Aerobic alcohol oxidations catalyzed by oxorhenium complexes containing redox-active ligands

Article abstract of DOI:10.1002/ejic.201101044

The capacity of five-coordinate oxorhenium(V) anions with redox-active catecholate ligands to homolyze O₂ and afford dioxorhenium(VII) products is utilized for the development of new aerobic alcohol oxidation catalysts. The reaction of [Re^VII(O)₂(cat) ₂]^- with benzyl alcohol (BnOH) affords the expected products of net H₂ transfer: [Re^V(O)(cat) ₂]^-, benzaldehyde, and presumably H₂O. However, mechanistic studies reveal that the formation of the active oxidant requires both the dioxo and monooxo species, so BnOH oxidation by[Re^VII(O) ₂(cat)₂]^- exhibits an unexpected catalytic dependence on [Re^V(O)(cat)₂]^-. Attempts to oxidize more thermodynamically challenging primary alcohols, which include CH₃OH, using the [Re^VII(O)₂(cat) ₂]^- + [Re^V(O)(cat)₂]^- system did not yield aldehyde products. However, experiments performed in CH₃OH allowed the observation of a catalytically active intermediate species, which provides an insight into the mechanism of catalytic action and catalyst degradation. Based on these observations, complexes that contain a more oxidatively robust [Br₄cat]^2- ligand were shown to exhibit higher catalytic activity as measured by total turnover number. The requirement for a redox-active ligand for catalyst function has both benefits and limitations that are discussed in the context of aerobic alcohol oxidation catalysis. Copyright

Full text of DOI:10.1002/ejic.201101044

FULL PAPER
DOI: 10.1002/ejic.201101044
Aerobic Alcohol Oxidations Catalyzed by Oxorhenium Complexes Containing
Redox-Active Ligands
Cameron A. Lippert,[a] Korbinian Riener,[a] and Jake D. Soper*[a]
Keywords: Redox-active ligands / Homogeneous catalysis / Oxidation / Rhenium / Proton-coupled electron transfer
The capacity of five-coordinate oxorhenium(V) anions with
redox-active catecholate ligands to homolyze O2 and afford
dioxorhenium(VII) products is utilized for the development
of new aerobic alcohol oxidation catalysts. The reaction of
[ReVII(O)2(cat)2]– with benzyl alcohol (BnOH) affords the ex-
CH3OH, using the [ReVII(O)2(cat)2]– + [ReV(O)(cat)2]– system
did not yield aldehyde products. However, experiments per-
formed in CH3OH allowed the observation of a catalytically
active intermediate species, which provides an insight into
the mechanism of catalytic action and catalyst degradation.
pected products of net H2 transfer: [ReV(O)(cat)2]–, benzalde- Based on these observations, complexes that contain a more
hyde, and presumably H2O. However, mechanistic studies
reveal that the formation of the active oxidant requires both
the dioxo and monooxo species, so BnOH oxidation by
[ReVII(O)2(cat)2]– exhibits an unexpected catalytic depen-
dence on [ReV(O)(cat)2]–. Attempts to oxidize more thermo-
dynamically challenging primary alcohols, which include
oxidatively robust [Br4cat]2– ligand were shown to exhibit
higher catalytic activity as measured by total turnover
number. The requirement for a redox-active ligand for cata-
lyst function has both benefits and limitations that are dis-
cussed in the context of aerobic alcohol oxidation catalysis.
Introduction
Recent progress in the elaboration of complexes with re-
dox-active ligands for selective bond making and break-
ing[1,2] has precipitated the development of an array of new
catalytic methods for small molecule transformations,
which include hydrogenation and dehydrogenation,[3,4]
alcohol oxidation,[5] nitrene transfer,[6] and C–C bond for-
mation.[3c,7,8] In addition to some of these, we are interested
in expanding the utility of redox-active ligands for selective
O2 reduction in aerobic oxidation catalysis.[5c] Towards this
goal, a recent study from our laboratory described the de-
tails of O2 homolysis by oxorhenium(V) anions with redox-
active ligands to generate dioxorhenium(VII) products
(Scheme 1a).[9] Notably, the capacity of catechol- and ami-
nophenol-derived ligands to undergo reversible 1e– redox
changes is a critical feature of O2 homolysis reactions be-
cause multielectron O2 cleavage proceeds by a series of 1e–
Re–O bond formation steps that generate superoxo and per-
oxo complex intermediates. Accordingly, the coordination
of a redox-active ligand lowers the kinetic barrier to O2
activation by allowing the 1e– Re–O bond formation reac-
tions to occur at the oxorhenium(V) centers without gener-
ating high-energy oxorhenium(VI) intermediates.
Scheme 1. Redox-active ligand-mediated O2 homolysis at [ReV(O)-
(cat)2]– for oxo transfer catalysis.
Because high-valent metal–oxo compounds are ubiqui-
tous in oxidation catalysis,[10] facile generation of a terminal
ReVII=O from O2 presaged the development of new aerobic
oxidation catalytic cycles. In our initial report, we noted
that the dioxorhenium(VII) anions were efficiently reduced
by PPh3 to yield OPPh3 and the corresponding monooxo-
rhenium(V) species (Scheme 1b).[9] The sum of these stoi-
chiometric reactions comprises a complete cycle for cata-
lytic aerobic phosphane oxidation (Scheme 1). However, the
practical utility of the rhenium complexes with redox-active
ligands as oxo transfer reagents is limited by the thermo-
dynamic stability of the ReVII=O bonds, which require
strong oxo acceptor substrates for catalytic turnover. In an
attempt to expand the scope of O2-derived oxo transfer ca-
[a] School of Chemistry and Biochemistry, Georgia Institute of
Technology,
Atlanta, GA 30332-0400, USA
E-mail: jake.soper@chemistry.gatech.edu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201101044.
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Aerobic Alcohol Oxidations Catalyzed by Oxorhenium Complexes
talysis, we turned our attention to oxidase-type oxidations,
in which the net addition of H2 (or 2e– + 2H+) to the oxo
fragment might provide a more accessible kinetic pathway
for reduction of the dioxorhenium(VII) complexes. We took
inspiration from previous reports of catalytic alcohol oxi-
dations that used high-valent rhenium complexes along
with stoichiometric cooxidants, such as H2O2 and PhIO,[11]
to pursue an aerobic variant of this chemistry based on fac-
ile O2 homolysis by oxorhenium(V) compounds with redox-
active ligands.[12]
This manuscript describes our first efforts to use O2-de-
rived dioxorhenium(VII) complexes with redox-active li-
gands for aerobic alcohol oxidation catalysis. The condi-
tions for stoichiometric and catalytic oxidations of benzyl
alcohol to afford benzaldehyde are reported. Attempts to
oxidize more challenging alcohols, including CH3OH, did
not yield aldehyde products, but allowed the observation
of a catalytically active intermediate species. Mechanistic
investigations of oxidations by the dioxorhenium(VII) com-
plexes revealed a surprising requirement for catalytic
oxorhenium(V) and highlighted the necessity of the redox-
active ligand for catalyst function. The utility and scope of
this new methodology within these constraints are dis-
cussed.
Results
Aerobic Oxidation of [ReV(O)(cat)2]– to [ReVII(O)2(cat)2]–
in CH3OH
Exposure of a green CH3OH solution of [Et4N][ReV(O)-
(cat)2] ([cat]2– = 1,2-catecholate) to dry air results in the
formation of a bright blue intermediate species (IB) in ca.
100 min followed by slow conversion to a bright purple
solution over 50 h (Figure 1). Comparison of the UV/Vis
spectrum of the resulting purple solution with that of an
authentic sample of [Et4N][ReVII(O)2(cat)2], confirmed that
[ReVII(O)2(cat)2]– was the only observable colored product
of the reaction. However, at UV/Vis concentrations (ca.
0.5 mm) in CH3OH the aerobic oxidation of [ReV(O)-
(cat)2]– produced [ReVII(O)2(cat)2]– in moderate yields (ca.
30%). The yield of [ReVII(O)2(cat)2]– increased to ca. 75%
when the reaction was performed using higher initial con-
centrations of [ReV(O)(cat)2]– (10–15 mm) in [D4]methanol,
Figure 1. Selected UV/Vis absorption data from a reaction of
4.5ϫ10–4 m [ReV(O)(cat)2]– with 1 atm dry air in CH3OH at 25 °C.
Spectra were selected at time points that correspond to the maxi-
mum yield of each observable species, which was measured by the
change in absorption intensity at their respective λmax values. (a)
The reaction occurs in two phases; the first phase converts [ReV(O)-
(cat)2]– (green line) into IB (blue line) in 100 min. The second phase
affords [ReVII(O)2(cat)2]– (purple line) in ca. 30% yield after
3000 min. (b) Time resolved absorption data at 592 nm for the for-
mation and decay of IB.
1
which was determined by H NMR spectroscopy. In con-
which appeared to correspond to the reverse of the oxi-
dation described above. These color changes were repro-
duced by addition of 1 equiv. of [ReV(O)(cat)2]– to a purple
solution of pure [ReVII(O)2(cat)2]– in CH3OH under N2,
which yielded a bright blue solution immediately with a
UV/Vis spectrum identical to that of IB observed in the re-
action of [ReV(O)(cat)2]– with air in CH3OH (vide supra).
Over several days the blue solution gradually reformed pale
trast, the same reaction performed in CH2Cl2 or CH3CN
gave quantitative conversion to [ReVII(O)2(cat)2]– at all con-
centrations without the observation of IB.[9]
Reduction of [ReVII(O)2(cat)2]– to [ReV(O)(cat)2]– in Air-
Free Alcohol Solutions
The UV/Vis spectra of CH3OH solutions of pure green [ReV(O)(cat)2]–, which was evidenced by its character-
[ReVII(O)2(cat)2]– were stable under N2 for weeks at ambient istic H NMR spectrum. Similarly prepared solutions that
1
temperature, but we observed that samples that contained contained [ReV(O)(cat)2]– and [ReVII(O)2(cat)2]– in CH3CN,
trace [ReV(O)(cat)2]– impurity underwent a series of color tetrahydrofuran, dimethylsulfoxide, or CH2Cl2 gave no re-
changes (from purple to blue to green) over several days, action that could be observed by UV/Vis or 1H NMR spec-
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C. A. Lippert, K. Riener, J. D. Soper
FULL PAPER
troscopy over several days, which suggests that CH3OH is at constant concentrations of [ReVII(O)2(cat)2]–, the yield of
a critical component of the reduction.
IB was independent of the initial concentration of [ReV(O)-
Because both [ReV(O)(cat)2]– and [ReVII(O)2(cat)2]– were (cat)2]– (Figure 2b) but its relative rate of formation was a
required for the reaction under N2, we initially suspected function of initial [ReV(O)(cat)2]– concentration (Figure 2c).
that the blue intermediate IB observed in CH3OH was a These data imply that [ReV(O)(cat)2]– is a catalyst for the
dimer of the oxorhenium(V) and dioxorhenium(VII) net reduction of [ReVII(O)2(cat)2]– Ǟ IB Ǟ [ReV(O)-
anions. All attempts to isolate and structurally characterize (cat)2]– in CH3OH under N2 but [ReV(O)(cat)2]– is not itself
IB were unsuccessful, which prompted us to pursue a spec- a component of IB.
troscopic investigation of its reaction kinetics and stoichi-
Interestingly, the blue spectrum characteristic of IB was
ometry.[13] Surprisingly, experiments performed with vary- independently generated by treating CH3OH solutions of
ing initial proportions of [ReV(O)(cat)2]– and [ReVII(O)2- pure [ReVII(O)2(cat)2]– with 1 equiv. of LiOCH3 under N2
(cat)2]–, but at constant total rhenium concentration, re- (Figure 3). This suggests that IB may be an [OCH3]– adduct
vealed that the yield of IB scales linearly with the initial to [ReVII(O)2(cat)2]– (discussed below), and the role of
[ReVII(O)2(cat)2]– concentration (Figure 2a). Furthermore, [ReV(O)(cat)2]– is to deliver a [OCH3]– fragment to [ReVII
(O)2(cat)2]–.
-
Figure 3. UV/Vis absorption spectra of 2.7ϫ10–4 m [ReVII(O)2-
(cat)2]– (purple line) and IB (blue line) in CH3OH obtained ca. 10 h
after addition of 1.0 equiv. LiOCH3 under N2 at 25 °C.
The addition of 0.1 equiv. of [ReV(O)(cat)2]– to a benzyl
alcohol (BnOH) solution of [ReVII(O)2(cat)2]– under N2 ef-
fected a color change from purple to green over 1 h at ambi-
ent temperature. Monitoring by UV/Vis spectroscopy gave
no evidence of a blue intermediate species analogous to IB
during the reaction. Analysis of the resulting green solution
1
by H NMR spectroscopy confirmed that [ReV(O)(cat)2]–
was the only observable catecholate-containing product,
which was isolated in 62% yield by selective precipitation.
The balance of the rhenium was apparently in the form of
1
perrhenate [ReO4]–, which does not have a H NMR spec-
trum but was observed at m/z = 250 in negative ion mode
ESI–MS analyses of the crude reaction mixture. Analysis of
the same product mixture by GC–MS showed benzaldehyde
as the only organic product, which was formed in 58% yield
based on limiting [ReVII(O)2(cat)2]– (Table 1, Entry 1).
The marked increase in the rate of the [ReVII(O)2(cat)2]–
Ǟ [ReV(O)(cat)2]– reduction in BnOH vs. CH3OH and the
observation of PhCHO in the BnOH reaction mixtures im-
plied that that [ReVII(O)2(cat)2]– oxidizes alcohols according
to Equation (1).
Figure 2. Selected UV/Vis absorption data from reactions of
[ReVII(O)2(cat)2]– with [ReV(O)(cat)2]– in CH3OH to produce the
blue intermediate IB with λmax = 600 nm. (a) Yield of IB as a func-
tion of initial [ReVII(O)2(cat)2]– concentration for reactions per-
formed at constant total [Re] = 4.0ϫ10–4 m. (b) Yield of IB from
reactions of 1.5ϫ10–4 m [ReVII(O)2(cat)2]– performed with varying
amounts of added [ReV(O)(cat)2]–. (c) Ratio of the yield of IB at t =
300 s to the maximum IB yield observed in reactions of 1.5ϫ10–4
m
[ReVII(O)2(cat)2]– performed with varying amounts of added
[ReV(O)(cat)2]–.
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Aerobic Alcohol Oxidations Catalyzed by Oxorhenium Complexes
perature. These data imply that alcohol oxidation results
from a reaction with an intact oxorhenium complex that
contains [cat]2– ligands and not from oxidative decomposi-
tion products, such as [ReO4]– or derivatives thereof, which
are produced during the course of the reaction. Consistent
with this hypothesis, the addition of excess BnOH to a pre-
formed blue solution of IB, which was generated from
[ReVII(O)2(cat)2]– and 0.1 equiv. [ReV(O)(cat)2]– in MeOH,
gave an immediate color change to pale green accompanied
by the formation of PhCHO, which suggested that IB is the
species responsible for BnOH oxidation, or a direct precur-
sor to the active oxidant.
Table 1. Stoichiometric and catalytic oxidations of BnOH to
PhCHO using dioxorhenium(VII) complexes with redox-active li-
gands.[a]
Aerobic Alcohol Oxidations Catalyzed by Catecholate-
Containing Oxorhenium Complexes
Entry
Oxidant[b]
[ReVII
]
[ReV]initial Yield
initial
[mm]
[mm]
[%][c]
The sum of the alcohol oxidation reaction in Equation
(1) and the O2 homolysis reaction in Scheme 1a comprises
a complete catalytic cycle for aerobic alcohol oxidation
based on the interconversion of [ReVII(O)2(cat)2]– and
[ReV(O)(cat)2]–. Accordingly, stirring a BnOH solution of
[ReV(O)(cat)2]– under 1 atm of dry air at ambient tempera-
ture gave a slow color change from pale green to dark pur-
ple. Analysis of the reaction mixture by 1H NMR spec-
troscopy after 24 h showed [ReVII(O)2(cat)2]– and no reso-
nances for [ReV(O)(cat)2]–, and GC–MS analysis of the
same reaction mixture indicated that PhCHO was produced
in 118% yield, which corresponds to slightly Ͼ1 turnover
(Table 1, Entry 2). Although this is approximately double
the PhCHO obtained in the stoichiometric reactions of
BnOH with [ReVII(O)2(cat)2]– (Table 1, Entry 1), the cata-
lytic activity is poor.
1
2
3
4
[ReVII(O)2(cat)2]– + N2
[ReVII(O)2(cat)2]– + O2
[ReVII(O)2(Br4cat)2]– + N2
[ReVII(O)2(Br4cat)2]– + O2
9.7
0
9.4
0
1.0
8.8
0.9
8.4
58
118
93
707
[a] All reactions were performed in neat BnOH at 25 °C. [b] Cata-
lytic oxidations using O2 were performed under 1 atm dry air. [c]
The yield was determined by GC analysis vs. a calibrated internal
standard as described in the Experimental Section. All yields are
the average of at least two independent experiments and are calcu-
lated based on limiting rhenium.
Consequently, NMR tubes containing [ReVII(O)2(cat)2]–
and 0.1 equiv. of [ReV(O)(cat)2]– in [D4]methanol with
5 equiv. of BnOH as an oxidizable substrate rapidly pro-
duced a blue solution under N2 that slowly faded to a pale
green over several days at ambient temperature. Formation
of the blue intermediate was accompanied by the appear-
ance of new resonances at 6.74 and 6.41 ppm in the 1H
NMR spectrum, which do not match those expected for
[ReV(O)(cat)2]– or [ReVII(O)2(cat)2]– and cannot be assigned
to free [cat]2– or its oxidized derivatives. The new reso-
nances subsequently decreased in intensity as [ReV(O)-
(cat)2]– was produced with concomitant formation of
PhCHO. Integration of the 1H NMR resonances for
PhCHO and [ReV(O)(cat)2]– indicated that 50% of the ini-
tial [ReVII(O)2(cat)2]– was converted to [ReV(O)(cat)2]– and
a corresponding 0.5 equiv. of aldehyde was formed, which
is consistent with the reaction stoichiometry in Equation
(1) (Figure S1). H2O was not found in the reaction mixture,
but rapid H/D exchange with [D4]methanol likely obscured
its observation by 1H NMR spectroscopy. As above, the fate
of the residual rhenium was apparently [ReO4]–, as evi-
denced by its presence in the ESI–MS spectra of the reac-
tion mixtures and the observation free [cat]2– oxidation
The observation of [ReO4]– and catechol-derived oxi-
dation products in the reactions of BnOH with [ReVII
-
(O)2(cat)2]– suggested that the yields and rates of alcohol
oxidation may be improved by substitution of the redox-
active ligand with one that is more difficult to oxidize. Ac-
cordingly, addition of 0.1 equiv. of [ReV(O)(OPPh3)-
(Br4cat)2]– ([Br4cat]2– = tetrabromo-1,2-catecholate) to a
BnOH solution of [ReVII(O)2(Br4cat)2]– under N2 produced
a color change from dark purple to pale green over 90 min
at ambient temperature. Analysis of the resulting green
solutions by UV/Vis spectroscopy and ESI-MS confirmed
that [ReVII(O)2(Br4cat)2]– was completely consumed in the
reaction, and GC–MS of the same reaction mixture showed
that PhCHO was the only organic product, which was
formed in 93% yield (Table 1, Entry 3). Performing the re-
action under dry air gave PhCHO in Ͼ700% yield after 7 d,
based on the initial [ReV(O)(OPPh3)(Br4cat)2]– concentra-
tion, which corresponds to a sixfold increase in catalytic
efficacy compared to the parent catecholate complexes.
1
products in the H NMR spectra at 6.64 and 6.55 ppm. To
verify that these decomposition products were not the
active species in the alcohol oxidation, solutions of
[ReO4]–, [ReO4]– + [ReV(O)(cat)2]–, and [ReO4]– + [ReVII(O)2-
(cat)2]– in BnOH or MeOH were independently prepared
and monitored by 1H NMR, UV/Vis spectroscopy, and
GC–MS. In all cases, no aldehyde was produced and no
Discussion
Formation and Composition of the Blue Oxidant IB
Formation of IB in CH3OH solution required at least two
blue intermediate was observed within 15 h at ambient tem- components: [ReVII(O)2(cat)2]– and 1 equiv. [OCH3]– or
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C. A. Lippert, K. Riener, J. D. Soper
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Scheme 2. Proposed mechanism for aerobic alcohol oxidations catalyzed by [ReV(O)(cat)2]– + [ReVII(O)2(cat)2]– via IB.
trace [ReV(O)(cat)2]–. Accordingly, the same IB species was to [ReVII(O)2(cat)2]–. In this respect, it is noteworthy that
generated both during O2 homolysis by [ReV(O)(cat)2]– and the color of the [ReV(O)(cat)2]– anion is visibly different in
alcohol reduction by [ReVII(O)2(cat)2]– because at interme- CH3OH (green) vs. noncoordinating solvents (pale tan). We
diate reaction times both CH3OH solutions contain mix- have previously discussed the capacity of [ReV(O)(cat)2]– to
tures of [ReV(O)(cat)2]– and [ReVII(O)2(cat)2]–. The observa- accommodate a sixth ligand,[9] and the reported single-crys-
tion that IB was obtained from the reaction of [ReVII
-
tal X-ray data for a CH3OH adduct to the perchlorinated
(O)2(cat)2]– with either stoichiometric [OCH3]– or catalytic homolog [ReV(O)(CH3OH)(Cl4cat)2]– ([Cl4cat]2– = tetra-
[ReV(O)(cat)2]– suggests that the function of [ReV(O)- chloro-1,2-catecholate) provides strong support for the ca-
(cat)2]– is to supply a [OCH3]– fragment to [ReVII(O)2- pacity of the [ReV(O)(cat)2]– to bind alcohols in this man-
(cat)2]–. As a result, mixtures of [ReV(O)(cat)2]– and [ReVII
-
ner.[15] It is conceivable that such a coordinated CH3OH
(O)2(cat)2]– produce IB only in CH3OH because the solvent ligand might hydrogen bond to a catechol oxygen atom of
itself is a component of the reaction. Addition of 1 equiv. [ReVII(O)2(cat)2]–, and the close contact interaction leads to
of [OCH3]– to [ReVII(O)2(cat)2]– did not afford IB in solvents methanol transfer and the generation of IB. This hypothesis
such as CH2Cl2 or CH3CN, perhaps due to the lack of is supported by previous reports of isolated dinuclear
accessible H+, as discussed below.
rhenium complexes that are bridged by Lewis bases and/or
A blue species analogous to IB was also not observed in hydrogen bonding interactions.[16]
any reaction that contained BnOH, and the addition of
As discussed above, the formulation of IB as an alkoxide
BnOH to a blue solution of IB resulted in the immediate complex dianion, [ReVII(O)2(OCH3)(cat)2]2–, is consistent
formation of PhCHO and [ReV(O)(cat)2]–. These data ratio- with the observation that IB is produced by the addition
nalize why IB only accumulated to observable concentra- of [OCH3]– to [ReVII(O)2(cat)2]–. Accordingly, generation of
tions in CH3OH: because IB is competent to oxidize BnOH, [ReVII(O)2(OCH3)(cat)2]2– from a reaction of [ReVII(O)2-
generation of IB (or its [OBn]– congener) is followed by ra- (cat)2]– with [ReV(O)(CH3OH)(cat)2]– would require the loss
pid reduction to [ReV(O)(cat)2]–. With the less easily re- of a proton to solvent or adventitious base. An alternative
duced CH3OH, IB is unable to generate the corresponding assignment for IB (shown in Scheme 2) is [ReVII(O)-
aldehyde, so slow catabolism of the oxorhenium complex (OH)(OCH3)(cat)2]– derived from a net [2+2] addition of
affords [ReO4]– and free oxidized ligand.
CH3OH across one Re=O bond. In this formulation for IB,
Although the structure of IB is not known, it likely con- the dianionic [ReVII(O)2(OCH3)(cat)2]2– complex generated
tains both [ReVII(O)2(cat)2]– and [OCH3]– or CH3OH. Alk- from [ReVII(O)2(cat)2]– + 1 equiv. [OCH3]– must be a rea-
oxide addition to [ReVII(O)2(cat)2]– requires the oxorheni- sonably strong Brønsted base.
um(VII) center to accommodate a seventh ligand, which is
not unreasonable. Computational studies of O2 activation
at [ReV(O)(cat)2]– suggest that a stable seven-coordinate η2-
Alcohol Oxidation by IB
peroxo complex [ReVII(O)2(O22–)(cat)2]– is the minimum en-
ergy dioxygen adduct,[9] and seven-coordinate oxorhenium-
The generation of aldehyde and [ReVII(O)2(cat)2]– prod-
(VII) complexes have been inferred as intermediates in ucts from the putative [ReVII(O)(OH)(OCH2R)(cat)2]– inter-
H2O2/CH3ReO3 oxidations.[14] mediates requires the formal removal of [H]– from the
Scheme 2 presents a mechanism for aerobic alcohol oxi- bound alkoxide fragment by transfer of 2e– to the oxorheni-
dation by [ReVII(O)2(cat)2]– + [ReV(O)(cat)2]–, which is con- um(VII) center and 1H+ to the coordinated hydroxide li-
sistent with all of our experimental observations. In this gand. A plausible mechanism for this transformation is pre-
mechanism, the [ReV(O)(cat)2]– catalyst is proposed to me- sented in Scheme 3. It invokes a five-membered ring transi-
diate the formation of IB by binding and delivering CH3OH tion state that is strongly reminiscent of alcohol oxidation
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Aerobic Alcohol Oxidations Catalyzed by Oxorhenium Complexes
by chromic acid and Dess–Martin periodinane.[10] It is (Br4cat)2]– significantly retard the O2 homolysis reaction as
likely that generation of the aldehyde product occurs in a compared to the [cat]2– congener, which makes the turnover
single 1H+ + 2e– step because protonation of the coordi- frequency exceptionally sluggish. The turnover-limiting step
nated hydroxide ligand significantly enhances the oxidizing changes based on the nature of the ligand employed, but
power of the oxorhenium(VII) center, and reduction of the this presents a different problem. For the [cat]2– ligand, the
rhenium(VII) ion makes the hydroxide ligand a much turnover-limiting step is apparently BnOH oxidation by
stronger base. Accordingly, coupling proton and electron [ReVII(O)2(cat)2]–. Because the alcohol oxidation by
transfer allows the net reaction to occur without the genera- [ReVII(O)2(cat)2]– requires a steady-state, nonzero concen-
tion of high-energy species that would result from a step- tration of [ReV(O)(cat)2]–, the rapid reaction of [ReV(O)-
wise process.
(cat)2]– with O2 effectively deactivates the catalyst by deplet-
ing the solution of [ReV(O)(cat)2]–.
Conclusions
The capacity of oxorhenium(V) anions that contain re-
dox-active catecholate ligands to homolyze O2 and generate
dioxorhenium(VII) products has been shown to facilitate
oxidase-type aerobic oxidations of BnOH to PhCHO. Al-
though [ReVII(O)2(cat)2]– is the net H2 acceptor in the reac-
tion, it does not react directly with alcohols. Rather, cata-
lytic quantities of [ReV(O)(cat)2]– are required to generate
the active oxidant, which we speculate may be a hydroxo-
alkoxo rhenium(VII) complex, formed by the intramolecu-
lar delivery of alcohol from [ReV(O)(CH3OH)(cat)2]– to
[ReVII(O)2(cat)2]–. Demonstration of net H2 addition to
[ReVII(O)2(cat)2]– establishes PCET as a method for direct
production of [ReV(O)(cat)2]– that does not require a strong
O atom acceptor substrate, opening new avenues to utilize
the [ReVII(O)2(cat)2]–/[ReV(O)(cat)2]– oxo transfer couple for
aerobic oxidation catalysis. However, [ReVII(O)2(cat)2]– is a
poor catalyst for BnOH oxidation because it undergoes oxi-
dative decomposition during the alcohol oxidation reaction
and because the rapid reoxidation of [ReV(O)(cat)2]– by O2
is actually detrimental to the catalytic function. The use of
a perbrominated [Br4cat]2– ligand provides enhanced cata-
lyst stability and increased turnover number, but turnover
frequency is low due to the slow kinetics of O2 homolysis
by [ReV(O)(Br4cat)2]–. The development of more useful
oxidase-type alcohol oxidations based on O2 homolysis by
[ReV(O)(cat)2]–-like species will require a balance of these
competing trends so that the rate of O2 activation matches
the rate of the alcohol oxidation.
Scheme 3. Intramolecular proton transfer to a hydroxide ligand in
[ReVII(O)(OH)(OCH2R)(cat)2]– (IB) for [OCH2R]– oxidation.
The generation of PhCHO from BnOH solution estab-
lishes net H2 addition and specifically proton-coupled elec-
tron transfer (PCET) as a viable alternative to strong O
atom acceptors for the production of [ReV(O)(cat)2]– from
[ReVII(O)2(cat)2]–. However, [ReVII(O)2(cat)2]– does not ef-
ficiently oxidize CH3OH to formaldehyde. Assuming that
both [ReVII(O)(OH)(OCH2R)(cat)2]– intermediates have
similar steric constraints for the formation of the five-mem-
bered ring transition state shown in Scheme 3, the inability
of [ReVII(O)2(cat)2]– to oxidize CH3OH cannot arise from
kinetic factors. Rather, its disparate reactivity with CH3OH
vs. BnOH is apparently a function of the relative reducing
power of the two alcohols.[17]
Limitations of Redox-Active Ligands for Rhenium Aerobic
Alcohol Oxidation Catalysis
Because of their role in mediating O2 homolysis by
[ReV(O)(cat)2]–, redox-active ligands are a crucial compo-
nent of oxidase- or oxygenase-type catalytic cycles based on
the interconversion of [ReV(O)(cat)2]– and [ReVII(O)2-
(cat)2]– with dioxygen. Unfortunately, the data presented
here suggest that the capacity of catecholate ligands to be
oxidized at modest potentials also limits the practical utility
of the [ReVII(O)2(cat)2]– + [ReV(O)(cat)2]– system for cata-
lytic alcohol oxidations.
Experimental Section
Foremost among these limitations is catalyst stability.
Under all of the conditions examined, alcohol oxidation by
General Considerations: Unless otherwise specified, all manipula-
[ReVII(O)2(cat)2]– yielded at most 50% of the intact, reduced tions were performed under anaerobic conditions using standard
[ReV(O)(cat)2]– and substantial quantities of nonalcohol
vacuum line techniques or in an inert atmosphere glove box under
oxidation products. Substitution of [cat]2– with electron-
purified nitrogen. Routine NMR spectra were acquired with a Var-
ian Mercury 300 spectrometer (300.323 MHz for 1H). All chemical
poor [Br4cat]2– increased the yield of PhCHO to Ͼ90% in
shifts are reported in ppm relative to Me4Si with the residual sol-
stoichiometric reactions of BnOH with [ReVII(O)2-
vent peak serving as an internal reference. UV/Vis absorption spec-
(Br4cat)2]– and gave a substantial increase in catalyst sta-
tra were acquired with a Varian Cary 50 spectrophotometer. Unless
bility as measured by total turnover numbers. However,
otherwise noted, all electronic absorption spectra were obtained at
even the more robust [ReVII(O)2(Br4cat)2]– anion preferen-
ambient temperatures in 1 cm quartz cells. Mass spectra were re-
tially oxidizes a [Br4cat]2– ligand over CH3OH. Moreover,
corded at the Georgia Institute of Technology Bioanalytical Mass
the perbrominated catalyst system has its own drawbacks.
Specifically, the oxidizing [Br4cat]2– ligands in [ReV(O)- tions with a Micromass Quattro LC spectrometer. GC–MS was
Spectrometry Facility. ESI-MS was carried out with BnOH solu-
Eur. J. Inorg. Chem. 2012, 554–561
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
559

C. A. Lippert, K. Riener, J. D. Soper
FULL PAPER
performed with an Agilent 6890 GC equipped with an autosampler
and a Restek Rxi-5ms column (30 mϫ0.25 mm internal dimen-
sions, 0.25 μm film thickness). 1 μL injections were made at a 50:1
split ratio. The GC oven program consisted of a hold at 30 °C for
1 min, followed by a 15 °C min–1 ramp to 300 °C, and a hold at
300 °C for 11 min. The mass spectrometer used in tandem was a
Micromass AutoSpec electroionization (EI) detector.
scribed above for the stoichiometric variants, but the reaction mix-
tures were stirred under 1 atm dry air rather than N2.
Supporting Information (see footnote on the first page of this arti-
cle): 1H NMR spectra from a reaction of [ReVII(O)2(cat)2]– and
0.1 equiv. of [ReV(O)(cat)2]– with 6 equiv. BnOH at 25 °C in
[D4]methanol.
Methods and Materials: Anhydrous CH3CN and CH2Cl2 for air-
and moisture-sensitive manipulations were purchased from Sigma–
Aldrich and further dried by passage through columns of activated
alumina, degassed by at least three freeze–pump–thaw cycles, and
stored under N2 prior to use. CH3OH (anhydrous, 99.0%) was pur-
chased from Honeywell Burdick and Jackson. [D4]Methanol was
used as received from Cambridge Isotope Laboratories and stored
under a dry N2 atmosphere prior to use. (Et4N)[ReV(O)(cat)2],[18]
(Et4N)[ReVII(O)2(cat)2],[18] (Et4N)[ReV(O)(OPPh3)(Br4cat)2],[15] and
(Et4N)[ReVII(O)2(OPPh3)(Br4cat)2][9] were prepared by literature
methods. All characterization data matched those referenced. All
other reagents were purchased from Sigma–Aldrich and were used
as received.
Acknowledgments
We gratefully acknowledge financial support from a Defense Ad-
vanced Research Projects Agency (DARPA) Young Faculty Award
(Grant N6600 1-09-1-2094 to J. D. S.), the Fulbright Program
(K. R.), and the Georgia Institute of Technology. We thank David
Bostwick for mass spectrometry analyses.
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Reactions with Dioxygen in CH3OH: In a representative procedure,
a 1.0 cm quartz cuvette fitted with a Kontes high-vacuum Teflon
valve was charged with [ReV(O)(cat)2]– (2.0 mL, 0.90 mmol,
0.45 mm) in CH3OH and sealed under N2. The solution was de-
gassed on a high-vacuum line by three freeze–pump–thaw cycles
and an initial spectrum was acquired. The cuvette was then back-
filled with 1 atm of dry air, shaken vigorously, placed in a Peltier
UV/Vis cell holder with a magnetic stirrer, and stirred to maintain
a constant concentration of dissolved O2 throughout the reaction.
The reaction progress was monitored by UV/Vis spectroscopy
(200–900 nm) for 3000 min at 5 min intervals. For NMR kinetics
studies, a J. Young NMR tube with a Teflon screw cap was used
in place of the cuvette.
Determination of IB Yields with Varying Ratios of [ReVII(O)2-
(cat)2]–/[ReV(O)(cat)2]–: A 1.0 cm quartz cuvette fitted with a
Kontes high-vacuum Teflon valve was charged with freshly pre-
pared CH3OH solutions of [ReVII(O)2(cat)2]– and [ReV(O)(cat)2]–
to make the final concentrations 0.15 mm and 0.02–0.24 mm,
respectively. The cuvette was sealed under N2, shaken vigorously,
and placed in a Peltier UV/Vis cell holder. The reaction progress
was monitored by UV/Vis spectroscopy (300–1000 nm) at 2 min
intervals, and the yield of IB was determined by measurement of
the maximum absorbance at 600 nm (λmax for IB).
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Stoichiometric Alcohol Oxidations: In a representative procedure,
[ReVII(O)2(cat)2]– (4.0 mL, 48 mmol, 12.1 mm) in BnOH and
[ReV(O)(cat)2]– (1.0 mL, 5.0 mmol, 5 mm) in BnOH were combined
in a 20 mL scintillation vial, sealed under N2, and stirred at ambi-
ent temperature for 15 h. For GC–MS analysis a 1 mL aliquot was
transferred to a 2 mL GC autosampler vial and n-decane (1 μL)
was added to serve as an internal standard. The vial was sealed
under N2 with a Teflon-lined septum cap prior to GC–MS analysis
as described above. The organic products were quantified by com-
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derived from pure materials and by comparison to the detector
response for the n-decane internal standard. For NMR studies,
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[ReVII(O)2(cat)2]– (20 mm) and BnOH (100 mm) in a J. Young NMR
tube with a Teflon screw cap. The reaction was monitored over
48 h.
Catalytic Alcohol Oxidations: For catalytic alcohol oxidation ex-
periments, solutions were prepared and analyzed exactly as de-
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Eur. J. Inorg. Chem. 2012, 554–561

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Received: September 30, 2011
Published Online: November 24, 2011
Eur. J. Inorg. Chem. 2012, 554–561
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