Synthesis and oxidation of d6 tungsten pincer complexes: A complete series of tungsten(ii) hydridocarbonyl and halocarbonyl pincer complexes

Article abstract of DOI:10.1039/c2dt30848g

Reaction of the neutral P^HNP ligand [HN(SiMe₂CH ₂PPh₂)₂] with tungsten hexacarbonyl resulted in coordination of P^HNP through both phosphorus donor atoms to form the tungsten complex [W(P^HNP)(CO)₄] (1). Reaction of P ^HNP with tris(acetonitrile)tricarbonyl tungsten gave both facial and meridional tridentate isomers [W(P^HNP)(CO)₃] (2-fac and 3-mer). These three d⁶ tungsten complexes could be interconverted under appropriate conditions. The thermodynamically favored isomer 3 was protonated to form seven-coordinate [W(P^HNP)(CO)₃H] [BF₄] (4). A related series of cationic tungsten(ii) halide complexes was synthesized, [W(P^HNP)(CO)₃X]⁺ (6, X = I; 7, X = Br; 8, X = Cl; 9, X = F), by various routes. All of the tungsten(ii) complexes underwent deprotonation at the amine site of the P^HNP ligand when triethylamine was added, resulting in neutral seven-coordinate complexes. Variable temperature ¹H, ³¹P{¹H}, and ¹³C{¹H} NMR spectroscopy showed fluxional behavior for all the seven-coordinate complexes reported here. Analysis of IR and NMR spectroscopic data showed trends through the series of coordinated halides. Crystal structures of tetracarbonyl 1, meridional tricarbonyl 3, and cationic hydride 4 were determined to confirm the coordination mode of the P ^HNP ligand.

Full text of DOI:10.1039/c2dt30848g

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Synthesis and Oxidation of d6 Tungsten Pincer Complexes:
A Complete Series of Tungsten(II) Hydridocarbonyl
and Halocarbonyl Pincer Complexes†
Leah A. Wingard, Peter S. White, and Joseph L. Templeton*
W. R. Kenan Laboratory, Department of Chemistry, University of North Carolina,
Chapel Hill, North Carolina 27599ꢀ3290
†
Dedicated to Professor David ColeꢀHamilton on the occasion of his retirement and for his
outstanding contribution to transition metal catalysis.
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Abstract
Reaction of the neutral PHNP ligand [HN(SiMe2CH2PPh2)2] with tungsten
hexacarbonyl resulted in coordination of PHNP through both phosphorus donor atoms to
form the tungsten complex [W(PHNP)(CO)4] (1). Reaction of PHNP with
tris(acetonitrile)tricarbonyl tungsten gave both facial and meridional tridentate isomers
[W(PHNP)(CO)3] (2ꢀfac and 3ꢀmer). These three d6 tungsten complexes could be
interconverted under appropriate conditions. The thermodynamically favored isomer 3
was protonated to form sevenꢀcoordinate [W(PHNP)(CO)3H][BF4] (4). A related series
of cationic tungsten(II) halide complexes was synthesized, [W(PHNP)(CO)3X]+ (6, X = I;
7, X = Br; 8, X = Cl; 9, X = F) by various routes. All of the tungsten(II) complexes
underwent deprotonation at the amine site of the PHNP ligand when triethylamine was
added, resulting in neutral sevenꢀcoordinate complexes. Variable temperature 1H,
31P{1H}, and 13C{1H} NMR spectroscopy showed fluxional behavior for all the sevenꢀ
coordinate complexes reported here. Analysis of IR and NMR spectroscopic data showed
trends through the series of coordinated halides. Crystal structures of tetracarbonyl 1,
meridional tricarbonyl 3, and cationic hydride 4 were determined to confirm the
coordination mode of the PHNP ligand.
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Introduction
Halocarbonyl complexes of tungsten(II) have been studied for over 50 years1ꢀ5.
Oxidation to tungsten(II) is most commonly achieved by addition of elemental halogens
to zeroꢀvalent carbonyl complexes containing at least one nonꢀcarbonyl donor ligand6,7.
Halogenation by iodine8, bromine9, and chlorine10 are common in the literature.
Oxidative decarboxylation with hydrofluoric acid11 or addition of fluoride to a neutral
tungsten (II) dicarbonyl complex12 can provide access to the less common tungsten
fluoride complexes. Despite decades of study, we are only aware of one literature report
that includes the entire series of halocarbonyl tungsten complexes for a single ligand
system11, and the hydridocarbonyl analogs of the halide complexes are seldom
mentioned. Halocarbonyl complexes can be used as catalysts for wideꢀranging reactions
in homogeneous catalysis13,14, and they are also used as catalyst precursors for
heterogeneous polymerization15. Chelating donor ligands anchor the coordination sphere
of many sevenꢀcoordinate d4 complexes with halide ligands11,16ꢀ20
.
Phosphineꢀbased pincer complexes have become popular due to their ability to sit
at the pivot point of stability and reactivity in metal complexes21ꢀ24. Chelating
phosphines are known to increase the basicity of the metal center25ꢀ27, and for tridentate
ligands this donor behavior can also be affected by the bonding geometry adopted by the
chelate. Meridional pincer coordination typically increases basicity at the metal center
compared to the facially coordinated isomer26,28ꢀ30
.
First synthesized in 1981 by Fryzuk, HN(SiMe2CH2PPh2)2 (PHNP) is a neutral
multidentate hybrid pincer ligand31. This silazane PNP ligand has two soft phosphine
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donor sites linked to a hard amine donor nitrogen. The flexible silazane backbone can
bind either as a tridentate meridional or tridentate facial ligand (PHNP) or in a bidentate
κ2ꢀP2 (PHNP) chelation mode31ꢀ34. The amine can be deprotonated to give an anionic
tridentate amido ligand (PNPꢀ). We report here coordination of PHNP to a d6 tungsten
center and subsequent oxidation to form cationic and neutral d4 tungsten(II) hydride and
halide complexes.
Results and Discussion
Tungsten(0) Complexes. Upon refluxing a chlorobenzene solution of equimolar
W(CO)6 and neutral ligand (HN(SiMe2CH2PPh2)2, PHNP) for 24 hours, complex 1 with a
bidentate PHNP ligand was synthesized (Scheme 1). An alternative method for the small
scale preparation of 1 was the reaction of W(CO)6 with HN(SiMe2CH2PPh2)2 in a
methylene chloride solution in a photolysis chamber for one hour. Complex 1 slowly
decomposed in air, but this tungsten (0) complex was stable under inert gas both in the
solid state and in solution. The infrared spectrum showed three absorbances in the
carbonyl region (2016, 1922, and 1885 cmꢀ1), typical for a sawhorse arrangement of the
carbonyl ligands in a tungsten(0) tetracarbonyl complex35.
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Scheme 1. Preparation of a bidentate tungsten(0) complex (1).
Yellow, needleꢀlike crystals of 1 were obtained by slow evaporation at 0 °C. The
molecular structure of 1 determined by singleꢀcrystal Xꢀray diffraction is depicted in
Figure 1. The crystal structure showed a tetracarbonyl complex with a distorted
octahedral geometry where the phosphine groups were cis to one another while the amine
was not coordinated to tungsten (Figure 1). The cis phosphines were related by an obtuse
bond angle of 104.2 º, while the carbonyl groups trans to the phosphines and cis to one
another were squeezed to a decreased bond angle of 81.7 º (C(3)ꢀW(1)ꢀC(7)). The
carbonyls trans to one another were bent off axis as a result of the bulky bidentate ligand,
with a trans angle of 173.3 º (C(1)ꢀW(1)ꢀC(5)). Both WꢀP bonds were 2.54 Å long. The
carbonyl groups trans to the phosphines had shorter WꢀC bonds, at 1.97 and 1.98 Å, than
the carbonyls trans to one another, at 2.02 and 2.04 Å.
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Figure 1. ORTEP plot of 1.
Table 1. Bond lengths and bond angles of 1.
__________________________________________________________
Bond distances (Å)
Bond angles (deg)
W(1)ꢀC(7)
W(1)ꢀC(3)
W(1)ꢀC(1)
W(1)ꢀC(5)
W(1)ꢀP(1)
W(1)ꢀP(2)
1.973(5)
1.980(6)
2.024(6)
2.039(6)
2.5351(14)
2.5431(13)
P(1)ꢀW(1)ꢀP(2)
C(1)ꢀW(1)ꢀC(5)
C(3)ꢀW(1)ꢀC(7)
C(1)ꢀW(1)ꢀP(1)
104.20(4)
173.31(19)
81.7(2)
90.43(16)
__________________________________________________________
The 1H NMR spectrum of 1 showed one sharp singlet for the four silazane methyl
groups, indicating equivalence of the methyl groups on both sides of the eightꢀmembered
ligand metallocycle. A doublet with splitting of 10.4 Hz due to geminal phosphorus
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coupling was observed for the four methylene protons. The amine proton signal for the
NꢀH was not observed. The 31P NMR spectrum showed a single peak at ꢀ0.37 ppm with
one bond tungstenꢀphosphorus coupling of 233 Hz indicating that both phosphines
experience the same magnetic environment. The tungstenꢀphosphorus coupling was
observed as a doublet satellite due to 183W, 14% abundant with I = ½ superimposed over
the dominant singlet. Overall, the 1H and 31P NMR data suggested an effective C2v
molecular symmetry on the NMR timescale as the eightꢀmembered metallocycle rapidly
flips through planarity, thus equilibrating both sides of the ring.
Use of facial tris(acetonitrile)tricarbonyl tungsten as a starting material resulted in
clean metallation of the neutral PHNP ligand. Reaction times of one hour or less resulted
in formation of primarily facial W(PHNP)(CO)3 (2) while longer reaction times resulted
in meridional W(PHNP)(CO)3 (3) (Scheme 2). In solution, facial isomer 2 converted to
the more thermodynamically stable meridional isomer 3 over time. The isomerization
could be catalyzed by addition of a trace of acid (Scheme 3). 3 could also be formed by
photolysis of tetracarbonyl 1, and the reverse reaction to form 1 from 3 could be
accomplished by bubbling carbon monoxide through a solution of 3. Since carbonyl
stretching frequencies are exquisitely sensitive to metalꢀligand bonding, coordination of
the amine to tungsten resulted in an IR spectrum of 2 with lower carbonyl absorbance
frequencies of 1923, 1831, and 1800 cmꢀ1 relative to 1. Isomerization to the meridionally
coordinated PHNP ligand resulted in carbonyl stretching frequencies of 1957, 1847, and
1798 cmꢀ1, with the highest frequency no doubt predominantly due to the trans
carbonyls. The 31P NMR spectrum of 2 showed a singlet at 13.09 ppm with 1JWꢀP = 238
Hz, which was comparable to the 1JWꢀP coupling constant for 1, which also had cisꢀ
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phosphines. The transꢀphosphines in 3 resulted in a downfield shift to 20.4 ppm and the
one bond tungstenꢀphosphorus coupling increased significantly to 311 Hz.
Scheme 2. Synthetic route to tridenate facial W(PHNP)(CO)3 (2)
and tridentate meridional W(PHNP)(CO)3 (3).
Scheme 3. Interconversion of tungsten (0) complexes (1), (2), and (3).
Orange rodꢀlike crystals of 3 were obtained by slow diffusion at ꢀ32 °C. The
crystal structure of 3 showed coordination of the amine to tungsten in a distorted
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octahedral tricarbonyl complex (Figure 2). This locked the ligand in the tridentate
meridional orientation, and the pyramidal amine forced puckering of the bicyclic ring
system. Furthermore, the pyramidal amine group created distinctly different chemical
environments on each side of the ring system, resulting in diastereotopic methyl groups
on each silicon and diastereotopic methylene protons. The trans phosphines span an
angle of 166º, as do the trans carbonyl ligands. The acute PꢀWꢀN bond angles averaged
83.3º. The WꢀP bond lengths had decreased slightly from those in the bidentate complex
1 to 2.46 and 2.47 Å. A weak trans influence from the coordinated amine was seen in the
shortened WꢀC1 bond length of 1.95 Å compared to the other WꢀC bonds of 2.04 Å.
Another approach to examine the trans influence of the amine is to compare the WꢀO
distances where O is the carbonyl oxygen since the carbon is not always reliably located
and the C≡O distance varies only slightly, so the sum of WꢀC and CꢀO distances is a
more reliable xꢀray parameter than either distance alone. Here for the transꢀcarbonyl, the
WꢀO1 distance was 3.11 Å, compared with the cisꢀcarbonyl WꢀO distances of 3.19 and
3.16 Å. This further reflected a weak trans influence by the nitrogen of the meridional
PHNP ligand.
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Figure 2. ORTEP diagram of meridional W(PHNP)(CO)3 (3).
Table 2. Bond lengths and bond angles of 3.
__________________________________________________________
Bond distances (Å)
Bond angles (deg)
W(1)ꢀC(1)
W(1)ꢀC(3)
W(1)ꢀC(2)
W(1)ꢀN(1)
W(1)ꢀP(1)
W(1)ꢀP(2)
1.947(7)
2.009(6)
2.043(6)
2.443(5)
2.4606(16)
2.4731(17)
P(1)ꢀW(1)ꢀP(2)
P(1)ꢀW(1)ꢀN(1)
P(2)ꢀW(1)ꢀN(1)
C(2)ꢀW(1)ꢀC(3)
C(1)ꢀW(1)ꢀC(2)
C(1)ꢀW(1)ꢀC(3)
P(1)ꢀW(1)ꢀC(1)
P(2)ꢀW(1)ꢀC(1)
166.42(6)
83.28(13)
83.53(13)
166.1(3)
82.5(3)
83.8(3)
96.69(19)
96.45(19)
__________________________________________________________
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Tungsten(II) Hydride Complexes. Addition of tetrafluoroboric acid to a
methylene chloride solution of 3 resulted in an immediate color change from orange to
pale yellow consistent with protonation at the tungsten center to formally generate a
tungsten(II) hydride complex (Scheme 4). The IR spectrum showed full conversion to 4
with absorbance frequencies of 2034, 1935, and 1914 cmꢀ1 for the cation, a shift of almost
100 cmꢀ1 to higher energy for the three carbonyl frequencies.
Scheme 4. Preparation of [W(PHNP)(CO)3H][BF4] (4).
Yellow rodꢀshaped crystals of 4 were obtained by slow evaporation of methylene
chloride from a solution of 4 in an NMR tube at 0 °C. The crystal structure showed the
tridentate PHNP ligand bound meridionally with three carbonyl ligands filling the
remaining three sites. The carbonyl trans to nitrogen was pushed toward one of the
phosphine ligands, presumably to accommodate the seventh ligand, a hydride not located
by crystallography. The tetrafluoroborate counterion was crystallized in the cell. All
three tungstenꢀcarbon bonds were slightly lengthened relative to the neutral
W(PHNP)(CO)3 tungstenꢀcarbon distances (3). The two tungstenꢀphosphorus bonds were
also lengthened (by ~0.05 Å from 3), while the tungstenꢀnitrogen bond was shortened
from 2.443 Å in 3 to 2.386 Å in 4. The longer bond lengths around tungsten presumably
reflected expansion of the sixꢀcoordinate geometry to sevenꢀcoordinate and the
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accompanying crowding from seven ligands. The PꢀWꢀP bond angle was decreased by
2° from 3 to 164.42° while one of the PꢀWꢀN bond angles stayed about the same at 83.7°.
The other P(1)ꢀWꢀN bond angle decreased to 81.4°, presumably due to placement of the
hydride ligand near that phosphorus. The most noticeable change in the geometry of the
coordination sphere is that the two PꢀWꢀC(1) angles, nearly identical at 96.5° in 3, differ
by twenty degrees in 4. The C(1)ꢀWꢀP(2) bond angle at 87.3° is much smaller than the
C(1)ꢀWꢀP(1) bond angle at 107.5°, surely opened to accommodate the hydride.
Figure 3. ORTEP diagram of [W(PHNP)(CO)3H][BF4] (4).
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Table 3 Bond lengths and bond angles of 4.
__________________________________________________________
Bond distances (Å)
Bond angles (deg)
W(1)ꢀC(1)
W(1)ꢀC(5)
W(1)ꢀC(7)
W(1)ꢀN(24) 2.386(8)
W(1)ꢀP(2)
W(1)ꢀP(1)
1.965(13)
2.025(9)
2.044(9)
P(1)ꢀW(1)ꢀP(2)
P(1)ꢀW(1)ꢀN(24)
P(2)ꢀW(1)ꢀN(24)
C(5)ꢀW(1)ꢀC(7)
C(1)ꢀW(1)ꢀC(5)
C(1)ꢀW(1)ꢀC(7)
N(24)ꢀW(1)ꢀC(1)
P(1)ꢀW(1)ꢀC(1)
P(2)ꢀW(1)ꢀC(1)
164.42(9)
81.4(2)
83.7(2)
176.8(4)
88.5(7)
88.3(7)
171.0(6)
107.5(6)
87.3(6)
2.505(2)
2.529(2)
__________________________________________________________
Both the crystal structure and the low temperature NMR data showed hydride
complex 4 to be a C1 symmetric molecule. The fluxional behavior of 4 was evident in
variable temperature 1H NMR spectra (Figure 4). At room temperature, the four PHNP
methyl groups appeared as singlets at 0.6 and 0.0 ppm, while the methylene protons
appeared as a broad signal between 2.3 and 2.5 ppm. The hydride resonance appeared
upfield at ꢀ5.03 ppm as a 76 Hz doublet with a broad central hump indicative of a
dynamic process involving coupling to the trans phosphines. Each peak of the doublet
also had tungsten satellites reflecting one bond WꢀH coupling of 16 Hz, a value
comparable to reported one bond tungsten(II)ꢀhydride couplings36. The fluxional
behavior of 4 at room temperature was further evident in the 31P{1H} NMR spectrum, in
which the inequivalent phosphines appeared as a broad doublet of doublets (Figure 5).
When cooled to 242K, the methylene protons resolved into four doublets of
doublets in the region of 2.2 to 2.6 ppm with twoꢀbond coupling to phosphorus of 10.5
Hz (Figure 4). The diastereotopic methylene protons had a geminal HꢀH coupling of 15
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Hz. The PHNP methyls between ꢀ0.1 and 0.6 ppm also resolved from two singlets to four
singlets, indicating a distinct environment for each methyl group in the sevenꢀcoordinate
tungsten coordination sphere. The hydride resonance resolved into a doublet of doublets
while maintaining the same tungsten coupling of 16 Hz. The C1 symmetry differentiated
the two phosphorus atoms resulting in different geminal couplings between the hydride
and the two phosphorus atoms of 53.0 Hz and 20.5 Hz. At 242K, the 31P{1H} NMR
resolved into two roofed doublets at 19.7 and 14.7 ppm (Figure 5). The geminal
phosphorusꢀphosphorus coupling was 85.4 Hz while the oneꢀbond tungstenꢀphosphorus
couplings were 200.2 and 211.6 Hz. This general description of fluxionality on the NMR
timescale applied to all of the cationic and neutral sevenꢀcoordinate tungsten(II)
complexes reported here.
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Figure 4. Variableꢀtemperature 1H NMR spectra of 4, alkyl and hydride regions.
Figure 5. Variableꢀtemperature 31P{1H} NMR spectrum of 4.
Addition of one equivalent of triethylamine to a methylene chloride solution of
the cationic hydride complex 4 at 195 K resulted in selective kinetic deprotonation of the
amine proton to form W(PNP)(CO)3H (Scheme 5). The resulting lone pair on the
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nitrogen atom may have provided some electron density to the tungsten center, but all of
the original ligands remained bound to the 18 electron tungsten in the neutral product.
The carbonyl stretching frequencies dropped an average of 22 cmꢀ1 to 2008, 1923, and
1885 cmꢀ1. When warmed to room temperature, the metal hydride 1H NMR signal
disappeared as the proton was transferred to the amide nitrogen to reform the
thermodynamically favored isomer of 5, W(PHNP)(CO)3 (3), in a formal reduction to
tungsten(0).
Scheme 5. Synthesis of W(PNP)(CO)3H (5).
The low temperature NMR data for 5 showed a C1 symmetric molecule much like
4. The methylene protons shifted upfield and had a comparable geminal coupling to
phosphorus of 14.5 Hz. The hydride resonance shifted slightly downfield about one half
of a ppm from the hydride in 4 to ꢀ4.53 ppm in 5 as a doublet of doublets. The two
geminal couplings between the hydride and phosphorus were slightly smaller than in 4, at
51.0 Hz and 14.5 Hz. The low temperature 31P{1H} NMR showed a roofed doublet of
doublets pattern as in 4. The signals were shifter downfield to 21.7 ppm and 19.3 ppm
with a larger geminal phosphorusꢀphosphorus coupling of 136 Hz. The oneꢀbond
tungstenꢀphosphorus couplings had increased as well to 213 Hz and 230 Hz in 5 from
200 Hz and 212 Hz in 4.
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Tungsten(II) Halide Complexes. A complete series of cationic and neutral
tungsten(II) halide complexes was established. All of the cationic sevenꢀcoordinate
halide complexes were fluxional at room temperature on the NMR timescale and
displayed C1 symmetry when cooled. Given the differing reactivities of the halogens,
different synthetic routes were required to access the full series of complexes. None of
the tungsten(II) halide complexes could be isolated in the solid state or crystallized. They
were thermally unstable and decomposed rapidly upon exposure to air.
To a methylene chloride solution of 3, one equivalent of iodine was added, and
the solution was stirred to dissolve the reagents (Scheme 6). The IR spectrum shows that
the carbonyl frequencies shifted to 2031, 1950, and 1928 cmꢀ1 as the tungsten center was
oxidized to W(II). The increase in CO stretching frequencies from 3 was comparable to
that observed in forming 4 by protonation, as formal oxidation to a cationic tungsten(II)
species occurred in both cases. Complex 6 decomposed over time, possibly passing
through a dicarbonyl intermediate (Scheme 7). The nucleophilic iodide counterion may
have participated in the decomposition reaction. By IR spectroscopy, CO stretches at
1931 and 1838 cmꢀ1 grew in as the carbonyl frequencies for 6 faded away. Perhaps the
iodide counterion displaced a carbonyl ligand and formed a neutral W(PHNP)(CO)2(I)2
intermediate. Further decomposition occurred rapidly, so identification of the
intermediate was not possible. Addition of excess sodium tetraphenylborate to complex
6 resulted in precipitation of sodium iodide and a longerꢀlived tungsten(II) cationic
complex [W(PHNP)(CO)3I][B(Ph)4].
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Scheme 6. Preparation of [W(PHNP)(CO)3I][I] (6).
Scheme 7. Proposed decomposition pathway of 6.
[W(PHNP)(CO)3Br][Br] could be synthesized by addition of bromine to 3,
analogous to the synthesis of 6. However, elemental bromine could oxidize a phosphine
ligand, and the nucleophilic bromide counterion may have promoted rapid
decomposition. Surprisingly, a cleaner twoꢀstep synthetic method was available that
began with a oneꢀelectron oxidation to tungsten(I) followed by addition of a bromine
radical source to give the same cationic bromide complex 7. To a methylene chloride
solution of 3 cooled to 195K, excess silver triflate was added, and the solution was stirred
for one hour (Scheme 8). The solution was cannula filtered, and one equivalent of Nꢀ
bromosuccinimide was added to the filtrate and stirred for 30 minutes at 195K. The pink
tungsten(I) intermediate, [W(PHNP)(CO)3][OTf], was characterized by IR spectroscopy,
but this paramagnetic intermediate could not be analyzed by NMR. The carbonyl
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frequencies moved higher (2025, 1925, 1899 cmꢀ1) as the cationic tungsten(I)
intermediate was formed, then they shifted even higher (2033, 1956, and 1928 cmꢀ1)
when the bromine radical was added to form 7. The net increase in CO frequencies from
3 to 7 was comparable to those observed for the other cationic tungsten(II) complexes
described above.
Scheme 8. Synthetic route to [W(PHNP)(CO)3Br][OTf] (7).
To form the chloride analogue, excess tris(4ꢀbromophenyl)ammoniumyl
hexachloroantimonate ([N(PhBr)3][SbCl6]) was added to a methylene chloride solution of
3 cooled to 195K and the reaction mixture was stirred for one hour. The solution was
then cannula filtered. Since the tris(4ꢀbromophenyl)ammoniumyl hexachloroantimonate
is known to function as a radical oneꢀelectron oxidant37,38, it would presumably initially
oxidize the tungsten(0) complex 3 to a cationic tungsten(I) intermediate (Scheme 9). The
hexachloroantimonate counterion could then provide a chlorine radical to convert the
tungsten(I) to tungsten(II) and form 8, similar to the reaction scheme outlined for the
bromo analogue 7.
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Scheme 9. Synthetic route to [W(PHNP)(CO)3Cl][Cl] (8).
Addition of excess Selectfluor (1ꢀchloromethylꢀ4ꢀfluoroꢀ1,4ꢀ
diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) to an acetonitrile solution of 3
resulted in fluorination of the tungsten center to produce 9 (Scheme 10). The reaction
proceeded more slowly in methylene chloride, perhaps due to the lower solvent polarity.
Selectfluor can serve as a net source of F+ as a reagent, so in some ways this reaction is
analogous to addition of a proton to 3 to form the cationic hydride complex 4. The IR
spectrum showed conversion of 3 to fluoride complex 9 over one hour with an increase in
absorbance frequencies to 2024, 1939, and 1846 cmꢀ1. Although both protonation and
fluorination occurred in this system, addition of methyl triflate to 3 did not lead to
methylation.
-
2BF4
Cl
BF4
PPh2
CO
CO
N
PPh2
CO
W
CO
Si
Si
N
W
HN
HN
F
OC
Si
OC
F
PPh2
Si
PPh2
NCCH3
3
9
Scheme 10. Synthetic route to [WPHNP)(CO)3F][BF4] (9).
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All four of the cationic tungsten(II) halide complexes described here (6ꢀ9) could
be deprotonated at the pincer ligand amine nitrogen by addition of triethylamine (Scheme
11).
Scheme 11. Synthetic route to neutral W(PNP)(CO)3X, where
X = I (6a), Br(7a), Cl (8a), and F (9a).
The IR spectra were consistent throughout the series of halide and hydride
tungsten complexes and provided a scheme for identifying the oxidation state and charge
of the tungsten pincer complex in question (Table 4). The highest carbonyl frequencies
were indicative of cationic tungsten(II) complexes, including 4, 6, 7, 8, and 9. These
cationic complexes had a weak but distinctive carbonyl stretch around 2030 cmꢀ1. When
deprotonated to form the neutral tungsten(II) complex, that weak symmetric stretch
shifted down about 25 cmꢀ1 to an average of 2005 cmꢀ1. The anomaly in the series was
found with the fluoride complexes. While the highest frequency carbonyl stretches were
comparable to the rest of the series, the two lower stretches came at much lower
frequencies than those in the rest of the series.
Table 4. IR, 31P NMR, and 13C carbonyl NMR data for all complexes.
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2
Complex
νCO (cmꢀ1)
δP (ppm) 1JWꢀP (Hz) JPꢀP (Hz)
δCO (ppm)
207, 204
2016, 1922, 1885
1923, 1831, 1800
1957, 1847, 1798
2034, 1935, 1914
2008, 1923, 1885
2031, 1956, 1928
2033, 1956, 1928
2033, 1953, 1914
2024, 1939, 1846
2003, 1928, 1886
2007, 1928, 1883
2009, 1927, 1880
2008, 1915, 1807
ꢀ0.4
233
1
2
13.1
238
218, 215, 214
220, 211, 208
206, 204, 197
209, 200
20.4
311
3
19.7, 14.7
21.7, 19.3
14.4, ꢀ0.2
22.8, 3.0
23.6, 4.3
37.9, 35.4
19.5, 17.9
24.8, 21.3
27.2, 20.9
43.1, 38.5
200, 212
213, 230
180, 136
170, 142
152, 150
156, 160
189, 143
192, 138
189, 141
170, 162
85
136
141
160
151
32
4
5
231, 215
6
220, 217, 206
234, 218, 216
222, 221
7
8
9
207
215
223
34
236, 217, 212
238, 219, 214
239, 220, 215
228, 227
6a
7a
8a
9a
The phosphorus NMR data also showed a pattern for cationic versus neutral
complexes (Table 4). The 31P signals were consistently downfield for the neutral
complexes compared to the cationic complexes. Additionally, the neutral complexes all
had larger geminal phosphorusꢀphosphorus coupling and larger oneꢀbond tungstenꢀ
phosphorus couplings compared to their cationic analogues. An examination of the
halide series revealed that as the halides became more electronegative, the chemical shift
of the phosphorus atoms moved downfield while the tungstenꢀphosphorus couplings
generally decreased. As seen in the IR data, the irregularity was with the tungstenꢀ
fluoride complexes, which had very small geminal coupling (32 Hz and 34 Hz). The 13C
NMR data for the carbonyl carbons also showed the trend in which the neutral
tungsten(II) complexes consistently had signals further downfield compared to their
cationic counterparts.
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In summary, we have reported the synthesis and characterization a series of PNP
tungsten complexes. When coordinated to tungsten(0), the ligand could adopt three
different coordination modes. Formal oxidation by protonation of the metal center
occurred to form a cationic tungsten hydride species. Oxidation through different routes
resulted in a series of cationic tungsten(II) halide complexes. The cationic species could
all be deprotonated at the amine to form neutral tungsten(II) hydride or halide species.
The structures of complexes 1, 3, and 4 were determined by Xꢀray single crystal analyses.
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Experimental
General Information. Reactions were performed under a dry nitrogen
atmosphere using standard Schlenk techniques. Methylene chloride, hexanes, pentane,
and diethyl ether were purified by passage through an activated alumina column under a
dry argon atmosphere. Tetrahydrofuran (THF) was distilled from sodium benzophenone
ketyl under argon atmosphere. Methylene chlorideꢀd2 was dried over CaH2 and degassed
using standard freezeꢀpumpꢀthaw techniques. HN(SiMe2CH2PPh2)2 was synthesized
according to a literature method31.
NMR spectra were recorded on Bruker DRX400, 500, or AVANCE400
spectrometers. Infrared spectra were recorded on an ASI Applied Systems ReactIR 1000
FTꢀIR spectrometer.
HN(SiMe2CH2PPh2)2W(CO)4 (1). Thermal Synthesis. In a 200mL Schlenk
flask, W(CO)6 (0.50 g, 0.14 mmol) was added to chlorobenzene (40 mL).
HN(SiMe2CH2PPh2)2 (0.75g , 0.14 mmol) was dissolved in a small amount of
chlorobenzene (5 mL) and cannula transferred into the hexacarbonyl solution. The
mixture was heated to 120 ºC for 24 hours. The solvent was removed in vacuo and the
orangeꢀbrown oil was washed several times with pentane. The product was extracted
with ether, and was purified by dissolving in methylene chloride and layering with
hexanes (1:10) in the freezer overnight (0.17 g, 14.5%). IR (diethyl ether):
ν
= 2016,
CO
1
1922, 1885 cmꢀ1. H NMR (CD2Cl2,
δ): 7.56 (m, 8H, Ph CH), 7.40 (m, 12H, Ph CH),
1.92 (d, 4H, 2
J
PH = 10.4 Hz, CH2), ꢀ0.28 (s, 12H, CH3). 31P NMR (CD2Cl2,
δ): −0.38 (s,
1JWꢀP = 233.0 Hz). 13C{1H} NMR (CD2Cl2,
δ
): 206.6 (t, CO, 2JPꢀC = 10 Hz, 1JWꢀC = 70.4
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Hz),
δ
): 203.8 (t, CO, 2JPꢀC = 7.5 Hz, 1JWꢀC = 65.4 Hz), 141.2−128.4 (Ar C), 20.6 ( ꢀCH2ꢀ
), 2.7 (ꢀCH3).
HN(SiMe2CH2PPh2)2W(CO)3 Facial (2). HN(SiMe2CH2PPh2)2 (1.5 g, 2.9
mmol) was combined with W(CH3CN)3(CO)3 (1.1 g, 2.8 mmol) in THF (30 mL) and
stirred at room temperature for 1 hour. Isolation was accomplished by precipitation of
product with ether and subsequent pentane washes (740 mg, 33%). IR (THF):
ν
=
CO
1
1923, 1831, 1800 cmꢀ1. H NMR (C6D6,
δ): 8.02 (t, 4H, Ph CH), 7.23 (t, 4H, Ph CH),
7.11 (t, 4H, Ph CH), 6.94 (t, 2H, Ph CH), 6.66 (m, 6H, Ph CH), 1.83, 1.10 (each a dd, 2H,
2
CH2, JHꢀH = 14.0 Hz, 2JPꢀH = 9.2 Hz), 0.05, 0.01 (each a s, 6H, CH3). 31P NMR (C6D6,
δ
): 13.09 (s, 1JWꢀP = 238.1 Hz). 13C{1H} NMR (CD2Cl2, ): 218.0 (t, CO, 2JPꢀC = 5.7 Hz),
δ
214.9, 214.7 (each a t, CO, 2JPꢀC = 16.6Hz), 139.4, 139.0, 138.9, 138.8, 138.7, 130.4,
129.6, 129.0, 128.6, 128.5 (aryl C), 16.9 (s, ꢀCH2ꢀ), 3.9 (t, 1JPꢀC = 3.1Hz, ꢀCH2ꢀ), 1.8 (s, ꢀ
CH3). HRMS (m/z, ESI+, CHCl3): found 798.1338, calcd for [M + H]+, 798.1375.
HN(SiMe2CH2PPh2)2W(CO)3 Meridional (3). Synthesis from
W(CH3CN)3(CO)3. HN(SiMe2CH2PPh2)2 (7.0 g, 13.2 mmol) was combined with
W(CH3CN)3(CO)3 (4.9 g, 12.5 mmol) in THF (50 mL) and stirred at room temperature
overnight. Isolation of 3 was accomplished by precipitation of product with ether and
subsequent pentane washes (6.58 g, 65.7%). IR (THF):
ν
= 1957, 1847, 1798 cmꢀ1.
CO
1H NMR (CD2Cl2,
2.65, 1.61 (each a dt, 2H, 2JHꢀH = 14.4 Hz, 2JPꢀH = 8.8 Hz, CH2), 0.36, 0.04 (each a s, 6H,
CH3). 31P NMR (CD2Cl2, ): 20.40 (s, 1JWꢀP = 310.8 Hz). 13C{1H} NMR (CD2Cl2,
): 219.8 (t, CO, 2JP,C = 9.3 Hz), 210.9, 208.0 (each a t, CO, 2JP,C = 14.6 Hz),
δ): 7.81 (m, 6H, Ph CH), 7.40 (m, 8H, Ph CH), 7.31 (m, 6H, Ph CH),
δ
δ
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142.2, 139.7 (each a t, ipso Ar C, 1JPꢀC = 36.9Hz), 131.6, 131.4 (each a t, ortho Ar C, 2JPꢀC
= 13.2Hz), 129.3, 129.3 (each a s, para Ar C), 128.5 (t, meta Ar C, 3JPꢀC = 9.2 Hz), 21.4 (t,
1JPꢀC = 6.0 Hz, ꢀCH2ꢀ), 2.7 (t, 1JPꢀC = 8.6Hz, ꢀCH2ꢀ), −0.3 (s, ꢀCH3). Analysis: Calculated
for 3: C, 49.69; H, 4.68; N, 1.76. Found: C, 49.72; H, 4.62; N, 1.71. HRMS (m/z, ESI+,
MeOH): found 930.0397, calcd for [M + Cs]+, 930.0351.
[HN(SiMe2CH2PPh2)2W(CO)3H][BF4] (4). In an NMR tube, 3 (0.010 g, 0.013
mmol) was dissolved in CH2Cl2 (0.5 mL). Tetrafluoroboric acid (1.6 mg, 0.20 mmol)
was added and immediately the orange solution faded to a pale yellow. IR and NMR
indicated quantitative conversion to the sevenꢀcoordinate hydride. IR (CH2Cl2):
ν
=
CO
1
2034, 1935, 1914 cmꢀ1. H NMR (242K, CD2Cl2,
δ): 7.89 (m, Ph CH), 7.62 (s, Ph CH),
7.54 (s, Ph CH), 7.34 (m, Ph CH), 2.58, 2.34 (each a dd, 1H, CH2, 2JPꢀH = 15Hz), 2.42,
2.23 (each a dd, 1H, CH2, 2JHꢀH = 14.5 Hz, 2JPꢀH = 10.5 Hz), 0.61, 0.58, ꢀ0.05, ꢀ0.10 (each
a s, 3H, CH3), ꢀ5.02 (dd, 1H, WH , 1JWꢀH = 16.5 Hz, 2JPꢀH = 53.0 Hz, 2JPꢀH = 20.5 Hz).
31P{1H} NMR (242K, CD2Cl2, ): 19.7 (d, 1JWꢀP = 200.2 Hz, 2JPꢀP = 85.4 Hz),
δ δ): 14.7 (d,
1JWꢀP = 211.6 Hz, 2JPꢀP = 85.4 Hz). 13C{1H} NMR (230K, CD2Cl2, δ): 205.5, 203.9, 196.6
(each a dd, CO, 2JPꢀC = 6.5 Hz), 137.3ꢀ124.6 (Ar C), 13.2, 12.7 (each a s, ꢀCH2ꢀ), 1.8, 1.7,
0.8, ꢀ0.7 (each a s, ꢀCH3). HRMS (m/z, ESI+, MeOH): found 820.1195, calcd for [M – H
+ Na]+, 820.1194.
N(SiMe2CH2PPh2)2W(CO)3H (5). [HN(SiMe2CH2PPh2)2W(CO)3H][BF4]
(0.013 mmol) was prepared in situ and cooled to 195K. Triethylamine (5.0 ꢁL, 0.036
mmol) was added and mixed. IR and NMR showed quantitative conversion to yellow
1
product. IR (CH2Cl2): νCO = 2008, 1923, 1885 cmꢀ1. H NMR (185K, CD2Cl2,
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δ
): 7.51 (s, Ph CH), 7.37 (s, Ph CH), 1.94, 1.83 (each a d, 2H, CH2, 2JPꢀH = 10.5Hz),
0.006, ꢀ0.19 (each a s, 3H, CH3), ꢀ4.53 (dd, 1H, WH , 2JPꢀH = 51.0 Hz, 2JPꢀH = 14.5 Hz).
31P{1H} NMR (185K, CD2Cl2, ): 21.7 (d, 1JWꢀP = 213 Hz, 2JPꢀP = 136 Hz),
1JWꢀP = 230 Hz, 2JPꢀP = 136 Hz). 13C{1H} NMR (185K, CD2Cl2,
δ
δ): 19.3 (d,
δ): 208.6, (dd, CO),
200.3 (t, CO, 1JWꢀC = 125Hz), 138.0, 135.6 (each a d, ipso Ar C, 1JPꢀC = 42Hz), 131.9 (dd,
ortho Ar C, 2JPꢀC = 10Hz, 2JPꢀC = 16Hz), 129.6 (d, para Ar C, 4JPꢀC = 13Hz), 127.9 (d, meta
Ar C, 3JPꢀC = 9Hz), 24.6, (d, 1JPꢀC = 14.2Hz, ꢀCH2ꢀ), 22.6 (d, 1JPꢀC = 11Hz, ꢀCH2ꢀ), 4.3, 4.2
(each a s, ꢀCH3).
[HN(SiMe2CH2PPh2)2W(CO)3I][I] (6). In an NMR tube, 3 (30 mg, 0.038 mmol)
was dissolved in CH2Cl2 (0.5 mL). Iodine (0.010 g, 0.038 mmol) was added and shaken
to dissolve. IR and NMR showed quantitative conversion to yellow product. IR
1
(CH2Cl2):
ν
= 2031, 1950, 1928 cmꢀ1. H NMR (200K, CD2Cl2,
δ): 7.68 (s, Ph CH),
CO
7.47 (m, Ph CH), 2.78, 2.62, 2.58, 2.09 (each a t, 1H, CH2, 2JHꢀH = 16.5Hz), 0.76, 0.60,
0.38, 0.08 (each a s, 3H, CH3). 31P{1H} NMR (200K, CD2Cl2, ): 14.4 (d, 1JWꢀP = 180 Hz,
2JPꢀP = 140.5 Hz), ): −0.24 (d, 1JWꢀP = 136Hz, 2JPꢀP = 140.5 Hz). 13C{1H} NMR (200K,
δ
δ
CD2Cl2, δ): 230.8, 214.9 ( CO), 133.5, 132.7, 131.9, 131.4, 129.4, 128.7, 127.9 (Ar
C), 17.5, 14.7 ( ꢀCH2ꢀ), 3.9, 3.6, 0.8, 0.12 (ꢀCH3). HRMS (m/z, ESI+, MeOH): found
930.0446, calcd for [M – I + Cs]+, 930.0351.
N(SiMe2CH2PPh2)2W(CO)3I (6a). [HN(SiMe2CH2PPh2)2W(CO)3I][I] (0.038
mmol) was prepared in situ and cooled to 195K. Triethylamine (10 ꢁL, 0.076 mmol) was
added and mixed. IR and NMR showed quantitative conversion to yellow product. IR
1
(CH2Cl2):
ν
= 2003, 1928, 1886 cmꢀ1. H NMR (195K, CD2Cl2,
δ): 7.29−7.72 (Ph
CO
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CH), 2.82, 2.56, 2.22, 1.89 (each a t, 1H, CH2, 2JPꢀH = 12Hz), 0.59, 0.30, 0.23, .0.01 (each
a s, 3H, CH3). 31P{1H} NMR (195K, CD2Cl2,
δ
): 19.5 (d, 1JWꢀP = 189 Hz, 2JPꢀP = 207 Hz),
17.9 (d, 1JWꢀP = 143Hz, 2JPꢀP = 207 Hz). 13C{1H} NMR (195K, CD2Cl2,
δ): 235.7 (dd,
CO, 2JPꢀC = 25.8Hz, 2JPꢀC = 9.3Hz) , 216.5 (dd, CO, 2JPꢀC = 23.4Hz, 2JPꢀC = 9.3Hz), 212.3
(dd, CO, 2JPꢀC = 15.2Hz, 2JPꢀC = 5.8Hz), 135.8ꢀ127.3 (aryl C), 23.7, 23.0 (each a d, ꢀCH2ꢀ
), 7.8, 5.5, 5.4, 5.2 (each a s, ꢀCH3). HRMS (m/z, ESI+, MeOH): found 795.1451, calcd
for [M – I – H]ꢀ, 795.1140.
[HN(SiMe2CH2PPh2)2W(CO)3Br][OTf] (7). In a schlenk flask, 3 (30 mg, 0.038
mmol) was dissolved in CH2Cl2 (10 mL) and cooled to 195K. Excess silver triflate (0.10
g, 0.39 mmol) was added and stirred for one hour. The solution was cannula filtered into
a cold Schlenk flask. Nꢀbromosuccinimide (0.0070 g, 0.038 mmol) was added and
1
stirred 30 minutes at 195K. IR (CH2Cl2):
ν
= 2033, 1956, 1928 cmꢀ1. H NMR
CO
(230K, CD2Cl2,
0.59, 0.31, ꢀ0.19 (each a s, 3H, CH3). 31P{1H} NMR (230K, CD2Cl2,
170 Hz, 2JPꢀP = 160 Hz), 3.0 (d, 1JWꢀP = 142Hz, 2JPꢀP = 160 Hz). 13C{1H} NMR (230K,
δ
): 7.88−7.43 (Ph CH), 3.02, 2.74, 2.47, 2.16 (each a m, 1H, CH2), 0.78,
δ
): 22.8 (d, 1JWꢀP
=
CD2Cl2, δ): 219.5, 216.9, 205.7 (each a CO), 135.8ꢀ127.9 (aryl C), 20.5, 17.9 (each a d, ꢀ
CH2ꢀ), 6.2, 1.86, 0.5, −0.1 (each a s, ꢀCH3).
N(SiMe2CH2PPh2)2W(CO)3Br (7a). [HN(SiMe2CH2PPh2)2W(CO)3Br][OTf]
(0.038 mmol) was prepared in situ and cooled to 195K. Triethylamine (10 ꢁL, 0.076
mmol) was added and mixed. IR and NMR showed quantitative conversion to yellow
1
product. IR (CH2Cl2):
ν
CO
= 2007, 1928, 1883 cmꢀ1. H NMR (195K, CD2Cl2,
δ): 7.68−7.26 (Ph CH), 2.56, 1.89, 1.20 (each a dd, CH2), 0.56, 0.22, 0.20, 0.03 (each a s,
29