3
3
8
). None of the quaternary ammonium salts of HPA-n was
uptake without an induction period. The ‘‘VOSO ·5H O/
4
2
active in these oxidations (not shown here) and only the
heteropolyacids HPA-n, which are known to be strong ac-
ids, catalyse CϪC bond cleavage (entries 10 and 11). How-
ever, these results with alkylammonium may be ambiguous,
because these salts are not only less acidic but also less effi-
MeOH/O ’’ system gives a blue solution, which turns green
2
immediately after the addition of 2-methylcyclohexanone
(8). This change in colour is apparently due to the forma-
tion of a complex between vanadium-containing species
and 8.
cient oxidants (change of redox potentials and stabilisation
ϩ
We also investigated the oxidation of 8 in acetonitrile,
which is aprotic, although residual water cannot be ex-
cluded. It turned out that, in the absence of any other addit-
ives, catalyst precursor 3 induces slow consumption of mo-
lecular oxygen (Figure 4, curve 1). The oxidation occurs
with auto-acceleration after an induction period (ca. 4
hours) during which the colour of the reaction solution
changes from very pale yellow to intense orange-yellow. Ad-
dition of acetic acid (ഠ 4 equivalents relative to 3) acceler-
ates the reaction insignificantly, the induction period re-
maining the same (curve 3, see Figure 4). The reaction also
proceeds very similarly (curve 4, see Figure 4) in the pres-
ence of a chelating amino acid (pyrazine-2-carboxylic acid,
PCA) that has been shown to accelerate V-catalysed oxy-
genations with hydrogen peroxide or air dramatically.[
Addition of PTSA (only 2 equivalents) considerably in-
creases the rate, and the induction period is eliminated
of the [VO ] moieties in the Keggin unit). Addition of only
2
2
Ϫ10 equivalents (relative to one vanadium) of para-tolu-
enesulfonic acid (PTSA) to the reaction solutions con-
taining catalyst precursors 3 or 4a results in the formation
of 6-oxoheptanoic acid in the form of its methyl ester (see
Scheme 2; Table 4, entries 7 and 9). However, it should be
noted that the yields for all mononuclear dioxo catalysts are
lower than those of HPA-n (n ϭ 4 or 3; see entries 10 and
1
1, respectively). Hence, the redox and acid properties of
IV
V
HPA-3 and mixed valence HPA-4 [[V /V ] are better tuned
than those of the mononuclear species. In the case of the
‘
‘4a/O /MeOH’’ system at 60 °C in the absence of added
2
acid, Ph SiOMe was formed and characterised by GC-MS
3
(
see Exp. Sect.). We conclude that even without addition of
38]
strong acid the Ph SiO ligands in 4a are partly replaced
3
to generate oxohydroxo- and/or alkoxo-anionic or neutral
vanadium() species. As 2-hydroxypinan-3-one is not ox-
idised by this system at 20 °C, probably due to the bulkiness
of this molecule and of 4a (see above), it is certain that this
catalyst precursor does not decompose with formation of
small mononuclear species at room temperature (vide su-
pra: vibrational studies).
(curve 2; see Figure 4). If 4 equivalents of PCA are added
It is noteworthy that vanadyl precursors (entries 1 and 3)
exhibit some activity even in the absence of an acid. Dioxy-
Table 4. Oxidation of 2-methylcyclohexanone (8) to 6-oxoheptanoic
acid (Scheme 2)
Entry[a] Catalyst
[PTSA]/[V] Conversion (%)[b] Yield (%)[c]
1
2
3
4
5
6
7
8
9
0
1
VOSO
4
·5H
2
O (1) 0
34
41
20
66
100
0
48
0
50
100
99
26
21
18
34
50
0
25
0
40
90
97
10
[VO(acac)
2
] (2)
0
10[
10
0
d]
[nBu
4
N]VO
3
(3)
10
0
4a
2
0
IV
V
1
1
HPA-4 (V V )
HPA-3
[
e]
0
[
a]
Conditions: vanadium complex, 0.076 mmol; 2-methylcyclohexa-
none, 7.6 mmol; methanol, 5 mL; dioxygen, 0.1 Mpa; reaction tem-
[
b]
Figure 4. Consumption of O
7.6 mmol) in MeCN (total volume of reaction solution 6 mL),
catalysed by [nBu N]VO (0.076 mmol) at 63 °C without additives
2
in reaction of 2-methylcyclohexanone
perature 60 °C; reaction time 24 h. See corresponding footnotes
to Table 3. See corresponding footnotes to Table 3. The cata-
lytic species was prepared by the reaction between CF SO H and
(
[
c]
[d]
4
3
3
3
(
3
curve 1) and with PTSA (0.155 mmol) (2), CH COOH
[
e]
[
VO(acac)
2
] (ratio 1.1) in methanol at 60 °C during 8 h. Reaction
(0.32 mmol) (3), PCA (0.306 mmol) (4), PTSA (0.155 mmol) ϩ
in water; reaction time 8 h.
PCA (0.306 mmol) (5), and pyridine (0.755 mmol) (6)
Eur. J. Inorg. Chem. 2003, 908Ϫ917
913

M. Vennat, P. Herson, J.-M. Br e´ geault, G. B. Shul’pin
FULL PAPER
to the same mixture the oxidation proceeds at practically would be a carbon-centred radical. In the presence of mo-
the same rate (curve 5). It may thus be concluded that acids lecular oxygen species, 11 would react rapidly with O . In-
2
[
48]
accelerate the oxidation of 8, the rate of this reaction rising deed, it is well known
that carbon-centred radicals add
significantly on going from weak acids to strong ones. to oxygen molecules in a very fast reaction (Scheme 3).
When pyridine is added, there is a long induction period
(
curve 6; see Figure 4), a reduced initial rate and a reduc-
2
R· ϩ O Ǟ ROO·
tion in the yield of keto acid (Յ 15%).
ϩ
2ϩ
A study of immobilised [VO ] /[VO] on several perflu-
2
It is reasonable to assume that the peroxyl radical formed
after addition of O to the carbon atom in 11 would add to
orinated supports with highly acidic and noncoordinating
Ϫ
ϩ
[11]
groups such as -SO3
H
has been presented.
This sys-
2
IV
V
the V ion of the same complex to form V peroxo derivat-
tem produces highly selective CϪC bond cleavage and low-
IV
V
ive 12. In this step, radical ROO· oxidises V to V . An
example of the direct formation of a vanadium() peroxo
complex in the reaction between a vanadyl() complex and
leaching catalysts; mononuclear vanadium(V/IV) moieties
Ϫ
3
are the most likely active species interacting with SO
groups.
[
11,47]
[
49]
molecular oxygen was recently described.
On the Mechanism of the Oxidative Cleavage of C؊C
Bonds
2 [VIV(ϭO)(OH)] ϩ O2 Ǟ [V (ϭO) ]
ϩ
ǟ
V
ϩ
ϩ
2
V
ϩ
[
V (ϭO)(O
2
)] ϩ H
2
O
On the basis of our results and others in the literature we
propose a mechanism as depicted in Scheme 3 for CϪC
bond cleavage of 2-hydroxycyclohexanone. This scheme
Vanadium() oxo complexes can also be reoxidised by
represents a formally closed catalytic cycle, although it is the usual four-electron overall oxidation process shown be-
likely that other steps occur and that the equilibria are more low.
complex. The reaction begins with the formation of a cata-
lytically active oxovanadium() species 9, depicted as
[VIV(ϭO)]2ϩ ϩ 2 H
ǟ
V
ϩ
ϩ
4
2
O ϩ O2 Ǟ 4 [V (ϭO) ] ϩ 4 H
2
ϩ
Ϫ
[
VO ] X , this species containing additional ligands such
2
as coordinated water or organic solvent. Species 9 coordin-
ates to the α-ketol monoanion through its hydroxy oxygen
to produce complex 10. One-electron and proton transfers
from the carbon atom to the vanadium moieties give rise to
the formation of complex 11. Naturally, the proton transfer
occurs not intramolecularly as schematically depicted as
step 2, but through deprotonation and protonation with
participation of the solvent, substrate or water (the latter is
present at relatively low concentrations in all reaction solu-
tions). The one-electron transfer from the electron-rich or-
Vanadium() oxo and peroxo complexes have been re-
ported to hydroxylate aromatic compounds in the presence
[
50,51]
of acids or/and under light irradiation.
The mechanism
of this oxidation appears to include an electron-transfer
step.
Vanadium() peroxo complexes are unstable and can de-
[
52,53]
compose by homolytic fission of OϪO bonds.
In our
case, we propose that homolytic CϪC bond cleavage in 12
occurs simultaneously, possibly through a six-membered
transition state. Structure 13 represents one of the species
on the reaction coordinate for this electron redistribution.
As a result we obtain structure 14 with an acylium cation,
which could explain the formation of adipaldehyde and lac-
V
ganic moiety to the V -oxo part of the complex produces
species 11, which contains the organic part of the complex
IV
σ, n-coordinated to V . There is an unpaired electron on
the organic ligand, and one of the mesomeric forms of 11
[
45]
tones.
Addition of hydroxy anion (or methoxy anion
when methanol is used as a solvent) gives rise to the forma-
tion of adipic acid (or its methyl ester) in Step 6. The hy-
droxy anion from the VV species in Scheme 3 is used only
conventionally to close the catalytic cycle.
An alternative is that in the interaction between 11 and
O the latter adds to the two carbon atoms to produce an
2
endoperoxide species 15 (Scheme 4). It can be seen that this
IV
V
process is also accompanied by the oxidation of V to V .
The simultaneous fission of CϪC and OϪO bonds in 15
gives adipic acid directly coordinated to vanadium() (16).
A similar mechanism has previously been proposed for
oxidative CϪC bond cleavage in acetylacetonate ligand co-
[
48,54]
ordinated to palladium() to afford acetate.
In this
case the reaction proceeds smoothly at room temperature
under irradiation by visible light. Another ligand, ortho-
II
metallated azobenzene, coordinated to Pd plays the role of
an ‘‘antenna’’, absorbing light energy necessary for electron
Scheme 3
transfer from the acac ligand to the palladium centre. It
914
Eur. J. Inorg. Chem. 2003, 908Ϫ917

Vanadium-Catalysed Aerobic Cleavage of CϪC Bonds in Substituted Cyclohexanones
FULL PAPER
Conclusion
Despite the advantage of the use of air or dioxygen as
the oxidant, there are relatively few known methods that
use homogeneous catalysts under very mild conditions. It
has been demonstrated here that CϪC bond cleavage can
be performed efficiently in the presence of dioxygen with
simple vanadium-containing precursors or catalysts. Novel
anionic vanadium() complexes [nBu N][{Ph SiO} VO ]
4
3
2
2
(
4a) and [nBu N] [{Ph SiO VO } ] (5a) have been syn-
4 2 2 2 2 2
thesised and used as models of catalytic homogeneous sys-
ϩ
tems in order to generate [VO2] moieties.
Scheme 4
Comparison of the activities of substituted heteropolyac-
ids H3ϩn[PMo12ϪnV O ]·aq., HPA-n, with these pervana-
n
40
dyl precursors and other vanadyl complexes with different
substrates has caused us to refine our previous mechanism
involving mononuclear vanadium() and () oxo species
should be noted that such a mechanism is related to that of
CϪC bond cleavage in substituted phenols and catechols
ϩ
[3]
and substrate coordination to these [VO2] species. HPA-
under the action of O and catalysed by non-heme iron di-
2
n species are practical and highly efficient reagents for vana-
oxygenase enzymes.[
48,55]
For example, gentisate 1,2-dioxy-
ϩ
dium-based catalysts because their counterions are H O ;
3
genase induces the ring cleavage shown in Scheme 5, and so
the oxidative systems described in this paper can be re-
garded as models of dioxygenases.
they are reversible oxidants, capable of undergoing repeated
cycles of reduction and of re-oxidation with dioxygen.
Moreover, they can readily be formed from inexpensive re-
[
7]
agents: V O , MoO , H PO and H O . We believe that
2
5
3
3
4
2
2
these results can help understanding of systems based on
ϩ
2ϩ
the [VO ] /[VO] couple or on modified HPA-n species for
2
linear alkane and cyclohexane oxidation under study in our
group. These systems also give oxygenated intermediates
Scheme 5
(keto acids, α,ω-diacids, alcohols, ketones, etc.) and will be
further developed.
For the oxidation of 2-methylcyclohexanone, direct σ-co-
ordination of the substrate with vanadium-containing cata-
lytically active species is unlikely. However (see above), this
compound could react in the enol form, since addition of
weak or, especially, strong acids (see Figure 4) dramatically
Experimental Section
Synthesis
[56]
accelerates the oxidation. The reaction starts with the coor- [nBu
4
N][{Ph
3
SiO}
2
VO
2
] (4a): [nBu
4
N]VO
3
(0.68 g; 2 mmol) in
3
SiOH (1.1 g; 4 mmol)
ethanol (5 mL) was added to a solution of Ph
dination of the enol species to produce species 17
Scheme 6). Subsequent electron transfer and dioxygen ad-
in the same solvent (10 mL). After addition of aqueous ammonia
1.25 ; 3.2 mL), the solution was slowly stirred for 45 min. The
white precipitate was isolated by filtration, and then washed with
diethyl ether and dried over P 10 (yield 50Ϫ70%). C52
Si V (876.22): calcd. C 71.28, H 7.59, N 1.60, Si 6.41, V 5.81; found
(
(
dition give rise to species 19 or/and 20. After the cleavage of
the CϪC and OϪO bonds, 6-oxoheptanoic acid is formed.
4
O
H66NO4-
2
C 71.47, H 7.55, N 1.63, Si 5.58, V 5.40. The structure shows dis-
order of the tetrabutylammonium cations, but the tetraphenylphos-
phonium salt gave crystals suitable for an X-ray diffraction study.
[
4
Ph
a (0.90 g; 1 mmol) in methanol (10 mL). The complex anion was
then precipitated by slow addition of an aqueous solution (10 mL)
of PPh Cl (0.42 g; 1 mmol). The white solid was isolated by filtra-
tion, washed with diethyl ether and then dried in air (yield 95%).
PSi V (973.15): calcd. C 74.05, H 5.18, P 3.18, Si 5.77, V
.23; found C 73.52, H 5.07, P 2.35, Si 5.40, V 5.09.
4 3 2 2
P][{Ph SiO} VO ] (4b): This salt was prepared by dissolving
4
C
5
60
H
50
O
4
2
[56]
[
nBu
ethanol (10 mL) was added to a solution of Ph
mmol) in the same solvent (10 mL). After 45 min the white pre-
cipitate was filtered off, washed with diethyl ether and then dried
4
N]
2
[{Ph
2
SiO
2
VO
2
}
2
] (5a): [nBu
4
N]VO
3
(1.36 g; 4 mmol) in
2
Si(OH) (0.86 g;
2
4
over P
4
O10 (yield 50Ϫ75%). C56
H
92
N
2
O
8
Si
2
V
2
(1079.42): calcd. C
62.31, H 8.59, N 2.59, Si 5.20, V 9.44; found C 62.29, H 8.76, N
Scheme 6
2.61, Si 5.30, V 9.38.
Eur. J. Inorg. Chem. 2003, 908Ϫ917
915

M. Vennat, P. Herson, J.-M. Br e´ geault, G. B. Shul’pin
FULL PAPER
‘
‘H
prepared by described procedures. PMo
calcd. P 1.64, Mo 45.65; V 8.08; found P 1.70, Mo 45.50, V 7.75. 5S)-2-hydroxypinane-3-one (7a), its (1R,2R,5R) enantiomer 7b, and
Solid HPA-n species and their aqueous solutions are multicompon- 2-methylcyclohexanone (8), were better than 99% pure by GC and
ent systems: they contain several polyanions, positional isomers of NMR analysis; they were used without further purification. H and
6
[PMo
9
V
3
O
40]·11H
2
O": The heteropolyacids ‘‘HPA-3’’ were
diol Ph
2 2
Si(OH) are commercial products; they were purified by
[7]
9 3 51
V O H28 (1891.45):
fractional crystallisation. 2-Hydroxycyclohexanone (6), (1S, 2S,
1
ϩ
IV
13
these, [VO
2
]
and often traces of V . The identification is sup-
C NMR spectra were recorded at ambient temperature, on a
[5,7,47]
ported by solutions prepared with 2 р n р 10 (see refs.
). In-
Fourier transform apparatus (Bruker WP200), using CDCl
3
as
complete Keggin species or lacunary HPA-n are also formed and
solvent.
only equilibrium exchange-average resonances are observed.
The 31P NMR spectra were recorded with 10 mm o.d. tubes on a
Bruker WM 250 apparatus operating at 101.2 MHz. From these
NMR measurements it was found that the HPA-n precursors (n Ն
Structure Determinations
Single crystals of 4b and 5a were grown from solutions in methanol
at room temperature.
2) are not pure heteropolyacids: they contains a certain fraction of
3 4
HPA-1, HPA-2, Ϫ3 and Ϫ4 with several isomers and excess H PO .
We were unable to prepare a 99% pure isomer.
[
Ph
clinic, space group P2
c ϭ 2268.2(7) pm, β ϭ 118.59(2)°, V ϭ 4.976 (2) nm , Z ϭ 4,
4
P][{Ph
3
SiO}
2
VO
2
] (4b): C60
50 4 2 r
H O PSi V, M ϭ 973.14, mono-
1
/n (no. 14); a ϭ 2233.7(4), b ϭ 1118.5(3),
Raman spectra were recorded at room temperature with a
JobinϪYvon U1000 spectrometer with a resolution of 4 cm
Solid samples of the complexes were mounted on a disk rotating
at about 1000 rpm in order to avoid decomposition and/or photore-
duction by the laser beam (excitation line, 514.5 nm of an Ar
laser; source power 30Ϫ50 mW; scanning time ca. 2 h). Solutions
3
Ϫ1
.
Ϫ3
ρ
calcd. ϭ 1.30 Mg·m , F(000) ϭ 2034, λ ϭ 71.069 pm, T ϭ 293 K,
Ϫ1
3
α
µ (Mo-K ) ϭ 0.317 mm , crystal dimensions 0.2 ϫ 0.3 ϫ 0.5 mm ,
2
Յ 2θ Յ 50°, 9487 measured reflections, 8729 were unique; 614
parameters refined in full-matrix, R ϭ 0.046 with 3730 reflections
ϭ 0.054, w ϶ 1 [weighting scheme of the form
ϩ
1
[I Ͼ3σ (I)], wR
2
(
(
0.1 ) of the mononuclear complex 4a were studied. IR spectra
resolution: 2 cm ) were measured in a Fourier transform appar-
2
2
w ϭ wЈ[1Ϫ(||F
o
| Ϫ |F
c
||) / 6σ (Fo)) ] with wЈ ϭ 1/Σ
r
A
r
T
r
(X) with
Ϫ1
coefficients 3.55, Ϫ0.579 and 2.62 for a Chebyshev series for which
X ϭ Fc/Fc(max)].
3
atus (Bruker Vector 22) in Nujol suspension either between two
caesium bromide plates or in KBr pellets. Elemental analyses (C,
H, N, Si, V) were carried out at the Service Central dЈAnalyses
CNRS, Lyon.
[
57]
Min./max. residual electron density Ϫ 250/
Ϫ3
20 e nm
.
[
nBu
monoclinic, space group P2
567.8(2), c ϭ 1778.6(2) pm, β ϭ 114.28(1)°, V ϭ 3.068 (2) nm ,
4
N]
2
[{Ph
2
SiO
2
VO
2
}
2
] (5a): C56
92 2 8 2 2 r
H N O Si V , M ϭ 1079.42,
1
/n (no.14); a ϭ 1207.2(2), b ϭ
Oxidation of 6, 7a or b, or 8 by Dioxygen in the Presence of a
Vanadium Precursor
3
1
Ϫ3
Z ϭ 2, ρcalcd. ϭ 1.17 Mg·m , F(000) ϭ 1167, λ ϭ 71.069 pm, T ϭ
2
0
Ϫ1
Typical experiments were performed in an all-glass reactor vessel
(40 mL) attached to a vacuum line, with a manometer and a gas
inlet. This was charged with the vanadium precursor (0.076 mmol
V), the solvent (5 mL) and the substrate (ca. 7.6 mmol), and the
93 K, µ (Mo-K
α
) ϭ 0.379 mm , crystal dimensions 0.1 ϫ 0.3 ϫ
.4 mm , 2 Յ 2θ Յ 50°, 5896 measured reflections, 5388 were
ϭ 0.047 with 2386
ϭ 0.056, w ϶ 1 (weighting scheme of
3
unique; 317 parameters refined in full-matrix, R
reflections [I Ͼ3σ (I)], wR
the form w ϭ wЈ{1Ϫ[(||F
(X) with coefficients 3.16, 0.109 and 2.29 for a Chebyshev
1
2
2
2
2
mixture was stirred magnetically under O for 24 h at 60 °C. Dioxy-
o
c
| Ϫ |F ||)/6σ (Fo)] } with wЈ ϭ 1/
gen uptake was determined by use of a gas burette system. The
mixture was cooled and analysed by GC (OV 1701). An aliquot
portion (usually 1 mL) including a standard (methyl heptanoate or
naphthalene) was esterified by addition of ethereal diazomethane
to the crude mixture or after removal of the vanadium on active
charcoal (GC-MS experiments). Products were identified by GC-
MS.
r r r
Σ A T
[57]
series for which X ϭ Fc/Fc(max)).
density Ϫ290/300
Min./max. residual electron
Ϫ3
e
nm
.
The data were collected on
a
EnrafϪNonius CAD4 four-circle diffractometer (ω-2θ scans). All
measurements were made at room temperature; two standard
reflections showed no significant variation in intensity. Corrections
were made for Lorentzian and polarisation effects; an extinction
correction was also applied;[ an empirical absorption correction
on the basis of Ψ scan data was introduced. The structures were
solved by direct methods (SHELX-86 program) and subsequent
Fourier difference techniques. All non-hydrogen atoms were refined
anisotropically. All hydrogen atoms were found on difference Four-
ier maps; their positions were not refined and they were given one
58]
The GC-MS analyses were performed on a TRACE GC 2000 gas
chromatograph (50 m capillary column) interfaced with a mass se-
[59]
1
13
lective detector (RTX5-MS). H and C NMR spectra and MS
[45]
(
70 eV or CI, NH
3
) data correspond to our previous findings or
to literature references.
overall isotropic thermal parameter. Refinements were carried out
2
by minimising the function Σw(|F
o
| Ϫ |F
c
|) , where F
o
and F
c
are Acknowledgments
the observed and calculated structure factors (program CRYS-
G. B. ShulЈpin expresses his gratitude to the Pierre and Marie Curie
University in Paris for an Invited Professorship. We thank Dr. J. S.
Lomas for constructive discussions and for correcting the manu-
script.
[
60]
TALS).
CCDC-167421 (4b) and CCDC-167420 (5a) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge at www.ccdc.cam.ac.uk/conts/retriev-
ing.html [or from the Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge CB2 1EZ, UK; Fax: (internat.)
ϩ44Ϫ1223/336Ϫ033; E-mail: deposit@ccdc.cam.ac.uk].
[
[
[
[
1]
2]
3]
4]
J.-M. Br e´ geault, B. El Ali, J. Mercier, J. Martin, C. Martin, C.
R. Acad. Sci. Paris, S e´ rie II 1988, 307, 2011Ϫ2014.
J.-M. Bregeault, F. Launay, A. Atlamsani, C. R. Acad. Sci.
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