Further insights into the chemistry of niobium and tantalum pentahalides with 1,2-dialkoxyalkanes: Synthesis of bromo- and iodoalkoxides, spectroscopic and computational studies

Article abstract of DOI:10.1016/j.poly.2011.03.005

The room temperature reactions of a series of 1,2-dialkoxyalkanes ROCH ₂CH(R′)OR′′ with MX₅ (M = Nb, Ta; X = Br, I) in 1:1 ratio result in single C-O bond cleavage and high-yield formation of the halo-alkoxides MBr₄[κ²-OCH ₂CH(R′)OR′′] or [NbI₄{κ ¹-OCH₂CH(R′)OR′′}]₂, and equimolar amounts of the corresponding alkyl halides RX. The reaction of NbBr₅ with 1,2-dimethoxyethane, dme, proceeds with preliminary formation of the ionic species [NbBr₄(κ²-dme) (κ¹-dme)][NbBr₆], 3b, which has been identified by solution NMR at low temperature and conductivity analyses. The gas-phase structure of 3b has been optimized by DFT calculations, confirming that the dme ligands adopt bidentate and monodentate coordination, respectively. Although the formation of NbOBr₃(dme), 4b, 1,4-dioxane and MeBr from NbBr ₅/dme (ratio 1:2) is an exoergonic process (calculated ΔGr° = -115.96 kcal mol^-1), it is inhibited at room temperature. High temperature conditions enhance the production of 1,4-dioxane at the expense of selectivity. The dinuclear species NbOBr₃(dme)NbBr₅ (Nb-O-Nb), 5b, (X-ray) has been isolated in modest yield as byproduct of the room temperature reaction of NbBr₅ with dme. In general, the 1:2 molar reactions of NbX₅ (X = Br, I) with ROCH₂CH(R′) OR′′ occur with the exclusion of nearly one equivalent of organic reactant.

Full text of DOI:10.1016/j.poly.2011.03.005

Polyhedron 30 (2011) 1412–1419
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Further insights into the chemistry of niobium and tantalum pentahalides
with 1,2-dialkoxyalkanes: Synthesis of bromo- and iodoalkoxides, spectroscopic
and computational studies
c
Riccardo Bini a, Fabio Marchetti b,1, Guido Pampaloni b, , Stefano Zacchini
⇑
a Dipartimento di Chimica, Università di Firenze, Via della Lastruccia 3, I-50019 Sesto Fiorentino, Italy
b Università di Pisa, Dipartimento di Chimica e Chimica Industriale, Via Risorgimento 35, I-56126 Pisa, Italy
c 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
The room temperature reactions of a series of 1,2-dialkoxyalkanes ROCH2CH(R0)OR00 with MX5 (M = Nb,
Ta; X = Br, I) in 1:1 ratio result in single C–O bond cleavage and high-yield formation of the halo-alkoxides
MBr4[j2-OCH2CH(R0)OR00] or [NbI4{j1-OCH2CH(R0)OR00}]2, and equimolar amounts of the corresponding
alkyl halides RX. The reaction of NbBr5 with 1,2-dimethoxyethane, dme, proceeds with preliminary for-
Article history:
Received 6 December 2010
Accepted 3 March 2011
Available online 10 March 2011
mation of the ionic species [NbBr4(j j
2-dme)( 1-dme)][NbBr6], 3b, which has been identified by solution
Keywords:
Niobium
Tantalum
Diethers
CO bond activation
Oxygen abstraction
NMR at low temperature and conductivity analyses. The gas-phase structure of 3b has been optimized by
DFT calculations, confirming that the dme ligands adopt bidentate and monodentate coordination,
respectively. Although the formation of NbOBr3(dme), 4b, 1,4-dioxane and MeBr from NbBr5/dme (ratio
1:2) is an exoergonic process (calculated
D
Gꢀr = ꢁ115.96 kcal molꢁ1), it is inhibited at room temperature.
High temperature conditions enhance the production of 1,4-dioxane at the expense of selectivity. The
dinuclear species NbOBr3(dme)NbBr5 (Nb–O–Nb), 5b, (X-ray) has been isolated in modest yield as
byproduct of the room temperature reaction of NbBr5 with dme. In general, the 1:2 molar reactions of
NbX5 (X = Br, I) with ROCH2CH(R0)OR00 occur with the exclusion of nearly one equivalent of organic
reactant.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
amounts of O-donors lead to coordination adducts which, under
appropriate conditions, may evolve as result of C–Y bond activa-
Among early transition metal halides, the coordination chemis-
try of the pentahalides of niobium and tantalum, MX5 (M = Nb:
X = F, 1a; X = Cl, 1b; X = Br, 1c; X = I, 1d; M = Ta: X = F, 1e; X = Cl,
1f; X = Br, 1g) [1], has not been exhaustively explored so far. This
is probably related to the moisture-sensitivity of these oxophilic
materials, which makes their preparation and storage difficult.
Nevertheless, an increasing interest in the use of 1 in metal-
mediated synthesis has grown up in the recent years [2]; this is
consequence of the unusual and striking behaviour exhibited by
such complexes, in comparison with other early transition metal
halides [3].
tion (Y = H, O) [4b–e,5]. The outcomes of these reactions are strictly
associated with the nature of the halide, while the metal (niobium
or tantalum) does not play a crucial role. More remarkably, we
have reported that the reactions of MX5 (M = Nb, Ta; X = F, Cl) with
simple bifunctional O-donors may proceed giving uncommon
transformations. In particular, the room temperature reaction of
1b,f with a twofold excess of dme leads to selective conversion
into MOCl3(dme), methyl chloride and 1,4-dioxane [5d–e]. This
outcome is unprecedented, on account of the fact that 1,4-dioxane
formation implies the establishment of new C–O bonds, other than
C–O cleavages. Actually the activation of ethers or diethers, by
means of oxophilic metal derivatives, is generally limited to the
breaking down of the organic substrate into smaller fragments
[5d,e].
On the other hand, the fluorides 1a,e are able to activate dme
affording 1,4-dioxane and dimethyl ether at high temperature
[5a]; the process does not affect the [MF5] frame and occurs also
in catalytic conditions.
In consideration of the interesting results achieved by studying
the direct interaction of 1 with limited amounts of bifunctional
In view of the paucity of information available on the reactivity
of 1, some years ago we started a research project aimed to clarify
the main features of the coordination chemistry of 1 with oxygen
donor ligands [4]. We have found that the reactions with limited
⇑
Corresponding author. Tel.: +39 050 2219 219; fax: +39 050 2219 246.
E-mail addresses: fabmar@dcci.unipi.it (F. Marchetti), pampa@dcci.unipi.it (G.
Pampaloni).
1
Born in 1974.
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.03.005

R. Bini et al. / Polyhedron 30 (2011) 1412–1419
1413
Br
Br
oxygen substrates, we moved to see the reactivity of the heavier
Br
Br
O
Br
Br
O
niobium and tantalum pentahalides, i.e. 1c,d,g, with diethers of
the type ROCH2CH(R0)OR00. It has to be noted that such metal spe-
cies were even less studied in the past than the lighter congeners
[3a,b,6]. This is certainly associated with the extreme moisture-
sensitivity, due to the relatively low M–X (M = Nb, Ta; X = Br, I)
bond energy.
Nb
H
Me
Nb
+
O
O
Me
Br
Br
H
Me
Me
MeOCH2CH(Me)OMe
2d
NbBr
5
Br
Me
Br
Br
O
Nb
O
Br
Our purpose is to expand the knowledge on the role possibly
played by 1 in organic reactions, with the aim to contribute to de-
velop the use of these oxygen-affine materials in metal-directed
syntheses. Advancements in this field should be desirable also tak-
ing into account that niobium and tantalum are essentially dispos-
able [7] and are considered biocompatible elements [8]. The
experimental results discussed herein have been accompanied by
computational analyses, carried out by B3LYP/LANL2DZ method;
the latter has previously revealed to be effective in predicting some
chemistry of MX5 (M = Nb, Ta; X = F, Cl) with dme [5a].
Me
Scheme 2. The reaction of NbBr5 with 1,2-dimethoxypropane.
the methylene unit in 2c resonates at 4.21 (1H) and 72.8 (13C)
ppm respectively, to be compared with the values 3.54 (1H) and
66.7 (13C) ppm reported for uncoordinated 1,2-diethoxyethane.
All these information suggest that, as well as 2a,b, the alkoxide
compounds 2c,d exist in solution as monomers comprising a
bidentate –OCH2CH(R0)OR00 ligand (see Scheme 1).
The reaction of NbBr5 with 1,2-dimethoxypropane might pro-
ceed in principle with cleavage of either Me–OCH2 or Me–OCH(Me)
bond, resulting in formation of two different regioisomers (see
Scheme 2). Notwithstanding, according to NMR data, the reaction
is almost regiospecific, and compound NbBr4[OCH2CH(Me)OMe],
2d, forms selectively (no traces of NbBr4[OCH(Me)CH2OMe] have
been detected). NMR analysis of 2d has evidenced the presence
of two stereoisomers differing in the orientation of the substitu-
ents (H and Me) on the asymmetric carbon of the five-membered
cycle (Scheme 2). This feature reflects what seen for the analogous
2. Results and discussion
2.1. The 1:1 reactions of MX5 (M = Nb, Ta; X = Br, I) with 1,2-
dialkoxyalkanes: synthesis of haloalkoxides
A
number of mixed haloalkoxides having formula
MX4[OCH2CH(R0)OR00] have been efficiently prepared by room-
temperature treatment of the parent compounds MX5 (1c,d,g) with
one molar equivalent of 1,2-dialkoxyalkanes, ROCH2CH(R0)OR00, see
Scheme 1.
chloro-complex NbCl4[j
2-OCH2CH(Me)OMe] [5e].
NMR experiments have pointed out that the formation of 2
takes place with contextual production of equimolar amount of al-
kyl halide, RX (see Section 4). The products 2a–g have been charac-
terized by spectroscopic and analytical techniques.
The preparation of the iodo-derivatives 2e–g has required much
longer reaction times with respect to 2a–d, in agreement with the
inertness usually exhibited by niobium and tantalum pentaiodides
on reacting with ethers [12]. A careful analysis of the NMR spectra
of 2e–g suggests that the [OR00] unit does not coordinate the metal
centre. For example, the –OMe unit in the 1H NMR spectrum of 2e
resonates at 3.60 ppm, which is relatively close to the value
3.39 ppm related to uncoordinated 1,2-dimethoxyethane, see
above. Since the iodo-derivatives 2e–g contain monodentate oxy-
gen ligands, they presumably hold dinuclear structure in order to
provide octahedral coordination around each niobium, with the
two metal centres sharing two bridging ligands. It is noteworthy
that dinuclear structures are generally adopted by M(OR)5 alkox-
ides [13] and MXn(OR)5ꢁn halo-alkoxides (M = Nb, Ta; n = 1/3). In
the latter, monodentate oxygen ligands occupy preferentially
bridging sites rather than terminal ones [5c,14].
The NMR spectra (CDCl3) of 2a,b display the resonance due to
the two equivalent methoxy units at ca. 4.1 ppm (1H) and
66 ppm (13C), respectively. On considering that the –OMe moiety
resonates at 3.39 (1H) and 59.0 ppm (13C) in uncoordinated dme,
and at 3.52 ppm (1H) and 59.5 ppm
NbF5[
(
13C) in the complex
j
1-O(H)CH2CH2OMe] (where –OMe does not coordinate nio-
bium) [9], the methoxy fragment in 2a,b should be involved in the
coordination to the metal centre. Thus, compounds 2a,b are mono-
nuclear with the [–OCH2CH2OMe] group acting as a bidentate li-
gand. This is coherent with what found for the chloro derivatives
MCl4[j
2-OCH2CH2OMe] (M = Nb, Ta) [5e,10,11].
In a similar manner, the [OR00] frame of 2c,d gives NMR reso-
nances which are significantly shifted to high frequencies, with re-
spect to what observed in the corresponding uncoordinated
diethers [EtOCH2CH2OEt and MeOCH2CH(Me)OMe]. For instance,
The reactions of 1c,d,g with ROCH2CH(R0)OR00 described in this
Section occur selectively with breaking of one terminal C–O bond.
In view of this, the reaction of NbI5, 1d, with the unsymmetrical
MeO(CH2)2OCH2Cl might proceed basically with cleavage of either
Me–O or ClCH2–O bond, leading to a mixture of products. In reality
selective cleavage of the Me–O bond takes place, resulting in exclu-
sive formation of NbI4[OCH2CH2OCH2Cl], 2g, and MeI. The alterna-
tive products 2e/CH2ClI (derived from ClCH2–O cleavage) have not
been recognized in the reaction medium. In agreement with these
observations, the [OCH2Cl] group has been found by NMR in the fi-
Br
Br
Br
O
O
M
X=Br
R'
R'
Br
R"
− RX
X=I
MX
+
2a-d
5
O
O
R
R"
[d(1H,
CDCl3) = 5.80 ppm;
d(13C,
1
nal
metal
compound
NbI [κ -OCH CH(R')OR"]
4
2
2
CDCl3) = 83.0 ppm].
2e-g
The mononuclear structure adopted by 2a–d is the consequence
of the possibility for the –OCH2CH(R0)OR00 ligand to behave as
bidentate, so forming a stable five-membered ring. On the other
hand, in 2e–g the five-membered ring is not stable with respect
to the dinuclear structure containing monodentate alkoxyethers,
presumably due to the steric hindrance exerted by the iodides.
In conclusion, we would like to remark that the reactions of
MX5 (X = Cl, Br, I) with 1,2-dialkoxyalkanes (see Scheme 1 and
Ref. [5e]) represent a new and straightforward route for the prep-
Halide
Dialkoxyalkane
Compound
NbBr5
TaBr5
NbBr5
NbBr5
NbI5
MeOCH2CH2OMe
MeOCH2CH2OMe
EtOCH2CH2OEt
MeOCH2CH(Me)OMe
MeOCH2CH2OMe
EtOCH2CH2OEt
2a
2b
2c
2d
2e
2f
NbI5
NbI5
MeOCH2CH2OCH2Cl
2g
Scheme 1. Preparation of mixed halo-alkoxides of niobium and tantalum.

1414
R. Bini et al. / Polyhedron 30 (2011) 1412–1419
+
O
O
O
Cl
b
Cl
O
dme
Cl
Cl
O
O
dme
−
Nb
+
O
Nb
NbCl
1b 5
NbCl
6
− MeCl
O
O
Cl Cl
Cl
4a
3a
dme
= dme
a
− MeCl
O
O
Cl
Cl
Cl
O
O
Nb
major
minor
Cl
2h
O
O
Fig. 1. B3LYP/LANL2DZ-optimized structure of [NbBr4(j j
1-dme)( 2-dme)][NbBr6],
3b, in the gas phase.
+
5
NbOCl (dme)NbCl
3
5a
Scheme 3. Reactions of NbCl5 with dme.
Table 1
Selected computed bond distances (Å) and angles (°) for [NbBr4(
dme)][NbBr6], 3b.
j
1-dme)(j2
-
aration of a vast series of monoalkoxo-tetrahalo compounds of
Nb(V) and Ta(V). The simple synthetic procedure that we propose
may give an impulse to the development of the use of Nb(V) and
Ta(V) alkoxy-derivatives in metal-directed reactions.
Nb(2)–Br(21)
Nb(2)–Br(22)
Nb(2)–Br(23)
Nb(2)–Br(24)
Nb(2)–O(3)
Nb(2)–O(4)
Nb(2)–O(5)
Nb(2)–O(6)
O(3)–C(7)
2.653
2.597
2.529
2.677
2.292
4.350
2.213
2.199
1.489
1.497
1.522
1.447
O(4)–C(10)
O(5)–C(11)
O(5)–C(12)
C(12)–C(13)
O(6)–C(13)
O(6)–C(14)
O(3)–Nb(2)–Br(21)
O(5)–Nb(2)–O(6)
Br(24)–Nb(2)–Br(21)
Br(24)–Nb(2)–Br(22)
Br(21)–Nb(2)–O(5)
1.457
1.494
1.488
1.538
1.490
1.503
71.73
66.72
141.87
87.13
75.56
2.2. The reactivity of MX5 (M = Nb, Ta; X = Cl, Br, I) with dme: NMR,
computational and crystallographic results
We have previously reported that the room-temperature reac-
tion of NbCl5 with 1,2-dimethoxyethane leads to different out-
comes depending on the stoichiometry employed. More
specifically, the mononuclear compound NbCl4(OCH2CH2OMe),
2h, is obtained in good yield by using strictly 1:1 molar ratio
(Scheme 3, pathway a) [5e]. On the other hand, the use of an excess
(two or more equivalents) of dme gives selectively NbOCl3(dme),
4a, and 1,4-dioxane, via multiple C–O bond activation (Scheme 3,
pathway b) [5d]. The initial formation of the ionic intermediate
[NbCl4(dme)2][NbCl6], 3a, is common to the two pathways.
Although the way to 2h is largely favoured when equimolar
quantities of reagents (i.e. NbCl5 and dme) are engaged, advanced
ether activation occurs to a low extent under these conditions
and minor amounts of 1,4-dioxane and of the oxo-bridged complex
NbOCl3(dme)NbCl5, 5a, may form (see Scheme 3).
With the aim to see the possibility to increase the production of
1,4-dioxane in the 1:1 reaction of NbCl5 with dme, we have carried
out this reaction in CDCl3 in sealed tubes at progressively increas-
ing temperatures but a reasonable dme ? 1,4-dioxane conversion
has not been achieved (see Section 4 for details). We also have
tested the reactivity of 2h with dme in CDCl3 (see Scheme 3). Clean
formation of 4a/1,4-dioxane has not been observed even at high
temperatures, suggesting that 2h cannot be considered as interme-
diate in the course of the 1:2 Nb/dme reaction.
O(3)–C(8)
C(8)–C(9)
O(4)–C(9)
be satisfactorily extended to the solid state and to solutions of
non-polar solvents.
The computer outcome obtained for 3b is in agreement with the
NMR data (see above), showing the two dme ligands in the cation
adopting respectively monodentate and bidentate coordination
mode. As a matter of fact, three calculated Nb–O distances are
within bond-length [Nb(2)–O(3) 2.292, Nb(2)–O(5) 2.213, Nb(2)–
O(6) 2.199 Å]. Conversely, the remaining Nb–O distance value
points the absence of interaction [Nb(2)ꢂ ꢂ ꢂO(4) 4.350 Å]. This situ-
ation resembles well that described for the chloro analogues 3a
[5e], and contrasts with what found for the fluoro [NbF4(dme)2]+
where both dme ligands are bidentate [5a]. The structure differ-
ence between [NbF4(dme)2]+ and [NbX4(dme)2]+ (X = Cl, Br) seems
to be the result of the higher steric demand of the heavier halides
with respect to the small fluoride ion.
The oxo-complex NbOCl3(dme), 4a, can be obtained expedi-
tiously by reaction of NbCl5 with two equivalents of dme (see
above). In principle, the analogous reaction of NbBr5 with dme
could afford NbOBr3(dme), 4b. Indeed DFT results indicate that
the formation of 4b from NbBr5 is exoergonic with D
Gꢀr comparable
In the previous Section, we have seen that NbBr5 reacts with
dme in 1:1 molar ratio yielding the monoalkoxo complex 2a. The
reaction proceeds with preliminary formation of the ionic adduct
to that calculated for the transformation 1b ? 4a (Eq. (1)).
[NbBr4(j j
2-dme)( 1-dme)][NbBr6], 3b, as indicated by 1H NMR
Nb2X10 þ4dme ! 2NbOX3ðdmeÞ þ1; 4-dioxane þ 4MeX
ð1Þ
1b;c
4a;b
experiment in CDCl3 at ꢁ60 °C. The spectrum displays two reso-
nances due to bidentate dme [d = 4.00 (CH2), 3.68 (CH3) ppm]
and four resonances ascribable to monodentate dme ligand
[d = 4.61 (NbOCH2), 4.24 (CH3ONb), 3.90 (CH2), 3.41 (CH3) ppm].
Moreover, the ionic nature of 3b has been corroborated by solution
conductivity measurement (see Section 4).
In view of the fact that the B3LYP/LANL2DZ computational
method allowed to predict the structures of compounds 3a and
4a [5a], we have optimized the gas-phase structure of 3b by the
same computational method. Making reference to Fig. 1, a selec-
tion of bond lengths (Å) and angles (°) is reported in Table 1.
Although the calculations referred to the gas phase, they could
D
D
Gꢀr ðX ¼ ClÞ ¼ ꢁ106:25 kcal molꢁ1
Gꢀr ðX ¼ BrÞ ¼ ꢁ115:96 kcal molꢁ1
Somehow surprisingly, the combination of NbBr5 with a two-
fold excess of dme leaves one equivalent of organic material unre-
acted, thus 2a is obtained as main metal product (see Scheme 4). In
other terms, the pathway leading to 4b and 1,4-dioxane from 1c/
dme (Eq. (1)) is almost not practicable at room temperature. In-
deed very little 1,4-dioxane has been recognized after hydrolysis
of the CDCl3 mixture maintained for 15 days at room temperature
(see Section 4 and Scheme 5).

R. Bini et al. / Polyhedron 30 (2011) 1412–1419
1415
+
Br
Br
O
O
O
O
dme
O
O
Br
Br
dme
−
O
Nb
Br Br
Nb
+
NbBr
1c
NbBr
X
5
6
25°C
O
O
− MeBr
Br
3b
− MeBr
4b
= dme
O
O
dme
− MeBr
Br
Br
Br
O
O
Nb
Br
major
2a
O
O
minor
+
5
NbOBr (dme)NbBr
3
5b
Fig. 2. View of the molecular structure of NbOBr3(dme)NbBr5, 5b. Displacement
ellipsoids are at the 30% probability level. Only one of the two independent
molecules present in the unit cell is represented.
Scheme 4. Room temperature reactions of NbBr5 with dme.
Table 2
MeO(CH ) Br + MeBr +
H2O
15 d, 25 °C
CDCl3
2 2
2a + MeBr
1:1
Selected experimental and calculated bond distances (Å) and angles (°) for
NbOBr3(dme)NbBr5, 5b.
1,4-dioxane
CDCl3
molar ratio 9:9:1
dme
2 eq.
NbBr
5
Experimental
Calculated
1) 12 h, 80 °C
2) H2O
dme + MeO(CH ) Br + MeBr + dioxane
2 2
+ Br(CH ) Br + MeOH
Molecule 1
Molecule 2
2 2
molar ratio 8:5:16:3:7:6
CDCl3
Nb(1)–O(1)
Nb(2)–O(1)
Nb(2)–O(2)
Nb(2)–O(3)
Nb(2)–Br(6)
Nb(2)–Br(7)
Nb(2)–Br(8)
C(2)–O(2)
C(2)–C(3)
C(3)–O(3)
Nb(1)–Br(1)
Nb(1)–Br(2)
Nb(1)–Br(3)
Nb(1)–Br(4)
Nb(1)–Br(5)
Nb(1)–O(1)–Nb(2)
O(1)–Nb(2)–O(3)
O(1)–Nb(2)–O(2)
O(1)–Nb(1)–Br(1)
2.137(5)
1.747(5)
2.199(6)
2.307(6)
2.120(6)
1.755(6)
2.202(6)
2.283(6)
2.184
1.783
2.239
2.363
2.525
2.598
2.600
1.475
1.519
1.472
2.511
2.599
2.609
2.560
2.559
172.95
167.24
96.39
179.06
Scheme 5. Reactions of NbBr5 with two equivalents of dme.
2.4274(12)
2.4863(12)
2.4998(12)
1.440(10)
1.488(12)
1.412(10)
2.4147(11)
2.4752(12)
2.4825(11)
2.4829(12)
2.4807(12)
170.2(3)
2.4076(13)
2.4833(14)
2.5098(13)
1.440(11)
1.455(14)
1.410(11)
2.4169(12)
2.4812(12)
2.4832(12)
2.4868(12)
2.4724(13)
170.6(3)
The mixture obtained from treatment of NbBr5 with an excess
of dme was layered with pentane, hence few X-ray quality crystals
of the dinuclear complex NbOBr3(dme)NbBr5 (Nb–O–Nb), 5b, were
collected (see Scheme 4). The low-yield isolation of 5b, together
with the NMR identification of small amounts of 1,4-dioxane in
the same reaction (see above), confirm that the NbBr5-mediated
transformation dme ? 1,4-dioxane is operative to a limited extent
only. It has to be remarked that the l-oxo unit found in complexes
of the type MOX3(O–O)MX5 (M = Nb, Ta; X = Cl, Br; O–O = bidentate
oxygen ligand) commonly results from oxygen-abstraction from
the organic reactant rather than being imputable to adventitious
water [5b].
164.2(2)
93.4(2)
179.75(17)
164.6(2)
92.6(2)
179.67(17)
Salient spectroscopic feature of 5b is given by the IR absorption
at 857 cmꢁ1, accounting for the Nb@O interaction (the correspond-
ing value calculated for the gas phase is 892 cmꢁ1). In addition, the
1H NMR spectrum (in CDCl3) shows the two peaks due to the dme
ligand, falling at 4.30 (CH2) and 3.93 ppm (CH3).
The molecular structure of 5b is in Fig. 2, whereas selected bond
lengths and angles are reported in Table 2. The asymmetric unit of
5b contains two independent molecules located in general posi-
tions. In view of the very similar bonding parameters, only the
parameters of one molecule will be discussed. The structure is clo-
sely related to the ones previously reported for MOCl3(dme)MCl5
(M = Nb, Ta) [5e], and it can be viewed as the result of the coordi-
nation of the oxo complex [Nb(@O)Br3(dme)] to the Lewis acid
NbBr5. In agreement with this, the oxo-bridge displays a consider-
able asymmetry, being double bonded to Nb(2) [Nb(2)–O(1)
1.747(5) and 1.755(6) Å for the two independent molecules] and
single bonded to Nb(1) [Nb(1)–O(1) 2.137(5) and 2.120(6) Å for
the two independent molecules].
Fig. 3. B3LYP/LANL2DZ-optimized structure of NbOBr3(dme)NbBr5, 5b, in the gas
phase.
Compound 5b represents one of the rare examples of crystallo-
graphically-characterized M(V) derivative (M = Nb, Ta) containing
bromide ligands [5d,14].
Fig. 3 depicts the computer-optimized structure of compound
5b. A comparison of geometric parameters (X-ray versus DFT) is
shown in Table 2, evidencing good agreement between experimen-
tal data for the solid state and calculated values for the gas phase.
Attempting to force the thermodynamically-allowed conversion
of dme into 1,4-dioxane (Eq. (1)), a 1:2 mixture of NbBr5 and dme
in CDCl3 was heated at ca. 80 °C for 12 h. Although the final
mixture resulted enriched in 1,4-dioxane, significant amounts of
1,2-dibromoethane and methanol were detected after hydrolysis
(see Scheme 5 and Section 4), thus suggesting that high tempera-
tures favour the conversion dme ? 1,4-dioxane but, under in these
conditions, several fragmentation routes turn to be operative. The
halide MeOCH2CH2Br plausibly generates from 2a by reaction of

1416
R. Bini et al. / Polyhedron 30 (2011) 1412–1419
the [OCH2CH2OMe] ligand with H2O/HBr, HBr being produced dur-
ing the hydrolysis. Actually the formation of 1,2-dibromoethane
and methanol appears to be the consequence of the scission of
internal CꢁO bonds. Methanol may be produced upon hydrolysis
of niobium–methoxide species [Nb–OMe].
The mixtures obtained by addition of a twofold excess of
diether to NbI5, 1d, in CDCl3, could be analyzed after hydrolysis.
Thus, the outcomes of the reactions with 1,2-dimethoxyethane or
1,2-dimethoxypropane resemble what discussed above for NbBr5:
one equivalent of RI (R = Et, Me) generates per metal atom, indicat-
ing the occurrence of a single C–O breaking process. Besides, one
equivalent of diether comes unreacted. Interestingly, minor
amounts of CH2ICH2I, MeOH and 1,4-dioxane have been found as
products of the treatment of NbI5 with dme/H2O. This indicates
that advanced fragmentation of the ligand may be operative
although to a low extent.
In agreement with the higher reactivity usually exhibited by
MeO(CH2)2OCH2Cl when reacting with M(V) pentahalides [5e],
the room temperature interaction between NbI5 and MeO
(CH2)2OCH2Cl leads to quantitative and non-selective degradation
of the organic material. Hence, MeI, CH2ICH2I, CH2ClI, ClCH2
OCH2OCH2Cl, dme and Me2O have been clearly detected after
hydrolysis. The presence of ClCH2OCH2OCH2Cl, dme and Me2O is
remarkable, since it indicates that C–O bond formation somehow
takes place in the course of the reaction. Moreover, the prevalence
of MeI with respect to CH2ClI points that the less hindered terminal
C–O bond may be broken preferentially, analogously to what
shown above for the 1:1 reaction of NbBr5 with MeO(CH2)2OCH2Cl.
The different reactions of NbX5 (X = Cl, 1b; X = Br, 1c) with a
twofold excess of dme (Eq. (1)) might be explained on the basis
of steric factors. We can suggest that the interaction of 1b,c with
one equivalent of dme produces the intermediate species
[NbX4(dme)2][NbX6] (X = Cl, 3a; X = Br, 3b), and that only com-
pound 3a is able to react efficiently with a second equivalent of
dme, providing NbOCl3(dme), 4a (Scheme 3, pathway b). Com-
pound 3b is substantially inert towards addition of further dme,
hence the formation of NbOBr3(dme), 4b, is inhibited (Scheme 4).
The observed inertness of 3b could be the consequence of the steric
hindrance of the bromide ligands, which may favour a rearrange-
ment with distribution of the two dme fragments between the
two metal centers, rather than the addition of further dme required
for the formation of 4b.
The hypothesis that steric factors play a crucial role in the reac-
tivity of 1 with 1,2-dialkoxyalkanes finds support in the fact that
the 1:2 molar reactions of niobium pentachloride with 1,2-dialkox-
yalkanes more encumbered than dme (O–O) do afford alkoxide
complexes analogous to 2h and not NbOCl3(O–O) [5e].
3. Conclusions
2.3. The 1:2 molar reactions of MX5 (M = Nb, Ta; X = Br, I) with 1,2-
dialkoxyalkanes
In this paper, we have presented some new aspects concerning
the chemistry of niobium and tantalum pentahalides with 1,2-dial-
koxyalkanes. The 1:1 molar reactions of MX5 (X = Br, I) with
ROCH2CH(R0)OR00 proceed with single C–O bond cleavage and lead
to selective formation of uncommon monoalkoxo-tetrahalo com-
pounds, whose nuclearity appears to be determined by the dimen-
sion of the halide. This novel synthetic procedure may give further
impulse to the use of Nb(V) and Ta(V) derivatives in metal-directed
synthesis.
In the present Section, we deal with the 1:2 molar reactions of
Nb(V) bromides and iodides with 1,2-dialkoxyalkanes (see Table 3).
They have been performed in NMR tubes in CDCl3 solution, using
CH2Cl2 as NMR standard.
The 1:2 reactions of NbBr5, 1c, with ROCH2CH(R0)OR (R = Et,
R0 = H; R = R0 = Me) give the alkoxyether compounds 2c,d, in
admixture with ca. one equivalent of RBr per metal atom. After
treatment with water, CH2Cl2, unreacted diether [EtOCH2CH2OEt
and MeOCH2CH(Me)OMe], RBr and BrOCH2CH(R0)OR have been de-
tected in comparable amounts in the respective mixtures. The bro-
mide BrOCH2CH(R0)OR reasonably origins from hydrolysis of the
alkoxyether ligand; in fact, treatment of NbBr4[OCH2CH(R0)OR]
with water in excess should generate niobium oxides, HBr and
HOCH2CH(R0)OR, which in turn may react with HBr to give
BrOCH2CH(R0)OR. The experimental evidences indicate that the
two reactions basically proceed with the cleavage of one terminal
C–O bond per molecule to give the adducts 2, thus leaving one
equivalent of diether unreacted. Traces of 1,4-dioxane have been
recognized in the reaction mixture of 1c with 1,2-diethoxyethane.
The 1:2 reactivity of MX5 (X = Cl, Br, I) with ROCH2CH(R0)OR00 is
mainly regulated by kinetic factors, associated with the steric de-
mand of X, R and R0. In the case of low impediment, advanced acti-
vation of the organic material may occur affording oxo-niobium
compounds and 1,4-dioxane in high yield. This is the case of the
1:2 reaction of NbCl5 with dme. On the other hand, increased steric
encumbrance, due to either the halide ligand or the organic reac-
tant, inhibits the pathway leading to 1,4-dioxane. In such case,
monoalkoxo-tetrahalo compounds are obtained prevalently, with
nearly one equivalent of organic material remaining unreacted.
For instance, the generation of NbOBr3(dme)/1,4-dioxane is not ob-
served from NbBr5/excess dme, although computer calculations
suggest that this process is highly exoergonic.
Table 3
Products of the 1:2 reactions of 1c,d with ROCH2CH(R0)OR00, detected after hydrolysis
(CH2Cl2 used as standard, in 1:1 ratio with the metal compound).
4. Experimental
4.1. General procedures
Diether
Products (relative molar ratio)
NbBr5
All manipulations of air and/or moisture sensitive compounds
were performed under atmosphere of pre-purified Argon using
standard Schlenk techniques. The reaction vessels were oven dried
at 150 °C prior to use, evacuated (10ꢁ2 mmHg) and then filled with
argon. Compounds MX5 (M = Nb, X = Cl, 1b; M = Nb, X = I, 1d) were
purchased from Aldrich and stored under argon atmosphere as re-
ceived. Compounds MBr5 (M = Nb, 1c; M = Ta, 1g) were prepared
according to literature procedures and stored under argon atmo-
sphere [15], CH2Cl2, CHCl3, CD2Cl2, CDCl3 and MeO(CH2)2OMe
(dme) were distilled before use under argon atmosphere from
P4O10, while pentane was distilled from LiAlH4. The commercial
MeOCH2CH(Me)OMe, EtO(CH2)2OEt and MeO(CH2)2OCH2Cl were
MeOCH2CH(Me)OMe CH2Cl2 (1), BrCH2CH(Me)OMe (1), MeBr (1),
MeOCH2CH(Me)OMe (1)
EtO(CH2)2OEt
CH2Cl2 (20), Br(CH2)2OEt (15), EtBr (20), EtO(CH2)2OEt
(16), 1,4-dioxane (1)
NbI5
MeO(CH2)2OMe
CH2Cl2 (20), CH2ICH2I (3), MeOH (2), MeI (15), dme
(13), MeO(CH2)2OH (12), 1,4-dioxane (3)
CH2Cl2 (10), EtI (8), EtO(CH2)2OEt (10), EtOCH2CH2OH
(7)
EtO(CH2)2OEt
MeOCH2CH(Me)OMe CH2Cl2 (1), MeI (1), MeOCH2CH(Me)OMe (1),
ICH2CH(Me)OMe (1)
MeO(CH2)2OCH2Cl
CH2Cl2 (16), MeI (12), CH2ICH2I (4), CH2ClI (2),
ClCH2OCH2OCH2Cl (4), MeO(CH2)2OMe (11), Me2O (1)

R. Bini et al. / Polyhedron 30 (2011) 1412–1419
1417
distilled before use under argon atmosphere from appropriate dry-
ing agents. Infrared spectra were recorded at 293 K on a FT-IR-Per-
CHMe); d (minor isomer) = 5.30/5.07 (m, NbOCH2), 4.45 (m, CH),
3
3.93 (s, OMe), 1.62 ppm (d, JHH = 5.86 Hz, CHMe). Isomer ratio
kin–Elmer Spectrometer, equipped with
a
UATR sampling
2:1. 13C{1H} NMR (CDCl3): d (major isomer) = 86.4 (NbOCH2),
84.6 (CH), 62.5 (OMe), 13.5 ppm (CHMe); d (minor isomer) = 87.2
(NbOCH2), 84.2 (CH), 62.5 (OMe), 14.9 ppm (CHMe).
accessory. NMR measurements were recorded on Mercury Plus
400 instrument at 293 K, unless otherwise specified. The chemical
shifts for 1H and 13C were 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. Conductivity measurements
were carried out with Eutech Con 700 Instrument (cell con-
stant = 1.0 cmꢁ1) [16]. Carbon and hydrogen analyses were per-
formed on Carlo Erba mod. 1106 instrument. The halide content
was determined by the Volhardt method [17] after exhaustive
hydrolysis of the sample. The metal was analyzed as M2O5
(M = Nb, Ta), obtained by hydrolysis of the sample followed by cal-
cination in a platinum crucible. The halogen and the metal analyses
were repeated twice in order to check for reproducibility.
4.2.5. NbI4[OCH2CH2OMe], 2e
Red solid, 58% yield from 1d (0.195 g, 0.268 mmol) and dme
(0.030 mL, 0.29 mmol). Time: 170 h. Anal. Calc. for C3H7I4NbO2: C,
5.33; H, 1.04; Nb, 13.75; I, 75.13. Found: C, 5.26; H, 1.10; Nb,
3
13.61; I, 74.22%. 1H NMR (CDCl3): d = 5.03 (t, 2H, JHH = 5.13 Hz,
3
CH2ONb), 3.88 (t, 2H, JHH = 5.13 Hz, CH2OMe), 3.60 ppm (s, 3H,
Me).
4.2.6. NbI4[OCH2CH2OEt], 2f
Red solid, 61% yield from 1d (0.240 g, 0.330 mmol) and 1,2-
diethoxyethane (0.055 mL, 0.39 mmol). Time: 120 h. Anal. Calc.
for C4H9I4NbO2: C, 6.97; H, 1.32; Nb, 13.47; I, 73.61. Found: C,
7.04; H, 1.24; Nb, 13.31; I, 73.30%. 1H NMR (CDCl3): d = 4.92 (t,
4.2. Preparation of MX4[OCH2CH(R0)OR00] (M = Nb, X = Br, R0 = H,
R00 = Me, 2a; M = Ta, X = Br, R0 = H, R00 = Me, 2b; M = Nb, X = Br, R0 = H,
R00 = Et, 2c; M = Nb, X = Br, R0 = R00 = Me, 2d; M = Nb, X = I, R0 = H,
R00 = Me, 2e; M = Nb, X = I, R0 = H, R00 = Et, 2f; M = Nb, X = I, R0 = H,
R00 = CH2Cl, 2g)
3
2H, JHH = 5.86 Hz, CH2ONb), 4.32 (m, 2H, CH2OEt), 3.65 (q, 2H,
3
3JHH = 8 Hz, CH2CH3), 1.41 ppm (t, 3H, JHH = 8 Hz, CH2CH3).
4.2.7. NbI4[OCH2CH2OCH2Cl], 2g
Orange solid, 62% yield from 1c (0.265 g, 0.364 mmol) and
MeO(CH2)2OCH2Cl (0.042 mL, 0.37 mmol). Time: 48 h. Anal. Calc.
for C3H6ClI4NbO2: C, 5.07; H, 0.85; Nb, 13.08; I, 71.49. Found: C,
5.13; H, 0.74; Nb, 13.00; I, 70.56%. 1H NMR (CDCl3): d = 5.80 (s,
The preparation of NbBr4[OCH2CH2OMe]2, 2a, is described in
detail; compounds 2b–g were obtained by analogous procedure.
3
4.2.1. NbBr4[OCH2CH2OMe], 2a
2H, CH2Cl), 4.99 (t, 2H, JHH = 5.8 Hz, CH2ONb), 4.46 ppm (t, 2H,
A suspension of NbBr5 (0.180 g, 0.366 mmol) in CH2Cl2 (10 mL)
was treated with dme (0.038 mL, 0.37 mmol), and the resulting
mixture was stirred for 30 min at room temperature. Progressive
dissolution of the solid was observed. Compound 2a was obtained
as a red microcrystalline precipitate upon addition of pentane.
Yield: 0.161 g (90%). Anal. Calc. for C3H7Br4NbO2: C, 7.39; H, 1.45;
Nb, 19.05; Br, 65.55. Found: C, 7.26; H, 1.37; Nb, 18.46; Br,
3JHH = 5.8 Hz, CH2OCH2Cl). 13C{1H} NMR (CDCl3): d = 83.0 (CH2Cl),
75.4 (CH2OCH2Cl), 74.9 ppm (CH2ONb).
The reactions of MX5 (0.30 mmol) with 1,2-dialkoxyalkanes
(0.30 mmol) were also performed in NMR tubes (CDCl3 solutions,
0.65 mL ca.). Compounds 2a–g formed cleanly together with MeBr
(2a,b,d), EtBr (2c), MeI (2e,g), or EtI (2f) respectively, in about 1:1
ratio (1H NMR).
3
65.09%. 1H NMR (CDCl3): d = 5.31 (t, 2H, JHH = 5.86 Hz, CH2ONb),
3
4.36 (t, 2H, JHH = 5.86 Hz, CH2OMe), 3.97 ppm (s, 3H, Me).
4.3. NMR studies on the reactivity of NbCl5, 1b, with dme
13C{1H} NMR (CDCl3): d = 79.6 (CH2ONb), 78.6 (CH2OMe),
65.9 ppm (Me). KM = 0.080 S cm2 molꢁ1
.
(a) Formation of 1,4-dioxane in the 1:1 reaction of NbCl5 with
dme. To a suspension of NbCl5 (0.250 mmol) in CDCl3 (0.75 mL)
in an NMR tube, CH2Cl2 (0.250 mmol) and dme (0.250 mmol) were
added in the order. The tube was sealed, shaken in order to homog-
enize the mixture, and stored at variable temperature for variable
time. Subsequent 1H NMR spectrum did not give useful informa-
tion. Then the tube was opened and the yellow solution inside
was treated with water (ca. 5 mmol). A light-yellow solution was
separated from a colourless precipitate, thus the former was ana-
lyzed by NMR/GC–MS.
4.2.2. TaBr4[OCH2CH2OMe], 2b
Yellow solid, 82% yield from 1g (0.210 g, 0.362 mmol) and dme
(0.038 mL, 0.37 mmol). Time: 1 h. Anal. Calc. for C3H7Br4O2Ta: C,
6.26; H, 1.23; Ta, 31.43; Br, 55.52. Found: C, 6.32; H, 1.28; Ta,
3
31.20; Br, 54.77%. 1H NMR (CDCl3): d = 5.40 (t, 2H, JHH = 5.86 Hz,
3
CH2OTa), 4.45 (t, 2H, JHH = 5.86 Hz, CH2OMe), 4.16 ppm (s, 3H,
Me). 13C{1H} NMR (CDCl3): d = 81.0 (CH2OTa), 74.9 (CH2OMe),
66.8 ppm (Me).
From sample stored at 20 °C for 24 h: CH2Cl2, 1,4-dioxane, MeCl
and MeO(CH2)2Cl (ratio 20:2:18:18 ca.).
4.2.3. NbBr4[OCH2CH2OEt], 2c
Red solid, 77% yield from 1c (0.190 g, 0.39 mmol) and 1,2-dieth-
oxyethane (0.055 mL, 0.39 mmol). Time: 1 h. Anal. Calc. for
C4H9Br4NbO2: C, 9.58; H, 1.81; Nb, 18.52; Br, 63.71. Found: C,
9.50; H, 1.72; Ta, 18.36; Br, 63.21%. 1H NMR (CDCl3): d = 5.29 (t,
From sample stored at 90 °C (temperature of the external oil-
bath) for 3 h: CH2Cl2, 1,4-dioxane, MeCl and MeO(CH2)2Cl (ratio
12:1:8:8 ratio).
From sample stored at 120 °C (temperature of the external oil-
bath) for 5 h: CH2Cl2, MeOH, ClCH2CH2OMe, CH2ClCH2Cl, 1,4-diox-
ane and MeCl (ratio 7:2:2:2:1:4).
(b) Reaction of NbCl4(OCH2CH2OMe), 2h, with dme. A solution
of compound NbCl4(OCH2CH2OMe) (2 h, 0.60 mmol) in CDCl3
(0.80 mL) was treated with CH2Cl2 (0.60 mmol) and then with
dme (0.90 mmol). The tube was sealed, shaken in order to homog-
enize the mixture, and stored at variable temperature for variable
time. Subsequent 1H NMR spectrum did not indicate clearly forma-
tion of 4a. Then the tube was opened and the yellow solution in-
3
2H, JHH = 5.86 Hz, CH2ONb), 4.21 (m, 2H, CH2OEt), 3.67 (q, 2H,
3
3JHH = 8 Hz, CH2CH3), 1.45 ppm (t, 3H, JHH = 8 Hz, CH2CH3).
13C{1H} NMR (CDCl3): d = 81.2 (CH2ONb), 76.1 (CH2OEt), 72.8
(CH2CH3), 18.6 ppm (CH2CH3).
4.2.4. NbBr4[OCH2CH(Me)OMe], 2d
Red microcrystalline solid, 86% yield from 1c (0.165 g,
0.335 mmol) and 1,2-dimethoxypropane (0.042 mL, 0.34 mmol).
Time: 6 h. Anal. Calc. for C4H9Br4NbO2: C, 9.58; H, 1.81; Nb,
18.52; Br, 63.71. Found: C, 9.50; H, 1.74; Nb, 18.63; Br, 63.21%.
1H NMR (CDCl3): d (major isomer) = 5.24/5.11 (m, 2H, NbOCH2),
4.61 (m, 1H, CH), 3.96 (s, 3H, OMe), 1.59 ppm (d, 3H, 3JHH = 6.23 Hz,
side was treated with water (ca. 5 mmol).
A solution was
separated from a colourless precipitate, thus the former was ana-
lyzed by NMR/GC–MS.

1418
R. Bini et al. / Polyhedron 30 (2011) 1412–1419
From sample stored at 20 °C for 24 h: CH2Cl2, MeO(CH2)2Cl,
4.6. Reactions of NbX5(X = Br, I) with 1,2-dialkoxyalkanes in 1:2 molar
ratio
MeCl, dme and 1,4-dioxane (ratio 10:8:8:15:1).
From sample stored at 90 °C (temperature of the external oil-
bath) for 12 h: CH2Cl2, MeO(CH2)2Cl, MeCl, dme and 1,4-dioxane
(ratio 10:7:10:13:3).
From sample stored at 120 °C (temperature of the external oil-
bath) for 5 h: MeO(CH2)2Cl, MeCl, dme, 1,4-dioxane, MeOH and
CH2ClCH2Cl (ratio 2:4:2:3:2:1).
General procedure: A suspension of MX5 (0.200 mmol) in CDCl3
(0.70 mL) in an NMR tube, was treated with CH2Cl2 (0.200 mmol)
and with the appropriate diether (0.400 mmol). The tube was
sealed, shaken in order to homogenise the content, and stored at
room temperature for 48 h. Then 1H NMR spectrum was recorded.
Afterwards, the tube was opened and water (ca. 3 mmol) was
added to the mixture, giving the precipitation of a colourless solid
from a solution. The latter was analyzed by 1H NMR/GC–MS.
From NbBr5 and MeOCH2CH(Me)OMe: CH2Cl2, 2d, MeBr and
MeOCH2CH(Me)OMe (ratio ca. 1:1:1:1). After hydrolysis: CH2Cl2,
BrCH2CH(Me)OMe, MeBr and MeOCH2CH(Me)OMe (ratio 1:1:1:1).
From NbBr5 and EtO(CH2)2OEt: CH2Cl2, 2c, EtBr and EtO(-
CH2)2OEt (ratio ca. 1:1:1:1). After hydrolysis: CH2Cl2, Br(CH2)2OEt,
EtBr, EtO(CH2)2OEt and 1,4-dioxane (ratio 20:15:20:16:1).
From NbI5 and MeO(CH2)2OMe (dme)/H2O: CH2Cl2, CH2ICH2I,
MeOH, MeI, dme, MeO(CH2)2OH and 1,4-dioxane (ratio
20:3:2:15:13:12:3).
4.4. NMR studies on the reactivity of NbBr5 with dme
(a) Characterization of [NbBr4(j j
1-dme)( 2-dme)][NbBr6], 3b. A
mixture of NbBr5 (0.450 mmol), CDCl3 (0.75 mL) and CH2Cl2
(0.450 mmol) was introduced into an NMR tube, then dme
(0.350 mmol) was added. The tube was sealed, shaken in order to
homogenize the content, and stored at ꢁ10 °C for 10 min. Subse-
quent NMR spectrum at ꢁ60 °C evidenced the absence of uncoor-
dinated dme and the clean formation of 3b. 1H NMR (CDCl3,
213 K): d = 4.61 [m-br, 2H, NbOCH2
MeONb(
1-dme)], 4.00 [s, 4H, CH2
CH2
1-dme)], 3.68 [s, 6H, CH3 2-dme)], 3.41 ppm [s, 3H, Me
1-dme)]. Similar NMR pattern was obtained upon reaction of
(j
1-dme)], 4.24 [s, 3H,
2-dme)], 3.90 [m-br, 2H,
j
(j
From NbI5 and EtO(CH2)2OEt/H2O: CH2Cl2, EtI, EtO(CH2)2OEt
and EtOCH2CH2OH (ratio 10:8:10:7).
(j
(j
(j
From NbI5 and MeOCH2CH(Me)OMe/H2O: CH2Cl2, MeI,
MeOCH2CH(Me)OMe and ICH2CH(Me)OMe (ratio ca. 1:1:1:1).
From NbI5 and MeO(CH2)2OCH2Cl/H2O: CH2Cl2, MeI, CH2ICH2I,
CH2ClI, ClCH2OCH2OCH2Cl, dme and Me2O (ratio ca.
16:12:4:2:4:11:1).
1c (0.450 mmol) with dme (0.950 mmol).
In a different experiment, a suspension of 1c (0.900 mmol) in
CH2Cl2 (25 mL) was cooled at ꢁ10 °C and treated with dme
(0.910 mmol). An orange solution formed upon stirring the mix-
ture for 10 min at ꢁ10 °C. KM (CH2Cl2, 263 K) = 8.1 S cm2 molꢁ1
The solution was allowed to warm up to room temperature and
stirred for additional 30 min. Subsequent conductivity measure-
ment resulted in agreement with the formation of NbBr4[OCH2-
CH2OMe], 2a (KM = 0.25 S cm2 molꢁ1).
(b) Reactions of NbBr5 with dme in 2:1 molar ratio. To a suspen-
sion of NbBr5 (0.250 mmol) in CDCl3 (0.75 mL) in an NMR tube,
CH2Cl2 (0.250 mmol) and dme (0.500 mmol) were added in the or-
der. The tube was sealed, shaken and maintained at room temper-
ature for 15 days. Subsequent 1H NMR analysis evidenced the
presence of CH2Cl2, complex 2a and MeBr in ca. 1:1:1 ratio. Then,
the tube was opened and water was added inside (ca. 4 mmol).
Immediate precipitation of a colourless solid from a pale-yellow
solution occurred. NMR/GC–MS analyses on the solution revealed
the presence of CH2Cl2, MeOCH2CH2Br, MeBr and 1,4-dioxane (ra-
tio ca. 10:9:9:1).
4.7. X-ray crystallographic study
Crystal data and collection details for NbOBr3(dme)NbBr5, 5b,
are listed in Table 4. The diffraction experiment was carried out
on a Bruker APEX II diffractometer equipped with a CCD detector
and using Mo Ka radiation. Data were corrected for Lorentz polar-
ization and absorption effects (empirical absorption correction SAD-
ABS) [18]. The structure was solved by direct methods and refined
by full-matrix least-squares based on all data using F2 [18]. All
non-hydrogen atoms were refined with anisotropic displacement
parameters. H-atoms were placed in calculated positions and trea-
ted isotropically using the 1.2-fold Uiso value of the parent atom ex-
cept methyl protons, which were assigned the 1.5-fold Uiso value of
the parent C-atom.
In a different experiment, an NMR tube was charged with NbBr5
(0.200 g, 0.406 mmol), CDCl3 (0.85 mL), CH2Cl2 (0.410 mmol) and
dme (0.83 mmol), in the order given. Then the tube was sealed,
and the mixture was heated at 80 °C (temperature of the external
oil-bath) for 12 h. Hence, the tube was cooled to ꢁ20 °C and
opened. A large excess of water (ca. 5 mmol) was added, thus the
solution obtained was analyzed by GC/MS and NMR spectroscopy:
CH2Cl2, dme, MeOCH2CH2Br, MeBr, CH2BrCH2Br, MeOH and 1,4-
dioxane were finally recognized in 25:8:5:16:7:6:3 ratio.
Table 4
Crystal data and structure refinement for NbOBr3(dme)NbBr5, 5b.
Formula
Formula weight
k (Å)
Temperature (K)
Crystal system
Space group
a (Å)
C4H10Br8Nb2O3
931.22
0.71073
100(2)
orthorhombic
Pna21
36.552(6)
10.1067(15)
10.0757(15)
90
3719.1(10)
8
3.326
18.415
3360
1.12–28.00
30406
8706 (Rint = 0.0647)
8706/2/311
1.022
0.0393
0.0850
b (Å)
c (Å)
b (°)
4.5. Isolation of the complex NbOBr3(dme)NbBr5 (Nb–O–Nb), 5b
Cell volume (Å3)
Z
Dcalc (g cmꢁ3
)
A
mixture of NbBr5, 1c (0.700 g, 1.42 mmol), and dme
l
(mmꢁ1
F(0 0 0)
h Limits (°)
)
(2.10 mmol), in CH2Cl2 (9 mL) in a Schlenk tube, was stirred at
room temperature for 4 h. The resulting red solution was layered
with pentane and stored at room temperature. Hence, dark-red
crystals of compound NbOBr3(dme)NbBr5, 5b, formed in 12 h.
Yield: 0.038 g (6% yield). Anal. Calc. for C4H10Br8Nb2O3: C, 5.16;
H, 1.08; Br, 68.65. Found: C, 5.06; H, 1.13; Br, 67.86%. IR (solid
state): 2962w, 2896w, 2825vw, 1467w, 1443m, 1403w, 1259m,
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness on fit on F2
R1 (I > 2
r(I))
wR2 (all data)
1067s, 1015s, 857m-s (
d = 4.30 (s, 4H, CH2), 3.93 ppm (s, 6H, CH3).
m .
Nb@O), 792vs cmꢁ1 1H NMR (CDCl3):
Largest difference in peak and hole (e Åꢁ3
)
1.460 and ꢁ1.476

R. Bini et al. / Polyhedron 30 (2011) 1412–1419
1419
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for NbOBr3(dme)NbBr5, 5b. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
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