Photochemically generated transients from κ2- and κ3-Triphos derivatives of group 6 metal carbonyls and their reactivity with olefins

Article abstract of DOI:10.1021/om300044k

The synthesis and characterization of (κ²-Triphos)M(CO) ₄ derivatives, where M = Mo, W and Triphos = MeC(CH ₂PPh₂)₃, are reported. Photolyses of these metal carbonyls in dichloromethane or CO₂-saturated dichloromethane readily afford the (κ³-Triphos)M(CO)₃ complexes with no evidence of significant solvent or carbon dioxide interactions with the site vacated by CO. However, in the presence of 1-hexene a transient (κ²-Triphos)M(CO)₃(1-hexene) adduct was observed, which subsequently releases the olefin with formation of the stable κ³-tricarbonyl species. In the case of M = W the kinetic parameters for this process were assessed, with the rate of olefin replacement being inversely proportional to [1-hexene]. A dissociative rate constant of 25.6 ± 1.1 s^-1 at 298 K was determined for olefin loss, with the selectivity for 1-hexene vs free phosphine arm addition to the unsaturated intermediate being somewhat surprisingly large at 22. The activation parameters measured were δH^? = 26.1 ± 0.4 kcal/mol and δS^? = 36 ± 3 eu, which are consistent with a dissociative substitution reaction. The kinetic parameters for this transformation were unaffected in the presence of excess quantities of CO ₂. Although no interaction of CO₂ with the transient species resulting from CO loss in the κ² complex was noted on the time scale of 50 ms, an intermediate described as an η²- HSiEt₃ complex was observed upon addition of triethylsilane. This latter transient species underwent dissociation with κ³-complex formation about 15 times as fast as its 1-hexene analogue. X-ray structures of the κ² complexes of Mo and W where the dangling phosphine arm has undergone oxidation are also reported.

Full text of DOI:10.1021/om300044k

Article
pubs.acs.org/Organometallics
Photochemically Generated Transients from κ2- and κ3-Triphos
Derivatives of Group 6 Metal Carbonyls and Their Reactivity
with Olefins
Samuel J. Kyran,† Sohail Muhammad,‡ Matthew Knestrick,† Ashfaq A. Bengali,*,‡
and Donald J. Darensbourg*,†
†Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
‡Department of Chemistry, Texas A&M University at Qatar, Doha, Qatar
S
* Supporting Information
ABSTRACT: The synthesis and characterization of (κ2-Triphos)M(CO)4 derivatives, where
M = Mo, W and Triphos = MeC(CH2PPh2)3, are reported. Photolyses of these metal carbonyls
in dichloromethane or CO2-saturated dichloromethane readily afford the (κ3-Triphos)M(CO)3
complexes with no evidence of significant solvent or carbon dioxide interactions with the site
vacated by CO. However, in the presence of 1-hexene a transient (κ2-Triphos)M(CO)3(1-
hexene) adduct was observed, which subsequently releases the olefin with formation of the
stable κ3-tricarbonyl species. In the case of M = W the kinetic parameters for this process were
assessed, with the rate of olefin replacement being inversely proportional to [1-hexene]. A
dissociative rate constant of 25.6 1.1 s−1 at 298 K was determined for olefin loss, with the
selectivity for 1-hexene vs free phosphine arm addition to the unsaturated intermediate being
somewhat surprisingly large at 22. The activation parameters measured were ΔH⧧ = 26.1 0.4
kcal/mol and ΔS⧧ = 36 3 eu, which are consistent with a dissociative substitution reaction.
The kinetic parameters for this transformation were unaffected in the presence of excess
quantities of CO2. Although no interaction of CO2 with the transient species resulting from CO loss in the κ2 complex was noted
on the time scale of 50 ms, an intermediate described as an η2-HSiEt3 complex was observed upon addition of triethylsilane. This
latter transient species underwent dissociation with κ3-complex formation about 15 times as fast as its 1-hexene analogue. X-ray
structures of the κ2 complexes of Mo and W where the dangling phosphine arm has undergone oxidation are also reported.
INTRODUCTION
■
Alternative sources of chemical carbon for the production of
fuels and useful organic chemicals will be required as the supply
of petroleum declines during the current century. Carbon
dioxide and biomass will serve as the major replacement feed-
stocks to fill this demand.1 Relevant to this issue, the current
major utilization of CO2 in the chemical industry is the produc-
tion of urea. Other less consuming applications involve the
synthesis of salicylic acid, cyclic carbonates and polycarbonates,
methanol, and inorganic carbonates.
Over the past decades we and numerous other research
Triphos = (Ph2PCH2CH2)2PPh, with CO2 (eq 4), we wish to
groups have investigated the alternating copolymerization of
communicate herein some of our results covering the reactivity
CO2 and epoxides to provide polycarbonates.2 Earlier we un-
of phosphine derivatives of group 6 carbonyls with terminal
olefins.7 Subsequent studies of these systems with carbon
successfully studied the alternating coupling of CO2 and olefins
to provide polyesters. However, unlike the CO2/cyclic ether
dioxide will also be described.
copolymerization process, this transformation is thermodyna-
mically unfavorable except at low CO2 incorporation.3 Never-
theless, there have been several investigations of CO2 coupling
with ethylene to provide either metallalactones4 or metal acrylate
derivatives (eqs 1 and 2).5 Furthermore, promising reactions of
metallalactones with methylating reagents to afford methyl
acrylate have recently been reported (eq 3).6
Inspired by the kinetic and mechanistic studies reported by
Bernskoetter and Tyler on the reaction of (Triphos)Mo(N2)2(C2H4),
Received: January 19, 2012
Published: March 20, 2012
© 2012 American Chemical Society
3163
dx.doi.org/10.1021/om300044k | Organometallics 2012, 31, 3163−3170

Organometallics
Article
Figure 1. Three-dimensional stack plots (left) for the substitution reaction of cis-Mo(CO)4(pip)2 with Triphos in CHCl3 at 308 K, where the
starting material is subtracted out, and reaction profiles (right) for the formation of (κ2-Triphos)W(CO)4 (1891 cm−1) and (κ3-Triphos)W(CO)3
(1842 cm−1).
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The reactions of cis-
M(CO)4(pip)2 (M = Mo, W and pip = piperidine) with the
tripodal version of Triphos, i.e., 1,1,1-tris(diphenylphosphinomethyl)-
ethane, provided κ2 or κ3 coordination, depending on reaction
conditions. Although the κ2-Triphos derivatives of the group 6
tetracarbonyls have been prepared by direct substitution of the
parent hexacarbonyl in refluxing ethanol,8 replacement of the
labile piperidine ligands in cis-M(CO)4(pip)2 derivatives
represents milder reaction conditions for the synthesis of κ2-
[MeC(CH2PPh2)3]M(CO)4 complexes (M = Mo (1a), W
(1b)).9,10 In a similar manner, when the reaction temperature
and time were increased, the κ3-Triphos derivatives of molyb-
denum and tungsten tricarbonyls were formed. Interestingly,
during the synthesis of the bidentate coordination of Triphos to
Figure 2. 31P NMR spectra: (A) purified sample of (κ3-Triphos)W-
(CO)3; (B) isolated sample of (κ2-Triphos)W(CO)4 from synthesis
under mild conditions.
either molybdenum or tungsten carbonyls under mild
conditions, small quantities of the tridentate chelated ligand
were observed. This was true in the short-term reactions carried
out in refluxing dichloromethane (313 K) or over longer reac-
tion times at ambient temperature. Figure 1 depicts the three-
dimensional stack plots for the substitution reaction of cis-
Mo(CO)4(pip)2 with Triphos in CHCl3 at 308 K, along with
the reaction profiles for the growth of κ2 and κ3 Mo carbonyl
species. As is readily visible from the in situ infrared monitoring
of the substitution reaction, formation of this κ3 complex occurs
concurrently with that of the κ2 species, albeit at a slower rate.
Further evidence for simultaneous formation of the κ3 deriva-
tive is noted in the 31P NMR spectrum (see Figure 2). This
behavior is suggestive of a phosphine-assisted (Ia) pathway for
the displacement of CO in the κ2-Triphos complexes.11 Support
for such a process is noted from our observation that CO
substitution in Mo(CO)4(diphos) by PPh3 takes place much
more slowly by a dissociative mechanism.
X-ray analysis. X-ray crystallography established these cube-
shaped crystals to be the [(Ph2PCH2)2CMeCH2PPh2(O)]M-
(CO)4 (M = Mo (3a), W (3b)) complexes, where the free phos-
phine arm was oxidized despite all efforts to exclude oxygen and
H2O. 31P NMR revealed the mixture of crystals to be composed
of approximately 80% of complex 3a or 3b, with the other
unsuitable crystals for analysis being (κ2-Triphos)M(CO)4.
Previously, these derivatives have been synthesized by oxidation
of the free phosphine arm in the κ2 species with H2O2.10 The
structures of these two metal complexes are shown in Figure 3. It
is worth noting that PPh3O has been shown to greatly aid the
crystallization of organic materials and hence may account for the
preferential well-defined crystals isolated of the oxidized form of
the κ2 species.13 Table 1 contains selected bond distances and
angles for the two derivatives 3a,b. The nonbonding distances
between the phosphine oxide and the nearest CO ligand,
O(5)···C(1), of 3.943(4) and 3.88(1) Å for the Mo and W
derivatives, respectively, are greater than the sum of the van
der Waals radii of 3.25 Å.
The crystal structures of the (κ3-Triphos)M(CO)3 (M = Mo
(2a), W (2b)) complexes have been reported.12 However, in all
of our attempts at obtaining X-ray-quality crystals of the κ2-
Triphos metal tetracarbonyls we obtained two crystalline forms,
a cluster of needles and cubes; only the latter were suitable for
3164
dx.doi.org/10.1021/om300044k | Organometallics 2012, 31, 3163−3170

Organometallics
Article
Figure 3. X-ray structures: (A) thermal ellipsoid representation of complex 3a with ellipsoids at 50% probability surfaces (hydrogen atoms have been
omitted for clarity); (B) ball-and-stick drawing of complex 3b.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
Complexes 3a,b
Scheme 1
M = Mo (3a)
M = W (3b)
M(1)−C(1)
M(1)−C(2)
M(1)−C(3)
M(1)−C(4)
M(1)−P(1)
M(1)−P(2)
O(5)−P(3)
C−Oave
2.020(3)
1.999(3)
2.001(3)
2.054(3)
2.5368(12)
2.5041(12)
1.483(2)
1.150(3)
2.048(12)
2.003(10)
1.993(9)
2.076(12)
2.529(3)
2.498(3)
1.464(8)
1.139(12)
As shown in Figure 4, photolysis of (κ2-Triphos)W(CO)4 in the
presence of 1-hexene results in spectral changes that are consistent
with alkene coordination to the W center following CO loss. Upon
photolysis, peaks are observed at 1944 and 1860 cm−1 which have
band structures similar to those of the (κ3-Triphos)W(CO)3 com-
plex, suggesting the formation of a transient tricarbonyl species.
However, since the CO absorptions are blue-shifted (more electron
poor metal center) relative to those of (κ3-Triphos)W(CO)3, it is
reasonable to suggest that the third phosphine arm of the Triphos
ligand remains uncoordinated in this intermediate complex.
This transient species undergoes a first-order exponential
decay, and the (κ3-Triphos)W(CO)3 complex grows in at the
same rate. The most likely candidate for this intermediate
species is the (κ2-Triphos)W(CO)3(η2-1-hexene) complex, which
then undergoes intramolecular displacement of the 1-hexene
ligand by the free phosphine arm of the Triphos ligand within a few
seconds at room temperature. Further evidence for this assignment
comes from the observation that the reaction rate varies inversely
with [1-hexene]. As shown in Figure 5, an increase in [1-hexene]
results in a slower displacement reaction, as evidenced by the longer
lifetime of the (κ2-Triphos)W(CO)3(η2-1-hexene) complex. The
lifetime of the η2-alkene species is unaffected by increasing the
concentration of the parent tetracarbonyl complex, thereby ruling
out an intermolecular process in the displacement of the alkene.
A plot of kobs, obtained from a single-exponential fit to the
observed decay of the η2-hexene complex, as a function of
[1-hexene] is shown in Figure 6.
P(1)−M−P(2)
P(1)−M−C(1)
P(1)−M−C(2)
P(1)−M−C(3)
P(1)−M−C(4)
P(2)−M−C(1)
P(2)−M−C(2)
P(2)−M−C(3)
P(2)−M−C(4)
87.14(5)
84.41(8)
93.51(8)
171.85(8)
99.93(8)
85.57(8)
173.02(7)
88.85(8)
95.72(8)
87.11(9)
84.3(3)
93.2(3)
170.8(3)
99.8(3)
84.8(3)
173.2(3)
89.1(2)
96.4(3)
Kinetic Measurements. Kinetic studies designed to
investigate the reactivity of terminal olefins with the Triphos
derivatives of the group 6 metal carbonyls have been undertaken.
Photolysis of a CH2Cl2 solution of (κ2-Triphos)W(CO)4 in the
absence of 1-hexene resulted in the loss of a CO ligand, yielding
only the (κ3-Triphos)W(CO)3 complex absorbing at 1933 and
1841 cm−1, which has been isolated and characterized (Scheme 1).
It is well established that UV photolysis of metal carbonyls in
the solution phase results in the photoejection of a CO ligand
and solvent coordination to the vacant site occurs within
picoseconds of CO loss.14 On the time scale of the present
study, however (a few seconds), binding of CH2Cl2 to the W
center upon photolysis is not observed. This finding is not
surprising, since CH2Cl2 is expected to bind weakly to the W
center and may undergo facile intramolecular displacement by
the uncoordinated phosphine arm in the κ2 structure.
3165
dx.doi.org/10.1021/om300044k | Organometallics 2012, 31, 3163−3170

Organometallics
Article
is correct. Thus, at low [1-hexene] the rate-determining step
becomes the dissociation of the 1-hexene ligand, while the
displacement rate approaches 0 at high [1-hexene], as the
Figure 4. Spectral changes observed upon photolysis of
(κ2-Triphos)W(CO)4 in the presence of [1-hexene] = 3.2 M at 288 K.
The peaks at 1944 and 1860 cm−1 assigned to the (κ2-Triphos)W-
(CO)3(η2-1-hexene) complex decrease in intensity, while those at
1933 cm−1 and 1841 cm−1 due to the (κ3-Triphos)W(CO)3 complex
grow at the same rate (see inset). The spectra were obtained 0, 5.8,
17.3, and 46.2 s after photolysis.
Figure 6. Plot of kobs vs [1-hexene] at various temperatures, illustrating
the inverse dependence of kobs on the concentration of 1-hexene.
presence of a large amount of free alkene inhibits the
dissociation of the η2 coordinated 1-hexene molecule. A fit of
the experimental kobs vs [1-hexene] data to eq 5 yields values of
k1 and k′ given in Table 2.
Surprisingly, the selectivity ratio k′ is ∼20, which suggests
that the 16-electron (κ2-Triphos)W(CO)3 intermediate reacts
faster with 1-hexene than with the phosphine arm of the
Triphos ligand. The temperature independence of k′ further
suggests that the enthalpic barriers for the reaction of (κ2-
Triphos)W(CO)3 with either 1-hexene or phosphine are similar
or that they are both barrierless reactions. Previous studies have
indicated that several 16-electron organometallic complexes
react with a number of ligands without a significant enthalpic
barrier, and so it is likely that this is the case in the present
system as well.15 If so, then the kinetic preference of 1-hexene
over intramolecular phosphine coordination may be due to en-
tropic factors, although this is surprising, since ΔS⧧ is ex-
−1
pected to be considerably more negative than ΔS⧧ . However,
2
steric reorganization of the Triphos ligand involved in κ2 → κ3
coordination may result in a small barrier for the k2 step. Given
the relatively small temperature range over which the reactions
were studied, the difference in ΔG⧧ for the two reactions may
be as much as 2 kcal/mol, which could easily account for the
observed selectivity ratio.
Figure 5. Effect of increasing 1-hexene concentration upon the decay
rate of the (κ2-Triphos)W(CO)3(η2-1-hexene) complex at 288 K.
The inverse dependence of kobs on [1-hexene] is consistent
with a displacement mechanism that involves reversible
dissociation of the 1-hexene ligand prior to formation of the
final (κ3-Triphos)W(CO)3 product complex. A plausible
mechanism is presented in Scheme 2.
Applying the steady state assumption to the 16-electron
(κ2-Triphos)W(CO)3 intermediate yields the following depen-
dence of kobs on [1-hexene]:
An Eyring plot obtained from the temperature dependence
of k1, shown in Figure 7, yielded values of ΔH⧧ = 26.1 0.4
1
kcal/mol and ΔS⧧ = 36 3 eu. The strongly positive ΔS⧧ is
1
1
consistent with the dissociative nature of the transition state
expected for the k1 step, and ΔH⧧ therefore provides an
1
estimate for the W−(η2-1-hexene) bond dissociation enthalpy
(BDE). A BDE of 26.1 kcal/mol is consistent with an experi-
mental determination of 24.8 kcal/mol for the (CO)5Cr−
(η2-C2H4) BDE16−18 and a theoretical estimate of 26.2−28.9
kcal/mol for the (CO)5W−(η2-C2H4) BDE.19,20
k
k
k
2
1
−1
k
=
, where k′ =
obs
k′[1‐hexene] + 1
(5)
According to eq 5, limiting kobs values of k1 at low [1-hexene]
and 0 at high [1-hexene] are expected if the above mechanism
3166
dx.doi.org/10.1021/om300044k | Organometallics 2012, 31, 3163−3170

Organometallics
Article
Scheme 2
(η2-C2H4) BDE is almost 4 kcal/mol less for M = Mo than for
M = W.19,20
Table 2. Kinetic Parameters Obtained from a Fit of the kobs
vs [1-hexene] Data to Eq 5
a
Of particular interest in this study are photochemical
reactions of (κ2-Triphos)W(CO)4 in the presence of excess
quantities of carbon dioxide. As is evident in Figure 9A from
the difference spectrum obtained upon photolysis of a CO2-
saturated solution of (κ2-Triphos)W(CO)4 in dichloromethane
at 296 K, only (κ3-Triphos)W(CO)3 is observed. This obser-
vation suggests that either CO2 does not bind to the vacant site
or that it binds weakly, resulting in a transient species that has a
lifetime that is too short to detect on this time scale (<50 ms).
Similarly, the difference spectra obtained upon photolysis of a
CO2-saturated solution of (κ2-Triphos)W(CO)4 in CH2Cl2
with [1-hexene] = 1.6 M at 298 K revealed spectral charac-
teristics and reaction rate constants identical with those of the
process determined in the absence of CO2 (Figure 9B).21
Taken together, these results strongly support the lack of signi-
ficant CO2 interactions with the vacant site in (κ2-Triphos)-
W(CO)3, especially in comparison with the corresponding
binding of an olefin ligand.
T (K)
k1 (s−1
)
k′ (=k−1/k2)
278
288
293
298
303
0.96 0.04
5.3 0.2
24
17
18
22
21
1
1
1
1
2
10.8 0.5
25.6 1.1
52.5
3
a
Rate constants were obtained using KaleidaGraph version 4.1.0,
Synergy Software, Reading, PA, USA. The R2 values for the fits at
various temperatures were all greater than 0.999.
By way of contrast, a rather strong interaction is observed
between triethylsilane and the transient tricarbonyl species
afforded upon photolysis of (κ2-Triphos)W(CO)4. This is illus-
trated in Figure 10, where the two (κ2-Triphos)W(CO)3-
(silane) νCO bands are similar to those seen in the 1-hexene
analogue, although as expected these are red-shifted by 5−8 cm−1.
However, the initial species reacts with the free phosphine arm
to afford (κ3-Triphos)W(CO)3 almost 15 times more quickly
than the 1-hexene complex. Given the position of the νCO
bands in the intermediate and its reactivity, this species is most
likely (κ2-Triphos)W(CO)3(η2-HSiEt3) and not a Si−H
activated complex.
Figure 7. Eyring plot obtained from the temperature dependence of
k1. The equation for the trend line was y = 41.776 − 13190x, with an
R2 value of 0.9993.
Preliminary results on the interactions of 1-hexene in
(κ2-Triphos)W(CO)3(1-hexene) and (κ3-Triphos)W(CO)2
(1-hexene) reveal olefin binding in these complexes to be
quite similar: i.e., the lifetimes of the two hexene complexes are
comparable. Further studies to examine these interactions as
well as the binding of olefins with metal carbonyl transients
photochemically generated from more electron-rich precursors
in the presence and absence of CO2 are planned.
A few experiments were conducted with the analogous
Mo system, and as shown in Figure 8, it was found that
under similar conditions of temperature and [1-hexene], the
(κ2-Triphos)Mo(CO)3(η2-1-hexene) reacted almost 200 times
faster than the W complex. This is perhaps not surprising, since
the Mo−(η2-1-hexene) BDE is expected to be weaker than that
in the W system. Thus, for example, the calculated (CO)5M−
3167
dx.doi.org/10.1021/om300044k | Organometallics 2012, 31, 3163−3170

Organometallics
Article
Figure 10. Difference spectra obtained upon photolysis of a CH2Cl2
solution of (κ2-Triphos)W(CO)4 with [HSiEt3] = 1.25 M at 288 K.
Figure 8. Spectral changes observed upon photolysis of
(κ2-Triphos)Mo(CO)4 in the presence of [1-hexene] = 2.4 M at 278 K.
much shorter time scale. The activation parameters for the
replacement of the 1-hexene ligand in the tungsten complex
were determined to be ΔH⧧ = 26.1 kcal/mol and ΔS⧧ = 36 eu.
The enthalpy of activation closely relates to the estimated value
of the BDE, as expected for a dissociative reaction. Attempts to
observe a CO2 adduct with the transient provided by photo-
lysis of (κ2-Triphos)W(CO)4 were unsuccessful, indicative of
no binding or weak binding with a lifetime of binding less than
10−3 s. Similarly, the kinetic parameters for 1-hexene binding
were unaffected by the presence of excess quantities of CO2 in
solution. That is, CO2 coordination at the vacant metal site is
not competitive with olefin binding. On the other hand, the
transient complex (κ2-Triphos)W(CO)3(HSiEt3) was observed
upon photolysis of (κ2-Triphos)W(CO)4 in the presence of
CONCLUDING REMARKS
■
We have shown herein that photolysis of (κ2-Triphos)W(CO)4
in the presence of 1-hexene affords a short-lived (κ2-Triphos)-
W(CO)3(1-hexene) complex which subsequently undergoes
loss of the olefin with concomitant formation of the stable (κ3-
Triphos)W(CO)3 derivative. The transient 1-hexene adduct
was found to sustain intramolecular displacement of the olefin
ligand by the free phosphine arm with a reaction rate which
varied inversely with [1-hexene], indicative of a dissociative
process. Furthermore, the unsaturated intermediate was shown
to be ∼20 times more selective for olefin coordination as
compared to the uncoordinated phosphine. The molybdenum
analogue was shown to undergo these transformations on a
Figure 9. (A) Difference spectrum obtained upon photolysis of a CO2-saturated solution of (κ2-Triphos)W(CO)4 in CH2Cl2 at 296 K. (B)
Difference spectra obtained upon photolysis of a CO2-saturated solution of (κ2-Triphos)W(CO)4 in the presence of 1.6 M 1-hexene at 298 K. The
strong band in the 2330−2350 cm−1 region is due to CO2.
3168
dx.doi.org/10.1021/om300044k | Organometallics 2012, 31, 3163−3170

Organometallics
Article
1
31P{1H} δ −1.4 (s, JP−W = 211 (satellite), 3 P); H δ 1.43 (m, 3 H,
triethylsilane at ambient temperature. On the basis of the νCO
bands in the HSiEt3 adduct and its rapid dissociation, about 15
times more labile than 1-hexene, this species is assumed to be
(κ2-Triphos)W(CO)3(η2-HSiEt3).
3
CH3), 2.35 (m, 6 H, CH2), 7.09 (t, JH−H = 7.2, 12 H, Hm), 7.18 (t,
3JH−H = 7.2, 6 H, Hp), 7.34 (m, 12 H, Ho); 13C NMR data could not be
obtained due to low solubility of the compound. Anal. Calcd for
C44H39O3P3W: C, 59.2; H, 4.4. Found: C, 59.0; H, 4.3.
Kinetic Measurements. In a typical experiment, the photolysis
solution was ∼4 mM in (κ2-Triphos)W(CO)4 dissolved in dichloro-
methane with varying amounts of 1-hexene. The reactions were
conducted over a 25 K temperature range from 278 to 303 K, and the
1-hexene concentration was varied over a 50-fold range (0.08−4 M). A
Bruker Vertex 80 FTIR equipped with both step-scan and rapid-scan
capabilities was used to obtain the IR spectra using a temperature-
controlled 0.75 mm path length transmission cell with CaF2 windows.
Spectra were obtained at 8 cm−1 resolution. Photolysis was conducted
with 355 nm light from a Nd:YAG laser (Continuum Surelite I-10).
Spectral acquisition was initiated following a single shot of the UV
laser (30 mJ/pulse), which was collinear with the IR beam and
defocused to ensure irradiation of the entire cell volume.
EXPERIMENTAL SECTION
■
Methods and Materials. All reactions were carried out under an
argon atmosphere. Dichloromethane was purified by an MBraun
Manual Solvent Purification System packed with Alcoa F200 acti-
vated alumina desiccant. Decalin (Aldrich) was degassed before use.
Methanol and chloroform were used as purchased from EMD
chemicals. The ligand 1,1,1-tris(diphenylphosphinomethyl)ethane
(Triphos) was purchased from Sigma-Aldrich, and the metal−carbonyl
precursors M(CO)4(pip)2 (M = Mo, W) were prepared as per the
literature.9 NMR spectra were recorded on a Varian INOVA 300
(operating at 299.96 and 121.43 MHz for H and 31P, respectively)
1
and Varian MERCURY 300 (operating at 75.42 MHz for 13C) spectro-
1
meters. H and 13C NMR spectra were referenced to residual solvent
resonances, while 31P NMR spectra were referenced to an external
H3PO4 in D2O at 0.0 ppm. Infrared spectra were obtained on a Bruker
Tensor 27 FTIR spectrometer. In situ IR monitoring was carried
out using a Mettler Toledo iC10 ReactIR with an AgX fiber conduit
probe. Elemental analyses were determined by Atlantic Microlab
(Norcross, GA).
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files giving X-ray structural data for 3a,b. This material is
available free of charge via the Internet at http://pubs.acs.org.
Synthesis of (κ2-Triphos)Mo(CO)4. Mo(CO)4(pip)2 (0.894 g,
2.36 mmol) and Triphos (1.500 g, 2.40 mmol) in dichloromethane
(100 mL) were heated to reflux for 30 min. The resulting orange
solution was filtered through Celite, and the filtrate was reduced in
volume to ca. 5 mL and treated with methanol to give a tan precipitate.
Recrystallization from chloroform/methanol yielded a white product
(1.550 g, 79%). IR data in CH2Cl2 (νCO): 1870 (sh), 1901 (s), 1920
(sh), 2020 (m). NMR data in CDCl3: 31P{1H} δ −27.7 (t, JP−P = 2, 1 P),
19.7 (d, JP−P = 2, 2 P); 1H δ 0.75 (s, 3 H, CH3), 2.15 (m, 2 H, CH2), 2.43
(dd, 2JH−H = 14, 2JH−P = 4, 2 H, CH2), 2.71 (dd, 2JH−H = 14, 2JH−P = 9, 2
H, CH2), 7.25−7.46 (m, 26 H, C6H5), 7.60−7.70 (m, 4 H, C6H5);
selected 13C{1H} δ 209.6 (t, 2JC−P = 8, 1 C, CO), 211.1 (t, 2JC−P = 9, 1 C,
CO), 215.3 (m, 2 C, trans-CO). Anal. Calcd for C45H39MoO4P3: C, 64.9;
H, 4.7. Found: C, 64.4; H, 4.6.
AUTHOR INFORMATION
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This publication was made possible by NPRP Grant No. 09-
157-1-024 from the Qatar National Research Fund (a member
of Qatar Foundation). The statements made herein are solely
the responsibility of the authors. M. K. was a participant in the
NSF REU Program (No. CHE-1062840) at College Station, TX.
REFERENCES
Synthesis of (κ3-Triphos)Mo(CO)3. Mo(CO)4(pip)2 (0.897 g,
2.37 mmol) and Triphos (1.504 g, 2.41 mmol) in decalin (110 mL)
were heated to reflux for 19 h. The resulting brown solid was isolated
by filtration and washed with methanol until the filtrate ran clear.
Recrystallization from chloroform/methanol yielded a white product
(1.410 g, 74%). IR data in CH2Cl2 (νCO): 1844 (s), 1938 (s). NMR
data in CDCl3: 31P{1H} δ 18.2 (s, 3 P); 1H δ 1.44 (m, 3 H, CH3), 2.26
(m, 6 H, CH2), 7.08 (t, 3JH−H = 7.2, 12 H, Hm), 7.18 (t, 3JH−H = 7.2, 6
H, Hp), 7.35 (m, 12 H, Ho); 13C NMR data could not be obtained due
to low solubility of the compound. Anal. Calcd for C44H39MoO3P3: C,
65.7; H, 4.9. Found: C, 65.5; H, 4.7.
■
(1) (a) Aresta, M. In Carbon Dioxide as Chemical Feedstock; Aresta,
M., Ed.; Wiley-VCH: Weinheim, Germany, 2010. (b) Mathers, R. T.
J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 1.
(2) For reviews in this area, see: (a) Coates, G. W.; Moore, D. R.
Angew. Chem., Int. Ed. 2004, 43, 6618. (b) Darensbourg, D. J.;
Mackieweicz, R. M.; Phelps, A. L.; Billodeaux, D. R. Acc. Chem. Res.
2004, 37, 836. (c) Darensbourg, D. J. Chem. Rev. 2007, 107, 2388.
(d) Sugimoto, H.; Inoue, S. J. Polym. Sci., Part A: Polym. Chem. 2004,
42, 5561. (e) Kember, M. R.; Buchard, A.; Williams, C. K. Chem.
Commun. 2011, 141. (f) Klaus, S.; Lehenmeier, M. W.; Anderson,
C. E.; Rieger, B. Coord. Chem. Rev. 2011, 255, 1460.
(3) Price, C. J.; Reich, B. J. E.; Miller, S. A. Macromolecules 2006, 39,
2751.
(4) (a) Hoberg, H.; Schaefer, D. J. Organomet. Chem. 1982, 236, C28.
(b) Hoberg, H.; Schaefer, D. J. Organomet. Chem. 1983, 251, C51.
(c) Walther, D.; Dinjus, E.; Sieler, J.; Anderson, L; Lindqvist, O.
Synthesis of (κ2-Triphos)W(CO)4. W(CO)4(pip)2 (0.700 g,
1.50 mmol) and Triphos (1.000 g, 1.60 mmol) in dichloromethane
(100 mL) were heated to reflux for 4 h. The resulting clear yellow
solution was reduced in volume to ca. 5 mL and treated with methanol
to give a yellow precipitate. Recrystallization from chloroform/methanol
yielded a pale yellow product (0.597 g, 43%). IR data in CH2Cl2 (νCO):
1861 (sh), 1892 (s), 1915 (sh), 2011 (m). NMR data in CDCl3:
31P{1H} δ −27.7 (t, JP−P = 2, 1 P), 0.15 (d, JP−P = 2 (JP−W = 226,
J. Organomet. Chem. 1984, 276, 99. (d) Walter, D.; Braunlich, G.;
̈
Kempe, R.; Sieler, J. J. Organomet. Chem. 1992, 436, 109.
(5) (a) Alvarez, R.; Carmona, E.; Colc-Hamilton, D. J.; Galindo, A.;
Gutierrez-Puebla, E.; Monge, A.; Poveda, M. L.; Ruiz, C. J. Am. Chem.
Soc. 1985, 107, 5529. (b) Alvarez, R.; Carmona, E.; Galindo, A.;
Gutierrez, E.; Marin, J. M.; Monge, A.; Poveda, M. L.; Ruiz, C.;
Savariault, J. M. Organometallics 1989, 8, 2430. (c) Galindo, A.; Pastor,
A.; Perez, P.; Carmona, E. Organometallics 1993, 12, 4443.
(6) Lee, S. Y. T.; Cokoja, M.; Drees, M.; Li, Y.; Mink, J.; Herrmann,
W. A.; Kuhn, F. E. ChemSusChem 2011, 4, 1275.
1
satellite), 2 P); H δ 0.76 (s, 3 H, CH3), 2.13 (m, 2 H, CH2), 2.52
(dd, 2JH−H = 14, 2JH−P = 5, 2 H, CH2), 2.81 (dd, 2JH−H = 14, 2JH−P = 10,
2 H, CH2), 7.27−7.45 (m, 26 H, C6H5), 7.58−7.68 (m, 4 H, C6H5);
selected 13C{1H} δ 202.1 (t, 2JC−P = 7, 1 C, CO), 203.5 (t, 2JC−P = 7, 1
C, CO), 206.0 (m, 2 C, trans-CO). Anal. Calcd for C45H39O4P3W: C,
58.7; H, 4.3. Found: C, 58.1; H, 4.2.
Synthesis of (κ3-Triphos)W(CO)3. W(CO)4(pip)2 (1.101 g, 2.36
mmol) and Triphos (1.504 g, 2.41 mmol) in decalin (100 mL) were
heated to reflux for 20 h. The resulting gray solid was isolated by
filtration and washed with ether (2 × 25 mL). Recrystallization from
chloroform/methanol twice yielded a white product (0.874 g, 42%).
IR data in CH2Cl2 (νCO): 1841 (s), 1933 (s). NMR data in CDCl3:
(7) Bernskoetter, W. H.; Tyler, B. T. Organometallics 2011, 30, 520.
(8) Chatt, J.; Leigh, G. J.; Thankarajan, N. J. Organomet. Chem. 1971,
29, 105.
(9) Darensbourg, D. J.; Kump, R. L. Inorg. Chem. 1978, 17, 2680.
3169
dx.doi.org/10.1021/om300044k | Organometallics 2012, 31, 3163−3170

Organometallics
Article
(10) Fernandez, E. J.; Gimeno, M. C.; Jones, P. G.; Laguna, A.;
Laguna, M.; Olmos, E. J. Chem. Soc., Dalton Trans. 1996, 3603.
(11) (a) Keiter, R. L.; Ye, P.; Keiter, E. A.; Benson, J. W.; Lin, W.;
Brandt, D. E.; Southern, J. S.; Rheingold, A. L.; Guzei, I.; Wheeler,
K. A.; Cary, L. W. Inorg. Chim. Acta 2010, 364, 176. (b) Darensbourg,
D. J.; Wiegreffe, H. P. Inorg. Chem. 1990, 29, 592. (c) Darensbourg, D.
J.; Joyce, J. A.; Bischoff, C. J.; Reibenspies, J. H. Inorg. Chem. 1991, 30,
1137.
(12) (a) Walter, Q.; Klein, T.; Huttner, G.; Zsolnai, L. J. Organomet.
Chem. 1993, 458, 63. (b) Dilsky, S. J. Organomet. Chem. 2007, 692,
2887.
(13) Etter, M. C.; Baures, P. W. J. Am. Chem. Soc. 1988, 110, 639.
(14) (a) Lee, M.; Harris, C. B. J. Am. Chem. Soc. 1989, 111, 8963.
(b) Yu, S. C.; Xu, X.; Lingle, R. Jr.; Hopkins, J. B. J. Am. Chem. Soc.
1990, 112, 3668. (c) Joly, A. G.; Nelson, K. A. Chem. Phys. 1991, 152,
69. (d) Hall, C.; Perutz, R. N. Chem. Rev. 1996, 3125. (e) Xie, X.;
Simon, J. D. J. Am. Chem. Soc. 1990, 112, 1130.
(15) Zheng, Y.; Wang, W.; Lin, J.; She, Y.; Fu, K.-J. J. Phys. Chem.
1992, 96, 7650.
(16) An average value from the references in ref 15.
(17) (a) McNamara, B.; Becher, D. M.; Towns, M. H.; Grant, E. R.
J. Phys. Chem. 1994, 98, 4622. (b) McNamara, B.; Towns, M. H.;
Grant, E. R. J. Am. Chem. Soc. 1995, 117, 12254. (c) Wells, J. R.;
House, P. G.; Weitz, E. J. Phys. Chem. 1994, 98, 11256.
(18) Cedeno, D. L.; Weitz, E. J. Am. Chem. Soc. 2001, 123, 12857.
̃
(19) Schlappi, D. N.; Cedeno, D. L. J. Phys. Chem. A 2009, 113, 9692.
̃
(20) Cedeno, D. L.; Sniatynsky, R. Organometallics 2005, 24, 3882.
̃
(21) It is important to note here that this experiment involves
complex 1b leading to 2b plus CO upon photolysis in the presence of
CO2 and does not involve the uncoordinating phosphine oxide
complex 3b.
3170
dx.doi.org/10.1021/om300044k | Organometallics 2012, 31, 3163−3170