Decomposition of Vanadium(V) Alkylidenes Relevant to Olefin Metathesis

Article abstract of DOI:10.1021/acs.organomet.0c00610

Vanadium alkylidenes can be highly active initiators for ring-opening metathesis polymerization of cyclic olefins; however, attempts to expand their use to cross metathesis have been unsuccessful due to catalyst decomposition. Detailed knowledge of the decomposition reaction is imperative to guide future catalyst design. Herein, we demonstrate that β-hydride elimination is the dominant decomposition pathway during cross metathesis. The isolated vanadium decomposition products [N-2,6-(CH3)2C6H3](OC6Cl5)[P(CH3)3]2VCl (10) and [N-2,6-(CH3)2C6H3]V(OC6F5)2[P(CH3)3]2 (11), generated from two separate catalysts, agree with this pathway. Compounds 10 and 11 were shown computationally to form via bimolecular routes after β-hydride elimination from the metallocyclobutane and reductive elimination of propylene, which was itself demonstrated to be exergonic with low thermodynamic barriers to metallocyclobutane formation. Relative conversions in the self-metathesis of 1-hexene with the series of vanadium alkylidenes of various sterics and electronics (N-2,6-X2C6H3)(OC6Y5)[P(CH3)3]2V[CHSi(CH3)3] [X = CH3, Y = Cl (1); X = CH(CH3)2, Y = Cl (6), X = CH(CH3)2, Y = F (7)] were determined. It was found that bulkier, yet more electron-donating ligands decreased conversions, indicating that β-hydride elimination is favored over bimolecular decomposition. Analysis of organic reaction products demonstrated that reductive elimination of propylene occurs before insertion of ethylene into the newly formed vanadium-hydrogen bond.

Full text of DOI:10.1021/acs.organomet.0c00610

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Article
Decomposition of Vanadium(V) Alkylidenes Relevant to Olefin
Metathesis
Wesley S. Farrell,* Christine Greene, Pokhraj Ghosh, Timothy H. Warren, and Peter Y. Zavalij
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ABSTRACT: Vanadium alkylidenes can be highly active initiators for ring-opening
metathesis polymerization of cyclic olefins; however, attempts to expand their use to cross
metathesis have been unsuccessful due to catalyst decomposition. Detailed knowledge of
the decomposition reaction is imperative to guide future catalyst design. Herein, we
demonstrate that β-hydride elimination is the dominant decomposition pathway during
cross metathesis. The isolated vanadium decomposition products [N-2,6-(CH3)2C6H3]-
(OC6Cl5)[P(CH3)3]2VCl (10) and [N-2,6-(CH3)2C6H3]V(OC6F5)2[P(CH3)3]2 (11),
generated from two separate catalysts, agree with this pathway. Compounds 10 and 11
were shown computationally to form via bimolecular routes after β-hydride elimination
from the metallocyclobutane and reductive elimination of propylene, which was itself
demonstrated to be exergonic with low thermodynamic barriers to metallocyclobutane formation. Relative conversions in the self-
metathesis of 1-hexene with the series of vanadium alkylidenes of various sterics and electronics (N-2,6-X2C6H3)(OC6Y5)[P-
(CH3)3]2V[CHSi(CH3)3] [X = CH3, Y = Cl (1); X = CH(CH3)2, Y = Cl (6), X = CH(CH3)2, Y = F (7)] were determined. It was
found that bulkier, yet more electron-donating ligands decreased conversions, indicating that β-hydride elimination is favored over
bimolecular decomposition. Analysis of organic reaction products demonstrated that reductive elimination of propylene occurs
before insertion of ethylene into the newly formed vanadium−hydrogen bond.
dispersity were obtained.27 Our group has sought to expand
upon the foundational work of Nomura, specifically through
expansion in the use of vanadium to include CM. This reaction
was selected as a target due to the fact that success will have
INTRODUCTION
Olefin metathesis (OM) plays a critical role in synthetic
■
chemistry through the formation of CC bonds with high
precision.1 The reaction is useful in small molecule synthesis,
implications for future possibilities of ADMET, RCM, and
ROCM with vanadium, as all require the ability for transient
methylidenes ([V]CH2, [V] = vanadium-ligand complex) to
exist, even fleetingly. Although Nomura reported the use of
terminal olefins in low concentrations as chain transfer agents
in ROMP,28 systematic investigations into this reaction are
lacking. Thus far, we disclosed that the vanadium(V)
alkylidenes (N-2,6-X2C6H3)(OC6Cl5)[P(CH3)3]2V[CHSi-
(CH3)3] [X = CH3 (1), F (2)] (Chart 1) can in fact catalyze
the self-metathesis (SM) of various terminal olefins; however,
premature catalyst death prevents the reaction from achieving
high conversions.29
with implications for natural product and drug discovery
chemistry2,3 via cross metathesis (CM), ring-closing metathesis
(RCM), and ring-opening cross metathesis (ROCM),4 as well
as polymer synthesis via ring-opening metathesis polymer-
ization (ROMP),5−7 acyclic diene metathesis polymerization
(ADMET),7−12 and ring-expansion metathesis polymerization
(REMP).13 Since the initial reports of Schrock14 and Grubbs15
on well-defined OM catalysts, the field has been dominated by
myriad catalysts based on molybdenum and ruthenium.16,17
Variation in ligands has allowed for remarkably high activity
and selectivity, and low catalyst loadings (as low as 1 ppm in
the case of cyclic alkyl amino carbene supported ruthenium
catalysts).18 Incorporation of substrates once thought beyond
the scope of OM, such as alkenyl halides or others bearing
functional groups directly on the olefinic carbon, is also now
possible due to advancements with group 6 catalysts.19−22
Recently, Nomura and co-workers reported efforts to add
the more earth-abundant and inexpensive vanadium to the OM
toolbox. Vanadium(V) alkylidenes demonstrated impressively
high activity in ROMP of cyclic olefins.6,23,24 In the case of
highly strained norbornene, both living and Z-selective
polymerization were achieved.25,26 When low-strain cyclo-
pentene, cycloheptene, and cis-cyclooctene were employed,
high molar mass polyalkenemers of moderately narrow
One can consider three species present during SM as the
sources of decomposition, as depicted in Scheme 1. Alkylidene
A appeared unlikely to be the culprit for decomposition, given
its similarity to alkylidenes generated during polymerization of
Received: September 14, 2020
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position of transient methylidenes was at play during the SM
previously reported.29 Indeed, Fogg demonstrated that the
sterics of the substituents on the alkylidene carbon greatly
influence the rate of bimolecular decomposition in ruthenium
catalysts, with methylidenes decomposing the fastest.41
Nomura also showed that reaction of the ketimide containing
vanadium(V) alkylidene [N-2,6-(CH3)2C6H3]{NC[C-
(CH3)3]2}[P(CH3)3]V[CHSi(CH3)3] (3) with 1 equiv of
styrene yielded the metallocyclopropane species [N-2,6-
(CH3)2C6H3]{NC[C(CH3)3]2}[P(CH3)3]V[η2-CH2CH-
(C6H5)] (4) in 24.5% isolated yield (Scheme 2).42 The
Chart 1. Vanadium(V) Alkylidenes Screened in Self-
Metathesis
Scheme 1. Mechanism and Intermediates of Self-Metathesis
Scheme 2. Decomposition of Vanadium(V) Alkylidenes
monomers such as cyclopentene, cycloheptene, and cis-
cyclooctene, which proceed to extremely high conversions
with no indication of catalyst decomposition.27 On the other
hand, methylidene B was considered the species most prime
for decomposition. Within group 5, decomposition of
methylidenes is well-known, usually occurring through a
bimolecular mechanism. Schrock first demonstrated this
bimolecular process with the tantalum(V) complex Cp2Ta-
(CH3)(CH2) (Cp = cyclopentadienyl), which decomposed to
form Cp2Ta(CH3)(η2-CH2CH2).30 Rothwell later demonstra-
ted spectroscopically that tantalum(V) methylidenes may be
generated photolytically from the tantalum(V) trimethyl
precursors (OAr)2Ta(CH3)3 {Ar = 2,6-[C(CH3)3]C6H3, 2,6-
[C(CH3)3]-4-(OCH3)C6H2}, but that these compounds
decomposed in a unimolecular fashion through cyclometala-
tion of a tert-butyl group on an aryloxide ligand.31,32 Arnold
reported the synthesis of the amidinate-supported tantalum(V)
methylidene complexes (Am)2Ta(CH2)(CH3) {Am = η2-
ArC[NSi(CH3)3]2, Ar = phenyl, tolyl} which were not
reported to undergo bimolecular decomposition,33 but did
demonstrate methylidene group transfer to pyridine N-oxides,
yielding tantalum(V)-oxo and methyl-substituted pyridine
products.34 Presumably, the larger amidinates (relative to
Cp)35 prevented bimolecular decomposition. Additional
reactivity of these species was later demonstrated with nitriles,
isocyanides, aldehydes, thiols, and propylene sulfide.36 Other
examples of stable tantalum methylidenes were reported by
Ozerov and Mindiola, also supported by sterically demanding
ligands (e.g., PNP-pincers and aryloxides).37,38 Bulky aryl-
oxides were also employed to support niobium methylidenes
immune to bimolecular decomposition.39,40
authors reported that while mechanistic investigations were
not performed, bimolecular decomposition was likely at work.
Lastly, and with these examples in mind, the observation of fast
decomposition in SM but not in ROMP also leads one to
consider bimolecular decomposition as the most likely
scenario.
In our previous work, the assumption of bimolecular
decomposition was challenged upon observing that the
reaction of vanadium(V) alkylidenes 1 and 2 with ethylene
produced propylene and unidentified paramagnetic vanadium
1
products, as demonstrated by H nuclear magnetic resonance
(NMR) spectroscopy (Scheme 2).29 This observation strongly
indicated that rather than bimolecular decomposition, β-
hydride elimination from the metallocyclobutane intermediate
C (Scheme 1) was the cause of decomposition. Propylene was
also observed in decomposition of both tungsten and
molybdenum molecular alkylidenes upon treatment with
ethylene, likely via β-hydride elimination from the metal-
locyclobutane, although the authors in these cases simply
referred to the process as rearrangement of the metal-
locyclobutane.43,44 Products associated with β-hydride elimi-
nation were observed with heterogeneous rhenium,45 molyb-
denum,46 and tungsten alkylidenes as well.47 In these cases,
computations demonstrated low barriers to decomposition in
square pyramidal metallocyclobutane intermediates via β-
hydride elimination.48 Lastly, propylene generation via β-
hydride elimination was shown to occur within ruthenium
olefin metathesis catalysts upon treatment with ethylene.49,50
The conflict between the previous report that decom-
position of a vanadium(V) alkylidene upon treatment with a
terminal olefin occurs through a bimolecular pathway42 and
our own observations29 led us to examine the decomposition
more thoroughly, as knowledge of which decomposition pathway
is present and dominant is imperative to guide future catalyst
design. Specifically, we sought to isolate and characterize the
Given that bulky ligands are certainly needed to stabilize
group 5 methylidenes and prevent bimolecular decomposition,
and that a vanadium methylidene has yet to be isolated, it
would not be surprising to learn that bimolecular decom-
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paramagnetic vanadium products in order to shed light on the
decomposition mechanism, aiming to determine if they are
consistent with β-hydride elimination from the metallocyclo-
butane rather than bimolecular decomposition of the
methylidene (Scheme 3) and, if so, what the subsequent
species [N-2,6-(CH3)2C6H3](OC6X5)V[P(CH3)3]2 [X = Cl
(8), F (9)]. In both cases, oxidation to the isolated and fully
characterized vanadium(IV) compounds [N-2,6-
(CH3)2C6H3](OC6Cl5)[P(CH3)3]2VCl (10) and [N-2,6-
(CH3)2C6H3]V(OC6F5)2[P(CH3)3]2 (11), respectively,
(Scheme 4) occurred. Mechanisms for the formation of both
species are proposed and supported by DFT calculations.
Scheme 3. Possible Decomposition Routes Considered
Scheme 4. Synthesis of Decomposition Products 10 and 11
RESULTS AND DISCUSSION
■
Isolation of Decomposition Products. In order to
determine the identity of the products of decomposition
during OM with vanadium catalysts, compound 1 was treated
with ethylene in benzene-d6, and the reaction was monitored
reactivity of these products are. This latter point is of great
importance given computational and empirical evidence that
after β-hydride elimination in rhenium, molybdenum, and
tungsten metallocyclobutanes, the subsequent hydride species
may insert ethylene to form bis(alkyl) compounds, which can
then undergo α-hydride abstraction to regenerate alkylidenes,
which would be catalytically active (infra).48 If reductive
elimination of propylene follows β-hydride elimination,
however, catalytically inactive vanadium(III) species would
be expected. The first vanadium(V) alkylidene was prepared
from a vanadium(III) precursor,51 so one could imagine
regeneration of catalytically active species if no further reaction
takes place. This would be unlikely, however, given the known
instability of coordinatively unsaturated vanadium(III)-imido
species. The first stable example of such a compound was
reported by Teuben and co-workers, in which a vanadium(III)
alkylidene complex bearing the relatively large Cp ligand was
treated with tert-butylnitrile, generating the imido ligand.52
Teuben then reported a more direct synthesis of a similar
vanadium(III) imido species, also bearing a Cp ligand.51
Galindo and co-workers demonstrated the synthesis of a Cp-
free vanadium(III) imido complex bearing carbon monoxide
(CO) and phosphine ligands; however, decomposition could
not be prevented, even when under a CO atmosphere.53 More
recently, Bouwkamp reported the generation of a vanadium-
(III) imido complex bearing a Cp ligand via thermolysis of a
vanadium(V) precursor.54
1
by H NMR (Scheme 4). Immediately after the addition of
ethylene, no vanadium species were observed; however, vinyl-
trimethylsilane (vinyl-TMS) and propylene were present, as
previously reported.29 After removal of the solvent and volatile
byproducts products in vacuo, the resulting crude oily material
was dissolved in minimal dichloromethane (DCM), layered
with pentane, and cooled to 0 °C. Deep red crystals suitable for
single crystal X-ray diffraction were obtained, which con-
clusively demonstrated the paramagnetic product to be the 13
electron, vanadium(IV) chloride complex 10 (Figure 1a).55
Compound 10 is distorted trigonal bipyramidal, with a P1−
V1−P2 angle of 171.4(5)°. The sum of all bond angles in the
trigonal N−O−Cl plane is 359.8° [N1−V1−O1 100.1(3)°,
N1−V1−O1 109.2(4)°, and O1−V1−Cl1 150.7(5)°]. The
VN and V−O bond lengths to the imido and aryloxide
ligands are longer in 10 than in 1, 1.692(11) Å vs 1.687 Å and
Herein, we report the results of our investigation into the
decomposition of vanadium(V) alkylidenes 1 and [N-2,6-
(CH3)2C6H3](OC6F5)[P(CH3)3]2V[CHSi(CH3)3] (5) upon
treatment with excess ethylene gas. Density functional theory
(DFT) calculations indicated that β-hydride elimination is
exergonic with low (≤13.4 kcal/mol) thermodynamic barriers
to metallocyclobutane formation. Analysis of the organic
products indicated that reductive elimination occurs before
insertion of ethylene into the newly formed [V]−H bond.
Conversions for SM of 1-hexene with 1 are reported and
compared to the less electron deficient, more sterically
hindered isopropyl-substituted complexes {N-2,6-[CH-
(CH3)2]C6H3}(OC6X5)[P(CH3)3]2V[CHSi(CH3)3] [X = Cl
(6), F (7)], providing additional experimental support for β-
hydride elimination being the dominant decomposition
pathway, rather than bimolecular routes. Reductive elimination
and release of propylene yielded the unobserved vanadium(III)
Figure 1. Molecular structures (50% thermal ellipsoids), EPR spectra
(black trace, 25 °C in toluene, frequency = 8.993584 GHz, power =
2.00 mW, Mod Width = 0.2 mT, time-constant = 0.03 s), and
simulated EPR spectra (red trace) for (a) 10 and (b) 11. Hydrogen
atoms have been omitted from structures for sake of clarity.
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Scheme 5. β-Hydride Elimination from Vanadium(V) Metallocyclobutanes to Yield Vanadium(III) Complexes
Figure 2. Free energy profile and mechanism for formation of 10. All values reported in kcal/mol at 25 °C at the B3P86/6-311+G(d) level of
theory.
2.044(7) Å vs 1.983 Å, respectively.27 When synthesized on a
preparative scale with DCM as the reaction solvent (Scheme
4), 10 was isolated in 33% crystalline yield. The X-band
electron paramagnetic resonance (EPR) spectrum of com-
pound 10 recorded at 25 °C in toluene displayed an overall
isotropic multiline spectrum due to hyperfine coupling to
multiple neighboring nuclei. The simulated EPR spectrum
showed giso= 1.881, which is lower than g factor of a free
electron (ge = 2.0023) due to spin−orbit coupling as observed
for early transition metal complexes (electronic configurations:
d1−d4).56 The hyperfine contributions from five distinct
nuclei: Aiso(V) = 218 MHz, Aiso(N) = 40 MHz, Aiso(P1 and
P2) = 75 MHz, and Aiso(Cl) = 5 MHz (Figure 1a).55
Initially, we presumed that compound 10 formed via
abstraction of a chlorine atom from DCM by an unobserved
vanadium(III) compound 8, which resulted from the initial
decomposition via β-hydride elimination (Scheme 5). Chlorine
abstraction from chlorinated solvents (or other aliphatic
chlorides) is not uncommon, and has been observed with
copper,57,58 rhenium,59 manganese,59 iron,60−62 silver,63
cerium,64 and titanium.65 The reaction of 1 with ethylene
was repeated in toluene on a preparative scale in the absence of
DCM in an attempt to isolate 8. Compound 10 was isolated
again, however, as determined by elemental analysis, in 32%
crystalline yield (Scheme 4). This nearly identical yield
strongly suggested that the chlorine atom must have originated
from the chlorinated aryloxide ligand of another molecule after
decomposition, rather than the chlorinated solvent. Chlorine
atom abstraction from aryl chlorides is far less common than
from aliphatic chlorides, but has been observed,60,61,66,67
although not for vanadium.
In order to prevent the undesired chlorine atom abstraction
and possibly isolate the proposed vanadium(III) decomposi-
tion product, compound 5,25 bearing a fluorinated aryloxide
ligand, was prepared and subjected to identical reaction
conditions in the absence of DCM (Scheme 4). Rather than
the anticipated vanadium(III) product 9 (Scheme 5), the 13
electron, vanadium(IV) bis(aryloxide) complex 11 was
produced, as confirmed by single crystal X-ray diffraction
(Figure 1b) and elemental analysis, again in <50% crystalline
yield. Similar to compound 10, 11 exhibits distorted trigonal
bipyramidal geometry, with a P1−V1−P1A angle of
177.09(2)°, and the sum of all angles in the trigonal N−O−
O plane is 360.00° [N1−V1−O1 119.53(4)°, N1−V1−O1A
119.53(4)°, O1−V1−O1A 120.94(9)°]. Compound 11
displayed a broad resonance in the 1H NMR spectrum
centered at −5.81 ppm, and one signal in the 19F NMR
spectrum at −167.9 ppm. No resonances were observed by 31P
nor 51V NMR. EPR for compound 11 also displayed an overall
isotropic multiline spectrum, with simulations providing giso
=
1.877 (<ge) with hyperfine contributions from four distinct
nuclei: Aiso(V) = 225 MHz, Aiso(N) = 40 MHz, and Aiso(P1
and P2) = 74 MHz (Figure 1b).55 In order to probe whether
exposure to a chlorine atom source would lead to a
vanadium(IV) chloride species similar to 10, compound 11
was treated with 2 equiv of DCM. After 12 h at room
1
temperature, no changes were observed by H and 19F NMR.
An EPR spectrum of compound 11 prepared in situ from 5
with DCM as the solvent provided a similar multiline spectrum
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Figure 3. Free energy profile and mechanism for formation of 11. All values reported in kcal/mol at 25 °C at the B3P86/6-311+G(d) level of
theory.
Scheme 6. Synthesis of Sterically Hindered Vanadium(V) Alkylidenes
to the isolated product, with distinct hyperfine coupling
(Figure S20).55 When prepared with DCM as the solvent,
single crystals of 11 were isolated, and single crystal X-ray
diffraction again confirmed its identity.
manner to donate a chlorine atom to form the final product.
After delivery of the chlorine atom, the vanadium(IV)
compound 13 is also proposed to form; however, it was not
detected (Figure 2).
Mechanisms. With the identity of the decomposition
products established, we next sought to determine if they are
consistent with β-hydride elimination being the main mode of
decomposition. For both compounds 1 and 5, DFT
calculations demonstrated that treatment with ethylene
generates the expected methylidene species, which then
undergo [2 + 2] cycloaddition with another equivalent of
ethylene to generate metallocyclobutanes. β-Hydride elimi-
nation and reductive elimination then follow to form a
coordinated propylene species reminiscent of the styrene
complex 4 reported by Nomura,42 followed by release of
propylene to generate the vanadium(III) complexes 8 and 9
(Scheme 5, Figure S22 and S23).55
In the proposed vanadium(III) intermediate 8, bearing the
chlorinate aryloxide ligand, DFT revealed that coordination of
an orthochlorine to vanadium may occur with little energetic
cost (1.1 kcal/mol) (Chart S4). This is reminiscent of the
ruthenium alkylidene complex bearing a brominated aryloxide
ligand (OC6Br5)(IMes)(py)Ru(Cl)[CH(C6H5)] (IMes = 1,3-
dimesitylimidazol-2-ylidene, py = pyridine), in which DFT
analysis revealed a ruthenium−bromine interaction.68 In the
case of 8, the chlorine coordination allows for facile oxidative
addition to generate the vanadium(V) chloride species 12
(Figure 2). Compound 12 may then act as the source of the
chlorine atom observed in the product, 10, via a
comproportionation reaction with another equivalent of the
vanadium(III) species 8 (Figure 2). The fact that yields of
<50% are observed in the preparation of 10 are in agreement
with half of the vanadium present serving in a sacrificial
In the case of the fluorinated catalyst, no such coordination
of a fluorine to the vanadium center after decomposition was
found in the proposed intermediate 9. Two possible routes for
the generation of 11 from 9 were explored. In the first
possibility, reaction occurs between 2 equiv of 9, leading to the
observed product 11 and the vanadium(II) complex 14. The
second possibility, which is more likely and overall lower in
energy, involves the abstraction of an aryloxide ligand from
unreacted starting material 5, producing the product 11 and
the unobserved vanadium(IV) alkylidene complex 15 (Figure
3). Compound 15 then likely decomposes further, as
vanadium(IV) alkylidenes are quite rare, with the only
examples reported by Mindiola. These complexes were
supported by sterically encumbering β-diketiminate ligands,
and decomposed upon gentle heating.69 Preferential abstrac-
tion of an aryloxide from 5 over 9 was in agreement with the
calculated V−OAr bond dissociation free energies calculated as
well: 65.8 and 105.7 kcal/mol for 5 and 9, respectively. Again,
the yield of <50% is consistent with half of the vanadium
species serving in a sacrificial manner.
Taken together, the generation of compounds 10 and 11
with the computationally supported mechanisms provide
strong support for β-hydride elimination, followed by reductive
elimination of propylene, being the dominant decomposition
pathway.
Synthesis of New Catalysts and Relative Conversions
in SM. In order to further probe the favored decomposition
pathway, compounds 6 and 7 bearing isopropyl groups on the
arylimido ligand were prepared. Similar niobium species were
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shown by Nomura to polymerize cyclic olefins by ROMP,70
while Murphy described the synthesis and reactivity of
vanadium(V) trialkyl complexes bearing the same arylimido
ligand.71 If β-hydride elimination is the dominant decom-
position pathway, as described above, one would expect 6 and
7, bearing the more electron donating arylimido ligand, to
decompose more readily, resulting in decreased conversion
during SM relative to methyl-substituted 1, which we have
previously disclosed as a catalyst for the SM of various terminal
olefins.29 If, however, bimolecular decomposition occurred,
higher conversions would be expected for 6 and 7, given the
increased steric bulk.
dimethylaryl imido species [N-2,6-(CH3)2C6H3](OC6F5)[P-
(CH3)3]2V[CHSi(CH3)3]25 has not been reported, so no
comparisons may be made at this time.
SM with 1-hexene was performed under identical conditions
with compounds 1, 6, and 7, as summarized in Scheme 7. The
Scheme 7. Self-Metathesis of 1-Hexene with Vanadium(V)
Alkylidenes
Compounds 6 and 7 were prepared from the vanadium(V)
trichloride complex 1672 as summarized in Scheme 6,
following well-established procedures.27 The molecular
structures determined by single crystal X-ray diffraction are
depicted in Figure 4. Compound 6 is distorted trigonal
Table 1. Self-Metathesis of 1-Hexene Using Catalysts 1, 6,
and 7
a
b
entry
catalyst
conversion (%)
1
2
3
1
6
7
15.6
13.0
9.1
a
All runs performed with 5 mol % catalyst in a J. Young NMR tube
equipped with a Teflon seal at 25 °C. [Catalyst]0 = 10.1 mM, [1-
b
hexene]0 = 199.9 mM. Determined by 1H NMR analysis, average of
two separate experiments.
1
conversions determined by H NMR are reported in Table 1.
Conversions are not high, and are not meant to imply that
these catalysts may be effectively used for SM in synthetic
procedures. Rather, their relative values help shed light on the
decomposition pathway. In this way, these values may be
viewed as a proxy for rate of decomposition, which occurs too
quickly to measure through standard means. Complex 1,
bearing the less electron-donating and less sterically hindered
dimethyl arylimido ligand, showed higher conversion than
complex 6, containing the more electron-donating and more
sterically demanding isopropyl-substituted imido ligand. Both
species are identical aside from this difference. Complex 7, in
which a fluorinated aryloxide is employed, demonstrated the
lowest conversion. One may initially predict that 7 would
provide conversions higher than the chlorinated analogue 6,
given the fact that the fluorinated aryloxide should be more
electron-withdrawing. In comparing the solid state structures
of these species, however, it is apparent that the increased bulk
of the chlorine atoms of the aryloxide in 6 result in a longer V−
O bond length than in 7 [1.969(4) Å and 1.949(4) Å,
respectively]. This decreases the amount of π-donation from
the aryloxide in 6 relative to 7, a trend that has been observed
in other vanadium alkylidenes bearing this chlorinated
aryloxide ligand,27 making it less electron-donating. Overall,
faster decomposition (and therefore lower conversions) was
observed as the ligands became more electron-donating and
sterically demanding, providing evidence that β-hydride
elimination is the dominant decomposition pathway, rather
than bimolecular decomposition.
Figure 4. Molecular structures (50% thermal ellipsoids) for (a) 6
(chlorinated aryloxide) and (b) 7 (fluorinated aryloxide). Hydrogen
atoms have been omitted from structures for sake of clarity.
bipyramidal with a P1−V1−P2 bond angle of 168.92(3)°,
similar to 168.66(4)° in 1.27 The trigonal O−N−C plane has a
sum of all bond angles totaling 359.67° [N1−V1−C19
111.30(12)°, N1−V1−O1 133.37(15)°, and O1−V1−C19
115.0(2)°], again nearly identical with those in 1.27 The syn
isomer for the alkylidene is observed in the solid state
Analysis of Organic Decomposition Products. On the
basis of reports for the treatment of OM catalysts based on
rhenium, molybdenum, and tungsten with ethylene, in which
1-butene and 2-butene are observed along with propylene,48 it
is worthwhile to consider similar reactions in the present
system. The dominant decomposition path is clearly the one
depicted in Scheme 5, as evidenced by propylene being the
1
structure, and is the dominant species in solution (95% by H
NMR).55 For 7, the structural features are rather similar to
those in 6; however, the anti isomer is observed for the
alkylidene ligand. In solution, the anti isomer is dominant
55
1
(70% by H NMR). The structure for the analogous 2,6-
F
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Scheme 8. Possible Reaction Pathways after β-Hydride Elimination
major organic product by 1H NMR. The other possible routes
reminiscent of those reported for rhenium, molybdenum, and
tungsten, are illustrated in Scheme 8. In these reactions,
insertion of ethylene into the newly formed [V]−H bond after
β-hydride elimination but before reductive elimination of
propylene leads to an ethyl complex, [V](C2H5)(CH2CH
CH2), which may undergo α-hydrogen abstraction by the
propenyl ligand, generating a new ethylidene complex. This
ethylidene may reenter productive catalysis, or also decompose
via β-hydride elimination after addition of another equivalent
of ethylene, yielding 2-butene. On the other hand, another
insertion of ethylene may occur, followed by another α-
hydrogen abstraction to yield 1-butene or 2-butene. In order to
determine if such decomposition paths were at all present in
SM with vanadium, the gaseous reaction products of catalyst 1
with ethylene were examined by gas chromatography with
flame-ionization detection (GC-FID). Propylene was ob-
served; however, no other gaseous organic products (e.g., 1-
butene and 2-butene) were detected, indicating that insertion
of ethylene into the [V]−H bond does not occur. Rather,
reductive elimination and release of propylene is the exclusive
decomposition route.
hydride elimination. Similar decomposition is likely not
observed in ROMP23 given that cycloreversion of the
metallocyclobutane to generate a new alkylidene is highly
favored over β-hydride elimination, given that cycloreversion
involves the release of ring-strain in the cyclic monomer. More
systematic investigations into the possibility of β-hydride
elimination in ROMP with vanadium alkylidenes are
warranted, and will be investigated in subsequent work by
our group. Insertion of ethylene into the newly formed [V]−H
bond after β-hydride elimination is not competitive with
reductive elimination of propylene, as evidenced by the
absence of butenes in the product mixture. Lastly, SM of 1-
hexene with three catalysts of varying sterics and electronics
demonstrated that bulkier and more electron-donating ligands
result in decreases in conversion, which strongly indicated that
β-hydride elimination is the dominant decomposition pathway.
Therefore, in order to effectively employ vanadium in OM
beyond ROMP, catalysts more immune to β-hydride
elimination must be prepared and/or employed. Possible
candidates for such compounds should be increasingly
electron-poor relative to the species employed in this study.
Some potential compounds have recently been described in
the literature, such as [N-2,6-Cl2C6H3](OC6Cl5)[P(CH3)3]2V-
[CHSi(CH3)3],27 [N-C6F5](OC6F5)[P(CH3)3]2V[CHSi-
(CH3)3], and [N-C6F5](OC6Cl5)[P(CH3)3]2V[CHSi-
(CH3)3].87 These compounds have shown incredibly high
activities in ROMP of low ring-strain cyclic olefins due to their
electron-poor nature. Decreasing π-donation from the
aryloxide ligand through the use of more sterically demanding
aryloxides may also be useful in this regard. Previously, the
preparation of vanadium compounds bearing large aryloxides
was found to be quite challenging;88 however, recent work with
niobium has illuminated new pathways that may be useful in
CONCLUSION
■
We demonstrated previously that vanadium(V) alkylidenes are
capable of catalyzing SM; however, decomposition prevented
complete conversion.29 In this work, we provided additional
thorough evidence that the primary mode of decomposition is
β-hydride elimination from the metallocyclobutane intermedi-
ate, rather than bimolecular routes as was previously
reported.42 This was accomplished computationally, and
through the isolation of the downstream vanadium products,
which form via bimolecular routes after decomposition by β-
G
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generating these species.89 The synthesis and activity of such
compounds is beyond the scope of this work, which sought
solely to firmly establish the dominant decomposition pathway.
However, we are currently investigating such species, which
will be reported in due course in their own dedicated study.
Synthesis of [N-2,6-(CH3)2C6H3]V(OC6F5)2[P(CH3)3]2 (11) in
Toluene. Compound 5 (0.1266 g, 0.2144 mmol) was dissolved in 4.0
mL toluene and distributed across two J. Young NMR tubes equipped
with Teflon seals ([5] = 54 mM). The headspaces were evacuated and
charged with ethylene (10 psi). The tubes were sealed and agitated for
10 min, at which point the contents were transferred to a vial, and
volatiles were removed in vacuo. 1H NMR revealed the presence of 5.
The crude material was redissolved in 4.0 mL toluene, and treated
with ethylene again in the same fashion, before again removing
volatiles in vacuo. The resulting material was dissolved in minimal
toluene and filtered through Celite/Kimwipe in a pipet. Volatiles were
removed in vacuo, and then dissolved in minimal toluene, layered
with pentane, and cooled to −30 °C, furnishing 11 as a bright red
crystalline material (0.0612 g, yield = 42%). For 11: Anal. Calcd for
VNO2P2F10C26H27: C, 45.36; H, 3.95; N, 2.03. Found: C, 45.11; H,
EXPERIMENTAL SECTION
■
General Methods. All air and moisture sensitive manipulations
were carried out under nitrogen using standard glovebox or Schlenk-
line techniques. Pentane and toluene were distilled under nitrogen
from sodium/benzophenone. Dichloromethane (DCM) was distilled
from calcium hydride under nitrogen. Benzene-d6 and 1-hexene were
stirred over sodium/potassium alloy, degassed, and isolated by
transfer under static vacuum. Ethylene (>99.5%) was purchased from
Sigma-Aldrich and used as received. Celite was oven-dried for several
days before use in the glovebox. Compounds 1,27 5,25 and 1672 were
prepared as described in the literature. All other reagents were used as
received. Elemental analyses were performed by Midwest Microlab,
LLC. Nuclear magnetic resonance (NMR) spectroscopy was
1
3.92; N, 2.08. H (400 MHz, benzene-d6, 25 °C) NMR −5.18 (br).
19F (376.5 MHz, benzene-d6, 25 °C) NMR −169.6. EPR: See Figure
S20.
Synthesis of [N-2,6-(CH3)2C6H3]V(OC6F5)2[P(CH3)3]2 (11) in
DCM for XRD Analysis. Compound 5 (0.0196 g, 0.0332 mmol) was
dissolved in 1.0 mL DCM in a J. Young NMR tube equipped with a
Teflon seal ([5] = 33 mM). The headspace was evacuated and
charged with ethylene (10 psi). The tube was sealed and agitated for
20 min, at which point the contents were transferred to a vial, and
volatiles were removed in vacuo. The crude material was redissolved
in minimal DCM, layered with pentane, and cooled to −30 °C,
furnishing single crystals which were analyzed by XRD, confirming the
identity as compound 11.
1
performed using a JEOL 400 MHz spectrometer. H NMR spectra
were referenced to the residual solvent signal (7.16 ppm for benzene-
d6). 13C NMR spectra were referenced to the residual solvent signal
(128.1 ppm for benzene-d6). 19F and 51V NMR spectra were
referenced to hexafluorobenzene (−164.9 ppm) and VOCl3 (0.00
ppm), respectively. Electron paramagnetic resonance (EPR) spectra
were recorded using a JEOL continuous wave spectrometer JES-
FA200 equipped with a cylindrical mode cavity having X-band Gunn
oscillator bridge. Experiments were performed at 298 K. Freshly
prepared solutions (0.5−2 mM in toluene) were used to measure EPR
spectra, except where noted otherwise. Overall the instrument features
for spectral measurements for compounds 1 and 2 were: Frequency =
8.993584 GHz, power = 2.00 mW, Mod Width = 0.2 mT, time-
constant = 0.03 s. Pure toluene was used to measure background
spectra at the same measurement conditions. Spectral simulations
were carried out using QCMP 136 program by Prof. Dr. Frank Neese
from the Quantum Chemistry Program Exchange as used by Neese et
al.73 The “chi by eye” approach was used for fitting using collinear g
and A tensors (see Figure S19). CG-FID was performed on a
Shimadzu GC-2010 Plus Gas Chromatograph instrument with a
Restek Rt-Alumina BOND/Na2SO4, 30 m, 0.32 mmID column (Cat#
19757), with flame-ionization detection. The temperature was held
constant at 130 °C, and a flow rate of 51 cm/s was used. The injection
and detector temperatures were held constant at 200 °C. Retention
times were compared to quality assurance reports provided by Restek.
Synthesis of [N-2,6-(CH3)2C6H3](OC6Cl5)[P(CH3)3]2VCl (10) in
Toluene. Compound 1 (0.1045 g, 0.1678 mmol) was dissolved in 3.0
mL toluene in a J. Young NMR tube equipped with a Teflon seal ([1]
= 56 mM). The headspace was evacuated and charged with ethylene
(10 psi). The tube was sealed and agitated for 10 min, at which point
the contents were transferred to a vial, and volatiles were removed in
Synthesis of {N-2,6-[CH(CH3)2]2C6H3}V[CH2Si(CH3)3]3 (17).
Compound 16 (2.0781 g, 6.248 mmol) was dissolved in 100 mL n-
hexane and cooled to −30 °C, at which point 1.0 M LiCH2Si(CH3)3
in hexane (19.0 mL, 19.0 mmol) was added dropwise. After addition
was complete, the flask was sealed, allowed to warm to room
temperature, and stirred overnight (13 h). The resulting solution was
filtered through Celite, and volatiles were removed in vacuo to yield a
reddish-brown oil (2.8669 g, yield = 94%). Freshly prepared 17 was
used as the precursor for the synthesis of the alkylidenes described
1
below. H (400 MHz, benzene-d6, 25 °C) NMR 7.11 (d, 2H, J = 8.0
Hz), 7.00 (t, 1H, J = 8.0 Hz), 4.46 (sp, 2H, J = 7.2 Hz), 1.97 (br, 6H),
1.38 (d, 12H, J = 6.8 Hz), 0.17 (s, 27H). 51V (105 MHz, benzene-d6,
25 °C) NMR 1074.15.
Synthesis of {N-2,6-[CH(CH3)2]2C6H3}(OC6Cl5)[P(CH3)3]2V
[CHSi(CH3)3] (6). Compound 17 (1.3743 g, 2.817 mmol) was
dissolved in 100 mL n-hexane and cooled to −30 °C, at which point
pentachlorophenol (0.7796 g, 2.927 mmol) was added as a
suspension in 10 mL n-hexane. The flask was sealed, and the reaction
was allowed to warm to room temperature and stir overnight (14.5 h).
Volatiles were removed in vacuo, yielding the dialkyl species 18 as a
brownish-red oil (1.7425 g, yield = 93%) which was identified by its
diagnostic 51V chemical shift.27 51V (105 MHz, benzene-d6, 25 °C)
NMR 669.24. 18 was used in the next step without further
purification.
1
vacuo. H NMR revealed the presence of 1. The crude material was
Compound 18 (1.7425 g, 2.616 mmol) was dissolved in 100 mL n-
hexane and cooled to −30 °C, at which point trimethylphosphine
(800 μL, 7.865 mmol) was added dropwise. The flask was sealed, and
the solution was warmed to room temperature and stirred overnight
(14 h), at which point an orange precipitate was visible. The solution
was filtered through Celite with n-hexane until the filtrate was clear.
Volatiles were removed in vacuo, and the resulting reddish-orange
solid was cooled to −30 °C in excess pentane. The red supernatant
was removed, and additional pentane was added and cooled again.
Removal of the red supernatant again yielded alkylidene 6 as an
orange crystalline solid (0.6776 g, yield = 35%). For 6: Anal. Calcd for
VSiP2ONCl5H45C28: C, 46.08; H, 6.21; N, 1.92. Found: C, 46.27; H,
redissolved in 3.0 mL toluene, and treated with ethylene again in the
same fashion, before again removing volatiles in vacuo. The resulting
red oil was dissolved in minimal toluene and filtered through Celite/
Kimwipe in a pipet. The solution was concentrated, layered with
pentane, and cooled to −30 °C, furnishing 10 as a deep red crystalline
material (0.0332 g, yield = 34%). For 10: Anal. Calcd for
VNOP2Cl6C20H27: C, 38.56; H, 4.37; N, 2.25. Found: C, 38.24; H,
4.37; N,2.03. EPR: See Figure S19.
Synthesis of 10 in DCM. Compound 1 (0.0502 g, 0.0745 mmol)
was dissolved in 1.0 mL DCM in a J. Young NMR tube equipped with
a Teflon seal ([1] = 75 mM). The headspace was evacuated and
charged with ethylene (10 psi). The tube was sealed and agitated for
30 min, at which point the contents were transferred to a vial, and
volatiles were removed in vacuo. The resulting red oil was dissolved in
minimal toluene layered with pentane, and cooled to −30 °C,
furnishing 10 as a deep red crystalline material (0.0154 g, yield =
33%).
1
5.98; N, 1.87. H (400 MHz, benzene-d6, 25 °C) NMR 16.60, 15.49
(br, 1H), 7.08 (m, 2H), 6.93 (t, 1H, J = 6.8 Hz), 4.84 (sp, 2H, J = 6.8
Hz), 1.36 (d, 12H, J = 6.8), 0.78 (s, 18H), 0.23 (s, 9H). 13C (100
MHz, benzene-d6, 25 °C) NMR 146.8, 131.3, 131.1, 124.8, 123.1,
122.4, 121.9, 118.3, 27.2, 25.3, 15.4, 2.2. 51V (105 MHz, benzene-d6,
H
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1
25 °C) NMR 36.58, −130.29 (t, JVP = 285 Hz). 31P (162 MHz,
rubber septum. An aliquot (25 μL) of the headspace was removed
with a syringe and injected in the GC-FID for analysis.
1
benzene-d6, 25 °C) NMR −8.08 (oct, JVP = 293 Hz).
Synthesis of {N-2,6-[CH(CH3)2]2C6H3}](OC6F5)[P(CH3)3]2V
[CHSi(CH3)3] (7). Compound 17 (1.4680 g, 3.009 mmol) was
dissolved in 100 mL n-hexane and cooled to −30 °C, at which point
pentafluorophenol (0.5841 g, 3.173 mmol) was added as a suspension
in 10 mL n-hexane. The flask was sealed, and the reaction was allowed
to warm to room temperature and stir overnight (19.5 h). Volatiles
were removed in vacuo, yielding the dialkyl species 19 as a brown oil
(1.7481 g, yield = 99%) which was identified by its diagnostic 51V
chemical shift.25 51V (105 MHz, benzene-d6, 25 °C) NMR 667.45. 19
was used in the next step without further purification.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00610.
■
sı
Computational data and structures, crystallographic
data, and supporting spectra and chromatogram (PDF)
Accession Codes
Compound 19 (1.7041 g, 2.920 mmol) was dissolved in 100 mL n-
hexane and cooled to −30 °C, at which point trimethylphosphine
(900 μL, 8.849 mmol) was added dropwise. The flask was sealed, and
the solution was warmed to room temperature and stirred overnight
(12 h). The solution was filtered through Celite with n-hexane until
the filtrate was clear. Volatiles were removed in vacuo, and the
resulting orange solid and brown oil was cooled to −30 °C in excess
pentane. The red supernatant was removed, and additional pentane
was added and cooled again. This process was repeated three times,
yielding alkylidene 7 as an orange crystalline solid (1.0531 g, yield =
56%). For 7: Anal. Calcd for VSiP2ONF5H45C28: C, 51.93; H, 7.00;
CCDC 2013007−2013008 and 2026031−2026032 contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
1
N, 2.16. Found: C, 52.19; H, 6.77; N, 2.41. The H NMR spectrum
Wesley S. Farrell − Chemistry Department, United States Naval
Academy, Annapolis, Maryland 21402, United States;
orcid.org/0000-0003-0158-2463; Email: wfarrell@
usna.edu
exhibited broad resonances for the isopropyl groups for one of the
1
isomers. Both are reported. See Figure S10 and S11. H (400 MHz,
benzene-d6, 25 °C) NMR 16.42, 15.31 (br, 1H), 7.10 (m, 2H), 6.95
(m, 1H), [5.44 (br), 4.67 (sp, J = 6.4 Hz), 4.373 (br), total = 2H],
[1.42 (br), 1.39 (d, J = 6.8 Hz), total = 12H], 0.78 (s, 18H), 0.39,
0.26 (s, 9H). 13C (100 MHz, benzene-d6, 25 °C) NMR 147.6, 146.6,
141.3, 140.7, 140.2, 139.0, 138.3, 137.7, 132.7, 130.4, 124.7, 123.1,
123.0, 27.5, 25.1, 24.7, 15.5, 15.2, 2.4, 2.3. 19F (376 MHz, benzene-d6,
25 °C) NMR −166.9 (dd), −168.0 (dd), 169.2 (m), −180.1 (m),
−180.4 (m). 51V (105 MHz, benzene-d6, 25 °C) NMR −2.87, −87.56
Authors
Christine Greene − Department of Chemistry, Georgetown
University, Washington, D.C. 20057, United States
Pokhraj Ghosh − Department of Chemistry, Georgetown
University, Washington, D.C. 20057, United States
Timothy H. Warren − Department of Chemistry, Georgetown
University, Washington, D.C. 20057, United States;
orcid.org/0000-0001-9217-8890
1
(t, JVP = 275 Hz). 31P (162 MHz, benzene-d6, 25 °C) NMR −8.88
1
(oct, JVP = 279 Hz).
Computational Details. The Gaussian 16 suite of programs was
used for all optimizations and frequency calculations. Visualization
and structure analyses were done using CCDC-Mercury.74 The
B3P86 functional75 with the 6-311+G(d) basis set76−82 was used for
all the optimization of geometries and frequency calculations.83−85 At
the B3P86/6-311+G(d) stationary points, single point energies were
calculated using B3P86 with the 6-311++G(d,p)77−82,85 basis set, in
benzene or acetonitrile using the PCM86 solvent model. Four small
molecules (ethylene, propene, ·OC6F5, and vinyl-TMS) and six
complexes {(OC6Cl5)[V](CH-TMS), (OC6Cl5)[V](Cl), (OC6F5)-
[V](CH-TMS), (OC6F5)2[V], [V](CH-TMS), and [V], where [V] =
[N-2,6-(CH3)2C6H3][P(CH3)3]2V} were calculated to analyze
thermodynamics of the reaction mechanism. Two isomers were
calculated for six of the complexes {(OC6Cl5)[V](CH2), ((OC6F5)-
[V](CH2), (OC6Cl5)[V](CH2−CH2−CH2), (OC6F5)[V](CH2−
CH2−CH2), (OC6Cl4)[V], and (OC6Cl4)[V]Cl}. Four isomers
were calculated for three of the complexes {(OC6Cl5)[V](CH2−
CH−CH3), (OC6F5)[V](CH2−CH−CH3), and (OC6F5)[V]}. Five
isomers were calculated for complex (OC6Cl5)[V].
Peter Y. Zavalij − Department of Chemistry and Biochemistry,
University of Maryland, Maryland 20742, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00610
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Defense Threat Reduction
Agency Service Academy Research Initiative, Grant CB3260.
The summer salary for W.S.F. was provided by the Naval
Academy Research Council. Funding for T.H.W. was provided
by the National Science Foundation, Grant CHE-1665348.
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