Improved synthetic routes to tungsten(IV) bromide complexes

Article abstract of DOI:10.1007/s11243-015-9954-x

Abstract Convenient synthetic routes to the compounds WBr₄ (1), WBr₄(MeCN)₂ (2), WBr₄(THF)₂ (3), WBr₄(PPh₃)₂ (4), WBr₄(bpy) (6), and WBr₄(dppe) (

Full text of DOI:10.1007/s11243-015-9954-x

Transition Met Chem (2015) 40:613–621
DOI 10.1007/s11243-015-9954-x
Improved synthetic routes to tungsten(IV) bromide complexes
Mohammad A. I. Bhuiyan1 Willie R. Hargrove1 Clyde R. Metz2
•
•
•
Peter S. White3 Shawn C. Sendlinger1
•
Received: 20 May 2015 / Accepted: 1 June 2015 / Published online: 16 June 2015
Ó Springer International Publishing Switzerland 2015
Abstract Convenient synthetic routes to the compounds
WBr4 (1), WBr4(MeCN)2 (2), WBr4(THF)2 (3), WBr4
(PPh3)2 (4), WBr4(bpy) (6), and WBr4(dppe) (7) are
described via the solution-phase oxidation of W(CO)6
using two equivalents of bromine. These one-pot syntheses
use inexpensive and readily available starting materials and
produce analytically pure compounds with high yields
under mild conditions. Attempts to grow crystals of 1 by
heating in a sealed tube at *200 °C resulted in formation
of the previously reported compound WOBr4. Attempts to
recrystallize 4 from dichloromethane solution produced
[HPPh3]2[WBr6] (5). X-ray crystallographic studies
showed that 5 consists of an array of [WBr6]2- anions and
[HPPh3]? cations and that 7ÁCH2Cl2 has the expected six-
coordinate tungsten center. The synthesis of tungsten(IV)
bromide compounds via oxidation of W(CO)6 is simple and
provides better yields than previously reported methods.
This synthetic route also has many advantages over the
syntheses of similar tungsten(IV) chloride compounds
which involve reduction of WCl6.
Introduction
Synthetic approaches to complexes that contain tungsten in
the (II), (III), or (IV) oxidation states typically involve
initial reduction of WCl6 to WCl4 by various methods [1].
Further reduction of WCl4 to tungsten(II), in the form of
the cluster compound (W6Cl8)Cl4, is possible via solid-
state disproportionation of WCl4 [2] or via direct reduction
of WCl6 using Al [3, 4]. The majority of these reductions
involve tube furnace reactions where high pressures and
temperatures are used. Many require a two-zone furnace
with careful temperature control, and irreproducibility has
been a problem. To avoid such difficulties, a solution-phase
route using tin as the reducing agent has been developed,
which provides WCl4 in high yield. However, the large-
scale version of this route involves 3 days of reaction time
with repeated washing/centrifugation steps to remove the
SnCl4 by-product [1]. Early syntheses of tungsten(III)
chloride complexes, in the form of the anionic cluster
[W3Cl9]3-, used WO3 as the starting material [5].
Tungsten bromide complexes have received less atten-
tion, partly due to the limited commercial availability of
tungsten(V) or (VI) bromides as compared with readily
found WCl6. Preparations of WBr6 have been reported
through direct treatment of W(CO)6 with liquid bromine
[6–8] or via solid-state halide exchange reactions using
WCl6 and BBr3 [9, 10]. Preparation of WBr5 proceeds from
direct oxidation of tungsten metal using Br2 at high tem-
peratures (*600 °C or higher) with subsequent isolation of
the WBr5 through repeated sublimation steps from a mix-
ture that also contains WBr6 [8, 11–13]. A more recent
report indicates that heating of this mixed product under
vacuum can yield pure WBr5 [14]. A low-yield (*10 %)
synthesis of WBr4 has been reported from the high-tem-
perature reduction of WBr5 using W [15] or Al [2] in a
& Shawn C. Sendlinger
ssendlin@nccu.edu
1
Department of Chemistry, North Carolina Central University,
1801 Fayetteville Street, Durham, NC 27707, USA
2
Department of Chemistry and Biochemistry, College of
Charleston, 66 George Street, Charleston, SC 29424, USA
3
Department of Chemistry, University of North Carolina at
Chapel Hill, 131 South Road, Chapel Hill, NC 27599, USA
123

614
Transition Met Chem (2015) 40:613–621
uniform temperature gradient. McCarley and Brown [15]
have reported the synthesis of WBr3 using WBr2 and Br2
heated in a sealed tube at 50 °C for 2 weeks. When the
WBr3 was heated to higher temperature, it decomposed to
form WBr2 and Br2. Another preparation of WBr2 involves
disproportionation of WBr4 in sealed tubes in a furnace
with a temperature gradient [2]. As with the tungsten
chloride compounds, the tungsten bromide syntheses
involve sealed tubes where high pressures and tempera-
tures, as well as careful temperature control, are necessary.
Our previous work with molybdenum(IV) bromide as a
precursor to higher-oxidation-state molybdenum nitrido
compounds [16] led us to investigate the solution-phase
oxidation of W(CO)6 using bromine. We report here
improved synthetic routes and characterization of the
known compounds WBr4 (1), WBr4(MeCN)2 (2), WBr4
(THF)2 (3), WBr4(PPh3)2 (4), WBr4(bpy) (6), and WBr4
(dppe) (7). Although mentioned in the literature, several of
these compounds (3, 4, and 7) have never been sufficiently
characterized. Structural characterization of the tung-
sten(IV) compound WBr4(dppe)ÁCH2Cl2 (7ÁCH2Cl2) is also
discussed. The formation and structural characterization of
the tungsten(IV) compound [HPPh3]2[WBr6] (5) are also
addressed. The synthetic methods reported avoid the dif-
ficulties associated with previous approaches and provide
analytically pure compounds with high yields under mild
conditions using readily available starting materials.
Services. Infrared spectra were obtained using Nujol mulls
or KBr pellets on a Shimadzu Advantage FTIR-8900 or on
1
a Perkin-Elmer 1600 FTIR spectrophotometer. H-NMR
data were collected on a Varian Unity INOVA instrument
operating at 299.950 MHz. Chemical shifts were refer-
enced to the residual protons of the solvent. Magnetic
susceptibilities were measured with a Johnson-Matthey
MS-1 Magnetic Susceptibility Balance.
Synthesis
Tungsten(IV) bromide, WBr4 (1)
Tungsten hexacarbonyl (1.00 g, 2.84 mmol) was dried under
vacuum for *45 min. Freshly distilled dichloromethane
(40 mL) was added, and the mixture was placed in a cold
bath at -18 °C (orthodichlorobenzene/liquid nitrogen bath).
After ca. 10 min, Br2 (0.28 mL, 5.70 mmol) was added
dropwise via syringe over a period of 2 min. As the Br2
reacted, bubbles of gas (CO) were seen, and the suspension
turned black (Warning: This reaction should be performed in
an efficient fume hood). After stirring the mixture for several
minutes at -18 °C, the cold bath was removed, and a black
solid formed at room temperature. After stirring for 3 h, the
black solid was isolated by filtration, washed with
3 9 10 mL portions of CH2Cl2, and was thoroughly dried
under vacuum. Yield 1.35 g (94 % based on W). Decompo-
sition point: ca. 190 °C (see text). IR (KBr pellet): No peaks
observed from 4000 to 450 cm-1. Anal. Calcd. for WBr4: W,
36.52; Br, 63.48. Found: W, 36.50; Br, 63.43.
Experimental
Materials and methods
Tungsten bis(acetonitrile) tetrabromide, WBr4(CH3CN)2
(2)
Compounds were manipulated in an Innovative Technology
System One glovebox under prepurified N2 or through use
of standard Schlenk techniques. Dichloromethane, 1,2-
dichloroethane, and acetonitrile solvents were distilled from
CaH2 under nitrogen. Diethyl ether and tetrahydrofuran
were distilled from purple Na/benzophenone ketyl under
nitrogen. Deuterated chloroform was dried with CaH2 and
was vacuum-transferred to an airtight vessel containing
activated molecular sieves. Tungsten hexacarbonyl, triph-
enylphosphine, 1,2-bis(diphenylphosphino)ethane (dppe),
and 2,20-bipyridine (bpy) were purchased from Aldrich
Chemical Company and were dried under vacuum before
transfer to the glovebox. Bromine was purchased from
Aldrich and was distilled from CaO prior to use.
To a flask containing WBr4 (1) (4.55 g, 9.04 mmol) was
added ca. 70 mL of freshly distilled acetonitrile. A dark red
suspension formed instantly in an exothermic reaction. The
mixture was degassed, placed under a nitrogen atmosphere,
and was allowed to stir for 24 h. A dark reddish-brown
solid was isolated by filtration and was dried under vac-
uum. Yield 3.40 g (64 % based on W). Decomposition
point: 145–148 °C. IR (Nujol mull, cm-1): 2307(m),
2299(m), 2269(s), 1352(m), 1260(w), 1015(m), 802(b), and
662(w). Anal. Calcd. for C4H6N2Br4W: C, 8.20; H, 1.03;
N, 4.78; Br, 54.59. Found: C, 8.13; H, 1.02; N, 4.61; Br,
54.98.
Tungsten tetrabromide bis(tetrahydrofuran), WBr4(THF)2
Physical measurements
(3)
Decomposition points were measured by a Mel-Temp II
apparatus in sealed glass tubes and are uncorrected. Ele-
mental analyses were performed by Columbia Analytical
To a nitrogen-purged flask containing WBr4(CH3CN)2 (2)
(1.00 g, 1.71 mmol) was added 35 mL of freshly distilled
tetrahydrofuran. The flask was degassed, placed under
123

Transition Met Chem (2015) 40:613–621
615
nitrogen, and the mixture was allowed to stir for 3 h. After
this time, the mixture had turned brownish orange. The
product was isolated by filtration and dried under vacuum.
Yield 0.76 g (69 % based on W). IR (Nujol mull, cm-1):
1400(s), 1350(s), and 1240(s). Decomposition point:
130–132 °C. Anal. Calcd. for C8H16O2Br4W: C, 14.82; H,
2.49; N, 0.00; Br, 49.36. Found: C, 14.68; H, 2.33; N, 0.08,
Br, 49.15.
the crystals were partially dried under vacuum. Recrystal-
lization of WBr4(PPh3)2 from dichloromethane solutions at
-10 °C over a period of several weeks also yielded
orange-red crystals of 5, contaminated with deeper red
crystals of WBr4(PPh3)2 (4). 1H-NMR (CDCl3): d 11.4
(broad, 12H), 8.5 (broad, 12H), 8.0 (broad, 6H).
Tungsten tetrabromide (2,20-bipyridine), WBr4(bpy) (6)
Tungsten tetrabromide bis(triphenylphosphine),
Method A A 100 mL Schlenk flask was charged with WBr4 (1)
(1.00 g, 1.99 mmol), and *35 mL freshly distilled 1,2-
dichloroethane was added to form a black suspension. A sep-
arate solution of 2,20-bipyridine (0.34 g, 2.18 mmol) in 35 mL
of freshly distilled 1,2-dichloroethane was added to the flask
containing the WBr4. The black suspension turned brown. The
reaction mixture was allowed to stir for 24 h. The brown
product was isolated by filtration, washed with 3 9 10 mL
portions of 1,2-dichloroethane, and was extensively dried under
vacuum. Yield 0.806 g (61 % based on W).
WBr4(PPh3)2 (4)
Method A To a nitrogen-purged flask containing WBr4 (1)
(0.500 g, 0.993 mmol) and PPh3 (2.08 g, 7.94 mmol) was
added ca. 40 mL of freshly distilled 1,2-dichloroethane at
room temperature. An orange-red suspension formed
immediately, and the mixture was allowed to stir for sev-
eral hours under nitrogen. A dark orange-red solid was
isolated by filtration, washed with 3 9 10 mL portions of
1,2-dichloroethane, and dried under vacuum. Yield 0.555 g
(54 % based on W).
Method B To a stirred suspension of W(CO)6 (1.00 g,
2.84 mmol) in ca. 30 mL of CH2Cl2 at -18 °C was added
Br2 (0.29 mL, 5.66 mmol) dropwise with a syringe over a
period of 2 min. Gas evolution (CO) was observed as a
black suspension formed (Warning: This reaction should
be performed in an efficient fume hood). After 10 min, the
cold bath was removed and the reaction was allowed to stir
at room temperature for 7 h. At this point, 2,20-bipyridine
(0.598 g, 3.83 mmol) was added to the mixture using a
solid addition funnel. The black suspension turned brown.
The reaction mixture was allowed to stir for 24 h. The
product was isolated by filtration, washed with 3 9 10 mL
portions of CH2Cl2, and extensively dried under vacuum.
Yield 1.67 g (89 % based on W). Decomposition point:
330–334 °C. IR (KBr pellet, cm-1): 1601(s), 1442(m),
1315(s), 969(s), 765(s), 668(w), 647(w), 519(w), 503(w),
494(w), and 473(w). Magnetic susceptibility: Corrected
molar susceptibility, 1.16 9 10-3 emu, effective magnetic
moment, 1.65 lB. Anal. Calcd. for C10H8N2Br4W: C,
18.19; H, 1.22; N, 4.25. Found: C, 17.87; H, 1.19; N, 4.08.
Method B To a stirred suspension of W(CO)6 (1.01 g,
2.87 mmol) in 30 mL CH2Cl2 at -18 °C (or-
thodichlorobenzene/liquid nitrogen bath) was added Br2
(0.29 mL, 5.66 mmol) dropwise via syringe over a period
of two minutes. Evolution of CO gas was evident as a black
suspension formed. (Warning: This reaction should be
performed in an efficient fume hood). The reaction mixture
was allowed to stir for 1 h at room temperature, and then,
PPh3 (1.64 g, 6.26 mmol) was added using a solid addition
funnel. As the PPh3 reacted, the black suspension turned to
an orange-red suspension. The reaction mixture was stirred
for several hours, and the red-orange product was isolated
by filtration, washed with 3 9 15 mL portions of CH2Cl2,
and extensively dried under vacuum. Yield 2.93 g (70 %
based on W). Decomposition point: 152–156 °C. IR (KBr
pellet, cm-1): 3150(w), 2017(b), 1940(b); 1896(b),
1601(m), 1435(s), 1088(s), 997(m), 519(s), 508(s), 492(w),
472(w), and 452(w). Magnetic susceptibility: Corrected
molar susceptibility: 1.11 9 10-3 emu, Magnetic moment,
1.61 lB. 1H-NMR (CDCl3): d 12.4 (broad, 12H), 8.4
(broad, 12H), 7.8 (broad, 6H). Anal. Calcd. for C36H30P2-
Br4W: C, 42.04; H, 2.94; N, 0.00; P, 6.03. Found: C, 42.34;
H, 2.99; N, \0.05; P, 5.64.
Tungsten tetrabromide [bis(diphenylphosphino)ethane],
WBr4(dppe) (7)
Method A A 100 mL Schlenk flask was charged with WBr4
(1) (0.50 g, 0.99 mmol) and 1,2-bis(diphenylphos-
phino)ethane (0.435 g, 1.09 mmol). To the flask was added
80 mL of freshly distilled 1,2-dichloroethane via syringe.
The resulting dark suspension was allowed to stir for 4 h at
room temperature. The reaction mixture was placed in a
freezer (-10 °C) for 16 h after which shiny, golden-brown
crystals were isolated by filtration, washed with 10 mL of
1,2-dichloroethane, and dried under vacuum. Yield 0.479 g
(54 % based on W).
Triphenylphosphonium hexabromotungstate(IV),
[HPPh3]2[WBr6] (5)
Approximately 20 mL of the filtrate from Method A of the
WBr4(PPh3)2 synthesis was placed in a Schlenk tube and
layered with 10 mL of freshly distilled diethyl ether. After
10 days, shiny orange-red crystals had formed at the bot-
tom of the tube. The supernatant liquid was removed, and
123

616
Transition Met Chem (2015) 40:613–621
Method B To a stirred suspension of W(CO)6 (2.02 g,
on the phosphorus atom in compound 5 was located on an
electron density difference map as a Fourier peak appended
to the least-squares refinement output file and was refined
isotropically. Crystal data and selected details of the
structure determinations are given in Table 1.
5.74 mmol) in ca. 40 mL CH2Cl2 at -18 °C was added Br2
(0.58 mL, 11.3 mmol) dropwise via syringe. Gas evolution
(CO) was observed as a black suspension formed (Warn-
ing: This reaction should be performed in an efficient fume
hood). The cold bath was removed after 10 min, and the
suspension was allowed to stir at room temperature for 5 h.
At this point, 1,2-bis(diphenylphosphino)ethane (2.97 g,
7.46 mmol) was added to the mixture with a solid addition
tube. The black suspension turned brown and was allowed
to stir for 16 h at room temperature. Golden-brown crystals
were isolated by filtration, washed with 10 mL CH2Cl2,
and were dried extensively under vacuum. Yield 4.04 g
(78 % based on W). IR (KBr pellet, cm-1): 1262(w),
1097(b), 867(w), 707(w), 688(s), 516(s), 497(m), and
472(w). Decomposition point: 274–278 °C. Magnetic sus-
ceptibility: Corrected molar susceptibility, 1.78 9 10-3
emu, effective magnetic moment 2.04 lB. 1H-NMR
(CD2Cl2): d 16.430 (s, 8H), 14.762 (dd, 8H), 13.791 (t,
4H), -8.880 (s, 4H). Elemental analysis indicated the
unsolvated form. Anal. Calcd. For C26H24P2Br4W: C,
34.61; H, 2.68; N, 0.00; P, 6.87. Found: C, 34.56; H, 2.49;
N, \0.05; P, 6.49.
Computational studies
Calculations were performed using the GAUSSIAN 09 Rev
A.02 programming package [17]. Geometry optimizations
in the gas phase were performed using density functional
theory (DFT) with the B3LYP hybrid functional [18–20].
The LANL2DZ basis set was used.
Results and discussion
Synthesis
Using W(CO)6 as the starting material, the synthesis of
WBr4 (1) involves a stoichiometric oxidation in dichlor-
omethane solvent using Br2, as shown in Scheme 1. Due to
the generation of CO gas, care should be taken to use an
effective fume hood and to cool the reaction mixture before
the bromine is added. The black product is easily isolated
via filtration and was extensively dried under vacuum.
Typical yields were over 90 %. Previous solid-state syn-
theses of WBr4 via reduction of WBr5 using W or Al
involved sealed tubes, high temperatures, and 10-day
reaction times [2, 15].
X-ray data collection and refinement
A
crystal suitable for structure determination of
[HPPh3]2[WBr6] (5) was obtained from the filtrate illus-
trated in Scheme 1 after layering with diethyl ether.
Crystals of WBr4(dppe)ÁCH2Cl2 (7ÁCH2Cl2) were produced
by layering the filtrate obtained from the reaction illus-
trated in Scheme 2 with diethyl ether. Crystallographic
data were obtained on a Bruker SMART diffractometer
using the x-scan mode and graphite-monochromated Mo
Ka radiation. All crystals were mounted in grease on a
glass fiber and were quickly placed in the cold stream
(173 K) of the goniometer. Refinement was carried out
with the full-matrix least-squares method based on F2
(SHELX-97) with anisotropic thermal parameters for all
nonhydrogen atoms. The H atoms were geometrically
Addition of acetonitrile to solid WBr4 under an inert
atmosphere produces WBr4(MeCN)2 (2) as a dark red
suspension in an exothermic reaction, as shown in
Scheme 1. The mixture was filtered to collect the black
product with yields of 65 %. Other syntheses of WBr4(-
MeCN)2 have been reported. Fowles reported that reaction
of WBr5 with excess acetonitrile produced WBr4(MeCN)2.
No yield was given, but the elemental analysis (C, H, and
N) was within acceptable range [12]. McCarley reports that
oxidation of W(CO)6 using bromine in acetonitrile with
subsequent extraction of the product with acetonitrile gave
WBr4(MeCN)2 in yields of 70–80 %. Elemental analysis
data for both W and Br were consistent, but no C, H, and N
data were reported [21]. Marshalsea synthesized this
compound in acetonitrile solvent according to Eq. (1):
˚
placed and treated as riding atoms with C–H = 0.96 A and
U
iso(H) = (1.2) Uiso(parent C atom). The hydrogen atom
Scheme 1 Synthesis and reactivity of WBr4 (1)
Scheme 2 One-pot syntheses of compounds (4), (6), and (7)
123

Transition Met Chem (2015) 40:613–621
617
Table 1 Crystallographic data for 5 and 7ÁCH2Cl2
5
7ÁCH2Cl2
Reference
CCDC 902600
C36H32Br6P2W
CCDC 902599
C27H26Br4Cl2P2W
986.81
Empirical formula
Formula weight
T (K)
1189.87
173
173
Crystal system
Space group
Monoclinic
P21/n
Monoclinic
P21/c
Unit cell dimensions
˚
a (A)
10.2295(3)
13.5330(5)
13.9489(5)
90.00
11.4017(4)
19.6307(6)
14.6629(5)
90.00
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
91.283(1)
90.00
108.143(1)
90.00
3
˚
V (A )
1930.5(1)
2
3118.7(2)
4
Z
l (mm-1
)
9.309
9.119
˚
k (A)
0.71073
1124
0.71073
1864
F (0 0 0)
Crystal size (mm)
Dcalc (g cm-3
0.15 9 0.15 9 0.10
2.047
0.35 9 0.15 9 0.15
2.102
)
Theta range for data collection (°)
Index ranges
2.10–25.15
1.79–25.15
-11 B h B 12, -16 B k B 15, -16 B l B 16 -13 B h B 13, -23 B k B 23, -17 B l B 17
Reflections collected
Independent reflections
Reflections with I [ 2r(I)
Data/restraints/parameters
GOF on F2
18,926
35,937
3460 [(R(int) = 0.0533]
2938
5616 [(R(int) = 0.0420]
5030
3460/0/209
5616/0/325
1.057
1.175
Final R indices [I [ 2r(I)]
R1 = 0.0284
wR2 = 0.0715
R1 = 0.0364
wR2 = 0.0715
0.620 and -1.408
R1 = 0.0271
wR2 = 0.0707
R1 = 0.0321
wR2 = 0.0729
1.244 and -1.060
R indices (all data)
-3
)
˚
Largest difference in peak and hole (e A
WðCOÞ6 Reflux WðCOÞ ðMeCNÞ þ2 Br ; RefluxWBr ðMeCNÞ
McCarley reported [21] the synthesis of WCl4(MeCN)2 via
the reaction shown in Eq. (2):
2
ƒƒ!
ƒƒƒƒƒƒ!
4
3
3
2
ð1Þ
4WCl5 þ WðCOÞ6 þ 10MeCN ! 5WCl4ðMeCNÞ2 þ 6CO
ð2Þ
The yield was 60–70 %, and elemental analysis data (W,
Cl) were consistent. Messerle placed WCl4 (prepared by
reduction of WCl6 with Sn) in MeCN to give WCl4
(MeCN)2 in 81 % yield [1]. Elemental analysis data (W,
Cl) were consistent.
No yield was reported, and the elemental analysis data
were not within the accepted level of 0.4 % [22]. Her-
rmann reports that oxidation of W(CO)6 in acetonitrile
using PBr5 gives the product in 98 % yield with consistent
elemental analysis data [23]. This method gives the best
yield of those reported. The chloro analog of compound
(2), WCl4(MeCN)2, was produced in a reaction analogous
to Eq. (1) using a stream of chlorine gas in place of the
bromine [22]. No yield was reported, and the elemental
analysis results were not within an acceptable range.
An attempted synthesis of WBr4(THF)2 (3) via addition
of tetrahydrofuran to solid WBr4 resulted in the formation
of blue-green products, usually indicative of the presence
123

618
Transition Met Chem (2015) 40:613–621
of tungsten oxo compounds. Alternatively, the acetonitrile
ligands of (2) could be replaced with THF as shown in
Scheme 1. The brown-orange product was isolated via
filtration in ca. 70 % yield. The chloro analog of (3),
WCl4(THF)2, was synthesized by reduction of WCl6 in
CH2Cl2 using cyclopentene, followed by addition of THF.
A low yield (36 %) was obtained [24].
Another bidentate ligand employed was 1,2-
bis(diphenylphosphino)ethane, (dppe). The synthesis of
WBr4(dppe) (7) proceeds in 1,2-dichloroethane as solvent as
shown in Scheme 1. Golden-brown crystals of the product
were isolated by filtration in 53 % yield. This yield could be
increased by employing a one-pot synthesis using dichlor-
omethane as solvent, summarized in Scheme 2. Since CO gas
is generated, care should be taken to use an effective fume
hood and to cool the reaction mixture before the bromine is
added. This method produced analytically pure (7) in 95 %
isolated yield. Lorenz has reported the synthesis of compound
(7) using THF as solvent, as shown in Eq. (6):
The synthesis of WBr4(PPh3)2 (4) proceeds using 1,2-
dichloroethane as solvent, as shown in Scheme 1. The
orange-red product was isolated by filtration in 54 % yield,
but a one-pot synthesis provided better results. The sepa-
rate reactions shown in Scheme 1 can be combined to yield
compound (4) directly using dichloromethane as solvent, as
shown in Scheme 2. Due to the generation of CO gas, care
should be taken to use an effective fume hood and to cool
the reaction mixture before the bromine is added. This
method produced analytically pure (4) in 70 % yield.
Cotton has reported an alternative synthesis of (4) using
dibromomethane as solvent via Zn reduction of WBr5 as
shown in Eq. (3):
0ꢀC
WðdppeÞ2N2 þ 2SOBr2 À! WBr4ðdppeÞ þ 2N2 þ SO
þ dppe
ð6Þ
An 82 % yield is reported, but the elemental analysis
results are outside acceptable limits [28]. A report on the
synthesis of the chloro version of compound (7), WCl4
(dppe), involved reaction of K2WCl6 with fused dppe at
220 °C [29]. However, the elemental analysis of the com-
pound was not within acceptable range. An alternative
synthesis [30] is shown, using toluene as solvent, in Eq. (7):
Zn
WBr5 þ excess PPh3 À! WBr4ðPPh3Þ2
ð3Þ
The reported yield was 61 %, but no characterization
data of the product were included [25]. The chloro version
of compound (4), WCl4(PPh3)2, has been reported and is
synthesized in dichloromethane via the reduction shown in
Eq. (4):
70ꢀC
WCl4ðPPh3Þ2 þ dppe À! WCl4ðdppeÞ þ 2PPh3
ð7Þ
The reported yield was 89 %, but this route requires the
additional step of preparing WCl4(PPh3)2, which in turn
involves reduction of WCl6 as shown in Eq. (4).
Zn
WCl6 þ 2PPh3 À! WCl4ðPPh3Þ2 þ ZnCl2
ð4Þ
The product is isolated in low yield (34 %), and sepa-
ration from the unused Zn can be problematic [26, 30].
Bidentate ligands can also be used. Synthesis of
WBr4(bpy) (6) proceeds in 1,2-dichloroethane as shown in
Scheme 1. The dark-brown product was filtered from the
reaction mixture in 63 % yield. A one-pot synthesis using
dichloromethane as solvent, as shown in Scheme 2, pro-
vided an increased yield. Since CO gas is generated, care
should be taken to use an effective fume hood and to cool
the reaction mixture before the bromine is added. This
method produced analytically pure (6) in 98 % yield.
Stiddard has reported the synthesis of compound (6) via the
oxidation of W(CO)4(bpy) in dichloromethane, as shown in
Eq. (5):
Compound characterization
Tungsten(IV) bromide (1) proved to be quite sensitive to
hydrolysis and required an inert atmosphere for manipu-
lation. Previous work indicates that WBr4 is isomorphous
with WCl4 [2]. Messerle has shown that the WCl4 structure
consists of polymeric chains of edge sharing, distorted
˚
bioctahedra with alternating short (2.688 A) and long
˚
(3.787 A) distances between the tungsten centers [1]. More
recent work has confirmed a similar structure for WBr4
˚
˚
with alternating short (2.794 A) and long (4.044 A) W/W
distances [8]. The black WBr4 solid displayed slight solu-
bility in dichloromethane and 1,2-dichloroethane.
An attempt to determine the decomposition temperature
of solid WBr4 in a sealed capillary tube led to observed
sublimation and deposition of black, shiny crystals in the
cooler portion of the tube. It was initially thought that WBr4
might be decomposing to form a lower–oxidation-state
bromide, as is observed with MoBr4 [31–33]. Structural
determination of the deposited crystals by X-ray diffraction
revealed the known compound, WOBr4, and further
refinement of the data was not pursued. Adventitious
oxygen apparently reacted with WBr4 to form the
WðCOÞ ðbpyÞ þ2Br ; D WBr ðbpyÞ þ 4CO
ð5Þ
2
ƒƒƒƒ!
4
4
A 68 % yield was reported along with acceptable
element analysis results [27]. Stiddard also reported syn-
thesis of the chloro version, WCl4(bpy), that was pro-
duced by replacing the Br2 shown in Eq. (5) with a slow
stream of Cl2 gas. An 83 % yield was reported, but the
elemental analysis results were not within acceptable
range [27].
123

Transition Met Chem (2015) 40:613–621
619
Fig. 1 ORTEP representation
of [HPPh3]2[WBr6] (5) showing
the atom numbering scheme
˚
Table 2 Bond distances (A) and angles (°) for compound 5
W(1)–Br(1)
W(1)–Br(2)
W(1)–Br(3)
P(1)–H(1)
2.538(1)
2.534(1)
2.532(1)
1.341(4)
Br(2)–W(1)–Br(3)
Br(2)–W(1)–Br(4)
Br(3)–W(1)–Br(4)
C(1)–P(1)–H(1)
89.81(2)
90.47(2)
89.16(2)
105.3(4)
product. WOBr4 has been previously synthesized by reac-
tion of WBr5 with SO2 [34] or by a solid-state synthesis
using WBr6 and WO3 [7]. The structure of WOBr4 has been
shown to consist of square pyramidal WOBr4 units associ-
ated via the O atoms to form linear W/O chains [35].
The compound WBr4(MeCN)2 (2) was sparingly soluble
in dichloromethane or chloroform and was more soluble in
polar solvents with which it can undergo ligand exchange
(THF, DMSO, Pyridine). The infrared spectrum revealed
the expected m(CN) frequency near 2300 cm-1 for the
coordinated acetonitrile. Earlier work with low-tempera-
ture far-infrared spectra identified both WCl4(MeCN)2 and
WBr4(MeCN)2 as the cis isomers, based on the number of
tungsten halide stretching modes [17].
The compound WBr4(THF)2 (3) had similar solubility
characteristics to compound 2. The infrared spectrum was
typical of the coordinated tetrahydrofuran ligand. This
compound has not been previously reported in the
literature.
Fig. 2 ORTEP representation of WBr4(dppe), 7ÁCH2Cl2 showing the
atom numbering scheme. Hydrogen atoms have been omitted for
clarity
The compound WBr4(PPh3)2 (4) was soluble in both
dichloromethane and 1,2-dichloroethane. The infrared
spectrum was consistent with bound triphenylphosphine.
Previous work identified WCl4(PPh3)2 as the trans isomer,
based on single-crystal X-ray diffraction [37]. Compound 4
exhibited broadened and paramagnetically shifted peaks in
the 1H-NMR spectrum. Attempts to recrystallize 4 by
redissolving in dichloromethane or by layering of the
reaction filtrate eventually led to isolation of crystals of the
tungsten(IV) compound [HPPh3]2[WBr6] (5), the structure
of which is shown in Fig. 1. Bond distances and angles are
given in Table 2. Compound 5 crystallizes in the
123

620
Transition Met Chem (2015) 40:613–621
˚
paramagnetically shifted peaks that retained the expected
splitting pattern. Crystals of 7ÁCH2Cl2 were grown from the
reaction mixture. Compound 7 crystallizes in space group
P21/c, and the structure is shown in Fig. 2. The structure
revealed a distorted octahedral tungsten center, as well as a
noninteracting molecule of dichloromethane solvent. Bond
distances and angles are given in Table 3. The two trans Br
atoms [Br(3) and Br(4)] appear to be bent toward the cleft
formed by the two phenyl rings of the chelating phosphine
ligand. The Br(3)–W(1)–Br(4) angle is 161.98(2)°, while the
Br(1)–W(1)–Br(2) angle is 92.15(2)°. The average W–Br
Table 3 Bond distances (A) and angles (°) for compound 7ÁCH2Cl2
W(1)–Br(1)
W(1)–Br(2)
W(1)–Br(3)
W(1)–Br(4)
W(1)–P(1)
W(1)–P(2)
2.512(1)
2.509(1)
2.451(1)
2.493(1)
2.547(1)
2.561(1)
Br(1)–W(1)–Br(2)
Br(1)–W(1)–Br(3)
Br(1)–W(1)–Br(4)
Br(2)–W(1)–Br(3)
Br(2)–W(1)–Br(4)
Br(3)–W(1)–Br(4)
P(1)–W(1)–P(2)
P(1)–W(1)–Br(2)
P(2)–W(1)–Br(1)
P(1)–W(1)–Br(3)
P(1)–W(1)–Br(4)
92.15(2)
97.01(2)
95.11(2)
96.10(2)
96.71(2)
161.98(2)
81.47(4)
175.66(3)
172.85(3)
82.85(3)
83.42(3)
˚
˚
bond distance is 2.491 A, between the values of 2.445(2) A
˚
from the tungsten(VI) compound (WOBr4) and 2.535(1) A
from the tungsten(IV) compound 5 [HPPh3]2[WBr6].
Magnetic measurements
Table 4 Effective magnetic moment (leff) values, units of Bohr
magnetons
Effective magnetic moments from this and previous work
are presented in Table 4. All values fall within the range
(1.5–2.2 lB) previously reported for a variety of tung-
sten(IV) compounds [21].
Compound
X = Br
X = Cl
WX4(MeCN)2
WX4(bpy)
1.89a
1.78a
1.64c
1.72d
1.74e
1.65b; 1.69c
1.61b
2.04b
d
WX4(PPh3)2
WX4(dppe)
a
b
c
e
Conclusion
Ref. [12], this work, ref. [27], ref. [21], ref. [29]
In summary, the solution-phase oxidation of W(CO)6 using
bromine is a simple, high yield route to a number of
tungsten(IV) bromide compounds. One-pot syntheses have
been described that avoid several difficulties involved with
the preparation of similar tungsten(IV) chloride compounds
that involve reduction of WCl6. Our approach is also
superior to previously reported syntheses of tungsten bro-
mide compounds that involve sealed tubes, high tempera-
tures, and the requirement of careful temperature control.
The methods described here are easily modified to allow
incorporation of other ligands. We are currently investi-
gating the electrochemistry and reactivity of various
tungsten(IV) bromide compounds and will report our
findings in due course.
monoclinic space group P21/n and consists of two triph-
enylphosphonium cations and the octahedral [WBr6]2-
dianion. It was initially thought the compound was
cocrystallized, neutral molecules of triphenylphosphine
and tungsten(VI) hexabromide, WBr6. Further analysis
1
revealed paramagnetically shifted peaks in the H-NMR
spectrum, and a hydrogen atom was located on the phos-
phorus atom via an electron density difference map. Also,
˚
the average W(IV)–Br length of 2.535(1) A in 5 is sig-
˚
nificantly longer (0.081 A) than the reported W(VI)–Br
˚
length in WBr6 of 2.454(3) A [38]. Calculated W–Br bond
distances in the gas phase at the B3LYP/LANL2DZ level
˚
of theory show a difference of 0.115 A between
[W(IV)Br6]2- (average W–Br of 2.656 A) and [W(VI)Br6]
˚
˚
(W–Br of 2.541 A). The structure of 5 shows the closest
Supplementary data
approach between cation and anion (along BrÁÁÁH–P) is
˚
over 2.8 A.
CCDC 902599 and 902600 contain the supplementary crys-
tallographic datafor5 and7ÁCH2Cl2. Thedatacanbeobtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev
ing.html, or from the Cambridge Crystallographic Data
Centre, 12Union Road, Cambridge CB2 1EZ, UK; fax: (?44)
1223 336 033; or email: deposit@ccdc.cam.ac.uk.
Bidentate ligands can also be attached to the WBr4
moiety. The compound WBr4(bpy) (6) was insoluble in
dichloromethane, but dissolved in acetonitrile. The infrared
spectrum of 6 revealed the expected peaks due to the
coordinated 2,20-bipyridine ligand.
The compound WBr4(dppe) (7) was soluble in both
dichloromethane and 1,2-dichloroethane. The infrared
spectrum displayed peaks due to the 1,2-bis(diphenylphos-
phino)ethane ligand. The 1H-NMR spectrum had sharp,
Acknowledgments W.R.H. acknowledges the Partnership for
Minority Access to Doctoral Degrees program of the National Insti-
tutes of Health for financial support. This work was supported by the
123

Transition Met Chem (2015) 40:613–621
621
Donors of the Petroleum Research Fund administered by the Amer-
ican Chemical Society, the Research Corporation (Cottrell College
Science Award to S.C.S.), and the North Carolina Space Grant
Consortium. The Varian Unity INOVA NMR spectrometer was pur-
chased with funds provided by the US Office of Naval Research.
JA, Jr., Peralta JE, Ogliarov F, Bearpark M, Heyd JJ, Brothers E,
Kudin KN, Staroverov VN, Kobayashi R, Normand J, Ragha-
vachari K, Rendell A, Burant JC, Iyengar SS, Tomasi J, Cossi M,
Rega N, Millam JM, Klene M, Knox JE, Cross JB, Bakken V,
Adamo C, Jaramillo J, Gomperts R, Stratmann RE, Yazyev O,
Austin AJ, Cammi R, Pomelli C, Ochterski JW, Martin RL,
Morokuma K, Zakrzewski VG, Voth GA, Salvador P, Dannen-
berg JJ, Dapprich S, Daniels AD, Farkas O, Foresman JB, Ortiz
JV, Cioslowski J, Fox DJ (2009) Gaussian, Inc., Wallingford
18. Becke AD (1988) Phys Rev A 38:3098–3100
References
1. Kolesnichenko V, Swenson DC, Messerle L (1998) Inorg Chem
37:3257–3262
19. Becke AD (1993) J Chem Phys 98:5648–5652
20. Schmider HL, Becke AD (1998) J Chem Phys 108:9624–9631
21. Schaefer King MA, McCarley RE (1973) Inorg Chem
12:1972–1979
22. Edwards DA, Marshalsea J (1975) Syn React Inorg Met Org
Chem 5(2):139–149
23. Scheer M, Nam TT, Herrmann E, Fedin VP, Ikorski VN, Fedorov
VE (1990) Z Anorg Allg Chem 589:214–220
24. Persson C, Andersson C (1993) Inorg Chim Acta 203:235–238
25. Cotton FA, Eglin JL, James GA (1993) Inorg Chem 32:681–686
26. Butcher AV, Charr J, Leigh GJ, Richards RL (1972) J Chem Soc
Dalton Trans 10:1064–1069
2. McCarley RE, Brown TM (1964) Inorg Chem 3:1232–1236
3. Hogue RD, McCarley RE (1970) Inorg Chem 9:1354–1360
4. Mussell RD, Nocera DG (1990) Inorg Chem 29:3711–3717
5. Salliant R, Hayden JL, Wentworth RAD (1967) Inorg Chem
6:1497–1501
6. Shchukarev SA, Kokovin GA (1964) Zhur Neorg Khim
9(6):1309–1315
7. Crouch PC, Fowles GWA, Gerald WA, Walton RA (1970) J
Inorg Nucl Chem 32(1):329–333
¨
8. Strobele M, Meyer H-J (2012) Russ
38(3):178–182
J
Coord Chem
27. Hull CG, Stiddard MHB (1966) J Chem Soc (A) 11:1633–1635
¨
28. Stoll C, Lorenz I-P, Noth H, Ponikwar W (2000) J Organomet
Chem 602:24–28
9. Druce PM, Lappert MF, Riley PNK (1967) Chem Comm
10:486–487
10. Druce PM, Lappert MF (1971) J Chem Soc (A) 22:3595–3599
11. Shchukarev SA, Kokovin GA (1964) Russ J Inorg Chem
9(6):715–718
12. Allen EA, Brisdon BJ, Fowles GWA (1964) J Chem Soc
11:4531–4534
29. Boorman PM, Greenwood NN, Hildon MA (1968) J Chem Soc
(A) 10:2466–2467
30. Dilworth JR, Richards RL (1990) Inorg Synth 28:33–43
31. Cannell PJH, McCarley RE, Hogue RD (1967) Inorg Synth
10:49–54
13. Brown TM, Ruble B (1967) Inorg Chem 6:1335–1338
14. Brisdon AK, Hope EG, Levason W, Ogden JS (1989) J Chem Soc
Dalton Trans 2:313–316
15. McCarley RE, Brown TM (1962) J Am Chem Soc 84:3216
16. Holmes M, Sendlinger SC (1999) Syn React Inorg Met Org Chem
29(1):143–153
17. Gaussian 09, Revision A.02, Frisch MJ, Trucks GW, Schlegel
HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone
V, Mennucci B, Petersson GA, Nakatsuji H, Caricato M, Li X,
Hratchian HP, Izmaylov AF, Bloino J, Zheng G, Sonnenberg JL,
Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M,
Nakajima T, Honda Y, Kitao O, Nakai H, Vreven T, Montgomery
32. Vasilkova IV, Efimov AI, Pitrimov BZ (1964) Khim Redk Elem
47:44–50
33. Vasil’kova IV, Efimov AI, Pitrimov BZ (1975) Chem Abstr
83:171891j
34. Fowles GWA, Frost JL (1967) J Chem Soc (A) 4:671–675
35. Mu¨ller U (1984) Acta Cryst C40:915–917
36. Shiskin NY, Zharskill IM, Novikov GI (1981) J Mol Struct
73:249–252
37. Drew MGB, Page EM, Rice DA (1983) J Chem Soc Dalton Trans
61–63
38. Willing VW, Mu¨ller U (1987) Acta Cryst C43:1425–1426
123