Oxygen atom transfer with organofunctionalized polyoxovanadium clusters: O-atom vacancy formation with tertiary phosphanes and deoxygenation of styrene oxide

Article abstract of DOI:10.1039/c9sc02882j

We report a rare example of oxygen atom transfer (OAT) from a polyoxometalate cluster to a series of tertiary phosphanes. Addition of PR₃ (PR₃ = PMe₃, PMe₂Ph, PMePh₂, PPh₃) to a neutral met

Full text of DOI:10.1039/c9sc02882j

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DOI: 10.1039/C9SC02882J
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Oxygen atom transfer with organofunctionalized
polyoxovanadium clusters: O-atom vacancy formation with
tertiary phosphanes and deoxygenation of styrene oxide
Received 00th January 20xx,
Accepted 00th January 20xx
Brittney E. Petel,a Rachel L. Meyer,a William W. Brennessel,a and Ellen M. Matson*a
DOI: 10.1039/x0xx00000x
We report a rare example of oxygen atom transfer (OAT) from a polyoxometalate cluster to a series of tertiary
phosphanes. Addition of PR3 (PR3 = PMe3, PMe2Ph, PMePh2, PPh3) to a neutral methoxide-bridged polyoxovanadate-
alkoxide (POV-alkoxide) cluster, [V6O7(OMe)12]0, results in isolation of a reduced structure with phosphine oxide datively
coordinated to a site-differentiated VIII ion. A positive correlation between the steric and electronic properties of the
phosphane and the reaction rate was observed. Further investigation of the steric influence of the alkoxy-bridged clusters
on OAT was probed through the use of POV clusters with bridging alkoxide ligands of varying chain length ([V6O7(OR’)12]; R’
n
= Et, Pr). These investigations expose that steric hinderance of the vanadyl moieties has significant influence on the rate
of OAT. Finally, we report the reactivity of the reduced POV-alkoxide clusters with styrene oxide, resulting in the
deoxygenation of the substrate to generate styrene. This result is the first example of epoxide deoxygenation using
homometallic polyoxometalate clusters, demonstrating the potential for mono-vacant Lindqvist clusters to catalyze the
removal of oxygen atoms from organic substrates.
properties of these assemblies are credited to their molecular
composition, containing early transition metal ions in their
highest oxidation states (M= MoVI, WVI, VV, NbV), bridged by
oxide (O2-) ions. Previous work focusing on the reactivity of
POMs have revealed that these clusters are effective catalysts
for a variety of oxidative chemical transformations, ranging
from C-H oxidation16-19 and C-E (E = C, Sn) bond cleavage,20-22
to OAT to arenes, alkylarenes, and sulphides.23-25
Introduction
Oxygen atom transfer (OAT) is a key step in catalytic
transformations with relevance to the activation of small
molecules and organic substrates involving metal oxide
systems. Specifically, heterogeneous reducible metal oxide
materials have been demonstrated to facilitate OAT reactions
via the Mars-van Krevelen mechanism.1-8 In chemical reactions
of this type, the metal oxide surface is activated by a substrate,
resulting in the formation of an oxygen-atom defect
(“vacancy”). These reactive sites are thought to be key in the
activation of chemically inert, gaseous substrates, including
CO2 and NO, serving as O-atom acceptors from these
substrates in small molecule activation.3-7
Given the prevalence of metal oxide materials capable of
mediating OAT reactions, chemists have long sought well-
defined, molecular species that can serve as structural and
functional models of these heterogeneous catalysts.
Polyoxometalates (POMs), are ideal molecular entities for
modelling OAT reactivity with metal oxide systems due to their
modularity, stability, and ability to undergo reversible multi-
electron redox processes.9-15 The inherent physicochemical
These seminal examples of oxidation catalysis mediated by
POMs have sparked extensive research in OAT chemistry of
these clusters over the past thirty years. In particular, the
reactivity of the phosphovanadomolybdate clusters of the -
Keggin framework, [H3+xPVxMo12-xO40] (x = 0, 1, 2), popularized
19, 26-28
29
by Neumann18,
and others,25,
has dominated this
chemical space, illustrating novel OAT chemistry for soluble,
metal oxide assemblies. The authors have proposed that these
clusters mediate OAT through
a
Mars-van Krevelen
mechanism, in analogy to heterogeneous bulk metal oxide
17
systems.16,
Theoretical investigations, supported by
spectroscopic analysis, have revealed that OAT occurs via an
initial one-electron reduction of the polyoxoanion by the
substrate, resulting in the formation of a reactive ion pair. In a
subsequent mechanistic step requiring water, the oxidation of
21, 30-33
said substrate occurs.16,
Density functional theory
a. Department of Chemistry, University of Rochester, Rochester, New York 14627,
United States
supports the transient formation of
a coordinatively
unsaturated vanadium ion at the surface of the cluster that
rapidly reacts with a terminal oxidant to restore the vacant
site. Given the significance of the proposed reduced species in
these molecular transformations, extensive efforts have
focused on the isolation and characterization of oxygen-
Electronic Supplementary Information (ESI) available: 1H NMR, ESI-MS (+ve), UV-
vis, and IR spectroscopic data for all complexes, crystallographic parameters and
BVS calculations of complexes 2-OPMe3, 2-OPMe2Ph, 2-OPMePh2, 2-OPPh3, and 4-
OPPh3 (CCDC 1914250-1914254), attempted synthesis of 4-OPMePh2, 4-OPPh3,
and propoxide derivatives, 1H NMR analysis of styrene oxide OAT reactions with 2-
OPMe3 and 4-OPMe3. See DOI: 10.1039/x0xx00000x
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deficient POMs. However, to date, examples of well-defined
polyoxoanions containing coordinatively unsaturated,
reduced ion, generated via reduction of the parent metal oxide
cluster, remain scarce.29, 34-37
Results and Discussion
O-atom vacancy formation with tertiary phosphanes: Reduction of
[V6O7(OMe)12] with PMe3
View Article Online
DOI: 10.1039/C9SC02882J
a
Recently, we disclosed O-atom vacancy generation at the
surface of a neutral POV-alkoxide scaffold, demonstrating that
the addition of a single equivalent of V(Mes)3(THF) (Mes =
2,4,6-trimethylphenyl) to the parent cluster, [V6O7(OMe)12]0
(1), results in OAT from the POV-alkoxide to the reductant
(Scheme 1, left).34 The product mixture included both the
mono-vacant complex [V6O6(OMe)12(MeCN)]0, 2-MeCN) and
di-vacant cluster [V6O5(OMe)12(MeCN)2]0, whose similar
solubilities made separation challenging. The “over-reduction”
of 1 prompted a search for improved methods for selective
generation of a neutral species containing a single O-atom
defect-site.
Here, we report a rare example of OAT from homometallic
polyoxovanadate-alkoxide
(POV-alkoxide)
clusters
to
phosphane substrates in nonaqueous media. These
hydrophobic, organofunctionalized metal oxide assemblies are
completely insoluble/immiscible with water, thus eliminating
the possibility of oxygen-atom abstraction occurring via
water/proton assisted pathways. Furthermore, the bridging
alkoxide ligands isolate oxygen atom vacancy formation to
terminal oxide positions, preventing formation of
coordinatively unsaturated species with defects at
thermodynamically
preferred
bridging
sites.
Using
paramagnetic 1H NMR spectroscopy, electrospray ionization
mass spectrometry (ESI-MS), and X-ray crystallography, we
Trivalent alkyl- or aryl-phosphanes (PR3) constitute a class
of oxygen atom acceptors that have been demonstrated to
efficiently remove O-atoms from a variety of inorganic metal-
oxo complexes.42-49 As such, we began our efforts toward
probing OAT by adding trimethylphosphine (PMe3) to complex
1 (Scheme 1, right). Analysis of the crude reaction mixture
resulting from addition of 1 equiv of PMe3 to complex 1 at
room temperature (~ 21 ºC) via 1H NMR spectroscopy showed,
after 24 hours, small amounts of formation of a new product,
with three paramagnetic resonances slightly shifted from
those of the previously reported, neutral mono-vacant cluster,
2-MeCN (Figure S1).35 Optimization of reaction conditions
revealed that addition of 4 equiv PMe3 to 1 at 70 ºC results in
the complete consumption of the parent cluster after 19 hours
(Figure S1-S2). ESI-MS(+ve) of the resulting species contained a
single prominent signal, corresponding to the molecular mass
of the parent ion plus the phosphane substrate (Figure S3, m/z
= 866). This result, coupled with the increased number of
unambiguously
confirm
formation
of
a
reduced
polyoxovanadium cluster via OAT to a phosphane. The
resulting site-differentiated vanadium(III) centre of the
Lindqvist core is coordinated to a dative phosphine oxide
ligand, as confirmed by crystallography and IR spectroscopy.
Subsequent investigations surrounding structure-activity
relationships of both the phosphane and the POV-alkoxide
cluster are undertaken, revealing the importance of steric
accessibility for the direct transfer of an oxygen atom from the
cluster to the substrate. This work marks the first example of
such a study, unveiling not only new routes for the formation
of reactive POV-alkoxide architectures, but also considerations
in system design for efficient OAT.
Additionally, we present an extension of the reactivity of
our reduced POV-alkoxide scaffolds, summarizing their utility
in the multielectron deoxygenation of styrene oxide. While
POMs are well-established for the mediation of the chemical
reverse of this transformation (oxidation of alkenes),25, 29, 38-40
there have been limited reports of the reduction of epoxides
as mediated by reduced metal ions supported by a metal-oxide
framework.41 This distinct example of the deoxygenation of an
oxidized organic substrate by oxygen deficient clusters
completes a water-independent, stoichiometric OAT cycle,
establishing new reactivity for homometallic POM assemblies.
1
signals and slight shift in H NMR resonances, supports that
the product formed is the trimethylphosphine oxide (OPMe3)
adduct of a POV-alkoxide cluster containing a single oxygen-
atom vacancy, [V6O6(OMe)12(OPMe3)]0 (2-OPMe3).
Further support for the generation of the reduced,
phosphine oxide adduct of the cluster is observed in the
infrared (IR) and electronic absorption spectra of the product
Scheme 1. Previously reported synthesis of the mono-vacant neutral cluster, 2-MeCN, via addition of VMes3(THF) addition (left)35 and synthesis of
[V6O6(OMe)12OPMe3] (2-OPMe3) via OAT to PMe3 reported in this work (right).
2 | J. Name., 2012, 00, 1-3
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a)
trimethylphosphine oxide ligand datively coordinated to the
site-differentiated, reduced vanadium ion (Figure 2, Table 1;
View Article Online
DOI: 10.1039/C9SC02882J
for full structural parameters and BVS calculations, see Tables
S1-S2). The asymmetric unit of the crystal contained two
independent molecules of 2-OPMe3, with one of the molecules
being highly disordered over two positions (Figure S6). Bond
metrics of 2-OPMe3 were therefore obtained from the
molecule in the asymmetric unit which exhibited no disorder.
Complex 2-OPMe3 contains a lengthened V1–O1 bond in
comparison to the parent POV-alkoxide cluster (2-OPMe3:
2.026(5) Å; 1 (avg): 1.589 Å, Table S3).14 Additionally, the short
P=O bond distance in 2-OPMe3 (1.467(6) Å) is comparable to
the theoretically determined P=O bond length in free
trimethylphosphine oxide (Me3P=O, 1.48 Å).51
The lengthened V1–O1 bond and similarity in P=O bond
distances between free and bound OPMe3 collectively provide
support for a purely dative interaction between a reduced,
vanadium(III) centre and a phosphine oxide ligand. Indeed,
these bond metrics refute the alternative product of one
electron transfer, namely the ion-paired species, [Me3P•+---
V6O7(OCH3)12].21, 30, 31 Further support for complete OAT can be
garnered from examination of V1–Oc bond distances.
Previously we have reported that reduced vanadium ions
2-OPMe2Ph
2-OPPh3
2-OPMe3
2-OPMePh2
3200
2700
2200
Wavenumber (cm-1
1700
)
1200
700
b)
2000
1600
6000
1200
800
5000
4000
3000
2000
400
0
500
600
700
800
900
1000
Wavelength (nm)
2-OPMe3
2-OPMe2Ph
2-OPMePh2
2-OPPh3
1000
0
within
oxygen-deficient
POV-alkoxide
clusters,
300
400
500
600
700
800
900
1000
[V6O5(OMe)12(MeCN)2]35 and [V6O6(OMe)12OTf]34 (OTf=
trifluoromethylsulfonate), possess a shortened V1–Oc bond
(Oc = central oxygen atom, 2.0173 (avg) Å, 2.08(4) Å,
Wavelength (nm)
Figure 1. a) Infrared spectra of complexes 2-OPMe3, 2-OPMe2Ph, 2-OPMePh2, and
2-OPPh3 b) Electronic absorption spectra of complexes 2-OPMe3, 2-OPMe2Ph, 2-
OPMePh2, and 2-OPPh3 collected in acetonitrile. See Table S7 for bond vibrations.
respectively) and contracted V1–Ob–Vn bond angles (Ob
=
bridging oxygen atom, avg ~105º, Table S3). Similar deviations
in bond lengths and angles are observed in the X-ray structure
of 2-OPMe3, with a V1–Oc bond length of 2.120(5) Å and V1–
Ob–Vn bond angles averaging ~105º. Bond valence sum (BVS)
calculations revealed an oxidation state distribution of
vanadium ions of VVVIV4VIII, consistent with that reported for 2-
MeCN (Table S2).35
(Figure 1). 14, 15 In the IR spectra of 2-OPMe3, two strong, broad
absorption bands centred at 1032 cm-1 and 962 cm-1 are
observed, corresponding to ν(Ob—CH3) (Ob = bridging oxo) and
ν(V=Ot) (Ot = terminal oxo), respectively (Figure 1a). These
features resemble those reported for 2-MeCN (ν(Ob—CH3)
1030 cm-1; ν(V=Ot) 968 cm-1), consistent with the formation of
the reduced species (Figure S4).35 An additional absorbance is
observed at 1163 cm-1, assigned to ν(O=P) for the bound
OPMe3 ligand (free O=PMe3; 1170 cm-1).50 The similarities in
energy between the ν(O=P) of bound and dissociated OPMe3
suggests complete OAT from the cluster to the substrate.
The electronic absorption spectrum of 2-OPMe3 features
two pronounced intervalence charge transfer (IVCT) bands,
located at 390 nm ( = 3766 M-1 cm-1) and 1000 nm ( = 655 M-
Direct OAT between a metal-oxide cluster and phosphane
52
is exceedingly rare.49,
An original report by Kawafurane
described OAT from the Keggin ion [PMo12O40]3-, resulting in
the formation of a series of reduced clusters with O-atom
3-
vacancies, [PMo12O40-x
]
(x = 1, 2, 3).53 However, Mattes54 and
Proust55 have separately revisited this work, clarifying that
OAT to triphenylphosphine actually occurs via reductive
quenching of a transiently formed PPh3 species by water,
•+
with no evidence for the formation of an oxygen-atom vacancy
at the surface of the cluster. To date only one example of
direct OAT from a polyoxoanion to a tertiary phosphane has
been reported. In the case of their chromium-functionalized
tungstate ion, [XW11O39CrVO]n- (X = P, Si; n = 4, 5, respectively),
Hill and coworkers cite reduction of the CrV heteroion to CrIII
upon addition of triphenylphosphine.29 While this work is of
seminal importance, the OAT reactivity of this cluster is
localized to the heteroion embedded within the
polyoxotungstate structure. Indeed, heterometals in these
types of clusters are well-established to be significantly
1
cm-1) (Figure 1b). These features are diagnostic of a mixed
15
valent (VIV/VV) Lindqvist core.14,
Comparison of the
intensities of these absorptions in the starting material (1) and
product (2-OPMe3) reveals a two-fold decrease in molar
absorptivity (Figure S5). This result further supports reduction
of the hexavanadate core upon OAT, as the change in
oxidation state distribution from VV VIV to VVVIV4VIII
2
4
corresponds to a two-fold decrease in probability of IVCT
occurrence between VIV and VV ions in the product, 2-OPMe3.
Crystals suitable for structural analysis were grown from
slow diffusion of pentane into a concentrated solution of 2-
OPMe3 in tetrahydrofuran. Resolution of crystallographic data
revealed the expected product; a hexavanadate core with a
decoupled from the electronic structure of the whole, with
56-58
redox events localized to the heteroatom.11,
This fact
differentiates our work from that reported previously, as we
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present the OAT reactivity of a homometallic POM to an were required for complete reduction of the parent cluster, 1.
View Article Online
DOI: 10.1039/C9SC02882J
P1
O1
P1
O1
P1
O1
P1
O1
V1
V1
Ob
Vc
V1
V1
Ob
Vc
Ob
Vc
Ob
Vc
OC
OC
Vt
OC
Vt
OC
Vt
Vt
V6O6(OMe)12OPMe3
(2-OPMe3)
V6O6(OMe)12OPMe2Ph
(2-OPMe2Ph)
V6O6(OMe)12OPMePh2
(2-OPMePh2)
V6O6(OMe)12OPPh3
(2-OPPh3)
BVS: VIIIVIV4VV
BVS: VIIIVIV4VV
BVS: VIIIVIV4VV
BVS: VIIIVIV4VV
Figure 2. Molecular structures of 2-OPMe3, 2-OPMe2Ph, 2-OPMePh2, and 2-OPPh3. 2-OPMe2Ph, 2-OPMePh2, and 2-OPPh3 shown with 50% probability ellipsoids, 2-OPMe3
shown with 20% probability ellipsoids due to high disorder over two positions (see Figure S6 for more information). Hydrogen atoms removed for clarity.
Table 1. Selected bond lengths and angles for complexes 2-OPMe3, 2-OPMe2Ph, 2-OPMe2Ph, and 2-OPPh3. For full bond metrics and structural parameters, see supporting
information file.
Bond
V1–O1
P1–O1
1
--
--
--
--
2-OPMe3
2.062(5) Å
1.467(6) Å
2.120(5) Å
105.1º
2-OPMe2Ph
2.0403(19) Å
1.5032(19) Å
2.0982(17) Å
105.3º
2-OPMePh2
2.052(14) Å
1.487(15) Å
1.981(13) Å
104.2º
2-OPPh3
2.049(3) Å
1.506(3) Å
2.117(3) Å
104.7º
V1–Oc
V1–Ob–Vn (avg)
V=Ot (avg)
V–Oc (avg)
1.60 Å
2.25 Å
1.591 Å
2.314 Å
1.600 Å
2.3245 Å
1.600 Å
2.318 Å
1.595 Å
2.317 Å
organophosphane.
These results follow kinetic trends reported previously for OAT
between PR3 and various metal-oxo complexes (Table S8)42, 60
.
Tuning oxygen atom transfer through variation of PR3
The only exception to the aforementioned trend is noted in
comparing the qualitative reaction rate of OAT in the case of
PMe3 and PMe2Ph. Despite its smaller cone angle and
increased nucleophilicity, reduction of 1 by PMe3 took 19
hours, whereas consumption of the starting material was
Given the ability to tune the reactivity of phosphanes by
altering their steric or electronic properties, we shifted our
focus to the implications of phosphane identity in OAT.59
Addition of excess phosphane (2-4 equiv of PMe2Ph, PMePh2,
or PPh3) to 1 in THF at 70 ºC resulted in the formation of a
series of alkyl- and aryl-phosphine oxide-bound clusters in
good yield (2-OPMe2Ph; 83 %, 2-OPMePh2; 87 %, and 2-OPPh3;
76 %), as indicated by 1H NMR spectroscopy and ESI-MS
(Scheme 2, Figures S7-S10). The formation of the desired
phosphine-oxide adducts was confirmed via X-ray
crystallography of complexes 2-OPMe2Ph, 2-OPMePh2 and 2-
OPPh3 (Figure 2, Table 1; for full structural parameters and
BVS calculations, see Tables S4-S6). BVS calculations,
supported by subsequent analytical investigations (IR and UV-
vis spectroscopies), established retention of the Lindqvist core
and indicate similar oxidation state distributions of vanadium
ions in the reduced clusters (Figure 1a-1b, Table S7).
observed after only
7 hours in the case of PMe2Ph.
Furthermore, synthetic optimization for 2-OPMe2Ph revealed
that modelling reaction conditions after 2-OPMe3, through use
of 4 equiv of reductant, resulted in a mixture of the desired
product and the over-reduced, (OPMe2Ph)2-bound molecule,
Scheme 2. Synthesis of complexes 2-OPMe2Ph, 2-OPMePh2, and 2-
OPPh3.
While nominal changes in the molecular and electronic
structures of the reduced phosphine-oxide adducts of the
POV-methoxide clusters were observed when altering the
basicity or cone angle of the tertiary phosphine, large
variations in reaction times were obtained. Reaction
optimization revealed that as phosphine nucleophilicity
decreased, and cone angle increased, longer reaction times
[V6O5(OMe)12(OPMe2Ph)2]0,
confirmed
by
1H
NMR
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spectroscopy and ESI-MS(+ve) after 2 hours (Figure S11-S12). could drive partial dissociation of the phosphine VoiexwidAerticlliegOannlidne,
DOI: 10.1039/C9SC02882J
Selective formation of 2-OPMe2Ph required a decrease in OPMe2Ph, in coordinating solvents (MeCN-d3). This would
reductant equivalents and time (2 equiv, 7 hours). We result in a mixture of products, with one set of paramagnetic
attribute the discrepancy in reaction rate to the volatility of resonances corresponding to the expected product
the PMe3 (b.p. = 38 ºC) versus that of PMe2Ph (b.p. = 75 ºC); at ([V6O6(OEt)12OPMe2Ph]0 4-OPMe2Ph), and the other belonging
,
the requisite elevated reaction temperatures, PMe3 spends a to a cluster containing a single oxygen-atom vacancy with
majority of its time in the headspace of the pressure vessel, acetonitrile bound to the axial coordination site,
rendering this reaction heterogeneous.
([V6O6(OEt)12(MeCN)]0, 4-MeCN). This hypothesis was
confirmed via independent synthesis of 4-MeCN (Figure S16,
see experimental for details). Further support for fluxional
coordination of OPR3 was obtained by H NMR analysis of 4-
OPMe2Ph in a non-coordinating solvent (DCM-d2, Figure S17).
Oxygen atom transfer on extended alkoxy-bridged POV-alkoxide
clusters
1
In an effort to decouple the steric and electronic influences of
OAT from the POV-alkoxide cluster to tertiary phosphanes, we
extended our investigations to Lindqvist architectures with
longer bridging alkoxide chains. Our group and others have
previously reported that increasing the chain length of the
bridging alkoxides has no effect on the electronic structure of
the Lindqvist core.15, 61, 62 Thus, increasing the length of the
bridging ligands surrounding the vanadyl moieties would serve
to deconvolute the electronic and steric influences on OAT.
We initiated these efforts by investigating the reactivity of
PMe3 with the ethoxide-bridged Lindqvist cluster,
[V6O7(OEt)12]0 (3). Addition of 4 equiv of PMe3 to the POV-
ethoxide cluster resulted in a gradual colour change from
green to brown, after 4 days at 70 ºC (Scheme 3). The
increased steric bulk at the vanadyl moieties of the POV-
ethoxide cluster results in a 4-fold increase in reaction time
required for complete starting material consumption as
compared to its POV-methoxide analogue. Spectroscopic
characterization of the reduced product was consistent with
formation of [V6O6(OEt)12OPMe3]0 (4-OPMe3; Figure S13-S15,
Table S9).
This spectrum revealed
a single set of paramagnetic
resonances, with a five-peak pattern similar to that of 4-
OPMe3 (for full characterization of 4-MeCN and 4-OPMe2Ph
see Figures S14-S15, S18 Table S9-S10).
While OAT was possible for complex 3 using PMe2Ph as a
reductant, the steric bulk imparted by the alkoxide ligands to
the vanadyl moiety at the surface of the POV-ethoxide cluster
significantly affected the reaction rate (4 days) and promoted
phosphine oxide dissociation in the presence of coordinating
solvents. Attempts to reduce complex 3 with more sterically
encumbered tertiary phosphanes (PMePh2, PPh3) resulted in
limited OAT after prolonged heating (Figures S19-S20). A
similar lack of reactivity was observed when OAT was
attempted from the POV-propoxide analogue of the
hexavanadate assemblies [V6O7(OnPr)12]0 (Figure S21). These
results highlight the sensitivity of this reaction to the steric
bulk of both phosphane and POV-alkoxide, indicating that the
efficiency of OAT can be influenced by limiting the accessibility
of the vanadyl moieties.
Selective deoxygenation of styrene oxide using
[V6O6(OR)12OPMe3]0 (R=Me, Et)
Scheme 3. Synthesis of complexes 4-OPMe3 and 4-OPMe2Ph.
While the reactivity of POMs in an oxidative capacity is well
understood, comparatively less is known about their ability to
mediate reductive organotransformations.23, 63-67 As such, with
the reduced POV-alkoxide structures in hand, we shifted our
focus toward elucidation of new reductive transformations
using these oxygen-atom deficient clusters. In particular, we
became interested in the deoxygenation of epoxides, resulting
in the formation of an alkene. The described reaction is an
important chemical process in organic synthesis, as oxirane
rings are commonly used for the chemical protection of an
alkene fragment.68-71 Indeed, deprotection reactions of
epoxides are often performed through stochiometric
Isolation of 4-OPMe3 prompted further investigation of the
reactivity of 3 with the previously described family of mixed
alkyl- and aryl-phosphanes (PMenPh3-n; n = 0-3). Thus, we next
explored the reactivity of 3 with PMe2Ph (Scheme 3). After
four days at 70 ºC the expected colour change from green to
brown was observed. Analysis of the crude product by 1H NMR
spectroscopy in MeCN-d3 revealed the expected paramagnetic
resonances of the reduced product, however each possessing
a single shoulder (Figure S16). This observation suggests the
presence of multiple species in solution. Given the increased
bulk of both the phosphane and the bridging moieties on the
POV-alkoxide cluster, we hypothesized that steric hinderance
homogeneous processes that produce large amounts of toxic
72, 73
by-products.70,
Only recently have nonprecious, metal-
oxide derived materials (e.g. TiO2) been implemented for the
deoxygenation of this class of substrates.74 Notably,
mechanistic investigations have revealed that O-atom vacancy
formation at the surface of metal-oxide materials plays a key
role in the activation of substrate.
Given that our reduced POV-alkoxide clusters bear oxygen-
atom deficient sites, we hypothesized that similar C—O bond
activation reactions could afford epoxide reduction at the
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The difference in rate and yield of the reaction of styrene
View Article Online
oxide with the two reduced POV-alkoxiDdOeI:c1l0u.s1t0e3r9s/C(92S-CO0P2M88e2J3
and 4-OPMe3) suggests that the steric bulk of the bridging
alkoxide moieties around the active VIII centre influence OAT.
In contrast to the changes in rate observed in the case of
oxygen-atom vacancy formation with tertiary phosphanes, the
reaction of the POV-ethoxide cluster and styrene oxide is
completed in less than 10% of the time required for its
methoxide congener (Table 2). We justify these observations
through consideration of the lability of the dative phosphine
oxide ligands in the case of the methoxide- and ethoxide-
bridged POV clusters. For deoxygenation of the styrene oxide
to occur, the phosphine oxide ligand must dissociate from the
reduced vanadium centre, resulting in the formation of a
coordinatively unsaturated metal ion (Scheme 5). We recall
that in the case of complex 4-OPMe2Ph, the OPR3 ligand
readily dissociates from the cluster in the presence of
coordinating solvents (substrates). The observed increase in
rate of epoxide deoxygenation with 4-OPMe3 (as compared to
2-OPMe3) can therefore be attributed to an increased rate of
phosphine oxide dissociation from the POV-ethoxide scaffold.
Thus, we can conclude that the rate limiting step in alkene
formation is the generation of the coordinatively unsaturated
vanadium(III) centre via dissociation of OPR3.
Scheme 4. Deoxygenation of styrene oxide using (OPMe3)-bound
methoxide and ethoxide POV-alkoxide clusters.
Table 2. Deoxygenation of styrene oxide with 2-OPMe3 and 4-OPMe3.a,b
Complex
Time (h)
Styrene oxide
Styrene
(%)c
46 %
(%)c
0 %
2-OPMe3
4-OPMe3
48
4
22 %
83 %
aDeoxygenation was carried out using the (OPMe3)-bound clusters (1 equiv)
and styrene oxide (1.1 equiv). bGCMS analysis revealed selective formation of
styrene as product (no evidence for the formation of 1-phenethanol or 2-
phenethanol. cYield determined by 1H NMR spectroscopy vs. an internal
standard in CD2Cl2 (propylene carbonate, 1 equiv, Figures S22-S26)
Prior to this work, only one example of the reductive
reactivity of a series of homometallic POMs containing a
spectroscopically well-defined defect site had been reported.36
The investigation pertaining to the oxidation of these reduced
polyoxotungstate clusters, [XW12O37(OH2)3]n- (X=H2, B, Si; n= 4-
6) detailed the reactivity of these systems with OAT reagents,
including Ph3AsO, PhNO, and R2SO (R = Me, Ph). In all cases,
OAT is facilitated by substrates bearing oxygen-heteroatom
reactive surface sites of the Lindqvist core. Thus, we opted to
investigate the reactivity of the (OPMe3)-bound methoxide
cluster, 2-OPMe3, with an equivalent of styrene oxide (Scheme
4, Table 2). Indeed, exposure of the substrate to 2-OPMe3 at
70 ºC resulted in complete consumption of styrene oxide after
2 days (Figure S22). However, only 46 % of the deoxygenated
product, styrene, was formed (1H NMR yield vs. propylene
carbonate as an internal standard, Figure S22). Further analysis
1
of the paramagnetic region of the H NMR spectrum revealed
Scheme 5. Stochiometric oxygen atom transfer facilitated by an oxygen-
atom vacancy on a POV-alkoxide cluster.
a mixture of resonances that correspond to both the reduced
and oxidized vanadate clusters, complexes 2-OPMe3 and 1,
respectively (Figure S23). The presence of unreacted, reduced
cluster, coupled with the complete consumption of styrene
oxide, suggests that substrate decomposition (polymerization)
is competitive with OAT, preventing full conversion of the
reduced POV-alkoxide to 1.75-77 To verify this hypothesis, we
added a subsequent equivalent of styrene oxide to the crude
reaction mixture, affording further oxidation of 2-OPMe3. This
experiment confirms that OAT from styrene oxide is hindered
by substrate degradation under the prescribed reaction
conditions (Figure S24).
Intrigued by whether we could extend OAT to the reduced
longer-alkoxy bridged cluster, we added styrene oxide (1
equiv) to 4-OPMe3. In contrast to the results obtained with the
methoxide-bridged species, complete consumption of 4-
1
OPMe3 was observed by paramagnetic H NMR spectroscopy
after 4 h at 70 ºC (Scheme 4, Table 2, Figure S25).
Quantification of styrene produced from this reaction was
1
confirmed by H NMR spectroscopy, revealing formation of 83
% styrene (vs. propylene carbonate as an internal standard,
Figure S26).
6 | J. Name., 2012, 00, 1-3
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multiple bonds, resulting in oxidation of the cluster via the Deoxygenation of styrene oxide via OAT to the (OPMe3)-bound
View Article Online
well-defined multielectron chemistry of the substrate. In scaffolds, [V6O6(OR)12OPMe3]0 (R = Me, DEOt)I,: 1p0r.1e0s3e9n/Cts9StCh0e28fi8r2sJt
contrast, the work presented here summarizes the example of OAT to a reduced, homometallic polyoxometalate
homogeneous deoxygenation of an epoxide by a reduced system. Comparative investigations between the reactivity of
homometallic metal oxide cluster, resulting in formation of the methoxide- and ethoxide-bridged clusters provides insight into
corresponding alkene. The reduction of styrene oxide the mechanism of deoxygenation by the cluster. The higher
highlights a rare example of the activation of a purely organic yield of styrene observed in the case of the (OPMe3)-bound
substrate by a structurally characterized, oxygen-deficient ethoxide species suggests that increased lability of the
POM. This work, taken in conjunction with the reductive phosphine oxide ligand lowers the kinetic barrier for epoxide
transformations of homometallic polyoxotungstate clusters deoxygenation. Thus, we propose that styrene formation
summarized in the 1980s by Piepgrass and Pope, prompts proceeds via a disassociate mechanism, with the rate limiting
further investigation into the ability of reduced POM step being disassociation of the phosphine oxide ligand from
architectures bearing oxygen-atom defect sites to facilitate the surface of the cluster (Scheme 5). Future work
reductive transformations of relevance to organic chemistry investigating the reactivity of the reduced POV-alkoxide
and small molecule activation.36, 37
clusters will expand upon the deoxygenation of various kinds
of epoxides including less activated aliphatic substrates.
Conclusions
Experimental
We have demonstrated an alternative route for O-atom
vacancy formation at the surface of alkoxy-bridged POV
clusters via OAT to a series of tertiary phosphanes (PR3 = PMe3,
PMe2Ph, PMePh2, PPh3). While the reactivity of
polyoxometalates with phosphane substrates has been
reported previously,29, 30, 53 evidence suggesting an important
role of surface-bound water ligands has created ambiguity as
to whether oxygen-atom vacancies are truly formed during the
course of the reaction.55, 78 In contrast, this work offers clear
indication of OAT between cluster and substrate, resulting in
the formation of mono-vacant POV-alkoxide clusters with a
datively coordinated phosphine oxide ligand bound to the
oxygen deficient moiety. The rigorously nonaqueous reaction
environment imparted to the polyoxovanadate by its bridging
alkoxide ligands eliminates the possibility of proton/water
assisted phosphane oxidation. Structural and spectroscopic
characterization of the reduced clusters, [V6O6(OR)12OPR’3]0,
provides unambiguous support for complete OAT from the
polyoxometalate to the organophosphane substrate.
Furthermore, we have presented an investigation of
structure-function relationships, elucidating the role of sterics
and electronics on OAT between POV-alkoxide clusters and
tertiary phosphanes. While similar investigations regarding the
reactivity of phosphanes with mononuclear metal-oxo
complexes have been reported previously, this work presents
the first example of such a study with polynuclear assemblies.
The implication of nucleophilicity and cone angle of the
phosphane reductant on OAT was probed, both by varying
substituents of the phosphane as well as through the
investigation of the reactivity of more sterically encumbered
POV-alkoxide architectures. These results reveal a positive
correlation between the sterics of both substrate and metal-
oxide cluster and reaction time. Collectively, our observations
indicate that increased bulk limits the rate of OAT between the
metal-oxide cluster and phosphane. In sum, this manuscript
provides key insight into the mechanism of formation of O-
deficient sites in POV-alkoxide clusters.
General Considerations. All manipulations were carried out in the
absence of water and dioxygen using standard Schlenk techniques,
or in a UniLab MBraun inert atmosphere drybox under a dinitrogen
atmosphere except where specified otherwise. All glassware was
oven-dried for a minimum of 3 hours and cooled in an evacuated
antechamber prior to use in the drybox. Anhydrous tetrahydrofuran
was purchased from Sigma-Aldrich and stored over activated 3 Å
molecular sieves. All other solvents were dried and deoxygenated
on a Glass Contour System (Pure Process Technology, LLC) and
stored over activated 3 Å molecular sieves purchased from Fisher
Scientific prior to use. [V6O7(OMe)12]0 (1),14 [V6O6(OMe)12]0 (2),35
[V6O7(OEt)12]0 (3),15 and [V6O7(OnPr)12]0,62 were prepared according
to
published
procedures.
Trimethlyphosphine,
dimethylphenylphosphine, triphenylphosphine, and propylene
carbonate were purchased from Sigma Aldrich and used as
received. Diphenylmethylphosphine was purchase from Alfa Aesar
and used as received.
All 1H NMR spectra were recorded at 500 and 400 MHz on
Bruker DPX-500 and Bruker DPX-400 MHz spectrometers locked on
the signal of deuterated solvents. All chemical shifts were reported
relative to the peak of residual H signal in deuterated solvents.
CD3CN and CD2Cl2 were purchased from Cambridge Isotope
Laboratories, degassed by three freeze−pump−thaw cycles, and
stored in the drybox over activated 3Å molecular sieves. THF-d8 was
purchased from Cambridge Isotope Laboratories and stored over
activated 3Å molecular sieves. Infrared (FT-IR, ATR) spectra of
complexes were recorded on a Shimadzu IRAffinity-1 Fourier
Transform Infrared Spectrophotometer and are reported in
wavenumbers (cm-1). Electronic absorption measurements were
recorded at room temperature in anhydrous acetonitrile in a sealed
1
cm quartz cuvette with an Agilent Cary 60 UV-Vis
spectrophotometer. Mass spectrometry analyses were performed
on an Advion ExpressionL Compact Mass Spectrometer equipped
with an electrospray probe and an ion-trap mass analyser. Direct
injection analysis was employed in all cases with a sample solution
in acetonitrile. Cyclic Voltammetry experiments were recorded with
Finally, we present an example of
a
reductive
organotransformation mediated by a polyoxometalate cluster.
This journal is © The Royal Society of Chemistry 20xx
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a Bio-Logic SP200 potentiostat/galvanostat and the EC-Lab software
suite. All measurements were performed in a three electrode
system cell configuration that consisted of a glassy-carbon (ø = 3.0
mm) as working electrode (CH Instruments, USA), a Pt wire as the
counter electrode (CH Instruments, USA), and an Ag/Ag+ non-
aqueous reference electrode with 0.01 M AgNO3 in 0.05 M
[nBu4N][PF6] in acetonitrile (BASi, USA). All electrochemical
measurements were performed at room temperature in a N2-filled
drybox. Anhydrous dichloromethane that contained [nBu4N][PF6]
was used as the electrolyte solution. Gas chromatography-mass
spectrometry analyses were performed on a Shimadzu GCMS QP
2010.
pressure to give a brown solid. The remaining residual solid was
View Article Online
stirred in pentane (10 mL) for 30 min and DthOeI:n10fi.1lt0e3r9e/dC.9TSCh0e2s8o8l2idJ
was washed with excess pentane (5 mL, 3x) until the filtrate ran
clear. The solid was then extracted with dichloromethane (2 mL,
3x), and any volatiles were removed under vacuum to yield the
product, 2-OPMe2Ph, as a dark brown solid (0.042 g, 0.045 mmol,
87 %). Crystals suitable for X-ray analysis were grown from slow
diffusion of pentane into a concentrated solution of the product in
1
tetrahydrofuran. H NMR (400 MHz, CD3CN)  = 24.58 (fwhh = 136
Hz), 20.06 (fwhh = 156 Hz), 8.99, 8.54 (fwhh = 68 Hz), 7.69 (fwhh =
68 Hz), 3.42 (fwhh = 136 Hz), -12.52 (fwhh = 372 Hz) ppm. FT-IR
(ATR, cm-1): 1159 (P=O), 1036 (O-CH3), 962 (V=O), 943 (P=O). UV-Vis
(CH3CN): 390 nm ( = 4224 M-1cm-1), 1000 nm (  = 721 M-1cm-1).
Elemental analysis for C20H47O19V6P (MW 928.20 g/mol) Calcd (%):
C, 25.88; H, 5.10. Found (%): C, 26.08; H, 4.92.
Single crystals of 2-OPMe3, 2-OPMe2Ph, 2-OPMePh2, 2-OPPh3
and 4-OPMe3 were mounted on the tip of a thin glass optical fiber
(goniometer head) and mounted on a XtaLab Synergy-S Dualflex
diffractometer equipped with a HyPix-6000HE HPC area detector
for data collection at 100.00(10)-192.99(10) K, respectively. The
structure was solved using SHELXT-2018/279 and refined using
Synthesis of [V6O6(OCH3)12OPCH3(C6H5)2] (2-OPMePh2): In
glovebox, a 15 mL pressure vessel was charged with [V6O7(OCH3)12]
(1) (0.042 g, 0.053 mmol) and mL tetrahydrofuran.
a
6
Diphenylmethylphosphine (0.04 mL, 0.22 mmol, 4 equiv) was added
dropwise to the solution. The reaction vessel was sealed, removed
from the glovebox, and placed in an oil bath where it was left to stir
for 28 hours at 70 ºC. After this time, the reaction vessel was
brought back into the glovebox, and solvents were removed under
reduced pressure to give a brown-green oil. The remaining oil was
stirred in pentane (10 mL) for 30 min and then filtered. The solid
was washed with excess pentane (5 mL, 3x) until the filtrate ran
clear. The solid was then extracted with dichloromethane (2 mL,
3x), and any volatiles were removed under reduced pressure to
yield the product, 2-OPMePh2, as a dark brown solid (0.044 g, 0.043
mmol, 84 %). Crystals suitable for X-ray analysis were grown from
slow diffusion of pentane into a concentrated solution of the
product in tetrahydrofuran. 1H NMR (400 MHz, CD3CN)  = 25.13
(fwhh = 172 Hz), 20.17 (fwhh = 136 Hz), 8.52 (fwhh = 36 Hz), 7.71,
7.57, 3.42, -12.52 (fwhh = 304 Hz) ppm. FT-IR (ATR, cm-1): 1159
(P=O), 1036 (O-CH3), 1013 (P=O), 964 (V=O). UV-Vis (CH3CN): 390
nm ( = 3195 M-1cm-1), 1000 nm ( = 546 M-1cm-1). Elemental
analysis for C25H49O19V6P (MW 990.27 g/mol) Calcd (%): C, 30.32; H,
4.99. Found (%): C, 30.45; H, 4.67.
SHELXL-2018/3.60 Elemental analyses were performed on
a
PerkinElmer 2400 Series II Analyzer, at the CENTC Elemental
Analysis Facility, University of Rochester.
Synthesis of [V6O6(OCH3)12OP(CH3)3] (2-OPMe3): In a glovebox, a 15
mL pressure vessel was charged with [V6O7(OCH3)12] (1) (0.051 g,
0.065 mmol) and 10 mL tetrahydrofuran. A 1.0 M solution of
trimethylphosphine in tetrahydrofuran (0.26 mL, 0.26 mmol, 4
equiv) was added dropwise to the reaction mixture and the
pressure vessel was sealed, removed from the glovebox, and placed
in an oil bath and set to stir for 19 hours at 70 ºC. After this time,
the reaction vessel was brought back into the glovebox, and
solvents were removed under reduced pressure to give
a
green/brown solid. The remaining residual solid was stirred in
pentane (10 mL) overnight and then filtered. The solid was washed
with excess pentane (5 mL, 3x) until the filtrate ran clear. The solid
was then extracted with tetrahydrofuran (2 mL, 3x), and volatiles
were removed under vacuum to yield the product, 2-OPMe3, as a
dark green solid (0.047 g, 0.055 mmol, 84 %). Crystals suitable for X-
ray analysis were grown from slow diffusion of pentane into a
concentrated solution of the product in tetrahydrofuran. 1H NMR
(500 MHz, CD3CN)  = 28.38 (fwhh = 690), 23.13 (fwhh = 170 Hz),
19.74 (fwhh = 145), -12.25 (fwhh = 269) ppm. FT-IR (ATR, cm-1):
1163 (P=O), 1032 (O-CH3), 962 (V=O), 945 (P=O). UV-Vis (CH3CN):
Synthesis of [V6O6(OCH3)12OP(C6H5)3] (2-OPPh3): In a glovebox, a 15
mL pressure vessel was charged with [V6O7(OCH3)12] (1) (0.048 g,
0.055 mmol), triphenylphosphine (0.061 g, 0.233 mmol, 4 equiv),
and 10 mL tetrahydrofuran. The reaction vessel was sealed,
removed from the glovebox, and placed in an oil bath where it was
left to stir for 15 days at 70 ºC. After this time, the reaction vessel
was brought back into the glovebox, and solvents were removed
under reduced pressure to give a brown solid. The solid was stirred
in pentane (10 mL) overnight and then filtered. The remaining solid
was washed with excess pentane (5 mL, 3x) until the filtrate ran
clear. The solid was then extracted with dichloromethane (2 mL,
3x), and any volatiles were removed under vacuum to yield the
product, 2-OPPh3, as a dark brown solid (0.055 g, 0.052 mmol, 82
%). Crystals suitable for X-ray analysis were grown from slow
diffusion of pentane into a concentrated solution of the product in
390 nm ( = 3766 M-1cm-1), 1000 nm ( = 655 M-1cm-1). Elemental
analysis for C15H45O19V6P  ½ C4H8O (MW 902.17 g/mol) Calcd (%): C,
22.63; H, 5.47. Found (%): C, 22.50; H, 5.19.
Synthesis of [V6O6(OCH3)12OP(CH3)2C6H5] (2-OPMe2Ph): In
glovebox, a 15 mL pressure vessel was charged with [V6O7(OCH3)12]
(1) (0.041 g, 0.052 mmol) and mL tetrahydrofuran.
a
6
Dimethylphenylphosphine (15 L, 0.10 mmol, 2 equiv) was added
dropwise to the solution. The reaction vessel was sealed, removed
from the glovebox, and placed in an oil bath where it was left to stir
for 7 hours at 70 ºC. After this time, the reaction vessel was brought
back into the glovebox, and solvents were removed under reduced
1
tetrahydrofuran. H NMR (400 MHz, CD3CN)  = 25.62 (fwhh = 360
Hz), 20.39 (fwhh = 280 Hz), 8.49, 7.37, -13.06 (fwhh = 520 Hz) ppm.
FT-IR (ATR, cm-1): 1157 (P=O), 1040 (O-CH3), 966 (V=O). UV-Vis
8 | J. Name., 2012, 00, 1-3
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(CH3CN): 390 nm ( = 2298 M-1cm-1), 1000 nm (  467 M-1cm-1). MeCN.35 In a glovebox, two 20 mL scintillation vials VwieewrAerticchleaOrngleinde

Elemental analysis for C30H51O19V6P ¼ C4H8O (MW 1070.35 g/mol) with [V6O7(OC2H5)12] (3) (0.101 g, 0.10D6OI:m10m.1o0l3)9/aCn9dSC02882J
4
mL
Calcd (%): C, 34.79; H, 4.99. Found (%): C, 35.09; H, 5.01.
tetrahydrofuran. In separate vials, VMes3(THF) (0.0559 g, 0.117
mmol, 1.1 equiv) was dissolved in 8 mL tetrahydrofuran, and these
solutions were added dropwise to the corresponding 3 solution.
The reaction mixtures were heated to 50 ºC for an hour, affording a
red-brown solution. The reaction mixtures were united, and the
volatiles were removed under vacuum. The resulting brown solid
was washed with acetonitrile (4 mL), then washed with pentane
until the pentane went from red-orange to yellow (12 mL, 6x). The
solid was extracted with acetonitrile (12 mL x 6) and filtered over a
bed of Celite (1 cm) on a medium-porosity glass frit. The volatiles
were removed under vacuum to yield the product, 4-MeCN, as a
Synthesis of [V6O6(OC2H5)12OP(CH3)3] (4-OPMe3): In a glovebox, a
15 mL pressure vessel was charged with [V6O7(OC2H5)12]0 (3) (0.122
g, 0.128 mmol), trimethylphosphine (1M in tetrahydrofuran, 0.51
mL, 0.51 mmol, 4 equiv), and 12 mL tetrahydrofuran. The reaction
vessel was sealed, removed from the glovebox, and placed in an oil
bath where it was left to stir for 4 days at 70 ºC. After this time, the
reaction vessel was brought back into the glovebox, and volatiles
were removed under reduced pressure to give a green-brown solid.
The solid was extracted with acetonitrile (5 mL, 3x), and any
volatiles were removed under vacuum. The solid was extracted with
diethyl ether (5 mL, 3x), and any volatiles were removed under
vacuum to yield the product, 4-OPMe3, as a green-brown solid
(0.068 g, 0.066 mmol, 52 %). The product can be further purified via
recrystallization from slow diffusion of pentane into a concentrated
solution of 4-OPMe3 in tetrahydrofuran (0.040, 0.038 mmol, 30 %).
1H NMR (400 MHz, CD3CN):  = 29.16 (fwhh = 272 Hz), 22.08 (fwhh
= 200 Hz), 4.69 (fwhh = 120 Hz), 2.26 (fwhh = 64 Hz), -0.53 (fwhh =
76 Hz), -2.41 (fwhh = 84 Hz), -22.62 (fwhh = 424 Hz) ppm. 1H NMR
(400 MHz, CDCl3):  = 33.62 (fwhh = 288 Hz), 18.15 (fwhh = 276 Hz),
1
dark brown solid (0.099 g, 0.100 mmol, 47 %). H NMR (400 MHz,
CD3CN):  = 30.53 (fwhh = 476 Hz), 20.74 (fwhh = 304 Hz), 3.07, -
0.21 (fwhh = 148 Hz), -2.55 (fwhh = 160 Hz), -24.21 (fwhh = 548 Hz)
ppm. FT-IR (ATR, cm-1): 1040 (O-C2H5), 964 (V=O). UV-Vis (CH3CN):
394 nm ( = 2740 M-1cm-1), 1000 nm (  407 M-1cm-1). UV-Vis
(CH2Cl2): 394 nm ( = 2790 M-1cm-1), 1000 nm (  417 M-1cm-1).

Elemental analysis for C24H60O19V6
¾ (CH3)CN ¼ C4H8O (MW
991.20 g/mol) Calcd (%): C, 32.11; H, 6.53; N, 1.06. Found (%): C,
32.33; H, 6.55; N, 1.18.
4.40 (fwhh = 192 Hz), 2.40 (fwhh = 128 Hz), -0.02 (fwhh = 124 Hz), - Oxygen atom transfer from styrene oxide to 2-OPMe3: In a
3.03 (fwhh = 144 Hz), -22.43 (fwhh = 536 Hz) ppm. FT-IR (ATR, cm-1): glovebox, a J-young tube was charged with 2-OPMe3 (0.016 g,
1171 (P=O), 1043 (O-C2H5), 964 (V=O), 943 (P=O). UV-Vis (CH3CN): 0.019 mmol, 1 equiv), styrene oxide (2.25 L, 0.020 mmol, 1.1
396 nm ( = 4300 M-1cm-1), 1000 nm (  610 M-1cm-1). UV-Vis equiv), propylene carbonate (1.50 L, 0.018 mmol, 1 equiv), and
(CH2Cl2): 394 nm ( = 4010 M-1cm-1), 1000 nm (  628 M-1cm-1). 0.7 mL CD2Cl2. The J-young reaction was placed in a 70 ºC oil bath
1
Elemental analysis for C27H69O19V6P (MW 1034.45 g/mol) Calcd (%): and removed for time points. H NMR yields were determined by
C, 31.35; H, 6.72. Found (%): C, 31.75; H, 6.68.
integration against an internal standard, propylene carbonate.
Synthesis of [V6O6(OC2H5)12OP(CH3)2C6H5] (4-OPMe2Ph): In
a
Oxygen atom transfer from styrene oxide to 4-OPMe3: In a
glovebox, two 15 mL pressure vessels were charged with glovebox, a J-young tube was charged with 4-OPMe3 (0.204 g,
[V6O7(OC2H5)12] (3) (0.102 g, 0.106 mmol), 0.020 mmol, 1 equiv), styrene oxide (2.50 L, 0.022 mmol, 1.1
dimethylphenylphosphine (0.062 g, 0.44 mmol, 4 equiv), and 12 mL equiv), propylene carbonate (1.75 L, 0.021 mmol, 1 equiv), and
tetrahydrofuran. The reaction vessels were sealed, removed from 0.7 mL CD2Cl2. The J-young reaction was placed in a 70 ºC oil bath
1
the glovebox, and placed in an oil bath where they were left to stir and removed for time points. H NMR yields were determined by
for 4 days at 70 ºC. After this time, the reaction vessels were integration against an internal standard, propylene carbonate.
brought back into the glovebox, the solutions were united, and
volatiles were removed under reduced pressure to give a green-
brown solid. The solid was extracted with acetonitrile (5 mL, 3x),
Conflicts of interest
and any volatiles were removed under vacuum. The solid was
extracted with diethyl ether (5 mL, 3x), and any volatiles were
removed under vacuum to yield the product, 4-OPMe2Ph, as a dark
brown solid (0.061 g, 0.055 mmol, 26 %). The product can be further
purified via recrystallization from slow diffusion of pentane into a
concentrated solution of 4-OPMe2Ph in tetrahydrofuran (0.027,
0.025 mmol, 12 %). These crystals were suitable for X-ray analysis.
1H NMR (400 MHz, CD2Cl2)  = 32.18 (fwhh = 288 Hz), 19.09 (fwhh =
356 Hz), 8.54, 8.33 (fwhh = 72 Hz), 7.54 (fwhh = 52 Hz), 3.82 (fwhh =
172 Hz), 2.55 (fwhh = 128 Hz), -0.14 (fwhh = 116 Hz), -2.86 (fwhh =
124 Hz), -22.05 (fwhh = 556 Hz) ppm. FT-IR (ATR, cm-1): 1157 (P=O),
1040 (O-CH3), 964 (V=O), 934 (P=O). UV-Vis (CH2Cl2): 394 nm ( =
4710 M-1cm-1), 1000 nm (  638 M-1cm-1). Elemental analysis for
C32H71O19V6P (MW 1096.51 g/mol) Calcd (%): C, 35.05; H, 6.53.
Found (%): C, 35.43; H, 6.39.
There are no conflicts to declare.
Acknowledgements
This research was funded by the National Science Foundation
through grants CHE-1653195 and CHE-1725028. The authors
also acknowledge financial support from the University of
Rochester.
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