Visible light generation of chromium(V)-oxo salen complexes and mechanistic insights into catalytic sulfide oxidation

Article abstract of DOI:10.1016/j.ica.2020.119681

Visible light irradiation of the photo-labile salen-chromium(III) chlorate or bromate precursors produced salen-chromium(V)-oxo complexes that were spectroscopically and kinetically indistinguishable from those formed by chemical oxidation of chromium(III) salens with PhI(OAc)₂. The photochemistry observed in this work is ascribed to the heterolytic cleavage of the O-X bond in the apical counterion that results in a two-electron oxidation of the metal to form the chromium(V)-oxo species. Second-order rate constants for oxidation reactions of 3 with organic substrates were determined under pseudo-first order condition, and particularly low level of reactivity for sulfide oxidations was observed. In this study, chromium(III) salen complexes effectively catalyzed the oxidation of aryl sulfides into sulfoxides with PhI(OAc)₂ in the presence of a small amount of water. The competition product studies with the Hammett correlation plot indicated that the observed chromium(V)-oxo species is not likely to serve as the major oxidant for the sulfide oxidations catalyzed by chromium(III) salens with PhI(OAc)₂.

Full text of DOI:10.1016/j.ica.2020.119681

InorganicaChimicaActa509(2020)119681
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Research paper
Visible light generation of chromium(V)-oxo salen complexes and
mechanistic insights into catalytic sulfide oxidation
Seth Klaine, Ngo Fung Lee, Angeline Dames, Rui Zhang⁎
Department of Chemistry, Western Kentucky University, Bowling Green, KY 42101-1079, USA
A R T I C L E I N F O
A B S T R A C T
Keywords:
Chromium(V)-oxo
Salen
Visible light
Oxidation
Kinetics
Visible light irradiation of the photo-labile salen-chromium(III) chlorate or bromate precursors produced salen-
chromium(V)-oxo complexes that were spectroscopically and kinetically indistinguishable from those formed by
chemical oxidation of chromium(III) salens with PhI(OAc)2. The photochemistry observed in this work is as-
cribed to the heterolytic cleavage of the O-X bond in the apical counterion that results in a two-electron oxidation
of the metal to form the chromium(V)-oxo species. Second-order rate constants for oxidation reactions of 3 with
organic substrates were determined under pseudo-first order condition, and particularly low level of reactivity
for sulfide oxidations was observed. In this study, chromium(III) salen complexes effectively catalyzed the
oxidation of aryl sulfides into sulfoxides with PhI(OAc)2 in the presence of a small amount of water. The
competition product studies with the Hammett correlation plot indicated that the observed chromium(V)-oxo
species is not likely to serve as the major oxidant for the sulfide oxidations catalyzed by chromium(III) salens
with PhI(OAc)2.
1. Introduction
widely applicable methods for the epoxidation of unfunctionalized
olefins [22,23].
In industry and laboratories, the catalytic oxidation of organic
compounds is one of most important processes conducted daily on a
large scale for the production of valuable chemicals, remediation of
pollutants, and the production of energy [1–4]. Meanwhile, oxidations
mediated by a variety of enzymes play a key role in the oxidative
transformation of endogenous and exogenous molecules in all forms of
life [5]. An ubiquitous type of monooxygenase is cytochrome P-450
enzymes (P450s) that contain a metalloporphyrin core and catalyze a
wide variety of oxidation reactions with exceptionally high reactivity
and selectivity [6,7]. Many synthetic metal complexes have been syn-
thesized to develop enzyme-like oxidation catalysts [8–13]. In this
context, metallosalen complexes (salen = N,N'-bis(salicylidene)ethyle-
nediamine) have received considerable attention in view of their en-
ormous utility of catalytic transformations [14–17]. With tetradentate-
binding motif, metallosalens have features in common with metallo-
porphyrins with respect to their catalytic properties [18]. Notably,
metal-salen complexes are more easily synthesized and manipulated to
create an asymmetric environment around the active metal site than
porphyrin analogues [18,19]. In the 1990s, the groups of Jacobsen and
Katsuki independently introduced chiral manganese–salen catalysts to
achieve highly enantioselective epoxidations [20,21]. To date, the Ja-
cobsen–Katsuki catalysts are accepted as one of the most successful and
Despite great success in catalytic transformations, mechanistic elu-
cidation of these salen systems have so far been hampered by the fact
that the catalytically active species appear only as fleeting putative
intermediates [18,24]. The mechanistic scheme adopted for oxygen
atom transfer to organic substrates by salen complexes is mainly based
on the isolation and characterization of a chromium(V)-oxo species
[14]. Limited evidence for the existence of most salen-metal-oxo species
is responsible for the lack of mechanistic insights [25,26]. In this re-
gard, we recently introduced visible light-induced ligand cleavage re-
actions that successfully generated a variety of metal-oxo species sup-
ported by porphyrin and corrole ligands [27–32]. Unlike the commonly
used chemical oxidations, the use of visible light (sunlight) rather than
a chemical reagent offers an ideal means to produce metal-oxo species
with much higher temporal resolution, and permits direct studies of
their oxidation reactions in real time [33–35]. In addition, kinetics of
oxidation reactions of the photo-generated transients of interest are not
convoluted with the kinetics of reactions that form the transients from
chemical methods [36]. With the aim to probe the nature of active
oxidizing species and elucidate mechanistic courses in metal-salen
catalysis, we have applied the promising photo-induced ligand cleavage
approaches to generate salen-metal-oxo intermediates. Herein, we re-
port our ongoing progress on visible light-generation of chromium(V)-
⁎ Corresponding author.
E-mail address: rui.zhang@wku.edu (R. Zhang).
https://doi.org/10.1016/j.ica.2020.119681
Received 28 February 2020; Received in revised form 9 April 2020; Accepted 15 April 2020
Availableonline18April2020
0020-1693/©2020ElsevierB.V.Allrightsreserved.

S. Klaine, et al.
InorganicaChimicaActa509(2020)119681
Scheme 1. Visible light-induced formation of salen-chromium(V)-oxo complexes.
oxo complexes bearing the well-known Jacobsen salen ligand and one
of its derivatives (Scheme 1). The results attest to the fact, for the first
time, that photochemical production of metal-oxo species is not ex-
clusive to porphyrin or corrole systems, and it can be achieved using
simple, easily constructed salen-based complexes. Meanwhile, we re-
port rate constants for the reactions of generated salen-chromium(V)-
oxo intermediates with organic substrates. The kinetic and competition
studies provide mechanistic insights into the identity and reactivity of
the active oxidant involved in the catalytic sulfide oxidations.
green, and accompanied by the formation of a broad peak from 550 to
800 nm. Following previous work, the same [CrV(salen)(O)] was also
produced by chemical oxidation of precursors 1 with PhI(OAc)2 (2.5–5
equiv.) as the sacrificial oxidant, and the time-resolved absorption
spectra exhibited the same characteristic broad peak.
2.3. Kinetic studies of chromium(V)-oxo salens
Reactions of high-valent salen-chromium(V)-oxo species with excess
amounts of organic substrates were conducted in solutions at
2. Experimental
23
2 °C. The approximate concentrations of 3 were estimated by
assuming 100% conversion of chromium(III) precursor in the photo-
chemical reactions. The rates of the reactions which represent the rates
of oxo group transfer from [CrV(salen)(O)] to substrate were monitored
by the decay of the broad absorption band of the oxo-species 3. The
kinetic traces at λmax of 680 nm displayed good pseudo-first-order be-
havior for at least four half-lives, and the data was solved to give
pseudo-first-order observed rate constants, kobs. Plots of these values
against the concentration of substrate were linear in all cases. The
second-order rate constants for reactions of the oxo species with the
organic substrates were solved according to Eq. (1), where k0 is a
background rate constant found in the absence of added substrate, kox is
the second-order rate constant for reaction with the substrate, and
[Sub] is the concentration of substrate. Most second-order rate con-
stants are averages of 2–3 determinations consisting of 3 independent
kinetic measurements. Errors in the rate constants were weighted and
are at 2σ.
2.1. Materials and instruments
All commercial reagents were of the best available purity and were
used as supplied unless otherwise specified. Acetonitrile was obtained
from Sigma Aldrich (HPLC grade) and use as such. All organic sub-
strates for kinetic studies were passed through a flash chromatography
column of active alumina (Grade I) before use. CrIII(salen)Cl (1a)
(salen = N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohex-
anediamino) was purchased from Sigma Aldrich and use without fur-
ther purification. Complex 1b (salen = N,N′-bis-(2-hydroxy-1-naph-
thalidene)-1,2-cyclohexanediamine) was prepared according to the
well-established procedure [37] and its purity was checked by UV–vis
and 1H NMR.
UV–Vis spectra were conducted on an Agilent 8454 diode array
spectrometer using standard 1.0-cm quartz cuvettes. Visible light was
produced from a SOLA SE II light engine (Lumencor) configured with a
liquid light guide (output power ranging from 6 to 120 W). Electrospray
ionization-mass spectroscopy (ESI-MS) data was collected using an
Agilent 500 LCMS Ion Trap System. Kinetic measurements were per-
formed on an Agilent 8454 diode array spectrophotometer using stan-
dard 1.0-cm quartz cuvettes.
kobs = k0 + kox[Sub]
(1)
2.4. General procedure for catalytic sulfoxidations
Unless otherwise indicated, all catalytic reactions were typically
carried out in a 10 mL reaction vial which was charged with the catalyst
(10 µmol, 2.0 mol%), sulfide substrate (0.5 mmol) in methanol (2 mL)
with a small amount of H2O (5.0 µL). PhI(OAc)2 (0.75 mmol) was then
added at 23 °C to trigger the reactions. Aliquots of the reaction solution
at constant time interval were diluted (> 200 times) and analyzed by
GC/MS to determine the formed products with an internal standard
(1,2,4-trichlorobenzene). The conversions were calculated based on the
substrate consumed and the selectivities based on the percentage ratios
of sulfoxides and sulfones, which were only oxidized products detected.
All reactions were run 2 to 3 times, and the data reported represent the
average of these reactions. Products including sulfoxides and sulfones
from the over oxidation were identified by GC–MS. Since the
2.2. General procedure for synthesis of chromium(V)-oxo salens
Axial ligand exchange of the salen complexes [CrIII(salen)Cl] (1)
with excess of Ag(ClO3) or Ag(BrO3) in CH3CN or CH2Cl2 gave the
corresponding chlorate or bromate complexes [CrIII(salen)(XO3)]
(X = Cl or Br) (2), which were photo-labile and subsequently used for
photochemical reactions. The solution of 2 with concentration of ca.
1.0 × 10−3 M was irradiated with visible light from a SOLA engine
(output power 120 W) at ambient temperature. The formation of
chromium(V)-oxo salen complexes (3) was complete in the range of 12
to 15 min, as monitored by UV–vis spectroscopy. The photo transfor-
mation is characterized by a distinct color changes from yellow to dark
2

S. Klaine, et al.
InorganicaChimicaActa509(2020)119681
sulfoxidation reactions were not affected by molecular oxygen, all the
reactions presented in Table 2 were performed in air under ambient
light.
formation of [CrV(salen)(O)](pyNO) with the well-defined broad band
extending beyond 800 nm in the visible absorption spectrum [14].
Control experiment showed that species 3a was not formed in the
absence of light. As expected, the transformation of 2a to 3a was more
rapid using more intensive visible light (120 W). The use of the non-
coordinating solvents such as CHCl3 gave similar results (data not
shown). However, no formation of 3a was observed in CH3OH solution.
Clearly, the relatively chlorate bound to chromium metal can be readily
disassociated by the strong coordinating solvent. As monitored by
UV–vis spectroscopy, we did not observe any significant photo-de-
gradation of the complexes in all studies under visible light irradiation.
Of note, the long tail band of photo-generated 3a was not significantly
changed when adding pyridine N-oxide as the additional donor ligand.
Visible light irradiation of salen-chromium(III) bromate also gave
the formation of species 3a in a faster rate (see Fig. 2A), implying a
more efficient photochemical process. In a fashion similar to that de-
scribed for the generation of 3a, 3b with a derived salen ligand (N,N′-
bis-(2-hydroxy-1-naphthalidene)-1,2-cyclohexanediamine) was also
formed by visible light photolysis of corresponding chlorate or bromate
precursors (Fig. 2B). Again, the spectral signature of product 3b was
further confirmed by forming same species with the characteristic
broad bands from 600 to 800 nm from the chemical oxidation of 1b by
PhI(OAc)2.
2.5. Competition and Hammett correlation studies
A CH3OH solution containing equal amounts of two substrates, e.g.
thioanisole (0.2 mmol) and substituted thioanisoles (0.2 mmol), chro-
mium(III) salen catalyst (10 µmol) and an internal standard of 1,2,4-
trichlorobenzene (0.1 mmol) was prepared (final volume = 2.0 mL).
PhI(OAc)2 (0.1 mmol) as the limiting reagent, was added and the
mixture was stirred at ambient temperature (23
2 °C) in the pre-
sence of a small amount of H2O (5.0 µL) for 10 to 20 min. Relative rate
ratios for catalytic oxidations were determined by GC based on the
amounts of sulfoxide products as measured against an internal stan-
dard. In this work, all the catalytic sulfoxidations proceeded with good
yields (> 95%). Thus, the ratio of product formation should reasonably
reflect the relative sulfide reactivity toward the salen-chromium(III)-
catalyzed oxidations.
3. Results and discussion
3.1. Visible light photolysis of [CrIII(salen)(ClO3)] and [CrIII(salen)(BrO3]
Photolysis of porphyrin-MnIII(ClO4) complexes was previously re-
ported to give porphyrin-MnV(O) species by heterolytic cleavage of an
O-Cl bond [33]. Recent studies showed that heme iron(III)–hy-
droperoxo complexes thermodynamically led through a heterolytic
OeO bond cleavage to a high-valent iron(IV)–oxo heme cation radical
intermediate [38]. Our photolysis studies of porphyrin–iron(III) bro-
mates also showed that the electron-releasing porphyrin system favored
a heterolysis of O-Br bond to give porphyrin-iron(IV)-oxo radical ca-
tions (compound I models) [29,39]. Thus, the generation of the chro-
mium(V)-oxo species 3 upon visible light irradiation of chlorate or
bromate 2 can be rationalized by photo-induced hetereolytic cleavage of
O-X (X = Cl or Br) bonds in the apical counterion, which results in two-
electron photo-oxidation reactions as expected. For comparison, we
found that photochemical cleavage of the bromate complexes was
considerably more efficient than cleavages of chlorate complex to
generate the salen-chromium(V)-oxo species under identical conditions,
similar to our previously reported work on the photochemical forma-
tion of trans-dioxoruthenium(VI) porphyrins [40].
As shown in Scheme 1, treatment of CrIII(salen)Cl (1a) with excess
Ag(ClO3) (10 equiv.) resulted in rapid exchange of the axial ligand to
form the corresponding chlorate CrIII(salen)(ClO3) (2a) with observa-
tion of AgCl precipitate. The formation of 2a was further indicated by
the UV–vis spectra with a blue-shifted Soret band at 422 nm (Fig. 1A).
Species 2a was highly photo-labile and thus, not isolated and im-
mediately used for photochemical reactions after preparation. Irradia-
tion of chlorate complex 2a in anaerobic CH3CN with visible light from
a SOLA engine (output power 60 W) resulted in formation of a new
species with a distinct color change. Over a period of 12 min., the
yellow species 2a was decayed and a dark-green species 3a was formed
(Fig. 1A), exhibiting a broad band ranging from 550 to 800 nm that is
characteristic for chromium(V)-oxo salen complexes [14]. The photo-
generated 3a was metastable and can be further characterized by ESI-
MS. As shown in the inset of Fig. 1A, the ESI-MS spectrum (positive
mode) exhibited a prominent peak at a mass-to-charge ratio (m/z) of
612, matching the molecular composition of [Cr(salen)(O)]+. Accord-
ingly, species 3a was assigned as [CrV(salen)(O)](ClO3) on the basis of
its distinct UV–vis absorption and ESI-MS. The spectra signature of the
chromium(V)-oxo salen was further confirmed by production of the
same species in the mixing chemical oxidation of 1a with a mild oxidant
of PhI(OAc)2 (Fig. 1B). It is noteworthy that the presence of donor li-
gands such as excess pyridine N-oxide (pyNO) in this study led to the
3.2. Kinetic studies of [CrV(salen)(O)] species
As expected, photo- and chemical-generated salen-chromium(V)-
oxo species 3 is reactive towards organic substrates, and its character-
istic broad peak with λmax at 680 nm undergoes exponential decay at a
Fig. 1. (A) UV–visible spectrum: [CrIII
(salen)Cl] (1a, dotted) and [CrIII(salen)
(ClO3)] (2a, dash), and [CrV(salen)(O)]
(ClO3) (3a, solid) formed upon visible light
photolysis of 2a in CH3CN; Inset showing
ESI-MS spectrum of photo-generated 3a in a
positive mode. (B) UV–visible spectrum: 1a
(dashed), 3a formed by oxidation of 1a with
PhI(OAc)2 (2.5 equiv.) in CH3CN, and the
adduct of 3a (solid) in the presence of pyr-
idine N-oxide (10 equiv.)
3

S. Klaine, et al.
InorganicaChimicaActa509(2020)119681
Fig. 2. (A) Time-resolved formation spectra of 3a (red line) following irradiation of [CrIII(salen)(BrO3)] (1.0 mM) over 5 min with visible light in anaerobic CH3CN at
23 °C; (B) Time-resolved spectra of the photo-generated 3b decaying in the absence of substrate in CH3CN over 10 min. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
Fig. 3. (A) Time-resolved spectra of the photo-generated 3a reacting with cyclohexene (0.15 M) in CH3CN over 10 min. (B) Kinetic plot of observed rate constants
versus concentration of cyclohexene. Inset shows the kinetic traces at 680 nm for the reactions of 3a with cyclohexene at different concentrations (0.15, 0.25, 0. 5 and
0.75 M from the top to bottom).
rate dependent on the nature and concentration of the organic sub-
strate, permitting direct kinetic studies of their oxidations. In the pre-
sence of organic substrate such as cyclohexene, the time-resolved
spectra show the clean conversion of 3a to regenerate the chromium
(III) species (1a) with a λmax of 420 nm (see Fig. 3A). In kinetic studies,
we monitored the absorbance in the Q band region at 680 nm that
decayed over the course of reaction (Inset in Fig. 3B). The traces were
fit to single-exponential decay, as expected for reactions under pseudo-
first-order conditions. When organic substrates such as cyclohexene
were present, the decay of photochemically generated species 3a ac-
celerated linearly with the substrate concentrations with a near-zero
intercept (Fig. 3B). The plot slope gave a second-order rate constant of
Table 1
Second-order rate constants for chromium(V)-oxo salens 3a.
Entry
Substrate
kox (M−1s−1) × 103
3a
3b
1
cyclohexene
8.7
0.9
3.0
3.0
0.02
0.08
0.02
0.04
0.02
35.0
70.0
78.5
58.5
1.5
6.0
10.0
6.0
diphenylmethane
1-phenylethanol
thioanisole
4-fluorothioanisole
4-chlorothioanisole
4-methylthioanisole
4-methoxylthioanisole
33.0
32.0
0.41
1.1
0.33
0.37
0.37
2
3
4
5
6
7
(8.7
0.9) × 10−3 M−1 s−1, almost identical to the kox for oxidant 3a
a
Generated either by photochemical or chemical methods in CH3CN at
produced from chemical oxidation by PhI(OAc)2. Rate constants for the
reactions of other organic substrates with generated oxo species 3 were
collected in the Table 1.
23
2 °C. Reported values are the average of 2–3 runs with a deviation of 2σ.
4

S. Klaine, et al.
InorganicaChimicaActa509(2020)119681
Table 2
Catalytic oxidation of thioanisoles by chromium(III) salens (1) and PhI(OAc)2.a
Entry
1
Catalyst
Substrate
Time (min)
30
Conv.b (%)
100
Product
Selectivity%b (sulfoxide : sulfone)
91:09
1a
2
1b
1a
1a
10
30
60
100
100
99
85:15
92:08
87:13
3c
4
5
6
7
1a
1a
1a
60
20
20
100
92
89:11
84:16
92:08
97
a
Unless otherwise specified, all reactions were performed in CH3OH (2 mL) at ca. 23 °C with 1.5 equiv. of PhI(OAc)2 (0.75 mmol), sulfide substrate (0.5 mmol),
2.0 mol% catalyst in the presence of H2O (5.0 µL). In all cases, only sulfoxides and small amounts of sulfone were detected by GC–MS analysis of the crude reaction
mixture.
b
Based on the substrate consumed and products ratios determined by GC–MS analysis on the crude reaction mixture after the reaction; material balance > 95%. c
Carried out in the presence of pyridine N-oxide.
Inspection of Table 1 revealed several noteworthy aspects of the
salen-chromium(V)-oxo kinetics. It is clear to note the difference in the
reactivity of two different [CrV(salen)(O)] complexes toward the same
substrate, i.e. the less sterically encumbered complex 3b reacted faster
with a given substrate than the complex 3a, apparently due to steric
effects. Although both complexes have essentially the similar square-
pyramidal configuration about the chromium(V) center, 3b carries a
more conjugated and planar naphthalene structure than 3a. Of note,
thioanisoles exhibited lower level of reactivity in comparison to hy-
drocarbons such cyclohexene and diphenylmethane (Ph2CH2). In gen-
eral, a remarkable rate acceleration of sulfide oxidation versus hydro-
carbons by electrophilic metal-oxo species is expected in view of the
enhanced nucleophilicity and easy access of sulfur for oxidation. For
example, our previously studies with iron(IV)-oxo porphyrins [41] and
manganese(V)-oxo corroles [30] showed that sulfoxidation reactions
were 3 to 4 orders of magnitude faster than those alkene and activated
CeH bonds oxidations by the same oxo species. However, the k2 values
determined in this work did not show such rate acceleration of sulfide
oxidation by salen-chromium(V)-oxo species, reflecting the operation of
a different mechanism for sulfide oxidations. The small rate constants
observed in this study suggested the sulfide oxidation did not proceed
through nucleophilic attack of sulfide on the chromium center, as it is
difficult to generate the sulfoxide through that formation [42]. Indeed,
the strong coordination of the sulfides to the metal center suppressed
the reactivity of electrophilic salen-chromium(V)-oxo species.
thioanisoles. The kinetic data from the reaction of para-substituted
thioanisoles with 3a permitted Hammett analysis. However, there is no
linear correlation between the logkrel [krel = k(substituted thioanisole)/
k(thioanisole)] and σ or σ+, implying that no appreciable charge de-
veloped on the sulfur during the oxidation process by chromium(V)-oxo
species. Of note, non-linear Hammett plots have also been observed for
the reactions of para-substituted thioanisoles with trans-dioxor-
uthenium(IV) and iron(IV)-oxo porphyrins [41,43], which were as-
cribed to substantial development of radical character in the transition
states.
3.3. Catalytic and competition studies of sulfoxidation reactions
The selective oxidation of organic sulfide to sulfoxide (sulfoxida-
tion) is of importance particularly in the preparation of optically active
sulfoxides for a variety of medicinal and pharmaceutical compounds
[44]. To date, various catalysts including polyoxometalates have been
reported in the literature for the catalytic oxidation of sulfides com-
monly with H2O2 [45–47]. Recently we and others have found that PhI
(OAc)2 with a small amount of water is an efficient oxygen source for
metalloporphyrins and metallocorroles catalyzed oxidations [48–52].
According to these studies, the trace amount of water can induce the
steady and slow formation of more oxidizing PhIO from the PhI(OAc)2,
at the same time, allow for the release of the stable AcOH (detected by
GC) instead of anhydride Ac2O under anhydrous conditions, thus, re-
sulting in an accelerated reaction.
For a given oxo species (3a), we observed a small kinetic effect that
the substituent at para-phenyl position of substrates has on the rates of
reactions at room temperature, and the second-order rate constants for
the oxidation of para substituted thioanisoles typically showed little
variation (Table 1, entries 4–7), except that only 4-fluorothioanisole
showed increased reactivity in comparison to other substituted
In this study, we also found that chromium(III) salens catalyzed
highly efficient oxidation of aryl sulfides. Under previously established
conditions in methanol solutions [53], thioanisoles can be efficiently
oxidized with quantitative conversions and the good chemoselectivity
for sulfoxides versus sulfones (Table 2, entry1). In comparison, 1b
5

S. Klaine, et al.
InorganicaChimicaActa509(2020)119681
Table 3
the directly observed chromium(V)-oxo species 3 is not the active
oxidant under catalytic turnover conditions [55]. Reaction of the salen-
chromium(III) catalysts with the sacrificial oxidant PhI(OAc)2 possibly
formed a more reactive intermediate to oxidize the sulfide substrate
through a different mechanism. Alternatively, the chromium(V)-oxo
salen may function as Lewis acid to activate the PhI(OAc)2 in a similar
way as the stable corralizine-manganese(V)-oxo served in the catalytic
sulfoxidation with PhIO [56]. A clear mechanism of catalytic sulfox-
idation by chromium salen with PhI(OAc)2 requires the definitive de-
tection and characterization of the elusive oxidant that is the direction
for future studies.
Relative rate constants from kinetic studies and competition catalytic oxida-
tions.a
b
Substrates
Method
krel
p-F-PhSMe/PhSMe
p-Cl-PhSMe/PhSMe
p-Me-PhSMe/PhSMe
p-MeO-PhSMe/PhSMe
kinetic results
PhI(OAc)2
kinetic results
PhI(OAc)2
kinetic results
PhI(OAc)2
kinetic results
PhI(OAc)2
2.68
0.40
0.80
0.23
0.90
2.36
0.90
6.30
a
4. Conclusion
A reaction solution containing equal amounts of two substrates, e.g.,
thioanisole (0.2 mmol) and substituted thioanisole (0.2 mmol), manganese(III)
porphyrin catalyst (10 µmol) and an internal standard of 1,2,4-trichlorobenzene
was prepared in CH3OH (2 mL). Iodobenzene diacetate PhI(OAc)2 (0.1 mmol)
was added with 5.0 µL H2O, and the mixture was stirred for ca. 10 to 15 min at
In conclusion, we report here a new and facile photochemical entry
to produce and study high-valent salen-chromium(V)-oxo derivatives
by visible light irradiation of the corresponding chromium(III) chlorate
or bromate complexes. The photochemistry observed in this work is
rationalized by the heterolytic cleavage of the O-X bond in the apical
counterion that results in a two-electron oxidation to afford the chro-
mium(V)-oxo species. We have confirmed the photo-generated [CrV
(salen)(O)] is spectroscopically and kinetically indistinguishable from
the species formed by the chemical oxidation of chromium(III) pre-
cursors with PhI(OAc)2. The small second-order rate constants for sul-
fide oxidations along with competition product studies in this work
suggested that the observed chromium(V)-oxo species in kinetic studies
is unlikely to serve as the major oxidant for the catalytic sulfide oxi-
dations by chromium(III) salen catalysts with PhI(OAc)2. On the basis
of these findings, we anticipate that analogous salen-metal-oxo species
such as manganese(V)-oxo salens could be photochemically generated
and kinetically studied in more important alkene expoxidation and
CeH bond oxidation, which is currently under investigation in our la-
boratory.
23
2 °C.
b
Relative ratios of absolute rate constants from kinetic results with chro-
mium(V)-oxo salen complex (3a) and for competitive oxidations with the
chromium(III) salen catalyst (1a) at ambient temperature.
showed higher catalytic activity albeit with reduced selectivity for
sulfoxide under identical conditions (entry 2). Addition of pyridine N-
oxide (10 equiv. to the catalyst) showed no appreciable effect on the
reactivity and selectivity of the products (entry 3). Control experiments
showed that no oxidized products (< 1% by GC) was formed in the
presence of PhI(OAc)2 and water without catalyst. Of note, use of other
solvents such as CH3CN, CHCl3 and CH2Cl2 resulted in a reduced re-
activity in comparison to MeOH (data not shown here). Under the usual
catalytic conditions, monitoring reaction by UV–vis spectroscopy before
and after reactions indicated that no significant degradation of the salen
catalysts was found after catalytic reactions. Similar to our previous
studies [49,51], the excellent catalytic activities for sulfide oxidations is
rationalized in part by the enhanced stability of chromium salens
against the catalytic degradation by using the mild oxygen source. In a
sharp contrast, a sluggish reactivity (< 10% convn) was observed after
12 h for the unfunctionalized alkene such as cyclohexene or styrene
(data not shown).
Conflicts of interest
There are no conflicts to declare.
CRediT authorship contribution statement
In the catalytic oxidation of substituted thioanisoles, we observed a
significant effect of substituents in the aryl ring of substrates. As evident
in the courses of substituted thioanisole oxidations, the introduction of
electron-donating groups like methyl or methoxy groups resulted in
increased reactivities compared to thioanisole, while reduced reactiv-
ities were observed in the presence of electron-demanding groups like
Cl or F groups (entries 4–7 in Table 2). To assess the nature of the active
oxidant involved in the catalytic reactions, the competitive studies of
sulfoxidations catalyzed by the chromium(III) salen (1a) with PhI
(OAc)2 as sacrificial oxygen source were conducted as described in
experimental section. Notably, the log krel [krel = k(substituted thioa-
nisole)/k(thioanisole)] versus Hammett substituent constant (σp) gave
an excellent linear correlation (R = 0.99) for the catalytic oxidation of
substituted sulfides under competitive conditions. The slope (ρ) of the
plot is –2.73, which indicates transition states for rate-limiting steps of
sulfide oxidation involves a significant development of positive charge.
As described early in Fig. 1B, PhI(OAc)2 was able to oxidize the
chromium(III) complexes (1) to produce salen-chromium(V)-oxo 3,
which then reacted with substrate and decayed back to chromium(III)
product. Catalytic aerobic oxidation that relies on a corrole-based
chromium(V)-oxo/chromium(III) cycle was also known [54]. However,
the directly observed salen-chromium(V)-oxo species 3 in above kinetic
studies is not necessarily the active oxidant under catalytic turnover
conditions. As evident in Table 3, the ratios of absolute rate constants
found in direct kinetic studies differed dramatically from the oxidation
ratios for competition oxidation reactions of the two substrates under
turnover conditions. The obvious explanation for this behavior is that
Seth Klaine: Investigation, Methodology, Validation. Ngo Fung
Lee: Methodology, Investigation. Angeline Dames: Investigation. Rui
Zhang: Conceptualization, Project administration, Supervision,
Funding acquisition, Writing - review & editing.
Acknowledgments
We greatly acknowledge the National Science Foundation (CHE
1764315) for support of this research. S. Klaine is thankful to the WKU
Office of Research and Graduate Studies for awarding internal grants
(FUSE and GSRG).
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7