Unusual room temperature activation of 1,2-dialkoxyalkanes by niobium and tantalum pentachlorides

Article abstract of DOI:10.1039/b810210d

The outcome of the reactions of MCl₅ (M = Nb, 1a; M = Ta, 1b) with 1,2-dialkoxyalkanes [i.e. MeO(CH₂)₂OMe (dme), EtO(CH₂)₂OEt, MeOCH₂CH(Me)OMe, MeO(CH ₂)₂OCH₂Cl, MeO(CH₂) ₂O(CH₂)₂OMe (diglyme)] depends strictly on the stoichiometry. In the 1:1 molar ratio reactions, single C-O bond cleavage occurs, resulting in formation of alkyl chloride and of the complexes MCl ₄[O(R)CH(R′)CH₂O] [M = Nb, R = Me, R′ = H, 3a; M = Ta, R = Me, R′ = H, 3b; M = Nb, R = Et, R′ = H, 5; M = Nb, R = R′ = Me, 6; M = Nb, R = CH₂Cl, R′ = H, 7; M = Nb, R = (CH₂)₂O(Me)NbCl₅, R′ = H, 8], which have been characterized spectroscopically. Moreover, minor amounts of the oxo-bridged adducts MOCl₃(dme)MCl₅ (M = Nb, 4a; M = Ta, 4b) have been isolated in the reactions involving dme. On the other hand, compounds 1 react with two (or more) equivalents of dme mainly via a multiple C-O bond cleavage process, affording MOCl₃(dme) (M = Nb, 2a; M = Ta, 2b), 1,4-dioxane and methyl chloride. The oxychloride compounds MOCl₃ (M = Nb, 11a; M = Ta, 11b) have been efficiently obtained by addition of TiCl₄ to 2. Compound 2a is reduced in high yield to the Nb(iii) species NbCl ₃(dme), 12, upon treatment with SnBu₃H. The oxychloride NbOCl₃(diglyme), 9, 1,4-dioxane, CH₃Cl and the hexachloroniobate salt [MeOCH₂CH₂OCH₂CH ₂O(H)Me][NbCl₆], 10, have been identified as products of the reaction of 1a with two equivalents of diglyme. The 1 : 2 molar ratio reaction of 1a with MeOCH₂CH(Me)OMe gives 2,5-dimethyl-1,4-dioxane. Compound 1a reacts with two equivalents of EtO(CH₂)₂OEt or MeO(CH₂)₂OCH₂Cl yielding Cl(CH ₂)₂OCH₂CH₃ or O(CH ₂Cl)₂ and diglyme, respectively, but not dioxane, suggesting that fragmentation pathways different from that found for dme are operating. The X-ray molecular structures of 4a, 4b and 10 have been determined.

Full text of DOI:10.1039/b810210d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Unusual room temperature activation of 1,2-dialkoxyalkanes by niobium and
tantalum pentachlorides†
Fabio Marchetti,‡a Guido Pampaloni*a and Stefano Zacchinib
Received 17th June 2008, Accepted 8th September 2008
First published as an Advance Article on the web 30th October 2008
DOI: 10.1039/b810210d
The outcome of the reactions of MCl5 (M = Nb, 1a; M = Ta, 1b) with 1,2-dialkoxyalkanes
[i.e. MeO(CH2)2OMe (dme), EtO(CH2)2OEt, MeOCH2CH(Me)OMe, MeO(CH2)2OCH2Cl,
MeO(CH2)2O(CH2)2OMe (diglyme)] depends strictly on the stoichiometry. In the 1 : 1 molar ratio
reactions, single C–O bond cleavage occurs, resulting in formation of alkyl chloride and of the
complexesMCl4[O(R)CH(R¢)CH2O] [M = Nb, R = Me, R¢ = H, 3a; M = Ta, R = Me, R¢ = H, 3b;
M = Nb, R = Et, R¢ = H, 5; M = Nb, R = R¢ = Me, 6; M = Nb, R = CH2Cl, R¢ = H, 7; M = Nb,
R = (CH2)2O(Me)NbCl5, R¢ = H, 8], which have been characterized spectroscopically. Moreover, minor
amounts of the oxo-bridged adducts MOCl3(dme)MCl5 (M = Nb, 4a; M = Ta, 4b) have been isolated
in the reactions involving dme. On the other hand, compounds 1 react with two (or more) equivalents
of dme mainly via a multiple C–O bond cleavage process, affording MOCl3(dme) (M = Nb, 2a; M = Ta,
2b), 1,4-dioxane and methyl chloride. The oxychloride compounds MOCl3 (M = Nb, 11a; M = Ta, 11b)
have been efficiently obtained by addition of TiCl4 to 2. Compound 2a is reduced in high yield to the
Nb(III) species NbCl3(dme), 12, upon treatment with SnBu3H. The oxychloride NbOCl3(diglyme), 9,
1,4-dioxane, CH3Cl and the hexachloroniobate salt [MeOCH2CH2OCH2CH2O(H)Me][NbCl6], 10, have
been identified as products of the reaction of 1a with two equivalents of diglyme. The 1 : 2 molar ratio
reaction of 1a with MeOCH2CH(Me)OMe gives 2,5-dimethyl-1,4-dioxane. Compound 1a reacts with
two equivalents of EtO(CH2)2OEt or MeO(CH2)2OCH2Cl yielding Cl(CH2)2OCH2CH3 or O(CH2Cl)2
and diglyme, respectively, but not dioxane, suggesting that fragmentation pathways different from that
found for dme are operating. The X-ray molecular structures of 4a, 4b and 10 have been determined.
aliphatic cyclic ethers different reactions can take place, beyond the
formation of the expected acid–base adducts MX5(L), depending
Introduction
The activation of C–O bonds by low valent and electron-rich
on the nature of the halide and the O-donor to metal molar ratios.
transition metal complexes has represented a key-step for a huge
In the light of these considerations, we became interested in the
variety of organic syntheses.1 Alternatively, the use of electron-
poor metal derivatives is a less employed approach:2 a significant
reactivity of MCl5 (M = Nb, 1a; Ta, 1b)7 with open-chain ethers
and we recently reported a short communication on the room
example is given by the synthesis of esters by acylative cleavage of
temperature reaction of NbCl5 with 1,2-dimethoxyethane, dme,
ethers, promoted by low valent early transition metal complexes.3
Otherwise, it has been reported that the stoichiometric reac-
tions of ethers with compounds of strongly oxophilic metals
which cleanly affords NbOCl3(dme), 2a, CH3Cl and 1,4-dioxane
(eqn (1), C4H10O2 = dme).8
(lanthanides, early transition elements in high oxidation states)
NbCl5 + 2C4H10O2 Æ NbOCl3 C H O + 2CH Cl + 1 C4H8O2
(
)
usually afford stable metal alkoxides.4 In such cases, the ether
undergoes breaking into smaller fragments, rather than affording
functionalized species. For instance, 1,2-dimethoxyethane, dme,
a diether widely used as solvent in synthetic chemistry, reacts
with various oxophilic metal complexes with cleavage of one or
two O–CH2 bonds, affording methoxy-derivatives.4e–g,5 Ethylene
production has been observed in some cases.4e,g,5c–e
4
10
2
3
2
2a
(1)
The result schematized in eqn (1) is notable for several reasons:
(i) niobium pentachloride is a simple system effective in promoting
the unusual transformation of dme into 1,4-dioxane and methyl
chloride under mild conditions; (ii) the reaction proceeds through
the unprecedented cleavage of three dme C–O bonds; (iii) pre-
liminary studies indicate that the same reaction is extensible to
1,2-dialkoxyalkanes different from dme; (iv) other preparations
previously reported for niobium oxytrihalide adducts mostly
require high temperatures;9 (v) some reductive complexations
of group 5 metal pentahalides10 and some organic syntheses
mediated by MCl5 (M = Nb, Ta)2e,11 have been performed in dme
solution, thus these processes might take place via the intermediate
formation of oxo-complexes.
We recently showed6 that when niobium and tantalum pen-
tahalides are allowed to react with ketones or aldehydes or
aUniversita` di Pisa, Dipartimento di Chimica e Chimica Industriale,
Via Risorgimento 35, I-56126, Pisa, Italy. E-mail: pampa@dcci.unipi.it;
Fax: +39 50 2219246; Tel: +39 50 2219219
bUniversita` di Bologna, Dipartimento di Chimica Fisica e Inorganica, Viale
Risorgimento 4, I-40136, Bologna, Italy
† CCDC reference numbers 691886–691888. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b810210d
‡ Born in Bologna (Italy) in 1974.
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◦
˚
With respect to these points of interest, we decided to carry out
a detailed study of the reactions of 1 with dme, and to extend
this study to a variety of 1,2-dialkoxyalkanes. In the present
paper, both the metal containing compounds and the organic
products obtained by the different reactions will be presented, with
particular reference to the stoichiometry employed in each case.
Moreover, a novel room temperature synthesis of the oxychloride
compounds MOCl3 (M = Nb, Ta) will be described, and the role
possibly played by oxo-compounds during the course of reductive
reactions (see point v above) will be discussed.
Table 1 Selected bond lengths (A) and angles ( ) for MOCl3(dme)MCl5
(M = Nb, 4a, M = Ta, 4b)
4a
4b
M(1)–O(1)
M(1)–O(2)
M(1)–O(3)
M(1)–l(8)
M(1)–Cl(7)
M(1)–Cl(6)
M(2)–O(1)
M(2)–Cl(2)
M(2)–Cl(5)
M(2)–Cl(1)
M(2)–Cl(4)
M(2)–Cl(3)
1.7579(14)
2.1661(15)
2.2898(14)
2.2913(6)
2.3417(6)
2.3536(6)
2.1353(14)
2.2778(6)
2.3120(6)
2.3284(6)
2.3311(6)
2.3423(6)
1.800(3)
2.158(3)
2.232(3)
2.2671(10)
2.3351(10)
2.3424(10)
2.030(3)
2.3272(10)
2.3201(10)
2.3407(10)
2.3348(10)
2.3521(10)
Results and discussion
Reactivity of niobium and tantalum pentachlorides with
1,2-dialkoxyalkanes in 1 : 1 molar ratios
O(1)–M(1)–O(2)
O(1)–M(1)–O(3)
O(2)–M(1)–O(3)
M(1)–O(1)–M(2)
O(2)–C(2)–C(3)
C(3)–O(3)–M(1)
O(1)–M(2)–Cl(2)
O(1)–M(2)–Cl(5)
O(1)–M(2)–Cl(1)
O(1)–M(2)–Cl(4)
O(1)–M(2)–Cl(3)
96.67(6)
168.53(6)
72.37(5)
167.07(9)
107.09(17)
112.86(11)
178.76(4)
83.48(4)
94.21(11)
167.03(11)
72.84(10)
164.15(16)
105.8(3)
114.1(2)
178.71(8)
87.47(8)
The reaction of TaCl5, 1b, with one equivalent of dme affords
the alkoxyether adduct TaCl4[O(Me)CH2CH2O], 3b, and methyl
chloride in ca. 1 : 1 ratio, according to NMR evidence (see
Experimental). The formation of 3b is the result of single dme
C–O bond cleavage, resembling what we reported in a recent
communication about the 1 : 1 molar ratio reaction of 1a with
dme, see Scheme 1.8
86.67(4)
85.32(4)
85.23(4)
88.77(8)
86.91(8)
87.38(8)
Scheme 1
It is noteworthy that the complex NbOCl3(dme), 2a (see eqn
(1)), formally derives from compound 3a by addition of one dme
and removal of one MeCl molecule and half dioxane unit. Actually
we found that the treatment of 3a with an excess of dme did cause
partial transformation of 3a into 2a, CH3Cl and 1,4-dioxane (see
Experimental). This fact suggests that complexes 3 might act as
intermediates during the reaction of 1 with large quantities of dme.
Compounds 3a–b were obtained in optimal yields (ca. 80%)
by reacting MCl5, 1, with one equivalent of HOCH2CH2OMe,12
and were fully characterized by IR, NMR and elemental analyses.
On the other hand, the reaction of 1 with one equivalent of dme
afforded lower yields of 3 (55–60%). This is due to the fact that
3 is not the only metal-containing product obtainable by reacting
1 with dme in 1 : 1 molar ratio: another complex containing dme
was isolated in 15–20% yield. This complex was characterized
as the dinuclear species MOCl3(dme)MCl5 (M–O–M) (M = Nb,
4a; M = Ta, 4b), and more satisfactory yields for this were gained
by reacting dme with a ca. 30% molar excess of 1. The X-ray
molecular structures of 4a–b are represented in Fig. 1, and selected
bond lengths and angles are reported in Table 1.
Fig. 1 View of the molecular structure of MOCl3(dme)MCl5 (M = Nb,
4a, M = Ta, 4b). Displacement ellipsoids are at the 30% probability level.
At variance to what is generally observed for m-oxo derivatives of
transition metals [see for example (MCp2X)2O,14 (Ta2OCl10)2-,13d,15
[TaCl2(NMe2)2(HNMe2)]2O16], the M(1)–O(1)–M(2) angle devi-
ates significantly from linearity [167.07(9)◦ in 4a and 164.15(16)◦
in 4b]; moreover the two M–O lengths differ considerably from
˚
each other [Nb(1)–O(1): 1.7579(14) A vs. Nb(2)–O(1): 2.1353(14)
˚
˚
˚
A; Ta(1)–O(1): 1.800(3) A vs. Ta(2)–O(1): 2.030(3) A]. The bond
distances and angles regarding the bridging oxygen in 4 are better
comparable to those reported for NbOCl3,17 the structure of which
consists of dimeric units linked by asymmetric [Nb–O: 2.203(3)
◦
˚
and 1.758(3) A] and bent (Nb–O ◊ ◊ ◊ Nb: 170.56 ) bridges. The
asymmetry of the bridge further corroborates the fact that 4a–b
can be viewed as the result of coordination of MOCl3(dme) to the
Lewis acid MCl5.
The molecules can be viewed as adducts of MCl5 (M =
Nb, Ta) with 2, resembling in this the oxo bridged tetramers
[M4O2Cl18]2-, M = Nb, Ta.13 The geometry and bonding param-
eters of the MOCl3(dme) units in 4a–b are closely related to the
ones discussed for 2a,8 apart from a detectable elongation of the
We believe that the oxo-unit within compounds 4 originates
via a O-abstraction reaction from dme (see eqn (1)). Accordingly,
the formation of 4, in the course of the 1 : 1 reaction between
1 and dme, should be accompanied by the formation of 1,4-
dioxane. Although no dioxane was observed by monitoring these
˚
˚
M(1)–O(1) interaction [1.7579(14) A in 4a vs. 1.6939(18) A in 2a],
which is a consequence of the further coordination of the oxo
ligand to M(2).
1
reactions via H NMR, small amounts of that cyclic ether were
detected (GC/MS) on the reaction mixtures after hydrolysis
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(see Experimental). Presumably, under anhydrous conditions (i.e.
the conditions of the NMR experiment), the sparingly soluble
MCl5(dioxane)18 is formed, the lower solubility of this complex
with respect to 3 preventing the detection of the former in solution.
In summary, the reactions of 1 with limited amounts of
dme proceed mainly via single C–O bond cleavage affording 3:
nevertheless, further dme fragmentation takes place to a limited
extent producing minor amounts of MOCl3(dme), which gives 4
in the presence of excess MCl5.
With the aim to extend the chemistry of 1 with dme to similar
organic substrates, we decided to study the reactivity of NbCl5, 1a,
with a series of 1,2-dialkoxyalkanes. The 1 : 1 molar ratio reactions
proceed with cleavage of one external C–O bond, similar to the
behaviour of dme (Scheme 1). In particular, compound 1a reacts
with EtO(CH2)2OEt or MeOCH2CH(Me)OMe to give in good
yield compounds of general formulaNbCl4[O(R)CHR1CH2O] (5,
R = Et, R1 = H; 6, R = R1 = Me), and alkyl chloride (in about
Scheme 2
elemental analyses, is the result of the coordination of one diglyme
to two NbCl5 moieties, accompanied by cleavage of one O–Me
bond and one Nb–Cl bond. In agreement, methyl chloride has
been detected as a product of this reaction, via both NMR and
GC/MS. In other words, the initial coordination of diglyme
to 1a, occurring presumably via two oxygen atoms in a “dme-
fashion” (see Scheme 1), is then followed by MeCl elimination
and consequent formation of a fragment which is still able to
coordinate further NbCl5.
1
1 : 1 ratio with respect to the inorganic product, according to H
NMR evidence), see eqn (2).
NbCl5 + ROCHR1CH2OR
→ NbCl4[O(R)CHR1CH2O] + RCl
(2)
R = Et, R1 = H, 5
R = R1 = Me, 6
Compounds 5 and 6 have been characterized by NMR spec-
troscopy and elemental analyses. On the basis of the NMR
features, the reaction of NbCl5 with 1,2-dimethoxypropane af-
fording 6 is highly chemoselective, since the alternative complex
NbCl4[O(Me)CH2CH(Me)O], deriving from the cleavage of the
O–Me bond at the most hindered side of the diether, has not been
detected in solution. Complex 6 has been obtained as a mixture
of two inseparable isomers, in 2 : 1 ratio, differing in the relative
orientations of the substituents at the chiral centre (i.e., H and Me).
Scheme 3
Complexes containing a bidentate, intact, diglyme ligand have
been reported in the literature, and examples are: [Pb(hexa-
3
1
fluoroacetyl-acetonato)2(m-h -h -diglyme)]2,19
TiCl2(OBut)2-
(O,O¢-diglyme),20 and TiCp2(O,O¢-diglyme).21
Reactivity of niobium and tantalum pentachlorides with
1,2-dialkoxyalkanes in 1 : 2 molar ratios
1
The H NMR spectrum of the major isomer exhibits resonances
at 5.24 and 5.08, 4.68, 3.98, 1.60 ppm, which have been assigned
to the NbOCH2, CH, OMe and CHMe protons, respectively. The
1H NMR spectrum of the less abundant isomer displays a similar
pattern.
Furthermore, the reactivity of NbCl5, 1a, with 1,2-
dialkoxyethanes of generic formula MeO(CH2)2OR [R = CH2Cl,
(CH2)2OMe (diglyme)] has been explored in order to establish a
possible influence of different R substituents on the reactivity.
According to 1H NMR data, the addition of one equiv-
It is worthy of mention that when a CH2Cl2 suspension of 4a was
reacted with dme, compound 2a was isolated in very good yield.
This indicates that the [MCl5] moiety in 4 can be easily displaced
from MOCl3(dme) by addition of further dme, giving 2 equivalents
of MOCl3(dme), methyl chloride and 1,4-dioxane (eqn (3)).
4a + 2C4H10O2 Æ 2NbOCl3 C H O + 2CH Cl + 1 C4H8O2
(
)
4
10
2
3
(3)
2
2a
The oxytrichloride species MOCl3(dme) (M = Nb, 2a;8 M = Ta,
2b) were isolated in good yields by direct combination of MCl5, 1,
with two equivalents of dme, eqn (4).
alent of MeO(CH2)2OCH2Cl to 1a gives
a mixture of
NbCl4[O(Me)CH2CH2O], 3a, and NbCl4[O(CH2Cl)CH2CH2O],
7, in ca. 5 : 2 ratio, which we have not been able to sepa-
rate. The formation of these compounds is the consequence
of the alternative cleavage of one of the two different ex-
ternal C–O bonds within 1-chloromethoxy-2-methoxyethane,
Me–O(CH2)2O–CH2Cl. In other words, two distinct potential
fragmentation routes, consisting of breaking of one external
C–O bond, are possible for this diether, and both are operating.
Methyl chloride and methylene chloride should be expected as
organic products, see Scheme 2, and these two substances have
been actually found in the mixture at the end of reaction.
MCl5 + 2C4H10O2 → 2 + 2CH3Cl + 1 1,4-C4H8O2
(4)
2
The spectroscopic features of 2b resemble those already reported
for 2a.8 The IR spectrum (recorded in the solid state) displays an
intense absorption related to the Ta O bond, at 910 cm-1 (954 cm-1
=
8,22
=
for Nb O in 2a). The shift towards higher wavenumbers of the
metal–oxygen stretching vibration in 2 with respect to 4 (876 cm-1
in 4a and 872 cm-1 in 4b) is a consequence of the enhancement
of the metal–oxygen bond order, due to loss of the M–O–M
interaction, which exists in 4.
Otherwise, the reaction of 1a with diglyme has yielded a
complicated mixture containing prevalently the dinuclear complex
8, Scheme 3. This compound, which has been isolated by fractional
crystallization and identified by 1H NMR spectroscopy and
Moreover, the 1H NMR resonances (CDCl3 solution) account-
ing for the dme unit appear as singlets at 4.14 (CH2) and 3.95 (CH3)
ppm. In agreement with the X-ray solid state structure determined
for 2a, the dme frame within complexes 2 is asymmetrically
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coordinated to the metal centre; therefore, two pairs of resonances
for non-equivalent CH2 and CH3 groups would be expected in
the NMR spectra. The equivalence of the two CH2 and of the
two CH3 groups suggests that a fast exchange phenomenon is
operating in solution at room temperature. Analogous fluxionality
was observed in similar mononuclear niobium adducts.23 Unfortu-
nately, low temperature NMR experiments could not be performed
on complexes 2, due to the low solubility in suitable deuterated
solvents below 0 ◦C.
In contrast to that described about the reaction of 1a with two
equivalents of dme, which leads to the formation of 2a, the reac-
tions of 1a with two (or more) equivalents of 1,2-diethoxyethane,
1,2-dimethoxypropane, 1-chloromethoxy-2-methoxyethane and
diglyme appear to be less selective, and the inorganic products
could not be identified, the only exception being NbOCl3(O,O¢-
=
diglyme), 9, which is characterized by a strong Nb O stretching
vibration at 934 cm-1.
Concerning the organic products, 2,5-dimethyl-1,4-dioxane has
been discovered by GC/MS in the reaction of 1a with a two-fold
excess of 1,2-dimethoxypropane: although a precise quantification
of the yield has not been possible hitherto, a new potential way for
the facile preparation of methyl-substituted dioxanes is given.
The nature of the alkoxy groups is crucial in determining
the fragmentation pathway followed by the 1,2-dialkoxyalkane
ROCH2CH2OR, when this is reacted with NbCl5 in 2 : 1 molar
ratio. Interestingly, MeO(CH2)2OCH2Cl gives O(CH2Cl)2 and
diglyme, while Cl(CH2)2OCH2CH3 has been recognised in the
final mixture of the reaction of 1a with two equivalents of
EtO(CH2)2OEt (see Experimental). As a matter of fact, neither
1,4-dioxane nor 1,2-dichloroethane have been found as products
in these two cases.
On the other hand, 1,4-dioxane is produced in good yield when
1a is reacted with diglyme in ca. 1 : 2 molar ratio. Nevertheless,
when monitoring the 1 : 2 reaction of 1a with diglyme by NMR,
a complicated 1H NMR spectral pattern was seen, indicating
that, under these conditions, diglyme probably reacts with 1a
following several fragmentation pathways. In particular, we ob-
served a downfield resonance (d = 8.72 ppm, see Experimental),
which suggested to us the possible formation of some stabilized
O-protonated organic species. Indeed, the study of the chemistry
of the halides MX5 (M = Nb, Ta) with ketones and acetylacetones
has demonstrated that [MX6]- salts of the protonated organic
substrates can form as secondary products, for which the O–H
protons can give rise to 1H NMR downfield resonances.6 In
order to verify the possibility, we made several attempts to grow
X-ray quality crystals from the reaction liquor. A successful
attempt gave crystals suitable for X-ray analysis which were
identified as the hexachloroniobate salt of protonated diglyme, i.e.
[MeOCH2CH2OCH2CH2O(H)Me][NbCl6], 10, which OH hydro-
gen atom might come from some diglyme C–H bond activation.
Water cannot be excluded in principle as the source of the OH
hydrogen, although this possibility appears unlikely since the
reaction has been carried out in strictly anhydrous conditions.
The molecular structure of 10 is represented in Fig. 2, and
selected bond lengths and angles are reported in Table 2. Further
characterization of 10 was supported by IR spectroscopy (in solid),
showing the O–H stretching vibration at 3175 cm-1.
In order to focus on the formation of organic products deriving
from the fragmentation of dme, the reactions of MCl5 with two
equivalents of dme were carried out in NMR tubes (CDCl3 as
1
solvent) and monitored by H and 13C NMR spectroscopies, in
the presence of a known quantity of CH2Cl2 used as reference.
Compounds 2, CH3Cl, MCl5(1,4-dioxane)18 and 1,4-dioxane,
24–26
together with minor amounts of [MCl3(OCH3)2]2
and 1,2-
dichloroethane, were seen in the final solutions (see Experimental).
Coherently with the NMR features, the main reaction occurring
when 1 is mixed with two equivalents of dme may be outlined
according to eqn (4). This reaction implies the breaking of
three of the four C–O bonds of dme. Beside that reaction, a
secondary pathway of dme fragmentation seems to be operative
concurrently: this pathway requires the cleavage of the two O–CH2
bonds and would explain the formation of 1,2-dichlorethane and
of MCl3(OCH3)2 species, which have been reported to exist as
dimers26,25 (eqn (5)).
MCl5 + C4H10O2 → 1 [MCl3(OCH3)2]2 + ClCH2CH2Cl
(5)
2
The formation of CH3Cl, 1.4-dioxane and 1,2-dichloroethane
has been confirmed by GC/MS analyses. On the other hand, the
presence in solution of MCl5(1,4-dioxane), detected by NMR, is
reasonable, since the dioxane produced by the main reaction (eqn
(4)) should compete with dme in the coordination to the MOCl3
moiety or to the still unreacted MCl5. Interestingly, GC/MS and
NMR analyses on the mixture obtained by reacting MCl5 with
a large excess of dme (10 equivalents or more) have allowed the
identification of 2, CH3Cl and free dioxane, thus indicating that
the main route (eqn (4)) is still effective even in correspondence of
high dme/MCl5 molar ratios. As expected, under these conditions,
MCl5(1,4-dioxane) is not generated, in view of the fact that dme
is now largely predominant with respect to dioxane, and thus
significantly favoured in coordinating the metal centre.
As stated in the Introduction, a number of examples have been
reported of the activation of 1,2-dimethoxyethane by oxophilic
metal complexes, which gives methoxy derivatives, as a conse-
quence of the cleavage of two O–CH2 bonds.4,5
Also, in connection with the chemistry described herein,
some aluminium species promote the cleavage of the dme
O–CH3 bonds: for instance, [(Me3Si)2CH]2Al–Al[CH(SiMe3)2]2
reacts with potassium/dme affording the glycolato derivative
[(Me3Si)2CH]2Al(OCH2CH2O)K(dme);27 a similar aluminium
compound, [(Me3SiCH2)2Al(OCH2CH2O)]2, has been isolated in
attempting to prepare aluminium amides in dme solution.28 More-
over, the species [AlIMe(OCH2CH2OMe)]2 forms by reaction of
dme with the “high-temperature molecule AlI”, via single O–CH3
bond breaking.29
The molecular structure of 10 consists of an octahedral
[NbCl6]- anion and a protonated diglyme cation [MeOCH2CH2O–
CH2CH2O(H)Me]+. The unit cell contains twelve anions and
twelve cations, one and a half of which being independent (the
complete second anion and cation are generated by an inversion
centre). The O-bonded hydrogen atoms in the protonated diglyme
have been located in the Fourier map and their positions are in
agreement with the formation of an extended network of hydrogen
bonds, which generates an infinite chain of H-bonded cations.
In order to obtain this polymeric structure, protonation occurs
on the terminal –OMe groups of each diglyme molecule [O(1),
Notwithstanding this, the dme fragmentation process summa-
rized in eqn (4) is unprecedented in the sense it takes place via
cleavage of three ethereal C–O bonds.
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◦
˚
H(5). Therefore, hydrogen bonds exist between all O(3) ◊ ◊ ◊ O(3)
and O(1) ◊ ◊ ◊ O(5) contacts, but because of disorder protons are
alternatively present on one or the other oxygen atom. As a
consequence of protonation, all C–O bonds involving protonated
Table 2 Selected bond distances (A), angles ( ) and hydrogen bond
parameters for [MeOCH2CH2OCH2CH2O(H)Me][NbCl6], 10
Bond distances
˚
˚
oxygens [C(1)–O(1) 1.446(3) A, C(2)–O(1) 1.455(3) A, C(5)–O(3)
Nb(1)–Cl(2)
Nb(1)–Cl(1)
Nb(1)–Cl(5)
Nb(2)–Cl(7)
Nb(2)–Cl(8)
2.3297(8) Nb(1)–Cl(6)
2.3422(8) Nb(1)–Cl(4)
2.3562(8) Nb(1)–Cl(3)
2.3483(8) Nb(2)–Cl(9)
2.3542(7)
2.3356(8)
2.3471(8)
2.3768(8)
2.3462(8)
˚
˚
1.444(3) A, C(6)–O(3) 1.446(3) A, C(8)–O(5) 1.461(3) and C(9)–
O(5) 1.454(3) A] are markedly elongated compared to the non
protonated ones [C(3)–O(2) 1.423(3) A, C(4)–O(2) 1.422(3) A,
C(7)–O(4) 1.423(3) A]. All other C–C and C–O contacts are in
˚
˚
˚
˚
keeping with normal single bonds present in unsaturated organic
molecules.30
Angles
Cl(2)–Nb(1)–Cl(6)
Cl(6)–Nb(1)–Cl(1)
Cl(6)–Nb(1)–Cl(4)
Cl(2)–Nb(1)–Cl(5)
Cl(1)–Nb(1)–Cl(5)
Cl(2)–Nb(1)–Cl(3)
Cl(1)–Nb(1)–Cl(3)
Cl(5)–Nb(1)–Cl(3)
C(1)–O(1)–C(2)
O(2)–C(3)–C(2)
O(2)–C(4)–C(5)
C(5)–O(3)–C(6)
O(5)–C(8)–C(7)
90.19(3) Cl(2)–Nb(1)–Cl(1) 90.71(3)
91.17(3) Cl(2)–Nb(1)–Cl(4) 90.25(3)
91.36(3) Cl(1)–Nb(1)–Cl(4) 177.29(3)
179.23(3) Cl(6)–Nb(1)–Cl(5) 89.16(3)
89.72(3) Cl(4)–Nb(1)–Cl(5) 89.35(3)
91.04(3) Cl(6)–Nb(1)–Cl(3) 178.73(3)
89.11(3) Cl(4)–Nb(1)–Cl(3) 88.34(3)
89.60(3)
Room temperature synthesis of niobium and tantalum
oxytrichlorides
The oxygen-abstraction reaction leading to MOCl3(dme), 2,
occurs smoothly at room temperature, whereas the syntheses of
the oxyhalides MOX3 (M = Nb, Ta; X = Cl, Br) generally require
temperatures between 60 and 130 ◦C,9,31 as described for analogous
compounds of the sp-block, such as AlOX.32
Therefore, we became interested in finding the way to prepare
the known MOCl3 from 2. Interestingly, we saw that the addition
of an excess of TiCl4 to dichloromethane solutions of crystalline
2 resulted in the almost instantaneous precipitation of the colour-
less oxychloride compounds MOCl3, 11, from yellow solutions
containing TiCl4(dme) (eqn (6)).
116.2(2) O(1)–C(2)–C(3)
109.2(2) C(4)–O(2)–C(3)
108.7(2) O(3)–C(5)–C(4)
115.3(2) O(4)–C(7)–C(8)
110.4(2) C(9)–O(5)–C(8)
111.4(2)
111.9(2)
110.6(2)
107.6(2)
116.4(2)
Hydrogen bonds parameters
D–H ◊ ◊ ◊ A
d(D–H) d(H ◊ ◊ ◊ A)
d(D ◊ ◊ ◊ A) ∠(DHA)
O(3)–H(3) ◊ ◊ ◊ O(3)#3 0.845(10) 1.550(16)
O(5)–H(5) ◊ ◊ ◊ O(1)
O(1)–H(1) ◊ ◊ ◊ O(5)
2.388(4) 171(8)
2.413(2) 170(3)
2.413(2) 170(7)
0.844(10) 1.578(15)
0.848(10) 1.574(15)
MOCl3 dme + TiCl Æ MOCl + TiCl dme
(
)
(
)
4
3
4
(6)
11
2
Symmetry transformations used to generate equivalent atoms: #3 - x,
y,-z + 1/2.
This observation indicates a room temperature pathway for
the preparation of MOCl3 (M = Nb, 11a; Ta, 11b), starting
from MCl5. Unfortunately, attempts to obtain 11 by a one-
pot synthesis, i.e. treating MCl5 with excess dme and then with
TiCl4, led to clean solutions of TiCl4(dme), but the solids 11a–b
resulted contaminated by organic impurities, according to IR and
elemental analyses data.
Niobium and tantalum oxytrichlorides as precursors of M(III)
species
The fact that 2 may possibly play a role in synthetic procedures
which make use of niobium and tantalum pentahalides and
dme10 has been already mentioned (see Introduction, point v).
According to the literature, the tantalum(III)–alkyne complexes
TaCl3(alkyne) are obtained by zinc reduction of TaCl5 in the
presence of dme, and successive treatment with alkyne. These
compounds, which represent important precursors for a variety
of interesting organic preparations,2e,11 might derive from initial
formation of the oxychloride species TaOCl3dme, 2b, in agreement
with eqn (4). Furthermore, with reference to the reduction of
NbCl5 to NbCl3(dme) by SnBu3H in dme,10a we observed that the
treatment of a suspension of crystalline 2a with SnBu3H resulted
in the fast formation of a brick-red solid, identified as NbCl3(dme),
12 (eqn (7)).
Fig.
2 View of the molecular structure of [MeOCH2CH2OCH2-
CH2O(H)Me][NbCl6], 10: (a) the octahedral [NbCl6]- anion (only
one of the two independent anions has been drawn). (b) the
[MeOCH2CH2OCH2CH2O(H)Me]+ cation for which part of the infinite
H-bonded chain is represented. Displacement ellipsoids are at the 30%
probability level. Only independent atoms are labelled.
NbOCl3 dme + SnBu H Æ NbCl dme + "SnBu OH"
(
)
(
)
3
3
12
3
(7)
2a
O(3) and O(5)], and not on the central oxygens [O(2) and O(4)].
In order to have one H+ per diglyme molecule and to avoid the
simultaneous presence of two OH groups on the same H-bond,
an occupancy factor of 0.5 has been assigned to H(1), H(3) and
This evidence supports the idea that the formation of
NbOCl3(dme) may be really the first step in the course of the
tin hydride reduction of Nb(V) to Nb(III) by SnBu3H in dme.
7030 | Dalton Trans., 2008, 7026–7035
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referenced to the non-deuterated aliquot of the solvent. GC/MS
analyses were performed on a HP6890 instrument, interfaced with
MSD-HP5973 detector and equipped 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, paying particular attention
to the more sensitive compounds which were weighed and directly
introduced into the analyzer. The chlorine content was determined
by the Volhard method33 after exhaustive hydrolysis of the sample.
The metal was analyzed as M2O5 obtained by hydrolysis of
the sample followed by calcination in a platinum crucible. The
halogen and the metal analyses were repeated twice in order to get
reproducible results.
Conclusions
Although the chemistry of niobium and tantalum pentahalides
with oxygen-containing organics was scarcely developed in the
past,6 the results presented herein show that such halides
are efficiently capable of promoting clean reactions on 1,2-
dialkoxyalkanes at room temperature, proceeding through the
cleavage and the establishment of C–O bonds. The fact that 1,2-
dimethoxyethane, dme, is not stable in the presence of MCl5
is particularly meaningful when considering that dme has been
widely employed as solvent in preparative inorganic chemistry and
in metal-directed organic synthesis involving group 5 pentahalides.
Our observations suggest that M(V) oxo-complexes (M = Nb, Ta)
might behave as intermediates in reactions involving the reduction
of MCl5, carried out in dme solution.
We have shown that the outcomes of the reactions of MCl5
(M = Nb, Ta) with 1,2-dialkoxyalkanes depend strictly on the
stoichiometry used. In more detail, when MCl5 and the diether
react in ca. 1 : 1 molar ratio, single C–O bond activation occurs,
with consequent formation of alkoxyether complexes and alkyl
chlorides. Minor amounts of the dinuclear m-oxo bridged species
MOCl3(dme)MCl5 have been isolated in the reactions involving
dme.
On the other hand, the 1 : 2 ratio reactions between MCl5 and
1,2-dialkoxyalkanes are strongly influenced by the nature of the
latter. In the cases of dme and diglyme, unprecedented activation
of three ethereal C–O bonds takes place, resulting in formation
of the oxychloride species MOCl3(O,O¢-ligand) and 1,4-dioxane.
Analogously, the 2 : 1 ratio reaction of 1,2-dimethoxypropane with
NbCl5 indicates a potential strategy for the expeditious synthesis
of a methyl-substituted dioxane. Alternative fragmentation path-
ways, which do not lead to dioxanes, are followed by different
1,2-dialkoxyalkanes.
Preparation of MOCl3(dme), M = Nb, Ta. The preparation
of 2b is described in detail; compound 2a was obtained by
analogous procedure.8 A suspension of 1b (0.180 g, 0.502 mmol)
in CH2Cl2 (20 mL), was treated with dme (0.12 mL, 1.15 mmol),
and the resulting mixture was stirred at room temperature for
5 h, during which progressive dissolution of the solid and
darkening of the solution were noticed. Hence, the volatile
materials were removed under vacuum. Crystallization of the
residue from CH2Cl2/pentane at room temperature gave a pale
yellow microcrystalline solid corresponding to 2b. Yield: 0.136 g
(69%). Anal. Calcd for C4H10Cl3O3Ta: C, 12.21; H, 2.56; Ta, 45.99;
Cl, 27.03. Found: C, 12.10; H, 2.69; Ta, 45.61; Cl, 26.45%. IR (solid
1
state): 910 s [nTa O] cm-1. H NMR (400 MHz, CDCl3): d = 4.14
=
(s, 2H, CH2), 3.95 ppm (s, 3H, CH3).
In a different experiment, CH2Cl2 (0.0321 mL, 0.500 mmol)
and dme (0.055 mL, 0.53 mmol) were introduced into a
NMR tube containing a suspension of compound 1b (0.090 g,
0.251 mmol) in CDCl3 (0.80 mL). The tube was sealed,
and a light brown solution formed in 3 h time. Subsequent
1H and 13C NMR analyses (recorded at 243 K and at 298
K) revealed the presence of CH2Cl2, compound 2b, CH3Cl
(1H NMR: 3.00 ppm34), TaCl5(1,4-dioxane),18 1,4-dioxane, 1,2-
dichloroethane and [TaCl3(OCH3)2]224,25 in ca. 20 : 9 : 16 : 3 : 2 : 1 : 1
ratio. CH3Cl, 1,4-dioxane and 1,2-dichloroethane were also de-
tected by GC/MS analysis carried out on an aliquot of the mixture
at the end of reaction. When complex 1b (0.25 mmol) was reacted
with dme (2.8 mmol) in CDCl3, compound TaCl5(1,4-dioxane)
was not ascertained by 1H NMR spectroscopy.
Finally, the oxytrichlorides MOCl3(dme) are revealed as con-
venient precursors for MOCl3, thus indicating a novel room
temperature route for the preparation of MOCl3 starting from
MCl5.
Experimental
General
Analogous results were achieved by NMR and GC/MS investi-
gations on the reactions of 1a with dme in different stoichiometric
ratios.
NbOCl3(dme), 2a. Anal. Calcd for C4H10Cl3O3Nb: C, 15.71;
H, 3.30; Nb, 30.44; Cl, 34.82. Found: C, 15.93; H, 3.24; Nb, 29.82;
Cl, 33.97%. IR (solid state): 2946 w, 1466 m, 1448 m-s, 1277 w-m,
All manipulations of air and/or moisture sensitive compounds
were performed under an atmosphere of pre-purified argon using
standar◦d Schlenk techniques. The reaction vessels were oven dried
at 150 C prior to use, evacuated (10-2 mmHg) and then filled
with argon. MCl5 (M = Nb, 1a; M = Ta, 1b) were commercial
products (Aldrich), sublimed at 100–110 ◦C and 10-2 Torr before
use. TiCl4, SnBu3H, 2-methoxyethanol and 1-chloromethoxy-
2-methoxyethane were commercial products (Aldrich), stored
under argon atmosphere as received. CH2Cl2, CD2Cl2,
CDCl3, 1,2-dimethoxyethane (dme), 1,2-dimethoxypropane, 1,2-
diethoxyethane and MeO(CH2)2O(CH2)2OMe (diglyme) were
distilled before use under argon atmosphere from P4O10, while
pentane was distilled from LiAlH4. Infrared spectra were recorded
at 298 K on a FT IR-Perkin Elmer Spectrometer, equipped
with a UATR sampling accessory. NMR measurements were
recorded on Mercury Plus 400 instrument at 298 K, unless
otherwise specified. The chemical shifts for 1H and 13C were
1238 w-m, 1188 w, 1123 w, 1071 m, 1016 s, 984 s, 954 vs [nNb O],
=
1
927 ms, 854 vs, 818 s cm-1. H NMR (400 MHz, CD2Cl2): d =
4.20 (s, 2H, CH2), 3.98 ppm (s, 3H, CH3). 13C NMR (400 MHz,
CD2Cl2): d = 74.0 (CH2), 63.5 ppm (CH3).
Preparation of MCl4[O(Me)CH2CH2O], M = Nb, Ta. The
preparation of 3a is described in detail; compound 3b was obtained
by analogous procedure. Liquid MeO(CH2)2OH (0.068 mL,
0.86 mmol) was added to a suspension of 1a (0.230 g, 0.851 mmol)
in CH2Cl2 (10 mL). The mixture was stirred for 30 min at room
temperature, during which progressive dissolution of the solid and
gas evolution (HCl) were observed. Compound 3a was obtained
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as a colourless microcrystalline solid from a dichloromethane
solution layered with pentane, at -20 ◦C. Yield: 0.211 g (80%).
Anal. Calcd for C3H7Cl4NbO2: C, 11.63; H, 2.27; Nb, 29.99; Cl,
45.77. Found: C, 11.48; H, 2.35; Nb, 30.25; Cl, 44.98%. 1H NMR
(400 MHz, CDCl3): d = 5.23 (t, 3JHH = 5.86 Hz, 2H, CH2ONb),
Reaction of NbOCl3(dme)NbCl5 (Nb–O–Nb) with dme. The
reaction of crystalline 4a (0.050 g, 0.087 mmol) with dme
(0.020 mL, 0.19 mmol), in CH2Cl2 (8 mL), afforded a dark yellow
solution in 3 h time. The solution was layered with pentane:
crystals of compound 2a formed overnight in nearly 80% yield. The
identity of 2a was checked by IR (strong absorption at 954 cm-1),
whereas GC/MS analysis of the liquor pointed out the presence
of CH3Cl and 1,4-dioxane.
4.43 (t, 3JHH = 5.86 Hz, 2H, CH2OMe), 4.02 ppm (s, 3H, Me). 13
C
NMR (400 MHz, CDCl3): d = 78.7 (CH2ONb), 78.3 (CH2OMe),
65.2 ppm (Me).
TaCl4[O(Me)CH2CH2O], 3b. Colourless, air sensitive solid;
79% yield from 1b (0.180 g, 0.502 mmol) and MeO(CH2)2OH
(0.040 mL, 0.51 mmol). Anal. Calcd for C3H7Cl4O2Ta: C, 9.06; H,
1.77; Ta, 45.48; Cl, 35.64. Found: C, 9.21; H, 1.82; Ta, 45.30; Cl,
34.95%. IR (solid): 2944 w, 2891 w, 2846 w, 1597 w-m, 1455 m,
1260 m-s, 1228 m, 1061 vs, 1007 s, 988 s, 932 s, 843 vs, 797 vs cm-1.
Reactivity of NbCl5 with 1,2-diethoxyethane. Preparation of
NbCl4[O(Et)CH2CH2O], 5.
A suspension of 1a (0.150 g,
0.555 mmol) in CH2Cl2 (10 mL) was treated with 1,2-
diethoxyethane (0.070 mL, 0.50 mmol), and stirred at room
temperature for 30 min. The final mixture was filtered in order to
remove some solid, then it was layered with pentane. Compound
5 was obtained as a yellow solid after 12 h at -20 ◦C. Yield:
0.125 g (77%). Anal. Calcd for C4H9Cl4NbO2: C, 14.84; H,
2.80; Nb, 28.69; Cl, 43.79. Found: C, 14.65; H, 2.71; Nb, 28.45;
Cl, 43.21%. 1H NMR (400 MHz, CDCl3): d = 5.25 (t, 2H,
3
1H NMR (400 MHz, CDCl3): d = 5.34 (t, JHH = 5.86 Hz, 2H,
3
CH2OTa), 4.51 (t, JHH = 5.86 Hz, 2H, CH2OMe), 4.19 ppm (s,
3H, Me). 13C NMR (400 MHz, CDCl3): d = 81.2 (CH2OTa), 74.1
(CH2OMe), 66.2 ppm (Me).
3
3JHH = 5.86 Hz, CH2ONb), 4.53 (q, 2H, JHH = 6.59 Hz,
Reactivity of MCl5, 1a–b, with dme (dme/M molar ratio = 1 : 1).
The reactions of 1 (0.40 mmol), in CH2Cl2 (7 mL), with dme
(0.40 mmol) in Schlenk tubes afforded clear solutions, which
were layered with pentane. Compounds 3a–b were obtained as
microcrystalline solids after 12 h at -20 ◦C, in 55–60% yields.
In a different experiment, the pentachloride (0.30 mmol) was
suspended in CDCl3 (0.8 mL) in a NMR tube, and then treated
with dme (0.30 mmol). The tube was sealed, and progressive dis-
solution of the solid was observed. 1H-NMR spectrum, recorded
30 min after mixing, showed the presence of CH3Cl (3.00 ppm)
and 3, in about 1 : 1 ratio. GC/MS carried out on aliquots of
the solutions, after hydrolysis and filtration on alumina, showed
the presence of 2-methoxyethanol, methyl chloride and minor
amounts of 1,4-dioxane.
3
OCH2CH3), 4.52 (t, 2H, JHH = 5.86 Hz, CH2OEt), 1.51 ppm
3
(t, 3H, JHH = 6.59 Hz, OCH2CH3). 13C NMR (400 MHz,
CDCl3): d = 78.9 (CH2ONb), 76.0 (CH2OEt), 40.9 (OCH2CH3),
19.1 ppm (OCH2CH3). GC/MS analysis on an aliquot of the
mixture at the end of reaction revealed the presence of ethyl
chloride. The reaction of NbCl5 (0.105 g, 0.389 mmol) with 1,2-
diethoxyethane (0.12 mL, 0.85 mmol), in a mixture of CDCl3
(0.65 mL) and CH2Cl2 (0.0256 mL, 0.400 mmol), gave a solution
1
which was analyzed by H NMR and GC/MS: CH2Cl2, EtCl
and Cl(CH2)2OCH2CH3 were detected in ca. 10 : 20 : 15 ratio (1H
NMR), together with some unreacted 1,2-diethoxyethane.
Reactivity of NbCl5 with 1,2-dimethoxypropane. Preparation of
NbCl4[O(Me)CH(Me)CH2O], 6. To a suspension of 1a (0.205 g,
0.759 mmol) in CH2Cl2 (15 mL) 1,2-dimethoxypropane (0.079 mL,
0.65 mmol) was added, and the mixture was stirred at room
temperature for additional 30 min. The final red mixture was
filtered in order to remove some solid, then the filtrated solution
was layered with pentane. Dark-red microcrystalline compound
6 was obtained as a mixture of two isomeric forms after 12 h at
Reaction of 3a with dme: formation of NbOCl3(dme), 2a, CH3Cl
and 1,4-dioxane. Compound 3a (0.25 mmol) was dissolved
in CDCl3 (0.7 mL) in a NMR tube and treated with dme
(0.50 mmol). After 3 d, a mixture of 3a, 2a,8 CH3Cl and 1,4-
dioxane (10 : 20 : 15 : 8 ratio) was detected.
◦
Preparation of MOCl3(dme)MCl5 (M–O–M), M = Nb, Ta.
The preparation of 4a is described in detail; compound 4b
was obtained by analogous procedure. A suspension of NbCl5
(0.350 g, 1.30 mmol) in CH2Cl2 (25 mL) was treated with dme
(0.10 mL, 0.96 mmol), and the yellow mixture was stirred at room
temperature for 45 min, during which progressive dissolution of
the solid occurred. Hence, the mixture was filtered and layered
with pentane. Pale yellow crystals of 4a suitable for X-ray analysis
were obtained after 24 h at room temperature. Yield: 0.058 g (21%
yield). Anal. Calcd for C4H10Cl8Nb2O3: C, 8.34; H, 1.75; Nb, 32.28;
Cl, 49.28. Found: C, 8.09; H, 1.81; Nb, 32.05; Cl, 48.72%. IR (solid
state): 2943 w, 2885 w, 2833 w, 1462 m, 1454 m-s, 1447 m-s, 1274
m, 1239 m, 1185 m, 1126 w, 1062 m-s, 1008 s, 996 s, 976 s, 956 m-s,
876 s [nNb–O–Nb], 836 vs, 811 vs cm-1.
-20 C. Yield: 0.181 g (82%). Anal. Calcd for C4H9Cl4NbO3: C,
14.14; H, 2.67; Nb, 27.34; Cl, 41.73. Found: C, 14.04; H, 2.70;
1
Nb, 26.91; Cl, 41.02%. H NMR (400 MHz, CDCl3): d (major
isomer) = 5.24, 5.08 (m, 2H, NbOCH2), 4.68 (m, 1H, CH), 3.98
(s, 3H, OMe), 1.60 ppm (d, 2H, 3JHH = 6.23 Hz, CHMe); d (minor
isomer) = 5.29, 5.02 (m, 2H, NbOCH2), 4.45 (m, 1H, CH), 4.02
(s, 3H, OMe), 1.53 ppm (d, 2H, 3JHH = 5.86 Hz, CHMe). Isomer
ratio = 2 : 1. 13C NMR (400 MHz, CDCl3): d (major isomer) = 87.7
(NbOCH2), 84.6 (CH), 63.1 (OMe), 15.6 ppm (CHMe); d (minor
isomer) = 87.5 (NbOCH2), 83.6 (CH), 62.4 (OMe), 16.8 ppm
(CHMe).
When the reaction of 1a (0.65 mmol) with 1,2-
dimethoxypropane (0.52 mmol) was carried out in CDCl3 inside
1
a NMR tube, H NMR spectroscopy revealed the presence of
TaOCl3(dme)TaCl5 (Ta–O–Ta), 4b. Colourless, air sensitive
solid; 15% yield from 1b (0.360 g, 1.00 mmol) and dme (0.080 mL,
0.77 mmol). Anal. Calcd for C4H10Cl8O3Ta2: C, 6.39; H, 1.34; Ta,
48.15; Cl, 37.73. Found: C, 6.25; H, 1.38; Ta, 47.80; Cl, 37.19%.
IR (solid): 2962 w, 2916 vw, 1400 br-m, 1260 ms, 1074 br-s, 1013
vs, 872 s [nTa–O–Ta], 797 vs cm-1.
methyl chloride, in ca. 1 : 1 ratio with respect to 6.
The reaction of 1a (0.110 g, 0.407 mmol) with 1,2-
dimethoxypropane (0.11 mL, 0.90 mmol), in CDCl3, gave a
mixture which was analyzed by 1H NMR and GC/MS: MeCl, 2,5-
dimethyl-1,4-dioxane and some unreacted 1,2-dimethoxypropane
were detected.
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Reactivity of NbCl5 with 1-chloromethoxy-2-methoxyethane.
Preparation of NbCl4[O(CH2Cl)CH2CH2O], 7. A suspension of
1a (0.209 g, 0.774 mmol) in CH2Cl2 (15 mL) was treated with 1-
chloromethoxy-2-methoxyethane (0.070 mL, 0.61 mmol), and the
mixture was stirred at room temperature for additional 45 min.
The final yellow mixture was filtered in order to remove some
solid, then it was layered with pentane. Compounds 3a and 7
were◦obtained in admixture as a dark-yellow solid, after 12 h at
Preparation of MOCl3, M = Nb, Ta. The preparation of 11a
is described in detail; compound 11b was obtained by analogous
procedure. Crystalline NbOCl3(dme), 2a (0.120 g, 0.393 mmol),
was added to CH2Cl2 (15 mL), hence the mixture was treated
with TiCl4 (0.065 mL, 0.59 mmol). Precipitation of colourless 11a
occurred nearly instantaneously from a yellow solution. Yield:
0.071 g, 84%. Anal. Calcd for Cl3NbO: Nb, 43.16; Cl, 49.41.
Found: Nb, 43.02; Cl, 48.90%. The yellow solution was dried
under vacuum, and an aliquot of the residue was dissolved in
CDCl3 (0.5 mL): 1H NMR analysis revealed the presence of
TiCl4(dme) [d = 4.28 (s, 2H, CH2), 4.08 ppm (s, 3H, CH3)].35
A second aliquot of the residue was dissolved in CH2Cl2, and the
solution was layered with pentane. Crystals of TiCl4(dme), which
were identified by X-ray analysis, were collected after 24 h at room
temperature.
1
-20 C. Complex 7. H NMR (400 MHz, CDCl3): d = 5.60 (s,
3
2H, CH2Cl), 5.28 (t, 2H, JHH = 5.86 Hz, CH2ONb), 4.64 ppm
3
(t, 2H, JHH = 5.86 Hz, CH2OCH2Cl). 13C NMR (400 MHz,
CDCl3): d = 84.8 (CH2Cl), 79.7 (CH2OCH2Cl), 74.0 ppm
(CH2ONb).
When the reaction of 1a (0.70 mmol) with ClCH2O(CH2)2OCH3
(0.60 mmol) was carried out in CDCl3 in a NMR tube, 1H NMR
spectroscopy revealed the presence of methyl chloride, 3a and 7 in
ca. 3 : 5 : 2 ratio.
The reaction of 1a (0.110 g, 0.407 mmol) with
ClCH2O(CH2)2OCH3 (0.10 mL, 0.88 mmol), in a mixture
of CDCl3 (0.70 mL) and CH2Cl2 (0.0260 mL, 0.405 mmol), gave a
solution which was analyzed by 1H NMR and GC/MS: CH2Cl2,
O(CH2Cl)2, diglyme (100 : 18 : 17 molar ratio) and unreacted
1-chloromethoxy-2-methoxyethane were detected.
TaOCl3, 11b. Colourless, air sensitive solid; 80% yield from 2b
(0.160 g, 0.407 mmol) and TiCl4 (0.070 mL, 0.64 mmol). Anal.
Calcd for Cl3OTa: Ta, 59.66; Cl, 35.07. Found: Ta, 59.20; Cl,
34.95%.
Preparation of NbCl3(dme), 12. Crystalline NbOCl3(dme), 2a
(0.060 g, 0.20 mmol), was added to CH2Cl2 (8 mL), thus the
mixture was treated with SnBu3H (0.12 mL, 0.45 mmol). A brick-
red solid, corresponding to NbCl3(dme), 12, quickly formed. The
solid was recovered by filtration, washed with pentane (2 ¥ 10 mL)
and dried under reduced pressure. Yield: 0.050 g, 87%. Anal. Calcd
for C4H10Cl3NbO2: C, 16.60; H, 3.48; Nb, 32.10; Cl, 36.75. Found:
C, 16.78; H, 3.56; Nb, 31.77; Cl, 36.31%.
Reactivity of MCl5 with diglyme. Isolation and characterization
of NbCl4[OCH2CH2O(CH2)2O(Me)NbCl5], 8, NbOCl3(diglyme),
9, and [MeO(CH2)2O(CH2)2O(H)Me][NbCl6], 10. A suspension
of 1a (0.140 g, 0.518 mmol) in CH2Cl2 (15 mL) was treated with
diglyme (0.055 mL, 0.38 mmol), and the mixture was stirred
at room temperature for additional 20 min. The final yellow
mixture was filtered in order to remove some solid, then the
filtrated solution was layered with pentane. Compound 8 was
obtained as an ochre-yellow microcrystalline solid, after 48 h
at room temperature. Yield: 0.079 g (49%). Anal. Calcd for
C5H11Cl9Nb2O3: C, 9.62; H, 1.78; Nb, 29.78; Cl, 51.13. Found: C,
9.71; H, 1.75; Nb, 29.54; Cl, 50.81%. 1H NMR (400 MHz, CDCl3):
d = 5.51 (m, 2H, CH2ONb), 4.8 (m-br, 6H, CH2), 4.31 ppm (s,
3H, Me).
When the reaction of 1a (0.50 mmol) with diglyme (0.40 mmol)
was carried out in CDCl3 in a NMR tube, 1H NMR spectroscopy
revealed the presence of 8, 3a and methyl chloride, in about
10 : 1 : 20 ratio.
The reaction of 1a (0.085 g, 0.31 mmol) with diglyme
(0.095 mL, 0.66 mmol), in CDCl3/CH2Cl2 (CDCl3 5.0 mL,
CH2Cl2 0.308 mmol), afforded a solution which was analyzed
by 1H NMR and GC/MS: CH2Cl2, MeCl, MeOCH2CH2Cl, 1,4-
dioxane, 1,2-dichloroethane were detected in 20 : 30 : 5 : 7 : 1 ratio
(1H NMR). Compound 9 was isolated by layering the CDCl3
solution with pentane, at -20 ◦C. Yield: 0.043 g (40%). Anal.
Calcd for C6H14Cl3NbO4: C, 20.62; H, 4.04; Nb, 26.59; Cl, 30.44.
X-Ray crystallography
Crystal data and collection details for NbOCl3(dme)NbCl5
(Nb–O–Nb), 4a, TaOCl3(dme)TaCl5 (Ta–O–Ta), 4b, and
[MeO(CH2)2O(CH2)2O(H)Me][NbCl6], 10, are listed in Table 3.
The diffraction experiments were carried out on a Bruker APEX
II diffractometer equipped with a CCD detector and using Mo-
Ka radiation. Data were corrected for Lorentz polarization and
absorption effects (empirical absorption correction SADABS).36
Structures were solved by direct methods and refined by full-
matrix least-squares based on all data using F2.36 The asymmet-
ric unit of 10 contains one independent [NbCl6]- anion located
on a general position and half of a second [NbCl6]- anion
located on an inversion centre, as well as one fully independent
[MeO(CH2)2O(CH2)2O(H)Me]+ cation and a second one located
on an inversion centre. All non-hydrogen atoms were refined with
anisotropic displacement parameters.
H-atoms bonded to carbons were placed in calculated positions
and treated isotropically using the 1.2 fold Uiso value of the
parent atom except methyl protons, which were assigned the 1.5
fold Uiso value of the parent C-atom. The O-bonded hydrogen
atoms in 10 [H(1), H(3) and H(5)] were located in the Fourier
map and refined isotropically using the 1.2 fold Uiso value of the
parent O-atom. Restraints were applied to the O–H distances. The
O(3)–H(3) group is located close to an inversion centre and forms
a hydrogen bond to a symmetry generated O(3) atom. Therefore,
H(3) appears to be disordered over two symmetry related positions
with 0.5 occupancy factor each. As a consequence of this fact,
in order to maintain one protonated oxygen atom per diglyme
molecule, the hydrogen atom involved in the H-bond between O(1)
and O(5) has been split into two equally populated positions, one
Found: C, 20.51; H, 4.11; Nb, 26.22; Cl, 29.88%. IR (solid): 934
1
vs [nNb O] cm-1. H NMR (400 MHz, CDCl3): d = 4.42 (m, 2H,
=
CH2), 3.95 (m, 4H, CH2), 3.7 ppm (m-br, 8H, CH2 and CH3).
In a second attempt of crystallization, at -20 ◦C, X-ray
quality crystals of 10 were collected: yield 12% from 1a (0.090 g,
0.33 mmol) and diglyme (0.049 mL, 0.34 mmol). Anal. Calcd for
C6H15Cl6NbO3: C, 16.35; H, 3.43; Nb, 21.08; Cl, 48.26. Found:
C, 16.02; H, 3.34; Nb, 21.05; Cl, 48.43%. IR (solid): 3175 w
[nO–H] cm-1.
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Table 3 Crystal data and experimental details for NbOCl3(dme)NbCl5 (Nb–O–Nb), 4a, TaOCl3(dme)TaCl5 (Ta–O–Ta), 4b, and [MeO(CH2)2O-
(CH2)2O(H)Me][NbCl6], 10
Complex
4a
4b
10
Formula
Fw
l/A
T/K
C4H10Cl8Nb2O3
575.54
0.71073
C4H10Cl8O3Ta2
751.62
0.71073
C6H15Cl6NbO3
440.79
0.71073
˚
100(2)
100(2)
100(2)
Crystal system
Space group
Orthorhombic
P212121
7.0287(5)
13.5297(9)
17.5555(12)
90
1669.5(2)
4
2.290
2.644
1104
0.21 ¥ 0.15 ¥ 0.12
1.90–27.00
9274
Monoclinic
P21/n
Monoclinic
C2/c
˚
a/A
6.6388(3)
26.0083(13)
9.9726(5)
103.1160(10)
1676.99(14)
4
2.977
14.305
1360
21.528(3)
15.3879(19)
17.492(2)
124.665(2)
4766.1(10)
12
1.843
1.756
2616
˚
b/A
˚
c/A
b/◦
3
˚
Cell volume/A
Z
Dc/g cm-3
m/mm-1
F(000)
Crystal size/mm
0.16 ¥ 0.14 ¥ 0.12
1.57–27.00
18476
0.22 ¥ 0.19 ¥ 0.15
1.75–25.03
11126
q limits/◦
Reflections collected
Independent reflections
3564
3666
4066
(Rint = 0.0173)
3564/0/156
1.068
0.0138
0.0327
(Rint = 0.0354)
3666/0/156
1.053
0.0194
0.0428
(Rint = 0.0276)
4066/2/228
1.017
0.0247
0.0499
Data/restraints/parameters
Goodness on fit on F2
R1 [I > 2s(I)]
wR2 (all data)
-3
˚
Largest diff. peak/hole, e A
0.255/-0.401
1.336/-0.702
0.407/-0.399
directly bonded to O(1) and the other to O(5). Therefore, one O-
bonded H+ is present on each diglyme molecule, which is located
alternatively on one or the other of the two terminal methoxy
groups in each independent cation.
CCDC reference numbers 691886–691888 contain the supple-
mentary crystallographic data for this paper. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b810210d
H. Schumann and M. N. Bochkarev, Eur. J. Inorg. Chem., 2001, 2405;
(h) C. A. Bradley, L. F. Veiros, D. Pun, E. Lobkovsky, I. Keresztes and
P. J. Chirik, J. Am. Chem. Soc., 2006, 128, 16600.
5 (a) Y. K. Gun’ko, P. B. Hitchcock and M. F. Lappert, J. Organomet.
Chem., 1995, 499, 213; (b) M. C. Cassani, Y. K. Gun’ko, P. B. Hitchcock,
A. G. Hulkes, A. V. Khvostov, M. F. Lappert and A. V. Protchenko,
J. Organomet. Chem., 2002, 647, 71; (c) S. Le Cae¨r, M. Heninger, J.
Lemaire, P. Boissel, P. Maˆıtre and H. Mestdagh, Chem. Phys. Lett.,
2004, 385, 273; (d) S. Le Cae¨r, M. Heninger, P. Pernot and H. Mestdagh,
J. Phys. Chem. A, 2006, 110, 9654; (e) C. A. Bradley, L. F. Veiros and
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6 F. Marchetti, G. Pampaloni and S. Zacchini, Dalton Trans., 2007, 4343.
7 The fluorides of niobium(V) and tantalum(V) have tetranuclear struc-
tures 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 Inorganic Chemistry, 5th Edition, Clarendon Press,
Oxford, 1993]. For sake of simplicity, in this paper the general formula
MX5, when dealing with the niobium(V) and tantalum(V) halides, will
be used.
Acknowledgements
The authors wish to thank the Ministero dell’Istruzione,
dell’Universita` e della Ricerca (MIUR, Roma), Programmi di
Ricerca Scientifica di Notevole Interesse Nazionale 2004–2005,
for financial support.
8 F. Marchetti, G. Pampaloni and S. Zacchini, Chem. Commun., 2008,
3651.
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The Royal Society of Chemistry 2008
Dalton Trans., 2008, 7026–7035 | 7035
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