Synthesis and characterization of oxo-imido and alkyl-imido complexes of molybdenum(VI)

Article abstract of DOI:10.1016/S0022-328X(99)00457-X

Treatment of the complex Mo(Nmes)(O)Cl₂(dme) (mes=2,4,6-trimethylphenyl; dme=1,2-dimethoxyethane) with KTp^Me2, NaCp and bipy gives the corresponding derivatives (Tp^Me2)Mo(Nmes)(O)Cl (1), CpMo(Nmes)(O)Cl (2) and Mo(Nmes)(O)

Full text of DOI:10.1016/S0022-328X(99)00457-X

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 590 (1999) 202–207
Synthesis and characterization of oxoꢀimido and alkylꢀimido
complexes of molybdenum(VI)
Francisco Montilla, Antonio Pastor, Agust´ın Galindo *
Departamento de Qu´ımica Inorga´nica, Uni6ersidad de Se6illa, Aptdo 553, E-41071 Se6ille, Spain
Received 22 June 1999; accepted 29 July 1999
Abstract
Treatment of the complex Mo(Nmes)(O)Cl2(dme) (mes=2,4,6-trimethylphenyl; dme=1,2-dimethoxyethane) with KTpMe2
,
NaCp and bipy gives the corresponding derivatives (TpMe2)Mo(Nmes)(O)Cl (1), CpMo(Nmes)(O)Cl (2) and Mo(N-
i
mes)(O)Cl2(bipy) (3). Other oxoꢀimido compounds of composition Mo(Nmes)(O)(S2CNR2)2 (R2=C4H4 4, C5H10 5, Pr2 6) can
be obtained by reacting Mo(Nmes)(O)Cl2(dme) with the appropriate dithiocarbamate salt. The NMR properties of 4–6 are
consistent with the presence of two rapidly equilibrating dithiocarbamate ligands. The reaction of Mo(Nmes)(O)Cl2(dme) with
different Grignard reagents, Mg(R)X, produces the trialkyl imido complexes Mo(Nmes)R3Cl (R=Me 7, CH2C(Me)2Ph 8,
CH2SiMe3 9). © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Imido; Oxo; Molybdenum; Alkyl
bipyridine (bipy) and dithiocarbamate ligands, as well
as that of trialkyl imido compounds of formula Mo(N-
1. Introduction
mes)R3Cl. As discussed below, these new imido species
Interest in the chemistry of transition metal com-
have been prepared using the complex Mo(N-
pounds that contain multiple bonded ligands has in-
mes)(O)Cl2(dme) as the starting material.
creased greatly in the last decades [1], due mainly to
their involvement in many important reactions. Of par-
ticular importance are the high-valent organo-imido
and -oxo molybdenum derivatives, which have been
2. Experimental
subjected to intense investigations. A large number of
All preparations and other operations were carried
bis(imido) [2] and dioxo complexes [3] of this metal
out under a dry, oxygen-free, nitrogen atmosphere,
are known, but information on the related mixed-
following conventional Schlenk techniques. Solvents
terminal oxoꢀimido compounds is still scarce [4]. We
were dried and degassed before use. Microanalyses were
carried out by the Microanalytical Service of the IIQ
have recently shown [5] that the compound
Mo(Nmes)(O)Cl2(dme) (mes=2,4,6-trimethylphenyl;
dme=1,2-dimethoxyethane) can be prepared from
(Seville). Infrared spectra were recorded on Perkin–
Elmer model 883 spectrophotometer. 1H-, 13C- and
31P-NMR spectra were run on Bruker AMX-300 and
Bruker AMX-500 spectrometers. 31P shifts were mea-
sured with respect to external 85% H3PO4. 13C-NMR
spectra were referenced using the 13C resonance of the
solvent as an internal standard but are reported with
respect to SiMe4. The petroleum ether used had b.p.
40–60°C. Compound MoCl2(Nmes)(O)(dme) was pre-
pared according to the literature procedure [5].
Mo(Nmes)2Cl2(dme) and Mo(O)2Cl2(dme). As an ex-
tension of this work [6], we describe here the synthesis
and characterization of several oxoꢀimido complexes of
molybdenum that contain TpMe2 (TpMe2=hydro-
tris(3,5-dimethylpyrazolyl)borate), C5H5 (Cp), 2,2%-
* Corresponding author. Fax: +34-95-4557156.
E-mail address: galindo@cica.es (A. Galindo)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00457-X

F. Montilla et al. / Journal of Organometallic Chemistry 590 (1999) 202–207
203
2.1. Preparation of TpMe2Mo(Nmes)(O)Cl (1)
2.4. Synthesis of Mo(Nmes)(O)(S2CNR2)2 (R2=C4H4
4, C5H10 5, Pr2 6) complexes
i
A mixture of MoCl2(Nmes)(O)(dme) (0.1 g, 0.25
mmol) and KTpMe2 (0.08 g, 0.25 mmol) was dissolved
in THF (25 ml) and stirred at room temperature (r.t.)
overnight. The solvent was removed under vacuum and
the solid extracted with a 1:1 Et2O–petroleum ether
mixture. The solution was concentrated and cooled at
−20°C. Compound 1 was isolated as a purple solid
A mixture of MoCl2(Nmes)(O)(dme) (0.20 g, 0.49
mmol) and KS2CNC4H4 (0.20 g, 1.1 mmol) was dis-
solved in THF (25 ml). The solution was stirred at r.t.
overnight. The solvent was then removed and the re-
sulting solid recrystallized from a Et2O–petroleum
ether mixture. Compound 4 was collected as orange
crystals (0.16 g, 61% yield). 1H-NMR (300 MHz,
C6D6): l 7.35 (pt, 2.3 Hz, 4, pyrrole), 6.53 (s, 2, CH,
Ph), 5.89 (pt, 2.3 Hz, 4, pyrrole), 2.85 (s, 6, o-CH3),
2.04 (s, 3, p-CH3). 13C{1H}-NMR (75 MHz, C6D6): l
210.9 (CS2), 154.6 (C ipso), 142.0, 141.5 (p- and o-C),
128.5 (m-C), 119.4, 115.7 (CH, pyrrole), 21.1 (p-CH3),
19.3 (o-CH3). Anal. Calc. for C19H19N3MoOS4: C, 43.1;
H, 3.6; N, 7.9. Found: C, 43.1; H, 3.6; N, 8.0.
1
(0.07 g, 48% yield). H-NMR (500 MHz, C6D6): l 6.63,
6.39 (s, 1, CH, Ph), 5.55, 5.52, 5.32 (s, 1, CH, TpMe2),
3.39, 3.15, 2.59, 2.32, 2.09, 2.06, 2.03, 1.97, 1.73 (s, 3,
CH3). 13C{1H}-NMR (75 MHz, C6D6): l 153.5, 153.3,
153.1, 153.0, 145.3, 143.2, 142.6, 141.2, 138.1, 133.5 (C,
TpMe2 and Ph), 129.0, 127.8 (m-C), 107.0, 106.7, 106.5
(CH, TpMe2), 20.7, 18.5, 16.9, 15.1, 14.6, 14.3, 12.4,
12.0, 11.9 (CH3, TpMe2, p- and o-CH3). Anal. Calc. for
C24H33N7BClMoO 1/4 Et2O: C, 50.4; H, 6.0; N, 16.5.
Found: C, 50.5; H, 5.9; N, 16.5.
Compounds 5 and 6 were obtained as red and orange
crystals in 50 and 35% yield, following the procedure
described for 4, but using NaS2CNC5H10 and
NaS2CNiPr2, respectively. Mo(Nmes)(O)(S2CNC5H10)2
2.2. Preparation of CpMo(Nmes)(O)Cl (2)
1
(5): H-NMR (300 MHz, C6D6): l 6.58 (s, 2, CH, Ph),
To a solution of MoCl2(Nmes)(O)(dme) (0.18 g, 0.44
mmol) in THF (30 ml) was added one equivalent of
NaCp (0.67 M solution in THF) and the mixture stirred
at r.t. overnight. The solvent was removed in vacuum
and Et2O (30 ml) was added to dissolve the resulting
green solid. The suspension was filtered and the solu-
tion concentrated and cooled at −20°C. Compound 2
was obtained as an orange crystalline solid (0.05 g, 33%
3.38 (br, 8, CH2), 3.09 (s, 6, o-CH3), 2.01 (s, 3, p-CH3),
0.90 (br, 12, CH2). 13C{1H}-NMR (75 MHz, C6D6):
l 199.9 (CS2), 154.3 (C ipso), 140.2, 138.5 (p- and
o-C), 128.2 (m-C), 49.1, 24.9 (4 CH2), 23.2 (2 CH2),
20.9 (p-CH3), 19.6 (o-CH3). Anal. Calc. for
C21H31N3MoOS4 1/4 Et2O: C, 45.2; H, 5.7; N, 7.2.
1
Found: C, 45.7; H, 5.8; N, 7.0. H-NMR (300 MHz,
213 K, toluene-d8): l 6.45 (s, 2, CH, Ph), 3.6 (br m, 4,
CH2), 3.07 (s, 6, o-CH3), 3.0 (br m, 4, CH2), 1.99 (s, 3,
p-CH3), 0.83 (br, 12, CH2). 13C{1H}-NMR (75 MHz,
213 K, toluene-d8): l 199.4 (CS2), 198.0 (CS2), 154.6 (C
ipso), 141.2, 139.3 (p- and o-C), 49.9, 49.4, 48.9 (br,
CH2), 25.2 (br, 4 CH2), 23.6 (br, 2 CH2), 21.6 (p-CH3),
20.4 (o-CH3), m-C is obscured by toluene-d8. Mo(N-
1
yield). H-NMR (300 MHz, C6D6): l 6.56 (s, 2, CH,
Ph), 5.83 (s, 5, CH, Cp), 2.50 (s, 6, o-CH3), 2.07 (s, 3,
p-CH3). 13C{1H}-NMR (75 MHz, C6D6): l 154.7 (C
ipso), 139.1, 138.2 (p- and o-C), 128.7 (m-C), 111.2
(Cp), 21.3 (p-CH3), 18.7 (o-CH3). Anal. Calc. for
C14H16NClMoO: C, 48.6; H, 4.6; N, 4.0. Found: C,
47.6; H, 4.5; N, 4.0.
1
mes)(O)(S2CNiPr2)2 (6): H-NMR (300 MHz, 298 K,
C6D6): l 6.58 (s, 2, CH, Ph), 3.04 (s, 6, o-CH3), 2.02 (s,
3, p-CH3), 0.95 (br, 4, CH3). 13C{1H}-NMR (75 MHz,
C6D6): l 201.0 (CS2), 198.3 (CS2), 154.2 (C ipso), 139.8,
138.0 (p- and o-C), 128.2 (m-C), 52.1 (br, CH), 20.9
2.3. Preparation of MoCl2(Nmes)(O)(bipy) (3)
A solution of MoCl2(Nmes)(O)(dme) (0.1 g, 0.25
mmol) in Et2O (20 ml) was treated with a solution of
bipy (0.04 g, 0.25 mmol) in Et2O (15 ml). A red solid
was formed immediately, which was collected by filtra-
tion, washed with Et2O and extracted with hot toluene
(90°C). The resulting solution was cooled to r.t. and
compound 3 was obtained as a red crystalline solid
1
(p-CH3), 19.5 (o-CH3), 19.3 (br, CH3). H-NMR (300
MHz, 340 K, C6D6): l 6.62 (s, 2, CH, Ph), 4.37 (br, 1,
CH), 4.20 (br, 1, CH), 2.98 (s, 6, o-CH3), 2.05 (s, 3,
p-CH3), 1.06 (d, 6 Hz, 6, CH3), 0.97 (d, 6 Hz, 6, CH3).
2.5. Preparation of alkyl imido Mo(Nmes)R3Cl
(R=Me 7, CH2C(Me)2Ph 8, CH2SiMe3 9) complexes
1
(0.09 g, 76% yield). H-NMR (300 MHz, acetone-d6): l
9.77, 9.28 (m, 1, bipy), 8.68, 8.35 (m, 2, bipy), 7.96, 7.84
(m, 1, bipy), 6.93 (s, 2, CH, Ph), 2.83 (s, 6, o-CH3), 2.34
(s, 3, p-CH3). 13C{1H}-NMR (75 MHz, acetone-d6): l
153.0, 152.3 (C ipso, bipy), 150.9 (C ipso, Ph), 142.0,
141.6 (C, bipy), 139.2, 137.4 (p- and o-C), 129.3 (m-C),
127.55, 127.5, 124.4 (C, bipy), 21.1 (p-CH3), 19.0 (o-
CH3). Anal. Calc. for C19H19N3Cl2MoO: C, 48.3; H,
4.0; N, 8.9. Found: C, 49.1; H, 3.9; N, 8.3.
A solution of MoCl2(Nmes)(O)(dme) (0.2 g, 0.49
mmol) in Et2O (40 ml) was treated, at −60°C, with
three equivalents of Mg(Me)I (0.5 M solution in Et2O).
The mixture was allowed to warm to r.t. and the
stirring was continued for 3 h. Filtration and evapora-
tion of the volatiles under vacuum gave compound 7 as
1
an orange oil (0.09 g, 60% yield). H-NMR (500 MHz,

204
F. Montilla et al. / Journal of Organometallic Chemistry 590 (1999) 202–207
C6D6): l 6.41 (s, 2, CH, Ph), 2.02 (s, 6, o-CH3), 1.94 (s,
3, p-CH3), 1.78 (s, 9, CH3). 13C{1H}-NMR (125 MHz,
C6D6): l 150.7 (C ipso), 140.5, 140.0 (p- and o-C),
129.2 (m-C), 52.6 (CH3), 21.0 (p-CH3), 19.3 (o-CH3).
The use of similar procedures, but using three equiv-
alents of Mg(CH2CMe2Ph)Cl (0.87 M solution in Et2O)
and of Mg(CH2SiMe3)Cl (1.4 M solution in Et2O),
allowed the preparation of compounds 8 and 9, respec-
tively. They were also obtained as orange oils (60%
yield). This makes difficult to obtain reliable analytical
data but they were fully characterized by solution
NMR. Spectroscopic data for Mo(Nmes)(CH2-
at room temperature, nine signals for the methyl groups
(six due to the TpMe2 ligand and three for the ortho and
para methyl groups of the aromatic ring). Two signals
are additionally observed for the meta hydrogens.
These data are in agreement with the absence of rota-
tion of the aryl ring. Restricted rotation of the
arylimido group has been reported in other systems [4d,
6a, 8] and it is generally attributed to steric factors. In
our case, the presence of the bulky TpMe2 ligand causes
a steric congestion that hinders the rotation. In com-
parison, free rotation of the aryl imido ligand has been
observed [5] in complex TpMo(Nmes)(O)Cl. This is in
agreement with the differences in the cone angles re-
ported [9] for these ligands (199°, Tp; 236°, TpMe2).
Addition of one equivalent of bipy to a solution of
Mo(Nmes)(O)Cl2(dme) in Et2O produces the complex
MoCl2(Nmes)(O)(bipy) (3) by substitution of the coor-
dinated dme. This compound is obtained as a red
crystalline solid from hot toluene solutions. The NMR
data are in agreement with the structure represented in
1
CMe2Ph)3Cl (8): H-NMR (300 MHz, C6D6): l 7.33–
7.01 (m, 15, CH2CMe2Ph), 6.43 (s, 2, CH, Ph), 1.50 (s,
3, p-CH3), 1.39 (s, 6, o-CH3), 1.21 (s, 6, CH2CMe2Ph),
1.12 (s, 18, CH2CMe2Ph). 13C{1H}-NMR (75 MHz,
C6D6): l 151.3 (C ipso), 150.7, 149.0 (CH2CMe2Ph),
140.2 (C, Ph), 129.2 (m-C), 87.6 (CH2CMe2Ph), 37.2
(CH2CMe2Ph), 31.7 (p-CH3), 30.5 (o-CH3), 28.9
(CH2CMe2Ph). Spectroscopic data for Mo(Nmes)-
1
(CH2SiMe3)3Cl (9): H-NMR (300 MHz, C6D6): l 6.56
1
Scheme 1. The H-NMR spectrum is consistent with a
(s, 2, CH, Ph), 2.58 (s, 6, CH2Si(CH3)3), 2.50 (s, 6,
o-CH3), 2.03 (s, 3, p-CH3), 0.23 (s, 27, CH2Si(CH3)3).
13C{1H}-NMR (75 MHz, C6D6): l 148.3 (C ipso),
140.2, 138.1 (p- and o-C), 129.5 (m-C), 66.2
(CH2Si(CH3)3), 20.3 (p-CH3), 20.1 (o-CH3), 1.6
(CH2Si(CH3)3).
1:1 ratio of the organoimido and bipy ligands. The
related tungsten compound, W(NtBu)(O)Cl2(bipy), is
known [10] and has been structurally characterized by
X-ray crystallography.
The reaction of MoCl2(Nmes)(O)(dme) with two
equivalents of the potassium 1-pyrrole-carbodithio-
ate, KS2CNC4H4, sodium 1-piperidine-carbodithioate,
3. Results and discussion
3.1. Synthesis and characterization of oxoꢀimido
molybdenum(VI) complexes
We have recently reported [5] that the treatment
of Mo(Nmes)2Cl2(dme) with Mo(O)2Cl2(dme) gives
the complex Mo(Nmes)(O)Cl2(dme). This synthetic
methodology constitutes a suitable entry to the chem-
istry of oxoꢀimido derivatives of molybdenum. Thus,
the reaction of Mo(Nmes)(O)Cl2(dme) with one molar
equivalent of KTpMe2 or NaCp leads to the formation
of the complexes (TpMe2)Mo(Nmes)(O)Cl (1) and Cp-
Mo(Nmes)(O)Cl (2), respectively (Scheme 1). Analytical
and NMR data are consistent with the proposed for-
mulation. Related tris(pyrazolyl)borate [4a,f,5] and cy-
clopentadienyl [4h,5,7] oxoꢀimido derivatives of
molybdenum and tungsten(VI) have been reported
previously.
The facility with which the arylimido ligand of these
compounds rotates around the nitrogenꢀcarbon bond
can be deduced from spectroscopic data. For complex
2, free rotation around the MꢀC bond takes place in
solution since only two methyl and one aromatic CꢀH
1
resonances are observed in the H-NMR spectrum (l
2.07 p-CH3, 2.50 o-CH3, 6.56 m-H; relative intensities:
3:6:2). In contrast, the 1H-NMR spectrum of 1 displays,
Scheme 1.

F. Montilla et al. / Journal of Organometallic Chemistry 590 (1999) 202–207
205
NaS2CNC5H10, or sodium diisopropyldithiocarbamate,
NaS2CNiPr2, in THF gives the complexes Mo(N-
mes)(O)(S2CNR2)2 (R2=C4H4 4, C5H10 5, Pr2 6) (Eq.
25°C, shows two resonances (l 201.0, 198.3 ppm) for the
S2C groups, in accord with the presence of two inequiv-
alent dithiocarbamate ligands. At the same temperature,
i
1
(1)).
the H-NMR spectrum displays a broad signal (l 0.95)
for the methyl groups of the iPr units. Moreover, the CH
group resonances of the four isopropyl units cannot be
discerned. Upon increasing the temperature (340 K), a
broad hump centered at 4.37 ppm, attributable to the
CH groups is observed. A similar situation is found
when variable temperature NMR studies of the
analogous compound 5 are undertaken. Relevant infor-
mation is provided by the 13C{1H}-NMR spectra. At 213
K, two S2C carbon resonances, centered at 199.4 and
198.0 ppm, and several broad signals due to the CH2
groups bonded to the nitrogen atom of the piperidine
fragments are observed. The splitting clearly indicates
the existence of non-equivalent dithiocarbamate ligands,
the low-temperature spectrum actually being consistent
with the octahedral structure I. The magnetic equiva-
lence of the two dithiocarbamate ligands, at the fast
interchange limit, may then be proposed to be due to the
mechanistic path depicted in Scheme 2.
Considering the strong trans influence exerted by the
oxo and imido ligands and assuming that their trans
effect follows the same trend, a facile dissociation of the
sulfur atoms occupying the trans position with respect to
these oxo and imido functionalities is likely to occur in
solution. This process would create a vacant coordina-
tion position (IV) and the adoption of a five-coordinate
configuration (V) containing (S,S%)-S2CNR2 and (S)-
S2CNR2 ligands. A fast concerted exchange of the
mono- and the bidentate dithiocarbamate ligands ren-
ders these groups equivalent. A similar equilibration of
the two dithiocarbamate units has been observed in the
13C{1H}-NMR spectra of Pt((S,S%)-S2CNR2)((S)-
S2CNR2)(PR3) complexes, at room temperature [14] and
an analogous explanation, based on mono- and biden-
tate ligand exchange through five-coordinated species,
has been proposed to account for their dynamic NMR
behavior. Moreover, interconversion of monodentate
and bidentate coordination modes have also been no-
ticed in square-planar gold-dithiocarbamate compounds
[15,16] and in complexes of O-alkyldithiocarbonate
(xanthate) ligands [17]. Nevertheless, a mechanism in-
volving the equilibration of the ligands through struc-
tures of type II or III can not be excluded on the basis
of the NMR data.
(1)
Analytical data are consistent with the proposed formu-
lation but the NMR features of these complexes are
unexpected. Thus, for compounds 4 and 5, 1H- and
13C{1H}-NMR data are in agreement with the presence
1
of two equivalent dithiocarbamate ligands. Similar H-
NMR characteristics have been reported for other
oxoꢀimido complexes Mo(NR)(O)(S2CNEt2)2 [4i,j],
and oxoꢀhydrazido(2-) complexes Mo(NNR2)(O)-
(S2CNR’2)2 [11], but no attempts have been made to
investigate their solution dynamics. The room tempera-
1
ture H-NMR spectrum of 4, recorded in benzene-d6
displays two pseudo-triplets (l 7.35 and 5.89) each for
four hydrogen nuclei of the pyrrole fragment. Further-
more, the 13C{1H}-NMR spectrum shows two sharp
signals in the pyrrole region (l 119.4 and 115.7) and
only one resonance for the two S2C units (l 210.9). In
the case of complex 5, the 13C{1H}-NMR spectrum
shows again only one resonance for the two S2C frag-
ments (l 199.9). A rigid distorted octahedral structure,
with a cis distribution of the two multiple bonded
ligands [4i], (see I) cannot account for these observa-
tions.
Other coordination geometries, such as trigonal prism
II or skew–trapezoidal–bipyramidal III could account
for the NMR properties of 4 and 5. It should be noted,
however, that whilst structure type III has been reported
for some dioxo compounds [12], as far as we are aware
the structure II has no precedent in complexes of this
kind and it is electronically less favorable than the
octahedral geometry [13]. Since in addition compound 6
exhibits detectable fluxional behavior in solution it is
reasonable to assume that a dynamic process is respon-
sible for the NMR properties of 4 and 5. At variance
with the data already described for these two com-
pounds, the 13C{1H}-NMR spectrum of 6, recorded at
3.2. Synthesis of trialkyl imido molybdenum(VI) com-
plexes
The reaction of the complex Mo(Nmes)(O)Cl2(dme)
with three equivalents of the Grignard reagents
Mg(Me)I, Mg(CH2C(Me)2Ph)Cl, and Mg(CH2SiMe3)Cl
produces the alkyl compounds Mo(Nmes)R3Cl (R=

206
F. Montilla et al. / Journal of Organometallic Chemistry 590 (1999) 202–207
Scheme 2.
Me 7, CH2C(Me)2Ph 8, CH2SiMe3 9) (Eq. (2)).
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As is evident from the composition of compounds 7–9,
their formation requires the abstraction of the oxo and
one of the chloride ligands from the coordination
sphere of the metal. Similar synthetic methodology was
employed previously by Osborn and co-workers [18],
using dialkylmagnesium reagents. Derivatives 7–9 are
orange oils, very soluble in hydrocarbon solvents. The
C36 symmetry [12,19] proposed for these molecules is
readily apparent from the NMR data. For instance, the
1H- and 13C{1H}-NMR spectra of 7 show only one
resonance for the three methyl groups (1H-NMR: l
1.78; 13C{1H}-NMR: l 52.6 ppm). For the complexes 8
and 9 one signal due to the three methylene groups of
the CH2R alkyl ligands is also observed (1H-NMR: l
1.21, 8; 2.58, 9; 13C{1H} NMR: l 87.6, 8; 66.2 ppm, 9).
Analogous trialkyl imido complexes of tungsten were
reported earlier by Pedersen and Schrock [20] and, very
recently, Royo and co-workers have characterized re-
lated trialkyl imido cyclopentadienyl complexes of
tungsten [21].
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Financial support from the DIGICYT and Junta de
Andaluc´ıa are gratefully acknowledged. We thank the
Junta de Andaluc´ıa for a research studentship (F.M.).
Helpful discussions with Professor Ernesto Carmona
are gratefully acknowledged.
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