Functionalization of carbon dioxide with ethylene at molybdenum hydride complexes

Article abstract of DOI:10.1021/om400448m

The molybdenum tetrahydride species (Triphos)MoH₄PPh₃ (Triphos = PhP(CH₂CH₂PPh₂)₂) generated from sodium triethylborohydride addition to (Triphos)MoCl₃ was found to promote CO₂ functionalization to afford acrylate, propionate, and formate species. The formation of (Triphos)MoH ₄PPh₃ occurs via a (Triphos)Mo(H)Cl(PPh₃) intermediate followed by dismutation of an unobserved six-coordinate molybdenum(II) dihydride complex. Addition of dihydrogen to the dismuation product mixture affords a nearly quantitative yield of (Triphos)MoH ₄PPh₃. The molybdenum tetrahydride species facilitates CO₂ insertion into a metal hydride to produce a formate complex, (Triphos)Mo(H)(κ²-CHO₂)(PPh₃), with an observed rate constant of [2.9(2)] × 10^-4 s^-1 (25 C), which is independent of CO₂ pressure. Selective formation of acrylate and propionate carbon dioxide-ethylene coupling products, (Triphos)Mo(H)(κ²-C₃H₃O ₂)(PPh₃) and (Triphos)Mo(H)(κ²-C ₃H₅O₂)(PPh₃), was achieved by sequential addition of olefin and heterocumulene to (Triphos)MoH ₄PPh₃. A formally zerovalent TriphosMo(η²- C₂H₄)₃ intermediate was characterized by NMR spectroscopy and computational analysis along the pathway for carbon dioxide-ethylene coupling.

Full text of DOI:10.1021/om400448m

Article
pubs.acs.org/Organometallics
Functionalization of Carbon Dioxide with Ethylene at Molybdenum
Hydride Complexes
Yuanyuan Zhang,† Brian S. Hanna,† Andrew Dineen,‡ Paul G. Williard,† and Wesley H. Bernskoetter*,†
†Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States
‡The Charles Stark Draper Laboratory, Inc., Cambridge, Massachusetts 02139, United States
S
* Supporting Information
ABSTRACT: The molybdenum tetrahydride species (Triphos)MoH4PPh3
(Triphos = PhP(CH2CH2PPh2)2) generated from sodium triethylborohydride
addition to (Triphos)MoCl3 was found to promote CO2 functionalization to afford
acrylate, propionate, and formate species. The formation of (Triphos)MoH4PPh3
occurs via a (Triphos)Mo(H)Cl(PPh3) intermediate followed by dismutation of an
unobserved six-coordinate molybdenum(II) dihydride complex. Addition of
dihydrogen to the dismuation product mixture affords a nearly quantitative yield of (Triphos)MoH4PPh3. The molybdenum
tetrahydride species facilitates CO2 insertion into a metal hydride to produce a formate complex, (Triphos)Mo(H)(κ2-
CHO2)(PPh3), with an observed rate constant of [2.9(2)] × 10−4 s−1 (25 °C), which is independent of CO2 pressure. Selective
formation of acrylate and propionate carbon dioxide−ethylene coupling products, (Triphos)Mo(H)(κ2-C3H3O2)(PPh3) and
(Triphos)Mo(H)(κ2-C3H5O2)(PPh3), was achieved by sequential addition of olefin and heterocumulene to (Triphos)-
MoH4PPh3. A formally zerovalent TriphosMo(η2-C2H4)3 intermediate was characterized by NMR spectroscopy and
computational analysis along the pathway for carbon dioxide−ethylene coupling.
examined the mechanism and product extrusion pathway for
similar reactions at group VI metals using PhP(CH2CH2PPh2)2
(Triphos) and other ligand platforms.10 These investigations
suggested that the (Triphos)Mo fragment could be useful in
other CO2 functionalization reactions which also incorporate
hydrogen (or hydride) reducing agents, a potential step for
accessing either fuels or commodity chemicals from CO2.
Herein we report the reductive functionalization of CO2 to
acrylate, proprionate, and formate by a single, easily accessed
molybdenum polyhydride species. Multiple intermediates along
these stoichiometric CO2 activation routes have been
characterized and the pathways for the reactions examined.
INTRODUCTION
■
The rising cost and environmental impact of fossil fuel
utilization has renewed interest in carbon dioxide functionaliza-
tion for use in carbon-neutral energy supplies and commodity
chemical production.1,2 The reduction of CO2 to fuels, such as
methane, methanol, and formate, has received substantial
investigation, with many notable successes in activating CO2
with hydrogen equivalents.3 Analogous reductions of CO2 to
commodity chemicals have been less explored, despite the
potential to leverage a renewable and inexpensive C1 chemical
feedstock.4 The contrasting scope of examination between
these two endeavors is no doubt influenced by the higher
consumption of petroleum as a fossil fuel than as an industrial
chemical precursor. Nevertheless, the ability to convert CO2
into industrially desirable chemicals would still contribute
significantly to harnessing one of the world’s most underutilized
renewable resources.5 Additionally, transition-metal-mediated
CO2 reduction pathways toward fuels and commodity
chemicals likely share many elementary reaction features and
could be pursued in unison.1,2
Currently the chemical functionalization of carbon dioxide in
commercial processes is limited to a small collection of
products, including urea, salicylic acids, and carbonates,6
though many other thermodynamically viable products have
been proposed and studied.2−4,7 Our laboratory has recently
become interested in the reductive functionalization of CO2
using ethylene and other small molecules which could furnish
valuable acrylates and propionates used in applications ranging
from polymeric materials to food preservatives.8 Inspired by
seminal reports of carbon dioxide−ethylene coupling from
Hoberg and Carmona in the 1980s,9 our laboratory has
RESULTS AND DISCUSSION
■
Synthetic Utility of (Triphos)MoCl3 Reduction with
NaEt3BH. Traditional synthetic routes to obtain low-valent
molybdenum− and tungsten−phosphine complexes used in
CO2 functionalization reactions apply alkali-metal sources (e.g.,
sodium amalgam, potassium graphite) to metal chloride
precursors in the presence of modest stabilizing ligands (e.g.,
dinitrogen, phosphines).11 These alkali-metal reduction meth-
ods work well but are challenging to perform on a large scale,
require the use of toxic mediators (e.g., mercury), and present
significant safety hazards. Homogeneous hydride reducing
agents offer a relatively mild and convenient alternative to
accessing lower valent metals, by either direct reduction of a
metal halide precursor or reductive elimination from
intermediate metal−hydride species.12 Given our interest in
Received: May 21, 2013
Published: June 28, 2013
© 2013 American Chemical Society
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incorporating hydrogen atom sources into CO2 activation
reactions at low valent molybdenum, the use of organic soluble
hydride reducing agents, such as sodium triethylborohydride
(NaEt3BH), seemed a reasonable starting point for inves-
tigation. Gratifyingly, treatment of a tetrahydrofuran suspension
of (Triphos)MoCl3 (1-Cl3) with 2 equiv of NaEt3BH in the
presence of triphenylphosphine rapidly afforded a
molybdenum(II) species, (Triphos)Mo(H)Cl(PPh3) (2-HCl),
as a deep green solid in excellent yield (eq 1). The pathway for
Information. The data were of sufficient quality such that all
hydrogens, including that attached to molybdenum, were
located and refined from the Fourier map. The solid-state
structure of 2-HCl exhibits a coordination environment highly
distorted from octahedral symmetry. This feature is probably
induced by the strong trans influence ligand and the steric
congestion of the multiple phenylphosphine substituents. This
distortion is particularly evident in the Cl(1)−Mo(1)−H(1)
and P(3)−Mo(1)−P(4) angles of 157(2) and 107.49(2)°,
respectively, which draw the PPh2 groups down toward the
relatively open molybdenum−hydride pocket. The diamagnet-
ism of 2-HCl is likely a consequence of the hydride-induced
distortion, as the corresponding molybdenum(II) dichloride
complex (Triphos)MoCl2(PPh3) and related species with
octahedral symmetry exhibit the expected magnetic moments
for S = 1 metals.15
Complex 2-HCl has proven a useful synthetic precursor to
several new molybdenum species relevant to our studies in CO2
functionalization (vide infra), as well as zerovalent molybde-
num complexes our laboratory has previously employed in the
preparation of acrylate from CO2 and ethylene coupling. Prior
studies demonstrate that both trans-(Triphos)Mo(N2)2(C2H4)
(1-N2) and trans-(Triphos)Mo(N2)2PPh3 (2-N2) are capable of
functionalizing CO2 with ethylene to afford mono- and dimeric
forms of molybdenum(II) acrylate hydride species (Figure
2).10a Vinylation of 2-HCl can provide alternate access to these
this transform likely proceeds via well-precedented hydride for
chloride substitution and dinuclear reductive elimination of H2,
though the mechanistic details of this reaction have not been
examined here.13 Complex 2-HCl was characterized by a
combination of NMR spectroscopy, combustion analysis, and
X-ray diffraction, as well as by comparison to a closely related
molybdenum(II) hydrido chloride species isolated by George
and co-workers during protonolysis of molybdenum dinitrogen
compounds.14 The H NMR spectrum of diamagnetic 2-HCl
1
exhibited a characteristically upfield shifted Mo−H resonance at
−2.42 ppm with a multiplicity consistent with coupling to an
A2BC 31P spin system. Three corresponding peaks with a 2:1:1
integration ratio were observed in the 31P{1H} NMR spectrum
at 132.2 (dd), 100.1 (dt), and 47.5 (dt) ppm and assigned to
the PPh2, PPh, and PPh3 fragments, respectively. The molecular
structure of 2-HCl was also confirmed by single-crystal X-ray
diffraction (Figure 1), with relevant metrical parameters for this
and all subsequent crystal structures given in the Supporting
Figure 2. Acrylate formation from CO2 and ethylene promoted by 1-
N2 and 2-N2.10a
same active species without the sodium amalgam reductions
employed previously.10a,11b Addition of vinylmagnesium
chloride to a tetrahydrofuran solution of 2-HCl in the presence
of dinitrogen immediately afforded 1-N2 and free PPh3.
Subsequent substitution of ethylene by PPh3 to yield 2-N2
required at least 30 min at ambient temperature to give
concentrations detectable by NMR spectroscopy (Figure 3).16
However, it should be noted that quick isolation and multiple
recrystallizations are often necessary to purify samples of 1-N2
from free triphenylphosphine and the resultant magnesium
halide salts. These issues have been resolved through an
improved synthesis of 1-N2, as described in the Experimental
Section.
Figure 1. Molecular structure of (Triphos)Mo(H)Cl(PPh3) (2-HCl)
with ellipsoids at the 30% probability level. All hydrogen atoms, except
that attached to molybdenum, all phenyl carbons, except those
attached to phosphorus, and a cocrystallized diethyl ether molecule are
omitted for clarity.
The molybdenum(II) hydrido chloride species provided
more convenient access to 1-N2 for CO2−ethylene coupling
reactions but also opened an attractive synthetic route directly
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Figure 3. Preparation of 1-N2 and 2-N2 via vinylation of a molybdenum(II) hydrido chloride species.
to 2-acrylate, via treatment with silver acrylate (eq 2). This
method does not derive the acrylate from CO2 and ethylene
of the acrylate ligand, which retains a carbon−carbon double
bond, indicated by the C(2)−C(3) bond length of 1.36(2) Å.
Formation of (Triphos)MoH4PPh3. Having successfully
employed 2-HCl in the synthesis of complexes which couple
CO2 and ethylene to acrylate, additional pathways for CO2
functionalization were sought from this platform. To induce
further reactivity, the remaining chloride ligand in 2-HCl was
substituted for hydride using 1 equiv of NaEt3BH (Figure 5).
but afforded substantial quantities of 2-acrylate with relative
ease and permitted isolation of crystals suitable for X-ray
diffraction. The molecular structure of 2-acrylate (Figure 4)
Figure 5. Hydride substitution and dismutation from 2-HCl.
Interestingly, pale yellow crystals of (Triphos)MoH4PPh3 (2-
H4) were isolated from the reaction mixture instead of the
expected molybdenum(II) dihydride species. Complex 2-H4
may also be obtained by addition of 3 equiv of NaEt3BH
directly to 1-Cl3.17,18 Similar eight-coordinate molybdenum
polyhydride species have been reported which, along with
variable-temperature NMR spectroscopy and X-ray crystallog-
raphy, aided in the characterization of 2-H4.19 The H NMR
1
spectrum of 2-H4 in benzene-d6 at ambient temperature
featured a broad metal−hydride signal at −3.49 ppm with an
integration corresponding to four hydrogens. Three resonances
comprising an A2BC spin system were also observed in the
31P{1H} NMR spectrum at 119.8, 84.1, and 74.4 ppm. Cooling
tetrahydrofuran-d8 solutions of 2-H4 to −70 °C produced
virtually no change in the 31P NMR signals but resulted in
decoalescence of the Mo−H signal in the 1H NMR spectrum.20
At low temperature, the hydride resonances appear as three
sharp but complex multiplets centered at −2.71, −4.03, and
−5.44 ppm with a 1:2:1 integration ratio (Figure 6). This
variable-temperature behavior indicates the four hydrogens
attached to the metal interchange rapidly at ambient temper-
ature through either geometric rearrangement or reversible
dihydride−dihydrogen equilibria.21 Due to the exchange of the
metal−hydride positions and the inability to obtain spectra at
lower temperatures, measurement of true T1(min) values for
each resonance was obviated, though a T1 value of 780 ms (400
MHz) was measured for the three peaks at −70 °C. Detailed
prior studies on closely related molybdenum and tungsten
polyhydride complexes imply 2-H4 may best be described as a
classical tetrahydride molybdenum(IV) structure.22
Figure 4. Molecular structure of (Triphos)Mo(H)(κ2-C3H3O2)(PPh3)
(2-acrylate) with ellipsoids at 30% probability. All hydrogen atoms,
except that attached to molybdenum, and all phenyl carbons, except
those attached to phosphorus, are omitted for clarity.
confirms the characterization previously reported using NMR
and IR spectroscopy. The data reveal an approximate
pentagonal-bipyramidal geometry about the molybdenum,
with P(1)−Mo(1)−P(2) (173.99(7)°) constituting the axis
and the metal−hydride, carboxylate oxygens, and other
phosphines comprising the equatorial pentagon. The data
were again of sufficient quality such that all hydrogens,
including that attached to molybdenum, were located and
refined from the Fourier map. As with 2-HCl, the Triphos PPh2
groups in 2-acrylate are drawn toward the molybdenum−
hydride site with a P(3)−Mo(1)−P(4) bond angle of
110.90(6)°. The structure also illustrates the κ2 coordination
The solid-state structure of 2-H4 was confirmed by X-ray
diffraction (Figure 7) and was consistent with the symmetry
expected from low-temperature NMR spectra. Despite the
incorporation of an isotropic, disordered solvent molecule with
no hydrogens which could be located and refined, the data for
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1
1
Figure 6. Partial H NMR (top) and H{31P} NMR (bottom) spectra of 2-H4 at −70 °C in tetrahydrofuran-d8.
copy in the presence of an internal integration standard.
Treatment of a tetrahydrofuran solution of 2-HCl with 1 equiv
of NaEt3BH immediately bleached the dark green color from
the solution and afforded 2-H4 as the only diamagnetic product
(neglecting minor amounts of free PPh3) in approximately 33%
yield based on molybdenum. The low yield suggested that two-
thirds of the molybdenum was converted to a paramagnetic
1
product (Figure 5). Examination of the H NMR spectrum
revealed no signals between 150 and −150 ppm assignable to a
paramagnetic molybdenum species. Conducting the NMR
experiment with a ferrocene standard confirmed the presence
of a paramagnetic product by the Evans method,24 but without
a reliable magnetic susceptibility measurement, given the
mixture of products. Despite this frustration in characterizing
the paramagnetic product, exposure of the in situ reaction
mixture to 1 atm of dihydrogen induced growth in the NMR
signals of 2-H4 equivalent to a >90% total yield. No additional
organic or molybdenum species were observed upon H2
addition, and control experiments using greater amounts of
NaEt3BH did not change the initial yield of 2-H4.
The limited data regarding the paramagnetic product of the
2-HCl and NaEt3BH reaction prevented definitive character-
ization, but several features regarding the species may be
inferred from its reactivity and stoichiometry. The 1:2 product
ratio between 2-H4 and the uncharacterized species suggests a
molybdenum(I) oxidation state. Additionally, the ability to
form 2-H4 without extrusion of other detectable organics
during hydrogenation implies that the species has only
phosphine and hydride ligands. One pathway that fits the
observations is hydride dismutation from an unstable
(Triphos)MoH2PPh3 intermediate.25 (Triphos)MoH2PPh3
was initially the expected product from 2-HCl and NaEt3BH,
but in fact, examples of six-coordinate molybdenum(II)
dihydride species are exceptionally rare,26 especially in
comparison to the corresponding molybdenum tetrahydride
complexes.19,22,23,27 This hypothesis leads to a speculative
assignment of (Triphos)Mo(H)PPh3 for the paramagnetic
species. However, the relevance of this paramagnetic species to
the CO2 functionalization studies described below is minimized
by the ability to hydrogenate this uncharacterized species to 2-
H4 and recrystallize with elemental purity.
Figure 7. Molecular structure of (Triphos)MoH4PPh3 (2-H4) with
ellipsoids at the 30% probability level. All hydrogen atoms, except
those attached to molybdenum, all phenyl carbons, except those
attached to phosphorus, and a cocrystallized solvent molecule are
omitted for clarity.
2-H4 did permit refinement of four hydrogens attached to
molybdenum from the Fourier map. Complex 2-H4 exhibits a
generalized dodecahedral structure consisting of two inter-
penetrating tetrahedra of MoH4 and MoP4 fragments, with the
MoP4 fragment significantly distorted toward planarity. These
structural attributes are similar to those reported for
MoH4(PMePh2)4 and MoH4(dppe)2.23 Though not crystallo-
graphically equivalent, H(97) and H(98) bisect an idealized
mirror plane passing through P(1)−P(2)−H(95)−H(96). This
feature accounts for the two equivalent (H(97) and H(98))
and inequivalent (H(95) and H(96)) hydride environments in
the solution NMR spectra (vide supra). The X-ray diffraction
data show long H−H interaction distances (>1.80(6) Å)
consistent with a dominantly molybdenum(IV) tetrahydride
ground state, though caution must be exercised when using
these X-ray-determined distances in dihydride−dihydrogen
structural determinations.21
Carbon Dioxide Functionalization. The molybdenum-
(IV) tetrahydride complexes isolated from sodium triethylbor-
ohydride addition to 2-HCl exhibited a higher valence state
than our laboratory initially sought for CO2 functionalization
Isolation of 2-H4 from the combination of 2-HCl and
NaEt3BH cannot account for the full mass balance of the
reaction and motivated in situ monitoring by NMR spectros-
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Figure 8. CO2 reductive functionalization from 2-H4.
with ethylene, but 2-H4 was nevertheless examined for
reactivity. Treatment of 2-H4 with 1 atm of a 5:1 ethylene to
carbon dioxide mixture produced what initially appeared to be
an intractable mixture of products. However, subsequent
experiments which altered the ratio and sequence of ethylene
and carbon dioxide addition permitted isolation of (Triphos)-
Mo(H)(κ2-CHO2)(PPh3) (2-formate), (Triphos)Mo(H)(κ2-
C3H5O2)(PPh3) (2-propionate), and 2-acrylate, three distinct
CO2 activation products (Figure 8). Application of a 5:1
ethylene to carbon dioxide mixture to 2-H4 produced 2-
formate as the major product of CO2 functionalization;
however, increasing the carbon dioxide fraction to a 1:1
mixture gave virtually quantitative formation of the
molybdenum(II) formate hydride species over 2 h, as judged
by NMR spectroscopy. In preparative-scale experiments, 2-
formate was isolated in good yield by treating 2-H4 with an
atmosphere of pure CO2.
Table 1. Pressure Dependence of Rate Constants for
Formation of 2-formate from 2-H4
a
PCO (atm)
kobs (10−4 s−1
)
2
0.5
1.0
1.5
2.0
2.7(2)
2.9(2)
2.9(2)
2.8(2)
3.0(2)
2.5
a
Rate constants measured at 25 °C in well-mixed tetrahydrofuran
solutions.
precedes CO2 insertion (Figure 9).31 An alternative route
necessitating dissociation of a chelating phosphine ligand could
The complex 2-formate was obtained as a red powder and
characterized by NMR and IR spectroscopy, isotopic labeling,
and elemental analysis. The 1H and 31P{1H} NMR spectra of 2-
formate bear several similarities to those reported for 2-
acrylate, including a Mo−H signal at −4.74 ppm and three
1
phosphorus resonances at 122.6, 103.5, and 50.1 ppm. The H
NMR spectrum also exhibited a signal at 7.11 ppm originating
from the formate hydrogen. Its identity was confirmed by
13CO2 isotopic labeling, which resulted in enhanced coupling in
1
the H NMR signal and growth in a resonance at 169.13 ppm
in the 13C{1H} NMR spectrum. 13C labeling also altered the
solid-state infrared spectrum, with bands at 1550 and 1362
cm−1 for 2-formate being red-shifted down to 1511 and 1340
cm−1 for the labeled isotopologue.
Figure 9. Proposed pathway for CO2 insertion into 2-H4.
The insertion of carbon dioxide into transition metal−
hydride bonds is a key step in the hydrogenation of CO2 to
formate and has previously been observed for other
molybdenum complexes.19a,26a,28 Recent advances in the
catalytic hydrogenation of carbon dioxide under basic
conditions at late transition metals have enhanced the potential
of CO2 hydrogenation as an energy carrier for molecular
hydrogen and an input for carbon fuel cells.3d,e,29 The insertion
of carbon dioxide into 2-H4 occurred with loss of dihydrogen
(detected by 1H NMR spectroscopy) and no evidence of
organic formate products. Previous studies in the field have
concluded that CO2 insertion into a metal−hydride may occur
without precoordination of the heterocumulene.30 Thus, it is
reasonable to consider the direct insertion of CO2 into 18-
electron 2-H4, followed by reductive H2 elimination, as a
possible route to 2-formate. However, monitoring the
conversion of 2-H4 to 2-formate over a range of CO2 pressures
(Table 1) showed no influence on the observed rate constant of
the reaction. The persistent [2.9(2)] × 10−4 s−1 (25 °C) rate
constant indicates that incorporation of CO2 occurs after the
rate-limiting step and suggests that elimination of dihydrogen
also account for a lack of CO2 dependence but was deemed less
likely, given the relative affinity of carbon dioxide and
phosphine for molybdenum. Attempts to experimentally
distinguish these routes by rate inhibition from excess
dihydrogen were obviated by further reaction of 2-formate
with H2, which afforded an intractable mixture of products. The
reversibility of carbon dioxide insertion was evidenced by
isotopic scrambling of 2-formate and 1 atm of 13CO2 at
ambient temperature over 18 h.
Completing the characterization of 2-formate shifted our
focus toward selective synthesis of the CO2 functionalization
products which incorporated ethylene, 2-propionate, and 2-
acrylate. The formation of 2-formate, 2-propionate, and 2-
acrylate in the same reaction suggested that at least two distinct
CO2 reduction pathways from 2-H4 were competitive.
Divergence between the formate and acrylate/propionate
production routes could be influenced by the relative rates of
2-H4 (or species derived from 2-H4) reacting with carbon
dioxide versus ethylene. However, the observation that addition
of a 5:1 ethylene to carbon dioxide mixture to 2-H4 still
afforded 2-formate as the major product prompted sequential
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addition of ethylene and carbon dioxide instead of experiments
which further altered the ratio of gases.
monitoring of the reaction of 2-H4, ethylene, and CO2.
Exposure of a tetrahydrofuran-d8 solution of 2-H4 to 3 atm of
ethylene resulted in complete decay of the molybdenum
tetrahydride complex over 4 h at ambient temperature.34 The
31P{1H} NMR spectrum displayed a new doublet and triplet at
76.4 and 105.9 ppm, respectively, along with a large peak
Treatment of 2-H4 with 1 atm of ethylene for 5 h followed by
addition of 0.1 atm of CO2 and stirring for a further 14 h
afforded an approximately 7:1 mixture of 2-propionate and 2-
acrylate with no detectable formation of 2-formate. Previously
reported characterization data for 2-acrylate were used to
differentiate the signals corresponding to 2-propionate.10a
Many of the NMR signals for 2-propionate closely resemble
those of 2-acrylate, including 31P{1H} NMR resonances at
110.9, 101.5, and 48.2 ppm and a Mo−H peak centered at
1
corresponding to free PPh3. The H NMR spectrum revealed
six new signals arrayed between −1.32 and 2.13 ppm along with
dissolved ethane and peaks from the Triphos ligand. These
1
resonances appear broad in the ambient-temperature H NMR
spectrum but sharpen considerably upon 31P decoupling and
cooling to −20 °C (Figure 10). 1H−13C HSQC NMR
spectroscopy indicated that these six equal integration signals
corresponded to a total of four methylene 13C NMR
resonances, with two of the methylene carbons bearing
1
−4.70 ppm in the H NMR spectrum. The proton spectrum
also revealed broad triplet and quartet signals at 0.47 and 1.10
ppm, respectively, assigned to the ethyl moiety of the
propionate ligand. The assignments were confirmed by
1H−13C HSQC NMR spectroscopy, which exhibited correla-
tions to methyl and methylene resonances at 8.85 and 30.43
ppm in the 13C NMR spectrum. 13CO2 isotopic labeling
induced additional coupling in the propionate methyl signal in
inequivalent hydrogens.20 The H COSY NMR spectrum also
1
1
indicated three pairwise H−1H coupling interactions among
the six resonances. These data support the assignment of the
immediate product from 2-H4 and ethylene reaction as a
formally zerovalent molybdenum tris(ethylene) complex,
(Triphos)Mo(η2-C2H4)3 (1-C2H4) (Figure 11). Computational
analysis of 1-C2H4 indicated the lowest energy geometry
orients the mutually trans ethylene ligands such that the C−C
bonds are nearly perpendicular to each other with the third
ethylene C−C bond approximately coplanar with the Triphos
chelate (Figure 12). This geometry is consistent with the
number of ethylene 13C and 1H NMR resonances observed for
1-C2H4 as well as the correlations in the 2D NMR spectra.20
Attempts to isolate 1-C2H4 were unsuccessful, as the species
was moderately unstable in the absence of an ethylene
atmosphere.35
1
the H NMR spectrum, confirming coupling of the saturated
ethylene and 13CO2. Observation of JC−H coupling was expected
for the methylene signal as well but was not clearly resolved due
to the broad peak width and overlap with residual solvent.
13CO2 labeling also enhanced a peak at 182.68 ppm in the
13C{1H} NMR spectrum. Additionally, bands at 1514 and 1445
cm−1 in the infrared spectrum of 2-propionate were red-shifted
to 1486 and 1414 cm−1 upon 13C labeling.
The origin of 2-propionate was initially hypothesized as CO2
insertion into a transient molybdenum ethyl complex, but
subsequent observations suggested that the complex more
likely derives from hydrogenation of 2-acrylate. Repeated
syntheses of the 2-propionate and 2-acrylate mixture were
found to increasingly favor 2-propionate at the expense of 2-
acrylate over longer time courses. This is consistent with 2-
acrylate serving as an intermediate to 2-propionate. Addition-
ally, several previous reports of CO2 insertion indicate that the
rates of reactivity with metal−hydrides outpace those of metal−
alkyls in carboxylate formation.30b,32 Given the near-certain
presence of molybdenum−hydride species during this syn-
thesis, it was deemed unlikely that CO2 insertion into a
molybdenum−ethyl would be sufficiently rapid to preclude
formate formation. The origin of 2-propionate was confirmed
by addition of dihydrogen to the 2-propionate and 2-acrylate
mixture, which completed the conversion to 2-propionate.
Identical observations were made upon addition of an
atmosphere of dihydrogen to isolated samples of 2-acrylate
(eq 3). During CO2 functionalization reactions, the dihydrogen
The pathway for formation of 1-C2H4 from 2-H4 likely
involves a sequence of reductive H2 elimination followed by
ethylene hydrogenation. Although dissolved dihydrogen could
1
not be detected by H NMR spectra in the presence of an
excess of ethylene, earlier experiments (vide supra) which
altered the subsequent 2-propionate:2-acrylate ratio by
evacuating the reaction head space strongly support the
presence of H2. Additionally, the ethylene hydrogenation
(confirmed by the presence of ethane) would necessitate a
coordination vacancy to allow ethylene to bind, insert, and
reductively eliminate in the ubiquitous olefin hydrogenation
sequence. Unlike insertion of CO2, there is little evidence for
direct olefin insertion of ethylene into 18-electron transition-
metal−hydride complexes such as 2-H4.36
In situ monitoring of the reaction by NMR spectroscopy was
continued following addition of an atmosphere of CO2 to the
1-C2H4 solution. After 30 min the resonances of 1-C2H4 had
completely disappeared from the 31P{1H} NMR spectrum and
were replaced by minor amounts of 2-propionate and 2-
acrylate along with more significant doublet and triplet signals
at 64.9 and 95.5 ppm, respectively. These resonances match
those previously reported for (Triphos)Mo(CO2)(C2H4), a
transient intermediate observed in kinetic studies of CO2−
ethylene coupling to acrylate (Figure 11).10a Over the next 8 h
the 31P NMR resonances of 2-propionate and 2-acrylate
continued to grow at the expense of (Triphos)Mo(CO2)-
(C2H4). Over the first 2 h following CO2 addition, 2-acrylate
was observed as the major CO2 functionalization product.
However, over longer time courses, 2-propionate became the
major product while the resonances for 2-acrylate diminished.
After 12 h, all (Triphos)Mo(CO2)(C2H4) was consumed,
leaving an approximate 1:3 ratio of 2-acrylate to 2-propionate.
probably originates from 2-H4 reductive elimination. This is
supported by observation that 2-propionate formation was
suppressed when the head space of the reaction vessel was
evacuated between the ethylene and carbon dioxide additions.33
Interest in these remarkable CO2 reductive functionalization
pathways motivated further investigation by in situ NMR
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1
Figure 10. Partial H{31P} NMR spectrum of 1-C2H4 at −20 °C in tetrahydrofuran-d8.
reaction pathway to a broader array of substrates for CO2−
olefin coupling and examine the prospects for catalytic versions
of these reactions. Additionally, we hope to apply the in situ
NaEt3BH reduction procedure in pursuing a wider survey of
ligand−Mo architectures for CO2 functionalization without the
restriction of isolating suitable zerovalent metal complexes.
CONCLUSIONS
■
The wealth of CO2 functionalization chemistry obtained from
2-H4 highlights the relationship between CO2 reduction efforts
toward molecular energy targets and renewable commodity
chemicals. In the case of the (Triphos)Mo platform, reactions
progressing toward each of these goals occur from a single
molybdenum polyhydride species. The reactive 2-H4 complex
appears to form, in part, by dismutation of an unstable six-
coordinate molybdenum(II) dihydride complex produced by
treating 2-HCl with NaEt3BH. Complete conversion of the
dismutation reaction mixture to 2-H4 may be induced by
dihydrogen addition. Our findings demonstrate that CO2
functionalization from 2-H4 most likely occurs via this same
transient molybdenum(II) dihydride complex, with competitive
insertion of ethylene and carbon dioxide ultimately leading to
formate, acrylate, and propionate products. The CO2
functionalization pathways to produce formate and acrylate/
propionate may be selected by utilizing specific reaction
conditions with sequential addition of carbon dioxide and
ethylene. The observed rate constant of formate formation
from 2-H4, [2.9(2)] × 10−4 s−1 (25 °C), was found to be
independent of CO2 pressure (0.5−2.45 atm). The pathway for
CO2 functionalization with olefin proceeds via a formally
zerovalent molybdenum tris(ethylene) complex, 1-C2H4, which
as characterized by in situ NMR spectroscopy and computa-
tional analysis. Coupling of carbon dioxide to 1-C2H4 affords an
acrylate product which may then be rapidly hydrogenated to
produce propionate. In addition, the remarkable array of CO2
functionalization chemistry observed herein may be accessed by
use of a convenient and mild reducing agent, obviating the
harsh alkali-metal reductions commonly employed to prepare
zerovalent group IV metal complexes. This observation
suggests that screening a wider landscape of supporting ligands
for improved rates and selectivity in CO2 functionalization may
Figure 11. Observed pathway for the coupling of carbon dioxide and
ethylene from 2-H4.
Figure 12. Geometry optimized structure for 1-C2H4.
The lack of complete hydrogenation of 2-acrylate may
originate from loss of H2 into the reaction headspace or
through ethylene hydrogenation. These observations create a
partial description of the reaction sequence for CO2
functionalization with ethylene from 2-H4 (Figure 11). Our
laboratory is currently engaged in studies to transfer this
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Then 81 μL (0.08 mmol) of a NaEt3BH solution (1 M in THF) was
added to the suspension, resulting in a color change from dark green
to red-orange over 30 min. After addition of 1 atm of hydrogen gas,
the reaction mixture was stirred a further 14 h at ambient temperature.
The volatiles were then removed in vacuo, the residue was extracted
with toluene, and the filtrate was concentrated to approximately 3 mL.
Careful layering of pentane onto the toluene solution and chilling at
−35 °C afforded 76 mg (92%) of 2-H4 as a yellow powder. Anal.
Calcd for C52H52MoP4 + C7H8 (one cocrystallized toluene): C, 71.65;
be approached by rapid in situ generation of group VI metal
hydrides or other low-valent species.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out using
standard vacuum, Schlenk, cannula, or glovebox techniques. Ethylene
and carbon dioxide were purchased from Corp Brothers and stored
over 4 Å molecular sieves in heavy-walled glass vessels prior to use.
[(Ph2PCH2CH2)2PPh]MoCl3 was obtained as previously described.37
All other chemicals were purchased from Aldrich, Fisher, VWR, Strem,
or Cambridge Isotope Laboratories. Solvents were dried and
deoxygenated using literature procedures.38
1
H, 6.11. Found: C, 71.32; H, 6.05. H NMR (25 °C, C6D6): δ −3.49
(br m, 4H, Mo-H), 1.07 (m, 2H, PCH2), 1.63 (m, 2H, PCH2), 2.12
(m, 2H, PCH2), 2.49−2.77 (m, 2H, PCH2), 6.74−6.82 (m, 6H, C6H5),
6.87 (m, 10H, C6H5), 7.20−7.25 (m, 13H, C6H5), 7.69 (m, 6H,
1
1H, 13C, and 31P NMR spectra were recorded on Bruker DRX 400
MHz Avance, 300 MHz Avance, and 600 MHz Avance spectrometers.
1H and 13C chemical shifts are referenced to residual protio solvent
C6H5), 8.14 (m, 3H, C6H5), 8.28 (t, 2H, C6H5). H NMR (−70 °C,
THF-d8): δ −5.44 (m, 1H, Mo-H), −4.03 (m, 2H, Mo-H), −2.71 (m,
1H, Mo-H), 1.10 (m, 2H, PCH2), 1.26 (m, 2H, PCH2), 2.46−2.57 (m,
2H, PCH2), 2.86−2.99 (m, 2H, PCH2), 6.88 (q, 8H, C6H5), 6.96−7.03
(m, 8H, C6H5), 7.20−7.23 (m, 3H, C6H5), 7.30 (t, 5H, C6H5), 7.39−
7.43 (m, 11H, C6H5), 8.23 (m, 3H, C6H5), 8.32 (t, 2H, C6H5).
13C{1H} NMR (23 °C, C6D6): δ 30.62 (PCH2), 38.12 (PCH2),
signals; 31P chemical shifts are referenced to an external standard of
H3PO4. Probe temperatures were calibrated using ethylene glycol and
methanol as previously described.39 IR spectra were recorded on Jasco
4100 FTIR and Mettler Toledo React IR spectrometers. X-ray
crystallographic data were collected on Bruker Smart Apex I and D8
QUEST diffractometers. Samples were collected in inert oil and
quickly transferred to a cold gas stream. The structures were solved by
direct methods and Fourier syntheses and refined by full-matrix least-
squares procedures with anisotropic thermal parameters for all non-
hydrogen atoms. Crystallographic calculations were carried out using
SHELXTL. Crystal structure data have been deposited with the
CCDC under reference numbers 936630, 936631, and 936632.
Elemental analyses were performed at Atlantic Microlab, Inc., in
Norcross, GA.
Preparation of [(Ph2PCH2CH2)2PPh]Mo(H)(PPh3)(Cl) (2-HCl).
A 20 mL scintillation vial was charged with 0.075 g (0.10 mmol) of 1-
Cl3, 0.025 g (0.10 mmol) of PPh3, and approximately 5 mL of
tetrahydrofuran. Then 204 μL (0.20 mmol) of NaEt3BH solution (1 M
in toluene) was added to the suspension, resulting in a rapid color
change from yellow to dark blue-green. After the mixture was stirred at
ambient temperature for 1 h, the volatiles were removed in vacuo to
afford a dark residue. The residue was washed with pentane to remove
free PPh3 and extracted with toluene, and the extract was dried to
afford 91 mg (97%) of 2-HCl as a dark blue-green solid of good purity,
which can be used in subsequent procedures without further
purification. Analytically pure material can be obtained by recrystal-
lization at −35 °C from toluene solution by layering with diethyl ether.
Anal. Calcd for C52H49MoP4Cl + C4H10O (one cocrystallized diethyl
ether): C, 67.13; H, 6.09. Found: C, 67.30; H, 6.12. 1H NMR (25 °C,
C6D6): δ −2.42 (tdd, 75.04, 42.02, 12.01 Hz, 1H, Mo-H), 0.79 (m,
2H, PCH2), 1.30 (m, 2H, PCH2), 1.56−1.79 (m, 2H, PCH2), 2.42−
2.65 (m, 2H, PCH2), 6.74 (m, 6H, C6H5), 6.87 (m, 7H, C6H5), 6.93−
7.04 (m, 13H, C6H5), 7.20 (m, 5H, C6H5), 7.37 (m, 3H, C6H5), 7.77
(m, 4H, C6H5), 7.95 (t, 2H, C6H5). 13C{1H} NMR (23 °C, C6D6): δ
27.86 (PCH2), 35.91 (PCH2), 127.88, 128.27, 128.49, 128.81, 129.60,
131.41 133.24, 134.31, 135.33, 137.31, 137.61, 141.32, 147.80 (Ar),
three quaternary signals not located. 31P{1H} NMR (23 °C, C6D6): δ
47.5 (dt, 12.1, 163.4 Hz, 1P, PPh3), 100.1 (dt, 24.3, 163.4 Hz, 1P,
PPh), 132.2 (dd, 12.1, 24.3 Hz, 2P, PPh2).
127.53, 127.84, 128.09, 128.31, 128.68, 129.41 132.44, 134.46, 134.55,
135.34, 141.30, 143.28, 144.56 (Ar), three quaternary signals not
located. 31P{1H} NMR (23 °C, C6D6): δ 74.4 (dt, 20.7, 47.4 Hz, 1P,
PPh3), 84.1 (d, 20.7 Hz, 2P, PPh2), 119.8 (d, 47.4 Hz, 1P, PPh).
Preparation of [(Ph2PCH2CH2)2PPh]Mo(H)(PPh3)(κ2O,O-
CHO2) (2-formate). Method A. A heavy-walled glass reaction vessel
was charged with 0.060 g (0.067 mmol) of 2-H4 and approximately 4
mL of tetrahydrofuran. On a high-vacuum line, 10 equiv of carbon
dioxide (460 Torr in 28.9 mL) was admitted to the reaction mixture at
−196 °C via a calibrated gas bulb. After the mixture was stirred for 2 h
at ambient temperature, the volatiles were removed in vacuo, the
residue was extracted with diethyl ether, and the extract was filtered
through Celite and dried to afford 57 mg (99%) of 2-formate as a red-
orange solid with excellent purity. Crystalline material may be obtained
through layering pentane on a concentrated diethyl ether solution and
chilling at −35 °C.
Method B. A heavy-walled glass reaction vessel was charged with
0.075 g (0.10 mmol) of 1-Cl3, 0.025 mg (0.10 mmol) of PPh3, and
approximately 5 mL of tetrahydrofuran. Then 305 μL (0.30 mmol) of
NaEt3BH solution (1 M in toluene) was added to the suspension and
stirred for 30 min. On a high vacuum line, 10 equiv of carbon dioxide
(187 Torr in 101 mL) was admitted to the reaction mixture at −196
°C via a calibrated gas bulb. After the mixture was stirred for a further
18 h at ambient temperature, the volatiles were removed in vacuo, the
residue was extracted with diethyl ether, and the extract was filtered
through Celite and dried to afford 83 mg (87%) of 2-formate as a red-
orange solid which contained minor amounts of free PPh3. The crude
product may be redissolved in diethyl ether, and this solution may be
concentrated to approximately 3 mL, layered with pentane, and chilled
at −35 °C to afford 72 mg of 2-formate as analytically pure red
crystals in 75% yield based on molybdenum. Anal. Calcd for
C53H50MoP4O2: C, 67.81; H, 5.37. Found: C, 68.00; H, 5.49. 1H
NMR (25 °C, C6D6): δ −4.74 (tdd, 74.02, 44.01, 14.00 Hz, 1H, Mo-
H), 1.22 (m, 2H, PCH2), 1.43 (m, 2H, PCH2), 1.94 (m, 2H, PCH2),
2.46−2.65 (m, 2H, PCH2), 6.80 (t, 6H, C6H5), 6.88 (t, 3H, C6H5),
7.07 (m, 17H, C6H5), 7.11 (s, 1H OCHO), 7.14 (m, 2H, C6H5) 7.30
(m, 6H, C6H5), 7.55 (m, 4H, C6H5), 7.80 (t, 2H, C6H5). 13C{1H}
NMR (23 °C, C6D6): δ 26.15 (PCH2), 34.52 (PCH2), 127.72, 127.92,
128.30, 128.66, 128.78, 131.57, 133.57, 133.83, 135.36, 137.78, 141.52,
143.86, 144.66 (Ar), 169.13 (s, OCHO), three quaternary signals not
located. IR (KBr): νC−O 1550, 1362 cm−1. 31P{1H} NMR (23 °C,
C6D6): δ 50.1 (dt, 11.5, 151.9 Hz, 1P, PPh3), 103.5 (dt, 20.0, 151.9 Hz,
1P, PPh), 112.6 (dd, 1.5, 20.0 Hz, 2P, PPh2). Partial spectral data for
Preparation of [(Ph2PCH2CH2)2PPh]MoH4PPh3 (2-H4). Method
A. A heavy-walled glass reaction vessel was charged with 0.15 g (0.20
mmol) of 1-Cl3, 0.05 g (0.20 mmol) of PPh3, and approximately 10
mL of tetrahydrofuran. Then 610 μL (0.30 mmol) of a NaEt3BH
solution (1 M in THF) was added to the suspension, resulting in a
rapid color change from yellow to dark green over the first few
minutes and finally to red- orange over 30 min. After addition of 1 atm
of hydrogen gas, the reaction mixture was stirred a further 14 h at
ambient temperature. The volatiles were removed in vacuo, the residue
was extracted with toluene, and the filtrate was concentrated to
approximately 4 mL. Careful layering of pentane onto the toluene
solution and chilling at −35 °C afforded 164 mg (90%) of 2-H4 as
yellow crystals.
2-formate labeled with 13CO2 are as follows. 13C{1H} NMR (23 °C,
13
C6D6): δ 169.13 (s, OCHO). IR (KBr): ν
1511, 1340 cm−1.
Preparation of [(Ph2PCH2CH2)2PPCh−]OMo(H)(PPh3)(κ2O,O-
C3H5O2) (2-propionate). Method A. A 50 mL heavy-walled glass
reaction vessel was charged with 137 mg of 2-H4 and approximately 5
mL of tetrahydrofuran. On a high-vacuum line, 3 atm of ethylene was
added to the reaction mixture at −196 °C via a calibrated gas bulb.
Method B. A heavy-walled glass reaction vessel was charged with 75
mg (0.08 mmol) of 2-HCl and approximately 5 mL of tetrahydrofuran.
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After the mixture was stirred for 3 h at ambient temperature, the
ethylene was partially removed and 4 equiv of carbon dioxide was
added via a calibrated gas bulb (393 Torr in 28.9 mL) followed by 0.25
atm of dihydrogen at −196 °C. After the mixture was stirred for a
further 14 h at ambient temperature, the volatiles were removed in
vacuo, the residue was extracted with diethyl ether, and the extract was
filtered through Celite and dried to afford 64 mg (43%) of 2-
propionate as a red-orange solid.
acrylate. The solids were dissolved in approximately 5 mL of a 3:1
diethyl ether to toluene mixture, and the mixture was stirred at
ambient temperature for 1 h. The dark reaction mixture was then
filtered through Celite, which was further washed with 3 mL of
toluene. The solvent was then removed by vacuum and the resulting
red powder suspended in 10 mL of a 1:1 diethyl ether to pentane
mixture. A red-orange powder was obtained by filtration of the
suspension, affording 134 mg of 2-acrylate. Concentration of the
filtrate and cooling to −35 °C afforded a further 49 mg of 2-acrylate as
red crystals, for a total yield of 67% based on molybdenum. The
material from both crops was examined by NMR spectroscopy and
found to be identical with the previously reported characterization
data.10a
Method B. A heavy-walled glass reaction vessel was charged with
0.075 g (0.10 mmol) of 1-Cl3, 0.025 g (0.10 mmol) of PPh3, and
approximately 5 mL of tetrahydrofuran. Then 305 μL (0.3 mmol) of a
NaEt3BH solution (1 M in THF) was added to the suspension and the
mixture stirred for 6 h. On a high-vacuum line, 25 equiv of ethylene
(470 Torr in 101 mL) was admitted to the reaction at −196 °C via a
calibrated gas bulb. After the mixture was stirred for 30 h at ambient
temperature, an additional 4 equiv of carbon dioxide (75 Torr in 101
mL) was admitted to the reaction mixture at −196 °C via a calibrated
gas bulb. After the mixture was stirred for a further 18 h at ambient
temperature, the volatiles were removed in vacuo, the residue was
extracted with diethyl ether, the extract was filtered through Celite, and
the filtrate was concentrated to approximately 3 mL. The solution was
layered with pentane and chilled at −35 °C to afford 41 mg (41%) of
2-propionate as red crystals. Anal. Calcd for C55H54MoO2P4 + C5H12
(one cocrystallized pentane): C, 69.36; H, 6.40. Found: C, 69.20; H,
Improved Preparation of trans-[(Ph2PCH2CH2)2PPh]Mo(η2-
C2H4)(N2)2 (1-N2). A heavy-walled glass reaction vessel was charged
with 0.300 g (0.41 mmol) of 1-Cl3 and approximately 10 mL of
tetrahydrofuran. Then 1.22 mL (1.22 mmol) of NaEt3BH solution (1
M in THF) was added to the suspension at room temperature,
resulting in an immediate color change from yellow to red-orange.
After addition of 1 atm of dihydrogen gas, the reaction mixture was
stirred at ambient temperature for 14 h. The excess dihydrogen gas
1
was removed in vacuo and replaced with approximately /2 atm of
ethylene and stirred a further 3 h, affording a color change to yellow.
The volume of the reaction mixture was then concentrated by half in
vacuo, and the vessel was placed under an atmosphere of dinitrogen.
After the mixture was stirred for 1 h, the remaining solvent was
removed in vacuo over 15 min with five periodic reintroductions of N2
for several seconds to avoid decomposition. The residue was then
extracted with toluene, and the filtrate was concentrated to
approximately 2 mL, layered with pentane, and chilled at −35 °C to
afford 221 mg (76%) of 1-N2 as a yellow powder, as previously
characterized.10a Unlike the previously reported synthesis of 1-N2, the
above method circumvents the use of mercury and was found to work
better on larger scale syntheses.
General Procedure for the Determination of Kinetics of 2-
Formate Formation. A J. Young NMR tube was charged with 500
μL of a tetrahydrofuran solution of 2-H4 of known concentration (ca.
0.02 M) and a capillary of triethyl phosphite for use as an integration
standard. Then varying amounts of carbon dioxide were added via a
calibrated gas bulb at −196 °C. The reaction mixture was thawed,
shaken, and inserted into a temperature-controlled NMR probe. The
sample was quickly ejected from the NMR probe and shaken between
measurements to ensure complete mixing of the carbon dioxide into
solution. The reaction progress was monitored by 31P NMR
spectroscopy over greater than 2 half-lives. The decays of the
resonances for 2-H4 were converted to concentrations and fitted to a
first-order plot of ln [2-H4] versus time, which gave the observed rate
constant as the slope. Example graphs may be found in the Supporting
Information.
1
6.37. H NMR (25 °C, C6D6): δ −4.70 (tdd, 71.97, 43.98, 14.00 Hz,
1H, Mo-H), 0.47 (t, 7.47 Hz, 3H, CH3), 1.10 (br q, 7.34 Hz, 2H,
CH2), 1.21 (m, 2H, PCH2), 1.51 (m, 2H, PCH2), 1.97 (m, 2H, PCH2),
2.51−2.65 (m, 2H, PCH2), 6.80 (t, 6H, C6H5), 6.89 (t, 4H, C6H5),
7.02 (m, 7H, C6H5), 7.09−7.14 (m, 11H, C6H5) 7.32 (m, 6H, C6H5),
7.58 (m, 4H, C6H5), 7.81 (t, 2H, C6H5). 13C{1H} NMR (23 °C,
C6D6):
δ 8.85 (OCOCH2CH3), 26.16 (PCH2), 30.43
(OCOCH2CH3), 34.57 (PCH2), 127.70, 127.85, 128.31, 128.47,
128.68, 131.53, 133.64, 133.88, 135.40, 138.21, 142.68, 144.25, 144.78
(P-Ar), 182.67 (s, OCOCH2CH3), three quaternary signals not
located. 31P{1H} NMR (23 °C, C6D6): δ 48.2 (dt, 30.4, 392.1 Hz, 1P,
PPh3), 101.5 (dt, 48.0, 392.1 Hz, 1P, PPh), 110.9 (dd, 30.4, 48.0 Hz,
2P, PPh2). IR (KBr): νC−O 1514, 1445 cm−1. Partial spectral data for 2-
propionate labeled with 13CO2 3are as follows. 1H NMR (25 °C,
3
C6D6): δ 0.47 (td, JH−H = 7.47; JC−H = 5.20 Hz, 3H, CH3), 13C−1H
coupling to methylene group obscured by signal broadening. 13C{1H}
13
NMR (25 °C, C6D6): 182.64 (s, CO2Et). IR (KBr): ν C−O 1486, 1414
cm−1.
Observation of [(Ph2PCH2CH2)2PPh]Mo(η2-C2H4)3 (1-C2H4)
from the Reaction of 2-H4 with Ethylene. Method A. A J.
Young NMR tube was charged with 0.010 g (0.011 mmol) of 2-H4,
approximately 0.5 mL of tetrahydrofuran-d8, and 25 equiv (180 Torr in
28.9 mL) of ethylene. The reaction was monitored by NMR
spectroscopy, showing complete conversion to a yellow solution of
1-C2H4 after 4 h. 1-C2H4 proved unstable in the absence of an
1
ethylene atmosphere. The product was characterized by H, 31P{1H},
1H−13C HSQC, and 1H−1H COSY NMR spectroscopy without
isolation.
ASSOCIATED CONTENT
■
S
* Supporting Information
Method B. A J. Young NMR tube was charged with 0.015 g (0.02
mmol) of 1-Cl3 and approximately 0.5 mL of tetrahydrofuran. Then
61 μL (0.30 mmol) of a NaEt3BH solution (1 M in THF) was added
to the suspension, and the mixture was thawed, shaken, and allowed to
stand for 1 h. Then 25 equiv of ethylene was added via a calibrated gas
bulb (328 Torr in 28.9 mL) at −196 °C on a high-vacuum line. After 6
h at ambient temperature conversion to 1-C2H4 was complete. Partial
1H NMR (25 °C, THF-d8): δ −1.32 (br, 2H, CH2CH2), −0.22 (br,
2H, CH2CH2), 0.28 (br, 2H, CH2CH2), 0.47 (br, 2H, CH2
CH2), 1.10 (br, 2H, CH2CH2), 2.12 (br, 2H, CH2CH2), 2.32 (m,
2H, PCH2), 2.55 (m, 2H, PCH2), 2.71 (m, 2H, PCH2), 3.65 (m, 2H,
PCH2). 31P{1H} NMR (23 °C, THF-d8): δ 75.5 (d, 9.7 Hz, 2P, PPh2),
Text, figures, tables, and CIF files giving crystallographic data
for 2-HCl, 2-acrylate, and 2-H4, selected spectral data, and
computational details. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*e-mail: wb36@brown.edu.
Notes
1
105.3 (t, 9.7 Hz, 1P, PPh). Partial 13C NMR taken from H−13C
The authors declare no competing financial interest.
HSQC (25 °C, THF-d8): δ 27.16 (PCH2), 34.08 (PCH2), 36.87,
38.58, 41.93, 46.52 (CH2CH2).
ACKNOWLEDGMENTS
■
Alternative Preparation of [(Ph2PCH2CH2)2PPh]Mo(H)(PPh3)-
(κ2O,O-C3H3O2) (2-acrylate). A 20 mL scintillation vial was charged
with 0.256 g (0.28 mmol) of 2-HCl and 0.050 g (0.28 mmol) of silver
We gratefully acknowledge funding by the U.S. Department of
Energy-National Energy Technology Laboratory (Award No.
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