The synthesis of tungsten imido hydride complexes by the hydrogenolysis of dialkyl complexes

Article abstract of DOI:10.1016/S0022-328X(99)00484-2

The reaction of the complexes [(TMS)₂pda]W(=NPh)(R)₂, (R=CH₂CMe₃, CH₂CH₃) ([(TMS)₂pda]={o-[Me₃SiN]₂C₆H ₄}^2-) with dihydrogen in the presence of trialkyl phosphines results in the formation of a series of W(VI) hydride complexes of the general formula [(TMS)₂pda]W(=NPh)(H)₂(PR₃)₂ (PR₃=PMe₃, PMe₂Ph, 1/2Ph₂PCH₂CH₂PPh₂, 1/2P(C₆H₁₁)₃). The reactions of these complexes with olefins are also described.

Full text of DOI:10.1016/S0022-328X(99)00484-2

www.elsevier.nl/locate/jorganchem
Journal of Electroanalytical Chemistry 591 (1999) 8–13
The synthesis of tungsten imido hydride complexes by the
hydrogenolysis of dialkyl complexes
James M. Boncella *, Shu-Yu S. Wang, Daniel D. VanderLende
Department of Chemistry and Center for Catalysis, Uni6ersity of Florida, Gaines6ille, FL 32611-7200, USA
Received 8 June 1999; accepted 9 August 1999
Abstract
The reaction of the complexes [(TMS)2pda]W(ꢀNPh)(R)2, (R=CH2CMe3, CH2CH3) ([(TMS)2pda]={o-[Me3SiN]2C6H4}2−
)
with dihydrogen in the presence of trialkyl phosphines results in the formation of a series of W(VI) hydride complexes of the
general formula [(TMS)2pda]W(ꢀNPh)(H)2(PR3)2 (PR3=PMe3, PMe2Ph, 1/2Ph2PCH2CH2PPh2, 1/2P(C6H11)3). The reactions of
these complexes with olefins are also described. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Tungsten; Imido; Hydride; Alkyl; Hydrogenolysis
1. Introduction
tungsten imido group. We have found that these hydride
complexes react rapidly with olefins to give the corre-
sponding alkane and W(IV) olefin complexes, and they
behave as homogeneous olefin hydrogenation catalysts.
Recently, we have studied a series of tungsten and
molybdenum complexes with the general for-
mula
[(TMS)2pda]M(ꢀNPh)(R)2((TMS)2pdaꢀ[(Me3-
SiN)2C6H4]2−; M=W, Mo; R=alkyl, aryl) [1–5]. Al-
though our initial studies were directed toward the syn-
thesis of metal alkylidene complexes for use as olefin
metathesis catalysts, the ease with which a wide variety of
dialkyl complexes could be synthesized has led us to
study the chemistry of these d0 dialkyl complexes in more
detail. We have found that isocyanides readily insert into
the metal carbon bonds of these complexes [6] and that
the W complexes with b-H containing alkyl groups are
stable at and above room temperature (r.t.), despite their
electronic and coordinative unsaturation [1].
In this paper, we report the reaction of several tungsten
dialkyl complexes with H2 in the presence of trialkyl
phosphines. These reactions result in the formation of a
series of new W(VI) imido hydride complexes. Despite
the fact that the hydride ligand is the simplest anionic lig-
and in coordination chemistry, there are relatively few ex-
amples of transition metal complexes that contain both
the imido and hydride ligands [7–16]. The compounds re-
ported herein afford us the opportunity to study the
chemistry of the hydride ligand in the presence of the
2. Results and discussion
2.1. Synthesis
The reaction of the complex [(TMS)2pda]W(ꢀNPh)-
(CH2CMe3)2 (1) with H2 (ca. 2 atm) and two equivalents
of trialkyl phosphine results in the formation of the
seven-coordinate dihydride complexes, [(TMS)2-
pda]W(ꢀNPh)(H)2(PR3)2 (2–4), as shown in Eq. (1). In
the case of PCy3, only one phosphine can coordinate to
the metal for steric reasons and the product is
[(TMS)2pda]W(ꢀNPh)(H)2(PCy3) (5).
(1)
* Corresponding author. Fax: +1-352-392 3255.
E-mail address: boncella@chem.ufl.edu (J.M. Boncella)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00484-2

J.M. Boncella et al. / Journal of Organometallic Chemistry 591 (1999) 8–13
9
Although single crystals of 2 have been obtained,
their quality has not resulted in an X-ray diffraction
study that is completely refined and therefore publish-
able. The results of the structural study do however
indicate that the phenyl imido group and one amide
from the TMS2pda ligand are trans with respect to one
another. The other silylamide group is cis to the NPh
ligand as are the two PMe3 ligands which are mutually
transoid. A structure that is derived from a pentagonal
bipyramid with the imido group occupying an axial
position is consistent with these observations, as shown
in Fig. 1.
The NMR spectra of complex 4 are also consistent
with a structure that is derived from a pentagonal
bipyramid, as shown in Fig. 1. This structure has an
axial phenylimido group with the TMS2pda ligand
amido groups occupying equatorial sites, while the
phosphines of the DPPE group occupy one equatorial
site and the axial site trans to the imido group. The
hydrides are symmetrically disposed in the equatorial
plane. This structure has a mirror plane that contains
the phosphorus atoms of the DPPE group as well as the
imido N atom. This mirror plane renders the SiMe3
protons of the TMs2pda ligand chemically equivalent as
is observed in the proton NMR spectrum. The PPh2
groups of the DPPE ligand are chemically inequivalent
and are observed as a pair of doublets in the 31P{1H}-
NMR spectrum of 4. The hydride ligands give rise to a
four line pattern with 183W satellites that is the A2 part
of an A2XY spin system where X and Y are the 31P
nuclei of the DPPE ligand.
The NMR spectra of 5 are consistent with a structure
that is derived from an octahedron in which the PCy3
group and the phenyl imido ligand are mutually trans.
This leaves the hydride ligands in cis positions as
proposed in Fig. 1. The proton NMR spectrum of 5
displays a doublet for the chemically equivalent hydride
protons (2JPꢁH=83 Hz) and a single peak that is as-
signed to the Me3Si protons that are equivalent due to
the mirror plane that contains the N, W and P atoms.
Futhermore, the value of JWꢁP (56 Hz) is significantly
smaller than that of 2 and 3 as would be expected given
the strong trans influence of the WꢀN group.
1
The r.t. H-NMR spectrum of 2 displays two peaks
that are assigned to inequivalent Si(CH3)3 groups as
well as a hydride resonance at 9.26 ppm that integrates
to two protons. The PMe3 protons appear as a broad
singlet at 1.04 ppm. The inequivalence of the Me3Si
groups is consistent with a structure that has the
NSiMe3 groups occupying sites that are cis and trans to
the imido group, while the broadness of the hydride
and PMe3 peaks suggests that the compound is flux-
ional at r.t.
The low-temperature NMR spectra are consistent
with this structural assignment. At −50°C the hydride
peak of 2 is a sharp doublet of doublets because the
hydrides are the AA% part of an AA%XX% spin system
(the 31P nuclei being the XX% part.). The PMe3 protons
resolve into a virtual triplet with a peak separation of 3
Hz. This is consistent with the pseudotrans disposition
of the PMe3 groups in the equatorial plane of the
pentagonal bipyramid. The 31P{1H} spectra of 2 display
a singlet with tungsten satellites (1JWꢁP=188 Hz) from
25 to −50°C.
2.2. Reacti6ity studies
The NMR spectral parameters of 3 are similar to
those of 2 and are also consistent with the proposed
structure. The difference is that 3 is more fluxional than
2. At r.t., the hydride signal is observed as a broad
singlet, and at −50°C, this resonance has only decoa-
lesced into a broad triplet that is similar to the one
observed for 2 at r.t. It is likely that the fluxionality in
2 and 3 involves dissociation of the PR3 ligand and the
larger steric demands of PMe2Ph versus PMe3 lead to a
lower activation barrier for dissociation.
Although our initial discovery of these complexes
occurred when we were examining the interaction of
complex 1 with H2 in the presence of phosphines, a
more efficient and economical synthesis involves the
reaction of the complex [o-(Me3SiN)2C6H4]W-
(ꢀNPh)(CH2CH3)2 (7) with H2 in the presence of the
desired phosphine ligand. As part of this procedure,
crude 7 was allowed to react with an excess of the
phosphine in the presence of ca.
2
atm of
Fig. 1. Proposed structures of compounds 2–5.

10
J.M. Boncella et al. / Journal of Organometallic Chemistry 591 (1999) 8–13
Scheme 1.
H2 over the course of several hours at r.t. This reaction
sequence is an efficient synthesis of 2 from [o-
(Me3SiN)2C6H4]W(ꢀNPh)(Cl)2 (6). It appears that the
reaction sequence proceeds as shown in Scheme 1.
Addition of PR3 to the diethyl complex rapidly induces
b-H abstraction to generate the ethylene complex 8 [4].
Hydrogenation of 8 in the presence of PR3 leads to the
desired hydride products. We have confirmed that 8
reacts with H2 under the same conditions as employed
here to generate complex 2.
An alternative route to complex 2 involves hy-
drogenolysis of the alkylidene complex W(NPh)[o-
(Me3SiN)2C6H4](CHCMe3)(PMe3) (9) in the presence of
one equivalent of PMe3. In this reaction, the alkylidene
is completely hydrogenated to form neopentane and the
metal ends up as 2. Reactions that were performed in
an NMR tube reveal essentially quantitative conversion
of 9 to 2. If the reaction was performed in the absence
of added PMe3, then a 50% yield of 2 was observed.
The remaining W containing material is the metallated
TMS2pda complex 10 [3], shown in Scheme 2.
does in fact react with H2 much more slowly to give a
material that has a peak in the hydride region of the
1H-NMR spectrum that is converted to 2 when it is
allowed to react with PMe3. The direct hydrogenolysis
of the alkylidene group of 9 could also lead to 2.
We have explored the reaction of various hydride
sources with the dichloride complex 6 as an alternative
route to these complexes. Unfortunately, the use of
KH, LiAlH4, LiBEt3H, LiAl(O-t-Bu)3H and NaBH4
did not give satisfactory results. While hydride com-
plexes were formed in all cases, there were significant
quantities of impurities that proved extremely difficult
to separate from the desired products. The most effi-
cient synthesis of these phosphine hydride complexes
appears to be generating the diethyl complex 7, fol-
lowed by its in situ hydrogenolysis in the presence of
excess phosphine ligand.
When a solution of compound 2 was exposed to
ethylene at r.t. and 1 atm pressure, the ethylene com-
plex 8 was observed along with one equivalent of
ethane. Compound 2 does function as an olefin hydro-
genation catalyst though the turnover frequency is very
low. Thus, styrene is hydrogenated to form ethylben-
zene (TONꢀ3/h) and cyclohexadiene is converted to
cyclohexene with 98% selectivity. We believe that the
low turnover frequency and the inability to hydro-
genate cyclohexene at an appreciable rate is due to the
inhibitory effect of the PMe3 and the formation of a
stable cyclohexene complex, respectively. Both of these
One possible mechanism for the conversion of 9 to 2
involves initial dissociation of PMe3 from 9. Hy-
drogenolysis of the intermediate base-free alkylidene
complex would generate a base-free hydrido neopentyl
complex that could be trapped by PMe3 to give 2 by
reductive elimination, followed by oxidative addition of
H2. If there is not enough PMe3 present in the reaction
mixture, then 10 is formed along with 2. Complex 10

J.M. Boncella et al. / Journal of Organometallic Chemistry 591 (1999) 8–13
11
Scheme 2.
factors could inhibit the oxidative addition of H2 to the
metal center and would thereby inhibit the hydrogena-
tion reaction.
Indeed, when 2 is exposed to an excess of ethylene over
the period of weeks at r.t., 11 is the final product of the
reaction. In all cases, the driving force for the forma-
tion of W(VI) leads to the observed thermodynamic
product, the W(VI) metallacyclopentane complex.
This work demonstrates the viability of W(VI) imido
hydride complexes. One interesting aspect of the reac-
tivity of these compounds is their ability to cycle be-
tween the W(IV) and W(VI) formal oxidation states.
Unlike typical d0 complexes that are olefin hydrogena-
tion catalysts [17], the hydrogenation reaction appears
to be proceeding via a mechanism that involves both
oxidative addition and reductive elimination steps. It is
possible that the imido group somehow facilitates the
d2/d0 couple that is necessary for such reactions. We are
actively investigating the oxidative addition and reduc-
tive elimination reactivities of this class of compounds.
When complex 4 was exposed to excess ethylene, a
rapid reaction occurred that produced ethane and the
metallacyclopentane complex 11 [5] (Eq. (2)). We were
surprised by the identity of the W complex formed in
this reaction. Evidently, the DPPE ligand in 4 is dis-
placed once the complex is exposed to excess ethylene.
Presumably, the formation of a W(VI) ethylene com-
plex occurs as it does with 2 and 3, but in this case, the
DPPE does not bind strongly enough to prevent the
formation of the W(VI) metallacyclopentane complex
11.
3. Materials and experimental methods
(2)
All syntheses were carried out under a dry argon
atmosphere using standard Schlenk techniques. The
compounds W(ꢀNPh)Cl4(OEt2) (6) [18], [o-(Me3SiN)2-
C6H4]W(ꢀNPh)(CH2CH3)2 (7) [1], W(NPh)[o-(Me3Si-
N)2C6H4](C2H4)(PMe3)2 (8) [5], and W(NPh)[o-
(Me3SiN)2C6H4](CHCMe3)(PMe3) (9) [2], were synthe-
sized according to literature procedures. Tetra-
hydrofuran (THF), diethyl ether (Et2O), and pentane
were distilled from sodium benzophenone ketyl. Ben-
zene was distilled from sodium. NMR solvents were

12
J.M. Boncella et al. / Journal of Organometallic Chemistry 591 (1999) 8–13
stored over molecular sieves and degassed prior to use.
NMR spectra were acquired on either Varian VXR 300
or Gemini 300 spectrometers. 1H and 13C chemical
shifts are referenced to the residual proton peaks of the
deuterated solvents and are reported relative to TMS.
Elemental analyses were performed by Atlantic Micro-
labs, Inc., or by the analytical services of this
department.
Over a period of less than 2 h, complete conversion of
9 to 2 was observed by H-NMR.
1
3.1.3. Method 3
The compound [(TMS)2pda]M(ꢀNPh)(Et)2 (7) was
generated by the addition of 2.1 equivalents of EtMgCl
to
a
diethyl ether solution of [(TMS)2pda]-
M(ꢀNPh)(Cl)2 at −78°C. The solution was allowed to
warm to r.t. and was stirred for 30 min. The Et2O was
removed under reduced pressure and the resultant
residue was extracted with pentane (3×20 ml) and the
combined extracts were then filtered into a tube with a
Teflon valve. At this time, 2.1 equivalents of PMe3 was
added to the solution resulting in a change from red to
purple. The reaction mixture was stirred for 1 h and
then frozen in liquid N2 and evacuated. The flask was
then charged with H2 (ca. 10–15 PSIG) as described
above. The solution was allowed to warm to r.t. and
was stirred for 24 h. The solution turned the character-
istic magenta color of compound 2. The crystals that
deposited were collected and the mother liquors were
cooled to −20°C to harvest further compound from
the reaction. The yield of 2 using this procedure is ca.
80% based on the [(TMS)2pda]W(ꢀNPh)(Cl)2 starting
3.1. W(NPh)[o-(Me3SiN)2C6H4](H)2(PMe3)2 (2)
3.1.1. Method 1
In a glass tube with a Teflon Young’s joint,
W(NPh)[o-(Me3SiN)2C6H4](CH2C(CH3)3)2 (10) (1.66 g,
2.48 mmol) was dissolved in 25 ml of hexanes. PMe3
(0.64 ml, 6.20 mmol) was added via syringe. The solu-
tion was then placed in liquid nitrogen while a vacuum
was applied. Once the solution was frozen solid under
vacuum, the flask was sealed. The neck of the flask was
then purged with hydrogen gas and the H2 hose was
wired securely to the flask. The flask was opened until
the H2 reached a pressure of 10 PSIG. The flask was
then resealed and the H2 line removed. The reaction
was then allowed to warm to r.t. After 4 h of stirring,
the color of the solution changed from brown to ma-
genta, and magenta crystals precipitated from solution.
The remaining solution was transferred to a Schlenk
tube and cooled to −10°C to give additional magenta
crystals. The mother liquors were concentrated and
cooled to give another crop of crystals. Total yield of 2:
1.48 g (88%). 1H-NMR (25°C, C6D6): 0.79 (s, 9H,
ꢁSi(CH3)3); 0.81 (s, 9H, ꢁSi(CH3)3); 1.04 (s, 18H,
ꢁPMe3); 6.80–7.45 (m, 9H, aromatic); 9.26 (br t, 2H,
W-H). 13C{1H}-NMR (C6D6, 25°C): 4.92 (Si(CH3)3);
6.43 (ꢁSi(CH3)3); 15.99 (s, ꢁPMe3); 115.21, 116.53,
118.17, 119.20, 123.31, 126.71, 128.68, 151.13 (aro-
matic). 1H-NMR (−50°C, C7D8): 0.69 (s, 9H,
ꢁSi(CH3)3); 0.75 (s, 9H, ꢁSi(CH3)3); 0.84 (t, separa-
tion=3 Hz, 18H, ꢁPMe3); 6.74–7.38 (m, 9H, aro-
matic); 9.0 (d of d, 2H, 2JPꢁH=37, 41 Hz, W-H).
1
material. The identity of 2 was confirmed by H-NMR
spectroscopy.
3.2. W(NPh)[o-(Me3SiN)2C6H4](H)2(PMe2Ph)2 (3)
This compound was synthesized according to the
procedure used for 2 by substituting PMe2Ph for PMe3
and following either Method 1 or Method 3, as de-
1
scribed above. The yield was ca. 85%. H-NMR (25°C,
C6D6): 0.53 (s, 18H, ꢁSi(CH3)3); 1.22 (s, 12H,
ꢁPMe2Ph); 6.72–7.29 (m, 9H, aromatic); 9.63 (br s 2H,
W-H). 1H-NMR (−50°C, C7D8): 0.53 (s, 9H,
ꢁSi(CH3)3); 0.59 (s, 9H, ꢁSi(CH3)3); 1.19 (t, peak sepa-
ration=4 Hz, 12H, ꢁPMe2Ph); 6.72–7.29 (m, 9H, aro-
matic); 9.56 (br t, 2JPꢁH=40 Hz, 2H, W-H).
13C{1H}-NMR (C6D6, 25°C): 4.92 (Si(CH3)3); 6.43
(ꢁSi(CH3)3); 15.99 (s, ꢁPMe3); 115.21, 116.53, 118.17,
119.20, 123.31, 126.71, 128.68, 151.13 (aromatic). 31P-
1
31P{1H}-NMR (C6D6, 25°C): −24.46 (s, JWꢁP=188
Hz). Anal. Calc. for C24H47N3P2Si2W: C, 42.41; H,
6.97; N, 6.18. Found: C, 42.18; H, 6.79; N, 6.03.
1
NMR (C6D6, 25°C): −17.4 (s, JWꢁP=164 Hz). Anal.
Calc. for C34H51N3P2Si2W: C, 50.81; H, 6.39; N, 5.22.
Found: C, 50.52; H, 6.19; N, 5.03.
3.1.2. Method 2
W(NPh)[o-(Me3SiN)2C6H4](CHCMe3)(PMe3) (9) (50
mg, 0.07 mmol) was dissolved in C6D6 in an NMR tube
fitted with a Teflon Young’s joint. One equivalent of
PMe3 (7 ml, 0.07 mmol) was added via a microliter
syringe. The NMR tube was then fitted with a Schlenk
adapter, frozen in liquid nitrogen, and placed under
vacuum. The tube was then sealed while frozen under
vacuum. Hydrogen gas was then purged through the
Schlenk adapter for 5 min. The Teflon seal was opened
to allow the H2 to fill the vacuum in the NMR tube.
The NMR tube was charged with ca. 15 PSIG of H2.
3.3. W(NPh)[o-(Me3SiN)2C6H4](H)2(DPPE) (4)
In a glass tube with a Teflon Young’s joint,
W(NPh)[o-(Me3SiN)2C6H4](CH2C(CH3)3)2 (10) (0.65 g,
0.98 mmol) and DPPE (0.39 g, 0.98 mmol) were dis-
solved in 30 ml of hexanes. The solution was then
placed in liquid nitrogen while a vacuum was applied.
H2 gas was introduced as described for 2. The reaction
was then allowed to warm to r.t. After 8 h of stirring,
the color of the solution had changed from brown to

J.M. Boncella et al. / Journal of Organometallic Chemistry 591 (1999) 8–13
13
purple. Purple solid had also precipitated from solu-
Acknowledgements
tion. The solution was transferred to a Schlenk tube,
concentrated to 10 ml, and cooled to −10°C to give
additional purple solid. Total yield: 0.71 g (78%).
1H-NMR (C6D6, 25°C): 0.49 (s, 18H, ꢁSi(CH3)3); 2.23
(m, 2H, PCH2CH2P); 2.42 (m, 2H, PCH2CH2P); 6.72–
7.69 (aromatic); 10.56 (m, 2H, W-H). 31P{1H}-NMR
(C6D6, 25°C): 30.43 (d, 2JPꢁP=83 Hz); −12.70 (d,
2JPꢁP=83 Hz). Anal. Calc. for C44H53N3Si2P2W: C,
57.08; H, 5.77; N, 4.54. Found: C, 57.33; H, 5.84; N,
4.54.
We wish to acknowledge the National Science Foun-
dation CHE-9523279 for support of this work.
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W(NPh)[o-(Me3SiN)2C6H4](CH2C(CH3)3)2 (10) (1.06 g,
1.58 mmol), and PCy3 (0.476 g, 1.70 mmol) were dis-
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placed in liquid nitrogen while a vacuum was applied.
Once the solution was frozen solid under vacuum, the
flask was sealed. The neck of the flask was then purged
with hydrogen gas and the H2 hose was wired securely
to the flask. The flask was opened until the H2 reached
a pressure of 10 PSIG. The flask was then resealed and
the H2 line removed. The reaction was then allowed to
warm to r.t. After 12 h of stirring, the color of the
solution changed from brown to red. The solution was
transferred to a Schlenk tube and concentrated to ca.
10 ml and cooled to −10°C to give red crystals. The
mother liquors were concentrated and cooled to give a
second crop of crystals. Total yield: 1.04 g (82%).
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1.93 (m, 33H, PC6H11); 6.81 (t, 1H, p-Ph H); 6.95 (m,
2H pda ring H); 6.99 (t, 2H m-Ph-H) 7.18 (d, 2H
o-Ph-H); 7.47 (m, 2H, pda ring H); 11.35 (d, 2JPꢁH=85
Hz, 1JWꢁH=64 Hz, W-H). 31P{1H}-NMR (C6D6,
25°C): 66.71 (s, 1JWꢁP=56 Hz). Anal. Calc. for
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53.98; H, 6.64; N, 5.23.
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