Ancillary ligand effects on carbon dioxide-ethylene coupling at zerovalent molybdenum

Article abstract of DOI:10.1021/om500324h

A series of zerovalent molybdenum complexes bearing triphosphine ligands, [Ar₂PCH₂CH₂]₂PPh, have been synthesized and evaluated for reductive functionalization of CO₂ with ethylene. The ability to form dimeric triphosphine molybdenum(II) acrylate hydride species from CO₂-ethylene coupling was found to be highly sensitive to steric encumbrance on the phosphine aryl substituents. Trapping of triphosphine molybdenum(II) acrylate hydride species using triphenylphosphine afforded isolable monomeric CO₂ functionalization products with all ancillary ligands studied. Kinetic analysis of the acrylate formation reaction revealed a first-order dependence on molybdenum, but no influence from CO ₂ pressure or the triphenylphosphine trap. Systematic attenuation of steric and electronic features of the triphosphine ligands showed a strong CO₂ functionalization rate influence for ligand size with [(3,5- ^tBu-C₆H₃)₂PCH₂CH ₂]₂PPh coupling nearly four times slower than with [(3,5-Me-C₆H₃)₂PCH₂CH ₂]₂PPh. A considerably milder electronic effect was observed with complexes bearing [(4-F-C₆H₄) ₂PCH₂CH₂]₂PPh reducing CO ₂ at approximately half the rate as with [Ph₂PCH ₂CH₂]₂PPh.

Full text of DOI:10.1021/om500324h

Article
pubs.acs.org/Organometallics
Ancillary Ligand Effects on Carbon Dioxide-Ethylene Coupling at
Zerovalent Molybdenum
Brian S. Hanna,† Alex D. MacIntosh,† Steven Ahn,‡ Brian T. Tyler,† G. Tayhas R. Palmore,‡
Paul G. Williard,† and Wesley H. Bernskoetter*,†
‡
†Department of Chemistry and School of Engineering, Brown University, Providence, Rhode Island 02912, United States
S
* Supporting Information
ABSTRACT: A series of zerovalent molybdenum complexes bearing
triphosphine ligands, [Ar2PCH2CH2]2PPh, have been synthesized and
evaluated for reductive functionalization of CO2 with ethylene. The ability
to form dimeric triphosphine molybdenum(II) acrylate hydride species from
CO2-ethylene coupling was found to be highly sensitive to steric
encumbrance on the phosphine aryl substituents. Trapping of triphosphine
molybdenum(II) acrylate hydride species using triphenylphosphine afforded
isolable monomeric CO2 functionalization products with all ancillary ligands
studied. Kinetic analysis of the acrylate formation reaction revealed a first-
order dependence on molybdenum, but no influence from CO2 pressure or the triphenylphosphine trap. Systematic attenuation
of steric and electronic features of the triphosphine ligands showed a strong CO2 functionalization rate influence for ligand size
with [(3,5-tBu-C6H3)2PCH2CH2]2PPh coupling nearly four times slower than with [(3,5-Me-C6H3)2PCH2CH2]2PPh. A
considerably milder electronic effect was observed with complexes bearing [(4-F-C6H4)2PCH2CH2]2PPh reducing CO2 at
approximately half the rate as with [Ph2PCH2CH2]2PPh.
carbon dioxide and ethylene to acrylate at transition metals
has garnered the attention of numerous researchers6 since the
first reported example from Carmona and co-workers.7 More
recently, our laboratory has followed this seminal work with the
discovery of a CO2-ethylene coupling reaction mediated by
trans-(Triphos)Mo(N2)2(C2H4) (1-C2H4) (Triphos =
(Ph2PCH2CH2)2PPh) (Figure 1).8 NMR and IR spectroscopic
studies of the CO2-ethylene coupling reaction indicate that 1-
C2H4 generates acrylate through an intermediate species in
which carbon dioxide and ethylene are co-localized on the
metal (1-INT). Additional kinetic and isotopic labeling analysis
of the conversion of 1-INT to the dimeric molybdenum(II)
acrylate hydride species (1-dimer) suggests that the reaction
proceeds with a rate-limiting oxidative coupling to form a C−C
bond, followed by a relatively swift β-hydride elimination
(Figure 1).
Isolation of an acrylate product from molybdenum-mediated
CO2-ethylene coupling augurs well for its ability to utilize CO2;
however, the poor lability of the metal-acrylate interaction and
the sluggish pace of the coupling reaction (half-life of ∼8 h) will
require advancements in order to make significant contribu-
tions to chemical synthesis. Given the implication that
metalalactone formation from 1-INT limits the rate of acrylate
formation, the most obvious target is to attenuate the ligands to
optimize this reaction. Unfortunately, the structural or
electronic environments that may facilitate the oxidative
coupling reaction were not self-evident. While this trans-
INTRODUCTION
■
Expanding evidence for the deleterious effects of anthropogenic
carbon dioxide production has brought CO2 capture,
utilization, and storage (CCUS) efforts to the forefront of
chemical and engineering research. The challenge of mitigating
greenhouse gas emissions while satisfying society’s energy
demand is immense and will most certainly require
technological advances across many scientific fields. One area
in which homogeneous transition-metal-mediated processes are
poised to contribute is the utilization of carbon dioxide for
commodity chemical production.1 Although the chemical
fixation of CO2 to value added chemicals alone will not
meaningfully impact carbon emissions, it is one of the few
CCUS endeavors that offer to both create an intrinsic economic
gain and safely sequester CO2 for an extensive lifetime.2
Chemical CO2 utilization thus presents an attractive comple-
ment to larger scale carbon capture activities where high costs,
safety, and storage permanence remain evolving challenges.3
Current utilization of CO2 as a feedstock for commodity
chemicals remains quite limited. Carbonates, salicylic acid, urea,
and (to a limited extent) methanol are the only direct CO2
fixation products now manufactured on significant industrial
scale, leaving CO2 as one of the world’s largest unharnessed
carbon resources.4 The production of acrylates from the
coupling of CO2 with ethylene is a potential utilization process
that could substantially expand the role of carbon dioxide as a
renewable synthon. Acrylates, which are currently manufac-
tured from propylene oxidation, are utilized on a more than 4
million ton annual scale for a variety of applications including
fabrics and superabsorbent polymers.5 The conversion of
Received: March 25, 2014
© XXXX American Chemical Society
A
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Figure 1. Probable pathway for acrylate formation from 1-C2H4.
formation is formally oxidative, the effective metal valence of 1-
INT and the metalalactone are probably quite similar due to
Dewar−Chatt−Duncanson resonance forms.9 Likewise, the
role of steric constraint in an intramolecular bond formation
such as this was not entirely obvious. These questions, as well
as the desire to accelerate acrylate formation, have motivated a
systematic investigation of the influence of ancillary ligand
substituents on the rate of CO2-ethylene coupling.
−1.5 V relative to ferrocene, which has previously been
assigned as the reduction of molybdenum(III) trichoride to the
molybdenum(II) trichoride anion.13 The potential of this
reduction was used to establish a spectrum for relative metal
electrophilicity engendered by each ligand (Table 1). As
Table 1. MoIII/MoII Reduction Potentials of
[(Ar2PCH2CH2)2PPh]MoCl3
a
complex
E° (V)
RESULTS AND DISCUSSION
■
1-Cl3
2-Cl3
3-Cl3
4-Cl3
5-Cl3
6-Cl3
7-Cl3
−1.49
−1.42
−1.30
−1.51
−1.53
−1.56
−1.55
Examination into the role of the ancillary Triphos ligand on
CO2 functionalization began with preparation of a family of
related tridenate ligands described in Figure 2. For synthetic
a
Cyclic voltammetry recorded at ambient temperature using 2 mM
compound THF solution with 0.2 M nBu4NPF6 under N2 atmosphere
at 20 mV/s. All potentials are referenced to ferrocene/ferrocenium.
expected, compounds 6-Cl3 and 7-Cl3 bearing 3,5-dialkylaryl
substituents were found to be most electron-donating, while
fluoro- and trifluoromethyl-substituted 2-Cl3 and 3-Cl3
displayed the greatest electrophilicity.
Figure 2. Synthesis of [(Ar2PCH2CH2)2PPh]MoCl3 complexes.
convenience the aryl substituents on the terminal phosphines
were selected for attenuation, and the ligands were prepared via
radical or base-catalyzed coupling of divinylphenylphosphine
and the corresponding diarylphosphine.10 Coordination of the
tridentate ligands was accomplished by treatment with
MoCl3(THF)3 to afford the [(Ar2PCH2CH2)2PPh]MoCl3 (2-
to 7-Cl3) complexes in good yields, analogous to the procedure
previously employed for (Triphos)MoCl3 (1-Cl3).11 These
paramagnetic species were characterized by combustion
Following the synthesis of an array of molybdenum(III)
trichoride compounds, the preparation of corresponding
molybdenum(0) species suitable for CO2-ethylene coupling
was targeted. For comparison to our previous studies of acrylate
formation, the trans-[(Ar2PCH2CH2)2PPh]Mo(C2H4)(N2)2
(2-C2H4−7-C2H4) complexes were selected and prepared
either by sodium amalgam or sodium triethylborohydride
reduction of the molybdenum trichloride species (Figure 3).8
Complexes 2-C2H4−7-C2H4 were typically isolated in moder-
ate yields as yellow powders from chilled diethyl ether solutions
layered with pentane. Application of this isolation method to
the previously reported 1-C2H4 occasionally afforded small
crystalline samples, which permitted confirmation of its
structure by X-ray diffraction (Figure 4).8a The solid state
structure of 1-C2H4 exhibits an idealized Cs symmetry with
trans positioned dinitrogen ligands and ethylene carbons (C(1)
1
analysis, H NMR spectroscopy, and cyclic voltammetry. In
some cases it was possible to discern the presence of both mer
and fac isomers by NMR spectroscopy. This geometric
flexibility has been observed in previous studies with
(Triphos)MoCl3 complexes.12 Electrochemical analyses of 2-
to 7-Cl3 indicated strong similarities to those reported by
George and co-workers for 1-Cl3.13 A key feature conserved
throughout 1- to 7-Cl3 is a partially reversible feature at ca.
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been structurally characterized.14,15 The NMR and IR spectra
for 2-C2H4−7-C2H4 were also comparable to those observed
for 1-C2H4.8a Each species exhibited a doublet and triplet
resonance in the 31P NMR spectrum at ca. 75 and 95 ppm,
respectively, with resonances for the bound ethylene observed
between 1.42 and 2.42 ppm in the 1H NMR spectra. The most
notable features in the solid-state infrared spectra were two
strong bands with an approximate 2:1 intensity assigned to the
NN stretching frequencies. The frequency of these bands
(Supplementary Table S1) offer further assessment of the
electronic character engendered by the chelating ligands and
are generally consistent with the trend observed from the cyclic
voltammetry of the molybdenum(III) trichloride species.
With a collection of trans-[(Ar2PCH2CH2)2PPh]Mo(C2H4)-
(N2)2 complexes in hand, measurement of the relative rates of
CO2-ethylene coupling to acrylate became the focus of
investigation. Initial experiments to measure these rates
employed a technique our laboratory has previously used
successfully in studies with 1-C2H4. This involved treatment of
benzene-d6 solutions of 2-C2H4−7-C2H4 with 4 atm of carbon
dioxide to rapidly generate a molybdenum(0) ethylene carbon
dioxide intermediate (2-INT−7-INT) and then monitoring the
relatively slow conversion of the intermediate species into a
dimeric molybdenum(II) acrylate hydride complex. Interest-
ingly, though addition of CO2 to 2-C2H4−7-C2H4 did produce
facile formation of 2-INT−7-INT, only the p-fluorophenyl-
substituted species converted to an observable molybdenum
acrylate hydride complex (2-dimer) (Figure 5). The other
molybdenum(0) ethylene carbon dioxide intermediates, 3-
INT−7-INT, proved stable under carbon dioxide for greater
than 2 days at ambient temperature and slowly degraded upon
heating at 45 °C without any acrylate formation detectable by
NMR spectroscopy.
Figure 3. Synthetic routes to the trans-[(Ar2PCH2CH2)2PPh]Mo-
(C2H4)(N2)2 species.
Figure 4. Molecular structure of 1-C2H4 with ellipsoids at 30%
probability. All hydrogen atoms and a co-crystallized toluene molecule
are omitted for clarity.
Noting that the two systems with the smallest ancillary ligand
substituents (1-INT and 2-INT) successfully formed isolable
acrylate hydride species, it was hypothesized that steric
repulsions may destabilize or kinetically block access to the
dimeric molybdenum complex for the larger ligands. In order to
circumvent this barrier, complexes 2-INT−7-INT were
generated in the presence an exogenous trapping agent,
triphenylphosphine, which has been employed to stabilize
and C(2)) lying nearly coplanar with the Triphos phosphorus
atoms (P(1)−P(3)). The metrical parameters exhibit modest
elongation of the bound ethylene C(1)−C(2) bond length,
1.41(1) Å, along with minimal activation of the dinitrogen N−
N bond lengths (1.09(1) and 1.11(1) Å). Such bond distances
are quite similar to those observed for related ethylene and
trans-bis(dinitrogen) molybdenum(0) complexes that have
Figure 5. CO2-ethylene coupling reactions for the trans-[(Ar2PCH2CH2)2PPh]Mo(C2H4)(N2)2 complexes.
C
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other monomeric Triphos molybdenum carboxylate hydride
species.8 Gratifyingly, the addition of PPh3 afforded isolable
[(Ar2PCH2CH2)2PPh]Mo(H)PPh3(CO2CHCH2) (2-acryl-
ate−7-acrylate) for all the Triphos variants studied. Complexes
2-acrylate−7-acrylate were characterized by a combination of
combustion analysis and NMR and IR spectroscopy, as well as
by comparison to the previous characterized 1-acrylate
congener. These species were typically isolated as red-orange
powders, exhibiting modest solubility in arene or ethereal
solvents. The 1H NMR spectra of each complex displayed three
diagnostic resonances between 4.5 and 5.5 ppm for the acrylic
olefin protons and a 12-line resonance at ca. −5 ppm
originating from the metal-hydride. The corresponding 31P
NMR spectra exhibited three resonances, a doublet of doublets
at ca. 110 ppm, and two doublet of triplets at ca. 100 and 50
ppm.
The ability to prepare molybdenum(II) acrylate hydride
triphenylphosphine compounds with all ligands 1−7 made this
species the preferred target for assessing the ancillary ligand
influence on CO2-ethylene coupling. Kinetic analysis of the
conversion of each molybdenum(0) ethylene carbon dioxide
intermediate (1-INT−7-INT) to the corresponding
molybdenum(II) acrylate hydride triphenylphosphine species
(1-acrylate−7-acrylate) was performed using 31P NMR
spectroscopy. In a typical experiment, 1 equiv of PPh3 was
added to a frozen benzene-d6 solution of 2-C2H4, followed
addition of 4 atm of carbon dioxide. Upon thawing, the sample
quick conversion of 2-C2H4 to 2-INT (<30 min) was observed.
Following complete consumption of 2-C2H4, the conversion of
2-INT to 2-acrylate was monitored over 2−3 half-lives to
obtain a rate constant for CO2-ethylene coupling (Figure 5).
Control experiments using 1-C2H4, 6-C2H4, and 7-C2H4
showed no observed rate constant influence for varied additions
of PPh3 (1−10 equiv) or carbon dioxide (1−4 atm). For
experiments using electron-withdrawing ligand variants (e.g., 2-
C2H4 and 3-C2H4), it was critical to keep the reaction mixture
frozen until the excess carbon dioxide was added to avoid
competing formation of an alternate species tentatively
identified as trans-[(Ar2PCH2CH2)2PPh]Mo(N2)2PPh3, the
product of ethylene substitution with PPh3 (Figure 5). This
side reaction is likely more prevalent for electron-poor species
due to a weaker metal-ethylene π-bonding interaction.16
Repeating these kinetic measurements at least three times for
each analogue afforded the set of rate constants listed in Table
2.
attenuate the reaction with the smallest complex (1-C2H4)
achieving the fastest rate and the largest (7-C2H4) the slowest
rate. Perhaps a better separation of the steric and electronic
influences comes from pairwise comparison of 6-C2H4 and 7-
C2H4. These two 3,5-dialkyl aryl-substituted Triphos complexes
exhibit reduction potentials within a scant 9 mV of each other,
t
but the Bu-substituted congener couples CO2 and ethylene
roughly four times more slowly than the Me analogue. The
precise origin of this steric influence remains speculative since
no dependence on incoming PPh3 ligand was observed. It is
possible that the crowded environment slows bond rotations
about the CO2 and ethylene that may be required to achieve
lactone formation, or that larger substituents influence the
flexibility of the Triphos ligand backbone. However, the data in
hand relegate these possible sources to hypotheses.
Most likely an electronic preference exists for the acrylate
formation reaction, but this influence is largely obscured by
steric or other factors. One comparison that provides some
limited insight in this area is the relative rate of phenyl- and p-
fluorophenyl-substituted 1-C2H4 and 2-C2H4. Since these two
species were the only studied variants capable of forming the
sterically hindered dimeric molybdenum acrylate hydride
complexes, their steric impetus is likely minimized. Notably,
reduction potential of the faster 1-C2H4 is 66 mV more
negative than 2-C2H4, suggesting a mild preference for
electron-donating groups in the CO2-ethylene coupling
reaction. Together, this study of ancillary ligand effects suggests
optimized rates for coupling CO2 and ethylene to acrylate may
be obtained using related ligand platforms bearing smaller and,
to a lesser degree, more electron-donating substituents. Our
laboratory is currently investigating ligands of this type, as well
as methods to induce reductive acrylate extrusion to enable
catalytic turnover of the CO2 functionalization process.
CONCLUDING REMARKS
■
The rare ability of molybdenum(0) to couple CO2 and ethylene
into acrylate appears to be general across a wide range of
tridentate phosphine ligands of the class (Ar2PCH2CH2)2PPh.
Surprisingly, the formation of dimeric molybdenum(II) acrylate
hydride species can be disfavored by even modest changes in
the steric impetus of the phosphine aryl substituents. However,
the use of triphenylphosphine as an intermolecular trap serves
to access monomeric acrylate hydride complexes in each case.
Kinetic examination of the acrylate formation reaction indicates
that neither the pressure of CO2 nor the concentration of PPh3
plays a direct role in the rate of reaction, suggesting that the C−
C bond formation between CO2 and ethylene is likely still the
rate-limiting step in the presence of added phosphine.
Alteration of aryl substituents on the tridentate phosphine
shows a strong steric influence on the rate of acrylate formation
despite observation that CO2 and ethylene are already bound at
the metal. This, along with the absence of [PPh3] dependence,
hints that flexibility of the chelate ligand or rotation of the
unsaturates at the metal may be important factors in C−C bond
formation. The influence of ligand electron donation on the
rate of coupling is more difficult to detect, but comparisons of
the phenyl- and p-fluorophenyl-substituted species indicates a
mild enhancement for more electron-rich metals. These
observations suggest the examination of smaller, more
election-donating ligands on zerovalent molybdenum could
substantially accelerate the rate of CO2 reduction, and that
issues such as the freedom of motion for any chelating forms of
these ligands ought to be closely examined.
Surprisingly, the data do not indicate a strong correlation
between the rate of coupling and the reduction potential of the
metal center. While the range of observed rate constants spans
only a factor of 5, the steric impetus of the ancillary ligand does
Table 2. Observed Rate Constants for Acrylate Formation
a
from [(Ar2PCH2CH2)2PPh Mo](C2H4)(CO2)
complex
kobs × 105 (s−1
)
1-C2H4
2-C2H4
3-C2H4
4-C2H4
5-C2H4
6-C2H4
7-C2H4
5.2(1)
3.2(1)
4.05(4)
3.26(4)
2.3(1)
4.15(8)
0.8(1)
a
Rate constants measured by NMR spectroscopy at 24 °C in benzene.
D
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Hz, 2P, PAr2). 19F NMR (23 °C, C6D6): δ −62.60 (s). (ESI) m/z
calcd for C38H29F12P3 [M + H]+: 807.132, found 807.136.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out using
standard vacuum, Schlenk, cannula, or glovebox techniques. Under
standard glove conditions, purging was not performed between uses of
pentane, diethyl ether, benzene, toluene, and THF; thus, traces of all
of these solvents were in the atmosphere and could be found
intermixed in the solvent bottles while reactions were conducted.
Ethylene and carbon dioxide were purchased from Corp Brothers and
stored over 4 Å molecular sieves in heavy-walled glass vessels prior to
use. MoCl3(THF)3 was obtained as previously described.17 [(4-F-
C6H4)2PCH2CH2]2PPh (2), [(4-CH3C6H4)2PCH2CH2]2PPh (4), [(4-
OMe-C6H4)2PCH2CH2]2PPh (5), [(3,5-Me-C6H3)2PCH2CH2]2PPh
(6), (3,5-tBu-C6H3)2PH, and (4-CF3C6H5)2PH were also prepared
according to literature procedures.18 All other chemicals were
purchased from Aldrich, Fisher, VWR, Strem, or Cambridge Isotope
Laboratories. Tetrabutylammonium hexafluorophosphate (nBu4NPF6,
electrochemical grade) was dried at 60 °C under vacuum for 24 h and
stored in the glovebox. Solvents were dried and deoxygenated using
literature procedures.19 1H, 13C, 19F, and 31P NMR spectra were
recorded on Bruker DRX 400 MHz and Avance 300 and 600 MHz
Preparation of [(3,5-tBu-C6H3)2PCH2CH2]2PPh (7). A high
pressure Schlenk vessel was charged with 0.300 g (0.73 mmol) of
(3,5-tBu-C6H3)2PH, 0.053 g (0.33 mmol) of divinylphenylphosphine,
0.014 g (0.15 mmol) of NaOtBu, and approximately 5 mL of THF.
The reaction was heated at 60 °C for 4 h, after which the volatiles were
removed in vacuo, and the resulting oil was purified by silica gel
chromatography with 90:10 hexane to ethyl acetate elutent to give
0.200 g of 7 (57%) as colorless oil. 1H NMR (23 °C, C6D6): δ 1.23 (d,
3 Hz, 72H, C(CH3)3), 1.97 (m, 4H, PCH2) 2.25−2.39 (m, 4H,
PCH2), 7.02−7.06 (m 3H, aryl) 7.32−7.35 (m, 2H, aryl), 7.47−7.49
(m, 4H, aryl), 7.47−7.59 (m, 8H, aryl). 13C {1H} NMR (23 °C,
C6D6): δ 23.91, (PCH2) 24.12, (PCH2) 30.91, (ArC(CH3)3) 34.63,
(ArC(CH3)3) 120.92, 126.02, 128.39, 128.79, 132.66, 132.85, 138.75,
139.07. 31P {1H} NMR (23 °C, C6D6): δ 16.16 (t, 30 Hz, 1P, PPh),
10.18 (d, 30 Hz, 2P, PAr2).
G e n e r a l P r o c e d u r e f o r t h e P r e p a r a t i o n o f
[(Ar2PCH2CH2)2PPh]MoCl3 (2-Cl3−7-Cl3). Complexes 2-Cl3−7-Cl3
were prepared in a manner analogous to that reported for 1-Cl3. In a
typical synthesis 1 equiv each of [(Ar)2PCH2CH2]2PPh and
MoCl3(THF)3 were stirred in tetrahydrofuran for 16 h at ambient
temperature, over which time the reaction mixture turned from orange
to yellow-green. Pentane was added to the reaction mixture to
precipitate the product from solution. The suspension was then
filtered, washed with additional pentane, and dried to afford desired
product as yellow powders.
1
spectrometers. H and 13C chemical shifts are referenced to residual
solvent signals; 19F and 31P chemical shifts are referenced to the
external standards C6H5CF3 and H3PO4, respectively. Probe temper-
atures were calibrated using ethylene glycol and methanol as
previously described.20 IR spectra were recorded on Jasco 4100
FTIR and Mettler Toledo React IR spectrometers. Cyclic voltammetry
was performed on a Pine Research AFCBP1 bipotentiostat. The three-
electrode system consisted of a platinum disk working electrode
(BASi, 1.6 mm diameter), a platinum wire counter electrode, and a
Ag/AgNO3 reference electrode (BASi). The reference was filled with
0.01 M AgNO3 and 0.2 M nBu4NPF6 in THF, +0.09 V vs the
ferrocene/ferrocenium couple. Prior to each cyclic voltammetry
experiment, the platinum disc was successively polished with 1-, 0.3-,
and 0.05-μm alumina slurry to obtain a mirror surface and then
sonicated in and rinsed with Milli-Q water and acetone. Ferrocene was
added at the end of experiments, and the potential was converted from
Ag/Ag+. X-ray crystallographic data were collected on a Bruker D8
QUEST diffractometer. Samples were collected in inert oil and quickly
transferred to a cold gas stream. The structures were solved from
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. Irradiated reactions were performed in a Rayonet
Photochemical Reactor using an array of 350 nm wavelength bulbs
or a Biotage Microwave Initiator apparatus. Elemental analyses for
select compounds in each series were performed at Atlantic Microlab,
Inc., in Norcross, GA or Robertson Microlit Laboratory in Ledgewood,
NJ. Several of the [(Ar2PCH2CH2)2PPh]Mo(C2H4)(N2)2 complexes
repeatedly afford elemental analyses that were well matched for carbon
and hydrogen, but drastically low in nitrogen. This suggests a loss of
N2 ligand during shipping. Those [(Ar2PCH2CH2)2PPh]Mo(H)-
PPh3(CO2CHCH2) complexes prepared via Method A were not
subjected to elemental analysis. Evidence for the purity of these
complexes has been provided by NMR spectroscopy, which is detailed
in the Supporting Information.
{[(4-F-C6H4)2PCH2CH2]2PPh}MoCl3 (2-Cl3). Yield 83%. Anal.
Calcd for C34H29Cl3F4MoP3: C, 50.49; H, 3.61. Found: C, 50.78; H,
1
3.61. H NMR (23 °C, CD2Cl2): two isomers δ 73.67 (w1/2 = 762
Hz), −40.60 (w1/2 = 706 Hz), −35.5 (w1/2 = 1273 Hz), −23.9 (w1/2
=
856 Hz), −16.9, −14.6, −13.0 7.4, 8.8, 9.9, 10.1, 10.8, 11.4, 14.4, 14.7,
15.3 (w1/2 = 104 Hz), 17.2, 17.9, 20.3, 22.9, 23.8. 19F{1H} (23 °C,
CH2Cl2): two isomers δ −111.22, −106.10, −105.37, −103.17.
{[(4-CF3-C6H5)2PCH2CH2]2PPh}MoCl3 (3-Cl3). Yield 75%. Anal.
Calcd for C38H29F12P3MoCl3: C, 45.24; H, 2.90. Found: C, 44.90; H,
1
2.66. H NMR (23 °C, CD2Cl2): two isomers δ −73.98 (w1/2 = 137
Hz), −42.94 (w1/2 = 81 Hz), −41.03, −35.23, −23.56, −14.51, −13.32
(w1/2 = 483 Hz), 8.68, 10.12, 10.77, 12.06, 14.58, 15.38, 16.30, 17.63,
18.34 21.35 (w1/2 = 1230 Hz), 22.57, 26.76. 19F{1H } NMR (23 °C,
CH2Cl2): two isomers δ −65.48, −63.25, −61.53, −60.25.
{[(4-CH3-C6H5)2PCH2CH2]2PPh}MoCl3 (4-Cl3). Yield 90%. Anal.
Calcd for C38H41P3MoCl3: C, 57.12; H, 4.92. Found: C, 57.33; H,
5.20. 1H NMR (23 °C, CD2Cl2): two isomers δ −75.59, −42.85,
−41.70, −35.81, −23.83, −17.73, 0.20, 1.31, 1.83, 2.84, 3.18, 3.45,
4.74, 8.38, 9.61, 10.23, 11.42, 13.32, 14.69, 15.29, 15.99, 16.86, 19.59,
22.14, 23.95.
{[(4-OCH3-C6H4)2PCH2CH2]2PPh}MoCl3 (5-Cl3). Yield 72%. Anal.
Calcd for C38H41Cl3O4MoP3: C, 53.26; H, 4.82. Found: C, 52.73; H,
1
5.43. H NMR (23 °C, C6D6): two isomers δ −73.9, −39.5, −41.8,
−22.6, −17.0, 3.71, 4.0, 4.2, 4.3, 9.3, 9.7, 10.3, 10.8, 15.4, 17.4, 22.5.
{[(3,5-CH3-C6H3)2PCH2CH2]2PPh}MoCl3 (6-Cl3). Yield 87%. Anal.
Calcd for C42H49Cl3MoP3: C, 59.41; H, 5.82. Found: C, 59.63; H,
1
5.98. H NMR (23 °C, C6D6): two isomers δ −66.3, −41.8, −37.2,
−19.6, −19.6, −12.1, 3.0, 3.2, 3.6, 3.9, 8.0−8.9, 9.9, 11.1, 17.4, 21.3,
23.4.
{[(3,5-tBu-C6H3)2PCH2CH2]2PPh}MoCl3 (7-Cl3). Yield 89%. Anal.
Preparation of [(4-CF3C6H5)2PCH2CH2]2PPh (3). A microwave
vial was charged with 1.50 g (4.66 mmol) of (4-CF3C6H5)2PH, 0.27 g
(1.66 mmol) of divinylphenylphosphine, and 0.032 g (0.19 mmol) of
azobis(isobutyronitrile) in approximately 1 mL of benzene. The
reaction mixture was placed in a microwave reactor and irradiated at
88 °C for 24 h. The resulting oil was purified by silica gel
chromatography with 95:5 hexane to ethyl acetate eluent to give
Calcd for C66H97Cl3MoP3: C, 66.85; H, 8.25. Found: C, 66.57; H,
1
7.99. H NMR (23 °C, C6D6): two isomers δ −79.7, −43.3 (w1/2
664.2), 1.3 (w1/2 = 121.84), 1.8 (w1/2 = 57.82), 7.2, 7.5, 8.1, 9.9 (w1/2
45.32), 10.9 (w1/2 = 81.11), 16.7 (w1/2 = 1516), 22.7, 32.3 (w1/2
1283).
=
=
=
General Procedure for the Preparation of trans-
[(Ar2PCH2CH2)2PPh]Mo(C2H4)(N2)2 (2-C2H4−7-C2H4). Complexes
2-C2H4−7-C2H4 were prepared by procedures analogous to those
previously reported for 1-C2H4 using either sodium amalgam (Method
A) or sodium triethylborohydride (Method B) as a reducing agent.
Method A: A heavy-walled glass reaction vessel was charged with
[{(Ar)2PCH2CH2}PPh]MoCl3, 10 equiv of 0.5% sodium amalgam,
and approximately 10 mL of tetrahydrofuran. On a vacuum line,
1
1.08 g of 3 (81%) as a white solid. H NMR (23 °C, CDCl3): δ 1.71
(m, 4H, PCH2), 1.88−1.98 (m, 2H, PCH2), 2.05−2.13 (m, 2H,
PCH2), 7.34 (m, 11H, aryl), 7.52 (m, 10H, aryl). 13C {1H} NMR (23
°C, CDCl3): 23.46 (PCH2), 23.77 (PCH2), 123.44, 123.50, 128.92,
130.07, 131.04, 131.37, 132.86, 133.23 (aryl), 142.32 (CF3). 31P {1H}
NMR (23 °C, C6D6): δ −17.6 (t, 30.8 Hz, 1P, PPh), −12.8 (d, 30.8
E
dx.doi.org/10.1021/om500324h | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
approximately 0.5 atm of ethylene and 1 atm of dinitrogen were added
at −196 °C. The resulting reaction mixture was stirred at ambient
temperature for 16 h, the volatiles were removed in vacuo, and the
residue was extracted through Celite with diethyl ether. The product
solution was concentrated, layered with pentane, and chilled to −35
°C to afford the desired product as a yellow powder. Method B: A
heavy-walled glass reaction vessel was charged with
[{(Ar)2PCH2CH2}PPh]MoCl3, 3 equiv of NaEt3BH solution (1 M
in THF), and approximately 10 mL of tetrahydrofuran. The red-
orange reaction mixture was then treated with of 1 atm of dihydrogen
gas and stirred at ambient temperature for 14 h. The reaction mixture
was then frozen at −196 °C. the excess dihydrogen removed in vacuo
and replaced with approximately 0.5 atm of ethylene. The vessel was
allowed to warm to ambient temperature and stirred a further 3 h. The
volume of the reaction mixture was reduced by half, and the vessel was
placed under an atmosphere of dinitrogen. After standing under
dinitrogen for 1 h, the remaining solvent was removed, and the desired
product obtained by the purification procedure described above.
trans-{[(4-F-C6H4)2PCH2CH2]2PPh}Mo(C2H4)(N2)2 (2-C2H4).
39.41 (C2H4), 129.12, 129.97, 130.16, 130.91, 130.98, 131.71, 131.83,
132.31, 136.18, 136.95, 137.63, 138.25 (aryl). 31P {1H} NMR (23 °C,
C6D6): δ 74.36 (d, 16.0 Hz, 2P, PAr2), 97.16 (t, 16.0 Hz, 1P, PPh). IR
(KBr): νNN = 1982, 2048 cm−1 (approximately 1:2 relative intensity).
trans-{[(3,5-tBu-C6H3)2PCH2CH2]2PPh}Mo(C2H4)(N2)2 (7-C2H4).
Yield 71% (from Method A). 1H NMR (23 °C, C6D6): δ 1.23 (s, 72H,
ArC(CH3)3), 2.00 (s, 4H, C2H4), 2.37 (m, 2H, PCH2) 2.51 (m, 2H,
PCH2), 2.74 (m, 2H, PCH2), 3.16 (m, 2H, PCH2), 7.02 (m, 1H, aryl)
7.11 (m, 2H, aryl), 7.48 (m, 8H, aryl), 7.66 (m, 2H, aryl), 7.74 (m, 4H,
aryl). 13C{1H} NMR (23 °C, C6D6): δ 30.18 (PCH2), 31.90
(C(CH3)3), 31.99 (C(CH3)3), 35.45 (PCH2), 39.75 (C2H4), 128.98,
129.81, 130.84, 131.49, 131.63, 132.26, 136.83, 137.52, 138.13, 139.58
(aryl), 2 aryl and C(CH3)3 resonances not located. 31P{1H} NMR (23
°C, C6D6): δ 80.18 (d, 15.0 Hz, 2P, PAr2), 97.99 (t, 15.0 Hz, 1P, PPh).
IR (KBr) νNN = 1981, 2046 cm−1.
General Procedure for the Determination of Kinetics of
Acrylate Formation. In a typical experiment a J. Young tube was
charged with 0.5 mL of a ca. 0.03 M benzene-d6 solution of 1-C2H4
and a capillary of triethyl phopshite in benzene-d6 for use as an
integration standard. The sample was frozen at −35 °C, and 1 equiv of
PPh3 (0.1 M solution in benzene-d6) was added. Immediately
following PPh3 addition, the sample was further cooled to −196 °C,
and on a high vacuum line 4 atm of carbon dioxide was added via a
calibrated gas bulb. The tube was then thawed, shaken, and inserted
into a temperature-controlled NMR probe. The reaction progress was
monitored by 31P NMR spectroscopy over greater than 2 half-lives
following complete conversion of 1-C2H4 to 1-INT (approximately 30
min). 31P {1H} NMR spectroscopy was performed by taking 128 scans
with a delay time of 2 s at each time interval of 15 min. The decay of
the resonances of 1-INT was converted to concentration and fitted to
a first-order plot of ln [1-INT] versus time, which gave the observed
rate constants as the slope. Sample graphs may be found in the
Supporting Information. The kinetic measurements were repeated no
fewer than three times, using at least two independent synthetic
batches for each complex. Spectral features for (2-INT - 7-INT) are
listed below.
1
Yield 66% (from Method A) or 64% (from Method B). H NMR
(23 °C, C6D6): δ 1.70 (m, 2H, PCH2), 2.03 (m, 2H, PCH2), 2.15 (br
s, 4H, C2H4), 2.61 (m, 2H, PCH2), 2.84−2.99 (m, 2H, PCH2), 6.97−
7.01 (m, 11H, aryl), 7.37 (m, 4H, aryl), 7.65 (m, 2H, aryl), 7.72 (m,
4H, aryl). 13C{1H} NMR (23 °C, C6D6): δ 25.83 (C2H4), 26.6
(PCH2), 34.0 (PCH2), 114.3, 114.5, 115.12, 115.4, 128.8, 129.6, 132.9,
133.2, 134.9, 138.3 (aryl) two aryl resonances not located. 31P{1H}
NMR (23 °C, C6D6): δ 73.59 (d, 16.8 Hz, 2P, PAr2), 96.85 (t, 16.8 Hz
1P, PPh,). 19F{1H} NMR (23 °C, C6D6): δ −112.72 (s), −110.87 (s).
IR (KBr) νNN = 1982, 2052 cm−1 (approximately 1:2 relative
intensity).
trans-{[(4-CF3-C6H4)2PCH2CH2]2PPh}Mo(C2H4)(N2)2 (3-C2H4).
1
Yield 62% (from Method A). H NMR (23 °C, C6D6): δ 1.55 (m,
2H, PCH2), 1.85 (m, 2H, PCH2), 2.42 (br s, 4H, C2H4), 2.68 (m, 4H,
PCH2), 6.91 (m, 1H, aryl), 7.02 (m, 6H, aryl), 7.34 (m, 14H, aryl).
13C{1H} NMR (23 °C, C6D6): δ 30.11 (PCH2), 33.97 (PCH2), 39.35
(C2H4), 124.01, 125.19, 125.76, 125.88, 129.20, 130.65, 131.73, 133.67
(aryl), 142.19 (CF3), 143.99 (CF3). 31P{1H} NMR (23 °C, C6D6): δ
76.0 (d, 16.2 Hz, 2P, PAr2), 98.6 (t, 16.2 Hz, 1P, PPh). 19F{1H} NMR
(23 °C, C6D6): δ −62.6 (s), −62.5 (s). IR (KBr): νNN = 2062, 1992
cm−1 (approximately 1:2 relative intensity).
{[(4-F-C6H4)2PCH2CH2]2PPh}Mo(C2H4)(CO2) (2-INT). 1H NMR
(23 °C, C6D6): δ 0.70 (m, 4H, C2H4), 2.02 (m, 4H, PCH2), 2.21−2.69
(m, 4H, PCH2), 6.85 (m, 9H, aryl), 7.02 (m, 5H, aryl), 7.45 (m, 7H,
aryl). 31P{1H} NMR (23 °C, C6D6): δ 65.1 (d, 3.7 Hz 2P, PAr2), 94.3
(t, 3.7 Hz, 1P, PPh). 19F{1H} (23 °C, C6D6): δ −111.53 (s), −112.21
(s).
trans-{[(4-CH3-C6H4)2PCH2CH2]2PPh}Mo(C2H4)(N2)2 (4-C2H4).
Yield 82% (from Method A). Anal. Calcd for C40H45P3MoN4·C7H8:
{[(4-CF3-C6H4)2PCH2CH2]2PPh}Mo(C2H4)(CO2) (3-INT). 1H
NMR (23 °C, C6D6): δ 0.52 (m, 4H, C2H4), 2.21 (m, 2H, PCH2),
2.43 (m, 4H, PCH2), 3.05 (m, 2H, PCH2), 6.85 (m, 2H, aryl), 6.90
(m, 3H, aryl), 6.94 (m, 1H, aryl), 7.06 (d, 7.7 Hz, 2H, aryl), 7.09 (m,
2H, aryl), 7.12 (d, 7.7 Hz, 2H, aryl), 7.21 (d, 7.7 Hz, 2H, aryl), 7.25 (d,
7.7 Hz, 2H, aryl), 7.43 (m, 5H, aryl). 31P{1H} NMR (23 °C, C6D6): δ
68.96 (d, 7.3 Hz, 2P, PAr2), 96.80 (t, 7.3 Hz, 1P, PPh). 19F{1H} NMR
(23 °C, C6D6): δ −62.7 (s), −62.6 (s).
1
C, 65.42; H, 6.19; N, 6.49. Found: C, 64.53; H, 5.69; N, 6.21. H
NMR (23 °C, C6D6): δ 1.51 (br s, 4H, C2H4), 1.78 (m, 2H, PCH2),
1.98 (s, 6H, CH3), 2.01 (s, 6H, CH3), 2.66 (m, 4H, PCH2), 2.92−3.07
(m, 2H, PCH2), 6.92−7.04 (m, 11H, aryl), 7.33 (m, 4H, aryl), 7.53
(m, 2H, aryl), 7.64 (m, 4H, aryl). 13C{1H} NMR (23 °C, C6D6): δ
20.88 (CH3), 29.58 (PCH2), 33.74 (PCH2), 37.94 (C2H4), 128.34,
129.03, 131.27, 131.31, 131.59, 133.09, 137.42, 138.91 (aryl). 31P{1H}
NMR (23 °C, C6D6): δ 73.2 (d, 17.0 Hz, 2P, PAr2), 97.9 (t, 17.0 Hz,
1P, PPh). IR (KBr): νNN = 2048, 1982 cm−1 (approximately 1:2
relative intensity).
{[(4-CH3-C6H4)2PCH2CH2]2PPh}Mo(C2H4)(CO2) (4-INT). 1H
NMR (23 °C, C6D6): δ 0.30 (m, 4H, C2H4), 1.84 (m, 2H, PCH2),
1.94 (s, 6H, CH3), 2.02 (s, 6H, CH3), 2.21 (m, 2H, PCH2), 2.69 (m,
2H, PCH2), 2.95 (m, 2H, PCH2), 6.90 (m, 9H, aryl), 7.06 (m, 5H,
aryl), 7.66 (m, 7H, aryl). 31P{1H} NMR (C6D6): δ 64.9 (d, 5.2 Hz, 2P,
PAr2), 96.1 (t, 5.2 Hz, 1P, PPh).
trans-{[(4-OCH3-C6H4)2PCH2CH2]2PPh}Mo(C2H4)(N2)2 (5-C2H4).
1
Yield 35% (from Method A). H NMR (23 °C, C6D6): δ 1.41 (br s,
4H, C2H4), 1.88 (m, 4H, PCH2), 2.08 (m, 2H, PCH2), 2.15 (m, 2H,
PCH2), 3.23 (s, 6H, Ar-OCH3) 3.24 (s, 6H, Ar-OCHH3) 6.69 (m, 8H,
Ar-OCH3) 7.05 (m, 3H, aryl), 7.29−7.35 (m, 8H, Ar-OCH3), 7.37 (m,
2H, aryl). 13C{1H} NMR (23 °C, C6D6): δ 21.20 (PCH2), 31.24
(PCH2), 36.61 (C2H4), 48.21 (OCH3), 49.15(OCH3), 125.46, 128.32,
128.88, 129.09, 132.78, 133.11, 134.23, 137.86, 148.91, 149.27 (aryl).
31P{1H} NMR (23 °C, C6D6): δ 72.5 (d, 16.4 Hz, 2P, PAr2), 96.7 (t,
16.4 Hz, 1P, PPh). IR (KBr) νNN = 2049, 1982 cm−1 (approximately
1:2 relative intensity).
{[(4-OCH3-C6H4)2PCH2CH2]2PPh}Mo(C2H4)(CO2) (5-INT). 1H
NMR (23 °C, C6D6): δ 0.97 (m, 4H, C2H4), 2.18 (m, 4H, PCH2),
2.55 (s, 6H, OCH3), 3.29 (s, 6H, OCH3), 3.09 (m, 4H, PCH2).
31P{1H} NMR (23 °C C6D6): δ 67.7 (d, 3.1 Hz, 2P, PAr2) 99.5 (t, 3.1
Hz, 1P, PAr).
{[(3,5-Me-C6H3)2PCH2CH2]2PPh}Mo(C2H4)(CO2) (6-INT). 1H
NMR (23 °C, C6D6): δ 1.00 (m, 4H, C2H4), 2.01 (s, 12H, CH3),
2.06 (s, 12H, CH3), 3.23 (m, 4H, PCH2), 3.58 (t, 6 Hz, 4H, PCH2),
6.62 (d, 5 Hz, 4H, aryl), 6.75 (s, 2H, aryl), 6.98 (d, 5 Hz, 4H, aryl),
7.04 (d, 5 Hz, 2H, aryl), 7.39 (m, 1H, aryl), 7.66 (m, 2H, aryl), 8.05 (t,
8 Hz, 2H, aryl). 13C {1H} NMR (23 °C, C6D6): 21.8 (CH3), 22.0
(CH3), 29.3 (C2H4), 66.3 (PCH2), 68.1 (PCH2), 123.1, 127.8, 129.6,
129.8, 130.0, 130.1, 131.0, 131.1, 136.7, 138.2 (aryl) one aryl
resonance and carbonyl not located. 31P{1H} NMR (23 °C, C6D6):
trans-{[(3,5-CH3-C6H3)2PCH2CH2]2PPh}Mo(C2H4)(N2)2 (6-C2H4).
Yield 72% (from Method A). 1H NMR (23 °C, C6D6): δ 1.60 (s, 4H,
C2H4), 1.91 (m, 2H, PCH2), 2.07 (s, 6H, CH3), 2.08 (s, 6H, CH3),
2.21 (m, 2H, PCH2), 2.68 (m, 2H, PCH2), 2.98−3.13 (m, 2H, PCH2),
6.70 (d, 11 Hz, 2H, aryl), 6.84−7.12 (m, 5H, aryl), 7.22 (m, 1H, aryl),
7.47 (m, 1H, aryl), 7.52 (t, 8.9 Hz, 2H, aryl). 13C{1H} NMR (23 °C,
C6D6): δ 21.22 (CH3), 21.36 (CH3), 30.07 (PCH2), 34.05 (PCH2),
F
dx.doi.org/10.1021/om500324h | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
65.84 (d, 18 Hz, 2P, PAr2), 96.83(t, 18 Hz, 1P, PPh) IR (KBr): νCO
1699 cm−1.
7H, aryl), 6.90 (m, 5H, aryl), 7.03 (m, 9H, aryl), 7.33 (m, 7H, aryl),
7.58 (m, 5H, aryl), 7.85 (m, 3H, aryl). 13C{1H} NMR (23 °C, C6D6):
δ 21.61 (CH3), 21.65 (CH3), 26.33 (PCH2), 34.70 (PCH2), 123.30
(CHCH2), 131.81 (CHCH2), 127.60, 127.68, 129.13, 129.20,
131.5, 131.64, 133.68, 134.04, 134.44, 134.64, 135.45, 135.57, 137.78,
138.24 (aryl) two aryl signals not located.31P{1H} NMR (23 °C,
C6D6): δ 49.3 (dt, 11.9, 156.8 Hz, 1P, PPh3), 99.4 (dt, 18.5, 156.9 Hz,
1P, PPh), 108.6 (dd, 12.0, 20.0 Hz, 2P, PAr2). IR (KBr): νCO = 1515
cm−1.
{[(3,5-tBu-C6H3)2PCH2CH2]2PPh}Mo(C2H4)(CO2) (7-INT). 1H
NMR (23 °C, C6D6): δ 0.45 (m, 4H, C2H4), 1.27 (s, 36H, CCH3),
1.29 (s, 36H, CCH3), 3.12 (m, 4H, PCH2), 3.47 (m, 4H, PCH2),
6.60−6.72 (m, 4H, aryl), 6.77 (s, 2H, aryl), 6.98−7.07 (m, 6H, aryl),
7.70 (m, 3H, aryl), 8.07 (m, 2H, aryl). 31P{1H} NMR (23 °C, C6D6):
67.70 (d, 2.7 Hz, 2P, PAr2), 98.76 (d, 2.7 Hz, 2P, PPh).
G e n e r al P r oc e d u r e f o r t h e P re p ar a ti o n of
[(Ar2PCH2CH2)2PPh]Mo(H)PPh3(CO2CHCH2) (2-acrylate−7-
acrylate). Complexes 2-C2H4−7-C2H4 were prepared by procedures
analogous to those previously reported for 1-acrylate either by carbon
dioxide addition to trans-[(Ar)2PCH2CH2}2PPh]Mo(C2H4)(N2)2 in
the presence of triphenylphosphine (Method A) or transmetalation of
silver acrylate with in situ generated [(Ar)2PCH2CH2}2PPh]Mo(H)-
PPh3(Cl) (Method B). In a typical synthesis using Method A, a J.
Young tube was charged with [(Ar)2PCH2CH2}2PPh]Mo(C2H4)-
(N2)2, 1 equiv of PPh3, and approximately 0.5 mL of benzene-d6. On a
vacuum line, 4 atm of carbon dioxide was admitted to the sample via
calibrated gas bulb at −196 °C. The tube was warmed to ambient
{[(4-OCH3-C6H4)2PCH2CH2}2PPh}Mo(H)PPh3(CO2CHCH2) (5-
acrylate). Yield 79% (Method B). Anal. Calcd for C34H29MoO6P4: C,
1
65.31; H, 5.57. Found: C, 65.59; H 5.43. H NMR (23 °C, C6D6): δ
−4.73 (tdd, 73.7, 42.3, 12.5 Hz, 1H, Mo-H), 1.16−1.22 (m, 2H,
PCH2), 2.56−2.68 (m, 4H, PCH2), 3.33 (s, 6H, CH3), 3.35 (s, 6H,
CH3), 4.74 (dd, 2.3, 10.6, 1H, CHCH2), 5.26 (dd, 10.6, 17.4 Hz,
1H, CHCH2), 5.55 (dd, 2.3, 17.4 Hz, 1H, CHCH2), 6.74 (m, 7H,
aryl), 6.91 (m, 5H, aryl), 7.04 (m, 9H, aryl), 7.40 (m, 7H, aryl), 7.64
(m, 5H, aryl), 7.89 (m, 3H, aryl). 13C{1H} NMR (23 °C, C6D6): δ
49.35 (COCH3), 51.34 (COCH3), 27.52 (PCH2), 35.41 (PCH2),
122.20 (CHCH2), 131.92 (CHCH2), 128.11, 128.68, 130.72,
130.92, 131.20, 132.44, 133.86, 134.24, 134.64, 134.64, 134.78, 135.27,
137.58, 137.64 (aryl) three aryl resonances not located. 31P{1H} NMR
(23 °C, C6D6) δ 49.31 (dt, 10.1, 155.3 Hz, 1P, PPh3) 98.93 (dt, 19.1,
155.3 Hz, 1P, PAr2) 106.41 (dd, 10.1, 19.1 Hz, 2P, PPh). IR (KBr):
νCO = 1518 cm−1
1
temperature, shaken thoroughly, and left to stand overnight. H and
31P{1H} NMR spectroscopy revealed complete conversion along with
a small quantity of free PPh3. In a typical synthesis using Method B, a
20 mL scintillation vial was charged with [(Ar)2PCH2CH2}2PPh]-
MoCl3 and 1 equiv of PPh3 in approximately 5 mL of THF. Then 2
equiv of NaHBEt3 (1 M in tetrahydrofuran) was added via syringe, and
the reaction stirred at ambient temperature for 2 h, changing rapidly
from yellow to dark green. The volatiles were removed in vacuo,
affording a dark residue that was washed with pentane, extracted with
toluene, and dried to afford a dark green solid. The solid was then
treated with 1 equiv of silver acrylate in a cosolvent of toluene/diethyl
ether and stirred for 1 h at ambient temperature. The solution was
then filtered, concentrated to ca. 1 mL, and crystallized from toluene/
diethyl ether at −35 °C to afford the desire product as reddish solids.
{[(4-F-C6H4)2PCH2CH2]2PPh}Mo(H)PPh3(CO2CHCH2) (2-
acrylate). Yield 56% (Method B). Anal. Calcd for C34H29MoO2P4:
C, 63.71; H, 4.67. Found: C, 63.49; H, 4.40. 1H NMR (23 °C, C6D6):
δ −4.93 (tdd, 1H, 13.6, 42.3, 73.4 Hz Mo-H), 1.28 (m, 2H, PCH2),
1.84, (m, 2H, PCH2), 2.35−2.47 (m, 4H, PCH2) 4.70 (dd, 1H, 2.1,
10.3 Hz, CHCH2), 5.13 (dd, 10.3, 17.2 Hz, 1H, CHCH2), 5.44
(dd, 2.1, 17.2 Hz, 1H, CHCH2), 6.70−6.83 (m, 16 H, aryl), 6.90 (t
8.1 Hz, 3H, aryl), 7.03 (m, 6H, aryl), 7.29, (m, 2H, aryl), 7.38 (m, 6H,
aryl) 7.71 (t, 7.5 Hz, 3H aryl). 13C {1H} NMR (23 °C, C6D6): δ 26.26
(PCH2), 34.77 (PCH2), 124.03, 131.41, (CHCH2)) 114.87, 127.81,
127.94, 129.14, 129.20, 134.54, 135.08, 135.08, 135.08, 135,20, 135.72,
137.29 (aryl), three aryl signals not located. 31P{1H} NMR (23 °C,
C6D6): δ 47.2 (dt, 12 Hz, 154 Hz, 1P, PPh3), 99.7 (dt, 20 Hz, 154 Hz,
1P, PPh), 107.2 (dd, 12 Hz, 20 Hz, 2P, PAr2). 19F{1H} (23 °C, C6D6):
δ −112.84 (s), −113.41 (s). IR (KBr): νCO = 1587 cm−1.
{[(3,5-CH3-C6H3)2PCH2CH2]2PPh}Mo(H)PPh3(CO2CHCH2) (6-
1
acrylate). Yield 60% (Method A). H NMR (23 °C, C6D6): δ −4.37
(ddt, 74.3, 38.1, 13.4 z, 1H, Mo-H), 1.20 (m, 2H, PCH2), 1.52 (m, 2H,
PCH2), 2.03 (s, 12H, Ar−CH3), 2.15 (s, 12H, Ar−CH3), 2.76−2.89
(m, 4H, PCH2), 4.71, (dd, 10.3, 2.3 Hz, 1H, CHCH2), 5.28 (dd,
17.3, 10.3 Hz, 1H, CHCH2), 5.56 (dd, 17.3, 2.3 Hz, 1H, CH
CH2), 6.80−6.94 (m, 11H, aryl), 7.02−7.07 (m, 6H, aryl), 7.20−7.42
(m, 12H, aryl) 7.86 (t, 7.8 Hz, 3H, aryl). 13C{1H} NMR (23 °C,
C6D6): δ 20.03 (ArCH3), 20.63 (ArCH3), 26.20 (PCH2), 34.20,
(PCH2), 121.95, (CHCH2) 127.07, 127.31, 128.44, 128.61, 130.37,
130.48, 130.90, 130.98, 133.01, 133.27, 134.80, 137.12, 137.34, 141.16,
141.69 (aryl) two aryl resonances not located. 31P{1H} NMR (23 °C,
C6D6): δ 52.09 (dt, 11.5, 156 Hz, 1P, PPh3) 102.96 (dt, 17, 156 Hz,
1P, PAr2) 109.60 (dd, 11.5, 17 Hz, 2P, PPh). IR (KBr): νCO = 1518
cm−1
{[(3,5-tBu-C6H3)2PCH2CH2]2PPh}Mo(H)PPh3(CO2CHCH2) (7-
1
acrylate). Yield 16% (Method A). H NMR (23 °C, C6D6): δ −4.74
(ddt, 72.9, 41.0, 15.2 z, 1H, Mo-H), 1.56 (m, 2H, PCH2), 1.99 (m, 2H,
PCH2), 1.56 (s, 36H, Ar−CCH3), 1.63 (s, 36H, Ar−CCH3), 2.62 (m,
4H, PCH2), 4.73, (dd, 9.9, 2.4 Hz, 1H, CHCH2), 5.26 (dd, 17.5, 9.9
Hz, 1H, CHCH2) 5.55 (dd, 17.5, 2.4 Hz, 1H, CHCH2), 6.80−
6.94 (m, 11H, aryl), 7.02−7.07 (m, 6H, aryl), 7.20−7.42 (m, 12H,
aryl) 7.86 (t, 7.8 Hz, 3H, aryl). 13C NMR (23 °C, C6D6): δ 15.90,
(ArCCH3) 16.21 (ArCCH3), 26.14, (PCH2) 34.39 (PCH2), 35.28,
(ArCCH3) 121.57, 123.45, 126.45, 127.69, 127.87, 129.18 129.65,
131.55, 133.59, 133.85, 134.46, 135.34, 137.83, 138.38, 153.71 (aryl),
177.42 (O2CC2H3) three aryl signals not located. 31P{1H} NMR (23
°C, C6D6): δ 56.03 (dt, 12, 157 Hz, 1P, PPh3), 99.14 (dt, 20, 157 Hz,
1P, PAr2), 108.42 (dd, 12, 20 Hz, 2P, PPh). IR (KBr): νCO = 1513
cm−1
{[(4-CF3-C6H4)2PCH2CH2]2PPh}Mo(H)PPh3(CO2CHCH2) (3-
acrylate). Yield 51% (Method A). 1H NMR (23 °C, C6D6): δ
−4.79 (tdd, 1H, 14.2, 44.0, 72.0 Hz, Mo-H), 1.78 (m, 4H, PCH2),
2.29−2.45 (m, 4H, PCH2), 4.70 (dd, 1H, 2.0, 10.3 Hz, CHCH2),
5.09 (dd, 1H, 10.3, 17.3 Hz, CHCH2), 5.40 (dd, 1H, 2.1, 17.2 Hz,
CHCH2), 6.67 (m, 4H, aryl), 6.81 (m, 6H, aryl), 7.04 (m, 12H,
aryl), 7.32 (m, 2H, aryl), 7.29 (m, 9H, aryl), 7.46 (m, 3H, aryl).
13C{1H} NMR (23 °C, C6D6): 26.15 (PCH2), 34.78 (PCH2), 125.11
(CHCH2), 131.09 (CHCH2), 133.28 (CF3), 133.93 (CF3),
127.83, 127.92, 129.20, 129.49, 131.34, 131.43, 134.44, 134.63,
134.84, 134.95, 136.35, 138.32, 138.44 (aryl). 31P{1H} NMR (23
°C, C6D6): δ 46.5 (dt, 12.5, 151.8 Hz, 1P, PPh3), 102.3 (dt, 18.5, 152.3
Hz, 1P, PPh), 112.0 (dd, 12.5, 18.1 Hz, 2P, PAr2). 19F{1H} NMR (23
°C, C6D6): δ −62.5 (s), −62.3 (s). IR (KBr): νCO = 1606 cm−1
{[(4-CH3-C6H4)2PCH2CH2]2PPh}Mo(H)PPh3(CO2CHCH2) (4-
acrylate). Yield 68% (Method B). Anal. Calcd for C59H48F12P4MoO2:
C, 69.41; H, 5.92. Found: C, 69.13; H, 6.14. 1H NMR (23 °C, C6D6):
δ −4.71 (tdd, 73.9, 41.5, 13.4 Hz, 1H, Mo-H), 1.59 (m, 2H, PCH2),
2.00 (m, 4H, PCH2), 2.15 (s, 6H, CH3), 2.20 (s, 6H, CH3), 2.59−2.67
(m, 2H, PCH2), 4.41 (dd, 1.9, 9.7, 1H, CHCH2), 5.23 (dd, 9.7, 17.2
Hz, 1H, CHCH2), 5.56 (dd, 1.9, 17.2 Hz, 1H, CHCH2), 6.83 (m,
S p e c t r o s c o p i c C h a r a c t e r i z a t i o n o f { [ [ ( 4 - F -
C6H4)2PCH2CH2]2PPh]Mo(H)(CO2CHCH2)}2 (2-dimer). A reac-
tion vessel was charged with 200 mg of [{(4-CF3-C6H4)2PCH2CH2}-
PPh]Mo(N2)2(C2H4) in toluene. The reaction vessel was then filled
with 2 atm of CO2, left to stir at room temperature for 24 h, and then
dried in vacuo. The resultant brownish-red mixture that resulted was
extracted with pentane followed by diethyl ether and toluene. The
diethyl ether fraction was then reduced in volume and layered with
pentane, which precipitated 115 mg of an orange-red solid as a crude
reaction product, which we were unable to purity further from residual
free ligand. Two isomers: 1H NMR (23 °C, C6D6) δ −7.09 (ddd, 13.9,
55.9, 94.3 Hz, Mo-H), −6.70 (ddd, 12.5, 66.4, 97.8 Hz, Mo-H), 1.7−
2.2, 2.18, 3.65, (m, PCH2 and CHCH2) 6.91−7.14 (m, aryl) 7.56
(m aryl), 8.15, (m aryl) 8.23 (m aryl). 31P {1H} NMR NMR (23 °C,
C6D6): δ 80.3 (dd, 14.5, 27.6 Hz), 79.5 (dd 14.5, 27.6 Hz), 86.7 (m),
G
dx.doi.org/10.1021/om500324h | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
87.9 (m), 99.9 (m), 105.8 (m), 110.8 (m), 118.7 (m). 19F{1H} NMR
(23 °C, C6H6): δ −111.22(s), −106.10(s), −105.37(s), −103.17(s).
(7) (a) Alvarez, R.; Carmona, E.; Cole-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. (d) Collazo,
C.; del Mar Conejo, M.; Pastor, A.; Galindo, A. Inorg. Chim. Acta 1998,
272, 125.
(8) (a) Bernskoetter, W. H.; Tyler, B. T. Organometallics 2011, 30,
520. (b) Zhang, Y.; Hanna, B. S.; Dineen, A.; Williard, P. G.;
Bernskoetter, W. H. Organometallics 2013, 32, 3969.
(9) Hartwig, J. In Organotransition Metal Chemistry: From Bonding to
Catalysis; University Science: Sausalito, 2010; pp 21−22.
(10) See Experimental Section for details.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic information file for 1-C2H4 in cif format and
selected spectral and kinetic data. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wb36@brown.edu.
(11) George, A. T.; Kovar, R. A. Inorg. Chem. 1981, 20, 285.
(12) Cole, A. A.; Mattamana, S. P.; Poli, R. Polyhedron 1996, 15,
2351.
Notes
The authors declare no competing financial interest.
(13) Talarmin, J.; Al-Salih, T. I.; Pickett, C. J.; Bossard, G. E.; George,
T. A.; Duff-Spence, C. M. J. Chem. Soc., Dalton Trans. 1992, 2263.
(14) For selected examples of molybdenum bis(dinitrogen)
structures, see: (a) Carmona, E.; Marin, J. M.; Poveda, M. L.;
Atwood, J. L.; Rogers, R. D. J. Am. Chem. Soc. 1983, 105, 3014.
(b) Yuki, M.; Miyake, Y.; Nishibayashi, Y.; Wakji, I.; Hidai, M.
Organometallics 2008, 27, 3947. (c) Ning, Y.; Sarjeant, A. A.; Stern, C.
L.; Peterson, T. H.; Nguyen, S. T. Inorg. Chem. 2012, 51, 3051.
(15) Carmona, E.; Galindo, A.; Marin, J. M.; Gutierrez, E.; Monge,
A.; Ruiz, C. Polyhedron 1988, 7, 1831.
(16) The trans-[(Ar2PCH2CH2)2PPh]Mo(N2)2PPh3 species are
competent for CO2-ethylene coupling but proceed far more slowly
at such low concentrations of ethylene. In control experiments many
of these species produced acrylate compounds (X-acrylate) without
building up a detectable concentration of the intermediate species (X-
INT). In addition, no reversion to trans-[(Ar2PCH2CH2)2PPh]
Mo(N2)2PPh3 was observed from any intermediate complex (X-
INT) under the conditions of the kinetic experiments.
ACKNOWLEDGMENTS
■
This material is based upon work supported by the National
Science Foundation under the Center for Chemical Innovation:
“Center for the Capture and Conversion of CO2” (CHE-
1240020) and the Air Force Office of Scientific Research
(Award No. FA9550-11-1-0041). S.A. would like to thank the
U.S. Dept. of Education for support through a GAANN Award
(P200A120064). We would like to thank Yuanyuan Zhang for
assistance in obtaining electrochemical data.
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dx.doi.org/10.1021/om500324h | Organometallics XXXX, XXX, XXX−XXX