Epoxide ring opening and insertion into the M-X bond of niobium and tantalum pentahalides: Synthesis of dihalide-tris(2-haloalcoholato) complexes

Article abstract of DOI:10.1016/j.poly.2009.02.037

The Group 5 pentahalides MX₅ (M = Nb, Ta; X = Cl, Br), 1, react with a variety of epoxides (1,2-epoxybutane, styrene oxide, 2,3-dimethyl-2,3-epoxybutane, epoxycyclohexane, ethylene oxide) in a 1:3 molar ratio to afford the dinuclear dihalide-tr

Full text of DOI:10.1016/j.poly.2009.02.037

Polyhedron 28 (2009) 1235–1240
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Epoxide ring opening and insertion into the M–X bond of niobium and tantalum
pentahalides: Synthesis of dihalide-tris(2-haloalcoholato) complexes
b
Fabio Marchetti a,1, Guido Pampaloni a, , Stefano Zacchini
*
a Università di Pisa, Dipartimento di Chimica e Chimica Industriale, Via Risorgimento 35, I-56126 Pisa, Italy
b Dipartimento di Chimica Fisica e Inorganica, Università di Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The Group 5 pentahalides MX5 (M = Nb, Ta; X = Cl, Br), 1, react with a variety of epoxides (1,2-epoxybu-
tane, styrene oxide, 2,3-dimethyl-2,3-epoxybutane, epoxycyclohexane, ethylene oxide) in a 1:3 molar
ratio to afford the dinuclear dihalide-tris(2-haloalcoholato) complexes [MX2(OR)2(l-OR)]2 [M = Nb,
X = Cl, R = –CH(Et)CH2Cl, 2a; M = Nb, X = Cl, R = –CH2CH(Ph)Cl, 2b; M = Nb, X = Cl, R = –C(Me)2C(Me)2Cl,
Received 27 November 2008
Accepted 13 February 2009
Available online 11 March 2009
2c; M = Nb, X = Br, R = –CH2CH(Ph)Br, 2d; M = Nb, X = Br, R = –C(Me)2C(Me)2Br, 2e; M = Ta, X = Cl, R =
Keywords:
Niobium
Tantalum
Pentahalides
Epoxide insertion
Halo-alkoxides
Epoxide isomerisation
–CH(Et)CH2Cl, 2f; M = Ta, X = Cl,
, 2g; M = Ta, X = Br, R = –CH CH Br, 2h], in moderate
CH(CH2)4CHCl
2
2
to good yields. The products, 2a–h, result from multiple epoxide insertion into metal–halide bonds,
and the reactions involving 1,2-epoxybutane and styrene oxide proceed with high regioselectivity. The
molecular structure of 2h has been elucidated by X-ray diffraction. Differently, NbF5 adds one equivalent
of 2,3-dimethyl-2,3-epoxybutane to give the monomer NbF5[O@C(Me)(But)], 3, in high yield, as a result of
epoxide to ketone isomerization. The reactions of MF5 (M = Nb, Ta) with other epoxides proceed non-
selectively according to various pathways, including formation of C–O and C–F bonds and C–C and C–
H cleavages.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
der to attain transformations on the organic substrate, we decided
to study the reactivity of 1 with epoxides. We present herein the
Epoxides have been employed as feasible materials for organic
syntheses, and recent examples refer to the preparations of natural
products [1], lactones [2] and amino-alcohols [3]. The reactions of-
ten proceed via epoxide ring opening and, in a number of cases,
they require mediation by metal species. Indeed, the opening of
epoxide rings promoted by transition metal and lanthanide deriv-
atives is a widely described process [4].
results of these studies, which have indicated an unprecedented
route for the preparation of novel dihalide-tris(2-haloalcoholato)
complexes. Moreover, alternative reactions of epoxides mediated
by MF5, including epoxide to ketone isomerization, will be
discussed.
2. Results and discussion
In the recent past, we have been involved in studying the coor-
dination chemistry of Group 5 pentahalides, MX5 (1; M = Nb, Ta;
X = F, Cl, Br) [5], with potentially oxygen-donor ligands [6]. The
strong acidic character of 1 is responsible for promoting C–O bond
cleavage [6a] under mild conditions, and the ring-opening poly-
merization of tetrahydrofuran is a significant example [6b]. It is
noteworthy that these C–O bond breaking processes may lead, in
some cases, to interesting clean organic transformations [6c,6d].
In contrast to the chemical behaviour exhibited by THF when
reacted with 1, different cyclic systems such as 1,4-dioxane and
tetrahydropyran do not undergo C–O cleavage at room tempera-
ture in the presence of 1 [6b]. Thus, with the aim to extend the
chemistry of 1 with cyclic oxygen-containing molecules and in or-
2.1. Reactivity of MX5 (M = Nb, Ta, X = Cl, Br) with epoxides: synthesis
of dihalide-tris(2-haloalcoholato) complexes
Dichloromethane suspensions of MX5 (M = Nb, Ta; X = Cl, Br)
react vigorously with epoxides (1:3 molar ratio) to afford the
dinuclear compounds [MX2(OR)2(l-OR)]2 [M = Nb, X = Cl, R =
–CH(Et)CH2Cl, 2a; M = Nb, X = Cl, R = –CH2CH(Ph)Cl, 2b; M = Nb,
X = Cl, R = –C(Me)2C(Me)2Cl, 2c; M = Nb, X = Br, R = –CH2CH(Ph)Br,
2d; M = Nb, X = Br, R = –C(Me)2C(Me)2Br, 2e; M = Ta, X = Cl, R =
–CH(Et)CH2Cl, 2f; M = Ta, X = Cl, R =
, 2g; M = Ta,
CH(CH2)4CHCl
X = Br, R = –CH2CH2Br, 2h], in 55–70% yield, Scheme 1. On the other
hand, the reactions of NbCl5 with one, two, four and five equivalents
of 1,2-epoxybutane, respectively, gave complicated mixtures of
products containing mainly 2a, suggesting that the compounds 2
are the most stable species obtainable by reacting 1 with epoxides,
independent of the stoichiometry. Similar conclusions were reached
* Corresponding author. Tel.: +39 50 2219 219; fax: +39 50 2219 246.
E-mail address: pampa@dcci.unipi.it (G. Pampaloni).
1
Born in Bologna in 1974.
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.02.037

1236
F. Marchetti et al. / Polyhedron 28 (2009) 1235–1240
OR
M
OR
OR
R
O
R2R1C
CR3R4
RHC
CH2
RO
X
MX4
R
O
O
O
MX5
M
RHC
CH2
A
B
CH2Cl2
X
X
X
MX4
O
O
R
MX5
X
X
R = C(R1)(R2)-C(R3)(R4)-X
RHC
CH2
O
X
O
2a - h
X4M
R
X
X4M
Scheme 1. The reactivity of MX5 (M = Nb, Ta; X = Cl, Br) with epoxides.
Scheme 2. Possible routes of formation of the haloalcoholato fragment.
by studying the reactions of NbCl5 with styrene oxide and epoxycy-
clohexane (vide infra).
The compounds 2a–h have been characterized spectroscopi-
cally (IR and NMR), by elemental analyses, and by X-ray diffraction
in the case of 2h (Fig. 1 and Table 1).
Compounds 2 add to the family of transition metal alcoholato
complexes [7], which have found various applications (film-depo-
sition [8], nanotechnology [9], polymerization catalysis [10] and
the preparation of materials with notable properties [11]); and in
more detail, cases of structurally characterized metal complexes
containing 2-haloalcoholates are available in the literature, includ-
ing both late [12] and early transition [13] metal species.
Regarding niobium(V) and tantalum(V), although examples of
pentaalcoholates are numerous [14a–e], only a few mixed halide-
alcoholates [MXn(OR)5Àn]2 have been crystallographically charac-
terized so far [14f–i]. Complexes 2 belong to this rather uncommon
class of compounds [15], and represent the first case of structurally
characterized coordination compounds of the heavier Group 5 ele-
ments containing halo-substituted alcoholates.
The NMR spectra of 2a, c, e and f exhibit slightly distinct reso-
nances for the terminal and bridging ligands (see Section 4) [13],
The unit cell contains discrete dinuclear [TaBr2(OCH2CH2Br)2(l-
OCH2CH2Br)]2 molecules, which reside on a crystallographic inver-
sion centre. The dimeric species adopts an edge sharing bioctahe-
dral geometry, with four terminal bromide ligands in the axial
positions (trans geometry). Conversely, the equatorial plane is
occupied by four terminal and two bridging haloalcoholates. All
bonding distances are comparable with those previously reported
for Ta(V)–Br and Ta(V)–OR containing complexes [6]. The bridging
ligand is almost symmetric [Ta(1)–O(1) 2.091(11) Å; Ta(1)–O(1)#1
2.107(10) Å], and these contacts are considerably elongated com-
pared to the ones referring to the terminal haloalcoholates
[Ta(1)–O(2) 1.818(9) Å; Ta(1)–O(3) 1.822(10) Å].
and agree with the dinuclear structure [MX2(OR)2(l-OR)]2. For in-
stance, the methyl units belonging to the OC(Me)2 moieties in 2c
give rise to resonances at 1.70 (bridging ligands) and 1.58 ppm
(terminal) in the 1H NMR spectrum, and at 29.1 (bridging) and
28.9 ppm (terminal) in the 13C NMR spectrum. On the other hand,
the 1H NMR spectra of 2b, d and g display only broad resonances,
for which discrimination between terminal and bridging ligands
has not been possible. In contrast to the previous observations
on some niobium or tantalum alkoxides, no dimer–monomer equi-
libria [13,16] have been detected in solution at room temperature
for compounds 2.2
It is worthy noting that the formal insertion of 1,2-epoxybutane
into the metal–halide bond could in principle generate two iso-
meric forms, according to the site of attack of the halide, see
Scheme 2.
A careful reading of the 1D and 2D NMR spectra of 2a and f has
evidenced the absence of resonances attributable to –OCH2-
CH(Et)Cl units [6c,6d], suggesting that multiple 1,2-epoxybutane
insertion into the metal–chloride bonds of MCl5 (M = Nb, Ta) oc-
curs in a highly regioselective mode (Scheme 2, path B), placing
the halide on the less hindered carbon atom.
Fig. 1. Molecular structure of [TaBr2(OCH2CH2Br)2(l-OCH2CH2Br)]2, 2h. Thermal
ellipsoids are drawn at the 30% probability level. Only independent atoms are
labelled.
Table 1
The NMR spectra of 2b and d, obtained by reacting NbX5 (X = Cl,
Br) with styrene oxide, do not allow, unambiguously, the formation
of any of the species obtainable by the two potential modes of
insertion to be ruled out (see Scheme 2). Nevertheless, the hydro-
lysis of CDCl3 solutions of complexes 2b and d, and subsequent
GC–MS and NMR analyses on the organic phase, have pointed
out that either 2b or 2d contain the [OCH2CH(Cl)Ph] fragment (en-
try 2, Table 2). In other words, the insertion of styrene oxide to give
2b and d seems to proceed, differently from the insertion of 1,2-
epoxybutane, according to path A of Scheme 2, the halide going
to the more hindered carbon atom. This feature may be attribut-
able to the steric hindrance of the phenyl ring, which would force
this ligand to occupy the farthest position from the metal centre.
Selected bond distances (Å) and angles (°) of [TaBr2(OCH2CH2Br)2(
2h.
l
-OCH2CH2Br)]2,
Ta(1)–Br(1)
Ta(1)–Br(2)
Ta(1)–O(1)
Ta(1)–O(1)#1
Ta(1)–O(2)
2.459(2)
2.466(3)
2.091(11)
2.107(10)
1.818(9)
1.822(10)
1.50(2)
C(2)–Br(3)
O(2)–C(3)
C(3)–C(4)
C(4)–Br(4)
O(3)–C(5)
C(5)–C(6)
C(6)–Br(5)
1.97(2)
1.43(2)
1.52(3)
1.93(2)
1.418(19)
1.50(2)
Ta(1)–O(3)
O(1)–C(1)
1.950(16)
C(1)–C(2)
1.48(3)
O(2)–Ta(1)–O(3)
O(2)–Ta(1)–O(1)
O(3)–Ta(1)–O(1)
O(2)–Ta(1)–O(1)#1
O(3)–Ta(1)–O(1)#1
O(1)–Ta(1)–O(1)#1
O(2)–Ta(1)–Br(1)
O(3)–Ta(1)–Br(1)
105.8(5)
92.4(4)
161.8(4)
160.3(4)
93.8(4)
68.0(5)
91.8(3)
89.2(3)
O(1)–Ta(1)–Br(1)
O(1)#1–Ta(1)–Br(1)
O(2)–Ta(1)–Br(2)
O(3)–Ta(1)–Br(2)
O(1)–Ta(1)–Br(2)
O(1)#1–Ta(1)–Br(2)
Br(1)–Ta(1)–Br(2)
Ta(1)–O(1)–Ta(1)#1
89.8(3)
90.0(3)
88.4(3)
91.4(3)
89.6(3)
89.6(3)
179.34(8)
112.0(5)
2
Symmetry transformations used to generate equivalent atoms: #1 Àx + 1/2,Ày + 1/
2,Àz + 2.
NMR experiments could not be run on complex 2h, because of its low solubility in
suitable deuterated solvents (CDCl3, CD2Cl2).

F. Marchetti et al. / Polyhedron 28 (2009) 1235–1240
1237
Table 2
enced by the metal–halide bond energy [6a,6b], and the fluorides
often show a chemistry quite different from that of the other ha-
lides. Therefore, we decided to investigate the reactivity of MF5
(M = Nb, Ta) with epoxides, with the aim to see whether insertion
reactions could take place, despite the high value of the metal–
fluoride bond energy [17].
Products of hydrolysis of compounds 2a–h.
Entry
1
Epoxide
Haloalkoxo derivative
Hydrolysis product
2a, fa
Et
OH
Cl
O
Niobium pentafluoride and 2,3-dimethyl-2,3-epoxybutane re-
act to afford the ketone complex NbF5[O@C(Me)(tBu)], 3, in ca.
90% yield. This reaction is strictly stoichiometric: the same reac-
tion, carried out by using variable epoxide to metal ratios greater
than 1, resulted in the formation of 3 in an admixture with the
unreacted epoxide. Complex 3, which belongs to the restricted
family of carbonylic adducts of Group 5 pentahalides [6a], has been
characterized spectroscopically (1H, 13C and 19F NMR, IR) and by
GC–MS analysis carried out on the organic material obtained by
hydrolysis of 3 (entry 1, Table 3). A salient spectroscopic feature
is given by a strong IR absorption at 1647 cmÀ1, ascribed to the car-
bon–oxygen double bond (uncoordinated t-butylmethylketone
shows the C@O stretching vibration at 1708 cmÀ1, recorded as a li-
quid film). The 1H NMR spectrum displays one set of two reso-
nances, attributed to the methyl unit (s, 2.84 ppm) and the t-
butyl group (s, 1.41 ppm). The major 13C NMR aspect is given by
the carbonylic resonance, seen at 245.6 ppm, i.e. significantly
downfield-shifted with respect to the resonance of the uncoordi-
nated ketone (d = 213.81 ppm). Furthermore, the 19F NMR spec-
Cl
Cl
Cl
X
2
3
2b, db
Ph
OH
Ph
O
2g
OH
Cl
O
4
2c, eb
Me Me
O
Me
Me
X
Me
Me
HO
Me
Me
t-But
Me
trum shows uniquely
a
broad resonance at 145.8 ppm,
accounting for the five fluorines of the [NbF5] moiety, and indicat-
ing the monomeric, neutral nature of 3.
O
In contrast to that discussed about the reaction of NbCl5 with
2,3-dimethyl-2,3-epoxybutane, which affords some t-butylmeth-
ylketone as result of HCl release and subsequent hydrolysis (vide
infra), the formation of t-butylmethylketone in the case of NbF5
is probably the consequence of direct epoxide isomerisation pro-
moted by niobium fluoride. This latter rearrangement has been re-
ported to occur under the assistance of various metal species,
including high valent vanadium [18], cobalt [19], erbium [20], bis-
muth [21] and lithium [22] compounds.
5
2h
OH
O
Br
a
1,2-Dichlorobutane and 1-chloro-2-butene have been detected probably due to
the action of HCl, product of the hydrolysis, on 1-chloro-2-butanol.
b
X = Cl, Br.
The identities of products 2a–h have been corroborated by com-
bined GC–MS and NMR analyses on concentrated CDCl3 solutions
of these compounds after treatment with H2O, see Section 4 and
Table 2. The addition of water causes the quick precipitation of a
white solid (probably metal oxides), and release of the organic
material. The hydrolysis of 2c (entry 4, Table 2) deserves more
We tried to extend the chemistry of niobium and tantalum
pentafluorides to a series of epoxides (1,2-epoxybutane, styrene
oxide and epoxycyclohexane). At variance with that reported
Table 3
Reactants and main products (after hydrolysis) of the reactions of MF5 with epoxides.
comment:
beside
2,3-dimethyl-3-chloro-2-butanol,
minor
amounts of t-butylmethylketone have been observed by NMR
and GC–MS analyses, suggesting that pathways alternative to the
insertion (see Scheme 1) may be operating, at least for the reaction
of 2,3-dimethyl-2,3-epoxybutane with NbCl5.
Entry
1
M
Epoxide
Hydrolysis product
Nb
Me Me
t-But
Me
Me
Me
O
With the purpose of extending the series of halo-2-haloalcoho-
lato complexes of type 2, we studied the reactivity of NbBr5 with
1,2-epoxybutane and with epoxycyclohexane. Unfortunately, these
reactions afforded mixtures of compounds which could not be sep-
arated and characterized. Nonetheless, GC–MS and NMR analyses
on the reaction mixtures treated with water led to the identifica-
tion of 1,2-dibromobutane and 1-bromo-2-butene from NbBr5
and 1,2-epoxybutane, and of 2-bromocyclohexanol and 1,2-dibro-
mocyclohexane from NbBr5 and epoxycyclohexane. These results
indicate that the reactions of 1,2-epoxybutane and epoxycyclohex-
ane with NbBr5 proceed via epoxide insertions, analogously to that
discussed for 2a–h.
O
2
3
Nb, Ta
Et
Et
Me
O
O
Ta
OEt
Ph
O
;
4
Ta
O
OH
F
2.2. Reactivity of MF5 (M = Nb, Ta) with epoxides: isomerisation of 2,3-
dimethyl-2,3-epoxybutane to 3,3-dimethyl-2-butanone
According to our recent reports, the reactivity of niobium and
tantalum pentahalides with oxygen-donor ligands is strongly influ-

1238
F. Marchetti et al. / Polyhedron 28 (2009) 1235–1240
above about the synthesis of 3, the reactions of MF5 with epoxides
other than 2,3-dimethyl-2,3-epoxybutane led to complicated mix-
tures of products. We did not succeed in the characterization of the
inorganic species, but we were able to identify some of the organic
compounds produced in the course of the reactions, and released
from the metallic frames upon hydrolysis (see Table 3). Thus, eth-
ylmethylketone has been recognized in the mixture obtained from
NbF5 or TaF5 and 1,2-epoxybutane (entry 2, Table 3): accordingly,
this reaction proceeds with epoxide rearrangement, similar to that
described for 2,3-dimethyl-2,3-epoxybutane (entry 1, Table 3).
Conversely, no traces of acetophenone and cyclohexanone have
been found in the hydrolyzed mixtures generated from the reac-
tions of TaF5 with styrene oxide and epoxycyclohexane, respec-
tively. Therefore, alternative pathways are likely to be operating
in these two cases. More precisely, benzene and ethoxybenzene
have been recognized (GC–MS and NMR) as the main products
from the treatment of TaF5 with styrene oxide followed by hydro-
lysis (entry 3, Table 3). Furthermore, GC–MS and NMR analyses on
a sample obtained by addition of epoxycyclohexane to TaF5, and
successive hydrolysis, revealed the presence of small amounts of
2-fluorocyclohexanol (entry 4, Table 3). All these organics prove
that C–C and C–H bond activation, together with C–O and C–F bond
formation, are possible processes taking place in the course of the
reactions of MF5 (M = Nb, Ta) with epoxides.
with a Phenonex Zebron column. Carbon and hydrogen analyses
were performed at the Dipartimento di Chimica Farmaceutica of
the University of Pisa on a Carlo Erba mod. 1106 instrument, pay-
ing particular attention to the sensitive compounds, which were
weighed and directly introduced into the analyzer. The halide con-
tent refers to the metal-bound halides, and was determined by the
Volhardt method [25] after exhaustive hydrolysis of the sample.
Metals were analyzed as M2O5, obtained by hydrolysis of the sam-
ple followed by calcination in a platinum crucible. Every elemental
analysis was repeated twice in order to get reproducible results.
4.2. Preparation of [MX2(OR)2(l-OR)]2 [M = Nb, X = Cl, R =
–CH(Et)CH2Cl, 2a; M = Nb, X = Cl, R = –CH2CH(Ph)Cl, 2b; M = Nb,
X = Cl, R = –C(Me)2C(Me)2Cl, 2c; M = Nb, X = Br, R = –CH2CH(Ph)Br,
2d; M = Nb, X = Br, R = –C(Me)2C(Me)2Br, 2e; M = Ta, X = Cl, R =
–CH(Et)CH2Cl, 2f; M = Ta, X = Cl, R =
X = Br, R = –CH2CH2Br, 2h]
, 2g; M = Ta,
CH(CH2)4CHCl
General procedure: A suspension of MX5 (M = Nb, Ta; X = Cl, Br;
1.00 mmol), in CH2Cl2 (15 mL) inside a Schlenk tube, was treated
with the appropriate epoxide (3.00 mmol). In the case of the syn-
thesis of 2h, ethylene oxide was added as a CH2Cl2 solution
(50 mg/mL). Dissolution of the solid and a colour change occurred
rapidly and exothermically. The mixture was stirred for an addi-
tional 30 min, then the final solution was concentrated (3 mL), lay-
ered with pentane (15 mL) and stored at À20 °C. The product was
obtained as a solid after 48–72 h. In a different experiment, MX5
3. Conclusions
(0.50 mmol) was introduced into
a NMR tube, then CDCl3
The strong oxophilicity of niobium and tantalum pentahalides
is tested in the room temperature reactions with epoxides, in a
1:3 molar ratio, which proceed rapidly, exothermically and regio-
selectively. Multiple epoxide ring opening and insertion into me-
tal–chloride (or bromide) bonds give novel stable dinuclear
species containing both halide and 2-haloalcoholato ligands.
Although MF5 (M = Nb, Ta) are very reactive towards epoxides,
the relatively high metal–fluoride bond energy inhibits insertion
reactions. Several alternative pathways appear to be operative con-
currently; among them, epoxide to ketone isomerization prevails
in some cases.
(0.85 mL) and epoxide (1.50 mmol) were added in the order given.
The 1H NMR analysis was performed once the solid dissolved: com-
plete consumption of the epoxide occurred. Afterwards, the mix-
ture was treated with a large excess of H2O (ca. 20 mmol):
precipitation of a colourless solid took place nearly instanta-
neously. The content of the resulting solution was determined by
means of 1H and 13C NMR spectroscopies, and by GC–MS analysis.
2a: orange, from NbCl5 and 1,2-epoxybutane. Anal. Calc. for
C24H48Cl10Nb2O63: C, 29.63; H, 4.97; Nb, 19.10; Cl, 14.58. Found:
C, 29.44; H, 5.06; Nb, 18.90; Cl, 14.19%. Yield: 0.312 g, 64%. 1H
NMR (CDCl3) d = 5.05–4.81 (m-br, 6H, CH); 4.74–3.82 (m-br, 12H,
ClCH2); 1.95, 1.90, 1.63 (m-br, 12H, CH2CH3); 1.06 (m-br, 18H,
CH3) ppm. 13C NMR (CDCl3) d = 63.4, 62.8 [(CH)terminal and
(CH)bridging]; 46.3, 45.2 [(ClCH2)terminal and (ClCH2)bridging]; 27.9,
26.9 [(CH2CH3)terminal and (CH2CH3)bridging]; 10.9 (CH3) ppm. IR (solid
4. Experimental
4.1. General procedures
state):
m .
= 2975w-m, 1448 m, 1378 m, 1051s, 903s, 783vs cmÀ1
All manipulations of air and/or moisture sensitive compounds
were performed under an atmosphere of pre-purified argon using
standard Schlenk techniques. The reaction vessels were oven dried
at 150 °C prior to use, evacuated (10À2 mm Hg) and then filled with
argon. All the reagents, including MX5 (M = Nb, Ta, X = Cl; M = Nb,
Ta, X = F), were commercial products (Aldrich) of the highest purity
available. NbBr5 and TaBr5 were prepared according to published
procedures [23]. Epoxides were liquid commercial products stored
under an argon atmosphere as received. Solvents were distilled be-
fore use under an argon atmosphere from appropriate drying
agents: CH2Cl2 and CDCl3 from P4O10, pentane from LiAlH4.
Infrared spectra were recorded at 298 K on a FT-IR Perkin–El-
mer Spectrometer equipped with a UATR sampling accessory (solid
samples). NMR measurements were performed at 298 K on Varian
Gemini 200BB and Mercury Plus 400 instruments. The chemical
shifts for 1H, 13C and 19F NMR spectra were referenced to TMS
and to CFCl3, respectively. The chemical shifts were assigned via
DEPT experiments and 1H, 13C correlation through gs-HSQC and
gs-HMBC experiments [24]. NMR assignments related to terminal
and bridging ligands have been specified only where a distinction
has been possible. GC/MS analyses were performed on a HP6890
instrument, interfaced with a MSD-HP5973 detector and equipped
Hydrolysis (entry 1, Table 2): 1-chloro-2-butanol, 1,2-dichlorobu-
tane, 1-chloro-2-butene (NMR and GC–MS); ratio 4:2:1 (1H NMR).
2b: orange, from NbCl5 and styrene oxide. Anal. Calc. for
C48H48Cl10Nb2O63: C, 45.71; H, 3.84; Nb, 14.73; Cl, 11.24. Found:
C, 45.66; H, 3.97; Nb, 14.43; Cl, 11.01%. Yield: 0.366 g, 58%. 1H
NMR (CDCl3) d = 7.69–6.89 (br, 30H, arom CH); 5.18 (m-br, 6H,
CHCl); 4.16 (br, 12H, CH2) ppm. 13C NMR (CDCl3) d = 137.9, 137.2
[(ipso-Ph)bridging and terminal]; 128.6, 127.4 (arom CH); 65.2 (CHCl);
60.7, 59.9 [(CH2)terminal and (CH2)bridging] ppm. Hydrolysis (entry
2, Table 2): 2-phenyl-2-chloroethanol (NMR and GC–MS).
2c: yellow, from NbCl5 and 2,3-dimethyl-2,3-epoxybutane.
Anal. Calc. for C36H72Cl10Nb2O63: C, 37.89; H, 6.36; Nb, 16.28; Cl,
12.42. Found: C, 37.77; H, 6.19; Nb, 16.10; Cl, 12.23%. Yield:
0.337 g, 59%. 1H NMR (CDCl3) d = 1.70 [s, 12H, (OCMe2)bridging];
1.58 [s, 24H, (OCMe2)terminal]; 1.27 (s, 36H, CMe2Cl) ppm. 13C
NMR (CDCl3) d = 80.7 (OCMe2); 75.8, 74.4 [(CMe2Cl)terminal and
(CMe2Cl)bridging]; 29.1 [(OCMe2)bridging]; 28.9 [(OCMe2)terminal];
25.2, 24.2 [(CMe2Cl)terminal and CMe2Cl)bridging] ppm. IR (solid state):
3
Due to the fact that the carbon-bonded halide does not precipitate with silver
nitrate under the conditions used for the analysis, the halide content refers to the
metal-bonded halide ion.

F. Marchetti et al. / Polyhedron 28 (2009) 1235–1240
1239
m
= 2979w, 2936w, 2886w, 1447wm, 1384m, 1321wm, 1119m,
tion, which was analyzed by both GC–MS and 1H NMR. From NbBr5
and 1,2-epoxybutane: 1-bromo-2-butene, 1,2-dibromobutane (ra-
tio 1:1). From NbBr5 and epoxycyclohexane: 2-bromocyclohexa-
nol, 1,2-dibromocyclohexane (ratio 3:1).
1097s, 1072s, 1043vs, 977m, 951s, 932vs, 831s, 811m, 674vs,
661s cmÀ1. Hydrolysis (entry 4, Table 2): 2,3-dimethyl-3-chloro-
2-butanol, t-butylmethylketone (NMR and GC–MS); ratio 5:1 (1H
NMR).
In a NMR tube, 2,3-dimethyl-2,3-epoxybutane (1.50 mmol) was
added to NbCl5 (0.50 mmol) in CDCl3 (0.70 mL). An exothermic
reaction took place accompanied by gas (HCl) evolution, and the
solid quickly dissolved.
2d: light yellow, from NbBr5 and styrene oxide. Anal. Calc. for
C48H48Br10Nb2O63: C, 33.80; H, 2.84; Nb, 10.89; Br, 18.74. Found:
C, 33.90; H, 2.88; Nb, 10.71; Br, 18.55%. Yield: 0.512 g, 60%. 1H
NMR (CDCl3) d = 7.69–7.25 (br, 30H, arom CH); 5.28 (m-br, 6H,
CHCl); 4.67 (br, 12H, CH2) ppm. Hydrolysis (entry 2, Table 2): 2-
phenyl-2-bromoethanol (NMR and GC–MS).
2e: yellow, from NbBr5 and 2,3-dimethyl-2,3-epoxybutane.
Anal. Calc. for C36H72Br10Nb2O63: C, 27.27; H, 4.58; Nb, 11.71; Br,
20.16. Found: C, 27.11; H, 4.66; Nb, 11.60; Br, 19.95%. Yield:
0.484 g, 61%. 1H NMR (CDCl3) d = 1.84 [s, 12H (OCMe2)bridging];
1.75 [s, 24H, (OCMe2)terminal]; 1.36, 1.32 [s, 36H, (CMe2Br)terminal
and (CMe2Br)bridging] ppm. 13C NMR (CDCl3) d = 80.7 (OCMe2);
4.4. Preparation of NbF5[O@C(Me)(But)], 3
A suspension of NbF5 (0.120 g, 0.639 mmol) in CH2Cl2 was trea-
ted with 2,3-dimethyl-2,3-epoxybutane (0.082 mL, 0.64 mmol).
The resulting mixture was stirred for 90 min, then the volatile
materials were removed under vacuum. Compound 3 was obtained
as a crystalline colourless solid by storing a CH2Cl2 solution (3 mL),
layered with pentane (10 mL), at À20 °C for one week. Yield:
0.166 g, 90%. Anal. Calc. for C6H12F5NbO: C, 25.02; H, 4.20; Nb,
32.25. Found: C, 24.90; H, 4.26; Nb, 32.11%. 1H NMR (CDCl3)
d = 2.84 (s, 3H, Me); 1.41 (s, 9H, But) ppm. 13C NMR (CDCl3)
d = 245.6 (CO); 48.5 (CMe3); 26.7 (Me); 26.2 (CMe3) ppm. 19F
NMR (CDCl3) d = 145.8 (br, 5F) ppm. IR (solid state):
m = 2979w-
m, 2947w, 2881w, 1647s (C@O), 1482m, 1466m, 1404w-m,
1368m, 1293w, 1226w, 1146vs, 1054m, 994m, 923s, 797vs,
684vs cmÀ1. In a different experiment, NbF5 (0.55 mmol), CDCl3
(0.85 mL) and 2,3-dimethyl-2,3-epoxybutane (0.50 mmol) were
added in the order given in a NMR tube and the tube was sealed.
The 1H NMR spectrum, recorded when the solid dissolved (about
2 h), evidenced the clean formation of 3. The tube was opened,
and an excess of water was introduced (ca. 30 mmol). Precipitation
of a colourless solid occurred nearly instantaneously. 3,3-Di-
methyl-2-butanone was recognized in solution by both 1H NMR
and GC–MS.
The reaction of NbF5 (0.05 mmol) with 2,3-dimethyl-2,3-epox-
ybutane (0.50 mmol), in CDCl3 inside a sealed NMR tube, resulted
in the clean formation of 3, in an admixture with a large quantity
of unreacted epoxide (ratio: 1:80 ca.). The ratio did not change
on heating the solution at 120 °C for 30 min.
75.8,
[(OCMe2)bridging]; 28.9 [(OCMe2)terminal]; 25.2, 24.2 [(CMe2Br)terminal
and CMe2Br)bridging] ppm. IR (solid state): = 2979w, 2936w,
2886w, 1447wm, 1384m, 1321wm, 1119m, 1097s, 1072s,
1043vs, 977m, 951s, 932vs, 831s, 811m, 674vs, 661s cmÀ1. Hydro-
lysis (entry 4, Table 2): 2,3-dimethyl-3-bromo-2-butanol (NMR
and GC–MS).
74.4
[(CMe2Br)terminal
and
(CMe2Br)bridging];
29.1
m
2f: dark yellow, from TaCl5 and 1,2-epoxybutane. Anal. Calc. for
C24H48Cl10O6Ta23: C, 25.09; H, 4.21; Ta, 31.50; Cl, 12.34. Found: C,
25.21; H, 4.07; Ta, 31.29; Cl, 12.12%. Yield: 0.368 g, 64%. 1H NMR
(CDCl3) d = 5.26–4.78 (m-br, 6H, CH); 4.62–3.53 (m-br, 12H,
ClCH2); 1.95, 1.82, 1.67, 1.54 (m-br, 12H, CH2CH3); 1.02 [t,
3JHH = 6.59 Hz, 12 H, (CH3)terminal]; 0.90 [t, JHH = 8.05 Hz, 6 H,
3
(CH3)bridging] ppm. 13C NMR (CDCl3) d = 62.9, 62.4 [(CH)terminal and
(CH)bridging]; 46.4, 45.3 [(ClCH2)terminal and (ClCH2)bridging]; 27.4,
27.3 [(CH2CH3)terminal and (CH2CH3)bridging]; 10.4 [(CH3)terminal];
9.3 [(CH3)bridging] ppm. Hydrolysis (entry 1, Table 2): 1-chloro-2-
butanol, 1,2-dichlorobutane, 1-chloro-2-butene (NMR and GC–
MS); ratio 5:2:1 (1H NMR).
4.5. Reactivity of MF5 (M = Nb, Ta) with epoxides (1,2-epoxybutane,
styrene oxide, cyclohexene oxide)
General procedure: To a suspension of compound MF5 (M = Nb,
Ta; 0.50 mmol) in CDCl3 (0.80 mL) inside a NMR tube, the appropri-
ate epoxide was added (0.50 mmol). The tube was sealed, and left
at room temperature overnight. A 1H NMR spectrum revealed the
complete consumption of the epoxide and the formation of a com-
plicated mixture of unidentifiable products. The tube was opened,
then the mixture was treated with a large excess of water (ca.
15 mmol), resulting in the quick precipitation of a solid from the
pale yellow solution. The latter was analyzed by both GC–MS
and 1H NMR. From NbF5 or TaF5 and 1,2-epoxybutane (entry 2, Ta-
ble 3): ethylmethylketone. From TaF5 and cyclohexene oxide (en-
try 3, Table 3): 2-fluorocyclohexanol. From TaF5 and styrene
oxide (entry 4, Table 3): benzene, ethoxybenzene (ratio ca. 1:1).
2g: orange, from TaCl5 and epoxycyclohexane. Anal. Calc. for
C36H60Cl10O6Ta23: C, 33.13; H, 4.63; Ta, 27.73; Cl, 10.86. Found:
C, 33.03; H, 4.68; Ta, 27.60; Cl, 10.61%. Yield: 0.359 g, 55%. 1H
NMR (CDCl3) d = 4.92 (m-br, 6H, CHCl); 4.22 (m-br, 6H, OCH);
2.32, 1.74, 1.31 (m-br, 48H, CH2) ppm. IR (solid state):
m = 2944w,
2863w, 1595m, 1448m, 1362m, 1259s, 1205w, 1079s, 1050s,
1023s, 977ms, 794vs, 738s, 666s cmÀ1. Hydrolysis (entry 3, Table
3): 2-chlorocyclohexanol (NMR and GC–MS).
2h: light yellow, from TaBr5 and ethylene oxide. Anal. Calc. for
C12H24Br10O6Ta23: C, 10.11; H, 1.70; Ta, 25.39; Br, 22.42. Found:
C, 10.13; H, 1.66; Ta, 25.23; Br, 22.19%. Yield: 0.499 g, 70%. IR (solid
state):
m = 2965w, 2906w, 1470w-m, 1434m, 1414m, 1368w-m,
1278m, 1214w, 1172m-s, 1122s, 1084vs, 1042s, 1014vs, 996m-s,
934vs, 861s, 763m, 737m, 697s, 678vs cmÀ1. Hydrolysis (entry 5,
Table 2): 2-bromoethanol (NMR and GC–MS).
4.6. X-ray crystallographic study
Crystal data and collection details for [TaBr2(OCH2CH2Br)2(l-
OCH2CH2Br)]2, 2h, are reported in Table 4. The diffraction experi-
4.3. Reactivity of NbBr5 with 1,2-epoxybutane and epoxycyclohexane
ments were carried out on a Bruker APEX II diffractometer
equipped with a CCD detector using Mo K
a radiation. Data were
General procedure: A suspension of NbBr5 (1.00 mmol), in CDCl3
(0.80 mL) inside a NMR tube, was treated with the appropriate
epoxide (3.00 mmol). The tube was sealed, and left at room tem-
perature overnight. Then, a 1H NMR spectrum revealed that com-
plete consumption of the epoxide had occurred, and formation of
a complicated mixture of products had taken place. This mixture
was treated with water (ca. 30 mmol), resulting in quick formation
of a white precipitate. The precipitate was separated from the solu-
corrected for Lorentz polarization and absorption effects (empirical
absorption correction SADABS) [26]. The structure was solved by
direct methods and refined by full-matrix least-squares based on
all data using F2 [27]. The asymmetric unit contains only half of
the molecule, whereas the second half is generated by an inversion
centre. Hydrogen atoms bonded to C-atoms were fixed at calcu-
lated positions and refined by a riding model. All non-hydrogen
atoms were refined with anisotropic displacement parameters.

1240
F. Marchetti et al. / Polyhedron 28 (2009) 1235–1240
Inorganic Chemistry, 5th Ed., Clarendon Press, Oxford, 1993. For the sake of
Table 4
simplicity, in this paper the general formula MX5, when dealing with the
niobium(V) and tantalum(V) halides, will be used.
Crystal data and experimental details for 2h.
Complex
Formula
Formula weight
T (K)
2h
[6] (a) F. Marchetti, G. Pampaloni, S. Zacchini, Dalton Trans. (2007) 4343;
(b) F. Marchetti, G. Pampaloni, S. Zacchini, Inorg. Chem. 47 (2008) 365;
(c) F. Marchetti, G. Pampaloni, S. Zacchini, Chem. Commun. (2008) 3651;
(d) F. Marchetti, G. Pampaloni, S. Zacchini, Dalton Trans. (2008) 7026;
(e) F. Marchetti, G. Pampaloni, S. Zacchini, Polyhedron 27 (2008) 1969;
(f) F. Marchetti, G. Pampaloni, S. Zacchini, Eur. J. Inorg. Chem. (2008) 453;
(g) F. Marchetti, G. Pampaloni, T. Repo, Eur. J. Inorg. Chem. (2008) 2107.
[7] (a) As general references see: J.A. McCleverty, T.J. Meyer (Eds.),
Comprehensive Coordination Chemistry II, Elsevier Pergamon, London, 2005;
(b) A. Singh, R.C. Mehrotra, Coord. Chem. Rev. 248 (2004) 101.
[8] (a) N. Huesing, B. Launay, G. Kickelbick, S. Gross, L. Armelao, G. Bottaro, M.P.
Feth, H. Bertagnolli, G. Kothleitner, Appl. Catal. A 254 (2003) 297;
(b) T.J. Boyle, T.M. Alam, D. Dimos, G.J. Moore, C.D. Buchheit, H.N. Al-Shareef,
E.R. Mechenbier, B.R. Bear, J.W. Ziller, Chem. Mater. 9 (1997) 3187;
(c) P.A. Shcheglov, D.V. Drobot, G.A. Seisenbaeva, S. Gohil, V.G. Kessler, Chem.
Mater. 14 (2002) 2378.
C12H24Br10O6Ta2
1425.31
100(2)
0.71073
monoclinic
C2/c
12.2223(13)
10.4276(12)
23.080(3)
90
95.602(2)
90
2927.5(6)
4
3.234
21.144
2560
k (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
Cell volume (Å3)
Z
Dcalc, g cmÀ3
[9] (a) J.H. Jung, T. Shimizu, S. Shinkai, J. Mater. Chem. 15 (2005) 3979;
(b) M. Niederberger, N. Pinna, J. Polleux, M. Antonietti, Angew. Chem., Int. Ed.
43 (2004) 2270.
l
(mmÀ1
)
F(000)
Crystal size (mm)
h limits (°)
Reflections collected
Independent reflections [Rint
Data/restraints/parameters
Goodness on fit on F2
0.17 Â 0.15 Â 0.14
2.57–25.03
13415
2595 [0.0379]
2595/100/192
1.051
0.0561
0.1835
2.569/À4.554
[10] (a) Some recent references are: E.C.E. Rosenthal, H. Cui, J. Koch, ACS Symp. Ser.
974 (2007) 70;
(b) T. Gueta-Neyroud, B. Tumanskii, M. Kapon, M.S. Eisen, Macromolecules 40
(2007) 5261;
(c) J. Yamada, K. Nomura, Organometallics 24 (2005) 3621;
(d) E.C.E. Rosenthal, H. Cui, J. Koch, P. Escarpa Gaede, M. Hummert, S. Dechert,
J. Chem. Soc., Dalton Trans. (2005) 3108;
(e) A.V. Grafov, C.L. Firme, I.A. Grafova, F. Benetollo, M.L. Dias, M.J.M. Abadie,
Polymer 46 (2005) 9626;
(f) B. Lian, Y. Qian, W. Zhou, J. Huang, Appl. Organomet. Chem. 19 (2005) 621;
(g) E.C.E. Rosenthal, H. Cui, K.C.H. Lange, S. Dechert, Eur. J. Inorg. Chem. (2004)
4681;
]
R1 (I > 2
wR2 (all data)
Largest difference in peak and hole (e ÅÀ3
r
(I))
)
(h) Z. Janas, L.B. Jerzykiewicz, K. Przybylak, P. Sobota, K. Szczegot, Eur. J. Inorg.
Chem. (2004) 1639.
[11] (a) U. Schubert, Acc. Chem. Res. 40 (2007) 730;
(b) G.J.deA.A. Soler-Illia, C. Sanchez, New J. Chem. 24 (2000) 493;
(c) J.H. Thurston, K.H. Whitmire, Inorg. Chem. 41 (2002) 4194.
[12] R.M. Trend, B.M. Stoltz, J. Am. Chem. Soc. 126 (2004) 4482.
[13] (a) A. Decken, G.B. Nikiforov, J. Passmore, Dalton Trans. (2006) 4328;
(b) F. Wolff, R. Choukroun, C. Lorber, B. Donnadieu, Eur. J. Inorg. Chem. (2003)
628;
The bridging –CH2CH2Br group and a terminal one are disordered.
Disordered atomic positions were split and refined using one occu-
pancy parameter per disordered group and restrained to have sim-
ilar geometries (SAME line in SHELXTL). Similar U restraints were
applied to the C, Br and O atoms.
Supplementary data
(c) Y. Li, A. Turnas, J.T. Ciszewski, A.L. Odom, Inorg. Chem. 41 (2002) 6298;
(d) T. Carofiglio, P.G. Cozzi, C. Floriani, A. Chiesi-Villa, C. Rizzoli,
Organometallics 12 (1993) 2726;
(e) D.C. Bradley, H. Chudzynska, M.E. Hammond, M.B. Hursthouse, M.
Motevalli, W. Rouwen, Polyhedron 11 (1992) 375;
CCDC 710805 contains the supplementary crystallographic data
for [TaBr2(OCH2CH2Br)2(l-OCH2CH2Br)]2, 2h. These data can be
(f) C.H. Winter, P.H. Sheridan, M.J. Heeg, Inorg. Chem. 30 (1991) 1962.
[14] (a) A.A. Pinkerton, D. Schwarzenbach, L.G. Hubert-Pfalzgraf, J.G. Riess, Inorg.
Chem. 15 (1976) 1196;
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk.
(b) L.N. Lewis, M.F. Garbauskas, Inorg. Chem. 24 (1985) 363;
(c) R.N. Kapoor, F. Cervantes-Lee, C.F. Campana, C. Haltiwanger, K. Abney, K.H.
Pannell, Inorg. Chem. 45 (2006) 2203;
(d) S.C. Goel, J.A. Hollingsworth, A.M. Beatty, K.D. Robinson, W.E. Buhro,
Polyhedron 17 (1998) 781;
(e) R. Wang, K. Folting, J.C. Huffman, L.R. Chamberlain, I.P. Rotwell, Inorg.
Chim. Acta 120 (1986) 81;
(f) J.R. Clark, A.L. Pulvirenti, P.E. Fanwick, M. Sigalas, O. Eisenstein, I.P.
Rothwell, Inorg. Chem. 36 (1997) 3623;
(g) T. Matsuo, H. Kawaguchi, Inorg. Chem. 41 (2002) 6090;
(h) H. Kawaguchi, T. Matsuo, J. Organomet. Chem. 690 (2005) 5333;
(i) C. Redshaw, D.M. Homden, M.A. Rowan, M.R.J. Elsegood, Inorg. Chim. Acta
358 (2005) 4067.
Acknowledgments
The authors wish to thank the Ministero dell’Istruzione,
dell’Università e della Ricerca (MIUR, Roma), Programmi di Ricerca
Scientifica di Notevole Interesse Nazionale, Cofinanziamento
2004–2005, and the Consiglio Nazionale delle Ricerche (C.N.R.)
Progetto CNR-MIUR ‘‘Sviluppo di microcelle a combustibile”, for
financial support.
[15] Niobium(V) fluoroalkoxides have been reported as being obtained from the
reaction of NbOCl3 and the corresponding fluoroalcohols L.G. Hubert-Pfalzgraf,
J.G. Riess, Inorg. Chim. Acta 41 (1980) 111.
[16] D.C. Bradley, C.E. Holloway, J. Chem. Soc. (A) (1968) 219.
[17] J.E. Huheey, E.A. Keiter, R.L. Keiter, Inorganic Chemistry, 4th Ed., Principles of
Structure and Reactivity, Harper and Collins College Publisher, New York,
1993.
[18] F. Martinez, C. del Campo, E.F. Llama, J. Chem. Soc., Perkin 1 (2000) 1749.
[19] J.-Y. Kim, T. Kwan, Chem. Pharm. Bull. 18 (1970) 1040.
[20] A. Procopio, R. Dalpozzo, A. De Nino, M. Nardi, G. Sindona, A. Tagarelli, Synlett
14 (2004) 2633.
[21] A.M. Anderson, J.M. Blazek, P. Garg, B.J. Payne, R.S. Mohan, Tetrahedron Lett. 41
(2000) 1527.
[22] B. Rickborn, R.M. Gerkin, J. Am. Chem. Soc. 93 (1971) 1693.
[23] F. Calderazzo, P. Pallavicini, G. Pampaloni, P.F. Zanazzi, J. Chem. Soc., Dalton
Trans. (1990) 2743.
[24] W. Wilker, D. Leibfritz, R. Kerssebaum, W. Bermel, Magn. Reson. Chem. 31
(1993) 287.
[25] D.A. Skoog, D.M. West, Fundamentals of Analytical Chemistry, 2nd Ed., Holt,
Rinehart and Winston, Chatham, UK, 1974. p. 233.
[26] G.M. Sheldrick, SADABS, University of Göttingen, Göttingen, Germany.
[27] G.M. Sheldrick, SHELX97, University of Gottingen, Göttingen, Germany.
References
[1] (a) Y. Shichijo, A. Migita, H. Oguri, M. Watanabe, T. Tokiwano, K. Watanabe, H.
Oikawa, J. Am. Chem. Soc. 130 (2008) 12230;
(b) J. Justicia, A.G. Campaña, B. Bazdi, R. Robles, J.M. Cuerva, J.E. Oltra, Adv.
Synth. Catal. 350 (2008) 571.
[2] H.S. Park, D.W. Kwon, K. Lee, Y.H. Kim, Tetrahedron Lett. 49 (2008) 1616.
[3] (a) T. Ollevier, E. Nadeau, Tetrahedron Lett. 49 (2008) 1546;
(b) M. Curini, F. Epifano, M.C. Marcotullio, O. Rosati, Eur. J. Org. Chem. 21
(2001) 4149.
[4] (a) A. Gansäuer, A. Barchuk, F. Keller, M. Schmitt, S. Grimme, M. Gerenkamp, C.
Mück-Lichtenfeld, K. Daasbjerg, H. Svith, J. Am. Chem. Soc. 129 (2007) 1359;
(b) A.W. Eppley, N.I. Totah, Tetrahedron 53 (1997) 16545;
(c) K. Fujiwara, T. Tokiwano, A. Murai, Tetrahedron Lett. 36 (1995) 8063;
(d) M. Emziane, P. Lhoste, D. Sinou, J. Mol. Catal. 49 (1988) L23;
(e) A.E. Vougioukas, H.B. Kagan, Tetrahedron Lett. 28 (1987) 6065.
[5] The fluorides of niobium(V) and tantalum(V) have tetranuclear structures in
the solid state, while the corresponding heavier halides are dinuclear in the
solid state and mononuclear in the vapour phase A.F. Wells, Structural