Synthesis and structure of boron-bithiazole complexes

Article abstract of DOI:10.1002/ejic.201000940

The syntheses of the first reported p-block complexes of the bithiazole moiety are reported, namely [(tBuBTz)BBr₂][Br] (3), and [(PhBTz)BBr₂][Br] (4) (tBuBTz = 4,4′-di-tert-butyl-2,2′- bithiazole and PhBTz = 4,4′-diphenyl-2,2′-bithia

Full text of DOI:10.1002/ejic.201000940

SHORT COMMUNICATION
DOI: 10.1002/ejic.201000940
Synthesis and Structure of Boron–Bithiazole Complexes
[a]
[a]
[a]
Michael Findlater,* Nicholas S. Swisher, and Peter S. White
Keywords: Boron / Boronium cations / Bithiazole ligands
The syntheses of the first reported p-block complexes of the
3
quantities of the appropriate bithiazole ligand with BBr . The
bithiazole moiety are reported, namely [(tBuBTz)BBr
2
][Br]
][Br] (4) (tBuBTz = 4,4Ј-di-tert-butyl-
,2Ј-bithiazole and PhBTz = 4,4Ј-diphenyl-2,2Ј-bithiazole).
molecular structures of 3, and the known compound PhBTz
(2) were established using single-crystal X-ray diffraction
techniques.
(
3), and [(PhBTz)BBr
2
2
Compounds 3 and 4 were prepared via reaction of equimolar
Introduction
with alkyl- and haloboranes have been prepared and char-
[
4]
acterized. The stabilities of these boronium complexes
vary greatly depending on the nature of the Lewis base and
the substituent on the boron atom. Despite the fact that
the first stable cationic chelate boronium complex had been
The coordination chemistry of bithiazole ligands with
transition metal elements is well established.[ These neu-
tral, four-electron, N-donor bidentate chelating ligands,
have also found widespread use in materials chemistry as
components in conjugated organic polymers.[ The stereo-
electronic properties of the ligand architecture are readily
modified, for instance alkyl, carbocyclic or heterocyclic
substituents are easily introduced in either the 4- or 5-posi-
tions (Figure 1). It is thus surprising that the deployment
of such a versatile ligand framework within the realm of s-
block chemistry has been limited, and the p-block chemistry
completely overlooked.
1]
[5]
reported in 1906, relatively few complexes involving neu-
2]
tral bi- and polydentate Lewis bases have been reported.
Rund et al. prepared and characterized a series of com-
plexes of the type [LLBX ][BX ], where X = F, Cl and LL =
2
4
[6]
1
,10-phenanthroline, 2,2Ј-bipyridine. Since these studies, a
[
3]
plethora of new chelating ligands containing diimine sub-
units have been disclosed, though surprisingly few examples
[4]
remain structurally characterized.
Crystals of 2, suitable for X-ray diffraction studies, were
grown from a concentrated chloroform solution at low tem-
perature. Ligand 2 crystallized in the monoclinic space
group C2/c and there are no unusually short intermolecular
contacts. The thiazole rings are disposed in an overall trans
fashion, consistent with previously described examples,[
and the molecular structure is shown in Figure 2. Perhaps
the most striking feature of 2 is the near co-planarity of the
phenyl and bithiazole rings. The phenyl ring is twisted out
of planarity by a mere 8.8°. Details of data collection and
refinement are given in Table 1.
7]
Figure 1. Numbering scheme of bithiazole ligand system.
In the present contribution we seek to redress this over-
sight by reporting the syntheses and molecular structures
of two novel boron–bithiazole complexes. To the best of our
knowledge these compounds represent the first examples of
any p-block coordination complexes of the bithiazole
moiety.
Results and Discussion
A considerable number of stable cationic boron com-
plexes involving the interaction of monodentate Lewis bases
[
a] Department of Chemistry, Campus Box 3290, Caudhill and
Kenan Laboratories, The University of North Carolina at
Chapel Hill,
Figure 2. Molecular structure of PhBTZ (2). Selected bond lengths
[
Å] and angles [°]: C(1)–N(1) 1.302(2), N(1)–C(3) 1.384(2), C(3)–
C(2)1.363(3), C(2)–S(1) 1.7086(18), S(1)–C(1) 1.7287(17), C(1)–
N(1)–C(3) 110.55(14), N(1)–C(3)–C(2) 114.31(15), C(2)–S(1)–C(1)
88.64(8), S(1)–C(1)–N(1) 115.34(13).
Chapel Hill, NC 27599-3290, USA
Fax: +1-919-962-2476
E-mail: findlate@email.unc.edu
Eur. J. Inorg. Chem. 2010, 5379–5382
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5379

M. Findlater, N. S. Swisher, P. S. White
SHORT COMMUNICATION
Table 1. Crystal structure determination of 2 and 3.
Compound
2
3
Formula
Formula weight
Crystal system
Space group
a /Å
b /Å
c /Å
α /°
β /°
C
320.42
18
H
12
N
2
S
2
C
14 20 2 2 3
H N S BBr
530.98
triclinic
P1
10.4059(5)
12.9470(5)
16.7728(7)
83.200(2)
78.822(2)
74.700(3)
2132.87(16)
4
1.654
1040
monoclinic
C2/c
18.7867(5)
7.7364(2)
11.4354(3)
90
116.127(2)
90
1492.21(7)
4
¯
γ /°
V /Å
3
Z
–1
ρcalcd. [gcm ]
1.426
664
F(000)
Crystal size /mm
θ range [°]
Number of reflections
collected
0.25ϫ0.10ϫ0.10
6.29–66.50
6514
0.10ϫ0.20ϫ0.10
2.69–66.05
7045
Scheme 1. Preparation of complexes 3 and 4.
Independent reflections
1309
6226
R
1
[IϾ2σ(I)]
wR (all data)
Peak and hole [eÅ ]
0.0314
0.0868
0.396 and –0.189
0.0531
0.1352
0.356 and –0.167
2
–3
puckered out of the plane by 0.195 Å. The average B–N,
N–Cimino and C–Cring bond lengths are 1.581(9), 1.335(9),
and 1.360(10) Å, respectively, thus indicating retention of
the N=C imino linkages and binding of the nitrogen atoms
in a purely dative fashion. Intriguingly, upon close examina-
tion of the extended packing of monomers a weak interac-
tion between the outer-sphere bromide ion and chelating
sulfurs of the ligand framework was evident. This interac-
tion (3.577 Å), illustrated in Figure 3 (b), falls within the
In order to gain additional insight into the electronic
structure of ligand 2, DFT calculations were carried out at
[8]
the B3LYP level of theory using the Gaussian 03 suite of
[
9]
programs. The fractional coordinates from the X-ray crys-
tal structure was used as input data. The 6-31G* basis set
was employed for all elements. In general, there is a very
good agreement between the theoretical and experimental
values. For example, the computed C=N bond length of
1
1
2
.305 Å compares well with the experimental value of
.302(2) Å. Furthermore, the examination of the HOMO of
revealed extensive delocalization throughout the ligand
framework, thus explaining the origin of the near co-plan-
arity of the phenyl and thiazole rings.
Reaction of either tBuBtz (1) or PhBtz (2) with BBr3
(
1 equiv.) in dichloromethane solution affords, after work
up, the cationic bithiazole complexes 3 and 4, respectively
Scheme 1). These salts are isolated as yellow powders in
nearly quantitative yields. Characteristic, four-coordinate,
(
1
1
B NMR signals were observed for both 3 and 4 (δ =
1.23, and 4.76 ppm, respectively).[ Both H and C{ H}
10]
1
13
1
1
NMR studies indicate that an overall C2v symmetric struc-
ture is retained in solution. Crystals of 3 suitable for an
X-ray diffraction study were grown from dichloromethane
solution at –40 °C.
Boronium cation 3 crystallizes in the triclinic space group
¯
P1 and the solid-state structure consists of arrays of indi-
vidual molecules with no unusually short intermolecular
contacts. The molecular structure is depicted in Figure 3
Figure 3. (a) Molecular structure of 3. Selected bond lengths [Å]
(a) and data collection/refinement details are presented in
and angles [°]: C(1)–N(1) 1.324(9), C(4)–N(2) 1.347(9), C(2)–C(3)
Table 1. The closest contact between the outer sphere bro- 1.370(10), C(5)–C(6) 1.351(9), C(1)–C(4) 1.446(10), C(1)–S(1)
mide and the boron center is 5.612 Å and thus exceeds the 1.683(7), S(1)–C(3) 1.716(7), C(4)–S(2) 1.682(7), S(2)–C(6) 1.714(6)
sum of the van der Waals radii for boron and bromine
N–B(av.) 1.581(9), B–Br(av.) 1.989(7), C(1)–N(1)–C(2) 112.0(5),
C(4)–N(2)–C(5) 110.7(5), C(1)–S(1)–C(3) 88.1(4), C(4)–S(2)–C(6)
8
(
4.0 Å). In contrast to closely related diimine-supported
7.6(3), S(1)–C(1)–N(1) 116.2(5), S(2)–C(4)–N(2) 116.2(5), N(1)–
B(1)–N(2) 99.2(5). (b) Interaction of outer sphere bromine ion with
tion of the [BBr2] is not planar, with the boron center sulfur-chelate.
+
[11]
[
BX ] cations, the five-membered ring formed by chela-
2
+
5380
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 5379–5382

Synthesis and Structure of Boron–bithiazole Complexes
sum of the van der Waals radii of sulfur and bromine anisotropic thermal motion, and hydrogen atoms were placed in
fixed, calculated positions using a riding model (C–H: 0.96 Å). Se-
(
3.65 Å). Such an interaction raises the possibility of using
lected crystal data, and data collection and refinement parameters
are listed in Table 1. Elemental analyses were performed by Robert-
son Microlit Laboratories Inc. in Madison, NJ.
the sulfur chelate to coordinate a second metal center and
current efforts are focused at the introduction of gold cen-
ters, for instance.
CCDC-750087 (for 2) and -750088 (for 3) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
DFT calculations were carried out on the cation 3 at an
identical level of theory as indicated above. The computed
charge (Mulliken) at the boron center is +0.504. This par-
tial quenching of charge at a boronium cation by donor
ligands is observed for both borenium and boronium sys-
tems. However, as noted recently, care should be taken
in the interpretation of such charge values and more exten-
sive electronic structure calculations are currently being un-
dertaken.
4
,4Ј-Di-tert-butyl-2,2Ј-bithiazole (1): Following a literature pro-
[14]
[
4]
[12]
cedure, a mixture of 1-bromopinacolone (3.759 g, 21 mmol), di-
thioxamide (1.202 g, 10 mmol) and ethanol (60 cm ) was refluxed
3
for 4 h to afford a red supernatant liquid with a pale solid mixture.
After cooling, the reaction mixture was placed in a freezer (–40 °C)
and left overnight. The pale yellow solid was isolated via filtration
and dried under reduced pressure (2.200 g, 79%). Spectroscopic
characterization was in agreement with literature values.
Conclusions
4,4Ј-Diphenyl-2,2Ј-bithiazole (2): Using a similar procedure as
The first coordination complexes of the bithiazole ligand above, 2 was prepared in 80% yield from 2-bromoacetophenone.
Crystals suitable for study via X-ray diffraction experiments were
class with boron have been prepared and characterized. The
1
grown from concentrated chloroform solutions. H NMR (CDCl
3
,
boron complexes [(tBuBTz)BBr ][Br], and [(PhBTz)BBr ]-
2
2
400 MHz): δ = 7.97 (d, J = 7.2 Hz, 4 H, aryl ortho); 7.58 (s, 2 H,
[
Br] could be easily obtained by treatment of the appropri-
thiazole CH); 7.45 (t, J = 14.8 Hz, 4 H, aryl meta); 7.36 (t, J =
ate bithiazole ligand with an equimolar quantity of BBr3.
The molecular structure of the novel boronium cation 3,
obtained via boron–bromine bond heterolysis, was deter-
mined using single-crystal X-ray diffraction experiments.
Further studies are currently underway to expand the scope
of the main-group coordination chemistry of this hitherto
neglected ligand class.
13
1
7
.2 Hz, 2 H, aryl para) ppm. C{ H} NMR (CDCl
δ = 161.3 (thiazole, C-thiazole); 156.7 (C-Ph); 134.0 (Ph); 128.8
Ph); 128.5 (Ph); 126.5 (Ph, C2); 114.7 (thiazole, C-H) ppm.
HRMS: expected [M + H]+ 321.0523, observed 321.0520.
3
, 75.5 MHz):
(
Preparation of [(tBuBtz)BBr ][Br] (3): To a dichloromethane
10 cm ) solution of 1 (280 mg, 1 mmol), was added boron tribro-
2
3
(
3
mide as a dichloromethane solution (1 cm , 1.0 m) at ambient tem-
perature. The yellow solution was allowed to stir at this tempera-
ture for 12 h before the solvent was removed under reduced pres-
sure to yield 3 as a free flowing yellow powder (501 mg, 95%).
Crystals suitable for study by X-ray diffraction were grown from
Experimental Section
General Procedures: All manipulations were carried out using stan-
dard vacuum, Schlenk, cannula or glovebox techniques. Argon gas
was purified by passage through columns of BASF R3–11 and 4-
Å molecular sieves. THF was distilled from sodium benzophenone
ketyl under nitrogen. Deuterated solvents were purchased from
Cambridge Isotope Laboratories and dried with 4-Å molecular
1
concentrated dichloromethane solutions at –30 °C. H NMR
(
CD
2 2
Cl , 500 MHz): δ = 7.55 (s, 2 H, thiazole CH); 1.56 [s, 18
1
3
1
H, (CH
3
)
3
] ppm. C{ H} NMR (CD
2
Cl
2
, 75.5 MHz): δ = 164.3
(
(
thiazole, C=N); 159.5 (thiazole, C–C); 114.7 (thiazole, C–H); 35.4
1
1
tBu); 29.5 (tBu) ppm. B NMR (CD
2
Cl
2
, 96 MHz): δ = 11.23 (s)
+
ppm. HRMS: expected [M ] 450.9507, observed 450.9510.
20BBr ·C (623.11): calcd. C 40.48, H 4.53; found C
0.86, H 5.03.
sieves ([D
6
]benzene, [D
2
]dichloromethane) and distilled prior to
and distilled prior
C
4
14
H
3
N
2
S
2
7 8
H
use. Methylene chloride was dried with CaH
2
[15].
to use. All other reagents were purchased from Aldrich, or Acros
chemicals and used as received.
Preparation of [(PhBtz)BBr
5% yield from 2 in a manner similar to that described above. H
NMR (CD Cl , 500 MHz): δ = 8.09 (d, J = 7.0 Hz, 4 H, aryl ortho),
.87 (s, 2 H, thiazole CH), 7.55 (t, J = 15.5 Hz, 4 H, aryl meta),
2
][Br] (4): Compound 4 was prepared in
1
8
Physical Measurements: High-resolution ESI mass spectra were re-
corded on a Bruker Daltonics, Inc., Billerica, MA, USA, BioToF
instrument. All analyses were performed on samples that had been
2
2
7
7
1
3
1
2 2
.36 (m, 2 H, aryl para) ppm. C{ H} NMR (CD Cl , 75.5 MHz):
1
13
sealed in glass capillaries under an argon atmosphere. H, C, and
δ = 162.4 (thiazole, C-thiazole), 156.3 (thiazole, C-Ph), 134.0 (Ph),
1
1
B NMR spectra were recorded on DRX 400 and 500 MHz spec-
1
1
1
28.2 (Ph), 127.9 (Ph), 125.5 (Ph), 116.2 (thiazole, CH) ppm.
NMR (CD Cl , 96 MHz): δ = 4.76 (s) ppm. HRMS: expected
M ] 490.8881, observed: 490.8903. 12BBr ·C
B
1
13
trometers. H and C chemical shifts are referenced to residual
solvent signals; 11B chemical shifts are referenced to an external
2
2
+
[
C
18
H
3
N
2
15]
S
2
7
H
8
3 2
standard of BF ·OEt .
[
(
663.09): calcd. C 45.28, H 3.04; found C 45.21, H 3.27.
X-ray Crystallography: For compounds 2, and 3 a crystal of suit-
able quantity was removed from a Schlenk flask under positive
argon pressure, covered immediately with degassed hydrocarbon oil
and mounted on a glass fiber. The X-ray diffraction data were col-
lected at 173 K on a Nonius Kappa CCD diffractometer equipped
with an Oxford Cryostream low-temperature device and a graphite-
Acknowledgments
The authors thank Professor Maurice Brookhart for helpful dis-
cussions. We are grateful to the National Science Foundation
(
NSF) for funding (CHE-0615794).
α
monochromated Mo-K radiation source (λ = 0.71973 Å). Correc-
tions were applied for Lorentz and polarization effects. All struc-
[
13]
tures were solved by direct methods
and refined by full-matrix
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2
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Eur. J. Inorg. Chem. 2010, 5379–5382
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5381

M. Findlater, N. S. Swisher, P. S. White
SHORT COMMUNICATION
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Received: September 3, 2010
Published Online: October 26, 2010
5382
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Eur. J. Inorg. Chem. 2010, 5379–5382