Complexes of BX 3 with EMe 2 (X = F, Cl, Br, I; E = Se or Te): Synthesis, multinuclear NMR spectroscopic and structural studies

Abstract

The [BX3(EMe2)] (X = Cl, Br, I; E = Se or Te) have been prepared by reaction of BX3 with the EMe2 in hexane under anhydrous conditions. The X-ray crystal structures of [BX3(TeMe2)] (X = Cl, Br, I) and [BX3(SeMe2)] (X = Cl, Br) have been determined; all are pseudo-tetrahedral monomers and show d(B−E) decreases with halogen, Cl > Br > I. Multinuclear NMR data (¹H, ¹¹B, ⁷⁷Se and ¹²⁵Te) are reported and compared with data on the corresponding [BX3(SMe2)], and the trends discussed. The unstable [BF3(SeMe2)], prepared from BF3 and SeMe2 in the absence of a solvent, has been similarly characterised by multinuclear NMR spectroscopy, and evidence for the existence of unstable [BF3(TeMe2)] obtained for the first time, although it could not be obtained pure. The results are discussed in the light of recent theoretical modelling of boron halide adducts.

Full text

Accepted Manuscript
Complexes of BX3 with EMe2 (X=F, Cl, Br, I; E=Se or Te): Synthesis, multinuclear
NMR spectroscopic and structural studies
Coco K.Y.A. Okio, William Levason, Francesco M. Monzittu, Gillian Reid
PII: S0022-328X(17)30484-9
DOI: 10.1016/j.jorganchem.2017.08.004
Reference: JOM 20057
To appear in: Journal of Organometallic Chemistry
Received Date: 16 June 2017
Revised Date: 27 July 2017
Accepted Date: 4 August 2017
Please cite this article as: C.K.Y.A. Okio, W. Levason, F.M. Monzittu, G. Reid, Complexes of BX3 with
EMe2 (X=F, Cl, Br, I; E=Se or Te): Synthesis, multinuclear NMR spectroscopic and structural studies,
Journal of Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.08.004.
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Complexes of BX
3
with EMe
2
(X= F, Cl, Br, I; E = Se or Te): synthesis,
multinuclear NMR spectroscopic and structural studies.
Coco K.Y.A. Okio
a
, William Levason
∗,b
, Francesco M. Monzittu
b
and Gillian Reid
b
a. Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia,
Bogotá, Colombia.
b. Chemistry, University of Southampton, Southampton SO17 1BJ, UK; email: wxl@soton.ac.uk
Abstract
The [BX
3
(EMe
2
)] (X = Cl, Br, I; E = Se or Te) have been prepared by reaction of BX
3
with the EMe
2
in hexane under anhydrous conditions. The X-ray crystal structures of [BX
3
(TeMe
2
)] (X = Cl, Br, I)
and [BX
3
(SeMe
2
)] ( X = Cl, Br) have been determined; all are pseudo-tetrahedral monomers and show
d(B−E) decreases with halogen, Cl > Br > I. Multinuclear NMR data (
1
H,
11
B,
77
Se and
125
Te) are
reported and compared with data on the corresponding [BX
3
(SMe
2
)], and the trends discussed. The
unstable [BF
3
(SeMe
2
)], prepared from BF
3
and SeMe
2
in the absence of a solvent, has been similarly
characterised by multinuclear NMR spectroscopy, and evidence for the existence of unstable
[BF
3
(TeMe
2
)] obtained for the first time, although it could not be obtained pure. The results are
discussed in the light of recent theoretical modelling of boron halide adducts.
Keywords: boron halide, selenoether, telluroether, boron-11 NMR, crystal structures
1. Introduction
The study of boron Lewis acids remains a very active area of research and has been discussed in
several recent reviews [1-6].
The boron(III) halides are good Lewis acids whose acidity generally
increases BF
3
< BCl
3
< BBr
3
< BI
3
, which is counter-intuitive on electronegativity grounds, and was
for many years attributed to significant X→B π bonding which decreased down Group 17. Much
computational work over the past fifteen years has been devoted to exploring the factors involved,
almost all focussed on N- or O-donor ligands. The results of the DFT calculations may differ in fine
details, however it is generally agreed that the order of Lewis acidity results from the varying strength
of the σ-interactions, and the π-bonding explanation has been discounted [5,7-9]. It is important to
note that the properties of both the Lewis acid and Lewis base in a complex must be taken in
consideration, and that calculations deal with gas phase species, meaning solvation or solid state
effects may mask Lewis acidity trends in solution or in the solid state. Due to their innate Lewis
acidity, boron(III) halides have found widespread use in organic syntheses and catalysis [6,10]. There
∗
Corresponding author.

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is also much current interest in the chemistry of boron-based frustrated Lewis pairs [11],
and the use
of boron compounds in fluoride sensing and PET imaging [12].
We recently reported a systematic study of the coordination chemistry of the neutral diphosphines,
R
2
P(CH
2
)
2
PR
2
(R = Me or Et) and o-C
6
H
4
(PR’
2
)
2
(R’ = Me or Ph), and the diarsine, o-C
6
H
4
(AsMe
2
)
2
towards BX
3
(X = F, Cl, Br and I). The studies revealed that whilst flexible ligands produced [X
3
B(µ-
L−L)BX
3
] complexes, with o-phenylene linked diphosphines and diarsines, very rare dihaloboronium
cations, [BX
2
{o-C
6
H
4
(EMe
2
)
2
}]
+
(E = P, As) were obtained [13]. In these complexes and in the
corresponding [BX
3
(PMe
3
)] [14]
and [BX
3
(AsMe
3
)] [15] the d(B−P) and d(B−As) bond lengths
follow the expected order with X (F < Cl < Br <I), although data on fluoride complexes are rather
limited.
Boron trihalide complexes of thio- and seleno-ethers were reported many years ago and are all of type
[BX
3
(ER
2
)] (X = F, Cl, Br or I; E = S or Se, R most often Me, sometimes Et or
i
Pr, R
2
= c-(CH
2
)
n
)
Typical syntheses involved direct reaction of the BX
3
and ER
2
in the absence of a solvent, whilst
others used alkanes, CCl
4
, CH
2
Cl
2
or CS
2
as solvents [16-25]. Generally, the complexes were
characterised by microanalysis,
1
H and sometimes
11
B NMR and IR spectroscopy. X-Ray structural
data are surprisingly rare with only the crystal structures of [BX
3
(tht)] (X = Cl, Br or I; tht =
tetrahydrothiophene) reported [26]. In these, the d(B−S) bond length increased with halide Br > Cl >
I, the anomalous position of the bromide was suggested by the authors to be due either to solid state
effects or a consequence of the disorder of the tht molecule [26].
The unit cell data for several
[BX
3
(EMe
2
)] (E = S, Se) were reported in a conference paper [27], but the full structures have never
appeared. Very much less is known about TeMe
2
adducts [22,23], and no complex was reported to
form between BF
3
and TeMe
2
at ambient temperatures [28].
Here, we report a detailed study of the complexes [BX
3
(EMe
2
)] (E = Se or Te) focussing on
multinuclear NMR data and the X-ray crystal structures of [BX
3
(TeMe
2
)] (X = Cl, Br, I) and
[BX
3
(SeMe
2
)] ( X = Cl, Br). Multinuclear NMR spectroscopic data are also reported for [BX
3
(SMe
2
)]
for comparison. Comparisons between the boron complexes and chalcogenoether complexes with
other Group 13 halides are also described.
2. Experimental
Infrared spectra were recorded as Nujol mulls between CsI plates using a Perkin-Elmer
Spectrum 100 spectrometer over the range 4000–200 cm
−1
.
1
H,
11
B,
19
F{
1
H},
77
Se{
1
H}, and
125
Te{
1
H} NMR
spectra were recorded from CH
2
Cl
2
/CD
2
Cl
2
solutions using a Bruker AV400
spectrometer and
referenced to the residual solvent resonance, external [BF
3
(OEt
2
)], CFCl
3
,
neat SeMe
2
and neat TeMe
2
respectively. Microanalyses were undertaken by Medac Ltd.
Hexane was dried prior to use by distillation from sodium and CH
2
Cl
2
from CaH
2
, and all
preparations were carried out under rigorously anhydrous conditions via a dry dinitrogen

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atmosphere and standard Schlenk and glovebox techniques. Boron trifluoride was obtained
from Fluorochem. Other boron trihalides were obtained from Sigma-Aldrich and used as
received. SMe
2
and SeMe
2
(Sigma-Aldrich) were stored over molecular sieves. TeMe
2
was
made as described [29]. [BF
3
(SMe
2
)] and [BCl
3
(SMe
2
)] were commercial samples (Sigma-
Aldrich), and [BX
3
(SMe
2
)] (X = Br, I) were made as described, by reaction of the constituents
in n-hexane [23].
2.1 [BCl
3
(TeMe
2
)]
TeMe
2
(0.14 g, 0.9 mmol.) was dissolved in n-hexane (10 mL), the solution cooled in an ice bath, and
a slow stream of BCl
3
passed in, producing a white precipitate. The mixture was stirred for 30 min.
and then the solid filtered off and dried in vacuo. Yield: 0.185 g, 61%. Anal. Required for C
2
H
6
BCl
3
Te
(274.8): C, 8.74; H, 2.20%. Found: C, 9.19; H, 2.13%.
1
H NMR (CDCl
3
, 295 K): δ 2.10 (s).
11
B NMR
(CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ −20.01 (s),
125
Te NMR (CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ +242.9 (s). IR (Nujol):
ν = 721 (br) (BCl) cm
−1
. Colourless crystals were grown by slow evaporation of a CH
2
Cl
2
solution of
the product.
2.2 [BBr
3
(TeMe
2
)]: To an n-hexane solution (10 mL) of BBr
3
(0.20 g, 0.8 mmol) in an ice bath, was
added dropwise TeMe
2
(0.126 g, 0.8 mmol), leading to the formation of an immediate white
precipitate. After stirring for 30 min, the white powder was isolated by filtration and dried in vacuo.
Yield: 0.230 g, 70%. Anal. Required for C
2
H
6
BBr
3
Te (408.2): C, 5.99; H, 1.48 %. Found: C, 6.94; H,
1.81%.
1
H NMR (CDCl
3
, 295 K): δ 2.10 (s).
11
B NMR (CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ −20.01 (s),
125
Te
NMR (CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ +242.9 (s). IR (Nujol): ν = 622 (s) (BBr) cm
−1
. Small colourless
crystals were grown by slow evaporation of a CH
2
Cl
2
solution of the product.
2.3 [BI
3
(TeMe
2
)]
Powdered BI
3
(0.20 g, 0.5 mmol) was added to n-hexane (15 mL) and TeMe
2
(0.08g, 0.5 mmol) was
added dropwise to the stirred solution. A pale cream solid formed immediately and after stirring for
20 min. the white solid was filtered off and dried in vacuo. Yield: 0.19 g, 69%. C
2
H
6
BI
3
Te (549.2): C,
4.37; H, 1.10%. Found: C, 5.14; H, 1.68%.
1
H NMR (CDCl
3
, 295 K): δ 1.91 (s).
11
B NMR
(CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ −86.80 (s).
125
Te NMR (CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ +290.2 (q,
1
J
BTe
= 121
Hz). IR (Nujol): ν = 560(m), 543(s) (BI) cm
−1
.
2.4 [BCl
3
(SeMe
2
)]
SeMe
2
(0.16 g, 1.5 mmol) was dispersed in stirred n-hexane (15 mL), and BCl
3
gas slowly bubbled
into the solution for 5 min, resulting in the rapid formation of a white powdery precipitate. The BCl
3
was stopped, and the mixture stirred for 30 min, after which the white precipitate was isolated by
filtration and dried to a white powder in vacuo. Yield: 0.120 g, 36%. C
2
H
6
BCl
3
Se (226.2): C, 10.62; H,

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2.67%. Found: C, 10.61; H, 2.65%.
1
H NMR (CDCl
3
, 295 K): δ 2.36 (s).
11
B NMR (CH
2
Cl
2
/CD
2
Cl
2
,
295 K): δ +7.36 (s).
77
Se NMR (CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ +170.5 (s). IR (Nujol): ν = 765 (m), 723
(s) (BCl) cm
−1
.
2.5 [BBr
3
(SeMe
2
)]
To a solution of SeMe
2
(0.087 g, 0.8 mmol) in n-hexane (8 mL) was added dropwise BBr
3
(0.20 g, 0.8
mmol) which immediately led to the precipitation of a white solid. The reaction was stirred for 30
min, and then the white powder was isolated by filtration and dried in vacuo. Yield: 0.134 g, 47%.
C
2
H
6
BBr
3
Se (359.6): C, 6.68; H, 1.68%. Found: C, 6.76; H, 2.60%.
1
H NMR (CDCl
3
, 295 K): δ 2.33
(s).
11
B NMR (CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ −12.94 (s).
77
Se NMR (CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ +194.4
(s). IR (Nujol): ν = 680(m), 640 (s) (BBr) cm
−1
.
2.6 [BI
3
(SeMe
2
)]
To a n-hexane (10 ml) solution of BI
3
(0.05 g, 0.13 mmol) was added dropwise Me
2
Se (0.014 g, 0.13
mmol) leading to the immediate formation of a white precipitate. After stirring the mixture for a
further 30 min. the white powder was filtrated and dried in vacuo. Yield: 0.022 g (34%). C
2
H
6
BI
3
Se
(500.6): C, 4.80; H, 1.21%. Found: C, 4.87; H, 1.33%.
1
H NMR (CDCl
3
, 295 K): δ 2.30 (s).
11
B NMR
(CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ −73.96 (s),
77
Se NMR (CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ +203.7 (q,
1
J
BSe
= 52
Hz). IR (Nujol): ν = 560 (m), 550 (s) (BI) cm
−1
.
2.7 [BF
3
(SeMe
2
)]
Neat SeMe
2
(0.22 g, 2.0 mmol) was cooled in an ice-bath and a slow stream of BF
3
bubbled in for 5
min. The product was a clear straw coloured liquid which fumes in air and is hydrolysed by trace
moisture. The liquid was stored under a nitrogen atmosphere and measurements made on freshly
prepared samples. The complex has a significant vapour pressure of BF
3
at room temperature and the
microanalysis cannot be obtained.
1
H NMR (CDCl
3
, 295 K): δ 2.04 (s).
11
B NMR (CH
2
Cl
2
/CD
2
Cl
2
,
295 K): δ +5.31 (s); (183 K): δ +3.25 (s).
19
F{
1
H) NMR (CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ −132.8 (s); (183
K): δ −134.2 (s).
77
Se NMR (CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ +1.03 (s); (183 K): δ +0.53 (s).
2.8 [BF
3
(TeMe
2
)]
Treatment of TeMe
2
with BF
3
gas either at ambient temperature or in an ice bath gave a yellow oil
with some orange-red solid. Similarly, adding BF
3
gas to a solution of TeMe
2
in CH
2
Cl
2
gave a yellow
solution and some red precipitate. Both the neat liquid and the solution decompose in a few hours at
room temperature, turning orange and then dark red. It was not possible to produce a pure sample, and
the freshly made yellow CH
2
Cl
2
solution was used for the spectroscopic measurements.
1
H NMR
(CD
2
Cl
2
, 295 K): δ 1.98 (s); (183 K): 1.86 (s).
11
B NMR (CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ +5.31 (s); (183

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K): δ −1.52 (s).
19
F{
1
H) NMR (CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ −144.5 (s); (183 K): δ −148.3 (s).
125
Te
NMR (CH
2
Cl
2
/CD
2
Cl
2
, 295 K): δ −4.80 (s); (183 K): δ −29.2 (s).
After 12 h at room temperature the CH
2
Cl
2
solution had become deep orange with much orange-red
precipitate and had singlet
1
H NMR resonances at 1.92, 2.68 and 5.41, tentatively assigned to free
TeMe
2
and [Me
2
TeCH
2
Cl]
+
(or [Me
2
Te(CH
2
Cl)Cl]), with corresponding
125
Te{
1
H} resonances at −13
and +418. The
19
F{
1
H} NMR showed [BF
4
]
−
as the major species, with some smaller amounts of F
−
.
2.8 X-ray experimental
Crystals of the complexes were grown from CH
2
Cl
2
solutions of the complexes allowed to evaporate
slowly in the glove box. Data collections used a Rigaku AFC12 goniometer equipped with an
enhanced sensitivity (HG) Saturn724+ detector mounted at the window of an FR-E+ SuperBright
molybdenum (λ = 0.71073 Å) rotating anode generator with VHF Varimax optics (70 micron focus)
with the crystal held at 100 K. Structure solution and refinement were performed using
SHELX(S/L)97, SHELX-2013 or SHELX-2014/7 [30]. H atoms bonded to C were placed in
calculated positions using the default C−H distance, and refined using a riding model. Details of the
crystallographic parameters are given in Table 1. CCDC reference numbers in cif format are
[BCl
3
(TeMe
2
)]: CCDC 1554690; [BBr
3
(TeMe
2
)]: CCDC 1554688; [BI
3
(TeMe
2
)]: CCDC 1554689;
[BCl
3
(SeMe
2
)]: CCDC 1554686; [BBr
3
(SeMe
2
)]: CCDC 1554687. 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, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.

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Table 1. X-ray crystallographic data.
a
Compound [BCl
3
(TeMe
2
)] [BBr
3
(TeMe
2
)] [BI
3
(TeMe
2
)] [BCl
3
(SeMe
2
)] [BBr
3
(SeMe
2
)]
Formula C
2
H
6
BCl
3
Te C
2
H
6
BBr
3
Te C
2
H
6
BI
3
Te C
2
H
6
BCl
3
Se C
2
H
6
BBr
3
Se
M 274.85 408.21 549.18 226.19 359.57
crystal system monoclinic monoclinic monoclinic monoclinic monoclinic
Space group (No) P2
1
/m (11) P2
1
/m (11) P2
1
/m (11) P2
1
/m (11) P2
1
/m (11)
a [Å] 5.76470(10) 6.08639(17) 6.6604(2) 5.78171(18) 6.1384(3)
b [Å] 10.8546(2) 11.0548(3) 11.3864(9) 10.6641(2) 10.8664(4)
c [Å] 6.5636(2) 6.69386(18) 6.96470(10) 6.4752(3) 6.6455(3)
α
°
90 90 90 90 90
β ° 104.676(3) 104.876(3) 105.488(2) 107.398(4) 108.280(5)
γ
°
90 90 90 90 90
U [Å
3
] 397.308(17) 435.29(2) 509.01(4) 380.97(2) 420.90(3)
Z 2 2 2 2 2
µ
(Mo Kα) [mm
–
1
] 4.645 17.089 11.938 5.868 18.600
total no. reflns 4289 3420 11377 216 324
unique reflns 822 904 468 8451 9534
R
int
0.0163 0.020 0.039 0.035 0.055
no. of params, restraints 38, 0 38, 0 38, 0 38, 0 38, 0
F(000) 252 360 1059 781 876
GOF 1.089 1.074 1.165 1.140 1.104
Largest peak and hole e /Å
3
0.379, -0.235 0.923, -0.623 0.566, -0.741
0.523, -0.249 0.798, -0.498
R
1
b
[I
o
> 2
σ
(I
o
)] 0.010 0.017 0.014 0.016 0.021
R
1
[all data] 0.027 0.044 0.014 0.016 0.023
wR
2
b
[I
o
> 2
σ
(I
o
)] 0.011 0.017 0.035 0.039 0.051
wR
2
[all data] 0.027 0.044 0.035 0.039 0.052
a. Common items: temperature = 100 K; wavelength (Mo-K
α
) = 0.71073 Å; θ(max) = 27.5°.b R
1
= ΣF
o
| – F
c
/ ΣF
o
; wR
2
= [Σw(F
o2
- F
c2
)
2
/Σ wF
o4
]
1/2

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3. Results and Discussion.
3.1 [BX
3
(TeMe
2
)] (X = Cl, Br, I)
Although [BX
3
(TeMe
2
)] (X = Cl, Br, I) have been briefly mentioned in larger studies of boron halide
complexes [22,23], they lack detailed characterisation. All three complexes were made by reaction of
the appropriate BX
3
and TeMe
2
in dry n-hexane at 0°C, when they separated as white powders
(Scheme 1).
BX3hexane, 0oC
X = Cl, Br, I
E = S, Se
E*= E, Te
TeMe2
EMe2
hexane
[BX3(EMe2)]
[BF3(E*Me2)] E*Me2
X = F [BX3(TeMe2)]
Scheme 1. Synthesis of the complexes.
The same compounds are produced using excess ligand. The solids slowly darken and decompose at
room temperature and are significantly decomposed after 24 h in solution in chlorocarbons.
Colourless crystals of [BX
3
(TeMe
2
)] (X = Cl, Br, I) were grown by evaporation of CH
2
Cl
2
solutions
in the glove box and the structures are shown in Fig. 1.

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Fig. 1 The structure of [BX
3
(TeMe
2
)] (X = Cl, Br, I) showing the numbering scheme. Ellipsoids are drawn at the
50% probability level and H atoms are omitted for clarity. Symmetry operation: i = x, ½ -y, z. Selected bond
lengths (Å) and angles (°):
X = Cl: Te1−B1 = 2.298(2), Cl1−B1 = 1.8302(13), Cl2−B1 = 1.834(2), C1−Te1−C1
i
= 95.00(9), Cl1−B1−Te1
= 106.29(8), Cl2−B1−Te1 = 108.87(10), Cl2−B1−Cl1 = 111.18(8), Cl2−B1−Cl2
i
= 112.73(11).
X = Br: Te1−B1 = 2.266(4), Br2−B1= 1.986(4), Br1−B1 = 1.996(2), C1−Te1−C1
i
= 95.08(16), Br2−B1−Te1 =
110.60(17), Br2−B1−Br1 = 110.92(12), Br1−B1−Te1 = 106.29(12), Br1−B1−Br1
i
= 111.63(18).
X = I: I1−B1= 2.215(4), I2−B1 = 2.227(2), Te1−B1 = 2.262(4), C1−Te1−C1
i
= 95.84(17), I1−BI−I2 =
111.15(12), I1−B1−Te1 = 111.66(18), I2−B1−I2
i
= 111.12(18), I2−B1−Te1 = 105.75(13).
The crystals are isomorphous and contain tetrahedrally coordinated boron centres with <X−B−X
slightly greater than the idealised tetrahedral angle and one of the <X−B−Te slightly smaller. The
d(B−X) (Table 2) are very similar to those in the corresponding PMe
3
or AsMe
3
adducts [14,15]. The
d(B−E) bonds increase I ~ Br < Cl. The B-X bonds are not significantly affected by the
chalcogenoether present. The <C−Te−C angles are ~95°, consistent with a large tellurium p-character
in the C−Te bonds [31]. Examination of the packing diagrams shows no Te
···
X interactions between
neighbouring molecules within the sum of the van der Waals radii, and hence no hypervalent
interactions at Te.
Table 2 Bond length comparisons.
[BCl
3
(TeMe
2
)]
[BBr
3
(TeMe
2
)]
[BI
3
(TeMe
2
)]
[BCl
3
(SeMe
2
)]
[BBr
3
(SeMe
2
)]
B-E/ Å
2.298(2)
2.266(4)
2.262(4)
2.106(3)
2.088(4)
B-X /Å
1.8302(13)
1.834(2)
1.986(4)
1.996(2)
2.215(4)
2.227(2)
1.8287(15)
1.825(3)
1.987(5)
1.993(3)
The
11
B NMR resonances (
11
B, 80%, I = 3/2, Ξ = 32.084 MHz, Q = 3.55 x 10
−30
m
2
, R
c
= 754) move
to high frequency with halide, I < Br < Cl, the same trend as found with Group 15 donor ligands
[13,32]. The
11
B chemical shifts of the telluroether complexes are all much lower frequency than
those of the parent trihalides, reflecting both the change in coordination number and the different
electronic environment at B (Table 3).
Table 3. Multinuclear NMR Data on [BX
3
(ER
2
)]
a,b
.
1
H
11
B
c
19
F
d
77
Se/
125
Te
e
[BF
3
(SMe
2
)] 2.25 +3.52 −138.8 -
[BF
3
(SeMe
2
)] 2.01 +5.34 −132.8 +1.04
[BF
3
(TeMe
2
)] 1.93 +1.90 −144.2 −4.8
[BCl
3
(SMe
2
)] 2.49 +8.49 - -

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[BCl
3
(SeMe
2
)] 2.36 +7.36 - +170.5
[BCl
3
(TeMe
2
)] 2.10 +4.78 - +182.9
[BBr
3
(SMe
2
)] 2.57 −9.84 - -
[BBr
3
(SeMe
2
)] 2.33 −12.94 - +194.4
[BBr
3
(TeMe
2
)] 2.10 −20.01 - +243.2
[BI
3
(SMe
2
)] 2.56 −67.55 - -
[BI
3
(SeMe
2
)] 2.30 −73.96 - +203.7 (q)
[BI
3
(TeMe
2
)] 2.05 −86.80 - +290.2 (q)
a. all data from CH
2
Cl
2
/CD
2
Cl
2
solution at 295 K. b. BF
3
:
19
F NMR (CD
2
Cl
2
): −127.8;
11
B NMR (CD
2
Cl
2
): BF
3
+11.03; BCl
3
+41.9, BBr
3
+39.5, BI
3
−5.5 from Ref. 34. c. relative to external [BF
3
(OEt
2
)]. d. relative to
external CFCl
3
. e. relative to neat SeMe
2
or TeMe
2
singlets except q = four line pattern. Since the zero
references are SeMe
2
and TeMe
2
, the coordination shifts (∆) in these cases are numerically the same as the
observed chemical shifts.
The
125
Te{
1
H} NMR resonances observed for X = Cl or Br, are singlets with no resolved coupling to
11
B; this can be attributed to a combination of fast ligand exchange in solution and quadrupolar
relaxation of the
11
B in the significant electric field gradients about the boron centre [20,22]. Cooling
the samples stepwise down to 190 K resulted in small changes in the line width and small drifts in
chemical shifts, but did not resolve couplings. In contrast, the
125
Te{
1
H} NMR spectrum of
[BI
3
(TeMe
2
)] at 295 K is a four line pattern with the
11
B coupling clearly resolved (Fig. 2). Couplings
to the
10
B (I = 3, Ξ = 10.75 MHz, Q = 7.4 x 10
−30
m
2
) were not resolved due to its larger quadrupole
moment, and account for the broad absorption underlying the four line pattern. The coordination shifts
are large and increase Cl < Br < I, suggesting increasingly strong B−Te interaction in this order,
although care should be taken in interpreting such changes as due to a single effect [32,33].
Fig. 2 The
125
Te NMR spectrum of [BI
3
(TeMe
2
)] in CH
2
Cl
2
at 295K showing the
11
B−
125
Te coupling,
1
J = 120
Hz. The broad feature under the resonance is due to unresolved
10
B couplings.
Chemical Shift (ppm)305 300 295 290 285

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3.2 [BX
3
(SeMe
2
)] (X= Cl, Br, I)
The dimethylselenide complexes are white powders and unlike the telluroether analogues (Scheme 1),
are stable for many weeks in the solid state and show no decomposition in chlorocarbon solution after
several days. X-ray structures were determined for [BX
3
(SeMe
2
)] (X = Cl or Br) (Fig. 3) and again
are isomorphous. The trends in bond length with X and E are as seen in the telluroether complexes.
Fig. 3
The structure of [BX
3
(SeMe
2
)] (X = Cl, Br) showing the numbering scheme. Ellipsoids are drawn at the
50% probability level and H atoms are omitted for clarity. Symmetry operation: i = x, ½ -y, z. Selected bond
lengths (Å) and angles (°):
X = Cl: B1−Se1 = 2.106(3), B1−Cl2 = 1.8287(15), B1−Cl1 = 1.825(3), Cl2−B1−Se1 = 105.30(10),
Cl2−B1−Cl2
i
= 112.58(14), Cl1−B1−Se1 = 109.57(13), Cl1−B1−Cl2 = 111.82(10), C1−Se1−C1
i
97.93(11).
X = Br: B1−Se1 = 2.088(4), B1−Br2 = 1.987(5), B1−Br1 = 1.993(3), Br2−B1−Se1 = 110.7(2), Br1−B1−Br1
i
=
112.1(2), Br1−B1−Se1 = 105.14(14), Br1−B1−Br2 = 111.66(14), C1−Se1−C1
i
98.3(2).
The
11
B NMR chemical shifts occur at slightly higher frequency than those found in the telluroether
complexes for common X (Table 3) and the trend to higher frequency Cl > Br > I is observed. The
77
Se NMR spectra also show coordination shifts with X (Cl < Br < I). At 295 K the
77
Se NMR
resonances of [BX
3
(SeMe
2
)] (X = Cl or Br) are broad singlets, but that of [BI
3
(SeMe
2
)] (Fig. 4) shows
a sharp four line pattern due to coupling to
11
B,
1
J
BSe
= 52 Hz. Cooling a CH
2
Cl
2
solution of
[BBr
3
(SeMe
2
)] to 183 K caused the singlet resonance to significantly broaden, but even at this
temperature
11
B−
77
Se coupling was not resolved.

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Fig 4. The
77
Se NMR spectrum of [BI
3
(SeMe
2
)] in CH
2
Cl
2
at 295 K showing the
11
B−
77
Se coupling,
1
J = 52 Hz.
3.3 [BX
3
(SMe
2
)] (X= Cl, Br, I)
Multinuclear NMR data for these three complexes are given in Table 3. The data are generally in
good agreement with literature data [18,21,22,23] when allowance is made for the use of different
solvents and sometimes different boron zero references. The trends in chemical shift with X mirror
those described above.
3.4 [BF
3
(EMe
2
)] (E = Te, Se, S)
The BF
3
adducts are very different to those formed by the other boron halides. The commercially
available [BF
3
(SMe
2
)], which is a convenient source of BF
3
in organic synthesis,
is a colourless oil,
which fumes in air and has a significant vapour pressure at 298 K [16,18]. The
1
H,
11
B and
19
F NMR
data (Table 3) obtained from CH
2
Cl
2
solution at 295 K are in good agreement with literature data [20].
Cooling the solution results in small low temperature drifts of the chemical shifts and below ~ 200 K
the resonances broaden, but even at 183 K,
11
B-
19
F coupling is not resolved. [BF
3
(SeMe
2
)] was
obtained as a very pale yellow liquid by saturating neat SeMe
2
in an ice-bath with BF
3
. It is more
volatile and more extensively dissociated than the SMe
2
analogue [16], and has a significant vapour
pressure of BF
3
at ambient temperature. The
1
H,
11
B and
19
F NMR data (Table 3) obtained from
CH
2
Cl
2
solution at 295 K are similar to those observed for [BF
3
(SMe
2
)]. The
77
Se chemical shift of
[BF
3
(SeMe
2
)] in CH
2
Cl
2
solution is very different to those found with the other three boron halides.
At ambient temperatures it is a broad singlet at ~ +1 ppm, which is little changed on cooling the
solution to 183 K. It is worth pointing out here that the zero reference is neat SeMe
2
(δ = 0), and that
SeMe
2
in CH
2
Cl
2
solution has (δ ~ −7) [34], so the observed value is some ~ 8 ppm to high frequency
of the free selenoether in this solvent; nonetheless a very small coordination shift. The most obvious
explanation is that the SeMe
2
is very weakly bound to the BF
3
centre and fast neutral ligand exchange
may be taking place even at the lowest temperatures. The [BF
3
(SeMe
2
)] fumes in air and is very

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sensitive to moisture which generates [BF
4
]
−
and a species with δ (
1
H) = 2.59 and δ (
77
Se) = 247,
which is tentatively identified as [Me
3
Se]
+
(lit. δ (
77
Se) = 253 (H
2
O) [34], 259 (acetone) [35]).
The only report [28] found of the reaction of BF
3
with TeMe
2
described “immediately upon addition
of BF
3
the solution assumed a brilliant red hue and a volatile orange coloured sublimate quickly
formed”. We observed that on passing BF
3
gas into TeMe
2
in an ice bath, a yellow liquid with small
amounts of orange-red solid formed. A yellow solution and some red solid formed on conducting the
reaction in anhydrous CH
2
Cl
2
solution. The amount of red solid increases over time, and both the
liquid and the CH
2
Cl
2
solution seem extensively decomposed after 12 h at room temperature. The
multinuclear NMR data (Table 3) suggest the yellow liquid is [BF
3
(TeMe
2
)], but this has not been
obtained pure, always seeming to contain some orange impurity. The
11
B and
19
F NMR spectra are not
dissimilar to those obtained from the SMe
2
and SeMe
2
adducts and are little affected by cooling the
solution to 183 K. The
125
Te{
1
H} NMR spectrum in CH
2
Cl
2
at 295 K is a sharp resonance at δ = −4.8,
which shifts to δ = −29 at 183 K. Like the case of the selenium analogue discussed above, the
chemical shift of free TeMe
2
is solvent and temperature sensitive [34] and in CH
2
Cl
2
solution at 295
K has δ = −15 relative to the zero reference of neat TeMe
2
. A CH
2
Cl
2
solution which had become
deep orange with much orange precipitate after ~20 h at room temperature showed new resonances
identified as free TeMe
2
, [BF
4
]
−
, and possibly [Me
2
Te(CH
2
Cl)Cl] (see Section 2.9). The
[Me
2
Te(CH
2
Cl)Cl] (or possibly[Me
2
Te(CH
2
Cl)][BF
4
] is formed by the quaternisation of the
telluroether by the solvent, promoted by the strong Lewis acidic BF
3.
We have observed similar Lewis
acid promoted quaternisation of chalcogenoether ligands in gallium and aluminium systems [36,37].
3.5 Comparisons and Conclusions
The X-ray crystallographic data described for the [BX
3
(EMe
2
)] (E = Se or Te; X = Cl, Br, I) above
(Table 2) shows systematic trends with d(B−E) falling with X, Cl > Br > I, the same order as found
with most other neutral donor systems, and consistent with BI
3
being the strongest Lewis acid [3,5].
Although the X-ray data are less complete, the heavier elements of Group 13 (Al, Ga and In) show the
reverse trends, with the metal chloride forming the shortest M−E bonds [37-40]. The multinuclear
NMR data (Table 3) also show some consistent trends, for example the
11
B chemical shifts for a fixed
chalcogenoether move to low frequency Cl → Br → I, whilst for a fixed halide the trend to low
frequency is S → Se → Te. For complexes of these three boron halides with SeMe
2
the
77
Se
coordination shifts (∆ = δ
complex
– δ
ligand
) are also large and positive (to high frequency) with ∆ I > Br
> Cl. The
125
Te coordination shifts follow the same order. In many series of organoselenium and
organotellurium compounds the ratio δ(Te)/δ(Se) ~ 1.8 is observed [33,34,41], and this empirical
observation also holds for some d-block metal complexes in medium oxidation states [33]. However,
in low valent organometallic or carbonyl complexes the ratio of the coordination shifts are much
larger, δ(Te)/δ(Se) > 2.3, rationalised as due to greater R
2
Te→M donation in the soft metal centres

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and with expanded metal d-orbitals resulting from the low oxidation states providing good orbital
overlap with Te centres [33,42-44]. In the case of the boron halide adducts the opposite trend is
observed with δ(Te)/δ(Se) ~ 1.1−1.4, consistent with the stronger interaction being B−SeMe
2
.
The chalcogenoether adducts of BF
3
are clearly different to those with the other boron halides, and
whilst this work has shown that [BF
3
(TeMe
2
)] does exist, it is unstable and could not be obtained
pure. The
11
B NMR spectra show the chemical shifts move to low frequency Cl > F > Br > I for a
fixed EMe
2
, the anomalous position of the fluoride also being observed with Group 15 donor ligands
[3,13]. The very small coordination shifts observed in the
77
Se and
125
Te NMR spectra of the fluoride
complexes also suggest weak interactions. Overall the data confirm the trends in Lewis acidity of the
boron halides predicted by recent theoretical studies [5-9].
3.5 Acknowledgements
C.K.Y.A.O. thanks the Universidad Nacional de Colombia for authorising sabbatical leave. We also
thank the EPSRC (EP/ L505651/1, EP/K039466/1) and the Newton Fund for support.
Supplementary Information.
ESI for this work including X-ray crystallographic data, and multinuclear NMR spectra may be found
at DOI:…..

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Highlights
Synthesis of boron halide-dimethyltelluride complexes
X-ray structures of TeMe
2
and SeMe
2
complexes of boron halides
Multinuclear NMR data on boron halide-chalcogenoethers
Comparisons of S, Se and Te ligand complexes with boron halides

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Highlights
Synthesis of boron halide-dimethyltelluride complexes
X-ray structures of TeMe
2
and SeMe
2
complexes of boron halides
Multinuclear NMR data on boron halide-chalcogenoethers
Comparisons of S, Se and Te ligand complexes with boron halides