Angewandte
Communications
Chemie
OsIVO to OsIII and OsII species, but the exact composition of
these species are uncertain.
Unlike the corresponding ruthenium complexes, OsVO
does not oxidize water either stoichiometrically or catalyti-
cally using CAN. However, OsVO readily oxidizes various
organic substrates in CH3CN, and reacts cleanly and rapidly
with PPh3, as monitored by UV/Vis spectrophotometry
(Figure 3). The final spectrum indicates quantitative forma-
tion of OsIII. PPh3O could be detected by GC/MS and ESI/MS
[as (PPh3O + H)+, m/z = 279]. When 18O-enriched OsVO was
used, PPh318O could be detected (see Figure S7). These results
demonstrate O-atom transfer from OsVO to PPh3. The
reaction is first-order in [OsVO] and [PPh3], and the second-
order rate constant k2(PPh3) = (1.08 Æ 0.02) 103 mÀ1 sÀ1 at
298.0 K. A similar O-atom-transfer reaction occurs between
OsVO and thioanisole, and k2(PhSCH3) was found to be
(9.29 Æ 0.13) 10À3 mÀ1 sÀ1 at 298.0 K (see Figures S8 and S9).
OsVO also undergoes O-atom transfer to cyclooctene to give
cyclooctene oxide (yield 40%) with k2(C8H8) = (1.14 Æ 0.05)
10À4 mÀ1 sÀ1 (see Figure S10). These results show that OsVO
can react by a two-electron O-atom transfer pathway, which
bypasses the unstable OsIV state.
V
À
Figure 5. Plot of log k’2 vs. C H BDEs for the reaction between Os O
and alkylaromatics in CH3CN at 298.0 K. Slope=-(4.26Æ0.26)10À1
y-intercept=(3.25Æ0.21)101; r2 =0.9776.
;
alkylaromatic compounds is linear (slope = 0.43; Figure 5),
and consistent with a mechanism involving initial rate-limit-
ing H-atom transfer (HAT) from the alkylaromatic substrate
V
V
IV
À
to Os O (Os O + R H!Os OH + RC), with subsequent
rapid O-rebound (OsIVOH + RC!OsIII + ROH), as observed
À
in C H activation by cytochrome P450 and other metal oxo
species.[16] The OsIII product could be detected by ESI/MS
(see Figure S13). The product yields and rates were not
affected by the presence of air or a radical scavenger such as
BrCCl3, thus indicating that O-rebound to the carbon radicals
is very efficient (see Table S5). The HAT mechanism is
further supported by the observed large deuterium isotope
effects in the oxidation of [D2]DHA, [D10]ethylbenzene, and
[D8]toluene, with kH/kD = 5.6 (Figure 4b), 10 (see Figure S11),
and 9.1 (see Figure S12), respectively. Similar linear correla-
OsVO also reacts readily with various alkylaromatic
compounds (RH) at ambient conditions. Oxidation of 1,10-
dihydroanthracene (DHA) produces anthracene in 88%
yield, while oxidation of cumene and diphenylmethane give
the alcohol (44% yield) and ketone (76% yield), respectively
(see Table S4). The yields are based on OsVO functioning as
a two-electron oxidant. However OsVO does not react with
alkanes. Kinetic studies have been carried out with a variety
of alkylaromatic substrates (Figure 4; see Figures S11 and
S12). The reactions are first-order in [OsVO] and [RH]. The
second-order rate constants span over six orders of magni-
À
tions between logk’2 and C H BDE were also observed in the
oxidation of alkylaromatics by other metal oxo species such as
[Ru(O)(bpy)2py]2+[17] and [Ru(O)2(N2O2)]2+.[18]
In conclusion, we have reported the first example of
tude. Xanthene, which has the smallest a C H bond
a
group 8 seven-coordinate oxo species, [OsV(O)(qpy)-
À
dissociation energy (BDE) of 75.5 kcalmolÀ1, is the most
reactive substrate [k2 = (3.65 Æ 0.14) mÀ1 sÀ1 at 298.0 K], while
toluene, with the largest BDE of 89.8 kcalmolÀ1, is least
reactive [k2 = (1.05 Æ 0.01) 10À5 mÀ1 sÀ1]. The plot of logk’2
(pic)Cl]2+. This complex readily undergoes O-atom transfer
to phosphine and thioanisole, and H-atom transfer to
alkylaromatics. Although a number of osmium(VI) dioxo
species, such as [Os(O)2(TMC)]2+ (TMC = 1,4,8,11-tetra-
methyl-1,4,8,11-tetraazacyclotetradecane)[19] and trans-[OsVI-
(O)2(4,4’-Me2bipy)(CN)2] (4,4’-Me2bipy = 4,4’-
À
(rate constant per active H) versus C H BDE of the
dimethyl-2,2’-bipyridine)[20] are strong photo-
oxidants, OsVO can thermally abstract H atoms
from alkylaromatics with
À1
À
C H BDE as high as 90 kcalmol , which to the
best of our knowledge has not been reported for
other osmium oxo species. Our work suggests
that highly active oxidants may be designed
based on group 8 seven-coordinate metal-oxo
species.
Acknowledgements
Figure 4. a) Spectral changes at 20 s intervals for the reaction between OsVO
(1.7110À4 m) and DHA (5.1510À3 m) in CH3CN at 298.0 K. Inset shows the
corresponding absorbance-time traces at 357 and 509 nm. b) Plot of kobs vs. [DHA] for
the reaction between OsVO and DHA in CH3CN at 298.0 K [for DHA (solid circle):
slope=(2.23Æ0.07); y-intercept=(4.59Æ4.50)10À4; r2 =0.9965. For [D4]DHA (open
circle): slope=(3.95Æ0.31)10À1; y-intercept=(2.68Æ3.43)10À4; r2 =0.9819].
KIE=(5.6Æ1).
The work described in this paper was supported
by Hong Kong University Grants Committee
(AoE/P-03-08), the Research Grants Council of
Hong Kong (CityU 101811), and the Shenzhen
Science and Technology Research Grant
(JCYJ20120613115247045).
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Angew. Chem. Int. Ed. 2016, 55, 288 –291