Photocatalytic and aerobic oxidation of saturated alkanes by a neutral luminescent trans-dioxoosmium(vi) complex [OsO2(CN)2(dpphen)]

Article abstract of DOI:10.1039/a700702g

A neutral trans-dioxoosmium(vi) complex [OsO₂(CN)₂(dp-phen)] (dpphen = 4,7-diphenyl-1,10-phenanthroline) is prepared and characterized by X-ray crystal analysis; the excited state is long-lived, emissive and oxidizing [E°(Os^VI-V) = 2.15 V vs. NHE; lifetime = 0.95 μs; quantum yield = 5 × 10^-4; λ_em(max.) = 650 nm]; under UV-VIS irradiation and in the presence of dioxygen, cyclohexane is oxidized to cyclohexanol and cyclohexanone with a turnover number of 16; secondary C-H bonds of propane and primary C-H bonds of n-hexane are also oxidized.

Full text of DOI:10.1039/a700702g

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Photocatalytic and aerobic oxidation of saturated alkanes by a neutral
luminescent trans-dioxoosmium(vi) complex [OsO2(CN)2(dpphen)]
Jack Y. K. Cheng,a Kung-Kai Cheung,a Chi-Ming Che*a and Tai-Chu Laub
a Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong
b Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong
A neutral trans-dioxoosmium(VI) complex [OsO2(CN)2(dp-
phen)] (dpphen 4,7-diphenyl-1,10-phenanthroline) is
and in solution at room temp. Emission occurs at 635 nm with
=
an excited state lifetime of 0.95 ms in MeCN. The E0–0 value,
estimated from the 77 K solid-state emission spectrum is 2.11
eV and the E°(OsVI–V) by cyclic voltammetry in MeCN is 0.04
V vs. NHE. Using the equation, E°(OsVI*–V) = E°(OsVI–V) +
E0–0, the excited state is oxidizing with E°(OsVI*–V) = 2.15 V
vs. NHE. Upon excitation with UV–VIS light, 1 can oxidize
saturated hydrocarbons such as cyclohexane, n-hexane, pro-
pane, adamantane and cumene. This features the first well
defined metal–oxo complexes capable of oxidizing such
unreactive C–H bonds. Results of photoreactions undertaken in
MeCN or CH2Cl2 are summarized in Table 1. The organic
products were either alcohols, ketones or aldehydes. The
product yield for adamantane oxidation is low, presumably due
to competitive secondary photochemical oxidation of the
solvent by the excited osmium–oxo complex, the concentration
of adamantane in CH2Cl2–MeCN being limited by its solubility.
It is striking to find that the secondary C–H bonds of propane
and primary C–H bonds of n-hexane could also be oxidized.
Importantly, the oxidation of cyclohexane and n-hexane became
catalytic in the presence of air. For cyclohexane oxidation, 16
turnovers were found after 12 h of reaction. The excited
osmium–oxo species in this work has a significantly different
reactivity from other highly oxidizing metal–oxo complexes. It
is known that oxidation of adamantane by highly oxidizing
RuNO complexes takes place almost exclusively at the tertiary
position.8,9 In this work, both secondary and tertiary C–H bonds
of adamantane were photochemically oxidized by 1. Fur-
prepared and characterized by X-ray crystal analysis; the
excited state is long-lived, emissive and oxidizing
[E°(OsVI–V) = 2.15 V vs. NHE; lifetime = 0.95 ms; quantum
yield = 5 3 1024; lem(max.) = 650 nm]; under UV–VIS
irradiation and in the presence of dioxygen, cyclohexane is
oxidized to cyclohexanol and cyclohexanone with a turnover
number of 16; secondary C–H bonds of propane and primary
C–H bonds of n-hexane are also oxidized.
The search for new metal catalysts for aerobic oxidation of
saturated alkanes has continued to receive general interest.1 One
approach to this area is to develop highly oxidizing metal–oxo
complexes through irradiation with UV light as represented in
Scheme 1.2,3
hn
O
VI
R
R
H
Os
O
O2
IV
Os
OH
O
Scheme 1
Previous studies revealed that trans-[OsVI(tmc)O2]2+ (tmc =
1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) is ca-
pable of oxidizing a wide variety of organic substrates in
acetonitrile upon UV–VIS irradiation.2,4 However, the photo-
reduced product trans-[OsII(tmc)(MeCN)2]2+ is stable with
regard to aerobic oxidation back to the starting [OsVI-
(tmc)O2]2+.5 Herein is described a neutral trans-dioxo-
C(20)
C(21)
C(22)
C(19)
C(17)
C(18)
osmium(vi) complex [OsO2(CN)2(dpphen)] 1 (dpphen
=
C(16)
4,7-diphenyl-1,10-phenanthroline) which can mediate photo-
catalytic oxidation of saturated alkanes in the presence of
dioxygen.
C(14)
C(13)
C(15)
C(23)
C(10)
C(11)
C(25)
C(24)
Addition of dpphen in CH2Cl2 to a freshly prepared aqueous
solution of K2[OsO2(OH)2(CN)2]6 and in the presence of
NBu4Cl at room temp. gave complex 1 in 85% yield.† A
perspective view of the molecule is shown in Fig. 1.‡ Similarly
to most octahedral trans-dioxoosmium(vi) complexes,2d the
OsO2 moiety is close to linear with the average Os–O distance
and ONOsNO angle being 1.722(6) Å and 172.6(2)°, respec-
tively.
C(9)
N(4)
C(12)
C(5)
O(2)
C(26)
C(6)
N(3)
Os
C(1)
C(2)
N(1)
C(8)
C(7)
C(4)
C(3)
O(1)
Complex 1 is soluble in acetonitrile, dichloromethane and
benzene. In MeCN, it shows an intense high energy absorption
at l = 290 nm (e = 35840 dm3 mol21 cm21) and a shoulder at
360 (970). Compared with trans-[OsVI(tmc)O2]2+, it has a much
higher absorptivity in the UV region and this is attributed to the
mixing of intraligand p–p* and spin-allowed pp(O22)–
dp*(OsVI) charge-transfer transitions.7 In this regard, dpphen
serves as an antenna for UV light. As with other trans-
dioxoosmium(vi) complexes,2 1 is emissive in the solid state
N(2)
Fig. 1 Perspective view of 1 (50% thermal ellipsoids): Os–O(1) 1.721(5),
Os–O(2) 1.723(6), Os–N(3) 2.127(7), Os–N(4) 2.157(6), Os–C(1) 2.02(1),
Os–C(2) 2.053(8); O(1)–Os–O(2) 172.6(2), O(1)–Os–N(3) 87.0(2),
O(1)–Os–C(1) 92.7(3), O(2)–Os–N(3) 87.5(3), O(2)–Os–C(1) 94.4(4),
N(3)–Os–N(4) 77.5(2), N(3)–Os–C(1) 176.3(3), N(3)–Os–C(2) 95.3(3),
C(1)–Os–C(2) 88.4(3)
Chem. Commun., 1997
1443

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Table 1 Photochemical oxidations of various organic substrates by 1a
Yield (%)
(degassed)
Yield (%)
(aerobic)
Substrate
Solvent
Product(s)
n-Hexane
MeCN (5 ml)–
CH2Cl2 (2 ml)
Hexan-3-one
Hexan-2-one
Hexan-3-ol
Hexan-2-ol
Hexaldehyde
Hexan-1-ol
26
32
5
160
195
113
171
10
®
©
0.5
20
Cyclohexane
Cyclohexane
Propane
MeCN (5 ml)
Cyclohexanol
Cyclohexanone
4
35
227b
225
MeCN (5 ml)–
CH2Cl2 (2 ml)
Cyclohexanol
Cyclohexanone
6
30
765c
800
MeCN (4 ml)–
propane (2 atm) Propan-2-ol
Propanone
16.4
7.3
Cumene
CH2Cl2 (5 ml)
2-Phenylpropan-2-ol
Acetophenone
115
77
Adamantaned
CH2Cl2 (8 ml)–
MeCN (2 ml)
1-Adamantanol
2-Adamantanol
10.6
10.2
a Complex 1 (30 mg for degassed and 10 mg for aerobic) was used with 5 ml of organic substrate; reaction time = 12 h. Yield based on 1 by GC analysis
with an internal standard. b The yield for cyclohexanol and cyclohexanone after 2 days was 396 and 370%, respectively. c 2 atm O2. d 0.2 g of adamantane
used.
was used for data collection on a Rigaku AFC7R diffractometer with
thermore, a small primary kinetic isotope effect (kH/kD) of ca.
graphite monochromatized Mo-Ka radiation (l = 0.71073 Å) using w–2q
2.2 was found for cyclohexane oxidation,which is smaller than
that of 5.3 for [RuVL(O)]2+ {HL = [2-hydroxy-2-(2-pyr-
idyl)ethyl]bis[2-(2-pyridylethyl)]amine}.10 The small kinetic
isotopic effect and the lower tertiary C–H/secondary C–H
selectivity suggests that the excited OsNO species could be
regarded as a highly reactive ‘ligated oxygen atom’ species and
Sawyer used the term ‘oxene’ to denote such a species.11 A trace
amount of cyclohexyl chloride was detected in the photo-
chemical oxidation of cyclohexane in the presence of CCl4.
An orange–brown solution was usually obtained after the
photochemical reactions of 1 in MeCN, which slowly re-
oxidized back to 1 by air. Interestingly, it gave an intense orange
emission at 590 nm, the intensity of which is 20 times higher
than the emission of 1 recorded under similar conditions. Our
previous study revealed that [OsII(CN)2(X)2L] complexes are
highly emissive (quantum yield = 0.4–0.06) at similar energy
(L = dpphen; lmax/nm, X = Me2SO, 625; PPh3, 660).12 We
tentatively assign the photoproduct as [OsII(CN)2(dpphen)-
(MeCN)2]. It is likely that the coordinated MeCN in
[OsII(CN)2(dpphen)(MeCN)2] is more labile than in trans-
[OsII(tmc)(MeCN)2]2+, thus accounting the observed aerobic
reoxidation of the former back to the starting trans-dioxoos-
mium(vi) complex. For the aerobic photochemical oxidations,
there could be other reactive pathway(s) besides the one by the
excited osmium–oxo species, since the cyclohexanone/cyclo-
hexanol ratio for the aerobic and anaerobic oxidation of
cyclohexane are different (1:1 cf. 10:1).
scans with w-scan angle (0.63 + 0.35 tanq)° at a scan speed of 4.0 min21
.
4452 reflections were uniquely measured, of which 2307 reflections with I
> 1.5s(I) were considered observed and used in the structural analysis. A
crystallographic asymmetric unit consists of one molecule and all 33 non-H
atoms were refined anisotropically. 16 H atoms at calculated positions with
thermal parameters equal to 1.3 times that of the attached C atoms were not
refined. Convergence for 298 variable parameters by least-squares refine-
ment on F with w = 4 Fo2/s2 (Fo2), where s2 (Fo2) = [s2(I) + (0.018Fo2)2]
for 2307 reflections with I > 1.5s(I) was reached at R = 0.035 and Rw
0.030 with a goodness-of-fit of 1.32 (D/smax = 0.01). CCDC 182/509.
=
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Footnotes
* E-mail: cmche@hkucc.hku.uk
† Proton NMR (CDCl3): d 10.17 (d, 2), 8.24 (s, 2), 8.10 (d, 2), 7.69–7.52 (m,
5). IR (Nujol): n (OsNO) 856.1 cm21. UV–VIS: l/nm (e/dm3 mol21 cm21),
290 (35835), 330(sh) (8492), 367(sh) (966).
‡ Crystal data: C26H16N4O2Os, Mr = 606.64, orthorhombic, space group
Pbca (no. 61), a = 27.385(4), b = 13.025(2), c = 12.635(2) Å, U =
4508(1) Å3, Z = 8, Dc = 1.788 g cm23, m(Mo-Ka) = 56.86 cm21, F(000)
= 2336, T = 301 K. A brown crystal of dimensions 0.20 3 0.15 3 0.30 mm
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11 D. T. Sawyer, Comments Inorg. Chem., 1990, 10, 129.
12 J. Y. K. Cheng, K. K. Cheung and C. M. Che, J. Chem. Soc., Chem.
Commun., 1997, 6, 623.
Received in Cambridge, UK, 30th January, 1997; 7/00702G
1444
Chem. Commun., 1997